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ISBN 978-952-62-2546-3 (Paperback)ISBN 978-952-62-2547-0 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2020

C 739

Panpan Li

DEEP EUTECTIC SOLVENTS IN THE PRODUCTION OF WOOD-BASED NANOMATERIALS

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF TECHNOLOGY

C 739

AC

TAPanp

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C739etukansi.fm Page 1 Tuesday, February 18, 2020 10:58 AM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 7 3 9

PANPAN LI

DEEP EUTECTIC SOLVENTS IN THE PRODUCTION OF WOOD-BASED NANOMATERIALS

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the Arina auditorium (TA105), Linnanmaa, on13 March 2020, at 12 noon

UNIVERSITY OF OULU, OULU 2020

Copyright © 2020Acta Univ. Oul. C 739, 2020

Supervised byAssociate Professor Henrikki LiimatainenDoctor Juho SirviöAssociate Professor Antti Haapala

Reviewed byProfessor Wolfgang Falk LiebnerDoctor Alistair William Thomas King

ISBN 978-952-62-2546-3 (Paperback)ISBN 978-952-62-2547-0 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

PUNAMUSTATAMPERE 2020

OpponentAssistant Professor Tiina Nypelö

Li, Panpan, Deep eutectic solvents in the production of wood-based nanomaterials. University of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 739, 2020University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Wood-based nanomaterials (WNMs) are renewable, biodegradable, and inexpensive greenalternatives for many current materials. WNMs have especially drawn significant interest due totheir potential for numerous high-end applications and replacement of petroleum-based products.The main obstacle in the industrial up-scaling of WNM production is the high energy consumptionduring the process of mechanical nanofibrillation. Chemical pretreatment methods are seen asbeneficial in decreasing energy demand and thus have been extensively investigated.

Deep eutectic solvents (DESs) are a novel and versatile class of systems that can be used assolvents, reagents, and catalysts in many fields due to their availability, low toxicity, typicallygreen origin, and relatively low cost. DESs can efficiently interact with biomass and are thereforeone of the most promising solvents in sustainable WNM production.

In this thesis work, three DES systems were designed and applied for the production of WNMs.DES pretreatments enhanced WNM production significantly and resulted in high-quality andstable (without sedimentation) WNM suspensions. DESs were used as a non-reactive medium thatsolely swells the fiber structure without deteriorating its degree of polymerization. In addition,reactive DESs were also developed to produce functionalized WNMs with high anionic or cationiccharge densities. The fabricated WNMs had a well-individualized structure and formedhomogenous films with high mechanical strengths. Furthermore, WNMs containing lignin orcombined with tannin extract resulted in biohybrid films with improved UV-shielding and anti-oxidative properties, while the films obtained from anionic WNM possessed some flame-retardancy. The results indicate that DESs are an up-and-coming pretreatment approach forproducing versatile WNMs with highly tailorable characteristics.

Keywords: films, functional biomaterials, hydrogen bonding, nanocellulose, nanopaper,pretreatment, urea

Li, Panpan, Syväeutektiset liuottimet puupohjaisten nanomateriaalien valmistuksessa. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 739, 2020Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Puupohjaiset nanomateriaalit ovat uusiutuvia, biohajoavia ja edullisia vihreitä vaihtoehtojamonille nykyisille materiaaleille. Ne ovat kiinnostavia erityisesti korkean jalostusasteen tuotteis-sa ja öljypohjaisten tuotteiden korvaamisessa. Yksi suurimmista haasteista näiden nanomateriaa-lien teolliselle valmistukselle on mekaanisen nanofibrilloimisen korkea energiankulutus. Erilai-sia kemiallisia esikäsittelymenetelmiä pidetäänkin tehokkaana tapana laskea valmistuksen ener-giankulutusta ja niitä on siksi tutkittu laajasti.

Syväeutektiset liuottimet (DES) ovat verrattain uusi ja monipuolinen kemikaaliryhmä, joitavoidaan käyttää liuottimina, reagensseina ja katalyytteinä eri kemianaloilla. Ne ovat helpostisaatavilla, myrkyttöminä, usein vihreitä alkuperältään, ja myös melko edullisia. DES:it vuoro-vaikuttavat tehokkaasti biomassan komponenttien kanssa ja ovat siksi yksi lupaavimmista liuot-timista puupohjaisten nanomateriaalien valmistukseen.

Tässä väitöstyössä tutkittiin kolmea erilaista DES-systeemiä puupohjaisten nanomateriaalienvalmistuksessa. DES-esikäsittelyt tehostivat valmistusprosessia merkittävästi ja tuottivat korkea-laatuisia ja stabiileita suspensioita, joissa nanopartikkelit eivät sedimentoidu. DES-liuottimiakäytettiin ei-reaktiivisina väliaineina, jotka turvottivat kuitumatriisin niiden polymeroitumisas-teen laskua. Lisäksi kehitettiin reaktiivisia DES-systeemejä tuottamaan funktionalisoituja nano-materiaaleja, joilla oli korkea anioninen tai kationinen pintavaraus. Valmistetut nanomateriaalitolivat hyvälaatuisia ja niistä valmistettiin filmejä, joilla oli hyvät mekaaniset lujuusominaisuu-det. Yhdistämällä puupohjaisia nanomateriaalit ligniinin tai tanniiniuutteen kanssa tuotettiin UV-suojaavia ja hapettumista estäviä biohybridifilmejä, kun taas anionisilla nanomateriaaleista val-mistetuilla filmeillä oli palonsuojaominaisuuksia. Saadut tulokset osoittavat, että syväeutektisetliuottimet ovat lupaavia esikäsittelykemikaaleja puupohjaisten ja ominaisuuksiltaan räätälöitävi-en nanomateriaalien valmistuksessa.

Asiasanat: esikäsittely, filmit, funktionaaliset biomateriaalit, nanopaperi, nanoselluloosa,urea, vetysidokset

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Acknowledgements The research work reported in this thesis was carried out at the Fibre and Particle Engineering Research Unit at the University of Oulu during the years 2016–2019. This thesis was funded by the SafeWood (Business Finland) and Bionanochemicals (Academy of Finland) projects, which are greatly acknowledged.

I would like to thank my principal supervisor, Associate Professor Henrikki Liimatainen, for offering me this doctoral study position as well as opportunities for international conferences and excellent courses. To me, his leadership always helps employees become a better version of themselves. A special thanks goes to my co-supervisor Docent Juho Sirviö. I always enjoy having fruitful discussions with him, which greatly expands the boundaries of my research. He is a professional researcher I respect from the bottom of my heart. He is also a trusted friend that I share all good and bad things with. He witnessed every single step of my research, always ready to offer his constructive thoughts. Without his efforts, this thesis would not have been possible.

I thank both revisors Professor Wolfgang Falk Liebner and Dr. Alistair William Thomas King for their instructive feedback concerning the manuscript. My co-supervisor Dr. Antti Haapala and the follow-up-group chairperson Dr. Nønne Prisle are thanked for their valuable advice and encouragement throughout the doctoral training process. Our laboratory technicians, Jarno Karvonen, Jani Österlund, and Elisa Wirkkala, are greatly acknowledged. I deeply appreciate their assistance in the lab and several good moments spent together on coffee breaks. I would also like to express my sincere gratitude to my current tutors Professor Tekla Tammelin and Dr. Katri Kontturi from VTT for their solid support on my academic career.

I am eternally grateful to the Mahrberg family from Joensuu and the Pylkkänen family from Lappeenranta. They take me as part of their family and have taken care of me these last six years. Importantly, to my lovely friends, Chen Li, Xiangxiang Zhou, Ruichi Zhang, Huifang Wen, Marie Gestranius, Ville Rissanen…Thank you for the encouragement and simply for the friendship!

Finally, I wish to express my deepest gratitude to my parents, Yulan and Xingshun, together with my sisters, Wenjing and Yuan, who love me unconditionally. To my wonderful girlfriend, Marleena, thank you for believing in me and picturing the beauty of life!

Oulu, July 2019 Panpan LI

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Abbreviations and symbols ANOVA One-way analysis of variance CNC Cellulose nanocrystal CNF Cellulose nanofibril or nanofibers CrI Crystallinity index of cellulose DAC Dialdehyde cellulose DES Deep eutectic solvent DP Degree of polymerization DPPH 2,2-diphenyl-1-picrylhydrazyl FESEM Field emission scanning electron microscopy LCNF Lignin-containing wood nanofiber LOI Limiting oxygen index PES-Na Polyethylene sulfonate PVDF Polyvinylidene fluoride TEM Transmission electron microscopy TEMPO 2,2,6,6-tetramethyl-1-piperidinyloxy TGA Thermogravimetric analysis WNM Wood-based nanomaterial XRD X-ray diffraction DADMAC Diallyldimethylammonium chloride Iam Peak intensity associated with the amorphous fraction I200 Peak intensity of the main crystalline plane p < 0.05 Suggested a significant difference p < 0.01 Suggested a critical difference T1 Background checking value T2 Total transmitted illumination T3 Beam checking value T4 Pure diffusive transmittance Asample The absorbance of the sample solution Acontrol The absorbance of DPPH solution without addition of the film TOnset Onset decomposition temperatures

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List of original publications This thesis is based on the following publications, which are referred throughout the text by their Roman numerals:

I Li, P., Sirviö, J. A., Haapala, A., & Liimatainen, H. (2017). Cellulose nanofibrils from nonderivatizing urea-based deep eutectic solvent pretreatments. ACS Applied Materials & Interfaces, 9(3), 2846–2855.

II Li, P., Sirviö, J. A., Asante, B., & Liimatainen, H. (2018). Recyclable deep eutectic solvent for the production of cationic nanocelluloses. Carbohydrate Polymers, 199, 219–227.

III Li, P., Sirviö, J. A., Hong, S., Ämmälä, A., & Liimatainen, H. (2019). Preparation of flame-retardant lignin-containing wood nanofibers using a high-consistency mechano-chemical pretreatment. Chemical Engineering Journal, 375, 122050.

IV Li, P., Sirviö, J. A., Haapala, A., Khakalo, A., & Liimatainen, H. (2019). Anti-oxidative and UV-absorbing biohybrid film of cellulose nanofibrils and tannin extract. Food Hydrocolloids, 92, 208–217.

The author of this thesis was the primary author of Papers I–IV and wrote the first draft of all the publications. Additionally, the author was principally responsible for designing and conducting the experimental laboratory work, data analysis, and reporting the results in the first draft of the publication. All initial manuscripts, I–IV were written by the author of this thesis, and co-authors participated by providing valuable comments and corrections.

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Contents Abstract Tiivistelmä Acknowledgements 9 Abbreviations and symbols 11 List of original publications 13 Contents 15 1 Introduction 17

1.1 Background ............................................................................................. 17 1.1.1 Cellulose nanocrystals .................................................................. 20 1.1.2 Cellulose nanofibers ..................................................................... 22 1.1.3 Lignin-containing wood-based nanofibers ................................... 23

1.2 Films from wood-based nanomaterials (WNMs) .................................... 24 1.3 Deep eutectic solvents in WNM production ........................................... 25 1.4 Aims and hypotheses of the thesis .......................................................... 28

2 Materials and methods 31 2.1 Materials ................................................................................................. 31 2.2 Pretreatment of wood-based fibers using DES systems .......................... 31

2.2.1 Non-derivatizing pretreatment using DES-I ................................. 32 2.2.2 Cationic pretreatment using DES-II ............................................. 32 2.2.3 Anionic pretreatment using DES-III ............................................. 33

2.3 Nanofibrillation of DES-treated fibers .................................................... 34 2.4 Characterization of WNMs ..................................................................... 34

2.4.1 Morphology .................................................................................. 35 2.4.2 Crystallinity .................................................................................. 35 2.4.3 Charge density .............................................................................. 36 2.4.4 Surface chemical characteristics ................................................... 36 2.4.5 Elemental analysis ........................................................................ 36

2.5 Fabrication of WNM films ...................................................................... 36 2.6 Characterization of the WNM films ........................................................ 37

2.6.1 Morphology .................................................................................. 37 2.6.2 Mechanical strength properties ..................................................... 38 2.6.3 Thermogravimetric analysis ......................................................... 38 2.6.4 UV and optical properties ............................................................. 38 2.6.5 Free radical scavenging activity ................................................... 39 2.6.6 Limiting oxygen index ................................................................. 40

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3 Results and discussion 41 3.1 Influence of DES systems on wood-based fibers .................................... 41

3.1.1 Dimensional changes .................................................................... 41 3.1.2 Functionalization .......................................................................... 42 3.1.3 Sustainability of using DES in WNM production ........................ 46

3.2 Characteristics of WNMs ........................................................................ 47 3.2.1 Morphology .................................................................................. 47 3.2.2 Crystallinity .................................................................................. 50 3.2.3 Thermal stability ........................................................................... 50

3.3 Functional properties of WNM films ...................................................... 51 3.3.1 UV-shielding and optical performance ......................................... 53 3.3.2 Anti-oxidant activity ..................................................................... 54 3.3.3 Flame-resistance ........................................................................... 55

4 Conclusions and future prospects 59 List of references 61 Original publications 73

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1 Introduction

1.1 Background

When you google the word “sustainability”, images and references containing of trees and forests are most often shown in the results. Living trees regulate the climate and atmosphere by reducing carbon dioxide and releasing oxygen, and, simultaneously, we may also get food, like fruits, from trees. Trees are also a source of renewable materials that have been rooted in our daily lives in the form of wooden buildings, furniture, paper products, textiles, etc. Wood-derived applications play a significant role in the process of carbon sequestration, and even if they are eventually burnt for heating or electricity, the course of a tree’s entire lifetime can still be regarded as a carbon-neutral lifecycle. (Salazar & Meil, 2009) Cellulose is the dominant chemical component in wood (cell wall) and has been often noted as the most abundant natural polymer in this planet after Anselme Payen first identified this carbohydrate in 1838 (Payen, 1838). Even though cellulose can be also obtained from bacteria, fungi, and some animals (tunicates), plants have been the most available resource for producing cellulose on a large scale (e.g., via the industrial pulping process).

Wood cell walls exhibit a hierarchical structure consisting of the middle lamella (ML), the outermost layer between the cell walls; a one-layer primary cell wall (P), and three layers of secondary cell walls (S1, S2, and S3) (Fig. 1) (Kekäläinen, 2016). In the secondary cell walls, the biological elementary fiber units, cellulose microfibrils, are biosynthesized in certain orientations (e.g., helix, twisting) and angles (e.g., angle to vertical). The microfibrils are composed of cellulose linear chains, in which hundreds to thousands of D-glucose units are linked via β-(1,4)-glycosidic bonds. Glucosyl residues in native cellulose chains are fixed by intramolecular hydrogen bonds, such as O3−H···O5 and O2−H···O6, which make the cellulose chain rigid. Furthermore, a sheet of cellulose is built when multiple chains parallelly align with lateral intermolecular hydrogen bonds (O6−H···O3 and O6−H···O2). Several sheets (with spacing of ca. 5 Å) become stabilized by van der Waals interactions and C−H···O hydrogen bonding to form a microfibril assembly (Phyo, Wang, Yang, O’Neill, & Hong, 2018); however, the number of cellulose chains required to form one microfibril varies (e.g., depending on the species of cellulosic resources) and the exact reaction remains unknown. For example, a 6 × 6 rosette cellulose chain model has been often used to explain the

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biosynthesis of cellulose microfibrils in wood, that is, 36 cellulose chains are simultaneously formed, linked, exported and crystallized (partially containing disordered areas) from the inner cell membrane (using uracil-diphosphate-glucose as a resource) to outer membrane (aligning one microfibril by rosettes (i.e. cellulose synthases)). (Endler & Persson, 2011) It should be noted that cellulose microfibrils are semi-crystalline and they are crystallized, disordered, and even twisted (based on computational simulations) as they are synthesized. Native cellulosic fiber is always assembled in the form of microfibrils, even though the quantities and models of cellulose chains in a microfibril can vary (e.g., 36 and 24 chains in the hexagon and rectangular shapes, respectively (Okita, Saito, & Isogai, 2010; Fernandes et al., 2011). Nevertheless, different techniques, including atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and X-ray diffraction, have been used to formulate and support different models (Park, Baker, Himmel, Parilla, & Johnson, 2010).

Fig. 1. Structural characterization of wood components (reprinted [adapted] with permission from Kekäläinen, 2016 © University of Oulu).

No matter what kind of models of cellulose microfibrils are used to describe their biosynthesis, cellulose crystallinity is a significantly important feature, as it decides the polymorph structure of cellulose and many fiber characteristics (e.g., mechanical strength, thermal stability, and chemical accessibility). Depending on the hydrogen bonding networks (i.e., intramolecular and/or intermolecular bonds)

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and the direction of cellulose chains, cellulose can be categorized into native cellulose (cellulose I) and man-made celluloses (cellulose II and III). There are two types of native cellulose I structures: allomorphs Iα and Iβ. (Atalla & Vanderhart, 1984). The crystalline structures of cellulose Iα and Iβ are composed by a triclinic one-chain unit cell and a monoclinic two-chain unit cell, respectively (Nishiyama, Langan, & Chanzy, 2002). The Iα is the predominant form existing in primitive organisms like algae or bacteria, whereas Iβ is the predominant form in higher plants (e.g., wood) and tunicates (Habibi et al. 2010). Nevertheless, cellulose Iα and Iβ often coexist in nature, even within the same cell.

The structural differences among native and man-made celluloses are significant. Cellulose II has mostly been regarded as regenerated cellulose (or mercerized cellulose), which is often obtained after the dissolution of native cellulose into alkaline. The hydrogen bonding networks in the chain sheet are more complicated (compared with cellulose I), because additional inter-sheet hydrogen bonding (O2−H···O2) is formed. In addition, cellulose chains are arranged parallelly (e.g., from the non-reducing end to the reducing end) in cellulose I, while cellulose II have an anti-parallel chain direction, which is thermodynamically more favorable than parallel arrangement. It should be noted that the transformation between cellulose I and II is irreversible. Cellulose III, including cellulose IIII and IIIII, is obtained from cellulose I or II using liquid ammonia or amine treatments, and the transformations are reversible (e.g., via hydrothermal treatment at c.a. 160 °C or thermal treatment above 200 °C) (Chanzy, Henrissat, Vincendon, Tanner, & Belton, 1987; Wada, 2001). Different than cellulose I but similar to cellulose II, hydrogen bonding also occurs between the cellulose chain sheets in cellulose III. Previously, researchers have reported that cellulose IV (with the orthogonal unit cell) can be obtained after the thermal treatment of cellulose II and III using glycerol at 260 °C, whereas cellulose I cannot be used as a starting material for its direct transformation into cellulose IV. However, recent evidence has revealed that cellulose IVI is most likely analogous with cellulose Iβ (Wada, Heux, & Sugiyama, 2004). In general, native cellulose and its polymorphs have different unit cells and hydrogen bonding network structures, which, in turn, affect their chemical reactivity (accessibility) and mechanical strength (e.g., native cellulose I is stiffer and stronger than man-made cellulose II and III) (Nishino, Takano, & Nakamae, 1995).

Native cellulose always exists in the form of microfibrils, which have a semi-crystalline and twisted structure in addition to chirality (right-handed) (Conley, Whitehead, & van de Ven, 2017). Disordered areas in the microfibrils are likely

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“defects” occurring between crystallites, yet they are more accessible for chemical-based treatments. For example, when cellulose is hydrolyzed using HCl, the majority of glycosidic bonds in the disordered areas of cellulose are cleaved, while the crystalline domains are released (a similar explanation can be applied to the “levelling-off degree of polymerization” concept). The released crystalline parts of the microfibrils are known as cellulose nanocrystals (CNC), cellulose nanowhiskers (CNW), or nanocrystalline cellulose (NCC), while the significantly longer cellulose microfibrils, along with its aggregates, are often referred to as cellulose nanofibrils/nanofibers (CNF), nanofibrillated cellulose (NFC), or microfibrillar cellulose (MFC). Traditionally, both CNC and CNF are denoted as “nanocelluloses”, usually obtained from purified lignocellulosic materials such as bleached pulp or dissolving pulp. Nevertheless, lignin can be partially present in CNF when coarser biomass grades, such as wood chips or sawdust, are applied as starting materials. Therefore, the terms “lignin-containing cellulose nanofibrils/nanofibers” (LCNF), “lignocellulosic nanofibers”, or even “wood nanofibers” (WNF) have been used to differentiate them from more pure cellulosic nanomaterials with lower amounts of lignin and extractive components as well as differing quantities of hemicelluloses (Kang et al., 2013). In the present work, all nanocelluloses derived from wood were referred to as wood-based nanomaterials (WNMs).

1.1.1 Cellulose nanocrystals

Acid or enzyme-catalyzed hydrolysis are widely used approaches to prepare CNCs (Dong, Revol, & Gray, 1998; Cui, Zhang, Ge, Xiong, & Sun, 2016). Chemically, active H+ ion hydrolyses the cellulose chain by protonating the oxygen atom in β-1,4-glycosidic bonds and then forming a carbenium ion. The newly formed carbonium ion reacts with water and releases glucose and the H+ ion. Traditionally, cellulose hydrolysis is regarded as a heterogeneous process, since water and H+ ions mostly cannot penetrate the cellulose crystalline structure (Kupiainen, Ahola, & Tanskanen, 2012); however, the cellulose surface and the more looser structures of its disordered sections have more accessible glycosidic bonds (and -OH groups) with which the chemicals can react. Therefore, the hydrolysis of cellulose releases cellulose crystals. It should be noted that the lengths of the CNCs are not only decided by reaction conditions (e.g., temperature, time, and acid concentration), but cellulose crystallinity and the particle size of the starting cellulosic materials also have a strong influence on cellulose hydrolysis (Yu & Wu, 2011).

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A notable problem of using HCl (solely) as the cellulose hydrolysis agent is that obtained CNCs have poor dispersion stability in aqueous environments because they are not charged on their surfaces. The instability of CNC suspension limits its applications (e.g., as a stabilizer in emulsion or as a homogenous matrix/reinforcement in composites. Hence, surface charged CNCs are often more desirable. Even though a further chemical modification following HCl-hydrolysis can introduce different functional groups to CNC and thus lead to stable CNC suspension (owing to its electrostatic repulsion), a one-step reaction using sulfuric acid as a hydrolysis agent has been mostly applied due to its high efficiency and convenience. For example, CNC with a crystal length of approximately 220 nm can be obtained when cotton linters are treated with 64–65 wt% of sulfuric acid at 45 °C for only 45 minutes; however, obvious problems with using sulfuric acid, including its low yield (< 30%), difficulties with acid recovery, and equipment corrosion need to be carefully addressed (Chen, Zhu, Baez, Kitin, & Elder, 2016).

In addition to strong mineral acids (e.g., HCl, sulfuric acid, and phosphoric acid), weak acids (e.g., oxalic acid and formic acid) or oxidation methods (e.g., periodate oxidation, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and ammonium persulfate oxidation) have also been developed to prepare CNCs with a surface charge; however, weak acids are usually not able to hydrolyze all cellulose, and CNCs typically need to be separated from the supernatant via centrifugation (Leung et al., 2011; Yang, Alam, & van de Ven, 2013; Jiang, Wu, Han, & Zhang, 2017). Notably, mechanical-assisted liberation (e.g., ultrasonication, homogenization, or milling) is often required, especially when weak acids are selected in CNC production. For example, CNCs with carboxylic groups can be obtained using maleic anhydride under ball-milling (Tang et al., 2013).

Recently, interesting approaches, such as the utilization of HCl vapor to produce CNC with a high yield (97%), have been highlighted. This high yield has been attributed to a rapid degradation with simultaneous crystallization caused by vapor phase acid (Kontturi et al., 2016). Follow-up studies have been able to improve CNC dispersion stability when TEMPO-mediated oxidation was addressed, which makes the preparation of CNC even more practical (Lorenz, Sattler, Reza, Bismarck, & Kontturi, 2017).

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1.1.2 Cellulose nanofibers

Different than rod-like CNCs, fibrillar CNFs consist of elongated, single microfibril entities that also have some disordered areas left. Moreover, microfibril aggregates/bundles are often referred to as CNFs. CNFs can be as narrow as 2–10 nm while having lengths up to several micrometers. In a water suspension, CNFs form highly viscous and translucent gels due to their fibrous and flexible morphologies and networked structure (large surface area, high aspect ratio, and possible surface charge). CNFs are also typically produced through intensive mechanical disintegration with or without chemical or enzymatic pretreatments (Henriksson, Henriksson, Berglund, & Lindström, 2007; Saito, Kimura, Nishiyama, & Isogai, 2007). Moreover, pure mechanical approaches have a processing drawback related to their high energy consumption (Wang et al., 2012).

Although enzymatic treatments have been widely applied to produce CNFs (Henriksson et al., 2007), chemo-mechanical combined processes seem to be more consistently used in the continuous production of CNFs, especially on the industrial scale (Khalil et al., 2014). Chemical pretreatments and modifications intend to primarily introduce functional groups on the cellulose surface to loosen the cellulose fiber structure through the interruption of hydrogen bonding networks. Mechanical techniques using grinders (Berglund, Noël, Aitomäki, Öman, & Oksman, 2016) or homogenizers (Siddiqui, Mills, Gardner, & Bousfield, 2011), for example, are based on the heavy impact and/or shearing forces that can liberate individual microfibrils from visible larger fiber and fibril bundles. Depending on the chemical modification methods as well as the cellulose materials used, the properties of CNFs, such as thermal stability, crystallinity, and morphological appearances, can vary significantly.

Among all the chemical pre-treatment approaches, TEMPO-mediated oxidation might be the most recognized and well-studied method for the production of anionic CNFs due to its preconceptions (pioneering work since 1998) (Isogai & Kato, 1998), convenience (water-medium at room temperature), low-energy consumption (> 90% reduction in energy consumption when compared to sole mechanical fibrillation) (Nakagaito & Yano, 2004; Siró & Plackett, 2010), and high CNF quality (highly monodispersed CNF with a width of few nm) (Saito, Nishiyama, Putaux, Vignon, & Isogai, 2006; Saito et al., 2007). In TEMPO-mediated oxidation (e.g. TEMPO/NaBr/NaClO), C6 primary hydroxyls of cellulose are selectively oxidized into carboxyl groups. TEMPO and NaBr act as catalytic agents under alkaline conditions, whereas oxidation is initiated by adding

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NaClO as a stoichiometric oxidant. The reaction can be controlled by the consumption of aqueous NaOH (neutralization of cellulose carboxylic groups), which is continuously added to the reaction to maintain the constant alkaline condition. The obtained anionic fibers can easily be further individualized to CNFs using rather mild mechanical treatments; however, the strong alkaline conditions degrade cellulose and reduces its DP (e.g., from 680 to 40 at pH 10 for two hours) (Isogai, Yanagisawa, & Isogai, 2009). Under slightly acidic conditions, the TEMPO system, composed of TEMPO/NaClO/NaClO2, can minimize the degradation of cellulose and decrease defects in the CNF (Hirota, Tamura, Saito, & Isogai, 2009; Isogai, Yanagisawa, & Isogai, 2009; Saito et al., 2009).

Many other chemical pre-treatments, such as the sequential periodate−chlorite oxidation method, have also widely been applied. In this reaction, cellulose hydroxyl groups at positions 2 and 3 are selectively oxidized, first to aldehyde groups and then carboxyl groups, which results in the production of dicarboxylic acid cellulose (Liimatainen, Visanko, Sirviö, Hormi, & Niinimaki, 2012). Other pre-treatment routes, including a variety of cellulose esterification-using acids, can also produce functionalized (anionic and cationic) cellulose nanofibers; however, these nano-sized fibers often exist as microfibril aggregates with a heterogeneous width, and to obtain nanoscale particles, some form of a mechanical fibrillation step is often employed (Chen et al., 2015). In such cases, TEMPO-mediated oxidation methods are favorable due to their ability to produce well-individualized microfibrils (Saito et al., 2006).

1.1.3 Lignin-containing wood-based nanofibers

Lignin-containing wood-based nanofibers (LCNFs) are complexes of cellulosic CNFs and amorphous lignin (and hemicellulose) and are often produced from unrefined lignocellulosic biomass, unbleached chemical pulp, or mechanical pulp using intensive mechanical grinding processes (nanofibrillation). Lignin can have notable influence on both CNF production and its properties. When lignin containing raw materials without delignification is used, production costs are reduced, toxic chemicals in pulping and bleaching processes are not required, and a very high production yield (85–95%) can be achieved (Sirviö & Visanko, 2017). More importantly, lignin is rich with phenolic groups, which provide functionalities to cellulosic materials, e.g., lignin can absorb light at the UV wavelength band Sadeghifar, Venditti, Jur, Gorga, & Pawlak, 2017), eliminate free radicals, and improve flame resistance (Sadeghifar & Argyropoulos, 2015). However, lignin also

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decreases the bonding between LCNFs, and, thus, the mechanical strength of the films produced from LCNF are often lower than nanocellulose films (Kalia et al., 2011). Besides, the production of nanofibers directly from wood is a more cumbersome process compared to bleached cellulose pulp, because additional energy is needed to disintegrate a strong biomass structure enhanced by a fiber-lignin complex.

1.2 Films from wood-based nanomaterials (WNMs)

WNMs are fascinating natural materials that have been used in biocomposites (Credou & Berthelot, 2014), food additives (Endes et al., 2016), and drug delivery (Pachuau, 2017). Among them, WNM films are considered one of the most appealing materials due to their attractive properties and practical significance. Moreover, the large specific surface area, various hydroxyl groups, and tight crystalline structure of WNMs result in films with a light-weight, high mechanical-strength, optical-transparency, and thermal stability. Simple processes for WNM film fabrication, such as vacuum-filtration, casting, solvent exchange, spraying, and spin-coating, can be used.

WNM suspensions form condense and tight networks by hydrogen bonding, and the dry films they afford exhibit a characteristic layered structure. Nevertheless, depending on the morphologies and functionalities of WNMs, the removal of water from WNM suspensions can be challenging and vary in duration from several minutes to hours. For highly viscous WNM suspensions, an additional degassing step is often required for removing entrapped gas bubbles, which negatively affect film properties like mechanical strength and barrier performance. WNM films usually have a porosity of 20–40%, which can be tailored using different material thicknesses or different solvents or drying processes (e.g., supercritical drying) (Henriksson, Berglund, Isaksson, Lindström, & Nishino, 2008; Benítez & Walther, 2017).

In addition to solely self-assembled WNM films, functional hybrid films can also be made when WNMs are incorporated with other inorganic (Ho, Zimmermann, Ohr, & Caseri, 2012) or organic materials (Sirviö, Honkaniemi, Visanko, & Liimatainen, 2015). Due to WNM fibrous networks, these hybrids can form layer-by-layer thin films with enhanced properties. For example, a flexible hybrid film made from aminoclays and carboxylated nanocellulose can have outstanding mechanical and optical properties (Ji et al., 2017); when introducing a small dosage of lignin to cellulose, a strong UV-shielding biohybrid film can be

25

fabricated (Sadeghifar et al., 2017). Moreover, even insulating nanocellulose film can become conductive when graphene (Valentini, Bittolo Bon, Fortunati, & Kenny, 2014) or carbon nanotubes are added (Jiang, Seney, Bayliss, & Kitchens, 2019). Therefore, WNM films can be used in many high-value applications.

1.3 Deep eutectic solvents in WNM production

Deep eutectic solvents (DESs) were first presented in 2003 by Abbott et al. (Abbott, Capper, Davies, Rasheed, & Tambyrajah, 2003). They found that a liquid, room-temperature solvent can be obtained when choline chloride (melting point 302 °C) was mixed with urea (melting point 133 °C) at a certain ratio. This major decrease in melting point (i.e., the eutectic point) occurred at a urea to choline chloride ratio of 2:1, for which the melting point of the mixture is only 12 °C. Since then, publications based on DES have been expanding due to their many advantages; for example, they are often inexpensive, easy to prepare, have a low toxicity (Yu et al., 2008; Ilgen et al., 2009), and can even be recyclable (Singh, Lobo, & Shankarling, 2011).

Commonly, DESs can be defined as mixtures of two or three components (often organic/metal halide salts together with hydrogen bond donors), in which interactions such as hydrogen bonding, electrostatic and van der Waals forces, lead to a mixture with a notably lower melting point when compared to their individual precursor components (Zhang, De Oliveira Vigier, Royer, & Jérôme, 2012; Fanjul-Mosteirín, Concellón, & del Amo, 2016; Mbous et al., 2017). For example, low lattice energies of individual DES components as well as the occurrence of charge delocalization via the formation of hydrogen bonding networks, have often been used to describe the low melting point of DESs (Smith, Abbott, & Ryder, 2014). DESs have also been reported as potential alternatives to traditional molecular solvents, and some DESs have been noted as a sub-class of ionic liquids (ILs) due to their low melting points, low volatility, non-flammability, and low vapor pressure (Zhang et al., 2012). Nevertheless, compared with ILs, DES systems do not require purification (Domínguez & Maugeri, 2011; Dai, van Spronsen, Witkamp, Verpoorte, & Choi, 2013).

DESs have also been applied in the production of different types of WNMs (Ma et al., 2019; Sirviö, Ukkola, & Liimatainen, 2019). In DESs, a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) can penetrate the disordered sections of a wood-based biomass and thus disrupt and rearrange the hydrogen bonding networks among the microfibrils and their aggregates. Therefore, cellulose

26

hierarchical structures and reactive hydroxyl groups can become more accessible for further modifications when DESs are assisted as the reaction medium. Furthermore, when DES components are reactive with hydroxyl groups, the DES medium becomes a reagent and can be simultaneously used for the production of functionalized WNMs. Currently, DESs have been applied as solely fiber-swollen agents (non-reactive DES), mediums (for carrying other reactive chemicals), and reagents (reactive DES) to produce functionalized WNMs.

Non-reactive DESs are often formed using e.g. urea as an HBD along with quaternary ammonium salts to loosen and swell the fiber structure without causing damage to the fibers (e.g., a decrease in DP, crystallinity, and thermal stability). WNMs obtained from this approach (through mechanical disintegration after DES treatment) have high mass yields (ca. 90%) but can still contain large (20–100 nm in width) microfibril aggregates. These DES-based WNMs have been used for the fabrication of strong films, as a reinforcement agent in composites, and for the formation of hydrophobic and super-absorbing aerogels (Hosseinmardi, Annamalai, Wang, Martin, & Amiralian, 2017; Laitinen, Suopajärvi, Österberg, & Liimatainen, 2017).

DESs can also be used as a reaction medium for lignocellulose biomasses. For example, acylation using succinic anhydride has been performed in two different DES systems: lithium chloride–urea (LcU) and triethylmethylammonium chloride–imidazole (TcI) (Selkälä, Sirviö, Lorite, & Liimatainen, 2016; Sirviö & Visanko, 2017). Both dissolving pulp (in LcU) and lignin-containing pulp (in TcI) were succinylated under mild conditions (below 90 °C for two hours) and resulted in high mass yields and carboxyl contents. Nevertheless, TcI DES seemed to be more chemically effective because of imidazole, which worked as a catalyst (Nawaz, Pires, & El Seoud, 2013).

It should be also noted that lignin can increase the content of acetylated WNMs due to its amorphous structure, which is more accessible to anhydride acid compared to rigid semi-crystalline, lignin-free cellulose fibers. Interestingly, a notable decrease in carboxyl contents indicates that LcU DES was less applicable for the cellulose sample at higher temperatures (e.g. 100 °C) or longer reaction times (24 hours). This might be due to the occurrence of a cross-linking reaction between the carboxyl and hydroxyl groups, and, thus, less vacant/accessible hydroxyl groups being available for succinylation (Yoshimura, Matsuo, & Fujioka, 2006; Liu, Zhang, Li, Yue, & Sun, 2010). In addition to anionization, TcI DES was also used for the cationization of cellulose using betaine hydrochloride and tosyl chloride as a reagent and coupling agent, respectively, under mild conditions (80 °C

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for four hours). In the reaction, both dissolving pulp and lignin-containing pulp were esterified with a high-charge density (1.95 mmol g-1). After cationization, individualized CNFs with a very small width of 3 to 5 nm were obtained via only four cycles of microfluidization (Sirviö, 2018).

Recently, DESs have been designed as a reactive medium, in which functional groups are introduced to the lignocellulosic biomass without using any additional agents. A typical example is the DES of sulfamic acid (Sa) and urea (U) (i.e., SaU DES) which works as an esterification reagent. Although sulfamic acid has been known as a sulfating agent with low toxicity and low-hygroscopicity, the high melting point of sulfamic acid (205 °C) and the dehydration (charring) of cellulose often limits its utilization unless additional co-solvents (e.g. diglyme) are used (Levdansky et al., 2014). The introduction of urea enables solid sulfamic acid to form a clear solution at 80 °C. This SaU DES forms a stiff gel and then crystallizes at room temperature, indicating SaU DES exhibits a supercooling behavior (Shahbaz et al., 2016). With a short sulfation duration (30 minutes), a variety of lignocellulosic materials (including groundwood pulp, sawdust, and dissolving pulp) obtained high contents of the sulfate groups (> 3.0 mmol g-1) (Sirviö et al., 2019; Sirviö & Visanko, 2019). More importantly, sulfated CNFs possessed a well-individualized structure with a homogeneous size distribution (ca. 4 nm in width), which is very similar to the carbonylated CNF obtained from TEMPO-mediated oxidations (Isogai, Saito, & Fukuzumi, 2011). However, lignocellulosic materials can greatly increase the viscosity of SaU DES, which prevents reaction progress and limits the amount of lignocelluloses. This problem was addressed in Paper III of the thesis, in which SaU DES was used in a mechano-chemical treatment. Besides SaU DES, reactive DES formed by dimethylurea and ZnCl2 was reported to produce cellulose methyl carbamate with good alkaline solubility (in a 3% NaOH solution) after freeze-thawing (Sirviö & Heiskanen, 2017). Moreover, anionic and cationic CNCs have been obtained from an acidic DES (formed by choline chloride and organic acids) and a recyclable DES system (formed by aminoguanidine hydrochloride and glycerol), respectively (Sirviö, Visanko, & Liimatainen, 2016). Very recently, DES composed of guanidine hydrochloride and anhydrous phosphoric acid was found to dissolve 5% of cellulose at room temperature. After regeneration with ethanol, cellulose II was obtained with some phosphate ester groups. This is different from the conventional approach, in which water is involved in the reaction (due to aqueous phosphoric acid) and which, in turn, hydrolyze cellulose phosphate groups back into phosphoric acid. After nanofibrillation, a homogenously distributed nanofiber with a width of ca. 6 nm was produced and

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successfully used as a reinforcement in a hybrid film (Sirviö, 2019). In this thesis, DES systems were designed and used solely as a pre-treatment medium to swell the fiber structure (DES-I in Paper I) and as a reactive medium for the production of cationic (DES-II in Paper II) and anionic (DES-III in Paper III) nanofibers (Table 1).

Table 1. DESs used in the thesis.

Publication DESs

(chemicals with molar ratios of 1:2)

Products Charge Application

Paper I DES-I (Iα and Iß)

(Iα: ammonium thiocyanate-urea)

(Iß: guanidine hydrochloride-urea)

WNM-I (CNF) Neutral Film-I

Paper II DES-II

(aminoguanidine hydrochloride-glycerol)

WNM-II

(CNF, CNC)

Cationic Film-II

Paper III DES-III

(sulfamic acid–urea)

WNM-III

(LCNF)

Anionic Film-III

Paper IV DES-II

(aminoguanidine hydrochloride-glycerol)

WNM-II

(CNF)

Cationic Tannin hybrid

Film-II

1.4 Aims and hypotheses of the thesis

The main aim of the thesis was to develop pretreatment methods for the production of WNMs using different DESs. Specifically, the objective was to obtain new information on the fabrication and performance of WNMs and their functionality as films.

More specific hypotheses are defined as follows:

Paper I:

– Does the urea-based DES pretreatment enable the nanofibrillation of cellulose fibers?

– How does the urea-based DES pretreatment affect the nanofiber properties?

Paper II:

– Can a glycerol-based DES be used to produce cationic nanocelluloses? – Is it possible to recycle glycerol-based DES in the synthesis of cationic

nanocelluloses?

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Paper III:

– Can a SaU DES be used in the pretreatment of sawdust in a combined mechano-chemical treatment for LCNF production?

– What are the properties of the produced lignin-containing wood nanofibers?

Paper IV:

– Can a biohybrid film be fabricated from DES-modified, cationic nanocelluloses and natural tannin extract via electrostatic attraction?

– How does tannin extract affect the properties of cationic nanocellulose film?

In Paper I, non-derivatizing DESs were used solely as the swelling media for the pretreatment of cellulose fibers and fabrication of CNF. DESs in Papers II and III were designed to be used as both a reaction medium and reagent in the production of functionalized WNMs. In Paper IV, multifunctional biohybrid films were fabricated using WNM-II (Paper II) and tannin extract. Different wood-based raw materials were used for WNM production, including chemically bleached birch cellulose pulp (Papers I, II and IV) and lignin-rich spruce timber sawdust (Paper III).

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2 Materials and methods

2.1 Materials

Bleached birch (Betula pendula) pulp obtained in dry sheets was used as the raw material for the fabrication of WNMs (Papers I–II, IV). The birch pulp consisted of cellulose (74.8%), xylan (23.6%), glucomannan (1.1%), lignin (0.4%), and the extractives (0.08%), which were determined in a previous study (Liimatainen, Sirviö, Haapala, Hormi, & Niinimäki, 2011). Spruce timber sawdust with a diameter of ca. 2 mm (Pölkky Oy, Finland) was used as the raw material for the fabrication of anionic lignin-containing nanofibers (Paper III). The sawdust contained 19.7, 28.6, and 1.2% of hemicellulose, lignin, and extractives, respectively (Sirviö & Visanko, 2019). The sawdust was sieved with a mesh (2 × 2 mm) prior to its use and then oven-dried at 105 °C for 24 hours. Commercial quebracho tannin extract (refined from Quebracho Colorado, Schinopsis lorenzii) in the form of dry powder was obtained from Haarla Limited (Tampere, Finland). Tannin extract containing 117 mg/g tannin and 10 mg/g residual carbohydrates was used for the biohybrid film in Paper IV.

Ammonium thiocyanate (NH4SCN; ≥ 97.5%), guanidine hydrochloride (NH2C(NH)NH2·HCl; ≥ 99%), and urea (NH2CONH2; > 99.0%) from Sigma-Aldrich (Taufkirchen, Germany) were used for non-reactive DES (i.e., DES-I). Aminoguanidine hydrochloride (> 98%) (Tokyo chemicals industry, Japan) and glycerol (97%) (VWR, France) were used to form DES-II. Sulfamic acid (98%) (Sigma-Aldrich, Germany) and urea (97%) (Biuron, Austria) were used to form DES-III (Table 1).

Ethanol (96%) (VWR, France) was used to wash the cationic fibers from DES-II. Films III to IV were fabricated on top of a Durapore polyvinylidene fluoride (PVDF) membrane obtained from Millipore, whereas Film I was prepared using a solvent casting method on a plastic Petri dish. Deionized water was used in all experiments, and all chemicals were used as received.

2.2 Pretreatment of wood-based fibers using DES systems

Three different DESs (DES I-III) were synthesized and used as pretreatment media for the swelling (Paper I), cationization (Paper II), and anionization (Paper III) of wood-based fibers. In Papers I–II, cellulose fibers were added after a clear liquid

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form of the DES mixture was obtained at a certain heating temperature. In Paper III, wood sawdust was added to DES components during a mechanical milling process, which was then followed with 30-minute heating.

After DES treatment, the deionized water was typically applied to remove DESs from the wood-based fibers (Papers I and III). Instead of deionized water, ethanol was used in Paper II, in which the recyclability of DES was investigated using a rotatory evaporator (Büchi rotavapor R114, Switzerland). DES-treated fibers were diluted into 0.5–1 wt% water suspensions (Papers I–III) prior to nanofibrillation with a microfluidizer (Microfluidics M-110EH-30, USA).

2.2.1 Non-derivatizing pretreatment using DES-I

DES-I (Iα and Iß) systems were formed from ammonium thiocyanate and urea, and guanidine hydrochloride and urea (both prepared at a molar ratio of 1:2), respectively. Birch pulp pieces (1 wt%) were added to clear DES-I at 100 °C and mixed for two hours (Fig. 2). Then, the DES-fiber suspension was filtered and washed with deionized water until the conductivity of the filtrate was below 20 μS cm−1.

Fig. 2. DES-I (Iα) before and after adding 1 wt% bleached birch pulp (reprinted [adapted] with permission from Paper I © 2017 American Chemical Society).

2.2.2 Cationic pretreatment using DES-II

The cationization of cellulose was conducted using a two-step process, and DES-II was recycled (via the evaporation of ethanol) and reused (Fig 3). Cellulose birch

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pulp fibers were first oxidized into dialdehyde cellulose fibers (DAC) using a previously reported method with slight modifications (instead of water, the current work used ethanol to wash the sample without a follow-up drying step) (Sirviö, Hyväkkö, Liimatainen, Niinimäki, & Hormi, 2011). DES-II was formed from aminoguanidine hydrochloride and glycerol at a molar ratio of 1:2. DAC fibers (5 wt% to DES-II) with different aldehyde contents (2.18 and 3.79 mmol g−1) were added into a clear DES-II system under different conditions (reaction temperatures were 70, 80, 90, and 100 °C and reaction times were 5, 10, 15, 30, and 60 minutes. Then, the DES-fiber suspension was filtrated and washed with ethanol (500 wt% to DES-II).

Fig. 3. Cationization of cellulose using sequential periodate oxidation and imidization with aminoguanidine hydrochloride (reprinted [adapted] with permission from Paper II © 2018 Elsevier).

2.2.3 Anionic pretreatment using DES-III

The anionization of sawdust (Fig. 4) was conducted during mechano-chemical pretreatment with DES-III formed from sulfamic acid and urea at a molar ratio of 1:2. Sawdust and DES chemicals were mechanically milled together using a planetary ball mill device (Retsch PM200, Germany), which was followed with an oven-heating process (105 °C for 30 minutes). Then, the DES-sawdust mixture was washed with deionized water until a neutral pH of sample suspension was achieved.

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During the mechano-chemical pretreatment, high mass consistencies of sawdust (20, 30, 40, 50, and 60 wt%) were applied.

Fig. 4. Sulfation of hydroxyl groups with sulfamic acid (reprinted [adapted] with permission from Paper III © 2019 Elsevier).

2.3 Nanofibrillation of DES-treated fibers

DES-treated wood-based fibers were prepared at a consistency of 0.5–1 wt% in water and dispersed with a Ultra-Turrax mixer (IKA T25, Staufen, Germany) for one minute at a speed of 11000 rpm. The nanofibrillation of the fiber suspensions were adjusted according to different combinations of chamber pairs and pressures using a microfluidizer (Table 2).

Table 2. Parameters of nanofibrillation using a microfluidizer.

Samples Consistency

(%)

Chamber pairs

(μm)

Pressures

Bar

Cycles WNM Products

DES-I-treated fiber 0.5 400 × 200 1300 3 WNM-I

(Paper-I) 400 × 100 2000 2 (CNFs)

200 × 87 2000 1

DES-II-treated fiber

(Paper II)

1.0 400 × 200 1000 2 WNM-II

(CNF and CNC)

DES-III-treated fiber 1.0 400 × 200 1000 1 WNM-III

(Paper III) 400 × 100 1500 1 (LCNF)

2.4 Characterization of WNMs

The morphological features of the WNMs, including their dimensions and shapes, were measured using transmission electron microscopy (TEM). In addition, an image-analyzer, FiberLab (Metso, Finland), was applied to measure the widths (μm) of the DES-treated fibers prior to nanofibrillation (Paper I). The crystalline

35

structures of the WNMs were investigated using wide-angle X-ray diffraction (Papers II and III). The introduced charged functional groups of the WNMs were determined using the polyelectrolyte titration method with a particle charge detector (Papers II and III).

2.4.1 Morphology

All the obtained WNMs (Papers I-III) were imaged using TEM (Tecnai G2 Spirit, FEI Europe, Netherlands and JEOL JEM-2200FS JEOL Ltd., Japan). The preparation of the TEM samples of the WNMs followed the same procedure, whereas the concentration of the WNM samples slightly varied (0.03–0.1 wt%). Highly diluted WNM samples were dosed on top of a carbon-coated copper grid, kept for one minute, and then wiped away using a filter paper. Adhesive agent polylysine and stain agent uranyl acetate (2% w/v) were also applied in the same manner prior and after the dosing of the WNM samples, respectively (Marsich et al., 2012). The widths of WNMs were measured using iTEM image analysis software (Olympus Soft Imaging Solutions GMBH, Germany). 100 individual WNM particles were measured in each sample, and the results were averaged with the standard deviations.

The widths (μm) of both the pristine and DES-I-treated cellulose fibers were measured with FiberLab, in which over 5000 individual fibers in each sample were captured with a scanning camera in the instrument, and the results were averaged with the standard deviations.

2.4.2 Crystallinity

The crystallinities of the original sawdust (Paper III), cationic (Paper II), and anionic (Paper III) WNMs were investigated using wide-angle X-ray diffraction. The measurements were conducted on a Rigaku SmartLab 9 kW rotating anode diffractometer (Japan) equipped with a Cu Kα radiation source (λ= 0.1542 nm) at 45 kV, 200 mA. Samples were dried prior to the formation of the tablet (thickness of 1 mm) with pressing. Scans were taken with a 2θ (Bragg angle) range from 5° to 50° at a scanning speed of 2° min−1 and a step of 0.05°. The peak intensity of the main crystalline plane diffraction (I200) and associated cellulose amorphous fraction (Iam) were located at ca. 22.5° and 18.0°, respectively. The degree of crystallinity in terms of the crystallinity index (CrI) was calculated according to the empirical Segal method (Segal, Creely, Martin, & Conrad, 1959) as following Equation (1):

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CrI = × 100%. (1)

2.4.3 Charge density

The surface charge densities of the WNMs were determined using the polyelectrolyte titration method with a particle charge detector (BTG Mütek PCD-03, Germany). Cationic (Paper II) and anionic (Paper III) WNMs were titrated with polyethylene sulfonate (PES-Na) and polydiallyldimethylammonium chloride (polyDADMAC), respectively. The WNM samples were then diluted with deionized water in a 0.01wt% solution and mixed with a magnetic stirrer at room temperature for 30 minutes. Then, 10 ml of well-dispersed WNM suspension was titrated with the associated polyelectrolyte (PES-Na or polyDADMAC). The charge density was calculated based on the consumption of the polyelectrolyte, and the results were the average of the two trials with very minor difference.

It should be noted that the polyelectrolyte titration method only indicates the total amount of functional groups located on the surface of WNMs, and, therefore, the results calculated from polyelectrolyte titration usually presented lower values than what the elemental analysis method indicates.

2.4.4 Surface chemical characteristics

The Fourier transform infrared spectroscopy spectra of birch cellulose, DAC, and WNM-II were recorded with a Bruker IR spectrometer (Bruker Tensor II FTIR Spectrometer, USA) equipped with an attenuated total reflection (ATR) accessory. The samples were prepared by pressing a 0.2 g (abs.) dried sample into a pellet.

2.4.5 Elemental analysis

Dried WNM-III samples (at 105 °C for 24 hours) were analyzed with PerkinElmer CHNS/O 2400 Series II elemental and LECO CS-200 carbon-sulfur analyzers to determine nitrogen and sulfur contents, respectively.

2.5 Fabrication of WNM films

WNM films were prepared using WNM suspensions through solvent-casting (Paper I) or vacuum-filtration (Papers II–IV) approaches. In the solvent-casting

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approach, WNM suspensions were degassed prior to drying under ambient conditions for two weeks. In the vacuum-filtration approach, WNM suspensions were dewatered through a PVDF membrane (Durapore, 0.65 µm), and a concentrated WNM-cake was obtained. The WNM films were prepared by hot-pressing of a WNM-cake using a sheet former (Estanit, Germany) at 93 °C for 10 minutes. Depending on the applications, the grammages of the WNM films varied from 60 (Paper I) to 80 g m-2 (Papers III and IV). The WNM films were stored at constant temperature and humidity conditions (23 ± 1 °C and 50 ± 2%, respectively) for further analysis.

Films made from these WNMs were coded as Film-I (Paper I), Film-II (Paper IV), and Film-III (Paper III), respectively (Table 1).

2.6 Characterization of the WNM films

The morphological characteristics of the surface and cross-sections of the WNM films were measured with field emission scanning electron microscopy (FESEM, Zeiss Sigma HD VP, Oberkochen, Germany, for Paper I, III, IV). Mechanical properties were tested using a universal material testing machine (Instron 5544, USA, for Paper I; D0724587, Switzerland, for Papers III and IV). Additionally, the thermal stabilities of the WNM films were analyzed using a thermal analyzer (Netzsch STA 449F3, Germany). Shimadzu UV-2600 spectrophotometers (Kyoto, Japan) were used to measure the UV–Vis spectra of the WNM films (Papers III and IV), and the antioxidant property of Film-II (Paper IV) was tested according to a DPPH (2,2-diphenyl-1-picrylhydrazyl, Sigma Aldrich, Germany) radical scavenging assay. (Byun, Kim, & Whiteside, 2010) Moreover, the flame-resistance of the WNM film (Paper IV) was represented as limiting the oxygen index (LOI) values using a JF-3 oxygen index meter (Jiangning Analysis Instrument Company, China).

2.6.1 Morphology

The fracture surfaces of WNM film cross-sections were obtained after immersing the film strips in liquid nitrogen. Both the film surfaces and obtained cross-sections were fixed to a carbon-coated carrier and then sputter-coated with platinum (with thickness of 5 nm). An accelerating voltage of 1–5 kV was applied during FESEM imaging.

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2.6.2 Mechanical strength properties

Before and during the measurement of the mechanical tensile test, the WNM films were always kept under constant temperature and humidity conditions (temperature and relative humidity were 23 ± 1 °C and 50 ± 2%, respectively) for 24 h. The films were then cut into uniform strips with a width of 5 mm. The thickness of each sample strip was measured using a precision thickness gauge (FT3; Hanatek Instruments, East Sussex, UK), which determined the average value of three random locations on the sample strip. For the tensile tests, a 40 mm gauge length was set at a strain rate of 4 mm min−1. After the stress–strain curve was plotted, Young’s modulus was calculated from the initial linear slope. The ultimate tensile strength was defined as stress upon specimen breakage.

For each WNM film sample, more than five strips were tested, and the results were averaged. Moreover, statistical evaluation was addressed using a one-way analysis of variance (ANOVA) test. Between the two samples, results with (★) p < 0.05 and (★★) p < 0.01 suggested a significant and critical difference, respectively.

2.6.3 Thermogravimetric analysis

The thermal stabilities of the WNM films were represented by the results of the thermogravimetric analysis (TGA). Approximately five mg of the room-temperature-dried nanofiber film sample was placed into an aluminum oxide pan and heated from ca. 30 to 600 °C with a heating rate of 10 °C min−1. Two separate atmospheres, including nitrogen and air flow (dynamic air), at a constant rate of 60 ml min−1, were also under investigation. The decomposition temperature (Tonset) was taken when the temperature at the onset point of the weight loss in the TGA curve was obtained. The maximum mass loss rates (Tmax) were obtained according to the first derivate curves of the TGA. Moreover, the differential scanning calorimetry (DSC) curve, associated with exothermic and endothermic peaks, indicated the occurrence of reactions when samples were heated with elevated temperatures (Paper III).

2.6.4 UV and optical properties

The transmittances of the WNM film samples (Papers III and IV) were measured using a spectrophotometer at wavelengths 200 to 800 nm with a step size of 1 nm.

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In addition, the optical haziness of the biohybrid films (Paper IV) were also calculated based on ASTM D1003 Standard (ASTM International, 2006) with the following equation:

Haze = − × 100%, (2)

where

T1 = background checking value,

T2 = total transmitted illumination,

T3 = beam checking value and,

T4 = pure diffusive transmittance.

The total transmitted illumination is the sum of specular transmittance and pure diffusive transmittance.

2.6.5 Free radical scavenging activity

The antioxidant activity of the tannin-WNM films (Paper IV) were measured based on the DPPH radical scavenging assay (Byun et al., 2010) method with slight modifications. Specifically, 100 mg of the film sample was mixed with 2 ml methanol with a magnetic stirrer for three hours at room temperature. Then, 500 µl of the supernatant liquid was obtained and vortexed with 2 ml of a methanolic solution of DPPH (0.06 mM) at room temperature in the dark for 30 minutes. The absorbance spectra (at a wavelength of 517 nm) of the film samples and control sample were then recorded. The final DPPH radical scavenging activity of the films was calculated using the following equation (Singh & Rajini, 2004):

Radical scavenging activity % = 1 − × 100, (3)

where

Asample = the absorbance of the sample solution and

Acontrol = the absorbance of the DPPH solution without the addition of the film.

The results were taken as the average of the three measurements.

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2.6.6 Limiting oxygen index

The flame-resistant performance was indicated by recording the limiting oxygen index of sample films using a JF‐3 oxygen index meter (Jiangning Analysis Instrument Company, China). Based on the ISO 4589-1996 standard, the WNM films (Film-III) were prepared as narrow strips (130 mm × 3 mm) and ignited with the required oxygen percentage using the oxygen index meter.

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3 Results and discussion Wood materials, including cellulose fibers, are rich in hydroxyl groups, through which intra- and interchain hydrogen bonds are built. DESs are often composed of two or more compounds that also interact through hydrogen bonding, resulting in a eutectic effect, which greatly reduces the melting point of the mixture when compared to individual precursor components (Smith et al., 2014). Consequently, DESs are hypothesized to interact and interrupt the hydrogen bonding network of wood and cellulose and therefore result in a loose and swollen cellulose fiber structure because DES elements like Cl, O, and N are strong hydrogen bonding competitors of the oxygen atom in cellulose (surfaces and disordered sections). (Chen, Zhang, You, & Xu, 2019) Nevertheless, the theory and mechanisms behind DES interaction with wood materials are still unclear and require intensive investigation.

3.1 Influence of DES systems on wood-based fibers

3.1.1 Dimensional changes

Results from FiberLab image analysis (Paper I) showed that the widths of the bleached birch cellulose fibers increased (1.2–3.4%) after DES-I pretreatment, without noticeable decrease in DP or the transformation of the cellulose I crystalline structure into other allomorphs. The swollen wood fiber structure had a positive influence on the mechanical nanofibrillation that followed. It is well-known that non-treated pristine cellulose suspension or mechanical pulp often cause chamber clogging during nanofibrillation (Siró & Plackett, 2010). In this work, both bleached cellulose fibers and lignin-containing cellulose fibers were able to smoothly pass through microfluidizer chambers after their DES-treatment.

Even though DESs are usually water-soluble and can easily be washed from the biomass after treatment (Papers I–IV), DES residues can precipitate as tiny particles in the fiber network during washing when lower temperatures than the melting point of DES are implemented. Therefore, additional mechanical stirring and a washing agent (water or ethanol) are often necessary for removing all the DES residues.

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3.1.2 Functionalization

Differ to DES-I which was used only for swelling the fiber structure, DES-II and -III were both used as a swelling medium and reactive reagent (i.e., the functional groups were introduced to the fibers through the DES-treatments).

The results from ATR–IR (Fig. 5) and polyelectrolyte titration both verified the successful cationization of DAC (Paper II). Moreover, WNM-II exhibited bands at 4000−2995 cm-1, 2900 cm-1, and 1430 cm-1, which correspond to the stretching of OH, and CH as well as the plane deformation vibrations of HCH (OCH) (Oh, Yoo, Shin, & Seo, 2005; Sirviö, Visanko, & Liimatainen, 2015). The stretching vibration of aldehyde carbonyl (1728 cm−1) in DAC was eliminated in WNM-II; however, new bands (at 1674 cm−1 and 1635 cm−1) occurred, indicating the carbon-nitrogen double bond of the imines and nitrogen-hydrogen bond bending, respectively (Zhang, Jiang, & Chen, 1999; Sirviö et al., 2014).

Fig. 5. ATR-IR spectra of birch cellulose, dialdehyde cellulose with the characteristic aldehyde band (1728 cm-1), and WNM-II with the characteristic carbon-nitrogen double bond (1674 cm−1) and nitrogen-hydrogen bond (1635 cm−1) (reprinted [adapted] with permission from Paper II © 2018 Elsevier).

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In the reaction (Fig. 6), DAC was cationized by DES-II in the reaction in which aminoguanidine hydrochloride was able to react with aldehyde groups to form a stable, conjugated imine structure. Meanwhile, the glycerol functioned as HBD, competing with hydrogen from DAC (e.g. C6-OH) in the formation of new hydrogen bonding networks (between glycerol and DAC) and, thus, loosening the fiber structure. The introduced cationic groups can cause electrostatic, internal repulsive forces in the fibers, which are beneficial for both following the nanofibrillation process and preventing the WNMs from aggregation (Sirviö, Honka, Liimatainen, Niinimäki, & Hormi, 2011; Sirviö et al., 2014).

Fig. 6. Cationization of DAC in DES-II.

DACs with aldehyde contents of 2.18 or 3.79 mmol g-1 were separately treated in the DES-II with variable reaction times (5, 10, 15, 30, and 60 minutes) and at different temperatures (70, 80, 90, and 100 °C). The titration results showed that DACs exhibited a high charge density (> 1 mmol g-1) after a short reaction time (< 15 minutes), and DAC with higher aldehyde contents exhibited overall higher charge densities (Fig. 7). It should also be noted that the high reaction temperatures (90 and 100 °C) had a clear negative influence on the mass yields of the reaction. Therefore, mild conditions (< 30 min; < 90 °C) are preferably applied in DES-II.

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These findings indicated the effective reaction occurring between the aldehyde groups in DAC and aminoguanidine hydrochloride in the DES-II (Paper II).

Fig. 7. The charge densities and reaction mass yields of the cationized DACs with different aldehyde contents of 2.18 (A, C) and 3.79 (B, D) mmol g-1 in the DES-II treatment (reprinted [adapted] with permission from Paper II © 2018 Elsevier).

Similar to DES-II, DES-III was also designed to be used as both a reaction medium and reagent. The reactive component in DES-III, sulfamic acid, had been used previously in the sulfation of the lignocellulosic biomass, but additional reaction mediums, such as dioxane (Vasileva et al., 2015), N,N-dimethylformamide, and diglyme (Levdansky et al., 2014) are also usually required. These chemicals increase the costs of reaction and cause chemical pollution and waste (Clarke, Tu, Levers, Bröhl, & Hallett, 2018). Recently, sulfamic acid composing DES was applied to the sulfation of cellulose without using any additional solvent (Sirviö et

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al., 2019); however, high temperatures (150 °C) and/or low biomass concentration (< 20 wt.%) were required due to the high viscosity of the DES-biomass mixture, which greatly reduced the feasibility and sustainability of DES use. Therefore, a mechano-chemical approach based on ball-milling was developed in the present work to overcome the high viscosity of the reaction system and simultaneously reduce the particle size of the sawdust, thus improving the reaction’s efficiency.

The results from the elemental analysis indicated that WNM-III exhibited high (up to 3.1 mmol g−1) sulfate group content based on both the N and S contents, but the S contents were overall lower than the N contents (Fig. 8). Theoretically, the N content associated with the ammonium salt is the same as the S content of the introduced sulfate groups (Fig. 4); however, as noted by other researchers, a side reaction between urea and cellulose take place in acidic environments, resulting in the production of cellulose carbamate (Segal & Eggerton, 1961; Fu et al., 2015; Willberg-Keyriläinen, Hiltunen, & Ropponen, 2018). Therefore, N from cellulose carbamates made a contribution to the total N contents.

Compared to the results from the elemental analysis, sulfate group contents measured by polyelectrolyte titration were lower, as the detection of the anionic charge occurred only on the surface of WNM-III (i.e. some anionic groups among the fiber networks could not be reached by a large polyelectrolyte). Nevertheless, similar trends of reduced sulfate group contents existed as the function of a decreased amount of DES-III (Fig. 8).

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Fig. 8. The contents of the sulfate groups in WNM-III, represented by elemental analysis (N and S) and polyelectrolyte titration, respectively (reprinted [adapted] with permission from Paper III © 2019 Elsevier).

3.1.3 Sustainability of using DES in WNM production

DES has been often reported as a more green and inexpensive medium compared to conventional solvents (Yu et al., 2008; Lobo, Singh, & Shankarling, 2012; Wang et al., 2017). In the production of WNMs, solvent properties, including costs, recyclability, and reaction efficiency are vitally important. In the current work, DES-II was mixed with water and/or ethanol following the washing step and further separated using a simple distillation process for DES reuse. DES-II was recycled and reused five times without any reduction in reaction efficiency compared to the original DES. Besides, no additional chemicals were added during the recycling, which further improved the feasibility of DES-II use as a cationization medium.

In Paper III, a high solid content was used during mechano-chemical DES treatment, which reduced the required chemical consumption significantly. It is well-known that DES systems often have a high viscosity when their

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lignocellulosic biomass consistency increases, and, thus, the mass content has often been limited to 1–5% in the production of non-derivatized (Suopajärvi, Sirviö, & Liimatainen, 2017) and functionalized (Sirviö, 2018; Sirviö & Visanko, 2017) WNMs, respectively. Despite the high solid content, a high sulfate content was still obtained in the mechano-chemical DES treatment. For example, the sawdust sample pretreated with DES-III at a mass ratio of 1:1 possessed 1.98 mmol g-1 of the sulfate group. Moreover, the reaction was conducted under mild reaction conditions (105 °C for 30 minutes) (Paper III).

The mass yields (Papers I–III) of all the materials remained high after DES pretreatments (Table 3); however, hemicellulose (Ray & Sarkar, 2001) and lignin can be partially dissolved in DESs during pretreatment and then washed away, which can slightly reduce the final yields.

Table 3. Reaction conditions, recyclability, and mass yields of DES pretreatments.

Processing parameters DES-I DES-II DES-III

Reaction time (min) 120 15 60

Reaction temperature (°C) 100 80 105

Washing agent Water Water+ Ethanol Water

Recyclability Not tested Yes Not tested

Product mass yield (%) > 90 > 90 > 90

In general, DESs offer efficient and sustainable routes for the production of functionalized WNMs. They have low chemical consumption, high reaction efficiency (short reaction time with high charge density) and mass yields and can potentially be recycled.

3.2 Characteristics of WNMs

3.2.1 Morphology

The morphological properties of WNMs were studied using TEM and FESEM imaging (Papers I–IV). The WNM morphology plays an important role in, for example, the stability of WNM suspensions (for instance, in sedimentation) and the mechanical strength of WNM films. In the literature, elemental nanofibers have been reported to have a typical width of 3–5 nm (Moon, Martini, Nairn, Simonsen, & Youngblood, 2011; Wang et al., 2012; Mishra, Sabu, & Tiwari, 2018), which can vary depending on raw materials and treatments.

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WNM-I contained mainly nanofiber aggregates, but both individual nanofibers (widths 3–5 nm) and large aggregates (widths > 100 nm) were observed. Generally, the widths of nanofiber aggregates ranged from 13.0 ± 11.8 to 19.3 ± 21.4 nm, which aligns with previously reported results in which non-derivatizing DES (choline chloride-urea) was applied (Sirviö et al., 2015). Different than the non-modified nanofiber produced form DES-I, surface-charged WNMs prepared using DES-II and DES-III exhibited more homogeneous and smaller size distributions, with widths ranging from 4.6 ± 1.1 to 5.7 ± 1.3 and from 2.4 ± 0.7 to 5.4 ± 1.7 nm, respectively (Fig. 9). These nanofiber sizes (from Papers II and III) were close to the widths of wood elemental nanofibers (3–5 nm). Since the WNM-II and -III were chemically modified, additional repulsive forces seemed to have a crucial role in the liberation of individual, narrow nanofibers despite being under more severe conditions during nanofibrillation with WNM-I.

Fig. 9. TEM images of WNM-I, -II, and -III (reprinted [adapted] with permission from Paper I © 2017 American Chemical Society, Paper II © 2018 Elsevier, and Paper III © 2019 Elsevier).

Films prepared from WNM suspensions using either the casting method (Paper I) or vacuum-filtration (Papers III and IV) all exhibited clear layered structures in their cross-sections (Fig. 10). This feature makes WNMs become a natural matrix that can be further utilized in the formation of functional hybrid films when mixed with small particles such as talc (Liimatainen et al., 2013), mica (Ho et al., 2012), and tannin extract (Paper IV). Furthermore, tight hydrogen bonding networks among nanofibers can occur when the water is removed, thus endowing the film with mechanical strength.

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Fig. 10. FESEM images of the Film-I, -II, and -III in cross-sections (reprinted [adapted] with permission from Paper I © 2017 American Chemical Society, Paper III © 2019 Elsevier, and Paper IV © 2019 Elsevier).

The smoothness of the top layer of the films (surface) was found to be affected by the number of passes of DES-treated fibers through the microfluidizer (i.e., the increase in passes resulted in a smoother film surface) (Fig. 11). Compared with Film-I or -III, Film-II had a smoother surface, and only a small amount of microfibril aggregates (with widths > 1µm) were observed. This is mostly because WNM-II had a more homogenous size distribution (compared to non-derivatizing WNM-I, WNM-II was addressed in a more severe chemical functionalization process) and did not contain lignin nanoparticles (compared with WNM-II).

Fig. 11. FESEM images of Film-I surfaces obtained from different severities (passes) of mechanical nanofibrillation with a microfluidizer (reprinted [adapted] with permission from Paper I © 2017 American Chemical Society).

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3.2.2 Crystallinity

XRD was used to indicate an alteration in the crystalline structure of wood-based fibers before and after DES-treatment. Previously, DES formed from guanidine hydrochloride and anhydrous phosphoric acid was used to dissolve 5 wt% of dissolving pulp at room temperature, and cellulose II was then observed in a XRD diffraction pattern (Sirviö, 2019). Thus far, DES systems applied in non-dissolution treatments have not been observed to alter the pristine crystalline structure of cellulose I to cellulose II. Nevertheless, the calculated crystallinity of DES-treated cellulose can change due to, for example, the dissolution of hemicelluloses (Ray & Sarkar, 2001; Wang et al., 2012) and lignin (when the raw materials are not pure cellulose) (Francisco, van den Bruinhorst, & Kroon, 2012), damage to the crystal sections caused by chemical and/or mechanical processes (Clark & Terford, 1955), and the introduction of a functional group on the fiber surfaces.

In the current work (Papers I–III), no rearrangement of the crystalline structure of cellulose was observed after DES treatments. Similar results have also been reported by other researchers when DESs were applied either solely as a fiber-swelling medium (Sirviö et al., 2015), a reaction medium (Selkälä et al., 2016; Sirviö, 2018), or as a reactive agent (Sirviö et al., 2016); however, changes in the degree of crystallinity (Papers II and III) were likely caused by the dissolution of the amorphous components of cellulose in addition to chemical (e.g., hydrolysis) as well as mechanical (e.g., milling and nanofibrillation) damage to the cellulose crystal structure (Table 4).

Table 4. The CrI values of WNMs and original raw materials.

Samples

Pristine cellulose

pulp

WNM-I Pristine

cellulose pulp

WNM-II Sawdust WNM-III

CrI values (%) 56.6 Not tested 56.6 63.2–64.9 61.9 27.4–48.0

3.2.3 Thermal stability

The fibers from DES-I treatment (before nanofibrillation) exhibited good thermal stability with onset decomposition temperatures (TOnset) at ca. 270 °C (air flow), which was 20 °C higher than the TOnset of its pristine birch pulp fibers. The slightly improved TOnset could be explained by the dissolution and removal of hemicellulose. Moreover, the TOnset value decreased after mechanical nanofibrillation (i.e., WNM-

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I), which was likely caused by the reduction of DP after mechanical disintegration (Li et al., 2012). Compared to WNM-I, WNM-II and -III exhibited lower reduced thermal stability than the original raw materials (i.e., birch pulp fibers and sawdust, respectively). This decrease in thermal stability was likely due to chemical (e.g., hydrolysis) and sequentially mechanical (e.g., disintegration) damage to the lignocellulosic biomass structures (Li et al., 2012; Dhar, Bhardwaj, Kumar, & Katiyar, 2014; Hideno, 2016; Yildirim & Shaler, 2017).

The introduced functional groups could also react when the samples were heated, thus, lowering the thermal stability. An example of such a reaction could include the conversion of newly introduced functional groups into strong acids (e.g., phosphoric acid and sulfuric acid), which leads to the dehydration and hydrolysis of the lignocellulosic biomass and reduces its thermal stability (TOnset). On the other hand, the dehydration reaction via the hydroxyl groups of lignocellulosic biomass generates non-flammable char and gases (e.g., water and carbon dioxide) (Liodakis, Statheropoulos, Tzamtzis, Pappa, & Parissakis, 1996; Roman & Winter, 2004). This feature causes various salts to be utilized as fire retardants for biomass, with a significant increase in the amount of char residue at high temperatures (> 500 °C). In Paper III, high contents of the introduced sulfate group of WNM-III led to the formation of sulfuric acid at an elevated temperature. Sulfuric acid is believed to hydrolyze lignocellulose fibers and therefore reduce the TOnset by ca. 100 °C when compared to the TOnset of pristine sawdust, which was ca. 265 (N2) °C or 235 (air) °C (Table 5). On the other hand, the release of sulphuric acid also leads to flame-retardant properties, which will be discussed later.

Table 5. Comparison of the onset decomposition temperatures between the starting materials (birch fibers and sawdust) and fabricated WNMs (I-III).

TOnset (°C) Brich fibers WNM-I WNM-II Sawdust WNM-III

Air flow 250 250 167 235 130

N2 flow 255 259 165 265 145

3.3 Functional properties of WNM films

Films I-III (Fig. 12) fabricated from WNMs (I-III) exhibited several superior properties such as being light-weight, having good mechanical strength, and possessing barrier (e.g., anti-UV or -flame) properties. More importantly, these properties could be tailored using different raw biomass materials, nanofibrillation methods, and chemical modification routes and, therefore, improve the feasibility

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of these natural nanomaterials as alternatives to petroleum-based applications. Film-I exhibited very good mechanical properties, such as tensile strength and elastic modulus of ca. 150 MPa and 7.1 GPa, respectively. These results are better than those previously reported (e.g., enzyme-pretreated CNF films [60−120 MPa]) (Qing, Sabo, Wu, Zhu, & Cai, 2015).

In addition to pure nanocellulose films, chemically modified cellulosic hybrid films containing additional components (tannin in Film-II or lignin in Film-III) were prepared. These films exhibited advanced functionalities without losing the inherent properties of cellulose.

Fig. 12. Film-I to -III fabricated from different WNMs: Film-I obtained from WNM-I with different severities (passes) of mechanical nanofibrillation with a microfluidizer; Film-II obtained from WNM-II with the introduction of different percentages of tannin extract; Film-III obtained from WNM-III with different charge densities (numbers refer to the reaction consistency of sawdust) (reprinted [adapted] with permission from Paper I © 2017 American Chemical Society, Paper III © 2019 Elsevier, and Paper IV © 2019 Elsevier).

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3.3.1 UV-shielding and optical performance

Nanocellulose films are often reported as having good optical transparency because of their condensed, thin layer structure (thickness of tens of micrometers) that is composed of nanosized particles. Transparent nanocellulose films can have various applications, such as packaging materials, electronic screens, and biomedical materials. Nevertheless, parameters such as the dimension and homogeneity of the nanofibers as well as the smoothness, porosity, and density of the films usually have a strong influence on optical properties (e.g., transparency and haziness) (Zhu et al., 2013; Jiang et al., 2018). Additives can also be used to tailor the optical properties. For example, aromatic compounds (e.g., phenolic groups) can provide UV-shielding properties for the film. Both tannin extract and lignin have high phenolic unit contents and can potentially be used as UV-absorbers in nanocellulose films.

In the present work, Film-II and Film-III were hybrid films fabricated from tannin and cationic WNM-II and lignin-containing WNM-III, respectively. In Paper IV, slightly anionic tannin extract was added, mixed, and absorbed with cationic nanofiber suspension through electrostatic attraction. Lignin-containing Film-III was, in turn, prepared from DES-modified sawdust directly after the nanofibrillation procedure.

The results (Table 6) show that Film-II exhibited a gradually improving UV-shielding property as a function of the tannin extract dosage (from 1 to 10 wt%). However, the optical transparency of the hybrid film decreased when the amount of tannin extract increased. Compared to the pure nanocellulose film which had optical transmittance at 320 and 550 nm of 40 and 58%, the film containing 5% of tannin extract had the values reduced to 8 and 44%, respectively. Therefore, the introduction of tannin extract can greatly improve the film’s UV-blocking ability, whereas its optical transparency is slightly decreased. However, the tannin hybrid films obtained ca. 40% more improved optical clarity (scattering of light) than the film without the tannin extract.

Different than Film-II, Film-III, made from anionic lignin-containing WNMs, had both good UV-shielding and optical transparency. Film-III was able to block 100% of UV-B and UV-C (wavelength below 320 nm) simultaneously while having a distinctly optical transmittance of 62% at a wavelength of 550 nm. The optical appearances of Film-III (Fig. 12) were slightly affected by the sulfate group content and morphology of WNM-III.

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Table 6. Transmittance of hybrid films at different wavelengths (nm).

WNM films Transmittance (%) at different wavelengths (nm)

280 320 400 550

Film-II (hybrid with 5 wt% tannin) 0.6 7.4 28 44

Film-III (sulfate group: 2.4 mmol g−1) 0 0 10 62

3.3.2 Anti-oxidant activity

Active free radicals, such as reactive oxygen species, can damage biomolecules (e.g. DNA) through propagation and, thus, cause degenerative diseases in living cells (Halliwell & Gutteridge, 1990). In food packaging applications, reactive oxidation reduces food quality and shortens its warranty period (Byun et al., 2010; Gemili, Yemenicioğlu, & Altınkaya, 2010; Dicastillo et al., 2011). Previously, barrier films made from cellulosic nanofibers have been used to prevent oxidation damages by limiting oxygen permeability. As a natural antioxidant, tannin extract containing phenolic groups can terminate the chain propagation of active free radicals by donating hydrogen atoms and forming resonance-stabilized aryloxyl radicals (Andrade et al., 2005; Gülçin, Mshvildadze, Gepdiremen, & Elias, 2006; Foti, 2007).

In Paper IV, the anti-oxidant activity of the tannin hybrid film was analyzed using the DPPH assay in which the violet free radical, DPPH, reacts with antioxidants and results in a color fading phenomena, which can be further recorded as absorbance at 517 nm using spectrophotometers (Gülçin, 2007; Gülçin, Huyut, Elmastaş, & Aboul-Enein, 2010). The results (Fig. 13) showed that the tannin-hybrid film (Film-II) had a 300% improved anti-radical activity compared to the pure nanocellulose film when 5 wt% tannin extract was used. This result is similar to previous research, in which tannin acid was tested at a higher concentration than in the current work (Gülçin et al., 2010). Furthermore, the increase in the concentration of the tannin extract to 10 wt% resulted in only a 10% improvement in anti-oxidant activity, yet the optical transparency dropped dramatically (29% of transmittance at 550 nm). Hence, small doses (ca. 5 wt%) of tannin extract can be used to obtain a film with a good balance between its antioxidation ability and optical transparency. Film-II was derived from renewable wood products, which offer an inexpensive and biodegradable approach to producing green alternatives for current petroleum products (e.g. plastic packaging materials).

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Fig. 13. Anti-oxidant activity of tannin-hybrided Film-II (reprinted [adapted] with permission from Paper IV © 2019 Elsevier).

3.3.3 Flame-resistance

Flame-retardants are widely used in various applications, such as packaging materials, textiles, and plastic composites (Alaee & Wenning, 2002). Conventional retardants (e.g., halogen-based chemicals) are considered problematic because they are toxic, corrosive, and bioaccumulative (Segev, Kushmaro, & Brenner, 2009). Fortunately, lignin itself has been reported as a natural flame-retardant additive that is able to extend the combustion duration, improve the char residue, and decrease the heat release rate when the material is heated at an elevated temperature. This is because the presence of a highly aromatic structure in lignin can form a char surface during heating.

The charred surface works as a barrier layer that protects the underlying material from the spreading of heat flux and flame, which, thus, enhances the fire-resistance towards material (Mandlekar et al., 2018). In addition, using lignin as a carbon source and its mixing with acidic agents (e.g., ammonium polyphosphate, melamine polyphosphate, metal phosphonate) were reported to form a intumescent system with enhanced fire retardant properties (De Chirico et al., 2003; Réti,

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Casetta, Duquesne, Bourbigot, & Delobel, 2008). In the present work, Film-III, produced from lignin-containing WNM-III, were tested in respect of potential nonflammability.

Table 7. Flame-resistance of Film-III.

WNM films Reference films Film-III

Film-I Lignin-containing film Sulfated lignin-containing films

Contents of sulfate group (mmol g-1) 0 0 3.1 2.8 2.4 2.0 1.1

LOI (%) 19.6 21.9 32 34 41 31 29

Ignitability in air Yes Yes No No No No No

The results from the LOI test indicated that Film-III had very good flame-resistance, that is, the LOI values of Film-III were 46–108%, higher than that of Film-I and even 31–87% higher than that of chemically non-derivatized lignin-containing reference film (Table 7) (Visanko et al., 2017). During the burning test, non-modified wood nanofiber film was ignited and easily burnt with little visible char residue, while the sulfated Films-III were not ignited when tested in the open-air using match fire (Fig. 14). The strong flame retardancy of Film-III was likely the result of a sulfation reaction, since high contents of acidic sulfate groups in Film-III can form sulfuric acid when the film is heated at an elevated temperature. This phenomenon dehydrates the film into char without burning, and producing non-flammable gases (e.g., water) (Liodakis et al., 1996; Roman & Winter, 2004). The formed char layers are able to reduce oxygen diffusion and heat transfer inside of the film. Also, non-flammable gases (e.g. ammonia, generated from ammonium sulfate and possibly also from carbamate) can dilute the oxygen concentration and thus enhances the fire retardancy (Liodakis, Vorisis, & Agiovlasitis, 2006; Mostashari & Mostashari, 2008). Previously, similar thermal performance and mechanisms had also been reported by other researchers when sulfated cellulose nanocrystal and microcrystalline cellulose (without lignin) were applied (Wang, Ding, & Cheng, 2007).

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Fig. 14. A comparison of non-sulfated lignin-containing film and Film-III: ignitability in the air atmosphere (video capture, reprinted [adapted] with permission from Paper III © 2019 Elsevier).

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4 Conclusions and future prospects Non-derivatizing and reactive DESs were studied as new pretreatment systems for the production of WNMs. In the present work, DES I-III served as fast (15–120 min), efficient (> 90% mass yield), and sustainable (e.g. recyclable) methods for swelling the fiber structure (Paper I) or functionalized lignocellulosic materials (Papers II–III) as well as producing WNM results with tailorable characteristics.

After DES pretreatment, wood-based fibers were able to pass through the microfluidizer without chamber clogging to yield stable (without sedimentation) WNM I-III suspensions. Non-derivatizing WNM-I reserved the original features of birch cellulose fiber, and Film-I obtained from WNM-I had good mechanical strength and thermal stability (Paper I). Cationic WNM-II was able to absorb the tannin component (extracted bark) and thus form a tree-based natural hybrid Film-II, which achieved good UV-shielding and anti-oxidant ability (Paper II). In Paper III, after sulfation using DES-III, the self-hybrid WNM-III obtained very strong UV-resistance. More importantly, flame-retardancy was also added to the Film-III due to its char formation (caused by releasing the sulphuric acid at an elevated temperature).

In general, Films (I-III) made from WNM (I-III) could be easily fabricated using a simple process of dewatering, and they all exhibited translucent and homogeneous appearances after being dried. Herein, the present work presents approaches to turning natural, renewable, and abundant woody materials into multi-functional and high value-adding WNM materials that can potentially be applied as advanced packaging materials, solar cell panels, and cosmetic additives, among others.

It should be noted that the parameters in DES pretreatments (e.g., molar ratio of the DES components) and nanofibrillation (e.g., pressure, chamber dimensions, and number of passes) were not optimized; however, depending on the application, the quality (e.g. dimensions, rheology) of WNMs can be adjusted by altering these parameters. In addition to the many advantages of using DES, their limitations must also be highlighted. For example, acidic DES systems can be corrosive to devices (Paper III), and hemicelluloses in bleached pulp can partly dissolve in DES and thus decrease their mass yields (Papers I and II). Moreover, DES modification and mechanical nanofibrillation often damage the crystallinity of cellulose, which negatively affects its thermal stability as well as reduces the film strength (Papers I–IV). Besides, side-reactions (e.g., carbamation between urea and hydroxyl groups) can reduce the yield and purity of the final products. Generally, DES-based systems

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are easy to prepare, can be recyclable, and are relatively inexpensive, indicating a green and up-and-coming pretreatment approach for producing WNMs with tailorable characteristics.

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List of references Abbott, A. P., Capper, G., Davies, D. L., Rasheed, R. K., & Tambyrajah, V. (2003).

Novel solvent properties of choline chloride/urea mixtures. Chemical Communications, 1, 70–71. doi: 10.1039/B210714G

Abdul Khalil, H. P. S., Davoudpour, Y., Islam, Md. N., Mustapha, A., Sudesh, K., Dungani, R., & Jawaid, M. (2014). Production and modification of nanofibrillated cellulose using various mechanical processes: A review. Carbohydrate Polymers, 99, 649–665. doi: 10.1016/j.carbpol.2013.08.069

Alaee, M., & Wenning, R. J. (2002). The significance of brominated flame retardants in the environment: Current understanding, issues and challenges. Chemosphere, 46(5), 579–582.

Andrade, R. G., Dalvi, L. T., Silva, J. M. C., Lopes, G. K. B., Alonso, A., & Hermes-Lima, M. (2005). The antioxidant effect of tannic acid on the in vitro copper-mediated formation of free radicals. Archives of Biochemistry and Biophysics, 437(1), 1–9. doi: 10.1016/j.abb.2005.02.016

ASTM International. (2006). Standard test method for haze and luminous transmittance of transparent plastics (Vol. D1003-11e1). Retrieved from http://file.yizimg.com/175706/2011120519401287.pdf

Atalla, R. H., & Vanderhart, D. L. (1984). Native cellulose: A composite of two distinct crystalline forms. Science, 223(4633), 283–285. doi: 10.1126/science.223.4633.283

Benítez, A. J., & Walther, A. (2017). Cellulose nanofibril nanopapers and bioinspired nanocomposites: A review to understand the mechanical property space. Journal of Materials Chemistry A, 5(31), 16003–16024. doi: 10.1039/C7TA02006F

Berglund, L., Noël, M., Aitomäki, Y., Öman, T., & Oksman, K. (2016). Production potential of cellulose nanofibers from industrial residues: Efficiency and nanofiber characteristics. Industrial Crops and Products, 92, 84–92. doi: 10.1016/j.indcrop.2016.08.003

Byun, Y., Kim, Y. T., & Whiteside, S. (2010). Characterization of an antioxidant polylactic acid (PLA) film prepared with α-tocopherol, BHT and polyethylene glycol using film cast extruder. Journal of Food Engineering, 100(2), 239–244. doi: 10.1016/j.jfoodeng.2010.04.005

Chanzy, H., Henrissat, B., Vincendon, M., Tanner, S. F., & Belton, P. S. (1987). Solid-state 13C-N.M.R. and electron microscopy study on the reversible cellulose I→cellulose IIII transformation in Valonia. Carbohydrate Research, 160, 1–11. doi: 10.1016/0008-6215(87)80299-9

Chen, L. H., Zhu, J. Y., Baez, C., Kitin, P., & Elder, T. (2016). Highly thermal-stable and functional cellulose nanocrystals and nanofibrils produced using fully recyclable organic acids. Green Chem., 18(13), 3835–3843. doi: 10.1039/C6GC00687F

62

Chen, W., Li, Q., Cao, J., Liu, Y., Li, J., Zhang, J., … Yu, H. (2015). Revealing the structures of cellulose nanofiber bundles obtained by mechanical nanofibrillation via TEM observation. Carbohydrate Polymers, 117, 950–956. doi: 10.1016/j.carbpol.2014.10.024

Chen, Y.-L., Zhang, X., You, T.-T., & Xu, F. (2019). Deep eutectic solvents (DESs) for cellulose dissolution: A mini-review. Cellulose, 26(1), 205–213. doi: 10.1007/s10570-018-2130-7

Clark, G. L., & Terford, H. C. (1955). Quantitative X-ray determination of amorphous phase in wood pulps as related to physical and chemical properties. Analytical Chemistry, 27(6), 888–895. doi: 10.1021/ac60102a006

Clarke, C. J., Tu, W.-C., Levers, O., Bröhl, A., & Hallett, J. P. (2018). Green and sustainable solvents in chemical processes. Chemical Reviews, 118(2), 747–800. doi: 10.1021/acs.chemrev.7b00571

Conley, K., Whitehead, M. A., & van de Ven, T. G. M. (2017). Probing the structural chirality of crystalline cellulose with induced circular dichroism. Cellulose, 24(2), 479–486. doi: 10.1007/s10570-016-1130-8

Credou, J., & Berthelot, T. (2014). Cellulose: From biocompatible to bioactive material. J. Mater. Chem. B, 2(30), 4767–4788. doi: 10.1039/C4TB00431K

Cui, S., Zhang, S., Ge, S., Xiong, L., & Sun, Q. (2016). Green preparation and characterization of size-controlled nanocrystalline cellulose via ultrasonic-assisted enzymatic hydrolysis. Industrial Crops and Products, 83, 346–352. doi: 10.1016/j.indcrop.2016.01.019

Dai, Y., van Spronsen, J., Witkamp, G.-J., Verpoorte, R., & Choi, Y. H. (2013). Ionic liquids and deep eutectic solvents in natural products research: Mixtures of solids as extraction solvents. Journal of Natural Products, 76(11), 2162–2173. doi: 10.1021/np400051w

De Chirico, A., Armanini, M., Chini, P., Cioccolo, G., Provasoli, F., & Audisio, G. (2003). Flame retardants for polypropylene based on lignin. Polymer Degradation and Stability, 79(1), 139–145. doi: 10.1016/S0141-3910(02)00266-5

Dhar, P., Bhardwaj, U., Kumar, A., & Katiyar, V. (2014). Cellulose nanocrystals: A potential nanofiller for food packaging applications. In ACS Symposium Series: Vol. 1162. Food Additives and Packaging (Vol. 1162, pp. 197–239). Retrieved from http://dx.doi.org/10.1021/bk-2014-1162.ch017

Domínguez de María, P., & Maugeri, Z. (2011). Ionic liquids in biotransformations: From proof-of-concept to emerging deep-eutectic-solvents. Current Opinion in Chemical Biology, 15(2), 220–225. doi: 10.1016/j.cbpa.2010.11.008

Dong, X., Revol, J.-F., & Gray, D. (1998). Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose. Cellulose, 5(1), 19–32.

63

Endes, C., Camarero-Espinosa, S., Mueller, S., Foster, E. J., Petri-Fink, A., Rothen-Rutishauser, B., … Clift, M. J. D. (2016). A critical review of the current knowledge regarding the biological impact of nanocellulose. Journal of Nanobiotechnology, 14. doi: 10.1186/s12951-016-0230-9

Endler, A., & Persson, S. (2011). Cellulose synthases and synthesis in arabidopsis. Molecular Plant, 4(2), 199–211. doi: 10.1093/mp/ssq079

Fanjul-Mosteirín, N., Concellón, C., & del Amo, V. (2016). l-Isoleucine in a choline chloride/ethylene glycol deep eutectic solvent: A reusable reaction kit for the asymmetric cross-aldol carboligation. Organic Letters, 18(17), 4266–4269. doi: 10.1021/acs.orglett.6b01989

Fernandes, A. N., Thomas, L. H., Altaner, C. M., Callow, P., Forsyth, V. T., Apperley, D. C., … Jarvis, M. C. (2011). Nanostructure of cellulose microfibrils in spruce wood. Proceedings of the National Academy of Sciences, 108(47), E1195–E1203. doi: 10.1073/pnas.1108942108

Foti, M. C. (2007). Antioxidant properties of phenols. Journal of Pharmacy and Pharmacology, 59(12), 1673–1685. doi: 10.1211/jpp.59.12.0010

Francisco, M., van den Bruinhorst, A., & Kroon, M. C. (2012). New natural and renewable low transition temperature mixtures (LTTMs): Screening as solvents for lignocellulosic biomass processing. Green Chemistry, 14(8), 2153. doi: 10.1039/c2gc35660k

Fu, F., Xu, M., Wang, H., Wang, Y., Ge, H., & Zhou, J. (2015). Improved synthesis of cellulose carbamates with minimum urea based on an easy scale-up method. ACS Sustainable Chemistry & Engineering, 3(7), 1510–1517. doi: 10.1021/acssuschemeng.5b00219

Gemili, S., Yemenicioğlu, A., & Altınkaya, S. A. (2010). Development of antioxidant food packaging materials with controlled release properties. Journal of Food Engineering, 96(3), 325–332. doi: 10.1016/j.jfoodeng.2009.08.020

Gülçin, I. (2007). Comparison of in vitro antioxidant and antiradical activities of L-tyrosine and L-Dopa. Amino Acids, 32(3), 431–438. doi: 10.1007/s00726-006-0379-x

Gülçin, İ., Huyut, Z., Elmastaş, M., & Aboul-Enein, H. Y. (2010). Radical scavenging and antioxidant activity of tannic acid. Arabian Journal of Chemistry, 3(1), 43–53. doi: 10.1016/j.arabjc.2009.12.008

Gülçin, I., Mshvildadze, V., Gepdiremen, A., & Elias, R. (2006). Screening of antiradical and antioxidant activity of monodesmosides and crude extract from Leontice smirnowii tuber. Phytomedicine: International Journal of Phytotherapy and Phytopharmacology, 13(5), 343–351. doi: 10.1016/j.phymed.2005.03.009

Halliwell, B., & Gutteridge, J. M. (1990). Role of free radicals and catalytic metal ions in human disease: An overview. Methods in Enzymology, 186, 1–85.

64

Henriksson, M., Henriksson, G., Berglund, L. A., & Lindström, T. (2007). An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. European Polymer Journal, 43(8), 3434–3441. doi: 10.1016/j.eurpolymj.2007.05.038

Henriksson, Marielle, Berglund, L. A., Isaksson, P., Lindström, T., & Nishino, T. (2008). Cellulose nanopaper structures of high toughness. Biomacromolecules, 9(6), 1579–1585. doi: 10.1021/bm800038n

Hideno, A. (2016). Comparison of the thermal degradation properties of crystalline and amorphous cellulose, as well as treated lignocellulosic biomass. BioResources, 11(3), 6309–6319. doi: 10.15376/biores.11.3.6309-6319

Hirota, M., Tamura, N., Saito, T., & Isogai, A. (2009). Oxidation of regenerated cellulose with NaClO2 catalyzed by TEMPO and NaClO under acid-neutral conditions. Carbohydrate Polymers, 78(2), 330–335. doi: 10.1016/j.carbpol.2009.04.012

Ho, T. T. T., Zimmermann, T., Ohr, S., & Caseri, W. R. (2012). Composites of cationic nanofibrillated cellulose and layered silicates: Water vapor barrier and mechanical properties. ACS Applied Materials & Interfaces, 4(9), 4832–4840. doi: 10.1021/am3011737

Hosseinmardi, A., Annamalai, P. K., Wang, L., Martin, D., & Amiralian, N. (2017). Reinforcement of natural rubber latex using lignocellulosic nanofibers isolated from spinifex grass. Nanoscale, 9(27), 9510–9519. doi: 10.1039/C7NR02632C

Ilgen, F., Ott, D., Kralisch, D., Reil, C., Palmberger, A., & König, B. (2009). Conversion of carbohydrates into 5-hydroxymethylfurfural in highly concentrated low melting mixtures. Green Chemistry, 11(12), 1948. doi: 10.1039/b917548m

Isogai, A., & Kato, Y. (1998). Preparation of polyuronic acid from cellulose by TEMPO-mediated oxidation. Cellulose, 5(3), 153–164. doi: 10.1023/A:1009208603673

Isogai, A., Saito, T., & Fukuzumi, H. (2011). TEMPO-oxidized cellulose nanofibers. Nanoscale, 3(1), 71–85. doi: 10.1039/C0NR00583E

Isogai, T., Yanagisawa, M., & Isogai, A. (2009). Degrees of polymerization (DP) and DP distribution of cellouronic acids prepared from alkali-treated celluloses and ball-milled native celluloses by TEMPO-mediated oxidation. Cellulose, 16(1), 117–127. doi: 10.1007/s10570-008-9245-1

Ji, S., Jang, J., Cho, E., Kim, S.-H., Kang, E.-S., Kim, J., … Park, J.-U. (2017). High dielectric performances of flexible and transparent cellulose hybrid films controlled by multidimensional metal nanostructures. Advanced Materials, 29(24), 1700538. doi: 10.1002/adma.201700538

Jiang, F., Li, T., Li, Y., Zhang, Y., Gong, A., Dai, J., … Hu, L. (2018). Wood-based nanotechnologies toward sustainability. Advanced Materials, 30(1), 1703453. doi: 10.1002/adma.201703453

65

Jiang, H., Wu, Y., Han, B., & Zhang, Y. (2017). Effect of oxidation time on the properties of cellulose nanocrystals from hybrid poplar residues using the ammonium persulfate. Carbohydrate Polymers, 174, 291–298. doi: 10.1016/j.carbpol.2017.06.080

Jiang, M., Seney, R., Bayliss, P. C., & Kitchens, C. L. (2019). Carbon canotube and cellulose nanocrystal hybrid films. Molecules, 24(14). doi: 10.3390/molecules24142662

Kalia, S., Dufresne, A., Cherian, B. M., Kaith, B. S., Avérous, L., Njuguna, J., & Nassiopoulos, E. (2011). Cellulose-based bio- and nanocomposites: A review. International Journal of Polymer Science, 2011, 1–35. doi: http://dx.doi.org/10.1155/2011/837875

Kang, Y., Ahn, Y., Lee, S. H., Hong, J. H., Ku, M. K., & Kim, H. (2013). Lignocellulosic nanofiber prepared by alkali treatment and electrospinning using ionic liquid. Fibers and Polymers, 14(4), 530–536. doi: 10.1007/s12221-013-0530-8

Kekäläinen, K. (2016). Microfibrillation of pulp fibres: The effects of compression-shearing, oxidation and thermal drying (Doctoral dissertation, University of Oulu). Acta Universitatis Ouluensis. C, Technica, 586. Retrieved from http://urn.fi/urn:isbn:9789526213668

Kontturi, E., Meriluoto, A., Penttilä, P. A., Baccile, N., Malho, J.-M., Potthast, A., … Sixta, H. (2016). Degradation and crystallization of cellulose in hydrogen chloride vapor for high-yield Isolation of cellulose nanocrystals. Angewandte Chemie International Edition, 55(46), 14455–14458. doi: 10.1002/anie.201606626

Kupiainen, L., Ahola, J., & Tanskanen, J. (2012). Distinct effect of formic and sulfuric acids on cellulose hydrolysis at high temperature. Industrial & Engineering Chemistry Research, 51(8), 3295–3300. doi: 10.1021/ie202323u

Laitinen, O., Suopajärvi, T., Österberg, M., & Liimatainen, H. (2017). Hydrophobic, superabsorbing aerogels from choline chloride-based deep eutectic solvent pretreated and silylated cellulose nanofibrils for selective oil removal. ACS Applied Materials & Interfaces, 9(29), 25029–25037. doi: 10.1021/acsami.7b06304

Leung, A. C. W., Hrapovic, S., Lam, E., Liu, Y., Male, K. B., Mahmoud, K. A., & Luong, J. H. T. (2011). Characteristics and properties of carboxylated cellulose nanocrystals prepared from a novel one-step procedure. Small, 7(3), 302–305. doi: 10.1002/smll.201001715

Levdansky, V. A., Kondracenko, A. S., Levdansky, A. V., Kuznetsov, B. N., Djakovitch, L., & Pinel, C. (2014). Sulfation of microcrystalline cellulose with sulfamic acid in N,N-Dimethylformamide and diglyme. Journal of Siberian Federal University, 2, 162–169.

66

Li, J., Wei, X., Wang, Q., Chen, J., Chang, G., Kong, L., … Liu, Y. (2012). Homogeneous isolation of nanocellulose from sugarcane bagasse by high pressure homogenization. Carbohydrate Polymers, 90(4), 1609–1613. doi: 10.1016/j.carbpol.2012.07.038

Liimatainen, H., Ezekiel, N., Sliz, R., Ohenoja, K., Sirviö, J. A., Berglund, L., … Niinimäki, J. (2013). High-strength nanocellulose–talc hybrid barrier films. ACS Applied Materials & Interfaces, 5(24), 13412–13418. doi: 10.1021/am4043273

Liimatainen, H., Sirviö, J., Haapala, A., Hormi, O., & Niinimäki, J. (2011). Characterization of highly accessible cellulose microfibers generated by wet stirred media milling. Carbohydrate Polymers, 83(4), 2005–2010. doi: 10.1016/j.carbpol.2010.11.007

Liimatainen, H., Visanko, M., Sirviö, J. A., Hormi, O. E. O., & Niinimaki, J. (2012). Enhancement of the nanofibrillation of wood cellulose through sequential periodate–chlorite oxidation. Biomacromolecules, 13(5), 1592–1597. doi: 10.1021/bm300319m

Liodakis, S. E., Statheropoulos, M. K., Tzamtzis, N. E., Pappa, A. A., & Parissakis, G. K. (1996). The effect of salt and oxide-hydroxide additives on the pyrolysis of cellulose and Pinus halepensis pine needles. Thermochimica Acta, 278, 99–108. doi: 10.1016/0040-6031(95)02790-4

Liodakis, S., Vorisis, D., & Agiovlasitis, I. P. (2006). Testing the retardancy effect of various inorganic chemicals on smoldering combustion of Pinus halepensis needles. Thermochimica Acta, 444(2), 157–165. doi: 10.1016/j.tca.2006.03.010

Liu, C. F., Zhang, A. P., Li, W. Y., Yue, F. X., & Sun, R. C. (2010). Succinoylation of cellulose catalyzed with iodine in ionic liquid. Industrial Crops and Products, 31(2), 363–369. doi: 10.1016/j.indcrop.2009.12.002

Lobo, H. R., Singh, B. S., & Shankarling, G. S. (2012). Deep eutectic solvents and glycerol: A simple, environmentally benign and efficient catalyst/reaction media for synthesis of N-aryl phthalimide derivatives. Green Chemistry Letters and Reviews, 5(4), 487–533. doi: 10.1080/17518253.2012.669500

López de Dicastillo, C., Nerín, C., Alfaro, P., Catalá, R., Gavara, R., & Hernández-Muñoz, P. (2011). Development of new antioxidant active packaging films based on ethylene vinyl alcohol Copolymer (EVOH) and green tea extract. Journal of Agricultural and Food Chemistry, 59(14), 7832–7840. doi: 10.1021/jf201246g

Lorenz, M., Sattler, S., Reza, M., Bismarck, A., & Kontturi, E. (2017). Cellulose nanocrystals by acid vapour: Towards more effortless isolation of cellulose nanocrystals. Faraday Discussions, 202(0), 315–330. doi: 10.1039/C7FD00053G

Ma, Y., Xia, Q., Liu, Y., Chen, W., Liu, S., Wang, Q., … Yu, H. (2019). Production of nanocellulose using hydrated deep eutectic solvent combined with ultrasonic treatment. ACS Omega, 4(5), 8539–8547. doi: 10.1021/acsomega.9b00519

67

Mandlekar, N., Cayla, A., Rault, F., Giraud, S., Salaün, F., Malucelli, G., & Guan, J.-P. (2018). An overview on the use of lignin and its derivatives in fire retardant polymer systems. In M. Poletto (Ed.), Lignin—Trends and Applications. Rijeka: Intech. doi: 10.5772/intechopen.72963

Marsich, L., Bonifacio, A., Mandal, S., Krol, S., Beleites, C., & Sergo, V. (2012). Poly-l-lysine-coated silver nanoparticles as positively charged substrates for surface-enhanced raman scattering. Langmuir, 28(37), 13166–13171. doi: 10.1021/la302383r

Mbous, Y. P., Hayyan, M., Hayyan, A., Wong, W. F., Hashim, M. A., & Looi, C. Y. (2017). Applications of deep eutectic solvents in biotechnology and bioengineering—Promises and challenges. Biotechnology Advances, 35(2), 105–134. doi: 10.1016/j.biotechadv.2016.11.006

Mishra, R. K., Sabu, A., & Tiwari, S. K. (2018). Materials chemistry and the futurist eco-friendly applications of nanocellulose: Status and prospect. Journal of Saudi Chemical Society, 22(8), 949–978. doi: 10.1016/j.jscs.2018.02.005

Moon, R. J., Martini, A., Nairn, J., Simonsen, J., & Youngblood, J. (2011). Cellulose nanomaterials review: Structure, properties and nanocomposites. Chemical Society Reviews, 40(7), 3941. doi: 10.1039/c0cs00108b

Mostashari, S. M., & Mostashari, S. Z. (2008). Combustion pathway of cotton fabrics treated by ammonium sulfate as a flame-retardant studied by TG. Journal of Thermal Analysis and Calorimetry, 91(2), 437–441. doi: 10.1007/s10973-007-8332-0

Nakagaito, A. N., & Yano, H. (2004). The effect of morphological changes from pulp fiber towards nano-scale fibrillated cellulose on the mechanical properties of high-strength plant fiber based composites. Applied Physics A: Materials Science & Processing, 78(4), 547–552. doi: 10.1007/s00339-003-2453-5

Nawaz, H., Pires, P. A. R., & El Seoud, O. A. (2013). Kinetics and mechanism of imidazole-catalyzed acylation of cellulose in LiCl/N,N-dimethylacetamide. Carbohydrate Polymers, 92(2), 997–1005. doi: 10.1016/j.carbpol.2012.10.009

Nishino, T., Takano, K., & Nakamae, K. (1995). Elastic modulus of the crystalline regions of cellulose polymorphs. Journal of Polymer Science Part B: Polymer Physics, 33(11), 1647–1651. doi: 10.1002/polb.1995.090331110

Oh, S. Y., Yoo, D. I., Shin, Y., & Seo, G. (2005). FTIR analysis of cellulose treated with sodium hydroxide and carbon dioxide. Carbohydrate Research, 340(3), 417–428. doi: 10.1016/j.carres.2004.11.027

Okita, Y., Saito, T., & Isogai, A. (2010). Entire surface oxidation of various cellulose microfibrils by TEMPO-mediated oxidation. Biomacromolecules, 11(6), 1696–1700. doi: 10.1021/bm100214b

Pachuau, L. (2017). Application of nanocellulose for controlled drug delivery. In Nanocellulose and Nanohydrogel Matrices (pp. 1–19). doi: 10.1002/9783527803835.ch1

68

Park, S., Baker, J. O., Himmel, M. E., Parilla, P. A., & Johnson, D. K. (2010). Cellulose crystallinity index: Measurement techniques and their impact on interpreting cellulase performance. Biotechnology for Biofuels, 3(1), 10. doi: 10.1186/1754-6834-3-10

Payen, A. (1838). Mémoire sur la composition du tissu propre des plantes et du ligneux. Comptes Rendus Hebdomadaires Des Séances de l’Académie Des Sciences, 7, 1052–1056.

Phyo, P., Wang, T., Yang, Y., O’Neill, H., & Hong, M. (2018). Direct determination of hydroxymethyl conformations of plant cell wall cellulose using 1 H polarization transfer solid-state NMR. Biomacromolecules, 19(5), 1485–1497. doi: 10.1021/acs.biomac.8b00039

Qing, Y., Sabo, R., Wu, Y., Zhu, J. Y., & Cai, Z. (2015). Self-assembled optically transparent cellulose nanofibril films: Effect of nanofibril morphology and drying procedure. Cellulose, 22(2), 1091–1102. doi: 10.1007/s10570-015-0563-9

Ray, D., & Sarkar, B. K. (2001). Characterization of alkali-treated jute fibers for physical and mechanical properties. Journal of Applied Polymer Science, 80(7), 1013–1020. doi: 10.1002/app.1184

Réti, C., Casetta, M., Duquesne, S., Bourbigot, S., & Delobel, R. (2008). Flammability properties of intumescent PLA including starch and lignin. Polymers for Advanced Technologies, 19(6), 628–635. doi: 10.1002/pat.1130

Roman, M., & Winter, W. T. (2004). Effect of sulfate groups from sulfuric acid hydrolysis on the thermal degradation behavior of bacterial cellulose. Biomacromolecules, 5(5), 1671–1677. doi: 10.1021/bm034519+

Sadeghifar, H., & Argyropoulos, D. S. (2015). Correlations of the antioxidant properties of softwood kraft lignin fractions with the thermal stability of its blends with polyethylene. ACS Sustainable Chemistry & Engineering, 3(2), 349–356. doi: 10.1021/sc500756n

Sadeghifar, H., Venditti, R., Jur, J., Gorga, R. E., & Pawlak, J. J. (2017). Cellulose-lignin biodegradable and flexible UV protection film. ACS Sustainable Chemistry & Engineering, 5(1), 625–631. doi: 10.1021/acssuschemeng.6b02003

Saito, T., Hirota, M., Tamura, N., Kimura, S., Fukuzumi, H., Heux, L., & Isogai, A. (2009). Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation using TEMPO catalyst under neutral conditions. Biomacromolecules, 10(7), 1992–1996. doi: 10.1021/bm900414t

Saito, T., Kimura, S., Nishiyama, Y., & Isogai, A. (2007). Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules, 8(8), 2485–2491. doi: 10.1021/bm0703970

Saito, T., Nishiyama, Y., Putaux, J.-L., Vignon, M., & Isogai, A. (2006). Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules, 7(6), 1687–1691. doi: 10.1021/bm060154s

69

Salazar, J., & Meil, J. (2009). Prospects for carbon-neutral housing: The influence of greater wood use on the carbon footprint of a single-family residence. Journal of Cleaner Production, 17(17), 1563–1571. doi: 10.1016/j.jclepro.2009.06.006

Segal, L., Creely, J. J., A. E. Martin, J., & Conrad, C. M. (1959). An empirical method for estimating the degree of crystallinity of native cellulose using the X-Ray diffractometer. Textile Research Journal, 29(10). doi: https://doi.org/10.1177/004051755902901003

Segal, L., & Eggerton, F. V. (1961). Some aspects of the reaction between urea and cellulose. Textile Research Journal, 31(5), 460–471. doi: 10.1177/004051756103100510

Segev, O., Kushmaro, A., & Brenner, A. (2009). Environmental impact of flame retardants (persistence and biodegradability). International Journal of Environmental Research and Public Health, 6(2), 478–491. doi: 10.3390/ijerph6020478

Selkälä, T., Sirviö, J. A., Lorite, G. S., & Liimatainen, H. (2016). Anionically stabilized cellulose nanofibrils through succinylation pretreatment in urea-lithium chloride deep eutectic solvent. ChemSusChem, 9(21), 3074–3083. doi: 10.1002/cssc.201600903

Shahbaz, K., AlNashef, I. M., Lin, R. J. T., Hashim, M. A., Mjalli, F. S., & Farid, M. M. (2016). A novel calcium chloride hexahydrate-based deep eutectic solvent as a phase change materials. Solar Energy Materials and Solar Cells, 155, 147–154. doi: 10.1016/j.solmat.2016.06.004

Siddiqui, N., Mills, R. H., Gardner, D. J., & Bousfield, D. (2011). Production and characterization of cellulose nanofibers from wood pulp. Journal of Adhesion Science and Technology, 25(6–7), 709–721. doi: 10.1163/016942410X525975

Singh, Lobo, H., & Shankarling, G. (2011). Selective N-alkylation of aromatic primary amines catalyzed by bio-catalyst or deep eutectic solvent. Catalysis Letters, 141(1), 178–182. doi: 10.1007/s10562-010-0479-9

Singh, N., & Rajini, P. S. (2004). Free radical scavenging activity of an aqueous extract of potato peel. Food Chemistry, 85(4), 611–616. doi: 10.1016/j.foodchem.2003.07.003

Siró, I., & Plackett, D. (2010). Microfibrillated cellulose and new nanocomposite materials: A review. Cellulose, 17(3), 459–494. doi: 10.1007/s10570-010-9405-y

Sirviö. (2018). Cationization of lignocellulosic fibers with betaine in deep eutectic solvent: Facile route to charge stabilized cellulose and wood nanofibers. Carbohydrate Polymers, 198, 34–40. doi: 10.1016/j.carbpol.2018.06.051

Sirviö. (2019). Fabrication of regenerated cellulose nanoparticles by mechanical disintegration of cellulose after dissolution and regeneration from a deep eutectic solvent. Journal of Materials Chemistry A, 7(2), 755–763. doi: 10.1039/C8TA09959F

70

Sirviö, & Heiskanen, J. P. (2017). Synthesis of alkaline‐soluble cellulose methyl carbamate using a reactive deep eutectic solvent. ChemSusChem, 10(2), 455–460. doi: 10.1002/cssc.201601270

Sirviö, Honkaniemi, S., Visanko, M., & Liimatainen, H. (2015). Composite films of poly(vinyl alcohol) and bifunctional cross-linking cellulose nanocrystals. ACS Applied Materials & Interfaces, 7(35), 19691–19699. doi: 10.1021/acsami.5b04879

Sirviö, J. A., Anttila, A.-K., Pirttilä, A. M., Liimatainen, H., Kilpeläinen, I., Niinimäki, J., & Hormi, O. (2014). Cationic wood cellulose films with high strength and bacterial anti-adhesive properties. Cellulose, 21(5), 3573–3583. doi: 10.1007/s10570-014-0351-y

Sirviö, J. A., Ukkola, J., & Liimatainen, H. (2019). Direct sulfation of cellulose fibers using a reactive deep eutectic solvent to produce highly charged cellulose nanofibers. Cellulose, 1–14. doi: 10.1007/s10570-019-02257-8

Sirviö, J. A., & Visanko, M. (2019). Highly transparent nanocomposites based on poly(vinyl alcohol) and sulfated UV-absorbing wood nanofibers. Biomacromolecules, 20(6), 2413–2420. doi: 10.1021/acs.biomac.9b00427

Sirviö, J. A., Visanko, M., & Liimatainen, H. (2015). Deep eutectic solvent system based on choline chloride-urea as a pre-treatment for nanofibrillation of wood cellulose. Green Chemistry, 17(6), 3401–3406.

Sirviö, J. A., Visanko, M., & Liimatainen, H. (2016). Acidic deep eutectic solvents as hydrolytic media for cellulose nanocrystal production. Biomacromolecules, 17(9), 3025–3032. doi: 10.1021/acs.biomac.6b00910

Sirviö, J., Honka, A., Liimatainen, H., Niinimäki, J., & Hormi, O. (2011). Synthesis of highly cationic water-soluble cellulose derivative and its potential as novel biopolymeric flocculation agent. Carbohydrate Polymers, 86(1), 266–270. doi: 10.1016/j.carbpol.2011.04.046

Sirvio, J., Hyvakko, U., Liimatainen, H., Niinimaki, J., & Hormi, O. (2011). Periodate oxidation of cellulose at elevated temperatures using metal salts as cellulose activators. Carbohydrate Polymers, 83(3), 1293–1297.

Sirviö, & Visanko, M. (2017). Anionic wood nanofibers produced from unbleached mechanical pulp by highly efficient chemical modification. J. Mater. Chem. A, 5(41), 21828–21835. doi: 10.1039/C7TA05668K

Smith, E. L., Abbott, A. P., & Ryder, K. S. (2014). Deep eutectic solvents (DESs) and their applications. Chemical Reviews, 114(21), 11060–11082. doi: 10.1021/cr300162p

Suopajärvi, T., Sirviö, J. A., & Liimatainen, H. (2017). Nanofibrillation of deep eutectic solvent-treated paper and board cellulose pulps. Carbohydrate Polymers, 169, 167–175. doi: 10.1016/j.carbpol.2017.04.009

Tang, L., Huang, B., Yang, N., Li, T., Lu, Q., Lin, W., & Chen, X. (2013). Organic solvent-free and efficient manufacture of functionalized cellulose nanocrystals via one-pot tandem reactions. Green Chemistry, 15(9), 2369. doi: 10.1039/c3gc40965a

71

Valentini, L., Bittolo Bon, S., Fortunati, E., & Kenny, J. M. (2014). Preparation of transparent and conductive cellulose nanocrystals/graphene nanoplatelets films. Journal of Materials Science, 49(3), 1009–1013. doi: 10.1007/s10853-013-7776-9

Vasil’eva, N. Yu., Levdansky, A. V., Kuznetsov, B. N., Skvortsova, G. P., Kazachenko, A. S., Djakovitch, L., & Pinel, C. (2015). Sulfation of arabinogalactan by sulfamic acid in dioxane. Russian Journal of Bioorganic Chemistry, 41(7), 725–731. doi: 10.1134/S1068162015070158

Visanko, M., Sirviö, J. A., Piltonen, P., Sliz, R., Liimatainen, H., & Illikainen, M. (2017). Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp. Cellulose, 24(10), 4173–4187. doi: 10.1007/s10570-017-1406-7

Wada, M. (2001). In situ observation of the crystalline transformation from cellulose III I to I β. Macromolecules, 34(10), 3271–3275. doi: 10.1021/ma0013354

Wada, M., Heux, L., & Sugiyama, J. (2004). Polymorphism of cellulose I family: Reinvestigation of cellulose IV I. Biomacromolecules, 5(4), 1385–1391. doi: 10.1021/bm0345357

Wang, J., Han, J., Khan, M. Y., He, D., Peng, H., Chen, D., … Xue, Z. (2017). Deep eutectic solvents for green and efficient iron-mediated ligand-free atom transfer radical polymerization. Polymer Chemistry, 8(10), 1616–1627. doi: 10.1039/C6PY02066F

Wang, N., Ding, E., & Cheng, R. (2007). Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups. Polymer, 48(12), 3486–3493. doi: 10.1016/j.polymer.2007.03.062

Wang, Q. Q., Zhu, J. Y., Gleisner, R., Kuster, T. A., Baxa, U., & McNeil, S. E. (2012). Morphological development of cellulose fibrils of a bleached eucalyptus pulp by mechanical fibrillation. Cellulose, 19(5), 1631–1643. doi: 10.1007/s10570-012-9745-x

Willberg-Keyriläinen, P., Hiltunen, J., & Ropponen, J. (2018). Production of cellulose carbamate using urea-based deep eutectic solvents. Cellulose, 25(1), 195–204. doi: 10.1007/s10570-017-1465-9

Yang, H., Alam, Md. N., & van de Ven, T. G. M. (2013). Highly charged nanocrystalline cellulose and dicarboxylated cellulose from periodate and chlorite oxidized cellulose fibers. Cellulose, 20(4), 1865–1875. doi: 10.1007/s10570-013-9966-7

Yildirim, N., & Shaler, S. (2017). A study on thermal and nanomechanical performance of cellulose nanomaterials (CNs). Materials, 10(7), 718. doi: 10.3390/ma10070718

Yoshimura, T., Matsuo, K., & Fujioka, R. (2006). Novel biodegradable superabsorbent hydrogels derived from cotton cellulose and succinic anhydride: Synthesis and characterization. Journal of Applied Polymer Science, 99(6), 3251–3256. doi: 10.1002/app.22794

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Yu, Y., & Wu, H. (2011). Effect of ball milling on the hydrolysis of microcrystalline cellulose in hot-compressed water. AIChE Journal, 57(3), 793–800. doi: 10.1002/aic.12288

Yu, Yinghao, Lu, X., Zhou, Q., Dong, K., Yao, H., & Zhang, S. (2008). Biodegradable naphthenic acid Ionic liquids: Synthesis, characterization, and quantitative structure-biodegradation relationship. Chemistry - A European Journal, 14(35), 11174–11182. doi: 10.1002/chem.200800620

Zhang, Q., De Oliveira Vigier, K., Royer, S., & Jérôme, F. (2012). Deep eutectic solvents: Syntheses, properties and applications. Chemical Society Reviews, 41(21), 7108. doi: 10.1039/c2cs35178a

Zhang, Y., Jiang, J., & Chen, Y. (1999). Synthesis and antimicrobial activity of polymeric guanidine and biguanidine salts. Polymer, 40(22), 6189–6198.

Zhu, H., Parvinian, S., Preston, C., Vaaland, O., Ruan, Z., & Hu, L. (2013). Transparent nanopaper with tailored optical properties. Nanoscale, 5(9), 3787. doi: 10.1039/c3nr00520h

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Original publications I Li, P., Sirviö, J. A., Haapala, A., & Liimatainen, H. (2017). Cellulose nanofibrils from

nonderivatizing urea-based deep eutectic solvent pretreatments. ACS Applied Materials & Interfaces, 9(3), 2846–2855.

II Li, P., Sirviö, J. A., Asante, B., & Liimatainen, H. (2018). Recyclable deep eutectic solvent for the production of cationic nanocelluloses. Carbohydrate Polymers, 199, 219–227.

III Li, P., Sirviö, J. A., Hong, S., Ämmälä, A., & Liimatainen, H. (2019). Preparation of flame-retardant lignin-containing wood nanofibers using a high-consistency mechano-chemical pretreatment. Chemical Engineering Journal, 375, 122050.

IV Li, P., Sirviö, J. A., Haapala, A., Khakalo, A., & Liimatainen, H. (2019). Anti-oxidative and UV-absorbing biohybrid film of cellulose nanofibrils and tannin extract. Food Hydrocolloids, 92, 208–217.

Reprinted with permission from the American Chemical Society (I) and Elsevier (II, III, and IV). Original publications are not included in the electronic version of the dissertation.

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