SURFACEMODIFICATIONOFCELLULOSEBY

COVALENTGRAFTINGANDPHYSICALADSORPTION

LinnCarlssonDoctoralThesis

AKADEMISKAVHANDLINGsommedtillstndavKungligaTekniskahgskolaniStockholmframlggstilloffentliggranskningfravlggandeavtekniskdoktorsexamenfredagenden 21:a februari 2014, kl. 10:00 i Kollegiesalen, Brinellvgen 8, KTH,Stockholm. Avhandlingen frsvaras p engelska. Fakultetsopponent:ProfessorMohamedNaceurBelgacem,Ecole InternationaleduPapier,delacommunication imprimeetdesbiomatriaux,Grenoble INPPagora,France.

Copyright2014LinnCarlssonAllrightsreserved PaperI 2012AmericanChemicalSocietyPaperII 2012TheRoyalSocietyofChemistryPaperIII 2013AmericanChemicalSocietyPaperV 2012TheRoyalSocietyofChemistry TRITACHEReport2014:2ISSN16541081ISBN9789175019871

ABSTRACTThe interest innew environmentally friendly cellulosebasedproductshas increased tremendously over the last years. At the same time theSwedish forest industry facesnew challenges in its strive to increase theutilizationof cellulose fibers inhighvalue endproducts.Theaimof thisstudywas to expand the toolbox for surfacemodificationof cellulosebyemploying covalent surfaceinitiated (SI) polymerizations or by physicaladsorptionofpolymers.SIringopening polymerization (ROP) of caprolactone (CL) wasperformed from filterpaper (FP) andhigh surface areananopaper (NP).Larger amounts of polycaprolactone (PCL) were grafted from NP,compared to FP, owing to the higher amount of available initiatinghydroxyl groups. Furthermore, the mechanical properties of PCL wereimprovedbythegraftingofFPandNP,ascomparedtopurePCL.Itischallengingtocharacterizeapolymergraftedfromasurface.Hence,quartz crystalmicrobalancewith dissipation (QCMD)was employed toinvestigateSIROPinrealtimefromacellulosemodelsurface.Furthermore,itwasshownbycolloidalprobeAFMthatincreasedlengthof grafted PCL, from cellulose microspheres, improved the interfacialadhesiontoapurePCLsurface,suggestingthatchainentanglementshavea significant impact on the interfacial properties. Increased temperatureandtimeincontactalsoimprovedtheadhesion.Inordertoinvestigatethedegreeofsubstitution(DS)andthedegreeofpolymerization (DP), PCLgrafted hydrolyzed cellulose cotton linters(HCCL)werestudiedbysolidstateNMR.ItwasfoundthatdespiteaDSofonly a few percent, the surface character changed considerably;furthermore,theDSwasvirtually independentof theDP.To increase theamount of grafted polymer, ringopening metathesis polymerization(ROMP) of norbornene was performed from FP. Short polymerizationtimesandlowtemperaturesresultedinhighlygraftedsurfaces.Alternatively, physical adsorption by electrostatic interactions wasemployed tomodify a cellulosemodel surface in theQCMD. Cationiclatexnanoparticlesofpoly(dimetylaminoethylmethacrylatecomethacrylicacid)blockpoly(methyl methacrylate) were produced by reversibleadditionfragmentation chaintransfer (RAFT)mediated surfactantfreeemulsionpolymerizationbypolymerizationinducedselfassembly(PISA).Thisstrategydoesnot requireanyorganicsolventsandcouldpotentiallybeintroducedinindustrialprocesses.

SAMMANFATTNINGIntresset fr nya miljvnliga cellulosabaserade produkter har katmarkantdesenasteren.Dettasamtidigtsomdensvenskaskogsindustrinstr infr nya utmaningar i sin strvan att ka anvndningen avcellulosafibrerifrdladeprodukter.Syftetmeddennastudievarattutkaverktygsldan fr ytmodifiering av cellulosa via kovalent ytinitieradepolymerisationerellergenomfysikaliskadsorptionavpolymerer.Ytinitierad ringppningspolymerisationav kaprolaktonutfrdes frnfilterpapper (FP) och nanopapper (NP) med hg ytarea. NP gav strreympad mngd p grund av fler initierande hydroxylgrupper. Demekaniska egenskaperna av polykaprolakton (PCL) frbttrades vidympningen frn FP och NP jmfrt med en ren PCL matris ochfuktadsorptionen reduceradesavsevrt i jmfrelsemedomodifieradeFPellerNP.Det r en stor utmaning att karakterisera ympade polymerer, drfrutvecklades en metod dr ytinitieradROP frn en cellulosamodellytastuderades med en kvartskristallmikrovg med dissipation (QCMD).Atomkraftsmikroskopimedkolloidalsondvisadeattkadympningslngdav PCL frbttrade vidhftningsfrmgan, vilket indikerar attkedjeintrasslingarharensignifikantpverkanpgrnsskiktsegenskaperna.kad temperatur och kontakttid resulterade ocks i kadvidhftningsfrmga.Frattutredainverkanavdenympadepolymerisationsgraden(DP)ochsubstitutionsgraden (DS) studerades PCLympade hydrolyseradecellullosabomullslintersmed fastfasNMR. Studierna visade att trots lgsubstitutionsgrad (ngra fprocent) s ndradesytkaraktren signifikantochsubstitutionsgradenvarrelativtkonstantoberoendeavDP.Fr att ka mngden ympad polymer utfrdes ringppningsmetatespolymerisation (ROMP) frn FP. Korta reaktionstider och lgapolymerisationstemperaturerresulteradeikraftigtympadeytor.Fysikaliskadsorptiongenomelektrostatiska interaktioneranvndes fratt modifiera en cellulosamodellyta i en QCMD. Katjoniskalatexnanopartiklar av sampolymeren poly(dimetylaminoetylmetakrylatmetakrylsyrametylmetakrylat) syntetiserades med reversibel additionsfragmenteringskedjeverfringsmedladtensidfriemulsionspolymerisationgenom polymerisationsinducerad sjlvorganisation. Denna strategikrveringaorganiskalsningsmedelochharpotentialattkunnatillmpasiindustriellaprocesser.

LISTOFPAPERSThisthesisisasummaryofthefollowingpapers: I. Facile Preparation Route for Nanostructured Composites: Surface

InitiatedRingOpeningPolymerizationof Caprolactone fromHighSurfaceAreaNanopaper,A.Boujemaoui,L.Carlsson,E.Malmstrm,M.Lahcini,L. Berglund,H. Sehaqui, andA.Carlmark,ACSAppliedMaterialsandInterfaces2012,4,31913198

II. Surfaceinitiated ringopening polymerization from cellulosemodelsurfacesmonitoredbyaQuartzCrystalMicrobalance,L.Carlsson,S.Utsel,L.Wgberg,E.Malmstrm,andA.Carlmark,SoftMatter2012,8,512517

III. NanobiocompositesAdhesion:RoleofGraftLengthandTemperature

in a Hybrid Biomimetic Approach, N. Nordgren, L. Carlsson, H.Blomberg, A. Carlmark, E. Malmstrm, and M. W. Rutland,Biomacromolecules2013,14,10031009

IV. SolidStateNMRinvestigationofhydrolyzedcottonlintersgraftedby

surfaceinitiated ringopening polymerization of caprolactone, L.Carlsson,P.T.Larsson,T.Ingverud,H.Blomberg,A.Carlmark,andE.Malmstrm,Manuscript

V. Surfaceinitiated ringopening metathesis polymerisation from

cellulosefibres,L.Carlsson,E.Malmstrm,andA.Carlmark,PolymerChemistry2012,3,727733

VI. Modification of cellulose surfaces by cationic latex prepared by

RAFTmediatedsurfactantfreeemulsionpolymerizationL.Carlsson,A. Fall, I. Chaduc, L. Wgberg, B. Charleux, E. Malmstrm, F.DAgosto,M.Lansalot,andA.Carlmark,Manuscript

Mycontributiontotheappendedpapers:I. Part of the experimentalwork, analyses, andminor part of the

preparationofthemanuscript.II. Amajority of the experimentalwork, analyses, andmost of the

preparationofthemanuscript.III. Part of the experimentalwork, analyses, andminor part of the

preparationofthemanuscript.IV. Partoftheexperimentalwork,partoftheanalyses,andmostofthe

preparationofthemanuscript.V. All the experimental work and analyses, and most of the

preparationofthemanuscript.VI. Alltheexperimentalwork,amajorityoftheanalyses,andmostof

thepreparationofthemanuscript.

Scientificcontributionsnotincludedinthisthesis:VII. Aligned Cellulose Nanocrystals and Directed Nanoscale

DepositionofColloidalSpheres,G.Nystrm,A.Fall,L.CarlssonandL.Wgberg,submitted

VIII. Nanohybrid selfcrosslinked PDMA/silica hydrogels, L.Carlsson, S. Rose, D.Hourdet, A.Marcellan, SoftMatter 2010, 6,36193631

ABBREVIATIONSAGU anhydroglucoseunitAFM atomicforcemicroscopyAIBA 2,2azobis(2methylpropionamidine)

dihydrochlorideATR attenuatedtotalreflectanceBCN bacterialcellulosenanofibersBET BrunauerEmmettTellerCA contactangleCCL cellulosecottonlintersCL caprolactoneCMC criticalmicelleconcentrationCMS cellulosemicrosphereCNC cellulosenanocrystalsCP/MAS crosspolarizedmagicanglespinningCNF cellulosenanofibersCTPPA 4cyano4thiothiopropylsulfanylthiocarbonyldL lateraldimensionM molarmassdispersityDCM dichloromethaneDLS dynamiclightscatteringDMAEMA N,NdimethylaminoethylmethacrylateDMAP 4(dimethylamino)pyridineDMF N,NdimethylformamideDMSO dimethylsulfoxideDP degreeofpolymerizationDS degreeofsubstitutionDSBM bulkmonomerdegreeofsubstitutionDSSP particlesurfacedegreeofsubstitutionDSC differentialscanningcalorimetryDVS dynamicvaporsorptionFESEM fieldemissionscanningelectronmicroscopyFRP freeradicalpolymerizationFTIR FouriertransforminfraredHCCL hydrolyzedcellulosecottonlintersMAA methacrylicacid

MMA methylmethacrylateNBE norborneneNMMO NmethylmorpholineNoxideNMR nuclearmagneticresonancePdI polydispersity(DLS)PEG poly(ethyleneglycol)PET polyelectrolytetitrationPISA polymerizationinducedselfassemblyPS polystyrenePVAm poly(vinylamine)QCM quartzcrystalmicrobalanceRAFT reversibleadditionfragmentationchaintransferRDRP reversibledeactivationradicalpolymerizationRH relativehumidityROMP ringopeningmetathesispolymerizationROP ringopeningpolymerizationRq surfaceroughnessRT roomtemperatureSEC sizeexclusionchromatographySI surfaceinitiatedSn(Oct)2 tin2ethylhexanoateSSA specificsurfaceareaTBD 1,5,7triazabicyclo[4.4.0]dec5eneTc crystallizationtemperatureTEM transmissionelectronmicroscopyTGA thermogravimetricanalysisTg glasstransitiontemperatureTi(OiPr)4 titaniumipropoxideTi(OnBu)4 titaniumnbutoxideUV ultravioletXc degreeofcrystallinity

TABLEOFCONTENTS

1. PURPOSE OF THE STUDY 12. INTRODUCTION 2

2.1CELLULOSE .................................................................................................. 22.1.1 Structure of cellulose ......................................................................... 22.1.2 Cellulosic substrates .......................................................................... 42.1.3 Surface modification of cellulose ....................................................... 6

2.2 RING-OPENING POLYMERIZATION TECHNIQUES ........................................... 92.2.1 Ring-opening polymerization (ROP) ................................................. 92.2.2 Ring-opening metathesis polymerization (ROMP) .......................... 122.3REVERSIBLE ADDITION-FRAGMENTATION CHAIN-TRANSFER (RAFT)

POLYMERIZATION ..................................................................................... 142.3.1 RAFT-mediated surfactant-free emulsion polymerization ............... 172.3.1.1 Polymer-induced self-assembly ....................................................... 19

2.4SURFACE MODIFICATION OF CELLULOSE BY COVALENT GRAFTING ............ 212.5SURFACE MODIFICATION OF CELLULOSE BY PHYSICAL ADSORPTION ......... 23

3. EXPERIMENTAL 263.1MATERIALS ............................................................................................... 263.2EXPERIMENTAL PROCEDURES .................................................................... 27

3.2.1 Cellulose surface modification by ring-opening polymerization ..... 273.2.2 Cellulose surface modification by ring-opening metathesis

polymerization ................................................................................. 283.2.3 Synthesis of cationic latex nanoparticles ......................................... 293.2.3.1 Aqueous RAFT polymerization of DMAEMA .................................. 293.2.3.2 RAFT-mediated surfactant-free emulsion polymerization of MMA . 303.2.3.3 Adsorption of cationic latex on cellulose model surfaces ................ 31

3.3CHARACTERIZATION METHODS ................................................................. 313.3.1 QCM-D ............................................................................................ 313.3.2 Colloidal probe AFM ....................................................................... 323.3.3 Solid state CP/MAS 13C-NMR .......................................................... 32

4. RESULTS AND DISCUSSION 334.1CELLULOSE SURFACE MODIFICATION BY COVALENT GRAFTING ................ 33

4.1.1 Cellulose surface modification by ring-opening polymerization ..... 334.1.1.1 SI-ROP of -CL from FP and NP. Comparison between Sn(Oct)2 and

Ti(OnBu)4 ........................................................................................ 33

4.1.1.2 Monitor grafting of -CL from a cellulose model surface in QCM-D ......................................................................................................... 37

4.1.1.3 PCL-grafted cellulose microspheres Impact of graft length and temperature on interfacial adhesion a study by colloidal probe by AFM ................................................................................................. 41

4.1.1.4 Solid state CP/MAS 13C-NMR investigation of HCCL grafted by SI-ROP of -CL .................................................................................... 45

4.1.2 Cellulose surface modification by ring-opening metathesis polymerization (ROMP) .................................................................. 50

4.2MODIFICATION OF CELLULOSE SURFACES BY PHYSICAL ADSORPTION ....... 534.2.1 Synthesis of cationic latex nanoparticles ......................................... 534.2.1.1 RAFT-mediated surfactant-free emulsion polymerization of MMA . 544.2.2 Adsorption of cationic latex on cellulose model surfaces ................ 57

5. CONCLUSIONS 616. FUTURE WORK 637. ACKNOWLEDGEMENTS 658. REFERENCES 67

PURPOSEOFTHESTUDY

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1. PURPOSEOFTHESTUDY

The interest indesigning anddeveloping new,more environmentallyfriendlymaterials from cellulose has increased immensely in the lastyears. Cellulose has several interesting properties, e.g., high stiffnessand low density compared to commonly employed glass fibers, butcellulose isalsohighlyhygroscopic,asa resultof the largenumberofhydroxyl groups. These can in turn be readily utilized for surfacemodification; hence, the cellulose properties can be tailored and newfunctionalities introduced, improving thecompatibilitywithnonpolarpolymermatricesforexample.The purpose of this study was to expand the toolbox for surfacemodificationofcelluloseandtherebypossiblyincreasetheutilizationofcellulosebased materials. Different techniques were employed toachieve a better fundamental knowledge of surface modification bypolymersand itseffecton thesurfaceproperties.Surfacemodificationhasbeenperformedbycovalentgraftingandphysisorptionofpolymers.The polymers have been synthesized by controlled polymerizationtechniques.

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2. INTRODUCTION

2.1 CELLULOSECelluloseisoneofourmostabundantpolymersonearth.Thepolymeris renewable, biocompatible, biodegradable, and inexpensive, and hasbothinterestingphysicalandchemicalproperties.1,2Thecellulosefibershavebothlowdensityandhighstrength.Themodulusofthecellulosecrystal is138GPa,whichcanbecompared tosteelwithamodulusof200GPa.3 The production of cellulose ismainly performed in plants,e.g.,flax,hempandjute.4However,therearealsoseveralothersourcesofcellulose,e.g.,wood,whichhasadrycontentof40%cellulose.5 Inaddition, wood contains significant amounts of lignin and differenttypesofhemicelluloses.5Therefore,thecelluloseneedstobeisolatedbyseparation from the other components prior to utilization. Bacteria,algae,andfungicansynthesizecellulosewithhighpurity,crystallinity,and very specific morphologies can be obtained.6 The total annualproductionofcelluloseisestimatedtobeover7.51010tons.1

2.1.1 StructureofcelluloseCellulose isa linear,polydispersebiopolymer;exhibitinganadvancedhierarchical structure that can be attributed to the hydrogen bondsbetweenthehydroxylgroups.Thedegreeofpolymerization(DP)variesbetween3001,700forwoodfibersand80010,000forcottonandotherplants,dependingonoriginandtreatmentofthecelluloserawmaterial.4Thecellulosefibersarelocatedinthecellwallofplantsandarebuiltupof aggregates ofmicrofibrils, see Figure 1. The bundle ofmicrofibrilscontains3040cellulosechainswithdifferentorientationsandisformedby extended cellulosemacromolecules that are organized in sheets withbothcrystallineandamorphousregionswhicharestabilizedbyintraandintermolecularhydrogenbonding.4,7

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Figure1.Hierarchicalstructureofcellulose;fromthetreeonamacroscopicscaledown to thecellulosemacromoleculeonnanoscalewith length (L)and lateraldimension (dL).4, 7 The schematic picture is adopted from Isogai et al.7 andHansson8.

ThebiopolymerconsistsofDglucosylunitsthatarelinkedtogetherby1,4glycosidic bonds. Each repeating unit in cellulose contains twoanhydroglucose units (AGU). The AGU, see Figure 2, has threehydroxylgroupswith aprimaryhydroxylgroup at the carbon 6 (C6)positionandtwosecondaryhydroxylgroupsattheC2andC3positions,respectively.Every second repeatingunit is rotated 180 in theplane.Cellulose is highly hydrophilic owing to the numerous hydroxylgroups;however,thestronginterandintramolecularbondingrenderscelluloseinsolubleinwater.Theexceptionalmechanicalpropertiesandthe lack of a melting point can also be ascribed to these hydrogenbonds.9

Figure2.Numberingof thecarbonatoms inarepeatingunitofcellulose (twoAGU)ofthecellulosechain.

O

1

45O

HOOH

6

O23

OH

O

1

45

HOOH

6

O23

OH

n

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2.1.2 CellulosicsubstratesCellulosecanbedividedintogroupsdependingonitscrystalstructure.The naturally occurring cellulose is denoted celluloseI where thepolymer chainsareorganized inaparallelarrangement.However,bytreatmentofcelluloseI,themostthermallystablestructure,celluloseII,isobtainedbychangeoftheorganizationofthemacromolecularchainsto an antiparallel arrangement.The transformation canbeperformedby conc. alkali aqueous solution or by regeneration from solutions orsemistablederivatives.RegeneratedcellulosehasaDPof250500.Theviscous process was developed over 100 years ago to regeneratecellulosic fibers.4This techniquehasbeenemployed toproduce fibers,films,membranes,sponges,andspheres.However, themethodsuffersfrommajor disadvantages since it requires hazardous chemicals andonlyhighlypurepulpscanbeutilized.AnimportantbreakthroughfortheregeneratedfiberswastheLyocellprocesswherethecellulosefiberssuccessfullyweredissolvedinNmethylmorpholineNoxide (NMMO)andthereafterspun,formingregeneratedfibers.10Inthisprocess,almostallemployedsolventscanberecycledandtheenvironmentalimpactislow. By employing the Lyocell process, Wgberg et al.11, 12 havedeveloped cellulosemodel surfaces. These surfaces are smooth, havegoodstabilityindifferentsolventsandthethicknessofthefilmscanbetuned, rendering the surfaces suitable for highresolution techniques,suchasatomicforcemicroscopy(AFM)andquartzcrystalmicrobalancewithdissipation(QCMD).13Thenanocellulosesareproducedbydisintegrationof thewood fibers,smaller fibrils can be liberated which increases the available surfacearea.14 For a summary of the characteristics for the most commonnanocelluloses,seeTable1.MicrographscanbeobservedinFigure3.

Figure 3. TEM images of a) CNF,15 b) CNC,16 and c) SEM image of BNC.14Reprintedwithpermissionfrom(Klemm,D.etal.,Angew.Chem.,Int.Ed.201150p.54385466).Copyright(2011)GermanChemicalSociety.

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Table1.CC

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CNFandtheothernanocellulosesarealreadycommercializedinseveralproducts,e.g.,absorbentmaterialinhygieneproducts,foodadditives,cosmetics,andpharmaceuticals.17CNFhaverecentlybeenemployedforproducingnanofoams,18,19hydrogels,20nanopapers,21,22andaerogels23.Cellulose cotton linters (CCL) are short (26mm), slightly curled,cylindrically shaped fibers with widths of 1727m.24 Furthermore,CCLpossesshighercrystallinityand smaller lateraldimensionsof themicrofibrilscomparedtocellulosefromwoodfibers.4HighqualityCCLare utilized to produce Whatman #1 filter paper which has a highcellulosecontent(>98%)andacrystallinityof68%.25

2.1.3 SurfacemodificationofcelluloseThe firstpatent thatreportedsurfacemodificationofcellulose isdatedtothe1870s.Thefiberswerereactedwithnitrosulfuricacid(amixtureof nitric and sulfuric acid) to form cellulose nitrate, and by adding aplasticizer, the first cellulosebasedmaterial, celluloid,wasproduced.4In recent years, the interest and utilization of cellulose fibers haveincreased tremendously with the aim to create new and sustainablematerialsdeveloped frombiobasedresources.4,6,14,26,27 It ispossible toobtainrenewablecompositeswithlowerdensityandcostcomparingtoother fillers or if reinforcements, such as glass or carbon fibers, areemployed. Moreover, a cellulosebased composite can be recyclable.Nanocelluloseswithhighcrystallinityareofparticularinterestowingtotheir high stiffness that can reinforcepolymermatrices.The extent ofamorphousdomainsisdependingonthepretreatmentofthefibers.Theamorphouspartsenhancetheflexibility.9Cellulose can be applicable in many products; recent publicationsdiscussawide rangeofexcitingareas,e.g., inexpensiveelectronics,2830paperbased medical diagnostics,3133 functional clothes,34, 35 andmembranes36,37.However, it isachallenge to incorporateanddispersecelluloseduetotheincompatibilityofthepolarcellulosewithnonpolarpolymermatrices.Goodcompatibilityisessentialtodrawbenefitsfromthefibersasreinforcement.Furthermore,thehydrophiliccharacterwillmost probably lead to moisture adsorption and swelling of thecomposite.However,theavailablehydroxylgroupsoncellulosecanbeemployed formodifications as they can act as chemical handles. Thenumber of available hydroxyl groups can be tuned by different

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pretreatmentmethods.Forexample,withmercerization, i.e., treatmentwithastrongbase followedbyneutralization, theamountofhydroxylgroups is increased by breaking the hydrogen bonds, resulting inswellingofthecellulosestructureand,thus,increasedsurfaceareaandstrengthisachieved.Still,itshouldbetakenintoconsiderationthatthemechanical properties of cellulosemay be affected upon breakage ofintermolecularhydrogenbonds.4Surface modification of cellulose can be performed by physicaltreatmentssuchassolventexchange,physicochemicalmodificationsbycorona or plasma discharges, laser, UV or irradiation, or physicaladsorption. Chemical, i.e., covalent, modification is performed byattachmentofsmallmoleculesorpolymers.26,27,3844Thedifferent typesofmodificationscanalsobecombined.Traditionally, surfacemodificationof celluloseby smallmoleculeshasbeenthemostutilizedtechniquetointroducevariousfunctionalitiesonthesurface.Anhydrides,isocyanates,organometallics,sulfates,andacidchlorides are examples of molecules that successfully have beenemployed to obtain functional groups on the fiber surface.6,26,27,38,4547Celluloseestersandethersarethemostcommonexamplesofcellulosederivatives. Thewater or organosoluble derivatives are obtained bysubstitutionofthehydroxylgroups.Celluloseacetate,themostcommonester, isproducedbyreactionwithaceticanhydride in thepresenceofsulfuricacidascatalyst.Celluloseestershave thermoplasticpropertieswhichareinfluencedbythecarbonnumberoftheacylresidues.Watersoluble cellulose ethers are formed by alkaline treatment andsubsequentsubstitutionofanalkylhalideoradditiontoanoxirane.4Freeradicalpolymerization(FRP)ofvinylmonomershavesuccessfullybeenperformedfromthecellulosebackbone.Theinitiatingradicalscanbeformedbyhydrogenabstraction(chaintransfer),byemployingredoxsystems or by utilizing substituents that can form radicals orpolymerize.48,49However,due topoorcontrol it is impossible to tailorthemolecularstructureorthemolarmassbyFRP.Otherdrawbackscanbe thepossibleoccurrenceof chain scissionof the cellulosebackbone,which may influence the strength of the final material50, as well asunattachedpolymerformedinthebulk51.Modification of cellulose by grafting polymer chains from its surfaceenables alteration of both the physical and chemical properties and

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couldpotentially introduce other functionalities.4144,5254The twomostcommonly employed grafting techniques are graftingfrom andgraftingto, seeFigure4. In the graftingfrommethod thepolymer isformed by propagation of monomer from initiating species (reactivecenters) on the surface, i.e., surfaceinitiated (SI) polymerization. Inordertofullycharacterizethepropertiesofthegraftedpolymerchains,cleavageofthechainsandsubsequentisolationisnecessary.Thiscanbeperformedbyacidhydrolysis,whichdecomposescellulose,leavingthepolymergrafts intact.Nevertheless, thismethodcannotbeapplied forpolymerscontaininghydrolyticallysensitivegroups.44,55Tocharacterizethe grafted polymer without cleaving it from the surface is a greatchallenge;therefore,thegraftingtotechniquecanbepreferable.

Figure 4. Schematic illustration of the graftingfrom and the graftingtotechnique.

In the graftingto technique, preformed polymers, with an activechainendthatcanattachcovalentlywithreactivecenterspresentonthecellulose surface.Recently,polymerswith two active chainendshavebeen reported,which enables furtherpostmodification of thegraftedpreformed polymer.1, 6 An advantage by employing the graftingtotechnique is thepossibilityof characterizing thepreformedpolymers,molar mass and the molarmass dispersity (M) prior to the surfaceattachment. However, previous studies have shown that covalent

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surfacemodificationbygraftingfromenableshighergraftingdensitiesduetolessstericalhindranceofmonomerscomparedtowhenlargeandbulky prepolymers are coupled to the surface.1, 42, 52, 53Nevertheless,therearesomereportsintheliteraturewheresimilargraftingdensitieshavebeenobtainedbythegraftingtoapproach.5658

2.2 RINGOPENINGPOLYMERIZATIONTECHNIQUESRingopening polymerization (ROP) and ringopening metathesispolymerization (ROMP) are two techniques that are suitable for SIpolymerizations.59 These techniques are widely utilized forpolymerization of cyclic monomers. The polymerization mechanisminvolves three reaction steps: initiation,propagation, and termination.Theemployed initiatorvariesdependingonthetechniqueappliedandtheringopeningof thecyclicmonomeroccursduring thepropagationstepinpresenceofacatalyst.Undesiredterminationmayoccurbyinteror intramolecular chain transfers. Intermolecular chain transfersoccurbetweendifferentchainswhereas intramolecularchain transfers (backbiting)occurswithin thepolymerchain, formingcyclicoligomers.Thechain transfer reactionswill give rise to deviation from the targeteddegreeofpolymerization(DP)andincreasetheM.

2.2.1 Ringopeningpolymerization(ROP)ROP, developed in the 1930s by Carothers et al.60, providingmacromoleculeswithwelldefined endgroups and highmolarmass.Sincethen,numerouscyclicmonomerssuchaslactones,lactides,cycliccarbonates,siloxanes,andethershavebeenpolymerizedemployingthistechnique.61Theeaseofpolymerizationof thecyclicmonomerscanbeattributedtobothkineticandthermodynamicfactors.Thepresenceofaheteroatom(oxygen,nitrogen,sulfur,etc.)intheringfacilitatestheringopening via a nucleophilic or electrophilic attack of the initiator.Moreover, thereactivityof themonomerscanbeattributed to theringsize, i.e., thermodynamic stability decreases reactivity due tounwillingnesstochangestate.62Poly(caprolactone) (PCL) is an aliphatic polyester, and its physical,mechanicalandthermalpropertiesaredependentonthemolarmass.63PCLpossessesadvantagessuchasgoodmiscibilitywithotherpolymers,biodegradability and biocompatibility. The utilized catalyst system

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determines whether an anionic, a cationic, monomeractivated orcoordinationinsertion mechanism occurs.63 Several different catalyticsystems have been explored; metalbased, organic or enzymatic.6366Three examples of catalysts are: the metalbased stannous 2ethylhexanoate (Sn(Oct)2), titanium nbutoxide (Ti(OnBu)4), and theorganic1,5,7triazabicyclo[4.4.0]dec5ene(TBD).

Scheme 1. ROP of CL by coordinationinsertion mechanism employingSn(Oct)2ascatalystandanalcoholinitiator(ROH).

Themostcommonlyutilizedcatalyst isSn(Oct)2,owing to itsexcellentperformance, good thermostability, low cost, reasonably low toxicity,and approval in food and drug applications (FDA approved).67 Theproposedmechanism forSn(Oct)2iscoordinationinsertionreaction,aspresented in Scheme 1. The initiation is divided into two steps formation of a metal oxide alkoxide (OctSnOR) from the catalyst(Sn(Oct)2) and the initiator (ROH) which subsequently initiates thepolymerization ofmonomer by the coordinationinsertionmechanismthroughcoordinationofthemonomertothecatalystandinsertionofthemonomer intoametaloxygenbondofthecatalyst.Themetalalkoxideremains as the active center throughout all the polymerization.However,therearesomedrawbacksofthiscatalystsystemintraandintermoleculartransesterificationreactionsmayoccurwhichwillleadtobroadermolarmassdispersitiesanddeviationsfromthetargetedmolarmass.Theextentoftheundesiredterminationreactionsisdependentonthe temperature and themonomer conversion.68Anotherdrawback isthe difficulty in removing the catalyst, and the presence of tin inbiomedicalor food applications ishighlyundesirable; therefore,othercatalystsystemsareofprimeimportance.Hence,inrecentyears,alotof

HO SnO

OR-O

O

O

HR-O

O-O

SnO

O

R-OO

O

O

OSn OR

OO

O

OSn OR

O

O

O

HO

O

OSn OR

OHR O

OSn

O

Otin 2-ethylhexanoate

(Sn(Oct)2)

++

Coordination Insertion

1. n e-CL2. H

n

+

+

poly(caprolactone)PCL

-CL

Preinitiation

Initiation

Propagation

Oct-Sn-O-R Oct-H

Oct-Sn-OR

Oct-Sn-OR

SnO

O

INTRODUCTION

11

research has been conducted to find new, more environmentallyfriendlyand less toxiccatalysts forROP.63,65,66,69,70Thishasresulted indevelopmentofbothnewmetalbased71andorganiccatalysts65forROP.In 2010,Prssinen etal.72utilized titanium ipropoxide (Ti(OiPr)4) andtitaniumnbutoxide(Ti(OnBu)4),forROPinairatmosphereatelevatedtemperatures, 70140 C. The chemical structure of Ti(OnBu)4 ispresentedinFigure5.Anadvantagewiththosecatalystsisthenontoxicdegradationproducts,titaniumoxideandalcohol,whichareapprovedfor internal use in humans.72 They report openair systems requiringhigher temperatures (140 C) for efficient initiation attributed todecreasing water content. However, after initiation the reactiontemperature couldbedecreased slightlyover themelting temperaturefor PCL (~60 C).73 Noteworthy, M was quite high and the valuesvariedfrom1.52,however,by1HNMRitwasconfirmedthatthePCLwasonlyformedfromoneofthearmsinthecatalyst.ROP has also been performed by employment of enzymatic catalystswhich enables mild reaction conditions.66 Several different catalystsystems,employingaminoacidsandsmallorganicmolecules forROPof cyclic lactones, have also been reported as successful.63, 70, 74, 75Furthermore,Hedrick etal.76havedeveloped efficientorganocatalyststhatenablepolymerizationatroom temperaturewithgoodaccordancebetween theoretical and experimentalmolarmass, andnarrowmolarmass dispersities. An example is 1,5,7triazabicyclo[4.4.0]dec5ene(TBD),whichpolymerizesbythedualactivationmechanism,includingbothmonomerandinitiatorinatransitionstatewiththecatalyst.76

Figure5.DifferentcatalystsemployedforROP.

SIROP has beenwidely utilized for surfacemodification of differentsubstrates,e.g.,silicaandgoldsurfacesbyselfassembledmonolayers,77clay minerals,78 nanoparticles of silica and cadmium sulfide,79magnetite,80 and awide rangeofdifferent cellulosic substrates,43 andmorerecently,inionicliquids75.

N

N

NH

O

O

Sn

O

O

TiO

O

On-Bu

On-Bu

n-Bu

n-Bu

tin 2-ethylhexanoate (Sn(Oct)2)

1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)

titanium n-butoxide (Ti(On-Bu)4)

INTRODUCTION

12

2.2.2 Ringopeningmetathesispolymerization(ROMP)ROMPhashada large impact inbothorganicandpolymerchemistryfor olefin metathesis over the last decade.8183 The polymerizationtechniqueisachaingrowthprocesswheremono,biorpolycyclicringsareopenedandconvertedintoapolymericmaterial.81In1957,Eleuterioetal.84atDuPontwerethefirsttopatentthistechnique.In1960,Truettetal.85 reported the first polymerization of norbornene(bicyclo[2.2.1]heptane) (NBE) byROMP.The ringopeningmechanismofnorbornenewasconfirmedbyDallAstaetal.88,89 inthebeginningofthe1970s.ThemonomerandpolymerstructurecanbeseeninFigure6.ROMPisahighlyinterestingpolymerizationtechniquesincehighmolarmasspolymerswithnarrowmolarmassdispersitiescanbeobtainedinshort reaction times and under mild reaction conditions. Moreover,preserved double bonds in the polymer backbone enable facile postmodification.

Figure6.Molecularstructureofnorbornene(NBE),andtherepeatingunitofthecorrespondingpolymerpoly(norbornene)(PNBE).TopolymerizeNBE,Grubbs1stgenerationcatalystisoftenutilized.

Initially,ROMPrequiredstringentreactionconditionsincludingtheuseof highly pure reagents and the total exclusion ofwater, oxygen andotherfunctionalgroups.However,owingtotheNoblePrizelaureatesinChemistryin2005Schrock86andGrubbs87andtheirdevelopmentofless sensitive catalysts for ROMP, the interest for the polymerizationtechniquehasincreasedremarkablyduetotheincreasedstabilityofthecatalysts and, thus, a wider range of polymerizable monomers withfunctional groups etc.81, 88Grubbs developed ruthenium alkyl carbene

n

Ru

P

ClP

ClPh

norbornene(NBE) polynorbornene

(PNBE)

Grubbs' 1st generationcatalyst

INTRODUCTION

13

thatareefficientinitiatorswithhighactivityforpolymerizationofNBEandsubstitutedNBEs.89 Inaddition toorganicsystems, thesecatalystscanalsobeemployedinaqueoussystemsformetathesis.90TheROMPmechanismofNBEisbasedonolefinmetathesiswheretheringopening occurs at themost stable site of themonomer, i.e., thedoublebond,seeScheme2.

Scheme2.SchematicoverviewoftheROMPprocess.Thecriticalstepuponpropagation istheformationofthe intermediatemetallocyclobutane by coordination of theNBE by the catalyticmetalcenterof thecarbenecomplex, followedby [2+2]cycloadditionwherethepropagating species is formed.Themetalatom separates from theolefinbondbyacycloreversion.82Commonly, transvinyleneunitsareformed during ROMP ofNBEwhenGrubbs catalysts are employed.Undesiredsecondarymetathesisreactionsleadtoterminationandoccurby intra or intermolecular chain transfer, yielding polymers withbroadermolarmassdispersitiesandilldefinedendgroups.Theextentof the termination reactions is dependent on many parameters, e.g.,bulkiness of themonomer, temperature,monomer concentration, andsolvent.83ThepioneerworkofSIROMP fromagold surfaceby the rutheniumbasedGrubbs1stgenerationcatalystwasperformedbyGrubbs etal.91in1999.Thereafter,numeroussubstrateshavebeensurfacemodifiedbySIROMP, e.g., silicon wafers,92 quartz,93 Wang resins,94 silicananoparticles,95carbonnanotubes,96gold,97clay,98andcellulose99.

LnM

R

LnM

R

LnM

LnMR

LnMR LnM

R

m

LnMR

m+2

LnMR

m+2X=Y MLn=X Y

R

m+2

coordination

metalalkyldiene

metallacyclobutane

Termination:

Propagation:

Initiation:[2+2]

INTRODUCTION

14

2.3 REVERSIBLEADDITIONFRAGMENTATIONCHAINTRANSFER(RAFT)POLYMERIZATION

During the last years,muchwork has been conducted in the field ofpolymersynthesiswiththeaimtodesignwelldefinedmacromolecularstructures with narrow M and endgroup functionalities.100 Tailoredpolymers can be synthesized by reversibledeactivation radicalpolymerization (RDRP) techniques. The most commonly employedRDRPtechniquesincludesnitroxidemediatedpolymerization(NMP),101atom transfer radical polymerization (ATRP),100, 102 and reversibleadditionfragmentation chain transfer (RAFT)103, 104. Among thosetechniques,RAFTisthemostrecentlydevelopedandprobablythemostversatile.105, 106 The RAFT technique has versatility for numerousdifferentmonomers and polymerization can be conducted in awiderange of reaction media, organic solution, aqueous solution, or indispersed phase.107, 108 Therefore, this technique has been extensivelyemployed for controlled polymerization of both, hydrophilic andhydrophobic monomers.104, 106, 108, 109 Moreover, a good control of themacromolecular structure was achieved and, as the RAFT chainendwaspreserved to a large extent, facilepostfunctionalization could beperformedofthepolymers.100,106,107InareviewonRAFT,Moadetal.104,morethan700publicationsfromthemiddleof2009untilthebeginningof 2012was cited,whichdemonstrate the intenseongoing research inthisfield.RAFTpolymerization includes the same steps as classical free radicalpolymerization (FRP): initiation, propagation and termination.However,inaRAFTsystemtheterminationreactionsaresuppressedbyfastchaintransfer.Thepolymerizationstartsfromradicalinitiatorsthatare activated in a classical manner, either by heating, redox,photochemistry,UVorgammairradiation.Theadditionofanexternalsourceofradicalsisrequiredtoinitiatepolymerizationandmaintaintherate of polymerization.105 However, RAFT polymerization is a moresophisticatedtechniquecomparedtoFRPandthecontrolisprovidedbydegenerative(i.e.,thermodynamicallyneutral)chaintransfer,involvingtransfer of an atom or a group from a covalently bond and dormantspecies (i.e., the chain transfer agent,RAFTagent) to the active chainend.TheRAFTagent,seeFigure7,iscomposedofanactivatinggroupZ and a good free radical leaving group, R. The RAFT agent

INTRODUCTION

15

provides controlof thepolymerizationand shouldbepresent in largeexcesstotheinitiator103,104,110tosuppressirreversibleterminationoftheactive,propagatingchains.102,103,109

Figure 7. An example of a RAFT agent, the thiocarbonylthio chain transferagent.105

The mechanism of RAFT polymerization employing thethiocarbonylthio RAFTagent is presented in Scheme 3. In the initialstep,afreeradical,I,isformedwhichpropagateswithmonomerunits,M.Thisisfollowedbythepreequilibriumstepwhichoccursassoonasthe RAFTagent reactswith the radical in the propagating oligomer,(Pn).TheradicalpositionisthentransferredtotheRAFTagent,formingarelativelystable (dormant)specie.Further, theradical iseither transferredbacktothepropagatingoligomer,whichenablespropagation,ortotheRgroup.Inthiscase,theRgroupwillactasaninitiator,andstartaddingmonomerunits (Pm),until itgoesback to thedormant sidebyreactingwiththeRAFTagent.Thisprocessisfastwhichisvitalforthecontroland,hence,themainequilibriumshouldbereachedearlyintheprocess.TherateconstantsarestronglydependentontheZgroup.Thethiocarbonylthio groups are highly efficientRAFT agents,which is ofhighimportancetoobtainagoodcontrolofthepolymerization.

Z

SS R

INTRODUCTION

16

Scheme 3. Mechanism of RAFT polymerization by employing thethiocarbonylthioRAFTagent.The versatility of theRAFT technique has permitted awide range ofmonomers for polymerization, for instance, acrylates, methacrylates,and vinyl acetate.104, 111, 112However, the environmental consciousnesshas increased and in this context, water is the preferred reactionmediumnotonlydue toeconomic reasonsbutalso forenvironmentaland health aspects.106 Hence, polymerization of watersolublemonomers,suchasN,Ndimethylaminoethylmethacrylate(DMAEMA),has gained significant attention. The structure of the monomer,DMAEMA, and corresponding polymer PDMAEMA are shown inFigure8.

Figure 8. Molecular structure of N,Ndimethyl aminoethylmethacrylate(DMAEMA) and the repeating unit of the corresponding polymer,P(DMAEMA).

InitiationInitiator I M M Pn

Initialization/pre-equilibrium

Pn

M kp Z

SS

R kaddk

-addS S

Z

RPnkbk

-bS S

Z

RPn +

Reninitiation

R M Pm

Main equilibrium

Pm

M kp Z

SS

PnkaddPk

-addPS S

Z

PnPmkaddPk

-addPS S

Z

PnPm

M kpTermination

Pn Pm+kt Dead polymer

+ +

+

kd kiMkp kp

P1

kiRMM

kp kpP1

O

N

O

DMAEMA

O

N

O

PDMAEMA

n

INTRODUCTION

17

The polymer is both thermo and pHresponsive. Stimuliresponsivepolymers can change conformation, properties and interactions inresponse to external stimuli, e.g., switching from hydrophilic tohydrophobic.113Themolarmassofthepolymerinfluencetheresponsiveproperties.114 Hence, they are interesting for a wide range ofapplications.115 PDMAEMA has been employed in biologicalapplications, forantibacterialactivity,116,117genedelivery,118 aswellasother fields,e.g.,wastewater treatment,119andpaints120. It is thepolartertiary amine sidechain group that offers water solubility and, ifdesired, canbe charged eitherwhenbelow itspKavalue (close to7.5,slightly chain length dependent)114 or irreversibly charged byquaternization.However, thereare few earlier investigationsavailablewhereDMAEMAhasbeenpolymerized inaqueousmediabyATRP121,122orbyRAFT123.SurfaceinitiatedRAFT (SIRAFT)hasbeenperformed fromnumeroussurfaces,forinstance,goldnanoparticles,124silicananoparticles,125siliconwafers,126proteins,127andvariouscellulosesurfaces52,55,128,129.

2.3.1 RAFTmediated surfactantfree emulsionpolymerization

RAFT polymerization, enables development and design of new,advanced and tailoredmacromolecular architectures, both for homoandblock copolymers.100Amphiphilicblock copolymers, composedofbothahydrophilicandahydrophobicblock,130132arehighlyinterestingin applications such as compatibilizers. However, the synthesis ofamphiphilic block copolymers is demanding due to the difference inpolaritybetweentheblockswhichtypicallycausesatleasttwo(ormore)steps, including thorough purification. The synthesis of blockcopolymerscanbeperformedbyvariousmethods;twoexamplescanbeseeninFigure9.

INTRODUCTION

18

Figure9.Synthesisofblockcopolymersinatwostepprocessbya)bycouplingtwopreformedmacromolecularchainstogetherorb)firstformingablockwithliving chainends that can be employed to initiate the growth of the secondblock.Previously, the RDRP techniques have primarily been performed inorganicsolventsorbulk.However, the interest in the fieldofaqueousdispersionshas increased tremendously.133,134All theRDRP techniqueshave successfully been employed in dispersion.135 RAFT has thus farbeen themost successful technique forwaterborne systems.134, 136 Byemploying dispersed systems, nanoparticles (d=20nm10m) withdifferentmorphologies(coreshellparticles,hollowparticlesorcomplexmultilayerstructures)areformedwhichmaybe interestingforvariousapplications,seeFigure10.133

Figure10.P(MAAcoPEOMA)bpolystyreneamphiphilicblockcopolymerselfassemblies.137Reprintedwithpermissionfrom(Zhang,W.etal.,Macromolecules2012,45,p.40754084).Copyright(2012)AmericanChemicalSociety.

Classicalemulsionpolymerizationhasbeenemployedtoalargeextentinindustrysincevolatileorganicsolventscanbeavoided,awiderangeofmonomers canbeutilized and this technique enablesprocessing athigh solid content without any influence on the viscosity.134 Thetechnique requires employment of an organosoluble monomer, awatersoluble initiatorandasurfactant.The threeregimesofemulsionpolymerizationaredemonstratedinScheme4.

INTRODUCTION

19

Interval I: Initially, an emulsion polymerization system consists of adispersion of monomers in an aqueous phase. The initiation alwaystakesplaceintheaqueousphase.Thereafter,twodifferentmechanismsmayoccur,micellarnucleationorhomogeneousnucleation,dependingoftheconcentrationofthesurfactant.Themonomerisentrappedinthemicelles if the concentration of surfactant is above the criticalmicelleconcentration (CMC). During the nucleation, particles are formedwhereinthepolymerizationwillproceed.IntervalII:Duringthesecondinterval,thepolymerizationisproceedingintheparticles.Thegrowingparticlesareconstantlyfedwithmonomerby diffusion from the droplets through the aqueous phase into theparticles.Theparticlesareincreasinginsizeduringthepolymerization.However, the number of particles and the polymerization rate areconstant throughout the process. When the monomer droplets areconsumed,thethirdintervalstarts.Interval III:The residualmonomers in theparticleand in theaqueousphase are consumed.At the end of interval III, the polymerization isterminated.

Scheme 4. Schematic illustration of intervals in an emulsion polymerization,adoptedfromZhang.138

2.3.1.1 PolymerinducedselfassemblyMore recently, the development of RAFTmediated emulsionpolymerizationhas led to the aqueous synthesis of amphiphilic blockcopolymers,whichselfassembletoformpolymerparticlesaccordingtothepolymerizationinduced selfassembly (PISA) strategyasoriginallydescribedbyHawkettetal.139141ThePISAprocesshassuccessfullybeenemployed for all three RDRP techniques.134, 140 In the RAFTmediated

INTRODUCTION

20

PISAprocess,itiscrucialtochooseasuitableRAFTagentthatprovidesgood control over formation of both blockswhen synthesizing blockcopolymers.107 Recently, new RAFT agents have been developed thatcan be employed for polymerization of both hydrophilic andhydrophobicmonomers,wherethefirstformedhydrophilicpolymerisutilized as a macroRAFT for formation of the second hydrophobicblock.133ThemacroRAFTisactingasanelectrostericsurfactantasitbothfunctionalizes the surface of the particles and controls the particlegrowth, see Scheme 5. No additional low molarmass surfactant isrequired, which is highly advantageous since the low molar masssurfactants have an undesired impact on the latex stability underfreezingconditionsanddiffusionuponfilmformation.142

Scheme5.AnoverviewofthePISAprocess.AdaptedfromChaduc.143

InFigure11,thestructureofathiocarbonylthioRAFTagent,4cyano4thiothiopropylsulfanylthiocarbonyl (CTPPA)142 is shown, which isobtained by reacting the radical initiator, 4,4Azobis(4cyanovalericacid) with bis(propylsulfanylthiocarbonyl) disulfide according to theliterature.144, 145 CTPPA permits polymerization in water despites itsinsolubilityinwater,asitissolubleinmanyhydrophilicmonomers.142,146151

INTRODUCTION

21

Figure11. The RAFT agent, 4cyano4thiothiopropylsulfanylthiocarbonyl(CTPPA).

Previously,thefirstdevelopedmacroRAFTagentswerepresynthesizedbefore being employed in water (i.e., a twopot process). However,DAgostoetal.148recentlyperformedPISAaccordingtoatwostep,onepot procedure. The synthesis of the hydrophilic macroRAFT isperformed in water up to full monomer conversion and directlyfollowedbyadditionofthehydrophobicmonomer inthesamereactorandbytheformationofthepolymerparticles,withoutpresenceofanyadditional molecular surfactant. This simple, robust and ecofriendlystrategy has successfully been applied to various hydrophilicmonomers,e.g.,acrylicacid,146methacrylicacid(MAA),152andamixtureofMAA and poly(ethylene oxide)methyl ethermethacrylate.137, 149, 150Various hydrophobicmonomers such as styrenics and (meth)acrylicshave been employed to synthesize the hydrophobic core, leading tonumerous compositionsofamphiphilicblock copolymers. Inaddition,differentmorphologieshavebeenobtained,137,150 similarly to thepostpolymerization selfassembly of amphiphilic block copolymersconductedatvery lowconcentrations (

INTRODUCTION

22

ThepreinitiatingstepcanbecircumventedbyinsteademployingROP,since this polymerization can be initiated from the hydroxyl groupsalready present in the cellulose structure.43 This technique has beenwidely utilized for numerous cellulose substrates: solid fibers andfibrils, dissolved celluloses, and cellulosic derivatives, employingheterogeneous and homogeneous systems aswell as graftingto andgraftingfrom.6,38,4547 Furthermore,more environmentally friendly SIROPsystemshavebeenconductedbyemployingenzymes.43HafrnandCordovaperformedpioneeringworkinthefieldofSIROPfrom cellulose surfaces, e.g., cotton and filter paper (FP), using anorganic catalyst.155 Belgacem et al.156 reported surface modification ofCNCandbleachedsoftwoodkraftpulp ina threestepprocesswhereone chainendofpreformedPCLwas reactedwithphenyl isocyanateand theotherwith2,4toluenediisocyanate.ThedifunctionalizedPCLwassubsequentlyattachedtocelluloseinamixtureofdichloromethane(DCM) and anhydrous toluene in a graftingto process.Despite lowamounts of grafted PCL, the surface energy of the resultingbiocomposite changed considerably.Hult etal.were the first to reportSIROPfromCNF,resultinginPCLgCNFthatcouldbedispersedinanorganicsolventwhich isnotpossible forunmodifiedCNF.157DufresneandDubois etal.158 have also reported improveddispersity ofPCLgCNC in pure PCL prepared by extrusion. They found a significantdifference inthermomechanicalperformanceandrheologicalbehaviorsbetween the PCLgCNC compared to the unmodified CNC.Furthermore,Dufresne etal.159have reportedgraftingof fattyacidsorpolymersontoCNC.ThegraftedCNCwere successfullydispersed inpolymermatrices,e.g.,PCL,polyethylene,andPLA160byextrusion.Hult etal.161utilizedPCLgCNF toproducebilayer laminates togetherwith pure PCL. By dynamic mechanical analysis (DMA) it could beconcluded that the graft lengths had a large influence on the finalpropertiesintermsoftoughnessoftheinterfaceandthepeelingenergy,whichwasascribedtotheincreasedentanglementsoccurringforlongerpolymergrafts.MalmstrmandRutlandetal.162employedthecolloidalprobetechniquebyatomicforcemicroscopy(AFM)tostudythefrictionforces between cellulosemicrospheres (CMS)with orwithout graftedPCL, to achieve an asymmetric system (CMS/PCLgCMS) or asymmetric system (PCLgCMS/PCLgCMS). This study showed that

INTRODUCTION

23

the interfacial adhesionwas improved for the symmetric system as aresultofPCLentanglements.Furthermore,measurementsperformedathighertemperatures, increasedthemobilityofPCL,thus,theadhesionwasimproved.162The graftingto technique can also be employed for covalent surfacemodification of cellulose. This approach has been examined forattachment of small molecules, such as isocyanates, acid chlorides,maleic or succinic anhydrides, or click functionalities to the cellulose,which subsequently enabled attachment of endfunctionalizedpolymers.1, 6, 38, 41, 45, 47Different click chemistries can be utilized, themost studied one being the copper (I)catalyzed 1,3dipolarcycloadditionofalkynesandazides (CuAAC),but thiolenechemistryhasalso frequentlybeenapplied.1,27,163 Click reactionsareconsideredtobeselectivereactionsproceedingwithhighyieldand intheabsenceof byproducts.164 In addition, the reactions are orthogonal,modular,wideinscope,andperformedundermildreactionconditions.Recently, click chemistry in water was conducted from alkyne oralkene groups attached to carboxymethylated cellulose which wasadsorbed onto a cellulose substrate.165, 166 Grafting of cellulose with(PCL) by click chemistry has been reported by several groups.167169Graftingto in ionic liquids170172 and employment of bifunctionalcouplingagents1,6hasalsobeenreported.

2.5 SURFACE MODIFICATION OF CELLULOSE BYPHYSICALADSORPTION

Polymers with charges can be adsorbed to an oppositely chargedsurfaceby electrostatic interactions.Thedriving force is the releaseofcounter ions, e.g., gain in entropy upon adsorption. Physical surfacemodification can often be performed undermild reaction conditions,commonly inwater,which is interesting fromanenvironmentalviewpoint.2 The pulp and paper industry has employed cationicpolyelectrolytesfordecadestosurfacemodifyfiberstoimprovesizing,paperstrength,retentionandcreepproperties.173Thisisfeasibleduetothe slightlynegative chargeofnativewood fibers.Thenanocelluloses(CNC and CNF) can obtain further negative charges, e.g., sulfate orcarboxylicgroupsduetopretreatmentsduringtheirproduction.7,14Itisof prime importance that the interactions between the physically

INTRODUCTION

24

adsorbedpolymerandthesurfacearesufficientlystrongsotheinterfaceis stable under conditions exposed to, such aswide temperature andpHranges,differentsaltconcentrationsaswellasdifferentsolvents.Amphiphilicblockcopolymers,140,174aresuitableforphysicaladsorptionto cellulose if one of the blocks are composed of a polymer that cananchortheentireblockcopolymertothesurface,i.e.,ifthehydrophilicblock is charged, or composed of ahemicelluloseor anotherpolymerwhichhasanaturalhighaffinityforcellulose.Oneexampleofahydrophilicblockwhicheasilyadsorbstonegativelycharged cellulose is quaternized poly(dimethylaminoethylmethacrylate) (PDMAEMA), which is a cationicpolyelectrolyte.Seppletal.175reportedphysicaladsorptionbetweenacationic block copolymer of P(ethylene oxide)bP(DMAEMA) andpapersheetswhich improved thepaperstrength.Surfacemodificationof paper sheets by adsorption of amphiphilic block copolymersconsisting of hydrophobicallymodified water soluble methacrylatecopolymers with one block of P(DMAEMA) generated paper sheetswith increasedhydrophobicity.176ByutilizingRDRP techniques, tailormadeblockcopolymersbetweenquaternizedPDMAEMAandanotherfunctionalblockcanbeobtained.Forexample,surfacemodificationofcellulose by amphiphilic block copolymers with PDMAEMA ashydrophilicblockandvarioushydrophobicblocks,e.g.,polystyrene,131or polybutadiene177 have successfully been performed. Several otherstudieshaveutilized thesameconceptwithdifferentcompositions forphysicaladsorptiontocellulose,suchaspolystyrenebpoly(Nmethyl4vinylpyridinium iodide)178andpoly(Nisopropylacrylamide)bpoly(3acrylamidopropyl)trimethylammoniumchloride.179PhysicaladsorptiontocellulosebyamphiphilicblockcopolymersconsistingofquaternizedPDMAEMAandPCLformedbyROPhasalsobeeninvestigated.130Allthesephysicalsurfacemodificationswereperformedinwater.Severalotheraqueoussystemsforsurfacemodificationofcellulosehavebeeninvestigated.Belgacemetal.180performedanonestepinsituminiemulsionpolymerizationofbutylmethacrylate inpresenceofCNC.AcationicsurfactantwasutilizedtoformmicellesforpolymerizationandalsotoenableelectrostaticinteractionbetweenthelatexparticlesandtheCNC.Filmswithgoodopticalpropertieswerepresentedasaresultofthewell dispersedmodifiedCNC. In another study, the same group

INTRODUCTION

25

surfacemodified CNF54 by alkyl ketene dimers (AKD), withoutstabilizer, inpresenceofacationicsurfactant,and subsequentlypapersheetswereprepared.ByemployingmodifiedCNF,themechanicalandbarrierpropertiesweresignificantlyimprovedcomparedtounmodifiedCNF.Anotherbenefitwas the increasedhydrophobicityof thepaper,despiteadditionofsmallamountsofmodifiedCNF.

EXPERIMENTAL

26

3. EXPERIMENTAL

3.1 MATERIALSDetailed information about employed chemicals can be found in thecorrespondingpapers.Filterpaper(FP)Whatmanno.1FPwasutilizedinpapersIandV.Thesubstrateswerecutintopieces2.53cm2andwashedinmethanolandacetoneanddriedinvacuumovenat50Cfor24hpriortouse.Nanopaper(NP)TheNP,utilizedinpaperI,wasproducedaccordingtoSehaquietal.22Anaqueoussuspensionofcellulosenanofibrils(CNF)was prepared by enzymatic pretreatment followed by high shearmechanical treatment in amicrofluidizer according to the literature.21ThehighsurfaceareaNPwasprepared froma0.1%CNFsuspension(300mg dry CNF) by vacuum filtration (0.65m membrane). Thefiltratewassolventexchangedtomethanolandplacedinacriticalpointdryer chamber (Autosamdri815, Tousimis, USA), and liquid carbondioxidewasinjectedintothechamberunderapressureofca.50barforsolventexchange.ThechamberwasthenbroughtabovetheCO2criticalpointconditions toapprox.100barand36 C.Thechamberwas thendepressurized,andCO2evaporatedtoformaporousCNFNP.Cellulosemicrospheres(CMS)TheCMS, inpaperIII,wereproducedfrom regenerated cellulose by the viscose process byKanebo (Japan).The diameters were 1015 m. Prior to use, the CMS were dried invacuumovenat50Cfor24h.Hydrolyzed cellulose cotton linters (HCCL) In paper IV, cellulosecotton linters (CCL) were hydrolyzed according to a proceduredeveloped by Larsson et al.181 Prior to the grafting, the HCCL wassolventexchanged inamultistepprocess,accordingtoKhnkeetal.182Initially, the water was replaced by acetone followed by npentane,repeatedtentimesforeachsolvent,respectively.TheHCCLweredriedunderArovernight.

EXPERIMENTAL

27

CellulosemodelsurfaceInpapersIIandVI,cellulosemodelsurfaceswere utilized. The QCM silica crystals with an active surface ofsputteredsilica(50nmthickness)wererinsedinwaterandethanolandcleaned inaplasmacleaner.Thereafter, thecrystalwas immersed toaPVAm solution (0.01 g L1, pH 7.4). The cellulosefibers (0.25 g)wereadded to a 100 mL glass beaker and dissolved in NMMO (12.5g,50wt%solution inwater)at120Cuntilall the fibersweredissolved(approx.1h).Thereafter37.5gofDMSOwasaddeddropwisetodilutethesolution.AdropofthedissolvedcellulosewasspincoatedontothePVAm coatedQCM crystals and regenerated by exposure toMilliQwater. The obtained cellulosemodel surfacewas assumed to have athicknessof3040nmaccordingtoearlierreports.11

3.2 EXPERIMENTALPROCEDURES3.2.1 Cellulose surface modification by ringopening

polymerizationSIROP of CL from various solid cellulose substrates has beenperformed (papers I IV). Herein, a short description of theexperimental procedures will follow. Detailed experimental setups,procedures and characterization data can be found in the appendedpapers/manuscripts.Table2.OverviewofemployedreactionconditionsforgraftingreactionsbySIROPofCLfromcelluloseinpaperIIV.Paper Cellulose

substrateCatalyst Sacrif.

init.aSolv.b T

(C)Atmosph. t

(h)I NP Sn(Oct)2 Yes 120 Inert 0.81 FP Sn(Oct)2 Yes 120 Inert 0.40.8 NP Ti(OnBu)4 120 Air 19.329 FP Ti(OnBu)4 120 Air 12.522II Cellulose

modelsurface

TBD RTc Inert 0.53

III CMS Sn(Oct)2 Yes tol.c 110 Inert 13IV HCCL Sn(Oct)2 Yes tol.c 90 Inert 514.5

aBenzylalcoholwasemployedassacrificialinitiator.DPtarget=600.bsolvent:toluene.croomtemperature.

EXPERIMENTAL

28

To purify PCLgrafted cellulose from ungrafted PCL, the precipitatedmixture was Soxhlet extracted in THF overnight and thereafter reprecipitatedinmethanolseparately.ThefreePCLandthePCLmodifiedcelluloseweredriedindividuallypriortocharacterization.SIROP from cellulosemodel surfaces (paper II) A solution of CLanddriedTBD(0.5mol%and1mol%)wasultrasonicatedandfiltered(45mTeflon filter)prior toadsorptionmeasurements in theQCMDperformedat25 C.Theexperimental setup canbe seen inFigure12andpaperII.

Figure12.ReactionsetupforperformingSIROPofCLfromacellulosemodelsurfaceintheQCM.

3.2.2 Cellulose surface modification by ringopeningmetathesispolymerization

The surfacemodification by ROMP from FPwas performed in threesteps,seeScheme6. Initially, theFPwas functionalizedbyattachinganorborn2ene5carboxylic acid chloride, step i, which was preparedaccording to literature.96Theresultingnorborn2ene functionalizedFPwas catalystfunctionalized by addition of Grubbs first generationcatalyst,stepii.TheSIpolymerizationofNBEwasconductedfromthecatalystfunctionalizedFP,stepii.Thepolymerizationwasconductedatthreedifferenttemperatures,RT,0Cor78Candfordifferentreactiontimesvaryingfrom1minto1h.

EXPERIMENTAL

29

Scheme 6. Reaction scheme for the surfaceinitiated ringopeningmetathesispolymerization (SIROMP) from FP of NBE. i) triethylamine/4(dimethylamino)pyridine/THF,RT;ii)heptane,RT;iii)DCM,RT,0C,or78C.

ThegraftedcellulosesurfaceswereextensivelywashedwithbothDCMandMeOHincombinationwithultrasonication.

3.2.3 SynthesisofcationiclatexnanoparticlesCationicamphiphilic latexparticlesweredesignedandsynthesizedforsurface modification of cellulose. A twostep process was developedwhereamphiphilicblockcopolymers,withachargedhydrophilicblock,composedofcationicP(DMAEMAcoMAA)andahydrophobicblock,PMMA,weredeveloped.

3.2.3.1 AqueousRAFTpolymerizationofDMAEMAAqueousRAFTpolymerizationofDMAEMAwasperformedat70 C,pH7, inpresenceof theRAFTagent4cyano4thiothiopropylsulfanylthiocarbonyl (CTPPA) and the watersoluble initiator 2,2azobis(2methylpropionamidine) dihydrochloride (AIBA). However, it wasfound that approximately 10% of DMAEMA repeating unitshydrolyzedintomethacrylicacid(MAA)duringpolymerization.Hencea copolymer of cationic poly(DMAEMAcoMAA) was formed,confirmed by 1HNMR spectroscopy. A schematic overview of thesynthesiscanbeseeninScheme7.ThedetailedexperimentalprocedurecanbefoundinpaperVI.

EXPERIMENTAL

30

Scheme7.ThesynthesisofcationicP(DMAEMAcoMAA)obtainedbyaqueousRAFTpolymerizationofDMAEMA.

3.2.3.2 RAFTmediated surfactantfree emulsion polymerizationofMMA

Amphiphilic block copolymers, composed of cationicP(DMAEMAcoMAA)macroRAFTasthehydrophilicandchargedblockandPMMAasthehydrophobicblockwerepreparedbyemploying thePISAprocess,seeScheme8.Theemulsionpolymerizationwasperformedat70C,pH6, in presence of the cationicmacroRAFT P(DMAEMAcoMAA) andAIBA.FourdifferentDPweretargetedforthePMMAblock,seeTable3.

Scheme 8.The synthesisof cationicP(DMAEMAcoMAA)bPMMAobtainedbyRAFTmediatedsurfactant freeemulsionpolymerizationofMMA.CationicP(DMAEMAcoMAA),seeScheme7wasemployedasmacroRAFT.Table3.ExperimentalconditionsofperformedRAFTmediatedsurfactantfreeemulsionpolymerizationofMMA.

Entry [macroRAFT](mMH2O)a

DPnb tc(min)

conv.d(%)

e(%)c

Latex1 15 176 90 90 25Latex2 7.3 353 80 83 20Latex3 3.7 705 90 86 20Latex4 1.9 1410 90 80 17

aConcentrationofmacroRAFT.bTargetedDPofMMA.cReactiontime.dMonomerconversiondeterminedbygravimetry.eSolidcontent.

S SS CN

OHO

O

NH

O

DMAEMA

ClO

NH

OCNmHO

O

Cl

HOO

S

SS

n

AIBA, waterpH 7, 70 C

CTPPA

P(DMAEMA-co-MAA)

OO

O

NH

OCNmHO

O

Cl

HOO

S

SS

n

AIBA, waterpH 6, 70 C

P(DMAEMA-co-MAA)

MMA O

NH

OCNmOH

O

Cl

HOO

n

P(DMAEMA-co-MAA)-b-PMMA

OO

pS S

S

EXPERIMENTAL

31

3.2.3.3 AdsorptionofcationiclatexoncellulosemodelsurfacesPriortomeasurements,waterdispersionsoflatexinMilliQwaterwereprepared (100mgL1).The adsorption in theQCMDwas performedwithacontinuous flowof fresh latexdispersionuntilastablebaselinewas obtained indicating that saturation had been reached. Theadsorptionwas followedbyarinsingstep toensure that theadsorbedlatexeswereattachedtothesurface.

3.3 CHARACTERIZATIONMETHODSDetailedinformationaboutthegeneralinstrumentsandcharacterizationtechniques can be found in the individual papers. However a shortdescription about somemore advanced characterizationmethods andtoolswillfollow.

3.3.1 QCMDTheQCMDwas employed as a flowthrough reactor tomeasure thechangeinresonancefrequencyofthecrystal,correspondingtoachangeinmass attached to the surface.Measurementswere performedwithcontinuous flow of reaction solution, 0.215mLmin1 (paper II) and0.15mLmin1(paperVI),respectively.Toconvertachangeinfrequencyintoachangeinattachedmassperareaunit,mA,theSauerbreymodel183wasused: [3]where C = the sensitivity constant of 0.177 (mg (m2Hz)1), f = thechangeinresonancefrequency(Hz),n=theovertonenumber.Thetheoreticaldefinitionofthedissipation,D,canbeseenbelowandisameasureoftheviscoelasticpropertiesoftheattachedcompounds.184 [4]Where Energy dissipated during one oscillation period, Energystoredintheoscillatingsystem.

EXPERIMENTAL

32

3.3.2 ColloidalprobeAFMAdhesion and frictionmeasurementsby the colloidalprobe techniquewere performed using aMultiModePicoforceAFMwith aNanscopeIIIacontroller.APCLgraftedcellulosemicrosphere(CMS)wasattachedto the end of a cantilever using a tiny amount of epoxy resin. Theadhesion was measured between the upper functionalized celluloseprobe (PCLgraftofMw=55kgmol1)anda lower sphereof eitheraneatCMSorPCLgCMSafterbeinginsurfacecontactatvaryingtimesat a constant applied load. Typical force and friction measurementswere conducted in air (20 C, 40 C, and 60 C). To ensurereproducibility, all measurements were repeated using differentsubstrates forall theconditions.Furtherdetailscanbe found inpaperIII.

3.3.3 SolidstateCP/MAS13CNMRSolidstateCrossPolarizationMagicAngleSpinningCarbon13NuclearMagneticResonance (CP/MAS 13CNMR)spectrawererecordedwithaBrukerAvance III AQS 400 SB instrument operating at 9.4 T. ThemonomerbulkdegreeofsubstitutionwasestimatedfromtherecordedCP/MAS13CNMRspectrabyspectralintegration.Furtherdetailsaboutthecalculationofthedegreeofsubstitution(DS)ofthepolymergraftedHCCLcanbefoundinpaperIV.

RESULTSANDDISCUSSION

33

4. RESULTSANDDISCUSSION

Thework in this thesishasbeen conducted to expand the toolbox forsurfacemodificationofcellulosebypolymers.Thefirstpartofthisstudyinvestigated surface modification by covalent grafting. SIpolymerizationviaROPofCLfromdifferentcellulosesubstrateswereperformed(paperIIV).Furthermore,SIROMPwasconductedfromacellulose surface (paper V). The last part of this thesis investigatessurface modification by physical adsorption where cationic latexnanoparticles were designed, synthesized and developed forelectrostaticinteractions/adsorptionwithcellulose(paperVI).

4.1 CELLULOSE SURFACE MODIFICATION BYCOVALENTGRAFTING4.1.1 Cellulose surface modification by ringopening

polymerizationHerein, results from papers I IV will be presented and discussedwhere SIROP of CL was conducted from various solid cellulosesubstrates,andsubsequentlycharacterized.

4.1.1.1 SIROP of CL from FP andNP.Comparison betweenSn(Oct)2andTi(OnBu)4

SIROPofCLwasexaminedusingtwodifferentcellulosicsubstrates,highsurfaceareananopaper (NP)22andconventional filterpaper (FP),respectively. SIROP from FP has been studied earlier,154, 182 thus, thesubstrate issuitable forcomparisonswith thenovelNP. In thisstudy,two different catalystswere examined and compared; the commonlyemployedSn(Oct)2andTi(OnBu)4 thatmore recently,successfullywasinvestigated for ROP of CL in air72. The less demandingpolymerization conditions facilitate the experimentalperformanceandare more interesting from an industrial viewpoint. The graftingreactionswerecarriedouttolowandhighconversions,seeTable4.

RESULTSANDDISCUSSION

34

Table4.SIROPconductedfromNPandFP.Characterizationof freePCLandtheweight percent of grafted PCL in grafted papers. The sample names arebasedonthecellulosicsubstrate,FPorNP,andtheemployedcatalystTi(OnBu)4or Sn(Oct)2 and finally theweightpercentage of graftedPCL, seePaper I forfurtherdetails.

SampleReactiontime(h)

Conv.a(%)

Mn,thb(gmol1)

Mnc(gmol1) M

GraftedPCLd(%)

FPTi33 12.5 33 16300 10700 1.16 33FPTi50 22.0 78 38600 25800 2.0 50NPTi50 19.3 32 15800 15000 1.15 50NPTi64 29.0 70 34600 27900 1.45 64FPSn2 0.4 35 16500 15700 1.16 2FPSn3 0.8 77 36300 36700 1.43 3NPSn61 0.8 45 21200 20300 1.13 61NPSn79 1.0 81 38100 39400 1.6 79FPbe 22.0 1 4NPbe 29.0 2 8aConversion (conv.) determined by 1H NMR. bEstimated from conv. by 1HNMR. cMolar mass of free PCL determined by CHCl3SEC. dPercentage ofgraftedPCLinthegraftedcellulosesubstratemeasuredbyweightdifferenceofthesubstratebeforeandaftergrafting.eBlankreaction(nocatalyst).InTable4,itcanbeseenthatthemolarmassincreaseswithconversion.TheM is broad for higher conversionswhen Ti(OnBu)4was utilizedwhich may be ascribed to the more frequently occurringtransesterification reactions at elevated temperatures and higherconversions63.ThegraftedamountofPCL issignificantlyhigherwhenusingNPcompared toFP.Thiscanbeattributed to the larger surfaceareaofNPi.e.moreavailable, initiatinghydroxylgroupscompared tothe FP. The grafting reactions performed fromNP by employing theSn(Oct)2catalystresulted inthehighestgraftingamounts.However,byutilizingFP,the lowestamountofPCLwasobtainedwhenemployingthe same catalyst,Sn(Oct)2.Thismaybe explainedby shorter reactiontime compared to Ti(OnBu)4 and less available hydroxyl groupscompared toNP.The same trend isnotobserved forTi(OnBu)4wheresignificantamountsofPCLareobtainedbothfromNPandFP,althoughsmaller amounts from FP. The results may be elucidated by thedifference in atmospheres the synthesis employing Sn(Oct)2 wasperformedinaninertatmosphereandtheTi(OnBu)4inairatmosphere.

RESULTSANDDISCUSSION

35

Furthermore,thepolymerizationsperformedwiththeTibasedcatalystareconsiderablyslowerandthusthesubstratesaresubjectedtoelevatedtemperaturesforlongertimes,whichmayleadtoanincreasednumberof transesterification reactions; subsequently this may lead todisintegrationofthecellulosesubstrate.Thiswillleadtoanincreaseinnumberofhydroxylgroupsavailable for initiationduring thegraftingreaction.A slight swelling in the structurewasobserved forNPwithhighamountsofgraftedPCL.CharacterizationbyATRFTIR ishighlyapplicable to evaluate surfacemodification owing to its simplicity, speed and the fact that thetechnique is nondestructive; therefore, it is commonly employed tomonitorchangesintheamountofsurfaceboundpolymerasaneffectofmonomer conversion. Neat cellulose does not contain any carbonylgroupsbutPCLcontainsonegroupperrepeatingunitwhichgivesrisetoasignalaround1730cm1.TheintensityofthissignalincreasesforthecellulosesubstratesgraftedwithhigheramountsofPCL,seeFigure13.Moreover,theintensityofthesignalfromthestretchofhydroxylgroups(~3300cm1) decreases with increased amount of PCL indicating thathydroxylgroupsareconsumedduringgrafting.

Figure13. FTIRspectraofgraftedNPandFP(enlargementoftheregions18001600 cm1 and 3600 3000 cm1, top) and full spectra of reference PCL andreferencecellulose(bottom).

By DSC, it was found that the degree of crystallinity (Xc) and thecrystallizationtemperature(Tc)increasedforhigheramountgrafted(i.e.higher conversion) PCLwhich is in accordancewith observations by

RESULTSANDDISCUSSION

36

Hult et al.161. Furthermore, FESEMwas employed to investigate andcompare samples of unmodifiedNP and FP to samples graftedwithvariousamountsofPCL,seeFigure14.Additionally,theporosity,poresize and specific surface area (SSA) were determined by BET, seeTable5. The SEM images showed that the PCLgrafting resulted in asmoothening of the surface, which is in accordance with the resultsobtainedbyBETmeasurements,showingthattheporosityandtheSSAdecreasedforthegraftedsamples.Interestingly,thisshowsthattheporesizeofananofibernetworkcanbesuccessfullycontrolledbytheamountofgraftedpolymer.

Figure14.FESEMimagesofunmodifiedFP(A),NP(D)andPCLgraftedFP(B,C),andPCLgraftedNP(E,F).Pleasenote;therearethreeordersofmagnitudedifference between the micrographs of unmodified FP (5 30 m), andNP(520nm).Table5.BETspecificsurfacearea,poresize,density,andporosityofthegraftedcellulosesubstrates.Sample Specificsurface

area(m2g1)Poresize(nm) Density

(kgm3)Porosity

(%)FPba 0.55 c 550 62FPTi50 0.39 c 670 47refNPb 304 35.8 380 74NPTi50 52 31.8 600 52NPTi64 28 28.1 685 47aFPblanksample(seeTable4).bUnmodifiedNP. cOutofmeasurementoftheequipment.

RESULTSANDDISCUSSION

37

Themechanicalpropertiesof thebiocompositeswerestudiedbyDMAat room temperature. The glass transition temperature (Tg) of PCL isapproximately60Cwhichexplainstherelativelylowstoragemodulusobtained forPCL,225MPa.Asignificantdecreaseofseveralordersofmagnitudes in storagemoduluswasnoticed from 60C, attributed tothemelting of the crystalline domains in PCL.However, the graftedsampleshave considerablyhighervalues;FPTi50andNPTi50havestoragemoduliof700MPaand800MPa,respectively.Thehighmoduliareretainedatelevatedtemperaturesandcanbeattributedtotheintraand intermolecularhydrogenbonds in cellulose.Hence, the stiffeningeffectofthenanofibernetworkwasclearlyconfirmed.Unmodifiedcellulosefibersarehighlyhygroscopicandinacomposite,themechanicalperformance is considerably reducedwhenexposed tomoisture.Consequently, themoistureuptake forPCLgraftedcelluloseis very relevant. Moisture uptake was studied by dynamic vaporsorption (DVS); experiments were performed at three differenthumidities(25,50and80%RH).ItwasfoundthattheNPTi50(50wt% grafted PCL) had 60 % lower moisture uptake, compared tounmodifiedNP, at the highest relative humidity (80%).Noteworthy,the water uptake is higher for the unmodified NP at all evaluatedhumidities.

4.1.1.2 MonitorgraftingofCLfromacellulosemodelsurfaceinQCMD

This study was conducted to increase the understanding of SIpolymerizationfromcellulosesubstrate,whichischallengingduetothelackofsuitablecharacterizationtechniquesforthegraftedpolymerandthegrafted substrate.Adeeperknowledge isofparticular interest forpolyesters, such as PCL, since selective cleavage by hydrolysis of theester linkage between the surface and the grafted polymer chain isdifficult.42, 43Herein, theQCMD techniquewas employed tomonitorthe insitu SIROP of CL from a cellulose model surface. The SIpolymerizationwas performed in bulk and catalyzed by the organiccatalyst 1,5,7triazabicyclo[4.4.0]dec5ene (TBD) at room temperature.InFigure15,itisshownhowthepolymerizationsgaverisetoachangeinfrequencyasafunctionofreactiontime.TheSIpolymerizationsfromthecellulosemodelsurfaceswereperformedwithtwodifferentcatalystconcentrations, a) 1 mol% (black line) and 0.5 mol% (grey line).

RESULTSANDDISCUSSION

38

Furthermore, theSIpolymerizationwas studied fromboth a cellulosemodel surface (black line) and a reference gold surface (grey line)utilizing 1 mol% catalyst, see Figure 15b. The gold surface has nohydroxylgroups to initiateROP,and thus,utilizedasa reference.Allthemeasurementswere performed under continuous flow. The CLwas pumped into the system under argon atmosphere until a stablebaselinewasobtained, thereafter the reactionsolution (CLandTBD)wasintroducedandaninstantdecreaseinfrequencywasnoticedwhichcan be ascribed to an undesired bulk polymerization taking place,despite the absence of sacrificial initiator.However, to solelymonitorthegraftedPCL;theexperimentwasperformedinthreecycles30minwith thorough rinsing in between the cycles, see Figure 15, whichpermittedanestimationoftheaccumulatedamountofgraftedPCL.

Figure 15.ROP of CLwith TBD as catalyst insitu in theQCMD. (a) Twodifferent catalyst concentrations, 0.5mol% and 1mol%, were usedrespectively,(b)1mol%TBD,employinggoldasareferencesurface(greyline)andacellulosemodelsurface(blackline).The three consecutive cycles shows that reinitiation is possible aftereachcycleandthatmostofthechainendsremainactivesincethemassincrease is approximately the same for each step.However, a slightdecreasewas noticed for the third cycle; see Figure 15 and Table 6.Calculations, by employing the Sauerbreymodel,were performed toassess the grafted amounts, see Table6. The higher catalystconcentration (1mol%) increased the amount of grafted PCL ascompared to when 0.5mol% of catalyst was utilized, but not theanticipateddoublingof theamountofpolymer.Hence, there isanonlinear relationshipbetweengrafted amountsofPCL in relation to theinvestigatedcatalystconcentrations.

RESULTSANDDISCUSSION

39

Table 6.Accumulatedincreaseinmassforeachcycleispresentedforthegoldsurface and for the twodifferent cellulose surfaceswithdifferent amounts ofcatalystinthereactionmixture.Surface Cycle1(mgm2) Cycle2(mgm2) Cycle3(mgm2)Gold(ref.) 1.4 2.7 4.4Cellulose,0.5mol%TBD

13.9 26.9 37.5

Cellulose,1mol%TBD

17.2 31.7 44.4

Inanattempttobetterunderstandtheeffectofthebulkpolymerization,thedecreaseinfrequencyforthegoldsurfacewasestimatedtoonlybeattributed to thepolymerization in thebulk.However,gold is ahighenergy surface which may explain the small amounts of physicallyadsorbedPCLorimmobilizedmonomer.Nevertheless,itconfirmedthata SI polymerization occurred from the cellulosemodel surfaces. Thehigherconcentration (1mol%)ofTBDalso increased the formationofbulkpolymer,whichcanbeseeninFigure15bytheimmediatedecreaseinfrequencyafter introducingthereactionsolution inthesystem.ThismostlikelyisattributedtoTBDundergoingunwantedsidereactionsasreported byHedrick et al.185.However, this nonlinearitymay also beexplained by decreased accessibility of the chain ends due to sterichindranceasthepolymerchainsaregrowing.TBDneedstoactivatethehydroxylgroupsonthesurface,andpossiblytheaccessibilityreachesamaximumwhichcanbeanotherreasonwhythegraftedamount isnotdirectlyproportionaltothecatalystamount.Alreadyatrelativelyshortgraftlengths,consequencesduetosterichindrancemaybenoticed.Thisis in accordancewith an earlier studywhere itwas reported that thenumberofgrowingchainsdecreasedasthefilmthicknessincreasedforSIATRP froma siliconwafer.186A similarbehaviorcouldbeexpectedforSIROP.It is also seen from the QCMD analyses that the dissipation isincreasingbetweentheconsecutivecycles,indicatingthattheamountofgraftedpolymer increased foreveryperformedcycle, seeTable7.Theobtained dissipation values are in good accordance to other studiedsystemswherepolymershavebeenattached toaQCM substrate.188,189Theratiobetweenthenormalizeddissipationandfrequencycanalsobeseen in Table 7, showing a change in viscoelastic behaviors betweeneachconsecutivecycle.Adecreasingtrendforeachcyclewasobserved

RESULTSANDDISCUSSION

40

whichcouldbeattributedtotheformationofamorecompactpolymerlayerandmayalsoindicatethatthepolymerstartstocrystallizeonthesurfacewhenthePCLchainshavereachedacriticalmolarmass.Table7.Dissipationaftereachcycle(D)andthedissipationnormalizedagainstthefrequency(D/f)aftereachcycleforbothcatalystconcentrations1mol%TBDand0.5mol%,respectively. Da Db D/fax106/Hz1 D/fbx106/Hz1Cycle1 6.6 5.4 0.068 0.064Cycle2 11.5 7.3 0.064 0.048Cycle3 12.6 9.5 0.051 0.045

a1mol%TBD,cellulose.b0.5mol%TBD,celluloseThepolymerformedinbulkwasisolatedandcharacterizedby1HNMRspectroscopy.Themonomerconversionaftereachcyclewassimilarforthe formed bulk polymer. Interestingly, the bulk polymerizationwasfour times faster for the higher catalyst concentration which can becompared to the SI polymerization which only was 1.2 times fasterwhen twice the amount of catalystwas employed. Thismay also beattributed to the behavior of the catalyst during polymerization, asdiscussedpreviouslyfortheSIpolymerizationabove.InFigure16,AFM images and contact angles (CA) are shown for thecellulosemodel surfaces before and after grafting of PCL and it canclearlybe seen that the surface topographyhas changedThe fibrillarlike surfaceprior to grafting has changed to a smoother surface aftergrafting.This isalsoreflectedbythesurfaceroughness,(Rq),thelowerthe value the smoother the surface. Hence, Rq is decreasing withincreasedamountofgraftedPCL.Furthermore, theCAs,presented inFigure 16, show that PCL grafted cellulose model surfaces haveconsiderably higherCA compared to the unmodified cellulosemodelsurface. This can be attributed to the nonpolar character of PCLcompared to cellulose. The CA is both affected of the chemicalcompositionof thesurfaceaswellas thesurface roughness.However,theCAsforthePCLmodifiedcellulosemodelsurfacesaresimilarwhichmay be explained by the similar composition in the outermost layerwherebothsurfacesarecompletelycoveredwithPCL.

RESULTSANDDISCUSSION

41

Figure16.Contactanglemeasurements (meanvaluesarepresentedbelow theimages) and AFM height (upper) and AFM phase (lower) images of QCMcrystalswith cellulosemodel surfaces before and after grafting of PCL. TheAFMimagesare2x2m,theheightimageshaveazrangeof100nmandthephase is90.Theroughness, (Rq),of thedifferentsurfaces isshownunder theimages.Experimentswereperformedat50%RHand23C.

4.1.1.3 PCLgrafted cellulose microspheres Impact of graftlengthand temperatureon interfacialadhesiona studybycolloidalprobebyAFM

The aim of this studywas to investigate the impact of polymer graftlengthsontheadhesion,whichisofsignificantinterestwhendesigningaheterogeneousmaterialasabiocomposite.Themodelsystem,studiedat a nanoscopic level by the colloidal probe technique, allows directquantification of surface interactions. In this investigation, cellulosemicrospheres(CMS)havebeengraftedwithdifferentlengthsofPCLvia

RESULTSANDDISCUSSION

42

SIROP.ThisstudywasacontinuationofaninitialworkbyMalmstrmetal.162whereitwasconcludedthatPCLgraftingoncelluloseimprovedtheadhesion significantlyespecially for the symmetric system,PCLgCMS/PCLgCMS. The AFM cantilever tipwas functionalizedwith aCMS(bothunmodifiedandPCLgrafted)allowinginvestigationsoftheinterface between two CMS. Two different systems; an asymmetricsystem(CMS/PCLgCMS)andasymmetricsystem(PCLgCMS/PCLgCMS) were examined. Furthermore, the impact of graft length andtemperature was elucidated. The adhesion measurements were alsoperformedatdifferenttimesofcontact.TheCMSweregraftedwithPCL via SIROPof CL inpresenceof asacrificial initiator. After Soxhlet extraction, the PCL and the PCLgCMS were characterized. The molar mass of the free PCL wasdeterminedforthesamples.Samplee,seeTable8,wassynthesizedandutilizedinapreviouslypublishedstudy.162Table 8. Sample denotation for PCLgrafted cellulose microspheres (PCLgCMS)andcorrespondingmolarmassesandmolarmassdispersities(M)offreePCL formed from sacrificial initiator simultaneously by SIROP of CL fromCMS.DPtarget=600forallperformedsyntheses.

Samples Mw(kgmol1)a MCMS PCL9kgCMS 9 1.2PCL21kgCMS 21 1.3PCL34kgCMS 34 1.5PCL55kgCMS 55 1.9

aDeterminedbyTHFSECwithpolystyrenestandards.ByATRFTIRspectroscopy, thecarbonylsignalat1730cm1confirmedthepresenceofPCL,an increased intensity insignalwasobserved forhighergraftedamounts,seeFigure17.

RESULTSANDDISCUSSION

43

Figure17.ATRFTIRspectra: (a)neatCMS;CMSsurfacegraftedwithPCLof;(b) PCL9kgCMS; (c) PCL21kgCMS; (d) PCL34kgCMS; (e) PCL55kgCMS. Themolecularweights refer to the corresponding values obtained from polymersformedinbulk(obtainedbySEC).

The adhesion was studied with a PCL55kgCMS (Sample e) probeattached to the cantilever.Asanticipated, the interaction isdependentonthegraftlengthascanbeseenintheforcedistanceprofilepresentedinFigure18.The interaction between thePCLmodifiedprobe and theunmodifiedCMSreflectsaforceminimumwithasharpjumptozeroforce.Thiscanbe attributed to the instability when the cantilever spring constantovercomes the adhesion force between the two surfaces. This forceminimum (pulloff force) is useful for composite design as a relativemeasure between different materials. When symmetrical systems arestudied, i.e. adhesion between two PCLgrafted CMS, the increasinggraftlengthwillincreasetheadhesionduetotheincreasedsurfaceareaavailableforinteractions,suchasentanglements,tooccur.

RESULTSANDDISCUSSION

44

Figure18. Normalized force profiles upon retraction between aCMS graftedwith PCL (Mw=55 kg mol1, Sample e in Table 8.) and (a) neat CMS; CMSsurfacegraftedwithPCLof; (b)PCL9kgCMS; (c)PCL21kgCMS; (d)PCL34kgCMS;(e)PCL55kgCMS.ThemolecularweightsrefertothebulkvaluesobtainedbySEC.

By integrationof the force curves inFigure 18 (the area isdefined asforce curve at negative loads) the total work of adhesion can becalculated.By studying theworkofadhesion asa functionof time incontactat60C,whichisclosetothemeltingtemperatureofPCL,itcanclearly be seen that the adhesion in a symmetric system is higher,probably due to the higher chainmobility of the PCL. Thework ofadhesion is increasing with longer PCL chains and time in contact.However,SamplebshowsasimilarbehaviorasunmodifiedCMSwhichmay be explained by the insufficient graft length to createentanglementsandinterpenetration.Themaximumpulloffdistance,i.e.thesurfaceseparationatwhichfinaldetachment occurs, for Sample ae was studied at three differenttemperatures(25C,40Cand60C),seeFigure19.Thismeasurementelucidatestherelationshipbetweenthemolecularweightthresholdandtemperature. For measurements performed at 25C, the significantdifference in the interactionbetween thedistancesdisappearsbetweenThesamplescandd,correspondingtomolarmassesbetween2134kgmol1,visualizedbythebluearrowinFigure19.However,byincreasingthe temperature, it canbe seen that the requiredmolarmass is lowerwhich is to be expected due to the increasing mobility at elevated

RESULTSANDDISCUSSION

45

temperatures.The threshold isbetween921kgmol1,see theorangearrowinFigure19.

Figure19.Maximumpulloffdistance(nm)asafunctionofmolecularweight(gmol1) at 25 C, 40 C, and 60 C afterbeing in contact for 100 s. Inset showsschematicof themacromolecular inducedadhesionmechanism.Themolecularweightthresholdsareindicatedbythebluearrow(25C)andtheorangearrow(40and60C).

Thelateral,slidingfrictionalforcesarealsoimportantinabiocomposite,providing information about structural changes. This colloidal probeAFM study showed that the results obtained for the frictionalmeasurementscorrelatewiththeobtainedresultsfornormalforces.Theresults elucidate the benefits of the grafted layer and increasedtemperaturewhich improves the interfacialadhesion,both in termsofnormal and frictional forces which is of great interest regardingintegrationandstabilityforcomposites.

4.1.1.4 Solid state CP/MAS 13CNMR investigation of HCCLgraftedbySIROPofCL

The aim of this study was to investigate the impact of monomerconversion on the degree of polymerization (DP) and degree ofsubstitution (DS),byemployingsolidstateCP/MAS 13CNMR.Herein,SIROPofCLwasperformedfromhydrolyzedcellulosecottonlinters(HCCL).PCLgHCCLswithdifferentlengthsofPCLwerepreparedtoelucidate if the polymer chains continued to grow with increasing

RESULTSANDDISCUSSION

46

monomer conversion, or if more chains were initiated. The graftingreactionswereperformed inbulkandasacrificial initiatorwasadded.TheresultingfreeformingPCLwascharacterizedbothbysolution 1HNMRandSEC,seeTable9.Bielaetal.68havereportedacorrectionfactorfor the molar mass of PCL with regard to the value obtained byemploying a SEC calibration with PS standards. This allows thedeterminationofmolarmassescloser to the truevalues.Thecorrectedmolar masses were assessed, and were found to be in rather goodagreementwiththemolarmassesdeterminedby1HNMR,seeTable9.Table9.ResultsfromcharacterizationoffreePCLproducedbyROPofCLat90CandDPtarget=600.Sample MWtheoa

(gmol1)MWb

(gmol1)DPb Mnc

(gmol1)DPSECc M

PCL20gHCCL 19200 2400 20 1900 15 1.18PCL31gHCCL 39200 3600 31 2600 22 1.26PCL35gHCCL 31400 4100 35 2700 23 1.31PCL39gHCCL 53600 4600 51 4200 36 1.35PCL64gHCCL 55300 6500 64 5100 43 1.38

aTheoreticalmolarmass based on conversion by 1HNMR. bEstimated fromendgroup analysis by 1HNMR. cMolecularweights and correspondingDPsobtainedbySECandcorrectedaccordingtoBielaetal.68Areference13CNMRspectrumforHCCLwithassignedcarbonsignalsisshown inFigure20.BasedonanalysesbyCP/MAS13CNMR, itwasfoundthatapproximately3%ofthesurfacehydroxylgroupsinHCCLwereavailableforsurfacemodification.Inaddition,thismethodcanbeutilized todetermine thebulkmonomerdegreeof substitution (DSBM)bymeasuringthespectralintegralsrecordedonPCLgHCCL.

RESULTSANDDISCUSSION

47

Figure 20. 13CNMR spectrumwith assigned carbon signals from theAGU inHCCL.ThenumberingofthecarbonsintheAGUispresentedinFigure2.

TheCP/MAS13CNMRspectraofunmodifiedHCCL,PCLgHCCL,andpurePCLare shown inFigure21.The shiftsof the carbonylgroupat~170 ppm, OCH2 at ~64ppm, and CH2 at 40 20 ppm arecharacteristicforPCL.Inthespectrumforthegraftedsample(PCL64gHCCL),thecharacteristicsignalsoriginatingbothfromHCCLandPCLarepresent,confirmingthesuccessfulgrafting.Itwasassumedthattheintensityinsignalsfromalltheatomsinthedrysampleswereequalandhomogeneouslydistributedinthedetectedvolume.

Figure21.CP/MAS13CNMRspectraofPCL(top),PCL64gHCCL(middle)andHCCL(bottom).

RESULTSANDDISCUSSION

48

TheDSSP, i.e. thenumberofgrafting sites,wasestimatedutilizing theDSBMandDP.TheDPvaluesweredeterminedeitherbySECorbyendgroup analysis of 1HNMR spectra (Table 9). Further details aboutassumptions and employed equations can be found in paper IV. InTable10, the integralvalue (Ip)determinedbyCP/MAS 13CNMRandthecorrespondingDSSPcanbe found for thePCLgraftedHCCLswithfivedifferentgraftlengths.DSSPwasdeterminedbyseparatelyutilizingtheDP achieved by SEC or 1HNMR. TheDP values from these twotechniquesdeviate slightlybut exhibit a linear relationship, seepaperIV.Therefore, a similar trend forDPversusDSSPwas anticipated, seeFigure22.Table10.IntegralvaluesbyCP/MAS13CNMR,Ip,andpolymerparticlesurfacedegree of substitution (DSSP) for PCLgHCCL of different graft lengths.DPsdeterminedbySECand1HNMR.

Sample Ip DSSP(SEC) DSSP(1HNMR)PCL20gHCCL 0.086 0.046 0.034PCL31gHCCL 0.133 0.048 0.034PCL35gHCCL 0.129 0.045 0.029PCL39gHCCL 0.222 0.049 0.044PCL64gHCCL 0.352 0.065 0.050

Figure 22. Degree of surface substitution (DSSP) of the PCLgHCCLs versusdegreeofpolymerization(DP)offreeformingPCL.

RESULTSANDDISCUSSION

49

Figure22showsthatDSSPwasroughlyconstantandindependentoftheDP of the grafted chains, suggesting that as themonomer conversionincreasesnoadditionalchainswereformed.ItisalsonoticeablethattheDSSPwas very low for all the PCLgHCCL samples.Despite the lownumberofavailablehydroxylgroups for surfacemodificationand thethin layer of grafted polymer, the surface properties changedconsiderably.AlthoughallthesamplesexhibitedrelativelylowDSSP,largedifferencescan be seen in ATRFTIR spectra recorded on the neat HCCL,homopolymerofPCLand thePCLgraftedHCCL,asshown inFigure23.However,onlyPCL64gHCCLisshowninFigure23a,sinceitisonlythecarbonylregionof the IRspectra that isstrongly influencedby thedifferent graft lengths. All the PCLgrafted HCCL are presented inFigure23b,and the increased intensityof thecarbonylsignalconfirmstheincreasingDPofthegraftedpolymer.

Figure23.(a)AT