Chemical modification of Cellulose Acetate by Diallylamine S. Abdel-Naby... · Int.J.Curr.Microbiol.App.Sci (2014) 3(6): 10-24 11 (Hon and Ou 1989), the plasticization of starch (Ma

Int.J.Curr.Microbiol.App.Sci (2014) 3(6): 10-24

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Original Research Article

Chemical modification of Cellulose Acetate by Diallylamine

Abir S. Abdel-Naby 1* and Azza A. Al-Ghamdi2

1Chemistry Department, Faculty of science, Fayium University, Fayium, Egypt 2Chemistry Department, Faculty of science, Dammam University, Dammam,

Saudi Arabia

*Corresponding author

A B S T R A C T

Introduction

The interest in polymers from renewable resources increased considerably in the last few decades. These materials are available in large quantities and they have numerous advantages (Mohanty et al 2000; Bledzki and Gassan 1999; Zhang et al 2005).

Natural polymers, mainly polysaccharides, consist of large, rigid molecules, thus they cannot be processed very easily with the usual processing technologies of

thermoplastic polymers (Schroeter and Felix 2005; Heinze and Liebert 2001; Pereira and Campana, 1997). As a consequence, they are often modified both to improve process ability and to adjust their properties to the intended application.

Natural polymers can be modified physically by plasticization, or chemically through the reaction of their active (-OH) groups (Joly et al 2005; Frisoni et al 2001; Nishio 2006). The benzylation of wood

ISSN: 2319-7706 Volume 3 Number 6 (2014) pp. 10-24 http://www.ijcmas.com

K e y w o r d s

Cellulose acetate, Diallylamine, Thermal properties, Morphology.

Cellulose acetate (CA) was modified using Diallylamine (DAA). The structure of the modified polymer was characterized by 13C-NMR. The chemical modification is based on the reaction between the active hydrogen of the amino group of Diallylamine molecule and acetyl group of the glucopyranose ring in cellulose acetate. The thermo gravimetry (TG) was used to investigate the thermal stability of the modified polymeric sample. The modified cellulose acetate exhibits higher thermal stability as compared to the unmodified CA, Which is attributed to the presence of thermally stable pyrrolidine rings throughout the polymeric chains. The crystallinity and morphology of the modified polymeric sample were investigated using X-Ray diffraction (XRD) and emission scanning electron microscope (ESEM) respectively. The presence of Diallylamine moieties (as pyrrolidine ring) in the cellulose acetate matrix improved its mechanical property. Also, the organic nature of Diallylamine moieties inside CA matrix reduced its wettability.

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(Hon and Ou 1989), the plasticization of starch (Ma et al 2005; Gomes et al 2001), and the grafting of cellulose (Mayumi et al 2006) or cellulose acetate are typical examples of such modifications (Lee and Shiraishi 2001; Yoshioka and Hagiwara 1999; Hatakeyama et al 2000; Guruprasad and Shashidhara 2004; Warth et al 1997; Teramoto et al 2004; Teramoto et al 2002; Teramoto and Nishio 2002; Abdel-Naby and Aboubshait 2013).

Cellulose acetate (CA) is often used in these attempts, since this polymer is a commercial product and can be handled relatively easily compared to cellulose or to most of the other natural polymers (Simon et al 1998).

Modification is carried out under a wide variety of conditions. Because of these wide range of methods and conditions, the effect of various parameters on grafting efficiency and on the structure product obtained are not completely clear yet (Sz mel et al 2008). Despite the advantages of low cost and high productivity, the serious problems facing cellulose acetate are its poor heat stability at high temperatures as well as its hydrophilic nature.

Cyclizaion or cyclopolymerization of diallylamine, as symmetrical non-conjugated diolefin, was found to be possible only in the presence of ultrasound and peroxodisulfate (Vivekanandam et al 2000). While, the cyclization and cyclopolymerization of N-substituted diallylamine salts was found to possible with the help of either ultrasound or chemical initiator (Zaikov et al 2001; Abdel-Naby and Al-Harthi 2013 ). In the present work, diallylamine is suggested to modify cellulose acetate aiming to improve its thermal and mechanical

properties. The choice of diallylamine is due to the presence of active hydrogen atom of the amino (-NH) group, which possesses the ability to attraact the acetyl group of glucopyranose ring, thus enhancing the chemical modification process.

Materials and Methods

Materials

Cellulose acetate (CA) was purchased from Aldrich with a degree of polymerization and degree of substitution 160 and 2,4 respectively. Diallylamine was purchased from Aldrich. Cyclohexanone purchased from Prolabo. Other reagents and solvents were of analytical grades.

Preparation of modified cellulose acetate

The modified polymers were prepared by gradually adding (0.2mol) of diallylamine to a solution of cellulose acetate (8 mol) dissolved in 100 ml of cyclohexanone. Both solutions had been prepared under nitrogen atmosphere. The reaction was allowed to occur for 10 h at 70 C, in ultrasonic bath of power 1050 watt. The reaction product was precipitated in cold methanol, filtered, and washed with hot methanol using soxhlet system. The recovered polymer was then dried under vacuum at 40 C.

Preparation of cellulose acetate samples films

The films were prepared by casting polymeric sample solution in THF on glass petri-dish, then allowing the solvent to evaporate at room temperature then dried at 40 C in vacuum oven. The thickness of the film was 0.5 ± 0.01mm


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Characterizations

Spectroscopic characterization

13C-NMR analysis

The 13C-NMR spectra of monomers and polymeric samples were collected on a Bruker Advance- 600 MHz spectrometer. All the chemical shifts were reported in part per million (ppm) using tetra methyl silane (TMS) as the internal standard. DMSO or CDCl3 or a mixture of both solvents was used.

Morphological surface analysis

Cellulose acetate surfaces were examined by Emission Scanning Electron Microscopy (ESEM) using a (FEG-SEM/EDS)or LEO 440 ZEISS/LEICA model.

X-Ray Diffraction

X-ray diffraction patterns were recorded using a Rigaku D/max 2500 v/pc X-ray diffractometer. The diffractograms were measured at 2 in the range of 5- 80° using Cu K as the monochromatic radiation source ( = 1.54 ) by applying a parabolic filter, at a tube voltage of 40 kV and a tube current of 200 mA.

Thermal characterization

Thermal analysis TGA / DTA

The thermal stability was examined, under nitrogen, using a Perkin-Elmer (TGA/DTA) thermogravimetric analyzer from room temperature to 500 °C at a heating rate of 10°C

Differential scanning calorimetry (DSC)

The glass transition temperature Tg of the CA/ PVC blend films was measured on a

Perkin-Elmer differential scanning calorimeter (Model DSC 2010, USA) by heating from 20 to 500 C, under nitrogen atmosphere. The DSC curves were recorded.

Mechanical properties characterization

Young s modulus of the polymeric films was measured using a Shimadzu autograph in air at room temperature.The correlation between stress ( in MPa) and elongation ( in %) was determined (El Hadi et al 2008).

= E where, E = Young s modulus in MPa

= ( Lo-L) / L Lo = original length L = length after elongation

Thermomechanical properties

The films were analyzed by dynamic mechanical thermal analysis (DMTA) using a Rheometric Solid Analyzer II (Rheometric Scientific, Piscataway, NJ) under nitrogen. A film sample of 19.28 mm × 6.35 mm was prepared and its thickness was measured. The sample was subjected to a small axial force of 5 g and a modulating frequency of 16 Hz and heated at a rate of 5 C/min until the length of the sample was stretched by 0.5 mm. The temperature at which the film was stretched by 0.5mm was recorded as the rejuvenated temperature (Tr).

Wettability

The wettability of the polymeric film was examined by measuring the contact angles of water droplets on the film with video contact angle system (VCA 2000Advanced surface technology, Billerica, MA) at three different locations on the film.


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Results and Discussion

Characterization of modified cellulose acetate

The modification of cellulose acetate by diallylamine (DAA) occurred according to the method previously described. The elemental analysis indicated the presence of nitrogen atom in the CA/DAA sample, which confirmed the modification process between CA and DAA, as CA lack the presence of nitrogen atom in its matrix.

13C-NMR spectroscopic analysis

13C-NMR spectroscopy was used to identify the reaction sites in both CA and DAA monomer involved in the modification process. The 13C-NMR spectral characteristic data of (CA/DAA) (Table 1) and (Figure 1) showed the following additional peaks as compared to that of CA (Figure 2).

1. A new peak at (58.5 ppm) indicating the formation of new bond between nitrogen atom of (-NH) group of DAA and carbon atom (1) of glucopyranose ring of cellulose acetate after the release of acetic acid molecule. Absence of vinyl C=C of the monomer confirms the cyclization process since this cyclization was known to occur only if the nitrogen atom of DAA was substituted (Zaikov et al 2001) ( Figure 3)( Eq. 1)

2. Appearance of a new peak at (41 ppm) (with double intensity), representing carbon atoms (2 and 2 ). The double intensity confirmed the symmetry of carbon atoms in the pyrrolidine ring.

3. Appearance of new peak at (30.9 ppm) corresponding to carbon atoms (3 and 3 ).

4. The presence of a peak in the methyl region ( 18.4 ppm) with considerable intensity confirms the structure indicated in ( Figure 3 )( Eq.2). Figure 3 represents the suggested reaction mechanism of the modification of CA by DAA.

Thermal properties of the modified cellulose acetate

Thermogravimetry (TG)

This work aims to improve the thermal stability of cellulose acetate at high temperatures through chemical modification by DAA. The thermal stability was examined using thermogravimetry (TG). The initial decomposition temperature (To) and the total weight loss percentage (Wt loss %) values of the modified sample as well as those of unmodified cellulose acetate are complied in Table 2.

The data indicated that the thermal stability of the modified CA sample is improved as shown by the decrease in weight loss % at (500 C) (85%) and the increase in the value of initial decomposition temperature (To)( 320 C) as compared to those of unmodified CA, where (To = 250 C) and the total weight loss % at (500 C) reached (89.3 %). The increase in the thermal stability of the modified sample is mainly attributed to the stability of the highly thermal stable pyrrolidine rings built throughout the cellulose acetate matrix.

The thermogravimetry curves of both modified sample and that of unmodified CA are illustrated in Figure 4. The figure emphasizes the extra thermal stability of the modified cellulose acetate as compared to the unmodified CA.


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Differential scanning calorimetry (DSC)

The differential scanning calorimetry (DSC) of CA/DAA is shown in Figue 5. The data show that the modified polymer exhibits only one glass transition temperature Tg ( 191.83), different from that of the unmodified CA ( 189.49). The single Tg confirmed the formation of a new modified polymer. Also, the increase in the Tg value of CA/DAA indicates the increase in its degree of of crystallinity as compared to CA.

Differential thermal analysis (DTA)

The DTA curve of CA/DAA sample exhibits a large sharp exotherm peak at temperature of flow ( Tf ) near (400 °C), as shown in Figure 6 , which is related to the crystal structure. While the DTA curve of CA exhibits a broad peak at lower temperature of flow Tf ( (388°C). This result implies that the degree of crystallinity of the modified sample is higher than that of the unmodified CA.

Characterization of crystallinity of modified CA/DAA sample

To confirm the efficiency of pyrrolidine rings inside the CA/DAA matrix in increasing the degree of crystallinity, it was of interest to check the X-Ray pattern of the modified CA/DAA sample and compare it to that of unmodified CA.

The XRD pattern of CA showed three sharp peaks at (2 = 9°, 10.6°, 13.4°) related to the crystalline part of the polymer and a broad peak at (2 = 18.2°) related to the amorphous part of the polymer. For this reason cellulose acetate is known to be a semicrystalline polymer (Abdel-Naby and Aboubshait 2013).

The XRD pattern of CA /DAA polymer exhibits higher crystallinity as shown from the sharpness and intensity of the peaks (Figure 7). The aforementioned results revealed that the modification of CA by DAA increased the percentage of crystallinity due to the presence of the pyrollidine rings inside the modified polymer matrix.

Morphological characterization

A half gram of the modified CA/DAA sample was dissolved in THF and then poured into suitable petri-dish. for some interval of time, then the later was put in a vacuum oven at 40 °C to obtain the film. The film was of 0.5 mm thick. The morphology of the surface of CA modified by DAA film shows the modified regions as compared to that of unmodified CA (Figure 8).

Mechanical properties characterization

To adjust the semi crystalline CA polymer for various fibre applications, its mechanical property should be improved. The aim of this work is to produce modified CA with high mechanical property. The measurement of the value of Young s modulus reflects the ability of the sample to retain its original form after release of the stress (Okui and Sakai 1987). CA is known to exhibit Young's modulus around (600-3000 MPa) depending on the degree of substitution and the average molecular weight (Wu et al 2008).

The results revealed that the modified CA sample by DAA possessed higher Young's modulus value than CA at room temperature (Table 3). Thus, the modification of CA by DAA increased Young's modulus value.


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Fig.1 13 C NMR spectrum of CA modified by DAA


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Fig.2 13C NMR spectrum of cellulose acetate


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Fig.3 Suggested mechanism of the modification of Cellulose acetate by diallylamine

Eq 1

Eq2

O

CH2OCOCH3

OOH

OCOCH3

N

H

CH3COOH

O

CH2OCOCH3

OOH

N

Ultrasonic waves

O

CH2OAc

OH

N

CH3

O

CH3


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Fig.4 Thermogravimetry (TG) , under nitrogen atmosphere, of (b) CA modified by DAA as

compared to that of (a). CA .

Fig.5 Differential scanning calorimetry (DSC) , under nitrogen atmosphere, of (a) CA and (b) CA/DAA.

CA-DAA


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Fig.6 Differential thermal analysis (DTA), under nitrogen atmosphere, of CA/DAA as

compared to that of CA

Fig.7 XRD patterns of (a) CA, (b) CA / DAA


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Fig.8 ESEM morphology of polymeric films (a) CA and (b) CA / DAA

a

b


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Table.1 13C NMR spectral characteristics of CA/ DAA modified polymer

Table.2 Thermal analysis of CA/DAA as compared to CA

Total weight loss at 500 C To Sample 89.3 % 250 C CA 85% 320 C CA/DAA

Table.3 Mechanical properties and wettability of CA/DAA as compared to CA

Contact angle of water droplets

Young's Modulus (MPa)

Rejuvenated temperature (Tr)

Sample code

64 1500 210 ºC CA 76 1850 333 ºC CA/DAA

Dynamic mechanical thermal analysis was used to determine the rejuvenated temperature (Tr) of a film sample. The films with higher degree of elasticity exhibited lower rejuvenated temperatures. The results revealed that the modified CA/DAA sample exhibited higher rejuvenated temperature (333 C) than that of the unmodified CA (210 C). The aforementioned results were consistent with the XRD results, confirming the increase in the percentage of crystallinity of the modified CA as compared to CA, which could adjust it for various fibre applications.

Wettability characterization

The wetting behavior of modified cellulose acetate film was examined by measuring the contact angles of water droplets on the film (Table 3). The results revealed that the modified CA/DAA film exhibited higher contact angle (76o) than unmodified CA film (64o). This is attributed to the presence of organic pyrrolidine rings throughout the backbone of CA. Apparently, the organic pyrrolidine rings reduced the wettability of CA.

13C NMR ( ppm)

Carbon atom in CA/DAA

Structure

58.5 1

41 2,2'

30.9 3,3'

18.4 4,4'

1

2

3

4

2'

3'

4'

O

CH 2OA c

O H

N

CH 3

O

CH 3


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Conclusions

1. This piece of work is an attempt to modify cellulose acetate structure in order to improve its thermal and mechanical properties. Increasing crystallinity and reducing wettability of the polymer are also considered as essential targets to enable the adjustment of the polymer for fibre applications.

2. Diallylamine, as an organic compound possessing active hydrogen, is suggested to modify CA.

3. The structures of the modified polymeric sample is confirmed by 13C NMR.

4. The incorporation of DAA into the cellulose acetate matrix led to the cyclization of the DAA moieties and formation of pyrrolidine rings throughout the modified polymer chains. These rings gave the modified polymer its extra thermal stability as compared to the CA.

5. The results of mechanical properties revealed that the modified CA sample by DAA possessed higher Young's modulus than CA at room temperature . Also, the presence of the organic pyrrolidine ring throughout the CA matrix reduced its wettability.

6. The modified CA/DAA sample was found to exhibit higher percentage of crystallinity as compared to the unmodified CA which could adjust it for various fibre applications.

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Documents

Chemical modification of Cellulose Acetate by Diallylamine S. Abdel-Naby... · Int.J.Curr.Microbiol.App.Sci (2014) 3(6): 10-24 11 (Hon and Ou 1989), the plasticization of starch (Ma