ISSN e-polymers 2004, no Ali Pourjavadi *, Hossein Ghasemzadeh, Hossein Hosseinzadeh

Transcription

1 e-polymers 2004, no ISSN Preparation and swelling behaviour of a novel anti-salt superabsorbent hydrogel based on kappa-carrageenan and sodium alginate grafted with polyacrylamide Ali Pourjavadi *, Hossein Ghasemzadeh, Hossein Hosseinzadeh Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Azadi Ave., P.. Box , Tehran, Iran; purjavad@sharif.edu (Received: March 7, 2004; published: May 12, 2004) Abstract: A novel superabsorbent hydrogel was synthesized via crosslinking graft copolymerization of acrylamide (AAm) onto kappa-carrageenan (κc) and sodium alginate (Na-Alg) backbones in a homogeneous solution. Methylenebisacrylamide (MBA) and potassium persulfate (KPS) were applied as water-soluble crosslinker and initiator, respectively. FTIR spectroscopy was used for confirming the structure of the final product. A mechanism for superabsorbent hydrogel formation was also suggested. The parameters affecting the swelling capacity of the synthesized hydrogel, i.e., κc-alg weight ratio, concentration of AAm, MBA and KPS, as well as reaction temperature were systematically optimized for obtaining maximum absorbency. The swelling capacity of hydrogels was also measured in various salt solutions (LiCl, NaCl, KCl, MgCl 2, CaCl 2, SrCl 2, BaCl 2, and AlCl 3 ). Due to their high swelling ability in salt solutions, the hydrogels may be referred to as anti-salt superabsorbent polymers. The overall activation energy for the graft copolymerization reaction was found to be 374 kj/mol. The swelling kinetics of the hydrogels in distilled water and in saline solution (0.9 wt.-% NaCl) was investigated. Introduction Superabsorbent hydrogels are three-dimensional hydrophilic crosslinked networks, which are able to absorb and retain many times their weight of water, saline or biological fluids, without dissolution [1,2]. Synthesis and characterization of these attractive materials have received significant attention because of their excellent applications in many fields. Hydrogels specially are used in disposable diapers, hygienic napkins, agricultural applications and in medicine as drug delivery carriers [2-5]. This accounts for an increase in the worldwide production of superabsorbent polymers (SAPs) from 6000 tons in 1983 to tons in 1996 [1]. Nowadays, the worldwide production of SAPs is more than one million tons per year. Natural-based superabsorbent hydrogels have attracted much interest from the viewpoint of improving the tissue tolerance of synthetic polymers and the mechanical properties of natural polymers. Because of their biocompatibility, biodegradability and non-toxicity, polysaccharides are the main part of the natural-based superabsorbent hydrogels. Free radical graft copolymerization of vinylic monomers onto polysaccharide backbones followed by crosslinking of their chains is a well-known method for the synthesis of these biopolymer-based networks [6-9]. The first industrial superabsorbent hydrogel was synthesized using this method via ceric-induced graft 1

2 copolymerization of acrylonitrile onto starch followed by crosslinking alkaline hydrolysis of the nitrile groups of the produced graft copolymer [10]. In our previous studies, we synthesized and characterized various polysaccharidebased superabsorbent hydrogels, i.e., ceric-initiated acrylonitrile grafting onto chitosan [11] and carboxymethylcellulose [12], graft copolymerization of mixtures of acrylic acid and acrylamide onto chitosan [13], and persulfate-induced graft copolymerization of acrylamide [14], acrylic acid [15], and methacrylic acid [16] onto kappacarrageenan. In the present work, we studied the synthesis of a novel polysaccharide-based superabsorbent hydrogel via graft copolymerization of acrylamide onto kappa-carrageenan and sodium alginate backbones. Carrageenans are a group of linear sulfated polysaccharides that are obtained from certain red seaweeds by extraction with hot water under alkaline conditions. The three commercially available types of carrageenans are designated lambda, kappa, and iota (Fig. 1a). Because of their exceptional properties, they are broadly used as ingredients in a variety of applications [17]. Alginates are linear anionic polysaccharides of (1 4) linked α-l-guluronate (G units) and β-d-mannuronic acid (M units) residues. The structure of alginates can therefore be represented schematically in Fig. 1b. Alginates are used in a wide range of applications. For example, in food industry alginates were used as suspension-stabilizing and gelling agents. An important and useful property of alginates is their ability to form gels by ionic crosslinking reaction with calcium cations. However, ionic crosslinked alginate gels show low absorbencies due to their high crosslinking density. So, in this work, we attempted synthesise chemically crosslinked hydrogels in the presence of methylenebisacrylamide as a crosslinker. - 3 S H H R H H - 3 S - CH 2 S 3 H - S 3 (a) CH H H H H CH CH H H H H CH HC H H H HC H HC H H H HC H (G units) (M units) Fig. 1. Repeating saccharide units of κc (R = H), ιc (R = S 3 - ) and λc (a) and Na-Alg (b) (b) Experimental part Materials The polysaccharides kappa-carrageenan (κc, from Condinson Co., Denmark) and sodium alginate (from Merck Co., Germany) were used without further purification. Methylenebisacrylamide (Fluka), potassium persulfate (Merck) and acrylamide (Fluka) were used as received. All other chemicals were also analytical grade. Double distilled water was used for hydrogel preparation and swelling measurements. 2

3 Synthesis of hydrogels In general, certain amounts of κc ( g) and Na-Alg ( g) were added to a 30 ml three-neck reactor equipped with a mechanical stirrer while stirring (600 rpm). The reactor was immersed in a thermostated water bath preset at 80 C. After homogenizing the mixture, AAm and MBA solution (AAm g, MBA g in 10 ml H 2 ) were simultaneously added to the reaction mixture. Then, KPS solution ( g in 5 ml H 2 ) was added. After 60 min, the produced hydrogel was allowed to cool to ambient temperature. Then the product was kept in ethanol (200 ml) for 24 h to dewater. The completely hardened gel particles were filtered, washed with fresh ethanol (2 50 ml) and dried in an oven at 50 C for 10 h. The final powdered superabsorbent hydrogel was stored away from moisture, heat and light. Swelling measurements An accurately weighed (0.2 ± g) portion of the powdered hydrogel with particle sizes between mesh ( µm) was put into a weighed tea bag and immersed in 100 ml doubly distilled water or in 50 ml salt solution and allowed to soak for 2 h at room temperature. The equilibrium swelling (ES) capacity was calculated using the following equation: ES W W W s d = (1) d where W s and W d are the weights of the swollen gel and the dry sample, respectively. FTIR analysis FTIR spectra of samples in the form of KBr pellets were recorded using an ABB Bomem MB-100 FTIR spectrophotometer. Results and discussion Synthesis and reaction mechanism Crosslinking and graft copolymerization of PAAm onto backbones of κc and sodium alginate substrates was carried out in an aqueous medium using KPS as a free radical initiator and MBA as a crosslinking agent. The proposed mechanism for this crosslinking graft copolymerization is illustrated in Scheme 1. The sulfate anionradical, produced from thermally decomposition of KPS, abstracts hydrogen from the hydroxyl groups of κc and Na-Alg to form corresponding alkoxy radicals on the substrates. Then the resulting macroradicals radically initiate graft copolymerization of AAm. Since a crosslinking agent, MBA, is present in the reaction mixture, a threedimensional network results. Spectral characterization Infrared spectroscopy was used to confirm the chemical structure of the grafted products. Fig. 2 shows the FTIR spectra of pure Na-Alg (Fig. 2a) and κc (Fig. 2b) substrates and the κc-alg based hydrogel (Fig. 2c). A new broad peak in the 3

4 spectrum of the hydrogel appeared at 1675 cm -1 that may be attributed to the C= stretching mode of amide groups [18]. o 80 C -. K 2 S S K + 2- S 3 2- S 3 kc kc H -. S 4. n CNH 2 H. Alg Alg NH HN - + CNa - + CNa (MBA) S 3 2- kc CNH 2 [ ] m NH HN CNH 2 [ ] n-m Alg (crosslinked kc/na-alg hydrogel) - + CNa Scheme 1. utline of the synthesis of the κc-alg based superabsorbent hydrogel Fig. 2. FTIR spectra of Na-Alg (a), κc (b), and crosslinked κc/alg-g-paam hydrogel (c) 4

5 To obtain an additional evidence of acrylamide grafting onto the polysaccharide backbones, a similar polymerization was conducted in the absence of crosslinker. After extracting the homopolymer, PAAm, an appreciable fraction of graft copolymer (70%) was found. The graft copolymer spectrum is very similar to Fig. 2c. According to preliminary measurements, the sol (soluble) content of the hydrogel networks was as little as 2%. This fact practically proves that all AAm is involved in the polymer network. So, the AAm fraction in the network will be very similar to that of the initial feed of the reaction. ptimizing the parameters affecting the swelling capacity The different variables affecting the ultimate swelling capacity, i.e., κc-alg weight ratio, concentration of MBA, AAm and KPS, as well as reaction temperature were systematically optimized. Fig. 3. Effect of crosslinker concentration on swelling capacity. Reaction conditions: κc 0.5 g, Alg 0.5 g, AAm 0.45 mol/l, KPS mol/l, 90 C, 60 min Effect of crosslinker concentration Fig. 3 demonstrates the effect of MBA concentration on water absorbency of the synthesized hydrogel. The maximum of swelling is achieved at mol/l of MBA. In fact, with mol/l MBA a slimy gel is formed so that the swollen gel strength is not sufficient to be referred to as a real superabsorbent hydrogel. According to the figure, a reverse relationship between swelling capacity and concentration of crosslinker was concluded (Eq. (2)). Swelling capacity K [MBA] -n (2) where K and n are constant values. Such a well-known behaviour was reported by pioneering scientists [2,19]. A power law behaviour of swelling vs. [MBA], with K = 8.1 and n = 0.29, is obtained from the curve fitted with Eq. (2). It is well known that a higher crosslinker concentration increases the extent of crosslinking of the polymeric 5

6 chains and decreases the free spaces between them; consequently the swelling capacity is decreased. Similar behaviours have been reported by other workers [20]. The n value represents the extent of the sensitivity of the hydrogel to the crosslinker content, while the K value (constant for individual hydrogel) makes it possible to compare the extents of swelling for a fixed crosslinker content. Effect of κc-alg weight ratio The swelling capacity of the hydrogels with various weight ratios of polysaccharides is shown in Fig. 4. With increasing the κc content of the hydrogel, water absorbency increased. This increasing absorbency value may be attributed to the higher hydrophilicity of sulfate groups in the κc backbones compared to the carboxylate anions in the Na-Alg chains. Subsequent swelling loss with further increase in κc amount is due to the increased viscosity of the reaction mixture with increasing κc content, which hinders the movement of reactants. To establish the argument, we measured the sol content of two hydrogels with κc/alg weight ratio of 0.6% (hydrogel A) and 1.2% (hydrogel B). As expected, the sol content of hydrogel B (4%) was higher than that of hydrogel A (1%). This proves that the higher κc content in the reaction mixture results in a higher viscosity prohibiting reactant movement and effective collision Swelling, g/g kc-alg Weight Ratio Fig. 4. Effect of κc content on swelling capacity. Reaction conditions: MBA mol/l, AAm 0.45 mol/l, KPS mol/l, 90 C, 60 min Effect of monomer concentration The effect of monomer concentration on the water absorbency of the hydrogel was investigated by varying the acrylamide concentration from 0.14 to 1.2 mol/l (Fig. 5). As show in Fig 5, with increasing the monomer concentration up to 0.6 mol/l, the swelling capacity is increased and then it is considerably decreased with further increase in monomer amount. The maximum absorbency (50 g/g) is obtained at 0.6 mol/l of AAm. The initial increase in water absorbency can be attributed to an 6

7 increase in the diffusion of AAm molecules in to the polysaccharide backbones resulting in an increase in swelling capacity. In addition, the higher AAm content enhances the hydrophilicity of the hydrogel, in turn, causes a stronger affinity for more absorption of water. The subsequent decrease after maximum absorbency may be ascribed to (a) an increase in viscosity of the reaction medium that hinders the movement of the macroradicals and monomer molecules, (b) preferential homopolymerization over graft copolymerization, and (c) the enhanced chance of chain transfer to monomer molecules. Such behaviours have been reported by other investigators [21-23] Swelling, g/g AAm, mol/l Fig. 5. Effect of monomer concentration on swelling capacity. Reaction conditions: κc 0.4 g, Alg 0.6 g, MBA mol/l, KPS mol/l, 90 C, 60 min Swelling, g/g KPS, mol/l Fig. 6. Effect of initiator concentration on swelling capacity. Reaction conditions: κc 0.4 g, Alg 0.6 g, AAm 0.6 mol/l, MBA mol/l, 90 C, 60 min 7

8 Effect of initiator concentration In this series of experiments, the swelling capacity as a function of initiator concentration was investigated (Fig. 6). Maximum swelling (82 g/g) has been obtained at mol/l of KPS. With increasing KPS content, a large number of active free radical sites was produced on the κc and Na-Alg backbones, which led to a higher chance of acrylamide graft copolymerization reaction onto the substrates. The swelling-loss after use of mol/l KPS may be attributed to an increasing terminating step reaction via bimolecular collision. This reason is referred to as self crosslinking by Chen and Zhao [20]. Also a decreasing molecular weight of grafted PAAm of the hydrogel [24] causes a decrease of the swelling value. In addition, the free radical degradation of κc backbones by sulfate radical-anions is an additional reason for swelling-loss at higher KPS concentration. The proposed mechanism for this possibility is reported in the previous work [14]. A similar observation was recently reported by Hsu et al. for the case of degradation of chitosan with potassium persulfate [25] Swelling, g/g Temperature, o C Fig. 7. Effect of reaction temperature on swelling capacity. Reaction conditions: κc 0.4 g, Alg 0.6 g, AAm 0.6 mol/l, MBA mol/l, KPS mol/l, 60 min Effect of reaction temperature The temperature dependence of the swelling capacity of the κc-alg hydrogel is shown in Fig. 7. According to this figure, the absorbency is increased with increasing reaction temperature from 70 C up to 80 C and then it is decreased with further increasing temperature of reaction. The thermally decomposing initiator, KPS, is completely dissociated to produce sulfate anion-radicals at temperatures higher than its dissociation temperature, viz., 70 C [26]. So, more active sites are created on the polysaccharide backbones at T > 70 C. In addition, the kinetic energy of the resulting macroradicals is increased at higher temperatures, which, in turn, results in faster graft polymerization reaction and consequently higher ultimate swelling of the hydrogel. Decreasing water absorbency with further increasing reaction temperature may be attributed to an increasing rate of termination and chain transfer reactions, and oxidative degradation of kc chains by sulfate radical-anions. In addition, decompo- 8

9 sition of KPS gives molecular oxygen (a radical scavenger), which consumes the produced free radicals (Eqs. (3) and (4)) [27], resulting in decreased molecular weight and decreased swelling. (The sulfate radical anions may react with water to produce hydroxyl radicals, Eq. (3), and finally oxygen, Eq. (4).) (3) S 4 + H 2 HS 4 + H. 2 H H 2 2 H 2 + 1/2 2 The rate of graft copolymerization (R p ) was calculated simply using the following empirical formula [28], for three initial points of Fig. 7: R p (in mol/l s) = m H /MTV (5) where m H (g) stands for the weight of produced dry hydrogel, M (g/mol) denotes the molecular weight of the acrylamide, T and V are the reaction time (in s) and total volume (in L) of the reaction mixture, respectively. The overall activation energy (E a ) of the graft polymerization reaction was calculated via the slope of the plot ln R p vs. 1/T based on the Arrhenius relationship [k p = A exp(-e a /RT)]. Therefore, E a for the graft copolymerization of AAm onto polysaccharide backbones was found to be 374 kj/mol. (4) Fig. 8. Plot of ln R p versus 1/T Swelling in salt solutions Generally, the swelling capacity of ionic hydrogels in salt solutions was significantly decreased compared to the absorbency values in distilled water. This undesired swelling-loss is often attributed to a charge screening effect of the additional cations causing a non-perfect anion-anion electrostatic repulsion [19]. Therefore, the osmotic pressure difference between gel and aqueous phase decreases and consequently the absorbency amounts decrease. In the case of salt solutions with multivalent cations, ionic crosslinking at the surface of the particles causes an appreciable decrease in swelling capacity. 9

10 However, the synthesized hydrogels in this work comprise lots of non-ionic amide groups and a little number of sulfate and carboxylate anions. Therefore, swelling capacity of these hydrogels in salt solutions was not significantly decreased in comparison with water absorbency values. To study various saline effects on the swelling behaviour of the synthesized hydrogels, the equilibrium swelling capacity was measured in 0.15 M chloride solutions of different cations (Tab. 1). Decrease in swelling amounts is strongly dependent on the type and concentration of salt added to the swelling medium. The effect of the type of salt on swelling capacity can be concluded from Tab. 1. Absorbency decreased with an increasing charge of the metal cations. With increasing cation charge, the degree of crosslinking increased and the swelling capacity consequently decreased (LiCl > MgCl 2 > AlCl 3 ). The effect of the cation radius on swelling may also be observed from the data given in Tab. 1. As reported by Tako et al. [29], the κc molecules form intermolecular cation-bridges between the sulfate group of an adjacent anhydro-d-galactose residue with large cations, such as K +, Rb + and Cs +, but not with small cations, e.g., Li + and Na +. Thus, κc has the strongest affinity for crosslinking with K + among monovalent cations of the studied salt solutions. As a result, swelling of the synthesized hydrogels in KCl solution is lower than in LiCl and NaCl solutions (swelling capacity in LiCl and NaCl solutions is approximately equal, but in KCl solution a decreased absorbency is found). Similar results are observed in the case of chloride solutions of Mg 2+, Ca 2+, Sr 2+, and Ba 2+ (Tab. 1). Tab. 1. Swelling and salt sensitivity of κc-alg based superabsorbent hydrogels in distilled water and different salt solutions (0.15 M) Swelling medium Swelling in g/g H LiCl NaCl KCl MgCl CaCl SrCl BaCl AlCl f To achieve a comparative measure of salt-sensitivity of the hydrogels, we defined a dimensionless salt sensitivity factor (f) as follows [30]: f = 1 - [(swelling in salt solution)/(swelling in distilled water)] (6) The f values are also given in Tab 1. The low values of f ( ) demonstrate clearly that the synthesized hydrogels show low salt-sensitivity. This anti-salt behaviour is due to the presence of lots of sulfate groups in κc parts with high ionization tendency and low salt-sensitivity characteristics. Similar conclusions were recently reported by Lim et al. [31] for the synthesis of sodium starch sulfate-g-polyacrylonitrile superabsorbent and by Barbucci et al. for the case of a sulfated carboxymethylcellulose hydrogel [32]. Since the sulfate ions do not keep cations in their 10

11 vicinity, the charge screening effect is not so effective. Therefore, the hydrogels containing sulfate groups show the lowest salt-sensitivity and lowest swelling loss. So, the main reason for high saline absorbency, in comparison with water absorbency, in our κc-based hydrogels certainly is a result of the presence of sulfate and non-anionic amide groups in its κc and PAAm parts of the networks. The swelling capacity of κc-alg hydrogels was also measured in various salt solutions with different concentrations (Fig. 9). The known relationship between swelling and concentration of salt solution is stated in the following equation [2]: swelling = k [salt] -n where k and n are constant values for an individual superabsorbent. Again, a charge screening effect is the main explanation for the intense loss of swelling in comparison with distilled water. But swelling capacity of the synthesized hydrogels is not strongly dependent on salt concentration so that the hydrogel absorbency at higher concentration of saline solutions levels off. Therefore, the k values (swelling at high concentration of salt) are almost the same for the hydrogels in salts solutions and n values are very low. Since the n value is also a measure of salt sensitivity, it indicates that the synthesized hydrogels show an anti-salt behaviour. (7) 80 Swelling, g/g MgCl2 CaCl2 SrCl2 BaCl Salt Concentration, mol/l Fig. 9. Swelling dependency on saline concentration for the κc-alg based hydrogel Swelling kinetics Fig. 10 represents the swelling capacity of the hydrogel in distilled water and a saline solution (0.9 wt.-% NaCl solution) at consecutive time intervals. According to this figure, the swelling rate sharply increases and then begins to level off. The data may be well fitted with a Voigt-based model equation as follows [33]: S t = S e (1 - e -t /τ ) (8) where S e and S t are equilibrium swelling and swelling at time t, respectively. τ (in s) stands for the rate parameter. According to Fig. 10 and using Eq. (8), the rate 11

12 parameters for swelling of the hydrogel in water and a saline solution are found to be 27 and 18 min, respectively. The hydrogel swollen in saline solution comprises the smaller rate parameter; this means with NaCl solution the swelling rate is higher Swelling, g/g Water NaCl Swelling Time, min Fig. 10. Swelling kinetics of CMC-Alg hydrogel in water saline solution. Reaction conditions: κc 0.4 g, Alg 0.6 g, AAm 0.6 mol/l, MBA mol/l, KPS mol/l (particle size of the dried gel was µm) Conclusion Crosslinking graft copolymerization of acrylamide onto kappa-carrageenan and sodium alginate polysaccharides was conducted in an aqueous medium using a persulfate initiator and a hydrophilic bifunctional crosslinker. The maximum water absorbency (82 g/g) was achieved under the optimum reaction conditions found: κc/alg weight ratio 0.67, MBA mol/l, AAm 0.6 mol/l, KPS mol/l, and reaction temperature 80 C. Swelling measurement in various salt solutions showed a known swelling-loss in the presence of multivalent metal cations. However, due to the high swelling capacity of these hydrogels in other salt solutions, they may be categorized in the anti-salt superabsorbent hydrogel family. [1] Buchholz, F. L.; Graham, A. T.; "Modern Superabsorbent Polymer Technology", Wiley, New York [2] Peppas, L. B.; Harland, R. S.; "Absorbent Polymer Technology", Elsevier, Amsterdam [3] Po, R.; Rev. Macromol. Chem. Phys. 1994, 34, 607. [4] Peppas, N. A.; Mikes, A. G.; "Hydrogels in Medicine and Pharmacy", vol. 1, CRC Press, Boca Raton, FL [5] Hoffman, A. S.; "Polymeric Materials Encyclopedia", vol. 5, p. 3282, Salamone, J. C., editor; CRC Press, Boca Raton, FL

13 [6] Yazdani-Pedram, M.; Retuert, J.; Quijada, R.; Macromol. Chem. Phys. 2000, 201, 923. [7] Sugahara, Y.; Takahisa,.; J. Appl. Polym. Sci. 2001, 82, [8] Patel, G. M.; Trivedi, H. C.; Eur. Polym. J. 1999, 35, 201. [9] Silong, S.; Rahman, L.; J. Appl. Polym. Sci. 2000, 76, 516. [10] Fanta, G. F.; "Polymeric Materials Encyclopedia", vol. 10, pp. 7901, 8051, Salamone, J. C., editor; CRC Press, Boca Raton, FL [11] Pourjavadi, A.; Mahdavinia, G. R.; Zohouriaan-Mehr, M. J.; J. Appl. Polym. Sci. 2003, 90, [12] Pourjavadi, A.; Zohuriaan-Mehr, M. J.; Ghasempoori, S. N.; Hossienzadeh, H.; React. Func. Polym., submitted. [13] Mahdavinia, G. R.; Pourjavadi, A.; Hosseinzadeh, H.; Zohouriaan-Mehr, M. J.; Eur. Polym. J., in press. [14] Hosseinzadeh, H.; Zohouriaan-Mehr, M. J.; Pourjavadi, A.; Mahdavinia, G. R.; Polym. Int., submitted. [15] Pourjavadi, A.; Harzandi, A. M.; Hosseinzadeh, H.; J. Biomater. Sci., Polym. Eds., submitted. [16] Pourjavadi, A.; Harzandi, A. M.; Hosseinzadeh, Eur. Polym. J., submitted. [17] Kirk, R. E.; thmer, D. F.; "Encyclopedia of Chemical Technology", vol. 4, p. 942, Kroschwitz, J. I.; Howe-Grant, M.; editors; John Wiley & Sons, New York [18] Silverstein, R. M.; Webster, F. X.; "Spectrometric Identification of rganic Compounds", 6 th edition, Wiley, New York [19] Flory, P. J.; "Principles of Polymer Chemistry", Ithaca, Cornell University Press, New York [20] Chen, J.; Zhao, Y.; J. Appl. Polym. Sci. 2000, 75, 808. [21] Lee, W. F.; Lin, G. H.; J. Appl. Polym. Sci. 2001, 79, [22] Athawale, V. D.; Lele, V.; Carbohydr. Polym. 1998, 35, 21. [23] Athawale, V. D.; Lele, V.; Starch/Stärke 1998, 50, 426. [24] dian, G.; "Principles of Polymerization", ch. 3, 2 nd ed., Wiley, New York [25] Hsu, S. C.; Don, T. M.; Chiu, W. Y.; Polym. Degrad. Stab. 2002, 75, 73. [26] Brandrup, J.; Immergut, E. H.; Polymer Handbook, 3 rd edition, Wiley, New York [27] Hebeish, A.; Cuthrie, J. T.; "Chemistry and Technology of Cellulosic Copolymers", p. 46, Springer-Verlag, Berlin [28] Pourjavadi. A.; Zohuriaan-Mehr, M. J.; Starch/Stärke 2002, 54, 140. [29] Tako, M.; Toyama, S.; Qi, Z. Q.; Yoza, E.; Food Res. Int. 1998, 31, 543. [30] Pourjavadi, A.; Zohuriaan-Mehr, M. J.; J. Polym. Mater. 2003, 20, [31] Lim, D. W.; Whang, H. S.; Yoon, K. J.; J. Appl. Polym. Sci. 2001, 79, [32] Barbucci, R.; Maganani, A.; Consumi, M.; Macromolecules 2000, 33, [33] midian, H.; Hashemi, S. A.; Sammes, P. G.; Meldrum, I.; Polymer 1998, 39,