Chitosan/carboxymethyl cashew gum polyelectrolyte complex: synthesis and thermal stability

doi:10.1016/j.eurpolymj.2005.05.009

European Polymer Journal

Volume 41, Issue 11, November 2005, Pages 2726-2733

https://doi.org/10.1016/j.eurpolymj.2005.05.009 Get rights and content

Abstract

Chitosan/carboxymethyl cashew gum polyelectrolyte complexes were synthesized using different proportions of chitosan (CH) and carboxymethyl cashew gum (CMCG). The optimum CH:CMCG ratio was 25:75, which produced highest product yield. Polyelectrolyte (PEC) samples were characterized by thermogravimetric analysis and FT-IR spectroscopy. Parameters such as initial and maximal degradation temperatures and activation energy (E_a) were determined. Activation energies follows the order CMCG > CH > PECs samples. Infrared analysis from residual products after heating at 280 °C in different times indicated that cleavage of the glycosidic bond and formation of unsaturated products occurred.

Introduction

Polyelectrolyte complexes (PEC) are usually formed by reacting two oppositely charged polyelectrolytes in aqueous solution [1]. This interaction can lead to coarcervate or gel formation. If the interaction between the opposite charge polymer is strong, precipitation can occur [2]. Interaction of chitosan with other polysaccharides such as carboxymethyl cellulose [3], [4], [5], alginate [6], [7], [8], [9], carrageenan [6], hyaluronic acid [10], [11], acacia gum [12], pectin [13], dextran sulphate [14], heparin [15], and carrageenan [16], promote the formation of polyelectrolyte complexes through the link between amine group of chitosan and anionic groups of those polysaccharides such as carboxylate or sulphate groups.

PECs from chitosan and polysaccharides have been investigated for uses in controlled release systems [6], [7], [8], tissue reconstruction [17], [18] as well as pH [5] and electrical sensitive [11] hydrogels.

Cashew gum (CG) is an exudate polysaccharide from Anacardium occidentale trees and has been previously characterized [19], [20]. It is reported to contain galactose (72%), glucose (14%) arabinose (4.6%), rhamnose (3.2%) and glucuronic acid (4.7%) [19], [20]. The polysaccharide has also been modified by carboxymethylation with monochloroacetic as etherifying agent, resulting in samples with degree of substitution (DS) between 0.10 and 2.21 [21].

Thermal stability studies of polysaccharides, its derivatives and blends have been investigated and can give information on the applications of these materials as films, adhesives, coating and also in pharmaceutical and food formulations. Thermal analysis of samples such as acacia gum [22], amylose [23], cashew gum [24], [25], cellulose derivatives [22], [26], [27], chitosan and derivatives [28], [29], [30], [31], [32], guar gum [33], [34], galactoxyloglucan [35], Sesbania gum derivative [36], sodium alginate [22] sodium hyaluronate [37], starch [38], [39], tragacanth gum [22] and xanthan [37] have been carried out. Few reports have been published on the thermal stability of polysaccharide polyelectrolyte complexes, except the studies on chitosan/hyaluronic acid [10], [40], [41] chitosan/chondroitin sulphate [10] and chitosan/carboxymethyl cellulose [3] complexes. In all cases, it is showed decrease of thermal stability of complexes in comparison with the original polysaccharides.

This paper reports on the synthesis and thermal characterization of chitosan/carboxymethyl cashew gum polyelectrolyte complexes. The energy of activation of degradation reaction was determined according to Broido [42] and MacCallum [43] procedures.

Section snippets

Materials

Chitosan was obtained from shrimp shells (deacetylation degree 82%, and molar mass M_v = 3.7 × 10⁵ g/mol). Carboxymethyl cashew gum was synthesized as described by Silva et al. [21] with degree of substitution of 0.86.

Preparation of PECs

Solutions of chitosan in 1% acetic acid were mixed with desired amount of CMCG aqueous solution to give CH/CMCG proportions as seen in Table 1. The complexes were left at room temperature for 12 h and then centrifuged. The solid complexes were washed with distilled water and dried with

General characterization

Polyelectrolyte complex reaction between carboxymethyl cashew gum (CMCG) and chitosan (CH) can be represented by: $CMCG – {COO}^{-} + CH – {NH}_{3}^{+} \to CMCG – {COO}^{- +} {NH}_{3} – CH$ Table 1 shows the product yield of PEC samples when the chitosan-CMCG ratio increases. The optimum mass ratio was taking at the point where a maximal yield of solid complex was obtained. The maximum yield was obtained when the ratio of CH:CMCG was 25:75 (w/w). Denuziere et al. [10] shows that the best polyanion–polycation ratio obtained using

Conclusions

Polyelectrolyte complexes formed by chitosan and carboxymethyl cashew gum start decomposition process at lower temperature than the original polysaccharides. Based on E_a values, the thermal stability follows the order carboxymethyl cashew gum > chitosan > PECs samples. Isothermal degradation shows that E_a values are almost constant with increasing degree of conversion for CH and CMCG samples and E_a increase with degree of conversion for PEC sample. Analysis of residual products revealed the

Acknowledgements

The authors would like to acknowledge CAPES, FUNCAP and CNPq for financial support.

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Chitosan/carboxymethyl cashew gum polyelectrolyte complex: synthesis and thermal stability

Abstract

Introduction

Section snippets

Materials

Preparation of PECs

General characterization

Conclusions

Acknowledgements

Eur J Pharm Biopharm

Polymer

Eur J Pharm Biopharm

Carbohydr Polym

Carbohydr Polym

Solid State Ionics

Int J Pharm

Mater Res Bull

Carbohydr Polym

Carbohydr Polym

Thermochim acta

Polym Degrad Stab

Polym Degrad Stab

Polym

Polym Degrad Stab

Polym Degrad Stab

Carbohydr Polym

Thermochim Acta

Thermochim Acta

Thermochim Acta

Angew, Makromol Chem

Polym Bull