Viscoelastic properties of chitosan solutions: Effect of concentration and ionic strength

Abstract

The dynamic rheological properties of chitosan solutions have been investigated in terms of ionic strength and chitosan concentration for entangled systems. The overlap concentration C* and the entanglement concentration C_e have first been determined to identify the dilute, semi-dilute and concentrated entangled regimes. In dilute solutions, the effect of ionic strength (I) on the intrinsic viscosity ([η]) has been interpreted in terms of the Debye electrostatic screening length. The intrinsic viscosity, the radius of gyration and the persistence length decreased with increasing ionic strength, due to the screening of the electrostatic charges on the chitosan chains by the salt resulting in increased chain flexibility. For concentrated solutions, dynamic measurements were performed and the relaxation spectra were calculated from the storage (G′) and loss (G″) moduli characterized in the linear viscoelastic region. For all chitosan concentrations and ionic strengths, the time-weighted stress relaxation spectra showed only one peak. However, the width of the spectra increased with increasing polymer concentration, indicating a larger distribution of relaxation modes due to more entanglements and interchain interactions. The mean relaxation time (τ_H), related to reptation, and the zero-shear viscosity (η₀) were found to follow power-law expressions of the chitosan concentration, with τ_H ∼ C^3.1 and η₀ ∼ C^4.1, respectively, with the high exponents indicating the associating character of the polymer. The non-Newtonian nature (elastic properties and shear-thinning behavior) of the chitosan solutions increased with increasing chitosan concentration and decreasing ionic strength.

Introduction

Chitin is the most abundant organic material after cellulose. It is extracted from the shells of crustaceans and the exoskeletons of arthropods. Applications for chitin are limited due to its poor solubility. However, it becomes soluble in aqueous acid solutions after deacetylation in an alkaline environment that results in the product called chitosan. Chitin and chitosan are linear binary heteropolysaccharides composed of 2-amino-2-deoxy-d-glucopyranose (glucosamine) and 2-acetamido-2-deoxy-d-glucopyranose (acetylglucosamine) units, and the two biopolymers are mainly distinguished from each other by their solubility in aqueous acidic solutions (Roberts, 1992). One important parameter of the molecular structure of these materials is the degree of deacetylation (DDA), or number percentage of glucosamine in the chitosan molecule. The copolymer is generally accepted as chitosan when the DDA is larger than 50% (Brugnerotto, Desbrières, Heux, Mazeau, & Rinaudo, 2001).

The increase solubility of chitosan with respect to that of chitin is related to its positively charged polyelectrolyte nature, due to the protonation of the free amine groups below a pH of 6.2 (Park, Choi, & Park, 1983). Positively charged chitosan has been attracting a great deal of attention because of its various bioactivities such as antifungal, antitumor, antiallergic and immune activating characters (Kasaai, 1999). In addition, chitosan is a non-toxic, biocompatible and biodegradable material. It has been used in many industries such as food, medical, cosmetic and pharmaceutical, and numerous international patents have claimed the applications of chitosan in these areas (Chaput and Chenite, 2001, Chenite et al., 2001, Jackson, 1987, Kasaai, 1999).

These characteristics and related potential applications have driven many studies on chitosan. Several have been focusing on the investigation of the physical properties of dilute solutions (Anthonsen et al., 1993, Chen and Tsaih, 2000, Tsaih and Chen, 1997, Wetton et al., 1991) looking at the effect of pH, ionic strength, DDA and molecular weight on the conformation of the chitosan molecule. Other studies have dealt with the rheological properties of concentrated chitosan solutions (Desbrières, 2002, Mucha, 1997, Nystrom et al., 1999, Wang and Xu, 1994). While the effect of pH on these solutions has been investigated (Nystrom et al., 1999), no studies have looked at the effect of ionic force at constant pH for concentrated solutions. Since pH controls the ionization of the chitosan molecule, it is interesting to isolate the effect of the solution ionic strength on a constant polymer charge density.

This study aims at evaluating the effect of chitosan concentration and ionic force at constant pH on the evolutionary aspect of the viscoelastic properties of chitosan solutions, towards gelation by salting out. The results are analyzed in the light of simple rheological models and compared to theoretical scaling law predictions.