Effect of soluble polysaccharides addition on rheological properties and microstructure of chitin nanocrystal aqueous dispersions

Highlights

• The effect of polysaccharides in chitin nanocrystals (ChN) dispersions was studied.

• The non-adsorbing polysaccharides cause formation of nematic ChN structures.

• Polysaccharides induce gelation in ChN dispersions, except pullulan and chitosan.

• Mixed ChN–κ-carrageenan (oppositely charged) dispersions form the strongest gels.

• Mixed ChN-chitosan dispersions exhibit liquid like behavior.

Abstract

Mixtures of chitin nanocrystal aqueous dispersions (at pH 3.0) with soluble polysaccharides of varying molecular features were examined rheologically and microscopically, under different conditions of biopolymer concentration, ionic strength, pH and temperature. The addition of non-adsorbing polysaccharides (guar gum, locust bean gum and xanthan) as well as oppositely charged (κ-carrageenan) to a chitin nanocrystal dispersion, resulted in a network formation and the gel strength increased with the chitin nanocrystal concentration. In contrast, the chitin nanocrystal – chitosan or – pullulan mixed dispersions did not show any network formation (tanδ > 1) at the concentration range examined. An increase in ionic strength and pH also resulted in an enhanced elasticity of the chitin nanocrystal–guar gum dispersions. Furthermore, an increase in the elastic modulus, which was irreversible upon cooling, was observed upon heating the chitin nanocrystal–polysaccharide mixed dispersions.

Introduction

Polysaccharide nanocrystals from various sources such as starch, cellulose and chitin have been receiving plenty of attention lately, due to their distinguished physical properties (Dufresne, 2011, Lin et al., 2012). Moreover, chitin has attracted a lot of interest because of its plethora of biological properties. Chitin is a structural biopolymer found in shellfish, insects, and microorganisms and is the second most abundant polysaccharide found in nature. It can be hydrolyzed with hydrochloric acid to produce a dispersion of colloidal chitin nanocrystals that are positively charged due to the protonation of the amino groups present in the chitin molecules (Belamie et al., 2004, Lin et al., 2012, Marchessault et al., 1959, Revol and Marchessault, 1993, Zeng et al., 2012). Chitin nanocrystals (also called nanofibrils or nanowhiskers) have been used in various biomedical applications, such as wound medicaments and anti-inflammatory agents (Azuma et al., 2012, Muzzarelli, 2012, Muzzarelli and Muzzarelli, 2005, Muzzarelli et al., 2007) or as mechanically reinforcing biodegradable particles (Dufresne, 2011, Zeng et al., 2012).

Because of the rod-like shape of the chitin nanocrystals, these dispersions display liquid crystalline behavior above a critical particle concentration, as proposed by Onsager's theory for rigid rod-like particles (1949). At low chitin nanocrystal concentrations the dispersions are isotropic, with a random arrangement of rods, whereas at high concentrations the dispersions are anisotropic, with the chitin rods developing a birefringent nematic-like structure. Just beyond the critical concentration for anisotropic phase formation is a biphasic region in which the isotropic and anisotropic phases coexist (Revol & Marchessault, 1993). In addition to the isotropic–anisotropic transition, another phenomenon may take place, the anisotropic–nematic gel formation, which is usually caused with a further increase of particle concentration (Tzoumaki et al., 2010, Wierenga et al., 1998). The physical origin of gelation of these dispersions is not fully understood, since it may involve different mechanisms depending on the type of interactions operating in such systems, either of repulsive or attractive nature (Buining et al., 1994, van Bruggen and Lekkerkerker, 2002, Wierenga et al., 1998). Some of the most important variables that affect the rheological behavior of such dispersions are particle concentration as well as factors which influence the strength of inter-particle interactions; i.e. the nature of particles themselves (size and size distribution, shape and surface charge), the ionic strength and pH of the aqueous medium (ten Brinke, Bailey, Lekkerkerker, & Maitland, 2007).

Mixing the anisotropic particle dispersions with soluble polymers is another way to modulate the phase behavior and the mechanical properties of these systems (Aarts et al., 2002, Beck-Candanedo et al., 2007, Buitenhuis et al., 1995, ten Brinke et al., 2007); the characteristics of such mixtures would mainly depend on differences in the physical properties and structure (size, shape, conformational flexibility, or charge) of the rod-like particles and the soluble polymer. For example, in the case of a non-adsorbing added polymer, it has been widely shown that if a structural property, such as shape or flexibility of the two components is different enough, a bulk demixing can occur in their mixed dispersions (Beck-Candanedo et al., 2007, Edgar and Gray, 2002, Flory, 1978). In this context, a random coil polymer will be excluded from an anisotropic phase consisting of rod-like particles. These thermodynamically unfavorable interactions arise mainly from excluded volume effects, governed by the physical volume occupied by one biopolymer molecule that is inaccessible to the other biopolymer molecules (Semenova, 2007). In general, depletion-type interactions occur from the imbalance in osmotic pressure that results when the polymer molecules are excluded from the area between two colloidal particles, where the inter-particle distances are smaller than the polymer effective diameter, resulting in an attractive force between the colloids (Adams et al., 1998, Asakura and Oosawa, 1954, Asakura and Oosawa, 1958, Tuinier et al., 2003, Tuinier et al., 2008). There are studies showing that the addition of a flexible polymer in rod-like particle dispersions, like cellulose nanocrystals (Beck-Candanedo et al., 2007, Edgar and Gray, 2002) and boehmite rods (Buitenhuis et al., 1995), may lead to an isotropic–nematic transition in systems of lower anisotropic particle content. Moreover, in other studies it was found that the addition of a relatively flexible polymer induces aggregation and gelation (van Bruggen & Lekkerkerker, 2000), and increases the elastic modulus of colloidal dispersions (Fan & Advani, 2007). These changes in rheological properties have been attributed to formation of a percolated filler network in the polymeric matrix (Fan and Advani, 2007, van Bruggen and Lekkerkerker, 2000). On the other hand, the addition of an oppositely charged polymer in a rod-like particle dispersion, would be expected to result in associative phase separation (or else called complex coacervation) (de Kruif and Tuinier, 2005, de Kruif et al., 2004).

In an effort to modulate the mechanical properties of chitin nanocrystal aqueous dispersions, the impact of soluble polysaccharides with varying molecular characteristics, like molecular conformation, charge and size, on the rheological behavior and microstructure of mixtures containing chitin nanocrystals was examined. The selected soluble polysaccharides were guar gum, locust bean gum and xanthan gum, κ-carrageenan (a negatively charged polysaccharide), chitosan (a positively charged polysaccharide) and pullulan as a particularly flexible neutral biopolymer. Solutions of these polysaccharides were mixed with aqueous chitin nanocrystal dispersions and the resulting mixtures were studied with dynamic rheometry under different conditions of soluble polysaccharide-particle concentrations, ionic strength, pH and temperature. Also, complementary polarized optical micrographs have been captured in an attempt to relate the rheological behavior and stability of the composite dispersions to microstructural changes. Some of the results presented herein are those for guar gum, unless otherwise stated, since similar behavior was observed for some of the soluble polysaccharides tested in this study.