62ª Reunião Anual da SBPC

A. Ciências Exatas e da Terra - 4. Química - 1. Físico-Química

RHEOLOGICAL PARAMETERS OF POLYMERIC SOLUTIONS: THE RELATIONSHIP BETWEEN CONSISTENCY INDEX AND POLYMER CONCENTRATION FOR CHITOSAN SOLUTIONS

Ítalo Guimarães Medeiros da Silva 1 Keila dos Santos Alves 1 Rosangela Balaban 1

1. Departamento de Química da UFRN

INTRODUÇÃO:

In the last years, many researches have been developed on polysaccharide-based materials for a wide range of applications. In general, it happens mainly due to the biocompatibility, biodegradability, abundance in nature and variety of functional properties of the polysaccharides. Chitin is the second most important natural polysaccharide (after cellulose) and can be found in the shell of crustaceans such as shrimps and crabs. Since chitin is not soluble in many solvents, it is usually deacetylated in alkali media reaching chitosan. In fact, this is the principal use of chitin. In acidic aqueous solutions, amino groups randomly distributed on chitosan chains are protonated, and the solubility in this media is achieved by electrostatic repulsion between the polycationic segments. Regarding the many chitosan applications in which rheological parameters are required and the strong shrimp farming in Brazil (specifically in Rio Grande do Norte state), the study of flow properties of chitosan solutions is noteworthy. The aim of this work was to evaluate the rheological description of chitosan solutions by the Ostwald-de Waele (OW) model and to investigate the interdependence between the consistence index (K), the flow behavior index (n), and the polymer concentration (Cp) for two chitosan samples with/without NaCl.

METODOLOGIA:

Chitosans, here denoted C1 (DA = 23% and Mv = 35.700 g.mol-1) and C2 (DA = 18% and Mv = 910.000 g.mol-1), were purchased from Polymar (Brazil). The Viscosity-average Molecular weights (Mv) as well as the Degree of Acetylation (DA) were determined following literature procedures. Before rheological analysis, chitosan was purified. Detailed purification proceedings can be found elsewhere (Desbrieres, J. Polymer, 45, 2004, p.3285-3295). All the others reagents used are of analytical grade and were used without further purification. A typical solution for rheological measurements was prepared by polymer dissolution in AcOH 0.25 M under constant stirring (in some cases, NaCl 1 or 2% was added after the polymer), at 25°C, for 24 h, at Cp = 5, 10, 15 and 20 g.L-1. The rheological behavior of chitosan solutions were analyzed by a Haake RheoStress RS150 rheometer with coaxial cylinders (DG41) geometry. Shear rate was varied from 0 up to 100 s-1. All experiments were carried out at 25oC, which was kept stable with a circulating water bath.

RESULTADOS:

Flow curves (shear stress versus shear rate) of C1 and C2 were obtained. Neither C1 nor C2 showed yield stress or considerable "minimum shear stress", which is a Bingham Plastic's characteristic. Viscosity curves (apparent viscosity versus shear rate) of C1 and C2 were also obtained. In all Cp studied, the aqueous solutions of C1 were less viscous than C2. This behavior was attributed to the higher molecular weight of C2 that increases the hydrodynamic volume and the interactions between the macromolecules, enhancing the viscosity. The effect of NaCl addition was studied in C2 solutions. In a given concentration, NaCl 1% added decreased remarkably the viscosity of C2 solution, but no significant variation was found doubling NaCl concentration. Chloride ions diminished the electrostatic repulsion between the positively charged segments, decreasing the viscosity. As OW is an appropriate model to linear flexible polymer solutions (without side-chain branching effects), it was fitted. All chitosan solutions studied were well described by OW model, exhibiting R² values higher than 0.997. Except for C1 5 g.L-1, all solutions showed pseudoplastic n-values. Toward K, we found a new empirical relationship between K and Cp. Fitting the developed mathematic relation, we found R² values higher than 0.95.

CONCLUSÃO:

The rheological behavior of two chitosan with different molecular weights was compared, and the role of NaCl addiction was verified on C2 solutions. Among the different rheological models dealing with the shear rate-viscosity dependence, OW model described satisfactorily all chitosan solutions studied, and was used to determine the K and n rheological parameters. The n values seemed to have no relation with any other parameter studied. In other hand, a new empirical relationship between K and Cp was established, and useful estimatives can be carried out. Nowadays, there is no mathematical relationship relating K, n and Cp for any system. Meanwhile, in order to contribute to the complete rheological modeling, efforts have been made to correlate at least the pairs of these parameters.

Instituição de Fomento: PRH-30 ANP/MCT.

Palavras-chave: chitosan, rheology, Ostwald-de Waele model.