String phase formation in biopolymer aqueous solution blends

In previous publications it has been shown that the flow-morphology relationships that apply to two-phase polymer blends or solutions, and to immiscible emulsions, also apply to two-phase aqueous solutions of mixed biopolymers, (referred to here as "biopolymer blends"). The relationships in question are classical theories of single drop deformation in shear flow, fibril breakup upon cessation of steady shear flow, and the Palierne model. In this paper, we report the experimental investigation of string phase formation in steady shear as another structural phenomenon which has been observed in polymer blends or polymer solutions, but has not been previously reported for biopolymer blends. The rheological and morphological behavior of gelatin-dextran mixtures was analyzed using conventional rotational rheometry and shear-microscopy techniques. It was found that the development of a string phase depends on the applied shear rate, the viscosity ratio and the phase volume of the minor phase. A rheological model by Kume and Hashimoto (1995) predicting string phase morphology was successfully applied for viscosity ratios not too dissimilar from unity. The application of a further model by Jeon and Hobbie (2001), however, gave ambiguous results with regard to the prediction of string phase formation. (C) 2003 The Society of Rheology.