PHASE-BEHAVIOR OF AQUEOUS-SOLUTIONS OF HYDROXYPROPYLCELLULOSE

The phase behavior of hydroxypropylcellulose (HPC) in water has been s tudied in the temperature range from -3 to +46 degrees C by video-enha nced contrast (VEC) optical microscopy. In the low temperature range ( below about 18 degrees C), a significant thermotropic effect was found in a narrow biphasic gap, with the critical HPC concentration for mes ophase formation rapidly increasing as a function of temperature. The effect can be explained in terms of an increase of chain rigidity upon lowering the temperature, as estimated from intrinsic viscosity measu rements. At higher temperatures (up to about 40 degrees C), phase sepa ration takes place in-a wider region, which extends to a polymer conce ntration of about 65% wt at 25 degrees C. Upon a further increase in t emperature, the material becomes white and turbid in a broad concentra tion range. When samples are examined at high magnification with the V EC optics, three distinct morphologies are observed in the turbid regi on, depending on concentration. At low concentrations, anisotropic sph erulites are dispersed in an anisotropic medium. At high concentration s, isotropic inclusions of irregular shape are observed in an anisotro pic continuous phase. A gel-like structure, characterized by a three-d imensional network, is displayed by solutions of intermediate concentr ations. The phase diagram here obtained is compared to previous result s on the HPC/water system and discussed in terms of the existing theor ies on liquid crystalline polymer thermodynamics.