Rheological characterization of aqueous hydroxypropyl cellulose solutions close to phase separation

The influence of temperature, concentration, shear rate and degree of substitution on the molecular structure of aqueous hydroxypropyl cellulose solutions (HPC) was investigated by means of rheo-mechanical and rheo-optical methods. During the investigations, it was possible to demonstrate the formation of supramolecular structures, even before visual or rheo-mechanical configuration, through the detection of dichroism in the shear field at T = 39 degreesC shortly before the flocculation temperature at T-F = 40 degreesC. In addition, birefringence experiments and mechanical oscillation investigations showed that this conformational change of the segments was dependent upon the polymer concentration and the shear rate. The aggregated structure of the 0.6% HPC was elastic and easily degradable under shear, whereas the 1.5% solution was less elastic and the aggregates could only be degraded at higher shear rates. At high shear rates it is presumed that a conformation change in the HPC segments for 0.6-1.5% solutions led to the development of a second shear-table aggregate structure. It was therefore not possible to perform a uniform normalization of the birefringence in the semi-concentrated solutions and compare it with phenomenological theories for the temperature range at T = 39 degreesC due to differences in conformation that were dependent upon concentration and shear stress. The influence of the molar substitution, MS, at T = 39 degreesC on the formation of supramolecular HPC structures was significant for 1.5% solutions. Beyond a MS > 6, the birefringence: increased by over a decade as a function of the shear rate.