Rheology-Structure Interrelationships of Hydroxypropylcellulose Liquid Crystal Solutions and Their Nanocomposites under Flow

A new approach to study the flow-induced orientation in LC polymer-based nanocomposites has been developed. This approach is a combination of optical, mechanical, and X-ray methods that allowed us to "visualize" physical processes taking place in these systems under flow. The HPC/water solution (matrices) concentrations were selected based on the accurate phase equilibrium lines determined with the optical interferometry method. Flow curves and the concentration dependence of the viscosity provide additional information on the phase state and structure of the samples. Rheo-X-ray data were obtained using a specially developed micro-Couette geometry consisting of a coaxial arrangement of two standard X-ray capillaries (the inner one rotates at different speeds). The orientation development of HPC in two-phase and full anisotropic solutions, as well as of the clay and HPC in the nanocomposites were measured (separately) as a function of shear rate. In addition, the decay of the orientation parameter with time after cessation of flow was analyzed to reveal several stages related to the disorientation and reformation of the cholesteric helix in HPC solutions. Crystalline nanoclay in flowing LC solutions also formed a mesophase structure. Specifically, the columnar mesophase of clay formed under certain conditions could be completely or partially converted to a discotic phase. Comparison of these results with optical images of the regular (circle-like) morphology observed under strong flow suggested that the elasticity is the general driving force for development of the regular instability in length scales of optical and X-ray scattering measurements.