Effects of particles on the steady state and transient rheology of lyotropic hydroxypropylcellulose solutions

The rheological effects of adding particles to main-chain liquid crystallin e polymers (LCPs) are studied. Suspensions of polystyrene particles in liqu id crystalline solutions of hydroxypropylcellulose in water are used. Even very small amounts of particles seem to eliminate negative normal stress di fferences. The complex and unsteady flow field in between particles might i nterfere with the director orientation and in this manner affect the occurr ence of negative normal stresses. Row reversal experiments on filled system s produce damped oscillatory responses for both the shear stress and the fi rst normal stress difference. In these transients the scaling of time with shear rate, a characteristic of LCPs, is preserved. This indicates that the LCP texture still determines the time scale of the rheological response of the filled system. The textural changes are more rapid when particles are present. This has been observed in recoil and relaxation experiments as wel l as in flow reversals. When adding particles the decrease in time scales c losely follows the increase in viscosity. The observed phenomena can be rat ionalized by considering how the presence of particles affects the macrosco pic textural length scale, the amount of LCP material present and the direc tor orientation during flow. (C) 2000 The Society of Rheology. [S0148-6055( 00)00606-4].