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

The rheological effects of adding particles to main-chain liquid crystalline polymers (LCPs) are studied. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose in water are used. Even very small amounts of particles seem to eliminate negative normal stress differences. The complex and unsteady flow field in between particles might interfere with the director orientation and in this manner affect the occurrence of negative normal stresses. Row reversal experiments on filled systems produce damped oscillatory responses for both the shear stress and the first 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 well as in flow reversals. When adding particles the decrease in time scales closely follows the increase in viscosity. The observed phenomena can be rationalized by considering how the presence of particles affects the macroscopic textural length scale, the amount of LCP material present and the director orientation during flow. (C) 2000 The Society of Rheology. [S0148-6055(00)00606-4].