Birefringence and flow visualization are used to study molecular orientation, texture, and the cholesteric nature of a 60
wt% aqueous hydroxypropylcellulose solution at low to moderate shear rates. There is a sharp transition in behavior at a shear
rate near 0.5 s
–1. Below this rate, the sample shows “Region I” shear thinning, takes on a frosted appearance, has low flow-induced orientation,
and exhibits faint optical diffraction characteristic of cholesteric liquid crystals. Upon flow cessation from low rates,
a highly organized cholesteric phase showing bright optical diffraction is re-formed in around 5min. Above the critical shear
rate, the sample exhibits much higher orientation and a striped texture which is readily apparent with or without polarized
light illumination. Upon flow cessation, molecular orientation decays significantly for up to around 400 s; however, the striped
textures established during shear persist. It is suggested that persistence of the cholesteric phase under shear is responsible
for the occurrence of Region I shear thinning at low shear rates in this solution.
Key words Liquid crystalline - polymer - flow visualization - birefringence - cholesteric phase - hydroxypropylcellulose
Received: 23 July 1997 Accepted: 1 December 1997