Molecular orientation, “Region I” shear thinning and the cholesteric phase in aqueous hydroxypropylcellulose under shear |
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Authors: | Kwan Hongladarom W R Burghardt |
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Institution: | Department of Chemical Engineering Northwestern University Evanston, IL 60208, USA E-mail: w-burghardt@nwu.edu, US
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Abstract: | 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. |
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