Flow-induced isotropic/liquid-crystalline transitation monitored by rheological measurements

Autori: N. Grizzuti, P. Aresta, P.L. Maffettone
Presentato al: Southern Europe Conference on Rheology
Anno: 1999
Volume: 2
Issue: Unico
Casa Editrice: SIR-Cuen Napoli
Lingua: English


Abstract
Recent theoretical work [1] on the pre-transitional behaviour of Liquid Crystalline Polymers (LCPs) has shown that an isotropic system can undergo a transition to a liquid crystalline phase under shear flow. Such a transition, however, is predicted to be quite complex, including a bifurcational behaviour leading to a hysterisis loop upon continuous changes in shear rate.
In this work, an experimental investigation of the shear induced transition of a polymer displaying liquid crystalline behaviour has been conducted. The system is an aqueous solution of Hydroxypropylcellulose (HPC), which presents both lyotropic and thermotropic features [2]. Here, in particular, the possibility of a phase change due to a temperature variation has been exploited. Both steady-state and transient shear flow experiments have been carried out on an isotropic HPC solution very close to the isotropic/LC transition temperature. The temperature-dependent rheological response of the system clearly indicates the presence of a transition from an isotropic-like behaviour to a liquid crystalline one. Preliminary tests around the transition temperature, where the system is practically fully isotropic, show that the viscosity undergoes a hysterisis loop, depending on the direction (increasing or decreasing) of the applied shear stress. The experimental results are discussed and compared with the above mentioned theoretical predictions.


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