Unexpected rheological behavior of solutions of aromatic polyamide in transient physical states

Abstract

The subject of this study was an aromatic polyamide in dimethylacetamide/LiCl solutions in a concentration range from 0.5 vol. % – 5 vol. %. Dilute and semi-dilute solutions of this polymer demonstrate a complex of unexpected time- and temperature-dependent rheological effects under shearing in a heating–cooling cycle. In a static state, all systems under study are transparent solutions and no temperature-dependent thermal or visual effects are observed. However, superposition of shearing radically changes the situation. Heating up to 100 °C–140 °C leads to the phase separation with the coexistence of the amorphous and LC phases. On cooling of low-concentrated solutions, a decrease in the temperature leads to a several-times increase in the viscosity, and the subsequent viscosity decrease takes place at further temperature decrease. Both changes are kinetic effects. The first one is treated as an order-to-disorder transition. The decrease in viscosity is accompanied by a heat release, which reflects the reverse process of the disorder-to-order state transition. The isothermal viscosity decline in time is described by the Maxwell relaxation law with temperature-independent relaxation time. Hence, this is a non-temperature-activated process. At higher concentrations, strong temperature thixotropic behavior with much lower viscosity values on cooling, in comparison with the heating, is characteristic of these solutions due to their tendency for undercooling. The shear-induced transition, conjugated with the heat excess, was observed in semi-dilute solutions at the same temperature similar to that observed for dilute solutions. So during cooling under shearing, the solutions under study display a rather unusual phenomenon of a first-order transition. This type of phenomenon has not been described before. The phase transitions become quite evident in the polarized light. The observed kinetic effects in transient physical states are discussed on the basis of the concept of delayed and latent structure and phase transitions, including the formation of the LC state, which are initiated by the shearing.