Unexpected rheological behavior of solutions of aromatic polyamide in transient physical statesстатья Исследовательская статья

Статья опубликована в высокорейтинговом журнале

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Дата последнего поиска статьи во внешних источниках: 19 августа 2020 г.

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[1] Unexpected rheological behavior of solutions of aromatic polyamide in transient physical states / A. Y. Malkin, V. G. Kulichikhin, M. Y. Polyakova et al. // Physics of Fluids. — 2020. — Vol. 32. — P. 073107–1–073107–13. 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 effectsunder shearing in a heating–cooling cycle. In a static state, all systems under study are transparent solutions and no temperature-dependentthermal or visual effects are observed. However, superposition of shearing radically changes the situation. Heating up to 100 ○C–140 ○C leadsto the phase separation with the coexistence of the amorphous and LC phases. On cooling of low-concentrated solutions, a decrease in thetemperature 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 heatrelease, which reflects the reverse process of the disorder-to-order state transition. The isothermal viscosity decline in time is described bythe Maxwell relaxation law with temperature-independent relaxation time. Hence, this is a non-temperature-activated process. At higherconcentrations, strong temperature thixotropic behavior with much lower viscosity values on cooling, in comparison with the heating, ischaracteristic of these solutions due to their tendency for undercooling. The shear-induced transition, conjugated with the heat excess, wasobserved 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 describedbefore. The phase transitions become quite evident in the polarized light. The observed kinetic effects in transient physical states are discussedon the basis of the concept of delayed and latent structure and phase transitions, including the formation of the LC state, which are initiatedby the shearing. [ DOI ]

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