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On the linearity of the pressure drop during flow of thermotropic LCPs in slits and capillaries

Langelaan, Hubertus Cornelis, 1964-, Gotsis Alexandros, Posthuma de Boer A.

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Περίληψη

The flow behavior of a thermotropic liquid‐crystalline polymer (LCP) was measured in a slit rheometer with nine pressure transducers flush mounted along its axis. Thus it was possible to measure the pressure drop and its (non)linearity. The rheometer was provided with a restriction at the end of the flow channel, enabling measurements at elevated pressures. The shape of the pressure profile was found not to depend on the position in the slit but to be entirely determined by the pressure. The model of deorientation of a LCP melt as proposed by Wissbrun [J. Rheol. 37, 777–797 (1993)] cannot account for the observed nonlinearities. The effect of pressure on the viscosity seems a better explanation. The value of β calculated from the slit rheometer measurements ranges between 7.1×10−9 Pa−1 for γ̇=151 s−1 and 5.3×10−9 Pa−1 for γ̇=2262 s−1, which is in the same order of magnitude as the reported values for flexible polymers. The influence of the measuring geometry on the viscosity was investigated by comparing the viscosities of the material in the slit with the value of the viscositymeasured in a capillary rheometer. When the pressure dependence of the viscosities is taken into account, it is found that the viscosities as measured in both dies have the same values. The value of β calculated from the slit rheometer measurements cannot describe the observed nonlinearity of the Bagley plots.

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