Transient rheological behavior of a thermotropic liquid-crystalline polymer. I. The startup of shear flow

Abstract

The transient rheological responses upon startup of shear flow were investigated for a well-characterized thermotropic liquid-crystalline polymer in the nematic region using a cone-and-plate rheometer. For the study, an aromatic polyester, poly[(phenyl sulfonyl)-p-phenylene 1,10-decamethylene-bis(4-oxybenzoate)] (PSHQ10), was synthesized via solution polymerization in our laboratory. The PSHQ10 was found via gel permeation chromatography to have the weight-average molecular weight of 45 000 relative to polystyrene standards and a polydisperity index of 2, and via differential scanning calorimetry to have (a) a glass transition temperature of 88 °C; (b) a melting point of 115 °C; and (c) a nematic–isotropic transition temperature (TNI) of 175 °C. The initial morphology (i.e., initial conditions) for the startup of shear flow of PSHQ10 in the nematic region was controlled by first heating a solvent-cast specimen to 190 °C (i.e., to the isotropic region), shearing at a rate of 0.085 s−1 for 5 min, and then cooling slowly down to a predetermined temperature in the nematic state (i.e., 130, 140, 150, or 160 °C). A fresh specimen was employed for each transient experiment, and the growths of shear stress σ+(t,γ̇) and first normal stress difference N1+(t,γ̇) were recorded as functions of time (t) for shear rates (γ̇) ranging from 0.107 to 1.07 s−1 and for temperatures ranging from 130 to 160 °C. It was found that thermal and deformation histories could be erased, and large overshoots in σ+(t,γ̇) and N1+(t,γ̇) were observed with only positive steady-state normal stress difference existing for all temperatures.