Effect of thermal history on the properties of thermostropic liquid crystalline polymers having phenyl sulfonyl substituent

Abstract

A series of triad type thermotropic polyesters based on phenylsulfonyl hydroquinone and $\alpha,\;\;\omega$-bis(4-carboxy phenoxy)alkanes was prepared and the effect of thermal history on the properties of the synthesized polyesters was studied. The effect of annealing temperature with constant annealing time(3h) as well as the effect of annealing time(at constant temperature) on the phase transitions of the polyesters was investigated. The phase transitions of all polyesters were found to be significantly affected by thermal history. For the NP3 polyester, which has octamethylene flexible units, the melting temperature increased with increasing annealing temperatures and in some cases, multiple transition endotherms were found in DSC thermograms. The maximum heat of transition was observed at an intermediate annealing temperature. When the samples were treated in the isotropic state, melting endotherm was observed after annealing but, compared to virgin samples, the heat of transition was significantly reduced. WAXD(Wide angle X-ray diffraction) patterns of NP8 fibers showed strong equatorial reflection with d spacing of 4.29A$^\circ$. Annealed samples showed an additional reflection with d=13.90A$^\circ$. The spin-spin relaxation time(T$_2$) was measured by pulsed NMR using solid echo method for both annealed and unannealed samples. After annealing, the T$_2$ value of the rigid structure as well as the soft structure simultaneously decreased. It was suggested that the rigid structure and the soft structure were coupled together so that the motion of soft structure was hampered by already formed rigid crystal structure. When the NP9 polyester, which has nonamethylene flexible units, were annealed as polymerized, the melting endotherm shifted to higher temperature with increasing annealing temperature up to $110\,^\circ\!C$ The maximum heat of melting was found in $100\,^\circ\!C$-annealed sample. WAXD patterns of NP9 fiber showed the intense equatorial reflect...