Studies on the effect of initial domain size on the physical properties of thermotropic polyester

Abstract

To see the effect of initial domain size on the physical properties of thermotropic liquid crystalline polyester, poly[(phenylsulfonyl)-p-phenylene 1,8-octamethylene bis(4-benzoate)](NP8) having low isotropization temperature at 178℃ was synthesized via solution polymerization. Previous thermal histories were erased by treating at isotropic temperature at 200℃ and the LC domain size was controlled by annealing at liquid crystal state temperature, at 170℃. LC domains grew with annealing time and the domain growth rate was studied by optical microscopy and light scattering experiment. The scaling exponents obtained from both studies were comparably the same, and the domain growth rate could be scaled as $d~t^{0.19}$ at LC state temperatures. The effect of the initial LC domain size on the crystallization rate was investigated. Crystallization temperature of 110℃ was chosen since the growth rate showed maximum at that temperature. Three crystal melting peaks were observed on the crystallized sample. Avrami treatment and WAXD analysis for the three different crystal structures revealed that the high temperature melting crystal has higher degree of orientation than the low temperature melting crystal. It was found that the NP8 with large domain size had the faster crystallization rate and higher isotropization temperature. Another crystallization temperature of 140℃ was chosen since the crystallized sample at 140℃ showed single crystal melting peak. However, unlike the crystallization at 110℃, the effect of initial domain size on the crystallization rate at 140℃ was negligible. The effect of the initial LC domain size on the transient rheological properties showed that the larger domain size had smaller stress overshoot and $N_1$ overshoot for the transient shear flow. The effect of the initial LC domain size on the steady state rheological properties showed that the larger domain size had the smaller steady state $N_1$ values at low shear rate region.