Effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer

Abstract

The effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer was investigated. For the study, an aromatic polyester, poly[(phenylsulfonyl)-p-phenylene-l,10decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized. PSHQ10 was found to have (a) a weightaverage molecular weight of 35 000 relative to polystyrene standards and the polydispersity index of 2.0, (b) a glass transition temperature of 80 °C, (c) a melting point of 110 °C, and (d) a nematic-isotropic transition temperature (Tni) of 175 6C. Dynamic storage and loss moduli, G' and G", in the oscillatory shear mode were measured during isothermal annealing for specimens of PSHQ10 having various thermal histories. We found that thermal history had a profound influence on the rheological behavior of PSHQ10; specifically, during isothermal annealing in the nematic region, the values of G', G", and complex viscosity |i)*| of the as-cast specimen increased with time much faster at 130 °C than at 135 and 140 °C, but at temperatures above 140 °C the values of G', G", and |i?*| initially decreased and then increased, finally leveling off after a very long annealing period (for instance, 500 h after the annealing began at 145 °C). Using information obtained from differential scanning calorimetry, we explained the seemingly peculiar behavior in terms of a slow process of "crystal-like phase" formation below ca. 140 °C. However, when the as-cast specimen was first heated to a temperature above Tni, subjected to steady shear flow at y = 0.085 s_1 for 5 min, and then cooled slowly down to the nematic region, the values of G', G", and |t?*| were found to change little during the subsequent annealing for a long period. Also conducted were light scattering experiments on as-cast specimens of PSHQ10 having different thermal histories. We found that during isothermal annealing the maximum intensity of scattered light increased faster as the annealing temperature increased, and the rate of "domain" coarsening, induced by isothermal annealing in the nematic range, increased as the annealing temperature increased. This finding was corroborated by the micrographs taken with a cross-polarized optical microscope.

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3-bis[4-(4′-aminophenoxyl) cumyl] benzene

“…These crystals are metastable and the formation is highly dependent on annealing conditions: temperature and time. Han et al 15 and Kim and Han 16 also found an intermediate melting endotherm before normal melting in poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylene bis-(4-oxybenzoate)] (PSHQ10) caused by annealing; the formation of this annealing-induced endotherm has a profound effect on the rheological properties of this polymer.…”

“…The double or multimelting phenomenon has long been observed in a number of crystalline polymers, such as flexible chain, semirigid chain, and rigid chain polymers: for instance, polyethylene (PE), 3 polypropylene (PP), 4 nylon, 5 poly(ethylene terephthalate) (PET), 6,7 poly(aryl ether ether ketone) (PEEK), 8 -10 poly(ether diphenyl ether ketone) (PEDEK), 11 and thermotropic liquid crystalline polyesters. [12][13][14][15][16][17][18] Among those polymers PET and PEEK have been extensively studied by a number of authors. A variety of explanations for the multimelting behavior have been put forward: (1) different crystal structures, which have been observed in trans-1,4-polyisoprene, 19 isotactic polypropylene 4 ; (2) different morphological characters with different lamellar thicknesses, which have been found in cis-1,4-polyisoprene 20 and PEEK; and (3) melting, reorganization, and remelting, which were observed in poly(⑀-caprolectone), 21 and PET.…”

Microphase transitions of block copolymer/homopolymer under shear flow

“…So, we consider that thermal history also has a certain effect on the difference between figure 6 and figure 8. Seung Su Kim [74] and M. L. Cerrada [75] have studied the effect of thermal history on the phase behavior of polymeric materials, and it showed that some contradictory results in the references are likely to be caused by the different thermal history of a sample.…”

Optical and Mechanical Rheometry of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers with Varying Pendant Groups

“…As demonstrated previously, [13][14][15][16] the initial morphology of a TLCP specimen can be controlled by heating the specimen above its clearing temperature (T NI ), shearing in the isotropic phase, and then cooling to a predetermined temperature in the anisotropic state. Such practice is only possible for TLCPs with T NI lower than the thermal degradation temperature.…”