BLENDS OF A LONGITUDINAL POLYMER LIQUID-CRYSTAL WITH POLYCARBONATE - RELATION OF THE PHASE-DIAGRAM TO MECHANICAL-PROPERTIES

This work is aimed at an evaluation of the capability to predict the m echanical behaviour of blends on the basis of the phase diagram. Conne ctions between phase structures, the phase diagram and mechanical prop erties were studied for binary blends of polycarbonate (PC) based on b isphenol-A with PET/0.6PHB, where PET = poly(ethylene terephthalate), PHB = p-hydroxybenzoic acid and 0.6 = the mole fraction of PHB in the copolymer. PET/0.6PHB is a longitudinal polymer liquid crystal (PLC) w ith LC sequences in the backbone along the main-chain direction. The t echniques used were differential scanning calorimetry (d.s.c.), thermo mechanical analysis (t.m.a.) in the penetration and three-point-bendin g modes, thermally stimulated depolarization (t.s.d.) and two kinds of dynamic mechanical analysis (d.m.a.) as a function of frequency omega and temperature T. The resulting phase diagram is plotted as a quasi- binary one (T versus concentration of PET/0.6PHB or PC) but provides i nformation on miscibilities of all three pairs of constituents; it con tains equilibrium as well as long-living non-equilibrium phases, inclu ding the quasiliquid. D.s.c. and d.m.a. results show that glass transi tion temperatures for the PC-rich phase drop by similar to 50 degrees C after 1 h of annealing, with evident consequences for mechanical pro perties. Variations of storage modulus and strain as a function of tim e are reported for different blend compositions. Isothermal storage mo duli and results of three-point bending as a function of composition a re connected to the phase diagram. Prediction of mechanical behaviour is possible from miscibilities of constituents in specific regions of the diagram. Copyright (C) 1996 Elsevier Science Ltd.