Characterization of thermotropic liquid crystalline polyester/polycarbonate blends: Miscibility, rheology, and free volume behavior

Miscibility, rheology, and free volume properties of blends of thermotropic liquid crystalline polymers (TLCPs) (Vectra A950) and polycarbonate (PC) a re studied in this work. Despite the unusual increase in T-g of the PC phas e, the blends are found to be generally immiscible. Transesterification may occur during blending and be the cause of the increase of T-g of the PC ph ase and the partial miscibility of the blends at high TLCP concentrations. With regard to the melt rheology of these materials, according to a three-z one model, dynamic moduli of Vectra A950 show plateau- and transition-zone behavior, while PC exhibits terminal-zone behavior. The blends show only te rmi nal-zone behavior at low Vectra A950 contents (less than or equal to 50 %) and terminal- and plateau-zone behavior at higher Vectra A950 contents. The relaxation time of Vectra A950 is much longer than PC and the blends ha ve relaxation times greater than additivity. Both the complex and steady sh ear viscosities of the blends increase with the addition of Vectra A950. Th is is attributed to interfacial association, which retards the reorientatio n and alignment of the Vectra A950 phase in the molten state. The Cox-Merz rule holds true for PC but not for Vectra A950 and the blends. Free volume properties on an angstrom scale evaluated by positron annihilation lifetime spectroscopy (PALS) indicate that Vectra A950 has smaller, fewer free volu me cavities than PC and the variation of free volume behavior in the blends can be explained in terms of blend miscibility. The measured densities of the blends agree well with the free volume fractions of the blends determin ed from PALS. (C) 2000 John Wiley & Sons, Inc.