Barrier Properties of Blends Based on Liquid Crystalline Polymers and Poly(ethylene terephthalate)

Abstract

Blends of poly(ethylene terephthalate) (PETP) and two different thermotropic liquid crystalline (LC) polymers of the Vectra-type were prepared by melt mixing. Oxygen and water vapor permeability, light transmission and welding properties were measured on compression-molded and film-blown specimens. SEM showed that the LC polymers were the disperse phase with a good phase adhesion to the PETP matrix in the majority of the compression-molded blends. The 50/50 blend based on the low melting point LC polymer showed possibly a continuous LC polymer phase. The film-blown specimens showed LC polymer spheres at low LC polymer content. Above a certain LC polymer content (10-30% LC polymer), fibrous and ellipsoidal LC polymer particles was the dominant morphological feature of the blends. Density measurements showed that the void content in the blends was low. The compression-molded blends based on the high melting point LC polymer showed permeabilities conforming to the Maxwell equation assuming low permeability (LC polymer) spheres in a high permeability (PETP) matrix. The compression-molded blends based on the low melting point Vectra showed lower permeabilities than predicted by the Maxwell equation, particularly at high LC polymer content. The film-blown blends showed extensive scattering in the permeability data. The blend with 30% low melting LC polymer exhibited a 96% lower oxygen permeability than PETP. This was due to a reduction in both oxygen diffusivity and solubility. Ellipsoidal and fibrous LC polymer particles increased the diffusional path and lowered the diffusivity. The transparency of the compression-molded samples was lost already at 1% LCP. The blends showed welding properties superior to those of PETP.