Liquid crystalline copolyester/polythylene in situ composite film: Rheology, morphology, molecular orientation, and tensile properties

A thermotropic liquid crystalline copolyester (TLCP) was blended with low density polyethylene using a corotating twin screw extruder and then fabricated by extrusion through a miniextruder as cast film. Rheological behavior, morphology, and tensile properties of the blends were investigated. Melt viscosities of neat components and blends measured by using plate-and-plate and capillary rheometers at 240°C, in the shear rate range 1-10⁴ s^-1, showed similar shear thinning effect. The viscosity values measured by the two techniques in the overlapping range of shear rate are found to be identical, which is in accord with the Cox-Merz rule. Addition of TLCP slightly reduces the matrix melt viscosity. TLCP dispersed phase in the extruded strand appeared in the form of spherical droplets. These droplets were elongated into fibrils with high aspect ratio (length to width) at the film extrusion step. As a result, the Young's modulus in machine direction (MD) of the composite film was greatly enhanced. At 20 wt % of TLCP, the MD Young's modulus was found to be about 16-fold increase compared to that of the neat polyethylene film. However, the elongation at break sharply dropped with the increase of TLCP content.