Multilayer coextrusion of rheologically modified main chain liquid crystalline polymers and resulting orientational order

Liquid crystalline polymers (LCP) are attractive candidates for use as barrier films in packaging and high performance membrane applications due to their extraordinary barrier properties and chemical resistance. However, LCPs usually display extremely low viscosities, which makes processing them into multilayer films using coextrusion a challenge due to the mismatch of their viscosities with other film components. In this study, a commercial grade diepoxy reagent (Heloxy 67) was used to modify the rheological properties of a thermotropic main chain LCP. The effects of diepoxy concentration and reaction time on the melt viscosity and liquid crystal structures of the LCP were investigated. The addition of small amounts of diepoxy (e.g., 1.5 wt. %) increased the viscosity of the LCP nearly 15-fold. However, addition of excessive diepoxy (e.g., 2 wt. % or greater) led to cross-linking of the LCP. More importantly, these modified samples were thermally stable when melt reprocessed at temperatures of up to 250 °C in air. High quality multilayer films were prepared by coextruding the rheologically modified LCP with polypropylene-graft-maleic anhydride. The orientational order of the LCP in the multilayer films was studied by polarized infrared spectroscopy as well as x-ray diffraction. Interestingly, instead of aligning along the shear direction, the LCP chains tended to orient perpendicular to the extrusion direction, presumably due to a 'log-rolling' effect during processing.