Interrogation of "surface," "skin," and "core" orientation in thermotropic liquid-crystalline copolyester moldings by near-edge X-ray absorption fine structure and wide-angle X-ray scattering

Injection molding thermotropic liquid-crystalline polymers (TLCPs) usually results in the fabrication of molded articles that possess complex states of orientation that vary greatly as a function of thickness. "Skin-core" morphologies are often observed in TLCP moldings. Given that both "core" and "skin" orientation states may often differ both in magnitude and direction, deconvolution of these complex orientation states requires a method to separately characterize molecular orientation in the surface region. A combination of two-dimensional wide-angle X-ray scattering (WAXS) in transmission and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to probe the molecular orientation in injection molded plaques fabricated from a 4,4′-dihydroxy-α-methylstilbene (DHαMS)-based thermotropic liquid crystalline copolyester. Partial electron yield (PEY) mode NEX-AFS is a noninvasive ex situ characterization tool with exquisite surface sensitivity that samples to a depth of 2 nm. The effects of plaque geometry and injection molding processing conditions on surface orientation in the regions on- and off- axis to the centerline of injection molded plaques are presented and discussed. Quantitative comparisons are made between orientation parameters obtained by NEXAFS and those from 2D WAXS in transmission, which are dominated by the microstructure in the skin and core regions. Some qualitative comparisons are also made with 2D WAXS results from the literature.