Characterization of the skin orientation of thermotropic liquid‐crystalline copolyester moldings with near‐edge X‐ray absorption fine structure

Abstract

ABSTRACT:The process of injection-molding net-shape parts from thermotropic liquid-crystalline polymers results in a skin-core macrostructure. The underlying orientation in the core and the skin may differ both in magnitude and direction. A combination of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and two-dimensional wide-angle X-ray scattering (2D WAXS) in transmission was used to characterize the orientation in injection-molded plaques fabricated from thermotropic liquid-crystalline copolyesters based on either 4,4Ј-dihydroxy-␣-methylstilbene or 6-hydroxy-2-naphthoic acid/6-hydroxybenzoic acid. NEXAFS is presented as a noninvasive in situ means of determining surface layer orientation that samples to a depth of as little as 2 nm and does not require slicing or ultramicrotoming of the samples. The effects of various processing conditions on the surface orientation in the region of the centerline of square injection-molded plaques are presented and discussed. Comparisons are made between orientation parameters obtained by 2D WAXS in transmission, which is dominated by the microstructure in the core, and the NEXAFS technique.

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

“…4), ''skin'' and ''core'' contributions to the orientation must be computed separately. As previously described by Rendon et al 12 and Bubeck et al, 7 bimodal azimuthal scans typically have two different sets of intensity peaks that are attributed to the strength and direction of skin (driven by shear flow) and core (driven by transverse extension) orientational contributions. To isolate each set of peaks we utilize an appropriate deconvolution technique (provided an appropriate baseline is used) that generates two separate azimuthal scans describing the distribution of orientation over two different depths in the sample (Fig.…”

“…This convention has been used by several authors, [15][16][17]20 including previous work on the surface orientation in TLCPs. 7 Under this convention, an average uniaxial orientation parameter, S a , is defined as:…”

“…6 A less invasive and more precise means of determining surface orientation directly presents itself in the form of NEXAFS spectroscopy. 7 NEXAFS is a synchrotron source soft X-ray spectroscopy technique that is sensitive to the orientation of phenyl groups via the intensity, I, of the partial electron yield (PEY) of Auger electrons of 1s?p* transition of the C¼ ¼C bonds. A depth of just 2-3 nm can be selectively characterized using the PEY mode of the NEXAFS measurement whereas depths of $100 nm can be probed using the fluorescence yield (FY) mode.…”

“…Preliminary work on DHaMS moldings adopted this method for computing near surface skin order parameters for relatively thin plaques. 7 Work presented here however, adopts a new analysis scheme proposed (and demonstrated) by Pattison et al 10 that defines S of the molecular axis, as related to the p* transition dipole moment of the C¼ ¼C bond segment in the direction of processing (surface rubbing direction in their case, melt flow direction in our case). This method results in determinations of S for molecular orientation parameters of aligned nematic polymers that are directly comparable with other techniques such as ATR-FTIR and WAXS.…”

Ultrasonic measurement of orientation in HDPE/iPP blends obtained by dynamic packing injection molding

“…Therefore, it is always an important issue on how to characterize the orientation of polymer chain quickly and conveniently. Up to now, a variety of methods have been developed to explore the orientation structure on-line or off-line, including 2D-WAXS [3][4][5][6][7][8][9], 2D-SAXS [3][4][5]10], optical birefringence [11][12][13], IR dichroism [14][15][16][17], Raman and Infrared spectroscopy [18] et al These conventional methods can well determine the orientation structure off-line, but they have rarely been applied to on-line detection during processing, especially in injection molding, partly due to certain technique difficulties. Although many methods can be utilized to determine the orientation of polymer chain, different results and phenomena may occur due to different principle based.…”