| The microstructure of high performance liquid crystal polymer (LCP) fibers has been intensively studied over the last few decades because of its influence on ultimate mechanical properties. LCP molecules are both straight and stiff leading to high tenacity (fiber tensile strength) and modulus only along the chain axis. The ability to narrow the distribution of molecular orientations within a whole fiber to the direction of the fiber axis is critical for exploiting these anisotropic properties. Essential to the orienting process is a means by which to assess orientation at relevant length scales. X-ray diffraction techniques, which currently provide micron-scale resolution at best, are insufficient for detecting sub-micron scale structural features since the data are averaged over the area from which it is collected.; In this work, an electron-optical technique has been developed to spatially resolve the molecular alignment of LCP on length scales as small as 100 nm. The technique is based on one used for X-ray diffraction. However, the effects of low signal-to-noise and convergence angle, which are a consequence of achieving high spatial resolution, must be considered. Experiments are carried out within the limitation of a critical radiative dose beyond which the integrity of the original structure is questionable. Orientational order parameters are calculated from the equatorial intensities in the diffraction patterns to quantify the distribution of molecular orientations. The effect of heat-treatment and chain straightness are studied in two types of fibers that are composed of hydroxybenzoic acid (HBA) hydroxynaphthoic acid (HNA) and HBA, HNA, terephalic acid, biphenodiol (COTBP). A bimodal phase is seen in the as-spun HBA/HNA fibers suggesting that a domain type structure is present. The effect of heat-treatment on the local orientation of LCP is observed in the heat-treated HBA/HNA fibers where a highly oriented continuous phase is seen. A structural gradient within the first 500 nm from the skin edge is found in the heat-treated fibers. This gradient has not yet been seen because of structural averaging associated with the poorer resolution of other scattering techniques. By combining spatially resolved diffraction with dark-field imaging, this research finds that crystallinity is less likely to be present near the edge of the fiber skin. Relatively high orientational order parameters are still found, however, indicating that the lack of observed crystallinity does not necessitate a large distribution of molecular orientations. The results of this study show that improvements in molecular orientation, through alleviation of the structural gradients and areas deplete of crystallinity, can still be made to fully exploit the unique properties of LCP fibers. |
