Correlative STM, FESEM, and TEM studies of fibrillar structures in liquid crystalline polymers

Abstract

Hierarchical fibrillar structures have been reported to be present in liquid crystalline polymer (LCP) extrudates such as fibers and tapes. Using conventional scanning and transmission electron microscopy (SEM, TEM), the smallest fibrils were shown to be about 50 nm wide and 5 nm thick. With the emergence of higher resolution imaging techniques such as scanning tunneling microscopy (STM) and field emission SEM (FESEM), we have been able to further explore the ultrastructures in the LCP fibrils and extend the LCP structural model.The LCP materials investigated consist of copolyesters of 4-hydroxybenzoic acid (HBA) and 2- hydroxy-6-napthoic acid (HNA) in the form of melt-spun fibers and extruded tapes. These materials exhibit unidirectionally-oriented structures as revealed by x-ray diffraction and microscopy studies. For STM and FESEM, samples were prepared by a peel-back method which reveals the internal structures of fibrils. In addition, ultrasonication was used to disintegrate the LCP fibers and tapes to provide fine “fibrillar” samples for STM, FESEM, and TEM studies. The original tape and fiber surfaces were also examined by STM and FESEM. Thin Pt coatings (ca. 5 nm) were deposited on the LCP samples using ion beam sputtering to provide a conducting surface for STM and FESEM imaging. These coatings have been shown to introduce minimal topography to the original sample surface.