Optical microscopy study on poly(p‐phenylene terephthalamide) fibers

Abstract

AbstractTwo highly oriented poly(p‐phenylene terephthalamide) (PPT) fibers of different grades, commercially known as Kevlar and Twaron, were examined at high magnification under a transmission optical polarizing microscope in the extinction position between crossed polars. An image with regions of regular fine bands is produced for both classes of fibers, which is interpreted to be due to the passage of depolarized scattered light from a material with regularly spaced discontinuities. These bands reflect the periodic pleat structure of the fibers. The fibers were chemically etched, coated with gold, and investigated under a Nomarski differential interference contrast microscope in the extinction position between crossed polars. The observation for Kevlar fibers shows a clear periodicity of transverse bands perpendicular to the fiber axis, but this is much less evident for Twaron fibers. The etchant gives smooth, nearly flat sections through the Kevlar fibers, whereas it is not very effective for the Twaron fibers. The variable wavelength interferometry (VAWI) technique, which is valuable for studying highly oriented polymeric fibers, is used to measure the mean refractive indices and birefringence of a fiber as a function of light wavelength. The results show that these fibers absorb the surrounding liquid, which may change their optical properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 360–369, 2003