Determination of the degree of radial order in Kevlar fibers using x-ray linear dichroism microscopy

Abstract

The Scanning Transmission X-ray Microscope (STXM) located at beamline X1A at the National Synchrotron Light Source at Brookhaven National Laboratory can be used to acquires high resolution, chemically and orientationally sensitive images with 50 nm spatial resolution, as well as point spectra from 0.1 μm2 areas. Chemical (valence) sensitivity can be achieved via the X-ray Absorption Near Edge Structure (XANES) at the carbon K edge. In addition, linear dichroism microscopy exploits the dependence of x-ray absorption resonances on the bond orientation relative to the linearly polarized x rays. Previous x-ray linear dichroism microscopy work has demonstrated the ability to determine the orientation of specific chemical groups within polymeric systems at high spatial resolution. Here, we have extended this technique to quantitatively determine the degree of radial order in various grades of poly(p-phenylene terephthalamide) (Kevlar™) fibers.The orientational dependence of the absorption cross-section yields “butterfly” patterns when thin sections of certain grades of Kevlar fibers cut at 45° with respect to the fiber axis are imaged at specific photon energies.