Low Dose High Resolution Electron Microscopy (HREM) Analysis of Regularly Twisted Poly(m-Phenylene Diisophthalamide) (MPDI) Fibers

Abstract

Poly(m-phenylene diisophthalamide) (MPDI) is an all-aromatic polyamide used to manufacture thermally stable high strength fibers (Nomex® by DuPont). The X-ray diffraction data of the commercial solution-spun fiber is in good agreement with a triclinic unit cell structure proposed by Kakida et al. with hydrogen bonds forming a two dimensional network approximately orthogonal to the polymer main chain. When MPDI dissolved in N,N-dimethylacetamide (DMAc) is crystallized over several weeks by exposure to a non-solvent (water) in a diffusion vessel, the polymer precipitates into uniform, regularly twisted crystalline bundles (Figure 1).TEM images show that the bundles consist of 1-8 individual strands and can reach a length of 20 microns. The pitch and the diameter of these bundles are uniform along the visible length for each bundle but depend on the number of strands per bundle. The diameter of an individual strand is 20-50 nm resulting in a diameter of 20-300 nm for a bundle and a “macroscopic” pitch of 200-1000 nm (Figure 2).