Effect of thermal annealing on the structure of Technora and Kevlar polyamide fibers

Abstract

This paper describes X-ray diffraction analysis for the structural changes of Technora and Kevlar polyaramid fibers as a result of thermal annealing. Technora fibers are based on the aromatic copolyamide prepared by reaction of terephthaloyl chloride (TPA) with a 50/50 mole ratio of p-phenylene diamine (PPD) and 3,4′-diaminodiphenyl ether (DPE); Kevlar is the alternating copolymer prepared from TPA and DPE. X-ray fiber diagrams of Technora contain a series of non-periodic layer lines that are characteristic of a completely random comonomer sequence. The structure consists of ordered arrays of chain segments that have extended but sinuous conformations. By matching the profiles of the meridional peaks with those predicted for molecular models, the segment length in unannealed Technora is determined to be 300 ± 15 Å with a sinuosity g of 1.52 ± 0.05 %, where g is analogous to the paracrystallinity parameter for homopolymer structures. The segment length is similar to that for unannealed Kevlar 29 fiber (350 ± 10 Å) and the sinuosity is lower than that for thermally annealed Kevlar 149 (g = 1.69 ± 0.08 %), for which the crystallite size has increased to 440 ± 10 Å. On annealing Technora fibers at 300 °C for 48 hours, both the chain segment length and chain sinuosity remain essentially the same. In contrast, annealing Kevlar 29 under the same directions results in an increase in the crystallite size along the fiber direction to 410 ± 10 Å with g declining to 2.05 ± 0.06 %. Both polymers show evidence for improved perfection in the lateral packing as a result of annealing. The g values determined for Technora indicate that despite the random copolymer sequence, the chain conformation is straighter in the Kevlar homopolymer. This linearity is probably maximized during spinning and is not improved by annealing, whereas the ordering of the homopolymer increases due to crystal growth along all three axes.