The effects of ionization on the structure of poly(p-phenylene terephthalamide) fibres You-Lo Hsieh Division of Textiles and Clothing, University of California, Davis, CA 95616-8722, USA (Received 14 September 1992; revised 26 March 1993) Extended periods of ionization of poly(p-phenylene terephthalamide) (PPTA), namely Kevlar 149, fibres by sodium methylsulfinyl carbanions leads to a significant deterioration in the fibre tensile properties. The non-chain-breaking nature of the ionization suggests that hydrogen bonds among the PPTA chains in the intercrystalline regions also contribute to the axial strength of the fibres. Rinsing with dimethylsulfoxide (DMSO) following the ionization initially improves the tensile strength. With DMSO immersions of 10 h or longer, the tensile strength loss resumes, and can be explained by hydrolytic chain scission of the PPTA molecules in the intercrystalline regions. Ionization which does not affect the fibre strength of Kevlar 49 is effective for structural differentiation between Kevlar 49 and 149 fibres. In a similar fashion to ionization, etching by argon glow discharge produces little change in Kevlar 49 fibre structure and strength. The fact that both ionization and glow discharge treatment affect PPTA fibre properties similarly demonstrates that both reactions depend on the accessibility and precise microstructure of these fibres. (Keywords: poly(p-phenylene terephthalamide); ionization; tensile properties) INTRODUCTION The ionization of poly(p-phenylene terephthalamide) (PPTA) can be achieved by using sodium methylsulfinyl carbanions, produced by mixing sodium hydride (Nail) in dimethylsulfoxide (DMSO). The formation of sodium methylsulfinyl carbanions (equation (1)) and the polyanions Na ÷ PPTA- (equation (2)), are illustrated as follows: CH CH 3 O=S + Nail • O=S + H 2 (1) L CH 3 Na + ~ H 3 H H O 0 + CH] Na t p ~ H 3 Na + O O • I ~ _ H / - ~ , 20=S + -~- N----( ( ) )---N--C~ ( ))~C CH3 (2) Short ionization reaction times followed by reactions with epoxy resin, have been shown to be very effective in improving the adhesive properties of Kevlar 149 fibresL Although the effects of ionization on the fibre strength can be minimized, a significant deterioration of the fibre tensile properties can result if ionized PPTA fibres are quenched in water instead of reacting with epoxy. The loss of strength of water-quenched fibres after short ionization treatment can be lessened by rinsing the ionized fibres with DMSO prior to quenching. It is believed that a DMSO rinse removes any residual sodium 0032-3861/94/06/1216-06 © 1994 Butterworth-Heinemann Ltd 1216 POLYMER Volume 35 Number6 1994 methylsulfinyl carbanions from the fibre surfaces, thus reducing their potential reactions with water. However, the fibre strength deterioration increases with increasing ionization time. The fact that brief DMSO rinses cannot recover the tensile strength loss of fibres that have been ionized for extended lengths of time suggests that both diffusion of sodium methylsulfinyl carbanions and ionization occur inside the fibres. Chemical diffusion and reactions of polymers are structure-dependent, and thus further understanding of the effects of ionization on PPTA fibres should provide additional information on their microstructure. This paper reports the effects of extended DMSO immersions on the structure and properties of ionized PPTA fibres, including differentiating the structural differences between Kevlar 49 and 149 fibres. Accessibility of these fibres to ionization are also compared with the effects of a base on its own and argon glow discharge. EXPERIMENTAL The two types of poly(p-phenylene terephthalamide) fibres used in this study were Kevlar 49 and 149 filaments, obtained from E. I. du Pont de Nemours Inc. Approximately 0.3 g of continuous filaments were wound into a loop with a circumference of about 20 cm. The wound filaments were cleaned in carbon tetrachloride for 10 min using bubbling nitrogen for agitation, and then oven dried at 100°C for 2 h. The filaments were cooled, weighed and stored in a desiccator under vacuum before use. The chemicals were reagent grade, and were used without further purification. Each ionization bath was prepared by stirring 0.24g of Nail into 30ml of DMSO at a 8 mg ml-1 NaH/DMSO concentration, or