Swelling studies of crosslinked poly(p-phenylene terephthalamide) copolymers in sulfuric acid

In an attempt to improve the mechanical properties of extended chain polymers such as poly(p‐phenylene terephthalamide) (PPTA), a crosslinkable terephthalic acid derivative (XTA) has been developed which can be incorporated into copolymers in various concentrations and activated after polymerization. The crosslinking of PPTA‐co‐XTA copolymer particles was investigated through a series of swelling experiments in concentrated H2SO4. The data show a systematic decrease in equilibrium swelling with increasing XTA content, indicating the XTA units are in fact acting as crosslink sites. Values for crosslink density were calculated from the Flory‐Rehner theory of polymer swelling and compared with previous findings on crosslinked rigid polymer network systems. The effective number of crosslinks per XTA unit (efficiency) predicted by the Flory‐Rehner theory increases and then decreases with % XTA. The decrease in crosslinking efficiency at high XTA concentrations is consistent with differential scanning calorimetry data which show the enthalpy of XTA reaction decreasing slightly with % XTA. The deviations at low % XTA may represent a failure of the Flory‐Rehner theory to properly describe the rubbery elasticity of extended chain polymers. © 1994 John Wiley & Sons, Inc.