Crystallization behaviour of poly( p-phenylene sulfide): Effects of molecular weight fractionation and endgroup counter-ion

Abstract

The crystallization behaviour of poly( p-phenylene sulfide) (PPS) has been studied. Two PPS samples with 〈 M W〉 = 44 K and 〈 M W〉 = 64 K were fractionated by a process which selectively removes a portion of the low molecular weight species yielding fractionated PPS samples with 〈 M W〉 = 56 K and 〈 M W〉 = 104 K respectively. The fractionated samples were then treated with an ion exchange process to allow control over the nature of endgroup counter-ion, i.e. to introduce Na + and Zn 2+ ions. Using a Hoffman-Weeks analysis, the equilibrium melting temperature of these polymers was estimated to be 320°C irrespective of endgroup counter-ion or polymer molecular weight. The nucleation density was observed to increase as a function of molecular weight by small-angle light scattering (SALS). The spherulitic growth rates and nucleation densities were studied as a function of the chemical nature of endgroup counter-ion for PPS with 〈 M W〉 = 56 K that had been fractionated to remove low molecular weight species. Additionally, isothermal rates of bulk crystallization were analysed as a function of molecular weight of PPS and chemical nature of the endgroup counter-ion. It was found that as a function of endgroup counter-ion, crystal growth rates and overall crystallization rates decreased in the following order: H + > Zn 2+ > Na +. The order of decreasing crystal growth rates corresponded to a similar increase in melt viscosity as a function of endgroup counter-ion, suggesting that the reason for decreasing growth rates could originate from increasing secondary interchain interactions. Optical microscopy studies showed the nucleation density decreased in the order H + > Na +. The ion-exchange reactions were shown to be reversible by differential scanning calorimetry (d.s.c.) and optical microscopy studies. Crystallinity determinations by wide angle X-ray diffraction (WAXD) and measurements of the heats of melting illustrated that higher molecular weight PPS attained lower levels of crystallinity than PPS of lower molecular weight when crystallized under identical conditions.