Polyacrylonitrile, an unusual linear homopolymer with two glass transitions

Abstract

The glass transition of polyacrylonitrile (PAN), unlike that of many amorphous and semi-crystalline polymers, is a source of considerable puzzlement. By DMA, it has been shown that there are two tan δ peaks at ~100°C and ~150°C in unoriented PAN, while in oriented fibres, there is only a single peak at ~100°C. Some authors take the peak at ~100°C while others assign the peak at ~150°C to the glass-rubber transition of PAN. The view taken here is that as the two DMA peaks are of similar intensity, they represent two glass transitions. Hitherto, by DSC, these two glass transitions have not been unequivocally recognised, mainly because the endothermic peaks arising from enthalpy relaxation mask the true nature of the transitions. In this work, the case for two DSC glass transitions in unoriented PAN is presented. In oriented PAN, however, a single DSC glass transition is observed at ~100°C. Thus, the results from the DSC, which is a less sensitive technique for measuring glass transitions, mirror the DMA findings. Whereas two <i>T</i>_gs<i> </i>can be expected in an

immiscible blend of two amorphous polymers, or in a block copolymer with two non-crystallisable units, it is more remarkable to find this in a linear homopolymer. The two <i>T</i>_gs<i> </i>in unoriented PAN arise from laterally ordered and

amorphous domains, both of which are in a glassy state at ambient. The single transition in oriented PAN is not the conventional glass-rubber transition found in other amorphous or semi-crystalline polymers. It represents a transition from a laterally-ordered glassy state to a more mobile but still laterally-ordered state.