Glass transition and the origin of poly(p-phenylene sulfide) secondary crystallization

The effect of low temperature cold-crystallization on quenched poly(p-phenylene sulfide) (PPS) amorphous phase behaviour was systematically investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) over the entire range of the process, from its early stage to the end. For the first time a well resolved double glass transition of partially cold-crystallized samples was evidenced. A Tg steady increase was observed during the primary crystallization process, due to the reduction of amorphous chain segmental mobility imposed by the growing rigid phase. A shift of the relaxation temperature of about 10°C was recorded at the end of the primary crystallization process. As the secondary crystallization takes place a new glass transition appears at higher temperature. For longer annealing time the lower Tg disappears while the intensity of the upper one increases. The upper temperature glass transition of semi-crystalline PPS is explained as a consequence of the PPS secondary cold-crystallization process. In the light of the thermal and dynamic mechanical results, an interpretation is given of step-wise double crystallization, evidenced in non-isothermal cold-crystallization experiments carried out at low heating rate and followed by means of FT-IR techniques.