Influence of plasticizers suggests role of topology in polymer solidification at high cooling rates

Although solidification in processing deter- mines short- and long-term properties, methods for under- standing polymer crystallization mostly rely on real time experiments. Their evidences being drawn on time scales farther apart with respect to those experienced in process- ing. Nor significant outcomes have been so far drawn with approaches mimicking the typical processing times, the Continuous Cooling Transformation methods. Use of these techniques has indeed been limited to a heuristic interpretation of the structure developed under extreme solidification conditions without suggesting alternative routes to the understanding or even clues to the many open questions on polymer crystallization under realistic solidification conditions. If the mechanisms inferred from realtime methods cannot in principle be translated to proc- essing conditions the oxymoron is not overcome. In this work, we compare the influence of a plasticizer on the density drop with cooling rate, and therefore on the disap- pearance of any long range order crystalline phases, observed after fast controlled quenching of polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). The plasticizer enhances significantly the maximum cool- ing rate at which PET still shows long range order crystal- line phases, whereas barely modifies the situation for PBT. On the basis of a series of concurrent clues, we suggest that the complex inherent topology of macromolecular melts introduces a timescale on polymer crystallization lying in the range of the solidification conditions experi- enced in processing. Although melt topology is known to determine in many instances polymer solidification, all approaches so far proposed to describe the development of polymer crystallization overlook or even neglect any explicit dependence of such mechanisms on parameters like the molecular weight between entanglements. Albeit the arguments are feeble they all concur to raise several questions even on the contradictory role of nucleating agents under the extreme conditions experienced in proc- essing. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: 4233-4242, 2012