Structure development during crystallization of polycaprolactone

In this paper the quiescent crystallization of Polycaprolactone (PCL) melts is studied by rheological measurements coupled to calorimetry and optical microscopy. Based on comparison between the different techniques, we find that the increase of viscoelastic properties during crystallization starts only when a relatively high degree of crystallinity is reached, which corresponds to a much developed crystalline microstructure. Like other semicrystalline thermoplastic polymers, the crystallization of PCL can be seen as a gelation process. In this case, however, we find a peculiar critical gel behaviour, as the liquid-to-solid transition takes place at a very high (20%) relative crystallinity and this value is independent of temperature. These facts, and the comparison with optical microscopy observations, suggest that the microstructure at the gel point is controlled by the interactions between the growing crystallites. The gel time (from rheometry) and the half-crystallization time (from DSC) both show an Arrhenius-like behaviour and have the same pseudo-activation energy. A practical implication of this parallel behaviour of tgel and t0.5 is that the rheological measurements can be used to extend to higher temperatures the study of crystallization kinetics where DSC is not sufficiently sensitive.