Relaxation and crystal nucleation in polymer glasses

Depending on the degree of undercooling, polymers crystallize from heterogeneous or homogeneous nuclei. The volume density of homogeneous nuclei at deep undercooling is often several orders of magnitude larger than that of heterogeneous nuclei; allowing crystallization even below the glass transition temperature. With the introduction of fast scanning chip calorimetry (FSC) detailed studies of nucleation and eventually crystallization at such low temperatures (deep undercooling) became possible. Homogeneous nucleation at temperatures below the glass transition shows an unexpected feature in regard to enthalpy relaxation occurring in the same temperature range. As a result of the often fast quench, the glass is initially in a high-enthalpy state causing enthalpy relaxation. Even if the mobility in the material is high, formation of overcritical nuclei is not immediately observed. Only after completion of the enthalpy relaxation, in other words, when the enthalpy of the supercooled liquid state is reached, homogeneous nucleation is observed. On the example of several polymers with different crystallization rates, we show that this behavior seems to be general.