Sequence of Enthalpy Relaxation, Homogeneous Crystal Nucleation and Crystal Growth in the Glass of Polyamide 6

摘要

Recent analysis of the crystallization rate in a wide temperature range between the glass transition temperature (Tg) and the melting temperature, using fast scanning chip calorimetry (FSC), revealed a bimodal distribution for the specific cases of poly(?-caprolactone) (PCL),1 isotactic polypropylene (iPP),2–4 poly(butylene terephthalate) (PBT),5 or polyamide 11 (PA 11).6 This observation has been explained by a change of the nucleation mechanism from heterogeneous nucleation at rather low supercooling of the melt to homogeneous nucleation at rather high supercooling of the melt. The interpretation of the low-temperature maximum of the crystallization rate to be related to homogeneous nucleation has been supported by morphological analyses providing information about the nucleation density,7–9 or crystallization experiments on heterogeneity-free droplets.10 Furthermore, analyses of the cold-crystallization behavior of samples subjected to a specific low-temperature annealing history allowed determination of the temperature dependence of the rate of homogenous nucleation.1,11,12 It has been observed that the rate of homogeneous crystal nucleation is maximum at about Tg, however, occurs also in the glassy state. The experimental observations obtained on PCL,1 isotactic poly(butene-1) (iPB-1),12 or poly(L-lactic acid) (PLLA)13 indicate a temperature-dependent dwell time/threshold before onset of formation of homogeneous nuclei in the glassy state, such that the rate of formation of homogeneous nuclei decreases with decreasing temperature. It has been suggested for the specific cases of PCL and iPB-1 that formation of homogeneous crystal nuclei is related to a prior densification of the glass.1,12 With the present work, we attempt to continue the research in the field of homogeneous crystal nucleation of crystallizable polymers in the glassy state, such to provide quantitative information about the sequence of the densification of glass, homogeneous crystal nucleation and crystal growth for the specific case of polyamide 6 (PA 6).