The effect of secondary crystallization on melting

[Display omitted] •Melting point increases with temperature and time due to thickening of lamellae.•Increase with temperature used to determine equilibrium melting point.•Growth is diffusion controlled depending on the square root of time.•Chain entanglements and impurities in the inter-lamellar regions slow to diffuse. The effect of isothermal crystallization temperature and time on the melting endotherms of poly (ethylene terephthalate), PET, has been measured using differential scanning calorimetry. Restricting the study to the primary stage of crystallization the melting endotherms were broad, frequently exhibiting multiple peaks. The temperature of the last trace of crystallinity increased linearly with crystallization temperature which could be interpolated to determine the equilibrium melting point of PET under equilibrium conditions. Heating beyond the primary stage of crystallization and into the secondary stage produced a narrowing of the melting endotherms with time with a loss of the low melting components and a progressive shift to higher temperatures. This was interpreted as an increase in thickness of the initial lamellae laid down in the primary stage by growth of the “fold surface” into adjacent amorphous material. By deriving the increase in lamellae stem length with time from the melting endotherms the growth was observed to be diffusion controlled, since it was dependent on the square root of time and a thermally activated process in that it increased with temperature following an Arrhenius dependence. The amorphous segments in the inter-lamellar regions were considered to have restricted mobility due to the presence of adjacent rigid crystalline regions and chain entanglements which were slow to diffuse from the growth front. An alternative suggestion which could also account for these observations was that growth was restricted by the diffusion of non-crystallizable low molecular weight impurities from the growth front.