Annealing of poly (ethylene terephthalate)

Poly (ethylene terephthalate), PET, crystallized isothermally at 234 °C has been annealed above the crystallization temperature in regimes II, below the onset of melting, and in regime III, in the range of observable melting . The mechanism has been analyzed by differential scanning calorimetry and FTIR spectroscopy and shown to be one of the limited recrystallization nucleated by crystalline regions which survived melting followed by a process of lamellae thickening. Annealing above the crystallization temperature in regime II where melting and recrystallization did not occur involved lamellae thickening by growth of the lamellae along the chain direction. This was a diffusion controlled mechanism involving either diffusion of non-crystallizable impurities or chain entanglements. The process of stem thickening was the same as that of secondary crystallization previously observed in isothermal crystallization and which is responsible for the observed melting point of the polymer being dependent on the annealing time as well as the temperature. In regime III melting of the polymer occurred initially followed by recrystallization on the residual lamellae acting as seeds by one dimensional growth along the original growth direction. This was later accompanied by thickening of the lamellae with the same characteristic time dependence of diffusion control as observed in regime II and with secondary crystallization. This involves reptation of the molecular chain from the entangled melt onto the growth face.