Influence of molecular weight on the ordered state in poly(ethylene terephthalate)

WAXS, DSC, and IR methods have been employed to follow the crystallization of poly(ethylene terephthalate) as a function of molecular weight. The degree of order, which is a measure of the extent of crystallization, decreased with increasing molecular weight. The degree of reduction in the molecular order with increasing molecular weight differed depending on the technique employed to measure it. Crystallinity indices obtained by x‐ray diffraction methods show an almost linear relationship with molecular weight. The Trans‐Gauche ratio inferred from the IR spectrum tends to decrease at a very rapid rate with increasing molecular weight. The heat of fusion, computed from the melting endotherm in DSC thermograms, shows a smaller but a definite decreasing trend with increasing molecular weight. Besides, the thermograms themselves showed distinct changes related to the molecular weight differences. The results have been discussed in terms of two different possible influencing factors known to affect the crystallization process in almost all polymers. Up to certain molecular weights, random coiling of molecules appears to be the more dominant factor, but in a much higher range of molecular weights, entanglement in the molecular network may become predominant.