| Abstract: | The two endotherms found during DSC analysis of annealed or drawn poly(ethylene terephthalate), PET, are discussed in greater. detail. Earlier workers proposed that the endotherms were the result of separate morphologies, i.e., extended-chain and folded-chain crystals, but more recently Roberts and others have presented data on the effect of DSC heating rate on annealed PET endotherm areas which indicate that the higher temperature endotherm is the result of recrystallization in the DSC. The present work explains the reasons for recrystallization, and presents data showing that samples cooled at various rates from the melt also exhibit recrystallization in the DSC, in much the same manner as samples annealed for different lengths of time. Further, by prolonged annealing before analysis, part of the recrystallization exotherm can be observed in the DSC scan. Drawn nylon 66 also exhibits recrystallization in the DSC, in a manner similar to annealed or slowly crystallized PET. The amount of material that recrystallizes is determined by the time and supercooling available between first melting and the ultimate recrystallization temperature, i.e., a temperature at which there is too little time and temperature driving force for further recrystallization to occur. Infrared absorption data show an increase in “regular” fold content during prolonged annealing of PET, while dynamic mechanical data show a marked decrease in a dispersion that is likely associated with the looser fold crystal morphologies. Annealed PET does superheat in the DSC, leaving unanswered the question as to whether any partially extended material is present along with the regular-fold material. For cold-drawn PET, the infrared data indicate disappearance of regular folds and the dynamic mechanical data indicate disappearance of the looser folds. Cold-drawn PET also superheats. These data indicate a likelihood of at least partially extended morphologies in cold-drawn PET; these observations do not apply to PET drawn at high temperatures or to polyethylene. |
