Study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

摘要

Poly(ethylene terephthalate) (PET) is a semi--crystalline polymer that can becrystallized to different degrees heating from the amorphous state. Even whenprimary crystallization has been completed, secondary crystallization can takeplace with further annealing and modify the characteristics of the amorphousinterlamellar phase. In this work we study the glass transition of highlycrystallized PET and in which way it is modified by secondary crystallization.Amorphous PET samples were annealed for 4 hours at temperatures between 140Cand 180C. The secondary crystallization process was monitored by differentialscanning calorimetry and the glass transition of the remaining interllamelaramorphous phase was studied by Thermally Stimulated Depolarization Currentsmeasurements. Non--isothermal window polarization is employed to resolve therelaxation in modes with a well--defined relaxation time that are subsequentlyadjusted to several standard models. Analysis of experimental results, showthat cooperativity is reduced to a great extend in the interlamellar amorphousregions. The evolution of the modes on crystallization temperature reveals thatlarge scale movements are progressively replaced by more localized ones, withhigher frequency, as crystallization takes place at higher temperatures. As aconsequence, the glass transition temperature of the amorphous interlamellarphase tends to lower values for higher annealing temperatures. Evolution ofcalorimetric scans of the glass transition are simulated from the obtainedresults and show the same behaviour. The interpretation of these results interms of current views about secondary crystallization is discussed.