The chemical heterogeneity of poly(ethylene terephthalate) (PET) and of poly(ethylene terephthalate- co -isophthalate) (PETI) was investigated using temperature rising elution fractionation (TREF). NMR and SEC results of the TREF fractions of the two polymers are compared. Both polymers show the same trend upon fractionation, confirming that diethylene glycol units (DEG) can be considered as a statistical comonomer in PET which influences the crystallizability of PET. PET and PETI were finally analyzed by the stepwise isothermal segregation technique (SIST). The occurrence of multiple melting peaks for both polymers, when crystallized isothermally by step from the melt, is related to the statistical inclusion of the comonomers in the chains.
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Lodefier et al. (1999) studied this question.
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