Determination of the crystalline fold period in poly(ethylene terephthalate)

The morphological characteristics of poly(ethylene terephthalate) (PET) have been inferred from the selective chemical degradation of amorphous regions, including chain folds, and subsequent analysis of the molecular weight distribution of the reaction products. Bulk PET of known thermal and orientation history was reacted with 40% aqueous methylamine at 35°C and the reaction products examined by gel permeation chromatography. It was determined that the average crystalline fold period increased from six to nine monomer units over the crystallization temperature range of 126°C to 229°C. Evidence for the existence of a paracrystalline form at the lower crystallization temperatures was suggested by the presence of residual high molecular weight polymer in the reaction products. Samples crystallized at the lower temperatures exhibited multiple melting during DSC analysis. Results obtained by heating such samples in the DSC to a temperature between the two endotherms and quenching, followed by aminolysis and GPC, lead one to conclude that the first endotherm represents the initiation of melting and recrystallization. The second endotherm at ∼255°C, characteristic of PET, represents the melting of folded chain lamellae containing nine repeat units between folds. This material is formed during the DSC run as a result of melting and thermally compensatory recrystallization.