Direct Evidence for Sequence Ordering of Random Semicrystalline Copolyesters during High-Temperature Annealing

In an effort to study sequence ordering during annealing in semicrystalline copolyesters, the carbonyl carbon of p -hydroxybenzoic acid was 13 C tagged and then solution polymerized with 2-hydroxy-6-naphthoic acid to form a low molecular weight, 50/50, random copolyester. The random copolyester was annealed at 40 deg below the crystal nematic transition temperature ( T cn ). DSC results indicated a large increase in crystallinity; however 13 C NMR analysis showed no change in the benzoic−naphthoic (BN) and benzoic−benzoic (BB) diad sequence peak ratios. When the random copolyester was annealed near the T cn, DSC results indicated a large increase in the transition temperature, no increase in crystallinity, and a shift in the BN:BB peak ratio indicating a significant increase in alternating sequences. On the basis of these results, the dramatic increase in transition temperature during annealing near T cn is best interpreted as sequence ordering via interchain transesterification reactions within the existing crystalline regions.