Sequence distribution and crystal structure of poly(ethylene/trimethylene terephthalate) copolyesters

The sequence distribution and the crystal structure of copolyesters synthesized from ethylene glycol, 1,3-propanediol, and dimethyl terephthalate with different molar volume ratios were investigated in this study. The triad sequence probabilities of ethylene/trimethylene terephthalate were characterized from the aromatic quaternary carbons by ¹³C NMR. The composition of the copolyesters was determined from the aromatic quaternary carbons by ¹³C NMR, and the methylene protons by ¹H NMR. Results show that 1,3-propanediol reacted faster with terephthalic acid in copolyester polymerization than ethylene glycol. The difference in monomer reactivity is significant in the polymerization. Although the constitutional units revealed a random distribution in the molecular chain by ¹³C NMR, crystallites formed across the full range of ethylene glycol/1,3-propanediol composition by use of differential scanning calorimetry, a hot stage polarizing microscope, and a wide angle X-ray diffraction method. The WAXD deconvolution results show that the major constitutional repeating unit in the molecular chain dominates the crystal structure as a host crystal. The crystal structure was examined by a scanning electron microscope after a solvent etching. Photomicrographs show that the random distribution of the third constitutional unit in the molecular chain of copolyester significantly disturbs the host crystal formation and lamellar orientation.