Thermal and mechanical properties of short‐segment block copolyesters and copolyether–esters

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Abstract
The effects of chain structure and processing variables on the microstructure and linear viscoelastic behavior of a series of copolyether–ester block polymers are described. In addition, the random copolyester analogs of the hard block are examined. The ester segments are composed of two isomers, poly(tetramethylene terephthalate) (PTMT) and poly(tetramethylene isophthalate) (PTMI), which possess significantly different crystallization kinetics. The ratio of PTMT to PTMI in the series has been systematically varied to alter the crystallizability without changing the chemical composition. The results of differential scanning calorimetry, wide‐angle x‐ray diffraction, and dynamic mechanical characterization are presented. Copolymerization of a second ester shortens the average sequence length of the first ester, resulting in melting‐point depression for crystals of the first polyester and substantial lowering of the dynamic mechanical storage modulus above the glass transition of the intercrystalline phase. The melting‐point depression may be predicted by using Flory's model for random copolymers, but the calculated heats of fusion are significantly lower than those obtained from diluent melting‐point depression.