Relationship between physical properties and chemical structures of poly(ethylene terephthalate-co-ethylene isophthalate)

A series of copolyesters based on different ratios of polyethylene terephth alate (PET) to polyethylene isophthalate (PEI) have been synthesized. With the involvement of PEI, the copolyesters become less crystallizable and eve n amorphous when PEI content is above 20%. The WAXD profiles of the crystal lizable copolyesters infer that the crystals come from PET homopolymer. DSC cooling runs indicated that the copolyesters with PEI no more than 15% are easily crystallizable, while the copolyester with 20% PEI is not easily cr ystallized at a cooling rate above 5 degrees C/min. Heating runs indicated that the copolyesters with PEI below 20% show melting processes. Nonuniform results were provided by WAXD and DSC, however, the effect of PEI on the a bility of crystallization was deduced similarly from WAXD and DSC. Glass tr ansition temperatures have been measured by DSC. Due to the flexibility of PEI chain, glass transition temperatures of the copolyesters decrease linea rly with increasing composition of PEI as predicted by the principle of add itive contribution. (C) 1999 John Wiley & Sons, Inc.