Isothermal crystallization behavior and crystal structure of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters

Abstract

Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters with different molar ratios of ethylene glycol (EG) to 1,4‐cyclohexanedimethanol (1,4‐CHDM) were investigated by their isothermal crystallization behavior, transparency, crystal structure and morphology changes in different isothermal crystallization process using differential scanning calorimetry (DSC), digital photos, wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM). The results revealed that P(ET/CT) copolyesters with ≤ 15 mol% 1,4‐CHDM and ≥ 50 mol% 1,4‐CHDM, such as P(ET/CT)(85/15), P(ET/CT)(50/50) and P(ET/CT)(30/70), were crystallizable, while that with 30 mol% 1,4‐CHDM, namely P(ET/CT)(70/30), was amorphous. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process, i.e. it first decreased and then increased remarkably with the increase of 1,4‐CHDM content. Accordingly, the crystal structure of these copolyesters changed from PET‐type lattice to PCT‐type lattice when the copolyester with around 30 mol% 1,4‐cyclohexylene dimethylene terephthalate (CT) unit. Interestingly, the small incorporation of 1,4‐CHDM into PET could lead to the formation of larger spherulite crystals than that of PET. But small and grainy crystallites appeared with further increase in the 1,4‐CHDM content.