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

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. Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate-co-1,4-cyclohexylene dimethylene terephthalate) copolyesters with different molar ratios of ethylene glycol to 1,4-cyclohexanedimethano (1,4-CHDM) were investigated using a variety of techniques. They were found crystaline in a specific range of that ratio, and amorphous outside of this range. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process with a corresponding change in crystal structure. Interestingly, the small incorporation of 1,4-CHDM into PET could lead to the formation of larger spherulite crystals than that of PET.