Poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate) random copolymers: effect of copolymer composition and microstructure on the thermal properties and crystallization behavior

A series of poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate) (PETG) random copolymers were synthesized and characterized using ¹H and ¹³C nuclear magnetic resonance, infrared spectroscopy and viscometry. Differential scanning calorimetry, wide-angle X-ray diffraction, and thermogravimetric analysis were used to probe the effects of copolymer composition and microstructure on the thermal properties, crystallization behavior, and thermal stability of the PETG copolymers, respectively. The mechanical properties were evaluated by tensile testing and dynamic mechanical measurements. The partial replacement in the polymerization feed of ethylene glycol by 1,4-cyclohexanedimethanol led to statistically random PETG copolymers with adjustable compositions and suitable molecular weights, which were thermally stable above 380 °C. The number-average sequence length of ethylene glycol terephthalate decreased with the increasing 1,4-cyclohexanedimethanol terephthalate (CT) content. In contrast, the number-average sequence length of CT increased gradually with the increasing CT content. The crystalline structure of the PETG copolymers changed from a PET-type lattice to a PCT-type lattice at a lower CT content. The crystallinity decreased at first, and then increased remarkably with the increasing CT content. It was interesting to notice that the rigid structure of the CT unit controlled the crystallization. The incorporation of CT in the PET chain, significantly altered the thermal transitions of the polyester. The glass transition temperature increased linearly with the increasing CT content. The melting temperature of the segments in crystalline domains strongly depended on the corresponding average sequence length. An increase in the average sequence length resulted in a higher melting temperature and an increase in the melting enthalpy.