Crystallization and properties of poly(ethylene terephthalate) copolymers containing 5- tert-butyl isophthalic units

Abstract

The influence of incorporating 5- tert-butyl isophthalic units ( t BI) in the polymer chain of poly(ethylene terephthalate) (PET) on the crystallization behavior, crystal structure, and tensile and gas transport properties of this polyester was evaluated. Random poly(ethylene terephthalate- co-5- tert-butyl isophthalate) copolyesters (PET t BI) containing between 5 and 40 mol% of t BI units were examined. Isothermal crystallization studies were performed on amorphous glassy films at 120 °C and on molten samples at 200 °C by means of differential scanning calorimetry. Furthermore, the non-isothermal crystallization behavior of the copolyesters was investigated. It was observed that both crystallinity and crystallization rate of the PET t BI copolyesters tend to decrease largely with the comonomeric content, except for the copolymer containing 5 mol% of t BI units, which crystallized faster than PET. Fiber X-ray diffraction patterns of the semicrystalline PET t BI copolyesters proved that they adopt the same triclinic crystal structure as PET with the comonomeric units being excluded from the crystalline phase. Although PET t BI copolyesters became brittle for higher contents in t BI, the tensile modulus and strength of PET were barely affected by copolymerization. The incorporation of t BI units slightly increased the permeability of PET, but copolymers containing up to 20 mol% of the comonomeric units were still able to present barrier properties.