Novel copolyester thermoplastics based on 4,4′-bibenzoate and 3,4′-bibenzoate moieties with ethylene glycol were synthesized via melt polycondensations. Crystallization behavior was modified by the additional incorporation of terephthalate or isophthalate units into the backbones. Copolyester compositions were verified by 1H NMR spectroscopy and molecular weights were assessed using inherent viscosity (ηinh). Thermogravimetric analysis (TGA) showed single-step weight losses in the range of 360–400 °C. Differential scanning calorimetry (DSC) was used to determine melting points and glass transition temperatures over a wide range of copolyester compositions. Observation of thermal data was used to identify amorphous windows in composition ranges containing 3,4′BB and 4,4′BB moieties. Dynamic mechanical analysis (DMA) provided information about thermal transitions and sub-T g relaxations. Mechanical data were obtained using tensile testing to expand structure-property- morphology relationships. Permeability analysis helped to understand how monomer symmetry affects oxygen diffusivity and solubility in selected amorphous film and biaxially oriented copolyester samples.
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