Amorphous copolyesters based on bibenzoic acids and neopentyl glycol

High T_g amorphous copolyester thermoplastics were synthesized by incorporating 4,4′-bibenzoate (4,4′BB) and 3,4′-bibenzoate moieties into the polyester backbone via melt polycondensation. The high levels of crystallinity typically associated with 4,4′BB containing polyesters were suppressed through copolymerization of ethylene glycol, 1,4-cyclohexane dimethanol, and neopentyl glycol (NPG) diols. NPG was shown to be highly effective in suppressing crystallization and was used to produce amorphous compositions with T_g’s as high as 129 °C. Diol ratios were determined by ¹H NMR spectroscopy and molecular weights were assessed with inherent viscosity (η_inh). Thermogravimetric analysis showed single-step weight losses in the range of 395–419 °C. Differential scanning calorimetry was used to determine melting points and glass transition temperatures over a wide range of copolyester compositions and identified amorphous compositions. Dynamic mechanical analysis confirmed T_g’s and was used to study β-relaxations below the T_g. Rheological analysis revealed the effect of NPG structures on shear thinning and thermal stability.