Hydroxyethylresorcinol- and hydroxyethylhydroquinone-containing poly(ethylene terephthalate) copolymers

Melt polycondensation incorporating meta-substituted hydroxyethylresorcinol (HER) or para-substituted hydroxyethylhydroquinone (HEH) regioisomers enabled the synthesis of poly(ethylene terephthalate) (PET)-based (co)polyesters with targeted comonomer mol %. Incorporation of either HER or HEH lowered the glass transition temperature (T_g) relative to PET, but the 5% weight loss temperature (T_d,5%) remained unchanged. Similarly, the semi-crystalline morphology varied as a function of comonomer selection and mol % comonomer. Dynamic mechanical analysis confirmed decreasing β-relaxation intensities upon HER and HEH incorporation, while HER impacted β-relaxation intensity more substantially at similar mol %. HER incorporation also shifted the β-relaxation temperature (T_β) to lower temperatures considerably more than HEH. Time-temperature-superposition (TTS) and Williams-Landel-Ferry (WLF) analysis of melt rheology probed various physical polymer properties including melt flow activation energy (E_a), fractional free volume at T_g (f_g), and molecular weight of entanglement (M_e). HER-based (co)polyesters exhibited lower E_a compared to HEH. Likewise, HER incorporation lowered f_g while HEH incorporation increased f_g. HER-based (co)polyesters possessed a higher M_e compared to HEH-based (co)polyesters, although both copolymers resulted in higher M_e compared to PET. Tensile testing revealed HER/HEH-based (co)polyesters ultimate mechanical performance compared to amorphous PET. Both HER and HEH incorporation increased Young's moduli and yield strengths, although comonomer incorporation above 25–30 mol % yielded minimal change. Establishing structure-property relationships provided insight into the role of regioisomeric variation at various comonomer levels on thermal, rheological, and mechanical performance for PET-based copolymers.