Glass transition temperature and shear viscosity of pre-copolyethers correlated with their end-groups

Research concerning polymer shear viscosity and the glass transition temperature has traditionally concentrated on parameters such as segmental structure and molecular weight, with comparatively limited attention devoted to the influence of end-group configurations. In this investigation, hydroxyl-terminated ethylene oxide-tetrahydrofuran copolyether (PET-OH) served as the precursor material to synthesize three copolyethers, namely, propargyl ester-terminated (PET-O₂CC [tbnd] CH), propargyl ether-terminated (PET-OCH₂C [tbnd] CH), and azide-terminated (PET-N₃), which possess nearly identical main chain architectures, molecular weights, and molecular weight distributions, differing solely in their end-group configurations. Differential scanning calorimetry (DSC) analysis reveals that the glass transition temperatures (T_g) of these four copolymer ethers decrease progressively in the order: PET-O₂CC [tbnd] CH > PET-OH > PET-OCH₂C [tbnd] CH > PET-N₃. Correspondingly, the crystallization enthalpy of the tetrahydrofuran microblocks increases gradually at low temperatures. Viscosity measurements and low-field nuclear magnetic resonance investigations demonstrate a consistent correlation between segmental mobility and viscosity, with the zero-shear flow activation energy decreasing accordingly. Variable-temperature infrared spectroscopy indicates the presence of hydrogen bonding among end groups in PET-O₂CC [tbnd] CH, PET-OH, and PET-OCH₂C [tbnd] CH, thus restricting segmental motion; conversely, PET-N₃ displays lower viscosity, enhanced segmental mobility, reduced T_g, and increased crystallinity owing to the absence of intermolecular end-group interactions. Density functional theory (DFT) calculations further corroborate that the hydrogen bonding strength at the terminal groups of these three modified copolyethers diminishes in the sequence: PET-CO₂C [tbnd] CH > PET-OH > PET-OCH₂C [tbnd] CH. Moreover, the extent of association exhibits a positive correlation with both segmental mobility and T_g. These findings offer new theoretical insights and practical approaches for modulating thermodynamic properties, rheological behavior, and aggregation structures in polymeric materials.