Molecular dynamics research on piperazine- and renewable cross-linker-based polyurethanes. Dielectric study of relaxation behavior

By using broadband dielectric spectroscopy over a wide frequency and temperature range, the effect of the chain extender and cross-linkers on molecular dynamics of polyurethanes based on piperazine as a heterocyclic compound was assessed. Three relaxations attributed to local motions of polar side groups and small segments of the polyurethane chain (β, γ-mode) and glass transition (α-mode) were detected with the increase in temperature from −120 to 100 °C, along with the conductivity process at high temperatures and low frequencies. Experimental data were examined using the impedance and electric modulus for the conductivity study. Due to the presence of dipole polarization of functional groups in the hard segments, cross-linked polyurethanes are characterized by higher dielectric constant value (≈11 at 1 kHz and 25 °C) compared to the linear ones (≈6–7 at 1 kHz and 25 °C). High conductivity was achieved for the Tween 20-cross-linked polyurethanes. The activation energy for the secondary γ relaxation was in the range of 34–37 kJ mol⁻¹ for all polyurethanes, regardless of the chain extender or cross-linker used. Piperazine-based polyurethanes had lower activation energies for α relaxation and high conductivity compared to cyclohexane-based polyurethanes. This indicates that α relaxation is more vulnerable to structural alterations.