The unique dynamics of water and polymer in polyethylene imine–water mixtures across the glass transition

Abstract

This study investigates the molecular dynamics of polyethyleneimine (PEI)-water mixtures (100–50 wt% PEI) at subzero temperatures without ice formation, leveraging broadband dielectric spectroscopy. PEI's electrolytic nature and hydrogen bonding capabilities influence relaxation times (τ), strength (Δε), glass transition temperature (T_g), and fragility (m). Pure PEI exhibits slower α-relaxation from cooperative polymer segment motion and faster Johari-Goldstein (JG)-β-relaxation. In addition, ν process due to water molecules was observed from 95 wt% of PEI downwards. PEI-water mixtures exhibit unique relaxation dynamics compared to other polymer-water systems, such as polyvinyl pyrrolidone (PVP) and poly (vinyl methyl ether) (PVME), showing minimal plasticization by water down to a critical PEI concentration of 70 wt%. Strong hydrogen bonding between PEI and water increases T_g and ν-process relaxation times, preventing ice crystallization. These interactions yield concentration-independent relaxation dynamics (τ_α, τ_ν, T_gν and T_gα) in 95-70 wt % PEI-water mixtures compared to those in PVP and PVME-water mixtures. X-ray diffraction (XRD) patterns confirm these findings: amorphous PEI shows a broad peak at 2θ = 20°, which weakens and splits at 70 wt% PEI into peaks at 2θ = 15° and 24°, indicating amorphous ice. Molecular dynamics (MD) simulations reveal a cohesive energy density of 1.50 × 10⁹ J/m³, dominated by electrostatic interactions (1.40 × 10⁹ J/m³) with minor van der Waals contributions (9.9 × 10⁷ J/m³). FTIR spectroscopy highlights a broad, intense peak (3750–2500 cm⁻¹), confirming hydrogen bonding's role in PEI's structure and dynamics. These findings underscore PEI's unique capacity to prevent ice formation and maintain water at low temperatures despite its low T_g.