Disentangling proton-transfer and segmental motion relaxations in poly-vinyl-alcohol aqueous solutions by means of ultrasonic relaxation spectroscopy

Abstract

A detailed temperature- and concentration-dependent ultrasonic relaxation study has been conducted in aqueous polyvinyl alcohol (PVA) solutions as a function of frequency under isobaric conditions. Concentration and temperature have similar effect on acoustic spectra. Two well-resolved relaxation processes were observed in the experimental acoustic spectra that seem to follow a Cole-Cole distribution function. The systematic analysis of the variation of the acoustic parameters with frequency, temperature and concentration revealed that both processes have collective and not local nature. The low-frequency relaxation mechanism is assigned to segmental motions of the polymer chains and the high-frequency relaxation to proton-transfer reaction, respectively. The characteristic relaxation frequencies were found independent of molecular weight, as expected for relative flexible vinyl-polymer chains with molecular weight above ~104. Based on Eyring's theory, the activation enthalpy of the low- and high-frequency relaxation were estimated equal to ΔΗ1* = 4.5 ± 0.2 kcal/mol and ΔΗ2* = 19.7 ± 0.8 kcal/mol, respectively. The contribution of the low-frequency lying normal mode relaxation has been quantitatively estimated in the context of relevant theories of the field and found almost four order of magnitude lower in amplitude compared to segmental motion process indicating that the experimentally detected relaxation is dominated by the latter. The experimental results were complemented with molecular mechanics calculations performed in parallel. The comparison between the experimental data and the outcome of the theoretical calculations evidenced that the length of the relaxing element in a PVA polymer chain is between 5 and 8 monomers in agreement with what is expected for vinyl-polymers and established the local segmental motions of the PVA polymer chains in the solution.