Dynamic heterogeneity and cooperativity in polyurethane-based vitrimers

Abstract

The complex, non-Maxwellian behaviour observed in the mechanical relaxation of polyurethane-based vitrimers, is analysed in the light of dynamical heterogeneity and cooperativity concepts recently formalized within a statistical mechanical framework. Cooperative effects are highlighted at temperatures significantly higher than calorimetric $T_g$ ($\approx -53$ °C) as an effect of transient dynamic links; the Maxwellian behaviour is recovered when the temperature is raised up to 120 °C. The analysis provides the number of monomers involved both in the collective precursory oscillatory modes, and in the subsequent structural rearrangements. The mean activation energies, which can be derived at each single temperature, are in the same order of those customarily estimated from Arrhenius plots assuming a Maxwellian behaviour. Making the dynamic network inactive at low temperatures results in larger values of both cooperativity and mean activation energy. Overall, a more complete description of the relaxation behaviour is achieved, particularly in the regime where complex segmental dynamics starts to interfere with the ideal Maxwellian behaviour expected at high temperatures.