The relaxation dynamics and role of dynamic facilitation in polymer glasses

The glass transition is a long-standing unsolved problem in materials physics. For polymers, our understanding remains relatively poor because of the added complexity of chain connectivity and chain flexibility. Here, we use data from broadband dielectric spectroscopy, calorimetry and rheology, combined with simple computer modelling, to determine how the relaxation dynamics varies with key polymer characteristics, such as molecular chain-length and flexibility. Based on our results, we propose that in polymers, a set of generic molecular relaxations are linked through the mechanism of dynamic facilitation [1], by which a `local’ relaxation facilitates adjacent relaxations, resulting in hierarchical dynamics; the important role of local conformational degrees of freedom paves the way for predictive polymer design based on monomer-scale metrics [1-2]. Importantly, we identify chain-length-dependent regimes where intra- and inter-molecular dynamics play different roles in defining the dynamics. Moreover, we demonstrate how dynamic heterogeneities (DH), a hallmark of glass-formation and of dynamic facilitation, depend on both chain-length and flexibility [3]: highly flexible polymers show a molecular weight (M) independent behaviour, whereas less flexible polymers show a complex regime behaviour, consistent with observations in both T_g(M) and conformational structure. Based on our DH results, we discuss and evaluate the detailed temperature-dependent evolution of relaxation dynamics in polymers. Finally, we illustrate how the fundamental relaxation behaviour of polymers link to important transport properties, with focus on ion transport relevant to the development of efficient polymer-based solid electrolytes in battery applications.

[1] Baker, D.L., Reynolds, M., Masurel, R., Olmsted, P.D., Mattsson, J., PRX 12, 021047 (2022).

[2] Brierley-Croft, S., Olmsted, P.D., Hine, P.J., Mandle, R.J., Chaplin, A., Grasmeder, J., Mattsson, J. arXiv: 2411.06461.

[3] Reynolds, M., Baker, D.L., Olmsted, P.D., Mattsson, J., arXiv: 2405.02733.