The formation and behaviour of glassy disordered non-equilibrium solids is one of the deepest unsolved questions in physics, as demonstrated by the seminal contributions to the problem by the 2021 Nobel laureate in physics, Giorgio Parisi. The fact that neither the microscopic mechanisms involved in glass formation, nor the behaviour of the glassy state, are well understood makes this a key problem both within the fundamental and applied sciences.
For long-chain molecules – polymers – our present understanding of glass-formation is particularly poor due to the complexity provided both by chain connectivity and chain flexibility; this means that the molecular motions linked to glass-formation involves an intricate interplay between intra- and inter-molecular molecular cooperativity [1]. In turn, this leads to fascinating observations including polymer-specific memory behaviour observed in the slow evolution (aging) of the out-of-equilibrium glassy state, and polymer-specific transport of ions important for the construction of safe and flexible polymer-based battery materials. Generally, polymer glasses are common for instance in construction materials, in medical implants, in optical components and in membranes for controlled transport of ions or gases. Thus, understanding polymer glass-formation directly impacts our ability to design better or totally new polymer-based applications.
Polymers in restricted geometries, such as in thin polymer films, often show dramatic changes in behaviour, which are not well understood. This include a remarkable reduction in the glass transition temperature of ~70 K for thin free standing polystyrene films [2]. The high surface-to-volume ratio of thin films means that interfacial interactions play a strong role and the change in molecular motions at the interfaces are transferred to the film interior; we do not presently understand how this transfer takes place. Thin film polymers are highly important for coatings, in microelectronics, and in a wide array of nanotechnology applications. For the development of better, and more sustainable, technologies of the future, it is thus essential to understand how geometric confinement changes the behaviour of polymers.
To address these questions, detailed experimental studies of model polymers both in thin film geometries and in the bulk are needed. Advanced experimental techniques including broadband dielectric spectroscopy, ellipsometry, calorimetry, (light, neutron and x-ray) scattering, rheology, and atomic force microscopy will be used to investigate both thin polymer films and the corresponding bulk polymers. We have recently [1] proposed a new framework for understanding polymer glass-formation, whereby the `local’ molecular motions are coupled to longer-range structural relaxations through so-called `Dynamic Facilitation’. These ideas, and their implications for the behaviour of both bulk polymers and thin polymer films will be investigated. In addition, we are interested in determining how the generated fundamental knowledge can be utilised in relevant important appllications.
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[1] Baker, D., Reynolds, M., Masurel, R., Olmsted, P.D., Mattsson, J., Phys. Rev. X 12, 021047 (2022). [2] Mattsson, J., Forrest, J.A., Börjesson, L., Phys. Rev. E 62, 5187 (2000).
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