Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Significance Diverse viewpoints have been developed to understand the scientifically fascinating and universal dynamics of glass-forming fluids. Currently, there are several prevailing models in the scientific literature based on seemingly different physical conceptions of glass formation, a fact that limits both theoretical and technological development in many scientific fields. We address this fundamental problem by simulating polymer glass-forming materials having a wide variation in the temperature dependence of structural relaxation (“fragility”), and we show by direct comparison that existing models equally describe our data, revealing deep relations between them. In this way, we achieve a greater theoretical unity of understanding glass-forming materials that should aid many applications in materials development and biology, the preservation and aesthetic properties of food, and medical science. The study of glass formation is largely framed by semiempirical models that emphasize the importance of progressively growing cooperative motion accompanying the drop in fluid configurational entropy, emergent elasticity, or the vanishing of accessible free volume available for molecular motion in cooled liquids. We investigate the extent to which these descriptions are related through computations on a model coarse-grained polymer melt, with and without nanoparticle additives, and for supported polymer films with smooth or rough surfaces, allowing for substantial variation of the glass transition temperature and the fragility of glass formation. We find quantitative relations between emergent elasticity, the average local volume accessible for particle motion, and the growth of collective motion in cooled liquids. Surprisingly, we find that each of these models of glass formation can equally well describe the relaxation data for all of the systems that we simulate. In this way, we uncover some unity in our understanding of glass-forming materials from perspectives formerly considered as distinct.

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