The Impacts of Interfacial Energy and 'Softness' of Confinement on the Glass Formation Behavior of Nanostructured Polymers

摘要

Numerous studies have demonstrated that polymers and other glass-forming materials confined to dimensions under 100 nm can exhibit large deviations from bulk glass formation behavior. These studies furthermore indicate that alterations in the glass transition under nanoconfinement depend on both the interfacial energy and the 'softness' of confinement. However, the exact functional form of this combined dependence has remained unknown, and the proper definition of 'softness' of confinement is presently unclear: some studies have focused on the relative structural or viscous relaxation times of the confined and confining materials, whereas other studies have emphasized their relative moduli. A richer understanding of the impact of these variables on nanoconfined Tg and other glass-formation-related properties, such as fragility of glass formation, would be of considerable value in the design of nanostructured materials with targeted properties. Here we describe molecular simulations of multinanolayered polymers in which we systematically vary the interfacial energy and the relative Tg's of the domains. Results suggest a simple functional form that describes the combined dependence of nanoconfined Tg on interfacial energy and the relative Debye- Waller factors of the two domains. These results indicate that the 'softness' of confinement should he defined in terms of the relative high frequency shear moduli, rather than low frequency moduli or structural relaxation times, of the confined and confining materials. Moreover, we find that the fragility of glass formation of the layers is depressed whenever the two films are dissimilar in terms of their bulk Tg's. These simulations qualitatively accord with available experimental results, suggesting that the resulting functional forms may provide new guidance towards the design and understanding of nanostructured materials. Moreover, results show that a 'compensation point' in interfacial energy, at which a confined material exhibits bulk Tg, may account for some measurements of bulk-like Tg in nanoconfined materials.