Effects of confinement on glass transition temperature in polymer films and nanocomposites: What are the effects and their origin and how can they be suppressed or eliminated?

摘要

Effects of nanoscale confinement on glass-forming materials have been studied for more than two decades. The seminal studies in 1994 by Keddie, Jones, and Cory on thin polymer films showed the importance of free surfaces in reducing the glass transition temperature, Tg, and attractive interactions (hydrogen bonds) between polymer and substrates in increasing Tg relative to Tg,bulk. Numerous confinement studies have shown that Tg values of some confined polymer systems can be perturbed from bulk by many tens of degrees. I shall not only describe the range of Tg-confinements effects but also consider the physical origin. In particular, I shall provide data that disproves one of the possible origins that is commonly considered important. Additionally, I shall provide evidence for the importance of a physical concept known as fragility, which is related to the extent of cooperative motion and the breadth of the relaxation distribution associated with Tg. Other things being equal, the greater the value of fragility (and hence the broader the relaxation distribution or greater the cooperativity required for relaxation), the greater is the effect of free surfaces in reducing Tg in films lacking attractive substrate interactions. In films that show reductions in Tg with confinement, there is also a reduction in fragility with confinement. We have hypothesized that this fragility reduction is the underlying physical cause of the Tg reduction. We support this hypothesis by demonstrating how fragility-confinement effects observed with linear polymers of a particular species, e.g., polystyrene or poly(methyl methacrylate), can be suppressed or eliminated by changes in polymer architecture, e.g., ring polymers, star polymers, cross-linked polymers, dense brushes, etc., or with plasticizing additives. In cases where fragility-confinement effects are strongly suppressed or eliminated, the Tg-confinement effects are also strongly suppressed or eliminated. Additionally, by going to linear polymers with very low bulk fragility values, e.g. some styrene-butadiene copolymers and versions of low molecular weight polystyrene, it is possible for even linear polymer chains to exhibit almost total elimination of the Tg-confinement effect. This knowledge is important in practical design of nanoconfined films or coatings or nanocomposites that can maintain desired Tg response.