John Torkelson1
Northwestern University1
For two decades, i.e, slightly more than half of my professorial career, my research group has been interested in nanoscale confinement effects and the important roles of polymer interfaces, including the polymer-air interface or free surface, polymer-substrate interface, and polymer-polymer interface, on polymer dynamics. Much of this work has concerned confinement effects on the glass transition temperature (Tg) and physical aging. With ultrathin polystyrene (PS) films, the free surface plays a significant role in perturbing polymer chain dynamics, with Tg decreasing significantly with decreasing thickness below ~100 nm. Past studies by my research group have shown that changes to the PS chain architecture, from linear PS coils to cyclic PS chains with no chain ends, dense PS brushes, or PS bottlebrushes can eliminate the Tg-confinement effect in PS films as thin as ~15 nm. However, such unusual polymer architectures are unlikely to be mass produced or adopted at large scale because of complex syntheses and purifiction steps. Recently, we have demonstrated that such Tg-confinement effects can be eliminated in simple styrene (S)-based random copolymers containing very low levels (2 mol%, 6 mol%) of 2-ethyl hexyl acrylate (EHA). Related results are obtained with ultrathin films of 4-methyl styrene/EHA random copolymers. The special nature of the EHA comonomer is made evident by the fact that the elimination of the Tg-confinement effedt is not generalizable to n-alkyl acrylate comonomers, e.g., n-butyl acrylate and n-octyl acrylate. A study of the Tg-confinement effect in 98/2 mol% and 94/6 mol% S/EHA copolymers on both piranha-treated and dicholorosilane-treated silicon wafers indicates that hydrogen bonding plays no significant role in eliminating the effect. Instead, we find a strong correlation between Tg-confinement and fragility-confinement effects, indicating that the presence of the very low levels of EHA comonomer can substantially reduce fragility in bulk copolymer, which is correlated with packing efficiency of polymer chain segments. Our results indicate that the presence of as little as 2-6 mol% EHA comonomer can fundamentally alter chain packing efficiency and the effect of a free surface on the cooperativity of polymer segmental mobility. We hypothesize that the EHA units can interdigitate due to van der Waals interactions, thereby reducing bulk fragility and the fragility-confinement effect. Capitalizing on this, we will also discuss a very recently discovered interfacial behavior of EHA-containing copolymers that is of significant technological and commercial importance. Finally, a brief comment will be made on the long-standing diversity within the materials community.