Eliminating the Tg-Confinement Effect in Polystyrene Films: Extraordinary Impact of a 2 mol % 2-Ethylhexyl Acrylate Comonomer

Abstract

Nanoconfined polystyrene (PS) films exhibit substantial reductions in glass transition temperature (Tg) from bulk Tg. By incorporating 2–6 mol % 2-ethylhexyl acrylate (EHA) into styrene (S)-based random copolymers and characterization via ellipsometry, we show that the Tg-confinement effect for films supported on silicon wafers is eliminated within experimental uncertainty down to a 15 nm thickness. Previous studies have neutralized this confinement effect by the copolymerization of minority levels of styrene with majority levels of a comonomer that can undergo hydrogen bonding with hydroxyl groups on a substrate surface, thus counteracting the free-surface-based Tg reduction with a Tg increase near the substrate interface. In contrast, the Tg-confinement effect is eliminated in our 2–6 mol % EHA copolymers independent of the presence of substrate surface hydroxyl groups. Thus, polymer–substrate interfacial hydrogen bonds play no significant role in neutralizing the Tg-confinement effect in the S-based copolymers with 2–6 mol % EHA. Instead, the neutralization must come from suppressing free-surface effects via very low levels of an EHA monomer in the copolymer. Importantly, this simple approach to eliminate the Tg-confinement effects with as little as 2 mol % EHA is accompanied by only a minor change in bulk Tg and no change in thermal expansivity within the experimental error relative to PS. Furthermore, this approach cannot be generalized to other acrylate comonomers, such as n-butyl acrylate. It neither requires complex syntheses to achieve dense brush, bottlebrush, or cyclic or ring polymer topologies nor the addition of a plasticizer or a surfactant to the polymer, earlier approaches that suppressed the Tg-confinement effect in PS. For a 99:1 mol % S/EHA copolymer, the confinement effect is nearly identical to that of PS, indicating that a critical yet low EHA level is needed to eliminate the effect.