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

Nanoconfined polystyrene (PS) films exhibit substantial reductions in glass transition temperature (T_g) from bulk T_g. By incorporating 2−6 mol % 2-ethylhexyl acrylate (EHA) into styrene (S)-based random copolymers and characterization via ellipsometry, we show that the T_g-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 T_g reduction with a T_g increase near the substrate interface. In contrast, the T_g-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 T_g-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 T_g-confinement effects with as little as 2 mol % EHA is accompanied by only a minor change in bulk T_g 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 T_g-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.