Important Roles of 2-Ethylhexyl Acrylate Comonomers at Interfaces in Nanoconfined and Bulk Styrene-Based Random Copolymers

The free surface, or polymer-air interface, plays a significant role in perturbing polymer chain dynamics in nanoconfined polymer films. Due to the free-surface effect, the glass transition temperature (T_g) decreases significantly with decreasing film thickness below ~100 nm for polystyrene (PS) films. We show that such effects can be eliminated down to 15 nm thickness for styrene (S) copolymers with low levels of 2-ethylhexyl acrylate (EHA). The elimination of the free-surface effect is not generalizable to n-alkyl acrylate comonomers, e.g., n-butyl acrylate and n-octyl acrylate. A study of confinement effects in 98/2 and 94/6 mol% S/EHA copolymers on both piranha-treated and dichlorodimethylsilane-treated silicon wafers indicates that hydrogen bonding plays no significant role in eliminating the effect. Instead, we find a strong correlation between T_g-confinement and fragility-confinement effects for these copolymers. We hypothesize that EHA units can form special associations via interdigitation to reduce the bulk fragility and thus suppress the fragility-confinement effect. Based on this hypothesis, other important effects and behavior of polymers related to the EHA units will also be discussed.