Eliminating the T_g- and Fragility-Confinement Effects in Polystyrene Films with Very Low Levels of 2-Ethylhexyl Acrylate Comonomer

With nanoconfinement below 50 nm thickness, polystyrene (PS) films exhibit glass transition temperature (T_g) values reduced relative to T_g(bulk). Such effects are eliminated down to 15 nm thickness with copolymers of styrene (S) with low levels of 2-ethylhexyl acrylate (EHA). Confinement effects in 98/2 and 94/6 mol% S/EHA copolymers were studied by ellipsometry on piranha-treated and dichlorodimethylsilane-treated silicon wafers. The T_g-confinement effect was eliminated in both cases. Thus, interfacial hydrogen bonds play no significant role in eliminating the effect. Dynamic fragility (m) of bulk films decreases strongly with EHA content; m decreases from 166 to 139 to 103 as EHA increases from 0 to 2 to 6 mol%. Nanoconfinement effects on m are very strong for PS, relatively weak for 98/2 mol% S/EHA copolymers and absent for 94/6 mol% S/EHA. This suggests that the elimination of confinement effect with 2-6 mol% EHA originates from EHA improving the bulk chain packing efficiency and reducing the susceptibility to free-surface perturbations. Notably, these results are not generalizable to other acrylate monomers, e.g., n-butyl acrylate.