Substantial spatial heterogeneity and tunability of glass transition temperature observed with dense polymer brushes prepared by ARGET ATRP

Dense polystyrene (PS) brushes were synthesized on silica wafers via a "grafting from" technique using activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). Spectroscopic ellipsometry and fluorescence spectroscopy were used to investigate the effect of nanoscale confinement on the glass transition temperature (T_g) and its distribution in dense PS brushes and bilayer constructs of brushes and freely deposited PS films. Single-layer PS brushes exhibit an overall average T_g that is nearly invariant with thickness down to 11 nm, with T_g values of the dense brushes differing by no more than 2 °C from those of bulk PS of the same molecular weight as that of the brush. These results differ greatly from those obtained with freely deposited PS films that show major T_g reductions with confinement. Distributions of T_gs were also studied by fluorescence in dense, ∼70-nm-thick PS brushes. Relative to T_g,bulk, chain segments within 10 nm of the substrate exhibit a ∼36 °C increase in T_g while segments within 5 nm of the free surface exhibit a 14 °C decrease in T_g. Fluorescence was also used to characterize the tunability of T_g within a single layer of bilayer constructs. As brush thickness increases from 13 to 94 nm, the T_g of a 15-nm-thick PS overlayer film decreases from 100 °C to 87 °C; in contrast, as the overlayer PS film thickness increases from 0 to 101 nm, the T_g of an 11-nm-thick underlayer PS brush increases from 98 °C to 126 °C. These results are compared with the few previous experimental reports of T_g-confinement effects of densely grafted PS brushes and bilayer constructs of brushes and films.