Grafting density dominant glass transition of dry polystyrene brushes

The effects of the grafting densities (sigma(p)), molecular weights (M-n) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T-g(brush))were investigated by ellipsometry. The results show that T-g(brush) strongly depends on the grafting density of the PS brushes. The T-g(brush) of the PS brushes with sigma(p) > 0.30 increases with decreasing M-n (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T-g(brush) of PS brushes with sigma(p) < 0.30 decreases with decreasing Mn (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (sigma(p) = 0.30), T-g(brush) becomes independent of Mn or brush thickness. The reason for this grafting density dependence of T-g(brush) is attributed to the transition of the PS brush conformation from mushroom-to-brush.