Grafting density dominant glass transition of dry polystyrene brushes
The effects of the grafting densities (σp), molecular weights (Mn) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (Tbrushg) were investigated by ellipsometry. The results show that Tbrushg strongly depends on the grafting density of the PS brushes. The Tbrushg of the PS brushes with σp > 0.30 increases with decreasing Mn (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 Tbrushg of PS brushes with σp < 0.30 decreases with decreasing Mn (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σp = 0.30), Tbrushg becomes independent of Mn or brush thickness. The reason for this grafting density dependence of Tbrushg is attributed to the transition of the PS brush conformation from mushroom-to-brush.