Dynamics and glass transition temperature of polystyrene films supported on hydroxyl terminated substrates

All atom molecular simulations are employed to systematically study the properties of polystyrene (PS) films supported on hydroxyl (–OH) terminated Si substrates. We modulate the –OH grafting density ϕG to probe its effect on polymer diffusion, local segmental relaxation, and glass transition temperature Tg. First, our results indicate a monotonic facilitation of polymer dynamics with increasing ϕG. The underlying mechanism is investigated in terms of energy and PS free volume change in detail, and a consistent scenario is obtained. The facilitated diffusion dynamics can be attributed to the –OH induced reduction of interaction energy between PS and the substrate. In addition, –OH grafting leads to the augment of the polymer free volume, which is responsible for the faster local segmental relaxation process. Second, we uncover a very intriguing increase–decrease non-monotonic tendency of glass transition temperature with increasing grafting density, where the maximum Tg occurs in a moderate grafting case. A wetting-friction transition is proposed to rationalize such an unusual behavior. As ϕG increases from a native case to a half grafting situation, polymer films undergo a sharply enhanced interpenetration (or wetting) into the substrate, resulting in the pronounced increment of Tg. As ϕG further increases, the polymer–substrate interaction energy experiences a heavy drop, which implies a greatly reduced friction effect, leading to the decreasing trend of Tg as observed in the range from half grafting to full grafting.