Network swelling competing with translational entropy in autophobic polymer dewetting

Autophobic dewetting of a thin polystyrene (PS) layer on a cross-linked PS film has been studied with optical microscopy and neutron reflection. For two different cross-linking densities, the increase in contact angle and the fast decrease of both dewetting velocity and slippage length as a function of dewetting time indicate that the dewetting dynamics is controlled by the penetration of free linear polymer chains into the cross-linked film and the concomitant stretching of the network mesh: the loss of entropy of the network favors dewetting, while partial chain interdiffusion between the layers increases friction and eventually stabilizes the top film. In a second series of experiment, we modified the entropy of the network by swelling it with linear chains from a reservoir of identical polymers prior to depositing the top PS film. Once equilibrated, a stable interface without considerable interdiffusion between network and top film was achieved. Without such interdiffusion, a constant contact angle and a constant slippage length were observed.