Competition between Disentanglement Effect and Interfacial Effect Determines the Chain Relaxation Dynamics of Entangled Ultrathin Polystyrene Films

Polymer–substrate interfaces can alter the dynamics of a confined polymer film. However, the influence of interfacial interactions on chain dynamics is unclear owing to difficulty in measurement. Herein, the effects of substrate–polymer interactions on the chain relaxation dynamics of confined-entangled polystyrene (PS) thin films were investigated. The chain relaxation time of a PS film on substrates with interfacial energies (γs–p) of 44.9 and 2.3 mJ m–2 monotonically decreased and increased, respectively, with a decrease in film thickness. The chain relaxation time of the PS film on substrates with γs–p ranging from 33.5 to 10.6 mJ m–2 decreased and then increased with decreasing film thickness, and this trend become monotonically increasing with the decrease of γs–p. A new model describing this particular relaxation behavior is proposed and was used to elucidate the origin of the nonmonotonic chain relaxation dynamics of PS chains, which is attributed to the competition between disentanglement and substrate effects. The former accelerates while the latter retards the chain relaxation dynamics of the PS films. The impact of the PS free surface on relaxation dynamics decreases rapidly with the decrease of γs–p. This phenomenon was further confirmed via Monte Carlo simulations. These results provide a new perspective for elucidating the mechanism of interface interactions affecting chain relaxation in the confined polymer film.