Competition\nbetween Disentanglement Effect and Interfacial\nEffect Determines the Chain Relaxation Dynamics of Entangled Ultrathin\nPolystyrene Films

Polymer–substrate interfaces can alter the dynamics\nof a\nconfined polymer film. However, the influence of interfacial interactions\non chain dynamics is unclear owing to difficulty in measurement. Herein,\nthe effects of substrate–polymer interactions on the chain\nrelaxation dynamics of confined-entangled polystyrene (PS) thin films\nwere investigated. The chain relaxation time of a PS film on substrates\nwith interfacial energies (γ<sub>s–p</sub>) of 44.9 and\n2.3 mJ m<sup>–2</sup> monotonically decreased and increased,\nrespectively, with a decrease in film thickness. The chain relaxation\ntime of the PS film on substrates with γ<sub>s–p</sub> ranging from 33.5 to 10.6 mJ m<sup>–2</sup> decreased and\nthen increased with decreasing film thickness, and this trend become\nmonotonically increasing with the decrease of γ<sub>s–p</sub>. A new model describing this particular relaxation behavior is proposed\nand was used to elucidate the origin of the nonmonotonic chain relaxation\ndynamics of PS chains, which is attributed to the competition between\ndisentanglement and substrate effects. The former accelerates while\nthe latter retards the chain relaxation dynamics of the PS films.\nThe impact of the PS free surface on relaxation dynamics decreases\nrapidly with the decrease of γ<sub>s–p</sub>. This phenomenon\nwas further confirmed via Monte Carlo simulations. These results provide\na new perspective for elucidating the mechanism of interface interactions\naffecting chain relaxation in the confined polymer film.