Enhancement of Suppressed Segmental Mobility in Thin PS Films Supported on Cyclic PS Brushes by Linear-Cyclic Threading Behavior

The chain mobility in thin polymer film mediated by interfacial chain conformations has attracted much attention over the past decades, but its mechanism is still unclear. In this paper, we modulated the grafting density (σ) of cyclic (c-) polystyrene (PS) brushes on their substrate to probe their effect on the glass transition temperature (T_g) in overlaying PS thin films. The results show that the segmental mobility of PS films supported on c-PS brushes with σ ≤ 0.22 nm^–2 was greatly suppressed compared with that on linear brushes. When σ > 0.31 nm^–2, the T_g of the supported PS film decreased with the increase of σ and was lower than that of its bulk T_g. The enhanced suppressed segmental mobility in thin PS films was attributed to the linear-cyclic threading behavior occurring in free chains in the PS film and c-PS brushes at the interface. When σ was relatively low, the PS chains in the film were threaded through cyclic brushes and their mobility was greatly suppressed. However, the PS chains were difficult to thread through the c-PS brushes at relatively high σ and the polymer melts were expelled from the polymer brushes by entropic effects, resulting in a topology of the PS brushes that had no effect on the T_g of the upper PS film. This work demonstrated that the T_g of thin polymers films could be enhanced through the threading behavior of upper free chains with c-polymer brushes anchored on their substrate, providing a novel method to improve the thermal stability of ultrathin polymer films and revealing the mechanism of the effect of adsorbed chains in which the chain mobility in the film should be affected by the threading of the free chains through the “loops” in adsorbed chains on the substrate.