Nanoconfined Polystyrene: A New Phase

Abstract

Transverse layering of molecular gyration spheres in spin-coated atactic polystyrene ($aPS$) films, for film thickness $R\le$ 4$R_g$ ($R_g$ = unperturbed gyration radius), causes an increase in free energy that does not follow the $(R_g/R{)}^{-2}$ dependence of planar confinement and is explained by invoking a fixed-range, repulsive, modified Pöschl-Teller intermolecular potential, its strength decreasing with increase in $R$. Vacuum ultraviolet spectroscopy reveals a change in 'physical dimers' of adjacent pendant benzene rings of $aPS$ from 'oblique' to 'head-to-tail' configuration as film thickness goes from 9$R_g$ to 2$R_g$. This reduces cohesion by reducing dimer dipole moment. Thus a new phase of $aPS$, the nanoconfined phase, ordered but with lower cohesion than bulk, is formed.

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