Intramolecular and intermolecular rearrangements in nanoconfined polystyrene

Vacuum ultraviolet spectroscopy reveals an intramolecular rearrangement involving a change in 'physical dimers' of adjacent pendant benzene rings of atactic polystyrene (aPS) from 'oblique' to 'head-to-tail' (ht) configuration in aPS films spin coated on fused quartz, as film thickness R goes below 4R(g) (R-g = unperturbed polystyrene gyration radius approximate to 20.4 nm). Simultaneously, transverse layering of molecular 'gyration spheres' for film thickness R <= 4Rg, causes an increase in free energy (reduction in cohesion) that follows a (R-g/R)(b) dependence with b approximate to 3, a clear deviation from planar confinement. The variation of in-plane and out-of-plane cohesive energy over a film of a given thickness is explained by invoking a fixed-range, repulsive, modified Poschl-Teller intermolecular potential, with the strength of this potential decreasing with increase in R. Possible reduction of 'dimer' dipole moment due to ht configuration is consistent with reduction of cohesion between aPS molecular gyration spheres.