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 4 R g ( R g = unperturbed polystyrene gyration radius ≈ 20.4 nm). Simultaneously, transverse layering of molecular “gyration spheres”, for film thickness R ≤ 4 R g, causes an increase in free energy (reduction in cohesion) that follows a ( R g / R ) b dependence with b ≈ 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 Pöschl-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.