Nanoconfined Polystyrene: A New Phase

TL;DR

This study uncovers a new nanoconfined phase of atactic polystyrene characterized by molecular layering, altered intermolecular interactions, and reduced cohesion, driven by film thickness below 4 times the gyration radius.

Contribution

It introduces the concept of a nanoconfined phase in polystyrene, explaining its formation through a modified intermolecular potential and spectroscopic evidence of molecular reorientation.

Findings

Layering of polystyrene molecules occurs at film thicknesses ≤ 4$R_g$.

Intermolecular potential deviates from classical confinement models.

Molecular dimers change configuration, reducing cohesion.

Abstract

Transverse layering of molecular gyration spheres in spin-coated atactic polystyrene ($aPS$) films, for film thickness $R \leq$ 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 P\"{o}schl-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.