Surface Chain Packing Density-Dominant Number of Arms-Dependent Free Surface Effect in Star-Shaped Polystyrene

The surface mobility of polymers is generally higher than that of the bulk (called the free surface effect), which is of fundamental and practical interest, affecting physical properties of nanoconfined polymer materials. Hence, understanding the origin of the free surface effect is essential to adjusting the physical performance (including surface properties) of nanoconfined polymers. Herein, the surface dynamics of star-shaped polystyrene with different numbers of arms ( f ) along with M arm = 57 kg/mol for each arm and the mechanism about its f dependence were investigated. The surface T g was observed to increase from 324 to 371 K with increasing f from 4 to 64, which was almost equal to its bulk T g when f is above 32. According to the experimental results from density distribution of the film surface via XRR and deuterium oxide vapor diffusion via neutron reflectivity, a diminished free surface effect with increasing f was attributed to an increase in surface density (the low free volume) and narrow free volume distribution on the surface, which resulted from both soft-colloid-like behavior and reduced fragility with larger f . Because of low levels of packing frustration, the chain packing efficiency on the free surface of low fragile polymers was enhanced during film formation. This study reveals directly the relationship between the free surface effect and surface density resulting from both the dynamic fragility index and the number of arms in star polystyrene.