Glass transition of small polystyrene spheres in aqueous suspensions

The glass transition temperature Tg of polystyrene spheres in aqueous suspension was investigated by differential scanning calorimetry. Spheres with diameters of 42–548 nm show an unambiguous glass transition very near the Tg of bulk polystyrene. The magnitude of the observed heat capacity jump ΔCp at the transition decreases as the sphere size decreases. These results are interpreted as indicating that the center portion of a sphere has bulklike dynamics while an outer shell has substantially faster dynamics than the bulk. The ΔCp values are consistent with a mobile layer approximately 4 nm thick. In contrast, free-standing polystyrene films with thicknesses similar to these sphere diameters have been reported to show a single glass transition substantially below the bulk Tg value.