Glass transition of small polystyrene spheres in aqueous suspensions

The glass transition temperature T-g 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 T-g of bulk polystyrene. The magnitude of the observed heat capacity jump DeltaC(p) 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 DeltaC(p) 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 T-g value. (C) 2003 American Institute of Physics.