Wall-Thickness Dependence of Cooling-Induced Deformation of Polystyrene Spherical Shells

Experiments on the wall-thickness dependence of the cooling-induced deformation (CID) of polystyrene (PS) spherical shells were carried out. For the experiments, the PS shells were fabricated by the density-matched emulsion method using the hand-shaken microencapsulation technique. The number-averaged and weight-averaged molecular weights of the PS were M_n = 1.1 × 10⁵ and M_w = 4.0 × 10⁵, respectively. The diameter of the PS shells was ~400-550 m. To investigate the wall-thickness dependence of the CID, the wall thickness of the PS shells was varied between 5 and 60 m. In the experiments, the PS shells were cooled by using liquid nitrogen, and their images were captured at 0 and -190°C. For the investigation of the CID, two shapes of each shell that were measured at 0 and -190°C were compared. The thinner PS shells showed larger CID. The maximum deformation was almost 1% of the outer radius when the shell aspect ratio (outer radius)/(wall thickness) was higher than 20. The repeatability of the CID was studied, and the results implied that residual stress in the PS shells had an influence on the CID.