In this thesis, extensive experiments were performed using Atomic Force Microscopy (AFM) in studying the relaxation of surface roughness of PS thin film induced by rubbing. The ridge/ditch patterns were created by rubbing with velvet cloth at room temperature, while the applied pressure by velvet cloth was just about 4 to 5 orders less than the yield stress of PS (~60MPa). The main driven force to flatten the surface roughness was the Laplace pressure with average value 0.125MPa, which was about 1/500 of yield stress.

In study of ageing effect to the surface roughness, we performed two tests: (1) ageing before rubbing and (2) ageing after rubbing. Although different thermal history was applied to the samples, they showed no notable difference in the change in surface roughness....[ Read more ]

In this thesis, extensive experiments were performed using Atomic Force Microscopy (AFM) in studying the relaxation of surface roughness of PS thin film induced by rubbing. The ridge/ditch patterns were created by rubbing with velvet cloth at room temperature, while the applied pressure by velvet cloth was just about 4 to 5 orders less than the yield stress of PS (~60MPa). The main driven force to flatten the surface roughness was the Laplace pressure with average value 0.125MPa, which was about 1/500 of yield stress.

In study of ageing effect to the surface roughness, we performed two tests: (1) ageing before rubbing and (2) ageing after rubbing. Although different thermal history was applied to the samples, they showed no notable difference in the change in surface roughness. Also, the shift rate for two samples with different in ageing duration of about 300 times was estimated to be 0.1, which was drastically difference with Struik’s result for the bulk properties of PS (shift rate ~ 0.7) . Generally, we observed little or even absence of ageing effect to the deformation of surface roughness. It might be due to ageing process was either extremely fast or slow that went beyond the time scale of our measurement.

The relaxation process below the glass temperature T_g was in general governed by the Kohlrausch-Williams-Watts (KWW) stretched exponential function. In the relaxation of surface roughness, the temporal evolution revealed that at a given annealing temperature, once the drop in surface roughness was observed, the further annealing at the same temperature could cause the further drop in surface roughness but decrease in magnitude. Based on this observation, the model of depth dependent T_g was proposed. The surface mobile layer was estimated about 1nm at temperature 102°C. The activation energy of the relaxation of surface roughness was estimated between 250kJ/mol and 800kJ/mol, which was in the range of α-relaxation.