Cooling rate dependence of the glass transition at free surfaces

In situ x-ray reflectivity measurements are used to determine the cooling rate dependent freezing of capillary waves on the oligomer poly(propylene glycol). Only above the glass transition temperature T-G can the surface roughness sigma be described by the capillary wave model for simple liquids, whereas the surface fluctuations are frozen-in at temperatures below T-G. As the state of a glass forming liquid strongly depends on its thermal history, this effect occurs for fast cooling rates already at a higher temperature than for slow cooling. For the fastest cooling rates a very large shift of T-G up to 240 K compared to the bulk value of 196 K was observed.