Cooling rate effects relaxation time

In 3.3 the effect of cooling rate on the free volume in the glassy state has been discussed. Rapidly cooled glassy polymers have a greater free volume, but they show volume retardation. This volume change, though very small, has a considerable effect on the creep behaviour all relaxation times for creep are shifted towards higher values. This phenomenon has been studied extensively by Struik (thesis Delft 1977). 

Thermal equilibration effects are also present in undoped a-Si H (Smith et al. 1986). The dangling bond density varies reversibly with temperature, as is shown in Fig. 6.8 for samples of different deposition conditions. The experiment is performed by rapidly cooling the material from different temperatures to freeze in the equilibrium configuration and measuring the ESR spin density of the g = 2.0055 resonance. The defect density increases four-fold between 200 C and 400 °C, with an activation energy of 0.15-0.2 eV. The relaxation times are slower than in the doped material at the same temperature, and the equilibration temperature of about 200 °C, for a cooling rate of... 

From this it can be seen that as the temperature of a supercooled liquid is being reduced, the time it takes for structural relaxations to occur increases very rapidly. If the cooling rate is kept constant, then at a certain temperature there will no longer be sufficient time for the liquid to return to equilibrium before the temperature is reduced. At this temperature the liquid undergoes a kinetic transition to the glassy phase and the structure is effectively fixed on an experimental time-scale. This is often defined [25, 44] as 100 s, which corresponds to a viscosity of -lO Pa s. It is also apparent from this definition of the glass transition that it is dependent on the rate at which the liquid is cooled a slower cooling rate will... 

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