Abstract
Small-angle X-ray scattering (SAXS) was utilized to measure the density fluctuation, in two samples of poly(methyl methacrylate) and a sample of polycarbonate, as a function of temperature on a constant rate cooling and heating; and also as a function of time on isothermal annealing. For the purpose of comparison, measurements were also made on the specific volume of poly(methyl methacrylate) under comparable conditions. In the transition region of 20°–40° interval around T g both the specific volume and the density fluctuation decreased with time on isothermal annealing. At temperatures below the transition region, however, the density fluctuation showed no observable change, while the specific volume continued to decrease linearly with the logarithm of time. In order to interpret the density fluctuation in terms of the concept of free volume, a simple model is presented in which holes of various sizes are randomly distributed in the continuous medium, and an equation is then derived to correlate the observed density fluctuation to the free volume fraction and the weight-average size of the holes.
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