| Abstract: | A diffusive gas-transport isotope effect is used to estimate the size of the free volume element above and below the glass transition for poly(ethyl methacrylate) and poly(vinyl fluoride). The cavity size, as measured by the hydrogen probe molecule, is apparently larger in the glassy region for both polymers than it is above Tg. It is postulated that the number of free volume elements essentially decreases below Tg, so that the total free volume, which is the sum of all such elements, is smaller below the glass transition, in accord with density measurements on bulk polymers. |
