DIFFUSION OF SMALL MOLECULES IN POLYMER SOLUTIONS NEAR THE GLASS TRANSITION TEMPERATURE.

The transient optical grating technique referred to as force Rayleigh scattering provides a means to measure the transport of small molecules in polymer systems, under a variety of equilibrium and nonequilibrium conditions. One appealing feature of the experiment is the possibility of measuring self-diffusion coefficients in the absence of macroscopic concentration gradients, in contrast to the more traditional sorption and permeation experiments. We have measured the diffusion coefficients of photochromic dye molecules present in trace amounts in polymer solutions, over a wide range of temperature and polymer concentration. The results are interpreted in terms of free-volume theories.