On Measuring the Distribution of Local Free Volume in Glassy Polymers by Photochromic and Fluorescence Techniques

A general technique, based on photoisomerization of photochromic and fluorescent probe molecules having no net dipole moment, has been devised for measuring the distribution of local free volume in glassy polymers such as polystyrene. In this technique, the amount of probe photoisomerization in a glassy polymer relative to that in dilute solution in a nonviscous model solvent, where free volume is not a constraint to isomerization, is measured as a function of the volume required for photoisomerization of the probe. A total of eight probes were used with required isomerization volumes ranging from a low of 127 ųfor azobenzene to about 571 ųfor 4,4'-diphenylstilbene, the volume requirements calculated from the volume swept by the van der Waals area of a probe molecule during excited-state geometrical rearrangement. The median of the local free volume in unannealed polystyrene found by this method is about 260–280 ų, with over 90% larger than about 100–120 ųand practically none larger than 400 ų. The size distribution of local free volume is sensitive to physical aging, with a greater reduction occurring in larger local free volume than smaller local free volume during sub-T_gannealing.