PHOTOCHROMIC AND FLUORESCENT-PROBE STUDIES IN GLASSY POLYMER MATRICES .3. EFFECTS OF PHYSICAL AGING AND MOLAR WEIGHT ON THE SIZE DISTRIBUTION OF LOCAL FREE VOLUME IN POLYSTYRENE

Changes in the distribution of local free volume in polystyrene glass during physical aging were measured by using photochromic probe molecules requiring different amounts of volume to photoisomerize. The fraction of local free volume in the size range 130-180 angstrom is constant as the polystyrene molar weight decreases to a critical value between 34,000 and 92,000. At lower molar weights the fraction of local free volume in the size range 130-280 angstrom3 decreases with decreasing molar weight. Physical aging in polystyrene glass reduces or leaves unchanged the fraction of local free volume larger than each probe needs to isomerize. During physical aging the greatest loss of local free volume occurs in the size range larger than 280 angstrom3, but the amount of loss varies with annealing temperature.