IMPACT OF PHYSICAL AGING ON THE CRAZE INITIATION AND PROPAGATION IN STYRENE ACRYLONITRILE CO-POLYMER. Mats Delin and Gregory B. McKenna, Polymers Division, Building 224, Room A209, NIST, Gaithersburg, MD 20899 (301-975-6843, email:delin@nist.gov)

Studies of the craze response of a styrene acrylonitrile co-polymer have been made in stress relaxation conditions after quenching from above the glass transition to below and then aging the sample for times up to 120 h. Measurements have been made of the time to craze as a function of the applied strain and of aging time. In addition craze propagation was investigated using length and length distribution measurements. It was found that the craze initiation is sensitive to aging time, strain and temperature. The average craze length and its distribution was also found to change as the sample aged and strain magnitude increased. Of particular interest is the observation that for all temperatures investigated the craze length saturated at substantially smaller sizes at long aging times than for freshly quenched materials. In addition, the density of crazes (#/mm2) was significantly greater for the long aging times than for freshly quenched materials.