SOME SELECTED FEATURES OF THE PHYSICAL AGING PROCESS FOR GLASSY POLYMERS
National Science Foundation Science and Technology Center:
High Performance Polymeric Adhesives and Composites, and
Department of Chemical Engineering Virginia Polytechnic Institute and State University
Blacksburg, Virginia
ABSTRACT
The effects of physical aging were examined in two separate studies: (1) The amorphous commercial polyimide Regulus® was subjected to physical aging and then tested thermally with differential scanning calorimetry. The enthalpic physical aging rate p rameter betaH increased, reached a maximum (0.69 J/g at Tg -20°C), and then decreased as aging temperature approached Tg (239°C). After aging for 100h at Tg -25°C, tensile testing revealed that modulus increased 19%, stress at yield increased 16%, but strain at break decreased 47% compared to an unaged sample. The mechanical physical aging rate parameter was measured with creep testing. The lar est value found was 0.81 at Tg -35°C. Thus, physical aging is expected to adversely affect impact toughness and strain at break of Regulus used as a composite matrix or structural adhesive. (2) Amorphous poly(ethylene terepht late) and isotactic polystyrene were physically aged at about 30°C below their respective Tg s for 0, 10, and 100h. After cooling to room temperature, the samples were immediately brought to a crystallization temperature and h ld for varying amounts of time. Such treatment did not significantly affect either the crystallization rate or maximum percent crystallinity versus unaged samples. These results oppose a recent literature study of isotactic polystyrene.