IMPEDANCE SPECTROSCOPY OF REACTIVE POLYMERS - 5 - IMPEDANCE AS A MEASURE OF CHEMICAL AND PHYSICAL CHANGES IN GLASS FORMERS

A study was conducted aimed at establishing the nature of chemical and physical phenomena in polymeric and nonpolymeric glass formers that c an be observed by impedance measurements. Various systems were investi gated that undergo a temporal evolution of structure as a result of ch emical reactions and physical processes such as crystallization, vitri fication, or phase separation. Distinct and systematic changes in impe dance during crystallization and vitrification confirmed that these ev ents could be monitored by impedance spectroscopy. Of particular inter est was the potential use of impedance measurements in detecting gelat ion in crosslinking polymers. It was shown that the experimentally obs erved ''knee'' in imaginary impedance during reaction shifts with freq uency and, hence, cannot be used to measure gelation. But a new insigh t at the molecular level was obtained by employing a novel experimenta l approach based on simultaneous dielectric-infrared measurements. Evi dence was generated to support the formation of a hydrogen-bonded comp lex in the vicinity of gel point in polymer networks, which affords a vehicle for the migration of intrinsic charges and provides a contribu tion to the overall conductivity. This finding should be explored furt her because it suggests the possibility of correlating dielectric resp onse with gelation. (C) 1998 John Wiley & Sons, Inc.