Self-referencing fluorescence sensor for monitoring conversion of nonisothermal polymerization and nanoscale mixing of resin components

Fluorescence sensor techniques have been developed for monitoring nonisothermal polymerization reactions as well as for mixing of resin components both at the nanoscale and macroscale. A polarity-sensitive probe, 4-(N,N-dimethylamino)-4′-nitrostilbene (DANS), was used to monitor the cure of an industrially relevant epoxy resin system through a fluorescence emission intensity ratio, which is self-referencing and independent of excitation intensity and monitoring geometry. The ratio demonstrated significant sensitivity to cure, increasing by a factor of 5.9 over 80% conversion. It was shown that the spectral shape, and thus the intensity ratio, was independent of temperature at a given state of cure. This insensitivity to temperature makes practical monitoring of nonisothermal reactions much simpler than many fluorescence techniques whose measurable is a function of both extent of reaction and temperature. In addition, the solvatochromatic nature of DANS was utilized to monitor nanoscale mixing of epoxy resin components, with spectral shape showing significant sensitivity to well-mixed resin composition. Fluorescence nonradiative energy transfer, which involves a nanoscale dipole–dipole interaction between excited-state donor and acceptor chromophores, has also been shown to be useful for monitoring molecular scale resin mixing. Further, a method of multiple, directly excited fluorescence probes was introduced to monitor overall stoichiometry in epoxy resin systems through spectral shape, showing sensitivity to changes of ±1% in composition.