Cure Characterization in Diamine-Cured Epoxy and Polyimide by UV Reflection Spectroscopy

Abstract

UV specular reflection spectroscopy has been investigated for the characterization of cure reactions in an aromatic diamine cured epoxies and their composites, as well as in the imidization characterization of an aromatic polyamic acid. The UV reflection peak due to the reaction of the curing agent bis(p-aminophenyl) sulfone (DDS) with epoxide shows a red shift of about 24 nm in the spectral range of 300 nm to 350 nm. After the Kramers-Kronig conversion of the reflection spectra which is shifted to longer wavelength than the transmission spectra, the DDS peak occurs at a similar position as the transmission peak. The spectral shifts obtained by three different techniques, UV reflection, UV transmission, and fluorescence excitation were similar as a function of cure time at 160 °C. Therefore, the peak position in the reflection spectra may be used without the Kramers-Kronig conversion in order to correlate to the extent of cure, as established for the excitation spectral shift. The UV reflection spectral analysis is also obtained in DDS containing epoxy reinforced with glass or carbon fiber. Since the reflection probes only the first 0.5 μm layer of the resin, the presence of the fiber does not influence the spectral quality. In following the imidization of a polyamic acid made from 1,4-phenylenediamine (PDA) and an aromatic dianhydride, the spectral deconvolution of the UV reflection spectra provides a way to determine the relative composition of polyamic acid and polyimide. The resulting extent of imidization was compared with the result by IR analysis. Thus, UV reflection technique can be a useful cure characterization method with the samples which have smooth surfaces, strong UV absorption, but weak fluorescence.