Curing of linear and crosslinked epoxy systems: A fluorescence study with dansyl derivatives

Abstract

The curing of diglycidyl ether of bisphenol A (DGEBA) with N,Nʹ-dimeth-ylethylenediamine (N,Nʹ-DMEDA) or ethylenediamine (EDA) was monitored by fluo-rescence spectroscopy and Fourier transform infrared (in the nearinfrared region). 5-Dimethylamino-naphthalene-1-sulfonamide (DNS) derivatives were used as probes (fluorophores added to the reaction mixture) and labels (fluorophores attached by covalent bonds to diglycidyl reactants). The term containing the ratio of the reaction rate constants for the addition of the secondary and primary amine hydrogens to the epoxide was included in the reduced reaction rate term for the autocatalyzed and catalyzed epoxide curing reactions. The changes in the integrated fluorescence inten-sities of the labels during the epoxy group conversion indicated, in some cases, the most important changes in the chemical transformations of the reaction mixture: the epoxy group conversion, during which a rapid increase in the tertiary amino group concen-tration was first observed; the gel point (for EDA); and the entry of the system into the glassy state (for N,Nʹ-DMEDA and EDA). The fluorescence probes monitored neither the gel point nor the threshold of the glassy state. For the DGEBA-N,Nʹ-DMEDA system, a wavy dependence of the integrated fluorescence intensities of the DNS labels on the epoxy group conversion might reflect the molar concentrations of polymer homologues (referred to the initial number of moles in the system) in the reaction mixtures of the diepoxide and secondary diamine.

Phase‐separation process in a poly(methyl methacrylate)‐modified epoxy system: A novel approach to understanding the effect of the curing temperature on the final morphology

“…Furthermore, to optimize the conditions of curing for thermosetting materials, to monitor simultaneously, nondestructively, and in situ, both the chemical conversion and changes in T g or any other parameter directly related to it would be very helpful. Although several techniques have been used to monitor the curing process in epoxy‐based materials, Fourier transform infrared (FTIR) spectroscopy20–25 and differential scanning calorimetry (DSC)26–30 have probably been the most widely used. However, none of these studies have achieved the objective of nondestructively monitoring the curing process and determining in every moment either the value of the T g of the system or any parameter directly related to it.…”

“…In general, the fluorescence emission changes when polarity and/or rigidity variations occur in the system where the fluorophore is immersed 31–40. For example, there has been an important amount of work done with fluorescence to follow polymerization processes 24, 25, 36, 41–52. In some studies, it has been proposed that during the polymerization, an enhancement in the microviscosity of the medium exists, which leads to a decrease in nonradiative decay rate and, consequently, an increase in the fluorescence quantum yield 36.…”

“…However, although this method seems to be very promising, mainly for monitoring the curing process at long times or postcuring processes, quantitative analysis of the curing characterization based on this method does not seem to be possible. González‐Benito and coworkers24, 25, 37, 47, 53, 55 reported a new method for accurately monitoring polymerizations. This method is based on the use of 〈ν〉 [〈ν〉 = Σ I F (ν F )ν F /Σ I F (ν F ), where I F (ν F ) is the fluorescence intensity at a specific wave number ν F ] 47.…”