No abstract

An investigation was conducted to determine how the advancement of chemical reactions and the prepolymer molecular weight affect the reorientational dynamics and intermolecular cooperativity in model epoxy−amine systems. Experimental results were obtained by dielectric spectroscopy over a wide range of frequency and temperature. A strong effect of the progress of reaction on reorientational dynamics was noted and an explanation was put forward within the framework of the coupling theory, marking the first time this concept was applied to reactive systems. It was proposed that the molecular-level characteristics that govern the intermolecular cooperativity of reactive systems can be classified into two categories:  (1) molecular architecture, determined by molecular symmetry, rigidity, and steric hindrance, and (2) dielectric architecture, determined by the type and concentration of all dielectrically active species. Both molecular and dielectric architecture vary in the course of chemical reaction, and the overall direction in which the cooperativity shifts is governed by the interplay between these two phenomena.

An investigation was carried out on the mechanism and kinetics of cure of a two-component bismaleimide formulation, composed of 4,4′-methylenebis[maleimidobenzene] and 2,2′-diallylbisphenol A. In-situ real time study of the progress of reaction was conducted in the temperature range from 140 to 250°C using remote fiber optic near-infrared spectroscopy. The obtained signal was clean, free of noise, and remarkably reproducible. The principal reaction observed was an alternating copolymerization involving maleimide and allyl double bonds. Maleimide homopolymerization was detected only in the initial stages of reaction at temperatures above 200°C. The extent of self-condensation (or etherification) of hydroxyl groups on the allyl component, which leads to cross-linking, was observed to vary with reaction temperature, suggesting a path to tailor-making networks with desired morphology and physical/ mechanical properties.

An examination was carried out of the reorientational dynamics of dipoles during the network formation in multifunctional epoxy−amine systems. Experimental results were generated by simultaneous dielectric and Fourier transform infrared (FTIR) measurements. The observed changes in reorientational dynamics during the advancement of reactions were utilized to (1) describe the origin of the α relaxation during the network formation, (2) propose a methodology for the evaluation of the kinetics of network formation, and (3) advance an interpretation of network dynamics in terms of intermolecular cooperativity based on the interplay between molecular and dielectric architecture. Fragility or cooperativity plots proved most informative in relating intermolecular cooperativity to the molecular characteristics of the growing network.

An investigation of the kinetics of two non-polymer-forming epoxy/amine model systems and a polymer-forming multifunctional epoxy/amine formulation was carried out by dielectric and nearinfrared (near-IR) spectroscopy. Dielectric measurements were performed in the frequency range where polarization by charge migration is the dominant mechanism and the extent of reaction was calculated from the measured variation in impedance during reactions. Near-IR spectroscopy was carried out in the frequency range between 7100 and 4000 cm-l, and the extent of reaction was evaluated from the changes in the characteristic absorption peaks. Kinetic results determined by dielectric and near-IR spectroscopy were in excellent agreement for all systems investigated.