Reorientational Dynamics and Intermolecular Cooperativity of Reactive Systems
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advancement of chemical reactions alters the very nature of the material and adds a new component to the study of dynamics of molecular motions in polymers. Changes in the chemical composition, glass transition temperature (Tg), crosslink density, and morphology, are accompanied by a continuous variation in the time scale of the segmental motions. Consequently, the origin and the characteristic relaxation times of various molecular processes, their temperature dependencies, activation energies, and the widths and breadths of their distributions, all change with the progress of reaction. The principal goal of this study is to examine how the advancement of chemical reactions in epoxyamine systems affects the reorientational dynamics within the general framework of intermolecular cooperativity. EXPERIMENTAL Materials - The six epoxy prepolymers utilized in this study were all based on diglycidyl ether of bisphenol A (DGEBA). Their chemical structure, degree of polymerization (x) and molecular weight (MW) are shown in the inset in Figure 1. The non polymer-forming model epoxy-amine system consisted of 1,2-epoxy-3-phenoxypropane, also known as phenyl glycidyl ether (or PGE), and aniline. The multifunctional polymer-forming formulation consisted of a DGEBA epoxy resin and methyelne dianiline (MDA). All 195 Mat. Res. Soc. Symp. Proc. Vol. 455 ©1 997 Materials Research Society
reactive mixtures contained the stoichiometric amount of epoxy group and amine hydrogen. Techniques - Our experimental facility for dielectric measurements consists of modified commercial and custom-made (in-house) instruments [2]. The three commercial instruments are: 1) Solartron 1260 Impedance/Gain Phase Analyzer (10 pHz -32 MHz); 2) Hewlett-Packard 4284A Precision LCR Meter (20 Hz - I MHz); and 3) HewlettPackard 8752C Network Analyzer (300 kHz- 1.3 GHz). Each instrument was modified by the addition of a temperature controlled chamber and interfaced to a computer.
RESULTS AND DISCUSSION 1. Effect of molecular weight of epoxy prepolymer on reorientational dynamics The first part of our study examines the reorientational dynamics of a homologous series of bifunctional epoxies of different molecular weights. Frequency sweeps at various temperatures were performed on all prepolymers and an example is given in Figure 1 for the prepolymer with the degree of polymerization of 2.3 (MW=690 g/mole). The a relaxation peak is clearly observed in the frequency domain; with increasing temperature it shifts to higher frequency, while its intensity and dielectric dispersion decrease.
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Next, we sought to relate the observed changes in the dielectric response to the molecular and dielectric architecture of prepolymers within the framework of the coupling theory. The interpretation afforded by the cooperativity plot, shown in Figure 2 for all si
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