POLYMERIZATION KINETICS OF THERMOSETS WITH AN EPOXIDE-FUNCTIONALIZED POLYHEDRAL OLIGOMERIC SILSESQUIOXANE

Abstract

<p>Effects of a nanometer-size, reactive polyhedral oligomeric silsesquioxane (POSS) on a thermoset’s polymerization kinetics and dielectric relaxation have been investigated by using differential scanning calorimetry (DSC) and dielectric spectroscopy. Four amines, hexylamine (HA), ethylene diamine (EDA), isophorone diamine (IPDA), and diethylene triamine (DETA) were used to crosslink epoxy resin DGEBA and a multi functionalized -glycidyl POSS. The polymers formed ranged from POSS-free to POSS-only network structure, while maintaining an equal ratio of reactive groups.</p> <p>Calorimetric studies of stoichiometric DETA and IPDA samples performed by ramp-heating showed that the polymerization kinetics increased with low POSS content, but progressively decreased upon further addition, and the POSS-only mixtures reacted the slowest. The same pattern of changes in the rate of polymerization was observed by isothermal polymerization of the DETA mixtures. The results have been interpreted based on changes in viscosity and diffusion coefficients according to the Stokes-Einstein and Smoluchowski equations. Furthermore, isothermal polymerization of stoichiometric DETA and EDA samples showed that the extent of reaction increased with POSS, indicating that it might be easier to form a higher crosslinked thermoset with higher amounts of POSS.</p> <p>Non-isothermal polymerization of POSS-IPDA mixtures was also investigated under off-stoichiometric conditions. Unlike the reaction mechanisms of non-stoichiometric DGEBA-IPDA mixtures, a significant difference in reactivity of primary and secondary amines in the POSS-IPDA reaction was not observed.</p> <p>Dielectric relaxation studies were performed in real time during polymerization. The distribution of relaxation rates increased with an increase in POSS, and the duration of polymerization required to reach a specific relaxation time increased. Furthermore, the time at which the dielectric loss minimum and peak occurred increased with POSS, however, when only POSS was reacted with the amine, an <em>a</em>-relaxation peak was not observed. A frequency-dependent increase in permittivity during polymerization was observed with the POSS-only EDA, DETA, and HA samples, and a corresponding change in the loss was not detected. It was attributed to a structure-dependent increase in interfacial polarization.</p> <p>Although not related to the polymerization study, work done on the residual entropy of glasses is included here. To help resolve the controversy of the residual entropy of glassy materials, specific heat data taken from the literature were analyzed. The changes in entropy of a glass on cooling and heating paths were determined, and it was found that the upper and lower limits of entropy differed by no more than 2 %. This showed that although the thermodynamic path through the glass transition is irreversible, this irreversibility has a negligibly small effect on determining the entropy.</p>