Free volume after cure vs. fractional conversion for a high‐Tg epoxy/amine thermosetting system

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Abstract
Isothermal glassy‐state properties of cured thermosetting materials pass through maximum and minimum values with increasing fractional chemical conversion. In this work, a diglycidyl ether of bisphenol A (a diepoxide) and trimethylene glycol di‐p‐aminobenzoate (a tetrafunctional aromatic diamine) system was investigated for the purpose of analyzing the complex behavior after cure of the isothermal properties of the glassy state with increasing conversion. The glass transition temperature (Tg) is used as a direct measure of conversion. Dilatometric, differential scanning calorimetry, torsional braid analysis, and positron annihilation spectroscopy techniques were used to monitor the density, Tg, modulus, and free volume of the material after cure with increasing conversion. The specific volume at 25°C after cure passes through a minimum and the modulus passes through a maximum with increasing conversion. The fractional free volume and the average radius of free volume at 25°C after cure pass through minimum values with respect to conversion. The specific volume, modulus, and fractional free volume at 25°C vs. conversion data qualitatively correlate. The anomaly of the increasing specific volume in the glassy state with increasing conversion is thus considered to arise from changes in free volume on a length scale corresponding to angstroms. The increasing free volume with increasing conversion is related to the phenomenon of antiplasticization. © 1995 John Wiley & Sons, Inc.

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