Anomalous behavior of thermosetting systems after cure vs. chemical conversion: A normalized conversion–temperature–property diagram

Isothermal properties of thermosetting materials after cure, such as density and modulus, pass through maximum and minimum values with increasing chemical conversion. In this report observed decreases in modulus and density at isothermal temperatures below the glass-transition temperature, T_g, are termed “anomalous.” Four diepoxide (diglycidyl ether of bisphenol A) and tetrafunctional diamine (trimethylene glycol di-p-aminobenzoate) high T_g thermosetting systems with different ratios of amine to epoxy were investigated for the purpose of analyzing the evolution of the isothermal properties with increasing conversion. The density, T_g, and modulus of the materials with increasing conversion were measured by a combination of dilatometric, differential scanning calorimetry, and torsional braid analysis techniques. The results are presented in the form of conversion–temperature–property (T_gTP) diagrams with modulus and density as the properties. T_g is used as a direct measure of conversion based on the one-to-one relationship between T_g and conversion. The property-conversion behavior of the systems with different ratios of amine to epoxy show similar behavior if T_g is used as the measure of conversion and the data are normalized with respect to T_g at a conversion corresponding to the lower limit of the conversion range at which a maximum in the isothermal modulus occurs. The conversion corresponding to molecular gelation, _gelT_g, correlates with the lower limit of the conversion range at which the maximum in isothermal modulus occurs; _gelT_g also marks a change in the behavior of the sub-T_g mechanical relaxations vs. conversion. The conversion corresponding to the maximum in isothermal modulus vs. conversion correlates with the conversion corresponding to the maximum in isothermal density vs. conversion. © 1995 John Wiley & Sons, Inc.