LIQUID-CRYSTALLINE EPOXIDE THERMOSETS - A DEUTERIUM NUCLEAR-MAGNETIC-RESONANCE STUDY

The orientational order for a reactive liquid crystalline epoxy-amine mixture was monitored in real time with the aid of deuterium nuclear magnetic resonance (H-2-NMR) and birefringence (DELTAn) measurements. The orientational order increased during the chain extension process, and it became irreversibly fixed as a result of the cross-linking reaction. The order parameter of the amine compound was more sensitive to the degree of the polymerization reaction than that of the epoxide. During the isothermal polymerization, the point where DELTAn started to level off corresponded well with the point of gelation as determined by solubility experiments. The relative increase of the orientational order according to H-2-NMR agrees with the value extracted from birefringence measurements. X-ray diffraction measurements revealed that the orientation of the network hardly changes upon heating and cooling in the absence of the magnetic field. The reaction was also measured in quasi-real time by gel permeation chromatography (GPC) and so the nature of various species, formed during the reaction, was determined. There was a good agreement between the reaction kinetics as determined by GPC and H-2-NMR experiments. The conversion (alpha) of the epoxy groups at the gel point agreed well with the value predicted by the statistical theory for network formation in isotropic stoichiometric epoxy-amine mixtures, namely, alpha almost-equal-to 55%. In addition, good agreement was found between the experimentally determined increase in the number-average molecular weight and theoretical predictions.