LIQUID-CRYSTALLINE EPOXIDE THERMOSETS - DYNAMIC-MECHANICAL AND THERMAL-PROPERTIES

Dynamic mechanical and thermal properties of a certain liquid crystall ine (LC) diepoxide cross-linked with three different aromatic diamines were studied. For one epoxy-amine mixture, the position of the gel po int was determined with the aid of frequency-dependent theological mea surements. The value of the critical relaxation exponent was 0.5. The gel point was also determined by solubility experiments. There was a c lear agreement between the two methods. The degree of conversion of th e epoxy groups at the gel point (55 +/- 3%) corresponded well with the value predicted by the statistical theory for network formation in is otropic stoichiometric epoxy-amine mixtures. Mechanical measurements w ere carried out on macroscopically ordered networks in the direction o f orientation. In highly ordered networks prepared from the LC diepoxi de and a rigid aromatic diamine, the value of the rubber modulus devia ted from the predictions of rubber elasticity theory by a factor of 30 times higher. In networks with the same high level of macroscopic ori entation prepared from the LC diepoxide and a relatively more flexible diamine, the deviation from the classical theory was much less (facto r of 1.7). In the rubbery region, the value of the Young's elastic mod ulus decreased as a function of temperature, which seems to be connect ed to the decrease of the order. This is confirmed by a theory, presen ted by T. Odijk, concerning the polymer nematic gels under tension (se e Appendix). The thermal expansion coefficient of the macroscopically ordered networks was highly anisotropic. It was indeed possible to com bine the good mechanical and thermal properties of conventional epoxy polymer networks with the special features that LC polymers offer.