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

Dynamic mechanical and thermal properties of a certain liquid crystalline (LC) diepoxide cross-linked with three different aromatic diamines were studied. For one epoxy-amine mixture, the position of the gel point was determined with the aid of frequency-dependent theological measurements. The value of the critical relaxation exponent was 0.5. The gel point was also determined by solubility experiments. There was a clear agreement between the two methods. The degree of conversion of the epoxy groups at the gel point (55 +/- 3%) corresponded well with the value predicted by the statistical theory for network formation in isotropic stoichiometric epoxy-amine mixtures. Mechanical measurements were carried out on macroscopically ordered networks in the direction of orientation. In highly ordered networks prepared from the LC diepoxide and a rigid aromatic diamine, the value of the rubber modulus deviated from the predictions of rubber elasticity theory by a factor of 30 times higher. In networks with the same high level of macroscopic orientation prepared from the LC diepoxide and a relatively more flexible diamine, the deviation from the classical theory was much less (factor of 1.7). In the rubbery region, the value of the Young's elastic modulus decreased as a function of temperature, which seems to be connected to the decrease of the order. This is confirmed by a theory, presented by T. Odijk, concerning the polymer nematic gels under tension (see Appendix). The thermal expansion coefficient of the macroscopically ordered networks was highly anisotropic. It was indeed possible to combine the good mechanical and thermal properties of conventional epoxy polymer networks with the special features that LC polymers offer.