Analysis of smectic structure formation in liquid crystalline thermosets

Smectic network formation using a twin liquid crystalline epoxy monomer (4-(oxiranylmethoxy)benzoic acid 1,8-octanediylbis(oxy-4,1-phenylene) ester) cross-linked with sulfanilamide was examined in real time by polarizing optical microscopy, X-ray diffraction, dynamic viscometry and differential scanning calorimetry. The initial curing reaction of the twin epoxy monomer and sulfanilamide was found to be diffusion controlled because of the heterogeneity of the system. An acceleration of the curing rate was observed a critical time after the start of the reaction. It was found that the adduct of the twin epoxy monomer and sulfanilamide produced a smectic phase even though the twin epoxy monomer itself only displayed a nematic phase. The strong interaction of the sulfonamide function may contribute to the formation of smectic order. A change in curing behaviour took place when the curing reaction was carried out above similar to 170 degrees C. Above this transition region, a smectic phase forms directly from an isotropic melt. On the other hand, below this transition, the smectic phase forms via a transient nematic phase. Cross-linking causes fine disclination lines in the optical texture as well as a decrease of the smectic layer spacing. (C) 1997 Elsevier Science Ltd.