Jimmy D. Earls

Phase transformations of a liquid crystalline epoxy during curing

Abstract The phase transformations of a novel liquid crystalline epoxy, diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene (DGEDHMS), from isotropic phase to liquid crystalline phase during curing have been studied using a cross-polarized optical microscope equipped with a hot-stage. Time-temperature-transformation (TTT) diagrams were constructed following the isothermal curing of the DGEDHMS in the isotropic phase with a difunctional amine, the aniline adduct of the DGEDHMS, and a tetrafunctional sulfonamido amine, sulfanilamide. Our results show that curing with the difunctional amine results in a nematic-like texture, while curing with the tetrafunctional sulfonamido amine leads to a smectic-like texture. Furthermore, curing with the difunctional amine gives rise to a faster development of the liquid crystalline phase, but with a less stable mesophase as compared to the tetrafunctional curing agent. These results are explained in terms of intermolecular interaction and the chain conformation anisotropy resulting from the curing reactions. The TTT diagram will be useful for providing guidelines for processing these new liquid crystalline epoxy systems.

Evolution of structure and properties of a liquid crystalline epoxy during curing

The evolution of structure, and thermal and dynamic mechanical properties of a liquid crystalline epoxy during curing has been studied with differential scanning calorimetry (DSC), polarized optical microscopy, x-ray scattering, and dynamic mechanical analysis. The liquid crystalline epoxy was the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene (DGEDHMS). Two curing agents were used in this study: a di-functional amine, the aniline adduct of DGEDHMS, and a tetra-functional sulfonamido amine, sulfanilamide. The effects of curing agent, cure time, and cure temperature have been investigated. Isothermal curing of the liquid crystalline epoxy with the di-functional amine and the tetra-functional sulfonamido amine causes an increase in the mesophase stability of the liquid crystalline epoxy resin. The curing also leads to various liquid crystalline textures, depending on the curing agent and cure temperature. These textures coarsen during the isothermal curing. Moreover, curing with both curing agents results in a layered structure with mesogenic units aligned perpendicular to the layer surfaces. The layer thickness decreases with cure temperature for the systems cured with the tetra-functional curing agent. The glass transition temperature of the cured networks rises with increasing cure temperature due to the increased crosslink density. The shear modulus of the cured networks shows a strong temperature dependence. However, it does not change appreciably with cure temperature.

Morphology of liquid crystalline epoxy composite matrices based on the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene

Abstract The morphologies of various diglycidyl ethers of 4,4′-dihydroxy-α-methylstilbene-based liquid crystalline epoxy (LCE) formulations have been studied as matrices for high performance composites. Reflected light optical microscopy, selected area electron diffraction, micro-Raman spectroscopy and transmission electron microscopy of thin sections and replicas of etched surfaces were employed to probe both the micrometer-scale and nanometer-scale LCE morphology in infusion moulded, uni-weave graphite fibre composites. The results suggest that various sizes and shapes of LCE domains can be formed in the matrix resin, depending on the cure schedule and the LCE curative composition. Preferred molecular orientation along the graphite fibres can be achieved if the LCE resin is cured so as to promote extensive linear chain extension, thus giving a high concentration of mesogenic segments. Approaches for aligning mesogenic segments in the composite are discussed as well as the potential benefits of doing so.