Optical Rheology of New Liquid Crystalline Thermosets (LCTs): Influence of Shear on Disclination Texture

Liquid crystalline thermosets (LCTs) have been under intensive study because of their outstanding mechanical performance and low viscosity during processing. We have successfully synthesized four kinds of nematic bismaleimide thermosets that differ in the nature of a pendant group substitution and that feature a thermally stable nematic phase. Importantly, the pendant groups were found to be efficient in depressing the melting point to a level that allows for flow and cure at reasonably low temperatures. By blending two particular monomers with monofunctional maleimide that we synthesized, N-(4-hydroxyphenyl) maleimide (HPMI), the melting point is depressed and the curing process is postponed to higher temperatures. Such a blend has a processing window amenable to much-needed thermal and rheological characterization, particularly the evolution of disclination density during shear flow as cure progresses. Our previous work on disclination density measurements during shear flow has revealed particular scaling of dimensionless disclination density with dimensionless shear rate. The influence of shearing on the disclination density, flow patterns, and molecular orientation though gelation of thermosetting liquid crystals remains unexplored. In this presentation, we will follow a description of material synthesis with a report on the results of optical rheology experiments applied to the ternary blends containing HPMI and two bismaleimide monomers, detailing the influence of shear on disclination density though gelation. INTRODUCTION: Because of their outstanding mechanical performance and low viscosity during processing, liquid crystalline thermosets (LCTs) have received intensive study over the past two decades. Control over the micron-scale texture in LCTs may provide particular improvement in fracture toughness behavior. In 1998, C. Ortiz et. al. studied one type of epoxy-based LCT. In curing the material under different conditions, they prepared epoxy resins with amorphous, nematic, or smectic-A structures and compared their fracture behavior. The results showed that the material with a smectic-A structure exhibited slow, stable crack propagation and so had the highest fracture toughness. In their proposed toughening mechanism for the smectic-A resin, it was envisioned that upon deformation, unfavorably oriented domains fail first leaving microscopic voids as defects ahead of the crack tip. As a result, domains adjacent to the voids undergo plastic deformation. This mechanism is similar to that of polycrystalline metals, which are characterized by admirable ductility. In light of these findings, we anticipate a significant role of disclination density in the toughening of LCTs, resulting in a need to control this parameter during processing. In previous work, we studied the dependence of disclination density on shear rate in simple shearing flow of two well-understood liquid crystalline Mat. Res. Soc. Symp. Proc. Vol. 709 © 2002 Materials Research Society