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Anthony Sullivan1 Anil Saigal1 Michael Zimmerman1

1, Tufts University, Medford, Massachusetts, United States

Liquid crystalline polymers (LCP’s) comprise a class of performance materials that exhibit high mechanical strength, chemical inertness, flame retardancy and frequency-stable dielectric response. These favorable properties make LCP’s ideal candidates for various engineering applications, such as semiconductor packaging, high-frequency electronics, and high strength-to-weight ratio components. The characteristic LCP behavior arises from a unique microstructure, which in contrast to conventional amorphous polymers, consists of aligned crystalline domains in both the melt and solid phases. The resulting long-range molecular ordering gives rise to anisotropic behavior of the bulk material, which can be problematic from an industrial perspective. Thus, the ability to simulate the driving forces behind this morphology is essential to the design of processes for isotropic material production.
Goldbeck-Wood, et al. introduced a method for modeling the directionality, or orientation, of LCP’s on a mesoscopic scale. This hybrid method is carried out in two steps: first, the LCP rheology is simulated with conventional numerical methods, and second, the calculation of directionality is performed, using the data from the rheological modeling, assuming the LCP can be approximated as small molecule liquid crystals. Although Goldbeck-Wood, et al. demonstrated the ability of this technique to simulate LCP orientation on structured lattice grids, the methodology can be extended to unstructured meshes using modern finite element or finite volume solvers, to simulate complex geometric domains.
In this investigation, the evolution of LCP directionality during an injection molding process was modeled. The commercial solver, ANSYS FLUENT, was used to simulate the flow of a commercial LCP during injection molding. The resulting rheological outputs were then applied to user-defined calculations in MATLAB governing LCP directionality. These calculations captured three primary factors driving the final orientation state, namely: rheological (shear stresses), elastic (distortional interactions between crystals), and translational (bulk fluid flow).
To validate the model, the simulation results were compared to wide-angle x-ray scattering (WAXS) measurements of orientation in injection molded LCP plaque samples. An order parameter and anisotropy factor were calculated from the scattering data, as well as from the modeling results, to serve metrics for comparison between the two. Plaques of two thicknesses were both simulated and fabricated for the analysis, and it was hypothesized that decreasing the plaque thickness would result in a larger shear gradient driving crystal ordering in the material. The WAXS analysis found that the thinner plaque exhibited a higher degree of orientation with respect to the the flow direction, and the corresponding model predicted greater crystalline alignment with the flow than in the thicker simulation.

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