Effects of material heterogeneities on the compressive response of thiol-ene pyramidal lattices
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A process of directed UV photo-curing was previously developed for producing periodic thiol-ene lattices, with potential for use in lightweight structures. The present study probes the compressive response of two families of such lattices: with either one or two layers of a pyramidal truss structure. The principal goals are to assess whether the strengths of the lattices attain levels predicted by micromechanical models and to ascertain the role of lattice heterogeneities. These goals are accomplished through characterization of the lattice geometries via X-ray computed tomography and optical microscopy, measurements of the mechanical properties of the constituent thiol-ene and those of the lattices, and strain mapping on the lattices during compressive loading. Comparisons are also made with the properties of the thiol-ene alone, produced in bulk form. We find two lattice heterogeneities: (i) variations in strut diameter, from smallest at the top surface where the incident UV beam impinges on the monomer bath to largest at the bottom surface; and (ii) variations in physical and mechanical properties, with regions near the top surface being stiffest and strongest and exhibiting the highest glass transition temperature. Finally, we find that the measured strengths of the lattices are in accord with the model predictions when the geometric and material property variations are taken into account in the micromechanical models.
This study was supported by the Institute for Collaborative Biotechnologies through Grant W911NF-09-0001 from the US Army Research Office. The content of the information does not necessarily reflect the position or the policy of the Government and no official endorsement should be inferred. Beamtime at the Advanced Light Source was acquired with proposal titled “X-Ray Tomography of Co-Continuous Polymeric Composite Materials for Blast Mitigation” (ALS-04549). The Advanced Light Source is supported by the Director, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors gratefully acknowledge Dr. Dula Parkinson for his assistance with the beamline experiments and post-processing of the data in generating the tomographic images. The authors also thank Prof. L. Chazeau and Dr. J.-M. Chenal of MATEIS Lyon for use of their facilities in performing the DMA measurements.
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