Measurement of anisotropic fracture energies in periodic templated silica/polymer composite coatings.

We report measurements of the fracture energies of hexagonal honeycomb structured silica/polymer composite films that were produced through an evaporation induced self-assembly process. These films exhibit large anisotropy with their hexagonal pore axes aligned with the dip-coating direction. The experimental strategy included depositing films onto a flexible Kapton substrate and then straining them, in situ, under a microscope. To study the effect of the anisotropic microstructure on the fracture energy, cracks were propagated both parallel and perpendicular to the cylindrical pore axis directions. For both cases, the geometries of the evolving crack patterns with loading were micrographically recorded and the desired energy release rates were calculated using a two-dimensional steady-state channeling crack model. The model was implemented using the ANSYS finite element program. The experimental observations showed significant inelastic film deformation prior to crack propagation. These deformations were fully captured in the model, with properties obtained directly from the experiments. The calculated energy release rates were 12.3±0.5 J/m² for the parallel direction and 6.7±0.5 J/m² for the perpendicular direction. These numbers are significantly larger than the bulk silica value of roughly 4 J/m², indicating the role of the local nanostructure in blunting and deflecting the crack tips. Experimental validation of the highly anisotropic energy release rates was obtained through transmission electron microscopy images of fractured films. [ABSTRACT FROM AUTHOR]