Order induces toughness in anisotropic colloidal crystal composites

Abstract

Spatial ordering of matter elicits exotic properties sometimes absent from a material's constituents. A few highly mineralized natural materials achieve high toughness through delocalized damage, whereas synthetic particulate composites must trade toughness for mineral content. We test whether ordering the mineral phase in particulate composites through the formation of macroscopic colloidal crystals can trigger the same damage resistance found in natural materials. Our macroscopic silica rod-based anisotropic colloidal crystal composites are processed fully at room temperature and pressure, reach volume fractions of mineral higher than 80%, and aided by a ductile interface, unveil toughness up to two orders of magnitude higher than bulk silica through the collective movement of rods and damage delocalization over millimeter. These composites demonstrate key design rules to break free from conventionally accepted structural materials' properties trade-off.

Keywords: bioinspiration; colloidal crystal; fracture toughness; particulate composites.

Fig. 1.

Fabrication of bulk ordered colloidal crystals from anisotropic building blocks. Schematic representation of the composite fabrication: (A) Sol–gel synthesis of rods and functionalization with γ-MPS, (B) templated entropy-driven assembly of the rods in DMSO-water mixture into cm-sized colloidal crystals, (C) infiltration with acrylate monomer after solvent-removal and cross-linking to form the a-C³. (D and E) SEM images of the as-synthesized rods. (F) Optical tracking of the crystal growth with and without template through a cross-polarized microscopy setup. (G) Position of the disorder-to-crystal interface as a function of time.

Fig. 2.

Short-range order of the rods in a-C³. SEM images of the structure of composites fabricated (A) with a template and (B) without. SEM images of ion-polished cross-section of the a-C³ grown from a template with cross-section taken (C) in the wedge direction and (D) perpendicular to the wedge direction. Insets are FFT of the images or of the highlighted area in the image. The dotted line at 91% in the histograms is the theoretical maximum volume fraction of rods.

Fig. 3.

Long-range order of the rods in a-C³. (A) Line representing the intensity at half maximum of the SAXS signal made at nine different spots on sample grown with and without template with the X-rays traversing the sample in the direction of the crystal growth. The line in the top image represents the orientation of the wedge of the template. (B) Integrated intensity as a function of the azimuthal angle $\varphi$ for all scans taken on the sample grown with a template. (C) Intensity as a function of wavenumber in the direction of the template and perpendicular to it. (D) Intensity as a function of wavenumber with the X-ray going along the direction of the template. (E) Comparison of the size of the crystal grown with the silica rods in this work with other methods to grow large-size colloidal crystals with spheres. (Inset) optical microscope image of an a-C³ taken in the direction of the sample growth before infiltration. Data from literature color coded based on the process used: templated growth in brown (44, 45, 50), directional evaporation/shear in green (47, 48, 51, 59, 60), sedimentation in blue (, –63), and triggered self-assembly in solvent in black (49, 64).

Fig. 4.

Mechanical macroscopic behavior of a-C³. (A) Stress–strain curves for a-C³ composites tested in bending with two polymer interfaces. (Inset) Stress–strain curves of the pure polymers used as interfaces in the composites tested in tension. (B) SEM images of the zone in tension taken at different strains during in situ bending test of a-C³|PBMA:PMMA. Cracks are highlighted in teal. (Scale bar 20 µm.) (C) Image of the a-C³|PBMA:PMMA after bending test. (D) FFT of the cracks network visible in the bottom part of the sample. (E) R-curve measured from single-edge notch bending tests of a-C³|PBMA:PMMA. Grayed symbols represent values beyond the ASTM-recommended crack extension limit. Each teal hue represents a sample tested. Gray lines represent the toughness of PBMA:PMMA and of silica. (F) SEM of the crack front taken at different crack lengths during the in situ fracture test. Microcracked area highlighted in teal. (G) Stress intensity factors of particulate composites $K_I$ or $K_J$ normalized with the polymer toughness $K^{\mathit{polymer}}$ for a-C³|PBMA:PMMA and composites made by mixing and centrifuging rods at various volume fractions of silica rods, compared with particulate composites made in the literature from mixing glass fibers in polymer resins (68, 69), centrifugation (70), and heat assisted slip casting (71).