Emily Asenath-Smith

Hierarchical Porous Polymer Scaffolds from Block Copolymers

A Complicated Scaffold, Simply Materials with tailored pore structures can be useful as catalysis supports and for lightweight materials. When preparing medical scaffolds, restrictive preparation conditions have to be met, which can prohibit multistep preparation procedures. Sai et al. (p. 530) describe a method for making porous polymers containing both relatively large (several microns) interconnecting pores and a second population of ∼ tens of nanometer pores. The process exploits spinodal decomposition of a block copolymer blended with small-molecule additives and requires a simple washing step with water, methanol, or ethanol. Spinodal decomposition of block copolymers and oligomeric additives produces three-dimensional hierarchical porous polymers. Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems. Here, we introduce a synthesis method combining well-established concepts of macroscale spinodal decomposition and nanoscale block copolymer self-assembly with porosity formation on both length scales via rinsing with protic solvents. We used scanning electron microscopy, small-angle x-ray scattering, transmission electron tomography, and nanoscale x-ray computed tomography for quantitative pore-structure characterization. The method was demonstrated for AB- and ABC-type block copolymers, and resulting materials were used as scaffolds for calcite crystal growth.

Hierarchically Structured Hematite Architectures Achieved by Growth in a Silica Hydrogel

Biomineralization strategies include the use of hydrogels to direct the formation of composite, single-crystal-like structures with unique structure-property profiles. Application of similar synthetic approaches to transition-metal oxides has the promise to yield low-temperature routes to hierarchically structured crystals that are optimized for a range of applications. Here, growth of hematite (α-Fe2O3) within a silica hydrogel resulted in hierarchical, mosaic crystals preferentially expressing catalytically active {110} facets, which are absent in solution-grown controls. Quantitative structural and compositional analysis reveals architectural changes that begin with the incorporation of silicon into the hematite lattice and propagate through to the nanoscale domain structure and assembly, leading to microscale morphologies that show improved photocatalytic performance. This work demonstrates the potential of applying bioinspired crystallization techniques to design functional oxides with multiscale architectural features.

Sectioning of Individual Hematite Pseudocubes with Focused Ion Beam Enables Quantitative Structural Characterization at Nanometer Length Scales

A dual-beam focused ion beam microscope equipped with a nanomanipulator was used to fabricate slices from within individual hematite (α-Fe2O3) pseudocubes with selected orientations with respect to the original pseudocubes. Transmission electron microanalysis through selected area electron diffraction enabled assignment of each thin section to a particular zone of the hematite lattice. While the pseudocubes are composed of numerous crystallites, 25-50 nm in size, they are not simply polycrystalline particles. Electron diffraction of thin sections showed that while the pseudocubic hematite particles are composed of numerous coherent domains, the individual thin sections display a net crystallographic orientation to the underlying hematite lattice. Quantitative analysis of the lattice misorientation between coherent domains was calculated from the azimuthal spread of electron diffraction peaks and is consistent with a structure that contains small-angle grain boundaries. Based upon this analysis, we conclude that the pseudocubic hematite particles are mosaic crystals, composed of highly oriented coherent domains.

Revealing the Internal Structure and Local Chemistry of Nanocrystals Grown in Hydrogel with Cryo-FIB Lift-Out and Cryo-STEM

Hydrogels, three-dimensional polymeric networks with entrapped solvents, have gained increasing interest in a number of fields, including novel crystal synthesis. Compared to solution-based processes, crystal growth in hydrogels opens new routes to controlling morphology and function. Additionally, hydrogels have found applications in biomedical and biological research due to their biomimetic properties, which allow them to imitate the conditions surrounding cells. Understanding processes in hydrogels requires gaining access to their internal structures. Commonly, this requires removal of the liquid from the sample, as the hydrogel will dehydrate upon entering the vacuum of the microscope. Artifacts due to drying, however, can prevent imaging of the samples’ native structure.

Mosaic anisotropy model for magnetic interactions in mesostructured crystals

We propose a new model for interpreting the magnetic interactions in crystals with mosaic texture called the mosaic anisotropy (MA) model. We test the MA model using hematite as a model system, comparing mosaic crystals to polycrystals, single crystal nanoparticles, and bulk single crystals. Vibrating sample magnetometry confirms the hypothesis of the MA model that mosaic crystals have larger remanence (Mr/Ms) and coercivity (Hc) compared to polycrystalline or bulk single crystals. By exploring the magnetic properties of mesostructured crystalline materials, we may be able to develop new routes to engineering harder magnets.