[Show abstract][Hide abstract] ABSTRACT: We report the synthesis and nanostructural development of polycrystalline and single crystalline LiFePO4 (LFP) nanostructures using a solvothermal media (i.e., water–tri(ethylene glycol) mixture). Crystal phase and growth behavior were monitored by powder and synchrotron X-ray diffraction, as well as transmission electron microscopy (TEM), while particle morphologies were examined using scanning electron microscopy (SEM). Initially, thin (100 nm) platelets of Fe3(PO4)2·8H2O (vivianite, VTE) formed at short reaction times followed by the nucleation of LFP (20 nm particles) on the metastable VTE surfaces. Upon decrease in pH, primary LFP nanocrystals subsequently aggregated into polycrystalline diamond-like particles via an oriented attachment (OA). With increasing reaction time, the solution pH further decreased, leading to a dissolution–recrystallization process (i.e., Ostwald ripening, OR) of the oriented polycrystalline LFP particles to yield evenly sized, single crystalline LiFePO4. Samples prepared at short reaction durations demonstrated a larger discharge capacity at higher rates compared with the single crystalline particles. This is due to the small size of the primary crystallites within larger secondary LiFePO4 particles, which reduced the lithium ion diffusion path while subsequently maintaining a high tap density. Understanding the relationship between solution conditions and nanostructural development as well as performance revealed by this study will help to develop synthetic guidelines to enable efficient lithium ion battery performance.
[Show abstract][Hide abstract] ABSTRACT: During mineralization, the hard outer magnetite-containing shell of the radular teeth of Cryptochiton stelleri undergoes four distinct stages of structural and phase transformations: (i) the formation of a crystalline α-chitin organic matrix that forms the structural framework of the non-mineralized teeth, (ii) the templated synthesis of ferrihydrite crystal aggregates along these organic fibers, (iii) subsequent solid state phase transformation from ferrihydrite to magnetite, and (iv) progressive magnetite crystal growth to form continuous parallel rods within the mature teeth. The underlying α-chitin organic matrix appears to influence magnetite crystal aggregate density and the diameter and curvature of the resulting rods, both of which likely play critical roles in determining the local mechanical properties of the mature radular teeth.
[Show abstract][Hide abstract] ABSTRACT: Systematic changes in the exocyclic substiution of core phthalocyanine platform tune the absorption properties to yield commercially viable dyes that function as the primary light absorbers in organic bulk heterojunction solar cells. Blends of these complementary phthalocyanines absorb a broader portion of the solar spectrum compared to a single dye, thereby increasing solar cell performance. We correlate grazing incidence small angle x-ray scattering structural data with solar cell performance to elucidate the role of nanomorphology of active layers composed of blends of phthalocyanines and a fullerene derivative. A highly reproducible device architecture is used to assure accuracy and is relevant to films for solar windows in urban settings. We demonstrate that the number and structure of the exocyclic motifs dictate phase formation, hierarchical organization, and nanostructure, thus can be employed to tailor active layer morphology to enhance exciton dissociation and charge collection efficiencies in the photovoltaic devices. These studies reveal that disordered films make better solar cells, short alkanes increase the optical density of the active layer, and branched alkanes inhibit unproductive homogeneous molecular alignment.
Journal of Materials Chemistry A 02/2013; 1(5):1557-1565. DOI:10.1039/C2TA00415A · 7.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Collagen in vertebrate animals is often arranged in alternating lamellae or in bundles of aligned fibrils which are designed to withstand in vivo mechanical loads. The formation of these organized structures is thought to result from a complex, large-area integration of individual cell motion and locally-controlled synthesis of fibrillar arrays via cell-surface fibripositors (direct matrix printing). The difficulty of reproducing such a process in vitro has prevented tissue engineers from constructing clinically useful load-bearing connective tissue directly from collagen. However, we and others have taken the view that long-range organizational information is potentially encoded into the structure of the collagen molecule itself, allowing the control of fibril organization to extend far from cell (or bounding) surfaces. We here demonstrate a simple, fast, cell-free method capable of producing highly-organized, anistropic collagen fibrillar lamellae de novo which persist over relatively long-distances (tens to hundreds of microns). Our approach to nanoscale organizational control takes advantage of the intrinsic physiochemical properties of collagen molecules by inducing collagen association through molecular crowding and geometric confinement. To mimic biological tissues which comprise planar, aligned collagen lamellae (e.g. cornea, lamellar bone or annulus fibrosus), type I collagen was confined to a thin, planar geometry, concentrated through molecular crowding and polymerized. The resulting fibrillar lamellae show a striking resemblance to native load-bearing lamellae in that the fibrils are small, generally aligned in the plane of the confining space and change direction en masse throughout the thickness of the construct. The process of organizational control is consistent with embryonic development where the bounded planar cell sheets produced by fibroblasts suggest a similar confinement/concentration strategy. Such a simple approach to nanoscale organizational control of structure not only makes de novo tissue engineering a possibility, but also suggests a clearer pathway to organization for fibroblasts than direct matrix printing.
[Show abstract][Hide abstract] ABSTRACT: Nature has evolved efficient strategies to synthesize complex mineralized structures that exhibit exceptional damage tolerance. One such example is found in the hypermineralized hammer-like dactyl clubs of the stomatopods, a group of highly aggressive marine crustaceans. The dactyl clubs from one species, Odontodactylus scyllarus, exhibit an impressive set of characteristics adapted for surviving high-velocity impacts on the heavily mineralized prey on which they feed. Consisting of a multiphase composite of oriented crystalline hydroxyapatite and amorphous calcium phosphate and carbonate, in conjunction with a highly expanded helicoidal organization of the fibrillar chitinous organic matrix, these structures display several effective lines of defense against catastrophic failure during repetitive high-energy loading events.
[Show abstract][Hide abstract] ABSTRACT: Objective: To determine whether material properties influence stem cell differentiation.
Method: C3H10T1/2-derived progenitor cells genetically engineered to express the differentiation inducer rhBMP-2, under control of the Doxycycline (Dox)-repressible promoter, Tet-Off, were plated on spun cast films of Polybutadiene (PB) and partially Sulfonated polystyrene (PSS28) polymers, whose moduli are 1.4MPa and 3 GPa, respectively. In order to probe the influence of mechanical cues, PB films, 20nm and 200nm, whose moduli vary by a factor 4, were used. The role of chemistry was probed by plating on PSS28 and Tissue Culture Petri-Dish. The cells were grown in DMEM, supplemented with 10% fetal-bovine serum, 0.2mM L-ascorbic acid 2-phosphate, 2mm glutamine, 10mM beta-glycerol phosphate 100 U/ml Penicillin, and 100ug/ml Streptomycin with or without 1 µg/mL Dox. The substrates were prepared by spin casting monodispersed PB or PSS28 films on HF treated Si wafers, which were then annealed in at P~10-6 and P=10 -3 Torr at T=170 °C. Differentiation was monitored over a 21 day period and biomineralization was characterized by SEM, EDAX, and qPCR.
Result: We show that the C3H10T1/2-derived progenitor cells expressing rhBMP-2, when grown on PB (20 or 200 nm), fail to show significant biomineralization even up to 21 days and express aggrecan typical of chondrogenesis. In contrast, those grown on SPS (200 nm) biomineralize as early as day 14. These cells do not express aggrecan but rather the osteogenic marker, bone sialoprotein. Addition of Doxycycline prevented biomineralization on all surfaces. Cells expressing rhBMP-2 had increased mRNA expression of collagen X, osteocalcin, alkaline phosphatase and osterix, as compared to the C3H10T1/2 and those grown with Doxycycline, regardless of the polymer substrate surface.
Conclusion: Surface chemistry can modify the response to rhBMP-2 in C3H10T1/2-derived progenitor-cells, and participate in the control of endochondral bone formation.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the effects of moderate static magnetic fields (SMFs) on murine MC3T3-E1 osteoblasts, and found that they enhance proliferations and promote differentiation. The increase in proliferation rates in response to SMFs was greater in cultures grown on partially sulfonated polytstyrene (SPS, degree of sulfonation: 33%) than in cultures grown on tissue culture plastic. We have previously shown that when the degree of sulfonation exceeded a critical value (12%) , spontaneous fibrillogenesis occured which allowed for direct observation of the ECM fibrillar organization under the influence of external fields. We found that the ECM produced in cultures grown on the SPS in the presence of the SMFs assembled into a lattice with larger dimensions than the ECM of the cultures grown in the absence of SMFs. During the early stages of the biomineralization process (day 7), the SMF exposed cultures also templated mineral deposition more rapidly than the control cultures. The rapid response is attributed to orientation of diamagnetic ECM proteins already present in the serum, which could then initiate further cellular signaling. SMFs also influenced late stage osteoblast differentiation as measured by the increased rate of osteocalcin secretion and gene expression beginning 15 days after SFM exposure. This correlated with a large increase in mineral deposition, and in cell modulus. GIXD and EDXS analysis confirmed early deposition of crystalline hydroxyapatite. Previous studies on the effects of moderate SMF had focused on cellular gene and protein expression, but did not consider the organization of the ECM fibers. Our ability to form these fibers has allowed us explore this additional effect and highlight its significance in the initiation of the biomineralization process.
[Show abstract][Hide abstract] ABSTRACT: Bone is a hierarchically structured composite which imparts it with unique mechanical properties and bioresorptive potential. These properties are primarily influenced by the underlying nanostructure of bone, which consists of nanocrystals of hydroxyapatite embedded and uniaxially aligned within collagen fibrils. There is also a small fraction of non-collagenous proteins in bone, and these are thought to play an important role in bone's formation. In our in vitro model system of bone formation, polyanionic peptides are used to mimic the role of the non-collagenous proteins. In our prior studies, we have shown that intrafibrillar mineralization can be achieved in synthetic reconstituted collagen sponges using a polymer-induced liquid-precursor (PILP) mineralization process. This led to a nanostructured arrangement of hydroxyapatite crystals within the individual fibrils which closely mimics that of bone. This report demonstrates that biogenic collagen scaffolds obtained from turkey tendon, which consist of densely packed and oriented collagen fibrils, can also be mineralized by the PILP process. Synchrotron X-ray diffraction studies show that the mineralization process leads to a high degree of crystallographic orientation at the macroscale, thus emulating that found in the biological system of naturally mineralizing turkey tendon.
[Show abstract][Hide abstract] ABSTRACT: All-conjugated block copolymers have significant potential for solution-processed optoelectronic applications, in particular those relying on a p/n junction. Herein, we report the synthesis and structure of all-conjugated diblock copolymers poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) and poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene-co-benzothiadiazole) in thin films and in the bulk. The diblock copolymers are prepared using a combination of Grignard metathesis polymerization and Suzuki polycondensation and characterized with NMR spectroscopy, size-exclusion chromatography, multiangle laser light scattering, and UV/vis spectroscopy. Structure in thin films and in the bulk is characterized using differential scanning calorimetry, X-ray diffraction, small-angle X-ray scattering, and atomic force microscopy. Diblock copolymer thin films self-assemble into a crystalline nanostructure with some long-range order after extended solvent annealing, and X-ray scattering measurements show that powder samples exhibit crystallinity throughout the bulk. By temperature dependent X-ray scattering measurements, we find that diblock copolymers self-assemble into crystalline nanowires with phase segregated block copolymer domains. These measurements show all-conjugated diblock copolymers may be useful for achieving solution-processed active layers in organic photovoltaics and light-emitting diodes with optimized structural and photophysical characteristics.
[Show abstract][Hide abstract] ABSTRACT: Engineering and Applied Sciences We present the first resonant x-ray reflectivity measurements from a liquid surface. The surface structure of the liquid Hg-Au alloy system just beyond the solubility limit of 0.14at% Au in Hg had previously been shown to exhibit a unique surface phase characterized by a low-density surface region with a complicated temperature dependence. In this paper we present reflectivity measurements near the Au LII1 edge, for 0.2at% Au in Hg at room temperature. The data are consistent with a concentration of Au in the surface region that can be no larger than about 30at%. These results rule out previous suggestions that pure Au layers segregate at the alloy surface.
[Show abstract][Hide abstract] ABSTRACT: Chitons are marine mollusks found worldwide in the intertidal or subtidal zones of cold water as well as in tropical waters. These organisms have evolved an amazing feeding structure called a Radula. The Radula is a ribbon-like structure that consists of abrasion resistant teeth anchored to a flexible stylus that the organism uses to abrade rocky substrates to reach endolithic and epilithic algae. In this work, we investigate the structure and mineralization process in Cryptochiton stelleri, the largest of the chitons. Using various microscopy and spectroscopy techniques as well as synchrotron analyses, we have uncovered critical structure-function relationships in the mineralized teeth as well as insights into the mineralization processes in these unique structures. Investigation of the mechanical properties of the fully mineralized teeth have revealed that the combination of ultrahard minerals and templating organics, architected in a unique microstructure, lead to a damage tolerant composite that is of the hardest known biominerals known in nature.
[Show abstract][Hide abstract] ABSTRACT: The extracellular matrix (ECM) is composed of mixed protein fibers whose precise composition affects biomineralization. New methods are needed to probe the interactions of these proteins with calcium phosphate mineral and with each other. Here we follow calcium phosphate mineralization on protein fibers self-assembled in vitro from solutions of fibronectin, elastin and their mixture. We probe the surface morphology and mechanical properties of the protein fibers during the early stages. The development of mineral crystals on the protein matrices is also investigated. In physiological mineralization solution, the elastic modulus of the fibers in the fibronectin-elastin mixture increases to a greater extent than that of the fibers from either pure protein. In the presence of fibronectin, longer exposure in the mineral solution leads to the formation of amorphous calcium phosphate particles templated along the self-assembled fibers, while elastin fibers only collect calcium without any mineral observed during early stage. TEM images confirm that small needle-shape crystals are confined inside elastin fibers which suppress the release of mineral outside the fibers during late stage, while hydroxyapatite crystals form when fibronectin is present. These results demonstrate complementary actions of the two ECM proteins fibronectin and elastin to collect cations and template mineral, respectively.
[Show abstract][Hide abstract] ABSTRACT: Recent analyses of the ultrastructural and mechanical properties of mineralized biological materials have demonstrated some common architectural features that can help explain their observed damage tolerance. Nature has accomplished this feat through the precise control of anisotropic crystal nucleation and growth processes in conjunction with nanoscale control over the self-assembly of spatially distinct organic and inorganic phases, resulting in effective inhibition of crack propagation through these materials. One such example is found in the hyper-mineralized and abrasion resistant radular teeth of the chitons, a group of herbivorous marine mollusks who have the surprising capacity to erode away the rocky substrates on which they graze. Through the use of modern microscopy and nanomechanical characterization techniques, we describe the architectural and mechanical properties of the radular teeth from Cryptochiton stelleri. Chiton teeth are shown to exhibit the largest hardness and stiffness of any biominerals reported to date, being notably as much as three-fold harder than human enamel and the calcium carbonate-based shells of mollusks. We explain how the unique multi-phasic design of these materials contributes not only to their functionality, but also highlights some interesting design principles that might be applied to the fabrication of synthetic composites.
[Show abstract][Hide abstract] ABSTRACT: The details of air nanobubble trapping at the interface between water and a nanostructured hydrophobic silicon surface are investigated using X-ray scattering and contact angle measurements. Large-area silicon surfaces containing hexagonally packed, 20 nm wide hydrophobic cavities provide ideal model surfaces for studying the morphology of air nanobubbles trapped inside cavities and its dependence on the cavity depth. Transmission small-angle X-ray scattering measurements show stable trapping of air inside the cavities with a partial water penetration of 5-10 nm into the pores, independent of their large depth variation. This behavior is explained by consideration of capillary effects and the cavity geometry. For parabolic cavities, the liquid can reach a thermodynamically stable configuration-a nearly planar nanobubble meniscus-by partially penetrating into the pores. This microscopic information correlates very well with the macroscopic surface wetting behavior.
[Show abstract][Hide abstract] ABSTRACT: Liquid crystal (LC) elastomers with bent-core side-groups incorporate the properties of bent-core liquid crystals in a flexible and self-supporting polymer network. Bent-core liquid crystal elastomers (BCEs) with uniform alignment were prepared by attaching a reactive bent-core LC to poly(hydrogenmethylsiloxane) and crosslinking with a divinyl crosslinker. Phase behavior studies indicate a nematic phase over a wide temperature range that approaches room temperature, and thermoelastic measurements show that these BCEs can reversibly change their length by more than a factor of two upon heating and cooling. Small-angle X-ray scattering studies reveal multiple, broad low-angle peaks consistent with short-range smectic C order of the bent-core side groups. A comparison of these patterns with predictions of a Landau model for short-range smectic C order shows that the length scale for smectic ordering in BCEs is similar to that seen in pure bent-core LCs. The combination of rubber elasticity and smectic ordering of the bent-core side groups suggests that BCEs may be promising materials for sensing, actuating, and other advanced applications.
[Show abstract][Hide abstract] ABSTRACT: Small angle X-ray diffraction from the uniaxial nematic phase of certain bent-core liquid crystals is shown to be consistent with the presence of molecular clusters possessing short-range tilted smectic (smectic-C) order. Persistence of these clusters throughout the nematic phase, and even into the isotropic state, likely accounts for the unusual macroscopic behavior previously reported in bent-core nematics, including an anomalously large flexoelectric effect ( 1000 times that of conventional calamitic nematics), very large orientational and flow viscosities ( 10–100 and 100–1000 times, respectively, typical values for calamitics), and an extraordinary flow birefringence observed in the isotropic state.
[Show abstract][Hide abstract] ABSTRACT: Current methods for synthesizing impact-tolerant and abrasion-resistant materials are traditionally inefficient and costly and often require the use of environmentally hazardous components and processes. In stark contrast to their industrial counterparts, however, biological systems are well known for their ability to synthesize a wide range of high performance composites at ambient temperatures and pressures, and near neutral pH without the use of caustic precursors of byproducts. One such example is found in the mineralized teeth of the chitons, a group of benthic marine invertebrates common along the North American Pacific Coast. The teeth are anchored to a flexible belt like structure, the radula that is used for scraping algae from rocks, on which the chitons feed. Because of their constant rasping motion, the teeth must be specifically adapted to persist under such harsh conditions.
Elemental mapping via Energy Dispersive Spectroscopy (EDS) in conjunction with electron and X-ray diffraction have revealed that each tooth is composed of two dominant biominerals, (an amorphous ferrous phosphate core and a thick magnetite veneer) that are intimately associated with the tooth organic matrix. Backscattered electron microscopy has also been used to investigate the interfaces between these two mineral phases and their roles in ultimately affecting the mechanical properties of the teeth. High-resolution imaging of this interface reveals that the transition between the two mineral phases is not abrupt as one would typically encounter in synthetic multi-material composites. Following mechanical loading of the teeth, cracks propagating through the core phase are deflected laterally as they encounter the harder magnetite phase, revealing the fact that not only is this architecture specifically adapted for abrasion resistance, but is also very effective in preventing the propagation of large cracks originating form contact-induced surface defects. Nanoindentation of the two mineral phases reveals that there is a gradual 4-fold increase in modulus from the core to the magnetite periphery of each tooth, a design strategy that has been shown through both experimental and modeling approaches to be very effective in increasing fracture toughness of related composite materials via crack deflection, without the problems associated with delamination of the two phases via complications arising from modulus mismatch. The radular teeth thus represent an excellent model system for investigating the properties of mechanically graded materials and future investigation will be aimed at elucidating the various stages of tooth maturation and mineralization. It is hoped that in the not too distant future, this and related research into the structural complexities of biological systems may ultimately guide the fabrication of a new generation of high performance synthetic materials for a wide range of technologically relevant applications.
[Show abstract][Hide abstract] ABSTRACT: The field-induced transitions between ferri-, antiferro-, and ferroelectric liquid crystal phases are interesting because although there are only small thermodynamic differences between them, each of these phases has different electrical and optical properties. We report an irreversible field-induced transition from an antiferroelectric phase to the ferrielectric phase in a liquid crystal device, and compare it to a system in which the transition is reversible. The two systems differ mainly in their spontaneous polarization (120 nC cm−2 for the former and 60 nC cm−2 for the latter) while the optical tilt is comparable (29° and 25°, respectively). We explain the observed transitions based on the relative magnitudes of the discrete flexoelectric and spontaneous polarizations.
[Show abstract][Hide abstract] ABSTRACT: Electrospinning of polymeric fibers has been attracted increased interest in recent years. However, the research for ethylene-vinyl acetate (EVA) and linear polyethylene (PE) is still limited, due to their relatively poor solubility in conventional solvent systems at ambient temperature. In this study, EVA and PE fibers were electrospun with different fiber diameter when the electrospinning solution was kept at a temperature greater than that of the solidification temperature of the polymer solutions. The effects of the fiber physical dimension to its crystallization and mechanical properties were thus detected. The morphology of the fibers was measured by scanning electron microscope (SEM) and atomic force microscope (AFM). The shear modulation force microscopy technique (SMFM) was used to measure the melting point, Tm, which was found to increase with increased fiber diameter and crystallinity. AFM three-point bending test demonstrated that the Young's modulus of the fibers drastically increased as fiber diameter decreased.Grazing-incidence small angle x-ray scattering (GISAX) showed that, compared to the bulk material, the crystallinity of the electrospun fibers had been changed.