[show abstract][hide abstract] ABSTRACT: Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation migration allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N2). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO2 over CH4 reported to date with important application to natural gas purification.
Journal of the American Chemical Society 10/2012; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The healing of a mendable epoxy resin containing polyethylene-co-methacrylic acid (EMAA) particles has been confirmed to occur via a pressure delivery mechanism of the healing agent, EMAA, into a crack plane during thermal activation. Internal pressure rises within a bubble, formed from interactions between EMAA particles causing expansion of the bubble which subsequently force the healing agent to flow into an available cavity. The use of X-ray ultra-microscopy (Xum), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) provides evidence for volatile formation, bubble expansion and delivery of healing agent. Spectroscopic studies of the interfacial reactions between epoxy and EMAA during post-cure, compared with adhesion and fracture toughness measurements, reveal that hydroxyl acid reactions catalysed via tertiary amine initiate the pressure delivery mechanism. Furthermore, adhesion and FTIR measurements suggest that the re-binding or healing of a crack interface is likely to be dominated by hydrogen bonding.
European Polymer Journal 01/2012; 48:524-531. · 2.56 Impact Factor
[show abstract][hide abstract] ABSTRACT: Resilin is an important elastomeric protein of insects, with roles in the storage and release of energy during a variety of different functional categories including flight and jumping. To date, resilin genes and protein function have been characterised only in a small number of flying insects, despite their importance in fleas and other jumping insects. Microscopy and immunostaining studies of resilin in flea demonstrate the presence of resilin pads in the pleural arch at the top of the hind legs, a region responsible for the flea's jumping ability. A degenerate primer approach was used to amplify resilin gene transcripts from total RNA isolated from flea (Ctenocephalides felis), buffalo fly (Haematobia irritans exigua) and dragonfly (Aeshna sp.) pharate adults, and full-length transcripts were successfully isolated. Two isoforms (A and B) were amplified from each of flea and buffalo fly, and isoform B only in dragonfly. Flea and buffalo fly isoform B transcripts were expressed in an Escherichia coli expression system, yielding soluble recombinant proteins Cf-resB and Hi-resB respectively. Protein structure and mechanical properties of each protein before and after crosslinking were assessed. This study shows that resilin gene and protein sequences are broadly conserved and that crosslinked recombinant resilin proteins share similar mechanical properties from flying to jumping insects. A combined use of degenerate primers and polyclonal sera will likely facilitate characterisation of resilin genes from other insect and invertebrate orders.
[show abstract][hide abstract] ABSTRACT: The outstanding rubber-like elasticity of resilin and resilin-mimetic proteins depends critically on the level of hydration. In this investigation, water vapor sorption and the role of hydration on the molecular chain dynamics and viscoelastic properties of resilin-mimetic protein, rec1-resilin is investigated in detail. The dynamic and equilibrium swelling behavior of the crosslinked protein hydrogels with different crosslink density are reported under various controlled environments. We propose three different stages of hydration; involving non-crystallizable water, followed by condensation or clustering of water around the already hydrated sites, and finally crystallizable water. The kinetics of water sorption for this engineering protein is observed to be comparable to hydrophilic polymers with a diffusion coefficient in the range of 10(-7) cm(2) s(-1). From the comparison between the absorption and desorption isotherms at a constant water activity, it has been observed that rec1-resilin exhibits sorption hysteresis only for the tightly bound water. Investigation of molecular mobility using differential scanning calorimetry, indicates that dehydrated crosslinked rec1-resilin is brittle with a glass transition temperature (T(g)) of >180 °C, which dramatically decreases with increasing hydration; and above a critical level of hydration rec1-resilin exhibits rubber-like elasticity. Nanoindentation studies show that even with little hydration (<10%), the mechanical properties of rec1-resilin gels change dramatically. Rheological investigations confirm that the equilibrium-swollen crosslinked rec1-resilin hydrogel exhibits outstanding elasticity and resilience of ∼ 92%, which exceeds that of any other synthetic polymer and biopolymer hydrogels.
[show abstract][hide abstract] ABSTRACT: Here we present a new type of approach for the mass production of metallorganic frameworks (MOFs). In particular, a heterogeneous process for MOF-5 growth involving amino functionalized silica nanoparticles is presented. The proposed sol-gel approach provides a promising route toward the large-scale production of colloidal MOFs by leveraging the enormous surface areas offered by nano-sized seeds used as nucleating agents. The synthesis of the ceramic seeds (SiO2 NPs) is performed at room temperature and considerable volumes of MOF crystals can be produced in 1/10 of the time required for the conventional solvothermal method. The growth of the crystals has been observed with optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and the crystal quality has been verified using X-ray powder diffraction.
IOP Conference Series Materials Science and Engineering 05/2011; 18(5):052006.
[show abstract][hide abstract] ABSTRACT: With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device fabrication, well-organized or patterned MOF growth is required, and thus conventional synthetic routes are not suitable. Moreover, to expand their applicability, the introduction of additional functionality into MOFs is desirable. Here, we explore the use of nanostructured poly-hydrate zinc phosphate (α-hopeite) microparticles as nucleation seeds for MOFs that simultaneously address all these issues. Affording spatial control of nucleation and significantly accelerating MOF growth, these α-hopeite microparticles are found to act as nucleation agents both in solution and on solid surfaces. In addition, the introduction of functional nanoparticles (metallic, semiconducting, polymeric) into these nucleating seeds translates directly to the fabrication of functional MOFs suitable for molecular size-selective applications.
[show abstract][hide abstract] ABSTRACT: Two novel recombinant proteins An16 and Dros16 have recently been generated. These recombinant proteins contain, respectively, sixteen copies of an 11 amino acid repetitive domain (AQTPSSQYGAP) observed in a resilin-like gene from Anopheles gambiae and sixteen copies of a 15 amino acid repetitive domain (GGRPSDSYGAPGGGN) observed in the first exon of the Drosophila melanogaster CG15920 gene. We compare structural characteristics of the proteins and material properties of resulting biopolymers relative to Rec1-resilin, a previously characterized resilin-like protein encoded by the first exon of the Drosophila melanogaster CG15920 gene. While the repetitive domains of natural resilins display significant variation both in terms of amino acid sequence and length, our synthetic polypeptides have been designed as perfect repeats. Using techniques including circular dichroism, atomic force microscopy, and tensile testing, we demonstrate that both An16 and Dros16 have similar material properties to those previously observed in insect and recombinant resilins. Modulus, elasticity, resilience, and dityrosine content in the cross-linked biomaterials were assessed. Despite the reduced complexity of the An16 and Dros16 proteins compared to natural resilins, we have been able to produce elastic and resilient biomaterials with similar properties to resilin.
[show abstract][hide abstract] ABSTRACT: It has been widely accepted that the gas diffusion through a glassy polymer can be related to the fractional free volume of the polymer through the Doolittle relation D = A exp(−B/f) where f is the fractional free volume and A and B are constants. As the free volume increases and pores become connected and bi-continuous the Doolittle relation does not adequately model the experimental data. By adding newly available high free volume polymers to Park and Paul's large database of 105 polymers, an empirically determined relation of the form D = α exp(βf), with α and β as constants, is shown to model all of the data well. The new relation also agrees with a variety of computer simulations. Plausible reasons for the better fit of this new relation over a wide range of f are postulated. The practical utility of the new relation is that it can be used as an efficient tool for predicting transport properties in the wide range of available polymers based on one readily obtainable material characteristic, namely, fractional free volume. In addition, in Part II of this work the new relation is shown to provide an exact solution to the vacancy diffusion equation allowing thin film physical aging to be accurately and easily modelled.
[show abstract][hide abstract] ABSTRACT: A new concept is described for methane and hydrogen storage materials involving the incorporation of magnesium-decorated fullerenes within metal-organic frameworks (MOFs). The system is modeled using a novel approach underpinned by surface potential energies developed from Lennard-Jones parameters. Impregnation of MOF pores with magnesium-decorated Mg(10)C(60) fullerenes, denoted as Mg-C(60)@MOF, places exposed metal sites with high heats of gas adsorption into intimate contact with large surface area MOF structures. Perhaps surprisingly, given the void space occupied by C(60), this impregnation delivers remarkable gas uptake, according to our modeling, which predicts exceptional performance for the Mg-C(60)@MOF family of materials. These predictions include a volumetric methane uptake of 265 v/v, the highest reported value for any material, which significantly exceeds the U.S. Department of Energy target of 180 v/v. We also predict a very high hydrogen adsorption enthalpy of 11 kJ mol(-1) with relatively little decrease as a function of H(2) filling. This value is close to the calculated optimum value of 15.1 kJ mol(-1) and is achieved concurrently with saturation hydrogen uptake in large amounts at pressures under 10 atm.
Journal of the American Chemical Society 08/2009; 131(30):10662-9. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nearly 800 cultures from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Collection of Living Microalgae (CCLM) were screened for exopolysaccharide (EPS) production by examining the viscosity of conditioned media supernatants. We first established dose-response relationships for the viscosity of reference polysaccharides dissolved in microalgal growth media. Then, using the 40 most viscous CCLM cultures, we confirmed that the viscosity of culture supernatants correlated well with their total sugar (and thus EPS) content. The conditioned medium with the highest viscosity (6.55 cP, equivalent to 1.16 g/L xanthan gum) was produced by a non-axenic isolate of the cyanobacterium Microcystis aeruginosa f. flos-aquae. Two types of bacteria, designated CSIRO501 (Gram-positive) and CSIRO505 (Gram-negative), were subcultured from it. At 20 dry wt% sugar, an exopolymer preparation from CSIRO505 contained substantially more EPS than its counterparts from CSIRO501 or from axenic Microcystis cultures, and it resembled a well-known bacterial EPS (xanthan gum) in being much more effective at bonding wood than PVC. It bonded wooden lap joints with a dry shear strength of 1.5 MPa, four times the value obtained in equivalent tests of a commercial polyvinyl acetate (PVA) glue.
[show abstract][hide abstract] ABSTRACT: It has been suggested that physical aging is a result of vacancies (cavities, holes or pores) travelling to the external surface where they disappear and therefore cause density increase. The Doolittle relation D=Aexp(−B/f) where f is the fractional free volume and A and B are constants, has been used in previous work to describe the diffusion coefficient D (diffusivity) for vacancy transport. In part I a new empirically determined relation has been suggested as an alternative to the Doolittle relation, and this new relation has been shown to accurately model gas permeability data over a wide range of polymeric free volume. This new relation takes the form D=αexp(βf) with α and β as constants. Here we show that when the Doolittle relation is replaced with the new relation an exact analytical solution exists to the differential equation that governs the fractional free volume behaviour throughout the sample during physical aging leading to an Empirically derived Vacancy Diffusion (EVD) model for physical aging. An approximate analytical solution based on the exact solution is then compared to experimental data and other popular models such as the Kovacs, Aklonis, Hutchinson and Ramos phenomenological model and the Curro, Lagasse and Simha vacancy diffusion model. This EVD model is also combined with a lattice contraction model to form a dual lattice contraction and vacancy diffusion model which is compared with McCaig, Paul and Barlow's experimental results, showing a good correlation. Further support for the new EVD model is revealed by its similarity with the early established constitutive kinetic equation. Previous aging models are complicated and difficult to implement, therefore, a model that is easy to implement and physically meaningful such as this EVD model is highly sought after. An application of the model reveals that vacancy diffusion to the external surface is the dominant aging mechanism within polysulfone thin films (1μm).
Journal of Membrane Science - J MEMBRANE SCI. 01/2009; 338(1):38-42.
[show abstract][hide abstract] ABSTRACT: Proresilin is the precursor protein for resilin, an extremely elastic, hydrated, cross-linked insoluble protein found in insects. We investigated the secondary-structure distribution in solution of a synthetic proresilin (AN16), based on 16 units of the consensus proresilin repeat from Anopheles gambiae. Raman spectroscopy was used to verify that the secondary-structure distributions in cross-linked AN16 resilin and in AN16 proresilin are similar, and hence that solution techniques (such as NMR and circular dichroism) may be used to gain information about the structure of the cross-linked solid. The synthetic proresilin AN16 is an intrinsically unstructured protein, displaying under native conditions many of the characteristics normally observed in denatured proteins. There are no apparent alpha-helical or beta-sheet features in the NMR spectra, and the majority of backbone protons and carbons exhibit chemical shifts characteristic of random-coil configurations. Relatively few peaks are observed in the nuclear Overhauser effect spectra, indicating that overall the protein is dynamic and unstructured. The radius of gyration of AN16 corresponds to the value expected for a denatured protein of similar chain length. This high degree of disorder is also consistent with observed circular dichroism and Raman spectra. The temperature dependences of the NH proton chemical shifts were also measured. Most values were indicative of protons exposed to water, although smaller dependences were observed for glycine and alanine within the Tyr-Gly-Ala-Pro sequence conserved in all resilins found to date, which is the site of dityrosine cross-link formation. This result implies that these residues are involved in hydrogen bonds, possibly to enable efficient self-association and subsequent cross-linking. The beta-spiral model for elastic proteins, where the protein is itself shaped like a spring, is not supported by the results for AN16. Both the random-network elastomer model and the sliding beta-turn model are consistent with the data. The results indicate a flat energy landscape for AN16, with very little energy required to switch between conformations. This ease of switching is likely to lead to the extremely low energy loss on deformation of resilin.
[show abstract][hide abstract] ABSTRACT: Understanding the process of particle transport is important for various applications such as separation, storage and blockage of selected particles within a polymer. The diffusivity of particles has been related to the fractional free volume f within a sample by the expression D(f)=A exp(−B/f) for certain constants A and B. Polymers are known to undergo physical aging such that the free volume distribution changes over time towards an equilibrium. This phenomenon has been well explained by the vacancy diffusion model established by Curro et al. [J.G. Curro, R.R. Lagasse, R. Simha, Macromolecules 15 (1982) 1621]. Using both the diffusion expression and the vacancy diffusion model, this paper models particle transport in aging, unaged and aged polymer samples.
Current Applied Physics 01/2008; 8(3):501-503. · 1.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Resilin is an elastic protein found in specialized regions of the cuticle of insects, which displays unique resilience and fatigue lifetime properties. As is the case with many elastomeric proteins, including elastin, gliadin and spider silks, resilin contains distinct repetitive domains that appear to confer elastic properties to the protein. Recent work within our laboratory has demonstrated that cloning and expression of exon 1 of the Drosophila melanogaster CG15920 gene, encoding a putative resilin-like protein, results in a recombinant protein that can be photochemically crosslinked to form a highly resilient, elastic biomaterial (Rec1 resilin). The current study describes a recursive cloning strategy for generating synthetic genes encoding multiple copies of consensus polypeptides, based on the repetitive domains within resilin-like genes from D. melanogaster and Anopheles gambiae. A simple non-chromatographic purification method that can be applied to these synthetic proteins and Rec1 is also reported. These methods for the design and purification of resilin-like periodic polypeptides will facilitate the future investigation of structural and functional properties of resilin, and the development of novel highly resilient biomaterials.
Protein Engineering Design and Selection 02/2007; 20(1):25-32. · 2.59 Impact Factor