Imperial College London

Prof Milo Shaffer

Faculty of Natural SciencesDepartment of Chemistry

Professor of Materials Chemistry
 
 
 
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Contact

 

+44 (0)20 7594 5825m.shaffer Website

 
 
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Assistant

 

Mr John Murrell +44 (0)20 7594 2845

 
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Location

 

M221Royal College of ScienceSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

277 results found

Lee C, Greenhalgh E, Shaffer M, Panesar Aet al., 2020, Optimized microstructures for multifunctional structural electrolytes, Multifunctional Materials, Vol: 2, ISSN: 2399-7532

Multifunctional structural materials offer compelling opportunities to realize highly efficient products. However, the need to fulfil disparate functions generates intrinsically conflicting physical property demands. One attractive strategy is to form a bi-continuous architecture of two disparate phases, each addressing a distinct physical property. For example, structural polymer electrolytes combine rigid and ion-conducting phases to deliver the required mechanical and electrochemical performance. Here, we present a general methodology, based on topology optimization, to identify optimal microstructures for particular design considerations. The numerical predictions have been successfully validated by experiments using 3D printed specimens. These architectures are directly relevant to multifunctional structural composites whilst the methodology can easily be extended to identify optimal microstructural designs for other multifunctional material embodiments.

Journal article

De Luca H, Anthony D, Greenhalgh E, Bismarck A, Shaffer Met al., 2020, Piezoresistive structural composites reinforced by carbon nanotube-grafted quartz fibres, Composites Science and Technology, Vol: 198, Pages: 1-12, ISSN: 0266-3538

Nano-engineered fibre/matrix interfaces can improve state-of-the-art fibre-reinforced composites. Grafting carbon nanotubes (CNTs) to high temperature quartz glass fibres produces “hairy” or “fuzzy” fibres, which combine reinforcements at micrometre and nanometre length scales. Fuzzy quartz fibres were produced continuously, reel-to-reel, on whole tows, in an open chemical vapour deposition reactor. The resulting uniform coverage of 200 nm long CNTs increased the interfacial shear strength with epoxy (90.3 ± 2.1 MPa) by 12% compared to the commercially-sized counterpart, as measured by single fibre pull-out tests. The improved interfacial properties were confirmed at the macroscale using unidirectional hierarchical bundle composites, which exhibited a delayed onset of fibre/matrix debonding. Although the quartz fibres are electrically insulating, the grafted CNT create a conductive path, predominantly parallel to the fibres. To explore the applicability for structural health monitoring, the resistivity was recorded in situ during mechanical testing, and correlated with simultaneous acoustic emission data. The baseline resistivity parallel to the fibres (ρ0 = 3.9 ± 0.4 × 10−1 Ω m) displayed a linear piezoresistive response (K = 3.64) until failure at ca. 2.1% strain, also referred to as "gauge factor”, a two-fold improvement over traditional resistance strain gauges (e.g. constantan). Hierarchical, fuzzy quartz fibres, therefore, simultaneously enhance both structural and sensing performance, offering multifunctional opportunities in large composite parts.

Journal article

Finley JM, Shaffer MSP, Pimenta S, 2020, Data-driven intelligent optimisation of discontinuous composites, Composite Structures, Vol: 243, Pages: 1-19, ISSN: 0263-8223

Fibre composites, and especially aligned discontinuous composites (ADCs), offer enormous versatility in composition, microstructure, and performance, but are difficult to optimise, due to their inherent variability and myriad permutations of microstructural design variables. This work combines an accurate yet efficient virtual testing framework (VTF) with a data-driven intelligent Bayesian optimisation routine, to maximise the mechanical performance of ADCs for a number of single- and multi-objective design cases. The use of a surrogate model helps to minimise the number of optimisation iterations, and provides a more accurate insight into the expected performance of materials which feature significant variability. Results from the single-objective optimisation study show that a wide range of structural properties can be achieved using ADCs, with a maximum stiffness of 505 GPa, maximum ultimate strain of 3.94%, or a maximum ultimate strength of 1.92 GPa all possible. A moderate trade-off in performance can be achieved when considering multi-objective optimisation design cases, such as an optimal ultimate strength & ultimate strain combination of 982 MPa and 3.27%, or an optimal combination of 720 MPa yield strength & 1.91% pseudo-ductile strain.

Journal article

Clancy AJ, Au H, Rubio N, Coulter GO, Shaffer MSPet al., 2020, Understanding and controlling the covalent functionalisation of graphene, Dalton Transactions, Vol: 49, Pages: 10308-10318, ISSN: 1477-9226

Chemical functionalisation is one of the most active areas of graphene research, motivated by fundamental science, the opportunities to adjust or supplement intrinsic properties, and the need to assemble materials for a broad array of applications. Historically, the primary consideration has been the degree of functionalisation but there is growing interest in understanding how and where modification occurs. Reactions may proceed preferentially at edges, defects, or on graphitic faces; they may be correlated, uncorrelated, or anti-correlated with previously grafted sites. A detailed collation of existing literature data indicates that steric effects play a strong role in limiting the extent of reaction. However, the pattern of functionalisation may have important effects on the resulting properties. This article addresses the unifying principles of current graphene functionalisation technologies, with emphasis on understanding and controlling the locus of functionalisation.

Journal article

Leung AHM, García-Trenco A, Phanopoulos A, Regoutz A, Schuster ME, Pike SD, Shaffer MSP, Williams CKet al., 2020, Cu/M:ZnO (M = Mg, Al, Cu) colloidal nanocatalysts for the solution hydrogenation of carbon dioxide to methanol, Journal of Materials Chemistry A, Vol: 8, Pages: 11282-11291, ISSN: 2050-7488

Doped-ZnO nanoparticles, capped with dioctylphosphinate ligands, are synthesised by the controlled hydrolysis of a mixture of organometallic precursors. Substitutional doping of the wurtzite ZnO nanoparticles with 5 mol% Mg(II), Al(III) and Cu(I) is achieved by the addition of sub-stoichiometric amounts of the appropriate dopant [(n-butyl)(sec-butyl)magnesium, triethylaluminium or mesitylcopper] to diethylzinc in the precursor mixture. After hydrolysis, the resulting colloidal nanoparticles (sizes of 2–3 nm) are characterised by powder X-ray crystallography, transmission electron microscopy, inductively-coupled plasma optical emission spectrometry and X-ray photoelectron spectroscopy. A solution of the doped-ZnO nanoparticles and colloidal Cu(0) nanoparticles [M:ZnO : Cu = 1 : 1] are applied as catalysts for the hydrogenation of CO2 to methanol in a liquid-phase continuous flow stirred tank reactor [210 °C, 50 bar, CO2 : H2 = 1 : 3, 150 mL min−1, mesitylene, 20 h]. All the catalyst systems display higher rates of methanol production and better stability than a benchmark heterogeneous catalyst, Cu–ZnO–Al2O3 [480 μmol mmolmetal−1 h−1], with approximately twice the activity for the Al(III)-doped nanocatalyst. Despite outperforming the benchmark catalyst, Mg(II) doping is detrimental towards methanol production in comparison to undoped ZnO. X-Ray photoelectron spectroscopy and transmission electron microscopy analysis of the most active post-catalysis samples implicate the migration of Al(III) to the catalyst surface, and this surface enrichment is proposed to facilitate stabilisation of the catalytic ZnO/Cu interfaces.

Journal article

Johannisson W, Nguyen S, Lindbergh G, Zenkert D, Greenhalgh E, Shaffer M, Kucernak Aet al., 2020, A residual performance methodology to evaluate multifunctional systems, Multifunctional Materials, Vol: 3, ISSN: 2399-7532

The development of multifunctional materials and structures is receiving increasing interest for many applications and industries; it is a promising way to increase system-wide efficiency and improve the ability to meet environmental targets. However, quantifying the advantages of a multifunctional solution over monofunctional systems can be challenging. One approach is to calculate a reduction in mass, volume or other penalty function. Another approach is to use a multifunctional efficiency metric. However, either approach can lead to results that are unfamiliar or difficult to interpret and implement for an audience without a multifunctional materials or structures background.Instead, we introduce a comparative metric for multifunctional materials that correlates with familiar design parameters for monofunctional materials. This metric allows the potential benefits of the multifunctional system to be understood easily without needing a holistic viewpoint. The analysis is applied to two different examples of multifunctional systems; a structural battery and a structural supercapacitor, demonstrating the methodology and its potential for state-of-the-art structural power materials to offer a weight saving over conventional systems. This metric offers a new way to communicate research on structural power which could help identify and prioritise future research.

Journal article

Valkova M, Anthony DB, Kucernak ARJ, Shaffer MSP, Greenhalgh ESet al., 2020, Predicting the compaction of hybrid multilayer woven composite reinforcement stacks, Composites Part A: Applied Science and Manufacturing, Vol: 133, ISSN: 1359-835X

A meso-scale finite element modelling strategy was developed to investigate the effect of hybridisation on the compaction response of multilayer stacks combining glass and carbon dry woven fabrics. It is expected that the electrochemical-mechanical properties of emerging multifunctional hybrid composites are strongly dictated by the morphology of the compacted reinforcements, yet no investigations into their compressibility have been reported. Model predictions were evaluated against compressibility measurements for monolithic and hybrid fabric stacks. The ply offset had a major influence on the predicted internal morphologies and fibre content, contributing to experimental variability thereof. Optical microscopy and micro X-ray computed tomography imaging indicated greater likelihood of intermediate ply offsets in physical specimens, over limit case model idealisations. Compressibility was slightly reduced in the hybrid multilayer stacks studied in this work. The model outputs presented are being used to analyse the electrochemical-mechanical response of hybrid woven structural power composites.

Journal article

Lee WJ, Paineau E, Anthony DB, Gao Y, Leese HS, Rouzière S, Launois P, Shaffer MSPet al., 2020, Inorganic nanotube mesophases enable strong self-healing fibers, ACS Nano, Vol: 14, Pages: 5570-5580, ISSN: 1936-0851

The assembly of one-dimensional nanomaterials into macroscopic fibers can improve mechanical as well as multifunctional performance. Double walled aluminogermanate imogolite nanotubes are geo-inspired analogs of carbon nanotubes, synthesized at low temperature, with complementary properties. Here, continuous imogolite based fibers are wet spun within a polyvinyl alcohol matrix. The lyotropic liquid crystallinity of the system produces highly aligned fibers with tensile stiffness and strength up to 24.1 GPa (14.1 N tex⁻¹) and 0.8 GPa (0.46 N tex⁻¹), respectively. Significant enhancements over the pure polymer control are quantitatively attributed to both matrix refinement and direct nanoscale reinforcement, by fitting an analytical model. Most intriguingly, imogolite-based fibers show a high degree of healability via evaporation induced self assembly, recovering up to 44%, and 19% of the original fiber tensile stiffness and strength, respectively. This recovery at high absolute strength highlights a general strategy for the development of high-performance healable fibers relevant to composite structures and other applications.

Journal article

Au H, Rubio N, Buckley DJ, Mattevi C, Shaffer MSPet al., 2020, Thermal decomposition of ternary sodium graphite intercalation compounds, Chemistry: A European Journal, Vol: 26, Pages: 6545-6553, ISSN: 0947-6539

Graphite intercalation compounds (GICs) are often used to produce exfoliated or functionalised graphene related materials (GRMs) in a specific solvent. This study explores the formation of the Na-tetrahydrofuran (THF)-GIC and a new ternary system based on dimethylacetamide (DMAc). Detailed comparisons of in situ temperature dependent XRD with TGA-MS and Raman measurements reveal a series of dynamic transformations during heating. Surprisingly, the bulk of the intercalation compound is stable under ambient conditions, trapped between the graphene sheets. The heating process drives a reorganisation of the solvent and Na molecules, then an evaporation of the solvent; however, the solvent loss is arrested by restacking of the graphene layers, leading to trapped solvent bubbles. Eventually, the bubbles rupture, releasing the remaining solvent and creating expanded graphite. These trapped dopants may provide useful property enhancements, but also potentially confound measurements of grafting efficiency in liquid-phase covalent functionalization experiments on 2D materials.

Journal article

Au H, Rubio N, Buckley DJ, Mattevi C, Shaffer MSPet al., 2020, Cover Feature: Thermal Decomposition of Ternary Sodium Graphite Intercalation Compounds (Chem. Eur. J. 29/2020), Chemistry – A European Journal, Vol: 26, Pages: 6291-6291, ISSN: 0947-6539

Journal article

Sehmi SK, Lourenco C, Alkhuder K, Pike SD, Noimark S, Williams CK, Shaffer MSP, Parkin IP, MacRobert AJ, Allan Eet al., 2020, Antibacterial surfaces with activity against antimicrobial resistant bacterial pathogens and endospores, ACS Infectious Diseases, Vol: 6, Pages: 939-946, ISSN: 2373-8227

Hospital-acquired bacterial infections are a significant burden on healthcare systems worldwide causing an increased duration of hospital stays and prolonged patient suffering. We show that polyurethane containing crystal violet (CV) and 3–4 nm zinc oxide nanoparticles (ZnO NPs) possesses excellent bactericidal activity against hospital-acquired pathogens including multidrug resistant Escherichia coli (E. coli), Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA), and even highly resistant endospores of Clostridioides (Clostridium) difficile. Importantly, we used clinical isolates of bacterial strains, a protocol to mimic the environmental conditions of a real exposure in the healthcare setting, and low light intensity equivalent to that encountered in UK hospitals (∼500 lux). Our data shows that ZnO NPs enhance the photobactericidal activity of CV under low intensity light even with short exposure times, and we show that this involves both Type I and Type II photochemical pathways. Interestingly, polyurethane containing ZnO NPs alone showed significant bactericidal activity in the dark against one strain of E. coli, indicating that the NPs possess both light-activated synergistic activity with CV and inherent bactericidal activity that is independent of light. These new antibacterial polymers are potentially useful in healthcare facilties to reduce the transmission of pathogens between people and the environment.

Journal article

Gonzalez-Castillo EI, Costantini T, Shaffer MSP, Boccaccini ARet al., 2020, Nanocomposite coatings obtained by electrophoretic co-deposition of poly(etheretherketone)/graphene oxide suspensions, Journal of Materials Science, Vol: 55, Pages: 8881-8899, ISSN: 0022-2461

Nanocomposite coatings were successfully prepared by electrophoretic deposition of poly(etheretherketone) (PEEK)/graphene oxide (GO) suspensions. The GO flakes developed a large-scale co-continuous morphology with the basal plane mainly aligned with the coating surface. However, the PEEK particles were also found to be wrapped by GO nanosheets when deposited on the stainless steel substrate. Both phenomena, the co-continuous morphology and the wrapping effect, were dependent on the initial GO content in the suspension and influenced the final morphological characteristics of the thermally treated coatings. The PEEK matrix developed a dendritic morphology during its cooling from the molten state because of transcrystallinity that was induced by the incorporation of GO. The preparation of suspensions involved tip ultrasonication (TS) to deagglomerate, disperse, and mill the PEEK particles. A detailed study of the microstructure revealed that TS tended not only to reduce PEEK particle size, but also to promote an elongated shape, favourable for the nanocomposite coatings.

Journal article

Finley J, Henry J, Shaffer M, Pimenta Set al., 2020, The influence of variability and defects on the mechanical performance of tailorable composites, Journal of Composite Materials, Vol: 54, Pages: 565-589, ISSN: 0021-9983

Aligned hybrid-fibre discontinuous composites offer the ability to tailor their mechanical response through careful microstructural design. However, with tailorability comes microstructural complexity, which in turn leads to many sources of variability and defects. A virtual testing framework was further extended to investigate the influence of variability and defects on the mechanical performance of various aligned discontinuous composite material systems. This approach identified the most critical sources of variability as (i) fibre strength, (ii) the distance between fibre ends, or (iii) the level of fibre-type intermingling, depending on the material system. Fibre vacancy defects were shown to have the most significant influence on the strength and ductility of aligned discontinuous composites, although this sensitivity can be reduced through hybridisation of the fibre types.

Journal article

Hart M, Chen J, Michaelides A, Sella A, Shaffer MSP, Salzmann CGet al., 2019, One-dimensional pnictogen allotropes inside single-wall carbon nanotubes, Inorganic Chemistry, Vol: 58, Pages: 15216-15224, ISSN: 0020-1669

The discovery of phosphorene, a single layer of black phosphorus, has accelerated the investigation of pnictogen nanomaterials, leading to the recent identification of arsenene and antimonene. These two-dimensional nanomaterials display physical properties superior to those of graphene for some applications. Recently, single-wall carbon nanotubes (SWCNTs) have been filled with P4 molecules from the melt and As4 molecules from the vapor phase. Confined within SWCNTs, polymerization reactions yielded new one-dimensional pnictogen allotropes. Here, we show using high-resolution electron microscopy that such nanostructures can also be observed upon filling SWCNTs from the vapor phase using red phosphorus as the source material. Using larger-diameter SWCNTs, the vapor phase favors the formation of double-stranded phosphorus zigzag ladders observed here for the first time. Overall, however, SWCNTs were generally found to fill more efficiently with liquid phosphorus; substantial decreases in the filling yields were observed for both phosphorus and arsenic filling of narrow SWCNTs using the vapor route. Attempts to extend the pnitogen series using molten antimony gave very low filling yields. However, the antimony zigzag ladder was observed on two occasions, suggesting that this structural motif dominates across the pnictogens. Computational predictions of the encapsulation energies of the various pnictogen nanostructures are consistent with the observed experimental trends, and band gap calculations predict that the single-stranded zigzag chains of all investigated pnictogens are fully metallic. Using SWCNTs with diameters of >1.5 nm revealed a plethora of complex new phosphorus nanostructures, which highlights an exciting new avenue for future work in this area.

Journal article

Gonzalez Carter D, Goode A, Kiryushko D, Masuda S, Hu S, Lopes Rodrigues R, Dexter D, Shaffer MSP, Porter AEet al., 2019, Quantification of blood-brain barrier transport and neuronal toxicity of unlabelled multiwalled carbon nanotubes as a function of surface charge, Nanoscale, Vol: 11, Pages: 22054-22069, ISSN: 2040-3364

Nanoparticles capable of penetrating the blood-brain barrier (BBB) will greatly advance the delivery of therapies against brain disorders. Carbon nanotubes hold great potential as delivery vehicles due to their high aspect-ratio and cell-penetrating ability. Studies have shown multiwalled carbon nanotubes (MWCNT) cross the BBB, however they have largely relied on labelling methods to track and quantify transport, or on individual electron microscopy images to qualitatively assess transcytosis. Therefore, new direct and quantitative methods, using well-defined and unlabelled MWCNT, are needed to compare BBB translocation of different MWCNT types. Using highly controlled anionic (-), cationic (+) and non-ionic (0) functionalized MWCNT (fMWCNT), we correlate UV-visible spectroscopy with quantitative transmission electron microscopy, quantified from c. 270 endothelial cells, to examine cellular uptake, BBB transport and neurotoxicity of unlabelled fMWCNT. Our results demonstrate that: i) a large fraction of cationic and non-ionic, but not anionic fMWCNT become trapped at the luminal brain endothelial cell membrane; ii) despite high cell association, fMWCNT uptake by brain endothelial cells is low (< 1.5% ID) and does not correlate with BBB translocation, iii) anionic fMWCNT have highest transport levels across an in vitro model of the human BBB compared to non-ionic or cationic nanotubes; and iv) fMWCNT are not toxic to hippocampal neurons at relevant abluminal concentrations; however, fMWCNT charge has an effect on carbon nanotube neurotoxicity at higher fMWCNT concentrations. This quantitative combination of microscopy and spectroscopy, with cellular assays, provides a crucial strategy to predict brain penetration efficiency and neurotoxicity of unlabelled MWCNT and other nanoparticle technologies relevant to human health.

Journal article

Nguyen S, Anthony DB, Qian H, Yue C, Singh A, Bismarck A, Shaffer MSP, Greenhalgh ESet al., 2019, Mechanical and physical performance of carbon aerogel reinforced carbon fibre hierarchical composites, Composites Science and Technology, Vol: 182, ISSN: 0266-3538

Carbon aerogel (CAG) is a potential hierarchical reinforcement to improve the matrix-dominated mechanical properties of continuous carbon fibre reinforced polymer (CFRP) composites in both multifunctional and purely structural applications. When using CAG to reinforce a polyethylene glycol diglycidyl ether (PEGDGE) matrix, the interlaminar shear strength, compressive modulus and strength increased approximately four-fold, whilst the out-of-plane electrical conductivity increased by 118%. These mechanical and electrical performance enhancements significantly improve the multifunctional efficiency of composite structural supercapacitors, which can offer weight savings in transport and other applications. However, CAG also has the potential to reinforce conventional continuous CF composites in purely structural contexts. Here, CAG reinforcement of structural epoxy resin composites marginally increased compressive (1.4%) and tensile (2.7%) moduli respectively, but considerably reduced compressive, tensile and interlaminar shear strengths. Fractographic analysis shows that the reduced performance can be attributed to poor interfacial adhesion; in the future, alternative processing routes may resolve these issues to achieve advances in both moduli and strengths over conventional structural CFRPs.

Journal article

Leo BF, Fearn S, Gonzalez-Carter D, Theodorou I, Ruenraroengsak P, Goode A, Mcphail D, Dexter DT, Shaffer MSP, Chung KF, Porter AE, Ryan MPet al., 2019, Label-free TOF-SIMS imaging of sulfur producing enzymes inside microglia cells following exposure to silver nanowires, Analytical Chemistry, Vol: 91, Pages: 11098-11107, ISSN: 0003-2700

There are no methods sensitive enough to detect enzymes within cells, without the use of analyte labelling. Here we show that it is possible to detect protein ion signals of three different H2S-synthesizing enzymes inside microglia after pre-treatment with silver nanowires (AgNW) using time of flight-secondary ion mass spectrometry (TOF-SIMS). Protein fragment ions, including the fragment of amino acid (C4H8N+ - 70 amu), fragments of the sulfur producing cystathionine-containing enzymes and the Ag+ ion signal could be detected without the use of any labels; the cells were mapped using the C4H8N+ amino acid fragment. Scanning electron microscopy imaging and energy dispersive x-ray chemical analysis showed that the AgNWs were inside the same cells imaged by TOF-SIMS and transformed chemically into crystalline Ag2S within cells in which the sulfur producing proteins were detected. The presence of these sulfur producing cystathionine-containing enzymes within the cells was confirmed by Western Blots and confocal microscopy images of fluorescently labelled antibodies against the sulfur producing enzymes. Label-free ToF-SIMS is very promising for the label-free identification of H2S-contributing enzymes and their cellular localization in biological systems. The technique could in future be used to identify which of these enzymes are most contributory.

Journal article

Anthony D, Nguyen S, Senokos E, Bismarck A, Kucernak A, Greenhalgh E, Shaffer Met al., 2019, Hierarchical carbon aerogel modified carbon fiber composites for structural power applications, 22nd International Conference on Composite Materials 2019 (ICCM22), Publisher: Engineers Australia, Pages: 1-7

The desire to reduce overall weight in devices is a key driver for perpetual material development; the ability to combine composites with energy storage functions/capabilities which simultaneously provide structural integrity has the potential to supersede monofunctional components. To achieve this ambition, the multifunctional structure must perform both mechanical and energy storage functions sufficiently, but often there is a trade off in performance which is a significant challenge to overcome. Carbon aerogels have been shown to contribute positively to (electro-chemical double layer) capacitive performance due to an increased surface area in multifunctional carbon fiber based composite electrodes, but have also been shown to reduce mechanical properties; the addition of nanoscale reinforcers, such as carbon nanotubes, graphene or alike, with their superlative electrical and mechanical properties are proposed to address these concerns and create a truly hierarchical structure suitable for structural power applications.

Conference paper

Basma N, Cullen PL, Clancy AJ, Shaffer MSP, Skipper NT, Headen TF, Howard CAet al., 2019, The liquid structure of the solvents dimethylformamide (DMF) and dimethylacetamide (DMA), Molecular Physics: An International Journal at the Interface Between Chemistry and Physics, Vol: 117, Pages: 3353-3363, ISSN: 0026-8976

The liquid structure of the commonly used solvents dimethylformamide (DMF) and dimethylacetamide (DMA)were measured using state-of-the-art state neutron diffraction augmented with isotopic substitution (NDIS) and interpreted with empirical potential structure refinement (EPSR). Both solvents are found to develop rich local ordering with similar local packing densities, though with differences related to their three-dimensional molecular structure. While DMF’s dipole preferentially orientates anti-parallel to maximise hydrogen bonding, DMA favours parallel arrangement maximising non-directional dispersive forces. The highly-developed local orientational structure found in these solvents rationalises their ability to solvate a range of charged and neutral nanomaterials and highlights that the understanding of nanomaterial dispersions is a multi-body problem in which the geometry of the molecule, as well its dipole moment, must be incorporated.

Journal article

Kennedy OW, White R, Shaffer MSP, Warburton PAet al., 2019, Vapour-liquid-solid growth of ZnO-ZnMgO core-shell nanowires by gold-catalysed molecular beam epitaxy, Nanotechnology, Vol: 30, ISSN: 0957-4484

Nanowire heterostructures, combining multiple phases within a single nanowire, modify functional properties and offer a platform for novel device development. Here, ZnO/ZnMgO core–shell nanowires are grown by molecular beam epitaxy. At growth temperatures above 750 °C, Mg diffuses into ZnO making heterostructure growth impossible; at lower shell-growth temperatures (500 °C), the core–shell structure is retained. Even very thin ZnMgO shells show increased intensity photoluminescence (PL) across the ZnO band-gap and a suppression in defect-related PL intensity, relative to plain ZnO nanowires. EDX measurements on shell thickness show a correlation between shell thickness and core diameter which is explained by a simple growth model.

Journal article

Lee W, Clancy A, Fernández-Toribio JC, Anthony D, White E, Solano E, Leese HS, Vilatela JJ, Shaffer MSPet al., 2019, Interfacially-grafted single wall carbon nanotube / poly (vinyl alcohol) composite fibers, Carbon, Vol: 146, Pages: 162-171, ISSN: 0008-6223

Nanocomposites are critically influenced by interfacial interactions between the reinforcement and matrix. Polyvinyl alcohol (PVOH) of varying molecular weights were prepared and grafted to single-walled carbon nanotubes (SWCNTs), to improve the interfacial interaction with a homopolymer PVOH matrix. Nanocomposite fibers were coagulation spun across a broad range of loading fractions, controlled by the spinning dope composition. An intermediate grafted-PVOH molecular weight (10 kDa) maximized grafting ratio, and the final composite mechanical performance; the positive effects were attributed to the increased degree of dispersion of the SWCNTs in the dope, as well as the favorable interface. The PVOH grafting increased the stability of the SWCNT loading fractions (up to 45 wt.%), offering increased strength (up to 1100 MPa) and stiffness (up to 38.5 GPa); at the same time, strain to-failures remained high (up to 23.3%), resulting in high toughness (up to 125 J g-1).

Journal article

Clancy A, Sirisinudomkit P, Anthony D, Thong A, Greenfield J, Salaken Singh MK, Shaffer Met al., 2019, Real-time mechanistic study of carbon nanotube anion functionalisation through open circuit voltammetry, Chemical Science, Vol: 10, Pages: 3300-3306, ISSN: 2041-6520

The mechanism of the functionalisation of reduced single walled carbon nanotubes with organobromides was monitored byopen circuit voltammetry throughout the reaction and further elucidated through a series of comparative reactions. Thedegree of functionalisation was mapped against the reagent reduction potential, degree of electron donation of substituents(Hammett parameter), and energies calculated, ab initio, for dissociation and heterolytic cleavage of the C-Br bond. Incontrast to the previously assumed reduction/homolytic cleavage mechanism, the reaction was shown to consist of a rapidassociation of carbon-halide bond to the reduced nanotube as a complex, displacing surface-condensed countercations,leading to an initial increase in the net nanotube surface negative charge. The complex subsequently slowly degradesthrough charge transfer from the reduced single-walled carbon nanotube to the organobromide, utilizing charge, and thecarbon-halide bond breaks heterolytically. Electron density on the C-Br bond in the initial reagent is the best predictor fordegree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Both themechanism and the new application of OCV to study such reactions are potentially relevant to wide range of related systems.

Journal article

Kennedy OW, White ER, Howkins A, Williams CK, Boyd IW, Warburton PA, Shaffer MSPet al., 2019, Mapping the origins of luminescence in ZnO nanowires by STEM-CL, Journal of Physical Chemistry Letters, Vol: 10, Pages: 386-392, ISSN: 1948-7185

In semiconductor nanowires, understanding both the sources of luminescence (excitonic recombination, defects, etc.) and the distribution of luminescent centers (be they uniformly distributed, or concentrated at structural defects or at the surface) is important for synthesis and applications. We develop scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements, allowing the structure and cathodoluminescence (CL) of single ZnO nanowires to be mapped at high resolution. Using a CL pixel resolution of 10 nm, variations of the CL spectra within such nanowires in the direction perpendicular to the nanowire growth axis are identified for the first time. By comparing the local CL spectra with the bulk photoluminescence spectra, the CL spectral features are assigned to internal and surface defect structures. Hyperspectral CL maps are deconvolved to enable characteristic spectral features to be spatially correlated with structural features within single nanowires. We have used these maps to show that the spatial distribution of these defects correlates well with regions that show an increased rate of nonradiative transitions.

Journal article

Liu B, Liu C, De Luca H, Raman Pillai SK, Anthony DB, Li J, Bismarck A, Shaffer M, Chan-Park MBet al., 2019, Synthesis of epoxidized poly(ester carbonate)-b-polyimide-b-poly(ester carbonate): reactive single-walled carbon nanotube dispersants enable synergistic reinforcement around multi-walled nanotube-grafted carbon fibers, Polymer Chemistry, Vol: 10, Pages: 1324-1334, ISSN: 1759-9954

Polyimides (PI) generally have a high affinity for single-walled carbon nanotubes (SWNTs), but they suffer from poor solubility in most low boiling point organic solvents and low compatibility with common resins (such as epoxy) used in composites, limiting their suitability as dispersants. PI block copolymer systems containing reactive poly(ester carbonate)s have not yet been reported and are expected to act as effective reactive dispersing agents of SWNTs. Herein, PI-derived block copolymers are synthesized via ring-opening copolymerization of lactide (LA) (a control monomer) and allyl-bearing 2-methyl-2-(allyloxycarbonyl)-propylene carbonate (MAC) from the OH-terminal ends of the PI block to produce PLA-PI-PLA (TB1, a control) and PMAC-PI-PMAC (TB2). The allyl pendant group of TB2 allows further facile functionalization to form a third series of epoxidized (EP) derivatives, i.e. PMACEP-block-PI-block-PMACEP (TB3). TB3 copolymer when mixed with a conventional structural epoxy resin forms blends that do not show inferior tensile properties compared with the epoxy, which is unusual. Furthermore, the mixing solvent tetrahydrofuran (THF) can be readily evaporated off after forming the blends. TB3-dispersed (2 wt%) SWNTs added to epoxy increased the tensile strength, modulus, and elongation at break of the resulting nanocomposite films by 40%, 34%, and 26% respectively, compared to the baseline epoxy resin. Furthermore, when TB3b triblock-dispersed SWNTs in epoxy were combined with fuzzy carbon fibers, i.e. carbon nanotube-grafted-carbon fibers (CNT-g-CF), a synergistic interfacial strength reinforcement was observed, together with shifting of the failure mode from the matrix interphase to the carbon fiber-grafted nanotube interface. The ultimate interfacial shear strength between the TB3-dispersed SWNT-epoxy matrix and the fuzzy carbon fibers (i.e., fibers having carbon nanotubes grown on them) measured via single fiber pull-out tests was 100 MPa, which was ca. 11% imp

Journal article

Stanier D, Radhakrishnan A, Gent I, Roy SS, Hamerton I, Potluri P, Scarpa F, Shaffer M, Ivanov DSet al., 2019, Matrix-graded and fibre-steered composites to tackle stress concentrations, Composite Structures, Vol: 207, Pages: 72-80, ISSN: 0263-8223

This paper studies the feasibility of improving structural performance of composites in the presence of stress concentrators. Matrix grading through local deposition of additive-enhanced matrices and fibre steering by varying fibrous architecture are examined independently and in combination on a glass-fibre triaxial braided composite subjected to open hole tensile test. Stiffened and toughened matrices were incorporated through precise point-wise injections of liquid reactive resin into dry preforms (Liquid Resin Printing). Fibre steering was implemented by varying the braiding angle along the length of the braided sleeve. It has been shown that these novel forms of architecture modification enable a significant improvement in composite strength through a variety of deformation mechanisms. This includes local stiffening of composite in the direct vicinity of the stress concentrator and damage accumulation away from the stress concentrators. The experimental observations are explained by using simple finite-element models.

Journal article

Finley JM, Shaffer MSP, Pimenta S, 2019, Intelligent optimisation of aligned discontinuous composites

© 2019 by DEStech Publications, Inc. and American Society for Composites. All rights reserved. Aligned discontinuous composites offer a tailorable structural response, as their mechanical behaviour can be tailored by adjusting their microstructure. However, the stochastic nature of their microstructure, and the myriad permutations of different constituent properties makes optimisation of these materials difficult. In this work, an accurate yet efficient virtual testing framework is combined with an intelligent Bayesian optimisation routine to maximise the initial stiffness, ultimate strain, and ultimate strength of aligned discontinuous composites.

Conference paper

Finley JM, Shaffer MSP, Pimenta S, 2019, Intelligent optimisation of aligned discontinuous composites

© 2019 by DEStech Publications, Inc. and American Society for Composites. All rights reserved. Aligned discontinuous composites offer a tailorable structural response, as their mechanical behaviour can be tailored by adjusting their microstructure. However, the stochastic nature of their microstructure, and the myriad permutations of different constituent properties makes optimisation of these materials difficult. In this work, an accurate yet efficient virtual testing framework is combined with an intelligent Bayesian optimisation routine to maximise the initial stiffness, ultimate strain, and ultimate strength of aligned discontinuous composites.

Conference paper

Zainol Abidin MS, Herceg T, Greenhalgh ES, Shaffer M, Bismarck Aet al., 2019, Enhanced fracture toughness of hierarchical carbon nanotube reinforced carbon fibre epoxy composites with engineered matrix microstructure, Composites Science and Technology, Vol: 170, Pages: 85-92, ISSN: 0266-3538

Journal article

Clancy AJ, Leese HS, Rubio Carrero N, Buckley DJ, Greenfield JL, Shaffer MSPet al., 2018, Depleting depletion: maintaining single-walled carbon nanotube dispersions after graft-to polymer functionalization, Langmuir, Vol: 34, Pages: 15396-15402, ISSN: 0743-7463

Grafting polymers onto single-walled carbon nanotubes (SWCNTs) usefully alters properties but does not typically yield stable, solvated species directly. Despite the expectation of steric stabilization, a damaging (re)dispersion step is usually necessary. Here, poly(vinyl acetate)s (PVAc) of varying molecular weights are grafted to individualized, reduced SWCNTs at different concentrations to examine the extent of reaction and degree of solvation. The use of higher polymer concentrations leads to an increase in grafting ratio (weight fraction of grafted polymer relative to the SWCNT framework), approaching the limit of random sequentially adsorbed Flory ‘mushrooms’ on the surface. However, at higher polymer concentrations, a larger percentage of SWCNTs precipitate during the reaction; an effect which is more significant for larger weight polymers. The precipitation is attributed to depletion interactions generated by ungrafted homopolymer overcoming Coulombic repulsion of adjacent like-charged SWCNTs; a simple model is proposed. Larger polymers and greater degrees of functionalization favor stable solvation, but larger and more concentrated homopolymers increase depletion aggregation. By using low concentrations (25 μM) of larger molecular weight PVAc (10 kDa), up to 65% of grafted SWCNTs were retained in solution (at 65 μg mL-1) directly after the reaction.

Journal article

Brandley E, Greenhalgh E, Shaffer M, Li Qet al., 2018, Mapping carbon nanotube orientation by fast fourier transform of scanning electron micrographs, Carbon, Vol: 137, Pages: 78-87, ISSN: 0008-6223

A novel method of applying a two-dimensional Fourier transform (2D-FFT) to SEM wasdeveloped to map the CNT orientation in pre-formed arrays. Local 2D-FFTs were integratedazimuthally to determine an orientation distribution function and the associated Hermanparameter. This approach provides data rapidly and over a wide range of lengthscales.Although likely to be applicable to a wide range of anisotropic nanoscale structures, themethod was specifically developed to study CNT veils, a system in which orientationcritically controls mechanical properties. Using this system as a model, key parameters forthe 2D-FFT analysis were optimised, including magnification and domain size; a model setof CNT veils were pre-strained to 5%, 10% and 15%, to vary the alignment degree. Thealgorithm confirmed a narrower orientation distribution function and increasing Hermanparameter, with increasing pre-strain.To validate the algorithm, the local orientation was compared to that derived from a commonpolarised Raman spectroscopy. Orientation maps of the Herman parameter, derived by bothmethods, showed good agreement. Quantitatively, the mean Herman parameter calculatedusing the polarised Raman spectroscopy was 0.42±0.004 compared to 0.32±0.002 for the 2DFFTmethod, with a correlation coefficient of 0.73. Possible reasons for the modest andsystematic discrepancy were discussed.

Journal article

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