Publications
154 results found
Sanz A, Wong HC, Nedoma AJ, et al., 2015, Influence of C-60 fullerenes on the glass formation of polystyrene, Polymer, Vol: 68, Pages: 47-56, ISSN: 0032-3861
We investigate the impact of fullerene C60 on the thermal properties and glass formation of polystyrene (PS) by differential scanning calorimetry (DSC) and dielectric spectroscopy (DS), for C60 concentrations up to 30% mass fraction. The miscibility and dispersibility thresholds of PS/C60 nanocomposites are first estimated by a combination of microscopy, small angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS) experiments, and these thresholds were found to be ≃1 mass% and ≃4 mass% C60, respectively. The addition of C60 increases the glass-transition temperature (Tg) of rapidly precipitated PS composites, up to a ‘threshold’ C60 concentration (≃4 wt%, in agreement with the dispersibility estimate). Beyond this concentration, the Tg reverts gradually towards the neat PS value. We present a comprehensive study for composites based on PS of molecular mass 270 kg/mol, and demonstrate the generality of the impact of C60 on Tg for PS matrices of 2 and 20 kg/mol. Thermal annealing or slowly evaporated composites largely reverse these effects, as the dispersion quality decreases. The dynamic fragility m of the composite is found to increase in the presence of C60, but the scaling of m with Tg for PS is retained. Similarly, physical ageing experiments show a reduction of relaxation enthalpy in the glass regime, which is largely accounted for by the increase of Tg with C60. The slowing down of the PS α-relaxation with C60 contrasts with the local ‘softening’ indicated by former Debye-Waller measurements and increase in fragility m. This effect is opposite to that of antiplasticizer additives, which both stiffen the material in the glassy state and reduce Tg, and simulations suggest this could be due to an increase in packing frustration. Finally, we review observations on the effect of nanoparticles on the Tg of PS and discuss the non-universal nature of Tg shifts by various types of nanoparticles.
Dattani R, Cabral JT, 2015, Polymer fullerene solution phase behaviour and film formation pathways, Soft Matter, Vol: 11, Pages: 3125-3131, ISSN: 1744-6848
We report the phase behaviour of polymer/fullerene/solvent ternary mixtures and its consequence for themorphology of the resulting composite thin films. We focus particularly on solutions of polystyrene (PS), C60fullerene and toluene, which are examined by static and dynamic light scattering, and films obtained fromvarious solution ages and thermal annealing conditions, using atomic force and light microscopy.Unexpectedly, the solution phase behaviour below the polymer overlap concentration, c*, is found to bedescribed by a simple excluded volume argument (occupied by the polymer chains) and the neatC60/solvent miscibility. Scaling consistent with full exclusion is found when the miscibility of the fullerenein the solvent is much lower than that of the polymer, giving way to partial exclusion with more solublefullerenes (phenyl-C61-butyric acid methyl ester, PCBM) and a less asymmetric solvent (chlorobenzene),employed in photovoltaic devices. Spun cast and drop cast films were prepared from PS/C60/toluenesolutions across the phase diagram to yield an identical PS/C60 composition and film thickness, resultingin qualitatively different morphologies in agreement with our measured solution phase boundaries. Ourfindings are relevant to the solution processing of polymer/fullerene composites (including organicphotovoltaic devices), which generally require effective solubilisation of fullerene derivatives and polymerpairs in this concentration range, and the design of well-defined thin film morphologies.
Lopez CG, Rogers SE, Colby RH, et al., 2015, Structure of Sodium Carboxymethyl Cellulose Aqueous Solutions: A SANS and Rheology Study, Journal of Polymer Science Part B-Polymer Physics, Vol: 53, Pages: 492-501, ISSN: 1099-0488
We report a small angle neutron scattering (SANS) and rheology study of cellulose derivative polyelectrolyte sodium carboxymethyl cellulose with a degree of substitution of 1.2. Using SANS, we establish that this polymer is molecularly dissolved in water with a locally stiff conformation with a stretching parameter inline image. We determine the cross sectional radius of the chain ( inline image 3.4 Å) and the scaling of the correlation length with concentration (ξ = 296 c−1∕2Å for c in g/L) is found to remain unchanged from the semidilute to concentrated crossover as identified by rheology. Viscosity measurements are found to be in qualitative agreement with scaling theory predictions for flexible polyelectrolytes exhibiting semidilute unentangled and entangled regimes, followed by what appears to be a crossover to neutral polymer concentration dependence of viscosity at high concentrations. Yet those higher concentrations, in the concentrated regime defined by rheology, still exhibit a peak in the scattering function that indicates a correlation length that continues to scale as
Nania M, Matar OK, Cabral JT, 2015, Frontal vitrification of PDMS using air plasma and consequences for surface wrinkling, Soft Matter, Vol: 11, Pages: 3067-3075, ISSN: 1744-6848
We study the surface oxidation of polydimethylsiloxane (PDMS) by air plasma exposure and its implications for the mechanically-induced surface wrinkling of the resulting glass–elastomer bilayers. The effect of plasma frequency (kHz and MHz), oxygen content (from O2 to air), pressure (0.5 ≤ P ≤ 1.5 mbar), as well as exposure time and power, is quantified in terms of the resulting glassy skin thickness h, inferred from wrinkling experiments. The glassy skin thickness is found to increase logarithmically with an exposure time t, for different induction powers p, and all data collapse in terms of a plasma dose, D ≡ p × t. The kinetics of film propagation are found to increase with the oxygen molar fraction yO2 and decrease with the gas pressure P, allowing both the wrinkling wavelength λ and amplitude A to be effectively controlled by gas pressure and composition. A generalised relationship for frontal vitrification is obtained by re-scaling all λ and h data by D/P. A coarse-grained wave propagation model effectively describes and quantifies the process stages (induction, skin formation and propagation) under all the conditions studied. Equipped with this knowledge, we further expand the capabilities of plasma oxidation for PDMS wrinkling, and a wavelength of λ ≈ 100 nm is readily attained with a modest strain εprestrain ≈ 20%.
Dattani R, Telling MTF, Lopez CG, et al., 2015, Rapid precipitation: An alternative to solvent casting for organic solar cells, Chemphyschem, Vol: 16, Pages: 1231-1238, ISSN: 1439-7641
Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.
Dattani R, Gibson KF, Few S, et al., 2015, Fullerene oxidation and clustering in solution induced by light, Journal of Colloid and Interface Science, Vol: 446, Pages: 24-30, ISSN: 1095-7103
We investigate the environmental stability of fullerene solutions by static and dynamic light scattering, FTIR, NMR and mass spectroscopies, and quantum chemical calculations. We find that visible light exposure of fullerene solutions in toluene, a good solvent, under ambient laboratory conditions results in C60 oxidation to form fullerene epoxides, and subsequently causes fullerene clustering in solution. The clusters grow with time, even in absence of further illumination, and can reach dimensions from ≈100 nm to the μm scale over ≈1 day. Static light scattering suggests that resulting aggregates are fractal, with a characteristic power law (df) that increases from approximately 1.3 to 2.0 during light exposure. The clusters are bound by weak Coulombic interactions and are found to be reversible, disintegrating by mechanical agitation and thermal stress, and reforming over time. Our findings are relevant to the solution processing of composites and organic photovoltaics, whose reproducibility and performance requires control of fullerene solution stability under storage conditions.
Cabral JP, Watanabe T, Martel A, et al., 2015, Microfluidic-SANS: flow processing of complex fluids, Scientific Reports, Vol: 5, ISSN: 2045-2322
Understanding and engineering the flow-response of complex and non-Newtonian fluids at a molecular level is a key challenge for their practical utilisation. Here we demonstrate the coupling of microfluidics with small angle neutron scattering (SANS). Microdevices with high neutron transmission (up to 98%), low scattering background, broad solvent compatibility and high pressure tolerance (≈3–15 bar) are rapidly prototyped via frontal photo polymerisation. Scattering from single microchannels of widths down to 60 μm, with beam footprint of 500 μm diameter, was successfully obtained in the scattering vector range 0.01–0.3 Å−1, corresponding to real space dimensions. We demonstrate our approach by investigating the molecular re-orientation and alignment underpinning the flow response of two model complex fluids, namely cetyl trimethylammonium chloride/pentanol/D2O and sodium lauryl sulfate/octanol/brine lamellar systems. Finally, we assess the applicability and outlook of microfluidic-SANS for high-throughput and flow processing studies, with emphasis of soft matter.
Vitale A, Hennessy MG, Matar OK, et al., 2014, Interfacial profile and propagation of frontal photopolymerization waves, Macromolecules, Vol: 48, Pages: 198-205, ISSN: 0024-9297
We investigate the frontal photopolymerization of a thiol–ene system with a combination of experiments and modeling, focusing on the interfacial conversion profile and its planar wave propagation. We spatially resolve the solid-to-liquid front by FT-IR and AFM mechanical measurements, supplemented by differential scanning calorimetry. A simple coarse-grained model is found to describe remarkably well the frontal kinetics and the sigmoidal interface, capturing the effects of UV light exposure time (or dose) and temperature, as well as the front position and resulting patterned dimensions after development. Analytical solutions for the conversion profile enable the description of all conditions with a single master curve in the moving frame of the front position. Building on this understanding, we demonstrate the design and fabrication of gradient polymer materials, with tunable properties along the direction of illumination, which can be coupled with lateral patterning by modulated illumination or grayscale lithography.
Schroeder BC, Li Z, Brady MA, et al., 2014, Enhancing fullerene-based solar cell lifetimes by addition of a fullerene dumbbell, Angewandte Chemie-International Edition, Vol: 53, Pages: 12870-12875, ISSN: 1521-3773
Cost-effective, solution-processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon-based solar cells. However, one of the major remaining challenges of OPV devices is their lack of long-term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM-based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability.
Dattani R, Michels R, Nedoma AJ, et al., 2014, Conformation and Interactions of Polystyrene and Fullerenes in Dilute to Semidilute Solutions, Macromolecules, Vol: 47, Pages: 6113-6120, ISSN: 0024-9297
We report the polymer conformation and fullereneaggregation in a ternary system containing polystyrene, C60, andtoluene measured by small angle neutron, static, and dynamic lightscattering. We investigate polymer concentrations across the diluteand semidilute regime for five polymer molecular weights (Mw =20 kg/mol to 1 Mg/mol), and fullerene concentrations below andabove its miscibility threshold in toluene. We find that the polymerradius of gyration (Rgpoly), hydrodynamic radius (Rh), and themixture correlation length (ξ) remain unchanged upon addition ofC60. The miscibility of C60 in toluene, however, decreases upon addition of polystyrene forming aggregates with a timedependentradius on the order of 100 nm, and this effect is amplified with increasing polymer Mw. Our findings are relevant tothe solution processing of organic photovoltaics, which generally require the effective solubilization of fullerene derivatives andpolymer pairs in this concentration range.
Dattani R, Bannock JH, Fei Z, et al., 2014, A general mechanism for controlling thin film structures in all-conjugated block copolymer: fullerene blends, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 2, Pages: 14711-14719, ISSN: 2050-7488
Gorgojo P, Karan S, Wong HC, et al., 2014, Ultrathin Polymer Films with Intrinsic Microporosity: Anomalous Solvent Permeation and High Flux Membranes, Advanced Functional Materials, Vol: 24, Pages: 4729-4737, ISSN: 1616-3028
Organic solvent nanofiltration (OSN) membranes with ultrathin separation layers down to 35 nm in thickness fabricated from a polymer of intrinsic microporosity (PIM-1) are presented. These membranes exhibit exceptionally fast permeation of n-heptane with a rejection for hexaphenylbenzene of about 90%. A 35 nm thick PIM-1 membrane possesses a Young's modulus of 222 MPa, and shows excellent stability under hydraulic pressures of up to 15 bar in OSN. A maximum permeance for n-heptane of 18 Lm−2h−1bar−1 is achieved with a 140 nm thick membrane, which is about two orders of magnitude higher than Starmem240 (a commercial polyimide-based OSN membrane). Unexpectedly, decreasing the film thickness below 140 nm results in an anomalous decrease in permeance, which appears to be related to a packing enhancement of PIM-1, as measured by light interferometry. Further, thermal annealing of the membranes formed from PIM-1 reveals that their permeance is preserved up to temperatures in excess of 150 °C, whereas the permeance of conventional, integrally skinned, asymmetric polyimide OSN membranes decreases significantly when they are annealed under the same conditions. To rationalize this key difference in response of functional performance to annealing, the concept of membranes with intrinsic microporosity (MIMs) versus membranes with extrinsic microporosity (MEMs) is introduced.
Watanabe T, Lopez CG, Douglas JF, et al., 2014, Microfluidic Approach to the Formation of Internally Porous Polymer Particles by Solvent Extraction, LANGMUIR, Vol: 30, Pages: 2470-2479, ISSN: 0743-7463
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- Citations: 40
Bayley FA, Liao JL, Stavrinou PN, et al., 2014, Wavefront kinetics of plasma oxidation of polydimethylsiloxane: limits for sub-mu m wrinkling, Soft Matter, Vol: 10, Pages: 1155-1166, ISSN: 1744-683X
We investigate the surface plasma oxidation of polydimethylsiloxane (PDMS) elastomers and its implication for the morphologies attainable by wrinkling of glassy-elastomer ‘bilayers’. The kinetics of glassy skin formation is found to follow a logarithmic dependence with plasma exposure time t and, for various plasma intensities I, the relevant control variable is shown to be dose (≡I × t). We model the mechanism and kinetics of glassy film formation by plasma oxidation with a frontal propagation coarse-grained model, describing the spatio-temporal evolution of a conversion order parameter (ϕ) orthogonal to the film surface. The model is validated by X-ray reflectivity experiments, which confirm the logarithmic growth and quantify the initial growth of a transient, incomplete, skin layer during the early stage of plasma exposure. Three regimes are identified as (I) induction, (II) formation and (III) propagation with a combination of X-ray and wrinkling experiments. The simultaneous increase in thickness and skin mechanical modulus is found to be responsible for an unexpected minimum wavelength λmin attainable, which depends on critical strain εc and is ultimately limited by mechanical failure of the elastomer (λmin ≃ 140 nm is demonstrated at ε = 200%). We conclude by establishing a 1D surface morphology diagram, in terms of wavelength λ and amplitude A, limitations and capabilities for producing highly ordered (sub-)micropatterns over macroscopic areas using plasma oxidised PDMS under uniaxial strain.
Wong HC, Li Z, Tan CH, et al., 2014, Morphological Stability and Performance of Polymer-Fullerene Solar Cells under Thermal Stress: The Impact of Photoinduced PC<sub>60</sub>BM Oligomerization, ACS NANO, Vol: 8, Pages: 1297-1308, ISSN: 1936-0851
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- Citations: 116
Livingston AG, Karan S, Gorgojo P, et al., 2014, Microporosity in organic solvent nanofiltration membranes - Intrinsic or extrinsic?, Pages: 127-128
Li Z, Wong HC, Huang Z, et al., 2013, Performance enhancement of fullerene-based solar cells by light processing, NATURE COMMUNICATIONS, Vol: 4, ISSN: 2041-1723
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- Citations: 109
Wong HC, Higgins AM, Wildes AR, et al., 2013, Patterning Polymer-Fullerene Nanocomposite Thin Films with Light, ADVANCED MATERIALS, Vol: 25, Pages: 985-991, ISSN: 0935-9648
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- Citations: 32
Rushworth CM, Davies J, Cabral JT, et al., 2012, Cavity-enhanced optical methods for online microfluidic analysis, CHEMICAL PHYSICS LETTERS, Vol: 554, Pages: 1-14, ISSN: 0009-2614
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- Citations: 27
James D, Oag B, Rushworth CM, et al., 2012, High-sensitivity online detection for microfluidics <i>via</i> cavity ringdown spectroscopy, RSC ADVANCES, Vol: 2, Pages: 5376-5384
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- Citations: 13
Wong HC, Cabral JT, 2011, Mechanism and Kinetics of Fullerene Association in Polystyrene Thin Film Mixtures, MACROMOLECULES, Vol: 44, Pages: 4530-4537, ISSN: 0024-9297
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- Citations: 21
Frosch T, Chan KLA, Wong HC, et al., 2010, Nondestructive Three-Dimensional Analysis of Layered Polymer Structures with Chemical Imaging, LANGMUIR, Vol: 26, Pages: 19027-19032, ISSN: 0743-7463
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- Citations: 34
Davies J, Rushworth C, Vallance C, et al., 2010, Drop kinetic analysis in real time by optical spectroscopy, Pages: 330-332
The spatio-temporal correspondence between microchannel position and reaction 'time' permits the study of kinetics of (chemical and physical) processes with unprecedented time resolution and dynamic range [1]. Monitoring reactions in real-time with non-invasive probes remains, hitherto, a major shortcoming of microchemical drop reactors due to the minute sample volumes (pL-nL) and fast travel speeds (1-1000 mm/s). We evaluate the potential of novel microdevice fabrication via frontal photopolymerisation (FPP) [2] integrated with Cavity Ring-Down Spectroscopy (CRDS) [3] for the online analysis of individual reaction travelling droplets.
Chua Y-C, Chan A, Wong H-C, et al., 2010, Thermodynamics of TMPC/PSd/Fullerene Nanocomposites SANS Study, MACROMOLECULES, Vol: 43, Pages: 9578-9582, ISSN: 0024-9297
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- Citations: 8
Wong HC, Cabral JT, 2010, Spinodal Clustering in Thin Films of Nanoparticle-Polymer Mixtures, PHYSICAL REVIEW LETTERS, Vol: 105, ISSN: 0031-9007
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- Citations: 29
Wong HC, Sanz A, Douglas JF, et al., 2010, Glass formation and stability of polystyrene-fullerene nanocomposites, JOURNAL OF MOLECULAR LIQUIDS, Vol: 153, Pages: 79-87, ISSN: 0167-7322
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- Citations: 64
Wong HC, Cabral JT, 2010, Nanoparticle aggregation behaviour in polymer nanocomposites: bulk vs. thin films, 14th International Conference on Small-Angle Scattering (SAS09), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
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- Citations: 8
Martin HP, Brooks NJ, Seddon JM, et al., 2010, Complex fluids under microflow probed by SAXS: rapid microfabrication and analysis, 14th International Conference on Small-Angle Scattering (SAS09), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
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- Citations: 15
Valera FE, Quaranta M, Moran A, et al., 2010, The Flow's the Thing ... Or Is It? Assessing the Merits of Homogeneous Reactions in Flask and Flow, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 49, Pages: 2478-2485, ISSN: 1433-7851
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- Citations: 152
Cabral JT, Higgins JS, 2009, Small Angle Neutron Scattering from the Highly Interacting Polymer Mixture TMPC/PSd: No Evidence of Spatially Dependent χ Parameter, MACROMOLECULES, Vol: 42, Pages: 9528-9536, ISSN: 0024-9297
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- Citations: 18
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