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Journal articleKlosowski MM, McGilvery CM, Li Y, et al., 2016,
Micro-to nano-scale characterisation of polyamide structures of the SW30HR RO membrane using advanced electron microscopy and stain tracers
, Journal of Membrane Science, Vol: 520, Pages: 465-476, ISSN: 1873-3123The development of new reverse osmosis (RO) membranes with enhanced performance would benefit from a detailed knowledge of the membrane structures which participate in the filtration process. Here, we examined the hierarchical structures of the polyamide (PA) active layer of the SW30HR RO membrane. Scanning electron microscopy combined with focused ion beam milling (FIB-SEM) was used to obtain the 3-D reconstructions of membrane morphology with 5 nm cross-sectional resolution (comparable with the resolution of low magnification TEM imaging in 2D) and 30 nm slice thickness. The complex folding of the PA layer was examined in 3 dimensions, enabling the quantification of key structural properties of the PA layer, including the local thickness, volume, surface area and their derivatives. The PA layer was found to exhibit a much higher and convoluted surface area than that estimated via atomic force microscopy (AFM). Cross-sectional scanning transmission electron microscopy (STEM) was used to observe the distribution of a tracer stain under various conditions. The behaviour of stain in dry and wet PA indicated that the permeation pathways have a dynamic nature and are activated by water. High resolution STEM imaging of the stained PA nano-films revealed the presence of <1 nm pore-like structures with a size compatible with free volume estimations by positron annihilation lifetime spectroscopy (PALS). This study presents a comprehensive map of the active PA layer across different length scales (from micro- to sub-nanometre) and mechanistic insight into their role in the permeation process.
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Journal articleFerretti GL, Cabral JP, 2016,
Phase behaviour and non-monotonic film drying kinetics of aluminium chlorohydrate-glycerol-water ternary solutions
, Journal of Colloid and Interface Science, Vol: 481, Pages: 263-270, ISSN: 0021-9797We study the drying and film formation of a model ternary system comprising an inorganic salt (aluminium chlorohydrate, ACH), a humectant (glycerol) and water. Employing viscometric, X-ray diffraction, calorimetric, dynamic vapour sorption, spectroscopic, gravimetric and adhesion measurements, we examine the roles of humectant concentration, temperature and relative humidity (RH) in the phase behaviour and kinetics of film formation. Equilibrium film compositions are found to be non-monotonic with glycerol content. Around 15:4 ACH:glycerol mass ratio, films exhibit enhanced, albeit slower, desiccation, with water content lower than that of binary ACH–water solutions. At higher glycerol content, drying is faster, yet the resulting films have higher water content and remain tackier. Water adsorption/desorption is shown to be fully reversible, and share a similar non-monotonic kinetic dependence on glycerol composition. These findings are rationalised in terms of the competitive binding of water and glycerol to ACH, the overall miscibility and glass formation within the ternary system. Our study is relevant to a range of salt formulations, employed in a variety of commercial applications, including lyoprotectants and personal care products.
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Journal articleUdoh C, Garbin V, Cabral JP, 2016,
Microporous polymer particles via phase inversion in microfluidics: impact of non-solvent quality
, Langmuir, Vol: 32, Pages: 8131-8140, ISSN: 0743-7463We investigate the impact of ternary phase behavior on the microstructure of porous polymer particles produced by solvent extraction of polymer solution droplets by a nonsolvent. Microfluidic devices fabricated by frontal photopolymerization are employed to produce monodisperse polymer (P)/solvent (S) droplets suspended in a carrier (C) phase before inducing solvent extraction by precipitation in a nonsolvent (NS) bath. Model systems of sodium poly(styrenesulfonate) (P), water (S), hexadecane (C), and either methyl ethyl ketone (MEK) or ethyl acetate (EA) as NS are selected. Extraction across the liquid–liquid interface results in a decrease in the droplet radius and also an ingress of nonsolvent, leading to droplet phase demixing and coarsening. As the concentration of the polymer-rich phase increases, droplet shrinkage and solvent exchange slow down and eventually cease, resulting in microporous polymer particles (of radius ≃50–200 μm) with a smooth surface. The internal structure of these capsules, with pore sizes of ≃1–100 μm, is found to be controlled by polymer solution thermodynamics and the extraction pathway. The ternary phase diagrams are measured by turbidimetry, and the kinetics of phase separation is estimated by stopped-flow small-angle neutron scattering. The higher solubility of water in MEK results in faster particle-formation kinetics than in EA. Surprisingly, however, the lower polymer miscibility with EA/water results in a deeper quench inside the phase boundary and small phase sizes, thus yielding particles with small pores (of narrow distribution). The effects of droplet size, polymer content, and nonsolvent quality provide comprehensive insight into porous particle and capsule formation by phase inversion, with a range of practical applications.
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Journal articlePoulos AS, Nania M, Lapham P, et al., 2016,
Microfluidic SAXS Study of Lamellar and Multilamellar Vesicle Phases of Linear Sodium Alkylbenzenesulfonate Surfactant with Intrinsic Isomeric Distribution
, Langmuir, Vol: 32, Pages: 5852-5861, ISSN: 1520-5827The structure and flow behavior of a concentrated aqueous solution (45 wt %) of the ubiquitous linear sodium alkylbenzenesulfonate (NaLAS) surfactant is investigated by microfluidic small-angle X-ray scattering (SAXS) at 70 °C. NaLAS is an intrinsically complex mixture of over 20 surfactant molecules, presenting coexisting micellar (L1) and lamellar (Lα) phases. Novel microfluidic devices were fabricated to ensure pressure and thermal resistance, ability to handle viscous fluids, and low SAXS background. Polarized light optical microscopy showed that the NaLAS solution exhibits wall slip in microchannels, with velocity profiles approaching plug flow. Microfluidic SAXS demonstrated the structural spatial heterogeneity of the system with a characteristic length scale of 50 nL. Using a statistical flow–SAXS analysis, we identified the micellar phase and multiple coexisting lamellar phases with a continuous distribution of d spacings between 37.5 and 39.5 Å. Additionally, we showed that the orientation of NaLAS lamellar phases is strongly affected by a single microfluidic constriction. The bilayers align parallel to the velocity field upon entering a constriction and perpendicular to it upon exiting. On the other hand, multilamellar vesicle phases are not affected under the same flow conditions. Our results demonstrate that despite the compositional complexity inherent to NaLAS, microfluidic SAXS can rigorously elucidate its structure and flow response.
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Journal articleMiller RM, Poulos AS, Robles ESJ, et al., 2016,
Isothermal Crystallization Kinetics of Sodium Dodecyl Sulfate–Water Micellar Solutions
, Crystal Growth & Design, Vol: 16, Pages: 3379-3388, ISSN: 1528-7505The crystallization mechanisms and kinetics of micellar sodium dodecyl sulfate (SDS) solutions in water, under isothermal conditions, were investigated experimentally by a combination of reflection optical microscopy (OM), differential scanning calorimetry (DSC), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The rates of nucleation and growth were estimated from OM and DSC across temperatures ranging from 20 to −6 °C for 20% SDS-H2O, as well as for 10 and 30% SDS-H2O at representative temperatures of 6, 2, and −2 °C. A decrease in temperature increased both nucleation and growth rates, and the combined effect of the two processes on the morphology was quantified via both OM and ATR-FTIR. Needles, corresponding to the hemihydrate polymorph, become the dominant crystal form at ≤ −2 °C, while platelets, the monohydrate, predominate at higher temperatures. Above 8 °C, crystallization was only observed if seeded from crystals generated at lower temperatures. Our results provide quantitative and morphological insight into the crystallization of ubiquitous micellar SDS solutions and its phase stability below room temperature.
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Conference paperVitale A, Hennessy M, Matar O, et al., 2016,
Unified approach for polymeric patterning via controlling the propagation of frontal photopolymerization waves
, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727 -
Journal articleFerretti GL, Nania M, Matar OK, et al., 2016,
Wrinkling measurement of the mechanical properties of drying salt thin films
, Langmuir, Vol: 32, Pages: 2199-2207, ISSN: 1520-5827 -
Journal articleMartin HP, Brooks NJ, Seddon JM, et al., 2016,
Microfluidic processing of concentrated surfactant mixtures: online SAXS, microscopy and rheology
, Soft Matter, Vol: 12, Pages: 1750-1758, ISSN: 1744-6848 -
Journal articleVitale A, Hennessy MG, Matar OK, et al., 2015,
A Unified Approach for Patterning via Frontal Photopolymerization
, ADVANCED MATERIALS, Vol: 27, Pages: 6118-6124, ISSN: 0935-9648- Author Web Link
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- Citations: 50
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Journal articleLi Z, Chiu K-H, Shahid RS, et al., 2015,
Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices
, Scientific Reports, Vol: 5, ISSN: 2045-2322Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a PEDOT:PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced “burn-in” effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.
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Journal articleTan CH, Wong HC, Li Z, et al., 2015,
Synergetic enhancement of organic solar cell thermal stability by wire bar coating and light processing
, Journal of Materials Chemistry C, Vol: 3, Pages: 9551-9558, ISSN: 2050-7534We demonstrate that organic solar cells can exhibit different morphological and performance stability under thermal stress depending upon the processing technique employed, without compromising initial device efficiency. In particular, we investigate benchmark PCDTBT:PC60BM solar cells fabricated by wire bar coating (a technique attractive for commercial manufacture) and the more widely employed, lab scale, technique of spin coating. For this system, wire bar deposition results in superior device stability, with lifetime improvements in excess of 20-fold compared to spun cast devices. Neutron reflectivity reveals that the enhanced PC60BM segregation to the top interface in the slower, wire bar, casting process is likely responsible for the hindered PC60BM nucleation at tens of nm length scale, characterized by atomic force microscopy (AFM), and thus enhanced morphological stability. Modest light exposure of the active layer (at approximately 10 mW cm−2), known to reversibly photo-oligomerize fullerenes and thus impart morphological stability, is found to further improve device stability by a factor of 10. The combined effects of wire bar coating and light processing are highly synergetic, resulting in solar cells which are overall 200 times more stable than devices prepared by spin casting without light processing.
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Journal articleHennessy M, Vitale A, Cabral JT, et al., 2015,
Role of heat generation and thermal diffusion during frontal photopolymerization
, Physical Review E, Vol: 92, Pages: 022403-022403, ISSN: 1539-3755Frontal photopolymerisation (FPP) is a rapid and versatile solidification process that can be used to fabricate complex three-dimensional structures by selectively exposing a photosensitive monomer-rich bath to light. A characteristic feature of FPP is the appearance of a sharp polymerisation front that propagates into the bath as a planar travelling wave. In this paper, we introduce a theoretical model to determine how heat generation during photopolymerisation influences the kinetics of wave propagation as well as the monomer-to-polymer conversion profile, both of which are relevant for FPP applications and experimentally measurable. When thermal diffusion is sufficiently fast relative to the rate of polymerisation, the system evolves as if it were isothermal. However, when thermal diffusion is slow, a thermal wavefront develops and propagates at the same rate as the polymerisation front. This leads to an accumulation of heat behind the polymerisation front which can result in a significant sharpening of the conversion profile and acceleration of the growth of the solid. Our results also suggest that a novel way to tailor the dynamics of FPP is by imposing a temperature gradient along the growth direction.
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Journal articleLeguy AMA, Frost JM, McMahon AP, et al., 2015,
Corrigendum: The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells
, Nature Communications, Vol: 6, ISSN: 2041-1723 -
Journal articleHennessy MG, Vitale A, Matar OK, et al., 2015,
Controlling frontal photopolymerization with optical attenuation and mass diffusion
, Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Vol: 91, ISSN: 1063-651XFrontal photopolymerization (FPP) is a versatile directional solidification process that can be used to rapidly fabricate polymer network materials by selectively exposing a photosensitive monomer bath to light. A characteristic feature of FPP is that the monomer-to-polymer conversion profiles take on the form of traveling waves that propagate into the unpolymerized bulk from the illuminated surface. Practical implementations of FPP require detailed knowledge about the conversion profile and speed of these traveling waves. The purpose of this theoretical study is to (i) determine the conditions under which FPP occurs and (ii) explore how optical attenuation and mass transport can be used to finely tune the conversion profile and propagation kinetics. Our findings quantify the strong optical attenuation and slow mass transport relative to the rate of polymerization required for FPP. The shape of the traveling wave is primarily controlled by the magnitude of the optical attenuation coefficients of the neat and polymerized material. Unexpectedly, we find that mass diffusion can increase the net extent of polymerization and accelerate the growth of the solid network. The theoretical predictions are found to be in excellent agreement with experimental data acquired for representative systems.
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Journal articleLeguy AMA, Frost JM, McMahon AP, et al., 2015,
The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells
, Nature Communications, Vol: 6, ISSN: 2041-1723Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ~14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a device’s built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ~0.1–1 ms, faster than most observed hysteresis.
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Journal articleSanz 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: 0032-3861We 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.
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Journal articleDattani R, Cabral JT, 2015,
Polymer fullerene solution phase behaviour and film formation pathways
, Soft Matter, Vol: 11, Pages: 3125-3131, ISSN: 1744-6848We 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.
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Journal articleLopez 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: 1099-0488We 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
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Journal articleNania 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: 1744-6848We 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%.
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Journal articleDattani 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: 1439-7641Rapid 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.
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Journal articleDattani 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: 1095-7103We 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.
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Journal articleCabral JP, Watanabe T, Martel A, et al., 2015,
Microfluidic-SANS: flow processing of complex fluids
, Scientific Reports, Vol: 5, ISSN: 2045-2322Understanding 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.
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Journal articleVitale A, Hennessy MG, Matar OK, et al., 2014,
Interfacial profile and propagation of frontal photopolymerization waves
, Macromolecules, Vol: 48, Pages: 198-205, ISSN: 0024-9297We 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.
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Journal articleSchroeder 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: 1521-3773Cost-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.
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Journal articleDattani 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: 0024-9297We 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.
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Journal articleDattani 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 -
Journal articleGorgojo 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: 1616-3028Organic 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.
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Journal articleWatanabe 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- Author Web Link
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- Citations: 40
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Journal articleBayley 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: 1744-683XWe 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.
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Journal articleWong 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- Author Web Link
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- Citations: 116
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