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Journal articlePont S, Durrant JR, Cabral JT, 2019,
Dynamic PCBM:dimer population in solar cells under light and temperature fluctuations
, Advanced Energy Materials, Vol: 9, Pages: 1-9, ISSN: 1614-6832Photoinduced dimerization of phenyl-C61-butyric acid methyl ester (PCBM) has a significant impact on the stability of polymer:PCBM organic solar cells (OSCs). This reaction is reversible, as dimers can be thermally decomposed at sufficiently elevated temperatures and both photodimerization and decomposition are temperature dependent. In operando conditions of OSCs evidently involve exposure to both light and heat, following periodic diurnal and seasonal profiles. In this work, the kinetics of dimer formation and decomposition are examined and quantified as a function of temperature, light intensity, blend composition, and time. The activation energy for photodimerization is estimated to be 0.021(3) eV, considerably smaller than that for decomposition (0.96 eV). The findings are benchmarked with a variety of conjugated polymer matrices to propose a descriptive dynamic model of PCBM:dimer population in OSCs, and a framework is proposed to rationalize its interplay with morphology evolution and charge quenching. The model and parameters enable the prediction of the dynamic and long-term PCBM:dimer populations, under variable temperature and light conditions, which impact the morphological stability of OSCs.
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Journal articleKhan H, Seddon JM, Law RV, et al., 2019,
Effect of glycerol with sodium chloride on the Krafft point of sodium dodecyl sulfate using surface tension
, Journal of Colloid and Interface Science, Vol: 538, Pages: 75-82, ISSN: 0021-9797The effect of glycerol with sodium chloride (NaCl) on the phase behaviour of sodium dodecyl sulfate (SDS) near the Krafft point was studied by surface tension analysis using the pendant drop method. The critical micelle concentration (CMC) and Krafft Temperature (TK) of SDS in water: glycerol mixtures, across the full composition range, and in NaCl solutions within 0.005–0.1 M were obtained. The pendant drop method successfully allowed us to determine the Krafft point of SDS in high glycerol systems where other traditional methods (e.g. conductivity) have been ineffective. Overall the addition of glycerol increases the CMC and the TK, thus shifting the Krafft point of SDS to higher temperatures (increasing crystallisation temperatures) and higher SDS content in the presence of glycerol, which is interpreted as a result of the reduction in solvent polarity which opposes micellization. The addition of NaCl to the SDS – water-glycerol systems brings the CMC back down, while having no significant effect on the TK. Our results establish a robust route for tuning the Krafft point of model surfactant SDS by adjusting solvent quality and salt content.
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Journal articleAoki Y, Wang H, Sharratt W, et al., 2019,
Small angle neutron scattering study of the thermodynamics of highly interacting PαMSAN/dPMMA blends
, Macromolecules, Vol: 52, Pages: 1112-1124, ISSN: 0024-9297Poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) form partially miscible blends with lower critical solution temperature (LCST) behaviour. We revisit this system using small angle neutron scattering (SANS), examining the effect of molecular weight (Mw) of deuterated PMMA (dPMMA), blend composition (φ) and temperature (T) in the homogeneous region. All data are well described by the Random Phase Approximation (RPA) theory, enabling us to determine thermodynamic and structural parameters, including the correlation length ξ, G00 (the second derivative of the free energy of mixing with respect to composition), and the statistical segment length a of each component. Phase boundaries are computed by extrapolation of G00 with temperature, to yield the spinodal, and inspection of Kratky plots to yield the binodal. For PαMSAN, a is determined to be 10.1±0.4 ˚A. Unsurprisingly, this system deviates strongly from Flory-Huggins expectations, exhibiting a minimal Mw dependence of the phase boundaries and φ-dependence of effective interaction parameter (˜χ). Comparison of G00 with values for other blend systems places PαMSAN/dPMMA in a class of highly interacting blends, expected from Cahn-Hilliard theory to yield small initial phase sizes upon spinodal demixing. This is confirmed experimentally, with an illustrative temperature jump resulting in an initial phase size of ' 30 nm.
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Journal articleBeber A, Taveneau C, Nania M, et al., 2019,
Membrane reshaping by micrometric curvature sensitive septin filaments
, Nature Communications, Vol: 10, ISSN: 2041-1723Septins are cytoskeletal filaments that assemble at the inner face of the plasma membrane. They are localized at constriction sites and impact membrane remodeling. We report in vitro tools to examine how yeast septins behave on curved and deformable membranes. Septins reshape the membranes of Giant Unilamellar Vesicles with the formation of periodic spikes, while flattening smaller vesicles. We show that membrane deformations are associated to preferential arrangement of septin filaments on specific curvatures. When binding to bilayers supported on custom-designed periodic wavy patterns displaying positive and negative micrometric radii of curvatures, septin filaments remain straight and perpendicular to the curvature of the convex parts, while bending negatively to follow concave geometries. Based on these results, we propose a theoretical model that describes the deformations and micrometric curvature sensitivity observed in vitro. The model captures the reorganizations of septin filaments throughout cytokinesis in vivo, providing mechanistic insights into cell division.
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Conference paperWang H, Aoki Y, Sharratt W, et al., 2019,
SANS study of the thermodynamics and demixing of highly interacting PaMSAN/dPMMA blends
, APS March Meeting 2019 -
Conference paperWang H, Khodaparast S, Cabral J, 2019,
Study of phase behavior and (meta)stability of model surfactant solutions by microfluidic thermal fluctuation platform
, APS March Meeting 2019 -
Journal articleMiller RM, Cabral J, Robles E, et al., 2018,
Crystallisation of sodium dodecyl sulfate–water micellar solutions with structurally similar additives: counterion variation
, CrystEngComm, Vol: 20, Pages: 6834-6843, ISSN: 1466-8033The effects of a series of structurally similar sodium dodecyl sulfate (SDS) additives on the crystallisation of SDS–water micellar solutions were investigated using a combination of differential scanning calorimetry, dynamic light scattering, optical microscopy and inductively coupled plasma optical emission spectroscopy. Seven different counterions were chosen from groups 1 and 2 of the periodic table to replace the sodium on SDS: LDS, (SDS), KDS, RbDS, CsDS, Mg(DS)2, Ca(DS)2 and Sr(DS)2. Two representative temperature profileswere employed – linear cooling ramps at rate of 0.5 °C min−1 to determine near-equilibrium kinetics and transitions and isothermal holds at 6 °C to elucidate morphological changes. Crystallisation of the reference solution 20% SDS–H2O with 0.25, 1.0 and 2.5% additive was generally promoted or inhibited even at the lowest concentrations. Melting points however remained largely unchanged, suggesting that the additives predominantly had a kinetic rather than thermodynamic effect. ICP-OES measurements for the solutions containing 1% additive indicated that most of the additives were integrated into the SDS crystals which was reflected by morphological changes, including the formation of hexagonal and oval shaped crystals. Our results both quantify and provide a morphological insight into the effect of a series of additives on the crystallisation of micellar SDS solutions, which can readily form due to preferential Na exchange.
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Journal articlePont S, Foglia F, Higgins A, et al., 2018,
Stability of polymer:PCBM thin films under competitive illumination and thermal stress
, Advanced Functional Materials, Vol: 28, ISSN: 1616-301XThe combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl‐C61‐butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV–vis spectroscopy is employed to quantify dimerization, time‐resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.
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Journal articleSharratt W, Brooker A, Robles E, et al., 2018,
Microfluidic solvent extraction of poly (vinyl alcohol) droplets: effect of polymer structure on particle and capsule formation
, Soft Matter, Vol: 14, Pages: 4453-4463, ISSN: 1744-683XWe investigate the formation of poly(vinyl alcohol) microparticles by the selective extraction of aqueous polymer solution droplets, templated by microfluidics and subsequently immersed in a non-solvent bath. The role of polymer molecular mass (18–105 kg mol−1), degree of hydrolysis (88–99%) and thus solubility, and initial solution concentration (0.01–10% w/w) are quantified. Monodisperse droplets with radii ranging from 50 to 500 μm were produced at a flow-focusing junction with carrier phase hexadecane and extracted into ethyl acetate. Solvent exchange and extraction result in droplet shrinkage, demixing, coarsening and phase-inversion, yielding polymer microparticles with well-defined dimensions and internal microstructure. Polymer concentration, varied from below the overlap concentration c* to above the concentrated crossover c**, as estimated by viscosity measurements, was found to have the largest impact on the final particle size and extraction timescale, while polymer mass and hydrolysis played a secondary role. These results are consistent with the observation that the average polymer concentration upon solidification greatly exceeds c**, and that the internal microparticle porosity is largely unchanged. However, reducing the initial polymer concentration to well below c* (approximately 100×) and increasing droplet size yields thin-walled (100's of nm) capsules which controllably crumple upon extraction. The symmetry of the process can be readily broken by imposing extraction conditions at an impermeable surface, yielding large, buckled, cavity morphologies. Based on these results, we establish robust design criteria for polymer capsules and particles, demonstrated here for poly(vinyl alcohol), with well-defined shape, dimensions and internal microstructure.
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Journal articleCabral JP, Higgins JS, 2018,
Spinodal nanostructures in polymer blends: on the validity of the Cahn-Hilliard length scale prediction
, Progress in Polymer Science, Vol: 81, Pages: 1-21, ISSN: 0079-6700Spinodal decomposition of partially miscible polymer blends has the potential to generate well-defined polymeric nanostructured materials, with precise control of length scale and connectivity, and applications ranging from membranes and scaffolds to photovoltaics. In this review, we briefly summarize the theoretical basis for describing spinodal decomposition in binary polymer blends, and the parameters that determine the accessible demixing length scales and the timescales over which they develop. We then examine experimentally the validity of the classical Cahn-Hilliard (CH) theory prediction for the initial spinodal length scale, where G′′ is the second derivative of the free energy of mixing with respect to composition, and k is the ‘square gradient’ parameter, accounting for changes in free energy arising from concentration gradients. Benefitting from the perspective of over 40 years of neutron and light scattering data, and noting (remaining) misconceptions in the literature when analyzing phase separation, we examine a large collection of Λ measurements, and independent -G′′(T) and k experimental estimates. Overall, we find the CH prediction for Λ to be remarkably accurate for all blends and self-consistent conditions examined. We then summarize design considerations for generating polymeric materials via spinodal decomposition, bound by thermodynamics of available polymer systems, coarsening kinetics governed by rheology, as well as by engineering constraints. The fulfillment of the potential of this approach in the development of real functional materials demands, however, improved thermodynamic theories for polymer blends, able to quantitatively predict G′′(T) and k in terms of molecular structure and interactions.
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Journal articleGonzalez Lopez C, Watanabe T, Adamo M, et al., 2018,
Microfluidic devices for small angle neutron scattering
, Journal of Applied Crystallography, Vol: 51, Pages: 570-583, ISSN: 0021-8898A comparative examination is presented of materials and approaches for the fabrication of microfluidic devices for small-angle neutron scattering (SANS). Representative inorganic glasses, metals, and polymer materials and devices are evaluated under typical SANS configurations. Performance criteria include neutron absorption, scattering background and activation, as well as spatial resolution, chemical compatibility and pressure resistance, and also cost, durability and manufacturability. Closed-face polymer photolithography between boron-free glass (or quartz) plates emerges as an attractive approach for rapidly prototyped microfluidic SANS devices, with transmissions up to ∼98% and background similar to a standard liquid cell (I ≃ 10−3 cm−1). For applications requiring higher durability and/or chemical, thermal and pressure resistance, sintered or etched boron-free glass and silicon devices offer superior performance, at the expense of various fabrication requirements, and are increasingly available commercially.
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Journal articleGonzalez Lopez C, Colby R, Cabral JP, 2018,
Electrostatic and hydrophobic interactions in NaCMC aqueous solutions: effect of degree of substitution
, Macromolecules, Vol: 51, Pages: 3165-3175, ISSN: 0024-9297The rheology of water soluble polyelectrolytesat intermediate and high concentrations is controlled byentanglements, hydrophobic and electrostatic interac-tions, whose influence is difficult to isolate. We investi-gate the rheology of semidilute solutions of sodium car-boxymethyl cellulose (NaCMC) with molecular weightMw'2.5×105g/mol and varying degree of substi-tution (D.S.) as a function of polymer concentration invarious solvent media: salt-free water (long ranged elec-trostatic interactions), 0.5M aqueous NaCl (screenedelectrostatics) and 0.5M aqueous NaOH (screened elec-trostatics, diminished hydrophobic interactions) in or-der to selectively probe the different interactions. De-creasing D.S. is found to decrease solubility and inducepartial aggregation and eventual gelation. In salt-freeand 0.5M NaCl solution, NaCMC with D.S.'1.2 ex-hibits hydrophilic polyelectrolyte and neutral polymerin good solvent behaviour respectively. Decreasing D.S.to'0.7-0.8 leads to hydrophobic behaviour in bothsolvents, becoming weak gels at high concentrations. In0.5M NaOH (pH = 13.5) the viscosities of samples withdifferent D.S. become identical when plotted againstthe overlap parameter, which we interpret as result-ing from the solubilisation of unsubstituted celluloseblocks. Small angle neutron scattering (SANS) data in-dicate that the polymer conformation is not stronglyaffected by hydrophobic interactions. By varying D.S.,ionic strength and pH, we demonstrate the tuning ofNaCMC-solvent interactions, controlling separately the electrostatic and hydrophobic effects on the solutionrheology.
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Conference paperVitale A, Hennessy M, Matar O, et al., 2018,
Controlling the evolution of frontal photopolymerization waves for 3D polymeric patterning
, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, ISSN: 0065-7727 -
Journal articleAdamo M, Poulos AS, G Lopez C, et al., 2018,
Droplet microfluidic SANS
, Soft Matter, Vol: 14, Pages: 1759-1770, ISSN: 1744-683XThe coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H2O/D2O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (∼1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.
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Journal articleUdoh C, CABRAL J, Garbin V, 2017,
Nanocomposite capsules with directional, pulsed nanoparticle release
, Science Advances, Vol: 3, ISSN: 2375-2548The precise spatiotemporal delivery of nanoparticles from polymeric capsules is required for applications ranging from medicine to materials science. These capsules derive key performance aspects from their overall shape and dimensions, porosity, and internal microstructure. To this effect, microfluidics provide an exceptional platform for emulsification and subsequent capsule formation. However, facile and robust approaches for nanocomposite capsule fabrication, exhibiting triggered nanoparticle release, remain elusive because of the complex coupling of polymer-nanoparticle phase behavior, diffusion, phase inversion, and directional solidification. We investigate a model system of polyelectrolyte sodium poly(styrene sulfonate) and 22-nm colloidal silica and demonstrate a robust capsule morphology diagram, achieving a range of internal morphologies, including nucleated and bicontinuous microstructures, as well as isotropic and non-isotropic external shapes. Upon dissolution in water, we find that capsules formed with either neat polymers or neat nanoparticles dissolve rapidly and isotropically, whereas bicontinuous, hierarchical, composite capsules dissolve via directional pulses of nanoparticle clusters without disrupting the scaffold, with time scales tunable from seconds to hours. The versatility, facile assembly, and response of these nanocomposite capsules thus show great promise in precision delivery.
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Journal articleHennessy M, Vitale A, Matar O, et al., 2017,
Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation
, Soft Matter, Vol: 13, Pages: 9199-9210, ISSN: 1744-683XFrontal photopolymerisation (FPP) is a directional solidification process that converts monomer-rich liquid into crosslinked polymer solid by light exposure and finds applications ranging from lithography to 3D printing. Inherent to this process is the creation of an evolving polymer network that is exposed to a monomer bath. A combined theoretical and experimental investigation is performed to determine the conditions under which monomer from this bath can diffuse into the propagating polymer network and cause it to swell. First, the growth and swelling processes are decoupled by immersing pre-made polymer networks into monomer baths held at various temperatures. The experimental measurements of the network thickness are found to be in good agreement with theoretical predictions obtained from a nonlinear poroelastic model. FPP propagation experiments are then carried out under conditions that lead to swelling. Unexpectedly, for a fixed exposure time, swelling is found to increase with incident light intensity. The experimental data is well described by a novel FPP model accounting for mass transport and the mechanical response of the polymer network, providing key insights into how monomer diffusion affects the conversion profile of the polymer solid and the stresses that are generated during its growth. The predictive capability of the model will enable the fabrication of gradient materials with tuned mechanical properties and controlled stress development.
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Journal articlePoulos AS, Jones CS, Cabral JT, 2017,
Dissolution of anionic surfactant mesophases
, Soft Matter, Vol: 13, Pages: 5332-5340, ISSN: 1744-683XLinear and circular solvent penetration experiments are used to study the dissolution of anionic SLE3S surfactant mesophases in water. We show that a lamellar (Lα) phase in contact with water will transit through a series of cubic, hexagonal, and micellar phase bands with sharp interfaces identified from their optical textures. In both linear and circular geometries, the kinetics of front propagation and eventual dissolution are well described by diffusive penetration of water, and a simple model applies to both geometries, with a different effective diffusion coefficient for water Df as the only fitting parameter. Finally, we show a surprising variation of dissolution rates with initial surfactant concentration that can be well explained by assuming that the driving force for solvent penetration is the osmotic pressure difference between neat water and the aqueous fraction of the mesophase that is highly concentrated in surfactant counterions.
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Journal articleBedoya-Lora FE, Hankin A, Holmes-Gentle I, et al., 2017,
Effects of low temperature annealing on the photo-electrochemical performance o tin-doped hematite photo-anodes
, Electrochimica Acta, Vol: 251, Pages: 1-11, ISSN: 0013-4686The effects of post-deposition annealing at 400 and 500 °C on the photo-electrochemical performance of SnIV-doped α-Fe2O3 photo-anodes are reported. Samples were fabricated by spray pyrolysis on fluorine-doped tin oxide (FTO) and on titanium substrates. Photo-electrochemical, morphological and optical properties were determined to explain the shift in photocurrent densities to lower electrode potentials and the decrease of maximum photocurrent densities for alkaline water oxidation after annealing. Annealing at 400 and 500 °C in air did not affect significantly the morphology, crystallinity, optical absorption or spatial distributions of oxygen vacancy concentrations. However, XPS data showed a redistribution of SnIV near SnIV-doped α-Fe2O3 | 1 M NaOH interfaces after annealing. Thus, electron-hole recombination rates at photo-anode surfaces decreased after annealing, shifting photocurrents to lower electrode potentials. Conversely, depletion of SnIV in the α-Fe2O3 bulk could increase recombination rates therein and decrease photon absorption near 550 nm, due to an increased dopant concentration in the semiconductor depletion layer. This accounted for the decrease of maximum photocurrents when electron-hole recombination rates were suppressed using HO2− ions as a hole scavenger. The flat band potential of SnIV-doped α-Fe2O3 remained relatively constant at ca. 0.7 V vs. RHE, irrespective of annealing conditions.
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Journal articleFoglia F, Karan S, Nania M, et al., 2017,
Neutron Reflectivity and Performance of Polyamide Nanofilms for Water Desalination
, ADVANCED FUNCTIONAL MATERIALS, Vol: 27, ISSN: 1616-301XThe structure and hydration of polyamide (PA) membranes are investigated with a combination of neutron and X-ray reflectivity, and their performance is benchmarked in reverse osmosis water desalination. PA membranes are synthesized by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), varying systematically reaction time, concentration, and stoichiometry, to yield large-area exceptionally planar films of ≈10 nm thickness. Reflectivity is employed to precisely determine membrane thickness and roughness, as well as the (TMC/MPD) concentration profile, and response to hydration in the vapor phase. PA film thickness is found to increase linearly with reaction time, albeit with a nonzero intercept, and the composition cross-sectional profile is found to be uniform, at the conditions investigated. Vapor hydration with H2O and D2O from 0 to 100% relative humidity results in considerable swelling (up to 20%), but also yields uniform cross-sectional profiles. The resulting film thickness is found to be predominantly set by the MPD concentration, while TMC regulates water uptake. A favorable correlation is found between higher swelling and water uptake with permeance. The data provide quantitative insight into the film formation mechanisms and correlate reaction conditions, cross-sectional nanostructure, and performance of the PA active layer in RO membranes for desalination.
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Journal articlePurnama AR, Hennessy MG, Vitale A, et al., 2017,
Cover Image, Volume 66, Issue 6
, Polymer International, Vol: 66, Pages: i-i, ISSN: 0959-8103 -
Journal articleLi Y, Klosowski MM, McGilvery CM, et al., 2017,
Probing flow activity in polyamide layer of reverse osmosis membrane with nanoparticle tracers
, JOURNAL OF MEMBRANE SCIENCE, Vol: 534, Pages: 9-17, ISSN: 0376-7388We investigate the flow activity of the nanostructured polyamide layer in reverse osmosis (RO) membrane, using gold nanoparticle (NP) tracers of 1–40 nm diameter. Following a detailed structural examination of a commercial SW30RH membrane selected for this study, NP solutions were infiltrated from either the polyamide front or the polysulfone support side. The permeate was then analyzed spectroscopically while the entrapment of NPs within the membrane was mapped by high resolution electron microscopy. Results show that back-filtered NPs exhibited a fractionated distribution according to size: 1 nm nanoparticles permeate across the polyamide-polysulfone interface reaching the interior of the polyamide corrugations, while the larger ones (>10 nm) are retained within the polysulfone and gradually arrested at approximately 100 nm below the polyamide-polysulfone interface. Intermediate-sized 5 nm nanoparticles reached the undulating folds just below the polyamide layer. Permeation pathways across polyamide layer appear to exclude all tracers above 1 nm, which become selectively distributed across the polyamide layer: positively charged NPs label the outer surface of the polyamide film (expected to be carboxylate-rich), while negatively charged particles are uniformly distributed within the layer. Diafiltration measurements quantify the transient kinetics of NP retention and permeation. Overall, our results establish the flow activity of the polyamide nodular surface and provide estimates for the dimensions of permeation pathways.
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Journal articleAdamo M, Poulos AS, Miller RM, et al., 2017,
Rapid contrast matching by microfluidic SANS
, Lab on a Chip, Vol: 17, Pages: 1559-1569, ISSN: 1473-0189We report a microfluidic approach to perform small angle neutron scattering (SANS) measurements of contrast variation and matching, extensively employed in soft and biological matter research. We integrate a low scattering background microfluidic mixer and serpentine channel in a SANS beamline to yield a single phase, continuous flow, reconfigurable liquid cell. By contrast with conventional, sequential measurements of discrete (typically 4–6) solutions of varying isotopic solvent composition, our approach continually varies solution composition during SANS acquisition. We experimentally and computationally determine the effects of flow dispersion and neutron beam overillumination of microchannels in terms of the composition resolution and precision. The approach is demonstrated with model systems: H2O/D2O mixtures, a surfactant (sodium dodecyl sulfate, SDS), a triblock copolymer (pluronic F127), and silica nanoparticles (Ludox) in isotopic aqueous mixtures. The system is able to zoom into a composition window to refine contrast matching conditions, and robustly resolve solute structure and form factors by simultaneous fitting of scattering data with continuously varying contrast. We conclude by benchmarking our microflow-SANS with the discrete approach, in terms of volume required, composition resolution and (preparation and measurement) time required, proposing a leap forward in equilibrium, liquid solution phase mapping and contrast variation by SANS.
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Journal articleMiller RM, Ces O, Brooks NJ, et al., 2017,
Crystallization of Sodium Dodecyl Sulfate-Water Micellar Solutions under Linear Cooling
, CRYSTAL GROWTH & DESIGN, Vol: 17, Pages: 2428-2437, ISSN: 1528-7483The crystallization kinetics of sodium dodecyl sulfate (SDS)-water micellar solutions, under linear cooling conditions, were experimentally investigated using optical microscopy, differential scanning calorimetry, and infrared spectroscopy. Cooling rates were systematically varied, from 0.1 to 50 °C min–1, encompassing environmental to near-“isothermal” temperature changes, between 22 and −5 °C, for a reference concentration of 20% SDS-H2O. The cooling rate was shown to determine the dominant crystal morphologies, with platelets and needles predominating at the lowest and highest rates, respectively. The results were rationalized in terms of isothermal crystallization data and the time–temperature cooling profile. Rates 0.1, 5.0, and 10 °C min–1 yield morphologies and kinetics analogous to those of isothermal quenches at the corresponding crystallization temperature window. Nontrivial deviations were observed for intermediate rates (0.5, 1.0 °C min–1), due to commensurate changes in temperature and crystallization mechanism, accompanied by solute depletion. The polythermal metastable zone width was estimated, and the non-isothermal nucleation described by the Nývlt equation, while the Avrami and Kissinger models described overall crystallization kinetics. Our measurements quantify the impact of temperature gradients in the crystallization of ubiquitous SDS micellar solutions, for a range of practically relevant profiles incurred during manufacturing and storage.
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Journal articlePurnama AR, Hennessy MG, Vitale A, et al., 2017,
Coarse-grained models for frontal photopolymerization with evolving conversion profile
, POLYMER INTERNATIONAL, Vol: 66, Pages: 752-760, ISSN: 0959-8103We introduce a series of ‘minimal’ models to describe a common light-driven directional solidification process, known as frontal photopolymerization (FPP), focusing on experimental observables: the solidification kinetics, light attenuation and spatiotemporal monomer-to-polymer conversion. Specifically, we focus on FPP propagation that yields conversion profiles that are not invariant with time, and which cannot be simply described by the presence of mass or heat diffusion. The models are assessed against experimental data for the photopolymerization of a model trimethacrylate system. We find that the simplest model, comprising a single equation of motion for the conversion fraction ϕ and a generalized Beer–Lambert law, can only describe the experimental data by assuming an unphysical variation in optical absorption. Introducing a ϕ-dependent reaction constant Keff is found to require a time dependence, regardless of the functional form in ϕ. We conclude by introducing a ‘minimal’ chemical model, which is based on a simple three-step reaction scheme involving the spatiotemporal evolution of the photoinitiator fraction, relative fraction of radicals and monomer conversion fraction, that is able to capture the experimental data with a small number of parameters and under reasonable FPP assumptions. Our framework provides important predictive ability for ubiquitous solidification and patterning processes, including three-dimensional printing, via photopolymerization.
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Journal articleNania M, Foglia F, Matar OK, et al., 2017,
Sub-100 nm wrinkling of polydimethylsiloxane by double frontal oxidation
, Nanoscale, Vol: 9, Pages: 2030-2037, ISSN: 2040-3364We demonstrate nanoscale wrinkling on polydimethylsiloxane (PDMS) at sub-100 nm length scales via a(double) frontal surface oxidation coupled with a mechanical compression. The kinetics of the glassy skinpropagation is resolved by neutron and X-ray reflectivity, and atomic force microscopy, combined withmechanical wrinkling experiments to evaluate the resulting pattern formation. In conventional PDMSsurface oxidation, the smallest wrinkling patterns attainable have an intrinsic lower wavelength limit dueto the coupling of skin formation and front propagation at fixed strain εprestrain, whose maximum is, inturn, set by material failure. However, combining two different oxidative processes, ultra-violet ozonolysisfollowed by air plasma exposure, we break this limit by fabricating trilayer laminates with excellent interfacialproperties and a sequence of moduli and layer thicknesses able to trivially reduce the surface topographyto sub-100 nm dimensions. This method provides a powerful, yet simple, non-lithographicapproach to extend surface patterning from visible to the deep UV range.
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Journal articleLopez CG, Colby RH, Graham P, et al., 2016,
Viscosity and Scaling of Semiflexible Polyelectrolyte NaCMC in Aqueous Salt Solutions
, Macromolecules, Vol: 50, Pages: 332-338, ISSN: 0024-9297We investigate the viscosity dependence on concentration and molecular weight of semiflexible polyelectrolyte sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and NaCl solutions. Combining new measurements and extensive literature data, we establish relevant power laws and crossovers over a wide range of degree of polymerization (N) as well as polymer (c) and salt (cs) concentrations. In salt-free solution, the overlap concentration shows the expected c* ∝ N–2 dependence, and the entanglement crossover scales as ce ∝ N–0.6±0.3, in strong disagreement with scaling theory for which ce ∝ c* is expected, but matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (ηsp ∝ c3.5±0.2), commonly assigned to the concentrated regime, is shown to follow c** ∝ N–0.6±0.2 (with c**/ce ≃ 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c* and ce in 0.01 and 0.1 M NaCl shows neutral polymer in good solvent behavior, characteristic of highly screened polyelectrolyte solutions. This unified scaling picture enables the estimation of viscosity of ubiquitous NaCMC solutions as a function of N, c, and cs and establishes the behavior expected for a range of semiflexible polyelectrolyte solutions.
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Journal articleBarnes PRF, Vaissier V, Garcia Sakai V, et al., 2016,
How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study
, Scientific Reports, Vol: 6, ISSN: 2045-2322Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.
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Journal articleGuilbert AAY, Cabral JT, 2016,
Impact of solution phase behaviour and external fields on thin film morphology: PCBM and RRa-P3HT model system
, Soft Matter, Vol: 13, Pages: 827-835, ISSN: 1744-683XWe report the impact of the ternary solution phase behaviour on the film morphology and crystallization of a model polymer:fullerene system. We employ UV-Vis absorption spectroscopy, combined with sequential filtration and dilution, to establish the phase diagram for regio-random poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene. Films are systematically cast from one- and two-phase regions decoupling homogeneous and heterogenous nucleation, and the role of pre-formed aggregates from solutions. Increasing annealing temperature from 120 to 200 °C reveals a highly non-monotonic nucleation profile with a maximum at 170 °C, while the crystal growth rate increases monotonically. UV ozonolysis is employed to vary substrate energy, and found to increase nucleation rate and to promote a binary crystallization process. As previously found, exposure to light, under an inert atmosphere, effectively suppresses homogeneous nucleation; however, it has a considerably smaller effect on heterogeneous nucleation, either from solution aggregates or substrate-driven. Our results establish a quantitative link between solution thermodynamics, crystallization and provide insight into morphological design based on processing parameters in a proxy organic photovoltaic system.
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Journal articleHennessy MG, Ferretti GL, Cabral JT, et al., 2016,
A minimal model for solvent evaporation and absorption in thin films
, Journal of Colloid and Interface Science, Vol: 488, Pages: 61-71, ISSN: 0021-9797We present a minimal model of solvent evaporation and absorption in thin films consisting of a volatile solvent and non-volatile solutes. An asymptotic analysis yields expressions that facilitate the extraction of physically significant model parameters from experimental data, namely the mass transfer coefficient and composition-dependent diffusivity. The model can be used to predict the dynamics of drying and film formation, as well as sorption/desorption, over a wide range of experimental conditions. A state diagram is used to understand the experimental conditions that lead to the formation of a solute-rich layer, or “skin”, at the evaporating surface during drying. In the case of solvent absorption, the model captures the existence of a saturation front that propagates from the film surface towards the substrate. The theoretical results are found to be in excellent agreement with data produced from dynamic vapour sorption experiments of ternary mixtures comprising an aluminium salt, glycerol, and water. Moreover, the model should be generally applicable to a variety of practical contexts, from paints and coatings, to personal care, packaging, and electronics.
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Journal articleVitale A, Cabral JT, 2016,
Frontal Conversion and Uniformity in 3D Printing by Photopolymerisation
, Materials, Vol: 9, ISSN: 1996-1944We investigate the impact of the non-uniform spatio-temporal conversion, intrinsic to photopolymerisation, in the context of light-driven 3D printing of polymers. The polymerisation kinetics of a series of model acrylate and thiol-ene systems, both neat and doped with a light-absorbing dye, is investigated experimentally and analysed according to a descriptive coarse-grained model for photopolymerisation. In particular, we focus on the relative kinetics of polymerisation with those of 3D printing, by comparing the evolution of the position of the conversion profile (zf) to the sequential displacement of the object stage (∆z). After quantifying the characteristic sigmoidal monomer-to-polymer conversion of the various systems, with a combination of patterning experiments, FT-IR mapping, and modelling, we compute representative regimes for which zf is smaller, commensurate with, or larger than ∆z. While non-monotonic conversion can be detrimental to 3D printing, for instance in causing differential shrinkage of inhomogeneity in material properties, we identify opportunities for facile fabrication of modulated materials in the z-direction (i.e., along the illuminated axis). Our simple framework and model, based on directly measured parameters, can thus be employed in photopolymerisation-based 3D printing, both in process optimisation and in the precise design of complex, internally stratified materials by coupling the z-stage displacement and frontal polymerisation kinetics.
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