Publications

Journal article

Seddon D, Müller EA, Cabral JT, 2022,

Machine learning hybrid approach for the prediction of surface tension profiles of hydrocarbon surfactants in aqueous solution

, Journal of Colloid and Interface Science, Vol: 625, Pages: 328-339, ISSN: 0021-9797

HYPOTHESIS: Predicting the surface tension (SFT)-log(c) profiles of hydrocarbon surfactants in aqueous solution is computationally non-trivial, and empirically challenging due to the diverse and complex architecture and interactions of surfactant molecules. Machine learning (ML), combining a data-based and knowledge-based approach, can provide a powerful means to relate molecular descriptors to SFT profiles. EXPERIMENTS: A dataset of SFT for 154 model hydrocarbon surfactants at 20-30 °C is fitted to the Szyszkowski equation to extract three characteristic parameters (Γmax,KL and critical micelle concentration (CMC)) which are correlated to a series of 2D and 3D molecular descriptors. Key (∼10) descriptors were selected by removing co-correlation, and employing a gradient-boosted regressor model to rank feature importance and carry out recursive feature elimination (RFE). The hyperparameters of each target-variable model were fine-tuned using a randomised cross-validated grid search, to improve predictive ability and reduce overfitting. FINDINGS: The ML models correlate favourably with test experimental data, with R2= 0.69-0.87, and the merits and limitations of the approach are discussed based on 'unseen' hydrocarbon surfactants. The incorporation of a knowledge-based framework provides an appropriate smoothing of the experimental data which simplifies the data-driven approach and enhances its generality. Open-source codes and a brief tutorial are provided.

Journal article

Tan A, Pellegrino L, Ahmad Z, Cabral JTet al., 2022,

Tunable structural color with gradient and multiaxial polydimethylsiloxane wrinkling

, Advanced Optical Materials, Vol: 10, ISSN: 2195-1071

The generation of structural color from wrinkled polydimethylsiloxane (PDMS) surfaces, fabricated by plasma exposure, subjected to uni- and multi-axial, and sequential strain fields is examined. The approach is based on the well-known, mechanically-induced, buckling instability of a supported bilayer, whereby the top glassy “skin” is formed by plasma oxidation. Surface periodicities 200 nm ≲ d ≲ 3 μm, encompassing the visible spectrum, are investigated in terms of the observed color, intensity spectrum, and color mixing from different diffraction orders, exhibiting good agreement with model predictions. By contrast with complex fabrication methods, color tunability and mechanochromic response are readily achieved by adjusting plasma and strain parameters, and by dynamically varying strain (ε ≲ 50%). Prescribed strain directionality, employing uniaxial, isotropic, gradient strain, and wave-sum wrinkling superposition, as well as skin thickness (and thus d) and amplitude gradients, using facile and scalable fabrication approaches, yield striking spatial color variation, homogeneity, and directionality.

Journal article

Pellegrino L, Kriem LS, Robles ESJ, Cabral JTet al., 2022,

Microbial response to micrometer-scale multiaxial wrinkled surfaces.

, ACS Applied Materials and Interfaces, Vol: 14, ISSN: 1944-8244

We investigate the effect of micrometer-scale surface wrinkling on the attachment and proliferation of model bacteria (Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli K12) and fungi (Candida albicans). Specifically, sinusoidal (1D), checkerboard (C), and herringbone (H) patterns were fabricated by mechanical wrinkling of plasma-oxidized polydimethylsiloxane (PDMS) bilayers and contrasted with flat (F) surfaces. Microbial deformation and orientation were found to correlate with the aspect ratio and commensurably with surface pattern dimensions and local pattern order. Significantly, the proliferation of P. aeruginosa could be described by a linear scaling between bacterial area coverage and available surface area, defined as a fraction of the line integral along each profile with negative curvature. However, in the early stages of proliferation (up to 6 h examined), that C and H patterns disrupt the spatial arrangement of bacteria, impeding proliferation for several hours and reducing it (by ∼50%) thereafter. Our findings suggest a simple framework to rationalize the impact of micrometer-scale topography on microbial action and demonstrate that multiaxial patterning order provides an effective strategy to delay and frustrate the early stages of bacterial proliferation.

Journal article

Tyagi G, Sharratt WN, Erikson S, Seddon D, Robles ESJ, Cabral JTet al., 2022,

Solution structures of anionic-amphoteric surfactant mixtures near the two-phase region at fixed pH

, Langmuir: the ACS journal of surfaces and colloids, Vol: 38, Pages: 7198-7207, ISSN: 0743-7463

We examine the solution structures in a mixed surfactant system of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO) in water, on both sides of the two-phase boundary, employing dynamic light scattering, small-angle neutron scattering, and Fourier transform infrared spectroscopy. The precipitate phase boundary was accessed by lowering pH to 8, from its floating pH 9.5 value, and was experimentally approached from the monomeric and micellar regions in three ways: at fixed DDAO or SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We characterize the size, shape, and interactions of micelles, which elongate approaching the boundary, leading to the formation of disk-like aggregates within the biphasic region, coexisting with micelles and monomers. Our data, from both monomeric and micellar solutions, indicate that the two phase structures formed are largely pathway-independent, with dimensions influenced by both pH and mixed surfactant composition. Precipitation occurs at intermediate stoichiometries with a similar SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant transition, returning to the mixed micelle phase. Overall, our findings demonstrate the effect of stoichiometry and solution pH on the synergistic interaction of mixed surfactants and their impact on phase equilibrium and associated micellar and two-phase structures

Journal article

Foglia F, Frick B, Nania M, Livingston A, Cabral Jet al., 2022,

Multimodal confined water dynamics in reverse osmosis polyamide membranes

, Nature Communications, Vol: 13, ISSN: 2041-1723

While polyamide (PA) membranes are widespread in water purification and desalination by reverse osmosis, a molecular-level understanding of the dynamics of both confined water and polymer matrix remains elusive. Despite the dense hierarchical structure of PA membranes formed by interfacial polymerization, previous studies suggest that water diffusion remains largely unchanged with respect to bulk water. Here, we employ neutron spectroscopy to investigate PA membranes under precise hydration conditions, and a series of isotopic contrasts, to elucidate water transport and polymer relaxation, spanning ps-ns timescales, and Å-nm lengthscales. We experimentally resolve, for the first time, the multimodal diffusive nature of water in PA membranes: in addition to (slowed down) translational jump-diffusion, we observe a long-range and a localized mode, whose geometry and timescales we quantify. The PA matrix is also found to exhibit rotational relaxations commensurate with the nanoscale confinement observed in water diffusion. This comprehensive ‘diffusion map’ can anchor molecular and nanoscale simulations, and enable the predictive design of PA membranes with tuneable performance.

Journal article

Pellegrino L, Tan A, Cabral JT, 2022,

Ripple patterns spontaneously emerge through sequential wrinkling interference in polymer bilayers

, Physical Review Letters, Vol: 128, ISSN: 0031-9007

We report the formation of “ripple” patterns by the sequential superposition of nonorthogonal surfacewaves excited by the spontaneous buckling of polymeric bilayers. Albeit of a different nature and micronscale compared to the familiar sedimentary ripples caused by gentle wave oscillations, we findcommonalities in their topography, defects, and bifurcations. The patterns are rationalized in terms ofa defect density that depends on the relative angle between generations, and a constant in-plane bendingangle that depends on skin thickness. A minimal wave summation model enables the design of ripple andcheckerboard surfaces by tuning material properties and fabrication process.

Journal article

O'Connell RA, Sharratt WN, Aelmans NJJ, Higgins JS, Cabral JTet al., 2022,

SANS Study of PPPO in mixed solvents and impact on polymer nanoprecipitation

, Macromolecules, Vol: 55, Pages: 1050-1059, ISSN: 0024-9297

We investigate the conformation of poly(2,6-diphenyl-p-phenylene oxide) (PPPO) in good and mixed solvents by small-angle neutron scattering (SANS) across its ternary phase diagram. Dichloromethane was selected as a “good” solvent and heptane as a “poor” solvent whose addition eventually induces demixing and polymer precipitation. Below the overlap concentration c*, the polymer conformation is found to be well described by the polymer-excluded volume model and above by the Ornstein–Zernike expression with a correlation length ξ which depends on the concentration and solvent/nonsolvent ratio. We quantify the decrease in polymer radius of gyration Rg, increase in ξ, and effective χ parameter approaching the phase boundary. Upon flash nanoprecipitation, the characteristic particle radius (estimated by scanning electron microscopy, SEM) is found to scale with polymer concentration as well as with nonsolvent content. Significantly, the solution volume per precipitated particle remains nearly constant at all polymer concentrations. Overall, our findings correlate ternary solution structure with the fabrication of polymer nanoparticles by nonsolvent-induced phase separation and precipitation.

Journal article

Donina L, Rafique A, Khodaparast S, Porcar L, Cabral JTet al., 2021,

Lamellar-to-MLV transformation in SDS/octanol/brine examined by microfluidic-SANS and polarised microscopy

, Soft Matter, Vol: 17, Pages: 10053-10062, ISSN: 1744-683X

The lamellar-to-multilamellar vesicle (MLV) transformation in a model surfactant system, sodium dodecyl sulfate (SDS), octanol and brine, is investigated under continuous and oscillatory microfluidic contraction–expansion flows, employing polarised optical microscopy and small angle neutron scattering (SANS), with sample volume probed down to ≃20 nL. We determine the lamellar-to-MLV transition requirements at varying flow velocity, oscillation amplitude, frequency, and number of oscillatory cycles. The spatio-temporal evolution of the hierarchical fluid structure is elucidated: lamellar sheets initially align with flow direction upon entering a constriction and then perpendicularly upon exiting; the formation of MLVs at the nanoscale is first observed by SANS within a few (<5) oscillatory cycles, followed by the gradual appearance of a regular (albeit not crystalline) MLV arrangement, at the micronscale, by optical microscopy after tens of cycles, under the conditions investigated. Once MLVs form under flow, these remain metastable for several days.

Journal article

Khodaparast S, Sharratt WN, Dalgliesh RM, Cabral JTet al., 2021,

Growth of myelin figures from parent multilamellar vesicles

, Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 12512-12517, ISSN: 0743-7463

We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as dMLV ∝ t1/2, myelins grow linearly in time as LM ∝ t1, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium.

Journal article

Tan A, Pellegrino L, Cabral JT, 2021,

Tunable phase gratings by wrinkling of plasma-oxidized PDMS: gradient skins and multiaxial patterns

, ACS Applied Polymer Materials, Vol: 3, Pages: 5162-5170, ISSN: 2637-6105

Wrinkling instabilities in polymeric bilayers have been exploited as optical phase gratings with tunable performance. Here, we report strain modulated 1D and 2D phase gratings fabricated by the ubiquitous process of plasma-oxidation of polydimethylsiloxane (PDMS). While surface oxidation provides a remarkably facile glassy skin formation approach, minimizing delamination and debonding, it inherently results in a gradient conversion profile emanating from the top film interface. We examine and quantitatively model the consequences of this gradient layer on the optical properties of the resulting strain-tunable phase gratings. Diffraction efficiencies up to 48% are demonstrated. We then develop and validate a surface reconstruction methodology based on the diffraction pattern of our sinusoidal gratings and our model, which we extend to the high deformation regimes and to 2D gratings, obtained by superposition of two wrinkling generations, where both amplitudes and wavelengths can be independently tuned. Overall, this approach provides a rapid, robust and predictive framework for the design and fabrication of tunable, single, and multiaxial surface gratings.

Journal article

Sharratt WN, Lee VE, Priestley RD, Cabral JTet al., 2021,

Precision polymer particles by flash nanoprecipitation and microfluidic droplet extraction

, ACS Applied Polymer Materials, Vol: 3, Pages: 4746-4768, ISSN: 2637-6105

We comparatively review two versatile approaches employed in the precise formation of polymer particles, with length scales from 10s of nm to to 100s μm, from ternary polymer(s), solvent and nonsolvent mixtures. Flash nanoprecipitation (FNP) utilizes an opposing jet arrangement to mix a dilute polymer solution and a nonsolvent in confinement, inducing a rapid (∼millisecond) chain collapse and eventual precipitation of nanoparticles (NPs) of 10–1000 nm diameters. FNP of polymer mixtures and block copolymers can yield a range of multiphase morphologies with various functionalities. While droplet solvent extraction (DSE) also involves the exposure of a polymer solution to a nonsolvent, in this case the polymer solution is templated into a droplet prior to solvent extraction, often using microfluidics, resulting in polymer particles of 1–1000 μm diameter. Droplet shrinkage and solvent exchange are generally accompanied by a series of processes including demixing, coarsening, phase inversion, skin formation, and kinetic arrest, which lead to a plethora of possible internal and external particle morphologies. In the absence of external flow fields, DSE corresponds effectively to nonsolvent induced phase separation (NIPS) in a spherical geometry. In this review, we discuss the requirements to implement both approaches, detailing consequences of ternary solution phase behavior and the interplay of the various processes underpinning particle formation and highlighting the similarities, differences, and complementarity of FNP and DSE. In addition to reviewing previous work in the field, we report comparative experimental results on the formation of polystyrene particles by both approaches, emphasizing the importance of solution phase behavior in process design.

Journal article

Sharratt WN, Lopez CG, Sarkis M, Tyagi G, O'Connell R, Rogers SE, Cabral JTet al., 2021,

Ionotropic gelation fronts in sodium carboxymethyl cellulose for hydrogel particle formation

, Gels, Vol: 7, ISSN: 2310-2861

Hydrogel microparticles (HMPs) find numerous practical applications, ranging from drug delivery to tissue engineering. Designing HMPs from the molecular to macroscopic scales is required to exploit their full potential as functional materials. Here, we explore the gelation of sodium carboxymethyl cellulose (NaCMC), a model anionic polyelectrolyte, with Fe3+ cations in water. Gelation front kinetics are first established using 1D microfluidic experiments, and effective diffusive coefficients are found to increase with Fe3+ concentration and decrease with NaCMC concentrations. We use Fourier Transform Infrared Spectroscopy (FTIR) to elucidate the Fe3+-NaCMC gelation mechanism and small angle neutron scattering (SANS) to spatio-temporally resolve the solution-to-network structure during front propagation. We find that the polyelectrolyte chain cross-section remains largely unperturbed by gelation and identify three hierarchical structural features at larger length scales. Equipped with the understanding of gelation mechanism and kinetics, using microfluidics, we illustrate the fabrication of range of HMP particles with prescribed morphologies

Journal article

Tyagi G, Seddon D, Khodaparast S, Sharratt WN, Robles ESJ, Cabral JTet al., 2021,

Tensiometry and FTIR study of the synergy in mixed SDS:DDAO surfactant solutions at varying pH

, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol: 618, ISSN: 0927-7757

The interactions between a model anionic and amphoteric surfactant pair in aqueous solution are examined as a function of composition, at floating and fixed pH, employing a combination of tensiometry, regular solution theory analysis, and FTIR spectroscopy. An extensive series of pure and mixed ratios of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO), ranging from 0.0016 to 100 mM, yielding 77 data points below and above the critical micelle concentrations (CMC), is investigated. Compared to either pure surfactant solutions, the CMC of mixed SDS:DDAO solutions is found to decrease by up to 20-fold, and the surface tension (γ) at CMC down to ≃ 23 mN/m. At all concentrations, the most prominent effects are observed at equimolar SDS:DDAO ratios. Further, the pH of mixed micellar solutions is found to increase with respect to the pure surfactant solutions (from ≃ 7 up to ≃ 9.5), which is attributed to the enhanced protonation of DDAO in the presence of SDS, and supported by FTIR frequency shifts of isolated O‒H stretching vibrations. Vibrational responses from CH2 stretching of the methylene tails, and the S‒O stretching modes for the sulfate headgroups indicate strong lateral interaction and enhanced packing between SDS and DDAO. From regular solution theory analysis of tensiometry data, the molecular interaction parameters are found to have a larger magnitude (i.e., more negative) at the interface as compared to within micelles. At fixed solution pH, a decrease from pH 9.5 to 7.5 results in minimal changes in both interfacial and micellar parameters, indicating the intrinsic origin of these pairwise interactions. Overall, our findings demonstrate a pronounced synergistic interaction between SDS and DDAO, arising from diminished electrostatic and steric repulsions in, respectively, SDS and DDAO, accompanied by enhanced lateral surfactant packing.

Journal article

Khodaparast S, Sharratt WN, Tyagi G, Dalgliesh RM, Robles ESJ, Cabral JTet al., 2021,

Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition

, Journal of Colloid and Interface Science, Vol: 582, Pages: 1116-1127, ISSN: 0021-9797

Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0–60 °C), on the 20 wt% SDS micellar solutions with varying DDAO (5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations.

Journal article

Khodaparast S, Marcos J, Sharratt WN, Tyagi G, Cabral JTet al., 2021,

Surface-induced crystallization of sodium dodecyl sulfate (SDS) micellar solutions in confinement

, Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 230-239, ISSN: 0743-7463

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10–30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18–65 mN/m) and substrate roughness (Rα ≃ 0–60 μm) on the kinetics of surface-induced crystallization through measurements of induction time (ti) under isothermal conditions. While ti is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ∼40 fold. For substrates with the lowest surface energy and longest ti, microscale surface roughness is found to enhance crystal nucleation, in particular for Rα ≥ 10 μm. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Journal article

Wong HC, Wang Q, Speller EM, Li Z, Cabral JT, Low HYet al., 2020,

Photoswitchable solubility of fullerene-doped polymer thin films

, ACS Nano, Vol: 14, Pages: 11352-11362, ISSN: 1936-0851

Controlling polymer film solubility is of fundamental and practical interest and is typically achieved by synthetically modifying the polymer structure to insert reactive groups. Here, we demonstrate that the addition of fullerenes or its derivatives (C60 or phenyl-C61-butyric acid methyl ester, PCBM) to polymers, followed by ultraviolet (UV) illumination can change the film solubility. Contrary to most synthetic polymers, which dissolve in organic solvents but not in water, the fullerene-doped polymer films (such as polystyrene) can dissolve in water yet remain stable in organic solvents. This photoswitchable solubility effect is not observed in either film constituents individually and is derived from a synergy of photochemistries. First, polymer photooxidation generates macroradicals which cross-link with radical-scavenging PCBM, thereby contributing to the films' insolubility in organic solvents. Second, light exposure enhances polymer photooxidation in the presence of PCBM via the singlet oxygen pathway. This results in polymer backbone scission and formation of photooxidized products which can form hydrogen bonds with water, both contributing to water solubility. Nevertheless, the illuminated doped polymer thin films are mechanically robust, exhibiting significantly increased modulus and density compared to their pristine counterpart, such that they can remain intact even upon sonication in conventional organic solvents. We further demonstrate the application of this solubility-switching effect in dual tone photolithography, via a facile, economical, and environmentally benign solution-processing route made possible by the photoactive nature of polymer-PCBM thin films.

Journal article

Rafique AS, Khodaparast S, Poulos AS, Sharratt WN, Robles ESJ, Cabral JTet al., 2020,

Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt

, Soft Matter, Vol: 16, Pages: 7835-7844, ISSN: 1744-683X

We investigate the shape, dimensions, and transformation pathways of micelles of linear sodium alkylbenzenesulfonate (NaLAS), a common anionic surfactant, in aqueous solution. Employing Small Angle Neutron Scattering (SANS) and surface tensiometry, we quantify the effects of surfactant concentration (0.6–15 wt%), temperature (5–40 °C) and added salt (≤0.35 M Na2SO4). Spherical micelles form at low NaLAS (≤2.6 wt%) concentration in water, and become elongated with increasing concentration and decreasing temperature. Addition of salt reduces the critical micelle concentration (CMC) and thus promotes the formation of micelles. At fixed NaLAS concentration, salt addition causes spherical micelles to grow into cylindrical micelles, and then multilamellar vesicles (MLVs), which we examine by SANS and cryo-TEM. Above a threshold salt concentration, the MLVs reach diameters of 100 s of nm to few μm, eventually causing precipitation. While the salt concentrations associated with the micelle-to-cylinder transformation increase only slightly with temperature, those required for the cylinder-to-MLV transformation exhibit a pronounced, linear temperature dependence, which we examine in detail. Our study establishes a solution structure map for this model anionic surfactant in water, quantifying the combined roles of concentration, temperature and salt, at practically relevant conditions.

Journal article

White RP, Aoki Y, Higgins JS, Keddie JL, Lipson JEG, Cabral JTet al., 2020,

Thermodynamics of model PαMSAN/dPMMA blend: a combined study by SANS, ellipsometry, and locally correlated lattice (LCL) theory

, Macromolecules, Vol: 53, Pages: 7084-7095, ISSN: 0024-9297

We combine experiment and theory to elucidate how small, local, structural changes can impact miscibility in polymer blends. Small-angle neutron scattering (SANS) experiments yield both the phase boundaries and the temperature dependence of the second derivative of the free energy of mixing. We demonstrate here, for the first time, that a fundamental characterization of pure component properties can be achieved through ellipsometry measurements on films of pure polymers (thickness ∼200 nm) to provide key data on the volume (or thickness)–temperature relationships; this development is significant given the scarcity of precise pressure–volume–temperature (PVT) data on pure polymers and blends. The experimental measurements allow us to undertake a detailed thermodynamic analysis of mixing using the locally correlated lattice (LCL) theory, which has been shown to be effective in rationalizing blend miscibility in terms of the pure component properties. We focus here on polymer blends of poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) with deuterated poly(methyl methacrylate) (dPMMA), which differ in the degree of tacticity in the dPMMA component (atactic or syndiotactic), leading to an increase in miscibility for the latter. By combining LCL analysis of pure and mixed systems, we are able to connect tacticity changes to shifts in local nonbonded interactions, in free volume, and in thermal expansion coefficients, which in turn impact the thermodynamic compatibility of the blend components.

Journal article

Higgins JS, Cabral JT, 2020,

A thorny problem? spinodal decomposition in polymer blends

, Macromolecules, Vol: 53, Pages: 4137-4140, ISSN: 0024-9297

Journal article

McGilvery CM, Abellan P, Klosowski MM, Livingston AG, Cabral JT, Ramasse QM, Porter AEet al., 2020,

Nanoscale chemical heterogeneity in aromatic polyamide membranes for reverse osmosis applications

, ACS Applied Materials & Interfaces, Vol: 12, Pages: 19890-19902, ISSN: 1944-8244

Reverse osmosis membranes are used within the oil and gas industry for seawater desalination on off-shore oilrigs. The membranes consist of three layers of material: a polyester backing layer, a polysulfone support and a polyamide (PA) thin film separating layer. It is generally thought that the PA layer controls ion selectivity within the membrane but little is understood about its structure or chemistry at the molecular scale. This active polyamide layer is synthesized by interfacial polymerization at an organic/aqueous interface between m-phenylenediamine and trimesoyl chloride, producing a highly cross-linked PA polymer. It has been speculated that the distribution of functional chemistry within this layer could play a role in solute filtration. The only technique potentially capable of probing the distribution of functional chemistry within the active PA layer with sufficient spatial and energy resolution is scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS). Its use is a challenge because organic materials suffer beam-induced damage at relatively modest electron doses. Here we show that it is possible to use the N K-edge to map the active layer of a PA film using monochromated EELS spectrum imaging. The active PA layer is 12 nm thick, which supports previous neutron reflectivity data. Clear changes in the fine structure of the C K-edge across the PA films are measured and we use machine learning to assign fine structure at this edge. Using this method, we map highly heterogeneous intensity variations in functional chemistry attributed to N—C═C bonds within the PA. Similarities are found with previous molecular dynamics simulations of PA showing regions with a higher density of amide bonding as a result of the aggregation process at similar length scales. The chemical pathways that can be deduced may offer a clearer understanding of the transport mechanisms through the membrane.

Journal article

Wang H, Khodaparast S, Carroll J, Kelly C, Robles ESJ, Cabral JTet al., 2020,

A microfluidic-multiwell platform for rapid phase mapping of surfactant solutions

, Review of Scientific Instruments, Vol: 91, Pages: 1-11, ISSN: 0034-6748

Measurement of the phase behavior and (meta)stability of liquid formulations, including surfactant solutions, is required for the understanding of mixture thermodynamics, as well as their practical utilization. We report a microfluidic platform with a stepped temperature profile, imposed by a dual Peltier module, connected to an automated multiwell plate injector and optical setup, for rapid solution phase mapping. The measurement protocol is defined by the temperature step ΔT ≡ T1 − T2 (≲100 °C), volumetric flow rate Q ≡ ΔV/Δt (≲50 μl/min), which implicitly set the thermal gradient ΔT/Δt (≃0.1–50 °C/min), and measurement time (which must exceed the intrinsic timescale of the relevant phase transformation). Furthermore, U-shaped microchannels can assess the reversibility of such transformations, yielding a facile measurement of the metastable zone width of the phase diagram. By contrast with traditional approaches, the platform precisely controls the cooling and heating rates by tuning the flow rate, and the absolute temperature excursion by the hot and cold thermal profile, which remain stationary during operation, thus allowing the sequential and reproducible screening of large sample arrays. As a model system, we examined the transition from the micellar (L1) to the liquid crystalline lamellar phase (Lα), upon cooling, of aqueous solutions of sodium linear alkylbenzene sulfonate, a biodegradable anionic surfactant extensively employed in industry. Our findings are validated with quiescent optical microscopy and small angle neutron scattering data.

Journal article

Sharratt WN, OConnell R, Rogers SE, Lopez CG, Cabral JTet al., 2020,

Conformation and phase behavior of sodium carboxymethyl cellulose in the presence of mono- and divalent salts

, Macromolecules, Vol: 53, Pages: 1451-1463, ISSN: 0024-9297

We report a small-angle neutron scattering (SANS) study of semidilute aqueous solutions of sodium carboxymethyl cellulose (NaCMC), in the presence of mono- (Na+) and divalent salts (Mg2+, Ca2+, Zn2+, and Ba2+). A degree of substitution of 1.3 is selected to ensure that, in salt-free solution, the polymer is molecularly dissolved. We find that Na+ and Mg2+ salt addition yields H-type phase behavior, while Ca2+, Zn2+, and Ba2+ instead yield a mixed H/L-type phase behavior dependent on the NaCMC concentration (cp), in the decreasing order of the salt concentration required to induce turbidity (at a fixed cp). Charge screening by addition of NaCl induces the disappearance of the characteristic polyelectrolyte correlation peak and eventually yields scattering profiles with a q–1 dependence over nearly 3 decades in the wavenumber q. By fitting a descriptive model to data with excess Na+, we obtain a correlation length ξ′ = 1030 cp–0.72 Å with cp in g L–1. Addition of Mg2+, which does not interact specifically with NaCMC carboxylate groups, yields an analogous screening behavior to that of Na+, albeit at lower salt concentrations, in line with its higher ionic strength. At low salt concentration, addition of specifically interacting Ca2+, Zn2+, and Ba2+ yields a comparatively greater screening of the polyelectrolyte correlation peak, and at concentrations above the phase boundary, results in excess scattering at low-q, compatible with the formation of 20–40 nm clusters. This behavior is interpreted as due to the reduction in charge density along the chain, promoting interchain association and multichain domain formation resulting in visible turbidity. Overall, drawing analogies with NaCMC at a lower degree of substitution, where hydrophobic association takes place, our findings provide a framework to describe the solution structure and phase behavior of NaCMC in salt-free and salt solutions.

Journal article

Pellegrino L, Khodaparast S, Cabral JT, 2020,

Orthogonal wave superposition of wrinkled, plasma-oxidised, polydimethylsiloxane surfaces

, Soft Matter, Vol: 16, Pages: 595-603, ISSN: 1744-683X

We report a versatile approach to generate 2D dual-frequency patterns on soft substrates by superposition of 1D single-frequency wrinkles. Wave superposition is achieved by applying sequential orthogonal strains to elastomeric coupons, as opposed to the application of a (simultaneous) biaxial strain field. First, a 1D wrinkling pattern is induced by the well-known mechanical instability of a bilayer formed by oxygen plasma-oxidation of a (pre-strained) polydimethylsiloxane elastomer. The wrinkled surface formed upon strain release is then replicated to obtain a stress-free substrate, and stretched in the direction perpendicular to the first generation. Subsequent plasma exposure and mechanical relaxation (with independent process parameters) yield a prescribed second-generation wrinkling, whose profile and dependence on the first generation we examine in detail. By independently varying plasma oxidation and strain parameters in both directions, we demonstrate the formation of a wide array of topographies, including arrays of symmetric 2D checkerboard patterns with exceptional area coverage with respect to those formed by simultaneous 2D wrinkling. While the resulting topographies cannot be explained in terms of a simple orthogonal wave superposition, we show that, by accounting for the orthogonal prestrain experienced by the first wrinkling generation, the resulting 2D patterns can be readily calculated from 1D wrinkling behaviour.

Journal article

Aoki Y, Sharratt W, Wang H, O'Connell R, Pellegrino L, Rogers S, Dalgliesh RM, Higgins JS, Cabral JTet al., 2020,

Effect of tacticity on the phase behavior and demixing of p alpha MSAN/dPMMA blends investigated by SANS

, Macromolecules, Vol: 53, Pages: 445-457, ISSN: 0024-9297

We investigate the effect of polymer tacticity on the phase behavior and phase separation of polymer mixtures by small-angle neutron scattering (SANS). Poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) and deuterated poly(methyl methacrylate) (dPMMA) with two degrees of syndiotacticity were selected as a model partially miscible blend, as one of the most highly interacting systems known (defined by the temperature dependence of the blend’s interaction parameter). One-phase (equilibrium) and time-resolved, spinodal demixing experiments were analyzed by de Gennes’ random phase approximation (RPA) and Cahn–Hilliard–Cook (CHC) theory, respectively. The second derivative of the Gibbs free energy of mixing with respect to composition (G″ ≡ ∂2ΔGm/∂ϕ2) and corresponding χ parameter were obtained from both RPA and CHC analysis and found to correlate well across the phase boundary. We find that blends with higher PMMA syndiotacticity exhibit greater miscibility and a steeper G″ temperature dependence by ∼40%. The segment length of dPMMA with higher syndiotacticity was found to be a = 7.4 Å, slightly larger than 6.9 Å reported for lower syndiotacticity dPMMA. Consideration of thermal fluctuations is required for the self-consistent analysis of the nontrivial evolution of the spinodal peak position q* over time, corroborated by CHC model calculations. The temperature dependence of the mobility parameter, M, can be described by a “fast-mode” average of the diffusion coefficients of the blend constituents, except for quenches originating near the glass transition. A minimum demixing length scale of Λ ≈ 40 nm is obtained, in agreement with the theory for deeper quenches, but deviates at shallower quenches, whose origin we discuss. CHC correctly describes demixing length and time scales, except for quenches into the vicinity of the spinodal boundary. Our data demonstrat

Book chapter

Sharratt WN, Cabral JT, 2020,

Design and fabrication of polymer microparticles and capsules using microfluidics

, Polymer Colloids: Formation, Characterization and Applications, Editors: Priestley, Prud'homme, Pages: 100-147

Since the advent of microfluidics in the late 1990s, microfluidic approaches to polymer microparticle and capsule formation have become widespread. They benefit from the precise spatio-temporal control attainable over single and multi-phase channel flows, coupled with a range of solidification strategies, which enable the predictive and reproducible design and manufacture of unprecedented polymeric and composite particles. The control over particle shape, microstructure and architecture, monodispersity and regularity, provides unique chemical, biological, bio-medical and physical opportunities for the complex assembly and functionality of these materials. In this chapter, we summarise recent developments of the use of microfluidics for particle and capsule formation, providing an overview of the main approaches available for their manufacture. We describe key mechanistic and design considerations, including system compatibility and demonstrated capability, seeking to establish limitations and identify unexplored opportunities for these methods. We conclude with an outlook on future directions in terms of scalability, functionality, phase space mapping and commercial and societal impact, of this creative and rapidly evolving soft matter field.

Journal article

O'Connell RA, Porter AE, Higgins JS, Cabral JTet al., 2019,

Phase behaviour of poly(2, 6-diphenyl-p-phenylene oxide) (PPPO) in mixed solvents

, Polymer, Vol: 180, ISSN: 0032-3861

The solution phase behaviour of poly(2, 6-diphenyl-p-phenylene oxide) (PPPO) is investigated by a combination of turbidimetry, infrared spectroscopy, dynamic light scattering and densitometry, combined with calorimetry and X-ray scattering. We select dichloromethane (DCM) and heptane as, respectively, representative good and poor solvents for the polymer. This ternary system results in a miscibility gap which can be utilised for the design and fabrication of PPPO porous materials, membranes and scaffolds via phase inversion. We establish the phase diagram and resolve the kinetic solidification condition arising from the intersection between the coexistence and glass transition curves. PPPO exhibits a high 230 ∘C and is found to crystallise at 336 ∘C, and melt at 423, 445 ∘C with a double endotherm. The kinetics of demixing and (buoyancy-driven) stratification are quantified by optical imaging and the PPPO-rich phase analysed by SAXS/WAXS to resolve both amorphous and crystalline phases. Equipped with this knowledge, we demonstrate the controlled formation of nodular, bicontinuous and cellular morphologies by non-solvent induced demixing.

Journal article

Hynes EL, Cabral JT, Parnell AJ, Gutfreund P, Welbourn RJL, Dunbar ADF, Mon D, Higgins AMet al., 2019,

Interfacial width and phase equilibrium in polymer-fullerene thin-films

, Communications Physics, Vol: 2, ISSN: 2399-3650

Domain composition and interfacial structure are critical factors in organic photovoltaic performance. Here, we report neutron reflectivity, grazing-incidence X-ray diffraction and atomic force microscopy measurements of polymer/fullerene thin-films to test a hypothesis that these partially miscible blends rapidly develop composition profiles consisting of co-existing phases in liquid-liquid equilibrium. We study a range of polymer molecular weights between 2 and 300 kg mol−1, annealing temperatures between 120 and 170 oC, and timescales up to 10 min, yielding over 50 distinct measurement conditions. Model bilayers of fullerene-derivatives and polystyrene enable a rigorous examination of theoretical predictions of the effect of polymer mass and interaction parameter on the compositions, ϕ, and interfacial width, w, of the coexistent phases. We independently measure ϕ and w and find that both Flory-Huggins mean-field-theory and key aspects of self-consistent-field-theory are remarkably consistent with experiment. Our findings pave the way for predictive composition and interface design in organic photovoltaics based on simple experimental measurements and equilibrium thermodynamic theory.

Journal article

Pont S, Osella S, Smith A, Marsh AV, Li Z, Beljonne D, Cabral JT, Durrant JRet al., 2019,

Evidence for strong and weak phenyl-C61-butyric acid methyl ester photodimer populations in organic solar cells

, Chemistry of Materials, Vol: 31, Pages: 6076-6083, ISSN: 0897-4756

In polymer/fullerene organic solar cells, the photochemical dimerization of phenyl-C61-butyric acid methyl ester (PCBM) was reported to have either a beneficial or a detrimental effect on device performance and stability. In this work, we investigate the behavior of such dimers by measuring the temperature dependence of the kinetics of PCBM de-dimerization as a function of prior light intensity and duration. Our data reveal the presence of both “weakly” and “strongly” bound dimers, with higher light intensities preferentially generating the latter. DFT simulations corroborate our experimental findings and suggest a distribution of dimer binding energies, correlated with the orientation of the fullerene tail with respect to the dimer bonds on the cage. These results provide a framework to rationalize the double-edged effects of PCBM dimerization on the stability of organic solar cells.

Journal article

Udoh CE, Garbin V, Cabral JT, 2019,

Polymer nanocomposite capsules formed by droplet extraction: spontaneous stratification and tailored dissolution

, Soft Matter, Vol: 15, Pages: 5287-5295, ISSN: 1744-683X

We report the formation of polymeric and nanocomposite capsules via droplet solvent extraction, focusing on the interplay between solvent exchange and removal, demixing and directional solidification kinetics. We investigate a model system of sodium poly(styrene sulfonate), NaPSS and silica nanoparticles in aqueous solution, whose phase behaviour is experimentally measured, and examine a series of selective extraction solvents (toluene, butyl acetate, ethyl acetate and methyl ethyl ketone), ranging from 0.04 to 11% v/v water solubility. Tuning the rate of solvent exchange is shown to provide an effective means of decoupling demixing and solidification timescales, and thereby tunes the internal microstructure of the capsule, including hollow, microporous, core–shell, and bicontinuous morphologies. In turn, these determine the capsule dissolution mechanism and kinetics, ranging from single to pulsed release profiles of nanoparticle clusters (at intermediate solubilities), to minimal dissolution (at either extremes). These findings provide facile design and assembly strategies for functional capsules with time-varying release profiles.

Journal article

Khodaparast S, Sharratt W, Wang H, Robles ESJ, Dalgliesh R, Cabral JTet al., 2019,

Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS)

, Journal of Colloid and Interface Science, Vol: 546, Pages: 221-230, ISSN: 0021-9797

We report the spontaneous formation of multilamellar vesicles (MLVs) from low concentration (<30 wt%) aqueous micellar solutions of sodium linear alkylbenezene sulfonate (NaLAS) upon cooling, employing a combination of optical microscopy (OM), Small Angle Neutron Scattering (SANS), and Cryo-TEM. Upon cooling, MLVs grow from, and coexist with, the surfactant micelles, attaining diameters ranging from hundreds of nanometers to a few micrometers depending on the cooling rate, whilst the d-spacing of internal lamellae remains unchanged, at 3 nm. While microscale fluid and flow properties of the mixed MLVs and micellar phase depend on rate of cooling, the corresponding nanoscale structure of the surfactant aggregates, resolved by time-resolved SANS, remains unchanged. Our data indicate that the mixed MLV and micellar phases are in thermodynamic equilibrium with a fixed relative volume fraction determined by temperature and total surfactant concentration. Under flow, MLVs aggregate and consequently migrate away from the channel walls, thus reduce the overall hydrodynamic resistance. Our findings demonstrate that the molecular and mesoscopic structure of ubiquitous, low concentration NaLAS solutions, and in turn their flow properties, are dramatically influenced by temperature variation about ambient conditions.

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