Stuart W. Prescott

University of Greenwich, Londinium, England, United Kingdom

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Publications (44)161.35 Total impact

  • Zengyi Wei · Stuart W. Prescott
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    ABSTRACT: The confinement of soft materials between surfaces is central to the interactions that lead to adhesion, lubrication, and colloidal stability. Force measurements have long been able to describe the potential that exists between various coated surfaces, but it is only more recently that scattering techniques have permitted the detailed study of surface layers under confinement. By focusing on the characterization of interfacial layer structures, there is a growing understanding of the structural basis for the previously reported interaction potentials. Work to date has illustrated that the nanoscale confinement of adsorbed polymers, polymer brushes, polyelectrolyte multilayers, liquid crystals and lipid bilayers frequently leads to unexpected structural features. Approaches to probing interfacial structures with X-ray reflection and neutron reflection are described along with some of the advances in sample environment design that have facilitated these explorations.
    No preview · Article · Sep 2015 · Current Opinion in Colloid & Interface Science
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    ABSTRACT: A series of dumbbell shape nanocomposite materials of polydimethylsiloxanes (PDMS) and polyhedral oligomeric silsesquioxanes (POSS) was synthesized through hydrosilylation reactions of allyl- and vinyl-POSS and hydride-terminated PDMS. The chemical structures of the dumbbell shaped materials, so called POSS-PDMS-POSS triblocks, were characterized by 1H NMR and FT-IR spectroscopy. The molecular weights of the triblock polymers were de- termined by gel permeation chromatography (GPC). Their size was analysed by small-angle neutron scattering (SANS) and pulsed-field gradient stimulated echo (PFG STE) NMR ex- periments. The impact of POSS on the molecular mobility of the PDMS middle chain was observed by using 1H spin-spin (T2) relaxation NMR. In contrast to the PDMS melts, the tri- blocks showed an increase in mobility with increasing molecular weight over the range studied due to the reduced relative concentration of constraints imposed by the end-tethered nanopar- ticles. The triblock systems were used to compare the impact of tethered nanoparticles on the mobility of the linear component compared to the mobility of the polymer in conventional blended nanocomposites. The tethered nanoparticles were found to provide more reinforce- ment than physically dispersed particles especially at high molecular weights (low particle concentration). The physical blends showed an apparent percolation threshold behavior.
    No preview · Article · Jul 2015 · Langmuir
  • David Fairhurst · Terence Cosgrove · Stuart W. Prescott
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    ABSTRACT: Solvent relaxation NMR has been used to estimate the surface areas and wettability of various types of nanostructured carbon materials in a range of solvents including water, ethanol, and tetrahydrofuran. We illustrate the application of the technique through several short case studies using samples including nanocarbon blacks, graphene oxide, nanographites, and porous graphenes. The technique is shown to give a good measure of surface area, correlating well with conventional surface area estimates obtained by nitrogen adsorption, transmission electron microscopy, or light scattering for the non-porous samples. NMR relaxation has advantages in terms of speed of analysis and being able to use concentrated, wet, and opaque samples. For samples that are porous, two distinct surface areas can be estimated assuming the two environments ('inner' and 'outer') have the same surface chemistry, and that there is a slow exchange of solvent molecules between them. Furthermore, we show that differences in wettability and dispersability between samples dispersed in water, ethanol, and cyclopentanone can be observed, along with changes to the surface chemistry of the interface. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    No preview · Article · May 2015 · Magnetic Resonance in Chemistry
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    ABSTRACT: The structures of polymer brushes under confinement were measured using a combination of neutron reflectivity and a surface force type apparatus. The samples were either poly(ethylene oxide), PEO, used to investigate the effect of the grafting density or poly(acrylic acid), PAA, used to determine the effect of charge on the structure of a polymer brushes under confinement. Without confinement both PEO and PAA brushes are found to be highly swollen with water, >50% v/v, with the expected parabolic brush structure. Compression of the PEO brushes with as little as 0.5 bar of confinement is found experimentally to reduce the brush to a polymer block of uniform density that is significantly dehydrated, <12% v/v. Further subsequent increases in the confinement pressure only marginally decrease the hydration and thickness of the polymer block. The grafting density of the brush does not significantly influence this behavior. PAA polymer brushes with little (pH 3) or an intermediate level of charging (pH 5.5) are also found to be compressed into a single uniform density polymer block with a confinement of 5 bar. However, with a high level of charge (pH 9) the brush structure is believed to be partially retained due to the repulsion between the internal charges. These experimental results are compared against a theoretical model based on numerical self-consistent field (nSCF) theory as well as to osmotic, SFA, and AFM data. While the nSCF model correctly predicts the observed transition from a brush to a block profile, experimentally it occurs at a pressure 2 orders of magnitude lower than the simulations would suggest. The results acquired through simulation are consistent with available osmotic pressure data; however, SFA and AFM measurements are consistent with the neutron reflection experimental data presented here. This significant disagreement between the two data sets indicates that in a confined system the effective osmotic counter pressure to an applied mechanical pressure is much less than the osmotic pressure expected from the local polymer concentration.
    No preview · Article · Apr 2015 · Macromolecules
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    ABSTRACT: The adsorption of dopants at the surface of 5CB has been studied using neutron reflection. The dopants were versions of 11OCB with partly fluorinated chains, and the 5CB was interfaced with air or with silica treated with fluorocarbon or hydrocarbon coatings. At the air interface, the F17-11OCB adsorbed homeotropically, and the amount increased on cooling into the nematic phase of 5CB. At low temperatures, the adsorption from the nematic phase appeared to saturate at a bilayer. At the solid interfaces, there was some evidence for a thin planar layer at the surface, but most of the adsorbed dopant was in the form of diffuse layers. In both cases, an increase in dopant concentration caused higher adsorbed amounts. For F17-11OCB, the amount absorbed was much greater for a fluorinated coating as opposed to a hydrocarbon-coated surface. Also no adsorption of the F3-11OCB could be detected, suggesting that the adsorption process is driven by the oleophobic effect. The results demonstrate that it is possible to manipulate surface properties of liquid crystals using surfactant-like dopants.
    No preview · Article · Mar 2015 · Liquid Crystals
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    ABSTRACT: The interactions between the strong polyelectrolyte sodium poly(styrene sulfonate), NaPSS, and the neutral polymer poly(vinyl pyrrolidone), PVP, were investigated in bulk and at the silica/solution interface using a combination of diffusion NMR, SANS, solvent relaxation NMR and ellipsometry. We show for the first time that complex formation occurs between NaPSS and PVP in solution; the complexes formed were shown not to be influenced by pH variation, whereas increasing the ionic strength increases the complexation of NaPSS but does not influence the PVP directly. The complexes formed contained a large proportion of NaPSS. Study of these interactions at the silica interface demonstrated that complexes also form at the nanoparticle interface where PVP is added in the system prior to NaPSS. For a constant PVP concentration and varying NaPSS concentration, the system remains stable until NaPSS is added in excess, which leads to depletion flocculation. Surface complex formation using the layer-by-layer technique was also reported at a planar silica interface.
    No preview · Article · Mar 2015 · Langmuir
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    ABSTRACT: Methacrylate-terminated poly(dimethylsiloxane)s in both linear and star architectures have been produced through a time-efficient 1 pot, 2 stage reaction which involved hydrosilylation of small molecule silanes with allyl methacrylate and subsequent equilibration of the product with octamethylcyclotetrasiloxane (D4) in the presence of an acid catalyst. This synthetic route required only one work-up procedure and the products were comparable to those produced by 2 step processes typically reported in literature. All methacrylate-terminated products were approximately double the molar masses anticipated based on reagent loadings. This is thought to be due to redistribution of siloxane bonds in the presence of the platinum hydrosilylation catalyst accompanied by a loss of silicon from the reaction by evaporation of dimethylsilane. It is believed that this is the first report of such siloxane equilibration occurring at room temperature.
    No preview · Article · Jan 2015
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    ABSTRACT: Optical flow cell reflectometry was used to study the adsorption of poly(vinylpyrrolidone) (PVP) to a silica surface and the subsequent surfactant adsorption and polymer desorption upon exposure to the anionic surfactant sodium dodecyl sulfate (SDS). We have studied these effects as a function of pH and surfactant concentration, but also for two different methods of silica preparation, O2 plasma and piranha cleaning. As a function of pH, a plateau in the amount adsorbed of ∼0.6 mg/m(2) is observed below a critical pH, above which the adsorption decreases to zero within 2-3 pH units. An increase in pH leads to dissociation of surface OH groups and a decreased potential for hydrogen bonding between the polymer and surface. For the plasma- and piranha-cleaned silica, the critical pH differs by 1-2 pH units, a reflection of the much larger amount of surface OH groups on piranha-cleaned silica (for a given pH). Subsequent rinsing of the adsorbed layer of PVP with an SDS solution leads to total or partial desorption of the PVP layer. Any remaining adsorbed PVP then acts as an adsorption site for SDS. A large difference between plasma- and piranha-cleaned silica is observed, with the PVP layer adsorbed to plasma-cleaned silica being much more susceptible to desorption by SDS. For a plasma-cleaned surface at pH 5.5, only 30% of the originally adsorbed PVP is remaining, while for piranha-cleaned silica, the pH can be increased to 10 before a similar reduction in the amount of adsorbed PVP is seen. For a given pH, piranha-cleaned silica has a higher surface charge, leading to a smaller amount of adsorbed SDS per PVP chain on a piranha-cleaned surface compared to a plasma-cleaned surface under identical conditions. In that way, the high negative surface charge makes desorption by negatively charged SDS more difficult. The high surface charge thus protects the neutral polymer from surfactant-mediated desorption.
    No preview · Article · Jul 2014 · Langmuir
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    ABSTRACT: Using a combination of neutron reflectivity and a surface force type apparatus, the hydration of polyelectrolyte multilayers, PEMs, is investigated under mechanical confinement. The samples consist of poly(styrenesulfonic acid), PSS, and poly(allylamine hydrochloride), PAH, which were formed by spin-coating with 11, 23, and 47 bilayers. The terminating layer polyelectrolyte of the PEMs was also varied. Selectively deuterating some layers and using a model that separately considers the last layer from the other strata representing the material structure of the PEM allow a low-resolution hydration distribution within the PEM to be determined. When swollen with H2O and unconfined, PSS-terminated PEMs are approximately 26% v/v water, and this decreases to 15% with an applied pressure of 5 bar. By comparison, PAH-terminated PEMs are 20% water when unconfined and dehydrate to 10% water at 5 bar. For all samples, the final adsorbed layer is significantly more hydrated at approximately 40% v/v. The water is generally uniformly distributed within the bulk of a PEM; however, there is a significant increase in hydration in the bilayers adjacent to the final layer in PAH-terminated PEMs. Also considered is an experiment where the inherent charge of the final adsorbed layer of a PEM is neutralized by compressing it with an oppositely charged PEM. This determines the hydration for an uncharged PEM and demonstrates that the influence of the potential of the final layer on a PEMs’ internal hydration is a perturbation. A detailed scheme is then presented whereby the amount and distribution of internal charge are directly responsible for the hydration of a PEM.
    No preview · Article · May 2014 · Macromolecules
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    ABSTRACT: Novel amphiphilic graft copolymers composed of a polyisoprene (PIP) backbone with Pluronic side chains, polyisoprene-g-Pluronic, have been synthesized using a "graft onto" technique. Small-angle neutron scattering (SANS) has been used to characterize the conformation of the P123 and P103 Pluronic graft copolymers in selective solvents such as ethanol and hexane and in a non-selective solvent, THF. The results indicated that, in a selective solvent for the side chain Pluronics (e.g. ethanol), "crew-cut" micelles were formed with a large core of radius ~ 120 Å; data were fitted with a core-shell model. In a good solvent for the backbone (e.g. hexane) "flower-like" micelles were formed with a small inner radius of ~ 64 Å. In the non-selective solvent a swollen polymer coil was found, which was described using the Guinier-Debye model. As THF/ethanol, and THF/hexane can be prepared in any ratio, it was possible to vary the solvent composition gradually in order to study the transition from swollen coil to micelle. When going from 100% THF to 100% ethanol, the transition to micellar behaviour was observed at a ratio of 20:80 (v/v %) THF/ethanol for both grafted copolymers and 40:60 (v/v %) THF/hexane for grafted P123 copolymers.
    Full-text · Article · May 2014 · Langmuir
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    ABSTRACT: The weak polyelectrolyte sodium polyacrylate, NaPA, and the neutral poly(vinyl pyrrolidone), PVP, were combined and the effects of complexation both in solution and at the nanoparticle interface were studied under basic pH conditions. Using a combination of SANS and PCS, we demonstrate that attractive interpolymer interactions occur between PVP and NaPA in solution (for polymers with molecular weights equal to 40 K and 60 K respectively); typically, no attractive interactions between PVP and NaPA are reported above a critical pH of 4-5. Polymer interactions in the bulk are observed between the larger molecular weight polymers studied. The mass ratios of polymer also affect the interactions in the bulk, indicating that polymer interactions are influenced by the charges present in the system. The addition of NaPA to a silica dispersion with PVP previously adsorbed to the particles is shown to led to polymer desorption and the destabilisation of the system leading to particle aggregation or even flocculation. SANS data show that the interparticle potential changes from being repulsive with bare silica or PVP-coated silica to attractive on addition of NaPA. The molecular weight of PVP is seen to affect the amount of polymer desorbed, whereas the molecular weight of NaPA had an effect on the aggregation of particles.
    No preview · Article · May 2014 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    ABSTRACT: Reports as early as 1974[1] have indicated that polymer nanocomposites(PNCs) can exhibit viscosities that decrease with particle addition counter to the classical behavior for filled systems originally described by Einstein[2]. Schmidt et. al.[3] previously reported that PNC's prepared from trimethylsiloxy-treated polysilicate nanoparticles of 0.75≤ Rg ≤2.1nm and a series of narrow molecular weight distribution polydimethylsiloxane (PDMS) polymers exhibited a critical size ratio (Rg(particle)/Rg(polymer) ∼0.13) for viscosity reduction behavior. In this paper we will discuss potential physical phenomena responsible for the viscosity behavior. Also, some initial observations on the impact of tethered particles compared to blended particles on the mobility of the host polymer will be made, using mono-functional polyoctahedral silsesquioxane (POSS) particles tethered to PDMS chain ends and NMR relaxation analyses.
    No preview · Article · Jan 2014
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    ABSTRACT: The competition between poly(vinyl pyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace pre-adsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems, however only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4 % w/v with 1.1 % w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.
    No preview · Article · Sep 2013 · Langmuir
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    ABSTRACT: The self-assembly behaviour, structure, and consequently the electronic properties of electroactive organic molecules can differ significantly from those of the bulk material when confined to thin films. Here we have examined the self-organised in-plane and out-of-plane structures of aniline oligomers in thin films using surface-sensitive grazing-incidence X-ray scattering (GIXS). Thin films of the aniline tetramer (TANI) and octamer (OANI) were prepared both in their native emeraldine base (EB) oxidation state and in the doped emeraldine salt (ES) state (combined with the acid surfactant bis(ethyl hexyl)phosphate (BEHP)), using a simple drop-casting and solvent annealing process. It was found that the presence of the acid surfactant induced self-organisation into highly ordered structures. The details of these structures, such as the morphology, orientation relative to the underlying substrate and the degree of orientation were found to depend on the molecular architecture of the oligomer. The BEHP-doped TANI system formed a highly oriented hexagonal unit cell (lattice parameters: a ¼ b ¼ 2.53 nm, c ¼ 2.91 nm, g ¼ 120), whereas the BEHP-doped OANI complex adopted a randomly oriented lamellar structure (d-spacing ¼ 2.25 nm). Such detailed structural information reveals that the self-assembly behaviour and the packing of oligomer–BEHP complexes, when confined to thin films, are indeed different to that of the bulk phase materials. Furthermore, the molecular architecture of the oligomers directly influenced the structural changes of the doped films in response to in situ thermal treatment. These results demonstrate that through a simple processing route the morphology of electroactive oligomer films can be tailored by molecular design. These findings are important to future applications where thin film structure is a crucial consideration for device function and performance.
    Full-text · Dataset · Jul 2013
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    ABSTRACT: Solvent relaxation nuclear magnetic resonance has been widely used to study the interactions of polymers and surfactants with nanoparticles, an important area of research for use in a range of industrial formulations, especially with regards to competition effects between components. The ability of the solvent relaxation technique to distinguish between solvent molecules at the surface and those in the bulk solution has been used to obtain valuable information on the interfacial interactions and structure. We focus on systems containing combinations of polymer, surfactant and colloidal particles and illustrate how solvent relaxation measurements have addressed problems of stabilisation, flocculation and depletion in both academically and industrially relevant systems.
    No preview · Article · Jun 2013 · Soft Matter
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    ABSTRACT: We have developed a unique surface force type apparatus that allows direct measurements of the structure of a polymer layer under a confining pressure using neutron reflection. We have used this device to study the structure and the mechanical properties of a water swollen poly(styrenesulfonic acid) (PSSA) and poly(allylamine) PAH polyelectrolyte multilayer on silicon under confining pressures. A multilayer of 23 bilayers (60 nm) in thickness and terminated with a PAH layer swells to about 21% (v/v) of H2O. When the layer is then confined between a flexible plastic film and the silicon substrate, increased confining pressure gradually decreases the amount of water in the multilayer until, at 5 bar, only 10% of H2O remains. For a layer of the same number of bilayers but terminated with PSSA, the layer swells to about 28% of water, but between a confining pressure of 0–1, this is reduced to 16%, after which higher confining pressures do not lead to changes in hydration. The bulk salt concentration does not affect the structure and hydration for the layers without confinement, but a much larger degree of hydration is retained under confining pressures. Clearly both the final layer of our multilayer and the bulk salt concentration have a large effect on the layers mechanical properties. Another significant difference between the PSSA and PAH terminated multilayer is how the water is internally distributed under confining pressure. For the PAH terminated multilayer, all the H2O is squeezed out of the separate layers closer to the silicon interface, while the layers at the edge of the multilayer retain their H2O. For the PSSA terminated multilayer, the H2O is far more evenly distributed throughout the whole structure, even under pressure. An increase in bulk ionic strength weakens these effects, making the hydration in the PAH terminated layer somewhat more uniformly distributed and the PSSA terminated layer somewhat less uniformly distributed. These observations can be explained by the effect that the high electrostatic potential of the final adsorbed layer has on the dissociation of an excess of weakly charged PAH monomers in the multilayer and supports that as the mechanism behind odd–even effects in polyelectrolyte multilayers in general.
    No preview · Article · Feb 2013 · Macromolecules
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    ABSTRACT: Binary mixtures of colloidal particles of sufficiently different sizes or shapes tend to demix at high concentration. Already at low concentration, excluded volume interactions between the two species give rise to structuring effects. Here, a new theoretical description is proposed of the structure of colloidal sphere-plate mixtures, based on a density expansion of the work needed to insert a pair of spheres and a single sphere in a sea of them, in the presence or not of plates. The theory is first validated using computer simulations. The predictions are then compared to experimental observations using silica spheres and gibbsite platelets. Small-angle neutron scattering was used to determine the change of the structure factor of spheres on addition of platelets, under solvent contrast conditions where the platelets were invisible. Theory and experiment agreed very well for a platelet/sphere diameter ratio D∕d = 2.2 and reasonably well for D∕d = 5. The sphere structure factor increases at low scattering vector Q in the presence of platelets; a weak reduction of the sphere structure factor was predicted at larger Q, and for the system with D∕d = 2.2 was indeed observed experimentally. At fixed particle volume fraction, an increase in diameter ratio leads to a large change in structure factor. Systems with a larger diameter ratio also phase separate at lower concentrations.
    Full-text · Article · Nov 2012 · The Journal of Chemical Physics
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    ABSTRACT: The competitive adsorption of poly(vinyl pyrrolidone) onto silica and alumina-modified silica particles was studied using solvent relaxation nuclear magnetic resonance. The additive nature of the measured relaxation rate enabled predictions to be made of the relaxation rate in different polymer adsorption scenarios. Preferential adsorption of the poly(vinyl pyrrolidone) onto the unmodified silica particles occurred when there was insufficient polymer in the system to coat the entire available surface area. Desorption was also found to occur when the polymer was initially adsorbed upon the alumina-modified particle and silica particles were added.
    No preview · Article · Nov 2012 · Langmuir
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    ABSTRACT: A unique surface force type apparatus that allows the investigation of a confined thin film using neutron reflection is described. The central feature of the setup consists of a solid substrate (silicon) and a flexible polymer membrane (Melinex(®)). We show that inflation of the membrane against the solid surface provides close and even contact between the interfaces over a large surface area. Both heavy water and air can be completely squeezed out from between the flexible film and the solid substrate, leaving them in molecular contact. The strength of confinement is controlled by the pressure used to inflate the membrane. Dust provides a small problem for this approach as it can get trapped between membrane and substrate to prevent a small part of the membrane from making good contact with the substrate. This results in the measured neutron reflectivity containing a small component of an unwanted reflection, between 10% and 20% at low confining pressures (1 bar) and between 1% and 5% at high confining pressures (5 bar). However, we show that this extra signal does not prevent good and clear information on the structure of thin films being extracted from the neutron reflectivity. The effects of confinement are illustrated with data from a poly(vinyl pyrollidone) gel layer in water, a polyelectrolyte multilayer in water, and with data from a stack of supported lipid-bilayers swollen with D(2)O vapor. The data demonstrates the potential of this apparatus to provide information on the structure of thin films under confinement for a known confining pressure.
    No preview · Article · Nov 2012 · The Review of scientific instruments
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    ABSTRACT: Structural changes in the micellization of Pluronics P103 and P123, as a function of temperature, cosolvent (ethanol, 10 v/v %), and the addition of the hydrophobic drug flurbiprofen, were investigated by SANS and tensiometry. Addition of ethanol increases the critical micellization concentration of the Pluronics (making the polymer more soluble), while increasing the repulsive interactions between the flurbiprofen-Pluronic spherical complexes. However, increasing temperature and addition of drug increases both the aggregation number and core radius and leads to a more dehydrated core. The addition of flurbiprofen to Pluronic P103 was also found to reduce the critical micellization temperature from between 15 and 20 °C to below 10 °C and at higher drug concentrations leads to an attractive interaction between micelles and eventually phase separation.
    Full-text · Article · Aug 2012 · The Journal of Physical Chemistry B

Publication Stats

814 Citations
161.35 Total Impact Points


  • 2015
    • University of Greenwich
      • Department of Pharmaceutical, Chemical & Environmental Sciences
      Londinium, England, United Kingdom
  • 2013-2015
    • University of New South Wales
      • School of Chemical Engineering
      Kensington, New South Wales, Australia
  • 2009-2014
    • University of Bristol
      • School of Chemistry
      Bristol, England, United Kingdom
  • 2005-2008
    • University of Melbourne
      • • The Particulate Fluids Processing Centre (PFPC)
      • • School of Chemistry
      Melbourne, Victoria, Australia
  • 2001-2005
    • University of Sydney
      • • School of Chemistry
      • • Key Centre for Polymers and Colloids (KCPC)
      Sydney, New South Wales, Australia