Stuart W Prescott

University of Bristol, Bristol, England, United Kingdom

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Publications (20)78.31 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Langmuir : the ACS journal of surfaces and colloids. 07/2014;
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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.
    Macromolecules 05/2014; 47(10):3263–3273. · 5.93 Impact Factor
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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.
    Langmuir 05/2014; · 4.38 Impact Factor
  • Colloids and Surfaces A Physicochemical and Engineering Aspects 01/2014; 449:57–64. · 2.11 Impact Factor
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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.
    Langmuir 09/2013; · 4.38 Impact Factor
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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.
    Soft Matter 06/2013; · 4.15 Impact Factor
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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.
    Macromolecules 02/2013; 46(3):1027-1034. · 5.93 Impact Factor
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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.
    The Journal of Chemical Physics 11/2012; 137(20):204909. · 3.12 Impact Factor
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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.
    Langmuir 11/2012; · 4.38 Impact Factor
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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.
    The Review of scientific instruments 11/2012; 83(11):113903. · 1.52 Impact Factor
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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.
    The Journal of Physical Chemistry B 08/2012; 116(37):11545-51. · 3.61 Impact Factor
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    ABSTRACT: The micellization of Pluronic triblock copolymers (P103, P123, and L43) in the presence of flurbiprofen at different pH was studied by small-angle neutron scattering (SANS), pulsed-field gradient stimulated-echo nuclear magnetic resonance (PFGSE-NMR), and surface tension measurements. Addition of flurbiprofen to the Pluronic at low pH leads to an increase in the fraction of micellization, aggregation number, and the core radius of the micelles. However, changing the pH to above the pK(a) of flurbiprofen in an ethanol/water mixture (∼6.5) reduces the fraction of micellization and results in a weaker interaction between the drug and micelles due to the increased drug solubility in aqueous solution.
    Langmuir 04/2012; · 4.38 Impact Factor
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    ABSTRACT: The effects of a nonionic alcohol ethoxylate surfactant, C(13)E(7), on the interactions between PVP and SDS both in the bulk and at the silica nanoparticle interface are studied by photon correlation spectroscopy, solvent relaxation NMR, SANS, and optical reflectometry. Our results confirmed that, in the absence of SDS, C(13)E(7) and PVP are noninteracting, while SDS interacts strongly both with PVP and C(13)E(7) . Studying interfacial interactions showed that the interfacial interactions of PVP with silica can be manipulated by varying the amounts of SDS and C(13)E(7) present. Upon SDS addition, the adsorbed layer thickness of PVP on silica increases due to Coulombic repulsion between micelles in the polymer layer. When C(13)E(7) is progressively added to the system, it forms mixed micelles with the complexed SDS, reducing the total charge per micelle and thus reducing the repulsion between micelle and the silica surface that would otherwise cause the PVP to desorb. This causes the amount of adsorbed polymer to increase with C(13)E(7) addition for the systems containing SDS, demonstrating that addition of C(13)E(7) hinders the SDS-mediated desorption of an adsorbed PVP layer.
    Langmuir 03/2012; 28(15):6282-90. · 4.38 Impact Factor
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    ABSTRACT: The surfactant-mediated desorption of adsorbed poly(vinylpyrrolidone), PVP, from anionic silica surfaces by sodium dodecyl sulfate, SDS, was observed. While photon correlation spectroscopy shows that the size of the polymer-surfactant-particle ensemble grows with added SDS, a reduction in the near-surface polymer concentration is measured by solvent relaxation NMR. Volume fraction profiles of the polymer layer extracted from small-angle neutron scattering experiments illustrate that the adsorbed polymer layer has become more diffuse and the polymer chains more elongated as a result of the addition of SDS. The total adsorbed amount is shown to decrease due to Coulombic repulsion between the surfactant-polymer complexes and between the complexes and the anionic silica surface.
    Langmuir 12/2011; 28(5):2485-92. · 4.38 Impact Factor
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    ABSTRACT: Pulsed-field gradient stimulated-echo nuclear magnetic resonance (NMR) and surface tension measurements have been used to study the effect of drug addition on the micellization behavior of pluronic triblock copolymers (P103, P123, and L43). The addition of 0.6 wt% flurbiprofen to Pluronic P123 and P103 solutions reduced their cmc and promoted micellization. Also, a substantial increase in the hydrodynamic radius of Pluronic P103 from 5 to 10 nm was observed, along with an increased fraction of polymer micellized, demonstrating that the polymers solubilize this nonsteroidal anti-inflammatory drug.
    Langmuir 06/2011; 27(13):8054-60. · 4.38 Impact Factor
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    ABSTRACT: In addition to containing spherical pigment particles, coatings usually contain plate-like clay particles. It is thought that these improve the opacity of the paint film by providing an efficient spacing of the pigment particles. This observation is counterintuitive, as suspensions of particles of different shapes and sizes tend to phase separate on increase of concentration. In order to clarify this matter a model colloidal system is studied here, with a sphere-plate diameter ratio similar to that found in paints. For dilute suspensions, small angle neutron scattering revealed that the addition of plates leads to enhanced density fluctuations of the spheres, in agreement with new theoretical predictions. On increasing the total colloid concentration the plates and spheres phase separate due to the disparity in their shape. This is in agreement with previous theoretical and experimental work on colloidal sphere-plate mixtures, where one particle acts as a depleting agent. The fact that no large scale phase separation is observed in coatings is ascribed to dynamic arrest in intimately mixed, or possibly micro-phase separated structures, at elevated concentration.
    Journal of Physics Condensed Matter 05/2011; 23(19):194109. · 2.22 Impact Factor
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    ABSTRACT: Numerical self-consistent field theory is used to study the structural characteristics of a polymer brush consisting of end-grafted comb-polymers. A comb-polymer brush is shown to retain the parabolic density profile characteristic of the unbranched brush. Increasing either the number of branches or the length of the side-chains leads to an increase in the height (H) of the brush. This is partly because of the branched structure: it is more favorable to stretch the polymer backbone, while leaving the side-chains almost unstretched. The other reason for the increased stretching is simply because of the extra polymer in the brush due to the branches. We find that it does not matter how the side-chains are distributed along the backbone for the predicted density profile; it is only the total amount of polymer in the side-chains that is important. The effect of branching on the brush height can be captured in a simple scaling law: H Nb(NT/Nb)2/3, where Nb is the chain length of the polymer backbone, NT is the total chain length, thus including the side-chains, and their ratio NT/Nb is a measure for the amount of branching. The structure of the branched brush is less suitable for keeping away small particles from the grafted interface. It is, however, a good way of increasing the polymer density, and thus its antifouling properties, when the grafting density of the brush is limited
    Macromolecules 01/2011; 44(7):2334-2342. · 5.93 Impact Factor
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    ABSTRACT: Numerical self-consistent field theory is used to study the structural characteristics of a polymer brush consisting of end-grafted comb-polymers. A comb-polymer brush is shown to retain the parabolic density profile characteristic of the unbranched brush. Increasing either the number of branches or the length of the side-chains leads to an increase in the height (H) of the brush. This is partly because of the branched structure: it is more favorable to stretch the polymer backbone, while leaving the side-chains almost unstretched. The other reason for the increased stretching is simply because of the extra polymer in the brush due to the branches. We find that it does not matter how the side-chains are distributed along the backbone for the predicted density profile; it is only the total amount of polymer in the side-chains that is important. The effect of branching on the brush height can be captured in a simple scaling law: H~ Nb(NT/Nb)2/3, where Nb is the chain length of the polymer backbone, NT is the total chain length, thus including the side-chains, and their ratio NT/Nb is a measure for the amount of branching. The structure of the branched brush is less suitable for keeping away small particles from the grafted interface. It is, however, a good way of increasing the polymer density, and thus its antifouling properties, when the grafting density of the brush is limited.
    Macromolecules 01/2011; 44(7):2334-2342. · 5.93 Impact Factor
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    ABSTRACT: Understanding the gelation of liquids by low molecular weight solutes at low concentrations gives an insight into many molecular recognition phenomena and also offers a simple route to modifying the physical properties of the liquid. Bis-(α,β-dihydroxy ester)s are shown here to gel thermoreversibly a wide range of solvents, raising interesting questions as to the mechanism of gelation. At gelator concentrations of 5-50 mg ml⁻¹, gels were successfully formed in acetone, ethanol/water mixtures, toluene, cyclohexane and chloroform (the latter, albeit at a higher gelator concentration). A range of neutron techniques - in particular small-angle neutron scattering (SANS) - have been employed to probe the structure of a selection of these gels. The universality of gelation in a range of solvent types suggests the gelation mechanism is a feature of the bis-(α,β-dihydroxy ester) motif, with SANS demonstrating the presence of regular structures in the 30-40 Å range. A correlation between the apparent rodlike character of the structures formed and the polarity of the solvent is evident. Preliminary spin-echo neutron scattering studies (SESANS) indicated the absence of any larger scale structures. Inelastic neutron spectroscopy (INS) studies demonstrated that the solvent is largely unaffected by gelation, but does reveal insights into the thermal history of the samples. Further neutron studies of this kind (particularly SESANS and INS) are warranted, and it is hoped that this work will stimulate others to pursue this line of research.
    Beilstein Journal of Organic Chemistry 01/2010; 6:1079-88. · 2.80 Impact Factor
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    ABSTRACT: The influence of hydroxypropyl guar (HPG), with and without boric acid, on dodecyltrimethyl ammonium bromide (DTAB) micellization was characterized by surface tension measurements, isothermal titration calorimetry, and small-angle neutron scattering. Although HPG is a nonionic water-soluble polymer, borate ions form weak bonds with HPG, transforming it into an anionic polyelectrolyte, HPG-borate. Surprisingly, the three independent measurements showed that HPG-borate does not promote DTAB micellization or phase separation normally seen when mixing oppositely charged polyelectrolytes and surfactants. However, the neutron scattering results suggested that HPG-borate binds to and flocculates existing DTAB micelles. The unusual behavior of HPG-borate with DTAB was underscored by showing that carboxymethyl guar (CMG) formed precipitates with DTAB.
    Langmuir 06/2009; 25(24):13712-7. · 4.38 Impact Factor