Stuart W. Prescott

Gold nanorod extinction spectra

Geometrical factors suitable for use in Mie–Gans theory are derived from discrete dipole aproximation generated spectra for a number of pseudonanorods permitting the rapid calculation of extinction spectra. It is shown that the rod width, rod end-cap geometry (flat, oblate spheroid, and sphere) and the rod size distribution all have a significant effect on the position of the peak absorbance. Moreover, it is shown that spectrometric characterization of nanorods is possible given an independent measure of rod width and suitable assumptions regarding the end-cap geometry and the form of the rod-length distribution; under such conditions the full width half maximum and the extinction peak uniquely determine the average rod length and the breadth of the distribution.

Surfactant-mediated desorption of polymer from the nanoparticle interface.

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.

Flurbiprofen Encapsulation Using Pluronic Triblock Copolymers

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.

Growth and shrinkage of pluronic micelles by uptake and release of flurbiprofen: variation of pH.

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 pKa 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.

Competition between Polymers for Adsorption on Silica: A Solvent Relaxation NMR and Small-Angle Neutron Scattering Study

The competition between poly(vinylpyrrolidone) 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 preadsorbed 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.

Structure of colloidal sphere-plate mixtures

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.

Synthesis of temperature responsive poly(N-isopropylacrylamide) using ultrasound irradiation.

Ultrasound was employed to synthesize poly(N-isopropylacrylamide) [poly(NIPAM)] either as aqueous solutions or microgels in the absence of a chemical initiator. Poly(NIPAM) of different microstructures can be readily prepared via ultrasound irradiation by varying the reaction temperature. At a preparation temperature of 20 degrees C, poly(NIPAM) was formed in aqueous solutions, whereas, at a higher preparation temperature (beyond the lower critical solution temperature of approximately 32 degrees C), poly(NIPAM) microgels were formed. In addition, the high shear gradients generated by the acoustic cavitation process aid to control or vary the molecular weights of poly(NIPAM) formed in aqueous solutions. The swelling behavior of poly(NIPAM) at different concentrations of sodium dodecyl sulfate (SDS) was also studied. An increase in the transition temperature and hydrodynamic size of the particles was attributed to SDS binding to the polymer network through hydrophobic interactions. Light scattering data reflected the formation of larger microgels at low cross-linker concentrations. The encapsulation of rhodamine B within the microgels was achieved by sonicating the monomer containing the dye. The subsequent release of the dye was consistent with Fickian diffusion and the diffusion coefficient of the dye was estimated as 4.0 x 10(-12) and 3.6 x 10(-11) m(2) s(-1) at 20 and 40 degrees C, respectively. From the diffusion coefficients, the viscosities of the polymer samples at 20 and 40 degrees C were determined using the Stokes-Einstein equation to be 77 and 10 cP, respectively.

The use of solvent relaxation NMR to study colloidal suspensions

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.

Manipulating Interfacial Polymer Structures through Mixed Surfactant Adsorption and Complexation

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.

Not all anionic polyelectrolytes complex with DTAB.

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.