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Publications - 2002

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SANS from adsorbed polymer layers

Stephen King, Peter Griffiths, John Hone and Terence Cosgrove
ISIS Facility, Rutherford Appleton Laboratory, Didcot, UK

Macromolecular Symposia (2002), 190(Scattering Methods for the Investigation of Polymers), 33-42. Publisher: Wiley-VCH Verlag GmbH & Co.


One of the areas in which small-angle neutron scattering (SANS) has had a profound impact over the last twenty years is the investigation of polymers and surfactants adsorbed at the solid/liq. and liq./liq. interfaces. This is of course also an area of considerable com. importance. Control of the structure of an adsorbed layer is thus a key priority of the colloid scientist. Equally important, however, is the ability to probe the arrangement of the adsorbed polymer mols. and to ext. quant. information from what might well be a complex, multicomponent system. SANS in combination with the principle of deuterium contrast variation is uniquely placed to provide this information.

Computer simulation of water molecules at mineral surfaces

M.R. Warne, N.L. Allan and T. Cosgrove
Tripos Receptor Research, Cornwall, UK

Special Publication - Royal Society of Chemistry (2002), 282(Speciality Chemicals in Mineral Processing), 90-101. Publisher: Royal Society of Chemistry


A review of the authors' work on the behavior of water mols. on mineral (kaolinite) surfaces. Topics discussed include theor. methodol., static simulations, and mol. dynamics simulations (d. profiles, self-diffusion coeffs., dynamics of reorientation, av. NMR relation times).

NMR Self-Diffusion Studies on PDMS Oil-in-Water Emulsion

Tatjana Garasanin, Terence Cosgrove, Leon Marteaux, Axel Kretschmer, Andy Goodwin and Klaus Zick
School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK

Langmuir (2002), 18(26), 10298-10304


Std. spin-echo pulsed field gradient (SE-PFG) and stimulated spin-echo pulsed field gradient (STE-PFG) 1H NMR methods were used to study the translational diffusion of polymeric-oil-in-H2O (O/W) emulsions, made with poly(dimethylsiloxane) PDMS fluids and two nonionic surfactants polyoxethylene (4) lauryl ether (Brij-30) and polyoxethylene (23) lauryl ether (Brij-35p). A detailed anal. of the free (Gaussian) diffusion of the PDMS fluid, its restricted diffusion inside the emulsion droplets, and the Brownian diffusion of the droplets themselves are presented. A graphical representation, which relates the square root of the mean-squared displacement of the fluid and emulsion droplets with the exptl. diffusion times and the radius of the droplets, helps distinguish the different diffusion regimes. The Brownian diffusion of the emulsion particles becomes the dominant diffusion process with diln. of the system and increase in the mol. wt. (viscosity) of the PDMS. These different processes which contribute to diffusion in the system need to be considered if a correct size distribution of the emulsion droplets is to be detd. For a concd. emulsion system, the condition for restricted diffusion is reached by increasing the diffusion time. Under this condition, the Murday and Cotts model for diffusion inside a spherical droplet can be used to det. the size distribution of the emulsion. In very dil. systems by increasing the diffusion time, the Brownian diffusion becomes the dominant process so that the Stokes-Einstein model can be applied to obtain the size distribution.

Wetting of Water and Water/Ethanol Droplets on a Non-Polar Surface: A Molecular Dynamics Study

Mathias Lundgren, Neil L. Allan, Terence Cosgrove and Neil George
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(26), 10462-10466


Spreading of sessile water droplets on a solid graphite surface has been studied using atomistic mol. dynamics techniques. The system is in the partial wetting regime and a straightforward scheme is applied to ext. the contact angle from the atomistic simulations. Even for droplets contg. a thousand mols., the calcd. contact angle for water is 83°, which is close to the exptl. macroscopic value. On addn. of ethanol, the contact angle decreases to 30° and we approach complete wetting. Time-averaged structural analyses show that the ethanol mols. form a monolayer on top of the solid surface. A few .ANG. above the solid surface, the ethanol mols. are preferentially located at the liq./vapor interface rather than in the bulk of the droplet.

NMR Spin-Spin Relaxation Studies of Silicate-Filled Low Molecular Weight Poly(dimethylsiloxane)s

Terence Cosgrove, Claire Roberts, Tania Garasanin, Randall G. Schmidt and Glenn V. Gordon
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(26), 10080-10085


NMR spin-spin relaxation measurements were used to investigate the mobility of poly(dimethylsiloxane) (PDMS) polymers, below the mol. entanglement point, when mixed with trimethylsilyl-treated polysilicate nanoparticles. The results showed that a high mol. wt. polysilicate caused a dramatic redn. in the overall PDMS chain mobility at all concns. The relaxation decays were deconvoluted into multiple exponential decays using nonlinear least squares and the DISCRETE algorithm. The components of these decays were assocd. qual. with adsorbed and nonadsorbed polymer segments. When compared with differential scanning calorimetry measurements, the redn. in the mobility of the PDMS chains as seen in the NMR expts. corresponded to a shift in the glass transition to higher temps., a decrease in the sp. heat increment at the glass transition, and a loss in the ability of the polymer to crystallize at high concns. of polysilicate.

Relaxation Studies of High Molecular Weight Poly(dimethylsiloxane)s Blended with Polysilicate Nanoparticles

Terence Cosgrove, Claire Roberts, Yoon Choi, Randall G. Schmidt, Glenn V. Gordon, Andrew J. Goodwin and Axel Kretschmer
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(26), 10075-10079


NMR spin-spin relaxation (T2) measurements have been used to investigate the segmental mobility of high mol. wt. poly(dimethylsiloxane) (PDMS) polymers when blended with trimethylsilyl-treated polysilicate nanoparticles. The relaxation behavior was complex and could not be described by a single-exponential function. The results showed that a high mol. wt. polysilicate caused a dramatic redn. in the overall PDMS chain mobility above a crit. concn. However, a surprising increase in mobility was obsd. below this crit. concn. The origin of this phenomenon was hypothesized to be a combination of free vol., polymer adsorption, and entanglement effects at low particle concns. The apparent increase in mobility was also manifested by a decrease in the shear viscosity and plateau modulus relative to data for the base polymer. The crit. concn. was found to be dependent upon temp. and mol. wt.

Small-Angle Neutron Scattering of Gelatin/Sodium Dodecyl Sulfate Complexes at the Polystyrene/Water Interface

John C. Marshall, Terence Cosgrove, Kevin Jack and Andrew Howe
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(25), 9668-9675


Using isotopic substitution, the small-angle neutron scattering from a gelatin layer preadsorbed upon polystyrene latex has been measured as a function of the added concn. of sodium dodecyl sulfate (SDS). The data have been fitted using a model which includes an exponential vol. fraction profile and a term to account for the scattering from spatial concn. fluctuations. Measurements indicate that the gelatin layer thickness increases to a max. as a function of SDS concn., while the vol. fraction of gelatin adsorbed at the interface simultaneously decreases. Consequently, the adsorbed amt. of gelatin peaks at SDS concns. equiv. to approx. one SDS micelle/gelatin chain. Further addn. of SDS sees the adsorbed amt. decay below that of the system in the absence of SDS. At the highest SDS concns., the gelatin also develops a scattering form similar to that of SDS micelles.

Structure of Polymer/Surfactant Complexes Formed by Poly(2-(dimethylamino)ethyl methacrylate) and Sodium Dodecyl Sulfate

Robin D. Wesley, Terence Cosgrove, Laurie Thompson, Steven P. Armes and Fiona L. Baines
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(15), 5704-5707


The small-angle neutron scattering from poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was studied in the presence of the anionic surfactant, sodium dodecyl sulfate (SDS). At pH 9.1, the polymer is a Gaussian coil whose size is detd. primarily by nonelectrostatic interactions. The presence of the polymer induces micellization of the surfactant, even at surfactant concns. below the surfactants normal crit. micelle concn. The polymer decorates the surfactant micelles and in some respects resembles a polymer adsorbed on a spherical particle.

NMR Solvent Relaxation in Studies of Multicomponent Polymer Adsorption

Andrew Nelson, Kevin S. Jack, Terence Cosgrove and Darby Kozak
School of Chemistry, The University of Bristol, Bristol, UK

Langmuir (2002), 18(7), 2750-2755


NMR spin-spin solvent relaxation measurements were performed on a series of aq. dispersions of silica with physisorbed poly(ethylene oxide) (PEO) and poly(vinylpyrrolidone) (PVP). Enhancements in the specific relaxation rate const. as a result of polymer adsorption were obsd. Measurements of the relaxation rate const. as a function of the concns. of silica and polymer in the system have been used to det. the amt. of polymer trains adsorbed at the particle surface, at satn. Studies of PEO-PVP-silica dispersions have shown that preadsorbed PEO is completely displaced when the silica concn. is small enough to allow the added PVP to sat. the surface. When the concn. of silica is increased beyond this point, then PEO and PVP are able to coexist on the particle surface.

Structure of Physically Adsorbed Polymer Layers Measured by Small-Angle Neutron Scattering Using Contrast Variation Methods

John H.E. Hone, Terence Cosgrove, Marina Saphiannikova, Timothy M. Obey, John C. Marshall and Trevor L. Crowley
School of Chemistry, University of Bristol, Bristol, UK

Langmuir (2002), 18(3), 855-864


The scattering from poly(ethylene oxide) adsorbed on polystyrene latex particles was measured as a function of the isotopic compn. of the aq. solvent. Oscillations in the data were obsd. even when the particle was matched in scattering length d. to the solvent. These oscillations are a new and important characteristic of the scattering from an adsorbed polymer layer compared to the scattering from free chains. The contrast variation approach allowed the scattering from the interference between the layer and the particle to be measured explicitly. The data were fitted using a model function to find the vol. fraction profile of the adsorbed layer without the complications that can arise from d.-fluctuation contributions to the scattering. It was therefore possible to obtain the fluctuation contribution itself by simple subtraction. The fluctuation scattering was in good agreement with scaling predictions and with a new function derived for an exponential vol. fraction profile.

Interaction between a Novel Gemini Surfactant and Cyclodextrin: NMR and Surface Tension Studies

S. Abrahmsen-Alami, E. Alami, J. Eastoe and T. Cosgrove
School of Chemistry, University of Bristol, Bristol, UK

Journal of Colloid and Interface Science (2002), 246(1), 191-202


The interaction between cyclodextrins, hydroxypropyl-β-cyclodextrin (HPβCD), and hydroxypropyl-γ-cyclodextrin (HPγCD) and a novel type of nonionic surfactant synthesized from a fatty acid has been investigated. The so-called nonionic hetero-Gemini surfactant (NIHG750) contains two hydrophobic groups and two hydrophilic groups, composed of one monomethyl ethylene glycol and one secondary OH group, CH3(CH2)7-CH[OH]-CH[O(CH2CH2O)16CH3]-(CH2)7CN. Surface tension studies indicate that micelles form in NIHG750 systems in both the presence and the absence of small quantities (molar ratio (HPβCD:NIHG750) ≈ 2) of cyclodextrin (HPβCD or HPγCD). This gives NIHG surfactants an advantage compared to single-tailed nonionic surfactants, which generally lose their ability to micellize at much lower addns. of cyclodextrins. However, the interaction between HPβCD and NIHG750 results in a disruption of the micellar aggregates at higher levels of cyclodextrin. In the dil. systems (CNIHG750 < 0.1% (wt./wt.) approx.) prolate-shaped mixed aggregates (HPβCD and NIHG750) form, with a short and a long axis of the order of 8-9 and 17-20 .ANG., resp. These gradually aggregate into micellar-like structures at higher concns. In the aq. bulk phase HPβCD interacts mainly with the hydrophobic part of NIHG750, but both NMR and surface tension measurements indicate that an interaction with the hydrophilic part of NIHG750, as well, may exist. This interaction results in a better packing of NIHG750 at air-water interfaces. However, at elevated temps. results from turbidity measurements indicate that NIHG750 and HPβCD interact mainly through the hydrophilic part of the surfactant; a decrease in the cloud point temp. is obsd. The interaction of the larger cavity mol., HPγCD, with NIHG750, on the other hand, seems to be relatively weak. The interaction, when present, most probably takes place through inclusion of the hydrophilic EO part of NIHG750. The results suggest that HPγCD in combination with NIHG750 is a better solubilizing system than with HPβCD.

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