Publications - 2007
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Steric interactions between physically adsorbed polymer-coated colloidal particles: Soft or hard?
Dong Qiu and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
Andrew M. Howe
Kodak European Research, 332 Cambridge Science Park, Milton Road, Cambridge CB4 0FW, U.K.
Langmuir (2007) 23, 475–481
DOI: 10.1021/la062294t
Abstract
The steric interaction potential between colloidal particles imparted by adsorbed polymer layers is directly related to their structure. Due to the complexity of these interfacial structures, the steric potential may behave differently at different interparticle separations. In this study, we proposed a combined model of the equivalent hard-sphere model (EHS) and the Hayter-Penfold/Yukawa model (HPY) to describe the steric potential due to adsorbed homopolymers on colloidal particles. The EHS potential describes the dense train/small-loop region and the HPY potential the more diffuse tail/long-loop region. The steric potential was extracted from the structure factors measured by small-angle neutron scattering (SANS). It was found that this combined model gave better agreement with experimental data than either of its component models alone. This study also shows that the adsorbed polymer layer in a good solvent partially collapses when the layers approach one another, which is also supported by an NMR solvent relaxation study.
Rupturing polymeric micelles with cyclodextrins rupturing polymeric micelles with cyclodextrins
Julie Joseph, Cécile A. Dreiss and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Jan Skov Pedersen
University of Aarhus, Department of Chemistry and iNANO Interdisciplinary Nanoscience Center, Langelandsgade 140, DK-8000 Aarhus, Denmark
Langmuir (2007) 23, 460–466
DOI: 10.1021/la061850g
Abstract
Small-angle neutron scattering has been used to investigate the associative structures formed by triblock copolymers of poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-poly(ethylene oxide) (PEO) (also known as Pluronics) and to monitor the structural changes occurring upon complexation with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (h beta-CD) over the temperature range from 5 to 70 degrees C. At low temperature, the Pluronics are dispersed as unimers. Close to ambient temperature, the hydrophobicity of PPO causes the aggregation of the polymers into spherical micelles with core sizes between 40 and 50 A and a high inclusion of solvent. The aggregation number increases with temperature as the hydrophobicity of the core is gradually enhanced. h beta-CD spontaneously forms pseudopolyrotaxanes with the triblock copolymers either when in their unimer form or micellized. The complexation results in an increase in the effective critical micellar concentration. It is suggested that the cyclodextrins thread onto the polymer backbone to localize preferentially on the central PPO block, therefore improving its water solubility. At temperatures where the polymers exist in micellar form, complexation with h beta-CD gives rise to a complete disruption of the aggregates. These processes are highly temperature-dependent. Above 50 degrees C, the break-up of the aggregates is inhibited, and large-scale aggregation is observed.
Modeling of wetting: A study of nanowetting at rough and heterogeneous surfaces
Mathias Lundgren, Neil L. Allan and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Langmuir (2007) 23, 1187–1194
DOI: 10.1021/la060712o
Abstract
Molecular dynamics simulations were performed to study the behavior of nanoscale water droplets at solid surfaces. Simulations of droplets on heterogeneous patterned surfaces show that the relative sizes of the domains and the droplets play an important role as do the interactions between the solid and the liquid, particularly when the domain width is comparable to the droplet radius. For pillar surfaces, a transition is observed between the Wenzel and the Cassie and Baxter regimes with increasing pillar height. The effects of pillar width and the gap between the pillars were also examined. The simulations show clearly the importance of the detailed topography and composition of the solid surface.
The Influence of Surfactant and Electrolytes on Adsorbed Polymer Layers
Charlie Flood, Terence Cosgrove, Dong Qiu and Youssef Espidel
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Ian Howell and Patricia Revell
Unilever R&D Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, UK.
Langmuir (2007), 23, 2408-2413
DOI: 10.1021/la062034b
Abstract
Solvent relaxation NMR and small-angle neutron scattering have been used to characterize adsorbed poly(ethylene oxide) (PEO) layers on silica at a range of surfactant and electrolyte concentrations. Below the critical aggregation concentration (CAC), the results suggest that sodium dodecyl sulfate (SDS) interacts relatively weakly, perhaps analogously to a simple salt reducing the solvency of PEO. This is evidenced by a decrease in the adsorbed layer thickness combined with an increase in the bound fraction, although the total adsorbed amount is not greatly affected. The layer thickness goes through a minimum at the CAC, after which further SDS addition results in the formation of PEO-SDS aggregates that repel each other and hence tend to desorb. The adsorbed amount therefore decreases, from 0.7 mg m-2 initially to 0.2 mg m-2 with 32 mM SDS. The aggregates that remain adsorbed also repel and hence there is an increase in the layer thickness and the persistence length, while the bound fraction is reduced. In comparison, the effects of electrolyte at the ionic strength studied are relatively minimal. There is however evidence that the repulsions between adsorbed PEO-SDS aggregates are partially screened, allowing them to approach each other more readily. This leads to a contraction of the adsorbed layer when the SDS concentration is sufficiently high.
Nonaqueous suspensions of surface-modified kaolin
Yan Zhang, Terence Cosgrove, and Jeroen S. van Duijneveldt
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
David I. Gittins and David Skuse
Imerys Minerals, Ltd., Par Moor Centre, St. Austell, PL24 2SQ, U.K.
Langmuir (2007) 23, 3424-3431
DOI: 10.1021/la063033m
Abstract
A range of different stabilizers have been used to render natural kaolin clay particles hydrophobic and dispersible in nonpolar solvents such as heptane. Both silanol and aluminol groups are known to be present at the kaolin surface. Use of a Hammett indicator showed that silanes would not neutralize the acidic aluminol sites, whereas amines would neutralize these sites. Both types of stabilizer adsorbed chemically onto the clay. In addition, a combined silane + amine treatment and a polyisobutylene-based stabilizer with a succinimide/amine head group (SAP230) were also considered. Both would neutralize the acid sites. The final sediment density after settling under gravity was used to gauge suspension stability, which varied with the kaolin surface treatment as silanes < amines < silane + amine < SAP230. This behavior was very similar for suspensions in heptane and in a higher molecular weight branched alkane, polydecene. This trend of increasing stability correlated very well with an increase in surface coverage of the stabilizing moieties, a decrease in particle size found using small-angle light scattering, and a decrease in Bingham yield stress obtained by fitting rheological data.
Sodium polyacrylate adsorption onto anionic and cationic silica in the presence of salts
Charlie Flood, Terence Cosgrove and Youssef Espidel
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Ian Howell and Patricia Revell
Unilever R&D Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, UK.
Langmuir (2007) 23, 6191-6197
DOI: 10.1021/la070047z
Abstract
Sodium polyacrylate is well known for its application as a scale inhibitor in common household products, and the effects of both monovalent and divalent metal cations on its structure have been covered by a range of previous publications. In the present article, we extend this work by using solvent relaxation NMR to look at the adsorption of the polyelectrolyte onto both positively and negatively charged silica and how this is altered by calcium chloride. In the anionic case, we found that polyacrylate adsorption was predictably very weak, and interestingly, perhaps counterintuitively, it was further reduced by calcium ions. This is probably linked to NaPA-Ca2+ binding, which changes the conformation and charge of the polyelectrolyte. In contrast, NaPA adsorbs very strongly on cationic silica, to the point that precipitation often occurs, particularly on addition of salt.
Self-diffusion in solutions of carboxylated acrylic polymers as studied by pulsed field gradient NMR. 1. Solvent diffusion studies
Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Victor Rodin
now of Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford, GU2 7XH, UK.
Martin Murray and Richard Buscall
ICI Strategic Technology Group, The Wilton Centre, Redcar, TS10 4RF, UK.
Journal of Polymer Research (2007) 14, 167-174
DOI: 10.1007/s10965-006-9087-1
Abstract
The diffusion constants of solvent in solutions of carboxylated acrylic random copolymers in isopropanol (IPA) have been investigated using the PFG NMR technique. The copolymers are composed of butyl methacrylate and methacrylic acid with a range of molar ratios (BMA/MAA 100/0 through to 60/40 in 10 mol% steps). It was found that the echo attenuation function depended on the diffusion time, and experiments with short and long diffusion times have been analysed separately. Two questions are addressed: the first concerns the influence of concentration on the polymer aggregation in solution, and the second concerns the influence of the BMA/MAA molar ratio on the diffusion constants of the solvent. The data obtained have been discussed using established physical models for diffusion. Using the model of Wang gave the relative mass of bound solvent per mass of polymer which increased with the mol% of MAA in the copolymer.
Self-diffusion in solutions of carboxylated acrylic polymers as studied by Pulsed Field Gradient NMR. 2. Diffusion of macromolecules
Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Victor Rodin
now of Department of Physics, School of Electronics & Physical Sciences, University of Surrey, Guildford, GU2 7XH, UK.
Martin Murray and Richard Buscall
ICI Strategic Technology Group, The Wilton Centre, Redcar, TS10 4RF, UK.
Journal of Polymer Research (2007) 14, 175-180
DOI: 10.1007/s10965-006-9088-0
Abstract
The diffusion constants of carboxylated acrylic copolymers in isopropanol (IPA) have been investigated using the PFG NMR technique. A range of compositions of butyl methacrylate and methacrylic acid random copolymers (BMA-MAA 100/0 through to 60/40 in 10 mol% steps) have been studied. The polymer diffusivity is two orders of magnitude slower than that for the solvent. A maximum value of G(max)=10 T/m for the gradient pulse was used to suppress solvent signal and to measure the low value of the self-diffusion coefficient for the polymers. Polymer diffusion studies at different diffusion times showed that the diffusion constant decreased with increasing diffusion time. The influence of the concentration on the polymer aggregation in solution and the influence of the BMA/MAA molar ratio on the diffusion constants of the polymer have been studied. The diffusion constant for the polymer at long diffusion times increased with mol% BMA which is similar to the short diffusion time studies. The data obtained have been discussed using published results and models for "anomalous diffusion" of macromolecules in polymer solutions.
Effect of crosslinking on the mobility of PDMS filled with polysilicate nanoparticles: Positron lifetime, rheology and NMR relaxation studies
Cécile A. Dreiss, Terence Cosgrove and Natalie J. Benton
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Duncan Kilburn and M. Ashraf Alam
H.H. Will Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK.
Randall G. Schmidt and Glenn V. Gordon
Dow Corning Corporation, Midland, MI 48686-0994, USA.
Polymer (2007) 48 4419-4428
DOI: 10.1016/j.polymer.2007.05.070
Abstract
We study the effect of adding trimethylsilyl-treated polysilicate nanoparticles (Rg similar to 2.2 nm) to crosslinked poly(dimethylsiloxane) (PDMS) elastomers above the entanglement molecular weight. The results are compared to un-crosslinked PDMS of a similar molecular weight, reported in previous studies and filled with the same polysilicate nanoparticles.
Three techniques are used and compared to assess the enhancement or reduction in mobility with addition of filler: positron annihilation lifetime spectroscopy (PALS), rheology and nuclear magnetic resonance (NMR) spin-spin relaxation (T-2) measurements. PALS measurements do not show any clear effect of the filler on the mobility of the chains, as assessed by the size of free volume holes, but reveal a net increase in free volume with temperature increase (from 30 degrees C to 60 degrees C). A reduction in the dynamic shear storage modulus (measured at 1 rad s(-1)) is observed in the filled network relative to the unfilled polymer (from 63 kPa without filler to 44 kPa with 40 w/w% filter), attributed primarily to a partial inhibition of the chemical crosslinking reaction by the particles. The NMR relaxation measurements, instead, show a reinforcement of the polymer network with increasing addition of polysilicate particles, as revealed by the faster T2 decays at higher filler loadings, caused by increasing polymer bridging and particle flocculation. Similar trends are observed at higher temperatures (up to 80 degrees C), with a higher overall mobility. The apparent disagreement between rheology and NMR stems from the fact that theology reflects bulk mobility and is primarily sensitive to chemical crosslinks in the network, while NMR probes segmental dynamics, which are affected by the presence of particles.
In un-crosslinked PDMS instead, both rheology and NMR show an initial increase in mobility at low filler content, followed by reinforcement with further particle addition. These results strongly suggest that entanglements and filler-induced packing disruption, rather than free volume, play a major role in polymer dynamics.
Monodisperse emulsions from a microfluidic device, characterised by diffusion NMR
Andrew Woodward, Terence Cosgrove and Youssef Espidel
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Paul Jenkins and Neil Shaw
Unilever R&D Port Sunlight, Quarry Road East, Bebington, Wirral,
CH63 3JW, UK.
Soft Matter (2007) 3, 627-633
DOI: 10.1039/b616463n
Abstract
A microfluidic device has been used to create novel monodisperse polymeric oil-in-water emulsions of diameter 15-100 mu m, stabilised by surfactants and polymers. The pulse field gradient nuclear magnetic resonance (PFG-NMR) signal attenuation function showed minima that allow a simple calculation of the droplet size and polydispersity. The presence of the minima is due to the restricted diffusion of molecules within the droplets, which can be analysed using standard solutions of the diffusion equation with a spherical boundary condition. However, even when a small population of polymer is unrestricted, the deep troughs in the attenuation function are obscured. The simultaneous water NMR attenuation function gives structural information about the continuous-phase matrix. The NMR size measurements were compared with those obtained by optical and confocal microscopy, and laser diffraction analysis.
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