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Publications - 2008

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Nanoscale thin film ordering produced by channel formation in the inclusion complex of alpha-cyclodextrin with a polyurethane composed of polyethylene oxide and hexamethylene

Erol A. Hasan and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
Andrew N. Round
H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

Macromolecules (2008) 41, 1393–1400
DOI: 10.1021/ma071484n


A polyurethane consisting of six blocks of polyethylene glycol and five blocks of hexamethylene diisocyanate was synthesized. The influence of the addition of (x-cyclodextrin (alpha-CyD) on the lower critical solution temperature behavior of the polyurethane was investigated by "cloud point" measurements, and the dependence of the phase state (solution, suspension, and gel) of alpha-CyD/polyurethane mixtures on the concentration of the two components was determined. The results suggest that the polyurethane forms inclusion complexes with alpha-CyD and that close to the maximum number of alpha-CyDs was included. The associative constant of the alpha-CyD/polyurethane inclusion complex was determined by H-1 NMR shift titration using a modified Benesi-Hildebrand equation, and the complex was characterized in the solid state by C-13 cross polarization/magic angle spinning NMR and X-ray diffraction. These studies showed that the complexes adopted a channel-like structure. Finally, the morphology of (alpha-CyD/polyurethane complexes in the solid state was visualized by scanning electron microscopy and atomic force microscopy (AFM). AFM images of the inclusion complexes spun-cast on to silicon reveal the existence of ordered domains with heights commensurate with the existence of tetra- and higher-order a-CyD channels. The height quanta of these well-ordered, discrete plateaus point to the dominating influence of the size of the polyethylene glycol blocks within the polyurethane and suggest a route to the production of controlled subnanometer structured surfaces.

Use of Water Spin-Spin Relaxation Rate to Probe the Solvation of Cyclodextrins in Aqueous Solutions

Edvaldo Sabadini, Fernanda do Carmo Egํdio and Fred Yukio Fujiwara
Instituto de Quimica, Universidade Estadual de Campinas, Caixa Postal 6154, CEP 13084-862, Campinas, Brazil.
Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK

Journal of Physical Chemistry B (2008) 112, 3328–3332
DOI: 10.1021/jp710013h


H-1 spin-spin relaxation rate constant, R-2, of water was measured by using the Carr-Purcell-Meiboom-Gill sequence in aqueous solutions of native cyclodextrins (alpha, beta and gamma-CD) and chemically modified CDs in order to probe the structuring of the water surrounding these cyclic carbohydrate molecules. R-2 values for water in solutions containing glucose and dextran were also measured for comparison. A two-site model for bonded and free water molecules was used to fit the results for the dependence of R-2 on the solute concentrations. The order of relaxation rates for water in aqueous solution at a fixed specific hydroxyl group concentration is glucose > dextran congruent to CDs. No significant difference was observed for R-2 of water in solutions containing native CDs, which indicates that the size and nature of the cavity has a small effect on the spin-spin relaxation times of water. The lower relaxation rate for water in CD solutions was attributed to the intramolecular hydrogen bonding formed between the secondary hydroxyl groups that line the rim of the CDs. For comparison, the relaxation rates for water in solutions of two chemically modified CDs were also studied.

A small-angle neutron scattering study of adsorbed polymer structure in concentrated colloidal dispersions

Charlie Flood and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
Dong Qiu
now of School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK

Langmuir (2008) 24, 2983–2986
DOI: 10.1021/la800023d


Poly(ethylene oxide) (PEO) adsorption on colloidal silica particles was studied by small-angle. neutron scattering under the core-contrast-matching condition. The volume fraction profile of the adsorbed layer was derived by modeling the average layer scattering term. It was found that, with increasing colloid concentration, the adsorbed PEO layers collapse due to the repulsions between adsorbed layers on neighboring particles. At the same time, the correlation length in the adsorbed layer obtained by fitting the layer fluctuation scattering term was found to decrease, indicating that denser polymer layers are formed. These two observations are self-consistent.

Molecular modeling simulation and experimental measurements to characterize chitosan and poly(vinyl pyrrolidone) blend interactions

Krit Suknuntha and Vimon Tantishaiyakul
Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Sciences, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand
Visit Vao-Soongnern
School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
Youssef Espidel and Terrence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK

Journal of Polymer Science Part B: Polymer Physics (2008) 46, 1258-1264
DOI: 10.1002/polb.21460


Blends of chitosan and poly(vinyl pyrrolidone) (PVP) have a high potential for use in various biomedical applications and in advanced drug-delivery systems. Recently, the physical and chemical properties of these blends have been extensively characterized. However, the molecular interaction between these two polymers is not fully understood. In this study, the intermolecular interaction between chitosan and PVP was experimentally investigated using C-13 cross-polarization magic angle-spinning nuclear magnetic resonance (C-13 CP/MAS NMR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). According to these experimental results, the interaction between the polymers takes place through the carbonyl group of PVP and either the OH-C-6, OH-C-3, or NH-C-2 of chitosan. In an attempt to identify the interacting groups of these polymers, molecular modeling simulation was performed. Molecular simulation was able to clarify that the hydrogen atom of OH-C6 of chitosan was the most favorable site to form hydrogen bonding with the oxygen atom of C=O of PVP, followed by that of OH-C3, whereas that of NH-C2 was the weakest proton donor group. The nitrogen atom of PVP was not involved in the intermolecular interaction between these polymers. Furthermore, the interactions between these polymers are higher when PVP concentrations are lower, and interactions decrease with increasing amounts of PVP.

Effects of surfactants and electrolytes on adsorbed layers and particle stability

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
Port Sunlight Laboratory, Unilever Research, Quarry Road East, Bebington, The Wirral CH63 3JW, UK

Langmuir (2008) 24, 7323–7328
DOI: 10.1021/la800143x


Adsorbed polymer and polyelectrolyte layers on colloidal silica nanoparticles have been studied in the presence of various salts and surfactants using photon correlation spectroscopy and solvent relaxation NMR. Poly(ethylene oxide) (PEO; molar mass 103.6 kg mol(-1)) adsorbed with a relatively high affinity and gave a layer thickness of 4.2 +/- 0.2 nm: While the nonionic surfactant used only increased this thickness slightly, anionic surfactants had a much greater effect, mainly due to repulsions between adsorbed aggregates, leading to expansion of the layer. A nonionic/anionic surfactant mixture was also tested and resulted in a larger increase in layer thickness than any of the individual surfactants. The dominant factor on addition of salt was generally the reduced solvency of PEO, which resulted in a further increase in the layer thickness but in some cases caused flocculation. This was not the case when the surfactant was sodium dodecylbenzenesulfonate; instead screening of the intermicellar repulsions possibly combined with surfactant-cation binding resulted in a reduction in the layer thickness. In comparison the affinity between silica and sodium poly styrenesulfonate was very weak. Anionic surfactants and salts did not noticeably increase the strength of adsorption, but instead encouraged flocculation. The situation was different with a nonionic surfactant, which was able to adsorb to silica itself and apparently facilitated a degree of polyelectrolyte adsorption as well.

Fourier-transform Carr-Purcell-Meiboom-Gill NMR experiments on polymers in colloidal dispersions: How many polymer molecules per particle?

Charlie Flood, Terence Cosgrove, Youssef Espidel and Eloise Welfare
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
Ian Howell and Patricia Revell
Port Sunlight Laboratory, Unilever Research, Quarry Road East, Bebington, The Wirral CH63 3JW, UK

Langmuir (2008) 24, 7875–7880
DOI: 10.1021/la800144c


Fourier transform relaxation NMR has been used to study how the mobility of poly(ethylene oxide) is affected by its adsorption onto colloidal silica particles of various sizes. Novel results have been obtained which illustrate the unexploited potential of this method for the Study of interfacial species in complex systems. The results quantify how polymer mobility varies along an adsorption isotherm. When the particles are in excess, the polymer is strongly adsorbed and hence has a large spin-spin magnetic relaxation rate constant, R-2. The value of R-2 in this region increases with particle size, because the associated reduction in particle surface curvature results in a reduction in the mobility of the adsorbed polymer. This is accompanied by a reduction in the signal intensity, as a higher fraction of the polymer is adsorbed in the form of train segments too immobile to detect using the Carr-Purcell-Meiboom-Gill pulse sequence. When the polymer concentration reaches similar to 0.5 mg m(-2), the initial region of high affinity adsorption ends and so the polymer solution concentration increases. This is accompanied by a reduction in R-2, which then approaches the value for a simple polymer solution in the absence of particles. The results are corroborated by comparison with rheological Measurements and molecular dynamics simulations of an analogous particle-polymer system.

Mapping the positions of beads on a string: dethreading rotaxanes by molecular force spectroscopy

Alex Dunlop and Andrew N. Round
H H Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
Jirut Wattoom, Erol A Hasan and Terence Cosgrove
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK

Nanotechnology (2008) 19, 345706
DOI: 10.1088/0957-4484/19/34/345706


The direct manipulation by atomic force microscopy (AFM) of individual macrocycles within a rotaxane offers a potential route to a new sequencing tool for complex macromolecules such as polysaccharides, glycoproteins and nucleic acids. In this paper we demonstrate for the first time that a sliding contact made between a macrocycle, alpha-cyclodextrin, and its polymer axle by an AFM tip can be used to map the positions of specific groups along the polymer as if they were beads along a string, thereby generating sequence information. We find very good agreement (linear fit with slope = 1.03, R-2 = 0.968) between the calculated and measured positions of phenylenediamine and benzenetricarboxylic acid groups within polymers of polyethylene oxide (PEO). The rupture force profiles attributable to the dethreading interactions of phenylenediamine and benzenetricarboxylic acid differ observably from each other and, in the latter case, from the rupture of the corresponding host -guest complex. As well as opening the way to a macromolecular sequencing technique, the ability demonstrated by this method to manipulate the dethreading of a rotaxane offers a new tool for investigating the process energetics in a wide array of spontaneously forming and forced rotaxane systems.

Composites of Kaolin and Polydimethylsiloxane

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
Performance Minerals, New Technology Group, IMERYS Minerals Ltd., Par Moor Centre, St. Austell, PL24 2SQ, UK

Langmuir (2008) 24, 12032–12039
DOI: 10.1021/la8018259


Kaolin particles were surface-treated with isobutyltrimethoxysilane (IBTMS), hydrogenated tallow (HT), and a polyisobutyl chain-based stabilizer (SAP) to make composites with polydimethylsiloxane (PDMS). IBTMS did not cover the strong acid sites on the kaolin surface and as a result a cross-linking reaction occurred for silanol-terminated PDMS. The polyisobutyl chain of SAP was found to be incompatible with PDMS and this caused aggregation of the kaolin particles. HT was the most effective at dispersing the particles into silanol-terminated PDMS. The aggregation state of the composites was characterized using rheology and microscopy. Both showed the HT-treated particles were well-dispersed in low molecular weight silanol-terminated PDMS, and they were weakly flocculated in higher molecular weight silanol-terminated PDMS. However, the same particles aggregated when dispersed in methyl-terminated PDMS. It appears the silanol-terminated PDMS acted as costabilizer through interaction with the kaolin surface. Transverse relaxation NMR was used to probe mobility of the PDMS chains in the composites. This showed little dependence on surface treatment, aggregation state, or polymer end groups. For all samples, chain mobility decreased with increasing kaolin concentration.

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