Richard A. L. Jones

Self-motile colloidal particles: from directed propulsion to random walk

The motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.

Interface and surface effects on the glass-transition temperature in thin polymer films

We have measured the thickness dependence of the glass-transition temperature (Tg) of thin films of poly(methyl methacrylate)(PMMA) by using spectroscopic ellipsometry to detect the discontinuity in thermal expansivity occurring at Tg. We studied films on two surfaces: the native oxide of silicon, and evaporated gold. The Tg of PMMA on a gold surface decreases with decreasing film thickness, in accordance with previous results for polystyrene on silicon. We suggest that at the air surface a liquid-like layer exists whose size diverges as Tg is approached from below. For films of PMMA on the native oxide of silicon, however, we find a slight increase in Tg with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility and leads to an increase in Tg, outweighing the effect of the free surface.

Polymers at Surfaces and Interfaces

Preface 1. Introduction and overview 2. The surface of a simple polymer melt 3. Experimental techniques 4. Polymer/polymer interfaces 5. Adsorption and surface aggregation from polymer solutions and mixtures 6. Tethered polymer chains in solutions and melts 7. Adhesion and the mechanical properties of polymer interfaces at the molecular level 8. Polymers spread at air/liquid interfaces Index.

Anisotropic spinodal dewetting as a route to self-assembly of patterned surfaces

The ability to pattern surfaces on a microscopic length scale is of importance for technological applications such as the fabrication of microelectronic circuits and digital storage media. Devices fabricated entirely from polymers are now available, opening up the possibility of adapting polymer processing technologies to fabricate cheap, large-area devices using non-lithographic techniques—for example, by exploiting dewetting and phase separation in thin films. But the final pattern adopted by the polymer film using such approaches requires a template printed onto the substrate by optical lithography, microcontact printing or vapour deposition. Here we describe a simple process for patterning surfaces that does not require a template. Our method involves the spinodal dewetting of a polymer surface by a thin polymer film, in which a liquid film breaks up owing to the amplification of thermal fluctuations in film thickness induced by dispersion forces. A preferred orientation is imposed on the dewetting process simply by rubbing the substrate, and this gives rise to patterns of remarkably well-aligned polymer lines. The width of these lines is well-defined, and is controlled by the magnitude of the dispersion forces at the interface, which in turn can be varied by varying the thickness of the polymer substrate. We expect that further work will make it possible to optimize the degree of order in the final morphology.

Depletion of PCBM at the Cathode Interface in P3HT/PCBM Thin Films as Quantified via Neutron Reflectivity Measurements

[*] Dr. A. J. Parnell, A. J. Pearson, Dr. P. A. Staniec, A. J. C. Dennison, Dr. H. Hamamatsu, Prof. D. G. Lidzey, Prof. R. A. L. Jones Department of Physics and Astronomy, The University of Sheffield Hicks Building, Hounsfield Road, Sheffield, S3 7RH (UK) E-mail: a.j.parnell@sheffield.ac.uk; d.g.lidzey@sheffield.ac.uk Dr. A. D. F. Dunbar Department of Chemical and Process Engineering, The University of Sheffield Sir Robert Hadfield Building, Mappin St, Sheffield S1 3JD (UK)

Recent advances in the study of chemical surfaces and interfaces by specular neutron reflection

The use of specular neutron reflection to study a wide variety of problems in surface and interfacial chemistry is introduced and discussed. Recent developments in neutron reflectometry instrumentation, and their implementation in the design of the SURF reflectometer at the ISIS pulsed neutron source, are described. The design of the SURF reflectometer has been optimised for the surface chemistry of soft matter and new experimental results that exploit the novel features of this second generation neutron reflectometer are presented and discussed in the context of the opportunities for future studies that the technique and the new instrumentation presents. The examples from the broad programme in surface chemistry include surfactant and polymer adsorption at the air/liquid and liquid/solid interfaces, adsorption at the liquid/liquid interface; Langmuir–Blodgett films and liquid crystalline alignment layers, thin solid polymer films and interfaces; liquid mixtures and in situ electrochemistry.

Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract The microstructure of YBa2(Cu1−xCox)3O7−δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x≲0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x≳0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20A) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.

X-ray diffraction study of the structure of thin polyfluorene films

The molecular arrangement in thin films of poly(9,9-dioctylfluorene) and poly(9,9-dihexylfluorene) deposited on silicon substrates has been investigated with grazing incidence X-ray diffraction. In particular, the effect of the interface on the molecular orientation is highlighted. Both materials display a periodicity normal to the surface arising from stacked sheets of fluorene chains in both the crystalline and liquid crystalline phases. For the crystalline phase, a periodicity in the plane of the surface of 4.15 A is observed corresponding to half the fluorene ring repeat distance along the backbone, consistent with interdigitating side-chains. For crystalline films deposited onto rubbed polyimide films, strong orientation effects are observed. In the liquid-crystalline phase, this strong in-plane ordering of backbones is lost. Poly(9,9-dihexylfluorene) exhibits an additional degree of ordering in the plane of the interface, which is likely to arise from hexagonal ordering of the backbone chains.

The Form of the Enriched Surface Layer in Polymer Blends

The concentration profile at the surface in blends of deuterated and protonated polystyrene (d-PS and PS) is inferred from measurements of neutron reflectivity and secondary-ion mass spectrometry, using constraints provided by forward recoil spectrometry and X-ray reflectometry results on the same samples. The surface is enriched in d-PS, the volume fraction and the decay length of which are in good agreement with the predictions of mean-field theory but the form of the profile shows small, but statistically significant, deviations from that predicted by the theory.

Adsorption and displacement of a globular protein on hydrophilic and hydrophobic surfaces

Abstract The adsorption of the globular protein β -lactoglobulin on hydrophilic and hydrophobic surfaces has been investigated. Spectroscopic ellipsometry and Fourier transform infrared spectroscopy in the attenuated total reflection mode were used in our study. β -lactoglobulin adsorbed in surface concentrations less than or equal to those corresponding to a closely packed monolayer of molecules. The amount of β -lactoglobulin removed from hydrophilic surfaces upon elution with buffer solution, as a fraction of the final amount reached upon adsorption, was observed to decrease at longer adsorption times. This provides evidence that the conformation of the adsorbed protein changes relatively slowly once adsorbed, leading to more irreversibly adsorbed states with stronger binding to the surface. The fraction of protein irreversibly adsorbed to the surface was generally higher on hydrophobic surfaces than hydrophilic surfaces, confirming the general principle that globular proteins exhibit stronger binding to hydrophobic surfaces than hydrophilic ones. The displacement of pre-adsorbed layers of the protein from hydrophobic and hydrophilic surfaces by the non-ionic surfactant octaethylene glycol monododecyl ether (C 12 E 8 ) was also studied. We observed that the non-ionic surfactant caused partial displacement of adsorbed protein from hydrophobic surfaces. The displacement kinetics also reveal that the protein layers are more strongly bound to hydrophobic surfaces at longer than at shorter adsorption times, showing that slow protein binding changes and ageing effects are also important at hydrophobic surfaces.