David G. Bucknall

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.

Nanolithography and patterning techniques in microelectronics

Copolymer ordering Templated synthesis for controlled 3D structural control Surface-induced structure formation of polymer blends Rapid prototyping of functional microfabricated devices by soft lithography Control of polymer topography AFM based patterning Patterning of confined polymer thin films Ion beam patterning Nanoprinting techniques Photolithography beyond the diffraction limit Standard printing techniques for unconventional methods Microfluidics Manipulation of biomolecules and reactions Fabrication of organic display devices Printing techniques for plastic electronics.

Investigation of the melt interface between polyethylene and polystyrene using neutron reflectivity

Abstract The characteristics of the interface between immiscible polymers, semicrystalline polyethylene (PE) and amorphous deuterated polystyrene (dPS), were determined in the melt state, using the neutron reflectivity technique. The neutron reflectivity profiles of PE-dPS bilayer samples were measured in situ at 150°C in the melt regime giving a melt interfacial roughness, σ12, of 1.18 ± 0.33 nm. The measured interfacial width consists of a true interfacial width, lt, convoluted with a width, l0. The latter has been variously described by a number of authors as a capillary wave interfacial width or an initial width. From the true interfacial width the Flory-Huggins-Stavermann interaction parameter, χ, was calculated. The calculated values of χ were shown to differ by almost an order of magnitude from each other depending on the definition of l0 used. These values are compared with other values of χ obtained via theoretical interpretations of interfacial tension measurements and solubility parameter determination. It is shown that neither definition of l0 is totally satisfactory.

Modulation of cytochrome C coupling to anionic lipid monolayers by a change of the phase state: a combined neutron and infrared reflection study.

The effect of monolayer domain formation on the electrostatic coupling of cytochrome c from the subphase to a monolayer at the air/water interface was studied using a combination of neutron reflection (NR) and infrared reflection absorption spectroscopy (IRRAS) techniques. The monolayers consisted of a binary mixture of the zwitterionic phosphatidylcholine and the anionic phosphatidylglycerol. For a monolayer of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylglycerol (DMPG, 30 mol%), which exhibits a non-ideal mixing of the two lipid components, we observed a significantly higher protein coupling to the liquid-condensed phase compared to the liquid-expanded state. In contrast, this higher protein binding was not observed when the two lipids had identical chain lengths (nearly ideal mixing). Similarly, for an equimolar mixture of DPPC and DMPG, we did not observe significant differences in the protein binding for the two phase states. The results strongly suggest that the domain formation in a condensed monolayer under non-ideal lipid mixing conditions is crucial for the cytochrome c binding strength. Furthermore, this study demonstrates the significant advantages of gathering information on protein-monolayer coupling by the combined use of a dedicated IRRAS set-up with the NR technique.

Conformational Changes in SP-B as a Function of Surface Pressure

X-ray reflectivity of bovine and sheep surfactant-associated protein B (SP-B) monolayers is used in conjunction with pressure-area isotherms and protein models to suggest that the protein undergoes changes in its tertiary structure at the air/water interface under the influence of surface pressure, indicating the likely importance of such changes to the phenomena of protein squeeze out as well as lipid exchange between the air-water interface and subphase structures. We describe an algorithm based on the well-established box- or layer-models that greatly assists the fitting of such unknown scattering-length density profiles, and which takes the available instrumental resolution into account. Scattering-length density profiles from neutron reflectivity of bovine SP-B monolayers on aqueous subphases are shown to be consistent with the exchange of a large number of labile protons as well as the inclusion of a significant amount of water, which is partly squeezed out of the protein monolayer at elevated surface pressures.

Neutron reflectivity of polymer interfaces

Abstract Neutron reflectivity (NR) measurements have been used to investigate the interfacial composition profile between immiscible semi-crystalline polymers, from which values for the Flory–Huggins χ parameter and interfacial tension can be obtained. In addition, measurements of small molecule ingress into high molecular weight polymers, have successfully been performed in-situ in real time. In both cases, special sample environment equipment has been used.

Early stages of oligomer–polymer diffusion

Abstract We present results of neutron reflectivity measurements of plasticiser diffusion into high molecular weight deuterated poly(methyl methacrylate) (dPMMA). These measurements were made in situ and in real time, enabling the interfacial profile to be studied in detail in situations where the normal ‘anneal–quench’ procedure is impossible. From the velocity of the diffusion front, the diffusion coefficients for two molecular weights of oligo-methyl methacrylate (OMMA) have been determined. Oligo-poly(ethylene glycol) (PEG) diffusing into dPMMA has also been measured and limited solubility of PEG observed.

Surface segregation from polystyrene networks

We have used neutron reflectometry and carbon and neon forward recoil spectrometry to measure the surface segregation of deuterated polystyrene from a hydrogenous polystyrene network. We find that when the linear polymer is of a high molecular weight (~600 000), the surface segregated profile can be predicted by mean field theory. In these systems the segregation is a rather slow function of time, reflecting the large number of entanglements in such crosslinked mixtures. When the deuterated polystyrene is of a lower molecular weight (~100 000), the surface segregated layer does not evolve monotonically with time but the shape of the profile can be predicted by mean-field theory. However, when the network is significantly crosslinked, the linear polymer is expelled from the network.

Roughness correlation and interdiffusion in thin films of polymer chains

The surface morphology of thin bilayer polymer films on top of glass substrates was investigated. The bilayer consists of a blend film of protonated and deuterated polystyrene and an underlying deuterated polystyrene film. Choosing the thickness of the top film larger than 8 times and smaller than 2 times the radius of gyration of the chains enables the determination of film thickness and confinement effects. With diffuse neutron scattering at grazing incidence in the region of total external reflection, a depth sensitivity and a contrast even at the internal polymer-polymer interface was achieved. The underlying film is conformal to the substrate, and depending on the thickness of the top film two different types of roughness correlations are observed. Thin confined films nestle to the underlying polymer films, while the stiffness of thicker bulky films provides an independent morphology. In both cases, annealing above the glass-transition temperature yields an interdiffusion at the internal polymer-polymer interface, and the polymer-air surface remains essentially unchanged with respect to roughness correlations.

The ordering of semi-crystalline PS-b-hPB copolymers at a PS/PE interface and their effects on interfacial strength

The organisation and mechanical effectiveness of a series of polystyrene-block-hydrogenated polybutadiene (PS-b-hPB) diblock copolymers (where the hPB block is semicrystalline) when placed as thin layers between the immiscible polymers, polyethylene (PE) and polystyrene (PS), has been explored using neutron reflectivity, transmission electron microscopy and mechanical peel tests. For thin copolymer layers (expected to leave the interface unsaturated) the trend in variation of interfacial strength with copolymer molecular weight was found to be similar to that previously reported for amorphous copolymers. However, for thick layers (oversaturating the interface) the interfacial strength became in one case too large to separate the layers in the peel test. This may suggest a different mechanism for increasing interfacial strength when at least one of the copolymer blocks is crystallisable.