J.R.P. Webster

NEUTRON REFLECTIVITY DETERMINATION OF BURIED ELECTROACTIVE INTERFACE STRUCTURE : PBT/PPY AND PBT/PXV BILAYERS

Structure−property correlations of two electroactive polymer bilayers fabricated by different methods are investigated using neutron reflectivity to probe the buried polymer/polymer interfaces. Both bilayers utilize electropolymerized poly(2,2‘-bithiophene) as an inner, mediating layer, while the outer layer consists of either electropolymerized polypyrrole or electroprecipitated polyxylylviologen, respectively. These devices have very different current−potential behaviors, which neutron reflectivity shows is a consequence of the fabrication method and the resulting polymer/polymer interfacial structure. The polybithiophene/polypyrrole film is found to be of composite structure while polybithiophene/polyxylylviologen forms a film comprised of segregated polymer layers.

The measurement of volume fraction profiles for adsorbed polymers under compression using neutron reflectometry

Abstract A new surface force apparatus is described which enables simultaneous measurement of the interaction forces between adsorbed polymer layers as well as their surface structure by neutron reflectometry. The apparatus utilises two optically flat quartz plates that are brought together by applying a known external force. Surface force and structural profiles have been obtained for polystyrene adsorbed on quartz from cyclohexane, i.e. a poor solvent.

Study of Off-Specular Neutron Reflectivity Using a Model System

The specular reflection of neutrons gives information on the neutron refractive-index profile normal to an interface. The refractive index is simply related to the scattering-length density and hence the composition. However, in-plane inhomogeneities or corrugated surfaces can give rise to strong off-specular scattering. This has been observed in several systems. The difficulty arises in the interpretation of off-specular data from systems having poorly defined or unknown structure. We have measured off-specular reflection from a gold replica optical diffraction grating with 1 μm line spacing. A series of data was obtained using the CRISP neutron reflectometer at the Rutherford Appleton Laboratory in Oxfordshire, England. The analysis of specular and off-specular data, and the determination of the periodicity of the grating and the thickness of the metal film will be discussed. The maximum periodicity that could be observed was 42 μm. This is in agreement with the off-specular resolution which is determined by the horizontal divergence of the ribbon-shaped beam, but very much larger than the longitudinal coherence length of the neutron wave packet.

Surface phase separation in an amphiphilic block copolymer monolayer at the air-water interface

Abstract Deuterium labelled linear diblock copolymers of polyethylene oxide and polymethyl methacrylate were spread at the air-water interface and neutron reflectometry was used to determine the spatial organization of the copolymer. The mole fraction composition of the block copolymer was ca 0.5, but with some variation from copolymer to copolymer arising from the separate syntheses needed for each labelled copolymer. The neutron reflectometry data for two surface concentrations of the block copolymer (0.6 and 1.2 mg m−2) were analysed using the kinematic approximation. In addition to providing the composition and thickness of regions which are rich in one component or the other, this method also gives the separation between each of these regions. For the lowest surface concentration used the two blocks were mixed together near the surface with a Gaussian distribution of segments, the characteristic dimension of which was circa 8 A. At the higher surface concentration of 1.2 mg m−2, there is a clear separation between the two blocks with the polyethylene oxide exploring the aqueous subphase and part of the polymethyl methacrylate block being expelled into the air phase. However, there is no evidence for the formation of a stretched brush like layer of polyethylene oxide at the surface. Attempts to analyse for such a layer are discussed and the care needed in such analyses is pointed out. For both concentrations the near surface distribution of water takes on a tanh distribution.

Solution pH and Oligoamine Molecular Weight Dependence of the Transition from Monolayer to Multilayer Adsorption at the Air–Water Interface from Sodium Dodecyl Sulfate/Oligoamine Mixtures

Neutron reflectivity and surface tension have been used to investigate the solution pH and oligoamine molecular weight dependence of the adsorption of sodium dodecyl sulfate (SDS)/oligoamine mixtures at the air-water interface. For diethylenetriamine, triamine, or triethylenetetramine, tetramine mixed with SDS, there is monolayer adsorption at pH 7 and 10, and multilayer adsorption at pH 3. For the slightly higher molecular weight tetraethylenepentamine, pentamine, and pentaethylenehexamine, hexamine, the adsorption is in the form of a monolayer at pH 3 and multilayers at pH 7 and 10. Hence, there is a pH driven transition from monolayer to multilayer adsorption, which shifts from low pH to higher pH as the oligoamine molecular weight increases from tetramine to pentamine. This results from the relative balance between the electrostatic attraction between the SDS and amine nitrogen group which decreases as the charge density decreases with increasing pH, the ion-dipole interaction between the amine nitrogen and SDS sulfate group which is dominant at higher pH, and the hydrophobic interalkyl chain interaction between bound SDS molecules which changes with oligoamine molecular weight.

Adsorption studies of different end-functionalised linear poly(dimethylsiloxane)

Abstract The adsorption of end-functionalised poly(dimethylsiloxane) (PDMS), of both low and high molecular weight, on to silica and alumina from carbon tetrachloride has been investigated by Fourier transform infrared spectroscopy. It was found that the adsorbed amounts of these end-functionalised PDMS depend strongly on the composition of the polymer; samples with end groups having a high surface affinity were found to adsorb to a greater extent. Detailed information on the structure of the adsorbed layer obtained by the Scheutjens—Fleer theoretical model and a Monte Carlo simulation indicated that polymers with such end groups adsorb primarily as loops, with fewer segments in trains and tails compared with a homopolymer. Adsorption was found to be greater on alumina than on silica for both molecular weight series.

Unique density profile determination in thin films

Abstract A recent paper by D.S. Sivia and R. Pynn (SPIE Conf. Proc., Vol. 1738, 1992) has shown theoretically the use of polarised neutrons and a magnetised substrate to determine uniquely a density profile from neutron reflectometry data. Presented here are preliminary results of the technique applied to a polymer film on a magnetised nickel substrate.

Adsorption of Bovine Serum Albumin (BSA) at the Oil/Water Interface: A Neutron Reflection Study.

The structure of the adsorbed protein layer at the oil/water interface is essential to the understanding of the role of proteins in emulsion stabilization, and it is important to glean the mechanistic events of protein adsorption at such buried interfaces. This article reports on a novel experimental methodology for probing protein adsorption at the buried oil/water interface. Neutron reflectivity was used with a carefully selected set of isotopic contrasts to study the adsorption of bovine serum albumin (BSA) at the hexadecane/water interface, and the results were compared to those for the air/water interface. The adsorption isotherm was determined at the isoelectric point, and the results showed that a higher degree of adsorption could be achieved at the more hydrophobic interface. The adsorbed BSA molecules formed a monolayer on the aqueous side of the interface. The molecules in this layer were partially denatured by the presence of oil, and once released from the spatial constraint by the globular framework they were free to establish more favorable interactions with the hydrophobic medium. Thus, a loose layer extending toward the oil phase was clearly observed, resulting in an overall broader interface. By analogy to the air/water interface, as the concentration of BSA increased to 1.0 mg mL(-1) a secondary layer extending toward the aqueous phase was observed, possibly resulting from the steric repulsion upon the saturation of the primary monolayer. Results clearly indicate a more compact arrangement of molecules at the oil/water interface: this must be caused by the loss of the globular structure as a consequence of the denaturing action of the hexadecane.

Self-diffusion, viscosity and spin-spin relaxation in liquid poly(propylene oxide) melts

Abstract Self-diffusion, viscosity and spin-spin relaxation have been measured for a series of low molecular weight liquid poly(propylene oxide) melts. The data are discussed within the framework of theories commonly invoked for polymer dynamics including the Rouse chain and free volume effects. The average diffusion coefficients of a range of bimodal blends have also been measured and the results interpreted using the Rouse model.

High-temperature neutron reflectometry of liquid metal-ceramic interfaces

Neutron reflectometry has been used to study liquid metal-ceramic interfaces in situ. A high-temperature facility designed and commissioned for use on the SURF reflectometer at the ISIS beamline, Rutherford Appleton Laboratory, is described. Results from a study of two Sn-Ti alloys on sapphire up to 1173 K are presented and the interfacial composition established by interpretation of the recorded reflectivity profiles. Overall the results are in good agreement with previously published work inferred from the analysis of solidified structures and the neutron reflection technique, established as a unique method for the high temperature analysis of buried solid/liquid interfaces.