Neutron reflection system

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

The measurements of concentration gradients at surfaces or in multilayer specimens by neutron reflectivity requires contrast in the reflectivity fiDr the neutrons. Under most circumstances this means that one of the components must be labeled. Normally this is done is by isotopic substitution of protons with deuterons. This means that reflectivity studies are usually performed on model systems that are designed to behave identically to systems of more practical interest. In a few cases, however (for organic compounds containing fluorine, for example) sufficient contrast is present without labeling. 

The toughness of interfaces between immiscible amorphous polymers without any coupling agent has been the subject of a number of recent studies [15-18]. The width of a polymer/polymer interface is known to be controlled by the Flory-Huggins interaction parameter x between the two polymers. The value of x between a random copolymer and a homopolymer can be adjusted by changing the copolymer composition, so the main experimental protocol has been to measure the interface toughness between a copolymer and a homopolymer as a function of copolymer composition. In addition, the interface width has been measured by neutron reflection. Four different experimental systems have been used, all containing styrene. Schnell et al. studied PS joined to random copolymers of styrene with bromostyrene and styrene with paramethyl styrene [17,18]. Benkoski et al. joined polystyrene to a random copolymer of styrene with vinyl pyridine (PS/PS-r-PVP) [16], whilst Brown joined PMMA to a random copolymer of styrene with methacrylate (PMMA/PS-r-PMMA) [15]. The results of the latter study are shown in Fig. 9. 

Tarek et al. [388] studied a system with some similarities to the work of Bocker et al. described earlier—a monolayer of n-tetradecyltrimethylammonium bromide. They also used explicit representations of the water molecules in a slab orientation, with the mono-layer on either side, in a molecular dynamics simulation. Their goal was to model more disordered, liquid states, so they chose two larger molecular areas, 0.45 and 0.67 nm molecule Density profiles normal to the interface were calculated and compared to neutron reflectivity data, with good agreement reported. The hydrocarbon chains were seen as highly disordered, and the diffusion was seen at both areas, with a factor of about 2.5 increase from the smaller molecular area to the larger area. They report no evidence of a tendency for the chains to aggregate into ordered islands, so perhaps this work can be seen as a realistic computer simulation depiction of a monolayer in an LE state. 

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

In the last 10-15 years, neutron reflectometry has been developed into a powerful technique for the study of surface and interfacial structure, and has been extensively applied to the study of surfactant and polymer adsorption and to determine the structure of a variety of thin films [14, 16]. Neutron reflectivity is particularly powerful in the study of organic systems, in that hydrogen/deu-terium isotopic substitution can be used to manipulate the refractive index distribution without substantially altering the chemistry. Hence, specific components can be made visible or invisible by refractive index matching. This has, for example, been extensively exploited in studying surfactant adsorption at the air-solution interface [17]. In this chapter, we focus on the application of neutron reflectometry to probe surfactant adsorption at the solid-solution interface. 

Clearly, neutron reflectivity has contributed much to our understanding of the nature of surfactant adsorption at the solid-solution interface. It has already been successfully applied to an extensive range of systems, as illustrated in this chapter. 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

The present paper is organized as follows. In Sect. 2 we present an experimental system for which both chemically end-grafted layers with high grafting densities and irreversibly adsorbed surface anchored layers can be formed (Sect. 2.1). We then discuss how the internal structure of these two kinds of surface layers can be analyzed and compared to the different available models (Sect. 2.2). In Sect. 3, we review what is expected for the interdigitation between such surface-anchored layers and a polymer melt, and compare these expectations with neutrons reflectivity results. In Sects. 4 and 5, we analyze the experimental data and the different models which allow to understand under which conditions such surface anchored layers can be used to promote adhesion or to reduce friction. 

Polymer-polymer interfaces are an important area of study since the interfacial behaviour is fundamental to the bulk properties of the system. This is particularly true when two or more polymers are mixed to form a blend, but the interface also plays a dominant role in areas such as adhesion, welding, surface wetting and mechanical strength. To understand fully polymer behaviour in such applications, the interface must be characterised at a microscopic level. Through deuterium labelling the interface between otherwise indistinguishable polymers can be studied, and neutron reflectivity provides unprecedented detail on interfacial width and shape. In addition to the inherent interdiffusion between polymers at a polymer-polymer interface, the interface is further broadened by thermally driven capillary waves. Capillary waves... 

The time dependence of the early stages of polymer interdiffusion at interfaces is indicative of the diffusion process. The normal approach to study such interdiffusion by neutron reflectivity is to use an anneal/quench cycle where the sample is heated for a given time above the glass transition temperature (Tg) of the polymer, then quenched rapidly to room temperature, after which the reflectivity profile is measured. This has proved to be highly effective for a number of systems, but is difficult to apply when Tg is room temperature, or for small molecule ingress into a higher molecular weight polymer layer. 

Thomas, R.K. (1999) Neutron reflectivity at liquid-vapor, liquid-liquid and solid-liquid interfaces, in Modern Characterization Methods of Surfactant Systems (ed. B.P. Binks), Marcel Dekker, New York, pp. 417-479. 

Interfacial agents, such as block copolymers, are known to reduce the Interfaclal tension and hence are expected to Increase the degree of dispersion in blends. The measurement of Interfacial tension for polymer systems is not easy. Most measurements have been made by the pendant drop technique. Measurements of Interfacial thickness are also difficult. They have been made using electron microscopy and, mostly in the case of block copolymers, by x-ray and neutron scattering. Recent results using neutron reflection suggest that this will be a useful technique in the future. 

The supercooling is also observed with protein (BSA, casein, lactoglobulin) in addition to the aqneous phase-Cjg system, bnt the freezing point of hexadecane increases to 18.2°C. This indicates that the crystallization of the hexadecane is affected by the presence of surface-active molecules. The supercooling will have extensive dependence on various interfaces, such as emulsions, oil recovery, and immunological systems. The adsorption of proteins from aqueous solutions on snrfaces has been studied by neutron reflection. ... 

In this review we focus on polyelectrolyte-surfactant interactions at solid-liquid interfaces as studied with surface force measuring techniques. The last years have seen much progress in this area, and it is timely to recapitulate some main findings. It is, however, clear that in order to understand interfacial properties of polyelectrolyte-surfactant systems one needs to understand bulk association. Further, a multitude of experimental techniques needs to be applied. Recent advances have been made using ellipsometry [34,35], reflectometry [36,37], neutron reflectivity [38], and surface sensitive spectroscopic techniques [39,40], It is also our belief that the... 

The smallest critical sizes are obtained for homogeneous systems of pure fissile nuclides with maximum neutron reflection. For neutrons with the fission energy spectrum, the critical mass of a metallic sphere of pure is 22.8 kg, that of is 7.5 kg, and that of Pu is 5.6 kg, assuming a 20 cm uranium metal neutron reflector. For fission by thermal neutrons the smallest critical size of a spherical homogeneous aqueous solution of 1102804 without reflector requires 0.82 kg of in 6.3 1 of solution. The corresponding figures for are 0.59 kg in 3.3 1, and of Pu, 0.51 kg in 4.5 1. 

There has been increasing interest in polymer- polymer interfaces from both an academic and an applied point of view. Because most of the industrial relevant polymer blend systems have a multiphase morphology, the understanding of interfaces is essential to achieve desired product properties. Therefore, theoretical interest in polymer-polymer interfaces has increased recently and simultaneously a number of experimental methods to study them have been developed. Neutron reflectivity has an excellent depth resolution of about 0.2 nm but requires deuterated samples which are often not available for engineering... 

Figure 1 shows the interfacial thickness X, measured by ellipsometry for the blend systems PS/PMMA, PMMA/SAN-5.7 and PMMA/SAN-38.7. The systems containing random copolymers show a relatively thick interface. This is caused by their small polymer-polymer interaction parameter Xab and will be discussed below. The interfacial thickness in the system PS/PMMA increases slightly with temperature. The value at 120 °C was obtained by neutron reflectivity and was taken from ref. 4. 

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

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