Interplay between symmetry breaking and quasiperiodicity in spin chains

Summary An isotropicXY spin chain is studied in the presence of symmetry-breaking perturbations and a periodic modulation with wave vector incommensurate with the periodicity of the chain. The ground states of the system are obtained in terms of a two-dimensional area-preserving map defined on a cylinder of finite extent. The spin distributions are found to exhibit KAM and Cantorus phases with the onset of transition being the critical phase. The relationship between these three phases and the magnetic properties of the model is also studied. The presence of two competing easy axes for spin alignment results in incommensurate reentrant phase diagram with fractal boundary.     

Effect of density fluctuating supercritical carbon dioxide on polymer interfaces

We investigated an effect of CO2 sorption on the compatibility of immiscible polystyrene (PS) and polybutadiene (PB) bilayers by using in situ neutron reflectivity. By labeling either polymer with deuterium, we found that the excess CO2 molecules were adsorbed to both top PS and bottom PB layers when the bilayers were exposed to CO2 at the narrow T and P regime near the critical point of pure CO2. Furthermore, we clarified that this excess sorption of CO2 molecules increased the interfacial width between the layers up to 100 angstroms even near room temperature, while the interfacial width without CO2 exposure has been reported to be at most 40 A even at the highest temperature (T congruent with 175 degrees C).     

Comparison of critical adsorption scaling functions obtained from neutron reflectometry and ellipsometry

Carpenter et al. [Phys. Rev. E 59, 5655 (1999); 61, 532 (2000)] managed to explain ellipsometric critical adsorption data collected from the liquid-vapor interface of four different critical binary liquid mixtures near their demixing critical temperature using a single model. This was the first time a single universal function had been found which could quantitatively describe the surface critical behavior of many different mixtures. There have also been various attempts to investigate this surface critical behavior using neutron and x-ray reflectometries. Results have been mixed and have often been at variance with Carpenter et al. In this paper, the authors show that neutron reflectometry data collected from a crystalline quartz-critical mixture interface, specifically deuterated water plus 3-methylpyridine, can be quantitatively explained using the model of Carpenter et al. derived from ellipsometric data.     

Temperature-triggered micellization of block copolymers on an ionic liquid surface

In situ neutron reflectivity was used to study thermally induced structural changes of the lamellae-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer thin films floating on the surface of an ionic liquid (IL). The IL, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, is a nonsolvent for PS and a temperature-tunable solvent for P2VP, and, as such, micellization can be induced at the air-IL interface by changing the temperature. Transmission electron microscopy and scanning force microscopy were used to investigate the resultant morphologies of the micellar films. It was found that highly ordered nanostructures consisting of spherical micelles with a PS core surrounded by a P2VP corona were produced. In addition, bilayer films of PS homopolymer on top of a PS-b-P2VP layer also underwent micellization with increasing temperature but the micellization was strongly dependent on the thickness of the PS and PS-b-P2VP layers.     

Characterization of Langmuir-Blodgett organoclay films using X-ray reflectivity and atomic force microscopy

Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of 35 degrees +/-6 degrees from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites.     

Synthetic polypeptide adsorption to Cu-IDA containing lipid films: a model for protein-membrane interactions

Adsorption of synthetic alanine-rich peptides to lipid monolayers was studied by X-ray and neutron reflectivity, grazing incidence X-ray diffraction (GIXD), and circular dichroic spectroscopy. The peptides contained histidine residues to drive adsorption to Langmuir monolayers of lipids with iminodiacetate headgroups loaded with Cu2+. Adsorption was found to be irreversible with respect to bulk peptide concentration. The peptides were partially helical in solution at room temperature, the temperature of the adsorption assays. Comparisons of the rate of binding and the structure of the adsorbed layer were made as a function of the number of histidines (from 0 to 2) and also as a function of the positioning of the histidines along the backbone. For peptides containing two histidines on the same side of the helical backbone, large differences were observed in the structure of the adsorbed layer as a function of the spacing of the histidines. With a spacing of 6 A, there was a substantial increase in helicity upon binding (from 17% to 31%), and the peptides adsorbed to a final density approaching that of a nearly completed monolayer of alpha-helices adsorbed side-on. The thickness of the adsorbed layer (17 +/- 2.5 A) was slightly greater than the diameter of alpha-helices, suggesting that the free, unstructured ends extended into solution. With a spacing of 30 A between histidines, a far weaker increase in helicity upon binding was observed (from 13% to 19%) and a much lower packing density resulted. The thickness of the adsorbed layer (10 +/- 4 A) was smaller, consistent with the ends being bound to the monolayer. Striking differences were observed in the interaction of the two types of peptide with the lipid membrane by GIXD, consistent with binding by two correlated sites only for the case of 6 A spacing. All these results are attributed to differences in spatial correlation between the histidines as a function of separation distance along the backbone for these partially helical peptides. Finally, control over orientation was demonstrated by placing a histidine on an end of the sequence, which resulted in adsorbed peptides oriented perpendicular to the membrane.     

Neutron reflectivity characterization of the photoacid reaction-diffusion latent and developed images of molecular resists for extreme ultraviolet lithography

Lithographic feature size requirements have approached a few radius of gyration of photoresist polymers used in thin-film patterning. Furthermore, the feature dimensions are commensurate with the photoacid diffusion length that defines the underlying latent image. Smaller imaging building blocks may enable reduced feature sizes; however, resolution limits are also dependent upon the spatial extent of the photoacid-catalyzed reaction diffusion front and subsequent dissolution mechanism. The reaction-diffusion front was characterized by neutron reflectivity for ccc stereoisomer-purified, deuterium-labeled tert-butoxycarbonyloxy calix[4]resorcinarene molecular resists. The spatial extent of the reaction front exceeds the size of the molecular resist with an effective diffusion constant of (0.13 ± 0.06) nm(2)/s for reaction times longer than 60 s, with the maximum at shorter times. Comparison to a mean-field reaction-diffusion model shows that a photoacid trapping process provides bounds to the spatial and extent of reaction via a reaction-limited mechanism whereas the ratio of the reaction rate to trapping rate constants recovers the effective diffusion peak. Under the ideal step-exposure conditions, surface roughness was observed after either positive- or negative-tone development. However, negative-tone development follows a surface-restructuring mechanism rather than etch-like dissolution in positive-tone development.     

Simultaneous CO concentration and temperature measurements using tunable diode laser absorption spectroscopy near 2.3 μm

Simultaneous measurements of carbon monoxide (CO) mole fraction and temperature using tunable diode laser absorption spectroscopy (TDLAS) near 2.3 μm are reported. The measurement method uses ro-vibrational transitions [R(27): v″ = 1 → v′ = 3] and [R(6): v″ = 0 → v′ = 2] in the first overtone band of CO near 2.3 μm (~4,278 cm^?1). The measurements were performed in the post flame environment of fuel rich premixed ethylene–air flames with a N₂ co-flow, stabilized over a water cooled McKenna burner. Non-uniformity in the temperature and CO mole fraction, along the absorption line of sight, in the mixing layer of the co-flow, was considered during data analysis. The TDLAS based temperature measurements (±80 K) were in good agreement with those obtained using N₂ vibrational coherent anti-Stokes Raman scattering (±20 K), and the CO mole fraction measurements were in good agreement with the equilibrium values, for equivalence ratios lower than 1.8. A signal to noise ratio of 45 was achieved at an equivalence ratio of 1 for a CO concentration of 0.8 % at 1,854 K.