Use of the successive self-nucleation and annealing technique to characterize <Superscript>60</Superscript>Co gamma irradiated HDPEs

The application of the Successive Self-nucleation and Annealing (SSA) thermal fractionation technique can yield detailed information of the structural changes induced in linear polyethylene by irradiation. The production of tertiary carbons during the crosslinking reactions can be equivalent to the structural heterogeneity present in branched polyethylenes since in both cases interruption of the linear crystallizable sequences occurs, and these are structural differences that can be easily detected by thermal fractionation. We demonstrate how correlations between melting point and short chain branching content employed for branched polymers can be useful to characterize the distribution of chain heterogeneity produced by crosslinking. As the radiation dose is increased and the crosslinking content also increases, the distribution of chain heterogeneity gets broader as detected by SSA. When the results are coupled with morphological observations made by transmission electron microscopy, valuable information on the morphological changes produced by crosslinking can also be ascertained, since the distribution of lamellar thicknesses substantially broadens with crosslinking. Such a broad distribution can also be predicted from SSA by simple calculations performed employing a modified version of the Gibbs–Thomson equation and is expected on the basis of random crosslinking reactions.     

Activation energy for polypyrrole oxidation: film thickness influence

The oxidation rates of polypyrrole films at different temperatures fit Arrhenius plots, allowing the obtention of the activation energy for the reaction. The activation energy increases for rising thicknesses, up to 4 μm, of the polymer film and decreases for rising film thicknesses. Those values include the constant chemical activation energy and the energy required to relax the polymeric structure allowing the entrance of anions from the solution. The existence of a maximum on the polymeric relaxation energy points to a parallel change on the film molecular structure during the electropolymerization time. The variation of the diffusion coefficient per degree of temperature for the counterions, as a function of the film thickness, is similar to that obtained for the activation energy. Diffusion coefficients were obtained from the electrochemical stretched exponential describing a range of relaxation behaviors in disordered and non-equilibrium systems.     

Evolution of an iron passive film in a borate buffer solution (pH 8.4)

The evolution under open-circuit conditions of iron passive films formed at 0.8 V_SCE in a borate buffer solution at pH 8.4 was investigated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composition of the freshly formed passive film as determined by X-ray photoelectron spectroscopy (XPS) was found to be in agreement with a bilayer model, where the inner layer is composed mainly of iron oxide and the outer layer consists of a hydrated material. Results of XPS measurements also showed that the open-circuit breakdown of passive films was consistent with a reductive dissolution mechanism. When the iron electrode reached an intermediate stage in the open-circuit potential decay (approximately −0.3 V_SCE), the oxide film, containing both Fe(II) and Fe(III), was still protective. The impedance response in this stage exhibited a mixed control by charge transfer at the metal/film and film/solution interfaces and diffusion of point defects through the film. At the final stage of the open-circuit potential decay (approximately −0.7 V_SCE), the oxide film was very thin, and the ratio of Fe³⁺/Fe²⁺ and O²⁻/OH⁻ had decreased significantly. The impedance response also exhibited a mixed charge-transfer–diffusion control, but the diffusion process was related to transport of species in the electrolyte solution resulting from dissolution of the oxide film.     

Liquid-crystalline hybrid materials based on [60]fullerene and bent-core structures

What a core-ker! By the appropriate combination of promesogenic bent-core structures and the C(60) unit, lamellar polar liquid-crystal phases were induced. The supramolecular organization of the functional fullerene-based assemblies, the temperature range of the soft phase, the stabilization of the mesophase-like order at room temperature, and the molecular switching under an electric field can be tuned, depending on the molecular structure.     

Preparation of polymer-supported gold nanoparticles based on resins containing ionic liquid-like fragments: easy control of size and stability

Crosslinked polymers containing covalently attached functional sites with chemical structures related to those present in ILs and having macroscopic properties very similar to the ones of the corresponding bulk ILs can be advantageously applied to the preparation of gold nanoparticles (AuNPs). The size and morphology of the resulting metal nanoparticles (MNPs) can be easily modulated through a proper combination of the synthetic method used for the generation of the NPs and the structural elements of the polymer. Additionally, the resulting supported AuNPs are demonstrated to be stable for at least eight months with the vials open to the atmosphere. Several synthetic methodologies have been studied, as well as different structural parameters for the functional polymers. Those include the chemical nature (anions and cations) and loading of the ionic liquid-like fragments and the morphology of the polymeric matrix. The results obtained show the potential to prepare supported AuNPs with narrow size distributions and small diameters that can have potential interest for application in different fields.     

An analysis of two local measures of the electronic localization: a comparison with the ELF and the exchange-correlation density results

In this work we have explored the performance of two functions, recently proposed by Ayers [J. Chem. Sci., 2005, 117, 441], with the purpose of quantifying local electron localization. The first function, ζ(h), measures the total fluctuation per electron in the number of electrons at a given position r(1), while the second one, ζ(R), is a local representation of the minimum fluctuation criterion for electron localization. The study is carried out through a set of diatomic molecules that covers a wide range of covalent/polar character. Additionally, we have also calculated the electron localization function and the exchange-correlation hole along the internuclear axis. We have found that, for all the studied molecules, the numerical integration involved in computing ζ(h) did not converge. We think that this is so because the hole correlation calculations are not able to yield its correct asymptotic decaying behavior for large absolute values of the internuclear distances. On the other hand, the calculation of ζ(R) has proved to be feasible, and the information obtained from it has been concluded to be compatible to that rendered by the electron localization function (ELF) and the exchange-correlation density. Moreover, it has been also found that the results for ζ(R) allow to quantify the relative degree of electron localization within different molecular regions.     

Regio-selective decoration of nanocavity metal arrays: contributions from localized and delocalized plasmons to surface enhanced Raman spectroscopy

Spherical cap gold nanocavity arrays with internal diameters of 240, 430, 600 and 820 nm were fabricated on smooth gold films using nanosphere lithography with electrochemical metal deposition. Each array was prepared to the same normalized film thickness to diameter ratios, t(N), of 0.8 ± 0.04. Selective modification of the top surface and interior walls of the gold nanocavity arrays with [Ru(bpy)(2)(Qbpy)](2+), where bpy is 2,2'-bipyridyl and Qbpy is 2,2':4,4':4,4'-quarterpyridyl, was accomplished using a two step adsorption process exploiting the assembled polystyrene spheres as masks. This selective modification approach permitted direct quantitative comparison, for the first time, of plasmonic enhancement of Raman signal and luminescence signal from a monolayer adsorbed at the top surface versus interior walls of all-gold nanocavity arrays. For all cavity sizes, significantly greater Raman and luminescence signal enhancement was observed from [Ru(bpy)(2)(Qbpy)](2+) monolayer adsorbed at the top surface of the array compared with the cavity walls. This disparity in Raman intensity from top versus cavity interior increased as the cavity dimensions decreased. For example, the Raman signal intensity from [Ru(bpy)(2)(Qbpy)](2+) adsorbed at the top surface of 240 nm gold arrays was 170 times greater than SERS signal for this material adsorbed at the interior walls of this array, whereas the relative Raman signal enhancement was 6 from top versus interior for the 820 nm internal radius arrays under 785 nm excitation. The origin of the relatively greater signal at the top surface is discussed in the context of plasmonic distribution at each surface.     

First principles study of photostability within hydrogen-bonded amino acids

The photochemistry and photophysics of a two-glycine minimal model is studied at the CASPT2//CASSCF level of theory. Different photoinduced processes are discussed, on the basis of the calculated minimum energy paths and the characterization of the electronic state crossings. Two main processes could provide UV-photostability to the hydrogen-bonded peptide system: (i) forward-backward photoinduced electron/proton transfer involving the H in the hydrogen bond, (ii) singlet-singlet energy transfer between two amino acids, providing ultrafast population of the low-energy n,π* state.     

Structures and electronic properties of silicene clusters: a promising material for FET and hydrogen storage

Structures and electronic properties of clusters of an all-Si analogue of graphene, silicene, have been studied through quantum chemical calculations. The structures of the six-membered rings show interesting chair like puckering, which for large sheet-like clusters form ordered ripples. Binding energies, HOMO-LUMO gaps and polarizabilities for the silicene clusters show interesting monotonic trends analogous to polyacenes. Stacking of two silicene layers leads to the formation of closed 3D clusters with high symmetry and strong Si-Si bonds. The heat of hydrogenation of silicene to form silicanes is overwhelmingly exothermic and leads to the opening up of the HOMO-LUMO gaps. Thus, analogous to graphanes, silicanes are predicted to be interesting materials for hydrogen storage and for their band engineering properties.