Exploration of the (ethanol)4–water heteropentamers potential energy surface by simulated annealing and Ab initio molecular dynamics

A DFT electronic structure study of the (ethanol)₄–water heteropentamers at the B3LYP/6‐31+G(d) model chemistry was carried out. To get determine possible configurations, the potential energy surface (PES) was explored with two methods: simulated annealing and ab initio molecular dynamics. The results suggest that the PES is very flat. A total of 81 stable structures were determined. These structures were classified into 16 different geometric patterns according to geometric criteria like the number of hydrogen bonds and their spatial arrangement: cyclic, bicyclic, or lineal patterns. Thermodynamic stability was used for defining the order of such classification. Hydrogen bonds are mutually disturbed due to the existence of cooperative effects. Cooperativity affects the nature of the hydrogen bonds and the overall stability of the ethanol–water system given that the strongest interactions are markedly covalent and the most stable geometric pattern corresponds to the pentagonal arrangement. These observations were supported by the analysis of the loss of atomic charge of the hydrogen atoms involved in hydrogen bonds. These hydrogen bonds were classified as primary and secondary hydrogen bonds: O? H ··· O and C? H ··· O, respectively. For comparative purposes, some (ethanol)₅, (methanol)₅, and (methanol)₄–water clusters were characterized in this study. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

On the stabilization of unphysical pressure oscillations in MPS method simulations

In this paper, pressure stability through the suppression of high‐frequency pressure oscillations in the moving particle semi‐implicit (MPS) method is presented. To obtain a stable pressure field, we improve the free‐surface particle search algorithm. Pressure stability follows from the suppression of high‐frequency pressure oscillations due to a correction in the Laplacian operator of the Poisson pressure equation and from the correction of the pressure gradient operator. The three proposed modifications are applied gradually and compared with the MPS method to show the improvements in the hydrostatic pressure and dam‐breaking problems. To validate the suppression of the high‐frequency numerical pressure oscillations, modified MPS methods with and without a removable wall are compared with published dam‐breaking experiment pressure measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

Morphological study and photo-addressing in poly(styrene-b-butadiene-b-styrene) block copolymers with azobenzene groups and polystyrene matrix: Influence of chemical bonding

The main goal of this work was the synthesis of new azo-functionalized block copolymers (BCP) from epoxidized poly(styrene-b-butadiene-b-styrene) modified with azobenzene groups by one-step facile reaction between the epoxy groups and an azo-amine. The epoxy/amine reaction was verified by Fourier transform infrared spectroscopy. Additionally, we studied the effect of covalent attachment of the azobenzene moieties by analyzing the morphology and the optical anisotropic response of the resulting azo-containing BCP, with respect to solution mixing of the azobenzene as a guest in the BCP host without chemical bonding. Self-assembly of all modified BCP resulted in phase-separated morphologies on the nanometer scale. Nonetheless, segregation of azobenzene aggregates onto the BCP surface was observed in guest–host systems. In relation to the optical anisotropic behaviour of the resulting materials, two distinct optical responses were observed depending on the existence or not of covalent attachment of the azo-chromophores to the BCP.     

Electrostatic force microscopy measurements of CdSe-PS nanoparticles and CdSe-PS/poly(styrene-b-butadiene-b-styrene) nanocomposites

Electrostatic force microscopy (EFM) measurements were performed to analyze the conductive properties of CdSe nanoparticles functionalized with polystyrene (PS) brushes and embedded in a poly(styrene-b-butadiene-b-styrene) triblock copolymer. CdSe nanoparticles were synthesized aqueously and functionalized with polystyrene chains by the grafting through technique. CdSe-PS nanoparticles obtained after 5 and 8 h of polymerization were analyzed, in order to study the effect of the molecular weight of PS chains on conductive properties. EFM results showed the maintenance of the conductive properties of CdSe nanoparticles through functionalization reactions and even when they were confined in the block copolymer. Due to the low differences between the values obtained in the response of the samples to the charged tip, no effect of the molecular weight of brushes was confirmed.     

Intermolecular interactions on amine‐cured epoxy matrices with different crosslink densities. Influence on the hole and specific volumes and the mechanical behavior

The architecture of an epoxy matrix was modified by curing the resin with mono‐/diamine mixtures having identical chemical structures. Both hole volume and specific volume variations were studied by positron annihilation lifetime spectroscopy and pressure‐volume‐temperature/density measurements, respectively. The average hole volume of the networks at room temperature slightly increased when the monoaminic chain extender content increased. The increment in the intermolecular interactions between functional groups of the networks chains, due to the less hindered nitrogen introduced by the monoamine, appears to be the responsible for the observed behavior. Besides, only small variations on the specific volume were observed on increasing the monoamine content, which points out that for a cured epoxy system, the chemical structure of the curing agent is mainly responsible for chain packing in the networks. On the other hand, intermolecular interactions between chains were considered as the key factor for fixing stiffness and strength. Thus, it was observed that the increase of the intermolecular interactions with the monoamine content produced a decrease in the sub‐T_g small‐range cooperative motions, which increased the low‐deformation mechanical properties at temperatures between β and α relaxations. This conclusion could be applied to previous investigations with epoxy matrices not fully crosslinked (nonstoichiometric or noncompletely cured formulations). Finally, it was found that fracture properties do not significantly depend either on the hole volume or on the intermolecular interactions. Fracture properties are more dependent on the crosslink density and the glass transition temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1240–1252, 2009     

Photodegradation of Luminescence in SiO2 : Rh B Gels Exposed to YAG : Nd Laser Pulses

Silicate gels doped with organic dyes have been proposed as solid-state tunable lasers. Photobleaching of the dye under laser excitation is an important phenomenon in this application. The optical absorption and luminescence of SiO2 silica gels doped with Rhodamine B exposed to the second harmonic pulses of a YAG : Nd laser have been studied. In addition to the characteristic exponential decay with the number of pulses, overlapping oscillations in the intensity were observed. This behavior is explained in terms of a long lived metastable electronic excited state of the dye molecules.     

Influence of migration of plasticizer during melt mixing on the transitions and properties of PC/CAB blends

Static and dynamic mechanical properties, morphology, and thermal behavior of polycarbonate (PC)/plasticized cellulose acetobutyrate (CAB) blends were investigated to determine whether the plasticizer of the CAB modifies the miscibility of the blend and the mechanical properties of this essentially incompatible blend. In spite of the lack of transparency of the blends, both dynamic mechanical and thermal analysis results show the presence, at all blend compositions studied, of a single glass transition temperature which varies with the composition of the blend. Considering the ternary nature of the blends, we propose that plasticizer migration and the difficulty of discerning the presence of one or two peaks in a narrow temperature range may account for the observed behavior. Scanning electron microscopy confirms the immiscibility of the blends. The blends show large positive deviations of the tensile moduli from linearity and very low ductility. The reported tensile strength data are discussed in terms of several different equations for composites. This mechanical behavior is explained as a consequence of the migration of the plasticizer and of its subsequent antiplasticizing effect on the properties of the blend.     

Isoconversional kinetic analysis of novolac-type lignophenolic resins cure

Several phenomenological models (including simple models by Ozawa and Kissinger and the Kissinger–Akahira–Sunose isoconversional method) have been used to compare the cure kinetics of two lignin-based novolac-type phenolic resins with those from a commercial novolac system. When 45 wt% phenol is substituted by a sulfonated kraft lignin, an important reduction in the activation energy is obtained. This behaviour has been attributed to the incorporation of an extra amount of hydroxymethyl groups in the formulation, as they are present in important quantities in the original lignin structure. KAS isoconversional model shows that the rise in viscosity derived from lignin introduction leads to a moderate change in the limiting stage from a kinetic to a diffusion regime, while condensation reactions, which are favoured by the abundance of lignin hydroxymethyl groups, acquire high relevance in LPF-45 system cure. Finally, competition with other mechanisms initiated at high temperature is reported at high conversion grades for all cases.     

Surfactant addition effects on dispersion and microdomain orientation in SBS triblock copolymer/alumina nanoparticle composites

The aim of this study was to investigate the effects of surfactant addition on the dispersion of 1–3 wt.% alumina nanoparticles on the self-assembled morphology of poly(styrene-b-butadiene-b-styrene) (SBS) linear triblock copolymer. The neat triblock copolymer microphase separated into PS cylinders self-assembled on a hexagonal array in the PB matrix, being the orientation of domains dependent on the annealing conditions. UV–vis and AFM analyses showed an improvement on dispersion of Al₂O₃ nanoparticles into SBS matrix by adding dodecanethiol as organic surfactant, due to its miscibility with PS block. Interactions between surfactant-coated Al₂O₃ nanoparticles with PS block were demonstrated by the variation on the glass transition temperature of this block. AFM analysis showed that the incorporation of surfactant-coated Al₂O₃ nanoparticles in the SBS matrix have great influence on the orientation of microphase separated domains in SBS nanocomposites.