Mean-Field-Theory for Polymers in Mixed Solvents Thermodynamic and Structural Properties

Theoretical aspects of polymers in mixed solvents are considered using the Edwards Hamiltonian formalism. Thermodynamic and structural properties are investigated and some predictions are made when the mixed solvent approaches criticality. Both the single and the many chain problems are examined. When the pure mixed solvent is near criticality, addition of a small amount of polymers shifts the criticality towards either enhanced compatibility or induced phase separation depending upon the value of the parameter describing the interaction asymmetry of the solvents with respect to the polymer. The polymer-solvent effective interaction parameter increases strongly when the solvent mixture approaches criticality. Accordingly, the apparent excluded volume parameter decreases and may vanish or even become negative. Consequently, the polymer undergoes a phase transition from a swollen state to an unperturbed state or even take a collapsed configuration. The effective potential acting on a test chain in strong solutions is calculated and the concept of Edwards screening discussed. Structural properties of ternary mixtures of polymers in mixed solvents are investigated within the Edwards Hamiltonian model. It is shown that the effective potential on a test chain in strong solutions could be written as an infinite series expansion of terms describing interactions via one chain, two chains etc. This summation can be performed following a similar scheme as in the Ornstein-Zernike series expansion.Comment: accepted in Macromol. Theory Simu

Phonon-modulated magnetic interactions and spin Tomonaga-Luttinger liquid in the p-orbital antiferromagnet CsO2

The magnetic response of antiferromagnetic CsO2, coming from the p-orbital S=1/2 spins of anionic O2- molecules, is followed by 133Cs nuclear magnetic resonance across the structural phase transition occuring at Ts1=61 K on cooling. Above Ts1, where spins form a square magnetic lattice, we observe a huge, nonmonotonic temperature dependence of the exchange coupling originating from thermal librations of O2- molecules. Below Ts1, where antiferromagnetic spin chains are formed as a result of p-orbital ordering, we observe a spin Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting phenomena, which provide rare simple manifestations of the coupling between spin, lattice and orbital degrees of freedom, establish CsO2 as a model system for molecular solids.Comment: 9 pages, 5 figures (with Supplemental Material), to appear in Physical Review Letter

Coeficientes culturais de consórcio milho-feijão e milho-braquiária.

Para estabelecer os riscos climáticos para consórcios milho-feijão e milho-pastagens, é necessário que se conheça os seus respectivos coeficientes culturais. Objetivando determinar esses coeficientes, foram instalados, em três épocas, ensaios de campo onde determinou-se, o consumo de água pelas cultura no consórcio por meio do balanço hídrico do solo e a demanda potencial de água por meio da metodologia proposta por Penman-Monteith. Os resultados mostraram que os coeficiente culturais dos consórcios foram muito maiores que das culturas isoladamente, o que permite concluir que o consórcio demanda maior disponibilidade de água no solo. Portanto, a implantação do consorcio dependente grandemente da distribuição das chuvas na região e da a capacidade de retenção de umidade do solo. Assim sendo haverá restrição da época e da área a ser plantada por esse sistema produtivo comparado com a cultura isolada

One-dimensional quantum antiferromagnetism in the p−p-orbital CsO2_2 compound revealed by electron paramagnetic resonance

Recently it was proposed that the orbital ordering of $\pi_{x,y}^*$ molecular orbitals in the superoxide CsO$_2$ compound leads to the formation of spin-1/2 chains below the structural phase transition occuring at $T_{\rm{s1}}=61$~K on cooling. Here we report a detailed X-band electron paramagnetic resonance (EPR) study of this phase in CsO$_2$ powder. The EPR signal appears as a broad line below $T_{\rm{s1}}$, which is replaced by the antiferromagnetic resonance below the N\'{e}el temperature $T_{\rm N}=8.3$~K. The temperature dependence of the EPR linewidth between $T_{\rm{s1}}$ and $T_{\rm{N}}$ agrees with the predictions for the one-dimensional Heisenberg antiferromagnetic chain of $S=1/2$ spins in the presence of symmetric anisotropic exchange interaction. Complementary analysis of the EPR lineshape, linewidth and the signal intensity within the Tomonaga-Luttinger liquid (TLL) framework allows for a determination of the TLL exponent $K=0.48$. Present EPR data thus fully comply with the quantum antiferromagnetic state of spin-1/2 chains in the orbitally ordered phase of CsO$_2$, which is, therefore, a unique $p-$orbital system where such a state could be studied.Comment: 6 pages, 3 figure

Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure

Requerimento de água das culturas.

bitstream/CNPMS/16172/1/Circ_20.pd

Características físico-hídricas e disponibilidade de água no solo.

bitstream/CNPMS/15588/1/Circ_21.pd

MYOCARDIAL STRUCTURE AND VASCULARIZATION OF THE HEART VENTRICLE IN HOLOCEPHALI: IMPLICATIONS FOR HEART EVOLUTION

El resumen aparece en el Program & Abstracts of the 10th International Congress of Vertebrate Morphology, Barcelona 2013. Anatomical Record, Volume 296, Special Feature — 1: P-075.It has been classically assumed that the ventricle of the primitive vertebrate heart is composed of spongy myocardium, supplied exclusively by oxygen-poor, luminal blood. This idea is on two facts: (1) extant agnathans have a spongy ventricular myocardium, and (2) in avian and mammalian embryos, the formation of trabeculated myocardium precedes the appearance of compact myocardium. Recently, it has been proposed that, like elasmobranchs, the early gnathostomes possess a fully vascularised ventricle composed of mixed myocardium. We tested this idea by studying the structure and vascularisation of the ventricular myocardium in four holocephalan species of the families Chimaeridae and Rhinochimaeridae. Chimaera monstrosa, Hidrolagus affinis and Harriotta raleighana have a spongy myocardium covered by a thin layer of cardiac muscle. In H. raleighana, the compacta is reduced to an extremely fine rim. In all three species there is a well-developed coronary artery system consisting of subepicardial vessels which give off branches that penetrate the myocardial trabeculae. Rhinochimaera atlantica has no compacta and its ventricular coronary artery system is reduced to subepicardial vessels that do not enter the spongy layer. This report is the first to show that in wild living vertebrates, a coronary artery system supplying the whole myocardium exists in the absence of a well-developed compacta, which supports experimental work that shows that myocardial cell proliferation and coronary vascular growth rely on genetically separated programs. We conclude that the mixed ventricular myocardium is primitive for chondrichthyans, and that the lack of compacta in some holocephalans is a derived character. Moreover our results support the hypotheses that the mixed myocardium is the primitive condition in gnathostomes, and that the absence of a compacta in different actinopterygian taxa is the result of its repeated loss during evolution.Proyecto CGL2010-16417/BOS; Fondos FEDE

The bulbus arteriosus of the holocephalan heart

El resumen aparece en el Program & Abstracts of the 10th International Congress of Vertebrate Morphology, Barcelona 2013.Anatomical Record, Volume 296, Special Feature — 1: P-074.Previous work has shown that the outflow tract of the elasmobranch heart, namely the cardiac portion intercalated between the ventricle and the ventral aorta, does not consist of a single component, the conus arteriosus, as has classically been assumed, but two, the myocardial conus arteriosus and the non-myocardial bulbus arteriosus. From the evolutionary perspective, knowledge of the anatomy of the cardiac outflow tract of the holocephali is important, as they are the sister group of elasmobranchs. Our aim is to describe the cardiac outflow tract of four holocephalan species, two of them, Chimaera monstrosa and Hydrolagus affinis of the family Chimaeridae, and the other two, Harriotta raleighana and Rhinochimaera atlantica, of the family Rhinochimaeridae. The cardiac outflow tract of the four species consisted of a myocardial conus arteriosus, furnished with valves, and a bulbus arteriosus devoid of cardiac muscle. Both the bulbus and conus are tubular in shape. The length of the bulbus relative to the total length of the outflow tract is somewhat smaller in the rhinochimaerids (15%-19%) than in the chimaerids (19%-23%). The bulbus is covered by epicardium and is crossed by the main coronary artery trunks. Histologically, the bulbus is mainly composed of elastin and collagen, and, to a lesser extent, by smooth muscle. This suggests that in holocephalans, the bulbus actively helps to protect the gill vasculature from exposure to high-pressure pulses of blood. Our results prove that the bulbus arteriosus is common to chondrichthyans. In addition, they support the hypothesis that the cardiac outflow tract consisted of a conus arteriosus and a bulbus arteriosus from the beginning of the jawed vertebrate radiation, contributing to our understanding of the morphological changes that have occurred at the arterial pole of the heart in both actinopterygians and sarcopterygians.Proyecto CGL2010-16417/BOS; Fondos FEDE