Aerogels as diverse nanomaterials

Aerogels are 3-D light-weight nanoporous materials pursued for their low thermal conductivity, low dielectric constant and high acoustic attenuation. Those exceptional macroscopic properties of aerogels are dependent on the chemical nature of nanoparticles, complex hierarchical solid skeletal framework and porosity. Also, the free space can become host for functional guests such as pharmaceuticals. In chapter I, we investigated randomly mesoporous bio-compatible polymer-crosslinked dysprosia aerogels as drug delivery vehicles and demonstrated storage and release of drugs under physiological conditions. Comparative study with ordered and randomly mesoporous silica showed high drug uptake and slower release rate for random nanostructures (silica or dysprosia) relative to ordered silica. Drug release data from dysprosia aerogels showed that drug is stored successively in three hierarchical pore sites on the skeletal framework. In chapter II, we developed flexible polyurethane-acrylate aerogels from star monomer containing urethane linkage and terminal acrylate bonds by free-radical polymerization. Lower density samples were flexible, while higher density samples were mechanically strong. Those results were dependent on the particle size and interparticle connectivity of skeletal framework, pointing to a nanoscopic origin for their flexibility, rather than to a molecular one. Further, the acrylate bonds were converted to norbornene moieties and the gelation process was brought down to room temperature by using ring opening metathesis polymerization (ROMP). In chapter III, we developed polydicyclopentadiene (pDCPD) based aerogels using two different Grubbs catalysts (GC-I and GC-II) with different catalytic activity towards ROMP. The different behavior of pDCPD aerogels was traced to a different polymer configuration at molecular level. --Abstract, page v

Hyperfine Interactions in the Heavy Fermion CeMIn_5 Systems

The CeMIn_5 heavy fermion compounds have attracted enormous interest since their discovery six years ago. These materials exhibit a rich spectrum of unusual correlated electron behavior, and may be an ideal model for the high temperature superconductors. As many of these systems are either antiferromagnets, or lie close to an antiferromagnetic phase boundary, it is crucial to understand the behavior of the dynamic and static magnetism. Since neutron scattering is difficult in these materials, often the primary source of information about the magnetic fluctuations is Nuclear Magnetic Resonance (NMR). Therefore, it is crucial to have a detailed understanding of how the nuclear moments interact with conduction electrons and the local moments present in these systems. Here we present a detailed analysis of the hyperfine coupling based on anisotropic hyperfine coupling tensors between nuclear moments and local moments. Because the couplings are symmetric with respect to bond axes rather than crystal lattice directions, the nuclear sites can experience non-vanishing hyperfine fields even in high symmetry sites.Comment: 15 pages, 5 figure

Crystalline free energies of micelles of diblock copolymer solutions

We report a characterization of the relative stability and structural behavior of various micellar crystals of an athermal model of AB-diblock copolymers in solution. We adopt a previously devel- oped coarse-graining representation of the chains which maps each copolymer on a soft dumbbell. Thanks to this strong reduction of degrees of freedom, we are able to investigate large aggregated systems, and for a specific length ratio of the blocks f = MA/(MA + MB) = 0.6, to locate the order-disorder transition of the system of micelles. Above the transition, mechanical and thermal properties are found to depend on the number of particles per lattice site in the simulation box, and the application of a recent methodology for multiple occupancy crystals (B.M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)) is necessary to correctly define the equilibrium state. Within this scheme we have performed free energy calculations at two reduced density {\rho}/{\rho}\ast = 4,5 and for several cubic structures as FCC,BCC,A15. At both densities, the BCC symmetry is found to correspond to the minimum of the unconstrained free energy, that is to the stable symmetry among the few considered, while the A15 structure is almost degenerate, indicating that the present sys- tem prefers to crystallize in less packed structures. At {\rho}/{\rho}\ast = 4 close to melting, the Lindemann ratio is fairly high (~ 0.29) and the concentration of vacancies is roughly 6%. At {\rho}/{\rho}\ast = 5 the mechanical stability of the stable BCC structure increases and the concentration of vacancies ac- cordingly decreases. The ratio of the corona layer thickness to the core radius is found to be in good agreement with experimental data for poly(styrene-b-isoprene)(22-12) in isoprene selective solvent which is also reported to crystallize in the BCC structure

Direct current measurements of a shelf-edge frontal jet in the southern Benguela system

A bathythermograph section generated during the outward run along a roo mile line running 255° from Cape Town was used to condition the positioning of a series of direct current measurements during the inward run along the same line. The latter produced the first direct measurement current velocity section across the Benguela system...

Collapse arrest and soliton stabilization in nonlocal nonlinear media

We investigate the properties of localized waves in systems governed by nonlocal nonlinear Schrodinger type equations. We prove rigorously by bounding the Hamiltonian that nonlocality of the nonlinearity prevents collapse in, e.g., Bose-Einstein condensates and optical Kerr media in all physical dimensions. The nonlocal nonlinear response must be symmetric, but can be of completely arbitrary shape. We use variational techniques to find the soliton solutions and illustrate the stabilizing effect of nonlocality.Comment: 4 pages with 3 figure

Burn septicaemia in Kuwait: associated demographic and clinical factors

Objective: To study the demographic and clinical factors associated with burn septicaemia patients in Kuwait. Materials and Methods: All burn in-patients, who developed septicaemia at the Burns Unit, Al-Babtain Centre for Burns and Plastic Surgery, Kuwait, during a 9-year period (June 1992 to May 2001) were included in the study. The data were recorded for age, sex, nationality, cause and percentage of burns, inhalation injury, resuscitation, number of episodes, septicaemia on post-burn day, the microorganisms responsible in each episode, treatment and outcome for statistical analysis. Using SPSS (PC version 11.0) software, a probability level of p<0.05 was considered significant. Results: Of the 2,082 patients treated in the Burns Unit, 166 [8%; 99 (60%) males and 67 (40%) females] with a mean age of 26 years (range 1-70) had septicaemia. Significantly higher (p <0.001) cases were recorded among Kuwaiti children (≤14 years) and non-Kuwaitis (25-59 years) than other corresponding age groups. The total body surface area burned ranged from 2 to 95% (mean 42%) and the main cause of burn was flame (77.1%). Inhalation injury was diagnosed in 39 (23.5%) patients. A total of 253 septicaemic episodes occurred in all patients. The majority, 123 (74.1%), had a single episode and the remaining 43 (25.6%) had multiple (2-10) episodes. One hundred and fifty-five (61.3%) episodes were due to gram-positive organisms, mainly methicillin-resistant Staphylococcus aureus, and 32 (12.7%) were polymicrobial. One hundred and twenty-four (74.7%) patients had wound excision and skin grafting procedures and their survival was significantly higher (OR=4.3; 95% CI: 1.98-9.31) than nonsurgically treated patients. Thirty-nine (23.5%) patients died mainly due to multi-organ failure. Conclusion: The findings indicate that the patients with extensive flame burns were prone to developing septicaemia due mainly to gram-positive bacteria. The surgical excision of eschar and wound covering improved the outcome of the patients while prophylactic antibiotic treatment had no role in the incidence and outcome of the burn patients

Interaction-induced localization of anomalously-diffracting nonlinear waves

We study experimentally the interactions between normal solitons and tilted beams in glass waveguide arrays. We find that as a tilted beam, traversing away from a normally propagating soliton, coincides with the self-defocusing regime of the array, it can be refocused and routed back into any of the intermediate sites due to the interaction, as a function of the initial phase difference. Numerically, distinct parameter regimes exhibiting this behavior of the interaction are identified.Comment: Physical Review Letters, in pres

Optical phonon scattering and theory of magneto-polarons in a quantum cascade laser in a strong magnetic field

We report a theoretical study of the carrier relaxation in a quantum cascade laser (QCL) subjected to a strong magnetic field. Both the alloy (GaInAs) disorder effects and the Frohlich interaction are taken into account when the electron energy differences are tuned to the longitudinal optical (LO) phonon energy. In the weak electron-phonon coupling regime, a Fermi's golden rule computation of LO phonon scattering rates shows a very fast non-radiative relaxation channel for the alloy broadened Landau levels (LL's). In the strong electron-phonon coupling regime, we use a magneto-polaron formalism and compute the electron survival probabilities in the upper LL's with including increasing numbers of LO phonon modes for a large number of alloy disorder configurations. Our results predict a nonexponential decay of the upper level population once electrons are injected in this state.Comment: 10 pages, 23 figure

Characteristics of the polymer transport in ratchet systems

Molecules with complex internal structure in time-dependent periodic potentials are studied by using short Rubinstein-Duke model polymers as an example. We extend our earlier work on transport in stochastically varying potentials to cover also deterministic potential switching mechanisms, energetic efficiency and non-uniform charge distributions. We also use currents in the non-equilibrium steady state to identify the dominating mechanisms that lead to polymer transportation and analyze the evolution of the macroscopic state (e.g., total and head-to-head lengths) of the polymers. Several numerical methods are used to solve the master equations and nonlinear optimization problems. The dominating transport mechanisms are found via graph optimization methods. The results show that small changes in the molecule structure and the environment variables can lead to large increases of the drift. The drift and the coherence can be amplified by using deterministic flashing potentials and customized polymer charge distributions. Identifying the dominating transport mechanism by graph analysis tools is found to give insight in how the molecule is transported by the ratchet effect.Comment: 35 pages, 17 figures, to appear in Phys. Rev.