Using optical microscopy, we have measured the formation and growth of holes in freely standing polystyrene films in the melt state. For all films, we observed exponential growth of the hole radius and uniform thicken...
Using optical microscopy, we have measured the formation and growth of holes in freely standing polystyrene films in the melt state. For all films, we observed exponential growth of the hole radius and uniform thickening. Our measurements are interpreted in terms of the dependence of the viscosity on the shear strain rate associated with the hole growth process. The measurement of hole growth in thin freely standing polymer films provides a unique probe of nonlinear viscoelastic effects in confined polymers.
We propose a potential for wormlike polymer chains which could be used to model the low-temperature polypeptide structures. We successfully reproduced helix ground states up to 6.5 helical loops, using the multicanoni...
We propose a potential for wormlike polymer chains which could be used to model the low-temperature polypeptide structures. We successfully reproduced helix ground states up to 6.5 helical loops, using the multicanonical Monte Carlo simulation method. We demonstrate that the coil-helix transition involves four distinct states: coil (gaslike), collapsed globular (liquidlike), and two helical states I and II (both solidlike). The helix I state is characterized by a helical ordering with dangling loose ends, and the helix II state corresponds to a near-perfect helical ordering in the entirely crystallized chain.
A dendrimer is a hyperbranched macromolecule that is grown from a central core generation by generation. We demonstrate through Monte Carlo simulations of a dendrimer model in two dimensions that when the terminal mon...
A dendrimer is a hyperbranched macromolecule that is grown from a central core generation by generation. We demonstrate through Monte Carlo simulations of a dendrimer model in two dimensions that when the terminal monomers and the interior monomers interact through an effective Flory-Huggins parameter χ, an angularly dependent domain structure forms in the molecule for sufficiently large χ. Based on the simulation data, we further show that the formation of the domain pattern is a smooth crossover, not a phase transition, from a mixed structure.
The dynamical interaction effects on the Coulomb explosion and the stopping power are studied for MeV H2+ ions in grazing collisions with a solid surface. The surface dynamical potential induced by the molecular ions ...
The dynamical interaction effects on the Coulomb explosion and the stopping power are studied for MeV H2+ ions in grazing collisions with a solid surface. The surface dynamical potential induced by the molecular ions is obtained by using a local frequency-dependent dielectric function for a semi-infinite electron gas with damping. A set of equations describing the motion of the center of mass and the Coulomb explosion is obtained by including the repulsive force of the surface atoms, the bare Coulomb force, and the dynamical forces. The total energy loss for the molecular ion is calculated, taking into account a contribution from both the collective excitations of the conduction electrons and the single-electron excitations of each of the substrate atoms.
The dissociation of a diatomic molecule by a chirped infrared laser pulse is modeled by a Morse oscillator interacting with a classical electric field with a time-dependent frequency. Our previous classical analysis i...
The dissociation of a diatomic molecule by a chirped infrared laser pulse is modeled by a Morse oscillator interacting with a classical electric field with a time-dependent frequency. Our previous classical analysis in terms of bucket dynamics, in which systems within the single-node separatrices (buckets) in phase space are trapped and undergo convection to highly excited states, is found to be applicable to the more general cases of nonlinear chirping and using a realistic dipole moment function for the molecule. This route of excitation leads to a much lower dissociation threshold laser intensity when compared to the chaotic diffusion route for monochromatic excitation. Time-dependent quantum mechanical calculations of the dissociation probability based on the split-operator method are performed. It is found that the classical and quantum results agree well, and the classical resonances appear also in the quantum probabilities. Hence the classical method can be used to investigate various characteristics of the chirped pulse excitation and dissociation processes.
In this work, a vector lattice model is employed to study binary mixtures of water and surfactants. The face-centered-cubic lattice is employed in order to best approximate a realistic liquid environment. The model is...
In this work, a vector lattice model is employed to study binary mixtures of water and surfactants. The face-centered-cubic lattice is employed in order to best approximate a realistic liquid environment. The model is analyzed by low-temperature expansion and Monte Carlo methods. It is shown that for sufficiently strong surfactant-water interactions the system exhibits a rich polymorphism where up to eight phases are stable. In addition to the disordered water-rich and surfactant-rich phases, liquid-crystalline phases such as the hexagonal and the lamellar phases as well as the inverse bicontinuous cubic, inverse hexagonal and inverse micellar cubic phases are stable in the model. It is shown that the inverse bicontinuous cubic structure in our model is remarkably similar to the gyroid phase. The formation of water channels in the surfactant bilayers of a lamellar phase is also examined.
We have used Brillouin light scattering (BLS) to measure the room-temperature, high-frequency mechanical properties of thin freely standing polystyrene (PS) films. We have investigated the effects of chain confinement...
We have used Brillouin light scattering (BLS) to measure the room-temperature, high-frequency mechanical properties of thin freely standing polystyrene (PS) films. We have investigated the effects of chain confinement and the free surface on the mechanical properties by measuring the velocity of film-guided acoustic phonons in films with thicknesses ranging from less than to greater than the average end-to-end distance of the unperturbed molecules REE. We find that the measured, room-temperature sound velocities are the same, to within ±1%, for all films that have glass transition temperatures that differ by as much as 65 K. Our results, which differ markedly from those of recent picosecond acoustic measurements, are discussed in terms of models proposed to explain anomalous glass transition behavior in thin polymer films. A careful analysis of the BLS data reveals that, at room temperature, the mechanical stiffness, mass density, and thermal expansion of thin, freely standing PS films in the glassy state are consistent with bulk values for all film thicknesses.
We have measured the phase separation morphology of polystyrene–poly (methyl methacrylate) (PS-PMMA) blend films of thickness h on a silicon oxide (SiOx) substrate with a SiOx capping layer. We observe a novel phase ...
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We have measured the phase separation morphology of polystyrene–poly (methyl methacrylate) (PS-PMMA) blend films of thickness h on a silicon oxide (SiOx) substrate with a SiOx capping layer. We observe a novel phase separation morphology for small capping layer thicknesses L and a transition from lateral to lamellar morphology as L is increased. We present a simple model that explains the observed lateral morphology and the transition in morphology in terms of a balance between the free energy increase associated with forming the interfaces between PS-rich and PMMA-rich domains and the free energy increase associated with the elastic bending of the SiOx capping layer. The simple model reveals the dependence of the transition capping layer thickness Lc on the polymer blend film thickness h, and gives a reasonable quantitative prediction of Lc.
Using a Si(Li) x-ray spectrometer, we have measured the respective fractions of L1, L2, and L3 x rays in the L x-ray spectrum emitted in the 157Tb−157Gd electron-capture decay. Using, in addition, our previously measu...
Using a Si(Li) x-ray spectrometer, we have measured the respective fractions of L1, L2, and L3 x rays in the L x-ray spectrum emitted in the 157Tb−157Gd electron-capture decay. Using, in addition, our previously measured value for the ratio of L and K x rays, we deduce values for the fluorescence and Coster-Kronig yields of the L1 subshell. These are ω1=0.101±0.005, f12=0.166±0.020, and f13=0.287±0.014. The f12 and f13 values are significantly below the predictions of the independent-particle model (IPM). The deduced L1 level width of 3.7 eV is also significantly below the IPM width of 4.6 eV.
The detailed response of an Si(Li) detector to the manganese K x-rays from a 55 Fe source was studied by least-squares fitting various spectrum models using a generalized variant of the GUPIX code. The peak-to-backgro...
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