Experimental investigations of coherently controlling molecular dynamics are currently hampered by the lack of high intensity, mid-infrared laser sources. This paper describes our approach to generating short pulse, h...
Experimental investigations of coherently controlling molecular dynamics are currently hampered by the lack of high intensity, mid-infrared laser sources. This paper describes our approach to generating short pulse, high energy mid-infrared laser pulses. The laser will be used to investigate chirped dissociation of molecules.
To probe the effects of mechanical confinement on freely standing polystyrene (PS) films, we have symmetrically capped freely standing PS films by thin, solid layers. Annealing of the trilayer films produces a novel l...
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To probe the effects of mechanical confinement on freely standing polystyrene (PS) films, we have symmetrically capped freely standing PS films by thin, solid layers. Annealing of the trilayer films produces a novel lateral morphology which is driven by the dispersion force between the capping layers. A simple model is presented which describes the scaling behavior of the morphology with the layer thicknesses. The same morphology is observed for freely standing and supported films in eight different systems. We also demonstrate reversibility of the morphology by manipulation of the dispersion force.
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 present a semianalytical approach to the determination of the dynamic properties of randomly branched polymers under the Rouse approximation. The principal procedure is based on examining an eigenvalue spectrum whi...
We present a semianalytical approach to the determination of the dynamic properties of randomly branched polymers under the Rouse approximation. The principal procedure is based on examining an eigenvalue spectrum which represents the average dynamic behavior of various structures. The calculated spectra show that the eigenvalue distribution is random even within a single structure, which in turn produces a continuous spectrum of values for the entire class. The autocorrelation function for the square of the radius of gyration was calculated based on these spectra, which confirms that the dynamics are nonexponential as earlier reported. A universal stretched exponent is also found in this study.
We describe the evolution of the charge state of a fast atomic projectile under the equilibrium conditions as a stationary continuous-time Markov process with discrete values. The time scale of the charge-state autoco...
We describe the evolution of the charge state of a fast atomic projectile under the equilibrium conditions as a stationary continuous-time Markov process with discrete values. The time scale of the charge-state autocorrelation function is governed by the rate of charge-changing collisions in the target. Describing the collective electron excitations in the target by a linear-response formalism, we show that the finite response time of the electron gas makes the mean stopping power and the mean self-energy of the projectile sensitive to the charge-state correlation effects. We evaluate these effects for light projectiles by using a two-level model for the Markov process and simple models for the dielectric function of the electron gas.
Transients for photoconductivity, for photoabsorption and for the photodielectric effect were simulated for an exponential density of states, and the transition between non-dispersive and dispersive transport was dete...
Transients for photoconductivity, for photoabsorption and for the photodielectric effect were simulated for an exponential density of states, and the transition between non-dispersive and dispersive transport was determined. A universal time-energy diagram was developed to predict these transitions. To observe dispersive transport when thermalization occurs to give a power-law decay, an individual trapping time must be less than the release time and both must be less than the recombination time. The effect of trap saturation on this transition is discussed.
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.
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 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.
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