Search results for "Optica"
showing 10 items of 7696 documents
Insight in NLO Polymer Material Behavior from Langevin Dynamic Modeling of Chromophore Poling
2011
Calculation of chromophore polar order, i.e. the polarization of nonlinear optically (NLO) active polymers poled by external field, is challenging. One possible solution is to reproduce the orientation polarization of chromophores under external field using Langevin dynamics (LD) molecular modeling. The present work investigates the influence of chromophore dipole moment, density and field extent on NLO efficiency and polarization/relaxation dynamics. Results of simulations convince us, that the method is applicable for modeling real NLO polymers. In the spotlight of this investigation we would like to describe a NLO polymer as a “ferrofluid” where chromophores are active particles or elect…
Stochastic dynamics of polymer brushes under shear deformation
1998
The dynamical properties of polymer brushes under shear deformation have been studied by computer simulation. Both the local and the global dynamic properties at various shear rates have been calculated. The distribution of orientational and translational mobilities of the different monomers along the chain have been obtained. It was shown that the local mobility of the brushes changes very slowly with increasing of shear rates up to the largest rates. Increase in grafting density leads to an increasingly step like dependence of the correlation times as a function of shear rate.
Development of core-shell colloids to study self-diffusion in highly concentrated dispersions
2007
To study single particle motion in highly concentrated colloidal dispersions, a host-tracer colloid system was developed, consisting of crosslinked polymer micronetwork spheres placed in a good solvent. The host colloid is made invisible to the experimental probe by matching its refractive index to that of the solvent. For the tracer particles a core-shell structure was chosen to ensure the interaction potential to be identical to that of the host particles. Therefore the shell was made of the same polymer as the host. The core differs in refractive index from the solvent and is therefore visible due to scattered light.
Electronic structure of polysilanes: influence of substitution and conformation
1993
Abstract The valence effective Hamiltonian (VEH) quantum-chemical approach is used to investigate the electronic properties of polysilane. The valence band structure calculated for this fully saturated polymer is analyzed in terms of orbital contributions and compared to that of the closely related carbon polymer, polyethylene. The effects of alkyl substitution and silicon backbone conformation are studied by elucidating the modifications that these structural changes induce on the electronic valence band structure of all-trans unsubstituted polysilane. The VEH results predict a decrease of the band gap upon alkyl substitution and on going from helical to all-trans conformations.
Photoinduced electron transfer in molecular organizates at the gas-water interface
1994
Abstract The influence of the molecular geometry is of fundamental importance for a better understanding of the photoinduced electron transfer mechanism. Because of their typical molecular structures, cyclophane rings have proved to be suitable for this purpose as electron acceptor molecules adsorbed under an amphiphile monolayer. We used a pyrene-labelled phospholipid derivative both as molecular anchor for the cyclophane ring and electron donor molecule. The co-spreading technique was used to prepare the complex monolayers. Surface pressure and surface potential measurements have indicated similar monolayer behaviour as with dimyristoylphosphatidic acid as anchor molecule, leading to the …
Chemical structure and internal diffusion within polymer chains in the melt
1985
The single chain dynamics of polydimethylsiloxane in the melt is studied by means of quasielastic neutron scattering. For this polymer the wave vector range 0.03 A−1≦q≦0.30 A−1 covers the regime of universal modes as well as local diffusive processes. A model is described which incorporates the specific chemical structure of the macromolecule and allows to interpret our data in the full wave vector regime. The only parameter which enters the model, the monomer diffusion constantDm, is found to be (1.2±0.2)·10−5 cm2s−1.
Residual order in amorphous dry films of polymer latices: indications of an influence of particle interaction
2002
Abstract We report small angle X-ray scattering and atomic force microscopy measurements on macroscopically thick, dry films of polymer latex particles. While the surface of dried droplets has long range order due to layering effects, the overall bulk structure is amorphous. This holds for both low charge polymethylmetacrylate particles and for highly charged polystyrene particles with additional stabilisation with sodiumdodecylsurfate. In the latter case, however, considerable amounts of residual crystal-like order is observed.
Measurement of the strain-optic coefficients of PMMA from 800 to 2000 nm
2020
The strain-optic coefficients of PMMA are measured in a broad wavelength range from 800 to 2000 nm. The sensitivity of the azimuthal whispering gallery mode resonances to the strain is exploited to measure the strain-optic coefficients of PMMA micro-rods. The technique is based on measuring the wavelength shift of the resonances of both polarizations states, the TE and TM, when an axial strain is applied to the polymer rods. This method enables the determination of the strain-optic coefficients of the material in a broad wavelength range. In particular, in the near-infrared range, the PMMA exhibits negligible dispersion and anisotropy, and the strain-optic coefficients show constant values …
Mechanism of Trichloroethene Hydrodehalogenation: A First-Principles Kinetic Monte Carlo Study
2014
A hydrodehalogenation (HDC) reaction of trichloroethene (TCE) has gained a lot of interest due to its possible application in water purification, but the reaction mechanism has been subject to much controversy. In this work, HDC of TCE on Pd(111) was examined by carrying out kinetic Monte Carlo simulations based on DFT-calculated thermodynamic and kinetic parameters. Obtained kMC results show that the HDC follows a so-called direct pathway, which means that, after adsorption on a catalyst, TCE quickly dechlorinates, producing CH–C and then, more slowly, hydrogenates to form hydrocarbon products. This is reflected in the surface coverage snapshots, where intermediates corresponding to the di…
Impact of aggregates on excitation dynamics in transparent polymer films doped by dipolar molecules
2008
Abstract Optical properties of transparent polymer films of polymethylmethacrylate doped (up to 25 wt.%) by dipolar N , N -dimethylaminobenzylidene 1,3-indandione (DMABI) molecules were studied. Formation of DMABI nanocrystallites, increasing in their density and size with dopant concentration, was revealed by optical microscopy. Transformation of the fluorescence spectrum from the molecular-like emission (for the low dopant concentration below 1 wt.%) to highly red-shifted fluorescence corresponding to self-trapped excitons in the crystallites was observed. It was shown, that due to the resonant energy transfer in the blend, the DMABI nanocrystals can serve as efficient fluorescence marker…