Static and dynamic properties of supercooled thin polymer films

The dynamic and static properties of a supercooled (non-entangled) polymer melt are investigated via molecular-dynamics (MD) simulations. The system is confined between two completely smooth and purely repulsive walls. The wall-to-wall separation (film thickness), D, is varied from about 3 to about 14 times the bulk radius of gyration. Despite the geometric confinement, the supercooled films exhibit many qualitative features which were also observed in the bulk and could be analyzed in terms of mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate time scale of the MCT β-process. An analysis of the temperature dependence of the α-relaxation time suggests that the critical temperature, T _c, of MCT decreases with D. If the confinement is not too strong ( D≥10monomer diameter), the static structure factor of the film coincides with that of the bulk when compared for the same distance, T - T _c(D), to the critical temperature. This suggests that T - T _c(D) is an important temperature scale of our model both in the bulk and in the films. Received 12 September 2001     

Static and dynamic quenching of biologically active coumarin derivative by aniline in benzene-acetonitrile mixtures

The fluorescence quenching of coumarin derivative, 4-(5-chloro-3-furan-2-yl-benzofuran-2-yl)-6-methyl-2H-chromen-2-one (ClFBMC), in the presence of aniline in different proportion of benzene-acetonitrile mixture was studied by means of steady-state measurement (296 K). The quenching process was characterized by Stern-Volmer (S-V) plots, which display positive (upward) deviation. The positive deviation from linearity suggests that the quenching is due to the simultaneous presence of dynamic and static quenching, which is interpreted in terms of the ground-state complex formation and the sphere of action static quenching model. The sphere of action static quenching model agrees very well with experimental results. Further with the use of finite sink approximation model, it is concluded that the bimolecular quenching reactions are diffusion-limited. Various rate parameters for the fluorescence quenching process have been determined. The value of quenching constant k_q increases with increase in dielectric constant of the mixed solvent, suggesting the charge transfer character of the excited complex.     

Static and dynamic properties of polymer brush with topological ring structures: Molecular dynamic simulation

By defining a topological constraint value(rn),the static and dynamic properties of a polymer brush composed of moderate or short chains with different topological ring structures are studied using molecular dynamics simulation,and a comparison with those of linear polymer brush is also made.For the center-of-mass height of the ring polymer brush scaled by chain length h~N~v,there is no significant difference of exponent from that of a linear brush in the small topological constraint regime.However,as the topological constraint becomes stronger,one obtains a smaller exponent.It is found that there exists a master scaling power law of the total stretching energy scaled by chain length N for moderate chain length regime,F_(ene)~Np~v,for ring polymer brushes,but with a larger exponent v than 5/6,indicating an influence of topological constraint to the dynamic properties of the system.A topological invariant of free energy scaled by(c)~(5/4) is found.     

Static and dynamic properties of grafted ring polymer: Molecular dynamics simulation

The static and dynamic properties of a system of end-grafted flexible ring polymer chains grafted to a flat substrate and exposed to a good solvent are studied by using a molecular dynamics method. The monomers are described by a coarse-grained bead-spring model. Varying the grafting density ρ and the degree of polymerization or chain length N, we obtain the density profiles of monomers, study the structural properties of the chain (radius of gyration, bond orientational parameters, etc.), and also present the dynamic characteristics such as chain energy and bond force. Compared with a linear polymer brush, the ring polymer brush exhibits different static and dynamic properties for moderate or short chain length, while it behaves like linear polymer brush in the regime of long chain length.     

New azo-dye-doped polymer systems as dynamic holographic recording media

Polymer materials show their impact on optical storage technology for developing high information density and fast access-type memories with a high read-out efficiency. New azo-dye-doped polymer materials have been developed and used for recording dynamic holograms. Dynamic holograms with reasonably high diffraction efficiency have been recorded at 468 nm, and the efficiency of the Write, Read and Erase (WRE) cycle in these azo-dye-doped polymer materials was determined. Significant observations in these materials are: (i) there is no need to change the polarization of the writing beam to erase the recorded holograms, (ii) the whole WRE cycle is very fast (3–6 s) and (iii) recording of more than 250 WRE cycles without any fatigue of the recording materials is possible.     

Migrationally accelerated quenching of luminescence in disordered media

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 5, pp. 739–744, November, 1991.     

IR spectroscopic determination of alcohols in quenching media

A spectroscopic method is suggested for quantitative determination of some alcohols (1-butanol, 2-methylpropanol-2, 1-hexanol, and glycerol) in mineral oil I-20A used as a quenching medium. The efficiency of the method suggested is tested both in model mixtures and in samples of quenching media. The method can be used in laboratory and industry. A. N. Sevchenko Research Institute of Applied Physical Problems, 7, Kurchatov St., Minsk, 220064, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 591–594, September–October, 1997.     

Theoretical analysis of the static quenching of optical excitations in luminescent nanoparticles

A compact analytical expression has been obtained for the kinetics of the donor-acceptor transfer of the energy and the quenching of optical excitations in spherical nanoparticles with luminescent impurities. A series of numerical experiments simulating the process have been performed using the Monte Carlo method for nanoparticles of various sizes and concentrations of acceptors. The analytical expression well describes the numerical simulation results.     

Effect of C-H bonds on the quenching of luminescent lanthanide chelates

The quenching of europium(III) and terbium(III) chelate luminescence by high-energy C-H vibrational manifolds was studied with two types of stable chelates, i.e., a seven-dentate phenylethynylpyridine derivative and a nine-dentate terpyridine derivative. The replacement of C-H bonds by C-D bonds in the chelating parts of the ligands had a clear positive effect on Eu³⁺ luminescence but a negligible effect on Tb²⁺ luminescence. In aqueous solution, however, the positive effect was undetectable, if the chelating ligand did not create complete shielding of the ion against aqueous quenching. In chelates, where the coordination of water molecules to the inner sphere is prevented, the residual quenching through C-H vibrational quanta can be avoided by replacement of all C-H bonds in the vicinity of the emitting ion by C-D bonds.     

Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer

The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom.     

Static and dynamic electro-optic properties of a SmC* phase in surface stabilized geometry and dispersed in the polymer matrix

Comparative electro-optical measurements have been made on a ferroelectric liquid crystal (FLC) in surface stabilized geometry and confined to an ellipsoidal cavity within a polymer matrix. The static and dynamic electro-optical characteristics were measured for both systems and show qualitatively similar behaviours. A fast switching and important bistability were observed and characterized as a function of the applied electric field strength. The switching time between the two stable states of the surface stabilized cell was found to be longer than that found for the composite films. We argue that the faster switching dynamic of the FLC in cavities is due to the enhance of the rotational mobility of the molecules, probably (and partly) because of the soft anchoring character of the molecules at the cavity walls. Using a collective switching model in the high field regime, which assume a linear coupling between the spontaneous polarization and the local cavity electric field, we give an estimate of the rotational viscosity of the FLC molecules in the droplets.Received: 5 October 2003, Published online: 5 February 2004PACS: 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 77.80.Fm Switching phenomena     

Cooperative luminescence quenching on many-particle acceptors in disordered media

Many-body cooperative energy transfer is an important process in biology, medicine, photosynthesis, rare-earth-doped laser materials, responsible for up- and down-conversion of energy, optical excitation sensitization and relaxation. We present an analytical solution for long-time asymptotic of static luminescence quenching kinetics due to cooperative energy transfer to ensembles of acceptors comprised of two-, three-, and more particles. For cooperative energy transfer and cooperative luminescence quenching to n-body acceptors we have discovered a new law of power d/(nS−(n−1)d)$d/(nS-(n-1)d)$ time dependence (d=1,2,3$d=1,2,3$ is the space dimension, S=6,8,10$S=6,8,10$ is the multipolarty of interaction: dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole). The detailed numerical simulation of cooperative quenching by Monte-Carlo method confirms the theoretical result.     

Static and dynamic properties of pseudoscalar mesons

We report the results of form factors, charge radii and decay constants of both light and heavy flavoured pseudoscalar mesons in a QCD inspired quark model. We use the quantum mechanical perturbation theory and discuss its limitations in the present problem. Several predictions are also made for bottom and top flavours.     

Static and dynamic properties of discrete systems with compressed state space. A polymer chain as an example

The method of the reduction of the size of the state space based on its symmetry is presented. An important property of the method is that in its static version, it preserves probabilities of states of the system. In the reduced state space, the probability of a new state (termed below as class) is equal to the probability of each of the states belonging to a given class multiplied by the number of states which form this class. We also present an appropriate method which allows to calculate time dependent probabilities of states of a given system, if multiple absorbing states are present. This is done with a continuous version of the exact enumeration method for weighted networks. The approach provides a new method of the analysis of non-equilibrium processes. As an application, the state space of a polymer molecule is analysed. We model circular polymer on a regular 2D square lattice. The reduction of the size of the system is presented, as dependent on external conditions. Finally, we evaluate the time of relaxation to and from the absorbing states.     

Static and dynamic double-exchange effects in doped manganites

It is shown that antiferromagnetic ordering in doped manganites with strong double-exchange interaction is transformed into ferromagnetic canted ordering with residual antiferromagnetic behavior in the basal plane as a result of hopping of mobile electron. The canting angle between the core magnetiztions is controlled by the competition of the Heisenberg antiferromagnetic exchange and double exchange. The temperatures of the paramagnet-antiferromagnet and paramagnet-canted ferromagnetic phase transitions are calculated. The results on the dependence of the magnetization in the canted phase and critical temperatures on the doping degree are in qualitative agreement with experiment. The form of uniform oscillations of core magnetiztions in the canted ferromagnetic phase of a doped manganite sample with hopping conduction is analyzed with and without allowance for relaxation of mobile electrons to the lattice. We propose a mechanism for the ferromagnetic resonance broadening and its resonance frequency shift in a ferromagnetic conducting sample (hopping conduction) of doped manganite due to double exchange. The resonance frequency shift and the ferromagnetic resonance damping constant (linewidth) are calculated in this model. In contrast to other relaxation mechanisms, the model is based on the fact that mobile electrons rapidly relax to the lattice (over a time on the order of the precession period).     

Special features of spectral and luminescent properties of squarilic dyes in polymer matrices

A study was made of spectral and luminescent properties of a squarilic dye on the basis of 1,3,3-trimethyl-3H-indoline in comparison with the cationic polymethine dye representing a derivative of the same heterocycle. The squarilic dye was introduced into polymer films of poly-N-epoxypropylcarbazole, which is capable of bearing photoproduced charges, and polystyrene, which lacks this ability. Luminescence spectra of films with different dye concentrations at temperatures of 4.2 and 300 K were studied. Spectral and luminescent properties of squarain in both types of polymers were found to be determined by the dye tendency to aggregation. For poly-N-epoxypropylcarbazole, this tendency is less pronounced than for polystyrene, which is associated with the enhancement of specific solvation of a bipolar squarain molecule by polar groups of poly-N-epoxypropylcarbazole. The structure with an angle between chromophore directions of 180° is shown to be the most likely spatial structure of associates. The advantage of this structure is that it is stabilized at both ends of bipolar molecules by electrostatic attractive forces between opposite charges.     

Effect of temperature and dynamic fluorescence quenching on proton transfer in 3-hydroxyflavone

The effect of temperature and a fluorescence quencher on the properties of the excited states of 3-hydroxyflavone is considered. The absorption spectra and the spectra of dual fluorescence exited by electromagnetic radiation in the region of the S ₁ absorption band of 3-hydroxyflavone in acetonitrile are measured and analyzed in the temperature range of 20–80°C. The fluorescence lifetimes are also measured at different temperatures. As a quencher of excited states we used the TEMPO spin quencher. The analysis of the fluorescence parameters shows that the heating of the solution to 60°C leads to a considerable (by a factor of 1.24) increase in the proton-transfer rate for the first absorption band. The introduction of a quencher decreases the yield of the two fluorescence bands by the diffusion mechanism and increases the proton-transfer rate with respect to the rate in the pure solvent by a factor of 1.16 at room temperature and 1.65 at T = 80°C.     

Static and dynamic bifurcations in semiconductor lasers for device applications

The properties of semiconductor lasers are reviewed within the framework of bifurcation theory and presented as a means for constructing new devices. Static bifurcations are shown to be related to two forms of non-reciprocity. One case of non-reciprocity is found to be analogous to effects in other non-linear waveguides; the second form appears to be novel. Dynamic bifurcations are considered in relation to delay-induced instabilities with particular attention given to multiple time-delay effects. The potential for exploiting properties of systems near bifurcations is examined and shown to be of great applications potential. The role to be played by semiconductor lasers in deepening knowledge of dynamic systems is indicated.