Influence of high pressure on optical impurity spectra

Expressions are derived for inhomogeneous band shapes of impurity spectra in highly compressed glassy matrices. Intermolecular guest-host interactions are approximated to isotropic two-body Lennard-Jones 6-12 potentials having different parameters in the ground state and the excited state. Calculated shifts and widths are compared to published values for the absorption spectra of phenanthrene, anthracene [B. Y. Okamoto and H. G. Drickamer, J. Chem. Phys. 61, 2870 (1974)], and several polymethine dyes [G. A. Samara et al., J. Chem. Phys. 37, 1482 (1962)] embedded in polymer hosts and subject to pressures up to 140 kbars. The magnitudes of barochromic shifts of the band maxima and the inhomogeneous broadening suggest that the equilibrium coordinates of the excited state are typically less by 5+/-2%.     

Effect of pressure and temperature on chromophore reorientation in a side-chain nonlinear optical polymer

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of the side-chain copolymer poly(disperse red 1 methacrylate-co-methyl methacrylate) (DR1-MMA). The decay of the SHG signal from poled films of DR1-MMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25°C below the glass transition temperature (T_g) to 35°C above it. The activation volume, ΔV^*, decreased with increasing temperature. The data suggests that the coupling between chromophore reorientation and the long-range motion of the polymer is stronger for the DR1-MMA side-chain system than in previously measured guest–host systems. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2793–2803, 1998     

High-pressure studies of optical dephasing in polymer glasses

The effect of high pressure on the optical dephasing of chromophores in organic polymers at low temperature is evaluated within the stochastic sudden jump two-level-system (TLS) model. The approximations within the "standard" TLS model cannot account for the observed pressure dependence of the pure dephasing rate without ad hoc assumptions about changes in the TLS density of states. However, the photon echo model of Geva and Skinner for disordered systems can be used to model pressure-dependent optical dephasing results for a variety of doped polymer systems without assuming changes in the TLS density of states. The relative importance of pressure-induced changes in TLS density, chromophore-TLS coupling, and TLS-phonon coupling is evaluated by fitting experimental high-pressure photon echo results to the TLS model.     

Relation between pressure shift and electric-field shift of single-molecule lines in a polymer glass

The pressure shifts and the electric-field shifts of individual chromophores in an amporphous matrix are--due to strong disorder--subject to broad distributions. By means of single-molecule spectroscopy we measured both the pressure and the electric-field shift of about 800 tetra-tert-butylterrylene molecules in polyisobutylene. We found a significant correlation of 0.52 (Kendall's correlation coefficient) between the two observables. Analytical calculations and Monte Carlo simulations based on a model by Laird and Skinner predict a nonzero, yet, distinctly smaller correlation. The Monte Carlo simulations showed that the usual assumptions of a spherical shape and isotropic polarizability of the chromophores in glassy systems is an oversimplification of the complex nanoscopic structure and cannot reproduce our experimental results. By taking the molecular anisotropy into account, we obtain agreement of the simulated and the measured correlation between pressure shift and electric-field shift parameter.     

Effect of pressure and temperature on chromophore reorientation in a new syndioregic main‐chain hydrazone nonlinear optical polymer

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume (ΔV*) of a syndioregic main‐chain hydrazone (SMCH) nonlinear optical polymer. The decay of the SHG signal from poled films of SMCH was recorded at hydrostatic pressures up to 2924 atm and at temperatures between 25 °C below the glass‐transition temperature (T_g) to 20 °C above it. ΔV* for pressures less than 500–1000 atm and T > T_g decreased as the temperature was increased. For pressures greater than 1000 atm, ΔV* was essentially constant for all temperatures. In addition, the size of ΔV* indicated that the chromophore in this main chain was internally flexible. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 895–900, 2001     

“Vanishing” structural effects of temperature in polymer glasses close to the glass-transition temperature

Positron annihilation lifetime (PAL) measurements were used for observation of structural effects of temperature in polystyrene (PS), super-cross-linked polystyrene networks (CPS), and in polyimides (PI) below and in the vicinity of glass-transition temperature T_g. “Vanishing” of these structural effects in the repeating cycles of the temperature controlled PAL experiments due to the slow relaxation processes in different conditions and details of chemical structure is demonstrated. Obtained results illustrate complex, dependent on thermal history, inhomogeneous character of the glass structure. In fact, structure of some polymer glasses is changing continuously. Calculations of the number density of free volume holes in these conditions are discussed.     

Influence of xenon difluoride on the optical properties of fluorozirconate and fluorohafnate glasses

To study the effect of xenon difluoride as a fluorinating agent on optical properties of glasses in ZBLAN (ZrF₄–BaF₂–LaF₃–AlF₃–NaF) and HBLAN (HfF₄–BaF₂–LaF₃–AlF₃–NaF) systems, their optical transmission in the range from UV to IR was investigated. The treatment of the initial fluorides with XeF₂ was shown to lead to a broadening of the transmission region of the obtained glasses both in the UV and IR ranges. Moreover, the treatment of the batch with xenon difluoride leads to the removal of oxygen-containing impurities that absorb in the region of 2.8 μm.     

Application of non-photochemical hole burning to the two-photon spectroscopy of impurity molecules in low temperature glasses

One-photon non-photochemical hole burning (NPHB) is shown to be a viable method for producing narrow-line two-photon spectra of molecules in solution at low temperature. The TPE spectrum of phenanthrene at 2 K in a glassy medium is presented and is shown to manifest one-photon NPHB effects.     

Dependence of impurity zero-phonon band thermal broadening and shift on impurity site and vibron level in organic crystals

Experimental results on the thermal broadening and shifts of impurity zero-phonon bands in some organic mixed crystals are reported and interpreted in terms of the electron-phonon interaction. Attention is focused on the variation of the thermal broadening and line shift parameters with (1) impurity site in a system exhibiting several energetically different impurity sites and with (2) the vibron level in a given final electronic state of the impurity. Experiments were conducted over a temperature range which ensures that hot vibron transitions are negligible. Data from the 5600 A ¹B₁ ← ¹A₁ transition of 2-phenyl-l-monoazaazulene in p-terphenyl show that the variations of the above parameters with impurity site are as large as the variations of the same between different chemical systems. No dependence of the thermal broadening parameters on the impurity vibron level is observed for the above system or the 7000 A ¹B₁ ← ¹A₁ transition of azulene in naphthalene. It is shown that, within the adiabatic (electron-nucleus) and harmonic approximations, the zero-phonon band spectral shape function arising from the electron-phonon interaction should be independent of the final vibron level of the impurity transition.     

聚合物驱弹性效应对注入压力的影响

引人综合粘度度量聚合物溶液的粘弹性,利用无限大地层达西定律,导出了粘弹性流体径向流的压力损失方程。采用数值方法研究了粘弹性流体在变截面孔隙模型中的阻力特性,结合实例分析了粘弹性流体径向流的压力损失随粘弹性的变化规律。研究结果表明,弹性效应是影响注入压力升高的一个重要因素。聚合物溶液的粘弹性越大,其在地层孔隙中的渗流阻力越大,弹性效应在增高注入压力中所占的份额也越大。增加地层渗透率、降低注入排量、减小聚合物溶液的粘弹性均有利于降低注入压力,提高注入能力,改善注入效果。     

Influence and correction of peak shift and band broadening observed by rank analysis on vibrational bands from variable-temperature measurements

Variable-temperature infrared (IR) spectra of cyclohexane and IR and Raman spectra of chlorocyclohexane have been investigated by graphic eigenvalue analysis. Thermal effects known as peak shift and band broadening combined with heteroscedastic noise in vibrational bands are found to have severe influence on the interpretation of the outcome of rank analysis. Methods for correction of frequency shifts and band broadening in the spectral profiles due to temperature variation are developed and tested.     

Influence of substrate temperature on the stability of glasses prepared by vapor deposition

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265 K indicated that the enthalpy was 8 J/g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20 K reduction in the fictive temperature. Deposition at 265 K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295 K for 7 months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.     

Strain hardening of polymer glasses: Effect of entanglement density,temperature, and rate

The strain hardening behavior of model polymer glasses is studied with simulations over a wide range of entanglement densities, temperatures, strain rates, and chain lengths. Entangled polymers deform affinely at scales larger than the entanglement length as assumed in entropic network models of strain hardening. The dependence of strain hardening on strain and entanglement density is also consistent with these models, but the temperature dependence has the opposite trend. The dependence on temperature, rate, and interaction strength can instead be understood as reflecting changes in the flow stress. Microscopic analysis of local rearrangements and the primitive paths between entanglements is used to test models of strain hardening. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3487–3500, 2006     

Theory of relaxation and elasticity in polymer glasses

The recently developed activated barrier hopping theory of deeply supercooled polymer melts [K. S. Schweizer and E. J. Saltzman, J. Chem. Phys. 121, 1984 (2004)] is extended to the nonequilibrium glass state. Below the kinetic glass temperature T(g), the exact statistical mechanical relation between the dimensionless amplitude of long wavelength density fluctuations, S(0), and the thermodynamic compressibility breaks down. Proper extension of the theory requires knowledge of the nonequilibrium S(0) which x-ray scattering experiments find to consist of a material specific and temperature-independent quenched disorder contribution plus a vibrational contribution which varies roughly linearly with temperature. Motivated by these experiments and general landscape concepts, a simple model is proposed for S(0)(T). Deep in the glass state the form of the temperature dependence of the segmental relaxation time is found to depend sensitively on the magnitude of frozen in density fluctuations. At the (modest) sub-T(g) temperatures typically probed in experiment, an effective Arrhenius behavior is generically predicted which is of nonequilibrium origin. The change in apparent activation energy across the glass transition is determined by the amplitude of frozen density fluctuations. For values of the latter consistent with experiment, the theory predicts a ratio of effective activation energies in the range of 3-6, in agreement with multiple measurements. Calculations of the shear modulus for atactic polymethylmethacrylate above and below the glass transition temperature have also been performed. The present work provides a foundation for the formulation of predictive theories of physical aging, the influence of deformation on the alpha relaxation process, and rate-dependent nonlinear mechanical properties of thermoplastics.     

Non-photochemical hole burning and impurity site relaxation processes in organic glasses

Laser-induced holes in the inhomogeneously broadened absorption spectra of aromatic hydrocarbons in frozen glasses are reported. The holes are shown to be of non-photochemical origin. Mechanisms for excited state site interconversion based on a metastable distribution of sites in asymmetric double well potentials are presented. Methods of eliminating non-photochemical holes and of distinguishing between photochemical and non-photochemical holes are described.     

Influence of pressure and temperature on mixing properties of Ar-Kr mixtures

The mixing properties or equimolar Ar-Kr mixtures are studied by Monte Carlo calculations in the isobaric-isothermal ensemble and by the van der Waals one-Fluid model. Results of excess volume and enthalpy at T = 300 K calculated by both methods are compared: these are in close agreement for excess volume while the conclusion appears less clear in the case of excess enthalpy.     

Influence of the polymer structure on the achievement of polar orientation in high glass transition temperature nonlinear optical polyimides by photo-assisted poling

We have used combinations of light, heat, and electrostatic fields to investigate the orientation of nonlinear azo-chromophores chemically incorporated into high glass transition temperature (T_g) polyimides. A number of nonlinear optical polyimides have been synthesized in which the interaction between the nonlinear optical chromophore and the polymer main chain was systematically altered to determine to what extent this steric interaction influences the orientation of the nonlinear chromophore. Chromophores in polymers may be oriented by a number of methods: (a) polarized light at room temperature (i.e., photo-induced orientation or PIO), (b) polarized light and electric fields (i.e., photo-assisted poling or PAP) at temperatures ranging from room temperature to the polymer T_g, and (c) electric fields at T_g (thermal poling). While thermal poling and PIO are usually possible, PAP depends strongly on the molecular structure of the polymer. Previously we have shown that PIO can be accomplished at room temperature in a system where the nonlinear chromophore is embedded into the polyimide main chain via the donor substituent, and this orientation can only be thermally erased at temperatures approaching T_g. In this article we show that, whereas photoisomerization can efficiently depole donor-embedded polyimides in a matter of few minutes at room temperature, PAP does not induce any polar order. This behavior is in marked contrast to a structurally related, side-chain, nonlinear polyimide, in which the azo chromophore is tethered via a flexible linkage to the polymer backbone. In this case some PAP occurs even at room temperature, while no PAP is observed for a donor-embedded system with a similar T_g. We suggest that the orientation during PAP below T_g in the side-chain polyimide is primarily due to the movement of the azo side chains, and there is a very little coupling of this motion to the main chain. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1669–1677, 1998     

Influence of box shape on the hard-disk pressure at high fluid density

The finiteness effects induced by periodic boundary conditions are examined for a hard-disk fluid. Besides the relatively simple N dependence there is a more complicated V dependence. Three different approaches are displayed and contrasted: (i) A rough estimate for the V dependence of the pressure, based on a theoretical equation of state. (ii) A more detailed and expensive treatment of the V dependence by a method due to Pratt and Haan, (iii) MC simulations, intending to evoke V dependence deliberately. The influence of varying box shape and particle number (N = 71.72 and 73) is investigated. All simulations have been carried out for high fluid density (packing FRACTION = 0.648). Finally, the consistency of these approaches is discussed.