Showing papers on "Relaxation (NMR) published in 2007"
TL;DR: In this paper, an interpolation function that precisely represents e(ν,t) at standard atmospheric pressure was derived for frequencies and temperatures in the ranges 0⩽ν⩻25THz and 0 ⩽t⩾100°C.
Abstract: All the currently available experimental permittivity data for pure water are used to derive an interpolation function that precisely represents e(ν,t,) at standard atmospheric pressure, for frequencies and temperatures in the ranges 0⩽ν⩽25THz and 0⩽t⩽100°C. The permittivity data is represented in terms of relaxations and resonances processes. There are three relaxations in the microwave region and two resonances in the far infrared. The temperature dependence of the relaxation and resonance parameters are determined. For example, at 25°C the three relaxation frequencies are 18.56GHz, 167.83GHz, 1.944THz and the two resonance frequencies are 4.03 and 14.48THz.
557 citations
TL;DR: In this article, the authors measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers and found that an initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range.
Abstract: Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.
413 citations
TL;DR: The physical content of three-point functions that relate the sensitivity of the averaged two-time dynamics to external control parameters (such as temperature or density) as well as their connection to the more standard four-point dynamic susceptibility associated with dynamical heterogeneities are discussed.
Abstract: We use recently introduced three-point dynamic susceptibilities to obtain an experimental determination of the temperature evolution of the number of molecules N corr that are dynamically correlated during the structural relaxation of supercooled liquids. We first discuss in detail the physical content of three-point functions that relate the sensitivity of the averaged two-time dynamics to external control parameters such as temperature or density, as well as their connection to the more standard four-point dynamic susceptibility associated with dynamical heterogeneities. We then demonstrate that these functions can be experimentally determined with good precision. We gather available data to obtain the temperature dependence of N corr for a large number of supercooled liquids over a wide range of relaxation time scales from the glass transition up to the onset of slow dynamics. We find that N corr systematically grows when approaching the glass transition. It does so in a modest manner close to the glass transition, which is consistent with an activation-based picture of the dynamics in glassforming materials. For higher temperatures, there appears to be a regime where N corr behaves as a power-law of the relaxation time. Finally, we find that the dynamic response to density, while being smaller than the dynamic response to temperature, behaves similarly, in agreement with theoretical expectations.
217 citations
TL;DR: The results show an approximately linear increase in relaxation rate R2* with field strength for all tissues, leading to a greater range of relaxation times across tissue types at 7 T that can be exploited in high-resolution T2*-weighted imaging.
216 citations
TL;DR: The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state and an Fe-N bond elongation of 0.2 A in the quintet state compared to the singlet ground state is delivered.
Abstract: Structural changes of the Fe(II)-tris-bipyridine ([FeII(bpy)3]2+) complex induced by ultrashort pulse excitation and population of its short-lived (?0.6 ns) quintet high spin state were detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined anal. of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 A in the quintet state compared to the singlet ground state. [on SciFinder (R)]
212 citations
TL;DR: In this paper, the topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness, and can be explained in terms of a simple electrostatic picture.
199 citations
TL;DR: Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298-443 K.
Abstract: Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298–443 K. The model asphalt systems were chosen previously to represent real asphalt. Green–Kubo and Einstein methods were used to estimate viscosity at high temperature (443.15 K). Rotational relaxation times were calculated by nonlinear regression of orientation correlation functions to a modified Kohlrausch–Williams–Watts function. The Vogel–Fulcher–Tammann equation was used to analyze the temperature dependences of relaxation time, viscosity, and diffusion coefficient. The temperature dependences of viscosity and relaxation time were related using the Debye–Stokes–Einstein equation, enabling viscosity at low temperatures of two model asphalt systems to be estimated from high temperature (443.15 K) viscosity and temperature-dependent relaxa...
172 citations
TL;DR: A method of two-dimensional infrared (2D IR) spectroscopy called relaxation-assisted 2D IR (RA 2DIR) is proposed that utilizes vibrational energy relaxation transport in molecules to enhance cross-peak amplitudes and is capable of identifying long-range connectivity patterns in molecules.
Abstract: A method of two-dimensional infrared (2D IR) spectroscopy called relaxation-assisted 2D IR (RA 2DIR) is proposed that utilizes vibrational energy relaxation transport in molecules to enhance cross-peak amplitudes. This method substantially increases the range of distances accessible by 2D IR and is capable of identifying long-range connectivity patterns in molecules. RA 2DIR is illustrated in interactions among CN and CO modes in 3-cyanocoumarin and 4-acetylbenzonitrile, where the distances between the CN and CO groups are ≈3.1 and ≈6.5 Å, respectively. A 6-fold increase in cross-peak amplitude was observed in 4-acetylbenzonitrile when the dual-frequency RA 2DIR method was used.
152 citations
TL;DR: Li diffusion in pure-phase microcrystalline Li(4)Ti(5)O(12) with an average particle size in the microm range was probed by (7)Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin-lattice relaxation (SLR) measurements.
Abstract: The cubic spinel oxides Li1+xTi2−xO4 (0 ≤
x
≤ 1/3) are promising anode materials for lithium-ion rechargeable batteries. The end member of the Li–Ti–O series, Li4Ti5O12, can accommodate Li ions up to the composition Li7Ti5O12. Whereas a number of studies focus on the electrochemical behaviour of Li insertion into and Li diffusion in the Li intercalated material, only few investigations about low-temperature Li dynamics in the non-intercalated host material Li4Ti5O12 have been reported so far. In the present paper, Li diffusion in pure-phase microcrystalline Li4Ti5O12 with an average particle size in the μm range was probed by 7Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin–lattice relaxation (SLR) measurements. Between T = 295 K and 400 K extremely slow Li jump rates τ−1 ranging from 1 s−1 to about 2200 s−1 were directly measured by recording the decay of spin-alignment echoes as a function of mixing time and constant evolution time. The results point out the slow Li diffusion in non-intercalated Li4Ti5O12·
τ−1 (1/T) follows Arrhenius behaviour with an activation energy EASAE of about 0.86 eV. Interestingly, EASAE is comparable to activation energies deduced from conductivity measurements (0.94(1) eV) and from SLR measurements in the rotating frame (0.74(2) eV) rather than from those performed in the laboratory frame, EAlow-T = 0.26(1) eV at low T.
142 citations
TL;DR: In this paper, the authors investigated the effect of the presence and mineralogic form of iron on measured proton nuclear magnetic resonance (NMR) relaxation rates and found that the presence of the iron oxides led to increases in the relaxation rates by increasing the surface relaxation rate.
Abstract: Using laboratory methods, we investigate the effect of the presence and mineralogic form of iron on measured proton nuclear magnetic resonance (NMR) relaxation rates. Five samples of quartz sand were coated with ferrihydrite, goethite, hematite, lepidocrocite, and magnetite. The relaxation rates for these iron-oxide-coated sands saturated with water were measured and compared to the relaxation rate of quartz sand saturated with water. We found that the presence of the iron oxides led to increases in the relaxation rates by increasing the surface relaxation rate. The magnitude of the surface relaxation rate was different for the various iron-oxide minerals because of changes in both the surface-area-to-volume ratio of the pore space, and the surface relaxivity. The relaxation rate of the magnetite-coated sand was further increased because of internal magnetic field gradients caused by the presence of magnetite. We conclude that both the concentration and mineralogical form of iron can have a significant impact on NMR relaxation behavior.
135 citations
TL;DR: In this paper, the translational and rotational diffusion of a series of gold nanorods with low aspect ratios was investigated by dynamic light scattering (DLS), and it was shown that the particle shape causes an anisotropy of the polarizability.
Abstract: The translational and rotational diffusion of a series of gold nanorods with low aspect ratios was investigated by dynamic light scattering (DLS). It is shown that the translational and rotational diffusion coefficients can be determined because the particle shape causes an anisotropy of the polarizability. This gives rise to two clearly distinguishable relaxation modes in the time correlation function of the scattered light. The particle length and aspect ratio were determined independently by transmission electron microscopy (TEM). Using a hydrodynamic model, these geometrical parameters were converted to diffusion constants, which agree well with the values determined by DLS. Additionally, it is possible to obtain an estimate of the particles' aspect ratio from the amplitude ratio of the two relaxation modes.
TL;DR: Comparison of the data of TSDC and dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC), 1H NMR spectroscopic with layer-by-layer freezing-out of bulk and interfacial water, adsorption/desorption of nitrogen, water and dissolved organics demonstrates high sensitivity and information content of the TSDC technique, allowing a deeper understanding of interfacial phenomena.
TL;DR: Measurements of lipid bilayer domain line tension based on two-dimensional thermal undulations of membranes with liquid ordered/liquid disordered phase coexistence and near-critical composition at room temperature are described.
TL;DR: In this paper, a simple approach to enhance sensitivity for 13 C high-resolution solid-state NMR for proteins in microcrystals by reducing 1 H T 1 relaxation times with paramagnetic relaxation reagents was discussed.
TL;DR: A pair of experiments are presented for measuring intra-methyl 1H-1H dipolar cross-correlated spin relaxation rates in highly deuterated, methyl protonated proteins with significantly improved sensitivity relative to previously developed experiments that measure dynamics via 1H spin relaxation.
Abstract: A pair of experiments is presented for measuring intra-methyl 1H-1H dipolar cross-correlated spin relaxation rates in highly deuterated, methyl protonated proteins with significantly improved sensitivity relative to previously developed experiments that measure dynamics via 1H spin relaxation. In applications to proteins with correlation times in the macromolecular limit, these cross-correlation rates are related directly to order parameters, characterizing the amplitude of motion of methyl-containing side-chains. The experimental approach is validated by comparing extracted order parameters with those obtained via 2H and 13C spin relaxation methods for both protein L (7.5 kDa) and malate synthase G (82 kDa), with excellent correlations obtained. The methodology is applied to study Ile, Leu, and Val side-chain dynamics in a 360 kDa "half-proteasome" complex. In particular, order parameters obtained from the WT complex and from a second complex where the proteasome gating residues are deleted establish that the relative levels of dynamics in each of the two molecules are very similar. It thus becomes clear that there is no communication between gating residues and other regions of the molecule involving pico- to nanosecond time-scale dynamics of these methyl-containing side-chains.
TL;DR: Some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead are discussed.
Abstract: The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins’ evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing th...
TL;DR: In this paper, the authors measured the Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He at 710 mK.
Abstract: NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He is measured to be 3.8+/-1.1 x 10(-19) cm(2) at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of gamma=7 x 10(4), in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. Klos, and G. Chalasinski, Phys. Rev. A 68, 051401 (2003)10.1103/PhysRevA.68.051401].
TL;DR: NMR measurements of 1H and 19F spin-lattice relaxation times (T1) and self-diffusion coefficients (D) as a function of temperature for ILs containing the TMSiM group and, for comparison, the analogous neopentyl group report the dynamics of the cations and anions.
Abstract: Trimethylsilylmethyl (TMSiM)-substituted imidazolium bis(trifluoromethylsulfonyl)imide (NTf2-), and tetrafluoroborate (BF4-) ionic liquids (ILs) have lower room-temperature viscosities by factors of 1.6 and 7.4, respectively, than isostructural neopentylimidazolium ILs. In an attempt to account for the effects of silicon substitution in imidazolium RTILs and to investigate the ion dynamics, we report nuclear magnetic resonance (NMR) measurements of 1H (I = 1/2) and 19F (I = 1/2) spin−lattice relaxation times (T1) and self-diffusion coefficients (D) as a function of temperature for ILs containing the TMSiM group and, for comparison, the analogous neopentyl group. The 1H and 19F nuclei probe the dynamics of the cations and anions, respectively. The low-temperature line shapes were determined to be Gaussian, and the onset of the rigid lattice line width is correlated with the measured glass transition temperature. The spin−lattice relaxation data feature a broad T1 minimum as a function of inverse temperatur...
TL;DR: In this article, the relaxation and creep behavior of polyurethane nanocomposites were investigated and the relaxation time spectrum and retardant time spectrum were derived according to the generalized Maxwell model and Voigt model with a Tikhonov regularization method.
Abstract: The microphase separation of polyurethane (PU) nanocomposite was studied. The result suggests that the addition of clay leads to a decrease in the size of hard domain and an increase in the degree of microphase separation. The stress relaxation and creep behavior of blank PU and PU/clay nanocomposites were investigated. The relaxation time spectrum and retardant time spectrum were derived according to the generalized Maxwell model and Voigt model with a Tikhonov regularization method. The characteristic relaxation time was identified with the corresponding relaxation process. At a small strain, the relaxation was mainly attributed to uncoiling/disentangling of soft segment chain network in the soft phase, with a single characteristic relaxation time in the range of 5~100s. The increase in the hard segment content leads to a decrease in the relaxation time, and the addition of clay leads to an increase in the relaxation time. At large strains, the multi-peak relaxations occurred, and they were attributed to the breakup of interconnected hard domains and pull-out of soft segment chains from hard domains, together with the disentangling of soft segment chain network in the soft phase. The creep results are in consistent with that of the stress relaxation. The relaxation and creep behavior were related to microphase separation of polyurethane. This study suggested that the relaxation spectrum H(i´) can be used to examine the complicated relaxation processes for a multi-phase and multi-component polymer system.
TL;DR: In this paper, the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the microstructure (specific surface area and pore size distribution) throughout the progressive hydration of cement-based materials.
TL;DR: The well-known red emitting complex Ir(btp)2(acac) (bis(2'-benzothienyl)-pyridinato-N,C3')iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix and it can be concluded that the emitting state T1 is a 3LC (ligand centered) state with significant admixtures of 1
Abstract: The well-known red emitting complex Ir(btp)2(acac) (bis(2-(2‘-benzothienyl)-pyridinato-N,C3‘)iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix. The studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B = 10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I → 0 at 16268 cm-1), the three substates I, II, and III of the T1 triplet state are separated by ΔEII-I = 2.9 cm-1 and ΔEIII-I = 25.0 cm-1, respectively. ΔEIII-I represents the total zero-field splitting (ZFS). The individual decay times of these substates are τI = 150 μs, τII = 58 μs, and τIII = 2 μs, respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for relaxation from state II to state I (spin−lattice relaxation, SLR) is as long as 22 μs at T = 1.5 K,...
TL;DR: This work explicitly considers evolution of an initially spin-polarized Fermi gas in a two-dimensional harmonic trap and derive nonequilibrium behavior of the spin polarization.
Abstract: We consider a trapped atomic system in the presence of spatially varying laser fields. The laser-atom interaction generates a pseudospin degree of freedom (referred to simply as spin) and leads to an effective spin-orbit coupling for the fermions in the trap. Reflections of the fermions from the trap boundaries provide a physical mechanism for effective momentum relaxation and nontrivial spin dynamics due to the emergent spin-orbit coupling. We explicitly consider evolution of an initially spin-polarized Fermi gas in a two-dimensional harmonic trap and derive nonequilibrium behavior of the spin polarization. It shows periodic echoes with a frequency equal to the harmonic trapping frequency. Perturbations, such as an asymmetry of the trap, lead to the suppression of the spin echo amplitudes. We discuss a possible experimental setup to observe spin dynamics and provide numerical estimates of relevant parameters.
TL;DR: In this article, a noncontact scanning probe microscopy method of probing local frequency-dependent dielectric spectroscopy is described, in which a small reduction in glass transition temperature and a moderate narrowing of the distribution of relaxation times are found within 20nm of a free surface.
Abstract: A noncontact scanning probe microscopy method of probing local frequency-dependent dielectric spectroscopy is described. Dielectric spectra can be measured with this technique from 0.01to100Hz, in nanometer-scale near-surface regions of materials. The technique is applied to polymer films (polyvinyl acetate), in order to determine if polymer glassy dynamics are altered near a free surface. A small reduction in glass transition temperature and a moderate narrowing of the distribution of relaxation times are found within 20nm of a free surface.
TL;DR: A mathematical schematization of a multylayer model was proposed that allowed qualitative reproduction of the observed thickness dependences of the static and dynamic properties, and the upper limit for the thickness of the reduced mobility layer was estimated as 20 nm.
Abstract: The structural dynamics of ultrathin polymer films of poly(ethylene terephthalate) capped between aluminum electrodes have been investigated by dielectric relaxation spectroscopy. A deviation from bulk behavior, appearing as an increase of the relaxation time at a fixed temperature, is observed for films of thickness below 35 nm. The slowing down acts as a constant shift factor independent from the temperature, and the fragility is constant. The interfacial energy between aluminum and poly(ethylene terephthalate) is calculated to be 3 mJ/m2, confirming a strong interaction between polymer and substrate, which leads to the presence of a layer characterized by a reduced mobility at their interfaces. We proposed a mathematical schematization of a multylayer model that allowed qualitative reproduction of the observed thickness dependences of the static and dynamic properties. In terms of such a model, the upper limit for the thickness of the reduced mobility layer was estimated as 20 nm. The conditions to ext...
TL;DR: In this article, a statistical model for stress-induced acceleration of thermally driven barrier hopping and segmental relaxation, dynamic yielding and devitrification in polymer glasses is proposed, based on the concept of a nonequilibrium free energy determined by the amplitude of temperature-dependent collective density fluctuations.
Abstract: A statistical mechanical theory is proposed for stress-induced acceleration of thermally driven barrier hopping and segmental relaxation, dynamic yielding and devitrification in polymer glasses The approach is built on the concept of a nonequilibrium free energy determined by the amplitude of temperature-dependent collective density fluctuations, and the idea that stress induces an instantaneous force on a segment Multiple predictions that fundamentally differ from the phenomenological Eyring model are made for the temperature and strain rate dependence of the dynamic yield stress and deformation modified segmental relaxation time, which are in accord with experiments and simulations
TL;DR: In this paper, a recently proposed expression to describe the temperature and volume dependences of the structural (or α-) relaxation time is discussed, which satisfies the scaling law for the relaxation times, τ (T, V ) = I ( TV γ ), where T is temperature, V the specific volume, and γ a material-dependent constant.
Abstract: A recently proposed expression to describe the temperature and volume dependences of the structural (or α-) relaxation time is discussed. This equation satisfies the scaling law for the relaxation times, τ ( T , V ) = I ( TV γ ) , where T is temperature, V the specific volume, and γ a material-dependent constant. The expression for the function I ( TV γ ) is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover, providing a description of the dynamics with a minimal number of parameters. The results herein can be reconciled with previously found correlations of the isochoric fragility with both the isobaric fragility at atmospheric pressure and the scaling exponent γ.
TL;DR: In this paper, a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique is presented, which shows that UNCD particles consist of a diamond core that is partially covered by a sp2-carbon fullerene-like shell.
Abstract: We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the 13C and 1H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp2-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.
TL;DR: Control of magnetic anisotropy in epitaxial (Ga,Mn)As by anisotropic strain relaxation in patterned structures using reciprocal space mapping by x-ray techniques is reported.
Abstract: We report control of magnetic anisotropy in epitaxial $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ by anisotropic strain relaxation in patterned structures. The strain in the structures is characterized using reciprocal space mapping by x-ray techniques. The magnetic anisotropy before patterning of the layer, which shows biaxial easy axes along [100] and [010], is replaced by a hard axis in the direction of large elastic strain relaxation and a uniaxial easy axis in the direction where pseudomorphic conditions are retained.
TL;DR: In this paper, the structural relaxation time of glass-forming liquids and polymer melts under high pressure has been studied and shown to increase with lowering temperature in a non-Arrhenius fashion.
Abstract: A glass is usually formed by cooling a liquid at a rate sufficient to avoid crystallization. In the vicinity of the glass transition the structural relaxation time increases with lowering temperature in a non-Arrhenius fashion and the structural relaxation function reveals a non-Debye behaviour. However, liquid can be also vitrified by keeping it at a constant temperature and increasing the pressure. This pressure-induced transition to the glassy state is also accompanied by dramatic changes in the relaxation dynamics. Herein we discuss the behaviour of the structural relaxation times of glass-forming liquids and polymer melts under high pressure.