Aging of oriented polymer glasses

Dimensional (D) and enthalpy relaxation (ΔH) of oriented polymer glasses (PS and PC) have been studied as function of temperature, between T_g and T_g−20 °C, and aging time t, ranging to several weeks. The dimensional relaxation (shrinkage) and enthalpy relaxation curves verify the logarithm law D(t) ∼ H(t) ∼ log t, between an incubation τ_i and a final relaxation time τ_f. The time τ_f to reach the equilibrium (D and ΔH) follows the Vogel–Tamann–Fulcher (VFT) law. Enthalpy relaxation and shrinkage exhibit important differences. Enthalpy relaxation of oriented and isotropic polymers follows the same logarithm law, independent of the draw ratio λ and the mode of deformation, the relaxation time τ_f coincides with the relaxation time of the α segmental motions. Shrinkage depends on λ and the mode of deformation, the relaxation time τ_f is attributed to the normal mode, the relaxation time of the whole chain. Finally the shrinkages of PS and PC show some differences. PC at short aging times presents another type of dimensional relaxation which would be due to the β motions. This would be in close connection with the ductile (PC) and fragile (PS) behavior of these two polymers far below T_g.     

Free induction decays in entangled polymer melts

The relaxation of the transverse magnetization components caused by both dipolar interactions between the spins of different polymer chains and the dipolar coupling between CH-protons on an isolated Kuhn segment along a single polymer chain have been calculated. Explicit expressions for the transverse relaxation function are given in terms of the absolute mean squared displacement of the Kuhn segment during melt g_r(t), the tangent vector dynamical correlation function 〈b_n(t)b(0)〉, the segmental relaxation time τ_s, the Kuhn segment length b, the bond length a₀, the internuclear distance d, and the spin number density ρ_s. It is shown that the functional dependence of the intramolecular relaxation function on 〈b_n(t)b(0)〉 is fairly weak. The time-dependence of the intramolecular contribution to the transverse relaxation function is dominated by the probability density distribution function of the end-to-end vector of the Kuhn segment. The long-time decay of the intramolecular contribution to the transverse relaxation function is found to scale as t^? 3/2 for τ_s < < t < < τ_max, where τ_maxis the maximum relaxation time of polymer chains in melts. For times much less than the spin–spin relaxation time, T₂ ≈ 10^? 3 ? 10^? 2s, we show that the intermolecular contribution to the relaxation function is given by the following expression: exp(? λ₁(b, τ_s, ρ_s)t²/g_r^3/2(t)). Both the numerical coefficient and the functional dependence of λ₁on b, τ_s and ρ_s reproduce the expression obtained from the frequently used second cumulant approximation. For longer times (T₂ ≤ t < < τ_max), the intermolecular contribution is determined by the following relation: exp(? λ₂(b, τ_s, ρ_s, t)g_r(t)). We show that λ₂ increases logarithmically with t. The molecular mass independence of λ₁and λ₂ shows that, in polymer melts with molecular masses M_w far above the critical value M_c, the relevant experimental window for the decay of the intermolecular relaxation function is connected with the anomalous diffusion regime. Comparison with the experimental data suggests that the intermolecular contribution plays a significant role in the NMR relaxation process in polymer systems close to the melting point.     

Stabilization of metallic glass studied by internal friction measurement

The internal friction Q^?1 and the oscillation frequency f of Zr–Ti–Cu–Ni–Be metallic glass specimens were measured using an inverted torsion pendulum with the free decay method. A single-roller melt-spinning apparatus was used for preparing the specimens. Isothermal annealing near the glass transition temperature T_g was performed to investigate the stabilization of the specimens. Q^?1 decreased with annealing time t due to the stabilization. Q^?1-vs-t was measured at various annealing temperatures T_a, and the values of relaxation time τ for the stabilization process were determined. The dependence of τ on T_a showed that the hydrodynamic behavior represented by the Vogel–Tammann–Fulcher form and the hopping behavior represented by the Arrhenius form were observed in high- and low-temperature regions, respectively. The crossover of the two behaviors was seen at a temperature near and somewhat higher than T_g. The result was discussed on the basis of the viscoelastic relaxation in glassy materials.     

Analysis of composition fluctuation lifetimes in a critical oxide mixture by volume relaxation spectroscopy

The distribution of volume relaxation times in a critical oxide mixture is calculated from longitudinal and shear modulus measurements. Ultrasonic relaxation allows the investigation of two distinct effects in the distribution of relaxation times as the temperature is lowered toward the critical point. At high temperatures, a coupling between the compressional component of the ultrasonic wave and supercritical fluctuations in composition leads to an anomalous broadening of the distribution of volume relaxation times. This interaction allows a measure of the average composition fluctuation lifetime, τ_D, as a function of temperature. At lower temperatures, as τ_D becomes longer than the volume relaxation times, the behavior of the distribution follows closely the predictions of an environmental relaxation model proposed by these authors for the analysis of shear relaxation processes in the same material.     

The role of stress development and relaxation on crystal growth in glass

The role of internal stresses energy is usually neglected when crystallization kinetics is considered. The common argument is that stress is relaxing too fast to affect the process. In this article we develop a generalized formalism to describe steady-state growth kinetics in viscoelastic media. The residual stress energy results from interplay between the rate of stress development (due to the propagation of a crystal throughout the matrix) and rate of stress dissipation (due to relaxation of the viscous matrix). The degree to which the stress energy can relax depends on the ratio of τ_c/τ_r the characteristic time for crystal growth, τ_c, and the relaxation time, τ_r. The present results challenge the widespread fallacy that internal stresses relax too fast to affect crystal growth. Our model explains the often observed lack of agreement between the theoretical predictions (without taking into account the stresses energy development) and experimental data.     

Dispersive transport from negatively correlated defect centers

The transient current response arising from time-of-flight experiments on a material whose trapping levels are predominantly associated with a well-defined negatively correlated defect is calculated. The resulting behavior is controlled by the relative magnitude of two characteristic times, the transit time, τ_t, and the time necessary for interconversion of a neutral acceptor to a neutral donor, τ. If τ_t < τ, nondispersive transport results. In contrast, if τ_t >τ, two kinks characterize I(t). The current is initially independent of time, but eventually becomes dispersive, decaying as $\text{t}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$ prior to τ_t and as $\text{t}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$ afterwards. The dispersion results from the spread of times necessary to interconvert the neutral defects.     

Fragility in mixed alkali tellurites

Linear viscoelastic stress relaxation and calorimetric measurements were performed on a series of mixed alkali tellurite glasses of composition 0.3([xNa₂O+(1−x)Li₂O])+0.7TeO₂ at temperatures near and above the glass transition temperature, T_g. The stress relaxation data were well described by the stretched exponential function, G(t)=G₀exp[−(t/τ)^β], where τ is the relaxation time, β is the distribution of relaxation times and G₀ is the high frequency modulus. The fragility, determined from the temperature dependence of τ, exhibited a minimum in the middle of the mixed alkali composition. A possible connection between the kinetic and the thermodynamic dimensions of this system was established, wherein the heat capacity change at the T_g, ΔC_p(T_g), and the fragility are correlated.     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Crystallization in stone wool fibres

The crystallization behavior of stone wool fibres (SWFs) is studied by isothermally heat-treating the SWFs under atmospheric conditions, and by determining the crystallization enthalpy (ΔH_ht) released from the samples during heat-treament. The dependences of the degree of crystallization (D_c) on the heat-treatment temperature (T_h) and time (t_h) are clarified using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and electron scanning microscopy (SEM). These dependences can be well described by a phenomenological model proposed in this work. The morphology of the crystals is analyzed using SEM, whereas the structure of the crystals and the maximum fraction of crystalline phase in the SWFs are determined using XRD. From a direct comparison between the XRD and DSC data, it is found that the T_h and t_h dependences of ΔH_ht overlap with those of D_c. This overlapping leads to the possibility of predicting D_c(T_h, t_h) for the SWFs from the heat-treatment conditions using the above-mentioned model.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

Space and time scaling in glycerol–water mixtures

The dielectric response of glycerol/water (G/W) mixtures in a wide frequency and temperature ranges is considered in terms of spatial dynamic heterogeneities. The minimal cooperative relaxation time τ₀ extracted from the dielectric ‘excess wing’ is associated with the main structural mesoscopic size L_m obtained by X-ray diffraction thereby providing clarification of the minimal unit structure responsible for the cooperative dynamics. The space and time scaling can be interpreted in terms of anomalous diffusion in the spanned percolation H-bonded networks.     

Persistent photoconductivity in high-temperature superconductors with nano-scale fragmented copper-oxide chains

We present studies of the relaxation of persistent photoconductivity in 60-K YB₂C₃O_x. By using thin films grown on vicinal SrTiO₃ substrates, measurements of the in-plane (a–b plane) conductivity and of the out-of-plane (c-axis) conductivity were performed. The photo-induced enhancements of both conductivities showed the well-known stretched-exponential relaxation. We observed a small anisotropy of the relaxation rates of the two conductivity components. The relaxation of the c-axis conductivity appeared somewhat delayed compared to the a–b plane conductivity. In a simple thermal relaxation picture this finding implies different energy barriers for the c-axis and a–b plane relaxations, which is not consistent with generally accepted models of persistent photoconductivity in YB₂C₃O_x. We interpret the observed anisotropy as a consequence of the structural modifications of the copper-oxide chains that are induced by light illumination. Structural changes such as the re-ordering of the oxygen ions are slow processes compared to electronic excitations. Hence, the c-axis conductivity, which is more influenced by (small) structural modifications of the chain layer, needs a longer time to return to the equilibrium state after the cessation of the illumination.     

On a chaotic potential field in vitreous As2Se3

Some experimental results of the vitreous As₂Se₃ investigation (the photoconductivity spectrum of volume examples, the temperature dependence of the thermoelectric E.M.F., the shift of the optical edge under the temperature and pressure) may be explained by the use of fluctuations of an internal potential field. A hypothesis about the nature of chaotic potential field is proposed, which is based on a negligible magnitude of the gap deformation potential D_g = D_c ? D_v, where D_c and D_v are deformation potentials of band edges. By this hypothesis the fluctuations of the substance density in glass generate the chaotic potential field of magnitude about a half of the gap E_g, while the gap magnitude fluctuates only slightly within some per cent of E_g. It is shown that the displacement of the optical edge at As₂Se₃ amorphization may be a demonstration of the deformation nature of the chaotic potential field in this material.     

Raman line shape analysis as a mean characterizing molecular glass-forming liquids

Raman scattering spectra in glass forming toluene were studied in the temperature range 50–323 K with the goal of extracting information about homogeneous, inhomogeneous and orientational broadening. It was found that the temperature dependence of inhomogeneous line width allows one to depict two peculiar temperatures: T_A and T_g, where T_g is the glass transition temperature and T_A is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature, τ_α(T). Temperature dependence of the orientational phase loss time τ_OPL was found to correspond well to τ_α at T > T_A and continues approximately Arrhenius behavior for lower temperature in contrast to τ_α(T). Also, a comparative analysis of homogeneous broadening of polarized and depolarized lines was done, which provided an estimation of the orientational broadening γ_NL(T). The found γ_NL(T) decreases linearly as the temperature decreases and goes to zero at T ~ T_c, where T_c is the critical temperature in framework of the mode-coupling theory (note that T_c is close to other peculiar temperatures T_B and T_β, but we did not intent to distinguish among them in the present work). Thus, it was shown that the Raman line shape analysis in molecular glass forming materials allows one to extract peculiar temperatures: T_A, T_g, and, probably, T_c.The test of the possibility to use a probe molecule for the Raman line shape analysis has revealed that the extracted data for probe molecule lines do not characterize the host matrices, at least in the low-viscous state (T > T_A).     

Crystal and molecular structure of 13-oxolupanine

The title compound, C₁₅H₂₂N₂O₂, is monoclinic:P2₁,a=10.876(2),b=8.620(2),c=7.390(2) Å, β=99.2(2)°,Z=2. TheA/B ring junction configuration isquasi-trans, and theC/D junction istrans. RingA has a conformation intermediate between sofa and half-chair; ringsB,C, andD are in chair, boat, and distorted chair conformations, respectively.     

Structural investigation of 1-amino-2,4,6-trinitrobenzenes in the solid state and in solution

The crystal structure of 1-pyrrolidino-2,4,6-trinitrobenzene3, 1-morpholino-2,4,6-trinitrobenzene4 and 1-piperidino-2,4,6-trinitrobenzene5 have been determined by single-crystal X-ray diffraction data and the structure in solution was investigated by UV-visible spectrophotometry and¹³C nmr. The three compounds are monoclinic, space groupP2_i/n. Unit cell dimensions area=6.6660(1),b=8.612(1),c=20.696(3) Å, β=90.73(1)^o, andD _c =1.58 g cm^?3 for compound3;a=7.090(2),b=21.493(9),c=8.298(3) Å, β=101.27(1)^o, andD _c 1.60 g cm^?3 for compound4 anda=10.426(1),b=10.038(1),c=12.291(1) Å, β=90.04(1)^o withD _c =1.53 g cm^?3 for compound5 forZ=4. In the solid state, differences regarding the planarity of the aromatic ring in the three substrates were found. Rotation of theortho-nitro groups and of the amino group out of the aromatic plane was observed both in the solid state and in the solution. Greater coplanarity of the two rings was found in3 than in4 and5.     

The structure of 2R-(3′S-benzyloxy-5′S-hydroxy-2′R-hydroxymethyl-1′-oxacyclohexan-6′S-yl)-propionic acid-1,5′-lactone,C16H20O5

The title compound is C₁₆H₂₀O₅, MW=292.3, orthorhombic,P2₁2₁2₁,a=9.741(2),b=29.391(7),c=5.354(1) Å from diffractometer measurements,V=1532.8 ų,Z=4,D _c =1.267 g cm^?3,D _o =1.271 g cm^?3 (ether/1,1,2,2-tetrabromoethane), λ(Mo Kα)=0.71069 Å,F(000)=624,μ=1.02 cm^?1, crystal dimensions 0.23×0.23×0.40 mm,R=0.049 for 1164 observed reflections. The molecule contains a possibly significant asymmetric ether linkage between the oxacyclohexane ring and the highly anisotropic benzyl ring. The packing consists of zigzagged chains parallel to thea-axis formed by hydrogen bonds. The chains are separated by van der Waals contacts.     

Positron annihilation and broadband dielectric spectroscopy: A series of propylene glycols

We report a thorough joint analysis of the behavior of the ortho-positronium lifetime as obtained from positron annihilation lifetime spectroscopy and of the dipolar relaxation spectra investigated by broadband dielectric relaxation spectroscopy in a series of glass-forming propylene glycols including propylene glycol, dipropylene glycol and tripropylene glycol. A number of empirical correlations between the temperature dependence of the ortho-positronium annihilation lifetime, τ₃(T), and the various spectral and relaxational quantities have been found. The phenomenological evaluation of the quasi-sigmoidal τ₃(T) dependence reveals three characteristic temperatures: T_g^PALS, T_b1 = (1.23 − 1.27)T_g^PALS and T_b2 = (1.46 − 1.53)T_g^PALS, which are found to decrease with increasing fragility. The slighter change of slope in the PALS response at T_b1 in this series of propylene glycols appears to be related to the crossover from the α-process to the excess wing or secondary relaxation, found in the dielectric spectra. The onset of the high-temperature plateau in the τ₃(T) plot at T_b2 occurs when τ₃ matches the average relaxation time of the primary α process. Moreover, the plateau region lies in the vicinity of the crossover in the dielectric parameters of the structural relaxation in all the samples, i.e. spectral width and relaxation strength. In addition it is approximately related to a crossover of the α relaxation time τ_α(T) from non-Arrhenius to Arrhenius regime. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in the series of propylene glycols.     

Dielectric study of the segmental relaxation of low and high molecular weight polystyrenes under hydrostatic pressure

In this work we analyze by means of dielectric spectroscopy the dynamics of the α-relaxation process of low and high molecular weight polystyrene over a wide range of pressures and temperatures. The results are interpreted in terms of a recently proposed equation which describes the behavior of the structural relaxation time, τ(T, P), as a function of both pressure and temperature. This equation has been derived from the Adam–Gibbs (AG) theory by writing the configurational entropy, S_c, in terms of the excess thermal heat capacity and of the excess thermal expansion. Consequently, the molecular dynamic of glass-forming liquids can be linked to its thermodynamic properties. The pressure dependence of the segmental dynamics for both polymers is here measured and analyzed in the AG framework for the first time. τ(T, P) was found to be very well described using the extended AG equation. Additionally, the pressure dependence of the fragility and glass transition temperature (T_g) is analyzed and discussed in terms of the role of chain length and end groups.     

Relaxation times and viscosity

On the basis of parallel Maxwell models having partial shear moduli G_i and relaxation times τ_i, mechanical relaxation processes including viscous flow can be easily described. Viscosity, e.g., equals the sum of the products of the above quantities, i.e. η = ΣG_iτ_i. The temperature dependence of each i term rigorously follows Arrhenius' las as logτ_i = logτ_{i, ∞} + Q_i/MRΘ. The assumption that a relation logτ_∞ = f(Q) exists between the activation energy Q_i and the relaxation time limit τ_∞ correlated to Θ = ∞ and that it varies with composition and cooling rate of the glass, proves useful. The usual function for the distribution of shear moduli over relaxation times [φ(logτ) ≡ G^?1dG/dlogτ] can thus be complemented by a corresponding one for the distribution over activation energies [Ψ(Q) ≡ G^?1dG/dQ]; which is temperature independent — the same as logτ_∞(Q)The corresponding functions Ψ (Q) are determined for the solid (?100 deg/h) and ‘liquid’ (L) glass states. Measurements of relaxation and viscosity are employed for that purpose.Application of the temperature-dependent functions logτ_∞ (Q) and Ψ (Q) are useful in several respects, as exemplified by calculating the freezing temperature, the change of activation energy with cooling rate, and the damping of mechanical vibrations in the transformation range.