Localized Motion in Supercooled Glycerol as Measured by 2 H-NMR Spin-Lattice Relaxation and Incoherent Neutron Scattering

Selectively deuterated glycerol has been subjected to 2H-NMR spin-lattice relaxation and quasi-elastic neutron scattering experiments. The measurements yield relaxation rates and a non-Gaussian Q-dependence of the Debye-Waller factor which are different for the two hydrogen sites. The data analysis shows that below the onset of the glass transition α-process the hydrogens perform a local motion (≈ 10-12 s) in addition to what is expected from harmonic phonons. The resulting mean-square displacements are highly temperature dependent but are significantly smaller than those found in van der Waals glasses. Amplitudes and activation energies of the carbon-bonded and oxygen-bonded hydrogens are …

Inelastic Neutron Scattering Experiments on Van der Waals Glasses - A Test of Recent Microscopic Theories of the Glass Transition

Etude realisee sur un verre d'o-terphenyle afin de montrer l'existence d'une relaxation secondaire presentant des caracteristiques inhabituelles et le comportement Kohbrausch de la fonction de correlation de densite decrivant la relaxation structurale

Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering

It has been shown that translational diffusion coefficients, Dt, in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T ≲ 1.2Tg and can be described by Dt ∼ η−χ with χ < 1 whereas Dr ∼ η−1 applies for the mean rotational diffusion coefficients, Dr, down to the glass transition temperature, Tg. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to Tg.

Dynamic anomaly in the glass transition region of orthoterphenyl

We report on incoherent and coherent neutron scattering results in the supercooled liquid and the glassy regime of the van der Waals fluido-terphenyl using the backscattering and spin echo technique, respectively. A critical comparison of both techniques is presented. The data are analysed in the time domain assuming that microscopic correlation times (τ) scale with the viscosity η according to τ(T)∼η(T)/T. With this assumption we obtain an agreement with several predictions of mode coupling theory: the existence of a critical temperatureT c is shown, independently for both incoherent and coherent data, by a cusp in the temperature dependence of the Debye-Waller factorf Q (T). BelowT c fQ(T…

Dynamic anomalies at the glass transition of organic van der Waals liquids

Abstract The paper discusses the question of whether there is a characteristic temperature T c above the calorimetric glass transition temperature T g . Mode-coupling theory (MCT) predicts a crossover from liquid- to solid-like dynamics at T c . Neutron scattering and gradient NMR experiments have been carried out to test MCT using the molecular van der Waals liquid ortho -terphenyl as a model system. A significant anomaly of the Debye—Waller factor and a “decoupling” of self-diffusion from viscosity support the MCT predictions. A critical discussion of the relevance of such tests and of the limitations of neutron scattering is presented.

The glass transition in (KI)0.5(ND4I)0.5 mixed crystals as studied by deuteron spin-lattice relaxation

Abstract Nuclear spin resonance has been used to study the deuteron magnetization recovery in (KI)0.5(ND4I)0.5 mixed crystals. At high temperatures the spin-lattice-relaxation is exponential. For T ⪅ 45 K deviations from this simple behaviour occur, signalling the onset of spatial inhomogeneities due to the formation of an orientational glass. The results demonstrate that the transition of (KI)0.5(ND4I)0.5 into the glassy state is driven by the freezing of random bonds.

Static and MAS 35CI NMR and Molecular Motions of ClO −4 Ions in the Various Phases of Multimethylammonium Perchlorates

Static and MAS*) 35CI NMR data of CIO−4 ions in the low-, intermediate- and high-temperature phases of trimethylammonium, dimethylammonium and monomethylammonium perchlorates are reported. The observed solid-solid phase transitions are interrelated with the motional state of the perchlorate ions. In the low-temperature phase III of trimethylammonium perchlorate there is an anisotropic motion of the ClO−4 ions with a constant quadrupole frequency of 185 kHz and an increasing value of the asymmetry parameter when approaching phase transition III-II. The activation energy for this motion is found to be 44.3 kj/mol. At the transition III-II the ClO−4 ions gain orientational degrees of freedom m…

Nonexponential 2H spin-lattice relaxation as a signature of the glassy state

Abstract High-precision measurements of 2H spin-lattice relaxation on several molecular glass-forming liquids have been performed. As a general feature the following can be stated: At temperatures more than ten to twenty degrees above the calorimetric glass transition temperature Tg the 2H spin-lattice relaxation is exponential; below that temperature regime the relaxation is nonexponential. This crossover from exponential to nonexponential magnetization recovery implies that no common spin temperature caused by spin diffusion exists in a 2H glass. This contrasts 1H spin-lattice relaxation which is found to be strictly monoexponential throughout. The occurrence of nonexponential 2H relaxati…

Relaxation and phonons in viscous and glassy orthoterphenyl by neutron scattering

We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from α-to β-relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureT c [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a m…

Fast Local Motion around T g in a Molecular Glass as Observed by Incoherent Neutron Scattering

Incoherent quasi-elastic neutron scattering experiments on a molecular glass (1,3,5-tri-α-naphtylbenzene, C36H24) are reported. We find clear evidence for an anomalously strong decrease of the elastic and a corresponding increase of the inelastic scattering around the glass transition temperature. The line shape of this extra inelastic intensity gives evidence for a quasi-elastic scattering implying the existence of a localized and fast (τc ≈ 4.10-12s) molecular motion. From the elastic incoherent structure factor (EISF) a mean jump length ≤ 0.6 A is estimated. The significance of this motion as precursor of the glass instability is discussed within the framework of dynamic glass transition…

Secondary relaxation in the glass-transition regime of ortho-terphenyl observed by incoherent neutron scattering.

We report on incoherent-neutron-scattering measurements in the supercooled regime of the van der Waals liquid ortho-terphenyl. A secondary localized relaxational process on the picosecond time scale is found. In accordance with mode-coupling theories of the glass transition, the relaxational dynamics around a critical temperature ${\mathit{T}}_{\mathit{c}}$ decomposes into two time regimes.

2H spin relaxation times, T1 and T2 and solid echo line shapes were investigated in 2-propanol (2-POH) solutions of poly(butyl methacrylate) (PBMA) in a wide concentration and temperature range, covering the solution, gel and glass states. From the results in deuterated 2-POH-d6 we found that the polymer influence upon solvent mobility is largely suppressed in the gel and glass states in contrast to normal polymer solutions where below the glass transition an increasing amount of solvent slows down with decreasing temperature. From 2H NMR of deuterated butyl side groups in the PBMA-d9/2-POH system we found that the side group motion is anisotropic and largely independent of solvent concentr…

Dynamic Anomalies and their Relation to the Glass Transition: A Neutron Scattering Study of the Glass Forming Van der Waals Liquid Ortho-terphenyl

Neutron scattering experiments on the molecular glass former ortho-terphenyl reveal a dynamic anomaly at a temperature Tc ≈ 290 K well above the calorimetric glass temperature Tg = 243 K. Close above Tc the density autocorrelation function ΦQ(t) shows a two step decay over 4–5 decades in time. The slower component obeys the time-temperature superposition principle. Its line shape can be well parametrized by a Kohlrausch law and is strongly temperature dependent as its relaxation time scales with the shear viscosity. Thus this component is identified with the structural relaxation (α-process). The faster component (β-process) is much less temperature dependent. Its line shape factorizes in a…

2 H-NMR-Stimulated Echo Study of Ultraslow Reorientational Motion in Viscous Glycerol near Its Glass Transition Temperature

2H-NMR stimulated echo experiments have been performed in order to study the molecular basis of the ?-process in viscous glycerol near its glass transition temperature. Decay functions following modified Jeener-Broekaert pulse sequences were compared with predictions from different models for molecular reorientation. Rotational diffusion, rotational random jumps, rotational fixed-angle jumps and combinations of diffusive and jump motions have been tested. All data are fitted with a log-Gaussian distribution of correlation times. Thereby, small-but finite-angle reorientation processes turn out to dominate in the 10-3 s.. 100 s regime. Pure large-angle rotational jumps can be ruled out with h…

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toD t =3·10−14 m2s−1 using a1H-NMR technique applying static field gradients up to 53T m−1 In a range of more than two decades theD t values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureT g . A change of mechanism is found for translational diffusion atT c ≈1.2T g whereD t is proportional to the inverse shear viscosityη −1 atT>T c butD t ∼η ξ with ξ=0.75 atT<T c . Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal toη −1 down toT g . Our re…

Anisotropic diffusion in etched particle tracks studied by field gradient NMR

Etched particle tracks produced after heavy ion irradiation of polymer foils are used as model systems to test the performance of NMR in a newly developed ultrahigh magnetic field gradient system. The stimulated NMR echo decay of molecules diffusing in the channels, formulated in terms of the self part of the intermediate scattering function, is anisotropic and yields the form factor of the channels.