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

Abstract

Selectively deuterated glycerol has been subjected to 'H-NMR spin-lattice relaxation and quasi-elastic neutron scattering experiments. The measurements yield relaxation rates and a non-Gaussian &-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 aprocess the hydrogens perform a local motion (= lO-"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 different. A possible mechanism is discussed.

Microscopic dynamics of glycerol in its crystalline and glassy states

“…6͑c͒ compares the average ͗u 2 ͘ atomic mean-square displacements ͑m.s.d.͒ as calculated for the glass and crystal from the relevant frequency distributions and data taken from a neutron scattering study. 32 As expected, the glass data lay above the harmonic curve within the whole range of temperatures, and the marked change in slope about 180 K unveils the presence of the glass transition at T g Ϸ185 K. A comparison between the previous estimates for ͗u 2 ͘ from neutron scattering and the present set shows an extremely good agreement for temperatures above 100 K, whereas the neutron data below such temperature are somewhat below the present ones for both glass and crystal. Since the present LD model adequately reproduces both the crystal structure and thermodynamics within such range of temperatures, it seems clear that the previous neutron data somewhat underestimate the value of the atomic displacement below 100 K.…”

Molecular dynamics in solid polymers

“…In particular, the Stimulated-Echo experiment (STE) [5,9,70] has been widely employed for studying the rates, amplitudes, and heterogeneities of slow motions in organic and inorganic solids. The pulse sequence for the basic stimulated-echo experiment is shown in Fig.…”

“…2 H spin alignment echoes have been used for many years in the study of a-and b-relaxation in supercooled liquids [5,94,95,118]. Important information about the motional rates, motional geometry, and distribution of correlation times have been obtained using this technique.…”

“…Dynamical aspects very often play little or no role in the characterization of the substances. However, solid materials and in particular polymeric systems exhibit a sometimes astonishing variety of segmental motions as well as molecular reorientations which have important effects on mechanical, transport and optical properties of polymers [1], the activity of proteins [2], the stability of pharmaceuticals [3], the transport properties in zeolites [4], behavior of amorphous materials near the glass transition [5,6], ion transport in organic and inorganic ionic conductors [7][8][9], and other structural properties of organic and inorganic systems [10,11]. Solid-state NMR provides powerful techniques for elucidating details of segmental dynamics and local conformation in solid materials [12].…”

Communication: Crystallite nucleation in supercooled glycerol near the glass transition

“…[1][2][3] Heterogeneity, which has been revealed by different experimental methods, [4][5][6] is a key feature which must be considered when dealing with the glass transition. Fluorescence microscopy of single molecules confirms the presence of large variations in the local viscosities.…”