S. Walker

Comparison of energy barrier data from dielectric, n. m. r. and ultrasonic techniques

Abstract Energy barriers for intramolecular motions have been determined for eight compounds in polystyrene matrices by dielectric absorption techniques in the

Dielectric and viscosity studies of the principal relaxation process of liquid 1-alkanols and their solutions

Abstract Dielectric studies have been made in the frequency and temperature ranges 10 6 –10 9 Hz and 170–320 K on all the pure liquid 1-alkanols of the general formula C n H 2 n +1 OH (where n is varied from 1 to 12) and a variety of their solutions in inert and weakly interacting solvents. A comprehensive study of these systems has been made on the variation of the principal relaxation time (τ 1 ) and enthalpy of activation (Δ H E ) as n is increased. For the pure 1-alkanols and their solutions the work demonstrates that the principal relaxation process cannot be attributed solely to either molecular or intramolecular motion of individual 1-alkanol molecules, and that between about n = 5 and 9 the presence of the alkyl chain is even more important than intermolecular hydrogen bonds in determining the magnitude of Δ H E . Some limitations of the Sagal theory have been examined, and, in addition, it has been found that τ 1 /viscosity versus concentration does not lead to a plot close to a single “master” curve as has been suggested in the literature. Interpretation of the mechanism based on a single “master” curve and employment of the Sagal theory is no longer valid. It is striking that the 1-alkanols (with the exception of methanol) in the most dilute solutions all have similar values of the principal relaxation time whereas their enthalpies of activation differ appreciably.

Dielectric relaxation of some rigid polar molecules in a polystyrene matrix

Dielectric absorption studies have been carried out on solutions of twelve polar solutes dispersed in an atactic polystyrene matrix over a range of temperatures in the frequency region 50 to 105Hz and in one case also in the range of 104 to 107Hz. Rigid molecules of varying size and molecular dipole moments halobenzenes, p-halotoluenes, p-halobiphenyls, p-nitrobiphenyl and p-bromoethylbenzene - have been examined. All these solute molecules have in common the fact that the molecular dipole moment lies along the main principal axis. The relaxation times and energy barrier parameters for molecular dipole relaxations are determined. The Eyring enthalpy of activation, ΔHE, is found to range from 9 for fluorobenzene to 102 kJ mol−1 for p-iodobiphenyl. Relaxation data of these molecules indicate a linear correlation between enthalpy and entropy of activation, and the former is found to depend upon the volume needed for the reorientation of molecules. Present results for halobenzenes and p-halotoluenes reveal a linear dependence of log (relaxation time) on the mean moment of inertia. The enthalpy data for these rigid molecules are useful in the study of flexible molecules when a decision has to be made as to whether the absorption is to be ascribed to molecular or intramolecular or overlap of the two relaxation processes.

Relaxation processes of some aromatic sulfides, sulfoxides, and sulfones in a polystyrene matrix

Dielectric absorption studies have been made of a number of aromatic sulfides, sulfoxides, and sulfones in a polystyrene matrix at a variety of temperatures in the frequency range of 102–105 Hz, and in three cases between 104–107 Hz. One compound, bis (4‐pyridyl) sulfide, was also examined as a pure compressed solid in the frequency range of 104–107 Hz. The Eyring parameters have been determined and used to characterize the relaxation processes. Low enthalpies of activation (∼15 kJ mol−1) have been estimated for an intramolecular relaxation process of diphenyl, bis (4‐tolyl) and bis (4‐pyridyl) sulfide, and rotation about the C–S bond in these symmetrical aromatic sulfides would, thus, be expected to occur quite readily. These low barriers are to be contrasted with those for the molecular relaxation values of 60–78 kJ mol−1 for diphenyl, bis(4‐tolyl) and bis(4‐chlorophenyl) sulfoxides and sulfones. The enthalpies of activation (∼39 kJ mol−1) for the molecular relaxation of the disklike molecules, dibenzot...

Reorientational motions of dipolar solutes in glassy o‐terphenyl

The low frequency (102–105 Hz) dielectric relaxation behavior of small dipolar molecules dispersed in supercooled o‐terphenyl has been studied below the glass transition temperature. Broad asymmetric loss curves are obtained for both rigid and nonrigid solute molecules. The relaxation times and Eyring activation enthalpies for molecular and intramolecular group reorientation processes are compared with the analogous values for the same solutes in a polystyrene matrix.

The nature of the β process for polar solutes in four glassy forming solids

Dielectric constant and loss data have been obtained for several rigid dipolar substituted benzenes in up to four glass forming solvents. Measurements were made between 10 and 105 Hz from about 80 K up to room temperature. The broad loss curves were analyzed to give relaxation times which were then used to determine Eyring activation enthalpies ΔHE. For the glassy systems dipole reorientation, at temperatures well below the glass transition, Tg is essentially independent of the solvent and is identified as a β process. The ΔHE values of the β process increase appropriately with molecular size but are largely independent of the glassy medium. The β process for these systems is ascribed to molecular relaxation which may possibly occur in a ‘‘solvent cavity.’’

Dielectric studies of the switch-over mechanism in the principal relaxation process of alkan-1-ols

Dielectric absorption studies have been made on the alkan-1-ols in inert and weakly interacting solvent over a wide range concentration at various temperatures and in the frequency range 106–109 Hz. Some representative alcohols have also been studied in the strongly hydrogen bonding solvents, diethyl ether and di-n-butyl ether. It has been found that not only does the flexible chain of the alkan-1-ol influence the enthalpy of activation on its dilution with a dialkyl ether, but the chain length of the latter also exerts an influence. Altogether the work strongly supports a switch-over type of mechanism as being involved in the principal relaxation process of the alkan-1-ols. The data for the alkan-1-ols in n-heptane and toluene solutions have been employed to test more quantitatively a switch-over theory, and for the shorter-chain alkan-1-ols this has been found to yield reasonable agreement between the measured and estimated principal relaxation time (τ1) over a wide range of concentration and temperature.

Dipole reorientation of solute molecules dispersed in organic glasses

Dielectric loss curves (102–105 Hz) have been obtained for a variety of organic molecules dispersed in a six‐ring polyphenyl ether over a range of temperatures below the glass transition. Characteristic broad loss curves were obtained for rigid molecules, such as nitrobenzene, and molecules, such as 1,4‐dimethoxybenzene, which contain a rotatable polar group that can contribute to the dielectric absorption. Eyring activation enthalpies determined from the temperature dependence of the relaxation times for these systems are compared with earlier results using o‐terphenyl and polystyrene glasses. The time scale of the group relaxation of a particular solute molecule in different organic glass solvents seems to be fairly independent of the nature of the solvent despite substantial differences in their glass transition temperature.

Internal rotation in some alkylamides in a polystyrene matrix

Abstract Dielectric absorption studies have been made of the intramolecular group relaxation of nine alkylamides dispersed in a polystyrene matrix. The group relaxation process has been found to occur near room temperature in all the nine molecules studied. The Δ H E value for the group process lies in the range 52–75 kJ mol −1 from butyramide to N -tert-butyl-acetamide. For these alkylamides variations of the steric effect of the groups attached to the nitrogen atom appear to have a greater influence on the barrier to internal rotation than alteration of the electronic factors.

Eyring activation parameters for the relaxation processes of some asymmetric diaryl molecules in a polystyrene matrix

Measurements of the dielectric absorption of a series of asymmetric diaryl ketones and ethers of the type X–C6H4–Y–C6H5 (Y=0, CO, and X=OH, F, Cl, Br, NO2, and C6H5) and 2‐benzoylpyridine have been made in polystyrene in the frequency range 102–105 Hz over a wide temperature range (77–330 K). The Eyring activation parameters have been determined, and these indicated the occurrence of a molecular relaxation process in all cases, except 4‐biphenyl phenyl ether, where only phenoxy group rotation has been detected. An intramolecular energy barrier has also been determined which is slightly greater than that for the rotation of a methoxy group in aromatic methoxy compounds. The enthalpies of activation have been estimated for molecular relaxation. Correlations have been obtained for the molecular relaxation processes, between ΔGE and ΔHE and the volume swept out on reorientation. The factors governing the relative magnitude of the molecular activation energy parameters have been considered.