David W. McCall

Diffusion in Liquids

Self‐diffusion coefficients have been measured for water, nitromethane, acetone, benzene, cyclohexane, isopentane, and neopentane. Temperature and pressure dependences have been determined for nitromethane, acetone, cyclohexane, and isopentane. The temperature dependence has been determined for neopentane and the pressure dependence has been determined for water and benzene. The results are discussed in terms of a semiempirical theory for transport in liquids and it is shown that semiquantitative predictions of self‐diffusion, viscosity, and thermal conductivity coefficients can be made.

Dielectric Properties of Linear Polyamides

The dielectric constant and loss have been measured for several linear polyamides over a range of frequency and temperature. At low frequencies and elevated temperatures, proton conduction through the amorphous regions gives rise to a Maxwell‐Wagner loss, while at high frequencies and high temperatures dipole relaxation of the amide groups contained in the amorphous regions gives rise to a typical loss peak. Nylons 6, 6–6, 6–10, 7–9, 6–9, and 10–10 yield very similar results. The N‐methylated derivative of nylon 10–10 exhibits a much smaller direct current conductivity than nylon 10–10 (when compared at temperatures of equivalent chain activity) owing to the absence of amide protons.

Molecular Motion in ortho‐Terphenyl

Nuclear magnetic relaxation measurements have been made for ortho‐terphenyl in the liquid, crystalline, and glassy form. Self‐diffusion coefficients have been measured in the liquid. Molecular correlation frequencies derived from these measurements are shown to correlate well with published viscosity data. The results were found to be described satisfactorily by an equation of the form νc = A″ exp[−B′ / (T−Tg)], but not by an equation of the form νc = A exp(− C / TSc), where Sc is the configurational entropy. Comments are made on the derivation of these functions and the nature of the molecular motions.

Nuclear Magnetic Resonance in Solid Adamantane

Proton magnetic resonance studies of solid adamantane reveal the existence of a rotational transition at about —130°C. The activation energy associated with the rotational transition is found to be about 5 kcal/mole. Theoretical and experimental second moments agree if it is assumed that the molecules are rigidly fixed below the transition and rotate freely above the transition.Proton magnetic resonance studies of solid adamantane reveal the existence of a rotational transition at about —130°C. The activation energy associated with the rotational transition is found to be about 5 kcal/mole. Theoretical and experimental second moments agree if it is assumed that the molecules are rigidly fixed below the transition and rotate freely above the transition.

Diffusion in Ethylene Polymers. IV

Self‐diffusion coefficients have been measured for two low molecular weight fractions (M = 4100 and 5800) of a linear polyethylene. The activation energy for the M = 4100 fraction is 5.3 kcal/mole which is near the value 5.6 kcal/mole found for n — C32H66. At 150°C, for this fraction, D = 1.6×10—7 cm2/sec and as 10—7 is near the lower limit of detectability the activation energy is probably not accurately determined. The diffusion coefficient for the M = 5800 fraction is 1.1×10—7 cm2/sec at 150°C. These results, combined with previous results, show that D = κn—5/3, where D is the coefficient of self‐diffuison for the normal paraffin CnH2n+2. κ is a constant at a given temperature, 4.5×10—3 cm2/sec at 200°C and 2.5×10—3 cm2/sec at 150°C.

Self‐Diffusion in Liquids: Paraffin Hydrocarbons

The pressure dependence of the self‐diffusion coefficients of several normal paraffin hydrocarbons and the isomers of hexane have been determined. In addition, the temperature dependence of self‐diffusion coefficients has been measured for the isomers of hexane. Energies and volumes of activation have been calculated. With the exception of 2, 2‐dimethyl butane, these parameters do not change significantly with branching in the isomers of hexane. The volumes of activation are about 10–20% of the molar volumes in all cases. Diffusion coefficients computed from a semiempirical modification of an equation derived by Longuet‐Higgins and Pople are compared with the experimental results.

Self‐Diffusion of Nearly Spherical Molecules. Neopentane and Tetramethyl Silane

Self‐diffusion coefficients have been measured for neopentane and tetramethyl silane by means of the nuclear magnetic resonance spin‐echo technique. Both pressure and temperature dependences have been determined, thus allowing evaluation of energies and volumes of activation; neopentane, ED=3.5 kcal/mole and ΔVΔ=31 cm3/mole tetramethyl silane, ED=1.16 kcal/mole and ΔVΔ=22 cm3/mole. The numbers given for neopentane are average values as the lnD vs 1/T and lnD vs p plots were not straight lines. Kirkwoods theoretical relation for the self‐diffusion coefficient of spherical molecules has been tested for the quasi‐spherical neopentane. At 0°C the theoretical value is 6×10—5 cm2/sec while the experimental value is 2.6×10—5 cm2/sec. The Kirkwood equation for the self‐diffusion coefficient is discussed in detail.

Nuclear Magnetic Resonance of Polymer Fibers

Nuclear magnetic relaxation times T2, T1, and T1ρ are reported for a tetrafluoroethylene–hexafluoropropylene copolymer (FEP), both in bulk and in drawn fiber form. The measurements cover a temperature range from − 200 to + 250°C. Relaxation times T2 and T1ρ exhibit marked anisotropy with fiber orientation in the magnetic field. The effect is less pronounced in the T1 case. Theoretical expressions for T2, T1, and T1ρ have been derived for the fiber case, and the results are presented. Lattice sums have been computed for rotation and rotation–translation modes of molecular activity and the values used to predict theoretical relaxation times. Quantitative agreement between theory and experiment is excellent for T2 and satisfactory for T1 and T1ρ, in support of the proposed molecular models. Relaxation effects previously labeled γ or Glass II are assigned to molecular reorientation about the helix axis, with the motion occurring in both the amorphous and crystalline regions of the polymer. The α or Glass I re...

Self‐Diffusion in Linear Dimethylsiloxanes

The pressure and temperature dependences of the self‐diffusion coefficients of the linear dimethylsiloxanes, containing from two to nine silicon atoms have been determined by the NMR spin‐echo technique. Activation energies and volumes are presented as derived quantities; the constant‐volume activation energy is estimated from these data. Extensive comparisons between the diffusion properties of the dimethylsiloxanes and equivalent hydrocarbons are made. From these comparisons, it is concluded that diffusion in siloxanes is controlled to a much larger extent by chain configurational effects. This concept is formalized by expressing the chain‐size effects in an entropy of activation model.

Nuclear Magnetic Resonance Studies of Molecular Relaxation Mechanisms in Polymers

N RECENT YEARS substantial progress has been made in the understanding of the Istructures and dynamics of high polymers. Crystal structures and chemical composition are well known for most common polymers [1, 2] . The fringed micelle model for crystalline-amorphous organization has been supplemented by the discovery and elucidation of chain folding and the relationship of chain-folded lamellae to spherulitic structures [2, 3]. Relaxation phenomena have been related to molecular motions, and qualitative and semiquantitative interpretations of diverse relaxation experiments show gratifying consistency [4, 5]. In this paper I