Effect of orientation,anisotropy, and water on the relaxation behavior of nylon 6 from 4.2 to 300°K

The relaxation behavior of nylon 6 from 4.2 to 300°K was investigated as a function of orientation, anisotropy and moisture content by using an inverted free-oscillating torsion pendulum. Three new relaxations, δ at 53°K, ? below 4.2°K, and ζ at 20°K, were discovered. The characteristics of these new relaxations strongly depend on the orientation anisotropy, and concentration of adsorbed water in the specimens. The results suggest that the mechanism of the γ process is associated with the motions of both the polar and methylene units. The mechanism of the β relaxation is postulated to originate with motions of both non-hydrogen-bonded polar groups and polymer—water complex units. The behavior of the α peak is consistent with the hypothesis that it originates with the rupture of interchain hydrogen bonding due to the motions of long-chain segments in the amorphous regions. Finally, the data strongly support the proposition that two types of water, tightly bound and loosely bound, exist in nylon 6.     

Dielectric loss of high-density polyethylene below 4.2°K

Drawing of oxidized high-density polyethylene (HDPE) and subsequent annealing greatly reduce the low-temperature dielectric loss when the electric field is applied perpendicular to the draw direction. This supports our model (J. Polym. Sci. Polym. Phys. Ed., 15 , 43 (1977)) of the proton moving parallel to the c axis of the PE crystal. A particular antioxidant (Ionox 330) in unoxidized HDPE induces a dielectric loss with a frequency and temperature dependence which differs from that for the loss in oxidized HDPE. The antioxidant loss seems to be an overlapping of tunneling and a thermal activation process. The possibility that the hydroperoxide group in the PE crystal is the origin of the loss in oxidized HDPE was theoretically examined in a manner similar to that used for the hydroxyl group in the previous paper. Results suggest that the hydroperoxide group is less probable than hydroxyl as the origin of the low-frequency loss in HDPE.     

Dielectric loss of polyethylene below 4.2°K arising from proton tunneling

The dielectric loss of high-density polyethylene (HDPE: melt-crystallized films and single-crystal mats), low-density polyethylene (LDPE), and copolymers of ethylene and vinyl alcohol (PEVA) was measured at 1.5 to 4.2°K in the frequency range from 10 Hz to 10 kHz. Results for HDPE show dispersion curves almost corresponding to a single relaxation and are interpreted in terms of phonon-assisted tunneling of the protons of hydroxyl groups which are accidentally attached to tertiary carbons (carbons with a short branch). Dispersion curves for LDPE and PEVA are quite broad, indicating a wide distribution of relaxation times. The loss level of PEVA passes through a maximum at 7.5% vinyl alcohol, suggesting that interaction between a couple of neighboring hydroxyl groups depresses the loss. A potential calculation for the rotation of a hydroxyl group in the orthorhombic lattice of polyethylene yielded a double-minimum potential for the tunneling when the lattice is assumed to have a distortion which is acceptable from x-ray analysis. The potential quantitatively explains the observed values of relaxation time and relaxation strength of HDPE. The concentration of hydroxyl groups in HDPE, which varies among the samples, is estimated to be of the order of 10¹⁶ cm^?3.     

Dielectric relaxations in polymers with pendent phenyl or pyridine groups at temperatures from 4°K to 80°K

Isochronal measurements of dielectric loss are made for polystyrene (PS), poly(4-vinyl pyridine) (P4VP), poly(2-vinyl pyridine) (P2VP), poly(L-phenylalanine) (PLPA), and poly(γ-benzyl-L-glutamate) (PBLG) at temperatures ranging from 4°K to 80°K and at frequencies from 10 Hz to 100 kHz. PS, P4VP, and PLPA show loss peaks around 50°K (10 kHz) while P2VP exhibits a loss peak around 20°K (10 kHz). PBLG has no detectable peak in this temperature range. The 50°K and 20°K peaks are ascribed to wagging and rotation, respectively, of phenyl or pyridine groups between two energy minima. The barrier height and energy difference between the minima evaluated from the experimental data are reasonably explained by assuming that the double minima are caused by interaction between a pair of phenyl or pyridine groups, each belonging to adjacent chains which pack irregularly.     

Low-temperature specific heat of polystyrene and related polymers (1.6° to 4°K)

The specific heat of polystyrenes of different origin and molecular weight, of α-substituted and ortho-substituted polystyrenes, and of polystyrenes crosslinked with different amounts of divinylbenzene have been measured between 1.6 and 4°K. The specific heat of all samples shows a temperature dependence that can not be explained by assuming a Debye frequency spectrum for the vibrational modes in these polymers. Good agreement is obtained by fitting the data to a superposition of a Debye T³ term and an Einstein specific heat with a characteristic temperature of 15–18°K. This localized frequency mode may have its origin in the one-dimensional nature of the polymer chain. A simple calculation of the length of a polystyrene chain necessary to obtain these characteristic temperatures shows reasonable agreement with the number of Einstein oscillators observed in the samples.     

Dielectric properties of polystyrene and some polychlorostyrenes from 4°K to room temperature

Dielectric measurements of carefully purified specimens of polystyrene and poly(2,3,4 or 3,4-chlorostyrene) have been obtained at audio frequencies ranging from 0.1 to 20 kHz and at temperatures between 4 and 300°K. Each of the samples exhibits a dielectric loss maximum in the range 15–50°K. The temperature of the maximum loss decreases with the addition of a substituent which lowers the symmetry of the pendant phenyl group. The results are explained by a model which invokes a coupling mechanism between two distinct modes of side group motions. This same model also explains some results of previously reported measurements of mechanical losses in similar polymers.     

Heat capacity of amorphous polyhexene-1 between 20 and 300°K: Intramolecular vibrational contribution to Cv below the glass transition

The heat capacity of polyhexene-1 was measured between 20 and 300°K. The apparatus, an adiabatic calorimeter giving results with a random error of 0.2–0.4%, is briefly described. The characterization of the sample by x-ray diffraction patterns established that it was amorphous at all temperatures. Gold foil was incorporated with the sample to increase the apparent thermal diffusivity and so to decrease the time needed for the measurements. The glass transition temperature was found to be 215.5 ± 1°K. On the C_p curve, no subglass anomaly was detected, unlike the results of experiments described elsewhere. The calculation of C_v is discussed, and an explanation is given for the choice of the number of intramolecular vibrational modes per monomer which are assumed to contribute to C_v. A linear continuum model with characteristic temperature θ₁ = 736°K allows us to fit the experimental curve over a temperature range of 140°K.     

The effect of strain rate on craze yielding,shear yielding,and brittle fracture of polymers at 77°K

The tensile strength of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen over the strain rate range of 2 × 10^?4 to 660 min^?1. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and then decreased with strain rate. Above a critical strain rate of about 200 min^?1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The nonlinear behavior of stress versus log strain rate at low strain rates was associated with a decrease in activation volume with increasing strain rate whereas the nonlinear behavior at high strain rates was associated with an increase in density and decrease in length of the crazes with strain rate. The strain rate effect was the basis for calculating the diffusion coefficient of nitrogen into the polymers at 77°K. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 ų. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength. The brittle fracture stress of PMMA, in the absence of crazing, in compression and in tension, did not vary with strain rate.     

Phenomenology of EPR Trapped Electron Spectra Produced by Photoionization in Alkalines at 4.2°k and 77°k

Trapped electrons, e_c^?, are produced by photoionization of K₄Fc(CN)₆ in 10M NaOH/H₂O and 10M NaOD/D₂O glasses at 4.2°K. The linewidth, ΔH_ms, of e_c^? changes reversibly between these two temperatures in contrast to the observations for organic matrics. Three proposed mechanisms including dipole relaxation of pendent molecules surrounding the electron cavity, contraction-expansion model of cavity, and electron retrapping process are each discussed. It appears that no one physical model satisfactorily accounts for the present observations. The electron retrapping process, however, might be a better condidate for interpreting the experimental results in compatible with the optical absorption data.     

Pyrolysis of vinylacetylene between 300 and 450°C

Vinylacetylene was pyrolyzed at 300–450°C in a packed and an unpacked static reactor with a pinhole bleed to a quadrupole mass spectrometer. The reactant and C₈H₈ products were monitored continuously during a reaction by mass spectrometry. In some runs, the products were also analyzed by gas chromatography after the run. In these runs CH₄, C₂H₆, C₃H₆, and C₂H₄ were also detected. The reaction for vinylacetylene removal and C₈H₈ formation is homogeneous, second order in reactant, and independent of the presence of a large excess of N₂ or He. However, C₈H₈ formation is about half-suppressed by the addition of the free-radical scavengers NO or O₂. The rate coefficient for total vinylacetylene removal is 1.7 × 10⁶ exp(?79 ± 13 kJ/mol RT) L/mol · s. The major reaction for C₄H₄ removal is polymerization. In addition four C₈H₈ isomers, carbon, and small hydrocarbons are formed. The three major C₈H₈ isomers are styrene, cyclooctatetraene (COT), and 1,5? dihydropentalene (DHP). The C₈H₈ compounds are formed by both molecular and free-radical processes in a second-order process with an overall k ? 3 × 10⁸ exp(?122 kJ/mol RT) L/mol · s (average of packed and unpacked cell results). The molecular process occurs with an overall k = 8.5 × 10⁷ exp (?118 kJ/mol RT) L/mol · s. The COT, DHP, and an unidentified isomer (d), are formed exclusively in molecular processes with respective rate coefficients of 4.4 × 10⁴ exp(?77 kJ/mol RT), 1.7 × 10⁵ exp(?89 kJ/mol RT), and 3.1 × 10⁹ exp(? 148 kJ/mol RT) L/mol · s. The styrene is formed both by a direct free-radical process and by isomerization of COT.     

Fluorescence of solid tetracene in the temperature range 4.2 to 300 K

The fluorescence spectrum of polycrystalline tetracene has been measured in the temperature range 4.2 to 300 K. The 0,0 transition occurs at 18700 ± 20 cm^?1 independent of temperature and crystal structure. The unusually large Stokes shift between absorption and fluorescence is traced back to structural relaxation.     

Partial Optical Bleaching of Trapped Electrons in Glasses of 2-Methyltetrahydrofuran at 4° K and 77° K

The microscopic view of trapped electron concentration gradient established within a few molecular diameters of 2-methyltetrahydrofuran after partial bleaching is known to be considerably small that there will be essentially no effects on the relative distribution of electrons in the sample. A weakly bound excited state of trapped electrons in the system is deduced from the Arrhenius activation energies determined from the bleaching data.     

v---v pumping in H2: estimate of induced dissociation at 300°K

By means of the stimulated Raman effect one may produce in H₂, D₂, and in several other gases, a substantial population in the ν = 1 state. Subsequent to the pulse excitation v---v energy transfers rapidly generate a vibrational distribution which is approximately equivalent to (3–4) × 10³°K. Then, in H₂ + D₂ mixtures metathetic reactions occur. Here we report on a computer simulation of an experiment described elsewhere, designed to estimate whether a measurable fraction of the hydrogen molecules reach the upper vibrational level and dissociate. Solutions of the coupled differential equations show that the anticipated hydrogen atom concentrations are too low by a factor of 10⁷-10⁸ to account for the observed H/D exchanges. These calculations also show that the conventional phenomenological rate equation for dissociation does not apply to this highly non-thermal distribution.     

Photoviscoelastic behavior of amorphous polymers during transition from the glassy to rubbery state

Tensile stress‐relaxation experiments with simultaneous measurements of Young's relaxation modulus, E, and the strain‐optical coefficient, C_?, were performed on two amorphous polymers—polystyrene (PS) and polycarbonate (PC)—over a wide range of temperatures and times. Master curves of these material functions were obtained via the time‐temperature superposition principle. The value of C_? of PS is positive in the glassy state at low temperature and time; then it relaxes and becomes negative and passes through a minimum in the transition zone from the glassy to rubbery state at an intermediate temperature and time and then monotonically increases with time, approaching zero at a large time. The stress‐optical coefficient of PS is calculated from the value of C_?. It is positive at low temperature and time, decreases, passes through zero, becomes negative with increasing temperature and time in the transition zone from the glassy to rubbery state, and finally reaches a constant large negative value in the rubbery state. In contrast, the value of C_? of PC is always positive being a constant in the glassy state and continuously relaxes to zero at high temperature and time. The value of C_σ of PC is also positive being a constant in the glassy state and increases to a constant value in the rubbery state. The obtained information on the photoelastic behavior of PS and PC is useful for calculating the residual birefringence and stresses in plastic products. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2252–2262, 2001     

The impedance behavior of the cell Au/HClO4 · 5.5 H2O/Au in the temperature range 4.2–300 K

The impedance of the cell Au/HClO₄-5.5 H₂O/Au was investigated in the frequency range 1 to 10⁵ Hz between 4.2 and 300 K. The analysis of the data enables an evaluation of important electrolyte properties such as conductivity and dielectric constant in a wide range of temperatures, predominantly in the solid state of the electrolyte HClO₄-5.5 H₂O (T_f = 228 K). The double layer capacity of the gold electrodes was also determined; it shows a qualitatively similar result compared with previous measurements. In the solid state, the ionic conductivity exhibits two distinct activation energies of 0.37 and 0.54 eV corresponding to the two phases present in HClO₄-5.5 H₂O above and below 170 K. Below 120 K the activation energy becomes very small and tends to zero around 80 K indicating possible tunneling processes in the rigid H₂O structure. At about the same temperature the dielectric constant reaches its low temperature limit with a value _∞ ≈ 11 which is considerably higher than the value of pure ice of _∞ ≈ 3.     

Synthetic Reactions of Metal Atoms at Temperatures of 10 to 273°K

Metal atoms, easily formed by vaporizing metals at high temperatures under vacuum, are reacted with inorganic and organic compounds in solution at 80 to 273°K or by co-condensing the atoms and substrate vapors at 10 to 80°K. Gram quantities of products containing metals in low valency states can be isolated in many cases, and some of the compounds formed are inaccessible by conventional synthetic routes.     

Thermodynamic functions of α,α-trehalose dihydrate and of α,β-trehalose monohydrate at temperatures from 13 K to 300 K

Thermodynamic properties at low temperatures were investigated for α,α-trehalose dihydrate and α,β-trehalose monohydrate. The heat capacities were measured using an adiabatic calorimeter at temperatures between 13 K and 300 K. The heat capacity data were expressed as a function of temperature, T, by a polynominal of forth to sixth order, with which thermodynamic functions, enthalpy, entropy, and Gibbs free energy, were determined.     

Heat capacities for atactic polystyrene of narrow molecular weight distribution to 360°K.

Precise heat capacity values are reported over the temperature range from 10 to 360°K. for a sample of atactic polystyrene having a narrow molecular weight distribution. This sample was taken from the stock from which National Bureau of Standards Standard Sample 705, Narrow Molecular Weight Distribution Polystyrene, was established. Data are reported for the sample as received, and after an annealing procedure. At temperatures below about 60°K. a systematic difference comparable with the limits of experimental precision appears between the values obtained for the present sample as received and after the annealing, although at higher temperatures the values for the two conditions showed no systematic difference beyond the limits of precision of the measurements. At temperatures above 100°K., previously published values for atactic polystyrene samples of various molecular weight distributions and for isotactic polystyrene agree within about 0.5% of the values from this investigation. At temperatures below 100°K. significant heat capacity differences appear, especially between values for the atactic and the isotactic isomers, and even between atactic samples of different molecular weight distribution.