Elastic scattering of vector polarized deuterons by 4He

The vector analyzing power iT₁₁ for deuterons elastically scattered by ⁴He has been measured for c.m. angles between 51.8° and 158.9° and deuteron energies in the range 11.5 to 17 MeV. The results show that d-α scattering is a convenient analyzer for deuteron vector polarization in the energy range investigated, since the angular dependence of iT₁₁ is fairly insensitive to energy and the magnitude of iT₁₁ is quite large at some angles.     

STABILITY PROPERTIES IN A MODEL OF FORAGE-UNGULATE-PREDATOR INTERACTIONS

This paper explores stability in a three differential equation model of ungulates, their forage and their predators. Stability most depends on the feeding efficiency of exploiters relative to the growth potential of those exploited. Stable, natural systems probably require relatively inefficient exploitation, refuges for the exploited population or prey switching by generalist exploiters. Increased exploitation of ungulates enhances ungulate-forage stability but, lacking prey refugia, eventually overexploits the ungulates. Multiple, stable equilibria representing sustained ungulate yield and ungulate extinction probably characterize big game. Under traditional management, game populations will approach the zone of attraction of the second of these equilibria.     

Four-wave interactions in plasmas

The interaction of four waves in a warm magnetized plasma is studied using a Lagrangian formalism. General expressions for coupling coefficients are presented and the coupled-mode equations describing the interaction are derived.     

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH₂) and chlorination (14–22 Cl/1000 CH₂). Both linear and branched polymers were studied. All of the relaxations between the melt and ?196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (T_max at constant frequency or log f_max at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log f_max and T_max on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing ε_max with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (～10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.     

Dielectric evidence for thermal creation of defects in polyethylene crystals

A recent study of the effect of pressure on the dielectric α process in slightly oxidized linear polyethylene showed that the intensity of the process decreases with increasing temperature. This phenomenon was attributed to thermal generation of conformational defects. In the present work a quantitative interpretation of that data is carried out to arrive at defect creation energy parameters. It is found that the creation energy lies in the range of 3–6 kcal/mole and that it has a high degeneracy or statistical weight (in the range 40–800). The statistical weight is interpreted as largely arising from the number of sites in a given crystalline chain for a defect to occur, that is, the number of CH₂ units spanning the crystal. It is speculated that kinks are the likely conformational defect.     

Dielectric relaxation in the odd-numbered polyamides: Nylon 7-7 and nylon 11

Odd-numbered polyamides have dipoles with components that point in the same direction in the planar zigzag conformation (rather than alternating as in the even-numbered polyamides). An investigation of the dielectric properties of two odd nylons was undertaken here to see if dipole correlation resulting from this effect could be detected. It was found that the dielectric constant and relaxation strength associated with the α loss process (amorphous phase glass–rubber relaxation) when normalized for dipole concentration was indeed markedly higher in the odd nylons. The effect of uniaxial orientation induced by solid-state extrusion on the isotropy of the dielectric properties was also studied. A previous study on nylon 6–10 found a reduction in relaxation strength of the α process in both the parallel and perpendicular directions but with the perpendicular direction being much stronger than the parallel. A similar result was found here for the odd polyamides. Thus the reduction in intensity appears to be due to effective reduction in the amount of amorphous material and not due to any change in dipole correlation induced by orientation. The anisotropy in relaxation strength can be explained by a composite material effect resulting from parallel stacking of crystal lamellas and interlamellar amorphous material and without invoking any anisotropy in the amorphous-phase dielectric strength. Thus there is no reason to conclude that the amorphous phase undergoes orientation as a result of the crystal-phase orientation.