摘要:

Vacuum audio frequency complex admittance measurements have been performed on poly(ethylene oxide) using a fully automated dielectric spectrometer. Measurements have been made over the temperature range 5.5–320 K for molecular weights from 1.85×104to 4×106. The &ggr; relaxation is essentially unchanged over the range of molecular weights studied. This represents evidence against assigning the &ggr; relaxation to the motion of end groups. Next, previously reported data for the &ggr; relaxation are reanalyzed using a reduced plot. The activation volume is found to be 3–4 cm3/mol in good agreement with the previous work and the small value implies that the relaxation is associated with the motion of very small segments of the polymer chain. Next, the relaxation associated with the glass transition &agr;ais only observed in temperature for some samples but is observed in both frequency and temperature for others. The strength of &agr;adecreases monotonically as molecular weight decreases. Finally, &agr;awas studied up to pressures of about 0.2 GPa for 5×106molecular weight material. The results for the shift of &agr;awith pressure are in qualitative agreement with recent theories.

ISSN:0021-8979    

DOI:10.1063/1.335834

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The technique of determining the longitudinal compliance over a wide dynamic range (from 10−1to 106Hz) using Rayleigh–Brillouin scattering has been studied. The method of computing the longitudinal compliance spectrum from the time correlation of density fluctuations of poly(vinyl acetate), as determined by the photon correlation spectroscopic technique, is illustrated.

ISSN:0021-8979    

DOI:10.1063/1.335835

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Structure‐property relationships for (bio)polymer systems, especially for crosslinked network systems, are discussed and calculated. Physical properties of the system, e.g., modulus of elasticity, gel fraction, etc., are expressed as certain functionals of the system’s actual structure. A new topological gelation criterion for the sol‐gel transition point is proposed. For the first time the gel fraction for a system with closed cycles is calculated, using a new method of equivalent crosslinks. In contrast to many existing theories, the present theory may easily be applied to crosslinking systems far from thermodynamic equilibrium and to nonrandom systems. It is suggested that a new kind of auto‐oscillatory phenomenon, namely an oscillating sol‐gel transition, is to be found in open crosslinked systems, e.g., in actomyosin solutions maintained far from equilibrium by a constant influx of ATP. Such phenomena may be of great importance in living cells. The theory is easily adaptable to a broad spectrum of systems to which probabilistic‐topological notions similar to the ones introduced here may be applied, e.g., to entanglement networks for systems of flexible chains or to contact networks for systems of stiff molecules.

ISSN:0021-8979    

DOI:10.1063/1.335836

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We examine a reciprocating heat engine which necessarily operates far from equilibrium and about an unstable steady state. The piston of the engine is driven by the nonlinear coupling of the working fluid to an external light source which provides high quality heat and to the environment into which waste heat is dumped. We determine the piston trajectories that optimize two different criteria of process performance, the maximization of work output, and the minimization of entropy production. The trajectories optimizing different performance goals are qualitatively different. In engines not dominated by friction losses, the cycle optimizing work output requires that the expansion stroke begins with a slight compression and the temperature of the working fluid increases briefly.

ISSN:0021-8979    

DOI:10.1063/1.336281

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In electron guns, three lenses are present: the cathode lens, projecting a crossover in front of it, the main lens that images the crossover on the target, and the prefocusing lens situated in between, adapting the aperture angle to the main lens. The spot size at the target is calculated for different positions and strengths of the prefocusing lens. The analysis is based on a phase space representation of the rotationally symmetric beam. It is shown that by proper choice of the position and strength of the prefocusing lens, the resulting selective prefocusing lens (SPL) reduces the combined spherical aberration of the cathode lens and main lens by 45%. The intensity distributions in the spot with and without the SPL differ greatly and a more uniform spot intensity is obtained.

ISSN:0021-8979    

DOI:10.1063/1.335837

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Level populations of the 2s3pand 2s23pconfigurations in ions of the Be I and B I isoelectronic sequences can be inverted, with respect to populations of the 2s3sand 2s23sconfigurations, by electron collisional pumping. In the case of B I ions, the process is analogous to the process for Ne‐like ions, which has lately received considerable attention. In the case of Be I ions, the inversion is a consequence of the slow radiative decay of the 2s3p3P2and 2s3p3P0levels. Level populations are calculated for Be‐ and B‐like ions with atomic numberZbetween 18 and 36. For each sequence, 20 levels are involved with principal quantum numbernequal either to two or three. Using the level populations, gain coefficients are calculated for transitions of the type 2s3s−2s3pand 2s23s−2s23p. Finally, the opacity of the 2s2p−2s3sand 2s22p−2s23stransitions are discussed, for plasma parameters for which a reasonable gain can be achieved.

ISSN:0021-8979    

DOI:10.1063/1.335838

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Intense laser light diffraction is observed from a low‐density spark (ne≲nc/100) outside the focal cone up to an angle of 14°. About 10% of the laser pulse energy is spread by diffraction. Temporal and spatial features of this effect are discussed in terms of self‐focusing and hydrodynamics plasma motion.

ISSN:0021-8979    

DOI:10.1063/1.335839

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The lateral‐mode behavior of a semiconductor‐laser array is analyzed after including the effect of injected charge carriers on the active‐layer dielectric constant. The wave equation is solved self‐consistently and the lowest‐threshold lateral supermode is obtained by treating the whole array as one unit. The resulting mode profile incorporates the effects of gain guiding, carrier‐induced index antiguiding, and built‐in index guiding. The analysis is therefore applicable to gain‐guided as well as index‐guided arrays; near‐ and farfields for both kinds of arrays are obtained and compared to those obtained without gain‐loss considerations using the plane‐wave diffraction theory or the coupled‐mode theory. For gain‐guided arrays, carrier‐induced antiguiding plays an important role and leads to a transition from twin‐lobe to three‐lobe farfields for closely spaced, narrow array elements. For weakly index‐guided arrays the in‐phase or out‐of‐phase coupling between neighboring elements can occur depending on whether the gain is laterally homogeneous or not. The calculated results are in qualitative agreement with the reported experimental observations. The present analysis is useful to understand the role of various guiding mechanisms as well as to provide guidelines for the device optimization.

ISSN:0021-8979    

DOI:10.1063/1.335840

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In InGaAsP/InP buried heterostructure lasers, a failure mode connected to the active region, not to the burying region, is observed. The degradation is related to a decrease in the carrier lifetime or a decrease in nonradiative lifetime due to degradations at the edges of the active region. Practically, the bad devices may be screened out by the hard screening method, however, the failure mode could determine the reliability of the devices under high‐temperature or high‐power operation, so care should be taken during forming the mesa and a regrowth of the burying layer.

ISSN:0021-8979    

DOI:10.1063/1.336298

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Wave propagation in a porous column consisting of sand particles is studied by means of a shock‐tube technique. Experimental results are presented for both air and water as pore fluids. Quantitative information on pore‐pressure amplitudes, wave speeds, and damping is compared with theoretical predictions. Good agreement is obtained for the air‐filled pores. For the water‐saturated pores the two‐wave structure is observed, and the corresponding pressure‐amplitude ratio is consistent with the observed first‐wave speed. The transient permeability is approximately one‐third the steady‐state value. Added mass is significant.

ISSN:0021-8979    

DOI:10.1063/1.335841

出版商:AIP    

年代:1985

数据来源: AIP