摘要:

A comparison between steady‐state and vibrational techniques for measuring the elastic properties of polymer solutions has shown an unexpected anomaly at high frequencies. The steady‐state technique used was flow birefringence which relates elasticity (recoverable shear,s) to the extinction angle &khgr;:s=2 cot 2&khgr;. The vibrational technique used, both the ultrasonic crystal and the Birnboim instruments, relates elasticity to the loss angle, &dgr;′: cot &dgr;m′=&eegr;2/(&eegr;1−&eegr; solv). The solutions investigated included a monodisperse polystyrene, S111, at concentrations of 2% and 4% in a chlorinated biphenyl (Aroclor 1248), a polydisperse polystyrene, PF‐139 and polyisobutylenes in oil. The same solutions were used in both kinds of instruments.In the lower range of frequencies (taken as equivalent to low shear rates) the steady‐state and dynamic instruments showed excellent agreement for the monodisperse polystyrene. In this range the curve agreed with the Zimm theory. But at higher frequencies the results from the two techniques diverged rapidly. The steady‐state measurements showedsto increase continually, whereas the dynamic measurements showed cot &dgr;m′ to pass through a flat maximum and then decrease slowly out to 2×106sec−1. This lack of agreement was unexpected and is unexplained. It does show that steady‐state elasticity cannot always be predicted from high‐frequency dynamic experiments. The steady‐state viscosity could be satisfactorily predicted from &eegr;1using Pao's theory, but only for the monodisperse polymers.

ISSN:0021-8979    

DOI:10.1063/1.1702923

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The time dependence of melting of morphologically different crystals of polyethylene was investigated. New DTA and calorimeter techniques were developed. A microscope hot stage allowing heating rates of up to 3000°C/min was built for melting‐point determinations. The superheating of extended‐chain crystals was clearly demonstrated by qualitative DTA and isothermal quantitative calorimetry. Smaller crystals and folded‐chain crystals showed decreasingly less superheating. Crystals which are less perfect and have large surfaces show increasing reorganization. A region of approximately zero entropy production melting seems to lie between reorganization at slow heating rates and superheating at faster heating rates.

ISSN:0021-8979    

DOI:10.1063/1.1702924

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

In an effort to understand the molecular mechanism of cold drawing, the tensile creep of unoriented nylon 66 monofilaments as a function of load has been obtained at three different combinations of temperature and moisture content. Qualitatively, an increase in either load, temperature, or moisture content will shift the creep curve to shorter times. These data can be quantitatively correlated by plotting the elongation &egr; against logt−a+bx+cT+c1&sgr;/(c2+&sgr;). Heretis time,xis % H2O,Tis temperature in °K, &sgr; is an average stress in dynes/cm2, anda, b, c, c1, andc2are constants. The parametersbandcare identical with those previously found to apply to the moisture and temperature dependence of the elastic extension of oriented nylon 66 fibers. The free‐volume theory frequently employed for amorphous polymer behavior is shown to be inapplicable since moisture sorption decreases the free volume although it quickens cold drawing. Application of dislocation concepts enables an estimate of the stress dependence of dislocation velocity to be calculated.

ISSN:0021-8979    

DOI:10.1063/1.1702925

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The direct observation of networks of dislocations between platelets of folded‐chain polyethylene crystals has been accomplished using diffraction contrast electron microscopy. The networks are composed of dislocations, primarily in their screw orientation, having Burgers vectors [110], [100], and [010]. The [110] dislocations are invariably dissociated into ribbons and there is evidence that the other two also tend to dissociate. Singularities in the networks are interpreted in terms of interactions between the partial edge dislocations previously detected by moire´ patterns and glissile screw dislocation ribbons. These observations prove that folded‐chain surfaces are sufficiently regular to pack crystallographically and that slip between folded‐chain platelets can occur by the motion of dislocations.

ISSN:0021-8979    

DOI:10.1063/1.1702926

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The deformation of glassy polystyrene under a fixed tensile dead load can take place in three different modes: (a) drawing by formation and propagation of a neck, (b) drawing by proliferation of deformation bands without necking, and (c) homogeneous creep. Which mode is observed depends primarily on stress level and temperature. In homogeneous creep, deformation bands are not found; however, they are always observed in connection with the two types of drawing. At any given temperature a boundary stress is observed above which drawing takes place and below which only creep is observed. This critical stress decreases with increasing temperature. It is believed that this is the stress necessary either for the formation or propagation of deformation bands. It seems likely that drawing by shear band proliferation is the ideal mode which would be obtained in a perfectly homogeneous and uniform sample; however, drawing usually goes over into the necking mode because of nonuniformities in specimen geometry or uneven temperature. Neck initiation is delayed for a certain time interval after application of the dead load. The logarithm of the delay time is found to be a linear function of stress and, also, an approximately linear function of temperature.

ISSN:0021-8979    

DOI:10.1063/1.1702927

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Birefringence measurements on solvent‐cast films of polyacrylonitrile homopolymer under dead load are reported as a function of temperature and as a function of time at constant temperature. It is shown that a large positive birefringence effect is associated with the 240°C aromaticization reaction in PAN. The high negative birefringence state produced by temperature cycling and first reported in Part I of the present series is found to require a small dead load on the sample during cooling for its formation, and is observed to form under conditions of almost constant length. From the interaction between the large negative effect and the positive effects of the nitrile conjugation reaction, it is deduced that the magnitude of the large negative effect is directly proportional to the concentration of nitrile groups present. It is proposed that the 140°C transition in PAN is a result of the association‐dissociation equilibrium between nitrile dipoles on adjacent chains. The temperature dependence of the equilibrium constant for this association is calculated from the birefringence ``cooling curve,'' giving an enthalpy change of 48 kcal/mole, which suggests that association of dipole multiples is involved. Creep, dynamic mechanical, and dielectric loss data from the literature are correlated on the basis of this proposal concerning the nature of the transition.

ISSN:0021-8979    

DOI:10.1063/1.1702928

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The low‐angle light scattering for anisotropic disks (representing two‐dimensional polymer spherulites) is calculated for the cases of (1) the optic axis lying in the plane of the disk and tilted at the angle &bgr; to the radius, (2) the optic axis rotating cylindrically about the radius through the angle &ohgr; while making a constant angle &bgr; to the radius where (a) &ohgr; rotates randomly and (b) &ohgr; varies linearly with the distancerfrom the center of the disk, and (3) affine deformation of the disk to an ellipsoid. For case (1) the scattering pattern is shown to rotate azimuthally through the angle &bgr; from that previously described where &bgr;=0° in agreement with experimental observations on polytetrafluoroethylene. Case (2) is shown to predict an azimuthal dependence of the wide‐angle scattering maxima of a type experimentally observed for banded spherulites. Case (3) predicts a deformation of the scattering pattern of a type observed for deformed polyethylene spherulites.

ISSN:0021-8979    

DOI:10.1063/1.1702929

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Results of probe, spectroscopic, and microwave studies are used to describe properties of a novel cold‐cathode discharge. The use of a brush cathode allows stable operation in the abnormal glow region where some of the usual cold‐cathode mechanisms are greatly exaggerated and determine the features of the produced plasma. The cathode fall is about an order of magnitude larger than for the corresponding normal cold cathode. The brush cathode generates a uniform electron beam in the energy range 1 to 10 kV and a corresponding negative glow with a longitudinal dimension (reaching distance of the beam) one to two orders of magnitude larger than for the normal cathode glow. The large dimensions of the negative glow and the fact that it is well‐behaved (no instabilities and no striations) make it ideal for a whole series of investigations in plasma physics and spectroscopy. The electron density in the helium plasma is in the range 1010to 1014cm−3, the electron temperature in the range 0.05 to 0.10 eV. The negative glow is beam generated (essentially field free) and recombination dominated making it a practically uniform plasma ideal for the study of rate and transport coefficients. Preliminary measurements of the particle balance give a recombination rate of 5×10−10cm3sec−1at 1100°K and an electron density of 3×1012cm−3in agreement with the collisional‐radiative recombination theory developed by Bates, Kingston, and McWhirter. The recombination light emitted by the plasma is sufficiently strong to make spectroscopic methods useful for measurements of the electron density and the electron temperature. Lines in the helium series 2s3S—np3P0are observed up to the quantum leveln=30.

ISSN:0021-8979    

DOI:10.1063/1.1702930

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Two epitaxial defects, the raised triangle and the tripyramid, which appear in silicon and germanium films, are discussed with respect to their origin, crystallography, and electrical characteristics. These crystalline defects propagate from substrate surface contamination, such as refractory particles, and grow more rapidly than the defect‐free film. Referred to the substrate lattice, the raised‐triangle defect is bound by the inclined {111} planes; the defect lattice is rotated 180° around the [111] direction. The raised‐triangle defect provides the conditions necessary for the formation of the tripyramid structure. The three crystallite lattices of the tripyramid are rotated relative to the raised‐triangle lattice's [111] direction such that their [1¯1¯1¯] directions are tilted 4° in the [211] direction. The major exposed faces of the tripyramid crystallites are {1¯1¯0} planes. Both the raised triangle and the tripyramid are centers for soft electrical junctions.

ISSN:0021-8979    

DOI:10.1063/1.1702931

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Instability thresholds for parallel‐pumped magnons in a two‐sublattice ferrimagnet are derived by means of a perturbation method. Cases are considered in which the pump frequency is twice a particular magnon frequency or else the sum of two distinct magnon frequencies—one belonging to each branch of the spinwave dispersion relation. The calculations are made within the molecular field approximation for a single crystal of ellipsoidal shape, and include the effects of the volume dipolar energy as well as those due to unequalgfactors of the sublattices. The thresholds appropriate to an ``unflopped'' antiferromagnet are also given when the dc magnetic field is applied parallel to the anisotropy axis.The prospects for experimentally reaching the thresholds, particularly those occurring at submillimeter wavelengths, are evaluated insofar as is possible; in such cases the possibility exists of utilizing a laser for the pump.

ISSN:0021-8979    

DOI:10.1063/1.1702932

出版商:AIP    

年代:1965

数据来源: AIP