摘要:

The shear wave velocity and attenuation of GR‐S, Butyl, Hevea, Hycar, and Paracril rubbers have been measured in the frequency range 0.2–7 Mc/sec and the temperature range −60°C to 20°C. To minimize errors arising from interfaces in the acoustic path, a double path technique was used. By combining the shear wave data with bulk wave data the dynamic shear, bulk, and Young's moduli, and their associated viscosities, were calculated. The results were coupled with low‐frequency Young's modulus data on the same materials to give information over many decades of logarithmic frequency, and from this the distribution of relaxation times was determined.To a fair approximation, in the range investigated, it was found that the real and imaginary parts of the dynamic Young's modulus could be taken as equal to three times the corresponding values of the dynamic shear modulus. It was found that the classical Stokes assumption, that the bulk viscosity is negligible in comparison to the shear viscosity, was reasonable for some of the rubbers, but not for all of them.

ISSN:0021-8979    

DOI:10.1063/1.1722551

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The Zimm‐Dandliker equation on the effect of light scattering upon the refractive index of colloidal dispersions of spheres is found to be in good agreement with experimental facts. It is transformed to give a new mixture rule from which the refractive index of colloidal spheres, to be expected in absence of light scattering, can be calculated. The latter is needed for particle size determinations in colloidal solutions from light‐scattering measurements to be evaluated on the basis of the Mie theory. For the same purpose, a graphical method is worked out which is less time consuming and yields very accurate results. The graphical method allows one to obtain simultaneously both the refractive index and the diameter of the particles. The method is tested successfully against data obtained by alternate procedures used heretofore.

ISSN:0021-8979    

DOI:10.1063/1.1722552

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

High speed motion pictures were taken of silicone rubber, irradiated polyethylene, Plexiglas II, and aluminum foil while they were being broken. Fractures started internally in some of the silicone samples but at the edges of all of the others. The rates of crack growth and the rates of retraction of the ends of the rubber samples were measured. The velocities with which the cracks grew were compared with the results of the theories of Poncelet, Yoffe, Mott, and Roberts and Wells. Their prediction, that the velocities should be about one half those of transverse waves in the media, was found to represent the data for materials with moduli differing by five decades. After fracture, the ends of the rubber samples contracted with velocities approximately equal to the velocities of longitudinal waves in these samples.

ISSN:0021-8979    

DOI:10.1063/1.1722553

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Simultaneous measurements of flow birefringence, shear and normal stresses on a 15% solution of polyisobutylene in decalin were performed. They proved that the extinction angle measured in birefringence is identical to the orientation of the principal tensile stress calculated from the normal stress measurements and that the birefringence was proportional to the difference in principal stresses in flow. A detailed discussion of the significance of this result is given with a possibility of a far‐reaching generalization of the mechanical behavior of high polymer solutions.

ISSN:0021-8979    

DOI:10.1063/1.1722554

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The changes in average birefringence during retraction have been measured at 85°C for four different oriented polystyrene monofilaments. The relationship between birefringence decay and length decrease is not linear: the birefringence decay is relatively more rapid than the length decrease, in all cases. Also, the birefringence‐length relationship seems to be essentially independent of retraction temperature. Birefringence decay rate can be reduced by a factor of 1000 by not allowing the filament to retract. One of the filaments shows an interesting change in sign of the birefringence, from negative to positive, in the final stages of the retraction. The birefringence decay data for the four filaments could not be reduced to any universal curve by plotting either birefringence or relative birefringence against any simple functions of length or extension (all the filaments were made from the same polymer, so that a reduction to some such common basis should be possible). Analyzing the data in terms of a distribution of retardation times, by assuming that birefringence decay and length decrease are in some constant ratio for each retardation time (where the additional parameter, time, is introduced), also failed to reduce the data to a single common curve. Internal stress may be an essential parameter for a proper characterization of the birefringence.

ISSN:0021-8979    

DOI:10.1063/1.1722555

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The radial distribution of birefringence, &Dgr;(r), has been measured in Filament # 16 at a series of time values up to 600 hrs, during retraction at 85°C. The birefringence distribution was obtained by cutting wedges on the end of the filament samples and analyzing the optical interference pattern down the center line of the wedges, under a polarizing microscope. The rates of birefringence decay at different points along the radius are not the same, or even proportional; the interrelationship is apparently fairly complex. Birefringence values near the center of the filament change from negative to positive sign in the long‐time region, while values near the exterior surface remain negative throughout. Values of the diameter average birefringence calculated from &Dgr;(r) are in close agreement with values obtained from direct experimental measurement. A cross‐sectional‐area average birefringence was also calculated from &Dgr;(r); this quantity decreases monotonically toward zero, with no transition to positive values in the long‐time region, and therefore corresponds more closely than the diameter average birefringence with the observed length changes. The area average is nearly proportional to the diameter average, however, except for values near zero.

ISSN:0021-8979    

DOI:10.1063/1.1722556

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The effective dipole moment of each polar group in polar‐polar or polar‐nonpolar copolymers is expressed in terms of the moment of an equivalent isolated polar group and the average of the vector sum of the moments of all of the polar groups surrounding it. If effects of short‐range forces leading to angular correlations of dipole pairs are neglected except for interactions between nearest neighbors along the chain, an expression for the average of the effective moments of the two types of polar groups present can be written as a function of the composition of the copolymer in the form⟨&mgr;2⟩Av=x1(&mgr;12)eff+(1−x1)(&mgr;22)eff=L+Mx1+Nx1P11(x1).Here,x1is the mole fraction of one of the monomers in the copolymer, (&mgr;12)effand (&mgr;22)effare the squares of the effective moments of the two polar groups.L, M, andNare constants involving the dipole‐moments of the equivalent isolated groups and the average cosines of the angles between nearest‐neighboring dipole pairs of types 1 − 1, 1 − 2, and 2 − 2. The quantityP11(x1) is the probability of a monomer of type 1 adding to a free radical of its own kind at the end of a growing chain in the polymerization mixture. It can be calculated fromr1andr2, the reactivity ratios of the two monomers, and the composition of the monomer feed mixture. Interactions extending farther along the chain than nearest neighbors introduce terms in higher powers ofP11(x1).Onsager's equation for the dipole moments of molecules in mixtures of polar liquids has been modified to express the average of the effective moments of the polar groups present in terms of the dipolar contribution to the dielectric constant,&egr;s−&egr;∞. Using this equation, values of ⟨&mgr;2⟩Avhave been calculated from measurements of&egr;s−&egr;∞for several copolymers of poly‐(p‐chlorostyrenestyrene) representing the whole range of copolymer composition. Measurements were made on bulk copolymers at temperatures above the glass‐transition temperature. A curve of the formL+Mx1+Nx1P11(x) has been fit within experimental error to the calculated values of ⟨&mgr;2⟩Avover the whole range of copolymer composition. This indicates that within the limits of error of this work only nearest‐neighbors on the polymer chain contribute to the effective moments of the polar groups. The average cosines have been calculated for the angles between each of the three kinds of dipole pairs in the copolymer.

ISSN:0021-8979    

DOI:10.1063/1.1722529

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The conductivity induced in polyethylene and Teflon by bombardment with x‐rays from a 2‐Mev Van de Graaff and gamma rays from Co60has been investigated as a function of time, temperature, geometry, exposure rate, and applied electric field. Within the range of the variables studied, the observed photocurrents were directly proportional to the exposure rate and the applied electric field. The photocurrent could be divided into three components, the current due to the action of monodirectional photons, the increase in this current due to the influence of the electric field, and a component of less than 10% made up of all other possible charge carriers. During irradiation, the conductivity increased by a factor of about 103. Between 78°K and 273°K the photocurrent was nearly independent of temperature. In general, the conductivity of Teflon was greater than that of polyethylene.

ISSN:0021-8979    

DOI:10.1063/1.1722531

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

It is shown that reagent CP‐4 plus ferric ions can be used to detect the positions of dislocations in LiF crystals. The technique is highly selective and both edge and screw (110) [11¯0] dislocations can be detected and distinguished. A double etching method is used to observe, for the first time, the glide and climb of individual dislocations in LiF.

ISSN:0021-8979    

DOI:10.1063/1.1722532

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The coercivity and form of iron deposited from the vapor were found to be influenced by the strength of a magnetic field applied during condensation. The intrinsic coercivities of powders deposited at −196°C and at −78°C in fields of 800 to 900 oersteds were about 120 oersteds. The greater fraction of particle diameters ranged from 10 to 20&mgr;. No appreciable quantity of submicron particles was detected in several samples examined by electron microscopy. Line‐broadening in the diffraction patterns of the vapor‐deposited iron was attributed to strain, the magnitude of which was theoretically sufficient to account for the observed coercivities.

ISSN:0021-8979    

DOI:10.1063/1.1722533

出版商:AIP    

年代:1956

数据来源: AIP