Volume 26, Issue 9, 01 September 1955
Index of content:
- SPECIAL ISSUE ON HIGH POLYMER PHYSICS
26(1955); http://dx.doi.org/10.1063/1.1722151View Description Hide Description
When oriented polystyrene monofilaments are heated to temperatures in the neighborhood of the transition temperature they retract to an unoriented state at conveniently measurable rates. Curves of length as a function of time have been measured at five different temperatures (75°–95°C, at 5° intervals) for two filaments with different initial amounts of orientation.
When the retraction curves are plotted as length vs log time, it is found that the curves at different temperatures are not superposable by lateral shift along the logarithmic time scale, as would be true for simple viscoelastic behavior. Activation energies calculated from these curves are a function of time as well as of temperature; the activation energy at any given temperature tends to level out to a final constant value with increasing time, however. The rate of change of activation energy with time increases with increasing temperature, so that the final value is attained more quickly; the final value also decreases systematically with increasing temperature.
Final activation energy values increase rapidly with decreasing temperature in this region and are very high (300 kcal at 75°). It is postulated that specific volume changes are responsible for the time‐dependent activation energies observed in these experiments. It is also of interest that these experiments appear to isolate the viscoelastic mechanism involving molecular configuration change from that involving local molecular distortions.
26(1955); http://dx.doi.org/10.1063/1.1722152View Description Hide Description
In general, extension of an elastomer results in a degree of preferred orientation of the molecular chains composing the amorphous phase. Therefore the amorphous fraction of a partially crystalline elastomer must be related to the integrated intensity of the amorphous diffraction halo rather than to the intensity at any one azimuth. A noteworthy exception is natural rubber, for which simple meridional measurements suffice.
A Geiger‐counter apparatus with beam monitor and temperature‐controlling accessories is described for making accurate measurements of the x‐ray intensities scattered at any azimuth and at small or moderate Bragg angles. Measurements of crystallinity in natural rubber are in essential agreement with the findings of previous workers. When polybutadiene is extended at room temperature, molecular orientation occurs, but little if any crystallization.Measurements at lowered temperatures show that the crystalline fraction becomes appreciable at about 0°C and that it increases with further reduction in temperature and with increasing extension ratio. Preferred orientation of the crystalline regions in extended polybutadiene has been measured quantitatively with the object of providing jointly with birefringencemeasurements a value of the birefringence of a single crystal of polybutadiene.
26(1955); http://dx.doi.org/10.1063/1.1722153View Description Hide Description
The semitheoretical expression, ,where fc =volume fraction crystalline, Δn 0=observed birefringence,c=the stress‐optical coefficient, s=the stress on actual cross section, F=a known function of crystallite orientation, and Δnc 0=the birefringence of a single polymer crystal, is proposed for relating birefringence, stress, and degree of crystallinity in polycrystallineelastomers under tensile stress. This expression is shown to yield results in substantial agreement with x‐ray results for the degree of crystallinity of 5°C polybutadiene when Δnc 0=0.157. Birefringencedensity data on natural rubber (due to L. R. G. Treloar) are shown to be a special case of the foregoing expression, yielding a value Δnc 0=0.218 for natural rubber which is in good agreement with the theoretical value for this quantity.
26(1955); http://dx.doi.org/10.1063/1.1722154View Description Hide Description
Evidence is given showing that the equation, logη=3.4 logZ+K, represents an empirical law of flow which holds generally for sufficiently long flexible chain molecules in bulk or in solution. Here η is the viscosity,Z is the number of atoms in the chain, and K is a constant which is dependent on the polymer type and on the temperature. All of the available data which cover wide molecular weight ranges for linear (and branched) molecules in bulk or in solution, including results on polyesters, polyamides, polystyrene, polyisobutylene, polydimethyl siloxane, and polymethyl methacrylate, support this conclusion. For chains having fewer than a critical number, Zc , of chain atoms (Zc being characteristic of the polymer species) the dependence of η on Z is less severe but more complex. These results are in semiquantitative agreement with the recent theory of F. Bueche. On the basis of this theory, values for the concentration of chain entanglements in various polymers are obtained from the observed values of Zc .
26(1955); http://dx.doi.org/10.1063/1.1722155View Description Hide Description
The problem considered is that of two bodies coming into normal contact over smooth curved surfaces. The initial relative velocity and the total kinetic energy involved is low. Contact is, however, confined to such small volumes of the objects involved that very high concentrations of energies are obtained at those places. The rates of application of stress are correspondingly high. The Hertz solution to this type of problem provides a useful approximation in the case of elastic objects.
In the present treatment one of the impinging bodies is of viscoelastic material. Two viscoelastic bodies may also be treated if they are of the same material. The Laplace transform method is used to obtain the viscoelastic expression for the force developed between the two surfaces. This expression is then applied to the impact case. The expression can also be applied to other truly static cases; e.g., contact between gear tooth surfaces.
The results are of technological interest, since it is not possible to say if a plastic is suitable for a certain category of impact applications, unless the rates of straining or stressing obtained in those applications can be estimated.
26(1955); http://dx.doi.org/10.1063/1.1722156View Description Hide Description
The low coefficient of friction for Teflon sliding on Teflon, as observed previously by others, has been found in this study to hold only if low sliding speeds and newly prepared surfaces are used. At high sliding speed, the nature of the surface is irreversibly changed such that subsequent sliding at low speed reveals a two‐ to threefold increase in the friction coefficient.
In addition to the effect of high speed sliding, there are reversible changes in the friction of Teflon on Teflon dependent in part on temperature. As the temperature of Teflon is increased from below room temperature, a sharp and pronounced increase in the coefficient of friction is observed in the vicinity of 20°C. The existence of a phase transition in Teflon at this temperature indicates a close correlation between the frictional and the structural properties of this material.
26(1955); http://dx.doi.org/10.1063/1.1722157View Description Hide Description
Glass temperatures (Tg ) of pure and plasticized elastomers have been obtained from dilatometric measurements and from the temperature dependence of angular response to torsional stress. The depression of Tg by a diluent, determined by either method, is a convenient measure of its plasticizing ability. Some compounds which are normally crystalline above Tg of the pure polymer, but nevertheless compatible with the polymer, depressed the glass temperature. The polymer‐diluent compatibility at low temperatures results either from supercooling, the depression of the melting point of diluent by polymer, or possibly a combination of both phenomena. Tg decreased as the diluent loading increased to a more or less well defined limit, beyond which further dilution had negligible effect. For diester type plasticizers both ΔT gmax and the limiting diluent concentration were linear functions of compatibility, as defined by the extent to which standard polymer vulcanizates were swollen by plasticizer. The viscosities and apparent activation energies for flow (Ev ) of hydrocarbon oils correlated with their low temperature plasticizing efficiencies. Pour points and glass temperatures may also be significant properties of the diluents. It is concluded that low Ev and high solvency for polymer are desirable properties for low temperature plasticizers.
26(1955); http://dx.doi.org/10.1063/1.1722158View Description Hide Description
The protonmagnetic resonance absorption has been observed in several linear polyamides from 77°K to 400°K. The absorption line narrows gradually with increasing temperature up to about 280°K. A marked decrease in line width with increasing temperature occurs over a span of 20 or 30 degrees within the range of 280 to 400°K, and is presumed to result from the onset of rotation of chain segments. In a series of polyamides possessing an even number of carbon atoms between polar groups, this transition occurs at a lower temperature as the hydrocarbon portions are made longer (polydecamethylene octadecanediamide compared to polyhexamethylene adipamide). In the copolymer polyhexamethylene adipamide‐sebacamide, the transition region is at a lower temperature than in the pure adipamide or pure sebacamide. The x‐ray diffraction studies on polyamide fibers show that the chain packing becomes more symmetrical over these same temperatures. The resonance results are interpreted in terms of simple models.
26(1955); http://dx.doi.org/10.1063/1.1722160View Description Hide Description
One‐dimensional compression measurements have been made on samples of natural rubber gum stock. The experimental stress‐strain curve and the theoretical relation f=G(L−1/L 2) agreed quite well over the entire range (0%–32%) studied. No hysteresis loop was evident.
The friction at the faces of the sample was eliminated by a new method of lubrication. The sample essentially floated between two thin films of a liquid which flowed under pressure between the sample and compressing plates allowing uniform lateral expansion of the sample.
26(1955); http://dx.doi.org/10.1063/1.1722161View Description Hide Description
An instrument has been constructed to allow simultaneous measurements of stress and birefringence in polymer samples to be made as a function of time and temperature. The stress is measured by attaching one end of the sample to a cantilever beam near the fulcrum; the small deflection of the beam is indicated by means of a differential transformer, which in turn is attached to an automatic recorder.
The birefringence is measured with a Senarmont compensator (quarter‐wave plate method). Measurements can be made either manually or with a continuously rotating analyzer. This latter method permits the automatic recording of the birefringence and is also adaptable to the study of rapid birefringence changes. The sample is housed in a copper pipe surrounded by a thermostatically controlled liquid bath.
Some data are presented showing the use of this apparatus in measuringbirefringence and stress changes during stress relaxation experiments (sample held at constant elongation), birefringence changes during creep under constant load, and stress‐optical coefficients.
- REGULAR CONTRIBUTED ORIGINAL RESEARCH
26(1955); http://dx.doi.org/10.1063/1.1722162View Description Hide Description
The effectiveness of acoustic absorbing wall materials in delaying the onset of detonation in gases is explored. Experiments are reported for stoichiometric H2–O2 mixtures in two‐inch diameter cylindrical tubes. The results show that the detonation induction distance may be increased by as much as a factor of two by making the walls of the tube of porous sintered bronze.
26(1955); http://dx.doi.org/10.1063/1.1722163View Description Hide Description
Radial distribution functions of the microcrystalline particles in fibers are derived from the x‐ray scattering at small angles. Particle distribution functions are obtained by application of the method of Fourier synthesis to the x‐ray data, by inversion of the intensity scattered under conditions where interparticle interference plays a part.
Different distribution curves are obtained with different fibers, investigated under the same conditions.
The distribution curves obtained with the same fiber investigated under different conditions of swelling, are consistent with the wider separation of the particles at higher degrees of swelling. A quantitative explanation is thus given of the differences between the scattered intensities at different degrees of swelling.
The degree of crystallinity is calculated from the distribution curves, so obtained, in combination with the particle sizes determined in preceding papers.
26(1955); http://dx.doi.org/10.1063/1.1722164View Description Hide Description
The general utility of an approximate expression for the first Townsend coefficient of a gas, derived several years ago by Druyvesteyn and Penning, has been tested by introducing into the theoretical treatment somewhat more realistic assumptions regarding the relevant scattering cross sections of the gas molecule. It has been demonstrated by numerical calculations that the functional dependence of the Townsend α upon the electric field and the gas pressure is, within certain limits, insensitive to the assumed energy dependence of the elastic and inelastic cross sections. The functional form of the inelastic cross section does, however, influence the way in which the ionization energy of the gas molecule enters in the exponential part of the expression.
26(1955); http://dx.doi.org/10.1063/1.1722165View Description Hide Description
Experimental techniques for measuring the magnetic susceptibilitytensor components in microwavecavities are described. General expressions for the perturbation of microwavecavities by the magnetic properties of ferrites are developed. Experimental results are given for Ferramic 1331 measured at 9165 Mcps. Since these results indicate that both the real and imaginary parts of the tensor components χ and κ are approximately equal to each other in the resonance region, only measurement of the diagonal components may suffice to specify the complete tensor. However, whenever the small differences between χ and κ are desired, measurement of the tensorsusceptibility in a degenerate cavity will be necessary. An attempt has been made to compare the experimental results with the Bloch‐Bloembergen magnetic resonance theory as applied to circularly polarized excitation.
26(1955); http://dx.doi.org/10.1063/1.1722166View Description Hide Description
A theory is presented for the tensile strength of rubbery plastics under dynamic test conditions. It is found that a nearly universal curve may be plotted relating the tensile strength of rubbers to the time of application of the load in a simple creep test or to the shear rate in a conventional type tensile test. The dependence on chain length, degree of cross linking, and temperature is also predicted.
Experimental results for the tensile strength of poly butyl methacrylate in the range 30–95°C are presented. These are shown to agree with the theory within the experimental error. The experimental testing scale extends over a range of eight decades when use is made of a temperature—time superposition principle predicted by the theory.
By comparing the tensile creep and tensile strength curves for poly butyl methacrylate, it is possible to find the size of a segment as defined in the theory of viscoelasticity. One segment is found to contain about six monomer units.
26(1955); http://dx.doi.org/10.1063/1.1722167View Description Hide Description
A light microscope study of etchedgermanium crystals has shown the dislocation distribution in them. The dislocations form a network in the crystal.
The formation of small angle grain boundaries is discussed.
Screw dislocations with large Burgers vectors have been found in some germanium crystals.
26(1955); http://dx.doi.org/10.1063/1.1722168View Description Hide Description
The effect of plates of nitrides on the coercive force of iron has been determined. A pronounced particle size effect was observed. The maximum effect on the coercive force was produced by particles whose largest dimension was of the order of the domain wall thickness, δ. For particles much larger than δ, an expression given by Néel for the effect of inclusions is found to be valid. The results of this study are in good agreement with earlier results reported for the effect of carbides on the coercive force of iron.
26(1955); http://dx.doi.org/10.1063/1.1722169View Description Hide Description
A new construction for an electromechanical transducer makes use of the Wiedemann effect to produce a magnetostrictive coupling between an input and an output winding, which are orthogonally disposed so that there is no direct inductive coupling between them. A first‐order theory of the operation of the transducer is given, and is used to derive an equivalent electrical circuit. The more important characteristics of the transducer are discussed in terms of the equivalent circuit, and are shown to be verified by experimental models. A method for deriving the equivalent circuit components by experiment is outlined, and the performance of a representative model is presented.
26(1955); http://dx.doi.org/10.1063/1.1722170View Description Hide Description
In electron tubes utilizing long electron beamsionization of residual gases can produce a positive space charge which can partially or wholly neutralize the negative space charge of the beam. Such neutralization has, under special conditions, been successfully utilized in focusing electron beams. In certain electron devices requiring long electron beams, this type of focusing is inadequate and an axial magnetic field is employed to focus the electron beam. However, when magnetic focusing is used, the assumption is usually made that the electronic space‐charge fields of the beam are not neutralized by positive ions. Measurements have been made on a magnetically focused electron beam yielding information on (a) the longitudinal drainage of positive ions from the electron beam and (b) the degree of positive‐ion neutralization of the electron beam. It is pointed out that localized ion traps may be present in the beam if the beam diameter varies periodically along its length because of the applied magnetic field.
26(1955); http://dx.doi.org/10.1063/1.1722171View Description Hide Description
A new cathode is described consisting of an oxide‐impregnated nickel matrix. A sinterednickel matrix of controllable porosity is prepared and impregnated with solutions which precipitate a mixed carbonate within the matrix. The resulting cathode is a dispenser type with a minimum of surface coating. The cathodes can be activated by schedules which approximate those of a normal oxide‐coated cathode. Pulse‐emission tests of these cathodes show properties intermediate between the oxide‐coated cathode and the L‐cathode, and of the same order as the molded cathode described by McNair, Lynch, and Hannay.