Volume 21, Issue 6, 01 June 1950
Index of content:
21(1950); http://dx.doi.org/10.1063/1.1699688View Description Hide Description
When thin films of polar plastic materials are subjected to d.c. potentials of 10 to 300 volts per mil they exhibit current fluctuations (with frequencies in the range of 60 to 1000 c.p.s.) which last for minutes and which even persist for a few seconds on removing the voltage. These fluctuations are about 1000 fold above the noise level of the circuit and are about 1 percent of the steady d.c. current. They eventually die out but can usually be restored by reversing the polarity. This noise is greater the more polar the nature of the polymer and the higher its moisture content. The effect seems to arise from water or impurity ions which can jump whenever a hole opens between polymer chains and is thus an activated rate process sufficiently slowed down by the highly viscous nature of the polymeric medium to be readily observable. On an oscillograph screen it is possible to see both the forward and the backward diffusion. This noise is most pronounced in Cellophane, Nylon, and copolymers of vinylidene chloride with acrylonitrile, but also appears with polystyrene stored at 86 percent relative humidity. Films of methylcellulose do not show the effect. Numerically the effect seems to arise from the random motion of groups of several hundred ions as a unit. As an extension of these ideas it is suggested and demonstrated that the moisture vapor diffusion constants for a series of polymers increase with the frequency of the loss factor maximum while the moisture vapor permeability increases with the direct current conductivity.
21(1950); http://dx.doi.org/10.1063/1.1699689View Description Hide Description
Emulsion‐type polymerization reactions have been accelerated by irradiation with ultrasonic energy.
A magnetostriction oscillator at 15 kc gave essentially the same results as a piezoelectric oscillator at 500 kc. The rate of formation of polystyrene was doubled and higher final yields obtained using about 0.03 watt of vibrational power per cc of latex. Experiments were made to separate local heating effects from those due to vibrational energy.
A thermostatted stainless steel cell was developed for irradiating butadiene emulsions under pressures up to 60 p.s.i. About ½ watt of vibrational power per cc of latex doubled the rate of reaction for GR‐S, Redox, and Redsol formulas.
The ultrasonic intensity apparently must exceed a critical value before any appreciable accelerative effect is observed. Above the critical intensity, the time to reach a given yield of polymer is approximately inversely proportional to the power used.
21(1950); http://dx.doi.org/10.1063/1.1699690View Description Hide Description
The evidence for and against the interpretation of the ``second‐order'' transition in polymers in terms of a thermodynamic transition of the second order is reviewed and some new experimental data concerning transition in mixtures of polymers are presented. It is shown that the relaxation theory is insufficient to explain the transition, as observed by mechanical, electrical and thermal measurements. The usual objections which are raised against the assumption of a thermodynamic transition are shown to be invalid. Various molecular processes responsible for a thermodynamic transition are discussed and it is shown that the general mechanism of ``hindered'' rotation (changes in rotational configuration) is the most likely.
21(1950); http://dx.doi.org/10.1063/1.1699691View Description Hide Description
Cast films of polystyrene were oriented by stretching at temperatures above the softening temperature of the material and then cooled. A factorially designed experiment was carried out in which the amount of stretching, temperature of stretching, time held in the stretched state before cooling, and effect of several cycles of stretching before cooling were investigated as they affect the birefringence, the stress‐strain properties, dynamic modulus and damping, and the softening temperature of polystyrene.
The values of many of the mechanical properties are more closely related to the birefringence than to the amount of hot stretching. In general, the value of the stress‐strain properties in the direction of hot stretch, the dynamic modulus, and the birefringence all increase with an increase in the amount of stretching, a decrease in temperature, an increase in rate of cooling after stretching, and a decrease in the number of times the film was stretched before finally cooling. The softening temperature is decreased in direct proportion to the birefringence.Annealed films give higher softening temperatures than films which have been stretched and then allowed to shrink before cooling.
21(1950); http://dx.doi.org/10.1063/1.1699692View Description Hide Description
The slope of the force‐extension curve in the Hooke's law region can be measured non‐destructively, for animal fibers, at three to fifteen minute intervals. The change of this Hookean slope with time is used to follow the course of the heterogeneous reaction between wool and a reagent.
Data are presented on the interaction between concentrated neutral salt solutions and wool fibers. These results indicate that wo processes occur: (a) Relatively rapid hydration or dehydration of the fiber until equilibrium with the activity of water in the salt solution is attained; (b) Relatively slow absorption of salt ions by the fiber. Very concentrated salt solutions dehydrate the fiber so completely that absorption of salt ions is not possible. Swelling experiments and experiments involving other mechanical properties confirm these hypotheses. The interaction between wool fibers and aqueous solutions of large organic molecules follows a similar pattern.
21(1950); http://dx.doi.org/10.1063/1.1699693View Description Hide Description
An explicit relationship is set forth for the time‐temperature dependence of the viscoelastic phenomena in the superelastic polymers. An empirical equation that was found to represent adequately the above‐mentioned relationship over the entire multiple‐temperature curve is of the form: logtc = [C α/(T−b)]+C β. Experimental verification is given for the equivalent influence of time and temperature on the creep and recovery of compounds based on Hevea, GR‐S, Neoprene GN, Butaprene, and butyl rubber. A comparison of the empirical equation with that of a theoretical reaction‐rate equation of Tobolsky and Eyring indicates a temperature dependence of the energy of activation.
21(1950); http://dx.doi.org/10.1063/1.1699694View Description Hide Description
The softening of a hard resin, polyvinyl chloride, by a liquid, dioctyl phthalate, is a set of processes occurring in sequence. Imbibition of the liquid is accompanied by volume reduction in the system. Dilatometer methods evaluate the amount and rate of volume decrease. The rate is temperature dependent, activation energy about 80 kcal. Solution of liquid into the resin follows with no further volume, but large dielectric change. Dielectric changes are measured on material in a cylindrical condenser, using a Schering bridge, and are rate dependent, activation energy about 110 kcal. Data are best described, assuming that segments of the polymer molecule ``react'' independently of one another.
Studies in Plasticization of Polyvinyl Chloride. II. Plasticizer Variations in PVC‐Plasticizer Systems21(1950); http://dx.doi.org/10.1063/1.1699695View Description Hide Description
Polyvinyl chloride resin is softened by certain liquids, called plasticizers, through a set of processes with high activation energies, 50–100 kcal. At fixed temperature, ``reaction'' rates are directly proportional to vapor pressure of the liquid. The dielectric change may be treated similarly to a chemical reaction between a solid and a vapor, and free energy of activation, ΔF**, evaluated. ΔF** is constant for various liquids with one resin. ΔE**=ΔH**+L in this system, where ΔE** is Arrhenius activation energy, ΔH** energy barrier, Llatent heat of vaporization of the liquid.
Mechanical Properties of Substances of High Molecular Weight. VIII. Dispersion of Dynamic Rigidity and Viscosity in Concentrated Polyvinyl Acetate Solutions21(1950); http://dx.doi.org/10.1063/1.1699696View Description Hide Description
The dynamic rigidities and viscosities of concentrated solutions of polyvinyl acetate in 1,2,3‐trichloropropane have been obtained from transducer and wave propagationmeasurements. Two polymer samples with number‐average molecular weights of 140,000 and 840,000 were studied in the lower audiofrequency range at concentrations from 3 to 40 percent and temperatures from −3 to 41°C. For each sample, when the reduced dynamic rigidity G′T 0/Tc and the reduced dynamic viscosity η′/η were plotted against the reduced frequency, ωηT 0/Tc, all the data superposed to give two composite dispersion functions. The distribution of Maxwellianrelaxation times can be derived from the dispersion of either G′ or η′, and the two calculations are in reasonable agreement.
21(1950); http://dx.doi.org/10.1063/1.1699697View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699698View Description Hide Description
1. Early stretch‐vibrometer data, of quite extensive character, on the frequency dependence of the internal friction μ of a Nylon monofil, has been found to conform to the relationwhich was deduced independently by Tobolsky and Eyring from considerations of molecular structure. Among other constants evaluated was the relaxation time for the secondary‐bond network τ2, which was found to have the value 9.3 sec.
2. By more sensitive graphing methods than had been used heretofore, it was confirmed by the Nylon data that the hyperbolic relationship between resonant frequency ω0 and internal friction μ holds equally as well as the foregoing equation. The two relationships are shown to be nearly equivalent in the present experimental range. On evaluation of graphical parameters, the equation μω0=2.47×109 ergs/cm3 was obtained.
3. Using the same graphical method with a set of data on 11/4/2 cotton cord the relation (μ+0.9)ω0=7.05 ×109 ergs/cm3 was established.
21(1950); http://dx.doi.org/10.1063/1.1699699View Description Hide Description
In a former paper the authors suggested the following equation of state, (P+π)(V−ω)=RT, where π is the internal pressure (or cohesive energy density) and ω is the volume at absolute zero. It was found that this equation was in good agreement with data on the thermal expansivity and isothermal compressibility of polystyrene.
The present paper reports isothermal compression measurements on polymethyl methacrylate, polyethylene, cellulose acetate‐butyrate, and ethyl cellulose at temperatures within the fabrication range. The above equation of state is shown to apply equally as well to these thermoplastics as to polystyrene and values are given for the constants. The usefulness of this relationship is illustrated in a brief discussion of some phases of the injection molding process.
21(1950); http://dx.doi.org/10.1063/1.1699700View Description Hide Description
In such fabricating processes as extrusion and injection molding it is frequently necessary to force a granular polymer through a channel. Frictional forces between polymer and channel walls lead to a drop in pressure through the granular material. Simultaneously, the applied pressure causes a compaction of the granules.
Assuming that the granular mass has certain fluid‐like properties, a simple theoretical expression for pressure transmission is derived. Comparison with data on saran powders and granular polystyrenes shows good agreement between theory and experiment. The effect of lubrication is shown also.
The compaction of granular polystyrene follows the same pattern as that observed in the earth's crust. The porosity decreases exponentially with increasing pressure. The ``compressibility coefficient'' for the porosity of granular polystyrene is comparable with that given for clays and shales.
21(1950); http://dx.doi.org/10.1063/1.1699701View Description Hide Description
This paper establishes the feasibility of applying the membrane analogy to the solution of two‐dimensional steady‐state heat‐conduction problems. The membrane analogy is established, and the construction and the use of the apparatus for measuring the film are described. Inexpensive apparatus and simplicity of execution are the predominating features of this type solution.
21(1950); http://dx.doi.org/10.1063/1.1699702View Description Hide Description
Like the chemical properties of the elements, the work function φ is a periodic function of atomic number, as shown clearly by a new compilation of published data for 57 elements. The progressive rise and fall of work function values throughout the table of the elements appears to be sufficiently regular to permit approximations to be made of values for the metallic and semimetallic elements on which no data has yet been published. Since the first ionization potentialEi and electrode potential E 0 exhibit a similar periodic function of atomic number, a striking resemblance exists among the respective plots of φ, Ei , and E 0.
21(1950); http://dx.doi.org/10.1063/1.1699703View Description Hide Description
The directional variation of body wave velocities has been computed for cubic and hexagonal metals where stiffness coefficients are known; the essential data is presented in the form of tables and graphs. The results for aluminum,copper,gold, iron(α), lead and silver are tabulated in terms of the directional indices, and a stereographic coordinate system has been devised for presenting these data in the form of iso‐velocity contours for the three wave vectors. The compressional and shear velocities for cadmium,magnesium, and zinc are plotted as a function of the angle formed by the direction of wave propagation and the hexagonal (c) axis from which have been derived the theoretical average velocities which may be compared with measured values for polycrystalline specimens.
21(1950); http://dx.doi.org/10.1063/1.1699704View Description Hide Description
The absolute energy of the light (ergs/cm2 per r‐unit) and its spectral distribution have been determined for four different types of x‐ray fluorescent screens: Patterson E‐2, Patterson B‐2, Patterson D, and Neossal. The investigation was performed in accordance with an objective spectrophotometric process with the aid of a tungsten ribbon filament lamp as a standard, a light attenuator of reflection type, monochromator, multiplier photo‐tube, and galvanometer.
The application of the method employed to the weak light intensities in question is made possible on account of the enormous light sensitivity of the multiplier photo‐tube and the possibility of reproducible continuous variation of this sensitivity within very wide limits. The method has proved speedy and accurate and appears to be well suited for investigations of similar character, within the spectral range which may be measured by the commercial types of multiplier photo‐tubes.
21(1950); http://dx.doi.org/10.1063/1.1699705View Description Hide Description
Theoretical calculations indicate that the unbalanced component of current on a two‐wire or shielded‐pair transmission line can be determined by comparing the different standing‐wave distributions on the line conductors. The measurement of reflected components from an unbalanced radiative termination on a slotted shielded‐pair line is described. The existence of a condition of minimum unbalanced reflected component from end‐coupled antennas is shown experimentally.
21(1950); http://dx.doi.org/10.1063/1.1699706View Description Hide Description
The method initiated by Fediaevsky for evaluating the shear stress distribution in a turbulent boundary layer under the action of an adverse pressure gradient is improved upon. Use is made of a more suitable polynomial and boundary conditions which include the concept that the turbulence which produces the shear at any point has its origin at the surface upstream from its present location. This ``history'' theory predicts that the transverse shear gradient in the outer portion of the boundary layer remains constant as the boundary layer develops. Comparison with shear distributions measured at the National Bureau of Standards along a simulated airfoil shows good qualitative agreement and appreciable improvement over Fediaevsky's theory.
21(1950); http://dx.doi.org/10.1063/1.1699707View Description Hide Description
The TM 01 mode in a circular dielectric‐filled wave guide, where dielectric losses and metal wall losses are considered simultaneously, is analyzed from the viewpoint of the boundary value problem. General expressions for the attenuation and phase constant are obtained, and the percentage errors introduced by (1) neglect of wall losses and (2) assumption that the total attenuation is the sum of dielectricattenuation and approximate wall loss attenuation are computed for a particular case.