Volume 19, Issue 5, 01 May 1948
Index of content:
19(1948); http://dx.doi.org/10.1063/1.1698151View Description Hide Description
Previous work on the effects of ionizing radiations on hydrocarbons has established that four competitive processes are usually operative: (1) dehydrogenation, (2) condensation or polymerization, (3) hydrogenation (action of nascent hydrogen on any unsaturated matter present), (4) decomposition (C‐C cleavage). The chain reacting nuclear pile offers an ideal means for subjecting relatively thick samples of matter (such as rubber) to uniformly high concentrations of radiation. Such studies on natural rubber, butyl rubber, and polyisobutylene allow one to draw the following conclusions. 1. Uncured natural rubber undergoes a slight curing action when exposed to pile radiations. 2. Polyisobutylene samples are appreciably degraded by pile radiations. 3. The same effects as noted in (1) and (2) are greatly enhanced by secondary alpha‐particles, produced by an (n,α) reaction on boron10 (milled into the elastomer). However, even a 2‐hour bombardment of natural rubber yields a product greatly inferior to sulfur vulcanizates. 4. Pile bombardment does not introduce measurable unsaturation in polyisobutylene and decreases the unsaturation in natural rubber but slightly. 5. A typical butyl rubber stock is permanently degraded by pile irradiation showing upon cure reduced tensile strength compared to that of a control sample. 6. Natural rubber shows a weak but measurable radioactivity days after bombardment, probably because of its mineral content. Polyisobutylene is not appreciably active.
19(1948); http://dx.doi.org/10.1063/1.1698152View Description Hide Description
The thermodynamic functions of principal interest in a strained elastomer are the entropy and energy of deformation. The volume is of secondary importance, and it can be assumed, with sufficient accuracy for most purposes, that the volume is linear in the temperature and the mean pressure. The basic partial differential equations of thermodynamics can then be integrated, yielding expressions for the energy and entropy of deformation in terms of observable quantities.
In the present analysis the volume effects caused by changes in mean pressure and crystallinity are taken into account, and the effects associated with change in shape are sharply separated from those associated with change in volume. It is shown that the superelastic functions of deformation previously published (M. Mooney, J. App. Phys. 11, 582–92 (1940)) have a very general validity.
19(1948); http://dx.doi.org/10.1063/1.1698153View Description Hide Description
Adiabatic compressibility coefficients were measured on a series of cured polymers of commercial types under pressures up to 5000 lb./in.2. In most cases, the second order as well as the first order coefficient could be determined from the data. The linear isothermal compressibility also was calculated from the measured linear adiabatic coefficient.
Preliminary measurements of the thermal expansion of a stretched Hevea gum stock show that there is a slight decrease in expansion at the 200 percent elongation, presumably due to partial crystallization.
The Thermodynamics of a Strained Elastomer. III. The Thermal Coefficient of Modulus and the Statistical Theory of Elasticity19(1948); http://dx.doi.org/10.1063/1.1698154View Description Hide Description
Published thermoelasticity data are critically analyzed by means of thermodynamic equations developed in the first paper of this series. Equations known to be of the proper form are fitted by least squares to the experimental data, which were first corrected to allow for the changes in shape when tested under variable temperature and constant length. If the measurements are accepted as truly reversible, the data show that several different elastomers depart considerably in some respects from the predictions of the statistical theory of elasticity.
Some thermoelasticity measurements in torsion are reported which seem to give the most reliable evidence that stress is proportional to absolute temperature.
19(1948); http://dx.doi.org/10.1063/1.1698155View Description Hide Description
A study was made of creep, recovery, and permanent set for Hevea and GR‐S over a 1000‐hr. period and a range of elongations at 35°C. Creep for GR‐S is greater than for Hevea. The stress for GR‐S is considered to be supported to a greater extent by a relatively unstable secondary bondstructure. This is consistent with the large creep values for GR‐S at low elongations and the large ratio of primary or recoverable creep to permanent set. Hevea, in contrast, shows low creep at both high and low elongations and maximum creep at intermediate elongations for which the structure is heterogeneous, consisting of amorphous and crystalline phases.
A procedure was worked out for determining the contribution of permanent set to the observed creep. The creep curves were concave to the strain axis when plotted against logarithmic time. After correction was made for permanent set, they were found to be approximately linear, thus extending the range of application of the Tobolsky‐Eyring theory of creep.
At the start of the creep test the flow appears to be due largely to the reversible yielding of relatively weak bonds which can reform under the action of the elastic network when the stress is removed. For longer periods of time, or for higher elongations, the flow involves more deep seated changes in structure. Larger units of structure are displaced or stronger bonds broken with resultant permanent molecular displacements upon removal of the stress. For Hevea at high elongations all of the flow was accounted for by permanent set.
19(1948); http://dx.doi.org/10.1063/1.1698156View Description Hide Description
A method has been devised for the application of the theory of the three‐element shear model of Eyring and co‐workers to textiles which does not necessitate the location of the ``basic spring line'' and thus is readily applicable to both yarn and fiber tests and to the commercial inclined‐plane and pendulum‐type testers. It is also applicable as a first approximation to those textiles which do not possess a ``basic spring line'' as such. The method employs the conventional spring constants or elastic moduli, but determination of the viscous constant involves the magnitude of the relaxation in constant rate of elongation experiments (or the magnitude of the creep in constant rate of loading experiments) from the upper envelope per interval of time. The relationship has been employed to establish the correlation between the viscoelasticproperties of ``slashed'' and ``unslashed'' viscose tire cord rayon yarns and also to show the identity of single filament stress‐strain relationships with those of yarns of normal twist of the same textile. The simplicity of technique which enables this method to be used as a routine laboratory test for the determination of the viscoelasticproperties of textiles affords it an advantage over previous methods, which were often unsatisfactory from both a theoretical and an experimental viewpoint.
19(1948); http://dx.doi.org/10.1063/1.1698158View Description Hide Description
A full account, including the theoretical basis, is given of a method whereby the dynamic stretch modulus, coefficient of internal friction, and hysteretic energy loss of textile yarns and cords, composed of either continuous filaments or staple fibers, can be accurately determined at longitudinal vibration frequencies above 100 cycles/sec. The method, which employs electromagnetic excitation, is applicable also to glass cords, and metallic wires and cables. Results have been obtained at frequencies extending to above 300 cycles/sec. A concise description is given of the improved stretch vibrometer. Results on a typical cotton cord show that energy absorption by this instrument is negligible, permitting the calculation of an accurate internal friction coefficient, and absolute hysteretic loss. Measurements on eight natural and synthetic textile fibers reveal that the dynamic moduli range from 6.4×1010 dynes/cm2 for Velon monofil to 32×1010 dynes/cm2 for a ramie cord. The dynamic moduli of Fiberglas and steel cords are found to be 54 and 106×1010 dynes/cm2, respectively. The internal friction coefficients are found to vary hyperbolically with frequency: in a 11/4/2 cotton cord, for example, from 16 to 3×106 poises between frequencies of 60 and 320 cycles/sec.
19(1948); http://dx.doi.org/10.1063/1.1698159View Description Hide Description
Measurements of the electric field on the surface of aircraft flying through all types of weather and clouds are summarized. The measuredelectric field inside a thunderstorm just prior to a lightning strike at 12,900 feet was 3400 volts/cm. The average maximum field for 9 different thunderclouds was 1300 volts/cm. It is noted that large fields usually occur only inside the cloud and at altitudes somewhat comparable to the freezing level. Frequent reversals of the field are observed in these areas. The electric fieldmeasured inside a stable non‐precipitating cloud is less than 10 volts/cm.
19(1948); http://dx.doi.org/10.1063/1.1698161View Description Hide Description
A method of converting the high repetition frequency used by echo sounding to a lower value by means of a converter between the receiver and the recorder is described. The method is adapted to recordings of radio‐echoes from the ionosphere, in which case the repetition frequency of 50 c/sec. of the pulse transmission after reception is converted into a repetition frequency of ½–⅕ c/sec. before being supplied to the recorder. Ionosphereechoes having a delay of the order of milliseconds and a pulse width of about 20–100 μsec. may thus be recorded mechanically.
The principle of repetition frequency conversion described here may also find applications in recording ultrasonic or radar reflections.
19(1948); http://dx.doi.org/10.1063/1.1698162View Description Hide Description
A quantitative investigation of the intensity of scatteredradiation in a series of native and regenerated cellulosic objects, including bacterial cellulose and fibers transformed into cellulose IV, is offered. The technique used involves the use of copperradiation monochromatized by reflection, specimens with randomized orientation of equal size and density, rotation of the plane film, and measurement of primary beam intensity by means of a device introduced by Goppel.
The procedure of evaluation consists of measuring integrated intensities of the crystalline peaks and that of the diffuse background. After correction of the latter for radiationscattered by air and for the background components due to Compton radiation and thermal scattering, as derived from exposures of sugar crystals and diamond, the diffuse background is represented by a curve exhibiting a flat maximum.
The height of this maximum was assumed to be a correct relative measure of the disordered portion and the total intensity of the coherently diffracted radiation of the crystalline peaks as a relative measure of the crystalline portion.
Within the series of native specimens the ordered and disordered fractions are found to be equal in cotton,cotton linters, ramie, and flax fibers. In woodpulp the disordered fraction is slightly higher and in bacterial cellulose considerably higher.
Within a series of rayons, including slightly and highly orientated viscose rayons, Cellophane, Lilienfeld rayon, and cuprammonium rayon, the ordered and disordered fractions were also found to be equal. Slightly less disordered substance is found in Fortisan, in mercerized ramie and in specimens where cellulose II has been transformed in cellulose IV by a heat treatment.
It is shown that the relative proportions of the fractions of disordered substance, for all objects investigated, are in satisfactory quantitative agreement with those deduced from the sorptive capacity for water vapor.
The absolute percentage of the crystalline portion calculated from the experimental data is 70±2 percent for the native fibers and 39±3 percent for the rayons. These figures are in reasonable agreement with those previously estimated from sorption isotherms (68 percent and 35 percent), from density determinations (60 percent and 25 percent), from birefringence and x‐ray orientation measurements in regenerated fibers (≤40 percent for rayon), and from recrystallization of amorphous cellulose powder (∼35 percent for rayon).
The observed relative intensities of the principal crystalline interferences give rise to a discussion of the limited scope of the concept ``crystallinity'' in cellulose fibers, and it is argued that the concept ``degree of lateral order'' may be preferable.
Comparison of observed relative line intensities with the theoretically calculated ones, reveals another marked difference between native and regenerated fibers, pointing to the fact that certain states of lateral order prevalent in the latter may be absent in the former.
19(1948); http://dx.doi.org/10.1063/1.1698163View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698164View Description Hide Description