Throughout the discussion, it has been stressed that the superior properties of USE rubber may be correlated with the production of an unspoiled,and unpolymerized hydrocarbon different from that found in ordinary market grades. Greater softness, greater solubility, improved flex cracking, faster rate of vulcanization, elimination of smoking, reduction of creping, heavy compression for shipment, the presence of formaldehyde as enzyme poison, antioxidant, and possible polymerization retarder-all these factors support the suggestion that USE may contain a more unsaturated rubber molecule than is present in usual market grades.

A precision research instrument for the measurement of stress-strain-time properties of tex tile materials, which was constructed in the research laboratory of the American Viscose Corpo ration, is described. Classification of this instrument as a research tool rather than as a routine testing instrument allows more flexibility in operating conditions. It is of the constant-rate-of- extension type, for use on all textile materials from single filaments to fabrics. The instrument consists essentially of a constant speed drive, a Statham gage, and a Brown recording potentiometer.

The rise of temperature on fast stretching of natural and synthetic rubber stocks was investigated recently by Dart, Anthony, and Guth (D.A.G.). Since then the experimental technique was considerably improved and the taking of data greatly simplified thereby. By D.A.G. emphasis was placed to record temperature rises of up to 15°C for extensions up to 700 percent by a fast galvanometer of moderate sensitivity. In the present work this high heat was investigated. In addition, however, a less fast but more sensitive galvanometer was employed to record changes of temperature as small as 0.001°C for 0–80 percent extension. In accordance with early work by Joule on natural gum (1859) it was found that also Butyl gum stocks show an initial cooling effect. This passes at a thermoelastic inversion point into a heating effect. In agreement with the recent theory by James and Guth the thermoelastic inversion point was found to depend solely upon the thermal expansion coefficient of the unstretched stock. The coefficient of thermal expansion was measured for Butyl stocks in the present work. Butyl tread stocks also show the thermoelastic inversion point. The work of D.A.G. showed that the rise in temperature on extension is a slowly rising function of the extension with a steep upward turn and almost linear continuation after the onset of crystallization. The samples were kept extended for a minute and then the cooling arising on retraction also measured. The negative of the cooling on retraction plotted against extension fall, according to the second law of thermodynamics as it should under the extension curve, but crosses it at the onset of crystallization. For Butyl gum stocks this crossing takes place at rather high (600 percent or more) extensions in agreement with x-ray work. More similarity was found between Butyl and Hevea tread stocks than between the corresponding gum stocks. Loading shifts the onset of crystallization to a range of smaller extensions. In addition to measuring the change of temperature on extension and retraction, the residual rise in temperature after an extension and immediate fast cycle retraction was also observed. This quantity is a measure of internal friction in Butyl rubber and is closely connected with rebound and free vibration tests. Summarizing, the method described has a twofold application for development work: 1. Changes of the temperature on extension and delayed retraction indicate in a simple manner the onset and progress of crystallization. 2. Changes of the temperature in a fast cycle give an estimate for internal friction. Both these applications will facilitate attempts to improve present synthetics.

Study of the rise in temperature by fast stretching of Butyl gum and tread stocks throws considerable light on crystallization, internal friction, and carbon black reënforcement as functions of elongation and other possible variables. For low elongations, the low-heat curves show the occurrence of thermoelastic inversion. They show also that internal friction is much larger in Butyl than in Hevea. For high elongations the high-heat curves enable one to follow the late and abrupt crystallization so characteristic for Butyl stocks. The carbon black reënforcement of Butyl is followed for the whole range of elongations up to the breaking point. Specific carbon-black reënforcement takes place in the range of elongations before crystallization of the gum stock, but the shortening of the breaking elongation by the black excludes to a large extent the added reënforcement through crystallization. Physical measurements of the type reported in this paper serve a manifold purpose. They enable one to establish a correlation between physical properties and chemical structure. Such a correlation is simpler for fundamental physical properties, as the ΔT vs. ε curve, than for more complicated physical tests, as data from flexometer, which were designed to approach service conditions. Comparing different gum stocks with Butyl polymers among themselves and with Hevea or other synthetics, these measurements may guide the development of better Butyl polymers. Finally, comparing tread with gum stocks, they may help to recognize the nature of reënforcing by carbon blacks and the action of other compounding ingredients.

A stress-strain study has been made of various fibers to determine their entropy force behavior at room temperature. The relative importance of the entropy and internal energy forces is shown with some illustrations of the use of these quantities in under standing the fine structure of polymers. Details are presented to show the effects of extension and godet stretch on some samples.