Fibering of Rubber. Time Lag and Its Relation to Rubber Structure

FOR several years it has been known that in the unstretched condition ordinary rubber acts toward x-rays like an amorphous material, but that, when it is sufficiently stretched, it acts toward the rays like a fibrous material. In 1931 Acken, Singer, and Davey (1) reported that at room temperature a time interval was required to build up the fibrous structure in cyclically stretched rubber. Investigation showed that the time-lag effect could not be accounted for in terms of a temperature change during the act of stretching. Even if all the mechanical energy of stretching were instantaneously changed into sensible heat, the temperature of the rubber sample could not have been increased momentarily by more than 5.2° C., whereas the fibering of rubber continuously stretched at 420 per cent elongation could be demonstrated up to a temperature of 47° C. Experimentally no temperature rise greater than 1.0° C. could be found in the samples used. Since the time-lag effect appeared, therefore, to be a real effect, it seemed worth while to study it in detail. It is the purpose of this paper to report: (1) typical data on the effects on the time lag, of temperature, previous temperature history, mechanical working and aging, time of relaxation, time of stretch, and rate of stretch; and (2) the relation of these results to the possible structure of the rubber fiber.