Measurement of the Isothermal Volume Dilation Accompanying the Unilateral Extension of Rubber

Using a torsion balance immersed in water and natural rubber ring specimens cured with di-tertiary-butyl-peroxide, measurements were made of the isothermal volume dilation of rubber for mean extensions ε=14, 33, and 51%, thus extending the results of Gee, Stern, and Treloar to lower strains. Chain molecular weights Mc=3000, 4400, 5100, and 5500 were employed. The chain molecular weights were determined by swelling in benzene, the uncertainty in each determination being about 10%. Observed fractional volume increases ranged from 3.2×10−5 for ε=14% and Mc=5500 to 14×10−5 for ε=51% and Mc=3000. Using Gee's expression for the volume dilation, but obtaining the slope of the stress-strain curve from the statistical theory, curves were fitted to the data. The fitting process constituted a determination of Young's modulus E for each rubber specimen. The resulting curves are in good agreement with those of Gee, Stern, and Treloar. Additional determinations of E were made from rough stress-strain curves and from the swelling data, the internal agreement between the three determinations being fair.