Tensile retraction measurements. IV. Determination of loose end pullout in gum vulcanizates

Abstract

SYNOPSISTensile retraction measurements have been made on unfilled rubbers cured with either sulfur or dicumyl peroxide to different levels of crosslink density. The rubbers chosen were emulsion and solution polymerized anionic SBRs, cis-polybutadiene, natural rubber, and anionic 1,4-polybutadienes of different molecular weight. The measured molecular weight between crosslinks (M,) at each extension gave a smooth curve when plotted against the maximum extension (A,,,) used for the measurement. This curve was separated into three linear regions that characterize the cured rubber. The first region, above about 30% elongation, gave a Amax = 1 intercept that decreased with either increased polymer M,, or increased level of curatives. Subtracting this calculated crosslink density from the measured crosslink density below about 30% elongation gave another smooth curve when plotted against A, , , .A second linear relationship was described in the general range of 5-30%elongation. Below about 5% elongation, a third linear relationship was found. This line was determined from the differences between the calculated and the observed crosslink densities in the lowest strain region of the intermediate strain plot. Possible explanations of these three linear regions have been made. Supporting evidence was gained from temperature-dependent experiments as well as from correlation of experimental results with polymers of different values of Tg. I NTRO D UCTlO NThe retraction portion of the stress-strain tensile elongation curve of elastomers has been investigated over the years as a means to determine the effective crosslink density (v,) of the rubbers.'r2 The very early work of Flory et a1.l was done at equilibrium using a triple-beam balance to measure the stress. They reported that as the test specimen increased in thickness to about 7 mil a deviation from a constant response was noted. A particular interesting deviation from theory was also noted for those samples prepared at the lowest level of crosslink density. These samples appeared to have a much higher value of ve than what would be predicted from the level of curative used. This was partially attributed to the contribution of entanglements.* To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 42, 1727-1738 (1991) Later, Fedors2 used an Instron stress-strain tester to determine the value of ve in a nonequilibrium method. This work was based on the retraction from X = 2.6, which was similar to the extension used in the work of Flory et al. ( A = 2.0). Fedors showed that the C2 of the Mooney-Rivlin equation had a value of zero under these conditions of testing, and as a result the data appeared to be equivalent to that collected under equilibrium conditions.In a more recent paper, Hergenrother and Doshak3 have demonstrated that tensile retraction ( T R ) can be used to determine the entanglement molecular weight ( M e ) of the rubbery center block of a thermoplastic elastomer. Here, the plot of the observed molecular weight between cros...