Rubber networks containing unattached macromolecules. IV. Stress relaxation in end‐linked polybutadiene with unattached styrene‐butadiene copolymer

Abstract

AbstractStress relaxation has been studied in networks of dihydroxy‐terminated polybutadiene (mostly cis:trans:vinyl = 34:40:26) crosslinked by triphenyl methane‐4,4′,4″‐triisocyanate and containing about 9.5% by weight of unattached linear random styrene‐butadiene copolymer with various molecular weights (from 1.4 to 3.3 × 105) and with styrene content and butadiene microstructure chosen to match the average solubility parameter of the end‐linked network. Stress relaxation measurements were made also on networks containing no unattached species and containing 9.3% hydrocarbon oil, and on the various uncrosslinked linear polymers. The stretch ratio was 1.25 and the Young's relaxation modulus was calculated from the neo‐Hookean stress‐strain relation. For the uncrosslinked linear polymers, the relaxation modulus E11(t) corresponds to a rather narrow distribution of relaxation times whose magnitudes were approximately proportional to the 3.4 power of viscosity‐average or weight‐average molecular weight; for one polymer, the time dependence agreed closely with the prediction of the Doi‐Edwards theory modified for a small degree of molecular weight distribution. The disengagement times calculated from the Doi‐Edwards theory as modified by Graessley appeared to be of the correct order of magnitude. The contribution of the unattached species in the networks E1(t) was calculated by difference; after multiplication by (1−v)−1, where v2 is the volume fraction of network, and correction for the difference in monomeric friction coefficient associated with the difference in fractional free volume in the two environments, E1(t) was compared with E11(t) for each linear polymer. The relaxation was slower in the network than in the uncrosslinked polymer by about an order of magnitude, but the form of the relaxation modulus was similar in both environments except for two linear polymers for which the relaxation in the network became very much slower at long times. This behavior appeared to be correlated with a broader molecular weight distribution.