Synthesis and relaxation dynamics of multiarm polybutadiene melts

We describe synthesis and nonlinear relaxation dynamics of various multiarm entangled polybutadiene molecules of the general type A(3)-A-A(3). In low-amplitude oscillatory shear, entangled multiarm polymers display broader relaxation spectra than linear polybutadienes of comparable molecular weight. Dramatic slowing down of cross-bar (A) relaxation by the entangled arms (A) is believed to be the source of this behavior. In nonlinear step strain experiments the A arms have a rather remarkable effect on polymer dynamics. At a critical shear strain gamma of around 6.0, [\E.u\] = 3.2, the nonlinear relaxation modulus G(t:gamma) abruptly decreases in value but retains similar time dependence to G(t;gamma) at strains below the critical value. The sudden drop in G(t;gamma) is reflected in the damping function and appears to be a consequence of arm withdrawal into the tube confining the cross-bar. This behavior is in near perfect agreement with a recent theoretical proposal for branched polymer dynamics. That this proposal is based on the notion of tensile forces on individual macromolecules due to tube confinement supports the existence of such forces and provides new circumstantial evidence for the existence of a mean-field tube. For all multiarm polymers studied we find time-strain separability at all strains with a separability time lambda(k) that appears insensitive to the arm withdrawal process. This last finding is not in agreement with current descriptions of multiarm polymer dynamics.