We describe synthesis and nonlinear relaxation dynamics of various mul
tiarm entangled polybutadiene molecules of the general type A(3)-A-A(3
). In low-amplitude oscillatory shear, entangled multiarm polymers dis
play broader relaxation spectra than linear polybutadienes of comparab
le molecular weight. Dramatic slowing down of cross-bar (A) relaxation
by the entangled arms (A) is believed to be the source of this behavi
or. In nonlinear step strain experiments the A arms have a rather rema
rkable 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 conseq
uence of arm withdrawal into the tube confining the cross-bar. This be
havior is in near perfect agreement with a recent theoretical proposal
for branched polymer dynamics. That this proposal is based on the not
ion of tensile forces on individual macromolecules due to tube confine
ment supports the existence of such forces and provides new circumstan
tial 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 wit
hdrawal process. This last finding is not in agreement with current de
scriptions of multiarm polymer dynamics.