Relaxation behavior of polymer blends after the cessation of shear

We investigated the relaxation of a binary polybutadiene/polyisoprene blend after the cessation of steady-state shear using both in situ optical microscopy (OM) and light scattering (LS). Through fast Fourier transforms of the micrographs, we were able to directly relate our LS patterns with the real space morphology. In the steady state we observed ellipsoidal domains and strings. Upon the cessation of shear, these morphologies relaxed to the equilibrium morphology via a variety of mechanisms. After low shear rates were applied, coalescence between domains of random spatial distribution occurred. If the shear rate previously applied was sufficiently high, the strings were found to break up into small droplets forming necklace-like structures. The nonrandom spatial distribution created by shearing resulted in a long-lasting anisotropy at rest. If the previously applied shear rate was further increased, thermodynamic effects became important and anisotropies relaxed very fast: as a consequence. A spinodal pattern in both real space and in the LS patterns was found to occur during the relaxation process.