(119c) Studies of Shear-Induced Demixing in Bi-Disperse and Polydisperse Entangled Blends of Linear Polymers

We present a new ‘multi-fluid’ generalization of the classic two-fluid approximation for flows of inhomogeneous polymer blends, which builds upon recent advancements in the constitutive modelling of bi-disperse and polydisperse entangled linear polymers, As an application of the model, we consider predictions for the linear and nonlinear dynamics of shear induced demixing (SID) instabilities in blends with bi-disperse and log-normal molecular weight distributions. We find that even in the absence of any chemical contrast between component chains, an imposed flow can induce a demixing instability provided there is sufficient contrast in the size of the chains. The lower bound polydispersity for SID coincides with the point where elastic forces (k_BT per entanglement) scaled by the contrast between chains (e.g. polydispersity index minus one) exceed the entropic forces for mixing (k_BT per chain). For bi-disperse blends, we show that the non-linear dynamics of SID strongly resemble what has previously been shown for SID in entangled polymer solutions.