Phase Inversion during Viscoelastic Phase Separation: Roles of Bulk and Shear Relaxation Moduli

We simulate viscoelastic phase separation of polymer solutions by solving numerically Langevin equations of a ``viscoelastic model'' that newly includes the bulk relaxation modulus in addition to the shear relaxation modulus. The results reproduce almost all the essential features of viscoelastic phase separation observed experimentally: (i) The existence of a frozen period, the nucleation of the solvent-rich phase, (ii) the volume shrinking of the polymer-rich phase, (iii) the transient formation of a networklike structure, and (iv) the phase inversion in the final stage. Our simulations clearly indicate that the bulk stress is responsible for (i), (ii), and (iv), while the shear stress for (iii).