Dynamics and pattern formation in thermally induced phase separation of polymer-solvent system

In this paper, the characteristic dynamics and pattern formation of polymer-solvent system, undergoing thermal induced phase separation (spinodal decomposition), have been studied using non-linear simulations. The modified Cahn-Hilliard equation with the incorporation of the Flory-Huggins free energy model is used. The development of a wide variety of initial and intermediate stage morphology is feasible either by introducing sharp/continuous spatial temperature variation or by altering the initial mean solvent concentration (Co) in the vicinity of the minima (Cm) of the spinodal parameter (second derivative of excess Gibbs free energy w.r.t concentration). For Co < Cm initial stage of phase separation proceeds by the formation of columns/island of the solvent rich phase dispersed in polymer rich continuous phase. On the contrary, Co > Cm, phase separation leads to formation of holes of polymer rich phase dispersed in the continuous solvent rich phase. For an intermediate mean concentration very close to Cm inter-connecting structures are evolved. A continuous variation of spatial temperature leads to the directional (rather than random) evolution of phase separated pattern. The initial stage of evolution always propagates from the lowest to the highest temperature regime. A spatial step variation of temperature leads to formation of localized novel phase separated patterns. For a flow system, the dynamics and pattern formation can be controlled by tuning the different timescales: timescale of convection, de-mixing and coalescence.