Predicting microstructures in polymer blends under two-step quench in two-dimensional space

Phase separation of mixtures after a second quench: composition heterogeneities

We investigate binary mixtures undergoing phase separation after a second (deeper) temperature quench into two- and three-phase coexistence regions. The analysis is based on a lattice theory previously developed for gas-liquid separation in generic mixtures. Our previous results, which considered an arbitrary number of species and a single quench, showed that, due to slow changes in composition, dense colloidal mixtures can phase-separate in two stages. Moreover, the denser phase contains long-lived composition heterogeneities that originate as the interfaces of shrunk domains. Here we predict several new effects that arise after a second quench, mostly associated with the extent to which crowding can slow down 'fractionation', i.e. equilibration of compositions. They include long-lived regular arrangements of secondary domains; wetting of fractionated interfaces by oppositely fractionated layers; 'surface'-directed spinodal 'waves' propagating from primary interfaces; a 'dead zone' where no phase separation occurs; and, in the case of three-phase coexistence, filamentous morphologies arising out of secondary domains.

Morphological-evolution pathway during phase separation in polymer solutions with highly asymmetrical miscibility gap

Microstructural evolution during thermally induced phase separation in a polymer solution with a highly asymmetrical miscibility gap is analyzed using Flory-Huggins thermodynamics and nonlinear Cahn-Hilliard kinetics. Computer simulation results demonstrate that, in contrast to systems with symmetric miscibility gaps, interesting morphological-evolution pathways (MEPs) including cluster-to-percolation and percolation-to-cluster transitions are identified. These MEPs are rationalized according to asynchronous evolution of the two product phases. For a highly asymmetric polymer system, the initial solution composition is also found to play a critical role in determining the MEPs. According to the simulation results a map of MEPs in terms of initial solution composition and aging time of phase separation is established to guide future microstructural design in asymmetrical polymer solutions.