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In light of recent advancements in the constitutive modeling of bidisperse and polydisperse entangled linear polymers, we present a new "multifluid" generalization of the classic two-fluid approximation for flows of inhomogeneous polymer blends. As an application of the model, we consider predictions for the linear and nonlinear dynamics of shear induced demixing (SID) instabilities in blends with bidisperse 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 two chains. The lower bound polydispersity for SID coincides with the point where elastic forces (k B T per entanglement) scaled by the contrast between chains (e.g., polydispersity index minus one) exceed the entropic forces for mixing (k B T per chain). For bidisperse blends, we show that the nonlinear dynamics of SID strongly resemble what has previously been shown for SID in entangled polymer solutions. [ABSTRACT FROM AUTHOR]

In the present paper, we continue our recent studies on a two-fluid Rolie-Poly approximation of entangled polymer solutions in simple shear flows. We review the existing literature on shear induced demixing (SID) in two-fluid models and highlight the apparent similarities to a thermodynamic model of demixing. Focusing on steady unidirectional simple shear flows driven by a constant applied shear stress, we show that when the frictional drag between solvent and polymer is asymptotically large, our two-fluid model is mathematically equivalent to a thermodynamic model in terms of its long-time concentration dynamics. In particular, we show that SID minimizes a Lyapunov functional L distinct from the system's free energy F. We apply our asymptotic model to make predictions regarding nucleation (finite amplitude instabilities) and coalescence phenomena in SID. Numerical calculations with the full model corroborate the asymptotic model predictions. Finally, we apply the same asymptotic analysis to two flows in which a thermodynamic mapping fails. First, we consider steady simple shear flows driven by a constant boundary velocity (as opposed to a constant boundary stress), wherein the hydrodynamic instability appears to have the same character but can no longer be mapped to an equivalent thermodynamic instability. Second, we consider unsteady simple shear flows driven by an oscillating boundary stress, wherein the nonlinear dynamics of demixing are totally distinct from what a thermodynamic model predicts. [ABSTRACT FROM AUTHOR]

Using both theory and continuum simulation, we examine a system comprising a mixture of polymer chains formed from 100 hard-sphere (HS) segments and HS colloids with a diameter which is 20 times that of the polymer segments. According to Wertheim's first-order thermodynamic perturbation theory (TPT1) this athermal system is expected to phase separate into a colloid-rich and a polymer-rich phase. Using a previously developed continuous pseudo-HS potential [J. F. Jover, A. J. Haslam, A. Galindo, G. Jackson, and E. A. Muller, J. Chem. Phys.137, 144505 (2012)], we simulate the system at a phase point indicated by the theory to be well within the two-phase binodal region. Molecular-dynamics simulations are performed from starting configurations corresponding to completely phase-separated and completely pre-mixed colloids and polymers. Clear evidence is seen of the stabilisation of two coexisting fluid phases in both cases. An analysis of the interfacial tension of the phase-separated regions is made; ultra-low tensions are observed in line with previous values determined with square-gradient theory and experiment for colloid–polymer systems. Further simulations are carried out to examine the nature of these coexisting phases, taking as input the densities and compositions calculated using TPT1 (and corresponding to the peaks in the probability distribution of the density profiles obtained in the simulations). The polymer chains are seen to be fully penetrable by other polymers. By contrast, from the point of view of the colloids, the polymers behave (on average) as almost-impenetrable spheres. It is demonstrated that, while the average interaction between the polymer molecules in the polymer-rich phase is (as expected) soft-repulsive in nature, the corresponding interaction in the colloid-rich phase is of an entirely different form, characterised by a region of effective intermolecular attraction. [ABSTRACT FROM PUBLISHER]

We study fluid–fluid phase separation in a colloid–polymer mixture adsorbed in a colloidal porous matrix close to the θ point. For this purpose we consider the Asakura–Oosawa model in the presence of a quenched matrix of colloidal hard spheres. We study the dependence of the demixing curve on the parameters that characterize the quenched matrix, fixing the polymer-to-colloid size ratio to 0.8. We find that, to a large extent, demixing curves depend only on a single parameter f, which represents the volume fraction which is unavailable to the colloids. We perform Monte Carlo simulations for volume fractions f equal to 40% and 70%, finding that the binodal curves in the polymer and colloid packing-fraction plane have a small dependence on disorder. The critical point instead changes significantly: for instance, the colloid packing fraction at criticality increases with increasing f. Finally, we observe for some values of the parameters capillary condensation of the colloids: a bulk colloid-poor phase is in chemical equilibrium with a colloid-rich phase in the matrix. [ABSTRACT FROM AUTHOR]

Electrokinetic demixing is attempted for faster separation of aqueous two-phase systems. Electrokinetic demixing is carried out employing the PEG/potassium phosphate phase system for the separation of biomolecules like Bovine serum albumin and C-phycocyanin. The effect of field polarity, field strength, and concentration of proteins on partitioning are studied at two different tie lines. In spite of the application of the electric field, BSA as well as C-phycocyanin partitioned to their preferred phases. Demixing time decreased by 50% at 4.4 V/cm when compared to gravity. Electrokinetic demixing could decrease the phase demixing time without affecting the partitioning of the target protein. [ABSTRACT FROM AUTHOR]

Abstract: We construct a functional for amphiphile–polymer mixtures and investigate the demixing transition by using a proposed version of density functional theory. It is found that increase of the amphiphilic size ratio and polymer length can effectively promote phase separation of the systems. Phase diagrams are plotted to clarify these influences. The results provide an effective way of controlling the stability of the fluid–fluid phase equilibrium of the mixtures. [Copyright &y& Elsevier]

Abstract: Sodium alginate is a polyelectrolyte consisting of the monomer units β-d-mannuronate and α-l-guluronate. Mainly based on the theory of Khokhlow et al., the state diagram of the binary system alginate/water has been calculated using different sets of parameters like degree of ionization, degree of polymerization and interaction function. The calculations comprise miscibility gaps, liquidus curves, eutectic points and the behaviour at temperatures below the melting point of water. Also gel and swelling curves have been treated, where gels are physically crosslinked. The DSC diagram of a 0.5 by wt.% polymer sol shows a double melting peak, which is explained by a heterogeneity above 0°C. The crystallization of water seems to concentrate the gelled system irreversibly. [Copyright &y& Elsevier]

Entropic contribution to the interaction parameter χeff in the model incompressible polymer/oligomer system is calculated by the lattice cluster theory (LCT). It is found that in the oligomer solvent, there exists a wide concentration range that the non-combinatorial "entropic interaction" term χeffϕ1ϕ2 perceptibly counteracts the mean field combinary entropy ぉSMF. With the increase of the solvent size, both χeffand the ratio χeffϕ1ϕ2/ぉSMF first reach their maximum and finally become trivially to zero. It is worth noting that no any demixing was found in the current calculation. This makes the controversial idea "entropically driven demixing" even elusive. However, we propose that further work on compressible polymer solution with structured monomer will witness the demixing owning to an increased configurational correlation. [ABSTRACT FROM AUTHOR]