Surface tension, polymer blend phase separation

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

The surface chemistry of the substrate, even if the substrate is homogeneous, will affect the film phase segregation (see Fig. 6.2). In a two component blend film of two polymers—let us call them polymer A and polymer B— if the surface tension of polymer A and the substrate is less than the surface tension of polymer B and the substrate, polymer A will tend to sink towards the substrate and polymer B will tend to rise from the substrate, producing a vertically phase segregated film. Substrate interactions tend to dominate the atmospheric interactions, though a similar process can lead to vertical phase separation at the atmospheric interface. 

In practice, it is not easy to control the phase separation process in polymer blend films. The phase separation process in the polymer blends is very complicated, and the final morphology in the blend films is highly sensitive to many factors, such as solvent evaporation rate, solubility parameter, film-substrate interaction, surface tension of each component, and film thickness. Thus, the vertical phase separation can take place only under very extreme conditions [4B, 41], and, alternatively, lateral phase separation is more typical than vertical phase separation because the forces that contribute to the formation of lateral structures minimize the interface area. 

Most polymers are immiscible with each other. When two or more immiscible polymers are melt blended, without any planned compatibilization process, the components of the blend form different phases, which are separated from each other in the final product. This phase separation is due to the high surface tension between the immiscible polymer components in the interface region. The compatibilization of an immiscible polymer blend relies on the reduction of this interfacial tension. Normally, a blend that has been compatibilized can still be phase separated, but with a finer, stabilized dispersion of minor phase in the matrix. Improvement in mechanical properties of compatibilized blends are often observed due to a better adhesion between different phases and/or a better stress distribution in the solid state morphology when the material is under stress. 

If one uses a coarse-grained approach, one has to identify the relevant properties that the description on the coarser scale has to capture. In the following, we specifically consider wetting phenomena in a binary AB polymer blend that exhibits liquid-liquid phase separation between an A-rich and a B-rich phase. The thermodynamics of the surface enrichment layers is dictated by the free energies of the solid in contact with the two coexisting phases. Yaw Ybw> their interfacial tension,... 

The interphase is a separate phase with its own characteristics and two interfacial tension coefficients, Vj -f V2 = Vjj, with Vj2 being the experimental quantity. The lattice theories predict that in binary blends (1) there is a reciprocity between Vi2 and the interphase thickness, VjjA/ = constant (2) the surface energy is proportional to (3) polymer chain ends concentrate at the interface and (4) any low molecular weight component migrates to the interface. In consequence, the inter-phase is characterized by low entanglement density and viscosity, often evidenced by the interlayer slip [3]. 

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