Small molecules, miscibility, thermodynamics

To be able to measure the osmotic pressure n, a semipermeable membrane that permits passage of the solvent molecules but not the solute molecules is needed. This can, in practice, be realized only when there is a large disparity between the sizes of the solute and solvent molecules, as in a solution of a polymer in a small-molecule solvent. However, the existence of osmotic pressure can be envisioned, at least mentally, with any kind of solution, such as a solution of two small-molecule liquids or a miscible blend of two polymers. Equation (6.6) is thus valid for any two-component (amorphous) system, as long as it is in equilibrium and classical thermodynamics is applicable to it. For applications to these general cases, it is more convenient if Equation (6.6) is reformulated in terms of the free energy of mixing and no explicit reference to osmotic pressure is made in it. 

Compatibilization by the achievement of thermodynamic miscibility is a concept that has been exploited in only a handful of situations to produce a commercial blend. The miscibility between polymers is determined by a balance of enthalpy and entropic contributions to the free energy of mixing. While the entropy of small molecules is high enough to ensure miscibility, the entropy of polymers is almost zero, which means that the entropy will determine the miscibility. " ... 

A polymier molecule is much larger than a typical small solvent molecule, often by more than a thousandfold. This difference in size has important consequences for the thermodynamics of pol>mier solutions. First, the coUigative attraction of a polymer solution for soWent small-molecule molecules is much higher than would be expected on the basis of the mole fractions. Rather the attraction is proportional to the volume fraction. Second, polymers are typically not miscible with other pol miers because of small mixing entropies. Third, the size as>Tnmetry between a polymer and a small-molecule solvent translates into an asymmetry in miscibility phase diagrams. 

Enthalpy AH depends on the attraction/repulsion between the two polymers usually, unlike molecules repel each other, so AH is generally positive (unfavorable to mixing). Entropy AS results from the randomization which occurs on mixing small solvent molecules produce large randomization, so most solvents are miscible, whereas large polymer molecules produce very modest randomization on mixing, not enough to overcome the repulsion between unlike molecules (+AH). Thus most polymer blends do not have thermodynamic miscibility, so they separate into two or more micro-phases. 

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