Polymer Solutions and Thermodynamics

Like low-molecular-weight solutes, a polymer dissolves in a solvent when solvation lowers the free energy. A good solvent lowers the free energy substantially. A nonsolvent increases the free energy. 

Amorphous polymers (transparent in the solid state to be precise, it is not a solid but rather a supercooled liquid) are usually easy to dissolve in the good solvent. In contrast, crystalline and semicrystalline polymers (opaque in the solid state) are sometimes not easy to dissolve. Within a crystallite, polymer chains are folded into a regular, thermodynamically stable arrangement. It is not easy to unfold the chain from the self-locked state into a disordered state in solution even if the latter state is thermodynamically more stable. Heating may help the dissolution because it facilitates the unfolding. Once dissolved, polymer chains take a random-coil conformation unless the chain is rigid. 

TABLE 2.1 Good Solvents and Nonsolvents for Some Polymers 

Poly(methyl methacrylate) Amorphous Tetrahydrofuran Methanol 

Poly(ethylene glycol) Crystalline Water (cold) Ether 

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