A macromolecule becomes a single phase in solution according to the same
thermodynamics laws as the micromolecular solution substances. However, differences in
molecular size influence the solubility properties of both these substances. For example, a
macromolecule dissolves slowly due to its entangled constitution opposing the penetration
of the solvent; but most polymer-solvent pairs can mix in all of the proportions. On
contrary, though micromolecular substances normally are much easier to dissolve than the
polymer, the solubility of micromolecules is generally limited to saturated solution.
The solvent of a macromolecule can be considered good by one of these two standards, i.
e. the kinetic standard or the thermodynamic standard. A good solvent from the kinetic
point of view is a solvent that easily dissolves the polymer in a short time. The solubility
can be made faster by decreasing the particle size of the polymer sample because more
of its surfaces are exposed to the solution. From the thermodynamic point of view, a good
solvent has to be able to strongly interact with the solution substances due to it similarity in
their chemical structure. Thus, a good solvent in terms of kinetics is fast while that in terms
of thermodynamics is efficient.
An amorphous polymer contains a network of entangled, flexible chains in a continuous
motion. When the polymer is sunk in a solvent, the polymer network will swell from the
osmotic activity of the solvent; the segmental motion will finally increase. Provided the
solvent is still available, the polymer will continue to expand and increase its freedom to
move. When the solvation process has sufficiently advanced to allow transitional
movement to the chains, the substances will separate out to form a solution.
Consequently, as the solution becomes more diluted, the intermolecular forces that exist
between the polymer chains will become less and finally the solution properties will only
exhibit polymer-solvent interaction forces.