Solubility process
The dissolving process in polymer is very slow and occurs in two steps as
1) The solvent dissolves into the polymer to give a swollen gel. This process is slow because of cross-linking, crystallinity or hydrogen bonding.
2) The gel gradually disintegrates into true solution. Agitation can speed up this process.
For very high molecular weight, the solution process takes days or weeks.
The dissolving process in polymer is shown in fig below
The dissolving process in polymer is shown in fig below
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| Solubility process. |
Solubility Parameter
A polymer is soluble when the Gibbs free energy of mixing i.e
ΔG = ΔH - TΔS is negative.
Since the entropy of mixing ΔS is always positive. Thus the sign of ΔG is dependent on the sign and magnitude of heat of mixing ΔH. For reasonably non polar molecules and in the absence of hydrogen bonding ΔH is positive. In this case heat of mixing per unit volume
∆H= v₁v₂ ( δ₁-δ₂)²
Here v₁ and v₂ are volume fraction of solvent and polymer respectively and δ² is the cohesive energy density or for small molecules energy of vaporisation per unit volume. The quantity δ is called solubility Parameter.
Since the entropy of mixing ΔS is always positive. Thus the sign of ΔG is dependent on the sign and magnitude of heat of mixing ΔH. For reasonably non polar molecules and in the absence of hydrogen bonding ΔH is positive. In this case heat of mixing per unit volume
∆H= v₁v₂ ( δ₁-δ₂)²
Here v₁ and v₂ are volume fraction of solvent and polymer respectively and δ² is the cohesive energy density or for small molecules energy of vaporisation per unit volume. The quantity δ is called solubility Parameter.
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