Physical properties of polymers, including solubility, are related to the strength of covalent bonds, the stiffness of the segments in the polymer backbone, the amount of crystallinity or amorphous nature of the polymer, and the intermolecular forces between the polymer chains. The strength of the intermolecular forces is directly related to the cohesive energy density (CED), which is the molar energy of vaporization per unit volume. Because intermolecular attractions of solvent and solute must be overcome when a solute (here a polymer) dissolves, CED values can be used to predict solubility.
This has led to the concept of a solubility parameter δ, introduced by Hildebrand, which is the square root of CED. The solubility parameter δ for nonpolar solvents is equal to the square root of the heat of vaporization per unit volume.
δ = (ΔE/V)1/2 = (CED)1/2, or δ2 = CED
V is the molar volume, and ΔE is the molar energy of vaporization. Units for the solubility parameter are (MPa)½ = (J cm-3)½ = 0.4887(cal cm-3)½. The polymer community often uses the unit Hildebrand (H) = (cal cm-3)1/2.
The heat of mixing a solute and a solvent, ΔHm, is proportional to the square of the difference in solubility parameters, as shown below, where φ is the partial volume of each component, namely solvent φ1 and solute (polymer) φ2. Because typically the entropy term favors solution and the enthalpy term acts counter to solution, the objective is to match solvent and solute so that the difference between their δ values is small, resulting in a small enthalpy acting against solubility occurring.
ΔHm = φ1 φ2 (δ1 - δ2)2
The energy of vaporization is not accessible for polymers, but cohesive energy density of polymers can be determined from PVT data. However, common ways for determining polymer solubility parameters use thermodynamic properties of polymer solutions and their relations to excess enthalpy or excess Gibbs energy per unit volume.
The solubility parameter concept predicts the heat of mixing for liquids and amorphous polymers. It has been experimentally found that generally any nonpolar amorphous polymer dissolves in a liquid or mixture of liquids having a solubility parameter δ that does not differ by more than ±1.8 (cal cm-3)1/2.
Sometimes the Flory-Huggins solvent-polymer interaction parameter is applied. References 1–3 give details for such procedures as well as extensive tables of polymer solubility parameters. Methods for calculating solubility parameters can be found in References 4–7.
Table 1 gives solubility parameters for a variety of typical solvents, including those that are poorly hydrogen bonding, moderately hydrogen bonding, and strongly hydrogen bonding. Table 2 provides solubility parameter values for selected common polymers covering all three hydrogen bonding scenarios. Table 3 gives solubility characteristcs of a number of common polymers with various solvents.
Solvent | δ (cal cm-3)1/2 |
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Poorly Hydrogen Bonding | |
Dimethylsiloxane monomer | 5.5 |
Dichlorodifluoromethane | 5.5 |
Neopentane | 6.3 |
1-Nitrooctane | 7.0 |
Pentane | 7.0 |
Octane | 7.6 |
Turpentine | 8.1 |
Cyclohexane | 8.2 |
1-Isopropyl-4-methylbenzene | 8.2 |
Tetrachloromethane | 8.6 |
Propylbenzene | 8.6 |
4-Chlorotoluene | 8.8 |
Decahydronaphthalene (unspecified isomer) | 8.8 |
Xylene (unspecified isomer) | 8.8 |
Benzene | 9.2 |
Styrene | 9.3 |
1,2,3,4-Tetrahydronaphthalene | 9.4 |
Chlorobenzene | 9.5 |
1,2-Dichloroethane | 9.8 |