Solubility Paramaters

Solubility parameter (SP for short) is a physical constant that measures the compatibility of liquid materials (including macromolecules because they are liquid under processing conditions). Its physical meaning is the square of the cohesive energy density of the material.

SP = (E / V) 1/2

Among them, SP is the solubility parameter, E is the cohesive energy, V is the volume, and E / V is the cohesive energy density.

The solubility parameters of different polymers vary in size and vary widely. Generally, the greater the polarity of a polymer, the greater its solubility parameter. Mastering the solubility parameters means mastering the degree of compatibility between different polymers and providing a basis for their success. The solubility parameter values ​​of different polymers can be used as a reference for selecting polymer solvents and blending components.

(1) Small molecule polymers

For the addition of small molecular components, the solubility parameter difference ∣δA-δB∣ <1.5 is considered to be good compatibility. For example, δA = 8.6 for CCl4, δB = 7.95 for natural rubber, and ∣δA-δB∣ <1.5, so Natural rubber is soluble in CCl4.

(2) Polymer

The compatibility law between polymer components is: ∣δA-δB∣ <0.5, and the larger the relative molecular mass, the smaller the difference requirement, which means that different components with high relative molecular mass are more difficult to phase. For example, the δA of PVC is 9.4 ~ 9.7, and the δB of NBR is 9.3 ~ 9.5, ∣δA-δB ， <0.5, so PVC / NBR has good compatibility and can be used in PVC toughening systems; for example, PS and PB The solubility parameter difference is greater than 0.7, the compatibility between the two is poor; the solubility parameter difference between PVC and PS is greater than 1, and the two are basically incompatible.

The closer the solubility parameters of the two polymer materials are, the better the blending effect. If the difference between the two exceeds 0.5, it is generally difficult to blend uniformly, and it is necessary to increase the compatibilizer. The function of the compatibilizer is to reduce the surface tension of the two phases, so that the surface at the interface is activated, thereby increasing the degree of compatibility. A compatibilizer is often a polymer that acts as a bridge intermediary.

In addition, when designing the formula, when choosing a liquid additive for a certain glue, it must also consider whether the SPs of both sides are close to ensure that the components are evenly dispersed.

For the principle of similar compatibility of solubility parameters, it is only suitable for the principle of judging the alloys between non-polar resins and amorphous resins, but not the principle of judging the polymers between polar resins and crystalline resins. The reason is that the solubility parameter only characterizes the dispersion force between non-polar molecules, but does not characterize the dipole force and hydrogen bond between polar molecules, so it cannot complete the characterization of the force between polar molecules. For polar resins, the three-parameter similarity principle should be used, that is, the principles of similar dispersion forces, similar dipole forces, and similar hydrogen bonds.