Question

Solubility parameters (5) have developed to characterize miscibility of polymers with various solvents in applications from paints to coatings to membrane permeation. And, predicting the solubility of a polymer in a given solvent is one of the most important aspects of the use of solubility parameter data. describe how the selection of a solvent might depend on the molecular weight of a given polymer and the temperature at which the solvation takes place.

Answer 1

The solubility parameter (δ) provides a numerical estimate of the degree of interaction between materials. It is the square-root of the cohesive energy density.And can also be a good indication of solubility, particularly for polymers. Polymers with similar values of δ are likely to be miscible. Solubility parameters also helps in the selection of solvents and to predict the compatibility of polymers​.

Hence , predicting a solubility of a polymer in a given solvent is one of the most important aspects of the use of solubility parameter .

​SELECTION OF SOLVENT ON THE MOLECULAR WEIGHT OF POLYMER :

A solvent is suitable for a polymer if both have similar values of the solubility parameter. That is ,the closer the solubility parameters of the solute and the solvent are, the more likely the solubility of the solute in the given solvent. Also if the polymer (p) and the solvent (s) have similar polar and hydrogen bonding parameter.

i.e.    |δ_s - δ_p| ≤ 3.6 MPa^1/2 ​{MPa = MegaPascal}

For example, polystyrene that has a solubility parameter of 9.1 cal^0.5cm^{− 1.5} is likely to be soluble in ethyl-acetate (solubility parameter = 9.1)​ , toluene (solubility parameter = 8.9) but not in n-hexane (solubility parameter = 7.24) because value of solubility parameter ​of n-hexane is not close to that of our polymer polystyrene .

Also as a general rule, the lower the molecular weight of a polymer, the greater its solubility in a solvent.

Also it is important to note that the higher the molecular weight of a polymer, the closer the solubility parameter of the solvent and polymer need to be to dissolve the polymer in the solvent.

TEMPERATURE AT WHICH SOLVATION TAKES PLACE :

The effects of temperature are relatively straightforward, in that the solubility generally increases as the temperature increases.

But the solubility parameters of polymers do not change much with temperature, whereas those of low molecular weight compounds often decrease noticeably with increasing temperature. In some cases, a solvent passes through soluble conditions to once more to become a non-solvent as the temperature increases .

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