Question

Polyester is known to be hydrophobic as shown by it's low moisture regain.

However, esters are at least somewhat hydrophilic and soluble in water. This decreases with the hydrocarbon chain length increase, but in the case of polyesters, this is accompained with a proportionate increase in the number of ester groups.

My attempts to explain this were as follows

The reason nonpolar molecules are not soluble in water is the H-bonded structure of water is disrupted - there's a loss in entropy; water molecules have to orient themselves in the form of a cage around the non polar regions.
In the case of a polymer chain of PET, the loss in entropy is greater than if we tried dissolving the individual monomer esters that constitute it in water. Therefore one is somewhat soluble, the other is not.

Since like dissolves like, and a polar aprotic solvent would not have the H-bonded structure of water that would be disrupted (and in fact since they are both polar aprotic, one would presumably become well incorporated into whatever bonding structure the other has), I would think a polar aprotic solvent would dissolve polyester.
Turns out that this isn't the case. In fact, polyester fibres are generally resistant to organic solvents (A. Bendak and El-Marfasi; pdf link)

Perhaps, I thought, this might be because of the relatively crystalline structure of polyester that doesn't allow solvent molecules to penetrate it below its glass transition temperature.

However, polyester fibres are soluble in phenol and m-cresol. They here also appear to be soluble in THF and DMF, but not the remaining organic solvents.

Why wouldn't the same issues that were occurring with water occur with these polar protic solvents (phenol and m-cresol), and why are polyesters soluble in THF&DMF but not other polar aprotic solvents.

Also, what is a good general framework to be thinking about the solubilities of substances in various solvents?

Answer 1

Polyesters like PET are hydrophobic due to their large nonpolar regions. Smaller esters are more soluble in water due to their ability to form hydrogen bonds.

Step-by-step explanation:

The solubility of a substance in any solvent, including water, is greatly influenced by the substance's ability to form hydrogen bonds and by its overall polarity.

Polyesters such as polyethylene terephthalate (PET) are known to be hydrophobic because they have large nonpolar regions that repel water, which disrupts water's hydrogen-bonded structure and results in a loss of entropy. Smaller esters with shorter carbon chains can dissolve in water due to their ability to form hydrogen bonds with water. However, as the size of the hydrocarbon chain increases, these polar interactions become less significant, and the molecule's solubility in water decreases.

Substances that are polar or ionic do tend to dissolve in water, as similar intermolecular forces can form between the solute and the solvent. Ester molecules, such as methyl propionate, demonstrate some level of solubility in water due to their ability to interact with water molecules through a molecular dipole. On the other hand, nonpolar molecules, such as propane, have very limited solubility in water since they lack substantial molecular dipoles.

Polyesters' solubility in certain solvents such as phenol, m-cresol, tetrahydrofuran (THF), and dimethylformamide (DMF) is due to the nature of these solvents and their ability to penetrate and interact with polyester chains. Not all polar aprotic solvents will dissolve polyesters because solvents must disrupt the crystalline structure and interact effectively with the polymer chains.

Phenol and m-cresol are polar protic solvents, able to interact with the ester groups in polyesters. THF and DMF work well with polyesters due to their strong solvation capabilities. A general framework for understanding solubility involves considering the molecular structure, polarity, ability to form hydrogen bonds or other specific interactions, and the size of both the solute and solvent molecules.