Keywords
hydrogen bond
It should be pointed out that not all polymers can be dissolved, and even though when they can, the dissolution process may take up to several days or weeks. According to Rosen (1982), there is an assembly of general rules for polymer solubility, based on experimental observations, from which interesting conclusions can be obtained.
Thus, it is well known that the dissolution of polymers depends not only on their physical properties, but also on their chemical structure, such as: polarity, molecular weight, branching, crosslinking degree, and crystallinity. The general principle that states like dissolves like is also appropriate in the case of polymers. Thus, polar macromolecules like poly (acrylic acid), poly (acrylamide) and polyvinyl alcohol, among others, are soluble in water. Conversely, nonpolar polymers or polymer showing a low polarity such as polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and poly(isobutylene), are soluble in nonpolar solvents.
On the other hand, the molecular weight of polymers plays an important role in their solubility. In a given solvent at a particular temperature, as molecular weight increases, the solubility of a polymer decreases. This same behavior is also noticed as crosslinking degree increases, since strongly crosslinked polymers will inhibit the interaction between polymer chains and solvent molecules, preventing those polymer chains from being transported into solution.
A similar situation occurs with crystalline macromolecules, although in such a case the dissolution can be forced if an appropriate solvent is available, or warming the polymer up to temperatures slightly below its crystalline melting point (Tm). For example, highly crystalline linear polyethylene (Tm = 135�C) can be dissolved in several solvents above 100�C. Nylon 6,6 (Tm = 265�C), a crystalline polymer which is more polar than polyethylene, can be dissolved at room temperature in the presence of solvents with enough ability to interact with its chains, through for example, hydrogen bonding. Branched polymer chains generally increase solubility, although the rate at which this solubility occurs, depends on the particular type of branching. Chains containing long branches, cause dense entanglements making difficult the penetration of solvent molecules. Therefore the rate of dissolution in these cases becomes slower than if it was short branching, where the interaction between chains is practically non-existent.