Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Synthetic Polymers and Biopolymers
20.6 Properties of Polymers
Since polymers play an essential role in our everyday lives, it is very important to know about the properties of the different polymers. The properties of polymers are typically divided into two main categories, their solubility in various solvents and properties that they exhibit at elevated temperatures.
Figure 20.1 Illustration of the molecular arrangements of crystalline and amorphous solid polymers. (a) Molecular arrangement of crystalline solid. (b) Molecular arrangement of amorphous solid.
20.6.1 Solubility of Polymers
You will recall from general chemistry that the solubility of solutes depends primarily on possible intermolecular interactions between the solute and solvent. If the solute and solvent have similar features to allow for effective intermolecular interactions, the solute will be soluble in the solvent. Thus, polystyrene, for example, which is a non-polar molecule, is more likely to be soluble in toluene than water owing to the similar van der Waals interactions between polystyrene and the organic solvent, toluene. On the other hand, if there are polar or ionic side chains on polymers, they will likely be soluble in water and other polar solvents.
20.6.2 Thermal Properties of Polymers
Polymers are typically solids, and they can be classified into two very broad categories: crystalline and amorphous. You will recall from general chemistry that crystalline solids such as sodium chloride have a regular arrangement of the ions, in this case, sodium cation and chloride anion are arranged in a regular manner. Since polymers consist of molecules and not ions or atoms, the possibility exists for the molecules to have regular orderly arrangements or not. The molecules of crystalline polymers have regular arrangement, and crystalline polymers are typically rigid and incompressible and hold a definite and fixed shape. On the other hand, amorphous solids do not have a regular arrangement of the molecules and, as a result, are not as rigid as crystalline solids. Figure 20.1 gives an illustration of the molecular arrangements of both types of solid polymers.
Crystalline solids have distinct precise melting points, whereas amorphous solids typically melt over a temperature range at which point they become rubbery and viscous. This temperature is typically referred to as glass-transition temperature (Tg).