Nylon is a synthetic fiber known for its strength, elasticity, and durability. It was the first fully synthetic fiber, originally developed as a substitute for silk, and is now widely used in textiles, carpets, and engineering plastics. Unlike simple molecules, nylon does not have a single, simple chemical formula. Nylon is a very long chain molecule, and its exact composition varies depending on the specific type. Understanding the formula requires looking at its fundamental building blocks and the process used to link them together.
The Chemical Definition of a Polymer
Nylon is categorized chemically as a polymer, a large molecule formed from many smaller, identical units linked together. These small building blocks are known as monomers, which connect repeatedly to form the long polymer chain. The constantly repeated section is called the repeating unit.
Polymers do not have a fixed molecular formula because the chain length varies significantly. Therefore, the chemical formula for a polymer represents the repeating unit enclosed in brackets, with a subscript ‘\(n\)‘ placed outside. This subscript indicates the variable number of repeating units and the polymer’s indefinite length. Nylon is a polyamide, characterized by the presence of amide bonds connecting these repeating units.
The Repeating Unit of Nylon 6,6
The most common form is Nylon 6,6, created from two distinct monomers: hexamethylenediamine and adipic acid. The two ‘6’s in the name refer to the six carbon atoms present in each starting material. Hexamethylenediamine has six carbon atoms, and adipic acid, a dicarboxylic acid, also contains six carbon atoms.
When these monomers react, they form a repeating unit held together by an amide linkage (\(\text{—NH—CO—}\)). The generalized empirical formula for the repeating unit of Nylon 6,6 is \(\text{—}[\text{NH}(\text{CH}_2)_6\text{NH}\text{CO}(\text{CH}_2)_4\text{CO}]\text{—}_n\). This formula shows a six-carbon chain derived from the diamine and a four-carbon chain derived from the diacid, connected by amide bonds. This arrangement gives Nylon 6,6 its high melting point and mechanical strength. The full molecular formula for the repeating unit is \(\text{C}_{12}\text{H}_{22}\text{N}_2\text{O}_2\).
Structural Differences in Nylon 6
Not all types of nylon are synthesized from two different monomers; Nylon 6 is formed from a single monomer, a cyclic compound called caprolactam. The name Nylon 6 indicates that the caprolactam molecule contains six carbon atoms.
When caprolactam undergoes polymerization, its ring structure opens to form the linear polymer chain. The resulting repeating unit contains all six carbon atoms derived from the original monomer. The generalized empirical formula for the repeating unit of Nylon 6 is \(\text{—}[\text{NH}(\text{CH}_2)_5\text{CO}]\text{—}_n\). The resulting polymer chain still contains the amide linkage, which is the defining feature of all polyamides. This shared structure is why Nylon 6 and Nylon 6,6 exhibit similar properties, such as toughness and high tensile strength.
The Condensation Reaction
The long polymer chains of nylon are formed through condensation polymerization. This reaction joins two molecules while simultaneously releasing a small, stable molecule, typically water (\(\text{H}_2\text{O}\)). For Nylon 6,6, the reaction occurs between the amine functional group (\(\text{—NH}_2\)) on the hexamethylenediamine and the carboxylic acid functional group (\(\text{—COOH}\)) on the adipic acid.
The amine group loses a hydrogen atom, and the carboxylic acid group loses a hydroxyl group (\(\text{—OH}\)). These components combine to form the water molecule released as a byproduct. The remaining portions of the two monomers link together to form the amide bond, connecting the repeating units in the polymer chain. This process repeats many times to create the final long-chain nylon polymer.