From studying the page on polyesters we looked at how ester linkages could be formed in a condensation reaction; that is a reaction where two or more small molecules combine to form a larger molecule and release a small molecule such as water (H2O) or hydrogen chloride (HCl) gas. In the case of polyesters the two small molecules or monomers which reacted together to form the polymer polyester were a dicarboxylic acid and a diol.
Polyamides are formed in a similar way to polyesters however this time the monomers used to make the polyamide polymer are a dicarboxylic acid and a diamine to form an amide bond. Amines are molecules containing the amino functional group (-NH2); so diamines are molecules containing two amino groups. The formation of a typical polyamide is outlined below:
It is important to note that the product of the above reaction still has a reactive amino group (-NH2) and a reactive carboxyl group (-COOH) present on the ends of the new larger molecule formed, this means that the reactive amino and carboxyl functional groups present can react further and build up an even larger molecule. Or to put it another way the small dicarboxylic acid and diamine monomers can react together to build up a larger molecule containing lots of amide groups, that is a polyamide polymer can be made.
One of the first synthetic polyamides to be produced was nylon. Nylon was first made in the 1930 by Wallace Carothers while he was working for the American pharmaceutical company Du Pont. One of the first uses for this new wonder
material was to make ladies stockings. Ladies stocking had previously been made of silk which unfortunately made them very expensive and not particularly hard wearing or long lasting.
When the first nylon stocking went on sale in America they were so popular that millions of pairs sold in only a few hours and shops quickly ran out of stock.
However during the Second World War production of nylon was switched to war time and military uses such as parachutes so there was a shortage of nylon stocking. After the war ended, DuPont announced the return of nylon stockings.
This sparked a period of intense demand and even "nylon riots" at stores.
Women lined up in long queues outside shops, there were even reports of fights breaking out as people scrambled to get their hands on the coveted nylon stockings.
Nylon is a thermoplastic polymer; this means it will soften and melt when heated. If molten nylon is forced through a device called a spinneret (as shown in the image opposite); this simply resembles a large shower head; then long filaments of nylon thread will emerge. If these filaments are then cooled by simply being exposed to warm air they will fully solidify and form long filaments of nylon. These are then stretched and drawn out to form long filaments or threads which can be spun onto bobbins and the filaments or threads can them be used to make fabrics or other items as necessary. Drawing or stretching the nylon filaments results in the formation of long parallel polymer chains which will pack closely together which enables them to form lots of intermolecular bonds to neighbouring polymer chains; this intermolecular bonding between the polymer chains dramatically increases the strength of the nylon filaments formed.
Amino acids contain the acidic carboxyl group (-COOH) and the basic amino group (-NH2) in one molecule. So instead of using two separate monomers; one with the acidic carboxyl group and one with the basic amino group to make a polyamide as saw above why not just use a single monomer with both these reactive functional groups on the ends of each monomer? Well in the case of amino acids that is exactly what happens when they form polypeptides and proteins; for example the amino acids alanine and glycine can undergo a condensation reaction to form a dipeptide molecule as shown below. This dipeptide molecule contains an amide or peptide bond. However the dipeptide molecule still has reactive amino (-NH2) and carboxyl groups (-COOH) on the ends of the molecule and can readily undergo more condensation reactions to form a polypeptide molecule or a protein.
It is not even necessary to start with different amino acids. Heating a single amino acid monomer will result in the formation of a polyamide. We can show this simply as follows where the amino acid glycine can polymerise to form the polymer polygylcine when heated.
