Thermoplastic Polymers: The individual chains of thermoplastic polymer are held together by van der Waals forces. They are strong if the polymer chains are lined up in an ordered, closely packed array. This region is called crystallites and the other where polymer chains are oriented randomly is called amorphous. Thermoplastic polymers have both ordered crystalline regions and amorphous noncrystalline regions. Thermoplastic polymers are hard at room temperature, but become soft when heated, because on heating individual polymer chains slip from one another. Polyethylene, polystyrene, polypropene and teflon are some examples for thermoplastic polymer.
Thermosetting Polymers: The greater the degree of cross-linking makes the polymer more rigid. Such cross-linked polymers are called thermosetting polymers. Thermosetting polymers are generally stronger than thermoplastic polymers due to strong covalent linkage (cross-linking) between polymer chains not by weak intermolecular van der Waals forces. They are more brittle in nature and their shape is permanent. Once it is hardened they cannot be recycled. Melmac®, a highly cross-linked thermosetting polymer of melamine and formaldehyde, used to make lightweight dishes. Bakelite and polyurethanes are examples for thermosetting polymers.
Elastomers: An elastomer is a randomly oriented amorphous polymer, which stretches and then reverts back to its original shape. To avoid slipping of polymer chains it must have some cross-linking. When elastomers are stretched, the random chains stretch out where as the van der Waals forces are not strong enough to maintain them in that arrangement and retains its original shape when the stress is removed. Rubber is an example of an elastomer.
Fibers: These are thread like polymers which can be used as fabrics. The strong intermolecular forces like hydrogen bond provide high tensile strength and high modulus to fibers. A few examples are cotton, wool, silk, nylon etc.
Generally, polymers can be divided into two broad groups depending on the source, either natural polymers or synthetic polymers.
16.3 Natural Polymers
Polymers are naturally available. A few examples are:
Polysaccharides: Polysaccharides are important class of biopolymers which are made up of long chains of monosaccharide units bound together by glycosidic bonds. Polysaccharides contain more than ten monosaccharide units. Starch and glycogen are very common examples for storage polysaccharides. Similarly, cellulose and chitin are examples for structural polysaccharides.
Nucleic Acids: Nucleic acids, such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from nucleotide monomers. Nucleic acids are found in all living things, where they function in encoding, transmitting and expressing genetic information.
Proteins: Proteins are large biological molecules, made up of the smallest units called α-amino acids. They are building blocks of plant and animal cells. Many proteins are enzymes that catalyze biochemical reactions and are essential to metabolic functions.
Natural Rubber: Natural rubber is the polymer of isoprene. This is mainly harvested as a sticky milky colloidal form called latex from the bark of the rubber tree.
