Polymer
Polymer are natural or synthetic substances composed of very large molecules, called macromolecules consisting of long chains or networks of smaller molecules or monomers. Sometimes, the words polymer and macromolecule are used interchangeably. The number of monomer units is undefined in polymers, and with a high number of monomers, polymers are often called high polymers. The long-chain nature of polymers is responsible for their characteristic property and makes them different from other molecules. Natural polymers include silk, hair, cellulose, proteins, nucleic acids, etc., whereas synthetic polymers include plastics, rubbers, glass, paper, graphene, etc. Polymers are essential to enhance our day-to-day life due to their vast applicability and use in the industry, economy, health, and biomedical sectors. Humans use a wide variety of polymerization techniques to synthesize polymers. However, a polymer's mechanical, structural, and functional application largely depends on the conformation, type, molecular size, weight, and distribution of the monomer units into that polymer. Polymers are widely used in every aspect of human life, from grocery bags, water bottles, toys, textile, electronics, and auto parts to water desalination plants, modern drug delivery systems, artificial tissue, organ development, etc.
Based on the composition, polymers are of two types. Homopolymers are made of one species of monomers, and copolymers contain more than one species of monomers connected to a long chain or network via covalent bonds. According to the skeletal structure, polymers can be (i) linear, represented by chains with two open ends, (ii) branched, with side chains of significant length connected to the main chain at specific branch points (iii) cyclic, with no open ends and possess remarkably different properties than polymers with open chains and, (iv) network polymers in which linear chain are crosslinked with the other chains by specific junction points, and properties of network polymers vary with crosslink density and chain length.
Polymers can be classified into three major classes according to physicochemical properties and their underlying molecular structures. Thermoplastics, commonly known as plastics, are linear or branched polymers that melt to liquid form when heat is applied. Thermoplastics can be molded or remolded into virtually any shape by using appropriate procedures. This group of polymers is the most significant proportion of commercially produced polymers. Elastomers are network polymers of low crosslink density with elastic or rubbery properties. Thermosets are also networking polymers with high crosslink density and are therefore rigid. The variations in molecular structure, length of polymer chains or networks, and degree of cross-linking result in varying degrees of strength, flexibility, heat resistance, and recyclability in different polymer types. [Sohidul Islam]
