What are the types of polymerization?
When we talk about the fiber properties it is important to gain mindful knowledge of why certain fiber behaves in a particular manner and answer to this is the molecular structure of that fiber. Molecular structure talks about the monomers and polymers a fiber is composed of.
What are monomers and polymers?
Textile fibers, like most substances, are composed of molecules. The word monomer can be broken into mono means ‘one’ and mer means ‘units’. So, the monomer is a single and very small unit but is chemically reactive. These monomers combine to form a polymer (poly means ‘many’ and mer means ‘units’). Thus, a polymer is a long and linear chain molecule but is chemically unreactive. A polymer is often referred to as the building block or backbone, consisting of atoms that are linear and bonded to one another.
The process or chemical reaction that involves the joining of monomers end-to-end to form a polymer is referred to as polymerization. However, the degree of polymerization is defined as the number of monomers that combine to form one polymer. It helps to determine the length of these long-chain polymers relatively easily.
Also, some fibers may consist of two or more different monomers to form a polymer that repeats in the entire structure.
Let us now understand the monomer for some common fibers: 1. Cotton- cellobiose (basic unit of cellulose and consist of two units of glucose) 2. Flax- cellobiose 3. Silk- fibroin (consist of 16 different amino acids) 4. Wool- keratin (composed of twenty different amino acids) 5. Rayon- cellobiose 6. Acetate- cellobiose (about two hydroxyl groups per glucose unit) 7. Nylon 6- caprolactam 8. Nylon 6,6- Adipic acid and Hexamethylene diamine 9. Polyester- Ethylene glycol and Terephthalic acid 10. Acrylic- Acrylonitrile 11. Modacrylic- Acrylonitrile
TYPES OF POLYMERISATION:
The process of polymerization is still unknown for natural fibers like cotton, flax, wool and silk. But the polymerization in the case of regenerated and synthetic fiber is well studied. The man-made or regenerated fibers, polymerization is of two types: addition and condensation.
Addition polymerization is also referred to as chain-growth polymer formation. As the name suggests, addition polymerization is a process by which two identical monomers add or combine end-to-end without producing any by-product.
The fibers which undergo addition polymerization are acrylic, modacrylic, polypropylene, polyethylene, and polyvinyl chloride.
Condensation polymerization is also referred to as step-growth polymer formation. This type of polymerization involves the end-to-end joining of two different monomers and produces by-product. Water and sometimes hydrogen chloride or ammonia, depending on the monomer involved, are some of the by-products liberated as a result of condensation polymerization.
The fibers which undergo condensation polymerization are nylon, polyester and elastomer.
A knowledge of polymerization helps to understand the synthesis and manufacturing process involved in the production of textile fibers.