Polymer Engineering Lecture 1: Introduction to Polymers

Introduction to Polymers

• Polymer is a large molecule comprising repeating structural units joined by covalent bonds.
• The word "polymer" comes from the Greek words "poly" meaning many and "mer" meaning unit or parts.
• Monomer is a small molecule or repeating unit in the structure of a polymer, which has a functionality of at least two.

Polymerization

• Polymerization is a chemical reaction in which two or more molecules of one or more substances combine to form a molecule of high molecular weight.
• Degree of polymerization (DP) is the number of monomer or repeating units in a polymer chain.

Classification of Polymers

Based on Source

• Natural polymers: found in nature, examples include proteins, cellulose, starch, resins, and rubber.
• Semi-synthetic polymers: obtained by simple chemical treatment of natural polymers, examples include starch and silicones.
• Synthetic polymers: obtained by polymerization of simple chemical molecules in a laboratory, examples include nylon, polyethene, polystyrene, synthetic rubber, PVC, and Teflon.

Based on Structure

• Linear polymers: monomers are linked to form a long straight chain with no side chains, examples include polyethene, PVC, and nylons.
• Branched chain polymers: have a straight long chain with different side chains, examples include polypropylene, amylopectin, and glycogen.
• Crosslinked or network polymers: two linear chains are joined together by covalent bonds, examples include Bakelite, melamine, formaldehyde resins, and vulcanized rubber.

Based on Mode of Polymerization

• Addition polymers: formed by the repeated addition of monomer molecules possessing double or triple bonds, examples include polythene and polypropene.
• Condensation polymers: formed by the combination of two monomers by removal of small molecules, examples include nylon 6, 6.

Based on Molecular Forces

• Elastomers: rubber-like solids with elastic properties, examples include buna-S, buna-N, and neoprene.
• Fibers: thread-forming solids with high tensile strength, examples include polyamides (nylon 6, 6) and polyesters (terylene).
• Thermoplastic polymers: can be softened on heating and hardened on cooling, examples include polythene, polypropene, and PVC.
• Thermosetting polymers: hard and infusible on heating, examples include bakelite, urea-formaldehyde resins, and melamine.

Based on Type of Monomers

• Homopolymers: contain a single type of repeat unit, examples include polystyrene.
• Copolymers: made up of two different monomers, examples include butadiene-styrene rubber.

Based on Morphology

• Crystalline polymers: have crystalline and amorphous regions, characterized by their melting temperature (Tm).

Based on End Use

• Rubbers: dimensionally unstable, characterized by low molecular cohesion and poor mechanical properties.
• Plastics: stronger than rubbers, exhibit tensile strength between 4000-13000 psi.
• Fibers: strongest of the three, exhibit tensile strength between 20000-150000 psi.

Bonding in Polymers

• Various types of bonding may exist in polymeric materials, including covalent bonds, van-der waals forces, and hydrogen bonding.