Molecular Structure of Polymers:           

The physical characteristics of a polymer depends on its:

• Molecular weight

• Shape

• Structure

1. Molecular Weight:

   • Extremely large molecular weights are to be found in polymers with very long chains.

2. Molecular Shape:

   • Polymer chain molecules are strictly straight that the zig-zag arrangement of the backbone atoms is disregarded. Single chain bonds are capable of rotation and bending in three dimensions.

   • Some polymer consists of large number of molecular chains, each of which are may bend, coil and kink, leading to extensive intertwining and entanglement of neighboring chain molecules. These random coils and molecular entanglements are responsible for a number of important characteristics of polymers.

   • Some of the mechanical and thermal characteristics of polymers are a function of the ability of chain segments to experience rotation in response to applied stresses or thermal vibrations.

3. Molecular Structures:

   • Linear Polymers: Linear polymers are those in which the mer (the group of atoms that constitutes a polymer chain repeat unit) units are joined together end to end in single chain. These long chains are flexible. For linear polymers, there may be extensive van der Waals bonding between the chains. Some of the common polymers that form with linear structures are polyethylene, polyvinyl chloride, polystyrene, polymethyl merthacrylate, nylon and the fluorocarbons

   • Branched Polymers: Polymers may be synthesized in which side-branch chains are connected to the main ones. These are called branched polymers. The branches, considered to be the part of the main-chain molecules result from side reactions that occur during the synthesis of the polymer. The chin packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density

   • Cross linked Polymers: In cross linked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds. The process of cross linking is achieved either during synthesis or by a nonreversible chemical reaction that is usually carried out at an elevated temperature. This cross linking is accomplished by additive atoms or molecules that are covalently bonded to the chains. Many of the rubber elastic materials are cross linked. In case of rubbers, it is called vulcanization

   • Network Polymers: Trifunctional mer units, having three active covalent bonds, form three-dimensional networks instead of the linear chain framework assumed by bifunctional mers. Polymers composed of a trifunctional units are termed network polymers. A polymer that is highly cross linked may be classified as a network polymer. These materials have distinctive mechanical and thermal properties. The epoxies and phenol-formaldehyde belong to this group.