The properties of a polymer largely depend on its structure. As already explained, polymers. are long-chain molecules and are formed by polymerization process which consists of linking and cross-linking of different polymers. 

Polymers may be 

(a) linear polymers, (b) branched polymers, (c) cross-linked polymers or (d) network polymers (Fig.). Linear polymershave chainlike structure which is a sequential type structure as found in some thermoplastics (such as acrylics, polyethylene, nylons and PVC). Branched polymers have side branch chains attached to the main chain. Since branches create interference to relative movement of the molecular chain, the arrangement helps increasing resistance to deformation (and thus increase in strength of the polymer). 

Branched polymers have lower density since branches create interference with packing efficiency of the polymer chains. Cross-linked polymers give three-dimensional structure and are the characteristic structure f most thermosetting plastics, rubbers or elastomers. The network polymer structure comprises three-dimensional networks of three or more active covalent bonds and is a highly cross-linked polymer, examples include epoxies and phenolics. When loge-chain. molecules in a polymer are cross-linked in a network for three-dimensional arrangement the structure results into one giant molecule having strong covalent bonds.

These polymer are called thermosetting plastics (or thermosets) The curing reaction or cross-linking in thermoset plastics is irreversible and is unlike that of thermoplastics which can be softened and remolded into various shapes as many times as needed. The thermoset polymers produced by cross-linking are not affected by temperature rise to that extent as thermoplastics are affected. Also, the cross linked polymers (thermosets have better mechanical strength thermal and electrical resistance, chemical properties and dimensional stability than do the thermoplastics (which in most cases are linear polymers).

Structure of Polymers

Showing polymer chains: (i) Linear structure, Examples: acrylics, mylons, polyethylene and PVC, (ii) Branched structure, Examples: polyethylene, (iii) Cross-linked structure, Examples elastomers and rubbers, (iv) Network structure, Examples: epoxies and phenolics.

The monomers get linked together by covalent bond (or primary bonds) in forming a polymer chain during polymerization. In polymerization, polymers are bonded together by secondary bonds also (such as hydrogen bond, ironic bond or van der Waals bonds) which are weaker than primary bonds but they decide overall strength of the polymer. Linear and branched polymers have weak secondary bonds.

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