21.6: Polymer Classification: Architecture
Polymers are classified as linear or branched on the basis of their chain architecture. The polymer chains in linear polymers have a long chain-like structure with minimal to no branching at all. Even if a polymer features large substituent groups on the monomer, which appear as branches to the skeleton, it is not considered a branched polymer. A branched polymer contains secondary polymer chains that arise from the main polymer chain. The branching occurs when the polymer growth shifts from the edge of the growing polymer to other areas in the chain. Similar to small molecules, branching prevents close packing, and the open structure minimizes the locations where dispersion forces can act between two polymer chains.
Consider the example of ethylene polymerization. Different grades of polyethylene are prepared by altering the polymerization conditions. High-density polyethylene (HDPE) is one of them. As the name indicates, it exhibits high density due to the close packing of linear polymer chains with minimum branching. This polymer melts at 135 °C and is used to manufacture relatively hard objects such as bottle caps, television cabinets, etc. Low-density polyethylene (LDPE) is another grade of polyethylene—a low-density polymer due to the extensive branching in the polymer chain. The melt transition temperature of this polymer is 120 °C, lower than high-density polyethylene. It is used to make flexible objects such as squeeze bottles, plastic carry bags, etc. The structures of high-density polyethylene and low-density polyethylene are shown in Figure 1.
Figure 1: Skeletal structures of high-density polyethylene (top) and low-density polyethylene (bottom)
Some modifications to the polymer chain structures are achieved by post-processing; for example, vulcanization of rubber. During vulcanization, sulfur reacts with polyisoprene to replace some C–H bonds with disulfide bonds. These disulfide bonds can connect different polyisoprene chains, and this type of bond is known as cross-linking. The cross-linking increases the rigidity of the polymer because most of the chains are linked. As a result, the relative movement of adjacent chains is diminished. So, the rigidity and elasticity of the rubber are tuned by controlling the amount of sulfur used for vulcanization.