D13. Briefly discuss how the molecules of a polymer are held together and explain why polymers do not have a high degree of crystallization (long-range ordering) except those with simple or no side groups.

Linear polymer chains are held together by polar bonds formed between the chains. The strongest polar bonds are based on hydrogen bonding, while the weaker are those formed by van der Waal forces. In addition to the polar bonds, polymer chains can also be entangled into each other, which results in a mechanical interlocking. These two bond mechanisms explain why a polymer that is stretched very fast fails in brittle way, while if it is exposed for loads over time it tends to "creep." A fast load stretches the chains and tightens some loops. Under these conditions, the material fails by breaking some of the entangled chains and their covalent bonds. This explains the brittle failure during fast loading. If, however, the resin withstand the load over some time, chains start to slip and rearrange and the resin deforms plastically.  

The above figure shows how short chains are lined up and held together by van der Waal's forces (top left section of figure). As the chain length increases, the van der Waal's forces remain holding the chains together (top right section of figure), but at a certain point, the chains may start folding and lining up to form organized regions (crystalline lamellae) shown in the lower section of the above drawing. The crystalline lamellae are separated by entanglements forming amorphous regions. In order to crystallize, the polymer chains must be linear and without side groups. An example regarding crystalline polymers is high-density polyethylene, a resin that has superior properties compared to the amorphous polyethylene.

Since most polymers have side groups, crystallization is limited in most polymers.