Macromolecular order in plastic kingdom

Pentad. The relative abundance of pentads — isotactic five-unit segments in a polymer chain — is a good measure of the polymer’s overall isotacticity. Credit: MIPT

A team of researchers at the Institute of Synthetic Polymer Materials of the Russian Academy of Sciences, MIPT and elsewhere has determined how the regularity of polypropylene molecules and thermal treatment affect the mechanical properties of the end product. Their new insights make it possible to synthesize a material with predetermined properties such as elasticity or hardness. The paper detailing the study was published in Polymer.

In terms of production volume, polypropylene it is second only to polyethylene. By tweaking its molecular structure, polypropylene can be used to manufacture materials with a wide range of features, from elastic bands to high-impact plastic. However, the relationship between the polymer‘s chemical structure and its mechanical properties is not fully understood.

What makes the properties of polymer materials so variable is their makeup. A polymer molecule is a long  of repeating units of unequal length. If these molecules are jumbled up more or less at random in a material, it is said to be amorphous. Such polymers are soft. In other materials, the polymer chains form interconnections called crosslinks. This gives rise to regions of highly regular atomic structure (fig. 1), similar to that of crystals, hence the name crystallites. They hold the whole molecular network together, and the more crystallites there are in a material, the harder it is. To form crosslinks, molecular chains need to possess a certain structural regularity called isotacticity.

Macromolecular order in plastic kingdom
Fig. 1. Polymer chains with the crystallites circled. Credit: MIPT

A polypropylene chain consists of a backbone of carbon atoms with attached hydrogen atoms. Every other carbon atom in the chain has a methyl group attached to it. Two adjacent carbon atoms in the chain with the  and the methyl group bonded to them constitute a repeating unit called propylene, or propene. The spatial configuration of the macromolecule—the polymer chain—is determined by the mutual orientation of the methyl groups in the chain (fig. 2): If they are all on one side, the molecule is said to be isotactic. If they alternate between facing one way and the other, the arrangement is known as syndiotactic. The absence of any consistent pattern is referred to as atacticity. Isotactic chain segments are very effective at forming crosslinks. Therefore, a higher degree of polypropylene isotacticity results in a stronger material. Chemists can synthesize polypropylene with predetermined isotacticity. The authors set out to establish the precise relationship between the material’s  and isotacticity.

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