A team of researchers at the Institute of Synthetic Polymer Materials of the Russian Academy of Sciences, MIPT, and elsewhere has found out 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.
Polypropylene is so ubiquitous one might call it the king of plastics. In terms of production volume, 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 was not fully understood.
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 hydrogen atoms 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 : 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 mechanical properties and isotacticity.
Using the results of mechanical tests, researchers plotted a stress-strain curve for each of the samples. They found that the behavior of the samples under strain was related to their isotacticity and thermal prehistory — that is, whether they were cooled slowly or rapidly. The researchers depicted this relationship as a dependence of the elastic modulus on the degree of crystallinity. Higher elastic moduli correspond to tougher materials. The degree of crystallinity is the percentage of the crystallites — as opposed to amorphous regions — in the volume of the material. The team also showed that the crystallites in quenched and slowly cooled samples differed in their form.
• A way to precise control the mechanical properties of polymers was proposed.
• Isotactic polypropylenes with different number of stereodefects were studied.
• Haward and Doi and Edwards models were successfully applied.
• Role of entanglements of different nature was elucidated.
Studies of polypropylene with different degrees of isotacticity have shown a way of the rational design of material with predetermined mechanical properties starting from the synthesis stage already – controlled introducement of stereodefects will allow the smooth adjustment of the Young’s modulus and elasticity in the range from plastic to elastomer materials. It was also revealed that modern theoretical models of the elasticity can be successfully applied not only for the description of the mechanical behavior of polymers, but also for better understanding of the mechanism of elasticity in them. While in the low crystalline materials deformation has Gaussian nature, in the materials of the intermediate crystallinity (30–40%) percolation takes place, and the cross-linking network becomes harder, manifesting the switch to the thermotropic behaviour of the material. Simultaneously the divide between cross- and slip-links becomes substantial, as an extensibility grows sharply.
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