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Materials

Polypropylene Found In New Form

Plastics: Fifth crystal form of commercial polymer could find industrial use

by Stu Borman
November 6, 2014 | A version of this story appeared in Volume 92, Issue 45

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Credit: Courtesy of Bernard Lotz
A unit cell of the new ε crystal phase of iPP. Gray is carbon in main chain and in isotactic methyl groups, yellow is carbon in stereodefective methyl groups, white is hydrogen, and blue dashed lines are unit-cell axes.
Unit-cell structure of new form of polypropylene. Blue lines are unit-cell axes. Full caption TK.
Credit: Courtesy of Bernard Lotz
A unit cell of the new ε crystal phase of iPP. Gray is carbon in main chain and in isotactic methyl groups, yellow is carbon in stereodefective methyl groups, white is hydrogen, and blue dashed lines are unit-cell axes.

Isotactic polypropylene (iPP), the number-two commercial polymer, was known for decades to adopt three crystalline forms, α, β, and γ, and in the past decade a fourth form, δ, was identified. Now, a researcher in France has found a fifth crystal form, ε. If ε-iPP can be better characterized and its synthesis scaled up, it could have industrial applications—for lightweight iPP formulations, for example.

In iPP, all of the polymer’s methyl groups, one per propylene unit, reside on the same side of the main chain. The polymer was discovered in the 1950s. Since then, it has seen wide commercial adoption, by itself and combined with other plastics, to create products ranging from chairs to carpets to diapers.

The α, β, and γ forms have varied properties and are used industrially in different ways. Scientists are still looking for commercial applications for the δ form. Now, polymer chemist Bernard Lotz of the National Center for Scientific Research (CNRS) and the University of Strasbourg, in France, reports a fifth form, ε-iPP, which he found and characterized using electron microscopy and electron diffraction (Macromolecules 2014, DOI: 10.1021/ma5009868).

Lotz found the ε crystals in a stereodefective sample, in which some methyl groups are on the wrong side of the chain. The new phase was a minor component—probably less than 1%—of a very small sample. And the sample was lost when Lotz’s lab was moved to a new location. “These circumstances are quite unfortunate, but they do not weaken the major message conveyed by this work—observation of a new crystal modification,” Lotz writes.

“The new polymer has not yet been characterized as much as would be desirable,” says polymer chemist Stefano Valdo Meille of Polytechnic University of Milan, in Italy. Still, he concurs with Lotz that substantial stereodefectivity is most likely a key factor in its formation. Lotz’s work, Meille says, will stimulate scientists “to reproduce the data and provide complementary results that will allow them to gauge the value of the proposed structure and its relevance.”

“The work is a tour de force of structural modeling and single-crystal electron diffraction analysis,” says polymer and materials chemist and dean of engineering Edwin L. Thomas of Rice University. “The fact that the crystal is about 10% less dense than any of the other crystal polymorphs of iPP, and even 2% less dense than the amorphous phase, has obvious consequences for physical properties—a lower melting point.” Whether this lower-density form “of a hugely important polymer would bring a new lighter-weight iPP to the market” remains to be seen, Thomas says.

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