Crystals of polystyrene

Chemists in Japan and Italy have created a polymer-based material that has a crystalline structure. The material, which achieves its crystallinity with crosslinks between its polymer chains, is expected to have a high mechanical strength that will lend itself well to engineering applications.

The structure of fibrous materials has a big impact on their properties; the high-performance material Kevlar, for instance, owes its high tensile strength to aligned polymers that are anchored together with numerous hydrogen bonds. For this reason, chemists have been keen to hone the structure of polymers that do not naturally crystallise.

To achieve this, liquid-crystal polymers can be aligned simply by rubbing the surface on which they are applied, while π-conjugated polymers – those in which electrons are shared over multiple atoms or bonds – can achieve alignment through the addition of guiding molecules. On the other hand, long strands of DNA can be folded into different shapes by ‘stapling’ them with shorter strands. The trouble, however, is that these methods are either too specific or cannot produce alignment that withstands heat or solvents.

Takashi Uemura at Kyoto University, together with colleagues from there and the University of Milan, Bicocca, has developed a hardy method to crystallise polymer materials. The researchers embed ‘crosslinkers’ of 2,5-divinyl-terephthalate in the walls of a porous coordination polymer (PCP), before introducing a vinyl monomer – styrene – into the PCP’s parallel channels. They then polymerize the monomer and crosslinkers together, before dissolving the PCP in the acid EDTA. The result is crosslinked, crystalline polystrene.

This new material has a density of 1.13 g/cm³, some 8% greater than the normal density of polystyrene, which is about 1.05 g/cm³ and can also withstand organic solvents and temperatures of up to 200°C. Uemura believes that the material will have enhanced mechanical strength, and direction-dependent optical properties. ‘Our polymers may find an application [as] engineering plastics,’ he says.

Macromolecular chemist Jean-François Lutz at the University of Strasbourg in France calls the work a ‘beautiful example of precision polymer chemistry’. ‘In nature, chemistry in confined environments is very often used to regulate biomaterial synthesis, [but] this strategy is under-explored in manmade materials,’ he says. ‘Here, a porous coordination polymer was used to template the synthesis of a polystyrene network.’

Materials chemist Masayuki Takeuchi at the National Institute for Materials Science in Tsukuba, Japan, agrees. ‘Uemura [and colleagues] have gone beyond the place where many scientists, including me, wanted to go,’ he says. ‘This paper clearly demonstrates that conventional polymers still have a potential to be real structural materials which only contain organic polymers, when they are highly aligned and dense.’