Fullerene ‘peas’ in a polymer ‘pod’

Fullerenes wrapped up in helical polymer bundles form highly ordered one-dimensional arrays.

Making arrays and controlling the molecular order of fullerenes is not an easy task. One-dimensional arrangements of fullerenes encapsulated within carbon nanotubes can been produced but difficulties in reproducibility and processing the peapod-like structures still remain. These problems may limit the development of fullerenes for opto-electronic and devices.

Fig. 1: Helical bundles of PMMA with C₆₀ encapsulated withinReproduced with permission. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Now, in an innovative approach, Eiji Yashima and co-workers, at ERATO, JST and Nagoya University, Japan have encased fullerenes inside helical cavities made of poly(methyl methacrylate) (PMMA)¹.

Syndiotactic PMMA, when heated in aromatic solvents, forms a gel that is composed of helical strands. Here, when C₆₀—a fullerene—was added to this mixture, a purple-colored gel was formed on cooling, which was made of helical polymer bundles with C₆₀ encapsulated inside the bundles. The isotactic form of PMMA did not encapsulate C₆₀, showing that the syndiotactic arrangement was essential for the formation of polymer-fullerene composites.

The crystallinity of the C₆₀-encased bundles increased with higher C₆₀ concentration. By varying the solvent and the ratio of C₆₀ and PMMA, a maximum of over 20% weight of C₆₀ was encased within the bundles. A monolayer of the helical polymer-C₆₀ composites formed crystalline rod-like structures and, on compression, a lamellar arrangement was produced with intermolecular distances between C₆₀ molecules of about one nanometre.

“The easy and inexpensive preparation of the fullerene-encapsulated PMMA into a processable and homogeneous film without phase segregation is an advantage,” says Yashima.

The Nagoya group was able to control which way the polymeric helix twists around the fullerenes by adding chiral molecules at the initial stage of synthesis. One enantiomer of a chiral alcohol induced a right-handed twist, and the other a left-handed twist, and on removal of all trace of the alcohol, the helicity remained unchanged—that is, the polymer had ‘memorized’ the induced chirality.

It was also possible to encase larger fullerenes—C₇₀ and C₈₄—within the helical PMMA and as a result of their increased size, it was found that the helical tubes expanded to fit them inside. Further experiments showed that even in the absence of fullerenes, a chiral solvent induces optically active PMMA helices, opening up the possibility of encapsulating a wide range of small molecules and macromolecules in their cavities.