NEW: Amer. Chem. Soc. PhD studentship available at Sheffield to study structure and crystallization - contact Dr. G. Ungar

NEW: Postdoctoral position available at Durham to synthesise new compounds - contact Dr. G.M. Brooke (g.m.brooke@durham.ac.uk)

Ultralong n-Alkanes: The Ultimate Model Polymers

Polymerization always produces a distribution of molecular weights - polymers are said to be polydisperse. However, in order to study the fundamentals of polymer behaviour we need clean (monodisperse) systems with uniform chain length. Such molecules can only be synthesised by a series of well controlled chemical coupling reactions, involving end-group protection and deprotection and careful purification of intermediate products. Very long chain monodisperse n-alkanes have been successfully synthesised in this way, the longest one so far being n-C390H782.1-3

chain-folded crystal of a polymerOne of the initial aims of the synthesis was to establish if n-alkanes would exhibit chain-folded crystallization characteristic of polymers. An idealized lamellar chain-folded polymer crystal is shown on the right.

Chains longer than 120-150 C-atoms were indeed found to be able to crystallize in a chain folded form. Depending on crystallization temperature, alkane n-C390H782 can adopt any conformation, from fully extended to folded in five.4

It was found that long alkanes have a tendency to fold in such a way as to leave the chain ends at the lamellar surface; this means that a chain makes an integer number of complete traverses through the crystal, i.e. 1 for extended, 2 for once-folded etc. This "integer" folding enables the long alkanes to achieve a high degree of crystallinity in spite of chain folding. Polydisperse polymers are, in contrast, always incompletely crystalline, or "semicrystalline".

The monodispersity of the long alkanes allows the study of the fold surface unconcealed by the usual amorphous polymer layer. New crystallization kinetics phenomena have been discovered in long alkanes and current research focuses on establishing their implication on crystallization (solidification) of polymers in general.

Real-time X-ray scattering experiments of alkane crystallization, using synchrotron radiation, has shown that the integer folded forms are the products of secondary rearrangements. The initial lamellar structure is non-integer folded (NIF) and its structure has recently been established by Fourier synthesis of electron density profiles . The initially formed low-crystallinity NIF transforms into the high-crystallinity integer forms via solid state rearrangements. These are believed to model the post-crystallization phenomena in polymers (cf. mould shrinkage),

Thanks to the new synthesis of long alkane by Dr. Brooke et al.5 we can now also investigate the behaviour of mixtures of long alkanes. Novel complex structures have already been established in binary long alkanes (unpublished), and this area is currently attracting much interest.

Dr. Brooke's team have recently also been synthesising different long chain alkane derivatives, such as branched, star-shaped long alkanes, carboxylic acids, as well as labelled compounds. Some unique modes of molecular organisation are expected in such system, hopefully leading to new nanostructured materials. Thus e.g. 40 nm thick bilayers of alkanoic acid have been obtained.6

  1. Bidd, I., Whiting, M. C. J. Chem. Soc. Chem. Commun. 1985, 543
  2. Bidd, I., Holdup, D. W., Whiting, M. C. J. Chem. Soc. Perkin Trans. 1987, 1, 2455
  3. Lee, K.-S., Wegner, G., Makromol. Chem., Rapid Commun. 1985, 6, 203
  4. Ungar, G., Stejny, J., Keller, A., Bidd, I., Whiting, M.C. Science 1985, 229, 386
  5. Brooke, G.M., Burnett, S., Mohammed, S., Proctor D., Whiting, M.C., J. Chem. Soc., Perkin Trans. 1 1996, 1635
  6. G Ungar and X B Zeng, Macromolecules, to appear May 1999.

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