Materials Science

Getting Packed

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Science  29 Nov 2002:
Vol. 298, Issue 5599, pp. 1681
DOI: 10.1126/science.298.5599.1681b

Many spherical objects, including colloids, surfactants, and polymer micelles, will pack into lattices under the right conditions of temperature and concentration. Many of these systems will undergo an order transition with changing temperature, transforming, for example, between closed packing [i.e., face-centered cubic (fcc)] and body-centered cubic (bcc) packing. For atomic and colloidal systems, the number of particles is conserved, so that the transformation must occur by particle rearrangement. However, micelles can undergo fusion, fission, or shrinkage or simply dissolve.

Bang et al. examined a polystyrene-polyisoprene diblock copolymer as it was thermally driven between the fcc and bcc phases. The transformation occurred epitaxially, through slippage along close-packed planes, and not by a substantial change in the composition of the micelles. The transition was probably driven by a decrease in the solvent selectivity with increasing temperature. The solvent entered the isoprene-rich micellar core, which would predominantly cause the micelles to swell, pushing the system to the looser bcc lattice. The similarity of the polymeric transitions to other atomic systems supports the notion that the bcc phase is thermodynamically preferred near the melting temperature. — MSL

Phys. Rev. Lett.89, 215505 (2002).

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