Materials Science

Solidifying into Shape

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Science  07 Sep 2007:
Vol. 317, Issue 5843, pp. 1297
DOI: 10.1126/science.317.5843.1297a

Finding materials that spontaneously form specific ordered patterns is a key for bottom-up fabrication strategies at the nanometer scale. Block copolymers, in which two or more dissimilar polymers are chemically linked, have the advantage in this respect of being thermodynamically driven toward regularly spaced patterns. One shortcoming though is that tuning the pattern often requires synthesis of a new block copolymer. Ejima et al. show that directional crystallization, a technique that has been used to enhance the patterning of block copolymers, can also be applied to a mixture of dissimilar homopolymers, which would normally phaseseparate into large disordered regions. They combined poly(L-lactic acid) (PLLA), a crystallizable polymer, with poly(vinyl acetate) (PVAc), one of many amorphous polymers with which PLLA is miscible. The two were dissolved at high temperature in hexamethylbenzene (HMB) and afterward transferred to a heating stage below the melting temperatures of HMB and PLLA but above the glass transition temperatures of both polymers. As the HMB crystallized along a growing front, the concentration of PLLA increased until it too formed axial crystals in the same direction (as distinct from the usual spherulitic morphology). Pitch spacings of 69 nm were observed over large sample areas and were tunable by variation of the PVAc fraction. — MSL

Macromolecules 40, 6445 (2007).

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