Materials Science

Flip-Flops

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Science  29 Aug 2003:
Vol. 301, Issue 5637, pp. 1159
DOI: 10.1126/science.301.5637.1159c

Most mixtures of polymers are unstable and form separate phases if the chains are sufficiently mobile. Separation can be hindered by linking the two polymers chemically or by grafting the chains to a common surface; even so, segregation can occur over the length of the polymer chain. Julthongpiput et al. combine these two restraints by connecting polystyrene (PS), which is hydrophobic, and polyacrylic acid (PAA) to a common stem that is then grafted to a silicon surface. Under dry conditions, the arms collapse to give a generally smooth surface with intermingled chain segments. When exposed to toluene, which solvates PS, the grafted chains segregate locally, and the resulting grainy surface structure is proposed to represent a cluster of local chains coming together to form a micelle with a PAA core enveloped by PS arms. On exposure to water, the chains reorganize, and the micelle turns inside out. As there is a higher volume fraction of PS, the PS core is not quite covered by PAA segments, and the micelles take on a dimpled or cratered structure. In a nonselective solvent, both arms should be fully extended, and this sort of surface may be of use in assembly on nanometer-sized objects and in microfluidics. — MSL

Langmuir 10.1021/la035007j (2003).

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