Shuaiyuan Han, Jacques Jestin, Erwan Nicol, Olivier Colombani, Laurent Bouteiller

[ Hal | Nature ]

Janus cylinders are one-dimensional colloids that have two faces with different compositions and functionalities and are useful as building blocks for advanced functional materials. Such anisotropic objects are difficult to prepare with nanometric dimensions. We recently described a robust and versatile strategy to form micrometer long Janus nanorods with diameters in the 10-nanometer range, by self-assembly in water of end-functionalized polymers. The Janus topology is driven by the interactions between unsymmetrical and complementary hydrogen bonded stickers (in green on the scheme). Therefore, even compatible polymers (in blue and red) can be used to form these Janus objects.

Small angle neutron scattering (SANS) was used to prove the segregation of the chains, by taking advantage of the contrast between hydrogen and deuterium. Particles (named J) were prepared with hydrogenated poly(dimethylacrylamide) (PDMAc) on one side and partially deuterated PDMAc (PDMAc(D6)) on the other side. In deuterated water, PDMAc(D6) has a contrast that is 14 times lower than PDMAc, so that its contribution to scattering can, in first approximation, be neglected.

Probing the segregation of polymer chains on Janus nanorods by SANS
Probing the segregation of polymer chains on Janus nanorods by SANS

The particles J are nanorods as shown by a q-1 dependence at low q. A fit with the form factor of a cylinder of elliptical cross-section and homogeneous contrast yields the diameters dminor = 7.6 nm and dmajor = 9.4 nm. The question is : how are the deuterated chains distributed on the nanorods J ? The answer is obtained by comparing with reference particles made from fully hydrogenated PDMAc (i.e. symmetrical particles S). If the deuterated chains were randomly distributed on J, the cylinders should have the same cross-section as S, but with a lower (homogeneous) contrast. Therefore, the normalized scattering data for both experiments should be identical. This is not the case. Conversely, if the deuterated chains are segregated in one half of the cylinder, this part of the object becomes virtually invisible to SANS but the other cylinder half should have the same contrast as S. Actually, the data for S can be fitted quite well with the form factor of a cylinder with the same homogeneous contrast and the same minor diameter (dminor = 7.6 nm) as J, but a much larger major diameter (dmajor = 12 nm), confirming that the deuterated chains on J are strongly segregated. These SANS results are in strong support of the Janus structure.

These results have been confirmed by electron microscopy and NMR data. To illustrate their applicative potential, we have shown that these Janus nanorods can stabilize oil-in-water emulsions more efficiently than both the corresponding block copolymers and the homogeneously covered particles.