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How to shuffle a nano-deck of cards


X-ray scattering measurements reveal that the orientation and stacking state of nanoplatelets confined within a polymer matrix can be reversibly controlled simply by pulling on the material.

A very promising strategy for obtaining new materials consists in dispersing nanoparticles with interesting properties in a medium that is easy to shape and manipulate ; in such nanocomposites, the main difficulty is controlling the aggregation state of the particles, as well as their position and orientation. A team of researchers from the Laboratoire de Physique des Solides and the SOLEIL synchrotron has just shown that all these parameters can be reversibly tuned by a stretching deformation [2].

The employed nanocomposite consists in a matrix of block-copolymer chains (polystyrene ─ polybutadiene ─ polystyrene) doped with fluorescent CdSe platelets, eight atoms thick and fifteen times as wide. The platelets assemble spontaneously in stacks, confined in the polybutadiene domains (Figure 1a). Using a traction device, the material is submitted to controlled strain, while being exposed to the strong X-ray beam of the SWING beamline of the SOLEIL synchrotron (Figure 1b). The scattering pattern thus recorded provides information on the evolution of the lamellar micro-structure and of the platelet stacks.

Figure 1 : a) Electron microscopy image of the composite system (from [1]). The scale bar is 50 nm. In the inset, a few platelet stacks (zoom x3). b) Diagram of the experimental setup.

In agreement with previous results on these systems, we detect a reorientation of the lamellar domains under strain. The surprising result is that this motion destabilizes the platelet stacks, shears them and tilts them on the side, as a deck of cards cut in two (Figure 2a). This transformation is reversible since, after releasing the stress, the stacks return to their initial size and orientation (Figure 2b). Moreover, the process can be inhibited by reinforcing the interaction between platelets in the stack using excess oleic acid, a molecule that renders them compatible with the matrix.
Figure 2 : a) Evolution of a platelet stack under strain : the initial pack is sheared and split into two smaller packs, perpendicular to the initial orientation. b) This transformation is reversible : the number of platelets (measured from the X-ray scattering patterns) returns to its initial value once the stress is released.

This discovery not only raises fundamental questions concerning the interaction of nano-objects with their environment, but also suggests new ways of formulating and processing composite materials with original properties.

References

[1] Strain-controlled fluorescence polarization in a CdSe nanoplatelet–block copolymer composite, E. Beaudoin, B. Abécassis, D. Constantin, J. Degrouard and P. Davidson. Chemical Communications 51, 4051-4054 (2015).
[2] Reversible strain alignment and reshuffling of nanoplatelet stacks confined in a lamellar block copolymer matrix, E. Beaudoin, P. Davidson, B. Abécassis, T. Bizien, and D. Constantin. Nanoscale 9, 17371 (2017) doi:10.1039/C7NR05723G.

Contacts

Emmanuel Beaudoin
Doru Constantin