Filming the solution self-assembly of block copolymers in slow motion

When surfactants self-assemble in water, the structures formed, known as micelles, are at thermodynamic equilibrium, i.e. the system succeeds in minimizing its free energy. In the case of large molecules such as polymers, self-assembled objects are often kinetically frozen in metastable states associated with a wide variety of structures. Indeed, the relaxation mechanisms of polymer chains are so slow that the self-assembled structures observed are not the most thermodynamically stable but those that form most rapidly, and the experimental conditions therefore play an important role. Slowing down self-assembly should allow polymer molecules to relax at each stage and reach their equilibrium state. But how can such a process be slowed down, and how can it be observed?

To answer these questions, physicochemists from four laboratories used a dialysis device developed at ILL by the Large Scale Structures group, allowing slow diffusion of water in a polymer solution prepared in a water-miscible solvent. This enables the hydrophobic effect responsible for self-assembly to be activated very gradually. While the dialysis technique is well known to physicochemists, the originality of the device lies in the possibility of monitoring the assembly mechanisms in situ by combining light scattering and small-angle neutron scattering, both of which are suitable for studying nanostructures ranging from 1 to 100 nm. Under these conditions, the whole self-assembly process of amphiphilic diblock copolymers made of poly(ethylene glycol) and poly(dimethylsiloxane) (PEG-b-PDMS) could be followed and the phase diagrams established under conditions of quasi-thermodynamic equilibrium. The study was rounded off by cryo-electron microscopy and surface tension analysis of the structures formed.

This method, presented in ACS Macro Letters, opens up interesting prospects for researchers seeking to understand the assembly mechanisms of synthetic or biological macromolecules. The dialysis technique implemented at the Institut Laue-Langevin on the large dynamic range SANS diffractometer D22 spectrometer enables the slow diffusion not only of solvents but also of any other molecular entity capable of triggering self-assembly, such as acids or bases, ions, hydrogen bond donors/acceptors or biological ligands.