As described in our surfactant explainer article, polymers can aggregate into structures called micelles, with their orientation dependant on their interaction with the solvent they are in. Polymers that have two hydrophobic ends, but a hydrophilic centre, are called telechelic polymers, and can form flower-like micelles when both ends aggregate at the centre of the micelle.
This study, published in Physical Review Letters, shows that the exchange mechanism of polymers from one micelle to another involves the formation of a bridge to a nearby micelle. This mechanism was determined after the research team were able to carry out a series of well-planned experiments that proved which of several conceivable mechanisms was responsible.
Due to the opposite neutron scattering properties of hydrogen and deuterium, the group were able to use neutron scattering to watch when proteated and deuterated micelles exchanged enough molecules for the signal to be averaged out to zero, as shown below. This enabled them to monitor the rate of the reaction under different conditions such as concentration and temperature. They also compared the results with the same experiment carried out with a single-ended polymer.
Illustration of the kinetic zero average contrast technique to monitor chain exchange in diblock and triblock mixed micelles.
Adapted from Phys. Rev. Lett. 124, 197801
They were able to show that the exchange mechanism did not involve the simultaneous release of both ends of the polymer from the original micelle. By developing a kinetic model to explain their experimental findings, they could provide a complete picture of the exchange mechanism, and how it is dependent on the collision with a second micelle that is then bridged by the polymer.
Visualization of the exchange mechanism of telechelic chains in flowerlike micelles and clusters of micelles via a sequence of consecutive equilibrium steps involving collision-induced exchange. Diffusion of free telechelic chains as an important pathway is excluded. Reproduced from Phys. Rev. Lett. 124, 197801
"We looked at how the molecules move between micelles formed by self-assembly of "double ended polymers". We discovered a novel mechanism that involves collisions of the micelles on the nanoscale," Explains Nico König and Reidar Lund from the University of Oslo; “This was only possible with neutrons that allow us to track molecules over time without disturbing the equilibrium. The results might be relevant for transport mechanism in biological systems, as well as for applications in nano-medicine and cosmetics."