Simulating polymer crystallization inside nanodroplets

During the crystallization process, polymer molecules transform from a disordered state into an ordered state. The ability to develop plastic materials with improved properties depends on the manipulation and processing of polymers, and hence knowledge about this underlying physics.

While there has been a large effort to study polymer crystallization, there is not yet a unified theory for how it proceeds. Experimental techniques cannot produce results at the necessary nanometer scale. Hasan Zerze developed molecular simulations to monitor what happens inside a polymeric droplet during crystallization.

Nucleation, the initial stage of crystallization, occurs when a small seed or nucleus begins to form in the droplet. Growth begins after the seed overcomes a free energy barrier between the disordered fluid and the ordered crystalline phases.

The simulations examined polyethylene droplets, ranging from 6 to 11 nanometers, in a vacuum. The system was cooled to an optimal temperature for crystallization, then monitored over time.

By varying droplet size, Zerze was able to decouple the effects of surface and bulk on the crystallization process. The nucleation rate per unit volume increased with decreasing droplet size, and the nucleation occurred primarily at the droplet surface.

“It has been assumed so far that the number of nucleation events per unit time increases linearly with increasing droplet volume,” he said. “This critical assumption has never been tested, although it is used in many studies to interpret experimental findings. Our study indicates that this assumption is most likely not valid and that the previous experimental findings might require some reinterpretation.”

In the future, Zerze hopes to uncover the reason why nucleation rate increases with decreasing droplet size.

Source: “Nucleation and growth of crystals inside polyethylene nano-droplets,” by Hasan Zerze, Journal of Chemical Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0105466 .