Polymer physics. Nucleation phenomena. Phase transitions. Engineering Education.
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Research Summary:
Polymer crystallization is the transformation of polymer molecules from a disordered (amorphous) state into an ordered (crystalline) one. At conditions suitable for polymer crystallization, as commonly modeled, there exists a free energy barrier between the amorphous and crystalline states. Nucleation is the initial stage of the crystallization in which a small sized crystalline domain, called as seed or nucleus, start to form until the time the seed passes the free energy barrier. Growth is the second stage of crystallization, and it starts when the free energy barrier is overcome by the seed growing in size.

Our capacity in developing polymeric materials with improved properties, depend heavily on our ability to manipulate the physical and chemical events occurring during the processing of polymers, which relies on a better understanding of the physics behind these events. For instance, the extent of crystallization is known to be related to the durability of a polymeric product and thus exploring how the polymer crystallization happens most likely help us produce high strength polymeric materials.

Polymer crystallization is one of the most important problems in polymer science. There has been a tremendous amount of scientific work devoted to understanding the polymer crystallization phenomenon. Yet, there has not yet been a unified understanding of how it proceeds in spite of this huge effort. Although with the advance of experimental techniques we learned a lot about this process, it is still not possible to produce experimental results in the nanometer length scale resolution. One can experimentally count in how many samples, each having a size of tens of nanometers, crystallization happens, but it is not possible with the current technology to see what is happening inside a nanometer sized droplet. Molecular simulations provide a fantastic set of tools to monitor what may happen inside a polymeric droplet during the crystallization. Results from such simulations to uncover polymer crystallization in nanodroplets, have been reported to a limited extent.

In our work performed with polymer droplets of various sizes, we remarkably found that the nucleation rate per unit volume increased with decreasing droplet size. Furthermore, we found that the nucleation primarily occurred near the surface of the droplets. The reason why the nucleation rate increases with decreasing droplet size remains to be uncovered. We need more work on the simulation of polymer crystallization within nanodroplets to better understand this process in such systems.