2D polymer single crystals have attracted much interests recently due to their unique mechanical and optical properties. Recently, Prof. Wenbing Hu’ group at Nanjing University, coauthored with Prof.Marcus Müller at University of Göttingen and Prof. Christopher Y. Li at Drexel University, published their collaborative work on Physical Review Letters with the title of “Anomalous Ostwald Ripening enables 2D Polymer Crystal via Fast Evaporation” (Physical Review Letters, 123, 207801 (2019)). Using dynamic Monte Carlo simulations, they predicted a unique crystalline behaviour within the skin layer induced by the fast evaporation on top of the polymer solutions. Simulation results demonstrates that at the suitable chain length and evaporation rate, the crystalline nuclei parallel to the free surface grows while other nuclei shrinks, eventually the nuclei grows toward large-scale 2D polymer single crystal. The grow kinetics of nuclei shows an anomalous Ostwald ripening process, which can be attributed to the interplay between the thermodynamically driven diffusion of noncrystalline fragments toward the growing nuclei and the diffusive current away from the free surface caused by the nonequilibrium skin layer. The findings might be beneficial for fabricating large-scale 2D polymer single crystals, and are also helpful for understanding the crystallization kinetics during the uni- and bi-axial stretching, and further tuning the structures and properties of semi-crystalline polymer films and fibers. Dr. Qiyun Tang was the first author.
Figure 1 shows the monomer densities and the snapshots of the crystalline nuclei during the fast evaporation process. One can see the skin layer at the free surface of the polymer solutions. For crystalline polymers, their crystalline ability in solutions are highly related to the monomer densities: inset of Fig. 1(a) shows that as monomer density increases from 0.05 toward 0.65, the reduced crystalline temperature increases from 2.0 to 3.1. Therefore at suitable temperatures (such as 3.0), the bulk solution of 0.1 can not crystallize, whereas the polymers in skin layer might crystallize once the densities approach 0.65, and the crystallization will solely be constrained in the 2D region. Figure 1(b) shows the typical snapshots of crystalline nuclei during the fast evaporation process, where large nuclei grow and small ones shrink, showing a typical Ostwald ripening process. Eventually the nuclei grow to 2D polymer single crystal at t=60.0τ.
