Interfaces and soft confinement promote crystallization in polymer nanodroplets

We apply all-atom molecular dynamics simulations to study the crystallization in nanodroplets of polyethylene in water. We show that these small nanoplastic particles exhibit semicrystalline morphologies under soft nanoconfinement. The polymer-water interface induces orientational ordering of PE at the droplet surface and enhances crystal nucleation kinetics. However, the droplet curvature hinders crystal nucleation and growth. In all the simulations, crystal nucleation prefers to occur near the droplet surface, and crystalline order propagates inward. In sufficiently large droplets, the growth of multiple crystalline nuclei leads to polycrystalline polymer-water interfaces. The growth of randomly orientated crystallites often results in spherical nanoparticles with amorphous centers. In small droplets, however, the reduced numbers of nuclei lead to the formation of large crystals, which can deform the PE nanoparticle from spheres into elongated ellipsoids.