Interfaces and soft confinement promote crystallization in polymer nanodroplets

We employ all-atom molecular dynamics simulations to study polyethylene (PE) crystallization under soft nanoconfinement in water. The hydrophobic PE phase separates from water and forms nanodroplets above the crystal melting temperature. By cooling the PE nanodroplets of different sizes in simulations, we show that the polymer-water interface can induce partial orientational order to polymer segments in the precursor state and enhance crystal nucleation. Curvature imposed by nanoconfinement, however, hinders PE crystallization, resulting in rare occurrences of crystalline atoms at highly curved droplet centers. We observe crystals nucleate at the PE-water interface and grow inward. For large droplets, the growth of multiple nuclei formed at the droplet interfaces leads to polycrystalline droplets after crystallization. Sufficiently small droplets can deform and elongate to accommodate the formation of large single crystallites and high crystallinity.