Microgravity crystal growth in a magnet

Abstract

Polymer crystallization is notoriously difficult to control at a local scale because the growth of polymeric crystals is complicated by the need for cooperativemovements of a large number of connected monomers, resulting in a lack of long-range order and rapid loss of orientational correlation over short distances. Typically, polymeric crystals organize intomicroscale, isotropic, spherical superstructures (spherulites), which are comprised of splaying and branching thin lamellae interleaved with amorphous layers. Here, we report on a fast and easy method to control the orientation of crystals of semicrystalline polymers at the nanometer scale by using nanoimprint lithography [1]. Whenever a hardmold that contains nanoscale features defined on its surface (typical lateral size 50-250 nm; typical height 100 nm), is pressed against a molten film of a crystallizable polymer cast on a substrate under controlled temperature and pressure conditions, themolten polymer is forced to flow into the cavities of the mold. The isolation of polymer in separate nano-cavities, which strongly decreases the importance of nucleation by impurities; the confinement of the polymer in nano-sized cavities, which hinders the transport of polymer chains between cavities and controls the fast crystal growth direction; the partial orientation of polymer chains near the vertical walls on themold due to elongational flow, which eases the formation of nuclei with the crystallographic c axis parallel to the vertical walls on the mold, lead to a fine control over crystallographic orientation, crystal size and shape of polymer crystals. This is shown for a series of geometries, from simple straight lines tomore complex shapes, such as concentric circles and small squares, and for a variety of polymers.