Core/Shell and Hollow Ultra High Molecular Weight Polyethylene Nanofibers and Nanoporous Polyethylene Prepared by Mesoscopic Shape Replication Catalysis

Polyvinyl alcohol (PVA) nanofibers, produced by electrospinning, represent attractive high-surface-area supports for olefin polymerization catalysts. Tethered with metal alkyls, PVA nanofibers immobilize a great variety of transition metal compounds, thus producing highly active nanofiber-supported Ziegler-, metallocene-, and post-metallocene catalysts. Whereas most conventional heterogeneous polymerization catalysts form particles, PVA-nanofiber-supported catalysts enable polyolefin nanofiber and nanostructure formation by mesoscopic shape replication using electrospun nanofibers as templates. At low ethylene pressure, linear correlation between average PE/PVA core/shell fiber diameter and polymerization time are made. At elevated pressure, this control is lost, accounting for the formation of reactor blends consisting of PE granules and built-in PE/PVA nanofibers. Whereas conventional catalysts produce micrometer-sized particles of ultrahigh molecular weight PE (UHMWPE), PVA-nanofiber supported chromium catalysts afford new families UHMWPE materials. They range from UHMWPE/PVA core/shell nanofibers and nonwovens to hollow UHMWPE fibers and nanoporous UHMWPE, obtained by removing the PVA component.