13C Solid State NMR Characterization of Structure and Orientation Development in the Narrow and Broad Molar Mass Disentangled UHMWPE

Abstract

The ultimate draw ratio in semicrystalline polymers, and the resultant mechanical properties of the polymer, strongly depends on entanglement density in the amorphous region of the polymer. The influence of entanglement density becomes more pronounced with the increasing molar mass, for an example in the ultrahigh molecular weight polyethylene (UHMW-PE, having weight-average molar mass greater than a million g/mol) the solid-state deformation (draw ratio >7) is feasible on crystallization of the polymer from dilute solution or during controlled polymerization using a single-site catalytic system. Here we address the influence of the molar mass distribution, associated with the polymerization conditions, on structural changes during solid-state deformation in the crystalline and the noncrystalline regions of UHMW-PE. With the help of various solid state NMR methods, differences in the deformation behavior of the mobile-amorphous, rigid-amorphous, and crystalline polymorphs have been followed in the broad and narrow molar mass UHMW-PEs. Orientation parameters arising at the segmental length scales, within different regions of the semicrystalline polymers, have been addressed. 2D 13C exchange NMR methods have been employed to follow the spatial proximity between the methylene segments of the noncrystalline regions (mobile- and rigid-amorphous phases) with the crystalline regions (crystalline core and crystal surface) during deformation of the two polymers. Distinct differences in the orientation parameters of the methylene segments in the noncrystalline and the crystalline regions, arising with the deformation of the broad and the narrow molar mass distributed UHMW-PE, have been observed and addressed.