Characterization of ultradrawn polyethylene fibers by NMR: crystallinity, domain sizes and a highly mobile second amorphous phase

The structure of ultradrawn ultra-high molecular weight polyethylene fibers has been investigated by solid-state NMR. A crystallinity of (88 +/- 2)% was determined by traditional H-1 NMR lineshape decomposition, and by a new adaptation of C-13 NMR crystallinity determination for polyethylenes with extremely long crystalline T-1 relaxation times. H-1 spin diffusion yields amorphous domain sizes of 10 +/- 5 nm, and crystalline regions of 100 +/- 50 nm diameters. A second, highly mobile, amorphous phase, making up (0.8 +/- 0.2)% of the sample, was detected by H-1 NMR. In spite of its 1.8 kHz H-1 line width, it shows little spin diffusion to the other phases, even on a 500-ms time scale; this suggests domains of more than 3 nm thickness or chains extending into voids. Being undetectable in the extruded precursor material and in the fibers after melting, this highly mobile phase must have been induced by the drawing process. C-13 NMR confirms that no low-molecularweight additives are present on a level above 0.01%. A similar highly mobile component has also been detected in drawn medium-molecular-weight polyethylenes. The fraction of partially mobile, oriented interfacial material or tie-molecules in the fiber was found to be similar to 5%, while rigid gauche conformers could not be detected (concentration < Ire). Altogether, five morphological components have been identified: 83% crystal core, of which 80% is orthorhombic and 3% monoclinic, with thickness of similar to 100 nm; 5% disordered all-trans interfacial and/or tie molecules; 11% mobile amorphous regions, with diameters of similar to 10 nm; and 1% highly mobile segments, probably at void surfaces. On this basis, a structural model for ultradrawn PE fibers is proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.