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

Abstract

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 1H NMR lineshape decomposition, and by a new adaptation of 13C NMR crystallinity determination for polyethylenes with extremely long crystalline T 1 relaxation times. 1H 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 1H NMR. In spite of its 1.8 kHz 1H 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. 13C NMR confirms that no low-molecular-weight 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 ∼5%, while rigid gauche conformers could not be detected (concentration <1%). Altogether, five morphological components have been identified: 83% crystal core, of which 80% is orthorhombic and 3% monoclinic, with thickness of ∼100 nm; 5% disordered all- trans interfacial and/or tie molecules; 11% mobile amorphous regions, with diameters of ∼10 nm; and 1% highly mobile segments, probably at void surfaces. On this basis, a structural model for ultradrawn PE fibers is proposed.