Structure-property analysis for gel-spun, ultrahigh molecular mass polyethylene fibers

The structures of four gel-spun, ultrahigh molecular mass polyethylene fibers have been studied with the techniques of full-pattern x-ray diffraction, small-angle x-ray scattering, powder x-ray diffraction, solid-state C-13 nuclear magnetic resonance, differential scanning calorimetry, and optical microscopy. A high molecular mass polyethylene fiber is also studied for comparison. At room temperature these fibers show mainly the common orthorhombic crystals and a small amount of monoclinic crystals in addition to an intermediate, oriented phase and the amorphous phase. The structure parameters of the orthorhombic phase change slightly with fiber processing history. The chains of the intermediate phase have largely a trans-conformation and are oriented preferentially parallel to the fiber axis, but are disordered laterally. The mobility (correlation time) of the carbon atoms of the intermediate phase is higher than that of the crystalline phase by 2 orders of magnitude, but lower than that of the amorphous phase by 3 orders of magnitude. The intermediate phase can contribute significantly to the meridional x-ray diffraction peaks 00l, but not to the equatorial peaks hk0. A simple structural model is proposed to reconcile the experimental results. The domain which consists mainly of intermediate phase extends much longer in the fiber direction than the orthorhombic ones, which are more regularly interrupted. Fiber tensile strength is mainly determined by the mass fraction and orientation of the intermediate phase, approximated by the ratio of diffraction intensities 002/110. The initial modulus of the fibers depends also on the axial connection between the domains of the orthorhombic phase. Endothermic double peaks of fibers analyzed with free ends result from the melting of the orthorhombic crystals of different degree of restraint by the surrounding material. Both peaks occur above the equilibrium melting temperature of polyethylene. Unusually small x-ray long periods between 4.2 and 6.0 nm have been found for two of the fibers. Other long periods and the phase transformations of the monoclinic phase with temperature and on compression are also discussed.