STUDY OF CRYSTALLINE ORIENTATION IN DRAWN ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE FILMS

A wide-angle x-ray diffraction (WAXD) study of the development of molecular orientation in the crystalline phase of ultra-high-molecular weight polyethylene films prepared by the gelation-crystallization method is presented. WAXD scans of the undrawn films show that the lamellae are oriented in the plane of the films. Upon drawing at 130-degrees-C, the orientation of the molecular chains changes from the direction normal to the film surface (ND) to the elongation direction. The decrease of the 200/020 intensity ratio at low draw ratio (lambda < 10) indicates that double orientation develops during the transformation from the lamellar to the fibrillar morphology, with the a-axis oriented parallel to ND. The orientation distributions of the 110, 200, 020, and 002 planes of the orthorhombic unit cell of polyethylene were studied and characterized by the coefficients of a Legendre polynomial series. At a draw ratio of 4.5, the second-order coefficient, [P2(cos chi], already gets close to its limiting value, but it is shown that higher order coefficients of the polynomial series can be used to describe the evolution of the orientation, even up to lambda = 50. The coefficients relative to the molecular chain orientation, [P(n)(cos chi)]c, can be calculated from different crystalline reflections. Curve-fitting calculations were made in order to improve the correlation between the results obtained from the orientation distribution of the 110, 020, and 002 planes. A Pearson VII function was found to give a better fit of the experimental curves than Gaussian or Lorentzian equations.