CRYSTAL ORIENTATION AND TWINNING IN COLD-ROLLED ULTRAHIGH MOLECULAR-WEIGHT POLYPROPYLENE

The mechanisms of plastic deformation in cold-rolled ultrahigh molecular weight polypropylene (UHMWPP) have been explored by means of wide angle X-ray (WAXD) pole figures. The melt crystallized UHMWPP slabs were rolled unidirectionally in a two-roll mill at 408 K as well as in the cross (transverse) direction. In the unidirectional rolling, all reciprocal lattice vectors of (110), (040), and (130) were found to orient preferentially in the film normal (thickness) direction. The cross-rolling of UHMWPP shows the preferential orientation of the above plane normals in the film thickness direction, but the distribution of poles broadens toward the transverse direction. Such orientation behavior is very different from that of conventional cold-rolled polypropylene. Various orientation mechanisms involving the orientation of lamellae in the film plane, twinning of the (110) plane and slippage mechanisms have been taken into consideration to account for the observed pole figures. A theoretical simulation has been carried out based on an orientation distribution function approach by introducing a slippage angle along the maximum shear stress in conjunction with intralamellar slip around the transverse direction. This model simulation conforms closely with the experimental WAXD pole figures of the (110), (040), and (130) planes.