Small‐angle X‐ray diffraction studies of plastically deformed polyethylene. III. Small draw ratios

Linear polyethylene drawn at 0, 110, and 120°C to a draw ratio between 1.1 and 6 was investigated by small‐ and wide‐angle X‐ray scattering. Besides a small increase of meridional long period at low draw ratios one observes a decrease of diagonal (at an angle φ to the meridian) and equatorial long period at high draw temperatures. This decrease is the consequence of thinning of lamellae by chain tilt in the compressional field of the uniaxial draw deformation. The early decrease of the meridional scattering intensity indicates the rapidly increasing loss of packing order as a consequences of the disappearance of lamellae oriented perpendicular to the draw direction due to rotational motion towards parallel orientation. The intensity decreases in the equatiorial scattering, however, is caused by the rapid destruction of lamellae parallel to the draw. By micronecking they are transformed into microfibrils of the new fiber structure. With increasing draw ratio the diagonal maximum shifts closer to the equator and to larger diffraction angle. It reflects the maximum of lamella population at an angle 90°−φ as a consequence of dynamic equilibrium between new arrivals from the equatorial area and continuous loss by micronecking of lamellae close to the pole. The chain tilt within the lamellae increasing with draw ratio shifts the optimum orientation for chain slip and lamellae fracture from 45° (corresponding to chains perpendicular to lamella surface) to smaller angles. At high temperature of drawing (120°C) the plastic deformation of lamellae by chain tilt and slip showing up in thinning of lamellae before the final micronecking persists up to high draw ratios (λ ∼ 4). At 0°C with practically blocked chain mobility in the crystal lattice, this contribution is limited to λ ≤ 1.5 with practically no reduction of LD or LE. The lamellae get destroyed at low λ by sliding motion of chains in the “noncrystalline” boundary layers between mosaic blocks without any substantial chain tilt in the blocks. Drawing at 110°C is much closer to the low than to the high temperature case. This temperature dependence is very similar to long period growth which is quite rapid at 120°C, just observable at 110°C, and not observable at all at temperatures below 80°C. This similarity seems to originate from the fact that in both cases one has to do with the longitudinal chain mobility in the crystal lattice.