EFFECT OF THE VOLUME FRACTION OF TAUT TIE MOLECULES ON DYNAMIC MECHANICAL-BEHAVIOR OF ORIENTED POLYPROPYLENE

When isotactic polypropylene material was uniaxially cold-drawn and then annealed with free ends, folded-chain lamellar crystals formed almost perpendicular to the c-axis, i.e., to the draw direction. When stress is applied in the draw direction, the forces are primarily transmitted by the taut tie molecules connecting adjacent lamellar crystal layers. In spite of the high crystallinity, this oriented material has a low dynamic modulus along the draw direction. These facts are due to the existence of only a few taut tie molecules between the crystal layers. When this oriented material was redrawn cold and annealed with fixed ends parallel to the draw direction, it tended to separate between the stacked lamellae so that the highly oriented fibrils bridging the stacked lamellae and the microvoids between the displaced lamellae could be observed by electron microscopy. The fraction of taut tie molecules that was determined on the basis of the mechanical two-phase model was substantially lower than one would guess from the fraction of material that is fibrillar as estimated by morphological observation. Molecular models have been proposed to account for the observed phenomena. © 1979, Taylor & Francis Group, LLC. All rights reserved.