(306c) Shear-Induced Conformational Changes of Flexible and Semi-Rigid Engineering Thermoplastics and Their Influence on Crystallization

Brief intervals of shear flow at rates exceeding the reciprocal of the Rouse time of the longest chains create precursors that nucleate orders of magnitude more crystals and change the morphology from ~30 μm spherulites to far smaller ~1 μm crystallites. This flow-induced crystallization (FIC) at farily low shear rates builds with shearing time and eventually saturates. In contrast, at much higher stress levels that might occur in processing flows, a second morphology transition to shish-kebabs is observed when a critical shear stress (~0.14 MPa) is exceeded. These two transitions are studied in detail for isotactic polypropylenes and poly(ether ether ketone)s representing flexible and semi-rigid polymers, and for Nylon6,6 representing a flexible polymer with strong hydrogen bonding, to see which aspects of FIC are universal to all polymers and which aspects are polymer-specific. The fact that the precursors are quite stable allows the sheared samples to be removed from the rheometer and studied extensively with DSC and optical microscopy, wherein annealing at elevated temperatures allows the study of precursor stability.