Shear-enhanced crystallization in isotactic polypropylene. 1. Correspondence between in situ rheo-optics and ex situ structure determination

The effects of "short term shearing" on the subsequent crystallization of a polydisperse Ziegler-Natta isotactic polypropylene are observed using in s itu optical measurements and ex situ microscopy. Imposition of brief interv als of shear (0.25-20 s, less than a thousandth of the quiescent crystalliz ation time) can reduce the crystallization time by 2 orders of magnitude (e .g., at 141 degrees C with a wall shear stress of 0.06 MPa). With increasin g shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes i n the transient behavior during flow and the semicrystalline morphology obs erved ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all con ditions that induce subsequent growth of highly oriented crystallites. In t urn, the development of oriented crystallites observed in situ after cessat ion of flow correlates with development of a "skin-core" morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with in creasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent cryst allization.