Thermal Stability of Shear-Induced Precursor Structures in Isotactic Polypropylene by Rheo-X-ray Techniques with Couette Flow Geometry RAJESH H. SOMANI, 1 IGORS SICS, 1 BENJAMIN S. HSIAO 1 Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400 Received 29 May 2006; revised 21 August 2006; accepted 6 September 2006 DOI: 10.1002/polb.21009 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic poly- propylene (iPP) at around its normal melting point (162 8C). Although SAXS results corroborated the emerging consensus about the formation of ‘‘long-living’’ metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantita- tive measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate ¼ 60 s 1 and duration t s ¼ 5 s, the oriented iPP structures survived a temperature of 185 8C for 1 h after shear, while no stable structures were detected at and above 195 8C. Following Keller’s concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate ¼ 30 s 1 and duration, t s ¼ 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 8C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty ‘‘phase’’ diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar ‘‘hourglass’’ shape, similar to that found in many two-phase poly- mer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear tem- perature (155 8C), the unoriented crystals crystallized so rapidly that they over- whelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystal- line phase. V V C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006 Keywords: couette flow; crystallization; isotactic polypropylene; phase diagram; shear; thermal stability; X-ray INTRODUCTION Polymer crystallization is a kinetically controlled process, in which molecules in random conforma- tions are organized into an ordered solid crystal- line structure. Crystallization of entangled poly- mer melts under quiescent conditions usually leads to spherulites, consisting of randomly ori- ented folded chain lamellae, and the process is, more or less, well understood. 1–4 Typically, it occurs in two stages: nucleation and growth. The primary nucleation step can be described by the concepts based on Boltzmann’s law of probability, adopted from the classical nucleation theory for Correspondence to: B. S. Hsiao (E-mail: bhsiao@notes.cc. sunysb.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 3553–3570 (2006) V V C 2006 Wiley Periodicals, Inc. 3553