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 polypropylene (iPP) at around its normal melting point (162 degrees C). 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 quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s(-1) and duration t(s) = 5 s, the oriented iPP structures survived a temperature of 185 degrees C for 1 h after shear, while no stable structures were detected at and above 195 degrees C. 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 = 30 s(-1) and duration, t(s) = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 degrees C, 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 polymer-polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 degrees C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. (c) 2006 Wiley Periodicals, Inc.