Thermal stability of shear‐induced precursor structures in isotactic polypropylene by rheo‐X‐ray techniques with couette flow geometry

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 polypropylene (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 quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate ¼ 60 s À1 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 À1 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 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 8C), 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.

Mechanistic Insights into the Shear-Inducedβ-Form Crystal Formation of iPP

“…Comparing the latter two samples (“b” and “c”), we consider the slightly increased melting temperature from T mb2 = 170 °C to T mc2 = 172.2 °C is attributed to the perfection of partial α crystal during the second crystallization, and the lower T mc1 = 163.1 °C represents some less ordered newly‐formed α crystal caused by the spatial restriction of the original α solids. The flow‐induced oriented structures have been proved to linger for hours at temperatures as high as 185 °C, well above the actual melting point of the α crystal . Thus, the original α ‐row nuclei could survive after the β ‐melting at 160 °C, and were supposed to induce the surrounding iPP melt to recrystallize into β crystal at the β ‐favorable crystallization temperature T c = 135 °C.…”

Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

“…15,34,35,48 Right after the cessation of shear, the linear density of these bundles, i.e. their number per unit length, is the highest; thus, as depicted on the left side of Figure 7, the closely spaced lamellae are bounded to develop in a perpendicular direction to form a cylindritic type of morphology.…”

“…32 Mesoscopic structures, consisting in a scaffold of row-nuclei onto which lamellae may develop, were also observed by means of rheo-SAXS in sheared i-PP melts. 15,34,35 These bundles of aligned chain segments can be generated by flow and linger for hours at temperatures as high as 185°C, well above the actual melting point of the crystals. 35 The formation of a metastable suspension of shishes by imposing the shear close to the thermodynamic melting temperature was also reported by Balzano et al for a bimodal HDPE.…”

“…15,34,35 These bundles of aligned chain segments can be generated by flow and linger for hours at temperatures as high as 185°C, well above the actual melting point of the crystals. 35 The formation of a metastable suspension of shishes by imposing the shear close to the thermodynamic melting temperature was also reported by Balzano et al for a bimodal HDPE. 36 The presence of threadlike precursors was proved with rheo-optical techniques by Kornfield et al 17,37 in i-PP melt submitted to duct flow.…”

“…This notwithstanding, several direct evidences of the existence of long-living metastable structures in sheared polymer melts have been reported. [30][31][32][33][34][35][36][37][38] On segmental scale, rheo-Raman spectra of sheared polyethylene melts indicate the presence of chain sequences in trans conformation, featuring the ordered crystalline state, which persist for several hours after cessation of the shear. 30 Recently, shear-induced conformational ordering in the molten state was also observed in samples of isotactic polypropylene by Li et al [31][32][33] In this case, flow enhances the concentration of long sequences of repeating units in the 3/1 helical conformation required to form the monoclinic crystalline structure.…”