No abstract

In-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) studies were carried out to investigate the nature of shear-induced precursor structures in isotactic polypropylene (iPP) melt at 165 °C, near its nominal melting point. Immediately upon the cessation of shear, SAXS patterns clearly showed an evolution of oriented structures in hundreds of angstroms, while the corresponding WAXD patterns did not exhibit any crystal reflections. SAXS patterns at later times showed that the shish-kebab morphology was developed, and the kebabs possessed only a small amount of crystallinity (3%). The combined SAXS and WAXD results indicate that, at the early stages of crystallization, a scaffold (network) of oriented structures is formed. These structures contain (1) primary nuclei (through homogeneous nucleation) that may be crystalline or mesomorphic but having linear connectivity along the flow direction, which form the shish entity, and (2) shish-induced layered crystalline lamellae (kebabs) oriented perpendicularly to the flow direction that have poor lateral connectivity. Subsequent polymer crystallization takes place in the framework of the scaffold, which is probably dominated by the lower molecular weight species. Amounts of the crystalline primary nuclei and the layered crystalline lamellae in the precursor structures were estimated. The results verified, quantitatively for the first time, the well-known concept that minor amounts of linear nuclei induce multiple secondary nucleation sites for the growth of a large quantity of lamellae that grow radially outward from the central core. A mechanistic pathway for the early stages of crystallization in polymer melts under flow is proposed.

The early stages of polymer melt crystallization using fractionated isotactic polypropylene (iPP) as a model system were investigated via simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) and laser light scattering (LS) techniques. Since the crystallinity in the early stages is very low, the issue of the crystallinity detection limit of WAXD was addressed. This was done by using solutions of n-paraffin (C 33H68) in dodecane (C12H26) at different concentrations (as low as 1%). The precipitated fraction simulated the degree of crystallinity in polyethylene since n-paraffin essentially completely crystallized and dodecane remained liquid at the measurement temperature. A modeling method was also used to simulate the WAXD profiles to check the effect of crystal size at low crystallinity. With these two methods, we conclude that our WAXD procedure is capable of detecting crystallinity from 0.5% to 1%. During the early stages of iPP isothermal crystallization, noticeable short-range density fluctuations with average periods from 20 to 24 nm (by SAXS) were seen prior to the observation of three-dimensional ordering of the crystalline R-form (by WAXD). The spacing associated with the peak of the SAXS was found not to increase with time, being constant or a possible initial decrease, which is consistent with the formation of a finite lamellar structure. Furthermore, larger objects with dimensions growing from 300 nm were observed with the more sensitive technique of polarized light scattering, prior to the detection of the lamellar period by SAXS. The development of the crystallinity as measured by WAXD as well as SAXS and light scattering are all consistent, which follow the same Avrami equation, suggesting that the early stages of crystallization as measured here follow classical nucleation and growth.

Studies of the temperature dependence of spherulitic growth rates, in the context of the Lauritzen−Hoffman secondary nucleation theory, and investigations of the crystallization time and temperature dependence of the melting behavior, in the context of the nonlinear Hoffman−Weeks (HW) extrapolation, are reported for isotactic polypropylene prepared with Ziegler−Natta catalysts. The quantitative agreement between estimations of the equilibrium melting temperature, T m, through an analysis of spherulitic growth rate data and use of the nonlinear HW approach provides strong support for the latter method. The results of these studies indicate specifically that T m (it-PP, α-phase) is in the vicinity of 212−215 °C, some 30 °C above the most commonly quoted value. This new value of the equilibrium melting temperature of it-PP leads to predictions of the crystallization temperature dependence of the initial lamellar thickness and of the correlation between the lamellar thickness and observed melting temperature, which are in good agreement with the limited morphological information available. The present studies confirm the existence of a regime II/III transition at T x = 139.5 °C. The results of this study furthermore cast some doubts as to the current value of the small-angle X-ray scattering technique for the analysis of morphological parameters in quiescently crystallized isotactic polypropylene.

The microstructures of two poly(propylene)s with matched molar masses and overall defect concentrations are inferred from the crystallization behavior of their narrow molar mass fractions. One poly(propylene) was produced with a MgCl2-supported Ziegler-Natta catalyst and the other with a metallocene catalyst. The fractions obtained from the metallocene isotactic poly(propylene) display a range in molar masses but each has the same defect concentration indicating a uniform intermolecular concentration of defects in the parent metallocene isotactic poly(propylene). These fractions provide direct evidence of the "single site" character of the metallocene catalyst. The variations of crystallization rates with molar mass reflect different chain diffusion/transport phenomena that are governed by the remnant entanglement state of the melt during crystallization. The molar mass fractions obtained from the ZN-iPP confirm that the interchain distribution of the nonisotactic content is broad in this polymer. The stereodefects are more concentrated in the low molar mass fractions. Furthermore, the invariance of the linear growth rates among the ZN fractions and the lack of formation of any significant content of the γ polymorph, even in the most defected fraction, is consistent with a nonrandom, blocky intramolecular distribution of defects in the ZN-iPP molecules. In contrast to the growth rates, the overall crystallization rates are a direct function of the primary nucleation density, which varies among the fractions and the unfractionated iPPs. Hence, the measured overall crystallization rates would be correlated with nucleation density and not necessarily with the microstructure of the iPP molecules. The crystallization data are also interpreted in light of results from pentad/heptad distributions predicted by two-state and threestate statistical models. Parameters from the models allow the prediction of sequence distribution curves that could be used to evaluate each of the models as to their consistency with the crystallization rate data.