Shear-Enhanced Crystallization in Isotactic Polypropylene. 3. Evidence for a Kinetic Pathway to Nucleation

In situ rheo-optical techniques are combined with synchrotron wide-angle X-ray diffraction (WAXD) to investigate the oriented crystallization precursors that develop upon strongly shearing an isothermal melt of polydisperse isotactic polypropylene (iPP). The “short-term shearing” experimental protocol, developed by Janeschitz-Kriegl and co-workers, is used under conditions previously determined to induce oriented crystallite growth. Surprisingly, the time for these precursors to appear decreases with increasing temperature, tracking the melt dynamics of the polymer moleculesa behavior unanticipated by current models. Thus, nucleation proceeds via a “nonclassical” kinetic pathway that effectively eliminates the activation barrier for nucleation. To characterize the importance of chain length distribution for the formation of nucleation precursors, experiments were performed with model bidisperse systems containing a small percentage of high molecular weight iPP blended with lower molecular weight iPP of matched stereoregularity. Oriented crystallization was not observed for the individual components of the blend under the most extreme experimental conditions investigated, but it was readily observed for the bidisperse blend. This suggests that, under intense shear, nucleation of oriented crystallites is governed by the rheologically determined formation of a critical anisotropic configuration of polymer chains in the melt

Phase Structures and Morphologies Determined by Competitions Among Self-Organization, Crystallization, and Vitrification in a Disordered Poly(Ethylene Oxide)-B-Polystyrene Diblock Copolymer

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer having a number-average molecular weight ((M) over bar(n)) of 11000 g/mol in the PEO blocks and an (M) over bar(n) of 5200 g/mol in the PS blocks has been synthesized (with a volume fraction of the PEO blocks of 0.66 in the molten state). Differential scanning calorimetry results show that this copolymer possesses a single endotherm, which is attributed to the melting of the PEG-block crystals. Based on real-time resolved synchrotron small-angle x-ray scattering (SAXS) observations, the diblock copolymer is in a disordered state above the glass transition temperature of the PS-rich phase (T-g(PS)) which has been determined to be 44.0 degrees C during cooling using dilatometer mode in thermomechanical measurements. The order-disorder transition temperature (T-ODT) for this diblock copolymer is thus experimentally inaccessible. Depending upon different isothermal crystallization temperatures quenched from the disordered state (T(q)s), four cases can be investigated in order to understand the phase relationships among self-organization, crystallization of the PEO blocks, and vitrification of the PS-rich phase: the region where the T-q is above the T-g(PS), the regions where the T-q is near but slightly higher or lower than the T-g(PS) ; and the region where the T-q is below the T-g(PS) . Utilizing simultaneous SPXS and wide angle x-ray-diffraction experiments, it can be seen that lamellar crystals of the PEO blocks in the first case grow with little morphological constraint due to initial disordered phase morphology. As the T-q approaches but is still slightly higher than the T-g(PS) , as in the second case, the PEG-block crystals with a greater long period (L) than that of the disordered state start to grow. The initial disordered phase morphology is gradually destroyed, at least to a major extent. When the T-q is near but slightly lower than the T-g(PS), the crystallization takes place largely within the existing phase morphology. Only a gradual shift of the L towards smaller q values can be found with increasing time, which implies that the initial phase morphology is disturbed by the crystallization of the PEO blocks. In the last case, the PEO blocks crystallize under a total constraint provided by the disordered phase morphology due to rapid vitrification of the PS-rich phase. Substantial decrease of crystallinity can be observed in this case. This study also provides experimental evidence that the PS-rich phase size, which is down to 7-8 nm, can still retain bulky glassy properties. [S0163-1829(99)01138-8]

Dislocation-Controlled Perforated Layer Phase in a Peo-B-Ps Diblock Copolymer

Small angle x-ray analyses show that the shear-induced hexagonal perforated layer phase in a poly(ethylene oxide)-b-polystyrene diblock copolymer consists of trigonal (R3(overbar)m) twins and a hexagonal (P6(3)/mmc) structure, with trigonal twins being majority components. Transmission electron microscopy reveals that the hexagonal structure is generated through sequential intrinsic stacking faults on the second layer from a previous edge dislocation line, while the trigonal twins are formed by successive intrinsic stacking faults on neighboring layers due to the plastic deformation under mechanical shear

Shear-Enhanced Crystallization in Isotactic Polypropylene. In-Situ Synchrotron SAXS and WAXD

In-situ synchtrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) are used to follow the isothermal crystallization (lamellar thickness, crystallinity, orientation, and parent-to-daughter ratio) of a polydisperse isotactic polypropylene subjected to “short term shearing” as a function of imposed shear stress, shearing duration, and crystallization temperature. The X-ray data are interpreted in view of the real-space morphological information from ex-situ microscopy. Under “mild” shearing conditions (shear stress less than a critical value and shearing duration less than a critical time), needlelike nuclei are induced during shear but are so far apart that crystallites splay substantially as they grow to form somewhat distorted spherulites; the X-ray results show weakly oriented growth on a time scale that is rapid compared to quiescent crystallization and show that the orientation distribution broadens as crystallization progresses. Stronger shearing leads to the elaboration of these nuclei into threadlike structures that template the formation of highly oriented crystals with fiberlike orientation. The parent-to-daughter ratio is influenced by both temperature and flow. As expected, increasing the crystallization temperature leads to fewer daughter crystals relative to the parents. Shear also enhances the formation of parents relative to daughters:  as parent crystals form with their chain axis along the flow direction, the epitaxial daughter crystals have their chain axis in an unfavorable direction, perpendicular to the flow