The role of long chains in shear-mediated crystallization was studied by in situ rheo-optical measurements and ex situ microscopic observations. To elucidate the effects of long chains, we prepared model blends in which fractionated isotactic polypropylene (iPP) (denoted L-PP) with high molecular weight (MW) and narrow molecular weight distribution was blended with a metallocene iPP (Base-PP) with lower molecular weight. The concentration of L-PP (c) was varied ranging from 0 to twice the concentration (c*) at which L-PP coils overlap. The crystallization of all blends after cessation of transient shearing was accelerated, while the quiescent crystallization kinetics were not affected by the addition of L-PP. A distinctive change in the development of birefringence after shearing was observed when the wall shear stress (σ w) exceeded a critical value (σ*). Below σ*, irrespective of c, the birefringence after transient shearing increased gradually, reaching a small value at the end of crystallization. Above σ*, a brief interval of shear induced highly oriented growth, manifested in the birefringence after cessation of flow and growing stronger and reaching a large value as crystallization proceeded. Further, the rate of growth of the birefringence exhibited a strong, nonlinear c dependence. The morphology of the skin layer showed a shish kebab type structure observed by TEM for samples subjected to stresses above σ*. The number density and thickness of shish were affected by c and changed drastically at c near the overlap concentration of the long chains. This indicates that the role of long chains in shear-induced oriented crystallization is cooperative (rather than a single chain effect), enhanced by long chain-long chain overlap.

The addition of small concentrations (2 wt % or less) of ultrahigh molecular weight isotactic polypropylene (M L ~ 3500 kg/mol) to a matrix of lower molecular weight chains (M S ~ 186 kg/mol, e.g. M L /M S ~ 20) substantially decreases the critical stress for inducing a highly oriented skin under flow-induced crystallization conditions-significantly more than for blends of M L /M S ~ 5 (Seki et al.)-and promotes the formation of point precursors and oriented "sausage-like" structures not observed for M L /M S ~ 5. These differences correlate with the onset of long chain stretching during shear: the ratio of long chains' Rouse time to short chains' disengagement time indicates that 3500 kg/mol chains can easily stretch if tethered onto a point nuclei and even when untethered. Adding 3500 kg/mol chains has strong effects that saturate beyond the overlap concentration, suggesting that an uninterrupted supply of long chains greatly accelerates formation of threads. A conceptual model is proposed that distinguishes between a critical stress for shish initiation and that for propagation.

SynopsisTransient structure development at a specific distance from the channel wall in a pressure-driven flow is obtained from a set of real-time measurements that integrate contributions throughout the thickness of a rectangular channel. This "depth sectioning method" retains the advantages of pressure-driven flow while revealing flow-induced structures as a function of stress. The method is illustrated by applying it to isothermal shear-induced crystallization of an isotactic polypropylene using both synchrotron x-ray scattering and optical retardance. Real-time, depth-resolved information about the development of oriented precursors reveals features that cannot be extracted from ex-situ observation of the final morphology and that are obscured in the depth-averaged in-situ measurements. For example, at 137°C and at the highest shear stress examined ͑65 kPa͒, oriented thread-like nuclei formed rapidly, saturated within the first 7 s of flow, developed significant crystalline overgrowth during flow and did not relax after cessation of shear. At lower stresses, threads formed later and increased at a slower rate. The depth sectioning method can be applied to the flow-induced structure development in diverse complex fluids, including block copolymers, colloidal systems, and liquid-crystalline polymers.

Syndiotactic polypropylene (sPP) exhibits a complex crystalline morphology, characterized by unique annealing- and deformation-induced changes. Rheooptical FTIR spectroscopy, wide-angle X-ray diffraction (WAXD), and Raman spectroscopy are used to characterize morphology and orientation responses of highly syndiotactic sPP to tensile drawing. Solid-state thin films of different initial morphology, either quenched or slowly cooled from the melt, are studied. Results suggest that a gradual transition in macromolecular chain conformation, from gauche−gauche−trans−trans helical to all-trans planar, is observed at room temperature for quenched samples that are drawn up to 400% strain. This transition is marked initially by the gradual disappearance of helical chains (disordered form I) and the subsequent emergence of a mesophase, which may transform into form III crystals at even greater strains. Our primary investigational tool, the rheo-FTIR spectrometer, allows us to monitor the presence and orientation of amorphous, mesomorphic, and crystalline domains directly, simultaneously, and sensitively. Results from all of the techniques used are correlated in an effort both to assign IR peaks to characteristic sPP moieties and to generate a plausible physical model of the deformation dynamics in melt-quenched sPP.

A thermoplastic olefin blend consisting of isotactic polypropylene (PP) and an ethylene-butene copolymer (EBR) impact modifier (25 wt % EBR) was subjected to a short, high-shear pulse within the flow channel of a pressure-driven microextruder following low-shear channel filling from a reservoir of the melt. The resulting morphology was examined by laser scanning confocal fluorescence microscopy (LSCFM), with contrast provided by a fluorescent tracer in the EBR minor phase. Shear experiments were performed under isothermal conditions with a known wall shear stress for a specified duration, providing a well-defined thermal and flow history. Low-shear channel filling produces small droplets across the central region of the channel and large droplets, consistent with steady-state shear, in the regions near the channel walls. After cooling the molten blend to a crystallization temperature of 153°C, a brief interval (5 s ϳ 1/2000 of the quiescent crystallization time) of high shear (wall shear stress: 0.1 MPa) induces rapid, highly oriented crystallization and a stratified morphology. Ex situ LSCFM reveals a "skin" at the channel walls (ϳ70 m) in which greatly elongated fiberlike droplets, oriented along the flow direction, are embedded in highly oriented crystalline PP. Further from the walls but directly beside the skin layers are surprising zones in which EBR domains show no deformation or orientation. Several zones of intermediate deformation and orientation at an angle to the flow direction are located closer to the center of the channel. At the center of the channel, EBR droplets are spherical, as expected for channel flow. The various strata are explained by the interplay of droplet deformation, breakup, and coalescence with the shear-induced crystallization kinetics of the matrix.

The crystallization behavior of isotactic propylene-1-hexene (PH) random copolymer having 5.7% mole fraction of hexene content was investigated using simultaneous timeresolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. For this copolymer, the hexene component cannot be incorporated into the unit cell structure of isotactic polypropylene (iPP). Only a-phase crystal form of iPP was observed when samples were melt crystallized at temperatures of 40 C, 60 C, 80 C, and 100 C. Comprehensive analysis of SAXS and WAXD profiles indicated that the crystalline morphology is correlated with crystallization temperature. At high temperatures (e.g., 100 C) the dominant morphology is the lamellar structure; while at low temperatures (e.g., 40 C) only highly disordered small crystal blocks can be formed. These morphologies are kinetically controlled. Under a small degree of supercooling (the corresponding iPP crystallization rate is slow), a segmental segregation between iPP and hexene components probably takes place, leading to the formation of iPP lamellar crystals with a higher degree of order. In contrast, under a large degree of supercooling (the corresponding iPP crystallization rate is fast), defective small crystal blocks are favored due to the large thermodynamic driving force and low chain mobility.

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