The effects of the placement of short-chain branches (SCB) on crystallization kinetics, morphology and mechanical properties of high-density polyethylene (HDPE) are examined using bimodal blends of short (S ∼40 kg/mol) and long (L ∼400 kg/mol and 550 kg/mol) polyethylenes with SCB (1-hexene comonomer) incorporated in either the high or the low molecular weight component. A pair of blends that has nearly matched molecular weight distribution and average SCB content shows that placement of branches preferentially on the longer molecules results in slower crystallization kinetics at high crystallization temperatures relative to the material with branches on the short chains. Blends with SCB on the high molecular weight components have superior ultimate mechanical properties, and their resistance to slow crack growth is tremendously enhanced. These improved mechanical properties are attributed to an increase in the amount of tie chains that form when branches are placed on the long chains. A conceptual model based on the interplay of inherent crystallization kinetics of each species and their competition at the growth front in binary blends qualitatively explains the effects of SCB distribution on crystallization kinetics, lamellar thickness and inferred formation of tie chains.

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.

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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.