In-situ synchrotron small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear under isothermal conditions. The melt was subjected to a shear strain of 1428% at three different shear rates (10, 57, and 102 s-1) using a modified Linkam shear stage. The SAXS patterns showed strong meridional reflections due to the rapid development of oriented polymer crystallites within the melt. On the basis of the SAXS data, a schematic representation of nucleation and growth in orientation-induced crystallization of i-PP is proposed. During flow, orientation causes alignment of chain segments of polymer molecules and results in the formation of primary nuclei in the flow direction. These nuclei facilitate the growth of oriented crystal lamellae that align perpendicular to the flow direction. The half-time of crystallization was calculated from the time evolution profiles of the total scattered intensity. Th...

Structure Development during Shear Flow-Induced Crystallization of i-PP: In-Situ Small-Angle X-ray Scattering Study

Flow-induced shish kebab precursor structures in entangled polymer melts

Shear-induced crystallization of isotactic polypropylene (iPP) homo-, block, and random copolymers was studied and compared to that in quiescent melt. It was evidenced by means of the thermo-optical technique that melt-shearing, caused by fiber pulling, is associated with the development of α-row-nuclei. The surface of the in situ formed α-row-nuclei may induce the growth of the β-modification of iPP resulting in a cylindrite of polymorphous composition. The polymorphous composition is controlled by the temperature-dependent relative growth rate of the α- and β-iPP for which a model explanation was given. The β-nucleation ability of the α-row-nuclei is lost by melt-shearing at high temperature or remelting. This was attributed to a coverage of the β-nuclei by the α-phase. The structural memory of the supermolecular structures was studied in repeated melting and crystallization cycles and discussed. It was found that the quality of the fiber did not influence the mechanisms concluded. The shear-induced crystallization of propylene block copolymers was highly analogous to the homopolymers. In case of the random copolymers, however, crystallization in sheared melt resulted in an α-cylindritic structure, because for propylene random copolymers the growth rate of the α-modification is always higher than that of the β. It was also demonstrated that the mechanism of shear-induced crystallization was unaffected when the crystallizing PP melt contained selective β-nucleants. © 1996 John Wiley & Sons, Inc.

Rules of supermolecular structure formation in sheared isotactic polypropylene melts

In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65−70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generate...

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Structure Development during Shear Flow Induced Crystallization of i-PP: In Situ Wide-Angle X-ray Diffraction Study

The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization.

Shear-Enhanced Crystallization in Isotactic Polypropylene. 1. Correspondence between in Situ Rheo-Optics and ex Situ Structure Determination

Crystallization processes in quiescent and moving polymer melts under heat transfer conditions

With short term shearing at low degrees of supercooling a method has been developed by which nucleation during shearing can effectively be separated from the subsequent crystal growth, whi...

On the Kinetics of Shear Induced Crystallization in Polypropylene

A survey is given of the experimental evidence of shear induced lamellar crystallization of polymers. This survey forms the basis for a theory by two of the authors. In this theory elements of polymer melt rheology and polymer crystallization kinetics are combined. In turns out that the capability of the melt of crystallizing in the typical lamellar form, is only gradually developed during flow. Similarly, relaxation occurs after cessation of flow. The obtained expressions are introduced into Avrami’s formalism in order to define an induction time. In principle the model can be applied to any flow and temperature history. The case of isothermal flow at constant shear rate and at a certain level of supercooling is worked out in more detail. The obtained result favorably compares with experiments by Lagasse and Maxwell. Implications for the application in industrial processes are discussed.

Shear induced crystallization, a relaxation phenomenon in polymer melts

It turns out that a classification of the conditions under which certain types of texture occur when a crystallizable polymer melt is unilaterally quenched cannot be given on the basis of theory only. Further development of theory seems possible only under the guidance of properly designed experiments. In the light of the wide variety of crystallization phenomena this should not be surprising, the more so since polymers of major industrial interest (such as polypropylene or polyethylene) are not the easiest to investigate. In the course of the experiments it became apparent that it was very difficult to create well-defined quenching conditions and shearing histories. A report is given of the results obtained with two types of newly-developed apparatus: one of them operating at low shear rates (down to zero) and long contact times, the other using realistically high shear rates and short contact times.

The stefan problem in polymer processing: Theory and experimentation

A relatively simple experiment is described: In a “unilateral” quench a melted disk-like polymer sample is partially solidified. The conditions are discussed which lead either to a sharp crystallization front or to the occurrence of dispersed spherulitic crystallites. The importance of this basic experiment for the understanding of industrial moulding processes is stressed.

H. Janeschitz-Kriegl

Papers

Crystallization processes in quiescent and moving polymer melts under heat transfer conditions

On the Kinetics of Shear Induced Crystallization in Polypropylene

Shear induced crystallization, a relaxation phenomenon in polymer melts

The stefan problem in polymer processing: Theory and experimentation

Crystallization fronts in quenched polymer samples