Quiescent and Flow-Induced Crystallization Polypropylene-Clay Nanocomposites
James P. Oberhauser, Ph.D.
Polymer R&D Manager
Bioabsorbable Polymer Solutions
Polymer-clay nanocomposites (PCNs) have recently attracted significant attention in both academic and industrial settings due to the profound enhancement of material properties (e.g., tensile strength, flammability reduction, reduced gas permeability) obtained with clay loadings substantially less than those required for traditional, micron-sized fillers. As a result, a fundamental understanding of the physics and chemistry governing these property changes over a hierarchy of morphological length scales is of paramount interest.
The first portion of this presentation discusses the compounding methods used to exfoliate and disperse clay domains in a polymer matrix, which is essential to achieving desired property improvements. Melt-blending in a twin-screw extruder was conducted to prepare 1, 3, and 5 wt % polypropylene (PP)-clay nanocomposites in the presence and absence of a maleic anhydride-based polypropylene compatibilizer (PP-g-MA). A single-screw extrusion system used in conjunction with in-situ addition of supercritical carbon dioxide (scCO2) to facilitate clay dispersion was also employed. Rheology, X-ray diffraction, and transmission electron microscopy demonstrate that the compatibilizer and the high shear forces in a twin-screw process are necessary to successfully exfoliate and disperse clay domains.
Secondly, we examine the terminal rheology of these PCNs, which were found to be time-dependent and highly sensitive to deformation history. Specifically, the PCNs exhibited logarithmic increases in storage modulus and complex viscosity with time during small-amplitude oscillatory shear at fixed low frequency, behavior that is analogous to soft glassy dynamics observed in colloidal glasses and gels. These time-dependent properties are believed to derive from the formation of a heterogeneous, mesoscale clay network driven by weak van der Waals attractions between clay domains. The fact that the time-dependent response is sensitive to the deformation history suggests that the randomization of flow-induced clay orientation is arrested prior to complete disorientation, perhaps due to the formation of the mesoscale network. Interestingly, these rheological characteristics are ubiquitous for a variety of clay loadings and dispersion states.
Finally, we focus upon the crystallization behavior of PCNs under both quiescent and flow conditions. Differential scanning calorimetry and polarizing optical microscopy were used in the quiescent state to show that the clay generally hinders the overall kinetics of crystallization and spherulitic growth rates. However, a late-stage nucleation of tiny spherulites occurs in the most highly loaded and well-dispersed PCNs, indicating that the high clay surface area in these systems may contribute to the buildup of local stresses and trigger subsequent nucleation. To examine the flow-induced crystallization (FIC) behavior, a custom-built mini-extruder was utilized to subject the samples to a finite shear pulse. Crystallization was monitored in-situ by measuring the evolution of turbidity and birefringence following shear pulse deformations. The kinetics are examined as a function of both the shearing time and applied wall shear stress, while tracking the birefringence allows quantification of the critical wall shear stress required to form the classic “skin-core” morphology. Wide-angle X-ray diffraction was used to further probe the kinetics and orientation fraction of the crystallites formed during FIC. Results show that at higher clay loadings, the FIC kinetics are significantly accelerated relative to those of the neat polymer, indicating that the clay filler increases the distortion of the melt during flow and enhances homogeneous nucleation. Data also reveal that the poorly dispersed and agglomerated clay domains of the uncompatibilized PCN induce considerably more melt distortion than their more exfoliated, compatibilized counterpart, behavior that is attributed to the fact that the mechanical rigidity of clay domains increases with size. Consequently, the uncompatibilized PCN exhibits a higher oriented fraction of crystallites along with a commensurate reduction in the critical wall shear stress.
Dr. Oberhauser received his B.S. from U.C. Davis, M.S. from Stanford University, and Ph.D. from U.C. Santa Barbara, all degrees in chemical engineering. His dissertation work, under the mentorship of Prof. Gary Leal, focused on polymer rheology in the nonlinear viscoelastic flows characteristic of industrial polymer processing. Subsequently, Dr. Oberhauser did postdoctoral work on shear-enhanced crystallization of polypropylene with Prof. Julia Kornfield at the CalTech. In 2001, Dr. Oberhauser joined the faculty of the Department of Chemical Engineering at the University of Virginia, where his research interests remained strongly influenced by polymer processing applications but expanded into nanostructured polymeric fluids. Dr. Oberhauser joined the Bioabsorbable Vascular Solutions group at Abbott Vascular in 2007 as Polymer R&D Manager, where the focus is the development of a fully bioabsorbable, polymeric coronary stent.
Tuesday, September 16
Michael's Restaurant at Shoreline Park
2960 N Shoreline Blvd
Mountain View, CA 94043
6 PM social hour
7 PM dinner
8 PM lecture
$30 with advance registration
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broiled salmon with a lemon buerre blanc
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