The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Christopher W Macosko - One of the best experts on this subject based on the ideXlab platform.
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processing property relationships of polycarbonate graphene composites
Polymer, 2009Co-Authors: Christopher W MacoskoAbstract:Polycarbonate composites reinforced with graphite and functionalized graphene sheets (FGS) were produced using Melt compounding. Composite samples with different degrees of graphite orientation were processed via injection, compression molding and long-term annealing. Electron microscopy and X-ray scattering revealed that FGS was nearly exfoliated. However, graphite remained multi-layer even after Melt processing. Flow induced orientation of graphite was observed from both injection and compression molded samples. Graphite particles in samples after long-term annealing exhibited more random orientation. Composites with the exfoliated FGS required a smaller amount of reinforcement for rigidity and connectivity percolation, as determined by Melt Rheology and electrical conductivity measurements. FGS also showed better performance in suppressing gas permeability of polycarbonate. However, improvements by FGS dispersion in tensile modulus and dimensional stability were not as significant. This may be due to defects in the sheet structure formed during oxidation and pyrolysis used to exfoliate.
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morphology and properties of polyester exfoliated graphite nanocomposites
Macromolecules, 2008Co-Authors: Hyunwoo Kim, Christopher W MacoskoAbstract:Nanocomposites reinforced with graphite platelets were compared to those with functionalized graphite sheets (FGS) prepared by partial pyrolysis of graphite oxide. Melt dispersion in poly(ethylene-2,6-naphthalate) (PEN) was quantified using a range of characterization techniques: electron microscopy, X-ray scattering, Melt Rheology, electrical conductivity, gas barrier, and mechanical properties. Conductivity percolation was obtained with as little as 0.3 vol % FGS, whereas 3 vol % was required for graphite. The threshold concentrations of FGS and graphite for rigidity percolation determined with Melt Rheology were in good agreement with conductivity percolation. Hydrogen permeability of PEN with 4 wt % FGS was decreased by 60% while the same amount of graphite reduced permeability only 25%. Structural differences between graphite and FGS were characterized with atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The highly exfoliated morphology of FGS was mainta...
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linear low density polyethylene lldpe clay nanocomposites part i structural characterization and quantifying clay dispersion by Melt Rheology
Polymer, 2007Co-Authors: Ali Durmus, Ahmet Kasgoz, Christopher W MacoskoAbstract:In this study, linear low density polyethylene (LLDPE)/clay nanocomposites with various clay content were prepared by Melt processing using two different compatibilizers, maleic anhydride grafted polyethylene (PE-g-MA) and oxidized polyethylene (OxPE). Effects of structure and physical properties of the compatibilizers on the clay dispersion and clay amount on the microstructure and physical properties of the nanocomposites were investigated. The OxPE was shown to significantly create interfacial interactions between the polymer phase and clay layers. Rheological behavior of the samples was examined by a dynamic oscillatory rheometry in linear viscoelastic region. Percolation threshold (fp) and corresponding aspect ratio (Af) values were determined by analyzing the improvement in storage modulus at low frequency region depending on the clay loading. Lower percolation and higher aspect ratio values were obtained for the sample series prepared with the PE-g-MA than that prepared with the OxPE. Moreover, fractal size of the clay network above the percolation point was determined by the scaling law for physical gelation of colloidal flocks to quantify clay dispersion depending on the compatibilizer structure. It was found that the PE-g-MA yielded better clay dispersion and more exfoliated structure compared to the OxPE. Microstructural characterization of the samples was also characterized by XRD and TEM. 2007 Elsevier Ltd. All rights reserved.
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quantifying dispersion of layered nanocomposites via Melt Rheology
Journal of Rheology, 2007Co-Authors: Jan Vermant, Simona Ceccia, Michail K Dolgovskij, Pier Luca Maffettone, Christopher W MacoskoAbstract:Rheological measurements are used to compare clay nanocomposites prepared through Melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data.
John R Dorgan - One of the best experts on this subject based on the ideXlab platform.
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Melt Rheology of variable l content poly lactic acid
Journal of Rheology, 2005Co-Authors: John R Dorgan, Jay Janzen, Michael P Clayton, Sukhendu B Hait, Daniel M. KnaussAbstract:Polylactides (PLAs) have been known for several decades and have recently gained considerable commercial significance. This development makes it highly desirable to have the rheological properties of these materials well characterized and reduced to useable mathematical models. However, comprehensive rheological characterization of PLAs is not yet available from the literature. In this study, rheological and thermal measurements were made on a comprehensive and well-characterized set of homopolymers and copolymers spanning wide ranges of molecular mass and stereoisomer proportions (L content). For all compositions within the weight average molecular weight range of 105–106(g∕mol) and a reference temperature of 180 °C, the zero shear viscosity is described by the relationship log(η0)=−14.26+3.4log(Mw), the plateau modulus is 1.0±0.2MPa, and average WLF parameters are c1=3.241∕K and c2=164.9K; the later correspond to a Vogel temperature of 288.25 K. The values of the glass transition temperatures at infinit...
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Melt Rheology of poly lactic acid consequences of blending chain architectures
Polymer Engineering and Science, 2001Co-Authors: Hans J Lehermeier, John R DorganAbstract:The Rheology of blends of linear and branched poly(lactic acid) (PLA) architectures is comprehensively investigated. Measurement of the Melt rheological properties of PLA is complicated by degradation effects but the addition of 0.35 wt% tris(nonylphenyl) phosphite (TNPP) provides excellent stabilization over a range of temperatures. Master curves of dynamic viscosity constructed using time-temperature superposition show significant dispersion for unstabilized samples; this behavior is accompanied by a loss of molecular weight. TNPP stabilized samples show excellent superposition throughout the entire frequency range and minimal loss in molecular weight. For the linear architecture, the Cox-Merz rule is valid for a large range of shear rates and frequencies. The branched architecture deviates from the Cox-Merz equality and blends show intermediate behavior. Both the zero shear viscosity and the elasticity (as measured by the recoverable shear compliance) increase with increasing branched content. The viscosities of both the unstabilized samples and the TNPP stabilized samples roughly obey a log additivity mixing rule. The recoverable shear compliance is monotonic in blend composition and a mixing rule for this property is also presented. For the linear chain, the compliance is independent of temperature but this behavior is apparently lost for the branched and blended materials. Tensile and thermal properties of the blends are also measured and found to be roughly equal within the statistical error of the experiments. The results suggest that excellent control over rheological behavior of PLA is possible through blending chain architectures without compromising mechanical properties.
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Melt Rheology of high l content poly lactic acid
Macromolecules, 2001Co-Authors: Liviuiulian Palade, Hans J Lehermeier, John R DorganAbstract:The Melt Rheology of linear poly(lactic acid)s (PLA) characterized by a high content of the l-form of the monomer is comprehensively investigated. Measurements of dynamic, steady, and transient shear viscosities are presented. Extensional data on PLA are presented for the first time and show a strong strain hardening behavior. The Cox−Merz relationship is obeyed over a particularly wide range (roughly 3 decades of shear rate). Results for high molecular weight samples suggest that the plateau modulus is approximately 5 × 105 Pa. In addition, the zero shear viscosity, η0, for these materials is found to roughly scale with the expected 3.4 power vs molecular weight. The transient shear results are satisfactorily predicted using a truncated form of the K−BKZ constitutive equation and a set of Maxwell modes (Gk, λk) derived from the dynamic spectra. However, to capture the observed extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate...
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Melt Rheology of poly lactic acid entanglement and chain architecture effects
Journal of Rheology, 1999Co-Authors: John R Dorgan, Joshua S Williams, David N LewisAbstract:Poly(lactic acids) (PLAs) are a family of polyesters available via fermentation from renewable resources and are the subject of considerable recent commercial attention. In this study, the Melt rheological properties of a family of poly(lactic acid) stars are investigated and compared to the properties of the linear material. For polymers made from a 98:2 ratio of the L to D enantiomeric monomers it is found that the entanglement molecular weight is in the range of 9000 g per mole (Me≈8700 g/mol) while the molecular weight for branch entanglement is inferred to be approximately 3500 g per mole (Mb≈34 600 g/mol). In addition, the zero shear viscosity of the linear material increases with the 4.6 power of molecular weight. These results may suggest that PLA is a semistiff polymer in accordance with other recent findings. The increase in zero shear viscosity for the branched materials is measured and quantified in terms of appropriate enhancement factors. Relaxation spectra show that the transition zone for ...
Daniel M. Knauss - One of the best experts on this subject based on the ideXlab platform.
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Linear Melt Rheology of Pom-Pom Polystyrenes with Unentangled Branches
Macromolecules, 2007Co-Authors: Evelyne Van Ruymbeke, M. Kapnistos, Dimitris Vlassopoulos, Tianzi Huang, Daniel M. KnaussAbstract:We measured the linear viscoelastic properties of a series of polystyrene Melts with pom-pom architecture consisting of backbones ranging from marginally to well-entangled (of molecular weight Mb), endgrafted with q unentangled branches (Mbr) per backbone end. In this case, the branches relax very fast and act primarily as solvents for the backbone. Using a time-marching tube-based model, we showed that, in such a case only, the pom-pom polymer is equivalent to a blend of long (ML Mb + 2Mbr) and short (MS Mbr) linear chains with respective proportion (Mb + 2Mbr)/2(q - 1)Mbr.
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Melt Rheology of variable l content poly lactic acid
Journal of Rheology, 2005Co-Authors: John R Dorgan, Jay Janzen, Michael P Clayton, Sukhendu B Hait, Daniel M. KnaussAbstract:Polylactides (PLAs) have been known for several decades and have recently gained considerable commercial significance. This development makes it highly desirable to have the rheological properties of these materials well characterized and reduced to useable mathematical models. However, comprehensive rheological characterization of PLAs is not yet available from the literature. In this study, rheological and thermal measurements were made on a comprehensive and well-characterized set of homopolymers and copolymers spanning wide ranges of molecular mass and stereoisomer proportions (L content). For all compositions within the weight average molecular weight range of 105–106(g∕mol) and a reference temperature of 180 °C, the zero shear viscosity is described by the relationship log(η0)=−14.26+3.4log(Mw), the plateau modulus is 1.0±0.2MPa, and average WLF parameters are c1=3.241∕K and c2=164.9K; the later correspond to a Vogel temperature of 288.25 K. The values of the glass transition temperatures at infinit...
Pitt Supaphol - One of the best experts on this subject based on the ideXlab platform.
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Melt Rheology and extrudate swell of titanium iv oxide nanoparticle filled isotactic polypropylene effects of content and surface characteristics
Polymer Testing, 2008Co-Authors: Rapeephun Dangtungee, Pitt SupapholAbstract:Melt Rheology and extrudate swell of isotactic polypropylene (iPP) filled with titanium (IV) oxide (TiO2) nanoparticles of varying content (i.e., 5-30 wt.%) and surface characteristics (i.e., uncoated, silica (SiO2)-coated, and stearic acid-coated) during capillary Melt-extrusion were investigated. The wall shear stress of both the iPP Melt and the Melts that contained nanoparticles increased in a non-linear, decreasing-rate manner with increasing apparent shear rate. The incorporation of the nanoparticles generally increased the wall shear stress of the Melts, with stearic acid-coated ones exhibiting the least variation in the property values. The shear viscosity, when plotted with the real shear rate on a log-log scale, exhibited a linear, decreasing function with the real shear rate. Clearly, the iPP Melts containing stearic acid-coated TiO2 nanoparticles exhibited the lowest shear viscosity values. Lastly, the extrudate swell of the iPP Melt and the Melts that had been filled with nanoparticles was found to increase with increasing apparent shear rate, while it was found to decrease generally with the addition and increasing amount of nanoparticles.
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Melt Rheology and extrudate swell of calcium carbonate nanoparticle filled isotactic polypropylene
Polymer Testing, 2005Co-Authors: Rapeephun Dangtungee, Jimmy Yun, Pitt SupapholAbstract:Abstract Melt Rheology and extrudate swell of isotactic polypropylene (iPP) filled uncoated and stearic acid-coated CaCO3 nanoparticles in various amounts of filler (i.e. from 5 to 25 wt%) during capillary Melt-extrusion were investigated. The wall shear stress for both neat iPP and iPP compounds increased in a non-linear manner with increasing apparent shear rate. The apparent shear viscosity for all of the samples investigated was found to decrease in a non-linear manner with increasing apparent shear rate. Addition of uncoated CaCO3 nanoparticles was responsible for increasing the apparent shear viscosity of the compounds, while stearic acid-coated ones did not affect the apparent shear viscosity of the iPP matrix as much. The percentage of extrudate swell was found to increase with increasing apparent shear rate in a non-linear manner, while it was found to increase linearly with increasing wall shear stress. Lastly, the percentage of extrudate swell was found to be a decreasing function of the filler loading.
D J Groves - One of the best experts on this subject based on the ideXlab platform.
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predicting low density polyethylene Melt Rheology in elongational and shear flows with pom pom constitutive equations
Journal of Rheology, 1999Co-Authors: Nathanael J Inkson, T C B Mcleish, O G Harlen, D J GrovesAbstract:A recent constitutive equation derived from molecular considerations on a model architecture containing two branch points a “pom-pom” captures the qualitative rheological behavior of low density polyethylene (LDPE) in shear and extension for the first time [, J. Rheol. 42, 82 (1998)]. We use a hypothetical Melt of pom-poms with different numbers of arms to model the behavior of LDPE. The linear relaxation spectra for various LDPE samples are mapped to the backbone relaxation times of the pom-pom modes. Data from start-up flow in uniaxial extension fixes the nonlinear parameters of each mode giving predictions for shear and planar extension with no free parameters. This process was carried out for data in the literature and for our own measurements. We find that multimode versions of the pom-pom equation, with physically reasonable distributions of branching, are able to account quantitatively for LDPE Rheology over four decades in the deformation rate in three different geometries of flows. The method suggests a concise and functional method of characterizing long chain branching in polymer Melts.