The Experts below are selected from a list of 21303 Experts worldwide ranked by ideXlab platform
Doros N Theodorou - One of the best experts on this subject based on the ideXlab platform.
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topological analysis of Linear Polymer melts a statistical approach
Macromolecules, 2006Co-Authors: Christos Tzoumanekas, Doros N TheodorouAbstract:We introduce an algorithm for the reduction of a computer generated atomistic Polymer sample to an entanglement network of primitive paths. These networks are structural representations of the topology underlying a Polymer melt. By examining network ensembles of polyethylene and cis-1,4-polybutadiene melts, we provide topological measures and statistical properties of primitive paths. We present the radial distribution function of entanglements and the distribution of the number of monomers between entanglements. A renewal point process that generates entanglement events along the monomer sequence of a chain is found to describe the statistics of detected topological constraints. We discuss chain thickness effects on topological measures and provide a method for detecting persistent chain contacts in melt configurations. A suitable scaling of acquired data leads to a unifying microscopic topological description of the melts studied.
Andrew J Boydston - One of the best experts on this subject based on the ideXlab platform.
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comparison of mechanochemical chain scission rates for Linear versus three arm star Polymers in strong acoustic fields
ACS Macro Letters, 2014Co-Authors: Derek C Church, Gregory I Peterson, Andrew J BoydstonAbstract:The effect of star versus Linear Polymer architecture on the rates of mechanochemically induced bond scission has been explored. We determined rate constants for chain scission of parent Linear and star Polymers, from which daughter fragments were cleanly resolved. These studies confirm a mechanistic interpretation of star Polymer chain scission that is governed by the spanning rather than total molecular weight. We further demonstrate the preserved rate of site-selective mechanophore activation across two different Polymer structures. Specifically, we observed consistent activation rate constants from three-arm star and Linear Polymer analogues, despite the Mn of the star Polymer being 1.5 times greater than that of the Linear system.
Dimitris Vlassopoulos - One of the best experts on this subject based on the ideXlab platform.
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Stress Relaxation in Symmetric Ring-Linear Polymer Blends at Low Ring Fractions
Macromolecules, 2020Co-Authors: Daniele Parisi, Dimitris Vlassopoulos, Junyoung Ahn, Taihyun Chang, Michael RubinsteinAbstract:We combine Linear viscoelastic measurements and modeling in order to explore the dynamics of blends of the same-molecular-weight ring and Linear Polymers in the regime of the low volume fraction (0.3 or lower) of the ring component. The stress relaxation modulus is affected by the constraint release (CR) of both rings and Linear components because of the motion of Linear chains. We develop a CR-based model of ring-Linear blends which predicts the stress relaxation function in the low fraction regime of the ring component in excellent agreement with experiments. Rings trapped by their entanglements with Linear chains can only relax by Linear-chain-induced CR, with much slower relaxation of rings compared to Linear chains. The relative viscosity η(ϕR*)/ηL of the blend with respect to the Linear melt viscosity ηL at a ring overlap volume fraction ϕR* is predicted to increase proportionally to the square root of the ring molecular weight Mw,R. Our experimental results clearly demonstrate that it is possible to enhance the viscosity and simultaneously the structural relaxation time of Linear Polymer melts by adding a small fraction of ring Polymers. These results not only provide fundamental insights into the physics of the CR process but also suggest ways to fine-tune the flow properties of Linear Polymers by means of adding rings.
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transition from confined to bulk dynamics in symmetric star Linear Polymer mixtures
Macromolecules, 2019Co-Authors: Daniele Parisi, Domenico Truzzolillo, Vishnu D Deepak, Mario Gauthie, Dimitris VlassopoulosAbstract:We report on the Linear viscoelastic properties of mixtures comprising multiarm star (as model soft colloids) and long Linear chain homoPolymers in a good solvent. In contrast to earlier works, we ...
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Osmotic Interactions, Rheology, and Arrested Phase Separation of Star-Linear Polymer Mixtures
Macromolecules, 2011Co-Authors: Domenico Truzzolillo, Dimitris Vlassopoulos, Mario GauthierAbstract:Starting from a glassy suspension of star Polymers in molecular solvent, we add Linear homoPolymer with a fixed size ratio and ever increasing concentration, hence diluting the glass and eventually approaching the regime of stars in Polymer matrix. We show that we can quantitatively decompose the rheology of the mixtures into colloidal star and Linear Polymer contributions by accounting for the osmotic shrinkage of the stars due to the added Polymers. We also estimate the effective star overlap concentration in the mixtures and show how the rheological properties change at the crossover concentration, where the number of star-particle contacts decreases and the star repulsions weaken and eventually become attractive upon increasing the Linear Polymer concentration. The attraction is accompanied by a phase separation, pointing to the presence of unstable regions in the star/Linear Polymer phase diagram, where gelation results from an arrested phase separation. The crossover concentration is also probed by ...
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osmotic shrinkage in star Linear Polymer mixtures
European Physical Journal E, 2010Co-Authors: Agnieszka Wilk, Sebastian Huismann, Emmanuel Stiakakis, J Kohlbrecher, Dimitris Vlassopoulos, Christos N Likos, G Meier, Jan K G Dhont, G PetekidisAbstract:Multiarm star Polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added Linear homoPolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding Linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small-angle neutron scattering study of star/Linear Polymer mixtures with different size ratios (varying the Linear Polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the Linear/star Polymer size ratio increases for the same effective Linear volume fraction (c/c ∗ with c ∗ the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher Linear Polymer volume fractions. A theoretical description of the force balance on a star Polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the Linear chains, accurately predicts the experimental findings of reduced star size as a function of Linear Polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.
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Osmotic shrinkage in star/Linear Polymer mixtures
The European Physical Journal E, 2010Co-Authors: Agnieszka Wilk, Emmanuel Stiakakis, J Kohlbrecher, Dimitris Vlassopoulos, Christos N Likos, G Meier, Jan K G Dhont, G Petekidis, Sebastian Huißmann, R. VavrinAbstract:Multiarm star Polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added Linear homoPolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding Linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small-angle neutron scattering study of star/Linear Polymer mixtures with different size ratios (varying the Linear Polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the Linear/star Polymer size ratio increases for the same effective Linear volume fraction (c/c ∗ with c ∗ the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher Linear Polymer volume fractions. A theoretical description of the force balance on a star Polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the Linear chains, accurately predicts the experimental findings of reduced star size as a function of Linear Polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.
John M Torkelson - One of the best experts on this subject based on the ideXlab platform.
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fragility confinement effects apparent universality as a function of scaled thickness in films of freely deposited Linear Polymer and its absence in densely grafted brushes
Macromolecules, 2016Co-Authors: Tian Lan, John M TorkelsonAbstract:Ellipsometry measurements as a function of cooling rate are used to study nanoscale confinement effects on dynamic fragility (kinetic fragility), m, in supported films of freely deposited, Linear Polymer. Polymers include neat polystyrene (PS), neat polycarbonate (PC), and PS + 2 wt % 1,10-bis(1-pyrene)decane (BPD) as small-molecule diluent; in each case, the substrate/Polymer interface lacks significant attractive interactions. In terms of both the length scale at which confinement effects become evident and the percentage reduction in m from its bulk value, the magnitude of the m-confinement effect increases with increasing bulk Polymer system m. Additionally, for films of Linear Polymer lacking significant attractive interactions with the substrate surface, m-confinement effects are evident at larger onset thicknesses than those commonly reported in the literature for the glass transition temperature (Tg)-confinement effect. Evans et al. [Macromolecules 2013, 46, 6091] found that the Tg-confinement eff...
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novel synthesis of branched polypropylene via solid state shear pulverization
Polymer, 2015Co-Authors: Mirian F Diop, John M TorkelsonAbstract:Abstract As synthesized by Ziegler–Natta or metallocene catalysis, commercial polypropylene (PP) is Linear and has low melt strength. A common approach for improving melt strength is to incorporate long-chain branches (LCBs). We describe the discovery of a novel approach to prepare LCB PP by subjecting Linear PP to solid-state shear pulverization (SSSP) with benzoyl peroxide (BPO) as the lone additive. Depending on BPO content, LCB PP can be formed by radical reactions during SSSP or post-SSSP melt extrusion. Using shear rheology, we demonstrated that LCB PP was formed during SSSP of samples with 4 and 6 wt% BPO. Relative to Linear PP, the post-SSSP sample (purified of residual BPO) made with 6 wt% BPO exhibited enhanced shear thinning behavior, a decreased dependence of storage modulus ( G ′) on frequency ( ω ) at low ω , and a deviation from Linear Polymer behavior in its van Gurp–Palmen curve. For samples that were prepared with low levels of BPO (0.5–1.5 wt%), LCBs were not formed (within error) during SSSP; instead, LCBs were formed during post-SSSP melt extrusion (with residual BPO). While a sample with 1.5 wt% BPO showed no deviation from Linear Polymer behavior when tested immediately after SSSP, the same sample after post-SSSP melt extrusion demonstrated enhanced shear thinning behavior, decreased dependence of G ′ on ω at low ω , deviation from Linear Polymer behavior in its van Gurp–Palmen plot, and improved crystallization and tensile properties (as is expected for branched PP). We also showed that the LCB formation is achieved by taking advantage of the near-ambient temperature conditions associated with SSSP and is unattainable via conventional melt processing of PP.
Shiqing Wang - One of the best experts on this subject based on the ideXlab platform.
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entangled Linear Polymer solutions at high shear from strain softening to hardening
Macromolecules, 2016Co-Authors: Shiqing WangAbstract:The present rheological study reveals for the first time that entangled Polymer solutions made of Linear polystyrene or poly(methyl methacrylate) can exhibit strain hardening due to non-Gaussian stretching during startup shear at sufficiently high rates and temperatures well above their overall glass transition temperatures: Tg,solute > Texp > Tg,solution. The solutions made of high-Tg Polymers only show partial yielding in the sense that both shear and normal stresses grow monotonically in time until a point of rupture, signified by an emergent cusp in the stress vs strain curve and macroscopic breakup along a shear plane. The shear softening-to-hardening transition, which occurs as a function of the applied shear rate, happens at lower equivalent rate with decreasing temperature, violating the time–temperature superposition principle.