Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W21: Polymer Melts and Solutions |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Rob Hoy, University of South Florida Room: 406 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W21.00001: Multi-level slip-link modeling Jay Schieber That the dynamics of concentrated, high-molecular-weight polymers are largely governed by entanglements is now widely accepted, and typically understood by the tube model. Although the original idea for slip-links was proposed at the same time as tubes, only recently have detailed, quantitative mathematical models arisen based on this picture. We argue here for the use of a slip-link model that has strong connections to atomistic, multichain levels of description, agrees with non-equilibrium thermodynamics, applies to any chain architecture and can be used in linear or non-linear rheology. We present a hierarchy of slip-link models that are connected to each other through successive coarse graining. One might choose a particular member of the hierarchy depending on the problem at hand, in order to minimize computational effort. In particular, the most detailed level of description has four parameters, three of which can be determined directly from atomistic simulations. The least-detailed member is suitable for predicting non-linear, non-uniform flow fields. We will show how using this hierarchy of slip-link models we can make predictions about the nonlinear rheology of monodisperse homopolymer melts, polydisperse melts, or blends of different architectures. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W21.00002: Synthesis of amphiphilic diblock copolymer for surface modification of Ethylene-Norbornene Copolymers Simon Levinsen, Winnie Edith Svendsen, Andy Horsewell, Kristoffer Almdal The aim of this work is to produce polymer modifiers in order to develop hydrophilic polymeric surfaces for use in microfluidics. The use of hydrophilic polymers in microfluidics will have many advantages e.g. preventing protein absorbance. Here we present an amphiphilic diblock copolymer consisting of a bulk material compatible block and a hydrophilic block. To utilize the possibility of incorporating diblock copolymers into ethylene-norbornene copolymers, we have in this work developed a model poly(ethylene-1-butene) polymer compatible with the commercial available ethylene-norbornene copolymer TOPAS. Through matching of the radius of gyration for the model polymer and TOPAS the miscibility was achieved. The poly(ethylene-1-butene) polymer was synthesized from a hydrogenated anionic polymerized polybutadiene polymer. As hydrophilic block poly(ethylene oxide) was subsequently added also with anionic polymerization. Recent miscibility results between the model polymer and TOPAS will be presented, as well ongoing efforts to study the hydrophilic surface. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W21.00003: Pulling a Polymeric Chain through Tiny Grommets Xiaorong Wang, Yongli Mi In this model, the N beads of mass m are connected in a chain by entropy springs. The friction of the chain is described by a viscous-force for energy dissipation. The interaction of the chain with the grommets is represented by a narrow Gaussian potential. The equations of motion for this model are Langevin type. It is shown that in this model the chain displays oscillated motions under a constant pulling condition. This stick-slip dynamics in a polymer system has not been discussed previously, but may really exist in some systems such as polymer nano-composites and polymer-entangled fluids. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W21.00004: Unravelling Popular Myths in the Rheology of Entangled Polymer Melts Richard P. Wool \underline {Myth No 1}: \textbf{Constraint release and Chain End Fluctuation} coupled with Reptation dominate stress relaxation of highly entangled chains. Fact: Experiments show that Percolation processes account for about 50{\%} of the relaxation, coupled with deGennes Reptation dynamics. In fact, the random coils are not relaxed (via Neutrons) when the stress (Birefringence) goes to zero, a critical prediction of the percolation mechanism, which is not in violation of the stress-optical law.. \underline {Myth No 2}: The \textbf{Packing Length Entanglement Model }for the critical entanglement molecular weight, M$_{\mathrm{e}} =$ 354 p$^{3}$, provides a fundamental description of entanglements at the molecular level. Fact: Experiments show that the Packing Length model is fundamentally incorrect in all its predictions of rheological properties via M$_{\mathrm{e}}$ $\sim$ [ M$_{\mathrm{o}}$/C$_{\mathrm{\infty }}$]$^{\mathrm{3}}$, especially at the nanoscale, as well as the bulk. This is due to an incidental relationship between the monomer molecular weight M$_{\mathrm{o}}$ and the characteristic ratio C$_{\mathrm{\infty }}$ for vinyl type polymers. The correct entanglement model is given by M$_{\mathrm{e}}$ $\sim$ C$\infty $M$_{\mathrm{o}}$. \underline {Myth No 3}: \textbf{The Glass Transition T}$_{\mathrm{\mathbf{g}}}$\textbf{ is dominated by Segmental Dynamics and Free Volume}: Fact: Experiments show that T$_{\mathrm{g}}$ is dominated by the cluster dynamics of anharmonically interacting fractal aggregates which range from 1-100 nm in size (ave $\sim$ 5 nm), as described by the TFT. In nanoconfined thin films, the segmental dynamics does not change much while huge decreases in T$_{\mathrm{g}}$ can be observed due to cluster size effects. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W21.00005: Shear and extensional rheology of model branched polymer melts (H shaped and grafted) Gengxin Liu, Konstantinos Ntetsikas, Kostas Misichronis, Namgoo kang, Jimmy Mays, Apostolos Avgeropoulos, Shi-Qing Wang While nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effect of chain architecture remains the last frontier in polymer rheology. Here we study two H-shape and one grafted-polyisoprene (3 branches) using startup and step extension and shear. Long chain branches (LCB) impede yielding and prevent entangled network from full disentanglement. Thus, nonlinear rheological behavior of LCB polymers forms a sharp contrast to that of linear chains. We will demonstrate these striking differences. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W21.00006: Visualize space-dependence of viscosity Lingxiang Jiang, Boyce Tsang, Steve Granick The space-time dependence of viscosity plays a fundamental, crucial role in a number of natural and industrial processes, where the time dependence has been extensively studied by conventional methods, yet its spatial counterpart has not been directly determined. Here, we propose an imaging based method to measure the space-time dependent cooperative viscosity and confirm its validity in a biopolymer, F-actin solution. A space dependent master curve of cooperative viscosity is identified with an exponential growth at short distance (correlation length 8 times of mesh size) and a plateau at long distance (surprisingly large crossover distance 18 times of mesh size), therefore visualizing the discrete-to-continuum transition of viscosity in real space. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W21.00007: Synthesis and rheological behavior of atactic polypropylene molecular bottlebrushes Samuel Dalsin, Frank Bates, Marc Hillmyer Molecular bottlebrushes are branched polymer structures characterized by an extremely high density of polymeric side chains emanating from a central backbone. Due to unique conformational and rheological properties, molecular bottlebrushes have become attractive candidates for developing new photonic bandgap materials, nanotubes and nanowires, and rheological modifiers. In this study, bottlebrushes comprised of atactic polypropylene (aPP) side chains were synthesized via ring-opening metathesis polymerization of norbornenyl-terminated aPP macromonomers. A series of bottlebrush polymers with fixed side chain length and variable backbone length was prepared using Grubbs' third-generation catalyst, yielding products with low dispersity in less than five minutes reaction time. Small-amplitude oscillatory shear measurements were performed to examine linear viscoelastic properties. Master curves of all bottlebrush polymers exhibited relaxation spectra devoid of any entanglement plateau, despite high molecular weights (up to 892 kg/mol). Lack of entanglement was further confirmed by zero shear viscosity experiments, which displayed a nearly linear dependence on molecular weight. These rheological properties are compared directly with a linear aPP control sample. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W21.00008: Structure of a bottlebrush melt Jaroslaw Paturej, Sergei Sheiko, Sergey Panyukov, Michael Rubinstein A bottlebrush polymer is a branched macromolecule composed of a linear chain (backbone) with side chains densely tethered to it. High grafting density of side chains gives rise to various unique structural properties, such as highly extended conformations of their backbones and tunable character of their stiffness and rheological properties with degree of polymerization of the side chains. We conducted coarse-grained molecular dynamics simulations to determine how the number of Kuhn segments in a bottlebrush backbone $L$ and in the side chains $N$ affect size, stiffness, and structure of these molecules. We found that the size (root-mean-squared radius of gyration and end-to-end distance) and persistence length of bottlebrushes in a melt state scales as $N^{1/2}$. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W21.00009: Role of the Entanglements and Bond Scission in High Strain-Rate Fracture of Polymer Melts Yelena Sliozberg, Robert Hoy, Randy Mrozek, Joseph Lenhart, Jan Andzelm We present coarse-grained molecular dynamics simulations of the effects of solvent molecular weight on the toughness of entangled and non-entangled polymer gels. Our results demonstrate that higher molecular weight solvents enhance gel toughness, and that mechanical properties including strength and toughness can be related to bond scission. We find a remarkable two-step gel fracture mechanism: network chains undergo scission first (and well before fracture), followed by scission of solvent chains. Even after the network chains break, long highly entangled solvent chains provide fracture resistance by effectively increasing the number of chains that must be broken as a crack propagates [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W21.00010: Controlling solubility of pNIPAM in aqueous solutions using hydrophobic and photoresponsive molecular units Rahul Singh, Sanket Deshmukh, Subramanian Sankaranarayanan, Ganesh Balasubramanian The structural properties of pNIPAM (poly-N-isopropylacrylamide), which is a thermally sensitive polymer, are investigated by copolymerizing it with molecular units that are either (1) hydrophobic (polystyrene) or (2) photoresponsive (spiropyran-merocyanine pair). We employ molecular dynamics (MD) simulations to examine aqueous solutions of pNIPAM (modified with these molecules) across a temperature range below and above the LCST of pure pNIPAM to understand the fundamental physics underlying the coil-to-globule transition in pNIPAM and the contribution of the attached constituents on the LCST. The LCST can be tuned by copolymerizing pNIPAM with polystyrene (PS), a hydrophobic molecule. We prepare a number of copolymers with different chain lengths of the hydrophobic units (PS) and observe the lowering of the LCST of the modified pNIPAM by computing the radius of gyration and end-to-end distances across the temperature range. Also, the aqueous solubility of pNIPAM can be controlled by functionalizing it with a photoresponsive moiety as this new copolymer exhibits a shifted LCST phase transition. Thus, the temperature sensitive behavior of pNIPAM can be tuned by copolymerizing it with varying molecular lengths of hydrophobic block units or attaching reversibly switchable photoresponsive moieties. Our work demonstrates the controllability of pNIPAM solubility aqueous solutions and recommends strategies to design complex programmable polymers that have wide-ranging applications in several biomedical and industrial processes. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W21.00011: Effect of Chain Architecture on the Structural and Rheological Properties of Dilute Polymer Solutions: A Molecular Simulation Study Fardin Khabaz, Rajesh Khare Advances in chemistry have allowed synthesis of polymer chains of specified architecture. The effect of chain architecture on the structure and rheology of dilute polymer solutions is not fully understood. Furthermore, possible breakage of polymer chains at high shear rates is important for several industrial applications. In this work, these effects are investigated by performing molecular dynamics simulations. Structure and rheology of dilute polymer solutions containing four types of chains - linear, comb shaped, H-shaped and star - are compared. Simulations are used to determine the scaling of the intrinsic viscosity with molecular weight for chains of different architectures. These simulation results are also compared with theoretical predictions from literature. In addition, structural stability of the chains is studied by monitoring their local deformation at high shear rates. Possible sites for chain scission are identified from these high shear rate simulations for each type of chain architecture. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W21.00012: Direct observation of polymer dynamics in semi-dilute solutions Kai-Wen Hsiao, Christopher Brockman, Charles M. Schroeder In this work, we use single molecule techniques to study polymer dynamics in semi-dilute solutions. Here, we study the steady state extension and relaxation time dynamics of polymer molecules in semi-dilute DNA solutions in extensional flow. Polymer chain dynamics are complicated in semi-dilute solutions due to chain overlap, hydrodynamic interactions, and excluded volume interactions. We use single molecule fluorescence microscopy and a microfluidic-based hydrodynamic trap to directly observe the dynamics of polymers in non-dilute solutions. We report the scaling of polymer relaxation time as a function of polymer concentration, and we observe a crossover in chain behavior from the dilute to semi-dilute regime. Interestingly, we observe a pronounced center-of-mass drift of single polymer chains in directions orthogonal to flow in semi-dilute solutions, which is characterized as a function of concentration and flow rate. By using the automated hydrodynamic trap coupled with a piezoelectric stage, we are able to track the 3-D position of single polymer molecules and deduce the relationship between strain rate and polymer extension. Overall, our work reports on a key advance in the field of polymer dynamics via direct observation of dynamics in semi-dilute solutions in strong flows [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W21.00013: Locality of entangled polymer dynamics Chi Hang Boyce Tsang, Lingxiang Jiang, Steve Granick A combination of sparse and full fluorescence labeling of entangled actin solutions (filaments about 15 $\mu $m long at 1 mg/ml concentration) allowed us to probe both filament-scale polymer dynamics and effectively monomer dynamics. On the filament scale, the reptation tube idea of classical polymer physics works well. However, on a local scale comparable to mesh size, local tube width fluctuation becomes important. For the first time, the dependence of longitudinal diffusion on local tube width was quantified. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W21.00014: Shear-induced irreversible breakdown of shear thickening fluids Jonathan Seppala, Kirk Rice, Gale Holmes Amorphous fumed silica/polypropylene glycol (PPG) suspensions were subjected to multiple steady shear and oscillatory shears above the critical strain rate and critical strain amplitude. After each strain sweep, the steady shear viscosity and oscillatory shear moduli decreased over the entire measured range, and the on-set of shear thickening occurred at increasingly higher critical strain rates or strain amplitudes. Analysis of the oscillatory intra-cycle stress-strain (Lissajous-Bowditch) curves indicated a single-cycle shear thickening occurred at strain amplitudes below the traditionally defined critical strain and only during the first pass. The changes in the material properties appear to be irreversible and are attributed to breakdown of fumed silica-PPG agglomerates. Simultaneous rheology and small angle neutron scattering (RheoSANS) was also used to test this hypothesis. Finally intra-cycle and non-linear responses for fumed silica-PPG on parallel plate and cone and plate were analyzed via the MITlaos package. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W21.00015: Application of scaling model to investigate solvent quality and functionality in star polymers Durgesh Rai, Gregory Beaucage, Ramanth Ramachandran, Kedar Ratkanthwar, Nikos Hadjichristidis, Hong Kunlun, David Uhrig, Andy Tsou Symmetric star polymers serve as model systems to understand branching effects in long chain macromolecules. Generally, the solution properties of stars have been modeled based on ideal Gaussian statistics or using empirical approaches that incorporate fractal scaling neither of which provide satisfactory complete understanding of thermodynamic or structural details across different solvent quality and temperature ranges. A coupling of the unified scattering function with the RPA equation and Benoit's approach to model inter-arm and intra-arm interactions is proposed to analytically quantify thermodynamic effects along with topological variations using the proposed scaling model. Detailed topological quantification of star polymers systems have been able to describe both, good and theta solvent conditions along with effects of functionalities, as well as resolve deviations in chain conformations due to steric interactions between star arms. The scaling model quantifies the distinction between invariant topological features for star polymers and chain tortuosity, which changes with functionality as well as goodness of solvent and steric interactions. [Preview Abstract] |
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