Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P20: Melts and Solutions |
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Sponsoring Units: DPOLY Chair: Chang Ryu, Rensselaer Polytechnic Institute Room: 321 |
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P20.00001: Dynamics and rheology of high molar mass polyethylene oxide solutions Abhishek Shetty, Michael Solomon We report dynamic light scattering (DLS), bulk rheology and turbulent drag reduction (TDR) measurements that investigate the structure and dynamics of high molar mass PEO solutions. Steady shear rheology of high molar mass PEO solutions, when modeled by the FENE-P constitutive equation, was consistent with viscoelastic relaxation times much larger than predicted by single polymer, dilute solution theory. DLS of dilute PEO solutions showed a single relaxation mode in the decay time distribution, which scales as q$^{-3}$ rather than the q$^{-2}$ scaling expected of diffusive dynamics. We interpret this result as consistent with the internal dynamics of large multichain domains, clusters or aggregates in the high molar mass PEO solutions. By means of DLS, we also show that the aggregation state of dilute solutions of high molar mass PEO can be manipulated by addition of the chaotropic salt guanidine sulfate or the divalent salt magnesium sulfate. Addition of these salts shifts the power law scaling of the relaxation time from q$^{-3}$ to q$^{-2}$. This shift of relaxation time scaling from one indicative of aggregate dynamics (q$^{-3})$ to one characteristic of polymer center-of-mass diffusion (q$^{-2})$ shows that these salts are effective de-aggregation agents for PEO. We discuss the results in light of the potential connection between aggregation behavior and polymer TDR of high molar mass PEO. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P20.00002: Diffusive Properties of Dilute HPC Solutions: Comparative Study with DLS and FPR Ryan McDonough, Kiril Streletzky, Paul Russo The dynamics of HPC (Hydroxy-propyl-cellulose) solutions were studied by two fundamentally different methods: FPR (Fluorescence Photo-bleaching and Recovery) and DLS (Dynamic Light Scattering). FPR captures diffusive processes by establishing a photo-bleached boundary and ``seeing'' only tagged particles diffusing back into bleached area, which yields a contrast function. DLS auto-correlates scattered light intensity from particles in order to determine a statistical decay function. Inverse Laplace transform (CONTIN) and stretch exponential line shape analysis (LSA) serve to quantitatively decompose decay data into different diffusion processes or modes. The first finding is that the CONTIN and LSA results on the same sample are fairly consistent. The second finding is that the modal distributions for FPR and DLS spectra on the same sample show consistent dissimilarities. This indicates a comparative limitation or sensitivity in range of detectable diffusive processes between FPR and DLS in a complex system. The third finding is that the fluorescent tag and tagging process seem to alter the diffusion processes seen by DLS in a way that is consistent; there is a slower mode apparent in non-tagged sample which does not appear in the tagged sample. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P20.00003: Helix formation via kinetic assembly of charged block copolymer cylinders in solution Sheng Zhong, Ke Zhang, Karen Wooley, Darrin Pochan A multi-micrometer-long, cylinder with helical superstructure is created from coassembly of poly(acrylic acid)-\textit{block}-poly(methyl acrylate)-\textit{block}-polystyrene (PAA-$b$-PMA-$b$-PS) triblock copolymers with excessive triethylenetetramine or diethylenetriamine in the mixture of 67{\%} volume ratio of water in tetrahydrofuran (THF). The stable pitch distance of the formed helices is due to the balance of long range electrostatic association and uniaxial tension along the cylinder, which can be efficiently tuned by varying the type and amount of the multivalent amine molecules. Double and triple helices are also formed with characteristic interhelical cynlinder distances similar to what is observed as the pitch in single helices. A kinetic study shows that the formation of a helix undergoes a complex, but reproducible, nanostructure evolution, including a starting stacked structure, a transition state of very short helices with the length of tens of nanometers and a final multi-micrometer-long mature helix by connecting those short helices. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P20.00004: Enhancing Polymer-Fullerene Miscibility Through Enthalpic Interactions Katie Campbell, David Bucknall, Yonathan Thio, Haskell Beckham, Uwe Bunz, Adam Hannon, Andrew Zappas, Bilge Hatiboglu, Michael Kempf Using both theoretical and experimental methods, the use of functional groups in controlling the miscibility between various polymers and fullerenes has been investigated. Molecular dynamics simulations with unmodified C$_{60}$ and C$_{60}$ dimers indicated that the number and connectivity of phenyl rings as functional groups, polymer backbone spacing, and aromaticity are all factors in fullerene miscibility. To distinguish between entropic and enthalpic factors, UV-visible spectroscopy was used to determine fullerene solubility with a variety of solvents and to also determine association constants with solvents and polymers. A distinct time dependency for complex formation with many of the fullerene- organic materials investigated was observed as evidenced by a change in solution color with time. Stern-Volmer approximations and fluorescence quenching were used to examine the association of C$_{60}$ with a series of poly(\textit{para}-phenylene ethynylene)s, cyclic polystyrene (PS), and linear PS. The fluorescence quenching of these materials by C$_{60}$ indicates an association between C$_{60}$ and the polymer. WAXS studies have shown the formation of C$_{60}$ aggregates in PS at concentrations of C$_{60}$ as low as 1 wt{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P20.00005: Modeling Thermodynamic Behavior of Nonionic Surfactants in Water Valeriy Ginzburg, Pierre Varineau Aqueous solutions of nonionic surfactants exhibit a non-trivial phase behavior known as lower critical solution temperature (UCST), where solutions are homogeneous at lower temperatures but become cloudy (two-phase) at higher temperatures. Conventional Flory-Huggins\footnote{M. L. Huggins, \textit{J. Chem. Phys. }\textbf{9}, 440 (1941); P. J. Flory, \textit{J. Chem. Phys. }\textbf{9}, 660 (1941).} theory of polymer solutions fails to describe such phase behavior. We utilize the approach suggested by Dormidontova\footnote{E. E. Dormidontova, \textit{Macromolecules }\textbf{35}, 987 (2002).} and modify Flory-Huggins theory by explicitly accounting for water-water and water-alkylene oxide hydrogen bonding. While the Dormidontova model was restricted to aqueous solutions of polyethylene oxide (PEO), we extend it to include other monomers and their copolymers. With the new approach, we can semi-quantitatively predict cloud points of various nonionic surfactants (Tergitol{\texttrademark} L and Ecosurf{\texttrademark} series) as functions of their molecular structures. We also discuss extensions of this model to calculate micellar phase behavior and oil/water/surfactant interfacial tensions. {\texttrademark} Trademark of The Dow Chemical Company [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P20.00006: Role of surfactants on the assembly of amphiphilic copolymers through instabilities of organic/water interfaces Jintao Zhu, Ryan C. Hayward We have studied the influence of aqueous surfactants on the assembly of amphiphilic copolymers through hydrodynamic instabilities of organic/water interfaces. Micropipette aspiration measurements on evaporating chloroform droplets containing polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers revealed that the onset of interfacial instability and subsequent growth in surface area corresponded to a near vanishing of the interfacial tension. By adding another surfactant, such as sodium dodecyl sulfate (SDS), to the aqueous phase, the chloroform/water interfacial tension was reduced and the onset of instability shifted to lower concentration of PS-PEO. Varying amounts of SDS also led to qualitatively different mechanisms of growth in interfacial area and correspondingly different morphologies of the resulting copolymer assemblies. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P20.00007: Influence of Intermolecular Interactions on Fragility of Polymers Kumar Kunal, Alexei Sokolov Glass transition in polymers is a result of slowing down of segmental relaxation. Steepness of the temperature-dependence of segmental relaxation times close to the glass transition temperature, Tg is expressed in terms of fragility parameter. A strongly non-Arrhenius temperature dependence of segmental relaxation times with steep variations close to Tg is called a `fragile' behavior, and a nearly Arrhenius behavior is called `strong'. The existing theoretical models and experimental investigations on polymers with weak van der Waal's interactions suggest that fragile behavior of polymers may be linked to their poor packing ability. However, the effect of strong intermolecular interactions on fragility such as polar interactions and hydrogen bonds remains unexplored. It has been predicted that polymers composed of polar monomers are likely to be highly fragile. We have studied polymers with strongly polar interactions and hydrogen bonds and found that although polar polymers do seem to have a higher Tg than their non-polar counterparts, no such conclusion can be drawn about their fragility. The different effects of polar interactions on different classes of polymers may be attributed to the difference in their Tgs. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P20.00008: Random Walk of Chain Molecules Along Pore Axis Guiduk Yu, Sergei Obukhov, Jiun-Tai Chen, June Huh, Yoontae Hwang, Soonchun Mok, Priyanka Dobriyal, Pappannan Thiyagarajan, Thomas P. Russell*, Kyusoon Shin* We investigated the overall conformation of polymer chain in cylindrical nanopores using small-angle neutron scattering. The mixture of hydrogenous PS and deuterated PS is confined in nanopores. Surprisingly, the overall conformation of polymer chains along the pore axis is observed to be the same as that in bulk. Even though the chain dimension is larger than the radius of the pores, the chains along the pore axis are not stretched, but sustain to be in unperturbed state. The SANS results implicate that the interpenetration of polymer chains decreases as polymer enters nanopores. We expect the reduction of intermolecular entanglement possibly alters other physical properties of polymer under nanoconfinement. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P20.00009: Dynamics of Cyclic Molecules Threaded into a Linear Polymer Chain Koichi Mayumi, Hitoshi Endo, Michihiro Nagao, Noboru Osaka, Hideaki Yokoyama, Mitsuhiro Shibayama, Kohzo Ito Dynamics of polyrotaxane (PR), in which cyclic molecules, cyclodextrins (CDs), are threaded on an axis linear polymer chain, poly(ethylene glycol) (PEG), are first studied by contrast variation neutron spin echo (CV-NSE). By comparing PRs of hydrogenated and deuterated PEG with different scattering contrasts, we successfully extract two diffusive modes of CDs, corresponding to self diffusion and relative motion to the axis PEG in PR. The self-diffusion constant of CD in PR is determined to be about one-third of the free one in the absence of the axis polymer, which would reflect the space dimension of diffusion with the topological restriction on the axis chain path. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P20.00010: Diffusion of Water through Methyl- and Hydroxyl-Terminated Poly(Dimethylsiloxane) Ahmed E. Ismail, Gary S. Grest, David R. Heine, Mark J. Stevens, Mesfin Tsige Both experimental and numerical reports of the diffusion constant of water through poly(dimethylsiloxane) (PDMS) show variances of nearly an order of magnitude. We use molecular dynamics simulations to calculate the diffusion constant for both methyl- and hydroxyl-terminated PDMS chains. We examine the effects of both concentration and chain length. For a single water molecule, we find that diffusion depends strongly on the initial location of the molecule, as the ``caging'' phenomenon reported by Müller-Plathe can occur. At intermediate concentrations, we observe the formation of dimer and trimer water clusters, leading to lower diffusion rates; at concentrations above the reported aggregation limit of 0.1 wt \%, we observe the onset of phase segregation. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P20.00011: Effect of Stereochemistry and Polydispersity on Diffusion in Polypropylene Ernst von Meerwall, Numan Waheed, Wayne Mattice We have performed dynamic Monte-Carlo (MC) simulations and pulsed-gradient diffusion (D) experiments to study the effect of stereochemical composition in linear polypropylene (PP) melts. The coarse-grained simulations were based on the rotational isomeric state model and Lennard-Jones potentials. For the proton NMR diffusion measurements we obtained three PP specimens of differing molecular weight M and dispersity, with the probability of a meso diad Pm = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). The experiment supplied the fixed conversion between MC steps and real time; no dependence on Pm is expected. Both simulation and M-scaled experiment found D at high Pm several times faster than at low Pm. The constant-M simulation also showed a maximum near Pm = 0.8 due to quenched randomness. To find the source of the remaining disagreement with experiment, new simulations tracked the samples' Pm, mean M, and polydispersity, producing modest improvement. We suspect that the GPC determination of M and its distribution, based on linear polyethylene calibration, is somewhat dependent on PP stereochemistry (via D), generating misleading results. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P20.00012: On chain statistics and entanglement of flexible linear polymer melts Shi-Qing Wang In this work the chain statistics of most linear flexible polymers have been found to be rather universal, allowing chain entanglement to be depicted with few parameters. We first show, to our surprise, based on the literature data of most familiar linear polymers that (a) at the same number of backbone bonds most linear polymers have comparable coil size and are similarly flexible in spite of widely varying chain thickness and (b) the Kuhn length involves a similar number of backbone bonds. The packing model is found to describe the onset molecular weight Me obtained from the elastic plateau modulus whereas all other models in the literature fail to provide good correlation. It is chain thickness not stiffness that correlates with Me for over one hundred flexible linear polymers. On the other hand, other models such as percolation model appear to provide some crude correlation for Mc, to which the packing model does not apply well, where Mc denotes the point of departure in the molecular weight scaling from Rouse like to reptation like. Thus, our analysis clarified the apparent contradiction among the various models. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P20.00013: Translational Diffusion in a Confined Polymer Melt Janet Wong, Liang Hong, Sung Chul Bae, Steve Granick At the University of Illinois, a new experimental platform has been developed that combines the surface forces apparatus with spatially-resolved fluorescence recovery after photobleaching, giving direct measurements of translational diffusion when polymer melts are confined between mica sheets to controlled thicknesses comparable to the size of the molecules themselves. Applying this platform to polydimethylsiloxane (PDMS), we find not only the anticipated dependence on film thickness but also a dependence on the local pressure: when mica sheets are pressed together so that they flatten, the diffusion of chains confined between them depends on the local pressure, being slowest near the center of the contact. The confined chains split into two populations: those that are immobile on the scale of hours, and those whose mobility is close to that of the unperturbed polymer melt. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P20.00014: Entangled Polymer Melt Dynamics Studied By Low-Field NMR Fabian Vaca Chavez, Patrick Huebsch, Ronald Zirbs, Wolfgang Binder, Kay Saalwaechter Proton Multiple-Quantum (MQ) NMR is a powerful technique to investigate polymer dynamics due to its sensitivity to molecular motions on very different timescales. Entangled melts exhibit dynamic processes that cover a wide range of timescales, starting from fast ps-scale segmental reorientation up to diffusive and cooperative motions on the ms-s-scale. In this work, we apply MQ NMR to linear poly(cis-1,4-isoprene) and poly(isobutylene) of different molecular weight above the glass transition over suitable ranges of temperature, in order to establish the dynamic regimes predicted by the tube model, and, for the first time, to extract actual time scale information. This directly complements many neutron scattering studies, which are restricted to the sub-$\mu$s-timescale. Measurements on PIB-grafted silica particles with different molecular weights and different chain densities on the surface of the particle are also shown. The data is analyzed by establishing scaling laws which can be directly associated with different dynamic regimes predicted by the tube/reptation model. Full analytical analyses based on a correlation function which explicitly includes segmental, Rouse, and reptation dynamics are discussed. [Preview Abstract] |
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