Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y24: Polymer Melts & Solutions: Structure & Solubility |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Ferenc Horkay, National Institutes of Health Room: Baltimore Convention Center 321 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y24.00001: Why Good Solvents Are Seldom All That Good S. T. Milner, M.-D. Lacasse, W. W. Graessley The strength of interactions between polymers and solvents is most commonly summarized in a chi parameter. Chi greater than 0.5 leads to phase separation, while a value of zero indicates an ``ideal'' solvent. In fact, chi values less than 0.25 are quite rare, even among so-called ``good solvents'' for a given polymer. Why should this be? The origin of energetic interactions between nonpolar polymers and solvents is primarily dispersive forces, which are proportional to the square of the difference in solubility parameter; hence it should be possible to choose polymer and solvent with matched solubility parameters to achieve an ideal solvent. We shall argue that there is also a generic entropic contribution to chi. One way of describing it is to say that polymer and solvent have a different propensity to explore free volume; when obliged to mix, there is an entropic penalty resulting from the necessary compromise in free volume per mer. One may also say that the solvent molecules induce a depletion attraction between chain segments, analogous to the attraction between colloidal particles induced by micellar solutions. This effect can be observed in simulations of hard-sphere chains in identical hard sphere solvent. There one finds that the radius of gyration of such chains in solvent are significantly smaller than in vacuum, though still scaling as self-avoiding walks. From this result, a chi value of about 0.3 may be inferred. [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y24.00002: Re-examination of the slow mode in semidilute solutions Chi Wu Dynamics of semidilute solutions has been well described by two different motions, respectively, related to the ``blobs'' and the reptation of an entire chain. In the past, dynamic LLS results revealed that besides the fast relaxation related to the ``blobs,'' there existed an additional slow relaxation. In the earlier time, such a slow mode was wrongly identified as the reptation. Later, this slow mode was attributed to possible problems in the sample preparation, such as dust particles or a concentration gradient. Whether this slow mode is real has remained a challenging problem since 80's. Recently, we found that it appears only when the solvent quality is less good. To avoid problems in the sample preparation, we, respectively, used the coil-to-globule transition of long polystyrene chains, the high-vacuum anionic polymerization of styrene in cyclohexane, and the living bulk polymerization of MMA to alternate the size of polymer chains (i.e., the overlap concentration) to induce an \textit{in-situ} dilute-semidilute transition. Our results confirm that this slow mode is real with no ambiguity. In theory, we can demonstrate that this slow mode appears whenever segments (monomers) in different ``blobs'' start to interact with each other and its characteristic relaxation time is related to the correlation length of these interacting ``blobs.'' [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y24.00003: On Fluctuations in Polymer Systems: Field Theoretic Simulations Kirill Katsov, Erin Lennon, Glenn Fredrickson We use variety of approaches to sample equilibrium fluctuations in polymersystems within a field theoretic framework. Comparison of (complex) Langevin and (smart) Monte Carlo sampling techniques is presented with special attention paid to their efficient implementation. We apply these methods to study effect of fluctuations on phase behavior in diblock copolymer melts and correlations in semidilute polymer solutions. [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 8:48AM |
Y24.00004: Molecular Dynamics Simulations of a Dendritic Polyelectrolyte with Flexible Spacers in Salt-free Solution Qi Liao, Yong Lin, Xigao Jin, Charles C. Han We present the results of molecular dynamics simulation of a dendritic polyelectrolyte in a dilute salt--free solution. The dendritic polyelectrolyte is modeled as an ensemble of bead spring regular-branched chain of charged Lernard-Jones particles with explicit counterions. The simulations were performed covering a wide range of molecular variables of the dendritic polyelectrolyte such as generation number, spacer length, and charge density. The relaxation time of dendrimer size, the conformation of spacers, the size depend of dendrimer on the generation and charge density are discussed and compared with a Flory-type theory. We determined the osmotic coefficients of the dilute dendritic polyelectrolyte solution, and compared with the prediction of cell model of charged sphere. [Preview Abstract] |
Friday, March 17, 2006 8:48AM - 9:00AM |
Y24.00005: Solution and Melt Rheology of Polypropylene Comb and Star Polymers Arnav Ghosh, Ralph H. Colby, Jeffrey M. Rose, Anna E. Cherian, Geoffrey W. Coates Syndiotactic polypropylene macromonomer arms have been prepared by coordination-insertion polymerization. These arms have been made into polypropylene star polymers by the homopolymerization of the syndiotactic arms with a living alkene polymerization catalyst. The macromonomer arms have also been randomly copolymerized with propylene using rac-dimethylsilyl(2-methyl-4-phenylindenyl) zirconium dichloride catalysts to make polypropylene combs. Consequently we have star polymers and a series of comb polymers with different backbone lengths that are all made from the same macromonomer arms. We compare linear viscoelastic data on star and comb polypropylene melts and solutions in squalane to predictions of the tube dilation model and the tube model without tube dilation. The ratio of comb terminal relaxation time to star terminal relaxation time eliminates the friction coefficient and allows determination of the extent of tube dilation the backbone experiences when it relaxes. The concentration dependence of the comb/star terminal relaxation time ratio can be described by either model, owing to adjustable parameters that are not known \textit{apriori}, so independent means to evaluate those parameters will be discussed$. $ [Preview Abstract] |
Friday, March 17, 2006 9:00AM - 9:12AM |
Y24.00006: Watching nucleation and growth of chain disentanglement in large-amplitude oscillatory shear of entangled polymer solutions P. Tapadia, A. Philips , Shi-Qing Wang Our recent stress-controlled measurements show that a sufficient yet moderate level of chain deformation due to shear can produce spatial topological rearrangement to free chains from mutual dynamic constraint [1]. The chain disentanglement seems to occur catastrophically, resulting in an inhomogeneously sheared sample during rate-controlled shear [2]. Thus, chain disentanglement is also expected to take place during large amplitude oscillatory shear (LAOS) at frequencies higher than the overall chain relaxation rate. We applied our particle-tracking velocimetric technique to probe the state of chain entanglement during LAOS. We found that the chain orientation produced by LAOS led to a new environment in which the initially well-entangled chains managed to disentangle inhomogeneously in space. A layer free of chain entanglement developed to take the load of the imposed strain. As a result of this nonlinearity, the rest of the sample avoided significant deformation and its chain entanglement remained intact. [1] Tapadia, P.; Wang, S. Q. \textit{Phys Rev. Lett}, \textbf{91}, 198301 (2003); Tapadia, P.; Wang, S. Q. \textit{Macromolecules }\textbf{37}, 9083 (2004). [2] Tapadia, P.; Wang, S. Q. \textit{Phys. Rev. Lett. }, in press (2005). [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y24.00007: Anomalous Sorption of Carbon Dioxide in Polymer Thin Films Xiaochu Wang, Isaac Sanchez Unusual sorption has been reported in thin polymer films exposed to near-critical CO2. When the supercritical fluid approaches the critical point the film appears to thicken, but it is not clear whether the film swells or if there is adsorption on the film surface A combination of the gradient model of inhomogeneous systems and the lattice fluid model have been used to investigate this phenomenon. It is shown that gas surface adsorption on an attractive surface is proportional to the compressibility of the fluid. We have also investigated numerically the sorption of supercritical CO2 on PDMS and PIB, and supercritical 1,1-difluorethane on PS. By calculating the Gibbs adsorption and adsorption layer thickness of the supercritical fluids, we found in all cases that maximum adsorption occurred when the supercritical fluid was near its compressibility maximum. [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y24.00008: Gas Diffusion in Polyethylene Terepthalate By Molecular Dynamics Simon Butler, David Adolf Molecular dynamics simulations of the diffusion of small penetrants through PET have been performed utilising the anisotropic united atom model [1] and a virtual liquid technique. [2] The accuracy and reliability of these two approaches has been assessed in terms of the improvement in equation of state behaviour and of diffusion co-efficients and solubilities. The effect of the diffusion of nitrogen, carbon dioxide, and oxygen on the local dynamics of PET have been investigated as a result. Attention has been focused on the dual mode effect [3] observed during mixed gas diffusion. \newline \newline [1] \textit{Molecular dynamics calculation of the equation of state of alkanes}, J. Chem. Phys. \textit{93}, 6 (1990) \newline [2] Kikuchi, Kuwajima, Fukada, \textit{Novel method to estimate the solubility of small molecules in cis-polyisoprene by molecular dynamics simulations}, J. Chem. Phys, \textit{115}, 13 (2001) \newline [3] Lewis, Duckett, Ward, Fairclough, Ryan, \emph{The barrier properties of polyethylene terephthalate to mixtures of oxygen, carbon dioxide and nitrogen}, Polymer, \emph{1631}, 44 (2003) [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y24.00009: Overcoming the difficulty in performing large step-strain experiments: A first reliable comparison with Doi-Edwards tube model Paula X. Wang, Shi-Qing Wang Large step shear has been a popular way to interrogate nonlinear viscoelastic responses of polymeric materials. In absence of any severe interfacial failure, the experimental data [1] were found to agree with the Doi-Edwards model of entangled chains. A separate set of experimental studies [2-4] produced strain-softening and showed disagreement with the D-E model. We have successfully prevented interfacial breakdown for the first time to show that the strain-softening is an interfacial artifact [5] and that the stress relaxation behavior of entangled melts and solutions can be reliably depicted experimentally and accounted for within the D-E model. \newline [1] Osaki, K. \textit{et al Macromolecules} \textbf{15}, 1068 (1982). \newline [2] Osaki, K.; Kurata, M. \textit{Macromolecules }\textbf{13}, 671 (1980). \newline [3] Vrentas, C. M.; Graessley, W. W. \textit{J. Rheol. }\textbf{26}, 359 (1982). \newline [4] Osaki, K. \textit{Rheol. Acta }\textbf{32}, 429 (1993). \newline [5] Venerus, D. \textit{J. Rheol. }\textbf{49}, 277 (2005). [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y24.00010: Simulations of the dynamics of polymer solutions in unidirectional flows Berk Usta, Jason Butler, Tony Ladd We investigate migration and dispersion of polymer chains in unidirectional channel flows by numerical simulation. The algorithm combines the fluctuating lattice-Boltzmann equation with a bead-spring model of a flexible polymer. The method has been shown to be very efficient, capable of simulating polymers in excess of 1000 beads with Oseen level hydrodynamic interactions. The grid-based solution of the fluid equations makes it straightforward to incorporate complex confining boundaries, but here we examine flows in a narrow channel. We observe lateral migration of a single polymer chain in both shear and Poiseuille flows. The direction and extent of migration depend on the degree of confinement as well as the Peclet number, contrary to previous reports. Numerical results show that the longitudinal dispersion deviates from the Taylor dispersion theory at high Peclet numbers, resulting in smaller dispersion coefficients. We will discuss the underlying mechanisms for the reduced dispersion, and also the possibility of improved separation scenarios. [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y24.00011: Heat Capacity of Liquid Poly(vinyl methyl ether) With and Without Water Marek Pyda, B. Wunderlich, K. Van Durme, B. Van Mele The liquid heat capacities at constant pressure Cp, of amorphous poly(vinyl methyl ether), PVME, without and with water have been computed as the sum of vibrational, external, and conformational contributions. The vibrational contribution was calculated from the heat capacity arising from group and skeletal vibrations. The external contribution was estimated from experimental data of the thermal expansivity and compressibility in the liquid state. The conformational heat capacity was evaluated from a fit of experimental part to a one-dimensional Ising model for two discrete states, characterized by parameters linked to stiffness, cooperativity, and degeneracy. For the PVME-water system the additional changes in the conformational heat capacity arising from the interaction of the PVME chains with water. The experimental liquid Cp agrees with these calculations to better than 3{\%}. The calculated liquid Cp was employed in the interpretation of the thermal analysis of the apparent Cp in the region of the melting, demixing, remixing, and crystallization within the PVME-water system. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y24.00012: Thermodynamically Constrained Inverse Monte Carlo Determination of Effective Pair Interactions Henry Ashbaugh, Lu Yang, Shekhar Garde, Sanat Kumar Bridging length scales with molecular simulations requires the development of effective interactions in which degrees of freedom are integrated out to reduce computational expense. Such coarse graining strategies often lose critical thermodynamic information as molecular detail is washed out. Inverse Monte Carlo methods which focus on the reproduction of fluid structure, for example, typically have heats of vaporization and vapor pressures which are significantly greater than experiment. We propose a new method, Thermodynamically Constrained Inverse Monte Carlo (TCIMC), which systematically constrains empirical potential functions to reproduce known energetics and pressures while minimizing the difference between experimental and model pair correlations. We demonstrate the application of TCIMC to the recovery of model pair interactions for the Lennard Jones fluid. This method is subsequently extended to the development of united atom potentials for the alkanes from more complex all atom descriptions. The development of thermodynamically consistent coarse grained potentials for the modeling of polymers is discussed. [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y24.00013: Equivalence of particle and field representation of coarse-grained polymer models Kirill Titievsky, Kenneth Beers Particle-based and field theoretic coarse-grained models of heterogeneous polymer melts have been difficult to compare quantitatively because of the differences in state variables and simulation parameters. We reconcile the two approaches by noting that assumptions involved in discretizing a polymer field theory allow each molecule to be treated as several particles connected by spring bonds. In effect, these particles are the grid points on which the fields are discretized. The interaction potentials of the points, therefore, are obtained directly from the energy model (e.g. Flory) used in the field theory. The equilibrium phase behavior of a melt of such coarse-grained chains is calculated by stochastic dynamics or Monte Carlo simulations. Unlike field theory which becomes infeasible away from the mean field limit due to complex terms in the Hamiltonian, our simulations converge for realistic chain lengths because the potentials are smooth and real. Also, unlike in existing particle-based methods, all parameters in our model are explicitly related to experimentally measurable quantities. Our simulations of diblock copolymers, furthermore, are quantitatively consistent with experimental and field theoretic results. [Preview Abstract] |
Friday, March 17, 2006 10:36AM - 10:48AM |
Y24.00014: Coarse-graining and dynamics of complex macromolecular liquids: melts and blends Marina Guenza Processes of scientific interest in macromolecular liquids can involve more than ten orders of magnitude in space and time variables, impairing our ability of performing atomic-level simulations in the long-time regime. A way to overcome this problem is to resort to multiscale modeling procedures. Here the challenge is to have a formally rigorous, possible analytical, coarse-graining procedure that enables accurate transfer of information between different lengthscales of interest. Starting from the Ornstein-Zernike equation we derived (G.Yatsenko et al. PRL 93, 257803 (2004)) an analytical procedure to coarse-grain structure and dynamics of macromolecular liquids (homo- and diblockco-polymers) and their mixtures. Our procedure maps macromolecules into interacting soft-colloidal particles and provides the effective soft-core potentials input to mesoscale simulations of the coarse-grained systems. Because analytical, our procedure provides a universal formalism easily implemented to treat different systems of interest. Our procedure efficiently extends the range of length- and timescales accessible in simulations of macromolecular liquids. The soft-core mean-force potential enters the Langevin Equation for Cooperative Dynamics (M.Guenza PRL 88, 025901 (2002)), which predicts anomalous subdiffusive center-of-mass motion in excellent agreement with simulations and recent experimental data. [Preview Abstract] |
Friday, March 17, 2006 10:48AM - 11:00AM |
Y24.00015: Kac-Dirac propagators modeling crossover between entangled and unentangled conformations in polymer melts Yitzhak Shnidman We will discuss the advantages of using the Kac stochastic process on a lattice for modeling the statistics of chain conformations in polymers melts. Certain projections of the master equation governing such a process assume the form of the telegrapher's equation in the continuum limit, while other projections assume the form of Dirac's equation for free fermions that can also be recast as the Klein-Gordon equation. The statistics of ideal chain conformations and the onset of entanglement in a dense polymer melt will be related to the finite path statistics and the onset of quantum entanglement in a system of free fermions. A persistence length above the Kuhn length scale emerges naturally in the entangled regime without assuming a confining tube as in the Doi-Edwards theory. Kac-Dirac propagators may also provide a better description than Wiener-Schroedinger propagators for conformations of interacting chains across interfaces in inhomogeneous polymer fluids. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700