Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L14: Invited Session: Understanding Ion Containing Polymer Systems using Computer Simulations |
Hide Abstracts |
Sponsoring Units: DPOLY DCOMP Chair: Gary Grest, Sandia National Laboratories Room: 301-303 |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L14.00001: Complexation of Oppositely Charged Polyelctrolytes and Diblock Polyampholytes Invited Speaker: Michael Rubinstein The conformational properties of both symmetric and asymmetric flexible diblock polyampholytes and oppositely charged polyelectrolytes are investigated by molecular dynamics simulations and scaling theory. The electrostatically driven coil-globule transition of a symmetric diblock polyampholyte consist of three regimes identified with increasing electrostatic interaction strength: the folding regime, the weak association regime dominated by the fluctuation-induced attractions between oppositely charged sections of the chains, and the ion binding regime that starts with direct binding of oppositely charged monomers (dipole formation), followed by a cascade of multipole formation leading to multiplets analogous to those found in ionomers. In asymmetric block polyampholytes we find the globule to tadpole transition with the increase of charge asymmetry. In the weak association regime this transition is controlled by the balance of net charge and surface tension of the complex and characterized by the ratio of the difference in the number of electrostatic ``blobs'' between oppositely charged blocks and one third power of the total number of electrostatic blobs. We find the maximum overcharging of the complexes formed by either asymmetric diblock polyampholytes or by pairs of oppositely charged polyelectrolytes is by 50{\%} independent of system parameters. We use scaling theory to estimate the average size of the complex and the electrostatic correlation length as functions of chains length, strength of electrostatic interactions, charge fractions, and solvent quality. The theoretically predicted scaling laws of these conformational properties are in good agreement with our simulation results. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L14.00002: Where Scattering and Computations Meet: Structure and Dynamics of Ionic Co-Polymers Invited Speaker: Dvora Perahia Ion transporting polymers constitute vital components in clean energy generation and storage devices including electrolytic media in fuel cells and ion conducting separators in batteries. While different polymers are currently in use, achieving controlled ion transport and storage ability while retaining mechanical and chemical stability remains a challenge: under the conditions which optimize the transport and storage for specific application, either mechanical or chemical stabilities are compromised. Designing block-co-polymers with ion transporting blocks bound to blocks that enhance mechanical and chemical stability would mitigate the challenge. Tailoring block copolymers with blocks that exhibit various desired properties, results in a new set of open questions that pertain to new complex materials including defining the phase diagram and understanding the interfacial regions of the muliblocks. Here we present the first molecular-level computational insight of the behavior of a pentablock, A-B-C-B-A, co-polymer that consists of an A block of poly(t-butyl-styrene), a B block of ethylene-r-propylene and a C block of a randomly sulfonated styrene, in solution in comparison with neutron scattering data. Neutron studies have shown that in hydrophobic solvents this pentablock forms elongated micelles in dilute solutions where the ionic block dominates the solution structure. These studies provide ensemble average of structure and properties. The computational studies provided further molecular-level insight. Here we will discuss the interrelations between scattering results and computational studies to provide remarkable understanding of a complex ionic system. Pathways to advance this molecular understanding to an actual membrane will be then discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L14.00003: Atomistic Simulations of Aggregation in Ionomer Melts Invited Speaker: Amalie L. Frischknecht Ionomers, polymers containing a small fraction of covalently bound ionic groups, are of interest as possible electrolytes in batteries. A single-ion conducting polymer electrolyte would be safer and have higher efficiency than currently-used liquid electrolytes. However, to date ionomers do not have sufficiently high conductivities for practical application, most likely because the ions tend to form aggregates, leading to slow ion transport. An understanding of the relationships between ionomer chemistry, morphology, and ion transport is needed to design ionomers with improved conductivity. To provide insight into the ionic aggregate morphology, we have performed molecular dynamics simulations of a series of polyethylene-based model ionomer melts, in which the spacing between functional groups is precisely controlled. We vary the counterion type, the neutralization level, and the length of the spacer. The simulations provide new insights into the shape, size and composition of ionic aggregates. In particular, we observe a wide variety of aggregate morphologies, ranging from small spherical aggregates to string-like shapes and large percolated networks. The structure factors calculated from simulation agree well with X-ray scattering data. Depending on the morphology, the simulation and experimental scattering curves can be well fit with either a modified hard sphere or a modified hard cylinder model. These fits, combined with the simulation data, provide the first (indirect) experimental evidence of string-like aggregate morphologies in ionomer melts. We speculate that stringy, percolated aggregates may enhance ionic conduction. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L14.00004: Molecular Dynamics Simulations of Polyelectrolyte Solutions Invited Speaker: Andrey Dobrynin Polyelectrolytes are polymers with ionizable groups. In polar solvents, these groups dissociate releasing counterions into solution and leaving uncompensated charges on the polymer backbone. Examples of polyelectrolytes include biopolymers such as DNA and RNA, and synthetic polymers such as poly(styrene sulfonate) and poly(acrylic acids). In this talk I will discuss recent molecular dynamics simulations of static and dynamic properties of polyelectrolyte solutions. These simulations show that in dilute and semidilute polyelectrolyte solutions the electrostatic induced chain persistence length scales with the solution ionic strength as $I^{\mathrm{-1/2}}$. This dependence of the chain persistence length is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing the salt concentration as \textit{R $\sim$ I}$^{-1/5}$. This is in agreement with the scaling of the chain persistence length on the solution ionic strength, $l_{p}$\textit{ $\sim$ I}$^{-1/2}$. In semidilute solution regime at low salt concentrations the chain size decreases with increasing polymer concentration, \textit{R $\sim$ c}$_{p}^{-1/4}$. While at high salt concentrations one observes a weaker dependence of the chain size on the solution ionic strength, \textit{R $\sim$ I}$^{-1/8}$. Analysis of the simulation data throughout the studied salt and polymer concentration ranges shows that there exist general scaling relations between multiple quantities $X(I)$ in salt solutions and corresponding quantities $X(I_{0})$ in salt-free solutions, $X(I)=X(I_{0})(I/I_{0})^{\beta }$. The exponent $\beta =$ -1/2 for chain persistence length $l_{p}, \beta =$ 1/4 for solution correlation length, $\beta =$ -1/5 and $\beta =$ -1/8 for chain size $R$ in dilute and semidilute solution regimes respectively. Furthermore, the analysis of the spectrum and of the relaxation times of Rouse modes confirms existence of the single length scale (correlation length) that controls both static and dynamic properties of semidilute polyelectrolyte solutions. These findings confirm predictions of the scaling model of polyelectrolyte solutions. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L14.00005: Effective interactions and aggregation of rodlike polyelectrolytes Invited Speaker: Erik Luijten Rodlike polyelectrolytes are known to exhibit various aggregation phenomena, assembling into structures that range from bundles to rafts. Such self-assembly is important in numerous biological and synthetic applications. Here, I provide an overview of various important aspects of such phenomena. In particular, I will highlight computational work on the free-energy landscape of various rod configurations. Furthermore, the role of many-body effects will be discussed. First, ionic excluded-volume effects lead to correlations, which can become particularly important in the dense environment within an aggregate. Second, induced polarization charges arise from the dielectric mismatch between the polyelectrolyte and the surrounding solvent. The latter are rarely taken into account, owing to the computational complexity of solving the bound charges, but can significantly alter the electrostatic interactions that are responsible for aggregation in the first place. New, efficient techniques now make it possible to incorporate these effects in standard molecular dynamics or Monte Carlo simulations. Using these techniques, I examine the role of both types of many-body effects on bundle configuration and stability. [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