Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R42: Small Molecule Transport in Polymers and Polymer Nanocomposites IIFocus
|
Hide Abstracts |
Sponsoring Units: DPOLY Chair: William Phillip, Unviersity of Notre Dame Room: 345 |
Thursday, March 17, 2016 8:00AM - 8:12AM |
R42.00001: Statistical Mechanical Theory of Penetrant Diffusion in Polymer Melts and Glasses Rui Zhang, Kenneth Schweizer We generalize our force-level, self-consistent nonlinear Langevin equation theory of activated diffusion of a dilute spherical penetrant in hard sphere fluids [1] to predict the long-time diffusivity of molecular penetrants in supercooled polymer liquids and non-aging glasses. Chemical complexity is treated using an a priori mapping to a temperature-dependent hard sphere mixture model where polymers are disconnected into effective spheres based on the Kuhn length as the relevant coarse graining scale. A key parameter for mobility is the penetrant to polymer segment diameter ratio, R. Our calculations agree well with experimental measurements for a wide range of temperatures, penetrant sizes (from gas molecules with R$\sim $0.3 to aromatic molecules with R$\sim $1) and diverse amorphous polymers, over 10 decades variation of penetrant diffusivity. Structural parameter transferability is good. We have also formulated a theory at finite penetrant loading for the coupled penetrant-polymer dynamics in chemically (nearly) matched mixtures (e.g., toluene-polystyrene) which captures well the increase of penetrant diffusivity and decrease of polymer matrix vitrification temperature with increasing loading. [1] R. Zhang and K. S. Schweizer, J. Chem. Phys., 143, 144906 (2015). [Preview Abstract] |
Thursday, March 17, 2016 8:12AM - 8:24AM |
R42.00002: Molecular Dynamics Simulations of Penetrants in Microphase Separated Tapered Diblock Copolymers Youngmi Seo, Jonathan R. Brown, Lisa M. Hall Tapered AB diblock copolymers contain pure A and B monomer blocks on the ends with a tapered midblock of intermediate composition, providing taper length as an additional tuning parameter to control microphase separation and interfacial behavior. We model the midblock as a statistical linear gradient from pure A to pure B. Recent experiments with salt dissolved in one of microphases show that a certain length of taper increases ion conductivity while the same length of inverse taper lowers conductivity. We perform coarse-grained molecular dynamics simulations of tapered copolymers with monomer sized penetrants, which have preferential interactions with one microphase, to better understand this observation and the general effects of tapering on dynamics. We calculate penetrant diffusion, polymer relaxation times, and other quantities over the range from 0{\%} (diblock) to 100{\%} (full gradient) taper length, with the taper direction either normal or inverse (with the A side of the taper connected to the pure B block). Normal taper results typically lie between those of diblocks and full gradients, while inverse tapers show strong nonmonotonic behavior as a function of taper length. For intermediate length inverse tapers, penetrant and monomer dynamics are significantly slower than those of diblocks or normal tapers, and this relates to the folding of the inverse chains back and forth across the interface. To provide further insight, we also compare to the dynamics of random copolymers of various compositions. [Preview Abstract] |
Thursday, March 17, 2016 8:24AM - 8:36AM |
R42.00003: Mechanism of Concentration Dependence of Water Diffusivity in Polyacrylate Gels. Sriramvignesh Mani, Fardin Khabaz, Rajesh Khare Membrane based separation processes offer an energy efficient alternative to traditional distillation based separation processes. In this work, we focus on the molecular mechanisms underlying the process of separation of dilute ethanol-water mixture using polyacrylate gels as pervaporation membranes. The diffusivities of the components in swollen gels exhibit concentration dependence. We have used molecular dynamics (MD) simulations to study the correlation between the dynamics of solvent (water and ethanol) molecules, polymer dynamics and solvent structure in the swollen gel systems as a function of solvent concentration. Three different polyacrylate gels were studied: (1) poly n-butyl acrylate (PBA), (2) copolymer of butyl acrylate and 2-hydroxyethyl acrylate P(BA50-HEA50), and (3) poly 2-hydroxyethyl acrylate (PHEA). Simulation results show that solvent concentration has a significant effect on local structure of the solvent molecules and chain dynamics; these factors (local structure and chain dynamics), in turn, affect the diffusivity of these molecules. At low concentration, solvent molecules are well dispersed in the gel matrix and form hydrogen bonds with the polymer. Solvent mobility is correlated with polymer mobility in this configuration and consequently water and ethanol molecules exhibit slower dynamics, this effect is especially significant in PHEA gel. At high solvent concentration, water molecules form large clusters in the system accompanied by enhancement in mobility of both the gel network and the solvent molecules. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 9:12AM |
R42.00004: Structure/property relationships in polymer membranes for water purification and energy applications Invited Speaker: Geoffrey Geise Providing sustainable supplies of purified water and energy is a critical global challenge for the future, and polymer membranes will play a key role in addressing these clear and pressing global needs for water and energy. Polymer membrane-based processes dominate the desalination market, and polymer membranes are crucial components in several rapidly developing power generation and storage applications that rely on membranes to control rates of water and/or ion transport. Much remains unknown about the influence of polymer structure on intrinsic water and ion transport properties, and these relationships must be developed to design next generation polymer membrane materials. For desalination applications, polymers with simultaneously high water permeability and low salt permeability are desirable in order to prepare selective membranes that can efficiently desalinate water, and a tradeoff relationship between water/salt selectivity and water permeability suggests that attempts to prepare such materials should rely on approaches that do more than simply vary polymer free volume. One strategy is to functionalize hydrocarbon polymers with fixed charge groups that can ionize upon exposure to water, and the presence of charged groups in the polymer influences transport properties. Additionally, in many emerging energy applications, charged polymers are exposed to ions that are very different from sodium and chloride. Specific ion effects have been observed in charged polymers, and these effects must be understood to prepare charged polymers that will enable emerging energy technologies. This presentation discusses research aimed at further understanding fundamental structure/property relationships that govern water and ion transport in charged polymer films considered for desalination and electric potential field-driven applications that can help address global needs for clean water and energy. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R42.00005: Ion transferring in polyelectrolyte networks in electric fields. Honghao Li, Aykut Erbas, Jos Zwanikken, Monica Olvera de la Cruz Ion-conducting polyelectrolyte gels have drawn the attention of many researchers in the last few decades as they have wide applications not only in lithium batteries but also as stretchable, transparent ionic conductor or ionic cables devices. However, ion dynamics in polyelectrolyte gels has been much less studied analytically or computationally due to the complicated interplay of long-range electrostatic and short-range interactions. Here we propose a coarse-grained non-equilibrium molecular dynamics simulation to study the ion dynamics in polyelectrolyte gels under external electric fields. We found a nonlinear response region where the molar conductivity of polyelectrolyte gels increases with external fields. We propose counterion redistribution under electric fields as the driving mechanism. We also found the ionic conductivity to be modulated by changing polylelectrolyte network topology such as the chain length. Our discovery reveals the essential difference of ion dynamics between electrolytes and polyelectrolyte gels. These results will expand our understanding in charged polymeric systems and help in designing ion-conducting devices with higher conductivity. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R42.00006: Thermodynamics of Ionic Transport through Functionalized Membranes Vikramjit Rathee, Siyi Qu, Theodore Dilenschneider, William A. Phillip, Jonathan K. Whitmer Through microphase separation of block copolymers, highly porous solid membranes may be assembled. Further functionalization with amine and sulfonic acid groups has demonstrated promise in exquisitely controlling the flux of charged species, and in particular multivalent ions. Using coarse-grained molecular simulations, we explore the essential thermodynamics underlying salt rejection in charge-functionalized membranes, and develop a model capable of linking the performance of these membranes to their molecular character through free energy calculations. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R42.00007: Theoretical model of Case-II diffusion based on molecular-dynamics study of methanol in PMMA Jiayuan Miao, Mesfin Tsige, Philip Taylor In Case-II diffusion, a sharp diffusion front moves at a nearly constant speed. An obstacle to the theoretical prediction of the form of this front lies in the large mismatch between the time scale of atomic motion, which is measured in femtoseconds, and the time scale of diffusion in a macroscopic sample, which is measured in millions of seconds. We attempt to overcome this limitation by using the short-time results of atomistic molecular-dynamics simulations to construct a stochastic model valid over all time scales. The ability of this model to yield Case-II diffusion behavior was confirmed, and it was then developed into a continuum mathematical model in which the diffusivity D has a strong dependence on the concentration of the penetrant. We anticipate that solution of the appropriate non-linear diffusion equation will yield an accurate portrayal of the characteristics of the diffusion process. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R42.00008: Multicomponent Diffusion of Penetrant Mixtures in Rubbery Polymers: A Molecular Dynamics Study Stefan Bringuier, Mark Varady, Craig Knox, Jerry Cabalo, Thomas Pearl, Brent Mantooth The importance of understanding transport of chemical species across liquid-solid boundaries is of particular interest in the decontamination of harmful chemicals absorbed within polymeric materials. To characterize processes associated with liquid-phase extraction of absorbed species from polymers, it is necessary to determine an appropriate physical description of species transport in multicomponent systems. The Maxwell-Stefan (M-S) formulation is a rigorous description of mass transport in multicomponent solutions, in which, mutual diffusivities determine the degree of relative motion between interacting molecules in response to a chemical potential gradient. The work presented focuses on the determination of M-S diffusivities from molecular dynamics (MD) simulations of nerve agent O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate (VX), water, and methanol mixtures within a poly(dimethylsiloxane) matrix. We investigate the composition dependence of M-S diffusivities and compare the results to values predicted using empirical relations for binary and ternary mixtures. Finally, we highlight the pertinent differences in molecular mechanisms associated with species transport and employ non-equilibrium MD to probe transport across the mixture-polymer interface. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R42.00009: ABSTRACT WITHDRAWN |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R42.00010: Predicting the solubility of gases in Nitrile Butadiene Rubber in extreme conditions using molecular simulation Musab Khawaja, Nicola Molinari, Adrian Sutton, Arash Mostofi In the oil and gas industry, elastomer seals play an important role in protecting sensitive monitoring equipment from contamination by gases - a problem that is exacerbated by the high pressures and temperatures found down-hole. The ability to predict and prevent such permeative failure has proved elusive to-date. Nitrile butadiene rubber (NBR) is a common choice of elastomer for seals due to its resistance to heat and fuels. In the conditions found in the well it readily absorbs small molecular weight gases. How this behaviour changes quantitatively for different gases as a function of temperature and pressure is not well-understood. In this work a series of fully atomistic simulations are performed to understand the effect of extreme conditions on gas solubility in NBR. Widom particle insertion is used to compute solubilities. The importance of sampling and allowing structural relaxation upon compression are highlighted, and qualitatively reasonable trends reproduced. Finally, while at STP it has previously been shown that the solubility of CO2 is higher than that of He in NBR, we observe that under the right circumstances it is possible to reverse this trend. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R42.00011: Analysis of surface segregation in polymer mixtures: A combination of mean f\/ield and statistical associated f\/luid theories Jaroslaw Krawczyk, Salvatore Croce, Buddhapriya Chakrabarti, Jos Tasche The surface segregation in polymer mixtures remains a challenging problem for both academic exploration as well as industrial applications. Despite its ubiquity and several theoretical attempts a good agreement between computed and experimentally observed profiles has not yet been achieved. A simple theoretical model proposed in this context by Schmidt and Binder combines Flory-Huggins free energy of mixing with the square gradient theory of wetting of a wall by f\/luid. While the theory gives us a qualitative understanding of the surface induced segregation and the surface enrichment it lacks the quantitative comparison with the experiment. The statistical associating f\/luid theory (SAFT) allows us to calculate accurate free energy for a real polymeric materials. In an earlier work we had shown that increasing the bulk modulus of a polymer matrix through which small molecules migrate to the free surface causes reduction in the surface migrant fraction using Schmidt-Binder and self-consistent field theories. In this work we validate this idea by combining mean field theories and SAFT to identify parameter ranges where such an effect should be observable. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R42.00012: Continuum Model for Decontamination of Chemical Warfare Agent from a Rubbery Polymer using the Maxwell-Stefan Formulation Mark Varady, Stefan Bringuier, Thomas Pearl, Shawn Stevenson, Brent Mantooth Decontamination of polymers exposed to chemical warfare agents (CWA) often proceeds by application of a liquid solution. Absorption of some decontaminant components proceed concurrently with extraction of the CWA, resulting in multicomponent diffusion in the polymer. In this work, the Maxwell-Stefan equations were used with the Flory-Huggins model of species activity to mathematically describe the transport of two species within a polymer. This model was used to predict the extraction of the nerve agent O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate (VX) from a silicone elastomer into both water and methanol. Comparisons with experimental results show good agreement with minimal fitting of model parameters from pure component uptake data. Reaction of the extracted VX with sodium hydroxide in the liquid-phase was also modeled and used to predict the overall rate of destruction of VX. Although the reaction proceeds more slowly in the methanol-based solution compared to the aqueous solution, the extraction rate is faster due to increasing VX mobility as methanol absorbs into the silicone, resulting in an overall faster rate of VX destruction. [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