Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T45: Transport in Polymer Membranes |
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Sponsoring Units: DPOLY Chair: Rajiv Taribagil, Infineum Room: 216AB |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T45.00001: Three-Step Water Sorption of Thin Nafion Films Daisuke Kawaguchi, Yudai Ogata, Norifumi Yamada, Keiji Tanaka Nafion has been widely used as a proton exchange film in polymer electrolyte fuel cell (PEFC). Although downsizing PEFC is one of the interesting developments in the near future, it appears that most studies conducted so far are limited to bulk systems. Here we examined water sorption behavior in thin Nafion films based on optical and neutron reflectivity measurements. Nafion films were prepared on silver and silicon oxide substrates. It was found that the thicknesses of Nafion thin films increased with time after contacting water in three steps. The asymptotic swelling ratios in regimes I, II and III were 1.05, 1.26 and 1.41, respectively. These values were in-dependent of the substrate species, and were coincident with the transition points of different hydration states in the bulk Nafion; water binding to sulfonic acid groups, the formation of sphere-like ionic clusters, and bridge formation between clusters. The swelling was much slower in thin films than in the bulk due to the mobility restriction of Nafion near the substrate. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T45.00002: Impact of Ageing on Properties of PFSA Ionomers Ahmet Kusoglu, Meron Tesfayr, Shouwen Shi, Will Tong, Adam Weber Perfluorosulfonic-acid (PFSA) ionomers are widely used as the solid-electrolyte in electrochemical energy applications due to their remarkable conductivity and chemical/mechanical stability. However, in these various applications, it is not uncommon for the ionomer to be subjected to various operational environmental stressors that could impact their morphology and properties. In this talk, the impact of hygrothermal ageing on both bulk ionomer and ionomer thin films will be discussed. The intrinsic changes to the morphology and properties will be presented including water uptake and swelling behavior as well as nanostructure using both transmission and grazing-incidence small- and wide-angle x-ray scattering. It will be shown how ageing at intermediate humidities (50 to 70{\%} relative humidity) induces higher mechanical properties and subsequently lower water uptake for both bulk and thin film ionomers due to a loss of accessible ionic sites, which are thought to form crosslinks. Ageing at higher and lower relative humidities shows a decrease in these changes. The ionomers to be studied include both Nafion and the short-side-chain analogue by 3M with higher ion-exchange capacity. Our findings provide new insights into how ageing alters the structure/function relationship of ionomers. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T45.00003: In-situ measurement of swelling induced stress of thin Nafion films during hydration cycles Bradley Frieberg, Kirt Page, Gery Stafford, Christopher Soles Perfluorinated ionomers, in particular Nafion, are a critical component in hydrogen fuel cells, as the binder within the membrane electrode assembly in which it can be confined to thicknesses on the order of ten nanometers. During normal operation of a hydrogen fuel cell the ionomer will progressively swell and de-swell in response to the changes in hydration, resulting in a mechanical fatigue and ultimately failure of the fuel cell with time. In this study we have developed and implemented a cantilever bending technique in order to investigate the swelling induced stresses in Nafion thin films. By monitoring the deflection of a cantilever beam coated with a polymer film as it is exposed to varying humidity environments, the swelling induced stress-thickness of the polymer film can be measured. By combining the stress-thickness results with a measurement of the film thickness as a function of humidity, the swelling stress can be determined. Using this technique we have measured the shear modulus and estimated the Young's modulus of thin Nafion films as a function of film thickness (ranging from 30 nm to 200 nm), processing conditions and humidity. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T45.00004: Elucidating the Role of Confinement on Structure and Water Transport in Nafion Thin Films Eric Davis, Nichole Nadermann, Edwin Chan, Christopher Stafford, Kirt Page Perfluorinated ionomers, specifically Nafion, are the most widely used polymer membranes for fuel cell applications. For these devices, Nafion is utilized in both a bulk (hundreds of microns) and confined (tens of nanometers) state. Therefore, a more complete understanding of the structure-processing-property relationships of these thin ionomer films, compared to the bulk membranes, is needed. In this study, water transport and structure of a series of Nafion thin film thicknesses were measured using a variety of experimental techniques. Water diffusion was measured via time-resolved in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and poroelastic relaxation indentation (PRI). The former probes through-thickness diffusion (direction of confinement), while the latter probes in-plane diffusion in the Nafion thin films. Suppressed water diffusion relative to the bulk was observed in both the through-thickness and in-plane directions of the thin film Nafion. Additionally, the structure of hydrated Nafion thin films was captured using small-angle neutron scattering (SANS). These results suggest that the nanostructure of the Nafion changes as the thickness of the film is decreased, resulting in a decrease in the effective water diffusivity of these thin ionomer films compared to the bulk membranes. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T45.00005: Kinetics of swelling enhancement of polyelectrolyte brushes Xiao Chu, Jingfa Yang, Guangming Liu, Jiang Zhao The swelling of polystyrene sulfonate brushes is enhanced due to the break-up of the multiplets formed by dipole-dipole interactions inside, when external salt concentration is increased to moderate value. By quartz crystal balance with dissipation, the kinetics of this process is investigated and a systematic study on the effect of grafting density and salt concentration is conducted. The results demonstrate the effect of spatial hindrance due to uneven distribution of segments and osmotic pressure of the external ions. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T45.00006: Nano Aggregation of Structured Ionic Copolymers: Molecular Dynamics Simulation Study Dipak Aryal, Dvora Perahia, Gary S. Grest Driven by mutual segregation, block copolymers exhibit a fascinating ability to self-assemble into a variety of ordered mesoscopic structures. Incorporating an ionizable block enhances incompatibility that together with tailoring blocks for specific functions presents an immense step towards engineering controlled transport systems. Here the interplay between the interactions of solvents with the specific blocks of a pentablock with a randomly sulfonated polystyrene center, tailored for transport, tethered to flexible poly (ethylene-r-propylene) end-capped with poly (t-butyl styrene) is studied by fully atomistic molecular dynamics simulations. The assembly of 2 to 30 macromolecules, in water results in a spherical tightly packed aggregate in which the ionizable blocks dominates the water interface. Transferred to a cyclohexane-heptane mixture, the hydrophobic blocks migrate to the interface. Surprisingly however, the ionic blocks form a nano-network rather than a corona. Further, this network also develops when assembled from hydrophobic solvents, where now the hydrophobic blocks dominate the interface. This network only slightly contracts or expands as the solvent is changed while concurrently the hydrophobic blocks migrate towards or a way from the solvent interface. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T45.00007: Cluster Morphology in Lightly Sulfonated Polystyrene Anupriya Agrawal, Dvora Perahia, Gary S. Grest Aggregation of ionic groups into clusters in ionomers renders their unique properties that drive the use in energy applications. The ionic clusters however have a dramatic impact on the rheology of this polymer. Even small fractions of ionic groups (less than 5{\%}), constrain their dynamics, making them difficult to process. Using molecular dynamics simulations, we show that in lightly sulfonated polystyrene melts, the ionic groups aggregate into ladder type clusters. These ladder morphologies prevail for a broad temperature range and degree of sulfonation, although with some variation due to steric effects of the chain. Reducing the electrostatic strength by tuning the dielectric constant changes the cluster morphology from ladder to spherical, which in turn, greatly increases the diffusion of the polymer. The effect of changing the electrostatic strength is comparable to the effect of addition of diluents to this melts. Enhancing the electrostatic screening results in breaking the clusters, whereas increasing temperature within the measured range, results in faster diffusion of the entire polymers while the clusters remain intact. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T45.00008: Salt uptake and dynamics in thin, highly crosslinked polyamide membranes Kathleen Feldman, Edwin Chan, Christopher Stafford Water purification membranes have historically been developed through an Edisonian, trial and error approach. It has been challenging to establish fundamental structure-property-performance relationships, including salt$\backslash$water permselectivity, in large part due to the typically rough and poorly-defined nature of the polyamide membrane active layer. By using molecular layer by layer (mLbL) deposition, we are able to produce thin, well-defined polyamide films with controlled chemistry analogous to commercial reverse osmosis and nanofiltration membranes. While water permeation in polyamide membranes is reasonably straightforward to measure, salt permeation is more challenging, particularly in ultrathin films. We measure both equilibrium salt uptake and dynamic behavior in nanofiltration-type membranes using x-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and quartz crystal microbalance (QCM). By connecting intrinsic material properties such as the salt permeability and permselectivity with macroscopic performance measures we can begin to establish design rules for improving membrane efficiency while lowering energy requirements. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T45.00009: Fabrication of ultrafiltration membranes using dynamic thermal annealing of Block Copolymer films Alamgir Karim, Yan Luo, Yan Sun Block copolymer (BCP) thin films have attracted immense attention for fabrication of ultrafiltration membranes due to their potential to provide dense nanostructured pores giving high flux and good efficiency. We have demonstrated generation of well-ordered hexagonally packed perpendicular cylindrical BCP films with greater than 90 percent of perpendicular cylinders over large areas with high cylinder density and desired porosity at the nanoscale via facile dynamic thermal annealing. The films are then transferred to a support membrane and subsequently the vertically oriented minority block is selectively etched to form a nanoporous membrane. The porosity of film and pore density is tuned by addition of selective homopolymer block component. The structure of nanoporous BCP membranes is characterized by Atomic Force Microscopy(AFM), Grazing Incidence Small Angle X-ray Scattering and Transmission Electron Microscopy(TEM). We further demonstrate the application of these membranes for separation of emulsions and correlate flux and efficiency to parameters such as porosity, membrane thickness and tortuosity. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T45.00010: Super stretchy polymer multilayer thin films with tunable gas barrier Fangming Xiang, Sarah Ward, Tara Givens, Jaime Grunlan Super stretchy multilayer thin film assemblies with tunable gas barrier were fabricated using layer-by-layer (LbL) assembly. Unlike ionically-bonded gas barrier coatings that exhibit mud-cracking after 10{\%} strain, hydrogen-bonded polyethylene oxide (PEO) and polyacrylic acid (PAA) multilayer thin films show no cracking after 100{\%} strain due to low modulus. It is believed that the exceptional elasticity of this thin film originates from the intrinsic elasticity of PEO and the moderate hydrogen bond strength between PEO and PAA. The oxygen transmission rate (OTR) of a 1.58 mm thick natural rubber sheet can be reduced 10 times with a 367-nm-thick PAA/PEO nanocoating. This gas barrier improvement is largely retained after 100{\%} strain. The modulus and oxygen permeability of PAA/PEO assembly can be tailored through altering the assembling pH. By setting the assembling pH to 2.75, a 50{\%} reduction in permeability can be achieved, while maintaining the elasticity of the assembly. These findings mark the first super stretchy gas barrier thin film, which is useful for elastomeric substrates designed to hold air pressure. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T45.00011: A molecular study of gas solubility in nitrile rubber Musab Khawaja, Arash Mostofi, Adrian Sutton One of the most important uses of elastomers in the oil industry is for seals to encase and protect sensitive monitoring equipment from contamination by gases and liquids at the high pressures and temperatures in the well. Failure of such seals sometimes occurs on decompression when they are returned to the surface. The conditions in the well lead to gases being absorbed by Nitrile rubber (NBR) seals. NBR exhibits a strong permselectivity towards CO2 compared to other gases; something attributed experimentally to the enhanced solubility of CO2. In this study an explanation is sought at the molecular level for this phenomenon. A series of molecular mechanics calculations are performed to compute solubilities of different gases in NBR. The effect of acrylonitrile content on their solubilities is studied for the first time by simulation, and we discuss the important issue of convergence with respect to the sampling of different elastomer configurations. It is observed that the presence of cyano groups has a marked impact on the solubility of CO2 and an explanation is offered. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T45.00012: Liquid Crystalline Block Copolymers with Brush Type Architecture: Toward Functional Membranes by Magnetic Field Alignment Youngwoo Choo, Manesh Gopinadhan, Lalit Mahajan, Rajeswari Kasi, Chinedum Osuji We introduce a novel liquid crystalline block copolymer with brush type architecture for membrane applications by magnetic field directed self-assembly. Ring-opening metathesis of n-alkyloxy cyanobiphenyl and polylactide (PLA) functionalized norbornene monomers provides efficient polymerization yielding low polydispersity block copolymers. The molecular weight of the PLA side chains, spacer length of the cyanobiphenyl mesogens are systematically varied to form well-ordered BCP morphologies at varying volume fractions. Interestingly, the system features morphology dependent anchoring condition where mesogens adopt planar anchoring on cylindrical interface while homeotropic anchoring was preferred on a planar block interface. The minority PLA domains from highly aligned materials can be readily degraded by hydrolysis to produce vertically aligned nanoporous polymer films which exhibit reversible thermal switching behavior. The polymers introduced here provide a versatile platform for scalable fabrication of aligned membranes and further functional materials based on such templates. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T45.00013: Bubble Growth and Dynamics in a Strongly Superheated Electrolyte within a Solid-State Nanopore Edlyn Levine, Gaku Nagashima, Michael Burns, Jene Golovchenko Extreme localized superheating and homogeneous vapor bubble nucleation have recently been demonstrated in a single nanopore in thin, solid state membranes. Aqueous electrolytic solution within the pore is superheated to well above its boiling point by Joule heating from ionic current driven through the pore. Continued heating of the metastable liquid leads to nucleation of a vapor bubble in the pore followed by explosive growth. Here we report on the growth dynamics of the vapor bubble after nucleation in the strongly superheated liquid. The process is modeled by numerically solving the Rayleigh-Plesset equation coupled with energy conservation and a Stefan boundary condition. The initial temperature distribution, peaked at the pore center, is taken to be radially symmetric. Energy conservation includes a Joule heating source term dependent on the bubble radius, which grows to constrict ionic current through the nanopore. Temperature-dependent properties of the electrolyte and the vapor are incorporated in the calculation. Comparison of the model to experimental results shows an initial bubble growth velocity of 50m/s and total bubble lifetime of 16ns. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T45.00014: Self-assembly of Spherical Macroions in Solution: A Coarse-grained Molecular Dynamics Study Zhuonan Liu, Tianbo Liu, Mesfin Tsige Macroions (such as polyoxometalates) in solution can form a stable hollow spherical super-molecular structure called blackberry when they have moderate surface charge density and size (1-10 nm). Depending on the surface charge density of macroions, the size of the blackberry can be from 20 to more than 100 nm. Other macroions such as dendrimers can also self-assemble into similar super-molecular structure in solution. Existing theories such as Debye-H\"{u}ckel and DLVO theories cannot explain this phenomenon and we are not aware of any other theory that can explain this. Previous studies using all-atom Molecular Dynamics simulations have shown identical macroions forming oligomers mediated by counterions. Due to the limitations in all-atom simulation and available computational capabilities, these studies handled only small systems with simple macroions, leading to less conclusive but still relevant results on the self-assembly behavior. To overcome these limitations, in this work large-scale coarse-grained modeling of macroions in solution is used. In order to understand the origin of the attractive force that is responsible for the self-assembly of macroions, different types of macroions in different solution conditions are studied. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T45.00015: Effects of polymer hydrophobicity on the diffusivity of water and ethanol in acrylate copolymer gels Fardin Khabaz, Sriramvignesh Mani, Rajesh Khare Pervaporation is an energy efficient process for separating dilute alcohol-water mixtures. The efficiency of the pervaporation process is governed by the solubility and diffusivity of the water and alcohol molecules in the polymer. Molecular simulations can be used to provide detailed insights on the dependence of the diffusivity on the molecular structure of the polymer. Polyacrylate systems with varying degree of hydrophobicity are built by changing the relative concentrations of butyl acrylate and 2-hydroxy ethyl acrylate monomers which are hydrophobic and hydrophilic, respectively. In order to create the membrane structure, a random copolymer of these monomers that is cross-linked with pentaerythritol tetracrylate, is obtained using the simulated annealing polymerization technique. The volumetric properties of the systems such as density and glass transition temperature (T$_{\mathrm{g}})$, are compared with the experimental values to validate the model structures. The diffusivity of the water and ethanol molecules inside the membrane is characterized by determining their mean squared displacement (MSD) in systems with varying degree of hydrophobicity and cross-linker concentration. The calculated diffusion coefficients of water and ethanol from simulations will be compared with available experimental diffusion data. The correlation between the diffusivities and the degree of hydrophobicity as well as the molecular packing in these systems will be identified. [Preview Abstract] |
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