Structured Ionomer Thin Films at Water Interface - ACS Publications

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Structured Ionomer Thin Films at Water Interface: Molecular Dynamics Simulation Insight Dipak Aryal, Anupriya Agrawal, Dvora Perahia, and Gary S. Grest Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02485 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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Structured Ionomer Thin Films at Water Interface: Molecular Dynamics Simulation Insight Dipak Aryal,1† Anupriya Agrawal,1‡ Dvora Perahia,1#* and Gary S. Grest 2+* 1

Department of Chemistry, Clemson University, Clemson, South Carolina 29634, USA

2

Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

Abstract Controlling the structure and dynamics of thin films of ionizable polymers at water interfaces is critical to their many applications. As the chemical diversity within one polymer is increased, controlling the structure and dynamics of the polymer, which is a key to their use, becomes a challenge. Here molecular dynamics simulations (MD), is used to obtain molecular insight into the structure and dynamics of thin films of one such macromolecule, at the interface with water. The polymer consists of an ABCBA topology with randomly sulfonated polystyrene (C), tethered symmetrically to flexible poly(ethylene-r-propylene) blocks (B), and end capped by a poly (t-butyl styrene) block (A). The compositions of the interfacial and bulk regions of thin films of the ABCBA polymers are followed as a function of exposure time to water. We find that interfacial rearrangements take place where buried ionic segments migrate towards the water interface. The hydrophobic blocks collapse and rearrange to minimize their exposure to water. The water that initially drives interfacial reengagements breaks the ionic clusters within the film forming a dynamic hydrophilic internal network within the hydrophobic segments.

1. Introduction Enhancing molecular complexity of polymers by tailoring function-enabling blocks such as ionizable segments presents an immense step towards engineering soft materials.1-4 The

ACS Paragon Plus Environment

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structure of co-polymers strongly depends on the dimensions of each of the blocks, their mutual affinity and the topology of the macromolecule. van der Waals co-polymers for a rich variety of long range ordered structures including lamellar, cylindrical, and cubic phases.5 The presence of ionizable groups however drives clustering and often become the dominating force in determining both the structure and dynamics of the co-polymers6-10 In parallel, introducing ionizable groups results in unique interfacial behavior which is critical to the current and potential uses of these polymers. Examples include biotechnology and electrochemical applications, where the polymers often reside at the interface with solids and liquids.11-15 Increasing the chemical diversity of polymers to incorporate functionality, results in a coupled set of interactions that affects the structure and dynamics of the macromolecules in bulk and at interfaces.

Ongoing studies of structured ionic polymers have made big strides towards

controlling the structure and dynamics in the bulk. The interfacial structure and dynamics of these polymers however often evolve with time and exposure to their environment. As a result, understanding the interfacial regime, particularity on a molecular level, remains a challenge. Here, using large scale atomistic molecular dynamics (MD) simulations we can obtain a detailed evolution of the interfacial regions of ionizable co-polymers in contact with water. In most of their applications, ionomers reside at interfaces with either solid supports such as electrodes and are often in contact with guest molecules with water being the most prevalent substance at the core of the numerous applications of these macromolecules. Hence understanding the changes at the interfacial regions in these types of polymers becomes critical. Similar to van der Waals polymers, the interfacial behavior of copolymers containing ionizable segments is a delicate balance of the block compatibility, affinity of each block to the interface, all coupled with entropic considerations. Incorporating ionizable segments introduces


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stronger long range interactions that affects this balance and impacts the interfacial regions. Further complexity is also introduced when films of these polymers are exposed to solvents, in particular water, either as an integral part of the function of the polymer or as part of the environment. Being polar, water molecules associate with the ionic aggregates, modulating their assemblies, thus affecting one of the fundamental domains that determine the properties of ionomers.16-19 Understanding the interfacial regions requires control over the chemistry of the blocks and their topology. The current study focuses on one macromolecule, a symmetric linear copolymers that consist of ionizable blocks in the center tethered to other hydrophobic blocks in the presence of water using atomistic MD simulations. Large scale MD studies enable capturing the molecular insight into the dynamic-interfacial charateristics of structured polymers at water interfaces. We were able to resolve the nature of rearrangements of hydrophobic and hydrophilic segments at the polymer-water interfaces. One such family of multifunctional copolymers is a pentablock of the form ABCBA. This linear polymer consists of an ionizable block of randomly sulfonated atactic polystyrene (PSS) (C) in the center tethered symmetrically to hydrophobic blocks of polyethylene-r-ethylene (PErP) (B) that allow control over tactility of the polymer, and terminated by a bulky atactic tertbutyl polystyrene (t-b-PS) (A). The rational that underline the topology of this polymer is to facilitated ion transport through the center block and simultaneously provide mechanical stability by tethers that are highly incompatible with the center and yet not brittle. The potential technological impact of this polymer has driven numerous studies both experimental20 and computational8 of structure, dynamics and transport characteristics21-23 of membranes of this polymer as well as their solution structure. These studies found that while well-defined


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aggregates are formed in solutions, these membranes do not exhibit long range order.20 Recent MD studies8 have shown that the interface of melts is dominated by hydrophobic blocks. The melt consists of intertwined domains of PSS and t-b-PS, segregated from each other, forming cocontinuous domains across the melt which are segregated from the PErP domains. The significance of interfaces in ionic polymers has been realized experimentally by several groups.24-29 Zawodzinski et al.24 found a time lag at the onset of water penetration into Nafion™ membranes. This time lag was attributed to surface rearrangements that take place upon contact with water. Similar effects were observed by Benziger et al.26 With the realization that the interfaces play a significant part, Elabd et al.30 probed thin Nafion films and He et al.29 have investigated water penetration into thin sulfonated polyethylene ionomer films using neutron reflectometry. They found a delay time for the onset of diffusion of water into the film which they interpreted in terms of interfacial barrier to diffusion. The technological significance has driven immense synthetic efforts to optimize ionomers and polyelectrolytes for different applications,31-33 with numerous endeavors to incorporate targeted blocks to tethered to ionizable ones to control their topology.34-36 Here we are set to resolve the structure and dynamics of thin film of this pentablock at the interface with water, above and below the glass transition temprature of the ionc clusters, probing interfacial dynamics and follow water molecules as they penetrate into films. This effort is set to resolve one critical aspect of integrating structured polymers into different uses, where the interface structure and stability determines functionality. We find that upon exposure to water, slow interdiffusion occurs while rearrangements take place, followed by water diffusion along percolating dynamic pathway formed by the ionic center block. 2. Model and Methodology


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The All Atoms Optimized Potential for Liquid Simulations (OPLS-AA) force fields by Jorgensen et.al.37-41 with updated parameters for alkanes42 were used to model the pentablock and the Na+ counterions. These force fields are chosen since they were optimized to capture well the properties, such as densities, diffusion constants, surface tensions and heat of evaporation, of hydrocarbons. Here, this model captures well the densities of the individual blocks.8 The OPLSAA potential includes both bonded and non-bonded interactions. The bonded potential consists of a sum of intermolecular bond, angle and dihedral interactions.37 The non-bonded interactions include a 12-6 Lennard-Jones potential and an electrostatic potential. LJ 12-6 potentials are commonly used to describe van der Waals type inter atomic interactions.

All Lennard-Jones

interactions are cutoff at rc = 1.2 nm. Coulomb interactions are long range and are therefore computationally expensive. The coulomb interactions are treated with particle-particle particlemesh algorithm (PPPM)43 Ewald with a real space cutoff of 1.2 nm and a precision of 10-4. The cutoff value was chosen to optimize computational efficiency while maintaining precision. Water molecules were modeled using the TIP4P/2005 model.44 This model was chosen since it reproduces the properties of water including the liquid/vapor equilibria and critical temperature, bulk diffusion constant, and surface tension much better than other non-polarizable models.45 The only exception being the dielectric constant ε =58, which is somewhat lower than the experimental value of 78.4 at 298K. Thirty unique, randomly sulfonated pentablock molecules with sulfonation fraction f = 0.30 and 0.55 were built using Polymer Builder and Amorphous Cell modules in Accelrys Materials Studio©.46 The number of molecules chosen is to optimize computational time while capturing the essential physics of the system. The total number of atoms Nch in each chain was 12,052 for f = 0.3 and 12181 for f = 0.55. The total number of atoms in the polymer membrane


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is 30 Nch. The total wt% of the center sulfonated block is ~40%, while each of the randomly substituted polyethylene blocks is ~20% and each of poly-t-butyl styrene blocks is ~10%. The counterion is Na+ in both cases. Experimentally a small fraction of propylene is introduced to prevent PE crystallization. The ratio of the blocks was chosen to match experimental studies.47 This ratio was optimized by Kraton Polymers LLC for balancing transport and tacticity of this polymer. The total molecular weight in the current study is ~33% less of the experimental one, as a compromise with computational cost. The chains were placed in a cubic periodic simulation cell of initial size L3 with L=30 nm. The energy of each molecule was initially minimized with the polymer described by the polymer consistent force field (pcff), available in Molecular Studio, followed by conversion to OPLS-AA potentials, using an in-house code. OPLS-AA potentials were used for production runs since they are optimized for alkanes and captures well the properties of hydrocarbon chains.42 They are not available in Materials Studio; therefore a conversion code was used. The number of molecules and box dimensions were chosen to make a reasonably thick membrane. The two systems were first run at constant volume temperature T = 500 K for 40 ns followed by compression to melt densities at a constant pressure of P = 1 atm and T=700 K using the NoseHoover barostat/thermostat for 30 ns.8 The systems were run at constant volume for 200 ns at 700 K, above the glass transition of sulfonated polystyrene. Since morphology and distribution of ionic clusters did not change after 200 ns we consider these systems as our starting membranes. Thin films were made from those membranes by unfolding chains in the z-directions and extending the length of the simulation cell Lz to 40 nm, more than twice the length in the other two periodic directions L|| = Lx = Ly = 16 nm. This effectively excluded any interaction between


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the upper and lower interface of the thin films. The films were further equilibrated for 100 ns at 700 K prior to the introduction of the water film. The thickness of the resulting film is ~ 17 nm. For the water film, we first equilibrated a system of 64,000 water molecules in a cubic simulation cell with dimensions L|| matching that of the pentablock film. A water slab 6 nm thick (52,520 water molecules) was cut out of the bulk simulation and placed in contact with one surface of the pentablock film as shown Figure 1a. Smooth, repulsive walls, modeled by a purely repulsive 9-3 LJ potential, were placed at upper and lower edge of the simulation cell to keep water molecules crossing directly from one side of the membrane to the other. There is a small region between the top of the water film and the upper wall as seen in Figure 1a to facilitate formation of a vapor. Since water evaporates into this gap, the diffusion into the membrane is under ambient pressure. All simulations were carried out using the LAMMPS classical MD code.48 The Newton equations of motions were integrated using a velocity-Verlet algorithm. The reference system propagator algorithm (RESPA)49 with multi-timescale integrator with a time step of 1.0 fs for the bond, angle, dihedral, van der Waals interactions and the direct interactions part of the electrostatic interactions was used. For the long range electrostatic interactions, a time step of 2.0 fs was used. Temperature of the system was maintained by using a Langevin thermostat with a 100 fs damping constant. All the simulations were performed at constant volume for 500-600 ns. 3. Results and Discussion 3.1 Water Transport and Mesoscopic Rearrangement An initial state of the membrane with f = 0.55 in contact with water is shown in Figure 1a. A top view of the interfaces at 400K and 500K at representative water exposure times are shown in


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Figure 1b and 1c respectively. These tempratures represents samples that are above and below the glass transition temprature of the ionic segments where the rest of the blocks are above Tg. Additional images of the time evolution of the interface for f = 0.55 are provided in Figure S1. The film interface of the polymer prior to exposure to water consists predominantly of hydrophobic PErP and t-b-PS blocks. After approximately 20 ns, the sulfonated groups immerge at the surface and their number increases with time. The composition evolution at the film interface is similar for f = 0.30 as demonstrated in Figure S2.

Figure 1. (a) A cross sectional view of a membrane with f = 0.55 with water at 10 ns.; (b) and (c) correspond to top view of film interface in contact with water at 400 K and 500 K respectively at indicated water exposure times. The t-butyl polystyrene block is shown in orange, ethylene-propylene block in green, polystyrene block in blue, oxygen atoms in red, sulfur atoms in yellow, sodium atoms in gray, and water molecules in violet.

The changes in the films are captured through calculation of the mass density variations as a function of water exposure time for all constituents including the entire co-polymer, individual blocks and the water molecules. The profiles measured from the center of membrane perpendicular to the interface for f = 0.55 at 500 K are presented in Figure 2. At time t=0, when the membranes were first exposed to water, the interface is rather rough, as expressed in the width of the interface. With time, the interface becomes smoother (Figure 2a) and the density of the PSS blocks at the water interface increases (Figure 2b). Surprisingly, the density of PErP blocks at interface slightly decreases with time while the density of the t-b-PS blocks hardly


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changes. Following a short delay, water molecules penetrate the membrane however they are not homogenously distributed either in plane or perpendicular to the interfaces, as is depicted in Figure 3.

Figure 2. Mass density profiles of the (a) co-polymer (full) and water (open), (b) PSS, (c) PErP, and (d) t-b-PS block as a function of distance z from the center of a film of f = 0.55 at 500 K at indicated exposure times. The water profiles are presented in light colors in the plots of the partial densities.

Water molecules diffuse across the membrane along a hydrophilic network that consists of the ionic block. The ionic clusters rearrange as the number of water molecules increases. At 500K, it takes ~140 ns for first few water molecules to diffuse across the entire membrane and reach the other side for both values of f.


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Figure 3. Water Penetration into the membranes at the indicated times (a) f = 0.30 and (b) f = 0.55 both at 500 K. The hydrophobic blocks are shown in light green, oxygen atoms in red, sulfur atoms in yellow, and water molecules in blue.

The water uptake is quantified by calculating the number of water molecules per unit area in the membrane as a function of time. The results are shown in Figure 4a.

A slow uptake is

observed at early times as the water molecules transverse the hydrophobic rich interface. At this initial stage the surface composition changes and the hydrophilic blocks slowly immerge at the interface. Following the early surface rearrangements, the water molecules diffuse faster along the percolating network formed by the sulfonated center blocks. With time, these transport pathways are dynamic as the percolating network changes as water penetrates into the film as shown in the images presented in Figure 4 for representative times. Overall the water uptake is intricate with molecular rearrangements that take place at the interface and across the film where a percolating hydrophilic network change within a rubbery PErP matrix, with only small changes in the end block. The water molecules are strictly associated with the hydrophilic network, where


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the size and shape of the hydrophobic domains change concurrently with the hydrophilic one. At later times, the network slowly changes and the overall uptake slows down.

Figure 4. (a) Number of water molecules per unit area inside the membrane N as a function of time for f = 0.30 and 0.55 at 400 K (open) and 500 K (full). (b) Images of SO3-Na+ groups with water molecules after 10 and 600 ns (left). Magnified images of sample ionic cluster associated with water molecules at indicated times for f = 0.55 at 500 K (right). Other blocks are not shown for clarity.

The morphological changes with water penetration are captured in representative clusters shown in Figure 4b for a film with f = 0.55. The dry film consists of one large ionic cluster that percolates in all three directions as shown in the top image at 10ns. Subsequently as more water molecules penetrate the membrane and associate with ionic groups, the ionic clusters break into smaller clusters, forming a new set of ionic networks. A similar result is observed for f = 0.30. The slow and fast stages are conceptually similar to experimental observations [24,25.26.29]. However, the values measured here are significantly faster since in contrast to experimental studies on thick membranes below Tg, here all blocks except the ionic one are above Tg. Note that similar simulation at 300K in the glassy state did not show any appreciable water penetration over the time scales accessible computationally.


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Figure 5. Average cluster size of sulfur atoms (left axis), and number density of water molecules (right axis) as function of time for f = 0.30 and f = 0.55 at 500 K for a slab taken from center of membrane as shown in insert.

The average cluster size of sulfur atoms and number density of water molecules are calculated for both f as a function of time for a slab of thickness 4 nm taken from the center of membrane as shown in insert image of Figure 5. Two sulfur atoms are considered to be in the same cluster if they are separated by less than 0.6 nm.8 The closest distance between two sulfur atoms in a multiplet is 4.6Å, accounting for the dimensions of sulfur, oxygen and the counter ion. We find that results for the cluster size distribution are identical taking 5Å or 6Å as the separation distance. At early stages, the ionic clusters are large and spans the entire slab. With the time, the ionic clusters break into smaller ones, resulting in decrease of average cluster size with time as shown in Figure 5. Further, for both f, the number density of water molecules within the slab increases with time.


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Figure 6. (a) Interfacial width of membrane, (b) number of sulfur atoms at interface as a function of time for f =0.30 and 0.55 at 400 K (open) and 500 K (closed).

Water is highly polar solvent and clearly dissociates the ionic clusters. This is consistent with our previous studies of sulfonated polystyrene, where we showed that the polarity of the solvent affects clustering. We demonstrated that increasing the dielectric constant ε to 5 is sufficient to dissociate the clusters.50, 51 For this ABCBA co-polymer however, while the ionic clusters dissociate in the presence of water, the ionic groups remain within the hydrophilic domains that consist of PSS, the Na+ counterion and the water. These domains remain segregated from the rest of the polymer forming a percolating network, within the time of the measurement.


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The mesoscopic evolution of the film was captured through calculating the interfacial width ∆ of the density profiles. The interfacial width as extracted from fitting to an error function erf (z/√2∆), as a function of exposure time, is presented in Figure 6a. It decreases over the first 100 ns for both sulfonation fraction and temperatures and then levels off. These results are consistent with the visual observation that the interface becomes smoother as the time the membrane is in contact with water increases. The time scale for the plateau is attributed to changes within the boundaries regions where potential changes within the hydrophobic domains occur at a significant larger time scales. The number of sulfur atoms at the membrane/water interface was counted as a function of time and the results are shown in Figure 6b. The number of sulfur atoms increases with increasing time for above and below the glass transition temperature. Surprisingly, more sulfur groups appear on surface for f = 0.30 than that for f = 0.55. We attribute the higher density for the lower sulfonation levels to a higher mobility within this films compared with that in the f = 0.55 one, as has been observed in the melts of this copolymer.8 This study revealed that the ionic clusters are near their percolation threshold for f = 0.30 whereas melts with f = 0.55, a percolating ionic networks are formed. The percolating network results in strong electrostatic interactions that limit the mobility of the ionic groups. Both the interfacial width and number of sulfur atoms at the surface increase with increasing temperature. These results reveal for the first time the molecular processes that take place as water penetrate into ionic polymers. Further they demonstrate that the hydrophobic domains conform to accommodate the dynamic network formed by the hydrophilic block and the water. The next section examines into the dynamics of the polymer.


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3.2 Dynamics of Polymer Segments The positions of individual segments of the pentablock in bulk and at interface were followed after exposure of the films to water. These results present visualization of the processes that underline the macroscopic measurements presented above on the molecular level. The images depict one segment of each film, however measurements carried out on several segments within a given distance from the interface show similar results. Results for f = 0.55 at 500 K are shown in Figure 7. For each of the blocks, one cluster or segment has been highlighted to enable visualization of their motion. A sample ionic cluster which was initially away from the surface migrates towards the water interface with exposure time as seen in Figure 7a. However, those clusters in the center of the film hardly change their average position though their morphology changes. In contrast to the ionic segments, the PErP segments, shown in Figure 7b collapse when exposed to water and move away from surface whereas their structure remain similar to those in the bulk. Further, the structure of the other hydrophobic t-b-PS blocks hardly change with time as seen in Figure 7c. However, a t-b-PS block at the interface moves slightly away from the membrane/water interface.


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Figure 7. Images of (a) a SO3- cluster (b) a chain of PErP, and (c) a chain of t-b-PS at interface (upper) and in bulk (lower) for f = 0.55 at 500 K at indicated time. One segment of each block is marked in a different color for visualization.

The motion of the center of mass of the different blocks as a function of time is shown in Figure 8. The distances are presented relative to the position of the species at t = 0. Two distinctive regions of motion, at the interface and in the bulk, are observed during the 600 ns run as shown in Figure 7. The interface includes 1.2 nm away from the interface. In the interfacial region during the first 100 ns the ionic clusters move towards the water interface and remain there for the duration of the run while their morphology slowly changes. Their dimension slowly increases as captured by changes in the radius of gyration Rg as function time as shown insert


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Figure in 8. The hydrophobic segments including the PErP and t-b-PS which are at interface slowly retreat from the water interface with time, as shown in Figure 8. The radius of gyration of the PErP block decreases significantly during the first 100ns and then remains constant. While the t-b-PS retreats from the water interface, Rg of t-b-PS blocks remain unchanged. Surprisingly, within the bulk of the film, none of the segments move significantly. The dimensions of the clusters slightly increase with water penetration, those of the PErP slightly decrease and the t-bPS does not change, as demonstrated in Figure S3.

Figure 8. Center of mass of motion as a function of time for SO3- cluster (blue), a chain of PErP (green), and a chain of t-b-PS (orange) in z for f = 0.55 at 500K. ZCOM is normalized by ZCOM(t) - ZCOM (0) of cluster. Membrane is shown in light green. Insert shows radius of gyration for highlighted sample as a function time.

A closer look into the interfacial rearrangements demonstrate that within the time of our measurements the changes take place are within one molecular layer of the water interface. However, as water penetrates the film, the hydrophilic-hydrophobic internal interfaces rearrange.


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Conclusions Here we have obtained a molecular level understanding of the structure and dynamics of ionic copolymers at the interface with water. Similar to melts, the thin films consist of segregated hydrophilic and hydrophobic domains with the two hydrophobic blocks mutually phase segregated. The interface of the dry membrane is rather rough and is dominated by the hydrophobic blocks. Upon exposure to water the interfacial region becomes significantly smoother and the polymers rearrange, exposing the ionic blocks to the water. This interfacial response of ionic polymers provides the first direct molecular level observation of interfacial reengagements asserted by transport studies in ionic polymers.25,

27

With increasing exposure

time, water molecules diffuse into the membrane. At early times, a slow diffusion concurrent with the interfacial compositional changes is observed, followed by a fast stage in which the water propagates within a dynamic percolating network. This network changes its morphology with time. At this stage the ionic clusters change their shape and cohesiveness as previously observed in polystyrene sulfonate.50

However the continuity of the hydrophobic blocks is

sufficient to maintain hydrophilic domains that percolate across the samples, even though the tight clusters observed in the membranes are dissociated. These results present a first atomistic insight into the interfacial region of ionic structured polymers. They show that both the interface with the environment and internal hydrophilic hydrophobic interfaces remain dynamic.

The degree of interfacial rearrangement can be

controlled by tuning the affinities of the blocks to the interface, which in turn will enable the design of ion containing block co-polymers for specific applications. Acknowledgements


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The authors gratefully acknowledge financial support from DOE Grant No. DE-FG0212ER46843. This support has enabled to develop computational techniques for computational preparation of the membranes. Under this support, membranes were made and the static structure was calculated. We gratefully acknowledge NSF grant No. DMR 1611136. This funding has enabled the study of the water interpenetration. This research used resources at the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the United States Department of Energy under Contract No. DE-AC02-05CH11231. This work was made possible in part by advanced computational resources deployed and maintained by Clemson Computing and Information Technology. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy and Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA-0003525.

References †

Present address: Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, USA ‡ Present address: Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, USA # email: [email protected] + email: [email protected] 1. Pan, H. M.; Yu, H.; Guigas, G.; Fery, A.; Weiss, M.; Patzel, V.; Trau, D., Engineering and Design of Polymeric Shells: Inwards Interweaving Polymers as Multilayer Nanofilm, Immobilization Matrix, or Chromatography Resins. ACS Applied Materials & Interfaces 2017, 9, 5447-5456.


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Table of Contents Figure

Image for a cross sectional view of a thin film of ionic pentablock co-polymer with sulfonation fraction f = 0.55 with water, and corresponding top view of film interface in contact with water at 10 and 600 ns.


Structured Ionomer Thin Films at Water Interface - ACS Publications

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