Session A31: Polymer Membranes for Clean Energy and Water I

Conductivity Scaling Relationships for Nanostructured Block Copolymer/Ionic Liquid Membranes

Nanostructured membranes containing structural and ion-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled with high thermal stability. To optimize the properties of such membranes, it is essential to understand scaling relationships between composition, structure, temperature, and ionic conductivity. The conductivity behaviors of mixtures of two block copolymer chemistries with two different ionic liquids have been investigated. The conductivities of all the mixtures are described by a single expression, which combines the Vogel-Tamman-Fulcher (VTF) equation with percolation theory. The VTF equation takes into account the effect of the glass transition temperature of the conducting phase on the temperature dependence of conductivity, while percolation theory reflects the power law dependence of conductivity on the overall volume fraction of ionic liquid in the membrane. The dominance of the overall volume fraction of ionic liquid in determining conductivity indicates that there is incredible flexibility in designing highly conductive block copolymer/ionic liquid membranes.   

Ionic Block Copolymers for Anion Exchange Membranes

Anion exchange membrane (AEM) fuel cells have regained interest because it allows the use of non-noble metal catalysts. Until now, most of the studies on AEM were based on random polyelectrolytes. In this work, Poly(vinylbenzyltrimethylammonium bromide)-b- (methylbutylene) ([PVBTMA][Br]-b-PMB) was studied by SAXS, TEM and dielectric spectroscopy to understand the fundamental structure-conductivity relationship of ion transport mechanisms within well-ordered block copolymers. The ionic conductivity and the formation of order structure were dependent on the casting solvent. Higher ion exchange capacity (IEC) of the membranes showed higher conductivity at as IEC values below 1.8mmol/g, as above this, the ionic conductivity decreases due to more water uptake leading to dilution of charge density. The humidity dependence of morphology exhibited the shifting of d-spacing to higher value and the alteration in higher characteristic peak of SAXS plot as the humidity increase from the dry to wet state. This phenomenon can be further explained by a newly developed polymer brush theory. Three ionic conduction pathways with different conduction mechanism within the membranes can be confirmed by broadband electric spectroscopy.   

Anion Exchange Membranes Based on Reactive Block Copolymers

The unmet needs for polymeric AEMs include high hydroxide conductivity, chemical stability under strongly basic conditions, and sufficient mechanical properties to withstand the temperature and humidity fluctuations in a fuel cell. This presentation will include our most recent findings from an effort to develop cation-containing polymers based on phosphonium and ammonium derivatives of styrene using co-polymerization of reactive, ion-containing block copolymers with a small molecule ``matrix'' monomer. By creating polymer membranes with co-continuous cation-containing domains in a cross-linked matrix, we hope to demonstrate high conductivity simultaneously with the robust mechanical properties required in the fuel cell environment. Morphological data from SAXS and TEM, mechanical property measurements, in- and through-plane charge transport measurements, and the results of fuel cell testing will be presented. It was found that the surface transport characteristics of these materials differ from the through-plane properties, that chemical crosslinks may not produce membranes with the required toughness, and that a polymerization technique that is highly sensitive to reaction kinetics is not ideal for the production of AEMs.   

Morphology and Proton Transport in Sulfonated Block Copolymer and Mesoporous Polymer Electrolyte Membranes

In an effort to understand the fundamentals of proton transport in polymer electrolyte membranes (PEMs), we have developed a series of poly(styrene-b--ethylene-b--styrene) (SES) membranes. The SES membranes were subsequently sulfonated to yield proton conducting S-SES membranes. We examine the effects of sulfonation level, temperature and thermal history on the morphology of S-SES membranes in both dry and hydrated states. The effects of these parameters on water uptake and proton transport characteristics of the membranes are also examined. Furthermore, building upon the strategy we deployed in sulfonating the SES membranes, we fabricated mesoporous S-SES membranes, with pores lined up with the proton conducting channels. These membranes have three distinct phases: structural block, proton-conducting block, and void. We examine the effects of pore size, domain structure and sulfonation level on water uptake and proton conductivity of the mesoporous PEMs at different temperatures.   

Characterization of Hybrid Polyhedral Oligomeric Silsesquioxane (POSS)-Polybenzimidazole (PBI)-Phosphoric Acid (PA) Materials Intended for Proton Exchange Membranes (PEM)

Isophthalic acid and 3,3'-diaminobenzidine (DAB) were polymerized in the presence of polyphosphoric acid (PPA) and various additives, degree of polymerization was monitored by viscosity and torque change measurements, and membranes were prepared by casting the reaction solution and allowing PPA to hydrolyze to PA under ambient conditions. As a function of relative humidity, the membranes were characterized for (1) acid content, (2) in-plane conductivity and (3) complex shear modulus G* obtained via oscillatory parallel plate dynamic mechanical spectroscopy. The addition of sulfonated octaphenyl polyhedral oligomeric silsesquixane (S-POSS) to $m$-polybenzimidazole (PBI)-phosphoric acid (PA) membranes resulted in increased in-plane proton conductivity at high temperatures (120-150 $^{\circ}$C) and increased G* relative to a $m$-PBI control membrane and to $m$-PBI control membranes carrying comparable weight loadings of non-proton conducting octaphenyl-POSS nanoadditive or silica.   

Molecular Dynamics Simulation of Polysulfone-Based Anion Exchange Membrane Fuel Cell

In this study, we investigate the nanophase-segregated structures and transport properties of quaternary ammonium grafted polysulfone membranes using molecular dynamics simulation method. For this, we develop a new force field from a reference density functional theory modeling with B3LYP and 6-31G** in order to describe the hydroxide anion. The bond stretching force constant is determined to reproduce the quantum mechanical vibrational frequency. The atomic charges are determined by Mulliken population analysis. Through the annealing procedure, the nanophase-segregated structure is developed as a function of water contents such as 10 and 20 wt {\%}. The extent of nanophase-segregation is evaluated by the structure factor analysis, which can be compared with the experimental small angle scattering data. Once the equilibrium structures are obtained, we run long MD simulations to analyze the diffusion of water and hydroxide using the mean-square displacement analysis with an assumption of Gaussian diffusion. The nanophase-segregated structures and the transport properties will be compared to the proton exchange membrane consisting of the same polymer backbone except for the acidic functional group.   

Predicting inhomogeneous water absorption in an ionic diblock polymer membrane

Fuel cells convert fuel directly into electrical power. Their performance depends on a permeable (yet strong) membrane to allow ion conduction (while preventing combustion). Anion-exchange membrane fuel-cells are especially economical to produce, but technological hurdles currently limit durability and $\mathrm{OH}^{-}$ conductivity of the membrane. One solution to these problems is a diblock morphology. Layers of stiff hydrophobic polymer provide structure, while interspersed layers of polyelectrolyte provide avenues for conduction. Previously, little was known about the structure within the conducting layer. We adapted Scheutjens-Fleer polymer-brush theory to a lamellar geometry. The calculation tells where the polyelectrolytes congregate within a lamella, and hence how conduction occurs. This talk focuses on a new diblock material, PMB-PVBTMA. We show how the features of the material determine the intra-lamellar structure. We conclude that at low humidity, the bulkiness of PVBTMA causes it to adopt a near-uniform distribution within the conducting block. At high humidity, however, a phase separation may induce abrupt water channels. Understanding the architecture within the conducting layer will help guide research into better anion-exchange membranes materials.   

Swelling of ultrathin crosslinked polyamide water purification membranes

Polyamide (PA) ultrathin films represent the state-of-the-art nanofiltration and reverse osmosis membranes used in water desalination. The performance of these materials, such as permselectivity, is intimately linked with extent of swelling of the PA network. Thus, quantifying their swelling behavior would be a useful and simple route to understanding the specific network structural parameters that control membrane performance. In this work, we measure the swelling behavior of PA ultrathin films using X-ray reflectivity as a function of water hydration. By applying the Flory-Rehner theory used to describe the swelling behavior of polymer networks, we quantify the PA network properties including Flory interaction parameter and the monomer units between crosslinks. Finally, we demonstrate application of this measurement approach for characterizing the network properties of different types of PA ultrathin films relevant to water purification and discuss the relationship between network and transport properties.   

Molecular Dynamics Simulations of a Single Chain Pentablock Ionomer in Dilute Solutions

Co-polymers are in the core of many applications such as fuel cells, batteries and purification membranes that require transport across membranes. The challenge remains however that under the condition that transport is optimized, the stability of the membranes is compromised. To surmount this challenge, co-polymers with blocks targeting specific roles have been designed. Using molecular dynamics simulations we have studies the structure and dynamics of ionic single chain pentablock copolymer \textit{(A-B-C-B-A)} containing randomly sulfonated polystyrene in the center, tethered to poly-ethylene-r-propylene end-capped by poly$-t-$butyl styrene. The ionic block facilitates transport while the A and B componenet are incorporated for mechanical stability. The conformation and dynamics of single pentablock ionomer of molecular weight M$_{\mathrm{w}} \quad =$ 50,000g/mol in an implicit poor solvent with dielectric constant of 1 and 77$.$7, water, and mixture (1:1) of cyclohexane and n-heptane at 300K and 500K will be presented. The effect of solvents on conformation of a single molecule of pentablock was determined and compared with experiment, providing a stepping stone to the understanding phase behavior of this polymer.   

Gas Permeation through Polystyrene-Poly(ethylene oxide) Block Copolymers

Lithium air batteries are a potential technology for affordable energy storage. They consist of a lithium metal anode and a porous air cathode separated by a solid polymer electrolyte membrane, such as PEO/LiTFSI (PEO $=$ poly(ethylene oxide), LiTFSI $=$ lithium bis-trifluoromethane sulfonimide). For extended operation of such a battery, the polymer electrolyte must conduct lithium ions while blocking electrons and gases present in air. In order to maintain a pressure difference the membrane must be mechanically robust, which can be achieved by incorporating the PEO into a block copolymer with a glassy block such as PS (PS $=$ polystyrene). To protect the lithium electrode, the membrane must have low permeability to gases in air such as CO$_{\mathrm{2}}$, N$_{\mathrm{2}}$, and O$_{\mathrm{2}}$. We have therefore studied the permeation of pure gases through a PS-PEO block copolymer. A high molecular weight, symmetric block copolymer with a lamellar morphology was used to cast free-standing membranes. Gas permeability was measured through these membranes with a standard, pressure-based technique. A model was developed to account for transport through the polymer membrane consisting of semi-crystalline PEO lamellae and amorphous PS lamellae. PEO crystallinity was extracted from the permeation model and compares well with values from differential scanning calorimetry measurements.   

Nanoporous thin films from nanophase-separated hybrids of block copolymer/metal salt

Block copolymers self-assemble into periodic nanostructures, i.e. nanophase-separated structures, which can be scaffolds for nano-applications such as nanoporous membranes, nanolithographic masks, photonic crystals, etc. In this study, we report facile preparation to achieve nanoporous thin films from nanophase-separated hybrids comprising polystyrene-$b$-poly(4-vinylpyridine) (PS-P4VP, $M_{\mathrm{n}}=$54k, PDI$=$1.13, $f_{\mathrm{s}}=$0.61) and water-soluble iron(III) chloride (FeCl$_{\mathrm{3}})$, where FeCl$_{\mathrm{3}}$ are incorporated into a P4VP phase via metal-to-ligand coordination. To obtain a nanoporous film, firstly a hybrid thin film was prepared by microtoming. Then, the film was immersed into water to remove metal salts, this simple procedure can produce nanoporous thin film. Morphological observations were conducted by using transmission electron microscopy (TEM). Ordered cylindrical nanopores were observed in the thin film of the water-immersed hybrid, which originally presents cylindrical nanodomains. The nanoporous film was modified by loading another metal salt, samarium(III) nitrate, into nanopores via coordination between the metal salt and P4VP tethered to the pore walls. The structure of the sample after modification was evaluated by TEM and an energy dispersive X-ray spectroscopy.   

Highly-Ordered Thin Films from Photocleavable Block Copolymers

A robust route for the preparation of nanoscopic dot/line patterns with long range lateral order from poly(styrene-block-ethylene oxide) (PS-b-PEO) with an o-nitrobenzyl ester junction (PS-h$\nu $-PEO) is demonstrated. Solvent annealing condition is optimized to achieve the highly ordered cylindrical block copolymer (BCP) microdomains oriented normal or parallel to the silicon substrates. Following a very mild UV exposure and successive washing with methanol, PS-hv-PEO thin films were transformed into highly ordered porous or trench templates. Afterwards the pores or trenches were either filled with PDMS by spin-coating or exposed to direct metal deposition of Au. After a plasma etching or lift-off process to remove the polymer templates, highly ordered arrays of silica or Au nanopatterns were obtained. This represents the first template application example from highly ordered nanoporous thin films derived from block copolymers featuring a photocleavable junction.   

Understanding the Internal Structure of Layered Organic Compounds deposited on mineral surface using Neutron Reflectivity

Organic carbon (OC) stabilization in soils plays a significant role in the global C cycle, therefore the understanding of the structure and function of the OC-soil mineral interface is of high importance. To study the internal structure, films with different combination of simple OC compounds, natural organic matter (NOM), Bi-layers of SA (Stearic Acid) on Glucose and NOM/Hydrophilic-NOM/Hydrophobic-NOM were deposited onto sapphire using spin coating. The phobic and phylic fractions of the NOM are operationally separated by exchange resins. We obtained detailed structural depth profile of the films using the depth-sensitive technique of the neutron reflectometry. The neutron reflectivity data were collected at the MAGICS Reflectometer at Spallation Neutron Source at the ORNL. Self-assembled ordering of SA in a repeating bi-layer structure was observed when it was deposited on NOM, phylic-NOM and Glucose. However, when SA was added to phobic-NOM no ordering of SA was detected. The formation of distinct, immiscible layers is due to insolubility of SA with NOM/Hydrophilic-NOM and Glucose. Our results reveal that the OC-mineral interface form complex layering and that the sequence of the layering depends on the compounds.