SOLID-STATE TRANSFORMATIONS OF BLOCK COPOLYMERS: SIGNIFICANTLY INCREASING Χ AND IMPARTING FUNCTIONALITY

To achieve small-scale features in semiconductors, storage devices, or porous membranes, self-assembly of block copolymers (BCPs) have been considered as a promising bottom-up platform, since BCPs offer a tremendous potential to push feature sizes into the single nanometer scale with highly-ordered periodic dot or line patterns. In this dissertation, a high χ−low N system, where χ is the Flory−Huggins segmental interaction parameter and N is the degree of polymerization, was developed for a self-assembled BCP morphology with a sub-10 nm period through an acid-catalyzed hydrolysis of symmetric poly(solketal methacrylate-b-styrene) (PSM-b-PS) copolymers. The acid-catalyzed hydrolysis transforms PSM-b-PS, having two hydrophobic blocks, into poly(glycerol monomethacrylate-b-styrene) (PGM-b-PS), having one hydrophilic and one hydrophobic block. This simple transformation significantly enhances χ such that a phase-mixed PSM-b-PS can be transformed in the solid-state into a microphase separated BCP without the use of any additives. Small-angle X-ray scattering (SAXS) measurements as functions of the degree of polymerization and PSM conversion were performed to examine the lamellar microdomain features. Using a mean-field correlation-hole analysis of the scattering, χ for PSM and PS was determined before and after the conversion of PSM to PGM. With the large increase in χ, even smallest synthesized PGM-b-PS copolymers underwent microphase separation, allowing us to achieve a center-to-center lamellar microdomain spacing of 5.4 nm. We also investigated the two-step chemical transformation of symmetric poly(styrene-b-solketal acrylate) (PS-b-PSA) as another responsive high χ BCP. Through an acid-catalyzed hydrolysis, the PSA block is converted into a poly(glycerol acrylate) (PGA), which subsequently can be hydrolyzed to a poly(acrylic acid) (PAA) block. With this two-step conversion, the responsive PSA block becomes increasingly polar as the reaction proceeds, improving the strength of segmental interactions. As a result, lamellar and cylindrical microdomain spacings of 7.4 nm and 6.9 nm were achieved after conversion to PS-b-PGA and PS-b-PAA, respectively, demonstrating that the size scale of the microdomains was reduced to the sub-10 nm level as well. Consequently, it is evident that PSM-b-PS and PS-b-PSA copolymers have a high potential for advanced nano-patterning as a template with a single nanometer feature size through a simple chemical transformation

Hydrocarbon-Based Composite Membrane Using LCP-Nonwoven Fabrics for Durable Proton Exchange Membrane Water Electrolysis

A new hydrocarbon-based (HC) composite membrane was developed using liquid crystal polymer (LCP)-nonwoven fabrics for application in proton exchange membrane water electrolysis (PEMWE). A copolymer of sulfonated poly(arylene ether sulfone) with a sulfonation degree of 50 mol% (SPAES50) was utilized as an ionomer for the HC membranes and impregnated into the LCP-nonwoven fabrics without any surface treatment of the LCP. The physical interlocking structure between the SPAES50 and LCP-nonwoven fabrics was investigated, validating the outstanding mechanical properties and dimensional stability of the composite membrane in comparison to the pristine membrane. In addition, the through-plane proton conductivity of the composite membrane at 80 °C was only 15% lower than that of the pristine membrane because of the defect-free impregnation state, minimizing the decrease in the proton conductivity caused by the non-proton conductive LCP. During the electrochemical evaluation, the superior cell performance of the composite membrane was evident, with a current density of 5.41 A/cm2 at 1.9 V, compared to 4.65 A/cm2 for the pristine membrane, which can be attributed to the smaller membrane resistance of the composite membrane. From the results of the degradation rates, the prepared composite membrane also showed enhanced cell efficiency and durability during the PEMWE operations

Dynamic modeling and simulation of hydrogen supply capacity from a metal hydride tank

The current study presents a modeling of a LaNi5 metal hydride-based hydrogen storage tank to simulate and control the dynamic processes of hydrogen discharge from a metal hydride tank in various operating conditions. The metal hydride takes a partial volume in the tank and, therefore, hydrogen discharge through the exit of the tank was driven by two factors; one factor is compressibility of pressurized gaseous hydrogen in the tank, i.e. the pressure difference between the interior and the exit of the tank makes hydrogen released. The other factor is desorption of hydrogen from the metal hydride, which is subsequently released through the tank exit. The duration of a supposed full load supply is evaluated, which depends on the initial tank pressure, the circulation water temperature, and the metal hydride volume fraction in the tank. In the high pressure regime, the duration of full load supply is increased with increasing circulation water temperature while, in the low pressure regime where the initial amount of hydrogen absorbed in the metal hydride varies sensitively with the metal hydride temperature, the duration of full load supply is increased and then decreased with increasing circulation water temperature. PID control logic was implemented in the hydrogen supply system to simulate a representative scenario of hydrogen consumption demand for a fuel cell system. The demanded hydrogen consumption rate was controlled adequately by manipulating the discharge valve of the tank at a circulation water temperature not less than a certain limit, which is increased with an increase in the tank exit pressure. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.N

Multi-Block Copolymer Membranes Consisting of Sulfonated Poly(<i>p</i>-phenylene) and Naphthalene Containing Poly(arylene Ether Ketone) for Proton Exchange Membrane Water Electrolysis

Glassy hydrocarbon-based membranes are being researched as a replacement for perfluorosulfonic acid (PFSA) membranes in proton exchange membrane water electrolysis (PEMWE). Here, naphthalene containing Poly(arylene Ether Ketone) was introduced into the Poly(p-phenylene)-based multi-block copolymers through Ni(0)-catalyzed coupling reaction to enhance π-π interactions of the naphthalene units. It is discovered that there is an optimum input ratio of the hydrophilic monomer and NBP oligomer for the multi-block copolymers with high ion exchange capacity (IEC) and polymerization yield. With the optimum input ratio, the naphthalene containing copolymer exhibits good hydrogen gas barrier property, chemical stability, and mechanical toughness, even with its high IEC value over 2.4 meq g−1. The membrane shows 3.6 times higher proton selectivity to hydrogen gas than Nafion 212. The PEMWE single cells using the membrane performed better (5.5 A cm−2) than Nafion 212 (4.75 A cm−2) at 1.9 V and 80 °C. These findings suggest that naphthalene containing copolymer membranes are a promising replacement for PFSA membranes in PEMWE

Poly(p-phenylene)-based membranes with cerium for chemically durable polymer electrolyte fuel cell membranes

A poly(p-phenylene)-based multiblock polymer is developed with an oligomeric chain extender and cerium (CE-sPP-PPES + Ce3+) to realize better performance and durability in proton exchange membrane fuel cells. The membrane performance is evaluated in single cells at 80 °C and at 100% and 50% relative humidity (RH). The accelerated stability test is conducted 90 °C and 30% RH, during which linear sweep voltammetry and hydrogen permeation detection are monitored periodically. Results demonstrate that the proton conductivity of the pristine hydrocarbon membranes is superior to that of PFSA membranes, and the hydrogen crossover is significantly lower. In addition, a composite membrane containing cerium performs similarly to a pristine membrane, particularly at low RH levels. Adding cerium to CE-sPP-PPES + Ce3+ membranes improves their chemical durability significantly, with an open circuit voltage decay rate of only 89 μV/h for 1000 h. The hydrogen crossover is maintained across accelerated stability tests, as confirmed by hydrogen detection and crossover current density. The short-circuit resistance indicates that membrane thinning is less likely to occur. Collectively, these results demonstrate that a hydrocarbon membrane with cerium is a potential alternative for fuel cell applications

Perfluorocyclobutyl-containing multiblock copolymers to induce enhanced hydrophilic/hydrophobic phase separation and high proton conductivity at low humidity

A multiblock copolymer containing a highly sulfonated poly(phenylene sulfide sulfone) (sPPSS) hydrophilic oligomer and a partially fluorinated perfluorocyclobutyl (PFCB)-containing hydrophobic oligomer was synthesized. The sharp contrast between the hydrophilic and hydrophobic moieties induced a well-developed phase separation, which was observed in the transmission electron microscopy (TEM) images within the polymer electrolyte membrane (PEM). The increased chain mobility from the flexible ether and PFCB groups afforded facile thermal annealing of the membrane. Thermal annealing induced polymer chain packing of the hydrophobic moieties, enhancing the hydrophilic/hydrophobic phase separation. The fabricated membranes exhibited higher proton conductivity compared with those of conventional hydrocarbon PEM possessing a random copolymer architecture, while their dimensional swelling was suppressed. Additionally, under low humidification (a relative humidity (RH) of 50%), the sulfonated-fluorinated membrane achieved a high proton conductivity of up to 41.9 mScm(-1). A high adhesion strength of 32.7 mNem(-1) was also observed, indicating strong interfacial compatibility in the membrane electrode assembly (MEA) due to its structural affinity for the contacting perfluorosulfonated binder. The enhanced hydrophilic/hydrophobic phase separations facilitated fuel cell performances of 1.13 and 0.61 Acm(-2) at 0.6 V and 65 degrees C under 100% and 50% RH conditions, respectively, in addition to achieving stable chemical and physical durabilities.N

Aluminum Diethylphosphinate-Incorporated Flame-Retardant Polyacrylonitrile Separators for Safety of Lithium-Ion Batteries

Herein, we developed polyacrylonitrile (PAN)-based nanoporous composite membranes incorporating aluminum diethylphosphinate (ADEP) for use as a heat-resistant and flame-retardant separator in high-performance and safe lithium-ion batteries (LIBs). ADEP is phosphorus-rich, thermally stable, and flame retardant, and it can effectively suppress the combustibility of PAN nanofibers. Nanofibrous membranes were obtained by electrospinning, and the content of ADEP varied from 0 to 20 wt%. From the vertical burning test, it was demonstrated that the flame retardancy of the composite membranes was enhanced when more than 5 wt% of ADEP was added to PAN, potentially increasing the safety level of LIBs. Moreover, the composite membrane showed higher ionic conductivity and electrolyte uptake (0.83 mS/cm and 137%) compared to those of commercial polypropylene (PP) membranes (Celgard 2400: 0.65 mS/cm and 63%), resulting from interconnected pores and the polar chemical composition in the composite membranes. In terms of battery performance, the composite membrane showed highly stable electrochemical and heat-resistant properties, including superior discharge capacity when compared to Celgard 2400, indicating that the PAN/ADEP composite membrane has the potential to be used as a heat-resistant and flame-retardant separator for safe and high-power LIBs

Evaluation of the Interaction Parameter for Poly(solketal methacrylate)-<i>block</i>-polystyrene Copolymers

A series of symmetric poly­(solketal methacrylate-<i>b</i>-styrene) (PSM-<i>b</i>-PS) copolymers with varying molecular weights that can transform a hydrophobic PSM block to a hydrophilic poly­(glycerol monomethacrylate) (PGM) block through an acid hydrolysis were investigated. This simple chemical transformation significantly enhances the segmental interaction parameter (χ), enabling a phase-mixed block copolymer (BCP) to microphase separate without any additives. Temperature-dependent small-angle X-ray scattering (SAXS) measurements as a function of the degree of polymerization (16 ≤ <i>N</i> ≤ 316) and PSM hydrolysis conversion were conducted to characterize the order-to-disorder transition (ODT) behavior as well as the lamellar microdomain features. Using a mean-field correlation-hole analysis of the scattering, the χ value for PSM and PS was determined as a function of the conversion of PSM to PGM. For 100% conversion of PSM to PGM, the χ with PS was found to be given by χ = 0.3144 + 36.91/<i>T</i>, with χ = 0.438 at 25 °C, which is ∼13 times larger in magnitude than χ parameter for PSM-<i>b</i>-PS copolymer (∼0.035 at 25 °C) calculated using a 118 ų reference volume. With this large increase in χ, even the smallest synthesized PGM-<i>b</i>-PS copolymers underwent microphase separation, allowing us to achieve a center-to-center lamellar microdomain spacing (commonly referred to as the full pitch) of 5.4 nm, obtained for the lowest molecular weight sample (<i>M</i>_n = 2200 g/mol, <i>N</i> = 16)

Studies on the 3-Lamellar Morphology of Miktoarm Terpolymers

The morphologies of ABC miktoarm star terpolymer consisting of polystyrene (PS), polyisoprene (PI), and poly­(2-vinylpyridine) (P2VP) were studied by combining small-angle neutron and X-ray scattering (SANS and SAXS) and transmission electron microscopy. We find that this system displays a rich morphological behavior including an alternate lamellar (ALT.LAM), cylinder in undulated lamellar (CYLULAM), and 3-lamellar (LAM-3) phase. While the SAXS data alone were insufficient to conclusively differentiate between possible phases, we show that the use of selective deuteration and SANS is essential to unambiguously identify the morphology. In particular, the primary peak in SANS for the miktoarm polymer containing deuterated PS was found to be lower than the next two higher order peaks. Such an unusual scattering pattern for a lamellar morphology was verified by self-consistent field theory calculations when the relative strength of the interaction between PI–P2VP over PS–PI is equal to or greater than 2 (χIV/χSI ≥ 2)