Details

Autor(en) / Beteiligte

• Senathiraja, Thivani

Titel

Tuning the Intermolecular Interactions of Pentablock Copolymers: Morphology, Physical Properties, Gas Separation, and Ion Conduction

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2024

Link zum Volltext

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Tuning the interactions of solid multi-block polymer electrolytes is a cost-effective approach to target and advance specific properties for sustainable ion-conducting applications such as fuel cells, polymer actuators, gas separation, vanadium flow batteries, water desalination using electrodialysis, and water electrolysis to produce H2 and O2. The accelerated research interest in tailoring the properties of multi-block ionomers is fueled by their ability to assemble into various heterogeneous nanoscale structures. These structures' spatial arrangement and connectivity define film strength, durability, and molecule transport. Systematically designing the spatial arrangement is a key to understanding the structure and property relationships of proton-exchange membranes (PEM)/ionomers. Such a design may solve global challenges and contribute to renewable technologies like water and energy. In this dissertation, the morphological transitions of a multi-block ionomer known as a pentablock copolymer (PBC) are investigated to advance the basic science related to the design of functional materials and the development of morphology-composition-property-transport relationships related to random and multi-block ionomers. The intermolecular interactions of PBC were altered by manipulating their free energy, thermodynamics and equilibrium via polymer blending, systematic incorporation of metal salt, thermal annealing and impregnation of inorganic SiO2/TiO2 sols.A synergistic polymer blend consisting of PBC with ion-exchange capacities (IECs) of 1.0 and 2.0, and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) revealed that PBC(2.0) exhibited greater clustering of ion groups and phase separation compared to PBC(2.0)/PPO blends, as evidenced by an increase in domain spacing from 33.1 nm to 37.0 nm. Notably, the PBC(2.0)/PPO (90/10) blend membrane showed potential in hydrogen fuel cells as they exhibited comparable proton conductivity (110ms/cm) to Nafion even when the water uptake (WU) was reduced by 58 wt%. Moreover, the PBC(1.0)/PPO blend demonstrated an H2 permeability ranging from 36.7 to 43.3 Barrers and an O2 permeability of 6.84 to 7.10 Barrers, with an ideal H2/O2 selectivity of 5.81 to 6.60, representing a hydrogen fuel cell crossover improvement over unmodified PBC(1.0).Tailoring the structure and function of PBC(1.5) with monovalent lithium and trivalent aluminum cations via both in situ and ex situ methods demonstrated that film properties were impacted by their Lewis acid strength, cation size, and interaction energy between the metal ion and sulfonated group. The Li+ formed the most stable pair with sulfonic acid groups in PBC(1.5) due to their comparable Lewis acid strength (LAS) with a Lewis base strength of 0.25 to 0.29. Lower interaction energies of 22.7 to 45.5 J/mol were exhibited by Al3+ as compared to Li+, indicating incomplete neutralization with sulfonic acid groups. At a metal salt concentration of 0.01 M, PBC(1.5)-Li-[0.01] and PBC(1.5)-Al[0.01] exhibited either 43.4% higher conductivity or similar conductivity to PBC(1.5)-H+ at all hydration levels (10% RH, 50% RH, 90% RH, and 100% wet). The results showed that metal cations incorporated via the in situ method in the PBC matrix allowed systematic tunability of these membranes for CO2/CH4 separations, elevating CO2 selectivity by 303%.Additionally, the PBC membranes with IEC 1.0 and 2.0 were blended to investigate the interplay between the simultaneous increase in the polymer backbone and sulfonic acid groups. Proton conductivity results demonstrated higher conductivity (164-224 mS/cm) in blend membranes compared to Nafion, even at 90% relative humidity (RH), with PBC(1.0)/PBC(2.0) (20/80) and (10/90) blends exhibiting superior performance. Under wet saturated conditions, PBC(1.5) showed excellent proton conductivity at 52% lower water uptake (WU) compared to PBC(2.0). Gas transport studies indicated higher selectivity for αH2/O2 (10.2-13.2) and αCO2/CH4 (4.32-36.2) in blend membranes with IECs between 1.6-1.7 mmol/g, making them potential candidates for preventing hydrogen fuel cell crossover and applications in natural gas separation.Further, the effects of manipulating the distribution of SiO2/TiO2 inorganic sols within a PBC(1.0) matrix through thermal annealing and incorporation in different physical states (solid/liquid) showed that the degree of crystallinity varied from 1.10-10.34%, with increased crystallinity correlating with decreased water uptake (WU%). Surface energy analysis revealed the enhancement of proton conductivity through the rearrangement of sulfonate domains at the surface of PBC(1.0) membranes, facilitating a more efficient path for ion conduction and varied from 17.8-30.5 mJ/m2. Notably, PBC(1.0) exchanged with liquid TiO2 sols outperformed control and benchmark Nafion membranes, exhibiting superior proton conductivity and dimensional stability, achieving 138 mS/cm conductivity even with a 12.6% reduction in WU and a 61% increase in crystallinity.Overall, this dissertation delves into the crucial role of optimizing the intermolecular interactions of PBC in achieving targeted properties. The findings hold tremendous promise for the development of sustainable ion-conducting applications, such as hydrogen fuel cells, as they enable high proton conductivity without sacrificing dimensional stability. Furthermore, these PBC membranes exhibited the potential to enhance membrane-based H2/O2 and CO2/CH4, gas separations.