MORPHOLOGY OF POLYMERS FOR APPLICATION IN ION EXCHANGE MEMBRANES
Open Access
- Author:
- Saikia, Nayan
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 08, 2022
- Committee Members:
- Nasim Alem, Outside Unit & Field Member
Enrique Gomez, Major Field Member
Christian Pester, Major Field Member
Michael Hickner, Chair & Dissertation Advisor
Robert Rioux, Program Head/Chair - Keywords:
- Morphology
Ion exchange membranes
Electron Microscopy
Membranes
small angle x-ray scattering
Nafion - Abstract:
- Ion exchange membrane (IEMs) are poised to play an important role in emerging energy conversion devices such as batteries, soft actuators and electrochemical cells, especially polymer-electrolyte-membrane fuel-cells (PEMFCs). IEMs are typically composed of a hydrophobic backbone which is covalently attached to an ionic group with a mobile counter-ion. The ion functionalized group dissociates in the presence of water releasing the counter ion and hence is responsible for ionic conduction. These materials are often called single ion conductors since only one charge is mobile, versus a salt-based ionic conductor where both anions and cations are mobile in the matrix. In IEMs conduction of counterions rely on a network of water-fille domains to transport the ion through the polymeric matrix. An effective network in the bulk provides for faster transport than an unorganized system, and hence high ionic conductivity of the ensuing material. Researchers have been interested in investigating these ionic domain networks (or microphase separated morphology) since the early days of small angle x-ray scattering (SAXS). SAXS is a fast technique that generates contrast based on electronic density between the phases. This allowed them to investigate the morphology of Nafion and propose representative structures that explain the superior ionic conductivity of the material. However, scattering alone cannot unambiguously elucidate the morphology since it relies on models to fit the experimental data, and hence TEM should be used in tandem with SAXS. In this dissertation, we will employ transmission electron microscopy (TEM) as a tool to understand the phase morphology of ionic polymers and directly visualize size, shape, and distribution of domains in the materials. Apart from this, we will use the different modalities such as field/dark field (BF), scanning transmission electron microscopy (STEM) and energy filtered TEM (EFTEM) to optimize contrast and resolution of the images. It has been well-established that polymeric materials undergo beam damage under irradiation. Radiation damage changes morphology due to heating, electrostatic charging, and sputtering and can produce artifacts that are not inherent to the unaltered material. To mitigate damage, we begin our experiment at low dose rates (< 500 e/nm2 s) and slowly increase the beam intensity to make sure radiation damage is minimized. STEM modes were controlled using spot sizes and lenses to achieve variable current. We observed that STEM mode is a more efficient way to image polymers as the contrast can be switched based on atomic number of the elements present or the density variations between the phases. Density variations allowed us image ionic block copolymers based on two common backbone architectures: polystyrene-b-polybutadiene-b--polystyrene (SBS) and polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS). The SEBS-based polymer demonstrated long range perforated lamellar structures with average lamellar thickness of 55 nm while the morphology of SBS polymers were heterogenous without widespread long-range order. Some local ordering was observed in the SBS samples of length scales of about ~150 nm which indicates non equilibrium conditions for the bulk film, but block architecture and ion content can also play a big role in nanophase morphology. In general, these ionic block copolymers demonstrated deviation in phase morphology from traditional block copolymers, which can be attributed to the additional interaction that the ions have on the polymer system. We also investigated the electrochemical properties of the block copolymers and established that larger cations and ion exchange capacity dictate ionic conductivity. In fact, bulkier cations shielded the ionic group from nucleophilic attack of the hydroxide ion increasing the lifetime of the membranes. For random copolymers, imaging conditions were highly dependent on the polymer sample. These materials were beam sensitive and would often tear and create holes. To avoid this degradation, ion exchange with tetrachloroplatinate was pursued. These larger inorganic counterions reduced the amount of charging as well as provided a shield to excess mass loss in the material. Ionic polymers based on ionic comb-shape functionalized poly (2,6-dimethyl phenylene oxide (PPO) was imaged using this method and we observed that despite the variability in 2D scattering, the TEM size distribution was found to be 2.04 ± 1.5 nm for all the comb-shaped PPO-based polymers. This is a because of swelling and counterion fill up in the ionic domains. Sulfonated polymers were also investigated with and without ion exchange. In case of the undoped material, the contrast was low, and the materials’ microphase separated morphology could not be visualized. Ion exchange with lead acetate improved the contrast between the ionic domains and the hydrophilic phase but likely increased the ionic domain size. Nevertheless, the size distribution in Nafion ion exchanged with lead was found to be 2.8 ± 1.7 nm whereas SPES displayed a broad range of domain sizes (1-6 nm) with the formation of co-continuous structures. The morphology of Nafion and SPES were found to be maintained on hydration, however counterions did change the x-ray scattering densities in the ionic group, resulting in the absence of the ionomer peak in Fe2+, Ca2+ and Na+. The addition of inorganic fillers did not change the electrochemical properties of the material. In fact, the permeability increased by 40% because of the formation of clusters larger than 5 nm diameter. Blend membranes synthesized using Nafion and SPES were not ideal either as the polymers phase separated, forming brittle membranes in the dry state.