Measuring Hydration of Cationic Polymers using Azide as a Probe

Open Access
Author:
Lopez-Hallman, Raymond John
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 28, 2017
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
  • James Patrick Runt, Committee Member
  • Robert John Hickey III, Committee Member
Keywords:
  • Anion Exchange Membrane
  • Azide
  • Hydration
  • FTIR
  • Hydration Probe
  • Polymer Membrane
Abstract:
As clean alternative energy conversion devices, fuel cells using proton exchange membranes (PEM) are a promising technology. However, high-cost noble-metal catalysts (typically platinum) hinder their widespread commercialization. Anion exchange membrane (AEM) fuel cells employ cationic moieties tethered to polymer chains and do not require expensive noble-metal catalysts due to their high internal device pH. Thus, AEMs are considered a next generation of fuel cells for stationary and transportation applications. To date, AEMs have their own set of challenges that need to be understood, including inferior conductivity and stability compared to PEMs. Water interactions within the cationic polymer membranes remain unexplored and are important for understanding and improving the ion conductivity of these materials. The main goal of thesis was to measure the hydration of cationic polymers with Fourier transform infrared (FTIR) spectroscopy using the azide anion as a vibrational probe. Using azide (N3-) as a probe has the advantage of being a small molecule, is soluble in water, and has a strong IR absorption band. With the objective of understanding hydration in AEMs, this thesis work was subdivided into two parts. The first part of the work focused on understanding the interaction of the azide (as TBAN3) in different solvent mixture environments as a foundation for its use as a vibrational probe. Studies were conducted on four sets of mixtures: dimethyl sulfoxide (DMSO)-H2O, tetrahydrofuran (THF)-H2O, chloroform-methanol (MeOH), and DMSO-chloroform. The FTIR spectra for these four sets of mixtures were analyzed, fitted using the appropriate number of mixed Lorentzian/Gaussian functions, and used to interpret the polymer spectra. Overall, it was observed that hydrogen bonding played a more dominant role for the peak position of the azide asymmetric stretch band, rather than the dielectric constant of the solvents and mixtures. The azide asymmetric stretch band blueshifted to a higher frequency contingent on each solvent’s ability to form hydrogen bonds with the azide. Based on these findings, polar protic solvents, such as H2O and MeOH, which are strong hydrogen bond donors, were able to interact strongly with the azide, which is a hydrogen bond acceptor. Also, the polar aprotic solvents, such as DMSO and THF, interacted weakly with the azide. Interestingly, chloroform showed an intermediate interaction because of its ability to form a bifurcated hydrogen bond with the azide. The second part of the work focused on studying a series of quaternary ammonium containing poly(2,6-dimethyl-1,4-phenylene oxide) (QA-PPO) cationic polymers, as a model of an AEM experimental system, to measure their hydration behavior. Three comb-shaped QA-PPO polymers were synthesized with a pendant n-alkyl side chain of six, ten and sixteen carbons; respectively, benzyldimethylhexyl ammonium, benzyldimethyldecyl ammonium, and benzyldimethylcetyl ammonium; or C6, C10, and C16, in short. A control polymer was also synthesized with the same degree of bromination (0.4 bromomethyl groups per PPO repeat unit) but without a pendant alkyl side chain, referred to as benzyltrimethyl ammonium (BTMA). Thin films of the QA-PPO polymer samples were saturated with sodium azide (NaN3) and measured in-situ by attenuated total reflectance (ATR) FTIR, during hydration from 0 to 90% relative humidity (RH). All RH data was calculated to their respected hydration number (λ), taking into consideration their ionic exchange capacity (IEC). All recorded FTIR spectra of the azide asymmetric stretch bands for the different QA-PPOs were presented, with the frequency of the azide band expressed as a function of λ. QA-PPO BTMA, C6, C10, and C16 had different interactions with the azide and sodium azide species as evidenced by the position and intensity of their characteristic FTIR bands. For BTMA the wavenumber of the azide group was at ~2000 cm-1 and 2026 cm-1 at 0% RH (λ=0) and 90% RH (λ=4.3), respectively. For C16, the azide absorbance band was observed at approximately 2000 cm-1 and 2020 cm-1 at 0% RH and 90% RH, respectively. The frequency of the azide band for the other comb-shaped polymers (C6 and C10) fell in-between the two parameters discussed above. Overall, these results indicate that the interaction of the QA-PPOs with water becomes stronger as the alkyl pendant becomes shorter; as evidenced by the observation that BTMA had more bulk-like water values at each λ (or % RH), and possibly indicative of higher water mobility within BTMA than within the comb-shaped polymers. When the fitted spectra data for BTMA and C16 (representing the two limits) at their respective RH was presented, the results better illustrated the behavior of the QA-PPOs. With the dry samples, the bands for the “free” azides (at 2000 cm-1) were most prominent; but with increasing λ, the bands for the “free” azides and the azides interacting with the QA groups (at ~2007 cm-1) decreased while the azides interacting with water increased. Upon comparison between BTMA and C16, it was noted that for C16 a higher hydration level (>25% RH) was required to eliminate the “free” azide band. It is assumed that the presence of alkyl side chains in C16 made it more difficult for the mobility of the water molecules within the membrane, and to promptly access all “free” azides in the system. In conclusion, the azide anion was successfully employed as a vibrational probe to measure the changes in water and polymer interactions and was effectively measured by ATR-FTIR. Further expansion of research in this area will lead to a better understanding of the ionic transportation and conductivity properties of cationic polymer membranes, such as the QA-PPOs. Understanding these properties is one of many steps needed to be taken in order to improve AEM technology and develop useful applications, such as energy and water purification systems.