Investigating molecular arrangements at gas-liquid and liquid-substrate interfaces

Abstract

Improving the efficiency and lowering the cost of materials-based carbon capture will prove imperative for its widespread use. This thesis will cover two different materials, aminopolymers that are loaded onto mesoporous oxide supports (solid-supported amines) and liquid crystal materials. Solid-supported amines are already in use for direct air capture (DAC) applications due to the high surface area of the composites, the low volatility of the polymer, and the excellent cyclability of the system. As polymer segmental mobility is coupled with CO2 uptake and diffusion, understanding how that mobility is influenced by nanoconfinement will ultimately be critical to the development of more efficient DAC systems. Liquid crystals have been proposed as novel carbon capture solvents but work in this field is limited. The ordered liquid crystal phases have been shown to lower solubility of gases compared to the isotropic (liquid) phase, which could be promising for energy efficient gas sorption/desorption between phases at these low enthalpy phase transitions. However, solubility differences shown so far have been too small for practical application and there is not a clear understanding of how arrangements of molecules in ordered liquid crystal phases affects gas uptake. This thesis will work towards addressing knowledge gaps for both materials. The first part of my thesis focused on the development of a fluorescent probe based on tetrakis(4-hydroxyphenyl)ethylene (TPE) whose emission spectra is strongly dependent on the viscosity of its supporting medium. As viscosity increases the probe’s spectra show an increase in intensity (via aggregation induced emission) along with a shift to shorter wavelengths. We demonstrate that the probe is a reliable indicator of polymer glass transition and/or melting temperatures across a wide range of temperatures (-100˚C to +100˚C) and has suitable photo and cycling stability under specified experimental conditions. Additionally, tracking the shifting spectra at two distinct wavelengths can give additional relative mobility information at specified temperatures when compared to standard calorimetry measurements. The mobility of bulk PEI as a function of molecular weight and polymer architecture will be presented along with the mobility of PEI confined in mesoporous silica as a function of polymer loading and pore functionality. The potential of this probe to study mobility at various locations in the polymer (including polymer-substrate and polymer-gas interfaces) upon exposure to CO2, humidity, or other environmental factors will be discussed. The second part of my work investigates CO2 uptake in the liquid crystal 4’-(octyloxy)-4-biphenylcarbonitrile (8OCB). Solubility of CO2 in 8OCB was studied by temperature-programmed desorption (TPD) and showed very small changes in CO2 capacity between phases, ~0.003wt% in 1 bar CO2. This indicated that gas absorption into the bulk of the liquid crystal is not greatly affected by molecular arrangements of molecules in different liquid crystal phases. However, CO2 uptake determined by a gravimetric microbalance showed a larger sorption between liquid crystal phases (0.13 – 0.32wt%) that was dependent on surface to volume ratio of the sample. This was attributed to gas adsorption on the surface of the sample. The possible reasoning for this adsorption, based on arrangements of molecules at the gas-liquid interface, will be discussed. This adsorption has the potential to be useful in thin films of liquid crystal for greater CO2 uptake.