(110e) Effects of Processing Conditions On Properties of Poly(arylene ether sulfone) Films for Polymer Electrolyte Membrane Applications

Nafion® is a widely used material for membranes in polyelectrolyte membrane fuel cells due to its high proton conductivity, material strength, and stability. While these characteristics make Nafion® attractive for this application, it is expensive and shows diminished properties at temperatures above 80oC. Operation of fuel cells at elevated temperatures minimizes gas crossover through the membrane and can increase current density by increasing the rate of oxidation of hydrogen. Recently, efforts have been focused on developing new materials for fuel cell membranes that offer better thermal stability than Nafion®. It is further desired that the proton conductivity of membranes composed of the material remains stable at low relative humidity. High water uptake in the membrane causes the membrane to swell, imparting mechanical stresses on the bipolar plates surrounding the membrane and limiting the fuel cell lifetime.

Various poly(arylene ether sulfone) candidates are being explored as a possible PEMFC membrane material. Earlier studies have shown solution-cast, vacuum dried PAES membranes can achieve comparable or better proton conductivity than Nafion®. It is recognized that drying temperature and rate of solution-cast films can play an important role in determining the final film morphologies, which in turn can affect film properties, including proton conductivity in the case of PAES membranes. This research is interested in studying the effect of polymer chain and casting solution composition, along with drying conditions, on final film properties, including proton conductivity and water uptake. Connections between drying conditions, morphology evolution, and film properties are also being investigated.

PAES membranes are cast from 15 to 30% w/v solutions of copolymer in either dimethyl acetamide (DMAC) or N-methyl pyrrolidone (NMP) onto a stainless steel substrate using an adjustable doctor blade. The former solvent is selective toward the sulfonated blocks while the latter is neutral. The initial casting thickness is set to achieve a final film thickness of 25-micrometers. The films are dried using the batch convection drying apparatus with adjustable air speed and air temperature controls. Films are subsequently dried in a vacuum oven to remove all residual solvent, then acidified with sulfuric acid. Proton conductivity is calculated by measuring the resistance across the films at 30oC. Water uptake was determined from the ratio of the weight change in the film following a 24 hour soak in room temperature deionized water to the initial dry weight.

In manipulating the drying conditions, the temperature and speed of the convection air are of particular interest. In addition to drying conditions, the benefit of block copolymers over random copolymers and the effect of block length on film properties have been studied. Further, solvent selectivity has been observed to play a role in determining proton conductivity. It has been found that increasing block length and casting with a selective solvent result in improved proton conductivity. Temperature has been found to be of little consequence in affecting film properties. While the proton conductivity is improved at high relative humidity, as compared to Nafion®, it has also been found that water uptake is high and proton conductivity decreases significantly as relative humidity is decreased. It is believed that the dependence of conductivity on water is due to poor phase separation. Protons are conducted through channels containing ion clusters. Poor connectivity between channels causes the membrane to rely on water to connect channels. Therefore, the better the phase separation, the better the membrane maintains its conductivity over a range of relative humidity values. TEM images are being used to reveal the morphologies formed and their orientation in the films to help better explain the observed phenomenon.

In an effort to improve the phase separation new fluorinated PAES polymers have been synthesized and are being studied for PEMFC applications. Fluorinating the non-sulfonated block is believed to impart greater phase separation by increasing incompatibility between blocks. Similar experiments to those described above are being carried out using the fluorinated PAES. To date, results show a decrease in proton conductivity and water uptake using the fluorinated copolymer. Different drying schemes are currently being explored to target the cause of the change in properties. TEM is also being used to identify any improvement or reduction in phase separation.