Molar mass distributions and viscosity behavior of perfluorinated sulfonic acid polyelectrolyte aqueous dispersions

Abstract

Perfluorinated sulfonic acid polyelectrolyte aqueous dispersions originating from similar polymer feed stocks and having similar compositions can have order‐of‐magnitude viscosity differences that are dependent on the manufacturing process. To better understand this phenomenon at the molecular level, a size exclusion chromatography method incorporating static light scattering detection was developed. The initial apparent mass distributions were broad and bimodal for all dispersions. A high‐molar‐mass shoulder was consistent with a previously postulated aggregate structure, and the evidence suggested that molecular aggregation accounted for viscosity variability. The apparent weight‐average molar masses ranged from 1.3 × 106 to 3.9 × 106 g mol−1. Upon the heating of the dispersions at or above 230 °C, the aggregate structure was broken down, and this resulted in similar low‐viscosity dispersions that had monomodal mass distributions. The weight‐average molar masses were reduced to approximately 2.5 × 105 g mol−1, and the polydispersities were approximately 1.7–1.8. Shear thinning with higher viscosities and apparent molar masses was rationalized with intrinsic viscosity and other measurements, which supported an anisotropic aggregate structure, with particles that could be significantly overlapped at nominal 11% concentrations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 421–428, 2005

Size-exclusion chromatography of perfluorosulfonated ionomers

“…In the case of Nafion with EW = 1200, Yeo [15] determined the cohesive energy density from swelling and calculated a solubility parameter ı 1 ∼ 9.68 (cal/cm 3 ) 1/2 for the tetrafluoroethylene segments and a second solubility parameter ı 2 ∼ 16.71 (cal/cm 3 ) 1/2 for the sulfonic acid functionalized comonomer units. The solvents selected by Lousenberg [10] (DMF ı = 12.1 and DMSO ı = 12.0) have solubility parameters between the two Nafion values, but closer to ı 1 . Both solvents have larger solubility parameters than polystyrene (ı = 9.1) which is the most common SEC packing for organic eluents.…”

“…Curtin and Lousenberg examined Nafion dispersions by SEC in DMF with light scattering detection [9], and Lousenberg subsequently examined the SEC behavior of Nafion using DMSO with surfactants and an amine as eluent modifiers [10]. A pronounced shoulder was observed in SEC chromatograms at early retention volumes in both DMF and DMSO eluents for dispersion samples that were diluted directly with SEC sample solvent.…”

Static light scattering and transmission microscopy study of dilute Nafion solutions

“…It might be the reason of the aggregations of Nafion in alcohol and water; few papers reported Nafion molecular weight measurements. Lousenberg [30], using a size exclusion chromatography (SEC) incorporating static light scattering (SLS) detection and viscosity measurements reported aggregation behavior of Nafion (EW=1000) molecules in aqueous solutions with a Nafion concentration of ~2 mg/ml. He reported broad apparent weight-average molar masses ranging from 1.3x10 6 to 3.9x10 6 g .…”

“…The Nafion concentrations in these dilute solutions were in the range of 0.2 ~ 1.0 mg/ml, which were lower than the Nafion concentration (2 mg/ml) used by Lousenberg [30] for a molecular weight distribution measurement using a SEC incorporating SLS detection. The δ=12.2 (cal/cm 3 ) 1/2 of DMF and δ=10.8 (cal/cm 3 ) 1/2 of DMAc are close to the δ 1 = 9.7 (cal/cm 3 ) 1/2 of Nafion perfluorocarbon backbones.…”

“…In the TEM micrographs, particle sizes distributions were observed in both solvents. Lousenberg data [30] showed the polydispersity of Nafion molecular weight was M w /M n = 1.7 ~ 1.8 with polymer molecular weight distributed from 2. Fig.…”