Anisotropy and Transport in Poly(arylene ether sulfone) Hydrophilic−Hydrophobic Block Copolymers

Abstract

Designing tailored block copolymers represents a viable strategy for building polymer membranes with fruitful combinations of properties, such as the high ionic or small molecule conductivity and high mechanical strength needed for applications such as fuel cells and reverse-osmosis water purification. Here we present a systematic study of water transport and morphological alignment in a class of poly(arylene ether sulfone) hydrophilic−hydrophobic multiblock copolymer membranes and compare these with Nafion 212. Multiaxis pulsed-field-gradient NMR yields diffusion anisotropy, the ratio of diffusion coefficients measured both in plane (D ∥) and through plane (D ⊥), as a function of water uptake and block lengths. As block mass increases, diffusion anisotropy exhibits an increasing dependence on water uptake, in contrast to Nafion 212, where diffusion is isotropic and displays no dependence on water uptake. 2H NMR spectroscopy on absorbed D2O further probes membrane alignment modes. Both types of measurements corroborate uniformly ordered planar structures oriented through the membrane plane in accordance with a lamellar morphology previously observed locally with microscopy. The combination of these two measurements also provides insights into average defect distributions.

Hot-Pressing Effects on Polymer Electrolyte Membrane Investigated by2H NMR Spectroscopy

“…Strong dependence of the Δν 0 value on the membrane water content (Figure 4) was consistent with the results of earlier reports 15,19 that showed Δν values at a certain orientation, rather than Δν 0 values. Here a single Δν 0 value extracted from a set of Δν values does not depend on magnetic field orientation.…”

“…For example, pulsed field gradient (PFG) methods have been used to detect self diffusions of water and to correlate them with proton conductivities. 3,12,15,19,20 Field-cycling NMR relaxometry was used to detect water dynamics in ionomer membranes.…”

“…Peak splitting data of 2 H NMR spectra 3,15,19,20,[22][23][24][25][26][27][28][29] have been used to investigate hydrophilic channel alignments, and they naturally detect the influence of water used as a probing molecule. Although similar information can be obtained with 1 H NMR spectra, 30 2 H NMR spectra are better since they do not have background signals.…”

“…Thus, to date, peak splitting of 2 H NMR spectra has been widely used to determine the anisotropic dynamics of probe molecules and/or anisotropic structures of the matrix. 3,15,19,20,[22][23][24][25][26][27][28][29] In this work, the average O-2 H director of 2 H 2 O in PEM was employed to extract the structural information from 2 H NMR data and applied to detect the structural change of Nafion induced by hot-pressing, which is one of the representative procedures for preparing MEAs for low temperature fuel cells. For the first time, the dependence of 2 H NMR data on the water content in PEM was systematically interpreted in terms of channel shape, size, and ordering.…”

“…Likewise if the water content variation changes the size but not the shape of the channels and if the Δν 0 value is related with the surface-to-volume ratio of hydrophilic channels, 15,19 the Δν 0 value becomes proportional to the inverse of a channel radius r (refer to the explanation of the relationship between Δν 0 and water content in Supporting Information for the explanation in detail). Here each straight segment of channels is regarded as an individual channel so that channel tortuosity is reflected in the Δν 0 value.…”