Architecture dependent water uptake in model polyelectrolyte membranes

Abstract

The effect of polymeric architecture on the water uptake in ionomer membranes is modelled by Dissipative Particle Dynamics. We consider four polymers (a-d) of similar ion exchange capacity that contain hydrophobic A and hydrophilic C beads. For architecture a (A[A]C[A][A2])5 and b (A[A]C[A3])5 the C beads are located along the backbone that contain respectively 3 (a) and 2 (b) hydrophobic side chains per repeat unit. For architectures c ((A[C]A4])5) and d ((A[C]A[A3])5) each C bead represents a short hydrophilic side chain. Water is modelled by W beads. For ~20 nm thick membranes the water uptake is followed. The equilibrium water volume fraction varies between ~0.075 (architecture a) and ~0.4 (architecture d), and increases with (1) decreasing amount of C-A bonds per repeat unit and (2) the average number of bonds between hydrophobic beads and the nearest C bead, <Nbondphob-phil>. The potential energy per bead at equilibrium decreases as function of <Nbondphob-phil>. Water uptake is anticipated to show an increasing trend with increase of <Nbondphob-phil> for various architecture families. Some supporting evidence from experiments in literature is discussed. These results are of relevance for the optimization of proton and hydroxide ion conducting pores in electrochemical devices.