How hydrophobic side chain design affects water cluster connectivity in model polymer electrolyte membranes: Linear versus Y-shaped side chains

Abstract

Dissipative Particle Dynamics is used to model water clustering in ion exchange membranes. We consider polymers with hydrophobic backbone alternatingly grafted with short hydrophilic ([C]) and hydrophobic linear (A6) or Y-shaped side chains (ie. Ap [As][As]; p(s) = 6(0), 4(1), 2(2), 0(3)). At 16% water volume fraction water cluster connectivity (DMC(W), water diffusivity, and inter water domain distance increase with length p and average number of bonds (<NbondA−C>) separating A and nearest C beads. Exchange of the pendent A and C beads for (A [C]A [A6])10 to form (A [A]A [A5C])10 decreases water cluster connectivity. This is because the lower number of bonds between nearest C beads for (A [C]A [A6])10 forces them to join the same water domain. This cooperative mechanism also explains deviations from the increase of DMC(W) with <NbondA−C> for similar ion exchange capacity (IEC). At high IEC (A [Ax+1C])10 architectures favour good connected pores which is surpassed by the H-phobic side chain (A [C]A [Ax])10 architectures at low IEC.