Ionic Clustering and Nonlinear Flow Response in Sulfonated Diblock Copolymers: A Molecular Dynamics Study

This study investigates the structure and flow response of sulfonated polystyrene-poly(ethylene-r-propylene) (SPS-PEP) diblock ionomer melts using atomistic molecular dynamics simulations. SPS-PEP ionomers combine a hydrophilic, ion conducting SPS block with a hydrophobic PEP block, enabling tunable phase segregation and mechanical reinforcement relevant to energy, membrane, and elastomeric materials.  In their quiescent state these polymers often form far from equilibrium structure as ionic clusters are formed.The flow response of this system will be presented, uncovering the molecular origins of nonlinear viscoelasticity. Systems with 107 chains (Mₙ ≈ 15 000 g mol⁻¹; SPS ≈ 6 000 g mol⁻¹) and sulfonation fractions f = 0-0.95 with Na⁺ counterions were probed under extensional flow. Increasing sulfonation strengthens ionic associations, forming transient clusters that restrict chain motion and raise the elastic modulus.  The extensional viscosity increases with sulfonation but decreases with strain rate, showing extensional thinning. Stress–strain curves exhibit strain hardening, with the hardening slope decreasing at high sulfonation as rigid ionic domains limit chain stretch. These results link ionic clustering to the nonlinear flow response of ionomeric copolymers.