Conformation of Ionomers in Uniaxial Elongation Deformation

Incorporating a small number of bonded ions into a polymer matrix to form polyampholyte ionomers extends the relaxation time and results in a wide range of nonlinear rheological behaviors. In this work, nonequilibrium molecular dynamics simulations are used to examine the extensional behavior of ionomer melts, emphasizing the interplay between chain length, ionic clustering, and deformation rate. Ionomer chains with varying degrees of polymerization (20 ≤ N ≤ 100) show characteristic strain-hardening behavior, evidenced by a notable deviation from linear viscoelastic predictions in the transient extensional viscosity. For intermediate chain lengths (N = 40 and 60) a distinct two-step plateau in the stress growth coefficient appears at slow deformation rates, indicating that the rearrangements between transient ionic contacts and the subsequent network reformation occur in consecutive stages. At high extension rates, rapid bond dissociation gives rise to stress overshoots, whereas slower rates of deformation allow local rearrangements in ionomers that strengthen ionic interactions, leading to the formation of new ionic bonds. Analysis of chain orientation reveals that, at low deformation rates, shorter chains quickly relax and fail to achieve substantial alignment, while longer chains maintain their orientation once stretched. Meanwhile, tracking end-to-end distances and ionic clusters indicates that partial breakage and reformation of ionic bonds sustain repeated cycles of stretching, recoiling, and tumbling. Although macroscopic metrics such as transient extensional viscosity and average chain orientation appear to reach steady states, individual chains’ conformation shows cyclic fluctuations throughout the extension process. This study clarifies how chain length and ionic associations govern the response of polyampholyte ionomers in extensional flow.