Interlaminar delamination is a prevalent and insidious damage mode limiting the mechanical integrity and lifetime of fiber-reinforced composites. Conventional resolution involves over-design, laborious inspection, and repair/replacement at cost to the economy and environment. Self-healing via in situ thermal remending of thermoplastic interlayers offers a promising solution. However, better understanding of the healing agent and related mechanisms is necessary to tailor healing performance. Here, we compare non-neutralized (copolymer) and metallic-ion neutralized (ionomer) poly(ethylene-co-methacrylic acid) (EMAA) thermoplastics for healing interlaminar fracture. We reveal (i) how EMAA chemistry affects the interfacial reactions driving healing and (ii) the influence of molten viscosity on repair efficiency. At fixed viscosity, higher methacrylic acid content, chain mobility, and lower neutralization positively influence healing, where lower melt viscosity at fixed temperature improves delamination recovery. Thus, this study deepens scientific understanding of key variables for healing interlaminar fracture with EMAA, providing new insight for the design of multifunctional composites.