Interplay between Cross-Linker Nucleophilicity and Diffusion in Polystyrene Vitrimer Dynamics

Over the past decade, imine chemistry has emerged as a critical tool in the design of vitrimers – polymer networks featuring dynamic associative cross-links. Imine-based vitrimers offer significant utility for a variety of applications, ranging from recyclable thermosets to separations membranes. In this study, we examine how cross-linker nucleophilicity and diffusivity influence the static and dynamic properties of polystyrene (PS) vitrimers containing imine linkages. PS vitrimers were synthesized through imine condensation between a precursor polymer bearing pendent aldehyde groups and a series of small molecule diamine cross-linkers. The cross-linker set, comprising both aromatic and aliphatic diamines, spans a broad range of nucleophilicities (pKa = 3.0 to 10.5) and diffusion coefficients. For static properties, cross-linkers with larger molar volumes lowered the glass transition temperature (Tg), while the plateau modulus remained largely unaffected by the diamine structure. For dynamic properties, the temperature dependence of fast segmental motions is correlated with Tg. The rheological activation energy exhibited a linear relationship with both the pKa and diffusion activation energy, indicating that these factors jointly govern the temperature dependence of the slow relaxation regime. pKa, however, primarily dictated the rate of slow relaxation, as confirmed by the creep and SAOS measurements. The cross-link exchange attempt time showed no direct correlation with either pKa or diffusivity. Overall, this work demonstrates that cross-linker nucleophilicity and diffusion are key determinants of the structural and rheological behaviors of imine-based vitrimers.