When, How, and Why Curing Conditions Change the Mechanical Properties of Silica-Free Silicone Elastomers

Commercial RTV-2 silicones, such as Sylgard 184 or RTV 141, are widely utilized in various fast-growing academic fields (e.g., microfluidics, soft robotics, biomedical devices), thanks to their ease of processing, biocompatibility, good mechanical properties, and transparency. This study primarily investigates the structural origins of the significant enhancement in elastomer stiffness and large strain response observed in these silica-free but MDVQ resin-filled systems, as the curing temperature increases. We explicitly reveal the complex structural heterogeneity present within these elastomer networks. First, the evolution of mechanical and swelling behavior of commercial RTV-2 silicones, as a function of curing temperature and time, was monitored. High-resolution solid-state 29Si NMR analysis revealed that hydrosilylation between cross-linkers and MDVQ resins proceeds slowly at room temperature and requires a prolonged curing time for complete cross-linking, which contrasts with the typically prescribed 2 days curing time at 23 °C. Double-quantum NMR results stressed that cross-link density evolves over 4 months at room temperature, while no difference is detected among specimens cured at different T ≥ 60 °C. Additionally, low-field solid-state NMR also indicated an increase in the rigid phase fraction of the silicone as the curing temperature rises, matching the trends of stiffness and swelling responses. Furthermore, small-angle X-ray scattering measurements exhibited a significant scattering peak, indicating progressive aggregation with increasing curing temperature, which we attributed to the macroscopic self-structuring of MDVQ resins. These findings demonstrate that variations in stiffness cannot be attributed to an enhanced cross-linking reaction in these RTV-2 systems but rather imply strengthening through nanostructuration at increasing curing temperature.