CO2-Curable Silicone Elastomers with Tunable Mechanical Properties Based on Amine-Functionalized PDMS

   Carbon dioxide (CO₂)-curable elastomers are attractive gas-responsive materials because their stiffness can be increased upon exposure to CO₂. We previously reported Polyethyleneimine/polydimethylsiloxane (PEI/PDMS) elastomers that undergo a large CO₂-induced transition from soft elastomers to rigid plastics. However, this design has two limitations: PEI does not react with CO₂ when the PEI-rich domains are large. Moreover,  the stiffness of the CO₂-cured domains is difficult to tune.

   Here, we use amine-functionalized polydimethylsiloxane containing 52 mol% amine units, denoted a52, as an alternative CO₂-curable component. Elastomers were prepared by crosslinking a52 with epoxy-terminated PDMS at different a52 contents. The resulting a52/PDMS elastomers formed a disordered microphase-separated structure consisting of a52-rich and PDMS-rich domains. FTIR measurements showed that CO₂ reacts with the amine groups to form mainly ammonium carbamate, together with a small amount of carbamic acid. TEM and SAXS revealed that the large-scale morphology remains almost unchanged after CO₂ exposure, whereas a new local structure with a spacing of 2.3 nm appears in the a52-rich domains.

   The PDMS-rich domains provide pathways for CO₂ transport, enabling CO₂ to reach and cure the a52-rich domains throughout the bulk material. After CO₂ exposure, the glass transition temperature of the a52-rich domains increased to around 40 to 50 °C, while the PDMS-rich domains remained soft. Consequently, the Young's modulus increased from several MPa to 800 MPa, depending on the a52 content. The modulus, strain at break, and toughness after CO₂ curing could be tuned by changing the amount of a52. These results show that amine-functionalized PDMS is a useful CO₂-curable component for silicone elastomers with tunable gas-triggered mechanical properties.