Preparation and characterization of silicone rubber cured via catalyst-free aza-Michael reaction

Abstract

This paper proposes for the first time a strategy for preparing silicone rubber cured by aza-Michael reaction. A novel compound, γ-piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP), was synthesized to produce high-molecular-weight poly[(piperazinylpropyl)methylsiloxane-co-dimethylsiloxane] (PyP-PDMS) by a base equilibration reaction with octamethylcyclotetrasiloxane (D₄). Then, silicone rubber was obtained with PyP-PDMS as the gum, oligo[(acryloxypropyl)methylsiloxane-co-dimethylsiloxane] (AP-PDMS) as the crosslinker, and silica as the filler. The crosslinking mechanism was validated by solid-state ¹³C NMR spectroscopy. The curing process was analyzed by cure-curves performed by rheometry. The effects of various factors, such as post-cure temperature and time, crosslinker and filler amounts, molecular weight, and piperazine group content, on the mechanical properties of the novel silicone rubber were investigated in detail. Vulcanizate with tensile and tear strengths of 11.43 MPa and 30.72 kN m⁻¹, respectively, was obtained. The cure was free of catalysts, and the cure temperature is not too high (120 °C). Our method yielded silicone rubber of high strength and stable dimension. The synthesized silicone rubber also exhibited favorable thermal stability, low temperature performance, and hydrophobic properties, as characterized by Thermogravimetric Analysis, Differential Scanning Calorimetry and static contact-angle analysis.