Improving Sustainability of Silicone Materials Using Natural Products via Imine Chemistry

Silicones are commonly used to make materials that play an important role in our everyday lives. Their broad uses are attributed to the vast array of unique properties that these materials boast, including thermal stability, biocompatibility and gas permeability, to name a few, attributed to the unique structure and stability of the dimethylsiloxane bonds. Materials made with silicones are not without their faults, particularly as it pertains to their environmental impact. There are negative environmental implications of synthesizing and disposing of silicone elastomers; synthesis requires very high heat and results in the release of greenhouse gases, in addition to the use of expensive and/or toxic metal catalysts, such as platinum or tin. This is made worse by the fact that the resulting elastomers resist degradation in the environment, unlike linear silicone oil, due to the formation of permanent bonds resulting in robust, thermosetting materials. As such, the need to improve the end-of-life of silicone elastomers and develop methods to recycle the silicone oil to reduce environmental impact is critical for the future of silicone materials. The use of dynamic bonds has been a recent area of interest for promoting better circularity of materials. Dynamic bonds, both covalent and non-covalent, could allow for the formation of materials with stable but reversible crosslinks, allowing for facile reprocessing and degradation at end-of-life. Silicone-hydrogels are unique materials that have important applications in the medical field, including wound dressings and contact lenses. As the rates of degradation of silicones in crosslinked matrices are unclear, it was reasoned that, if incorporated into a protein hydrogel, degradable materials which exhibit silicone-like properties could be obtained. Classic methods used to crosslink proteins exploit amine reactions with aldehydes; for this reason, formaldehyde was chosen as the crosslinking agent in this system. Aqueous protein solutions of gelatin were combined with telechelic and pendent aminopropyl silicones in different concentrations, to yield gelatin-silicone hydrogels. Using this method, hydrogels composed of >90% hydrophilic material could be prepared while still retaining silicone-like properties. The mechanical properties of the gels were tuned by altering the crosslinker concentration and the ratio of gelatin:silicone to yield robust materials that, after dehydration, formed tough elastomers that resisted rehydration and exhibited low surface energies. Enzymatic degradation of the hydrated gelatin-silicone hydrogels was facile using bromelain, a protease derived from pineapple stems. The use of gelatin allowed the target silicone materials to be enzymatically degraded, yielding silicone oil bound to amino acids or peptides. The ability to make silicone materials that could be reprocessed to reform the original silicone oil for reuse was of interest. The potential to use vanillin, a phenolic aldehyde, to make vanillin-silicone elastomers via Schiff-base chemistry was investigated. Vanillin-silicone oils were first synthesized by a green, catalyst-free process to form oils that were stable under inert conditions. The resulting oils were used to produce a library of vanillin-silicone elastomers that varied in hardness, rate of cure and heat and chemical reprocessability. These variables were shown to be highly tunable depending on the composition of the vanillin-silicone oil ([NH2], % vanillin functionalization, and molecular weight of the silicone) and curing conditions (including humidity and heat). Although stable in neutral aqueous solutions, hydrolysis under acidic conditions was possible, however very slow. Transimination with butylamine was shown to be significantly faster and successful in degrading all elastomers, resulting in the ability to recover the starting silicone oil.

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