Visible and ultraviolet light responsive silicones and their formulations toward functional and circular cosmetics

While synthetic silicone elastomers are produced on a large scale and utilized for a broad spectrum of applications that cannot be replaced by carbon-based polymers, redesigning them to realize energy-saving solventless functionalization, property modification by exploiting stimuli in the living environment, and end-of-use facile chemical recycling reactions are critically lacking. In addition, silicone elastomers should ideally have an inherent closed-loop lifecycle or reversible controllability in their physical properties upon applying spatial and temporal external stimuli. To tackle this, here we developed a new class of star-shaped poly(dimethyl siloxane)s (PDMSs) with visible- and ultraviolet light-responsive styrylpyrene (StP) end groups and examined their applicability as functional and circular cosmetics. The storage and loss moduli (G’ and G”) of the star-shaped PDMS immediately increased upon irradiation with visible light (λ = 436 nm) (VIS), demonstrating that star-shaped PDMS networks (SPNs) were formed upon intermolecular dimerization between StP end groups. However, subsequent ultraviolet light (λ = 330 nm) (UVA) irradiation to neat SPNs resulted in negligible decreases in G’ and G”, suggesting inhibited network scission upon dimer dissociation. On the other hand, depending on the adjacent functional groups of the introduced StPs, the photoreversibility of a mixture formulated from star-shaped PDMS and a cosmetic oil was dramatically improved, and the mixture displayed repetitive and drastic increases and decreases in G’ and G” upon alternating VIS and UVA irradiation even at room temperature.