Photoresponsive polymeric actuator cross-linked by an 8-armed polyhedral oligomeric silsesquioxane

Abstract A photoresponsive hydrogel consisting of stilbene dye (Sti; 4,4′-disubstituted stilbene), an 8-armed core molecule (POSS; polyhedral oligomeric silsesquioxane having eight amino groups), and bifunctional poly(ethylene glycol) (PEG) showed large and fast deformation derived from its highly symmetrical structure in aqueous media. The polymeric hydrogel with an 8-armed POSS (POSS-Sti; cross-linked polymer obtained from polycondensation reaction between POSS and PEG-modified Sti) showed a large deformation upon photostimulation (UV-A (λ = 350 nm) and UV-C (λ = 280 nm)). The bending deformation can be repeated at least ten times. On the other hand, the control hydrogel without Sti dye (POSS-PEG; polymer prepared from the polycondensation reaction between POSS and PEG) did not show any deformation upon photoirradiation. However, the TPEG-Sti hydrogel (Sti cross-linked with a 4-armed core molecule (TPEG; tetra-armed poly(ethylene glycol))) showed a smaller deformation compared to the POSS-Sti hydrogel with the 8-armed core molecule. The larger deformation of the POSS-Sti hydrogel was discussed in detail. The photoisomerization of the Sti units drives swelling/shrinking and the change in the Young’s modulus of the POSS-Sti hydrogel upon UV irradiation, where a stacked assembly of the Sti dye molecules acts as a cross-linking point. The photoisomerization of the Sti dye results in the association/dissociation of the Sti assembly, and the increase/decrease in the cross-linking density dramatically alters the volume of the POSS-Sti hydrogel, causing the efficient assembly of the Sti units because of their highly symmetric structure and the high density of the Sti dye derived from the 8-armed POSS core structure.