Ultrafast Photo-Crosslinking of Thiol–Norbornene Opaque Silicone Elastomer Nanocomposites in Air

Abstract

Photocured silicone elastomers have become increasingly popular in a variety of advanced materials applications including soft robotics, biomedical devices, microfabrication, functional coatings, and additive manufacturing. Oxygen inhibition, however, remains a major challenge for conventional photoradical curing, limiting applications and restricting processing. Here, we report a thiol−norbornene-based silicone system using a polydimethylsiloxane (PDMS) displaying terminal norbornene groups and a highly functionalized thiolated PDMS to achieve very fast curing speeds in an ambient atmosphere. Complete curing, without any unreacted oily residue on the surface of the silicone elastomer, was achieved in the absence of inert atmosphere protection. The impact of a formulation and photoinitiating system, including the use of visible-light initiators, on the kinetics and mechanical properties of the silicone networks was studied. Despite their opacity, nanocomposites incorporating fumed silica and graphene oxide (GO) retained fast cure rates (gelation times below 1 and 5 s for silica and GO composites, respectively) with tack-free surfaces. After thermal conversion, this afforded composites with conductivities above 0.1 S/m. Finally, combination with a conventional room-temperature vulcanization system enabled the formulation of effective dual-cure composites. The fast crosslinking thiol−norbornene silicones reported are attractive for a wide range of applications where ambient curing is required or where processing requires ultrafast reaction rates.

Mechanical Evaluation of Hydrogel–Elastomer Interfaces Generated through Thiol–Ene Coupling

“…Another interesting feature of the thiol–ene chemistry for the design of hybrid structures is the relative insensitivity of this chemistry to oxygen concentrations, therefore limiting variations in the apparent moduli that are observed in conventional radical polymerization-based hydrogels. Indeed, hydrophobic resins crosslinked via thiol–ene processes display little oxygen inhibition, 19 , 20 , 62 and thiol–ene coupling in aqueous buffers was found to remain high even without degassing at moderately low thiol concentrations. 63 The sensitivity of the ultimate tensile stress on the chemistry of the interface even in the absence of marked changes in the apparent Young’s modulus remains unclear, but we suggest that it could arise from changes in the ultimate strains at break observed, possibly reflecting slight changes in the network formation and associated energy dissipation or potentially highlighting the partial network interpenetration at the interfaces.…”

“…Similarly, a number of thiol–ene-based elastomers have been developed, making use of the rapid crosslinking of the associated residues in air. − Hence, an increasing number of reports proposed the crosslinking of polyurethanes, polycarbonates, and silicones using thiol–ene coupling and enabling their application in 3D printing platforms such as filament extrusion and stereolithography as well as for microfabrication. ,,− Taking advantage of the off-stoichiometry of thiol–ene-based resins also enables the control of bonding of microfluidic platforms and integration of microarrays, allowing the mechanical stimulation of cell cultures . In addition, thiol–ene and thiol–yne reactive polymers and elastomers allow us to confer biofunctionalization to traditional rigid degradable polymers such as poly­(ε-caprolactone) (PCL) and poly­(lactic acid).…”

Photocrosslinkable natural polymers in tissue engineering

“…Thiols react with various ene functional groups such as alkene, vinyl ether, and acrylate. Among them, however, norbornene moiety is commercially valuable because it is most reactive and progresses quickly (Hoyle and Bowman, 2010;Blasco et al, 2017;Nguyen et al, 2021). Norbornene is incorporated into natural polymers through amide reactions (Devaraj et al, 2008).…”

Photocatalyst-Incorporated Cross-Linked Porous Polymer Networks

“…Covalent cross-linking aims to form a linkage between independent polymer chains. Some synthetic examples on covalent cross-linking are (i) carbon–carbon bond formation, − (ii) carbon–heteroatom bond formation, − and (iii) various other approaches, such as S–S bond and tertiary amine-halogen quaternization . It is important to note that some covalent cross-linking can possess dynamic reversible nature, as in the case of S–S bonds.…”