Controlled Oxidation of a Tetrafunctional Thiol to Obtain a Poly (Thiol- disulfide) Oligomer and Its Use as Self-healing Agent for Epoxy/Thiol-ene Photocurable Systems
In this work the pentaerythritol tetrakis (3-mercaptopropionate) was partially oxidized to obtain an oligomer with both thiol and disulfide functional groups in its structure, by using iodobenzene diacetate (IBDA) as oxidant. This oligomer was used to introduce disulfide bonds into the crosslinked network of a polyether-polythioether matrix obtained by photopolymerizing an epoxy/thiol-ene system. The presence of the disulfide bonds induced self-healing properties bonds in the obtained co-network. The oligomer was a joint component of the thiol-ene system along with a tetraallyl-functionalized ditertiary amine curing agent and a radical photoinitiator. The thiol groups of the oligomer reacted with the double bonds of the curing agent to form polythioethers, while the tertiary amine groups of the curing agent induced the anionic ring opening polymerization of the oxirane groups of an epoxy resin to produce polyethers. The kinetics of photopolymerization of the epoxy resin diglycidylether of bisphenol A (DGEBA), revealed that conversions of the epoxy groups as high as 80 % were achieved in only fifteen minutes by increasing the concentration of the thiol-ene system in the formulation. The disulfide bonds introduced in the co-network by using the thiol-disulfide oligomer, allowed the repairing of the test specimens in as little as 10 minutes when the specimens were heated at 80 °C or for 500 minutes at room temperature. The analysis of mechanical properties using dynamic mechanical analysis (DMA) showed that the healed specimens, retained their mechanical properties depending on the amount of polythioethers present in the co-network.