(5ax) Photochemical and Thermally Adaptable Networks

Traditionally, thermosets are synthesized using irreversible covalent bonds, and as a consequence cannot be melted, molded, or dissolved. Here, we have explored a new class of materials, known as ?adaptable networks', which incorporate reversible covalent linkages throughout the network backbone to facilitate controllable bond rearrangement and material reshaping. These linkages are activated either photochemically or thermally as in, for example, the photo-induced addition-fragmentation of the mid-chain allyl sulfide functionality or the thermo-reversible cycloaddition of furan and maleimide, respectively. We have explored both post- and during-polymerization stress reduction utilizing the mid-chain allyl sulfide functionality within the network chemistry. The novel stress relieving properties of fully cured networks containing allyl sulfides within their backbone were demonstrated by the reshaping and even actuation of the material. Furthermore, we have studied the ability of the allyl sulfide functionality to undergo reversible addition-fragmentation chain transfer during photo-polymerization of thiol-ene monomers, demonstrating significant stress relaxation via bond rearrangement in the latter stages of photo-polymerization (i.e., post gel-point). Thermally reversible Diels-Alder networks allow an avenue for complete reverse gelation, and are thus fully recyclable and can be ?healed' upon material cracking or failure. We have employed rheometry and Fourier transform infrared (FTIR) spectroscopy to characterize the mechanical properties, reaction rate, and thermodynamic equilibrium over a broad temperature range. The gel-point temperature, as determined by rheometry using the Winter-Chambon criterion, corresponded to a gel-point conversion of 71%, as determined by FTIR, which is consistent with the Flory-Stockmayer equation. Furthermore, the material exhibited a low frequency relaxation, which compares well with the characteristic time-scale of bond rearrangement determined from the FTIR kinetic studies.