Thermosets, thermoplastics, and elastomers are established polymers with fixed properties. To expand the properties of materials and fill performance gaps, materials engineers are investigating adaptable, dynamic networks and their responsiveness to stimuli. Christopher N. Bowman, University of Colorado, Boulder, USA, and colleagues have synthesized a bistable material that can change its phase persistently when irradiated with light. This behavior relies on the on/off switching of a simple catalyst embedded in the network. It could be useful for the precise adjustment of fluid or solid regions in a material.
A very helpful tool to control industrial and laboratory polymerization processes is thiol–thioester chemistry. By employing the thiol–thioester exchange reaction in combination with radical chemistry, polymer chemists can precisely adjust chain lengths and fragmentation patterns. The researchers have adopted this method to create dynamic, adaptive networks of a thiol–thioester-based polymer, which are either fluid or solid depending on additives and stimuli.
The team used a short molecule containing a thioester function in the center and allyl esters at both ends as one monomer. The comonomer was a commercial tetrafunctional thiol. After the addition of a weak base and a photoinitiator, the mixture polymerized into a heavily cross-linked polymeric network upon irradiation with light.
Despite its high degree of cross-linking, the material was not hard. The reason for this is an excess of residual thiol groups. Many thiol residues remain unreacted after polymerization and are involved in exchange reactions with the thioester-bound thiol groups, producing fluidity. However, this exchange reaction was only possible if the catalyst base was present and active.
If the catalyst is inactive, the material remains in a “frozen” state resembling that of typical cross-linked solids, according to the researchers. The catalyst was initially inactive because it was tethered to a photosensitive leaving group. Irradiation with light of higher energy than that used for polymerization unmasked the catalyst to promote the thiol–thioester exchange reaction. Accordingly, the material transformed from hard to bendable, from solid to fluid. And it remained so even after switching off the light, as the team pointed out.
The reverse was possible as well: Inactivation of the base turned the material from bendable to hard, from fluid to solid. For inactivation, the scientists adapted a very simple chemical principle: neutralization. Phenylacetic acid was present in the initial mixture, but was masked by a UV (ultraviolet)-sensitive leaving group. UV irradiation after the polymerization reaction released the acid, which neutralized the base, and the fluid structure transformed into a solid one.
Light irradiation enables scientists to control the location, energy, and duration of a transformation with extraordinary precision. To demonstrate this, the researchers created solid patterns in a fluid environment and vice versa just by employing a physical mask blending off the light in the desired regions.
The most intriguing feature of this system is its simplicity: A finely balanced catalytic reaction using well-known sulfur chemistry, an established polymerization procedure, protection group chemistry from the toolkit, simple pH control, and a light source all add up to inspiring advances in basic materials research.
- Bistable and photoswitchable states of matter,
Brady T. Worrell, Matthew K. McBride, Gayla B. Lyon, Lewis M. Cox, Chen Wang, Sudheendran Mavila, Chern-Hooi Lim, Hannah M. Coley, Charles B. Musgrave, Yifu Ding, Christopher N. Bowman,
Nat. Commun. 2018.