The effect of photolatent catalysts on the exchange kinetics of dual-wavelength 3D printable and photopatternable thiol-click vitrimers

Abstract

The use of photolatent transesterification catalysts provides a unique way to locally control dynamic bond exchange reactions in vitrimers. In the non-illuminated state, the polymer network behaves like a permanent crosslinked duromer. Upon light exposure, the catalysing species is formed on demand facilitating topology arrangements above the network's topological freezing temperature (T_v). For photopolymer networks, photoacid generators provide distinctive advantages as latent catalysts. They are well soluble, comprise a high temperature stability and release strong Brønsted acids upon light exposure, which are able to efficiently catalyse thermo-activated transesterifications. In addition, they are typically transparent in the visible light region, which enables the radical induced curing of photopolymers by light irradiation at 405 nm, without premature release of Brønsted acids. Herein, a library of photoacid generators is comprehensively studied for the design of photocurable vitrimers applicable for dual-wavelength 3D printing and photopatterning. Stress relaxation measurements revealed that the bond exchange rate is affected by the thermal stability of the released Brønsted acid, the size of the counter-anion and the quantum yield of the cleavage reaction. As the photolatent catalysts neither affect pot life nor cure kinetics of the photocurable resins, it is possible to fabricate 3D printed objects via vat photopolymerization. By carrying out the printing at two different wavelengths (405 and 365 nm), the catalyst is locally activated during the layer-by-layer build-up of the 3D structures. In the UV exposed areas, the dynamic network is able to undergo triple shape memory by using the glass transition temperature (T_g) and T_v, whilst in the non-exposed parts it performs like a simple T_g-based shape memory polymer. This enables a controlled locking of defined areas during the thermal programming step and offers exciting ways towards the additive manufacturing of functional devices. In addition, the local activation of the catalyzing species was exploited to inscribe positive-tone microstructures in the photopolymer networks by photolithography. In the exposed areas, the network was selectively depolymerized during the development step in organic solvents at room temperature. Without any opimization, structures with a feature size of around 500 μm could be obtained.