Microscale manipulation of bond exchange reactions in photocurable vitrimers with a covalently attachable photoacid generator

Abstract

Vitrimers are polymer networks with covalent bonds that undergo reversible exchange reactions and rearrange their topology in response to an external stimulus. The temperature-dependent change in viscoelastic properties is conveniently adjusted by selected catalysts. In these thermo-activated systems, the lack in spatial control can be overcome by using photolatent catalysts. Herein, we advance this concept to locally manipulate bond exchange reactions on a single digit microscale level. For this, we synthetize a linkable non-ionic photoacid generator, which is covalently attached to a thiol-click photopolymer. UV induced deprotection of the photoacid yields a strong immobilized sulfonic acid species, which is able to efficiently catalyze transesterification reactions. Covalent attachment of the formed acid prevents migration/leaching processes and enables a precise tuning of material properties. As proof of concept, positive toned microstructures with a resolution of 5 μm are inscribed in thin films using direct two-photon absorption laser writing and subsequent depolymerization. In addition, the possibility to locally reprogram bulk material properties is demonstrated by performing a post-modification reaction with ethylene glycol and carboxylic acids. The Young's modulus is varied between 3.3 MPa and 11.9 MPa giving rise to the versatility of the newly introduced catalysts for creating light processable and transformable materials.

Fig. 1. (a) Components used in the preparation of the thiol–ene formulations. (b) Preparation of dynamic photopolymers by visible light induced radical curing (405 nm) of the thiol–ene monomers and immobilization of the photoacid generator (PAG-Vi). (c) Selective activation of the immobilized PAG-Vi by exposure with (i) two photon absorption laser light at 780 nm or (ii) 365 nm LED light.

Fig. 2. (a) Synthesis route for PAG-Vi and PAG-Me; (b) UV-vis absorption spectra of synthesized PAGs; (c) ¹H NMR spectra of PAG-Vi dissolved in deuterated acetonitrile, prior to and after UV light (365 nm) irradiation; (d) photocleavage mechanism of naphtalimide-based PAGs; (e) normalized signal integral of the residue water peak obtained by ¹H NMR spectroscopy in solution upon irradiation (365 nm), followed over time; (f) change of UV-vis absorption spectra upon photopolymerization (405 nm) and subsequent PAG activation (365 nm) of a formulation containing 5 mol% of PAG-Vi.

Fig. 3. Stress relaxation curves of a UV-light activated thiol–ene photopolymer with (a) 5 mol% PAG-Vi or (b) 5 mol% PAG-Me measured at temperatures in the range 90–130 °C in comparison to the relaxation behavior of a non-activated (= only cured at 405 nm) and a reference sample (containing no PAG) at 100 °C; (c) Arrhenius plot of UV-activated formulations containing 5 mol% PAG-Vi or PAG-Me derived from measured relaxation times; (d) scheme of the thermo-activated transesterification in dynamic polymer networks.

Fig. 4. (a) Chromatograms of synthetized PAGs, and extracts from cured formulations; (b) mass-spectra of peak corresponding to PAGs (21 min retention time) in GC analysis; (c) reaction scheme of matrix depolymerization upon treatment with EG; analysis of a film with 5 mol% PAG-Vi patterned via photomask (365 nm) and subsequent development in EG: (d) 3D reconstruction of surface topography scans, (e) surface topography scans with marked profile pathway, (f) related surface profile; analysis of a film with 5 mol% PAG-Vi patterned via direct TPA laser writing (intensity: 20 mW, scanning speed: 500 μm s⁻¹) and subsequent development in EG: (g) 3D reconstruction of surface topography scans; (h) surface topography scans with marked profile pathway; (i) related surface profile.

Fig. 5. (a) Reaction schemes of an additional crosslinking of the polymer network upon treatment with adipic acid and a decrease of the crosslinking degree upon treatment with EG; comparison of the (b) Young's modulus and (c) elongation at rupture of activated (cured at 405 nm and exposed at 365 nm) and non-activated (cured at 405 nm) thiol–ene photopolymers with 5 mol% PAG-Vi in comparison to a reference network containing no PAG after post-curing, and treatment with either EG or adipic acid.