No abstract

Photo-responsive thiol-epoxy click networks with spatially controllable solubility and surface wettability were prepared and characterized in detail.

Epoxy-anhydride vitrimers are covalent adaptable networks, which undergo associative bond exchange reactions at elevated temperature. Their service temperature is influenced by the glass transition temperature (Tg) as well as the topology freezing transition temperature (Tv), at which the covalent bond exchange reactions become significantly fast. The present work highlights the design of high-Tg epoxy-anhydride vitrimers that comprise an efficient stress relaxation at elevated temperature. Networks are prepared by thermally curing aminoglycidyl monomers with glutaric anhydride in different stoichiometric ratios. The tertiary amine groups present in the structure of the aminoglycidyl derivatives not only accelerate the curing reaction but also catalyse the transesterification reaction above Tv, as shown in stress relaxation measurements. The topology rearrangements render the networks recyclable, which is demonstrated by reprocessing a grinded powder of the cured materials in a hot press. The epoxy-anhydride vitrimers are characterised by a high Tg (up to 140 °C) and an adequate storage modulus at 25 °C (~2.5 GPa), which makes them interesting candidates for structural applications operating at high service temperature.

The present work highlights a new approach to write, erase, and rewrite micropatterns into the same region of covalent adaptable polymer networks. Thermal curing of an epoxy‐terminated o‐nitrobenzyl ester (o‐NBE) derivative with hexahydrophthalic anhydride in the presence of 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene yields a dynamic covalent network, whose solubility is locally controlled by irradiation with ultraviolet (UV) light. The photolysis of the o‐NBE chromophores enables a well‐defined cleavage of the epoxy‐anhydride network, and the formation of soluble photolysis products is confirmed by sol‐gel analysis. The photo‐induced change in solubility is exploited to inscribe micropatterns by photolithographic techniques and after development in an organic solvent positive tone structures with a feature size of 20 µm are obtained. Due to the thermo‐activated exchange reactions of the hydroxyl ester links and the related macroscopic reflow, the polymer patterns are fully erased at temperatures well above the topological freezing transition of the vitrimer network. The regenerated film has a smooth surface topology and can be reused to inscribe new micropatterns via mask lithography.

Exploiting the salient features of o‐nitrobenzyl chemistry, this work aims at the preparation of polydimethylsiloxane networks that undergo well‐defined bond cleavage in response to either UV light or different pH values as external trigger. Polydimethylsiloxane (PDMS) oligomers with terminal anhydride groups are thermally crosslinked in the presence of a bi‐functional epoxy monomer containing a photolabile o‐nitrobenzyl ester (o‐NBE) group. The susceptibility of the ester groups toward hydrolytic cleavage reactions is used for a pH‐triggered network degradation. Sol–gel analysis confirms that the degradation time can be easily controlled by the base concentration over different time scales varying between hours and weeks. Along with classical ester hydrolysis, photoinduced degradation of the elastomeric networks is carried out by photoisomerization and cleavage of the o‐NBE links upon exposure with UV light. Positive‐tone patterns with a structure size of 100 μm are obtained by photolithographic techniques confirming the spatial control of solubility properties. However, by increasing the exposure dose, recrosslinking of the networks is observed due to side reactions of the primary photocleavage products. As the reformed crosslinks are less sensitive to ester hydrolysis, the side reactions are employed to inscribe negative‐tone microstructures in the PDMS network. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 2319–2329