No abstract

Vitrimers are cross-linked networks through reversible dynamic covalent bonds. Usually, self-healing of vitrimers are realized by rearrangement of the networks under high temperature (>150 °C) and high pressure. The vitrimers that can be recovered under milder conditions have rarely been reported. In this work, interesting solvent-induced recyclable polyimine vitrimers derived from vanillin were first developed. Their recovery processes can be carried out through an unexpectedly fast and unusual gel–sol transition (less than 30 min) with the help of a few solvents. Thanks to this special ability, the polyimine vitrimers not only can be reprocessed by traditional hot-press and degradation, but also exhibit solvent-induced welding and self-healing abilities at room temperature. Moreover, aldehyde monomers can be recycled through chemical degradation without further separation in the mixture of recycled monomers, and the effects of aldehyde monomers on the properties of as-prepared polyimine vitrimers were first studied in detail.

A novel dielectric polysiloxane with low dielectric constants (<2.77) at frequencies ranging from 0.1 to 30.0 MHz was synthesized. The monomer was prepared from renewable eugenol using an extremely low amount of B­(C6F5)3 catalyst. After thermal curing, the polysiloxane displayed high thermostability with degradation onset and glass transition temperatures of 400 and 201 °C, respectively. The coefficient of thermal expansion was 64 ppm/°C, and it displayed a low water uptake (<0.15%) when immersed in deionized water at room temperature for 144 h. These properties are comparable to those of other high-performance synthetic polymers, and thus, eugenol could be considered as a sustainable feedstock to prepare high-performance polymers.

Silicon-based nonlinear metasurfaces were implemented only with third-order nonlinearity due to the crystal centrosymmetry and the efficiencies are considerably low, which hinders their practical applications with low-power lasers. Here, we propose to integrate a two-dimensional GaSe flake onto a silicon metasurface to assist high-efficiency second-order nonlinear processes, including second-harmonic generation (SHG) and sum-frequency generation (SFG). By resonantly pumping the integrated GaSe-metasurface, which supports a Fano resonance, the obtained SHG is about two-orders of magnitude 1 arXiv:1904.06027v2 [physics.optics] 22 Apr 2019 stronger than the third-harmonic generation from the bare silicon metasurface. In addition, thanks to the resonant field enhancement and GaSe's strong second-order nonlinearity, SHG of the integrated structure could be excited successfully with a lowpower continuous-wave laser, which makes it possible to further implement SFG. The high-efficiency second-order nonlinear processes assisted by two-dimensional materials present potentials to expand silicon metasurface's functionalities in nonlinear regime.