Cationic photopolymerization of bisphenol A diglycidyl ether epoxy under 385 nm

Fused Carbazole–Coumarin–Ketone Dyes: High Performance and Photobleachable Photoinitiators in Free Radical Photopolymerization for Deep Photocuring under Visible LED Light Irradiation

In this work, three dyes based on fused carbazole–coumarin–ketone structures were designed and synthesized. These dyes were named CCK–Me, CCK–Ph, and CCK–Tol in accordance with their different substituents. Their excellent...

3D printing of functional microrobots

3D printing (also called "additive manufacturing" or "rapid prototyping") is able to translate computer-aided and designed virtual 3D models into 3D tangible constructs/objects through a layer-by-layer deposition approach. Since its introduction, 3D printing has aroused enormous interest among researchers and engineers to understand the fabrication process and composition-structure-property correlation of printed 3D objects and unleash its great potential for application in a variety of industrial sectors. Because of its unique technological advantages, 3D printing can definitely benefit the field of microrobotics and advance the design and development of functional microrobots in a customized manner. This review aims to present a generic overview of 3D printing for functional microrobots. The most applicable 3D printing techniques, with a focus on laser-based printing, are introduced for the 3D microfabrication of microrobots. 3D-printable materials for fabricating microrobots are reviewed in detail, including photopolymers, photo-crosslinkable hydrogels, and cell-laden hydrogels. The representative applications of 3D-printed microrobots with rational designs heretofore give evidence of how these printed microrobots are being exploited in the medical, environmental, and other relevant fields. A future outlook on the 3D printing of microrobots is also provided.

Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers

Ultraviolet (UV)-induced cationic frontal polymerization has emerged as a novel technique that allows rapid curing of various epoxy monomers upon UV irradiation within a few seconds. In the presence of a diaryliodonium salt photoinitiator together with a thermal radical initiator, the cationic ring opening polymerization of an epoxide monomer is auto-accelerated in the form of a self-propagating front upon UV irradiation. This hot propagating front generates the required enthalpy to sustain curing reaction throughout the resin formulation without further need for UV irradiation. This unique reaction pathway makes the cationic frontal polymerization a promising route towards the efficient curing of epoxy-based thermosetting resins and related composite structures. This review represents a comprehensive overview of the mechanism and progress of UV-induced cationic frontal polymerization of epoxy monomers that have been reported so far in literature. At the same time, this review covers important aspects on the frontal polymerization of various epoxide monomers involving the chemistry of the initiators, the effect of appropriate sensitizers, diluents and fillers.

Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

Substituted stilbene-based oxime esters used as highly reactive wavelength-dependent photoinitiators for LED photopolymerization

Different substituents on stilbene-based oxime esters play an important role in the relationship between their structure and properties.

A substituentpara-to-orthopositioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

Dibenzothiophene-based oxalate derivatives were synthesized as type I photoinitiators, and their photoinitiation properties depend on the substituentpara-to-orthopositioning effect of the oxalates.

UV-Triggered Self-Healing of a Single Robust SiO2 Microcapsule Based on Cationic Polymerization for Potential Application in Aerospace Coatings

UV-triggered self-healing of single microcapsules has been a good candidate to enhance the life of polymer-based aerospace coatings because of its rapid healing process and healing chemistry based on an accurate stoichiometric ratio. However, free radical photoinitiators used in single microcapsules commonly suffer from possible deactivation due to the presence of oxygen in the space environment. Moreover, entrapment of polymeric microcapsules into coatings often involves elevated temperature or a strong solvent, probably leading to swelling or degradation of polymer shell, and ultimately the loss of active healing species into the host matrix. We herein describe the first single robust SiO2 microcapsule self-healing system based on UV-triggered cationic polymerization for potential application in aerospace coatings. On the basis of the similarity of solubility parameters of the active healing species and the SiO2 precursor, the epoxy resin and cationic photoinitiator are successfully encapsulated into a single SiO2 microcapsule via a combined interfacial/in situ polymerization. The single SiO2 microcapsule shows solvent resistance and thermal stability, especially a strong resistance for thermal cycling in a simulated space environment. In addition, the up to 89% curing efficiency of the epoxy resin in 30 min, and the obvious filling of scratches in the epoxy matrix demonstrate the excellent UV-induced healing performance of SiO2 microcapsules, attributed to a high load of healing species within the capsule (up to 87 wt %) and healing chemistry based on an accurate stoichiometric ratio of the photoinitiator and epoxy resin at 9/100. More importantly, healing chemistry based on a UV-triggered cationic polymerization mechanism is not sensitive to oxygen, extremely facilitating future embedment of this single SiO2 microcapsule in spacecraft coatings to achieve self-healing in a space environment with abundant UV radiation and oxygen.

A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure

A strong drawback to overcome for photoinitiators of cationic polymerization or photoacids is their low photosensitivity for long wavelengths.

Recent Developments of Versatile Photoinitiating Systems for Cationic Ring Opening Polymerization Operating at Any Wavelengths and under Low Light Intensity Sources

Photoinitiators (PI) or photoinitiating systems (PIS) usable in light induced cationic polymerization (CP) and free radical promoted cationic polymerization (FRPCP) reactions (more specifically for cationic ring opening polymerization (ROP)) together with the involved mechanisms are briefly reviewed. The recent developments of novel two- and three-component PISs for CP and FRPCP upon exposure to low intensity blue to red lights is emphasized in details. Examples of such reactions under various experimental conditions are provided.

Photopolymerization upon LEDs: new photoinitiating systems and strategies

The use of LEDs as novel and efficient light sources for the photopolymerization of various monomers (acylate, epoxy), interpenetrating polymer networks and thiols–ene, leads to the development of novel photoinitiating systems adapted for the LED emission.