Recent Advances and Challenges in Long Wavelength Sensitive Cationic Photoinitiating Systems

With the advantages offered by cationic photopolymerization (CP) such as broad wavelength activation, tolerance to oxygen, low shrinkage and the possibility of "dark cure", it has attracted extensive attention in photoresist, deep curing and other fields in recent years. The applied photoinitiating systems (PIS) play a crucial role as they can affect the speed and type of the polymerization and properties of the materials formed. In the past few decades, much effort has been invested into developing cationic photoinitiating systems (CPISs) that can be activated at long wavelengths and overcome technical problems and challenges faced. In this article, the latest developments in the long wavelength sensitive CPIS under ultraviolet (UV)/visible light-emitting diodes (LED) lights are reviewed. The objective is, furthermore, to show differences as well as parallels between different PIS and future perspectives.

Recent Advances on Furan-Based Visible Light Photoinitiators of Polymerization

Photopolymerization is an active research field enabling to polymerize in greener conditions than that performed with traditional thermal polymerization. At present, a great deal of effort is devoted to developing visible light photoinitiating systems. Indeed, the traditional UV photoinitiating systems are currently the focus of numerous safety concerns so alternatives to UV light are being actively researched. However, visible light photons are less energetic than UV photons so the reactivity of the photoinitiating systems should be improved to address this issue. In this field, furane constitutes an interesting candidate for the design of photocatalysts of polymerization due to its low cost and its easy chemical modification. In this review, an overview concerning the design of furane-based photoinitiators is provided. Comparisons with reference systems are also established to demonstrate evidence of the interest of these photoinitiators in innovative structures.

Patterned Magnetofluids via Magnetic Printing and Photopolymerization for Multifunctional Flexible Electronic Sensors

Liquid conductor-based flexible sensors with high mechanical deformability and reliable electrical reversibility have aroused great interest in electronic skin, soft robotics, environmental monitoring, and other fields. Herein, we develop a novel strategy to fabricate liquid conductor-based flexible sensors by combining ionic liquid-based magnetofluids (IL-MFs), magnetic printing, and photopolymerization techniques. The as-prepared sensors exhibit excellent electromechanical properties, such as a wide detection range, low hysteresis, fast response time, good durability, etc. Moreover, the gauge factors (GFs) of the sensor could be easily adjusted by changing the modulators with different line widths or patterns, and the strain sensors can also be designed for anisotropic monitoring. Apart from serving as strain sensors, the magnetofluid-based flexible sensors can be used to detect external pressure, human activities, and changes in temperature, illumination, and magnetic field as well. This work provides a facile strategy to fabricate liquid conductor-based multifunctional sensors. Such a magnetofluid-based sensor has a great promising future.

Radical Photopolymerization Using 1,4-Dihydropyrrolo[3,2-b]pyrrole Derivatives Prepared via One-Pot Synthesis

Radical photopolymerization has attracted significant attention for manufacturing products with complicated structures. Herein, the synthesized 1,4-bis(4-bromophenyl)-2,5-bis(4-nitrophenyl)-1,4-dihydropyrrole[3,2-b]pyrrole (PyBN) is found to show varying photoactivity upon irradiation at different wavelengths. PyBN affords two main absorption bands, and its maximum absorption peak is at 462 nm, attributing to its strong intramolecular charge transfer property based on the donor-acceptor structure. It efficiently photoinitiates the radical photopolymerization of different (meth)acrylate materials under 365 and 395 nm LED irradiation. The highest double bond conversion of 99.86% is achieved for these materials. Under 470 nm LED, PyBN does not show molecular structure change from photolysis results as a result of intramolecular charge transfer. Therefore, PyBN shows wavelength-selective photoactivity with potential application in dual-wavelength volumetric additive manufacturing. A unique solid product is successfully fabricated using a 365 nm LED with co-irradiation of a 470 nm LED. Additionally, PyBN incorporating camphorquinone (CQ) as a two-component visible light photoinitiator system is investigated under 470 nm LED irradiation. As PyBN has a charge transfer activity at 470 nm, the combination with CQ exhibits a good synergistic interaction. Besides nitro-based PyBN, a methyl-based PyBC was prepared as a reference compound.

Cost-Effectively 3D-Printed Rigid and Versatile Interpenetrating Polymer Networks

Versatile acrylate–epoxy hybrid formulations are becoming widespread in photo/thermal dual-processing scenarios, especially in 3D printing applications. Usually, parts are printed in a stereolithography or digital light processing (DLP) 3D printer, after which a thermal treatment would bestow the final material with superior mechanical properties. We report the successful formulation of such a hybrid system, consisting of a commercial 3D printing acrylate resin modified by an epoxy–anhydride mixture. In the final polymeric network, we observed segregation of an epoxy-rich phase as nano-domains, similar to what was observed in a previous work. However, in the current work, we show the effectiveness of a coupling agent added to the formulation to mitigate this segregation for when such phase separation is undesired. The hybrid materials showed significant improvement of Young’s modulus over the neat acrylate. Once the flexible, partially-cured material was printed with a minimal number of layers, it could be molded into a complex form and thermally cured. Temporary shapes were readily programmable on this final material, with easy shape recovery under mild temperatures. Inspired by repairable 3D printed materials described recently, we manufactured a large object by printing its two halves, and then joined them covalently at the thermal cure stage with an apparently seamless union.

Ketone Number and Substitution Effect of Benzophenone Derivatives on the Free Radical Photopolymerization of Visible-Light Type-II Photoinitiators

Three novel visible-light absorbing benzophenone-based hydrogen acceptors (BPD-D, BPDM-D and BPDP-D) were designed on the basis of a donor-benzophenone-donor structural backbone. Mono or diketone units and double diphenylamine electron-donating groups in para-or meta-positions were introduced to comprehend the electronic and structural effects on free radical photopolymerization (FRPP). Such a structural change leads not only to a red-shift of the absorption maxima but strongly enhances their molar extinction coefficients compared to the commercial phototinitiators such as benzophenone (BP) and 4,4'-bis(diethylamino) benzophenone (EMK). In addition, excellent melting points and thermal decomposition temperatures were achieved for those novel compounds. Further, the photochemical reaction behavior was studied by cyclic voltammograms (CV), photolysis and electron spin resonance (ESR) spectroscopy. Finally, benzophenone derivatives in combination with an amine (TEA, triethylamine) as a co-initiator were prepared and initiated the FRPP of trimethylolpropane trimethacrylate (TMPTMA) using a UV lamp as a light source. When used in stoichiometric amounts, the BPDP-D/TEA had the best double bond conversion efficiency among all the compounds studied, and were even superior to the reference compounds of BP/TEA and EMK/TEA. The results and conclusions could provide the fundamental rules applicable for the structural design of benzophenone derivative-based photoinitiators.

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

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.

The NIR-sensitized cationic photopolymerization of oxetanes in combination with epoxide and acrylate monomers

NIR-sensitized photopolymerisation at 805 nm facilitates the cationic polymerization of oxetanes. This can additionally be combined with free-radical polymerization.

Synthesis and free radical photopolymerization of triphenylamine-based oxime ester photoinitiators

Four visible light triphenylamine-based oxime ester photoinitiators (TP-1–4) were synthesized successfully. Photochemical reaction, photoreactivity and 3D pattern experiments were also conducted.

Upconversion nanoparticle-assisted cationic and radical/cationic hybrid photopolymerization using sulfonium salts

Several sulfonium salts as unimolecular photoinitiators can efficiently initiate UCNP-assisted direct cationic and radial/cationic photopolymerization under 980 nm NIR light irradiation.

Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts

The photoinduced polymerization of monomers is currently an essential tool in various industries. The photopolymerization process plays an increasingly important role in biomedical applications. It is especially used in the production of dental composites. It also exhibits unique properties, such as a short time of polymerization of composites (up to a few seconds), low energy consumption, and spatial resolution (polymerization only in irradiated areas). This paper describes a short overview of the history and classification of different typical monomers and photoinitiating systems such as bimolecular photoinitiator system containing camphorquinone and aromatic amine, 1-phenyl-1,2-propanedione, phosphine derivatives, germanium derivatives, hexaarylbiimidazole derivatives, silane-based derivatives and thioxanthone derivatives used in the production of dental composites with their limitations and disadvantages. Moreover, this article represents the challenges faced when using the latest inventions in the field of dental materials, with a particular focus on photoinitiating systems based on iodonium salts. The beneficial properties of dental composites cured using initiation systems based on iodonium salts have been demonstrated.