Synthesis of a Liquid Lignin-Based Methacrylate Resin and Its Application in 3D Printing without Any Reactive Diluents

3D printing of bio-based and renewable polymers such as lignin has gained research attention during the last few decades. We report on the synthesis and characterization of a liquid lignin-based photopolymer and its application in additive manufacturing (AM). Wheat straw soda lignin is liquified in an oxyalkylation reaction with propylene oxide under alkaline conditions and modified with methacryloyl chloride to obtain a lignin-based methacrylate resin. Ninety percent of the functional hydroxyl groups are grafted during the synthesis. The photopolymerization efficiency was evaluated by real-time-NIR-photorheology experiments with two different photoinitiators, leading to double bond conversions (DBC) of ≥80%. 3D-printing experiments of the methacrylated lignin were performed with the hot lithography technology. For the first time, a light-curable lignin derivative with a lignin content of over 30% was successfully 3D printed via vat photopolymerization without any reactive diluents, which is a significant improvement over current state-of-the-art solutions. This outstanding result is a motivating proof of concept and a promising starting point for the in-depth evaluation of bio-based precursors as an alternative to nonrenewable derivatives for 3D printing.

Holographic characterization of diffraction grating modulation in photopolymers

In general, the holographic grating refractive index profiles in photopolymer materials are not identical to the exposing pattern. During exposure, high harmonics of the fundamental refractive index period are generated within the layer volume. A set of equations to calculate the amplitudes of the higher harmonics of refractive index induced in the grating is introduced. Then, an algorithm involving the use of the 3D nonlocal photopolymerization-driven diffusion model is presented and applied to calculate the resulting grating diffraction efficiencies. The experimental observation that the grating diffraction efficiency cannot reach the theoretical maximum value (η_max=100%) and that, in the case of over-modulation, the minimum value (η_min=0%) is also never achieved, are explained theoretically. The predictions of the simulations are also fit to experimental data for an acrylamide/polyvinyl alcohol photopolymer material with good agreement being achieved.

Real Time-NIR/MIR-Photorheology: A Versatile Tool for the in Situ Characterization of Photopolymerization Reactions

In photopolymerization reactions, mostly multifunctional monomers are employed, as they ensure fast reaction times and good final mechanical properties of the cured materials. Drawing conclusions about the influence of the components and curing conditions on the mechanical properties of the subsequently formed insoluble networks is challenging. Therefore, an in situ observation of chemical and mechanical characteristics during the photopolymerization reaction is desired. By coupling of an infrared spectrometer with a photorheometer, a broad spectrum of different photopolymerizable formulations can be analyzed during the curing reaction. The rheological information (i.e., time to gelation, final modulus, shrinkage force) can be derived from a parallel plate rheometer equipped with a UV- and IR-translucent window (glass for NIR and CaF₂ window for MIR). Chemical information (i.e., conversion at the gel point and final conversion) is gained by monitoring the decrease of the corresponding IR-peak for the reactive monomer unit (e.g., C═C double bond peak for (meth)acrylates, H-S thiol and C═C double bond peak in thiol-ene systems, C-O epoxy peak for epoxy resins). Depending on the relative concentration of reactive functional groups in the sample volume and the intensity of the IR signal, the conversion can be monitored in the near-infrared region (e.g., acrylate double bonds, epoxy groups) or the MIR region (e.g., thiol signal). Moreover, an integrated Peltier element and external heating hood enable the characterization of photopolymerization reactions at elevated temperatures, which also widens the window of application to resins that are waxy or solid at ambient conditions. By switching from water to heavy water, the chemical conversion during photopolymerization of hydrogel precursor formulations can also be examined. Moreover, this device could also represent an analytical tool for a variety of thermally and redox initiated systems.

Holographic grating stability: influence of 4,4'-azobis (4-cyanopentanoic acid) on various spatial frequencies

This paper presents the results obtained when holographic gratings were stored with a spatial frequency of 954 and 2663 lines/mm in transmission geometry and 4600 lines/mm in reflection geometry in a polyvinyl alcohol/acrylamide-based material. Photopolymers are materials that give good results at low frequencies, but their diffraction efficiency (DE) decreases at high frequencies. A chain transfer agent, 4,4'-azobis (4-cyanopentanoic acid) (ACPA) was incorporated in the material composition to improve spatial resolution. Furthermore, a curing process was applied to the stored gratings in order to maintain the DE stable over time. The DE and shrinkage for symmetric holographic transmission and reflection gratings were measured to evaluate their quality and quantify the improvement produced by ACPA.

Surface Modification of ZrO2 Nanoparticles as Functional Component in Optical Nanocomposite Devices

We have recently shown the successful synthesis of zirconia nanoparticles that can be optimized for use in volume phase holography by a post-functionalization surface treatment. Here, we present further investigations on the surface modification treatment with the aim of providing tools to tailor the nanoparticle compatibility to the photocurable organic matrix. Highly crystalline ZrO2 nanoparticles with a mean diameter of 5nm are synthesized in multigram yield through a one-pot solvothermal reaction of zirconium (IV) n-propoxide in benzyl alcohol. It is shown that the yield of the ZrO2 nanoparticles and stability of the nanoparticle dispersions are strongly dependent on the synthesis temperature. Post-synthetic surface modification of ZrO2 nanoparticles using several aliphatic ligands with different surface binding groups such as carboxylate (-COO−), amine (−NH2), phosphate (−PO4) and methoxysilane (−SiOCH3) was performed in order to compare the binding ability of these functional groups to the nanoparticle surface and therefore provide a new rational approach for nanoparticle stabilization with organic ligands.

Influence of CdSe/ZnS quantum dots in the polymerization process and in the grating recording in acrylate materials

The initiation step of the polymerization of acrylate materials is first studied in detail by UV-visible spectroscopy, showing the involvement of each species of the three-component photosensitizer. Then, the implementation of a combined holographic and physicochemical investigation approach is used to determine the influence of photoluminescent CdSe/ZnS quantum dots (QDs) in the photopolymerization and grating recording process in composites containing those QD nanoparticles. The fluorescence microscopy evidences the dynamic distribution profile of QDs due to their diffusion from the irradiated zones to the interface between the bright and the dark zones and, finally, their accumulation in nonirradiated zones. At the same time, the infrared spectroscopy shows that the presence of QDs provides a noticeable decrease of the polymerization rate, which favors the diffusion of the monomer and QDs. These two phenomena contribute to the enhancement of the refractive index modulation depth.