LIBS as a novel tool for the determination of the imidization degree of polyimides.

Due to their outstanding chemical and physical properties, polyimides are widely used in industrial applications. The degree of imidization of polyimides significantly influences their properties, making it an important factor in tailoring the material for specific applications. Imidization refers to the process of converting a precursor poly(amic acid) by removing water, and it is essential to analyze this process in detail to tune the final structure and properties of the material. Conventional techniques for this task include Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), or differential scanning calorimetry (DSC), but they lack the possibility of spatially and/or depth-resolved analysis or do not enable in-line monitoring capabilities. To overcome these limitations, we propose laser-induced breakdown spectroscopy (LIBS) as a powerful tool for the monitoring of the imidization reaction. To establish a measurement method, a total of 130 in-house prepared, self-synthesized samples were thermally cured to exhibit varying imidization degrees. IR spectroscopy served as a reference technique during method development, and a novel formula for calculating the degree of imidization, based on the C2 and H signal trends, was introduced. The calculated imidization degrees of model thin films based on LIBS were in good accordance with the IR reference method although minor differences between the two methods were expected due to varying information depth and the size of the sampled area. Additionally, the robustness of the procedure was demonstrated by depth profiling of a stacked model polymer, spiking with commercially available additives and, ultimately, by analyzing industry-relevant polymer samples.

Solvent-Free Ultrasonic Dispersion of Nanofillers in Epoxy Matrix.

Dispersion of carbon nanotubes and carbon nanofibers is a crucial processing step in the production of polymer-based nanocomposites and poses a great challenge due to the tendency of nanofillers to agglomerate. One of the most effective methods for dispersion is the use of a three-roll mill, which is a well-established method and results in agglomerates below 5 µm. Nevertheless, this process is time-consuming and thus a limiting factor for industrial applications. Our aim was to establish an easy and efficient ultrasonic dispersion process, characterize the dispersion parameters, and compare both methods, ultrasonication and the three-roll mill. We applied rheological tests and analyzed the agglomerate sizes by an image fit of the microscopy images. All these analyses combined deliver a valuable set of information about the dispersion's quality and, therefore, allows the improvement and further adaptation of the dispersion process.

Microstructural study of epoxy-based thermosets prepared by "classical" and cationic frontal polymerization.

A microstructural study of bisphenol-A diglycidyl ether (BADGE), prepared via both "classical" and novel photo- and thermally-induced cationic frontal polymerization, can help to understand the relationships between the microstructure of epoxides and their material properties, as well as the propagation of frontal polymerization waves. Microstructural PALS characteristics, such as the ortho-positronium lifetime (τ o-Ps), lifetime distribution, and void fraction, were investigated in relation to the extension of H bonds obtained from ATR/FTIR and the bulk density. The thermal profiles of differently-induced RICFP revealed that photo-triggered propagation is twice as fast as thermally-induced RICFP, with a comparable maximal reaction temperature (∼283 °C) and heat conductivity. Both RICFP-based samples, induced by UV light and heat, showed a lower τ o-Ps, narrower lifetime distributions, and a reduced void fraction, in comparison to the "classical" cured anhydride-based epoxy sample. These may be the main factors which result in better material properties. In addition, both their radial and angular profiles of free volume fraction confirmed experimentally the rotational movement of the propagating frontal waves and their influence on the microstructural inhomogeneities, and the final material properties.

UV-Induced Cationic Ring-Opening Polymerization of 2-Oxazolines for Hot Lithography.

The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.

Acylstannanes: Cleavable and Highly Reactive Photoinitiators for Radical Photopolymerization at Wavelengths above 500†nm with Excellent Photobleaching Behavior.

Within this work, a novel acylstannane-based photoinitiator (PI) is presented. Tetrakis(2,4,6-trimethylbenzoyl)stannane (1) displays outstanding properties compared to state-of-the-art acylgermane-based initiators. Most importantly, the initiator shows absorption up to 550 nm, which allows higher penetration depths, especially in highly filled photopolymers. Besides that, 1 shows extremely high photoinitiating activity towards (meth)acrylic double bonds, as well as very fast photobleaching. Furthermore, unlike many organotin compounds, 1 shows surprisingly low cytotoxicity.

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.