We propel photopolymerizable liquid crystalline (LC) shape memory materials from solely elastomeric performance to the thermomechanical performance of tough, yielding thermosets. LC elastomers are at the forefront of smart, stimuli-responsive materials development. To apply their properties to mechanically superior thermosets, we demonstrate main-chain incorporation of high quantities of preordered LC motifs into a densely crosslinked network via thiol-ene photopolymerization to achieve a new material class hybridizing the advantages of LC elastomers and liquid crystalline networks. A terminal alkene mesogen with a robust LC phase is combined with multiple trithiol comonomers and selected based on resulting polymer crystallinities (13-37%). In-depth crystallinity characterization is presented (dynamic scanning calorimetry, X-ray diffraction analysis). The bulk materials exhibit high strength, stiffness and pronounced yielding under stress with elongations around 200%. Their excellent thermomechanical properties were explained by phase separation observed in atomic force microscopy. Furthermore, we demonstrate shape memory of these materials with fast, near-perfect shape imprinting (99%) and recovery (97%) over at least 20 cycles, and their light-based 3D printing at high temperature.

The inter-layer strength of 3D-printed photopolymers is a crucial parameter for the mechanical performance of parts manufactured via vat photopolymerization (VPP). In VPP, uncured or partially cured resin remains present during layer formation, making diffusion processes a decisive factor in the development of interfacial strength and the resulting mechanical properties. Before light exposure, the resin can diffuse into the partially cured preceding layer and, depending on the resin level in the vat and the immersion depth of the component, it may also penetrate the component's exterior surfaces. In a recently introduced multi-material approach, a methacrylate formulation was combined with an allyl-thiol formulation. Reliable layer adhesion in such systems depends on chemical compatibility between the monomers and physical bonding mechanisms such as network interpenetration and chain entanglement, both of which are influenced by diffusion across the interface. This study investigates the dependence of diffusion width on immersion time and temperature. The sulfur content in one formulation enables quantification of the diffusion zone via EDX, complemented by nanoindentation. Results show that diffusion width is controlled by printing parameters and directly influences tensile properties. These findings provide valuable structure-property relationships for optimizing VPP processing conditions.

In modern stereolithography, photopolymer chemistries are rapidly diversifying while geometric constraints remain minimal, pushing the technique from prototyping to manufacturing. Consequently, mechanical performance is critical. Tough bulk networks are difficult because high crosslink density and network heterogeneity yield stiff, brittle parts, whereas very soft materials print poorly due to low crosslinking. Although high crosslink density enables fast, high-resolution printing, it narrows thermomechanical tunability. We address this trade-off with a cleavable, siloxane-containing crosslinker that lowers crosslink density on demand after printing. Parts are printed efficiently with a highly crosslinked network, then photodegraded to tailor properties from stiff and tough to PDMS-like softness. Cleavage is triggered by light: a UV-only photoacid generator, latent under the longer wavelengths used for 3D printing, catalytically cleaves siloxane bonds, removing siloxane-derived crosslinks. We evaluated formulations containing 10–50 double-bond percent (db%) siloxane crosslinker in an ethyl-hexyl methacrylate matrix via tensile testing and dynamic mechanical analysis before and after cleavage. Swelling and leaching experiments quantified the extent of siloxane degradation. Finally, we demonstrated high-precision printing with the optimal formulation at 50 db% crosslinker, validating a pathway to decouple printability from post-print thermomechanical performance.

Radical-Induced Cationic Frontal Polymerization (RICFP) was successfully adapted for rapid and energy-efficient curing of carbon fiber-reinforced composites using industrial filament winding. Two aerospace-grade epoxy systems based on BADGE and BFDGE were formulated following a broad screening of reactive diluents. To meet manufacturing requirements, the curing process was modified into a thermally supported frontal polymerization (TSFP) approach, wherein a self-sustaining curing front is triggered locally and maintained via continuous external heat input to compensate for thermal losses. Using this method, composites with fiber volume fractions exceeding 58% were cured in significantly less time and with reduced energy consumption compared to standard industrial oven processes. Mechanical characterization showed increased interlaminar shear strength, lower outgassing, and comparable axial tensile properties relative to a reference BADGE system. Other properties (transverse tensile, torsion, compression, and glass transition) were lower but may be improved through further resin optimization. Finally, process scalability and performance were demonstrated through the manufacture and testing of a composite overwrapped pressure vessel, which withstood internal pressures of up to 400 bar.

OBJECTIVE:To evaluate the influence of the nature of silane coupling agents on the consistency of dental composites at various temperatures. METHODS:Silanes SI 1-4 were synthesized in one single step. They were characterized by 1H and 13C NMR spectroscopy. SI 1-4, as well as 3-methacryloyloxypropyltrimethoxysilane (MPTS), 8-methacryloyloxyoctyltrimethoxysilane (MOTS) and n-dodecyltrimethoxysilane were then used to functionalize a barium aluminum borosilicate glass filler (d50 = 1.0 µm). Silanizations were carried out in cyclohexane in the presence of a catalytic amount of n-propylamine. Each silane was used in an equimolar amount. Composites containing 67 wt% of silanized fillers and packable composites exhibiting a similar consistency at room temperature were subsequently formulated. The consistency of the uncured composites was determined at various temperatures (23 °C, 30 °C, 50 °C and 60 °C) using a texture analyzer. The flexural strength and modulus of the cured composites were assessed according to ISO 4049. RESULTS:The structure of the silane was shown to strongly influence the consistency of composites. The spacer length between silyl and methacrylate groups, as well as the presence of a urea or a urethane moiety, were demonstrated to be key parameters. Heating of each composite resulted in a drop of the consistency. The decrease was however significantly stronger if coupling agents with long spacers were selected. Especially, the use of SI 3 provided a packable composite which exhibited a packable consistency at 30 °C and flowable consistency at 60 °C. Regarding mechanical properties, it was shown that the coupling agent must be able to copolymerize with the monomers of the organic matrix to obtain high flexural strength and modulus values. The silanization of glass fillers using silanes bearing a long spacer was shown to have an additional advantage: packable composites having a higher filler content, and consequently improved flexural modulus, can be formulated. SIGNIFICANCE:The use of filler particles functionalized with silanes containing long alkyl spacers between the silyl and methacrylate moiety is a promising strategy for the development of packable dental composites which exhibit good mechanical properties and a strong drop in consistency upon heating.

We describe herein the facile preparation and characterization of new photobase generators based on carboxylate-functionalized chromophores. They pair high wavelength absorption with the liberation of an extremely strong phosphazene superbase. We additionally demonstrate their outstanding capacity in initiating a Brønsted base mediated oxa-Michael addition through a systematic optimization study. To the best of our knowledge, this work marks the first report on the light triggered initiation of an oxa-Michael addition reaction between an alcohol and acrylamide via photobase generators.

In lithography-based additive manufacturing, step-growth polymerization is a highly desired mode as the resulting polymer networks are usually more homogenous and therefore tougher than ones obtained by free radical chain growth polymerization. Therefore, thiol-ene chemistry sees widespread use, however, the employed thiols are accompanied by strong odor, limited availability and limited storage stability of the formulation. Replacing the thiols with alcohols resolves these problems as a wide variety of odorless alcohols is available. The oxa-ene reaction presented here is a base-catalysed Michael-type reaction for which a highly active Lewis base catalyst is known. Our work shows the preparation of photocaged Lewis base catalysts for this oxa-Michael addition, its implementation into photochemistry and the accompanying new mechanism compared to the regular thermal catalysis. Additionally, the storage stability of such formulation was investigated at different temperatures. Finally, the developed system is applied in additive manufacturing using hot lithography approaches with both linear and non-linear absorption of light.

Multi-material 3D printing concerns the use of two or more 3D printable materials within a single printed part. The result is a composite that benefits from the combined properties of the individual 3D printed materials. Typically, a distinct differentiation between material properties can only be achieved using multiple feedstocks and advanced engineering solutions. In this work, we create multi-material 3D printed photopolymer parts from a single monomer mixture through simple adjustments in printing temperature and light intensity. We achieve this by employing a liquid crystalline (LC) monomer that forms a highly stable LC phase in conjunction with a trifunctional thiol crosslinker. A drastic change in mechanical and optical properties was achieved depending on the presence of an LC phase during polymerization. The proof of principle from bulk experiments could be translated fully into 3D printing, achieving pixel-to-pixel resolution of the material properties solely guided by changing the printing parameters temperature and light intensity. The versatility of produced multi-material composite parts is demonstrated in shape memory applications and methods for chemical data storage and encryption.

This work investigates the development of photopolymerization-induced phase separation (PhIPS) in methacrylate-based photopolymers. Bisphenol A ethoxylated dimethacrylate (BisEMA) was copolymerized with urethane-based diacrylate diluted in isobornyl acrylate (UI) under UV irradiation, using diphenyl-(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as a photoinitiator. Optical changes in the cured copolymers were monitored in real time with a custom setup. NIR-photorheometry, photo-differential scanning calorimetry, dynamic mechanical analysis, scanning electron microscopy, and atomic force microscopy in both topography and phase imaging modes were employed to characterize the PhIPS effect on network development and microstructure. The correlation between the microstructure of copolymers with varying BisEMA/UI ratios and their bulk properties was studied through tensile and dynstat impact tests. Results showed that up to 20% UI enhances the elongation at break of BisEMA without compromising strength. At higher monomer/oligomer ratios, phase separation led to a strength-ductility trade-off. The study also addresses the influence of AFM sample preparation on correlating phase imaging results to the local stiffness distribution.

Conventional photopolymers used in light-based additive manufacturing are typically brittle materials with thermoset characteristics. Here we introduce a one-step synthesis of hyperbranched polyethylene rubbers functionalized with pendant methacrylic groups and their application as tougheners of a model brittle photopolymer based on non-volatile styrene and maleimide derivatives. The rubber tougheners can be tailored to tune their compatibility with the matrix, influencing the morphology and the thermomechanical properties of the final printed resins. The resulting polymer structures were analysed by atomic force microscopy, revealing various degrees of phase separation related to the rubber molar mass and methacrylate functionalization. Further, the analysis of the prepared toughened materials revealed the ability of functionalized hyperbranched polyethylene rubbers to improve the mechanical properties significantly (doubled stress at break and improvement of strain at break by a factor of 103 compared to the matrix), while glass transition temperatures around 100 degrees C could be maintained. Notably, even tensile behaviour mimicking typical thermoplastic yield strain comparable to ABS was observed in one of the prepared materials. This monomer/rubber system appeared to be the most promising and was therefore selected for in-depth analysis of the curing process using photo-rheology and photo-DSC. Finally, this material was used for hot lithography and several highly detailed objects were prepared, demonstrating the good printability of this toughened material.

Over the last years, stereolithography has developed to be one of the most promising fabrication techniques in tissue engineering. Posing the possibility of fabricating patient-specific, porous implants, it became especially attractive for scaffold fabrication for the treatment of critical sized bone defects. State-of-the-art photopolymer systems mostly consist of potentially cytotoxic compounds, such as (meth)acrylates, that furthermore show insufficient degradation and lead to acidic degradation products that could induce adverse tissue reactions. Herein, we introduced trifunctional monomers comprising cleavable silyl ether groups for thiol-ene photopolymerization to enlarge the material platform for printed bone grafts. Polymer networks comprising a high number of silyl ether moieties typically tend to be mechanically weak and exhibit low Tg values, especially when combined with thioether bonds, which are a direct result of polymerization via thiol-ene click reaction. To push thermomechanical properties to a level where they are sufficient for bone grafting (Tg > 37 °C), we introduced rigid bridged alicyclic structures in the form of norbornane-derived motifs into the silyl ether monomers, resulting in a norbornene-containing double bond monomer and a norbornane-derived thiol monomer. Together with noncleavable comonomers, we were able to demonstrate a substantial increase in Tg up to 62 °C, which is well above the values reported until now for similar thiol-ene networks. Furthermore, in this study, we demonstrated high photoreactivity for some of the monomers and also successfully performed proof-of-concept printing using a DLP setup. Besides excellent thermomechanical behavior, the mechanical strength of the silyl ether-based polymer network was shown to be outstanding. Cleavability of the silyl ethers was displayed with a quasi-linear degradation rate of 6.5% per month with moderate swelling. Additionally, the degradation product of the silyl ether-based network was isolated and shown to exhibit no relevant cytotoxicity to mouse fibroblast cells.

Photoinitiators are crucial components in the polymerization processes of lightcurable materials, with-TPO (diphenyl(2,4,6-trimethylbenzoyl) phosphinoxide) being a widely used example. Given the limited number of alternative photoinitiators suitable for the wavelength range in which TPO is employed, this study will investigate the potential of new variants of monoacylphosphine oxide (MAPO) photoinitiators. For this purpose, a series of structures were synthesized in which the phosphorus was modified with different substituents. Subsequently, these structures were analyzed with regard to their absorptive behavior and reactivity. Three derivatives, featuring different alkyl substituents, were selected for detailed investigation. In this detailed study, the addition of emerging phosphorus radicals to double bonds and quantum yields were also considered. It was demonstrated that introducing sterically demanding tert-butyl groups in 2,4,6-trimethylbenzoyl-phenyl( tert-butyl) phosphine oxide (2) and 2,4, 6-trimethylbenzoyl-di-tert-butylphosphine oxide (3) results in absorptions shifted to longer wavelengths. Additionally, using 2,4,6-trimethylbenzoyl- cyclohexylphenylphosphine oxide (1), it was shown that replacing the phenyl ring of TPO with a cyclohexyl ring does not significantly alter the absorption behavior, but that alkyl substituents reduce the reactivity of the corresponding P-centered radicals.

We introduce trioxanes as a new motif for on-demand photolysis of 3D printed polymer networks via an orthogonal light source using photoacid generators (PAGs). The acid-induced degradation of otherwise very stable trioxanes in combination with the versatility of generating trioxane-containing monomers makes the motif particularly interesting. Herein, we demonstrate the use of the motif as a trifunctional ene-crosslinker in combination with a trifunctional alicyclic thiol. Thereby, a latent PAG (UVI 6976, absorption <= 400 nm) is included into the photopolymerizable formulation and its latency during photocuring at longer wavelengths (>= 400 nm) is proven. Upon photopolymerization, the PAG-containing material exhibits equal (thermo)mechanical performance to a reference network excluding PAG. Upon UV light irradiation, degradation of trixoanes is proven via NMR, DSC and solubility tests. Finally, facile printing of the trioxane-containing formulation has been shown.

Photoinitiators (PIs) represent the key molecules within a photopolymerizable resin, due to their ability to generate the initiating species. However, the majority of state-of-the-art PIs comprise aromatic chromophores, known to produce cytotoxic photoproducts, whose migration out of the cured resin poses both environmental and human health threats. Herein, we present a set of multifunctional, aliphatic free radical photoinitiators based on the alpha-ketoester moiety, which exhibit low cytotoxicity even after irradiation. By systematically increasing the number of PI moieties, purely aliphatic molecules comprised of up to four radical-generating units have been synthesized. High miscibility in both organic and water-based formulations, combined with excellent photoreactivity and no discoloration upon irradiation with broadband (320-500 nm) and LED (385 nm) light sources, are demonstrated. The developed alpha-ketoester derivatives outperform the benchmark Norrish type II benzophenone/amine system and can be used for advanced applications, including UV-nanoimprint lithography as well as additive manufacturing technologies (DLP 3D printing).

Photolabile hydrogels have gained tremendous interest for a wide range of applications in materials and life sciences. Usually, photodegradability is introduced via chromophores and labile bonds, making such materials intrinsically light sensitive. In recent years, disulfide bonds have emerged as an innovative alternative, as they can be selectively cleaved in the presence of (photo)generated radicals. However, such materials suffer from limited network stability and high swelling as a result of thiol-disulfide metathesis reactions. Herein, we present two strategies to counteract such phenomena by network stabilization either via physical or chemical incorporation of (un)modified gelatin macromers to norbornene-modified poly(vinyl alcohol) networks. Photolabile behavior was introduced by a simple disulfide-containing dithiol cross-linker. Tunable material properties were investigated by means of in situ photorheology, in vitro swelling, and degradation experiments. Finally, we demonstrate an innovative method for localized disulfide cleavage via two-photon micropatterning.

Biodegradable thermoplastic polyurethanes (TPUs) are promising materials for vascular grafts, due to their excellent mechanical properties and the possibility to incorporate degradable moieties. Especially degradability is important in the field of tissue engineering (TE) or regenerative medicine, because the synthetic scaffold should be replaced by human tissue after a certain time. We synthesized thermoplastic polythiourethanes (TPTUs) based on degradable chain extenders and tested their mechanical, thermal and degradation characteristics. As soft-block we used polytetrahydrofuran (pTHF) and as hard-block the aliphatic hexamethylenediisocyanate (HMDI) in combination with a variety of chain extenders. The focus was set on the boronic acid-based dithiol 2,2 '-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane] (BDB). As dithiol-based reference 1,2-bis(2-mercaptoethoxy)ethane (BMEE) was used and bis(2-hydroxyethyl)-terephthalate (BHET) as well-known ester-based reference chain extender. By using BDB as degradable chain extender we could accelerate the degradation rate in terms of mass erosion at 37 and 90 degrees C compared to BHET-based TPUs. However, a strong increase in molecular weight was observed under degradation conditions. Further investigation by ATR-FTIR revealed a release of benzene-1,4-diboronic acid (BDBA) in the polymer. This leads to the formation of a hydroxy-terminated polyol, which then undergoes an intramolecular functional group metamorphosis with the thiourethane group to form a crosslinked polymer. BMEE-based TPTUs in contrast show excellent mechanical properties, even better than BHET-TPUs but a slightly lower mass erosion at 37 and 90 degrees C.

The copolymerization behaviour of four epoxide-containing ene-functionalized monomers with ethene using palladium alpha-diimine catalyst was investigated. The linker separating oxirane and double bond played a crucial role in the process. High molar mass copolymers of ethene rich in glycidyl 10-undecenoate were obtained without a significant loss of catalyst activity. Allyl glycidyl ether was able to form copolymers with high comonomer incorporation as well, however, the copolymer molar mass and catalyst activity decreased severely. Therefore, we tuned the reaction conditions in order to achieve a better compromise between comonomer incorporation and product molar mass. Adjustments of reaction temperature, ethene pressure and used solvent were shown to impact the copolymer properties. Furthermore, the prepared copolymers were used in subsequent post-polymerization modifications. We have demonstrated that despite the steric hinderance of the polymer chain, pendent epoxide rings can be reacted with acids, O, C, N and Snucleophiles quantitatively to prepare hyperbranched molecules with a diverse range of functional groups. Finally, the prepared epoxide-functionalized hyperbranched polyethylene was tested as rubber toughening agent of brittle photopolymers revealing its potential for hot lithography 3D printing.

Conventional photopolymers used in light-based additive manufacturing are typically brittle materials with thermoset characteristics. Here we introduce a one-step synthesis of hyperbranched polyethylene rubbers functionalized with pendant methacrylic groups and their application as tougheners of a model brittle photopolymer based on non-volatile styrene and maleimide derivatives. The rubber tougheners can be tailored to tune their compatibility with the matrix, influencing the morphology and the thermomechanical properties of the final printed resins. The resulting polymer structures were analysed by atomic force microscopy, revealing various degrees of phase separation related to the rubber molar mass and methacrylate functionalization. Further, the analysis of the prepared toughened materials revealed the ability of functionalized hyperbranched polyethylene rubbers to improve the mechanical properties significantly, while glass transition temperatures around 100 °C could be maintained. Notably, even tensile behaviour mimicking typical thermoplastic yield strain comparable to ABS was observed in one of the prepared materials. This monomer/rubber system appeared to be the most promising and was therefore selected for in-depth analysis of the curing process using photo-rheology and photo-DSC. Finally, this material was used for hot lithography and several highly detailed objects were prepared, demonstrating the good printability of this toughened material.