Dynamic polymers for hot lithography

Abstract

The market of additive manufacturing technologies is growing continuously and expanding its application field over various industries. Its main advantage compared to other manufacturing technologies is the precise creation of complex structures without additional product-specific manufacturing tools. Additionally, this technology can process different materials, like ceramics, metals, and especially photopolymers, the most widespread material class in the additive manufacturing industry. However, a key challenge of photopolymers is finding the balance between desired material behavior and sustainability. One chance to close this gap is the use of dynamic polymers, which have the potential to combine the excellent mechanical properties of thermosets with the recyclability of thermoplastics. This approach requires innovative materials incorporating a dynamic character within their photopolymerizable structure, which can be triggered through an external stimulus, such as temperature or pH. Herein, several photopolymerizable systems using different stimuli are explored to induce changes in photopolymer structures at their end of life stage.In the first part of this thesis, pH change was exploited as a degradation trigger. Several approaches are known to make pH degradable polymers, specifically photopolymers. Herein, it was aimed to enlarge the spectrum by utilizing classical protecting group chemistry for step growth photopolymerization via photoacid generators, which generate acid on demand. An additional advantage of implementing protecting group chemistry into polymer structures is their pH triggerable cleavability after polymerization, enabling special features such as recyclability or post polymerization-modification of the polymer on demand. Therefore, a novel catalytic step growth polymerization mechanism utilizing photoacids for initiation was investigated, where the polymer structure was formed through the protecting group tetrahydropyran (THP), primarily used in organic chemistry to protect alcohols in basic environments. The deprotection can be performed by strong acidic treatment and elevated temperatures.Consequently, this investigation focuses not only on designing novel monomers for light-induced polymerization and on the characterization of a novel mechanism but also on finding a balance between monomer reactivity and polymer stability in the presence of activated photoacids. Herein, the light-induced polymerization mechanism could be confirmed. However, the established photoacids were suspected to diffuse in the cured polymer, causing trigger-free cleavage of the created structure. In the second part of this thesis, temperature was investigated as a potent trigger to influence the behavior of non-covalent crosslinks in photopolymers compared to linear and covalently crosslinked polymer structures. By copolymerizing commercial methacrylates with hybrid bonding monomers, which combine classical covalent reactivity with the supramolecular bonding motif 2 ureido-4[1H]-pyrimidinones (UPy), thermoset like behavior is expected at room temperature, while thermoplastic one at elevated temperatures. Herein, the reactivity and shrinkage during light induced copolymerization of such hybrid bonding monomers with common methacrylates and the resulting mechanical properties were investigated. After the curing process, the temperature triggered dynamic behavior of the UPy interaction was analyzed compared to linear and covalently crosslinked photopolymers. In a final step, the UPy-containing resin was processed in bulk with excellent shape accuracy and resolution in an additive manufacturing process at elevated temperatures, known as Hot Lithography. These investigations revealed that incorporating UPy into polymer structures enhances reactivity and reduces shrinkage during light-induced radical photopolymerization. Furthermore, the dynamic characteristics of the UPy-based polymer network could be induced through a thermal stimulus. This allowed the 3D printed object to be reprocessed, underscoring the versatile advantages of the dynamic features facilitated by hybrid bonding monomers.