TY - BOOK

T1 - Orthogonal Photoreactions for the Realization of Multifunctional Photopolymers

AU - Müller, Stefanie Monika

N1 - no embargo

PY - 2024

Y1 - 2024

N2 - Additive manufacturing and the quest for novel, smart and renewable materials for 3Dprinting and light-based applications has become a major focus in polymer science. Theaim of this thesis was the further development of 3D printing resins with regard to thesecharacteristics, as well as the development of a novel multifunctional resin system byimplementation of orthogonal photoreactions into the 3D printing process.In a first approach the introduction of network disparity through the use of two orthogonalphotoinitiators was investigated. In a thiol-acrylate resin with excess acrylate a photo base(PB) and a radical photoinitiator (PI) are used in combination with a visible light source(405 nm) and UV light (365 nm). Activating the PB at 405 nm forms crosslinks betweenthiol and acrylate units, consuming both groups equally. The step-growth manner ofthe polymerization produces a well defined network with a late gel-point and a low glasstransition temperature (Tg), as the uncured acrylate in the system acts as plasticizer. Furthercuring the sample with 365 nm activates the radical PI, the remaining acrylate is cured andthe Tg is increased significantly. Very similar material properties can be achieved by solelycuring the resin with UV light, activating both photoinitiators and proceeding in a mixedmodepolymerization consuming all monomers simultaneously. While a stepwise curing witha relatively large difference in thermo-mechanical properties could be demonstrated, thehypothesis of the molecular structure having a significant effect on the material propertiescould not be confirmed.In a second, more comprehensive study, a dual-cure, single-vat resin was developed, basedon radical polymerization of a thiol-methacrylate monomer system containing covalentlybound chalcone units as dimerizable crosslinkers. Thermo-mechanical properties can bespatially and temporally controlled via the λ-orthogonal [2+2] cycloaddition reaction of thechalconyl groups during printing or post-processing. Using defined doses of light (405 nm)after polymerization (450 nm), the Tg and elastic modulus can be altered in a continuousway, generating not only two but numerous distinct material properties. Shape memoryexperiments as well as multi-wavelength 3D printing was shown on macro- and micro-scaleto present the vast opportunities for this novel 3D-printable material. Further functionalgroups were investigated upon their reactivity upon irradiation with the most commonwavelengths creating a library of photo-crosslinkable moieties. Notably, the reactivity didnot always align with the recorded ultraviolet-visible absorption spectrum, confirming areactivity analysis crucial for all light-induced processes especially if orthogonality is desired.In all studies photo-DSC and FTIR kinetics were used as a tool to investigate and characterizecuring behavior. These methods were used to contribute to detailed investigations on anumber of novel photoinitiators and biobased molecules for advanced applications in futureadditive manufacturing were additionally evaluated. Thereby, two bio-based methacrylates,eugenyl methacrylate (EM) and vanillyl alcohol methacrylate (VAM) were investigatedupon their curing behavior along a range of temperatures to optimize processing conditionsin paper coating. Finding VAM to be the more reactive of the two bio-based alternatives,exhibiting a water contact angle (106°) comparable to existing coatings if PDMS-ECEMSis used as an additive (10 wt%). This makes vanillyl alcohol methacrylate a suitable sustainable alternatives for hydrophobic paper coatings. Additionally, Novel type I radicalphotoinitiators, based on tin or germanium, were investigated upon their curing behavior.They can be used in future applications as an alternative to state-of-the-art phosphorous PIswith reduced toxicity and pronounced reactivity in the energy-efficient, innocuous visiblelight region. Lastly, bio-based monomers with methacrylate, vinyl and alkyne functionalitieswere evaluated for their applicability in novel resins for 3D-printable biological scaffolds orcoatings.Finally, a literature review is given as a comprehensive overview on single-molecule(type I) radical photoinitiators. The focus is put on visible light activation, comprising notonly phosphorous but also silicon, germanium and tin compounds, as a way to more benignand energy-conscious curing.The three main topics of this thesis are photoinitiators and their characterization, bio-based monomers for radical photo-polymerization and molecules for the implementationinto dual-cure 3D printing resins for smart materials. They all contribute to a betterunderstanding of all constituents used, their reaction mechanism, curing behavior andinfluence on the final material properties, crucial for sophisticated material developmenttowards advanced 3D printing of smart materials.

AB - Additive manufacturing and the quest for novel, smart and renewable materials for 3Dprinting and light-based applications has become a major focus in polymer science. Theaim of this thesis was the further development of 3D printing resins with regard to thesecharacteristics, as well as the development of a novel multifunctional resin system byimplementation of orthogonal photoreactions into the 3D printing process.In a first approach the introduction of network disparity through the use of two orthogonalphotoinitiators was investigated. In a thiol-acrylate resin with excess acrylate a photo base(PB) and a radical photoinitiator (PI) are used in combination with a visible light source(405 nm) and UV light (365 nm). Activating the PB at 405 nm forms crosslinks betweenthiol and acrylate units, consuming both groups equally. The step-growth manner ofthe polymerization produces a well defined network with a late gel-point and a low glasstransition temperature (Tg), as the uncured acrylate in the system acts as plasticizer. Furthercuring the sample with 365 nm activates the radical PI, the remaining acrylate is cured andthe Tg is increased significantly. Very similar material properties can be achieved by solelycuring the resin with UV light, activating both photoinitiators and proceeding in a mixedmodepolymerization consuming all monomers simultaneously. While a stepwise curing witha relatively large difference in thermo-mechanical properties could be demonstrated, thehypothesis of the molecular structure having a significant effect on the material propertiescould not be confirmed.In a second, more comprehensive study, a dual-cure, single-vat resin was developed, basedon radical polymerization of a thiol-methacrylate monomer system containing covalentlybound chalcone units as dimerizable crosslinkers. Thermo-mechanical properties can bespatially and temporally controlled via the λ-orthogonal [2+2] cycloaddition reaction of thechalconyl groups during printing or post-processing. Using defined doses of light (405 nm)after polymerization (450 nm), the Tg and elastic modulus can be altered in a continuousway, generating not only two but numerous distinct material properties. Shape memoryexperiments as well as multi-wavelength 3D printing was shown on macro- and micro-scaleto present the vast opportunities for this novel 3D-printable material. Further functionalgroups were investigated upon their reactivity upon irradiation with the most commonwavelengths creating a library of photo-crosslinkable moieties. Notably, the reactivity didnot always align with the recorded ultraviolet-visible absorption spectrum, confirming areactivity analysis crucial for all light-induced processes especially if orthogonality is desired.In all studies photo-DSC and FTIR kinetics were used as a tool to investigate and characterizecuring behavior. These methods were used to contribute to detailed investigations on anumber of novel photoinitiators and biobased molecules for advanced applications in futureadditive manufacturing were additionally evaluated. Thereby, two bio-based methacrylates,eugenyl methacrylate (EM) and vanillyl alcohol methacrylate (VAM) were investigatedupon their curing behavior along a range of temperatures to optimize processing conditionsin paper coating. Finding VAM to be the more reactive of the two bio-based alternatives,exhibiting a water contact angle (106°) comparable to existing coatings if PDMS-ECEMSis used as an additive (10 wt%). This makes vanillyl alcohol methacrylate a suitable sustainable alternatives for hydrophobic paper coatings. Additionally, Novel type I radicalphotoinitiators, based on tin or germanium, were investigated upon their curing behavior.They can be used in future applications as an alternative to state-of-the-art phosphorous PIswith reduced toxicity and pronounced reactivity in the energy-efficient, innocuous visiblelight region. Lastly, bio-based monomers with methacrylate, vinyl and alkyne functionalitieswere evaluated for their applicability in novel resins for 3D-printable biological scaffolds orcoatings.Finally, a literature review is given as a comprehensive overview on single-molecule(type I) radical photoinitiators. The focus is put on visible light activation, comprising notonly phosphorous but also silicon, germanium and tin compounds, as a way to more benignand energy-conscious curing.The three main topics of this thesis are photoinitiators and their characterization, bio-based monomers for radical photo-polymerization and molecules for the implementationinto dual-cure 3D printing resins for smart materials. They all contribute to a betterunderstanding of all constituents used, their reaction mechanism, curing behavior andinfluence on the final material properties, crucial for sophisticated material developmenttowards advanced 3D printing of smart materials.

KW - Photochemie

KW - Thiol-ene Systeme

KW - Orthogonalität

KW - Shape Memory Effekt

KW - Reaktionskinetik

KW - photochemistry

KW - thiol-ene systems

KW - orthogonality

KW - shape memory

KW - reaction kinetics

U2 - 10.34901/mul.pub.2024.204

DO - 10.34901/mul.pub.2024.204

M3 - Doctoral Thesis

ER -