TY - BOOK

T1 - New approaches towards stimuli-responsive polymers

AU - Giebler, Michael

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - The aim of the present work was to develop new concepts towards "stimuli-responsive polymers". On the one hand, ortho-nitrobenzyl ester (o-NBE) chemistry was used to spatially control network properties by UV induced cleavage reactions. On the other hand, thermo-activated transesterifications were exploited to introduce dynamic properties in epoxy-based networks. In particular, switchable polydimethylsiloxane networks were prepared, which were degraded in a controlled manner upon either UV exposure or in alkaline media. For this purpose, polydimethylsiloxane (PDMS) oligomers with terminal anhydride groups were thermally crosslinked with a bifunctional epoxy monomer containing a photolabile o-NBE- group. The sensitivity of the ester groups to hydrolytic cleavage reactions was used for pH-triggered network degradation. In addition, the photosensitivity of the o-NBE-group was employed for light-induced cleavage reactions. This allowed comparisons between the two cleavage mechanisms and a defined degradation of the network as a function of pH value and light intensity. Furthermore, it could be shown that by selective (over)-exposure and secondary photoreactions (leading to re-crosslinking of the networks), a switch between positive- and negative-tone photoresists is possible. In a further study, o-NBE chromophores were introduced in epoxy-based thermosetting resins. Mechanical and degradation properties of the networks were adjusted by the applied anhydride hardeners. Thus, a wide range of glass transition temperature, chain mobility, hardness and contrast of positive-tone photoresists was established. In the second part of this thesis, new covalent adaptable networks (e.g. vitrimers) were developed and investigated. By combining the chemistry of o-NBE with thermo-activated and reversible transesterification reactions, it was possible to generate a rewritable vitrimeric network. Positive-tone structures were inscribed by photolithography using the photoinduced degradation of the network across the o-NBE groups. Subsequently, a macroscopic reflow and erasing of the polymer patterns were induced by thermo-activated exchange reactions of the hydroxyl ester links at temperatures well above the topological freezing transition of the vitrimer. The regenerated film had a smooth surface topology and could be reused to inscribe new micropatterns via photolithography. Parallel with the introduction of photo-responsive properties, further research was carried out to increase the glass transition temperature (Tg) of vitrimers for structural applications. In particular, epoxy-anhydride vitrimers with a Tg of up to 140 °C were prepared, which additionally exhibited efficient stress relaxation at elevated temperature. The networks were obtained by thermal curing of aminoglycidyl monomers with glutaric anhydride in various stoichiometric ratios. The tertiary amine groups in the structure of the aminoglycidyl derivatives enabled acceleration of the curing reaction and also catalyzed the transesterification reaction, which was presented in stress relaxation measurements. In addition, a remolding of milled samples was feasible demonstrating the recyclability of the high Tg vitrimer networks. In subsequent work, the catalytic activity of selected amines for the transesterification of hydroxyl ester moieties was studied in a thermally cured epoxy- anhydride system using 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate as epoxy monomer and glutaric anhydride as hardener. The influence of the catalyst on the network density, the network mobility and the rate for the transesterification reactions within the polymer matrix was studied. Depending on the amine catalyst, networks with a Tg ranging from 66 to 166 °C were obtained. Subsequent stress relaxation measurements demonstrated the activity of the compounds for catalyzing transesterificatio

AB - The aim of the present work was to develop new concepts towards "stimuli-responsive polymers". On the one hand, ortho-nitrobenzyl ester (o-NBE) chemistry was used to spatially control network properties by UV induced cleavage reactions. On the other hand, thermo-activated transesterifications were exploited to introduce dynamic properties in epoxy-based networks. In particular, switchable polydimethylsiloxane networks were prepared, which were degraded in a controlled manner upon either UV exposure or in alkaline media. For this purpose, polydimethylsiloxane (PDMS) oligomers with terminal anhydride groups were thermally crosslinked with a bifunctional epoxy monomer containing a photolabile o-NBE- group. The sensitivity of the ester groups to hydrolytic cleavage reactions was used for pH-triggered network degradation. In addition, the photosensitivity of the o-NBE-group was employed for light-induced cleavage reactions. This allowed comparisons between the two cleavage mechanisms and a defined degradation of the network as a function of pH value and light intensity. Furthermore, it could be shown that by selective (over)-exposure and secondary photoreactions (leading to re-crosslinking of the networks), a switch between positive- and negative-tone photoresists is possible. In a further study, o-NBE chromophores were introduced in epoxy-based thermosetting resins. Mechanical and degradation properties of the networks were adjusted by the applied anhydride hardeners. Thus, a wide range of glass transition temperature, chain mobility, hardness and contrast of positive-tone photoresists was established. In the second part of this thesis, new covalent adaptable networks (e.g. vitrimers) were developed and investigated. By combining the chemistry of o-NBE with thermo-activated and reversible transesterification reactions, it was possible to generate a rewritable vitrimeric network. Positive-tone structures were inscribed by photolithography using the photoinduced degradation of the network across the o-NBE groups. Subsequently, a macroscopic reflow and erasing of the polymer patterns were induced by thermo-activated exchange reactions of the hydroxyl ester links at temperatures well above the topological freezing transition of the vitrimer. The regenerated film had a smooth surface topology and could be reused to inscribe new micropatterns via photolithography. Parallel with the introduction of photo-responsive properties, further research was carried out to increase the glass transition temperature (Tg) of vitrimers for structural applications. In particular, epoxy-anhydride vitrimers with a Tg of up to 140 °C were prepared, which additionally exhibited efficient stress relaxation at elevated temperature. The networks were obtained by thermal curing of aminoglycidyl monomers with glutaric anhydride in various stoichiometric ratios. The tertiary amine groups in the structure of the aminoglycidyl derivatives enabled acceleration of the curing reaction and also catalyzed the transesterification reaction, which was presented in stress relaxation measurements. In addition, a remolding of milled samples was feasible demonstrating the recyclability of the high Tg vitrimer networks. In subsequent work, the catalytic activity of selected amines for the transesterification of hydroxyl ester moieties was studied in a thermally cured epoxy- anhydride system using 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate as epoxy monomer and glutaric anhydride as hardener. The influence of the catalyst on the network density, the network mobility and the rate for the transesterification reactions within the polymer matrix was studied. Depending on the amine catalyst, networks with a Tg ranging from 66 to 166 °C were obtained. Subsequent stress relaxation measurements demonstrated the activity of the compounds for catalyzing transesterificatio

KW - Epoxid-anhydrid Epoxidharz Vitrimer o-Nitrobenzylester photosensitiv wiederbeschreibbar CANs kovalent adaptierbare Netzwerke Photoresist

KW - Epoxy‐anhydride resin vitrimer o‐nitrobenzyl ester photopatternable photo‐responsive rewritable CANs covalent

KW - adaptable networks photoresists

M3 - Doctoral Thesis

ER -