TY - BOOK

T1 - Design and characterization of elastomers with dynamic covalent bonds

AU - Kaiser, Simon Johannes

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - Conventional elastomers comprise covalent cross-links, which endows them with outstanding chemical and physical properties such as their exceptionally high elasticity. However, similar to thermosets, the permanent nature of the linkages brings along certain problems, preventing any further adaptability or (re)processing of the materials after curing. The aim of this thesis was the preparation of stimuli-responsive elastomers from an industrially relevant hydrogenated carboxylated nitrile butadiene rubber (HXNBR). The incorporation of reversible cross-links was envisaged to enable a temporally controllable adaptability of the prepared networks. A detailed characterisation of the material’s structure-property relationship was conducted, revealing a macroscopic responsiveness towards external stimuli. In the first part of the thesis, the incorporation of reversible covalent bonds into HXNBR based on a dissociative bond exchange mechanism was explored. Covalently attached anthracene moieties were exploited for a light-induced cross-linking of the rubber via the [4+4] photocycloaddition. The photodimers were shown being susceptive to controlled cleavage by exposure to UV-light or heat, and a repeatable switching over multiple cross-linking and cleavage cycles was demonstrated. The associated modulation of the viscoelastic properties was employed for switchable, spatially controllable bonding properties, which paves the way for the application of the materials as reversible dry adhesive. The second part of the thesis focused on the introduction of dynamic covalent bonds into HXNBR relying on an associative bond exchange mechanism following vitrimer chemistry. Exchangeable β-hydroxyl-ester cross-links were incorporated into HXNBR for the first time, and in the presence of catalyst, thermo-activated transesterifications allowed for a distinctive stress relaxation of the networks. Thermal adaptability of the elastomers was demonstrated by lap shear tests, shape change experiments and stress-rupture tests. The data confirmed the susceptibility of the prepared rubbery materials for weldability, reprocessability, and repairability. The study constitutes a straightforward and scalable methodology towards vitrimer-like elastomers, with the synthesis relying on facile chemical strategies and readily available materials. In the third part of the thesis, the step towards filled vitrimer-like HXNBR composites was taken, since the reinforcement of rubbers is essential to ensure high mechanical properties. Combining cross-linker and reinforcing filler in a single compound, a significant improvement in mechanical properties was achieved. Yet, the exchangeable nature of the linkages at the rubber-filler interface enabled thermo-activated and catalytically controlled topology rearrangements, rendering the cured composites thermally adaptable. Interestingly, compared to the unfilled analogue, the network dynamics could almost be preserved, which was remarkable considering the high filler content of up to 30 phr. Finally, a new analysis strategy for the determination of the topology freezing transition temperature (Tv) of vitrimers was presented. The direct and precise analysis of the Tv of vitrimers is challenging because measurement methods such as dynamic mechanical analysis (DMA) and dynamic scanning calorimetry (DSC) are not suitable. On the example of different literature-known epoxy-vitrimers, it was shown that the estimation of Tv from elongational creep measurements is feasible. However, a screening of various stresses at increasing temperature revealed the significance of external force in order to obtain precise Tv values. The method constitutes a facile measurement routine and allows a direct acquisition of the Tv.

AB - Conventional elastomers comprise covalent cross-links, which endows them with outstanding chemical and physical properties such as their exceptionally high elasticity. However, similar to thermosets, the permanent nature of the linkages brings along certain problems, preventing any further adaptability or (re)processing of the materials after curing. The aim of this thesis was the preparation of stimuli-responsive elastomers from an industrially relevant hydrogenated carboxylated nitrile butadiene rubber (HXNBR). The incorporation of reversible cross-links was envisaged to enable a temporally controllable adaptability of the prepared networks. A detailed characterisation of the material’s structure-property relationship was conducted, revealing a macroscopic responsiveness towards external stimuli. In the first part of the thesis, the incorporation of reversible covalent bonds into HXNBR based on a dissociative bond exchange mechanism was explored. Covalently attached anthracene moieties were exploited for a light-induced cross-linking of the rubber via the [4+4] photocycloaddition. The photodimers were shown being susceptive to controlled cleavage by exposure to UV-light or heat, and a repeatable switching over multiple cross-linking and cleavage cycles was demonstrated. The associated modulation of the viscoelastic properties was employed for switchable, spatially controllable bonding properties, which paves the way for the application of the materials as reversible dry adhesive. The second part of the thesis focused on the introduction of dynamic covalent bonds into HXNBR relying on an associative bond exchange mechanism following vitrimer chemistry. Exchangeable β-hydroxyl-ester cross-links were incorporated into HXNBR for the first time, and in the presence of catalyst, thermo-activated transesterifications allowed for a distinctive stress relaxation of the networks. Thermal adaptability of the elastomers was demonstrated by lap shear tests, shape change experiments and stress-rupture tests. The data confirmed the susceptibility of the prepared rubbery materials for weldability, reprocessability, and repairability. The study constitutes a straightforward and scalable methodology towards vitrimer-like elastomers, with the synthesis relying on facile chemical strategies and readily available materials. In the third part of the thesis, the step towards filled vitrimer-like HXNBR composites was taken, since the reinforcement of rubbers is essential to ensure high mechanical properties. Combining cross-linker and reinforcing filler in a single compound, a significant improvement in mechanical properties was achieved. Yet, the exchangeable nature of the linkages at the rubber-filler interface enabled thermo-activated and catalytically controlled topology rearrangements, rendering the cured composites thermally adaptable. Interestingly, compared to the unfilled analogue, the network dynamics could almost be preserved, which was remarkable considering the high filler content of up to 30 phr. Finally, a new analysis strategy for the determination of the topology freezing transition temperature (Tv) of vitrimers was presented. The direct and precise analysis of the Tv of vitrimers is challenging because measurement methods such as dynamic mechanical analysis (DMA) and dynamic scanning calorimetry (DSC) are not suitable. On the example of different literature-known epoxy-vitrimers, it was shown that the estimation of Tv from elongational creep measurements is feasible. However, a screening of various stresses at increasing temperature revealed the significance of external force in order to obtain precise Tv values. The method constitutes a facile measurement routine and allows a direct acquisition of the Tv.

KW - Rubber

KW - Elastomers

KW - Stimuli-responsive polymers

KW - Covalent adaptable networks

KW - Dynamic covalent bonds

KW - Vitrimers

KW - Topology freezing transition

KW - Composites

KW - Elastomere

KW - Kovalent adaptive Netzwerke

KW - Dynamische kovalente Bindungen

KW - Stimuli-responsive Polymere

KW - Vitrimere

KW - Vitrifizierungsübergang

M3 - Doctoral Thesis

ER -