TY - BOOK

T1 - Synthesis and characterization of ionic liquid-based epoxides for advanced applications

AU - Wanghofer, Florian

N1 - embargoed until 13-05-2027

PY - 2024

Y1 - 2024

N2 - Ionic liquids (IL) are a comparably new class of materials with numerous emerging applications. These partially organic salts with low melting temperatures exhibit many interesting properties due to their almost limitless different ion combinations. The aim of the present thesis is to investigate the utilization of ILs in epoxy formulations for advanced applications. In particular, epoxy-functionalized imidazolium cations with varying ligands and anions were synthesized and employed to develop matrix materials for type V hydrogen gas pressure vessels. Initially, a screening of hardeners was performed and cycloaliphatic anhydride hardeners were found to be able to yield networks with sufficiently high glass transition temperatures (Tg). Implementation of aromatic groups in the cation structure increased the Tg of the materials, while decreasing the hydrogen gas permeability. The same effect, even more pronounced, was observed when the anion size of the IL was reduced. However, the anhydride cured networks exhibited very low fracture toughness values and were ruled out as candidates due to detrimental impact of microcracks on the gas barrier of the liner-less pressure vessels. Consequently, a formulated IL copolymerized with bisphenol A glycidyl ether (DGEBA) and an aromatic amine hardener was chosen as most promising candidate. The resulting network exhibited superior thermal, mechanical and gas barrier properties compared to a commercial reference resin, while also providing a significantly improved fracture toughness. Even higher toughness values could be achieved by adding varying contents of graphene oxide particles into the formulations leading to GIC values of up to 740 J/m2. In a further study, epoxidized imidazolium ILs were used to manufacture vitrimers and the influence of ligands and anions on the thermal and mechanical properties as well as on the transesterification reaction was investigated. The results revealed a significant impact of the IL structure on the properties of the final networks. Addition of aromatic groups to the cation increased the mechanical strength of the networks but significantly reduced the bond exchange reaction rate compared to aliphatic side groups. Nonetheless, a synthesized aliphatic, dicationic IL resulted in very soft networks while the transesterification reaction was still rather slow. The anion selection was determined to be a very powerful tool to tailor the properties of the vitrimers. An exchange to smaller anions allowed to raise the Tg from 21 °C to 59 °C and the tensile strength from 363 kPa to 23900 kPa, while simultaneously accelerating the transesterification reaction. Lastly, butyl functionalized ILs were synthesized and added to dynamic DGEBA-thiol formulations to produce reversible adhesives. The ILs served a threefold function in the vitrimers; as catalyst for the curing reaction, as transesterification catalysts and to enable electrodelamination of the joints. The anions of the ILs were varied and their influence on the thermal and dynamic properties of the networks were assessed. Higher coordinating anions proved to be more efficient catalysts of the transesterification reactions. In particular, dicyanamide anions resulted in the fastest bond exchange reactions. The IL loading in the adhesive amounted to 20 wt.% to obtain electrically debondable adhesives. Application of 120 V to aluminium substrates resulted in an exponential decline of the bond strength, dropping from 10.2 MPa cohesive failure to 2.4 MPa adhesive failure after five minutes. The adhesive connection was then successfully rebonded under heat and pressure exploiting the vitrimeric properties of the material to reach 8.1 MPa.

AB - Ionic liquids (IL) are a comparably new class of materials with numerous emerging applications. These partially organic salts with low melting temperatures exhibit many interesting properties due to their almost limitless different ion combinations. The aim of the present thesis is to investigate the utilization of ILs in epoxy formulations for advanced applications. In particular, epoxy-functionalized imidazolium cations with varying ligands and anions were synthesized and employed to develop matrix materials for type V hydrogen gas pressure vessels. Initially, a screening of hardeners was performed and cycloaliphatic anhydride hardeners were found to be able to yield networks with sufficiently high glass transition temperatures (Tg). Implementation of aromatic groups in the cation structure increased the Tg of the materials, while decreasing the hydrogen gas permeability. The same effect, even more pronounced, was observed when the anion size of the IL was reduced. However, the anhydride cured networks exhibited very low fracture toughness values and were ruled out as candidates due to detrimental impact of microcracks on the gas barrier of the liner-less pressure vessels. Consequently, a formulated IL copolymerized with bisphenol A glycidyl ether (DGEBA) and an aromatic amine hardener was chosen as most promising candidate. The resulting network exhibited superior thermal, mechanical and gas barrier properties compared to a commercial reference resin, while also providing a significantly improved fracture toughness. Even higher toughness values could be achieved by adding varying contents of graphene oxide particles into the formulations leading to GIC values of up to 740 J/m2. In a further study, epoxidized imidazolium ILs were used to manufacture vitrimers and the influence of ligands and anions on the thermal and mechanical properties as well as on the transesterification reaction was investigated. The results revealed a significant impact of the IL structure on the properties of the final networks. Addition of aromatic groups to the cation increased the mechanical strength of the networks but significantly reduced the bond exchange reaction rate compared to aliphatic side groups. Nonetheless, a synthesized aliphatic, dicationic IL resulted in very soft networks while the transesterification reaction was still rather slow. The anion selection was determined to be a very powerful tool to tailor the properties of the vitrimers. An exchange to smaller anions allowed to raise the Tg from 21 °C to 59 °C and the tensile strength from 363 kPa to 23900 kPa, while simultaneously accelerating the transesterification reaction. Lastly, butyl functionalized ILs were synthesized and added to dynamic DGEBA-thiol formulations to produce reversible adhesives. The ILs served a threefold function in the vitrimers; as catalyst for the curing reaction, as transesterification catalysts and to enable electrodelamination of the joints. The anions of the ILs were varied and their influence on the thermal and dynamic properties of the networks were assessed. Higher coordinating anions proved to be more efficient catalysts of the transesterification reactions. In particular, dicyanamide anions resulted in the fastest bond exchange reactions. The IL loading in the adhesive amounted to 20 wt.% to obtain electrically debondable adhesives. Application of 120 V to aluminium substrates resulted in an exponential decline of the bond strength, dropping from 10.2 MPa cohesive failure to 2.4 MPa adhesive failure after five minutes. The adhesive connection was then successfully rebonded under heat and pressure exploiting the vitrimeric properties of the material to reach 8.1 MPa.

KW - polymer

KW - ionic liquid

KW - epoxy

KW - adhesive

KW - hydrogen

KW - vitrimer

KW - Polymere

KW - ionische Flüssigkeit

KW - Epoxy

KW - Klebstoff

KW - Wasserstoff

KW - Vitrimer

M3 - Doctoral Thesis

ER -