Stefan Zechel

A healing ionomer crosslinked by a bis-bidentate halogen bond linker: a route to hard and healable coatings

In this work we present the first incorporation of bis-bidentate halogen bond linkers into an organic healing ionomer system resulting in the formation of crosslinked supramolecular networks. The obtained supramolecular coatings feature an excellent healing ability as well as an enhanced mechanical performance.

Do You Get What You See? Understanding Molecular Self-Healing

The self-healing ability of self-healing materials is often analyzed using morphologic microscopy images. Here it was possible to show that morphologic information alone is not sufficient to judge the status of a self-healing process and molecular information is required as well. When comparing molecular coherent anti-Stokes Raman scattering (CARS) and morphological laser reflection images during a standard scratch healing test of an intrinsic self-healing polymer network, it was found that the morphologic closing of the scratch and the molecular crosslinking of the material do not take place simultaneously. This important observation can be explained by the fact that the self-healing process of the thiol-ene based polymer network is limited by the mobility of alkene-containing compounds, which can only be monitored by molecular CARS microscopy and not by standard morphological imaging. Additionally, the recorded CARS images indicate a mechanochemical activation of the self-healing material by the scratching/damaging process, which leads to an enhanced self-healing behavior in the vicinity of the scratch.

A translation of the structure of mussel byssal threads into synthetic materials by the utilization of histidine-rich block copolymers

Mussel byssal threads are well-known due to their self-healing ability after the mechanical stress caused by waves. The proposed mechanism demonstrates the importance of reversible histidine–metal interactions as well as the block copolymer-like hierarchical architecture of the underlying protein structure. Taking these two aspects as inspiration for the design of synthetic analogs, different histidine-rich block copolymers were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The hard domain was mimicked using polystyrene and the soft domain consists of n-butyl acrylate (BA) as well as histidine moieties as ligands. The block copolymers were crosslinked using different zinc(II) salts and the resulting metallopolymers were investigated with respect to their self-healing abilities. The observed two-step mechanism of the self-healing process was studied in detail. Furthermore, the mechanical properties were determined by nanoindentation and were correlated with other results.

Palladium-SCS Pincer Complexes as Cross-Linking Moieties in Self-Healing Metallopolymers

Metallopolymers combine properties of metal complexes and polymers and are thus interesting materials for a wide field of different applications. One current major field is the utilization as self-healing polymers, and for this purpose, palladium-SCS pincer complexes are synthesized. Well-defined copolymers featuring those metal complexes in the side chain are obtained using the reversible addition-fragmentation chain-transfer polymerization technique. By the addition of a tetravalent pyridine cross-linkers, reversible cross-linked supramolecular networks are prepared, enabling self-healing properties. By utilizing density functional theory calculations, IR, and Raman spectroscopy, as well as isothermal titration calorimetry, the complex formation, reversibility, and stability are studied. The information of these experiments also enables further optimizations for the design of similar reversible systems in the future.

Influence of Aspartate Moieties on the Self-Healing Behavior of Histidine-Rich Supramolecular Polymers

Aspartate incorporated into the protein structure of mussel byssal threads is believed to play an important role, besides the reversible histidine-zinc interactions, in the self-healing behavior of mussel byssal threads. Therefore, copolymers containing both aspartate and histidine moieties are synthesized in order to investigate the influence of aspartate on the complexation of zinc(II) as well as on the self-healing behavior and the mechanical properties of the resulting supramolecular networks. For this purpose, isothermal titration calorimetry measurements of a model aspartate compound as well as of these copolymers are performed and the thermodynamic parameters are utilized for the design of self-healing copolymers. For this purpose, n-lauryl methacrylate-based copolymers containing histidine and aspartate are synthesized and crosslinked with zinc(II) acetate. The self-healing behavior of the supramolecular networks is investigated using scratch healing tests and the mechanical properties by nanoindentation.

Hydrogel-Embedded Model Photocatalytic System Investigated by Raman and IR Spectroscopy Assisted by Density Functional Theory Calculations and Two-Dimensional Correlation Analysis

The presented study reports the synthesis and the vibrational spectroscopic characterization of different matrix-embedded model photocatalysts. The goal of the study is to investigate the interaction of a polymer matrix with photosensitizing dyes and metal complexes for potential future photocatalytic applications. The synthesis focuses on a new rhodamine B derivate and a Pt(II) terpyridine complex, which both contain a polymerizable methacrylate moiety and an acid labile acylhydrazone group. The methacrylate moieties are afterward utilized to synthesize functional model hydrogels mainly consisting of poly(ethylene glycol) methacrylate units. The pH-dependent and temperature-dependent behavior of the hydrogels is investigated by means of Raman and IR spectroscopy assisted by density functional theory calculations and two-dimensional correlation spectroscopy. The spectroscopic results reveal that the Pt(II) terpyridine complex can be released from the polymer matrix by cleaving the C═N bond in an acid environment. The same behavior could not be observed in the case of the rhodamine B dye although it features a comparable C═N bond. The temperature-dependent study shows that the water evaporation has a significant influence neither on the molecular structure of the hydrogel nor on the model photocatalytic moieties.