It was a popular song from the 90s that said “Life in plastic, it’s fantastic” - and indeed, the premises of the “Polymer Age” are fantastic. By now, polymers have become ubiquitous and are used in basically every conceivable way. This is an ongoing trend, and as polymers become even smarter, they might one day completely eliminate traditionally used materials from industrial and everyday applications from the market.
However, polymers – and by extension, all materials – have one inherent flaw: they break, losing their functionality, rendering them useless. Traditionally, material design was only concerned with improving the resistance of polymers towards damage, which in turn resulted in longer lifespans. This “damage prevention” approach still cannot fully prevent material failure. The “damage management” approach, on the other hand, presents a conceptually different way to handle material damage. Instead of avoiding the accumulation of damage, it tries to cope with it and, by means of self-healing materials, only looks at the net usage time of the material. However, self-healing research is still in its infancy: as of now, it cannot heal more sophisticated material failures, such as the photooxidative cleavage of bonds, that lead to material failure not on a mechanic, but on an optoelectronic level. Additionally, spectroscopic methods are rarely applied in the investigation of self-healing systems, and are mainly used to only uncover the chemistry behind the healing event, and not to probe the material properties.(Ahner et al.)
During my PhD studies, I investigated polymeric systems which included dynamic bonds and chromophoric units. By applying a thermal trigger, the chromophores within the polymer chain reshuffle and change their orientation towards each other. These changes are readily visible as changes in absorption (when the conjugated system length changes) or fluorescence spectra, quantum yield, or lifetimes (when two chromophores are adjacent enough to undergo energy transfer).(Micheel)
Energy transfer in dynamically linked polymers
Dynamically linked polymers (termed dynamers by Nobel laureat Jean-Marie Lehn) are polymers, in which structural units are connected by reversible bonds. In my PhD studies, these were basically always Diels Alder bonds which can be cleaved by changing the temperature and initiating the retro Diels Alder reaction. We introduced resonance energy acceptors and donors into these dynamers and investigated the intra- and inter-polymer exchange of these chromophores via fluorescence spectroscopy. (Ahner et al.; Micheel et al.)
Covalent reversible bonds are not the only possibility to introduce dynamic behavior to polymers. We also investigated inter-polymer hydrogen bonds by using different functional groups (urea vs. urethane). The urea bond, which introduces much stornger and/or more hydrogen bonds, brings polymers into much closer contact, and we observed the formation of very planar neighboring polymer chains. (Ahner et al.)
Restoration of absorption
The most ambitious goal of this project was to restore the absorption properties of a photo-oxidized conjugated polymer. For this, we used imine bonds as a reversible, conjugated linker (in contrast to the non-conjugated Diels Alder bonds mentioned above). At elevated temperatures, polymers undergo imine metathesis and exchange their chromophoric subunits. The increasing conjugation results in a redshift of the absorption.
Photo-oxidation destroys these conjugated polymers. However, we still have lots of different monomers or oligomeric chromophores in the film - if we again heat the film, these undergo exchange with the photo-oxidized chromophores! This will increase the absorption as long as we have monomers in the film. (Ahner et al.) While this exchange worked really well, we failed to incorporate the chromophores in an actual useful device, because conjugated imines are notorious for being extremely good light absorbers, but extremely bad ad doing anything with this light. I planned to write a review article Why conjugated imines suck and cannot get anything done, but then started my postdoc and this article is still in writing hell. It is an interesting story, going back to early experiments by Ciamician and the early photochemistry community, but maybe that is for another day…
References
2015
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Self-Healing Functional Polymeric Materials
Johannes Ahner , Stefan Bode , Mathias Micheel, and 2 more authors
In Self-healing Materials , 2015
Self-healing materials have been intensively investigated in recent decades, whereby the healing process was mostly based on the restoration of mechanical properties after mechanical damage. However, self-healing functional polymeric materials have now become the focus of research. In recent years, several approaches have been developed for self-healing of conductivity as well as the restoration of optical properties. In contrast to the healing of mechanical properties, such as stiffness and strength, the self-healing of functional materials focuses on the restoration of functionalities after damage caused by harmful environments (e.g., high temperatures or irradiation). The ultimate goal is the investigation or mimicking of a multifunctional self-healing system (e.g., biological material). In this review, the current state of the art in self-healing functional polymeric materials is summarized. In particular, we discuss self-healing conductive materials, healable optoelectronics, and functional coatings.
2016
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Thermally triggered optical tuning of π-conjugated graft copolymers based on reversible Diels–Alder reaction
J. Ahner , M. Micheel, J. Kötteritzsch , and 2 more authors
RSC Advances, Oct 2016
Publisher: The Royal Society of Chemistry
In order to design a π-conjugated polymer film with tunable optical properties by thermally triggered activation of energy transfer after processing, two monodisperse phenylene ethynylene based oligomers with different optical properties were synthesized and attached to aliphatic polymers as π-conjugated side chains. Subsequently, the exchange of the side chain chromophores between the prepared donor and acceptor graft polymers in the solid state based on a reversible Diels–Alder reaction was studied in detail. The resulting donor–acceptor graft copolymer exhibits intra polymer energy transfer upon excitation of the donor moiety. The photophysical properties of the original and exchanged graft copolymers were investigated by means of absorption and emission spectroscopy. This novel concept opens the possibility for optical tuning of π-conjugated polymer films after processing as well as applications as thermally triggered sensor systems.
2017
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Directed Orientation of Oligo(phenylene ethynylene)s Using Ureas or Urethanes in Rod–Coil Copolymers
Johannes Ahner , Mathias Micheel, Marcel Enke , and 4 more authors
Macromolecular Chemistry and Physics, 2017
_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/macp.201700343
Interchromophoric interactions between oligo(phenylene ethynylene)s incorporated in copolymers in solution as well as in solid state are investigated using UV/Vis and IR spectroscopy. One promising strategy to enable strong interchromophoric interactions is the introduction of hydrogen-bonding moieties. Together with the tendency of the chromophores to self-assemble via π–π-stacking a dense packing of the chromophores within a polymer film can be achieved. In order to demonstrate this strategy, oligo(phenylene ethynylene)s are used as monomers for copolymers using the highly efficient blocked isocyanate route and polymer films are prepared by spin coating technique. The copolymers are prepared by an in situ deprotection of a pyrazol protected bis-isocyanate functionalized rigid chromophore followed by subsequent polyaddition using a bis-amine or bis-alcohol linker unit. The specific introduction of urea as well as urethane moieties enhances coplanarization of the chromophores in the polymer films into well-organized structures, which can be characterized based on their specific optical properties.
2017
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Self-healing Functional Polymers: Optical Property Recovery of Conjugated Polymer Films by Uncatalyzed Imine Metathesis
Johannes Ahner , Mathias Micheel, Robert Geitner , and 4 more authors
Macromolecules, May 2017
Publisher: American Chemical Society
The implementation of a self-healing functionality into materials has become a prevalent approach for materials which require long-term reliability. As of today, the restoration of mechanical properties has dominated the research on self-healing materials, whereas research on healing of other functionalities (e.g., conductivity or optical properties) is still in its infancy. Here, the first conjugated polymer, which can restore its optical properties after photodamage is reported. The proposed self-healing mechanism relies on a thermally triggered imine metathesis between the conjugated polymer and additional macromolecular healing agents with no catalyst needed.
2019
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Photophysics of a Bis-Furan-Functionalized 4,7-bis(Phenylethynyl)-2,1,3-benzothiadiazole: A Building Block for Dynamic Polymers
Mathias Micheel, Johannes Ahner , Martha Frey , and 3 more authors
ChemPhotoChem, 2019
Dynamically linked polymers (dynamers) have attracted attention as a versatile building block for self-healing polymers, for example. However, research on combining optically active materials with dynamic chemistry is still in its infancy. Here, we report a highly emissive arylene ethynylene oligomer bearing a bis-furan functionalization, which makes it suitable for utilization in Diels−Alder dynamers. By combining steady-state and time-resolved absorption and emission spectroscopy from the femto- to the nanosecond timescale, the molecular dynamics of the excited state of the monomeric building block and the corresponding Diels−Alder polymer have been investigated in detail. Although in solution no pronounced differences in their photophysics were observed, thin films of the polymer exhibit a distinct dual emission with single chromophore and excimer character, respectively.
2019
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Photophysical characterization of dynamically linked polymers for self-healing applications
Mathias Micheel
2019
The thesis at hand deals with the spectroscopic and photophysical characterization of dynamically linked polymers, so called dynamers. Dynamers have already found extensive use in the fields of sensor systems or self-healing materials, but the combination of their dynamic chemistry with optical properties such as absorption or emission is still an open field of research. The present thesis is divided in three main parts. First, dynamers that rely on the Diels-Alder functionality were investigated by steady-state and time-resolved, i.e. transient absorption and time-resolved emission, spectroscopies. The chromophores embedded in the polymer scaffold belonged to the class of oligo(arylene ethynylene)s, which are known for their pronounced emission properties. In particular, the influence of the dynamer structure on their ground- and excited state properties both in solution as well as in thin films was studied. Additionally, energy transfer experiments in different polymer compositions were conducted. Second, an imine based polymer was probed with regards to its photostability. Utilizing different excitation energies and solvent properties, different photochemical deactivation pathways were found. Last, an imine-based polymer system was investigated that could partially self-heal its absorption properties after photodamage. Different polymer sructures which affected polymer mobility in films were tested and general rules for the design of optically active self-healing polymers were derived. Keywords: dynamic polymers, dynamic chemistry, self-healing, time-resolved spectroscopy, transient absorption spectroscopy, energy transfer, arylene ethynylene, phenylene ethynylene, photoisomerization, photooxidation.
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