German Research Foundation
The mechanical deformation of a polymer material typically starts with the macroscopic application of force and leads to conformational, configurational, and constitutional covalent and non-covalent bond rearrangements on the molecular level. The entire process of deformation thus covers around seven to ten orders of magnitude in length scale, rendering its analysis and understanding a conceptual and instrumental challenge. A comprehensive insight into the mechanical properties of a polymer material requires suitable analytical tools and techniques that allow collecting and mapping of mechanical information across these scales. In the context of polymer mechanochemistry, the force-induced activation of a latent molecular motif can be tailored to cause a detectable change in the optical properties, rendering the motif an optical force probe (OFP). There is a strong need for new OFPs with precisely designed optical responses, to overcome fundamental detection limitations, such as optical orthogonality and complementarity that are currently still in place. For example, further shifting of the excitation and emission wavelengths into the red and IR region is a standing challenge. In addition, a rational approach toward fluorescence lifetime-based OFPs is unknown. More generally, the fundamental question how force affects the excited state landscape in OFPs in the context of polymer mechanochemistry is unanswered, yet will be a prerequisite for future rational OFP developments. The goal of this project is to develop spectrally modulated annihilators and sensitizers for triplet-triplet annihilation photon-upconversion (TTA UC) as well as hot exciplex emitters for lifetime modifications that respond to the force-induced alteration of the energy levels of triplet and charge transfer (CT) excited states. First, this will modularize the available annihilators and sensitizers for force-induced TTA UC and shifted locally excited (LE) to CT emission. Secondly, the unprecedented mechanochemical activation of triplet sensitizers and CT exciplex to LE bridge emitters will be demonstrated. Together, these individual force-activated photophysically active moieties will enable their combination in polymer materials to understand fracture in complex, non-uniform environments. Specifically, the spectral features of TTA UC will allow the implementation of conjunctive fracture analysis, which would be difficult to achieve using regular OFPs. Concomitantly, lifetime-based OFP analysis will allow the use of non-intensity-based microscopy (FLIM) and allow additional insight into the local microenvironment of the fracture zone.
In this project, we will devise and synthesize several thermally stable, spectrally tunable optical force probes (OFPs) with expected thermal decomposition temperatures above the processing temperatures of technically used polymers. Therefore, [4+4] photodimers of anthracenes, [4+2] photodimers of π-extended anthracenes, [4+2] cycloadducts of benzyne and anthracene (triptycenes), and [4+2] cycloadducts of triazolinedione and anthracene will be investigated. After the successful synthesis of the aforementioned OFPs, thermal analysis of the OFP samples will reveal their thermal stability at elevated temperatures, which will also provide thermal dissociation rate constants, activation energies, dissociation onset temperatures, and possible solvent- as well as concentration-dependent characteristics as parameters to describe their thermal stability. Afterwards, the OFPs will be incorporated into diverse linear polymer chains including technically important polystyrene (PS) and polyethylene (PE). To obtain polymers with narrow dispersity, we will use Cu0-mediated controlled radical polymerization for acrylate and methacrylate polymers, especially poly(methyl acrylate) (PMA), living anionic polymerization for PS, and entropy-driven ring-opening metathesis polymerization for PE. To examine the mechanochemical reactivities of acquired OFP-functionalized linear polymer chains, those with a single central mechanophore unit (PMA, PS) will subsequently be subjected to sonochemical treatment, which will lead to chain scission and eventually provide the apparent scission rate constants. Then, OFP-containing polymers will be prepared under different technical conditions. We will blend OFP-functionalized linear poly(methyl methacrylate) (PMMA) either into MMA monomers with following cell casting process, or into commercial PMMA granules with subsequent extrusion at 180-240 °C and injection molding. OFP-functionalized linear PS will be blended with commercial high molar mass PS either through extrusion and following injection molding (150-220 °C), or through solution blending with subsequent hot pressing (150 °C). The latter approach is suitable for both general purpose PS (GPPS) and composites such as high-impact PS (HIPS). For PE, besides the aforementioned methods, we will also produce OFP-PE doped PE fibers by blending with ultra-high-molecular-weight PE (UHMWPE) and spinning fibers thereof. Finally, by combining subsequent mechanical testing of the obtained polymer samples with photon quantitative confocal laser scanning microscopy (CLSM), we expect to shed light on the yielding and fracture behavior of these authentically manufactured technical polymers.
The precondition for a Heisenberg Programme funding is high scientific quality and originality of the research project at international level and suitability for further qualification as a university teacher. Applicants need to meet all the requirements for appointment to a permanent professorship.The aim of this programme is to enable outstanding scientists to prepare for a scientific leadership function, and simultaneously work on further research topics. This research does not necessarily need to be planned and carried out in the form of a project.For this reason, and unlike the procedure in other funding programmes, both the abstracts of applications and final reports are not required and will therefore not be published in GEPRIS.
To understand the origin of complex physical behaviors in natural and synthetic polymers, conventional mechanical measurements are inadequate, because they merely provide a macroscopic relation between stress and deformation. In recent years, spectacular developments in experimental techniques and mechanochemistry have enabled access to real time micro-structural changes of polymeric materials, thereby considerably enhancing the knowledge of polymer physics. In this project, external radiation (scattering of X-rays) and internal radiation (mechanochemistry-induced visible fluorescence) based measurement techniques will be coupled with computational mechanics to investigate the generalized mechanics of polymers -- especially, double-network hydrogels. While the former measurement technique can detect spatial distribution of damage domains in polymeric samples, the latter one allows for the direct connection between the intensity of emitted light and the damage evolution in double-network hydrogels. These two approaches provide sufficient ingredients for constructing a micromechanical damage model for double-network hydrogels. Here, the ill-posedness induced in computation of continuum damage model is treated by a physically-based regularization theory. Then, numerical results will be validated by comprehensive experimental setups using confocal laser scanning microscopy (CLSM) and small-angle X-ray scattering (SAXS).
Federal Ministry of Education and Research
Within BoostLab 2, which has set itself the goal of developing switchable bonding technologies for a sustainable circular economy, the MultiGlue sub-project deals with bonds that are released in response to an external stimulus (light, mechanical force, heat). Here, new solutions are being developed for the pure recovery of materials from bonded composite materials in order to increase the recycling rate of plastics in composite materials with other plastics, aluminum or paper.
In MultiGlue, a conceptually innovative adhesive system is being developed that enables simple recycling of bonded components, in this case composite cardboard and multilayer composite films, and can recycle the individual components of the value chain. The adhesive will also be based on renewable raw materials. For the successful implementation of MultiGlue, the DWI - Leibniz Institute for Interactive Materials is pooling its expertise in the fields of biotechnology, polymer chemistry and materials science and addressing new value chains and market potential in recycling through cooperation with the University of Bonn, bio.IMPACT and Aachen Proteineers GmbH (as a strategic subcontracted partner).
Leibniz Association
The biomedical application of self-immolative polymers has spawned exciting applications for sensors and drug delivery utilizing different physicochemical stimuli, such as pH, enzymes, or light. In a biomedical context, commercial microbubbles (MBs) with poly(butyl cyanoacrylate) shells are employed that upon US application deform and fracture. Although these MBs would benefit from optimized approach to the mechanical degradation of the MB shells and the associated clearance from the body, research in this direction has not yet been performed. We will design and synthesize self-immolative polymers that respond to force in the form of ultrasound for an improved performance of MBs in the context of sonopharmacology.
