Mechanochemical transduction processes are omnipresent in nature. Efforts to exploit this general principle in artificial materials include the incorporation of so-called “mechanophores” into the macromolecules. These molecular motifs transduce mechanical forces into chemical reactions or structural changes that lead to optical changes, i.e., mechanochromic responses. Many mechanophores operate on the basis of covalent bond cleavage, and this renders their activation irreversible. An alternative approach is the use of supramolecular mechanophores, which can impart polymers with reversible mechanochromic behaviors. Several types of motifs that rely on this general design approach will be discussed, including rotaxanes, cyclophanes, and loop-forming motifs. These mechanophores all contain multiple, optically active, electronically interacting moieties, whose proximity is mechanically controlled. The operating principles exploited include processes such as fluorescence quenching, excimer formation, energy transfer, as well as charge-transfer formation. The solid-state mechanoresponsive characteristics of polymers containing these elements will be discussed and attempts to correlate mechanophore design, polymer topology, and mechanoresponse will be presented.

Vitrimers have emerged as a captivating class of materials with exchangeable covalent bonds, ingeniously combining the benefits of thermosets and thermoplastics[1,2]. These dynamic thermosetting materials can undergo network reconfiguration under specific stimuli, enabling welding, reprocessing and recycling[3]. However, the high-scale/high-throughput production of vitrimers remains complex, particularly with the need to conform to conventional thermoplastic processing techniques[4]. To address these challenges, incorporating a thermoplastic phase as a processing aid has proven effective for reactive processing of kg-scale vitrimer materials[5].
Herein, this strategy was applied to PVDF, an industrially widespread thermoplastic exhibiting outstanding chemical resistance, thermal stability, mechanical properties, blended with a transesterification vitrimer based on DGEBA and Pripol (Figure 1). PVDF/vitrimer blends with vitrimer contents in the 24-75 vol.% range were prepared by reactive blending using an internal mixer and a tailor-made compatibilizer. Morphologies were investigated by SEM/EDX, while viscoelastic and dynamic behaviors were assessed by rotational rheology. DSC and tensile tests further evaluated thermal and mechanical properties.
Mechanical characterizations revealed that blends with over 48 vol.% vitrimer exhibited superior elongation properties, surpassing both pure vitrimer and brittle PVDF. This tunability allows optimization of stress resistance or elongation by varying vitrimer content. Rheological analyses show two distinct relaxation behaviors: below 75 vol. % vitrimer, the thermoplastic phase dominates, while above this threshold, vitrimer controls relaxation. This shift is attributed to a percolated vitrimer network, altering the relaxation dynamics. This interplay of tunable mechanics and controlled relaxation dynamics makes PVDF/vitrimer blends highly promising for advanced applications requiring both durability and adaptability.

Dynamic processes are the essence of life, abundantly mimicked in chemistry and physics by supramolecular & dynamic covalent chemistry. When embedded in polymers, dynamic bonds impede dynamic properties therein, transferring properties, such as self-healing, or vitrimeric reprocessability; or allow to transport dynamic properties over distances which are far beyond the length scale of the individual dynamic bonding system.
In this talk I will address dynamicity in polymeric materials on two different levels : the first example is directed to (solvent-free) materials science, bearing embedded supramolecular or dynamic covalent bonds, responsible for self-healing& vitrimeric properties. The lifetimes, activation energies, and overall behavior of selected bonding systems (H-bonds, dynamic covalent bonds) in “stressful” environments, such as solid electrolytes or phase-segregated polymers, will be addressed. Therefrom, vitrimeric electrolytes, or self-healing/reversibly-adhesive materials can be designed and realized.

Supramolecular concepts in polymer science are further extended towards a fundamental property of life, chirality, and its dynamic transfer over specific distances. We recently have developed a helical polymer, transferring chirality via (a) synthesis and (b) light-switches. The underlying molecules, based on ring-opening polymerization also in solution & the solid state will be discussed further effecting the chiral-induced spin-selectivity-effect (CISS).

The development of hydrogels for cell culture applications has focused on controlling their linear elasticity and viscoelasticity. As cells exert tensile and compressive forces on their surroundings, it is crucial to also consider the non-linear responses of these materials. However, the complexities of non-linear deformation and hydrogel fracture remain inadequately understood. Our goal is to correlate linear and non-linear bulk hydrogel responses with the molecular properties of the hydrogel constituents. Employing a bottom-up approach, we synthesize hydrogels entirely composed of molecularly characterized building blocks. Using star-shaped polyethylene glycol with terminal crosslinking units, we create a material where the contour length of each crosslinked network chain is similar. The crosslinks consist of self-assembling coiled coil forming peptides. Utilizing a library of coiled coils with tunable thermodynamic, kinetic, and mechanical stabilities, we investigate how these parameters correlate with the linear and non-linear properties of the resulting hydrogels. Oscillatory shear rheology in the linear viscoelastic range reveals that the relaxation times are governed by the equilibrium properties of the crosslinks. More importantly, rotational stress-strain experiments reveal that fracture can be tuned when using crosslinks with different non-equilibrium bond rupture properties. These parameters are reproduced using atomic force microscope-based single-molecule force spectroscopy, underscoring our ability to link molecular and bulk mechanical properties. Our next step is to introduce a fluorescent reporter system to monitor the crosslink assembly state. This will ultimately enable the visualization of network topology and crosslink rupture in real-time.

Using light to control structural/mechanical properties and the formation of polymers is one of the most powerful tools in polymer science. In our work, we have successfully demonstrated that we can use light to introduce dynamic properties locally and on demand by varying the wavelength of the light during curing and activation of the exchange reactions. Applying photoacid generators (PAGs) as latent transesterification catalysts results in the local formation of Brønsted acids upon UV exposure, which can efficiently catalyze thermo-activated transesterification reactions. To control transesterifications on the microscale, we synthesized a covalently attachable PAG. Due to the covalent linkage of the catalyst, its migration and leaching was prevented, allowing precise tuning of material properties. As proof of concept, we inscribed positive-toned microstructures with a resolution of 5 µm in thin films by direct laser writing and subsequent depolymerization.
While latent catalysts are already well-established for spatiotemporally controlling bond exchange reactions, there is a lack in inherently tailorable systems. Thus, our latest work deals with a thiol-ene network based on disulfide exchange that alters its dynamic properties as a function of the color of light used during the curing reaction. At 450 nm, photocuring yields a dynamic network with disulfide bonds, which relaxes to 63 % of its original stress within 112 s at 160 °C. In contrast, curing with 365 nm light induces disulfide scission yielding photopolymers, which contain predominately permanent links which prevents the relaxation of the polymer within a reasonable period of time.