Visualization of Solvent-Induced Structure Evolution in Cyclodextrin Polyrotaxane Gels

Covalent Adaptable Poly[2]rotaxane Networks via Dynamic C-N Bond Transalkylation

Covalent adaptable networks (CANs), a novel class of crosslinked polymers with dynamic covalent bonds, have gained significant attention for combining the durability of thermosets with the reprocessability of thermoplastics, making them promising for emerging applications. Here, we report the first example of poly[2]rotaxane-type covalent adaptable networks (PRCANs), in which oligo[2]rotaxane backbones characterized by densely packed mechanical bonds, are cross-linked through dynamic C-N bonds. The oligo[2]rotaxane backbones could guarantee the mechanical properties of the CANs. Under an external force, the synergy of numerous microscopic motions of the cascade [2]rotaxane units, progressively introducing the initially hidden short chains, expands the polymer network, imparting good stretchability to the PRCANs (217 %). On the other hand, the dissociation of host-guest recognition, followed by the motion of mechanical bonds, constitutes a unique energy dissipation pathway, ultimately enhancing the toughness of PRCANs (7.6 MJ/m3). In contrast, the control CAN, which lacks movable mechanical bonds, demonstrates significantly lower stretchability (40 %) and toughness (1.5 MJ/m3). Moreover, the dynamic C-N bond can undergo high efficiency of 1,2,3-triazole alkylation and trans-N-alkylation exchanges at 1,2,3-triazolium sites at elevated temperatures, with good reprocessability and without compromising their mechanical performance. This work demonstrates the great potential of oligo[2]rotaxanes as a novel polymer backbone for the development of sustainable materials with excellent mechanical properties.

Mechano-Responsive Fluorescent Gel based on Tetraphenylethylene-Crosslinked Dynamic Covalent Network

The rapid development of mechano-responsive fluorescence has been driven by its promising applications in the fields of sensors, information encryption, and anti-counterfeiting. However, designing mechanophores that can exhibit fluorescence changes under relatively low force remains challenging. In this study, a mechano-responsive fluorescent gel was developed using a tetraphenylethylene derivative as a cross-linker, producing a dynamic covalent network that exhibits increased fluorescence under tensile stress. Based on controlled experimental studies and molecular modeling calculations, the fluorescence enhancement by external forces was attributed to the restriction of intramolecular motion in tetraphenylethylene by macromolecular chain orientation. In time-dependent experiments, due to the exchange of dynamic covalent bonds, the stress relaxation and the decrease in fluorescence intensity of the gel at fixed strain occurred simultaneously, demonstrating the potential of this fluorescence as an indication of internal stress through aggregation-induced emission (AIE) type mechanophore.