Response of a Tethered Zn-Bis-Terpyridine Complex to an External Mechanical Force: A Computational Study of the Roles of the Tether and Solvent

Polymeric materials containing weak sacrificial bonds can be designed to engineer self-healing and higher toughness, improve melt-processing, or facilitate recycling. However, they usually exhibit a lower mechanical strength and are subject to creep and fatigue. For improving their design, it is of interest to investigate their mechanical response on the molecular scale. We report on a computational study of the response to a mechanical external force of a Zinc(II) bis-methyl phenyl-terpyridine ([Zn-bis-Terpy]2+) complex included in a cyclic poly(ethylene glycol) (PEG) tether designed to maintain the two partners of the metal-ligand bonds in close proximity after the rupture of the complex. The mechanical response is studied as a function of the pulling distortion by using the CoGEF isometric protocol, including interactions with a polar solvent (DMSO). We show that tethering favors recombination but destabilizes the complex before bond rupture because of the interactions of the PEG units with Terpy ligands. Similar effects occur between the DMSO molecules and the complex. Our results on the molecular scale are relevant for single-molecule force spectroscopy experiments. Interactions of the complex with solvent molecules and/or with the tether lead to a dispersion of the rupture force values, which could obscure the interpretation of the results.

In-situ monitoring of polymer mechanochemistry: what can be learned from small molecule systems

Using mechanical energy to drive chemical transformations is an exciting prospect to improve the sustainability of chemical reactions and to produce products not achievable by more traditional methods. In-situ monitoring of reaction pathways and chemical transformations is vital to deliver the reproducible results required for scale up to realize the potential of mechanochemistry beyond the chemistry lab. This mini review will discuss the recent advances in in-situ monitoring of ball milling and polymer mechanochemistry, highlighting the potential for shared knowledge for scale up.

Oligo[2]catenane That Is Robust at Both the Microscopic and Macroscopic Scales

Polycatenanes are extremely attractive topological architectures on account of their high degrees of conformational freedom and multiple motion patterns of the mechanically interlocked macrocycles. However, exploitation of these peculiar structural and dynamic characteristics to develop robust catenane materials is still a challenging goal. Herein, we synthesize an oligo[2]catenane that showcases mechanically robust properties at both the microscopic and macroscopic scales. The key feature of the structural design is controlling the force-bearing points on the metal-coordinated core of the [2]catenane moiety that is able to maximize the energy dissipation of the oligo[2]catenane via dissociation of metal-coordination bonds and then activation of sequential intramolecular motions of circumrotation, translation, and elongation under an external force. As such, at the microscopic level, the single-molecule force spectroscopy measurement exhibits that the force to rupture dynamic bonds in the oligo[2]catenane reaches a record high of 588 ± 233 pN. At the macroscopic level, our oligo[2]catenane manifests itself as the toughest catenane material ever reported (15.2 vs 2.43 MJ/m³). These fundamental findings not only deepen the understanding of the structure-property relationship of poly[2]catenanes with a full set of dynamic features but also provide a guiding principle to fabricate high-performance mechanically interlocked catenane materials.

Action of Mechanical Forces on Polymerization and Polymers

In this review, we summarize recent developments in the field of the mechanochemistry of polymers. The aim of the review is to consider the consequences of mechanical forces and actions on polymers and polymer synthesis. First, we review classical works on chemical reactions and polymerization processes under strong shear deformations. Then, we analyze two emerging directions of research in mechanochemistry-the role of mechanophores and, for the first time, new physical phenomena, accompanying external impulse mechanical actions on polymers. Mechanophores have been recently proposed as sensors of fatigue and cracks in polymers and composites. The effects of the high-pressure pulsed loading of polymers and composites include the Dzyaloshinskii-Moriya effect, emission of superradiation and the formation of metal nanoparticles. These effects provide deeper insight into the mechanism of chemical reactions under shear deformations and pave the way for further research in the interests of modern technologies.

Terraced and Three-dimensional Pyramid-shaped Polymer Single Crystal via low temperature-Assisted Microfluidic Technology

Three-dimensional pyramidal polymer single crystals provide spatial gradient variations within the crystal molecules, and these variations facilitate the study of the relationship between structure and properties within the molecules of various complexes with anisotropic structures. As described herein, we propose a low-temperature-assisted microfluidic pore channeling approach to prepare structurally ordered polymer single crystals. A mixture of dichloromethane and dimethyl sulfoxide was used as a prepolymer, and a liquid microfluidic technique was employed to grow the end-functionalized polymers into three-dimensional polymer single crystals. Through the ordered growth of single crystals, a personalized pyramidal pattern with a homogeneous structure was formed. To evaluate the mesh node density, low-temperature growth time and substrate type were also investigated. Rectangular, pyramidal, and dendritic patterns were synthesized via low-temperature single crystal growth. This work shows that low temperature-assisted microfluidics provides a novel means to tune the three-dimensional structure of polymer single crystals. This article is protected by copyright. All rights reserved.

Rotaxane-Based Dual Function Mechanophores Exhibiting Reversible and Irreversible Responses

Mechanochromic mechanophores permit the design of polymers that indicate mechanical events through optical signals. Here we report rotaxane-based supramolecular mechanophores that display both reversible and irreversible fluorescence changes. These responses are triggered by different forces and are achieved by exploiting the molecular shuttling function and force-induced dethreading of rotaxanes. The new rotaxane mechanophores are composed of a ring featuring a luminophore, which is threaded onto an axle with a matching quencher and two stoppers. In the stress-free state, the luminophore is preferentially located in the proximity of the quencher, and the emission is quenched. The luminophore slides away from the quencher when a force is applied and the fluorescence is switched on. This effect is reversible, unless the force is so high that the luminophore-carrying ring slips past the stopper and dethreading occurs. We show that the combination of judiciously selected ring and stopper moieties is crucial to attain interlocked structures that display such a dual response. PU elastomers that contain such doubly responsive rotaxanes exhibit reversible fluorescence changes over multiple loading-unloading cycles due to the shuttling function, whereas permanent changes are observed upon repeated deformations to high strains due to breakage of the mechanical bond upon dethreading of the ring from the axle. This response allows one, at least conceptually, to monitor the actual deformation of polymer materials and examine mechanical damage that was inflicted in the past on the basis of an optical signal.