Tomoyuki Ohishi

Mechanophores with a Reversible Radical System and Freezing-Induced Mechanochemistry in Polymer Solutions and Gels†

Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone-based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross-linking led to efficient propagation of the force of more than 80 kJ mol(-1) to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.

Network Reorganization of Dynamic Covalent Polymer Gels with Exchangeable Diarylbibenzofuranone at Ambient Temperature

Reversible bonds and interactions have been utilized to build stimuli-responsive and reorganizable polymer networks that show recyclability, plasticity, and self-healing. In addition, reorganization of polymer gels at ambient temperature, such as room or body temperature, is expected to lead to several biomedical applications. Although these stimuli-responsive properties originate from the reorganization of the polymer networks, not such microscopic structural changes but instead only macroscopic properties have been the focus of previous work. In the present work, the reorganization of gel networks with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages in response to the ambient temperature was systematically investigated from the perspective of both macroscopic and microscopic changes. The gels continued to swell in suitable solvents above room temperature but attained equilibrium swelling in nonsolvents or below room temperature because of the equilibrium of DABBF linkages, as supported by electron paramagnetic resonance measurements. Small-angle X-ray scattering measurements revealed the mesh sizes of the gels to be expanded and the network structures reorganized under control at ambient temperature.

Repeatable mechanochemical activation of dynamic covalent bonds in thermoplastic elastomers

Repeated mechanical scission and recombination of dynamic covalent bonds incorporated in segmented polyurethane elastomers are demonstrated by utilizing a diarylbibenzofuranone-based mechanophore and by the design of the segmented polymer structures. The repeated mechanochemical reactions can accompany clear colouration and simultaneous fading.

Dynamic covalent polymer brushes: reversible surface modification of reactive polymer brushes with alkoxyamine-based dynamic covalent bonds

Reactive poly(methacrylate)-based polymer brushes with radically exchangeable alkoxyamine units in the side chains were prepared on flat silicon substrates and their reactivity and surface properties were investigated. The reactive polymer brushes were prepared by surface-initiated atom transfer radical polymerization (ATRP) and the graft density of the brushes was estimated to be approximately 0.36 chains per nm2. Radical crossover reactions among the alkoxyamine units in the side chains of the polymer brushes and at the ends of the fluorinated polymer chains were carried out to produce polymer brushes with low surface free energies. Angular-dependent X-ray photoelectron spectroscopy (XPS) measurement suggested that the fluorinated polymer chains were predominantly attached to the outermost surface because of the size exclusion from the high graft density. In addition, the de-grafting reaction of the fluorinated polymer chains was also performed to confirm the reversibility of the radical crossover reactions. The surface properties of the polymer brushes after the grafting and de-grafting processes were characterized by XPS and contact angle measurements. The composition calculated from the XPS indicated that the reversible grafting of the fluorinated polymer chains proceeded successfully. The surface wettability of the polymer brushes also changed after the grafting of the fluorinated polymers. Furthermore, after the de-grafting of the fluorinated polymer chains, the peaks attributed to fluorine atoms completely disappeared from the XPS spectrum and the wettability of the surface after the de-grafting returned to that obtained before grafting.

Biobased Polymer Coating Using Catechol Derivative Urushiol

We have investigated the mechanism of the superior mechanical robustness of coated thin films of the catechol derivative urushiol. We synthesized hydrogenated urushiol (h-urushiol) by hydrogenating the double bonds in the long alkyl side chain of urushiol, and the physical properties of thin films of mixtures of urushiol and h-urushiol were evaluated. Atomic force microscopy observations revealed that these coated thin films have a homogeneous surface with no phase separation, regardless of the h-urushiol content, arising from the similarity of the chemical structures. The films showed good adhesive properties because the adhesion originates from the catechol structure. In contrast, curing time depended strongly upon the h-urushiol content. The curing of the h-urushiol thin film took 12 h, whereas the urushiol thin film was cured within 10 min. Moreover, the strain-induced elastic buckling instability for mechanical measurements test and the bulge test confirmed that the increase in the h-urushiol content decreased the mechanical strength. Because the double bonds in the urushiol side chain contribute to forming the highly cross-linked structure, the lack of double bonds in h-urushiol resulted in the slow curing and low mechanical strength. Interestingly, the mechanical robustness started to increase over 80 mol % h-urushiol. The saturated long alkyl side chain of h-urushiol faced the surface, and the regular structure of the uniform side chain may improve the mechanical properties of the coated film. Our results will help to develop biomimetic catechol-based coatings.

Solvent free oxidative coupling polymerization of 3-hexylthiophene (3HT) in the presence of FeCl3 particles

A solvent free oxidative coupling reaction of 3-hexylthiophene (3HT) within a nanocavity is reported. It is found that 3HT can be encapsulated in nanocavities larger than 1 nm, which corresponds to the size of the molecule. In this case, the side reaction at the 4-position in 3HT is regulated.