Jian-Cheng Lai

A Stiff and Healable Polymer Based on Dynamic-Covalent Boroxine Bonds

A stiff and healable polymer is obtained by using the dynamic-covalent boroxine bond to crosslink PDMS chain into 3D networks. The as-prepared polymer is very strong and stiff, and can bear a load of more than 450 times its weight. When damaged, it can be completely healed upon heating after wetting.

Mechano-induced luminescent and chiroptical switching in chiral cyclometalated platinum(II) complexes

A couple of enantiomeric chiral cyclometalated platinum(II) complexes [Pt((−)-L1)(Dmpi)]Cl ((−)-1) and [Pt((+)-L1) (Dmpi)]Cl ((+)-1) [(−)-L1 = (−)-4,5-pinene-6′-phenyl-2,2′-bipyridine, (+)-L1 = (+)-4,5-pinene-6′-phenyl-2,2′-bipyridine, Dmpi = 2,6-dimethylphenylisocyanide] were synthesized. Two polymorphs (Form-Y and Form-R) of the complex (−)-1 were obtained. The crystallographic studies of both forms revealed that the emission and electronic circular dichroism (ECD) spectra of these complexes at solid state were sensitive to the intermolecular effects and to the molecular surrounding environments. The yellow forms (Form-Y) of the complexes (−)-1 and (+)-1 were found to undergo crystal-to-amorphous transformation upon mechanical grinding, resulting in luminescent and chiroptical switching behaviours as evidenced by emission and ECD spectra. The mechanochromic process can be reversed repeatedly by the addition of a few drops of dichloromethane. When the counteranion Cl− in 1 was replaced with trifluoromethanesulfonate (OTf−), complexes [Pt((−)-L1)(Dmpi)]OTf ((−)-2) and [Pt((+)-L1)(Dmpi)]OTf ((+)-2) were obtained. Complexes (−)-2 and (+)-2 showed a more pronounced luminescent switching behaviour, suggesting that the switching properties can be tuned by the counteranions.

A Highly Stretchable Polymer that Can Be Thermally Healed at Mild Temperature

Combining stretchability and self-healing properties in a man-made material is a challenging task. For an efficient self-healing material, weaker dynamic or reversible bonds should be presented as crosslinks so that they will first break upon damage and then reform after healing, which is not favorable when developing elastic materials. In this work, by incorporating dynamic Fe(III)-triazole coordination bonds into polydimethylsiloxane (PDMS) backbone, a highly elastic polymer is obtained that can be thermally healed at mild temperature. The as-prepared polymer can be stretched to 3400% strain at low loading speed (1 mm min(-1) ). When damaged, the polymer can be thermally healed at 60 °C for 20 h with a healing efficiency of over 90%. The good mechanical and healable properties of this polymer can be ascribed to the unique coordination bond strength and coordination conformation of Fe(III)-triazole coordination complex.

Electrochromic properties of novel octa-pinene substituted double-decker Ln(III) (Ln = Eu, Er, Lu) phthalocyanines with distinctive near-IR absorption

Octa-pinene substituted double-decker lanthanide(III) phthalocyanines LnPc*2 (Ln = Eu, Er, Lu) were prepared and their spectral, electrochemical, spectroelectrochemical and electrochromatic properties were studied. The introduction of the bulky and rigid pinene groups into phthalocyanines provides several advantages for the resulting double-decker lanthanide(III) phthalocyanine complex. First of all, the intermolecular interaction in the new LnPc*2 complex was weakened, leading to excellent solubility. Secondly, intramolecular distances between the macrocycles in LnPc*2 molecules were also increased, resulting in a significant red shift of the ring-to-ring intervalence charge transfer bands. Particularly, the intervalence bands of EuPc*2, ErPc*2 and LuPc*2 were observed at 1944, 1693 and 1620 nm, respectively, which appear to be the most red-shifted absorption in comparison with the literature values of mononuclear double-decker Eu(III), Er(III) and Lu(III) phthalocyanines. Thirdly, given the red-shifted near-IR (NIR) absorption and the various colorful oxidation states, the solution of LnPc*2 exhibits electrochromic behavior both in the UV-Vis and near-IR regions. Notably, spectral change of EuPc*2 covers almost the whole range of the NIR region. Finally, the pinene groups also enhance the film-forming ability. Therefore, we were able to fabricate solid state electrochromic devices through a solution processable method. The as-fabricated devices show reversible electrochromic behavior with high color efficiency and good stability.

Distinct Mechanical and Self-Healing Properties in Two Polydimethylsiloxane Coordination Polymers with Fine-Tuned Bond Strength

Coordination bonds are effective for constructing highly efficient self-healing materials as their strength is highly tunable. To design self-healing polymers with better performance, it is important to get a profound understanding of the structure-property relationships. However, this is challenging for self-healing polymers based on coordination bonds, because many parameters, such as bond energy, bond dynamics, and coordination number will have an essential effect on the mechanical and self-healing properties of the polymer. In this work, we synthesized two poly(dimethylsiloxane) (PDMS) polymers cross-linked by different Zn(II)-diiminopyridine coordination complexes (denoted as PDMS-NNN-Zn, PDMS-MeNNN-Zn respectively). The two cross-linking Zn(II)-diiminopyridine complexes are similar in coordination modes, but differ in coordination dynamics. As manifested by ITC, rheology, and tensile experiments, we confirm that the coordination bond in PDMS-MeNNN-Zn polymer films is weaker but more dynamic. Consequently, the PDMS-MeNNN-Zn polymer has poorer mechanical strength but higher stretchability and better self-healing properties. The inflicted cracks on PDMS-MeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%. Such fast self-healing properties have never been achieved in self-healing polymers based on coordination bonds. Our results also demonstrate the important impact of the thermodynamic stability and kinetic lability of coordination complexes on the mechanical and self-healing properties of polymers. Such a comprehensive understanding is helpful for further design of novel synthetic polymers, which can achieve an optimal balance between the mechanical strength and self-healing performance.

An Elastic Autonomous Self-Healing Capacitive Sensor Based on a Dynamic Dual Crosslinked Chemical System

Adopting self-healing, robust, and stretchable materials is a promising method to enable next-generation wearable electronic devices, touch screens, and soft robotics. Both elasticity and self-healing are important qualities for substrate materials as they comprise the majority of device components. However, most autonomous self-healing materials reported to date have poor elastic properties, i.e., they possess only modest mechanical strength and recoverability. Here, a substrate material designed is reported based on a combination of dynamic metal-coordinated bonds (β-diketone-europium interaction) and hydrogen bonds together in a multiphase separated network. Importantly, this material is able to undergo self-healing and exhibits excellent elasticity. The polymer network forms a microphase-separated structure and exhibits a high stress at break (≈1.8 MPa) and high fracture strain (≈900%). Additionally, it is observed that the substrate can achieve up to 98% self-healing efficiency after 48 h at 25 °C, without the need of any external stimuli. A stretchable and self-healable dielectric layer is fabricated with a dual-dynamic bonding polymer system and self-healable conductive layers are created using polymer as a matrix for a silver composite. These materials are employed to prepare capacitive sensors to demonstrate a stretchable and self-healable touch pad.

A rigid and healable polymer cross-linked by weak but abundant Zn(II)-carboxylate interactions

Achieving a desirable combination of solid-like properties and fast self-healing is a great challenge due to slow diffusion dynamics. In this work, we describe a design concept that utilizes weak but abundant coordination bonds to achieve this objective. The designed PDMS polymer, crosslinked by abundant Zn(II)-carboxylate interactions, is very strong and rigid at room temperature. As the coordination equilibrium is sensitive to temperature, the mechanical strength of this polymer rapidly and reversibly changes upon heating or cooling. The soft–rigid switching ability σ, defined as G’max /G’min, can reach 8000 when ΔT = 100 °C. Based on these features, this polymer not only exhibits fast thermal-healing properties, but is also advantageous for various applications such as in orthopedic immobilization, conductive composites/adhesives, and 3D printing.Combining solid-like properties with fast self-healing is a great challenge due to slow diffusion dynamics. Here the authors demonstrate a rigid and healable material by using weak but abundant coordination bonds to crosslink a PDMS polymer.