Bio-inspired molecular design and synthesis of high-performance and recyclable cross-linked polymers is reported. Reversible cross-links between hard segments are incorporated into linear segmented polyurethane via Diels-Alder reaction between maleimide pendant group and furan cross-linker. The materials form hierarchical structure and exhibit excellent properties with high stiffness, strength and toughness, and can be easily thermally reshaped and re-mended.

In this paper, an asymmetric plasmonic structure composed of a MIM (metal-insulator-metal) waveguide and a rectangular cavity is reported, which can support double Fano resonances originating from two different mechanisms. One of Fano resonance originates from the interference between a horizontal and a vertical resonance in the rectangular cavity. And the other is induced by the asymmetry of the plasmonic structure. Just because the double Fano resonances originate from two different mechanisms, each Fano resonance can be well tuned independently by changing the parameters of the rectangular cavity. And during the tuning process, the FOMs (figure of merit) of both the Fano resonances can keep unchanged almost with large values, both larger than 650. Such, the transmission spectra of the plasmonic structure can be well modulated to form transmission window with the position and the full width at half maximum (FWHM) can be tuned freely, which is useful for the applications in sensors, nonlinear and slow-light devices.

Polymeric materials combining good mechanical performances with self-healing ability and malleability have attracted dramatic attention, but it presently remains a challenge for the facile fabrication of such high-performance materials, not to mention the atomic-level characterization for understanding the molecular origin of the macroscopic properties. Herein, we proposed a facile strategy to fabricate a dual-cross-linked poly­(n-butyl acrylate) polymer material, in which the self-complementary quadruple hydrogen bonding interactions between 2-ureido-4­[1H]-pyrimidinone (UPy) dimers were utilized as the dynamic sacrificial cross-linkages, and thus to enhance the mechanical strength and toughness. The hydrogen bonding interactions between UPy dimers in such synthetic cross-linked polymer material were revealed in detail by selective saturation double-quantum (DQ) solid-state NMR spectroscopy under ultrafast magic-angle-spinning beyond 60 kHz. In the meantime, the self-healing capability and recyclability were achieved by utilizing dynamic fast boronic ester transesterification at an elevated temperature. A novel symmetrical diboronic ester cross-linker was developed and employed to enhance the probability of bornoic ester transesterification at an elevated temperature. The boronic ester transesterification was verified on a small molecular model and polymer materials by solution 1H NMR spectroscopy and swelling experiments, respectively, and the cross-linking structure of polymer materials was addressed by low-field proton multiple-quantum NMR spectroscopy and T 2 relaxometry. Overall, it is well demonstrated that a combination of diboronic ester bonds and UPy dimers as the chemical and physical cross-linkage, respectively, can impart the rubbery materials with enhanced mechanical stiffness and toughness, good healing and recycling efficiency, and elucidation of the structure–property relationship here can further provide piercing insights into the development of high-performance polymeric materials.

Thermoresponsive hydrogel-based actuators are highly important for fundamental research and industrial applications, while the preparation of temperature-driven bilayer hydrogel actuators with rapid response to bend and recover properties remains a challenge. To date, most temperature-driven bilayer hydrogel actuators are based on polymers only with a lower critical solution temperature (LCST) or with an upper critical solution temperature (UCST), which need more time to bend and recover just in a small range of bending angle. Herein, we propose a new strategy to design and synthesize a fully temperature-driven bilayer hydrogel actuator, which consists of a poly­(N-acryloyl glycinamide) (NAGA) layer with a UCST-type volume phase change and a poly­(N-isopropyl acrylamide) (NIPAM)-Laponite nanocomposite layer with an LCST-type volume phase change. Due to the complementary UCST and LCST behavior of the two selected polymers, both layers have opposite thermoresponsive swelling and shrinkage properties at low and high temperatures; this imbues the hydrogel actuator with rapid thermoresponsive bending and recovery ability, as well as a large bending angle. In addition, the incorporation of Laponite nanosheets in PNIPAM layer not only improves the mechanical property of actuators but also provides the excellent bonding ability of the two-layer interface, which prevents delamination caused by excessive local stress on the interface during the bending process. Thanks to high-performance behavior, the actuator can act as an effective and sensitive actuator, such as a gripper to capture, transport, and release an object, or as an electrical circuit switch to turn on and off a light-emitting diode (LED). Overall, such hydrogel actuator may provide new insights for the design and fabrication of artificial intelligence materials.

Hydrogels have received considerable attention as an innovative material due to their widespread applications in various fields. As a soft and wet material, its mechanical behavior is best understood in terms of the viscoelastic response to the periodic deformation, which is closely related to the microscopic chemically/physically cross-linked structures. Herein, a dualcross-linked (DC) hydrogel, where a physically cross-linked network by ionic coordination (Fe 3+ ) is imposed on a chemically cross-linked poly(acrylamideco-acrylic acid) network, was studied in detail by rheology and proton multiplequantum (MQ) NMR spectroscopy. Rheology experiments revealed the diverse temperature-and strain-frequency-dependent viscoelastic behaviors for DC hydrogels induced by the dynamic Fe 3+ coordination interactions, in contrast to the single chemically cross-linked (SC) hydrogels. During the shear experiment, the trivalent Fe 3+ complex with moderate/weak binding strength might transform to those with strong binding strength and serve as permanent-like cross-linkages to resist the periodic deformation when a large strain frequency was applied. The viscoelastic behaviors of the DC hydrogels were strongly affected by the monomer ratio (C AAc / C AAm ) and Fe 3+ concentrations; however, the chemically cross-linked density did not change with C AAc /C AAm , while the physically cross-linked density was greatly enhanced with increasing Fe 3+ concentrations. Besides, the DC hydrogels have less contents of network defects in comparison to the SC hydrogels. The heterogeneous structural evolution with increasing the Fe 3+ concentration and monomer ratio was also quantitatively determined and elucidated by proton MQ NMR spectroscopy. In addition, the moduli (G′, G″) of DC hydrogels were almost an order magnitude higher than that of the corresponding SC hydrogels, demonstrating the significant contribution of Fe 3+ coordination to the mechanical properties, in consistent with the high activation energy of viscoelasticity for the physically cross-linked network as obtained from the variable-temperature shear rheology experiments. The experimental findings obtained from the rheology and proton MQ NMR experiments can be correlated with and complementary to each other. Herein, a combination of rheology and proton solid-state NMR is well demonstrated as an effective and unique way for establishing the relationship between microscopic structures and macroscopic viscoelastic properties.

Vitrimers are fascinating thermoset polymers to the industry since they can be recycled and reprocessed without compromising their mechanical strength and solvent resistance. Particularly, transesterification reaction in the epoxy resin (ER) vitrimers usually requires the incorporation of a catalyst, whereas most reports only focused on investigating the catalytic effect of small molecules, such as zinc acetate (Zn­(OAc)2) and triazabicyclodecene (TBD). In this work, a polymer catalyst, poly­(acrylonitrile-co-zinc methacrylate) (Zn-PAM), was synthesized by the random copolymerization of zinc methacrylate and acrylonitrile, where the transesterification catalysis efficiency of Zn-PAM was systematically investigated and compared to that of Zn­(OAc)2 and TBD by temperature-dependent stress relaxation experiments. It was revealed that Zn-PAM has a higher catalysis efficiency than Zn­(OAc)2 and TBD under the same loading between Zn2+ and TBD. Besides, only a tiny amount of polymer catalyst is needed to efficiently catalyze the transesterification reaction while in the meantime enhance the mechanical properties of ER.

Metasurfaces have found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantenna-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1 ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

Various types of hydrogen bonds, their temperature dependence and water-polymer interaction in hydrated poly(acrylic acid) (PAA) were systematically investigated using 1 H CRAMPS solid-state NMR techniques in the temperature range from 25 to 110 °C. The 1 H CRAMPS NMR methods are based on a recently developed continuous phase modulation technique for 1 H-1 H homonuclear dipolar decoupling. The 1 H CRAMPS experiments revealed four types of protons in hydrated PAA which are assigned to protons from the mutually hydrogen-bonded COOH groups (1), from the free COOH groups (2), from the COOH groups bounded with water or from water bounded with COOH groups which are undergoing fast chemical exchange mutually (3), and from main chain groups (4), respectively. Furthermore, we proposed double-quantum filtered and dipolar filtered 1 H CRAMPS experiments to further assign the protons according to their dipolar coupling strength. In addition, high-resolution spin echo 1 H CRAMPS experiments were further employed to accurately determine the chemical shift of these protons. These NMR techniques were also used to elucidate the molecular mobility of the different groups. It was found that dehydration in PAA promotes the formation of hydrogen bonds between COOH groups. Variable-temperature 1 H CRAMPS experiments demonstrated that the dissociation of the hydrogen bonds between COOH groups occurs dramatically at lower temperature in hydrated PAA and slowly over a wide range of temperature in dehydrated PAA. It was also found that the dehydration of water bounded with COOH groups in hydrated PAA occurs significantly at high temperature. The NMR results were compared with previous work using DSC and other techniques. Besides undergoing fast chemical exchange, the adsorbed water was also demonstrated in proximity with the free COOH groups and far from the hydrogen bonds between COOH groups by using two-dimensional 1 H-1 H spin-exchange NMR experiments.