The effective control of crystallinity of covalent organic frameworks (COFs) and the optimization of their performances related to the crystallinity have been considered as big challenges. COFs bearing flexible building blocks (FBBs) generally own larger lattice sizes and broader monomer sources, which may endow them with unprecedented application values. Herein, we report the oriented synthesis of a series of two-dimensional (2D) COFs from FBBs with different content of intralayer hydrogen bonds. Studies of H-bonding effects on the crystallinity and adsorption properties indicate that partial structure of the COFs is "locked" by the H-bonding interaction, which consequently improves their microscopic order degree and crystallinity. Thus, the regulation of crystallinity can be effectively realized by controlling the content of hydrogen bonds in COFs. Impressively, the as-prepared COFs show excellent and reversible adsorption performance for volatile iodine with capacities up to 543 wt %, much higher than all previously reported adsorbents, although the variation tendency of adsorption capacities is opposite to their crystallinity. This study provides a general guidance for the design and construction of highly/appropriately crystalline COFs and ultrahigh-capacity iodine adsorbents.
Although Raman spectral fingerprints of Co 3 O 4 have been well established, the infrared spectrum of Co 3 O 4 is less understood due to its dependence on sample morphologies and experimental configurations. The same is true for both Raman and infrared spectra of CoO. In this study, we present a comprehensive optical characterization of Co 3 O 4 and CoO with Raman scattering and Fourier transform infrared spectroscopy (FTIR). Two of the transverse optical (TO) phonons and their corresponding longitudinal optical (LO) phonons of Co 3 O 4 above 500 cm −1 are observed in both transmission and diffuse reflectance with LO/TO intensity ratios depending on particle size and the incident angle of FTIR beam. CoO is featured by a broad infrared band around 510 cm −1 . In contrast to many previous reports, no Raman-active phonon line is observed, which is in agreement with the selection rule for rock-salt CoO. Nevertheless, CoO can still be characterized by Raman scattering from magnetic excitations in its antiferromagnetic phase at low temperature and a two-phonon Raman band at ∼1060 cm −1 .
Supramolecular chemistry has provided versatile and affordable solutions for the design of intelligent soft materials, but it cannot be applied in stiff materials. This paper describes a new concept for the design of high-performance supramolecular thermosets by using the noncovalent cation-π interaction as cross-linking. These supramolecular thermosets are a class of infusible and insoluble stiff polymers having excellent mechanical properties even at temperatures exceeding 300 °C. The cation-π interaction can be locally and reversibly installed and removed by aqueous treatments at high or low pH, respectively. Local manipulation of cross-linking confers these thermosets with multiple stimuli-responsive functions, such as recyclability, healability, adhesion, and nondestructive detection of cross-linking and mechanical properties.
A window of opportunity: A general copper-catalyzed C-H bond-activation path allows arylation of heterocycles with a wide range of aryl bromides (see scheme). The reaction shows excellent regioselectivity and exhibits good functional group tolerance. The 8-aryl xanthines exhibit fluorescence in a variety of solvents and show promise as reagents for biological imaging.
following freezing/thawing under strain, which induces the second cross-linking due to crystallization of PVA chains. The reversible formation and melting of PVA crystallites enables the temporary shape fixation and recovery. Chen et al. also constructed a shape memory hydrogel based on stronger and weaker H-bonding by introducing tannic acid (TA) into the PVA hydrogel networks. The stronger multiple H-bonding between PVA chains and TA acts as the permanent cross-links and the weaker H-bonding among PVA chains works as the temporary cross-links. [20] However, the shape memory and selfhealing PVA hydrogel still encounters some challenges. The first is how to control the SM&SH process spatially and temporally by appropriate stimulus. Most SM&SH hydrogels were thermally activated, but one disadvantage is that the direct heating will lead to temperature rising of the entire material. [21,22] Other stimulating means such as air, water, and pH can also trigger the shape recovery, but those stimuli are limited in practical applications. [23][24][25][26] In contrast, the remote stimulations such as ultrasound and light are very easy to achieve local focus and quantitative stimulation. [27,28] Previously we firstly proposed the use of therapeutic ultrasound to trigger the shape memory of a PVA hydrogel containing a small amount of melamine. [19] In this study, we will try to use near-infrared (NIR) to trigger the SM&SH process by employing the photothermal effect of polydopamine. As a safe and noncontact stimulus, NIR light is widely used in the field of biomedicine. Huang et al. proposed a one-pot method to produce a double network containing both permanent and temporary cross-linking structures through chemically cross-linked polyacrylamide (PAM) and physically cross-linked gelatin. Graphene oxide (GO) was introduced into the hydrogel matrix to serve as energy convertor to convert NIR irradiation to thermal energy. [29] Although GO has been widely used as nearinfrared light energy convertor, the dispersion problem and the interaction with the matrix are the bottlenecks that limit its application. [30,31] The second challenge is the contradiction between mechanical strength and self-healing performance. In most cases, the hydrogel with self-healing function has a lower mechanical strength, which in practice means no useful in many circumstances. There are some approaches to improve the mechanical properties of hydrogels, such as introducing physical and chemical cross-linkers or reinforced fillers. [32][33][34] HydrogelsThis study focuses on developing a facile approach to prepare biocompatible poly(vinyl alcohol) (PVA) composite hydrogels containing polydopamine particles (PDAPs) with ultrafast near-infrared (NIR) light-triggered shape memory and self-healing capability. The PVA-PDAPs composite hydrogels with excellent mechanical properties can be achieved after freezing/thawing treatment, and the formation of physically cross-linked networks from the hydrogen bonding (H-bonding) between PVA and PDAPs. Due to the exce...
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