Highly Ordered Supramolecular Assembled Networks Tailored by Bioinspired H-Bonding Confinement for Recyclable Ion-Transport Materials

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES4 Jul 2022Highly Ordered Supramolecular Assembled Networks Tailored by Bioinspired H-Bonding Confinement for Recyclable Ion-Transport Materials Chen-Yu Shi, Qi Zhang, Bang-Sen Wang, Dan-Dan He, He Tian and Da-Hui Qu Chen-Yu Shi Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , Qi Zhang Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , Bang-Sen Wang Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , Dan-Dan He Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , He Tian Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author and Da-Hui Qu *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202158 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Controlling dynamic molecular self-assembly to finely tune macroscopic properties offers chemical solutions to rational material design. Here we report that combining disulfide-mediated ring-opening polymerization with β-sheet-like H-bonding self-assembly can drive a direct small-molecular assembly into a layered ionic network with precise architectural tunability and controllable functions as ion-transport membranes. This strategy enables a one-step evaporation-induced self-assembly from discrete small molecules to layered ionic networks with high crystallinity. The interlayer distances can be readily engineered with nanometer accuracy by varying the length of the oligopeptide side chain. The synergy of the layered structure and hydrophilic terminal groups facilitates the formation and ordering of interlayer water channels, endowing the resulting membranes with high efficiency in transporting ions. Moreover, the inherent dynamic nature of poly(disulfide)s allows chemical recycling to monomers under mild conditions. We foresee that the robust strategy of combining dynamic disulfide chemistry and noncovalent assembly can afford many opportunities in designing smart materials with unique functions and applications. Download figure Download PowerPoint Introduction Controlling molecular self-assembly to construct precisely organized supramolecular architectures and materials has been a vital topic that attracts broad interests in multidisciplinary fields.1–5 Dynamic chemistry, including supramolecular noncovalent chemistry and dynamic covalent chemistry, provides many versatile and reliable chemistry tools for molecular self-assembly and materials design.6–9 One of the most intriguing strategies involves subtle elaboration of the molecular structure of building blocks. It results in assembled materials with desired architectures from microscopic to macroscopic scale through self-organization and self-repair.10,11 Some systems based on various building blocks (e.g., frameworks,12–14 cages,15–17 supramolecular polymers,18–21 amphiphiles,8,22,23 and molecular machines24–26) are established, and they are versatile and promising tools for dynamic chemistry and related materials. Therefore, expanding the toolbox of dynamic chemistry—especially those based on simple small molecules—would induce exponential development of functional supramolecular materials with diverse applications.27,28 As the most representative dynamic chemistry strategies, dynamic covalent chemistry and noncovalent chemistry have been extensively developed in the past decades to produce many functional materials, such as self-healing polymers,29–33 responsive materials,34–39 reconfigurable crosslinked networks,40–45 and chemically recyclable plastics.46–48 Despite these fascinating efforts being made by independently using dynamic covalent or noncovalent methodologies, combining the advantages of both remains rarely explored, that is, simultaneous dynamic covalent and noncovalent self-assembly49,50 to engender supramolecular materials with unique architectures and functions. The fundamental challenges lie on the orthogonalization or synergic cooperation of two different dynamic chemistries within a single system, which requires precise spatiotemporal controllability, that is, simultaneous kinetic and thermodynamic control and synchronous external environmental conditions. Here we report that combining reversible disulfide chemistry with β-sheet-inspired H-bonding chemistry can produce dually dynamic materials that exhibit high structural order, ion conductivity, and intrinsic reconfigurability. Simply coupling oligopoly(glycine) units with thioctic acid (TA) produces side chain-engineerable amphiphilic monomers for self-assembly (Figure 1a), which can be assembled into architecturally precise supramolecular networks by directly evaporating the water solvent on a substrate. The unique interfacial effect under evaporating environments enables spatiotemporal control of the sequential equilibrium transition (from monomers to polymers, and to final layered networks) (Figure 1b). Moreover, the combination of layered order and hydrophilicity induces the formation of water channels in the bulk phase of the assembled materials, which further enables efficient cation conductivity of the resulting films. This leads to the first example of high-performance polyelectrolytes with closed-loop chemical recyclability. We foresee this simple and robust molecular engineering strategy offering many opportunities in the design of polyelectrolytes, conductive polymers, sensors, and actuators featuring dynamic and good recycling capabilities. Figure 1 | Conceptual illustration of self-assembled supramolecular ionic networks by combining dynamic covalent and noncovalent chemistry in a single system. (a) Monomer molecular design and the one-step integrative self-assembly involving disulfide-mediated polymerization and H-bonding self-assembly. The morphology of poly(STGly-n) was semi-crystalline consisting of amorphous poly(disulfide) main chain phase, β-sheet-like H-bond stacking domains, and ordered ion channels. (b) Schematic illustration of the highly ordered layered network formed by evaporation-induced self-organization of poly(STGly-n) (n = 1–6) at the air/water/solid interface. Download figure Download PowerPoint Experimental Methods Small-angle X-ray scattering Small-angle X-ray scattering (SAXS) was performed on the BL19U2 SAXS beamline at Shanghai Synchrotron Radiation Facility (Shanghai, China). Bragg’s law: n λ = 2 d sin θ The scattering angle, which we define by convention as 2θ, defines a “probe length” expressed as d = 2π/q, where q = 4πsin(θ)/λ, in which λ is the wavelength of the X-rays. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) experiments were performed on the Metrohm Autolab M204. The free-standing poly(STGly-n) films were cut into squares (10 mm × 10 mm × 0.5 mm) and clipped between conductive glasses in a sandwich-like manner. The tunable frequency of impedance measurements ranges from 0.1 Hz to 1 × 106 Hz and an alternating potential of 100 mV. For the relative humidity (RH)-dependent EIS experiment, the films were stabilized at desired humidity conditions for at least 6 h before testing. The temperature-dependent conductivities were tested in situ on a constant heating plate from 20 to 80 °C with an insulation chamber, dwelling for 10 min at each temperature before measurement to let the polymer equilibrate. Impedance spectra were fitted with an equivalent circuit model (a parallel combination of a resistor and a constant phase element in series with the high-frequency resistance) to determine the resistance R. The conductivities were calculated by using the equation: σ = L R · S where σ (siemens per centimeter) is conductivity, L (centimeters) is the thickness of film, R (ohms) is the resistance calculated from Nyquist plots, and S (square centimeters) is the cross-sectional area of film. The Ea was fitted according to the Arrhenius equation: ln ( σ T ) = ln σ 0 + E a R T where Ea is the activation energy for ion transport, σ0 is a pre-exponential factor, R is the gas constant, and T is temperature. UV–vis absorption spectra UV–vis absorption spectra were recorded on a Shimadzu UV-2600 spectrophotometer (Shimadzu Corporation, Tokyo, Japan). For the aqueous solution, a quartz cell with specific optical path (10, 5, 2, 1, and 0.5 mm) was used to measure the sample solution according to the concentration, and then the absorbance was normalized with the optical path as 1 cm. For the film sample, a quartz slice was used as the substrate. STGly-n aqueous solution (100 μL, 200 g/L) was deposited on the substrate and placed in ambient condition to perform evaporation-induced interfacial self-assembly. The dry film attached on a quartz slice was obtained and used as the test sample for the UV–vis spectra. Water absorption and release experiments All the poly(STGly-n) films were shaped as regular samples (10 mm × 10 mm × 0.5 mm). The polymer film was pre-dried at 20 °C and 0% RH for 12 h to remove residual moisture. For D2O adsorption experiment, the dry film was exposed above heated D2O vapor for 5 min to obtain the deuterated film. For the water release experiment, the dry film was first exposed above heated H2O vapor for 5 min to obtain the hydrated film, followed by subsequent drying at 20 °C and 0% RH for 12 h to obtain dry film again. Mechanical tension experiments All the tensile tests were carried out on an HY-0580 tension machine (HENGYI Company, Shanghai, China). The data were recorded in real time by a wire-connected computer system. The film was shaped as a rectangle sample (20 mm × 10 mm × 0.5 mm). The initial length was 10 mm. Unless otherwise noted, samples were tested at a fixed tensile speed of 20 mm/min. Chemical closed-loop recycling experiments The aqueous phase basification-acidification two-step depolymerization strategy included decrosslinking in alkaline solution and precipitation in acid solution. In the decrosslinking process, 1 g poly(STGly-n) film fragments were dissolved in 20 mL 0.1 M alkaline aqueous solution (NaOH, Na2CO3) and were stirred for 5 min to obtain a yellowish solution. Subsequent acidification by 0.1 M HCI aqueous solution to pH 3 precipitated yellowish powders in yields of over 80%. A small amount of viscous oligomers could be easily separated. The recovered monomers were further polymerized to original-quality polymers following the evaporation-induced interfacial self-assembly strategy as described in the Supporting Information. Results and Discussion Design principle The building blocks feature a typical amphiphilic structure. The hydrophobic part contains a 1,2-dithiolane ring as a dynamic polymerizable unit while the polar carboxylate end acts as the hydrophilic end with an oligo-polypeptide linkage of tunable length. Glycine was introduced to form β-sheet-like H-bonds as interchain noncovalent crosslinkers. Monomers were prepared via a simple amide coupling reaction at gram scale (see experimental procedures and molecular characterizations in the Supporting Information). This led to thioctic-glycine derivatives with varied side chain length (TGly-n) (n = 1–6) for systematic investigation ( Supporting Information Figure S1). The polymerization of these amphiphilic 1,2-dithiolanes followed our previously reported method, that is, evaporation-induced interfacial self-assembly.34 A free-standing dry film was obtained by slowly evaporating the solvent (water) from the monomer solutions in atmospheric conditions. In this case, the presence of β-sheet H-bonds should facilitate the assembly process of monomers when concentrating during evaporation because the formation of intermolecular H-bonds essentially counteracts partial entropic decreases during self-assembly. Consequently, the evaporation process slowly increases the concentration of the system, thus shifting the monomer/polymer equilibrium toward the polymer side. Meanwhile, the interfacial effect of evaporation templates the secondary ionic-bonding stacking of the polyelectrolytes, thus resulting in highly ordered layered architectures. The interlayer distance as well as the water channel is apparently key for ion transport.51,52 Thus, this unique hierarchically assembled system was used as a research model for the structure-property relationships of cation conductivity within the layered water channels crosslinked by ionic bonds. Structural characterization The resulting free-standing poly(STGly-n) films were translucent ( Supporting Information Figure S2), exhibiting strong birefringence under polarized optical microscopy. This suggests the existence of crystallized domains ( Supporting Information Figure S3). The polymeric nature of the film was first confirmed by several experimental tests: (1) broad and shifted peaks attributed to polymeric species were observed in the 1H NMR spectra ( Supporting Information Figure S4); (2) the disappearance of a monomeric absorption band at 330 nm in the UV–vis absorption spectra indicates the monomer-to-polymer conversion ( Supporting Information Figure S5)53; (3) the increased peaks of ʋ(O–H) around 3300 cm−1 in the attenuated total reflectance infrared (ATR-IR) spectra reveal the presence of H-bonding in the polymeric networks ( Supporting Information Figure S6); and (4) the distinctive Raman signals of disulfide bonds shifted from 510 to 525 cm−1, thus proving the successful conversion of strained cyclic disulfides (monomers) into unstrained linear disulfides (polymers) ( Supporting Information Figure S7).54 A structural feature of this system rests on the stacking manner of H-bonds in the polymer network. An apparent consequence of introducing H-bonds is the significant increase in monomeric melting points from 60 (TA) to 253 °C (TGly-4) ( Supporting Information Figure S8). We luckily obtained an X-ray single-crystal structure of TGly-2 after many attempts (crystals are labile under ambient conditions due to polymerization) ( Supporting Information Table S1). In the solid state, the molecules assembled into rare β-sheet-like H-bonding arrays: every molecule connected with four other molecules by forming six H-bonds (1.759−2.067 Å), thus extending to a reticular H-bonding framework ( Supporting Information Figure S9). Notably, the crystal cell width (19.394 Å) (Figure 2a) representing the periodic interlayer distance was consistent with the result of SAXS. The sharp scattering signal at 0.33 Å−1 [full-width at half-maximum (FWHM) = 0.0014 Å−1] indicates high structural order with 19.04 Å ( Supporting Information Figure S10). The highly fitted data enabled SAXS to directly evidence further structural characterization of poly(STGly-n) (vide infra). Figure 2 | Structural characterization of ordered self-assembled supramolecular H-bonding networks. (a) The antiparallel molecular self-assembly of TGly-2 mediated by reticular H-bonds (along b axis) (a “side” evidence for the β-sheet H-bonds in poly(STGly-2)). (b) Synchrotron-radiation SAXS patterns of the poly(STGly-2) film. Inset image shows the distinctive scattering ring of the sample. (c) One-dimensional SAXS plots of poly(ST) and poly(STGly-1,2,3,4,5,6) polymer films. (d) The corresponding layer-distance relationship of poly(ST) and poly(STGly-1,2,3,4,5,6) films. (e) ATR-IR spectra of poly(STGly-1,2,3,4,5) polymer films. (f) ATR-IR spectra of the poly(STGly-2) polymer film before (red line) and after (cyan line) absorbing D2O vapor. Download figure Download PowerPoint SAXS analysis was used to further characterize the structural order of the resulting poly(ST) and poly(STGly-n) films (Figure 2b). All polymer films showed sharp scattering peaks, thus indicating a high packing order at the nanoscale. The broad and low peak in the SAXS plot of poly(ST) suggests an amorphous hydrophobic poly(disulfide) main chain phase due to the absence of H-bonding confinement, which was not observed in the SAXS plots of poly(STGly-n). The scattering vector (q) decreased from 0.299 to 0.109 Å−1 upon an increase in the number of amide bonds from 0 to 6 (Figure 2c). The corresponding periodic layer distances (from 2.10 to 5.76 nm) were nearly linearly related to the number of amide units (N) as calculated by the Bragg equation. Interestingly, an odd–even effect was observed when N = 0–3 (Figure 2d): The layer distance difference (Δd) was 0.47–0.48 nm between N and N + 1 when N was odd. It was 0.78–0.79 nm when N was even. This effect was absent when N = 4 or 5, which may be the result of less order as indicated by the relatively broad peaks when N = 4 or 5. Further analysis by SAXS of poly(STGly-1,2,3) showed distinct second-order scattering signals and minimal FWHM (<0.1 Å−1) versus poly(ST) and poly(STGly-4,5,6) ( Supporting Information Figures S11 and S12), thus revealing that overlong side chains led to some disorder—this was also suggested by the random orientation of crystalline domains in the isotropic two-dimensional X-ray scattering pattern ( Supporting Information Figure S11). A signature of β-sheet-like H-bonds is the IR vibration bands of amide shifted to short-wavenumbers. ATR-IR spectra of poly(STGly-n) films showed a distinctive vibration band (νN–H at 3290 cm−1; νC=O at 1635 cm−1) and stretching band (δN–H at 1540 cm−1) (Figure 2e and Supporting Information Figure S13). These observations are consistent with the literature.55 Hydrophilic ionic bonds made the resulting poly(STGly-n) films capable of water absorption. Using D2O as a probe, the IR band of the amide groups was shifted from 1635 to 1630 cm−1, thus proving the co-assembly of amide units with D2O. New vibration peaks (νO–Dfree at 2470 cm−1, νO–Dbonding at 2410 cm−1) appeared as well, indicating the existence of bound D2O in the polymeric networks (Figure 2f and Supporting Information Figure S14). The release process of H2O was performed to further confirm the reversible absorption/desorption. After drying at low humidity (<20% RH), the decreased peak around 3500 cm−1 and shifted peaks of the amide groups from 1635 to 1640 cm−1 proved the humidity-responsive H-bonding regulation ( Supporting Information Figure S15). Mechanical properties Rheological analysis was used to further study the mechanical properties of the resulting materials. The storage modulus (G′) was higher than the loss modulus (G″) over a broad range of strain and frequency. This suggested a typical crosslinked network with a typical rubbery plateau region ( Supporting Information Figure S16). In the oscillation amplitude curves, poly(STGly-1,2,3) samples manifested a broader single plateau region and enhanced G′ value from 0.1 up to 4 MPa with elongated peptide chains, thus indicating the strengthening effect of the β-sheet-like side-chain interactions ( Supporting Information Figure S16a). In the small-amplitude frequency sweeping curves ranging from 10−2 to 102 Hz, poly(STGly-3) retained a consistent maximum G′ value of approximately 3 MPa, thus suggesting a typical rubbery plateau with frequency-dependency. The G′ value of poly(STGly-2) exhibited a slight increase from 1 to 2 MPa, and that of poly(STGly-1) showed a distinct increase from 0.06 to 0.9 MPa ( Supporting Information Figure S16b). In the temperature-cycle experiments, the dynamic moduli exponentially decreased as temperatures increased from 20 to 80 recovered to the value upon to 20 This may be due to dynamic disulfide of polymer main chains and the side chain H-bonds ( Supporting Information Figure These the of the resulting ionic which may the materials dynamic functions such as and under mild conditions. tests were performed to further the mechanical properties of the resulting materials. N 2, the curves an initial region mechanical modulus followed by a in tensile with thus revealing the energy by H-bonds in the supramolecular networks. N a linearly process was observed in the with a maximum mechanical MPa and modulus This suggested the nature of the network ( Supporting Information Figure of the good and semi-crystalline properties of the the polymer film under a polarized optical exhibited varied the of tension ( Supporting Information Figure These data confirm the of H-bonding crystalline This facilitates further order and in the resulting layered network upon of external which is for material and applications. efficiency then to the of the resulting ionic water channels in the ordered layered network can be used for ion applications. with the water channels of amorphous polymer as the assembled water channels may facilitate ion efficiency and high-performance materials for potential applications. The ion efficiency of the resulting poly(STGly-n) films was via a used EIS based on a sandwich-like ( Supporting Information Figure to 80 the ion conductivities at RH an approximately linear increase from × to × (Figure and Supporting Information Figure These were higher than those of most previously reported solid polymer as well as solid polymer (Figure The Arrhenius was used to the activation energy of poly(STGly-1,2,3) which were and The low Ea value be attributed to ion-transport of highly ordered water channel by H-bonding Moreover, the films with as the cation of showed high conductivity, thus suggesting the of this material (Figure Figure 3 | ion to the water channels in the layered amphiphilic polymer networks. (a) Arrhenius plots of conductivities of poly(STGly-1,2,3) films at (b) A of conductivity of poly(STGly-1,2,3) films with other reported and at different (c) conductivity of poly(STGly-1,2,3) films at temperature. (d) and of the layered of poly(STGly-n) films 0.5 mm) the of the on The interlayer distance from to nm for from to nm for from to nm for Download figure Download PowerPoint ion conductivity ranging from RH to RH was further The ion conductivity increased exponentially from × to × in the range of RH to with the sharp increase from RH to which from the formation of H-bonding water networks ion-transport (Figure SAXS the structural of poly(STGly-n) films at different RH ( Supporting Information Figure The q value decreased with RH and confirmed the layer thus indicating increased layer distances after the adsorption of water molecules (Figure The conductivities reveal that the high hydrophilicity of the interlayer was for the formation of ordered water channels under conditions. In the highly hydrated the by water could the ionic thus the activation energy of the ion and the Chemical closed-loop A feature of the poly(disulfide)s main chain is reversible polymerization that enables chemical (Figure the depolymerization of the resulting materials, were used to the disulfide and water as an The poly(STGly-n) could be readily dissolved in alkaline aqueous solution within 5 min at temperature. Further depolymerization of poly(STGly-1,2,3) in 0.1 M alkaline aqueous solution (NaOH, Na2CO3) and water were by UV–vis absorption spectroscopy to the of H-bonding and on the depolymerization ( Supporting Information Figure The absorption peaks at and 330 nm were to poly(STGly-n) and STGly-n For distinct peaks were observed at thus suggesting the depolymerization to in The exhibited with the H-bonding and with solution ( Supporting Information Figure thus revealing polymeric networks and Figure 4 | dynamic properties chemical closed-loop recyclability. (a) Schematic and of the closed-loop recycling process of the poly(STGly-n) polymers and (b) 1H NMR spectra of poly(STGly-3) and recovered (c) SAXS patterns of

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