Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE7 Nov 2022Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes Mingxin Zhang†, Haitao Yu†, Qin Zou, Zi-Ang Li, Yuyan Lai, Linkun Cai and Panchao Yin Mingxin Zhang† State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 †M. Zhang and H. Yu contributed equally to this work.Google Scholar More articles by this author , Haitao Yu† State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 †M. Zhang and H. Yu contributed equally to this work.Google Scholar More articles by this author , Qin Zou State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Zi-Ang Li State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Yuyan Lai State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Linkun Cai State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author and Panchao Yin *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101718 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal–organic polyhedra (MOPs) possess a microporous framework and impose hierarchical constraints on their surface ligands, leading to the long-ignored, logarithmic ligand exchange dynamics. Herein, polymer networks with MOP as nanoscale cross-linkers (MOP-CNs) can integrate unique ligand exchange dynamics and microporosity, affording vitrimer-like gas separation membranes with promising mechanical performance and (re)processability. All the ligands on the MOP surfaces are confined and correlated via a 3D coordination framework and their neighboring spaces, giving rise to a high energy barrier for ligand exchange. Therefore, MOP-CNs demonstrate high mechanical strengths at room temperature due to their negligible ligand dynamics. The thermo-activated ligand exchange process with integrated network topology enables facile (re)processing and high solvo-resistance at high temperatures. This facilitates Arrhenius type temperature dependence of flowability and stress relaxation, giving rise to the simultaneous achievement of promising mechanical strengths and (re)processability. Finally, the cage topologies of MOPs endow the materials with a bonus microporous feature and spur their applications as gas separation membranes. Download figure Download PowerPoint Introduction Chemically cross-linked polymeric systems, though possessing high mechanical moduli, appreciable solvent resistance, and thermal stabilities, impose enormous difficulty for (re)processing and recycling.1–5 The introduction of an exchange and reversible covalent cross-linker leads to the so-called covalent adaptable networks (CANs), which demonstrate re-organizable network topologies upon the application of external stimuli and a resulting facile processability and recyclability.6 Generally, the exchangeable cross-linker chemistry can be achieved through either dissociative pathways via reversible chemical reactions or associative pathways with a bond exchange mechanism.7 The associative CANs, for example, vitrimers, show topological rearrangement driven by the bond exchange reactions without breaking the integrity of the network structures. They, therefore, exhibit a certain degree of solvo-resistance at high temperature and gradual thermal bulk viscosity decrease that obey Arrhenius temperature dependence.8 The processability of dissociative CANs is facilitated by the breaking and reforming of cross-linked bonds, which, theoretically, should lead to an abrupt bulk viscosity decrease and the dissolution of network structure in solutions at high temperatures.9 Actually, most dissociative CANs demonstrate similar reprocessing behavior to the associate ones that mainly originates from the interplay between the break and reform dynamics of the cross-linker and hierarchical polymer chain relaxation dynamics.8,10 Basically, the practical application of CANs highly relies on a moderate processing temperature between the materials’ characteristic service and decomposition temperatures.6 The enrichment of CAN systems with broad options of bond strengths and energy barriers for bond dynamics is of great significance to the material-by-design for structural materials with targeted processability and mechanical performances. Meanwhile, the effective integration of desired functionalities in CANs would be an important milestone for the design of functional materials upon the increasing requirements of sustainable materials.11 The introduction of function-bearing units, for example, nanoparticles (NPs), in CANs provides a convenient option for functionalization integration, which, however, could face an adverse effect to their characteristic properties from the spatially close contact of the two phases.12 Moreover, the delicate tuning of surface functionalization is required to ensure the compatibility between functional units and CANs for long-term stability. Therefore, the exploration of strategies for the synergic integration of functionalities in CANs is urgent. Especially, the incorporation of a microporous phase into CANs will be of high practical interest for the design of (re)processible and recyclable mixed matrix membranes (MMMs) for gas purification and separations. Recent work on the supramolecular complex of metal–organic polyhedra (MOPs) with polymer provide possible options.13 However, the lack of effective integration strategies hinders the development of MMMs from CANs. Coordination bonds can be imposed between organic ligands and metal ions with various geometrical topologies and tunable bond strengths, enabling their extensive applications in constructing dynamic and permanent supramolecular networks, respectively.14,15 Utilizing the fast dynamic nature of certain coordination bonds, the concept of sacrificial bonds has been proposed for circumventing the trade-off between stretchability and stiffness in elastomers.16 To strengthen the coordination bonds and the obtained networks, the introduction of bi- or multidentate ligands with appropriate geometry17–19 can lead to terminal assemblies with high symmetries, which can act as stable cross-linking agents for the facile construction of network structures.20,21 Typically, MOPs with cage topologies can be treated as cross-linkers with negligible bond dynamics, serving as analogues for multiple-site covalent cross-linking agents.18 Nevertheless, herein, the exploration of the structural dynamics of MOPs in their polymer composites enhances our understanding of MOPs: the ligand exchange on the MOP surface, actually, shows unique logarithmic kinetics and can be activated at high temperatures.22 Originating from the correlation of hierarchically constrained multiple coordination bonds,23 the MOP frameworks show extremely slow ligand exchange dynamics and high stability and bond strengths at temperatures <40 °C, while they possess fast ligand exchange dynamics with intact framework structures at high temperatures (<120 °C), which resembles typical dynamic covalent bonding in thermo-activated CAN systems.24 In our studies, 2.5 nm MOPs from isophthalic acid (IPA) ligands and Cu2+ are employed as both microporous units and the only dynamic cross-linkers in a polymer matrix to fabricate thermo-activated CAN gas separation membranes from hierarchically constrained clustering of coordination bonds. The temperature-dependence of the dynamic ligand exchange of MOPs in the polymer matrix has been evaluated through dielectric spectroscopy, scattering, and imaging techniques. The flowabilities and stress relaxation of MOP cross-linked polymer networks (MOP-CNs) possess Arrhenius-type temperature dependence, enabling high mechanical strengths, and robust structure at ambient conditions and convenient (re)processability at high temperatures. The coordination glassy networks’ protocol can be generally extended to typical polymeric systems, including polyacrylate and epoxy polymers, while the microporous feature of MOPs can promote MOP-CNs’ applications as gas separation membranes. In coordination chemistry, the enriched collection of ligands and metal ion centers would enable the rational tuning of topologies and critical processing temperatures of MOP-CNs.17 Experimental Methods The synthesis of macro-ligand (ML_IPA) ML_IPA was synthesized by the typical radical polymerization method. n-Butyl acrylate (24.2 mL (169 mmol), purified by CaH2) and acrylic-IPA (5 g, 21 mmol) were dissolved in 40 mL dimethyl sulfoxide (DMSO; purified by CaH2) in a 100 mL engineered flask (equipped with Teflon gaskets) and 50 mg (305 mmol) azodiisobutyronitrile (AIBN, recrystallized from hot alcohol). After three freeze–thaw cycles to eliminate dissolved oxygen and air, the mixture was placed under vacuum and stirred in a 70 °C oil bath for 10 h. Finally, the mixture was precipitated in water. Then the sediment was dissolved in tetrahydrofuran (THF), precipitated in hexane, and dried under vacuum at 80 °C to obtain ML_IPA. The fabrication of MOP-CNs MOP-CNs were fabricated with the following three steps. ML_IPA (3.0 g) was dissolved in dried THF (200 mL) with a dropwise addition of copper(II) acetate (Cu(OAc)2·H2O) dissolved in dried THF (100 mL) at a ligand (IPA) to copper ion ratio of 1:0.8. The mixed solution was heated to 70 °C to remove part of the solvent. In a slow evaporation step, the solution (about 50 mL) was transferred into a poly(tetrafluoroethylene) (PTFE) vessel (diameter is about 50 mm) which was placed in a semi-closed environment (50 °C) for 24 h to obtain a blue free-standing film. Next, for solvent- and thermal-annealing, the film was annealed in dried DMSO steam (120 °C) for 48 h. The postannealed MOP-CN was obtained after drying under vacuum at 80 °C for 12 h. All used samples were the post-annealed MOP-CNs ( Supporting Information Figure S7) unless otherwise noted. Recycling and remodeling of MOP-CNs MOP-CNs were reprocessed easily by hot-pressing, a widely-used polymer processing technique. MOP-CNs were cut into pieces and loaded into a mold with the desired shape. The recycled MOP-CNs were remodeled at 150 °C under 20 MPa pressure for 5 h. The reprocessing temperature was higher than the ideal Tp to shorten the processing time to an acceptable range. Results and Discussion Ligand exchange dynamics among MOPs in their bulks Originating from the hierarchically constrained dynamics of MOP ligands,25,26 the ligand exchange on the MOP surface demonstrates logarithmic kinetics, which is distinct from the ligand exchange kinetics of simple coordination complexes (usually first order reaction kinetics).27,28 This enables the unique thermally activated polymer network dynamics of MOPs rather than the fast dynamics of simple coordination complexes.29,30 To further confirm the potential of MOPs as dynamic cross-linkers, ligand exchange on the MOP surface as well as structural stability in the bulk state was evaluated in the mixtures of two types of MOPs with distinct sizes. PS-MOPs assembled from macromolecular ligands [polystyrene (PS) with IPA end, PS-IPA, Mw = 3.3 kg/mol] and C18-MOPs assembled from short alkyl ligands (C18-IPA, linear alkyl chain, C18H37, with IPA end) are blended and annealed at 160 °C in bulk while the size evolution of the two MOPs is monitored (Figure 1a). In size exclusion chromatography (SEC) studies, the continuously decreasing sizes of PS-MOPs were correlated with gradual increasing sizes of C18-MOPs (Figure 1c), suggesting the ligand exchange among MOPs. Interestingly, no side product was observed with the integrity of the MOP framework in the ligand exchange reaction. The ligand exchange kinetics in bulk studied here was slower than that in solution in our previous report due to slow polymer chain dynamics in the bulk state ( Supporting Information Figure S2).24 It is confirmed from small angle neutron scattering (SANS) studies (Figure 1b and Supporting Information Figure S3) that ligand exchange on MOPs was activated at high temperatures while the MOP frameworks were always intact. This implies a typical associative mechanism of the rearrangement of the coordination framework without disassembling of the MOP was responsible for the ligand exchange process. Figure 1 | The ligand exchange of MOPs in bulk. (a) The structure of MOP, C18-IPA, PS-IPA, C18-MOP, and PS-MOP; and the experiment design to monitor the ligand-exchange reaction between C18-MOP and PS-MOP. (b) SANS patterns of the final products (hybrid–ligand MOP with average six PS arms on one MOP, circular ring) and their fitting curves with core–shell model (solid line). (c) The stepwise exchange process between PS-MOP and C18-MOP (the initial molar ratio is 6–18) recorded by SEC. The merging of two peaks is attributed to the exchange between PS-MOP and C18-MOP at 160 °C. Download figure Download PowerPoint The design and synthesis of MOP-CNs To facilitate polymer cross-linking and the ligand exchange on the MOP surface, IPA ligands were randomly anchored as side chains of polymers with different backbones, for example, polyacrylates and epoxy polymers, and their further complexation with Cu2+ led to the formation of polymer networks, MOP-CNs (Scheme 1a and Supporting Information Figure S1). Meanwhile, a permanent covalent network (PCN) and weak coordination network (WCN) with the same cross-linking density as that of MOP-CNs were prepared for control studies (Schemes 1b and 1c and Supporting Information Figure S1). For MOP-CNs, the form factor of MOP was observed in small-angle X-ray scattering (SAXS) studies, suggesting the formation of MOPs, which was also confirmed by the evenly distributed MOP particles in transmission electron microscopy (TEM) studies (Figure 2a and Supporting Information Figures S5 and S6). Due to the multicross-linking node feature of MOP (24 strands on the 2.5 nm spherical surface) in polymer networks, the elastic modulus (26.5 MPa) of MOP-CN was much higher than that of WCN, although they possess a similar density of cross-linker units (Figure 2b). We attributed the high stretchability of WCN to the fast breaking and reformation dynamics of simple coordination bonds, whereas the comparatively rigid nature of MOP-CN was largely caused by the static dynamics of MOPs at room temperature (Figure 2b). This was further confirmed by the cycle tensile test where MOP-CN demonstrates superior mechanical stability that is almost comparable to PCN (Figure 2c and Supporting Information Figure S4a). The WCN irreversibly elongates upon strain increase, which demonstrates a similar fast relaxation to sacrificial linkages for energy dissipation (Figure 2d) in previous work.16,31 Meanwhile, similar to vitrimers,32 MOP-CN also exhibited the desired solvent resistance like PCN even after soaking over 2 days in DMSO at 120 °C (Figure 2e). Scheme 1 | The preparation and structure of coordination CANs. (a) MOP-CN, (b) PCN, and (c) WCN; the inset (right) is the remodeling behaviors comparing MOP-CN and PCN. Download figure Download PowerPoint Figure 2 | (a) X-ray scattering pattern of the merging of small angle and wide angle for MOP-CNs, simulated MOP in good solution, and pure polymer matrix (macroligands, ML_IPA). The inset is the TEM image of MOP-CN. The isolated MOP (about 4 nm) can be seen clearly. (b) Tensile test for MOP-CN and WCN. (c and d) Cycle-tensile behavior of MOP-CN and WCN, respectively. A 30 s interval was used between two cycles. (e) The solvent resistance testing of MOP-CN and WCN at 120 °C at different time. (f) Tensile test for samples as synthesized and after grinding into a powder and remolding eight times. Download figure Download PowerPoint Cross-linking bond exchange dynamics The network dynamics of MOP-CNs were further monitored using dynamic mechanical analysis (DMA) and rheometric technique under different temperatures to evaluate their potential as CANs. Interestingly, MOP-CNs showed relaxation dynamics similar to PCN at room temperature, while distinct dynamics were observed at high temperatures (<100 °C). Only the polymer strand relaxation can be detected for both samples at room temperature, suggesting a static feature of MOP cross-linker and the frozen dynamics of MOPs (see Supporting Information Figures S10 and S11). The dynamic character of MOPs was activated at high temperatures, the resulting MOP-CN stress relaxation data fit well to the Kohlrausch function (Figure 3a), and the characteristic relaxation time τ * was extracted G ( t ) G ( 0 ) = A e − ( t τ * ) β where G(t) and G(0) are the storage moduli at time t and at the start of the experiment, respectively; A is a pre-exponential constant; τ * is relaxation time; β ( 0 < β ≤ 1 ) is the stretching exponent that is related to the breadth of the distribution of relaxation modes.33 The flowing activation energy (Ea) for flowing dynamics was determined by studying the temperature dependence of τ * (Figure 3b), which should reflect the MOP ligand exchange dynamics modulated by chain segment relaxation dynamics.34 The ligand diffusion in MOP-CNs was harnessed by the polymer chain dynamics, and therefore, the flowing activation energies are higher than that previously reported for MOP ligand-exchange in solutions.24 In a typical Angell plot, MOP-CNs behave similarly to silica and demonstrate a gradual, Arrhenius-like viscosity reduction at elevated temperatures, confirming their promising (re)processability as CANs (Figure 3c and Scheme 2). Moreover, the storage moduli of MOP-CNs remained constant at high temperature range (T < 80 °C, Figure 3d), implying that the flow of these materials is facilitated by the topological rearrangements of the networks without changing the integrity of the networks. Such a mechanism is also supported by the long-term stability of MOP-CNs when soaked in DMSO, the good solvent for MOP-CN, at 120 °C (Figure 2e). For control studies, the polymer networks from a simple coordination complex (e.g. WCN) or coordination with NPs demonstrate high solubilities in solvents at high temperatures.15,35 The processing temperature Tps of MOP-CNs were determined as ca. 100 °C based on their flowability measurements (Discussion in the Supporting Information).36 Practically, temperatures higher than the measured Tp were usually adopted for CAN (re)processing and recycling to shorten the processing time to an acceptable range. Figure 3 | (a) Representative stress relaxation experiments of MOP-CNs for the temperature interval between 100 and 200 °C tested by DMA. Solid line is fitting curve with Kohlrausch function for the raw relaxation data. The detailed fitting data can be found in Supporting Information Table S1. (b) Arrhenius function fitting for relaxation time versus 1000/T. The fitting function is y = 21.34x − 45.25 (R2 = 0.99). (c) Angell fragility plot of MOP-CN, showing the viscosity as a linear function of the inverse temperature, scaled with Tp (Tp < The line is the linear fitting function = − = The data of MOP-CN and PCN are recorded under the small The almost constant storage modulus in the high temperature range the stable cross-linking in polymer (e) The broad dielectric relaxation of MOP-CN. The of modulus two relaxation process and β the temperature °C) and (f) The characteristic relaxation time of different relaxation process and β and their fitting with or Arrhenius The figure is one fitting of a different relaxation process at °C with two complex modulus Download figure Download PowerPoint The hierarchical relaxation process of MOP-CNs were via dielectric by their to external To possible the complex modulus ( * ( Figure was used of complex ( * the measured polymer chain relaxation was attributed to the dynamics between chain segment relaxation and ligand exchange dynamics of MOPs. Interestingly, a from to Arrhenius temperature dependence (Figure was observed at ca. 160 °C for the measured relaxation, which is with the Tp of The ligand exchange process of MOP is by the chain segment diffusion at a temperature whereas the chain diffusion than the chain relaxation at high temperatures, leading to the Arrhenius temperature This was by the that the activation energy of relaxation Supporting Information Table S3) at high temperature range was almost to that for the process of MOPs in solutions (26.5 All experiments that polymer network rearrangement is to the dynamics of MOPs in the MOP-CN matrix with the integrity of MOP a possible associative rearrangement mechanism for Scheme 2 | Ligand and topological rearrangement in Download figure Download PowerPoint The (re)processability of MOP-CNs All MOP-CNs were easily reprocessed via a typical at ca. 150 °C, while the stability of MOP-CNs under thermal processing was confirmed by the that no was observed after at 200 °C in either or environment for 2 h (see Supporting Information Figure The MOP-CNs were recycled and remodeled through (Scheme 1c), while the tensile strengths of MOP-CNs exhibited acceptable than Figure and Supporting Information Figure even after reprocessing eight times. The of the reprocessed MOP-CNs a constant and was no from possible permanent even after the side reactions in MOP-CN The reprocessing temperature can be through the addition of of DMSO ( Supporting Information Figures and Table The high thermal and stability and side feature of the networks as well as the thermo-activated ligand dynamics ensure MOP-CNs’ practical (re)processability. Gas separation performance The dynamic MOPs, are typical with Figure the MOP-CN with the integrated for gas The MOP-CN gas separation to that of PCN (Figure and the performance of most The surface of MOPs in MOP-CN can be to external gas giving rise to the of the MOP-CN membranes (Figure and Supporting Information Figure This leads to the of for to the high of for MOP-CN. Generally, membranes with both high and are desired for practical However, pure polymeric systems (Figure that are to are generally to gas mixtures and the of gas polymer membranes is by their process in The MOPs to a polymer network can break the trade-off the of of MOPs to external gas can the gas separation of the polymer matrix and provide of the obtained membranes (Figure Finally, the MOP-CN membranes for separation (Figure our MOP-CNs showed potential to as recyclable gas separation and in the will the integration of various functionalities to CAN Figure 4 | The behaviors of MOP-CN. (a) The of MOP-CN for gas (b) The gas performance of MOP-CN, and the distribution is in this (c) and diffusion for the of different through PCN or MOP-CN. separation by MOP-CN and PCN comparing with the reported in Download figure Download PowerPoint The unique thermo-activated ligand-exchange dynamics of MOPs was used to thermo-activated CANs from hierarchically constrained clustering of coordination bonds. The obtained MOP-CNs behave similarly to silica in their Arrhenius temperature dependence of their characteristic confirming the associative mechanism for network fast ligand exchange dynamics Tp can the network topology with the cross-linking density Moreover, MOP-CNs show robust without the mechanical even after recycling eight times. The of the MOP-CNs was upon the introduction of the side reactions promising the processing The reversible cross-linkers, MOPs, endow the materials with gas separation a facile for gas separation membranes with appreciable gas separation and mechanical as well as the applications of coordination complex for CAN design and provide options to integrate functionalities to CAN systems, which could be for the enrichment of CANs from various chemical and even bonding Supporting Information Supporting Information is and data and Figures and Supporting of The no The work is supported by the Science of China and and the Science of The would like to Zhang from University and Zhang from University for and The from for the of SANS measurements and of for the to the is also H. 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