MOF-Polymer Hybrid Materials: From Simple Composites to Tailored Architectures.

Metal-organic frameworks (MOFs) are inherently crystalline, brittle porous solids. Conversely, polymers are flexible, malleable, and processable solids that are used for a broad range of commonly used technologies. The stark differences between the nature of MOFs and polymers has motivated efforts to hybridize crystalline MOFs and flexible polymers to produce composites that retain the desired properties of these disparate materials. Importantly, studies have shown that MOFs can be used to influence polymer structure, and polymers can be used to modulate MOF growth and characteristics. In this Review, we highlight the development and recent advances in the synthesis of MOF-polymer mixed-matrix membranes (MMMs) and applications of these MMMs in gas and liquid separations and purifications, including aqueous applications such as dye removal, toxic heavy metal sequestration, and desalination. Other elegant ways of synthesizing MOF-polymer hybrid materials, such as grafting polymers to and from MOFs, polymerization of polymers within MOFs, using polymers to template MOFs, and the bottom-up synthesis of polyMOFs and polyMOPs are also discussed. This review highlights recent papers in the advancement of MOF-polymer hybrid materials, as well as seminal reports that significantly advanced the field.

Insights into the Structure and Dynamics of Metal-Organic Frameworks via Transmission Electron Microscopy.

Metal-organic frameworks (MOFs) are hybrid materials composed of metal ions and organic linkers featuring high porosity, crystallinity, and chemical tunability at multiple length scales. A recent advancement in transmission electron microscopy (TEM) and its direct application to MOF structure-property relationships have changed how we consider rational MOF design and development. Herein, we provide a perspective on TEM studies of MOFs and highlight the utilization of state-of-the-art TEM technologies to explore dynamic MOF processes and host-guest interactions. Additionally, we provide thoughts on what the future holds for TEM in the study of MOFs.

Hierarchical Fractal Assemblies from Poly(ethylene oxide- b-lysine- b-leucine).

Poly(ethylene oxide43- b-lysine62- b-leucine72) (wherein subscripts denote the degree of polymerization) was synthesized via ring-opening polymerization of N-carboxyanhydrides using an amine-terminated poly(ethylene oxide) macroinitiator, with polypeptide blocks produced by sequential monomer addition. Infrared and circular dichroism spectroscopy indicated that the peptide blocks in this polymer formed α-helices in the solid and solution states, respectively. In the aqueous solution, this polymer self-assembled into spherical micelles with a hydrodynamic radius of approximately 90 nm at concentrations between 0.05 and 0.20% w/w and pH values between 2 and 6.5. Upon preparation of transmission electron microscopy (TEM) grids, the micelles at pH 2 underwent hierarchical assembly to produce fractal assemblies, whereas small clusters were observed for micellar solutions at pH 6.5. Cryogenic-TEM of solutions showed spherical micelles, and dynamic light scattering showed no large (∼1 µm) aggregates in the solution, which suggests that fractal formation was a result of the drying process, and that fractals were not present in the solution. This system provides a facile route to nanostructured surfaces, which can be used for applications such as modulating cell adhesion or promoting the growth of neurons.

Nylon-MOF Composites through Postsynthetic Polymerization.

Hybridization of metal-organic frameworks (MOFs) and polymers into composites yields materials that display the exceptional properties of MOFs with the robustness of polymers. However, the realization of MOF-polymer composites requires efficient dispersion and interactions of MOF particles with polymer matrices, which remains a significant challenge. Herein, we report a simple, scalable, bench-top approach to covalently tethered nylon-MOF polymer composite materials through an interfacial polymerization technique. The copolymerization of a modified UiO-66-NH2 MOF with a growing polyamide fiber (PA-66) during an interfacial polymerization gave hybrid materials with up to around 29 weight percent MOF. The covalent hybrid material demonstrated nearly an order of magnitude higher catalytic activity for the breakdown of a chemical warfare simulant (dimethyl-4-nitrophenyl phosphate, DMNP) compared to MOFs that are non-covalently, physically entrapped in nylon, thus highlighting the importance of MOF-polymer hybridization.

Quantitative relationship between cavitation and shear rheology.

Cavitation rheology is a powerful, simple, and inexpensive technique to study the moduli of polymer gels, however its use has not yet become widespread because few studies to date have directly compared this technique to traditional oscillatory shear rheology. Herein, we report a quantitative relationship between the gel modulus determined using cavitation and shear rheology for three series of model gels whose networks are composed of (1) permanently covalent, (2) dynamic-covalent, and (3) physical hydrogen-bond crosslinks. We determine a simple proportionality constant that allows for conversion of the moduli obtained from both types of experiments and is highly dependent on the bond energy responsible for gelation. This study provides a framework for researchers in a broad range of disciplines who can exploit the ease of cavitation rheology and place their results in the context of traditional oscillatory shear rheology.

Supramolecular Metallopolymers: From Linear Materials to Infinite Networks.

The fields of coordination polymers (CPs) and supramolecular metallopolymers (SMPs) have been extensively studied for decades. Spectacular recent advances in both fields have created new compounds that lie at the interface between these two classes of organic-inorganic hybrid materials. At this interface, materials based on molecular weaving, supramolecular clusters, and metal-organic framework-polymer hybrids have emerged. This minireview provides a perspective on the intellectual emergence and connection between different supramolecular constructs and the state of the art with respect to this new materials interface.

Solvent effects on modulus of poly(propylene oxide)-based organogels as measured by cavitation rheology.

A series of novel organogels were synthesized from poly(propylene oxide) (PPO) functionalized with main chain urea moieties which provided rapid gelation and high moduli in a variety of solvents. Three different molecular weight PPOs were used in this study: 430, 2000, and 4000 g mol(-1), each with α,ω-amino-end groups. Four urea groups were introduced into the main chain by reaction with hexamethylene diisocyanate followed by subsequent reaction with a monofunctional alkyl or aromatic amine. This PPO/urea gelator was found to form gels in carbon tetrachloride, chloroform, dichloromethane, toluene, ethyl acetate, and tetrahydrofuran. Among these, carbon tetrachloride and toluene were found to be the best solvents for this system, resulting in perfectly clear gels with high moduli at low mass fraction for PPO-2000 systems. Flory-Huggins polymer-solvent interaction parameter, χ, was found to be a useful indicator of gel quality for these systems, with χCCl4/PPO-2000 < 0.5 and χtoluene/PPO-2000≈ 0.5. Systems with χ parameters >0.5 were found to form low moduli gels, indicating that for these systems, polymer-solvent interaction parameters can be a useful predictor of gel quality in different solvent systems.

Remote Detection of HCN, HF, and Nerve Agent Vapors Based on Self-Referencing, Dye-Impregnated Porous Silicon Photonic Crystals.

A one-dimensional photonic crystal is prepared from porous silicon (pSi) and impregnated with a chemically specific colorimetric indicator dye to provide a self-referenced vapor sensor for the selective detection of hydrogen fluoride (HF), hydrogen cyanide (HCN), and the chemical nerve agent diisopropyl fluorophosphate (DFP). The photonic crystal is prepared with two stop bands: one that coincides with the optical absorbance of the relevant activated indicator dye and the other in a spectrally "clear" region, to provide a reference. The inner pore walls of the pSi sample are then modified with octadecylsilane to provide a hydrophobic interior, and the indicator dye of interest is then loaded into the mesoporous matrix. Remote analyte detection is achieved by measurement of the intensity ratio of the two stop bands in the white light reflectance spectrum, which provides a means to reliably detect colorimetric changes in the indicator dye. Indicator dyes were chosen for their specificity for the relevant agents: rhodamine-imidazole (RDI) for HF and DFP, and monocyanocobinamide (MCbi) for HCN. The ratiometric readout allows detection of HF and HCN at concentrations (14 and 5 ppm, respectively) that are below their respective IDLH (immediately dangerous to life and health) concentrations (30 ppm for HF; 50 ppm for HCN); detection of DFP at a concentration of 114 ppb is also demonstrated. The approach is insensitive to potential interferents such as ammonia, hydrogen chloride, octane, and the 43-component mixture of VOCs known as EPA TO-14A, and to variations in relative humidity (20-80% RH). Detection of HF and HCN spiked into the complex mixture EPA TO-14A is demonstrated. The approach provides a general means to construct robust remote detection systems for chemical agents.

Inside polyMOFs: layered structures in polymer-based metal-organic frameworks.

In this report, we explore the internal structural features of polyMOFs consisting of equal mass ratios of metal-coordinating poly(benzenedicarboxylic acid) blocks and non-coordinating poly(ethylene glycol) (PEG) blocks. The studies reveal alternating lamellae of metal-rich, crystalline regions and metal-deficient non-crystalline polymer, which span the length of hundreds of nanometers. Polymers consisting of random PEG blocks, PEG end-blocks, or non-coordinating poly(cyclooctadiene) (COD) show similar alternation of metal-rich and metal-deficient regions, indicating a universal self-assembly mechanism. A variety of techniques were employed to interrogate the internal structure of the polyMOFs, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and small-angle synchrotron X-ray scattering (SAXS). Independent of the copolymer architecture or composition, the internal structure of the polyMOF crystals showed similar lamellar self-assembly at single-nanometer length scales.

Block co-polyMOFs: morphology control of polymer-MOF hybrid materials.

The hybridization of block copolymers and metal-organic frameworks (MOFs) to create novel materials (block co-polyMOFs, BCPMOFs) with controlled morphologies is reported. In this study, block copolymers containing poly(1,4-benzenedicarboxylic acid, H2bdc) and morphology directing poly(ethylene glycol) (PEG) or poly(cyclooctadiene) (poly(COD)) blocks were synthesized for the preparation of BCPMOFs. Block copolymer architecture and weight fractions were found to have a significant impact on the resulting morphology, mediated through the assembly of polymer precursors prior to MOF formation, as determined through dynamic light scattering. Simple modification of block copolymer weight fraction allowed for tuning of particle size and morphology with either faceted and spherical features. Modification of polymer block architecture represents a simple and powerful method to direct morphology in highly crystalline polyMOF materials. Furthermore, the BCPMOFs could be prepared from both Zr4+ and Zn2+ MOFs, yielding hybrid materials with appreciable surface areas and tuneable porosities. The resulting Zn2+ BCPMOF yielded materials with very narrow size distributions and uniform cubic morphologies. The use of topology in BCPMOFs to direct morphology in block copolymer assemblies may open new methodologies to access complex materials far from thermodynamic equilibrium.

Defect-Free MOF-Based Mixed-Matrix Membranes Obtained by Corona Cross-Linking.

Functionalized UiO-66 metal-organic frameworks (MOF) particles were covalently grafted with hydride-terminated poly(dimethylsiloxane) (PDMS) via postsynthetic modification. These PDMS-coated MOF particles (termed here "corona-MOF") were used in the preparation of mixed-matrix membranes (MMMs). Defect-free MMMs with weight loadings of 50% were achieved with corona-MOF particles, attributed to the improved dispersibility of the corona-MOF particles and covalent linkages between the corona-MOF particles and the polymer matrix. The PDMS MMMs showed distinct separation features in single gas permeation tests, displaying much higher CO2 gas permeation with no decrease in selectivity when compared to MMMs prepared with unmodified UiO-66 particles. Single gas separation tests with CO2, N2, and propane were performed to probe the separation mechanism of the corona-MOF MMMs, demonstrating that these MMMs avoid nonideal "sieve-in-a-cage" and "plugged sieves" scenarios. Additionally, due to covalent bond formation between both the MOF and the polymer matrix in corona-MOF MMMs, particle aggregation is negligible during film curing, allowing for the formation of flexible, self-standing MMMs of <1 µm in thickness. Low quantities of polymer covalently attached to the MOF surface (<5 wt %) are sufficient to fabricate thin, defect-free, high MOF-loading MMMs.

Multiple functional groups in UiO-66 improve chemical warfare agent simulant degradation.

A library of 26 mixed ligand UiO-66 analogs was synthesized, characterized, and screened for catalytic degradation of the chemical warfare agent (CWA) simulant dimethyl 4-nitrophenylphosphate (DMNP). The MOFs were screened and compared to physical mixtures of the same single component MOFs. Several of the MOFs display higher catalytic activity than the parent UiO-66 and other single ligand UiO-66 analogues.

Halogen bonding in UiO-66 frameworks promotes superior chemical warfare agent simulant degradation.

Herein, a series of halogenated UiO-66 derivatives was synthesized and analyzed for the breakdown of the chemical warfare agent simulant dimethyl-4-nitrophenyl phosphate (DMNP) to analyze ligand effects. UiO-66-I degrades DMNP at a rate four times faster than the most active previously reported MOFs. MOF defects were quantified and ruled out as a cause for increased activity. Theoretical calculations suggest the enhanced activity of UiO-66-I originates from halogen bonding of the iodine atom to the phosphoester linkage allowing for more rapid hydrolysis of the P-O bond.

Multicomponent metal-organic framework membranes for advanced functional composites.

The diverse chemical and structural properties of metal-organic frameworks (MOFs) make them attractive for myriad applications, but their native powder form is limiting for industrial implementation. Composite materials of MOFs hold promise as a means of exploiting MOF properties in engineered forms for real-world applications. While interest in MOF composites is growing, research to date has largely focused on utilization of single MOF systems. The vast number of different MOF structures provides ample opportunity to mix and match distinct MOF species in a single composite to prepare multifunctional systems. In this work, we describe the preparation of three types of multi-MOF composites with poly(vinylidene fluoride) (PVDF): (1) co-cast MOF MMMs, (2) mixed MOF MMMs, and (3) multilayer MOF MMMs. Finally, MOF MMMs are explored as catalytic membrane reactors for chemical transformations.

Probing Membrane Hydration at the Interface of Self-Assembled Peptide Amphiphiles Using Electron Paramagnetic Resonance.

The relative hydrophilicity at the interface of a nanoparticle was measured utilizing electron paramagnetic resonance (EPR) spectroscopy. The supramolecular structure was assembled from spin-labeled peptide amphiphiles (PA) derived from N-carboxy anhydrides (NCA). Cyanuric chloride, or 2,4,6-trichloro-1,3,5-triazine (TCT), was used as a modular platform to synthesize the spin-labeled, lipid-mimetic macroinitiator used for the ring-opening polymerization of γ-benzyl-l-glutamic acid NCA to produce polyglutamate-b-dodecanethiol2. Through static and dynamic light scattering, as well as transmission electron microscopy, PAs with DP of 50 and 17 were shown to assemble into stable nanoparticles with an average hydrodynamic radius of 117 and 84 nm, respectively. Continuous wave EPR spectroscopy revealed that the mobility parameter (h-1/h0) and 2Aiso of the nitroxide radical increased with increasing pH, in concert with the deprotonation of the PE side chains and associated helix-coil transition. These results are consistent with an increase in the relative hydration and polarity at the nanoparticle interface, which would be dependent on the secondary structure of the polypeptide. This research suggests that a pH stimulus could be used to facilitate water diffusion through the membrane.