Crystalline Polymeric Materials Research Articles

Fast Synthesis of Olefin-Linked Covalent Organic Frameworks via Supercritically Solvothermal Polymerization.

Covalent organic frameworks (COFs) are newly emerging yet rapidly developed porous crystalline polymeric materials, which feature topological designability, structural diversity, and porosity tunability. However, most of the reported COFs hitherto are prepared by a solvothermal method, which needs a long reaction time, especially for the olefin-linked COFs, which hinders their practical applications. Herein, we report the fast growth of olefin-linked COFs synthesized by the supercritical solvothermal method under supercritical carbon dioxide conditions. Via optimization of the reaction conditions, olefin-linked COFs are prepared within 6 h, which is period much shorter than that of the traditional solvothermal method. The as-obtained COFs feature two distinct morphologies, namely, spherical and rod-like. Interestingly, the rod-like crystal shows polarized light extinction and rebrightening phenomena, indicating the high crystallinity. After being doped with I2, the obtained I2@COFTMT-PA shows a Hall mobility of 1.94 cm2 V-1 S-1, indicating its potential applications in optoelectronics. This work expands the application of the supercritically solvothermal method to the fast growth of olefin-linked COFs and solves a long-standing challenge in chemical science, which opens up opportunities for basic research as well as practical applications of olefin-linked COFs and other polymeric materials.

Modulating the macroscopic anisotropy of liquid crystalline polymers by polarized light

AbstractPhotoalignment technology is serving as an emerging technology for programming liquid crystalline polymer (LCP) materials due to its advantages including noncontact, high resolution, spatial control, programmability, and high efficiency. In this review, we report the research progress in implementing polarized light to design the anisotropy of LCPs, which is categorized based on the photoalignment mechanisms. The alignment approaches and the different stimulus‐responsive behaviors of the materials after photoalignment are discussed. Additionally, we have summarized the applications of photoaligned LCPs such as liquid crystal displays, optical components, intelligent soft actuators, and beyond. Finally, the challenges and future directions of the technology are outlined.

Covalent Organic Frameworks as Electrode Materials for Alkali Metal-ion Batteries.

Covalent organic frameworks (COFs) are a class of porous crystalline polymeric materials constructed by linking organic small molecules through covalent bonds. COFs have the advantages of strong covalent bond network, adjustable pore structure, large specific surface area and excellent thermal stability, and have broad application prospects in various fields. Based on these advantages, rational COFs design strategies such as the introduction of active sites, construction of conjugated structures, and carbon material composite, etc. can effectively improve the conductivity and stability of the electrode materials in the field of batteries. This paper introduces the latest research results of high-performance COFs electrode materials in alkali metal-ion batteries (LIBs, SIBs, PIBs and LSBs) and other advanced batteries. The current challenges and future design directions of COFs-based electrode are discussed. It provides useful insights for the design of novel COFs structures and the development of high-performance alkali metal-ion batteries.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications.

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Research progress on preparation and applications of covalent organic framework-based chromatographic stationary phases

Given continuous developments in industrial and scientific research, the separation and analysis of complex systems with high sensitivity, throughput, and selectivity is facing new challenges. Chromatography plays an irreplaceable role in separation science and is widely applied in environmental monitoring, pharmaceutical analysis, and food safety. Owing to their outstanding advantages, such as high loading capacity, precise quantification, and good reproducibility, chromatographic separation techniques based on various retention mechanisms have been utilized to detect different analytes. The stationary phase is the core material of chromatographic columns and has an extremely important influence on their separation performance. The selectivity and efficiency of separation largely depend on the chromatographic stationary phase. However, traditional stationary phases, such as silicon-based matrices, are characterized by complex preparation processes, poor permeability, large mass transfer resistance, and a narrow pH range. In addition, polymer matrices show poor mechanical stability and susceptibility to swelling, which limit their applications in the field of separation. Therefore, the development of novel stationary phases with the advantages of traditional stationary phases has become a research emphasis in the field of analytical science in efforts to meet separation requirements under different environments. Various stationary phases based on novel porous materials, such as metal organic frameworks (MOFs), porous organic cages (POCs), and covalent organic frameworks (COFs), are used for chromatographic separation. As mesh crystalline porous materials, MOFs have the advantages of a large surface area, adjustable structure, and easy functionalization; thus, they are widely used as chromatographic stationary phases in reverse-phase chromatography, hydrophilic-mode chromatography, mixed-mode chromatography, and other separation modes. However, because the pore size of MOFs is small and most MOFs demonstrate poor chemical stability under acidic or alkaline conditions, their applications in chromatographic separation are greatly limited. COFs are porous, crystalline polymer materials composed of light elements (H, O, C, N, B, and Si) connected via covalent bonds. Their advantages include a low density, large specific surface area, high porosity, good chemical and thermal stability, regular pores, and adjustable pore sizes. Because of their unique structures and properties, COFs are widely used in many fields such as catalysis, enrichment, gas capture, and sensing. COF materials are also suitable for separation analysis and considered ideal materials for novel chromatographic stationary phases. This review summarizes the latest research progress on the preparation and applications of COF-based chromatographic stationary phases over the past five years. First, the preparation of COF-based stationary phases (SiO2@COFs stationary phase, COFs monolithic stationary phase, pure COFs stationary phase and COFs-coated stationary phase) is introduced. The latest applications of COF-based stationary phases in the separation of organic compounds, isomers, and chiral compounds are then described in detail. Finally, the future development trends and challenges of chromatographic stationary phases based on COFs are discussed to provide new ideas for the future design and development of novel chromatographic stationary phases based on COFs.

Triggering dynamics of acetylene topochemical polymerization

Topochemical reactions are a promising method to obtain crystalline polymeric materials with distance-determined regio- or stereoselectivity. It has been concluded on an empirical basis that the closest intermolecular C⋯C distance in crystals of alkynes, d(C⋯C)min, should reach a threshold of ∼3 Å for bonding to occur at room temperature. To understand this empirical threshold, we study here the polymerization of acetylene in the crystalline state under high pressure by calculating the structural geometry, vibrational modes, and reaction profile. We find d(C⋯C)min to be the sum of an intrinsic threshold of 2.3 Å and a thermal displacement of 0.8 Å (at room temperature). Molecules at the empirical threshold move via several phonon modes to reach the intrinsic threshold, at which the intermolecular electronic interaction is sharply enhanced and bonding commences. A distance–vibration-based reaction picture is thus demonstrated, which provides a basis for the prediction and design of topochemical reactions, as well as an enhanced understanding of the bonding process in solids.

The directed self-assembly of reflective liquid crystalline polymer films to form polarization-independent diffractive optical elements

The directed self-assembly of liquid crystal polymers enables significantly enhanced optical properties compared to conventional isotropic optical materials. The transmissive properties of these thin polymeric films have been extensively studied, but the exploration of reflective liquid crystalline polymeric materials films has received little attention due to challenges in producing large, high-quality self-assembled films. We explore here for the first time stacked polymer cholesteric liquid crystal thin films as a pathway to generate reflective diffractive optics that act on both hands of circularly polarized light independently. We present 3 unique exemplar cases with increasing design complexity to communicate the widespread potential of these unique thin polymer films.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications.

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

Pore partition in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) have emerged as a kind of crystalline polymeric materials with high compositional and geometric tunability. Most COFs are currently designed and synthesized as mesoporous (2–50 nm) and microporous (1–2 nm) materials, while the development of ultramicroporous (<1 nm) COFs remains a daunting challenge. Here, we develop a pore partition strategy into COF chemistry, which allows for the segmentation of a mesopore into multiple uniform ultramicroporous domains. The pore partition is implemented by inserting an additional rigid building block with suitable symmetries and dimensions into a prebuilt parent framework, leading to the partitioning of one mesopore into six ultramicropores. The resulting framework features a wedge-shaped pore with a diameter down to 6.5 Å, which constitutes the smallest pore among COFs. The wedgy and ultramicroporous one-dimensional channels enable the COF to be highly efficient for the separation of five hexane isomers based on the sieving effect. The obtained average research octane number (RON) values of those isomer blends reach up to 99, which is among the highest records for zeolites and other porous materials. Therefore, this strategy constitutes an important step in the pore functional exploitation of COFs to implement pre-designed compositions, components, and functions.

Thermally Crosslinked Hydrogen-Bonded Organic Framework Membranes for Highly Selective Ion Separation.

The weak bonding energy and flexibility of hydrogen bonds can hinder the long-term use of hydrogen-bonded organic framework (HOF) materials under harsh conditions. Here we invented a thermal-crosslinking method to form polymer materials based on a diamino triazine (DAT) HOF (FDU-HOF-1), containing high-density hydrogen bonding of N-H⋯N. With the increase of temperature to 648 K, the formation of -NH- bonds between neighboring HOF tectons by releasing NH3 was observed based on the disappearance of the characteristic peaks of amino groups on FDU-HOF-1 in the Fourier transform infrared (FTIR) and solid-state nuclear magnetic resonance (ss-NMR). The variable temperature PXRD indicated the formation of a new peak at 13.2° in addition to the preservation of the original diffraction peaks of FDU-HOF-1. The water adsorption, acid-base stability (12 M HCl to 20 M NaOH) and solubility experiments concluded that the thermally crosslinked HOFs (TC-HOFs) are highly stable. The membranes fabricated by TC-HOF demonstrate the permeation rate of K+ ions as high as 270 mmol m-2 h-1 as well as high selectivity of K+/Mg2+ (50) and Na+/Mg2+ (40), which was comparable to Nafion membranes. This study provides guidance for the future design of highly stable crystalline polymer materials based on HOFs.

A DFT study on the selective adsorption and separation of CO2/CH4 on the Mg-decorated 2-D covalent organic framework (COF-5)

Covalent organic frameworks (COFs), an emerging class of crystalline polymeric materials, have garnered growing interest due to their ideal chemical and thermal stability and ordered microporous architectures, which make them effective agents for selective CH4/CO2 separation. In this work, adsorption and separation of methane and carbon dioxide molecules on the two-dimensional pristine and Mg-decorated COF-5 (MgCOF-5) was investigated using density functional theory, employing B3LYP. Both CH4 and CO2 molecules were found to weakly adsorbed through van der Waals interactions to the bare sheet via physisorption, releasing energies ranging from -3.8 to -5.6 and -8.7 to 12.8, respectively and the sheet's electrical characteristics don't alter all that much. To overcome this weak selectivity/sensitivity, multiple Mg atoms were decorated atop aromatic rings of COF-5. Our results show that up to four CO2 molecules can be adsorbed on each Mg atom exothermically, whereas Ead of CH4 is near zero so the theoretical CO2 capacity of a full Mg-covered sheet is 0.51 gCO2/g MgCOF-5. Also, the decorating of Mg atoms on the surface of COF-5 induces certain changes in the sheet's electrical characteristics and that the sheet's Eg changes up to 80% following the adsorption of several CO2 molecules, making it a potential candidate for CO2 detection.

Investigation of elastic properties of solid polymer materials by indentation method

The mechanical properties of solid (crystalline) polymer materials are proposed to be investigated simultaneously using two complementary methods based on controlled indentation of a spherical indenter. The first method consists in the direct analysis of the load curve without using the concept of contact stiffness; the second method is based on measurements of the size of prints depending on the magnitude of the static load. The study shows that the reinforcement of a solid polymer with carbon fibers can lead to a significant increase in its elastic-strength properties. Keywords: polymer composite materials, indentation, Hertz contact theory, spherical stamp, reduced modulus of elasticity, Brinell hardness, ultimate strength.

Исследование упругих свойств твердых полимерных материалов методом индентирования

The mechanical properties of solid (crystalline) polymer materials are proposed to be investigated simultaneously using two complementary methods based on controlled indentation of a spherical indenter. The first method consists in the direct analysis of the load curve without using the concept of contact stiffness; the second method is based on measurements of the size of prints depending on the magnitude of the static load. The study shows that the reinforcement of a solid polymer with carbon fibers can lead to a significant increase in its elastic-strength properties.

Friction behavior analysis of crystalline polymer materials containing hard particles using coarsegrained molecular dynamics method

Friction behavior analysis of crystalline polymer materials containing hard particles using coarsegrained molecular dynamics method

Surface charge enhanced synthesis of TpEB-based covalent organic framework (COF) membrane for dye separation with three typical charge properties

As a novel crystalline polymeric material, covalent organic framework (COF) has been paid the most attention by membrane separation researchers all over the world due to its pre-designable, regular pore structures, and reliable chemical stability. However, the fabrication of a homogenous, stable, thin COF layer on the support membrane remains challenging. In this work, the mechanism of electrostatic interaction between amino monomer and support was used to generate a stable and clear interface between the aqueous phase and the organic phase through the interfacial polymerization (IP) process. Specifically, the hydrophilicity and large pore size of polyacrylonitrile (PAN) mixed matrix membrane (MMM) allowed ethidium bromide monomers (EB) from the aqueous phase to penetrate and reach the negatively charged porous surface of the MMM. This assisted the fixation of the EB monomers at the interface with highly electrostatic interaction between the MMM’s surface and the EB monomers. The Tp (1,3,5-triformylphloroglucinol) monomer of organic phase was polymerized with surface entangle EB, resulting in the TpEB COF with very thin, hydrophilic, stable, homogeneously distributed tiny pores, and cationic surface charge. The prepared TpEB COF composite membrane had a water permeability of 32.34 L·m−2.h−1.bar−1, highly selective sieving performances for dye molecules of different charges and sizes. This membrane could reject ∼ 99.99 % of anionic dyes like Potassium permanganate (PP), Congo red (CR) and Coomassie brilliant blue G-250 (CBB), meanwhile maintain high solvent permeability for a long time. But for cationic or neutral dyes, the rejection rates mainly depended on their molecular sizes.

Ultrathin Metal-Organic Framework Nanosheets Exhibiting Exceptional Catalytic Activity.

Two-dimensional (2D) metal-organic framework nanosheets (MONs) or membranes are classes of periodic, crystalline polymeric materials that may show unprecedented physicochemical properties due to their modular structures, high surface areas, and high aspect ratios. Yet preparing 2D MONs from multiple components and two different types of polymerization reaction remains challenging and less explored. Here, we report the synthesis of MOF films via interfacial polymerization, which involves three active monomers for simultaneous polycondensation and polycoordination taking place in a confined interface. The well-defined lamellar structure of the MOF films allowed feasible and scalable exfoliation to produce free-standing 2D MONs with high aspect ratio up to 2000:1 and ultrathin thickness (∼1.7 nm). The pore structure was revealed by high-resolution TEM images with near-atomic precision. The imide-linkage of MONs provided superior thermal (up to 530 °C) and good chemical stability in the pH range from 3 to 12. More importantly, the MONs exhibited exceptional catalytic activity and superior reusability for the hydroboration reactions of alkynes, in which the turnover frequency (TOF) reached 41734 h-1, which is 2-4 orders of magnitude greater than that reported for homogeneous and heterogeneous catalysts.

On-water surface synthesis of charged two-dimensional polymer single crystals via the irreversible Katritzky reaction

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) are classes of structurally defined crystalline polymeric materials with exotic physical and chemical properties. Yet, synthesizing 2DP and 2D COF single crystals via irreversible reactions remains challenging. Here we report the synthesis of charged 2DP (C2DP) single crystals through an irreversible Katritzky reaction, under pH control, on a water surface. The periodically ordered 2DPs comprise aromatic pyridinium cations and counter BF4− anions. The C2DP crystals, which are composed of linked porphyrin and pyrylium monomers (C2DP-Por), have a tunable thickness of 2–30 nm and a lateral domain size up to 120 μm2. Single crystals with a square lattice (a = b = 30.5 Å) are resolved by imaging and diffraction methods with near-atomic precision. Furthermore, the integration of C2DP-Por crystals in an osmotic power generator device shows an excellent chloride ion selectivity with a coefficient value reaching ~0.9 and an output power density of 4 W m−2, superior to those of graphene and boron nitride. Large single crystals of charged 2D polymers are synthesized on a water surface, under kinetic control, by the irreversible Katritzky reaction. The crystals can act as an anion-selective membrane for osmotic energy generation.

Covalent Organic Frameworks: New Materials Platform for Photocatalytic Degradation of Aqueous Pollutants.

Covalent organic frameworks (COFs) are highly porous and crystalline polymeric materials, constructed by covalent bonds and extending in two or threedimensions. After the discovery of the first COF materials in 2005 by Yaghi et al., COFs have experienced exciting progress and exhibitedtheirpromising potential applications invarious fields, such as gas adsorption and separation, energy storage, optoelectronics, sensing and catalysis. Because of their tunablestructures, abundant, regular and customizable pores in addition to large specific surface area, COFs can harvest ultraviolet, visible and near-infrared photons, adsorb a large amount of substrates in internal structures and initiate surface redox reactions to act as effective organic photocatalysts for water splitting, CO2 reduction, organic transformations and pollutant degradation. In this review, we will discuss COF photocatalysts for the degradation of aqueous pollutants. The state-of-the-art paragon examples in this research area will be discussed according to the different structural type of COF photocatalysts. The degradation mechanism will be emphasized. Furthermore, the future development direction, challenges required to be overcome and the perspective in this field will be summarized in the conclusion.

Structural Characteristics and Environmental Applications of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are emerging crystalline polymeric materials with highly ordered intrinsic and uniform pores. Their synthesis involves reticular chemistry, which offers the freedom of choosing building precursors from a large bank with distinct geometries and functionalities. The pore sizes of COFs, as well as their geometry and functionalities, can be pre-designed, giving them an immense opportunity in various fields. In this mini-review, we will focus on the use of COFs in the removal of environmentally hazardous metal ions and chemicals through adsorption and separation. The review will introduce basic aspects of COFs and their advantages over other purification materials. Various fabrication strategies of COFs will be introduced in relation to the separation field. Finally, the challenges of COFs and their future perspectives in this field will be briefly outlined.

Bending Behaviors of Azobenzene-containing Liquid Crystalline Polymers (AZ-LCP): Factors Influencing Bending Direction, Frequency and Degree

Liquid crystal polymers (LCP) containing azobenzene and its derivatives are fascinating and topics of intense scientific curiosities. As the addition of photo-responsive azobenzene, the LCP is exerted with the photochemical response, which shows fascinating and useful photo-induced motions and consequently enables a variety of applications in a lot of fields. In this review, we detail the macroscale mechanical motion of azobenzene containing liquid crystalline polymer (AZ-LCP) materials upon irradiation, with emphasis on the external and internal factors of bending behaviors of AZ-LCP, including bending direction, frequency and degree. We end with an outlook of challenging and competitive application as soft actuators.