Covalent Interconnections Research Articles

Recent progress in degradation and recycling of epoxy resin

Epoxy resin is widely used in electrical equipment and electronic devices fields due to its excellent electrical, thermal, and mechanical properties. However, its internal three-dimensional covalent interconnection structure brings barriers to its degradability and recycling because covalent bonds cannot be broken easily. With the replacements of power equipment and electronic devices, there will be more and more epoxy resins and their composites in them to be treated and effective recycling is of great significance for resource conservation and environmental protection. In this review article, recent progress in degradation and recycling of epoxy resin is introduced and the effect of three traditional degradation methods is discussed. The drawbacks of these methods are thought to come from the intrinsic properties of these epoxy resins. So the urgency of developing new kinds of degradable epoxy resins is proposed. Then different types of new degradable epoxy resins are reviewed. Degradation mechanisms of the opened-loop recycling and recycling methods of the closed-loop recycling are summarized in detail. Finally, the challenges and perspectives are discussed based on their current developments. This review comprehensively considers both traditional degradation methods and new methods for developing degradable epoxy resins. It covers not only an overview of the state-of-the-art advances of degradation and recycling of epoxy resin but also the prospects that provide reference for the synthesis of degradable epoxy resin materials.

Macro‐Sized All‐Graphene 3D Structures via Layer‐by‐Layer Covalent Growth for Micro‐to‐Macro Inheritable Electrical Performances

AbstractCreating 3D‐engineered macroscopic architectures while inheriting the superior properties of individual building blocks remains one of the fundamental challenges in nanotechnology. Stable covalent interconnection between micro/nanoblocks is a desired but underexplored strategy to meet the challenges, rather than current dependently‐used weak physical forces or organic cross‐linking, which disrupts the continuity of chemical composition and electrical properties. Herein, a novel layer‐by‐layer covalent growth protocol is developed to construct all‐graphene macrostructures (AGM) with micro‐to‐macro inheritable electrical properties by laser‐assisted covalent linkage of polyethersulfone‐derived 3D porous graphene microblocks without introducing any catalysts, templates, and additives. Creatively, along with graphene generation and inter‐layer bonding, a quality optimization process is integrated into one‐step laser irradiation, which is unique and efficient for synthesizing high‐crystalline graphene. With the covalently nondestructive bridge and free of non‐graphene foreign phase impurities, AGM shows unprecedented electrical conductivity, especially a more than 100‐fold improvement in cross‐layer conductivity compared with non‐covalent assembly. Furthermore, the covalent growth mechanism of AGM is clarified by molecular dynamics simulations. Finally, the application efficacy of AGM with enhanced isotropic conductivity is verified by using it as a supercapacitor electrode. This methodology enables the as‐obtained AGM to possess the potential for high‐performance‐pursuing, multi‐disciplinary, or large‐scale applications.

Physical, Electrochemical, and Solvent Permeation Properties of Amphiphilic Conetwork Membranes Formed through Interlinking of Poly(vinylidene fluoride)-Graft-Poly[(2-dimethylamino)ethyl Methacrylate] with Telechelic Poly(ethylene glycol) and Small Molecular Weight Cross-Linkers.

We report the preparation of dense and porous amphiphilic conetwork (APCN) membranes through the covalent interconnection of poly(vinylidene fluoride)-graft-poly[(2-dimethylamino)ethyl methacrylate] (PVDF-g-PDMAEMA) copolymers with telechelic poly(ethylene glycol) (PEG) or α,α-dichloro-p-xylene (XDC). The dense APCN membranes exhibit varying solvent swelling and mechanical properties depending on the compositions and overall crystallinity. The crystallinity of both PVDF (20-47%) and PEG (9-17%) is significantly suppressed in the dense APCNs prepared through the interconnection of PVDF-g-PDMAEMA with reactive PEG as compared to the APCN membranes (48-53%) prepared with XDC as well as mechanical blend of PVDF-g-PDMAEMA plus nonreactive PEG. The dense APCN membranes exhibit a good transport number of monovalent ions and ionic conductivity. The APCN membrane interconnected with PEG and containing binary ionic liquids exhibits a room-temperature lithium ion conductivity of 0.52 mS/cm. On the other hand, APCN ultrafiltration (UF) membranes exhibit organic solvent-resistant behavior. The UF membrane obtained by interconnecting PVDF-g-PDMAEMA with telechelic PEG shows low protein fouling propensity, higher hydrophilicity, and water flux as compared to membranes prepared using XDC as the interconnecting agent. The significant effect of the covalent interconnection of the amphiphilic graft copolymers with telechelic PEG or XDC on the overall properties provides a good opportunity to modulate the properties and performance of APCN membranes.

Butyl rubber as a macro-cross-linker in the preparation of a shape-memory and self-healing polymer

Recently, a simple strategy was developed for preparing interconnected interpenetrating polymer networks (IPNs) based on butyl rubber (IIR) and poly(n-octadecyl acrylate) (PC18A). Solvent-free UV polymerization of n-octadecyl acrylate (C18A) monomer in the melt of IIR at ambient temperature resulted in IPNs with self-healing and shape-memory functions. Here, we demonstrate that the use of IIR grafted with acrylic acid, methacrylic acid, and 10-undecenoic acid instead of unmodified IIR provides a significant improvement in the mechanical properties of IPNs. Differential scanning calorimetry, small-angle x-ray scattering, and wide-angle x-ray scattering analysis reveal side-by-side packing of C18 side chains of PC18A to form lamellar crystals with a melting temperature Tm between 46 and 52 °C. Transmission electron microscopy analysis indicates the existence of quasispherical nanoparticles composed of crystalline domains, which are dispersed in a continuous interpenetrating rubber-PC18A matrix. This microstructure provides them a complete self-recovery behavior induced by heating and an efficient shape-memory function. IPNs exhibit around tenfold higher chemical cross-link density as compared to those prepared from the native IIR, reflecting the effect of pendant vinyl groups on the extent of covalent interconnections between the IIR and PC18A components. The type of the grafted monomers significantly affects the mechanical performance of IPNs, which can be explained with the individual contributions of chemical and physical cross-links to the total cross-link density. The amount of the grafted rubbers in IPN could be further increased up to 80 wt. % by the incorporation of toluene into the reaction system, resulting in IPNs with a wide range of tunable thermal and mechanical properties.

In-situ polymerization of different molecular weight polyethylene glycol and maleic acid onto carbon fiber for reinforcing polyamide 66

ABSTRACT The interfacial adhesion character highly depends on the molecular interactions on the interface, including bond combination and molecular entanglement, and mobility for sharing load. In situ polymerization of different molecular weight polyethylene glycol (PEG) and maleic acid (MA) is used to form a hyperbranched polyester (HBPE) layer on an electrochemically oxidized carbon fiber (OCF) surface. It is found that more terminal hydroxyl groups are formed under the reaction at 280°C than that at other temperature. The branch extent and molecule chain length on the HBPE sizing reach balance by choice of 2000 molecular weight PEG monomer. HBPE as a film has uniform coverage on the OCF surface. The interlaminar shear strength of polyamide 66 composite reinforced with the sized OCF by in situ polymerization of 2000 molecular weight PEG and MA at 280°C is the optimal, which is enhanced to 90.5 MPa by 169% compared to unoxidized fiber and by 70% compared to OCF. The significant interfacial property improvement is attributed to the abundant terminal hydroxyl groups and appropriate chain length of the formed HBPE sizing, which enhanced optimally the interaction of fiber/sizing and sizing/matrix by balancing the contribution of molecular entanglement, deformation and covalent interconnection to interface.

Effects of chain length of polyether amine on interfacial adhesion of carbon fiber/epoxy composite in the absence or presence of polydopamine bridging platform

To understand the complementary mechanisms between molecular interlocking and covalent interconnection that affects interfacial shear strength (IFSS), the adherence of polyether amine with varying chain length onto the surface of carbon fiber in the absence or presence of bio-derived adhesive polydopamine (PDA) as a bridge was subtly designed and studied. Without PDA, up to 25.6% and 34.7% increases in IFSS relative to untreated CF was achieved respectively for shorter and longer molecular chain of polyether amine. The longer chain length encourages molecular interlocking with epoxy molecules and produces a higher penetration depth into epoxy matrix, thus giving a larger increase in IFSS. In the presence of PDA, 39–42% IFSS increase by analogy to CF is observed due to the synergistic effects of hybrid interfacial interactions, the increasing rigidity of interphase and the improved wetting behavior. While, in this case, impact of chain length becomes less noticeable, which is mainly attributed to the trade-off between the enhanced chemical cross-linking points and the less molecular entanglement with resin.

High temperature quasistatic and dynamic mechanical behavior of interconnected 3D carbon nanotube structures

Carbon nanotubes (CNTs) are one of the most appealing materials in recent history for both research and commercial interest because of their outstanding physical, chemical, and electrical properties. This is particularly true for 3D arrangements of CNTs which enable their use in larger scale devices and structures. In this paper, the effect of temperature on the quasistatic and dynamic deformation behavior of 3D CNT structures is presented for the first time. An in situ high-temperature nanomechanical instrument was used inside an SEM at high vacuum to investigate mechanical properties of covalently interconnected CNT porous structures in a wide range of temperature. An irreversible bucking at the base of pillar samples was found as a major mode of deformation at room and elevated temperatures. It has been observed that elastic modulus and critical load to first buckle formation decrease progressively with increasing temperature from 25 °C to 750 °C. To understand fatigue resistance, pillars made from this unique structure were compressed to 100 cycles at room temperature and 750 °C. While the structure showed remarkable resistance to fatigue at room temperature, high temperature significantly lowers fatigue resistance. Molecular dynamics (MD) simulation of compression highlights the critical role played by covalent interconnections which prevent localized bending and improve mechanical properties.

Diamond nanothread-based 2D and 3D materials: Diamond nanomeshes and nanofoams

Diamond nanothreads (DNTs) are one-dimensional, fully sp3-bonded carbon nanostructures resulting of covalent bonding between stacked benzene molecules in a crystal, induced by application of high pressure, as demonstrated in experiments. In this work, we used classical Molecular Dynamics simulations to propose the synthesis of analogous two- and three-dimensional porous nanostructures, which we named diamond nanomeshes (DNM) and diamond nanofoams (DNF), consistently to the definition of DNTs, and computed some of their structural and mechanical properties. Two different approaches toward creation of such materials are proposed. One of them consists in interconnecting finite domains of conventional DNTs, achieved through partial surface dehydrogenation and subsequent C-C covalent bonding. The other approach considers that the formation of sp3 C-C bonds between stacked benzene molecules under high pressure could be extended to polycyclic aromatic hydrocarbon (PAH) molecules, generating crosslinked DNT-like structures. Different atomic configurations can be achieved by varying the morphology of DNTs used in their construction, the PAH molecules, and the nature of the DNT covalent interconnections. The resulting materials exhibit an interesting combination of mechanical strength, flexibility, lightness, high porosity and high specific surface area, enabling potential applications in reinforced nanocomposites, gas storage/separation, sensors, among others.

Nanoporous carbon superstructures based on covalent bonding of porous fullerenes

Highly porous and mechanically stable nanostructures are of great interest for applications in selective membranes, adsorbents, catalysts and sensors. In this study, we use Density Functional Theory calculations and Molecular Dynamics (MD) simulations to demonstrate the feasibility of a novel class of porous carbon-based nanostructures with uniform pore size distributions, formed by covalent bonding of porous fullerenes. Their corresponding mechanical and electronic properties are evaluated, and results show that they typically exhibit an outstanding mechanical strength and electronic behavior ranging from metallic to semiconducting, depending on the hybridization of the covalent interconnections and dimensionality. The efficacy of these materials as molecular sieves is also demonstrated using MD simulations of gas transport across the nanoporous structure. This combination of properties makes these nanostructures suitable for the development of novel porous functional materials with several potential applications.

Chemically interconnected light-weight 3D-carbon nanotube solid network

Owing to the weak physical interactions such as van der Waals and π-π interactions, which hold nanotubes together in carbon nanotube (CNT) bulk structures, the tubes can easily slide on each other. Creating covalent interconnection between individual carbon nanotube (CNT) structures could remarkably improve the properties of their three-dimensional (3D) bulk structures. The creation of such nanoengineered 3D solid structures with improved properties and low-density remains one of the fundamental challenges in real-world applications. Here, we report the scalable synthesis of low-density 3D macroscopic structure made of covalently interconnected nanotubes using free-radical polymerization method after functionalized CNTs with allylamine monomers. The resulted interconnected highly porous solid structure exhibits higher mechanical properties, larger surface area and greater porosity than non-crosslinked nanotube structures. To gain further insights into the deformation mechanisms of nanotubes, fully atomistic reactive molecular dynamics simulations are used. Here we demonstrate one such utility in CO2 uptake, whose interconnected solid structure performed better than non-interconnected structures.

3D carbon networks and their polymer composites: Fabrication and electromechanical investigations of neat Aerographite and Aerographite-based PNCs under compressive load

Aerographite is a lightweight 3D nanocarbon network which offers covalent interconnections for polymer nanocomposites (PNCs). Here, the electrical and mechanical properties of neat Aerographite and Aerographite-based PNCs are investigated in detail. The Aerographite filler networks consist of hollow, graphitic tubes of μm-sized diameters and nm-sized wall thicknesses. Different densities of Aerographite in the range of 0.6–13.9 mg/cm3 have been investigated towards their mechanical deformation behavior, electrical conductivities and piezoresistive response under compression. This basic characterization of filler networks is compared to resulting PNCs if the Aerographite is fully embedded in epoxy matrix. It can be shown that the use of 3D interconnected Aerographite results in high electrical conductivities at low filler contents, e.g., 2–8.7 S/m for weight fractions of 0.1–1.2 wt.-%. The neat Aerographite has been characterized in detail by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy techniques. To explain the observed piezoresistive behavior of these 3D nanocarbon-based PNCs, a qualitative micromechanical model is introduced. The model describes the internal graphitic wall slippage and loss of interconnections of the inner electrically conductive networks under load. The piezoresistive response of Aerographite-based PNCs can be directly correlated to the applied outer mechanical loads.

Enhancement of cross-plane thermal conductivity and mechanical strength via vertical aligned carbon nanotube@graphite architecture

A three-dimensional (3D) carbon nanotube/exfoliated graphite block (CNT/EGB) was prepared by growing vertical aligned carbon nanotube (VACNT) at the surface of SiO2-coated exfoliated graphite plate (EGP) through chemical vapor deposition followed by hot-pressing. In such 3D CNT/EGB, EGPs were bridged by the VACNTs in the cross-plane direction, and the interface between EGPs and VACNTs was covalently bonded by SiC which formed by reaction of SiO2 and the adjacent carbon of EGPs and VACNTs. The length and growth density of VACNTs were adjusted by the growth time and concentration of catalysts. Thermal conductivity and mechanical strength of CNT/EGB were controlled by the growth states of VACNTs and hot-pressing. CNT/EGB showed a maximum cross-plane thermal conductivity (k⊥) of 38 W/mK, which is more than twice as much as that of EGB (14 W/mK). A remarkable increase in k⊥ was attributed to the efficient heat flow of VACNTs bridging EGPs in the cross-plane direction and the thermal conductive SiC interface between VACNTs and EGPs. Additionally, the increased bending (76 MPa) and compressive strength (59 MPa) of CNT/EGB was due to the combination of strong pull-out effect of high-density nanotubes and the strong covalent interconnections between VACNTs and EGPs.

3D Nanocomposites of Covalently Interconnected Multiwalled Carbon Nanotubes with SiC with Enhanced Thermal and Electrical Properties

Synthesizing 3D carbon nanotube (CNT) networks with multifunctional characteristics has stimulated the interest from the scientific community since the 1990s. Here, the fabrication of a novel composite material consisting of 3D covalently interconnected multiwalled CNT with silicon carbide (SiC) nano and microparticles is reported. The material is synthesized by a two‐step process involving the coating of CNT with silicon oxide (SiO x ) via chemical routes, followed by spark plasma sintering (SPS). SPS enables carbothermal reduction of SiOx and subsequent densification of the material into 3D composite blocks. Covalent interconnections of CNT are facilitated by a carbon diffusion process resulting in SiC formation as SiOx coated CNT are subjected to high temperatures. The presence of SiC in the sintered composite has been confirmed by Raman spectroscopy, as well as through energy filtered transmission electron microscopy maps. Interestingly, the 3D CNT composite exhibits high thermal conductivity (16.72 W m−1 K−1); and also a semiconducting behavior with an electron hopping mechanism associated to a 3D variable range hopping model. These findings demonstrate that it is indeed possible to fabricate SiC–CNT composites with enhanced physical properties that can be used as multifunctional materials.

Building complex hybrid carbon architectures by covalent interconnections: graphene-nanotube hybrids and more.

Graphene is theoretically a robust two-dimensional (2D) sp(2)-hybridized carbon material with high electrical conductivity and optical transparency. However, due to the existence of grain boundaries and defects, experimentally synthesized large-area polycrystalline graphene sheets are easily broken and can exhibit high sheet resistances; thus, they are not suitable as flexible transparent conductors. As described in this issue of ACS Nano, Tour et al. circumvented this problem by proposing and synthesizing a novel hybrid structure that they have named "rebar graphene", which is composed of covalently interconnected carbon nanotubes (CNTs) with graphene sheets. In this particular configuration, CNTs act as "reinforcing bars" that not only improve the mechanical strength of polycrystalline graphene sheets but also bridge different crystalline domains so as to enhance the electrical conductivity. This report seems to be only the tip of the iceberg since it is also possible to construct novel and unprecedented hybrid carbon architectures by establishing covalent interconnections between CNTs with graphene, thus yielding graphene-CNT hybrids, three-dimensional (3D) covalent CNT networks, 3D graphene networks, etc. In this Perspective, we review the progress of these carbon hybrid systems and describe the challenges that need to be overcome in the near future.

Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions

The establishment of covalent junctions between carbon nanotubes (CNTs) and the modification of their straight tubular morphology are two strategies needed to successfully synthesize nanotube-based three-dimensional (3D) frameworks exhibiting superior material properties. Engineering such 3D structures in scalable synthetic processes still remains a challenge. This work pioneers the bulk synthesis of 3D macroscale nanotube elastic solids directly via a boron-doping strategy during chemical vapour deposition, which influences the formation of atomic-scale “elbow” junctions and nanotube covalent interconnections. Detailed elemental analysis revealed that the “elbow” junctions are preferred sites for excess boron atoms, indicating the role of boron and curvature in the junction formation mechanism, in agreement with our first principle theoretical calculations. Exploiting this material’s ultra-light weight, super-hydrophobicity, high porosity, thermal stability, and mechanical flexibility, the strongly oleophilic sponge-like solids are demonstrated as unique reusable sorbent scaffolds able to efficiently remove oil from contaminated seawater even after repeated use.

Theoretical investigation of electromechanical effects for graphyne carbon nanotubes.

We present a theoretical study of the electronic and mechanical properties of graphyne-based nanotubes (GNTs). These semiconducting nanotubes result from the elongation of one-third of the covalent interconnections of graphite-based nanotubes by the introduction of yne groups. The effect of charge injection on the dimensions of GNTs was investigated using tight-binding calculations. Low amounts of electron injection are predicted to cause qualitatively different responses for armchair and zigzag graphyne nanotubes. Although the behavior is qualitatively similar to the usual carbon nanotubes, the charge-induced strains are predicted to be smaller for the GNTs than for ordinary single walled carbon nanotubes.

Families of carbon nanotubes: Graphyne-based nanotubes

New families of carbon single-walled nanotubes are proposed and their electronic structures are investigated. These nanotubes, called graphynes, result from the elongation of covalent interconnections of graphite-based nanotubes by the introduction of yne groups. Analogously to ordinary nanotubes, armchair, zigzag, and chiral graphyne nanotubes are possible. We here predict the electronic properties of these unusual nanotubes using tight-binding and ab initio density functional methods. Of the three graphyne nanotube families analyzed here, two provide metallic behavior for armchair tubes and either metallic or semiconducting behavior for zigzag nanotubes. A diameter- and chirality-independent band gap is predicted for the other investigated graphyne family, as well as an oscillatory dependence of the effective mass on nanotube diameter.

Synthesis and characterization of PCL/PEG/PCL triblock copolymers by using calcium catalyst

Triblock copolymer PCL–PEG–PCL was prepared by ring-opening polymerization of ε-caprolactone (CL) in the presence of poly(ethylene glycol) catalyzed by calcium ammoniate at 60 °C in xylene solution. The copolymer composition and triblock structure were confirmed by 1H NMR and 13C NMR measurements. The differential scanning calorimetry and wide-angle X-ray diffraction analyses revealed the micro-domain structure in the copolymer. The melting temperature T m and crystallization temperature T c of the PEG domain were influenced by the relative length of the PCL blocks. This was caused by the strong covalent interconnection between the two domains. Aqueous micelles were prepared from the triblock copolymer. The critical micelle concentration was determined to be 0.4–1.2 mg/l by fluorescence technique using pyrene as probe, depending on the length of PCL blocks, and lower than that of corresponding PCL–PEG diblock copolymers. The 1H NMR spectrum of the micelles in D 2O demonstrated only the –CH 2CH 2O– signal and thus confirmed the PCL-core/PEG-shell structure of the micelles.

Graphyne nanotubes: New Families of Carbon Nanotubes

ABSTRACTFundamentally new families of carbon single walled nanotubes are proposed. These nanotubes, called graphynes, result from the elongation of covalent interconnections of graphite-based nanotubes by the introduction of yne groups. Similarly to ordinary nanotubes, armchair, zig-zag, and chiral graphyne nanotubes are possible. We present here results for the electronic properties of graphyne based tubes obtained from tight-binding and ab initio density functional methods.

Influence of a Classical Chelate Effect due to a Covalent Linkage of Ionophore Subunits on the Performance of Corresponding Ion Selective Electrodes

To investigate the influence of a covalent interconnection of ionophore subunits (e.g. to obtain so-called bis(crown) compounds) on the ion selectivity of corresponding electrodes, a theoretical treatment of the underlying chelate effect is presented. Entropic contributions given by standard-state and statistical terms, as well as enthalpic contributions resulting from a ligand preorganization, are discussed. The latter contributions are shown to reflect the intrinsic chelate effect, as redefined by Frausto da Silva, and to be decisive for a possible enhancement of the potentiometric selectivity of ion selective electrodes based on such ionophores. An experimental study of the ion selectivities as well as of the alkali-ion complex stabilities of covalently bridged and non-bridged ligands with Li+ selectivity uncovers no significant effects that could be attributed to a preorganization of these ligands.