Abstract
Carbon nanomaterials receive much attention due to enhanced mechanical and electrical properties that arise from nanoconfinement. Carbon nanotubes (CNTs) (3-dimensional) and graphene sheets (2-dimensional) have seen applications as sensors, high-strength fiber reinforcements, and adhesives. Carbyne, a 1-dimensional purely carbon structure consisting of alternating single and triple bonds, is projected to be the strongest material in the known world, with specific strength several times that of CNTs and graphene. Despite its desirable properties, carbyne’s extreme instability under standard conditions has inhibited its commercial development. Recent advancements in carbyne synthesis using CNTs as molecular scaffolds show that carbyne may finally be able to progress from theory to reality. Here, an approach for the preparation and stabilization of long-chain carbyne without the use of CNTs is proposed. Using multilevel pulse-voltage injection (MLPI) to create uniform, subnanometer diameter pores in silicon nitride as thermally stable nanoreactors, initial theoretical calculations suggest that carbyne may be synthesized under high temperature and vacuum. Carbyne chains may then be extracted from the pores and subsequently stabilized by immersion into a solution of poly(diallyldimethylammonium chloride) (PDDA). The long-chain carbyne is then free to be used for a multiplicity of applications, including low-threshold sensors and the realization of high-strength carbyne fibers.
1. Introduction
With the rapid development of nanoscience and nanotechnology came the discovery of several types of nanostructural conformations of carbon. Of particular interest was the tensile strength and Young’s modulus of these materials. For example, when carbon nanotubes (CNTs) were discovered by Iijima in 1991 [1], they were soon found to be the strongest material ever recorded, with Young’s moduli up to 1 TPa for single-walled carbon nanotubes (SWCNTs) and tensile strengths up to 150 GPa for multiwalled carbon nanotubes (MWCNTs). In 2004, Novoselov et al. were able to isolate single graphene sheets with a Young’s modulus similar to the stiffest carbon nanotubes of 1 TPa and tensile strength of 130.5 GPa [2]. Due to the combination of both an extraordinarily high modulus and tensile strength, graphene surpassed CNTs as the strongest material ever tested.
Carbyne, a relatively less researched carbon polymer consisting of alternating single and triple bonds, was first hypothesized in the 19th century. Based on the acetylenic configuration, this polymer was projected to have exceptional mechanical properties. However, the extreme reactivity of carbon atoms in adjacent carbyne chains results in the spontaneous and explosive formation of crosslinks, thereby transforming sp-hybrized carbons into sp2 and sp3 hybridization states. Therefore, for over a century, the prospect of isolating or synthesizing pure carbyne chains was inaccessible, preventing commercial translation. In 2015, Kotrechko et al. demonstrated that an acetylenic carbon chain of 44 units could be synthesized in a stable form both via simulations and initial experimental syntheses. Furthermore, they found that carbyne’s tensile strength (393 GPa) and Young’s modulus (4.6 TPa) were indeed the highest of any known material in the world [3]. Shortly after this work, Shi et al. (2016) devised a method for synthesizing carbyne in high purity, chain length, and yield; by growing carbyne inside of double-walled carbon nanotubes (DWCNTs), the acetylenic chains become highly stable and protected from the external environment by their nanoscaffolds [4].
The problem of reactivity is of utmost importance. Several approaches have been attempted to limit the crosslinking probability of long carbyne chains. One solution is the addition of bulky end groups that inhibit adjacent chains from coming within crosslinking distance [5]. The longest chains synthesized via end group addition outside of CNTs are only 40-50 acetylenic carbon atoms long, while the DWCNT-encapsulated carbyne chains can reach approximately 6000 units [4]. Clearly demonstrated by this comparison, the reactivity of carbyne molecules greatly inhibits their chain length when synthesized in conditions such that adjacent chains are allowed to come into contact. The DWCNT approach provides a solution to this problem at the expense of function, as the carbyne molecules are confined within the CNT scaffolds with little possibility for removal. Therefore, an alternative synthetic method of long-chain carbyne outside of CNTs that will result in stable conditions in an ambient environment is proposed and modeled. By deactivating the reactivity of the triple bonds while preserving their structure, a novel route to long-chain carbyne synthesis is illuminated with the possibility of functional applications.
The primary aspect of acetylenic carbons that render them highly reactive is the presence of triple bonds, composed of 6 bonding electrons, with four being shared via a sideways p-orbital overlap of electron density. Due to the labile nature of the two p-p orbital overlaps, which leave the triple bond susceptible to both nucleophilic attack (at the exposed antibonding -molecular orbitals (MO)) and electrophilic attack (due to the triple bond being electron-rich with two pairs of electrons in weaker orbitals available for bonding), triple bonds are highly reactive carbon-carbon linkages. When two triple-bonded acetylenic carbons from adjacent carbyne chains come into contact, a -MO from one chain overlaps with a -antibonding molecular orbital (ABMO) on the other chain with an activation energy barrier of 0.6 eV [5] such that two single-bond crosslinks are formed between the chains. This results in a four-carbon ring connecting the two carbyne molecules (see Scheme 1).
2. Results and Discussion
A model system for the synthesis of carbyne chains outside of CNTs was created by first considering the mechanism by which two carbyne chains can crosslink. First, crosslinking resulting in the formation of a four-membered ring connecting the chains is a favorable reaction due to the - overlap with an ABMO. Second, because of the ring structure, carbons located close to the source of a crosslink are unable to form crosslinks themselves. This phenomenon occurs because of the change in bonding state of crosslinked carbons such that the narrowed bond angle from approximately 180° to 120° forces the two carbyne chains away from each other at the crosslink source, thereby providing a distal barrier to further chain interaction near this point. Computer simulations and some experimental evidence have determined the average crosslink density for two parallel carbyne chains to be one crosslink per 17 carbon atoms or about one crosslink every 2.2 nm [6, 7]. Third, the crosslink spacing provides important information about how to prevent the mechanism from occurring. Theoretically, placement of a spacer molecule every 10 or so carbon atoms along a carbyne chain may sterically obstruct crosslinking from occurring while maintaining the structural integrity of the polymer (see Figure 1).
It is clear from a theoretical proposal of molecular spacers as pictured in Figure 1 that covalent bonding to the chain is not desired, as this would destroy the triple-bond structure and result in a similar loss of function as observed in the crosslinking of carbyne chains with themselves. Instead, the spacer molecules must be capable of a noncovalent attraction to the acetylenic carbons under specific synthetic conditions such that they may physically adhere to the polymer without chemical reaction. Some obstacles are faced in enacting this approach. First, the particle spacers must have a high affinity for acetylenic carbons while maintaining weak interactions with each other to prevent aggregation. One solution to this problem is to use a charged species such that electrostatic repulsion will prevent aggregation. Furthermore, a positive charge would produce an electrostatic attraction to the electron-rich triple bonds. However, a second problem arises due to chemical adhesion via covalent interactions through triple-bond reactivity instead of the desired physical adhesion. Additional complications that must be contended with include the determination of an ideal particle size, the optimal distance of separation between carbyne chains, and spacer density.
The devised model, tested by theoretical calculations, shows that long-chain carbyne strands may indeed be synthesized and stabilized as isolated molecules under inert conditions. Poly(diallyldimethylammonium chloride), known more commonly as polyDADMAC, or simply PDDA, is used as the stabilizing agent such that synthesized carbyne chains cannot come into contact and react with each other (see Figure 2). The 5-membered ring in PDDA, containing a quaternary ammonium salt, solves three problems: (1) the bulky ring structure with methyl groups on the end ensures adequate separatory distance between carbyne chains by steric effects, (2) the positively charged polymer will not favor aggregation, and (3) quaternary ammonium salts are exceptionally unreactive even toward strong electrophiles, most nucleophiles, oxidants, and acids. Therefore, with the theoretical model of synthesized carbyne chains in an inert environment, the proposed stabilization method allows carbyne transfer into oxidative conditions for use as a sensor or reinforcer. A coating of PDDA as a spacer between carbyne molecules achieves crosslink inhibition, while simultaneously allowing for carbyne processing. To achieve this coating, single carbyne chains may be immersed into a low-density PDDA solution. With a coating method successful in preserving C-C triple-bond integrity and in adhering PDDA to the carbyne surface, the stabilized chains can be processed into a fiber or other functional form.
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An issue with this method is that, similar to the case of synthesis within CNTs, the mechanical properties of the stabilized carbyne material will suffer from the addition of a filler molecule. However, the proposed approach results in a higher density of carbyne chains with a closer packaging arrangement than the case of CNT preparation, and does so without the need for an organic scaffold. Additionally, problems concerning the CNT-based synthesis of carbyne include the exposure of chains upon opening of the CNTs and the CNT structure itself preventing the attachment of spacer molecules to the carbyne surface before the nanotube is opened; the reagents used to open the nanotubes would themselves react with and decompose the carbyne structure. The aim of the proposed approach is to better remove, isolate, and coat the freshly synthesized carbyne chains for practical applications. Therefore, proposed solutions are offered only to the additional obstacles that arise in preparing the PDDA coating (see Figure 2), and in achieving the PDDA coating after a long-chain carbyne synthesis. These proposed solutions are supported with theoretical calculations.
The primary advantage to the CNT synthesis method is that the scaffold provides an ideal chamber for the growth of a truly linear carbon chain. It has been shown that the inner DWCNT reactor diameter must be approximately 0.7 nm to achieve linear carbyne growth; larger dimensions yield spiraling carbon structures or no defined carbon structure at all [4]. However, many of the benefits of the CNT synthesis method are also disadvantages, as the carbyne chains are entirely confined to the CNTs (see Figure 3). As discussed above, removal from this protective sheath results in the destabilization and resultant crosslinking already described as inherent to free carbyne.
The DWCNT synthesis method does, however, offer information about the requirements of carbyne synthesis. First, a linear scaffold of specified width appears to be required to synthesize long-chain carbyne as demonstrated from Shi et al.’s synthesis using DWCNTs of varying inner diameters. Without a proper scaffold, amorphous or disorganized carbon structures result. Second, as with the chemical vapor deposition (CVD) method for CNT synthesis, high temperature and high vacuum (HTHV) are required for carbyne synthesis [4]. Therefore, four essential conditions for carbyne synthesis appear to be (1) a carbon source (e.g., methane, methanol, or ethanol), (2) a thermally resistant linear scaffold, (3) polymerization under HTHV, and (4) a mechanism for keeping individual carbyne chains out of contact with each other. CNTs satisfy all of these conditions which allow them to be successful nanoreactors in the synthesis of long-chain carbyne. However, from these criteria and evidence from Wong et al., it is also concluded that carbon nanotubes may not be necessary in carbyne synthesis; rather, they satisfy a set of conditions that appear to be needed for carbyne production [8]. Thus, the proposed mechanism sought to satisfy these four essential conditions without the use of CNTs, suggesting a possible route toward highly processable long-chain carbyne.
The use of nanopores is a versatile method to produce long and narrow polymeric nanomaterials. Nanopores themselves are readily synthesized by directing a concentrated beam of electrons into a chemically inert material, such as silicon nitride, to produce small channels with nanoscale diameters [9]. A channel-compatible monomer and initiator can then be inserted into these pores to synthesize a desired polymer. For the purposes of carbyne production, whose optimal temperature as reported by Shi et al. is approximately 1500°C [4], silicon nitride can be used as a suitable substrate since, under inert conditions, it has a maximum use temperature of 1600°C [10]. However, while traditional nanopore template polymerization uses an electron beam to produce pores with radii in the range of tens to hundreds of nanometers, carbyne polymerization requires subnanometer pore sizes of approximately 0.7 nm as previously reported [4]. A recent method by Yanagi et al. demonstrates that subnanometer-sized pores in silicon nitride can be produced by using multilevel pulse-voltage injection (MLPI) [11]. Therefore, MLPI is a suitable method of constructing porous templates of approximately 0.7 nm diameter in silicon nitride with a high level of precision.
Upon creation of the MLPI-produced pores, theoretical calculations given below suggest that carbyne chains may be synthesized by CVD. Then, after clearing the surface of amorphous carbon, extraction techniques can be employed to remove the carbyne chains from the ceramic mold. One method is to apply a surface layer of a common trifunctional polymer such as polyacetylene to cover the open ends of the nanopores, as polyacetylene can readily form crosslinks with the carbyne triple bonds exposed at the interface (see Figure 4). By covalently bonding the polyacetylene layer to the tips of carbyne chains in the silicon nitride pores, removal from the ceramic mold may be achieved to expose the freshly synthesized carbyne. The rigidity of the linear carbyne chain allows for the preservation of the carbyne structures such that they can be subsequently stabilized by PDDA.
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By this method, as evidenced from CNT brushes ordered in this manner, grafted carbyne as a polymer brush may have applications as sensors with lower thresholds and adhesives with higher strengths than CNTs. Next, a system of removing the carbyne chains from the polymer base is proposed, allowing for the processing of carbyne into fibers, which may see future use as a carbon fiber reinforcer in aerospace and automotive industries. A crude but immediate solution of physical separation is suggested to separate the carbyne chains from the polyacetylene substrate. With high uniformity of carbyne chains several nanometers in length, physical severing of the covalent bonds to the polymer base is feasible with an ionizing laser or even by mechanical methods. Here, the disadvantage of an inevitable loss of product to ensure clearance from the polymer base is justified to prevent disruption of the carbyne structure. By stabilizing long-chain carbyne with PDDA before cleavage, the chains may finally be wound and packed together into a carbyne fiber or processed for a variety of other applications.
This methodology is validated and supported by a theory-based analysis of molecular interaction energies. First, the emission of a photon, represented as was used to determine the energy of electrons in the hypothetical system, where is the Hamiltonian of an electron interacting with an electromagnetic field, is a unit vector describing direction, is the electron charge, is the electron mass, is the electron velocity, is the angular frequency, is the linear momentum, is a creation operator, and is the wave function of the photon emitted. From Equation (1), the interaction of two particles or molecules can be described with respect to the energy at which electrons are excited and photons are emitted. For example, at high vacuum and temperature, the interaction of methane gas and silicon nitride can be calculated to assess the likelihood of carbyne formation. Here, interaction energy can be defined as where is the number of photons, is the speed of light, is an applied magnetic potential, and is an applied electric potential.
Under proper electromagnetic control, Equation (2) suggests that high velocity molecular motion under HTHV conditions may result in an interaction energy favorable for acetylenic carbon-carbon interactions. Since angular momentum is regulated by pressure and temperature, and the applied magnetic and electric potentials can be adjusted, electrons on a carbon molecule can be “emitted” in a hydrogen-bound form such that hydrogen gas is subsequently evolved from the interaction of two produced hydrogen atoms. In other words, methane gas will condense to an amorphous carbon structure with an H2 byproduct.
These claims are supported by initial theoretical calculations. The energy required to remove hydrogen atoms from CH4 is calculated by substituting relevant values for the electrons contained in the molecule into Equation (1), resulting in a range of about 105 kcal/mol to remove the first hydrogen (CH3-H) and about 81 kcal/mol dissociation energy for the removal of the final hydrogen (C-H). The difference arises from the different hybridization states and different electromagnetic interactions that result as additional hydrogen atoms are removed. The resultant carbon atoms of various dissociation states can then be calculated to interact with each other with an energy of about -132 kcal/mol for two fully dissociated carbons to about -60 kcal/mol for two CH2 molecules. The energy for Si-N dissociation, on the other hand, can be calculated to range widely depending on the external conditions, from about 158 to 263 kcal/mol, and can therefore be made highly unfavorable under experimental control. The reaction of a synthesized carbyne chain with PDDA has a calculated activation energy of 63 kcal/mol, suggesting that under standard conditions, carbyne can be effectively stabilized by the polymer without undergoing reaction.
Therefore, an electromagnetic system can be employed where the energy in free electrons moving at high temperature and pressure supplies the activation energy to break the C-H bonds while simultaneously providing the energy and electrons necessary to form C-C single and triple bonds. Saturation of the silicon nitride pores with free electrons by electromagnetic means will, therefore, act synergistically to provide both a fine-tuned energy source and a fully reduced carbon molecule as the reaction goes to completion. It is important to note that the ability to fine-tune the electron number and energy through the application of an electromagnetic field can be further controlled to favor the formation of acetylenic carbon chains over cumulenic linkages. For example, higher field strength will result in increased probability of more stable cumulenic bond formation, whereas a field strength just above threshold dissociation energy is calculated to favor less stable acetylenic bond formation [12, 13]. Therefore, under thermodynamic control, the model of carbyne chains grown in MLPI-cut silicon nitride is supported by initial theoretical calculations and demonstrates a novel route to bulk carbyne synthesis.
3. Conclusion
It has been the purpose of this discussion to propose a theoretical method of long-chain carbyne synthesis that may lead to the reliable production of carbyne fibers or other processed carbyne materials. By modeling the DWCNT-assisted growth of carbyne fibers with MLPI-manufactured silicon nitride nanoreactors, a novel approach to carbyne synthesis is illuminated. This method has several advantages over DWCNT-assisted carbyne growth, including increased processability and packing density. Upon grafting to a polymeric sheet capable of networking with the exposed carbyne ends, the silicon nitride mold may be carefully removed to expose the carbyne chains under an inert atmosphere. Furthermore, with a large separatory distance between the produced nanopores, the distance between carbyne chains can be made large enough to prevent interaction. Then, by immersion of the carbyne brush into a solution of a dilute unreactive polymer with an electrophilic attraction, a protective coating is formed such that the carbyne chains become stabilized. The polymer base is subsequently removed, and the free, stabilized, long-chain carbyne can be spun into a fiber or processed for other purposes. Initial theoretical calculations support these claims. Carbyne fibers or grafts produced by this method may see use in both industrial reinforcement and electronic applications. Further research is planned to experimentally test and perfect this theoretical design.
Abbreviations
| CNT: | Carbon nanotube |
| SWCNT: | Single-walled carbon nanotube |
| MWCNT: | Multiwalled carbon nanotube |
| DWCNT: | Double-walled carbon nanotube |
| MLPI: | Multilevel pulse injection |
| PDDA: | Poly(diallyldimethylammonium chloride) |
| MO: | Molecular orbital |
| ABMO: | Antibonding molecular orbital. |
| HTHV: | High temperature and high vacuum. |
Data Availability
All data generated or analyzed during this study are included in this article.
Conflicts of Interest
The author declares no competing financial interest.
Authors’ Contributions
The manuscript was prepared in its entirety by the author.
Acknowledgments
Inspiration for the project is attributed to the Department of Macromolecular Science and Engineering, Case Western Reserve University. ChemDraw Professional 17.0 was used to create all chemical structure representations.