Comparative studies on mechanical properties and dehydrochlorination of chlorinated natural rubber-based composites containing carbon nanotubes and graphene through DFT and experiments

Abstract

This study employed a combination of density functional theory (DFT) and experimentation to compare the mechanical properties of chlorinated natural rubber (CNR)/carbon nanotubes (CNT) and CNR/graphene composites and to illuminate their thermal stability differences related to dehydrochlorination. The results showed that, in tensile tests, the Young's modulus of CNR/graphene was greater than that of CNR/CNT, while in pull-out simulation tests, the interfacial shear strength of CNR/graphene was smaller than that of CNR/CNT. In thermal stability tests, both CNT and graphene made the CNR dehydrochlorination reaction rate constant larger, with the thermal stability of CNR/graphene better than that of CNR/CNT. The performance change trends of the two composites were calculated using DFT and the experimental results were consistent with these trends.

Keywords

Chlorinated rubber carbon nanotube graphene density functional theory composites Young's modulus interfacial shear strength dehydrochlorination

Introduction

Chlorinated natural rubber (CNR), as an important coating additive, has been used in ships and offshore facilities for decades due to its excellent application properties, including good adhesion, good wear resistance, corrosion resistance, and fast drying [1-3]. CNR is one of the most useful chlorinated derivatives of natural rubber, produced by chlorination reactions between natural rubber and chlorine [4]. In chlorination reactions, hydrogen atoms of natural rubber are replaced by chlorine atoms and the actual Cl content of CNR can reach 62–68 wt-% [5]. The introduction of Cl atoms transforms the flexible linear structure of natural rubber into a more rigid CNR six-membered ring structure and, thus, CNR has stronger mechanical properties than natural rubber [4]. It has been reported that, at low deformation rates, the tensile strength of CNR can reach 39.24 MPa [6], but this is still lower than that of traditional polymer materials, such as PVC (∼60 MPa) [7]. Moreover, CNR has a tendency to suffer dehydrochlorination in humid and hot environments, resulting in covalent bond formation in its intramolecular structure [8]. After dehydrochlorination reactions, CNR forms a gel, which usually yields the CNR material more fragile, thereby impairing its mechanical properties and processing properties [4]. Therefore, it was of great significance to explore means for improving the mechanical properties and stability of CNR composites.

Graphene and carbon nanotubes (CNT) are currently the two most striking nanomaterials that belong to the same family of carbon nanomaterials. Graphene has a two-dimensional lamellar honeycomb structure, consisting of sp ²-bonded carbon atoms forming a single-atom thick plane [9, 10]. CNT is a kind of one-dimensional tube with a hollow inner cavity, which is formed by bending one or more sheets of graphene into a closed cylinder with two open ends [11]. Both kinds of carbon nanomaterials can be combined with polymers to prepare new composite materials with excellent properties [11]. It is worth noting that the unique structural characteristics of graphene and CNT also make composites containing them possess different properties. For example, when the graphene content in silicone rubber/graphene composites reaches 3.0 wt-%, the tensile strength of these composites reaches 6.25 MPa [12]. However, when the CNT content in silicone rubber/CNT composites is also 3.0 wt-%, the composite tensile strength only reaches 1.72 MPa, which is much lower than the former composite [13]. From this, it appears necessary to compare the effects of the two kinds of carbon nanomaterials in composite materials, thus determining which carbon nanomaterial is more suitable for preparing composite materials with better performance. Although CNR is a kind of very important industrial polymer and its composite materials have great practical application value, the comparative studies of the role of CNT and graphene in CNR-based composite materials have not yet to be reported.

Owing to the lack of comparative studies regarding physical and chemical properties of CNR/CNT and CNR/graphene composites, our interest was aroused to determine which carbon nanomaterials are more suitable for preparing CNR composites with excellent properties. In this study, density functional theory (DFT) and related experiments were employed to investigate the mechanical properties of CNR/CNT and CNR/graphene composites, including binding energy (BE), Young's modulus, and interfacial shear strength. Dehydrochlorination kinetics of the two composites were also examined to reveal their thermal stabilities. The theoretical and experimental results of CNR/CNT and CNR/graphene composites in this study provided valuable guidance for their practical applications in coatings and other fields.

Calculation and experimental details

Calculation details

All constructed models in this study, corresponding to studied systems, had first been optimised using the density-functional tight-binding (DFTB) method implemented in the DFTB+ package [14]. DFTB is a non-orthogonal tight-binding method that has been parameterised from DFT. The accuracy of the method has been improved using the self-consistent charge extension in DFTB (SCC-DFTB), which is comparable to that of full DFT calculations with a double-ζ plus polarisation basis set [14]. The Slater–Koster parameters were obtained using the 3OB parameter set [15], which numerically describes s and p atomic orbitals for carbon and chlorine and the s orbital for hydrogen, which was used in all calculations. The Brillouin zone was sampled using 2 × 2 × 1 k-points and the Monkhorst–Pack scheme [15]. Geometries were optimised using the conjugate-gradient method until the atomic forces were below 10⁻⁵ eV Å⁻¹ and SCC tolerance set to 10⁻⁵au.

For all optimised models, their total energies were successively calculated based on DFT theory using Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code [16, 17]. The exchange and correlation potential were treated using the generalised gradient approximation within the scheme of the generalised gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE). The basis set was set to a double-ζ plus polarisation (DZP) and an energy shift of 100 meV. The same Brillouin zone settings (Monkhorst–Pack scheme, k-points of 2 × 2 × 1) as the above DFTB method were adopted and the mesh cut-off energy representing the electron density set to 150 Ry [18]. To avoid an overestimation of calculated adsorption energies owing to basis set superposition errors, the counterpoise scheme of Boys and Bernardi was used to correct interaction energies between CNR and substrate [19].

Construction of CNR/CNT and CNR/graphene models. First, the (4,4) CNT model [20] with the size of 25.52 × 25.52 × 37.15 Å and the graphene model with the size of 54.12 × 14.76 × 30.00 Å were optimised to the lowest energy state and selected as the CNR adsorption matrix. Then, a series of CNR/CNT or CNR/graphene composite models were obtained by placing a CNR molecule with different numbers of monomers (1, 2, 3, 5, and 8) on the CNT or graphene surface. These complex models were respectively placed in a box with periodic boundary conditions. The influence of periodic images on the calculation results was avoided by setting the lateral distance between the central axes of the CNT or graphene surfaces to 27 Å. Geometrical optimisation was then carried out on the established complex models.

DFT theory, embedded in Gaussian software, was used to study dehydrochlorination reaction kinetics of CNR/CNT and CNR/graphene and the effects of the two carbon nanomaterials on the thermal stabilities of CNR-based composites compared. To investigate dehydrochlorination, a model of CNR monomer adsorbed on a CNT or graphene surface was established. The Gibbs free energies of initial states, transition states, and the final products in dehydrochlorination reactions occurring between the C1 and C2 atoms of the CNR monomer (Figure 1) at different temperatures were calculated, as dehydrochlorination in CNR was likely to occur between the C1 and C2 atoms outside the six-membered ring [21]. This effect was because the formation of a double bond between two carbon atoms outside the six-membered ring has less effect on the entire molecular conformation, experiences a lower energy barrier, and more easily occurs than inside the six-membered ring [21]. Reaction rate constants were calculated using the following equation: [22] (1) where k is the reaction rate constant; k_B is the Boltzmann constant, 8.62 × 10⁻⁵eVK⁻¹; T is the Kelvin temperature, K; h is the Planck constant, 6.58 × 10⁻¹⁶eVs⁻¹; and ΔG _b is the energy barrier in Gibbs energy, eV. In dehydrochlorination processes of CNR, transition states for all involved systems were searched using ONIOM QM1(M06-2X/6-31G**):QM2(PM6), where the M06-2X/6-31G** basis set, as high calculation level, was used for QM1 region and PM6 as the low calculation level for the QM2 region [23]. The QM1 subregion included the CH–CHCl part outside the six-member ring in the CNR monomer and several carbon atoms of substrate near the CH–CHCl part, and the QM2 subregion contained the rest of the system excepting the QM1 region. Final optimised structures of searched transition stations and their single-point energies were calculated using the calculation level of M06-2X/6-31G**. The M06-2X function exhibited excellent performance for accurately predicting main-group thermochemistry, kinetics, noncovalent interactions, electronic excitation energies, and aromatic-aromatic stacking interactions [24].

Experimental details

Reagents and materials

CNR (65.3 wt-% Cl, molecular-weight (M _W) 170 kDa, molecular-weight distribution (M _w/M _n) of 2.3, average degree of polymerisation of 48 and dynamic viscosity at 26–35 mPa s at 25°C) was obtained from Jiangsu Ruihe New Material Co., Ltd (Jiangsu, China). Graphene powder (carbon content 99.85%, thickness 1–2 nm, size of flake 2–3 μm, surface area 500–800 m²/g), single-walled carbon nanotubes (carbon content 95%, outside diameter 1–2 nm and length 5–30 μm), and dimethylbenzene (analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).

Composite preparation and tensile test

The preparation methods of CNR/CNT and CNR/graphene composite materials are described below. First, CNR were dissolved in xylene with the 1/4 mass ratio of CNR to solvent to obtain a viscous solution. Then, a certain amount of solid CNT or graphene was added to the solution and ultrasonically dispersed at 25°C for 6 h. The prepared mass ratios of solid CNT or graphene and CNR in solution were 0.5/99.5, 1/99, and 1.5/98.5. The ultrasonically dispersed mixture was placed in a vacuum drying oven and dried at a constant temperature of 50°C for 6 h for solvent removal, thereby obtaining composite materials. After drying, composite samples were prepared for scanning electron microscopy analysis (SEM, JSM-7600F, Nippon Electronics Corporation, Japan) and also were cut into the sizes of 50 × 20 × 2 mm for tensile testing.

Samples of CNR/CNT and CNR/graphene composites were tested according to the ASTM D412 standard [25] with a universal testing machine (Shenzhen SANS Testing Machine Co., Ltd, Shenzhen, China), with the tensile rate at 20 mm min⁻¹ and temperature at 25°C. Three tensile tests were conducted for each sample and the average Young's modulus was obtained from the linear (lower than 10%) regions of the stress–strain curves and taken as the sample's final Young's modulus [26].

Experiments of dehydrochlorination reactions

A thermogravimetric analyzer TG209 F1 (Netzsch Instruments North America, Burlington, MA, U.S.A.) was used to measure the main weight-loss temperature ranges, maximum weight-loss rates, and maximum decomposition velocity temperatures of CNR, CNR/CNT, and CNR/graphene. The thermal stabilities of CNR, CNR/CNT, and CNR/graphene and their dehydrochlorination were compared using characteristic parameters during the thermal decomposition. Thermal analysis samples of 6–8 mg masses of CNR, CNR/CNT, and CNR/graphene were individually loaded into an alumina crucible and analysis performed with nitrogen protective gas, flow rate at 20 mL min⁻¹, temperature range of 30–500°C, and heating rate at 5°C min⁻¹.

Results and discussion

Binding energies and mechanical properties

First, the interfacial bonding performance of CNR on CNT or graphene was examined considering steric hindrance and calculation effort, with the number of CNR monomers adsorbed on the substrate (CNT or graphene) at not > 8 [27]. The BEs between CNR molecules and the substrate in the composites showed that all calculated BEs between CNR and substrate were negative whether the substrate was CNT or graphene, which indicated that there was an attractive interaction between CNR and CNT or graphene (Figure 2). The absolute BE between CNR and substrate increased with increased CNR monomers, indicating that a longer CNR molecular chain was helpful for stabilising the entire composite system. Furthermore, the absolute BE between CNR and CNT was always greater than that between CNR and graphene and the difference between the former and latter tended to increase with increased CNR monomers. The possible reason for this trend was that the larger the number of CNR monomers was, the longer the molecular chain and the more flexible. Also, the natural form of the flexible polymer molecular chain usually tends to be spiral in order to maintain its low energy configuration [28]. The curved tubular configuration of CNT should be more suitable for CNR chains with more monomers to form a spiral shape than the planar configuration of graphene. This allowed more atoms in the CNR chain to come into close contact with the CNT, enhancing interactions between the CNR chain and CNT.

Young's modulus is an important quantity for measuring material stiffness and the resistance of a material to deformation resulting from external compressive or tensile forces [29, 30]. In this study, the theoretical Young's modulus of CNR/CNT and CNR/graphene along an extension direction of the substrates (the extension direction parallel with the CNT central axis and with the graphene surface) were obtained by gradually compressing or elongating in small increments. The length variation of complexes along the extension direction at every step during compression and elongation was set to 0.007 Å. Plots of the strain energy (E _s) versus ϵ in the harmonic region of the complexes with different numbers of CNR monomers adsorbed on the CNT and graphene are shown in Figure 3. The relationship between Young's modulus (Y), E _s, and strain value (ϵ) was expressed in the following equation [27] as (2) where V ₀ is the system equilibrium volume, E _s is the system strain energy, and ϵ is the strain on the system. The variation of Young's modulus of CNR/CNT or CNR/graphene composite, calculated by DFT as the number of monomers in CNR increased, showed that the moduli of only CNT and graphene (without CNR) were 981 and 773 GPa, respectively (Figure 4(a)). These values were close to the experimental values (1050 GPa for CNT, 740 GPa for graphene) reported in the literatures [31, 32], which indicated that DFT calculation results here were reliable. DFT Young's modulus of CNR/CNT and CNR/graphene composites decreased with increased monomers in CNR, such that, when the CNR monomer number exceeded five (or the CNR content in composites was > 21.4 wt-%), the DFT Young's modulus of CNR/graphene was greater than that of CNR/CNT. To verify these DFT results, the Young's modulus of CNR/CNT and CNR/graphene composites were also measured with different CNR contents (98.5, 99, and 99.5 wt-%, respectively; Figure 4(b)). The experimental values of Young's modulus of these CNR/CNT and CNR/graphene composites were all larger than those of CNR, indicating that CNT and graphene had significant enhancement effects on CNR mechanical properties. In addition, for CNR/CNT and CNR/graphene composites containing different CNR mass percentages (ω _CNR), the order of their experimental Young's modulus from small to large was ω _CNR= 99.5% < ω _CNR= 99% < ω _CNR= 98.5%. In the range of CNR content > 98.5 wt-%, the experimental Young's modulus of CNR/graphene composite was greater than that of CNR/CNT composite. Clearly, this experimental trend was consistent with the theoretical DFT trend, which confirmed that the DFT method could be effectively applied to comparative studies on the mechanical properties of CNR/CNT and CNR/graphene composites. Figure 5 shows the SEM images of CNR/CNT and CNR/graphene composites. By comparing the two images, it can be seen that the CNR/CNT composite had more hole-like defects than the CNR/graphene composite. This is also one of the reasons why the experimental Young's modulus of the former was lower than the experimental Young's modulus of the latter.

Interfacial shear strength is another important mechanical property of composites that determines material interface failure under dynamic loading [33]. Interfacial shear strength also affects other mechanical properties of composites, such as elastic modulus, tensile strength, and fracture toughness [34]. The interfacial shear strengths of CNR/CNT and CNR/graphene composite materials were calculated by performing pullout simulations. This simulation procedure involved CNT (or graphene) in a CNR/CNT composite (or CNR/graphene) being pulled completely out of (or away from) CNR molecules along the Z-axis of composites and then the pullout energy calculated during the process using DFT (Figure 1). The pullout energy was defined as the difference between the structural energy corresponding to the CNT (or graphene) completely embedded in the CNR molecule and the structural energy corresponding to the CNT (or graphene) completely pulled out from the CNR molecule. According to Equations (3) and (4) [27], the calculated pullout energy was used to calculate the interfacial shear strengths of CNR/CNT and CNR/graphene composites. (3a) (3b) where b is the width of graphene monolayer, L is the length of graphene drawn from CNR, and Z is the length of CNR moving along the Z-axis. (4a) (4b) where r is the radius of CNT, L is the length of CNT drawn from CNR, and Z is the length of CNR moving along the Z-axis. To yield the interfacial shear strengths of composite materials calculated by DFT close to the actual interfacial shear strengths of the present composite materials, 8-monomer CNR, as the longest CNR chain used here, was adsorbed onto the CNT or graphene surfaces for pullout simulation experiments. In the CNR/CNT model, the energies of the model before and after CNT was completely pulled away from the CNR molecule were −26173.36 and −26173.86 eV, respectively. The calculated interfacial shear strength between CNR and CNT from Equation (3) was 1.63 MPa. In the CNR/graphene model, the energies of the model before and after graphene was completely separated from the 8-monomer CNR molecule were −21795.13 and −21795.18 eV, respectively, and the calculated interfacial shear strength between CNR and graphene from Equation (4) was 1.14 MPa. Clearly, the interfacial shear strength of CNR/CNT was greater than that of CNR/graphene, which might have been because the interfacial shear strengths of the two composite materials were mainly affected by the aforementioned BE factors [34]. The absolute BE between CNR and CNT was greater than that between CNR and graphene, such that the CNR/CNT system was more stable than the CNR/graphene system, making CNR/CNT possess stronger resistance to interfacial shear forces than the CNR/graphene. The performance differences shown by CNR/CNT and CNR/graphene composites suggested that the appropriate composite should be chosen according to specific needs when actually considering the two composites. For example, when a high Young's modulus adhesive is required for bonding metal materials [35], CNR/graphene might be a better choice than CNR/CNT based on the present research results. In the preparation of composite materials with high interfacial shear strength for hydraulic impermeable engineering [36], CNR/CNT material might be a better choice than CNR/graphene.

Kinetics of dehydrochlorination

The dehydrochlorination reaction of CNR not only leads changed CNR colours, molecular structures, and performances but also the HCl released by decomposition causes serious steel corrosion, which is not conducive to the practical applications of CNR-based composite materials at higher temperatures [37]. Therefore, studying the effects of CNT or graphene on the kinetics of CNR dehydrochlorination was helpful for understanding the thermal stabilities of CNR/CNT and CNR/graphene composites and provided a guide for the practical use of these composites. DFT calculations were used to examine changes in the reaction rate constant k for dehydrochlorination reactions of CNR molecules adsorbed on CNT or graphene as a function of temperature [37]. The rate constant k is a quantitative expression of chemical reaction rate, which represents the reaction rate when the concentration of every reactant is at unit concentration [36]. The larger the k value is, the higher the dehydrochlorination reaction rate [38]. The traditional transition state theory method [39] was used to calculate the k value of the CNR dehydrochlorination reaction in CNR, CNR/CNT, and CNR/graphene and the relationship between the reaction rate constant k and temperature T fitted using the Arrhenius equation ( , Figure 6). The k values of the CNR dehydrochlorination reaction in CNR, CNR/CNT, and CNR/graphene all increased with increased temperature, indicating that the higher the temperature was, the faster the CNR dehydrochlorination reaction rate. Meanwhile, under the same temperature, the dehydrochlorination k of CNR, CNR/CNT, and CNR/graphene was in the order (from small to large) of CNR < CNR/graphene < CNR/CNT. This indicated that both the CNR/CNT and CNR/graphene composites had lower thermal stabilities than CNR alone and the CNR/graphene composites had better thermal stabilities than the CNR/CNT composites.

The dehydrochlorination reaction of CNR/CNT and CNR/graphene was also examined by thermal analysis. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of CNR, CNR/CNT, and CNR/graphene showed that the main weight-loss temperature range of CNR, CNR/CNT, and CNR/graphene was from 140°C to 370°C (Figure 7(a)). This was the CNR dehydrochlorination reaction temperature range reported in the literature [8]. It was noted that the pure CNR also had a weight loss of about 5% at 200°C, which was consistent with the experimental results reported in the literature [40]. This weight loss at 200°C was attributed to the dehydrochlorination reaction of CNR [40, 41]. And, the weight loss of the CNR/graphene composite (or the CNR/CNT composite) at 200°C reached 6.4% (or 8.9%), which was slightly higher than that (5%) of pure CNR. This was because for the CNR/graphene composite (or the CNR/CNT composite), the introduction of graphene (or CNT) caused a decrease in the energy barriers of the dehydrochlorination reaction of CNR and an increase in the reaction rate constants. From 140°C to 370°C, the weight losses of CNR, CNR/CNT, and CNR/graphene were 64.7%, 68.3%, and 65.2%, respectively, with the weight loss of CNR/CNT larger than that of CNR/graphene. The maximum thermal decomposition rates of CNR, CNR/CNT, and CNR/graphene from DTG results appeared at 307.8°C, 268.2°C, and 274.5°C, respectively (Figure 7(b)). Clearly, the temperature corresponding to the maximum pyrolysis rate of CNR/CNT was lower than that of CNR/graphene. That is to say, CNR/CNT composites were more prone to dehydrochlorination reactions than CNR/graphene composites and the thermal analytical results agreed with previous DFT calculation results.

Conclusions

The results of DFT simulations and experiments showed that, for CNR/CNT and CNR/graphene composites, each had its own advantages and disadvantages in mechanical properties and thermal stabilities. Specifically: (1) both CNT and graphene significantly improved CNR mechanical properties, such as Young's modulus and interfacial shear strength, but CNT and graphene also accelerated the CNR dehydrochlorination reaction rate, thereby reducing thermal stabilities of CNR-based composites; (2) when the CNR content was high (> 21.4 wt-%), the Young's modulus of CNR/graphene composite was greater than that of CNR/CNT composite; (3) the interfacial shear strength of CNR/graphene composites was lower than that of CNR/CNT composites, which might have been because the BE between graphene and CNR was smaller than that between CNT and CNR; and (4) the thermal stability of CNR/CNT composites caused by dehydrochlorination was lower than that of CNR/graphene composites. As the mechanical properties and thermal stability of a certain composite material determine its applicable fields and application value, the present research results provided theoretical and experimental basis for guiding the practical application of CNR/CNT and CNR/graphene composite materials.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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