Hidden proton pathways emerge as ultrathin polymer film method splits interface signals

Understanding how protons move at the interface between polymers and electrode materials is essential for improving fuel cells and related energy devices. However, conventional impedance measurements under inert conditions have long masked these interfacial contributions, showing only a single, merged signal. Now, researchers at the Japan Advanced Institute of Science and Technology, in collaboration with Tokyo University of Science and University of Calgary, have developed a method to experimentally separate and quantify proton transport at individual interfaces in ultrathin ionomer films. The study was published in the journal ACS Applied Materials & Interfaces.

Abstract

IThis study highlights how the transition from uncured to cured state in rubber is characterized by the formation of a particular network structure, so-called critical gel. Critical gel is formed in the first stages of vulcanization and is independent by the macromolecular structure and crosslinking.

Keywords: Curing cycle design; Under-cure phase; Spongelike region; Blowing point; Shrinkage; Critical Gel time; Elastomer; Compound; Viscoelasticity

Introduction

Vulcanization proceeds through three different stages, as shown in Figure 1. The first stage is characterized by an induction phase in which the reactions between the curatives and macromolecules of the polymeric matrix take place. In the second, commonly known as curing stage, cross-linking reactions between the macromolecular chains take place [1-2]. The initial sol-gel transition is characterized by the formation of a “critical gel” in which each polymer chain is chemically linked to at least another one. The reaction then proceeds until the full vulcanization is reached. In the final stage, so-called over-curing reactions take place and they can cause the partial destruction of the network. This phenomenon is called reversion and, in this case, a partial loss of product performance generally occurs [3].

Keywords: The Sol-Gel Transition & Volumetric Contraction

Rheology is commonly used to study the behaviour of phase transitions in polymeric systems [4]. The formation of the network during vulcanization, in which the transition from a rubbery to a solid state takes place, is characterized by the presence of permanent cross-links and by a sol-gel transition with the formation of a critical gel [5-6]. Critical gel shows a relaxation modulus, G(t) described by a power law, as follows:

G (t ) = S.t -n

where parameters S and n represent the rigidity and the critical relaxation exponent of the gel. While it is difficult to measure the G(t) modulus from step shear strain (stress-relaxation) tests, it is much easier to obtain it from simple frequency sweep tests. Applying the Laplace transform [7] of the function describing G(t), we can immediately obtain the complex modulus, G*, as follows:

G* = sL (G (s)) = S.G(1- n).sn

where s= î ω and î is the imaginary unit and ω the frequency. Noticing that G*=G’+ î G’’ the equation 3 is obtained:

Experimental

In this study we investigated three different compounds named C1 C2 and C3. C1 and C2 have the same vulcanizing system, based on dicumyl peroxide (DCP) at 1.3 phr (taken as pure), without the addition of fillers. C1 is high cis polyisoprene (IR0310 Kraton) and C2 is NBR nitrile rubber (N3345). C3 is a typical tread rubber compound recipe for truck tire application, with NR, BR and e-SBR as given in Table 1. The elastic modulus G’ (MPa) was measured during isothermal vulcanization at 3 different temperatures, 150°C, 160°C, and 170°C, at a frequency of 1 Hz at 1% of deformation. Simultaneously the volumetric shrinkage of the compound under isothermal vulcanization conditions. The studies were made using a Premier™ RPA Plus analyser, commercialized by Alpha Technologies (company of the Roper Technologies group, Hudson, OH, USA). The resulting torque is measured and through the appropriate built-in data treatment, split into real and imaginary components in order to yield dynamic viscoelastic data, such as G’ and G”. The testing part consists in a biconical test chamber with grooved dies to avoid slippage. The cavity is closed through the action of a ram [8-9]. The torque measuring system is attached to the upper die and a direct drive motor move the lower die sinusoidally over a wide range of strains and frequencies as shown in Figure 2. The critical gel point, that is the time at which the sol-gel transition takes place at a given temperature, has been determined by carrying out a continuous sequence of frequency sweep tests (in the range between 1 and 30Hz) at a given curing temperature.

Figure 3 shows the results of these frequency sweep measurement obtained for C1 (IR compound) during isothermal vulcanization at 150°C. Data shows that at 250s, the formation of the critical gel is observed: at the critical gel point tand is no frequency independent as given in eq. 3. As shown inf Figure 1 the time in which the sol-gel transition takes place (tc, time in which critical gel formation is observed), it is characterized by a degree of vulcanization of 0.25-0.30, regardless of the temperature at which the curing reaction occurs. The formation of the critical gel can therefore be related to the development of a crosslinked structure during vulcanization and its dependence on the curing temperature. Figure 3 shows the dependence of the critical gel time on the reciprocal of the vulcanization temperature for all compounds analysed. As expected, the formation of the critical gel occurs faster at higher temperatures, where the rate of vulcanization is faster. The dependence between the critical gel time and the vulcanization temperature follows the Arrhenius' equation as shown by the linear behaviour of in Figure 4. The calculated activation energy values are in excellent agreement with the experimental data available in the literature [5-6].

The time to reach the sol-gel transition is faster for the C1 (IR) than for the nitrile compound (C2), while it becomes practically equal at approximately 135°C, which is the temperature at which the DCP reaction rate becomes very slow. For the sulphur cured compound C3, the critical gel time is significantly longer than C1 and C2 at the same temperature. In other words, the same tc has been reached for C3 at a temperature 20°C higher than the peroxide cured compounds. Figure 5, shows the relaxation exponent vs. critical gel stiffness as a function of temperature. It is interesting to note that the stiffness of the critical gel slightly depends on temperature, and is characterized by particularly low values of n, especially at high temperatures (in the order of 0.10). These results suggest that the critical gel structure is qualitatively similar, independently of the elastomers and of the curing system and tend to be stiffer when formed in a curing reaction at high temperature, showing a quite limited time dependent relaxation modulus G(t).

Results and Conclusion

The formation of the network during vulcanization, in which the transition from a rubbery to a solid state takes place, is characterized by a sol-gel transition with the formation of a critical gel. Frequency sweep test has been used to determine the time require to form the critical gel during vulcanization and has been found to takes place at a degree of vulcanization of 0.25-0.30, regardless of the temperature at which the curing reaction occurs. In this paper, it has been also shown experimentally that critical gel time follows the Arrhenius' equation and the stiffness of the critical gel slightly depends on temperature, and is characterized by particularly low values of n, especially at high temperatures (in the order of 0.10). These results suggest that the critical gel structure is qualitatively similar, independently of the elastomers and of the curatives.

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Abstract

Ion exchange membranes (IEMs) are used for exchanging or removing ions from the solutions so that they are fabricated by polymers rather than metals or ceramics. When metal is in contact with the solutions, some reaction or corrosion occurs, and ceramic is difficult to use in membrane form because it is brittle. Also, normal metals and ceramics do not have high ion-exchange properties so that polymers are used widely as a host material of the IEM. The conventional IEMs are either layered membrane consisted of ion exchange material and support layer, and thick bulk membrane. There are no critical problems to use them for water treatment, but they are not suitable for electrochemical application such as reverse electrodialysis (RED). RED is a method of making electricity by sequential ion transport through semi-permeable membranes and electrochemical reaction at the electrode part so that the IEMs need to be developed for getting the structure not to interfere with ion movement through the IEM body. In the RED system, the IEMs are involved in separating ions to create the chemical potential which makes a voltage in the electrochemical system. IEMs for RED has not yet been widely adopted, and most of them are the membrane for electrodialysis (ED). Therefore, we need to study engineering approaches to improve the IEM properties.

Keywords: Ion exchange membrane; Cation exchange membrane; Anion exchange membrane; Polymer; Composite; Structure engineering; Reverse electrodialysis

Introduction

Ion exchange membrane (IEM) is a semi-permeable membrane that consisted of various polymers. When giving the driving force of ion transport, IEM allows the counter-ions and rejects the co-ions [1]. The IEM determines the mobility and selectivity of the ions with assumed that the same driving force is given in the system. There are two types of IEM according to the fixed charge in their body, cation exchange membrane (CEM) and anion exchange membrane (AEM). CEM has a negative charge, thereby the anions are rejected and the cations are exchanged by CEM. On the contrary, since AEM has a positive charge, the cations are rejected and the anions are exchanged by AEM (Figure 1). In addition, IEMs are classified into homogeneous and heterogeneous membranes according to their features. Homogeneous membranes can be fabricated by polymerization of monomers or blending and grafting polymers with different moieties [2-4]. Homogeneous membranes have evenly distributed fixed charges throughout the entire body so that uniform properties appear in all regions. Heterogeneous membranes can be obtained by blending charged resin with a binder and casting them on support [5,6].

Usually, the fixed charge distribution of heterogeneous membrane is uneven compared to homogeneous membrane due to the intrinsic drawback of the manufacturing process. In general, therefore, homogeneous membranes have better ion exchange properties than heterogeneous membranes, but heterogeneous membranes are mechanically strong compared to homogeneous membranes. However, both types of membranes can improve their properties through material or structural engineering. There are many high-quality commercial products in the market (Table 1). In the case of CEMs, the most representative commercial product is the Nafion from DuPont. It has a thickness of 178 nm, IEC of 0.84 meq/g, and exact permselectivity is unknown [7]. NEOSEPTA CM-1 of ASTOM, Selemion CMV of Asahi glass, RALEX CM(H)-PES of MEGA, and CEM type 1 of FUJIFILM are also well-developed CEMs. These products showed a thickness of 100-450 nm, IEC levels of 0.84-2.30 meq/g, and permselectivity of 92.0 -98.8% [5,8]. In the case of AEM, CJMA-2 of Hefei chemjoy, NEOSEPTA AM-1 of ASTOM, Selemion AMV of Asahi glass, RALEX AM(H)-PES of MEGA, and AEM type 1 of FUJIFILM exhibited thickness of 110- 450 nm, IEC values of 0.80-2.20 meq/g, and permselectivity of 90.0-95.0% [5,9-13].

Features of the conventional ion exchange membranes

The conventional IEMs have been developed in an aspect of IEC and mechanical endurance because the conventional products are generally used for adsorbing/exchanging ions to the membrane surfaces in a field of water treatment or ED. These are either layered structure that consisted of ion exchange and support layers or the thick bulk membrane that composed with ion-exchange resin and binder polymer (Figure 2). These type of membranes showed excellent performances for exchanging ions in solution or eliminating specific ions from water, but not suitable to maximize the power density of the applications that ions need to move in an opposite area through the IEM, such as RED. The ions would be hindered physically by a support layer or membrane body in conventional cases, no matter how much other properties are excellent. Thus, the idea that finding a novel material, developing a new surface treatment technique, and conducting structural engineering needs to be considered for fabricating novel IEMs for RED. The RED can be an alternative to overcome the dependency of climate and time for producing electricity because it uses saline water as an energy source. Also, there is no pollutant emission during power generation from the RED system.

The important parameters of the IEM for RED system are the thickness, membrane structure, IEC, areal resistance, and permselectivity, etc. [14-16]. Among them, the thickness and structure of the IEM influence critically the performance of the RED system because it affects how ions move smoothly by penetrating the IEM body. Another thing to consider is the selectivity between cation and anion. If the ion penetrates the IEM easily, the selectivity of IEM is low because it has a trade-off relation between ion conductivity and selectivity. Therefore, it is very important to develop IEMs which has high ion conductivity and selectivity, simultaneously, for expanding the application area of IEMs to electrochemical energy generation.

Operating mechanism and stack configuration of the RED system

The RED system can convert ionic to electric current by circulating two feed solutions that have different salinity, it is just an inverse procedure of the ED system. Salts or solutes in feed solutions are adsorbed to IEM by the induced potential difference between electrodes in the ED process so that the dilute and the concentrate are obtained [14,17]. However, RED makes the ions transport through the IEMs without an applying potential, and current is generated by a redox reaction at the electrode parts. In other words, the concentration difference between the two feeds is the driving force to move the ions, and the alternating CEM and AEM make the ions separate. Thus, high and low salinity solutions need to be supplied alternately across the IEMs for inducing concentration difference. The separation of cation and anion creates a chemical potential in the system so that electrochemical reactions occur to generate electric current (Figure 3) [18-21]. The larger the concentration difference, the higher the power density can be obtained theoretically, but the power density would be decreased owing to an increase of internal resistance in the real case. To enhance the maximum power density, it needs to maintain a high concentration difference between the two feeds with preventing the increase of internal resistance at the same time. IEM is the key to solve this issue. If the IEM performance is improved, the internal resistance of the system would be maintained at a low level even the concentration difference is increased. Therefore, the power density of RED can be determined by the properties of IEM assuming that all conditions of the systems are constant.

Tailor-made ion exchange membranes

Many commercial IEMs are on the market, but they still need to develop for adopting in electrochemical energy generation by overcoming their drawbacks. The methods related to cross-linking (or grafting) other polymers, and incorporating hetero materials on the membrane have been studied. Pal S, et al. [22] evaluated homogeneous phase crosslinked poly(acrylonitrile-co-2-acrylamido-2-methyl-1-propanesulfonic acid) (PAN-co-PAMPS) network CEM with controlling the mol ratio of monomers (AN and AMPS) [22]. By using a PAN-PAMPS-1 membrane including 15 mol% PAMPS, pretty good mechanical ability (tensile strength of 16.4 Mpa) and electrochemical performance (FCD of 0.757 meq/cm3 and transport number of 0.92) were obtained due to its uniform phase mixing of ionic domains and hydrophobic domains. Y. Chen’s group attempted to blend nanoparticles in IEM that were organic-inorganic nanocomposite CEMs such as sulfonated iron (III) oxides (Fe2O3-SO42‒) [23,24] and sulfonated silica (SiO2-SO3H) [25] to improve the IEC, resistance, and permselectivity. The Fe2O3-SO42‒/sPPO nanocomposite CEM was prepared by phase inversion method using the mixture of Fe2O3-SO42‒ particles and sPPO solution. This IEM showed AR of 0.82 Ω cm2, permselectivity of 85.6%, and enhanced gross power density of 1.4 W/m2 [23]. SiO2-SO3H/sPPO nanocomposite CEM was fabricated by phase inversion using the mixture solution. It exhibited AR of 0.85 Ω cm2 and higher gross power density than that of commercial CEM (1.3 W/m2) [25].

Yang S, et al. [26] incorporated ion exchangeable materials into the porous membrane. This is a roll-to-roll (R2R) pore-filling process consisting of pretreatment, impregnation, photo-polymerization, and polishing. The filler material was synthesized by mixing anionic/cationic electrolyte, crosslinking agent, and a photoinitiator. Thus, both CEM and AEM can be fabricated, respectively, depending on which electrolyte is selected and mixed. The pore-filled CEM (PCEM) showed IEC of 1.839 meq/g, AR of 0.475 Ω cm2, and permselectivity of 96%. Also, the pore-filled AEM (PAEM) exhibited IEC of 1.645 meq/g1, AR of 0.661 Ω cm2, and permselectivity of 94.3% [26,27]. Gao H, et al. [9] engineered the surface of AEMs via layer-by-layer (LBL) method with negatively charged poly(styrenesulfonate) (PSS) and positively charged poly(ethyleneimine) (PEI) polyelectrolytes. This membrane has 7.5 bilayers showed excellent anti-organic fouling characteristics and permselectivity. the antifouling potential also increased 30.29% higher than the pristine membrane and the ratio of transport number between Cl- and SO42- was improved from 1.10 (pristine) to 2.44. These results consequently have been enhanced the power density generation of RED by up to 17% [9]. Hong JG, et al. [28] fabricated the hybrid IEMs by the chemical reaction using poly (diallyldimethylammonium chloride) (PDDA) for AEMs and the sulfated polyvinyl alcohol (sPVA) as a hydrophilic polymer with sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (sPPO) for CEMs to improve AR, permselectivity, and power density [28-30]. R.A. Tufa et al. fabricated a polymer layer consisted of polypyrrole (PPy) and chitosan (CS) to prohibit the passage of the multivalent ions. The modified IEM showed improvement of open circuit voltage (OCV) and power density of 20% and 42%, respectively, compared to the pristine IEM [31].

Conclusion

Since the past, IEMs have been developed with the application of various polymer materials. As a result, high-quality commercial IEMs are being distributed on the market in recent years. These are quite suitable for application in the water treatment field, but their disadvantage is poor ionic conductivity for electrochemical energy applications. Therefore, as the researches introduced briefly in this review, it is necessary to continuously try to apply various materials on IEM, and further studies to suggest a novel structure of the membrane are also needed.

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Abstract

Most of the studies about ion exchange membrane (IEM) have been focused on the analysis of commercial IEM, material engineering on the tailor-made membrane, and system engineering related to the IEM applications. However, the structural engineering of IEM is quite effective to improve drastically IEM properties because the structure can affect the movement and hindrance of ion in the membrane body. It is worth taking this opportunity to recall the need for IEM structural studies. As one idea of structural engineering research, an IEM that simulates the concrete structure of a building can be very effective to overcome the limitation of conventional IEMs.

Keywords: Ion exchange membrane; structural engineering; nanofiber; concrete; composite

Introduction

The role of IEM is quite important in various research or industrial area, but its development has been stagnant in recent years. At this point, attempts to fabricate IEMs using a wider range of material are needed, but improving performance with this approach is nearing the limit so that it is necessary to think more fundamental engineering. The most important point that limits the performance of conventional IEMs is the nanostructure of membrane. It is difficult to obtain great effects in electrochemical applications that require smooth transport of ions since the conventional IEMs have a nanostructure that interferes with the movement of ions. The number of papers dealing with IEM related to the electrochemical application is increasing (Figure 1a), but the most of studies were still focused on electrode part or electrochemical system [1]. And, the studies of physical/chemical properties using commercial IEMs or well-known tailor-made IEMs were also intensively conducted. Only a few research focused on the IEM structure even though structural engineering is a quite effective way to overcome the limitation of IEM property for electrochemical applications (Figure 1b) [1]. Among the studies related to IEM structure, very a few studies reported a drastic change of the entire membrane structure, and most of them are the results that introducing nanopores or channels in the IEM. Therefore, we need to consider more fundamental and innovative ideas based on a clear understanding of the IEMs.

Suggestion for developing a novel ion exchange membrane

To overcome the technical limitations of previous IEMs, it is necessary not only to find novel materials but also to structurally modify the IEM. The layered membrane or a thick mixture of resin and polymer were intensively used in various fields because mechanical strength is prioritized in conventional IEMs. In advanced forms, filling porous support or mesh-shaped spacer with an electrolyte having ion exchange properties has been tried. These IEMs can show improved areal resistance and permselectivity, but these are not suitable because the resistance in the system was increased when stacking many IEMs which resulted from the hindering effect against ions. Therefore, the most ideal structure is that the membrane is thin and there is no support layer. If the mechanical strength of membrane is severely degraded without support, making open-structured support can be an option. By considering the above-mentioned issues, I suggest the concrete-structured membrane inspired by the building. The concrete structure consists of a hard skeleton and filler material, and it can endure stress greater than the capacity of the individual material [2]. Skeleton gives basic strength to the structure and filler acts as a buffer for distributing stress and reducing displacement of the skeleton. Based on this principle, if nanofibers are used as a support and filled with a material having ion exchange property, it can be expected that the movement of ions will be smooth while maintaining the minimum mechanical strength (Figure 2). In addition, it will be easy to modify the overall membrane thickness, unlike the conventional method of forming an ion exchange layer on a support having a predetermined thickness. Moreover, if appropriate nanomaterials which have high Zeta potential are introduced, it is expected that the high ion selectivity can be achieved even the membrane thickness thin.

Conclusion

Abstract

In this study, a brief review of the molecular dynamic’s simulations application in research of tribological properties of polymer composites were presented. To better classify and aggregate, particular attention is given to the frictional factor analysis and the enhancement of polymer composites on the tribological properties. The review of the latest research progress of the tribological research by molecular dynamics simulations and some proposals for further research may have implications for the relevant research and applications, which is practically meaningful and helpful for the related researchers to have a better understanding of this field.

Keywords: Molecular Dynamics Simulation; Polymer Composites; Tribological Properties

Introduction

Polymer composites are widely applied in various aspects of scientific and engineering applications, such as industrial production, petrochemicals, automotive industry, military, aeronautics and astronautics, and daily life owing to some of their excellent properties such as wear and corrosion resistance [1,2]. In particular, some important components often face severe working conditions such as high temperature and pressure, corrosion, swelling, and abrasive wear et al. [3,4]. Researchers have done a lot to improve the properties of polymer composites. As is known, simulation has become one of the important methods to study the properties of materials with the development of computer technology. Molecular dynamics simulations have been recognized and widely used due to the unique microscopic perspective as a new simulation method in recent years [5]. As an important index to characterize polymer composites, tribological properties are also one of the key research contents of molecular dynamics simulations. Therefore, many researchers are committed to the research of tribological properties by molecular dynamics simulation.

Discussion

Factor Analysis

The influencing factor analysis of the tribological systems by molecular dynamics methods are first aggregated. Kreer et al. [6] studied the tribological properties of two polymer brushes at different relative sliding speeds. It is found that the lateral force between the polymer brushes decreases logarithmically with an increase in the relative sliding speed v0 in a range that is more than ten times V0. Li et al. [7] developed three-layer molecular frictional pair models between Fe atoms and polymer/carbon nanotube composite to study the friction coefficient and wear rate with different sliding speed of iron nanorods. The results show that the friction coefficient and wear rate of CNT / polymer composites are reduced from 0.356 to 0.322 and 26% -6% at sliding speeds of 0.01 to 0.11 Å / ps. Heo et al. [8] studied the effect of sliding orientation on the tribological properties of polyethylene (PE) by using classical molecular dynamics simulations. The results show that the friction coefficient of sliding in the parallel direction is smaller than that of sliding in the vertical direction. Yuan et al. [9] studied the effect of scratch speed on nano scratching behavior. It was found that when the local temperature in the scratch region exceeds the glass transition temperature Tg, the polymer material in the scratch region around the indenter is removed in a tough manner. Larger scraping rates result in more material deformation and higher pile-up. For larger scraping speeds, the tangential and normal forces are larger, and the coefficient of friction is almost independent of the scraping speeds studied.

Lubrication condition during friction is also one of the focuses of tribology research. Braun et al. [10] used a simple two-dimensional model to study the rolling of spherical lubricant molecules’ effect on the friction system. Molecular dynamics simulations show that the rolling of spherical lubricant molecules exists only when the concentration is lower than that of the closepacked layer. Dai et al. [11] used molecular dynamics simulations to analyze the lubrication effect of perfluoropolyether (PFPE) films to reveal the mechanism behind the interface lubrication behavior of nanoelectromechanical (NEMS) systems and understand the performance of the films. The study found that due to the linear fluidity of the PFPE film, it can promote surface reconstruction and provide good lubricity. The simulation results match the experimental characterization well. Berro et al. [12] used molecular dynamics simulations to study the tribological properties of lubricant mixtures containing cetane base oil and 5% zinc dithiophosphate (ZDDP) under molecular confinement constraints. The results show that mechanical and thermal slip can be reduced by increasing the surface energy. Compared with pure cetane, the migration of ZDDP molecules and their adsorption on solid surfaces can significantly inhibit mechanical slip and leading to an increase in friction coefficient.

Material enhancement

To improve the tribological properties of polymer composites, MD simulations also involve studying from atomic views. Li et al. [13] established and studied a three-layer molecular model that contains Fe atoms as the top and bottom layers and uses the nitrile-butadiene rubber and carbon nanotube matrix as the core to understanding how to improve nitrile-butadiene rubber composition by incorporating carbon nanotubes as a reinforcing agent. Chawla [14] Developed graphene-reinforced natural rubber composites to study the improvement of mechanical and tribological properties of natural rubber by introducing graphene as a reinforcing material. The results show that after strengthening natural rubber with a single-layer graphene oxide sheet, Young’s modulus increased by 185%, the shear modulus increased by 32%, and the hardness increased by 48%. Moreover, the friction coefficient and wear rate obtained by introducing graphene oxide sheets in the natural rubber matrix were reduced by 28% and 36%. He et al. [15] used molecular dynamics simulations to study the enhanced tribological properties of polymer / nano- SiO2 composites by sliding a top iron layer on the surface of the polymer composite. The results show that by introducing nano- SiO2 particles, Young’s modulus is increased by about 190%. The average friction coefficient and wear rate of the polymer / nano- SiO2 composites were reduced by approximately 27% and 47.4%, respectively. Li et al. [16] established a molecular model with Fe atoms as the top nanorods and the bottom layer and a polymer/ carbon nanotube composite as the core to check the friction by sliding the top Fe nanorods on the surface of a polymer matrix under normal load coefficient and wear rate. Simulation results show that by introducing carbon nanotubes, the average friction coefficient of the composite material is reduced by about 38%, and the average wear rate is reduced by 60%. Chawla et al. [17] used molecular dynamics simulations to study Young’s modulus and tribological properties of pure styrene-butadiene rubber (SBR) polymers and carbon nanotube-reinforced polymer composites. The mechanism of using CNTs to enhance friction performance was studied and discussed. When carbon nanotubes are used as the reinforcing material, the obtained Young’s modulus indicates that the mechanical properties of the SBR polymer are improved.

Conclusion

Molecular dynamics has been widely used in the study of tribological properties of polymer composites for its excellent advantages. However, due to the large amount of calculation and the small calculation scale, the limitations of large calculation time, fluctuation in results and the susceptibility to size effects cannot be ignored. In the foreseeable future, the research on the mesoscopic scale, the improvement of atomic-scale research methods and the combination with macro-scale simulation methods will be some areas worth further exploration for the molecular dynamics study of the polymer composites on the tribological and some other properties. The contribution of this study is expected to broaden the research area of molecular dynamics and conduct a more thorough and comprehensive study of rubber materials.

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