Thermodynamic approach to enhance the dispersion of graphene in epoxy matrix and its effect on mechanical and thermal properties of epoxy nanocomposites

Graphene-reinforced polymer nanocomposites are under intense investigation in recent years. In this work, graphene nanosheets have been prepared using chemical reduction method of graphene oxide. Graphene-reinforced epoxy nanocomposites show an enhancement in mechanical and thermal properties at 0.05 wt.% of graphene in epoxy matrix. Modification of graphene with polyvinylpyrrolidone (PVP) shows the significant enhancement in mechanical and thermal properties of epoxy nanocomposites. PVP-modified graphene nanosheets reduces the gap of enthalpic and entropic penalties and facilitates improved dispersion of graphene in epoxy matrix. In addition, enhanced dispersion of PVP-modified graphene in epoxy matrix results in better load transfer across graphene–epoxy interface. Glass transition temperature (T_g) of PVP-modified graphene epoxy nanocomposites increases as compared to pure epoxy because there exist an interaction between epoxy and PVP. Fractography study reveals the localized ductile fracture.     

Effects of different parameters on thermal and mechanical properties of aminated graphene/epoxy nanocomposites connected by covalent: A molecular dynamics study

This paper is devoted to studying the thermal and mechanical properties of aminated graphene (AG)/epoxy nanocomposites connected by covalent bond using molecular dynamics (MD) simulation. The effects of crosslinking degree, mass fraction and functionalized graphene (FG) type on AG/epoxy nanocomposites are considered. The elasticity modulus (E), the glass transition temperature (T_g), the coefficient of thermal expansion (CTE) and the interfacial energy (E_int) are also investigated. The MD simulation results indicate that, when the mass fraction of AG is between 1.2% and 3.1% and crosslinking degree reaches about 70%, the E, T_g, E_int and CTE of AG/epoxy nanocomposites are significantly improved compared with those of pure epoxy and graphene/epoxy nanocomposites. The reason is that AG not only possesses some excellent thermodynamic properties of graphene, but also has the function of curing agent to crosslink with epoxy monomer to form the carbon-nitrogen (C–N) covalent bond. A better interfacial interaction between nanoparticles and epoxy is essential in enhancing the thermal and mechanical properties of nanocomposite materials, which will provide a microscopic theoretical basis for the study of epoxy nanocomposites.     

Effect of sodium alginate modification of graphene (by ‘anion-π’ type of interaction) on the mechanical and thermal properties of polyvinyl alcohol (PVA) nanocomposites

Layered aligned dispersion of graphene in graphene/polyvinyl alcohol (PVA) nanocomposites is prepared in the form of films through simple solution processing route. The results indicate that there exist an interfacial interaction between PVA and graphene because of hydrogen bonding. This is responsible for the change in structure of PVA (such as decrease in the level of crystallization) and exhibiting ductile PVA nanocomposite film with improved tensile modulus, tensile strength, and thermal stability. Moreover, to improve the mechanical properties of PVA nanocomposites, graphene is successfully modified using a non-covalent modifier, sodium alginate (SA) and there exist an ‘anion-π’ type of interaction in between SA and graphene. The modification results in finer dispersion of the graphene in PVA/SA-m-graphene nanocomposites. In addition, there exist a hydrogen bonding in between PVA and SA. This has resulted in the remarkable improvement in mechanical properties of PVA/SA-m-graphene nanocomposites as compared to pure PVA and PVA/graphene nanocomposites. The increase in mechanical properties of PVA/SA-m-graphene nanocomposites is achieved through better load transfer from graphene to polymer matrix, despite decrease in crystallinity of PVA. Improvement in tensile modulus and tensile strength is highest at 0.5 wt.% of SA-modified graphene in PVA/SA-m-graphene nanocomposites because of finer dispersion of graphene and is 62 and 40% higher than that of pure PVA. Addition of SA-modified graphene also improves the thermal stability of PVA/SA-m-graphene nanocomposites remarkably as compared to unmodified graphene PVA nanocomposites.     

Influence of functionalization on mechanical and electrical properties of carbon nanotube-based silver composites

In this study, we have extended the molecular-level mixing method to fabricate multiwall carbon nanotube (CNT)-reinforced silver nanocomposites. The multiwall nanotubes used in the synthesis process were dispersed by two ways viz. covalent and non-covalent functionalization techniques. To elucidate the comparative effects of functionalization, structural, mechanical and electrical properties of nanocomposites were evaluated before and after sintering. The structural characterization revealed that the nanotubes were embedded, anchored and homogenously dispersed within the silver matrix. Hardness and Young’s modulus of nanotube-reinforced nanocomposite were increased by a factor of 1–1.6 times than that of pure silver, even before and after the sintering. Covalently functionalized nanotube-based composites have shown more enhanced mechanical properties. The CNT reinforcement also improved the electrical conductivity of low-conducting nanosilver matrix before sintering. Non-covalently functionalized nanotube-based nanosilver composites showed more increased electrical conductivity before sintering. But a negative reinforcement effect was observed in high-conducting bulk silver matrix after the sintering. Thus, covalent functionalization might be appropriate for mechanical improvement in low-strength materials. However, non-covalent functionalization is suitable for electrical enhancement in low-conducting nanomaterials.     

The effective stiffness of an embedded graphene in a polymeric matrix

Modeling the real sizes of an embedded graphene and the surrounding polymer of a representative volume element in a molecular dynamics simulation is a tedious task. The less computational limitations made the continuum-based method a good candidate for modeling of nanocomposites. However, having a good knowledge of mechanical properties of the embedded graphene in a polymeric matrix is a challenge for employing a continuum-based method. Since the applied stress on the graphene/epoxy nanocomposites has not been directly transferred to the embedded graphene, it brings the following question to mind. Is the stiffness of the embedded graphene different from that of the isolated one? To answer to this question, a model was developed by combining the molecular dynamic simulation and the finite element method to calculate the stiffness of an embedded graphene in a polymeric matrix. The results show that the longitudinal stiffness of the embedded graphene is different from that of the isolated graphene and is a function of its length. The use of this relationship in the micromechanical method leads to consider the nanosize effect in macroscale. The results were compared with some available experimental data to validate the model.     

Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials

We review the thermal properties of graphene and multilayer graphene, and discuss graphene’s applications in thermal management of advanced electronics and optoelectronics. A special attention is paid to the use of the liquid-phase-exfoliated graphene and multilayer graphene as the fillers in the thermal interface materials. It has been demonstrated that addition of an optimized mixture of graphene and multilayer graphene to the composites with different matrix materials produces the record-high enhancement of the effective thermal conductivity at the small filler loading fraction (f≤10 vol%). The thermal conductivity enhancement due to the presence of graphene in the composites has been observed for a range of matrix materials used by industry. The hybrid composites where graphene is utilized together with metallic micro- and nanoparticles allow one to tune both the thermal and electrical conductivity of these materials. Theoretical considerations indicate that the graphene-based thermal interface materials can outperform those with carbon nanotubes, metal nanoparticles and other fillers owing to graphene’s geometry, mechanical flexibility and lower Kapitza resistance at the graphene–base material interface.     

Preparation and physical properties of polypropylene nanocomposites with dodecylated graphene nanoplatelets

The most important practical application of graphene nanoplatelets (GNPs) would be as nanofillers for polymer nanocomposites. However, the modification of GNPs is needed to improve the interfacial adhesion between GNPs and a polymer matrix. Therefore, in this study, the alkylation of GNPs by dodecylamine was carried out via chemical reactions between the amine groups of the alkyl amine and the carboxyl and epoxy groups of the oxidized GNPs’ surfaces. The dodecylation of the GNPs was confirmed by FTIR and TGA. The TGA data showed that the dodecyl-GNPs comprised alkyl groups 2.4%. Polypropylene nanocomposites with the dodecyl-GNPs were prepared in a platy shape by melt-blending followed by compression molding. The mechanical and thermal properties of the nanocomposites were measured by UTM, izod impact tester, DSC and DMA. Compared to the neat PP sample, the flexural modulus, flexural strength and impact strength of the PP nanocomposite with the dodecyl-GNPs 0.5 phr were increased by 38, 4 and 34% respectively. The fracture surfaces’ images of the nanocomposites taken by SEM showed that the dodecylation of the GNPs improved the interfacial adhesion between the GNPs and the PP matrix.     

Preparation of poly(3-hexylthiophene)/graphene nanocomposite via in situ reduction of modified graphite oxide sheets

Poly(3-hexylthiophene) (P3HT)/graphene nanocomposites were facilely prepared via an in situ reduction of modified graphite oxide (mGO) in the presence of P3HT. The chemical and aggregated structures of the P3HT/mGO nanocomposites were successfully characterized by means of atomic force microscope (AFM), transmission electron microscope (TEM), photoluminescence (PL), Raman spectra, X-ray photoelectron spectroscopy (XPS) and UV-vis measurements. Coated by P3HT, reduced modified graphite oxide (re-mGO) could significantly improve their processing problem in common organic solvent. It was shown that P3HT chains were attached to re-mGO sheets closely and there existed the pi-pi interaction between P3HT and re-mGO. The P3HT/mGO nanocomposites exhibited good dispersion in chloroform and show high storage stability (>20 days). This finding provides an efficient method for fabricating a light energy conversion materials with new optical and electrical properties, combining excellent mechanics, heat-stabilization properties of graphene and excellent optical, electrical, processing and film forming properties of soluble polythiophene materials.     

石墨烯/铝基复合材料在纳米压痕过程中位错与石墨烯相互作用机制的模拟研究

石墨烯因其优异的力学性能已成为增强金属基复合材料的理想增强体.然而,目前对石墨烯/金属基复合材料在纳米压痕过程中嵌入石墨烯与位错之间的相互作用仍不清晰.本文采用分子动力学模拟方法,对90°,45°和0°位向的石墨烯/铝基复合材料进行了纳米压痕模拟,研究了压痕加载和卸载过程中石墨烯/铝基复合材料的位错形核及演化,以获取不同位向的石墨烯与位错的相互作用机制,并分析其对塑性区的影响.研究发现,石墨烯可以有效阻碍位错运动,并且石墨烯会沿着位错滑移方向发生弹性变形.在纳米压痕过程中,位错与不同位向石墨烯之间的相互作用差异导致塑性区的变化趋势不同.研究结果表明,在石墨烯/铝基复合材料中,位向不同的石墨烯对位错阻碍强度和方式不同,且石墨烯位向为45°的复合材料的硬度高于其他模型.此外,石墨烯/铝基复合材料的位错线总长度的演化规律与石墨烯位向紧密相关.本文研究可为设计和制备高性能石墨烯/金属基复合材料提供一定的理论指导.     

Processing and properties of carbon nanofibers reinforced epoxy powder composites

Commercially available CNFs (diameter 30–300 nm) have been used to develop both bulk and coating epoxy nanocomposites by using a solvent-free epoxy matrix powder. Processing of both types of materials has been carried out by a double-step process consisting in an initial physical premix of all components followed by three consecutive extrusions. The extruded pellets were grinded into powder and sieved. Carbon nanofibers powder coatings were obtained by electrostatic painting of the extruded powder followed by a curing process based in a thermal treatment at 200 °C for 25 min. On the other hand, for obtaining bulk carbon nanofibers epoxy composites, a thermal curing process involving several steps was needed. Gloss and mechanical properties of both nanocomposite coatings and bulk nanocomposites were improved as a result of the processing process. FE-SEM fracture surface microphotographs corroborate these results. It has been assessed the key role played by the dispersion of CNFs in the matrix, and the highly important step that is the processing and curing of the nanocomposites. A processing stage consisted in three consecutive extrusions has reached to nanocomposites free of entanglements neither agglomerates. This process leads to nanocomposite coatings of enhanced properties, as it has been evidenced through gloss and mechanical properties. A dispersion limit of 1% has been determined for the studied system in which a given dispersion has been achieved, as the bending mechanical properties have been increased around 25% compared with the pristine epoxy resin. It has been also demonstrated the importance of the thickness in the nanocomposite, as it involves the curing stage. The complex curing treatment carried out in the case of bulk nanocomposites has reached to reagglomeration of CNFs.     

Role of curing conditions and silanization of glass fibers on carbon nanotubes (CNTs) reinforced glass fiber epoxy composites

Curing behavior of amino-functionalized carbon nanotubes (ACNT) used as reinforcing agent in epoxy resin has been examined by thermal analysis. Experiments performed as per supplier’s curing conditions showed that modification of the curing schedule influences the thermo-mechanical properties of the nanocomposites. Specifically, the glass transition temperature (T_g) of ACNT-reinforced composites increased likely due to the immobility of polymer molecules, held strongly by amino carbon nanotubes. Further, a set of composites were prepared by implementing the experimentally determined optimal curing schedule to examine its effect on the mechanical properties of different GFRP compositions, while focusing primarily on reinforced ACNT and pristine nanotube (PCNT) matrix with silane-treated glass fibers. From the silane treatment of glass fibers in ACNT matrix composition it has been observed that amino silane is much better amongst all the mechanical (tensile and flexural) properties studied. This is because of strong interface between amino silane-treated glass fibers and modified epoxy resin containing uniformly dispersed amino-CNTs. On the other hand, PCNT GFRP composites with epoxy silanes demonstrated enhanced results for the mechanical properties under investigation which may be attributed to the presence of strong covalent bonding between epoxy silane of glass fiber and epoxy–amine matrix.     

Fabrication of Graphene Nanoplatelet/Epoxy Nanocomposites for Lightweight and High‐Strength Structural Applications

Graphene nanoplatelets (GNPs), the most important mass‐produced graphene, are fabricated as a mechanical reinforcement for epoxy matrix nanocomposites. Current performance of GNPs as a reinforcing filler is limited by their agglomeration and weak interfacial interaction with certain polymer matrices. Herein, an approach to produce noncovalently functionalized GNPs (F‐GNPs) is reported that can be extended to the industrial level of mass production. The one‐step functionalization process uses melamine, a low‐cost chemical, to improve the interfacial adhesion and dispersion in an epoxy matrix. The mechanical properties of nanocomposites prepared with the F‐GNP flakes are much better (94.3% and 35.3% enhancements in Young's modulus and tensile strength, respectively) than those of the unfilled pure epoxy. Experimental data are analyzed using the Halpin–Tsai model. The fabrication process developed in this paper provides a strategy to use GNPs at the industrial level in lightweight and high‐strength structural applications.     

Geometry and temperature effects of the interfacial thermal conductance in copper- and nickel-graphene nanocomposites

Graphene has excellent mechanical, electrical and thermal properties. Recently, graphene-metal composites have been proposed as a means to combine the properties of metals with those of graphene, leading to mechanically, electrically and thermally functional materials. The understanding of metal-graphene nanocomposites is of critical importance in developing next-generation electrical, thermal and energy devices, but we currently lack a fundamental understanding of how their geometry and composition control their thermal properties. Here we report a series of atomistic simulations, aimed at assessing the geometry and temperature effects of the thermal interface conductance for copper- and nickel-graphene nanocomposites. We find that copper-graphene and nickel-graphene nanocomposites have similar thermal interface conductances, but that both cases show a strong performance dependence on the number of graphene layers between metal phases. Single-graphene-layer nanocomposites have the highest thermal interface conductance, approaching ~500 MW m(-2) K(-1). The thermal interface conductance reduces to half this value in metal-bilayer graphene nanocomposites, and for more than three layers of graphene the thermal interface conductances further reduces to ~100 MW m(-2) K(-1) and becomes independent with respect to the number of layers of graphene. This dependence is attributed to the relatively stronger bonding between the metal and graphene layer, and relatively weaker bonding between graphene layers. Our results suggest that designs combining metal with single graphene layers provide the best thermal properties.     

聚酰亚胺/功能化石墨烯复合材料力学性能及玻璃化转变温度的分子动力学模拟

应用分子模拟方法,建立了聚酰亚胺(polyimide,PI),石墨烯及羧基、氨基、羟基功能化石墨烯模型,探究了聚酰亚胺和石墨烯,聚酰亚胺和功能化石墨烯共混后复合材料的力学性能和玻璃化转变温度(T_g).研究结果表明,羧基修饰的石墨烯与PI复合后材料力学性能增加显著,其杨氏模量和剪切模量分别为4.946 GPa和1.816 GPa.不同官能团修饰的石墨烯引入PI后材料的T_g均有不同程度下降;未修饰的石墨烯与PI复合后,其T_g(559.30 K)较纯PI的T_g(663.57 K)降幅最大;而羧基修饰的石墨烯与PI复合后T_g(601.61 K)降幅最小.计算比较了PI/石墨烯复合材料体系密度、溶解度参数、相互作用能、弹性系数和氢键平均密度,研究发现羧基修饰石墨烯/PI复合材料的密度为1.396 g·cm~(-3),溶解度参数为23.51 J~(1/2)·cm~(-3/2),其相互作用能与氢键平均密度最大,弹性系数显示羧基修饰石墨烯与PI组成的复合材料内部最均匀.计算结果表明,羧基功能化石墨烯可以大幅度提高PI的力学性能,增强石墨烯与PI之间的相互作用可以减少复合材料T_g的降幅程度.此基体间相互作用的研究方法可以作为预测聚合物基纳米复合材料结构与性能的有效工具,以期为材料的设计与应用提供理论指导.     

Physical properties and device applications of graphene oxide

Graphene oxide (GO), the functionalized graphene with oxygenated groups (mainly epoxy and hydroxyl), has attracted resurgent interests in the past decade owing to its large surface area, superior physical and chemical properties, and easy composition with other materials via surface functional groups. Usually, GO is used as an important raw material for mass production of graphene via reduction. However, under different conditions, the coverage, types, and arrangements of oxygen-containing groups in GO can be varied, which give rise to excellent and controllable physical properties, such as tunable electronic and mechanical properties depending closely on oxidation degree, suppressed thermal conductivity, optical transparency and fluorescence, and nonlinear optical properties. Based on these outstanding properties, many electronic, optical, optoelectronic, and thermoelectric devices with high performance can be achieved on the basis of GO. Here we present a comprehensive review on recent progress of GO, focusing on the atomic structures, fundamental physical properties, and related device applications, including transparent and flexible conductors, field-effect transistors, electrical and optical sensors, fluorescence quenchers, optical limiters and absorbers, surface enhanced Raman scattering detectors, solar cells, light-emitting diodes, and thermal rectifiers.     

Properties of hydrophobically-modified graphene oxide (HG)/butyl rubber (IIR) nanocomposites prepared by shear mixing process

Abstract

Butyl rubber (IIR)/hydrophobically modified graphene oxide (GO) (HG) nanocomposites were prepared via shear-induced compounding. Hydrophilic GO was synthesized through the chemical oxidation of graphite (GP) and modified hydrophobically by octadecylamine which has a hydrophobic long alkyl chain. The obtained HG was characterized by Fourier transform infrared and wide-angle X-ray diffraction (WAXD) patterns. It was well dispersed in toluene for more than 30 days under stationary condition. The IIR/HG nanocomposites were prepared by the shear mixing process and followed by thermal vulcanization process through compression molding. Their properties were studied using oscillating disk rheometer, universal testing machine, differential scanning calorimetry, thermogravimetric analysis, WAXD patterns, and scanning electron microscope analysis. The hydrophobic HG was dispersed at the nanoscale within IIR matrix, and the resulting nanocomposites had significantly reduced curing time. The overall tensile properties were enhanced.     

Synthesis and characterization of graphene oxide,reduced graphene oxide and their nanocomposites with polyethylene oxide

This work describes the synthesis of GO, rGO and their nanocomposites with PEO. GO and rGO were prepared by the modified Hummers method and in-situ reduction of GO utilizing green reductant L (+) Ascorbic acid. The nanocomposites were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermogravimetric Analysis (TGA), and Universal Testing Machine (UTM). FT-IR and XRD confirmed the synthesis of GO and rGO. FE-SEM confirmed the uniformly exfoliated GO and rGO nanosheets in the polymer matrix. Hydrogen bonding was the main interaction mechanism for GO with PEO while no interaction was detected by FT-IR for rGO. Enhanced thermal stability was observed for both GO/PEO and rGO/PEO nanocomposites. The mechanical analysis showed an increase in Young's modulus, tensile strength, and elongation at break for GO/PEO nanocomposites, which is attributed to the homogeneous dispersion and hydrophilic hydrogen bonding interaction of GO with PEO.     

Investigation on thermal conductivity and physical properties of polythiophene/p-phenylenediamine-graphene oxide and polythiophene-co-poly(methyl methacrylate)/p-phenylenediamine graphene oxide composites

An efficient approach was employed to simultaneously functionalize and reduce the graphene oxide (GO) with p-phenylene diamine (PPD) using simple refluxing. There was a possibility of nucleophilic substitution of amino moieties of PPD with the epoxy groups of GO. The polythiophene (PTh) and polythiophene-co-poly(methylmethacrylate) (PTh-co-PMMA) nanocomposites with chemically modified GO were prepared using in situ polymerization technique. Two series of nanocomposites that is PTh/PPD-GO and PTh-co-PMMA/PPD-GO were designed. The nanocomposites were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), thermal conductivity, and electrical conductivity measurement. The FTIR spectra depicted the characteristic absorption peaks for the formation of copolymer and their composites with PPD-GO. The SEM micrographs showed that the PPD-GO nanosheets were homogeneously dispersed in copolymer matrix forming nano-granular morphology. The nanofluids were prepared by suspending modified GO particles inside the basefluid of polythiophene and PTh-co-PMMA. The thermal conductivity of nanocomposites was significantly improved even with low PPD-GO loading. The thermal conductivity of PTh-co-PMMA/PPD-GO with 1.5 wt.% filler was increased to 1.42 W/mK at a higher temperature. The XRD patterns confirmed the presence of chemical interactions between the copolymer and filler particles. The electrical conductivity of PTh-co-PMMA/PPD-GO was also found to increase in the range of 6.1 × 10^?3–2.5 × 10^?2 S/cm. Novel PTh-co-PMMA/PPD-GO-based nanocomposite is potentially significant in high-performance thermal systems.     

A comparison of the mechanical strength and stiffness of MWNT-PMMA and MWNT-epoxy nanocomposites

The surface of multi-wall carbon nanotubes (MWNTs) was functionalized by covalent linking of long alkyl chains. Such functionalization led to a much better tube dispersion in organic solvents than pristine nanotubes, favored the formation of homogenous nanocomposite films, and yielded good interfacial bonding between the nanotubes and two polymer matrices: a thermo-set (Epon 828/T-403) and a thermoplastic (PMMA). Tensile tests indicated, however, that the reinforcement was greatly affected by the type of polymer matrix used. Relative to pure PMMA, a 32% improvement in tensile modulus and a 28% increase in tensile strength were observed in PMMA-based nanocomposites using 1.0 wt% nanotube filler. Contrasting with this, no improvement in mechanical properties was observed in epoxy-based nanocomposites. The poorer mechanical performance of the latter system can be explained by a decrease of the crosslinking density of the epoxy matrix in the nanocomposites, relative to pure epoxy. Indeed we demonstrate that the presence of nanotubes promotes an increase in the activation energy of the curing reaction in epoxy, and a decrease of the degree of curing.     

Carbon nanotube (CNT)–epoxy nanocomposites: a systematic investigation of CNT dispersion

A systematic investigation of the dispersion of carbon nanotubes (CNTs), 1–6 nm in diameter and a few microns in length, in a bisphenol F-based epoxy resin has been presented. Several dispersing techniques including high-speed dissolver, ultrasonic bath/horn, 3-roll mill, etc. have been employed. Optical microscopy has been extensively used to systematically characterise the state of CNT dispersion in the epoxy resin during the entire processing cycle from mixing CNT with resin to adding and curing with hardener. Complimentary viscosity measurements were also performed at various stages of nanocomposite processing. A method to produce a good CNT dispersion in resin was established, but the state of CNT dispersion was found to be extremely sensitive to its physical and chemical environments. The cured nanocomposites were further tested for their thermo-mechanical properties by dynamic mechanical thermal analysis (DMTA), and for flexural and compressive mechanical properties. The measured properties of various nanocomposite plates were then discussed in view of the corresponding CNT dispersion.