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.     

Influence of covalent and non-covalent modification of graphene on the mechanical,thermal and electrical properties of epoxy/graphene nanocomposites: a review

Abstract

Graphene is emerged as a highly sought after reinforcing filler for epoxy matrix in view of its superior electrical, mechanical and thermal properties. Dispersion of low concentration of graphene can significantly enhance the epoxy/graphene nanocomposites properties. Dispersion of graphene in epoxy matrix depends on processing protocols used, and interfacial interaction between epoxy matrix and graphene. Interfacial interaction between epoxy matrix and graphene can be achieved by covalent and non-covalent modification of graphene. This paper comprehensively review the influence of different processing protocols adopted for the processing of epoxy/graphene nanocomposites, and its effect on mechanical, thermal and electrical properties. In addition, covalent and non-covalent strategies adopted for modification of graphene, and its influence on mechanical, thermal and electrical properties of epoxy/graphene nanocomposites are extensively discussed. The future challenges associated with graphene reinforced epoxy nanocomposites processing have been discussed.     

Morphology and thermal properties of clay/PMMA nanocomposites obtained by miniemulsion polymerization

Miniemulsion polymerization was used as the synthetic method to produce clay/poly(methyl methacrylate) nanocomposites. Two kinds of interfacial interactions clay–polymer particle were observed by electron microscopy, one where the polymer particles are adhered on the surface of the larger fragments of clay, and another where nanometric fragments of clay are encapsulated by polymer particles. Variations in the glass transition temperature (T_g) and thermomechanical properties of the matrix, as function of clay content, were observed. In particular, at the highest clay loading (1.0 wt%) depression of T_g and thermomechanical properties were observed. The increased clay–polymer matrix interfacial area appears to be the conditioning factor that determines such behavior.     

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.     

Effect of chemical ordering on the crystallization behavior of Se90Te10−xSnx (x=2, 4, 6, and 8) chalcogenide glasses

Ternary Se₉₀Te_10−xSn_x (x=2, 4, 6, and 8) chalcogenide glassy alloys have been prepared by melt quenching technique. Various crystallization parameters, such as onset (T_c) and peak (T_p) crystallization temperatures, activation energy of crystallization (E_c) and Avrami exponent (n) have been determined for these alloys. T_c and T_p have been determined directly from the non-isothermal differential scanning calorimeter (DSC) thermograms. The value of E_c has been calculated from the variation of both T_c and T_p with the heating rate (β) according to Kissinger, Takhor, Augis–Bennett and Ozawa models while Augis–Bennett method has been used to deduce the value of n for the studied samples. The obtained values of the crystallization parameters have been correlated with the character and the energy of the chemical bonds through the calculation of the heteronuclear bond energies of the constituent atoms using Pauling principle. In addition to that, Tichy–Ticha model was used to estimate the mean bond energy of the average cross-linking per atom 〈E_cl〉, the average bond energy per atom of the remaining matrix 〈E_rm〉, and the overall mean bond energy 〈E〉 of the studied glasses. Results reveal that both of T_c and T_p decreases with increases Sn content. This is may be attributed to the decreasing in the overall mean bond energy 〈E〉. Besides, the plot of E_c (and also T_g) against 〈E〉 was found to be non linear, which contradicts the well known linear correlation between E_c and T_g with 〈E〉 as suggested by Tichy–Ticha model. This discrepancy may be due to the fact that the Tichy–Ticha linear correlation model was based on the assumption of covalent glassy network, while in the present glassy alloys, Se–Te binary doped with heavy elements such as Sn exhibit iono-covalent bonding. The calculated values of the ionicity are in support of this argument.     

Relation between elastic modulus and glass softening temperature in the delocalized atom model

The ratio of softening temperature (glass transition temperature) to elastic modulus (T _g /E) is mainly determined by the limiting elastic deformation of an interatomic bond, which characterizes the transition of a structural microregion from an elastic into a viscous-flow state. In silicate glasses, this transition is caused by the limiting deformation of directed ionic-covalent Si-O-Si bonds. In the case of amorphous hydrocarbons, it is related to the relatively weak intermolecular bonds between regions in chain macromolecules, and the T _g /E ratio is significantly higher than in inorganic glasses. In glassy systems of one class, this ratio turns out to be constant (T _g /E ?? const), and a linear correlation is detected between softening temperature and elastic modulus, which can be explained in terms of the delocalized atom model. The values of T _g /E can be used to classify glasses similarly to the well-known Angell classification according to so-called fragility.     

Epoxy-silica hybrid materials synthesized via sol-gel process

We have successfully utilized the sol-gel method to synthesize epoxy-silica hybrid material at nanoscale. Using the sol-gel process we could overcome many disadvantages of conventional composite materials. In this research, two different methods are recommended — a one-step process and a two-step process — to synthesize epoxy-silica hybrid materials. The coupling agent, γ-glycidoxypropyl-methyldiethoxysilane (KBE-402), was utilized to modify the surface properties of the silica via the sol-gel process. The role of the coupling agent is to provide covalent bonding between the epoxy resin and silica. This method could reinforce the interfacial force of the hybrid material and promote the thermal properties of the materials. The thermal properties of the epoxy-silica materials were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). From these results, we noted that the best reaction time in the one-step process is 2 days and the addition of the inorganic component enhanced the thermal stability of the hybrid materials. On the other hand, the two-step process led to a phase separation phenomenon after mixing epoxy resin and precursor without coupling agent. The coupling agent could avoid the phase separation problem of hybrid materials and enhance the thermal stability of the materials through this process. At the same time, the T _g of the materials increased proportionally to the content of silica from 80°C to 113°C.     

Curing behavior and physical properties of an epoxy nanocomposite with amine-functionalized graphene nanoplatelets

Amine-functionalized graphene nanoplatelets (AGNPs) were prepared via an easy simple one-step process, treating graphite powder with 4-aminobenzoic acid in polyphosphoric acid, and then the effects of the AGNPs on the curing and physical properties of an epoxy resin were studied. The formation of the AGNPs was confirmed by scanning electronic microscopy (SEM), Fourier transform infrared spectroscopy, and thermogravimetric analyzer. Curing behavior of the epoxy/AGNPs nanocomposite was investigated by differential scanning calorimeter. The AGNPs made the epoxide curing reaction with amine groups slightly faster. The physical properties of the epoxy/AGNPs nanocomposite were investigated by dynamic mechanical analyzer, thermomechanical analyzer, and impact test. The AGNPs improved T_g by 21.4 °C, and storage modulus and impact strength of the epoxy resin 23 and 73%, respectively, much more effective than the graphite powder at the same filler loading of 1 phr. SEM images for the fracture surfaces of the epoxy/AGNPs nanocomposite showed improved interfacial bonding between the epoxy matrix and the nanofillers due to the amine functional groups of the AGNPs.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Spin-resolved correlations in the warm-dense homogeneous electron gas

We have studied spin-resolved correlations in the warm-dense homogeneous electron gas by determining the linear density and spin-density response functions, within the dynamical self-consistent mean-field theory of Singwi et al. The calculated spin-resolved pair-correlation function g _{σ σ′}(r) is compared with the recent restricted path-integral Monte Carlo (RPIMC) simulations due to Brown et al. [Phys. Rev. Lett. 110, 146405 (2013)], while interaction energy E _int and exchange-correlation free energy F _xc with the RPIMC and very recent ab initio quantum Monte Carlo (QMC) simulations by Dornheim et al. [Phys. Rev. Lett. 117, 156403 (2016)]. g _↑↓(r) is found to be in good agreement with the RPIMC data, while a mismatch is seen in g _↑↑(r) at small r where it becomes somewhat negative. As an interesting result, it is deduced that a non-monotonic T-dependence of g(0) is driven primarily by g _↑↓(0). Our results of E _int and F _xc exhibit an excellent agreement with the QMC study due to Dornheim et al., which deals with the finite-size correction quite accurately. We observe, however, a visible deviation of E _int from the RPIMC data for high densities (~8% at r _s = 1). Further, we have extended our study to the fully spin-polarized phase. Again, with the exception of high density region, we find a good agreement of E _int with the RPIMC data. This points to the need of settling the problem of finite-size correction in the spin-polarized phase also. Interestingly, we also find that the thermal effects tend to oppose spatial localization as well as spin polarization of electrons.     

Doping effect of copper in germanium tellurium semiconducting glass system

Addition of copper to the semiconducting glassy system Ge₁₅Te_85−xCu_x (x=2, 4, 5, 6, 8, and 10) at the tellurium site is seen to produce considerable changes in the glass transition temperature (T_g) of the system. For a particular value of copper percentage, the T_g shows a minimum critical value. We have made an attempt to explain the observed variation of T_g with Cu content on the basis of various models like the chemically ordered network model, the constraints theory, the covalent bond approach of Tichy and Ticha and former valence shell (FVS) model proposed by Liu and Taylor.     

Amplitude of photostructural changes in chalcogenide vitreous semiconductors

It is shown that a position of an optical absorption edge (OAE) of amorphous AsSe and As₂S₃ films irradiated by light up to saturation is independent on T_exp the temperature at which the sample is exposed, and the amplitude of a reversible photoinduced shift of OAE ΔE is determined by its thermal shift ΔE as it is heated from T_exp up to the glass-transition temperature T_g. So, in order to obtain maximum photoinduced changes we need to use materials with maximum thermal variations of the forbidden zone width, and to tend to a greater difference between T_g and T_exp. The obtained results are well explained within the scope of the local heating model.     

The nature of temperature dependence of energy gaps in layer semiconductors

The temperature dependences of direct and indirect energy gaps in layer semiconductors GaS, GaSe and GaS_xSe_1?x are investigated in the temperature range 5–150 K. The nonmonotonous behaviour of E_g(T) dependences is observed in these crystals. It is shown that the effect of thermal expansion cannot in itself explain the observed anomalies. A new model of electron-phonon interaction explaining the E_g(T) behaviour in layer crystals is proposed.     

Investigation of the interface between polyethylene and functionalized graphene: A computer simulation study

The effect of surface chemical functionalization of a single graphene layer on its thermodynamic work of adhesion (WA) with polyethylene (PE) chains has been investigated using molecular dynamics (MD) simulation. For this purpose, amine (NH₂), carboxyl (COOH), hydroxyl (OH), and methyl (CH₃) functional groups were distributed randomly throughout the graphene surface using a Monte Carlo (MC) algorithm to achieve graphene functionalized structures with minimized potential energies. The MD simulation results showed that the thermodynamic WA between the PE and the functionalized graphene was larger than that between the PE and the pristine graphene. In fact, the electronegativity of functional groups and Van der Waals forces play influential roles in the thermodynamic WA between the PE and the functionalized graphene. In addition, the amount of thermodynamic WA was increased with increasing the functional group surface density, except for the graphene functionalized with the methyl groups. The segmental density of the PE chains near the single sheet surface was determined based on the density profile calculation. The polymer segments exhibited strong ordering and sharp density variations near the PE/graphene interface. The dynamic of chains was quantitatively characterized by calculating mean square displacement (MSD). Furthermore, the influence of functionality on the glass transition temperature (T_g) of the PE at the PE/graphene interface region was investigated. The results showed that the T_g at the PE/graphene interface was much higher than that of the bulk polymer. In fact, the functionalization of the graphene surface seems to considerably enhance the T_g of the polymer due to lowering the chains mobility.     

A DFT study of formaldehyde adsorption on functionalized graphene nanoribbons

Density functional theory (DFT) based ab initio calculations were done to monitor the formaldehyde (CHOH) adsorptive behavior on pristine and Ni-decorated graphene sheet. Structural optimization indicates that the formaldehyde molecule is physisorbed on the pristine sheet via partly weak van der Waals attraction having the adsorption energy of about −15.7 kcal/mol. Metal decorated sheet is able to interact with the CHOH molecule, so that single Ni atoms prefer to bind strongly at the bridge site of graphene and each metal atom bound on sheet may adsorb up to four CHOH. The findings also show that the Ni decoration on graphene surface results in some changes in electronic properties of the sheet and its E_g is remained unchanged after adsorption of CHOH molecules. It is noteworthy to say that no bond cleavage was observed for the adsorption of CHOH on Ni-decorated graphene.     

Edge-modified zigzag-shaped graphene nanoribbons: Structure and electronic properties

Control of the band gap of graphene nanoribbons is an important problem for the fabrication of effective radiation detectors and transducers operating in different frequency ranges. The periodic edge-modified zigzag-shaped graphene nanoribbon (GNR) provides two additional parameters for controlling the band gap of these structures, i.e., two GNR arms. The dependence of the band gap E _g on these parameters is investigated using the π-electron tight-binding method. For the considered nanoribbons, oscillations of the band gap E _g as a function of the nanoribbon width are observed not only in the case of armchair-edge graphene nanoribbons (as for conventional graphene nanoribbons) but also for zigzag GNR edges. It is shown that the change in the band gap E _g due to the variation in the length of one GNR arm is several times smaller than that due to the variation in the nanoribbon width, which provides the possibility for a smooth tuning of the band gap in the energy spectrum of the considered graphene nanoribbons.     

New chalcogenide glasses in the Ag2Te-As2Se3-CdTe system

Chalcogenide glasses from the Ag₂Te-As₂Se₃-CdTe system were synthesized. The basic physicochemical parameters such as density (d), microhardness (HV) and the temperatures of phase transformations (the glass transition T_g, crystallization T_cr and melting T_m) were measured. Compactness and some thermomechanical characteristics such as volume (V_h) and formation energy (E_h) of micro-voids in the glassy network, as well as the module of elasticity E were calculated. The overall mean bond energy 〈E〉, the mean coordination number Z, the mean bond energy of the average cross-linking/atom and the average bond energy per atom of the “remaining matrix”— and , as well as the average heteropolar bond energy E_hb, the degree of “cross-linking/atom” P_p and the radial bond strength were determined.The correlation between the composition and properties of the Ag₂Te-As₂Se₃-CdTe glasses was established and comprehensively discussed.     

Synthesis of photosensitive poly(methyl methacrylate-co-glycidyl methacrylate) for optical waveguide devices

Photosensitive poly(MMA-co-GMA) for optical waveguide was synthesized, and the refractive index of the polymer film was tuned in the range of 1.481–1.588 at 1550 nm by mixing with bis-phenol-A epoxy resin. The film, which was made by spinning coated the poly(MMA-co-GMA) with photo initiator, had good UV light lithograph sensitivity, high glass transition temperature (T _g : 153°C, after crosslinking) and good thermal stabilities (T _d : up to 324°C, after crosslinking). The optical waveguides with very smooth top surface were fabricated from the resulting polymer by direct UV exposure and chemical development. For waveguides with cladding, the propagation losses of the channel waveguides were measured to be below 3 dB/cm at 1550 nm.     

Flame retardance and thermal stability of carbon nanotube epoxy composite prepared from sol-gel method

Carbon nanotubes (CNTs) are functionalized by vinyltriethoxysilane (VTES) to incorporate the -O-C₂H₅ functional group and become VTES—CNT. The VTES—CNTs are added to the modified DGEBA epoxy resin that contains silicon to induce the sol-gel reaction. The final products are organic/inorganic nanocomposites. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) are used to study the thermal property of nanocomposites. The T_g was increased from 118 to 160 °C and char yield of composites that contained 9 wt% CNT at 750 °C was increased by 46.94%. The integral procedural decomposition temperature (IPDT) was increased from 890 to 1571 °C. The limiting oxygen index (LOI) and UL-94 tests were classified as the flame retardance. The LOI of composites was increased from 22 to 27 and the UL-94 changed from V-1 to V-0 when the contents were increased to 9 wt%. The nanocomposites had a higher char yield and were highly flame retardant. The products can meet to the requirements of halogen-free and phosphorus-free ecological flame retardant.     

Size-dependent properties of Eu chalcogenide nanoparticles

The size effects on different properties, such as magnetization M, Curie temperature T_C and band gap E_g of ferromagnetic semiconducting EuS nanoparticles are studied based on the d-f model and using a Green's function technique. We have shown that M and T_C are decreased compared with the bulk material, whereas E_g is increased. The results are in qualitative agreement with the experimental data.