Intrinsic Insertion Limits of Graphene Oxide into Epoxy Resin and the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Intrinsic Insertion Limits of Graphene Oxide into Epoxy Resin, and the Dielectric Behavior of Composites Comprising Truly 2D Structures Ayrat M Dimiev, Albina Surnova, Ivan Lounev, and Artur Khannanov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07450 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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The Journal of Physical Chemistry

The Intrinsic Insertion Limits of Graphene Oxide into Epoxy Resin, and the Dielectric Behavior of Composites Comprising Truly 2D Structures Ayrat M. Dimiev,a* Albina Surnova,a Ivan Lounev,b Artur Khannanova aLaboratory

for Advanced Carbon Nanomaterials, bInstitute of Physics, Kazan Federal

University, Kremlyovskaya str. 18, Kazan 420008, Russian Federation Corresponding author: Ayrat Dimiev, e-mal: [email protected]

Abstract The composites of graphene oxide (GO) with epoxy resin were prepared via the homogeneous liquid phase transfer method, allowing uniform distribution and nearly fully exfoliated condition of GO in the matrix. The ~0.6% GO content is the absolute maximum that can be inserted into the epoxy matrix (at the flakes' size 5-20 μm) without sacrificing the exfoliation level of the 2D filler and the uniformity of the composition. Curing at 180 ºC causes the in-situ disproportionation, or so-called "thermal reduction" of GO in the matrix. As-induced conductivity of GO flakes alters dielectric properties of composites via the Maxwell-Wagner polarization. For the first time we experimentally demonstrate the dielectric properties of composite materials comprising truly 2D single-atomic-layer structures. They exhibit relatively low permittivity values that reach saturation at ~0.2% filling fraction, and classical relaxation peaks on the imaginary part function at 0.3-0.4% GO content. The presented experimental data strongly suggests that the Maxwell-Wagner polarization is sufficiently suppressed in composites comprising truly 2D structures due to their interaction with the matrix. And contrary, high permittivity values, reported simultaneously with the high loading fractions (>0.6% at the flakes' size 5-20 μm), are indicative of the non-single-layer character, and/or the aggregation of the 2D inclusion particles in the polymer matrix.

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1. Introduction Carbon nanomaterials are broadly used as fillers for a polymer matrix to alter the properties of resulted composite materials.1-2 The key point in attaining high quality polymer composites is the uniform distribution of the filler in the matrix, which in turn, depends, in large part, on the affinity of the filler toward the polymer matrix. Intact graphitic carbon has very low affinity toward the most of the known solvents, and does not readily disperse in the most known polymers. This is true for all the macro- and nano-forms of graphitic carbon, such as carbon fiber, carbon nanotubes (CNT), graphene nanoplatelets (GNP), graphene nanoribbons (GNR), etc. In polymer composites, they tend to form aggregates, especially at higher loadings.3 The uniform distribution of the filler is especially important in applications related to dielectric properties of the polymer composite materials. For this particular application, CNTs and GNRs gained much interest due to their excellent electrical conductivity and high aspect ratio.4-12 The latter is the main reason for relatively low percolation thresholds from 3% through 10% achieved with the use of CNTs, as opposed to >20% for the low aspect ratio fillers. Despite the apparent advantages, CNTs still tend to form aggregates, as do most of the known carbon fillers. Unlike many forms of carbon, graphene oxide (GO), due to its numerous functional groups, has high affinity to epoxy resins. Based on this property, we had recently developed a novel method for the homogeneous liquid phase transfer of GO into epoxy resins, affording fully exfoliated condition of GO in the epoxy matrix.13 GO is an insulator. This is why, usually, introduction of GO into a polymer matrix does not drastically change its dielectric properties. Thus, the reported permittivity values were in the range 3 - 7 at the loading fractions 1% - 5%.14-18 In several studies, not the increase, but the decrease in the permittivity was registered with GO insertion.14, 16 This is why, the reduced form of GO (RGO) is normally used instead of GO when high permittivity values are desired.19-26 Typically, the permittivity values up to 110 were reported with the RGO content 1.0% - 8.0%. However, being deoxygenated, RGO suffers from the low affinity toward the polymers, and high cohesive forces between the layers, as do most of the carbon materials. In the matrix, RGO aggregates and crumples, thus deviating from the pure 2D state. The multi-layered character of the particles is the main reason for the relatively high filling fractions needed to alter the


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dielectric properties.19-26 From the fundamental science perspectives it is interesting to investigate how the truly 2D materials will affect the dielectric properties of the polymer host. The physics behind the electromagnetic behavior of dielectric composites remains elusive. The formal approach, often employed, is the percolation theory, which proposes a power law behavior of the multi-component systems. Percolation theory explains the macroscopic parameters of composites by statistical distribution of inclusions, and predicts a sharp increase of the tested property in the vicinity of the percolation threshold.27-30 In particular, it was successfully applied to explain conductivity and permittivity of complex systems.27, 30-31 Despite the popularity among physicists, and satisfactory-to-good description of some complex materials such as microemulsions and porous glass,30-31 in general, experiments poorly support the universality of the power laws. This is especially true for the dielectric polymer composite materials. The percolation theory ignores the chemical nature of the two phases, i.e. the cohesive forces between the host and the inclusions. In real systems, when the affinity of a liquid host toward the inclusion material is high, it will always wet the surface of the inclusion particles. There will be always a thin polymer layer between the two neighboring particles, even at filling fractions >90%; formally, the system will never reach the actual percolation. Not to mention the complexity, introduced by the chemical interaction of the two phases at the interface, as it takes place in the system GO/Epoxy. At low conductive filler fractions, the real percolation threshold is not attained even in the lack of the cohesive forces; composites remain non-conductive for DC current. Still, researches broadly employ the term "percolation threshold" toward the filling fractions where the AC conductivity and/or permittivity exhibit saturation with the filling fractions.4, 32-36 To differentiate such behavior from the real percolation, earlier, we had introduced the term "pseudothreshold".11 In this report, to stay in line with the most recent literature mainstream, we will use the term "percolative behavior" toward the conditions where the permittivity values reach saturation level. In addition to percolation theory, the microcapacitors model was introduced recently to explain the dielectric behavior of composites.26, 35-36 This model considers composites as a network of microcapacitors randomly distributed in a dielectric host. For the reasons discussed above, this model offers better explanation for the observed phenomena than the percolation theory. In this



respect, composites comprising truly 2D filler particles, i.e. large capacitor plates arranged parallel to each other, are especially interesting. In this work, using the previously developed homogeneous liquid phase transfer method, we prepared the GO/epoxy composites with the highly exfoliated condition of GO in the matrix, and tested their dielectric properties. The GO content varied from 0.05% through 0.6%. The frequency range was from 10-1 Hz through 107 Hz. The tested temperatures were from -100 ºC through 150 ºC. 2. Methods 2.1. Materials The epoxy resin (NPEL-128, epoxy value 22.6, epoxide equivalent weight is 186.2 g eq.-1) which is manufactured from bisphenol-A and epichlorohydrin was obtained from the "Rus Chemicals Group" (Russia). The 4,4’-diaminodiphenylmethane (DDM) curing agent was supplied by Sigma-Aldrich. Isopropyl alcohol (IPA) and graphite GL-1 were obtained from “TatKhimProduct” LLC (Russia). All reagents and solvents were purchased from commercial suppliers and used as received. 2.2 Preparation of GO GO was prepared according to the modified Hummers method as described in our previous reports.37-39 Briefly, graphite flakes were dispersed in 96% sulfuric acid at room temperature using a mechanical stirrer. After a 10-minute stirring, 4 wt equiv. of KMnO4 were added sequentially in four portions after consumption of the previously added portion. The reaction was quenched with ice water, and a 30% H2O2 solution. The resulting mixture was centrifuged to separate GO from diluted acid. For purification, the separated GO precipitate was redispersed in DI water and centrifuged again for separation. Such cycles of GO purification were repeated four more times. The 3% HCl was used instead of water during the last three washing cycles. The GO precipitate was air dried after the last wash. 2.3 Preparation of GO dispersion in IPA To prepare the GO solution in IPA (GO-IPA), 100 ml of an aqueous 2 wt.% GO solution were mixed with 100 ml IPA. After 1 hour stirring, the mixture was centrifuged for 40 minutes at 8900 rpm. The clear supernatant was discarded. The precipitated gel was mixed with a fresh portion of IPA, bringing the volume up to 200 ml, stirred for 1 hour and centrifuged under the 4 ACS Paragon Plus Environment

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same conditions until GO separates from the supernatant. This procedure was repeated five more times. This sequence of operations led to the gradual replacement of water by IPA. The solution remained transparent and resistant to precipitation at all the stages of operation. After the last centrifugation, a GO-IPA gel with the GO content of 2.36 wt.% was obtained. According to the dilution factor, this gel should contain 0.048 wt.% water. The actual water content was not determined. 2.4 Preparation of the liquid GO/epoxy formulations In our previous study,13 GO was introduced into the resin directly in the form of the GOIPA gel with GO content ~2.36%. In this work, the GO-IPA gel was first diluted with IPA to achieve higher exfoliation degree of GO in IPA, and subsequently in the epoxy resin. The Epoxy/GO formulations were prepared as the following. The portions of 0.21 g, 0.42 g, 0.84 g, 1.69 g, 2.54 g of the 2.36 wt% GO-IPA gel were dissolved in 10 grams of isopropanol, using an ultrasonic disperser. Then each solution was blended with 10.0 g of epoxy resin in an ultrasonic bath for 30 minutes. Upon standing, the mixtures were separated into the two phases: epoxy resin with GO on the bottom, and colorless IPA on the top, indicating that all the GO has been transferred into the resin. The top IPA portion was decanted, and the GO-epoxy composition was agitated with the over-head stirrer for 18 h with the stirring rate of 500 rpm at 80 oC to completely remove all the traces of IPA. Thus, the GO-epoxy liquid formulations were obtained with the GO content from 0.05 wt% through 0.6 wt%. The samples with higher GO concentration cannot be prepared due to the very high viscosity of the formulation, beginning with the 0.6%: the polymer does not flow into the curing molds. 2.5 Curing the liquid GO/epoxy formulations and obtaining solid composites For curing, the DDM hardener was added into the formulation, stirred manually, and the mixture was kept with occasional stirring at 80 °C for forty minutes. Then, the mixture was poured into a silicone moulds and cured 2 h at 80 °C, 2 h at 150 °C, and post-cured for 1 h at 180 °C. This resulted in solid GO/Epoxy composites, which will be referred to as 0.05-GO/Epoxy 0.10-GO/Epoxy, etc., where the number refers to the percentage of GO in the composites. 2.6 Characterization of GO and GO-epoxy composites In this work, we refrain from detailed discussion of the GO characterization as that having little relation to the main topic. GO was prepared according to the protocol developed in our labs, and repetitively conducted in the highly reproducible manner with almost no batch-to-batch 5 ACS Paragon Plus Environment


variation. For the detailed characterization and discussion of the GO chemical structure we refer interested readers to our previous publications on this matter.37, 39-40 The important property of our GO with respect to this study is its oxidation level, allowing spontaneous and complete exfoliation to the single-atomic-layer sheets upon gentle agitation in water, which was repetitively demonstrated before. The viscosity of uncured epoxy resin and epoxy/GO blends was examined by the parallel plate rheometer DHR-2 (TA Instruments) in the flow-sweep mode. The samples were tested at 25 °C and shear rate varied between 0.1 and 1000 s−1 with the plate diameter 40 mm. 2.7 Dielectric measurements. The permittivity and conductivity values were calculated from the impedance, measured with the Novocontrol BDS Concept-80 impedance analyzer, supplied with the automatic temperature control provided by the cryo-system QUATRO (the temperature uncertainty is ±0.5°С). The samples were circle-shaped with a diameter of 30.0 mm and the thickness of 3.0 mm. A sample was placed between the two gold-plated electrodes of the capacitor. The capacitor was attached to the thermostated testing head. The measurements were conducted in the frequency range 0.1 Hz - 10 MHz. The temperature range was -100 ºC ÷ 150 ºC. 2.8 Conductivity measurements of GO films. A free standing GO film was fabricated by drop-casting on a microscope glass from an aqueous solution. Then, a rectangular strip 5 mm by 10 mm was cut from the film. The film thickness was measured with the profilometer Dektak XT and was 1.5 μm. The two electrodes were applied on the opposite sides of the strip from the graphite based glue. The measurements were performed with the use of the electrochemical interface POT/GAL 30V 3A of the Novocontrol BDS Concept-80 impedance analyzer. The measurements were conducted in the frequency range 1 Hz - 10 MHz at the temperatures changing from -100 ºC through 200 ºC with the step 10 ºC. After each temperature ramping the sample was equilibrated at new temperature for 15 min before being measured. 3. Results and Discussion The visco-elastic properties of the liquid GO/Epoxy formulations, before adding curing agent, are presented on Figure 1.


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Figure. 1. Dependence of complex viscosity on shear rate for pure epoxy and GO-Epoxy formulations with the 0.05% - 0.6% GO content (a), and the Y-axis expansion for the samples with the GO content 0.05% - 0.50% (b). The flow curves for neat epoxy as well as for compositions containing 0.05% - 0.15% GO demonstrate almost Newtonian behavior (Figure 1); the viscosity does not change significantly with adding GO up to 0.15%. Beginning with 0.2% GO content, the compositions exhibit very different behavior; at low shear rate their viscosity is significantly higher than that for the low GO content solutions. However, the viscosity rapidly decreases with shear rate, exhibiting strong shear thinning properties. Such behavior is typical for GO solutions in low-molecular-weight liquids.41-43 Thus, the GO aqueous solutions begin turning from isotactic to nematic phase at exactly the same 0.1% GO content as in our GO/Epoxy formulations.41-43. Apparently, at higher content, GO structurizes epoxy resin with formation of nematic phase. The decrease of viscosity with increasing the shear rate can be explained by disorientation of the established liquid crystalline phase, i.e. due to the rupture of the viscoelastic GO network and acquiring the liquid7 ACS Paragon Plus Environment


like behavior. At 0.6% GO content, viscosity sharply increases, the composition becomes more like a paste, rather than a liquid. It requires heating up to ~80 ºC and higher to liquefy to the conditions transferable to the curing molds. In the literature on GO/Epoxy systems, we have found only one paper that provides viscosity values.44 In that study, addition of 0.5% and 1.0% GO did not notably change the viscosity of the neat resin; addition of 2.0% and 3.0% GO increased viscosity by only ~25% and ~59% respectively. In our work, we registered 263%, 387%, and 529% increase in viscosity (Fig 1) with as little GO content as 0.2%, 0.3%, and 0.4%, respectively. At 0.6% loading, the increase is more than the two orders of magnitude. This apparent discrepancy between the two experiments most likely roots in the low exfoliation level of GO in the literature study.44 In our work, we observe the viscosity increase similar to that registered for aqueous GO solutions.43 The similar viscoelastic behavior suggests the similar level of GO exfoliation, i.e. nearly completely exfoliated condition of GO in the epoxy matrix. The highly exfoliated condition of GO is also confirmed by the optical micrographs of the liquid formulations (Figure S1). No particulate matter can be observed, suggesting uniform distribution of GO flakes with no aggregation. The GO flakes themselves are also non-visible, suggesting their single-layer character. One can only see some fluctuations, caused by refraction of light by sporadically distributed and oriented local areas of nematic phase, typical for the GO-based liquid crystals.43,

45, 46

At the same time, the color and the optical density of the formulations

change with GO content, confirming that GO is there. As comparison, we present micrographs of GO/Epoxy formulations, made by the alternative transfer method.47 The liquid formulation looks transparent with bare eye, however, the micrograph shows presence of numerous wellvisible multi-layered GO sheets, pointing at non-exfoliated condition of GO (Figure S2). As additional evidence we present the SEM images of the fracture surface of the solid GO/Epoxy composite (Figure S3). Again, no aggregates, no flake-like material can be distinguished on the surfaces. At the same time the difference in the fracture pattern of GO/Epoxy with that for neat epoxy suggests that GO is there, and that it notably altered morphology and mechanical properties of the resin. One can also conclude that in the liquid formulations, the GO flakes must be relatively flat, because only flat sheet-like inclusions, arranged parallel to each other, can form nematic phase.43, 48

The extremely high viscosity and the paste-like appearance of the 0.6% formulation suggests 8 ACS Paragon Plus Environment

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that composites with the higher GO content cannot be prepared (at the same lateral size of the flakes) without sacrificing the exfoliation level and uniformity of the composition. The formulations solidify before all the IPA is evaporated, and do not flow in the molds. In other words, higher loading fractions might be attained only at lower exfoliation level and/or with aggregation of the GO flakes. The curing of the GO/Epoxy formulations was conducted at 180 ºC, the temperature at which GO significantly decomposes. To differentiate the product of such decomposition from the original GO, we introduce the term "thermally processed" GO (tpGO). Starting with ~150 ºC, GO undergoes massive decomposition, loosing most of the oxygen functional groups. Annealing at sufficiently higher temperatures (>900 ºC), yields so-called "thermally reduced" GO that is electrically conducive. The electrical properties of tpGO, obtained by annealing at 100 Hz, the plots reach saturation at 0.2% GO content. When measured at 1 Hz, the well-pronounced maximum is registered at 0,3% GO content. Thus, in terms of the percolation theory, our samples exhibit percolative behavior in the 0.2% - 0.3% range of the filling fractions. These are rather low values. As comparison, for the composite materials containing MWCNTs and multi-layered GNR stacks, similar behavior was observed at 1.5% - 2.0% filling fractions.11,12 For the best of our knowledge, 0.2% is the lowest content, at which saturation was reported for the GO/polymer composites. Only ref. [32] claims reaching the percolation threshold at 0.033%. However, the reasons for this claim are not obvious; according to the presented data, permittivity gradually increases up to 0.2%, the highest filling fraction, tested in that work, without any saturation, and with nothing special registered at 0.033%.32 Most of the literature on GO/polymer composites reports the percolative behavior at 1.0% - 1.5% filling fractions.14-18 The ultra-low saturation level, registered in our work, indirectly confirms the highly exfoliated condition of GO flakes in the epoxy matrix. For better understanding macroscopic dielectric properties of the tested composites, as the next step, we investigate the conductive properties of the GO filler.



The as-prepared GO is non-conductive, with the band gap around 3.5 eV - 3.7 eV.49-50 Yu et al. demonstrated that annealing GO at progressively higher temperatures up to 160 ºC gradually decreased the band gap from 3.7 eV through ~1.0 eV, making it semiconductive.49 Recently, Lipatov et al.51 demonstrated that electrical conductivity of GO films increases five orders in magnitude when annealing at 150-160 ºC. Both observations were explained by formation of conductive channels between the graphenic domains of GO flakes. The characteristics of the parent GO, used in this study, are in full accordance with the literature data (Figure 4). The SEM image shows that, being deposited from an aqueous solution, GO flakes are in fully exfoliated condition (Figure 4a). The lateral size of the vast majority of the flakes is within the 5-20 μm range.

Figure 4. Characteristics of GO and tpGO. (a) The SEM image of GO flakes. (b) The TGA weight loss diagram, showing decomposition of GO during the heating protocol, identical to that used for curing GO/epoxy compositions. (c) The TGA curves for GO and tpGO. (d) The FTIR spectra for tpGO and parent GO. To investigate how the heating, applied during the curing procedure, affects the GO structure and conductivity, we subjected the free standing GO films to the heating protocol, used for the curing 12 ACS Paragon Plus Environment

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of the GO-epoxy formulations. The TGA curves (Figure 4b) demonstrate that decomposition of GO does not begin until 145 ºC. During the 2h processing at 150 ºC GO loses only 13% of its mass, while massive decomposition begins after reaching 180 ºC. However, even after annealing 1h at 180 ºC the total weight loss is only ~35%. This is significantly lower than ~65% weight loss when GO is annealed up to 900 ºC (Figure 4c). Subsequently, tpGO still loses ~10% of its weight in the 200-300 ºC interval (Figure 4c) suggesting that at the end of the heating protocol many functional groups survive. The FTIR spectrum (Figure 4d) shows significant decrease of absorbance in the 3680 - 2440 cm-1 spectral range, consistent with removal of almost all the water and, probably, the larger part of the alcohols. However, the two absorbance bands at 3688 cm-1 and 3614 cm-1 that we associate with the stretch of the O-H bond, point that some hydroxyl groups survive the thermal processing. Another confirmation for this statement is the survival of the 1043 cm-1 absorption band that we associate with the stretch of the C-O bonds in the alcohols. Finally, the 1723 cm-1 band, associated with carbonyls is still present. To conclude, the overall decomposition level of tpGO is consistent with the structure, where the conductive channels might indeed form between the graphenic domains of GO flakes, making them conductive. In a separate experiment, we investigated how thermal decomposition changes the conductivity of a pure GO sample. A thin GO film was heated up to 200 ºC in the 10 ºC steps, and the conductivity was measured after each heating step. The conductivity of the GO film slowly and gradually increases with temperature till 170 ºC (Figure S7). Note, in this experiment, the measured conductivity values increase for the two reasons simultaneously. The first is simply the temperature function, and the second is the transformation of GO into tpGO at temperatures >160 ºC. The rapid jump in conductivity, registered between the temperatures 170 ºC and 180 ºC, and especially between 180 ºC and 190 ºC, is caused mostly by the second factor. The difference between the conductivities of GO and tpGO is about five orders in magnitude, in full accordance with the literature data.50-51 Thus, the conductivity, introduced by the conversion of GO to tpGO during the curing of the GO/Epoxy formulations, explains the registered increase in the dielectric constant of the composite materials (Figure 2). Note, decomposition of GO in the epoxy matrix might occur differently from the decomposition of bulk GO in air or in the inert atmosphere due to the possible interaction between GO and epoxy resin or curing agent. This assumption seems reasonable, since against expectations, 13 ACS Paragon Plus Environment


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further annealing of the previously tested samples at 200 ºC did not change their dielectric properties, and annealing at 250 ºC slightly decreased the permittivity. Thus, at 250 ºC, GO flakes must be partially destroyed due to the interaction with epoxy matrix. The well-pronounced relaxation peaks, registered on the imaginary permittivity curves (Figure 2b), are not typical for composites with highly conductive inclusions such as CNTs and GNRs, where the conductivity component dominates over the Debye component.11,12 This observation suggests that the low conductivity of tpGO must be one of the reasons for the observed frequency function profile of the imaginary part. The curves for 0.3-GO/Epoxy and 0.4-GO/Epoxy exhibit the two well-pronounced relaxation regions (Figure 2b). In addition to the main process, registered in the 1 Hz - 100 Hz frequency range (process 1), discussed above, there is the low-amplitude relaxation process in the 104 Hz 105 Hz region (process 2). To further explore the origin of these two processes, we tested selected samples at varying temperatures from -100 ºC through 150 ºC. After the measurements, the complex permittivity

was approximated by superposition of the Cole-Cole function52 with the

Jonsher addition53 for conductivity. The complex permittivity can be represented by eq. (1),

 *     '    i "      



1  i 



 Bi 

n 1

i

0 ,  0 (1)

where ε'(ω) and ε"(ω) are the real and the imaginary parts of the complex permittivity; i is the imaginary unit; ε∞ is the permittivity at high frequency; ω is the cyclic frequency, Δε, τ, α are the magnitudes of the dielectric strength, relaxation time, and Cole-Cole broadening parameter, respectively; B is the magnitude of the Jonscher correction; 0 < n ≤ 1 is the Jonscher parameter; σ0 is the DC conductivity; ε0 = 8.85∙10-12 F/m is the permittivity of vacuum. Figure 5 shows the function of the relaxation time on reverse temperature for the two relaxation processes and for the three selected samples with 0.3%, 0.4% and 0.5% GO content.


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Figure 5. The function of the relaxation time (τ) on reversed temperature for two different processes (1) and (2), for the samples with 0.3%, 0.4% and 0.5% GO content. As evident from Figure 5, the logτ2(1000/T) curves for process (2) vary slightly with the GO content, however, they all situated in the same regions of times and temperatures. We attribute process (2) to the β-relaxation of the polymer matrix, since it is observed in all the tested samples. Quite different behavior is demonstrated by the curves for process (1). The relaxation times lay in the region 10-2 - 10+2 s, and increase with the GO content. The activation energies for process (1) are significantly lower than those for process (2), and also lower than for the polymer composites comprising MWCNTs and GNRs.12 To explain relatively low permittivity values, obtained in this study (Fig. 2), we discuss the specificity of the Maxwell-Wagner polarization in our materials. The Maxwell-Wagner polarization is normally observed in the systems comprising relatively large (compared to atomic level), conductive inclusions. The charges are thus separated over a considerable distance. Most of the nanocarbon fillers, used in polymer composites, are relatively large, even in their lowest dimension due to their multi-layered character: MWCNTs, GNPs, GNRs, etc. Even SWCNTs can be considered as quazi-multi-layered structures due to their existence in bundles, comprising tens and hundreds of individual nanotubes. The bundles are not easily separate even in the lowmolecular solvents with high-power sonication; SWCNT are definitely bundled in polymer matrix. The outer layers of multi-layered structures shield the inner layers from the matrix. 15 ACS Paragon Plus Environment


In this work, for the first time, we are reporting dielectric properties for the composite materials, comprising truly 2D single-carbon-layer flakes in the matrix. The fundamental question is if a single-atomic-layer flake can be considered as a true conductive inclusion, i.e. as a separate phase? It was demonstrated in numerous experiments that the measured conductivity of single layer graphene, laying flat on a substrate, is many times lower than that for the free-standing sheet. Even the second layer experiences the influence of the substrate.54 This must be especially true for a single-layer tpGO sheet sealed in the polymer. The interfacial interaction with the polymer simultaneously from both sides must strongly diminish conductivity and polarization of tpGO flakes. Thus, our experimental data brings us to the following fundamental conclusion: in composite materials, comprising truly 2D conductive inclusions, the Maxwell-Wagner polarization is notably suppressed due to the interaction of conductive inclusions with the matrix. Another reason for the low Maxwell-Wagner polarization is the low conductivity of tpGO compared to graphene. Unfortunately, existing methods do not allow insertion and uniform distribution of single-layer intact graphene into the polymer matrix. However, we hypothesize that even intact single-layer graphene would behave similar to tpGO, despite its significantly higher conductivity. This assumption is in good accordance with existing literature data, reporting the real part values up to 70 at filling fractions 1% - 8% with RGO as conductive inclusion.19-26 Both factors a) the possibility to insert that much RGO in the matrix, and b) relatively high permittivity values, strongly suggest the multi-layered character, and aggregation of RGO in the matrix, which is expectable for highly deoxygenated RGO sheets.

4. Conclusions The homogeneous liquid phase transfer method affords highly exfoliated condition of GO flakes in epoxy matrix. This, in turn, results in high viscosity of the liquid formulations, and in unique dielectric properties of the cured composites: the ultra-low saturation level registered at 0.2% filling fraction, and relatively low permittivity values. The ~0.6% GO content is the absolute maximum that can be inserted into the epoxy matrix (at the given lateral dimensions) without sacrificing the exfoliation level of the 2D filler and the uniformity of the composition. Our experimental data suggests that for single-atomic-layer conductive inclusions the MaxwellWagner polarization is sufficiently suppressed due to their interaction with the matrix. And 16 ACS Paragon Plus Environment

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contrary, the strong Maxwell-Wagner polarization, reported along with the loading fractions >0.6%, are the indicators of the non-single-layer character, and/or the aggregation of the inclusion particles. Supporting information Electronic supporting materials (SI) are available: microphotographs of liquid formulations, SEM images of fracture surfaces, additional permittivity and conductivity data. Acknowledgments This work was supported by the Russian Science Foundation; Grant # 16-13-10291. References (1) Ajayan, P. M.; Tour, J. M. Materials science: Nanotube composites. Nature 2007, 447, 10661068. (2) Terrones, M.; Martín, O.; González, M.; Pozuelo, J.; Serrano, B.; Cabanelas, J. C.; VegaDíaz, S. M.; Baselga, J. Interphases in graphene polymer-based nanocomposites: Achievements and challenges. Adv. Mater. 2011, 23, 5302-5310. (3) Gudarzi, M. M.; Aboutalebi, S. H.; Sharif, F. Graphene oxide-based composite materials. In Graphene oxide: fundamentals and applications; Dimiev, A. M., Eigler, S. Eds.; wiley: 2016; pp 314-363. (4) Coleman, J.; Curran, S.; Dalton, A.; Davey, A.; McCarthy, B.; Blau, W.; Barklie, R. Percolation-dominated conductivity in a conjugated-polymer-carbon-nanotube composite. Phys. Rev. B 1998, 58, R7492-R7495. (5) Grimes, C. A.; Mungle, C.; Kouzoudis, D.; Fang, S.; Eklund, P. C. The 500 MHz to 5.50 GHz complex permittivity spectra of single-wall carbon nanotube-loaded polymer composites. Chem. Phys. Lett. 2000, 319, 460-464. (6) Wu, J.; Kong, L. High microwave permittivity of multiwalled carbon nanotube composites. Appl. Phys. Lett. 2004, 84, 4956-4958. (7) Logakis, E.; Pandis, C.; Peoglos, V.; Pissis, P.; Pionteck, J.; Pötschke, P.; Mičušík, M.; Omastová, M. Electrical/dielectric properties and conduction mechanism in melt processed polyamide/multi-walled carbon nanotubes composites. Polymer 2009, 50, 5103-511. 17 ACS Paragon Plus Environment


(8) Dang, Z. M.; Wang, L.; Yin, Y.; Zhang, Q.; Lei, Q. Q. Giant dielectric permittivities in functionalized carbon-nanotube/ electroactive-polymer nanocomposites. Adv. Mater. 2007, 19, 852-857. (9) Yuan, J. K.; Yao, S. H.; Dang, Z. M.; Sylvestre, A.; Genestoux, M.; Bai, J. Giant dielectric permittivity nanocomposites: Realizing true potential of pristine carbon nanotubes in polyvinylidene fluoride matrix through an enhanced interfacial interaction. J. Phys. Chem. C 2011, 115, 5515-5521. (10) Dimiev, A.; Lu, W.; Zeller, K.; Crowgey, B.; Kempel, L. C.; Tour, J. M. Low-loss, highpermittivity composites made from graphene nanoribbons. ACS Appl. Mater. Interfaces 2011, 3, 4657-4661. (11) Dimiev, A.; Zakhidov, D.; Genorio, B.; Oladimeji, K.; Crowgey, B.; Kempel, L.; Rothwell, E. J.; Tour, J. M. Permittivity of dielectric composite materials comprising graphene nanoribbons. the effect of nanostructure. ACS Appl. Mater. Interfaces 2013, 5, 7567-7573. (12) Lounev, I. V.; Musin, D. R.; Dimiev, A. M. New details to relaxation dynamics of dielectric composite materials comprising longitudinally opened carbon nanotubes. J. Phys. Chem. C 2017, 121, 22995-23001. (13) Amirova, L.; Surnova, A.; Balkaev, D.; Musin, D.; Amirov, R.; Dimiev, A. M. Homogeneous liquid phase transfer of graphene oxide into epoxy resins. ACS Appl. Mater. Interfaces 2017, 9, 11909-11917. (14) Wang, J. Y.; Yang, S. Y.; Huang, Y. L.; Tien, H. W.; Chin, W. K.; Ma, C. C. M. Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situ polymerization. J. Mater. Chem. 2011, 21, 1356913575. (15) Wu, C.; Huang, X.; Wang, G.; Wu, X.; Yang, K.; Li, S.; Jiang, P. Hyperbranched-polymer functionalization of graphene sheets for enhanced mechanical and dielectric properties of polyurethane composites. J. Mater. Chem. 2012, 22, 7010-7019. (16) Liao, W. H.; Yang, S. Y.; Hsiao, S. T.; Wang, Y. S.; Li, S. M.; Ma, C. C. M.; Tien, H. W.; Zeng, S. J. Effect of octa(aminophenyl) polyhedral oligomeric silsesquioxane functionalized graphene oxide on the mechanical and dielectric properties of polyimide composites. ACS Appl. Mater. Interfaces 2014, 6, 15802-15812.


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Sadasivuni, K. K.; Ponnamma, D.; Kumar, B.; Strankowski, M.; Cardinaels, R.; Moldenaers, P.; Thomas, S.; Grohens, Y. Dielectric properties of modified graphene oxide filled polyurethane nanocomposites and its correlation with rheology. Compos. Sci. Technol. 2014, 104, 18-25. (18) Liu, P.; Yao, Z.; Zhou, J. Mechanical, thermal and dielectric properties of graphene oxide/polyimide resin composite. High Perform. Polym. 2016, 28, 1033-1042. (19) Wang, Z.; Nelson, J. K.; Hillborg, H.; Zhao, S.; Schadler, L. S. Graphene oxide filled nanocomposite with novel electrical and dielectric properties. Adv. Mater. 2012, 24, 3134-3137. (20) Wang, D.; Bao, Y.; Zha, J. W.; Zhao, J.; Dang, Z. M.; Hu, G. H. Improved dielectric properties of nanocomposites based on poly(vinylidene fluoride) and poly(vinyl alcohol)functionalized graphene. ACS Appl. Mater. Interfaces 2012, 4, 6273-6279. (21) Chen, Y.; Zhang, S.; Liu, X.; Pei, Q.; Qian, J.; Zhuang, Q.; Han, Z. Preparation of solutionprocessable reduced graphene oxide/polybenzoxazole nanocomposites with improved dielectric properties. Macromolecules 2015, 48, 365-372. (22) Li, M.; Huang, X.; Wu, C.; Xu, H.; Jiang, P.; Tanaka, T. Fabrication of two-dimensional hybrid sheets by decorating insulating PANI on reduced graphene oxide for polymer nanocomposites with low dielectric loss and high dielectric constant. J. Mater. Chem. 2012, 22, 23477-23484. (23) Cho, S.; Lee, J. S.; Jang, J. Poly(vinylidene fluoride)/NH2-treated graphene nanodot/reduced graphene oxide nanocomposites with enhanced dielectric performance for ultrahigh energy density capacitor. ACS Appl. Mater. Interfaces 2015, 7, 9668-9681. (24) Liu, H.; Xu, P.; Yao, H.; Chen, W.; Zhao, J.; Kang, C.; Bian, Z.; Gao, L.; Guo, H. Controllable reduction of graphene oxide and its application during the fabrication of high dielectric constant composites. Appl. Surf. Sci. 2017, 420, 390-398. (25) Zhang, T.; Huang, W.; Zhang, N.; Huang, T.; Yang, J.; Wang, Y. Grafting of polystyrene onto reduced graphene oxide by emulsion polymerization for dielectric polymer composites: High dielectric constant and low dielectric loss tuned by varied grafting amount of polystyrene. Eur. Polym. J. 2017, 94, 196-207. (26) Tong, W.; Zhang, Y.; Zhang, Q.; Luan, X.; Duan, Y.; Pan, S.; Lv, F.; An, Q. Achieving significantly enhanced dielectric performance of reduced graphene oxide/polymer composite by covalent modification of graphene oxide surface. Carbon 2015, 94, 590-598. 19 ACS Paragon Plus Environment


(27) Bergman, D. J.; Imry, Y. Critical behavior of the complex dielectric constant near the percolation threshold of a heterogeneous material. Phys. Rev. Lett. 1977, 39, 1222-1225. (28) Bánhegyi, G. Comparison of electrical mixture rules for composites. Colloid. Polym. Sci. 1986, 264, 1030-1050. (29) Nan, C. W. Physics of inhomogeneous inorganic materials. Prog. Mater. Sci. 1993, 37, 1116. (30) Feldman, Y.; Puzenko, A.; Ryabov, Y. Dielectric relaxation phenomena in complex materials. In Advances in chemical physics, fractals, diffusion and relaxation in disordered complex systems; T. Coffey, W. T., Kalmykov, Y. P. Eds.; wiley, 2005; pp 1-125. (31) Cametti, C. Dielectric spectra of ionic water-in-oil microemulsions below percolation: Frequency dependence behavior. Phys. Rev. E 2010, 81, 031403-8. (32) Wang, D.; Zhang, X.; Zha, J. W.; Zhao, J.; Dang, Z. M.; Hu, G. H. Dielectric properties of reduced graphene oxide/polypropylene composites with ultralow percolation threshold. Polymer 2013, 54, 1916-1922. (33) Moharana, S.; Mahaling, R. N. Silver (Ag)-Graphene oxide (GO) - Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanostructured composites with high dielectric constant and low dielectric loss. Chem. Phys. Lett. 2017, 680, 31-36. (34) He, F.; Lau, S.; Chan, H. L.; Fan, J. High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates. Adv. Mater. 2009, 21, 710-715. (35) Wan, Y. J.; Yang, W. H.; Yu, S. H.; Sun, R.; Wong, C. P.; Liao, W. H. Covalent polymer functionalization of graphene for improved dielectric properties and thermal stability of epoxy composites. Compos. Sci. Technol. 2016, 122, 27-35. (36) Yuan, J. K.; Li, W. L.; Yao, S. H.; Lin, Y. Q.; Sylvestre, A.; Bai, J. High dielectric permittivity and low percolation threshold in polymer composites based on SiC-carbon nanotubes micro/nano hybrid. Appl. Phys. Lett. 2011, 98, 032901. (37) Dimiev, A. M.; Polson, T. A. Contesting the two-component structural model of graphene oxide and reexamining the chemistry of graphene oxide in basic media. Carbon 2015, 93, 544554.


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Khannanov, A. A.; Valimukhametova, A. R.; Kiiamov, A. G.; Vakhitov, I. R.; Dimiev, A. M. The mechanistic details for the growth of palladium nanoparticles on graphene oxide support. Chemistry Select 2017, 2. (39) Amirov, R. R.; Shayimova, J.; Nasirova, Z.; Solodov, A.; Dimiev, A. M. Analysis of competitive binding of several metal cations by graphene oxide reveals the quantity and spatial distribution of carboxyl groups on its surface. Phys. Chem. Chem. Phys. 2018, 20, 2320-2329. (40) Dimiev, A. M.; Alemany, L. B.; Tour, J. M. Graphene oxide. Origin of acidity, its instability in water, and a new dynamic structural model. ACS Nano 2013, 7, 576-588. (41) Vallés, C.; Young, R. J.; Lomax, D. J.; Kinloch, I. A. The rheological behaviour of concentrated dispersions of graphene oxide. J. Mater. Sci. 2014, 49, 6311-6320. (42) Vallés, C. Rheology of Graphene Oxide Dispersions. In Graphene Oxide: Fundamentals and Applications; Dimiev, A. M., Eigler, S. Eds.; wiley: 2016; pp 121-146. (43) Kumar, P.; Maiti, U. N.; Lee, K. E.; Kim, S. O. Rheological properties of graphene oxide liquid crystal. Carbon 2014, 80, 453-461. (44) Vallés, C.; Beckert, F.; Burk, L.; Mülhaupt, R.; Young, R. J.; Kinloch, I. A. Effect of the C/O ratio in graphene oxide materials on the reinforcement of epoxy-based nanocomposites. J Polym. Sci. Part B 2016, 54, 281-291. (45) Shen, T. Z.; Hong, S. H.; Song, J. K. Electro-optical switching of graphene oxide liquid crystals with an extremely large Kerr coefficient. Nat. Mater. 2014, 13, 394-399. (46) Solodov, A.; Neklyudov, V.; Shayimova, J.; Amirov, R.; Dimiev, A.M. Magneto-optical properties of the magnetite-graphene oxide composites in organic solvents. ACS Appl. Mater. Interf. 2018, DOI: 10.1021/acsami.8b15129. (47) Bortz, D.R.; Heras, E.G.; Martin-Gullon, I. Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules, 2012, 45, 238-245. (48) Onsager, L., The effects of shape on the interaction of colloidal particles. In Annals of the New York Academy of Sciences, 1949; Vol. 51, pp 627-659. (48) Dimiev, A. M. Mechanism of formation and chemical structure of graphene oxide. In Graphene Oxide: Fundamentals and Applications; Dimiev, A. M., Eigler, S. Eds.; wiley: 2016; pp 36-84.



(49) Mathkar, A.; Tozier, D.; Cox, P.; Ong, P.; Galande, C.; Balakrishnan, K.; Leela Mohana Reddy, A.; Ajayan, P. M. Controlled, stepwise reduction and band gap manipulation of graphene oxide. J. Phys. Chem. Lett. 2012, 3, 986-991. (50) Zhu, Y.; Li, X.; Cai, Q.; Sun, Z.; Casillas, G.; Jose-Yacaman, M.; Verduzco, R.; Tour, J. M. Quantitative analysis of structure and bandgap changes in graphene oxide nanoribbons during thermal annealing. J. Am. Chem. Soc. 2012, 134, 11774-11780. (51) Lipatov, A.; Guinel, M. J. F.; Muratov, D. S.; Vanyushin, V. O.; Wilson, P. M.; Kolmakov, A.; Sinitskii, A. Low-temperature thermal reduction of graphene oxide: In situ correlative structural, thermal desorption, and electrical transport measurements. Appl. Phys. Lett. 2018, 112, 053103. DOI: 10.1063/1.4996337. (52) Cole, R. H. On the analysis of dielectric relaxation measurements. J. Chem. Phys. 1955, 23, 493-499. (53) Jonscher, A. K. Dielectric relaxation in solids. J. Phys. D 1999, 32, R57-R70. (54) Sinitskii, A.; Dimiev, A.; Kosynkin, D. V.; Tour, J. M. Graphene nanoribbon devices produced by oxidative unzipping of carbon nanotubes. ACS Nano 2010, 4, 5405-5413.

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Intrinsic Insertion Limits of Graphene Oxide into Epoxy Resin and the

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