Urethane-functionalized graphene oxide for improving compatibility

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Materials and Interfaces

Urethane-functionalized graphene oxide for improving compatibility and thermal conductivity of waterborne polyurethane composites Weining Du, Zetian Zhang, Hui Su, Hong Lin, and Zhengjun Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00656 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Industrial & Engineering Chemistry Research

Urethane-functionalized graphene oxide for improving compatibility and thermal conductivity of waterborne polyurethane composites Weining Du,†, ‡ Zetian Zhang,†, ‡ Hui Su,§ Hong Lin,§ and Zhengjun Li*, †, ‡ †

National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, China

‡

Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065, China §

Guangdong Huizhou Quality & Measuring Supervision Testing Institute, Huizhou 516003, China

Abstract This study provides an effective interface modification strategy for preparing graphene-based composite. Urethane-functionalized graphene oxide (FGO) was prepared by in situ polymerization and then was incorporated into a polyurethane binder (PUB) matrix to fabricate composites. The surface chemical composition and structural properties of FGO were systematically characterized by a series of measurements. It was found that urethane chains were covalently attached onto the graphene oxide surface, and the FGO possessed a comparatively regular lamellar structure and a larger interlayer distance. Additionally, amphipathic urethane chains on the FGO sheets not only improved the compatibility and interfacial interaction between the stiff FGO and the soft PUB matrix, but also alleviated the modulus mismatch between them. The most effective improvements in mechanical and thermal transport properties were achieved when 0.5 wt.% FGO was incorporated into the PUB matrix, leading to improvement of tensile strength, elongation at break, and thermal conductivity by 23.4%, 12.1%, and 61.5%, respectively, in comparison with the neat PUB matrix. We expect that this work will have potential applications in functional composite coatings.

Keywords: graphene oxide; urethane chains; polyurethane; compatibility; thermal conductivity

*

Corresponding author: [email protected] (Z. Li); Tel: +86-028-85408868

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1. Introduction Polyurethane (PU) is one of the most versatile materials used in coatings, adhesives, elastomers, and composites.1 The modification of polyurethane materials has predominantly focused on mechanical and thermal properties.2 To date, two main strategies are generally used for the modification of polyurethanes. One is to alter the molecular structure of polyurethane by decoration of its basic building blocks.3 Another is to introduce solid fillers into the polyurethane matrix.4 Ideally, the incorporation of small amounts of fillers into polymeric matrices can significantly improve the performance of composites, as predicted by the so-called generalized Maxwell model.5 Graphene is an efficient reinforcing filler for composites because of its intriguing characteristics which include high specific surface area, remarkable mechanical strength, and ultrahigh thermal conductivity.6-8 Unfortunately, graphene sheets have poor dispersibility in, and weak compatibility with, polymeric matrices attributed to the influence of the van der Waals' interactions.9 Significant efforts have been applied to decorate graphene by directly introducing functional groups to achieve enhanced interfacial bonding strength.10, 11 Although these methodologies have been effective in some cases, they often required harsh experimental conditions, and the grafting percentage was not significantly enhanced due to the absence of oxygen-containing groups on the solid surfaces.12 In addition, it is time consuming and demanding to produce high quality and defect-free graphene sheets.13 In this situation, graphene oxide (GO), an important graphene derivative, has been widely researched as an alternative to graphene. It is reasonable that oxidation followed by chemical functionalization will stabilize graphene to prevent agglomeration and facilitate good compatibility with polymeric matrices. GO is readily obtained from the oxidation and exfoliation of graphite, which contains abundant oxygen-containing groups including hydroxyl, carboxyl, and epoxide groups located on its basal planes and edges.14, 15 Despite being an oxidation state of graphene, GO itself exhibits prominent physical and extra-chemical properties.16,

17

Compared with other nano-fillers (e.g., carbon

nanotubes,18 graphite nanoplatelets,19 and graphene20) in fabricating composite materials, GO possesses considerable highly-reactive groups on surfaces, an indispensable property for triggering interaction between fillers and matrices as well as further surface modification.12, 21 Consequently, GO is useful as a reinforcing filler for fabrication of polymeric composites. GO can easily be 2


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dispersed in water and form a miscible suspension.22 Nevertheless, the dispersion of GO in polymer matrices has some limitations, probably assigned to the strong interlayer hydrogen bonds originating from the attached oxygen functional groups.13 Such strong interactions between adjacent GO sheets may prevent the penetration of macromolecular chains into the interlayer space, and thus decrease the interfacial interaction of GO with polymeric matrices. It is envisaged that non-covalent or covalent functionalization of GO sheets with reactive molecules and polymer chains may be able to solve this problem. Generally, GO, as a nanoscale cabon-based filler, can preserve polymeric matrix properties (e.g., strength, elasticity, and modulus) with the additional functionality of exceptional thermal conductivity.23 However, in practice, the enhancement of thermal conductivity by the addition of carbon-based fillers to polymeric composites does not reach theoretical expectations due to the high interfacial thermal resistance between fillers and polymeric matrices, which hinders phonon conduction at the interface.24, 25 Therefore, the big challenges encountered in preparing GO-based composites are achieving dispersion of GO in a polymeric matrix, and improving compatibility between them. Attempts have been made to improve the compatibility or interfacial interaction of GO sheets with polymeric matrices by modifying GO through covalent grafting, which in turn will improve mechanical and thermal properties.6,

12, 13, 26

Stankovich et al.27 functionalized graphite

oxides with isocyanate derivatives through formation of carbamate esters and exfoliation of graphene oxide nanoplatelets; the functionalized GO displayed stable dispersion in polar aprotic solvents. Jing et al.13 prepared functionalized graphene oxide with polyol and diisocyanate, and then blended it with a polyurethane matrix. They reported that the compatibility between the filler and polymer matrix was greatly enhanced, and the resulting GO-based composite also showed improved mechanical properties and thermal stability. In a similar study, Zhang et al.12 prepared a polyurethane composite using urethane chains modified graphene oxide as a cross-linker, resulting in enhanced mechanical properties and thermal stability. The main purpose of the aforementioned modification strategies was to improve the dispersion of functionalized GO in organic solvents or hydrophobic polymer matrices. Nevertheless, the covalent grafting of urethane chains on the GO surface is a simple matter. Therefore applications using GO to prepare composites in aqueous systems are limited. Currently, there have been few studies on functionalized graphene oxide with amphipathic urethane 3



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chains focused on the influence of compatibility between GO and the polymeric matrix on thermal conductivity. Our group previously synthesized encapsulated carbon black with amphipathic urethane chains embedded in a waterborne polyurethane matrix.28, 29 Not only did the encapsulated carbon black acquire good dispersibility and stability in water, but also the urethane chains attached on the solid surfaces facilitated strong interactions with the polyurethane matrix. Also, encapsulated carbon black-based composites exhibited improved mechanical properties. Both carbon black and graphene oxide belong to the family of carbon nano-materials, but graphene oxide possesses some important characteristics (e.g., low density, large specific area, high thermal and mechanical properties), not exhibited by carbon black. We reasoned that chemical functionalization of graphene oxide could be accomplished according to the method of our previous studies. The key objective of this work was to improve the dispersibility of graphene oxide in the polyurethane matrix and the interfacial interactions between them, and, in turn, enhance the mechanical properties, and especially the thermal conductivity, of the composites. To that end, we first synthesized urethane-functionalized GO with amphipathic chains, and determined their chemical and structural properties by a series of measurements. Subsequently, the prepared GO and FGO sheets were incorporated into a commercial waterborne polyurethane binder (PUB) matrix to fabricate composite films. The compatibility and interfacial interactions of these GO sheets with the polyurethane matrix were investigated. Their mechanical properties, in particular their thermal conductivity, were also studied and discussed.

2. Experimental section 2.1. Materials Detailed descriptions of the chemicals, reagents, and their purification methods are summarized in the Supporting Information. 2.2. Methods 2.2.1. Synthesis of graphene oxide Graphene oxide (GO) was prepared from graphite powder according to a modified Hummer's method.30, 31 Briefly, 0.5 g graphite powder and 15 mL H2SO4 were mixed in a glass flask and dispersed by ultrasonication at 100 Hz for 1.5 h. After that, 2 g KMnO4 was gradually added to the 4


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mixture with magnetic stirring in an ice bath, and the mixture was then ultrasonicated for 1 h. The resulting mixture was diluted by adding ~15 mL deionized water with magnetic stirring under ambient temperature. After completion of the oxidation reaction, the suspension was treated with 3 mL H2O2 solutions. Finally, the resulting mixture was centrifuged and washed with hydrochloric acid (1 M) and deionized water, respectively. This procedure was repeated five times, and the separated colloid product was dried using freeze-drying method. 2.2.2. Synthesis of urethane-functionalized graphene oxide Urethane-functionalized graphene oxide (FGO) was prepared using a two-step method, similar to our previous method.29 Firstly, 0.5 g GO was dispersed in 80 mL DMF with ultrasonication at room temperature. After 1 h, 0.5 g TDI and 30 µL DBTDL were added to the solution and stirred at 70 °C for 24 h under N2 protection. Secondly, 5 g TDI and 10.44 g PEG were gradually added to the mixture and continuously stirred at 60 °C. After 3 h, 0.35 g DMPA was put into the mixture, which was further maintained at 60 °C for 3 h. The resulting mixture was centrifuged, and the solid residue was dispersed in 30 mL deionized water containing 0.172 g EDA under ambient conditions. The final solution was centrifuged and washed completely with water at least five times. The functionalized graphene oxide product was lyophilized and abbreviated as FGO. 2.2.3. Preparation of waterborne polyurethane binder/graphene oxide composite films Polyurethane composite films containing 0.5 wt.% and 1.0 wt.% of FGO were fabricated via a solution mixing method and referred to as PUB/FGO1 and PUB/FGO2, respectively. In brief, FGO was first dispersed in water with ultrasonication for 15 min, and then blended with waterborne polyurethane binder (PUB) to achieve a uniform mixture. After that, the miscible solution was cast into Teflon molds and cured at ambient conditions overnight. Finally, the molds were placed in a convection oven and dried at 80 °C for 6 h. Another sample containing 0.5 wt.% of pristine GO was prepared using the same procedures, and referred to as PUB/GO. Pure polyurethane binder film (PUB) was also prepared without fillers. 2.3. Characterization Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet is10 spectrometer (Thermo Fisher Scientific, USA). Each sample was scanned in the range of 500-4000 cm−1 with a resolution of 4 cm−1. Thermo-gravimetric analysis (TGA) was performed on a TG209 (Netzsch, Germany). 5



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Each sample was heated at a rate of 10 K/min from 25 °C to 500 °C under nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an Escalab 220i spectrometer (Kratos, Japan) with monochromatic Al Kα (hν=1100 eV) at 15 kV. Elemental analysis (EA) was performed on an EA3000 (Leeman Labs, USA), which was one of the most accurate technique for measuring the elemental compositions. X-ray diffraction (XRD) patterns were determined on an X'Pert Pro MPD DY129 diffractometer (Panalytical, Holland) using CuKα radiation (λ= 1.541 Ǻ). Each sample was recorded at a scanning rate of 4°/min in the 2θ range from 4° to 60°. Raman spectra (RM) were performed using a LabRAM HR multichannel confocal microspectrometer (Horiba, France) with a 532 nm excitation laser. Transmission electron microscopy (TEM) was carried out on an H600 microscope (Hitachi, Japan) at 75 kV. Scanning electron microscopy (SEM) images were collected using JSM-7500F scanning electron microscope (Jeol, Japan) at 15 kV, and the fractured surface was recorded. The strain-stress tests were conducted using a UTM 6203 universal testing machine (Suns, China). Each sample was cut with a dumb-bell shape and determined by ASTM D-638 standard at a crosshead speed of 100 mm/min. Differential scanning calorimetry (DSC) curves were obtained on a DSC 200PC (Netzsch, Germany) under nitrogen protection. Each sample was heated at a rate of 5 K/min from -80 °C to 80 °C. Dynamic mechanical analysis (DMA) curves were obtained on a DMA 242C (Netzsch, Germany) in the tensile resonant mode, at a heating rate of 3 K/min from -80 °C to 50 °C. The storage modulus (E') and loss factor (tan δ) were obtained at 1 Hz frequency. 2.4. Thermal conductivity Thermal conductivity experiments were performed according to the laser flash method.32-35 The thermal conductivity (k) was calculated by fundamental parameters including thermal diffusivity (α), specific heat capacity (Cp), and density (ρ), as shown in Equation (1).32 For this purpose, thermal diffusivity was conducted on a LFA467 laser flash apparatus (Netzsch, Germany). Both sides of the samples (10 × 10 mm, ~0.5 mm thick) were coated by spraying a thin layer of graphite before analysis. The specific heat capacity was measured using a DSC-204F (Netzsch, Germany) with sapphire as a standard material.36 Each sample was analyzed from 275 K to 375 K with a heating rate of 5 K/min. The density was determined by an FA1004J electronic density balance (Yueping, China) at 25 °C. 6


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(1)

k = ρα C p

where k, ρ, α, and Cp are the thermal conductivity, density, thermal diffusivity, and specific heat capacity from each experimental sample, respectively. The above values were obtained at ~25 °C.

3. Results and Discussion 3.1. Synthesis of functionalized graphene oxide The procedures for preparing urethane-functionalized graphene oxide (FGO) are shown schematically in Figure 1. Graphite powder was first completely oxidized to graphene oxide in the presence of KMnO4, and the resulting product was purified using hydrochloric acid and deionized water to remove residual impurities. Subsequently, the graphene oxide (GO) was reacted with TDI. Because of the steric hindrance of the TDI molecule, nucleophilic hydroxyl groups on the GO surface of preferentially reacted with the para-isocyanate group of TDI, and the ortho-isocyanate group remained partially unreacted.29 Finally, decorated GO was covalently reacted with PEG and extenders such as DMPA and EDA by an in situ polymerization.

7



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Figure 1. Schematic representation of synthesis route of urethane-functionalized graphene oxide.

3.2. Characterization of functionalized graphene oxide FT-IR was employed to confirm the covalent functionalization of GO by polyurethane polymer (PUP). Figure 2a shows the characteristic peaks of GO, FGO, and PUP. For the pristine GO sample, the peaks at 3425 cm-1, 1727 cm-1, 1623 cm-1, 1226 cm-1, and 1060 cm-1 were designated to the -OH, -C=O, C=C, -COO, and C-O-C (epoxy) stretching vibrations, respectively.13, 30, 37 After chemical functionalization, FGO showed two new peaks at around 2958 cm-1 and 2871 cm-1, corresponding to the symmetric and asymmetric stretching vibrations of C-H, respectively.38 Moreover, the peak at 8


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1727 cm-1 of GO was shifted to a lower wavenumber at 1705 cm-1 that could be caused by the carbamate esters of the FGO. Incidentally, the new peak at around 1646 cm-1 could be attributed to an amide carbonyl-stretching mode.27 Furthermore, three other new peaks appeared at 1539 cm-1, 1223 cm-1, and 1108 cm-1, which were assigned to the stretching vibrations of N-H, C-N, and C-O, respectively.27 The new peaks were corresponded to the relevant peaks of the PUP spectrum, corresponding to our previous work.28 It is noteworthy that the peak at 2281 cm-1 (-N=C=O) was not found in FGO,39 suggesting the isocyanate groups of TDI had completely reacted and covalently functionalized onto the surface of GO.

Figure 2. (a) FT-IR, and (b) TGA curves of pristine GO, FGO, and PUP.

TGA was conducted to verify the presence of a PUP layer on the FGO surface. Figure 2b displays the TGA curves of GO, FGO, and PUP, and the corresponding DTG curves are illustrated in Figure S1. As shown in Figure 2b, weight loss for both GO and FGO samples below 100 °C was mainly due to the evaporation of absorbed water.37 As the temperature increased, the main decomposition of GO was observed from 100 °C to 300 °C, resulting from the pyrolysis of some oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy groups.40,

41

After chemical

functionalization, FGO became more thermally stable than GO at 250-425 °C, indicating the urethane chains attached on the GO surface reduced the degradation rate of oxygen functional groups.30 Weight loss observed from 100 °C to 250 °C was assigned mainly to the degradation of residual oxygen-containing functional groups in the FGO. The other two main weight loss processes appeared in the temperature range of 250-325 °C and 325-425 °C, mainly attributed to the degradation of the hard- and soft- components of the grafted urethane chains, respectively.42 These 9



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results were also consistent with the tendency of PUP, suggesting the successful functionalization of GO with urethane chains. XPS analysis was performed to further characterize the chemical structure of pristine GO and FGO. Figure 3a presents the XPS survey spectra of GO and FGO, and both samples possessed obvious C1s (284.6 eV) and O1s (531.8 eV) peaks.43, 44 In comparison with GO, a new peak at 399.6 eV of N1s (also seen in Figure 3b) was appeared in the spectrum of FGO.45 This could be ascribed to the covalently bonded urethane chains on the FGO surface. Moreover, the FGO sample showed an increase in intensity of the C1s peak and a reduction in intensity of the O1s peak compared with pristine GO (Table 1), indicating the polyurethane copolymer containing alkyl chains was successfully reacted with the hydroxyl groups on the GO surface by chemical functionalization. Table 1 also displays that the C/O ratio of FGO is higher than GO, which also demonstrates that the modification reaction occurred.12 In addition to the XPS analysis, quantitative elemental analysis was also conducted to measure the elemental compositions of the prepared samples (Table 1). As expected, the variation tendency of the main C, O, and N contents from elemental analysis was consistent with the data of XPS results. Figure 3c and 3d present the deconvoluted XPS C1s peaks of pristine GO and FGO, respectively. As shown in Figure 3c, the main peaks of C1s in the GO sample around 284.6 eV, 286.6 eV, and 288.5 eV corresponded to the C-C carbon components, C-O bond components of hydroxyl, and C=O bond components of carboxyl, respectively.46 For the FGO sample, a new peak at 285.6 eV appeared, which could be attributed to the C-N bond of the urethane structure (Figure 3d).47 In comparison with GO, the C-O peak intensity of FGO decreased from 46.00% to 32.73%, while the C=O peak intensity increased from 3.16% to 4.94%, indicating the urethane chains bonded onto the GO surface.

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Figure 3. XPS spectra of pristine GO and FGO: (a) survey, (b) N1s of FGO, (c) C1s of GO, and (d) C1s of FGO.

Table 1. Element content of pristine GO and FGO from XPS and elemental analysis. XPS

EA

Samples C (%)

O (%)

N (%)

C/O

C (%)

O (%)

N (%)

H (%) C/O

GO

63.77

36.23

-

1.76

50.54

45.82

-

3.64

1.10

FGO

69.87

23.96

6.17

2.92

54.55

33.01

7.76

4.68

1.65

XRD was conducted to study the structural change between GO and FGO; XRD patterns of each are presented in Fig 4a. As expected, GO exhibited a broad diffraction peak at approximately 2θ=9.81°, corresponding to the (002) reflection of GO, which was almost identical with previous literature.37 Moreover, the interlayer spacing of GO was 9.20 Ǻ. Upon chemical functionalization, the 2θ peak of FGO was downshifted to 2θ=6.63°, giving an interlayer spacing of 13.33 Ǻ. The enlarged d-spacing could be assigned to the urethane chains successfully grafted onto the interlayer of GO sheets. It is noteworthy that the covalently bonded urethane chains on the FGO surface effectively prevented the sheets from restacking. Moreover, FGO exhibited a much sharper diffraction band, owing to the orientation of graphenic sheets,30 implying that thermal conductivity was significantly enhanced. Raman spectroscopy was employed to confirm the interaction between graphene sheets. Figure 4b presents the Raman spectra of pristine GO and FGO. GO exhibited two characteristic peaks around 1348 cm-1 (D-band) and 1583 cm-1 (G-band). A D-band around 1346 cm-1 and a G-band around 1582 11



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cm-1 were observed for FGO. The D-band was attributed to sp3 hybridized carbon-carbon bonds assigned to the presence of oxygen-containing groups on the carbon backbones, while the G-band was ascribed to sp2 hybridized carbon-carbon bonds in the 2D hexagonal lattice.30, 48 Consequently, the presence of a G-band reflected the disordered lattice structures of carbon components in the GO or FGO sheets.49 The D- and G-bands of FGO were transferred to lower wavenumbers than those of pristine GO, indicating the strong interactions between urethane chains and the FGO surface.50 Moreover, the ID/IG ratio of FGO showed a slight increase than that of pristine GO. The result could be ascribed to the reduction of oxygen percentage for the FGO sample, similar to the results obtained by XPS and EA analyses (Table 1).

Figure 4. (a) XRD, and (b) Raman spectra of pristine GO and FGO.

From macroscopical photograph, the color of the GO aqueous dispersion changed from yellow brown to black after chemical functionalization (inset, Figure 1), due to the partial reduction of GO.51 That is to say, the reinforcing effect of the functionalized GO in polymeric composites may be enhanced. Additionally, TEM was carried out to provide direct observation of microscopic morphology of the pristine GO and FGO in an aqueous medium. Figure 5a and 5b illustrate that the pristine GO was wrinkled and almost transparent, and its surface was fairly smooth and clean. FGO exhibited a comparatively rough surface and were coated by a large quantity of polymeric substances (Figure 5c and 5d), coming from the polyurethane component. This phenomenon could be explained by the covalent functionalization of FGO by urethane chains, as confirmed by the FT-IR, TGA, and XPS results.

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Figure 5. Representative TEM images of (a, b) GO, and (c, d) FGO.

In summary, we first prepared graphene oxide via a modified Hummer's method, covalently functionalized the surface with urethane chains, and then analyzed the surface composition and structure using FT-IR, TGA, XPS, EA, XRD, Raman, and TEM. Significantly, the FGO possessed a relatively large structure with abundant urethane chains (proton donors and receptors) on its surface and enabling it to disperse well in the polyurethane matrix. It is reasonable that the FGO sheets could be used as a two-dimensional filler to enhance the performance of waterborne polyurethane binder (PUB) composite film. In order to investigate the compatibility between the FGO and the PUB matrix, and its promising applications in functional coatings, the morphology, mechanical properties, and especially thermal conductivity of composite films were systematically characterized and discussed.

3.3. Characterization of composite films The performance of composites depends on the dispersibility and compatibility of the fillers with the polymer matrix.29 Figure 6 presents SEM micrographs of PUB/GO, PUB/FGO1, and PUB/FGO2 composite films. The PUB/GO composite film exhibited relatively coarse surfaces (Figure 6a and 6d) and large aggregates (Figure 6g), owing to the incompatibility between pristine GO and the PUB matrix. By contrast, the surfaces of PUB/FGO1 and PUB/FGO2 films were much smoother (Figure 6b and 6c), demonstrating the FGO fillers could be uniformly dispersed in the PUB matrix. In addition, the urethane groups attached to the FGO sheets could supply more active-site to form hydrogen bonds, thus significantly enhancing the compatibility of FGO with the PUB matrix. Meanwhile, at a higher magnification (Figure 6e, 6f, 6h, and 6i), the roughness of cross-section 13



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surfaces was observed to increase slightly with increasing FGO content, and the PUB/FGO1 film containing 0.5 wt.% FGO possessed a much smoother surface. Below, we will find out what this implies.

Figure 6. Representative SEM micrographs of freeze-fractured surfaces of (a, d, g) PUB/GO, (b, e, h) PUB/FGO1, and (c, f, i) PUB/FGO2 composites.

In general, the glass transition temperatures (Tg) value of composites is dependent on both the dispersion level of the fillers and their adhesion to the polymer matrix.52 The effect of GO or FGO sheets in composite films on the Tg was investigated by DSC. Figure 7 presents the DSC curves of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composite films. The Tg of neat PUB was approximately -57.3 °C, while those of PUB/GO, PUB/FGO1, and PUB/FGO2 composite films were approximately -54.8 °C, -48.3 °C, and -48.6 °C, respectively. Such increments in Tg may be attributed to the introduction of graphene sheets, indicating the PUB segments near the GO and FGO surfaces were confined.53 Significantly, the amplitude change of Tg for PUB/FGO1 film (~9 °C) was much greater than that of PUB/GO film (~2.5 °C), due to the covalently grafted urethane chains on 14


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the FGO surface which could improve the interactions between the FGO and the PUB matrix. In addition, the Tg of the composite remained practically unchanged when the content of FGO further increased from 0.5 wt.% to 1.0 wt.%. This phenomenon may be attributed to the combined action of the confinement effect of functional graphene sheets and their lubrication effect at higher content.54, 55

These results suggest that the PUB segments were effectively confined even in the presence of 0.5

wt.% FGO, resulting in better compatibility between the two phases, similar to the observation from SEM micrographs (Figure 6). The compatibility or interfacial interaction of FGO sheets with the PUB matrix are illustrated schematically in Figure 7b. In addition, the strong interfacial interaction between FGO and the PUB matrix may help to increase the mechanical property and thermal transport capacity of PUB composites.

Figure 7. (a) Differential scanning calorimetry (DSC) thermograms of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composites; (b) schematic showing FGO sheets in PUB matrix.

The interfacial interaction between GO sheets and the PUB matrix was further confirmed by DMA measurement. Figure 8 presents the loss factor (tan δ) and storage modulus (E') versus temperature curves of the neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composite films. As shown in Figure 8a, the Tg peaks of PUB/GO (-34.9 °C), PUB/FGO1 (-30.1 °C), and PUB/FGO2 (-29.5 °C) films were shifted to higher temperatures as compared to the neat PUB (-35.1 °C), similar to the tendency shown by DSC curves. The variations in Tg values from DMA measurement can also be explained by the abovementioned reasons for the DSC results. Additionally, from the experimental point of view, DMA exhibited higher Tg values than DSC did, as reported previously.56 The main reason is that DSC is sensitive to the variable specific heat associated with Tg, while DMA is sensitive to 15



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mechanical relaxation, also associated with Tg.57 It is concluded that DMA is more sensitive to the glass transition phenomenon than the DSC employed in this work (i.e. Tg, DMA > Tg, DSC). As shown in Figure 8b, the PUB/GO, PUB/FGO1, and PUB/FGO2 composite films possessed higher storage modulus (E') than neat PUB through most of the test temperature range, indicating GO or FGO sheets could effectively improve the rigidity of the PUB matrix. Moreover, the E' of PUB/FGO1 was comparable with the E' of PUB/GO and PUB/FGO2 films below Tg, but close to the E' of neat PUB film above Tg. It makes senses that the PUB/FGO1 film was endowed with better rigidity and toughness, mainly due to the excellent dispersion of FGO sheets (0.5 wt.%) in PUB matrix and the relatively strong interfacial interaction between them.

Figure 8. (a) Loss factor (tanδ), and (b) storage modulus (E') at 1 Hz of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composites.

Figure 9a displays the representative stress-strain curves of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composite films; the relevant tensile strength and elongation at break are shown in Figure 9b. The tensile strength and elongation at break of the neat PUB film were 15.8 MPa and 731.6%, respectively. After introduction of GO or FGO into the PUB matrix, the mechanical properties of these composites were improved to some extent, except for the elongation at break of the PUB/GO film. Such reinforcement indicates an effective load transfer from the PUB matrix to graphene sheets under external stress.13 The most efficient improvement was achieved for the PUB/FGO1 film containing 0.5 wt.% FGO. Its tensile stress and elongation at break were 19.5 MPa and 820.2%, respectively, corresponding to increases by 23.4% and 12.1%, respectively, compared with the neat PUB film. By contrast, the PUB/GO composite with the same loading of GO (0.5 wt.%) 16


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only showed a 1.9% increase in tensile stress and (unexpectedly) a 5.5% decrease in elongation at break. This might be ascribed to the high dispersity of the FGO in the PUB matrix and the strong interfacial interaction between the two phases. However, further increment of FGO sheets into the PUB matrix resulted in a limitation of mechanical parameters, which were attributed to the aggregation as well as the lubrication effect exerted by FGO sheets.55 This phenomenon was also similar to some previous studies.53, 58

Figure 9. (a) Representative stress-strain curves, and (b) mechanical data of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composites. 3.4. Thermal conductivity Thermal conductivity is one of the most important parameters for functional coatings. In this work, laser flash method was carried out to determine thermal conductivity and relevant thermal properties of the prepared films. Figure 10 presents the thermal conductivity of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composite films. The corresponding fundamental parameters including thermal diffusivity, specific heat capacity (see Supporting Information, Figure S2), and density are listed in Table S1. The thermal conductivity of the neat PUB film was about 0.187 W/mK, which was similar to published values (~0.18 W/mK)59, 60. With the addition of 0.5 wt.% GO, 0.5 wt.% FGO, and 1.0 wt.% FGO to neat PUB, the thermal conductivity values increased by 10.7%, 61.5%, and 29.9%, respectively. It is well known that phonon conduction directly affects the thermal conductivity of materials.18 This increment may therefore be ascribed to the high thermal conductivity of the lamellar structure of graphene sheets (which have an experimental thermal conductivity of ~5000 W/mK7 when defect-free) that provided a route of lower resistance for phonons to spread. This result could also be explained by the high aspect ratio and large surface area 17



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of graphene sheets, and the interfacial interaction between filler and matrix.61, 62 Significantly, the thermal conductivity of PUB/FGO1 film was higher than that of PUB/GO film at the same loading (0.5 wt.%). One reason is that the plentiful urethane chains attached on the FGO sheets could achieve better compatibility with the PUB matrix, improving the interfacial interaction between FGO and PUB matrix, and thus decreasing its thermal resistance. Another reason is the fact that the covalently bonded urethane chains as a transition medium may alleviate the modulus mismatch between GO and PUB matrix, which could decrease the thermal resistance at the interface, therefore improving phonon transmission.63, 64 Surprisingly, adding further FGO did not exhibit higher thermal transport improvements, but rather brought a decrease of thermal conductivity (Table S1). This might be attributed to the aggregation of FGO sheets, leading to a reduction of lattice vibration or phonon propagation. These results indicate that the thermal conductivity of PUB film could be effectively improved at the loading of merely 0.5 wt.% FGO and yield higher thermal conductivity for PUB/FGO1 composites.

Figure 10. Thermal conductivity of neat PUB, PUB/GO, PUB/FGO1, and PUB/FGO2 composites.

4. Conclusions In this work, we first prepared functionalized graphene oxide (FGO) with urethane chains by an in situ polymerization method, and then incorporated FGO into a waterborne polyurethane binder (PUB) matrix to fabricate composites. The successful functionalization was demonstrated by FT-IR, TGA, 18


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XPS, EA, XRD, Raman, and TEM measurements. After chemical functionalization with urethane chains, the quantity of oxygen-containing functional groups on the GO sheets was reduced. Concomitantly, the FGO possessed a comparatively regular lamellar structure and its interlayer distance was also enlarged. In addition, covalently grafted urethane chains on the FGO sheets not only alleviated the modulus mismatch between the FGO and the PUB matrix, but also improved the dispersion of FGO in the PUB matrix and the compatibility between them. The introduction of only 0.5 wt% FGO to the PUB matrix significantly enhanced the mechanical and thermal transport properties of polyurethane composite film, which increased in tensile strength, elongation at break, and thermal conductivity by 23.4%, 12.1%, and 61.5%, respectively, in comparison with the neat PUB film. We expect that such composites will have applications in functional coatings and other fields.

Supporting Information Detailed description of chemicals or reagents, and their purification methods; DTG curves of GO samples; specific heat capacity curves of PUB composites; relevant data of thermal properties of PUB composites.

Acknowledgement This work was financially supported by the Sichuan Province Science and Technology Support Program (2012FZ0013).

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Urethane-functionalized graphene oxide for improving compatibility

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