Intercalation behavior and orientation structure of graphene oxide/polyethylene glycol hybrid material

Abstract

A series of graphene oxide/polyethylene glycol (GO/PEG) hybrid materials with different PEG molecular weights and contents were prepared, and the intercalation behavior and orientation structure were studied. It was found that compared with the XRD pattern of GO, the diffraction peaks of the GO/PEG hybrids shifted to lower angles due to the existence of PEG. With the increase of the molecular weight of PEG, the interlayer spacing of the hybrids first increased, reaching the maximum for the GO/PEG-20 000 hybrid, and then decreased. A similar variation trend can be observed for the hybrids with different PEG-20 000 contents, and the interlayer spacing and intercalation ratio reached the maximum for GO/80 PEG-20 000 sample. Remarkable red-shift of the O–H stretching vibration suggested the existence of hydrogen bonding interactions between the PEG-20 000 molecules and GO layers, leading to the enhancement of the thermal stability of GO. The intercalated PEG-20 000 molecules took a trans conformation and adopted an orderly structure of a monolayer arrangement as zigzag chains in the confined GO layers, while the skeletal structure of GO can be kept in the hybrids after intercalation. AFM and TEM analysis showed that the edges of GO/PEG-20 000 hybrids wrinkled more seriously than that of GO, and the maximum increased interlayer spacing (Δd) was approximately 0.4–0.5 nm, which agreed well with the XRD result.

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1. Introduction

Recently, increasing efforts have been made for the synthesis and improved properties of the polymer-intercalated layered materials which are expected to show new physical and chemical properties different from their starting materials such as catalytic performance, charge storage, ionic conductivity properties and so on.^1–7 Graphene oxide (GO) with a typical two-dimensional hexagonal arrangement of sp² carbon atoms, has received worldwide interest because of its unique properties, including excellent mechanical strength, and electrical and thermal conductive properties. There are abundant hydroxyl, carboxyl and epoxy functional groups on the GO layers,⁸ which make them easily absorb polar molecules and polar polymers by different means and react with them to form GO-intercalated nano-composites or exfoliated nano-composites.^9,10 And thus GO becomes a good choice for the design of lightweight intercalated materials to obtain wear-resistance, thermal, electrochemical and optical^11–13 properties, etc., which has received increasing attention in academia and industry. Moreover, the properties of the interlayer of GO such as polarity, size and free volume are important factors to determine the mobility of the introduced molecules and how many guest molecules can be introduced into the layers. Besides, polymer guests intercalated in the host layers of GO can serve as a geometric impediment to the agglomeration of exfoliated GO layers due to van der Waals interactions, and the unique properties of GO can be achieved.¹⁴

Polyethylene glycol (PEG) with flexible structure of C–O–C bonds is an important kind of thermoplastic polymer with high crystallinity, hydrophilicity, solubility in water and organic solvents and self-lubricating property. Additionally, PEG is a favorable candidate for the development of batteries and sensors as solid polymer electrolytes.^15,16 When its molecular weight (M_n) reached 20 000, polyethylene oxide (PEO) can be named. In terms of GO/PEG hybrid materials, strong hydrogen interaction exists between ether groups in PEG molecular chains and oxygen-containing groups on GO layers. The opportunity to combine PEG and GO appears as an attractive way to develop new organic–inorganic hybrid materials endowed with properties that are inherent to both types of components to obtain the synergistic effect in nanocomposites. Matsuo et al.⁴ prepared PEO-intercalated GOs via alkali solution mixing and studied the thermal decomposition process at various temperatures by DSC and XRD analysis. Li et al.¹⁵ prepared PEO/graphene composites by melt mixing, and DSC analysis showed that the PEO chains penetrated into the graphene layers were amorphous. However, few literatures can be available on the intercalation structure of GO/PEG hybrid materials with different PEG molecular weights and contents. The molecular weight and chain length of PEG could not only influence its intermolecular interaction with GO, but also influence the diffusion and intercalation behavior of PEG molecules into GO layers. Meanwhile, it is necessary to find the saturated intercalation ratio of PEG in the confined GO layers.

In our work, a series of GO/PEG hybrid materials were prepared by ultrasonic dispersion in an aqueous medium. The intercalation behavior and orientation structure of the GO/PEG hybrid materials with different PEG molecular weights and contents were systematically investigated in terms of intermolecular interaction, confined layered structure and intercalation morphology. The obtained hybrids are expected to be used as PEG-based polymer electrolytes to improve the electrical conductivity of PEG at the temperature below its melting point. Moreover, the GO/PEG hybrids can be used as lubricating materials to improve the wear-resistance and self-lubricating property of polymer materials.

2. Experimental

2.1 Materials

Graphene oxide (GO) powder with a particle size of micron grade was supplied by the Sixth Element Materials Technology Co. Ltd. (Changzhou, China). A series of analytical grade polyethylene glycol (PEG) with different molecular weights (PEG-X, X donated as M_n of PEG, M_n = 400–100 000) were purchased from the Kelong Chemical Reagents Co. Ltd. (Chengdu, China), and used as received without further purification. All experiments were carried out using deionized water.

2.2 Preparation of GO/PEG hybrid materials

The preparation procedure for the GO/PEG hybrid materials was carried out as follows: 0.5 g of GO was dispersed into 200 ml of deionized water under ultrasound for 30 min at room temperature, and in this process the stable GO/H₂O dispersion solution formed. Then PEG was added and the mixture solution was sonicated for another 1 h. Afterwards GO/PEG dispersion solution was centrifugated and washed with distilled water to remove the free PEG molecules. The resulting solids were freeze-dried overnight, finally yielding GO/PEG hybrids. The preparation process of GO/PEG hybrid materials was illustrated in Fig. 1.

Fig. 1 Preparation process of the GO/PEG hybrids via intercalation in an aqueous medium.

2.3 Measurements and characterization

The composition of GO/PEG hybrid materials were analyzed with a Nicolet-560 Fourier-transform infrared spectroscopy (FTIR) (USA). The scanning rate was 20 scans per s, and the resolution was 4 cm⁻¹. The pellet samples were prepared by pressing a mixture of the dried powder sample of the GO/PEG hybrids and KBr powder.

The interlayer spacing of the samples of GO and GO/PEG hybrids were measured at room temperature over the scanning range of 2θ = 5–40° with RigakuD/max III B X-ray diffraction equipment (Japan). Cu K_α radiation (λ = 0.154 nm) was used at a generator voltage of 40 kV and current of 35 mA. The d-spacing of the GO and GO/PEG hybrids were calculated with the Bragg equation:

2d sin θ = nλ (1)

where θ is the diffraction angle; n is the order of diffraction and λ is the incident wave length.

The thermogravimetric analysis (TGA) was used to characterize the thermal stability and intercalation ratio of GO/PEG hybrids. TGA was performed with a TA2950 thermobalance from TA Co. (USA) under nitrogen atmosphere with the flow rate of 50 ml min⁻¹. The granulated samples of about 10 mg were heated from ambient temperature to approximately 800 °C at a heating rate of 10 °C min⁻¹.

The skeletal structure of GO and obtained GO/PEG hybrid materials were investigated by Raman spectroscopy. Raman spectra was recorded from 1100 to 1800 cm⁻¹ on a Renishaw Invia Raman Microprobe (Britain) using a 514.5 nm argon ion laser.

The surface morphologies and thickness of GO and the as-synthesized GO/PEG hybrid materials were examined by atomic force microscopy (AFM). The AFM measurements were performed with a SPM-9700 Scanning Probe Microscope (Japan) in a tapping mode from Digital Instruments with a Nanoscope IV controller. Samples for AFM imaging were prepared by drop-casting the GO and GO/PEG dispersions onto freshly cleaved mica substrates, which were then allowed to dry in air at ambient temperature and pressure.

Transmission electron microscopy (TEM) analysis was carried out with a JEOL JEM 100CX II TEM equipment (Japan) operated at an acceleration voltage of 200 kV to observe the structure morphology of the samples of GO and GO/PEG hybrids. TEM sample was prepared by dropping the GO and GO/PEG dispersion onto 200-mesh copper grids.

3. Results and discussion

3.1 Effect of PEG molecular weight on the interlayer spacing of GO/PEG hybrids

The XRD patterns of GO and GO/PEG hybrids with different PEG molecular weights were illustrated in Fig. 2, and the characteristic parameters were listed intuitively in Table 1. The sharp characteristic peak for the (001) plane of GO could be observed at 2θ = 12.23°, corresponding to the interlayer spacing of ∼0.7228 nm. It is significantly larger than that of pristine graphite (2θ = 26.6°, corresponding to a d-spacing of 0.336 nm), due to the intercalation of oxide functional groups. Whereas the XRD patterns of GO/PEG hybrids were obviously different from that of GO. The diffraction peaks due to the existence of PEG shifted to lower angles and were observed at 2θ = 11.04°, 9.43°, 8.38°, 8.39°, 7.64° and 8.22° corresponding to the samples of GO/PEG hybrids with PEG-400, PEG-2000, PEG-6000, PEG-10 000, PEG-20 000 and PEG-100 000, respectively. With the increase of molecular weight of PEG, the interlayer spacing of GO/PEG hybrid materials first increased, reaching the maximum for GO/PEG-20 000 hybrid, and then decreased. Meanwhile, compared with GO, the relative intensity of the characteristic peak for GO/PEG hybrid materials decreased with the increase of PEG molecular weight, resulting from the destruction of orderly structure of GO due to the intercalation of PEG molecules into the GO layers. In addition, a new broad peak appeared at 2θ = 15–23° for the samples with PEG molecular weight from 2000 to 10 000, corresponding to the disorganization peak of PEG. However, it almost disappeared for GO/PEG-20 000 and GO/PEG-100 000 as a result of the strong interactions between PEG molecules and GO layers and entanglement of PEG chains, which prevent the orderly arrangement and crystallization of PEG molecular chains. The result suggested that interlayer structure of GO was indeed influenced by PEG molecules.

Fig. 2 XRD patterns of GO/PEG hybrids with varying PEG molecular weight and the corresponding samples.

Table 1 XRD parameters of GO and GO/PEG hybrids with varying PEG molecular weight

Sample	Angle (2θ/°)	Interlayer spacing (d/nm^−1)	Intensity (a.u.)
GO	12.23	0.7228	683 946
GO/PEG-400	11.04	0.8005	465 900
GO/PEG-2000	9.43	0.9367	464 629
GO/PEG-6000	8.38	1.0539	379 201
GO/PEG-10 000	8.39	1.0526	300 182
GO/PEG-20 000	7.64	1.1558	136 497
GO/PEG-100 000	8.22	1.0743	68 933

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XRD result demonstrated that molecular weight of PEG had an influence on the intercalation behavior and longer PEG molecular chains were beneficial for PEG molecules intercalation into the GO layers. However, excessively high molecular weight of PEG made the dispersion solution too viscous, resulting in the restriction of the movement of PEG chains and the impediment of intercalation behavior. Moreover, GO/PEG hybrids became light-colored, which confirmed the strong interaction between PEG molecules and GO.

3.2 Intercalation structure of GO/PEG-20 000 hybrid materials

3.2.1 Intermolecular interaction and intercalation ratio of GO/PEG-20 000 hybrid. The structural features of GO/PEG-20 000 hybrids with various PEG-20 000 contents were characterized by FT-IR, as shown in Fig. 3, together with those of GO and PEG-20 000. For GO/PEG-20 000 hybrid materials, the added content of PEG-20 000 was 20 wt%, 40 wt%, 60 wt%, 80 wt%, 100 wt%, 300 wt% and 500 wt% of GO respectively, which was denoted as GO/X₁ PEG-20 000 (X₁ was the weight percent of PEG-20 000 in GO). For GO sample, a strong and broad absorption at 3406 cm⁻¹ was attributed to O–H stretching vibrations. The characteristic stretching adsorption bands around 1727 cm⁻¹ corresponded to the carbonyl groups (C=O) in the COOH units situated at the edges of the GO sheets,¹⁷ whereas the bands at 1050 cm⁻¹ and 1223 cm⁻¹ were attributed to C–O or epoxy groups (C–OH/C–O–C). In addition, the peak at 1618 cm⁻¹ can be attributed to the O–H bending vibration, or C=C skeletal ring vibrations of unoxidized sp² carbon domain.^18,19 These vibrations revealed the presence of oxygen containing functional groups on the GO surface. For PEG-20 000 sample, the absorption peak at 3468 cm⁻¹ was attributed to O–H stretching vibrations. The bands at 2947 cm⁻¹, 2888 cm⁻¹ and 1470 cm⁻¹, 1342 cm⁻¹ were related to the asymmetric, symmetric stretching vibration and bending vibration of –CH₂–, respectively. The characteristic peaks at 960 cm⁻¹ and 1101 cm⁻¹ were assigned to symmetric and asymmetric vibration of C–O–C, respectively. For GO/PEG-20 000 hybrids, it was found that all samples exhibited characteristic absorption peaks of PEG-20 000 and GO, indicating that PEG-20 000 molecular chains were intercalated into GO layers.

Fig. 3 FT-IR spectra of (a) GO (b) GO/20 PEG-20 000 (c) GO/40 PEG-20 000 (d) GO/60 PEG-20 000 (e) GO/80 PEG-20 000 (f) GO/100 PEG-20 000 and (g) PEG-20 000.

Interestingly, compared with GO sample, the O–H stretching vibration of GO/PEG-20 000 hybrids red-shifted to lower wavenumber, suggesting that the hydrogen bonding interactions existed between PEG-20 000 molecules and GO layers. It should be mentioned that, with increasing PEG-20 000 content, the wavenumber corresponding to O–H vibration first decreased, and then increased, reaching the minimum for GO/80 PEG-20 000 sample. It red-shifted significantly from 3406 cm⁻¹ to 3371 cm⁻¹, indicating the presence of the strongest hydrogen bonding interaction, as shown in Fig. 4. The result was in agreement with XRD analysis above.

Fig. 4 The shift variation of O–H group of GO/PEG-20 000 hybrids in the vicinity of 3400 cm⁻¹.

Fig. 5 shows the typical Raman spectroscopy of GO and GO/PEG-20 000 hybrids with various PEG-20 000 contents. For GO, two characteristic peaks appeared at around 1360 and 1588 cm⁻¹, corresponding to the D band and G band, which were ascribed to structure defects and the first-order scattering of the E_2g vibration mode, respectively.^20,21 For GO/PEG-20 000 hybrids, the D and G band were observed at ∼1358 and ∼1592 cm⁻¹, and the slight shift of the G band should be attributed to the doping of PEG-20 000 molecules in comparison with GO,²² indicating the interaction between PEG-20 000 molecules and GO layer. The intensity ratio of I_D/I_G is the measure of the degree of disorder and the average size of the sp² domains. Meanwhile, the I_D/I_G value decreased slightly, and the skeletal structure of GO can be kept in the GO/PEG-20 000 hybrid materials after intercalation. PEG-20 000 intercalated in the GO layers led to the decrease of the relative content of sixfold aromatic rings of GO and reduction of its intensity of the D band.²³

Fig. 5 Raman spectra of GO and GO/PEG-20 000 hybrids with varying PEG-20 000 content.

The TGA was used to further characterize the thermal properties and the intercalation ratio of GO/PEG-20 000 hybrid materials with varying PEG-20 000 content, as shown in Fig. 6. It is noteworthy that, an improved initial thermal loss temperature and a decreased decomposition rate can be achieved for the GO/PEG-20 000 hybrids. GO showed a weight loss of 10% below 100 °C, which was attributed to the removal of the water that was held in the interlayer. A second large weight loss of 53.68% was observed in the range of 160–800 °C due to the removal of functional groups of –OH, –COOH from GO layer, leaving residual carbon. In contrast, obvious weight loss after 200 °C was observed for GO/PEG-20 000 hybrids and carbon residue ratio was improved. Introduction of PEG-20 000 into GO layers was of significance in enhancement of the thermal stability of GO due to the building-up of strong hydrogen bonding interactions.²⁴

Fig. 6 TGA curves of GO and GO/PEG-20 000 hybrids with varying PEG-20 000 content.

Based on the above analysis, assuming that the increased residual carbon content for GO/PEG-20 000 hybrids is due to the intercalated PEG-20 000, the mass percent of the intercalated PEG-20 000 molecules in the unit mass of GO (per gram) can be calculated according to the TGA data.

M₁ = M₀W_{GO/PEG-20 000} − (M₀ − M₁)W_GO (2)

that is,

D₂ = M₁/(M₀ − M₁)100% (4)

where W_GO and W_{GO/PEG-20 000} are the residual ratio of thermal loss of GO and GO/PEG-20 000, respectively, M₁ is the mass of PEG-20 000 intercalated in the GO layers, M₀ is the mass of the added GO/PEG-20 000 hybrid, and D₂ is the intercalation ratio of PEG-20 000 molecules for per gram of GO.

The intercalation ratio (D₂) of PEG-20 000 molecules in GO layers calculated according to eqn (4) was shown in Table 2. With increasing added content of PEG-20 000, the intercalation ratio (D₂) of GO/PEG-20 000 hybrid materials first increased, and then decreased, reaching the maximum and saturation for GO/80 PEG-20 000 sample, as high as 30.17%. Similarly, excessive addition of PEG-20 000 molecules made the viscosity of the dispersion solution increase due to entanglement of PEG-20 000 molecular chains, resulting in the impediment of intercalation behavior.

Table 2 The intercalation ratio of PEG-20 000 in the GO layer of GO/PEG-20 000 hybrids with varying PEG-20 000 content^a

Samples	GO	GO/20 PEG-20 000	GO/40 PEG-20 000	GO/60 PEG-20 000	GO/80 PEG-20 000	GO/100 PEG-20 000	GO/300 PEG-20 000	GO/500 PEG-20 000
a Note: D1 is the added content of PEG-20 000, W is the residual ratio of thermal loss of the samples.
D1 (%)	—	20	40	60	80	100	300	500
W (%)	36.32	41.30	43.53	46.35	51.08	42.27	41.84	42.95
D2 (%)	—	8.48	12.77	18.69	30.17	10.31	9.49	11.62

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3.2.2 Formation of a confined layered structure of GO/PEG-20 000. The interlayer spacing of GO layers in pristine GO and GO/PEG-20 000 hybrids with varying PEG-20 000 content were determined by XRD, as shown in Fig. 7. The corresponding characteristic parameters based on the XRD results were listed in Table 3. Compared with the XRD pattern of GO sample, the diffraction peaks of GO/PEG-20 000 hybrids shifted to smaller angles and were observed at 2θ = 8.78°, 8.56°, 8.32°, 7.79°, 8.80°, 8.78° and 8.79°, corresponding to the samples of GO/20 PEG-20 000, GO/40 PEG-20 000, GO/60 PEG-20 000, GO/80 PEG-20 000, GO/100 PEG-20 000, GO/300 PEG-20 000 and GO/500 PEG-20 000, respectively. Accordingly, the interlayer spacing reached the maximum for GO/80 PEG-20 000 sample.

Fig. 7 XRD patterns of GO and GO/PEG-20 000 hybrids with varying PEG-20 000 content and the corresponding samples.

Table 3 XRD parameters of GO and GO/PEG-20 000 hybrids with varying PEG-20 000 content

Sample	Angle (2θ/°)	Interlayer spacing (d/nm)	Increased interlayer spacing (Δd/nm)
GO	12.23	0.7228	—
GO/20 PEG-20000	8.78	1.0059	0.2831
GO/40 PEG-20 000	8.56	1.0317	0.3089
GO/60 PEG-20 000	8.32	1.0615	0.3387
GO/80 PEG-20 000	7.79	1.1335	0.4107
GO/100PEG-20 000	8.80	1.0037	0.2809
GO/300 PEG-20 000	8.78	1.0059	0.2831
GO/500 PEG-20 000	8.79	1.0048	0.2820

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Relative to the interlayer spacing of GO (∼0.72 nm), the increase of the interlayer spacing corresponded to the thickness of the intercalated PEG-20 000 chains (Δd). It can be seen that with the increase of PEG-20 000 content, the Δd values first increased, reaching the maximum for GO/80 PEG-20 000 sample, then decreased, and finally remained constant. Likewise, GO/PEG-20 000 hybrids became light-colored with increasing PEG-20 000 content due to the strong interaction between PEG-20 000 molecules and GO layers.

It is necessary to identify the conformation of PEG-20 000 molecular chains in the GO layer. It was well established that the –CH₂– deformation bands in PEG molecules revealed the trans or gauche conformations, corresponding to the FT-IR absorption peak at 1342 cm⁻¹ and 1360 cm⁻¹, respectively.^25,26 For GO/PEG-20 000 hybrids with different PEG-20 000 contents, the absorption peak at 1345 cm⁻¹ due to the deformation mode of a –CH₂– group was observed, suggesting that the –O–CH₂–CH₂–O– groups of PEG-20 000 took a trans conformation and the PEG molecules existed in the confined GO layers²⁷ as a zigzag chain, as shown in Scheme 1. It can be seen that the vertical height of PEG-20 000 molecular chains is about 0.375 nm.

Scheme 1 (a) Molecular structure of PEG-20 000 chain segment and (b) trans conformation of intercalated PEG-20 000.

The gallery expansion obtained for GO/80 PEG-20 000 was 0.4107 nm, larger than the vertical height value of PEG-20 000 molecular chains, but much smaller than double vertical height value of PEG-20 000 molecular chains. And PEG-20 000 chains can be speculated to adopt a monolayer arrangement as a zigzag chain in the interlayer of GO, as shown in Scheme 2. However, the gallery expansion obtained for the other intercalated samples was smaller than the vertical height of PEG-20 000 molecular chain, indicating that PEG-20 000 molecules existed in the form of curl, twist or rotation chains in the GO layer instead of the vertical monolayer orientation.

Scheme 2 Orientation of PEG-20 000 molecular chain in the GO layers for GO/80 PEG-20 000 hybrid.

3.2.3 Intercalation morphology of GO/PEG-20 000 hybrid materials. Fig. 8 showed the typical AFM height images and their section line analyses of GO and GO/PEG-20 000 hybrids. Generally, most GO individual nano-sheet usually had an average thickness of 0.7–1.2 nm due to the epoxy, carboxyl and hydroxyl groups on both sides. However, in this work the height of the GO sheet was measured to be ∼1.48 nm, which was attributed to water molecules trapped in the interlayer of GO and folding on the edge of GO sheets.²⁸ For GO/PEG-20 000 hybrids, the thickness was in the range of 1.7–1.9 nm. It first increased, reaching the maximum value of ∼1.93 nm for GO/80 PEG-20 000, and then decreased with increasing PEG-20 000 content. The increased thickness (Δd) of GO/PEG-20 000 hybrids agreed well with the increased interlayer spacing calculated from XRD data.

Fig. 8 Typical tapping-mode AFM images of GO (a) GO/20 PEG-20 000 (b) GO/40 PEG-20 000 (c) GO/60 PEG-20 000 (d) GO/80 PEG-20 000 (e) and GO/100 PEG-20 000 (f), and their section line analyses.

TEM was conducted to further characterize the exact structure morphologies of GO and GO/PEG-20 000 hybrids in the dispersion state, as illustrated in Fig. 9. Large and transparent GO sheets were clearly observed, and the sheets edges scrolled and folded slightly (Fig. 9a), resulting from thermodynamical stabilization via bending,^29,30 as a part of the intrinsic nature of GO sheets. From the image of Fig. 9a′ with high magnification, it can be observed that multi-layer GO sheets stacked together and the interlayer spacing of 2 layers of GO sheets was ∼0.77 nm, which was in consistence with XRD data above. For GO/PEG-20 000 hybrids, the microstructure of the GO sheets was not destroyed by intercalation. However, the edges of GO/PEG-20 000 hybrids wrinkled more seriously than that of GO (Fig. 9b–f), which was due to strong interactions between PEG-20 000 molecules and GO sheets. Specially, Fig. 9e′ revealed that the interlayer spacing for GO/80 PEG-20 000 was ∼1.27 nm and the increased thickness (Δd) was 0.5 nm in comparison with that of pristine GO, which was due to the intercalation of PEG-20 000 molecules into the GO layers. Similar result was consistent with XRD and AFM measurement.

Fig. 9 TEM images of GO (a and a′) GO/20 PEG-20 000 (b) GO/40 PEG-20 000 (c) GO/60 PEG-20 000 (d) GO/80 PEG-20 000 (e and e′) and GO/100 PEG-20 000 (f).

4. Conclusions

The GO/PEG hybrid materials were prepared with the intercalation method assisted by ultrasound in an aqueous medium. XRD result showed that with the increase of molecular weight of PEG, the interlayer spacing of GO/PEG hybrid materials first increased, reaching the maximum for GO/PEG-20 000 hybrid, and then decreased. FT-IR analysis showed that a remarkable red shift of O–H stretching vibration absorption peak from 3406 cm⁻¹ of GO sample to 3371 cm⁻¹ of GO/80 PEG-20 000 sample can be observed, indicating that strong hydrogen bonding interactions formed between PEG-20 000 molecules and GO sheets. Raman spectra showed that the skeletal structure of GO was still kept in the GO/PEG-20 000 hybrid materials after intercalation. TGA revealed that with increasing added content of PEG-20 000, the interlayer spacing and the intercalation ratio (D₂) of PEG-20 000 molecules in the GO layers first increased, and then decreased, reaching the maximum for GO/80 PEG-20 000 sample. Meanwhile, the intercalated PEG-20 000 molecular chains took trans conformation and adopted a monolayer arrangement as a zigzag chain in the interlayer of GO. AFM and TEM analysis showed that the maximum increased thickness (Δd) was approximately 0.4–0.5 nm corresponding to GO/80 PEG-20 000 sample, which was in good agreement with the XRD result. Besides, TEM analysis suggested that the edges of GO/PEG-20 000 hybrids wrinkled more seriously than that of GO due to strong interactions between PEG-20 000 molecules and GO sheets.

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