Preparation and Properties of Graphene/ODA-PMDA-NTADA Copolyimide Nanocomposites Based onKH550-modification

In this study, KH550-modified graphene/ODA-PMDA-NTADA copolyimide nanocomposites were successfully made in an organic medium using KH-550 chemically modified reduced graphene oxide (rGO-NH2) as a multi-functional comonomer combinied with commercially available ODA, PMDA, and NTADA monomers, and their dispersive, morphological, thermal, viscous, conductive properties were investigated. Compared to pure PI, the dispersion compatibility, thermal characteristics and electrical conductivity of the produced nanocomposites were improved significantly. The improved rGO/PI nanocomposites showed more than 20 °C and 10 °C increase of Td5% and Td10% values respectively. This research demonstrates a facile and efficient method for creating homogeneous and multi-functional PI-based engineering materials and advanced functional materials.


Introduction
Polyimide (PI) and its composites have attracted significant attention due to their remarkable characteristics as high strength, high heat resistance, and high-performance polymer engineering materials.These materials have been found extensive application in various technical domains such as aerospace, electrical equipment, biological detection, laser, adhesive, coating, and thin films [1][2][3].However, the comprehensive aromatic structure of PI restricts its subsequent application research due to its insoluble and infusible attributes [4][5][6].Consequently, numerous structural modifications on linear PI are requisite, including the addition of flexible diamine or dianhydride monomers [7], the synthesis of monomers with new structures (like asymmetric structures and fluorine-containing structures) [8], and the polymerization reaction.Furthermore, the copolycondensation polymerization reaction disrupts the regularity of the entire polymer molecular chain, thereby improving its solubility or other properties [9].The inclusion of organic or inorganic materials in PI to prepare hybrid or composite materials is also an approach employed by researchers to enhance polymer properties, thus offering new functions to composite materials [10,11].However, there are still practical problems with PI application, such as electrostatic charge accumulation and poor heat dissipation.These can be addressed by altering its structure and dispersing carbon nanotubes (CNTs), nano-silica particles, and nanometal particles in the matrix.
Graphene (GE), known for its excellent electrical, mechanical, and thermal properties, is frequently employed as a reinforcing phase.Despite its use as an additive to effectively modify or strengthen PI, poor compatibility with PI limits the overall performance improvement of the relative composite materials [12,13].Yang et al. [14] reported that 3-aminopropyltriethoxysilane (APTES, KH550 as its commercial code) was grafted onto the surface of graphene oxide (GO) nanosheets through the SN2 nucleophilic displacement reaction between amino groups and epoxy groups.Liao et al. [15] used different molecular weights D400 and D2000 grafting to the surface of GO to obtain two kinds of amino-functionalized GO (D400-GO/D2000-GO), then prepared -NH2 functionalized GO by in-situ polymerization to modify PI, and got high-performance composite nanofilms.Compared with the pure PI, the glass transition temperature (Tg) of 0.3 wt.% D400-GO/PI film was increased by 23.96℃, and the thermal expansion coefficient of 0.1 wt.% D400-GO/PI composite decreased significantly from 102.6 μm/℃ (pure PI) to 53.81 μm/℃ with the reduction of 48%.
The present study endeavours to address issues such as the accumulation of electrostatic charge and poor heat dissipation, and simultaneously enhance the heat resistance of PI by introducing modified graphene into its structure while improving its dispersibility.Our approach, illustrated in Scheme 1, is structured in three steps.Initially, the silanol groups hydrolyzed by 3-aminopropyltriethoxysilane (KH-550) are grafted to the hydroxyl groups on the surface of the GO nanosheets, and the side reactions between the silanol groups of hydrolyzed KH550 and the epoxy or carboxyl groups of GO nanosheets are minimized by controlling the pH value of the reaction system and continuous removal of the methanol produced by hydrolysis.In the second step, we add hydrazine hydrate to further deoxygenize most of the oxygen-containing functional groups on the surface of the GO nanosheets [16,17], obtaining the KH-550 modified rGO-NH2 containing free amino groups.Finally, rGO/PI nanocomposites are prepared by in-situ solution random polycondensation polymerization with commercially available diamine monomer 4,4'-diaminodiphenyl ether (ODA), dianhydride monomer pyromellitic dianhydride (PMDA), and 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTADA).
Based on the strategy mentioned above, the chemical modification of GO by KH-550 leads to rGO-NH2 with good organic solubility.The final products are rGO/PI nanocomposites with uniform dispersion, high antistatic properties, and improved heat resistance.Through detailed testing and analysis, we characterize the relationship between the structure and properties of the KH550-modified rGO-NH2/PI nanocomposite and analyze the influence of microscopic morphology.Additionally, the composite's electrical application is preliminarily explored to verify the feasibility of the in-situ solution polymerization method at the molecular design level.This research illuminates the potential for composites of modified graphene and oil-based polymer to obtain engineering materials with superior comprehensive performance and high practicality.Scheme 1. Synthetic process of KH550-modified GE/ODA-PMDA-NTADA type PI

Preparation of KH550-modified graphene oxide (rGO-NH2) dispersion in DMF
As shown in Scheme 1 (a), under the nitrogen atmosphere, GO (2.0 g) was added to a four-necked flask followed by the sequential addition of deionized water (1000 mL) and ethanol (1000 mL).This mixture was ultrasonicated for 1 hour and gradually heated to 60℃.When the temperature was stable, concentrated HCl (5.0 mL) was introduced, and then KH550 (24.7 g) was evenly dropped in 30 mins with constant temperature reaction for 16 h.Hereafter, ammonium hydroxide (5.0 mL) and hydrazine hydrate (20.0 mL) were sequentially added into the solution.The solution mixture was continued to raise the temperature up to 80 ℃ for 6 h reaction.The solution was filtered by suction, and the resulting filter residue was washed with absolute ethanol at least three times.The rGO-NH2 powder was obtained by drying the residue under vacuum at 80 ℃ for 48 h.The final product, rGO-NH2 powder (0.4 g), was dispersed in distilled DMF under ultrasonic treatment to afford an rGO-NH2/DMF dispersion with a concentration of 2 mg/mL for subsequent utilization.

Preparation of KH550-modified GE/ODA-PMDA-NTADA type PI
The GE/PI nanocomposites were synthesized via the procedures shown in Scheme 1 (b).Firstly, the preparation of the nanocomposite precursor solution (PAA) with different mass fractions was as follows: 0.25 times molar amount (based on ODA) PMDA and NTADA were added respectively into the DMF solution of rGO-NH2/DMF dispersion and ODA, then the mixture solution was placed in a four-necked flask for low-temperature random copolycondensation solution polymerization with the reaction temperature of 5℃ for 24 h.For example, by the previous steps, under the conditions of 0.4 g rGO-NH2/DMF dispersion (2 mg/mL), 8.89 g (0.0444 mol) ODA, 4.89 g (0.0224 mol) PMDA and 6.02 g (0.0224 mol) PMDA, 2 wt.% nanocomposite precursor solution were obtained from the reaction mixture for 24 h reaction at 20℃.Secondly, the GE/PI nanocomposite films with different rGO-NH2 content (about 0.15±0.02mm) was obtained by casting the precursor solution onto a levelled clean glass plate (15×15 cm) drying at 80℃ for 8 h, and then stripped after cooling by raising the temperature to 150℃, 200℃, 250℃ and 300℃ each for 1 h.

Measurements
Fourier transform infrared spectroscopic (FT-IR) analyses were performed on a Nicolet iS50 spectrometer (Thermo Fisher Scientific, USA) in KBr tablet mode or film reflection mode.Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were recorded on JSM-6510LV (JEOL, Japan) by placing analyte powder in conductive adhesive and spraying with gold.X-ray diffraction (XRD) pattern was analyzed on a D8-Advance diffractometer (Bruker, Germany) with a CuKα radiation source.Raman spectrum was conducted on a HR 800 UV Lab-RAM multi-channel confocal micro spectrometer (HORIBA Jobin Yvon, France) with 633 nm laser excitation.X-ray photoelectron spectroscopy (XPS) analysis was characterized with an MCD-9 detector on a PHOIBOS 150 analyzer (SPECS GmbH, Germany).Intrinsic viscosity ([η]) was measured by PXWSN-265B ubbelohde viscometer (Shanghai Pingxuan Scientific Instrument Co., Ltd., China) at 30℃ with 0.5 g/dL NMP standard solution.
The dynamic mechanical thermal analysis (DMTA) was tested by a Q800 DMA (TA Instruments, USA) at 1 Hz with a heating rate of 5℃/min in air and temperature range from 50 ℃ to 300 ℃ .Surface resistivity was examined by Victor 385 portable resistivity meter (Xi'an Victor Instrument Co., Ltd., China) with 0.6-0.7 mm sample preparation thickness.Thermogravimetric analysis (TGA) was performed by using an SII Diamond Model (Perkin-Elmer, USA) under both N2 and air atmosphere at a heating rate of 10℃/min from room temperature to 800℃.
As mentioned, the surface of GO contained a large number of various oxygen-containing functional groups, such as carboxyl, hydroxyl, epoxy, ester, ketone and others.The silanol groups of hydrolyzed KH550 were grafted to the surface of GO nanosheets , and the most of the oxygen-containing functional groups were reduced by hydrazine hydrate [19].The organic solubility and compatibility of the modified product rGO-NH2 could be improved by grafting the hydrophobic (3-aminopropyl) trisiloxane groups onto GO nanosheets and reducing the hydrophilic oxygen-containing functional groups on the surface.The dispersibility of rGO-NH2 in an appropriate organic solvent is a crucial step towards enabling the subsequent in-situ random copolycondensation reaction.The SEM images in Figure 2 reveal the micromorphology, structure, and aggregation of both GO and rGO-NH2.Consistent with the reported literature [19,20], GO appears to aggregate randomly with thick crumpled morphology, while rGO-NH2 displays obvious wrinkles and loose, silk like, ultrathin layer structures with the expected requirements.The EDX spectra of rGO-NH2 derived from SEM images are presented in Fig 4 .After modified with KH550, the mass percentage of carbon increases from 63.17 wt.% to 75.94 wt.%, while that of oxygen decreases from 27.25 wt.% to 16.02 wt.%, compared to GO.The mass fractions of silicon and nitrogen in rGO-NH2 are 5.63 wt.% and 2.41 wt.% respectively, which indirectly confirming the grafting of the KH550 siloxane group onto the GO nanosheets and the reduction of residual oxygen-containing groups on the nanosheets to transform GO into GE.), carbonyl group (1629 cm -1 ) and epoxy group (1101 cm -1 ) of GO, two distinctive absorption peaks at 2919 cm -1 and 2873 cm -1 appear in the FTIR spectrum of rGO-NH2 which are attributed to the alkyl group (C-H symmetric and asymmetric stretching vibration) in the KH550 fragment.The sharp peak at 1551 cm -1 represents the N-H deformation vibration peak of amino group, with another characteristic peak of the amino group coinciding with the peak of the hydroxyl group (near 3422 cm -1 ).Furthermore, the new peaks at 1050 cm -1 and 1150 cm -1 are assigned to Si-O-C and Si-O-Si stretching vibration absorption.In addition, the characteristic peaks of hydroxyl, carboxyl and epoxy groups of rGO-NH2 are significantly weaker than those of GO.These proved that the KH550 modifications are consistent as expected, and a small amount of oxygen-containing functional groups on the surface of the reduced rGO still exist [21].The Raman spectra of rGO-NH2 in Fig .6 exhibits the significant D-band at 1343 cm -1 , which can measure the number of sp3 carbon atoms in the plane of graphite and evaluate its degree of defects, and the G-band at 1596 cm -1 in which the carbon atoms in graphite split in E2g mode with varying degrees of displacement peaks [22].As an important index to evaluate the quality level and modification degree of graphite materials, the intensity ratio of the D band and G band (ID/IG) for GO and rGO-NH2 increase from 0.90 to 1.34 which indicates that the size of SP2 carbon atoms in rGO-NH2 have also been decreased, and the new crystallization points with small size and large number are formed.In addition, sharp displacement peaks of rGO-NH2 are observed at 2682 cm -1 , indicating that the GE nanosheets are stacked in disorder compared to the regular lamellar stacking structure of graphite [23].TGA stands as a dependable technique for determining the thermal stability of various structural forms of carbon materials and their respective modified derivatives.As illustrated in Fig 7, the TGA curve for GO depicts a preliminary weight loss of about 1.5 wt.% before 100℃, probably due to the volatilization of residual moisture in GO.A substantial weight loss of approximately 35 wt.% is noticeable at 250℃, attributable to the release of CO, CO2, and steam as a consequence of thermal decomposition of unstable oxygen-containing groups [24].At the final temperature of 800℃, there is a total weight loss of about 50 wt.%aligning with the thermal decomposition of the GO's graphitic carbon skeleton.
The curve of rGO-NH2 reveals about 1 wt.% loss before 150℃, suggesting complete removal of the residual water.The main weight loss occurs at two distinct points: 250℃ (about 13.5 wt.%) and 600℃ (about 50 wt.%),which is attributed to the deoxidation of most oxygen-containing functional groups and the decomposition of the grafted siloxane fragments [28], respectively.The rest of the weightlessness conditions are fundamentally consistent with GO.All the evidence above unequivocally indicate that KH550 is predominantly grafted onto GO nanosheets after hydrolysis, and successfully chemically reduced to rGO-NH2 containing free amino groups.

Preparation of KH550-Modified GE/ODA-PMDA-NTADA Copolyimide Nanocomposites
The surface reflection FTIR spectra of pure PAA (or PI) and rGO-NH2 modified PAA (or PI) nanocomposites are shown in Fig 8 (a) and (b).Three characteristic bands are observed at 3453 cm -1 (N-H stretching vibration), 1717 cm -1 (C=O stretching vibration) and 1246 cm -1 (C-N stretching vibration) in pure PAA, which proves the existence of the -CONH-structure [25].Furthermore, in the spectrum of pure PI, with the appearance of C=O stretching vibration peaks (1777 cm -1 and 1725 cm -1 ) and C-N stretching vibration peaks in imide bonds, the original amide absorption peak at 1543 cm -1 (amide band) disappears due to the dehydration and cyclization of PAA [26].With the addition of rGO-NH2 as comonomers, the nanocomposites are proved with total imidization by the appearance of the characteristic peaks of imide at 1775 cm -1 (C=O asymmetrical stretching), 1715 cm -1 (C=O symmetrical stretching), 1377 cm -1 (C-N stretching) and 745 cm -1 (C=O bending).Although the characteristic peaks of rGO-NH2 are not detected for its low content, the methylene group in the alkyl (-CH2-, 3046 cm -1 in rGO-NH2-PAA and 3048 cm -1 in rGO-NH2-PI), benzene group (C=C bond, 1497 cm -1 ) and Si-O-C bond (1090 cm -1 or 1092 cm -1 ) are appeared, while there is no such characteristic peak in pure PAA and PI spectra.The XRD pattern, as depicted in Figure 9, elucidates the physical aggregation morphology of pure PI and rGO-NH2 modified PI nanocomposites.As a multi-functional comonomer, the addition of rGO-NH2 (2 wt.%) doesn't significantly change the aggregation structure of the rGO/PI nanocomposites.In the figure, the pure PI and rGO/PI nanocomposites have a broad diffraction peak at 2θ of 5.6 o and 16.8 o respectively which indicates that both of them have some different degrees of crystalline aggregation characteristics.In addition, the diffraction peak of polymer is relatively blunt due to the low crystallinity of the synthesized polyimide.This is mainly because that the introduction of copolycondensation and -CH3 groups destroy the regularity of the polymer chain [27].The SEM photographs of pure PI (a) and corresponding KH550 modified rGO-NH2/PI nanocomposites (c) after low-temperature brittle fracture in liquid nitrogen are employed to observe the fracture surface morphology.It is found that the fracture cross-section images of pure PI show the unique deformation texture of engineering plastics, which is relatively smooth overall.However, with the increasement of rGO-NH2 content, the deformation texture of the fracture surface of the rGO-NH2 (2 wt.%)/PI nanocomposites becomes dense with tiny flakes of different sizes randomly scattering on the fracture surface which is obviously small voids around the flakes.This is mainly because the residual oxygen-containing functional groups on the surface of the rGO-NH2 nanosheets are difficult to escape and accumulate around them during thermal amination [19].Therefore, the subsequent structure and performance characterization of nanocomposites is mainly for comparative analysis between 0 and 2 wt.% of rGO-NH2 。

Intrinsic viscosity properties
The intrinsic viscosity ([η]) of the synthesized PI and nanocomposite precursor solution is determined by ubbelohde viscometer, and the results are shown in Table .1.With the increase of rGO-NH2 content, the [η] of the precursor solution shows a trend of increasing first and then decreasing.This is mainly because the molar ratio of total amino groups to entire anhydride groups in the reaction system gradually changes from less than 1 to equal to 1, and then gradually becomes greater than 1 which leads the same trend first to the molecular weight of polycondensation products.When the rGO-NH2 content is 2 wt.%, the [η] of the precursor solution reaches the maximum correspondingly.

Thermal properties
The thermal stability of polymer materials is evaluted on TGA by using Td5% and Td10% as indicators.
As shown in Fig. 11, the Td5% and Td10% values of pure PI in N2 atmosphere are 487.6℃and 538.0℃, while the corresponding Td5% and Td10% values of KH550 modified rGO-NH2/PI nanocomposites are 509.7℃and 548.3℃, which are 20℃ and 10℃ higher than that of pure PI, respectively.According to the literature [22,27], the Td5% and Td10% values of GE/PI composites are only slightly higher than the corresponding PI, while some others [24] report the values are even lower than that of the corresponding PI.The results of TGA show that the thermal stability of rGO/PI nanocomposites have been improved, which depends on the introduction of rGO-NH2 nanosheets with good thermal conductivity into PI through covalent bonds, and disperses or transfers the heat received by the rGO/PI nanocomposites rapidly through the good interfacial compatibility and interaction between rGO-NH2 nanosheets and PI.Furthermore, the carbon atoms on the GE surface in the rGO/PI nanocomposites can effectively terminate the generation of free radicals when the PI molecular chain is thermally broken [29], thereby delaying its weightless process.

Conductive properties
As we know, the surface resistivity can be defined as the ratio of the DC voltage drop per unit length to the current flowing per unit width on the surface of a material or substance.When the electrodes form opposite sides of a square, the surface resistivity will convert the measured resistance.The antistatic effect of polymer is related to its surface resistivity and volume resistivity.Generally speaking, the smaller the surface resistivity, the higher the volume resistivity, and the better the antistatic performance of polymer.Distinguishing by the surface resistivity, the values of the best antistatic property or certain conductivity that we call to polymer composites is between 10 3 ~10 6 Ω, and good antistatic property when the surface resistivity is between 10 6 ~10 8 Ω.The surface resistivity of pure PI and KH550 modified rGO-NH2/PI nanocomposites measured by portable resistivity meter were undetectable and 3×10 4 Ω respectively, which indicate that the pure PI exhibit the insulation characteristics of engineering plastics, and the KH550 modified rGO-NH2/PI nanocomposites exhibit the conductive properties due to graphene nanoparticles wrapping around its surface.The results are consistent with the SEM spectrum in Fig 10, which also proves that the modified graphene is successfully grafted into the PI molecular chain, and the graphene forms a uniform conductive network structure in the composite.Currently, the majority of engineering plastics or films improve their antistatic or electrical conductivity properties through the incorporation of antistatic agents during the blending process.However, the incorporation of modified graphene into the polymerization process of engineering plastics or films to yield antistatic or conductive properties presents a promising avenue for future development.

Conclusions
The present study reports the successful preparation of rGO/PI nanocomposites achieved by in-situ random solution copolycondensation utilizing a conventional two-step method.The methodology employed KH550-modified rGO-NH2 and commercial monomers including ODA, PMDA, and NTADA.The process of chemical modification and reduction on GO by KH550 not only rendered rGO-NH2 with enhanced solubility in organic solvents but also facilitated the preparation of uniform, high-content rGO-NH2/PI nanocomposites.KH550 served as a copolymerized functional monomer additionally, which contributed to excellent compatibility at the molecular level between rGO-NH2 and the PI matrix.
The chemical structure and morphology were characterized by FTIR, XRD and SEM to verify the participation of KH550-modified GO in the PI synthesis process.TGA analysis highlighted significant improvements in the thermal properties of the nanocomposites.The Td5% and Td10% values of the nanocomposite materials were found to exceed those of pure PI by more than 20 ℃ and 10 ℃ , respectively.In addition, the surface resistivity test indicated that the introduction of modified graphene into nanocomposites makes it conductive to a certain extent, which can fully satisfy the antistatic requirements of engineering plastics.This work developed a facile two-step reaction method to prepare uniformly dispersed high-performance rGO-NH2/PI nanocomposites by in-situ solution random polycondensation and thermal amination.

Fig 1
Fig 1 demonstrates the dispersion of 2 mg/mL KH550 chemically modified and reduced rGO-NH2 powder in water, ethanol, NMP and DMF after ultrasonication and standing for 72 h.Evidently, the rGO-NH2 powder is homogeneously dispersed in DMF and ethanol without stratification, but precipitate in water and NMP.This indicates the modification has caused the product to transition from hydrophilic to hydrophobic.Due to the low boiling point of ethanol and the presence of active hydroxyl groups, DMF was chosen to prepare the following nanocomposites in the further solution polymerization.

Fig 3
Fig 3 displays a broad diffraction peak at 2θ =12.1 o and 21.9 o for GO and rGO-NH2 respectively, which indicates that both of them have different degrees of crystalline aggregation characteristics, and the interlayer 002 crystal plane distance in GO is smaller than that in rGO-NH2.

Figure 4 .
Figure 4. EDX spectra of rGO-NH2The chemical structure change of GO to rGO-NH2 after KH550 modified modification and hydrazine hydrate reduction is investigated vis FTIR.As depicted inFig 5, comparing  to the characteristic absorption peaks of hydroxyl group (3422 cm -1 ), carbonyl group (1629 cm -1 ) and epoxy group (1101 cm -1 ) of GO, two distinctive absorption peaks at 2919 cm -1 and 2873 cm -1 appear in the FTIR spectrum of rGO-NH2 which are attributed to the alkyl group (C-H symmetric and asymmetric stretching vibration) in the KH550 fragment.The sharp peak at 1551 cm -1 represents the N-H

Figure 5 .
Figure 5. FTIR of (a) GO and (b) rGO-NH2The Raman spectra of rGO-NH2 in Fig .6exhibits the significant D-band at 1343 cm -1 , which can measure the number of sp3 carbon atoms in the plane of graphite and evaluate its degree of defects, and the G-band at 1596 cm -1 in which the carbon atoms in graphite split in E2g mode with varying degrees of displacement peaks[22].As an important index to evaluate the quality level and modification degree of graphite materials, the intensity ratio of the D band and G band (ID/IG) for GO and rGO-NH2 increase from 0.90 to 1.34 which indicates that the size of SP2 carbon atoms in rGO-NH2 have also been decreased, and the new crystallization points with small size and large number are formed.In addition, sharp displacement peaks of rGO-NH2 are observed at 2682 cm -1 , indicating that the GE nanosheets are stacked in disorder compared to the regular lamellar stacking structure of graphite[23].

Table 1 .
The intrinsic viscosity of rGO/PI nanocomposites with different rGO-NH2contents