Non-covalently Functionalized Graphene Oxide-Based Coating to Enhance Thermal Stability and Flame Retardancy of PVA Film
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KeywordsGraphene Non-covalent functionalization Layer-by-layer Flame retardant Poly(vinyl alcohol) (PVA)
Functionalized graphene oxide (FGO) containing phosphorus–nitrogen compound was prepared via a non-covalent strategy.
The multilayer FGO-based coating was deposited on a poly(vinyl alcohol) (PVA) film using the layer-by-layer assembly technique.
A significant synergistic effect between the FTO and conventional flame-retardant elements enhances thermal stability and fire retardancy of the coated PVA film.
Multifunctional polymer films have received much attention owing to their wide potential use in both research and practice applications. Conducting polymers such as polyaniline (PANI) [1, 2] or polypyrrole (PPy) [3, 4, 5] thin films with remarkable storage capacity have been widely applied as supercapacitor electrodes. Polyamide (PA) thin-film composite is considered a promising candidate for filtration membrane for water purification. A series of film products are also used in solar cells , sensors , and heavy metal detoxification . In addition, satisfactory flame-retardant property of polymeric membranes is crucial for their applications in numerous fields. In particular, as low-dimensional materials, the thin wall structure of polymer membranes leads to much higher combustibility compared to three-dimensional polymer products. Incorporation of flame-retardant additives into a film matrix is a conventional approach to reducing its flammability [9, 10]. However, the addition of flame retardants produces interior defects that deteriorate the film-forming ability and mechanical properties. Therefore, fabrication of high-performance flame-retardant polymer films via the conventional strategy remains a challenge worldwide.
Graphene, a two-dimensional carbon material, has remarkable properties such as superior mechanical strength and high thermal and electrical conductivities [11, 12, 13, 14]. These intrinsic properties of graphene have gained enormous interest in various fields including solar cells , flexible electrodes [16, 17], ultrasensitive sensors [18, 19], and reinforced nanocomposites [20, 21]. Besides, in the last few decades, graphene has been extensively applied in various polymer systems for improving their flame-retardant properties [22, 23, 24]. Due to its layered structure, graphene acts as a barrier retarding heat release and blocks the diffusion of pyrolysis products and the transfer of oxygen. However, because of combustibility in air atmosphere [25, 26, 27], satisfactory flame retardancy is hard to achieve via incorporation of individual graphene into the polymer matrix.
Flame-retardant materials contain one or more flame-retardant elements such as phosphorus [28, 29], nitrogen [30, 31], and silicon . These flame-retardant elements may be added in the form of an additive or chemically incorporated into the structure of the materials. Therefore, fabricating hybrid graphene-based compounds containing conventional flame-retardant elements may be an effective way to improving the fire resistance efficiency of graphene. In our previous work, owing to the barrier effect of graphene sheets in the initial combustion stage, a slower chemical charring behavior of phosphorus flame retardant (PFR) was observed. Thus, the system maintained a good shielding action during the entire process because of the double barrier effect of PFR-graphene oxide (GO) . Chiang et al.  prepared a novel phosphorus-containing reduced GO (rGO) flame retardant (DOPO-rGO) via a direct reaction between them. Resulting from the synergistic effect of DOPO-rGO, the epoxy resin was endowed with excellent flame retardancy.
Hexachlorocyclotriphosphazene (HCCP), a phosphorus–nitrogen compound, was chemically grafted onto the surface of GO. HCCP catalyzed the char formation from polymers, and graphene was protected from oxygen after being encapsulated by the HCCP-induced char. Thus, graphene did not burn out and acted as a graphitic char in the condensed phase [23, 25]. In this case, functionalized GO (FGO) with phosphorus–nitrogen elements will offer significantly enhanced flame retardancy [35, 36, 37]. It is worth noting that the oxygen-containing functional groups on the basal plane and along the edges of GO, the reactive sites in the chemical modification, will always be partially replaced. The hydrophilicity of FGO deteriorates to a certain extent, which renders its exfoliation in water into individual GO sheets forming a stable colloidal suspension difficult . Since homogeneous suspension of graphene sheets in water is crucial for processing and applications, it is necessary to develop a feasible method to prepare water-soluble graphene-based flame retardants. In contrast to chemical modification, which is often based on covalent linkages between GO and flame-retardant compounds, non-covalent modification has many advantages such as high efficiency and easy preparation process [38, 39, 40]. More importantly, non-covalent bonding through π–π interactions never degrade the physical and chemical properties of GO . In this way, non-covalently functionalized GO can be readily fabricated in aqueous solutions.
Since it was first proposed by Decher et al. , layer-by-layer (LBL) technique has proven to be very useful for assembling oppositely charged materials into thin films or coatings for membranes used in nanofiltration [43, 44], photo-catalysis , and controlled molecular release . GO contains a substantial amount of oxygen groups and exhibits a negative charge when dispersed in water, forming a suspension that can be used to prepare a thin GO film by the LBL self-assembly technique. In particular, this unique 2D structure of GO offers an exciting opportunity to create LBL membranes by stacking GO nanosheets, which can be used to fabricate multifunctional hybrid films with nanometer precision [47, 48, 49, 50, 51, 52]. Only a few studies deal with the preparation of LBL GO films or coatings for flame-retardant applications [53, 54, 55].
In this work, functionalized GO was prepared through non-covalent π–π stacking interactions with a flame-retardant compound containing phosphorus and nitrogen elements. The LBL-assembled GO-based flame-retardant multilayer films were deposited on the surfaces of poly(vinyl alcohol) (PVA) films with anionic functionalized GO and cationic polyethyleneimine (PEI). Thermal stability and flame-retardant properties of the coated PVA were studied systematically, and a detailed analysis of the mechanism is reported.
Graphite powder was kindly supplied by Nanjing XFNano Materials Tech Co., Ltd. Potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2), and sodium nitrate (NaNO3) were purchased from Aladdin Chemical Co., Ltd. Phenoxycycloposphazene (HPTCP) was friendly provided by Shengyi Technology Co., Ltd. PEI was purchased from Aladdin Chemical Co., Ltd. and PVA, with a polymerization degree of 1700 and an alcoholysis degree of 99, was supplied by Sichuan Vinylon Corporation (Chongqing, China). The deionized (DI) water used was prepared in our laboratory.
3.2 Preparation of Non-covalently Functionalized GO
3.3 Preparation of PVA Film Coated with FGO-Based Multilayer
Designing oppositely charged suspensions is crucial for successful LBL assembly. Intrinsically, GO is negatively charged when dispersed in water because of the existence of oxygen-containing groups on its surface. The LBL deposition process was used to distribute PEI and FGO on the PVA membranes. The PVA films were immersed into a PEI aqueous solution (2 mg mL−1) for 5 min, followed by 1 min of rinsing in a purified water bath and subsequent drying at 80 °C for 10 min under vacuum to remove the solvent. The prepared PEI-modified PVA membranes were then immersed into the FGO aqueous solution (2 mg mL−1) for 5 min, followed by the same rinsing and drying steps as mentioned above. These steps were repeated until the desired number of bilayer (FGO/PEI) was deposited on the PVA substrate.
The morphology and structure of GO and FGO were studied by transmission electron microscopy (TEM) using a Tecnai G2 F20 electron microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded using a DX-1000 diffractometer (Dandong Fangyuan Instrument Co., Ltd., China) with a Cu-K α generator system operated at 40 kV and 25 mA over a 2θ range of 5°–40° at a scanning rate of 1° s−1. Raman spectroscopy was conducted on a Labram HR spectrometer (HORIBA Jobin–Yvon) using 532-nm laser excitation with a power of 1 mW. Thermogravimetric analysis (TGA) was performed using a TA Q-500 TGA thermal analyzer at a heating rate of 10 °C min−1 over the temperature range of 30–650 °C with a nitrogen flow of 100 mL min−1. Approximately 8–10 mg of the sample was used in each test. UV–Vis absorption measurements were taken using a UV–visible spectrophotometer (Cary 100 Bio, Varian, USA). The surface morphology of the samples was observed using a scanning electron microscope (SEM) (JSM-5900LV, JEOL Ltd., Tokyo, Japan) with a conductive gold coating at an accelerating voltage of 10 kV. Microscale combustion calorimetry (MCC) analysis was carried out using an FAA-PCFC microscale combustion calorimeter (Fire Testing Technology Limited, UK) by heating about 2 mg of samples from ambient temperature to 800 °C at a heating rate of 1 °C s−1 under air atmosphere. Vertical burning tests of the samples were conducted on a HK-HVR vertical burning tester (Zhuhai Huake Testing Equipment Co., Ltd.). Elemental compositions of the coating residue were studied using a Shimadzu/Kratos AXIS Ultra DLD multifunctional X-ray photoelectron spectrometer (Manchester, UK).
4 Results and Discussion
4.1 Structural and Properties of Non-covalently Functionalized GO
The XRD patterns of GO and FGO are shown in Fig. 2c. The typical diffraction peak at 2θ = 10.4° was assigned to GO, indicating an interlayer distance of 0.85 nm (d2), which is in good agreement with previous results . After non-covalent functionalization, the peak shifted to 2θ = 9.2° in the FGO spectrum, suggesting that the interlayer distance increased to 0.96 nm (d1). As shown in Fig. 2c, the increase in interlayer distance (from d2 to d1) is due to the successful loading of HPTCP molecules on the surface of GO. Raman spectroscopy was conducted to further investigate the corrugated structures of GO and FGO. As shown in Fig. 2d, in-phase vibration of the sample’s lattice (G band) at 1570 cm−1 and the disorder band (D band) at approximately 1355 cm−1 was detected in the Raman spectra . FGO exhibits a pattern similar to GO; thus, the non-covalent modification did not destroy the layered structure of GO. The intensity ratio of the D and G bands is also a key parameter for evaluating the structure of graphene. The ID/IG ratios were 1.430 and 1.546 for GO and FGO, respectively. The slight increase in ID/IG ratio of the latter indicates an increase in amorphous carbon compared to the sp2-hybridized graphene due to the loading of HPTCP.
4.2 Structural and Properties of PVA Film Coated with FGO-Based Multilayer
TGA data of neat PVA, PVA/FGO and coated PVA samples
Temperature at 5% weight loss (°C)
Residual content at 600 °C (%)
4.3 Flame Retardancy and Mechanism of PVA Film Coated with FGO-Based Multilayer
The self-assembled FGO-based multilayer converted into an integrated char layer coating the PVA substrate after the flame treatment. We carefully cut the char layer from the burned sample. The surface exposed to the flame is regarded as the external surface of the FGO-based coating residue, while the surface in contact with the PVA substrate is the internal surface. The SEM images of the coating residue shown in Fig. 8e–j reveal that a more dense and continuous char layer with fewer cracks and holes is obtained with increasing number of bilayers from 10 to 30. It is noteworthy that there is a significant difference between the morphologies of the external and internal surfaces of the coating residue. The external surface appeared relatively smooth with a sheet-shaped structure, while the internal surface exhibited continuous sheets covered with a large number of small particles. From the combined results of XPS analysis, it is revealed that the outer layer is mainly composed of C and O elements, with additional P element observed in the inner layer of the coating residue. Accordingly, it is concluded that two different char structures were generated by different processes: the outer layer is a physical char mainly consisting of graphene sheets, while HPTCP catalyzed the PVA matrix into a chemical char that encapsulated the physical char, thus constituting the inner layer.
In summary, we fabricated a novel type of flame-retardant membrane by successively assembling negatively charged non-covalent FGO nanosheets and positively charged PEI on a PVA support based on the electrostatic LBL self-assembly technique. FGO was prepared by incorporating HPTCP onto the surfaces of GO sheets via a non-covalent strategy, thus facilitating the barrier effect of GO in the condense phase. HPTCP molecules were successfully loaded onto the GO surface without affecting its original structure and property. After a uniform deposition process, a compact and continuous multilayer coating based on FGO/PEI was constructed on the PVA surface. The initial decomposition temperature of the coated PVA improved by more than 30 °C compared to that of pure PVA, showing significantly enhanced thermal stability due to the effective barrier effect of the stacked FGO sheets. In addition, we evaluated the flame retardancy of the coated samples and a corresponding mechanism was proposed. The results indicate a substantial reduction in heat release for coated PVA, and the 30-bilayer-coated sample maintained its initial shape even after prolonged direct exposure to flame. A composite char, consisting of the physical char from graphene and the chemical char from the catalyzed matrix, formed on the PVA surfaces as a protective shield to effectively block heat and mass transfer. Therefore, the reported FGO-based LBL coating may have great potential applications in the flame-retardant treatment of various polymers.
This work was supported by National Natural Science Foundation of China (No. 51473095), the Program of Innovative Research Team for Young Scientists of Sichuan Province (2016TD0010).
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