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Effect of Functionalized Graphene Oxide with Organophosphorus Oligomer on the Thermal and Mechanical Properties and Fire Safety of Polystyrene Shuilai Qiu,† Weizhao Hu,*,† Bin Yu,†,‡ Bihe Yuan,†,‡ Yulu Zhu,† Saihua Jiang,§ Bibo Wang,† Lei Song,† and Yuan Hu*,†,‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, P. R. China § School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, Guangdong 510641, P. R. China ‡

S Supporting Information *

ABSTRACT: A novel organophosphorus oligomer was synthesized to functionalize graphene oxide. Subsequently, the functionalized graphene oxide (FGO) was incorporated into polystyrene (PS) to enhance the integration properties of the matrix. The effect of FGO on the thermal properties, fire safety, and mechanical properties of PS nanocomposites was investigated. The results showed that the introduction of FGO significantly increased the maximum decomposition temperature (Tmax) (25 °C increase), reduced the total heat release (20.8% reduction), and peak heat release rate (38.2% reduction) of PS. In addition, the thermogravimetric analysis/infrared spectrometry analysis results indicated that the amount of organic volatiles and toxic carbon monoxide of PS was remarkably reduced. The physical barrier effect of FGO and the synergistic effects between the organophosphorus oligomer and FGO were the main causations for these properties improvements. Homogeneous dispersion of FGO into the polymer matrix improved the mechanical properties of FGO/PS nanocomposites, as demonstrated by tensile tests results. medium.17 In addition, graphene sheets are easily stacked together and dispersed with difficultly because of the intrinsic strong van der Waals force.18,19 Therefore, improving the dispersion and compatibility between various organic solvents and materials with graphene is a challenging task. Surface functionalization of graphene by introducing specific functional groups via covalent or noncovalent methods is an effective approach to acquire several expectant achievements: to improve the dispersion of graphene; to improve surface activity of materials; to improve the compatibility with other substances.20−22 Graphene contains oxygen-containing functional groups, such as carboxyl, hydroxyl, and epoxy groups on the basal planes and edges, providing a platform for surface functionalization.23 Owing to toxic, corrosive, and halogenated gases during combustion, halogen-free flame retardants, like phosphoruscontaining compounds, either as additive or reactive fire retardants, are preferred.24−27 Phosphorus-containing flame retardants can form phosphoric radicals (PO·, PO2·, etc.) to capture the radicals and oxygen molecules during combustion and finally retard the combustion reactions.28 Moreover, phosphorus-containing flame retardants are able to catalyze the char formation of polymers. Generally, the addition of

1. INTRODUCTION Owing to many extraordinary physical properties such as electrical properties, high aspect ratio and specific surface area, Young’s modulus, fracture strengt,h and thermal conductivity,1−3 graphene, has attracted great interest in the fields of physics, chemistry, and materials in recent years. Graphene, a two-dimensional (2D) free-state atomic crystal,4,5 has broad application prospects in the composite materials, microelectronics, optics, energy, biomedical, etc.6−8 There have been numerous methods of preparing graphene, such as micromechanical exfoliation, epitaxial crystal growth, chemical vapor deposition and graphite oxidation−reduction.9−12 Of these methods, graphite oxidation−reduction is the most widely used method, since exfoliated graphite oxide is easily obtained under the action of strong acids and oxidants; then single-layer graphene oxide is obtained under strong shearing force or ultrasonic wave;13,14 finally, single or multilayer graphene sheets are achieved by using reducing agents. Similar to layered nanofillers, such as montmorillonite (MMT) and layered double hydroxide (LDH), graphene with a unique layered structure has been incorporated into polymeric materials to enhance their thermal stability and flame retardancy.15,16 Graphene may act as a physical barrier to reduce the heat release and inhibit transfer of combustible gases into the flame zone during combustion. However, owing to its integral structure composed of the benzene six-membered rings containing the stable bonds, graphene exhibits high chemical stability, an inert surface state, and weak interaction with other © XXXX American Chemical Society

Received: November 16, 2014 Revised: March 11, 2015 Accepted: March 19, 2015

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DOI: 10.1021/ie504511f Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Schematic Illustration of the Synthetic Routes of PMPPD and FGO

flammability and releases heavy smoke and toxic gases during combustion. In this work, to overcome these shortcomings of PS, PMPPD functionalized GO (FGO) was obtained, and then was incorporated into the PS matrix to prepare FGO/PS nanocomposites via masterbatch-melt blending approach. Thermal stability, fire safety, and mechanical properties of the FGO/PS nanocomposites were investigated.

phosphorus-containing flame retardants may endow polymeric materials with flame retardancy. However, conventional smallmolecule phosphorus-containing flame retardants exhibit main drawbacks, such as poor water resistance, high migration rate, deterioration of mechanical properties, and poor compatibility with the polymer matrix.29,30 Therefore, to overcome these drawbacks of small-molecular flame retardants, a novel organophosphorus oligomer (PMPPD) was synthesized. To achieve high-performance flame retardants, a novel strategy based on functionalization of graphene oxide (GO) was developed; that is, the PMPPD was covalently grafted on the surface of GO. As is well-known, only a tiny amount of graphene is required to significantly improve the mechanical and thermal properties of graphene/polymer nanocomposites (GPNCs) and reduce the release of heat and toxic gases during combustion. For example, the peak heat release rate (PHRR) and peak carbon monoxide release rate of 3 wt % FGO/PS nanocomposites were reduced by 53% and 66%, respectively.31 To date, there have been three main techniques to prepare GPNCs: (1) in situ polymerization, (2) solvent blending, and (3) melt blending.32−34 The dispersion of graphene and interfacial interaction with a polymer matrix are two key factors in enhancing the comprehensive properties of GPNCs.35,36 In-situ polymerization and solvent blending usually result in a good dispersion of graphene but need large amounts of organic solvents. Melt blending is the main processing technology in the plastics industry; however, the dispersion of nanoadditives in polymer materials is relatively poor. Of the melt blending methods, masterbatch-based melt blending gives rise to a much better dispersion, and the required amount of organic solvents is significantly reduced.37 Polystyrene (PS) is one of the most widely used thermoplastics in the electrical, decoration, construction, and transportation industries as well as in the military, etc., due to its excellent properties, such as water resistance, processability, and mechanical properties.38−40 However, PS exhibits high

2. EXPERIMENTAL SECTION 2.1. Materials. Expandable graphite (EG, 50 meshes, 99%) was supplied by Qingdao Tianhe Graphite Co., Ltd. (China). Tetrahydrofuran (THF), N, N-dimethylformamide (DMF), triethylamine (TEA), thionyl chloride (SOCl2), dicyclohexyl carbodiimide (DCC), hypophosphorous acid, paraformaldehyde, methylphosphonic dichloride (MDCP), methanol, and hydrochloric acid (HCl, 37% aq) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. PS (GPPS, 158 K) was purchased from Yangzi Petrochemical Co., Ltd., China. 2.2. Preparation of Organophosphorus Oligomer. Hypophosphorous acid (0.2 mol) was dissolved in deionized water (150 mL) at 80 °C in a three-necked flask equipped with a mechanical stirrer, nitrogen inlet, and reflux condenser. Then, paraformaldehyde (0.4 mol) was added in batches to the flask within 20 min, and simultaneously HCl aqueous solution was added as the catalyst. The reaction was performed for 12 h under nitrogen, and the temperature of the system was accurately controlled at 80 °C. After completion of the reaction, the solvent was removed by distillation under reduced pressure. Then, the transparent viscous liquid, bis(hydroxymethyl) phosphinic acid, was dried under vacuum at 80 °C for 12 h. The bis(hydroxymethyl) phosphinic acid (0.2 mol) was added to THF (40 mL) at 40 °C in a three-necked flask equipped with a mechanical stirrer, nitrogen inlet, and reflux condenser. Then, MDCP (0.2 mol) and TEA (0.45 mol) dissolved in THF (150 mL) was added to the flask and under stirring the reaction system was heated at 60 °C under nitrogen B


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Industrial & Engineering Chemistry Research

Figure 1. FTIR (a) spectrum and 1H NMR (b) spectrum of PMPPD.

for approximately 20 h. The precipitate was filtered after completion of the reaction. The solvent was removed by distillation under reduced pressure, and a yellow viscous liquid, organophosphorus oligomer modifier, called poly(methylphosphonyl bis(hydroxymethyl) hyposphate) (PMPPD), was obtained by being dried in vacuum. The schematic of the synthetic route of PMPPD is shown in Scheme 1. 2.3. Preparation of FGO. GO was prepared by a modified Hummers’ method41 and washed with THF to remove water. Then, GO was treated by the refluxing of SOCl2 in the presence of DMF at 70 °C for 24 h. After completion of the reaction, excess SOCl2 and solvent were removed by distillation. The as-prepared products (approximately 2 g) dispersed in THF (200 mL) was added into a 500 mL threenecked flask, equipped with a mechanical stirrer, nitrogen inlet, and reflux condenser. PMPPD (5 g) and DCC (10 mg) as the catalyst were added to the suspension with stirring, and the reaction was conducted at 50 °C for 12 h. The FGO was filtered, washed with methanol (approximately 50 mL) and deionized water (approximately 100 mL), and then dried under vacuum at 50 °C for 12 h. The schematic of the synthetic route of FGO is shown in Scheme 1. 2.4. Preparation of GO/PS and FGO/PS Nanocomposites. The FGO/PS masterbatch was prepared by the solvent blending method. First, the FGO was washed with THF to remove water, and then 1 g of FGO was dispersed into 100 mL of DMF with ultrasonication for 0.5 h. Then, 10 g of PS was added into the suspension by strong mechanical stirring under ultrasonication for 10 min; the system was heated to 60 °C with stirring for 1.5 h; then, the viscous black slurry was dried under vacuum at 80 °C for 10 h to obtain the FGO/PS masterbatch. FGO/PS nanocomposites (FGO contents of 0.1, 0.5, 1.0, 3.0, 5.0 wt %) were prepared by melt blending at 180 °C using the PS masterbatch on a twin roller mill (roller speed of 60 rpm, 10 min mixing). The samples were hot-pressed into sheets (thickness, 3 mm) at 185 °C on a vulcanizing machine. The preparation process of GO/PS nanocomposites was similar to that of FGO/PS nanocomposites. 2.5. Characterization. The 1H NMR spectrum was obtained from a Bruker AV400 NMR spectrometer (400 MHz) operating in the Fourier transform mode using DMSOd6 as solvent. Fourier transform infrared (FTIR) spectra of PMPPD, GO, and FGO were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). The samples were mixed with KBr powders and pressed into tablets before characterization.

Thermogravimetric analysis (TGA) was conducted on a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA) from 20 to 700 °C at a linear heating rate of 20 °C min−1 under nitrogen and air atmospheres. X-ray photoelectron spectroscopy (XPS) test was performed with a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK) to investigate the composition of the samples. The excitation source was an Al Kα ray at 1486.6 eV. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku Co., Japan), using Cu Kα radiation (λ = 0.15418 nm), at a scanning rate of 4° min−1. The morphologies of GO, FGO, and PS composites were examined by a JEM-2100F transmission electron microscope (Japan Electron Optics Laboratory Co., Ltd., Japan). GO and FGO were dispersed in ethanol with ultrasonication for 0.5 h, and then dripped onto copper grids for observation. The composites were microtomed to ultrathin sections with a diamond knife by a Du Pont MT-6000 Ultratome. The combustion properties were evaluated by a microscale combustion calorimeter (MCC) with an instrument model of Govmark MCC-2 (The Govmark Organization, Farmingdale, NY). Approximately 2−5 mg of sample was heated in an inert gas atmosphere at a heating rate of 1 K s−1, then the pyrolysis volatiles with carrier gases (nitrogen of 80 mL min−1; oxygen of 20 mL min−1) were mixed and burned at 900 °C in a combustion chamber. Recording the heat of combustion depends on the principle of oxygen consumption. The flammability of the samples was characterized by the cone calorimeter (Stanton Redcroft, UK) tests according to ISO5660. Each specimen, of dimensions 100 × 100 × 3 mm3, was wrapped in an aluminum foil, and irradiated horizontally to an external heat flux of 35 kW/m2. The limiting oxygen index (LOI) was measured by an HC-2 oxygen index meter 207 (Jiangning Analysis Instrument Co., China) according to ASTMD 2863. The dimensions of each specimen were 100 × 6.5 × 3 mm3. The microstructure of the char residue of PS composites was investigated by a LABRAM-HR laser confocal microRaman spectrometer (Jobin Yvon Co., Ltd., France) with an argon laser of 514.5 nm. The mechanical property of tensile strength was measured on an MTS CMT6104 universal testing machine (MTS Systems Co. Ltd., P.R. China) according to the Chinese standard of GB 13022-91. The stretching rate was 100 mm min−1. Each specimen was repeated for five times. Unnotchedimpact strength was measured on a pendulum impact testing C


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Figure 2. FTIR (a) spectra and XRD (b) patterns of GO and FGO.

Figure 3. XPS scan of GO and FGO (a), the C1s spectrum of FGO (b).

cm−1), O−H (3430 cm−1), CC and the adsorbed water (1620 cm−1). After the modifier PMPPD was grafted to the surface of GO, several new peaks are observed in the spectrum of FGO. The absorption peaks at around 2960 and 2930 cm−1 were assigned to the stretching vibration of CH3 and CH2 groups in the PMPPD. The peaks at 1256 and 1031 cm−1 are attributed to the PO absorption, and the P−O−C absorption, respectively. Furthermore, the intensity of CO stretching vibration at 1730 cm−1 is significantly reduced. These results demonstrate the occurrence of successful functionalization of GO with the PMPPD. XRD patterns of GO and FGO are shown in Figure 2b. GO exhibits a typical (002) diffraction peak at 2θ = 10.3°, which is shifted to 2θ = 9.2° in FGO, indicating that the d-spacing is increased from 0.85 nm (GO) to 0.96 nm (FGO). Simultaneously, the intensity of this diffraction peak significantly decreases, due to the PMPPD grafted onto the surface of GO, increasing the randomness of the stacking structure of GO sheets. The PMPPD with relatively high molecular weight effectively inhibits the reaggregation of GO sheets and promotes the homogeneous dispersion of GO in the PS matrix. The elemental compositions of the surface of GO and FGO were evaluated by XPS. The total spectra of GO and FGO obtained from XPS are shown in Figure 3a. Two new absorbance peaks at 134.7 and 190.9 eV, attributed to the characteristic peaks of P2p and P2s, respectively, of the PMPPD, are observed in the total spectrum of FGO relative to GO. Phosphorus loading is an important factor to affect flame retardancy of polymer materials. The phosphorus atom percentage on the surface of FGO is around 2.6%. The C1s XPS spectrum of FGO (Figure 3b) presents four deconvoluted peaks of the carbon species: CC/C−C at 284.0 eV, C−O at

machine (MTS Systems Co. Ltd., P.R. China), each specimen was repeated for five times. Thermogravimetric analysis/infrared spectrometry (TG-IR) was performed using a TGA Q5000 IR thermogravimetric analyzer, which was connected to a Nicolet 6700 FT-IR spectrophotometer through a stainless steel transfer pipe.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of PMPPD. Figure 1 shows the FTIR and 1H NMR spectra of the PMPPD. As can be seen from Figure 1a, the peak at 3432 cm−1 is attributed to the vibration of O−H and/or H2O moieties, indicating the existence of water; the peaks at 2960 and 2927 cm−1 are assigned to the stretching vibration of CH3 and CH2 groups in the PMPPD; the absorption at 1623 cm−1 is assigned to the adsorbed water. Moreover, some characteristic absorption bands at 1428 (P−C), 1295 (PO), and 1033 cm−1 (P−O− C) were observed, confirming the condensation reaction between the hydroxyl group and the phosphoryl chloride group.42,43 The structure of PMPPD is further certified by 1H NMR (Figure 1b). The PMPPD exhibits the chemical shifts at δ = 1.19−1.32 ppm [P-CH3] and δ = 3.56−3.68 ppm [P-CH2−], meanwhile, the absorption peak of P−OH in PMPPD at 10.55 ppm. These provide further supports for the occurrence of the condensation reaction between hydroxyl group and phosphoryl chloride group. 3.2. Structural and Morphological Characterization of FGO. FTIR was employed to evaluate the structural characteristics, and the FTIR spectra of pristine GO and FGO are presented in Figure 2a. In the spectrum of GO, there are several typical absorption peaks: C−O (1072 cm−1), CO (1730 D


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a result, the functionalization of GO with the PMPPD significantly improves the thermal stability of GO. 3.3. Flammability and Thermal Performance of PS Nanocomposites. Good dispersion can significantly improve the integration performances of polymer nanocomposites. Graphene sheets typically tend to reaggregate in a polymer matrix. Reaggregation influences the interfacial interaction and compatibility between the nanoadditive and polymer matrix, and thus has adverse effects on the integration performances of nanocomposites. Therefore, it is necessary to evaluate the dispersion of FGO in the PS matrix. The morphologies of PS nanocomposites with the nanoadditives loading of 1.0, 3.0, 5.0 wt % FGO and 5.0 wt % GO were investigated by TEM (Figure 6). From Figure 6d, reaggregation and nonuniform dispersion

286.0 eV, CO at 288.3 eV, and C−O−P at 286.4 eV. In comparison with that of the pristine GO in the previous literature,44 the appearance of the additional component at 286.4 eV ascribed to C−O−P, suggests the successful functionalization of GO with the PMPPD. TEM was employed to investigate the morphology and microstructure of GO and FGO. As shown in Figure 4a, GO

Figure 4. TEM photographs of GO (a) and FGO (b).

sheets exhibit a wrinkled and folded nanoplatelet shape with the diameter of several micrometers, similar to the previous report.45 However, the edge of FGO is relatively rougher than GO, and the surface of FGO is covered by the particulate addends, due to the covalent grafting of the PMPPD (Figure 4b). The morphological change indicates that PMPPD is successfully grafted onto the surface of GO, and the functionalization enhances steric exclusion and reduces the reaggregation of graphene sheets. Thermal properties of GO and FGO in a nitrogen atmosphere were investigated by TGA, and the results are displayed in Figure 5. As shown in Figure 5a, the initial mass loss of GO below 100 °C is attributed to the evaporation of the adsorbed water, and the maximum mass loss temperature (Tmax) occurs at around 200 °C, due to the decomposition of labile oxygen functional groups, such as hydroxyl and carboxyl groups. The steady weight loss occurs in the temperature range of 300−700 °C, due to the degradation of the residual char, and a char yield of 53% is obtained at 700 °C. For FGO (Figure 5a), the onset degradation temperature (Td) defined as the temperature at 5 wt % mass loss, increases from 112 to 212 °C, indicating that the labile oxygen functional groups have been partially removed. However, FGO shows two-step weight loss. The first stage is attributed to the degradation of oxygen functional groups on the surface of FGO and the grafted flame retardant. While the second mass loss between 450 and 550 °C is assigned to the further decomposition of the residual char. As

Figure 6. TEM observations of the ultrathin sections obtained from PS-FGO1.0 (a), PS-FGO3.0 (b), PS-FGO5.0 (c), and PS-GO5.0 (d).

of these GO nanosheets are observed for PS-GO5.0. However, from the inner structures of PS nanocomposites with 1.0 and 3.0 wt % FGO, it can be observed that FGO is homogeneously dispersed in the PS matrix (Figure 6a,b), because of the strong interfacial interaction and excellent compatibility between FGO and the PS matrix. Moreover, Figure 6c presents a TEM image (PS-FGO5.0) showing that FGO nanosheets are dispersed in the PS matrix with the form of not only multiple layers but also stacks. As a result, a relatively large additive amount of FGO leads to reaggregation of the nanosheets in the PS matrix. Therefore, the grafting modification of GO forms a strong interfacial interaction with the matrix, which improves the dispersion of the nanoadditives in the PS matrix.

Figure 5. TGA (a) and DTG (b) curves of GO and FGO under nitrogen atmosphere. E


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Figure 7. TGA (a and c) and DTG (b and d) curves of PS and its nanocomposites under air and nitrogen atmosphere, respectively.

thermal-oxidative degradation of FGO/PS nanocomposites.46 FGO sheets have high thermal conductivity, which improves the heat transfers in the matrix. Moreover, the PMPPD on the surface of GO with poor thermal stability simultaneously impacts the reduction of the Td of PS nanocomposites. However, as the amount of FGO increases from 0.1 to 5.0 wt %, the Tmax values are increased by 10−25 °C, the maximum increase being approximately 25 °C (PS-FGO5.0). The final char residues at 700 °C are also increased by 7−25 times, and the maximum increase for PS-FGO5.0 is 25 times than that of pure PS. Furthermore, the values of Td, Tmax, and char residue of PS-FGO5.0 are visibly higher than those of PS-GO5.0, suggesting that the functionalization of GO with the PMPPD results in the strong interfacial interaction and the reinforced barrier effect of FGO. Hence, the functionalization of GO is propitious to enhance the thermal stability of PS nanocomposites. When heated under nitrogen atmosphere (Figure 7c,d), PS nanocomposites exhibit similar mass-loss behavior as they do under air atmosphere. The thermal degradation process of PS nanocomposites in nitrogen atmosphere mainly occurs in the range of 380−470 °C, and the TGA curves show a single massloss stage, which is attributed to the degradation of macromolecular chains. The Td of pure PS is 379.4 °C, and its DTG curve has a single peak at 414.9 °C (Tmax). Different from that under air atmosphere, the Td values of all the FGO/PS nanocomposites increase, because of the expected thermal stability; FGO under the inert atmosphere without oxygen participates in thermal-oxidative degradation. In addition, the values of Tmax and char residue percentage of FGO/PS nanocomposites increase gradually as the amount of FGO increases. The maximum increase of Tmax is approximately 22 °C (PS-FGO5.0), and its final char residues at 700 °C are also increased by 1.6 wt %. However, the expected phenomenon is that GO/PS nanocomposites exhibit relatively lower thermal

TGA was employed to evaluate the thermal stability of PS nanocomposites. The TGA curves of PS and its nanocomposites under air and nitrogen atmosphere are shown in Figure 7, and the corresponding data are presented in Tables 1 Table 1. Related Data of TGA and DTG for PS and Its Nanocomposites under Air Atmosphere sample

Td (°C)

Tmax (°C)

residue at 700 °C (wt %)

PS PS-FGO0.1 PS-FGO0.5 PS-FGO1.0 PS-FGO3.0 PS-FGO5.0 PS-GO5.0

296.1 289.3 286.7 284.4 282.5 274.5 278.5

368.7 379.0 381.3 382.2 385.3 394.1 355.1

0.2 1.4 1.9 2.7 3.7 5.0 1.8

and 2. The thermal degradation process of FGO/PS nanocomposites in air atmosphere (Figure 7a,b) mainly occurs in the range of 320−400 °C. In comparison with that of pure PS, the Td of FGO/PS nanocomposites decreases at an earlier stage. Oxygen participates in the oxidation and thermaloxidative degradation of the materials, leading to the earlier Table 2. Related Data of TGA and DTG for PS and Its Nanocomposites under Nitrogen Atmosphere sample

Td (°C)

Tmax (°C)

residue at 700 °C (wt %)


379.4 380.9 382.4 385.8 391.9 392.7 384.1

414.9 415.6 420.1 426.9 433.6 436.8 419.5

1.7 1.8 2.3 2.4 2.8 3.3 2.9 F


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Figure 8. HRR (a) and THR (b) versus temperature curves of PS and its nanocomposites.

PHRR values of FGO/PS nanocomposites are gradually reduced by 13.3% (PS-FGO0.1) to 38.2% (PS-FGO5.0). In the case of PS-FGO5.0, incorporating 5 wt % FGO into a PS matrix gives rise to the lowest PHRR value (38.2% reduction), compared to that of pure PS. The THR curves (Figure 8b) exhibit a similar trend as PHRR, and the addition of 5.0 wt % FGO results in the lowest THR (20.8% reduction). The remarkable improvement in flame retardancy of PS is attributed to the synergism of the physical barrier effect of FGO sheets in the PS matrix, capture of oxygen molecules and free radicals, and the catalytic charring effect of the PMPPD. Furthermore, as compared to pure PS, the PHRR and THR values of the PSGO5.0 sample are reduced by 12.3% and 10.3%, respectively, which are far less than those of the PS-FGO5.0 sample. The results demonstrate that FGO is more efficient in enhancing the flame retardancy of the PS matrix than GO, because the functionalization of GO promotes the char formation of GO and enhances the physical barrier effect. Overall, the incorporation of FGO into PS significantly reduces the PHRR and THR of the matrix, and increases the temperature to the maximum value of heat release, thereby improving the flame retardancy of PS nanocomposites. The cone calorimeter is an efficient bench-scale tool for evaluating the flammability behavior of materials in real fire situation.47 The LOI provides an efficient measure of flammability for polymer materials over broad ranges. Figure 9 provides the HRR and THR versus time curves of selected samples, the related data and LOI values are summarized in Table 4. From Figure 9a, it can be observed that pure PS burns very rapidly after ignition, with a PHRR value of 1067.4 kW/

stability than the FGO/PS counterparts, reflected by the lower Tmax and final char residues. All these improvements indicate that the incorporation of FGO effectively enhances the thermal stability of the PS nanocomposites, attributed to the physical barrier effect of FGO nanosheets and the improved interfacial interaction between FGO nanosheets and PS matrix. Moreover, the increased char residues build a physical char barrier, which retards the penetration of external oxygen and the escape of volatile degradation products. As is well-known, graphene enhances the flame retardant properties of polymers, due to its unique 2D nanosheet structure. MCC was employed to characterize the combustion properties of the PS nanocomposites. The heat release rate (HRR) and total heat release (THR) versus temperature curves are given in Figure 8, and related data are summarized in Table 3. Pure PS burns rapidly, and exhibits high PHRR and THR Table 3. Related Data of MCC for PS and Its Nanocomposites under Nitrogen Atmosphere sample

PHRR (W/g)

THR (kJ/g)

Tmax (°C)


844.8 728.5 653.1 571.6 550.1 522.3 738.9

31.2 30.3 29.1 28.9 27.2 24.7 29.0

434.2 435.6 441.6 451.1 451.6 451.7 431.9

values of 844.8 W/g and 30.6 kJ/g, respectively. With the additive amount of FGO increasing from 0.1 to 5.0 wt %, the

Figure 9. HRR (a) and THR (b) versus time curves of PS and its nanocomposites. G


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nanocomposites. The THR value (Figure 9b) of the PSGO5.0 sample is slightly reduced by 2.6%. For FGO/PS nanocomposites, the THR value decreases as the additive amount of FGO increases. The lowest THR value of 97.3 MJ/ m2 (4.9% reduction) is observed for the PS-FGO5.0 sample. The results suggest that FGO enhances fire safety of PS nanocomposites. As can be observed from Table 4, the LOI value of pure PS is about 18.0%, and the LOI value increases with the incorporation of GO and FGO. The LOI value is gradually increased with increasing the additive amount of FGO, and the PS-FGO5.0 sample presents the highest LOI value (20.5%). Meanwhile, the LOI value of the PS-FGO sample is slightly higher than that of PS-GO sample with the same loading of nanoadditives. As a consequence, the FGO/PS nanocomposites perform with a slightly improved LOI value, because they selfextinguish with difficulty when the fuel combusts, although nanocomposites burn slowly in the ignition stage.48 3.4. Volatiles Analysis. To analyze the effect of FGO on the overflowed gas products during the pyrolysis process, TGIR was employed to investigate the volatile compounds of PS and its nanocomposites. Figure 10a presents the FTIR spectra of pyrolysis products of PS and its nanocomposites at

Table 4. Related Data of Cone Calorimeter and LOI for PS and Its Nanocomposites sample

PHRR (kW/m2)

THR (MJ/m2)

LOI (%)


1067.4 921.3 870.6 859.4 828.4 733.9 806.3

102.3 101.5 101.2 100.4 99.8 97.3 99.6

18.0 19.0 19.0 19.5 20.0 20.5 19.5

m2. Cone results of the PS samples show a similar alteration trend as MCC test results. As expected, introducing 5 wt % GO into PS matrix makes the PHRR decrease to 806.3 kW/m2. For the PS-GO5.0 sample, the PHRR value decreases by 24.4%, compared to that of pure PS. Moreover, the PHRR of the PSFGO5.0 sample exhibits further reduction relative to that of the PS-GO5.0 sample, the PS-FGO5.0 sample presents the lowest PHRR value of 733.9 kW/m2 (31.2% reduction). The better flame retardant properties of FGO/PS nanocomposites are attributed to two factors: first, the functionalization of GO by PMPPD reinforces the physical barrier effect of graphene; second, PMPPD facilitates the charring process of PS

Figure 10. FTIR spectra (a) of pyrolysis products of PS and its nanocomposites at maximum decomposition rates. Intensity of characteristic peaks for pyrolysis products of PS and its nanocomposites (b−f). H


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Figure 11. Data of tensile strength (a) and impact strength (b) of PS and its nanocomposites.

and smoke suppression effect of FGO/PS nanocomposites are superior to those of GO/PS nanocomposites. In summary, the introduction of FGO significantly reduces the total release of gaseous pyrolysis products, and the smoke toxicity delays the time of pyrolysis process of the materials and improves the fire safety of PS nanocomposites. 3.5. Mechanical Properties and Char Residues Analysis of FGO/PS Nanocomposites. Figure 11 lists the data of tensile strength and unnotched-impact strength of PS and FGO/PS nanocomposites. From Figure 11, it can be observed that the tensile strength of the nanocomposites at a low loading of FGO (0.1−3.0 wt %) is visibly increased. For example, the incorporation of 1.0 wt % FGO results in the maximum increase by 23.3% in tensile strength. Moreover, the unnotched-impact strength is slightly improved. When the loading of FGO is 5.0 wt %, the tensile strength and unnotched-impact strength of FGO/PS nanocomposites are decreased obviously, probably due to the relatively poor dispersion of FGO in the PS matrix. The dispersion of graphene was evaluated by the TEM observations of the ultrathin sections in Figure 6. A uniform dispersion of FGO in the PS matrix with the nanoadditives loading of 1.0 and 3.0 wt % is observed, but reaggregation and nonuniform dispersion of the FGO nanosheets are observed for the PS-FGO5.0 sample, which is consistent with the results of the mechanical properties. Therefore, FGO within a certain range of addition loading may improve the mechanical properties of the PS nanocomposites. Raman spectroscopy offers a powerful tool to characterize the structure of carbonaceous materials. Figure 12 presents the Raman spectra of the residual chars of FGO/PS nanocomposites. Two strong absorption peaks at 1595 and 1360 cm−1, which are named G peak and D peak,49 respectively, are observed in the spectra of all the samples. G peak reflects the E2g vibration of the graphite lattice network, belonging to an ordered carbons structure. D peak is the first-order Raman scattering of sp3 hybridized carbon atoms, belonging to a disordered carbons structure, which provides information on defects in the graphite layers. The intensity ratio of the G and D peaks (IG/ID) represents the relative content of graphitized carbons in the residual char. When the additive amount of FGO is increased, the ratio of IG/ID is gradually increased, indicating the improvement of the graphitized carbons in the residual char. High graphitization degree of the residual char can provide efficient thermal insulation, which results in an effective decrease in the degradation of the composites and heat transfer between the flame and the materials. Overall, the analysis of

maximum decomposition rates. As can be seen from Figure 10a, the similar characteristic peaks in the FTIR spectra indicate the similar composition of the pyrolysis products. The characteristic peaks of the pyrolysis products appear in the regions of 3500−4000 cm−1, 2750−3200 cm−1, 2200−2400 cm−1, 1750−1900 cm−1, 1250−1600 cm−1, and 600−1000 cm−1. The characteristic FTIR signals clearly identify the gaseous pyrolysis products of the PS. The peak at 3650 cm−1 is attributed to the vibration of O−H, indicating the existence of water; the peak at 2930 cm−1 is assigned to the stretching vibration of CH3 and CH2 groups in hydrocarbons; the peaks at 2360 and 2190 cm−1 correspond to the absorption of carbon dioxide (CO2) and carbon monoxide (CO), respectively. Moreover, the peaks at 1605 and 1510 cm−1 are assigned to the characteristic bands of aromatic rings. To further evaluate the evolution of the pyrolysis products, the absorbance of total release and four selected pyrolysis products for PS and its nanocomposites versus time curves are displayed in Figure 10b−f. As can be seen from Figure 10b, the total release of pyrolysis products of PS is in the regions of 1070−1300 s, while the gaseous pyrolysis products of the PSFGO5.0 sample releases in the range of 1120−1380 s, indicating that incorporating FGO delays the degradation process of the PS segment. It can be clearly found that the addition of FGO (5.0 wt %) decreases the maximum absorbance intensity of the pyrolysis products of PS, which is much lower than those for pure PS and PS-GO5.0. The decreased intensity of pyrolysis products is attributed to the reinforced physical barrier effect of FGO, which retards the escape of volatile degradation products. The absorption curves of the selected pyrolysis products, including CO2, CO, hydrocarbons, and aromatic compounds, are presented in Figure 10c−f. The introduction of FGO (5.0 wt %) in the PS matrix significantly reduces the maximum absorbance intensity of hydrocarbons and aromatic compounds during combustion, compared to pure PS, while the incorporation of GO (5.0 wt %) increases the intensity of those volatile products. The “fuels” of these organic volatiles are the major source of smoke particles, and the reduction in inorganic volatiles gives rise to the suppression of smoke. As shown in Figure 10d,e, the maximum absorbance intensity of nonflammable gas (CO2) and toxic gas (CO) of PS-FGO5.0 is lower than that of pure PS and PS-GO5.0, owing to the huge specific surface area of FGO, the physical adsorption effect, and barrier effect. The reduction in CO leads to the decrease in smoke toxicity during the combustion, which is beneficial for the improvement of fire safety. In addition, the thermal stability I


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2012CB719701), the National Natural Science Foundation of China (No. 51303167), and the China Postdoctoral Science Foundation (No. 2014M561838).

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Figure 12. Raman spectra of the char residues of FGO/PS nanocomposites.

Raman spectra suggests that FGO significantly improves the graphitization degree of the residual char layer, which is beneficial to improving the thermal stability and flame retardancy of PS nanocomposites.

4. CONCLUSIONS The organophosphorus oligomer PMPPD was designed and synthesized to functionalize GO, and then the FGO was introduced into the PS matrix to prepare a series of FGO/PS nanocomposites. The results of 1H NMR, FTIR, XRD, XPS, and TEM indicated that the PMPPD was successfully synthesized and grafted onto the surface of GO. TGA results showed that the incorporation of FGO in the PS matrix significantly improved the thermal stability of the PS nanocomposites. The fire safety of FGO/PS nanocomposites, some indexes such as LOI, PHRR, and THR, were significantly improved. Moreover, FGO added over a certain range improved the mechanical properties of the PS nanocomposites because of the homogeneous dispersion of FGO sheets in the PS matrix. TG-IR analysis indicated that the amount of organic volatiles of PS was reduced, and the toxic CO was suppressed by incorporating FGO. The incorporation of FGO into the PS matrix improved the thermal stability, mechanical properties, and fire safety performance of PS nanocomposites.

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ASSOCIATED CONTENT

S Supporting Information *

The mass of all samples and its residual chars from the cone calorimeter test (Table S1); the morphologies of the residual chars after the cone calorimeter test (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(Y.H.) E-mail: [email protected]. Fax/Tel: +86-55163601664. *(W.H.) E-mail: [email protected]. Fax/Tel.: +86551-63602353. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Basic Research Program of China (973 Program) (No. J


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