A New Strategy for Simultaneously Improved Flame Retardancy

Article pubs.acs.org/IECR

A New Strategy for Simultaneously Improved Flame Retardancy, Thermal Properties, and Scratch Resistance of Transparent Poly(methyl methacrylate) Saihua Jiang,*,†,‡ Guohua Chen,† Yuan Hu,‡ Zhou Gui,*,‡ and Zhijia Hu§ †

School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong 510641, P. R. China ‡ State Key Laboratory of Fire Science, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230027, P. R. China § School of Instrument Science and Optoelectronics Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China ABSTRACT: A novel poly(methyl methacrylate) (PMMA)-based nanocomposite combined with a reactive flame retardant (tetramethyl (3-(triethoxysilyl) propyl-azanediyl) bis(methylene) diphosphonate (TMSAP)) and organo-modified layered aluminophosphate (OLAP) was synthesized by the sol−gel method. The structure of the nanocomposite achieves maximal integration of both merits of each component, such as silane cross-linking function and dehydration charring effect of TMSAP and physical barrier effect and catalytic-charring effect of OLAP, which promotes cross-linked network formation, improves the quality and quantity of char, and inhibits the heat, oxygen, and mass transfer, leading to significant enhancements of scratch resistance, thermal stability, and flame retardancy. Compared to PMMA, the nanocomposites maintain high transparency and exhibit increased shore hardness by 90%, glass transition temperature by 13 °C, and half degradation temperature by 105.1 °C; in addition, peak heat released rate decreased by 59.1%. This work demonstrates the simultaneous enhancement of flame retardancy, thermal properties, and mechanical performance of polymers. effect.5 The other method is adding FRs by copolymerization with methyl methacrylate (MMA) or using some other chemical methods.6,7 In this case, the presence of chemical bonds between polymer matrixes and the FRs can maintain flame retardancy for a longer time, and the good compatibility between the FRs and monomers contributes to the maintenance of high transparency.5 However, the thermal properties and mechanical properties (e.g., scratch resistance) are often deteriorated because of the increased molecular flexibility and free volume of polymer chains.7,8 To date, there still remains an important challenge of obtaining transparent PMMA materials with simultaneously enhanced thermal properties, flame retardancy, and scratch resistance. Cross-linking, which can link the long chains of polymers to form a three-dimensional network, is an effective method of improving mechanical and thermal properties of polymers.9,10 On the basis of this concept, some researchers have combined cross-linking methods and halogen-free FRs to enhance thermal stability and mechanical performance of flame-retardant polymers. For example, Wilkie and co-workers prepared cross-linked Polyamide 6 (PA-6) materials containing phosphorus oxynitride or phospham under 60Co γ-ray irradiation.11 Chiang et al. synthesized cross-linked phosphorus-containing polymer composites through the sol−gel process. 5 In our previous study, we have also synthesized a series of reactive FRs

1. INTRODUCTION As a typical transparent polymer, poly(methyl methacrylate) (PMMA) has shown great potential for applications in building construction, optical fiber communication, and the electronics industry because of its good optical properties, convenience of processing and molding, outstanding mechanical properties, and low cost. However, the poor scratch resistance performance and thermal stability restrict its application in many desirable areas, and the high flammability of PMMA affects its fire safety in use.1 Consequently, developing flame-retardant PMMA with enhanced scratch resistance and thermal stability has attracted increasing attention recently. For a long time, people used chlorinated and brominated compounds to reduce polymer flammability. Nevertheless, halogen-containing flame retardants (FRs) often produce toxic, corrosive, and halogenated gases, which harm the environment and people’s health. Thus, there is a trend toward using halogen-free FRs in polymer resins.2,3 Phosphorus-, nitrogen-, or silicon-containing compounds have become the most widely used halogen-free FRs because of their high flame-retardant efficiency and extensive sources.4−7 In general, they are applied into PMMA by two methods. One method is adding FRs during the processing of PMMA.4 This method is simple and economic, but has several drawbacks. For example, the FRs are often used in high concentrations to be effective, which may deteriorate the mechanical and thermal properties and transparency of PMMA matrix. Additionally, because of the limited linkage between FRs and the polymer matrix, the FRs can leach from polymers during normal service and aging, leading to an environmental threat and a weakening of the flame-retardant © XXXX American Chemical Society

Received: December 30, 2014 Revised: March 20, 2015 Accepted: April 20, 2015

A

DOI: 10.1021/ie5050549 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration for the preparation of (MMA-co-MSMA)/TMSAP/OLAP nanocomposites: (a) reaction process and structure for TMSAP, (b) reaction process and structure for MMA-co-MSMA, (c) sol−gel process for (MMA-co-MSMA)/TMSAP/OLAP nanocomposites.

charring of polymers.22−24 The catalytic carbonization function is also a favorable factor for the flame retardant system, further making the OLAP a promising candidate for flame-retardant applications in polymeric nanocomposites.25,26 To achieve simultaneously enhanced scratch resistance, thermal stability, and flame retardancy of transparent PMMA, herein we first synthesized a new phosphorus-, nitrogen-, and silicon-containing reactive flame retardant (TMSAP), and then introduce the TMSAP as well as OLAP into PMMA by the sol−gel method, obtaining a novel cross-linked PMMA nanocomposite. Structure and morphology of the nanocomposites were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), and 29Si MAS NMR. Transparency and scratch resistance performance of the nanocomposites were evaluated by ultraviolet−visible (UV− vis) spectra and shore hardness test, respectively. In addition, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to investigate thermal properties of the nanocomposites. Microscale combustion calorimetry (MCC), limiting oxygen index (LOI), and real-time Fourier transform infrared spectra (RT-IR) were used to study flame retardancy of the nanocomposites. It is found that the combination of OLAP and TMSAP successfully endows PMMA nanocomposites with enhanced surface hardness, Tg, thermal stability, and flame retardancy. Detailed mechanisms for the improved performance are discussed.

and incorporated them into polymers through the sol−gel method to prepare some cross-linked polymer composites.12,13 These composites obtained exhibited enhanced flame retardancy, thermal stability, and mechanical performance, such as tensile strength and scratch resistance. However, to achieve effective flame retardancy, the strategy combining FRs and cross-linking requires high levels of FRs, which would adversely affect the transparency of PMMA. Polymer nanocomposite technology is a highly efficient strategy, which can achieve remarkable improvements in mechanical and thermal properties and flame retardancy of polymers by adding only very small amounts of layered nanomaterials.14−16 According to the literature, the introduction of layered double hydroxide (LDH) and clay (≤5 wt %) could retard thermal degradation of PMMA and decrease its heat release rate significantly while keeping the high transparency at an accepted level.16,17 Two main reasons for these enhancements are the physical barrier effect of nanolayers for heat and mass transfer and the interface interaction between nanolayers and polymer matrixes.14−17 OLAP is a typical layered nanomaterial which consists of macroanionic sheets built up from alternation of Al-centered polyhedra (AlO4, AlO5, AlO6) and P-centered P(Ob)n(Ot)4−n tetrahedra (b, bridging; t, terminal; n = 1−4). The protonated organic amine molecules or complex cations reside in the interlayer region. Benefiting from the inherent lamellar structure, OLAP shows many attractive characteristics including large surface area and aspect ratio, thermal and chemical stability, high elastic modulus, catalytic activity, and the potential to delaminate within polymers.18−21 These properties of OLAP are similar to those of LDH and clay, suggesting its potential application in improving flame retardancy and thermal stability of PMMA. Moreover, OLAP is a common solid acid catalyst, which could catalyze the dehydrogenation of polymers and promote the

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethyl phosphate (DMPP) was obtained from Kaijie chemical company (Hefei, China). Hydrochloric acid (HCl, CP) was provided by Shanghai Chemical Reagents Company of China. Tetramethylammonium hydroxide (TMAH), tetrahydrofuran (THF), 2-azobis(isobutyronitrile) (AIBN), orthophosphoric acid (H3PO4), 3-metha-cryloxyB


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Table 1. Composition, Hardness, and Tg of PMMA, MMAco-MSMA, and (MMA-co-MSMA)/TMSAP/OLAP Nanocomposites

propyltrimethoxysilane (MSMA), aluminum chloride hexahydrate (AlCl3·6H2O), dodecylamine (DDA), 3-aminopropylmethyldimethoxysilane (APTES), methyl methacrylate (MMA), and paraformaldehyde purchased from Sinopharm Chemical Reagent Co., Ltd. were of analytical grade. MSMA, MMA, and AIBN were used after further purification. All other chemicals were used as received. 2.2. Preparation of OLAP. The synthesis of OLAP has been described in our previous work.27 2.3. Synthesis of Tetramethyl (3-(Triethoxysilyl) propyl-azanediyl) Bis(methylene) Diphosphonate (TMSAP). TMSAP was prepared according to our previous work, as illustrated in Figure 1a.12 In a typical preparation, into a 150 mL three-necked flask equipped with an addition funnel and a mechanical stirrer was introduced APTES (22.1 g, 0.10 mol), paraformaldehyde (6.0 g, 0.20 mol), and THF (40 mL). The reaction mixture was stirred vigorously at 40 °C. DMPP (22.0 g, 0.20 mol) was then added dropwise within 3 h. The reaction was heated at 65 °C for 12 h and then rotary evaporated to remove the solvent. The TMSAP was obtained with 80% yield as a viscous lemon yellow liquid. FTIR (KBr, cm−1): 2959−2854 (νC−H), 1460 (δ−C−H), 1243 (νPO), 1184 (νC−N), 1118 (νSi−O), 1039 (νP−O). 1H NMR (400 MHz, CDCl3-d, ppm): 0.53 (2H, −Si−CH2−), 1.07 (9H, C−CH3), 1.46 (2H, C−CH2−C), 2.65 (2H, N−CH2−), 3.09 (4H, N−CH2−P), 3.45 (6H, O−CH2−C), 3.6 (12H, O−CH3). 31 P NMR (400 MHz, CDCl3-d, ppm): 27.0 (s). The chemical struture of TMSAP was confirmed in our previous work.12 2.4. Preparation of (MMA-co-MSMA)/TMSAP/OLAP Nanocomposites. The preparation of (MMA-co-MSMA)/ TMSAP/OLAP nanocomposites comprises two steps. The first step (Figure 1b) is the synthesis of the copolymers of MMA with MSMA (MMA-co-MSMA) by bulk radical copolymerization.12 MSMA content in this copolymer is 10 wt %. The second step is the preparation of (MMA-co-MSMA)/ TMSAP/OLAP nanocomposites by sol−gel method (Figure 1c). In detail, OLAP was first distributed in liquid TMSAP under ultrasound to form a homogeneous TMSAP/OLAP mixture, in which TMSAP could perform as an organic dispersant. Next, MMA-co-MSMA (17 g) and TMSAP/OLAP mixture (3 g) were fed into a THF solution; 5.0 g of H2O and 1.5 g of HCl used as the catalyst were added into the samples, and subsequently the solution was stirred mechanically at room temperature for 24 h to yield a transparent and pale yellow liquid. The products were casted into aluminum dishes to gel at room temperature for 24 h because of the hydrolysis. The wet gels were aged at room temperature for 48 h and then dried at 100 °C for 12 h in a vacuum oven. In the sol−gel process, with the catalysis of HCl solution, the inorganic spheres were formed in situ by hydrolysis and the condensation of metal alkoxides. The composition of the resulting samples is given in Table 1. 2.5. Characterization. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 6700 spectrometer (Nicolet Instrument Company). 1H NMR and 31P NMR spectra of synthetic compounds were obtained using an AVANCE 400 Bruker spectrometer at room temperature with chloroform-d as the solvent. 29Si MAS NMR of nanocomposites was conducted by a Bruker DSX-400WB instrument (Germany). XRD was performed using a Japan Rigaku D/Max-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) images were

sample PMMA MMA-co-MSMA (MMA-co-MSMA)/ TMSAP15 (MMA-co-MSMA)/ TMSAP14/ OLAP-1 (MMA-co-MSMA)/ TMSAP13/ OLAP-2 (MMA-co-MSMA)/ TMSAP12/ OLAP-3

TMSAP (wt %)

OLAP (wt %)

MMA-coMSMA (wt %)

hardness (Shore D)

Tg (°C)

0 100 85.0

32.0 28.0 65.0

102.0 93.0 116.9

0 0 15.0

0 0 0

14.0

1.0

85.0

62.5

115.8

13.0

2.0

85.0

61.5

115.2

12.0

3.0

85.0

61.0

115.0

recorded on a JEOL JEM-2100F transmission electron microscopy with a 200 keV accelerated voltage. Transmittance of nanocomposite plates (1.5 mm thick) was recorded by a UV/vis spectrometer (UV-240). Shore D hardness was obtained from an LX-D Rubber−Plastics Shore D Sclerometer (Mingzhu Instruments, Jiangdu) according to GB/T 24112008. TGA was performed on a Q5000 thermo-analyzer instrument (TA Instruments, United States) from 30 to 700 °C under an air flow of 60 mL/min, employing a linear heating rate of 10 °C/min. DSC was carried out on a DSC Q2000 instrument (TA Ltd., United States). Samples (2−3 mg) were heated from 0 to 170 °C at a linear heating rate of 10 °C/min under nitrogen atmosphere; the temperature was kept at 170 °C for 10 min and then decreased at a linear rate of 10 °C/min from 170 to 0 °C. The temperature was kept at 0 °C for 10 min, and then the heating−cooling cycle was performed again. Tg values were determined by the midpoint of the inflection curve of the typical second heating. Two parallel tests for each sample were performed under the same condition, and the average deviation of temperature is within ±1.2%. MCC tests were conducted on a Govmak MCC-2 Microscale Combustion Calorimeter, which is a pyrolysis-combustion flow calorimeter. Limit oxygen index values were measured by a LOI Analyzer instrument, according to the ASTM Standard D 2863. The vertical burning test of samples was carried out on a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Co., China), according to UL-94 test ASTM Standard D 3801-1996. Scanning electron microscopy (SEM) images were obtained using a Hitachi X650 scanning electron microscope. Raman spectroscopy (RS) measurements were conducted at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., United States). RT-IR spectra were conducted on the Nicolet 6700 FTIR spectrophotometer equipped with a heating device and a temperature controller. Powders of samples were mixed with KBr powders, and the mixture was pressed into a tablet, which was then placed in a ventilated oven. The oven was heated in air at a heating rate of 10 °C/min. RT-IR spectra were obtained in situ during the thermal degradation of test samples through 16 scans.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. XRD was used to characterize the structure of OLAP and the nanocomposites. As shown in Figure 2, the XRD pattern of OLAP exhibits two peaks in the range of 2θ = 1−10°, which correspond to (001) C


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composites. The left shift of the (001) peak location, corresponding to the increase of basal spacing, indicates the OLAP nanolayers has been intercalated. Furthermore, the intensity of the (001) peak is decreased sharply when compared to pristine OLAP, implying that the oriented distribution of OLAP nanolayers has been destroyed. Thus, here the OLAP nanolayers have been intercalated or partial exfoliated. When less OLAP is added, the signals of OLAP disappear in the XRD pattern. For example, no characteristic peaks can be observed in the case of (MMA-co-MSMA)/TMSAP14/OLAP-1, which implies that the OLAP has been fully exfoliated. The morphology of OLAP in (MMA-co-MSMA)/TMSAP14/ OLAP-1 is different from the other two nanocomposites. That is because inorganic nanolayers at a higher level tend to agglomerate in polymer matrixes because of the strong intermolecular interaction, which would retard the penetration of polymer chains into the interlayer and inhibit the exfoliation.28 To further investigate the morphology of nanocomposites, the nanocomposite samples were also characterized by TEM. Figure 3 shows typical TEM images of (MMA-co-MSMA)/ TMSAP14/OLAP-1 (Figure 3a,c) and (MMA-co-MSMA)/ TMSAP12/OLAP-3 (Figure 3b,d) at different magnifications. For (MMA-co-MSMA)/TMSAP14/OLAP-1, the images present a homogeneously distributed morphology at low magnification (Figure 3a) and fully exfoliated OLAP nanolayers at high magnification (Figure 3c). When more OLAP is incorporated, at low magnification (Figure 3b), OLAP nanolayers are also dispersed well in the matrix; but at high magnification, the nanolayers are partially exfoliated (Figure 3d). Some intercalated OLAP nanolayers with an enlarged basal spacing of approximately 3.4 nm can be observed, which is due to the increasing intermolecular interaction between OLAP nanolayers as OLAP content rises.28 That is consistent with the XRD analysis. The TEM results of (MMA-co-MSMA)/

Figure 2. XRD patterns of (a) OLAP, (b) (MMA-co-MSMA)/ TMSAP12/OLAP-3, (c) (MMA-co-MSMA)/TMSAP13/OLAP-2, and (d) (MMA-co-MSMA)/TMSAP14/OLAP-1.

and (002) diffractions of layered structure. The high intensity of the (001) peak indicates the highly ordered organization of OLAP. The basal spacing calculated from the (001) diffraction peak at 2θ = 3.0° using Bragg equation is around 2.94 nm, large enough for small TMSAP molecules to enter the gallery easily by self-diffusion. Because the TMSAP possesses an organosilane structure, it is more compatible with polymer chains than pristine OLAP. The predispersion of OLAP by TMSAP is thus expected to improve the interfacial properties between OLAP nanolayers and the polymer matrix. This point is beneficial to the exfoliation and distribution of OLAP in polymers. Small-angle XRD patterns of (MMA-co-MSMA)/TMSAP/ OLAP nanocomposites are also presented in Figure 2. In the patterns of both (MMA-co-MSMA)/TMSAP13/OLAP-2 and (MMA-co-MSMA)/TMSAP12/OLAP-3, a weak and broad (001) peak of OLAP can be found at around 2θ = 2.61°, which suggests the successful introduction of OLAP in the nano-

Figure 3. TEM images: (a) low magnification (50 000×) and (c) high magnification (200 000×) for (MMA-co-MSMA)/TMSAP14/OLAP-1 and (b) low magnification (50 000×) and (d) high magnification (200 000×) for (MMA-co-MSMA)/TMSAP12/OLAP-3. D


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condensation of TMSAP and MSMA after the sol−gel process. Compared to T1, the peaks of T2 and T3 are the major microstructures, implying the formation of strongly silica crosslinked networks in the nanocomposites. The cross-linked network as well as the good dispersion of TMSAP and exfoliated OLAP nanolayers would be beneficial to the following performance enhancements. 3.2. Scratch Resistance Performance. Hardness is an important parameter for evaluating the scratch resistance performance of materials. Table 1 lists the hardness (Shore D) of all samples. From Table 1, the hardness of MMA-coMSMA is lower than that of PMMA, in agreement with the earlier references. This is due to the increased molecular flexibility and free volume of polymer chains.7,8 With the addition of TMSAP or TMSAP/OLAP, the hardness of all nanocomposites is increased and is much higher than those of PMMA and MMA-co-MSMA (maximally by 90%), which means that the scratch resistance of the nanocomposites is enhanced. Cross-linked network in the nanocomposites, which links the polymer chains by chemical bonds and then restricts the polymer chain motion, is the main reason for this enhancement.11,12 In addition, from (MMA-co-MSMA)/ TMSAP15 to (MMA-co-MSMA)/TMSAP12/TMSAP3, it is evident that the decreasing TMSAP content reduces the crosslink degree, resulting in a slight hardness decrease of the nanocomposites. However, the decrements of hardness are reduced as OLAP content rises. According to the earlier work, interaction between OLAP nanolayers and polymer matrixes such as hydrogen bonds and electrostatic interaction can also inhibit the movement of polymer chains.14−17,28 The hardness decrease tendency is thus slowed. On the basis of the analysis above, both cross-linked network and the interaction between well-dispersed OLAP nanolayers and polymer matrix contribute to the scratch resistance performance enhancement of the nanocomposites. 3.3. Thermal Properties. Tg is an important thermal parameter of polymers.3,28,30 Tg values of all samples tested by DSC are listed in Table 1. In the nanocomposites, a shift in the Tg of PMMA toward higher temperatures is observed after the addition of TMSAP or TMSAP/OLAP mixture. According to our earlier work, the increase of Tg is due to the presence of silica cross-linked interaction among the polymer chains.12,13 With the decrease of TMSAP content, the Tg values are reduced slightly. For example, Tg of (MMA-co-MSMA)/ TMSAP14/OLAP-1 exhibits a 1.0 °C decrease in comparison to that of (MMA-co-MSMA)/TMSAP15. However, the Tg decrement is reduced with increasing OLAP content. This tendency of Tg is similar to that of hardness, which can also be attributed to the increasing strong interaction between OLAP nanolayers and the matrix.18,27,28,31,32 Therefore, the Tg enhancement of nanocomposites is also due to the combination of cross-linked network and the interaction between OLAP nanolayers and the polymer matrix. To evaluate thermal stability of the nanocomposites, TGA curves of all samples in air are shown in Figure 6, and the corresponding data are given in Table 2. The temperatures at which 10% (T0.1) and 50% (T0.5) mass loss occurs are used as the measure of initial degradation temperatures and half degradation temperatures, respectively. From Figure 6, the thermal degradation of TMSAP occurs in two stages: the first stage before 330 °C is due to the degradation of phosphite ester groups, while the second stage from 330 to 600 °C is ascribed to the decomposition of alkoxy groups, with a high char yield of

TMSAP13/OLAP-2 is the same as that of (MMA-co-MSMA)/ TMSAP12/OLAP-3. Moreover, Figure 4 presents UV−vis

Figure 4. UV−vis spectra and digital photos of PMMA, MMA-coMSMA, and the nanocomposites: (a) PMMA, (b) MMA-co-MSMA, (c) (MMA-co-MSMA)/TMSAP15, (d) (MMA-co-MSMA)/ TMSAP14/OLAP-1, (e) (MMA-co-MSMA)/TMSAP13/OLAP-2, and (f) (MMA-co-MSMA)/TMSAP12/OLAP-3.

spectra and digital photos of all samples. With the introduction of TMSAP or TMSAP/OLAP, the nanocomposites exhibit a slight decline in the transmission of visible light, but they still retain relatively high transparency. The relatively high transparency of the nanocomposites reflects good dispersion of OLAP nanolayers and TMSAP in the polymer matrix. On the basis of the results of XRD, TEM, and UV−vis spectroscopy, it is concluded that OLAP nanolayers have been successfully exfoliated in the matrix and dispersed well across the whole nanocomposites. 29 Si MAS NMR is an effective technique for characterizing silicon types and silica cross-linked network in the polymer resins.12,29 The representative 29Si MAS NMR spectrum of nanocomposite samples ((MMA-co-MSMA)/TMSAP13/ OLAP-2) are given in Figure 5. From Figure 5, three chemical shift peaks at −49, −58, and −66 ppm are assigned as T1, T2, and T3, respectively, which are in good agreement with the literature.12,29 These data indicate the complete hydrolysis and

Figure 5. 29Si MAS NMR spectrum of (MMA-co-MSMA)/TMSAP/ OLAP-2. E


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than what might be expected based only on the TMSAP and OLAP contents, indicating that the residue of nanocomposites consists of both char from the polymer matrix along with the residues of TMSAP and OLAP. Table 2 lists the char yields of all samples at 500 °C deducting the residues from MMA-coMSMA, TMSAP and OLAP. From (MMA-co-MSMA)/ TMSAP15 to (MMA-co-MSMA)/TMSAP12/OLAP3, the char yields of all (MMA-co-MSMA)/TMSAP/OLAP nanocomposites are higher than that of (MMA-co-MSMA)/ TMSAP15 and increase as the OLAP content increases. This result suggests that both OLAP and TMSAP catalyze charring of the nanocomposites and that the catalyzed efficiency of OLAP is higher than that of TMSAP within the same content. Additionally, compared to (MMA-co-MSMA)/TMSAP15, the T0.1 values of all (MMA-co-MSMA)/TMSAP/OLAP nanocomposites are smaller and decrease with increasing OLAP content. The early degradation of the nanocomposites further confirms the catalytic charring effect of OLAP because the OLAP catalyzes charring of polymers through dehydrogenation at relatively low temperature, ultimately improving the thermal stability of residues at high temperature.22−24,27 This point can be confirmed by the better thermal stability of the nanocomposites at high temperature: The temperatures at the 80% mass loss rate (T0.8) are in order of PMMA (336.1 °C) < (MMA-co-MSMA)/TMSAP15 (426.4 °C) < (MMA-coMSMA)/TMSAP14/OLAP-1 (432.5 °C) < (MMA-coMSMA)/TMSAP13/OLAP-2 (434.0 °C) < (MMA-coMSMA)/TMSAP12/OLAP-3 (441.5 °C), indicating the enhanced endurance of the nanocomposites against thermal oxidation at high temperature. Given the characterizations above, the thermal stability enhancement of (MMA-coMSMA)/TMSAP/OLAP nanocomposites results from the cross-linked network as well as the char formation caused by OLAP and TMSAP. 3.4. Flame Retardancy. MCC was used to investigate the flame retardancy of all samples. Figure 7 presents the heat

Figure 6. TGA curves of OLAP, TMSAP, PMMA, MMA-co-MSMA, and the nanocomposites in air.

Table 2. Thermal Properties of PMMA, MMA-co-MSMA, and the Nanocomposites sample PMMA MMA-co-MSMA (MMA-co-MSMA)/TMSAP15 (MMA-co-MSMA)/ TMSAP14/OLAP-1 (MMA-co-MSMA)/ TMSAP13/OLAP-2 (MMA-co-MSMA)/ TMSAP12/OLAP-3

T0.1 (°C)

T0.5 (°C)

residue (wt %)a

char (wt %)b

225.0 233.5 274.1 267.0

294.6 294.6 400.4 401.5

0 2.0 13.2 13.5

0 0 3.6 4.1

250.0

401.6

13.9

4.6

250.0

399.7

15.7

6.5

Residue yield of the nanocomposites obtained from TGA at 500 °C. Char yield of the nanocomposites obtained from TGA at 500 °C, deducting the residue of MMA-co-MSMA, TMSAP (50.4 wt %), and OLAP (37.0 wt %). a b

50.4 wt %.8 In the case of pristine OLAP, the first mass-loss stage below 190 °C is due to desorption of interlamellar and physically adsorbed water, while the main loss occurring at around 250 °C corresponds to the weight loss of DDA molecules.27 The thermal degradation of MMA-co-MSMA is similar to that of PMMA at low temperature, which may be attributed to the structure similarity between MMA-co-MSMA and PMMA. However, with the further increase of temperatures, the thermal degradation of MMA-co-MSMA is slowed by the cross-link reaction of alkoxysilane groups at high temperature, leaving a silica residue yield of 2.0 wt %.12 By combining with TMSAP and OLAP in MMA-co-MSMA, the (MMA-co-MSMA)/TMSAP/OLAP nanocomposites obtained exhibits significantly enhanced thermal stability: T0.1 and T0.5 of all nanocomposites are much higher than those of neat PMMA and MMA-co-MSMA. According to our previous work, the thermal stability enhancement of the nanocomposites can be ascribed to the cross-linked network and char formation.12 The cross-linked network formed by the sol−gel process improves thermal stability by restricting the movement of polymer chains and increasing the melt viscosity,33 and the char enhances the thermal stability by retarding the transfer of heat and degradation products.12,26 From TGA curves and Table 2, neat PMMA has no residue at 500 °C and MMA-co-MSMA has a small residue yield of 2.0 wt %, while the corresponding residue percentages of all nanocomposites are beyond 13.0 wt %. The residue amount of the nanocomposites is even larger

Figure 7. HRR curves of PMMA, MMA-co-MSMA, and the nanocomposites.

release rate (HRR) curves of PMMA, MMA-co-MSMA, and the nanocomposites versus temperatures. Peak heat release rate (PHRR) and total heat release (THR), corresponding to the largest HRR value and the integral area of HRR, respectively, are two key parameters for fire hazard evaluation of materials (Table 3). Because of the high flammability, the PHRR (294.0 W/g) and THR (22.0 kJ/g) of neat PMMA are very high. The introduction of MSMA brings a slight decline in PHRR and F


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exhibit higher LOI values. Compared to that of (MMA-coMSMA)/TMSAP15, the LOI values of (MMA-co-MSMA)/ TMSAP/OLAP nanocomposites within the same content of fillers are higher and increase with the increasing OLAP content. These results further confirm the flame-retardant synergistic effect between OLAP and TMSAP. As is wellknown, char formation of nanocomposites is an important factor for the flame retardancy enhancement. Figure 8 displays the residual char of PMMA and the nanocomposites after the LOI test. It is evident that neat PMMA has no residue after combustion. Conversely, the surface of all nanocomposite residues is covered with a compact char layer, which is due to the catalytic charring effect of TMSAP and OLAP on the polymer matrix. With increasing OLAP content, the char layer amount of the nanocomposites is enhanced, indicating that the OLAP may further promote the formation of char. This result is in good agreement with the TGA results above. Because the char on the surface can reduce the flammability of polymers by restricting the oxygen, mass, and heat diffusion, the increase of char benefits the further flame retardancy enhancement of nanocomposites.23 In addition to the char formation, the physical barrier effect of the OLAP nanolayers, which can slow the transfer of heat and flammable products, is another important reason for the flame retardancy enhancement of (MMA-co-MSMA)/TMSAP/OLAP nanocomposites being better than that of (MMA-co-MSMA)/TMSAP15.17,25,34 3.5. Char Analysis. To investigate the flame-retardant mechanism, the analysis of char residue for nanocomposites is required. In general, SEM was used to analyze the char structure, while Raman spectroscopy was used to analyze the char composition. From the results of TGA and LOI testing, the char formation of all nanocomposites is similar. (MMA-coMSMA)/TMSAP12/OLAP-3 has the best flame retardancy among the nanocomposites; its char analysis is thus most helpful for the explanation of flame-retardant mechanism.12,13,26 Figure 9 shows the SEM photographs of interior and exterior char of (MMA-co-MSMA)/TMSAP12/OLAP-3 after the LOI test. The exterior char of (MMA-co-MSMA)/TMSAP12/ OLAP-3 (Figure 9a,b) is compact and continuous, whereas the interior char (Figure 9c,d) exhibits some holes and many

Table 3. Flame Retardancy of PMMA, MMA-co-MSMA, and the Nanocomposites sample

LOI

UL-94 rating

dripping

PHRR (W/g)

THR (kJ/g)

yes

294.0

22.0

yes

260.8

20.2

no

170.2

14.9

PMMA

17.0

MMA-co-MSMA

17.5

(MMA-co-MSMA)/ TMSAP15 (MMA-co-MSMA)/ TMSAP14/OLAP-1 (MMA-co-MSMA)/ TMSAP13/OLAP-2 (MMA-co-MSMA)/ TMSAP12/OLAP-3

20.0

not classified not classified V-1

21.0

V-1

no

152.0

14.5

22.0

V-1

no

138.0

13.9

23.0

V-1

no

120.1

13.2

THR, but the obtained MMA-co-MSMA retains a relatively high flammability. In (MMA-co-MSMA)/TMSAP15, the addition of TMSAP reduces the PHRR and THR of MMAco-MSMA significantly, which is consistent with our previous work.12 When the TMSAP/OLAP mixture is introduced, the PHRR and THR of (MMA-co-MSMA)/TMSAP/OLAP nanocomposites are shifted to values even lower than those of (MMA-co-MSMA)/TMSAP15 within the same filler content, indicating the flame-retardant efficiency of TMSAP/OLAP is better than that of TMSAP. For instance, the nanocomposite with 15 wt % TMSAP alone ((MMA-co-MSMA)/TMSAP15) has a PHRR value of 170.2 W/g and a THR value of 14.9 kJ/g, whereas the nanocomposites with 15 wt % TMSAP/OLAP mixture presents a much lower PHRR (120.1−152.0 W/g) and THR (13.2−14.5 kJ/g). The maximum decrease of PHRR and THR are 59.1% and 40.0%, respectively. These results imply that there is a synergistic effect between OLAP and TMSAP in enhancing the flame retardancy of PMMA. To further study the flame retardancy of nanocomposites, Table 3 gives the LOI values and UL-94 test results of all samples. Neat PMMA is flammable, exhibiting a low LOI value of 17.0, serious dripping, and no rating in the UL-94 test. With the addition of TMSAP or TMSAP/OLAP, all nanocomposites can pass the V-1 rating of UL-94 test, without dripping, and

Figure 8. Photos of the sample residues at the end of the LOI test: (a) PMMA, (b) (MMA-co-MSMA)/TMSAP15, (c) (MMA-co-MSMA)/ TMSAP14/OLAP-1, (d) (MMA-co-MSMA)/TMSAP13/OLAP-2, and (e) (MMA-co-MSMA)/TMSAP12/OLAP-3. G


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Figure 9. SEM images for the char residue of (MMA-co-MSMA)/TMSAP12/OLAP-3: (a) exterior char (500×); (b) exterior char (2000×); (c) interior char (500×); (d) interior char (2000×).

latter peak (G band) corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline,35 which suggests the formation of graphitized char during nanocomposite combustion. The graphitized char has thermal stability higher than that of the amorphous char, which is more beneficial to the suppression of heat spread during combustion.36 Furthermore, the graphitization degree of the char can be estimated by a ratio of the intensity of the G and D bands (IG/ID), where IG and ID are the integrated intensities of the G and D bands, respectively. Basically, the higher the IG/ID ratio, the better the structure of char. From Figure 10, the IG/ID ratio follows the sequence (MMA-co-MSMA)/TMSAP14/ OLAP-1 (0.20) < (MMA-co-MSMA)/TMSAP13/OLAP-2 (0.22) < (MMA-co-MSMA)/TMSAP12/OLAP-3 (0.26). This result not only indicates the catalytic effect of OLAP on graphitic char formation but also suggests the highest graphitization degree and the most thermally stable char structure of the (MMA-co-MSMA)/TMSAP12/OLAP-3. The combination of catalytic charring effect of TMSAP and OLAP, graphitization of char catalyzed by OLAP, and physical barrier effect of OLAP nanolayers for heat and flammable degradation products makes the flame retardancy of (MMA-co-MSMA)/ TMSAP12/OLAP-3 better than that of any other nanocomposites. 3.6. Thermal Degradation Behavior. RT-IR was employed to study the thermal degradation process of the nanocomposites. Figure 11 shows the RT-IR spectra of PMMA and (MMA-co-MSMA)/TMSAP12/OLAP-3 at different degradation temperatures. From the RT-IR spectrum of PMMA (Figure 11a), the peak at 3442 cm−1 almost disappears at 280 °C because of the release of water. The other characteristic peaks, including 2956, 1730, 1438, 1438, 1242, and 1149 cm−1, are almost unchanged below 150 °C. That is consistent with the TGA results: little weight loss occurs below 150 °C. With

closed bubbles. On the basis of the analysis above, TMSAP and OLAP promote the char formation of the polymer matrix in the condensed phase, which can restrict the rapid volatilization of the degradation products, resulting in foamed char layers at high temperature. From Figure 9c,d, it is also found that some OLAP layers after degradation are embedded in the char layer, which would enhance the mechanical properties and compactness of the char layer. The foamed char combined with OLAP layers serve as a superior protective barrier for flammable gas products, oxygen, and heat and thus protects the polymers from further burning. Raman spectra of char residues for (MMA-co-MSMA)/ TMSAP/OLAP nanocomposites are displayed in Figure 10, exhibiting two peaks at 1356 and 1591 cm−1. The former peak (D band) represents disordered graphite or glassy carbons; the

Figure 10. Raman spectra for the char residues of (MMA-co-MSMA)/ TMSAP/OLAP nanocomposites. H


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reaches 400 °C, all characteristic peaks disappear, indicating that PMMA decomposes completely.37 The FTIR spectrum of (MMA-co-MSMA)/TMSAP12/OLAP-3 (Figure 11b) is similar to that of PMMA. The characteristic peaks of TMSAP and OLAP are not obvious because they probably coincide with the characteristic bands of the PMMA matrix. There is an overlapping of absorption bands assigned to Si−O and P−O bonds with those of PMMA at 950−1150 cm−1.5,38 Compared to that of PMMA, the RT-IR spectra of (MMA-co-MSMA)/ TMSAP12/OLAP-3 at above 340 °C exhibit some new peaks, including 1804, 1761, 1463, 1286, 1088, and 1012 cm−1. The peaks at 1600−2000 and 1463 cm−1 demonstrates the existence of aromatic structure in the char residue. The peak at 1088 cm−1 is assigned to the vibration of Si−O, suggesting the presence of Si−O−Φ in the char residue.5,38 The bands at 1283 and 1012 cm−1 correspond to the stretching vibrations of PO and PO, respectively, which are attributed to the OP OΦ structure. The Φ represents the graphite-like polynuclear aromatic structure. These peaks remain as the temperature reaches 500 °C, implying that the formed graphitic char includes SiOΦ and OPOΦ complexes and has very high thermal stability, which is consistent with the Raman results.12,38,39 3.7. Flame-Retardant Mechanism. Based on the analysis presented above, a schematic representation of flame-retardant mechanism of (MMA-co-MSMA)/TMSAP12/OLAP nanocomposites was proposed (Figure 12). The flame-retardant mechanism of the nanocomposites was believed to occur in the condensed phase because of the formation of a foamed char layer catalyzed by OLAP and TMSAP. The char combining with the physical barrier effect of OLAP nanolayers retards the transfer of flammable gas products, oxygen, and heat, and the flame retardancy of the nanocomposites is thus enhanced. Moreover, there are three beneficial factors for the barrier effect of the char layers: (a) the foamed structure of char, which has better heat insulation effect than normal char;26 (b) the presence of OLAP layers in char layer, which enhances mechanical property of the char; and (c) the graphitization of char by OLAP catalysis, which improves the thermal stability of

Figure 11. RT-IR spectra of (a) PMMA and (b) (MMA-co-MSMA)/ TMSAP12/OLAP-3 at different pyrolysis temperatures.

further increase in temperature, the relative intensity of these peaks decreases sharply, suggesting that the main decomposition of PMMA occurs at this stage. When the temperature

Figure 12. Schematic illustration of the mechanism for the enhanced flame retardancy of (MMA-co-MSMA)/TMSAP/OLAP nanocomposites. I


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Industrial & Engineering Chemistry Research the char.36 Because of the combination of flame-retardant strategies above, the flame-retardancy of nanocomposites with TMSAP/OLAP is significantly enhanced and is much better than that of PMMA and nanocomposites with TMSAP alone, exhibiting a flame-retardant synergistic effect of TMSAP and OLAP.

between metal oxide nanoparticles and phosphinates. Polym. Degrad. Stab. 2007, 92, 61. (5) Chiang, C. L.; Chiu, S. L. Synthesis, characterization and properties of halogen-free flame retardant PMMA nanocomposites containing nitrogen/silicon prepared from the Sol-Gel method. J. Polym. Res. 2009, 16, 637. (6) Vahabi, H.; Ferry, L.; Longuet, C.; Otazaghine, B.; NegrellGuirao, C.; David, G.; Lopez-Cuesta, J. M. Combination effect of polyhedral oligomeric silsesquioxane (POSS) and a phosphorus modified PMMA, flammability and thermal stability properties. Mater. Chem. Phys. 2012, 136, 762. (7) Wang, G. A.; Wang, C. C.; Chen, C. Y. The flame-retardant material - 1. Studies on thermal characteristics and flame retardance behavior of phosphorus-containing copolymer of methyl methacrylate with 2-methacryloxyethyl phenyl phosphate. Polym. Degrad. Stab. 2006, 91, 2683. (8) Tai, Q. L.; Song, L.; Hu, Y.; Yuen, R. K. K.; Feng, H.; Tao, Y. J. Novel styrene polymers functionalized with phosphorus-nitrogen containing molecules: Synthesis and properties. Mater. Chem. Phys. 2012, 134, 163. (9) Gheysari, D.; Behjat, A. The effect of high-energy electron beam irradiation and content of ATH upon mechanical and thermal properties of EVA copolymer. Eur. Polym. J. 2002, 38, 1087. (10) Smedberg, A.; Hjertberg, T.; Gustafsson, B. Effect of molecular structure and topology on network formation in peroxide crosslinked polyethylene. Polymer 2003, 44, 3395. (11) Balabanovich, A.; Levchik, S. V.; Levchik, G. F.; Schnabel, W.; Wilkie, C. A. Thermal decomposition and combustion of γ-irradiated polyamide 6 containing phosphorus oxynitride or phospham. Polym. Degrad. Stab. 1999, 64, 191. (12) Jiang, S. H.; Yu, B.; Zhou, K. Q.; Yang, H. Y.; Shi, Y. Q.; Lo, S. M.; Hu, Y.; Gui, Z. Sol-gel synthesis and enhanced properties of a novel transparent PMMA based organic-inorganic hybrid containing phosphorus, nitrogen and silicon. J. Sol-Gel Sci. Technol. 2014, 69, 418. (13) Jiang, S. H.; Hu, Y.; Gui, Z.; Dong, Y. Y.; Wang, X.; Zhou, K. Q.; Lo, S. M. Enhanced Thermal Properties and Flame Retardancy of a Novel Transparent Poly(methyl methacrylate)-Based Hybrid Prepared by the Sol-Gel Method. Ind. Eng. Chem. Res. 2012, 51, 9447. (14) Zhang, T.; Yan, H. Q.; Peng, M.; Wang, L. L.; Ding, H. L.; Fang, Z. P. Construction of flame retardant nanocoating on ramie fabric via layer-by-layer assembly of carbon nanotube and ammonium polyphosphate. Nanoscale 2013, 5, 3013. (15) Song, P. A.; Liu, L. N.; Huang, G. B.; Yu, Y. M.; Guo, Q. P. Largely enhanced thermal and mechanical properties of polymer nanocomposites via incorporating C-60@ graphene nanocarbon hybrid. Nanotechnology 2013, 24, 505706. (16) Chen, W.; Feng, L.; Qu, B. J. In situ synthesis of poly(methyl methacrylate)/MgAl layered double hydroxide nanocomposite with high transparency and enhanced thermal properties. Solid State Commun. 2004, 130, 259. (17) Wang, W. S.; Liang, C. K.; Chen, Y. C.; Su, Y. L.; Tsai, T. Y.; Chen-Yang, Y. W. Transparent and flame retardant PMMA/clay nanocomposites prepared with dual modified organoclay. Polym. Adv. Technol. 2012, 23, 625. (18) Covarrubias, C.; Quijada, R. Preparation of aluminophosphate/ polyethylene nanocomposite membranes and their gas permeation properties. J. Membr. Sci. 2010, 358, 33. (19) Sarkar, K.; Bhaumik, A. Hydrothermal transformation of a layered aluminophosphate into a mesoporous structure. J. Porous Mater. 2008, 15, 445. (20) Kimura, T.; Sugahara, Y.; Kuroda, K. Synthesis and characterization of lamellar and hexagonal mesostructured aluminophosphates using alkyltrimethylammonium cations as structure-directing agents. Chem. Mater. 1999, 11, 508. (21) Kongshaug, K. O.; Fjellvag, H.; Lillerud, K. P. Layered aluminophosphates III. Crystal structure and thermal properties of the novel layered aluminophosphate UiO-18. Microporous Mesoporous Mater. 2000, 38, 311.

4. CONCLUSIONS A novel PMMA-based nanocomposite combined with a reactive FR (TMSAP) and OLAP in PMMA matrix has been successfully prepared through the sol−gel method. Compared to neat PMMA, the as-fabricated nanocomposites keep relatively high transparency and exhibit significantly enhanced scratch resistance, Tg, thermal stability, and flame retardancy; shore hardness increased by 90%, Tg by 13 °C, and T0.5 by 105.1 °C, and PHRR decreased by 59.1%. Possible mechanisms for the performance enhancements of nanocomposites are proposed as follows: First, the improvements of scratch resistance and Tg in the nanocomposites are attributed to the cross-linked network and the strong interaction between the OLAP nanolayers and polymer matrix, which inhibit the motion of polymer chains. Second, the cross-linked network and char formation caused by TMSAP and OLAP retard the degradation of the polymer matrix; the thermal stability of nanocomposites is therefore enhanced. Third, the flameretardant synergistic effect between OLAP and TMSAP is responsible for the flame retardancy enhancements of the nanocomposites. This work not only provides a novel transparent PMMA material with excellent all-round properties but also demonstrates the simultaneous enhancement of the flame retardancy, thermal properties, and mechanical performance of polymers.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-020-22236321. Fax: +86-020-22236321. E-mail: [email protected]. *Tel.: +86-551-63601288. Fax: +86-551-63601669. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2012CB922002), the Opening fund of Guangdong Provincial Key Laboratory of fire science and technology (2013A01), and the Natural Science Foundation of Guangdong Province (2014A030310122).

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REFERENCES

(1) Kim, S.; Wilkie, C. A. Transparent and flame retardant PMMA nanocomposites. Polym. Adv. Technol. 2008, 19, 496. (2) Rakotomalala, M.; Wagner, S.; Doring, M. Recent Developments in Halogen Free Flame Retardants for Epoxy Resins for Electrical and Electronic Applications. Materials 2010, 3, 4300. (3) Chen, C. H.; Yen, W. H.; Kuan, H. C.; Kuan, C. F.; Chiang, C. L. Preparation, Characterization, and Thermal Stability of Novel PMMA/ Expandable Graphite Halogen-Free Flame Retardant Composites. Polym. Composite. 2010, 31, 18. (4) Laachachi, A.; Cochez, M.; Leroy, E.; Ferriol, M.; Lopez-Cuesta, J. M. Fire retardant systems in poly(methyl methacrylate): Interactions J


Article

Industrial & Engineering Chemistry Research (22) Saha, S. K.; Waghmode, S. B.; Maekawa, H.; Komura, K.; Kubota, Y.; Sugi, Y.; Oumi, Y.; Sano, T. Synthesis of aluminophosphate molecular sieves with AFI topology substituted by alkaline earth metal and their application to solid acid catalysis. Microporous Mesoporous Mater. 2005, 81, 289. (23) Sankar, G.; Gleeson, D.; Catlow, C. R. A.; Thomas, J. M.; Smith, A. D. The architecture of Mg(II) centres in MAPO-36 solid acid catalysts. J. Synchrotron Radiat. 2001, 8, 625. (24) Cora, F.; Catlow, C. R. A.; Civalleri, B.; Orlando, R. Acid strength of low-valence dopant ions in microporous zeolites and AlPOs. J. Phys. Chem. B 2003, 107, 11866. (25) Yang, D.; Hu, Y.; Song, L.; Nie, S.; He, S.; Cai, Y. Catalyzing carbonization function of α-ZrP based intumescent fire retardant polypropylene nanocomposites. Polym. Degrad. Stab. 2008, 93, 2014. (26) Chen, L.; Wang, Y. Z. A review on flame retardant technology in China. Part 1: development of flame retardants. Polym. Adv. Technol. 2010, 21, 1. (27) Jiang, S. H.; Gui, Z.; Hu, Y.; Zhou, K. Q.; Dong, Y. Y.; Shi, Y. Q. The intercalation of poly(methyl methacrylate)/aluminophosphate nanocomposites and the properties improvement. Mater. Chem. Phys. 2013, 141, 95. (28) Jiang, S.; Gui, Z.; Bao, C.; Dai, K.; Wang, X.; Zhou, K.; Shi, Y.; Lo, S.; Hu, Y. Preparation of functionalized graphene by simultaneous reduction and surface modification and its polymethyl methacrylate composites through latex technology and melt blending. Chem. Eng. J. (Amsterdam, Neth.) 2013, 226, 326. (29) Joseph, R.; Zhang, S.; Ford, W. T. Structure and Dynamics of a Colloidal Silica−Poly(methyl methacrylate) Composite by 13C and 29 Si MAS NMR Spectroscopy. Macromolecules 1996, 29, 1305. (30) Tress, M.; Mapesa, E. U.; Kossack, W.; Kipnusu, W. K.; Reiche, M.; Kremer, F. Glassy Dynamics in Condensed Isolated Polymer Chains. Science 2013, 341, 1371. (31) Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Model polymer nanocomposites provide an understanding of confinement effects in real nanocomposites. Nat. Mater. 2007, 6, 278. (32) Zhou, K.; Jiang, S.; Bao, C.; Song, L.; Wang, B.; Tang, G.; Hu, Y.; Gui, Z. Preparation of poly(vinyl alcohol) nanocomposites with molybdenum disulfide (MoS2): Structural characteristics and markedly enhanced properties. RSC Adv. 2012, 2, 11695. (33) Zhao, H. B.; Liu, B. W.; Wang, X. L.; Chen, L.; Wang, Y. Z. A flame-retardant-free and thermo-cross-linkable copolyester: Flameretardant and anti-dripping mode of action. Polymer 2014, 55, 2394. (34) Shi, Y.; Kashiwagi, T.; Walters, R. N.; Gilman, J. W.; Lyon, R. E.; Sogah, D. Y. Ethylene vinyl acetate/layered silicate nanocomposites prepared by a surfactant-free method: Enhanced flame retardant and mechanical properties. Polymer 2009, 50, 3478. (35) Dervishi, E.; Li, Z.; Watanabe, F.; Courte, A.; Biswas, A.; Biris, A. R.; Saini, V.; Xu, Y.; Biris, A. S. Versatile Catalytic System for the Large-Scale and Controlled Synthesis of Single-Wall, Double-Wall, Multi-Wall, and Graphene Carbon Nanostructures. Chem. Mater. 2009, 21, 5491. (36) Ma, H. Y.; Tong, L. F.; Xu, Z. B.; Fang, Z. P. Synergistic effect of carbon nanotube and clay for improving the flame retardancy of ABS resin. Nanotechnology 2007, 18, 375602. (37) Tripathi, S. N.; Saini, P.; Gupta, D.; Choudhary, V. Electrical and mechanical properties of PMMA/reduced graphene oxide nanocomposites prepared via in situ polymerization. J. Mater. Sci. 2013, 48, 6223. (38) Li, Q.; Jiang, P.; Su, Z.; Wei, P.; Wang, G.; Tang, X. Synergistic effect of phosphorus, nitrogen, and silicon on flame-retardant properties and char yield in polypropylene. J. Appl. Polym. Sci. 2005, 96, 854. (39) Zhu, S.; Shi, W. Thermal degradation of a new flame retardant phosphate methacrylate polymer. Polym. Degrad. Stab. 2003, 80, 217.

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A New Strategy for Simultaneously Improved Flame Retardancy

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