Enhanced Thermal Properties and Flame Retardancy of a Novel

Article pubs.acs.org/IECR

Enhanced Thermal Properties and Flame Retardancy of a Novel Transparent Poly(methyl methacrylate)-Based Hybrid Prepared by the Sol−Gel Method Saihua Jiang,†,‡,§ Yuan Hu,*,†,§ Zhou Gui,† Yangyang Dong,† Xin Wang,† Keqing Zhou,† and Siuming Lo‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China and USTC-CityU Joint Advanced Research Centre, Suzhou, P.R. China ‡ Department of Civil and Architectural Engineering, City University of Hong Kong and USTC-CityU Joint Advanced Research Centre, Suzhou, P.R. China § Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, Suzhou, P.R. China ABSTRACT: A novel monomer, poly(ethylene glycol) monoacrylate aminopropyltriethoxysilane phenyl phosphate (SNP), containing phosphorus, nitrogen, and silicon was synthesized and then incorporated into a poly(methyl methacrylate) (PMMA) matrix through copolymerization and the sol−gel method to produce organic−inorganic hybrids. The chemical structure of SNP was characterized by FTIR, 1H NMR, 29Si NMR, and 31P NMR spectroscopies. The 29Si MAS NMR results for the hybrid materials suggested the formation of cross-linked networks in the hybrids. A morphological study showed that the inorganic particles were well distributed in the PMMA matrix. The hybrids retained a high transparency and exhibited a significant improvement in glass transition temperature, thermal stability, hardness, and flame retardancy upon the incorporation of SNP into the PMMA matrix. The network structure, homogeneous distribution, and char formation during degradation were proposed as three key reasons for the improved properties.

1. INTRODUCTION As a typical transparent amorphous polymer, poly(methyl methacrylate) (PMMA) has been widely used as an important material for plastic optical fibers as well as optical lenses. It has several desirable properties such as high transparency, good flexibility, high strength, and excellent dimensional stability. However, its poor thermal stability and flammability limits its wide application.1,2 Consequently, improving the thermal properties and flame retardancy of PMMA has been attracting increasing attention. Traditionally, PMMA is treated to improve the thermal properties and flame retardancy through the physical incorporation of additives.3−5 However, physical incorporation often leads to the reduction of the transparency and mechanical properties due to poor distribution and causes an environmental threat because of the limited linkage between the additives and the polymer matrix.2,6 An alternative technique is the chemical incorporation of the flame retardants through copolymerization. The relatively low loadings required to achieve sufficient flame retardancy and careful selection of the comonomer can limit detrimental changes to the physical and mechanical properties of the polymer to an acceptable level. Additionally, the chemical bonds between the matrix and the flame retardants lead to a good distribution of the flame retardants without modification, and the flame retardancy of the polymer can be kept longer. Recently, most flame retardants used in PMMA resins have been halogenated compounds, which work by suppressing ignition and slowing the spread of the flame.7 However, because of the desire to avoid environmental pollution, there is a trend toward using halogen-free flame retardants in PMMA resins. Phosphorus-, © 2012 American Chemical Society

nitrogen-, or silicon-containing compounds as promising “green” flame retardants are mostly used to replace halogenated compounds in polymers.6,8−11 The sol−gel method has been widely used to produce organic−inorganic hybrids with homogeneous microstructures at relatively low temperatures. In sol−gel reactions, a crosslinked inorganic network is formed through the hydrolysis and condensation of metal alkoxides.12−14 High-performance or highly functionalized hybrid materials can be obtained by this effective combination of organic and inorganic components at a molecular level. These materials offer exceptional opportunities not only to combine the important properties of the organic and inorganic phases, but also to create entirely new compositions with truly unique optical, electrical, mechanical, and thermal properties.14−16 Various polymers, such as poly(vinyl acetate), polyimide, epoxy, and poly(vinylidene fluoride), have been successfully incorporated into inorganic networks by the sol−gel method, and the resulting hybrid materials have exhibited improved specific properties.17−20 In the present work, a novel organic−inorganic hybrid material was prepared by incorporating phosphorus, nitrogen, and silicon into a PMMA matrix through both copolymerization and sol−gel technologies. The hybrids exhibited enhanced hardness, glass transition temperature, thermal stability, and flame retardancy; moreover, they retained a high transparency. Received: Revised: Accepted: Published: 9447

March 26, 2012 June 27, 2012 June 28, 2012 June 28, 2012 dx.doi.org/10.1021/ie300803c | Ind. Eng. Chem. Res. 2012, 51, 9447−9455

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis Route and Structure of SNP

filtered, the solution was rotary evaporated to remove the solvent and the unreacted reactants under reduced pressure. A light yellow viscous liquid was obtained. This synthesis route is illustrated in Scheme 1. Yield 30.5 g (78.0%); 1H NMR (CDCl3, ppm) δ 7.1−7.3 (5H, aromatic proton), 6.4 and 5.8 (2H, vinyl), 6.1 (1H, CH CH2), 4.4 (2H, COOCH2), 4.3 (2H, CH2O PO), 4.1 (1H, NH), 3.8 (6H, CH2CH3), 3.0 (2H, NCH2), 1.6 (2H, SiCCH2), 1.1 (9H, CH3), 0.6 (2H, SiCH2C); 31P NMR (CDCl3, ppm), δ 4.9 (singlet peak); 29Si NMR (CDCl3, ppm), δ −46.0 (singlet peak); FTIR (neat liquid, cm−1) 3241 (s, NH), 3069 (w, aromatic C H), 2971−2886 (s, CH3 and CH2), 1730 (s, CO), 1637 (w, vinyl), 1264 (s, PO), 1100 (s, SiO). 2.3. Preparation of Poly(MMA-co-SNP) Copolymers. Copolymers of PMMA with various amounts of SNP were prepared by free-radical bulk copolymerization. Different amounts of SNP were dispersed in MMA monomers under ultrasound for 1 h; the total weight was 40 g. Once the mixture became completely transparent, 0.2 wt % of BPO was added. The mixture was then heated and stirred at 90 °C for 1 h until it formed a viscous paste. The paste was then transferred into a mold and held at 60 °C for 48 h to complete the polymerization process and remove the unconverted monomers. The charged monomer ratios of poly(MMA-co-SNP) copolymers are listed in Table 1. The reaction process is presented in Scheme 2a. 2.4. Preparation of PMMA-Based Organic−Inorganic Hybrids. The hybrids were prepared by a sol−gel method. Typically, adequate amounts of poly(MMA-co-SNP) copolymers were fed into a THF solution. H2O (4.0 g) and HCl (1.5 g) were used as the catalyst and added into the samples, and the solution was stirred mechanically at room temperature for 6 h to yield a transparent and pale yellow liquid. The products were

The enhanced effects of poly(ethylene glycol) monoacrylate aminopropyltriethoxysilane phenyl phosphate (SNP) on hardness and glass transition temperature were investigated. A mechanism for the improved thermal stability and flame retardancy of the hybrids is also proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. Phenyl dichlorophosphate (PDCP) was purchased from Deheng Chemical Corp. (Shijiazhuang, China). Hydrochloric acid (HCl, CP) was obtained from Shanghai Chemical Reagents Company of China. Tetrahydrofuran (THF), methyl methacrylate (MMA), benzoyl peroxide (BPO), triethylamine (TEA), 3-aminopropylmethyldimethoxysilane (APTES), and hydroxyethyl acrylate (HEA) were of analytical grade and were provided by Sinopharm Chemical Reagent Co., Ltd. THF was refluxed with natrium and then distilled before use. MMA was used after further purification including retarder removal, water removal, and reducedpressure distillation. BPO was further purified by recrystallization from methanol. PDCP, TEA, HEA, and APTES were purified by reduced-pressure distillation before use. All other chemicals were used as received. 2.2. Synthesis of the Functionalized Monomer (SNP). PDCP (16.88 g, 0.08 mol) was dissolved in 100 mL of dry THF in a 500 mL three-neck flask fitted with a mechanical stirrer. TEA (16.19 g, 0.16 mol) was added to this above blend, and the system was cooled to −5 °C. After 20 min, APTES (17.71 g, 0.08 mol) in 50 mL of THF was added dropwise over a period of 2 h, and the mixture was then held at constant temperature for 5 h. Subsequently, HEA (9.28 g, 0.08 mol) in 50 mL of THF was added dropwise over a period of 2 h and held at constant temperature for 4 h. Finally, the mixture was kept at room temperature and stirred for an additional 10 h. After the precipitated triethylamine hydrochloride had been 9448

dx.doi.org/10.1021/ie300803c | Ind. Eng. Chem. Res. 2012, 51, 9447−9455


Article

(Nicolet Instrument Company). For solid samples, KBr discs were prepared. Spectra of liquid samples were recorded on thin films of the neat material. Elemental analyses were carried out on an Elementar Vario EL-III instrument. 1 H NMR, 31P NMR, and 29Si NMR spectra of synthetic monomer were recorded using an AVANCE 400 Bruker spectrometer at room temperature with chloroform-d as the solvent. 29Si MAS NMR spectroscopy of the samples was conducted on a Bruker DSX-400WB instrument. Each sample was heated at 100 °C for 2 h and then ground into a fine powder. The high-performance liquid chromatography (HPLC) system used in this study consisted of a Shimadzu (Kyoto, Japan) LC-10AD HPLC pump and an SPD-M20A (254 nm) UV detector. Data were acquired with a Shimadzu Chromatopac C-R7A plus data processor. Chromatography was carried out at 30 °C with a Shimadzu CTO-10AC column oven at a flow rate of 1.0 mL/min. The mobile phase and HPLC column used were 20% acetonitrile solution and GLODS-3 (C18), respectively. Scanning electron microscopy (SEM) images of the fractured surfaces were recorded on a Sirion200 scanning electron microscope (FEI Co. Ltd.) at 5 kV acceleration voltage. The samples were cryogenically broken after immersion in liquid nitrogen. SEM images of cross sections of char residues were obtained using a Hitachi X650 scanning electron microscope.

Table 1. Compositions and Hardnesses of PMMA-Based Hybrids with Different SNP Contents

sample pure PMMA sample 1 sample 2 sample 3

MMA/ SNP (mass ratio)

SNP content in the hybrida (wt %)

100:0

0

97:3 95:5 90:10

8.8 14.6 29.2

N content (wt %)

P content (wt %)

Si content (wt %)

0

0

0

0.26 0.43 0.86

0.57 0.95 1.90

0.52 0.86 1.72

hardness (shore A) 81.0 93.2 96.1 101.3

a

SNP content in each hybrid obtained by dividing the nitrogen content in SNP by the nitrogen content in the hybrid.

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 6 h in a vacuum oven. In the sol−gel process (as shown in Scheme 2b), with the catalysis of HCl solution, inorganic spheres were formed in situ by hydrolysis and the condensation of metal alkoxides located at the end of the side chain of the copolymer. The contents of nitrogen, silicon, and phosphorus in the samples were obtained by elemental analyses. Details of the resulting samples with different concentrations of SNP are presented in Table 1. The reaction process is presented in Scheme 2b. 2.5. Characterization. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 6700 spectrometer

Scheme 2. Schematic Synthesis Procedure of Hybrids: (a) Reaction Process and Structure of Poly(MMA-co-SNP) Copolymers and (b) Sol−Gel Process for Organic−Inorganic Networks

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The specimens were previously coated with a conductive layer of gold. The shore A hardness values of the samples were obtained withan LX-A Rubber−Plastics Shore A Sclerometer (Jiangdu Mingzhu Instruments) according to standard test method GB T 531 92. The test methods are generally reproducible to an accuracy of ±0.8%. The transmittance values of 1.5-mm-thick PMMA and PMMA-based hybrid plates were measured using a UV/vis spectrometer (UV-240). Repeated experiments showed that there was no significant difference in transmittance for a small change in sample thickness. The glass transition temperature was determined using a TA2000 differential scanning calorimeter (DSC) at a heating rate of 10 °C/min from 0 to 170 °C under nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a TGA Q5000 thermogravimetric analyzer (TA Instruments), with samples of approximately 3 mg heated to 700 °C at a heating rate of 10 °C/min in air flow. Each sample was examined twice in DSC and TGA tests. Microscale combustion calorimetry (MCC) tests were carried out on a Govmak MCC-2 microscale combustion calorimeter, which is a pyrolysis-combustion flow calorimeter. Powder samples (4−6 mg) were heated to 650 °C at a heating rate of 1 °C/s in an inert gas steam (80 mL min−1), and each sample was examined three times. The pyrolysis products were mixed with oxygen (20 mL/min) before entering a 900 °C combustion furnace, and the heats of combustion of the pyrolysis products were measured by the oxygen consumption principle. Limiting oxygen index (LOI) values were measured using a limiting oxygen index analyzer instrument on test bars of size 100 × 6.5 × 3.0 mm3, according to ASTM standard D2863. X-ray photoelectron spectroscopy (XPS) of the char residue was performed with a VG Escalab Mark II spectrometer (VG Scientific Ltd.), using Al Kα excitation radiation (hν = 1253.6 eV).

Figure 1. (a) 1H NMR, (b) 31P NMR, and (c) 29Si NMR spectra of SNP.

3. RESULTS AND DISCUSSION The chemical structure of SNP was characterized by 1H NMR, 31 P NMR, 29Si NMR, and FTIR spectroscopies. As can be seen from Figure 1, the chemical shift at 1.1 ppm can be attributed to CH3, and the methylene resonance of ethyl groups appears at 3.8 ppm. The peaks at 0.6, 1.6, and 3.0 ppm are due to the methylene resonance of APTES. The methoxy protons in HEA appear at 4.3 and 4.4 ppm. The doublets of the  CH2 vinylic pattern appear at about 6.4 and 5.8 ppm, and the doublets of the CH vinylic pattern appear at 6.1 ppm. Aromatic signals appear at 7.3−7.1 ppm. The peak at 4.1 ppm can be attributed to NH. The chemical structure was further confirmed by 31P NMR and 29Si NMR spectra. Both the 31 P NMR and 29Si NMR spectra showed a single peak, as illustrated in parts b and c, respectively, of Figure 1. These results indicate that SNP was successfully synthesized. Figure 2 shows the FTIR spectrum of SNP, in which the stretching vibration peaks of CH2 and CH3 appear at 2971−2886 cm−1. The absorptions at around 3246 and 1730 cm−1 correspond to the stretching vibrations of NH and the double-bond absorption of CO, respectively. The absorptions at 1637 and 1594 cm−1 are attributed to the vinyl and aromatic vibration, respectively. The absorption band at 1264 cm−1 is assigned to PO. The characteristic peaks for

Figure 2. FTIR spectrum of SNP.

SiOC might coincide with other peaks at around 1100 cm−1.14 These results verify the structure of SNP, as shown in Scheme 1. The purity of SNP was determined by HPLC. In the chromatogram (Figure 3), four peaks can be seen at 1.66, 2.29, 2.85, and 3.41 min. The peak at 1.66 min corresponds to SNP; the other three peaks correspond to impurities. The purity of SNP in the product was calculated as 92.9%. 29 Si MAS NMR spectroscopy is an appropriate technique for describing the type of silicons present in the resin network. 9450



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Figure 3. HPLC chromatogram of the product.

Figure 4 shows the 29Si MAS NMR spectrum of the hybrid (sample 3). The spectrum includes three peaks, which are

Figure 5. SEM images of the fractured surfaces of (a) pure PMMA and (b) PMMA-based hybrid (sample 3) cryogenically broken after immersion in liquid nitrogen.

Figure 4. 29Si MAS NMR spectrum of PMMA-based hybrid (sample 3).

assigned as T1, T2, and T3. The chemical shifts of T1, T2, and T3 are −49, −58, and −66 ppm, respectively, which are consistent with the literature values.21 These data suggest the complete hydrolysis and condensation of SNP. The degree of condensation was estimated at around 78.0% by means of 29 Si MAS NMR spectroscopy.22 From Figure 4, T2 and T3 are the major microstructures compared to T1, indicating the formation of strongly cross-linked networks in the hybrids, just as shown in Scheme 2b. SEM of cryogenically broken surfaces is a common and useful method of determining the inner structure of the polymer/inorganic composites.14,23 Figure 5 presents representative SEM images of pure PMMA (Figure 5a) and sample 3 (Figure 5b). Comparison between parts a and b of Figure 5 suggests that the inorganic silica particles are formed in the hybrids after hydrolysis and condensation of SNP. As shown at higher magnification in Figure 5b, the inorganic particles (about 100 nm) have good interfacial adhesion with the polymer matrix and and are distributed uniformly in the fracture surface of polymer, because of the existence of chemical bonds between them. These results reveal that the organic/inorganic phases exhibit good compatibility and strong interaction in the hybrid materials. UV/vis transmission spectra and the digital images of pure PMMA and the hybrids (1.5 mm) with different SNP concentrations are shown in Figure 6. With increasing SNP concentration, the hybrids show a decline in transmission in the

Figure 6. UV/vis spectra and digital images of pure PMMA and PMMA-based hybrids.

visible light region, but they still retain a relatively high transparency in the visible light region. The relatively high transparency of the hybrids further verifies the good compatibility between the organic and inorganic phases and the good dispersion of the inorganic particles in the polymer matrix, which agrees well with the results from SEM. The good compatibility and excellent nanoscale dispersion of the inorganic particles are very helpful and important for the subsequent improvement in properties. Hardness is an important parameter for mechanical properties. Marked improvements in hardness (shore A) with increasing SNP content were measured, as reported in Table 1. Hardness for the hybrids varied from 93.2 to 101.3, whereas pure PMMA had a hardness of only 82.0, meaning that the addition of SNP to PMMA made the hybrids harder. It is speculated that the efficiently dispersed particles interacted 9451



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strongly with the polymer chains and formed networks in the matrix, leading to the enhancement of the hardness. The glass transition temperature (Tg) is an important thermal parameter that can be used to evaluate the segmental mobility of polymers. To verify the existence of the network and investigate its influence on segmental motions in the hybrid materials, the Tg values of all of the samples were tested and calculated by DSC. The DSC data and curves of the samples are presented in Table 2 and Figure 7, respectively. A significant Table 2. Thermal Properties and Flame Retardancy of PMMA-Based Hybrids with Different SNP Contents sample pure PMMA sample 1 sample 2 sample 3

Tg (°C)

T0.1 (°C)

Tmax (°C)

char residue at 500 °C (wt %)

LOI (%)

PHRR (W/g)

THR (kJ/g)

102

225

298

0

17.0

294

22.0

104 110 115

299 303 311

387 407 432

1.9 6.0 14.7

18.0 20.0 22.0

252 240 218

20.1 19.5 17.3

Figure 8. TGA and DTG curves of pure PMMA and PMMA-based hybrids with different SNP contents in air.

of the samples were promoted. The incorporation of only 8.8 wt % SNP increased T0.1 and Tmax by 74 and 89 °C, respectively. As the SNP content was further increased to 29.2%, T0.1 and Tmax of the hybrid reached 311 and 432 °C, which are 86 and 134 °C higher than the corresponding values for pure PMMA. These results indicate that the incorporation of SNP significantly enhanced the thermal stability of the PMMA-based hybrids. The cross-linked network structure and the intact phosphorus−silicon networks can promote the thermal stability of the hybrid materials.14 Therefore, it is proposed that the network structure in the PMMA hybrids is one of the main causes of the improved thermal stability. In the hybrids, the network and good interfacial interaction between the inorganic particles and the organic matrix make the segmental motion of the polymer in the hybrids more difficult than that in pure PMMA, as demonstrated by the increase in Tg. Then, the thermal stability of the PMMA-based hybrid was enhanced. Another possible reason is that the char formation of the hybrids during heating enhanced the thermal stability. According to the TGA curves, pure PMMA has almost no residues at 500 °C, whereas the corresponding residue amounts for samples 1−3 were about 1.9%, 6.0%, and 14.7%, respectively. The explanation of the observed behaviors is that the degradation of silicon- and phosphorus-containing groups generates heat-resistant residues to result in high char yields. The char would prevent heat transmission and heat diffusion, limit the production of combustible gases, and defer the degradation of the materials. The TGA and DSC tests indicated that the thermal stability of the polymer was effectively improved by incorporating SNP. The increase of Tg verifies the existence of the cross-linked network in the hybrids. A comparison of the char residue between pure PMMA and the hybrids suggests char formation during the degradation of the hybrid materials. Moreover, from Tables 1 and 2, the slight increases in phosphorus content (from 0.57% to 1.90%) and silicon content (from 0.52 to 1.72%) led to much higher increases in the residue amounts, thus verifying the high efficiency of the condensation reaction. The combined action of the network and char formation significantly retarded the thermal degradation of PMMA. To explain the effect of char formation on the improved thermal stability of the hybrids, the char residue of sample 3 (heated in a muffle furnace for 10 min at 600 °C) was investigated. Figure 9 shows the SEM images of the exterior and interior char residues of sample 3. It can be seen that the exterior char residue of sample 3 (Figure 9a,b) presented a

Figure 7. DSC curves of (a) pure PMMA, (b) sample 1, (c) sample 2, and (d) sample 3.

shift in the Tg values of the hybrids toward higher temperature was observed after the incorporation of SNP into PMMA. The Tg value increased gradually from 102 to 115 °C as the SNP content increased. The increase in Tg can be ascribed to the great restriction of the polymer chain motions, which suggests formation of a network and good interfacial interactions between the inorganic particles and the polymer chains. TGA is one of the most widely used techniques for the rapid evaluation of the thermal stability and thermal degradation behaviors of various polymers. Figure 8 presents the TGA and DTG curves of pure PMMA and hybrids with different SNP contents under a flow of air. The onset degradation temperatures of the samples were evaluated by the temperatures of 10 wt % weight loss (T0.1) from TGA curves, and the temperatures of the maximum weight loss rate (Tmax) of samples were obtained from the DTG curves. These data are listed in Table 2. From the TGA and DTG curves, a broad weight-loss step occurred between 180 and 450 °C, corresponding to the thermal decomposition of the polymer chains, which moved dramatically to higher temperatures with increasing SNP content. In detail, the T0.1 and Tmax values of the hybrids showed the same changing tendencies. With increased loading of SNP, the two characteristic temperatures 9452



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Table 3. XPS Results for the Char Residue from Sample 3 interior exterior

C

O

N

P

Si

Si/C

45.23 31.80

40.47 49.80

3.15 4.16

6.25 8.30

4.89 5.93

0.108 0.186

exterior char residue was highly oxidized under the richer oxygen conditions. In addition, the atom percentages of silicon in the exterior and interior char residues were 4.89% and 5.93%, respectively. The Si/C ratio of the exterior char increased by 72.2% (from 0.108 to 0.186) compared to that of interior char, implying that silicon atoms migrated to the surface during the combustion and carbonization of the hybrid materials. This process can be explained as follows: During the combustion process, the thermal oxidative degradation of hybrid materials proceeds quickly and violently at high temperature. The phosphorus element in SNP contributes to the formation of a char layer on the surface of the matrix. Meanwhile, the silicon element in the hybrids quickly undergoes a series of oxidation reactions to form SiO2. After “de-attachment” from the polymer chains, the silica becomes more hydrophilic and less compatible with the resin. Then, quite a few SiO2 particles are pushed to the surface of the matrix by numerous rising bubbles of degradation products and the associated convection flow.24 Under the interaction between particles, the silica migrates to the surface more easily. The presence of SiO2 on the surface protects the char layer from thermodegradation, enhances the stability of the char layer, and prevents the release of volatile products from the matrix.8 This is consistent with the SEM results for the char residues. The good char formation contributes not only to the enhancement of the thermal stability but also to the improvement of the flame retardancy of materials.12 To evaluate the flame retardancy of the hybrids, Table 2 reports the LOI values of all of the samples. Pure PMMA resin is highly combustible, and its LOI value is only 17.0%. With increasing SNP, the LOI values of the hybrids increased obviously. As the loading of SNP reached 29.2 wt % (phosphorus content of 1.90 wt % and silicon content of 1.72 wt %), the corresponding LOI value of the hybrid reached 22.0%. This observation indicates that SNP has a good flame-retardant effect on PMMA. For further investigate the flame retardancy of the hybrids, data on the heat release rate is required. Microscale combustion calorimetry (MCC) is a new, rapid, laboratory-scale test that uses thermal analysis methods to measure chemical properties related to fire. It provides key data such as peak of the heat release rate (PHRR) and the total heat release (THR).25,26 The curves of the heat release rate (HRR) of pure PMMA and the hybrids are shown in Figure 11, and the corresponding data are listed in Table 2. According to a previous report, PMMA depolymerizes and forms MMA monomers, which are highly flammable, during thermal degradation.23,27 Therefore, it is not surprising that pure PMMA gives high values of PHRR (294 W·g−1) and THR (22.0 kJ·g−1) during MCC tests. From Figure 11 and Table 2, the incorporation of SNP shifted the PHRR to lower values. As the amount of SNP increased, the PHRR and THR decreased obviously, suggesting that the incorporation of SNP significantly reduced the flammability of PMMA. On one hand, as can be seen in the SEM images of the char resiue and the TGA curves, the presence of SNP promoted the formation of a protective char, which can protect the underlying materials from further burning and reduce its heat release. On the other hand, the XPS results suggest that silicon atoms migrate to the

Figure 9. SEM images of char residues of sample 3: (a) exterior char residues (×200), (b) exterior char residues (×2000), (c) interior char residues (×200), (d) interior char residues (×2000).

compact and continuous char layer, whereas the interior char residue of sample 3 (Figure 9c,d) presented many cracked and closed bubbles. This means that the incorporation of SNP promoted the char formation of the hybrids, which restricted the rapid volatilization of the degradation products at the surface and protected the underlying matrix from further degradation. The exterior and interior char residues of sample 3 were also investigated by XPS analysis to further analyze their components. As shown in Figure 10, the peaks at 536.5,

Figure 10. XPS spectra of the exterior and interior char residues of the hybrid (sample 3).

408.2, 292.7, 139.3, and 107.1 eV are characteristic O 1s, N 1s, C 1s, P 2p, and Si 2p bands, respectively, of the char residue. The atom percentages are listed in Table 3. The atom percentage of oxygen in the exterior char residue was higher than that in the interior char residue. Moreover, the atom percentage of carbon in the exterior char residue was lower than that in the interior char residue. The reason is that the 9453



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-3601664. Fax: +86-551-3601664. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the joint fund of NSFC and CAAC (No. 61079015), and the Fundamental Research Funds for the Central Universities (WK2320000007).

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Figure 11. HRR curves of pure PMMA and PMMA-based hybrids.

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surface during combustion and carbonization, which protects the char layer from thermodegradation and enhances the stability of the char layer. Therefore, the migration of silicon is another important factor in the improvement of the flame retardancy of the hybrids. Based on the characterizations above, the possible mechanisms for enhanced thermal properties and flame retardancy are proposed as follows: First, the network structure in the hybrids plays as a key role in improving the thermal stability. The network produced by the sol−gel method can effectively restrict the mobility of the polymer chains, thus retarding the thermal degradation of the hybrids. Second, the degradation of phosphonate in SNP promotes the char formation of the hybrids. The compact and continuous char prevents heat transmission and diffusion, reduces the production of combustible gases, and defers the degradation of the polymer, all of which enhance both the thermal stability and the flame retardancy of the hybrids. Third, the migration of silicon to the surface during combustion and carbonization also plays an important role, protecting the char layer from thermal oxidative degradation and enhancing its stability. Therefore, compared with other modifying monomers for PMMA including traditional silane coupling agents such as OPS and TPM and phosphonates such as MEPP, SNP exhibits better efficiency in improving thermal properties and flame retardancy.6,11

4. CONCLUSIONS A flame retardant monomer (SNP) containing phosphorus, nitrogen, and silicon was successfully synthesized and characterized. The copolymerization and sol−gel technologies were used to incorporate the monomers into the PMMA matrix in different ratios, resulting in a novel organic−inorganic hybrid material. Because of the existence of chemical bonds between the inorganic and organic phases, the well-distributed particles exhibit strong interfacial interactions with the organic polymer chains. The network, good distribution, and strong interactions are believed to be three important factors in the enhanced hardness and thermal properties and the retention of the optical properties. Moreover, because of char formation and the migration of silicon during the combustion, the degradation of the hybrids is retarded, and the flame retardancy is improved. It was thus demonstrated that SNP is a promising and suitable monomer for PMMA hybrid materials with excellent combined properties of thermal stability, flame retardancy, and transparency. 9454



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Enhanced Thermal Properties and Flame Retardancy of a Novel

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