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An exfoliation nanocomposite (PE-g-MA/MgAl-LDH) was synthesized by solution ... Geoffrey I. N. Waterhouse , Kun Zhang , Shan Li , Peng Jin , and Geyan...
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Chem. Mater. 2003, 15, 3208-3213

Structural Characteristics and Thermal Properties of PE-g-MA/MgAl-LDH Exfoliation Nanocomposites Synthesized by Solution Intercalation Wei Chen and Baojun Qu* State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Received January 29, 2003. Revised Manuscript Received May 28, 2003

Exfoliated nanocomposites (PE-g-MA/MgAl-LDH) were synthesized by solution intercalation of polyethylene-grafted-maleic anhydride (PE-g-MA) into the galleries of organo-modified MgAl layered double hydroxide (OMgAl-LDH) under reflux in xylene. Their structural elucidation and thermal characterization were carried out by X-ray diffraction (XRD), Fourier transfer infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), and differential thermal analysis (DTA). The molecular dispersion of OMgAl-LDH layers within the PE-g-MA matrix has been verified by the disappearance of d001 XRD diffraction peak of OMgAl-LDH and the observation of TEM image. The OMgAl-LDH layers of about 70-nm length or width show a disordered phase in PE-g-MA matrix. The SAED pattern demonstrates that the MgAl hydroxide sheets of about 0.48-nm thickness have a hexagonal crystal structure with a ) 0.305 nm. TGA profiles of the PE-g-MA/MgAl-LDH nanocomposites show a faster charring process in temperature range from 210 to 360 °C and greater thermal stability above 370 °C than PE-g-MA. The decomposition temperature of the nanocomposites with 5 wt % OMgAlLDH can be 60 °C higher than that of PE-g-MA when 50% weight loss was selected as a measuring point. Dynamic FTIR spectra reveal that this nanocomposite has a slower thermooxidative rate than PE-g-MA from 200 °C to 320 °C. This kind of exfoliation nanocomposite is promising for use in flame-retardant polymeric materials.

Introduction In recent years, polymer/layered crystal nanocomposites have been recognized as one of the most promising research fields in materials chemistry because of their unique properties, such as enhanced mechanical property, increased thermal stability, improved gas barrier property, and reduced flammability.1-5 As far as we are aware, however, these kinds of nanocomposites have been mainly focused on the polymer/layered silicate systems, while the layered double hydroxide (LDH) systems have been much less reported in the literature.6-9 Several methods have been adopted to prepare polymer/layered silicate nanocomposites, such as in situ polymerization, self-assembly of exfoliated inorganic layers with polymers, template synthesis of layered crystals in the polymer solution, melting intercalation, and direct ion exchange of polyelectrolyte with hosts.5,10 * Corresponding author. Fax: +86-551-3607245. E-mail: qubj@ ustc.edu.cn. (1) Gu, A. J.; Chang, F. C. J. Appl. Polym. Sci. 2001, 79, 289. (2) Tyan, H. L.; Leu, C. M.; Wei, K. H. Chem. Mater. 2001, 13, 222. (3) Kim, D. W.; Blumstein, A.; Tripathy, S. K. Chem. Mater. 2001, 13, 1916. (4) Yeh, J. M.; Liou, S. J.; Lai, C. Y. Wu, P. C. Chem. Mater. 2001, 13, 1131. (5) Alexandre, M.; Dubois, P. Mater. Sci. Eng. R 2000, 28, 1, and references therein. (6) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (7) Messersmith, P. B.; Stupp, S. I. Chem. Mater. 1995, 7, 454. (8) Bubniak, G. A.; Schreiner, W. H.; Mattoso, N.; Wypych, F. Langmuir 2002, 18, 5967. (9) O’Leary, S.; O’Hare, D.; Seeley, G. Chem. Commun. 2002, 1506.

The LDHs can be represented by the ideal formula [M2+1-xM3+x(OH)2]x+An-x/n‚mH2O, where M2+ and M3+ are divalent and trivalent metal cations, such as Mg2+, Al3+, respectively, A is an anion, such as CO32-, SO42-, and NO3-. LDHs are important layered crystals due to their wide applications as catalysts, flame retardants, stabilizers, medical matericals, etc.11 Because of their highly tunable properties, LDHs are considered as a new emerging class of the most favorable layered crystals for preparation of multifunctional polymer/ layered crystal nanocomposites.6 Some organic or polymeric anions, such as alkyl carboxylates, alkyl sulfates, polyacylates, and poly(styrene sulfates) have been intercalated into the interlayer of LDHs by ion-exchange reaction or in situ polymerization. This was accompanied by an expansion of the basal spacing.12-15 LDHs have much higher charge density in the interlayer and stronger interaction among the hydroxide sheets than layered silicates, which makes the exfoliation of LDHs much more difficult.16-18 In addition, the (10) Lagaly, G. Appl. Clay Sci. 1999, 15, 1. (11) Vaccari, A. Appl. Clay Sci. 1999, 14, 161. (12) Carlino, S. Solid State Ionics 1997, 98, 73. (13) Hussein, M. Z. B.; Zainal, Z.; Ming, C. Y. J. Mater. Sci. Lett. 2000, 19, 879. (14) Rey, S.; Merida-Robles, J.; Han, K. S.; Guerlou-Demourgues, L.; Delmas, C.; Duguet, E. Polym. Int. 1999, 48, 277. (15) Wilson, O. C.; Olorunyolemi, T.; Jaworski, A.; Borum, L.; Young, D.; Siriwat, A.; Dickens, E.; Oriakhi, C.; Lerner, M. Appl. Clay. Sci. 1999, 15, 265.

10.1021/cm030044h CCC: $25.00 © 2003 American Chemical Society Published on Web 07/08/2003

PE-g-MA/MgAl-LDH Exfoliation Nanocomposites

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Table 1. Sample Compositions and Preparation Conditions sample code

loading of OMgAl-LDH (wt %)

refluxing condition

PEMgAl NC1 PEMgAl NC2 PEMgAl NC3

5 2 5

under air under nitrogen under nitrogen

basal spacing of pristine LDHs, such as hydrotalcite, is about 0.8 nm and its layer thickness is about 0.5 nm, so the gallery height is only about 0.3 nm, which is much smaller than the radius of gyration of general polymers. Moreover, the hydrophobic character of general polymers may also hinder the penetration of polymer chains into LDH layers. Therefore, to prepare polymer/LDH nanocomposites, it is necessary to modify the pristine LDHs in order to alter the surface properties and to facilitate the intercalation of polymers. In the present work we report the exfoliation of oganomodified MgAl layered double hydroxide (OMgAl-LDH) layers through intercalation of polyethylene-graft-maleic anhydride (PE-g-MA) in xylene solution to form PE-gMA/MgAl-LDH nanocomposites. Dodecyl sulfate (DS) ions were intercalated into the MgAl layered double hydroxide (MgAl-LDH) to weaken the electrostatic forces among the hydroxide sheets and render the LDH layers hydrophobic. The obtained polymer/LDH nanocomposites show enhanced flame-retardant properties and thermal stability. Experimental Section Materials. PE-g-MA (CMG9804) with 0.8 wt % maleic anhydride was supplied by Shanghai SUNNY New Technology Development Co., Ltd. Al(NO3)3‚9H2O and Mg(NO3)2‚6H2O (analytical pure) were supplied by Shanghai Zhenxing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS), NaOH, and Na2CO3 (analytical pure) were obtained from China Medicine (Group) Shanghai Chemical Reagent Corporation. All these commercial chemicals were used as received without further purification. Preparation. MgAl-LDH (also known as hydrotalcite) was obtained by adding 100 mL of Mg(NO3)2‚6H2O (0.075 mol) and Al(NO3)3‚9H2O (0.025 mol) aqueous solution into 100 mL of Na2CO3 (0.025 mol) and NaOH (0.2 mol) aqueous solution. The slurry was then aged for 12 h at 70-75 °C keeping the pH at 8.0-9.0 by adjusting with 1 mol/L NaOH. The precipitates were washed with distilled water and dried in oven at 75-80 °C. The OMgAl-LDH sample was obtained from the rehydration process of calcined MgAl-LDH. A 2-g MgAl-LDH sample was calcined at 500 °C for 6 h, and then suspended in 100 mL of aqueous solution containing 2 g of SDS under refluxing condition for 6 h, yielding a white powder OMgAl-LDH. The PE-g-MA/MgAl-LDH exfoliation nanocomposites were obtained by refluxing the mixture of desired amount of OMgAl-LDH and 3.80 g of PE-g-MA in 50 mLof xylene under air or nitrogen for 24 h. Table 1 shows the sample compositions and the details of preparation conditions. The solution was then poured out into 100 mL of ethanol. The precipitates, PE-g-MA/MgAlLDH nanocomposites, were filtered and dried under vacuum at 100 °C for 24 h. Characterization. The X-ray diffraction (XRD) was performed using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu KR tube and Ni filter (λ ) 0.1542 nm). The Fourier transfer infrared (FTIR) spectra were (16) Adachi-Pagano, M.; Forano, C.; Besse, J. Chem. Commun. 2000, 91. (17) Leroux, F.; Adachi-Pagano, M.; Intissar, M.; Chauviere, S.; Forano, C.; Besse, J. P. J. Mater. Chem. 2001, 11, 105. (18) Hibino, T.; Jones, W. J. Mater. Chem. 2001, 11, 1321.

Figure 1. FTIR spectra for samples of (A) PE-g-MA, (B) MgAlLDH, (C) OMgAl-LDH, and (D) PE-g-MA/MgAl-LDH exfoliation nanocomposite (PEMgAl NC1). recorded using a Nicolet MAGNA-IR 750 spectrometer using a KBr disk or thin film. The transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern were obtained on a Hitachi H-800 transmission electron microanalyzer with an accelerated voltage of 200 kV and a camera length of 0.8 m. The sample was ultramicrotomed with a diamond knife on an LKB Pyramitome to give 100-nm thick slices. The slices were transferred from water to a 200-mesh Cu grid. The thermogravimetric analysis (TGA) was preformed on a Shimadzu TGA-50H thermoanalyzer. In each case an 18-mg sample was examined under an air flow rate of 6 × 10-5 m3/min at a scan rate of 5 °C/min from room temperature to 800 °C. The differential thermal analysis (DTA) was carried out under an air flow rate of 6 × 10-5 m3/min, using a Shangping ZRY-1P Thermal Analyzer at a heating rate of 5 °C /min from room temperature to 800 °C. The real-time FTIR spectra were recorded using a Nicolet MAGNA-IR 750 spectrometer equipped with a heating device having a temperature controller as reported in the literature.19,20 The film samples were placed in a ventilated oven at a heating rate of 10 °C /min for the dynamic measurement of FTIR spectra in situ during the thermo-oxidative degradation. The FTIR software was used to measure peak intensity of the FTIR spectra in order to compare the degradation rate of different materials. The relative concentration of alkyl group can be calculated by the intensity ratio of related peak height to the maximum height of 2923 cm-1 peak at 150 °C. Repeated experiments showed that there was no real difference for a small change of sample thickness in measuring the relative peak intensity.

Results and Discussion FTIR Characteristics. Figure 1 compares the FTIR spectra of PE-g-MA, MgAl-LDH, OMgAl-LDH, and PE-g-MA/MgAl-LDH nanocomposite (PEMgAl NC1) samples. The PE-g-MA sample (Figure 1A) mainly shows the absorption bands of PE because the amount of maleic anhydride in the PE-g-MA sample is very small (0.8 wt %). The absorption bands at 2845-2960 cm-1 can be assigned to the stretching vibration for -CH-, -CH2-, or -CH3. The absorption at 1468 cm-1 is due to the deformation vibration of -CH2- or -CH3 groups. The absorption at about 720 cm-1 is due to (CH2)n rock when n g 4. The spectrum of MgAl-LDH sample (Figure 1B) has a strong and broad absorption band at about 3500 cm-1 due to O-H stretching mode. The band at about 1640 cm-1 is due to the δ (H-OH) vibration. The carbonate stretching modes appear at (19) Xie, R. C.; Qu, B. J.; Hu, K. L. Polym. Degrad. Stab. 2001, 72, 313. (20) Wu, Q.; Qu, B. J. Polym. Degrad. Stab. 2001, 74, 255.

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Figure 2. XRD patterns of (A) MgAl-LDH and (B) OMgAlLDH in the range of 2θ ) 5-65°.

632 cm-1 for ν4, 826 cm-1 for ν2, and 1384 cm-1 for ν3. The band at 1224 cm-1 has been assigned to the stretching vibration of sulfate in the OMgAl-LDH sample (Figure 1C). The above results are in good agreement with those reported in the literature.21 The stretching vibration bands of -CH2- or -CH3 in the aliphatic chains of DS appear at 2845-2960 cm-1. The weak bands at 632, 826, and 1384 cm-1 are due to a small amount of carbonate produced by CO2 dissolved in solution during the rehydration procedure of calcined MgAl-LDH. The XRD pattern in the next section also confirms the mixed phase of OMgAl-LDH with carbonate. Compared with the PE-g-MA sample (Figure 1A), the PE-g-MA/MgAl-LDH exfoliation nanocomposite sample, PEMgAl NC1, (Figure 1D) shows some new absorption peaks at 3100-3600 and 1640 cm-1 due to the O-H stretching vibration and δ (H-OH) vibration, respectively, which indicates the existence of OMgAlLDH layers in the PE-g-MA matrix. These FTIR assignments indicate that the OMgAl-LDH layers have been doped into the PE-g-MA matrix during the refluxing process and thus form the PE-g-MA/MgAl-LDH nanocomposites. XRD Characteristics. The XRD patterns in the range of 2θ ) 5-65° for MgAl-LDH and OMgAl-LDH samples are shown in Figure 2. Figure 2A represents a typical XRD pattern of MgAl-LDH sample as a pure hydrotalcite. Its diffraction peaks can be indexed by the JCPDS X-ray powder diffraction file of No. 22-700. The value of crystallographic parameter a is 0.305 nm, which is calculated from (110) diffraction following the literature.22 The first diffraction peak (003) at 2θ ) 11.2° corresponds to a basal spacing of 0.79 nm, which is close to the value of 0.78 nm reported by Chibwe et al.23 A new peak at 2θ ) 6.5° emerges in the XRD pattern of OMgAl-LDH, as shown in Figure 2B. That means the DS anions have been intercalated into the MgAl-LDH layers. The very weak peak at 2θ ) 11.2° indicates the presence of a small amount of MgAl-LDH. This is because the adsorption degree of monovalent anions is less than that of higher charged species in aqueous solution.23 Another reason is that the presence of a small amount of CO32- in the preparation of OMgAl-LDH is (21) Aramendia, M. A.; Borau, V.; Jimenez, C.; Luque, J. M.; Marinas, J. M.; Ruiz, J. R.; Urbano, F. J. Appl. Catal. A 2001, 216, 257. (22) Rives, V.; Kannan, S. J. Mater. Chem. 2000, 10, 489. (23) Chibwe, K.; Jones, W. Chem. Commun. 1989, 926.

Chen and Qu

Figure 3. XRD patterns of (A) OMgAl-LDH and (B) PE-MA/ MgAl-LDH nanocomposite (PEMgAl NC1) in the range of 2θ ) 1.5-10°.

Figure 4. Crystallographic structure of OMgAl-LDH.

very difficult to avoid. A similar observation has been reported.9,23 Actually, the very small amount of MgAlLDH has no effect on the exfoliation of OMgAl-LDH. Figure 3 shows the XRD patterns at the range of 2θ ) 1.5-10° for OMgAl-LDH and PE-g-MA/MgAl-LDH nanocomposite (PEMgAl NC1) samples. The basal spacing of OMgAl-LDH is determined to be 2.75 nm from the d001 diffraction peak at 2θ ) ∼3.2° as illustrated in Figure 3A. According to the 0.48-nm thickness of MgAl hydroxide sheets, i.e., [Mg2+1-xAl3+x(OH)2]x+ reported in the literature,15 the gallery height between the MgAl hydroxide sheets can be calculated as being 2.27 nm. This means the tails of DS anions are interdigitated in the interlamellar space because the whole DS molecule length is 2.08 nm.24 This crystallographic structure of OMgAl-LDH is schematically shown in Figure 4. However, the d001 diffraction peak from OMgAl-LDH component in the PE-g-MA/MgAl-LDH nanocomposite sample was not observed in Figure 3B, which indicates that the OMgAl-LDH layers, i.e., [Mg2+1-xAl3+x(OH)2]x+(DS)x, have been exfoliated in a PE-g-MA matrix. The other two PE-g-MA/MgAl-LDH nanocomposites (PEMgAl NC2 and PEMgAl NC3) show the same XRD patterns as that of PEMgAl NC1. Finally, all of these are supporting evidences for successful preparation of the PE-g-MA/ MgAl-LDH exfoliation nanocomposites. (24) Sundell, S. Acta Chem. Scand. A 1971, 31, 799.

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Figure 8. TGA profiles for (A) pure PE-g-MA; and PE-g-MA/ MgAl-LDH exfoliation nanocomposites: (B) PEMgAl NC1, (C) PEMgAl NC2, and (D) PEMgAl NC3.

Figure 5. TEM image of PE-g-MA/MgAl-LDH nanocomposite (PEMgAl NC1).

Figure 9. DTA curve of OMgAl-LDH sample.

Figure 6. Illustration of the OMgAl-LDH single layers in the PE-g-MA matrix.

Figure 7. SAED pattern of PE-g-MA/MgAl-LDH nanocomposite (PEMgAl NC1).

Morphological Structure. The typical morphology of PE-g-MA/MgAl-LDH exfoliation nanocomposite with 5 wt % OMgAl-LDH is shown in Figure 5. It is observed

that the TEM micrograph of the PE-g-MA/MgAl-LDH nanocomposite is very different from those of polymers/ layered silicates exfoliation nanocomposites in which the exfoliated clay layers are often face-face orientated because of the very high aspect ratio.25 In the case of PE-g-MA/MgAl-LDH exfoliation nanocomposite, the exfoliated MgAl hydroxide sheets combined with DS anions are dispersed disorderly in the PE-g-MA matrix. The positions marked ‘V’ in Figure 5 show some single hydroxide layers nearly vertical to the cutting section of the TEM specimen. Most of the exfoliated MgAl hydroxide nanolayers are tilted with respect to the cutting section at some large angle. These results give positive evidence of molecular dispersion of OMgAl-LDH layers in the PE-g-MA matrix. The TEM image shows that the length or width of these nanolayers is about 70 nm. O’Leary et al. have reported the similar results from polyacrylate/LDH exfoliation nanocomposites.9 The structure of an exfoliated single layer is illustrated in Figure 6. The thickness of the exfoliated single layer is about 4.64 nm, i.e., the sum of the MgAl hydroxide sheet thickness (0.48 nm) and twice the length of whole DS anion (2 × 2.08 nm). Figure 7 represents the SAED pattern of the PE-gMA/MgAl-LDH exfoliation nanocomposite. This SAED pattern reveals that the MgAl hydroxide sheets have a hexagonal crystal structure with a crystallographic parameter a ) 0.305 nm and about 0.48 nm thickness, which are in good agreement with the forenamed XRD data. (25) Triantafillidis, C. S.; LeBaron, P. C.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 4088.

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Figure 10. Dynamic FTIR spectra at different thermo-oxidative temperatures: (A) pure PE-g-MA; (B) PE-g-MA/MgAl-LDH exfoliation nanocomposite (PEMgAl NC1).

Thermal Oxidation Stability. Figure 8 illustrates the TGA curves of pure PE-g-MA and PE-g-MA/MgAlLDH exfoliation nanocomposites. About 5% weight loss of PE-g-MA in the temperature range from 225 to 370 °C is due to the volatilization of a small amount of thermo-oxidative product in the PE-g-MA sample. In the case of PE-g-MA/MgAl-LDH nanocomposites, about 15% weight loss is observed in each sample, PEMgAl NC1, PEMgAl NC2, or PEMgAl NC3 in the range from 210 to 360 °C that can be attributed to the decomposition of OMgAl-LDH and the thermo-oxidation degradation of PE-g-MA. During this weight loss step, PEMgAl NC1 loses more weight than PEMgAl NC3 because of the thermal oxidation of PE-g-MA by air during reflux. PEMgAl NC3 loses more weight than PEMgAl NC2 due to the higher loading of OMgAl-LDH in nanocomposites. It is found that an efficient charring process in flameretardant polymeric materials must occur at a temperature higher than the processing temperature of the polymer but much lower than the decomposition temperature of the polymer.26 Therefore, this kind of weight loss at the first step of decomposition is highly advantageous to promote the charring process and enhance the fire safety of nanocomposites. The weight loss rates of both pure PE-g-MA and PE-g-MA/MgAl-LDH nanocomposites increase rapidly with increasing decomposition temperature from 370 to 500 °C. However, as shown in Figure 8, TGA curve of PE-g-MA sample above 370 °C shows a step weight loss of 95% and almost no residue at 500 °C, whereas PEMgAl NC1, PEMgAl NC2, and PEMgAl NC3 have about 5%, 1%, and 5% white powder residue, respectively, above 500 °C persisting until 800 °C. In this decomposition step, PEMgAl NC1, PEMgAl NC2, and PEMgAl NC3 have higher thermal stability than PE-g-MA. When the selected point of comparison (26) Kashiwagi, T.; Gilman, J. W.; Nyden, M. R.; Lomakin, S. M. In Fire Retardancy of Polymers: the Use of Intumescence; Le Bras, M., et al, Eds.; The Royal Society of Chemistry: Cambridge, 1998; p 175.

was a 50% weight loss, the decomposition temperatures were 394, 418, 446, and 453 °C for pure PE-g-MA, PEMgAl NC1, PEMgAl NC2, and PEMgAl NC3, respectively. When comparing PEMgAl NC2 with PEMgAl NC3, it can be seen that the thermal stability above 370 °C increases with the increase of OMgAl-LDH loading. PEMgAl NC1 shows less thermal stability above 370 °C than PEMgAl NC3 or PEMgAl NC2 because of the thermal oxidation of PE-g-MA by air during reflux. Consequently, both the fast charring process at a low temperature and the enhanced thermal stability at a high temperature can improve the performance of PE-g-MA/MgAl-LDH nanocomposites as flame retardants. The above results indicate that the molecular dispersion of only a small amount of OMgAl-LDH layers in the PE-g-MA matrix can significantly impart the thermal stability of the nanocomposite. The decomposition of OMgAl-LDH plays an important role in the thermal decomposition of PE-g-MA/MgAlLDH exfoliated nanocomposites. Figure 9 shows the DTA curves of OMgAl-LDH sample. The endothermal peak at about 80 °C is due to the loss of water in the interlayer of OMgAl-LDH. The deshydroxylation of OMgAl-LDH observed at about 200 °C is shown as an endothermal peak. It can be seen that a sharp exothermic peak is centered at 300 °C. This exothermic event speeds up the weight loss of the nanocomposite around 300 °C, and thus promotes the formation of a charred layer to enhance the thermal stability of PE-g-MA/ MgAl-LDH nanocomposites at higher temperatures. Dynamic FTIR can be used to reveal the details of thermo-oxidative behavior of flame-retardant materials.19, 20 Figure 10 shows the changes of dynamic FTIR spectra obtained from the thermo-oxidative degradation of the pure PE-g-MA (Figure 10A) and PE-g-MA/MgAlLDH nanocomposite, PEMgAl NC1, (Figure 10B) samples in the condensed phase with increasing pyrolysis temperature from room temperature (R.T.) to 320 °C. There

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as that of pure PE-g-MA. But the relative intensities of the nanocomposite sample are much higher than those of the control PE-g-MA sample at the corresponding pyrolysis temperatures after 200 °C. These results indicate the PE-g-MA/MgAl-LDH nanocomposite has a slower thermo-oxidative rate than pure PE-g-MA in the range from 200 to 320 °C. Conclusion

Figure 11. Relative peak intensities of absorbance at 2925 cm-1 assigned to the CH2 or CH3 asymmetric vibration.

are two interesting regions in Figure 10. The first is located in the range of 2800-3000 cm-1, and the intensities of peaks at 2925 and 2854 cm-1 assigned to the CH2 or CH3 asymmetric and symmetric vibration, respectively, decrease rapidly with the increase of the pyrolysis temperature because of the thermo-oxidative degradation of PE-g-MA main chains. The second region of interest lies between 1700 and 1800 cm-1, and the peaks at 1716 and 1780 cm-1 are assigned to the various CdO thermo-oxidative products. The peak intensities of these oxidative products increase significantly for both PE-g-MA and PE-g-MA/MgAl-LDH nanocomposite samples when the pyrolysis temperature reaches 220 °C. Although the dynamic FTIR spectra in Figure 10 (A) and (B) show very similar features, the rates of change of their peak intensities with increasing pyrolysis temperature are completely different. Figure 11 shows the changes of relative peak intensities at 2925 cm-1 with the pyrolysis temperature for pure PE-g-MA and PE-g-MA/MgAl-LDH nanocomposite samples. From R.T. to about 200 °C, the thermooxidative rate of the nanocomposite is almost the same

The PE-g-MA/MgAl-LDH exfoliated nanocomposites were successfully synthesized by solution intercalation of PE-g-MA chains into an organo-modified MgAl-LDH in xylene under reflux. The exfoliated organo-modified LDH single layers are dispersed disorderly in the polymer matrix. These layers are about 4.64 nm thick and 70 nm wide or long. The MgAl hydroxide sheets of about 0.48-nm thickness have a hexagonal crystal structure with a ) 0.305 nm. The nanocomposites show a fast charring process in the temperature range from 210 to 360 °C and a higher thermal stability in the temperature range from 370 °to 500 °C than the pure PE-g-MA. The decomposition temperatures measured at 50% weight loss are 394, 418, 446, and 453 °C for pure PE-g-MA, PEMgAl NC1, PEMgAl NC2, and PEMgAl NC3, respectively. It is noteworthy that the thermal degradation behavior of OMgAl-LDH is of crucial importance, as it has a key role in the thermal decomposition of PE-g-MA/MgAl-LDH nanocomposites due to the molecular dispersion of OMgAl-LDH layers in PE-g-MA matrix. The rate of thermal oxidation of PE-g-MA chains in the nanocomposites is much slower than that in the pure PE-g-MA in the temperature range from 200 to 320 °C. Acknowledgment. This work was supported by the China NKBRSF project 2001 CB409600. CM030044H