Polypropylene/Montmorillonite Nanocomposites and Intumescent, Flame- Retardant Montmorillonite Synergism in Polypropylene Nanocomposites EXPERIMENTAL

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1 NOTE Polypropylene/Montmorillonite Nanocomposites and Intumescent, Flame- Retardant Montmorillonite Synergism in Polypropylene Nanocomposites YONG TANG, 1 YUAN HU, 1 BAOGUANG LI, 2 LEI LIU, 1 ZHENGZHOU WANG, 1 ZUYAO CHEN, 2 WEICHENG FAN 1 1 State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, , Anhui, People s Republic of China 2 Department of Chemistry, University of Science and Technology of China, Hefei, , Anhui, People s Republic of China Received 26 February 2004; accepted 25 July 2004 DOI: /pola Published online in Wiley InterScience ( clay; intumescent flame retardant (IFR); nanocomposites; poly(pro- Keywords: pylene) (PP) INTRODUCTION Polymer-layered silicate nanocomposites have aroused people s interest since the Toyota group reported polyamide-6/clay nanocomposites through in situ intercalative polymerization. 1 They showed dramatic improvements in the mechanical, thermal, and barrier properties with a small amount of a layered silicate. For most polar polymers, melt intercalation can be used to synthesize nanocomposites containing a low weight percentage of a layered silicate premodified with organic surfactants. 2 However, it is difficult to disperse silicate layers of montmorillonite (MMT) at the nanometer level in a nonpolar polymer because the silicate layers of clay mineral have polar hydroxyl groups and are incompatible with nonpolar polymers such as polypropylene (PP). To synthesize nonpolar polymer nanocomposites, researchers 3 5 usually use maleic anhydride, which can make polar the function adjacent to molecular chains of a nonpolar polymer, and then the modified polymer is melt-mixed with organically modified clay. PP clay nanocomposites have been successfully synthesized. Correspondence to: Y. Hu ( yuanhu@ustc.edu.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, (2004) 2004 Wiley Periodicals, Inc. In this study, a novel method 6 was used to prepare PP/MMT nanocomposites by melt intercalation with pristine MMT, PP, and maleic acid modified polypropylene (MAPP) (in this study, two types of MAPP were used). A cationic surfactant, that is, an ammonium salt bearing long alkyl chains [hexadecyl trimethyl ammonium bromide (C16)], was used. This study showed that the contents of C16 and MAPP influenced the final morphology and properties of the nanocomposites. Additionally, it has been reported that intumescent flame retardants (IFR) are efficient in polyolefins and are widely used as environmental, halogen-free additives. These additive systems consist of a precursor of a carbonization catalyst such as ammonium polyphosphate (APP) and a carbonization agent such as polyol and a blowing agent (melamine phosphate). In this study, IFRs were added to PP/MMT nanocomposites. The flammability properties of the nanocomposites were evaluated with cone calorimetry experiments. We examined the mechanisms of melt intercalation and synergism between MMT and IFR. EXPERIMENTAL Materials PP (F401; homopolymer, melt-flow rate 2.5 g/10 min) was supplied as pellets by Yangzi Petrochemical Co. 6163

2 6164 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Table 1. Compositions of the Samples Identification a Composition (wt %) PP1 PP MAPP1 MMT (4%) (PP/MAPP1 4:1) PP2 PP C16 MMT (4%) (MMT/C16 5:4) PP3 PP MAPP1 OMT (4%) (PP/MAPP1 4:1) PP4 PP MAPP2 MMT (4%) C16 (PP/MAPP2 4:1, MMT/C16 5:4) PP5 PP MAPP1 MMT (4%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP6 PP MAPP1 MMT (1%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP7 PP MAPP1 MMT (2%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP8 PP MAPP1 MMT (3%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP9 PP MAPP1 MMT (6%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP10 PP MAPP1 MMT (8%) C16 (PP/MAPP1 4:1, MMT/C16 5:4) PP11 PP MAPP1 MMT (4%) C16 (PP/MAPP1 4:1, MMT/C16 5:3) PP12 PP MAPP1 MMT (4%) C16 (PP/MAPP1 5:1, MMT/C16 5:3) PP13 PP MAPP1 MMT (4%) C16 (PP/MAPP1 7:1, MMT/C16 5:3) PP14 PP MAPP1 MMT (4%) C16 (PP/MAPP1 9:1, MMT/C16 5:3) PP15 PP IFR (25%) PP16 PP6 IFR (24%) PP17 PP7 IFR (23%) PP18 PP8 IFR (22%) PP19 PP5 IFR (21%) PP20 PP9 IFR (19%) PP21 PP10 IFR (17%) a Samples PP16 PP21 were based on PP/MMT nanocomposites. IFR was added to PP nanocomposites such as PP6 and PP7. The total mass percentage of IFR and MMT was 25%. (China). We used two types of MAPP. MAPP1 was supplied by Chemical Material Co., Ltd. The grafting yield of maleic anhydride groups was 4 phr. MAPP2 was prepared in our laboratory. The grafting yield of maleic anhydride groups was 1.1 phr. The pristine MMT (cation-exchange capacity 97 mequiv/100 g, average size 20 m) and organophilic montmorillonite (OMT) were kindly provided by KeYan Co. (HeiFei China). C16 was acquired from Shanghai Chemistry Co. Preparation of the Samples Before mixing, PP, MAPP, and MMT were dried in an oven at 100 C for 2 h and then cooled to room temperature. In this study, the composites were prepared in a single step. The pristine MMT (dried powder), C16, and PP with or without MAPP were added to a blender (a high-speed mixture machine, 730/1450 rpm), and then the blender was run at a high speed for about 5 min. The mixed powder (PP, MAPP, MMT, and C16) was added to a twin-screw extruder (TE-35, JiangShu, China; length-to-diameter ratio L/D 48, the screw speed: rpm). The compounding with the twin-screw extruder was carried out at C and at a screw speed of 200 rpm, and then we obtained the nanocomposites. After that, the dried IFR was added to the PP/MMT nanocomposites and extruded with a twinscrew extruder with the aforementioned method. Table 1 shows the mixing weight ratios of all the samples. Characterization Evaluation of the Dispersibility of the Clay in the PP Matrix The dispersion of the MMT was evaluated with X-ray diffraction (XRD) and transmission electron microscopy (TEM) or high-resolution electron microscopy (HREM). XRD analysis was carried out (Cu, Å, ) on the samples and pristine MMT. HREM specimens were cut from an epoxy block with the embedded films of compounds at room temperature with an ultramicrotome (Ultracut-1, United Kingdom) with a diamond knife. TEM images were obtained with a JEOL JEM-100SX with an acceleration voltage of 100 kv, and HREM images were obtained with a JEOL 2010 microscope at an acceleration voltage of 200 kv. Flammability Properties The flammability was characterized with a cone calorimetry test. The signals from the cone calorimeter were recorded and analyzed by a computer system. All

3 NOTE 6165 Figure 1. PP1 PP5. XRD patterns for pristine MMT, OMT, and the samples (10 cm 10 cm 0.3 cm) were exposed to a Stanton Redcroft cone calorimeter according to ASTM under a heat flux of 50 kw/m 2. The experiments were repeated three times, and the results were reproducible to within 10%. The cone data reported in this article are averages of three replicated experiments. RESULTS AND DISCUSSION Evaluation of the Dispersibility of MMT in the PP Matrix Figure 1 shows the XRD patterns of some samples of PP and pristine MMT. The d 001 peak of pristine clay at corresponds to a 1.4-nm interlayer spacing, but the d 001 peak of sample PP1 is not changed compared with that of pristine MMT. This indicates that PP did not intercalate into the silicate layers. The d 001 peak of sample PP2 can be observed at a lower angle than that of the pristine clay. The top of the (001) plane peak of the mixture is , and this angle indicates the average basal spacing is 3. 8 nm. Our previous study 7 showed that PP with C16 could intercalate into the interlayer and form a mixed nanocomposite in this system. A similar phenomenon can be observed in Figure 2(a). Figure 1(e) shows that the peaks of sample PP3 shift to a lower angle compared with that of OMT, and this clearly indicates the intercalation of PP, MAPP, or both between the silicate layers. The literature 3,8 reports that PP without MAPP cannot intercalate the interlayer of silicate. In this study, the use of mechanical shear (via a twin-screw extruder) promoted the dispersion and intercalation/ exfoliation of the OMT layers [Fig. 2(b)]. Figure 1(f,g) shows XRD patterns of samples PP4 and PP5, respectively. The shapes of the XRD patterns are dependent on the type of MAPP used. An apparent and strong peak of the (001) plane can be observed for sample PP4 (containing MAPP2); however, the diffraction peak of the (001) plane of PP5 (containing MAPP1) disappears. This suggests that MMT may be delaminated in PP5, but in PP4, the MMT layers still maintain a relatively strong ordering of the layered structures. TEM photographs of the two samples are shown in Figure 2(c,d). An individual silicate layer, along with two or three layer stacks, is well dispersed (exfoliated) in the polymer matrix [Fig. 2(c)]. The silicates are dispersed in a regular manner, which may be the shear flow in melt extrusion. Figure 2(d) shows that the silicates disperse uniformly. In addition, some large intercalated tactoids (multiplayer particles) are also visible in the TEM images. These results show that MAPP apparently affects the dispersibility of the silicate layers in the PP matrix. Moreover, the MAPP pretreatment with a high maleic anhydride content promotes the dispersibility of MMT in the PP matrix. In our system, the dispersion of MMT in PP requires sufficiently favorable enthalpic contributions to overcome any entropic penalties. MAPP is added to the PP matrix, and this makes the PP/MMT interactions more thermodynamically favorable than the surfactant (C16)/MMT interaction. A favorable enthalpy of mixing for PP/MMT is achieved. We describe the morphology evolution from a low concentration of MMT to a high concentration to understand clearly the process of the dispersion of MMT in the PP matrix. We vary the concentrations of the surfactants and the compatibilizer MAPP and want to know how alterations in one of the components affect the dispersion of bare MMT sheets within the PP matrix. We use the self-consistent field (SCF) method of Scheutjens and Fleer 9,10 and a density functional theory (DFT) 11,12 to analyze the dispersion of MMT. In this article, we use some ideas developed in the literature 13,14 for a polymer melt: m s p 1 (1) where m is the volume fraction of MMT, s is the volume fraction of the surfactant, and p is the volume fraction of the polymer (in this study, p contains the volume fraction of the compatibilizer MAPP). We keep the compatibilizer MAPP unchanged and vary m of MMT. Figure 3 shows the XRD patterns of samples PP5 PP10. The intensity of the diffraction increases when the MMT concentration is less than or equal to 3 wt %. When the MMT concentration is 4 wt % (PP5), the diffraction of the (001) plane disappears [Fig. 3(d)], and this means that the periodicity of MMT may be destroyed. However, above 4 wt %, the diffraction of the MMT appears as two peaks and three peaks in PP9 [Fig. 3(e)] and PP10 [Fig. 3(f)], respectively. The TEM photographs further confirm the different dispersions of MMT in the PP matrix. In Figure 4(a), MMT dis-

4 6166 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Figure 3. XRD patterns for PP/MMT nanocomposites: PP5 PP10. perses uniformly in the matrix (PP7). However, layers in stacks of more than 10 (intercalated tactoids) can still be seen. When the MMT concentration increases to 4 wt %, TEM [Fig. 2(c)] shows that the sheets are exfoliated in the PP matrix (PP5). Figure 4(b) shows that MMT is exfoliated or exfoliated and intercalated in the PP matrix. When the MMT concentration increases to 8 wt %, Figure 4(c) shows that intercalated structures exist in the matrix. The TEM photographs show that the layered silicates subjected to shear flow show orientation behavior: the silicate platelet normal is perpendicular to the flow direction. 15 The degree of orientational order changes as the concentration of MMT increases. These results show that 4 wt % is the optimal value for MMT dispersion in the PP matrix. According to the SCF DFT theory, to obtain the thermodynamic behavior of a system, we must minimize the free energy for all possible phases (isotropic, nematic, smectic, columnar, and crystal) for each value of and find the lowest energy state. The free energy is calculated as follows: F F id F ster F int (2) where F id is the free energy of an ideal gas of colloidal particles (clay) and polymers, F ster is the contribution due to the excluded volume effects for the colloidal sheets, F int represents the long-range (attractive or repulsive) interaction between clay sheets, and is equal to 1/kT where k is Boltzmann s constant; T is temperature. The first (ideal) term of the free energy in eq 2 can be written as the sum of two parts: F id F m F p (where F m is the ideal free energy of clay Figure 2. TEM photographs for (a) PP2, (b) PP3, (c) PP5, and (d) PP4.

5 NOTE 6167 F p includes only the translational (Flory Huggins) contribution: F p v/nv m 1 ln 1 (5) where N is the polymer chain length, v is the total volume of the system, v m is the monomer volume, and 1 is the volume fraction of the polymer. With the change in, eq 5 has the lowest values, which are favorable for creating thermodynamically stable exfoliated hybrids. In our study, the lowest value point is adjacent to the mass fraction of 4 wt %. However, the kinetic aspects influence the final structure of the hybrids. In this study, we have also found that the surfactant (C16) and compatibilizer (MAPP) influence the dispersion of MMT. Figure 5(b) shows that when the surfactant concentration decreases, there is a peak in the XRD pattern of PP11. However, when the surfactant concentration is zero, Figure 1(b) shows that there is a weak peak (it is the diffraction of pristine MMT), and this means that there are no polymer chains intercalated into the interlayer of MMT. A TEM image [Fig. 6(a)] shows the coexistence of exfoliated and intercalated morphologies in PP11. We kept the surfactant and MMT concentrations unchanged and varied the volume of the compatibilizer MAPP. Figure 5 shows strong diffraction peaks. When the ratio of PP to MAPP was 9, the peak of sample PP14 shifted to a higher angle than those of PP12 and PP13. From the TEM images, we can see that the sheets of MMT dispersed homogeneously in PP12 [Fig. 6(b)], PP13 [Fig. 6(c)], and the layers showed orientation behavior due to shear flow. The HREM image [Fig. 6(d)] shows that an individual sheet, 2 or 3 layer stacks (mark A), and about 10 layer stacks (mark B) coexisted in the matrix. When the MAPP concentration was further reduced, the image [Fig. 6(e)] shows the formation of a house-of-cards Figure 4. (c) PP10. TEM photographs for (a) PP7, (b) PP9, and particles and F p is the ideal free energy of the polymer melt; see the literature 9,10,16 19 for more details). In this study, according to the principles used by Gast et al. 20 and Lekkerkerker et al., 21 F m consists of translational and orientational terms: F m dr r)ln( (r r dnf n)ln(4 f n (3) where r m s (4) Figure 5. XRD patterns for PP/MMT nanocomposites: PP5 and PP11 PP14.

6 6168 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Figure 6. TEM photographs for (a) PP11, (b) PP12, (c) PP13, and HREM photographs for (d) PP13 and (e) PP14. structure, in which the clay sheets have some positional ordering and prefer edge-to-face configurations. A possible reason for the formation of a house-of-cards structure may be the interaction between silicate layers and a little MAPP. 15 We use SCF DFT theory 9,10 to illuminate these phenomena. To describe the free-energy function, we must calculate the interaction potential per unit of area [U(z)] for the clay sheets: U z U 1 z U 2 z (6) where U 1 (z) is the interaction between bare clay particles (due to electrostatic and van der Waals attraction) and U 2 (z) is the contribution from the polymer chains intercalated in the layers between adjacent clay sheets. U 1 (z) depends on the chemical structure of the clay, but U 2 (z) depends on N, the surfactant grafting density ( gr ), the surfactant length, and the different Flory Huggins interaction parameters ( ) between the polymer chain and surfactant. The literature 9,10 reports that when the interaction between the polymers and surfactants is repulsive ( 0), the polymers do not

7 NOTE 6169 penetrate the interlayers of clay, and the effective interaction between the interlayers is predominantly attractive. When is less than 0, the interaction between the polymers and surfactants is attractive, and the polymers can pull into the interlayers; this leads to the intercalation or exfoliation structure. We study the dependence of the dispersion on the surfactant density. By comparing the diffraction of PP11 (PP/MAPP 4:1, MMT/C16 5:3) and PP5 (PP/MAPP 4:1, MMT/C16 5:4) in Figure 5, we can see that, with an increasing surfactant concentration corresponding to an increasing gr value (the other conditions are fixed), the dispersion of MMT in PP is improved. The TEM photographs [Fig. 2(c)] shows the sheets exfoliated in the PP matrix. The results indicate that increasing the surfactant will reduce the value of U(z), and this means improved miscibility and enhanced thermodynamic stability of the polymer/clay system. This study also shows that the volume fraction of the compatibilizer (MAPP) influences the dispersion of clay. When the PP/MAPP is 9, we can obtain PP/MMT nanocomposites. Moreover, increasing the content of MAPP is better for the dispersion of MMT. As for the PP/MMT system, the compatibilizer MAPP may reduce the interfacial tension between PP and the surfactant, and this makes further negative and, therefore, lowers the value of the free energy for the system. 9,10 In our system, the compatibilizer MAPP can act as a highmolecular-weight surfactant; the functional group maleic anhydride anchored in the sheets of MMT and the nonreactive block of MAPP will attempt to gain entropy by pushing the sheets apart, and this promotes miscibility between PP and MMT. When the volume fraction of MAPP is increased, the sheets of MMT may be coated, and consequently, the surfaces are pushed apart by the absorbed compatibilizer chain; this further reduces the free energy in this system. Possible Dispersion Mechanism In our study, the system is a reactive process during melt mixing; modification and intercalation occur in one step. A prominent interaction arises between the three components 11 of the system: the silicate surface, the surfactant chains (C16), and the compatibilizer polymer MAPP. At first, some surfactant chains diffuse into the interlayer under physical absorption and shear. Because the negative charge originates in the silicate layer, the cationic head group of the surfactant will preferentially reside at the layer surface, and the aliphatic tail will radiate away from the surface. 22 This makes the MMT organophilic and reduces the interfacial energies. Our results have confirmed that without C16, MAPP cannot intercalate into the interlayers of MMT (PP1). However, there is some difference in this system because the surfactant does not diffuse into the interlayer at the same time; that is, there is some surfactant remaining in the polymer matrix (PP or PP and MAPP), which may enhance the compatibility when the polymer matrix is intercalated into the interlayer. In fact, there is an interaction between the polymer matrix and the surfactant, just like the interaction between the surfactant and the silicates. On the other hand, the MAPP in this system may be divided into two portions (if MAPP is used in the matrix). There may be some MAPP intercalated into the interlayer of MMT after the surfactant makes the silicates organophilic enough. At the same time, MAPP can act as a highmolecular-weight surfactant; the functional group maleic anhydride, anchored in the sheets of MMT, and the nonreactive block of MAPP will attempt to gain entropy by pushing the sheets apart under a strong shear field. The interlayer spacing of the clay increases, and the interaction of the layers should be weakened. At this time, if the miscibility of MAPP with PP is good enough to disperse at the molecular level, the exfoliation of intercalated MMT should take place. Scheme 1 shows the process of making PP/MMT nanocomposites. Flammability Properties Cone calorimetry is one of the most effective benchscale methods for studying the flammability properties of materials. The heat release rate (HRR), particularly the peak HRR, has been found to be the most important parameter for evaluating fire safety We have studied the flammability properties with cone calorimetry. Although polymer chains can be intercalated into the interlayer of layered silicate to form polymer/clay nanocomposites resulting in effective flame-retardant materials, sometimes it is difficult to meet the requirements for new standards or regulations related to the evaluation of fire hazards. It is necessary, therefore, to develop novel synergistic flame-retardant systems with high efficiency and acceptable environmental impact. In this study, we added an IFR to PP/MMT nanocomposites. The TEM image (Fig. 7) show the sheets still delaminated in the matrix. The flammability properties were evaluated with cone calorimetry experiments. Figure 8(a) shows that the peak HRRs are reduced as the mass fraction of MMT and C16 increases. When the mass of MMT increases to 8 wt %, the peak HRR of PP10 is 57.2% lower than that of pure PP. It is reasonable that as the fraction of clay increases, the amount of char that can be formed increases and the HRR decreases. IFR was added to the PP/MMT nanocomposites, and the mass fraction of MMT and IFR was 25 wt %. The HRR curves are shown in Figure 8(b). Figure 8(b) shows that when 25 wt % IFR was added to pure PP, the peak HRR was 385 kw/m 2. MMT was substituted for the IFR; when the mass fraction of MMT was 4 wt % (PP19), the curves show that the peak HRR of the sample was 293 kw/m 2, which was lowest of the samples. However, when the value (4 wt %) was exceeded, the peak HRR increased; when the mass frac-

8 6170 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Figure 7. TEM image for PP19. Scheme 1. Schematic representation of the production of PP/MMT nanocomposites. tion was increased to 8 wt %, the peak HRR increased to 460 kw/m 2. In Figure 8(b), we can see a synergistic effect between MMT and IFR, but the synergistic effect is related to the ratio of MMT to IFR, and Figure 8(b) shows the optimum value for the synergistic effect. We now explain the results of our study. In this system, the chemical reactions are complex. In IFR systems, 28,29 APP is used as the acid source; during heating, the formation of poly(phosphoric acid) provides an acid catalyst for organic reactions. 30,31 During the heating process of APP, ammonia is the main gaseous product, which volatilizes and makes the mixture of the carbonaceous residue and phosphocarbonaceous materials swell; this leads to the formation of the intumescent residue char. When IFR is added to PP/MMT nanocomposites, in addition to the aforementioned chemical reaction, APP reacts with MMT to form an aluminophosphate structure and a ceramic-like structure in the C temperature range. 26,32 These aluminophosphate species may thermally stabilize and lead to good fire performance in this temperature range. On the other hand, there is a catalytic role played by the layered silicates deriving from the Hoffman reaction 23 of C16. The decomposition of the amine silicate modifier leaves a strong acid catalytic site that may further favor the oxidative dehydrogenation crosslinking charring process and increase the char yield in the charring process. Moreover, during combustion, an ablative reassembling of the silicate layers may occur on the surface of a burning nanocomposite, creating a physical protective barrier on the surface of the material. 24 The physical process of layers reassembling acts as a protective barrier in addition to the intumescent shield and can limit the oxygen diffusion to the substrate or give a less disturbing low volatilization rate. In this study, when the mass fraction of MMT exceeded 4 wt %, the synergistic effect decreased and even dis- Figure 8. HRR for (a) PP/MMT nanocomposites and (b) PP/MMT nanocomposites with IFR.

9 NOTE 6171 Scheme 2. Chemical reaction in IFR. appeared. The main reason for this phenomenon may be that during the combustion process, the silicate layers have positive and negative roles. We know that silicate layers can limit the oxygen diffusion to the substrate or give a less disturbing low volatilization rate, which produces a positive effect on the fire properties. At the same time, an ablative reassembling of the silicate layers can hinder NH 3 from swelling, and this leads to a negative effect on the fire properties. Ammonia is the main gaseous product; it volatilizes and makes the mixture of the carbonaceous residue and phosphocarbonaceous materials swell, leading to the formation of the intumescent residue char. In particular, when the mass of MMT is increased, the negative effect may exceed the positive effect on the fire properties. Because of the reduction of the blowing agent, the value of HRR increases. The combustion mechanism is shown in Schemes 2 and 3. CONCLUSIONS This article shows that PP clay nanocomposites can be successfully synthesized with even pristine

10 6172 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Scheme 3. Mechanisms of PP/MMT nanocomposites with IFR. MMT (one step). The study shows that the structures of PP clay nanocomposites are sensitive to the compatibilizer and surfactant. Using MAPP1, we obtained exfoliated PP/MMT nanocomposites (PP5) because MAPP1 possessed a higher maleic anhydride concentration, which was beneficial for the PP/MMT nanocomposites. This study shows (under its conditions) that increasing the surfactant or compatibilizer concentration improves the dispersion of MMT in the PP matrix. The DFT SCF method indicates that increasing the compatibilizer and surfactant concentrations will reduce the free energy of the system, and this is favorable for thermodynamic stability. We have studied the synergistic effect between MMT and IFR and have found that there is an optimum value for the synergistic effect. The probable mechanism discussed in this article provides the reasons. This work was financially supported by the National Natural Science Foundation of China ( ), the China National Key Basic Research Special Fund project (2001CB409600), and the Tenth-Five-Years Tackle-Key-Problem Project of Anhui. REFERENCES AND NOTES 1. Fukushima, Y.; Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito, O. Clay Miner 1988, 23, Alexander, M.; Dubois, P. Mater Sci Eng 2000, 28, Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Macromolecules 1997, 30, Tjong, S. C.; Meng, Y. Z.; Hay, A. S. Chem Mater 2002, 14, Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. J Appl Polym Sci 1998, 67, Alexandre, M.; Beyer, G.; Henrist, C.; Cloots, R.; Jerome, A. R. R.; Dubois, P. Chem Mater 2001, 13, Tang, Y.; Hu, Y.; Wang, S.; Gui, Z.; Chen, Z.; Fan, W. Polym Int 2003, 52, Tang, Y.; Hu, Y.; Wang, S.; Gui, Z.; Chen, Z.; Fan, W. J Appl Polym Sci 2003, 89, Balazs, A. C.; Singh, C.; Zhulina, E. Macromolecules 1998, 31, 8370.

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