Morphology and Mechanical Properties of Nanocomposites

Jul 7, 2010 - S. G. Kang, J. H. Hong, and C. K. Kim*. School of Chemical Engineering and ... Louis Pilato. Reactive and Functional Polymers 2013 73, 2...
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Morphology and Mechanical Properties of Nanocomposites Fabricated from Organoclays and a Novolac Phenolic Resin via Melt Mixing S. G. Kang, J. H. Hong, and C. K. Kim* School of Chemical Engineering and Materials Science, Chung-Ang UniVersity, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Korea

Nanocomposites of novolac phenolic resin with a series of organoclays based on sodium montmorillonite exchanged with various amine compatibilizers were fabricated via melt processing. The effects of the amine structure on the resulting morphology and physical properties were investigated by employing wide-angle X-ray scattering, transmission electron microscopy, viscosity measurement, flexural strength measurement, and Izod impact strength testing. All of the composites containing organoclay showed evidence of the intercalated state with a shift in the gallery spacing. The extent of silicate platelet intercalation was increased by (1) decreasing the number of long alkyl tails from two to one, (2) using methyl rather than polar hydroxyethyl groups or bulky groups such as 2-ethylhexyl or benzyl groups, and (3) using an equivalent amount of compatibilizer with the montmorillonite. Novolac phenolic resin/organoclay nanocomposites cured with hexamethylenetetramine exhibited better flexural and Izod impact strength than the unmodified novolac phenolic resin. Nanocomposites with higher shifts in the platelet gallery height, i.e., higher platelet intercalation, showed better flexural and Izod impact strength. Introduction Polymer mixtures with montmorillonite (MMT) clay, which can form nanocomposites, have been studied extensively.1-19 Polymer-layered silicate nanocomposites afford enhanced physical properties and barrier properties, as well as flame-retardant behavior at very low filler loadings. Composites containing these nanofillers in the range of 3-5% often show properties comparable with those of conventional composites containing 20-30% microsized fillers.13-19 To achieve such a performance in nanocomposites, high levels of dispersion of the organoclay with high-aspect-ratio silicate platelets and exfoliation within the polymer matrix are required. A wide range of polymers such as polyolefins,1-6 epoxies,7-9 polystyrene,10 polyimides,11-13 polyamides,14,15 and polycarbonates16 have been explored to fabricate nanocomposites containing well-dispersed or wellexfoliated MMT. Using a variety of processing routes, wellexfoliated composites have been achieved only in a selected number of polymers such as nylon 6, polystyrene, certain polyimides, and epoxies.7-15 Phenolic resin is the oldest synthesized resin in history.20 A wide variety of commercially available phenolic resins have been developed after the first commercially available product was introduced by Baekeland.20,21 Phenolic resins are still employed in a wide range of applications from commodity construction materials to high technology applications. They are irreplaceable in many areas, especially in thermal insulation materials, molding compounds, foundry, wood products industry, coatings, and composite materials because of their thermal stability, ablative properties, high char yield, structural integrity, and solvent resistance. A new class of materials having better properties than phenolic resins is still required in various application fields such as molding compounds with low flammability and smoke generation, electronic package substrates with low coefficients of thermal expansion, and ablatives. It is interesting to introduce a layered silicate into phenolic resins because phenolic resin layered silicate nanocomposites may * To whom correspondence should be addressed. Fax: 822-824-3495. E-mail: [email protected].

afford improved flame-retardant behavior, as well as exceptional physical properties, at very low filler loadings. However, there is very little information available in the literature relating to nanocomposites containing phenolic resins.22-25 The reason is that general phenolic resins have a three-dimensional structure even if the resin is not cross-linked. Although the layered silicate can be intercalated or exfoliated by linear polymers, this threedimensional structure of the phenolic resin makes it very difficult to intercalate in the layered silicate gallery. Usuki et al. prepared a novolac resin composite with MMT by reacting phenol, formaldehyde, and oxalic acid in the presence of MMT modified with 4-aminophenol hydrochloride.23 Choi et al. reported phenolic resin/MMT composites prepared from novolac resin and three different organoclays.24 They found that the benzyldimethyloctadecylammonium-modified synthetic clay exhibited better intercalation than MMT modified with octadecylammonium. However, the effects of the chemical structure of the organoclay on the composite structure are poorly understood at this point. In this study, the effects of the ammonium ion compatibilizer used to treat the clay on the morphology and mechanical properties of novolac phenolic resin/MMT composites prepared via melt processing were explored. The structure of the compatibilizer headgroup, the number of long tail groups, and the compatibilizer exchange ratio were varied. The state of clay particle dispersion with organoclay structures was evaluated using wide-angle X-ray scattering (WAXS) and electron microscopy. Further, viscosity and other mechanical properties were measured. Materials and Procedure Linear novolac phenolic resin synthesized specifically for this study was kindly supplied by Kolon Chemical Co. Ltd. (Seoul, Korea). According to the supplier, this resin has a weightaverage molecular weight of 1400 g/mol and a number-average molecular weight of 760 g/mol. Sodium montmorillonite (Na+MMT) and organoclays formed by cation exchange between Na+MMT and various amine compounds were supplied

10.1021/ie1002872  2010 American Chemical Society Published on Web 07/07/2010

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Table 1. Organoclays Used in This Study

a

T ) tallow, HT ) hydrogenated tallow, M ) methyl, B ) benzyl, H ) hydrogen, EH ) 2-ethylhexyl, and HE ) hydroxyethyl.

by Southern Clay Products, Inc. (Gonzales, TX). Their pertinent properties, abbreviations, and amine compatibilizer structures used for cation exchange are provided in Table 1. To explore the effects of the compatibilizer structure on the dispersion of clay particles in novolac resin matrixes, organoclays containing various amine compatibilizers were examined. Hexamethylenetetramine (HMTA), purchased from Aldrich Chemicals (Milwaukee, WI), was used as a curing agent for the linear novolac resin and its composites. All of the chemicals used in this study were used without further purification. Melt-blended composites were prepared using a Haake internal mixer (Haake Record 90 connected with a Rheomix 900) at a temperature of 80 °C, a rotor speed of 70 rpm, and a mixing time of 10 min. Novolac resin was dried in a vacuum oven at 60 °C for 1 day prior to use, while the organoclay was used without drying. As described previously,14-19 comparison of the properties of nanocomposites formed from different organoclays should be made at a fixed MMT content rather than at a fixed organoclay content because the silicate portion of the organoclay is the reinforcing component. On the basis of the ignition loss percent in Table 1, MMT loadings of the composites were fixed to 2 wt % in the present experiments to explore the composite morphology and mechanical properties. To prepare specimens for flexural strength (ASTM D790) and Izod impact strength tests (ASTM D256), melt-blended composites were ground into fine powder and then mixed with HMTA (mixing ratio: composite/HMTA ) 10/1), which was used as a curing agent. The mixture was poured into a mold (length × width × thickness ) 120 mm × 13 mm × 3 mm) and placed in a vacuum oven at a temperature of 120 °C and a

pressure of 10 Torr for 1 h. Specimens were cured using a compression-molding machine (model 25-12; Carver, Inc., Wabash, IN) operating at a plate temperature of 180 °C, a holding pressure of 12 MPa, and a holding time of 2 h. After molding, the specimens were immediately placed in a vacuum desiccator for a minimum of 24 h prior to mechanical testing. Flexural tests were performed at room temperature according to ASTM D790 using a universal testing machine (model LR5K plus; Lloyd Instruments, West Sussex, U.K.) with digital data acquisition. Standard notched Izod impact strength tests were performed at room temperature according to ASTM D256 using an Izod impact strength tester (model 43-02 Monitor/Impact; TMI, Ronkonkoma, NY). Five specimens were tested, and the results were averaged to determine the mechanical properties. WAXS was conducted using a X-ray diffractometer (model XRD 2000; Scintag Inc., Cupertino, CA) operating in reflection mode using an incident X-ray (wavelength: 0.154 06 nm) at a scan rate of 1.0°/min. X-ray diffraction (XRD) measurements were performed on the organoclays, which were in powder form, and the composite samples before the curing reaction. Samples for transmission electron microscopy (TEM) analysis were taken from the core portion of an Izod bar. Ultrathin sections of approximately 100 nm in thickness were cut with a diamond knife at room temperature. The sections were examined by a transmission electron microscope (model Leo EM912 Ω; Zeiss, Oberkochenm, Germany) at an accelerating voltage of 120 kV. Rheological measurements were carried out using a rheometer (model Physica MCR 500; Anton Parr, Ostfildern, Germany; geometry ) 25 mm parallel plate). The measurements were made at 120 °C in the shear rate range of 1-400 s-1. The

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viscosities of composites composed of novolac phenolic resin and organoclays were evaluated from the data on shear stress and shear rates. Results and Discussion Changes in the Composite Morphology with the Organoclay Structure. The structure of the organic amine compound in the organoclay may affect the morphology of the nanocomposites fabricated from novolac resin and organoclay. The platelet gallery height of the MMT clay changes upon melt processing via intercalation, which refers to the incorporation of polymer molecules into the gallery, or exfoliation associated with a complete loss of the stack registry. It is known that the structure of the compatibilizer on the MMT clay surface significantly affects the shift in the platelet gallery height (∆d001).15-19 To explore the effect of the molecular structure of the ammonium ion headgroup on the extent of platelet intercalation (or exfoliation), the morphologies of the resulting composites were examined using WAXS and TEM as characterization tools. A summary of the shift in the gallery spacing for all of the composites from WAXS scans is listed in Table 1. All of the composites prepared here showed peaks characteristic of the intercalated state except for the composite containing Na+MMT. Note that none of the WAXS scans indicated a complete loss of the stack registry; i.e., no characteristic X-ray reflection associated with exfoliation was observed. As shown in Figure 1, the filler particles in composites prepared from organoclays are thinner than those prepared from Na+MMT. Figure 2 shows basal reflections of the WAXS scans for the pristine Na+MMT and composite containing Na+MMT. Both scans exhibit little difference in the basal reflections. These results indicate that intercalation or exfoliation might not be expected when novolac resin is blended with Na+MMT upon melt processing (see also Figure 1a). In what follows, details related to the effect of the organic modifier structure on the extent of platelet intercalation are described. To examine the effect of having one versus two long alkyl groups on the quaternary amine, composites were formed from novolac resin and organoclays M3(HT)1 (one alkyl tail) and M2(HT)2-95 (two alkyl groups). Figure 3 shows WAXS scans for the nanocomposites based on M3(HT)1 and M2(HT)2-95, while Figures 1 and 4 show their TEM results, respectively. Note that Figure 4 presents the stack registry in Figure 1 observed at high magnification. Upon blending with novolac resin, the organoclay having one alkyl tail shows a larger increase in the basal spacing than that containing two alkyl tails. The pristine organoclay with one tail, M3(HT)1, has a d001 spacing of 1.8 nm (2θ ) 4.98°) that increases by 2.18 nm (2θ ) 2.2°) upon blending. On the other hand, the organoclay with two tails, M2(HT)2-95, has a d001 spacing of 2.42 nm (2θ ) 3.68°) in the pristine state that increases by 1.14 nm (2θ ) 2.48°) upon blending. The composite containing one tail, M3(HT)1 (Figure 1g), exhibits smaller clay particles and better dispersed particles than that containing two tails, M2(HT)2-95 (Figure 1d). The TEM photomicrograph of the former reveals that most of the dispersed particles are thin tactoids containing several layers; there are a few single platelets that are partially exfoliated (Figure 4g). As described previously,13-17 the driving force for intercalation depends on three principle thermodynamic interactions (platelet-platelet, polymer-platelet, and polymer-compatibilizer). In general, a larger initial layer spacing may lead to easier intercalation because the platelet-platelet attraction is reduced. Polymer-platelet interactions may be favorable for intercalation, while polymer-compatibilizer interactions may be unfavorable for intercalation. Novolac resin, which contains polar hydroxyl groups,

Figure 1. TEM photomicrographs of novolac resin composites containing various organoclays: (a) Na+MMT; (b) M2B1(HT)1; (c) M2(HT)2-125; (d) M2(HT)2-95; (e) M2(EH)1(HT)1; (f) M1(HE)2(T)1; (g) M3(HT)1; (h) M1H1(HT)2.

Figure 2. WAXS scans of a novolac resin/Na+MMT composite and that of Na+MMT.

should have a relatively high affinity for the polar surface of the MMT. M2(HT)2, which has two long aliphatic tails, limits the favorable interactions of the novolac resin, with the silicate surface more than the single-tailed M3(HT)1. Thus, this shielding of

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Figure 3. WAXS scans of novolac resin/MMT composites and the pristine organoclay for compatibilizers (a) M3(HT)1 and (b) M2(HT)2-95.

Figure 4. TEM photomicrographs of the stack registry in the novolac resin/ organoclay composites observed at high magnification: (a) Na+MMT; (b) M2B1(HT)1; (c) M2(HT)2-125; (d) M2(HT)2-95; (e) M2(EH)1(HT)1; (f) M1(HE)2(T)1; (g) M3(HT)1; (h) M1H1(HT)2.

favorable interactions by M2(HT)2 results in an unfavorable situation for intercalation (see Figure 4d). To explore the effect of the substituent size of a compatibilizer on that of clay particles in the polymer matrix, composites containing organoclays (M)2X1(HT)1, where X is either a methyl, 2-ethylhexyl, or benzyl group, were compared. As shown in Figures 3a and 5, WAXS scans reveal that the d spacing of the pristine organoclay increases in the order of methyl (1.8 nm), 2-ethylhexyl (1.86 nm), and benzyl groups (1.92 nm). Upon blending with novolac resin, the basal spacing based on the methyl group increases by 2.18 nm, while those based on the 2-ethylhexyl and benzyl groups increase by 1.41 and 1.13 nm, respectively. Because the bulky substituent is nearest the positive end of the compatibilizer, i.e., near the MMT surface, it might interfere with access of the novolac phenolic resin to the MMT surface. Therefore, the nanocomposite made from the organoclay containing a bulkier substituent group exhibits a smaller increase in the basal spacing. This result is opposite of what is reported by Choi et al., who observed a greater increase in the basal spacing in composites with the benzyldimethyloctadecylammonium-modified clay than in composites with the octadecylammonium modified clay.24 Composites formed from (HE)2M1T1 organoclays (HE ) hydroxyethyl) contain large aggregates (Figure 1f), and the basal spacing increases by 0.80 nm. As

described previously,14 the hydroxyethyl groups occupy more surface space than the methyl groups, and therefore the polar hydroxyl moiety may prefer to reside flat on the surface because of the attraction to oxygen atoms in the clay. A reduction in the favorable novolac-platelet interactions leads to a reduction in the level of intercalation. To examine the effect of excess amine compatibilizer, the morphology of nanocomposites formed from M2(HT)2 organoclays [M2(HT)2-95 and M2(HT)2-125] was examined. WAXS scans for both composites containing M2(HT)2-95 (Figure 3b) and M2(HT)2-125 (Figure 6) exhibit basal reflections at approximately 2.48° (d001 ) 3.56 nm) and 2.30° (d001 ) 3.83 nm), respectively. Note that the pristine M2(HT)2-95 and M2(HT)2125 organoclays have spacings of 2.42 and 3.15 nm, respectively. This reveals a 1.14 nm increase in the gallery height for the equivalently exchanged organoclay composite, while the overexchanged system increases by only ∼0.68 nm. TEM photomicrographs also show that the M2(HT)2-125 composite (Figures 1c and 4c) has larger stacks of platelets and interparticle distances than the M2(HT)2-95 composite (Figures 1d and 4d). These results indicate that excess compatibilizer is disadvantageous with regard to dispersing MMT platelets. Figure 7 shows the effects of forming organoclays from quaternary [M2(HT)2] versus tertiary [M1H1(HT)2] ammonium

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Figure 7. WAXS scans of novolac resin/M1H1(HT)2 and novolac resin/ M2(HT)2-95 composites.

Figure 5. WAXS scans of novolac resin/MMT composites and the pristine organoclay for compatibilizers (a) M2(EH)1(HT)1 and (b) M2B1(HT)1.

Figure 6. WAXS scans of a novolac resin/M2(HT)2-125 composite and M2(HT)2-125.

compounds by ion exchange with Na+MMT. Nanocomposites showed a peak at 2.56° for M1H1(HT)2 and at 2.48° for M2(HT)2 due to polymer-intercalated galleries. The XRD scans show similar intercalation for both pairs with about a 1.1 nm increase in the gallery height upon blending. There is also very little difference between M2(HT)2 and M1H1(HT)2 in TEM images. In summary, novolac resin composites with organoclay having one long alkyl group (HT) with three methyl groups on the quaternary amine exhibit the best particle dispersion, containing thin tactoids and a few single platelets. However, an increase in the number of long alkyl groups in the amine compatibilizer and substitution of methyl groups for bulkier groups (benzyl or 2-ethylhexyl group) or polar groups (hydroxylethyl group) are disadvantageous with regard to dispersing MMT platelets and intercalation. Organoclay composites with thermoplastics such as nylon 6 and polycarbonate and thermosets such as epoxy resin also exhibited trends similar to those of novolac resin/ organoclay composites in changing intercalated galleries with the molecular structure of the amine compatibilizer.8,9,14-16 Changes in the Viscosity and Mechanical Properties with the Organoclay Structure. Figure 8 shows viscosity changes of selected composites as a function of the shear rate measured at 120 °C. Composites always exhibit higher viscosities than novolac resin regardless of the shear rate and kind of MMT. Among the composites, the Na+MMT composite, in which Na+MMT exists without intercalation or exfoliation upon melt processing, has the lowest viscosity. Viscosities of nanocomposites containing M2(HT)2-125 and M1(HE)2T1 are about 2 or 3 times higher than those of the Na+MMT composite at low shear rate, while those containing M3(HT)1 and M2(EH)1(HT)1 are about 9 times higher than those of the Na+MMT composite. To explore changes in the viscosity of the composite with the molecular structure of the amine compatibilizer, the average number of particles/(µm)2(MMT wt %) was calculated from four replicated photographs of each sample shown in Figure 1. Composites prepared from M3(HT)1 [1.4 ea/(µm)2(MMT wt %)] and M2(EH)1(HT)1 [1.1 ea/ (µm)2(MMT wt %)] contain more dispersed particles than those prepared from M2(HT)2-125 [0.41 ea/(µm)2(MMT wt %)] and (HE)2M1T1 [0.48 ea/(µm)2(MMT wt %)]. An increase in the number of particles at constant MMT percent results in an increase in the average aspect ratio of the dispersed phase. As suggested in a continuum mechanics theory, the increases in

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Figure 8. Changes in the viscosities of novolac resin and its nanocomposites prepared from various organoclays as a function of the shear rate.

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Figure 10. Flexural strength versus ∆d001 of novolac resin and novolac resin/MMT composites cured with HMTA.

may result in increased ductility (or Izod impact strength), whereas fully exfoliated platelets are easier to break.31-33 Improvement in the stiffness and Izod impact strength of novolac resin by blending with organoclays might stem from the existence of intercalated clay aggregates in the composite. Figure 10 shows a plot of the nanocomposite flexural strength versus ∆d001 observed in the nanocomposite. It seems that the degree of reinforcement increases as ∆d001 of the nanocomposite increases. Because ∆d001 reflects the affinity of the organoclay for novolac resin, the amount of novolac resin that is intercalated into the organoclay increases with an increase in ∆d001. These results indicate that the degree of reinforcement of the composite increases as the amount of novolac resin intercalated into the organoclay increases. Summary

Figure 9. Flexural vs Izod impact strength of novolac resin and novolac resin/MMT composites cured with HMTA.

the number of particles and the aspect ratio of the dispersed phase would lead to an increased mixture viscosity.26-28 Composites mixed with a curing agent (HMTA) were cured using a compression-molding machine to prepare specimens for the flexural and Izod impact strength tests. Figure 9 shows the flexural and Izod impact strength of the cured novolac resin and various composites. The flexural and Izod impact strength of the composites are higher than those of novolac resin except for the Na+MMT composite. Blending of novolac resin with organoclay results in reinforced composites, while that with Na+MMT, in which intercalation into the tactoids with novolac resin does not occur, deteriorates the mechanical properties of novolac resin. As shown in Figure 9, samples that show higher flexural strength exhibit higher Izod impact strength. Thus, blending with organoclay simultaneously increases the stiffness and Izod impact strength of novolac resin. Organoclayreinforced thermoplastics typically reveal lower ductility than the unfilled thermoplastic, while organoclay-reinforced thermosets such as epoxy resins often exhibit improvement in the stiffness and ductility.14-18,29-33 It was suggested previously that crack deflection induced by intercalated clay aggregates

Novolac phenolic resin composites with Na+MMT or organoclay were fabricated by melt processing to explore the effects of the amine compatibilizer structure on the morphology and physical properties of the composites. All of the composites prepared from organoclays show peak characteristics of the intercalated state, while intercalation is not observed in the Na+MMT composite. The structure of the amine compatibilizer on the clay was systematically varied to determine how specific groups affect the phenolic resin nanocomposite morphology and physical properties. Composites exhibit a greater extent of intercalation and higher flexural and impact strengths when the amine compatibilizer contains (1) one long alkyl tail on the amine compatibilizer rather than two long alkyl tails, (2) methyl groups on the amine compatibilizer rather than polar (2hydroxyethyl groups) or bulky groups (benzyl or 2-ethylhexyl group), and (3) an equivalent amount of the amine compatibilizer rather than an excess amount of the amine compatibilizer. As a consequence, the greatest shift in the platelet gallery height among the nanocomposites examined here is observed when phenolic resin nanocomposites are prepared from organoclay containing alkylammonium having three methyl groups and one long alkyl tail [M3(HT)1]. Phenolic resin/organoclay nanocomposites cured with HMTA exhibit better flexural and Izod impact strengths than the cured phenolic resin, while cured composites containing Na+MMT exhibit poorer flexural and Izod impact strengths. Composites prepared from M3(HT)1 organoclay show

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the most significant improvement in the mechanical strength, leading to partially exfoliated platelets. From these results, attractive phenolic resin nanocomposites can be produced with proper selection of the organoclay. Acknowledgment This Research was supported by the Chung-Ang University Research Grants in 2010. Literature Cited (1) Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. Preparation and mechanical properties of polypropylene-clay hybrids using a maleic anhydride-modified polypropylene oligomer. J. Appl. Polym. Sci. 1998, 67, 87. (2) Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Preparation and Mechanical Properties of Polypropylene-Clay Hybrids. Macromolecules 1997, 30, 6333. (3) Galgali, G.; Ramesh, C.; Lele, A. A Rheological Study on the Kinetics of Hybrid Formation in Polypropylene Nanocomposites. Macromolecules 2001, 34, 852. (4) Lertwimolnun, W.; Vergnes, B. Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix. Polymer 2005, 46, 3462. (5) Shah, R. K.; Cui, L.; Williams, K. L.; Bauman, B.; Paul, D. R. Nanocomposites from fluoro-oxygenated polyethylene: A novel route to organoclay exfoliation. J. Appl. Polym. Sci. 2006, 102, 2980. (6) Wang, K. H.; Choi, M. H.; Koo, C. M.; Choi, Y. S.; Chung, I. J. Synthesis and characterization of maleated polyethylene/clay nanocomposite. Polymer 2001, 42, 9819. (7) Messersmith, P. B.; Giannelis, E. P. Synthesis and Characterization of Layered Silicate-Epoxy Nanocomposites. Chem. Mater. 1994, 6, 1719. (8) Lan, T.; Pinnavaia, T. J. Clay-Reinforced Epoxy Nanocomposites. Chem. Mater. 1994, 6, 2216. (9) Wang, Z.; Pinnavaia, T. J. Hybrid Organic-Inorganic Nanocomposites: Exfoliation of Magadiite Nanolayers in an Elastomeric Epoxy Polymer. Chem. Mater. 1998, 10, 1820. (10) Weimer, M. W.; Giannelis, C. H.; Sogah, D. Y. Direct Synthesis of Dispersed Nanocomposites by in Situ Living Free Radical Polymerization Using a Silicate-Anchored Initiator. J. Am. Chem. Soc. 1999, 12, 1615. (11) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. Synthesis and properties of polyimide-clay hybrid. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2493. (12) Yano, K.; Usuki, A.; Okada, A. Synthesis and properties of polyimide-clay hybrid films. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2289. (13) Delozier, D. M.; Orwoll, R. A.; Cahoon, J. F.; Johnston, N. J.; Smith, J. G.; Connell, J. W. Preparation and characterization of polyimide/ organoclay nanocomposites. Polymer 2001, 43, 813. (14) Fornes, T. D.; Yoon, P. J.; Hunter, D. L.; Keskkula, H.; Paul, D. R. Effect of organoclay structure on nylon 6 nanocomposite morphology and properties. Polymer 2002, 43, 9915. (15) Fornes, T. D.; Yoon, P. J.; Keskkula, H.; Paul, D. R. Nylon 6 nanocomposites: the effect of matrix molecular weight. Polymer 2001, 42, 9929.

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ReceiVed for reView February 5, 2010 ReVised manuscript receiVed June 4, 2010 Accepted June 18, 2010 IE1002872