J. Phys. Chem. C 2007, 111, 10377-10381
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Effective Intercalation and Exfoliation of Nanoplatelets in Epoxy via Creation of Porous Pathways Woong J. Boo,† Luyi Sun,†,‡ Jia Liu,† Abraham Clearfield,*,‡ and Hung-Jue Sue*,† Polymer Technology Center, Department of Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012 ReceiVed: March 20, 2007; In Final Form: May 4, 2007
A new surface modification approach for improving intercalation and exfoliation efficiency of layered nanoplatelets is introduced on the basis of R-zirconium phosphate (R-ZrP) nanoplatelets. The intercalation of R-ZrP nanoplatelets was carried out using a number of different organic surface modifiers and their combinations. It is found that porous pathways, which can be created by incorporating a combination of a linear long-chain amine and a bulky, short-chain amine as intercalating agents, can lead to speedier and more effective intercalation. Consequently, fully exfoliated polymer nanocomposites can be more easily prepared. The intercalation/exfoliation mechanisms accounting for the observed effectiveness are discussed.
Introduction Since the original work on montmorillonite clay-modified nylon-6 nanocomposites was reported about 2 decades ago,1,2 research on inorganic layered compound based polymer nanocomposites has attracted worldwide attention because they exhibit significantly improved physical and mechanical properties compared with conventional polymer composites.3,4 In preparing polymer nanocomposites, no matter what layered compound is selected, intercalation and exfoliation have always been the two most critical steps for the preparation of polymer nanocomposites. Effective intercalation has been shown to be essential for the preparation of fully exfoliated polymer nanocompositeswithgreatlyimprovedmodulusandbarrierproperties.5-8 Among a number of routes to prepare polymer nanocomposites developed in the past 2 decades, the most traditional and probably also the most popular approach is to intercalate the layered compounds in advance with an organophilic surface modifier, followed by intercalation of monomers or polymers into the layers to achieve final exfoliation.1,3,5,9,10 To render hydrophilic nanoplatelet fillers to become miscible with polymer matrices, the hydrophilic nanoplatelet surfaces need to be converted to organophilic ones so that they can be more compatible with monomers or polymer chains. Generally, this surface modification process can be achieved by ion exchange reactions with a cation-based intercalating agent, such as ammonium or phosphonium salts, to lower the surface energy of the inorganic nanoplatelets and improve the wetting with the polymer matrix. Also, by employing appropriate intercalating agent(s), functional groups on the nanoplatelet surfaces may be tailored to trigger in-situ polymerization of monomers for improved interfacial strength between the inorganic nanoplatelets and the polymer matrix.11,12 When preparing intercalated layered compounds, most of the researchers emphasize the importance of interlayer distance to achieve exfoliation.7,13-15 As a result, the larger the interlayer * To whom correspondence should be addressed. Telephone: (979) 845-5024 (H.-J.S.); (979) 845-2936 (A.C.). Fax: (979) 862-3989 (H.-J.S.); (979) 845-4719 (A.C.). E-mail:
[email protected] (H.-J.S.); clearfield@mail. chem.tamu.edu (A.C.). † Department of Mechanical Engineering. ‡ Department of Chemistry.
distance for the surface-modified nanoplatelets, the easier the monomers or polymers can be introduced into the interlayer galleries to achieve full exfoliation. Typically, a larger interlayer distance leads to weaker interlayer binding strength, which allows for easier intercalation and exfoliation when external driving forces, such as mixing, shearing, and ultrasonication, etc., are applied. Interestingly, in some cases, the conformational freedom of the intercalating chains is more critical than a large interlayer distance to result in an improved intercalation and/or exfoliation.9,10,16 The reason for such an unexpected outcome may lie in the fact that when the interlayer galleries are fully occupied by intercalating molecules, there is no room for monomers or polymers to diffuse into the galleries. Thus, further intercalation and/or exfoliation cannot be easily realized. To avoid the above undesirable outcome, the present paper focuses on creating porous pathways in the interlayer galleries to facilitate diffusion of monomer or polymer chains. To demonstrate the importance of porous pathways for effective intercalation, a set of model intercalating agents containing longchain and short-chain amines, and their combination, were selected to study the intercalation process in R-zirconium phosphate (R-ZrP) nanoplatelets. The utilization of R-ZrP for preparation of polymer nanocomposites has been shown to be ideal for fundamental studies of intercalation processes and structure-propertyrelationshipsofpolymernanocomposites.5-8,17-20 The usefulness of porous pathways for the intercalation process in epoxy was monitored using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Approaches for achieving effective intercalation and exfoliation of polymer nanocomposites are also discussed. Experimental Section Materials. The R-ZrP was synthesized by refluxing 10.0 g of zirconium oxychloride octahydrate (ZrOCl2‚8H2O, 98%, Aldrich) in 100 mL of 3.0 M phosphoric acid (Aldrich) at 100 °C for 24 h. The detailed chemistry and procedures for the synthesis of R-ZrP can be found elsewhere.17,18 Cyclohexylamine (C6H11NH2, 99%, Aldrich), dodecylamine (CH3(CH2)11NH2, 99%, Aldrich), polyoxyalkyleneamine (Jeffamine M600, Huntsman Chemical), and tetra-n-butylammonium hydroxide (TBA, (CH3CH2CH2CH2)4N(OH), Aldrich) were used as surface
10.1021/jp072227n CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007
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Boo et al.
TABLE 1: Compositions of Surface Modifiers Utilized for Intercalation of r-ZrP
C-ZrP D-ZrP CD-ZrP
R-ZrP (mmol, dispersed in acetone)
cyclohexylamine (mmol, 0.02 M solution in acetone)
1 1 1
2 1
dodecylamine (mmol, 0.02 M solution in acetone) 2 1
modifiers to intercalate R-ZrP layers. The epoxy monomer used in this study was diglycidyl ether of bisphenol A (DGEBA) epoxy resin (DER332, The Dow Chemical Co.). The curing agent utilized was (4,4′-diaminodiphenyl)sulfone (DDS, Aldrich). All the chemicals, except the epoxy resin which was dried in a vacuum oven for 24 h prior to sample preparation, were used as received. Sample Preparation. All intercalation reactions were carried out at room temperature. Three sets of samples were prepared in round-bottom flasks. In each flask, 1.0 mmol of R-ZrP powder was mixed with 50 mL of acetone and ultrasonicated (1510R, Branson; 70 W, 42 kHz) for 30 min. Afterward, the R-ZrP powder was well-dispersed in acetone to facilitate intercalation reactions. A mixture of 1.0 mmol of cyclohexylamine and 1.0 mmol of dodecylamine was pre-dissolved in 50 mL of acetone and added dropwise into R-ZrP dispersion (CD-ZrP) during which the reactants were vigorously stirred. For comparison, the same amount of R-ZrP dispersion was intercalated separately by 2.0 mmol of cyclohexylamine and 2.0 mmol of dodecylamine, denoted as C-ZrP and D-ZrP, respectively, via the same procedure. After the drop-by-drop mixing was finished, each sample was ultrasonicated for 30 min. For clarity, the compositions of all the samples prepared for this study are given in Table 1. To investigate the effect of the interlayer distance of surfacemodified R-ZrP on diffusion, 20 mmol of epoxy monomer was pre-dissolved in 50 mL of acetone and added into the amineintercalated R-ZrP/acetone mixtures, followed by 15 min of ultrasonication and 30 min of stirring. X-ray diffraction (XRD) was performed after every step to monitor the changes of interlayer distance of the R-ZrP during intercalation and subsequent diffusion of epoxy monomers. Characterizations. XRD analysis was performed on a Bruker D8 diffractometer with Bragg-Brentano θ-2θ geometry (40 kV and 40 mA). For the R-ZrP powder sample, it was gently packed on a sample holder. The diffraction patterns were obtained for 2θ in the range from 3 to 60° with a step size of 0.04° and a count time of 1 s/step. For the intercalated R-ZrP samples, they were cast as a thin film on a clean silicon wafer and dried overnight at room temperature prior to XRD characterization. Their diffraction patterns were obtained for 2θ in the range from 1 to 15° with a step size of 0.04° and a count time of 2 s/step. Scanning electron microscopy (SEM) images were acquired using a Zeiss Leo 1530 VP field emission-SEM (FE-SEM). The samples were sputter-coated with a thin layer (ca. 3 nm) of Pt/ Pd (80/20) prior to SEM imaging. Transmission electron microscopy (TEM) was performed using a JEOL 1200EX, operated at 100 kV. A Reichert-Jung Ultracut-E microtome was utilized to prepare thin sections with ∼80 nm in thickness for TEM imaging. Results and Discussion Figure 1 shows the XRD pattern of the synthesized R-ZrP sample, which has an interlayer distance of 7.6 Å. The relatively
Figure 1. XRD pattern of R-ZrP.
Figure 2. SEM image of R-ZrP.
broad peaks in the XRD pattern indicate that the crystallinity of R-ZrP is relatively low. For example, the broad peak at 25° 2θ is a doublet.21 But the corresponding SEM image (Figure 2) clearly shows that sheet structures with lateral dimensions of 80-100 nm have formed. It has been confirmed by previous studies6,8,20 that R-ZrP with relatively low crystallinity is actually beneficial for the intercalation process. The low-crystallinity R-ZrP can be easily intercalated by a series of polyoxyalkyleneamines (Jeffamines, Huntsman) to an interlayer distance of about 70 Å, which is about 10 times its original interlayer distance.20 The XRD patterns of intercalated R-ZrP are present in Figure 3. XRD patterns 1 and 2 show the intercalated d-spacing of 18 and 36 Å for C-ZrP and D-ZrP, respectively. These interlayer distances correspond to the static state molecular sizes of cyclohexylamine and dodecylamine at room temperature. XRD pattern 3 shows a d-spacing of 28 Å for CD-ZrP, which has been intercalated by a mixture with an equal amount of cyclohexylamine and dodecylamine. This d-spacing is between 18 and 36 Å, which is expected. Generally, when certain aliphatic amines are used to intercalate layered compounds at a maximum stoichiometric ratio, the linear molecule chains tend to pack tightly with each other and form a highly ordered structure.22 However, by adding cyclohexylamine molecules as a co-intercalating agent with an aliphatic amine, it can prevent tight packing of intercalating molecules inside the galleries. As a result, a decreased d-spacing is observed. Compared with the XRD patterns of C-ZrP and D-ZrP, the peaks in CD-ZrP are broader. This also indicates that the structure of the CDZrP intercalated compound is less ordered and less uniform compared with those of C-ZrP and D-ZrP.
Intercalation and Exfoliation of Nanoplatelets
Figure 3. XRD patterns of intercalated R-ZrP with cyclohexylamine (C-ZrP), dodecylamine (D-ZrP), and a mixture of the two (CDZrP), with each followed by the mixing with epoxy monomer.
Figure 4. Schematic illustrations of interlayer spacing of R-ZrP treated with different organic modifiers: (a) cyclohexylamine, (b) dodecylamine, and (c) an equal mixture of cyclohexylamine and dodecylamine.
After addition of epoxy monomers into samples C-ZrP, D-ZrP, and CD-ZrP with ultrasonication and followed by stirring, XRD patterns were recorded and are shown as 1-E, 2-E, and 3-E in Figure 3, respectively. Compared with XRD patterns 1 and 2, it is noted that no appreciable change in d-spacing occurred in 1-E and 2-E after the addition of epoxy monomers. However, 3-E clearly shows an expanded d-spacing from 28 to 33 Å. To assist the description of the observed intercalation process, schematic illustrations of the intercalation mechanisms and d-spacing are presented in Figure 4. In both case a and case b, the interlayer spacing is fully occupied by cyclohexylamine and dodecylamine, respectively. Although an increased interlayer distance is achieved, there is little room for possible further intercalation. Therefore, there is little change in d-spacing after the addition of epoxy monomer, as shown in patterns 1-E and 2-E in Figure 3. In case c, even though the d-spacing is smaller than case b, a mixture of a linear chain and a bulky chain will
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10379 inevitably create porous pathways inside the galleries to facilitate fast diffusion of epoxy monomers into the gallery. It should be noted that Figure 4c is an idealized depiction of co-intercalation by cyclohexylamine and dodecylamine. In certain intercalated regions, domains of cyclohexylamine and dodecylamine should be present, thus resulting in nonuniform intercalation. This phenomenon is evidenced by the broad peaks on the XRD pattern of CD-ZrP shown in Figure 3. In fact, similar porous structure has also been observed in the mixed derivatives of R-ZrP,23 in which the porous structure is directly formed during synthesis. The difference in size and structure of the two types of amine molecules creates porous pathways to accommodate the epoxy monomer diffusion. Consequently, the d-spacing between intercalated layers is expanded by the diffusion of epoxy monomer into R-ZrP interlayer galleries, as illustrated in Figure 4c and supported by XRD pattern 3-E in Figure 3. To be noted, after further intercalation of epoxy monomer, the peak in XRD pattern 3-E becomes much broader and less intense. The widening of the peak is because of the loosening of the layered structure. The lowered intensity is probably due to the delamination of a portion of R-ZrP nanoplatelets, which results in exfoliation. Even though the layered structure in case 3-E of Figure 3 is still not completely exfoliated, the strategy of creating porous pathways clearly shows an advantage over the conventional intercalation approach, where only one intercalating agent is used. The above favorable intercalation phenomenon can be explained and supported by the work of Vaia and Giannelis9,10 and Pinnavaia et al.16,24 Vaia and Giannelis9,10 employed the concept of internal energy and entropic factors associated with intermolecular interactions to show the importance of conformational freedom of intercalating molecular chains for effective intercalation and exfoliation. Pinnavaia et al.16,24 reported that low charge density nanoplatelets are desirable to achieve nanocomposites with a high degree of layer exfoliation due to the low areal density of onium ion and prepolymer in the intergallery region. The porous pathway concept follows the preferences for intercalation and exfoliation described above. The porous pathways can be created by many choices of combination of intercalating agents, depending on the polymer matrices involved and the size and interfacial characteristics desired. Another significant side benefit of the porous pathway approach, in addition to the high level of exfoliation, is its fast intercalation and exfoliation rate. In all the samples prepared, all reach their maximum intercalation and exfoliation state within 30 min, having 15 min of ultrasonication and 15 min of magnetic stirring. Much longer time (>3 h) would be needed to achieve the same level of dispersion for the case where only one linear long-chain amine is utilized.25 This finding is consistent with the work of Ginzburg et al.,26 who uses a “kink model” to explain the fast intercalation rate due to the increased intergallery spacing created by the kinking of nanoplatelets. The fast intercalation and exfoliation rate phenomenon can also be supported by the free energy concept described by Vaia and Giannelis.9,10 On the basis of the aforementioned porous pathway concept, a mixture of TBA and polyoxyalkyleneamine (Jeffamine M600) were utilized as intercalating agents to maximize the sizes of the porous pathways for intercalation and exfoliation of R-ZrP nanoplatelets (TM-ZrP) in epoxy before curing. The molecular size difference between TBA and M600 is much larger than
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Figure 5. XRD of R-ZrP (a), R-ZrP intercalated with a mixture of TBA and Jeffamine M600 (b), and its corresponding epoxy nanocomposite (c) (before curing).
Boo et al. achieved. In this study, a new approach to facilitate effective diffusion of monomers, and possibly polymer chains, into the intergalleries of nanoplatelets is presented. In addition to the previously intercalating approaches proposed by others,9,16,22,24 the porous pathway approach reported in this study can be an effective alternative to achieve the preparation of exfoliated polymer nanocomposites. This present study also gives insights toward the intercalation/exfoliation mechanisms of nanoplatelets in polymer nanocomposites. It is evident that, in addition to the functionalities of the intercalating agents which affect the thermodynamic state, a kinetic pathway has to be created to facilitate a realistic preparation of exfoliated nanoplatelets in a polymer matrix. Although the present study utilizes R-ZrP as a model nanoplatelet to study intercalation and exfoliation mechanisms, the findings should be applicable to other layered compounds, such as montmorillonite clay, so long as appropriate choices of two or more intercalating agents having differences in sizes and amounts are made. It should be noted that the ion exchange capacity of montmorillonite clay is only 0.6-1.2 mequiv/g, which is much lower than that of R-ZrP (6.64 mequiv/g). Consequently, incomplete intercalation in montmorillonite clay is likely to occur, thereby creating more porous pathways for effective intercalation and exfoliation. It should also be noted that the compatibility between at least one of the intercalating agents and the host monomer (or polymer) needs to be sufficiently good to facilitate their subsequent intercalation and exfoliation. Therefore, the porous pathway concept via utilization of two or more intercalating agents is considered to be a versatile approach for preparing polymer nanocomposites. Conclusions
Figure 6. TEM image of highly exfoliated R-ZrP nanoplatelets (2.0 vol %) in epoxy.
that between cyclohexylamine and dodecylamine,25,27 which leads to more effective intercalation and exfoliation in epoxy. Figure 5 presents the XRD patterns of (a) pristine R-ZrP, (b) intercalated TM-ZrP, and (c) exfoliated TM-ZrP after the addition of epoxy monomers. The XRD pattern of TM-ZrP shows an increase in basal spacing from 7.6 (pristine R-ZrP) to 33 Å, which is between the d-spacing values of R-ZrP intercalated with only TBA or M600,19,20,28 meaning that the disruption of molecular packing took place and porous pathways, which are favorable for the diffusion of epoxy monomers, have been created. The XRD of epoxy/TM-ZrP indicates that exfoliation of R-ZrP has been achieved through the diffusion of epoxy monomer into the intercalated TM-ZrP galleries. It should be noted that the broad hump at 18° in 2θ corresponds to the amorphous halo of epoxy resin. To confirm the high degree of exfoliation of R-ZrP nanoplatelets in epoxy matrix, TEM investigations were performed after curing of epoxy/TM-ZrP with DDS curing agent. Figure 6 displays a representative TEM image showing exfoliated and well-dispersed TM-ZrP nanoplatelets (2.0 vol %) in epoxy with no sign of aggregation or intercalated nanoplatelets. To obtain the benefit of polymer nanocomposites, it is required that high degrees of exfoliation of nanoplatelets be
A simple and effective approach to achieve intercalation/ exfoliation of nanoplatelets in polymer matrices by using a mixture of intercalating agents with different sizes is introduced. The results show that the intercalation via creation of porous pathways using two or more intercalating agents with different sizes is an effective way to achieve intercalation and subsequent exfoliation. The approach allows for effective intercalation and exfoliation of nanoplatelets for manufacturing of polymer nanocomposites. Acknowledgment. The authors gratefully acknowledge partial financial support of the Defense Logistic Agency (Grant SP0103-02-D-0024), the State of Texas ARP Grant (00051200311-2003), and the National Science Foundation (Grant DMR-0332453). References and Notes (1) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 983-986. (2) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1179-1184. (3) Alexandre, M.; Dubois, P. Mat. Sci. Eng., R 2000, 28, 1-63. (4) Giannelis, E. P. AdV. Mater. 1996, 8, 29-35. (5) Boo, W.-J.; Sun, L.; Liu, J.; Clearfield, A.; Sue, H.-J. Compos. Sci. Technol. 2007, 67, 262-269. (6) Sue, H. J.; Gam, K. T.; Bestaoui, N.; Spurr, N.; Clearfield, A. Chem. Mater. 2004, 16, 242-249. (7) Boo, W. J.; Liu, J.; Sue, H. J. Mater. Sci. Technol. 2006, 22, 829834. (8) Sue, H. J.; Gam, K. T.; Bestaoui, N.; Clearfield, A.; Miyamoto, M.; Miyatake, N. Acta Mater. 2004, 52, 2239-2250. (9) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 79907999. (10) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 80008009.
Intercalation and Exfoliation of Nanoplatelets (11) Blumstei, A. J. Polym. Sci., Part A: Gen. Pap. 1965, 3, 26652673. (12) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728-1734. (13) Xu, B.; Zheng, Q.; Song, Y. H.; Shangguan, Y. Polymer 2006, 47, 2904-2910. (14) Oh, T. K.; Hassan, M.; Beatty, C.; El-Shall, H. J. Appl. Polym. Sci. 2006, 100, 3465-3473. (15) Hu, C.; Kim, J.-K. Compos. Interfaces 2005, 12, 271-289. (16) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144-2150. (17) Clearfield, A.; Medina, A. S.; Duax, W. L.; Garces, J. M. J. Inorg. Nucl. Chem. 1972, 34, 329-337. (18) Sun, L.; Boo, W. J.; Sue, H.-J.; Clearfield, A. New J. Chem. 2007, 31, 39-43. (19) Sun, L.; Boo, W. J.; Sun, D.; Clearfield, A.; Sue, H.-J. Chem. Mater. 2007, 19, 1749-1754.
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10381 (20) Sun, L.; Boo, W. J.; Browning, R. L.; Sue, H.-J.; Clearfield, A. Chem. Mater. 2005, 17, 5606-5609. (21) Clearfield, A. Annu. ReV. Mater. Sci. 1984, 14, 205-229. (22) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017-1022. (23) Wang, J. D.; Clearfield, A. Mater. Chem. Phys. 1993, 35, 208216. (24) Triantafillidis, C. S.; LeBaron, P. C.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 4088-4095. (25) Boo, W. J.; Sun, L.; Liu, J.; Moghbelli, E.; Clearfield, A.; Sue, H. J. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1459-1469. (26) Ginzburg, V. V.; Gendelman, O. V.; Manevitch, L. I. Phys. ReV. Lett. 2001, 86, 5073-5075. (27) Boo, W.-J.; Sun, L. Y.; Warren, G. L.; Moghbelli, E.; Pham, H.; Clearfield, A.; Sue, H.-J. Polymer 2007, 48, 1075-1082. (28) Kim, H. N.; Keller, S. W.; Mallouk, T. E.; Schmitt, J.; Decher, G. Chem. Mater. 1997, 9, 1414-1421.
