Amphiphilic Properties of Poly(oxyalkylene)amine-Intercalated

To compare with the MMT intercalation, the synthetic mica was subjected to the ionic exchange reaction with the same series of POP-amine salts, and it...
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Langmuir 2004, 20, 4261-4264

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Amphiphilic Properties of Poly(oxyalkylene)amine-Intercalated Smectite Aluminosilicates Jiang-Jen Lin* and Yu-Min Chen Department of Chemical Engineering, National Chung-Hsing University, Taichung 402, Taiwan Received December 4, 2003. In Final Form: February 26, 2004 Layered aluminosilicates, including synthetic fluorine mica and natural montmorillonite (MMT), were intercalated with poly(oxypropylene)-polyamine quaternary salts with a 230-5000 molecular weight range. The X-ray basal spacing of these silicates had been expanded from 13.5 to 83.7 Å for the synthetic mica and to 92.0 Å for MMT. The relative silicate dimensions (300-1000 nm for synthetic mica and 80-100 nm for MMT) were ascertained by direct TEM observations in the case of the co-intercalated synthetic mica and MMT mixtures with Mw ) 2000 quaternary ammonium salts. The tailored organic incorporation of synthetic mica and MMT clays could alter these hydrophilic clays, making them amphiphilic, and enable the lowering of toluene/water interfacial tension to 2.0 mN/m at the critical concentration of 0.1 wt %.

Introduction The naturally occurring layered silicates are conventionally applied in catalysts,1 adsorbants,2 and metal chelating agents.3 In recent years, silicate clays have found important uses in polymer/layered silicate (PLS) nanocomposites with significantly improved thermal and mechanical properties.4-6 The high aspect ratio and large surface area of the layered structure promote strong interactions with organic polymers on the molecular level. Utilization of these hydrophilic silicates requires an organic modification using low molecular weight ionic exchange agents such as alkylammonium derivatives5-9 or phosphonium salts.10-12 For example, the common montmorillonite clay is generally intercalated with C12-18 alkyl quaternary ammonium salts to become an organoclay with a 28 Å basal spacing.8 The organic incorporation renders the clay compatible with a polymer matrix. Recently, we have reported the use of poly(oxypropylene)-polyamine quaternary salt for the intercalation of sodium montmorillonite (Na+-MMT), creating a wide interlayer spacing enlargement up to 92 Å.13,14 To obtain * To whom correspondence should be addressed. Fax: +886-42287-1787. E-mail: [email protected]. (1) Cseri, T.; Bekassy, S.; Figueras, F.; Rizner, S. J. Mol. Catal. A 1995, 98, 101. (2) Celis, R.; Hermosin, M. C.; Carrizosa, M. J.; Cornejo, J. J. Agric. Food Chem. 2002, 50, 2324. (3) Kiraly, Z.; Veisz, B.; Mastalir, A.; Kofarago, G. Langmuir 2001, 17, 5381. (4) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 1755. (5) Pastorini, M. T.; Nunes, R. C. R. J. Appl. Polym. Sci. 1999, 74, 1361. (6) Fu, X.; Qutubuddin, S. Polymer 2001, 42, 807. (7) Klapyta, Z.; Fujita, T.; Iyi, N. Appl. Clay Sci. 2001, 19, 5. (8) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (9) Yang, J. H.; Han, Y. S.; Choy, J. H.; Tateyamab, H. J. Mater. Chem. 2001, 11, 1305. (10) Ijdo, W. L.; Pinnavaia, T. J. Chem. Mater. 1999, 11, 3227. (11) Imai, Y.; Nishimura, S.; Abe, E.; Tateyama, H.; Abiko, A.; Yamaguchi, A.; Aoyama, T.; Taguchi, H. Chem. Mater. 2002, 14, 477. (12) Maiti, P.; Yamada, K.; Okamoto, M.; Ueda, K.; Okamoto, K. Chem. Mater. 2002, 14, 4654. (13) Lin, J. J.; Chen, I. J.; Wang, R. C.; Lee, R. J. Macromolecules 2001, 34, 8832. (14) Chou, C. C.; Shieu, F. S.; Lin, J. J. Macromolecules 2003, 36, 2187.

such a wide basal spacing, the intercalating agents must have both abilities of ionic exchange with Na+-MMT and hydrophobic self-alignment within the layered confinement. The resultant modified clays are inorganic/organic hybrids with functionalities of hydrophilic silicates and hydrophobic organics. Furthermore, during the ionic exchange, the organic incorporation actually proceeds via a critical conformation change when filling up the silicate confinement,15 which is analogous to the critical micelle concentration (cmc) for surfactant aggregation in solution. Determining the reaction profile and chemical properties of these hybrids may assist the design and synthesis of various organoclays for different applications including nanocomposites and organic/layered silicate surfactants. In considering the utilization of hydrophilic silicate clays, the naturally occurring micas have received relatively little attention due to the inherent property of difficulty in swelling caused by the high affinity among the hydroxyl sheets. Studies have shown that the synthetic fluorine mica is dispersible in water, and, more importantly, synthetically controllable in its chemical composition.16,17 However, the property of large platelet dimension and high aspect ratio causes the organic exchange of these synthetic micas to be slow in reaction and results in a narrow interlayer widening. Among these reports, the exchange with C18-alkylammonium salt resulted in the intercalation of organics in the silicate interlayer with a 31.6-39.6 Å basal spacing but in low yield.8 In another example, phospholipid intercalation in layered synthetic mica results in a wide interlayer space of 72.3 Å.18,19 In this paper, we have compared the organic modification of the synthetic micas with the natural montmorillonite using the poly(oxyalkylene)amine as the modifying agent. Both hydrophilic and hydrophobic poly(oxyalky(15) Lin, J. J.; Chen, I. J.; Chou, C. C. Macromol. Rapid Commun. 2003, 24, 492. (16) Tateyama, H.; Nishimura, S.; Tsunematsu, K.; Jinnai, K.; Adachi, Y.; Kimura, M. Clays Clay Miner. 1992, 40, 180. (17) Kodama, T.; Higuchi, T.; Shimizu, K. I.; Komarneni, S.; Hoffbauer, W.; Schneider, H. J. Mater. Chem. 2001, 11, 2072. (18) Kanzaki, Y.; Kogure, M.; Sato, T.; Tanaka, T. Langmuir 1993, 9, 1930. (19) Kanzaki, Y.; Hayashi, M.; Minami, C.; Inoue, Y.; Kogure, M.; Watanabe, Y.; Tanaka, T. Langmuir 1997, 13, 3674.

10.1021/la0362775 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/14/2004

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Lin and Chen Table 1. Basal Spacing and Properties of Synthetic Mica and MMT by Modified Poly(oxyalkylene)amines organic fraction (wt %) intercalated agenta

Figure 1. Chemical structures of poly(oxyethylene)- and poly(oxypropylene)-backboned polyamines.

lene)-polyamines with molecular weights Mw ranging from 230 to 5000 are systematically employed. With similar cation exchange capacities for synthetic Na+-mica (1.20 mequiv/g) and Na+-MMT (1.15 mequiv/g), the differences in the intercalated gallery distances are correlated to the dimensions of clay platelet structure. Their amphiphilic properties are explored by measuring the toluene/water dispersing ability and interfacial tension. Experimental Section The synthetic fluorine mica (SOMASIF ME-100) having a chemical composition of Si (26.5 wt %), Mg (15.6 wt %), Al (0.2 wt %), Na (4.1 wt %), Fe (0.1 wt %), and F (8.8 wt %) and a cation exchange capacity (CEC) of 1.20 mequiv/g was obtained from CO-OP Chemical Co. (Japan). Sodium montmorillonite (Na+MMT) is a natural smectic aluminosilicate, obtained from Kunimine Industries, Inc. and was determined to have a cation exchange capacity of 1.15 mequiv/g. Poly(oxyalkylene)-backboned polyamines include hydrophilic poly(oxyethylene) (POE)-polyamine and various molecular weight poly(oxyproplylene) (POP)-amines. POE-D2000 is an R,ωdiamine of poly(oxypropylene)-b-poly(oxyethylene)-b-poly(oxypropylene) of 2000 Mw (abbreviated as POE-D2000) and watersoluble amine. The POP-amines, including diamines of Mw ) 230, 400, 2000, and 4000 (abbreviated as POP-D230, POP-D400, POP-D2000, and POP-D4000, respectively) and triamines of Mw ) 400, 3000, and 5000 (POP-T400, POP-T3000, and POP-T5000, respectively) were obtained from Huntsman Chemical Co. or Aldrich Chemical Co., structures shown in Figure 1. These POPamines of Mw ) 230-400 are hydrophilic and water-soluble, while their high Mw analogues (Mw ) 2000, 4000, and 5000) are waterinsoluble. The quaternary ammonium salts were formed by treating these (oxyalkylene)amines with equivalent amounts of hydrochloric acid. Detailed procedures for the quaternary ammonium salt intercalation have been reported previously.13 The intercalation was performed by mixing the intercalant salts with Na+-MMT or synthetic mica clay (both pretreated by swelling in water) in deionized water suspension at 80 °C and maintaining the agitation for 3-5 h. The resultant precipitates were collected at ambient temperature, washed thoroughly with toluene/ethanol mixtures, centrifuged, and dried in an oven at 50 °C overnight. The interlayer basal spacing of the prepared organoclays was analyzed by using an X-ray powder diffractometer (Schimadzu SD-D1 using a Cu target at 35 kV, 30 mA). The d spacing of the intercalated silicates was analyzed by using Bragg’s equation (nλ ) 2d sin θ). The value for n ) 1 was calculated from the observed values of n ) 2, 3, etc. The organic fractions were estimated by using a thermal gravimetric analyzer (TGA, PerkinElmer Pyris 1), with a temperature gradient that ramped from room temperature to 800 °C at a rate of 10 °C/min. The interfacial

basal spacingb (Å) mica MMT

based on TGAc mica MMT

based on CECd

none

12.6

12.4

POE-D2000 POP-D230 POP-D400 POP-T400

Water-Soluble Amines 18.0 19.5 52 13.5 15.0 16 17.2 19.4 22 15.0 16.0 18

43 17 26 18

71 22 32 32

POP-D2000 POP-T3000 POP-D4000 POP-T5000

Water-Insoluble Amines 41.6 57.3 53 45.8 62.6 56 68.2 92.0 66 83.7 82.0 67

63 69 72 75

71 78 83 86

a POE ) poly(oxyethylene); POP ) poly(oxypropylene); D and T ) diamine and triamine. b X-ray d spacing calculated on the basis of the Bragg equation (nλ ) 2d sin θ). c Organic fraction from TGA. d Organic fraction from CEC: [1.15 (MMT) or 1.20 (Mica) mequiv/g] × Mw.

tension was measured by the Wilhelmy method using a KrussK10 digital tensiometer equipped with a spherical ring. Transmission electron microscopy (TEM) was performed on a Zeiss EM 902A operated at 80 kV.

Results and Discussion Organic Modification and Basal Spacing Enlargement. The two types of layered aluminosilicates, synthetic mica and natural montmorillonite, were allowed to exchange with poly(oxyalkylene) quaternary ammonium salts. The use of hydrophobic POP quaternary ammonium salts for the intercalation of Na+-MMT to achieve basal spacing of 15.0, 19.4, 57.3, and 92.0 Å has been reported previously.13 The height of the interlayer widening was found to correspond to the POP molecular weights of 230, 400, 2000, and 4000, respectively (Table 1). To compare with the MMT intercalation, the synthetic mica was subjected to the ionic exchange reaction with the same series of POP-amine salts, and it was found to have basal spacing of 13.5, 17.2, 41.6, and 68.2 Å, respectively. It appears that the correlation of the intercalant molecular weight with the basal spacing for mica is similar to that for MMT. However, the gallery expansions are narrower for synthetic mica than for MMT intercalation throughout the POP-amine molecular weight range from 230 to 4000. The differences are attributed to the large platelet size of the synthetic mica and the hindrance for organic incorporation. As a derivative of naturally occurring calcium montmorillonite, sodium montmorillonite (Na+-MMT) has been well-recognized for its usefulness in the preparation of polymer nanocomposites. The primary structure is composed of approximately 8-10 platelets stacked in one unit20 with dimensions of 100 × 100 × 1 nm for each platelet.21 In contrast, the synthetic mica is only known for its aggregate size on the micrometer scale, but its primary structure dimensions are not well-documented.9,11,22 With both clays having similar cation exchange capacity (1.20 mequiv/g for synthetic mica and 1.15 mequiv/g for MMT), it seems that the aspect ratio is the dominant factor affecting the platelet adhesion and (20) Akelah, A.; Moet, A. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1994, 55, 153. (21) Usuki, A.; Hasegawa, N.; Kadoura, H.; Okamoto, T. Nano Lett. 2001, 1, 271. (22) McNally, T.; Murphy, W. R.; Lew, C. Y.; Turner, R. J.; Brennan, R. J. Polymer 2003, 44, 2761.

Amphiphilic Organic/Silicate Hybrids

Figure 2. X-ray diffraction patterns of (a) synthetic mica, (b) POP-D2000/synthetic mica, (c) POP-T5000/synthetic mica, and (d) POP-D2000/MMT.

the organic incorporation. For large platelets, the confined space could be spatially hindered for organic salts to access, and hence, the basal spacing enlargement is limited. Incorporated Organic Composition. The organic incorporation into the confinement can be analyzed using thermal gravimetric analysis (TGA). The POP-D4000 intercalation resulted in hybrids with a TGA of 66 wt % organics for synthetic mica and 72 wt % for MMT. This compares to the theoretical percentage of 83 wt % based on the clay cation exchange capacity (Table 1). These amounts of organic incorporation correspond with the basal spacing of 68.2 Å for synthetic mica and 92.0 Å for MMT. The differences are correlated to the relatively large platelet dimension for synthetic mica. The low organic incorporation for synthetic mica is consistent throughout the wide molecular weight range of the POP-amine intercalants. In contrast to the hydrophobic POP quaternary ammonium salts, it is interesting to note that the same molecular weight of hydrophilic POE-D2000 (Mw ) 2000) can only widen the basal spacing from the pristine 12.412.6 to 18.0-19.5 Å for MMT and synthetic mica, compared to 41.6-57.3 Å for POP-D2000 of the same molecular weight (Mw ) 2000). This implies that the POE-backboned amines have a high affinity for the silicate layer surfaces. As a result, the POE-D2000 backbones tend to horizontally align with the platelet surface. Low intercalating results were obtained similarly for both mica and MMT intercalation with the hydrophilic POE-amine. The encapsulated organic intercalations are 52 wt % for mica and 43 wt % for MMT intercalation, compared to the theoretical 71 wt %. The larger organic intercalation in mica than in MMT is perhaps due to the sticky sample, which caused a difficult washing and removing procedure for the adsorbed POE-D2000. Maximal Intercalation. The use of trifunctional POPamines of Mw ) 5000 has widened the synthetic mica gallery up to an unusual 83.7 Å, which is wider than that of the Mw ) 3000 POP-triamine (45.8 Å) and the Mw ) 4000 POP-diamine (68.2 Å). It appears that the trifunctional POP-amine salts with a relatively high molecular weight can effectively intercalate mica clay to reach a high basal spacing through the ionic exchange reaction of multiple quaternary ionic sites. The pattern of the diffraction peaks (n ) 2-6) in the XRD analysis obeys Bragg’s equation (nλ ) 2d sin θ), from which the n ) 1 d spacing can be calculated as indicated in Figure 2.

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Figure 3. (a) TEM image of POP-D2000-intercalated synthetic mica and MMT, prepared from co-intercalation, (b) magnified synthetic mica with an averaged 4-6 layered structure, and (c) magnified MMT with an averaged 8-10 layered structure.

Figure 4. TEM image of POP-D2000-intercalated synthetic mica.

Comparison of Silicate Platelet Dimensions. The mixture of synthetic mica and MMT was co-intercalated with POP-D2000 (Mw ) 2000) as the ionic exchanging agent. The co-intercalated sample was examined by using transmission electronic microscopy (Figure 3a), in which two sets of layered silicates were observed. The platelet dimensions of 300-1000 and 80-100 nm in length and 35-40 and 45-50 Å as the interlayer distance are assigned to the intercalated mica and MMT, respectively (Figure 3b,c). Both silicates have a platelet thickness of approximately 10 Å. The TEM interlayer spacing is consistent with the estimations from the X-ray Bragg peaks for the individual POP-D2000-intercalated mica (XRD, 41.6 Å) and MMT (XRD, 57.3 Å).14 For comparison, the individual POP-D2000-intercalated mica sample has been prepared and showed a clear 40 Å basal spacing by TEM (Figure 4), consistent with the XRD Bragg calculation (d spacing 41.6 Å). The large size of the synthetic mica, approximately 300-1000 nm, can also be observed in the TEM microgram of the pristine synthetic mica, which is organic-free and dispersible in water (Figure 5). Therefore,

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Figure 5. TEM image of synthetic fluorine mica swollen in water.

the primary structure of the synthetic mica has a much larger size than that of MMT with respect to their platelet surface. Amphiphilic Properties. In general, the degree of organic incorporation is dominated by the CEC of the clays. The ionic exchange reaction proceeds stoichiometrically between ammonium salts and exchangeable sodium ions. The theoretical amount of the organics incorporated into the silicate confinement can be calculated on the basis of CEC values. The actual amount of intercalated organics is measured by thermal gravimetric analysis and is summarized in Table 1. When comparing synthetic mica and MMT encapsulation, one can conclude that the organic fraction is generally lower for synthetic mica clay. The intercalants tend to alter the silicate hydrophilicity. In the cases of the water-soluble POE-D2000 and the POPamines both with a molecular weight range of 230-400, the intercalated hybrids are similar in their basal spacing to the pristine Na+-MMT or synthetic mica and in their ability to disperse in water without apparently observable aggregates. On the other hand, the hybrids that are formed when using the hydrophobic, high-Mw POP-amines (over 2000) are only dispersible in toluene. Furthermore, the hybrids formed by the combination of the silicate plates and the POP-amine salts are known to be amphiphilic in nature because of their surfactant behavior in solution. More specifically, the balance between the hydrophobic POP portions and hydrophilic silicates causes the hybrid to demonstrate the ability to reduce interfacial tension in toluene/water. By comparison, POP-D2000/silicate hybrids can reduce interfacial tension to a lower value than POE-D2000/clay (Figure 6). In the POE-D2000/silicates,

Lin and Chen

Figure 6. Interfacial tension of synthetic mica and MMT modified by POE-D2000 and POP-D2000, respectively.

both the organic and inorganic portions are too hydrophilic to interact with the toluene phase. On the other hand, the hydrophobic POP-D2000 intercalated the mica and MMT to afford amphiphilic hybrids that self-aggregate at the toluene/water interface. With a concentration as low as 0.1 wt %, the interfacial tension can be significantly decreased to 2 mN/m. In contrast, the large mica platelet hybrids showed a smaller decrease to 7 mN/m interfacial tension due to less appropriately matched hydrophobic balance. Conclusions The synthetic fluorinated Na+-mica and naturally occurring Na+-MMT were intercalated with various poly(oxypropylene) quaternary ammonium salts. Spatially enlarged silicates with up to a 83.7 Å basal spacing were obtained, but slightly different from that of montmorillonite intercalation. Depending on the hydrophobic POPor hydrophilic POE-amine and their molecular weights, the intercalated micas and MMT have amphiphilic properties and are capable of lowering the toluene/water interfacial tension. The difference in the platelet size of the mica (300-1000 nm) and the MMT (80-100 nm) affects the basal spacing enlargement, the organic incorporation, and consequently the hydrophilic/hydrophobic balance of the hybrids. Acknowledgment. This work was financially supported by the National Science Council (NSC) of Taiwan. LA0362775