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Emulsion Intercalation of Smectite Clays with Comb-Branched Copolymers Consisting of Multiple Quaternary Amine Salts and a Poly(styrene-butadiene-styrene) Backbone Yu-Chung Chang, Chih-Cheng Chou, and Jiang-Jen Lin* Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan Received April 10, 2005 Layered silicates were intercalated with comb-branched copolymers consisting of a hydrophobic polystyrene-b-poly(ethylene/butylene)-b-polystyrene (SEBS) backbone and multiple pendants of poly(oxyalkylene) (POA) quaternary ammonium salts. The requisite intercalating agents were synthesized by grafting POA amines on the maleated SEBS. The corresponding SEBS-POA amine salts were found to have two functions, the capability to emulsify toluene/water mixtures to fine particle sizes of 60-70 nm in diameter and to exchange ions with sodium montmorillonite. The resulting silicate hybrids were characterized by X-ray diffraction and transmission electron microscopy. Two types of intercalations with silicate d spacing of 18 and 50 Å were obtained, in which the dissimilarity is attributed to disparate polymer incorporations, POA pendants only or the combination of both the SEBS backbone and POA in the gallery. Furthermore, the two conformations of polymer-intercalated silicates are reversibly transformable by varying emulsion conditions, micelle sizes, and solvents.
Introduction The naturally occurring smectite clays are useful in various applications such as absorbents,1,2 modified electrodes,3-5 multilayer self-assembling substrates,6 and catalysts.7-10 Recently, layered silicates were mixed with polymers to form unique nanocomposites with advanced physical properties.11,12 For example, Imai et al.13 demonstrated that mica dispersion enhances the PET mechanical properties. The process for preparing such nanocomposites requires the alteration of the clay’s properties from hydrophilic to hydrophobic in order to be compatible with the target polymers. The layered silicate clays are organically modified by ionic exchange with various alkylammoniunm cationic surfactants.14-16 Different structures of incorporated surfactants may result * Corresponding author: fax 886-4-2287-1787; e-mail JJLin@ dragon.nchu.edu.tw. (1) Liu, W.; Gan, J.; Papiernik, S. K.; Yates, S. R. J. Agric. Food Chem. 2000, 48, 1935-1940. (2) Schulz, J. C.; Warr, G. G. Langmuir 2000, 16, 2995-2996. (3) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624. (4) Eckle, M.; Decher, G. Nano Lett. 2001, 1, 45-49. (5) Ozsoz, M.; Erdem, A.; Ozkan, D.; Kerman, K.; Pinnavaia, T. J. Langmuir 2003, 19, 4728-4732. (6) Umemura, Y.; Yamagishi, A.; Schoonheydt, R.; Persoons, A.; Schryver, F. D. J. Am. Chem. Soc. 2002, 124, 992-997. (7) Occelli, M. L.; Olivier, J. P.; Perdigon-Melon, J. A.; Auroux, A. Langmuir 2002, 18, 9816-9823. (8) Feng, J.; Hu, X.; Yue, P. L. Environ. Sci. Technol. 2004, 38, 269275. (9) Guo, J.; Al-Dahhan, M. Ind. Eng. Chem. Res. 2003, 42, 24502460. (10) Stackhouse, S.; Coveney, P. V.; Sandre, E. J. Am. Chem. Soc. 2001, 123, 11764-11774. (11) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539-1641. (12) Alexandre, M.; Beyer, G.; Henrist, C.; Cloots, R.; Rulmont, A.; Jerome, R.; Dubois, P. Chem. Mater. 2001, 13, 3830-3832. (13) Imai, Y.; Nishimura, S.; Abe, E.; Tateyama, H.; Abiko, A.; Yamaguchi, A.; Aoyama, T.; Taguchi, H. Chem. Mater. 2002, 14, 477479. (14) Alexandre, M.; Dubois, P. Mater. Sci. Eng., R: Rep. 2000, 28, 1-63. (15) Fu, X.; Qutubuddin, S. Mater. Lett. 2000, 42, 12-15. (16) Fu, X.; Qutubuddin, S. Polymer 2001, 42, 807-813.
in varied clay basal spacing as well as the changes in organophilic properties. The conventional intercalating agents have employed low-molecular-weight surfactants with ammonium salt functionalities because the clays are inherently hydrophilic and ionic in character. High homogeneity for the dispersion of silicate platelets in the matrix requires the intercalating agents to be highly compatible with silicate clay and the target polymers. Surface-active agents, including nonionic, anionic, cationic, and amphoteric compounds, are widely used to minimize the interfacial tension between two distinctly incompatible systems.17 For interacting with smectite clays, cationic surfactants are more suitable for exchange with the cationic silicates because of their similar ionic characters. Studies have also shown that the block copolymers with distinct functionalities may behave as surfactants and are capable of self-aggregation. Similar to the low-molecular-weight surfactants, the amphiphilic copolymers can serve as emulsifiers,18 polymer compatibilizers,19 crystal growth modifiers for inorganics,20 and templates for nanoparticles.21-23 Previously, we have synthesized different copolymers such as poly(etheramine)-grafted PP-g-MA24 and SEBS-g-MA25 and used them for antistatic materials. The common features for these amine-grafted copolymers are their comb-branched shape and multiple amine pendant groups. After treat(17) Binks, B. P. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry Press: Letchworth, U.K., 1998; Chapt. 1. (18) Kawaguchi, M.; Kubota, K. Langmuir 2004, 20, 1126-1129. (19) Lee, S.; Park, O. O. Polymer 2001, 42, 6661-6668. (20) Qi, L.; Colfen, H.; Antonietti, M. Chem. Mater. 2000, 12, 23922403. (21) Yuan, Y. J.; Hentze, H. P.; Arnold, W. M.; Marlow, B. K.; Antonietti, M. Nano Lett. 2002, 2, 1359-1361. (22) Xu, A. W. Chem. Mater. 2002, 14, 3625-3627. (23) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y. U.; Terasaki, O.; Park, S. E.; Stucky, G. D. J. Phys. Chem. B. 2002, 106, 2552-2558. (24) Lin, J. J.; Young, M. Y.; Shau, S. M.; Chen, I. J. Polymer 2000, 41, 2405-2417. (25) Lin, J. J.; Cheng, I. J.; Chen, C. N.; Kwan, C. C. Ind. Eng. Chem. Res. 2000, 39, 65-71.
10.1021/la050948c CCC: $30.25 © 2005 American Chemical Society Published on Web 06/08/2005
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Scheme 1. Synthetic Route and Chemical Structures of the SEBS-POA Copolymers
ment with HCl to convert the amine into ammonium salt, the copolymers have a cationic character and are amphiphilic due to the hydrophobic PP or SEBS backbone and the multiple pendant ions. Owing to the presence of these ionic sites, the amphiphilic copolymers are suitable for intercalating with the smectite clays. In previously reported work, we used alkylammonium salt surfactants and 4000 Mw poly(oxypropylene)-backboned amines to achieve an enlarged basal spacing as high as 92 Å for the sodium montmorillonite (Na+-MMT) intercalation.26,27 The factors for influencing d spacing include molecular weight, hydrophobicity, and the conformation of the intercalating agents. The basal spacing is almost linearly dependent on the intercalant’s molecular weight. However, the use of an extremely high-molecularweight polymer is limited because of the incompatibility between hydrophobic polymer and hydrophilic clay. In this study, we investigate the ionic exchanging intercalation of Na+-MMT by using an approximately 45 000 Mw comb-branch copolymer that is capable of forming emulsions to overcome the incompatibility problem. Since the ionic exchange reaction occurs in the silicate confinement, the size and character of colloidal copolymers in toluene/ water mixture correlates with the intercalation profile. Experimental Section Materials. Sodium montmorillonite (Na+-MMT), 1.40 mequiv/g cationic exchange capacity (CEC), was supplied from Nanocor Co. This clay is sheet unit aggregated particles with dimensions of approximately 100 × 100 × 1 nm for each sheet consisting of negatively charged silicate surfaces and positively charged sodium counterions. The charge-compensating ions on the layered surface may be easily exchanged by organic ammonium salts. The poly(oxyalkylene) diamines, including poly(oxyethylene) (POE) and poly(oxypropylene) (POP) backboned diamines, are purchased from Huntsman Chemical Co. or Aldrich Chemical Co. The POP diamines of 230, 400, and 2000 g/mol Mw (abbreviated as POP230, POP400, and POP2000, respectively) are amine-terminated poly(propylene glycols). The POE diamines of 2000 g/mol Mw (POE2000) are water-soluble poly(etheramines) based on a predominantly poly(ethylene oxide) backbone. The maleated polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymer (SEBS-g-MA, ∼2 wt % maleic anhydride; commercial trade name Shell Kraton 1901X, approximate 45,000 g/mol Mw (26) Lin, J. J.; Cheng, I. J.; Wang, R.; Lee, R. J. Macromolecules 2001, 34, 8832-8834. (27) Chou, C. C.; Chang, Y. C.; Chiang, M. L.; Lin, J. J. Macromolecules 2004, 37, 473-477.
with ∼28 wt % styrene content) was purchased from Shell Chemical Co. Synthesis of SEBS-Grafting POA Diamines as Intercalating Agents. The experimental procedures for synthesizing the series of copolymers were previously reported.25 To a 250 mL three-necked round-bottomed flask, equipped with a mechanical stirrer, nitrogen inlet-outlet lines, a thermometer, and a DeanStark trap, were added POE2000 (10.7 g, 5.35 mmol) and toluene (100 mL), and the mixture was stirred and heated to the toluene boiling temperature in order to azeotropically remove any water contamination. The solution was cooled to ambient temperature and SEBS-g-MA powder (26.2 g, 5.35 mmol of MA) was added. The reactants were further maintained at 120 °C for 3-4 h. During the process, most of the toluene was removed through a Dean-Stark trap. The reaction progress can be monitored by analyzing the characteristic absorptions in the Fourier transform infrared (FT-IR) spectrum at 1100 cm-1 (C-O-C of polyoxyalkylene) and 1653 and 1570 cm-1 (amide) and the disappearance at 1786 and 1782 cm-1 (maleic anhydride). The copolymer’s emulsion property was examined by varying their concentrations from 0.1 to 20 wt % to emulsify an equal volume of toluene/ water. The volume of emulsion against toluene or water phases was then measured. Intercalation of Na+-MMT with SEBS-POA Amines. Na+-MMT (3.0 g, 4.2 mequiv) was dispersed in a volume of deionized water equal to the following toluene volume and stirred vigorously at 80 °C for 3 h. The slurry was added to SEBS-POA2000 ammonium salts (30 g, 4.2 mmol of amine alkalinity in toluene), which had been previously treated with aqueous HCl (0.44 g, 4.2 mmol) at 80 °C for 3 h. Different concentrations of SEBS-POA2000 in toluene (6, 12, and 20 wt %) were explored for the intercalation. The resultant copolymer/ MMT hybrids were dried in a vacuum oven at 80 °C and analyzed by TEM. For the toluene-extracted samples, the hybrids were stirred vigorously in toluene at ambient temperature and then dried. Analytical Instruments. An X-ray powder diffractometer (Shimadzu SD-D1 with a Cu target at 35 kV, 30 mA) was used to measure silicate basal spacing. The d spacing of the intercalated silicates was determined by fitting Bragg’s equation (nλ ) 2d sin θ) and the n ) 1 peak was calculated on the basis of the observed n ) 2, 3, and so on. The organic fractions were measured by using a thermal gravimetric analyzer (TGA, Perkin-Elmer Pyris 1) at a heating rate of 10 °C/min up to 900 °C in air. The interfacial tension was measured by the Wilhelmy method on a Kruss-K10 digital tensiometer equipped with a spherical ring. Microstructure of the organoclay was characterized by a Zeiss 902A transmission electron microscope (TEM) operated at 120 kV. Heat analysis was carried out by a Seiko SII model SSC/5200 differential scanning calorimeter (DSC) from Seiko Instruments and Electronics Ltd. The size of the samples was approximately 3-8 mg
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on sealed aluminum pans. The analyses were performed at a cooling rate of 10 °C/min in 30 mL/min flowing nitrogen atmosphere. The enthalpy of crystallization (∆Hc) was determined by integration of the peak area under the linear baseline, and the peak value of the thermogram was reported as the crystallization point (Tc). The average particle diameters were estimated by use of a 90 Plus particle sizer (Brookhaven Instrument Corp.) equipped with a 15 mW solid-state laser (675 nm).
Results and Discussion Synthesis and Properties of Amphiphilic Copolymers. The maleated polystyrene-poly(ethylene/butylene)-polystyrene copolymer (SEBS-g-MA) is commercially available and ready for amine grafting. The comblike and amine pendant copolymers were prepared from the amidation of poly(oxyalkylene) diamines onto the copolymer. The preparation involves the maleic anhydride ringopening reaction and adapting different POA amines at a 120 °C reaction temperature. The synthesis and the amine chemical structures are described in Scheme 1. Two types of POA diamines, POE and POP diamines including POP2000, POP400, and POP230 of 2000, 400, and 230 Mw averaged molecular weight, are used for the grafting reaction. The resultant copolymers consist of a hydrophobic triblock backbone and multiple amine pendants. On the basis of the maleic anhydride content (∼2 wt %) and the provided molecular weight, the prepared copolymers were calculated to have an average of nine pendant amine functionalities on each polymer backbone of approximately 8000 Mw for PS endblocks and 29 000 Mw for a poly(ethylene/butadiene) midblock.28,29 The grafted POA amines could be POE2000 diamine, which is hydrophilic and water-soluble because of its backbone consisting of 2000 g/mol poly(ethylene glycol).30 By comparison, the low molecular weight POP230 and POP400 are water-soluble diamines, while POP2000 is water-insoluble due to its characteristic poly(propylene glycol) backbone at 2000 g/mol Mw. However, owing to the presence of diamine functionalities, POP2000 becomes water-soluble when treating with hydrochloric acid to convert the amine functional groups (-NH2) to ammonium salts (-NH3+Cl-). Hence, the hydrophilic/hydrophobic property is a relative term, depending on their chemical components, molecular weight, and pH environment. The grafting of these amines on the SEBS polymer afforded a copolymer with constituents of various hydrophilic POA amine pendants and carboxylic acids in the structure. When various POP and POE amines are grafted on the SEBS, the synthesized copolymers are amphiphilic in nature and capable of behaving as polymeric surfactants. Emulsified Behavior of Amphiphilic Copolymers in Toluene/Water without Clay. Due to the presence of highly hydrophobic backbone in the structure, the synthesized copolymers are toluene-soluble. The presence of a significant weight fraction of the pendant groups, for example, ∼29 wt % for 2000 g/mol POE2000, may contribute to the hydrophilicity and hence the copolymer’s amphiphilicity. It was found that both copolymers are capable of emulsifying the toluene/water mixture. With equal toluene and water volume and a copolymer concentration in toluene ranging from 0.1 to 20 wt %, the emulsion volumes were measured (Figure 1). Both copolymers exhibit some differences in emulsifying ability. (28) Ohlsson, B.; Hassander, H.; Tornell, B. Polymer 1998, 39, 67056714. (29) Gonzalez-Montiel, A.; Keskkula, H.; Paul, D. R. Polymer 1995, 36, 4587-4603. (30) Harris, J. M., Ed. Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications; Plenum: New York, 1992.
Figure 1. Emulsion properties of synthesized copolymers.
For example, 12 wt % SEBS-POP2000 in toluene can emulsify an equal portion of water completely, which corresponds to an emulsion volume fraction of 1.0. At the lower concentration of 6 wt %, the same copolymer had an emulsion volume fraction of 0.9 while leaving 0.1 fraction of toluene in the upper layer. When increasing the concentration up to 16 wt %, the emulsion portion dropped to 0.75 with the remaining 0.25 fraction as nonemulsified water in the bottom layer. In contrast, the analogous SEBS-POE2000 emulsified equal amounts of toluene and water at a lower concentration of 6 wt %. Below and above this optimal concentration, the emulsion volume deviated from 1.0 with toluene or water separated from the emulsion phase. As shown in Figure 1, the curves represent the copolymer’s amphiphilicity or their relative efficacy for emulsification. At the optimal concentration, the toluene/water emulsion is at 50/50 volume ratio. Below this equal point, the emulsion compositions are presumably an oil-in-water type of emulsion rather than a waterin-oil type because of the observed water- or toluene-rich emulsion compositions as in the continuous phases. By taking advantage of these emulsifying characteristics, the copolymers can be designed to interact with the waterswollen Na+-MMT clay for ionic exchanging. With the ability of self-emulsification, the copolymers are compatible with the clay/water system and will be able to ionically exchange or intercalate.
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Table 1. Na+-MMT Intercalation with Copolymers hybrid I intercalation agenta (wt %) SEBS-POE2000 (6) SEBS-POE2000 (12) SEBS-POE2000 (20) SEBS-POP2000 (6) SEBS-POP2000 (12) SEBS-POP2000 (20) SEBS-POP400 (5) SEBS-POP230 (5)
d
spacingb
TGAc
hybrid II
(Å)
(w/w)
d spacingb (Å)
TGAc (w/w)
50 24, 18 30 50 51 52 21, 17 13
93/7 91/9 88/12 90/10 92/8 90/10 83/17 90/10
17 18 17 22, 15 24, 15 20 17 13
94/6 89/11 88/12 73/27 75/25 70/30 40/60 47/53
a Intercalation agents in toluene (6, 12, or 20 wt %) and then added water to prepare emulsion for intercalation. b X-ray d spacing of copolymer/MMT hybrids (I, without toluene extraction; II, with toluene extraction). c Organic/inorganic weight ratio calculated from TGA (up to 900 °C in air).
Figure 3. TEM images for (a) SEBS-POP2000/MMT (XRD ) 52 Å) and (b) SEBS-POE2000/MMT (XRD ) 50 Å).
Figure 2. X-ray diffraction patterns of the copolymerintercalated MMTs: (a) SEBS-POP2000/MMT, (b) SEBSPOP2000/MMT (toluene-extracted), (c) SEBS-POE2000/MMT, and (d) SEBS-POE2000/MMT (toluene-extracted).
Intercalation of Emulsified Copolymers with Na+MMT. Previous results reveal that the POP2000 and POE2000 intercalation with Na+-MMT results in a d spacing increase from 12 Å to 58 Å and to 19 Å, respectively.26 The rationale for the difference in interlayer expansions is attributed to the hydrophobic POP backbone self-aggregating into a separated phase, which stretches out the basal spacing and the hydrophilic POE backbone binding flatly through chelating with metal ions on silicate surface. In the case of POP and POE amine-grafted polypropylene as the comb-shaped intercalants,31 the intercalation can only expand the interlayer space up to 19 Å d spacing because the incorporation into the silicate confinement only occurred for the pendant polyether side chains rather than the polypropylene backbone. The predominant portion of the copolymer hydrophobic backbone still remained outside of the silicate galleries. To enhance the backbone incorporation, the copolymer must (31) Lin, J. J.; Hsu, Y. C.; Chou, C. C. Langmuir 2003, 19, 51845187.
have a high emulsion ability to form fine particles and multiple ammonium ion exchanging sites. The SEBSbackboned copolymers prepared in this work are toluenesoluble and structured with an average of nine pendant amines. While the pristine SEBS-g-MA failed to intercalate with Na+-MMT, the presence of multiple ammonium salts (-NH3+Cl-) as the grafting pendants enables the copolymers to form a fine emulsion and increases their compatibility with silicate clay. Furthermore, the aminegrafted SEBS-POA copolymers are equipped with amine functionalities, which are required for the ionic exchanging with sodium ion in the silicates. The length of POA pendants may also affect the intercalating results. With a short chain of POA such as POP230 and POP400, the prepared copolymers are highly soluble in toluene and failed to emulsify water sufficiently to enter Na+-MMT. The resultant silicates had low d spacings of 17 and 13 Å, respectively (Table 1). On the other hand, upon mixing 6, 12, and 20 wt % SEBS-POE2000 or SEBSPOP2000 (2000 g/mol segmental weight for each POE or POP pendant) in toluene/water, the resultant emulsion could undergo the intercalation effectively. The XRD analyses (Figure 2) indicated a high d spacing of 5052 Å for both copolymers. The XRD results were confirmed by TEM analyses, as shown in Figure 3. The high d spacing was caused by the SEBS backbone participation since the POE2000 starting amine can only achieve a low 19 Å spacing. Although both copolymers had achieved a high gallery expansion, the SEBS-POP2000 appears to be more effective than that of SEBS-POE2000 by comparing the intercalations at different concentrations (Table 1).
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Figure 4. Conceptual illustration of the copolymer micelles (o/w and w/o) and the copolymer/MMT hybrids (SEBS- and POAembedded modes).
The effectiveness of forming a spatially expanded hybrid is explained by the copolymers’ emulsion ability. From the emulsion volume studies (Figure 1), it has been shown that these SEBS-derived copolymers exhibit excellent emulsion ability in the toluene/water system. The fine emulsion could be measured by a particle size analyzer. An average 60-70 nm particle size was obtained when the copolymers were dissolved in hot toluene. The fine size, similar to the clay primary particles, became much larger (370 or 790 nm) at ambient temperature. This result can be correlated to the fact that the intercalation required a high temperature such as 80 °C. It appears that the multiple pendant quaternary ammonium salts in an oilin-water emulsion form intercalated into the silicate galleries and brought along the SEBS backbones, resulting in a 50 Å d spacing. Another indirect verification of hydrophobic SEBS embedding is the shrinkage of the d spacing from 50 to 17 Å by a simple toluene extraction. The thermal gravimetric analyses (Table 1) indicated that either the entire copolymer or just the hydrophobic SEBS could be extracted out of the silicate spacing. A significant decrease of the organic fraction from 90/10 to only 70/30 accompanies the decrease in the basal spacing from 50 to 20 Å for the POP copolymer. However, the POE-derived copolymer hybrids shrunk the silicate interlayer after a toluene extraction, but the same TGA result remained. It is assumed that the hydrophilic POE segments still associated with the silicate platelet surface through the intensive POE/ionic silicate association, while the hydrophobic SEBS surrounded the silicate stack. The conceptual diagram for forming the o/w or w/o copolymer particles and their intercalation into SEBS-embedded or POA-embedded MMT are illustrated in Figure 4. The two silicate hybrids represent uniquely different intercalated intermediates with SEBS (50 Å) or POA (18 Å) embedding. The illustration is to explain the possible fine emulsion structures and the d spacing shrinkage of the intercalated hybrid. DSC Evidence of Two Different Modes of POE Pendant Intercalation. The DSC studies demonstrated different crystallization patterns for the POE2000 segments when the portion being grafted was in copolymers or embedded in clay. As shown in Figure 5, the POE2000 segment crystallized at 0 °C with 98 J/g enthalpy is different from the POE2000 diamine at 10 °C with 105 J/g enthalpy change. The lower temperature for the crystallization was caused by the chemical bonding connection between the POE segments and the SEBS backbone and
Figure 5. DSC analyses on crystallization behaviors of POE segments: (a) POE2000 starting amine, (b) SEBS-POE2000 copolymer, (c) SEBS-POE2000/MMT (50 Å), and (d) SEBSPOE2000/MMT (17 Å) hybrids.
Figure 6. Relative effectiveness of lowering the toluene/water interfacial tensions by copolymers (0, 4) and their MMT hybrids (9, 2).
consequently a decrease in crystallization rate. The same trend is observed for the copolymer-intercalated silicates with low d spacing (17 Å) at -14 °C. The restriction of the POE portions by the silicate platelet can be 2-fold, by ionic charge interaction and by the space restriction between two neighboring platelets. This phenomenon of hindering crystallization was reported previously in the case of poly(ethylene glycol) embedded in MMT.32 However, for the presumably SEBS-embedded hybrid (d spacing 50 Å), the restriction is less serious due to the wide spacing of the silicate interlayer. A temperature of 7 °C for crystallization was observed, significantly higher than -14 °C for the hybrid (17 Å). From the DSC peaks, the full width at halfmaximum (fwhm) was usually measured as an indication of crystal phase-forming behavior.33 The full width, in degrees Celsius, at half the maximum intensity (fwhm) of the DSC cooling curve was calculated. The hybrid (50 Å) with a narrow fwhm (10 °C) represents a fast rate of POE crystallization, similar to the pure POE2000 (8 °C). Relatively, a wide fwhm (15 °C) for the silicate (32) Strawhecker, K. E.; Manias, E. Chem. Mater. 2003, 15, 844849. (33) Strawhecker, K. E.; Manias, E. Chem. Mater. 2000, 12, 29432949.
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hybrid of 17 Å d spacing has a “solidlike”34 POE phase in comparison with the “liquidlike” phase of POE in the spacious gallery. The result suggests two different crystallization modes that correspond to the hybrids of 17 and 50 Å d spacing. Interfacial Property and Stable Dispersion Size of the Copolymer/MMT Hybrids. The surfactant property is explained by the noncovalent bonding forces in the copolymers. The POA backbones, amine, and quaternary ion charges are capable of associating with water molecules through hydrogen and ionic bonding, while the aromatic ring structures in the SEBS backbone attract themselves or aromatic solvents by π-π attraction. The two kinds of different characters bring up the miscibility between the originally immiscible systems, such as toluene and water. In measuring their interfacial tension, low tensions such as 5.0 and 2.8 mN/m were observed for SEBS-POP2000 and SEBS-POE2000, respectively. In the intercalated MMT hybrids (ca. 20 Å d spacing), the presence of ionic silicate component brings up a different amphiphilic balance. The structure consists of SEBS backbones bound with MMT through multiple quaternary charges. With a combined structure of the organic copolymers and the silicates, the hybrids form rigid and stable micelles. Their critical aggregation concentrations (CAC) at the toluene/water interface can be determined by measuring the trend change of the interfacial tension against the hybrid concentration in toluene. In Figure 6, two hybrids exhibited a critical aggregation at a lower concentration compared to their corresponding copolymers. The ability to decrease the interfacial tension, as low as 2-4 mN/m at the concentration of 0.01 wt %, has been achieved. This copolymer/ layered silicate hybrid is considered a rigid “micelle” consisting of the hydrophilic silicate core and the hydrophobic SEBS corona. Both hybrids demonstrate a fine dispersion in toluene with a particle size of 470 nm for SEBS-POP2000/MMT and 360 nm for SEBS-POE2000/ (34) Maitra, P.; Ding, J.; Huang, H.; Wunder, S. L. Langmuir 2003, 19, 8994-9004.
Chang et al. Table 2. Laser Particle Sizes of Copolymers and Their MMT Hybrids in Toluene at Different Temperatures avg. diameter (nm) copolymers and MMT hybrids
25 °C
60 °C
370 790 360 470
71 63 270 360
SEBS-POE2000 SEBS-POP2000 SEBS-POE2000/MMTa SEBS-POP2000/MMTa
a Intercalated MMTs: SEBS-POP2000/MMT (d spacing 20 Å) and SEBS-POE2000/MMT (17 Å).
MMT (Table 2). The particle sizes are relatively stable in the temperature range of 25-60 °C. In contrast, the original SEBS-POA copolymers are temperature-dependent for their emulsion particle sizes. Upon heating, the particle size became much smaller, possibly from 790 to 63 nm in average diameter. Conclusions The comblike copolymers, synthesized from the grafting reaction of maleated SEBS copolymer with amines, can serve as excellent intercalating agents for Na+-MMT. These synthesized copolymers consist of multiple amine pendants and a hydrophobic SEBS backbone, which enables it to emulsify the water/toluene mixture and ionically exchange with sodium ions in the silicate gallery. As a result, the natural clay is polymer-modified with a wide d spacing of ∼50 Å. When dispersed in a toluene suspension and vigorously extracted, the silicate layered configuration changed from 50 to 20 Å d spacing, indicating the embedding and extrusion of the SEBS portion in the hybrid structures. The result implies that the emulsion intercalation is a viable process for tailoring various hydrophobic organic/silicate hybrids. Acknowledgment. We acknowledge the financial support from National Science Council (NSC) of Taiwan. LA050948C
