Synthesis and Characterization of Polyoxyethylene Sulfate

Langmuir , 2003, 19 (14), pp 5570–5574 .... The elementary analysis shows that the average number of oxyethylene units (−CH2CH2O−) per molecule ...
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Langmuir 2003, 19, 5570-5574

Synthesis and Characterization of Polyoxyethylene Sulfate Intercalated Mg-Al-Nitrate Layered Double Hydroxide Qin Z. Yang, De J. Sun, Chun G. Zhang,* Xiao J. Wang, and Wei A. Zhao Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, 250100, People’s Republic of China Received March 27, 2003. In Final Form: May 21, 2003 The intercalation reaction of polyoxyethylene sulfate (PEGS) with Mg-Al-nitrate [Mg-Al-NO3] layered double hydroxide (LDH) was investigated by using X-ray diffraction, infrared spectroscopy, and transmission electron microscopy (TEM). X-ray diffractograms showed that the basal spacing of the [Mg-Al-NO3] LDH with PEGS as the intergallery anion expanded from 0.89 to 2.37 nm to form [Mg-Al-(PEGS)], but no expansion of the basal spacing was observed when poly(ethylene glycol) (PEG) reacted with [Mg-AlNO3]. The characteristic absorption bands of the organic anion in the infrared spectra were obtained after prolonged treatment with deionized water, demonstrating the stability of the intercalation compounds. Furthermore, TEM analysis revealed that [Mg-Al-NO3], which consisted of hexagonal particles approximately 100-120 nm in length, changed into monodisperse rigid spheres approximately 200 nm in diameter after the intercalation of PEGS.

Introduction Layered double hydroxides (LDHs), or the so-called hydrotalcite-like compounds, are important clay materials owing to their intercalation ability with anionic species and other physicochemical properties for applications as anion adsorbents, medicine stabilizers, ion exchangers, ionic conductors, catalysts, catalyst supports, and DNA reservoirs.1-6 LDHs can be represented by the general formula [MII1-xMIIIx(OH)2]x+ Xm-x/m‚nH2O, abbreviated as [MII-MIII-X]3, where MII is a divalent metal ion such as Mg2+, Ni2+, Cu2+, or Zn2+, MIII is a trivalent metal ion such as Al3+, Cr3+, Fe3+, or Ga3+, and Xm- is an anion charged m such as CO32-, Cl-, or NO3-. The net positive charge, due to substitution of trivalent by divalent metal ions, is compensated by an equal negative charge of the interlayer solvated anions. When LDHs are dispersed in aqueous solution, the dispersed particles have a positive zeta potential for the forming of a screening double layer due to the diffusion of anions. Compared to divalent anions such as CO32- and SO42-, monovalent anions such as NO3and Cl- are more easily replaced by almost any desired anions, organic or inorganic, by utilizing the ion-exchange method.7-10 For [Mg-Al-NO3], the NO3- has strong exchange capacity and the basal spacing could be adjusted easily by changing the ratio of Mg/Al, so in many studies, [Mg-Al-NO3] has been selected to be a precursor of polymer or biomolecule/LDH nanocomposites,4,11-12 and * To whom correspondence should be addressed. E-mail: [email protected]. (1) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2001, 105, 1743. (2) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (3) Legrouri, A.; Badreddine, M.; Barroug, A.; De Roy, A.; Besse, J. P. J. Mater. Sci. Lett. 1999, 18, 1077. (4) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (5) Lopez-Salinas, E.; Garcia-Sanchez, M.; Montoya, J. A.; Acosta, D. R.; Abasolo, J. A.; Schifter, I. Langmuir 1997, 13, 4748. (6) Zhao, H. T.; Vance, G. F. J. Chem. Soc., Dalton Trans. 1997, 1961. (7) Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5210. (8) Ookubo, A.; Ooi, K.; Hayashi, H. Langmuir 1993, 9, 1418. (9) Miyata, S. Clays Clay Miner. 1983, 31, 305. (10) Chibwe, K.; Jones, W. J. Chem. Soc., Chem. Commun. 1989, 926.

we also reported a simplified method to synthesize [MgAl-NO3].13 Because LDHs have a good anion-exchange property, many reports about the intercalation of organic anions such as anionic surfactants,14-17 large polyoxometalates,18 and various organic acids19-21 have been published. Recently, the research of LDH/polymer nanocomposites has attracted much attention due to their superior properties compared to the pure polymers.12,22-28 Three major methods were used to prepare the LDH/polymer nanocomposites: in situ polymerization, direct intercalation, and restacking.12 However, relatively few publications have reported the intercalation of polymers into LDHs directly by the ion-exchange method because LDH layers cannot be exfoliated for their high charge density and polymer molecules are too large to intercalate into (11) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J. J. Mater. Chem. 2001, 1671. (12) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (13) Yang, Q. Z.; Zhang, C. G.; Sun, D. J.; Guo, P. Z.; Zhang, J. Acta Chim. Sin. 2002, 9, 1712. (14) Kopka, H.; Beneke, K.; Lagaly, G. J. Colloid Interface Sci. 1988, 123, 427. (15) Crepaldi, E. L.; Pavan, P. C.; Valim, J. B. J. Mater. Chem. 2000, 1337. (16) You, Y. W.; Zhao, H. T.; Vance, G. F. J. Mater. Chem. 2002, 907. (17) You, Y. W.; Zhao, H. T.; Vance, G. F. Colloids Surf., A 2002, 205, 161. (18) Hu, C. W.; Liu, Y. Y.; Wang, Z. P.; Zhang, J. Y.; Wang, E. B. Sci. China, Ser. B 1995, 25, 916. (19) Kooli, F.; Chisem, I. C.; Vucelic, M.; Jones, W. Chem. Mater. 1996, 8, 1969. (20) Prevot, V.; Forano, C.; Besse, J. P. Appl. Clay Sci. 2001, 18, 3. (21) Millange, F.; Walton, R. I.; Lei, L. X.; Ohare, D. Chem. Mater. 2000, 12, 1990. (22) Wilson, O. C.; Olorunyolemi, T.; Jaworski, A.; Borum, L.; Young, D.; Siriwat, A.; Dickens, E.; Oriakhi, C.; Lerner, M. Appl. Clay Sci. 1999, 15, 265. (23) Whilton, N. T.; Vickers, P. J.; Mann, S. J. Mater. Chem. 1997, 1623. (24) Moujahid, E. M.; Besse, J. P.; Leroux, F. J. Mater. Chem. 2003, 258. (25) Rey, S.; Merida, R. J.; Han, K. S.; Guerlou, D. L.; Delmas, C.; Duguet, E. Polym. Int. 1999, 48, 277. (26) Moujahid, E. M.; Besse, J. P.; Leroux, F. J. Chem. Mater. 2002, 3324. (27) Hsueh, H. B.; Chen, C. Y. Polymer 2003, 44, 1151. (28) Bubniak, G. A.; Schreiner, W. H.; Mattoso, N.; Wypych, F. Langmuir 2002, 18, 5967.

10.1021/la034526j CCC: $25.00 © 2003 American Chemical Society Published on Web 06/07/2003

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the LDH gallery.29 Though there are some literature reports on the intercalation of poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) into LDHs with the help of anion surfactants,12,28 there is no report on the direct intercalation of PEG with the help of molecule modification. In this study, we prepared PEG derivatives, that is, PEGS, by a sulfating process. A PEGS/LDH nanocomposite was successfully synthesized by intercalation of PEGS into [Mg-Al-NO3] which was prepared by a nonsteady coprecipitation method. Experimental Section Materials. All the chemicals used in this work were of analytical grade and used without any further purification. Poly(ethylene glycol) (PEG 400) (M ) 370-460 g/mol) (HO(OCH2CH2)10OH) was purchased from Shanghai Chemical Reagent Co. The synthesis of PEGS (NaSO3(OCH2CH2)10OSO3Na) involves dissolving PEG 400 in chloroform at 10 °C, sulfating with 25% excess chlorosulfonic acid, and neutralizing with 18 N sodium hydroxide. The inorganic salt is separated, and the solvent is removed.30,31 The elementary analysis shows that the average number of oxyethylene units (-CH2CH2O-) per molecule is 10. Method. The synthesis of [Mg-Al-NO3] was described in an earlier paper.13 For instance, the synthesis of a [Mg-Al-NO3] with Mg/Al ) 2.0 was obtained as follows: Under a N2 atmosphere, a mixed solution containing 30.77 g (0.12 mol) of Mg(NO3)2‚6H2O, 22.51 g (0.06 mol) of Al (NO3)2‚9H2O, and 8.5 g (0.1 mol) of NaNO3 in 560 cm3 of water was prepared. Then diluted ammonia water (6 wt %) was added to the solution at a speed of 25 mL/min till the final pH value was 9.5. The precipitate was aged for 1.5 h in the mother solution at room temperature and then washed with deionized water. After that, the filter cake was peptized at a constant temperature of 80 °C, forming the positive sol, and dried at 65 °C to get [Mg-Al-NO3]. Polymer Intercalation. Dense suspensions of LDH (the molar ratio of Mg/Al ) 2, 3, 4) were diluted to 0.5 wt % solid content. To 0.02 mol/L of PEGS or PEG aqueous solutions (100 mL) was added 100 mL of LDH suspension. The pH values of the mixtures were adjusted to 9.0 with sodium hydroxide. Suspensions were aged at 65 °C for 4 days. The resulting precipitate was centrifuged, washed twice with methyl alcohol, and separated by centrifugation. The resulting white solid was thoroughly washed with deionized water, centrifuged, and dried in a vacuum at 60 °C. Characterization Techniques. The X-ray diffraction (XRD) instrument used was a D/max-γ B diffractometer with Cu KR radiation. Data were collected over the 2θ range from 2 to 70° in increments of 0.02°/s at room temperature. Differential thermal/thermogravimetric (DTA/TG) analysis was performed under a N2 atmosphere using a Schimadzu TGA-40 thermogravimetric analyzer. Curves were recorded at a rate of 10 °C/ min to a temperature of 1000 °C. Fourier transform infrared (FTIR) analysis was recorded on a Nicolet 50X spectrophotometer, in the range of 400-4000 cm-1. Transmission electron microscopy (TEM) analysis was performed using a JEM-100CX II electron microscope. TEM samples were prepared by dipping amorphous carbon coated copper TEM grids into dilute aqueous suspensions of the sample powder, which was washed only by deionized water. The electrophoretic mobility (µ) of samples was measured using a DXD-I microelectrophoresis instrument with a flow-through sample cell. Samples were prepared by dispersing 0.05 vol % of each sample powder. The chemical composition of LDH was determined by chemical analysis.

Results and Discussion Crystal Structure of the Polymer-LDH Nanocomposite. The X-ray diffraction patterns for the pristine LDH and its polymer/LDH nanocomposite are shown in (29) Hibino, T.; Jones, W. J. Mater. Chem. 2001, 1321. (30) Weil, J. K.; Bistline, R. G., Jr.; Stirton, A. J. J. Phys. Chem. 1958, 62, 1083. (31) Li, X. G.; Zhao, G. X. China Surfactants Deterg. Cosmet. 1989, 5, 6.

Figure 1. XRD patterns for LDH and LDH nanocomposites (Mg/Al ) 2): (a) [Mg-Al-NO3]; (b) [Mg-Al-(PEGS)]; (c) [MgAl-NO3]/PEG (15 days).

Figure 1. It is seen that the basal spacing (d003) of the pristine [Mg-Al-NO3] is 0.89 nm. The small impurity diffraction peak at 2θ ) 21° (the d value is about 0.42 nm) arises from a small quantity of Al(OH)3 which is produced in the preparation process; this phenomenon was explained by Han in an early publication32 (Figure 1a). The basal spacing of [Mg-Al-NO3]/PEG aged for 15 days is 0.82 nm (Figure 1c), which is quite close to that of the pristine [Mg-Al-NO3]. It shows that no intercalation of [Mg-Al-NO3] by PEG has occurred. However, the XRD pattern in Figure 1b shows that the basal spacing of [MgAl-(PEGS)] is 2.37 nm, indicating that the basal spacing of LDH has been expanded because of the intercalation of PEGS into the LDH galleries. The lamellar structure of the material is preserved upon intercalation. The length of a PEGS molecule is calculated as follows:

L ) 10 × 3 × 0.127 + 0.28 + 0.28 ) 4.37 nm In this equation, 10 is the number of oxyethylene units (-CH2CH2O-). One oxyethylene unit corresponds in length to about three methylene groups, and one methylene group is about 0.127 nm in length;14 the ion diameter of -OSO3- is about 0.28 nm. Since the brucite-like LDH sheet is 0.48 nm, the space occupied by PEGS would be approximately 1.89 nm, so the orientations of PEGS in the gallery of LDH may be monolayer or bilayer. In the monolayer structure, the PEGS lie inclined with the two -OSO3- groups linking with the adjacent layer of LDH. However, two -OSO3- groups of PEGS will link with the same layer to form a bilayer structure. A trimolecular layer is impossible because the PEGS molecule has two anion groups. The possible arrangements of the PEGS are shown in Figure 2. For the LDH of Mg/Al ) 3 or 4, the d value is 0.78 or 0.79 nm, respectively. And the electrophoretic mobilities of particles are 2.93 × 10-8 and 3.05 × 10-8 m2‚V-1‚s-1, respectively. In our experiment, the basal spacings of [Mg-Al-NO3](Mg/Al ) 3 or 4)/PEG and PEGS aged for 4 days are all 0.8 nm, which is quite close to that of the pristine [Mg-Al-NO3]; this means that PEG and PEGS cannot intercalate into the interlayers of the [Mg-Al-NO3], either Mg/Al ) 3 or 4, which shows the charge of LDH has an important effect on the intercalation of guest anions into layered materials. This is consistent with the results of previous studies.11,12 TEM Observations. Transmission electron micrographs for sample [Mg-Al-NO3] and polymer/LDH are (32) Han, S. H.; Zhang, C. G.; Hou, W. G.; Sun, D. J.; Wang, G. T. Chem. J. Chin. Univ. 1996, 11, 1785.

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Figure 2. Schematic illustration of anion structure intercalated in the LDH layer: (a) [Mg-Al-NO3]; (b) [Mg-Al-(PEGS)] (monolayer); (c) [Mg-Al-(PEGS)] (bilayer).

Figure 3. TEM photographs for LDH samples (Mg/Al ) 2): (a) [Mg-Al-NO3]; (b) PEG/LDH (4 days); (c) PEGS/LDH (1 h); (d) PEGS/LDH (3 days); (e) [Mg-Al-(PEGS)] (4 days).

shown in Figure 3. [Mg-Al-NO3] consisted of hexagonal particles with a length of about 100-120 nm. Compared to [Mg-Al-NO3], the samples of LDH/PEG and LDH/ PEGS both show some interesting morphological features. Spherical particles can be observed in the suspension when PEG was mixed with [Mg-Al-NO3] for 4 days (Figure 3b). It is indicated that PEG can be absorbed on the surface of [Mg-Al-NO3] though it cannot be intercalated into the gallery of [Mg-Al-NO3]. After the modification of PEG, the particles of [Mg-Al-NO3] turn into spheres due to the effect of solid-liquid interface energy, as the

formation of a water droplet on the surface of a solid. This phenomenon is also observed when PEGS reacts with LDH for less than 3 days. The edges of hexagonal LDH particles become indistinct when the aging time is just 1 h (Figure 3c). When the mixture of PEGS/LDH is aged for 3 days, the TEM image shows that regular spherical particles have been formed (Figure 3d). It also can be seen from Figure 3c,d that the particles have different sizes, which means the particles are polydisperse. When the mixture of PEGS/LDH is aged for 4 days, however, regular spherical particles of the same size, about 200 nm, are

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Figure 4. Schematic illustration of the intercalation process of polymer and LDH.

Figure 6. DTA/TG curve of LDH samples: (a) [Mg-Al-NO3]; (b) [Mg-Al-(PEGS)]. Figure 5. Electrophoretic mobility (µ) vs reaction time data for a [Mg-Al-(PEGS)] sample (Mg/Al ) 2).

observed, which indicates the particles are monodisperse. The different processes of adsorption and intercalation of polymer to LDH can lead to the polydispersity and monodispersity of the LDH particles, respectively, as illustrated by Figure 4. When PEG is added into the [MgAl-NO3] suspension, the polymer is absorbed on the surface of the LDH and the particles are modified into spheres, which leads to a polydispersity of LDH particles. The same results are observed when PEGS is absorbed on the surface of the [Mg-Al-NO3] while the aging time is less than 3 days. When the mixture of PEGS and [MgAl-NO3] is aged for 4 days, however, PEGS molecules can enter into the gallery of the [Mg-Al-NO3] layers to get the intercalated compound with almost the same size, which means a monodispersity of LDH particles. It is possible that a small quantity of PEG or PEGS also intercalated into the gallery of LDH at the beginning. The basal spacing of LDH, however, did not expand because there were still enough NO3- ions in the gallery and PEG or PEGS might lie parallel to the LDH surface as a monolayer. When the ion-exchange process was long enough, such as for 4 days, the NO3- ions were exchanged by PEGS totally, so the basal spacing of LDH expanded remarkably. Result of Electrophoretic Mobility. The electrophoretic mobility (µ) versus reaction time data for a [MgAl-(PEGS)] sample is displayed in Figure 5. The electrophoretic mobility of [Mg-Al-NO3] particles is +2.80 × 10-8 m2‚V-1‚s-1. When PEG was added into LDH and aged for 4 days, the electrophoretic mobility had a little change because of the change of LDH bulk after adsorption and the interaction between PEG and anion. The positive character of the products, however, remained the same. With the aging time increasing, the electrophoretic

mobility curve of PEGS/LDH showed a different behavior. The µ value decreased sharply within 6 h, followed by a slight decrease with the aging time, which produced a plateau. Then there was also a sharp decay of the µ value from 1 to 2 days. Henceforth, the µ value became nearly constant at a value slightly higher than the minimum which was observed at 2 days (Figure 5). There were some reports about the intercalation of LDH layers by polymer to get the polymer/LDH nanocomposite. Because of the positive zeta potential of LDH, the polymers which were selected to intercalate into the LDH layer were all anionic polymers. However, the polymer/LDH nanocomposite exhibited a negative µ value because the polymer adsorption occurred on the outer surface of nanocomposite particles. That is, anion functional groups from the adsorbed outer polymer layer dominated the electrokinetic behavior of the polymer/LDH nanocomposite.22 In the conditions of this experiment, the µ value of [Mg-Al(PEGS)] was slowly decreased compared to that of the pristine LDH and still was positive. In Figure 5, the electrophoretic mobility of [Mg-Al-(PEGS)] showed a sharp decay within 2 days, Henceforth, the µ value became nearly constant. However, the result of XRD showed that the basal spacing of PEGS/LDH had no expansion when PEGS reacted with LDH for 3 days. When PEGS aqueous solutions were added to the LDH suspension at the beginning, the polymer was adsorbed on the outer surface of LDH layers, which compressed the screening double layer, and made the µ value decrease clearly. After the mixture was aged for 2 days, the polymer PEGS adsorbed on the outer surface of LDH layers rearranged and entered into the intergallery of LDH layers. The spherical, rigid, and monodisperse particles were obtained finally. DTA/TG. DTA/TG profiles for samples [Mg-Al-NO3] and [Mg-Al-(PEGS)] are displayed in Figure 6. In the DTA curve of [Mg-Al-NO3] (Figure 6a), there are two endothermic peaks around 130 and 425 °C, with two corresponding steps of weight loss on the TG diagram.

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Figure 7. IR spectra of [Mg-Al-NO3] (a), PEG/LDH (b), pure PEGS (c), and [Mg-Al-(PEGS)] (d) compounds (Mg/Al ) 2).

The first step, around 130 °C, with a weight loss of 12.9%, is due to the loss of physically adsorbed and interlayer water. The second step, around 425 °C, with a weight loss of 32.1%, can be ascribed to the dehydroxylation of the LDH layers and the elimination of NO3-. The total weight loss for this region (70-800 °C) was 49.0%. In the DTA curve of [Mg-Al-(PEGS)] (Figure 6b), there are three endothermic peaks around 60, 370, and 620 °C, with three corresponding steps of weight loss on the TG diagram. The first step, around 60 °C, with a weight loss of 12.3%, corresponded to loss of adsorbed and interlayer water. The second and third steps, around 370 and 620 °C, with sum weight 48.7%, are due to the dehydroxylation of the LDH sheets and polymer degradation. The total weight loss for this sample (70-800 °C) was 62.5%. This was 13.5% higher than the weight loss for the [Mg-Al-NO3]; it was consistent with the results of elemental analysis. The differences between [Mg-Al-NO3] and [Mg-Al(PEGS)] nanocomposites in decomposition features are due to the polymer intercalated in the gallery of LDH layers. IR Spectroscopy. Figure 7a shows the FTIR spectrum of [Mg-Al-NO3]. A broad absorption band at around 3500 cm-1 was attributed to OH stretching due to the presence of hydroxyl groups of LDH. A strong absorption band at 1385 cm-1 was due to the presence of nitrates. The polymer PEG adsorption occurring on the outer surface of nanocomposite particles is evidenced by IR spectroscopy (Figure 7b). The presence of methyl groups of CH stretching vibrations can be implied by the observation of broadened bands at 2800-3000 cm-1. Another band at around 1110 cm-1 was assigned as due to C-O stretching vibrations. However, there is a strong absorption band around 13651385 cm-1 for PEG/LDH, which indicated that PEG has not been exchanged with the nitrates of LDH and cannot be intercalated into the gallery of LDH. The particles of LDH turned into spheres only due to the adsorption of PEG molecules, which was consistent with the result of XRD. The significant difference for FTIR spectra in Figure 7d is the disappearance of the absorption band around 1385 cm-1 for [Mg-Al-(PEGS)]. The presence of polymer in the intercalation compound is evidenced by the char-

acteristic C-H stretching bands of methylene groups over 2800-2900 cm-1. The presence of C-O stretching vibrations can be implied by the observation of bands at 1110 cm-1. The OSO3- asymmetric stretches are at 1247 cm-1. It is indicated that PEGS molecules have been intercalated into the gallery of LDH and substituted the nitrates in the gallery of LDH, which was consistent with the result of XRD. The characteristic absorption bands of the organic anion in the infrared spectra were still clear after prolonged treatment with deionized water, demonstrating the stability of the intercalation compounds. Conclusion PEG can easily intercalate into montmorillonite because -CH2CH2O- groups in the PEG have a strong interaction with the cation in the gallery of montmorillonite. However, there is no cation in the gallery of LDH, and PEG cannot easily intercalate into the galleries of LDH, though there are some -OCH2CH2- groups in the structure of the PEG molecule. For PEGS, it can be intercalated into LDH as a guest anion by the reaction of the polar group -OSO3and the surface of LDH, which is shown by the IR results. The basal spacing expands from 0.89 to 2.37 nm due to the intercalation. The orientations of PEGS in the gallery of LDH may be monolayer or bilayer. The FTIR results indicate that polymer can be stably intercalated into the gallery between the LDH layers. The TEM images show that the hexagonal LDH particle is modified into a rigid sphere with the same size under the coeffect of the polar group -OSO3- and the -CH2CH2O- chain. These results had not been reported in the former literature. Results of electrophoretic mobility show the intercalation compound, [Mg-Al-(PEGS)], is charged positive and the intercalation of polymer PEGS did not change the electrophoretic mobility of the particles. In conclusion, this nanocomposite, which consists of the structure of hydrotalcite (LDH) and water-soluble polymer PEGS, will be a new kind of functional material with novel properties and a candidate to act as an anisotropic solid electrolytic material. LA034526J