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Langmuir 2007, 23, 504-508
An Effective Way to Stabilize Colloidal Particles Dispersed in Polar and Nonpolar Media A. Kohut, A. Voronov,* and W. Peukert Institute of Particle Technology, Friedrich-Alexander UniVersity Erlangen-Nuremberg, Cauerstr. 4, 91058 Erlangen, Germany ReceiVed August 21, 2006. In Final Form: October 19, 2006 This article offers a new approach to building up self-adjustable invertible polymer coatings at solid surfaces. The approach is based on a two-step process. In the first step, the surface of dispersed TiO2 has been functionalized with the aid of toluene diisocyanate (TDI) as a coupling agent. In the second step, the chains of amphiphilic oligoester have been covalently grafted to the titanium dioxide surface functionalized with isocyanate groups. It is shown that the titania modified in this way can form stable suspensions in both polar (water) and nonpolar (toluene) media. Multiple redispersion cycles show the ability of the modified titanium dioxide particles, after their removal from one type of dispersion and consequent drying, to be redispersed in dispersing media strongly differing by polarity from the previous.
Introduction Hybrid materials containing both inorganic and organic moieties dispersed on the molecular level or in nanometric domains are of growing interest from the scientific and technological points of view.1-4 In particular, polymer composite materials are especially attractive in many fields such as surface coatings, adhesives, and manufacture of inks and paints.5-7 However, the common impending problems encountered in the field of polymer-inorganic nanocomposites are the agglomeration of inorganic nanoparticles and their incompatibility with organic matrices due to their mineral nature. Surface modification by grafting organic polymers onto inorganic fine particles is an effective way to improve their dispersibility in organic solvents and the compatibility with polymer matrices.8-10 On the other hand, the change from aqueous systems to organic media in pigment, filler, and coating compositions used in industries requires the development of new polymeric stabilizers with suitable amphiphilic properties which would be able to stabilize inorganic nanoparticles in both polar and nonpolar media. With the increasing demand for more sophisticated surfaces, one current approach is to fabricate materials with interfacial properties capable of undergoing reversible changes according to outside conditions or stimuli.11 In our previous work,10 we described a new approach to building up self-adjustable invertible polymer coatings at solid surfaces. This approach was based on the * To whom correspondence should be addressed.
[email protected]. (1) Warrick, E. L.; Pierce, O. L. Rubber Chem. Technol. 1979, 52, 437. (2) Schmidt, H.; Seiferling, B.; Philipp, G.; Deichmann, K. In Ultrastructure Processing of AdVanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; J. Wiley & Sons: New York, 1988; p 651. (3) Wilkes, G. L.; Huang, H. A.; Glaser, R. H. AdV. Chem. Ser. 1990, 224, 207. (4) Wei, Y.; Yang, D.; Tang, L.; Hutchins, M. K. J. Mater. Res. 1993, 8, 1143. (5) Tashiro, N.; Maruyama, O. European Patent EP 505 648 A1, 1992. (6) Balakrishnan, S.; Ramaswamy, D. J. Oil Colour Chem. Assoc. 1991, 74, 1177. (7) Nakatsuka, T.; Kawasaki, H.; Itadani, K.; Yamashita, S. J. Colloid Interface Sci. 1981, 82, 298. (8) Bourgeat-Lami, E.; Espiard, P.; Guyot, A.; Gauthier, C.; David, L.; Vigier, G. Angew. Makromol. Chem. 1996, 242, 105. (9) Hayashi, S.; Takeuchi, Y.; Eguchi, M.; Iida, T.; Tsubokawa, N. J. Appl. Polym. Sci. 1999, 71, 1491. (10) Kohut, A.; Ranjan, S.; Voronov, A.; Peukert, W.; Tokarev, V.; Bednarska, O.; Gevus, O.; Voronov, S. Langmuir 2006, 22, 6498. (11) Luzinov, I., Minko, S., Tsukruk, V. Prog. Polym. Sci. 2004, 29, 635.
functionalization of the solid surface with the reactive anchor copolymer poly(styrene-alt-maleic anhydride) due to the physical adsorption and on grafting the chains of amphiphilic oligoester to the modified solid surface through a reaction between the functional groups of the anchor and the oligoester. The present work deals with the covalent attachment of the amphiphilic oligoester onto the surface of titania particles via the “grafting to” approach utilizing toluene diisocyanate (TDI) as a coupling agent. The idea of using a diisocyanate to graft polymers onto inorganic surfaces has been reported for silica,12 glass fiber,13 nanosized apatite,14 and carbon black.15 Our approach is based on tethering the synthesized invertible oligoester to dispersed solid titania surface in a two-stage procedure. Immobilization of the coupling agent toluene diisocyanate onto the TiO2 surface is performed in the first stage. The attachment of switchable stabilizing oligoester chains is carried out in the second stage via a reaction of isocyanate groups immobilized on the surface with terminal functional groups of the oligoester. The invertible oligoester macromolecules are covalently bound to the titania surface in the case of using TDI as a coupling agent. Taking into account the high amount of hydroxyl groups on the titanium dioxide surface and the reactivity of toluene diisocyanate, we may expect the higher anchoring density of the invertible oligoester. In our previous work,10 the low grafting density of the oligoester on the reactive anchor polymer poly(styrene-altmaleic anhydride) has been explained by steric hindrance. We assumed that a conformation of the anchor macromolecules on the titania surface does not permit each reactive anhydride group to interact with the oligoester. The use of toluene diisocyanate as a low-molecular-weight coupling agent for the tethering of the invertible oligoester to the titania surface allows us to increase the anchoring density of the amphiphilic oligoester and, therefore, the efficiency of stabilization. We believe that this approach to the solid surface modification (12) Guo, Z.-X.; Liu, W.-F.; Li, Y.; Yu, J. J. Macromol. Sci. Pure Appl. Chem. 2005, 42, 221. (13) Yosomiya, R.; Morimoto, K.; Suzuki, T. J. Appl. Polym. Sci. 1984, 29, 671. (14) Liu, Q.; de Wijn, J. R.; de Groot, K.; van Blitterswijk, C. A. Biomaterials 1998, 19, 1067. (15) Tsubokawa, N.; Kobayashi, K.; Sone, Y. Polym. Bull. 1985, 13, 215.
10.1021/la062465u CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2006
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may find unique use in various technical applications as diverse as microelectronics, bioengineering, adhesive bonding, and so forth.
was determined by DLS after each dispersion. The same experiment was carried out with another titania sample; water, toluene, and again water were used as the dispersing media.
Experimental Section
Results and Discussion
Materials and Reagents. Submicrometer-size dispersed titania (rutile) with a mean particle size of 340 nm and a specific surface area of 8.5 m2/g was purchased from Sachtleben (Germany). Before use, it was dried in a vacuum at 115 °C for 12 h. Toluene diisocyanate (Merck, 80/20 mixture of 2,4- and 2,6-isomers) and toluene (Merck, grade “dried”) were used as received. Synthesis of Invertible Oligoester (PEG-SA). Oligoester has been synthesized, as reported earlier,10 via polycondensation of sebacic acid and poly(ethylene glycol) with a molecular weight of 600 g/mol in a toluene solution in the presence of trace amounts of sulfuric acid as a catalyst. Reaction of Toluene Diisocyanate with Titania. A mixture of titania (6.0 g) and TDI (0.06 g, 3.4 × 10-3 mmol) in toluene (60 mL) was heated to and held at 80 °C for 4 h under nitrogen. After cooling to room temperature, the suspension was centrifuged. To remove the unreacted TDI, the precipitate of modified titania was washed with a fresh portion of toluene (about 25 mL) and subsequently centrifuged; this procedure was repeated three times. The modified titania was dried in a vacuum oven at 60 °C until a constant weight was obtained. Attachment of the PEG-SA Oligoester to the Titania Surface Modified by TDI. A mixture of TiO2 particles (5.0 g) modified with TDI and the oligoester PEG-SA (1.0 g) in toluene (50 mL) was thoroughly stirred at 80 °C for 7 h under nitrogen. Then, the suspension was centrifuged, and the excess unreacted oligoester was removed as described above for the case of TDI grafting.
Recently, we reported16 on the synthesis of oligoesters having both hydrophilic poly(ethylene glycol) and hydrophobic aliphatic dicarboxylic acid moieties alternately distributed along the oligomer chains. The structure of the synthesized oligoester was proven by means of FTIR spectroscopy. The following absorption bands were found in its spectrum: an intensive duplet in the range 1000-1270 cm-1, which belongs to valence oscillations of C-O ester bonds; a broken but intense absorption band at 1730 cm-1, indicating the presence of ester carbonyl groups CdO; and a duplet of absorption bands in the range 2800-3000 cm-1 and intense absorption bands at 1450 cm-1 and 730 cm-1 correspond to the valence, deformation, and pendulum oscillations of -(CH2)n- groups, respectively. The presence of the ester bonds in the FTIR spectrum of the oligoester confirms the interaction of terminal carboxyl groups of the aliphatic acid with end hydroxyl groups of poly(ethylene glycol) to form an oligoester with alternating hydrophilic and hydrophobic blocks. The oligoesters have been grafted to disperse as well as flat solid substrates using a reactive copolymer poly(styrene-alt-maleic anhydride) adsorbed on the substrate surfaces.10 The grafted oligoesters have been found to reveal invertible behavior on the solid surface. It has been shown that an environmentally induced switching of the surface properties of the coatings from hydrophilic to lipophilic and vice versa is conditioned by the conformational changes in the oligoester macromolecules. In the present article, we suggest a new approach to the development of stabilizing coatings for both polar and nonpolar media consisting of covalent binding of the invertible oligoester chains to a solid disperse surface. A schematic representation of the two-step procedure for attaching an invertible oligoester to a titania surface is shown in Figure 1. In the first step, the solid surface is treated with a toluene solution of toluene diisocyanate as a coupling agent. It can be assumed that in this case TDI grafting occurs due to the interaction of hydroxyl groups usually present on the titanium dioxide surface with the isocyanate moieties of toluene diisocyanate.12-15 We considered the fact that there are two isocyanate groups in one toluene diisocyanate molecule. An excess amount of TDI has been used to modify the titania surface, so that only one isocyanate group per toluene diisocyanate molecule has reacted with a hydroxyl group of the titania. In this case, one could expect that, in the second step, the remaining isocyanate groups not involved in the interaction with the titania -OH groups are capable of reacting with the terminated hydroxyl and carboxyl functionalities of the invertible oligoester, that would lead to their tethering. The interaction of the oligoester -OH and -COOH groups with the TDI isocyanate groups is a typical reaction, resulting in formation of urethane and amide groups, respectively. At the same time, because isocyanate groups are immobilized at the solid surface, such interaction proceeds as a “grafting to” process. Both FTIR spectroscopy and thermogravimetric analysis have been employed to prove the reaction described above leading to the formation of the invertible coating at the solid surface. There is an appreciable difference among the spectra of bare titania, TDI-modified titania, and titania with grafted PEG-SA shown in Figure 2. As compared to the spectrum of bare titania, the new
Characterization The Fourier transform infrared (FTIR) spectra were recorded for the samples of virgin titania, TDI-modified titania, and PEG-SA-grafted titania pilled with KBr powder by a Varian Excalibur FTS 3100 spectrometer with a resolution of 4 cm-1 using the EasiDiff diffuse reflectance accessory. The obtained spectra were converted into the absorbance-like spectra by performing a Kubelka-Munk transform on the recorded data. Thermogravimetric Analysis (TGA). The constituents of the grafted TDI and the attached oligoester were calculated from TGA data by their weight loss in the temperature range from 150 to 600 °C. Thermogravimetric analysis was performed on a TA Q50 instrument with a heating rate of 10 °C/min in a nitrogen flow. Molecular Weight. The molecular weights of the PEG-SA oligoester was estimated via static light scattering in acetone using a Malvern Nano ZS instrument with a 633 nm “red” laser. Particle Size Distributions. Dynamic light scattering (DLS) was used for the determination of the titania particle size distributions employing a Malvern Nano ZS instrument with a He-Ne laser. Dispersions of about 1 mg of TiO2 in 5 mL of the selected solvent were ultrasonified for 3-5 min just before the measurements (sound with a frequency of 35 kHz). The Specific Surface Area of the titania powder was determined by nitrogen adsorption (BET method) using a Nova 2000 high-speed gas sorption analyzer (Quantachrome Corp.). A sample was outgassed at 350 °C for 6 h prior to measurement. Redispersion Cycles. A mixture of about 1 mg of titania with grafted PEG-SA oligoester and 5 mL of toluene was ultrasonified for 3-5 min using an ultrasonic bath Bandelin Sonorex TK 52 with 120 W of potency and 35 kHz of frequency. Then, the titania sample was isolated by centrifugation, dried, and redispersed in water. After the isolation in a similar manner, the titania was redispersed in toluene. The particle size distribution
(16) Voronov, A.; Kohut, A.; Peukert, W.; Voronov, S.; Gevus, O.; Tokarev, V. Langmuir 2006, 22, 1946.
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Figure 1. Reaction scheme of grafting PEG-SA onto a TiO2 surface via toluene diisocyanate coupling.
Figure 2. FTIR spectra of (1) titania, (2) TDI-modified titania, and (3) PEG-SA-grafted titania at 2000-1300 cm-1 (A) and 33002500 cm-1 (B). The urethane band at 1654 cm-1, the amide band at 1545 cm-1, and the aromatics bands at 1604 and 1508 cm-1 are all visible. The carbonyl peak at 1735 cm-1 and the -CH2- peaks at 2928 and 2870 cm-1 arose after the oligoester attachment.
characteristic absorption bands of urethane, amide II, and aromatics appear in the TDI-modified titania spectrum at 1654, 1545, 1604, and 1508 cm-1 (Figure 2A), indicating that the reaction of toluene diisocyanate with titania takes place through
the formation of a urethane bond. Apart from those bands mentioned above, in the spectrum of titania with grafted PEGSA, absorption peaks at 2928 and 2870 cm-1 arise, which belong to the CH2 stretch vibration (Figure 2B). A new absorption band at 1735 cm-1 indicates the appearance of carbonyl groups on the titania surface (Figure 2A). It should be mentioned that both CH2 and CdO groups are present in the structure of the PEG-SA oligoester. Therefore, we assumed that the presence of these new absorption peaks imply successful covalent grafting of the invertible oligoester onto the titania particulate surface via TDI coupling. The amounts of overall organic composition deposited on the titania particle surface have been estimated from the TGA analysis data. Figure 3 shows TGA diagrams of virgin titania (a), TiO2 modified with toluene diisocyanate (b), TiO2 with the grafted PEG-SA oligoester (c), and the PEG-SA oligoester (d). One can see from the TGA diagram of the oligoester (curve d in Figure 3) that its weight loss reaches almost 100% at about 450 °C. Therefore, all organic substances tethered to the particulate surface are removed in the temperature range 50-600 °C during the TGA analysis. The weight loss of the TDI-modified sample lies mainly in the region between 125 and 250 °C. This result is in good agreement with data reported by other authors,12 who found that TDI attached to a nanopaticulate silica surface decomposes within a similar temperature interval. According to TGA analysis, the amount of toluene diisocyanate grafted from the 1% toluene solution on the titania surface is 0.67% w/w. The sample of titania with grafted PEG-SA shows two distinct weight loss regions lying between 150 and 550 °C (curve c in Figure 3). The weight loss at the inflection point (about 230 °C) correlates quite well with that for TDI-modified titania (curve b). Therefore, it could be supposed that the second step at curve c (T > 230 °C) is attributed mainly to the decomposition of the grafted oligoester macromolecules. This result correlates with data reported in our earlier work.10 The total weight loss for PEG-SA-modified titania is about 1.55% relative to the initial TiO2 powder. This value, after substracting the contribution of TDI decomposition, has been used to calculate the percentage of PEG-SA grafted and the grafting efficiency of the PEG-SA reaction with toluene diisocyanate immobilized on titanium dioxide. The following parameters characterizing the formation process and the parameters of invertible polymer coatings on the titania
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Figure 3. TGA diagrams of (a) nonmodified titania, (b) TDI-modified titania, (c) PEG-SA-grafted titania, and (d) the PEG-SA oligoester. Table 1. Parameters of the Grafted Invertible Polymer Coatings on Titania Particles TDI grafting mmol/ga
PEG-SA grafting %
PEG-SA Mw kg/mol
PEG-SA grafting mmol/ga
ICGb efficiency %
3.85 × 10-2
0.88
9.2
9.57 × 10-4
2.5
a
The values expressed in mmol per 1 g of TiO2. b ICG: isocyanate groups.
particles, including the percentages of the overall attachment, toluene diisocyanate grafting, and invertible polyester PEG-SA grafting, have been estimated on the basis of TGA data using the expressions (eqs 1-5) given below:
total attachment (%) )
overall organic compositions (g) bare titania (g) × 100% (1)
grafted TDI (g) TDI grafting (%) ) × 100% bare titania (g)
TDI surface coverage ) (2)
PEG-SA grafting (%) ) total attachment (%) - TDI grafting (%) (3) TDI grafting (mmol/g) )
TDI grafting (%) × 10 TDI molecular weight
(4)
PEG-SA grafting (%) PEG-SA grafting (mmol/g) ) PEG-SA molecular weight × 10 (5) The ratio of the amount of isocyanate groups that have been reacted with the oligoester to their initial amount immobilized at the titania surface designated as the isocyanate group efficiency has been evaluated by the following expression:
ICG efficiency (%) )
conclusions. One can see that the grafting efficiency of the isocyanate groups is about 2.5%. A similar result has been obtained in our earlier work10 for grafting the invertible oligoester PEG-SA onto titania particles with an adsorbed reactive anchor polymer poly(styrene-alt-maleic anhydride). The main reason for this rather low value is steric hindrance with respect to the interaction of the hydroxyl and carboxyl groups of the oligoester with the -NCO functionalities immobilized on the titania surface. It could be supposed that some of the isocyanate groups are screened by the already-grafted oligoester macromolecules. Thus, other macromolecules present in the solution cannot reach reactive sites and interact with isocyanate groups. The surface coverage of toluene diisocyanate has been estimated from the grafted amount of TDI (TGA measurements) and the titania specific area (8.5 m2/g) evaluated in BET measurements. The following expression has been used for the calculation:
PEG-SA grafting (mmol/g) × 100% TDI grafting (mmol/g) (6)
The calculated values of the mentioned parameters are summarized in Table 1, which allows us to draw some
TDI grafting (mmol/g) × NA titania specific area (nm2/g) × 10-3 (7)
where NA ) Avogadro’s number (6.02 × 1023 mol-1). It has been found that a molecule of TDI occupies 0.37 nm2 (or 2.73 molecule/nm2) of titania. Similar calculations have been performed for the grafted oligoester macromolecules using the next expression:
PEG-SA surface coverage ) PEG-SA grafting (mmol/g) × NA titania specific area (nm2/g)
× 10-3 (8)
The anchoring density of the oligoester has been determined to be 0.07 nm-2. One oligoester macromolecule occupies, therefore, about 15 nm2. Hence, we observed a higher anchoring density than that of the oligoester grafted to the titania surface using poly(styrene-alt-maleic anhydride) (0.05 nm-2) as has been shown in our previous work.10 The higher value of the anchoring density of the oligoester in the case of the use of TDI as a coupling agent is connected with the higher density of the isocyanate groups immobilized on the TiO2 surface by covalent binding in comparison with the density of adsorbed reactive anhydride groups on the surface when poly(styrene-alt-maleic anhydride)
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Figure 4. Redispersion cycles of samples modified with PEG-SA titanium dioxide: toluene-water-toluene (A) and water-toluene-water (B).
has been used as an anchor polymer. Thus, the use of toluene diisocyanate as a coupling agent allows us to increase the amount of the switchable oligomer grafted to the surface of the titania particles as compared with the use of poly(styrene-alt-maleic anhydride). We believe that the higher anchoring density of the stabilizing chains improves the dispersibility of the modified titanium dioxide particles in organic solvents and their compatibility with polymer matrices. The effect of the grafted invertible oligoester coatings on the stability of titania suspensions in solvents of various polarity (toluene and water) has been studied. Figure 4 shows the change in the particle size distribution during the redispersion of a dried titania powder both in a polar medium (water) and in a nonpolar medium (toluene). In the absence of the polymer coating, the investigated titania particles do not form any stable suspension in toluene. On the contrary, the modified titania particles form a suspension with essentially enhanced stability in toluene and the maximum at about 700 nm on the normalized intensity density plot (curve 1 in Figure 4A) as it has been measured by the dynamic light scattering. Moreover, the modified titanium dioxide particles are able to be redispersed in a medium with diverse polarities. After the dispersion in toluene, the titania particles modified with the switchable oligoester have been isolated by the centrifugation, dried, and redispersed in water. We observed the formation of a stable suspension. The normalized intensity density plot (curve 2 in Figure 4A) shows the maximum at less than 400 nm. After the isolation from the aqueous medium and drying, these particles are able again to be redispersed in toluene to give a stable suspension with a particle size of 700 nm (curve 3 in Figure 4A). The same procedure has been repeated in another sequence: the titania with the grafted PEG-SA macromolecules has been step-by-step dispersed in water, toluene, and water. We observed the formation of stable suspensions in each case with maxima on the normalized intensity density plots (Figure 4B) at 420, 550, and 410 nm, respectively. The experiments confirm the ability of the invertible oligoester macromolecules grafted to the solid surface to switch their environmental appearance as a result of the proper orientation of their polar and nonpolar moieties by accepting those conformations that improve their compatibility with the surroundings, as has been discussed in greater detail in our previous paper.10 Briefly, upon exposure to toluene, the top of the polymer coating is covered with aliphatic
dicarboxylic acid moieties and becomes hydrophobic, and vice versa, upon exposure to water, the surface switches the energetic state to hydrophilic with a poly(ethylene glycol) top layer. This indicates that the randomly distributed polymer blocks can quickly respond to changes in the environment, and the top layer undergoes reorganization to adapt and enhance its compatibility with the surrounding media. The powder made of the colloidal titanium dioxide particles modified with the amphiphilic PEG-SA oligoester may be redispersed in water and toluene without any loss in colloidal stability.
Conclusions Our work gives a simple method to develop modified titania having self-adjustable invertible polymer coatings made of the amphiphilic oligoester covalently grafted to the surface. Oligoester comprising hydrophilic poly(ethylene glycol) and hydrophobic sebacic acid blocks alternately linked in macrochains have been successfully tethered by a two-step procedure onto the surface of submicrometer-size dispersed titania using toluene diisocyanate as a coupling agent. The grafting process proceeds in sequential reactions involving (i) interaction of TDI with titania to give a surface bearing isocyanate groups and (ii) following reaction of such functionalized titania with the oligoester. An essential increase in colloidal stability has been observed for the titania particles after their modification with the invertible oligoester chains. Redispersion cycles show the ability of the modified titanium dioxide particles to be redispersed in media differing in polarity from the previous medium. The obtained data support the capability of the polymer coatings covalently tethered to the titania surface to switch their environmental appearance and to behave as a self-adjustable invertible interfaces. It results in the ability of the grafted amphiphilic oligoester macromolecules to change their conformation in response to environmental changes. This elaborated process of the surface modification may potentially find application for the development of “smart” filler, for paint coatings or highly filled polymer composites. Acknowledgment. A.K. thanks the German Academic Exchange Service (DAAD) for the financial support. LA062465U
