Covalently Attached Monolayers of Oligo(dimethylsiloxane)s on Silica

Feb 28, 2002 - We note that our results are in good agreement with the results reported by Tripp and coauthors for the reaction of trichlorosilanes wi...
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Langmuir 2002, 18, 2665-2672

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Covalently Attached Monolayers of Oligo(dimethylsiloxane)s on Silica: A Siloxane Chemistry Approach for Surface Modification Alexander Y. Fadeev* and Yuri V. Kazakevich Department of Chemistry and Biochemistry, Seton Hall University, 400 South Orange Avenue, South Orange, New Jersey 07079 Received September 27, 2001. In Final Form: December 17, 2001 This work describes the synthesis of oligo(dimethylsiloxane) [-Si(CH3)2O-]n monolayers supported on silica. The fundamental properties of siloxane (Si-O-Si) backbones, for example, high flexibility and high thermal stability, set siloxane monolayers apart from self-assembled monolayers derived from long-chain alkylsilanes, alkylthiols, and so forth and provide attractive features for using “siloxane chemistry” for the modification of surfaces. Oligo(dimethylsiloxane) surfaces were prepared by reaction of low molecular weight R,ω-dichloro-dimethylsilane/oligo(dimethylsiloxane)s [Cl[Si(CH3)2O]n-Si(CH3)2Cl, n ) 0, 1, 2, 3, 4] with mesoporous precipitated silica (pore diameter 15 nm). Reactions with silicas of different degrees of hydration (dried at 200 °C in a vacuum, “as received”, and saturated with water vapors) were studied under two conditions: (1) in the vapor phase and (2) in solution. Reactions of Cl[Si(CH3)2O]n-Si(CH3)2Cl with dry silicas yield surfaces with bonding densities of 2.35 ( 0.15 [Si(CH3)2O] groups per nm2 corresponding to a single layer of dimethylsiloxy groups on the surface. Fourier transform infrared (FTIR) spectroscopy indicates that the major products of these reactions are poly(dimethylsiloxane) loops covalently attached to silica by two SiS-O-Si bonds. Reactions of Cl[Si(CH3)2O]n-Si(CH3)2Cl with wet silicas yield oligomeric dimethylsiloxane surfaces with apparent bonding densities up to 11.4 [OSi(CH3)2] groups per nm2. The bonding density of dimethylsiloxane can be controlled with high precision by varying of the amount of water preadsorbed to the silica; this provides a conventional method for manipulating the surface bonding process. In situ FTIR studies of reaction kinetics indicate that reactions of Cl[Si(CH3)2O]n-Si(CH3)2Cl with wet silica proceed through hydrolysis and polycondensation yielding linear OH-terminated oligomeric dimethylsiloxane. These oligomers reacted with the silica surface through SiS-O-Si bonds yielding covalently attached oligomeric dimethylsiloxane surfaces. The excess of poly(dimethylsiloxane) is adsorbed on the surface and is removed after rinsing with solvents. The thermal stability of dimethylsiloxane surfaces was studied using thermogravimetric analysis. Monomeric dimethylsiloxane surfaces on silica show remarkable thermal and oxidative stability, and no mass loss was observed below 550-650 °C in air. Oligomeric dimethylsiloxane surfaces on silica show a maximum rate of mass loss at 400-450 °C, which is close to that of poly(dimethylsiloxane) (silicones). It is suggested that degradation of the oligomeric surfaces proceeds by depolymerization of dimethylsiloxane chains, while oxidative destruction is a more likely pathway for the degradation of a monomeric dimethylsiloxane surface.

Introduction Nonpolar, lyophobic surfaces have drawn a significant amount of attention in the past several decades not only because they provide well-defined model systems for basic studies of interfacial phenomena but also because of growing applications as nonspecific adsorbents, waterrepellant and protecting coatings, and biomaterials.1-3 The only synthetic approach to making nonpolar monolayers that has been explored so far uses long-chain alkyl or fluoroalkyl groups assembled in closely packed twodimensional arrays on the surface. Well-defined lyophobic monolayers of alkylsilanes on metal oxides and semiconductors,4 alkyls on silicon,5 alkylthiols on noble metals,6 (1) Unger, K. K. Porous silica, its properties and use as support in column liquid chromatography; Journal of Chromatography Library, Vol. 16; Elsevier: Amsterdam, 1979. (2) Plueddemann, E. P. Silane coupling agents, 2nd ed; Plenum: New York, 1991. (3) Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985. (4) Boksanyi, L.; Liardon, O.; sz. Kova´ts, E. Adv. Colloid Interface Sci. 1976, 6, 95. Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759. (5) Sandoval, J. E.; Pesek, J. J. Anal. Chem. 1991, 63, 2634. Linford, M. R.; Chidsey, C. E. D.; et al. J. Am. Chem. Soc. 1995, 117, 3145. Buriak, J. M., et al. J. Am. Chem. Soc. 1999, 121, 11491. (6) Bain, C. D., et al. J. Am. Chem. Soc. 1989, 111, 321.

and alkylhydroxamic acids,7 alkylcarbonic acids,8 and alkylphosphonic acids9 on various metals and metal oxides have been reported. It is of interest to develop chemical strategies for making nonpolar lyophobic monolayers other than alkyls. One of the possible alternatives to CH2 and CF2 building blocks in constructing such surfaces is dimethylsiloxane [-Si(CH3)2O]n groups. Poly(dimethylsiloxane) (PDMS) has low surface energy, a robust backbone, and superior thermal properties. It is one of the most widely used polymers in biomedical, semiconductor, and automotive applications.10-13 Despite the great practical possibilities, the synthesis and proper(7) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11 (3), 813. (8) Baker, H. R.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1952, 56, 405. Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1069. Marguerettaz, X.; Fitzmaurice, D. Langmuir 1997, 13, 6769. Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 3294. (9) Woodward, J.; Ulman, A.; Schwartz, D. Langmuir 1996, 12, 3626. Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. Van Alsten, J. G. Langmuir 1999, 15, 7605. (10) Voronkov, M. G.; Mileshkevitch, V. P.; Yuzhelevskii, Yu. A. The Siloxane Bond; Consultants Bureau: New York, 1978. (11) Siloxane Polymers; Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993. (12) Noll, W. Chemistry and Technology of Silicones; Academic Press; New York, 1968. (13) Organosilicon chemistry IV: from molecules to materials; Auner, N., Weinheim, J. W., Eds.; Wiley-VCH: New York, 2000.

10.1021/la011491j CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002

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Fadeev and Kazakevich Scheme 1

ties of siloxane monolayers supported on solids have not been studied in detail. Preparation of PDMS surfaces via chemisorption of the end-functionalized PDMS on silica14,15 and thiol-derivatized copolymers of PDMS on gold16 has been reported. Wettability studies of PDMS surfaces prepared by the reaction of dichlorodimethylsilane with hydrated glass17 and Si wafers18 were also reported. The major commercial method for preparing poly(dimethylsiloxanes) (silicones) is polycondensation (step growth polymerization) of dichlorodimethylsilane (Cl2Si(CH3)2) with water in solution, in the gas phase, or in emulsion.11 R,ω-Dichloro-poly(dimethylsiloxanes) can be used in this reaction as well. This reaction has been the subject of many investigations, and an extensive literature is available.11-13 The reaction mechanism involves hydrolysis of chlorosilanes to the corresponding silanols and subsequent condensation to form the siloxane chain. Both linear and cyclic structures are formed, and the ratio between them can vary over a wide range depending on the monomer and the reaction conditions. The main focus of the work presented here is to use the reaction of difunctional silanes with water in pores of hydrated silica with the intention of preparing PDMS chains grown in situ that are capable of covalent attachment to silica. Scheme 1 shows the rational behind this work. In this paper, we report the synthesis of monomeric and oligomeric dimethylsiloxane surfaces via the reaction of R,ω-dichlorosilane/siloxanes of the general formula Cl[Si(CH3)2O]n-Si(CH3)2Cl (n ) 0, 1, 2, 3) with mesoporous precipitated silica and also the thermal properties of the resulting materials. Experimental Section General. All solvents (HPLC grade) were obtained from Aldrich (Milwaukee, WI) and used as received. All silanes and siloxanes were obtained from Gelest (Tullytown, PA) and were distilled before use. Infrared spectra were recorded with a Perkin-Elmer Spectrum One instrument with a DTGS detector. Spectra of modified silicas were collected in the reflectance mode using a Harrick Seagull accessory (50° angle of incidence, 200 scans, resolution 2 cm-1). Kinetics measurements of the reactions of silane vapors with silica and the spectra of dried silicas were taken using a highvacuum reaction chamber with a ZnSe dome (Harrick, Ossining, NY). Thermal analysis of the modified silicas was performed in air and in nitrogen using a TA Instruments Hi-Res 2950 thermogravimetric analyzer. Heating rates were varied from 2 to 25 (14) Patel, A.; Cosgrove, T.; Semylen, J. A.; Webster, J. R. P.; Scheutjens, J. M. H. M. Colloids Surf., A 1994, 87, 15. (15) Al-Maawali, S.; Bemis, J. E.; Akhremitchev, B. B.; Leecharoen, R.; Janesko, B. G.; Walker, G. C. J. Phys. Chem. B 2001, 105, 3965. (16) Tsao, M.-W.; Pfeifer, K.-H.; Rabolt, J. F.; Castner, D. G.; Haussling, L.; Ringsdorf, H. Macromolecules 1997, 30, 5913. (17) Herzberg, W. J.; Marian, J. E.; Vermeulen, T. J. Colloid Interface Sci. 1970, 33, 164. (18) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268.

°C/min and were found not to have an influence on the thermograms; the majority of the results were obtained using a heating rate of 10 °C/min. Carbon analysis was performed with a Perkin-Elmer 2400 CHN analyzer made by Schwarzkopf Microanalytical Laboratory (Woodside, NY) using the ASTM method. The apparent bonding density of the monolayers was calculated using the formula2

F)

6 × 105 (%C) 1 [2400 - MW × (%C)] S(BET)

(1)

where MW is the molecular weight of the dimethylsiloxy group (74 g/mol), %C is the carbon weight percentage of the modified silica, and S(BET) is the surface area of the original silica (m2/g). Equation 1 gives the number of [-Si(CH3)2O-] repeat units per 1 nm2 of the surface (F). Silica. Mesoporous silica gel, trade name Prodigy (Phenomenex, Torrance, CA), was studied. The surface area (375 m2/g), pore volume (1.16 cm3/g), and pore diameter (15 nm) of the nonmodified (bare) silica were determined from low-temperature nitrogen adsorption isotherms using the Brunauer-EmmettTeller (BET) method. Three samples with differing amounts of adsorbed water were prepared: (1) silica dried at 200 °C in a vacuum (it is well documented that such a treatment almost completely removes the molecularly adsorbed water); (2) silica “as received” containing ∼8% of water by weight; (3) silica saturated with water vapors at room temperature containing 22% of water by weight. The water content in silicas was determined by mass loss due to evacuation at 10-5 Torr at 200 °C for 2 h. Following the terminology suggested by Tripp and Hair,19 we refer to these samples as dehydrated, hydrated, and superhydrated silica, respectively. IR spectra of these silicas were almost identical to those presented elsewhere.19 The main features, which are well-documented, are a sharp peak at ∼3750 cm-1 (surface silanol groups), two broad bands at ∼3400-3550 and ∼1650 cm-1 (adsorbed water and hydrogen-bonded silanol groups), and a series of bands at ∼1100, 820, and 450 cm-1 (siloxane network of silica). Reaction of Dehydrated Silica with Dichlorodimethylsilane and with r,ω-Dichloro(dimethylsiloxanes) in Anhydrous Toluene. Three grams of dried silica (hot) was covered with anhydrous toluene (100 mL). The dichlorodimethylsilane/siloxane (10 µmol per 1 m2 of silica surface) was added by syringe. Reaction mixtures were shaken at room temperature for 14 days. After this time, the modified silica was transferred onto a glass filter and rinsed (in this order) with 3 × 100 mL of anhydrous toluene, 3 × 100 mL of acetone, 2 × 50 mL of acetonewater (4:1), and 3 × 100 mL of acetone. The modified silicas were dried on the filter until a free-flowing condition was achieved and then dried in a vacuum oven at 120 °C for 10 h. Reaction of Hydrated Silicas with Dichlorodimethylsilane and with r,ω-Dichloro(dimethylsiloxanes) in Toluene. Three grams of silica was covered with toluene (100 mL). Subsequent steps were performed as described in the procedure for dry solvent. The chosen reaction time was 24 h, as no further increase of bonding density was observed for longer reaction times. (19) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215.

Oligo(dimethylsiloxane) Monolayers on Silica

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Chart 1. Structures of the Precursors to Dimethylsiloxane Surfaces

Figure 1. Difference IR spectra of dehydrated silica before and after reaction with (a) Cl2Si(CH3)2, (b) Cl[Si(CH3)2O]2Si(CH3)2Cl, and (c) Cl[Si(CH3)2O]7-15-Si(CH3)2Cl.

Reaction of Silica with Dichlorodimethylsilane and with r,ω-Dichloro(dimethylsiloxanes) in the Vapor Phase. One gram of silica in an open small vial was placed in a larger vial, and the latter was sealed. Dichlorodimethylsilane/siloxane (0.5 mL) was added to the outer vial using a syringe and a long needle. All the silane liquid was confined outside the small vial, so silica was in contact only with vapors of silane. Reactions were carried out at room temperature, and the reaction time was chosen as 24 h, as no further increase of bonding density was observed for longer reaction times. The modified silicas were isolated, rinsed, and dried as described above for the solution-phase synthesis.

Results and Discussion Reactions of silicas with varying degrees of hydration with homologous series of R,ω-dichlorodimethylsiloxanes, Cl[Si(CH3)2O]n-Si(CH3)2Cl, were studied. Dichlorodimethylsilane (n ) 0) is the principal precursor for making silicones. The choice of the other oligomers was dictated by their commercial availability. By varying the size of the dichlorosilane, we intended to control the degree of polymerization (thickness) of siloxane monolayers. Chart 1 shows the structures of the compounds used. Besides the compounds shown in Chart 1, two mixtures of oligomers with n ) 4-7 and n ) 7-15 (as determined by gas chromatography-mass spectrometry) were used. Reaction of Dehydrated Silicas with Dichlorodimethylsilane, Cl2Si(CH3)2, and r,ω-Dichloro-oligo(dimethylsiloxanes), Cl[Si(CH3)2O]n-Si(CH3)2Cl. The reaction of dichlorodimethylsilane with dry silicas has been the subject of many previous works.20 Dichlorodimethylsilane reacts with silica covalently through SiS-OSi bonds; the reaction with rigorously dried silicas takes place only at high temperatures (∼300-350 °C). We began our work with the reaction of dichlorodimethylsilane with silica in solution. This reaction appears to be the simplest route to the dimethylsiloxane surfaces, and it also can serve as a model for the understanding of more complex reactions, for example, reactions of R,ω-dichlorosilane/ siloxanes with hydrated and superhydrated silicas. Figure 1a shows a difference IR spectra of dried silica before and after the reaction with dichlorodimethylsilane in toluene.21 In the difference spectra, bands that are positive are due to groups that have been formed on the (20) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 3947. Armistead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S. A.; Hockey, J. A. J. Phys. Chem. 1969, 73, 3974. Tiertykh, V. A.; Chuiko, A. A.; Mashchenko, V. M.; Pavlov, V. V. Zh. Phys. Chem. 1973, 47, 158. Tiertykh, V. A., et al. Teor. Eksp. Khim. 1975, 11, 174.

surface, while negative bands are due to groups that have been removed from the surface. The following characteristic features are present. Disappearance of the broad bands at 3400 and 1628 cm-1 indicates the removal of water from the surface. We interpret this as physisorbed water being replaced from the silica surface, which is consistent with the hydrophobic nature of dimethylsiloxy groups. The sharp negative band at 3740 cm-1 can be assigned to the surface silanol groups (SiS-OH). Disappearance of the 3740 cm-1 band from the spectra along with the appearance of the broad band centered at 1085 cm-1, which can be ascribed to asymmetrical stretching of the Si-O groupings, argues for the covalent attachment of dichlorosilanes to the surface via SiS-O-Si bonds. Absorption bands at 1265 and 808 cm-1 can be assigned to deformation and stretching modes of Si-CH3 groups. Bands at 2970, 1407, and 850 cm-1 are stretching, deformation, and rocking modes of CH3 groups in dimethylsiloxane moieties. We note that the results obtained agree well with previous work.20 The only inconsistency is that the reactions did take place at room temperature, perhaps due to traces of water in the solvent, as anhydrous toluene (Aldrich) was used without further drying. Difference IR spectra of silicas that were reacted with R,ω-dichloro-oligo(dimethylsiloxanes) in toluene are shown in Figure 1b,c. For the reactions of short dichlorosiloxanes (n ) 1, 2, 3, 4), the spectra are almost identical to the one observed for the reaction of Cl2Si(CH3)2 and exhibit a single broad band in the Si-O region at ∼1085 cm-1 (Figure 1b). For reactions of R,ω-dichloro-poly(dimethylsiloxanes) with longer chains (n ) 4-7 and n ) 7-15), two additional bands appear in the spectra at ∼1020 and 1100 cm-1 (Figure 1c). From the literature, it is known that the Si-O mode is present as a single band in spectra of cyclic dimethylsiloxanes with small rings (trimers, tetramers, and pentamers). For cyclodimethylsiloxanes with larger rings (six silicon atoms and more), this band splits (Table 1). This led us to conclude that the major products of the reaction of Cl[Si(CH3)2O]n-Si(CH3)2Cl with dry silica are cyclic dimethylsiloxanes attached to the surface via two SiS-O-Si bonds per molecule. The absence of the characteristic bands for Si(CH3)2OH groups (970, 840 cm-1, Si-OH stretching) argues for the absence of linear OHterminated PDMS species and suggests that the oligo(21) Prior to taking spectra, the samples were rigorously washed with solvents of different polarity. Such a treatment removes completely all the species that are not chemically bound to the surface, e.g., nonreacted silane monomers, unbound cyclic and linear siloxanes, and HCl.

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Fadeev and Kazakevich Scheme 2

Table 1. Characteristic Frequencies of the Si-O Bond in Infrared Spectra of Siloxanes structure

νas (cm-1)

ref

[(CH3)2SiO]3 [(CH3)2SiO]4 [(CH3)2SiO]5 [(CH3)2SiO]6 [(CH3)2SiO]7 ClSi(CH3)2O-Si(CH3)2Cl Cl[Si(CH3)2O]2-Si(CH3)2Cl Cl[Si(CH3)2O]3-Si(CH3)2Cl Cl[Si(CH3)2O]4-7-Si(CH3)2Cl silica reacted with Cl[Si(CH3)2O]n-Si(CH3)2Cl (n ) 0, 1, 2, 3) silica reacted with Cl[Si(CH3)2O]n-Si(CH3)2Cl (n > 4)

1020 1078 1082 1070, 1090 1060, 1090 1020, 1086 1054, 1095 1047, 1090 1012, 1125 1085

10 10 10 10 10 a a a a a

1020, 1100

a

a

This work.

(dimethylsiloxane)s are bonded to the surfaces by two ends (Scheme 2). Chemical analysis (C, H) provides quantitative data concerning the amount of organics grafted to silica and gives more insight into the structure of the monolayers. Table 2 shows areas per molecule in the monolayers as determined from carbon analysis and the BET surface area of bare silica. For convenience, these data are also presented as the number of dimethylsiloxy [Si(CH3)2O] (DMS) groups per 1 nm2 of bare silica (bonding density), permitting data for the surfaces prepared from different monomers to be compared directly. Table 2 shows that the bonding densities are practically the same for all of the R,ω-dichlorosiloxane used (∼2.35 ( 0.15 DMS group/ nm2). We note that the surfaces described in Table 2 are “saturated” with respect to the silane used, as they were prepared using a large excess of the silane and by long reaction times (2 weeks). No further bonding was observed by increasing either reaction time or concentration of the silane. Using 0.4 nm-2 for the cross section of the DMS group,22,23 one can obtain the bonding density for the closely packed single layer as 2.5 group/nm2, which is very close to the data shown in Table 2. The independence of the bonding density on the size of dichlorosiloxane used for the reaction, along with the spectral evidence of the presence of cyclic [Si(CH3)2O]n structures on the surface, led us to conclude that the major products of the reaction of Cl[Si(CH3)2O]n-Si(CH3)2Cl with dehydrated silica are dimethylsiloxane species that are attached to the surface by two SiS-O-Si bonds per molecule. Absolute values of bonding density of the monolayers suggest “horizontal” orientation of grafted oligo(dimethylsiloxane) molecules on the surface. Reactions of Hydrated and Superhydrated Silicas with Dichlorodimethylsilane, Cl2Si(CH3)2, and r,ω(22) Molecular modeling of the dimethylsiloxane chain using ACD software gives ∼0.4 nm2. This agrees well with the interface lateral cross section of dimethylsiloxy groups, ∼0.4-0.45 nm2, as obtained from the adsorption studies of methylsiloxane-terminated LangmuirBlodgett monolayers at the water-air interface in ref 23. (23) Ibn-Elhaj, M.; Mohwald, H.; Cherkaoui, M. Z.; Zniber, R. Langmuir 1998, 14, 504.

Table 2. Area per Molecule and Bonding Density for the Monolayers Prepared by Reacting Cl[Si(CH3)2O]n-Si(CH3)2Cl with Silica n in Cl[Si(CH3)2O]nSi(CH3)2Cl 0 1 2 3 0 0

reaction conditions SiO2 dried at 200 °C SiO2 dried at 200 °C SiO2 dried at 200 °C SiO2 dried at 200 °C SiO2 “as is” SiO2 saturated with water vapor

bonding density area per ([Si(CH3)2O] molecule groups/nm2) (nm2) 2.25 2.45 2.59 2.43 6.42 11.40

0.44 0.83 1.17 1.65 0.16 0.09

Dichloro-oligo(dimethylsiloxane)s, Cl[Si(CH3)2O]nSi(CH3)2Cl. Reactions of dichlorosilanes with wet silicas result in a substantial increase in bonding density (Tables 3 and 4). Maximum bonding densities are ∼5 times higher than a single-layer capacity, indicating formation of the poly(dimethylsiloxane) surfaces supported on silica. Assuming an even distribution of poly(dimethylsiloxane) chains on the surface, the average number of [OSi(CH3)2] repeat units per chain can be obtained by dividing the bonding density by the single-layer capacity. For instance, for the surface with a bonding density of 11.4 group/nm2, if all grafted material was spread evenly over the surface, it would form a closely packed layer of tetra- and pentasiloxane brushes (apparent oligomerization degree of 4.4). We note, however, that the actual distribution of the chain length can differ substantially from the average one. Adsorption studies of the oligo(dimethylsiloxane) surfaces provide more information about the homogeneity of their surfaces and are the focus of our paper.25 According to this work,25 oligomeric dimethylsiloxane surfaces are very homogeneous and show very low adsorption energy (CH3-termini). No accessible silanol groups (or other polar adsorption centers) can be detected on these surfaces, which proves the superior ability of short poly(dimethylsiloxane) chains to shield the silica substrate. In situ IR kinetics studies provide insight into the mechanism of the reaction of dichlorosilanes with silica. Figure 2a-d shows the evolution in time of the difference IR spectra as dichlorodimethylsilane vapor reacts with wet silica. In the initial stage of the reaction, the spectrum exhibits the characteristic features of dichlorodimethylsilane: 1265 cm-1 (CH3 deformation), a strong broad band centered at ∼830 cm-1 (CH3 rocking), and a strong band at 546 cm-1 (SiCl stretching). This indicates that the reaction begins by adsorption of dichlorodimethylsilane onto silica. After 30 s of the reaction, however, a series of new bands at ∼1100-1000 cm-1 (Si-O stretching in silanols and siloxanes10) appears in the spectrum, indicating the hydrolysis of dichlorodimethylsilane. Hydrolysis is also indicated by the disappearance of the band at 546 (24) Szabo, K.; Ha, N. L.; Schneider, P.; Zeltner, P.; sz. Kova´ts, E. Helv. Chim. Acta 1984, 67, 2128. (25) Kazakevich, Y. V.; Fadeev, A. Y. Langmuir, in press.

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Table 3. Bonding Density of the Dimethylsiloxane Surfaces Prepared by Reacting Cl2Si(CH3)2 with Silicas Containing Different Amounts of the Preadsorbed Water pretreatment of silica

wt % of water in silica

dried at 200 °C

0.8

as received

8.4

saturated with water vapor a

22.5

reaction conditionsa

bonding density ([-Si(CH3)2O-] groups/nm2)

toluene vapor phase toluene vapor phase toluene

2.85 4.10 5.05 6.42 4.40

vapor phase

11.40

Room temperature. Table 4. Bonding Density of the Dimethylsiloxane Surfaces Prepared by Reactinga Cl[Si(CH3)2O]n-Si(CH3)2Cl with Silica Saturated with Water Vapor

a

silane or siloxane

bonding density ([-Si(CH3)2O-] groups/nm2)

ClSi(CH3)2Cl ClSi(CH3)2O-Si(CH3)2Cl Cl[Si(CH3)2O]2-Si(CH3)2Cl Cl[Si(CH3)2O]3-Si(CH3)2Cl Cl[Si(CH3)2O]4-7-Si(CH3)2Cl Cl[Si(CH3)2O]7-15-Si(CH3)2Cl

4.40 4.48 4.45 4.03 5.10 5.50

Reaction in toluene at room temperature.

Figure 2. In situ FTIR spectra for the reaction of Cl2Si(CH3)2 vapor with silica containing 8 wt % of adsorbed water. Spectra a, b, c, and d were taken after 0, 0.5, 10, and 120 min of the reaction, respectively. Spectrum e is obtained for the sample in (d) after rinsing with solvent.

cm-1 (SiCl stretch) and appearance of the broad band at 3450 cm-1 (SiO-H). As the reaction proceeds further, the 1100 cm-1 band shifts by ∼20 cm-1 to a higher frequency and gradually increases with time. This band along with the strong band at 1012 cm-1, which also increases with time, can be ascribed to the asymmetric Si-O stretches in poly(dimethylsiloxane) species that are immobilized on the surface. These bands are present in the spectra when the reaction is complete, and they are not removed after evacuation or extensive extraction of the sample (Soxhlet). We note that our results are in good agreement with the results reported by Tripp and coauthors for the

reaction of trichlorosilanes with hydrated silica.26 These authors also observed the 1120 and 1016 cm-1 bands and ascribed them to a “siloxane network extended outward from the surface”. We note that the 1085 cm-1 band (covalent bonds with the surface, SiS-O-Si) is barely seen as a shoulder of the more intense 1012 cm-1 band (Figure 2). Two sharp bands at 1090 and 1031 cm-1 that appear in the first minute of the reaction can be ascribed to the Si-O stretches of poly(dimethylsiloxane), which adsorbs to the surface but is not chemically bound to it. The intensity of these bands goes through a maximum (attaining the maximum at ∼10 min of the reaction) and gradually decreases; they remain in the spectra even after 2 h of reaction. These bands, however, are completely removed from the spectrum after rinsing the sample with solvents, which proves that these species are not covalently bound to the surface. Rinsing with solvents also removes the bands at 840 and 980 cm-1 (Si-OH stretches) and at 3450 cm-1 (SiO-H, not shown), which indicates that these physisorbed poly(dimethylsiloxane)s have terminal OH groups. The absence of the bands at 3500 (SiO-H) and 970 and 840 cm-1 (Si-OH) in spectra of samples that were rinsed with solvents suggests that OH-terminated PDMS chains are not present on the surface. Spectral changes similar to those observed for Cl2Si(CH3)2 were also observed for the reactions of 1,3tetramethyldisiloxane and 1,5-hexamethylsiloxane vapors with hydrated silica. The reactions of the higher molecular weight siloxanes were not studied because of their low vapor pressures. To conclude this section, in situ IR analysis suggests that the reactions of dichlorosilanes with wet silica proceed through hydrolysis and polycondensation steps yielding linear OH-terminated oligomeric dimethylsiloxanes. These react with the silica surface forming SiS-O-Si bonds yielding covalently attached loops of oligomeric dimethylsiloxane. The excess poly(dimethylsiloxane) adsorbed on the surface can be removed by rinsing with solvents. The fact that the Si-O band is split into two components (∼1120 and 1012 cm-1) suggests that grafted poly(dimethylsiloxane) loops contain more than six repeat units. The Role of Adsorbed Water, Reaction Conditions, and the Size of r,ω-Dichloro-oligo(Dimethylsiloxane). Table 3 presents bonding densities for surfaces prepared by reaction of dichlorodimethylsilane with silicas containing different amounts of the adsorbed water. Two trends can be readily observed: (1) reactions with vapors of the silanes give substantially higher bonding densities than the reactions with the solution of silane in toluene and (2) bonding density increases as the amount of water increases. For the reactions carried in the vapor phase, the effect of water is profound and quite natural: the more water that is preadsorbed on silica, the higher the bonding density of siloxane (Figure 3). For the reactions carried in toluene, the bonding density, after an initial increase, levels off at ∼4.5 group/nm2. The most probable reason for such behavior is dilution of the reaction mixtures in the presence of solvent. Dilution is known to greatly increase the yield of cyclic dimethylsiloxane oligomers versus linear dimethylsiloxanes,11 which decreases the (26) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993, 9, 3518. Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693.

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Figure 3. Bonding density of the dimethylsiloxane surfaces prepared by reacting Cl2Si(CH3)2 with silica containing different amounts of the preadsorbed water. Open symbols, reaction in vapor phase; closed symbols, reaction in toluene.

concentration of the reactive species that can graft to the surface, resulting in a lower bonding density. Another possible explanation deals with the displacement of surface water. For reactions carried out in solution, solvent molecules compete with water for adsorption sites. Hence, the relative concentration of water near the surface is reduced, resulting in a slower polymerization of dichlorosilanes and a correspondingly lower bonding density. However, this hypothesis was not confirmed by these experiments. We tested a series of solvents with different water affinity (hexane, benzene, acetonitrile) and found no effect of the nature of solvent on the bonding density. By using R,ω-dichloro-dimethylsiloxane oligomers, we intended to increase the bonding density. Since less water is needed to make poly(dimethylsiloxane) from oligomeric siloxane than from dichlorodimethylsilane, one might expect an increase in the bonding density when oligomeric dimethylsiloxane is used for the reaction. However, the data in Table 4 show that the bonding density is similar for all R,ω-dichloro-poly(dimethylsiloxane)s and remains at ∼4-5 group/nm2. This can be explained as follows. As the length of R,ω-dichloro oligomeric dimethylsiloxane increases, the tendency to form cyclic dimethylsiloxane upon hydrolysis also increases.11 Apparently, this compromises the benefits of using of long oligomeric dimethylsiloxane with the purpose to increase the bonding density. Thermal Properties of the Dimethylsiloxane Surfaces Supported on Silica. The results of thermogravimetric analysis (TGA) of bare silica and silicas that had been reacted with Cl[Si(CH3)2O]n-Si(CH3)2Cl are presented in Figures 4 and 5. Nonmodified silica shows significant weight loss below ∼100 °C, which is due to desorption of weakly bound water from the surface. Further weight loss is attributed to the desorption of strongly bound water and, above ∼400 °C, to dehydroxylation of the surface silanols.27 The observed weight loss pattern agrees well with the previously reported data.27 Silicas that had been reacted with dichlorosilanes show significantly lower weight loss at low and moderate temperatures as compared to nonmodified silica and show several features at high temperatures. Thermal degradation of the “monomeric dimethylsiloxane surfaces”, that is, those prepared by the reaction of dichlorosiloxanes with dehydrated silica, will be discussed first (Figure 4). The differential TGA (DTGA) curve for dehydrated silica that was reacted with Cl2Si(CH3)2 (27) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.

Fadeev and Kazakevich

Figure 4. TGA and differential TGA graphs for bare silica (a, a′), for dehydrated silica reacted with Cl2Si(CH3)2 (b, b′), and for dehydrated silica reacted with Cl[Si(CH3)2O]2-Si(CH3)2Cl (c, c′).

Figure 5. TGA and differential TGA graphs for hydrous silica (8% of water) that had been reacted with Cl[Si(CH3)2O]2-Si(CH3)2Cl in the vapor phase (a, a′) and in toluene (b, b′).

shows a peak at 660 °C, which can be ascribed to the degradation of the dimethylsiloxy groups bound to silica. Two different pathways of the degradation can be considered: (1) thermal desorption of the DMS species from the surface as a whole and (2) combustion of the methyl groups yielding CO2 and H2O, leaving the silicon atom on the surface. These two mechanisms predict quite different weight losses for the samples. For instance, for a bonding density of 2.25 group/nm2, the desorption mechanism predicts a weight loss of ∼7%, whereas the weight loss due to combustion of methyl groups is only 1.1%. The weight loss observed is 0.9%, suggesting that combustion rather than thermodesorption is the principal mechanism for the degradation of this monolayer. DTGA curves for the surfaces prepared from R,ω-dichloro-poly(dimethylsiloxanes) show two peaks at ∼530-550 and ∼660 °C suggesting two different processes upon the decomposition of the monolayers. From the literature28 on thermal properties of poly(dimethylsiloxanes), it is known that their degradation proceeds by breaking of Si-O-Si bonds, which are more susceptible to thermal degradation than (28) Andrianov, K. A., et al. Vysokomol. Soedin. 1969, 11A, 2030. Howard Thomas, T.; Kendrick, T. C. J. Polymer Sci., Part A-2 1969, 7, 537. Clarson, S. J.; Semylen, J. A. Polymer 1986, 27, 91.

Oligo(dimethylsiloxane) Monolayers on Silica

Si-C bonds. This leads us to propose that the weight loss at 530-550 °C is due to breaking of siloxane bonds in the bonded dimethylsiloxane loops (followed by the formation of volatile cyclic or linear methylsiloxanes). The peak at 660 °C can be ascribed to the process of the decomposition of the DMS species directly bonded to silica. To test this hypothesis, we studied the thermal decomposition of methyl-terminated poly(dimethylsiloxane) adsorbed on silica. No covalent bonds with the surface can be formed in this case, and no 660 °C peak was observed in the DTGA. We also note that the area under the differential TGA peak at 530 °C (i.e., weight loss at this temperature) increases with n in Cl[Si(CH3)2O]n-Si(CH3)2Cl, while the area under the 660 °C peak remains approximately constant for all of the samples studied. This is consistent with the assignment of the 660 °C peak to the species chemically bonded to silica. The two different thermal decomposition processes of the monolayers, that is, combustion and thermally activated depolymerization, were further investigated by comparing TGA results in air and in nitrogen. DTGA curves in nitrogen show that the 530-550 °C peaks remain almost unchanged as compared to air, while the 660 °C peak is not present (more likely, it shifts to temperatures above 800 °C), indicating that the high-temperature process is due to oxidation on the surface. We point out that the decomposition temperatures of ∼530-660 °C in air are ∼100-150 °C higher than the decomposition temperatures reported for poly(dimethylsiloxanes).28 Such a remarkable stability of the dimethylsiloxane monolayers can be explained by the stabilization with the silica surface, which can abstract the excessive thermal energy from the grafted molecules and distribute it in the matrix. Thermal degradation of the “oligomeric dimethylsiloxane surfaces” (Figure 5) substantially differs from that of the monomeric ones. A new strong peak appears in the DTGA thermograms at ∼400-450 °C. The peak at 660 °C, which is identical to those observed for the monomeric surfaces, is also present. According to many studies,25 thermal degradation (weight loss) for poly(dimethylsiloxane) starts at ∼350 °C and attains a maximum rate at ∼400-500 °C. Our own measurement for the 24K poly(dimethylsiloxane) gave TMAX(DTGA) ) 425 °C. TMAX(DTGA) ) 430 °C was obtained for poly(dimethylsiloxane) adsorbed on silica-100. This suggests that the 400-450 °C peak corresponds to a degradation of the PDMS chains grafted to silica. The position of this peak is unchanged for the TGA carried out in nitrogen, arguing that, like PDMS polymers,28 depolymerization is the principal mechanism of thermal degradation of oligo(dimethylsiloxane)-silicas. As was the case for monomeric surfaces, the DTGA peak at 660 °C disappears if the analysis is done in nitrogen. This is consistent with the assignment of this peak to the oxidative decomposition of dimethylsiloxane species. The temperature corresponding to the maximum weight loss rate (not the weight loss itself) for surfaces with bonding densities greater than 3 group/nm2 is nearly constant and approaches ∼430 °C. This temperature is consistent with the average temperature of the maximum weight loss rate for PDMS polymers (dashed line in Figure 6). For the surfaces with bonding densities lower than 3 group/nm2, TMAX(DTGA) shifts toward higher temperatures, indicating that short PDMS chains are more thermally stable than the long ones. Conclusions. Monomeric and oligomeric dimethylsiloxane surfaces can be reproducibly prepared by the

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Figure 6. Temperature of maximal weight loss rate for silica that had been reacted with Cl[Si(CH3)2O]n-Si(CH3)2Cl plotted vs bonding density of the dimethylsiloxy groups. The dashed line (430 °C) is drawn at the temperature of the maximal weight loss for poly(dimethylsiloxane) (MW 24 000) adsorbed on silica.

Figure 7. TGA and differential TGA graphs for silicas modified with oligo(dimethylsiloxane) (solid lines) and octadecyldimethylchlorosilane (dashed lines).

reaction of dichloro-dimethylsilane and low molecular weight R,ω-dichloro-oligo(dimethylsiloxanes) with silica. According to FTIR and chemical analysis, Cl[Si(CH3)2O]nSi(CH3)2Cl reacts with dehydrated silicas covalently yielding monolayers of dimethylsiloxane attached to the surface via two SiS-O-Si bonds per molecule. Areas per molecule in these monolayers are close to the crosssectional areas of the molecules used for the reaction suggesting horizontal orientation of the grafted oligo(dimethylsiloxane) molecules on the surface. The reactions of Cl[Si(CH3)2O]n-Si(CH3)2Cl with wet silica are controlled by the amount of preadsorbed water in silica, providing an easy way of manipulating the surface bonding process. By varying the amount of adsorbed water from 0.1 to 22 wt %, one can prepare a series of oligomeric dimethylsiloxane surfaces with bonding densities ranging from 2.25 to 11.4 [Si(CH3)2O] group/nm2. According to the in situ FTIR studies, the reactions of Cl[Si(CH3)2O]nSi(CH3)2Cl with wet silica proceed through hydrolysis and polycondensation yielding linear OH-terminated oligomeric dimethylsiloxane. These oligomers react with the silica surface forming SiS-O-Si bonds yielding covalently attached oligomeric dimethylsiloxane surfaces. The excess poly(dimethylsiloxane) is physisorbed on the surface and is removed after rinsing with solvents.

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According to TGA, dimethylsiloxane surfaces have superior thermal and oxidative stability. Oligomeric surfaces show no weight loss until 350-380 °C. Monomeric surfaces are even more stable and show no weight loss until 550-650 °C in air. In contrast (Figure 7), covalently attached monolayers of alkylsilanes or fluoroalkylsilanes prepared on the same silica surfaces begin to lose weight at ∼170-190 °C, that is, several hundred degrees lower. The ease of preparation and the control of bonding density, accompanied by excellent thermal stability, make

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oligo(dimethylsiloxane) monolayers a superior alternative to conventional low-energy surfaces constituted of alkyl or fluoroalkyl groups. Acknowledgment. The authors acknowledge the use of the TA instrument supported by a grant from the New Jersey Commission on High Education. We also thank Phenomenex, Inc. for the samples of silica gels. LA011491J