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Packing and Thermal Stability of Polyoctadecylsiloxane Compared with Octadecylsilane Monolayers Rongwei Wang,† George Baran,‡ and Stephanie L. Wunder†,* Department of Chemistry and College of Engineering, Temple University, Philadelphia, Pennsylvania 19122 Received February 14, 2000. In Final Form: May 3, 2000 FTIR, Raman spectroscopy, and thermogravimetric analysis (TGA) were used to compare the structure and thermal properties of polyoctadecylsiloxane (POS) and octadecyl trichlorosilane (OTS) monolayers. The octadecyl chains in POS had higher conformational and intermolecular order than the same chains of OTS adsorbed as self-assembled monolayers (SAMs) on 106 nm hydrated silica beads. The latter were identical to the structures observed for OTS SAMs on superhydrated fumed silica that had primary particle sizes an order of magnitude smaller than the 106 nm beads, indicating that curvature did not affect the chain packing. The chains on POS were also more thermally stable to conformational and intermolecular disorder than the OTS chains, and this was attributed to increased free volume in the latter case. The differences in structure were partially determined by steric restrictions that arise because the Si-O-Si bond distance is less than the van der Waals radii of the alkyl chains. In POS, the most likely structure is therefore one in which the octadecyl chains are pointing in the same direction from every other Si atom, permitting good lateral chain packing. In the OTS SAMs, the octadecyl chains must all point in the same direction, away from the silica surface. Therefore, linear chains would be excluded but small clusters of dimers or trimers could be accommodated. These restraints increase the nearest neighbor distance between alkyl chains on SAMs and do not permit them to be as closely packed as for POS. This in turn contributes to the increased irreversible disordering of the chains with temperature.
Introduction Self-assembled monolayers (SAMs) have been of significant interest in applications such as electronics,1 microsensors,2 nonlinear optics,3 biological interfaces,4-6 and catalysis.7 SAMs are formed by spontaneous adsorption of molecules from solution onto a solid substrate. As the most common alkylchlorosilane for the preparation of SAMs, OTS adsorbed on a variety of substrates has been extensively studied.8-10 Several factors may affect the quality of SAMs on the surface of substrates, including their temperature of formation11 and subsequent curing processes,12,13 namely treatment at elevated temperatures. Well-ordered structures form only below a critical temperature, Tc, which is 28 ( 5 °C for OTS. In many applications, for example when silane treatments are used for chromatographic columns, the SAMs can be subjected to further thermal cycling. It is thus of some importance to investigate the thermal stability of SAMs. For comparison, the thermal stability of the three-dimensional structure formed from the hydrolysis of OTS, namely POS, is also of interest, since its organization is believed to be similar to that of well-ordered SAMs of OTS.14 * To whom correspondence should be addressed. † Department of Chemistry. ‡ College of Engineering. (1) Sugi, M. J. Mol. Electron. 1985, 1, 2. (2) Yongan, Y.; Bein, T. J. Phys. Chem. 1992, 96, 9387. (3) Kuriyama, K.; Oisi, Y.; Kajiyama, T. Rep. Prog. Polym. Phys. Jpn. 1994, 37, 553. (4) Uchida, M.; Tanizaki, T.; Oda, T.; Kajiyama, T. Macromolecules 1991, 24, 3238. (5) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (6) Dulcey, C. S.; et al. Science 1991, 252, 551. (7) Tundo, P. J. Chem. Soc., Chem. Commun. 1977, 18, 641. (8) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (9) Thompson, W. R.; Pemberton, J. E. Langmuir 1995, 11, 1720. (10) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (11) Flinn, D. H.; Guzonas, D. A.; Yoon, R. H. Colloids Surf. 1994, 87, 163. (12) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (13) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (14) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135.
Recently, steric effects on the condensation of silanols for OTS molecules have been discussed.15 When OTS molecules attach to a SiO2 substrate with SiOH groups, the OTS molecules need to be further apart than the van der Waals radii of the C-H backbone of the alkyl chains. Therefore, OTS cannot attach to SiOH groups connected to each other by only one oxygen atom, since the Si-O-Si bond distance is too short to accommodate all-trans alkyl chains due to repulsive interactions between adjacent chains. For the same reason, it is not possible for a linear backbone structure of -[Si(R,OH)-O-Si (R,OH)]n-, where R is CH3(CH2)17 , to have an all-trans octadecyl chain of OTS if adjacent chains are pointing in the same direction. Instead, the structure can accommodate the all-trans alkyl chain as a bilayer only if adjacent chains are pointing in opposite directions. These results suggest that the packing of alkyl chains in POS may be different than for OTS adsorbed on a substrate, and that these differences may affect the thermal stability of the structures formed. In our previous study,16 Raman spectroscopy was used to investigate the thermal behavior of POS below 100 °C. The results showed that at room temperature POS formed an hexagonal close packed structure with a DSC melting point at 63 °C, and that the intrachain and lateral packing structure of the alkane chains was reversible below 100 °C. Comparison of the Raman spectra of POS with wellhydrated fumed silica of 7 nm [Aerosil 380 (A380)] and 40 nm [Aerosil OX5O (OX50)] primary particle size indicated that both the lateral packing and conformational order of the alkyl chains were greater for POS. However, differences in the packing could have originated from curvature effects due to the small particle size, or incomplete coverage due to aggregation/agglomeration of the fumed silica.17 In this study, we confirm that the packing and order of the alkyl chains in POS are greater than for OTS adsorbed on larger, 106 nm, unaggregated/agglomerated silica with high (93%) surface coverage. The latter has a packing (15) Stevens, M. J. Langmuir 1999, 15, 2773. (16) Wang, R.; Guo, J.; Baran, G.; Wunder, S. L. Langmuir 2000, 16, 568. (17) Wang, R.; Wunder, S. L. Langmuir 2000, 16, 5008.
10.1021/la000206d CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000
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Figure 1. TGA plot of POS.
order comparable to that on well-hydrated fumed silica with smaller primary particle size and higher curvature, suggesting that steric factors affect the morphology of the alkyl chains. TGA, FTIR, and Raman spectroscopy are used to investigate the structure and thermal stability of POS and OTS monolayers. Experimental Section Materials and Sample Preparation. OTS (>95% purity) and anhydrous pentane were obtained from Aldrich and used as received. POS was directly made from polymerization of OTS. OTS is a water-sensitive compound that can hydrolyze and further polymerize in air to form a three-dimensional network. OTS was dropped with a pipet onto a glass slide or into a small vial and it spontaneously polymerized in air to form a white solid polymer. The 106 nm silica beads with a polydispersity index of 0.158 were obtained from micro caps (now called micromod Partikeltechnologie), and were prepared from tetraethylsilicate using a modified Sto¨ber process.18 Before silanation, the beads were treated with 1 M H2SO4 for 1 h and washed in doubly distilled water. This was accomplished by immersing the beads in excess water, centrifuging at 80 000 rpm, and repeating this process 6×. The beads were dried under reduced pressure at room temperature for 40 h and then stored at room temperature. More than a 10-fold excess of OTS was added to a given weight of silica in anhydrous pentane, and the mixture was stirred for 3 h. The beads were again centrifuged (3000 rpm) and washed/centrifuged 6× with anhydrous pentane. Raman Spectroscopy. Raman spectra were collected using a computer-controlled double monochromator (Spex 1403) with 1800 groove/mm gratings and a thermoelectrically cooled photomultiplier tube. The samples were excited with ∼30 mW of the 514.5 nm line of an argon-ion laser. All of the spectra were collected in a backscattering geometry with a 80× microscope objective, in 2 cm-1 steps with 1 s integration interval and 5 cm-1 resolution, and signals from more than 20 scans were averaged. FTIR Spectroscopy. FTIR spectra were recorded with 10 scans and a resolution of 1 or 0.5 cm-1 using a Mattson Research Series FTIR spectrometer. The sample compartment was purged with dry air and spectra were referenced to a background dry air spectrum. Measurements were performed at room temperature with KBr pellets. (18) Kaiser, C.; Hanson, M.; Giesche, H.; Kinkel, J.; Unger, K. K. NATO ASI Ser., Ser. 3 1996, 12, 71-84.
Thermogravimetric Analysis (TGA). TGA measurements were performed on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer using a ramp rate of 10 °C/min. Samples were held at 100 °C for 0.5 h and the temperature then increased at a rate of 10 °C/min up to 800 °C.
Results and Interpretation TGA Analysis. The TGA plot of POS in Figure 1 demonstrates that there are two observable weight loss regions, which are shown more clearly in two insets of the derivative curves of the TGA plot: one is located at ∼174 °C; the other at ∼484 °C. The first region has been assigned to the loss of physically bound water or water from condensation reactions between unreacted SiOH groups.14 In the present case the POS was prepared by directly exposing OTS to air; the sample was held at 100 °C for 0.5 h in the TGA in order to get rid of physically entrapped water and any residual HCl gas. Therefore the weight loss at 174 °C can be attributed only to condensation between silanols. The second region can be assigned to the degradation of the alkyl chain moiety. The weight losses are 0.97% and 79.4% for the first and second regions, respectively. The 19.6% residual weight is attributed to the SiOx moiety. Although the weight loss of 79.4% very closely matches the theoretical value of 80.6% expected for the C18H37 component of a linear -[CH3(CH2)17]Si(OH)O- (molecular weight of 314) chain, TGA analysis would not be sensitive to the presence or amount of threedimensional cross-link sites, i.e., siloxane, (R)Si(OSi)3 units. TGA plots for 106 nm silica beads and for OTS adsorbed on the 106 nm silica beads are shown in Figure 2a and b, respectively, where two weight loss regions are observed. In the case of the pure silica, the weight loss below 150 °C is attributed to the removal of physically adsorbed water, as discussed previously,16 and corresponds to a surface water coverage of 60 H2O/nm2. To cover a surface with water so that it has the same density as bulk water requires approximately 10 H2O/nm2. A layer of liquidlike water is thus adsorbed on the silica surface. Above 150 °C, dehydroxylation on the surface and interior of the silica contribute to the remaining weight loss. After silanation with OTS, there are again two weight loss regions. Below
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Figure 2. TGA plots of 106 nm silica beads (a) before and (b) after adsorption of OTS.
Figure 3. Room-temperature Raman spectra of POS after thermal treatment: (a) initial sample; (b) after 150 °C for 1 h; (c) after 200 °C for 1 h.
150 °C dehydration occurs. The existence of significant amounts of adsorbed water (similar to that observed for the unsilanated silica) even after silanation is consistent with previous literature reports.13,19 Above 150 °C, weight loss arises from dehydroxylation, as well as from pyrolysis of the octadecyl groups. Subtraction of the former contributions, normalized for the amount of silica, results in 93% coverage of the silica surface with OTS molecules. This calculation assumes that one OTS occupies 20 Å2 and that the density of the silica is 1.96 g/cm3, although it increases to 2.2 g/cm3 after heat treatment at 800 °C. Raman Spectroscopy. The Raman spectra of POS as a function of thermal treatment are given in Figure 3 and the assignments of the observed frequencies to vibrational modes are listed in Table 1. Figure 3a is the spectrum of (19) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647.
POS as first prepared, and is identical to that previously reported.14 The two strong bands at 1060 and 1126 cm-1 are characteristic of all-trans conformations of the carbon skeleton, and the weaker band at 1094 cm-1 occurs when a gauche conformation occurs in a predominantly all-trans chain, usually at the end. In the case of a liquid alkane there is a broad band centered at 1080 cm-1, indicative of random gauche conformational isomers. The CH2 twisting mode at 1297 cm-1 is narrow and at lower frequency than observed in the melt phase. The doublet vibrations at 1438 and 1459 cm-1 are due to the CH2 scissoring vibration and the CH3 antisymmetric bending mode, respectively. The 1459 cm-1 band has also been attributed to Fermi resonance between the scissoring vibrations with overtones of the 720 cm-1 CH2 mode, when the alkyl chain adopts an all-trans configuration. In the melt, this band is a broadened singlet. The extremely weak
Packing and Stability of POS and OTS Monolayers Table 1. Observed Frequencies for POS and Vibrational Assignments IR frequency, cm-1
Raman frequency, cm-1
1060 1098 1128 1170-1380 (1178.3, 1198.5, 1217.8, 1236.6, 1255.9, 1274.2) 1468 1628 2850 2919 2956 3434 a
assignment CH2 rocking (T)a
721 889 1029 1123
1294 1436 1456 2846 2882
Si-O-Si antisymmetric stretching Si-O-Si antisymmetric stretching C-C stretching (T) CH2 twist (T) C-C stretching (T) Coupled CH2 wag (T)
CH2 twisting CH2 scissoring CH3 antisymmetric bending bending mode from water CH2 symmetric stretching, d+ (T) CH2 antisymmetric stretching, d- (T) CH3 asymmetric stretching, rOH stretching from water
T stands for trans conformation of an alkyl chain.
vibration at 1418 cm-1 may arise from some orthorhombic packing of the alkyl chains.20 The symmetric, d+, and antisymmetric, d-, CH2 stretching modes are located at 2844 and 2878 cm-1, with the latter band the sharpest feature in the spectrum. In the octadecane spectrum, there is a splitting of the symmetric stretch into two peaks, one at 2844 cm-1 and one at 2856 cm-1, characteristic of the crystal field splitting of even numbered alkane chains that crystallize in a triclinic lattice.21 A weak feature is observed at 2856 cm-1 in the POS spectrum. The location and intensity distribution of the symmetric stretching mode of alkanes is complicated by Fermi resonance interactions with overtones of the CH2 bending vibrations, and intermolecular crystal effects, while the antisymmetric stretching mode is affected by coupling to the torsional and rotational motions of the chain. For alkanes or polyethylene in an all-trans conformation, the symmetric stretch mode appears as three bands, the sharpest at 2844 cm-1, and two broader bands centered at 2900 and 2930 cm-1, arising from Fermi resonance interactions with overtones of the bending modes. The antisymmetric stretching mode at 2878 cm-1 thus sits above the broad 2900 cm-1 mode. Crystal structure can affect the bandwidth of this vibration; in particular, the hexagonal crystal has a broadened d- mode, as do straight chain alkanes in a clatherate cage. The weak band at 2960 cm-1 is due to the terminal methyl vibrations. The POS spectrum is thus most similar to the hexagonal packing arrangement found in the premelt “rotator” phase, although the splitting of the d+ mode gives some indication for triclinic packing. The Raman spectra for POS heat-treated for 1 h at two temperatures 150 and 200 °C, during which additional silanol condensation occurs, is given in Figure 3b and c, respectively. Only small changes are observed in the Raman spectra. There is an increase in the intensity of the 1090 cm-1 band, indicating a slight increase in gauche conformational isomers. A slight increase in intensity at the high-frequency side of the CH2 twisting mode at 1297 cm-1 also indicates the presence of disordered structures. In the CH2 stretching region, there is a decrease in the Raman intensity ratio of the antisymmetric vs symmetric (20) Strobl, G. R.; Hagedorn, W. J. Polym. Sci., Part B: Polym. Phys. 1978, 16, 1181. (21) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395.
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stretching modes. A more detailed analysis shows that the frequency of the CH2 antisymmetric stretching mode remains the same, but that there is a small shift in the intensity distribution of the symmetric stretching mode to higher frequencies, also indicating a slight increase in gauche conformational isomers. The Raman spectra of OTS at 93% coverage adsorbed on the hydrated 106 nm silica beads as a function of thermal treatment are shown in Figure 4. The as prepared 106 nm beads have spectra (Figure 4a) identical to those observed for superhydrated A380 and OX50. There is increased intensity at 1090 cm-1 compared with POS (Figure 3a) indicating increased numbers of gauche conformational isomers. In the CH2 stretching region, there is a decrease in the Raman intensity ratio of the antisymmetric vs symmetric stretching modes, also indicative of decreased lateral packing of the alkyl chains for OTS. The spectrum appears similar to the 200 °C heattreated sample of POS. After thermal treatment at 150 and 200 °C there is an increase in the 1090 cm-1 band and a decrease in the Raman intensity ratio of the antisymmetric vs symmetric CH2 stretching modes, both indicative of increased conformational and packing disordering. However, although this disorder is greater than for POS after thermal treatment, the alkyl chains never approach the degree of disorder observed for alkane melts and for OTS adsorbed on hydrophobic, low surface coverage fumed silica.16 In the case of superhydrated A380, with approximately 63% OTS coverage, where the chains are initially ordered as for the 106 nm beads, thermal treatment at elevated temperature produces considerable conformational and lateral disordering of the alkyl chains.22 FTIR Spectroscopy. Figure 5 shows the IR spectrum of POS and the assignments of the observed frequencies to vibrational modes are listed in Table 1. The frequencies of the symmetric (d+) and antisymmetric (d-) CH2 stretching modes, presented in Table 2, indicate that high populations of the alkyl chains possess trans conformations since the CH2 stretching modes from all-trans crystalline n-alkanes are located at similar frequencies.23 Peak positions for d+ and d- range from 2846 to 2850 and 29152918 cm-1, respectively, for all-trans alkanes and at ∼2856 and ∼2928 cm-1, respectively, for disordered chains. The CH2 scissoring deformation mode at 1468 cm-1 appears to be a sharp singlet band with a full-width at halfmaximum (fwhm) of 10 cm-1, which is significantly smaller than ∼18-25 cm-1 fwhm values14 for disordered alkyl chains. This implies that there are highly ordered alltrans chains and only one type of alkyl chain per unit subcell24 in POS. The lowest band that appears at 721 cm-1 is attributed to the CH2 rocking mode. The singlet peak at this frequency with a fwhm of 10 cm-1 indicates that the crystal structure of the alkyl chains is a triclinic or hexagonal structure.25 The inset in Figure 5 shows the band progressions of coupled CH2 wag modes between 1150 and 1350 cm-1, which usually appear in the spectra of solid alkyl compounds with all-trans conformational sequences.26 Liquid alkanes possess a broad envelope in this spectral region. The number and interval of the wag modes are directly related to the length of the all-trans conforma(22) Wang, R. Ph.D. Dissertation, Temple University, 2000. (23) Snyder, R. G.; Schatschneider, J. H. Spectrochim. Acta, Part A 1963, 19, 85. (24) Casal, H. L.; Mantsch, H. H.; Cameron, D. G.; Snyder, R. G. J. Chem. Phys. 1982, 77, 2825. (25) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 1161. (26) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.
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Figure 4. Room-temperature Raman spectra of OTS adsorbed on 106 nm silica beads as a function of thermal treatment: (a) initial sample; (b) after 150 °C for 1 h; (c) after 200 °C for 1 h.
Figure 5. FTIR spectrum of POS, 0.5 cm-1 resolution. Table 2. Effect of Curing Temperature on IR CH2 Stretching Bands of POS and OTS Adsorbed on Hydrated Silica Beads OTS on hydrated silica beads
POS temperature, °C
d+ cm-1
d- cm-1
d+ cm-1
d- cm-1
no thermal treatment 150 200
2850 2850 2851
2919 2919 2921
2851 2853 2853
2919 2923 2923
a d+: CH symmetric stretching mode; d-: CH antisymmetric 2 2 stretching mode.
tional sequences.23 In the case of an octadecyl chain on a silicon atom, CH3(CH2)17Si-, the frequency interval ∆ν for an all-trans chain is given by the following formula:27
∆ν ) 326/(n + 1)
(1)
For a fully extended octadecyl chain, a 17-wag-mode
progression is expected. Only six wag modes can be clearly observed due to the broad tail of the antisymmetric SiO-Si stretching vibration. Based on this number of modes, the average value of ∆ν was calculated to be 19.2 cm-1. Using the above equation, the value n is determined to be 16, which is one less than the expected value. This result is consistent with the Raman data that indicates that the alkyl chains have a gauche defect, probably at their ends. The intensity and band shape of the coupled CH2 wag modes in the IR spectrum (not shown here) of 150 °Ctreated POS is quite similar to that in Figure 5. The IR spectrum of POS cured at 200 °C for 1 h is shown in Figure 6. The IR intensities of the coupled CH2 wag modes evidently decreased, but the number of CH2 units in an all-trans sequence (i.e., n in Equation 1) is 16 using an average value of ∆ν ) 19.4. As pointed out by Parikh et (27) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta, Part A 1963, 19, 117.
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Figure 6. FTIR spectrum of POS after thermal treatment at 200 °C for 1 h.
al.,28 analyzing this wag-twist feature only indicates that there are alkyl chains in POS that are well organized and exhibit an all-trans conformation. It is not possible to determine the relative population of these ordered chains in the whole system. The frequencies of the d- and d+ modes at 150 and 200 °C are presented in Table 2, and show that while thermal treatment at 150 °C did not cause band shifts in these vibrations, treatment at 200 °C did cause shifts toward higher wavenumber in the CH2 stretching modes. These shifts are not dramatic and most of the alkyl chains still possess their all-trans conformation as compared to the stretching bands (2856, 2928 cm-1) for liquid polymethylene chain.29 After the 200 °C thermal treatment, the OH stretching band at 3425 cm-1 increased in intensity and a new band at 1712 cm-1 was formed. One possible explanation for the latter was the oxidization of the alkyl chains of POS forming carboxylic acid. Moreover, the bending mode of water at 1635 cm-1 still exists (at room temperature) after the thermal treatment. This can be attributed to the adsorption and hydrogen-bonding of water with the hydroxyl groups of the carboxylic acids and the silanols that did not condense. The 150 °C treatment did not create a new band at 1712 cm-1, but the OH stretch and bending modes of water were observed. As discussed by Parikh, the wagging modes for OTS adsorbed on silica are obscured by the Si-O modes. The frequencies of the d- and d+ modes at 150 and 200 °C are presented in Table 2, and also indicate that both treatments caused shifts toward higher wavenumber in the CH2 stretching modes. Thus, the thermal stability of OTS SAMs to conformational and lateral disordering on the 106 nm hydrated silica beads is lower than that for POS. Discussion It is well-known that OTS molecules are spontaneously hydrolyzed either in a moist atmosphere or in water to (28) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. J. Phys. Chem. 1995, 99, 9996. (29) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
form a polymer. In the present study, the TGA plots of POS and the FTIR and Raman spectra of POS are in agreement with the previous results obtained by Parikh et al.,14 although different methods for preparation of POS were used. Their results14 indicate that when initially prepared at room temperature the cross-linking of the SiOx network of POS is incomplete. The majority of Si atoms exhibit two cross-links which result in extended linear siloxane chains and/or rings. Therefore, it is reasonable to assume that POS has the repeat structural unit [CH3(CH2)17]Si(OH)(OSi)2, with occasional [CH3(CH2)17]Si(OSi)3 cross-link sites, since the material cannot be dissolved. Both Raman and FTIR evidence suggest that the octadecyl chains are in a predominantly trans configuration. To account for their data, Parikh et al. proposed a multilayer structure consisting of alternating crosspolymerized monolayers with highly trans C18H37 chains. However, these monolayers are based on the structures that cannot occur due to steric limitations. Recently, after examining the possible packing of SAMs for alkyltrichlorosilanes and alkylmonochlorosilanes, Stevens15 pointed out that cross-polymerization of OTS, that is elimination of water between adjacent OTS molecules, possibly occurs for bilayers and multilayers, but is not possible for a monolayer at full surface coverage. This arises because the length of the Si-O bond is 1.6 Å,30 so that the distance between two Si atoms for Si-O-Si is less than 3.2 Å. Accordingly, steric constraints from van der Waals diameters of C and its bonded H (i.e., 3.5 and 2.5 Å, respectively) restrict the alkyl chains on these Si atoms from orienting in a parallel way or even possessing an all-trans conformation. In the case of the bilayer, Stevens suggested that half of the hexagon sites have chains pointing upward while the other half have chains pointing downward. Combining this information with the results of the present investigation as well as those of Parikh et al., a more likely structure for POS is shown in Figure 7. The octadecyl chains are pointing in the same direction from every other Si, rather than from (30) Sarnthein, J.; Pasquarello, A.; Car, R. Phys. Rev. Lett. 1995, 74, 4682.
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Figure 7. Schematic representation of the structure of POS: the Si-O-Si backbone is located in the plane.
Si separated by a single O, and there is a single Si-O-Si backbone in the same plane, rather than two back to back structures. For OTS adsorbed on silica surfaces this staggered packing arrangement is not possible, and the octadecyl chains must all point in the same direction, away from the silica surface. The Raman spectra observed for OTS on fully hydrated 106 nm silica beads indicate decreased conformational order and packing of the alkyl chains compared with POS. Furthermore, the Raman spectra for the fully hydrated 106 nm silica beads are the same as previously reported for both A380 (7 nm primary particle size) and OX50 (40 nm primary particle size) hydrated fumed silica. The similarity in conformation and packing for OTS on hydrated surfaces occurs despite many differences between them. The radius of curvature is significantly decreased for the 106-nm beads compared with the 7-nm particles. For silica prepared using the Sto¨ber’s method,31 the density of SiOH groups is high, about 12-14 SiOH/nm2,32 compared with fumed silica particles that have approximately 5 SiOH/nm2. There is increased water adsorption for the silica beads compared with fumed silica. Last, surface coverage of OTS ranged between 63% (A380) and 93% (106 nm silica beads), although the lower surface coverage for fumed silicas partially results from their aggregation/agglomeration (31) Sto¨ber, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62. (32) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695.
that decreases the effective surface area of the particles. These results suggest that the optimal arrangement of the octadecyl chains on silica surfaces is determined by the geometric and steric constraints cited above. Linear chains formed by condensation of adjacent OTS molecules with the octadecyl groups pointing away from the surface would be excluded, but small clusters of dimers or trimers could be accommodated. These restraints increase the nearest neighbor distance between alkyl chains and do not permit them to be as closely packed as for POS. The effects of thermal treatment of POS have shown that below 100 °C the intrachain order and lateral packing of the chains are reversible.16 Above this temperature, weight loss due to condensation between silanols occurs, with a TGA maximum at 174 °C. After thermal treatment at 150 °C, Raman and FTIR data show that there is not significant loss of intrachain order, but there is a slight decrease in the lateral packing order. After thermal treatment at 200 °C, both FTIR and Raman data show a slight increase in gauche conformational isomers. The Raman data is still consistent with a crystalline structure similar to that for the nonheat-treated samples, namely hexagonal or triclinic. The antisymmetric stretch is at the same frequency, and the crystal field splitting of the triclinic lattice at 2856 cm-1 is still observed. The intensity distribution of the symmetric stretch, which is affected by Fermi resonance interactions, is shifted to higher wavenumbers, as is expected for chains with gauche confor-
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Figure 8. Schematic representation of the structure of POS showing lateral packing. Two adjacent chains are in planes offset by about 1 Å.
mational isomers. These results suggest that the POS chains after thermal treatment become semicrystalline, with a small amorphous fraction in the predominantly crystalline material. The results from the TGA analysis indicate that the alkane chains are 79.4% of the total weight of the polymer, compared with 80.6% expected for a linear chain, and 83% for a completely cross-linked structure. This suggests that there are more terminal silanols, i.e., shorter chains, than in the POS prepared in solution. Parikh et al.14 using 29 Si single-pulse magic angle spinning nuclear magnetic resonance (MAS NMR) showed in fact that there were approximately 1.3% (HO)2(R)Si(OSi) [geminal silanol], 24.7% (R)Si(OSi)3 [siloxane], and 74.1% (HO)(R)Si(OSi)2 [isolated silanol] alkylsiloxane structures in solution prepared POS; the latter two are three-dimensional crosslink sites and linear chains, respectively. Their data suggested that the molar mass of the repeat unit in POS was 319. In the current investigation, the weight loss of ∼1% between 100 and 200 °C was attributed to elimination of water during condensation of silanols. These could either be cross-linking reactions or condensation of adjacent silanols to form longer linear chains. Using the data of Parikh, a theoretical weight loss of ∼2% is expected for a completely cross-linked network from the remaining 74.1% mole percentage of Si atoms (from the as prepared sample) which contain OH groups. Approximately ∼50% of the remaining silanols thus participate in subsequent condensation reactions. Although the alkane chains of POS melt at a temperature of 63 °C,16 the mobility of the silanols is restricted once some cross-linking of the siloxane backbone occurs, preventing the formation of a completely cross-linked network. The results from the Raman spectra of POS after the curing processes at 150 and 200 °C indicate that most of the alkyl chains are still well oriented and possess predominantly all-trans structures. The loss of some alltrans conformational order after the 200 °C treatment could be partially attributed to degradation of the alkyl chains, as evidenced by the appearance of the carbonyl group. The stability of POS with thermal treatment thus supports the structure shown in Figure 7. Siloxane linkages (3D cross-link sites) can occur between adjacent linear chains. As shown in Figure 8, the silanols from adjacent chains can condense without disruption of the octadecyl packing. Assuming tetrahedral bonding of Si, the planes of the two adjacent Si-O-Si chains are displaced vertically by about 1 Å. As shown in the figure, there is a difference in the tilt angle of alkyl moieties pointing in the same direction from adjacent Si-O-Si chains. It is possible that the gauche defects noted in the Raman
spectra of POS correct the different tilt angles and thus bring neighboring chains into crystallographic register. By comparison with POS, the OTS attached to the 106 nm particles shows poorer thermal stability, with increased disorder observed in the IR and Raman spectra after heat treatment above 150 °C. This irreversible disordering may be the result of the increased free volume available to the chains as the result of increased distance between the alkyl chains of OTS, as well as from the available free area (∼7%). Conclusions Using FTIR and Raman spectroscopy, it was found that POS has higher thermal stability from the point of view of chain conformation compared with OTS SAMs on 106 nm hydrated silica beads. These results were based upon the conformational order of the alkyl chains after thermal treatments at 150 and 200 °C. The difference in thermal stability was attributed to differences in the structures between POS and OTS adsorbed on surfaces. In POS, the octadecyl chains are pointing in the same direction from every other Si atom in -(Si-O-Si)- and there is a single Si-O-Si backbone (not a back-to-back structure) in the same plane as schematically depicted in Figures 7 and 8. Thermal curing results in condensation of silanols between adjacent chains in the same plane. This can occur without disruption of the packing of the alkyl chains. In the OTS SAMs, the octadecyl chains must all point in the same direction, away from the silica surface. In this case, steric constraints, namely, van der Waals repulsion between the alkyl chains if they are attached to adjacent silicon atoms in a -(Si-O-Si)- linkage (whose distance is 3.2 Å), must be considered. Linear chains with the octadecyl groups pointing away from the surface would be excluded, but small clusters of dimers or trimers could be accommodated in OTS SAMs. These restraints increase the nearest neighbor distance between alkyl chains and do not permit them to be as closely packed as for POS. The Raman spectra for the OTS SAMs indicate decreased conformational order and packing of the alkyl chains compared with those of POS. Furthermore, these Raman spectra are the same as those previously reported for OTS SAMs on superhydrated fumed silica (7 and 40 nm primary particle size), indicating that steric restraints and not the curvature differences between the particles determine the morphology of the chains. The decreased conformational stability of the OTS SAMs results from the increased free volume available to the chains. This is due to the increased distance between the chains as well as to the fact that ∼7% of the surface is not silanated. LA000206D
