Probing the Structure of Organosilane Films by Solvent Swelling and

ReceiVed: July 9, 2001; In Final Form: December 13, 2001. Thin films of organosilanes have great technological importance for adhesion promotion, dura...
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J. Phys. Chem. B 2002, 106, 2474-2481

Probing the Structure of Organosilane Films by Solvent Swelling and Neutron and X-ray Reflection H. Yim, M. S. Kent,* and J. S. Hall Department 1811, Sandia National Laboratories, Albuquerque, New Mexico 87185

J. J. Benkoski and E. J. Kramer Department of Materials, UCSB, Santa Barbara, California 93106 ReceiVed: July 9, 2001; In Final Form: December 13, 2001

Thin films of organosilanes have great technological importance for adhesion promotion, durability, and corrosion resistance. Although the cross-link density profile within such films is likely to strongly affect their performance, no reliable assay has been available to characterize this distribution. In this work we use solvent swelling combined with neutron and X-ray reflection to study the cross-link density distribution within ultrathin films (50-100 Å) of (3-glycidoxypropyl)trimethoxysilane (GPS) and bis(triethoxysilyl)ethane (BTSE) on silicon. The films are swelled with vapors of the good solvent nitrobenzene (NB). For GPS films, the solvent concentration profile has a strong maximum in the central region of the film. We conclude from this that the GPS films possess a high cross-link density near the silicon surface and a much lower cross-link density in the bulk of the film. The decreased solvent concentration at the air surface is more difficult to interpret. It appears to also indicate an elevated cross-link density, but may also involve contributions from other effects such as the difference in surface tension between GPS and NB. The degree of swelling reaches ∼50-60% in the bulk of the films. Good reproducibility is obtained for several swelling/deswelling cycles, indicating that swelling with d-NB does not irreversibly change the structure of the network. In contrast to GPS, highly cross-linked films of BTSE do not swell when exposed to d-NB, even though NB is also a good solvent for the BTSE monomers.

Introduction Organosilanes have achieved widespread use as adhesion promoters in composite materials.1 Originally applied to glass surfaces, they have also found use in a wide variety of carbon fiber, ceramic, and metal-reinforced composites. Organosilanes have more recently been used to attach biological materials onto inorganic supports for biocompatibilization of inorganic surfaces in the biomedical sciences.2,3 Organosilanes have the general structure X3Si(CH2)nY, where n ) 0-3. X is a group that can be hydrolyzed, typically ethoxy or methoxy, and Y is an organofunctional group chosen to react with a given organic material. Once hydrolyzed, the silanetriol is effectively a trifunctional monomer that can polymerize with itself via condensation or can react with an inorganic substrate. Such coupling agents are often deposited as thin films (∼100 Å).4-10 The cross-link distribution within such films affects their ability to interpenetrate with resins, as well as their ability to transfer stress. The distribution, then, can influence the mechanical properties, fracture mechanisms, and diffusion of penetrants. Experimental evidence indicates the presence of gradients in cross-link density within these films. For example, more highly cross-linked regions of silane near the substrate surface are more resistant to removal by solvents or boiling water.7 However, only indirect measurements of the cross-link density gradient within such films have been reported.11-16 * Author to whom correspondence should be addressed.

We have recently developed a new method to probe the crosslink density profile within thermosetting films.17 In this work we applied this method to measure variations in cross-link density within thin (3-glycidoxypropyl)trimethoxysilane (GPS) and bis(triethoxysilyl)ethane (BTSE) films on silicon substrates. GPS is widely used as an epoxy-compatible adhesion promoter in glass-fiber-reinforced composite materials. The good performance of GPS in this application suggests that there is substantial interpenetration of resin and GPS. This, in turn, implies that the cross-link density is sufficiently low to allow such interpenetration. On the other hand, bis(triethoxysilyl)ethane (BTSE) silane films have been known to impart corrosion resistance to metal surfaces under appropriate conditions.18-20 BTSE is believed to form highly cross-linked films due to the fact that it possesses 6-fold reactivity per molecule. It has been proposed that the high cross-link density provides an impenetrable barrier to water. In previous work we used neutron reflection to examine the interaction of water vapor with a BTSE film.21 The method is based on the fact that the equilibrium volume fraction of a swelling solvent is strongly dependent upon the local cross-link density.17 In this work, nitrobenzene (NB) was used to swell the GPS and BTSE films. Nitrobenzene is a good solvent for both GPS and BTSE monomers, and interacts with the films through physical interactions only. NB has a higher surface tension than most organic films which is important so that a wetting layer does not form. The profile of NB through the films was determined using neutron reflection. The change in film thickness upon swelling with NB was also examined by X-ray reflection and ellipsometry.

10.1021/jp0126006 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/16/2002

Probing the Structure of Organosilane Films Experimental Section Materials. GPS and BTSE were obtained from Gelest and used as received. The silicon wafers used as substrates were polished 3-in. diameter single crystals (111) obtained from Wafer World Co. Nitrobenzene-d5 (d-NB, 99.5 atom % D) was obtained from Aldrich and used as received. Procedures. The wafers were cleaned by immersion in piranha solution (3:1 sulfuric acid:hydrogen peroxide) for approximately 30 min, followed by a rinse with distilled water. The GPS and BTSE films were deposited by spin coating, cured at 90 °C for 1 h, and then rinsed with ethanol. The GPS solutions were prepared by dissolving GPS in an ethanol/water mixture (90/10 by wt) at concentrations of 0.3 and 0.5 wt %. The pH of these solutions was adjusted to 4.3 using glacial acetic acid. To allow for full hydrolysis, the GPS solutions were stirred for 1 h prior to spin coating. The quality of the films as measured by X-ray reflectivity was strongly dependent upon the hydrolysis time. The BTSE solution was prepared by dissolving BTSE in an ethanol/water mixture (2/1 by vol) at a concentration of 1.2 wt %. The pH was adjusted to 4.0. The solution was cloudy upon mixing but became clear after approximately 30 min. However, good quality films were obtained only after a delay period of about 4 h after mixing. This delay period was used in preparing the present samples. Neutron reflectivity (NR) measurements were performed on the SPEAR reflectometer (Los Alamos). The method of NR is described in detail elsewhere.22 NR probes the neutron scattering length density (SLD) profile normal to the substrate surface, which is determined by the density and atomic composition. Strong contrast is obtained using H/D substitution. During the NR measurements, the samples were maintained at room temperature in a sealed aluminum chamber. The reflectivity of an as-prepared sample was first measured with desiccant in the chamber. The desiccant was then removed and a reservoir inside the sample chamber was filled with d-NB. The sample chamber was then sealed and the NR measurements were initiated following a period of roughly 20 min for sample alignment. Ellipsometric measurements have shown that this time is sufficient to obtain equilibrium. X-ray reflectivity (XR) measurements were performed using a Scintag X1 powder diffractometer equipped with Cu KR radiation, an incident beam mirror, and a Peltier-cooled solidstate Ge detector. An incident beam mirror was used to generate a parallel beam (in the height dimension) from the divergent X-ray source. A variable slit system at the exit port of the mirror housing was adjusted to create a beam of 50 µm in height. The beam width was 10 mm. Two slits in front of the detector were used to reduce background scattering and limit the beam divergence. The technique determines the electron density profile normal to the surface. During the XR measurements, the samples were maintained at room temperature in a sealed aluminum chamber with a Mylar window. Profiles of the neutron or X-ray scattering length density (SLD) cannot be obtained by direct inversion of the reflectivity data, but must be obtained from a fitting procedure. This involves approximating the profiles by a series of slabs of constant SLD, and then calculating the reflectivity from the stack of layers using the optical matrix method.23 The effects of roughness and finite interfacial width are included by dividing each interface into a number of small slabs to form a smooth gradient. Both the interfacial width and the functional form of the gradient can be varied. The resolution, ∆q/q, where ∆q is the standard deviation of a Gaussian function, was fixed at 0.014

J. Phys. Chem. B, Vol. 106, No. 10, 2002 2475 for NR and 0.02 for XR. Best fit parameters were determined by the minimization of χ2 using the Marquardt algorithm. Absorption was considered in the fits for X-ray reflection, whereas absorption was negligible in the neutron experiments. The X-ray and neutron SLDs, denoted b/V, are defined as follows:

(b/V)X ) roNA Σ(FiZi/Ai) ) Felro (b/V)N ) NAΣ(Fi bi/Ai) where is NA Avogadro’s number, ro is the classical electrical radius (2.82 × 10-13 cm), Fel is the electron density, Fi is the density of element i with atomic weight Ai and atomic number Zi, and bi is the neutron scattering length of element i with density Fi and atomic weight Ai. Ellipsometry measurements were performed using a GAERTNER manual nulling ellipsometer with a fixed angle of incidence of 70°. The organosilane film thicknesses were estimated using a single layer model for the combination of the silicon oxide and the organosilane film, and then subtracting the previously measured thickness of the oxide. The index of refraction of the film and the silicon oxide layer was fixed at 1.46. The samples were maintained at room temperature in a sealed glass box. The thickness of an as-prepared sample was first measured in the glass box and then a reservoir inside the glass box was filled with NB. The presence of the glass case did not affect the measured dry film thickness values. Ellipsometry measurements have been applied previously to study the swelling of polymeric thin films by a solvent.23,24 Results The neutron reflectivity from a film spun from a 0.3% GPS solution as-prepared and after swelling to equilibrium with d-NB is shown in Figure 1a. The data cover nearly 6 orders of magnitude in reflectivity and are displayed as reflectivity × q4 to compensate for the q-4 decay of the reflectivity due to the Fresnel law. In the dry state, the minimum in the curve is a sensitive function of the overall thickness of the organosilane film. The curve through the data for the as-prepared sample corresponds to the best-fit using a single layer profile for the GPS film, shown by the solid line in Figure 1b. This simple model profile is entirely adequate to describe the data for the as-prepared sample. The dry film thickness was roughly 50 Å. In Figure 1a, the 0.3% GPS sample swollen with d-NB shows a large increase in reflectivity relative to the data for the asprepared sample. The increase is due to absorption of d-NB within the film. In addition, the number of oscillations within a given q range increases, which indicates an increase in film thickness. The best-fit SLD profile for the d-NB swollen sample is also shown in Figure 1b. The thickness and SLD of the swollen film have both increased substantially relative to that of the as-prepared film. Importantly, the reflectivity for the swollen sample is not consistent with a single layer profile as is the case for the as-prepared sample, but rather the SLD has a pronounced maximum well into the bulk of the film. We note that the SLD of the oxide layer increased from 2.96 to 3.30 (×10-6 Å-2) when the sample was swelled with d-NB. This effect must be included for a better fit at low q (q e 0.05 Å-1), but the essential feature of the profile, a pronounced maximum in the central region of the film, is unchanged regardless of the SLD value of the silicon oxide. The increase in SLD of the oxide layer for the swollen sample can be explained by the penetration of the d-NB into the pores.25

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Figure 1. (a) Neutron reflectivity data from a 0.3% GPS sample asprepared (O) and after swelling to equilibrium with d-NB (b). The curves through the data correspond to best fits using model scattering length density profiles. (b) Best-fit scattering length density profiles corresponding to the curves through the data in (a) for: as-prepared (-), after swelling (‚‚‚).

Figure 2. (a) Neutron reflectivity data from a 0.5% GPS sample asprepared (O) and after swelling to equilibrium with d-NB (b). The curves through the data correspond to best fits using model scattering length density profiles. (b) Best-fit scattering length density profiles corresponding to the curves through the data in (a) for: as-prepared (-), after swelling (‚‚‚).

Similar results were found with a thicker film spun from a 0.5% GPS solution. Figure 2a shows neutron reflectivity from a GPS film obtained from a 0.5% solution in the as-prepared state, and again after swelling with d-NB. A large increase in reflectivity is again observed after swelling with d-NB. The solid curves through the data in Figure 2a are best-fits which yield the SLD profiles shown in Figure 2b. The SLD profiles of the 0.5% sample are qualitatively similar to those of the 0.3% sample. The dry film thickness was found to be roughly 100 Å for this sample. We note that the SLD of the 0.5% dry film is slightly lower than that of the 0.3% dry film. This was a consistent finding, and indicates that either the thicker film is less dense or the atomic composition is slightly different for the two film thicknesses. A difference in atomic composition between the films could result from variation in the extent of hydrolysis or condensation. Regarding this interpretation, the lower SLD of the 0.5% film would imply a slightly lower degree of condensation (more SiOH groups). However, this is at odds with the slightly reduced swelling seen in the thicker film,

discussed further below. Thus we conclude that the lower SLD of the thicker film indicates a lower density. Data for both GPS films indicate that gradients in SLD exist at both the substrate and air surfaces. This is indicated by the fact that the oscillations in the reflectivity data for a swollen sample are suppressed compared to those of the dry sample. This is further illustrated in Figure 3 using the data for the 0.3% GPS film. First, a profile consisting of a single layer with an adjustable gradient at the air surface was considered. The single layer profile is clearly inadequate irrespective of the size of the gradient at the air surface, as indicated by the poor fit in Figure 3a. As the gradient at the air surface becomes larger, the reflectivity decreases at high q. Some damping of the oscillations also occurs at high q. However, the calculated curve falls below the experimental curve well before the oscillations become damped to the level seen in the data. Second, we considered a profile with an adjustable gradient near the substrate surface, a maximum in the bulk of the film, but only a small (fixed) gradient at the air surface. This is also shown in Figure 3a and

Probing the Structure of Organosilane Films

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Figure 4. X-ray reflectivity data from a 0.46% GPS sample: asprepared (O), 1st swelling with d-NB (b), after drying in air overnight (4), 2nd swelling with NB (2), after drying in air overnight (0). The curves have been shifted along the y-axis for clarity.

Figure 3. (a) Neutron reflectivity data from a 0.3% GPS sample. The data are compared with best-fit curves using (-) a model profile with adjustable gradients at both the substrate and air surfaces, (---) a single layer profile with an adjustable gradient only at the air surface, and (‚‚‚) a profile with an adjustable gradient at the substrate surface, but only a small fixed gradient at the air surface. (b) Best-fit profiles corresponding to the curves in (a). The lines have the same meaning as in (a). This comparison indicates that large gradients at both the substrate and the air surfaces are required to fit the data.

Figure 3b. In this case, the best-fit curve shows a relatively good fit at low q, but large oscillations and poor fitting at high q. Large gradients in SLD are required at both interfaces to produce the highly damped oscillations observed in the data. The swelling of GPS films with NB was further examined with X-ray reflectivity. For this system, X-ray reflectivity is very sensitive to roughness at the air surface, but only weakly sensitive to solvent concentration, in contrast to neutron reflectivity in which there is strong sensitivity to both effects. The insensitivity to solvent volume fraction in the X-ray experiment is due to the fact that the difference in electron density between NB and GPS is very small. Therefore, the X-ray data are not nearly as sensitive to gradients in solvent composition as the neutron data, where H/D substitution enhances contrast. Figure 4 shows X-ray reflectivity data for a GPS film spun from a 0.46% solution, measured for several swelling/ deswelling cycles. The increase in film thickness after swelling with NB is apparent from the shift in the Kiessig fringes to

Figure 5. Best-fit scattering length density profiles for the 0.46% GPS sample as-prepared (-) and after the first swelling with d-NB (‚‚‚). The X-ray reflectivity data and fits are shown in the inset: as-prepared (O), and after swelling (+).

lower q for q < 0.25 Å-1. The reflectivity returns to nearly that of the original dry sample upon removal from the saturated d-NB atmosphere. Furthermore, good reversibility was obtained for several swelling/deswelling cycles. This indicates that swelling with d-NB does not damage the structure of the film, and that d-NB does not react irreversibly with the GPS film. The subtle variations at high q in the data for the dried films are likely related to a small amount of residual solvent remaining in the films or perhaps to very small scale changes in structure. SLD profiles are shown in Figure 5 which correspond to the data of Figure 4 for the as-prepared sample and after swelling the first time. Since X-ray reflectivity is relatively insensitive to the solvent concentration but strongly sensitive to the roughness at the air surface, the SLD model profiles used to fit the data were composed of a single layer with a variable gradient at the air surface. This model profile is adequate to fit the data.

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Figure 6. Best-fit scattering length density profiles for the 0.46% GPS sample after the first swelling using a model profile without a gradient (-) and with a gradient (‚‚‚) in solvent distribution through the film. The X-ray reflectivity data and fits are shown in the inset: without gradient (-), with gradient (‚‚‚). The inset shows that the quality of fit is comparable.

The film thickness increases by a factor of 1.26 upon swelling, in reasonable agreement with the neutron study. The interfacial width (full width) at the air surface becomes only slightly larger upon swelling with NB. The full interfacial width for the asprepared sample is about 28 Å, but for the sample swelled with NB the interfacial width is 41 Å. The volume fraction of NB in the film calculated from the X-ray SLD profiles in Figure 5 is about 0.87, which is higher than the maximum obtained from the NR data. Part of this discrepancy may be accounted for by the large uncertainty of the value determined in the X-ray experiment. For example, fixing the volume fraction at 0.50 in the fitting corresponds to an increase in χ2 of only 10% relative to the best-fit. Another possibility is that the assumption of volume additivity may not be correct. A decrease in volume upon mixing would lead to a calculated solvent volume fraction that was higher than the true value. Solvent volume fractions obtained from neutron reflectivity would be relatively unaffected since most of the contrast comes from H/D substitution. In further analysis of the X-ray data, we attempted to extract the solvent distribution within the film by allowing a gradient at the substrate surface in the model profile. A comparable quality of fit was obtained with and without such a gradient, as shown in Figure 6. The χ2 value for the profile without a gradient near the silicon surface is 3.5, while χ2 for the profile with a gradient near the surface is 3.2. Thus, while the limited sensitivity in the X-ray experiment does not allow a reliable assay of the detailed solvent distribution, the measurement is useful to monitor changes in the film thickness and in the roughness at the air surface. Surface roughness can be probed more directly with either AFM or off-specular X-ray scattering. Off-specular X-ray scattering for the 0.46% GPS sample before and after swelling is shown in Figure 7. Little or no increase in off-specular scattering was observed upon swelling. This indicates very little change in surface roughness upon swelling, consistent with the specular X-ray data. The increase in film thickness upon swelling with NB was also examined using ellipsometry. Since the measurement time

Yim et al.

Figure 7. Detector scans measured at the angle of incidence 1.0° of the 0.46% GPS sample: as-prepared (O), after swelling (b).

Figure 8. X-ray reflectivity data from a 1.2% BTSE sample asprepared (O) and after swelling to equilibrium with NB (b).

for ellipsometry is very short, in principle the time scale to reach a steady state can be examined. However, since a manual nulling instrument was used, the minimum measurement time was roughly three minutes. By this time period, the films were already very close to the equilibrium thicknesses. Importantly, this indicates that all of the X-ray and neutron measurements correspond to equilibrium swelling since they were initiated at least 20 min after swelling. The fractional increase in film thickness upon swelling with NB measured by ellipsometry ranged from 1.20 to 1.23, in good agreement with that obtained by X-ray and neutron reflection. Finally, the degree of swelling of a BTSE film upon exposure to NB was also investigated using X-ray reflection. Figure 8 shows X-ray reflectivity data for the BTSE film spun from a 1.2% solution in the as-prepared state, and again after exposure to NB. In sharp contrast to the results for the GPS film, no change in reflectivity is observed upon exposure to NB vapors. Discussion A principle goal of this work is to determine solvent volume fraction profiles from the neutron SLD profiles, and from those,

Probing the Structure of Organosilane Films

Figure 9. The volume fraction profiles of d-NB calculated from the SLD profiles for the 0.3% sample (-) and 0.5% sample (‚‚‚). The inset shows the maximum effect that could be attributed to increased lateral roughness due to inhomogeneous swelling for the 0.5% sample: assuming no lateral roughness (s), assuming the maximum lateral roughness consistent with the specular X-ray data (‚‚‚).

extract profiles of the extension ratio. However, whereas the gradient in SLD at the substrate surface can be interpreted straightforwardly as a lower solvent concentration due to an elevated cross-link density, the interpretation of a gradient in SLD at the air surface is complicated by several factors. First, both in-plane roughness and a composition gradient can lead to a gradient in the model one-dimensional SLD profile. Thus, the gradual decrease in SLD from the maximum in the middle of the film to zero at the air surface can indicate either a gradient in solvent composition or a laterally inhomogeneous swelling which roughens the surface. However, the off-specular X-ray scattering data indicate little or no increase in roughness upon swelling. Therefore we conclude that the gradient in SLD at the air surface is due largely to a solvent composition gradient. A quantitative analysis is given below. Second, a gradient in solvent composition at the air surface could be attributed to the difference in surface tension between d-NB and GPS as well as to a gradient in the cross-link density. This will also be discussed further below. Volume fraction profiles of d-NB calculated from the NR SLD profiles for both GPS films are shown together in Figure 9. The volume fraction profiles of d-NB were determined from the SLD profiles of the swollen films relative to those of the dry films. This assumes volume additivity in which the SLD of a swollen film can be obtained by adding the contributions from the volume fraction of each component (SLDtotal ) φ SLDNB + (1 - φ)SLDGPS where φ is the NB volume fraction). The SLD of d-NB is 0.0555 × 10-4/Å2. The SLD profile of GPS in the swollen film was initially estimated by assuming a uniform one-dimensional swelling of the dry film. The solvent profile resulting from this is given in Figure 9. Recalculation of the solvent profile accounting for the inhomogeneous swelling of GPS as indicated by the profile in Figure 9 resulted in essentially the same solvent profile. As mentioned above, care needs to be taken regarding the effect of increased roughness at the air surface upon swelling. Whereas the analysis leading to the profiles in Figure 9 accounts

J. Phys. Chem. B, Vol. 106, No. 10, 2002 2479 for the roughness and density gradient at the air surface of the dry films, as well as the increase in interfacial width arising from uniform swelling (or uniform expansion) of the dry film, it does not account for increased lateral roughness at the air surface which may arise from inhomogeneous swelling. Accounting for such an effect would lead to a broader maximum and a sharper gradient at the air surface in the volume fraction profiles in Figure 9. However, the off-specular scattering data in Figure 7 indicate very little increase in roughness upon swelling. Furthermore, the specular X-ray reflectivity data in Figure 4 indicate an increase in interfacial width of only 13 Å upon swelling. The interfacial width in the model onedimensional X-ray SLD profile includes effects due to both inplane roughness and the gradient in electron density at the transition between the bulk film and air. So 13 Å is an upper limit to the increase in roughness due to inhomogeneous swelling. The X-ray off-specular scattering data suggest that the actual value is lower. However, even if the entire 13 Å were assigned to increased roughness due to inhomogeneous swelling, the effect on the solvent volume fraction profiles in Figure 9 is very minor. This is shown in the inset to Figure 9 for the 0.5% GPS sample. The gradients in solvent composition at the air surface in Figure 9 indicate a potential gradient of some form. The potential gradient may be due to a gradient in cross-link density. However, the higher surface tension of d-NB relative to GPS will also result in a potential gradient. Ellipsometric measurements have shown that a difference in surface tension produces composition gradients in small molecule mixtures of roughly 20 Å.26 Since the gradients in solvent composition in Figure 9 are observed to extend over much larger distances (∼60 Å and ∼95 Å for the 0.3% and 0.5% samples, respectively), it appears that this explanation cannot account for the entire effect. Also, the length scale of the gradient at the air surface is substantially larger for the thicker film, whereas that due to the surface tension effect should be independent of film thickness. Thus we conclude tentatively that an elevated cross-link density exists at the air surface. We suggest that the reduced cross-linking in the center of the films could be related to confinement or frustration imposed by the interfaces. The surface interactions at both the SiO2/GPS and GPS/air interfaces may have a strong influence on the molecular mobility within these thin films. For example, if GPS molecules order at these interfaces to lower their surface energy or if they cross-link more rapidly at the interfaces than in the center of the film, then the rearrangements necessary for cross-linking in the center of the film would become inhibited. We note that the cross-link density at the air surface cannot be too high or else the interpenetration of epoxy resin with the GPS film would be restricted, and that seems inconsistent with the well-known excellent durability and mechanical properties. The interpenetration of an epoxy resin with the GPS films is currently under study in our lab. Figure 10 shows the extension ratio as a function of depth for both the 0.3% and 0.5% films. The network extension ratio is calculated from the volume fraction profiles assuming purely 1-dimensional swelling. To calculate the curves in Figure 10, we assumed that the gradient in SLD at the air surface is due entirely to variations in cross-link density. While we conclude that the cross-link density is increased at the air surface, the fact that the calculated extension ratio drops to unity at the air surface is likely an artifact due to limited resolution in the data and the convoluting effects of roughness and the gradient arising from the difference in surface tension between NB and GPS. Notice that the maximum extension ratio is higher for the 0.3%

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Yim et al. combination of these two results indicated that the main effect of exposure to water vapor at 80 °C was the hydrolysis of residual ethoxysilyl groups, and that there was very little or no physical adsorption of water within the BTSE film. These two organosilane films show remarkably different swelling behavior. The thin GPS films show a large degree of swelling with NB, while the thin BTSE films show no swelling with NB. Since NB is a good solvent for both GPS and BTSE monomers, the differences are attributed to variation in the crosslink density. It is also worth noting that the two films show much different hydrophobicity. The equilibrium water and NB contact angles for the BTSE films are 104° and 51°, respectively. On the other hand, the water contact angle for the GPS films is 62°, but apparent complete wetting (contact angle e 10°) is observed for NB on the GPS films. The latter observation further supports the conclusion of solvent penetration into the GPS films but not the BTSE films. Conclusions

Figure 10. Extension ratio as a function of distance from the silicon oxide surface for the 0.3% (-) and 0.5% (‚‚‚) samples. The curves have been calculated from the volume fraction profiles in Figure 9 assuming that the gradient at the air surface is due entirely to variations in cross-link density. The fact that the calculated extension ratio drops to unity at the air surface is likely an artifact due to limited resolution in the data and the convoluting effects of roughness and the gradient in solvent concentration due to the difference in surface tension between NB and GPS.

film. The degree of swelling within the bulk of both films is rather high, about 0.5-0.6 volume fraction solvent corresponding to a network extension ratio of 2.5. Considering this high degree of swelling and high network extension ratio, it is clear that the thin GPS films do not have a tightly cross-linked, rigid network structure. For example, this degree of swelling substantially exceeds that observed for a model epoxy system at the stoichiometric ratio.17 We note that the GPS film with dry thickness of 100 Å is only 10-17 molecular layers thick, assuming a monomer dimension of 6-10 Å depending upon orientation. The maximum possible number of cross-links through the thickness of the film thus ranges from 5 to 8. The actual number must be reduced somewhat from this range to account for the observed swelling. While the general shape of the nonuniform swelling profile is our main concern in this work, we note that there appear to be subtle differences in structure for the two thicknesses examined. While the observation of reduced swelling indicates that the 0.5% film has a higher cross-link density, its SLD when dry suggests that it has a lower density than the 0.3% film. It would appear that cross-linking causes the GPS molecules in the film to pack less efficiently. The confinement/frustration effects postulated above to account for the reduced cross-linking in the center of the film would decrease in importance with increasing film thickness, accounting for the observed trend. Further results considering a wider range of film thickness would be useful. However, it is difficult to change film thickness over a wide range without substantially changing film chemistry or structure due to the competition between hydrolysis and condensation reactions in this system. The fact that the BTSE film does not swell with NB is consistent with conclusions from our previous work, in which the interaction of water with a BTSE film was examined by neutron reflection.20 In that work a large increase in reflectivity was observed upon conditioning with D2O, but no change in reflectivity was observed upon conditioning with H2O. The

We have developed a new method for studying the distribution of cross-links within thin thermosetting films. The method is based on solvent swelling combined with neutron and X-ray reflection, and was used to study the structure of GPS and BTSE films. Using H/D substitution, neutron reflection is very sensitive to the solvent distribution within the film. A large increase in neutron reflectivity was observed upon swelling GPS films with d-NB. The d-NB concentration profile has a strong maximum in the central region of the film. We conclude from this that the GPS films possess a high cross-link density near the silicon surface and a much lower cross-link density in the bulk of the film. The decrease in solvent concentration at the air surface is more difficult to interpret. It appears to also indicate an elevated cross-link density at the air surface, but may involve contributions from other effects such as the difference in surface tension between GPS and the solvent. The lower extent of cross-linking, or the greater number of network defects, in the center of the film may be a result of confinement or frustration caused by ordering or more rapid cross-linking at the interfaces. The extent of swelling in this system was substantially greater than that for a much thicker, stoichiometric epoxy film reported previously. Good reproducibility was obtained for several swelling/ deswelling cycles, indicating that swelling with d-NB does not damage the network. The fractional increase in thickness obtained by neutron reflectivity, X-ray reflectivity, and ellipsometry were in good agreement. In contrast to the GPS films, BTSE films do not swell with d-NB and we infer from this that they possess a much higher cross-link density. This is consistent with the significant moisture resistance and corrosion protection afforded by these films. Acknowledgment. This work was performed at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract DE-AC0494AL85000. J.J.B. and E.J.K. acknowledge the partial support from Xerox Corporation and the Semiconductor Research Corporation, as well as the use of the facilities of the MRL at UCSB which is funded by the DMR-MRSEC program. References and Notes (1) Pleuddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum Press: New York, 1992. (2) Elender, G.; Kuhher, M.; Sackmann, E. Biosens. Bioelectron. 1996, 11, 565.

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