Article pubs.acs.org/Langmuir
Density Profile in Thin Films of Polybutadiene on Silicon Oxide Substrates: A TOF-NR Study E. Tilo Hoppe,† Alessandro Sepe,†,§ Martin Haese-Seiller,‡ Jean-François Moulin,‡ and Christine M. Papadakis*,† †
Technische Universität München, Physik-Department, Physik weicher Materie, James-Franck-Str. 1, 85748 Garching, Germany Helmholtz-Zentrum Geesthacht, Outstation at FRM II, Lichtenbergstr. 1, 85747 Garching, Germany
‡
ABSTRACT: We have investigated thin films from fully deuterated polybutadiene (PB-d6) on silicon substrates with the aim of detecting and characterizing a possible interphase in the polymer film near the substrate using time-of-flight neutron reflectometry (TOF-NR). As substrates, thermally oxidized silicon wafers were either used as such or they were coated with triethylethoxysilyl modified 1,2-PB prior to deposition of the PB-d6 film. TOFNR reveals that, for both substrates, the scattering length density (SLD) of the PB films decreases near the solid interface. The reduction of SLD is converted to an excess fraction of free volume. To further verify the existence of the interphase in PB-d6, we attempt to model the TOF-NR curves with density profiles which do not feature an interphase. These density profiles do not describe the TOF-NR curves adequately. We conclude that, near the solid interface, an interphase having an SLD lower than the bulk of the film is present.
1. INTRODUCTION The properties of polymers near a solid interface have been in the focus for several years.1−17 This is due to the importance of this interface for applications, such as lubricants and adhesives. Also in polymer/(nano)composites, a very large polymer−solid interface area is usually present. The question whether and in which way the polymer properties change near a solid interface has been studied in numerous works focusing on, among others, the chain conformation, the mass density, and the chain mobility. The term “interphase” has been used for a near-solid layer in the polymer melt with properties different from the bulk. A number of theoretical works have addressed the structural properties of polymers near solid interface walls.1,13,18 A review has been given by Baschnagel et al. pointing to changes in, among others, crystallization or glass transition near solid interfaces.19 Simulations of linear chains near a solid and neutral interface using the cooperative motion method revealed that the cigar-shaped chains are reoriented in the vicinity of the interface, but do not change their dimensions and shapes.1 The chains align with their long axes parallel to the interface, and their centers of mass remain separated from the interface by a distance corresponding to the cigar radius. A flattening of the chains near the interface was also found in the work by Smith et al. for both attractive and repulsive surface-monomer interactions.18 Chemically realistic simulations of 1,4-polybutadiene (the polymer under investigation in the present work) near graphite (an attractive interface) revealed monomer layering near the interface which extends over a distance of roughly twice the polymer radius of gyration.13 For chains with 29 monomer units, the layering extends over a few nanometers away from the interface. © 2013 American Chemical Society
Experimentally, the polymer/solid interface is not easily accessible, unlike the polymer surface.9,20 A vast number of studies have therefore made use of the thin film geometry using supported thin films with a free surface. The properties of such polymer films are expected to be significantly determined by the presence of the solid substrate.3,6−8,10−12,14,15 Positron annihilation lifetime spectroscopy of polymer thin films revealed an increase of the free volume with decreasing film thickness which was attributed to an influence of both the substrate and the free surface on the chain packing.15 Thus, averaging the properties over the entire film, it is not straightforward to separate the influence of the substrate and the surface of the polymer film on the properties of the polymer film. Interface-sensitive methods specific to the polymer/solid interface are thus highly desired. X-ray (XR) or neutron reflectometry (NR) may give such information on the buried interface because of their high depth resolution.16,21−24 In the present work, we use time-of-flight NR (TOF-NR) to address the question of possible density changes in a polybutadiene (PB) film near an SiOx substrate. The choice of PB is due to its low glass transition temperature which minimizes the risk of having residual solvent from the preparation in the film; that is, it is constantly annealed. TOF-NR requires samples which are flat over a large area, therefore we chose a Si wafer as a substrate. Moreover, we wish to compare with our previous results obtained with surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS) on PB films on glass.25,26 A substrate similar Received: June 9, 2013 Revised: July 26, 2013 Published: August 14, 2013 10759
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formed on the SiO2 surface, resulting in a substrate which is chemically similar to the polymer film. We characterize the various substrates, consisting of thermally oxidized Si substrate with and without TEOS-PB layer, using contact angle measurements, atomic force microscopy (AFM), and TOF-NR and use the resulting characteristics for the interpretation of the TOF-NR curves of PB-d6 films on these substrates. The paper is structured as follows: After an experimental section describing materials and methods used, we present the results on the substrate characterization. Then, the TOF-NR curves of the PBd6 thin films on both substrates as well as their analysis using two different layer models are described and discussed, and an excess free volume in PB-d6 near the substrate is estimated from the reduced scattering length density found in the interphase. Finally, simple models not including this interphase are presented and are shown not to fit the data well. The results are eventually discussed.
to glass was obtained by using Si wafers with a 250 nm thick oxide layer as a substrate. The thickness of the SiOx layer is sufficiently high to shield the interaction of PB with Si efficiently.27 We note that TOFNR offers a much better depth resolution compared to SPR/OWS. A few studies have previously attempted to identify an interphase using XR or NR. Bollinne et al. used XR to investigate various polymer thin films on Si wafers having a native oxide layer which was hydrophilic.28 Changes of the electron density near the substrate were detected, and both the sign of the change and the width of the interphase were quantified. For polystyrene (PS) and poly(methyl methacrylate) (PMMA), a decrease of the electron density near the interface with the substrate was detected, whereas for poly(4vinylpyridine) (P4VP), the electron density near the interface with the substrate was found to increase. The interphase widths ranged between 11.7 and 26.9 Å. The authors attributed the interphase to changes of the molecular conformation of the polymers near the substrate, partly as a consequence of the interaction with the substrate. In a later study, Ahn et al. investigated various homo- and random copolymer thin films on Si wafers using synchrotron XR.17 Depletion layers near the interface with Si were found in PS, poly(α-methyl styrene) (PAMS), P4VP, and PMMA, with their amplitude being stronger in PS and PAMS (60−70% of their maximum density) than in P4VP and PMMA (80−90%). This was attributed to a higher fraction of free volume near the substrate leading to a gain in conformational entropy of the chains. Except for P4VP, the results agree qualitatively with the ones by Bollinne et al.28 We note that all polymers studied are at room temperature in the glassy state. For instance, Perlich et al. detected a toluene content of 10−15% in PS thin films spin-coated from toluene onto Si wafers as well as an enrichment of toluene at the Si/polymer interface, which may be a consequence of the high glass transition temperature (Tg) of PS.29 After annealing far above Tg, the solvent content was reduced by 3.2%.29 In contrast, the PB used in the present study has a Tg far below room temperature (approximately −89 °C); at ambient conditions, the film is thus constantly annealed and is not expected to contain solvent at the time of the TOF-NR measurement. In TOF-NR, the method used in the present work, neutrons covering a broad range of wavelengths impinge on the sample simultaneously, and the resolution in vertical momentum transfer can be decided on after the measurement; that is, it can be adapted to the statistics. Varying the incident angle, a large range of vertical momentum transfers, qz, is covered. The measuring geometry with the beam impinging through the substrate was chosen. This has the following advantages over the usual measuring geometry where the beam impinges onto the film surface: (i) The critical qz value of total external reflection, qcz, is shifted to lower qz, (qz denotes the component of the scattering vector along the film normal, z) thus more information on the polymer film is gained, (ii) the reflectivity is higher for qz > qcz, which results in better statistics, and (iii) the amplitude of the Kiessig fringes is higher, which allows to relax the resolution and again to gain statistics. Thus, a higher qz range is accessible and the statistics are improved when the neutron beam impinges through the substrate. As for PB, fully deuterated PB-d6 is used to minimize the incoherent background in TOF-NR and thus to increase both the dynamic and the qz-range. To investigate the influence of the substrate properties on the PB density profile, further experiments were conducted with the substrate additionally modified by silanization using a low molar mass triethylethoxysilyl modified 1,2-PB (TEOS-PB). A self-assembled TEOS-PB monolayer is
2. EXPERIMENTAL SECTION 2.1. Materials. (100) silicon wafers having a thickness of 4 mm and a diameter of 100 mm (4″) from Si-Mat, Germany, were used as substrates. For chemical cleaning and surface activation of the substrates, a mixture of deionized water, hydrogen peroxide (30%), and sulfuric acid (96%) was used (Carl Roth). A triethylethoxysilane modified 1,2-PB having a molar mass of 4.2 kg/mol (TEOS-PB) was purchased from ABCR, Germany, and was dissolved in HPLC grade toluene (Carl Roth) for silanization. Fully deuterated polybutadiene (PB-d6) having a molar mass of 55 kg/mol was purchased from Polymer Source Inc., Canada. The glass transition temperature Tg of PB-d6 is expected to be approximately −89 °C.30−32 The surface tension value of PB with alkyl end groups is expected at 33−36 mJ/m2 with a polarity close to zero.33 Here, the polarity of a substance xp is the ratio of the polar component of the surface tension and its absolute value, where the surface tension and its polar component are defined according to Owens and Wendt (see below). However, the surface tension of the PB-d6 used may be slightly lower than the literature value because of the methyl end groups which have a lower surface tension (∼30 mJ/m2) than the CH and CH2 groups of the chain backbone (∼43 mJ/m2).31 Fully deuterated toluene (toluene-d8, 99.96%) was purchased from Sigma Aldrich, Germany. For contact angle measurements, the following test liquids were used: low viscous paraffin oil (Carl Roth), diiodomethane (Sigma Aldrich), ethylene glycol (Carl Roth), formamide (Carl Roth), and deionized water. 2.2. Sample Preparation. Dry Thermal Oxidation. The Si wafers were cleaned wet chemically in an 80 °C warm acid bath according to
Figure 1. Temperature profile in the furnace used for dry thermal oxidation: predrying (light gray), oxidation (dark gray), and cooling (gray). Full line, actively controlled; symbols, measured temperatures during passive cooling; dashed line, guide to the eye. 10760
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Scheme 1. Schematic Overview of the Samples Measureda
a Thermally oxidized Si wafer I, thermally oxidized Si wafer I coated with a PB-d6 thin film (I/PB), thermally oxidized Si wafer coated with TEOS-PB (II) and with a PB-d6 thin film (II/PB). Si, light grey; SiOx, dark grey; TEOS-PB, yellow; PB-d6, orange.
therefore discarded). TOF-NR measurements were started within 30 min after preparation of the PB-d6 films. The steps of the sample preparation are compiled in Scheme 1. A commercially available silicon wafer was dry thermally oxidized (I) and coated by a thin PB-d6 film (I/PB). Another silicon wafer was dry thermally oxidized, silanized with TEOS-PB (II), and coated by a thin PB-d6 film (II/PB). TOF-NR measurements were carried out on the bare and the silanized substrates (I and II) and on the identical substrates coated by PB-d6 films (I/PB and II/PB). 2.3. Substrate Characterization. The substrate topographies were investigated using atomic force microscopy (AFM) on substrates prepared in the same way as the ones for TOF-NR but 0.25 mm thin. A JSPM 5200 instrument from JEOL was operated in tapping mode using NSC35/AIBS cantilevers from MikroMasch. Image sizes were chosen at 0.5 × 0.5 μm2 and 20 × 20 μm2. From these images, the surface ratio was deduced to calculate the effective contact angle θW of the liquids according to the method of Wenzel.36 Static contact angle measurements were carried out using an OCA 20 instrument from Dataphysics, Germany, using five test liquids with different polarities, respectively (deionized water, ethylene glycol, formamide, diiodomethane, and paraffin oil). The surface energy densities of the substrates, σS, and their dispersive and polar components, σDS and σPS, were calculated using the method of Owens and Wendt,37 where polarity is defined as xP = σPS /σS. 2.4. Time-of-Flight Neutron Reflectometry. For TOF-NR, 4″ wafer with a thickness of 4 mm were used as substrates. Measurements were carried out using the REFSANS instrument (Helmholtz-Zentrum Geesthacht) at FRM II (Garching, Germany).38 Neutron wavelengths λ = ∼2 to ∼15 Å were chosen together with incident angles αi = 0.2°, 0.8°, and 2.0°. Measuring times were 1, 6, and 37−40 h, respectively. This way, a qz range of ∼0.003 to ∼0.18 Å−1 was covered (qz = 4π sin αi/λ). The beam impinged through the side of the Si wafer. The maximum footprint on the polymer film was 60 mm × 60 mm, i.e., much smaller than the diameter of the Si wafer. This way, the edge of the sample was not illuminated. The instrument configuration was such that the overall qz resolution, Δqz/qz, was ∼7%. Data reduction was carried out using software provided by REFSANS which takes, among others, ballistic effects into account. The parts of the TOF-NR curves measured at different αi values were normalized such that the reflectivity is unity for qz < qcz, the critical value of qz. While the positions of the Kiessig fringes measured at different αi values coincide, their amplitude is affected by the resolution which depends on αi. The minimal layer thickness dmin that is resolved − qmin within the measured spectra is estimated using dmin ∝ π/(qmax z z ). For the setup used, a typical value of dmin is ∼20 Å. This value was used as a minimal layer thickness in the fitting models below. Motofit was used as software for fitting with a combined genetic and Levenberg−Marquardt algorithm with at least 2000 generations, a
ref 34 to remove possible surface contaminations. Then, the Si wafers were dry thermally oxidized to achieve a thick SiO2 layer. The oxidation of the Si wafer was carried out in a customized sealed hightemperature furnace with the samples being stacked. The temperature profile in the furnace displayed in Figure 1 comprises three steps: predrying of the sample, oxidation, and cooling. The wafers were placed in the furnace within 15 min after wet chemical cleaning and subsequent drying with nitrogen at 40% r.h. at 20 °C. During the entire oxidation process, the furnace was kept sealed until room temperature was reached at the end of the process. The temperature was increased with a rate of 1000 K/h to 150 °C and was kept there for 3 h to remove volatile adsorbed water and thus to avoid wet thermal oxidation during heating. Next, the temperature was increased to the oxidation temperature of 1200 °C with a rate of 1000 K/h and was kept there for 86 min. Then, the heating was switched off, and the samples were allowed to cool slowly to minimize the adsorption and implantation of impurities on the silicon oxide surface and the development of thermal tensions. Oxidation may still take place during cooling since temperature falls below 1000 °C only 70 min after switching off. The temperature in the furnace reached room temperature after additional 48 h. After the oxidation, the wafers were once more cleaned wet chemically to remove possible surface contaminations during oxidation or handling and to activate the surface chemically, i.e., to form hydroxyl groups at the SiO2 surface. Excess water on the substrate surface was removed by drying the sample at 120 °C for 2 h in vacuum. The wafers were stored under ambient conditions in a closed vessel sealed by Parafilm until use. Silanization. For the silanization, pristine glass vessels and solvents were used for each step to reduce possible contaminations of the substrate surface. Prior to use, every vessel was cleaned in a 2 vol. % solution of Hellmanex II (Hellma) and deionized water at 50 °C for 20 min, then rinsed with deionized water, and dried with N2. Excess water was removed from the substrate surface by drying it in vacuum for 2 h at 150 °C. The substrate was stored for 12 h in a 5 vol. % TEOS-PB solution in toluene, resulting in a monolayer of silane. Excess silane was removed by rinsing the substrate with toluene. The silanized substrate was stored under ambient conditions for curing in a vessel sealed with Parafilm for at least 24 h before further use. Film Preparation. Both substrates (nonsilanized and silanized) were rinsed with fully deuterated toluene (toluene-d8) before use to increase their wettability for PB-d6. Afterward, they were spin-coated for 60 s at 2000 rpm with a 2 wt % solution of PB-d6 in toluene-d8. According to the Schubert equation, the thickness of the PB-d6 film is under these conditions expected at ∼1100 Å.35 The samples were dried for 10 min in vacuum at room temperature to remove volatile solvent (further vacuum drying for about 3 h resulted in a partial dewetting of the PB-d6 film on the nonsilanized substrate and was 10761
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mutation constant of 0.7, a population size multiplier of 20, a recombination constant of 0.5 and a fractional tolerance of 0.05%.39 The resulting SLD interphase properties deviated per layer of approximately 0.2 Å−2. Two approaches to model the SLD profile in the polymer film were used: (i) Variable layer thickness model (Figure 3): A certain number of layers was fixed at the film/substrate interface and the film surface, and their layer thicknesses and scattering length densities (SLD) was allowed to vary. The minimum layer thickness was 20 Å, i.e., the resolution limit. (ii) Fixed layer thickness model: Layers of thickness 20 Å were used near the film/substrate interface and the film surfaces, and their number was increased until no more variation of the SLD of the outermost layers with respect to the neighboring bulk PB-d6 film or to air was observed. In both approaches, the thickness of the homogeneous part of the film was allowed to vary, while its SLD is given by qcz, the critical value of qz. Roughness of the individual layers was not considered in the modeling. The SLDs of Si and SiO2 are 2.07 × 10−6 and 3.47 × 10−6 Å−2.40 For PB-d6, a mass density of 0.98 g/cm3 (which corresponds to the value of 0.88 g/cm3 for nondeuterated PB)30−32 results in a SLD of 6.55 × 10−6 Å−2. Fully deuterated toluene has a SLD of 5.64 × 10−6 Å−2 (ref 40), a value very similar to the one of PB-d6 and is therefore invisible in the TOF-NR experiment. The properties of the substrates (I and II), e.g., their scattering length density profiles, were considered to be unaffected by the PB-d6 thin films (I/PB and II/PB). However, the silicon oxide surface is slightly rough, and these protrusions may be filled to a certain extent by PB-d6.
measurements of the substrates (I and II) and on the identical substrates coated with PB-d6 thin films (I/PB and II/PB) are presented. Finally, the presence of a PB-d6 interphase is scrutinized by fitting simplified models to the TOF-NR curves, such as a homogeneous film without interphase. 3.1. Substrate Characterization. Knowledge of the surface properties of the substrates is crucial for the discussion of polymer interphase properties. The chemical properties were determined using contact angle measurements with five test liquids (Table 1) and the analysis method of Owens and Wendt.37 The surface tension of dry thermally oxidized silicon(I) is found to
3. RESULTS Two substrates were used for studying the interphase of PB-d6 with a solid surface: A thermally oxidized Si wafer (I) and an additionally silanized thermally oxidized Si wafer (II). The substrates are characterized by AFM and contact angle measurements. Then, the results of the neutron reflectometry Table 1. Contact Angles of the Test Liquids on Substrates I (Nonsilanized) and II (Silanized) test liquid paraffin oil diiodomethane formamide ethyleneglycol water
θ (deg) I 54.9 61.3 62.2 58.7 83.3
± ± ± ± ±
3.8 1.4 3.5 2.5 2.0
θ (deg) II 29.6 56.1 52.2 51.3 72.3
± ± ± ± ±
6.1 1.0 4.0 4.0 1.7
Figure 3. TOF-NR results from the substrates. (a and b) Fresnel representation of the TOF-NR curves (symbols) with fits of the variable layer thickness model (solid lines) of the substrates, low (a) and high qz region (b). Thermally oxidized Si wafer (I, blue open circles, full blue line) and additionally silanized substrate (II, red open triangles, dashed red line). (c) Corresponding scattering length density (SLD) profiles of the thermally oxidized Si substrate (I, blue full line) and of the additionally silanized substrate (II, dashed red line). The profiles were shifted along z to match at the substrate surface. The SiO2 layers have slightly different thicknesses.
Figure 2. TOF-NR curves of a thermally oxidized Si wafer before (I, open blue circles) and after coating with a PB-d6 thin film (I/PB, closed blue circles, shifted by a factor of 3) and a thermally oxidized and silanized Si wafer before (II, open red triangles, shifted by a factor of 1000) and after coating with a PB-d6 thin film (II/PB, closed red triangles, shifted by a factor of 3000). 10762
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Figure 4. (a and b) Fresnel representation of the TOF-NR curves of the PB-d6 films on the two substrates (symbols) with fits of the variable layer thickness model (solid lines): low (a) and high qz region (b). Film on thermally oxidized Si wafer (I/PB, blue closed circles, full blue line) and on additionally silanized substrate (II/PB, red closed triangles, dashed red line). (c and d) Corresponding scattering length density (SLD) profiles of the PB-d6 film on the thermally oxidized Si substrate (I/PB, full blue line) and on the additionally silanized substrate (II/PB, dashed red line). The profiles were shifted along z to match at the substrate surface. (c) Full z range and (d) enlargement of the interfaces.
A clearer view is gained in Fresnel representation, Rqz4 vs qz (Figure 3). The critical qz value of total reflection qcz (Figure 3a) is for both substrates approximately 0.008 Å−1. This critical edge arises from total reflection at the interface between Si and SiOx, because the SLD of SiOx (3.47 × 10−6 Å−2 for SiO2)40 is higher than the one of Si (2.07 × 10−6 Å−2), and thus the refractive index is lower. Below qcz, the neutron beam is totally reflected at the Si−SiOx interface. From the periodicity of the Kiessig fringes, the thickness of the SiO2 layer in substrate I is estimated at approximately 2770 Å and the one of the SiO2 layer with TEOS-PB (II) at approximately 2790 Å. The difference may be due to the thickness of the TEOSPB layer and to slight differences in the thickness of the SiO2 layer because of different conditions during thermal oxidation. The TOF-NR curves of the two substrates differ mainly in the high qz region (qz > 0.1 Å−1, Figure 3b). Here, the substrate with a TEOS-PB layer (II) shows a lower reflectivity than the plain substrate (I). The monofunctional TEOS-PB with a PB molar mass of 4.2 kg/mol is expected to form a self-assembled monolayer having a thickness similar to the radius of gyration of PB, i.e., approximately 60 Å.41 The SLD of the (protonated) TEOS-PB (approximately 0.80 × 10−6 Å−2)40 is between the ones of SiO2 and air. Hence, it is difficult to resolve. The SLD profiles corresponding to the best fits of the variable layer thickness model are shown in Figure 3c. Three layers of variable layer thickness and variable SLD were
be approximately 30 mJ/m2 with a polarity of approximately 0.3. Thus, the surface tension of the thermally oxidized Si wafer is slightly lower than the one of PB-d6 (33−36 mJ/m2), but it has a much higher polarity (difference of xP approximately 0.3). The contact angles of the liquids on the thermally oxidized and TEOSPB coated Si wafer (II) are consistently lower (Table 1), especially the one of paraffin oil, which indicates a better wettability of the silanized substrate for hydrocarbons. The surface tension of the silanized substrate II is 36 mJ/m2 with a polarity of 0.3, the latter being unchanged compared to substrate I. Thus, silanization with TEOS-PB leads to a slight increase of the surface tension of the substrate, but it is still within the range of PB. However, the polarities of the substrates are higher than the one of PB. We assume that the TEOS-group is close to the surface of the SiOx layer whereas the PB chain covers the substrate. AFM measurements on thermally oxidized Si wafers in the as-prepared-state are featureless (not shown). A root-mean-square (RMS) roughness of 4 Å and a surface ratio of approximately 1.02 are deduced. The silanization with TEOS-PB only increases the RMS roughness to 13 Å; the surface ratio is unchanged. Thus, both substrates are very smooth. The TOF-NR curves of substrates I and II are presented in Figure 2. The curves have a very similar critical edge, qcz, regular Kiessig fringes with slightly different periods, presumably due to the thickness of the TEOS-PB layer. 10763
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Figure 5. (a and b) Fresnel representation of the TOF-NR curves of the PB-d6 films on the two substrates (symbols) with fits of the fixed layer thickness model (solid lines): low (a) and high qz region (b). (c and d) Corresponding scattering length density (SLD) profiles of the PB-d6 films. (c) Full z range and (d) enlargement of the interfaces. Same symbols as in Figure 4.
good agreement with the value expected from the Schubert equation (approximately 1100 Å).35 We have used both, the variable layer thickness model (Figure 4) and the fixed layer thickness model (Figure 5) to fit the TOF-NR curves of the PB-d6 films I/PB and II/PB. At this, the substrate parameters (I and II) were used as fixed parameters. In both models, the PB-d6 film consists of a homogeneous layer, the bulk part, and, near the SiO2 interface and the PB-d6 surface, layers of variable SLDs and either variable or fixed layer thickness. Both models show good agreement for both samples (I/PB and II/PB), only the fixed layer thickness model shows less pronounced fringes in the high qz region (Figure 5b) than the variable layer thickness model (Figure 4b) but fits the experimental data equally well. The corresponding SLD profiles mainly consist of the homogeneous bulk part of the PB-d6 films (Figures 4c and 5c) with SLD values of 6.55 × 10−6 Å−2 for sample I/PB and 6.48 × 10−6 Å−2 for sample II/PB. which are independent of the substrate properties. Both SLD profiles show a decrease of the SLD of PB-d6 at the interface to air (Figures 4d and 5d) which may be due to surface roughness. The overall thickness of these layers of decreasing SLD amounts to 96−100 Å for sample I/PB and to 40−49 Å for sample II/PB. Thus, the surface roughness seems to be higher when the film is spin-coated onto the nonsilanized substrate I, which may be due to less good wettability than on the silanized substrate II. We note that the spike in the fixed layer thickness model at the film surface
considered near each interface, having the lowest allowed layer thickness of 20 Å. The resulting overall thickness of the SiO2 layer is 2768 Å with a surface width of 46 Å for the thermally oxidized Si wafer (I) and 2849 Å with a surface width of 77 Å for the silanized substrate (II). These thickness values are in agreement with the value of 2600 Å measured using white light interferometry (not shown). The increase of surface width may be due to the TEOS-PB layer. The SLDs obtained from the fits are 2.07 × 10−6 Å−2 for Si (for both samples), i.e., exactly the literature value, and 3.49 × 10−6 Å−2 (sample I) and 3.46 × 10−6 Å−2 (sample II) for SiO2, i.e., values very close to the literature value (3.47 × 10−6 Å−2). Thus, reasonable values are obtained for all parameters. The depression of the SLD at the Si/SiO2 interface is in good agreement with theoretical assumptions.42 3.2. Characterization of PB-d6 Thin Films. Both substrates I and II were coated with a PB-d6 film of thickness ∼1000 Å. The TOF-NR curves of the PB-d6 coated substrates (I/PB and II/PB) are given in Figure 2. The Kiessig fringes have a larger period than the ones of the substrates, and their shape is more complex, which is due to the higher overall thickness of the SiO2/(TEOS-PB)/PB-d6 layers and the interference of the neutron beam at the interfaces in the system. The critical value qcz is now approximately 0.015 Å−1 for both samples (I/PB and II/PB), which corresponds to 6.55 × 10−6 Å−2, the expected value. From the period of the Kiessig fringes, we estimate that the thickness of the PB-d6 films is in the range 1140−1170 Å, independent of the substrate and in 10764
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where φSiO2 is the volume fraction of SiO2 in this layer and is determined by
(Figure 5d) is probably an artifact. Its origin remains at this point unclear. 3.3. Free Volume in PB-d6. The SLD profiles of both samples reveal a decrease of the SLD of PB-d6 near the substrate (Figures 4d and 5d) with slight differences according to the fitting model used. We attribute this decrease to an increase of the free volume in the PB-d6 melt and calculate therefore an average free volume of the interphase. (Since the PB-d6 films were prepared from solutions from fully deuterated toluene with similar SLD as PB-d6, residual solvent should not give rise to a decrease.) Figure 6a shows as an example the SLD
φSiO = 2
SLD1* SLDSiO2
(3)
where SLD*1 is the SLD of the surface layer in the measurement on substrate I. Overall, the interphase has a thickness d1 + d2 and an average SLD of SLDIP =
SLD1d1 + SLD2 d 2 d1 + d 2
(4)
Thus, the free volume is φFV =
SLDIP(d1 + d 2) − φSiO SLDSiO2 d1 2
[(1 − φSiO )d1 + d 2]SLDPB
(5)
2
For the thermally oxidized substrate I, d1 = 46 Å and φSiO2 = 6.9%, as determined from the SLD profile of the blank substrate. d2 calculates to 85 Å and, we get φFV = 5.7%. Neglecting the SiO2 protrusions in the bottom layer, φFV = 5.7% is found. The values from the fixed layer thickness model and the ones from the sample on the silanized substrate (II/PB) are compiled in Table 2. The value of 5.7% for sample Table 2. Fraction of Excess Free Volume in the PB-d6 Interphase φFV (%)
profiles of substrate I and sample I/PB near the SiO2/PB-d6 interface. It is seen that the SLD of the SiO2 surface decays over a height of approximately 46 Å which is attributed to surface roughness. The SLD of PB-d6, in contrast, increases over a wider range, namely approximately 131 Å. Figure 6b shows in a simplified way the corresponding structure of the interface: An interphase of PB-d6 containing higher free volume (indicated by the white circles) is present near the rough SiO2 surface. For simplicity, we distinguish two layers: The upper layer, having a thickness d2 and a SLD which is the average of PB and the free volume (i.e., vacuum) (1)
where φFV is the volume fraction of the free volume in PB-d6 and SLDPB is the scattering length density of PB-d6. The bottom layer has a thickness d1 and a SLD of SLD1 = φSiO SLDSiO2 + (1 − φSiO )SLD2 2
2
variable layer thickness model
fixed layer thickness model
I/PB II/PB
5.7 12.4
5.7 20.6
I/PB is confirmed by the fixed layer thickness model. The values for sample II/PB, however, lie unrealistically high, which may be due to the TEOS-PB layer. All values may be slightly biased because of unaccessible holes in the SiO2 layer which have not been taken into account. Assuming that the decay of the SLD near the substrate interface would be entirely due to an accumulation of residual solvent, we have carried out a calculation in line with the one shown above. It results in a volume fraction of deuterated toluene of 41%, a value which is unrealistically high. We therefore discard the possibility of the enrichment of residual solvent at the substrate interface. 3.4. Necessity of Two-Interphase Model. The question arises whether the interphase is really present or whether the TOF-NR curves can be fitted equally well by simpler models, which do not include an interphase near the substrate interface. We have therefore attempted fits with the following models: (i) The PB-d6 film is homogeneous with a flat surface, and the surface roughness of the substrate is taken into account; (ii) same as (i) but allowing for an interphase near the substrate; and (iii) same as (i) but allowing for a finite surface roughness of the PB-d6 film. These are described in the following. Figure 7a−c shows the fit of the TOF-NR curves with a homogeneous PB-d6 film. The decay of the SLD of PB-d6 near the interface with SiO2 is due to the surface roughness of SiO2 (Figure 7c). In this region, the SLD of PB-d6 was calculated using the bulk density, i.e., φFV = 0. Good agreement is obtained in the low qz region of the TOF-NR curves (Figure 7a); however, there is a clear mismatch at higher qz values (qz > 0.08 Å−1, Figure 7b):
Figure 6. (a) SLD profiles of the thermally oxidized Si wafer (I, full green line) and the PB-d6 film on this substrate (I/PB, dashed pink line) near the SiO2/PB-d6 interface. Both profiles are from the variable layer thickness model. (b) Sketch of the interface. Light gray, PB-d6; dark gray, SiO2; white, excess free volume. The rough surface of SiO2 is sketched as rectangles.
SLD2 = (1 − φFV )SLDPB
sample
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Figure 7. Alternative models. (a−c) Model of a homogeneous PB-d6 film with a flat surface, taking the surface roughness of the substrates into account. (d−f) Same as (a−c) but allowing for an interphase near the substrate. (g−i) Same as (a−c) but allowing for a finite surface roughness of the PB-d6 film. Fresnel representation of the TOF-NR curves of the PB-d6 films on the two substrates (symbols) with fits of the fixed layer thickness model (solid lines): low (a, d, g) and high qz region (b, e, h). (c, f, i) Corresponding scattering length density (SLD) profiles of the PB-d6 films. Same symbols as in Figure 4.
The model curve overestimates the reflectivity at high qz. Hence, the TOF-NR curves cannot be described by a homogeneous film. In another attempt, an interphase near the SiO2 interphase was included but the surface of the PB-d6 film was kept smooth (Figure 7d−f). Again, the best fits show deviations at qz > 0.08 Å−1 (Figure 7e). We conclude that the film surface is not smooth. Allowing the surface of the PB-d6 film to be rough but not including an interphase near the substrate surface (only the roughness of the substrate, Figure 7g−i), deviations are observed for qz > 0.06 Å−1 (Figure 7h), where the fitting curve has very low amplitude Kiessig fringes which are not present in the experimental data. In conclusion, none of the simpler models describes the experimental TOF-NR curves adequately. It is necessary to include both, an interphase of decaying SLD at the interface with the substrate and a certain roughness of the surface of the PB-d6 film, as described above (Figures 4 and 5). Our results demonstrate that measurements at high qz values having good statistics are needed to make this distinction.
which resulted in a SiO2 layer of 2770 Å thickness with a surface width of 46 Å. To alter the substrate properties, the thermally oxidized Si wafer was additionally coated with low molar-mass, triethylethoxysilyl modified 1,2-PB (TEOS-PB). The SiO2 thickness was now 2850 Å and the surface width had increased to 77 Å. Both the higher values of the thickness and surface width may be due to the coating with TEOS-PB but may also result from slightly different conditions during the oxidation process. The surface tension increases by silanization but is still within the range of surface tensions given in the literature for PB, and the surface polarity is unchanged; however, the wettability for hydrocarbons is improved after silanization. In both cases, the SiO2 layers are sufficiently thick to shield the interaction between the polymer PB-d6 and Si.25 They make the substrates similar to the glass substrates previously used by us previously in optical waveguide spectroscopy/surface plasmon resonance investigations of PB melts near the interface with glass.26 In these optical investigations, however, the depth resolution is not sufficient to characterize a possible interphase in detail. Thin films of fully deuterated polybutadiene (PB-d6) having a molar mass of 55 kg/mol were applied onto these two substrates by spin-coating from toluene-d8 solution. The film thicknesses amount to 1140−1170 Å on both substrates.
4. SUMMARY AND DISCUSSION In summary, thin films of PB-d6 near a SiO2 interface were investigated using time-of-flight neutron reflectometry. SiO2 interfaces were created by thermal oxidation of the Si wafers 10766
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Author Contributions
TOF-NR curves could be measured with excellent statistics (down to reflectivities of 1.5 × 10−6) and revealed a large number of Kiessig fringes having a complex shape. They were analyzed using various models allowing for scattering length densities (SLDs) near the film surface and the interface with the substrate different from the bulk of the film. To exclude artifacts, a number of thin layers were implemented into the model SLD profiles both near the film surface and near the substrate, and either the layer thickness was allowed to vary or the number of layers (having a fixed thickness). Indeed, decays of the SLDs are present near the interface with the substrates, and the thickness of this interphase is in both cases higher than the surface width of the substrates. We note that the literature value of the SLD is recovered in the center of the PB-d6 films. Attributing the decay of the SLD near the substrate interface entirely to an excess free volume, its volume fraction is estimated at 5−6% on the thermally oxidized Si wafer. Higher values are obtained on the silanized substrate, which seem unrealistic and may be prone to artifacts, presumably because of the TEOS-PB layer. We note that, to exclude artifacts, not the detailed shape of the decays was used for the analysis but only the average SLD of the interphase. Moreover, the surface roughness of the PB-d6 thin films increases upon silanization. To verify whether the interphase in PB-d6 near the substrate is not an artifact, we applied simpler models, such as a homogeneous film, but none of them describes the curves adequately. The relative decrease of the scattering length density in the interphase found by us is with 5.7% lower than the ones given in refs 17 and 28, and the interface width is with 85 Å larger even though the molar mass of PB-d6 (55 kg/mol) is smaller than the ones used in refs 17 (90−450 kg/mol) and 28 (66− 1030 kg/mol). The difference may lie in the chemical nature of the substrate and of the polymer: A thick SiO2 layer serves as the substrate in our study, whereas Si with a thin oxide layer was used in refs 17 and 28. Moreover, PB-d6 may equilibrate more easily because of its lower molar mass and its significantly lower Tg. It may therefore also behave differently from the glassy PS films studied by Perlich et al.29 A decay of the mass density has been observed in chemically realistic computer simulations on PB near graphite.13 This decay goes along with a layering and a preferential orientation of the chains along the wall but extends only over a few nm. In the system presented here, the interphase extends over a wider range, on the order of 10 nm. The difference may be due to the difference in substrate properties and in substrate/polymer interaction and may be related to the dewetting observed for PB on SiOx at later stages, similar to what was observed in PS thin films on SiOx.25 While the TOF-NR measurements cannot give molecular information, they allow quantifying the mass density profile near the substrate.
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Prof. P. Müller-Buschbaum for fruitful discussions and B. Wang for help with the experiments. This work was financially supported by Deutsche Forschungsgemeinschaft within the priority program SPP 1369 ‘PolymerSurface Contacts: Interfaces and Interphases’ (Pa771/7-1). We thank FRM II for providing beamtime.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Present Address §
Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom. 10767
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