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X-Ray and Neutron Reflectometry Study of Glow-Discharge Plasma Polymer Films Andrew Nelson,*,† Benjamin W. Muir,‡ James Oldham,‡ Celesta Fong,‡ Keith M. McLean,‡ Patrick G. Hartley,‡ Sofia K. Øiseth,‡,# and Michael James† Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Menai, New South Wales 2234, Australia, CSIRO Molecular and Health Technologies, Clayton South, Victoria 3169, Australia, and School of Chemistry, Monash UniVersity, Clayton South, Victoria 3169, Australia ReceiVed August 12, 2005. In Final Form: October 19, 2005 Radio-frequency glow-discharge plasma polymer thin films of allylamine (AA) and hexamethyldisiloxane (HMDSO) were prepared on silicon wafers and analyzed by a combination of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray reflectometry (XRR), and neutron reflectometry (NR). AFM and XRR measurements revealed uniform, smooth, defect-free films of 20-30 nm thickness. XPS measurements gave compositional data on all elements in the films with the exception of hydrogen. In combination with XRR and NR, the film composition and mass densities (1.46 and 1.09 g cm-3 for AA and HMDSO, respectively) were estimated. Further NR measurements were conducted with the AA and HMDSO films in contact with water at neutral pH. Three different H2O/D2O mixtures were used to vary the contrast between the aqueous phase and the polymer. The amount of water penetrating the film, as well as the number of labile protons present, was determined. The AA film in contact with water was found to swell by ∼5%, contain ∼3% water, and have ∼24% labile protons. The HDMSO polymer was found to have ∼6% labile protons, no thickness increase when in contact with water, and essentially no solvent penetration into the film. The difference in the degree of proton exchange within the films was attributed to the substantially different surface and bulk chemistries of the two films.
Introduction Radio-frequency glow-discharge plasma polymers films can be used to generate thin-film coatings for a number of applications, including the biomedical, microtechnology, aerospace, and automotive industries.1-9 The coatings are an attractive route to surface modification by virtue of their ability to introduce defined chemical functionalities at interfaces, within robust, thin films (typically 50-1000 Å). The coatings contour and adhere strongly to the surfaces of both organic and inorganic bulk materials and can be used as supports for further surface-grafting reactions. During the plasma process, an activated monomer introduced in the gas phase under vacuum undergoes fragmentation, excitation, and ionization and rearranges and reacts to form a cross-linked polymer matrix, which is deposited on the desired substrate. The physicochemical properties of a plasma polymer of a specific monomer may be quite different to those of a conventional polymer. For example, the hydrogen content for plasma polymers is low in comparison to the polymer of the corresponding monomer due to considerable cross-linking.10 * To whom correspondence should be addressed. E-mail: andrew.nelson@ ansto.gov.au. † Australian Nuclear Science and Technology Organisation. ‡ CSIRO Molecular and Health Technologies. # Monash University. (1) Suzuki, K.; Sawabe, A.; Yasuda, H.; Inuzuka, T. Appl. Phys. Lett. 1987, 50, 728. (2) Moisan, M.; Barbeau, C.; Claude, R.; Ferreira, C. M.; Margot, J.; Paraszczak, J.; Sa´, A. B.; Sauve´, G.; Wertheimer, M. R. J. Vac. Sci. Technol. B 1991, 9, 8. (3) Biederman, H.; Slavinska, D. Surf. Coat. Technol. 2000, 125, 371. (4) Favia, P.; D’Agostino, R. Surf. Coat. Technol. 1998, 98, 1102. (5) Musil, J. Vacuum 1996, 47, 145. (6) Clark, D. T.; Hutton, D. R. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 2643. (7) Biederman, H. In Plasma Polymer Films; Biederman, H. Ed.; Imperial College Press: London, 2004. (8) Hume, E. B. H.; Baveja, J.; Muir, B.; Schubert, T. L.; Kumar, N.; Kjelleberg, S.; Griesser, H. J.; Thissen, H.; Read, R.; Poole-Warren, L. A.; Schindhelm, K.; Willcox, M. D. P. Biomaterials 2004, 25, 5023. (9) Fo¨rch, R.; Zhang, Z.; Knoll, W. Plasma Processes Polym. 2005, 2, 351.
Moreover, plasma polymers generated from a particular monomer may possess different chemical composition depending on the operational conditions (monomer flow rate, pressure, radio frequency, and RF power).2 A variety of techniques have been used to characterize the physicochemical properties of plasma polymer films, including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, mass spectrometry methods (e.g., TOF-SIMS), contact angle goniometry (CAG), and atomic force microscopy (AFM).3,7 These techniques are well described in the literature and are suitable for analyzing the surface of plasma polymer films although AFM and CAG are limited to dry samples. In many cases, however, it is also desirable to probe the internal structure of plasma polymer coatings in order to optimize their properties for a given application. Reflectometry techniques are now becoming increasingly important in the characterization of nanoscalestructured interfaces.11-13 X-ray reflectometry (XRR) in particular is ideally suited to the study of the internal properties of layered film structures on surfaces, yielding data on subsurface structure and material properties. Neutron reflectometry (NR) meanwhile offers the ability to also characterize surface layers in aqueous environments. Isotopic substitution between hydrogen and deuterium, particularly in an aqueous phase in contact with a surface, provides contrast between the substrate, the film, and solution. Jeon et al. recently described the use of NR to investigate the microstructure properties of plasma-polymerized methyl methacrylate films.11 They demonstrated, by swelling in d(10) Gengenbach, T. R.; Chatelier, R. C.; Griesser, H. J. Surf. Interface Anal. 1996, 24, 271. (11) Jeon, H. S.; Wyatt, J.; Harper-Nixon, D.; Weinkauf, D. H. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2522. (12) Van der Lee, A.; Roualdes, S.; Berjoan, R.; Durand, J. Appl. Surf. Sci. 2001, 173, 115. (13) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikstro¨m, K. Polymer 2000, 41, 6851.
10.1021/la052196s CCC: $33.50 © 2006 American Chemical Society Published on Web 11/24/2005
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nitrobenzene, that the film had a uniform cross-link density and also showed it to have a lower scattering length density than a conventional PMMA film. The plasma-polymerized PMMA film swelled by 7.5% in comparison to the 36% swelling demonstrated by the conventional PMMA film, suggesting a difference in chemical structure between the plasma-polymerized and conventional polymer films. Using SPR measurements combined with micromechanical cantilever sensors, Forch et al.9 have demonstrated swelling of allylamine (AA) plasma polymer films and have noted differences in both vertical thickness and lateral expansion depending on deposition conditions. In this study, we have investigated the properties of AA and hexamethyldisiloxane (HMDSO) plasma polymer thin films using XRR and NR in air and aqueous environments. This choice of coatings demonstrates contrasting physicochemical properties, e.g., contact-angle, chemical functionality.9,10,14,15 Surface chemistry of the plasma polymer coatings was characterized using XPS, and AFM was used to characterize both roughness and local film thickness at step-edges on the films. Correlation of XPS and AFM data with XRR and NR measurements on the plasma polymer film versus air has allowed the stoichiometric composition and mass densities of the films to be obtained. Further, NR measurements were conducted with the AA and HMDSO films in contact with water to determine the amount of water penetrating the films. These studies demonstrate the utility of these techniques in understanding the nature of these films. Materials and Methods Substrates. Ultra-flat single-crystal, silicon wafers (〈111〉 , 10 cm diameter, 1 cm thick, Silrec Corporation, San Jose, CA and 〈100〉, 1 cm2 × 0.5 mm thick, from M. M. R. C. Pty Ltd, Melbourne, Australia) were used as substrates for the deposition of plasma polymer thin films. Prior to plasma deposition, the wafers were rinsed with ethanol and cleaned in aqua regia (3:1 HCl/HNO3) for an hour to remove any inorganic contaminants. The wafers were further treated with piranha solution (20% H2SO4 in concentrated H2O2) for 3 h to destroy residual organic contaminants. The wafers were thoroughly rinsed with Milli-Q water and blown dry with nitrogen gas after each step. This procedure did not introduce any measurable roughening of the surface as assessed by AFM. The cleaning protocol produces a hydrophilic surface, leaving a relatively dense layer of Si-OH groups on a silicon oxide layer.16 Two large wafers (10 cm diameter × 1 cm thick) were used for reflectometry measurements, and smaller wafers (1 cm2 × 0.05 cm thick) were used as substrates for AFM and XPS characterization. Plasma deposition on the large and small wafers was performed simultaneously. Plasma Polymer Deposition. Plasma polymerization of AA (98%, Sigma-Aldrich) and HMDSO (98.5%, Sigma-Aldrich) were carried out in a custom-built reactor described elsewhere.17 Briefly, the cylindrical reactor chamber is defined by a height of 35 cm and a diameter of 17 cm. Within this sit two circular electrodes 10.3 cm in diameter, spaced 15 cm apart. Samples were placed on the lower grounded electrode, and a continuous radio frequency pulse was generated at the upper electrode. The monomer vapors were supplied to the reactor chamber from the liquid reagents contained in a roundbottom flask via a stainless steel line and a manual valve for fine control of the flow. The AA and HMDSO monomer flasks were kept on ice and in ambient air, respectively, during the experiments. The monomer liquids were degassed before plasma deposition. (14) Gengenbach, T. R.; Chatelier, R. C.; Griesser, H. J. Plasmas Polym. 1999, 4, 283. (15) Gengenbach, T. R.; Griesser, H. J. Polymer 1999, 40, 5079. (16) Carim, A. H.; Dovek, M. M.; Quate, C. F.; Sinclair, R.; Vorst, C. Science 1987, 237, 630. (17) Griesser, H. J. Vacuum 1989, 39, 485.
Nelson et al. The plasma deposition of AA was performed using a frequency of 200 kHz, a power of 20 W, and initial monomer pressure of 20 Pa for a treatment time of 25 s. Similarly, deposition conditions for HMDSO were a frequency of 225 kHz, a power of 10 W, an initial monomer pressure of 10 Pa, and a treatment time of 10 s. Prior to plasma deposition, the reactor was evacuated to a base pressure of less than 0.1 Pa. After deposition, the reactor was immediately pumped down to base pressure before venting. The samples were stored in clean tissue culture grade Petri dishes under ambient conditions until further analysis. Atomic Force Microscopy. The topography and thickness of the plasma polymer films was determined by AFM using a Nanoscope Multimode atomic force microscope (Digital Instruments). Imaging was performed in tapping mode. Solvent-cast poly(D,L-lactide) (10% in acetone) was used to mask part of the Si wafers during plasma polymer deposition, allowing production of well-defined steps as analyzed by the AFM.18 Average step heights were obtained using the ‘step height analysis’ feature in the Nanoscope software for a typical scan size of 10 × 10 µm2. X-Ray Photoelectron Spectroscopy. To investigate the chemical composition of the plasma polymer coatings, XPS was employed. XPS analysis was performed using an AXIS Hsi spectrometer (Kratos Analytical Ltd.), equipped with a monochromated Al KR source at a power of 300 W (15 mA, 12 kV). Charging of the samples during irradiation was compensated by the internal flood gun. The pressure in the main vacuum chamber during analysis was typically 5 × 10-6 Pa. Spectra were recorded with the photoelectron detection normal to the sample surface. The elemental composition of the samples was quantified from survey spectra (320 eV pass energy). Highresolution spectra recorded from individual peaks at 40 eV pass energy gave equivalent atomic concentrations. It is important to note that XPS does not detect light elements such as hydrogen and helium. Contact Angle Measurements. Contact angles were determined using custom-built equipment. A video picture was captured using a Nikon Macro lens, the drop was digitized, and the profile fitted to the equation of Young and Laplace. The method used is based closely on the work of Jennings et al.19 Once the drop had been matched, the contact angle was calculated from the intersection of the theoretical profile with the baseline. The contact angles given are the average of three separate measurements on different regions of the same plasma polymer sample. Reflectometry Measurements. XRR and NR measurements were performed at the Australian Nuclear Science and Technology Organization (ANSTO), Sydney, Australia. Both the NR and XRR measurements were performed as a function of incident angle (θ), measuring the specularly reflected beam as a function of the momentum change perpendicular to the surface (Qz). The XRR measurements were performed in air on a Panalytical X’Pert Pro reflectometer operating with Cu KR (λ ) 1.54056 Å) radiation. X-rays from a (45 kV) tube source were focused with a Go¨bel mirror, collimated with pre- and postsample slits (0.2 mm), and detected with a Xe scintillator detector. NR data were collected over the Q range 0.007-0.200 Å-1 with a (∆Q/Q) resolution of 5% on the X172 reflectometer at the HIFAR facility using thermal neutrons (λ ) 2.43 Å).20 Reflectometry probes the structure of a thin film normal to the surface. By refining a structural model and fitting observed reflectivity data, the thickness (d), scattering length density (SLD, F), and interfacial roughness (σ) between layers may be determined. For radiation reflected at angles below the critical angle for total external reflection (θc), a plateau exists where R ) 1. For incident angles greater than θc, R rapidly decreases in intensity (as ∼Qz-4 where Qz ) 4π sin θ/λ). Superimposed on this “Fresnel” reflectivity are oscillations (Kiessig fringes) that are due to interference of the reflected radiation from the superphase-plasma polymer and plasma (18) Hartley, P. G.; Thissen, H.; Vaithianathan, T.; Griesser, H. J. Plasmas Polym. 2000, 5, 47. (19) Jennings, J. W.; Pallas, N. R. Langmuir 1988, 4, 959. (20) James, M.; Nelson, A.; Schulz, J. C.; Jones, M. J.; Studer, A. J.; Hathaway, P. Nucl. Instrum. Methods Phys. Res., Sect. A 2005, 536, 165.
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Table 1. Elemental Compositions (atomic %) of Native Silicon Wafer and Plasma Polymer Films Derived from High-Resolution XPS Spectraa % composition
C 1s
N 1s
O 1s
Si 2p
Si wafer AA monomer AA plasma polymer HMDSO monomer HMDSO plasma polymer
12 75 81.0 66.7 47.5
25 13.5 -
30 5.5 11.1 18.5
58 22.2 34
a
The theoretical monomer compositions are shown for comparison.
polymer-subphase interfaces. From the period of the Kiessig fringes (∆Qz), the thickness (d) of the film may be determined by d ≈ 2π/∆Qz. NR was also used to investigate the swelling behavior of plasma polymer films in various solvents. For this purpose, the plasma polymer-coated Si wafers were mounted in a Teflon-based solidliquid cell that has been described previously.20 Neutrons enter this cell through the side of the silicon wafer and are reflected from various interfaces before exiting through the other side of the wafer. The solid-liquid experiments were repeated at three different contrasts: specifically, the film was measured against D2O (Fsolv ) 6.36 × 10-6 Å-2), contrast-matched water (Fsolv ) 3.47 × 10-6 Å-2), and H2O (Fsolv ) -0.56 × 10-6 Å-2). This procedure helps in the search for an unambiguous film structure since scattering from different parts of the system is highlighted by matching of the “refractive index” for neutrons. The XRR and NR from the deposited AA and HMDSO plasma polymer films were modeled using the Parratt formalism,21 with the Motofit analysis package.22 The data were fitted as log(R) vs Q, with corrections for a linear background, resolution smearing, and Gaussian roughness at each interface. Refinements of the NR data were performed using a three-layer model that comprised a single homogeneous plasma polymer on a native silicon oxide layer with Gaussian roughness (typically 3 nm thick) on top of silicon. The neutron scattering length density values for Si and SiO2 were set at 2.07 × 10-6 and 3.47 × 10-6 Å-2, respectively. For XRR measurements, the scattering length density of the Si substrates was set at its literature value (20.1 × 10-6 Å-2). The native SiO2 layer was not included in these models, as the SLD is essentially the same as for Si and so offers no contrast for modeling as a discrete layer.
Results and Discussion Surface Chemistry. The results of XPS analysis of freshly deposited films of AA and HMDSO plasma polymers and an untreated Si wafer are shown in Table 1. The untreated wafer contains a small amount of carbon due to atmospheric hydrocarbon contamination. Elemental analysis by XPS reveals an AA film comprising carbon, nitrogen, and oxygen with atomic concentrations of 81.0%, 13.5%, and 5.5%, respectively, which is consistent with previous studies of these films.14 The chemical composition of the film differs significantly from the monomer, for which the nitrogen content is substantially higher (75% C, 25% N). The absence of silicon in the recorded spectra suggests a plasma polymer film thickness in excess of the XPS analysis depth of 10 nm. Deconvolution of the high-resolution carbon (C1s) spectra (not shown) reveals a film that is rich in hydrocarbon and carbonnitrogen and carbon-oxygen moieties such as amine- and amidecontaining species. The oxygen incorporation originates from plasma-activated air/water residues in the reactor and post-plasma oxidation reactions upon exposure of the film to ambient atmosphere.10 For the HMDSO plasma polymer film, XPS analysis (Table 1) indicates the presence of carbon, oxygen, and silicon. As (21) Parratt, L. G. Phys. ReV. 1954, 95, 359. (22) Nelson, A., MOTOFIT: Co-refinement of Neutron and X-ray reflectiVity data; http://motofit.sourceforge.net/, 2005.
Table 2. Film Thickness, Roughness, Scattering Length Density, and Composition of AA and HMDSO Plasma Polymer Films in Air as Determined by AFM, XRR, and NR AA SiO2 thickness (NR only) SiO2 roughness (NR only)
11 ( 2 5(3
HMDSO 9(4 4(3
AFM XRR NR
Film Thickness (Å) 267 278 ( 1 278 ( 8
191 213 ( 1 213 ( 3
AFM XRR NRa
Film Roughness (Å) 2.4 5(1 5
1.9 3(1 3
Scattering Length Density (F) (×106 Å-2) XRR 13.33 ( 0.08 9.98 ( 0.05 NR 2.03 ( 0.02 0.20 ( 0.02 composition C162O11N27H205 Si68O37C95H270 mass density (g cm-3) 1.46 1.09 a
Fixed to the value determined using XRR data
Figure 1. AFM images of a masked HMDSO plasma polymer film.
silicon is present in both the substrate and the coating, XPS cannot be used to definitively assess the homogeneity of coverage of the plasma polymer (see below). This was confirmed by AFM and reflectometry. The plasma polymer coating was enriched in both oxygen and silicon compared to the HMDSO monomer and is similar to poly(dimethylsiloxane) (PDMS) in chemical composition. The HMDSO plasma polymer film has a carbon/ silicon atomic ratio (C/Si) of 1.4 which is half that of the starting monomer (C/Si ) 3) but close to that of PDMS (C/Si ) 2). Although the atomic percents of O and Si are substantially different in the film and the monomer (Table 1), the oxygen/ silicon ratio of the plasma polymer (O/Si ) 0.6) is close to that of the monomer (O/Si ) 0.5). The chemical composition of the film and reduction in the carbon content suggests that methyl abstraction is a major fragmentation/activation pathway during plasma polymerisation. In contrast, Si-O bond scission does not appear to occur to an appreciable extent, as found by other workers.15,10 Characterization of the Films Using Atomic Force Microscopy (AFM). The thicknesses of the as deposited plasma polymer films were determined by AFM (Table 2). Figure 1 shows a typical AFM step height image recorded at the interface of a masked area of a HMDSO film. The image illustrates the flat, smooth, and defect-free nature of these films. A sharp boundary exists between the coated and uncoated regions of the Si wafer, demonstrating that minimal damage occurs to the plasma polymer film upon removal of the polymer mask. The HMDSO film thickness was 191 Å with a RMS roughness of 1.9 Å, while the AA film thickness was 267 Å with a RMS roughness of 2.4 Å.
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Figure 2. XRR and NR spectra from the air-AA-silicon system. The lines represent fits to the data. NR data offset by a factor of 10-2.
Figure 3. XRR and NR spectra from the air-HMDSO-silicon system. The lines represent fits to the data. NR data offset by a factor of 10-2.
Characterization of the Films Using XRR and NR in Air. The air-solid reflectivity measurements from the AA and HMDSO films are shown in Figures 2 and 3, respectively, while the refined structural parameters are given in Table 2. The symbols represent the observed reflectivity data, while the solid lines are calculated reflectivity profiles determined from the structural models. In each case, an excellent fit to these data was obtained using a single-layer model, based on polymer films of uniform density. The model also provides for a native SiO2 layer between the Si wafer and the polymer film. The lack of contrast between the Si and SiO2 layers with respect to X-rays means that the latter is only included as part of the model used in fitting the neutron data. The large number of Kiessig fringes in the XRR data (∼20), persisting to high Qz ()0.6 Å-1), where instrumental background usually dominates, confirms the extreme smoothness of these films and provides confidence for the precise determination of film thickness. The roughness values obtained from this X-ray analysis were used in all the subsequent analyses of the neutron datasets since the reflectivity is most sensitive to roughness at higher Q. In contrast, the neutron data were collected over a narrower Q-range, as no reflectivity signal above the instrumental background is observed beyond Qz ≈ 0.2 Å-1. The relatively weak fringes in the NR data (Figures 2 and 3) result from poor contrast between the neutron scattering length density of the polymer film (Ff) with either the substrate or the air layer above the film. For the AA plasma polymer, the refined scattering length density Ff ) 2.03 × 10-6 Å-2 (Table 2) is essentially the same as that for Si (FSi ) 2.07 × 10-6 Å-2), while
Nelson et al.
for the HMDSO polymer, the refined SLD (Ff ) 0.20 × 10-6 Å-2) is similar to the value (of zero) for air. Small discrepancies between AFM and reflectivity data likely result from the different analysis areas of the two techniques. The AFM thickness determination is performed by step height measurements on an area that is typically of the order of 10 µm × 10 µm, whereas the footprint of the reflectometry experiments are significantly larger, ∼1 cm × 1 cm for XRR and ∼8 cm × 3 cm for NR. By themselves, the XPS and XRR results are not sufficient to unambiguously apportion a composition and density for each of the plasma polymer films. When this information is used in conjunction with air-solid NR measurements, however, an accurate estimate of the composition and density of the plasma polymer film can be made. In particular, one may determine the hydrogen content of the plasma polymer layer, as XPS provides the elemental composition of all other atoms present. An assumption of this method is that the surface composition determined by XPS is similar to the bulk film composition. The coherent neutron scattering length (b) of H (-3.739 fm) is significantly different (both in sign and magnitude) to the other atoms (C, 6.646 fm; N, 9.36 fm; O, 5.803 fm; Si, 4.149 fm) found in these polymers. Notably, the neutron scattering length of deuterium (6.671 fm) is also very different to that of hydrogen, enabling isotopic labeling and contrast variation (see below). As the hydrogen content of these plasma polymer films is increased, the X-ray SLD increases and the neutron SLD decreases. By simultaneously fitting the composition and mass density to the X-ray and neutron SLD values determined for the AA films along with the XPS results (Table 1), a density of 1.46 g cm-3 and composition of C162O11N27H205 was determined (Table 2). A mass density of 1.09 g cm-3 and a composition Si68O37C95H270 were calculated using a similar protocol for the HMDSO plasma polymer. Interestingly, the density of PDMS (0.98 g cm-3) is close to that of the plasma-deposited HMDSO film, further highlighting the chemical and structural similarity of the two materials alluded to above. A similar mass density of organosilicon plasma polymer films has been reported by Van der Lee and co-workers.12 The density of the HMDSO film was determined to be lower than the AA plasma polymer, which can be explained in part by the greater bond length (1.64 Å) and angle (143°) of Si-O-Si species compared to the C-C tetrahedral geometry of (1.53 Å and 110°) the AA plasma polymer. The H/C ratio of 2.8 in the HMDSO plasma polymer film is close to that of HMDSO (H/C ) 3), indicating the retention of a significant concentration of methyl groups within the network of the plasma polymer film and a small degree of hydrogen abstraction during the plasma-deposition process. This correlates well with previous infrared studies of HMDSO plasma polymers by Gengenbach et al.15 Impact of Water on the Films Using Contrast Variation NR. NR provides an added dimension for probing the hydration behavior of these plasma polymer films in situ. The swelling behavior of plasma polymer films are of interest, as HMDSO films, in particular, are often touted as impermeable films and have been used in many applications where barrier properties have been required.12,13,15,23,24 In the current study, AA and HMDSO plasma polymer films were investigated by NR in different solvent systems: H2O, D2O, and a mixture of the two (HDmix). The three solid-liquid NR datasets obtained for each polymer film were co-refined. In this process, the film thicknesses, solvent penetration, and roughness are constrained to be the same for each dataset. The roughness of each of the layers and the thickness of the SiO2
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Table 3. Film Thickness, Roughness, Scattering Length Density, and Composition of AA and HMDSO Plasma Polymer Films in Contact with Water as Determined by NR AA SiO2 thickness (Å)a SiO2 roughness (Å)a film thickness (Å) film roughness (Å)a solvent penetration (% v/v) D2O - Fsolv Ff film composition HDmix - Fsolv Ff film composition H2O - Fsolv Ffa film composition a
HMDSO
11 5 293 ( 6 5 2.9 ( 0.7
9 4 213 ( 3 3 none b
Scattering Length Density (F) (×106 Å-2) 3.62 ( 0.05 C162N27O11H156D49 3.00 ( 0.01 C162N27O11H176.5D28.5
6.36 3.47 -0.56
2.03 C162O27N11H205
Fixed to the value determined using air-solid reflectivity data.
b
0.48 ( 0.13 Si68O37C95H254D16 0.46 ( 0.07 Si68O37C95H261D9 0.20 Si68O37C95H270
Within experimental error.
Figure 4. NR from the water-AA-silicon system. The HDmix data are offset by 10-1 and the H2O data are offset by 10-3 for clarity. The lines represent fits to the data.
Figure 5. NR from the water-HMDSO-silicon system. The HDmix data are offset by 10-1 and the H2O data are offset by 10-2 for clarity. The lines represent fits to the data.
layer were constrained to the values obtained for the air-solid measurement. To account for solvent penetration, the overall scattering length density of the polymer layer was fitted as a volume fraction weighted addition of the film scattering length density (Ff) and the solvent scattering length density (Fsolv). However, Ff was allowed to float independently for each contrast, allowing for proton/deuteron exchange between the solution and film. The exception was the H2O contrast, where the film scattering length density of the ‘wet’ film was assumed to be the same as that obtained from the ‘dry’ film. A further assumption is that all of the labile protons exchange rapidly, namely within ∼30 min of changing solvents. The NR profiles of the AA plasma polymer film against three different H2O/D2O contrasts are shown in Figure 4, with the refined model parameters given in Table 3. One immediately notices significant differences in the layer thicknesses and Ff compared to the data for the air-solid interface or dry film. There is an increase in the film thickness corresponding to ∼15 Å, which is attributed to solvent penetration. According to the structural refinements, the amount of solvent in the layer is 2.9% v/v. A comparison of the three reflectivity curves in the various solvents (Figure 4) highlights the effect of isotopic substitution of the solvent and serves to contrast the polymer film and the aqueous subphase. The film in contact with D2O, for example, has the best contrast between the aqueous phase (Fsolv ) 6.36 × 10-6 Å-2), the film (Ff ) 3.62 × 10-6 Å-2), and the Si wafer (FSi ) 2.07 × 10-6 Å-2). A further point to note in Figures 4 and 5 is that the reflectivity data for the H2O contrast does not
have a critical edge. The absence of a critical edge is typical for those systems where the subphase scattering length density (Fsolv -0.56 × 10-6 Å-2 for H2O) is less than that of the medium that contains the incident neutron beam (in this case, Si). The refined film scattering length densities (Ff) for the D2O and HDmix contrasts were both substantially higher than for the dry plasma polymer at Ff ) 3.62 × 10-6 and 3.00 × 10-6 Å-2, respectively (Table 3). Since solvent penetration has already been accounted for in this model, the variation in Ff is attributed to the labile exchange of protons/deuterons between the solvent and the polymer film. Given the empirical formula of the layer and its mass density (as calculated from the dry film above), as well as Ff for the wet film, the number of labile protons for each contrast variation can be determined. The empirical formula of the film arising from labile exchange was derived by substituting H atoms for D atoms in the NIST SLD calculator25 until the measured Ff was obtained. For the AA plasma polymer, this formula corresponds to C162N27O11H156D49, indicating that 49 of the 205 protons (24%) were labile. A consistency check for the model was made by considering the HDmix contrast. The bulk solution for this sample is made up of H2O/D2O in the ratio 0.418:0.582 (v/v). The chemical formula for this layer then becomes C162N27O11H176.5D28.5, which has a scattering length density of Ff ) 2.96 × 106 Å-2. This value agrees extremely well (23) Bonnar, M. P.; Wilson, J. I. B.; Burnside, B. M.; Reuben, R. L.; Gengenbach, T. R.; Griesser, H. J.; Beamson, G. J. Mater. Sci. 1998, 33, 4843. (24) Rats, D.; Hajek, V.; Martinu, L. Thin Solid Films 1999, 340, 33. (25) Available at http://www.ncnr.nist.gov/resources/sldcalc.html.
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with the value obtained by fitting the observed data (Ff ) 3.0 × 106 Å-2). Similarly, NR data for the HMDSO plasma polymer film in contact with the three different water contrasts were collected and modeled using analogous methodologies (Figure 5, Table 3). When in contact with D2O, the HMDSO plasma polymer was found to exchange 16 protons, giving a film composition of Si68O37C95H254D16. When in contact with H2O, the scattering length density was found to be the same as when the film was measured in air, indicating that no water penetrated the film. This was further supported by comparing the film thickness of the dry and wet films, which were identical. Further, if an empirical formula of Si68O37C95H261D9 is assumed for the HDmix contrast, then the calculated scattering length density of this film is Ff ) 0.36 × 10-6 Å-2. This value is in good agreement with the observed reflectivity data (Table 3). In summary, NR of AA and HMDSO plasma polymer films in different solvents has revealed that the AA plasma polymer film swells significantly while no swelling of the HMDSO plasma polymer was evident. Both films undergo proton/deuteron exchange between the film and solvent. However, the degree of this labile exchange is dependent upon the nature and hydrophilicity of the film. Specifically, 6% of the protons in the HMDSO film exchange with D2O, while appreciably more are capable of exchange in the AA film (24%). This suggests significant differences in the reactivity of the two plasma polymer films. AA plasma polymer films are frequently used as platforms for further grafting reactions due to the presence of residual amine functionalities.14 The large amount of proton exchange in this film is therefore attributed to reactive nitrogen- and oxygencontaining species both on the surface and within the bulk of the plasma polymer film. HMDSO films, however, are commonly used as barrier coatings and are known to be relatively inert and unreactive after deposition.23
Nelson et al.
Furthermore, the static water contact angles on the AA and HMDSO plasma polymer films are 55° and 99°, respectively. Due to the greater hydrophilicity of the AA film, it is not surprising that a small amount of water is able to penetrate the film. In contrast, the hydrophobic HMDSO film does not swell in the presence of water to any measurable extent. Instead, there is a small degree of proton exchange which most likely occurs at the film/water interface and is attributed to the hydrolysis of Si-Si species to silanols and/or from the oxidation and exchange of protons with Si-H species.15 We are currently examining plasma polymer films that have been formed under different deposition conditions and the structural properties of films formed by chemically similar monomers.
Conclusions Thin films of AA and HMDSO have been generated by plasma polymerization and studied using XPS, AFM, XRR, and NR. Using combinations of these techniques, we were able to accurately determine the film thickness, determine the RMS roughness, and decouple the composition from the mass density. Contrast variation of the aqueous subphase was employed along with NR to determine the penetration of water into the film, as well as to characterize the number of labile protons for each polymer. Acknowledgment. S.K.Ø. acknowledges Kristina Stenborg’s scientific foundation and the Swedish Royal Academy of Science for financial support. LA052196S
