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Nanomechanical Properties of Polysiloxane-Oxide Interphases Measured by Interfacial Force Microscopy H. Cabibil, H. Celio, J. Lozano, and J. M. White* Department of Chemistry and Biochemistry, Center for Nanomolecular Science and Technology, University of Texas, Austin, Texas 78712
R. Winter Department of Chemistry and Chemical Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 Received October 2, 2000. In Final Form: January 17, 2001 Using an interfacial force microscope (IFM), we have measured the elastic nanoindentation modulus of thin films of poly[(aminopropyl)siloxane] spin-coated on Na+-containing glass and native SiO2. The films were prepared by the hydrolytic polycondensation of γ-(aminopropyl)triethoxysilane (γ-APS). The elastic moduli of 500 Å thick films deposited on Na+-containing glass and native SiO2 are 8 ( 2 and 35 ( 3 GPa, respectively. IFM data were complemented with atomic force microscopy and infrared and X-ray photoelectron spectroscopy. We propose that the significantly smaller modulus of γ-APS on glass is related to the incorporation of Na+ ions from the glass into the siloxane network of the film. These incorporated Na+ ions act as Lewis acids and catalyze the depolymerization of the Si-O-Si network, resulting in a less rigid polysiloxane framework and a lower elastic modulus. A clue to the films’ structural and chemical difference is provided by the observation of time-dependent morphological changes for γ-APS on glass but not for γ-APS on SiO2.
1. Introduction γ-(Aminopropyl)triethoxysilane (γ-APS) belongs to a class of organofunctional trialkoxysilanes (YSi(OR)3), that hydrolyze and condense (see Scheme 1) forming products ranging from small oligomers to extensive cross-linked polymerized networks. When used as a surface treatment, the hydrolyzed monomers and oligomers undergo dehydration with hydroxyl groups on inorganic oxide substrates to form covalent attachments. The Y group in γ-APS contains a primary amine functionality (-(CH2)3NH2) capable of reacting with a number of chemical moieties, making γ-APS a good coupling agent for otherwise chemically incompatible materials. γ-APS is used in glassreinforced composites, as a film a few to a hundred monolayers thick, to enhance adhesion between the glass fiber/particulate and the polymer matrix and to improve the composites’ overall mechanical strength.1-3 The silane coupling agent is localized in the so-called interphase region, for example, between the glass fiber and epoxy matrix in fiberglass composites. The interphase is a complex region where the silane forms a structural and compositional gradient as it chemically and physically interacts with both glass reinforcement and polymer matrix. Previous studies4,5 using Fourier transform infrared spectroscopy and gel permeation chromatography have * Corresponding author. E-mail:
[email protected]. Fax: 512-471-9495. Tel: 512-471-3704. (1) Mittal, K. L. Silanes and Other Coupling Agents; VSP BV: The Netherlands, 1992. (2) Ishida, H. In The Interfacial Interactions in Polymeric Composites; Kluwer Academic Publishers: Norwell, MA, 1993; Chapter 8, p 169. (3) Plueddemann E. In Interfaces in Polymer, Ceramic, and Metal Matrix Composites; Ishida, H., Ed; Elsevier Science Publishing Co., Inc.: New York, NY, 1988; Part I, p 17. (4) Ishida, H.; Miller, J. D. Macromolecules 1984, 17, 1659. (5) Ishida, H.; Miller, J. D. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 2227.
provided some insights on the nature of the chemical interactions between a silane coupling agent and a variety of particulate inorganic substrates. Ishida and Miller4,5 found a strong correlation between the structural characteristics of the polysiloxane films and the acid-base properties of the substrate on which they were formed. The films comprised two well-defined layers (chemisorbed and physisorbed), and the relative proportion of these layers depended on the acid-base properties of the substrate surface. In addition, the average molecular weight of the physisorbed layer was influenced by the acid-base properties of the substrate surface. Intuitively, such molecular scale differences will be reflected in the films’ mechanical properties. To explore the relationship between interphase chemistry and nanomechanical properties, we measured the elastic nanoindentation modulus of two model siloxaneoxide interphases: thin films of γ-APS spin-coated on soda lime glass and on native SiO2 formed on Si(100). Nanoindentation measurements are reported for the elastic regime to avoid poorly understood complications related to plastic deformation of these oligomerized films, for example, generation of film defects and complex nonlocal movement of material. To complement interfacial force microscopy (IFM) results, atomic force microscopy (AFM), transmission infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements were made. Results indicate that the elastic nanoindentation modulus for γ-APS films deposited on soda lime glass is 1/4 that of similar films deposited on native SiO2. Because this variation could not be attributed to differences in film thickness or substrate mechanical properties, we look to structural and chemical differences within the films, influenced by properties of the two substrates. Specifically, we propose that the significantly lower modulus of the film spin-coated on soda lime glass is related to depolymerization of polysiloxane moieties brought on by
10.1021/la0013906 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/06/2001
Properties of Polysiloxane-Oxide Interphases Scheme 1. Reactions of γ-APS with Watera
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Substrates. The substrates were prepared from microscope glass slides (soda lime glass) and oxidized Si(100) cut into squares approximately 1 cm × 1 cm. For transmission IR measurements, NaCl disks (1 cm diameter) and Si(100) wafers were used as substrates. The glass slides were cleaned by (1) immersion in concentrated NH4OH for 60 min, (2) ultrasonication in deionized H2O (18 MΩ cm-1) for 10 min, (3) rinsing with deionized H2O, and (4) heating at 140 °C for 30-60 min. The surface composition (in atomic %) determined by XPS is silicon (18), oxygen (59), carbon (12), sodium (9), nitrogen (1), and aluminum (1). Potassium and beryllium were present in amounts below 0.5%.
Oxidized Si(100) was cleaned by (1) ultrasonication in piranha solution (50:50 concentrated H2SO4 and 30% H2O2) for 15 min, (2) ultrasonication in deionized H2O (18 MΩ cm-1) for 15 min, and (3) rinsing with deionized H2O. The surface composition determined by XPS indicates a 20 Å layer of SiO2 on Si(100). Films. The spin-coated films were made from fresh solutions (1-3 days old) of 5.0 wt % aqueous γ-(aminopropyl)triethoxysilane (γ-APS, 99% pure, Aldrich Chemical Co.). Spin-coating was done for 40 s at speeds of 500, 1000, and 2000 rpm. The films were either dried under ambient conditions (“air-dried”) or heated for 4 h in ambient air at 100 °C (“cured”). Film thickness was determined by measuring with AFM the depth of the groove created by carefully pressing a razor blade into the film. Nanoindentation experiments on γ-APS films deposited on glass are limited to fresh films (1-4 days old) because of morphological changes that develop upon aging in air.8 In contrast to γ-APS films on glass, γ-APS films spin-coated on native SiO2 are morphologically stable for months in ambient air. Reactions of γ-APS in Water: Hydrolysis and Condensation. The 5 wt % γ-APS solution is basic (pH ∼ 10.5). This is the result of equilibrium between -NH2 groups and H2O with -NH3+ and OH- groups, and the alkaline character catalyzes the hydrolysis and condensation of the γ-APS in water (Scheme 1). The γ-APS molecule (1) containing three ethoxy groups is quickly hydrolyzed to form trisilanol (2) and CH3CH2OH in aqueous solution.1 The trisilanol species readily condense among themselves eliminating H2O to form oligomerized networks (3).1 The stoichiometry of the γ-APS monomer (excluding H) is C9NSiO3 (Scheme 1). XPS analysis (not shown) of freshly prepared (1 day old) air-dried and cured γ-APS films gives a surface stoichiometry of C3.5N0.8SiO1.8. As anticipated, substantial amounts of carbon (60%) and oxygen (40%) have been removed, reflecting hydrolysis and removal of CH3CH2OH and H2O during film preparation. The absence of CH3CH2OH was confirmed by transmission IR measurements (not shown). A fully condensed poly[(aminopropyl)siloxane] network would have a stoichiometry of C3.0NSiO1.5 (see (4) in Scheme 1). With respect to this stoichiometry, the γ-APS film surfaces analyzed by XPS are slightly deficient in nitrogen but enriched in carbon and oxygen indicating, as anticipated, incomplete condensation. We conclude that the films are oligomeric and measurable concentrations of silanol groups remain. Interfacial Force Microscopy. IFM, which has been described in detail elsewhere,6 uses a novel electrostatically driven force-feedback system to ensure rigid displacement control during a loading experiment. With calibration, this design provides true force versus displacement profiles (both attractive and repulsive). The nanoindentation measurements were carried out under ambient conditions (typically between 40 and 50% relative humidity). Parabolic IFM probe tips were made by electrochemically etching 100 µm diameter W wire. The tip shape and radius of curvature (100-180 nm) were determined by field emission scanning electron microscopy. The paraboloid tips are convenient for analysis based on Hertzian mechanics9,10 which extracts elastic properties assuming continuum mechanics, isotropic materials, a parabolic indenter, and a planar sample. Adhesive and frictional interactions are not included. A typical force curve of a glass slide (Figure 1) is obtained by pushing a W tip (radius of curvature of 120 nm) into the surface and monitoring the load (force) as a function of tip-sample separation. After a predetermined repulsive force is achieved (5.0 µN in Figure 1), the W tip is retracted until the original tip position is reached. Positive force values refer to repulsive interactions, and negative values refer to attractive forces. Contact between the nanoindenter and the surface is loosely defined as the point at which the measured force starts to rise above zero. Long-range slightly negative forces reflect weak attractive forces that are not dealt with in this paper. At a maximum contact load of 5.0 µN, the W probe penetrates the substrate to a depth near 30 Å (6% of the film thickness). Upon
(6) Joyce, S. A.; Houston, J. E. Rev. Sci. Instrum. 1991, 62, 710. (7) Pawson, D.; Jones, F. R. J. Adhes. 1995, 52, 187. (8) Cabibil, H.; Pham, V.; Lozano, J.; Celio, H.; Winter, R.; White, J. M. Langmuir, in press.
(9) Johnson, K. L. Contact Mechanics; Cambridge University Press: Cambridge, 1996. (10) Timoshenko, S. P.; Goodier, J. N. Theory of Elasticity, 3rd ed.; McGraw-Hill: New York, 1970.
a When dissolved in H O, γ-APS (1) is quickly hydrolyzed to 2 form a silanetriol (2) and 3 CH3CH2OH. Silanetriols undergo condensation reactions (with the removal of H2O) to form a mixture of oligomeric structures (3) in solution. A schematic idealized γ-APS polymeric structure is shown (4) with a stoichiometry corresponding to C3NSiO1.5. This idealized structure assumes complete reaction of the three silanol groups of each monomer, forming a branched network with a Si-O-Si backbone. The average stoichiometry of freshly prepared airdried and cured APS films, determined using XPS, is C3.5N0.8SiO1.8.
incorporation of Na+ ions (absent in SiO2/Si(100)) from the glass into the film’s structure. One result of depolymerization is a less rigid, smaller elastic modulus -SiO-Si- film structure. Our results are consistent with a previous report7 documenting the reduced shear strength of γ-APS-treated glass fibers because of the presence of mobile Na+ ions within the composite interphase. The proposed difference in the structure and chemical nature of the films is supported by the observation of a highly organized fibrous growth, which occurs in aged films of γ-APS on glass but not on native SiO2/Si(100).8 2. Experimental Section
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Figure 1. A typical force versus displacement plot (force curve) of a glass slide (soda lime glass). After contact, the force increases as the 3/2 power of the deformation, in accordance with the Hertzian relation (fit shown as solid line). The W probe used has a radius of curvature of 1200 Å. tip retraction, the force behavior in Figure 1 closely retraces the loading curve, indicating that the sample was probed elastically. The unloading curve has a small offset (slight inverse looping of ∼2 Å), which is an instrumental artifact due to creep of the piezoscanner. Hertzian mechanics describes the repulsive portion of the force curve by the expression,10
4 F ) E*xRδ3/2 3
(1)
where F is the force applied to the nanoindenter, R is the tip radius, δ is the depth of deformation, and E* is the elastic modulus. The Hertzian fit (solid line in Figure 1) to the loading portion of the cycle is excellent. E* from the fit is reduced to the modulus of the indented material using the following expression:
(1 - νs2) (1 - νw2) 1 ) + E* Es Ew
(2)
where Es, Ew, νs, and νw refer to the elastic moduli and Poisson ratios of the indented sample (s) and the W probe (w). For W, Ew and νw are 400 GPa and 0.30, respectively.11 Assuming νs ) 0.17,12 the elastic nanoindentation modulus of our soda lime glass slides is 74 ( 6 GPa, based on eight force curves acquired on different regions, in good agreement with the literature value of 69 GPa.13 As a baseline to monitor tip conditions (shape and cleanliness), a glass slide measurement was made just prior to each nanoindentation experiment on a γ-APS film. Atomic Force Microscopy. The topography (roughness) of the films was measured with an atomic force microscope (CP Research Autoprobe, Thermomicroscopes, Sunnyvale, CA) operated in the intermittent contact mode (IC-AFM). Si probe tips were used with force constants between 2.0 and 3.5 N m-1 and a resonant frequency between 70 and 150 kHz. Typically, the root mean square (rms) roughness values of 5.0 × 5.0 µm images from five widely separated regions of the surface were averaged. Infrared and X-ray Photoelectron Spectroscopy. Chemical characterization of the whole film relied on transmission infrared spectroscopy (IR), and surface chemical characterization relied on XPS. IR data were collected using a Nicolet Magna 860 (11) Handbook of Physical Quantities; Radzig, A. A., Ed.; CRC Press: Boca Raton, FL, 1997. (12) Auld, B. A. Acoustic Field and Waves in Solids; Wiley: New York, 1973; Vol. 2, p 368. (13) Materials by Design at http://www.mse.cornell.edu/engri111/ modulus.htm.
Figure 2. Force versus displacement curves of 500 ( 75 Å thick air-dried γ-APS films spin-coated on a glass slide (upper) and on native SiO2 on Si(100) (lower) using a maximum contact load of 0.6 µN. The loading cycle was measured using a W probe with a radius of curvature of 1000 Å at an approach and retraction speed of 36 Å s-1. spectrometer with a room-temperature DTGS detector. XPS analysis was carried out using a Physical Electronics (PHI) model 5700 ESCA spectrometer equipped with an Al monochromatic source (Al KR energy of 1486.6 eV).
3. Results Figure 2 displays representative IFM force curves for air-dried γ-APS films, spin-coated on a glass slide (upper panel) and on native SiO2 (lower panel). Both films have an average thickness of 500 ( 75 Å. The W tip had a radius of curvature of ∼1000 Å and was moved at a constant probe speed of 36 Å s-1. Let us first compare the force curves of the γ-APS-covered glass (upper panel, Figure 2) and the soda lime glass shown in Figure 1. On the glass standard, a ∼5 µN load was required to push the W tip to an indentation depth near 30 Å. On the glass covered with γ-APS film, a loading force of only ∼0.55 µN was required to reach a similar indentation depth. Comparison of the force curves of γ-APS on glass and γ-APS on SiO2 indicates that for similar loads of ∼0.550.6 µN, the indentation depths significantly vary for these films: ∼30 Å for γ-APS/glass but only approximately half of this depth for γ-APS/SiO2. Thus, the γ-APS/SiO2 has a markedly higher nanoindentation modulus than the γ-APS/glass. The solid lines in Figure 2 indicate that, as expected, Hertzian mechanics gives an adequate description of the repulsive elastic behavior of these films, that is, the force varies as a 3/2 power of the deformation depth. As for the glass standard, the slight offset (inverse hysteresis of 3-4 Å) observed in the γ-APS on SiO2 curve is due to creep of the piezoscanner.
Properties of Polysiloxane-Oxide Interphases
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Table 1. Elastic Nanoindentation Moduli (Es) and RMS Roughness for Soda Lime Glass and Native SiO2 on Si(100) with and without Spin-Coated γ-APS Films (500 ( 75 Å Thick and Air-Dried)d film and/or substrate
elastic modulus,b Es (GPa)
rms roughnessc (Å)
γ-APS/glass γ-APS/SiO2 glass native SiO2
8(2 35 ( 3 74 ( 6 112 ( 17
2.2 ( 0.4 2.2 ( 0.5 7.4 ( 2.8 1.5 ( 0.4
a The film age varies from 1 to 7 days. The Poisson ratios (ν) used for γ-APS, glass, and native SiO2 are 0.25 (ref 14), 0.17 (ref 12),and 0.25 (ref 15), respectively. The Es and ν values used for the W nanoindenter are 400 GPa and 0.30 (ref 11). b Average of 7-10 loading cycles acquired in different locations of each surface. c The rms (root-mean-square) roughness is defined as the square root of the mean value of the squares of the distance of the points from the image mean value.
Table 1 summarizes the elastic moduli (Es) and rms roughness of the four systems examined. Average values of Es are based on Hertzian data analyses of 7-10 force curves acquired on different locations of each surface. The glass substrate, as discussed above, serves as a reference, and the Es (74 ( 6 GPa) is consistent with the literature.13 The native SiO2 surface, which consists of 20 Å of oxide on a Si(100) substrate, gave an average modulus of 112 ( 17 GPa (assuming a Poisson ratio of 0.2515), much higher than both bulk silica (73 GPa)12 and thick SiO2 films (5972 GPa).15-17 The measured modulus lies between that for Si(100) (168 GPa)18 and the thick oxides, indicating what was expected intuitively; the mechanical properties acquired by indenting a 20 Å film to a depth up to 20 Å will reflect the mechanical properties of the underlying material. For γ-APS on glass, nanoindentation experiments were limited to the first few days (
