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Synthesis and Micromechanical Properties of Flexible, Self-Supporting Polymer-SiO2 Nanofilms Richard Vendamme,† Takuya Ohzono,‡ Aiko Nakao,§ Masatsugu Shimomura,‡ and Toyoki Kunitake*,† Topochemical Design Laboratory, Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0198 Japan, DissipatiVe-Hierarchy Structures Laboratory, Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0198 Japan, and Surface Science DiVision, RIKEN, Hirosawa 2-1, Wako-shi, Saitama, 351-0198 Japan ReceiVed July 18, 2006. In Final Form: NoVember 22, 2006 Large-scale, self-supporting ultrathin films composed of an elastomeric polyacrylate network interpenetrated by a silica (SiO2) network were synthesized and characterized. The organic network was first photopolymerized and the silica structure was subsequently developed in situ in the preformed organic gel. Composition and morphology of the hybrid interpenetrated network (IPN) nanofilms were characterized by infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy and compared with the case of zirconia (ZrO2) hybrid IPN reported earlier. Young modulus, ultimate tensile strength, and ultimate tensile elongation were determined for different organic/inorganic molar ratios and give some insights on how the composition of the nanofilms influence their robustness and self-supporting properties.
Introduction Self-supporting ultrathin films of macroscopic size and nanometer thickness recently emerged as an important issue for many prospective nanotechnological applications such as sensors,1-5 actuators,6 and nanofiltration devices.7,8 Until now, the layer-by-layer (LbL) technique has been established as the most popular method for their preparation.9,10 For instance, LbL was employed for the preparation of self-supporting polyelectrolytes multilayers11-13 and hybrid nanoscale assemblies containing carbon nanotubes14 and inorganic colloids such as clay platelets.15,16 Besides LbL, several preparative techniques of free-standing nanofilms have been developed such as solvent * To whom correspondence should be addressed: e-mail, kunitake@ ruby.ocn.ne.jp; phone, +81-48-467-9601; fax, +81-48-464-6391. † Topochemical Design Laboratory, Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN). ‡ Dissipative-Hierarchy Structures Laboratory, Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN). § Surface Science Division, RIKEN. (1) Jiang, C.; Markutsya, S.; Pikus, Y.; Tsukruk, V. Nat. Mater. 2004, 3, 721-727. (2) Kato, D.; Masaike, M.; Majima, T.; Hirata, Y.; Mizutani, F.; Sakata, M.; Hirayama, C.; Kunitake, M. Chem. Commun. 2002, 2616-2617. (3) Roberts, M. M.; Klein, L. J.; Savage, D. E.; Slinker, K. A.; Friesen, M.; Celler, G.; Eriksson, M. A.; Lagally, M. G. Nat. Mater. 2006, 5, 388-393. (4) Jiang, C.; Markutsya, C.; Tsukruk, V. AdV. Mater. 2004, 16, 157-161. (5) Jiang, C.; Tsukruk, V. AdV. Mater. 2006, 18, 829-840. (6) O’Connel, P. A.; McKenna, G. B. Science 2005, 307, 1760-1763. (7) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. AdV. Mater. 2006, 6, 709-712. (8) Meier, W.; Nardin, C.; Winterhalter, M. Angew. Chem., Int. Ed. 2000, 39, 4599-4602. (9) Sugiyama Ono, S.; Decher, G. Nano Lett. 2006, 6, 592-598. (10) Huck, W. T.; Stroock, A. D.; Whitesides, G. M. Angew. Chem., Int. Ed. 2000, 39, 1058-1061. (11) Mallwitz, F.; Laschewsky, A. AdV. Mater. 2005, 17, 1296-1299. (12) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. J. Am. Chem. Soc. J. 2005, 127, 17228-17234. (13) Stroock, A.; Kane, R.; S., Weck, M.; Metallo. S. J.; Whitesides, G. M. Langmuir 2003, 19, 2466-2472. (14) Mamedov, A.; Kotov, N.; Prato, M.; Guldi, D.; Wicksted, J. P.; Kirsch, A. Nat. Mater. 2002, 1, 190-194. (15) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413-418. (16) Mamedov, A.; Kotov, N. Langmuir 2000, 16, 5530-5533.
casting,17 self-assembly of ABA triblock copolymers,8,18 crosslinking of Langmuir-Blodgett2,19 and self-assembled monolayers,20,21 or the surface sol-gel process.22,23 However, the preparation of truly nanoscale films with proven uniformity and macroscopic robustness is still very difficult and remains a major challenge to extend a nanometer structure laterally to millimeter or centimeter sizes. For various technological applications, mechanical and thermal properties of bulk polymer and rubber systems are commonly improved by addition of inorganic filler.24-26 Physical properties of such hybrids are determined by the respective properties of both organic and inorganic components and thus depend on the content of filler, its adhesion and interaction to the polymer matrix, uniformity of dispersion, etc.27,28 In the case of hybrid interpenetrated networks (IPNs), various polymerization procedures, including simultaneous or sequential developments of the organic and inorganic networks, result in different structures of the hybrid composite. We introduced very recently a facile method for the preparation of robust free-standing nanomembranes of organic/inorganic interpenetrating networks with unprecedented size and properties.29 To prepare these nanofilms, polyacrylate and zirconia (ZrO2) networks were generated from their precursors on a spin-coating plate. The in-plane interpenetration of the two materials affords outstanding (17) Mattsson, J.; Forrest, J. A.; Borjesson, L. Phys. ReV. E 2000, 62, 51875200. (18) Nardin, C.; Winterhalter, M.; Meier, W. Langmuir 2000, 16, 7708-7712. (19) Mallwitz, F.; Goedel, W. A. Angew. Chem., Int. Ed. 2001, 40, 26452647. (20) Eck, W.; Ku¨ller, A.; Grunze, M.; Vo¨lkel, B.; Go¨lzha¨user, AdV. Mater. 2005, 17, 2583-2587. (21) Xu, H.; Goedel, W. A. Langmuir 2002, 18, 2363-2367. (22) Hashizume, M.; Kunitake, T. Langmuir 2003, 19, 10172-10178. (23) Hashizume, M.; Kunitake, T. Soft Matter 2006, 2, 135-140. (24) Matejka, L.; Dukh, O.; Kolarik. J. Polym. 2000, 41, 1449-1459. (25) Sharp, K. G. AdV. Mater. 1998, 10, 1243-1248. (26) Matejka, L.; Dukh, O. Macromol. Symp. 2001, 171, 181-188. (27) Saegusa, T. Pure Appl. Chem. 1995, 67, 1965-1970. (28) Sperling, L. H.; Mishra, V. Polym. AdV. Technol. 1996, 7, 197-208. (29) Vendamme, R.; Onoue, S. Y.; Nakao, A.; Kunitake, T. Nat. Mater. 2006, 6, 494-501.
10.1021/la062084g CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
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Chart 1. Molecular Structures of the Sacrificial Layer of Poly(4-vinylphenol) (a), the Nanofilm Underlayer of Poly(vinyl alcohol) (b), the Inorganic Precursors Tetraisocyanatosilane (c), the Organic Monomers 4-Hydroxybutyl Acrylate (d), and Cross-Linking Agent 1,6-Hexanediol Diacrylate (e)
characteristics that are usually not compatible within the same film, e.g., macroscopic robustness and homogeneity, nanoscale thickness, sufficient mechanical strength, flexibility, and transparency. The aim of the present paper is 2-fold. First we demonstrate that our method initially established for the preparation of polymer-zirconia hybrids can be readily adapted to silica. The use of silica is particularly relevant because it is already commonly used as a reinforcing agent in conventional polymeric systems,30-32 and many forms of life, such as diatoms, also contain hybrid silica structures33,34 (biogenic silica). The interpenetration mechanism between the polyacrylate and the silica networks will be studied and compared with hybrid zirconia formation. In a second step, the micromechanical properties of the hybrid silica nanofilms will be determined by different techniques in order to clarify how the composition of the film influences the macroscopic robustness and stability of the nanofilms.
Results and Discussion Sequential Synthesis of Silica IPN Hybrid Nanofilms. Preparation of silica hybrid nanofilms was performed as follows. A 100 nm thick “sacrificial” layer of poly(vinylphenol) (Chart 1) was spin-coated onto a silicon wafer, followed by a 5 nm thick overlayer of poly(vinyl alcohol) (PVA). Then, a reactive chloroform formulation containing an IPN mix (the inorganic precursor consists of tetraisocyanatosilane Si(NCO)4 and the organic precursors are a combination of 4-hydroxybutyl acrylate (HOBuA), hexanediol diacrylate (HDODA), and a photoreactive mixture) was spin-coated in a nitrogen atmosphere and under UV light. UV radiation induced decomposition of the photoinitiator and the subsequent radical polymerization of the organic network. Development of the organic network was confirmed using IPN samples prepared on a gold-coated glass substrate and by monitoring the decrease in an IR band at 810 cm-1 characteristic of the acrylic unit (Figure 1a, spectrum i). A conversion of more than 95% of the acrylate double bond was achieved after 2 min of irradiation (Figure 1a, spectrum ii). (30) Sur, G. S.; Mark, J. E. Polym. Bull. 1985, 14, 325329. (31) Sanchez, C.; Soler-Illia, G.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061-3083. (32) Tamaki, R.; Naka. K.; Chujo, Y. Polym. Bull. 1997, 39, 303-310. (33) Coradin, T.; Durupthyl, O.; Livage, J. Langmuir 2002, 18, 2331-2336. (34) Nassif, N.; Bouvet, O.; Rager, M. N.; Roux, C.; Coradin, T.; Livage, J. Nat. Mater. 2002, 1, 42-44.
Figure 1. (a) Influences of UV irradiation and hydrolysis conditions on IR spectra of the sample Silica-1 synthesized on a gold-coated glass plate. (b) XPS spectrum of Silica-1 prepared on a silicon wafer and displaying the characteristic peaks of carbon, oxygen, nitrogen, and silicon.
After spin-coating, the IR band at 2300 cm-1 characteristic of the isocyanate unit of the silica precursor remains unchanged, denoting that Si(NCO)4 is still in the liquid state. This is different from our previous study using zirconia, in which the residual humidity of the PVA layer was sufficient to initiate the sol-gel synthesis of the metal oxide gel. This phenomenon arises from the lower reactivity of Si(NCO)4 as compared to Zr(BuO)4. Two strategies were implemented for the hydrolysis of Si(NCO)4. First, we left the sample in air for 1 day. The residual humidity of the air induced quasi-complete hydrolysis of the inorganic precursor (Figure 1a, spectrum iii). Another method was to dip the sample into water for a few seconds soon after spin-coating. In the later case, the conversion was complete, as demonstrated by the total absence of the isocyanate IR band (Figure, 1a spectrum iv). As silicon dioxide has an extremely weak spectrum, the development of the inorganic SiO2 structure could not be detected directly from Figure 1a. Three hybrid formulations with varying amounts of silica (denoted as Silica-1, Silica-2, and Silica-3) were used as displayed in Table 1. Although the preparative procedures of IPN hybrids containing zirconia or silica were quite similar, the molecular mechanisms on the origin of the interpenetrated structure appeared to be different. Zirconia hybrids were formed by a simultaneous mechanism, meaning that both the organic and inorganic networks were developed at the same time, during spin-coating. On the other hand, IPN nanofilms containing silica were formed by a sequential method (Figure 2). In its first step, only the organic network was developed. The sample after spin-coating consists of an organic polymer network swollen by the liquid inorganic precursor (Figure 2b). The degree of swelling varies as a function
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Table 1. Composition of the Reactive Formulations and of the Corresponding Silica Hybrid Nanofilmsa initial molar ratio of precursors (in mol %)
elemental composition of the IPN (in mol %) as obtained by XPS (and as calculated from molar ratios of reactive formulations)
sample
Si(NCO)4
organic monomers
carbon
oxygen
nitrogen
silicon
SiO2
Silica-1 Silica-2 Silica-3
50.8 67.4 75.6
48.2 36.6 24.4
57.8 (54.6) 56.6 (44.4) 50.3 (37.7)
32.9 (37.7) 31.7 (43.1) 36.6 (46.8)
1.9 (0.0) 2.96 (0.0) 1.67 (0.0)
7.32 (7.69) 8.72 (12.5) 11.5 (15.8)
21.9 (23.1) 26.2 (37.5) 34.5 (47.4)
a Calculations of the IPN compositions were performed from the precursors molar ratios in the initial solution, assuming that no preferential loss of monomers occurred during the spin-coating process and that the conversion of both organic and inorganic networks is complete (all monomers are fully reacted).
Figure 2. Schematic representation of the sequential and simultaneous hybrid network formation. Scheme 1. Possible Covalent Bonding between the Organic and Inorganic Networks in the Cross-Linked Hybrid IPN
of the organic/inorganic monomer ratio contained in the initial formulation and is estimated to be 2, 3, and 4 for the sample Silica-1, Silica-2, and Silica-3, respectively. After the sample was dipped into water, the inorganic network is formed in situ in the organic polymer matrix by the sol-gel process (Figure 2c). The growing inorganic structure appear to be distributed within the organic polymer on a molecular scale. As shown on Scheme 1, the silica gel can bind covalently to the organic network through the hydroxyl groups of the organic polymer,35,36 leading to overall strengthening of the hybrid nanofilm.31 After the organic and inorganic networks were fully developed, edges of the sample were scratched with a needle and the substrate was immersed in ethanol where the sacrificial underlayer was dissolved. The size of the self-supporting ultrathin film floating in ethanol is similar to the size of the substrate for all the silica specimens (Figure 3a), and no cracks can be observed. The films are transparent and uniform, float in ethanol like a piece of silk cloth, and change their shapes without disintegration, indicating remarkable flexibility and robustness. Self-supporting nanofilms can be transferred onto a wide variety of solid substrates, such as alumina membranes (Figure 3b), glass plates, silicon wafers, and TEM grids. (35) He, J.; Kunitake, T. Soft Matter 2006, 2, 119-125. (36) Ichinose, I.; Kawakami, T.; Kunitake, T. AdV. Mater. 1998, 10, 535-539.
Composition and Morphology of the Silica IPN Hybrids. Elemental analysis of the hybrid nanofilm was performed by X-ray photoelectron spectroscopy (XPS). A typical XPS spectrum (Figure 2b) of a silica IPN hybrid displays characteristic peaks of carbon, oxygen, nitrogen, and silicon. Increase in the fraction of the inorganic precursor in the reactive formulation leads to higher contents of silica in the nanofilm (Table 1). Theoretical elemental compositions of the IPNs were estimated using molar fractions of the precursors assuming quantitative conversion of all monomers. For an IPN film containing the smallest proportion of silica (Silica-1), experimental XPS compositions of all elements essentially agreed with the theoretical values. However, experimental molar concentrations of silica in the cured IPN are systematically lower than the theoretical ones, and discrepancies are enhanced for the samples containing higher amounts of silica. During the spin-coating process, intact Si(NCO)4 remains as liquid in the swollen organic gel, and some of the liquid must be spun away. The situation is quite different in the case of zirconia hybrid, where the two networks are developed simultaneously during the spin-coating process.29 Therefore, specific loss of monomers was not observed, giving excellent agreements between experimental and theoretical concentrations. The presence of nitrogen in the silica hybrid nanofilm (Table 1) indicates that the NCO moiety in Si(NCO)4 is left in the film as such or as reaction products, even after dipping in water. However, the conversion of the silica network estimated from the XPS data is at least 93% for all the samples, demonstrating the extensive development of the SiO2 network. An atomic force microscopy (AFM) study of the surface of silica hybrid IPNs as transferred onto silicon wafer denotes that the thickness is constant throughout the film with an accuracy of 10% and that its roughness is comparable to the substrate roughness. Cross sectional scanning electron microscopy (SEM) pictures of the nanofilm on porous alumina allow measurement of the membrane thickness, as shown in Figure 3c. For similar specimens, the thickness measured by SEM is slightly larger but essentially agrees with the AFM observation. It should be noted
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Young’s Moduli Determination via Stress-Induced Mechanical Buckling Instabilities. Moduli of the ultrathin silica hybrid films were analyzed using a recently reported37 bucklingbased metrology known as “strain-induced elastic buckling instability for mechanical measurements” (SIEBIMM). SIEBIMM exploits the fact that a thin, higher modulus material bonded to a semiinfinite, lower modulus substrate will buckle when subjected to compressive planar forces in order to relieve the strain energy of the system.38-40 To minimize this energy, buckling takes place at a wavelength, λ, given by eq 1, where h is thickness, Ef, νf, Es, and νs are Young’s moduli and Poisson’s ratios of the film and substrate, respectively.37,41 By convention, we assumed νf and νs to be 0.3 in this study.
λ ) 2πh[Ef(1 - νs2)/3Es(1 - νf2)]1/3
Figure 3. (a) Micrograph of a Silica-1 hybrid IPN nanofilm floating in ethanol. (b) Micrograph of a Silica-1 hybrid IPN nanofilm transferred on an alumina membrane. The arrow indicates a zone noncovered by the nanofilm. (c) SEM side-view image of a Silica-1 nanomembrane on an ANODISC. (d) SEM top-view image a Silica-1 nanomembrane covering the ANODISC. (e) TEM image of a freestanding Silica-1 nanomembrane on a Cu grid demonstrating the amorphous nature of the hybrid nanofilm.
that the thickness measured by SEM includes a few nanometers of platinum used for coating the sample and is a maximum value. The SEM top view image (Figure 3d) shows that the pores of ANODISC (200 nm at maximum) are fully covered with the nanofilm without any defects or cracks. Transmission electron microscopy (TEM) observation was performed on IPN nanofilms transferred onto a Cu grid. Figure 3e demonstrates that a smooth amorphous surface is formed. The organic and inorganic components are both amorphous and appear to be molecularly uniform, even for SiO2 concentration around 35 mol % (sample Silica-3). In contrast, a regular lattice with a periodicity of 0.29 nm and domain sizes of 5-10 nm was observed in zirconia hybrids. These different IPN structures may be caused by different interpenetration processes. In the sequential process, the preformed organic network acts as a template for the formation of a regular and well-dispersed nanoscopic inorganic structure and prevents nanoscale phase segregation between organic and inorganic elements. The sequential process also affords better control over hydrolysis condition and polymerization kinetics. On the other hand, the simultaneous method provides a better control of sample composition. A schematic representation of the hybrid network is given in Figure 2c. The theoretical number of monofunctional organic monomers between two cross-linking points of the poly(HOBuAco-HDODA) network (known as Nc in the literature) is estimated to be 19 from the molar composition in the reactive mixture. The connectivity of the inorganic network is much higher than that of the organic network because the individual inorganic monomer is tetrafunctional. The difference in connectivity between the two networks is highlighted by Figure 2c.
(1)
Hybrid silica nanofilms were transferred from the water surface to the poly(dimethylsiloxane) (PDMS) surface. The thickness of the “hard” IPN skin was adjusted by stacking one to three layers of nanofilms onto the PDMS. When subjected to lateral strain, all films exhibited uniform buckling over the entire sample surface (Figure 4) and any delamination was not observed between the PDMS substrate and the IPN film or between the stacked nanofilms. The uniform stress distribution throughout the whole sample and the strong adhesion of the nanofilms to the PDMS substrate are confirmed by the high reproducibility of the buckling patterns and the absence of microcracks, flat regions, and other defects that may arise from local delamination or other modes of stress release. The wavelengths of the buckling pattern were determined directly from the optical and AFM pictures by measuring an average wavelength over at least 10 wrinkles. Uniformity over the whole sample was confirmed by measuring the wavelength at different location of the substrate. Spacing and amplitude of the wrinkles depend on both the nanofilm composition and thickness. The wrinkle wavelength determined from optical micrographs for a bilayer of Silica-1 sample is 2.6 µm (Figure 4b), which is consistent with that obtained from AFM images (2.55 µm in Figure 4e). The buckling wavelength increases linearly with the IPN layer thickness (Figure 5a,b), as would be expected from eq 1 for a Young’s modulus that does not change with increasing film thickness. We also note from the AFM profiles presented in Figure 5a that the amplitude of the wrinkles also increases with the film thickness. The moduli of the specimens, Silica-1, Silica-2, and Silica-3, obtained using eq 1 are reported in Table 2. Decrease in of silica contents resulted in significantly lower elastic modulus. This effect can be accounted for by the filler toughening mechanism associated with the interpenetrated inorganic phase with a high elastic modulus as a second component of the multiphase nanocomposite.42-44 Samples reinforced with zirconia were also analyzed by SIEBIMM and the results are reported in Table 2. It appears that, for a similar inorganic content, the moduli of the zirconia hybrids are substantially higher than that of silica hybrids (37) Stafford, C. M.; Harrison, C.; Beers, K. L.; Karim, A.; Amis, E. J.; Vanlandingham, M. R.; Kim, H. C.; Volksen, W.; Millers, R. D.; Simonyi, E. E. Nat. Mater. 2004, 3, 545-550. (38) Genzer, J.; Groenewold, J. Soft Matter 2006, 2, 310-323. (39) Ohzono, T.; Shimomura, M. Langmuir 2005, 21, 7230-7237. (40) Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.; Genzer, J. Nat. Mater. 2005, 4, 293-297. (41) Nolte, A. J.; Rubner. M. F.; Cohen, R. E. Macromolecules 2005, 38, 5367-5370. (42) Mammeri, F.; Le Bourhis, E.; Rozes, L.; Sanchez, C. J. Mater. Chem. 2005, 15, 3787-3811. (43) Rajan, G. S.; Sur, G. S.; Mark, J. E.; Sharfer, D. W.; Beaucage, G. J. Polym. Sci., Part B 2003, 41, 1897-1901. (44) Sperling, L. H. Polymeric Multicomponent Materials; Wiley: New York, 1997.
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Figure 4. Buckling patterns of silica hybrids nanofilms (sample Silica-1) supported on a compressed PDMS block. Optical (a-c) and AFM (d-f) images of buckled hybrid IPN films. The strain was chosen at 6% for all the samples Arrows in image a indicate the direction of the strain. Stacking several layers of nanofilms increases the total thickness of the IPN “skin” and also the wavelength of the buckling instability. The first column of the figure corresponds to a single nanofilm, the second column to a stacking of two nanofilms, and the third column to a stacking of three nanofilms. (g-i) 3D AFM images of the buckling patterns.
(Silica-1 hybrid is reinforced with 21.9 mol % of SiO2 and has a modulus of 126 MPa, whereas Zirconia-1 hybrid contains only 11.2 mol % of ZrO2 but has a modulus of 350 MPa). Such difference may be attributed to the synthetic process (simultaneous or sequential interpenetration of the hybrid networks) as well as the morphology of the samples and the nature of the inorganic phase. The relative hardness/softness of the inorganic phase plays a crucial role in determining the physical properties of hybrid materials. In that framework, it is interesting to recall that TEM observations demonstrated different morphologies for the two samples: zirconia hybrids present a composite morphology with small domains of ordered ZrO2 layers dispersed in an amorphous polymer matrix,29 while silica hybrids always present an amorphous morphology. We assume here that the hard ZrO2 domains act as an efficient reinforcing agent for the rubbery polymer matrix. On the other hand, ultrathin networks of amorphous metal oxide act as soft matter.35 Therefore, the amorphous silica network must be less effective as a reinforcing agent for the hybrid nanofilm. The SIEBIMM has been generally used for single-layer films.37 In the present study, however, we extend it to the determination of a single nanofilm as well as stacked nanofilms. By doing so, we assume that the stacking of n nanofilms of thickness h can be assimilated to a thicker “monolayer” film of the same composition and of thickness n × h. This assumption is supported by the fact that the stacked nanofilms are similar (same
composition and thickness) and that no delamination was not observed between the stacked nanofilms (strong adhesion between nanofilms). A special method has been recently introduced to adapt the SIEBIMM technique to multilayered systems and stacked nanofilms.45 Nolte et al. described a two-plate buckling instability technique in order to make the SIEBIMM technique45 more applicable to a wide variety of thin film systems. In that model, a thin polystyrene (PS) film was introduced between the PDMS substrate and the nanofilm of unknown modulus. The role of the PS interlayer is to provide an interface promoting a good adhesion between the PDMS and the nanofilm. The twoplate method is particularly advantageous for nanofilms presenting poor adhesion to the PDMS substrate. The PS-nanofilm composite undergoes buckling instabilities like their homogeneous counterparts. Finally, by using the proper mechanical analysis, the unknown Young’s modulus of the nanofilm can be extracted from the overall mechanical properties of the two-plate composite film after mathematical deconvolution of the mechanical contribution of the PS interlayer.45 In this two-plate buckling technique, the use of a special theoretical model is made necessary because the PS layer and the nanofilm of the unknown modulus have widely different composition, thickness, and physical properties. This is quite different from the case of our stacked (45) Nolte, A. J.; Cohen, R. E.; Rubner, M. F. Macromolecules 2006, 39, 4841-4847.
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Figure 6. Optical image of the buckling pattern of stacked nanofilms at the interface between a monolayer and a bilayer. The black arrows indicate the direction of the strain (compression 6%). The two domains both display very uniform buckling patterns with distinct wavelengths, denoting good adhesion between the PDMS substrate and the nanofilms as well as between the stacked nanofilms. The defects of the buckling pattern (highlighted with circles) essentially appear at the interface between the monolayer and the bilayer domains. Figure 5. (a) Buckling profiles of silica hybrids nanofilms (sample Silica-1) supported on a compressed PDMS block (compression of 6%). The profiles are indicated for a single nanofilm and for stacking of two or three nanofilms. The AFM images corresponding to those profiles are indicated in parts d, e, and f of Figure 4, respectively. (b) Dependence of the buckling wavelength on the total thickness for Silica-1, Silica-2, and Silica-3 specimens. Solid lines are guides for the eyes. Table 2. Composition (mol %), Young’s Modulus (MPa), Ultimate Tensile Strain E (in %), and Ultimate Tensile Strength σ (MPa) of the Three Hybrid Silica Nanofilms sample
SiO2 or ZrO2 content (mol %)
modulus (MPa)
(%)
σ (MPa)
Silica-1 Silica-2 Silica-3 Zirconia-1 Zirconia-2 Zirconia-3
21.9 26.2 34.5 11.2 17.8 22.5
126 686 1126 350 400 a
4.9 2.1 1.3 2.6 1.4 a
73 65 45 105 68 a
a
Nanofilm brittle and fragmented into small pieces.
hybrid nanofilms, which are chemically and physically identical and display good adhesion properties. Therefore, the classical “monolayer” model of SIEBIMM was used in this work to obtain the modulus of the nanofilms. One of the striking advantages of the buckling metrology is its ability to characterize area-by-area the mechanical properties of nanofilms with the micrometer resolution.37 This concept is tested here by transferring a monolayer of nanofilm on the whole surface of the PDMS substrate and by subsequently transferring another layer of the same nanofilm that covers only partially the surface. After compression of the sample (Figure 6), it is clear that no delamination could be observed at the interface between the monolayer and the bilayer films. The monolayer domain and the bilayer domain both display homogeneous buckling patterns with distinct wavelengths, and the only few defects appear essentially at the interface between the monolayer
and the bilayer domains. Figure 6 demonstrates that, for a material with uniform modulus, a thickness map can be generated with micrometer resolution across the specimen with the buckling metrology. Similarly, Jiang et al. reported very recently that, for a nanofilm with uniform thickness, with micropatterned nanomembranes containing gold nanoparticle microarrays the SIEBIMM technique could generate a modulus map via complex buckling instability patterns.46 In another work, Wilder et al. use a uniform sensor film to map spatial differences and heterogeneity in the modulus within soft polymer networks substrates47 (PDMS or hydrogels possessing a gradient in the cross-linking density). Such a type of modulus or thickness mapping cannot be achieved with conventional tensile tests since the specimen would act as a composite material of average composition and thickness. Ultimate Tensile Stress and Ultimate Tensile Elongation. The ultimate tensile stress σ and ultimate tensile elongation of hybrid ultrathin films can be measured by applying an overpressure P to one side of a freely suspended film that covers a metal plate with a circular hole and measuring the resulting deflection of the film29,48-50 (bulging test). Side-view optical images of a free-standing Silica-2 nanofilm that is deformed by air pressures applied from below are given as Supporting Information and the results of σ and for the silica hybrid and zirconia hybrid nanofilms are presented in Table 2. Increasing amounts of silica in the film decrease both and σ. Apparently, the silica network acts as a giant quasi-two-dimensional cross-linking agent in the hybrid film, since the silica unit has a high connectivity of four. Nanofilms with a lower amount of silica (22 mol % for Silica-1) (46) Jiang, C.; Singamanemi, S.; Merrick, E.; Tsukruk, V. V. Nano Lett. 2006, 6, 2254-2259. (47) Wilder, E. A.; Guo, S.; Lin-Gibson, S.; Fasolka, M. J.; Stafford, C. M. Macromolecules 2006, 39, 4138-4143. (48) Poilane, C.; Delobelle, P.; Lexcellent, C.; Hayashi, S.; Tobushi, S. Thin Solid Films 2000, 379, 156-165. (49) Markutsya, S.; Jiang, C.; Pikus, Y.; Tsukruk, V. AdV. Funct. Mater. 2005, 15, 771-780. (50) Goedel, W. A.; Heger, R. Langmuir 1998, 14, 3470-3474.
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have a lower modulus, but are more “robust” in the free-standing state, because they are less brittle and they can sustain significant deformation. Their greater elasticity dissipates the fracture energy and prevents crack developments. Interestingly, this observation is consistent with hybrid IPN in the bulk state, for which a small volume fraction of inorganic filler is known to give the best reinforcing effect. We observe in Table 2 that, for comparable inorganic content, the zirconia hybrids have a higher tensile strength and a lower tensile elongation that the silica hybrids. Again, these observations can be accounted for by the relative “softness” of the amorphous silica network as compared to the more structured zirconia nanocrystalline domains, as we already discussed for the Young’s modulus values obtained with SIEBIMM. The outstanding micromechanical properties of the silica hybrid nanofilms arise mainly from their two-dimensional (2D) character and high homogeneity. For instance, it has already been reported that anisotropically cross-linked polymer networks have a tensile strength 3 times greater than that of the three-dimensional (3D) film and the ultimate elongation of the former is also 5 times greater.51 In the case of radically cross-linked 3D gels, formation of microgels leads to nonuniform cross-linking52 and results in limited strength and elongation. In contrast, the monomer molecules in an ultrathin film are uniformly distributed with thickness of a few tens of nanometers and cross-linking is expected to extend uniformly in the 2D space. These structural characteristics are conceivably expected to improve mechanical properties of the 2D films.
Conclusions Self-supporting ultrathin hybrid films composed of an acrylate network interpenetrated with silica were synthesized by a sequential process. The silica structure was developed in situ in the photopolymerised organic gel. The reactivity and the rate of hydrolysis of the inorganic precursor (faster or slower than the organic network development kinetics) are key parameters for controlling the interpenetration mechanism (simultaneous or sequential) during the formation of the hybrid-interpenetrated structure. The versatility of the sol-gel chemistry combined with UV curing enables creation of a wide range of hybrid ultrathin films with designed property. Numerous new applications in the field of advanced materials science are related to matter for which the surface-to-volume ratio is very high.53 In that framework, we note that a 10 cm2 and 40 nm thick freely suspended nanofilm has a surface-to-volume ratio of 5 × 109, which is comparable to the outstanding surface-to-volume ratio of silica aerogels. The combination of nanoscale thickness and macroscopic size in a single material opens the door to a new family of innovative multifunctional materials where robustness, softness, and interface are essential features, with promising applications like molecular imprinting, catalytic activity, or absorption of proteins. Experimental Section Materials. Poly(vinyl alcohol) (PVA) (98 mol % hydrolyzed, Mw ≈ 78 000 g/mol) was obtained from Polysciences. Poly(4vinylphenol) (Mw ≈ 8000 g/mol) was purchased from Aldrich. 4-Hydroxybutyl acrylate and 1,6-hexanediol diacrylate were purchased from Acros Organics and Alfa Aesar, respectively. A (51) Asakuma, S.; Okada, H.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 1749-1755. (52) Soppera, O.; Croutxe´-Barghorn, C. J. Polym. Sci., Part A 2003, 41, 716724. (53) Valle´, K.; Belleville, P.; Pereira, F.; Sanchez, C. Nat. Mater. 2006, 6, 107-111.
Vendamme et al. photoreactive mixture was prepared by mixing 2-hydroxy-2methylpropiophenone (Wako Chemicals) and 2,4,6-trimethylbenzoyldiphenylphosphine oxide (gift from Kyoritsu Chemicals) in a 1:1 weight ratio. Tetraisocyanatosilane Si(NCO)4 was obtained from Gelest. All chemicals were used as received without further purification. Preparation of Free-Standing Silica Hybrid Nanofilms. A 70 nm thick free-standing nanofilm Silica-1 specimen is fabricated as follows. A solution of the organic network precursors was first prepared by mixing 700 µL (5060 µmol) of 4-hydroxybutyl acrylate, 29 µL (130 µmol) of 1,6-hexanediol diacrylate, and 17.7 µL (130 µmol) of photoinitiator. Then 75 µL of this organic solution was diluted in 5 mL of chloroform together with 75 µL (5515 µmol) of tetraisocyanatosilane. This reactive formulation was filtrated before use with a disposable syringe filter unit (DISMIC-13HP, PTFE 0.20 µm, Advantec). Spin-coating was conducted with a MIKASA spincoater 1H-D7. An ethanol solution of poly(vinylphenol) (20 g/mol) was first spin-coated on a clean silicon wafer at a speed of 3000 rpm for 2 min. Then a PVA solution in water (5 mg/mL) was spin-coated at 3000 rpm for 2 min. The chloroform solution containing the IPN precursors was finally spin-coated at 4000 rpm during 2 min under a nitrogen atmosphere. The UV light was turned on after 10 s of spin-coating and maintained until the end of the spin-coating. The substrate was left in the air at room temperature for 1 day, dipped into water for 1 min, and dried under nitrogen flow. The specimen was finally placed in a petri dish, and ethanol was added in order to detach the nanofilm from the substrate. Instruments and Methods. Macroscopic images of self-supporting ultrathin films were photographed by a digital camera RICOH RDC-7, with 640 × 480 pixels. Fourier transform infrared spectroscopy measurements were performed using a Thermonicolet Nexus 870 FT-IR spectrometer. Irradiation of the sample during the spin coating was carried out with a Lightningcure LC5 (Hamamatsu). The irradiation system was equipped with a static UV lamp together with a light filter, allowing irradiation of the sample with a wavelength of 365 nm and an intensity of 23 mW/cm2 during spin coating. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-5200 field emission microscope. Specimens for SEM experiments were coated by a 2 nm thick platinum layer using an ion-sputtering coater (Hitachi, E-1030, 15 mA, 30 s). TEM observations were performed a JEOL JEM 2100 F/SP transmission electron microscope at 200 kV. XPS measurements were carried out on ESCALAB 250 (VG) using Al KR (1486,6 eV) radiation. AFM analysis was conducted with a MFP-3D-SA microscope (Asylum Research) in noncontact mode, using a silicon cantilever (Olympus OMLC-AC240TS-C2; f ) 52,38 Hz, k ) 0.8-2.4 N/m). Micromechanical Measurements. Bulge Test. The bulge test was conducted in accordance with the known routine.29,46,47 The tensile stress σ and tensile strain of ultrathin films can be measured by applying an overpressure P to one side of a freely suspended film that covers a metal plate with a circular hole and measuring the resulting deflection of the film. In the present study, the applied pressure was controlled with a digital manometer (AS ONE, M-832 manometer) and the membrane deflection was monitored with an optical microscope (Nikon OPTIPHOT-POL) equipped with a digital camera (Nikon COOLPIX 950). The ultimate tensile stress σ and the ultimate tensile strain of the freely suspended hybrid nanofilms were determined using the experimental setup and theoretical formalism described in our previous paper.29 Strain-Induced Elastic Buckling Instability for Mechanical Measurements. Poly(dimethylsiloxane) (PDMS) was prepared as suggested by the manufacturer (Sylgard 184, Dow Corning) at a ratio of 10:1 (by mass) of base to curing agent. Modulus of the PDMS substrate was measured by uniaxial compression (1.2 MPa). Self-supporting hybrid IPN nanofilms were first transferred from ethanol to the surface of water and finally onto a PDMS block (10
Polymer-SiO2 Nanofilms mm × 6 mm × 6 mm). Measurements were taken after samples were dried for several hours. Buckling wavelengths were determined using optical images acquired on an optical microscope equipped with a digital camera and by AFM.
Acknowledgment. This work was supported by the postdoctoral program for foreign researchers of the Japan Society for the Promotion of Science (JSPS) through a fellowship accorded
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to R.V. We are very grateful to Dr. Shin-ya Onoue for his help with TEM measurements. Supporting Information Available: Side view images of a 50 nm thick Silica-2 free-standing nanofilm that deformed by different pressures applied from below. This material is available free of charge via the Internet at http://pubs.acs.org. LA062084G
