The influence of ultraviolet (UV) irradiation on vinylsilsesquioxane (ViSiO3/2) films was ... Ultraviolet (UV) curing processes are becoming a very reliable.
Note
Journal of the Ceramic Society of Japan 120 [10] 442-445 2012
Influence of UV irradiation on mechanical properties and structures of sol–gel-derived vinylsilsesquioxane films Ainun Rahmahwati AINUDDIN,*,**,³ Tomonori ISHIGAKI,** Norio HAKIRI,** Hiroyuki MUTO,** Mototsugu SAKAI** and Atsunori MATSUDA** *Department
of Materials and Design Engineering, Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, Parit Raja, Batu Pahat, 86400 Johor, Malaysia **Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1–1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441–8580, Japan
The influence of ultraviolet (UV) irradiation on vinylsilsesquioxane (ViSiO3/2) films was investigated using a nanoindentation technique, and the structural changes were analyzed. Solid-state 13C cross-polarization magic-angle-spinning nuclear magnetic resonance and infrared spectra of the films confirmed the polymerization of C=C bonds and formation of a CC bridge structure upon UV irradiation. Since the vinyl group is a photosensitive organic component with a short organic chain, it is expected to show a large improvement in mechanical properties as a result of the formation of an organic network under the influence of UV irradiation. It is shown that varying the intensity of the UV irradiation leads to changes in the hardness and elasticity. The irradiation energy was found to determine the mechanical properties of the ViSiO3/2 films, regardless of the effects of different UV intensities on the hybrid films. ©2012 The Ceramic Society of Japan. All rights reserved.
Key-words : Sol–gel process, Inorganic–organic hybrid, UV irradiation, Indentation [Received July 4, 2012; Accepted August 27, 2012]
1.
Introduction
Silsesquioxanes are inorganicorganic hybrid materials with a specific type of organosilicate of formula RSiO3/2; they combine the mechanical, thermal, and chemical stabilities of ceramics and the flexibility of traditional soft materials in solution processing.1)6) Because they have three silicate linkages per silicon atom and one pendant organic substituent, silsesquioxane systems have a propensity to form cross-linked networks. This has led to widespread interest in their applications and it is evident that silsesquioxanes are a versatile class of hybrid materials. Ultraviolet (UV) curing processes are becoming a very reliable alternative to thermal processes, because of a particular characteristic, namely fast transformation of a liquid monomer into a thin solid cross-linked coating with tailored physical and chemical properties without the need to use a solvent, so they are considered as environmentally friendly processes. This technique is usually performed at room temperature, allowing energy saving.5)11) The first step in this method is the preparation of an inorganic network in a solgel synthesis by hydrolysis and condensation. In the second step, the organic network is generated by heat treatment and UV irradiation. Conventionally, hardness is measured by pressing an indenter of a certain shape into a test surface and imaging the indent after releasing the load and retracting the indenter. However, when working with films, especially those with thicknesses of a few microns, this method is not particularly convenient. An instrumental indentation technique, also known as nanoindentation, is therefore used, in which the displacement of a sharp indenter into the hybrid film is measured to investigate the mechanical ³
Corresponding author: A. R. Ainuddin; E-mail: ainun@uthm. edu.my
442
properties of surfaces by accurately measuring displacements of only a few nanometers using sensitive transducers.4),11),13)20) In the present study, vinylsilsesquioxane (ViSiO3/2) films, which have C=C bonds, were prepared using vinyltriethoxysilane [ViSi(OEt)3] as the starting material. The primary aim of this study is to clarify the features of ViSiO3/2 films that affect changes in the mechanical properties upon UV irradiation, compared with those of phenyl- and methyl-silsesquioxane hybrid films. Understanding these changes is essential for developing embossing and photolithographic micropatterning processes using hybrid films. Detailed studies of the effects of the UV irradiation time and intensity on structural changes in the films are also important in determining the most effective conditions for using the organosilsesquioxane films in micropatterning applications.
2.
Experimental procedure
In this study, ViSi(OEt)3, phenyltriethoxysilane [PhSi(OEt)3], or methyltriethoxysilane [MeSi(OEt)3] was hydrolyzed in ethanol (EtOH) with hydrochloric acid (0.1 wt % HCl) by stirring for 2 h at room temperature to allow hydrolysis of the trifunctional silicon alkoxide [RSi(OEt)3]. The molar ratios of RSi(OEt)3: EtOH:H2O were 1:3:4. All trifunctional silicon alkoxides were purchased from Shin-Etsu Chemical Co., Ltd. (Tokyo, Japan) and the other chemicals were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). RSiO3/2 hybrid films for mechanical characterization were prepared by dropping and spreading the sols on soda-lime silicate glass substrates. Films prepared on silicon substrates by dipping/withdrawing were used for structural and chemical characterizations. The thicknesses of the hybrid films were measured using a profilometer (SURFTEST SV-3000M4, Mitutoyo Co., Ltd., Kawasaki, Japan). The films coated on the substrates were placed in an oven and heat-treated at 80°C for 4 h. UV ©2012 The Ceramic Society of Japan
JCS-Japan
Journal of the Ceramic Society of Japan 120 [10] 442-445 2012
irradiation of the films was carried out using an ultra-highpressure mercury lamp (SP7-250DB, Ushio Inc., Tokyo, Japan). The RSiO3/2 hybrid films were irradiated through quartz glass and in a nitrogen atmosphere to prevent oxidation of the films. The intensity of the light was 5, 10, and 15 mW/cm2 at a wavelength of 365 nm. Changes in the mechanical properties of the films with UV irradiation were measured using a nanoindentation technique. Nanoindentation tests were carried out under ambient conditions using a specially designed nanoindentation apparatus. The penetrations of Berkovich and spherical (radius, R = 50 ¯m) indenters were controlled using a piezo actuator. The piezo actuator was used to control the penetration depth h as a function of time, using a personal computer (PC), and the load indentation P was monitored using a miniature load cell. The analog outputs were converted to digital data in their synchronized phases, and then stored and analyzed by the PC. Indentation tests were conducted on all the hybrid films before and after UV irradiation. Details of mechanical properties determination using nanoindentation have been reported in our previous papers.13)20) Fourier-transform infrared (FTIR) spectroscopy and solid-state 13 C cross-polarization magic-angle-spinning nuclear magnetic resonance (CPMAS-NMR) spectroscopy were used to analyze the structures of the films and to determine the relationship between the mechanical properties of the films and UV irradiation time. FTIR absorption spectra of the films coated on silicon substrates were measured in transmission mode before and after UV irradiation, using an FTIR spectrophotometer (FTIR-7300, Jasco, Tokyo, Japan). Changes in the structural units of the hybrid films before and after UV irradiation were examined using a 13C CPMAS-NMR spectrometer (Varian, UNITY-400P, CA, USA). Sample powders were measured using a 5-s decay between pulses and a spinning rate of 4000 Hz.
3.
(Color online) Indentation loaddisplacement cycles for Berkovich indentation of (a) ViSiO3/2, (b) PhSiO3/2, and (c) MeSiO3/2 hybrid films on soda-limesilicate glass substrates. Fig. 1.
Results and discussion
The thicknesses of the hybrid films were controlled to be 10 ¯m. Typical loaddisplacement cycles, √Ph, for the three kinds of hybrid films before UV irradiation are shown in Fig. 1. The results for the ViSiO3/2, PhSiO3/2, and MeSiO3/2 hybrid films are shown in Figs. 1(a)1(c), respectively. The linear √Ph lines confirm the quadratic relationship of P versus h for the hybrid films. In Fig. 1, the broken line indicates the √Ph relationship for the homogeneous hybrid films, with the quadratic relationship P = k1h2. The broken line and the √Ph cycles of the hybrid film plots are close and overlap with each other. This indicates that the indentation rheology affected the film area only, and the effect of the substrate on the deformation of the hybrid films was very limited when the film/substrate modulus mismatch was insignificant.20) The key features to note from the loaddisplacement plots are the differences in the maximum penetration depths and the recovery behaviors of the films during the unloading cycle. The ViSiO3/2 film, as shown in Fig. 1(a), initially displays highly elastic behavior with a minor plastic deformation, which is then followed by increasing deviations from the ideal elastic behavior, based on computation of stimulated loaddepth curves. The ViSiO3/2 film also shows the least amount of creep, which is attributable to a phenomenon in which the siloxane network predominantly provides the hybrid film with rigidity and hence there is less molecular movement under a constant load. UV light with a power density of 15 mW/cm2 was selectively irradiated onto the ViSiO3/2, PhSiO3/2, and MeSiO3/2 hybrid films coated on glass substrates, for different irradiation times. Figure 2
Fig. 2.
(Color online) Meyer hardness correlated with UV irradiation
time.
shows the changes in the Meyer hardness of the hybrid films upon UV irradiation. Circles, triangles, and squares represent the results for ViSiO3/2, PhSiO3/2, and MeSiO3/2 hybrid films, respectively. A significant increase in hardness is observed for the ViSiO3/2 film during UV irradiation. In contrast, a small increase in hardness is seen for the PhSiO3/2 film during UV irradiation, and almost no changes are seen for the MeSiO3/2 film. The significant increase in the hardness of the ViSiO3/2 film after irradiation is probably the result of opening and polymerization of C=C double bonds in the film. The small increase in hardness of the PhSiO3/2 hybrid film after irradiation can be ascribed to cleavage of SiC bonds in the film. The MeSiO3/2 film, which 443
JCS-Japan
Ainuddin et al.: Influence of UV irradiation on mechanical properties and structures of sol–gel-derived vinylsilsesquioxane films
(a)
(b)
Fig. 3.
(Color online) Elastic modulus correlated with UV irradiation
time.
does not have UV-sensitive functional groups such as C=C and SiPh bonds, shows no changes during irradiation. It is noteworthy that the ViSiO3/2 film shows much higher hardness and steeper increases in hardness than the other hybrid films with UV irradiation. This probably reflects significant densification of the film by the formation of Si(CH2)nSi bonds by UV irradiation. The elastic modulus data as a function of UV irradiation time follow the same trends and exhibit similar features, as shown in Fig. 3. For the ViSiO3/2 and PhSiO3/2 hybrid films, UV irradiation results in an increase in elastic modulus. The increase in the elastic modulus induced by UV irradiation is amplified systematically as polymerization and cleavage increase. The elastic modulus of the ViSiO3/2 hybrid film increases more rapidly than those of the other hybrid films. This might be explained by greater conversion of C=C bonds during the organic polymerization, generating a hybrid network with a higher cohesive energy density. A comparative experiment was also performed to study the influence of UV irradiation intensity on the ViSiO3/2 hybrid films in detail. The results indicate that the hardness and elastic modulus of the hybrid films rapidly increase as the UV exposure intensity increases from 5 to 10 and to 15 mW/cm2. Figure 4(a) shows the changes in Meyer hardness of the ViSiO3/2 films with UV irradiation time after irradiation with different UV intensities. A hardness value equivalent to that obtained with a strong intensity can be obtained using a weak intensity for a longer irradiation time. A high UV power is useful for shortening the exposure times of silsesquioxane hybrid films.21) Shortening the processing time is necessary for practical micropatterning applications. To examine the rate of increase of the hardness with intensity strength, the correlation between the Meyer hardness of the ViSiO3/2 films with UV irradiation energy is plotted in Fig. 4(b). The UV irradiation energy is the total irradiated energy delivered to an area. As shown in Fig. 4(b), the increases in the hardness were independent of the UV intensities applied to the hybrid films. The same trends and characteristic were also seen in the correlation of the elastic modulus values with UV irradiation energy. Similar results were also obtained from the changes in the normalized absorbance of the peak assigned to the C=C bond in the FTIR spectra of ViSiO3/2 films associated with UV light irradiation energy. The rapid increase in the hardness value with strong intensity therefore caused no struc444
Fig. 4. (Color online) Changes in Meyer hardness of ViSiO3/2 hybrid films irradiated with different UV intensity powers with (a) UV irradiation time and (b) UV irradiation energy.
Fig. 5. (Color online) FTIR spectra of ViSiO3/2 films before and after UV irradiation for 30 min.
tural changes in the hybrid films along with the changes in the mechanical properties. FTIR absorption spectra were measured for ViSiO3/2 films coated on silicon substrates and irradiated with UV light (15 mW/ cm2 at a wavelength of 365 nm). Figure 5 shows the FTIR spectra of the ViSiO3/2 films before and after UV irradiation. The band at around 1620 cm¹1 is assigned to C=C stretching.9),10),12),22),23) The band at around 750 cm¹1 is assigned to the SiC bonds. With UV irradiation for 30 min, the intensity of the band arising from C=C decreased and the band broadened. However, the band did not disappear completely. This shows that the C=C bonds polymerized with UV irradiation for 30 min were not completely
JCS-Japan
Journal of the Ceramic Society of Japan 120 [10] 442-445 2012
(a)
reactions, leading to significant improvements in the mechanical properties of ViSiO3/2 hybrid films. Among several kinds of hybrid films, ViSiO3/2 films showed significant improvements in mechanical properties as a result of the formation of an organic network upon UV irradiation. Since ViSiO3/2 has a short organic chain, UV irradiation caused the structure to be denser and increased the hardness of the film. The increase in hardness caused by irradiation can be quantified from the energy, regardless of differences in the intensity of the UV irradiation. Acknowledgments A. R. Ainuddin acknowledges financial support from the Ministry of Higher Education of Malaysia and Universiti Tun Hussein Onn Malaysia under the SLAI financial scheme. This work was partly supported by the Nippon Sheet Glass Foundation for Materials Science and Engineering.
(b)
References 1) 2) 3) 4) (Color online) 13C CPMAS-NMR spectra of ViSiO3/2 powders before and after UV irradiation for 1 h, in the range (a) 110 to 160 ppm and (b) ¹20 to 50 ppm. Fig. 6.
polymerized. The intensity of the band at 950 cm¹1 also decreased. This band is usually assigned to SiOH,12) but a band attributable to =CH2 wag may appear in this region. In the PhSiO3/2 films, the intensity of the band from SiC at around 750 cm¹1 also decreased (not shown in the figure). In addition, for MeSiO3/2, which does not have C=C bonds, the cleavage of SiC bonds was not observed. These results reflect small increases in the hardness of the PhSiO3/2 films and no changes in the hardness of the MeSiO3/2 films during UV irradiation, as shown in Fig. 2. 13 C CPMAS-NMR spectrometry was used to confirm the cleavage of the C=C bond and the formation of a bridge structure of CC bonds. Figure 6 shows 13C CPMAS-NMR spectra of ViSiO3/2 hybrid powders, which are originally from the peeling off film before and after UV irradiation for 1 h, in the ranges (a) 110 to 160 ppm and (b) ¹20 to 50 ppm. In Fig. 6(a), in the spectrum of the powder before UV irradiation, an intense band at 135 ppm, assigned to C=C bonds, was observed. The intensity of this C=C band was reduced after UV irradiation, indicating that the C=C bonds were polymerized and cleaved in the film. In Fig. 6(b), the peak assigned to CC bonds at 26 ppm increased after UV irradiation. This indicates that the C=C bonds in the ViSiO3/2 films were cleaved and formed a cross-linked structure on UV irradiation, and this structural change increased the hardness and elastic modulus of the film. This is supported by the study by Tadanaga et al.10),22) in which structural units with polymerized organic chains consisting mainly of T2 species were observed by 29Si CP-MAS NMR in a ViSiO3/2 film after UV irradiation.
4.
Conclusions
UV irradiation of ViSiO3/2 hybrid films caused significant increases in hardness and indentation modulus. The structural studies suggest that the opening and polymerization of C=C bonds, and formation of a cross-linked structure are the dominant
5)
6) 7)
8) 9) 10) 11) 12) 13) 14) 15)
16) 17) 18)
19)
20) 21) 22) 23)
H. W. Ro and C. L. Soles, Mater. Today, 14, 2033 (2011). D. B. Cordes, P. D. Lickiss and F. Rataboul, Chem. Rev., 110, 20812173 (2010). R. H. Baney, M. Itoh, A. Sakakibara and T. Suzuki, Chem. Rev., 95, 14091430 (1995). A. R. Ainuddin, N. Hakiri, H. Muto, M. Sakai and A. Matsuda, J. Ceram. Soc. Japan, 119, 490493 (2011). A. Matsuda, R. Ogawa, Y. Daiko, K. Tadanaga and M. Tatsumisago, J. Photopolym. Sci. Technol., 20, 101105 (2007). T. Sasaki, A. Matsuda, K. Tadanaga and M. Tatsumisago, J. Ceram. Soc. Japan, 113, 519524 (2005). M. Martin-Gallego, M. Hernández, V. Lorenzo, R. Verdejo, M. A. Lopez-Manchado and M. Sangermano, Polymer, 53, 18311838 (2012). D. Kim, K. Jeon, Y. Lee, J. Seo, K. Seo, H. Han and S. B. Khan, Prog. Org. Coat., 74, 435442 (2012). K. Tadanaga, K. Ueyama, T. Sueki, A. Matsuda and T. Minami, J. Sol-Gel Sci. Technol., 26, 431434 (2003). K. Tadanaga, T. Sueki, A. Matsuda, T. Minami and M. Tatsumisago, J. Ceram. Soc. Japan, 114, 125127 (2006). L. Sowntharya, S. Lavanya, G. R. Chandra, N. Y. Hebalkar and R. Subasri, Ceram. Int., 38, 42214228 (2012). P. S. G. Krishnan and C. He, Polym. Bull., 52, 251258 (2004). M. Sakai, M. Sasaki and A. Matsuda, Acta Mater., 53, 4455 4462 (2005). J. Q. Zhang, A. Matsuda, H. Muto and M. Sakai, Key Eng. Mater., 317–318, 317322 (2006). M. Sakai, “Principles and Applications of Indentation, in: Micro and Nano Mechanical Testing of Materials and Devices”, Springer, USA (2008) pp. 147. M. Sakai, Scr. Mater., 51, 391395 (2004). M. Sakai, J. Zhang and A. Matsuda, J. Mater. Res., 20, 2173 2183 (2005). A. Matsuda and M. Sakai, “Handbook of Nanoceramics and Their Based Nanodevices”, Ed. by T.-Y. Tseng and H. S. Nalwa, American Scientific Publishers, California (2009) pp. 85115. M. Sakai, “Elastoplastic Indentation Contact Mechanics of Homogeneous Materials and Coating-Substrate Systems”, Ed. by O. Tabata and T. Tsuchiya, Wiley-VCH, Germany (2008) pp. 2765. N. Hakiri, A. Matsuda and M. Sakai, J. Mater. Res., 24, 1950 1959 (2009). A. R. Ainuddin, N. Hakiri, H. Muto, M. Sakai and A. Matsuda, Phys. Status Solidi A, 20, doi:10.1002/pssa.201228139 (2012). K. Tadanaga, B. Ellis and A. B. Seddon, J. Sol-Gel Sci. Technol., 19, 687690 (2000). C. J. T. Landry, B. K. Coltrain, J. A. Wesson, N. Zumbulyadis and J. L. Lippert, Polymer, 33, 14961506 (1992).
445
