ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2011, том 53, № 10, с. 1800-1806
КОМПОЗИТЫ
УДК 541(64+14):535.3
TRANSPARENT POLY(METHYL METHACRYLATE)/TiO2 NANOCOMPOSITES
FOR UV-SHIELDING APPLICATIONS1
© 2011 г. Xiangfu Meng", Zhiwei Zhang", Nan Luo", Shengli Cao", and Mingshu Yang*
aDepartment of Chemistry, Capital Normal University, Beijing 100048, China bBeijing National Laboratory for Molecular Science, Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China e-mail: xfmeng@iccas.ac.cn Received October 27, 2010 Revised Manuscript Received February 20, 2011
Abstract — Transparent poly(methyl methacrylate) (PMMA)/TiO2 nanocomposites have been prepared by solution mixing PMMA with organically soluble titania xerogel. The organically soluble titania xerogel in the form of amorphous phase has been synthesized via a simple sol-gel method, involving hydrolysis of tetrabutyl titanate (TBT) in trifluoroacetic acid (TFA) and gelation. The obtained PMMA/TiO2 nanocomposites were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), thermogravimetry (TG) and ultraviolet-visible (UV-vis) absorption spectroscopy. The results showed that the interaction between titania nanoparticles and PMMA macromolecular chains led to a homogeneous dispersion of TiO2 in PMMA matrix. The resulting PMMA/TiO2 nanocomposites showed improved thermal stability, high transparency and high UV-shielding efficiency with a small amount of titania xerogel (<3.0 wt %). The present work is of interest for developing a series of transparent UV-shielding nanocomposites.
INTRODUCTION
Protection against ultraviolet radiation has become increasingly important in daily life and industry [1, 2]. The photo-oxidation effects of UV radiation can cause serious damages to both people and materials. For example, it produces DNA damage [3], immune suppression [4] and skin photoaging [5]. It is also responsible for the decomposition and degradation of organic compounds including polymers [6], dyes [7], pigments [8] and semiconductor devices [9]. UV light in the UVB range (290—320 nm) generally causes more serious damage than that in the UVA range (320— 400 nm) due to its higher energy. Therefore, there is a need to develop transparent UV-shielding materials that can decrease UV irradiation damage, especially in the UVB range. Such materials can be made by incorporating suitable UV-absorbing materials into a transparent polymeric matrix.
Organic UV absorbers have been used to protect organic materials against UV irradiation by transforming the absorbed radiation energy into the less damaging thermal energy through photophysical processes [10]. However, they are typically low in thermal stability and easy to migrate off the polymer matrix during uses [11]. In recent years, polymer-inorganic nano-composites have been considered to be novel and fast-growing class of materials, which have properties of both inorganic nanoparticles and polymers. Incorporation of a small amount of inorganic nanoparticles
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can dramatically improve the bulk performance of polymer matrix in mechanic, thermal, optical, electrical properties etc. [12—15]. Therefore, such nanocomposites have been considered as excellent candidates for UV-shielding applications. Zinc oxide (ZnO) and titanium dioxide (TiO2) are two wide band gap semiconductors that have been widely used as inorganic UV absorbers due to their remarkable optical properties and environmentally friendly nature. However, their inherent photocatalysis effect causes damages to the contacting organic materials, restricting their applications. Amorphous TiO2, as a UV absorber, has a much higher band gap (around 310 nm) than the crystalline TiO2 phase (around 400 nm) showing that it can effectively absorb the most damaging UV light, especially in the UVB range [16, 17]. Therefore, the amorphous TiO2 is a suitable candidate for a UV protective inorganic absorber.
Poly(methyl methacrylate) (PMMA) is one of the important transparent thermoplastic plastics, which has been utilized to make windows, lenses, and other optical devices. However, PMMA shows poor thermal stability, which restricts it from high-temperature applications. Furthermore, PMMA does not filter UV light down to 320 nm in wavelength. A number of studies have recently reported on the preparation of UV-absorbing PMMA/TiO2 or PMMA/ZnO nanocomposites [18—20]. Two general strategies were applied: in situ polymerization of polymer monomer in the presence of surface-modified inorganic particles
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and in situ generation of inorganic particles in the organic phase including bulk polymer, polymer solutions, and monomer systems. The in situ polymerization has a challenge to prepare nanoparticles with good dispersibility and long-term stability against aggregation in the monomer [21]. The in situ generation of inorganic particles strategy generally results in highly homogeneous nanocomposites with no phase separations at the molecular level [22]. Nevertheless, in situ generation of inorganic particles requires good control over both the sol-gel process to obtain TiO2 nanoparticles with desired properties and the polymerization process in the same reaction media, which only applies to limited systems. In addition, the complexity of the in situ approach limits its industrial applications.
In this work, we present a simple and effective approach to prepare transparent UV-shielding PMMA/TiO2 nanocomposites without any surface modification. Based on the synthesis of organically soluble titania xerogel, PMMA/TiO2 nanocomposite thin film is fabricated by casting the homogenous solution of titania xerogel and PMMA in tetrahydrofu-ran (THF). The transparent PMMA/TiO2 nanocom-posite thin film shows improved thermal stability and high UVB absorption efciency at low TiO2 concentrations (<3.0 wt %). Considering the simplicity of the fabrication of the nanocomposite materials, as shown in the experimental section, this work may shed light on the creation of transparent polymer/TiO2 nanocomposite materials with excellent UV-shielding properties.
conditions from 50°C to 600°C at a constant heating rate of 10°C/min. Ultraviolet-visible transmission spectra were measured with a UV-1600 UV/Vis Spectrophotometers (Shimadzu, Kyoto, Japan) in the wavelength range from 250 nm to 800 nm.
Synthesis of Organically Soluble Titania Xerogel
Organically soluble titania xerogel was synthesized via a modified sol-gel method, involving hydrolysis of tetrabutyl titanate (TBT) in trifluoroacetic acid (TFA) and gelation. In a typical synthesis, 15 ml of TBT was added into 15 ml of TFA with a molar ratio of 1/4. After stirring for 3 h at room temperature, a light yellow solution was obtained. The gelation process was carried out by placing the sol solution in air at room temperature for 2 d, and a light yellow gel was obtained. The gel was dried at 25°C in a vacuum oven for 8 h to obtain the titania xerogel.
Preparation of PMMA/TiO2 Nanocomposite Thin Films
The PMMA/TiO2 nanocomposite films were prepared via solution mixing method. PMMA and soluble titania xerogel were firstly dissolved in THF, respectively, and then mixed together under stirring. The film samples were obtained by casting the homogeneous solution on a Teflon Petri dishes. PMMA/TiO2 nanocomposite films with different contents of titania xerogel at 0.5 wt %, 1.0 wt % and 3.0 wt % were prepared. The film thickness was about 10 ^m measured by a micrometer.
EXPERIMENTAL
Materials
Tetrabutyl titanate (TBT, 98%), trifluoroacetic acid (TFA, 99.8%), and tetrahydrofuran (THF, 99.9%) was purchased from Beijing Chemical Reagent Co. and used without further purification. Poly(methyl meth-acrylate) (PMMA, Atoglas V920, Mw = 80 200 g/mol) was purchased from Atofina Chemicals, Inc.
Measurements
X-ray diffraction (XRD) pattern was recorded using CuZ"a radiation (X = 0.154 nm) at a generated voltage of 40 kV and current of 120 mA at room temperature, scanning at 8°/min. Fourier transform infrared (FTIR) spectra were obtained using a Perkin Elmer 2000 infrared spectrometer (PerkinElmer Inc., Walth-am, MA, USA) using the KBr pellet method. Transmission electron microscopy (TEM) measurement was carried out on a JEOL-2010 transmission electron microscopy operated at 200 kV. The TEM samples were prepared by dipping the solution on carbon-coated copper grids. Thermogravitational analysis (TGA) was measured on a Perkin-Elmer TGA-7 in nitrogen
RESULTS AND DISCUSSION
Structure Characterization of Soluble Titania Xerogel
The chemical sequence may be represented as shown in Scheme 1. According to the literature [23, 24], there are three possible structures of titania xerogel: monodentate, bidentate chelating and bidentate bridging. Carboxylate could be bound to one TiIV in a monodentate (Scheme 1A). Carboxylate could be also bound to one TiIV center in a bidentate chelating mode (Scheme 1B). Finally, the carboxyl group could bind with each of its oxygen atoms to a TiIV center of the surface yielding the bidentate bridging structure (Scheme 1C). It is well established that titanium alkoxides are highly reactive toward nucleophiles and that they can give rise to uncontrolled fast reactions. The presence of water in the reaction system often leads to instantaneous titania precipitation. Takahashi and co-workers thoroughly studied the reactivity of transition-metal alkoxides, and they demonstrated the possibility of running sol-gel reactions without water addition. In fact, the water, which is indeed required for hydrolysis to occur, can be obtained from air humidity, thus avoiding any undesired titania precipitation.
Intensity 100 I-
60 -
20 -
20
40
60
80
20, deg
Fig. 1. X-Ray diffraction pattern of titania xerogel. Insert is the digital graph of ti
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