Enhanced Degradation in Nanocomposites of TiO2 and

One solution to solve this problem is to make plastics using biodegradable polymers, which are degraded in soil, activated sludge, or compost after th...
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Environ. Sci. Technol. 2008, 42, 4551–4554

Enhanced Degradation in Nanocomposites of TiO2 and Biodegradable Polymer MASAHIRO MIYAUCHI,* YONGJIN LI, AND HIROSHI SHIMIZU Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan† AIST

Received January 16, 2008. Revised manuscript received March 27, 2008. Accepted March 31, 2008.

Nanocomposites of titamium dioxide (TiO2) particles and biodegradable poly (butylene succinate) (PBS) were fabricated by melt-blending using a high-shear extruder. TiO2 particles were highly dispersed in the PBS matrix by high-shear processing, and the addition of TiO2 particles into PBS did not decrease its mechanical strength. The photocatalytic decomposition and biodegradable properties of the nanocomposites were evaluated by UV irradiation or enzymatic degradation methods in vitro. It was found that both the esterase enzyme and UV irradiation decomposed the nanocomposites. Photocatalytic decomposition of PBS clearly depended on the size and dispersibility of TiO2 particles in PBS polymer. Higher dispersibility and smaller size of TiO2 particles were effective on the photocatalytic oxidation of PBS. In addition, decomposition rate under a simultaneous UV irradiation treatment and immersion in an enzyme solution was higher than those under UV irradiation or immersion in an enzyme solution. These results indicate that the nanocomposites can easily be decomposed not only by an enzyme in soil or compost, but also by photocatalytic oxidation of TiO2 under sunlight.

Introduction Recently, tremendous amounts of plastics have been produced from fossil fuels, and the environmental pollutions from these plastic wastes have become a global issue. Disposal of these plastics by incineration releases CO2 and toxic gas. One solution to solve this problem is to make plastics using biodegradable polymers, which are degraded in soil, activated sludge, or compost after the service life is over. Thus, enhancing the biodegradability is an important issue for industrial applications of biodegradable polymers. The addition of inorganic particles to a polymer matrix improves the biodegradability, and a high dispersion of inorganic nanoparticles in a polymer is key factor for enhancing the performance. Ray et al. have recently reported the enhanced biodegradability of layered silicate/poly (L-lactide) nanocomposites (1). In addition, Lee et al. have reported the improvement of biodegradability in aliphatic polyester/clay nanocomposites (2). In the present paper, we focus on nanoparticles of titanium dioxide (TiO2) as an inorganic filler in the polymer matrix. TiO2 is an efficient photocatalyst (3), which is nontoxic, resource abundant, and chemically stable. Thus, it has already been applied to various industrial items. * Corresponding author fax: +81-29-861-6299; e-mail: m-miyauchi@ aist.go.jp. 10.1021/es800097n CCC: $40.75

Published on Web 05/16/2008

 2008 American Chemical Society

TiO2 can decompose almost all organic compounds under UV irradiation. Cho et al. have reported the solid-phase photocatalytic degradation of poly (vinyl chloride) (PVC) and TiO2 composite under ambient air conditions (4). Kim et al. have recently reported the suppression of dioxin emittingincineration of post use PVC in PVC/ TiO2 nanocomposites (5). In addition to PVC, solid-phase photocatalytic degradations of polystyrene have also been reported (6, 7). Recently, the photoinduced degradation and antibacterial properties of TiO2/ethylene-vinyl alcohol copolymer composites have been reported (8). In the present paper, we focus on poly (butylene succinate) (PBS) because PBS has interesting properties such as biodegradability, melt processability, and thermal and chemical resistance (9, 10). We added photocatalytically active TiO2 nanoparticles into PBS to enhance the degradability of PBS by sunlight illumination. The dispersion of TiO2 nanoparticles in a polymer matrix is very important for enhancing the biodegradability of PBS because the surface of inorganic TiO2 has OH groups, which generally have less affinity to organic polymer. Our recent studies have found that highshear processing effectively fabricates nanodispersed polymer nanocomposites without using additives or surfactants (11, 12). In the present paper, we fabricated TiO2/PBS nanocomposites using a high-shear extruder, and evaluated their photoinduced decomposition and biodegradability in relation to the dispersion state of TiO2 particles.

Experimental Section Materials and Sample Preparation. Poly (butylene succinate) (PBS) used in this study was a commercial product (Bionolle no. 3001: Showa Highpolymer Co. Japan). PBS pellets were dried at 353 K in a vacuum oven prior to use. Two types of TiO2 particles were used in this study (ST-01: Ishihara Sangyo Co. Japan, and P-25: Degussa Co.). The particle sizes of ST01 and P-25 were 7 and 30 nm, respectively. High-shear processing was performed using a high-shear extruder, HSE3000mini (Imoto, Co. Japan) (13). A feedback-type screw was used in this extruder. The rotation speed of the screw was varied from 50 to 1000 rpm, which corresponded to an average shear rate of 75 to 1500 s-1, respectively. Pellets of PBS and TiO2 powder were melt-blended at 433 K under the desired screw rotation speed for two minutes. After the meltblend process, TiO2/PBS nanocomposites were extruded from a T-die. These nanocomposites were then converted into sheets with a thickness between 60-80 µm using a hot press at 423 K for three minutes. The TiO2/PBS nanocomposites were cut into 2.0 × 5.0 cm2 sheets. Evaluation. A scanning electron microscope (SEM) was used to characterizing the morphology of the TiO2/PBS nanocomposites. Nanocomposites were immersed in liquid nitrogen and subsequently fractured for the SEM observations. The fractured surfaces were coated with a thin layer of platinum and observed by SEM (S-4800, Hitachi Co. Japan) at an accelerating voltage of 5.0 kV. A transmission electron microscope (TEM; H-7600, Hitachi Co. Japan) was used to investigate the dispersion of TiO2 particles in nanocomposites. TEM images were obtained at an acceleration voltage of 100 kV using a two-step-staining method (14). Weight changes in the TiO2/PBS sheets were measured under UV irradiation, which was provided by a 20 W black light bulb (Toshiba Co., Japan) with a light intensity of 1.0 mW/ cm2 measured by a UV radiometer (UVR-2, Topcon Co. Japan). Fourier transform infrared spectra (FT-IR, Bio-Rad Co. UMA600) were recorded before and after UV irradiation. The biodegradability of TiO2/PBS sheets was also evaluated by VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. TEM images for TiO2/PBS composites with screw rotation speeds of 50 rpm (a) and 1000 rpm (b). TiO2 particles are ST-01, and the concentration of TiO2 is 5 wt% in a PBS matrix.

FIGURE 1. SEM images for TiO2/PBS composites with different screw rotation speed. TiO2 particles are ST-01, and the concentration of TiO2 is 5 wt% in a PBS matrix. an enzymatic degradation in vitro measurement. Lipase was used as a hydrolysis enzyme to decompose PBS (15) because this enzyme is widely found in animals, plants, and microorganisms. Lipase-PS (Amano Pharmaceutical Co. Japan) was dissolved in a phosphate buffer solution (pH 6.8) with a concentration of 0.1 wt%. The sheets of TiO2/PBS were immersed into this Lipase solution at a constant temperature of 300 K, and then removed from the solution at appropriate time intervals. After washing by pure water and drying, these TiO2/PBS sheets were weighed. In addition, the weight changes in TiO2/PBS under a simultaneous UV irradiation treatment and immersion in an enzyme solution were also measured. UV light irradiation was provided by a 20 W black light bulb (Toshiba Co., Japan) with a light intensity of 1.0 mW/ cm2 measured by a UV radiometer (UVR-2, Topcon Co. Japan). Since the enzyme molecules are insensitive to UV light, photons in UV region can reach the surface of the nanocomposites to excite the TiO2 in enzyme solution. The temperature of enzyme solution under UV irradiation was kept at 300 K. The experimental method for measuring the weight change of nanocomposites was the same as that for the above-mentioned method.

Results and Discussion Figure 1 shows SEM images for TiO2/PBS composites with different screw rotation speeds. The TiO2 particle in Figure 1 was ST-01 with a particle size of 7 nm, which was determined by Scherrer’s equation from the X-ray diffraction patterns. Aggregates of TiO2 particles, which were with several hundreds nanometers, were observed in low-shear processed samples below 100 rpm, whereas samples with high-shear processing above 200 rpm exhibited smooth surfaces. Figure 2 shows TEM images for nanocomposites with screw rotation speeds of 50 (a) and 1000 rpm (b). Aggregates of TiO2 nanoparticles with several hundreds nanometers were observed in nanocomposites with low screw rotation speed, whereas TiO2 particles were highly dispersed under the high screw rotation speed. The size of the TiO2 aggregates observed in the low-shear processed samples was much larger than 4552

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FIGURE 3. Weight loss in TiO2/PBS composites under UV irradiation. Closed diamond: 5wt% TiO2 in PBS, closed triangle: 2wt% TiO2 in PBS, closed square: 1wt% TiO2 in PBS, and cross: pure PBS, respectively. TiO2 particles are ST-01 with a particle size of 7 nm, and the screw rotation speed of these samples is 1000 rpm. UV irradiation is provided by a black light bulb with UV intensity of 1.0 mW · cm-2. that determined by Scherrer’s equation. The surface of TiO2 is hydrophilic, similar to the other inorganic metal oxides (16). It is known that there are OH groups on the TiO2 surface at terminal and bridging sites (17–19). Therefore, hydrophilic TiO2 particles aggregate in a hydrophobic polymer matrix when the screw rotation speed is low. TiO2 particles were highly dispersed in PBS under high-shear processing. Figure 3 shows the changes in weight loss of high-shear processed (1000 rpm) TiO2/PBS nanocomposites under UV irradiation. The weight of TiO2/PBS nanocomposites decreased under UV irradiation, whereas the pure PBS remained unchanged under the same conditions. These results indicate that PBS is decomposed by the photocatalytic activity of TiO2, but pure PBS is stable under UV irradiation. The photocatalytic decomposition rate of TiO2/PBS nanocomposites clearly depends on the amount of TiO2 in PBS. At an initial UV irradiation stage, the weight of the nanocomposites linearly decreased, indicating the photocatalytic decomposition of PBS proceeds under zero order reaction kinetics. The amount of TiO2 particles was below 5 wt%, which is much less than the PBS volume. In this case, the reaction rate does not depend on the volume of PBS. After a longer UV irradiation time, the photocatalytic reaction did not proceed under zero order kinetics, since the percentage of TiO2 particles in PBS matrix were increased. Furthermore, it is noteworthy that the weight loss in 5 wt% TiO2/PBS after 26 days UV irradiation corresponds to a 67% loss from the initial weight of the nanocomposite. Figure 4 shows the SEM images for the before and after 26 days of UV irradiation on a 5 wt% TiO2(ST-01)/PBS nanocomposite. After UV irradiation, the TiO2/PBS nanocomposites became porous and easily breakable. The solid-phase photocatalytic degradation of polymer

FIGURE 4. SEM images (a) before and (b) after UV irradiation for 26 days. TiO2 particles are ST-01, and the concentration of TiO2 is 5 wt% in a PBS matrix with a screw rotation speed of 1000 rpm.

FIGURE 6. Weight loss after 15 days under UV irradiation as a function of the screw rotation speed. Closed diamond: ST-01 (7 nm), open square: P-25 (30 nm). produced (22, 23), and these radical are effective to oxidize organic compounds. organic compound + O2 f CO2 + H2O + mineral acid (1) h+ + H2O f OH • + H+ -

(2)

+

e + O2 + H f HO2 •

FIGURE 5. FT-IR spectra for nanocomposites. (a) pure PBS, (b) TiO2/PBS before UV irradiation, and (c): TiO2/PBS after UV irradiation. by TiO2 particles is not suited for bulk materials, owing to the limitation of optical pass length in nanocomposites. The initial thicknesses of the nanocomposite films were about 60-80 µm in the present study, whereas the optical pass length at a wavelength of 360 nm in an indirect transition semiconductor was several micrometers (20). Cho et al. have reported that the light penetration depth of 1.5 wt% TiO2 contained poly (vinyl chloride) is 66 µm under a wavelength of 350 nm (4). PBS is crystalline polymer and the film is semitransparent by a light scattering effect. Although the apparent absorbance value of TiO2/PBS composites cannot be estimated, UV light is not absorbed in PBS but is scattered by crystalline PBS grains. In this case, UV light can reach and excite TiO2 particles in PBS matrix, since the optical pass length of TiO2 itself is several micrometers and the content of TiO2 particles is below 5 wt%. Figure 5 shows FT-IR spectra before and after UV irradiation on TiO2/PBS nanocomposites. The spectrum of the TiO2/PBS nanocomposite after UV irradiation revealed broad absorption ranged from 3000 to 3600 cm-1, which is assigned to hydroxyl or carboxyl groups. Further, the spectrum of the TiO2/PBS nanocomposite after UV irradiation had a broad absorption below 800 cm-1, which is originated in crystalline TiO2. These results indicate that the C-C or C-H bonds were oxidized to form carboxyl groups and OH groups by the photocatalytic reaction of TiO2. Irradiation of semiconductor TiO2 photocatalysts with photons, which have energies greater than the band gap, gives rise to excitedstate electrons in the conduction band and holes in the valence band. Photogenerated electrons and holes in TiO2 have strong reduction and oxidation power enough to mineralize organic compounds. Photocatalyzed reactions can be easily summarized by the mineralization of organic compound as the following eq 1 (21). During the mineralization reaction, intermediate species such as HO2• or OH• are

(3)

Kubo et al. have recently reported that these photogenerated radicals can diffuse in air and remotely oxidize organic compounds (24). PBS is composed of ester groups and alkyl chains, and it is easily mineralized by a photocatalytic reaction of TiO2 under UV irradiation. Figure 6 shows the weight loss under UV irradiation versus the screw rotation speed for the different size of TiO2 particles. The smaller TiO2 particles exhibited better photocatalytic performances than the large particles. P-25 is composed of rutile and anatase mixed phases, while ST-01 is pure anatase phase. Surface areas for P-25 and ST-01 are 50 and 200 m2/g, respectively. Sano et al. compared the photocatalytic activity of P-25 with that of ST-01 for decomposing vinyl chloride monomers, and the photocatalytic activity of ST-01 was higher than that of P-25, owing to the high surface area of ST-01 (25). The data in the present study also suggest that a higher screw rotation speed increases the photocatalytic decomposition rate of the nanocomposites. As is shown in Figure 1, TiO2 aggregates were observed in the PBS matrix below a screw rotation speed of 200 rpm. These results indicate that both a high dispersion and particle size of TiO2 in the polymer play important roles in the solid-phase photooxidation process of PBS. Our previous studies have revealed that immiscible polymer blends show a miscible region under a flow field with a high shear rate above 1000 s-1 (26). However, in the present study, the photocatalytic activity was saturated above 200 rpm, which corresponds to about 300 s-1. Thus, a high screw rotation speed above 200 rpm can lead to an efficient photocatalytic activity in TiO2/PBS nanocomposites without additives or surfactants due to the higher dispersion of TiO2 particles in the polymer. We also evaluated the mechanical strength of the nanocomposites, and found that the elastic modulus did not decrease upon the addition of TiO2 to PBS. Finally, we evaluated the effect of UV irradiation onto the biodegradability of TiO2/PBS nanocomposites. Figure 7 shows the weight loss of pure PBS and TiO2/PBS in an enzyme solution, under UV irradiation, and under simultaneous treatment of an enzyme and UV irradiation. Both the pure PBS and TiO2/PBS were decomposed under enzyme treatment. The Lipase enzyme in the present study is a well-known esterase, which can VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Weight loss after three days under enzymatic treatment, UV irradiation, and simultaneous treatment of enzyme and UV irradiation for pure PBS and TiO2/PBS nanocomposites. TiO2 particles are ST-01, and the concentration of TiO2 is 5 wt% in a PBS matrix with a screw rotation speed of 1000 rpm. decompose ester groups in PBS to form 1, 4- succinate and butanediol by a hydrolysis reaction as follows, HO-[-CO-(CH2)2-COO-(CH2)4-O-]n-H f n[HOOC-(CH2)2-COOH] + n[HO-(CH2)4-OH] (4) Previous studies reported that the molecular weight of biodegradable polymer was decreased by enzymatic treatment (27, 28). Produced succinate and butanediol are dissolved into aqueous enzyme solution, and the weight of nanocomposites is decreased under enzyme treatment. The decomposition rate of TiO2/PBS under the enzyme treatment was less than pure PBS, since TiO2 particles inhibit the diffusion and adsorption of enzyme molecules onto the PBS surface in an aqueous enzyme solution. However, it is noteworthy that the simultaneous treatment of the enzyme and UV irradiation onto the TiO2/PBS exhibited the highest degradability, indicating that UV irradiation enhanced the degradability of the TiO2/PBS nanocomposites in the enzyme solution. Pure PBS is photocatalytically inactive and is not decomposed under enzyme treatment, thus the decomposition rate under the simultaneous treatment of the enzyme and UV irradiation was nearly the same as that under only the enzyme treatment. Simultaneous treatment of the enzyme and UV irradiation in TiO2/PBS caused photogenerated radicals to further oxidize the PBS polymer, while the esterase reaction proceeded in PBS. While the enzyme decomposes the ester groups in PBS, TiO2 photocatalysts can completely mineralize the PBS polymer. It should be emphasized that the UV intensity in the present study is not very high, but is on the order of the UV intensity in natural sunlight. Hence, microorganisms found in soil as well as natural sunlight irradiation can decompose our nanocomposites. In the present study, PBS is used as a biodegradable polymer, but other polymers such as polylactide (PLA) are also easily decomposed by a TiO2 photocatalyst. The strategy in the present study is applicable to various eco-friendly film products with efficient degradability.

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ES800097N