Preparation and Characterization of Polypropylene Nanocomposites Containing Polystyrene-grafted Alumina Nanoparticles

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1 J. Ind. Eng. Chem., Vol. 12, No. 6, (2006) Preparation and Characterization of Polypropylene Nanocomposites Containing Polystyrene-grafted Alumina Nanoparticles Chan-Hee Jung, Jae-Hak Choi, Youn-Mook Lim, Joon-Pyo Jeun, Phil-Hyun Kang, and Young Chang Nho Radiation Application Research Division, Advanced Radiation Technology Institute-Jeongeup, Korea Atomic Energy Research Institute, Jeonbuk , Korea Received June 28, 2006; Accepted August 30, 2006 Abstract: After surface modification of γ-al 2 O 3 nanoparticles with a silane coupling agent, styrene was graftcopolymerized onto γ-al 2 O 3 nanoparticles using a simultaneous γ-ray irradiation technique. The influence of the irradiation dose and the grafting kinetics were investigated in detail. Polypropylene nanocomposites were fabricated by blending polypropylene, polystyrene-grafted γ-al 2 O 3 (γ-al 2 O 3 -g-ps), and a small amount of 1,4-butandiol dimethacrylate (1,4-BDDA) as a crosslinker, followed by e-beam irradiation. The polystyrene-grafted γ-al 2 O 3 and polypropylene nanocomposites were characterized by TGA, FT-IR spectroscopy, SEM, and UTM analyses. The polystyrene graft yield onto γ-al 2 O 3 increased with the absorbed dose. The graft yield was higher at 50 vol% of the styrene solution than at 70 vol%. The nanocomposite fabricated with 5 phr of γ-al 2 O 3 -g-ps and 3 phr of 1,4-BDDA showed the highest tensile strength. The homogeneous dispersion of γ-al 2 O 3 -g-ps into the polypropylene matrix and the crosslinking through e-beam irradiation improved the mechanical properties of the nanocomposites. Keywords: polypropylene, nanocomposite, nanoparticle, irradiation Introduction 1) Traditional polymer nanocomposites have improved mechanical properties, such as toughness, as a result of the incorporation of inorganic particulate fillers [1-2]. However, high filler loadings (up to 20 % by volume) are required for such an enhancement of performance, leading to a loss of the easy processability of the polymers. Consequently, polymer-based nanocomposites are attracting considerable attention because of the unique properties that result from their nano-scale microstructures [3,4]. They are much lighter in weight, more transparent, and easier to process than conventional inorganic particlereinforced polymers, in addition to displaying improved mechanical properties [5-8]. However, the homogeneous distribution of inorganic nanoparticles into the polymer matrix is required to obtain the desired polymer-based nanocomposites [9-13] because agglom- To whom all correspondence should be addressed. ( ycnho@kaeri.re.kr) eration of inorganic nanoparticles caused by immiscibility between the inorganic nanoparticles and the polymer matrix leads to a reduction, rather than an improvement, of the material s properties. To improve the mechanical properties through uniform dispersion of inorganic nanoparticles into the matrix, we synthesized polystyrene-grafted γ-al 2 O 3 through a highenergy irradiation method after surface modification of nano-γ-al 2 O 3. The polypropylene naocomposites were fabricated by blending surface-modified γ-al 2 O 3 nanoparticles and crosslinking agents, followed by e-beam irradiation. Their characterization is described. Experimental Materials Polypropylene (PP; B310; MW: 523,000; Honam Petrochemical Co., Ltd.) was employed as a polymer matrix. Micropolished γ-al 2 O 3, possessing an average diameter of 50 nm, was purchased from Buehler

2 Preparation and Characterization of Polypropylene Nanocomposites Containing Polystyrene-grafted Alumina Nanoparticles 901 Table 1. Graft Polymerizations onto TMSPM-modified Al 2O 3 Styrene Concentration a (vol%) Total Dose (kgy) Dose Rate (kgy/h) Graft Yield (%) b a Methanol used as solvent. b Weight of grafting polymer/weight of TMSPM-treated γ-al 2O 3 measured by TGA. Scheme 1. Surface modification of alumina particles: (a) silylation with TMSPM and (b) graft polymerization with styrene. Company. 3-(Trimethoxysilyl)propyl methacrylate (TM- SPM), xylene, methanol, and styrene were purchased from Aldrich Chemical Company and used as received. 1,4-Butanediol dimethacrylate (1,4-BDDA) and trimethylopropane triacrylate (TMPTA) were supplied by the Aldrich Chemical Company and used as crosslinking agents without further purification. Graft Polymerization onto γ-al 2 O 3 Nanoparticles Prior to a silylation, γ-al 2 O 3 nanoparticles (10 g) were dried in a vacuum oven (195 C) for 24 h and then dispersed in 250 ml of dry xylene with the aid of an ultrasonic probe. The nanoparticle/xylene mixture was added to a 500-mL round-bottom flask containing a stirrer a bar, and then a 10 % v/v TMSPM/xylene solution was added with stirring. The mixture was heated under reflux for 20 h under a nitrogen atmosphere. The mixture was cooled, filtered, and then dried in a vacuum oven at room temperature for 24 h. The dried TMSPM-modified γ- Al 2 O 3 (0.5 g) was immersed into various concentrations of styrene in methanol. The resulting solutions were flushed for 15 min with nitrogen and then irradiated using γ-rays from a 60 Co source at a dose rate of 2.5 kgy/h at room temperature. The resulting polystyrene-grafted γ-al 2 O 3 (γ-al 2 O 3 -g-ps) was washed thoroughly with hot benzene in a Soxhlet extractor to remove any residual monomer and homopolymer. The γ-al 2 O 3 -g-ps was dried in a vacuum oven at 80 o C for 24 h. The overall synthetic scheme is shown in Scheme 1. The weight loss of the grafting polymer was calculated using a TA thermogravimeter.

3 902 Chan-Hee Jung, Jae-Hak Choi, Youn-Mook Lim, Joon-Pyo Jeun, Phil-Hyun Kang, and Young Chang Nho Figure 1. FT-IR spectra of pure γ-al 2O 3, TMSPM-treated γ- Al 2O 3, and Al 2O 3-g-PS. Figure 2. Tensile strengths of nanocomposites plotted as a function of the amounts of the two kinds of crosslinking agents. Preparation and Characterization of Polypropylene Nanocomposites Polypropylene nanocomposites were fabricated by blending polypropylene pellets, γ-al 2 O 3 -g-ps, and crosslinking agents using a lab-scale Brabender instrument, followed by e-beam irradiation. E-beam irradiation of the mixed samples was performed in the EB-tech using an ELV-4 electron beam accelerator with an energy of 1.0 MeV. The integral irradiation dose levels were conducted at 4 kgy. The stress-strain properties of the prepared nanocomposite were determined using an Instron model 4411 testing machine according to ASTM D 638. The test procedure was performed at a crosshead speed of 50 mm/min at room temperature. The dispersion of γ-al 2 O 3 -g-ps in the nanocomposites was investigated using a scanning electron microscope (SEM; XL30S FEG, Philips Co.). Figure 3. Tensile strengths of nanocomposites plotted as a function of the filler content. Results and Discussion Graft Polymerization onto γ-al 2 O 3 Nanoparticles The FT-IR spectra of neat γ-al 2 O 3, TMSPM-modified γ-al 2 O 3, and γ-al 2 O 3 -g-ps are shown in Figure 1. After surface modification with TMSPM, a carbonyl peak at 1730 cm -1 and an aliphatic C-H band at 2950 cm -1 appeared in TMSPM. After graft polymerization, aromatic C-H bands (at 3010, 1600, and 1475 cm -1 ) resulting from the polystyrene were newly generated. The surface modification was also confirmed by performing a floating test on water. The surface-modified γ-al 2 O 3 nanoparticles did not wet because the hydrophilic surface of the alumina particles had become hydrophobic. Quantitative results of the graft polymerization on γ-al 2 O 3 are given in Table 1. The graft yield increased upon increasing the irradiation doses. The graft yield was higher at a 50 vol% monomer concentration than at 70 vol%. The highest graft yield was obtained when 50 vol% of styrene was irradiated at 10 kgy. In addition, neat γ-al 2 O 3, TMSPMtreated γ-al 2 O 3, and Al 2 O 3 -g-ps were characterized using an SEM. Characterization of Polypropylene Nanocomposites The tensile strengths of nanocomposites prepared with different crosslinking agents and 5 phr of TMSPMtreated γ-al 2 O 3 are shown in Figure 2. The tensile strengths of pure PP and 4 kgy-irradiated PP were 2.74 and 2.78 kgf/mm 2, respectively. Both the 1,4-BDDA- and TMPTA-crosslinked nanocomposites displayed similar tendencies. The tensile strengths were enhanced upon increasing the content of the crosslinking agents up to 3 phr, but they gradually decreased at contents of crosslinking agents over 3 phr. At higher concentration more than 3 phr the monomers may lead to the production of homopolymers, rather than reaction with PP, because so much monomer surrounds the radicals

4 Preparation and Characterization of Polypropylene Nanocomposites Containing Polystyrene-grafted Alumina Nanoparticles 903 of fillers. The tensile strengths of the prepared nanocomposites (Figure 3) increased upon increasing the γ-al 2 O 3 -g-ps content up to 5 phr. The nanocomposite containing 5 phr of γ-al 2 O 3 -g-ps exhibited the highest tensile strength. The SEM micrographs of the fracture surfaces of the composites are shown in Figure 4. γ-al 2 O 3 -g-ps formed comparatively smaller agglomerates than did the untreated γ-al 2 O 3 or TMSPM-treated γ-al 2 O 3. The reason for this phenomenon is that the polystyrene chain grafted onto the γ-al 2 O 3 interfered with the agglomerization of the nanoparticles. This result is in agreement with the results of the tensile strength measurements above. Conclusions In this study, we performed surface modification of neat γ-al 2 O 3 with TMSPM. The graft polymerization of styrene onto TMSPM-modified γ-al 2 O 3 was performed using the simultaneous irradiation polymerization technique under various conditions. We found that the graft yields increased upon increasing the absorbed dose; the graft yield at 50 vol% of monomer was higher than that at 70 vol%. The highest graft yield was obtained when 50 vol% of styrene was irradiated at 10 kgy. The nanocomposite fabricated with 5 phr of γ-al 2 O 3 -g-ps and 3 phr of 1,4-BDDA showed the highest tensile strength. The homogeneous dispersion of γ-al 2 O 3 -g-ps into the polypropylene matrix and the crosslinking by e-beam irradiation improved the mechanical properties of the nanocomposites. Additional studies are underway to incorporate various polymergrafted γ-al 2 O 3 samples into polymer matrixes for the preparation of nanocomposites with improved mechanical properties. Figure 4. Fracture surfaces of nanocomposites: (a) PP/5 phr of untreated γ-al 2O 3, (b) PP/1,4-BDDA/5 phr of γ-al 2O 3, and (c) PP/1,4-BDDA/5 phr of Al 2O 3-g-PS. formed during irradiation. The tensile strength of the 1,4- BDDA-crosslinked nanocomposite was higher than that of the TMPTA-crosslinked one. The reason may be that the rate of crosslinking of TMPTA is faster because of its higher reactivity for BDDA; as the crosslinking density of the matrix increases, however, the chain radical and TMPTA diffusion become restricted and, therefore, lead to a lower crosslinking efficiency [14,15]. The nanocomposites fabricated with 3 phr of 1,4-BDDA showed the highest tensile strength. From these results, we fixed the content of 1,4-BDDA at 3 phr and prepared nanocomposites according to various contents of the three kinds References 1. B. J. Ash, R. W. Siegel, and L. S. Schadler, Macrolmolecules, 37, 1358 (2004). 2. M. Hussan, A. Nakahra, and K. Niihara, Mater. Lett., 27, 21 (1996). 3. S. Sepeur, B. Werner, and H. Schmit, Thin Solid Films, 351, 216 (1999). 4. B. Jiang and G. Huang, Radiat. Phys. Chem., 69, 433 (2004). 5. F. Bauer, H. Ernst, U. Decker, E. Hartmann, and H. Langguth, Macromol. Chem. Phys., 201, 2654 (2000). 6. D. N. Bikiaris, A. Vassiliou, and G. P. Karaynannidis, Eur. Polym. J., 41, 1965 (2005).

5 904 Chan-Hee Jung, Jae-Hak Choi, Youn-Mook Lim, Joon-Pyo Jeun, Phil-Hyun Kang, and Young Chang Nho 7. M. Z. Rong, M. Q. Zhang, and H. M. Zeng, Polymer, 42, 167 (2001). 8. (a) C. L. Wu, M. Q. Zhang, and M. Z. Rong, Compos. Sci. Technol., 62, 1327 (2002). (b) C. H. Hong, Y. B. Lee, J. W. Bae, J. Y. Jho, B. U. Nam, and T. W. Hwang, J. Ind. Eng. Chem., 11, 76 (2005). (c) J. H. Sung, M. S. Cho, H. J. Choi, and M. S. Jhon, J. Ind. Eng. Chem., 10, 1217 (2004). 9. F. Bauer, V. Sauerland, and R. Mehnert, Macromol. Mater. Eng., 287, 546 (2002). 10. P. Matsuno, P, K. Yamaoto, and A. Takahara, Chem. Mater., 15, 3 (2003). 11. K. Mai, Z. Li, Y. Qui, and H. Zeng, J. Appl. Polym. Sci., 80, 2617 (2001). 12. S.C. Liufu, H. N. Xia, and Y. P. Li, Polym. Degrad. Stab., 87, 103 (2005). 13. F. Bauer, E. Hartmann, and R. Mehnert, Nucl. Instrum. Meth. B, 208, 267 (2003). 14. F. Yoshii, K. Makuuchi, S. Kikukawa, and K. Koyama, J. Appl. Polym. Sci., 60, 617 (1996). 15. S. Jonsson, P. Sundell, J. Hultgren, and C. E. Hoyle, Prog. Org. Coat. 27, 107 (1996).