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

J. Ind. Eng. Chem., Vol. 12, No. 6, (2006) 900-904

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 580-185, Korea

Received June 28, 2006; Accepted August 30, 2006

Abstract: After surface modification of γ-Al2O3 nanoparticles with a silane coupling agent, styrene was graft- copolymerized onto γ-Al2O3 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-Al2O3 (γ-Al2O3-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-Al2O3 and polypropylene nanocomposites were characterized by TGA, FT-IR spectroscopy, SEM, and UTM analyses. The polystyrene graft yield onto γ-Al2O3 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-Al2O3-g-PS and 3 phr of 1,4-BDDA showed the highest tensile strength. The homogeneous dispersion of γ-Al2O3-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 eration of inorganic nanoparticles caused by immisci- 1) bility between the inorganic nanoparticles and the poly- Traditional polymer nanocomposites have improved mer matrix leads to a reduction, rather than an improve- mechanical properties, such as toughness, as a result of ment, of the material’s properties. the incorporation of inorganic particulate fillers [1-2]. To improve the mechanical properties through uniform However, high filler loadings (up to 20 % by volume) are dispersion of inorganic nanoparticles into the matrix, we required for such an enhancement of performance, lead- synthesized polystyrene-grafted γ-Al2O3 through a high- ing to a loss of the easy processability of the polymers. energy irradiation method after surface modification of Consequently, polymer-based nanocomposites are at- nano-γ-Al2O3. The polypropylene naocomposites were tracting considerable attention because of the unique fabricated by blending surface-modified γ-Al2O3 nano- properties that result from their nano-scale micro- particles and crosslinking agents, followed by e-beam structures [3,4]. They are much lighter in weight, more irradiation. Their characterization is described. transparent, and easier to process than conventional inorganic particlereinforced polymers, in addition to dis- playing improved mechanical properties [5-8]. However, Experimental the homogeneous distribution of inorganic nanoparticles into the polymer matrix is required to obtain the desired Materials polymer-based nanocomposites [9-13] because agglom- Polypropylene (PP; B310; MW: 523,000; Honam Petrochemical Co., Ltd.) was employed as a polymer † γ To whom all correspondence should be addressed. matrix. Micropolished -Al2O3, possessing an average (e-mail: [email protected]) diameter of 50 nm, was purchased from Buehler Preparation and Characterization of Polypropylene Nanocomposites Containing Polystyrene-grafted Alumina Nanoparticles 901

Table 1. Graft Polymerizations onto TMSPM-modified Al2O3 Total Dose Dose Rate Graft Yield Styrene Concentrationa (vol%) (kGy) (kGy/h) (%)b 50 2.5 2.5 2.1 50 5 2.5 4.8 50 7.5 2.5 7.5 50 10 2.5 8.1 70 2.5 2.5 1.9 70 5 2.5 4.4 70 7.5 2.5 6.2 70 10 2.5 7.1 a b Methanol used as solvent. Weight of grafting polymer/weight of TMSPM-treated γ-Al2O3 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- for 20 h under a nitrogen atmosphere. The mixture was SPM), xylene, methanol, and styrene were purchased cooled, filtered, and then dried in a vacuum oven at room from Aldrich Chemical Company and used as received. temperature for 24 h. The dried TMSPM-modified γ- 1,4-Butanediol dimethacrylate (1,4-BDDA) and trimeth- Al2O3 (0.5 g) was immersed into various concentrations ylopropane triacrylate (TMPTA) were supplied by the of styrene in methanol. The resulting solutions were Aldrich Chemical Company and used as crosslinking flushed for 15 min with nitrogen and then irradiated us- agents without further purification. ing γ-rays from a 60Co source at a dose rate of 2.5 kGy/h at room temperature. The resulting polystyrene-grafted Graft Polymerization onto γ-Al2O3 Nanoparticles γ-Al2O3 (γ-Al2O3-g-PS) was washed thoroughly with Prior to a silylation, γ-Al2O3 nanoparticles (10 g) were hot benzene in a Soxhlet extractor to remove any residual dried in a vacuum oven (195 °C) for 24 h and then dis- monomer and homopolymer. The γ-Al2O3-g-PS was persed in 250 mL of dry xylene with the aid of an ultra- dried in a vacuum oven at 80 oC for 24 h. The overall sonic probe. The nanoparticle/xylene mixture was added synthetic scheme is shown in Scheme 1. The weight loss to a 500-mL round-bottom flask containing a stirrer a of the grafting polymer was calculated using a TA bar, and then a 10 % v/v TMSPM/xylene solution was thermogravimeter. added with stirring. The mixture was heated under reflux 902 Chan-Hee Jung, Jae-Hak Choi, Youn-Mook Lim, Joon-Pyo Jeun, Phil-Hyun Kang, and Young Chang Nho

Figure 3. Tensile strengths of nanocomposites plotted as a function of the filler content. Figure 1. FT-IR spectra of pure γ-Al2O3, TMSPM-treated γ- Al2O3, and Al2O3-g-PS. Results and Discussion

Graft Polymerization onto γ-Al2O3Nanoparticles The FT-IR spectra of neat γ-Al2O3, TMSPM-modified γ-Al2O3, and γ-Al2O3-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 ap- peared 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-Al2O3 nanoparticles did not wet because the hydrophilic surface of the alumina particles had become hydrophobic. Quantitative re- γ Figure 2. Tensile strengths of nanocomposites plotted as a fun- sults of the graft polymerization on -Al2O3 are given in ction of the amounts of the two kinds of crosslinking agents. Table 1. The graft yield increased upon increasing the irradiation doses. The graft yield was higher at a 50 vol% Preparation and Characterization of Polypropylene monomer concentration than at 70 vol%. The highest Nanocomposites graft yield was obtained when 50 vol% of styrene was ir- Polypropylene nanocomposites were fabricated by radiated at 10 kGy. In addition, neat γ-Al2O3, TMSPM- blending polypropylene pellets, γ-Al2O3-g-PS, and treated γ-Al2O3, and Al2O3-g-PS were characterized us- crosslinking agents using a lab-scale Brabender ing an SEM. instrument, followed by e-beam irradiation. E-beam irradiation of the mixed samples was performed in the Characterization of Polypropylene Nanocomposites EB-tech using an ELV-4 electron beam accelerator with The tensile strengths of nanocomposites prepared with an energy of 1.0 MeV. The integral irradiation dose different crosslinking agents and 5 phr of TMSPM- levels were conducted at 4 kGy. The stress-strain treated γ-Al2O3 are shown in Figure 2. The tensile properties of the prepared nanocomposite were strengths of pure PP and 4 kGy-irradiated PP were 2.74 2 determined using an Instron model 4411 testing machine and 2.78 kgf/mm , respectively. Both the 1,4-BDDA- and according to ASTM D 638. The test procedure was TMPTA-crosslinked nanocomposites displayed similar performed at a crosshead speed of 50 mm/min at room tendencies. The tensile strengths were enhanced upon temperature. The dispersion of γ-Al2O3-g-PS in the increasing the content of the crosslinking agents up to 3 nanocomposites was investigated using a scanning phr, but they gradually decreased at contents of electron microscope (SEM; XL30S FEG, Philips Co.). 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 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-Al2O3-g-PS content up to 5 phr. The nanocomposite containing 5 phr of γ-Al2O3-g-PS exhibited the highest tensile strength. The SEM micrographs of the fracture surfaces of the composites are shown in Figure 4. γ-Al2O3-g-PS formed comparatively smaller agglomerates than did the untreated γ-Al2O3 or TMSPM-treated γ-Al2O3. The reason for this phenomenon is that the polystyrene chain grafted onto the γ-Al2O3 interfered with the agglomer- ization 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-Al2O3 with TMSPM. The graft polymerization of styrene onto TMSPM-modified γ-Al2O3 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-Al2O3-g-PS and 3 phr of 1,4-BDDA showed the highest tensile strength. The homogeneous dispersion of γ-Al2O3-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-Al2O3 samples into polymer matrixes for the preparation of nanocomposites with improved mechanical properties.

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