Preparation and Characteristics of Liquid Polyorganosiloxane Rubber Containing Cadmium Selenide Nanoparticles

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1 Preparation and Characteristics of Liquid Polyorganosiloxane Rubber Containing Cadmium Selenide Nanoparticles Doo Whan Kang and Ji Young Kim Hyperstructured Organic Materials Research Center, Department of Polymer Science & Engineering, Dankook University, Seoul , Korea Received February 9, 2006; Accepted May 1, 2006 Abstract: Poly[(dimethyl-methylvinyl)siloxane] copolymer (PMViS) was prepared through the anionic polymerization of hexamethylcyclotrisiloxane (D 3 ) and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (D 4 Me,Vi )at0 o C for 24 h. Poly[(dimethyl-methylvinyl)siloxane]-phosphine oxide (PMViSPO) was prepared by adding phosphorus oxychloride (POCl 3 )topmvisdissolvedintolueneat0 o C under a nitrogen atmosphere. Cadmium selenide (CdSe) was prepared by reacting cadmium oxide (CdO), tetradecylphosphonic acid (TDPA), and trioctylphosphine oxide (TOPO) at 300 o C, after adding a solution of dispersed Se to the mixture of tributylphosphine (TBP) and trioctylphosphine (TOP). The CdSe-polyorganosiloxanephosphine oxide (CdSe-SPO) adduct was synthesized by adding PMViSPO to the CdSe preparing solution. CdSe and CdSe-SPO were characterized by UV-Vis spectroscopy and photoluminescent (PL) spectroscopy. α, ω-vinyl polydimethylsiloxane (VPMS), used as a base resin, was prepared from the equilibrium polymerization of D 3 and 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (MVS) at 90 o C for 2 h. α, ω-hydrogen polydimethylsiloxane (HPMS), as a crosslinker, was prepared from the equilibrium polymerization of D 3 and 1,1,3,3-tetramethyldisiloxane (MS) at 90 o C for 2 h. Liquid polyorganosiloxane rubber (LSR) composites containing the CdSe/CdSe-SPO adduct were fabricated from the compounding of VPMS, HPMS, and the CdSe/CdSe-SPO the adduct with a Pt catalyst. Through transmission electronic microscopy (TEM) analysis, the morphology of the LSR composites was confirmed to be that of CdSe nanoparticles with photoluminescence characteristics uniformly dispersed in the LSR. Keywords: anionic polymerization, equilibrium polymerization, liquid polyorganosiloxane rubber, cadmium selenide, polyorganosiloxanephosphine oxide Introduction 1)Silicone is the one of the most widely used polymers in adhesive, coating, and rubber materials in the electronic circuits of microelectronic devices. The silicone polymers used in preparing silicone rubber compositions include inorganic and organic portions, with each silicone chain unit having two organic groups. The properties of silicone include excellent weather and thermal stability, oxidation resistance, good electrical properties, extreme low-temperature flexibility, and curability at both elevated and ambient temperature [1,2]. We have reported nanocomposites of polyorganosiloxane, having To whom all correspondence should be addressed. ( kdoowh@dankook.ac.kr) various functional and polar groups in the backbone, with nanosilicate [3,4]. Nanostructured polymer materials that are composed of an organic polymer matrix containing inorganic particles with nanoscale dimensions exhibit unusual physical and chemical properties because of their extremely small size or large specific area [5]. Cadmium selenide nanoparticles are typically prepared from dimethyl cadmium, an extremely dangerous organometallic compound, as the cadmium precursor in a coordinating solvent composed of trioctylphosphine oxide (TOPO) and hexylphosphonic acid [6]. Recently, Peng [7] prepared cadmium selenide nanoparticles more easily by employing CaO, which is safe, inexpensive, and exhibits minimum toxicity. These methods provide welldefined nanoparticles containing aliphatic phosphines, phosphine oxides, and phosphonic acids bound to the

2 Preparation and Characteristics of Liquid Polyorganosiloxane Rubber Containing Cadmium Selenide Nanoparticles 547 surfaces of the cadmium selenide. Skaff and coworkers [8] prepared cadmium selenide- polyolefin composites from functional phosphine oxides. Research into organicinorganic nanocomposite materials continues to receive much attention. The methods used in the preparation of nanocomposite materials involve the compounding of metallic nanoparticles with processable matrices, such as polymers and ceramics. The metallic nanoparticles are usually synthesized first and then compounded with the polymer. Using this approach the metallic nanoparticles are not readily dispersed homogeneously in the polymer matrices. In this study, we attempted to prepare liquid polyorganosiloxane rubber (LSR) composites having luminescence characteristics by homogeneously dispersing CdSe nanoparticles in the matrix. Poly[(dimethyl- methylvinyl)siloxane] copolymer (PMViS) was prepared from the ionic polymerization of D 3 and D Me,Vi 4 using BuLi as the initiator. Poly [(dimethyl-methylvinyl) siloxane] phosphine oxide (PMViSPO) was prepared by adding PMViS to a POCl 3 solution. α, ω-vinyl polydimethylsiloxane (VPMS) and α, ω-hydrogen polydimethylsiloxane (HP- MS) prepolymers were prepared through equilibrium polymerization. Cadmium selenide nanoparticles having luminescence characteristics were prepared from CdO, tetradecylphosphonic acid (TDPA), and TOPO. Liquid polyorganosiloxane rubber composites containing homogeneously dispersed cadmium selenide nanoparticles were prepared; the luminescence characteristics and mechanical properties were measured by UTM. Experimental Materials Hexamethylcyltrisiloxane (D 3 ), 1,3,5,7-tetravinyl- 1,3,5, 7-tetramethylcyclotetrasiloxane (D Me,Vi 4 ), 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (MVS), 1,1,3,3-tetramethyldisiloxane (MS), POCl 3, cadmium oxide (CdO), tetradecylphosphonic acid (TDPA), trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), tributylphosphine (TBP), and Pt catalyst were obtained from Aldrich, Alfa, and Bayer Co. Tetramethylammonium siloxanolate (TMAS) was prepared by reacting D 3 with tetramethylammonium hydroxide. Nanosilica (A-200, Degussa Co.) having particle diameters of nm and a surface area of 200 m 2 /g was used. THF and other solvents were purified under vacuum after removing moisture with molecular sieves. Preparation of PMViS PMViS was prepared through ionic polymerization according to a reported method [9]. For example, in a four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer, and nitrogen inlet tube, D 3,D 4 Me,Vi and n-buli (10:10:1 mole ratio) were reacted for 24 h at 0 o C. After the reaction was complete, unreacted monomer was removed by vacuum stripping at 30 torr; a viscous pale-yellowish liquid was obtained (yield: 63 %). Preparation of PMViSPO PMViSPO was prepared by reacting a solution of POCl 3 and PMViS (3:1 mole ratio) in toluene for 5 h at 0 o C. After completing the reaction, the precipitated LiCl was removed by filtration; a pale-yellowish viscous liquid was obtained after removing the toluene by vacuum stripping at 30 torr (yield: 62 %). Preparation of CdSe Nanoparticles and CdSe-Polyorganosiloxanephosphine Oxide (CdSe-SPO) Adduct CdSe nanoparticles were prepared by heating CdO (1.6 mmol), TDPA (3.2 mmol), and TOPO (7.5 mmol) at 300 o C under argon flow. After it had been heated up to 320 o C, Se-TBP (25 % Se in mass, 0.8 mmol of Se) was mixed with TOP (3.9 mmol) and toluene (3 ml) to obtain the injection solution. After injecting this solution into the reaction flask, the temperature of the reaction mixture dropped to 250 o C and was maintained for 1 h at that temperature. Toluene and methanol were added into the reaction flask to precipitate the crystals at 50 o C. The crystals were separated by centrifugation and dried in vacuum. CdSe-SPO was prepared using a similar method as that for the preparation of CdSe, except that PMVi- SPO (3.5 mmol) was added to the reactants and that the unreacted monomers in product were removed by vacuum stripping at 30 torr. Preparation of α, ω-vinyl Polydimethylsiloxane (VP- MS) VPMS was prepared through equilibrium polymerization according to a reported method [10]. For example, in a four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer, and nitrogen inlet tube, D 3 and MVS (14:1 mole ratio) were reacted for 4 h at 90 o C in the presence of 0.02 wt% TMAS under a nitrogen atmosphere. After completing the reaction, the mixture was heated to 140 o C to decompose TMAS; the sample was then vacuum-stripped at 30 torr to remove oligomeric cyclic species and unreacted monomer to obtain the viscous VPMS (yield: 63 %). Preparation of α, ω-hydrogen Polydimethylsiloxane (HPMS) HPMS was prepared using a method similar to that for the preparation of VPMS. D 3 and MS (5:1 mole ratio) were reacted at 90 o C for 4 h in the presence of 0.02 wt% TMAS under a nitrogen atmosphere. After completing the reaction, the mixture was heated to 140 o C to

3 548 Doo Whan Kang and Ji Young Kim Table 1. Formulation of Liquid Polyorganosiloxane Rubber Composite Preparation (Units : phr) Materials LSR LSRC-1 LSRC-2 VPMS Silica CdSe CdSe-SPO Plasticizer (OH-oil) Cross-linker (HPMS) Pt (ppm) Time (min) Table 2. Molecular Weight of PMViS According to the Mole Ratio of Reactants Me,Vi D 3 : n-buli : D 4 (mole ratio) Mn Mw Polydispesity 2 : 1 : : 1 : : 1 : : 1 : Scheme 1. Synthesis of PMViS and PMViSPO. decompose TMAS, and then vacuum-stripped at 30 torr to remove oligomeric cyclic species and unreacted monomer (yield : 64 %). Preparation of Liquid Polyorganosiloxane Rubber (LSR) Composites The LSR composites were prepared from the compounding of VPMS, HPMS, catalyst, silica, and CdSe/ CdSe-SPO, according to the composition ratios shown in Table 1, using a high-speed dissolver (Linden Co. Model : K110) at 130 o C for 3 h under rmp. LSR composites containing CdSe of nanoscale dimensions were prepared by dispersing CdSe/CdSe-SPO adduct into organosiloxane matrix, respectively. Characterization 1 H-NMR spectra were obtained using a Varian EM- 360A NMR spectrometer. Tetramethylsilane (TMS) was used as an internal reference. FTIR spectra were obtained using a using Perkin-Elmer Spectra GX FTIR spectrometer. UV spectra were obtained using a Perkin-Elmer Lambda-20 UV spectrometer. Molecular weights were determined through gel permeation chromatography (Waters Breeze GPC System Model 1515) using tetrahydrofuran solutions at room temperature. Calibration was performed using polystyrene. Mechanical properties, tensile strength, hardness, and elongation were measured at room temperature using an Instron Universal Testing Machine (Model 4411) according to ASTM D412 and Scheme 2. Synthesis of CdSe-SPO TEM images were obtained using a JEOL JEM- 2000EXII microscope. Photoluminescence (PL) was measured using a Shimadzu RF-5310C spectrofluorophotometer. The morphology was examined using a JEOL JSM-6700F FE-SEM operated at an accelerating voltage, above 5.0 kv and a magnification of Results and Discussion Preparation of PMViS and PMViSPO Butyl and siloxane anion end-blocked polyorgan-

4 Preparation and Characteristics of Liquid Polyorganosiloxane Rubber Containing Cadmium Selenide Nanoparticles 549 Figure 1. FT-IR spectrum of PMViSPO. Figure 3. UV-Vis spectra of (a) CdSe and (b) CdSe-SPO. Figure 2. 1 H-NMR spectrum of PMViSPO. osiloxane copolymers containing the block units of dimethylsiloxane and methylvinylsiloxane were prepared. The molecular weight of the copolymer was controlled by the mole ratio of the reactants, D 3,D Me,Vi 4, and n-buli. The molecular weight increased upon decreasing the mole ratio of n-buli to D 3. The molecular weights obtained at various mole ratios of reactants are shown in Table 2. PMViS with the weight-average molecular weight of was reacted with POCl 3 to obtain PMViSPO having the Mw of The reaction sequence used for the preparation of PMViS and PMViSPO is shown in Scheme 1. The structures of PMViS and PMViSPO were confirmed by their FT-IR and 1 H-NMR spectra. The FT-IR spectra for PMViS and PMViSPO exhibited strong absorption peaks at 1100 to 1020 cm -1, due to Si-O-Si stretching vibrations, and at 1343, 1600, and 1950 cm -1, due to Si-vinyl vibrations. The FT-IR spectrum for PMViSPO (Figure 1) lacks the absorption peak due to P-Cl (at 590 cm -1 ) but displays one at 1260 cm -1 duetop O.The 1 H-NMR spectrum for PMViSPO (Figure 2) exhibits absorption peaks at Figure 4. PL spectra of (a) CdSe and (b) CdSe-SPO. Figure 5. FE-SEM image of CdSe ppm that are due to the Si-CH 3 protons and at 6 ppm that are due to Si-vinyl protons. Preparation of CdSe, CdSe-SPO Adduct Cadmium selenide nanoparticles having luminescence properties were synthesized from CdO, TDPA, TOPO,

5 550 Doo Whan Kang and Ji Young Kim Table 3. Mechanical Properties of LSR Nanocomposites Properties LSR LSRC-1 LSRC-2 Units Hardness Tensile Strength Elongation Figure 6. FE-SEM image of CdSe-SPO Figure 7. TEM image of LSR nanocomposite. Shore A kgf/cm 2 and selenide by referring to Peng s method [7]. Reactants were taken at given time intervals and dissolved into hexane for monitoring by UV-Visible spectrophotometry. Toluene and methanol were added into the reaction flask to precipitate the nanocrystals at 50 o C. The CdSe- SPO adduct was prepared by referring to Skaff s method [8] for introducing a polyolefin with vinyl groups to CdSe. The reaction used for the preparation of the CdSe-SPO adduct is shown in Scheme 2. UV spectra for the CdSe nanoparticles in the CdSe-SPO adducts are % shown in Figure 3, and photospectra measured using photoluminescence (PL) are presented in Figure 4. As shown in Figure 3, the strong absorption peaks due to CdSe (at 560 nm) shifted for 254 nm for CdSe-SPO, presumably as a result of the formation of a siloxane ligand to P = O. From these results, it appears that CdSe-SPO adducts were formed by the interaction of phosphine oxide with siloxane ligands on the surfaces of the CdSe nanoparticles. In Figure 4, the absorption peak at 565 nm, due to CdSe, is shifted to 535 nm in the CdSe-SPO adduct for the same reasons as the change in the UV spectra. FE-SEM images of CdSe and CdSe-SPO (Figures 5 and 6) were obtained to determine the sizes of the nanoparticles. The average size of the CdSe nanoparticles in CdSe-SPO was nm; they were distributed homogeneously in the siloxane polymer matrix. 1 H-NMR and FT-IR spectra confirmed the phosphine oxide to siloxane with vinyl groups attachement; the 31 P NMR spectrum exhibits the absorption peak of trialkylphosphine oxide at 46.3 ppm. Preparation of LSR Composites Containing CdSe and CdSe-SPO The CdSe nanoparticles modified with PMViSPO was treated with VPMS, HPMS (crosslinker), silicone oil (OH-oil) as plasticizer, silica, and Pt catalyst using the high-speed dissolver at 130 o C, for 3 h, under rpm. CdSe (0.15 phr) and CdSe-SPO adduct (0.15 phr to 100 phr of base resin of VPMS) were added, respectively. An uncured LSR composite was prepared by compounding for 3 h at 130 o C, according to the formulation shown in Table 1, using a high-speed dissolver. The LSR composites containing CdSe and the CdSe-SPO adduct are named LSRC-1 and LSRC-2, respectively. The TEM image of the surface of LSRC-2 is shown in Figure 7. The CdSe nanoparticles are uniformly distributed in the LSR; the particle sizes were nm. LSRC-2 exhibited an orange color but LSR was semitransparent, when UV light was applied to the composites. The color was consistent with the size of CdSe nanoparticles. Mechanical Properties The mechanical properties of LSRC-1 and -2 are presented in Table 3. The tensile strength, hardness, and elongation for LSRC-1 were 24 Kgf/cm 2, 14 Shore A, and 350 %, respectively; for LSRC-2 they were 25 Kgf/cm 2, 12 Shore A, and 370 %, respectively. The tensile strength and elongation for LSRC-2 were higher than those of LSRC-1 [11].

6 Preparation and Characteristics of Liquid Polyorganosiloxane Rubber Containing Cadmium Selenide Nanoparticles 551 Conclusion PMViS was prepared through the ionic polymerization of D 3 and D Mt,Vi 4 using n-buli as initiator at 0 o C for 24 h. PMViSPO was prepared by the reaction of phosphorus oxychloride and PMViS. VPMS presenting vinyl groups was prepared through the equilibrium polymerization of D 3 and MVS. HPMS was prepared through the equilibrium polymerization of D 3 with MS. Cadmium selenide nanoparticles were synthesized having diameters of nm. CdSe/liquid polyorganosiloxane rubber nanocomposites were fabricated with cadmium selenide dispersed well within the organosiloxane matrix; they exhibited photoluminescence properties. The tensile properties of LSRC-2 were higher than those of LSRC-1. Acknowledgments This study was conducted using the research fund of Dankook University in References 2. H. K. Chio and T. Yoon, Prospective Ind. Chem., 2, 41 (1999). 3. D.W.Kang,K.S.Lee,H.G.Yeo,andJ.S.Shim,J. Korean Ind. Eng. Chem., 13, 594 (2002). 4. D. W. Kang and K. S. Lee, Polymer(Korea), 29, 156 (2005). 5. T. J. Moon, C. G. Kang, and J. K. Jeong, Polymer(Korea), 10, 627 (1986). 6. X. G. Peng. L. Manna, W. D. Yang, J. Wickham, E. Scher, and A. P. Alivisatos, Nature, 404, 59 (2000). 7. Z. A. Peng and X. Peng. J. Am. Chem. Soc., 123, 183 (2001). 8. H. Skaff, M. F. Ilker, E. B. Coughlin, and T. Emrick. J. Am. Chem. Soc., 124, 5729 (2002). 9. G. Gordon and M. S. Chisholm, Polymer, 26, 437 (1985). 10. J. E. McGrath, J. S. Riffle, L. Yilgor, and A. K. Banthia, ACS Symp. Ser., No. 211, Ch., 2 (1983). 11. a)d.w.kangandh.g.yeo,j. Ind. Eng. Chem., 11, 567 (2005); b) H. B. Kim, C. H. Lee, J. S. Choi, B. J. Park, S. T. Lim, and H. J. Choi, J. Ind. Eng. Chem., 11, 769 (2005). 1. J. Joo and C. Y. Lee, J. Appl. Phys., 88, 513 (2000).