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CdS–SiO2 nanocomposites via ultrasonic process
Applied Surface Science 255 (2008) 2244–2250
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of polyurethane/CdS–SiO2 nanocomposites via ultrasonic process Jing Chen a, Yu-Ming Zhou a,*, Qiu-Li Nan a, Xiao-Yun Ye a, Yan-Qing Sun a, Zhi-Qiang Wang a, Shi-Ming Zhang b a
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2008 Received in revised form 7 July 2008 Accepted 7 July 2008 Available online 23 July 2008
In this study, the high-intensity ultrasound was applied in the preparation of chiral polyurethane/CdS– SiO2 nanocomposites. The polyurethane/CdS–SiO2 nanocomposites were analyzed by powder X-ray diffraction, thermogravimetric analysis (TGA), TEM and SEM. The results indicated that the heat stability of the nanocomposites was improved in the presence of CdS–SiO2 core-shell nanoparticles. The infrared emissivity (8–14 mm) study revealed that the nanocomposites possessed much lower infrared values compared with those of the neat polymers and nanoparticles, respectively. A possible mechanism of ultrasonic induced composite reaction was proposed based on the experimental results. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Polyurethane Binaphthyl Nanocomposites Infrared emissivity
1. Introduction High performance chiral polymers have attracted a great deal of interest in the applications of fields such as dentistry, drug delivery and tissue engineering [1,2] due to their promising properties. In recent years, many attentions have been paid for the usages of the optically active 1,10 -bi-2-naphthol (BINOL) such as a chiral building block [3,4], chiral catalyst [5–7] or chiral auxiliary [8– 10] in asymmetric synthesis, molecular recognition and enantioselective chromatographic separation [11–13]. A series of chiral polybinaphthalene polymers have also been prepared enantioselectively starting from BINOL because of its axial chirality and configutational stability [14–19]. As an important class of optico-active polymers [20–23], polyurethane has received an increasing amount of attention owing to their chemical and mechanical properties [24–26]. Nanocrystalline cadmium sulfide (CdS) has unique physical and chemical properties and can be used in electronic, optical, and magnetic aspects. Most applications are related to the CdS crystalline [27–29], however, it is labile and easy to decompose and therefore to be stabilized by fabricating the nanocrystallines
* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617. E-mail address: [email protected] (Y.-M. Zhou). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.089
with other materials such as SiO2. It is known that some core-shell structures such as Ag/SiO2, Au/SiO2 etc. have various potential applications [30]. Moreover, to allow CdS nanocrystals embedded in a polymer matrix, researches have been focused on the preparation of CdS-polymer nanocomposites. Chen et al. [31] produced transparent nanocrystal-polymer hybrids by grafting polyurethane onto functionalized CdS nanocrystals. Celebi et al. [32] reported a detailed study on poly(acrylic acid) stabilized CdS quantum dots and discussed the influence of reaction pH on the size and optical properties. Similar to other common polymers, the synthesis of PU nanocomposites has been studied through multifarious methods [20,33,34]. However, only a few papers reported the synthesis and properties of PU nanocomposites under ultrasonic irradiation. Ultrasonic irradiation has been widely employed in cleaning, jointing, machining, medicine, chemistry and preparing nanocomposites [35–39]. As known, this method can control the size distribution and morphology of the nanosized particles [40]. Kwang-Pill Lee reported the preparation of polydiphenylamine/ silica-nanoparticle composites under ultrasonication and extended this methodology to making conducting, processable nanocomposites with other types of conducting polymers [41]. However, as far as we knew, there is no report addressed the incorporation of core-shell nanostructures into polyurethanes. Hence, we sought to examine whether this incorporation under
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ultrasonic irradiation is feasible and, if so, how the structure, morphology, and properties of the nanocomposites are influenced upon the presence of nanoparticles. In this paper, optically active R-BPU/CdS–SiO2 and S-BPU/CdS– SiO2 nanocomposites were synthesized under ultrasonic irradiation. Although some nanoscale oxide is frequently used in low emissivity coating [42], PU/nanoparticles utilized in the study of infrared emissivity, to the best of our knowledge, has not yet been reported. 2. Experimental 2.1. Materials S (99.999%) and Cd powder (99.999%) were products of Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Isopropanol and ammonia (25%) were purchased from Shanghai Chemical Reagent Company. Tetraethylorthosilicate (TEOS), ethanediamine and toluene 2,4-diisocyanate (TDI) were purchased from Lingfeng Chemical Reagent Co., Ltd. (Nanjing, China); The coupling agent, g-amidopropyl-triethoxyl silicane (KH550), was obtained from Yaohua Chemical Plant(Shanghai, China); N,Ndimethylacetamide (DMAC), which was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), was redistilled under reduced pressure and used freshly. Chiral (R) and (S)1,10 -binaphthyl were synthesized according to the literature [43]. All reagents were commercial products of A.R. grade and used as received if not specified. 2.2. Synthesis of CdS and CdS–SiO2 nanoparticles CdS nanorods were synthesized by the solvothermal method [30]. S and Cd powders were mixed in appropriate proportions and put into a Teflon-lined stainless steel autoclave with a capacity of 100 mL. Then the autoclave was filled with ethylenediamine up to about 70% of the total volume. The autoclave was kept at 120– 190 8C for 3–6 h and then cooled to room temperature. The yellow precipitate was collected and washed with absolute ethanol and distilled water to remove residue of organic solvents. Finally, the product was desiccated in vacuum at 60 8C for 2 h. Core-shell structures of CdS–SiO2 were synthesized as follows: Pre-synthesized CdS nanorods (0.10 g) were mixed with isopropanol (200.0 mL), followed by high-intensity ultrasonic irradiation for 20 min. Then the mixture was put into a round-bottom flask with distilled water (18.0 mL) and stronger ammonia water (10.5 mL), and TEOS were quickly added in appropriate proportions, stirred at room temperature for 24 h. The product was filtered and washed with distilled water and absolute ethanol, then was dried at 70 8C for 4 h.
Fig. 1. XRD powder patterns of (a) CdS nanorods prepared with a S/Cd molar ratio of 1:1, (b–e) CdS/SiO2 core-shell nanocrystals with addition of TEOS: (b) 0.1 cm3; (c) 0.3 cm3; (d) 0.6 cm3; (e) 2.0 cm3.
Preparation of BPU/CdS–SiO2 nanocomposites was carried out in an ultrasonic irradiation process. The ultrasound source was a JY 92-2D ultrasonic cell crusher (2 104–109 Hz, 700 W, Scientz Biotechnology Co., Ltd., Ningbo, China) with the probe of the ultrasonic horn immersed directly in the mixture solution system. Nanoparticles were firstly treated with the KH550 silane coupling agent. A series of comparison experiments were done and optimal results were obtained when a weight percentage of KH550 to the nanoparticles was 10%. Different amounts of CdS– SiO2 nanoparticles (5, 10, 15, 20 wt.%) were mixed with the BPU and the mixture was dispersed in 20 mL absolute ethanol, followed by irradiation with high-intensity ultrasonic wave for 4 h at 30 8C. After irradiation, the resulted suspension was cooled to room temperature and then centrifuged, and the precipitate was washed twice with absolute ethanol and distilled water, respectively. The solid was dried in vacuum at room temperature for 6 h. The obtained product was kept for further characterization.
2.3. Synthesis of BPU/CdS–SiO2 nanocomposites The synthesis of S-BPU and R-BPU was reported in our previous work [44]. The R-BPU is the polymer synthesized from R-1,10 binaphthyl monomer and S-BPU is the polymer synthesized from S-1,10 -binaphthyl monomer. Here are the brief procedures. First, TDI (0.9 mL) was dissolved in 10 mL DMAc in a 250 mL flask with a magnetic stirrer at 80 8C under N2. Then (R)- or (S)-binaphthol (6 mmol) was added at the refuxing temperature for 16 h. A little NaOH was added to neutralize unreacted BINOL for 0.5 h. The mixture was cooled down to room temperature, poured to 150 mL anhydrous ethanol and precipitate out white flocculate. After neutralized with 10% aqueous HCl (150 mL), the product filtered, washed with anhydrous ethanol several times, dried at 80 8C. White polymer powders were obtained.
Fig. 2. XRD powder patterns of (a) R-BPU (b) S-BPU (c) CdS/SiO2 (d) R-BPU/CdS–SiO2 (15 wt.%) and (e) S-BPU/CdS–SiO2 (15 wt.%) nanocomposites.
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2.4. Characterization The surface morphology of the samples was monitored with scanning electron microscope (SEM) LEO-1530vp. X-ray diffraction measurements of polymer and nanocomposites were recorded using a Rigaku D/MAX-R with a copper target at 40 kV and 30 mA, in the range 5–808at the speed of 58/min. Thermal analysis experiments were performed using a thermogravimetric analysis (TGA) apparatus operated in the conventional TGA mode (TA Q600, TA Instrument) at the heating rate of 20 8C/min to simultaneously determine the correlation of temperature and weight loss in a nitrogen atmosphere. Infrared emissivity values of the samples were carried out on an IRE-I Infrared Emissometer of Shanghai Institute of Technology and Physics, China. 3. Results and discussion 3.1. X-ray diffraction data Fig. 1 showed the powder XRD patterns of (a) CdS nanorods and (b–e) CdS–SiO2. It is observed that the sample (a) was a pure
Fig. 3. TGA curves of R-BPU and R-BPU/CdS–SiO2 nanocomposites.
Fig. 4. TEM images of (a–b) CdS nanoparticles and (c–d) CdS–SiO2 core-shell nanocrystals with addition of TEOS: (c) 0.3 cm3; (d) 2.0 cm3.
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Fig. 5. SEM images of (a–d) CdS/SiO2 core-shell nanocrystals with addition of TEOS: (a–b) 0.3 cm3; (c–d) 2.0 cm3.
hexagonal CdS phase (wurtzite structure). There was a strong (0 0 2) peak, which indicated a preferential orientation of (0 0 1) in the CdS crystal. From the patterns of sample (b–e), no peak of SiO2 was detected, suggesting that SiO2 was amorphous which wraps the CdS nanorods. And the peak of CdS–SiO2 intensified with the concentration of the TEOS used, indicating that the morphology of CdS changed through the ultrasonic process. Fig. 2 was the XRD patterns of BPU, CdS–SiO2 and BPU/CdS–SiO2 nanocomposites. (R)- and (S)-BPU (a and b) were the amorphous polymers and did not exhibit any anisotropic behaviors. This may be due to the presence of the naphthyl ring and aromatic structures in the main chain, thus limiting the molecular mobility of the polymer. When adding CdS–SiO2 particles to BPU, as shown in Fig. 2, it appeared the characteristic peaks of CdS–SiO2. It also could be seen that two curves of the nanocomposites (d and e) were almost in the same position, indicating that the morphology of CdS–SiO2 particles had not been changed during the process. However, all the diffraction peaks were broadened. The average crystalline size of CdS–SiO2, determined by the Debye–Scherrer equation, was approximately 30 nm. Furthermore, ultrasonic irradiation reduced the crystallite size of CdS–SiO2 due to the generation of many localized hot spots in the solution, which further gave rise to the homogeneous formation of a large number of seed nuclei, leading to a smaller particle size [45–47].
3.2. Thermal properties Fig. 3 showed the thermogravimetric curves of the R-BPU and RBPU/CdS–SiO2 nanocomposites with the different core-shell nanoparticle contents. The thermal property of R-BPU/CdS–SiO2 nanocomposites was investigated because the thermal stability of R-BPU was better than S-BPU [48]. It was clear that the sample exhibited a very good thermal stability below 110 8C. It began to decompose around 120–130 8C in N2 gas. The 5% weight loss occurred in about 150 8C, which was attributed to the loss of residual water and organic solvent. Another weight loss started at around 250 8C, corresponded to the polymer degradation. Concerning the second peak, it was much higher than that of the common PU [49]. This increased in the thermal stability may result from the presence of hard naphthyl group unit [50]. It was evident that the R-BPU had a high decomposition temperature of about 250 8C. This thermal stability was further improved when CdS–SiO2 was introduced. It seemed that the initial temperature of weight loss did not increase with increasing CdS–SiO2 content. However, at a weight loss of 40%, CdS–SiO2 tended to increase the thermal resistance of the nanocomposites. This increase of the thermal stability may result from the high thermal stability of CdS–SiO2 nanoparticles, which limited the movement of the molecular chain of R-BPU.
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Fig. 6. SEM images of (a–d) R-BPU/CdS–SiO2 nanocomposites with the content of CdS–SiO2 5, 10, 15 and 20 wt.%, respectively.
3.3. Morphology In order to examine the microstructures and nanoparticles distribution within the nanocomposites, TEM and SEM analysis were
conducted. Fig. 4 showed the TEM micrographs of the CdS and CdS– SiO2 nanoparticles. According to the XRD results (Fig. 4), we inferred that the CdS crystal growth was oriented and morphologies are 1D rodlike. This inference was confirmed by TEM images. As shown in Fig. 4a, the average crystalline size of CdS was about 15–20 nm. Fig. 4c and d showed the CdS–SiO2 nanoparticles, it was apparent that CdS nanorods and SiO2 form the core-shell structure, and the average size of CdS–SiO2 was 50–60 and 70–80 nm, respectively. Fig. 5 showed the SEM micrographs of the CdS–SiO2 nanoparticles. It was observed that the thickness of SiO2 shell increased with the concentration of the TEOS used. As expected, in Fig. 6a–c, the nanocomposites presented a homogeneous microstructure. However, it should be noted that, as far as the content of CdS–SiO2 was 25 wt.%, Fig. 6d showed that the nanocomposites presented some irregularities in size and shape, which could be explained by the ultrasonic process. Ultrasonic cavitation can generate an environment of local temperature up to 5000 K and local pressure up to 500 atm, under such conditions the growth of particles had some difference. 3.4. Infrared emissivity
Fig. 7. Infrared emissivity values of (a) R-BPU/CdS–SiO2 (15 wt.%) emin = 0.50, (b) SBPU/CdS–SiO2 (15 wt.%) emin = 0.38.
IR emissivity testing results indicated that the R-PU, S-BPU and CdS–SiO2 possessed high emissivity of 0.90, 0.85, and 0.88, respectively. Polyurethane has a high infrared emissivity due to its strong absorbability at infrared wave band, commonly, an
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Fig. 8. Mechanism illustration of the ultrasonic induced reaction in the polyurethane formed on the nanoparticles.
increase in the absorbability results in an increase in the emissivity [51,52]. CdS–SiO2 has a high infrared emissivity value due to their high surface area. The IR emissivity of nanocomposites (0.38–0.70) was much lower than that of BPU or nanoparticles, which could be contributed to interfacial synergism forces such as hydrogen bonds or electrostatic interactions between the organic and inorganic components [53]. These interactions can alter the vibration mode of molecules, atoms or pendant groups on interface between organic and inorganic components [54], thus the composites possessed lower emissivity than that of two components. Fig. 7 showed the IR emissivity of BPU/CdS–SiO2 nanocomposites with different CdS– SiO2 nanoparticle content, as could be seen, with the increasing the ratio of CdS–SiO2 and BPU monomer, the IR emissivity increased initiatively and then decreased, the lowest value was 0.38 (S-BPU/ CdS–SiO2 (15 wt.%)) and 0.50 (R-BPU/ CdS–SiO2 (15 wt.%)), respectively. It can be indicated the nanocomposites with appropriate quantity CdS–SiO2 nanoparticles possessed higher surface area, surface area energy, quantity of surface atom and dangling bonds, thus possessed stronger surface effect, polarity effect, multiply dissociated radiation and lower IR emissivity. And these results were also in good agreement with the previous SEM analysis. On the other hand, the S-BPU/CdS–SiO2 had lower infrared emissivity values than those of R-BPU/CdS–SiO2, which could be contributed to the different optical properties between S-BPU and R-BPU. 3.5. Mechanism Based on the experimental results mentioned above, we proposed the mechanism of ultrasound induced composite reaction as follows: The process can be depicted as Fig. 8. Under ultrasonic, the coupling agent KH550 hydrolyzes to form hydroxyls, and these groups react with the hydroxyls of CdS–SiO2 surface through hydrogen bonding. Generally, the main effects of sonication are due to cavitation or the growth and explosive collapse of microscopic bubbles on a microsecond timescale [55]. At the same time, ultrasonic cavitation can generate a rigorous environment of local temperature up to 5000 K and local pressure up to 500 atm [50]. Under such conditions the modified nanoparticles might be dispersed absolutely and will combine with polyurethane via the amino of coupling agent. As a result, the dispersity of
nanoparticles increase with the decrease of surface energy of nanoparticles, and the polymer on the nanoparticle surface has a steric dispersing and stabilizing effect, which can be observed from the photographs of TEM and SEM. Meanwhile, due to the hydrogen bond effect the thermal property of nanocomposite is much higher than pure polymer, as seen from the TGA analysis. On account of the strong interactions occur between the –OH and –NH2 group of BPU and nanoparticles, it would lead the special surface effect of incidence wave and nanocomposite surface and change the transfer modes of dangling bonds lay in the surface of nanocomposite, which make the absorption bands existing in 8–14 mm are dissociated and weakened, thus the whole material possesses lower emissivity than those of two components. 4. Conclusion This work demonstrated a simple and effective route to synthesize optico-active polyurethane. Meanwhile a sonochemical method has been used for the preparation of BPU/CdS–SiO2 nanocomposites. TGA studies indicated that thermal stability of the nanocomposites had improved with increasing nanoparticles contents. Infrared emissivity study showed that the nanocomposites possessed lower emissivity value than those of BPU and nanoparticles, respectively. Finally, a possible mechanism of ultrasonic induced composite reaction was proposed. The ultrasonic method employed here may be a simple and inexpensive route to synthesize other polymer nanocomposites, which can be extended to prepare a novel low infrared emissivity material. Acknowledgements The authors are grateful to ‘‘the New Century Talents Program’’ of Ministry of Education of China (NCET-04-0482), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (06-A-033) and the National Nature Science Foundation of China (50377005) for financial supports. References [1] L. Feng, J.W. Hu, Z.L. Liu, F.B. Zhao, G.J. Liu, Polymer 48 (2007) 3616–3623.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of polyurethane/CdS–SiO2 nanocomposites via ultrasonic process Jing Chen a, Yu-Ming Zhou a,*, Qiu-Li Nan a, Xiao-Yun Ye a, Yan-Qing Sun a, Zhi-Qiang Wang a, Shi-Ming Zhang b a
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2008 Received in revised form 7 July 2008 Accepted 7 July 2008 Available online 23 July 2008
In this study, the high-intensity ultrasound was applied in the preparation of chiral polyurethane/CdS– SiO2 nanocomposites. The polyurethane/CdS–SiO2 nanocomposites were analyzed by powder X-ray diffraction, thermogravimetric analysis (TGA), TEM and SEM. The results indicated that the heat stability of the nanocomposites was improved in the presence of CdS–SiO2 core-shell nanoparticles. The infrared emissivity (8–14 mm) study revealed that the nanocomposites possessed much lower infrared values compared with those of the neat polymers and nanoparticles, respectively. A possible mechanism of ultrasonic induced composite reaction was proposed based on the experimental results. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Polyurethane Binaphthyl Nanocomposites Infrared emissivity
1. Introduction High performance chiral polymers have attracted a great deal of interest in the applications of fields such as dentistry, drug delivery and tissue engineering [1,2] due to their promising properties. In recent years, many attentions have been paid for the usages of the optically active 1,10 -bi-2-naphthol (BINOL) such as a chiral building block [3,4], chiral catalyst [5–7] or chiral auxiliary [8– 10] in asymmetric synthesis, molecular recognition and enantioselective chromatographic separation [11–13]. A series of chiral polybinaphthalene polymers have also been prepared enantioselectively starting from BINOL because of its axial chirality and configutational stability [14–19]. As an important class of optico-active polymers [20–23], polyurethane has received an increasing amount of attention owing to their chemical and mechanical properties [24–26]. Nanocrystalline cadmium sulfide (CdS) has unique physical and chemical properties and can be used in electronic, optical, and magnetic aspects. Most applications are related to the CdS crystalline [27–29], however, it is labile and easy to decompose and therefore to be stabilized by fabricating the nanocrystallines
* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617. E-mail address: [email protected] (Y.-M. Zhou). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.089
with other materials such as SiO2. It is known that some core-shell structures such as Ag/SiO2, Au/SiO2 etc. have various potential applications [30]. Moreover, to allow CdS nanocrystals embedded in a polymer matrix, researches have been focused on the preparation of CdS-polymer nanocomposites. Chen et al. [31] produced transparent nanocrystal-polymer hybrids by grafting polyurethane onto functionalized CdS nanocrystals. Celebi et al. [32] reported a detailed study on poly(acrylic acid) stabilized CdS quantum dots and discussed the influence of reaction pH on the size and optical properties. Similar to other common polymers, the synthesis of PU nanocomposites has been studied through multifarious methods [20,33,34]. However, only a few papers reported the synthesis and properties of PU nanocomposites under ultrasonic irradiation. Ultrasonic irradiation has been widely employed in cleaning, jointing, machining, medicine, chemistry and preparing nanocomposites [35–39]. As known, this method can control the size distribution and morphology of the nanosized particles [40]. Kwang-Pill Lee reported the preparation of polydiphenylamine/ silica-nanoparticle composites under ultrasonication and extended this methodology to making conducting, processable nanocomposites with other types of conducting polymers [41]. However, as far as we knew, there is no report addressed the incorporation of core-shell nanostructures into polyurethanes. Hence, we sought to examine whether this incorporation under
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ultrasonic irradiation is feasible and, if so, how the structure, morphology, and properties of the nanocomposites are influenced upon the presence of nanoparticles. In this paper, optically active R-BPU/CdS–SiO2 and S-BPU/CdS– SiO2 nanocomposites were synthesized under ultrasonic irradiation. Although some nanoscale oxide is frequently used in low emissivity coating [42], PU/nanoparticles utilized in the study of infrared emissivity, to the best of our knowledge, has not yet been reported. 2. Experimental 2.1. Materials S (99.999%) and Cd powder (99.999%) were products of Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Isopropanol and ammonia (25%) were purchased from Shanghai Chemical Reagent Company. Tetraethylorthosilicate (TEOS), ethanediamine and toluene 2,4-diisocyanate (TDI) were purchased from Lingfeng Chemical Reagent Co., Ltd. (Nanjing, China); The coupling agent, g-amidopropyl-triethoxyl silicane (KH550), was obtained from Yaohua Chemical Plant(Shanghai, China); N,Ndimethylacetamide (DMAC), which was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), was redistilled under reduced pressure and used freshly. Chiral (R) and (S)1,10 -binaphthyl were synthesized according to the literature [43]. All reagents were commercial products of A.R. grade and used as received if not specified. 2.2. Synthesis of CdS and CdS–SiO2 nanoparticles CdS nanorods were synthesized by the solvothermal method [30]. S and Cd powders were mixed in appropriate proportions and put into a Teflon-lined stainless steel autoclave with a capacity of 100 mL. Then the autoclave was filled with ethylenediamine up to about 70% of the total volume. The autoclave was kept at 120– 190 8C for 3–6 h and then cooled to room temperature. The yellow precipitate was collected and washed with absolute ethanol and distilled water to remove residue of organic solvents. Finally, the product was desiccated in vacuum at 60 8C for 2 h. Core-shell structures of CdS–SiO2 were synthesized as follows: Pre-synthesized CdS nanorods (0.10 g) were mixed with isopropanol (200.0 mL), followed by high-intensity ultrasonic irradiation for 20 min. Then the mixture was put into a round-bottom flask with distilled water (18.0 mL) and stronger ammonia water (10.5 mL), and TEOS were quickly added in appropriate proportions, stirred at room temperature for 24 h. The product was filtered and washed with distilled water and absolute ethanol, then was dried at 70 8C for 4 h.
Fig. 1. XRD powder patterns of (a) CdS nanorods prepared with a S/Cd molar ratio of 1:1, (b–e) CdS/SiO2 core-shell nanocrystals with addition of TEOS: (b) 0.1 cm3; (c) 0.3 cm3; (d) 0.6 cm3; (e) 2.0 cm3.
Preparation of BPU/CdS–SiO2 nanocomposites was carried out in an ultrasonic irradiation process. The ultrasound source was a JY 92-2D ultrasonic cell crusher (2 104–109 Hz, 700 W, Scientz Biotechnology Co., Ltd., Ningbo, China) with the probe of the ultrasonic horn immersed directly in the mixture solution system. Nanoparticles were firstly treated with the KH550 silane coupling agent. A series of comparison experiments were done and optimal results were obtained when a weight percentage of KH550 to the nanoparticles was 10%. Different amounts of CdS– SiO2 nanoparticles (5, 10, 15, 20 wt.%) were mixed with the BPU and the mixture was dispersed in 20 mL absolute ethanol, followed by irradiation with high-intensity ultrasonic wave for 4 h at 30 8C. After irradiation, the resulted suspension was cooled to room temperature and then centrifuged, and the precipitate was washed twice with absolute ethanol and distilled water, respectively. The solid was dried in vacuum at room temperature for 6 h. The obtained product was kept for further characterization.
2.3. Synthesis of BPU/CdS–SiO2 nanocomposites The synthesis of S-BPU and R-BPU was reported in our previous work [44]. The R-BPU is the polymer synthesized from R-1,10 binaphthyl monomer and S-BPU is the polymer synthesized from S-1,10 -binaphthyl monomer. Here are the brief procedures. First, TDI (0.9 mL) was dissolved in 10 mL DMAc in a 250 mL flask with a magnetic stirrer at 80 8C under N2. Then (R)- or (S)-binaphthol (6 mmol) was added at the refuxing temperature for 16 h. A little NaOH was added to neutralize unreacted BINOL for 0.5 h. The mixture was cooled down to room temperature, poured to 150 mL anhydrous ethanol and precipitate out white flocculate. After neutralized with 10% aqueous HCl (150 mL), the product filtered, washed with anhydrous ethanol several times, dried at 80 8C. White polymer powders were obtained.
Fig. 2. XRD powder patterns of (a) R-BPU (b) S-BPU (c) CdS/SiO2 (d) R-BPU/CdS–SiO2 (15 wt.%) and (e) S-BPU/CdS–SiO2 (15 wt.%) nanocomposites.
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2.4. Characterization The surface morphology of the samples was monitored with scanning electron microscope (SEM) LEO-1530vp. X-ray diffraction measurements of polymer and nanocomposites were recorded using a Rigaku D/MAX-R with a copper target at 40 kV and 30 mA, in the range 5–808at the speed of 58/min. Thermal analysis experiments were performed using a thermogravimetric analysis (TGA) apparatus operated in the conventional TGA mode (TA Q600, TA Instrument) at the heating rate of 20 8C/min to simultaneously determine the correlation of temperature and weight loss in a nitrogen atmosphere. Infrared emissivity values of the samples were carried out on an IRE-I Infrared Emissometer of Shanghai Institute of Technology and Physics, China. 3. Results and discussion 3.1. X-ray diffraction data Fig. 1 showed the powder XRD patterns of (a) CdS nanorods and (b–e) CdS–SiO2. It is observed that the sample (a) was a pure
Fig. 3. TGA curves of R-BPU and R-BPU/CdS–SiO2 nanocomposites.
Fig. 4. TEM images of (a–b) CdS nanoparticles and (c–d) CdS–SiO2 core-shell nanocrystals with addition of TEOS: (c) 0.3 cm3; (d) 2.0 cm3.
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Fig. 5. SEM images of (a–d) CdS/SiO2 core-shell nanocrystals with addition of TEOS: (a–b) 0.3 cm3; (c–d) 2.0 cm3.
hexagonal CdS phase (wurtzite structure). There was a strong (0 0 2) peak, which indicated a preferential orientation of (0 0 1) in the CdS crystal. From the patterns of sample (b–e), no peak of SiO2 was detected, suggesting that SiO2 was amorphous which wraps the CdS nanorods. And the peak of CdS–SiO2 intensified with the concentration of the TEOS used, indicating that the morphology of CdS changed through the ultrasonic process. Fig. 2 was the XRD patterns of BPU, CdS–SiO2 and BPU/CdS–SiO2 nanocomposites. (R)- and (S)-BPU (a and b) were the amorphous polymers and did not exhibit any anisotropic behaviors. This may be due to the presence of the naphthyl ring and aromatic structures in the main chain, thus limiting the molecular mobility of the polymer. When adding CdS–SiO2 particles to BPU, as shown in Fig. 2, it appeared the characteristic peaks of CdS–SiO2. It also could be seen that two curves of the nanocomposites (d and e) were almost in the same position, indicating that the morphology of CdS–SiO2 particles had not been changed during the process. However, all the diffraction peaks were broadened. The average crystalline size of CdS–SiO2, determined by the Debye–Scherrer equation, was approximately 30 nm. Furthermore, ultrasonic irradiation reduced the crystallite size of CdS–SiO2 due to the generation of many localized hot spots in the solution, which further gave rise to the homogeneous formation of a large number of seed nuclei, leading to a smaller particle size [45–47].
3.2. Thermal properties Fig. 3 showed the thermogravimetric curves of the R-BPU and RBPU/CdS–SiO2 nanocomposites with the different core-shell nanoparticle contents. The thermal property of R-BPU/CdS–SiO2 nanocomposites was investigated because the thermal stability of R-BPU was better than S-BPU [48]. It was clear that the sample exhibited a very good thermal stability below 110 8C. It began to decompose around 120–130 8C in N2 gas. The 5% weight loss occurred in about 150 8C, which was attributed to the loss of residual water and organic solvent. Another weight loss started at around 250 8C, corresponded to the polymer degradation. Concerning the second peak, it was much higher than that of the common PU [49]. This increased in the thermal stability may result from the presence of hard naphthyl group unit [50]. It was evident that the R-BPU had a high decomposition temperature of about 250 8C. This thermal stability was further improved when CdS–SiO2 was introduced. It seemed that the initial temperature of weight loss did not increase with increasing CdS–SiO2 content. However, at a weight loss of 40%, CdS–SiO2 tended to increase the thermal resistance of the nanocomposites. This increase of the thermal stability may result from the high thermal stability of CdS–SiO2 nanoparticles, which limited the movement of the molecular chain of R-BPU.
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Fig. 6. SEM images of (a–d) R-BPU/CdS–SiO2 nanocomposites with the content of CdS–SiO2 5, 10, 15 and 20 wt.%, respectively.
3.3. Morphology In order to examine the microstructures and nanoparticles distribution within the nanocomposites, TEM and SEM analysis were
conducted. Fig. 4 showed the TEM micrographs of the CdS and CdS– SiO2 nanoparticles. According to the XRD results (Fig. 4), we inferred that the CdS crystal growth was oriented and morphologies are 1D rodlike. This inference was confirmed by TEM images. As shown in Fig. 4a, the average crystalline size of CdS was about 15–20 nm. Fig. 4c and d showed the CdS–SiO2 nanoparticles, it was apparent that CdS nanorods and SiO2 form the core-shell structure, and the average size of CdS–SiO2 was 50–60 and 70–80 nm, respectively. Fig. 5 showed the SEM micrographs of the CdS–SiO2 nanoparticles. It was observed that the thickness of SiO2 shell increased with the concentration of the TEOS used. As expected, in Fig. 6a–c, the nanocomposites presented a homogeneous microstructure. However, it should be noted that, as far as the content of CdS–SiO2 was 25 wt.%, Fig. 6d showed that the nanocomposites presented some irregularities in size and shape, which could be explained by the ultrasonic process. Ultrasonic cavitation can generate an environment of local temperature up to 5000 K and local pressure up to 500 atm, under such conditions the growth of particles had some difference. 3.4. Infrared emissivity
Fig. 7. Infrared emissivity values of (a) R-BPU/CdS–SiO2 (15 wt.%) emin = 0.50, (b) SBPU/CdS–SiO2 (15 wt.%) emin = 0.38.
IR emissivity testing results indicated that the R-PU, S-BPU and CdS–SiO2 possessed high emissivity of 0.90, 0.85, and 0.88, respectively. Polyurethane has a high infrared emissivity due to its strong absorbability at infrared wave band, commonly, an
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Fig. 8. Mechanism illustration of the ultrasonic induced reaction in the polyurethane formed on the nanoparticles.
increase in the absorbability results in an increase in the emissivity [51,52]. CdS–SiO2 has a high infrared emissivity value due to their high surface area. The IR emissivity of nanocomposites (0.38–0.70) was much lower than that of BPU or nanoparticles, which could be contributed to interfacial synergism forces such as hydrogen bonds or electrostatic interactions between the organic and inorganic components [53]. These interactions can alter the vibration mode of molecules, atoms or pendant groups on interface between organic and inorganic components [54], thus the composites possessed lower emissivity than that of two components. Fig. 7 showed the IR emissivity of BPU/CdS–SiO2 nanocomposites with different CdS– SiO2 nanoparticle content, as could be seen, with the increasing the ratio of CdS–SiO2 and BPU monomer, the IR emissivity increased initiatively and then decreased, the lowest value was 0.38 (S-BPU/ CdS–SiO2 (15 wt.%)) and 0.50 (R-BPU/ CdS–SiO2 (15 wt.%)), respectively. It can be indicated the nanocomposites with appropriate quantity CdS–SiO2 nanoparticles possessed higher surface area, surface area energy, quantity of surface atom and dangling bonds, thus possessed stronger surface effect, polarity effect, multiply dissociated radiation and lower IR emissivity. And these results were also in good agreement with the previous SEM analysis. On the other hand, the S-BPU/CdS–SiO2 had lower infrared emissivity values than those of R-BPU/CdS–SiO2, which could be contributed to the different optical properties between S-BPU and R-BPU. 3.5. Mechanism Based on the experimental results mentioned above, we proposed the mechanism of ultrasound induced composite reaction as follows: The process can be depicted as Fig. 8. Under ultrasonic, the coupling agent KH550 hydrolyzes to form hydroxyls, and these groups react with the hydroxyls of CdS–SiO2 surface through hydrogen bonding. Generally, the main effects of sonication are due to cavitation or the growth and explosive collapse of microscopic bubbles on a microsecond timescale [55]. At the same time, ultrasonic cavitation can generate a rigorous environment of local temperature up to 5000 K and local pressure up to 500 atm [50]. Under such conditions the modified nanoparticles might be dispersed absolutely and will combine with polyurethane via the amino of coupling agent. As a result, the dispersity of
nanoparticles increase with the decrease of surface energy of nanoparticles, and the polymer on the nanoparticle surface has a steric dispersing and stabilizing effect, which can be observed from the photographs of TEM and SEM. Meanwhile, due to the hydrogen bond effect the thermal property of nanocomposite is much higher than pure polymer, as seen from the TGA analysis. On account of the strong interactions occur between the –OH and –NH2 group of BPU and nanoparticles, it would lead the special surface effect of incidence wave and nanocomposite surface and change the transfer modes of dangling bonds lay in the surface of nanocomposite, which make the absorption bands existing in 8–14 mm are dissociated and weakened, thus the whole material possesses lower emissivity than those of two components. 4. Conclusion This work demonstrated a simple and effective route to synthesize optico-active polyurethane. Meanwhile a sonochemical method has been used for the preparation of BPU/CdS–SiO2 nanocomposites. TGA studies indicated that thermal stability of the nanocomposites had improved with increasing nanoparticles contents. Infrared emissivity study showed that the nanocomposites possessed lower emissivity value than those of BPU and nanoparticles, respectively. Finally, a possible mechanism of ultrasonic induced composite reaction was proposed. The ultrasonic method employed here may be a simple and inexpensive route to synthesize other polymer nanocomposites, which can be extended to prepare a novel low infrared emissivity material. Acknowledgements The authors are grateful to ‘‘the New Century Talents Program’’ of Ministry of Education of China (NCET-04-0482), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (06-A-033) and the National Nature Science Foundation of China (50377005) for financial supports. References [1] L. Feng, J.W. Hu, Z.L. Liu, F.B. Zhao, G.J. Liu, Polymer 48 (2007) 3616–3623.
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