Applied Surface Science 255 (2008) 2244–2250

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Applied Surface Science

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Synthesis and characterization of polyurethane/CdS–SiO2 nanocomposites viaultrasonic 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, Chinab 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

A R T I C L E I N F O

Article history:

Received 26 May 2008

Received in revised form 7 July 2008

Accepted 7 July 2008

Available online 23 July 2008

Keywords:

Polyurethane

Binaphthyl

Nanocomposites

Infrared emissivity

A B S T R A C T

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.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

High performance chiral polymers have attracted a great deal ofinterest in the applications of fields such as dentistry, drug deliveryand tissue engineering [1,2] due to their promising properties. Inrecent years, many attentions have been paid for the usages of theoptically active 1,10-bi-2-naphthol (BINOL) such as a chiralbuilding block [3,4], chiral catalyst [5–7] or chiral auxiliary [8–10] in asymmetric synthesis, molecular recognition and enantio-selective chromatographic separation [11–13]. A series of chiralpolybinaphthalene polymers have also been prepared enantiose-lectively starting from BINOL because of its axial chirality andconfigutational stability [14–19].

As an important class of optico-active polymers [20–23],polyurethane has received an increasing amount of attentionowing to their chemical and mechanical properties [24–26].Nanocrystalline cadmium sulfide (CdS) has unique physical andchemical properties and can be used in electronic, optical, andmagnetic aspects. Most applications are related to the CdScrystalline [27–29], however, it is labile and easy to decomposeand therefore to be stabilized by fabricating the nanocrystallines

* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617.

E-mail address: fchem@seu.edu.cn (Y.-M. Zhou).

0169-4332/$ – see front matter � 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-shellstructures such as Ag/SiO2, Au/SiO2 etc. have various potentialapplications [30]. Moreover, to allow CdS nanocrystals embeddedin a polymer matrix, researches have been focused on thepreparation of CdS-polymer nanocomposites. Chen et al. [31]produced transparent nanocrystal-polymer hybrids by graftingpolyurethane onto functionalized CdS nanocrystals. Celebi et al.[32] reported a detailed study on poly(acrylic acid) stabilized CdSquantum dots and discussed the influence of reaction pH on thesize and optical properties.

Similar to other common polymers, the synthesis of PUnanocomposites has been studied through multifarious methods[20,33,34]. However, only a few papers reported the synthesis andproperties of PU nanocomposites under ultrasonic irradiation.Ultrasonic irradiation has been widely employed in cleaning,jointing, machining, medicine, chemistry and preparing nano-composites [35–39]. As known, this method can control the sizedistribution and morphology of the nanosized particles [40].Kwang-Pill Lee reported the preparation of polydiphenylamine/silica-nanoparticle composites under ultrasonication andextended this methodology to making conducting, processablenanocomposites with other types of conducting polymers [41].However, as far as we knew, there is no report addressed theincorporation of core-shell nanostructures into polyurethanes.Hence, we sought to examine whether this incorporation under

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.

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.

J. Chen et al. / Applied Surface Science 255 (2008) 2244–2250 2245

ultrasonic irradiation is feasible and, if so, how the structure,morphology, and properties of the nanocomposites are influencedupon 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 emissivitycoating [42], PU/nanoparticles utilized in the study of infraredemissivity, 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 ofSinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Iso-propanol and ammonia (25%) were purchased from ShanghaiChemical Reagent Company. Tetraethylorthosilicate (TEOS), etha-nediamine and toluene 2,4-diisocyanate (TDI) were purchasedfrom Lingfeng Chemical Reagent Co., Ltd. (Nanjing, China); Thecoupling agent, g-amidopropyl-triethoxyl silicane (KH550), wasobtained from Yaohua Chemical Plant(Shanghai, China); N,N-dimethylacetamide (DMAC), which was purchased from Sino-pharm Chemical Reagent Co., Ltd. (Shanghai, China), was redis-tilled 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 asreceived 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 andput into a Teflon-lined stainless steel autoclave with a capacity of100 mL. Then the autoclave was filled with ethylenediamine up toabout 70% of the total volume. The autoclave was kept at 120–190 8C for 3–6 h and then cooled to room temperature. The yellowprecipitate was collected and washed with absolute ethanol anddistilled water to remove residue of organic solvents. Finally, theproduct 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 isopro-panol (200.0 mL), followed by high-intensity ultrasonic irradiationfor 20 min. Then the mixture was put into a round-bottom flaskwith distilled water (18.0 mL) and stronger ammonia water(10.5 mL), and TEOS were quickly added in appropriate propor-tions, stirred at room temperature for 24 h. The product wasfiltered and washed with distilled water and absolute ethanol, thenwas dried at 70 8C for 4 h.

2.3. Synthesis of BPU/CdS–SiO2 nanocomposites

The synthesis of S-BPU and R-BPU was reported in our previouswork [44]. The R-BPU is the polymer synthesized from R-1,10-binaphthyl monomer and S-BPU is the polymer synthesized fromS-1,10-binaphthyl monomer.

Here are the brief procedures. First, TDI (0.9 mL) was dissolvedin 10 mL DMAc in a 250 mL flask with a magnetic stirrer at 80 8Cunder N2. Then (R)- or (S)-binaphthol (6 mmol) was added at therefuxing temperature for 16 h. A little NaOH was added toneutralize unreacted BINOL for 0.5 h. The mixture was cooleddown to room temperature, poured to 150 mL anhydrous ethanoland precipitate out white flocculate. After neutralized with 10%aqueous HCl (150 mL), the product filtered, washed withanhydrous ethanol several times, dried at 80 8C. White polymerpowders were obtained.

Preparation of BPU/CdS–SiO2 nanocomposites was carriedout in an ultrasonic irradiation process. The ultrasound sourcewas a JY 92-2D ultrasonic cell crusher (2 � 104–109 Hz, 700 W,Scientz Biotechnology Co., Ltd., Ningbo, China) with the probe ofthe ultrasonic horn immersed directly in the mixture solutionsystem. Nanoparticles were firstly treated with the KH550 silanecoupling agent. A series of comparison experiments were doneand optimal results were obtained when a weight percentage ofKH550 to the nanoparticles was 10%. Different amounts of CdS–SiO2 nanoparticles (5, 10, 15, 20 wt.%) were mixed with the BPUand the mixture was dispersed in 20 mL absolute ethanol,followed by irradiation with high-intensity ultrasonic wave for4 h at 30 8C. After irradiation, the resulted suspension wascooled to room temperature and then centrifuged, and theprecipitate was washed twice with absolute ethanol anddistilled water, respectively. The solid was dried in vacuum atroom temperature for 6 h. The obtained product was kept forfurther characterization.

Fig. 3. TGA curves of R-BPU and R-BPU/CdS–SiO2 nanocomposites.

J. Chen et al. / Applied Surface Science 255 (2008) 2244–22502246

2.4. Characterization

The surface morphology of the samples was monitored withscanning electron microscope (SEM) LEO-1530vp. X-ray diffractionmeasurements of polymer and nanocomposites were recordedusing 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 analysisexperiments were performed using a thermogravimetric analysis(TGA) apparatus operated in the conventional TGA mode (TA Q-600, TA Instrument) at the heating rate of 20 8C/min tosimultaneously determine the correlation of temperature andweight loss in a nitrogen atmosphere. Infrared emissivity values ofthe samples were carried out on an IRE-I Infrared Emissometer ofShanghai 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. 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.

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.

J. Chen et al. / Applied Surface Science 255 (2008) 2244–2250 2247

hexagonal CdS phase (wurtzite structure). There was a strong(0 0 2) peak, which indicated a preferential orientation of (0 0 1) inthe CdS crystal. From the patterns of sample (b–e), no peak of SiO2

was detected, suggesting that SiO2 was amorphous which wrapsthe CdS nanorods. And the peak of CdS–SiO2 intensified with theconcentration of the TEOS used, indicating that the morphology ofCdS 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 amorphouspolymers and did not exhibit any anisotropic behaviors. This maybe due to the presence of the naphthyl ring and aromatic structuresin the main chain, thus limiting the molecular mobility of thepolymer. When adding CdS–SiO2 particles to BPU, as shown inFig. 2, it appeared the characteristic peaks of CdS–SiO2. It also couldbe seen that two curves of the nanocomposites (d and e) werealmost in the same position, indicating that the morphology ofCdS–SiO2 particles had not been changed during the process.However, all the diffraction peaks were broadened. The averagecrystalline size of CdS–SiO2, determined by the Debye–Scherrerequation, was approximately 30 nm. Furthermore, ultrasonicirradiation reduced the crystallite size of CdS–SiO2 due to thegeneration of many localized hot spots in the solution, whichfurther gave rise to the homogeneous formation of a large numberof 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 R-BPU/CdS–SiO2 nanocomposites with the different core-shellnanoparticle contents. The thermal property of R-BPU/CdS–SiO2

nanocomposites was investigated because the thermal stability ofR-BPU was better than S-BPU [48]. It was clear that the sampleexhibited a very good thermal stability below 110 8C. It began todecompose around 120–130 8C in N2 gas. The 5% weight lossoccurred in about 150 8C, which was attributed to the loss ofresidual water and organic solvent. Another weight loss started ataround 250 8C, corresponded to the polymer degradation. Con-cerning the second peak, it was much higher than that of thecommon PU [49]. This increased in the thermal stability may resultfrom the presence of hard naphthyl group unit [50].

It was evident that the R-BPU had a high decompositiontemperature of about 250 8C. This thermal stability was furtherimproved when CdS–SiO2 was introduced. It seemed that theinitial temperature of weight loss did not increase with increasingCdS–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 highthermal stability of CdS–SiO2 nanoparticles, which limited themovement of the molecular chain of R-BPU.

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.

J. Chen et al. / Applied Surface Science 255 (2008) 2244–22502248

3.3. Morphology

In order to examine the microstructures and nanoparticlesdistribution within the nanocomposites, TEM and SEM analysis were

Fig. 7. Infrared emissivity values of (a) R-BPU/CdS–SiO2 (15 wt.%) emin = 0.50, (b) S-

BPU/CdS–SiO2 (15 wt.%) emin = 0.38.

conducted. Fig. 4 showed the TEM micrographs of the CdS and CdS–SiO2 nanoparticles. According to the XRD results (Fig. 4), we inferredthat the CdS crystal growth was oriented and morphologies are 1Drodlike. This inference was confirmed by TEM images. As shown inFig. 4a, the average crystalline size of CdS was about 15–20 nm.Fig. 4c and d showed the CdS–SiO2 nanoparticles, it was apparentthat CdS nanorods and SiO2 form the core-shell structure, and theaverage size of CdS–SiO2 was 50–60 and 70–80 nm, respectively.

Fig. 5 showed the SEM micrographs of the CdS–SiO2 nano-particles. It was observed that the thickness of SiO2 shell increasedwith 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 presentedsome irregularities in size and shape, which could be explained bythe ultrasonic process. Ultrasonic cavitation can generate anenvironment of local temperature up to 5000 K and local pressureup to 500 atm, under such conditions the growth of particles hadsome difference.

3.4. Infrared emissivity

IR emissivity testing results indicated that the R-PU, S-BPU andCdS–SiO2 possessed high emissivity of 0.90, 0.85, and 0.88,respectively. Polyurethane has a high infrared emissivity due toits strong absorbability at infrared wave band, commonly, an

Fig. 8. Mechanism illustration of the ultrasonic induced reaction in the polyurethane formed on the nanoparticles.

J. Chen et al. / Applied Surface Science 255 (2008) 2244–2250 2249

increase in the absorbability results in an increase in the emissivity[51,52]. CdS–SiO2 has a high infrared emissivity value due to theirhigh surface area. The IR emissivity of nanocomposites (0.38–0.70)was much lower than that of BPU or nanoparticles, which could becontributed to interfacial synergism forces such as hydrogen bondsor electrostatic interactions between the organic and inorganiccomponents [53]. These interactions can alter the vibration mode ofmolecules, atoms or pendant groups on interface between organicand inorganic components [54], thus the composites possessedlower emissivity than that of two components. Fig. 7 showed the IRemissivity of BPU/CdS–SiO2 nanocomposites with different CdS–SiO2 nanoparticle content, as could be seen, with the increasing theratio of CdS–SiO2 and BPU monomer, the IR emissivity increasedinitiatively and then decreased, the lowest value was 0.38 (S-BPU/CdS–SiO2 (15 wt.%)) and 0.50 (R-BPU/ CdS–SiO2 (15 wt.%)), respec-tively. It can be indicated the nanocomposites with appropriatequantity CdS–SiO2 nanoparticles possessed higher surface area,surface area energy, quantity of surface atom and dangling bonds,thus possessed stronger surface effect, polarity effect, multiplydissociated radiation and lower IR emissivity. And these results werealso in good agreement with the previous SEM analysis. On the otherhand, the S-BPU/CdS–SiO2 had lower infrared emissivity values thanthose of R-BPU/CdS–SiO2, which could be contributed to thedifferent optical properties between S-BPU and R-BPU.

3.5. Mechanism

Based on the experimental results mentioned above, weproposed the mechanism of ultrasound induced compositereaction as follows:

The process can be depicted as Fig. 8. Under ultrasonic, thecoupling agent KH550 hydrolyzes to form hydroxyls, and thesegroups react with the hydroxyls of CdS–SiO2 surface throughhydrogen bonding. Generally, the main effects of sonication aredue to cavitation or the growth and explosive collapse ofmicroscopic bubbles on a microsecond timescale [55]. At thesame time, ultrasonic cavitation can generate a rigorous environ-ment of local temperature up to 5000 K and local pressure up to500 atm [50]. Under such conditions the modified nanoparticlesmight be dispersed absolutely and will combine with polyurethanevia the amino of coupling agent. As a result, the dispersity of

nanoparticles increase with the decrease of surface energy ofnanoparticles, and the polymer on the nanoparticle surface has asteric dispersing and stabilizing effect, which can be observed fromthe photographs of TEM and SEM. Meanwhile, due to the hydrogenbond effect the thermal property of nanocomposite is much higherthan pure polymer, as seen from the TGA analysis. On account ofthe strong interactions occur between the –OH and –NH2 group ofBPU and nanoparticles, it would lead the special surface effect ofincidence wave and nanocomposite surface and change thetransfer modes of dangling bonds lay in the surface of nanocom-posite, which make the absorption bands existing in 8–14 mm aredissociated and weakened, thus the whole material possesseslower emissivity than those of two components.

4. Conclusion

This work demonstrated a simple and effective route tosynthesize optico-active polyurethane. Meanwhile a sonochem-ical method has been used for the preparation of BPU/CdS–SiO2

nanocomposites. TGA studies indicated that thermal stability ofthe nanocomposites had improved with increasing nanoparticlescontents. Infrared emissivity study showed that the nanocompo-sites possessed lower emissivity value than those of BPU andnanoparticles, respectively. Finally, a possible mechanism ofultrasonic induced composite reaction was proposed. Theultrasonic method employed here may be a simple andinexpensive route to synthesize other polymer nanocomposites,which can be extended to prepare a novel low infrared emissivitymaterial.

Acknowledgements

The authors are grateful to ‘‘the New Century Talents Program’’of Ministry of Education of China (NCET-04-0482), ‘‘Six TalentsPinnacle Program’’ of Jiangsu Province of China (06-A-033) and theNational Nature Science Foundation of China (50377005) forfinancial supports.

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