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Applied Surface Science 253 (2007) 9154–9158

Synthesis, characterization and infrared emissivity study

of polyurethane/TiO2 nanocomposites

Jing Chen, Yuming Zhou *, Qiuli Nan, Yanqing Sun, Xiaoyun Ye, Zhiqiang Wang

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

Received 8 March 2007; received in revised form 21 May 2007; accepted 21 May 2007

Available online 29 May 2007

Abstract

In this study, polyurethane/titania (PU/TiO2) nanocomposites were prepared in ultrasonic process and characterized by fourier transform IR

spectroscopy (FT-IR), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission

electron microscopy (TEM), scanning electron microscopy (SEM) and infrared emissivity analysis. The TEM and SEM results indicated that the

nanoparticles were dispersed homogeneously in PU matrix on nanoscale. TGA-DSC confirmed that the heat stability of the composite was

improved. Infrared emissivity study showed that the nanocomposite possessed lower emissivity value than those values of pure polymer and

nanoparticles.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Polyurethane; Nano-TiO2; Nanocomposites; Polymers; Emissivity

1. Introduction

Infrared low emissivity coating has found wide application in

civil and military fields such as camouflaging military equipment

or vehicles from infrared detection [1]. As a result of their

excellent physicochemical and optoelectronic properties, nano-

materials have received increasing attention [2] in recent years

and have been studied intensively in low emissivity coating [3].

In practical applications, the coating generally attains the low

emissivity by using nanocomposites based on indium oxide

owing to high emissivity of the coating made of single nanoscale

indium oxide. Few reports, however, have dealt with the coating

made of organic components and inorganic nanoparticles [4,5].

Polyurethanes are widely used in adhesives and coatings of

various materials, e.g., textiles, metals, and plastics [6–8]. In a

recent study, Crawford and Escarsega [9] systematically

reported the dynamic mechanical and durability properties of

novel polyurethane coating for military application. High

emissivity, however, can be found in native polyurethane and

nanooxide that will limit their practical applications. The

nanocomposites combining the merits of polymer and

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

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

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.05.046

inorganic nanoparticles [10] is recommended to deal with this

problem. Recently, greater attention has been paid to the

preparation of polymer/nanoparticles composites [11–13]. For

instance, Chen et al. [14] reported the preparation of polyester-

based polyurethane/titania hybrid films using a sol–gel process.

Lee and Chen [15] also reported the preparation of PMMA/

titania hybrid films with 3-(trimethoxysilyl)propyl methacry-

late as the coupling agent. However, the use of conventional

synthetic methods results in agglomerating during the reaction

and incompatibility after it [16,17]. Ultrasonic irradiation could

be employed to overcome these disadvantages [18], and Suslick

et al. [19] have explored a variety of applications of ultrasound

to materials chemistry.

In the present paper, the nanocomposite of polyurethane/

TiO2 was synthesized through a simple and convenient

ultrasonic wave dispersion process and infrared emissivity

values at wavelength of 8–14 mm of the composite and its

components were investigated.

2. Experimental

2.1. Materials

Nanoparticles (TiO2) were prepared according to the

literature [20]; toluene 2,4-diisocyanate (TDI) was used with

Fig. 1. FTIR spectra of (a) pure nano-TiO2, (b) pure PU and (c) PU/nano-TiO2.

J. Chen et al. / Applied Surface Science 253 (2007) 9154–9158 9155

low pressure distilled; coupling agent (g-amidopropyl-

triethoxyl silicane) (KH550), o-phenylenediamine, urea, amyl

alcohol, 37% formalin, 4-methylpentanone-2, and dimethyl

formamide were all purchased from Sinopharm Chemical

Reagent Co., Ltd (Shanghai, China). All solvents were

purchased from commercial A.R. grade.

2.2. Characterization

FT-IR spectra were recorded on a Nicolet Magna-IR 750

(USA) spectrometer. First, the samples (PU, TiO2, PU/TiO2

nanocomposites) were dried, and then mulled with KBr pellets.

X-ray diffraction measurements of the composite and its

components were recorded using a Rigaku D/MAX-R with a

copper target at 40 kV and 30 mA. The powder samples were

spread on a sample holder and the diffractograms were recorded

in the range 5–808 at the speed of 58/min. Thermal analysis

experiments were performed using a TG/DSC apparatus

operated in the conventional TG/DSC mode (TMDSC, TA

Q-600, TA Instrument) at the heating rate of 10 8C/min to

simultaneously determine the correlation of temperature and

weight loss in a nitrogen atmosphere. The surface morphology

of the samples was monitored with transmission electron

microscopy (TEM) and scanning electron microscopy (SEM).

TEM was performed on a JEM-1230 microscope operating at

an accelerating voltage of 100 kV. SEM was obtained on the

microscope of LEO-1530vp. Infrared emissivity values of the

samples were carried out on an IRE-I Infrared Emissometer of

Shanghai Institute of Technology and Physics, China.

2.3. Apparatus

The reaction was performed on a JY 92-2D ultrasonic cell

crusher purchased from Scientz Biotechnology Co., Ltd

(Ningbo, China). Ultrasound was a wave of frequency

2 � 104 � 109 Hz and power 700 W.

2.4. Polymer synthesis

The poly [l,3-bis(methylene) benzimidazolin-2-one, toluene

2,4-(biscarbamate)] was synthesized by the typical method

[21]. The diol, viz., l,3-bis(hydroxymethyl) benzimidazolin-2-

one (self-prepared, 0.01 mol, 1.94 g) was dissolved in dry DMF

under N2 atmosphere with constant stirring. Then, diisocyanate

(TDI, 0.0 lmol, 1.742 g) taken in dry 4-methylpentanone-2 was

added in this solution over a period of l h. The reaction mixture

was heated for 2.5 h at 115 8C to obtain the solid. The mixture

was stirred continuously for 3 h, cooled, poured into distilled

water, and then filtered, washed with distilled water, and dried

under reduced pressure at 90 8C.

2.5. Preparation of PU/TiO2 nanocomposites

The preparation of PU/TiO2 nanocomposites was carried out

in an ultrasonic process. First, nanoparticles (0.3 g) were added

into acetone (10 mL), and a small amount of KH550 was

dissolved in H2O (10 mL). In a series of controllable

experiments we found that 10% weight percentage of

KH550 was optimized. Then the mixture of nanoparticles

and KH550 was irradiated under ultrasonic radiation for 30 min

at 30 8C, cooled to room temperature, centrifuged and dried.

After that 0.3 g nano-TiO2 and 0.l g PU were dispersed in

20 mL ethanol and the mixture was irradiated with high-

intensity ultrasound radiation for 30 min at 30 8C. After

irradiation, the resulting 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 and was kept for further characterization.

3. Results and discussion

3.1. Spectral data

It can be seen, from the IR spectra of the pure TiO2, PU, and

PU/TiO2 shown in Fig. 1, that in pure TiO2, ı̂(OH) stretching

band and bending band are observed at 3423 and 1637 cm�1,

respectively. Following Knozinger [22], the observed broader

bands at 3500 and 3422 cm�1 were attributed to hydroxyl

groups on different sites and some varying interactions between

hydroxyl groups on TiO2, respectively. The bands at 643 and

540 cm�1 are assigned to the anatase nano-TiO2 [23]. However,

the carbonyl group of urethane and benzimidazolin-2-one is

shown in the region 1725 cm�1 [24]. In the case of the spectrum

of PU/TiO2, where the characteristic peaks of pure PU and TiO2

are still maintained, it may be proved that the structure pf PU

was affected by the presence of TiO2 implying that the TiO2 did

not react with the PU molecules.

3.2. Microstructure

Figs. 2–3 shows the transmission electron microscope and the

scanning electron microscopy micrographs of the pure TiO2

nanoparticles and PU/TiO2 nanocomposites. From Fig. 2 (a), it

can be clearly seen that the nanoparticles have some aggregation.

In contrast to this, in Fig. 2(b–d) and Fig. 3(a–d) the

Fig. 2. TEM micrographs of (a) pure nano-TiO2 and (b-d) PU/nano-TiO2.

J. Chen et al. / Applied Surface Science 253 (2007) 9154–91589156

nanocomposites with size of about 30–35 nm consist of the very

uniform particles. We can see clearly that the nanoparticles are

dispersed in PU matrix on nanoscale, which indicates the

formation of a nanocomposite in some way [25–27]. As

expected, through the ultrasonic technique nanoparticles, which

distribute in the polymer are homogeneous.

3.3. X-ray diffraction data

X-ray diffraction curves of the polymer, nanoparticle and

composite are shown in Fig. 4. PU (Fig. 4(a)) is amorphous

and does not exhibit any anisotropic behaviors [28]. From

Fig. 4(c), we can see that most of the characteristic peaks at

2u values of anatase TiO2 are kept intact. The average size

of the nanocomposites is calculated to be 30–35 nm

according to the Debye-Scherrer formula. However, the

intensities of anatase peaks become weaker in the presence

of PU and the peaks are widened. The findings indicate

that ultrasonic irradiation reduces the crystallite size of

anatase due to the generation of many localized hot spots in

the solution by the ultrasound irradiation, which further

gives rise to the homogeneous formation of a large number of

seed nuclei, leading to a smaller particle size [29–33]. This

feature also agrees with the previous TEM and SEM

measurements.

3.4. Thermal properties

A thermogravimetry-differential scanning calorimetry

(TGA-DSC) study of the obtained nanocomposite is shown

in Fig. 5. The TGA curve of PU/TiO2 composite shows a two-

step weight loss. The weight loss at about 100 8C in the first step

is attributed to the loss of residual water and organic solvent.

The second step, which starts at around 262 8C, corresponds to

polymer degradation. The second peak is much higher than that

of pure PU [28]. Meanwhile, it is obvious that at this

temperature, the total weight loss of the PU/TiO2 is 20%. As

can be seen, until 600 8C the total weight loss of the composite

is much lower than that of pure PU, suggesting that thermal

stability of the composite is improved considerably. The

possible reason for this behavior may be the reduced mobility

of urethane chains in the nanocomposite, which inhibits the

Fig. 3. SEM micrographs of PU/nano-TiO2 (�50000).

J. Chen et al. / Applied Surface Science 253 (2007) 9154–9158 9157

chain transfer reaction, thus slowing the degradation process

[34].

The DSC curve shows that the temperature of endothermic

peak at about 100 8C is attributed to the volatilization of

residual water and organic solvent. The exothermic peak

observed at 262 8C is related to the decomposition of the

polymer. In contrast to this, the decomposition of this kind of

pure PU is only 202 8C [28]. Meanwhile, some small

exothermic peaks observed from 359 to 518 8C are due to

the heat effect of the oxidation combustion of organic

substances [24]. Obviously, this result indicates a strong and

uniform interaction between PU and nanoparticles.

Fig. 4. X-ray diffraction patterns of (a) pure PU, (b) pure nano-TiO2 and (c) PU/

nano-TiO2.

3.5. Infrared emissivity

The infrared emissivity values at wavelength of 8–14 mm of

all the samples are tested (Table 1). It is clear that pure polymer

and TiO2 possess high emissivity values of 0.945 and 0.925,

respectively. Polyurethane has a high infrared emissivity value,

because this kind of polymer has strong absorbability at infrared

wave band [28], and commonly an increase in the absorbability

results in an increase in the emissivity [35]. TiO2 has a high

infrared emissivity value due to the high refractive index

(commonly larger than 0.8) [36]. Conversely, PU/TiO2

composite has the lower infrared emissivity value (0.538) than

Fig. 5. TGA-DSC curves of PU/nano-TiO2.

Table 1

Infrared emissivity values of samples

Samples Infrared emissivity

(eTIR at 8–14 mm)

PU 0.945

TiO2 0.925

PU/TiO2 0.538

J. Chen et al. / Applied Surface Science 253 (2007) 9154–91589158

both of its components. According to X-ray diffraction analysis,

there is no apparent difference in phase structure before and after

the measurement. Therefore, the decrease in infrared emissivity

may be owing to interfacial synergism forces such as hydrogen

bonds or electrostatic interactions between the organic and

inorganic components [37]. These interactions can alter the

original vibration mode of molecules, atoms or pendant groups

on interface between organic and inorganic components [38],

which have some effects on emissivity. Therefore, the decrease of

infrared emissivity of the nanocomposite is most likely to be a

direct consequence of the interfacial interactions.

4. Conclusions

A sonochemical method employed here may be a simple and

inexpensive route to synthesize the PU/nano-TiO2. Infrared

emissivity study at wavelength of 8–14 mn of nanocomposite

and its components showed that the composite possessed lower

emissivity value than those of pure PU and TiO2, and interfacial

interactions had great effect on emissivity of nanocomposites.

Thereby, the nanocomposites can be extended in application as

a novel low infrared emissivity material.

Acknowledgements

The authors are grateful to ‘‘Six Talents Pinnacle Program’’

of Jiangsu Province of China (06-A-033) and ‘‘the New

Century Talents Program’’ of ministry of education of China

(NCET-04-0482) for financial supports.

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