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F o r P e e r R e v i e w

Investigation on the modification to polyurethane by

multi-walled carbon nanotubes

Abstract

Purpose To investigate the influence of multi-walled carbon nanotubes (MWNTs) on the

mechanical, thermal and electrical conductivity properties of polyurethane via in situ

polymerization.

Design/methodology/approach A number of analytical techniques, including Fourier

transform infrared spectroscopy (FT-IR) measurement, X-ray photoelectron spectroscopy

(XPS) analysis were employed to assess the influence of acid treatment on multi-walled

carbon nanotubes, and the mechanical and thermal properties of the materials were

charactered by tensile tester machine and dynamic mechanical analysis (DMA), the electoral

conductivity properties of the materials were charactered by ohmmeter.

Findings It was found that modifications to multi-walled carbon nanotubes occurred after

modification by acid treatment, mainly carboxylic acid groups were introducing onto the

surface of multi-walled carbon nanotubes. And the acid treated multi-walled carbon

nanotubes could improve the mechanical, thermal and electrical conductivity properties of

polyurethane via in situ polymerization successfully.

Practical implications The investigation gave the method to synthesize multi-walled

carbon nanotubes and polyurethane composites via in situ polymerization. The mechanical,

thermal and electrical conductivity properties of polyurethane could be improved by

multi-walled carbon nanotubes.

Originality/value The investigation found the method to synthesize multi-walled carbon

nanotubes and polyurethane composites via in situ polymerization and the influence of

multi-walled carbon nanotubes on modifying mechanical, thermal and electrical conductivity

properties of polyurethane via in situ polymerization was investigated.

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Keywords multi-walled carbon nanotubes, polyurethane, composites, in situ polymerization,

properties

Paper type Research paper

Introduction

Carbon nanotubes (CNTs) was discovered by Iijima in 1991 (Iijima et al., 1991). CNTs are

cylinder-shaped macromolecules with a radius as small as a few nanometers, which can be

grown up to 20cm in length. The walls of these tubes are made up of a hexagonal lattice of

carbon atoms analogous to the atomic planes of graphite. They are capped at their ends by

one half of a fullerene-like molecule. In the most general case, a CNT is composed of a

concentric arrangement of many cylinders. Such multi-walled carbon nanotubes (MWNTs)

can reach diameters of up to 100nm (Balasubramanian et al., 2005).

Carbon nanotubes have attracted more and more attention due to their unique structure,

excellent electrical and thermal conductivity, together with their excellent mechanical

properties (Xia et al., 2006; Jung et al., 2006; Niu et al., 2007). The unique properties of CNTs

make them attractive for applications in many scientific and technological field such as

electronic structures, polymer composites, and biological systems (Liu et al., 2005; Jiang et al.,

2005; Chen et al., 2006; Xiong et al., 2006; Yang et al., 2006). And now intense research has

focused on CNTs composite materials with enhanced mechanical, electrical, and thermal

properties (Chen et al., 2005; Wang et al., 2007). Investigation of carbon nanotubes

improving the properties of polystyrene, poly(methyl methacrylate), epoxy resin and so on

were done (Qian et al., 2000; Jia et al., 1999; Schadler et al., 1998).

In order to make polymer/CNTs composites, chemical modification of carbon nanotubes is

necessary since CNTs are insoluble in common solvents and have a weak interfacial

interaction with the polymer matrix. Surface modification of CNTs has attracted significant

interest recently (Baskaran et al., 2005). Among these functional groups, the carboxyl group

is very attractive, because it can be used for further covalent and non-covalent

functionalization of CNTs (Chen et al., 2007; Saito et al., 2002; Liu et al., 2006). In another

words it generally starts from the functional carboxyl groups (COOH), which derived from

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the oxidation of CNTs (Cho et al., 2005).

In this paper the work about multi-walled carbon nanotubes and polyurethane composites

were done including acid treatment to MWNTs, the synthesis and characterisation of

composites to indicate the influence of multi-walled carbon nanotubes on the mechanical,

thermal and electrical conductivity properties of polyurethane.

Experimental

Materials and instrument

The multi-walled carbon nanotubes (MWNTs) was supplied by Shenzhen Nanotech Port Co.,

Ltd, the nanotubes have specified diameter 40-60nm and specified length 5-15m, with thepurity greater than 95%. Trimethylol propane (TMP) was supplied by Sinopharm

Chemical Reagent Co., Ltd. Toluene-2,4-diisocyanate (TDI) was supplied by Tianjin Yuanli

Chemical Engineer Co., Ltd. Castor oil was supplied by Tianjin Kermel Chemical Reagent

Co., Ltd. Dibutyl tin dilaurate was supplied by Shanhai Shanpu Chemical Engineer Co.,

Ltd. Isobutyl acetate (standard reagent grade) was supplied by Tianjin BASF Chemical

Engineer Co., Ltd. Sulphuric acid (95-98wt% standard reagent grade) and nitric acid

(65-68wt% standard reagent grade) were supplied by Harbin Xintian Chemical Reagent Co.,

Ltd.

Fourier transform infrared spectroscopy measurement was performed on a Magna-IR 560

E.S.P FT-IR spectrometer (supplied by Nicolet Co., Ltd) for infrared spectra of samples.

XPS analysis was carried out using a K-Alpha X-ray photoelectron spectrometer (supplied

by Thermo Fisher Scientific Co., Ltd). Mechanical properties of polyurethane and

composites were obtained on a CMT5504 tensile tester machine (supplied by Shenzhen

SANS Measurement Technology Co., Ltd) at room temperature using the 110 10 1mm 3

samples with a gauge length of 50mm and a crosshead speed of 50mm/min. Dynamic

mechanical analysis (DMA) was performed on a DMS6100 DMA unit (supplied by SII Nano

Technology Inc.) with an operating temperature range from 20 to 120 , the heating rate

was set at 3 /min, a frequency of 1Hz was applied, and the sample size was proximately 50

10 1mm 3. Volume electrical conductivity analysis was performed on an EST121



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ohmmeter supplied by Beijing labor safeguard science academe.

Procedures of acid treatment to multi-walled carbon nanotubes (MWNTs)

1g multi-walled carbon nanotubes (MWNTs) was dispersed in the mixture of 50ml HNO 3 and 150ml H 2SO4, and the mixture was sonicated at 40kHz in a water bath for 30min at room

temperature to be well dispersed, then stirred at 60 for 4h. After cooling to room

temperature that was diluted with deionized water and vacuum filtered, the solid left was

dispersed in deionized water and vacuum filtered for several times until no acid. The final

solid was dried in a vacuum oven at 80 to a constant weight (Jiang et al., 2009).

The schematic illustration of the acid treatment to multi-walled carbon nanotubes is shown

in Scheme 1.

(Take in Scheme 1)

Synthesis of polyurethane

5g trimethylol propane (TMP) was stirred and dissolved in 30ml isobutyl acetate at 55 ,

then 16ml toluene-2,4-diisocyanate (TDI) was introduced into the solvent. Then the system

was heated to 80 and stirred for 4h, then the isocyanates was got. The isocyanates and

30g castor oil were diluted with isobutyl acetate, respectively. Then the two systems and

Dibutyl tin dilaurate (0.03wt%) were mixed and stirred for 10min, then polymerized at room

temperature for 10 days.

The schematic illustration of the polyurethane synthesis is shown in Scheme 2.

(Take in Scheme 2)

Synthesis of multi-walled carbon nanotubes and polyurethane composites

Multi-walled carbon nanotubes (MWNTs) was sonicated in 10ml isobutyl acetate at 40kHz

in a water bath for 30min at room temperature to be well dispersed. 5g trimethylol

propane (TMP) stirring and dissolved in 20ml isobutyl acetate at 55 , then 16ml

toluene-2,4-diisocyanate (TDI) was introduced into the solvent, 10min later the MWNTs

dispersion was introduced to the system and the system was heated to 80 and stirred for



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4h, and the MWNTs isocyanates composites was got. The MWNTs isocyanates composites

and 30g castor oil were diluted with isobutyl acetate, respectively. Then the two systems

and Dibutyl tin dilaurate (0.03wt%) were mixed and stirred for 10min, then polymerized at

room temperature for 10 days.

The schematic illustration of the composites synthesis is shown in Scheme 3.

(Take in Scheme 3)

Results and discussion

Fourier transform infrared spectroscopy (FT-IR) analysis of raw and acid treated

multi-walled carbon nanotubes

The FT-IR analysis results of raw and acid treatment multi-walled carbon nanotubes for 4h

are shown in following figures, respectively.

(Take in Figure 1)

From the figures, it was clear that the FT-IR spectra of raw MWNTs showed peaks with very

low intensity at around 3440, 1711, and 1213cm -1, corresponding to OH, C=O, and C-C-O

stretching present in carboxylic groups (COOH), respectively. In the case of acid treated

MWNTs, these characteristic bands appeared with significantly higher intensity. This was

attributed to the increased number of carboxylic acid groups which had been successfully

attached to the surface of the MWNTs after acid treatment in H 2SO4/HNO 3 mixture. And

the peak at 1570cm -1 was attributed to the vibration of carbon skeleton (C-C stretch) of the

bulk MWNTs. Therefore, it was considered that carboxylic groups (-COOH) had been

attached onto the surface of the MWNTs by acid treatment successfully.

X-ray photoelectron spectroscopy (XPS) analysis of raw and acid treated multi-walled

carbon nanotubes

The XPS spectra of raw and multi-walled carbon nanotubes treated by acid for 4h are shown

in following figures, respectively.

(Take in Figure 2)



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The above figure showed the C(1s) and O(1s) spectra of the raw and acid treated MWNTs.

The atomic concentration of oxygen for a, b are 2.73%, 17.40%. It increased with the acid

treatment to MWNTs due to the carboxylic groups being introduced onto the surface of the

MWNTs.

The high-resolution XPS spectrum of C(1s) and O(1s) for acid treated multi-walled carbon

nanotubes are shown as the follows:

(Take in Figure 3)

The peaks of carbon atoms at 284.38eV, 285.38eV, 286.18eV, 288.88eV is corresponding to the

carbon atoms in C-C stretching, the carbon atoms bonding to carbonyl, the carbon atoms in

C-O stretching and the carbon atoms in C=O stretching, respectively. And also the peaks

of oxygen atoms at 531.14eV and 532.78eV are corresponding to the oxygen atoms in C=O

stretching, the oxygen atoms in C-O-H present in carboxylic groups (COOH), respectively.

Therefore, it was considered that carboxylic groups (-COOH) were introduced onto the

surface of the MWNTs by acid treatment.

From the results of FT-IR and XPS analysis, it was clear that the carboxylic groups (-COOH)

attached onto the surface of the MWNTs by acid treatment, and these groups could be used

to react with toluene-2,4-diisocyanate (TDI) to synthesize multi-walled carbon nanotubes

and polyurethane composites via in situ polymerization. And the acid treated

multi-walled carbon nanotubes could contribute the excellent mechanical, thermal and

electrical properties to the composites.

Mechanical properties analysis of polyurethane and composites

Mechanical properties analysis results of polyurethane and composites are shown in Table 1.

(Take in Table 1)

From the table above it was clear that elastic modulus and tensile strength of multi-walled

carbon nanotubes and polyurethane composites were improved compared to these of

polyurethane. And the composites with 0.03wt% acid treated MWNTs had the max values

of elastic modulus and tensile strength.



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The improved elastic modulus and tensile strength of composites were due to the excellent

mechanical properties of MWNTs, and via in situ polymerization the acid treated MWNTs

and PU could form copolymer, the excellent mechanical properties would perform

successfully. And the composites with 0.03wt% raw MWNTs had the lower values of

elastic modulus and tensile strength compared to the composites with 0.03wt% acid treated

MWNTs, it was because that there were no carboxylic groups (-COOH) attached onto the

surface of the raw MWNTs, the raw MWNTs could not to polymerize with polyurethane in

situ polymerization, so the excellent mechanical properties of MWNTs would not perform

very well. And also it found that higher concentration of MWNTs in the polyurethane

would decrease the elastic modulus and tensile strength of composites, it should beattributed to a decreased cross-linking density caused by addition of fillers for the

polyurethane matrix.

Dynamic mechanical analysis of polyurethane and composites

Dynamic mechanical analysis results of polyurethane and composites (raw and 0.03wt%

acid treated multi-walled carbon nanotubes) are shown in Figure 4 and 5.

(Take in Figure 4 and 5)

Compared to the PU the position of the loss factor (tan ) peak for the composite obviously

shifted toward high temperature, indicating that the glass transition temperature (Tg) of the

polymer was greatly increased. The improvement in Tg was mainly attributed to the

excellent mechanical and thermal properties of MWNTs and their direct connection with the

hard segments of PU. And the composites of acid treated MWNTs had the higher Tg than

the composites of raw MWNts due to the in situ polymerization between acid treated

MWNTs and PU matrix, it was to say that there were no carboxylic groups (-COOH)

attached onto the surface of the raw MWNTs, the raw MWNTs could not to polymerize with

polyurethane in situ polymerization, so the excellent thermal properties of MWNTs would

not perform very well.

From the storage modulus (E ) of PU and composite it is clear that there was some difference

between the storage modulus of PU and composite, above Tg there were no obvious



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difference in storage modulus, while below Tg the MWNTs had strongly influenced on the

elastic properties of the PU matrix with increasing the storage modulus, which may be

caused by extremely high modulus of MWNTs and their interactions with the PU matrix.

And the storage modulus of acid treated MWNTs composites had higher modulus than the

composites of raw MWNTs composites due to the in situ polymerization between acid

treated MWNTs and PU matrix, it was to say that there were no carboxylic groups (-COOH)

attached onto the surface of the raw MWNTs, the raw MWNTs could not to polymerize with

polyurethane in situ polymerization, so the excellent mechanical properties of MWNTs

would not perform very well.

Electrical conductivity properties analysis of polyurethane and composites

Electrical conductivity properties analysis results of polyurethane and composites are shown

in Table 2.

(Take in Table 2)

From the table above it was clear that volume resistivity of multi-walled carbon nanotubes

and polyurethane composites decreased with the concentration of MWNTs increasing

compared to that of polyurethane, it was said that the electrical conductivity properties of

the composites improved with the increasing of the MWNTs concentration due to the

excellent electrical conductivity of MWNTs. And the value of the volume resistivity of

composites with 1wt% acid treated MWNTs was higher than the composites with 1wt% raw

MWNTs, it was because that carboxylic groups (-COOH) attached onto the surface of the

MWNTs by acid treatment were favorable to improve the electrical conductivity of the

composites compared to the raw MWNTs, so the excellent electrical conductivity properties

of MWNTs would perform better.

Conclusion

Multi-walled carbon nanotubes could be modified by acid (H 2SO4/HNO 3 = 3:1 in volume)

treatment at 60 for 4h. After acid treatment carboxylic groups (-COOH) were

successfully introduced onto the surface of multi-walled carbon nanotubes.

Acid treated multi-walled carbon nanotubes and PU could form copolymer successfully via



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in situ polymerization. The mechanical and electrical conductivity of composites were

improved compared to PU, and the glass transition temperature and the storage modulus

were increased also.

Acknowledgements

The authors gratefully acknowledge the financial assistance of Heilongjiang Education

Committee Foundation (Grant No. 11541270) and Heilongjiang Natural Science Committee

Foundation (Grant No. B2007-03).

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Sheme 2 The schematic illustration of polyurethane synthesis

3TDI + TMP80

4hCH 3CH 2 C

CH 2 OC

O

NH CH 3

NCO

CH 2 OC

O

NH CH 3

NCO

CH 2 OC

O

NH CH 3

NCO

+

isocyanates

CH 2OCO CH 2 7 CHCH CH 2 CH

OH

CH 2 5 CH 3

CHOCO CH 27

CH CH CH 2 CH CH 2 5 CH 3OH

CH 2OCO CH 2 7 CHCH CH 2 CH

OH

CH 2 5 CH 3

room temperature

PU



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Sheme 3 The schematic illustration of the synthesis of acid treated multi-walled carbon

nanotubes (MWNTs) and polyurethane composites

COOH

COOH+ TDI + TMP

80

4hMWNTs isocyanates composites

+

CH 2OCO CH 2 7 CH CH CH 2 CH

OH

CH 2 5 CH 3

CHOCO CH 27

CH CH CH 2 CH CH 2 5 CH 3OH

CH 2OCO CH 2 7 CH CH CH 2 CH

OH

CH 2 5 CH 3

room temperature

PU PU PU

MWNTs MWNTs MWNTs



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Figure 1 The FT-IR analysis results of raw and acid treated multi-walled carbon nanotubes

(MWNTs) for 4h

1 5 7 0

35

40

45

50

55

60

65

70

% T

1000200030004000Wavenumber s (c m- 1)

(a) raw MWNTs

1 2 0 9

1 5 7 4

1 7 1 1

3 4 4 6

10

11

12

13

14

15

16

17

18

19

20

21

22

23

% T

100020003000Wavenumber s (c m- 1)

(b) MWNTs treated for 4h at 6 0 by acid



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Figure 2 The XPS spectra of raw and multi-walled carbon nanotubes (MWNTs) treated by

acid for 4h

(a) raw MWNTs

(b) MWNTs treated for 4h at 6 0 by acid

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

01002003004005006007008009001000110012001300

C o u n t s

/ s

Binding Energy (eV)

Survey

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

7.00E+05

01002003004005006007008009001000110012001300

C o u n t s

/ s

Binding Energy (eV)

Survey



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Figure 3 The high-resolution XPS spectrum of C(1s) and O(1s) for acid treated MWNTs

(a) XPS spectrum of C1s of acid treated MWNTs

(b) XPS spectrum of O1s of acid treated MWNTs

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

526528530532534536538540542544

C o u n t s

/ s

( R e s

i d .

1 )

Binding Energy (eV)

O1s Scan

O1s Scan A

O1s Scan B

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

1.40E+05

280282284286288290292294296298

C o u n t s

/ s

( R e s

i d .

0

. 5 )

Binding Energy (eV)

C1s Scan

C1s Scan A

C1s Scan B

C1s Scan CC1s Scan D



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Table 1 Mechanical properties analysis results of polyurethane and composites

Sample Elastic modulus Tensile strengthPolyurethane 204.48 34.55

Acid treated MWNTs (0.01wt%) and PU Composites 307.48 46.44 Acid treated MWNTs (0.03 wt %) and PU Composites 343.14 46.89 Acid treated MWNTs (0.05 wt %) and PU Composites 291.19 41.44 Acid treated MWNTs (0.1 wt %) and PU Composites 268.99 36.72 Acid treated MWNTs (0.2 wt %) and PU Composites 216.42 34.94 Acid treated MWNTs (0.4 wt %) and PU Composites 166.80 34.28 Acid treated MWNTs (1 wt %) and PU Composites 120.21 32.19

Raw MWNTs (0.03 wt %) and PU Composites 300.03 42.41



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Figure 4 Loss factors analysis results of polyurethane and composites

0 20 40 60 80 100 120

0

50

100

150

200

250

T a n

d e

l t a

Temperature( )

a) polyurethane; (b) raw MWNTs and polyurethane composites; (c) acid treated MWNTs and polyurethane composites



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Table 2 Electrical conductivity properties analysis results of polyurethane and composites

Sample Volume resistivity ( cm)Polyurethane 5.1110 13

Acid treated MWNTs (0.01%) and PUComposites 5.031013

Acid treated MWNTs (0.03%) and PUComposites

4.8510 13


4.0510 13


3.2610 13


2.9410 13


1.8710 13

Acid treated MWNTs (1%) and PUComposites

2.2310 12

Raw MWNTs (1%) and PU Composites 4.2610 12


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