preparation and properties of alkaline functionalized carbon nanotubes reinforced polyurethane...

Fibers and Polymers 2013, Vol.14, No.4, 571-577

571

Preparation and Properties of Alkaline Functionalized Carbon Nanotubes

Reinforced Polyurethane Composites

Jingrong Wang1*, Haiping Xu, Dandan Yang, and Yihua Wu

School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University,

Shanghai 201209, P.R. China

(Received March 26, 2012; Revised August 24, 2012; Accepted September 7, 2012)

Abstract: Composites consisting of polyurethane (PU)/carbon nanotubes (CNTs) have been successfully prepared bysolution mixing method. CNTs were modified through mechano-chemical reaction to increase the compatibility with PU viahydrogen bondings. SEM microphotographs proved that modified CNTs (M-CNTs) became shorter and FTIR spectrashowed that hydroxyl groups had been introduced to the surface of M-CNTs. SEM images of PU/M-CNTs composites alsoproved that M-CNTs were effectively dispersed in PU matrix. Mechanical property tests showed that addition of M-CNTscould significantly improve the tensile properties of PU/M-CNTs composite (breaking strength enhancement ratio forcomposite with 5.0 wt% M-CNTs was 103.81 %). The thermal stability of composites with M-CNTs was also improved. Theinitial degradation temperature enhancement was 19.9 oC for the composite with 0.5 wt% M-CNTs. Electrical property testsshowed that the electrical properties were improved by adding M-CNTs. The volume conductivities increased 3 and 5 ordersof magnitude for the composites with 5.0 wt% and 10 wt% M-CNTs, respectively. The addition of M-CNTs had little effecton the elastic properties of the composites.

Keywords: Composite, Carbon nanotubes, Mechano-chemical reaction, Polyurethane

Introduction

Carbon nanotube (CNTs) is a kind of nano-material with

high thermal stability, exceptional mechanical and electrical

property, etc [1-3]. The most important is that the molecular

structure of CNTs is very similar to the polymeric chains of

polymer. CNTs’ flexility is also excellent and they have

good compatibility with polymer. Therefore, one of the most

attractive applications is to incorporate CNTs into polymer

matrix to prepare high performance composites [4]. CNTs

are considered to be the most promising candidates as ideal

fillers to improve mechanical properties, thermal stability,

and electrical conductivity of composites [5-8].

Polyurethane (PU) is an important class of polymer materials

for a variety of applications due to its useful properties such

as fine flexibility, elasticity and damping ability. Nevertheless,

some properties such as thermal stability, electrical and

electrical conductivity, etc, could be improved with the addition

of CNTs. In order to combine the excellent properties of

CNTs and PU, PU/CNTs composites have been intensively

studied [9-12].

However, the intrinsic van der Waals attraction among

CNTs, in combination with their high surface area and high

aspect ratio, often leads to significant agglomeration, thus

hindering the development of homogeneous composites and

preventing efficient transfer of their superior properties to

polymer matrix [13]. Chemical oxidation under ultrasonication

or reflux conditions using mixtures of strong acids has been

shown to be very effective in modifying CNTs. After treatment,

CNTs’ dispersity and the properties of composites can be

improved [14,15]. Sahoo et al. [16] treated CNTs in a mixture

of concentrated H2SO4/HNO3 and prepared PU nano-

composites. They reported the tensile strength of the composite

(2.5 wt% of CNTs) was improved by 37 % compared with

pure PU. But author’s previous research also showed that the

acid treated CNTs were still very long and long CNTs

aggregated seriously in the matrix [17]. Chen et al. [18]

reported a mechano-chemical reaction method to functionalize

the CNTs and the resulted CNTs were further cut into short

ones by mechanical milling technology. Wang et al. [19]

prepared palmitic acid based composite with CNTs under

such treatment. They found that CNTs had been dispersed

well in PA matrix and the enhancement of the thermal

conductivity was about 30 % higher than that of the similar

composite containing CNTs treated by concentrated acid

mixture. However, other properties of the composites were

not shown in this paper.

In the present work, mechano-chemical reaction via

alkaline was employed to treat the pristine CNTs which have

poor dispersibility. PU/CNTs composites were prepared by

solution mixing method. The effects of modified CNTs on

the mechanical, thermal, electrical, and elastic recovery

properties were also studied.

Experimental

Materials

Pristine CNTs (P-CNTs) were commercially supplied by

Chengdu Organic Chemicals Co., Ltd., Chinese Academy of

Sciences. The purity, diameter and average length of the P-

CNTs were 95 wt%, 10-20 nm and 50 um respectively.

Diphenyl methane-4,4’-diisocyanate (MDI) and polytetra-*Corresponding author: [email protected]

DOI 10.1007/s12221-013-0571-z

572 Fibers and Polymers 2013, Vol.14, No.4 Jingrong Wang et al.

hydrofuran (PTHF, MW=2000 g/mol) were received from

BASF Co., Ltd., Shanghai, China. Potassium hydroxide

(KOH), 1,4-butanediol (BD), and N,N-dimethylacetamide

(DMAc) were obtained from Sinopharm Chemical Reagent

Co., Ltd., China.

Modification of P-CNTs

The mechano-chemical reaction treatment described in

Pan et al. [20] was applied to modify P-CNTs to enhance the

CNTs’ dispersibility. In a typical treatment, 1 g of P-CNTs

was mixed with 20 g of potassium hydroxide and the

mixture was ball-milled for 20 h as the rotating rate was

250 r/min. Then, the sample was washed by distilled water

and filtered. The processing was repeated until the used

water became neutral to ensure a complete removal of

potassium hydroxide residues, if any. Modified CNTs (M-

CNTs) were collected and dried at 100oC for 24 h to remove

the distilled water.

Preparation of the Composites

Pure PU was synthesized from its monomers, MDI and

PTHF, in a two-step process using BD as a chain extender. A

mole ratio of 3:1:2 of MDI:PTHF:BD was used, indicating

32 wt% of hard segment content. Here, MDI and BD acted

as hard segment in PU.

For the fabrication of PU/M-CNTs composite films, the

solution mixing method was adopted and the following

procedure was used: the necessary weight fraction of M-

CNTs was first dispersed in DMAC by sonication at room

temperature for 1.5 h using an ultrasonic homogenizer.

Thereafter, PU was added into this solution and the mixture

was stirred for 1.5 h. The mixture was then cast onto a clean

Teflon disk and dried completely in an oven. Free-standing

PU/M-CNTs composite film whose thickness was about

0.2 mm was obtained by peeling it from the disk. In the

composites, the weight ratios of M-CNTs were 0.5 %, 1 %,

5 % and 10 %, respectively. As a control, a pure PU film was

obtained using the same casting process.

Characterization

Scanning electron microscopy (Hitachi S-4800, Japan)

was conducted to observe the micro structure of CNTs and

the surfaces of PU and PU/M-CNTs composites which had

been fractured in liquid nitrogen. FTIR spectra were

recorded with spectrometer (Bruker V70, German) from

4,000 cm-1 to 400 cm-1 with a resolution of 2 cm-1.

Tensile tests were carried out on the universal material

testing machine (WDW3020, China). The specimens used

were dumb-bell samples with the length×width being 50×

5 mm and the thickness was measured by a micrometer. The

gauge length and strain rate were 10 mm and 50 mm/min,

respectively.

Thermogravimetric analysis was performed on a thermo

gravimetric-differential scanning calorimetry analyzer

(STA-449C Jupiter, Germany) heated from 35 to 700 oC at a

heating rate of 10oC/min in nitrogen.

The resistance of the specimen was measured by high

resistance tester (6517A, America). The diameter of the

sample was 10 mm. The volume conductivity was calculated

using the following equation:

In this equation, R, D and d indicates the resistance,

diameter and thickness of the sample, respectively.

The elastic recovery ratios were measured by a self-made

clamp. The samples were rectangular strips and the length

and width of the test specimens were 100 and 2 mm,

respectively. The sample was stretched to 300 %. After

being kept for 1 min, the sample was relaxed for 3 min and

the length of sample was measured. The elastic recovery

ratio was calculated using the following equation:

Elastic recovery ratio=(L1−L2)/(L1−L0) ×100 %

In this equation, L0 indicates the clamping length, L1

indicates the length after elongation and L2 indicates the

length after relaxation.

Results and Discussion

Characterization of M-CNTs

Figure 1 shows the scanning electronic microscope (SEM)

images of P-CNTs and M-CNTs. It is observed from Figure

1(a) that P-CNTs are very long and entangled with each

other. In comparison with the P-CNTs, M-CNTs which have

been cut into several hundred nanometers in length look

shorter and most of them are unentangled as shown in Figure

ρv

πD2R

4d-------------=

Figure 1. SEM images of P-CNTs (a) and M-CNTs (b).

Properties of PU/M-CNTs Composites Fibers and Polymers 2013, Vol.14, No.4 573

1(b). Compared with other modification method such as acid

oxidation method, the length of M-CNTs also became much

shorter after mechano-chemical treatment [16]. It indicates

that mechano-chemical reaction has obvious effect on the

micro-morphology of CNTs.

The chemical structures of P-CNTs and M-CNTs were

studied by micro-FTIR spectroscopy. As can be seen in

Figure 2, M-CNTs show a remarkably different FTIR

spectrum from that of P-CNTs. Consistent with previously

reported data [20,21], no detectable transmission band is

observed for P-CNTs in the wavenumber range covered in

this study. In contrast, the corresponding FTIR spectrum for

M-CNTs shows a very broad transmission band centered at

3,430 cm-1, characteristic of hydrogen bonded -OH. The

absorption peak at 1,170 cm-1 is corresponding to the stretching

band of C-O, while the peak at 1,372 cm-1 can be interpreted

to the bending stretching band of hydroxyl groups. The

strong transmission peak at about 1,580 cm-1 can be attributed

to the vibration of CNTs’ carbon skeleton and the peak at

1,625 cm-1 is assigned to the stretching mode of -C=C- in an

enol form [9,22]. It indicates that the -OH groups have been

introduced onto the M-CNTs’ surface.

As solution mixing method would be employed to prepare

PU/CNTs composite, the dispersion stability of P-CNTs and

M-CNTs in the solvent-DMAc was examined. The P-CNTs

and M-CNTs were respectively dispersed in DMAc under

ultrasonic condition for 2 h and then kept standing for a

certain period of time. The sediment of P-CNTs was observed

in half an hour while M-CNTs had good dispersion stability

after 15 days as shown in Figure 3. It indicates that the

hydroxyl groups on M-CNTs’ surface can improve the

interaction with the solvent. This provides a basis for preparation

of PU/CNTs composites through solution mixing method in

the presence of M-CNTs.

Characterization of PU and PU/M-CNTs Composite

Figure 4 shows the SEM images of PU and PU/M-CNTs

composites containing 5 and 10 wt% M-CNTs, respectively.

From the SEM images, M-CNTs’ dispersion in the polymer

matrix can be clearly observed with low magnification.

Figure 2. FTIR spectra of P-CNTs and M-CNTs.

Figure 3. Dispersion stability of CNTs in solvent-DMA; (a) P-

CNTs kept standing for 30 min and (b) M-CNTs kept standing for

15 days.

Figure 4. SEM images of PU and PU/M-CNTs composite with

different weight ratios of M-CNTs; (a) PU, (b) PU/5%M-CNTs,

and (c) PU/10%M-CNTs.


Figure 4(a) represents PU matrix having no any filler. From

Figure 4(b), it can be seen that M-CNTs are uniformly

dispersed in the polymer matrix in which 5 wt% M-CNTs

were added. It is important that all of the M-CNTs are

individual and there are no entanglements and agglomerations.

As the weight ratio of M-CNTs is increased to 10 %,

individual M-CNTs also can be clearly discerned and only a

small cluster is found in the composite as shown in Figure

4(c).

The FTIR of PU and PU/M-CNTs composite containing

5 wt% M-CNTs are also shown in Figure 5. It can be seen

from Figure 5 that FTIR spectra of these two materials are

very similar and there are no new bands appearing. But from

the magnified inset, we can see that the peak intensity at

1,700 cm-1 of the PU/M-CNTs composite is much stronger

than that of PU as the peak intensity at 1,730 cm-1 are close.

According to the reference, the band centered at around

1,700 cm-1 is assigned to hydrogen-bonded urethane carbonyl

groups while the band at 1,730 cm-1 is attributed to free

urethane carbonyl groups [23,24]. It indicates that solution

mixing method doesn’t change the chemical structure of the

composite, but increases the amount of the hydrogen bondings.

The increased hydrogen bondings may be formed by the

physical adhesion between the -OH group of M-CNTs and

>C=O group of the PU matrix and enhance the interfacial

interaction between the surface of M-CNTs with the hard

segment of PU. The increased hydrogen bonding degree of

composite would help improve microphase separation structures

within PU matrix and enhance the material properties of PU/

M-CNT composite as follows [25].

Properties of PU and PU/M-CNTs Composites

As PU is a kind of self-strengthened material, normal

fillers have no effect on its’ tensile strength. Nevertheless,

CNTs can increase the mechanical properties of the composites.

Figure 6 presents the stress-strain curves of PU and PU/M-

CNTs composites with different weight ratios of M-CNTs. It

is clearly observed that the breaking strength of the

composites is increased compared to the corresponding value

of PU. There are two reasons to explain the phenomenon.

One is that the CNTs’ theoretical strength is up to 10 GPa

which is 100 times higher than steel’s. The other one is that

the M-CNTs which contain many hydroxyl groups form

hydrogen bondings with urethane carboxyl groups. The

hydroxyl groups can transfer the outer energy to M-CNTs

and thus the composites can make use of the high mechanical

property of M-CNTs. Figure 6 also shows that the breaking

elongation of the samples is lower in the presence of M-

CNTs, which is probably because that the M-CNTs in the

matrix easily become the stress concentration points [26].

The breaking elongation of the composite with 10 wt% M-

CNTs is mostly decreased, but it is still higher than 550 %

and this value remains in the range of high elongation-at-

break.

In order to show clearly the effect for the mechanical

property with adding M-CNTs to PU, the breaking strength

enhancement ratios of the PU/M-CNTs composites have

also been calculated as shown in Figure 7. kc and k0 represent

the breaking strength of composite and PU, respectively.

(kc−k0)/k0 is breaking strength enhancement ratio of the

composite. It indicates that the breaking strength enhancement

ratios of the PU/M-CNTs composites firstly increase then

decrease with the mass fractions of the M-CNTs. For

example, the breaking strength enhancement ratio is 103.81 %

for the composite with 5 wt% M-CNTs while it is 70.17 %

for the composite containing 10 wt% M-CNTs. It indicates

that the M-CNTs can improve the composites’ mechanical

properties, but the degree of improvement is limited. It can

be explained from the distribution of M-CNTs in the matrix.

From the images of the composites as shown in Figure 4, all

Figure 5. FTIR spectra of PU and PU/M-CNTs composite

containing 5 wt% M-CNTs.

Figure 6. Stress-strain profiles of PU and PU/M-CNTs composites

with different weight ratios of M-CNTs.


of the M-CNTs are individual and dispersed evenly in the

matrix for the composite containing 5 wt% M-CNTs while

there is a small cluster in local matrix for the composite

containing 10 wt% M-CNTs. The agglomerates may reduce

the reinforcing effect of the M-CNTs as they are acting as

flaws in the matrix. The result has shown that the mechanical

properties of the composites are not only affected by the

amount of M-CNTs in polymer matrix, but also affected by

dispersion degree [16].

In our previous study, we prepared PU/CNTs composite in

which the pristine CNTs were pretreated by chemical

oxidation based on a concentrated acid mixture and the

CNTs mass fraction is 5 % [27]. Here we present the

mechanical property of the composite from Wang et al.

together with the measured data in this study. It is obvious

that the prepared PU/M-CNTs composites in this study have

much higher breaking strength than the composite described

in Wang et al. For example, the breaking strength enhancement

ratio for the prepared PU/M-CNTs composite containing

5 wt% M-CNTs in this study is 103.81 % while it is only

8.8 % for the composite containing the same amount CNTs

described in Wang et al. The marked discrepancy in the

breaking strength enhancement in these two composites

might be ascribed to the different length of the CNTs. As we

know that carboxylic acid groups are generated during the

acid treatment, which can enhance the dispersity of the

CNTs in the organic solvent, but the treatment has little

effect on the length of CNTs. Longer CNTs are easily

entangled with each other and agglomerated especially

during the solvent evaporation.

All polymers have a certain limit of service temperatures.

Improving the thermal stability can widen their application

areas. Figure 8 presents the thermal degradation behavior of

PU/M-CNTs composites with different M-CNTs contents.

Table 1 shows the initial degradation temperatures and the

residue ratios at 700oC of the samples. From Figure 8 and

Table 1, we can see that the initial degradation temperature is

increased. When M-CNTs content is 0.5 wt%, the initial

degradation temperature can be enhanced by 19.9 oC (from

280.8 to 300.7oC). It indicates that adding M-CNTs can

improve the composites’ thermal stability. The improvement

of the composites’ thermal stability might be attributed to

the hydroxyl bondings of the composites and the CNTs

themselves whose thermal stability can be up to 2800oC in

vacuum. Table 1 also indicates that the weight retention of

the composites at 700oC increases with the increase of M-

CNTs’ weight ratios. It is because that the M-CNTs can not

be decomposed at 700 oC. It is also observed that weight

retention is a little more than the theoretically retention. It is

probably because that some macromolecular segment may

enter the opened tubes of the cut M-CNTs or entangled

around the M-CNTs’ surface and such segments may have

been retained in the residue.

The effect of the weight ratios of M-CNTs on the electrical

properties of the composites is shown in Table 2. From the

table, we can see that the volume conductivities of the

composites increase with the increase of the weight ratios of

Figure 7. Breaking strength enhancement ratios of PU/M-CNTs

composites with different weight ratios of M-CNTs.

Figure 8. Thermal degradation curves of PU and PU/M-CNTs

composites with different weight ratios of M-CNTs.

Table 1. Initial degradation temperatures and weight retention at

700 oC of PU/M-CNTs composites with different weight ratios of

M-CNTs

Weight ratio of

M-CNTs (%)

Initial degradation

temperature (oC)

Weight retention at

700 oC (%)

0 280.8 5.51

0.5 300.7 7.57

1 300.7 9.15

5 295.7 12.98

10 300.7 18.70


the M-CNTs. As the weight ratios of the M-CNTs are 5 %

and 10 %, the volume conductivities increase 3 and 5 order

of magnitude, respectively. It is concluded that M-CNTs can

improve the composites’ electrical properties. It is probably

because that CNTs have better electrical conductivity than

copper as well as they have excellent compatibility with

polymer. Therefore, comparing with other inorganic fillers,

the electrical property of polymer with less CNTs can be

improved largely.

The elastic recovery ratios of PU and PU/M-CNTs

composites including different amounts of M-CNTs were

measured as the samples were stretched 300 % for 1 min.

The results are shown in Table 3 and we can see that all of

the elastic recovery ratios are more than 95 %. It indicates

that the M-CNTs have little effect on the composites’

application.

Conclusion

After modified by mechano-chemical reaction, CNTs

become shorter and are added hydroxyl groups onto their

surface. PU/M-CNTs composites were prepared by solution

method. M-CNTs can be well dispersed in the matrix as well

as hydrogen bondings have been formed in the composites.

M-CNTs have remarkable effect on the mechanical properties

of the composites (103.81 % in breaking strength enhancement

ratio for the composite with 5.0 wt% M-CNTs). Thermal

property tests show that adding M-CNTs improves the

composites’ thermal stability (the initial degradation temperature

is increased by 19.9oC when content of M-CNTs is 0.5 wt%).

M-CNTs can also increase the composites’ electrical properties

and have little effect on the elastic properties of the

composites.

Acknowledgements

This work was financially supported by Leading Academic

Discipline Project of Shanghai Municipal Education

Commission (J51803), Basic Key Research Programs of

Science and Technology Commission Foundation of Shanghai

City (09JC1406700), Natural Science Foundation of Shanghai

(11ZR141350), National Natural Science Foundation of

China (51207085) and Innovation Key Program of Shanghai

Municipal Education Commission (13ZZ140).

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Table 2. Electrical properties of PU/M-CNTs composites with

different weight ratios of M-CNTs

Weight ratio of M-CNTs (%) Volume conductivity (S/cm)

0 9.0×10-11

0.5 8.9×10-10

1 2.3×10-9

5 2.1×10-8

10 1.2×10-6

Table 3. Elastic recovery ratios of PU/M-CNTs composites with

different weight ratios of M-CNTs

Weight ratio of M-CNTs (%) Elastic recovery ratio (%)

0 96.09

0.5 96.07

1 95.97

5 95.68

10 95.28


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