Effect of Phenol Functionalized Carbon Nanotube onMechanical, Dynamic Mechanical, and ThermalProperties of Isotactic Polypropylene Nanocomposites

Selvin Thomas P.,1 Salihu Adamu Girei,1 Abdulhadi A. Al-Juhani,1 Khaled Mezghani,2 S.K. De,1

Muataz Ali Atieh1,3

1 Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, 31261 Dhahran,Kingdom of Saudi Arabia

2 Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, 31261 Dhahran,Kingdom of Saudi Arabia

3 Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals,31261 Dhahran, Kingdom of Saudi Arabia

In this work, multiwall carbon nanotubes (MWCNT)were functionalized with phenol and characterized byusing Fourier transform infrared spectroscopy (FTIR).Isotactic polypropylene (iPP)/MWCNT composites ofboth the unfunctionalized and functionalized MWCNTwere prepared by melt blending in a miniextruder atdifferent loadings of nanotubes (i.e., 0.1, 0.25, 1.0, and5.0 wt%). The tensile properties of the compositeswere found to increase with increase in nanotube load-ing with a maximum in Young’s modulus beingachieved at 1.0 wt% loading of phenol functionalizedMWCNT. The differential scanning calorimetry (DSC)studies reveal the nucleating effect of MWCNT on thecrystallization of iPP. Percentage crystallinity wasfound to increase on phenol functionalization ofMWCNT. Results of X-ray diffraction studies of thecomposites are in conformity with that of DSC studies.Dynamic mechanical studies reveal that the functional-ized MWCNT causes many fold increase in the storagemodulus, and the effect is pronounced in the case offunctionalized MWCNT. POLYM. ENG. SCI., 52:525–531,2012. ª 2011 Society of Plastics Engineers

INTRODUCTION

A great number of interesting carbon nanotube (CNT)

applications are related to polymer nanocomposites. Thus,

many efforts have been carried out to enhance nanocompo-

site preparation methods aiming the production of homoge-

nous materials with desirable properties. CNT surface

reactivity and polymer–nanotube interaction have been

improved by two main techniques, such as chemical func-

tionalization (by elemental doping and/or functional group

attachment) and surface polymer grafting. The ability to

manipulate CNTs enables their uniform dispersion in

solvents, the subsequent functionalization reactions, and,

the compatibilization with polymer, ceramic, or metallic

matrices for the fabrication novel composites [1–4].

Chemical functionalization of CNTs can be divided

into covalent, noncovalent, and other types of reactions

(endohedral). Among covalent functionalization methods,

two main strategies are used: modifications based on sur-

face-bound carboxylic acids on carbon nnaotubes and

direct sidewall modification [5, 6]. Oxidation, reduction,

and additions reactions of the nanotubes are reported in

the literature. The oxidation and subsequent functionaliza-

tion is a well-known route for surface functionalization of

nanotubes. In the first steps of oxidation, the strong

oxidizing agent intercalates reversibly in the nanotube

bundles; using longer oxidation times, the reactant disor-

ders and exfoliates the ropes into individual tubes.

Finally, the extremely aggressive reagents can disrupt the

aromatic ring system of the single wall nanotubes and

multiwall nanotubes. In particular, concentrated acid

solutions (nitric acid, HNO3; sulphuric acid, H2SO4;

hydrochloric acid, HCl; among others and their mixtures)

produce functionalized sites (such as carboxylic, sulfuric,

anhydride, ester, and hydroxyl etc), defects in hexagonal

lattice, and open ends in the nanotubes. Furthermore, the

strong acid treatments with the aid of sonication produces

Correspondence to: S.K. De or Muataz Ali Atieh; e-mail: sadhan@kfupm.

edu.sa, e-mail: motazali@kfupm.edu.sa

Contract grant sponsor: King Abdulaziz City for Science and Technology

(KACST), King Fahd University of petroleum and Minerals (KFUPM);

contract grant number: ARP-28-118.

DOI 10.1002/pen.22113

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

damage on the surface and results in the cutting of the

tubes. In addition, the temperature rise in acid reactions

increases the velocity and extent of structural damage

within nanotubes. The acid treatment is seen as a prelimi-

nary step to increase the CNT chemical reactivity by the

introduction of functional groups labile for posterior mod-

ification [7–9]. The carboxylic acid functional group can

be reacted with various compounds such as thionyl chlo-

ride for activation with subsequent reaction with groups

such as amines, alkyl amines, aniline derivatives, and

phenyl groups. The induced structural modification can

affect the mechanical, electrical, and thermal properties of

CNTs, thereby affect their applications in polymer com-

posites [10, 11].

This work reports the results of studies on phenolic func-

tionalization of CNTs and its effect on the mechanical,

dynamic mechanical, and crystallization characteristics of

polypropylene (PP).

EXPERIMENTAL

Materials

Commercially available isotactic polypropylene (iPP;

P510P, SABIC, Saudi Arabia) with density of 905 kg/m3

and MFR of 12 g/10 min (ASTM D4101-10) was used as

the matrix. Multiwall carbon nanotubes (MWCNT) with

nanotubes diameter of 20–40 nm, length 10–50 lm,

specific area 230 m2/g, and purity of more than 95%

were purchased from Nanostructure and Amorphous

Material, USA.

Phenol Functionalization of MWCNT

The phenol functionalization was done in two steps.

Treatment of multiwall carbon nanotubes (MWCNT) with

conc. HNO3: 1:10 ratio by weight of MWCNT to concen-

trated nitric acid were mixed in a round-bottomed flask

and refluxed at 1308C for 48 h with continuous stirring to

obtain acid-treated MWCNT. Upon cooling, the mixture

was thoroughly washed with deionized water to remove

the last traces of unreacted acid until the pH value was 7.

Treatment of acid-functionalized MWCNT with phe-

nol: acid-treated MWCNT was reacted with phenol in the

presence of a few drops of sulfuric acid at a MWCNT to

phenol ratio of 1:10 by weight at 428C, which is the melt-

ing temperature of phenol. The mixture was stirred for 6

h to yield phenol-functionalized MWCNT, hereafter

referred to as f-MWCNT (Scheme 1). Any traces of

unreacted sulfuric acid and phenol were removed via

repeated washing with deionized water and toluene,

respectively.

Preparation of MWCNT/iPP Composites

Isotactic polypropylene (iPP)/multiwall carbon nano-

tubes (MWCNT) nanocomposites were prepared by dry

blending of iPP and using different loadings (0.1, 0.25,

1.0, and 5.0 wt%) of the both modified and unmodified

MWCNT. The premix was then fed into a Haake mini

extruder and mixed for about 10 min at 60 rpm at 1908C.The mixed samples were compression molded under a

pressure of about 9 tons at 1908C for 15 min using a

Carver hydraulic hot press.

Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy (FTIR) spectra

were recorded between 400 and 4000 cm21 using Nicolet

6700 FITR spectrometer from the Nicolet Instrument Cor-

poration, USA. FTIR samples were prepared by grinding

dried CNT together with potassium bromide (KBr) to

make a pellet.

Scanning Electron Microscopy

The scanning electron microscopy study of the

carbon nanotubes and iPP/MWCNT nanocomposites

sample was conducted using scanning electron micro-

scope (SEM) Model JEOL JSM-840A using gold-coated

samples.

Differential Scanning Calorimetry

Nonisothermal differential scanning calorimetry

(DSC) analysis was performed using TA Q1000 instru-

ment equipped with liquid nitrogen cooling system and

autosampler. The standard procedure followed in noniso-

thermal scans was as follows: about 8 mg samples were

heated in nonhermetic aluminum pans from 20 to 2508Cat a scan rate of 58C/min and held for 5 min to erase

any thermal history. The samples were then nonisother-

mally crystallized from 250 to 2408C at a cooling rate

of 58C/min. Subsequently, the melting point was meas-

ured by raising the temperature to 2508C at a similar

rate.

X-Ray Diffraction Measurements

Wide-angle X-ray diffraction (WAXD) measurements

were carried out on a Shimadzu X-ray difrractometer

(40 kV and 40 mA) using Ni-filtered Cu Ka radiation

in 0.03 step from 5 to 35 (in 2y) with 15-s standing per

step.

SCHEME 1

526 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

Tensile Testing

The tensile testing was conducted using an Instron

machine (Model 5567) at room temperature with a speed of

5 mm/min according to ASTM-D3638. The test specimen

(dog bone) dimensions were 15 mm 3 3.9 mm 3 1 mm. A

minimum of five samples of each composition were tested.

Dynamic Mechanical Testing

Dynamic mechanical tests were conducted in tension

mode using DMA Q800 (TA instruments) fitted with liq-

uid nitrogen attachment from 260 to 608C at a frequency

of 1 Hz and an amplitude of 10 lm.

RESULTS AND DISCUSSION

Infrared Spectroscopy

Figure 1 shows the FTIR spectra of MWCNT and phe-

nol-modified MWCNT. The IR spectrum for the unmodi-

fied MWCNT shows absorption at (a) 2924 cm21, which

is attributed to symmetric and asymmetric CH2 stretching,

(b) 1698 cm21 assigned to carboxylic C¼¼O stretching for

acidic group, and (c) 1097 cm21 corresponding to C��O

stretch in alcohols (Fig. 1a). The presence of these func-

tional groups on the surface of MWCNT connotes their

introduction during removal of metal catalysts in nano-

tubes purification processes. On the other hand, upon

treatment with HNO3 acid, a new adsorption band ema-

nates with the characteristic absorption peak of carboxylic

acid at 3358 cm21. This peak is usually broad and can

obscure other peaks within the entire region of 3400–

2400 cm21 (Fig. 1b). Moreover, the carboxylic peak

(C¼¼O stretch) attributed to acid treatment was also

observed at 1730 cm21. Treatment of MWCNT-COOH

with phenol gives indicative peaks at 1596 cm21 for the

phenolic group, 1229 cm21 for the ether peak by the

interaction of ��OH of phenol and ��COOH moiety in

MWCNT, and strong peaks at 812, 748, and 692 cm21,

indicating the presence of phenoxide groups formed by

the reaction with COOH and phenol (Fig. 1b).

DSC Measurements

Figure 2 shows the thermograms obtained for iPP/

MWCNT and phenol-modified MWCNT iPP nanocompo-

sites, respectively. It is evident that crystallization of iPP

FIG. 1. FTIR spectra of (a) (1) CNT and (2) acid modified CNT and

(b) phenol funtionalized CNT.

FIG. 2. DSC thermograms of nonisothermal crystallization of PP and

composites of PP with MWCNT and phenol functionalized MWCNT (a)

cooling curves and (b) heating curves.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 527

begins at a higher temperature, and melting occurs at a

lower temperature on incorporation of MWCNT. Further-

more, the broadness of both melting and crystallization

curves decreases on incorporation of MWCNT, indicating

less heterogeneity in the crystallite sizes. This is indica-

tive of enhanced overall crystallization due to the nucleat-

ing effect of MWCNT [12]. Table 1 summarizes the crys-

tallization temperatures of iPP in the neat form and in the

composites, with both unmodified and modified MWCNT.

The DSC measurements were used to determine the

crystallinity (Xc) of the composites (Table 1). The Xc was

evaluated from heat evolved during crystallization (DHc)

using the relationship:

Xc ð%Þ ¼ DHc

ð1� wt%ÞDHm

� �100; (1)

where DHm is the heat of fusion for 100% crystalline PP

(DHm ¼ 209 J/g) and wt% is the weight fraction of

MWCNT in the composites [13]. Figure 3 shows an

increase in degree of crystallinity with the addition of

nanotubes of either type into iPP matrix, thus confirming

that the nanotubes act as nucleation sites. The results are

in agreement with some of the reports [14, 15] but in con-

tradiction to a few others [16, 17], which showed either

decrease or insignificant changes in the degree of crystal-

linity. Results of X-ray diffraction (XRD) studies pre-

sented below are in conformity with the results of DSC

measurements presented here.

XRD Measurements

Figure 3 shows the X-ray diffraction (XRD) pattern of

neat iPP and composites with 0.1 and 1 wt% of CNT

(both unfunctionalized and functionalized). The peaks at

2y ¼ 14.18, 16.98, 18.68, 21.28, 21.98, 25.58, and 28.68correspond to the planes (110), (040), (130), (111), (131,

(041), (150), (060), and (220), respectively, of the a form

of PP [18]. For PP and composites fabricated under the

experimental conditions, the b form of PP (peak at 2y ¼16.28) could hardly be detected. For PP, the (110) peak is

TABLE 1. Crystallization temperature (Tc), crystallization enthalpy DH (J/g), and the degree of crystallinity (Xc) of iPP, iPP/MWCNT, and iPP/

phenol functionalized MWCNT nanocompositesa.

MWCNT loading (wt%) Tc (8C) from DSC Tm (8C) from DSC DH (J/g) from DSC Xc (%) from DSC Xc (%) from XRD

0 119.1 161.1 97.0 45 43

0.10 122.1 (120.2) 161.9 (161.3) 96.3 (101.3) 46 (49) 44 (47)

0.25 125.9 (120.2) 164.4 (161.8) 96.5 (103.6) 46 (50) 45 (48)

1.00 129.4 (124.9) 165.3 (163.1) 102.0 (105.7) 49 (51) 47 (49)

5.00 132.1 (128.4) 165.9 (164.7) 98.7 (103.5) 50 (52) 48 (49)

a The values in parentheses correspond to the composites with phenol functionalized MWCNT.

FIG. 3. X-ray diffraction patterns of PP and composites of PP with (a)

0.1 and (b) 1 wt% loaded MWCNT and functionalized MWCNT.

FIG. 4. Stress–strain plots of PP composites of 0.1 and 1.0 wt% of

untreated and phenol functionalized CNT.

528 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

more intense than the (040) peak; for the various compo-

sites, the intensities of the two peaks are nearly the same.

The degree of crystallinity was calculated from XRD

peaks and is close to what was obtained from DSC meas-

urements (Table 1). The degree of crystallinity of samples

was quantitatively estimated following the method of

Nara and Komiya [19]. A smooth curve that connected

peak baselines was computer-plotted on the diffracto-

grams. The area above the smooth curve was taken as the

crystalline portion, and the lower area between smooth

curve and the linear baseline, which connected the two

points of the intensity 2y of 308 and 108 in the samples,

was taken as the amorphous section. The upper diffraction

peak area and the total diffraction area over the diffrac-

tion angle 10–30 [0] (2y) were integrated using Smad-

chrom software (Morgan and Kennedy Research, Aus-

tralia). The ratio of upper area to total diffraction was

taken as the degree of crystallinity.

The degree of crystallinity was calculated as follows:

Xc ¼ Ac

ðAc þ AaÞ ; (2)

where Xc refers to the degree of crystallinity, Ac refers to

the crystallized area on the X-ray diffractogram, and Aa

refers to the amorphous area on the X-ray diffractogram.

Mechanical Properties

The representative stress–strain plots of the composites

with 0.1 and 1 wt% untreated and phenol functionalized

CNT are given in Fig. 4. It is evident that the phenol-

modified MWCNT has remarkable reinforcing effect on

the mechanical properties of iPP when compared with the

untreated MWCNT. It is obvious that the Young’s modu-

lus and maximum stress increased significantly with the

increase in CNT loading, and the effect is pronounced in

the case of modified MWCNT. The mechanical properties

of iPP and to iPP/MWCNT composites are reported in

Table 2. However, the decrease in modulus at higher

loading of MWCNT is because of the agglomeration of

MWCNT at a loading of 5.0 wt%. Similar phenomena

was reported earlier [20, 21] at 1.0 wt% in the case of

iPP/SWCNT and iPP/grafted MWCNT composites. In

general, the tensile properties were improved with the

addition of phenol-modified MWCNT at all nanotube

loadings. It is apparent that phenol-modified MWCNT

promotes uniform dispersion of MWCNT into the iPP

matrix and helps to prevent agglomeration of the nano-

tubes. Moreover, it is believed that good dispersion of

MWCNT enables efficient load transfer from the polymer

FIG. 5. Plots of storage modulus versus temperature of PP and PP

composites with MWCNT and functionalized MWCNT.

FIG. 6. Plots of (a) loss modulus and (b) tan d versus temperature of

PP and PP composites with MWCNT and functionalized MWCNT.

TABLE 2. Summary of stress–strain properties of iPP and iPP/MWCNT

composites as a function of nanotube loadinga.

MWCNT loading

(wt%)

Young’s

modulus (MPa)

Maximum

stress (MPa)

% Elongation

at break

0 713.1 36.0 26.1

0.10 810.0 (909.5) 35.9 (39.5) 18.7 (20.0)

0.25 899.6 (1147.0) 37.3 (40.3) 16.4 (16.1)

1.00 933.8 (1479.0) 38.8 (43.8) 10.1 (13.2)

5.00 986.3 (1395.0) 43.1 (45.0) 8.1 (6.5)

a The values in parentheses correspond to the composites with phenol

functionalized MWCNT.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 529

matrix to the nanofillers [22, 23], resulting in improved

mechanical properties.

Dynamic Mechanical Properties

Dynamic mechanical properties of the composites with

respect to filler loading are given in Figs. 5 and 6. The

storage modulus increases with the increase in filler

loading, and the effect is pronounced in the case of

f-MWCNT composites. It is believed that the modification

of the MWCNT causes improvement in its dispersion in

the matrix, which, in turn, increases the storage modulus.

Table 3 shows the storage modulus of the 0.1 and 1 wt%

composites at 2208C and 208C. There is a significant

improvement in the storage modulus for the functional-

ized composites at both temperatures. Acid modification

of CNT causes its surface oxidation and the oxidized sites

in turn react with phenolic ��OH groups during function-

alization to form ether bonds. The steric effect of the

bulky phenolic groups facilitates deaggregation of CNT.

In other words, filler–filler interaction decreases on phenol

functionalization, which results in improved CNT disper-

sion in iPP matrix.

Figure 6 shows the loss modulus and tan d peaks

with respect to the filler loading. The loss moduli of the

TABLE 3. Results of dynamic mechanical studiesa.

Loading of

MWCNT

(wt%)

Storage modulus (MPa)

Tg (8C)(Tan d)max

at Tg2208C 208C

0 4556 2628 3.7 0.074

0.1 4776 (5158) 2889 (3161) 5.3 (7.2) 0.067 (0.057)

1.0 4801 (5531) 2618 (2886) 5.9 (8.1) 0.065 (0.054)

a The values in parentheses correspond to the composites with phenol

functionalized MWCNT.

FIG. 7. SEM images of fractured surfaces of (a) iPP, (b), and (c) composites of PP with 0.1 and 1 wt%

loading of MWCNT and (d) and (e) composites of PP with 0.1 and 1 wt% loading of phenol functionalized

MWCNT.

530 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

composites with functionalized MWCNT are higher when

compared with neat PP. In the case of tan d, iPP shows

the maximum value, and, with the incorporation of nano-

tubes, the tan d decreases, indicating better filler matrix

interaction. Table 3 shows the extent of decrease in tan dvalues with respect to the filler loading at the glass transi-

tion temperature. The decrease in tan d is more pro-

nounced in the case of phenol-functionalized MWCNT

composites. The shift in Tg to higher temperature (almost

58C) in the case of phenol functionalized CNT is believed

to be due to improved filler-matrix adhesion.

Microscopic Studies

Figure 7 shows the SEM images of the composites

with 0.1 and 1 wt% loading of both CNT and f-CNT. It

is observed that the fractographs of the composites show

less ductility than neat PP. This corroborates the findings

of stress–strain measurements.

CONCLUSION

Phenol functionalization of CNT results in improvement

in Young’s modulus and dynamic properties of CNT/iPP

composites due to greater dispersion of CNT in the polymer

matrix. Composites at low loadings can be useful under low

strain and in dynamic applications. At high-CNT loadings,

the ductility of the composites registers a drop.

REFERENCES

1. S. Banerjee, T. Hemraj-Benny, and S.S. Won, Adv. Mater.,17, 17 (2005).

2. S.B. Sinnott, J. Nanosci. Nanotechnol., 2, 113 (2002).

3. H. Peng, P. Reverdy, V.N. Khabashesku, and J.L. Margrave,

Chem. Commun., 363 (2003).

4. A. Hirsch, Angew. Chem. Int. Ed., 41, 1853 (2002).

5. Z. Jin, X. Sun, G. Xu, S.H. Goh, and W. Ji, Chem. Phys.Lett., 318, 505 (2000).

6. Y.-T. Shieh, G.-L. Liu, K.-C. Hwang, and C.-C. Chen, Poly-mer, 46, 10945 (2005).

7. Y.-T. Shieh and G.-L. Liu, PMSE Prepr., 90, 437 (2004).

8. M.S.P. Shaffer and K. Koziol, Chem. Commun., 2074 (2002).

9. N. Tsubokawa, Polym. J., 37, 637 (2005).

10. S. S. Sheiko and M. Moller, Chem. Rev., 101, 4099 (2001).

11. D. Tasis, N. TaJgamatarchis, A. Bianco, and M. Prato,

Chem. Rev., 106, 1105 (2006).

12. Z. Wang and D. Xu, Polymer, 49, 330 (2008).

13. J.E. Mark, Physical Properties of Polymers Handbook, AIP,New York (1996).

14. D. Tabuani, W. Granille, G. Camino, and M. Claes, e-Poly-mers, 6, 103 (2008).

15. W.H. Li, X.H. Chen, S.N. Li, L.S. Xu, and Z. Yang, Mater.Sci. Technol., 23, 1181 (2007).

16. A. Funck and W. Kaminsky, Compos. Sci. Technol., 67,906 (2007).

17. M.V. Jose, D. Dean, J. Tyner, G. Price, and E. Nyairo,

J. Appl. Polym. Sci., 45, 1616 (2007).

18. C.A. Mitchell, J.L. Bahr, S. Arepalli, J.M. Tour, and R.

Krishnamoorti, Macromolecules, 35, 8825 (2002).

19. S. Nara and T. Komiya, Starch-Starke, 35, 407 (1983).

20. L. Valentini, J. Biagiotti, J.M. Kenny, and S. Santucci, Com-pos. Sci. Technol., 63, 1149 (2003).

21. H.S. Goh, Y. Bing-Xing, H.J. Shi, Pramoda, Compos. Sci.Technol., 68, 2490 (2008).

22. R. Blake, J.N. Coleman, M.T. Bryne, J.E. McCarthy, T.S.

Perova, W.J. Blau, A. Fonseca, J.B. Nagy, and Y.K.

Gun’ko, J. Mater. Chem., 16, 4206 (2006).

23. B.X. Yang, K.P. Pramoda, G.Q. Xu, and S.H. Goh, Adv.Funct. Mater., 17, 2062 (2007).

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 531