microstructure and mechanical properties of carboxylated-cnt-pla comp

http://jcm.sagepub.comJournal of Composite Materials

DOI: 10.1177/0021998308092208 2008; 42; 1587 Journal of Composite Materials

Jiangtao Feng, Jiehe Sui, Wei Cai and Zhiyong Gao Nanotubes/Poly(L-lactic acid) Composite

Microstructure and Mechanical Properties of Carboxylated Carbon

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Microstructure and MechanicalProperties of Carboxylated Carbon

Nanotubes/Poly(L-lactic acid) Composite

JIANGTAO FENG, JIEHE SUI, WEI CAI* AND ZHIYONG GAOSchool of Material Science and Engineering

Harbin Institute of Technology, Harbin, China

ABSTRACT: Poly(L-lactic acid) (PLLA)/MWNTs composites were prepared bymixing solubilized PLLA with solutions of MWNTs treated by four kind of acids.Fourier transform infrared (FT-IR) spectra revealed that carboxyl groups weregrafted to the surface of MWNTs. The water solubility showed that the MWNTstreated by HNO3/H2O2 and HNO3/H2SO4 could suspend in the air at roomtemperature for more than 100 days. Thermogravimetric analysis (TGA) showedthat MWNTs treated by HNO3/H2O2 and HNO3/H2SO4 obtained relatively highCOOH content. Mechanical properties of composites showed that the Youngsmodulus of the carboxylated MWNTs/PLLA composites increased compared to thepure PLLA. Scanning electron microscopy (SEM) images of fracture morphologyconfirmed that the dispersion of carboxylated MWNTs was more homogeneous thanthe pristine MWNTs in polymer matrix. MWNTs treated by HNO3/H2O2 could getmore COOH group and less damage.

KEY WORDS: carbon nanotubes, poly(L-lactic acid) (PLLA), composite materials,mechanical properties.

INTRODUCTION

SINCE THE CARBON nanotubes (CNTs) were discovered by Iijima 1991 [1], they havebeen considered as ideal reinforcing fillers for polymer matrixes to achieve highperformance and multi-functions because of their nanometer size, high aspect ratio,extraordinary mechanical strength, and high eletrical conductivity [24].Considering the practical applications of carbon nanotubes, it is predicted that the area of

largest consumption of carbon nanotubes will be as one of the most promising candidatesfor the design of novel polymer composites [5]. CNTs/polymer composites could be usedas a scaffold for the tissue engineer in osteoblast proliferation and, bone formation [6,7].Li et al. [8] fabricated the composite based on CNTs/polycarbonate microfibrils reinforcedpolyethylene, and the results showed that the tensile strength increased from 20 to

*Author to whom correspondence should be addressed. E-mail: [email protected] 14 appear in color online: http://jcm.sagepub.com

Journal of COMPOSITE MATERIALS, Vol. 42, No. 16/2008 1587

0021-9983/08/16 15879 $10.00/0 DOI: 10.1177/0021998308092208 SAGE Publications 2008

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22.6MPa and the elastic modulus increased from 766 to 1197MPa. Kuan et al. [9] reportedthat the tensile strength of multiwalled carbon nanotubes reinforced polyurethane increasedby 370% compared to pure polyurethane. Blake et al. [10] functionalized MWNTs withchlorinated polypropylene (CPP), the following addition of the CPPMWNTs to the CPPpolymer matrix resulted in a significant increase of mechanical properties.Poly(L-lactic acid) (PLLA) is a linear aliphatic thermoplastic polyester, which has good

thermal plasticity, biodegradabililty, and biocompatibility. There is great interest in the useof PLLA in implant materials, drug delivery systems, and packaging [11,12]. However, oneof the drawbacks for PLLA is that the mechanical properties alone are insufficient for highload bearing applications [13]. To overcome the drawback, many approaches were carriedout to improve the mechanical properties of PLLA. Biphasic calcium phosphate (BCP) wasused as reinforcement to increase the mechanical properties of PLLA [14]. Shikinami et al.[15] have applied hydroxyapatite (HA) as filler to improve the mechanical properties ofPLLA. Zhang et al. [16] prepared PLLA/bioglass composites by phase separation ofpolymer solutions, and the elastic modulus of composites increased with the increase of glasscontent. The concept of adding fillers as reinforcement has been applied in an endeavour toproduce high strength composite. In this article, CNTs were used as filler in an attempt toimprove themechanical properties of PLLA.At present, the liquid oxiationmethod (usuallyrefluxing in nitric acid) is used to functionalize CNTs for attaching a variety of functionalgroups to increase the solubility in the organic solvent or polymer matirix [1719]. Consi-dering that the pristine CNTs easily entangle, different acids were used to treat the CNTs forobtaining fine dispersion of CNTs in polymer matrix. The CNTs/PLLA composites wereprepared by solution casting. The corresponding characterization and mechanical proper-ties were investigated by FT-IR, SEM, and tensile test. The dispersion of CNTs in polymermatrix and the interaction between carbon nanotubes and PLLA were also clarified.

METHOD

Materials

Multi-walled carbon nanotubes (MWNTs) were purchased from the Nanotech PortCompany, Shenzhen, China. (The diameter is 510 nm, the length is 510 mm, and specificsurface area is 40300m2/g). L-Lactic acid (PURAC Biochem.Spain) was used as received.Stannous octoate (Shanghai Chemical Reagent Company, China) was used as a catalyst.Chloroform and methanol were purchased from Kermel of China as analytic reagent.

Preparation of MWNTsCOOH/PLLA Composites

POLYMERIZATION OF PLLAL-Lactide was prepared by polycondensation of L-lactic acid to give lowmolecular weight

poly (L-lactic acid) which was subsequently depolymerized under vacuum to yield the rawlactide [20]. The lactide was purified by recrystallization from ethyl acetate. PLLA wassynthesized by ring-opening polymerization of L-lactide in the melt using stannous octoateas initiator. A typical procedure was followed: the prepared lactide (7 g) was placed intoa flask. 0.006mol stannous octoate was injected into the flask using a syringe. Then, theflask was sealed under vaccum and heated in an oil bath at 1308C for 48 h. The resultedproduct was dissovled in chloroform, precipitated in methanol, and dried under vacuum.

1588 J. FENG ET AL.


CARBOXYLATION OF MWNTsTypically, 40mL of HNO3 was injected into a 500mL flask loaded with 50mg pristine

MWNTs. And then, the mixture was stirred for 30min, sonicated in an ultrasonic bath(100 kHz) for 1 h and refluxed at 908C for 8 h to obtain MWNTs suspenstion solution. Thesolution was washed with deionized water until pH approaching approximately 7, anddried under vaccum at 608C for 8 h to obtain carboxylated MWNTs (MWNTsCOOH).The other MWNTsCOOH, where pristine MWNTs were treated by HNO3/H2SO4 (1 : 1by volume), HNO3/H2O2 (1 : 1 by volume) and HNO3/HCl (1 : 3 by volume), wereprepared in the same way.

FABRICATION OF MWNTsCOOH/PLLA COMPOSITESComposites were prepared by solution casting. A typical proceture for the MWNTs

COOH/PLLA composite was described as follows: PLLA was mixed with 3wt% ofMWNTsCOOH in chloroform at room temperature. The mixture was ultrasonicated for30min, then cast into glass mold and dried extensively at room temperature. As acomparation, the composite of pristine MWNTs/PLLA and pure PLLA were fabricatedby the same way.

Characterization

FT-IR spectra of MWNTs and MWNTsCOOH were recorded on a Perkin ElmerSpectrum One in the 4504000 cm1 region using KBr pellets. To investigate thehydrophilic property, the MWNTsCOOH and pristine MWNTs were ultrasonicated indeionized water for 30min, then remained at room temperature. The mechanicalproperties of composites as well as PLLA were performed by using a microcontrolelectronic tensionmeter (model WDW3100) according to the Chinese GB/T 5281998standard. The sample used was a dog-bone type dumb-bell sample. The gauge length andstrain rate were 16mm and 2.4mm/min, respectively. The dispersion of MWNTs inpolymer matrix was observed by SEM.

RESULTS AND DISCUSSION

Carboxylated MWNTs

Figure 1 shows the FT-IR spectra of pristine MWNTs and the MWNTsCOOH. Exceptthe MWNTsCOOH (treated by HNO3/HCl), the peaks of MWNTsCOOH at 1714 and3439 cm1 are attributed to CO stretching mode and OH stretching of carboxylic acidswith intermolecular hydrogen bonds, respectively [21], as shown Figure 1(b), (d) and (e).Even if the peaks of the HNO3/HCl treated MWNTs at 3439 cm

1 are not obvious,the presence of COOH groups are also manifested by the peak of 1714 cm1. Therefore,the presented spectrospcopic data makes clear that the COOH groups have beenintroduced onto the surface of MWNTs by different acids treatment.

The Hydrophilic Property of MWNTsCOOH

Figure 2 shows the hydrophilic property of MWNTsCOOH and pristine MWNTs indeionized water. The pristine MWNTs and MWNTsCOOH treated by HNO3/HCl begin

Properties of Carboxylated Carbon Nanotubes/Poly(L-lactic acid) Composite 1589


HNO3 HNO3 and HCl HNO3 and H2SO4 HNO3 and H2O2 CNT



(a)

(b)

(c)

Figure 2. Hydrophilic property of pristine MWNTs and MWNTsCOOH, (a) 8th; (b) 10 days and (c) 100 days.

2000 2500 3000 3500 4000Wavenumber (cm1)

e: MWNTs (HNO3/H2SO4)d: MWNTs (HNO3)c: MWNTs (HNO3/HCl)b: MWNTs (HNO3/H2O2)

a

e

d

b

c

1714 C=O 3439 OH

a: Pristine MWNTs

Tran

smitt

ance

Figure 1. FTIR spectra of pristine MWNTs and MWNTsCOOH.

1590 J. FENG ET AL.


to precipitate at the eighth hour. Then, 10 days later the MWNTsCOOH treated byHNO3 also precipitate. The MWNTsCOOH treated by HNO3/H2SO4 and HNO3/H2O2could suspend homogeneously in the deionized water for more than 100 days. It can beinferred from the following:

1. The existence of carboxy group. The COOH content of carboxylated MWNTs isdetermined by TGA (Figure 3). As shown in Figure 3, there is a continuous weight lossof all the carboxlated MWNTs, and the weight loss happens in the temperature rangefrom 200 to 5008C.The contents of COOH for MWNTsCOOH (HNO3/H2O2) andMWNTsCOOH (HNO3/H2SO4) are 17 and 16%, respectively.

2. The different length of MWNTsCOOH after being processed in different acidsolutions also lead to the difference in solubility.

Mechanical Properties

The stressstrain curves of pure PLLA, pristine MWNTs/PLLA, and MWNTsCOOH/PLLA composites are shown in Figure 4 and the characterized values of mechanicalproperties are listed in the Table 1. We prepare and measure three samples for each kind ofacid treatedMWNTs/PLLA composite, pristineMWNTs/PLLA composite, as well as purePLLA, and calculate the average value. Except for the composite of pristine MWNTs/PLLA, the Youngs modulus of PLLA is increased with the addition ofMWNTsCOOH incontrast to the pure PLLA. Specifically, the MWNTsCOOH (HNO3/H2SO4)/PLLAcomposite and MWNTsCOOH (HNO3/H2O2)/PLLA composite have a relatively highYoungs modulus with a value of 394 and 388MPa, separately. The ultimate tensile strength

0 100 200 300 400 500 600 700

0

50

100

pristine MWNTs

Temperature (C)

Wei

ght l

oss

(%)

MWNTs (HNO3)MWNTs (HNO3/HCl)

MWNTs (HNO3/H2SO4)MWNTs (HNO3/H2O2)

Figure 3. TGA traces of pristine MWNTs and MWNTsCOOH.



for the pristineMWNTs/PLLA composite andMWNTsCOOH/PLLA composites are notimproved, compared to the pure PLLA.Whereas, the ultimate tensile strength ofMWNTsCOOH (HNO3/H2O2) reaches 38.8MPa, which is the highest value except the pure PLLA.It is possible that the acid treatment changes the physical structure of MWNTs and deducesthe intrinsic mechanical properties of MWNTs, although the COOH on the surface ofMWNTs could anchor the polymer matrix. Alternatively, it can be inferred thatMWNTsCOOH (HNO3/H2O2) has less damage and more carboxy.The state of dispersion of MWNTs in PLLA matrix observed by SEM is shown in

Figure 5. The pristineMWNTs disperse randomly and are entangled on the fracture surfacein the PLLA matrix (Figure 5(a)), which leads to the decrease of Youngs modulus ofpristine MWNTs/PLLA composite. Figure 5(be) show that MWNTsCOOH dispersehomogeneously in PLLA matrix, which result in higher Youngs modulus than that of purePLLA. It can be inferred that carboxy groups on the MWNTs reduce the surfacial free

0 50 100 150 200 250

0

5

10

15

20

25

30

35

40

45

50MWNTs (HNO3/H2SO4) /PLLA

Stre

ss (M

Pa)

Strain (%)

MWNTs (HNO3/H2O2) /PLLAMWNTs (HNO3) /PLLAMWNTs/PLLA

MWNTs (HNO3/HCl) /PLLA PLLA

Figure 4. Stessstrain curve of PLLA, pristine MWNTs/PLLA and MWNTsCOOH/PLLA composites.

Table 1. Mechanical properties of PLLA andMWNTs/PLLA composites.

CompositeUltimate tensilestrength (MPa)

Modulus(MPa)

PLLA 40.9 234PLLA/CNTs 27.5 138PLLA/CNTs(HNO3/H2SO4) 36.6 394PLLA/CNTs(HNO3/H2O2) 38.8 388PLLA/CNTs(HNO3/HCl) 36.5 381PLLA/CNTs(HNO3) 35.4 320

1592 J. FENG ET AL.


energy of MWNTs through steric effect. It can be seen from Figure 5 that MWNTsCOOHare pulled out from the PLLA matrix in the process of tensile testing and the weakinteractions between fillers and polymer matrix always lead to a decrease of tensile strength.

CONCLUSIONS

The carboxyl groups have been successfully introduced onto the surface of MWNTs byacid treatment. The MWNTsCOOH treated by HNO3/H2SO4 and HNO3/H2O2 couldsuspend homogeneously in deionized water for more than 100 days. The acid treated

(a) (b)

(c) (d)

(e)

Figure 5. SEM of fracture morphology of (a) pristine MWNTs/PLLA composite, (b) MWNTsCOOH (HNO3/H2O2)/PLLA composite, (c) MWNTsCOOH (HNO3/H2SO4)/PLLA composite, (d) MWNTsCOOH (HNO3/HCl)/PLLA composite and (e) MWNTsCOOH (HNO3)/PLLA composite.



MWNTs could get good dispersion in the PLLA matrix, while the interaction force isweak between PLLA and MWNTsCOOH. The acid of HNO3/H2O2 has relativelypositive effects on the dispersion of MWNTs and interaction between MWNTs andpolymer matrix.

ACKNOWLEDGMENT

The authors acknowledge the financial support for the research from the National BasicResearch Program of China (No. 2006CB708609).

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microstructure and mechanical properties of carboxylated-cnt-pla comp

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