119 Wu Selective Localization of Multi Walled CNT

8/2/2019 119 Wu Selective Localization of Multi Walled CNT

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Selective Localization of Multiwalled Carbon Nanotubes inPoly(-caprolactone)/Polylactide Blend

Defeng Wu,*,, Yisheng Zhang,, Ming Zhang,, and Wei Yu

School of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu 225002, Peoples Republic

of China, Provincial Key Laboratories of Environmental Material and Engineering, Jiangsu 225002,

Peoples Republic of China, and Department of Polymer Science and Engineering, Shanghai Jiaotong

University, Shanghai 200240, Peoples Republic of China

Received October 20, 2008; Revised Manuscript Received December 11, 2008

Poly(-caprolactone)/polylactide blend (PCL/PLA) is an interesting biomaterial because PCL and PLA presentgood complementarity in their physical properties and biodegradability. However, the thermodynamic incompat-

ibility between two component polymers restricts further applications of their blend. In this work, we used

functionalized multiwalled carbon nanotube (MWCNT) to control the morphology of immiscible PCL/PLA blend.

The ternary PCL/PLA/MWCNTs composites were hence prepared by melt mixing for the morphology and the

properties investigation. It is interesting to find that the functionalized MWCNTs are selectively dispersed in the

matrix PCL phase and on the interface between two polymer phases, leading to simultaneous occurrence of

thermodynamically and kinetically driven compatibility. Those interface-localized MWCNTs prevent coalescence

of the discrete domains and enhance the phase interfacial adhesion as well. As a result, the phase morphology of

the ternary composites is improved remarkably in contrast to that of the blank PCL/PLA blend. Owing to thatunique selective interface-localization and improved phase morphology, the ternary composites present far lower

rheological and conductive percolation thresholds than those of the binary composites, and also present extraordinary

mechanical properties even at very low loading levels of the MWCNTs. Therefore, the amphiphilic MWCNTs

are believed to act as the reinforcements as well as the compatibilizer in the immiscible PCL/PLA blend.

1. Introduction

Recently, environmental concerns and a shortage of petroleum

resources have driven efforts aimed at bulk production of

biodegradable polymers such as aliphatic polyesters, polysac-

charides, unsaturated polyester, polyvinylalcohols, and modified

polyolefins.1 Among them, poly(-caprolactone) (PCL) hasreceived much attention as a new aliphatic polyester being

developed for a wide range of applications due to its thermo-

plastic, biodegradable, and biocompatible properties.2,3 Increas-

ing realization of the intrinsic properties of PCL, coupled with

knowledge of how such properties can be improved to achieve

the compatibility with processing, manufacturing, and end-use

requirements of thermoplastics, has hence fueled technological

and commercial interest.

Several approaches such as copolymerization4,5 and mixing

with the nanofiller6 have been used to improve and/or control

the properties of PCL. Another convenient strategy is to blend

PCL with many other polymers to obtain new biocompatible

materials with high performance.7 Among those blends, poly(-

caprolactone)/polylactide blend material (PCL-blend-PLA) isvery interesting due to the large difference in the physical

properties and biodegradability between two polymers, where

the glassy PLA with high degradation rate presents better tensile

strength, while the rubbery PCL with much slower degradation

rate shows better toughness. This property complementarity is

quite important for the PCL-blend-PLA material because one

can control or even design the performance by adjusting the

molecular characteristics of the polymers and blending ratio as

well as processing conditions to meet the requirements of various

applications. Therefore, much research work8-21 has hitherto

been reported on the preparation and the applications of this

kind of blend material. The performance such as enzymatic and

nonenzymatic degradation, thermal and mechanical properties

as well as drug release behavior has been studied extensively,aiming at relating those properties to the compositions and the

immiscible phase structures.8-13 The applications of such blend

system as a new type of microporous substrate or temporary

scaffold for tissue engineering and drug delivery have also been

explored.14-16

It is well-known that the performance of the polymer blends

depends not only on the properties of the matrix polymers, but

also highly on the phase morphology.22,23 However, PCL and

PLA are incompatible thermodynamically with each other and

can only form multiphase structure in their blend system with

poor interfacial adhesion, which restricts its further applications.

It is well accepted that the phase structure of the immiscible

polymer blends is mainly governed by the composition andrheological properties of the component polymers, which is

hence assumed to be a unique function of the flow history and

the properties of the polymers. Thus, the relations between

morphology and rheological properties within the PCL/PLA

blend have been studied,17,18 aiming at exploring appropriate

processing conditions to improve phase structure kinetically.

Much more research work, however, is concentrated on using

amphiphilic diblock or multiblock copolymers as compatibilizer

to improve the miscibility the PCL/PLA blend.19-21 It has been

reported that the third component, namely, those well-defined

copolymers whose chemical nature is identical to that of the

main components, can act as emulsifying agents, reducing the

* To whom correspondence should be addressed. Tel.: +86-514-87975590, ext. 9115. Fax: +86-514-87975244. E-mail: [email protected].

Yangzhou University. Provincial Key Laboratories of Environmental Material and Engineering. Shanghai Jiaotong University.

Biomacromolecules2009, 10, 417424 417

10.1021/bm801183f CCC: $40.75 2009 American Chemical Society

Published on Web 01/13/2009


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coalescence effect by lowering the interfacial tension between

the two components and finally leading to a well-dispersed

morphology.

In recent years, many researchers24-33 have found that the

anisotropic nanofiller, such as organoclay, can also be used as

a third component to improve phase morphology of the

immiscible polymer blends. It has been reported that addition

of the nanofiller may affect the interfacial properties in an

immiscible blend system with results similar to that of addition

of a compatibilizer. Two possible mechanisms33

have beenproposed on this morphological improvement. Mechanism I

(thermodynamic compatibility): The organic modifier of the

nanofiller is miscible or at least compatible with both phases,

thus the overall free-energy of mixing (Gmix) becomes negative

and thermodynamically driven compatibility is likely to occur

between the immiscible components. Mechanism II (dynamic

compatibility): The polymer pairs present large difference in

their polarity or rheological properties. In this case, the nanofiller

is mainly dispersed in the component with stronger polarity or

with lower viscosity. This selective localization not only changes

the viscosity ratio of two components but also prevents

coalescence of the domains during melt mixing, improving

compatibility between two phases kinetically. Therefore, the

addition of the anisotropic nanofiller may bring reinforcing and

compatible effects into an immiscible blend system simulta-

neously, which is a new approach to obtain polymer blend

nanocomposites with high performance.

As a new anisotropic one-dimensional nanomaterial, the

carbon nanotube (CNT) has become the next-generation rein-

forcements for nanostructured polymeric composite materials,

increasingly owing to its extraordinarily high elastic modulus,

strength, and resilience.34,35 Many conducting polymer/CNT

composites have been prepared successfully in recent years via

the approaches of melt mixing, film casting and/or polymeriza-

tion.36-46 As expected, small addition of CNTs can improve

mechanical properties and electrical conductivity of the matrix

polymers remarkably. Moreover, in vivo studies44-46 haveconfirmed good biocompatibilities of CNTs with various cells,

indicating that the CNTs can be used as new type of nano-

modifier to prepare biopolymeric composite materials.

In this work, therefore, we used CNTs as both the nanorein-

forcements and the compatibilizer to modify the immiscible

PCL/PLA blend. The weight ratio of PCL and PLA is 70/30,

in which the PCL component is continuous phase.18 The

distribution of the CNTs and the immiscible morphology of the

blend matrix were systematically investigated, aiming at relating

the morphological improvement to the selective localization of

the CNTs. The physical and mechanical properties were then

studied to further confirm the micro- and mescoscopic

structure-property relations proposed in the PCL/PLA/CNTternary nanocomposites.

2. Experimental Section

2.1. Materials Preparation. The poly(-caprolactone) (CAPA6500)

used in this study is a commercial product of Solvay Co. Ltd., Belgium.

Its Melt Index (MI) is about 7 g/10 min (160 C/2.16 Kg, ASTM

D1238), and the -OH value is lower than 2 mg KOH/g. The polylactide

(2002D) is also a commercial product of NatureWorks Co. Ltd., U.S.A.

Its residual monomer content is less than 0.3 wt %, and MI is about 8

g/10 min (190 C/2.16 Kg, ASTM D1238). The multiwalled carbon

nanotubes (MWCNT) were supplied by Chengdu Organic Chemistry

Co. Ltd., Chinese Academy of Sciences. The purified MWCNT

(M1203, purity > 95%) is a chemical vapor deposition material with

outside diameter of 10-20 nm, inside diameter of 5-10 nm and length

of 10-30 m. Its special surface area is higher than 200 m2/g. The

carboxylic MWCNT (MS1223, purity > 95%), which is functionalized

on M1203 by -COOH, has the identical dimension parameter and special

surface area with those of M1203. The rate of surface carbon atom on

MS1223 is about 8-10 mol % and the -COOH weight percent is about

1-6 wt % (measured by XPS).

The PCL/PLA/MWCNT ternary composites (PCL/PLAs, where s

denotes the weight of the MWCNT per hundreds weight of the blend

resin (phr)) were prepared by direct melt compounding various loadings

of carboxylic MWCNT with the blend matrix (the two matrixcomponents of PCL and PLA are in the proportion of 70/30 (w/w)) in

a HAAKE polylab rheometer (Thermo Electron Co., U.S.A.) at 170

C and 50 rpm for 6 min. All materials were dried at predetermined

temperature under vacuum before using. The sheet specimens in

thicknesses of about 1 mm for the rheological, morphological, and

electrical conductivity measurements were prepared by compression

molding at 180 C and 10 MPa. The dog-bone shaped specimens for

the tensile and the dynamic mechanical property characterizations were

prepared by injection molding at 180 C and 13 MPa using a RR/

PSMP2 test sample injection molding apparatus (Ray-ran Co., England).

The binary PCL nanocomposites with 1 wt % of carboxylic MWCNT

(PCL1) and the ternary composites with 1 wt % of purified MWCNT

(PCL/PLA1) were also prepared under the same processing conditions

for the properties comparison.2.2. Characterization. Microstructure and Morphology. The disper-

sion of MWCNT was observed using a Tecnai 12 transmission electron

microscope (TEM) (PHILIPS Co., Netherlands) with 120 kV accelerat-

ing voltage. The transmission electron micrographs were taken from

microtomed sections in thickness of 80-100 nm. The morphologies

of the fractured surfaces of the tensile samples and the brittlely fractured

samples were observed using a XL-30ESEM scanning electron

microscope (SEM) (PHILIPS Co., Netherlands) with 20 kV accelerating

voltage and a S-4800 field-emission scanning electron microscope (FE-

SEM; Hitachi Co., Japan) with 15 kV accelerating voltage. The number-

average diameter of the domains for sea-island morphology (a typical

phase separation structure, in which one polymer phase is dispersed in

another polymer phase as discrete domains) was determined according

to the following relation

Dn)NiDiNi (1)

where Ni is the number of dispersed domains with a diameter of Dicounted from the SEM images. The total number of particles is about

100 in the analysis.

Mechanical Properties. The tensile properties of the blank blend

and the nanocomposite samples were determined by an Instron

Mechanical Tester (ASTM D638) at a crosshead speed of 50 mm/min

at room temperature using the dog-bone shaped specimens. Property

values reported here represent an average of the results for tests run

on six specimens. The dynamic mechanical properties of the samples

were characterized using a DMA-242C dynamic thermal mechanicalanalyzer (NETZSCH Co., U.S.A.). The testing was performed in three-

point bending mode at the vibration frequency of 5 Hz in a N2atmosphere. The heating rate is predetermined as 5 C/min and the

temperature ranges from -100 to 100 C.

Rheological Properties. Rheological measurements were carried out

on a rheometer (HAAKE RS600, Thermo Electron Co., U.S.A.) using

a parallel plate geometry with 20 mm diameter plates. The samples

about 1.0 mm in thickness were melted in the parallel plate fixture at

180 C for 5 min to eliminate residual thermal history, and then carry

out the dynamic shear measurements immediately. The dynamic strain

sweep was first carried out to determine a common linear region, strain

level of 1%. Then, the small amplitude oscillatory shear (SAOS) was

applied and the dynamic frequency sweep was carried out.

Electrical Properties. The volume resistance of sheet samples was

measured by a four-point probe apparatus, ZC36 high resistance meter

418 Biomacromolecules, Vol. 10, No. 2, 2009 Wu et al.


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(Shanghai Precision and Scientific Co. Ltd., P. R. China) at room

temperature. Three specimens of each sample were tested taking four

data points.

3. Results and Discussion

3.1. Micro- and Mescoscopic Structure of the PCL/PLA/

MWCNTs Ternary Nanocomposites. Figure 1a-e gives the

SEM images of the fracture surface, respectively, for the blank

PCL/PLA blend and its ternary composite samples with various

carboxylic MWCNT loadings. Clearly, all samples show typical

sea-island morphologies, where the discrete PLA spherical

domains are dispersed in the PCL matrix. With addition of

MWCNTs, the size of the PLA domains reduces remarkably.

The average diameter of the domains decreases from 21.5 to

6.3 m as the MWCNT loadings achieve up to 1 phr. Moreover,the blend matrix presents better interfacial adhesion between

two phases in the presence of the carboxylic MWCNTs. Thisindicates that addition of the carboxylic MWCNTs improves

interfacial properties of the immiscible PCL/PLA blend with

similar results of adding a compatibilizer. However, it is seen

that the size of the discrete PLA domains also reduces evidently

with addition of the MWCNTs without functionalization,whereas the interfacial adhesion between the two matrix phases

is poor in this case. (Figure 1f). Thus, it is interesting to explore

the mechanism of the morphological change with addition of

the MWCNTs.

Figure 2a-c gives the TEM images of the PCL/PLA1 sample.

The typical two-phase structure can be seen in Figure 2a, in

which the light and dark gray parts correspond to discrete PLA

and matrix PCL phases, respectively. On the amplified images

of the parts marked in Figure 2a, it is seen clearly that the

carboxylic MWCNTs are mainly dispersed in the PCL phase

and on the phase interface (Figure 2b,c). The selective localiza-

tion of the CNTs in one polymer phase has also been observed

on some ternary nanosystems such as PA6/PPS/CNTs47 and PC/PP/CNTs composites48 in which the two matrix polymers show

Figure 1. SEM images of the brittlely fractured (a) blank PCL/PLA sample, (b-e) PCL/PLA samples with various carboxylic MWCNT loadingsof 0.2, 0.5, 1, and 2 wt %, and (f) PCL/PLA1 sample with purified MWCNT loadings of 1 wt %. The PCL and PLA are in the weight ratio of70/30.

Localization of Multiwalled Carbon Nanotubes Biomacromolecules, Vol. 10, No. 2, 2009 419


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a large difference in their polarity and in their affinity to the

CNTs. Thus, the CNTs are mainly located in one polymer phase

with stronger polarity. For the ternary systems in this work,

however, there is no remarkable difference in the affinity of

two polymers to the carboxylic MWCNTs because both the

PCL/MWCNTs42 and the PLA/MWCNTs binary composites43

present a good dispersion state of the MWCNTs. Actually, the

PLA shows more or less better affinity to the MWCNTs than

that of the PCL. The selective localization of MWCNTs,

therefore, is attributed to a large difference in the rheological

properties of two matrix polymers in which the PLA shows far

higher viscosity than that of the PCL (PLA/PCL 16). In this

case, the PCL chain can diffuse around and into the MWCNTaggregates more easily compared with that of the PLA at the

initial stage of melt mixing and, as a result, the MWCNTs prefer

to be detached and dispersed in the lower viscous PCL

continuous phase rather than the discrete PLA phase. With rapid

reduction in the viscosity ratio between two matrix polymers,

the higher viscous PLA phase can be broken into smaller

droplets more easily. On the other hand, many MWCNTs have

a tendency to be further dispersed on the phase interface driven

by the mixing flow because the carboxylic group on their surface

has nice affinity to both the PCL and the PLA phases. Finally,

they are distributed in the phase interface layer and ranged more

or less ordered along the surface of the PLA droplets, acting as

the emulsifier49 to enwrap the discrete domains well, as can beseen in Figure 2c.

Figure 2. TEM images for the samples of (a) PCL/PLA1 and (d) PCL/PLA1 . The parts indicated in (a) are amplified and shown in (b) and (c).

Figure 3. FE-SEM images of (a) tensile failure and (b) brittle fracture surface of the PCL/PLA1 sample.



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The mechanism proposed above on that selective localizationis further confirmed by the results of the TEM measurements

on the PCL/PLA1 sample that has identical MWCNT loadings

with that of the PCL/PLA1 sample, while the filled MWCNTs

in this ternary system are only purified without further func-

tionalized. As can be seen in Figure 2d, the purified MWCNTs

also present selective localization in the PCL phase, whichconfirms that the MWCNTs, as the filled component, show a

general tendency to be dispersed in the less viscous phase during

melt mixing. However, most of MWCNTs are still presented

as small aggregates or bundles in the PCL/PLA1 sample,

showing poor dispersion in contrast to that of the PCL/PLA1

sample. This is due to the poor affinity between the matrix

polymers and those MWCNTs without functionalization. The

migration of the MWCNTs in this case is hard to occur and

thus the selective interface distribution is nearly not observable

in the PCL/PLA1 sample. As a result, the immiscible blend

matrix of the PCL/PLA1 sample presents rather weak phase

interfacial adhesion compared with that of the PCL/PLA1

sample, although both samples have comparable size of discretePLA phase, as shown in Figure 1d and f. This confirms that

besides the rheological properties of the blend components, the

surface functionalization is also vital to the selective interface

localization of the MWCNTs. On the one hand, the surface

carboxylic group of the MWCNTs is compatible with both

the PCL and the PLA, which reduces the overall free energy of

mixing and increases the thermodynamical compatibility. On

the other hand, interfacial localization of the MWCNTs could

prevent the coalescence of the PLA domains effectively, which

helps compatibilization during melt mixing. Therefore, both the

thermodynamically and kinetically driven compatibility can

occur.25,26 Such interface localization of the carboxylic MWCNTs,

as a result, improves the interfacial adhesion of the PCL/PLA

blend matrix evidently, which is further confirmed by the FE-SEM images shown in Figure 3.

Figure 4. DTMA thermogram for the PCL/PLAs samples with variouscarboxylic MWCNT loadings.

Figure 5. Plots of (a) dynamic storage modulus (G) and (b) lossmodulus (G) vs frequency and (c) -GP plots of phase angle () vscomplex modulus (|G*|) obtained from dynamic frequency sweep onthe PCL/PLAs samples with various carboxylic MWCNT loadings.

Figure 6. Schematic diagrams of the percolated MWCNTs networkin (a) homogeneous matrix and (b) immiscible blend matrix. The whitepart is matrix PCL phase, the gray part is discrete PLA phase andthe short black curve is MWCNTs.

Figure 7. Plots of electrical conductivity vs weight fraction of thecarboxylic MWCNTs.



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To further evaluate the compatible effect of the carboxylic

MWCNTs on the PCL/PLA blend system, the dynamic thermal

mechanical analysis was performed. Figure 4 gives the DTMA

thermogram for the neat matrix polymers and PCL/PLAs

samples. It is seen that with the increase of MWCNT loadings,the glass transition temperatures (Tg) of the two component

polymers shift to each other gradually. This indicates that

emulsification occurs at the phase interface in the presence of

the amphiphilic carboxylic MWCNTs, leading to interface

stabilization thermodynamically. However, Both the PCL and

the PLA still present evident glass transition behavior, respec-

tively, in the ternary system and the shift degree of the two Tgis not remarkable, suggesting that thermodynamically driven

compatibility by the carboxylic MWCNTs is not as strong as

that by the well-defined copolymers.19,20 This is mainly due to

large difference of chemical structure between the MWCNTs

and the copolymers. In general, the copolymers used as

compatibilizer present the blocks structure similar to that of theparent homopolymers and, the length of the blocks should

exceed or at lest close to that of the homopolymers, while the

MWCNTs only have amphiphilic carboxylic group, which may

be too small to make mother-MWCNTs fully reptate at the phase

interface.50 In addition, the presence of MWCNTs restricts

motion of the matrix PCL chain, also counteracting the shift of

Tg somewhat. However, the overall compatible effect of the

carboxylic MWCNTs is still good, indicating that kinetically

driven compatibility is dominant in the melt mixing process.

In other words, the improvement of the phase morphology is

mainly due to the obstructing effect of those interfacial-located

MWCNTs on the coalescence of those discrete domains.

3.2. Percolation Behavior of the PCL/PLA/MWCNTs

Ternary Nanocomposites. Because the selective interfacelocalization of the MWCNTs improves the phase morphology

evidently, it may influence the final performance of the ternary

nanocomposites. The rheological behavior was first characterized

and the dynamic storage modulus (G) and loss modulus (G)

obtained from the dynamic frequency sweep are shown in Figure

5a,b. The shoulder presented on the modulus curve of the blank

PCL/PLA sample (see the arrow in Figure 5a) is attributed to

the shape relaxation of the discrete PLA phase in the PCL

matrix.51 During oscillatory shear flow, the total area of the

interface as well as the interfacial energy are changing periodi-

cally but with a much longer relaxation time than that ofcomponent polymers.52 This kind of long relaxation due to the

presence of interface leads to an additional transition shoulder

on the modulus curves. It is seen that the relaxation shoulder

shifts to low-frequency region with small addition of the

MWCNTs (0.2 phr). This indicates that the presence of

MWCNTs retards shape relaxation of the discrete PLA domains.

Because the interfacial relaxation time is proportional to the

ratio between droplet size and the interfacial tension, longer

interfacial relaxation process but with decreasing droplet size

means a large decrease of interfacial tension, which is also an

indication of the selective localization of MWCNTs on the

interface. It is notable that further addition of the MWCNTs

enhances the low-frequency modulus sharply. As the MWCNTloadings achieving up to 0.5 phr, the low-frequency G increases

even by about 4 orders as compared with that of the blank PCL/

PLA sample and the frequency dependence nearly disappears.

This nonterminal behavior is due to formation of the percolated

MWCNTs network,36-43 which highly restrains the long-range

relaxation of the matrix PCL chains.

van Gurp-Palmen plot53 is usually used to detect the rheo-

logical percolation of the filled polymeric composites, also

including polymer/CNTs systems.37,38,41 Figure 5(c) gives the

v-GP plots of phase angle () versus complex modulus (|G*|)for the PCL/PLAs ternary systems. The low-frequency of theblank PCL/PLA and the PCL/PLA0.2 samples are close to 80,

which is indicative of a flow behavior presented by the

viscoelastic fluid. As the MWCNT loadings increase to 0.5 phr,

the low-frequency decreases remarkably to lower than 45,indicating a rheological fluid-solid transition in that ternary

system. Accordingly, the rheological percolation threshold for

the PCL/PLAs ternary systems is lower than 0.5 phr. It is

interesting that the threshold of the ternary systems is far lower

than those reported on the PCL/MWCNTs and PLA/MWCNTs

binary systems prepared also by melt mixing, in which present

the critical values of about 2-3 wt %.42,43 This is due to the

selective localization of the MWCNTs consequentially. As

discussed in the section of morphology, the interface-localized

MWCNTs tend to be arranged more or less ordered along the

surface of the PLA droplets during melt mixing. In contrast to

the disorderly dispersion in the binary systems, the self-assemblylike behavior of the MWCNTs can enhance the particle-particle

interactions more effectively in the ternary systems, promoting

formation of the percolated MWCNTs network structure even

at lower volume concentration, as can be seen in the schematic

illustration shown in Figure 6. As a result, the PCL/PLAs ternary

systems present very low percolation threshold.

It is well-known that nonlinear electrical properties of the

polymeric composites filled with conductive filler, including

polymer/CNTs systems,36,37,39,40 also shows percolation behav-

ior.54 Owing to the selective localization of the MWCNTs, it

can be expected that the ternary systems may show lower

conductive percolation threshold than that of the binary ones.

Figure 7 gives the electrical conductivity as a function ofMWCNT loadings for the ternary composites. The curve clearly

Figure 8. (a) Stress/strain curves for the neat PCL, blank PCL/PLA,ternary PCL/PLAs, and PCL/PLA1 samples; (b) plots of tensile yieldstrength vs weight fraction of the carboxylic MWCNTs.



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presents three regions (dielectric, transition, and conductive),

which is indicative of a typical percolation behavior. At identical

MWCNT loadings of 1 phr, the conductivity of the ternary

composites is far higher than that of the binary ones by about

3-4 orders. This indicates that those well-ranged MWCNTs

improve structure of the conductive network. On the one hand,selective interface-localization inosculates the MWCNTs well

with one another, allowing direct electron transport more

smoothly. On the other hand, the increase of the phase interface

due to the decrease of the discrete phase size makes the

conductive network form more easily, even at relative lower

loading levels of the MWCNTs. Thus, the ternary systems

present higher conductivity and lower conductive percolation

threshold than those of the binary ones.

3.3. Mechanical Properties of the PCL/PLA/MWCNTs

Ternary Nanocomposites. The results above show that the

performance of the PCL/PLAs ternary composites highly

depends on the phase morphology of the blend matrix and the

dispersion state of the MWCNTs. The enhancement in themechanical properties, therefore, is foreseeable on those ternary

composites owing to the unique localization of the MWCNTs

and the improvement of phase structure as compared with that

of the blank PCL/PLA blend. Figure 8 gives the strength and

stress/strain curves for all samples. The tensile yield strength

is used here for better comparison because the PCL and its

blends are generally ductile.55,56 Although the PLA presents

higher strength than that of the PCL,2 their blend shows the

strength even lower than that of the neat PCL (Figure 8a). This

is due to the poor interfacial adhesion between matrix PCL and

minor PLA phase.11 Small addition of the MWCNTs, as

expected, enhances the blend materials remarkably, and the

strength of the ternary composites increases monotonically with

increase of the MWCNTs loadings within experimental loadingranges, as can be seen in Figure 8b.

Figure 9 gives the SEM images of those tensile samples. The

blank PCL/PLA sample clearly shows interface break (Figure

9a), while those ternary samples present both the interface and

bulk break (see the parts marked in Figure 9c). Addition of the

amphiphilic carboxylic MWCNTs improves the phase adhesion

due to the selective interface localization, resulting in good loadtransfer between two matrix phases. With increasing loading

levels, those well-dispersed MWCNTs selectively in the matrix

PCL phase begin to play an important role in bearing an extra

load. As a result, the tensile yield strength of the ternary

composites further increases, accompanied by an evident

reduction of the elongation at break because the presence of

MWCNTs impedes movement of the matrix molecule chain. It

is notable that the PCL/PLA1 sample (Figure 9d) shows poor

phase adhesion between PCL and PLA in contrast to that of

PCL/PLA1 sample (Figure 9b), although both present similar

sea-island morphologies and almost identical radius of discrete

domains. On the one hand, those MWCNTs without function-

alization can not be dispersed well in the matrix polymer andmerely act as a stress concentration point, leading to a decrease

of the strength of bulk PCL. The interfacial adhesion between

two polymers is very weak if the MWCNTs are not function-

alized and not selectively distributed on the interface. Thus, the

PCL/PLA1 sample presents a characteristic of brittle fracture

(most of domains are pulled out from fractured surface in the

tensile process). As a result, the breaking strength of the PCL/

PLA1 sample is far lower than the yield strength of the PCL/

PLA1 sample, even lower than the blank PCL/PLA blend

(Figure 8a). This also confirms that the surface functionalization

is a key point to the selective localization of the MWCNTs and,

only in the case of interface localization, addition of the

amphiphilic MWCNTs can bring reinforcing and compatibleeffects together into an immiscible blend system.

Figure 9. SEM images of the tensile samples of (a) blank PCL/PLA, (b) PCL/PLA0.2, (c) PCL/PLA1, and (d) PCL/PLA1 systems.



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4. Conclusions

PCL/PLA/MWCNTs ternary composites with various MWCNT

loadings were prepared by direct melt mixing. The weight ratio

of PCL and PLA is fixed in the proportion of 70/30 for all

samples in which PCL is the matrix phase. With addition of

the MWCNTs, the size of the discrete PLA domains reduces

remarkably due to the selective localization of the MWCNTs

in the blend matrix. The ternary systems containing the

carboxylic MWCNTs show the selective localization of MWCNTs

both in the matrix PCL phase and on the phase interface, while

the ternary systems containing the MWCNTs without function-

alization only show the selective localization in the matrix PCL

phase. This is attributed to a large difference in the affinity of

two MWCNTs to the polymers. Only in the case of interface

localization, the addition of the MWCNTs can bring reinforcing

and compatible effects together into the immiscible PCL/PLA

blend. The ternary systems containing carboxylic MWCNTs

hence present high improvement of the performance in terms

of rheological, conductive, and mechanical properties as com-

pared with those of PCL/PLA blend and binary composites.

Acknowledgment. This work was supported by the research

grants from the National Natural Science Foundation of China(No. 50803052), the Startup Program of Innovative Talent of

Jiangsu Province (No. BK2007559), and the Key Program of

Jiangsu Province (No. 06KJA15011).

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119 Wu Selective Localization of Multi Walled CNT

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