Nonisothermal crystallization kinetics of poly (lactide)—effect of plasticizers and nucleating...

DOI 10.1515/polyeng-2012-0124 J Polym Eng 2013; 33(2): 163–171

Yanhua Chen , Xiayin Yao * , Qun Gu and Zhijuan Pan *

Non-isothermal crystallization kinetics of poly (lactic acid)/graphene nanocomposites Abstract: Poly(lactic acid) (PLA)/graphene nanocompos-

ites were prepared by solution blending and the dispers-

ibility of graphene in the PLA matrix was examined by

transmission electron microscopy (TEM). The non-isother-

mal crystallization behaviors of pure PLA and PLA/gra-

phene nanocomposites from the melt were investigated

by differential scanning calorimetry (DSC). The results

showed that the graphene could play a role as a hetero-

geneous nucleating agent during the non-isothermal crys-

tallizing process of PLA, and accelerate the crystallization

rate. The non-isothermal crystallizing data were analyzed

with the Avrami, Ozawa and Mo et al. models and the

crystallization parameters of the samples were obtained.

It is demonstrated that the combination of the Avrami and

Ozawa models developed by Mo et al. was successful in

describing the non-isothermal crystallization process for

pure PLA and its nanocomposite. According to the Kiss-

inger equation, the activation energies were found to be

-154.3 and -179.5 kJ/mol for pure PLA and PLA/0.1 wt %

graphene nanocomposite, respectively. Furthermore, the

spherulite growth behavior was investigated by polarized

optical microscopy (POM) and the results also supported

the DSC data.

Keywords: graphene; kinetics; nanocomposites; non-iso-

thermal crystallization; poly(lactic acid).

*Corresponding authors : Xiayin Yao , Ningbo Institute of Materials

Technology and Engineering , Chinese Academy of Sciences,

Ningbo 315201, PR China , Tel.: +86-574-86686790,

Fax: +86-574-86685701, e-mail: [email protected] ; and

Zhijuan Pan, College of Textile and Clothing Engineering , Soochow

University, Suzhou 215006, PR China; and National Engineering

Laboratory for Modern Silk , Soochow University, Suzhou 215123,

PR China , Tel.: +86-512-67065713, Fax: +86-512-67246786,

e-mail: [email protected]

Yanhua Chen : College of Textile and Clothing Engineering , Soochow

University, Suzhou 215006, PR China

Yanhua Chen : National Engineering Laboratory for Modern Silk ,

Soochow University, Suzhou 215123, PR China

Qun Gu: Ningbo Institute of Materials Technology and Engineering ,

Chinese Academy of Sciences, Ningbo 315201, PR China

Yanhua Chen: China Textile Academy Jiangnan Branch , Shaoxing

312071, PR China

1 Introduction

Poly(lactic acid) (PLA), an important biodegradable

synthetic polymer made from renewable resources, has

received considerable attention in the field of environmen-

tal protection and biomedical applications, because of its

biodegradability and biocompatibility [1 – 4] . However,

due to its slow crystallization rate, as well as brittleness,

vulnerability to heat, and poor mechanical properties, the

applications of PLA in more fields are restricted. These

problems can be partly overcome through copolymer syn-

thesis and blending PLA with other polymers or nanofill-

ers [5 – 7] . Graphene, a new allotrope of carbon, has been

considered to be a kind of excellent inorganic filler for pre-

paring high performance polymer-based nanocomposites

at very low loading content, due to its remarkable elec-

tronic, thermal, and mechanical properties [8 – 10] . Until

now, many polymer/graphene nanocomposites have been

reported with improved properties [11 – 13] . Nevertheless,

only a few works focused on the PLA/graphene nanocom-

posites [14 – 16] .

The crystallization behavior of polymer is a basic

problem in polymer physics and has become one of the hot

research topics over the past few decades [17] . It is known

that the properties of a crystalline polymer depend on the

size, shape, orientation and perfection of the crystallites,

as well as the degree of crystallinity. Compared with that of

the isothermal crystallization process, the investigation of

non-isothermal crystallization process from the melt can

provide the fundamental theories for materials process-

ing, as this process is much closer to the practical indus-

trial applications, such as extrusion, injection molding,

film blowing etc. Furthermore, for PLA, the degradation

properties also mainly depend on the crystalline morphol-

ogy and crystallinity. Hence, the crystallization behav-

iors of pure PLA and its blends or copolymer have been

investigated by many researchers [18 – 24] . Liu et al. [19]

investigated the non-isothermal crystallization kinetics of

poly(L-lactide) and a polylactide stereocopolymer (PLA 98

),

containing 98 % L-lactyl and 2 % D-lactyl units, and found

that the PLA 98

exhibited a much reduced crystallization

ability compared with poly(L-lactide). Papageorgiou et al.

[20] prepared a series of PLA nanocomposites containing

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164 Y. Chen et al.: Crystallization kinetics of PLA/graphene nanocomposites

different types of inorganic filler nanoparticles, including

fumed silica nanoparticles, montmorillonite and oxidized

multi-walled carbon nanotubes. The effect of filler types

on non-isothermal crystallization of PLA nanocomposites

was studied. However, there is still little literature on gra-

phene induced crystallization of PLA [25 – 27] .

In our previous works, PLA/graphene nanocompos-

ites were prepared via solution blending, using chloro-

form as a mutual solvent. The introduction of a very low

loading of graphene could greatly enhance the thermal

stability of PLA. The temperature for 5 % weight loss for

PLA/0.1 wt % graphene nanocomposite was 18.1 ° C higher

than that for pure PLA [28] . The isothermal crystalli zation

behavior and crystal morphology of pure PLA and its

nanocomposites were also investigated, using differen-

tial scanning calorimetry (DSC) and polarizing optical

microscope (POM) [29] . In this work, the non-isothermal

crystallization behavior of graphene-filled PLA was

further investigated with DSC. Based on the DSC data,

the non-isothermal crystallization kinetics were analyzed

using various mathematical models, i.e., Avrami [30 – 32] ,

Ozawa [33] , and Mo et al. [34, 35] . The activation energy

describing the nonisothermal crystallization process was

calculated based on Kissinger ’ s method [36] . The main

objective of this work is to study the effect of graphene

on the non-isothermal crystallization behavior of PLA. It

is anticipated that the research reported herein would be

of great help for understanding the relationship between

the structure and properties of biodegradable polymer

nanocomposites.

2 Experimental

2.1 Materials

Pure PLA and PLA/0.1 wt % graphene nanocomposite were

used in the present study. Pure PLA was synthesized by

the melt polycondensation of lactic acid (Anhui BBCA

and GALACTIC Lactic Acid Co., Ltd., Anhui, China) in our

own laboratory. The weight-average molecular weight

and polydispersity were 12,776 and 1.38, respectively,

which were determined by gel permeation chromatogra-

phy (Waters 1515, Waters Corporation, Milford, MA, USA)

measurement. Graphene was prepared by a solution-

phase processing, followed by thermal reduction.

The PLA/0.1 wt % graphene nanocomposite was pre-

pared via solution blending and the coagulation method

[28] . In a typical procedure, 5 mg graphene was dis-

persed in chloroform (Shanghai Chemical Reagent Corp.,

Shanghai, China) with a mass concentration of 0.2 mg/ml

by ultrasonication (Scientz-IID, Ningbo Scientz Biotech-

nology Co., Ltd, Zhejiang, China; with an output power

650 W) for 2 h at room temperature and the stable disper-

sion was obtained. At the same time, 5 g PLA was also

dissolved in the chloroform. Then, the two solutions were

mixed together to form a homogeneous mixture, by mag-

netically stirring. After that, the mixture was dropwise

added to a 10 – 15 times excess of methanol (Shanghai

Chemical Reagent Corp. Shanghai, China) and the pre-

cipitate was collected by centrifugation. The pure PLA

was also prepared according to the same procedure, in the

absence of graphene, in order to eliminate experimental

error. Finally, the products were dried in a vacuum oven at

75 ° C for 12 h to remove the solvents totally and then stored

in a desiccator until used.

2.2 Characterization

Transmission electron microscopy (TEM) experiments

were performed on an FEI Tecnai G 2 transmission elec-

tron microscope (FEI Company, Oregon, USA), at an

accelerating voltage of 200 kV. The PLA/0.1 wt % gra-

phene nanocomposite was embedded in epoxy and cut

into ∼ 80 nm thick slices using an ultramicrotome for

TEM observation.

The DSC measurements were performed using a

Mettler Toledo DSC (Mettler-Toledo International Inc.,

Schwerzenbach, Switzerland). The temperature was cali-

brated by pure indium and zinc. All measurements were

carried out in a nitrogen atmosphere. The samples (5 – 6

mg) were first heated to 200 ° C and held there for 5 min to

remove the thermal history, and then cooled at a constant

cooling rate of 0.5, 1, 2, 3 and 4 ° C/min for nonisothermal

crystallization. The exothermal traces were recorded for

data analysis.

The spherulitic growth behavior of pure PLA and

PLA/0.1 wt % graphene nanocomposite during non-

isothermal crystallization was studied using a POM

(Olympus BX51, Olympus Corp., Tokyo, Japan) equipped

with a hot stage (Instec HCS601) controlling tempera-

ture accurately and a CCD camera recording the mor-

phology of the samples. The very thin samples, with a

thickness of around 40 μ m, were sandwiched between

two glass slides and heated to 200 ° C on the hot stage

and held there for 5 min to remove the thermal history.

Then, these molten films were non-isothermally crystal-

lized to 65 ° C at a cooling rate of 2 ° C /min from the melt.

The CCD camera was used to record the optical texture

images.



Y. Chen et al.: Crystallization kinetics of PLA/graphene nanocomposites 165

3 Results and discussion

3.1 Dispersibility of graphene in PLA

Figure 1 A shows the TEM image of graphene nanosheets

dispersed in chloroform. Extremely thin graphene

nanosheets with a wrinkled structure were detected.

After formation of the nanocomposite with PLA, by

solution blending, the graphene nanosheets could be

homogenously dispersed in the PLA matrix; no obvious

Figure 1 Transmission electron microscopy (TEM) images of

(A) graphene and (B) poly(lactic acid) (PLA)/0.1 wt % graphene

nanocomposite.

Figure 2 Differential scanning calorimetry (DSC) non-isothermal

measurement curves for (A) pure poly(lactic acid) (PLA) and (B)

PLA/0.1 wt % graphene nanocomposite from the melt at various

cooling rates.

aggregations were found in the nanocomposite, which

was confirmed by TEM, as shown in Figure 1B.

3.2 Nonisothermal crystallization behavior

The non-isothermal crystallization exotherms of pure

PLA and PLA/0.1 wt % graphene nanocomposite from

the melt at various cooling rates, ranging from 0.5 to

4 ° C/min, are presented in Figure 2 . As the cooling rate

increases, the exothermic traces become broader, and

the peak values shift to lower temperatures for both pure

PLA and PLA/0.1 wt % graphene nanocomposite, which

could be due to less time for the PLA molecular chains

to move and fold at the higher cooling rate. Furthermore,

the values of crystallization peak ( T p ) and onset tempera-

ture ( T c ) for the samples, under different cooling rates,

can be derived from Figure 2; these are listed in Table

1 . It can be seen that the T p and T

c for the PLA/0.1 wt %

graphene nanocomposite are higher than those of pure

PLA at the same cooling rates, which indicates that the

existence of graphene makes the crystallization of PLA

easier and also suggests that the graphene could act as a

heterogeneous nucleating agent and accelerate the crys-

tallization rate of PLA matrix during the non-isothermal

crystallization.

The relative crystallinity X ( T ), as a function of tem-

perature T , can be formulated as Eq. (1) [37, 38] :




( ) 0

0

d

d

T cT

T cT

H dTdTX T H dTdT

∞

⎛ ⎞⎜ ⎟⎝ ⎠

=⎛ ⎞⎜ ⎟⎝ ⎠

∫

∫

(1)

where T 0 and T

∞ represent the onset and end temperature

of crystallization, T is an arbitrary temperature, and d H c

is the enthalpy of crystallization released during an infini-

tesimal temperature interval d T . Figure 3 shows the rela-

tive crystallinity as a function of temperature for pure PLA

and PLA/0.1 wt % graphene nanocomposite, at various

cooling rates. The crystallization temperature in Figure 3

can be converted to crystallization time t using Eq. (2):

Figure 3 Relative crystallinity X(T) as a function of crystallization

temperature for (A) pure poly(lactic acid) (PLA) and (B) PLA/0.1 wt %

graphene nanocomposite during the non-isothermal crystallization.

Figure 4 Relative crystallinity X(t) as a function of crystallization

time for (A) pure poly(lactic acid) (PLA) and (B) PLA/0.1 wt % gra-

phene nanocomposite during the non-isothermal crystallization.

t = ( T 0 - T )/ φ (2)

where T is the crystallization temperature at time t and

φ is the cooling rate. The plots of the relative crystallin-

ity X ( t ) as a function of time t for pure PLA and its nano-

composite, during the non-isothermal crystallization, are

illustrated in Figure 4 . An important kinetic parameter

that can be obtained from Figure 4, is the half-time of

crystallization ( t 1/2

), which is defined as the time required

to reach half crystallinity ( X ( t ) = 0.5). The values of t 1/2

for

pure PLA and PLA/0.1 wt % graphene nanocomposite are

also listed in Table 1. Clearly, the t 1/2

values of the PLA/0.1

wt % graphene nanocomposite are smaller than those of

pure PLA at a given cooling rate, which further demon-

strates that the graphene acts as a nucleating agent in PLA

non-isothermal crystallization and leads the acceleration

of crystallization.

3.3 Nonisothermal crystallization kinetics based on various models

In order to elucidate the non-isothermal crystallization

process of pure PLA and PLA/graphene nanocomposites,

the non-isothermal crystallization kinetics were analyzed

by Avrami, Ozawa and Mo et al. models, based on the DSC

data. The Avrami equation is the most common approach

to analyze the isothermal crystallization kinetics, which

can be expressed as Eq. (3) or (4) [30 – 32] :

Table 1 Characteristic parameters obtained for non-isothermal crys-

tallization of pure poly(lactic acid) (PLA) and PLA/0.1 wt % graphene

nanocomposite based on differential scanning calorimetry (DSC)

measurements.

Samples φ ( ° C/min) T p ( ° C) T c ( ° C) t 1/2 (min)

Pure PLA 0.5 117.87 124.07 11.88

1 109.17 118.32 9.19

2 104.32 117.00 6.94

3 102.40 115.20 5.06

4 101.20 113.40 4.03

PLA/0.1 wt % graphene 0.5 119.23 125.23 11.44

1 114.26 123.25 8.97

2 109.67 120.13 5.73

3 106.48 118.50 4.55

4 104.41 116.07 3.73




X ( t ) = 1-exp(- kt n ) (3)

or

ln {-ln[1- X ( t )]} = ln k + n ln t (4)

where k and n are the Avrami rate constant and Avrami

exponent, respectively, depending on the nucleation

process and the geometry of the growing crystals. Based

on the assumption that the crystallization temperature

is constant, the Avrami equation can also be used to

describe the primary stage of non-isothermal crystalliza-

tion. Figure 5 shows the Avrami plots of ln{-ln[1- X ( t )]} as

a function of ln t for pure PLA and PLA/0.1 wt % graphene

nanocomposite. Each curve shows a linear part, as well as

Figure 5 Plots of In {-In[1- X ( t )]} as a function of ln t for (A)

pure poly(lactic acid) (PLA) and (B) PLA/0.1 wt % graphene

nanocomposite.

Table 2 Non-isothermal crystallization kinetics parameters for pure poly(lactic acid) (PLA) and PLA/0.1 wt % graphene nanocomposite

obtained from Avrami and Mo analyses.

Samples Avrami model Mo model

φ ( ° C/min) k (min -1 ) n X ( t ) % F ( T ) a

Pure PLA 0.5 0.00013 3.48 20 18.70 1.61

1 0.00062 3.16 40 43.33 1.80

2 0.0052 2.51 60 100.21 2.02

3 0.013 2.41 80 304.92 2.31

4 0.033 2.16 – – –

PLA/0.1 wt % graphene 0.5 0.00030 3.20 20 15.49 1.57

1 0.00064 3.21 40 32.04 1.72

2 0.0059 2.7 60 66.63 1.90

3 0.015 2.51 80 195.57 2.20

4 0.040 2.11 – – –

a nonlinear portion, that deviated from the Avrami equa-

tion at a high relative crystallinity region attributed to sec-

ondary crystallization caused by the spherulite impinge-

ment in the later stage. The deviation in the plots occurs at

a relative crystallinity of about 90 % . According to Eq.(4),

values of k and n are obtained from the intercept and the

slope of the linear parts of Avrami plots, corresponding to

approximately 5 – 90 % of the relative crystallinity and the

results are listed in Table 2 . It can be seen that the values of

the Avrami exponent n are similar for both pure PLA and

PLA/0.1 wt % graphene nanocomposite, which may indi-

cate that the addition of graphene into PLA matrix does

not affect the geometric dimension of PLA crystal growth

during the non-isothermal crystallization. The n values

for both samples are in the range 2.1 – 3.5, suggesting that

the crystallization mechanism is tridimensional spherical

growth during the non-isothermal crystallization process

[39 – 41] . The values of k for PLA/0.1 wt % graphene nano-

composite are larger than those of PLA at the same cooling

rates, indicating a higher crystallization rate for the nano-

composite, which is in accordance with the results of t 1/2

.

However, due to the variation of crystallization tempera-

ture, Avrami parameters lost their physical meanings in

the case of the non-isothermal crystallization process.

By taking into account the effect of cooling rate φ , Ozawa extended the Avrami theory to describe the non-

isothermal crystallization process. According to the Ozawa

theory, the non-isothermal crystallization process is the

result of an infinite number of small isothermal crystal-

lization steps and the developed model can be described

as Eq. (5) or (6) [33] :

X ( T ) = 1-exp [ - K ( T )/ φ m ] (5)

or

ln {-ln[1- X ( T )]} = ln K ( T )-mln φ (6)




where K ( T ) is the cooling crystallization function, which

is related to the overall crystallization rate and indicates

how fast the crystallization occurs, and m is the Ozawa

exponent that depends on the dimension of crystal

growth. The plots of In {-ln[1- X ( T )]} as a function of ln

φ , at a given temperature for non-isothermal crystalliza-

tion of pure PLA and PLA/0.1 wt % graphene nanocom-

posite, are shown in Figure 6 . It can be seen that some

non-parallel lines are observed for both samples, indicat-

ing that the Ozawa model fails to describe non-isothermal

crystallization in both pure PLA and its nanocomposite.

These results can be ascribed to the neglect of secondary

crystallization and the dependence of the fold length on

temperature in the Ozawa theory [17] .

In order to describe the non-isothermal crystallization

processes more accurately, Mo et al. [34, 35] proposed a

novel kinetic model by combining the Avrami and Ozawa

equations. They assumed that the degree of crystallinity

is correlated to the cooling rate and crystallization time.

Therefore, the modified equation can be expressed as Eq.

(7) or (8):

ln k + n ln t = ln K ( T )- m ln φ (7)

or

ln φ = ln F ( T )- a ln t (8)

where F ( T ) = [ K ( T )/ k ] 1/ m is the kinetics parameter, referring

to the value of the cooling rate chosen at the unit crystal-

lization time when the system has a defined degree of crys-

tallinity; a = n / m is the ratio of the Avrami exponent n and

Figure 6 Ozawa plots of In {-In[1- X ( T )]} as a function of In φ for

(A) pure poly(lactic acid) (PLA) and (B) PLA/0.1 wt % graphene

nanocomposite.

Figure 7 Plots of In φ versus ln t for (A) pure poly(lactic acid) (PLA)

and (B) PLA/0.1 wt % graphene nanocomposite.

the Ozawa exponent m . For a given degree of crystallinity,

the smaller the value of F(T) , the higher the crystallization

rate becomes. According to Eq. (8), plots of ln φ versus ln t at a given relative degree of crystallinity for pure PLA and

its nanocomposite, are shown in Figure 7 . Clearly, a series

of straight lines are obtained and the kinetic parameter

F(T) and a can be determined from the intercept and the

slope of the plots; the results are also listed in Table 2. As

expected, the F ( T ) values of PLA/0.1 wt % graphene nano-

composite are smaller than those of pure PLA at the same

degree of crystallinity, which further confirms the nuclea-

tion effect of graphene in the PLA matrix, and is consist-

ent with previous results from the Avrami analysis and t 1/2

.

Furthermore, the a values for PLA/0.1 wt % graphene nano-

composite change slightly compared with those of pure

PLA, indicating that the incorporation of graphene into the

PLA matrix affects the mechanism of nucleation and the

crystal growth of PLA slightly. Based on the above analy-

sis, Mo et al. model can well describe the non-isothermal

crystallization process of pure PLA and its nanocompos-

ite. Similar phenomena have also been reported for a PLA-

modified carbon black composite by Su et al. [21] .

3.4 Activation energy describing the overall crystallization process

It is known that the crystallization process of polymers

is controlled by two factors: one is the dynamic factor

relating to the activation energy for the transport of crys-

talline units across the phase boundary, and the other

is the static factor relating to the free energy barrier for




nucleation. Considering the effect of the cooling rate on

the non-isothermal crystallization process, Kissinger sug-

gested a method for determining the activation energy, as

shown in Eq. (9) [36] :

2ln( / )-

( 1 / )

p

p

d T Ed T R

φ⎡ ⎤ Δ⎣ ⎦ =

(9)

where Δ E is the non-isothermal crystallization activation

energy, R is the universal gas constant and T p is the peak

crystallization temperature at a certain cooling rate φ . By

plotting ( )ln / pTφ 2 against 1/ T p based on the data in Table 1,

two straight lines can be obtained, as shown in Figure 8 .

The Δ E values can be deduced from the slopes of the plots

in Figure 8 and are found to be -154.3 and -179.5 kJ/mol

for pure PLA and PLA/0.1 wt % graphene nanocomposite,

respectively. As the energy released during transforming

the molten state into the crystalline state, the obtained

values are negative. This shows that the addition of gra-

phene increases the non-isothermal crystallization activa-

tion energy of PLA, indicating an impeding effect on the

non-crystallization of PLA. However, the crystallization

rate of PLA/0.1 wt % graphene nanocomposite is faster

than that of pure PLA. The reason for this phenomenon

could be that the nucleation effect of graphene plays a

major role in the whole non-isothermal crystallization

process of PLA in this study.

3.5 Spherulitic growth behavior

In order to support the crystallization data analyzed by

DSC, the spherulite growth behavior was investigated by

POM. Figure 9 shows the POM images of pure PLA and

PLA/0.1 wt % graphene nanocomposite taken at 65 ° C

during non-isothermal crystallization from their melts,

Figure 8 Kissinger plots of ( / )φ 2

pln T versus 1/ T p for pure

poly(lactic acid) (PLA) and PLA/0.1 wt % graphene nanocomposite.

Figure 9 Polarized optical microscopy (POM) images of (A) pure

poly(lactic acid) (PLA) and (B) PLA/0.1 wt % graphene nanocompo-

site at 65 ° C during non-isothermal crystallization from their melts at

a cooling rate of 2 ° C/min.

at a cooling rate of 2 ° C/min. Obviously, the addition of

graphene into the PLA matrix had a significant effect on

the crystal morphology of PLA. The growth of spheru-

lites in pure PLA is fairly large and more perfectly grown

than those in the PLA/0.1 wt % graphene nanocomposite.

Besides, the PLA/0.1 wt % graphene nanocomposite shows

dense spherulites and the size of spherulite decreases

with the addition of graphene. These observations are

further evidence for the nucleating effect of graphene in

the PLA matrix.

4 Conclusions In this work, PLA/graphene nanocomposite were suc-

cessfully prepared by solution blending, and graphene




nanosheets were homogenously dispersed in the PLA

matrix; no obvious aggregations were found in the nano-

composite. The effect of graphene on the non-isother-

mal crystallization behavior of PLA was investigated by

DSC at various cooling rates. The results show that the

crystallization peak temperatures of the PLA/0.1 wt %

graphene nanocomposite are higher than that of pure

PLA at a given cooling rate. The values of halftime of

crystallization indicate that the graphene could act as a

heterogeneous nucleating agent and enhance the crys-

tallization rate during the non-isothermal crystallizing

process. Based on the DSC data, the non-isothermal crys-

tallization kinetics were analyzed by Avrami, Ozawa and

Mo et al. models. The Avrami and Ozawa models failed

to describe the whole non-isothermal crystallization

process for the samples, due to the omitting of the effect of

cooling rate and secondary crystallization, respectively.

However, the non-isothermal crystallization process of

pure PLA and the PLA/graphene nanocomposite can be

successfully described by the model developed by Mo

et al. The activation energies for the non-isothermal crys-

tallization process were calculated based on Kissinger ’ s

method and were found to be -154.3 and -179.5 kJ/mol for

pure PLA and PLA/0.1 wt % graphene nanocomposite,

respectively. Clearly, the presence of graphene has two

effects on the non-isothermal crystallization process of

PLA: one is the nucleation effect, which could acceler-

ate the crystallization rate, and the other is the impeding

effect, which could retard the transport of molecules to

the growing crystals, leading to a slower crystallization

rate. However, it worth noting that the nucleation effect

always plays a major role in the non-isothermal crystal-

lization process of PLA. Furthermore, POM results also

supported the DSC data and gave convincing evidence

for the nucleating effect of graphene in the PLA matrix.

Received September 18, 2012; accepted January 25, 2013; previously

published online March 21, 2013

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Nonisothermal crystallization kinetics of poly (lactide)—effect of plasticizers and nucleating...

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