Crystallization Behavior and Morphology of...

Crystallization Behavior and Morphology of Polylactideand PLA/Clay Nanocomposites in the Presence ofChain Extenders

N. Najafi, M.C. Heuzey, P.J. CarreauDepartment of Chemical Engineering, Center for Applied Research on Polymers and Composites (CREPEC),Ecole Polytechnique, Montreal, Quebec, Canada

The effect of clay and chain extender on the noniso-thermal, isothermal crystallization kinetics, and mor-phology of polylactide (PLA) was investigated in thisstudy. PLA and PLA-based nanocomposites containing2 wt% organoclay were prepared via melt compound-ing. Three commercially available chain extenders wereused: polycarbodiimide (PCDI), tris(nonylphenyl) phos-phite (TNPP), and Joncryl ADR4368F. The nanoclay par-ticles were found to act as nucleating agents. Chainextender incorporation, however, had diverse effectson both crystallization rate and degree of crystallinity.Nonisothermal DSC results revealed that the additionof PCDI increased the cold-crystallization temperature(Tc) from 106 to 1148C, reduced the degree of crystal-linity from 6.3 to 5.3%, and resulted in the formation ofbimodal melting peaks in PLA. On the other hand, thereduction of chain ends in the presence of TNPPresulted in a significant increase of the crystallizationrate and degree of crystallinity from 6.3 to 15.2%. Inthe case of Joncryl, its incorporation led to the forma-tion of a long-chain branching structure, which dis-rupted the chain packing. Therefore, the degree ofcrystallinity (from 6.3 to 1.6%) and the rate of crystalli-zation decreased, while Tc was increased from 106 to1228C in the presence of Joncryl. POLYM. ENG. SCI.,53:1053–1064, 2013. ª 2012 Society of Plastics Engineers

INTRODUCTION

The environmental impact of petroleum-based polymers

and their waste management has motivated their substitu-

tion by more environmentally benign products. The most

common biodegradable polymers are polylactic acid

(PLA), polycaprolactone (PCL), polybutylene adipate ter-

ephthalate (PBAT), and polyhydroxy butyrate (PHB) [1].

PLA has attracted the most attention among them, due to its

high strength, high modulus, good processability, transpar-

ency after processing, and commercial availability [2, 3].

PLA is a linear aliphatic thermoplastic polyester derived

from renewable plant sources such as starch and sugar beet

[4]. PLA can be synthesized either by direct condensation

polymerization of lactic acid (hence called polylactic acid)

[5] or by ring-opening polymerization of cyclic lactide (pol-

ylactide) [6], leading to the production of low and high-mo-

lecular weights, respectively. Despite all the advantages of

PLA, there are nevertheless some drawbacks such as poor

gas-barrier properties, low-mechanical resistance, and melt

strength [2] that restrict its practical applications. Many

attempts have been made to overcome such shortcomings

and improve PLA properties. Some of these efforts have

been directed toward the incorporation of nanoscale par-

ticles including organomodified layered silicate, carbon

nanotubes, and TiO2 [7–10]. Although the mechanical and

barrier properties of PLA can be improved by the incorpo-

ration of organically modified clay, thermal degradation of

PLA seems to be intensified with clay loading, leading to a

loss of molecular weight [11–13]. However, it has been

shown that the use of a chain extender during compounding

can compensate for the molecular weight decrease, with an

overall positive impact on mechanical and physical proper-

ties [12–15].

It is well known that crystalline properties play an

essential role in physical, mechanical, and gas-barrier

properties. Meanwhile, the crystallization rate and ulti-

mate crystalline morphology are impacted significantly by

the thermal history [16]. As a consequence, a considerable

attention has been devoted to the fundamental understand-

ing of PLA-crystallization kinetics [3, 7, 10, 16–20].

Crystallization is a process associated with partial align-

ment of polymer chains, starting from nucleation and fol-

lowed by subsequent growth. In addition to thermal his-

tory, other factors strongly affect the nucleation process.

Crystal nucleation is considerably influenced by impur-

ities, dyes, fillers, plasticizers, etc. Once the nuclei are

formed, segments of a chain pull out of the amorphous

phase, fold together, and sequentially attach to the growth

front, forming an ordered structure called lamellae. The

resulting lamellar crystals then organize themselves into

Correspondence to: M.C. Heuzey; e-mail: [email protected]

Contract grant sponsor: NSERC (Natural Science and Engineering

Research Council of Canada).

DOI 10.1002/pen.23355

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

VVC 2012 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2013

larger spheroidal entities named spherulites [21]. Detailed

investigation in both melt crystallization and cold crystal-

lization of PLA and its stereocomplex have been exten-

sively performed [17, 19, 22]. It has been reported that

the degree of crystallization, crystallization growth rate,

and the resulting crystalline morphology are highly influ-

enced by chain structure, concentration of the residual

monomer, nucleating agent, thermal history, and environ-

mental factors like shear/stretching flow [17, 19, 22].

Besides these factors, the molecular weight is also a key

parameter governing the crystallization kinetics.

As previously mentioned, the incorporation of organo-

clay particles into PLA leads to further thermal degrada-

tion, decreasing the molecular weight and mechanical

properties. To control such degradation in PLA nanocom-

posites, chain extenders can be used, resulting in a reat-

tachment of cleaved polymer chains [12]. The impact of

different chain extenders, polycarbodiimide (PCDI), tris(-

nonylphenyl) phosphite (TNPP), hexamethylene diisocya-

nate (HDI), pyromellitic dianhydride (PMDA), and

Joncryl 1ADR 4368F on the thermal degradation of PLA

and PLA/clay nanocomposites was investigated in our

previous works [12, 13]. The effect of chain extender and

processing conditions on clay delamination and the result-

ing rheological, mechanical, and gas-barrier properties

were also investigated in our previous studies [9, 12].

Although the impact of PCDI and TNPP on the noniso-

thermal crystallization of PLA was briefly reported in Refs.

[23, 24], to our knowledge, no comprehensive report has

been published on the influence of these chain extenders on

the nonisothermal and isothermal crystallization behavior

of PLA-based nanocomposites. The objective of this work

is to show how the presence of a chain extender affects the

crystallization behavior of PLA and PLA-based nanocom-

posites using differential scanning calorimetry (DSC) and

polarized optical microscopy (POM).

EXPERIMENTAL

Materials

The PLA investigated in this study, PLA 4032D, is a

semicrystalline grade from NatureWorks, USA, having a D-

LA content of 2 mol%. The organomodified clay is Cloi-

site1 30B, supplied by Southern Clay Products. In addition,

three different chain extenders were used in this work: PCDI,

a carboxyl-reactive chain extender, TNPP, and Joncryl1

ADR 4368F. The two former ones were purchased from

Sigma Aldrich, Canada, and the latter was supplied by

BASF, Germany. All these products were used as received.

The molecular formulae of the chain extenders and the orga-

nomodifier of Cloisite 30B are presented in Fig. 1.

PCDI (Fig. 1a) is a carboxyl reactive chain extender

where the carbodiimide (��N¼¼C¼¼N��) groups react

with the ��COOH groups of PLA to decrease the number

of active sites for degradation [12, 24]. TNPP (Fig. 1b)

has phosphate groups that react with the hydroxyl end

groups PLA and produce longer PLA chains through

transesterification [12, 23]. Joncryl (Fig. 1c) is a modified

acrylic copolymer with epoxy functions. The epoxy

groups of Joncryl can theoretically react with both

hydroxyl and carboxyl groups of the polyester, leading to

the formation of branched polymer chains [12, 25].

FIG. 1. Chemical structure of (a) polycarbodiimide (PCDI) [24], (b) tris(nonylphenyl) phosphate (TNPP)

[23], c) Joncryl ADR, where x, y, and z are all between 1 and 20 [25], and (d) the organomodifier of Cloisite

30B, where T is tallow [26].

1054 POLYMER ENGINEERING AND SCIENCE—-2013 DOI 10.1002/pen

Finally, Cloisite1 30B (Fig. 1d) is an organically modi-

fied clay with two hydroxyl groups. The interaction

between the C¼¼O groups of PLA and the hydroxyl

groups of the organomodifier makes it highly compatible

with the PLA matrix [26].

Material Processing

Melt compounding of PLA with clay and chain ex-

tender was carried out in a counter-rotating Brabender

Plasti-Corder1 internal mixer. Before mixing, PLA and

clay were dried at 708C in a vacuum oven for 48 h. The

dried PLA was compounded in the molten state with 2

wt% dried clay and chain extender in the internal mixer.

The nomenclature used for the nanocomposites is as fol-

lows: PLA-2C–PCDI, PLA-2C–TNPP, and PLA-2C–J for

the systems containing 2 wt% PCDI, 1 wt% TNPP, or 1

wt% Joncryl as a chain extender and PLA-2C for PLA

and clay only. For comparison purposes, PLA without

and with chain extender but without clay was also com-

pounded under the same conditions and are, respectively,

named neat PLA, PLA–PCDI, PLA–TNPP, and PLA–J.

The mixing was conducted under nitrogen atmosphere at

a rotation speed of 100 rpm for 11 min, while the temper-

ature was set at 1908C. After mixing, the various systems

were immediately immersed in liquid nitrogen to avoid

thermo-oxidative degradation during cooling. Thereafter,

the processed materials were placed in a vacuum desicca-

tor at ambient temperature for further use.

Characterization

The nonisothermal melt-crystallization behavior of the

various specimens was investigated using a TA Instru-

ments differential scanning calorimeter (DSC-Q 1000)

under nitrogen atmosphere. Samples (�10 mg) were

encapsulated in aluminum standard pans. To eliminate

any initial thermal history, the samples were heated at a

scanning rate of 108C/min from 30 to 2508C, held for 2

min and then cooled to 308C at the same rate. The crys-

tallization enthalpy (DHc), melting enthalpy (DHm), and

degree of crystallinity were determined from the second

heating cycle performed at the same heating rate. To

study the kinetics of isothermal cold-crystallization behav-

ior (from glassy state) in the temperature range of 80–

1208C, all the specimens were first heated at 608C/min to

2508C and held there for 5 min. Then the molten samples

were cooled at 608C/min to 308C and subsequently

reheated at the same rate to the desired crystallization

temperature (Tc). The protocol used to study the isother-

mal crystallization behavior was similar to Refs. [27, 28].

The samples were kept at Tc until the crystallization was

complete. To compare the kinetics of isothermal melt

with that of cold crystallization, the specimens were

heated to 2508C at 608C/min and held there for 5 min.

Then the molten samples were cooled at the same rate to

the desired crystallization temperature (Tc).The crystallization was monitored using a Nikon Opti-

phot-2 polarizing microscope to follow the formation and

growth of the spherulites. Thin films with a thickness of

�100 lm were prepared using compression molding. A

film was placed between two glass slides and heated on a

programmable hot stage at a heating rate of 508C/min to

1908C, where the sample was kept for 5 min to eliminate

the initial thermomechanical history. Subsequently, the

sample was cooled at the same rate to a set temperature

of 1308C to observe crystallization.

FIG. 2. DSC thermograms of the second heating for PLA and PLA-

based nanocomposites with and without chain extenders. The heat flow

axis has been shifted for clarity. The dotted vertical indicates neat PLA

cold-crystallization temperature.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 1055

Fourier transform infrared absorption spectra were

obtained using a Perkin Elmer FTIR spectrometer. The

attenuated total reflectance mode was used to measure the

IR absorption in the solution state. Samples (�4 g) were

dissolved in 10 mL of chloroform. The spectral resolution

and scanning speed were adjusted to 4 cm21 and 32 kHz,

respectively. The spectra were acquired after subtraction

of the chloroform absorption obtained under the same

conditions.

RESULTS AND DISCUSSION

Nonisothermal Analysis

PLA is known to crystallize slowly in comparison with

other polyesters such as PCL and polyethylene terephtha-

late (PET) due to the rigid segments in its main chain

[19]. The nonisothermal melt-crystallization behavior of

PLA and PLA-based nanocomposites with and without

chain extender is shown in the DSC thermograms in Fig.

2. The second heating is shown. From these thermograms,

the degree of crystallinity (Xc) is determined according to

Eq. 1:

Xc %ð Þ ¼ DHm � DHc

DH0m

fPLA

100

� �0@

1A� 100 (1)

where DHm and DHc are the measured melting and crys-

tallization enthalpies, respectively, and fPLA is the PLA

weight percent in the sample. For the enthalpy of fusion

(DHom) of a perfectly crystalline PLA, a value of 93.6 J/g

has been used [20, 29]. The main features of the DSC

thermograms are summarized in Table 1.

Crystallization of the neat PLA during the cooling pro-

cess was insignificant (not shown), whereas it easily crys-

tallized during the heating process (cold-crystallization).

A cold-crystallization exotherm followed by a melting

peak is observed for the neat PLA at 106 and 1678C,respectively, quite close to the values reported in previous

references [3, 17]. The incorporation of PCDI into PLA

increased the cold-crystallization temperature from 106 to

1148C, while it slightly decreased the extent of crystallin-

ity (from 6.3 to 5.3%). This behavior has also been

observed by Ding et al. [30] in a PCDI-poly(p-dioxanone)system. Based on their explanation, the molecular mobil-

ity is confined due to the introduction of rigid segments

(including phenyl groups) into the PLA, and hence the

crystallization ability is declined. To verify this explana-

tion, the FTIR spectra of PLA, PCDI, and PLA treated by

PCDI were obtained and presented in Fig. 3.

The reactive functional groups of PCDI, carbodiimide

(��N¼¼C¼¼N��) exhibit a characteristic infrared band at

2130 cm21 in the FTIR spectrum of PCDI alone. How-

ever, this peak is absent in the FTIR spectrum of PLA

treated by PCDI, indicating that the carbodiimide groups

have been consumed by the reaction with the chain ex-

tender. The phenyl rings exhibit a characteristic infrared

band at �1530 cm21 [31]. The FTIR results indicate the

presence of this group in the spectrum of PCDI alone and

the PLA–PCDI system, hence confirming the incorpora-

tion of PCDI into PLA.

For this system, a bimodal melting peak is observed in

Fig. 2 at 159 and 1678C. Such a multiple melting behav-

ior has also been observed by other authors for PLA sys-

tems [18, 22, 32] and reflects the melting process of crys-

tals having different degree of perfection. Considering

that the incorporation of PCDI into PLA further reduces

the segmental mobility of polymer chains, a more imper-

fect crystallites are expected to form in the PLA–PCDI

system when compared with neat PLA. Such small and

imperfect crystallites tend to melt at a lower temperature

in comparison with the more perfect crystals. Based on

this explanation, the low and high-melting peaks in the

second heating run of the PLA–PCDI system may corre-

spond to the melting points of less and more perfect crys-

tallites, respectively.

TABLE 1. Cold crystallization and melting temperatures, enthalpies,

and degree of crystallinity of neat PLA and PLA-based nanocomposites

with and without chain extender.

Tc (8C) Tm (8C) DHc (J/g) DHm (J/g) Xc (%)

Neat PLA 106 167 30.2 36.1 6.30

PLA-PCDI 114 159 and 167 31.1 36.0 5.30

PLA-TNPP 106 166 22.3 36.5 15.2

PLA-J 122 165 26.8 28.3 1.60

PLA-2C 103 166 29.0 36.9 8.70

PLA-2C-PCDI 107 167 26.5 34.0 8.30

PLA-2C-TNPP 103 168 24.1 38.0 15.4

PLA-2C-J 105 165 21.8 25.7 4.30

Data were obtained from the second heating cycle in DSC.

FIG. 3. FTIR spectra of PLA, PCDI, and PLA treated by PCDI

(adapted from [12]).


It has been shown that the addition of TNPP to PLA

increases the thermal stability and, in some cases, its mo-

lecular weight [12, 23, 33]. Theoretically, the crystalliza-

tion rate and degree of crystallization should decrease as

the molecular weight increases due to a reduction in chain

mobility. In contrast to expectations, a significant increase

in the degree of crystallinity (from 6.3 to 15.2%) is

reported in Table 1 for the PLA treated by TNPP. The

reason for this increase is discussed later.

The DSC thermograms of Fig. 2 and the data collected

in Table 1 indicate that the addition of Joncryl to PLA

decreases the extent of crystallinity from 6.3 to 1.6%,

while the crystallization temperature is shifted from 106

to 1238C. In our previous work, it was found that Joncryl

led to the formation of a long-chain branched (LCB)

structure in Joncryl-treated PLA and that it had a pro-

found effect on molecular weight [12]. The presence of

branches disrupts the packing of polymer chains, thus pre-

venting crystallization. The decreased chain mobility

caused by the increased molecular weight and LCB, on

the other hand, is responsible for the increased cold-crys-

tallization temperature.

The presence of clay nanoparticles is found to affect

the nucleation and crystal growth rate of PLA nanocom-

posites [7, 8, 10]. Figure 2 and Table 1 indicate that the

crystallization temperature (Tc) of PLA without chain ex-

tender was slightly reduced (from 106 to 1038C), and the

degree of crystallinity increased from 6.3 to 8.7% after

clay loading (PLA-2C). These findings suggest that the

nanosized dispersed clay particles may act as nucleating

agents in the PLA nanocomposites, leading to enhanced

nucleation and facilitating the crystallization process [7,

8, 10]. The nucleating effect of clay particles is more pro-

nounced in the PLA nanocomposite treated with PCDI

(PLA-2C–PCDI). The resulting nanocomposite has a

higher degree of crystallinity (8.3%) and a lower Tc(1078C) in comparison with the PLA–PCDI system (Xc ¼5.3% and Tc ¼ 1148C). However, the lower mobility of

polymer chains caused by the presence of PCDI rigid seg-

ments still leads to an increase in cold-crystallization tem-

perature from 103 to 1078C when compared with the

nanocomposite with no chain extender (PLA-2C).

A comparison of the nanocomposite containing TNPP

(PLA-2C–TNPP) with the PLA–TNPP system reveals that

their degree of crystallization is comparable, although the

Tc of the resultant nanocomposite is lower (1038C) than thatof TNPP-treated PLA (1068C). In spite of that, its degree of

crystallinity (15.4%) is higher than that of PLA (6.3%) and

the PLA nanocomposite without chain extender (8.7%).

The DSC results of the PLA nanocomposite with Joncryl

(PLA-2C–J) are also shown and reported in Fig. 2 and Table

1. On the basis of our last results [9, 12], the effect of Joncryl

in the PLA nanocomposite is not as spectacular as for neat

PLA due to the degradation of the matrix, highly favored by

the presence of the clay. Therefore, polymer chains of a

lower molecular weight and less LCB are expected to form

in the resulting nanocomposite when compared with the

PLA–Joncryl system. Such a decrease in molecular weight

accompanied by less LCB, as well as the nucleating effect of

the clay particles, is responsible for the decreased cold-crys-

tallization temperature from 122 to 1048C and the increased

degree of crystallization of the Joncryl-based nanocomposite

from 1.6 to 4.3% compared to the PLA–Joncryl system.

However, the LCB structure of the resultant nanocomposite

obstructs chain packing, leading to a reduction of its crystal-

linity (1.6%) in comparison with that of the neat PLA (6.3%)

and PLA nanocomposite without chain extender (8.7%).

Isothermal Analysis

The isothermal crystallization behavior of PLA and

PLA-based nanocomposites with and without chain ex-

tender was investigated by DSC in a temperature range

between 80 and 1208C. The results in the form of thermo-

grams for all systems at Tc ¼ 1108C and for Joncryl-based

nanocomposite at different crystallization temperatures are

presented in Fig. 4a and b, respectively. The thermograms

for the other crystallization conditions showed very similar

trends and, hence, are not presented, but all the correspond-

FIG. 4. Isothermal crystallization thermograms of (a) PLA and PLA

nanocomposites with and without different chain extenders at 1108C and

(b) Joncryl-based nanocomposite at different crystallization temperatures.


ing enthalpy values are listed in Table 2. Similarly to the

nonisothermal crystallization case, the addition of a chain

extender and clay particles strongly influenced the crystalli-

zation behavior. The addition of organoclay as a filler and

TNPP as a chain extender slightly increased the measured

values of DH, while the incorporation of PCDI and/or

Joncryl into PLA and the PLA-based nanocomposite

decreased the enthalpy values. To quantify how the chain

extenders affect the rate of crystallization, the relative

degree of crystallinity, Xt, versus the crystallization time, t,is considered and plotted in Fig. 5. The relative degree of

crystallinity is defined as Eq. 2:

Xt ¼DHðtÞDHð1Þ

(2)

where DH(t) is the enthalpy of isothermal crystalliza-

tion at time t and DH(!) is the enthalpy of complete crys-

tallization, both calculated from Eq. 3 and the data from

Fig. 4:

DHðtÞ ¼Z1

0

dH

dt

� �:dt

DHð1Þ ¼Z1

0

dH

dt

� �:dt

(3)

To further quantitatively describe the evolution of the

crystallinity during isothermal crystallization, we used the

well-known Avrami model, which is the most common

and easiest approach to obtain relevant parameters charac-

terizing the crystallization kinetics [34, 35]. The relative

crystallinity can be calculated from Eq. 4:

XðtÞ ¼ 1� expð�ktnÞ (4)

TABLE 2. Summary of isothermal crystallization data of PLA and PLA-based nanocomposites in the presence of different chain extenders, along

with parameters for Avrami equation.

Sample T (8C) t1/2 exp (min) DH (J/g) k (min2n) n m R2 t½ cal (min)

PLA 80 28.4 18.9 2.86 3 1027 4.41 0.41 0.998 28.1

90 5.40 21.5 5.24 3 1024 4.25 0.25 0.996 5.42

100 2.36 27.2 1.24 3 1022 4.61 0.61 0.992 2.37

110 1.67 26.1 9.85 3 1022 3.86 20.14 0.996 1.66

120 1.61 31.9 5.44 3 1022 5.27 1.27 0.990 1.62

PLA-PCDI 80 29.5 17.2 6.32 3 1029 5.47 1.47 0.997 29.5

90 6.69 21.3 5.64 3 1026 6.14 2.14 0.990 6.71

100 2.70 23.6 2.24 3 1023 5.75 1.75 0.992 2.70

110 1.61 27.1 7.54 3 1022 4.57 0.57 0.994 1.62

120 1.74 31.8 2.57 3 1022 5.63 1.63 0.987 1.77

PLA-TNPP 80 16.1 20.5 4.65 3 1027 5.10 1.1 0.996 16.2

90 2.56 24.6 1.19 3 1022 4.29 0.29 0.998 2.58

100 1.21 29.0 0.23 5.48 1.48 0.994 1.22

110 0.93 33.3 1.02 4.45 0.45 0.998 0.92

120 0.96 30.1 0.72 6.53 2.53 0.961 0.99

PLA-J 80 46.2 10.1 7.54 3 10210 5.39 1.39 0.999 46.1

90 10.8 20.0 2.83 3 1024 3.26 20.74 0.995 10.9

100 4.72 22.1 2.77 3 1023 3.55 20.45 0.996 4.73

110 2.75 17.3 1.03 3 1022 4.11 0.11 0.990 2.78

120 2.55 25.7 5.33 3 1023 5.15 1.15 0.988 2.56

PLA-2C 80 18.1 20.0 1.86 3 1027 5.23 1.23 0.998 18.1

90 3.38 23.1 6.04 3 1023 3.83 20.17 0.992 3.40

100 1.47 27.5 0.11 4.71 0.71 0.993 1.48

110 0.62 31.5 4.43 4.01 0.01 0.989 0.63

120 0.90 33.3 1.14 5.36 1.36 0.974 0.91

PLA-2C-PCDI 80 18.5 19.2 1.68 3 1027 5.21 1.21 0.999 18.5

90 3.40 21.6 1.99 3 1023 4.74 0.74 0.995 3.43

100 1.72 25.5 2.65 3 1022 5.96 1.96 0.995 1.73

110 0.92 26.1 0.96 4.37 0.37 0.998 0.93

120 1.11 32.1 0.31 6.06 2.06 0.978 1.14

PLA-2C-TNPP 80 12.6 21.1 4.99 3 1026 4.66 0.66 0.999 12.7

90 2.13 24.0 2.43 3 1022 4.38 0.38 0.998 2.15

100 1.24 29.6 0.22 5.12 1.12 0.997 1.24

110 0.61 34.6 4.53 4.27 0.27 0.989 0.64

120 0.93 30.4 0.91 6.04 2.04 0.967 0.96

PLA-2C-J 80 17.4 14.5 1.34 3 1026 4.60 0.6 0.998 17.5

90 2.98 14.6 5.44 3 1023 4.39 0.39 0.991 3.01

100 1.60 23.4 7.19 3 1022 4.60 0.6 0.981 1.63

110 0.83 27.6 1.30 3.52 20.48 0.993 0.83

120 1.01 23.8 0.59 5.79 1.79 0.974 1.02


where X(t) is the relative degree of crystallinity (from Eq.2), n is the Avrami index, and k is the overall crystalliza-

tion rate constant including nucleation and crystal growth

contributions. The Avrami constants k and n can be calcu-

lated by fitting the experimental data to Eq. 5, obtainedafter taking the double logarithm of Eq. 4:

lnð� lnð1� XðtÞÞÞ ¼ ln k þ n lnðtÞ (5)

Because the Avrami model rarely describes the whole

conversion range [36], the relative crystallinity data between

5 and 80% were used to calculate k and n. The plots of ln

[2ln (1 2 X (t)] as a function of ln (t) for the data obtainedat Tc ¼ 1108C for the PLA and PLA-based nanocomposites

with and without chain extenders are presented in Fig. 6.

The behavior at the four other crystallization tempera-

tures was similar, and, for conciseness, the results are not

presented here. However, the calculated k, n, and the cor-

relation coefficient of the fit (R2) are all summarized in

Table 2. Most correlation coefficient values are larger

than 99%, indicating good fits of the experimental data in

Fig. 5.

The Avrami index, n, is a constant with a typically in-

teger value between 1 and 4, depending on different fac-

tors such as nucleation type, nucleation density, crystal

growth dimension, and restriction of crystalline formation

due to surrounding fillers [17].

The index value of 1, 2, and 3 indicates one, two, and

three-dimensional spherulites growth, respectively. The

fractional Avrami index value or value greater than 4 can

theoretically be explained by the introduction of the

nucleation index, m (Eq. 6), describing the nucleation

mechanism throughout the crystallization process [37]:

m ¼ n� n0 � 1 (6)

where n0 is the dimensionality index (e.g., n0 ¼ 1 for rods,

n0 ¼ 2 for disks, and n0 ¼ 3 for spheres). m values

between 1 and 0 represent instantaneous and sporadic

nucleation gradually decreasing with time and approach-

ing a constant value at a certain time, while a value of 0

indicates sporadic nucleation steadily increasing with

time. An m value from 0 to 1 and greater than 1 indicates

a sporadic nucleation, which increases and markedly

FIG. 5. Relative crystallinity (Eq. 4) of (a) PLA and PLA nanocompo-

sites with and without different chain extenders at 1108C and (b)

Joncryl-based nanocomposite at different crystallization temperatures.

FIG. 6. Avrami fits of the isothermal crystallization data of (a) PLA

and PLA nanocomposites with and without different chain extenders at

1108C and (b) Joncryl-based nanocomposite at different crystallization

temperatures.


increases with time, respectively [37]. A large variety of

n values have been reported for PLA in the literature [17,

38]. The data summarized in Table 2 reveal that the

obtained Avrami exponent ranges from 3 to 6 in these

samples, suggesting a three-dimensional spherulite growth

(n0 ¼ 3). The calculated m values, reported in Table 2,

are mainly greater than zero but smaller than 1 indicating

that the nucleation process is most likely governed by

sporadic nucleation, which increases with time.

The k values of Table 2 as well as the data of Figs. 4

and 5 show that the addition of PCDI to the neat PLA

leads to a reduction in the crystallization rate. For exam-

ple, at the crystallization temperature of 1008C, the kvalue varies from 1.3 3 1022 min2n for the neat PLA to

2.2 3 1023 min2n for the PLA treated with PCDI. The

temperature of 1008C is chosen in the comparison of the

k values; however, the trend is the same at all crystalliza-

tion temperatures. The incorporation of rigid PCDI frag-

ments into the PLA confines the molecular mobility, as

shown by the FTIR results, and subsequently reduces the

rate of crystallization. It is interesting to note that the

crystallization of the PLA and PLA–PCDI system almost

terminates at the same time in Fig. 4, even though the

PLA crystallization begins sooner in comparison with that

in the PLA–PCDI system. This behavior indicates that the

crystallization rate of the PLA is gradually decreased at

later stage of crystallization when compared with that of

the PLA–PCDI system (see Fig. 5). The faster growth of

PLA crystallites at early stage of the crystallization may

lead to crystal impingements. Such impingements, there-

fore, retard the crystallization kinetics at later stage of the

crystallization. This explanation is in good agreement

with what is observed by optical microscopy as discussed

later.

In contrast with PCDI, the incorporation of TNPP to

PLA results in a significant increase of the PLA crystalli-

zation kinetics, even though the molecular weight is also

increased [12]. Based on the explanation of Carvalho

et al. [39], decreasing the number of chain ends in the

system as a consequence of short chains removal is re-

sponsible for the increased crystallization rate. Also,

Richter and coworkers [40] explained that chain ends are

much more mobile than mid-chain segments. The high

mobility of the chain ends decreases the likelihood of

their attaching to the growth front in comparison with a

stem from a mid-chain region.

Consequently, the chain ends are more resistant to

being folded into a compact crystalline form, acting as an

entropic defect at the crystal growth front, and subse-

quently decrease the crystal growth rate. As reported in

our previous work [12], the addition of TNPP decreases

the number of chain ends per mass in PLA, resulting in a

decrease of the defects at the growth front and promoted

crystallization kinetics when compared with neat PLA

(the k value changes from 1.3 3 1022 to 0.23 min2n).

The decreased number of chain ends per mass resulting

from the TNPP incorporation is also responsible for the

increased degree of crystallinity in PLA and PLA-based

nanocomposites containing TNPP during nonisothermal

crystallization, as shown in Table 1.

On the contrary, the addition of Joncryl to PLA signifi-

cantly reduces the crystallization kinetics (k value changes

from 1.3 3 1022 to 2.77 3 1023 min2n) as evidenced in

Table 2 and Figs. 4 and 5. The epoxy groups, present in

Joncryl (Fig. 1), react with hydroxyl and carboxyl groups

of the polyester and form a LCB structure [12]. The LCB

structure and branch points hinder the chain-folding phe-

nomenon, which is required for the incorporation of the

chains into growing crystalline lamellae.

As reported by other authors [7, 8, 10], the presence of

clay particles increases the crystallization kinetics of

PLA. Generally, two major factors, nucleation and mobil-

ity of chain segments, control the crystallization behavior.

As shown in Fig. 4 and Table 2, the inclusion of clay par-

ticles in neat PLA and PLA with chain extender decreases

the crystallization halftime from 2.36 to 1.47 min at Tc ¼1008C. This is due to heterogeneous nucleation that

results in narrower crystallization peaks, indicating that

the rate of crystallization is increased (k ¼ 0.11 min2n)

when compared with that of neat PLA (k ¼ 1.24 3

1022 min2n). In the case of the PLA-based nanocompo-

site-containing PCDI and Joncryl, the nucleating effect of

clay particles promotes the crystallization kinetics on one

hand. On the other hand, less chain mobility in PCDI-

enriched system and the LCB structure in the Joncryl

nanocomposite results in the retardation of the crystalliza-

tion process; the k value changes from 0.11 to 2.6 3

1022 and 7.2 3 1022 min2n in nanocomposite-containing

PCDI and Joncryl, respectively.

The comparison of the crystallization kinetics between

the nanocomposite-containing PCDI and those treated by

Joncryl is also interesting. Joncryl-based nanocomposite

shows a larger crystallization rate than the nanocompo-

site-containing PCDI at the early stage of the crystalliza-

tion, while a reverse trend is observed for the nanocompo-

site-containing PCDI as the relative crystallinity increases,

since the crystallization is finished at a shorter time (see

Fig. 5). Based on what is reported in Refs. [41, 42], the

crystallization rate is first controlled by nucleation and

then crystal growth and packing. We assume that the for-

mation of a branched structure in the nanocomposite-con-

taining Joncryl leads to an enhancement of the density of

nuclei, increasing the crystallization rate at the early stage

of crystallization. However, suppression or disruption of

chain packing within the crystallites, caused by a LCB

structure, then retards the crystal growth at later stages.

Comparing the k parameter of the various samples

shows that its value first increases with increasing crystal-

lization temperature, followed by a decrease above

1108C. Such a behavior is expected in polymer due to a

balance between two opposing effects. At low-crystalliza-

tion temperatures, close to the glass transition temperature

(Tg � 608C), the decreased chain mobility significantly

retards the crystallization rate, whereas at high-crystalliza-


tion temperatures, close to equilibrium melting tempera-

ture (Tm0), a considerable decrease in nucleation density

hinders the crystallization growth, although the chain mo-

bility is high. Moreover, the comparison of different sys-

tems at a given crystallization temperature shows that the

incorporation of PCDI and Joncryl into PLA decreases

the k value, especially at low-crystallization temperatures,

close to Tg, where the segmental mobility is the dominant

factor controlling the crystallization rate. However, the

TNPP and clay addition result in an increased k value, in

complete agreement with what is explained earlier.

To further verify whether these systems closely follow

the Avrami model, the obtained k and n are used to calcu-

late the crystallization halftime (t1/2) using Eq. 7.

t=1=2cal

ln 2

k

� �1=n (7)

The calculated t1/2 is also reported in Table 2. The

comparison between the experimental and calculated t1/2shows that the Avrami model can precisely predict t1/2,confirming that the crystallization kinetics of all the sys-

tems is correctly described by this model for the relative

degree of crystallinity between 5 and 80%.

To compare the overall crystallization rates from the

melt with glassy states, the isothermal crystallization of

the neat molten PLA and some nanocomposite samples

was also studied at 90 and 1108C. All the corresponding

data are listed in Table 3, although the thermograms are

not presented for the sake of brevity. The results indicate

that the overall crystallization rate in the molten state is

much slower than that from the glassy state (i.e., k value

of TNPP-enriched PLA decreased from 1.19 3 1022 and

1.02 to 1.9 3 1023 and 3.05 3 1023 min2n at 90 and

1108C, respectively). Similar results have been observed

in polypropylene by Supaphol [43].

Based on the Lauritzen and Hoffman’s theory [44], the

crystal growth rate is expected to be only a function of

the crystallization temperature, Tc. Supaphol [43] sug-

gested that faster crystallization from the glassy state

resulted from a higher contribution of nucleation mecha-

nisms (either as an enhancement of nucleation rate or

density). Indeed, the total number of activated nuclei that

can serve as predetermined homogeneous nuclei during

isothermal crystallization at Tc increased upon the

quenching process and led to an increase in the overall

crystallization rate. This argument can also be confirmed

by the calculated nucleation index, m, presented in Tables

2 and 3. Contrary to crystallization from the glassy state

(Table 2), m has a value less than zero in melt isothermal

crystallization (Table 3). This indicates that the instanta-

neous and sporadic nucleation gradually decrease with

time and approach a constant value in melt isothermal

crystallization, while such nucleation rates markedly

increase with time [37] in cold isothermal crystallization.

The crystal growth rate is usually defined as the

inverse of the crystallization halftime (G ¼ 1/t1/2), and

the variation of Gexp is plotted as a function of crystalliza-

tion temperature in Fig. 7. As expected, Gexp is strongly

dependent on the crystallization temperature. Gexp

increases first with increasing crystallization temperature

up to 1108C, passes through a maximum, and then is

reduced. The trend is consistent with the crystallization

kinetics (k values). A comparison of the neat PLA with

the PLA–PCDI and PLA–Joncryl systems reveals that the

restriction of chain movement in the PCDI and Joncryl-

enriched systems reduces Gexp when compared with that

of the neat PLA. However, the reduction of defects in

polymer chains promotes Gexp as TNPP is added to the

TABLE 3. Summary of melt-isothermal crystallization data of PLA and PLA-based nanocomposites in the presence of TNPP as a chain extender,

along with parameters for Avrami equation.

Sample T (8C) DH (J/g) t1/2 exp (min) k (min2n) n m R2 t1/2 cal (min)

PLA 90 22.3 6.18 6.12 3 1024 3.85 20.15 0.998 6.20

110 23.2 4.89 2.30 3 1023 3.58 20.42 0.998 4.92

PLA–TNPP 90 23.1 5.54 1.90 3 1023 3.45 20.55 0.999 5.53

110 26.2 4.48 3.05 3 1023 3.62 20.38 0.999 4.48

PLA-2C–TNPP 90 23.3 5.24 2.60 3 1023 3.36 20.64 0.999 5.27

110 27.6 4.05 4.58 3 1023 3.58 20.42 0.998 4.06

FIG. 7. Crystallization rate (G) of the PLA and PLA-based nanocom-

posites in the presence of different chain extenders as a function of crys-

tallization temperature.


neat PLA. The effect of clay loading on the crystallization

kinetics is evident in Fig. 7, especially at high-crystalliza-

tion temperature where nucleation is the dominant factor

controlling the crystallization kinetics. The formation of

additional nucleation sites by the clay particles is respon-

sible for such enhancement of Gexp.

Polarized Microscopy

POM was used to directly observe the effect of clay

and chain extenders on the crystal morphology of PLA.

Figure 8 displays the crystal morphology of PLA and

PLA nanocomposites with and without the presence of a

chain extender. The micrographs are arranged in three

columns. Columns 1 and 2 represent, respectively, the

polarized optical micrographs of samples isothermally

crystallized at 1308C for 10 and 15 min, while column 3

presents an approximate spherulite density and the occur-

rence of spherulite impingement.

The temperature of 1308C was chosen since the nucle-

ation process is slower at that temperature, and it allows

the observation of the spherulite grow. Figure 8a shows

that the spherulites are formed for the neat PLA after 10

min, and their size systematically increases with time

until the occurrence of spherulite impingement. The

impingement starts as adjacent growing spherulites meet

each other at a point, preventing spherulite growth at the

contact region. Figure 8b reveals that the spherulite

growth rate is clearly decreased as PCDI is added to

PLA, leading to the postponement of the spherulite

impingement. This is an expected observation, because

the introduced rigid segments as observed from the FTIR

results diminish the chain mobility and subsequently

retard the chain-folding process. On the contrary, the

incorporation of TNPP into PLA promotes the spherulite

growth (Fig. 8c) based on the reasons explained earlier in

relation to the results shown in Figs. 5 and 6. The crystal

morphology of Joncryl-treated PLA, presented in Fig. 8d,

on the other hand, shows the formation of dense finer

crystals in comparison with the neat PLA. This finding is

similar to what was observed in [13] and indicates that

the branching units obstruct packing of the polymer

chains and thus cause the cessation of the spherulite

growth.

The compounding of organoclay with PLA with and

without chain extender slightly enlarges the spherulite

size, as illustrated in Figs. 8e–h, and leads to the further

occurrence of spherulite impingement. Rangasamy et al.

[45] also observed a similar trend in PP–nanoclay system.

They believed that the increase of nucleation speed in the

nanocomposite promotes the crystallization rate, which

may change the crystal size. The comparison of the crys-

tal size of Joncryl-based nanocomposite (Fig. 8h) with

Joncryl-treated PLA (Fig. 8d) reveals that the spherulite

sizes in the nanocomposite appear to be larger than that

observed in the Joncryl–PLA system. As mentioned ear-

lier, a lower molecular weight and less long-chain branch-

ing (LCB), which favor the folding of polymer chains, are

responsible for the increased crystal size.

CONCLUSION

In this study, the nonisothermal and isothermal crystal-

lization behaviors of PLA and PLA–organoclay nanocom-

FIG. 8. Optical polarizing micrographs of (a) neat PLA, (b) PLA-

PCDI, (c) PLA-TNPP, (d) PLA-J, (e) PLA-2C, (f) PLA-2C-PCDI, (g)

PLA-2C-TNPP, and (h) PLA-2C-J isothermally crystallized at 1308C for

column 1: 10 min and column 2:15 min. Column 3 shows spherulite

impingement. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]


posites in the presence of three different chain extenders

(polycarbodiimide, PCDI, tris (nonylphenyl) phosphite,

TNPP, and Joncryl) were investigated using differential

scanning calorimetry (DSC) and polarized optical micros-

copy (POM). Both isothermal and nonisothermal crystalli-

zation studies suggested that organoclay particles act as

nucleating agents and yield a faster overall rate of crys-

tallization. The addition of PCDI increased the cold-crys-

tallization temperature and decreased the degree of crys-

tallinity. It was assumed that the lower mobility of poly-

mer chains, reacted with this chain extender, reduced

spherulite size and the crystallization growth rate, result-

ing in an increased crystallization halftime for the iso-

thermal case. In contrast, TNPP was found to signifi-

cantly intensify the crystallization process by reducing

the number of chain ends, acting as defects to incorpo-

rate into the crystals. The degree of crystallinity, crystal

growth rate, and subsequently spherulite size increased

as TNPP was added. The Joncryl incorporation, how-

ever, confined the crystallization process due to the for-

mation of a LCB structure, resulting in the disruption of

chain packing. Therefore, the degree of crystallinity and the

rate of crystallization decreased, while the cold-crystalliza-

tion temperature was dramatically increased in nonisother-

mal crystallization. The isothermal crystallization growth

rate of all the compounds was determined in the tempera-

ture range of 80–1208C. It first increased to attain a maxi-

mum at �1108C and was then followed by a reduction. The

Avrami analysis was used to quantify the effects of clay

and chain extenders on the isothermal crystallization behav-

ior. It was found that all systems satisfactorily follow the

Avrami equation and more likely exhibit a 3D spherulitic

growth.

A comparison of the overall crystallization rate param-

eters calculated from both cold and melt isothermal crys-

tallization processes revealed that the rate of crystalliza-

tion from the glassy state proceeded faster than that from

the melt state. This indeed suggests that the quenching

process increased the total number of activated nuclei that

can act as predetermined homogeneous nuclei upon subse-

quent crystallization at Tc.

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