crystallization behavior and nucleation analysis of poly(l...

Crystallization Behavior and Nucleation Analysis ofPoly(L-lactic acid) With a Multiamide Nucleating Agent

Ping Song, Zhiyong Wei, Jicai Liang, Guangyi Chen, Wanxi ZhangSchool of Automotive Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China

To accelerate the crystallization of poly(L-lactic acid)(PLLA) and enhance its crystallization ability, a multia-mide nucleator (TMC) was introduced into the PLLAmatrix. The thermal characteristics, isothermal andnonisothermal crystallization behavior of pure PLLAand TMC-nucleated PLLA were investigated by differ-ential scanning calorimetry. The determination of ther-mal characteristics shows that the addition of TMCcan significantly decrease the onset temperature ofcold crystallization and meanwhile elevate the totalcrystallinity of PLLA. For the isothermal crystallizationprocess, it is found that the overall crystallization rateis much faster in TMC-nucleated PLLA than in purePLLA and increases as the TMC content is increased,however, the crystal growth form and crystalline struc-ture are not influenced much despite the presence ofTMC. In the case of nonisothermal crystallization, thenucleation efficiency and nucleation activity were esti-mated and the results indicate that excellent nuclea-tion-promoting effect could be achieved when theweight percentage of TMC is chosen between 0.25%and 0.5%. Polarized optical microscopy observationreveals that the nuclei number of PLLA increases andthe spherulite size reduces greatly with the addition ofTMC. POLYM. ENG. SCI., 52:1058–1068, 2012. ª 2011 Societyof Plastics Engineers

INTRODUCTION

Poly(L-lactic acid) (PLLA) which is a high-strength,

high-modulus and eco-friendly thermoplastic, with poten-

tial to replace conventional petrochemical-based poly-

mers, has received increasing research interests owing to

its peculiar properties such as renewability, biodegradabil-

ity, biocompatibility, and less energy dependence during

the past few decades [1–3]. Based on the favorable physi-

cal features, PLLA can be processed by injection mold-

ing, film forming, fiber spinning, blow molding, extrusion

and expansion molding. PLLA offers great promise in

extensive application including packaging and containers,

agricultural and biomedical materials as well as engineer-

ing materials [4].

As a kind of semicrystalline polyester, the thermal,

mechanical, and degradation properties of PLLA are sig-

nificantly dependent on the level of crystallinity and crys-

tallization ability. However, it should be noted that the

crystallization rate of PLLA is extremely slow in contrast

with other commercial crystalline polymers [5]. In fact,

there are few crystals generating in PLLA resin and it

almost remains amorphous under the practical processing

and molding situation. This behavior has limited the

application of PLLA as an engineering material since the

modulus and thermal stability of amorphous products

remarkably drop above the glass transition temperature

[6]. Therefore, the promotion and improvement of crystal-

lization for PLLA is an essentially considered issue under

these circumstances.

As is well known, the most effective method to accel-

erate crystallization and enhance crystallinity is the incor-

poration of heterogeneous nucleator. Up to now, a number

of investigations have been conducted on nucleators for

PLLA, including various inorganic and organic additives.

Liao et al. studied the isothermal cold crystallization

kinetics of PLLA in the presence of CaCO3, TiO2, and

BaSO4 and found that the crystallinity of the sample

blended with 0.5 wt% BaSO4 was the highest whereas the

one with CaCO3 was the lowest [7]. Tsuji et al. reported

that the acceleration effect of additives on the overall

melt-crystallization of PLLA decreased in the following

order: poly(D-lactic acid) [ talc [ fullerene C60 [ mont-morillonite [ polysaccharides [8]. What’s more, Tsujiet al. found that biodegradable polyglycolide, polycapro-

lactone (PCL), and poly(3-hydroxybutyrate) can be effec-

tive crystallization-accelerating agents for PLLA [9].

Some other nucleators such as clay [10, 11], aliphatic

amide [12], hydrazide [13, 14], benzenetricarboxylamide

derivatives [15], myo-inositol [16], carbonated hydroxyap-

atite [17], carbon nanotube [18, 19], modified carbon

black [20], polyhedral oligomeric silsesquioxanes (POSS)

[21], and layered metal phosphonates [22, 23] were also

examined in terms of blending with PLLA. Very recently,

Correspondence to: Zhiyong Wei; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China;

contract grant numbers: 30870633, 31000427; contract grant sponsor:

National High Technology Research and Development Program of

China; contract grant number: 2009AA03Z319; contract grant sponsor:

China Postdoctoral Science Foundation Funded Project; contract grant

number: 20100481214; contract grant sponsor: Fundamental Research

Funds for the Central Universities.

DOI 10.1002/pen.22172

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

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

a multiamide compound (trademark: TMC-328, abbrevi-

ated as TMC in this article) has been used to control the

crystal superstructure of PLLA by self-organization in

PLLA melt and three characteristic crystal morphologies

including cone-like, shish-kebab, and needle-like struc-

tures have been successfully obtained [24]. To the best of

our knowledge, the crystallization behavior and nucleation

ability of TMC-nucleated PLLA have not been investi-

gated in detail. In this work, we prepared some PLLA

samples by using TMC as a nucleator from 0.25 to 1

wt%, and explored its crystallization behavior by differen-

tial scanning calorimetry (DSC) under both isothermal

and nonisothermal conditions. The main objective of this

study is to investigate the effects of TMC concentration,

crystallization temperature on the overall crystallization

rate and crystallization ability of PLLA, and offer a quan-

titative analysis in relation to the nucleation efficiency

(NE) and activity of PLLA nucleated with TMC.

EXPERIMENTAL

Materials

An injection grade PLLA resin (Biopla 305D) with a

density of 1.25 g/cm3 and melt flow index of 10–13 g/10

min (2.16 kg, 1908C) was purchased from Biopla Prod-ucts Factory, China. It had a weight-average molecular

weight of 1.2 3 105 g/mol and a polydispersity index of1.7. The multiamide compound (TMC-328), i.e. N,N0,N00-tricyclohexyl-1,3,5-benzenetricarboxylamide, was kindly

provided by Shanxi Provincial Institute of Chemical

Industry, China. Its chemical structure is shown in

Scheme 1.

Sample Preparation

The weight fraction of TMC in PLLA blends was

0.25, 0.5, 0.75, and 1 wt%. For brevity, the resultant

materials were denoted as TMC0.25, TMC0.5, TMC0.75,

and TMC1, respectively. Before mixing, PLLA was dried

at 608C for 24 h and TMC was dried at 1208C for 12 hunder vacuum. The samples of TMC-nucleated PLLA

were prepared in an internal mixer (SU-7013) at 1808Cand a rotor speed of 60 rpm for 15 min. In addition,

a pure PLLA sample was prepared with the identical

operated process for comparison.

Differential Scanning Calorimetry

The DSC experiments were performed in aluminum

pans using a Mettler-Toledo DSC1 instrument under the

nitrogen atmosphere. The instrument was calibrated by

means of high-purity indium and zinc standards, so as to

ensure accuracy and reliability of the data. Each sample

of about 6 mg was used. Relevant to different aspects,

four thermal programs were employed as follows:

Program A: The samples were first heated to 1858C at108C/min and held for 5 min to erase the previous ther-mal history. Then, they were quenched to room tempera-

ture at a cooling rate of 608C/min and reheated to 1858Cat 108C/min to determine the thermal characteristics.

Program B: For the isothermal crystallization tests,

each sample was initially melted at 1858C for 5 min toensure completely amorphous state, and then cooled rap-

idly at the rate of 608C/min to the designated crystalliza-tion temperature (Tc). When no more changes in the heatflow were observed, the samples were subsequently

heated to 1858C at 108C/min for exploring the meltingbehavior.

Program C: In the case of self-nucleation experiment

of pure PLLA, it was first heated at 108C/min to 1858Cand held for 5 min. The sample was then cooled to 808Cat 18C/min for the purpose of determining the crystalliza-tion peak temperature which was defined as the Tminp in-herent to PLLA without pre-existing nuclei. Subsequently,

it was heated again at 108C/min to the partial meltingtemperature (Tpm) and held for another 5 min to createself-nucleated sites. Finally, the sample was cooled to

808C at 18C/min and the crystallization peak temperatureof self-nucleated PLLA was determined. The graphic rep-

resentation of thermal program C is shown in Fig. 1.

Program D: For the nonisothermal crystallization, the

samples were heated to and kept at 1858C for 5 min, andthen cooled to room temperature at different cooling rates

of 0.5, 1, 1.5, 2, and 2.58C/min. The crystallization peaktemperature (Tp) of TMC-nucleated PLLA determined atthe cooling rate of 18C/min was used to estimate the NE.

Wide-Angle X-ray Diffraction

The wide-angle X-ray diffraction (WAXD) analysis

was carried out on a Rigaku D/Max-Ultimaþ X-ray dif-fractometer with Ni-filtered Cu Ka radiation (k ¼0.15418 nm). The operating target voltage was 40 kV and

SCHEME 1. Chemical structure of the multiamide compound (TMC-

328).

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 1059

the tube current was 10 mA. Scans were made between

58 and 308 at a scanning rate of 1.28/min.

Polarized Optical Microscopy

The spherulite morphology of pure PLLA and TMC-

nucleated PLLA were measured by a Zeiss Axio Lab. A1

polarized optical microscope equipped with a digital cam-

era. Thin film of each sample was sandwiched between

two glass slides. After melting in an oven at 1858C for5 min, it was quickly transferred to another oven preset to

1208C for isothermal crystallization.

RESULTS AND DISCUSSION

Thermal Characteristics

The second heating DSC curves of heat flow as a func-

tion of temperature for pure PLLA and TMC-nucleated

PLLA are shown in Fig. 2. The data of thermal character-

istics including glass transition temperature (Tg), onsettemperature of cold crystallization (Tcc,onset), peak temper-ature of cold crystallization (Tcc) and melting point (Tm),as well as the enthalpy of cold crystallization (DHcc) andmelting (DHm) for all the samples are obtained from thecorresponding curves and listed in Table 1. The crystallin-

ity (Xc1), which characterizes the total amount of crystal-line phase that develops in the sample below the melting

temperature, is calculated according to the following

equation:

Xc1ð%Þ ¼ DHmvPLLADHom� 100 (1)

where oPLLA is the weight fraction of PLLA in theblends, and DHom is the melting enthalpy of completely

crystalline PLLA, and its value is 93.7 J/g [25, 26]. It can

be seen that the incorporation of TMC scarcely affects Tg,but decreases Tcc,onset and Tcc of PLLA obviously. Thissuggests that TMC promotes the cold crystallization of

PLLA matrix, indicating an enhanced crystallization abil-

ity of PLLA. On the other hand, the increasing Xc1 (from17.6% to 36.2%) reveals that it behaves as an effective

nucleator once again.

To determine the degree of crystalline phase existing

in the sample prior to the second heating process, the

extra enthalpy absorbed by the crystallites formed during

heating (i.e. cold crystallization) has to be subtracted from

DHm. Thus, the crystallinity (Xc2) can be calculatedaccording to:

Xc2ð%Þ ¼ DHm � DHccvPLLADHom� 100 (2)

The values of Xc2 are approximately equal to zero,indicating that all the samples are nearly amorphous after

they are quenched at the rate of 608C/min. As seen inFig. 2, two melting peaks are observed for TMC-

nucleated PLLA, while only one for pure PLLA. Previous

studies [22, 23] have reported that the different melting

behavior of PLLA/nucleator systems is derived from the

effect of crystallization temperature. As for TMC-

nucleated PLLA, the cold crystallization temperature (Tcc)is relatively lower and less perfect crystals are formed,

thus the samples undergo the melting-recrystallization-

remelting process upon heating. The lower temperature

peak is attributed to the melting of primary crystals, and

the higher temperature peak corresponds to the melting of

the recrystallized crystals [27, 28]. In the case of pure

PLLA, Tcc is higher and more perfect crystals are formed,thus it melts directly without the melt-recrystallization

process, giving rise to the appearance of a single melting

peak.

FIG. 1. Thermal program C employed for the self-nucleation experi-

ment of pure PLLA. The dashed lines represent the partial melting zone

(160–1658C).

FIG. 2. DSC thermograms obtained during the second heating scan

(program A) for pure PLLA and TMC-nucleated PLLA.

1060 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

Isothermal Crystallization Behavior

Isothermal crystallization of TMC-nucleated PLLA was

examined at a temperature range of 90–1308C. Figure 3shows the isothermal crystallization exotherms of TMC0.25.

The relative crystallinity (Xt) can be obtained from theratio of the area of the exotherm at crystallization time tto the total area of the exothermal peak, i.e.

Xt ¼R t0ðdH=dtÞdtR1

0ðdH=dtÞdt (3)

where dH/dt is the heat flow rate. The evolution of therelative crystallinity with time for isothermal crystalliza-

tion of TMC0.25 is shown in Fig. 4. All the curves

exhibit a sigmoid dependence with crystallization time. It

is found that as the isothermal temperature increases from

90 to 1208C, the curves shift to a shorter crystallizationtime, and demonstrate an opposite trend with increasing

the temperature further (from 120 to 1308C). This indi-cates that the highest crystallization rate appears at around

1208C for the isothermal crystallization of TMC0.25.The Avrami method is the most common approach for

describing the isothermal crystallization process according

to the following equation: [29, 30]

Xt ¼ 1� expð�ktnÞ (4)

where k is the crystallization rate constant, and n is theAvrami exponent, which is dependent on the nature of

nucleation and the dimension of crystal growth. For

convenience, Eq. 4 is generally rewritten in a doublelogarithmic form as follows:

ln½� lnð1� XtÞ� ¼ ln k þ n ln t (5)

Plots of ln[2ln(1 2 Xt)] versus lnt were constructedand for the isothermal crystallization of TMC0.25 areshown in Fig. 5. It is apparent that a straight line isobtained at each crystallization temperature for TMC0.25with deviation from linearity in the parts close to the be-ginning and the end of crystallization, which is oftenattributed to initial nucleation and secondary crystalliza-tion processes. To minimize the possible errors, only therelative crystallinity data between 30% and 70% wereused to fit in this work. The evaluated results of n and kare summarized in Table 2. It can be observed that thevalue of n fluctuates within the range of 2.7–3.1 for purePLLA, which is similar to the literature (2.5–2.9 at Tc ¼90–1408C by Zhou et al. [17], and 2.3–3.2 at Tc ¼ 90–1258C by Tsuji et al. [31]), and within the range of 2.1–3.0 for blends with TMC, irrespective of its content.These results reflect a combinative crystal growth of two-dimension and three-dimension morphologies with athe-rmal nucleation.

TABLE 1. Thermal characteristics of PLLA with different contents of TMC as determined by the second heating scan (program A).

TMC content

(wt%)

Tg(8C)

Tcc,onset(8C)

Tcc(8C)

DHcc(J/g)

Tm(8C)

DHm(J/g)

Xc1(%)

Xc2(%)

0 59.0 114.6 128.7 16.6 152.4 16.5 17.6 0

0.25 58.8 106.1 121.0 24.9 150.9, 155.6 25.6 27.4 0.7

0.5 58.9 99.8 112.9 33.2 148.6, 155.5 33.3 35.7 0

0.75 58.6 98.2 112.2 33.3 148.0, 155.4 33.7 36.2 0.4

1 58.7 98.1 111.5 33.0 148.3, 156.0 33.6 36.2 0.6

FIG. 3. DSC exotherms of isothermal crystallization for TMC0.25 at

each designated temperature.

FIG. 4. Evolution of the relative crystallinity as a function of crystalli-

zation time for TMC0.25.


The crystallization half-time t1/2 can be directlyobtained from the curve of time dependence of relative

crystallinity, as the time needed for half of total crystal-

linity to develop. Generally, the reciprocal of t1/2, i.e. s1/2,can be employed to describe the overall crystallization

rate and is plotted in Fig. 6 as a function of crystallization

temperature (Tc). It shows that the crystallization of allthe samples is slow in the temperature ranges close to the

melting point and glass transition, and exhibits a maxi-

mum of s1/2 at around 105 and 1158C for pure PLLA andTMC-nucleated PLLA, respectively. This implies that

TMC can enhance the crystallization ability of PLLA to

shift the maximum of s1/2 to a higher temperature. Addi-tionally, it can be seen that the overall crystallization

rate presents a progressively rising trend with increasing

TMC content for a given Tc, indicating that TMC acceler-ates the crystallization of PLLA under the isothermal

condition.

From Fig. 6, it also can be found that the Tc–s1/2curves of TMC-nucleated PLLA are discontinuous in the

temperature range of around 100–1058C. Previous studies[28, 32, 33] have reported a similar crystallization behav-

ior of PLLA at temperatures between 100 and 1208C, andthey proposed that the discontinuity of crystallization

kinetics is related to crystal polymorphism of PLLA. To

examine whether TMC can favor crystallization in one of

the polymorphs of PLLA, the crystalline structure of pure

PLLA isothermally crystallized at Tc ¼ 80–1008C andTMC-nucleated PLLA isothermally crystallized at Tc ¼1008C was investigated by WAXD, and the results areshown in Fig. 7. It is found that the two strongest reflec-

tions of (200)/(110) and (203) planes shift to higher 2y,meanwhile, the intensities of (010) and (015) reflections

increase, and some weak reflections are present with

increasing Tc from 80 to 1008C for pure PLLA. Besides,only one reflection at nearby 2y ¼ 24.58, characteristic ofa’ crystal (indicated by the arrow), is observed for purePLLA crystallized at 808C, but two reflections of (016)and (206) planes of a crystal appear in this region as Tcwas increased to 90 and 1008C. These results suggest that908C is the critical temperature for polymorphous crystal-lization of the PLLA used in our work, that is, a0 and a

FIG. 5. Plots of ln[2ln(1 2 Xt)] versus lnt for isothermal crystalliza-tion of TMC0.25.

TABLE 2. Parameters obtained from the isothermal crystallization and subsequent heating scan (program B) of PLLA with different contents of

TMC.

TMC content (wt%) Tc (8C) n k (min2n) t1/2 (min) s1/2 (min

21) Tm (8C)

0 90 3.1 6.13 3 1025 20.26 0.049 142.7, 154.0100 2.9 6.30 3 1024 11.13 0.090 145.9, 155.1110 2.9 3.78 3 1024 13.32 0.075 149.6, 155.3120 2.7 9.35 3 1026 60.92 0.016 152.8

0.25 90 2.4 9.03 3 1024 15.34 0.065 142.6, 154.3100 2.3 1.30 3 1022 5.73 0.175 145.7, 155.3110 2.1 4.78 3 1022 3.51 0.285 149.4, 155.7120 2.3 6.39 3 1022 2.87 0.348 152.4130 2.6 1.30 3 1022 4.57 0.219 155.6

0.5 90 2.5 4.13 3 1023 7.74 0.129 142.4, 153.9100 2.6 6.20 3 1022 2.50 0.400 145.5, 154.9110 2.4 1.39 3 1021 1.96 0.510 149.0, 155.5120 2.7 1.15 3 1021 1.95 0.513 151.8130 3.0 8.48 3 1023 4.27 0.234 154.9

0.75 90 2.3 7.15 3 1023 7.09 0.141 141.9, 154.1100 2.4 1.04 3 1021 2.20 0.455 145.3, 155.1110 2.4 2.54 3 1021 1.52 0.658 148.6, 155.7120 2.6 2.14 3 1021 1.59 0.629 151.2130 2.7 2.13 3 1022 3.56 0.281 154.7

1 90 2.2 1.49 3 1022 5.65 0.177 141.6, 153.9100 2.4 1.65 3 1021 1.83 0.546 145.0, 154.8110 2.4 3.87 3 1021 1.28 0.781 148.2, 155.5120 2.7 2.84 3 1021 1.39 0.719 151.0130 2.8 3.05 3 1022 3.06 0.327 154.3


crystals are mainly formed at Tc \ 908C and Tc ‡ 908C,respectively. The critical temperature is ca. 208C lowerthan the one reported by Pan et al. [28, 32], which may

be the reason why no clear discontinuity can be observed

for the pure PLLA sample in the temperature range of

100–1208C.As for TMC-nucleated PLLA, it can be seen that TMC

has no discernible effect on the crystalline structure of

PLLA, and a-form crystals are produced in the samplesisothermally crystallized at 1008C, evidenced by theenlarged WAXD patterns in Fig. 7b. Therefore, the dis-

continuity of crystallization kinetic for TMC-nucleated

PLLA is not due to the polymorphous crystallization.

Generally, nucleating agent almost does not change the

spherulite growth rate of the polymers because it only

affects the primary nucleation but not the secondary

nucleation [22, 34]. The difference between the overall

crystallization rates of PLLA with and without the nucle-

ating agent is mostly due to the different nucleation rates.

The discontinuity of the curves is located in the tempera-

ture range of 100–1058C, where pure PLLA has the high-est crystallization rate. Additionally, the nucleation of

polymer without additional nucleating agents usually is

heterogeneous. Consequently, it is proposed that the pecu-

liar crystallization behavior of TMC-nucleated PLLA

probably arises from the combined nucleation effect of

the multiamide nucleating agent and intrinsic nucleation

sites existing in pure PLLA.

Melting Behavior After Isothermal Crystallization

After completion of isothermal crystallization, the sub-

sequent melting behavior of pure PLLA and TMC-

nucleated PLLA was measured by DSC at a heating rate

of 108C/min. Figure 8 presents the melting curves ofTMC0.5 after isothermal crystallization at each designated

temperature, and the resulting Tm values of all the sam-ples are listed in Table 2. The melting profile of the sam-

ples appears as two melting peaks in the case of Tc\ 1208C. Due to the fact that a crystals are mainlyformed at Tc ‡ 908C, the double melting behavior of iso-thermally crystallized PLLA and TMC-nucleated PLLA

also can be assigned to the melt-recrystallization mecha-

nism [35, 36]. The less perfect crystals generated at Tc gothrough the continuous melting/ structural reorganization/

crystal perfection processes, leading to the double endo-

thermic peaks. When the isothermal crystallization tem-

perature is at lower undercooling (i.e., Tc ‡ 1208C), onlya single endothermic peak appears due to the direct melt-

FIG. 6. Dependence of s1/2 on crystallization temperature (Tc) for purePLLA and TMC-nucleated PLLA.

FIG. 7. (a) WAXD patterns and (b) enlarged WAXD patterns of pure PLLA isothermally crystallized at Tc¼ 80 2 1008C and TMC-nucleated PLLA isothermally crystallized at Tc ¼ 1008C.


ing of more perfect crystals. Figure 9 shows the lower or

single melting temperature (Tm1) as a function of Tc forpure PLLA and TMC-nucleated PLLA. It can be seen that

Tm1 has a linear dependence on the crystallization temper-ature, agreeing well with the Hoffman–Weeks theory [37],

whereas the higher melting temperature (Tm2) remainsroughly constant in view of the Tm data in Table 2.

For a given Tc, it can also be seen that Tm1 decreasesslightly with the increase of TMC content. In light of the

Thomson–Gibbs equation [38, 39], a relationship exists

between the melting temperature (Tm) and the lamellarthickness (l) of the crystallites:

Tm ¼ T0m 1�2seDh

� 1l

� �(6)

where T0m is the melting point of an infinitely large crystal(equilibrium melting temperature), se is the fold surfacefree energy and Dh is the bulk heat of fusion. According toEq. 6, the fact that Tm1 decreases slightly with the increaseof TMC content suggests that the lamellar thickness of the

primary crystals of PLLA decreases somewhat with the

incorporation of TMC. This may be derived from the high

concentration of activated nuclei and restricted growing

space of crystallites in the presence of TMC, as will be

described in the later section of spherulite morphology.

Nucleation Efficiency

To directly understand the effectiveness of TMC to act

as a nucleator, the NE was evaluated based on a method

proposed by Fillon et al. [40]. The NE can be calculated

from the crystallization peak temperature by means of the

following equation:

NEð%Þ ¼ Tp � Tminp

Tmaxp � Tminp� 100% (7)

where Tp and Tminp are the crystallization peak temperature

of PLLA nucleated and non-nucleated with a nucleator,

respectively. Tmaxp is the highest achievable peak tempera-ture of self-nucleated PLLA. According to the self-nuclea-

tion theory [41], three domains of self-nucleation temper-

atures can be divided by Tmaxpm and Tminpm . The upper limit

of the partial melting zone (Tmaxpm ) is defined as the lowesttemperature at which complete melting occurs. Above this

temperature (Domain I), the number of nuclei in the sam-

ple is minimum and remains static. When the sample is

cooled from temperatures greater than Tmaxpm , the exother-mic crystallization peak is at Tminp . The lower limit of thepartial melting zone (Tminpm ) is defined as the temperatureat which the sample has the maximum achievable number

of nuclei, and the nuclei have the ideal interactions with

the crystallizing polymer. Tminpm therefore corresponds toTmaxp that can be reached for the sample by monitoring thenuclei concentration, dispersion, structure and compatibil-

ity within the melt. In the temperature range between Tminpmand Tmaxpm (Domain II), the polymer is partially melting,and a decrease in Tpm results in an increase of crystalliza-tion peak temperature on subsequent cooling. At tempera-

ture less than Tminpm (Domain III), the sample is insuffi-ciently melted so that self-nucleation and annealing of

unmelted crystals take place simultaneously.

To determine the values of Tminp and Tmaxp , the special

thermal program C was employed. The testing thermo-

grams of self-nucleated PLLA at the partial melting zone

of 160–1658C are shown in Fig. 10, and the data of peaktemperatures are recorded in Table 3. The results show

that the value of Tmaxp is 144.58C at Tminpm ¼ 1608C. In

addition, Tminp is determined to be 97.58C based on thecurve d in Fig. 10. It is noteworthy that two crystalliza-

tion peaks are present for PLLA after self-nucleation at

1608C. Multiple exothermic peaks after self-nucleation ofisotactic polypropylene have been reported by Fillon et al.

and attributed to entering Domain III [42]. Similar to the

FIG. 8. Melting curves of TMC0.5 after isothermal crystallization at

each designated temperature.

FIG. 9. Relationship between melting point (Tm1) and crystallization

temperature (Tc) for pure PLLA and TMC-nucleated PLLA.


curve a, two crystallization peaks were also observed for

homo-PLLA and PLLA-b-PCL diblock copolymers after

self-nucleation at the beginning of Domain II by Castillo

et al. However, the exact origin of the double exothermic

peaks is still unknown [43]. More in-depth investigations

need to be done further for clarifying this behavior.

Using Tp, which is obtained from the DSC exothermsof TMC-nucleated PLLA (as shown in Fig. 11), the value

of NE is calculated and listed in Table 4. The effect of

the TMC content on the NE is exhibited in Fig. 12. It can

be seen that the NE increases dramatically in the presence

of TMC, indicating that TMC is very effective even if its

content is only 0.25 wt%. However, the rising tendency

becomes inconspicuous as the TMC content is increased

further. It is reported that the NE of PLLA with 6 wt%

talc is 32% by Schmidt and Hillmyer [5], while the

obtained NE of PLLA nucleated with 1 wt% TMC is

66% in this study, showing that TMC is far superior to

talc.

Nucleation Activity

Another common measure of the efficacy of a nuclea-

tor is the nucleation activity (u). The value of nucleationactivity varies from 0 to 1, corresponding to extremely

active and inert foreign substrates, respectively. So the

more active the nucleator is, the lower the value of u

should be. According to Dobreva and Gutzow [44, 45],

the nucleation activity can be calculated from the ratio:

j ¼ B�

B(8)

where B is a parameter for pristine polymer, while B* isfor polymer/nucleator system. B and B* both can beexperimentally determined from the slope of the follow-

ing equation:

lnU ¼ Constant� BðorB�Þ

DT2p(9)

where F is the cooling rate, DTp denotes the degree ofsupercooling (DTp ¼ Tm – Tp).

Plots of lnF versus 1/DT2p for pure PLLA and TMC-nucleated PLLA are shown in Fig. 13. It is apparent that

a linear relationship is obtained for each sample. The val-

ues of B and B* are obtained from the slope of the fittedlines, and the nucleation activity is calculated from their

ratio. The results are listed in Table 4, and for clarity, the

variation of nucleation activity with the TMC content is

simultaneously presented in Fig. 12. It can be observed

that the value of u for TMC-nucleated PLLA is muchlower than that for pure PLLA, implying that TMC pro-

motes the nucleation of PLLA actively. However, after

the significant decline, the value of u levels off and is

FIG. 10. Testing thermograms from self-nucleation experiment con-

ducted on pure PLLA. Curves (a), (b), and (c) are recorded during the

cooling scan (step 6 in program C) after heating to the partial melting

temperature of 160, 163, and 1658C, respectively, while curve (d) isobtained at step 3.

TABLE 3. Results of DSC determination for self-nucleated PLLA.

Tpm (8C) Tp (8C) DHc (J/g)

160 144.5 27.4

163 116.7 31.4

165 97.7 14.1

FIG. 11. DSC exotherms of nonisothermal crystallization for TMC-

nucleated PLLA at a cooling rate of 18C/min.

TABLE 4. Results of NE and nucleation activity.

TMC content (wt%) NE (%) B (or B*) u

0 0 1.016 1

0.25 61.3 0.201 0.198

0.5 63.4 0.197 0.194

0.75 64.9 0.186 0.183

1 66.0 0.201 0.198


almost independent of the amount of TMC. These results

agree well with the estimation of NE, demonstrating that

excellent nucleation-promoting effect could be achieved

when the weight percentage of TMC is chosen between

0.25% and 0.5%.

Nevertheless, it is noted that the incorporation of 0.25

wt% of TMC leads to limited nucleation ability for

enhancing the cold crystallization of PLLA as evidenced

by the previous Fig. 2. It has been reported that TMC

could be completely dissolved in the PLLA melt and self-

associate into nucleation sites upon cooling when its con-

centration is relatively low (e.g., 0.2 wt%). However, it

becomes partly insoluble and large amount of undissolved

crystals remain in the PLLA melt as its concentration

reaches 0.5 wt%. In this case, the undissolved crystals

instead of self-organized structures serve as nucleation

sites [24]. It can be inferred that different from the other

three TMC-nucleated PLLA samples, the nucleation effec-

tiveness of TMC0.25 is significantly dependent on the

self-organizing ability of dissolved TMC, which is further

influenced much by the cooling rate. For determining the

thermal characteristics, the samples are first subjected to

rapid cooling and the dissolved TMC fails to achieve

self-organization under this condition. Furthermore, the

self-organization rate of TMC during the second heating

is comparatively slow due to the ‘‘cold’’ state. As a con-

sequence, the cold crystallization of TMC0.25 promoted

by the self-associated TMC is tardy in comparison with

the other three TMC-nucleated PLLA samples. In the

case of evaluation of NE and activity, the cooling rate is

lower than 38C/min, thus, the dissolved TMC has enoughtime to self-organize into nucleation sites upon cooling

before PLLA lamellae growing on their surface. This

may be the main reason why the addition of 0.25 wt%

TMC results in the close NE and activity compared to the

one of higher concentrations (‡0.5 wt%) as seen inFig. 12.

Spherulite Morphology

The spherulite morphology of TMC-nucleated PLLA

was compared with that of pure PLLA by polarized opti-

FIG. 13. Plots of lnF versus 1/DT2p to determine the nucleation activityfor nonisothermal crystallization of pure PLLA and TMC-nucleated

PLLA.

FIG. 12. Variation of NE and nucleation activity (u) with the TMCcontent.

FIG. 14. POM images of (a) pure PLLA and (b) TMC0.5 after com-

plete crystallization at 1208C. The scale bars are all 50 lm.


cal microscopy (POM) analysis. Figure 14 presents the

POM microphotographs of pure PLLA and TMC0.5 sam-

ples after complete crystallization at 1208C. In the ab-sence of nucleator, it can be seen that the spherulites of

pure PLLA grow drastically and impinge on each other,

in which the largest crystals reach 100 lm (Fig. 14a).With the incorporation of 0.5 wt% TMC, the spherulite

size is much smaller, and the spherulite number is rather

larger than that of pure PLLA (Fig. 14b), indicating that

TMC greatly increases the nucleation density and reduces

the size of PLLA spherulites. This observation further

confirms that the nucleation ability of TMC is very prom-

inent on the crystallization of PLLA, which is in accord-

ance with the aforementioned DSC results.

CONCLUSIONS

TMC-nucleated PLLA was prepared via the melt

blending method at various TMC contents from 0.25 to 1

wt%. The thermal characteristics, crystallization behavior,

and nucleation ability of pure PLLA and TMC-nucleated

PLLA were investigated in detail. The data of thermal

characteristics shows that the addition of TMC favors the

cold crystallization and elevates the total crystallinity of

PLLA remarkably. The two melting points of TMC-

nucleated PLLA can be attributed to the melt-recrystalli-

zation mechanism, suggesting that less perfect crystals

formed at lower crystallization temperature undergo the

melting–recrystallization–remelting process on heating.

The isothermal crystallization results indicate that the

overall crystallization rate is faster in TMC-nucleated

PLLA than in pure PLLA and increases with the TMC

loading level, however, the crystal growth form and crys-

talline modification are not altered despite the presence of

TMC. The quantitative analyses of NE and activity reflect

that a small amount of TMC can promote the nucleation

of PLLA effectively, and further increasing the TMC con-

centration has not led to a marked improvement in the

nucleation ability. The morphology observation by POM

reveals that the addition of TMC can greatly increase the

activated nuclei density and reduces the size of PLLA

spherulites.

REFERENCES

1. D. Garlotta, J. Polym. Environ., 9, 63 (2001).

2. S.F. Yan, J.B. Yin, Y. Yang, Z.Z. Dai, J. Ma, and X.S.

Chen, Polymer, 48, 1688 (2007).

3. R.M. Rasal, A.V. Janorkar, and D.E. Hirt, Prog. Polym.Sci., 35, 338 (2010).

4. K. Sudesh and T. Iwata, Clean-Soil Air Water, 36, 433(2008).

5. S.C. Schmidt and M.A. Hillmyer, J. Polym. Sci., Part B:Polym. Phys., 39, 300 (2001).

6. Z.Z. Su, Q.Y. Li, Y.J. Liu, W.H. Guo, and C.F. Wu, Polym.Eng. Sci., 50, 1658 (2010).

7. R.G. Liao, B. Yang, W. Yu, and C.X. Zhou, J. Appl. Polym.Sci., 104, 310 (2007).

8. H. Tsuji, H. Takai, N. Fukuda, and H. Takikawa, Macromol.Mater. Eng., 291, 325 (2006).

9. H. Tsuji, M. Sawada, and L. Bouapao, ACS Appl. Mater.Inter., 1, 1719 (2009).

10. Y. Di, S. Iannace, E.D. Maio, and L. Nicolais, J. Polym.Sci., Part B: Polym. Phys., 43, 689 (2005).

11. M. Day, A.V. Nawaby, and X. Liao, J. Therm. Anal. Calo-rim., 86, 623 (2006).

12. J.Y. Nam, M. Okamoto, H. Okamoto, M. Nakano, A. Usuki,

and M. Matsuda, Polymer, 47, 1340 (2006).

13. N. Kawamoto, A. Sakai, T. Horikoshi, T. Urushihara, and

E. Tobita, J. Appl. Polym. Sci., 103, 198 (2007).

14. N. Kawamoto, A. Sakai, T. Horikoshi, T. Urushihara, and

E. Tobita, J. Appl. Polym. Sci., 103, 244 (2007).

15. H. Nakajima, M. Takahashi, and Y. Kimura, Marcomol.Mater. Eng., 295, 460 (2010).

16. Y. Tachibana, T. Maeda, O. Ito, Y. Maeda, and M.

Kunioka, Polym. Degrad. Stabil., 95, 1321 (2010).

17. W.Y. Zhou, B. Duan, M. Wang, and W.L. Cheung, J. Appl.Polym. Sci., 113, 4100 (2009).

18. Y.T. Shieh, Y.K. Twu, C.C. Su, R.H. Lin, and G.L. Liu, J.Polym. Sci., Part B: Polym. Phys., 48, 983 (2010).

19. Y.L. Li, H.Y. Wu, Y. Wang, L. Liu, L. Han, J. Wu, and

F.M. Xiang, J. Polym. Sci., Part B: Polym. Phys., 48, 520(2010).

20. Z.Z. Su, W.H. Guo, Y.J. Liu, Q.Y. Li, and C.F. Wu, Polym.Bull., 62, 629 (2009).

21. H. Pan and Z.B. Qiu, Macromolecules, 43, 1499(2010).

22. P.J. Pan, Z.C. Liang, A.M. Cao, and Y. Inoue, ACS Appl.Mater. Inter., 1, 402 (2009).

23. S.S. Wang, C.Y. Han, J.J. Bian, L.J. Han, X.M. Wang, and

L.S. Dong, Polym. Int., 60, 284 (2011).

24. H.W. Bai, W.Y. Zhang, H. Deng, Q. Zhang, and Q. Fu,

Macromolecules, 44, 1233 (2011).

25. E.W. Fischer, H.J. Sterzel, and G. Wegner, Kolloid Z. Z.Polym., 251, 980 (1973).

26. D.R. Witzke, R. Narayan, and J.J. Kolstad, Macromolecules,30, 7075 (1997).

27. M. Yasuniwa, K. Sakamo, Y. Ono, and W. Kawahara, Poly-mer, 49, 1943 (2008).

28. P.J. Pan, W.H. Kai, B. Zhu, T. Dong, and Y. Inoue, Macro-molecules, 40, 6898 (2007).

29. M. Avrami, J. Chem. Phys., 7, 1103 (1939).

30. M. Avrami, J. Chem. Phys., 8, 212 (1940).

31. H. Tsuji, H. Takai, and S.K. Saha, Polymer, 47, 3826(2006).

32. P.J. Pan, B. Zhu, W.H. Kai, T. Dong, and Y. Inoue, J. Appl.Polym. Sci., 107, 54 (2008).

33. M.L. Di Lorenzo, Eur. Polym. J., 41, 569 (2005).

34. Y. He and Y. Inoue, Biomacromolecules, 4, 1865(2003).

35. Y. He, Z.Y. Fan, Y.F. Hu, T. Wu, J. Wei, and S.M. Li, Eur.Polym. J., 43, 4431 (2007).


36. M.L. Di Lorenzo, J. Appl. Polym. Sci., 100, 3145 (2006).

37. J.D. Hoffman and J.J. Weeks, J. Res. Natl. Bur. Stand. Sect.A, 66, 13 (1962).

38. M. Yasuniwa, K. Iura, and Y. Dan, Polymer, 48, 5398(2007).

39. P. Srimoaon, N. Dangseeyun, and P. Supaphol, Eur. Polym.J., 40, 599 (2004).

40. B. Fillon, B. Lotz, A. Thierry, and J.C. Wittmann, J. Polym.Sci., Part B: Polym. Phys., 31, 1395 (1993).

41. S. Schneider, X. Drujon, B. Lotz, and J.C. Wittmann, Poly-mer, 42, 8787 (2001).

42. B. Fillon, J.C. Wittmann, B. Lotz, and A. Thierry, J. Polym.Sci., Part B: Polym. Phys., 31, 1383 (1993).

43. R.V. Castillo, A.J. Muller, J.M. Raquez, and P. Dubois,

Macromolecules, 43, 4149 (2010).

44. A. Dobreva and I. Gutzow, J. Non-Cryst. Solids, 162, 1(1993).

45. A. Dobreva and I. Gutzow, J. Non-Cryst. Solids, 162, 13 (1993).


本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页文献云下载图书馆入口外文数据库大全疑难文献辅助工具

http://www.xuebalib.com/cloud/http://www.xuebalib.com/http://www.xuebalib.com/cloud/http://www.xuebalib.com/http://www.xuebalib.com/vip.htmlhttp://www.xuebalib.com/db.phphttp://www.xuebalib.com/zixun/2014-08-15/44.htmlhttp://www.xuebalib.com/

Crystallization behavior and nucleation analysis of poly(l-lactic acid) with a multiamide nucleating agent学霸图书馆link:学霸图书馆

crystallization behavior and nucleation analysis of poly(l...

Documents