trakpo

i, T

ng, Z

Received 21 December 2006; received in revised form 4 April 2007; accepted 4 April 2007Available online 14 April 2007

Keywords: Polypropylene; N,N,N 0,N 0-Tetraalkyl terephthalamide; Isothermal crystallization kinetics; Morphology; Evolution

phology of PP [1]. ties. However, b-PP is pseudostationary in thermo-dynamics, and it is dicult to form in dynamics[3]. Turner-Jones [4] reported that b-PP was formedwhen PP was melted between 190 C and 230 C andcooled abruptly to 100120 C. Fujiyama [5] got the

erved.

* Corresponding author. Tel./fax: +86 571 87951612.E-mail address: shentu@zju.edu.cn (B. Shentu).

European Polymer Journal 43

EUROPEANPOLYMER0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights res1. Introduction

Polypropylene (PP) is a most common commod-ity plastic, which is of practical use in many areas,such as home appliances, automotive, constructionand other industrial applications. The structureand morphology of PP have a direct impact on itsmechanical properties. Therefore, there is growinginterest in understanding the structure and mor-

PP is a typical semicrystalline polymer and hasthree crystalline forms: monoclinic a, hexagonal band triclinic c [2]. Under actual processing condi-tions PP usually crystallizes in a-form. However,PP with a-form shows poor impact strength at lowerand room temperature.

In comparison with a-PP, b-PP has high ductilityand strength. Therefore, PP with b-form hasattracted much attention due to its unique proper-Abstract

The inuence of N,N,N 0,N 0-tetraalkyl terephthalamide (TATA) on the isothermal crystallization kinetics of polypropyl-ene (PP) was studied using dierential scanning calorimetry (DSC). It was found that TATA shows a heterogeneous nucle-ation eect and leads to the formation of b-PP. TATA can not only shorten the crystallization time but also heighten thecrystallization temperature of PP. The crystallization rate constant of PP containing TATA is larger than that of pure PP.The evolution of crystalline morphology of PP was investigated on a polarized optical microscopy (POM) equipped with ahot stage and the results showed that the introduction of TATA into PP can quicken the crystallization of PP, which isconsistent with DSC results. TATA also leads to a substantial decrease in the spherulite size of PP and the boundariesof spherulites are hardly distinguished. 2007 Elsevier Ltd. All rights reserved.Inuence of N,N,N 0,N 0-teisothermal crystallization

evolution of

Baoqing Shentu *, Jipeng L

State Key Laboratory of Chemical Engineeridoi:10.1016/j.eurpolymj.2007.04.003alkyl terephthalamide oninetics and morphologylypropylene

engfei Gan, Zhixue Weng

hejiang University, Hangzhou 310027, China

(2007) 30363041

www.elsevier.com/locate/europolj

JOURNAL

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DSC-7 in a nitrogen atmosphere. The meltedmixed

The inuence of TATA on the melting behavior

B. Shentu et al. / European Polymer Journal 43 (2007) 30363041 3037nucleating agent to produce b-PP. However, thedegree of crystallinity for b-PP formed was low.Varga [8] found aliphatic dicarboxylic acids resultedin the high degree of crystallinity for stable b-PP.

Therefore, the addition of b-nucleating agents isone of the eective means to form b-PP. Kimurafound that N,N,N 0,N 0-dicyclohexyl-2,6-naphalene-dicarboxamide can be used as a b-nucleating agentof PP and induce PP to form b-form [9]. Recently,we observed that N,N,N 0,N 0-tetraalkyl terephthala-mide (TATA) was also an eective b-nucleatingagent. However, there are few reports about theinuence of TATA on the isothermal crystallizationkinetics and morphology evolution of PP.

The objective of this study was to analyze theinuence of TATA on isothermal crystallizationkinetics and morphology evolution of PP using dif-ferential scanning calorimetry (DSC) and polarizedoptical microscopy (POM), respectively. Avramisequation [10] was used to study the isothermal crys-tallization kinetics of PP and crystallization kineticsparameters of pure PP and PP containing 0.3 wt%TATA were estimated.

2. Experimental

2.1. Materials

Commercially available PP (grade F401, tactic-ityP 97%, melt index = 2 g/10 min, supplied byYangzi Petrochemical Corporation) was used in thisstudy.N,N,N0,N0-Tetraalkyl terephthalamide (TATA,supplied by Shanxi Institute of Chemical Engineer-ing) was a white powder with a particle size of

ando

(mW

)

3038 B. Shentu et al. / European Polymer Journal 43 (2007) 30363041PP, indicating that TATA can induce the formationof b-PP.

The values for the crystallinity degree (xa and xb)were calculated using DH0(a) = 209 J/g and DH0(b) =151.2 J/g [12]. The inuence of TATA on x(x = xa + xb), xa and xb of PP was shown in Table1. It is obvious that xb or PP containing 0.3 wt%TATA is very high in comparison with xa andTATA causes more PP chains to crystallize, result-ing in the increase of the total crystallization degreeof PP (x) due to the heterogeneous nucleation. xbincreases with the increase of TATA content. WhenTATA content is 0.3 wt%, xb remains constant,which indicates that the optimum TATA contentis 0.3 wt%. Therefore, the isothermal crystallizationkinetics and morphology evolution of PP containing0.3 wt% TATA were studied.

WAXD studies were also carried out to obtaininformation on the crystalline form of PP. The

130 140 150 160 170 180

b

T (C)

E

Fig. 1. Melting behavior of pure PP (a) and PP containing 0.3wt% TATA (b).diraction angle (2h) curves were shown in Fig. 2.Diraction peaks at 2h = 14.1, 18.4, and 21.7ascribe to the (110), (040), and (130) plane, respec-tively, suggesting that pure PP crystallizes only ina-form (monoclinic) because there are no peaks at2h = 16.1 associated with b-PP (hexagonal) and2h = 20.1 associated with c-PP (triclinic) [13].Whereas, there are two diraction peaks at2h = 16.1 and 21.2 ascribed to the (300) and

Table 1Inuence of TATA on x,xa and xb of PP

Samples x (%) xa (%) xb (%)

PP 46.3 46.3 0PP containing 0.3 wt% TATA 62.8 3.4 59.4(301) plane for PP containing TATA, which furtherindicates that TATA eectively induces the forma-tion of b-PP.

3.2. Inuence of TATA on the isothermal

crystallization behavior of PP

The isothermal crystallization behavior of PPwas investigated by DSC, and crystallization curvesof pure PP and PP containing 0.3 wt% TATA undervarious temperatures are shown in Fig. 3.

It was found from Fig. 3 that for both pure PPand PP containing 0.3 wt% TATA with the increaseof the crystallization temperature the crystallizationexothermal peak signicantly shifts to the right side,indicating that the crystallization time prolongs andthe crystallization rate decreases with the increase ofthe crystallization temperature. It takes PP about60 min to complete the crystallization process at132 C, while it only takes PP containing TATAabout 5 min to nish the crystallization process atthe same crystallization temperature, indicating that

2 ()10 15 20 25

(300)

(301)

(110) (130)(040)

a

b

Fig. 2. X-ray diraction proles of pure PP (a) and PPcontaining 0.3 wt% TATA (b).TATA can quicken the crystallization rate and sig-nicantly shorten the crystallization time of PPdue to the heterogeneous nucleation. PP containingTATA can crystallize at a temperature over 134 C,while there is no crystallization exothermal peak forpure PP at 134 C, which shows that TATA canwiden the scope of the crystallization temperature.

3.3. Inuence of TATA on the isothermal

crystallization kinetics of PP

The relative degree of crystallinity of PP aftertime t of the crystallization process (xt) can be calcu-lated according to Eq. (1) from the experimentalcurves of Fig. 3.

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0 15 30 45 60 75

a-125 Cb-127 Cc-129 Cd-132 Ce-134 C

t (min)

Endo

(mW

)

a

b

c

d

e

0 3 6 9 12 15

a-132 Cb-135 Cc-137 Cd-139 C

t (min)

Endo

(mW

)

a

b

c

d

Fig. 3. Isothermal crystallization curves of pure PP (left) and PP containing 0.3 wt% TATA (right) at various temperatures.

B. Shentu et al. / European Polymer Journal 43 (2007) 30363041 3039xt Z t0

dH=dtdtZ 10

dH=dt

; dt 1

where dH/dt is the heat ow.Fig. 4 showed the change of the relative crystal-

linity degree of PP (xt) with time (t). It was foundthat xt increases with the increase of time.

The isothermal crystallization kinetics of PP canbe characterized based on the Avrami Equation(Eq. (2)).

1 xt expKtn; 2where n is the Avrami exponent, K is the crystalliza-tion rate constant involving both the nucleation andgrowth rates of crystals.

Eq. (3) is obtained by taking the double loga-rithm of Eq. (2).

ln ln 1 xt lnK n ln t 3Then ln[ln[1 xt]] was plotted as a function of lnt(Fig. 5). If Eq. (3) is valid, the curve must be a

straight line whose slope is the exponent n and

0 10 20 30 40 50 60 70 800

20

40

60

80

100

a-125 Cb-127 Cc-129 Cd-132 C

x t (%

)

t (min)

a b c d

Fig. 4. Plots of xt versus t for pure PP (left) and PP containing 0.whose intercept is K. Fig. 5 showed good agreementbetween the experimental results and theoreticalpredication at various crystallization temperatures,indicating that the Avrami Equation can be usedto study the inuence of TATA on the isothermalcrystallization kinetics of PP.

In the case of xt = 1/2, the time which is termedthe half-time of crystallization (t1/2) can be calcu-lated according to the Avrami Equation, as shownin Eq. (4).

t1=2 ln 2K 1=n

4

The reciprocal of t1/2, represented by G1/2 (Eq. (5))can be used as a parameter characterizing the crys-tallization rate of PP.

G1=2 1=t1=2 5n, K, t1/2, and G1/2 were determined and listed inTable 2. It was shown from Table 2 that at the samecrystallization temperature (132 C) the value of nfor PP containing TATA is smaller than that for0 4 8 12 16 200

20

40

60

80

100

a-132 Cb-135 Cc-137 Cd-139 C

x t (%

)

t (min)

a b c d

3 wt% TATA (right) at various crystallization temperatures.

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2.0 2.5 3.0 3.5 4.0 4.5-0.2

0.0

0.2

0.4

0.6

0.8

1.0

a-125 Cb-127 Cc-129 Cd-132 C

ln[-l

n(1-x t

)]

ln t

a b c d

0.5 1.0 1.5 2.0 2.5-0.4

0.0

0.4

0.8

1.2

a-132 Cb-135 Cc-137 Cd-139 C

ln t

ln[-l

n(1-x t

)]

a b c d

Fig. 5. Plots of ln[ln (1xt)] versus ln t for pure PP (left) and PP containing 0.3 wt% TATA (right) at various crystallizationtemperatures.

3040 B. Shentu et al. / European Polymer Journal 43 (2007) 30363041Table 2Isothermal crystallization kinetic parameters of pure PP and PPcontaining 0.3 wt% TATA at various crystallization temperatures

Samples Tc/C n K/minn t1/2/min G1/2/min1

PP 125 3.00 0.0033 5.94 0.17127 2.68 0.0018 9.22 0.11129 2.44 0.0010 14.6 0.07pure PP due to the heterogeneous nucleation. Thevalues of K for all samples decrease with the in-crease of the crystallization temperature, which indi-cates that crystallization rate decreases with thecrystallization temperature within the experimentalrange. K for PP containing TATA is larger than thatfor pure PP, which further indicates that the crystal-

The morphological features of semi-crystalline

132 2.18 0.0004 30.6 0.03

PP containing0.3 wt%TATA

132 2.43 0.252 1.52 0.66135 2.25 0.080 2.61 0.38137 2.22 0.048 3.33 0.30139 2.18 0.013 6.20 0.16

Fig. 6. Evolution of the crystalline morphology during isothermal cry0.3 wt% TATA (right).polymers, such as the spherulite size, can aect themechanical properties. Morphology developmentduring the isothermal crystallization of PP under132 C was studied using a polarized optical micros-copy equipped with a hot stage. Fig. 6 shows POMmicrographs of pure PP and PP containing 0.3 wt%TATA. Clearly, the presence of TATA signicantlyinuences the crystallization process and morphol-lization of PP is accelerated by the addition ofTATA.

t1/2 for PP containing 0.3 wt% TATA is shorterthan that for pure PP at the same crystallizationtemperature, while G1/2 is longer, indicating thatthe crystallization rate of PP increases by the addi-tion of TATA.

3.4. Evolution of the crystalline morphology of PP

with timestallization at 132 C with time for PP (left) and PP containing

ogy of PP. Firstly, the introduction of TATA intoPP can quicken the crystallization of PP and shortenthe crystallization time, which is consistent with theDSC results. Secondly, the addition of TATA leadsto a substantial decrease in the spherulite size of PPand the spherulites in b-PP exhibits a distinct mor-phology. Sheaf-like aggregates of lamellar crystalsare generally observed. Furthermore, the bound-aries of spherulites are hardly distinguished. Thismorphological feature was consistent with Tjongsresults [14]. On the other hand, the spherulites ofa-form of pure PP consists of an aggregate of lamel-lae that radiates from the center outward and thespherulites have distinct boundaries.

4. Conclusions

TATA is an eective b-nucleating agent for PPand causes signicant changes in the isothermalcrystallization and melting characteristics of PP.

of TATA. TATA also results in a substantialdecrease in the spherulite size of PP.

Acknowledgement

The authors gratefully acknowledge nancialsupport from the Scientic Research Foundationfor the Returned Overseas Chinese Scholars, StateEducation Ministry.

References

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B. Shentu et al. / European Polymer Journal 43 (2007) 30363041 3041PP crystallizes mainly in b-form at a TATA concen-tration of 0.3 wt%. The total crystallinity degree ofPP containing TATA is markedly higher than thatof pure PP due to the heterogeneous nucleation.Avrami theory can be used to study the isothermalcrystallization of PP containing TATA. The crystal-lization rate constant of PP containing TATA is lar-ger than that of pure PP, which indicates that thecrystallization of PP is accelerated by the addition[8] Varga J, Mudra I. J Appl Polym Sci 1999;74:2357.[9] Kawai T, Iijima R, Yamamoto Y, Kimura T. Polymer

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Influence of N,N,N prime ,N prime -tetraalkyl terephthalamide on isothermal crystallization kinetics and morphology evolution of polypropyleneIntroductionExperimentalMaterialsSample preparationApparatus and experimental procedures

Results and discussionInfluence of TATA on the crystalline formof PPInfluence of TATA on the isothermal crystallization behavior of PPInfluence of TATA on the isothermal crystallization kinetics of PPEvolution of the crystalline morphology of PP with time

ConclusionsAcknowledgementReferences