crystallization kinetics and melting behavior of metallocene short-chain branched polyethylene...

$: Crystallization kinetics and melting behavior of metallocene short-chain branched polyethylene fractions$
Crystallization Kinetics and Melting Behavior ofMetallocene Short-Chain Branched Polyethylene Fractions

FANG-CHYOU CHIU,1 QIANG FU,2 YA PENG,2 HSI-HSIN SHIH3

1Department of Chemical and Materials Engineering, Chang Gung University, Kwei-San, Tao-Yuan, Taiwan 333,Republic of China

2Department of Polymer Science and Materials, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China

3Union Chemical Laboratories, Industrial Technology Research Institute, Hsin-Chu, Taiwan 300, Republic of China

Received 5 June 2001; revised 13 November 2001; accepted 15 November 2001Published online 00 Month 2001

ABSTRACT: Metallocene polyethylene (mPE) fractions are recognized as being morehomogeneous with respect to short-chain branch (SCB) distribution as compared withunfractionated mPEs. Differential scanning calorimetry and polarized optical micros-copy (POM) were used to study the influences of SCB content on the crystallizationkinetics, melting behavior, and crystal morphology of four butyl-branched mPE frac-tions. The parent mPE of the studied fractions was also investigated for comparativepurposes. mPE fractions showed a much simpler crystallization behavior as comparedwith their parent mPE during the cooling experiments. The Ozawa equation wassuccessfully used to analyze the nonisothermal crystallization kinetics of the fractions.The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined foreach fraction, indicating the crystal-growth geometry changed from three-dimensionalto two-dimensional. For isothermal crystallization, the fraction with a lesser SCBcontent exhibited a higher crystallization temperature (Tc) window. The results fromthe Avrami equation analysis showed the exponent n values were around 3 (with minorvariation), which implied that the crystal-growth geometry is pseudo-three-dimen-sional. Both of the activation energies for nonisothermal and isothermal crystallizationwere determined for each fraction with Kissinger and Arrhenius-type equations, re-spectively. Double melting peaks were observed for both nonisothermally or isother-mally crystallized specimens. The high-melting peak was confirmed induced via theannealing effect during heating scans. The Hoffman–Weeks plot was inapplicable inobtaining the equilibrium melting temperature (Tm°) for each fraction. The relationshipbetween Tc and Tm for the fractions is approximately Tm � Tc (°C) � 8.3. The POMresults indicated that the crystals of parent or fractions formed under cooling conditionsdid not exhibit the typical spherulitic morphology as a result of the high SCB content.© 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 325–337, 2002; DOI10.1002/polb.10094Keywords: metallocene catalysts; polyethylene; crystallization; morphology

INTRODUCTION

Polyethylene (PE) is recognized as one of the mostused polymeric materials in the world. The study

of its various properties is extensive, especiallyfor linear PE (in industry, this class of PE iscommonly called high-density polyethylene) andlong-chain branched PE (in industry, this class ofPE is commonly called low-density polyethylene(LLDPE)). Short-chain branched (SCB) PE (in in-dustry, this class of PE is commonly calledLLDPE) is a relatively newer material that is acopolymer of ethylene and �-olefins such as

Correspondence to: F.-C. Chiu (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 40, 325–337 (2002)© 2002 John Wiley & Sons, Inc.

325

butene-1, hexene-1, octene-1, or 4-methyl-pen-tene-1. Because of its versatile mechanical andthermal properties, it has received a lot of interestboth industrially and academically since its syn-thesis. SCB PE was originally manufacturedthrough the catalysis of Ziegler–Natta catalystsand has been proven to be a heterogeneous sys-tem caused by the molecular weight distribution(MWD) and SCB distributions molecularly. TheMWD and SCB distributions as well as othermolecular parameters including molecular weight(MW), type, and content of SCB result in theversatility of SCB PE.

In late 1980s, following the evolution of thesynthesis technique, a new generation of SCB PEwas synthesized successfully through metallo-cene catalysts.1,2 Metallocene catalysts are bestknown for their “single-sited” activity. As com-pared with traditional catalysts synthesized SCBPEs, metallocene SCB PEs set a new standard forpolymer purity with a narrower MWD and a moreuniform SCB distribution. Many industrial appli-cations refer to this class of SCB PEs as homoge-neous products. However, from previous work,3–5

inter- and intramolecular heterogeneity existswith respect to the SCB distribution for metallo-cene SCB PEs. The multiple melting endothermswere easily observed from differential scanningcalorimetric (DSC) experiments after the metal-locene whole polymers were subjected to a stagedannealing treatment during cooling. Further-more, phase separation induced by a designedthermal treatment was also evident from a studyof the overall crystallization kinetics as well ascrystal and phase morphological observations.Hsieh et al.6 also obtained direct proof of inter-molecular compositional heterogeneity in a met-allocene SCB PE by using a cross-fractionation(CF) technique.

Some important work7–14 on the thermal prop-erties and molecular structure characterizationsof SCB PEs have been reported. For instance,Wild et al.7 used a temperature-rising elutionfractionation (TREF) technique to determine theSCB distributions of various traditional SCB PEs.Using DSC and X-ray scattering techniques,Schouterden et al.8 observed the occurrence ofmolecular segregation upon isothermal crystalli-zation for traditional methyl- and hexyl- SCB PEs.Hosoda9 investigated the molecular and crystallinestructure of traditional SCB PEs using combinedTREF and size exclusion chromatography tech-niques. He concluded that the crystallinity andmelting temperature of SCB PE fractions decreased

with increasing comonomer content in the order ofoctene-1 � 4-methyl-pentene-1 � hexene-1� butene-1. Wilfong and Knight10 discerned thedifferences in crystallization temperature, meltingtemperature, spherulitic morphology, and the la-mellae long period among the traditional wholeSCB PEs (ethylene/octene copolymers) and theirfractions with various branch contents.

As compared with the traditional SCB PEs, thestudies on metallocene SCB PEs are limited. Inaddition to the already mentioned articles, Ben-sason et al.15 characterized metallocene SCB PEswith different octene comonomer contents pre-pared using an INSITE™ technique. The crystalmorphology, tensile properties, and dynamic me-chanical properties were determined. Marigo etal.16 used small- and wide-angle X-ray scatteringtechniques to compare the crystal lamellar thick-ness of traditional and metallocene SCB PEs. Themetallocene SCB PEs showed a narrower distri-bution of lamellar thickness because of its morehomogeneous structure. Alamo et al.17 and Kim etal.18 used thermal analysis and small-angle X-rayscattering techniques to assess the melting tem-perature as well as structural properties of cer-tain metallocene SCB PEs, independently. Wanget al.19 reported on the crystallization and melt-ing behaviors of a metallocene SCB PE (ethylene/butene copolymer) with a high branch content.We20 recently compared the molecular heteroge-neity between the metallocene whole SCB PEsand their CF fractions. Unlike the whole SCBPEs, the CFs showed less evidence of intermolec-ular SCB heterogeneity but slightly intramolecu-lar heterogeneity.

As reviewed previously, few works have beendone on the characterizations of metallocene SCBPE fractions. From either the theoretical or thepractical viewpoint, it is important to evaluate theSCB content impact on the thermal properties ofmore homogeneous metallocene SCB PE fractions.The main purpose of this study was to assess theeffect of the SCB content on the crystallization ki-netics and melting behavior of a well-defined met-allocene parent SCB PE and its fractions. In addi-tion, the crystal morphology of parent SCB PE andthe fractions formed under nonisothermal condi-tions were preliminarily investigated.

EXPERIMENTAL

Material

One butyl-branched metallocene parent SCB PEand its selected fractions with various branch

326 CHIU ET AL.

contents, supplied by the Phillips Petroleum Co.,were used in this study. The molecular character-istics of these specimens are listed in Table I. Thepreparation procedure for these materials is de-tailed in the literature.6 Briefly, the parent poly-mer was obtained by solvent gradient fraction-ation of a whole metallocene polymer. The parentpolymer was then subjected to further tempera-ture gradient fractionation to obtain its fractions.

DSC

Crystallization kinetics and the resulting meltingbehavior of the specimens were investigated us-ing a PerkinElmer DSC 7 analyzer equipped withan intracooler. The heat flow and temperature ofDSC were calibrated with standard materials, in-dium and zinc, before the investigations. To avoidthermal degradation of specimens upon thermaltreatments, nitrogen gas was always purged intothe DSC during the scans. The sample weightused in each DSC experiment was approximately3 mg to minimize the temperature gradient insidethe specimen during scanning. For nonisothermalcrystallization experiments, the specimens werefirst melted at 160 °C for 2 min to eliminate theprevious thermal history and then cooled to roomtemperature at various rates (1, 2.5, 5, 7.5, 10, 15,20, 25, 30, and 40 °C/min). For the isothermalcrystallization experiments, the specimens werequickly cooled from 160 °C to a preset tempera-ture (Tc) (e.g., 47, 48, 49 °C, and so forth). Thecrystallized specimens were subsequently heatedwith 20 °C/min or other heating rates for meltingbehavior investigations. For the nonisothermalcrystallization experiments, the thermal lags ex-isting inside the DSC would seriously affect theaccuracy of the thermal data obtained.21 There-fore, the thermal data obtained from the DSCwere corrected with a temperature calibration us-

ing a standard material (indium) at various scan-ning rates.

Polarized Optical Microscopy (POM)

POM was used to observe the crystalline mor-phology. An Olympus BH-2 PLM with an at-tached charge-coupled device camera was used inconjunction with a Linkam THMS 600 hot stage.The temperature of the hot stage was calibratedwith benzoic acid [melting temperature (Tm)� 122.5 °C], and the precision of the temperaturecontrol was � 0.2 °C. Thin-film specimens wereprepared by casting 4 wt % of SCB PE/1,2,4-tri-chlorobenzene solution on glass slides. The sol-vent was then evaporated completely after thespecimens were kept at 140 °C for 2 h undervacuum conditions. The specimens were thenthermally treated for morphological observations.

RESULTS AND DISCUSSION

Crystallization Kinetics

Normalized DSC thermograms of mPE-P and itsfractions cooled at 10 °C/min are illustrated inFigure 1. This clearly reveals that mPE-P exhib-its more complex crystallization kinetics as com-pared with its fractions. Because mPE-P pos-sesses high MW and a narrow MWD, this ob-served phenomenon is attributed to theheterogeneity with respect to the inter- and in-tramolecular SCB distribution. For the fractions,the nonisothermal crystallization behavior is

Table I. Molecular Characteristics of Parent SCBPE and Its Fractions

Sample IDMw

(g/mol) Mw/Mn

SCB(mol %)

mPE-P (parent) 184,000 1.30 8.1mPE-A (fraction) 260,000 1.21 10.0mPE-B (fraction) 216,000 1.19 9.0mPE-C (fraction) 187,000 1.21 8.2mPE-D (fraction) 173,000 1.21 8.0

Figure 1. DSC thermograms of mPE-P and its frac-tions cooled from the melt at a rate of 10 °C/min.

SHORT-CHAIN BRANCHED POLYETHYLENE FRACTIONS 327

comparable and simple. Each fraction first showsa dominant sharp exothermic peak at higher tem-peratures, followed by a trivial shallow tail atlower temperatures. This result confirms the pre-vious report20 that although the fractions aremore homogeneous than their parent polymer,they still possess some degree of intramolecularSCB distribution heterogeneity. Similar to theresults from traditional SCB PEs, the crystalliza-tion onset temperature (To) and the crystalliza-tion peak temperature (Tp, temperature at mini-mum crystallization exotherm) decreased as thebutyl content of the fraction increased. The sametrend is observed at other cooling rates. The butylSCB indeed hampers the crystallization ability(window) of mPE. Moreover, To and Tp both shiftto lower temperatures with increasing coolingrates (C) as expected. The values of To and Tp forthe mPEs under different cooling rates are tabu-lated in Table II. Disregarding the shallow low-temperature exothermic tail caused mainly by theintramolecular SCB distribution heterogeneity,Figure 2 takes mPE-C as a typical example for thedevelopment of relative crystallinity (XT) versustemperature in the high-temperature dominantpeak for the fractions cooled from the melt atvarious rates. The XT is defined as

XT � �T0

T dHc

dT dT��T0

T� dHc

dT dT (1)

where T� is the temperature at which the domi-nant sharp exothermic peak ends, and Hc is the

enthalpy of crystallization at temperature T. Allof the curves exhibit a common sigmoid shape.This indicates that the principal nonisothermalcrystallization goes through an initial inductionperiod, followed consecutively by a major fast“primary crystallization” process and a minorslow “secondary crystallization“ process. TheOzawa equation22 could be used to successfullyanalyze the nonisothermal crystallization kinet-ics of several polymers including poly(ethyleneterephthalate) (PET),22 polypropylene (PP),23,24

syndiotactic polystyrene (sPS),25 poly(phenylenesulfide) (PPS),26 and poly(aryl ether ketone)(PEDEKmK).27 The Ozawa equation is a modifi-cation of the well-known Avrami equation28–30

Table II. To and Tp of mPEs under Different Cooling Rates

Cooling Ratea

mPE-P mPE-A mPE-B mPE-C mPE-D

Tob Tp

b To Tp To Tp To Tp To Tp

1.0 103.4 73.2 55.7 53.1 60.9 58.0 66.9 64.3 70.7 68.12.5 101.5 70.9 53.6 51.0 59.3 56.2 64.8 61.5 69.9 65.95.0 98.1 69.4 52.8 49.7 58.0 54.8 63.3 59.8 68.8 64.37.5 96.5 68.7 52.1 48.4 57.8 53.8 62.6 58.7 67.9 63.2

10 96.0 67.8 51.4 48.1 57.0 52.9 62.1 58.0 67.3 62.415 95.8 66.5 51.0 46.3 55.6 51.7 61.6 56.9 66.3 61.020 95.2 65.4 49.8 45.9 54.6 50.7 60.4 55.5 65.7 59.925 93.8 64.4 49.5 44.9 54.0 50.0 59.5 54.0 65.3 59.130 93.2 63.4 49.4 43.2 53.7 48.6 58.8 52.9 64.2 58.040 91.5 61.5 42.6 36.9 50.2 42.0 56.4 49.3 63.3 55.5

a °C/min.b °C.

Figure 2. Development of relative crystallinity, XT,as a function of temperature for mPE-C in the domi-nant exothermic region.

328 CHIU ET AL.

1 � Xt � exp � � Ktn� (2)

where Xt is the fractional crystallinity at time t, Kis a kinetic function dependent on temperature,and n is the Avrami exponent. The Avrami expo-nent n is recognized as providing qualitative in-formation on the nature of the nucleation andcrystal-growth geometry. K is related to the crys-tal-growth rate as well as crystal geometry.Ozawa modified the Avrami equation by incorpo-rating a heating/cooling factor. After replacing twith T/C (in eq 2), the Ozawa equation is writtenas

1 � XT � exp [�K�T�

Cn ] (3)

where XT is a function of temperature instead,K(T) is a temperature-dependent parameter, andC is the cooling (or heating) rate. Upon analyzingthe nonisothermal crystallization kinetics, theOzawa equation is always cast into the followingdouble logarithmic form:

ln [�ln (1 � XT)] � ln K(T) � n ln (1/C)

� ln K(T) � n ln C (4)

Theoretically, if the Ozawa derivation can ade-quately follow the crystallization process, a plot ofln[�ln(1 � XT)] against �lnC should yield astraight line with slope n and intercept lnK(T).Typical Ozawa equation treated plots of two frac-tions in the primary exothermic regions are de-picted in Figure 3. A series of parallel lines areobtained. It is thus suggested that the Ozawaequation can be followed for the fractions inves-tigated. As reported by Eder and Wlochowicz,24

the Ozawa equation is not valid for linear PEbecause of a high degree of secondary crystalliza-tion. Therefore, the applicability of the Ozawaequation in our case of SCB PE fractions might beascribed to the exclusion of the high degree ofsecondary crystallization upon the analysis. TheOzawa exponent n and parameter K(T) deter-mined at different temperatures are tabulated inTable III. The n value decreases as the tempera-ture decreases for each fraction. This trend of ndecreases with a decline of temperature that hasbeen reported for other polymers.23,25–27 Accord-ing to Wunderlich,31 our result suggests that thenucleation might be a thermal-to-athermal typeduring the cooling processes. The nucleation is

then followed by a three-dimensional to a two-dimensional mixed crystal growth. The K(T)value, on the contrary, increases with decreasingtemperature. The definition of K(T) has been de-tailed by Ozawa.22 As suggested by Lopez andWilkes,26 K(T) is bulk crystallization rate related.The similar decreasing trend of K(T) with increas-ing temperature is always observed in other poly-mers as those shown in our SCB PEs. Conse-quently, our result implies the expected behaviorof the crystallization rate increases as the tem-perature decreases in the low supercooling region.However, because of the relatively faster crystal-lization rate of SCB PEs as compared with thepolymer such as PET,32 the crystallization kinet-ics study at a high supercooling region (nearbythe glass transition) is difficult to conduct. If thecrystallization kinetics at the lower temperaturescan be examined, the K(T) value will be expectedto increase with temperature.

As previously described, the crystallizationpeak temperature (Tp) is cooling rate dependent

Figure 3. Ozawa plots of ln[�ln(1 � XT)] versus�lnC for (a) mPE-B and (b) mPE-D at indicated tem-peratures.


for each fraction. According to Kissinger’s33 deri-vation, the apparent activation energy (Ea,N) for anonisothermal crystallization process can be ob-tained from the relationship between Tp and Cthrough the following equation:

dln�C/Tp2�

d�1/Tp��

Ea,N

R (5)

where R is the universal gas constant, and Ea,N isthe activation energy required for segmental dif-fusion as well as secondary nucleation of the mol-ten molecules under nonisothermal conditions. Ifa straight line could be obtained by plotting ln(C/Tp

2) versus 1/ Tp, the Kissinger equation is deemedadequate to model the crystallization kinetics.The slope of the straight line would be equal to�Ea,N/R, and thus Ea,N can be calculated. Figure4 describes the Kissinger plots of the four frac-tions. Each fraction can be fitted by a straight linewith the least-squares method. Therefore, the ap-parent activation energy of nonisothermal crys-tallization for each fraction can be calculated. TheEa,N values are also included in Table III; they aresimilar in magnitude, 306–356 kJ/mol.

In addition to the nonisothermal crystalliza-tion kinetics, the isothermal crystallization kinet-ics of the mPE fractions has also been studied.Figure 5 illustrates the representative DSC ther-

mograms of mPE-B isothermally crystallized atthree different Tc’s (50, 52, and 54 °C) from themelt. As expected, the time needed for crystalli-zation completion is longer at a higher Tc. More-over, primarily because of the intramolecularSCB distribution heterogeneity, an increase in Tccauses a decline in the heat of crystallization.Similar Tc-dependent crystallization behaviorsare also observed for the other fractions exceptthat different fractions show different crystalliza-tion windows (Tc range) that can be investigated(i.e., the mPE-D fraction possesses the highest

Table III. Ozawa Exponent n, Parameter K, and Activation Energy of Nonisothermal Crystallizationfor Each Fraction

Ea,N (kJ/mol)Temperature (°C)

mPE-A mPE-B mPE-C mPE-D

343.9 356.0 306.4 340.2

n K n K n K n K

47 1.9 164.0 — — — — — —48 2.2 134.3 — — — — — —49 2.4 66.7 2.5 445.9 — — — —50 3.0 49.4 3.1 270.4 — — — —51 3.5 24.5 3.2 164.0 — — — —52 — — 3.4 109.9 — — — —53 — — 3.3 81.5 — — — —54 — — 3.6 54.6 — — — —55 — — 3.8 44.7 2.5 1636.0 — —56 — — — — 2.6 1211.9 — —57 — — — — 2.6 665.1 2.4 3294.558 — — — — 3.0 601.8 2.5 2697.359 — — — — 3.2 365.0 2.5 2208.360 — — — — 3.4 134.3 2.7 1480.361 — — — — — — 2.9 1096.662 — — — — — — 3.2 601.8

Figure 4. Kissinger plots of the four fractions.

330 CHIU ET AL.

crystallization window then followed by mPE-C,mPE-B, and mPE-A). This is attributed to thedifference in crystallization ability as already ob-served in nonisothermal crystallization experi-ments. Figure 6 plots the tp, which is the time forthe minimum crystallization exotherm to occur,as a function of Tc for each fraction. Because tp isrecognized as being reciprocally proportional tothe overall crystallization rate, this result clearlyindicates that at the same Tc the fraction with thelower butyl content shows a higher overall crys-tallization rate.

Figure 7 illustrates the development of abso-lute crystallinity (Xt) as a function of time atvarious Tc’s for mPE-B and mPE-D fractions. Theabsolute crystallinity is calculated using the fol-lowing equation: Xt � �

0

t dHc

dt dt��Hf° (6)

where Xt is the crystallinity developed at time t,and dHc is the enthalpy change. The equilibriumheat of fusion (�Hf°) is taken as 293.6 J/g.34 All ofthe curves exhibit a similar sigmoid shape. Thecrystallinity developed decreases as Tc increases.The Avrami equation was used for analyzing theisothermal overall crystallization kinetics. TableIV gives the Avrami exponent n and K values. Ingeneral, n is around 3 with minor deviation, andK decreases as Tc increases. This finding mightsuggest that under the Tc range studied, the nu-cleation type of mPE fraction is mostly athermal,and the nucleation is followed by a pseudo-three-dimensional crystal growth. The apparent activa-tion energy, Ea,I, for the isothermal overall crys-tallization in the limited Tc range was estimatedusing the following Arrhenius-type equation:35

Figure 5. Representative DSC thermograms of mPE-B isothermally crystallized at indicated temperatures,Tc’s.

Figure 6. Isothermal crystallization peak time, tp,versus crystallization temperature, Tc, for the four frac-tions.

Figure 7. Development of absolute crystallinity, Xt,as a function of time for (a) mPE-B and (b) mPE-D atindicated Tc’s.


Vc � A exp ��Ea,I /RTc� (7)

where Vc is the crystallization rate correspondingto the linear portion of the Xt versus t plots asshown in Figure 7, A is a pre-exponential factor, Ris the universal gas constant, and Tc is the crys-tallization temperature. Here Ea,I is the activa-tion energy required for crystal growth duringisothermal conditions. If a straight line could beobtained by plotting lnVc versus 1/Tc, the slope ofthe straight line would then be equal to �Ea,I/R,and thus Ea,I could be calculated. The Ea,I’s deter-mined from the Arrhenius-type equation are in-cluded in Table IV. Comparing the data from Ta-bles III and IV, the activation energy of isother-mal crystallization is at the same magnitude asnonisothermal crystallization for each metallo-cene SCB PE fraction investigated.

Melting Behavior

Figure 8 indicates typical DSC melting curves formPE-A and mPE-C crystallized under differentcooling rates of 5, 10, 20, and 40°/min, respec-tively, from the melt. One minor low-melting peak(shoulder) as well as one major high-melting peakare observed for each curve. The low-melting peakshifts to a higher temperature as the precoolingrate decreases, whereas the high-melting peakremains almost constant. As the precooling ratedecreases, the low-melting peak increases its

Table IV. Avrami Exponent n, Parameter K, and Activation Energy of Isothermal Crystallizationfor Each Fraction

Ea,I (kJ/mol)Tc (°C)


371.8 361.7 407.2 323.5

n K n K n K n K

44 3.5 4.5 (E-5) — — — — — —46 3.6 1.5 (E-5) — — — — — —48 3.6 3.4 (E-6) 3.3 1.7 (E-4) — — — —50 3.3 8.3 (E-7) 3.1 5.5 (E-5) — — — —52 3.1 3.2 (E-6) 2.7 1.3 (E-4) — — — —54 2.8 6.8 (E-7) 2.5 3.0 (E-5) 3.4 2.0 (E-4) — —56 — — 2.6 1.7 (E-5) 3.4 2.5 (E-4) — —58 — — 2.5 6.1 (E-6) 3.6 8.9 (E-5) — —60 — — — — 3.5 1.4 (E-5) 3.1 6.1 (E-4)62 — — — — 3.3 1.3 (E-6) 3.4 2.5 (E-5)64 — — — — 2.7 3.4 (E-6) 3.5 1.1 (E-5)66 — — — — — — 3.1 9.6 (E-6)68 — — — — — — 3.0 4.5 (E-6)70 — — — — — — 2.7 1.1 (E-6)

Figure 8. DSC melting thermograms of (a) mPE-Aand (b) mPE-C crystallized under indicated coolingrates (heating rate: 20 °C/min).

332 CHIU ET AL.

share of the total heat of fusion. To disclose theorigin of the double melting peaks, the meltingbehavior under different heating rates (i.e.,2.5–60 °C/min) was also investigated. Increasingthe heating rate resulted in an increase in thelow-melting peak’s share of the total heat of fu-sion, and the gap between the two melting peakswas also shortened. These phenomena suggestthat the high-melting peak is due to an annealingeffect during heating36 that means recrystalliza-tion and/or reorganization processes occur duringthe DSC scans. The recrystallization and/or reor-ganization processes become less dominant as theprecooling rate declines. This is because the orig-inal crystals are formed more perfectly with adecrease in the precooling rate. A similar phe-nomenon was observed for mPE-B and mPE-D.Figure 9 shows the low-melting peak temperature(Tm,1) and high-melting peak temperature (Tm,2)as a function of the precooling rate for the fourfractions. We believe there is a distribution oflamellar thickness of the crystals formed. The

melting peak temperatures thus determined fromDSC are assumed to correspond to the averagelamellar thickness. Tm,1 rises with a decreasingprecooling rate, and this is even more evident atlow precooling rates. On the other hand, Tm,2 doesnot change much with the precooling rate. Addi-tionally, at the same precooling rate mPE-D ex-hibits both the highest low- and high-meltingpeak temperatures followed by mPE-C, mPE-B,and mPE-A. As recognized, a lower precoolingrate will result in a more stable crystal. It will bemore meaningful to compare the Tm’s of the crys-tals formed under an infinitely slow precoolingrate. The low- and high-melting peak tempera-tures for the fractions under an infinitely slowprecooling rate can be obtained as we extrapolatethe plots of melting temperatures versus the pre-cooling rate to a precooling rate of 0 °C/min. Theextrapolated Tm,1

* and Tm,2* are thus obtained. The

effect of the SCB content on the Tm,1* and Tm,2

* canbe directly observed from Figure 10. The Tm,1

* andTm,2

* decrease linearly with an increasing SCBcontent. As we extrapolate these two sets of datato an SCB content of 0%, Tm,1

* and Tm,2* intercept

almost at a common temperature around 137.5 °C(i.e., 137.2 and 137.8 °C, respectively). This tem-perature is commonly observed for a linear PE.This result also suggests the occurrence of thehigh-melting peak for each fraction is not fromthe crystals originally formed but caused by theheating process. From Figure 10, the relationsbetween the butyl SCB content and Tm

* ’s, Tm* �

137.5 °C � � � (SCB mol %), can be deduced. The� is 7.72 for Tm,1

* , and 7.03 for Tm,2* .

Figure 9. (a) Low-melting peak temperature, Tm,1,and (b) high-melting peak temperature, Tm,2, as a func-tion of precooling rate for the four fractions.

Figure 10. Plots of Tm* ’s versus SCB content of the

four fractions.


Figure 11 illustrates the melting curves formPE-A and mPE-C isothermally crystallized atdifferent Tc’s. Similar to nonisothermally crystal-lized specimens, a general feature of these curvesis the appearance of double melting peaks. Thedetails for these melting peaks depend on boththe SCB content and Tc. In this figure, two inter-esting features are noted. First, the total heat offusion for the double melting peaks decreases asTc increases. This is presumably ascribed to theintramolecular SCB distribution heterogeneityinstead of unfinished crystallization caused byinsufficient crystallization time at higher Tc’s. Be-cause the crystallization time at each Tc is at least10 times of tp. As Tc increases, the ethylene blockwith the shorter sequence length becomes lesscrystallizable, thus resulting in a decrease in thefinal heat of fusion. Second, the share of total heatof fusion for the low-melting peak increases withTc. As further confirmed by the results from thedifferent heating rates, the high-melting peaksalso originate from the annealing effect of thelow-melting peaks (like nonisothermally crystal-lized specimens). The low-melting peak tempera-ture (Tm,1) and high-melting peak temperature(Tm,2) values resulting from isothermal crystalli-zation are listed in Table V. Because the high-melting peak was formed through an annealingprocess, Tm,1 was used to estimate the equilib-rium melting temperature, Tm°, for each fractionthrough a conventional extrapolative method.37

Figure 12 depicts the result from the Tm/Tc ex-

Figure 11. DSC melting thermograms of (a) mPE-Aand (b) mPE-C crystallized at indicated Tc’s (heatingrate: 20 °C/min).

Table V. Melting Temperatures (Tm,1 and Tm,2) of Isothermally Crystallized Fractions

Tc� (°C)


Tm,1a Tm,2

a Tm,1 Tm,2 Tm,1 Tm,2 Tm,1 Tm,2

44 52.4 70.7 — — — — — —46 54.5 70.2 — — — — — —48 56.6 70.6 56.4 73.0 — — — —50 58.7 70.9 58.2 72.8 — — — —52 61.2 70.9 60.2 73.3 — — — —54 63.3 71.5 62.3 73.2 62.2 78.6 — —56 — — 65.1 73.9 64.5 79.0 — —58 — — 67.2 73.8 66.9 79.1 — —60 — — — — 68.8 79.5 68.0 82.662 — — — — 71.3 80.0 70.2 83.264 — — — — 73.5 80.2 72.4 84.366 — — — — — — 74.5 84.568 — — — — — — 76.4 85.570 — — — — — — 78.7 85.8

a °C.

334 CHIU ET AL.

trapolative analysis. The crystallinity developedat each Tc for each specimen is less than about10%. Each fraction yields a straight line, and thisstraight line falls nearly onto a master straightline. However, the slope of each fraction is slightlyhigher than 1, which means that there is no ex-pected intercept between each of the straightlines and the Tm � Tc line. This result is similarto that reported by Alamo et al.,17 Kim et al.,18

and Wang et al.19 on different SCB PEs, and the

possible reasons for these observations have beenreported. We can thus conclude that the Tm/Tcextrapolative method is inapplicable for obtainingthe equilibrium melting temperature in our case.To obtain the Tm° for each fraction, other tech-niques (e.g., scattering method and/or micros-copy) might be applied. Nevertheless, from theplots in Figure 12 we can deduce the relationbetween Tc and Tm (low-melting peak, arisingfrom the crystals formed at Tc) for the fractionsexamined. The relation was Tm � Tc (°C) � 8.3(�0.5).

Crystal Morphology

The crystal morphology of mPE-P and its frac-tions were observed through POM. Figure 13shows the micrographs for mPE-P and mPE-Ccrystallized under various cooling conditions (i.e.,1 and 20 °C/min as well as air-quenched from themelt). At least two observations are noteworthy inthis figure. First, for either mPE-P or mPE-C thesize of the crystalline superstructure decreases asthe cooling rate increases. This kind of cooling-rate-dependent morphology is commonly ob-served in crystalline polymers. It clearly suggeststhat the faster the cooling rate, the denser thenucleus density. This is reminiscent of the “re-gime” crystal-growth theory proposed by Hoffmanet al.38 The regimes (e.g., I, II, and III) in polymer

Figure 12. Plots of Tm,1 against Tc for the four frac-tions.

Figure 13. POM micrographs of mPE-P and mPE-C crystallized under various ratesfrom the melt: (a) mPE-P, 1 °C/min; (b) mPE-P, 20 °C/min; (c) mPE-P, air-quenched; (d)mPE-C, 1 °C/min; (e) mPE-C, 20 °C/min; and (f) mPE-C, air-quenched. [Color figure canbe viewed in the online issue, which is available at www.interscience.wiley.com.]


crystallization are defined by the relative ratesbetween the nucleation of polymer stems onto thesubstrate surface and the lateral spreading ofchain molecules across the substrate layer. Inregime I crystal growth, the lateral spreadingrate proceeds rapidly after surface nucleation iscompleted and covers the entire substrate surfacebefore another successive surface nucleates. Thisregime always occurs at higher Tc’s. As Tc de-clines, regimes II and III crystal growths are en-countered consecutively. In regime II crystalgrowth, the surface nucleation rate is faster thanthe lateral spreading rate; thus, it is a process ofmultiple nucleation growing onto a monocrystallayer. In regime III crystallization, the surfacenucleation is so fast that multiple nucleation oc-curs simultaneously on several crystal-growthlayers. From comparable POM analysis, increas-ing the cooling rate of nonisothermal crystalliza-tion is equivalent to lowering the Tc of isothermalcrystallization, thus increasing the number of ef-fective nuclei. The impingements between the su-perstructures occur more frequently, and the di-mensions of the superstructure decline. The sec-ond observation is that no typical spheruliticstructure was detectable in these micrographs,although Figure 13(a) barely exhibits a spheru-lite-like structure. This result indicates the highSCB content indeed hampers the PE chain mole-cules from forming a more regular crystalline en-tity. No special superstructure was developed inmPE-A under the air-quenched condition (onlyweak tiny bright spots are observed) because ofits high SCB content.

CONCLUSIONS

We compared the crystallization kinetics, meltingbehavior, and crystal morphology of four mPEfractions. From the nonisothermal crystallizationexperiments, the homogeneity in SCB distribu-tion for these fractions was confirmed. As the SCBcontent increases, the crystallization peak tem-perature (Tp) and crystallization onset tempera-ture (To) both shifted to lower temperatures dur-ing the cooling processes. The Ozawa analysisimplied that the nucleation processes for the frac-tions were thermal to athermal type, and thecrystal-growth geometry changed as the temper-ature declined. The activation energy for noniso-thermal melt crystallization was determined foreach fraction. This value was around 306–356kJ/mol. From the isothermal crystallization ex-

periments, we confirmed that the higher the Tc,the lower the crystallinity developed for each frac-tion. The investigated Tc range depended on thefraction. Avrami analysis revealed that the nucle-ation type for each fraction under isothermalcrystallization conditions was mostly athermal.The crystal-growth geometry is pseudo-three-di-mensional. The activation energy for isothermalcrystallization was determined for each fractionto be around 300–400 kJ/mol. Double meltingendotherms were observed for nonisothermallyand isothermally crystallized specimens, andthey are SCB content, precooling rate, and Tcdependent. The annealing effect during heatingscans led to the high-melting peak. The conven-tional Hoffman–Weeks’s approach failed to deter-mine the equilibrium melting temperature foreach fraction. However, the relation between Tcand Tm was determined to be Tm � Tc (°C) � 8.3.From the POM observations, we concluded thatthe typical spherulite was not formed for eachfraction as a result of the high SCB content. Thefraction with the highest SCB content could notform a large entity of crystals under a fast coolingcondition.

The authors express their sincerest appreciation to Dr.E. T. Hsieh of Phillips Petroleum Co. for supplying thestudied materials. The authors thank the work con-ducted by C. H. Chen. The financial support from theNational Science Council of the Republic of China un-der contract NSC 90-2216-E-182-003 is appreciated.

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