Quiescent Crystallization of Natural Fibers–Polypropylene Composites

Usa Somnuk,1 Gerhard Eder,2 Pranee Phinyocheep,3 Nitinat Suppakarn,1 Wimonlak Sutapun,1

Yupaporn Ruksakulpiwat1

1School of Polymer Engineering, Institute of Engineering, Suranaree University of Technology,Nakorn Ratchasima, Thailand2Institute of Polymer Science, Johannes Kepler University, Linz, Austria3Department of Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand

Received 1 March 2007; accepted 20 May 2007DOI 10.1002/app.26883Published online 9 August 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The effect of natural fibers (vetiver grassand rossells) on quiescent crystallization of polypropylene(PP) composites was analyzed in this study. Also, equilib-rium melting temperature (T0

m) of the composites was eluci-dated. Natural fiber-PP composites showed lower T0

m whencompared to neat PP. Thermal analysis was performed viadifferential scanning calorimeter to study the crystallizationkinetics. Natural fiber-PP composites exhibited higher rateof crystallization than that of neat PP. Furthermore, spheru-litic growth rate and transcrystallinity of the compositeswere investigated under a polarized light optical micro-

scope. It was found that the growth rates of the compositeswere lower than that of neat PP. The spherulitic growthrates combined with the crystallization rates were used tocalculate number of effective nuclei. It was shown that thenumber of effective nuclei of the composites was higherthan that of neat PP. This suggested that natural fiberscould act as a nucleating agent in the composite. � 2007Wiley Periodicals, Inc. J Appl Polym Sci 106: 2997–3006, 2007

Key words: crystallization; nucleation; polypropylene;composites; spherulites

INTRODUCTION

Polypropylene (PP) is one of the interesting com-modity thermoplastics, because of its high isotactic-ity, high cost-performance ratio, and its low process-ing temperature. Therefore, its worldwide produc-tion grows up very fast during years and there arepossibilities to modify PP to a wide range of finalproducts for several kinds of applications. Thermaland mechanical properties of polymeric materialscan be significantly improved using fillers.1 In recentyears, natural fibers have been increasingly used asalternative fillers in polymer composites.2–5 Theiradvantages over synthetic fibers are low cost, lesstool wear during processing, low density, environ-mental friendly, biodegradability, and renewability.6–8

The natural fibers have potential to dramaticallymodify the crystallization characteristics of a givenpolymer matrix. The ultimate properties of the natu-ral fiber–polymer composites depend on their micro-structure and crystallinity. Additionally, the failureof the materials takes place at the microscopic level.Recently, several efforts have been made to charac-

terize the effect of fibers, which served as fillers orreinforcements on crystallization of various thermo-plastic polymers.9–21 Experimental evidence confirmsthat the fibers affect the crystallization kinetics andmorphology of the matrix. However, those studieshave mentioned on the synthetic fibers9–17 or inor-ganic fillers.22–32

In this study, natural fibers used for polymer com-posite are vetiver grass and rossells. Vetiver grass isa tropical plant, which well adapt to different envi-ronments. According to His Majesty the King Bhumi-pol Adulyadej of Thailand’s Royal Initiative, themain purpose of vetiver grass cultivation is to con-serve soil and water, particularly for the steep slopeareas. Normally, leaves of the vetiver grass havebeen cut every few months to keep the vetiver rowsin order. Only a minor portion of the residues is re-served as animal feed or household fuel. On theother hand, huge quantities of the remaining residuesare burnt in fields or on the side of road. Rossells areplanted tremendously in the northern part of Thai-land. To help the farmer gain some extra income, ourprevious effort has been made to use vetiver grassand rossells as fillers in polymer composites.5,33 Itwas found that PP composites from vetiver grass androssells exhibited higher tensile strength, yield stress,and Young’s modulus than that of neat PP. This indi-cated that both vetiver grass and rossells could serveas reinforcements in PP composites.

Correspondence to: Y. Ruksakulpiwat (yupa@sut.ac.th).Contract grant sponsors: Thailand Research Fund (TRF)

and Suranaree University of Technology.

Journal of Applied Polymer Science, Vol. 106, 2997–3006 (2007)VVC 2007 Wiley Periodicals, Inc.

In the present work, the crystallization kinetics ofvetiver grass-PP composites and rossells-PP compo-sites were studied. The effect of vetiver grass androssells on equilibrium melting temperature, half-time of crystallization, and crystallization rate of thecomposites were examined. The spherulitic growthrates of neat PP and PP composites were deter-mined. In addition, nuclei concentration of the com-posites was explored.

EXPERIMENTAL

Materials

Vetiver grass and rossells were washed by water toeliminate dirt and dried in an oven at 1008C for24 h. After that, they were prepared into a length of2 mm. The aspect ratio of vetiver grass and rossellswas 6.15 and 27.82, respectively. The vetiver grasswas immersed in a solution of 4% (wt) NaOH for2 h at 408C and the vetiver-to-solution ratio is 1 : 25(w/v). The vetiver grass was then washed by waterand dried in an oven at 1008C for 24 h. The rossellsfibers were treated as follows. The fibers wereweighed about 700 g and put into a reactor. The 10 Lmethanol/benzene mixture (1 : 1) was then addedinto the reactor and heated to 808C for 3 h. Afterthat, the rossells fibers were immersed in 2% (wt)NaOH solution for 2 h at room temperature, subse-quently washed by water, and then dried overnightin an oven at 1008C.

A commercial grade of isotactic PP (700J) suppliedby Thai Polypropylene Co., Ltd. was mixed witheach natural fiber in an internal mixer (model HakkeRheomix Polylab) at 1708C. The ratio of natural fiberto PP is 20 : 80. Then, each composite sample wasprepared into a thin film with a thickness of 50 lmusing a compression molding.

Isothermal crystallization of naturalfibers-PP composites

Thermal properties of PP composites were deter-mined using a differential scanning calorimeter (DSC,Perkin–Elmer: model DSC7). The sample was heatedfrom 25 to 2008C and held at 2008C for 5 min to elimi-nate the thermal history of sample. Then, the samplewas cooled down with a rate of 408C/min to variouspredetermined crystallization temperatures (Tc) andmaintained at that temperature until the crystalliza-tion was completed. The enthalpy from DSC thermo-gram was used to obtain the relative degree of crys-tallinity, X(t), by using the following equation.29

XðtÞ ¼

Rt0

ðdH=dtÞdtR10

ðdH=dtÞdt(1)

where dH denotes the measured enthalpy of crystalli-zation during an infinitesimal time interval dt. Thelimits t and 1 are used to denote the elapsed timeduring the course of crystallization and at the end ofthe crystallization process, respectively.

After the crystallization completed, the sampleswere heated to the melting point at a rate of 108C/min. The melting temperature (Tm) of the sampleswas obtained from the maximum of the endothermicpeaks. The equilibrium melting temperatures (T0

m)was determined from Hoffman-Weeks plot (theextrapolation plot of Tc versus Tm).

11

Isothermal crystallization rate constantby Avrami plots

The relative degree of crystallinity, X(t), is related tothe crystallization time, t, according to Avrami equa-tion as shown in the following equation34–36:

XðtÞ ¼ 1� expð�ktnÞ (2)

where n is the Avrami exponent which is a functionof the nucleation process and k is the isothermal rateconstant of crystallization. The values of n and k canbe calculated by fitting to experimental data usingthe double logarithmic form as shown in eq. (3)

logf� ln½1� XðtÞ�g ¼ log kþ n log t (3)

A plot of log{2ln[1 2 X(t)]} as a function of log tyields a straight line with slope n and intercept log kcalled an Avrami plot.

Isothermal crystallization rate constant by half-timeof crystallization

The half-time of crystallization, t1/2, which is theelapsed time from the crystallization onset time untilthe relative degree of crystallinity reaches a valuesof 0.5, was determined and used to calculate the iso-thermal rate of crystallization from the followingequation.34–36

kðTÞ ¼ ln 2

ðt1=2Þn(4)

where n is assumed to be 3.

Nonisothermal rate constant from isothermalexperiment

Nonisothermal crystallization rate constant wasobtained by using Nakamura expression37,38:

XðtÞ ¼ 1� exp �Z

KðTÞdt� �n

(5)

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where K(T) is nonisothermal crystallization rate con-stant which is related to the isothermal crystalliza-tion rate constant as shown in eq. (6).

KðTÞ ¼ ½kðTÞ�1=n (6)

The overall rate of nonisothermal crystallization

The nonisothermal rate constant K(T), obtained fromeq. (6), can be expressed by Hoffman-Lauritzenexpression.39

KðTÞ ¼ ðln 2Þ1=n 1

t1=2

� �0

exp�U�

RðTc � T1Þ� �

exp�Kk

TcðDTÞf� �

(7)

From the expression, the kinetic model has four pa-

rameters: 1=t1=2� �

0is the pre-exponential factor that

includes all terms independent of temperature, Kk isthe nucleation exponent, U* and T1 are the Vogel-Fulcher-Tamman-Hesse (VFTH) parameters describ-ing the transport of polymer segments across the liq-uid/crystal interphase, DT denotes the supercooling

(DT 5 T0m 2 Tc) and f 5 2Tc/(Tc 1 T0

m) is a correc-tion factor accounting for the temperature depend-ence of the latent heat of fusion. R is the gas con-stant. The universal values used for the VFTH pa-rameters are U* 5 1500 cal/mol (6280 J/mol) andT1 5 (Tg 2 30) K.29 In this study, the Tg value of PPwas 270 K40 and the equilibrium melting tempera-

ture, T0m, was obtained from this study.

Spherulitic growth rate and numberof effective nuclei

Thin films (the thickness of 50 lm) of natural fibers-PP composites and neat PP were prepared by com-pression molding. Then, the samples were used tomeasure the spherulitic growth rate (G) by means ofa Hot Stage (Linkam TH600) under a Polarized Opti-cal Microscope connected with CCD video camcor-der system (Sony). The sample was heated fromroom temperature to 2008C with a heating rate of108C/min and held at that temperature for 5 min.Then, the sample was cooled down with a rate of508C/min to various Tc. The radius of spherulitewas measured as a function of time. The growth rateat various Tc was obtained from the slope of theplots of spherulite diameter versus time.

The kinetic data of isothermal crystallization canbe analyzed using the spherulitic growth rate in thecontext of Hoffman-Lauritzen secondary nucleationtheory. Accordingly, the growth rate G(T) is given asa function of the crystallization temperature by thefollowing bi-exponential equation41:

GðTÞ ¼ G0 exp�U�

RðTc � T1Þ� �

exp�Kg

TcðDTÞf� �

(8)

where G0 is the pre-exponential factor independentof temperature. Kg is a nucleation parameter. Theremaining factors have the same meaning as out-lined in eq. (7).

If spherulites are assumed to grow in a free spacewithout impingement, the total volume relative tothe unit volume transformed by time, t, is given bythe following equation30:

Vfree ¼X1i¼1

ni4p3

� �½Gðt� tiÞ�3 (9)

where ni is the number of nuclei per unit volumeactivated at time, ti, and G is the radial growth rate.If the mode of nucleation is simultaneous then,

Vfree ¼ N4p3

� �½GðtÞ�3 (10)

where N is the number of effective nuclei. For thereal space in which a number of spherulites impingewith each other and their growth is restricted, theAvrami-Evans theory proposed,

1� V

V1¼ expð�VfreeÞ (11)

where V1 is the transformable volume fraction andV is the volume fraction transformed by a certaintime. By representing V/V1 by the degree of crystal-lization, X(t), and replacing eq. (10) by Kt3, eq. (10)was transformed into the following equation:

1� XðtÞ ¼ expð�Kt3Þ (12)

where

N ¼ 3K

4pG3(13)

From eq. (13), the growth rates combined with thenonisothermal rate constants determined from eqs.(7) and (8), respectively, were used to estimate thenumber of effective nuclei as a function of tempera-tures.

RESULTS AND DISCUSSION

Equilibrium melting temperature

The Hoffman-Weeks plots of neat PP, vetiver grass-PP, and rossells-PP composites are shown in Figure 1.The equilibrium melting temperature (T0

m) of neatPP, vetiver grass-PP, and rossells-PP compositesobtained from the Hoffman-Weeks plot is listed in

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Table I. The addition of natural fibers to PP caused adecrease in T0

m. This may be due to the presence ofnatural fibers in the composite, leading to the forma-tion of unstable as well as less perfect spherulites. Itcan be observed that vetiver grass-PP compositeexhibited lower T0

m than rossells-PP composites.Lopez-Manchado and Arroyo11 also found that theaddition of synthetic fibers to PP resulted in adecrease of T0

m. Additionally, Arroyo et al.9 hasreported that adding more than 10% of glass fiberinto the PP composites resulted in decreasing T0

m ofthe composites.

Isothermal crystallization kinetics

Plots of relative crystallinity at various crystallizationtemperatures of neat PP, vetiver grass-PP, and ros-sells-PP composites were displayed in Figures 2–4,respectively. Half-time of crystallization obtainedfrom the time by which the relative degree of crys-tallization reached the value of 0.5 was shown inFigure 5. As expected, the half-time of crystallizationincreased when the crystallization temperatureincreased. It can be seen that the composites showed

a noticeable decrease in half-time of crystallizationwhen compared to that of neat PP. This may beattributed to the nucleating effect of the naturalfibers on PP crystallization. Moreover, rossells-PPcomposite gave the lower half-time of crystallizationthan vetiver grass-PP composite.

In addition, the relative degree of crystallinityshown in Figures 2–4 was used to determine Avrami

Figure 1 Hoffman-Weeks plots of neat PP and naturalfibers-PP composites. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

TABLE IEquilibrium Melting Temperature (T0

m) of Neat PP,Vetiver Grass-PP, and Rossells-PP Composites

Samples T0m (8C)

Neat PP 179.03Vetiver-PP 168.42Rossells-PP 175.34

Figure 2 Relative crystallinity (X(t)) of neat PP at variouscrystallization temperatures.

Figure 3 Relative crystallinity (X(t)) of vetiver grass-PPcomposite at various crystallization temperatures.

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exponent (n) using Avrami equation (eq. (2)). Theplots of Avrami exponent of neat PP, vetiver grass-PP, and rossells-PP composites are illustrated in Fig-ure 6. The Avrami exponent of neat PP was in arange of 1.57 to 2.66. The natural fiber-PP compositeshad lower n values than that of neat PP. Both vetivergrass-PP and rossells-PP composites showed n val-ues in the same ranges, 1.21–1.32. In this study, itwas observed that all Avrami exponents were nonin-teger values. The difference of n values may attrib-ute to the secondary crystallization process, complexnucleation modes, and the change in the materialdensity. Moreover, a study of cellulose-thermoplasticcomposites by Quillen et al. has shown that the pres-ence of a transcrystalline layer (TCL) changes then exponent obtained from the Avrami analysis.42 Intheir study, the change in n value related to thechange in shape of crystallites because of changes innuclei concentration with the inclusion of cellulosefiber. For the heterogeneous nucleation case, theshape lies between a diffusion-controlled sphere(n 5 3.0) and a truncated sphere (n 5 1.5).20 Theaddition of natural fibers tended to push the crystal-lites towards the truncated shape. This shift waslikely resulted from the increased nucleation sites onthe fiber surface, but not from a change in the crystalgrowth. The increased nuclei concentration causedthe impinging nuclei to truncate instead of complet-ing spherulitic structures.

The half-time of crystallization in Figure 5 wasused to calculate the isothermal rate constant accord-ing to eq. (4). Also, the relative degree of crystallinityin Figures 2–4 was used to determine crystallization

rate constant from Avrami equation referring to eq.(2). The isothermal rate constant from half-time ofcrystallization and from Avrami plot as a function ofcrystallization temperature are shown in Figure 7. Itwas observed that the isothermal crystallization rateconstant decreased when the crystallization tempera-ture increased. Additionally, the rate constants of thecomposites obtained from both Avrami plot andhalf-time of crystallization were higher than those ofneat PP. When compared between natural fiber-PPcomposites, rossells-PP composite showed the higherrate constant than vetiver grass-PP composite. Thisresult was in good agreement with the lower half-time of crystallization of rossells-PP composite aspreviously mentioned. Lopez-Manchado and Arroyo11

have also observed that the addition of syntheticfibers to PP resulted in the higher rate of crystalliza-tion. They have reported that both unmodified andmodified synthetic fibers led to the rise in rate ofcrystallization because of the nucleating effect offibers on PP crystallization. In case of the modifiedfibers, they suggested that the better affinity betweenthe fibers and polymer matrix was owing to thehigher nucleating effect of the fibers.

To compare the rate constants obtained from theAvrami plot and half-time of crystallization, the iso-thermal rate constants were converted to nonisother-mal rate constants by eq. (6). The nonisothermal rateconstant derived from both methods are presentedin Figure 8. The result from the plots showed thatthe nonisothermal rate constant obtained from Avramiplots and half-time of crystallization was not signifi-

Figure 4 Relative crystallinity (X(t)) of rossells-PP com-posite at various crystallization temperatures.

Figure 5 Half-time of crystallization (t1/2) as a function ofcrystallization temperatures for neat PP and natural fiber-PP composites.

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cantly different. Therefore, assuming an Avramiexponent of 3.0 and calculating the isothermal rateconstant by eq. (3) seems to be valid for neat PP andnatural fiber-PP composites.

The nonisothermal rate constant from half-time ofcrystallization obtained from Figure 8 was then fitted

to eq. (7) to obtain the nonisothermal rate constantin a wide range of crystallization temperatures ofneat PP and natural fiber-PP composites as illus-

Figure 6 Avrami exponent (n) as a function of crystalliza-tion temperatures for neat PP and natural fiber-PP compo-sites. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 7 Isothermal rate constant, k(T), from half-time ofcrystallization and from Avrami plot as a function of crys-tallization temperatures for neat PP and natural fiber-PPcomposites.

Figure 8 Nonisothermal rate constant, K(T), from half-time crystallization and Avrami plot as a function of crys-tallization temperature for neat PP and natural fiber-PPcomposites.

Figure 9 Nonisothermal crystallization rate constants ofneat PP and natural fiber-PP composites as a function ofcrystallization temperature. Symbols represent experimen-tal data while lines indicate the fitting to Hoffman-Laurit-zen expression, eq. (7). [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

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trated in Figure 9. The fitting parameters for noniso-thermal rate constants (Kk and (1/t1/2)0), which weredetermined from eq. (7), are summarized in Table II.The nonisothermal rate constant in the wide rangeof crystallization temperature would be combinedwith the growth rate to determine the number ofeffective nuclei, which would be discussed in thenext section.

Spherulitic growth rate and transcrystallization

Examples of optical micrographs of crystallized PPin the bulk and on vetiver fiber taken during crystal-lization process at Tc 5 1318C are shown in Figures10 and 11, respectively. Spherulitic growth rate of PPin the bulk was determined by measuring diameterof spherulites against time as the crystallization iso-thermally proceeded. Growth rate of transcrystalliza-tion (TC) on the fiber surface was evaluated bymeasuring a width of the TC region perpendicularlyto the fiber surface as a function of time. From thespherulitic growth rate study, it was observed that

embedding natural fiber into the polymer melt thefibers may act as nucleating sites for the spheruliticgrowth as well. As a result, the growing spheruliteswould be restricted in the lateral direction so that acolumnar layer, known as transcrystalline region,was developed on the fiber surface as seen in Figure11. The TC is possible if the energetic conditions ofnucleation are more favorable on the fiber surfacethan in the bulk of the melt.43 The growth of TC per-pendicularly proceeded to the fiber until the grow-ing front impinged with other spherulites nucleatedin the bulk. The mechanism which TC layers occurwas not fully understood and there was no rule forpredicting the appearance of TC in a particularfiber–matrix system. Besides, its effect on the me-chanical properties of the composites and on interfa-cial properties remained controversial.44 The abilityof cellulose-based fibers such as wood, flax, and sisalto induce transcrystallinity in PP composites hasbeen reported.19,20,45–47

The experimental data of spherulitic growth rateof PP in the bulk and transcrystalline region wererepresented as filled and unfilled symbols in Figures12 and 13, respectively. From Figure 12, in a case ofthe bulk, it was found that the growth rate of neatPP was noticeably higher than those of natural fiber-PP composites. This may be due to the restriction ofthe natural fiber on crystallization process. Thegrowth rate of PP in the bulk of vetiver grass-PPcomposites was not much different from that of

TABLE IIHoffman-Lauritzen Fitted Parameters for Nonisothermal

Rate Constants

Samples Kk (K2)

1t1=2

� �0

Neat PP 3.60 3 105 1.98 3 109

Vetiver-PP 2.17 3 104 2.24 3 107

Rossells-PP 3.00 3 104 3.52 3 108

Figure 10 Examples of optical micrographs of crystallizedPP in the bulk taken during the crystallization process atTc 5 1318C. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Figure 11 Examples of optical micrographs of TC of PPon vetiver grass fiber taken during the crystallization pro-cess at Tc 5 1318C. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

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rossells-PP composite. In addition, it was observedthat the growth rate of crystallized PP in the bulk ofall samples were greater than that of crystallized PPin TC region. When comparing between naturalfiber-PP composites, rossells-PP composite had thehigher growth rate in the TC region than vetivergrass-PP composite. This implied that the fiber to-pography of rossells fiber was more favorable forthe nucleation of TC than that of vetiver grass. Inaddition, TC strongly depended on thermodynamiccondition such as crystallization temperature or cool-ing rate because of its nucleation-controlled process.

The experimental spherulitic growth rates at vari-ous Tc were then fitted to eq. (8) as shown in Figure13. It can be verified that the eq. (8) used for deter-mining the spherulitic growth rate in the bulk couldalso be applied to the growth rate in TC region inthis study. The fitting parameters for the spheruliticgrowth rate, Kg and G0, in the bulk and in TC regionwere elucidated and then listed in Tables III and IV,respectively.

Figure 12 Experimental growth rates of crystallized PP inthe bulk and transcrystalline region as a function of crys-tallization temperatures. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 13 Growth rates of crystallized PP in the bulk andtranscrystalline region as a function of crystallization tem-peratures. Filled symbols represent experimental dataobtained in the bulk, and unfilled symbols indicate experi-mental data obtained on the fiber (TC). Lines refer to fit-ting to the Hoffman-Lauritzen growth equation, eq. (8).[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Figure 14 Number of effective nuclei (N) as a function ofcrystallization temperature for neat PP and natural fiber-PP composites calculated according to eq. (13). [Color fig-ure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

TABLE IIINucleation Constant (Kg) and Pre-exponent Factor (G0) of

Neat PP and Natural Fiber-PP Composites is Bulk

Sample Kg (K2) G0 (m/s)

Neat PP 1.32 3 105 8.82 3 1021

Vetiver-PP 6.88 3 104 5.90 3 1022

Rossells-PP 9.89 3 104 1.50 3 1021

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From Tables III and IV, it was found that both Kg

of natural fiber-PP composites in bulk and transcrys-talline region were lower than that of neat PP in thebulk. It is well known that a foreign surface fre-quently reduces the nucleus size needed for crystalgrowth. This is because the creation of the interfacebetween polymer crystal and substrate may be lesshindered than the creation of the corresponding freepolymer crystal surface. A heterogeneous nucleationpath makes use of a foreign pre-existing surface toreduce the free energy opposing primary nuclea-tion.29 Additionally, it revealed that the G0 of naturalfiber-PP composites were lower than that of neat PP.According to a study by Wang et al.,27 it has beenobserved that the incorporation of the BaSO4 into PPcaused a decrease of free volume available for PPchains to move, which certainly resulted in a de-crease in G0. Moreover, the interfacial modificationimproved the adsorption effects of macromolecularchains on particle surface, which resulted in an addi-tion barrier for the motion of the macromolecules.This situation is quite similar to the polymer matrixwith very high molecular weight, leading to veryhigh entanglement density. Therefore, a decrease inG0 with improved interfacial adhesion is possiblydue to retardant mechanism similar to the molecularweight dependence of G0.

From Figure 13, it was found that growth rates ofboth neat PP and natural fiber-PP composites wererepresented by a bell-shaped curve as a function oftemperature. The spherulitic growth rates combinedwith the nonisothermal rate constants determined byeqs. (7) and (8), respectively, were used to estimatethe number of effective nuclei as a function of tem-perature. Figure 14 displays the number of effectivenuclei as a function of crystallization temperature forneat PP and natural fiber-PP composites accordingto eq. (13). Typically, the number of effective nucleidecreased with increasing Tc. It was seen that thenumber of effective nuclei of natural fiber-PP com-posites was higher than that of neat PP. Moreover,rossells-PP composite showed the highest number ofeffective nuclei. This result gave good correspondingto the highest growth rate in the TC region of ros-sells-PP composite as discussed earlier.

Generally, the rate of crystallization depends onboth nucleation rate and growth rate. In this study,it was found that the growth rate of the compositeswere lower than that of neat PP. However, the

higher rate of crystallization of the composites thanthat of neat PP was observed. This implied that thenucleation rate dominated the growth rate. Further-more, it appeared that the number of effective nucleiof natural fiber-PP composites was higher than thatof neat PP. This suggested that natural fibers mayact as nucleating agents in the composites. Wangand Hwang48 have shown that fiber topography,chemical compositions of the surface, and surfaceenergy influenced the nucleation on the fiber surface.From our experimental observations it was interest-ing to point out that both natural fibers used in thisstudy have a-nucleating ability. Varga and Karger-Kocsis49 also reported that the high-modulus carbonfiber has strong a-nucleation ability in quiescent PPmelt. On the other hand, there are several additivesusing in PP melt having b-nucleating efficiency.50,51

However, it is worth to mention that the numberof effective nuclei strongly depends on the fittingparameters of rate constant equation (according toTable II) and growth rate equation (according toTable III). For the fact that the low crystallizationtemperatures or very high cooling rates cannot beachieved in the DSC and the spherulitic growth rateexperiments; therefore, the fitting of number of effec-tive nuclei at the low temperature range may not bevery accurate. At this range, some errors might beintroduced in the prediction of the number of effec-tive nuclei at very low temperature range.

CONCLUSIONS

From this study it can be concluded that naturalfibers i.e., vetiver grass and rossells, affected thecrystallization of PP in the composites. A decrease inequilibrium melting temperature, half-time of crys-tallization, and an increase in the rate of crystalliza-tion were observed in the PP composites when com-pared to those of neat PP. The presence of naturalfibers in the composites led to a decrease in Avramiexponent, which indicated that the fibers tended tocause impinging nuclei and led to truncated spheru-litic shapes. Moreover, the decrease in Avrami expo-nent was probably due to the development of TC.Transcrystallinity growth of PP could be seen on thefiber surfaces. According to an increase in the num-ber of effective nuclei when natural fibers wereincorporated into PP matrix, it can be suggested thatnatural fibers could serve as nucleating agents in thecomposites. Additionally, it was found that rossells-PP composite had the lowest half-time of crystalliza-tion resulting in the highest rate of crystallization. Inaddition, rossells-PP composite gave the highestgrowth rate in transcrystalline region. Also, the num-ber of effective nuclei of rossells-PP composites washigher than that of vetiver grass-PP composites.

TABLE IVNucleation Constant (Kg) and Pre-exponent Factor (G0) ofNeat PP and Natural Fiber-PP Composites in TC Region

Sample Kg (K2) G0 (m/s)

Vetiver-PP 6.22 3 104 1.95 3 1022

Rossells-PP 9.43 3 104 8.00 3 1022

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The authors thank the NEP Realty and Industry PublicCo., Ltd., for donating rossells fiber, and the Land Devel-opment Department (LDD), Nakorn Ratchasima, Thailandfor supplying vetiver grass.

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Journal of Applied Polymer Science DOI 10.1002/app