isothermal crystallization kinetics of poly(butylene terephthalate)/attapulgite nanocomposites

Isothermal Crystallization Kinetics of Poly(butyleneterephthalate)/Attapulgite Nanocomposites

XUEQIN CHEN,1 JIAJING XU,1 HONGBIN LU,1 YULIANG YANG1,2

1Department of Macromolecular Science, Key Laboratory of Molecular Engineering of Polymers,Ministry of Education of China, Fudan University, Shanghai 200433, China

2Department of Physics, Fudan University, Shanghai 200433, China

Received 19 December 2005; revised 27 February 2006; accepted 7 May 2006DOI: 10.1002/polb.20870Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Poly(butylene terephthalate) (PBT)/organo-attapulgite (ATT) nanocompo-sites containing 2.5 and 5 wt % nanoparticles loadings were fabricated via a simplemelt-compounding approach. The crystal structure and isothermal crystallizationbehaviors of PBT composites were studied by wide-angle X-ray diffraction and differen-tial scanning calorimetry, respectively. The X-ray diffraction results indicated that theaddition of ATT did not alter the crystal structure of PBT and the crystallites in all thesamples were triclinic a-crystals. During the isothermal crystallization, the PBT nano-composites exhibited higher crystallization rates than the neat PBT and the variedAvrami exponents when compared with the neat PBT. At the same time, the regime II/III transition was also observed in all the samples on the basis of Hoffman-Lauriztentheory, but the transition temperature increased with increasing ATT loadings. The foldsurface free energy (re) of polymer chains in the nanocomposites was lower than that inthe neat PBT. It should be reasonable to treat ATT as a good nucleating agent for thecrystallization of PBT, which plays a determinant effect on the reduction in re duringthe isothermal crystallization of the nanocomposites, even if the existence of ATT couldrestrict the segmental motion of PBT. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym

Phys 44: 2112–2121, 2006

Keywords: nanocomposites; differential scanning calorimetry (DSC); crystallization;attapulgite; PBT

INTRODUCTION

Polymer nanocomposites are a class of multiphasepolymer materials involving inorganic fillerswith at least one nanoscale dimension. In recentyears, various nanoparticles have been used toimprove the performance of polymers, includingspherical silica,1–4 layered silicates,5–11 carbonnanotubes,12–15 cubic polyhedral oligomeric silses-quioxane clusters,16–20 etc. It has been well recog-

nized that the anisotropy of nanoparticles playsan important role in optimizing the properties ofpolymer nanocomposites. A typical example is theremarkable improvement in the mechanical prop-erties of carbon nanotubes/nylon nanocomposites,where 214 and 162% of increases in elastic modu-lus and yield strength have been reported.21 Inaddition, we demonstrated recently the preferablemechanical enhancement of epoxy resin uponusing anisotropic rod-like silicates (attapulgite,ATT) as nanofillers, rather than layered sili-cates.22 Furthermore, the structure and morphol-ogy of materials seem easier to control than thoselayered silicate nanocomposites, due to the rela-tively weak interaction between the nanorods. In

Correspondence to: H. Lu or Y. Yang (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 2112–2121 (2006)VVC 2006Wiley Periodicals, Inc.

2112

the ongoing work, we attempt to further applythese organically modified nanorods to some com-mon engineering polymers.

Poly(butylene terephthalate) (PBT) is a semi-crystalline thermoplastic engineering materialwith a high crystallization rate, which is wellsuited for injection-molding because of its shortprocessing cycles and excellent dimensional stabil-ity. For semicrystalline polymers, the crystalliza-tion behavior is vital to the performance of materi-als. To optimize the morphology and properties ofmaterials, the crystallization kinetics of linearPBT,23 branched PBT,24 PBT ionomers,25 glassfiber reinforced PBT,26 as well as PBT blends withother polymers27 have been investigated. How-ever, the crystallization kinetics of PBT nanocom-posites has not been well understood up to date,although some PBT nanocomposites based on lay-ered silicates28–30 or carbon nanotubes31,32 havebeen reported.

For semicrystalline polymer nanocomposites,low volume percent addition of nanoparticles sig-nificantly affects the development of polymer crys-tallites. First, the presence of nanoparticles canalter the crystallization habit of polymers, induc-ing the occurrence of polymorphism. Recently, ithas been shown that the addition of layered sili-cates facilitates the nucleation and growth of thec-crystal in polyamide 6 nanocomposites, whereasthe c-crystal for neat polyamide 6 is often formedunder some specific conditions such as elonga-tional flow.33–36 The formation of c-crystal wasattributed to the fact that the added layered sili-cates force the amide group of polyamide 6 out ofthe plane formed by the molecular chains andrestrict the formation of hydrogen-bonded sheets.Second, the nanoparticles dispersed in matricescan accelerate or delay the crystallization of poly-mers. On the one hand, remarkably increasingsurface area of particles provides more nucleationsites so that polymer chains are able to stack andgrow more rapidly. For example, Mitchell andKrishnamoorti37 have showed recently that only0.35 wt % of single walled carbon nanotubes dra-matically increased the crystallization rate ofpoly(e-caprolactone) (PCL) but did not alter theunit cell of polymer crystals, melting temperatureas well as overall extent of crystallinity. On theother hand, nanoparticles may restrict the seg-mental motion of polymers during crystallizationso that the crystallization rate and overall extentof crystallinity of nanocomposites reduce, which isprobably true especially for those systems withstrong interaction between nanoparticles and poly-

mer chains. Several recent experimental resultshave demonstrated such a possibility, where modi-fied nanoparticles not only reduced the crystalliza-tion rate but also increased the equilibrium melt-ing temperature of polymer crystals.38–40 Third,nanoparticles may affect the crystallization re-gime of polymer chains. For example, the regime Iand II of linear polyethylene (PE) are usuallyobserved on the basis of the Lauritzen-Hoffman’ssecondary nucleation theory, while the regime IIIis difficult to observe.41 For an intercalated PE/clay nanocomposite, however, the crystallizationof PE may proceed primarily in the regime III.42

Fourth, the addition of nanoparticles may inducethe variation of fold surface free energy (re) of poly-mer chains during crystallization. For example,in PE/vermiculite systems, the nanoparticles canact as good nucleating agents, which cause sys-tematical re reduction with increasing solid load-ings.43 However, such a nucleating effect may besubjected to the competition of obstruction effect,which stems from the higher volume percent ofnanoparticles or strong interactions between pol-ymer chains and nanoparticles. In PE/vermiculitenanocomposites,43 PE displays the increase in rewith increasing vermiculite loadings. The hybrid-ization of silica networks with PCL also causes anincrease in re due to the hydrogen-bonding inter-action between the silicon oxide networks andPCL.44

For PBT nanocomposites, however, the knowl-edge related to their crystallization behaviors isstill scarce, although several kinds of nanopar-ticles have been used to enhance the performanceof PBT.28–32 In the present work, we examine theeffect of rod-like nanoparticles (ATT) on the crys-tallization behavior of PBT. There are three pri-mary purposes, that is, examine (1) whether theorganically modified ATT alters the crystal formof PBT, (2) whether the presence of ATT influen-ces the crystallization regime of PBT, and (3)whether ATT affects the crystallization rate andfold surface free energy of PBT during crystalliza-tion.

EXPERIMENTAL

Material and Sample Preparation

ATT (Attagel 50, Engelhard Co., USA) was firstpurified and its organic modification follows aprocedure we previously described.22 PBT (SHINITEPBT D201) pellets were purchased from the Shin-

ISOTHERMAL CRYSTALLIZATION KINETICS OF PBT/ATT 2113

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

kong Company (Taiwan). After dried at 120 8Cfor about 4 h, PBT pellets were mixed withorgano-ATT and extruded by a twin-screw ex-truder at a rotation speed of 90 rpm. The extrud-ing temperature profiles were 230, 250, 250, 250,240 8C from hopper to die. PBT/ATT nanocompo-sites with different weight ratios (2.5 and 5 wt %)would be denoted as PBT2.5 and PBT5. NeatPBTwas also extruded for the purpose of compar-ison. The extrudates were pelletized and dried.The resulting pellets were melted on a hot stageand then pressed into the thin films with the de-sired thickness for differential scanning calori-metry (DSC) use.

X-ray Diffraction Measurement

The samples for X-ray diffraction (XRD) measure-ments were prepared as follows: PBT and nano-composites were melted at 260 8C and thenpressed into the films (�1 mm). The temperaturewas maintained at 260 8C for 5 min to eliminatethe heat history. The films were quickly removedfrom the hot stage, and then annealed in an oilbath preset at 200 8C. After the isothermal crystal-lization for 2 h or more, the films were quenchedin ice water. XRD patterns were recorded with aPANalytical X’pert diffractometer equipped withNi-filtered Cu Ka radiation (k ¼ 0.154 nm) under avoltage of 40 kVand a current of 40 mA. The scan-ning rate employed was 18min�1. The as-preparedsamples were investigated over a diffraction angle(2h) range of 5–608.

Differential Scanning Calorimetry

DSC (PerkinElmer, Pyris 1) was used to recordthe isothermal crystallization of PBT and nano-composites. Calibration for the temperature scalewas carried out using both indium (Tm ¼ 156.6 8Cand DH 0

f ¼ 28.5 J g�1) and tin (Tm ¼ 231.9 8C) asstandards to ensure accuracy and reliability of thedata obtained. The PBT composite was sealed inan aluminum sample pan for DSC measurements.To minimize thermal lag between the polymersample and DSC furnace, each sample cut fromthe as-prepared films weighs around 10 6 0.1 mg.The measurements were performed following theprocedure: the samples were heated to 250 8C at20 K/min and maintained at this temperature for10 min to eliminate any previous thermal history,and then cooled rapidly (200 8C min�1) to thedesired crystallization temperatures, and isother-

mally kept for a period of time necessary to com-plete the crystallization. For each run, the heatreleased during the isothermal crystallizationwas recorded as a function of time, at differentcrystallization temperatures. All the experimentswere carried out in nitrogen atmosphere and theisotherms were constructed by integrating theareas under the exothermic peaks.

RESULTS AND DISCUSSION

Identification of Crystal Form

The crystallization behavior of PBT has beenextensively studied and two different crystalforms (a crystal with a ¼ 4.83 A, b ¼ 5.94 A,c ¼ 11.59 A and b crystal with a ¼ 4.95 A, b ¼ 5.67A, c ¼ 12.95 A) have been identified.45,46 Similarto the c-crystal of polyamide 6, the b-crystal ofPBT is also usually developed only under specialconditions, for example, application of stress tounoriented crystals.45–47 It has been demonstra-ted that the conversion between a and b forms ofPBT was reversible.46,48 For a neat sample, thea-crystal can be converted to the b-crystal uponstretching, meanwhile, the tensile modulus andstrength of materials will be improved with in-creasing fraction of b-crystal.49 However, the b-crystal would revert back to the a-crystal if theapplied stress is removed. Therefore, cold-draw-ing or melt-spinning can induce the formation ofb-crystal, and the latter can be retained in result-ing products. The unit cell parameter c of PBTcrystals is sensitive to the a–b transition, whichchanges from 11.59 to 12.95 A in going from a to bform. The planes (104) and (106) are almost per-pendicular to c-axis, thus the correspondingchanges in d-spacing in the course of externaldeformations can be used to identify the a–b tran-sition. The (104) and (106) reflections for a-crystaloccur at 2h values of 31.48 and 47.98 respectively.However, for b form, these two reflections appearat 2h values of 288 and 438.47,50–52

Herein, we try to examine whether organicallymodified ATT can alter the crystal form of PBT.Before the XRD measurements, samples werefirst melt for erasing previous thermal history,and then crystallized at 200 8C for 2 h or more sothat the maximum crystallinity was obtained. Allthe samples were measured at room temperature,without any stress application. Figure 1(A) showsthe XRD curves of PBT and nanocomposites(curves a and b). Both of them exhibit eight distin-

2114 CHEN ET AL.


guishable reflections, corresponding to (001),(011), (010), (111), (100), (111), (101), and (111)planes, respectively. The crystal structure of PBTwith similar reflections has been reported by someresearchers53 and determined to be triclinic crys-tal.54 For the convenience of comparison, the XRDcurve of the organo-ATT powder is also plotted inFigure 1(A). It can be seen that the peaks on thecurves a and b seem to be similar, except one weakpeak originated from the characteristic peak ofATT around 2h ¼ 26.58. The corresponding meridi-onal diffractograms a and b are also shown in Fig-ure 1(B). For both samples the angular position 2hof the (104) reflex locates at around 31.48, whichmeans that the crystallites are in the a form.47

In addition, it can also be seen that for thenanocomposites almost all diffraction peaks hori-zontally shift to lower angles (about 0.158). Whilewe carefully scale the distances between any twopeaks of nanocomposites and no apparent changesare found, which means the crystal axes areunchanged. The slight peak shift is thus probablycaused by the sample difference or systematicerrors of the instrument on measuring. As a re-sult, for the present PBT nanocomposites, we canconclude that ATT did not induce the polymorphictransition of PBT. This is similar to the experi-mental observations in MMT/PBT and ATT/PPnanocomposites,55,56 where MMT and ATT onlyaccelerated the crystallization of PBT and PP asnucleating agents.

For neat PBT, it has been argued that the a/btransition, as a first-order phase transition in-duced by the external stress, exhibits the follow-ing behaviors46: (1) when the applied stress F doesnot reach the critical value F*, only more stablea-crystals exist; (2) the transition is reversible; (3)when F ¼ F*, the a- and b-crystals coexist; (4) therelative amount of the b-crystals increases line-arly with strain. This implies that if only theexternal stress reaches a certain range in uniax-ially oriented samples, b-crystals could emerge. Inthe PBT/organo-ATT nanocomposites, ATT mayrestrict the motion of polymer chains, but thisrestriction effect could not be high enough to ori-ent the PBT chains in a way that occurred inMMT/polyamide nanocomposites.33 It is still notclear so far whether the presence of nanoparticlesaffects the conversion between different crystalforms under the action of external stress. How-ever, it is apparently important to clarify thispoint for better understanding the detailed mech-anism of crystallization and mechanical improve-ments in polymer nanocomposites.

Overall Kinetics of Isothermal Crystallization

The fractional degree of crystallinity Xt at time twas determined from the ratio of the peak area att to the total area of an exothermic peak.23 Toobtain the complete information of kinetics, thecrystallization temperatures of the nanocmopo-sites were adjusted to a range convenient for DSCobservations. From these fractional crystallinitycurves, the crystallization half-times (t1/2) (i.e. thetime needed for a sample to reach a fractionaldegree of crystallinity of 0.50) were extracted.

Figure 1. A: PBT isothermal (a) and PBT/ATT nano-composite (b) crystallized at 200 8C, wide-angle X-raydiffraction (WAXD) patterns of ATT functionalized withMDI (c). The Miller indices of the WAXD reflections arealso indicated. B: Meridional reflex (104) of the PBT (a)and nanocomposites (b) after isothermal crystallization.



The Avrami model provides a convenientapproach to analyze the overall crystallization ki-netics and its logarithmic form is expressed as,25

ln½� lnð1� XtÞ� ¼ ln kn þ n ln t ð1Þ

where Kn is the kinetic growth rate constant, n isthe Avrami exponent related to the type of nuclea-

tion and to the geometry of growing crystals, andXt is the fractional degree of crystallinity at timet. Figure 2 gives the Avrami plots (ln[�ln(1-Xt) vs.ln t) corresponding to PBT and nanocomposites.Linear regression of these straight lines yieldedthe Avrami exponents (n) and the rate constants(kn).

The t1/2 and n values of the neat PBT and nano-composites are shown in Figure 3. It can be seenthat the presence of ATT dramatically shortenedthe crystallization time, but the difference bet-ween the t1/2’s of two nanocomposites is not dis-tinct. Similar results were found in intercalatedpoly(trimethylene terephthalate)/MMT57 and poly(L-lactic acid)/MMT40 nanocomposites. These re-sults imply that the crystallization of these poly-mers may be accelerated with the addition of asmall mount of nanoparticles, and the accelera-tion effect probably reaches a maximum at somesolid loading.

It can also be seen in Figure 3 that the n valuesof the neat PBT is around 2.4, which is slightlylower than the values (n ¼ 2.6–2.9) reported inthe literature.24,58–60 The reason is probably dueto the uncertainty in determining the initial timeof crystallization processes24; however, this shouldnot influence the comparison of n values for differ-ent materials under the same conditions. Com-pared to the neat PBT, some n values for nano-composites are much lower than the theoreticalvalue 3. Especially for PBT2.5 at 193.8 8C, the nvalue is even lower than 2. Following Vyazovkin’ssuggestion,61 a disadvantage of isothermal runs isthat quick cooling from melting temperature to adesired temperature is followed by a period of

Figure 2. Avrami plots of ln[�ln(1-Xt)] versus ln t forthe crystallization of neat PBT and nanocomposites.

Figure 3. Half-time of crystallization (t1/2) andAvrami exponent (n) of PBT composite systems as afunction of crystallization temperature. Represented byfilled symbols for t1/2 and open symbols for n.

2116 CHEN ET AL.


temperature stabilization during which the crys-tallization kinetics remains inestimable. It shouldbe, therefore, reasonable to treat the uncertaintyin determining the initial time of crystallizationprocesses as the main reason for a decrease in nvalue.24 For those polymers with quick crystalli-zation rate, the experimental n value will be cer-tainly lower than the theoretical value 3, whichhas been confirmed by the isothermal crystalliza-tion of neat PBT (n ¼ 2.6–2.9).24,58–60 However,for PBT/ATT nanocomposites another factor influ-encing the n values is still necessary to consider,that is the promotion effect of nanoparticles onnucleation.

Through the analysis of the n values for nano-composites in Figure 3, the temperature 198 60.8 8C can be seen as a critical range, above whichthe nanocomposites exhibit higher n values thanthe neat PBT, whereas a contrary trend in n val-ues is obtained as below this temperature. Be-cause of the promotion effect of nanoparticles onnucleation, the crystallization kinetics in nano-composites are so quick in the lower temperaturerange that the crystallization has already begunduring the temperature stabilization, giving riseto the inaccurate determination of initial times.Furthermore, with the decrease of temperature,the crystallization rate of polymers becomesquicker. This should be true for the reduced n val-ues at lower crystallization temperatures. How-ever, at higher temperatures, especially as closeto the melting point, it is a little more difficult toform the crystal nuclei because of the high mobil-ity of chains,62 despite the existence of nanopar-ticles. Furthermore, at higher temperatures, onlya small part of nanoparticles could act as nucleifor crystallization, but the rest of them have retar-dation effect on the reptation of polymer chains.As a result, the crystallization process is not veryquick and the inducing time is longer relatively,which probably makes the n values close to 3. Inthe literature, there is a source of confusion in nvalues of polymer/ATT nanocomposites, for exam-ple, ATT-containing polyoxymethylene (POM)63

and PP64 where the n values of POM/ATT nano-composites located between 2.8 and 3.9 and all then values of PP/ATT nanocomposites are higherthan 4. For these nanocomposites, a common fea-ture is that the addition of small amount of ATT isenough to induce remarkable variation in n val-ues, while further addition exhibits very littleeffect on these two kinetic parameters (kn and n),which agrees with the results observed in thepresent PBT/ATTs. The difference of n values

between POM/ or PP/ATT and PBT/ATT mayderive from the quicker crystallization rate ofPBT and the larger temperature range, relative tothat of POM or PP systems.

To understand the apparent variation in n val-ues for PBT/ATT nanocomposites, it is necessaryto consider the characteristic structure of ATT.ATT is a hydrated magnesium aluminum silicatecontaining ribbons of a 2:1 phyllosilicate structuredifferent from other layered silicates. Each ribbonis connected to the next by the inversion of SiO4

tetrahedra along a set of Si��O��Si bonds andextends parallel to the x-axis to form rectangularchannels. Undoubtedly, this kind of special struc-ture can offer high specific surface area, whichshows a strong adsorption effect. Because of itsporous characteristics, ATT has been used as cata-lyst supports and environmental absorbents.65,66

In the present work, the structure characteristics(i.e. high specific surface area) also make poly-mers crystallize more rapidly, especially at lowertemperature ranges. This could be further verifiedby the following fact. Unlike ATT, for MMT/PBTnanocomposites, no significant n variation wasfound, relative to the neat PBT at the same tem-perature ranges.55

Isothermal Crystallization Regime

Hoffman and Lauritzen (LH) had developed a de-pendence of the linear growth rate of spherulites,G, on the crystallization temperature Tc, that is,

62

G ¼ G0 exp�U�

RðTc � T0Þ� �

exp�Kg

Tcð�TÞf� �

ð2Þ

where G0 is the pre-exponential factor, U* is thetransport activation energy, DT ¼ T 0

m � Tc is thesupercooling range (T 0

m is the equilibrium meltingtemperature). f is the correction factor related totemperature, usually described as f ¼ 2Tc/(T

0m þ

Tc) to account for the variation in the heat offusion per unit volume of crystals, Dhf. T0 is a hy-pothetical temperature below which all viscousflows cease, namely, T0 ¼ Tg � 30 K.62 Kg is thenucleation constant and can be expressed as

Kg ¼ jb0rreT0m=kBð�hf Þ ð2aÞ

where j ¼ 4 for regime I and III growth and j ¼ 2for regime II. b0 is the layer thickness, rre is theproduct of lateral and fold surface free energies,Dhf the enthalpy of fusion, kB Boltzmann’s con-stant.



Using the kinetic growth rate constant kn andthe Avrami exponent n, eq 2 can be rearranged asfollows:

1

nln knðTÞ þ U�

RðTc � T0Þ ¼ An � Kg

Tcð�TÞf ð3Þ

As aforementioned, the decreased n values areprobably due to the uncertainty in determiningthe initial times of crystallization processes. Inaddition, it should be noted that both kn and n areconstants for a given crystalline morphologyformed under a particular crystallization condi-tion.67 Furthermore, the same spherulitic mor-phology was observed in both PBT and its nano-composits by our optical microscopy measureme-nts. Therefore, for the convenience of comparison,kn can be corrected by assuming the tridimensionalcrystal growth (n ¼ 3)24: k3(T) ¼ ln 2/t 31/2. Thereby,eq 3a can be obtained24:

1

3lnk3ðTÞ þ U�

RðTc � T0Þ ¼ An � Kg

Tcð�TÞf ð3aÞ

An can remain constant approximately as theinvestigated Tc is very narrow, and other parame-ters are from eq 2. Therefore, Kg of PBT and nano-composites can be determined graphically fromthe slope of ln k3 (T)/3þU*/R(Tc�T0) versus 1/TcDTf1/TcDTf, and then the regime behavior of crystalli-zation can be analyzed.

According to the LH theory, three differentregimes can be developed during polymer crystalli-zation, depending on the competition between therate of secondary nuclei (i) and the rate of lateralsurface spreading (g).41 In regime I, the crystalliza-tion of polymers occurs at low supercooling temper-ature (DT) and i « g; in regime II, the crystalliza-tion occurs at moderate DT and i and g are thesame order of magnitude; and for regime III thecrystallization occurs at relative high DT and i >> g.

Figure 4 presents the above LH plot of PBT andnanocomposites in terms of eq 3a, where the uni-versal values of U* (6400 J mol�1)27 was used, andTg and T 0

m were taken as 42 and 233 8C,24 respec-tively. As seen in Figure 4, the experimental datacan be reasonably fitted with two straight linesfor the neat PBT, PBT2.5, and PBT5 nanocompo-sites. The alteration in slope qualitatively indi-cates a transition in crystallization regime. Theslope ratio for each sample, that is KgIII/KgII, isvery close to 2, which indicates a transition fromregime II to III. It can be found that the regimetransition temperatures of the neat PBT, PBT2.5,

and PBT5 nanocomposites are 464.5, 473.8, and475.8 K, respectively. In a recent work, Runt etal.27 had exhibited that the transition tempera-ture of regime II?III of PBT is 483.0 K by apply-ing the Lauritzen Z test, which is higher than thetransition temperature observed in the presentPBT. There are probably two reasons to cause thereduction in the experimental transition tempera-ture of the present PBT sample. One of them prob-ably arises from the difference in molecularweight of different samples.68 The other is likelyfrom the different values of T 0

m since Kg in eq 3ais quite sensitive to the T 0

m value (see eq 2a). Inthe present work, T 0

m ¼ 233 8C was used and 3 8Clower than the value used by Runt. In fact, how-ever, to obtain an accurate T 0

m value of PBT is rel-atively difficult because of its rapid crystallizationrate and crystal reorganization/thickening duringheating. Apparently, the slightly lower T 0

m doesnot interfere with our qualitative analysis for theeffect of ATTon the crystallization regime of PBT.

It can also be found from Figure 4 that theII?III transition temperature keeps increasingwith the addition of ATT, which is similar to theresult observed in PE/MMT nanocomposites,42

where the II?III transition observed is locatedaround 125 8C, a temperature higher than theII?III transition temperature (119 8C) of linearPE. These results show that the crystallizationtemperature range of regime III becomes broaderin the nanocomposites, which at least indicatesthat the relative rates of secondary nucleationand surface spreading are changed. This impliesthat the incorporation of nucleating agents couldlead to the alteration in crystallization behaviors.Although there is no evidence for the increase in

Figure 4. Hoffman-Lauritzen plots for PBT and nano-composites.

2118 CHEN ET AL.


secondary nucleation rate, at least the rate of sur-face spreading is decreased at a high nanoparticleloading.42

The obtained Kg values can be used to deter-mine the fold surface free energy (re) of PBT andits nanocomposites from eq 2a. The lateral surfacefree energy r can be evaluated from the followingempirical equation62:

r ¼ a�hf

ffiffiffiffiffiffiffiffiffiffia0b0

pð4Þ

where a is an empirical constant and usuallyassumed to be �0.1, a0b0 represents the cross-sec-tional area of polymer chains, Dhf is the volumet-ric heat of fusion. If we assume that PBT chainsfold along (010) planes, a0 ¼ 3.82 � 10�10 m andb0 ¼ 5.17 � 10�10 m can be used, respectively.27

From the mass heat of fusion (DHf ¼ 142 J g�1)and the density of PBT (d ¼ 1.3984 g cm�3), thevolumetric heat of fusion can be obtained by Dhf ¼DH 0

f � d ¼ 1.98 � 108 J m�3.24 As a result, wearrive at r ¼ 8.8 erg cm�2. Subsequently, re canbe split through the product of rre and the valuesof re for PBT and nanocomposites are shown inTable 1. For the neat PBT, the value of re variesin the range of 59.6–54.3 erg cm�2, which agreeswell with the results (re ¼ 57 erg cm�2) in theliterature.27

As seen in Table 1, the addition of nanopar-ticles gives rise to the decrease in re of PBT. Manyresearchers found that the value of re could bechanged by nanoparticles. On the one hand,nucleating agents such as filler particles andfibers tend to promote the nucleation of spheru-lites on their surfaces, decrease the thickness oflamina, and lead to epitaxial growth of the crys-tallites.43 As a result, the value of re is reduced,thereby giving rise to an increase in crystalliza-tion rate. As we previously described, in vermicu-lite-PE system Tjong and Bao43 demonstrated thiskind of trend and a 20% reduction in re (from 2.97to 2.37 J m�2) up to 2 wt % vermiculite loading.However, in our PBT/ATT systems, the reductionin re is not so high with the addition of 2.5 wt %

ATT, which perhaps indicate that the nucleatingeffect of vermiculite in PE is stronger than that ofATT in PBT. This possibility can be approved,since there is about 20% reduction in re when theloading of ATT in PBT reaches up to 5 wt %. Thereduction in re is found not only in nanocompo-sites but also in other systems. For example, athermoplastic elastomer (EPDM) uniformly dis-persed in PP also showed such a nucleating effectand caused systematical re reduction of PP.69

On the other hand, however, the contribution ofnucleation effect to re may be depressed becauseof remarkably reduced mobility of polymer chains,and the alteration in re with nanoparticle loadingshows a converse trend. Whether vermiculite-con-taining PE43 or EPDM-containing PP69 displaysan increase in re with further increasing nanopar-ticle loadings, which suggests a confined segmen-tal motion involved during their crystallizations.Unfortunately, we can only know the results inthe investigated range of nucleating agent load-ings, and usually these results seem to be differ-ent. For example, all the re values of PP/organo-clay nanocomposites are lower than that of theneat PP,70 whereas all the re values of the inor-ganic–organic hybrids involving PCL and silicanetworks are higher than that of the neat PCL.44

In fact, the highest organoclay loading is 5 wt %in the former system, while the lowest loading ofsilica network is 50 wt % in the later system. Forthese two kinds of composites, their re valuesshow the same increasing trend with the contentof nucleating agents.

In nanocomposites, there are two primary fac-tors possible to influence the re value of PBT, thatis, nucleating effect and the mobility of polymerchains. Apparently, the value of re depends on thecompetition between the nucleating effect andconfined segmental motions. Different nanopar-ticle may have different nucleating effects. Inaddition, this nucleating effect may be enhancedwith increasing the content of nucleating agentsuntil their volume percents in nanocompositesreach up to a critical value. At the same time, the

Table 1. The Parameters of the Hoffman-Lauritzen Theory

Specimens

Kg � 105 (K2)

KgIII/KgII

rre � 104 (J2 m�4)

re (erg cm�2)II III II III

PBT 1.11 2.02 1.82 5.84 5.32 59.6–54.3PBT2.5 1.10 1.93 1.75 5.82 5.09 59.4–51.9PBT5 0.88 1.72 1.94 4.67 4.53 47.7–46.2



confined motion originated from nanoparticlesalso may begin to work when a relative quantityof particles are incorporated into polymers. Forexample, in the PCL/silica system, the nucleatingeffect of silica particles may be concealed by theobstruction effect when the loading of particlesreaches up to 50%.44 In the previous investiga-tions,22 we had demonstrated such a confinementeffect of nanoparticles on the segmental motion ofa polymer, where epoxy networks were covalentlyconnected to the surface of layered silicates andthe reduced configurational entropies were ob-served. In the present work, this confinementeffect does not dominate the crystallizationkinetics, and the results show that ATT has sig-nificant nucleation effect in investigated nano-composites, which wins the confinement effect oncrystallization kinetics. Therefore, the nucleationeffect becomes more evident with increasing ATTloadings, which is probably the main reason forthe reduced re values.

CONCLUSIONS

In this study, the effects of ATT in nanocompositeson the crystal structure and the isothermal crys-tallization kinetics of PBT were studied. XRDmeasurements show that in spite of the additionof ATT the crystal form in PBT nanocompositesremains unchanged and is still triclinic a form.The overall crystallization kinetics by Avramianalysis reveals that ATT has an obvious acceler-ating effect on the crystallization kinetics of PBT.In terms of LH theory, both PBT and its nanocom-posites display a regime II?III transition. Thepresence of ATT in PBT appears to drive the re-gime transition to higher temperature. The foldsurface free energy of crystals in nanocompositesis lower than the value of the neat PBT. Thisreduction in fold surface free energy is attributedto the good nucleating effect of ATT, which con-ceals the possible confinement effect induced bythe nanoparticles.

This work was subsidized by the National BasicResearch Program of China (No. 2005CB623800) andthe NSF of China (Grant Nos. 20104002, 20234010,20374016, and 20221402).

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isothermal crystallization kinetics of poly(butylene terephthalate)/attapulgite nanocomposites

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