Competitive Crystallization of a Propylene/Ethylene RandomCopolymer Filled with a β‑Nucleating Agent and Multi-WalledCarbon Nanotubes. Conventional and Ultrafast DSC StudyDimitrios G. Papageorgiou,† George Z. Papageorgiou,‡ Evgeny Zhuravlev,§ Dimitrios Bikiaris,‡

Christoph Schick,§ and Konstantinos Chrissafis*,†

†Solid State Physics Department, School of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece‡Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24Thessaloniki, Greece§University of Rostock, Institute of Physics, Wismarsche Str. 43-45, 18051 Rostock, Germany

ABSTRACT: A propylene/ethylene polymeric matrix wasreinforced by the simultaneous addition of a β-nucleatingagent (calcium pimelate) and multi-walled carbon nanotubes(MWCNTs) in various concentrations. The present manu-script explores the competitive crystallization tendency that iscaused by the presence of the two fillers. On the one hand,calcium pimelate forces the material to crystallize predom-inantly in the β-crystalline form, while, on the other, the strongα-nucleating ability of MWCNTs compels the material todevelop higher α-crystalline content. An in-depth study hasbeen performed on the nanocomposite samples by means ofconventional, temperature-modulated, and differential fastscanning calorimetry (DFSC) under various dynamic and isothermal conditions. The results showed that β-crystals arepredominant at low MWCNT content (<2.5 wt %), while, at high MWCNT content, the material crystallizes mainly in the α-form. The recrystallization phenomenon during melting was confirmed with step-scan DSC, and the use of very high coolingrates by UFDSC made it possible to achieve and study the nucleation of the samples. The presence of MWCNTs enabled thenanocomposites to crystallize faster under both isothermal and dynamic conditions. The activation energy of the samples wasalso calculated according to Friedman’s theory.

1. INTRODUCTION

Polypropylene is one of the most extensively studiedthermoplastics with widespread applications due to its lowprice and excellent properties, which include enhancedmechanical properties, increased clarity, and recyclability.However, despite its advantages, it presents drawbacks suchas low impact strength, UV degradation, and poor weatheringresistance. For this reason, polypropylene is sometimescopolymerized with ethylene in order to improve clarity,impact strength, increase flexibility, and decrease the meltingpoint, which results in a lower heat-sealing temperature.1

Propylene/ethylene random copolymers (PPR) typicallycontain between 1 and 7 wt % ethylene, which is insertedrandomly between the propylene sequences in the polymerchain. The random distribution of ethylene unit into the longpropylene sequences affects the crystalline structure and thecrystallization ability of the polymer; thus, study of thecrystallization behavior of those polymers has attracted muchinterest.2

The most common crystalline forms of polypropylene arethe monoclinic α-form and the hexagonal β-form. The maindifference between them is the excellent ductility and toughness

that β-crystals present in contrast to α-crystals.3 The mosteffective way of inserting β-crystals in a polypropylene matrix isthe use of β-nucleating agents, such as dicarboxylic acidderivative type diamide compounds, quinacidrone typecompounds, quinacidronequinone type compounds, dicarbox-ylic acid salts of metals from group IIa of periodic system, andsalts of metals from group IIa of periodic system.4 However, thepresence of a PPR matrix makes it more difficult to induce highβ-crystal content. In our case, a β-nucleated polymeric matrix,consisting of PPR and calcium pimelate at 0.1 wt %concentration was produced.Polypropylene is also one of the most versatile polymer

matrixes due to its easy processability, which makes it an idealcandidate for reinforcement with the use of nanofillers. In thePP nanocomposites, the crystalline morphology and the finalproperties are greatly influenced by the morphology ofnanoparticles used such as the spherical silica particles, clayplatelets, graphite particles, and carbon nanotubes

Received: September 20, 2013Revised: November 12, 2013Published: November 13, 2013

Article

pubs.acs.org/JPCB

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(MWCNTs).5 Carbon nanotubes, which were used in thepresent manuscript as fillers, have received enormous attentionsince their discovery by Ijima in 1991.6 They possess an idealarrangement of sp2 C−C bonds, as found in the basal plane ofgraphite, which is the stiffest and strongest in nature, in theircylindrical and nearly defect-free structures.7 The mainadvantage of carbon nanotubes is their large surface area thatcan produce high quality adhesion with the polymeric matrix,which is an important factor for the effective enhancement ofthe properties of the nanocomposite.In nanocomposite materials, factors such as the degree of

crystallinity, the rate of crystallization, the growth direction, andthe crystalline morphology are greatly influenced by thepresence of fillers or nucleating agents. The present manuscriptaims to investigate the strong nucleating effect of carbonnanotubes on the melting behavior and crystallization underdynamic and isothermal conditions, of the complex system of aβ-nucleated propylene/ethylene polymeric matrix melt mixedwith carbon nanotubes. For this reason, conventional (DSC)and differential fast scanning calorimetry (DFSC) methodswere applied on the samples in order to observe in depth thenucleation, crystallization, and melting phenomena on thenanocomposite samples. The fact that all samples containheterogeneous nuclei makes it intriguing to examine thestarting point of crystallization with conventional DSC and tostudy in depth the heterogeneous nucleation with DFSC, sincevery fast cooling is a way of surpassing some nucleation eventsat high temperatures that are unavoidable at slow cooling rates.8

2. EXPERIMENTAL SECTION2.1. Materials and Methods. Polypropylene random

copolymer (PP-R) was supplied by Basell Polyolefins underthe commercial name Hostalen PP H5416 Random Copolymercontaining 7 wt % ethylene content and a melt flow rate of 0.29g/10 min (463 K/5 kg). Multi-walled carbon nanotubes werepurchased from Chendu Organic Chemicals Co. Ltd. Theirdensity was 2.1 g/cm3, and their surface area was almost 500m2/g.2.2. Synthesis of Calcium Pimelate Nucleator. The

calcium pimelate nucleator was synthesized by doubledecomposition.9 In detail, calcium pimelate was preparedthrough the reaction between equimolar CaCl2 and sodiumpimelate aqueous solution with stirring. The precipitation wasfiltered, washed twice with distilled water and ethanol,respectively, and dried in a vacuum at room temperature.The calcium pimelate was used at a concentration of 0.1 wt %as a β-nucleator for PP-R. The prepared β-nucleated polymericmatrix will be referred to in the current manuscript as PPR.2.3. Nanocomposite Preparation. PPR/MWCNTs nano-

composites containing 0.5, 1, 2.5, 3.5, and 5 wt % of multi-walled carbon nanotubes were prepared by melt-mixing in aHaake−Buchler Reomixer (model 600) at 490 K and 30 rpmfor 15 min. Prior to melt-mixing, the nanoparticles were driedby heating in a vacuum oven at 405 K for 24 h. Thecomponents were physically premixed before being fed into thereomixer. In order to achieve a better dispersion of thenanoparticles in PPR, a RETSCH planetary ball mill modelS100 was used. The PPR flakes along with the proper amountof MWCNTs were fed into a “C” type stainless steel grindingjar with a capacity of 25 mL. Five steel spheres were also addedas a grinding medium. The milling was set at 500 rpm for aperiod of 1 h. The nanocomposite samples that were preparedand studied will be referred to in the current manuscript as

PPR/MWCNTs 0.5 wt %, PPR/MWCNTs 1 wt %, PPR/MWCNTs 2.5 wt %, PPR/MWCNTs 3.5 wt %, and PPR/MWCNTs 5 wt %. Lower loadings of MWCNTs (like 0.25 wt%) were also prepared, but the fact that their melting andcrystallization behavior did not exhibit any significant differ-ences from the behavior of the sample filled with 0.5 wt %MWCNTs led us to present only the 0.5 wt % sample, with thelower filler content.

2.4. Differential Scanning Calorimetry (DSC). APerkin−Elmer, Pyris Diamond differential scanning calorimeter(DSC), calibrated with indium and zinc standards, was used forthe study of crystallization and melting of the two polymers.Samples of 5 ± 0.1 mg were used in the tests. They were sealedin aluminum pans and heated at a heating rate of 20 K/min to475 K, which is about 50 K above the melting point and quiteabove the equilibrium melting point of the studied polymers.The samples were held at 475 K for 2 min in order to erase anythermal history. For isothermal crystallizations, the sample wasrapidly cooled to 20 K above the crystallization temperatureand finally cooled to that at a rate of 50 K/min, to achieveequilibration of the instrument. The crystallization exothermicpeak was then recorded. Crystallization temperatures were inthe range between 373 and 398 K. Subsequent heating scanswere performed at a rate of 20 K/min. If some other procedurewas followed in specific tests, this will be described in thecorresponding part.For non-isothermal crystallization, the same melting

procedure described above was applied before the coolingscan. Cooling scans were performed at rates of 2.5, 5, 7.5, 10,15, and 20 K/min. A fresh sample was used in each run. Theheating rate was in most tests 20 K/min if not otherwise stated.Modulated temperature DSC experiments were also carried

out using the same Pyris Diamond DSC calorimeter and thePerkin-Elmer Step-Scan software. The samples were scanned atan average rate of 2.5 K/min (heating steps of 2.5 K performedat a rate of 5 K/min and isothermal steps of 0.5 min).

2.5. Differential Fast Scanning Calorimetry (DFSC).Conventional DSC cannot cool fast enough in order to preventnucleation on cooling for rapidly crystallizing polymers. Thus,for the detailed nucleation and growth study, DFSC wasapplied. The specific calorimeter was constructed in adifferential scheme of two fast scanning calorimeter sensorswith power compensation. The presence of an empty referencesensor reduces the influence of heat losses and addenda heatcapacity on the obtained data. For a better scanning ratecontrol, particularly in the transition regions, power compensa-tion was introduced. The details of the device and datatreatment are reported elsewhere.10 DFSC is able to performheat flow measurements during controlled heating and coolingup to 500 000 K/s. The power control circuit allows isothermalexperiments for times longer than 0.01 ms (with over orundershoots <1 K).During the experimental procedure, the samples were heated

from ambient temperature to 475 K and held there for 1 min inorder to erase their thermal history, cooled down at differentcooling rates, and then reheated with 10 000 K/s. In addition,the effect of isothermal crystallization for 1 min at 400 K wasobserved. After isothermal crystallization, the samples wereheated with heating rates ranging from 1000 to 20 000 K/s.

3. RESULTS AND DISCUSSION3.1. Melting Behavior of PPR/MWCNTs. The melting

behavior of PPR and its nanocomposites with carbon

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nanotubes was studied by means of differential scanningcalorimetry. The samples were first heated up to 475 K under anitrogen atmosphere, then cooled down with a cooling rate of20 K/min, and then reheated for a second time, at 10 K/min.This procedure was followed in order to erase the “thermalhistory” of the sample which typically comes from theprocessing conditions that are followed until the samples taketheir final form but plays a significant role in the crystallinestructure of the material.Consequently, the melting behavior of the materials under

study during the second heating is presented in Figure 1. The

normalization of the heat flow signal was performed in terms ofthe mass of the polymer and not the sample mass. Threedifferent peaks can be observed in the DSC thermograms.Initially, the first (low temperature) endothermic melting peakwhich is attributed to the melting of β-crystals (Tm1). Thewidespread area under the β-melting peak may be the result ofthe superposition of the melting of two categories of β-crystals[beta1 (β1) and beta2 (β2)] which correspond to imperfect andmore stable crystals, respectively. Also, the second exothermicpeak (Tβα) corresponds to βα-recrystallization, and as theMWCNT content increases, the phenomenon is less intense. Itis known that the β-crystals recrystallize partly to α-crystalswhen the isothermal crystallization temperature is low or theheating rate is small, as in this case.3a,4a,11 Finally, the thirdendothermic peak (Tm2), which is related to the melting of α-crystals, presents a broad peak which can be considered as adual melting peak, whose shape progressively evolves towardsingle melting with increasing MWCNT content. The dualmelting peak of α-crystals has been described in the literatureinitially by Hikosaka12 who attributed the splitting of the α-crystal peak into alpha1 (α1) and alpha2 (α2) forms. Themelting of the α1 form occurs at lower temperatures, and thisparticular form is considered a disordered metastable form, incontrast to the α2 form, which is a limiting ordered form and itsmelting occurs at temperatures higher than that of α1.Furthermore, the melting of the α2 crystal form at highertemperatures is a result of the melting of α-crystals formed bymelting and recrystallization of β-crystals during the heatingprocedure. Thus, the double melting of α-crystals can be

explained as a transition from disorder to more orderedsituations.Another very interesting observation that can be made from

Figure 1 is that the areas under the endothermic peaks, whichrepresent the values of the melting enthalpy, display diversitywith increasing MWCNT content. Initially, the neat polymericmatrix exhibits higher β- than α-crystal content, as it can bededuced from the areas under the melting peaks. However, asthe MWCNT content increases, after the concentration of 3.5wt %, the peak which represents the melting of α-crystals isconstantly developing. It seems that two competitivephenomena occur during the heating scan. On the one hand,the presence of a β-nucleating agent (at low MWCNT content)forces the materials to crystallize with higher β-crystal content,while, on the other hand, the strong α-nucleating effect ofMWCNTs as reported in the literature13 and observed hereforces the material to lower its β-crystal content due to themetastable nature of β-crystals and induces higher α-crystalcontent, as the MWCNT content increases.In order to confirm our previous conclusions about the dual

peak of α-crystals, the melting behavior of the polymeric matrixand the sample with the higher MWCNT content (5 wt %) wasstudied under different heating rates. The samples were firstheated up to 475 K, held there for 1 min, cooled down at 20 K/min, and then reheated with different heating rates. For thisreason, heating rates from 40 to 5 K/min were selected and theresults are presented in Figure 2. In the case of PPR (Figure2a), it is obvious that, at higher heating rates, the more perfectcrystals (α2 crystals) do not have enough time to develop, sothe less ordered form of α-crystals is displayed (α1). However,at lower heating rates (less than 10 K/min), the double meltingpeak of α-crystals is displayed as a disorder to order transition,since the slow heating rates allow partial melting of the α-crystals. The two peaks coexist for heating rates from to 5 K/min, and this is an indication that the time of the measurementin the partial melting temperature is comparable with the timeneeded for a recrystallization procedure. Since recrystallizationis a time-dependent process, the increase of heating rate resultsin the gradual surpassing of the recrystallization phenomenonduring heating. Also, at lower heating rates, the βα-recrystallization phenomenon is more intense, as is indicatedin Figure 2a. This tendency of the polymeric matrix for βα-recrystallization can be attributed to the formation of a verysmall amount of finely dispersed α-phase within the β-phasedue to a secondary crystallization in the cooling process.3a Theα-phase acts as an α-nucleant during the partial melting of theβ-phase and induces βα-recrystallization. Thus, the recrystalli-zation and melting phenomena are superimposed between eachother and the melting curves cannot be considered as accurateindicators of the polymorphic content. When the sample filledwith MWCNTs was studied, it exhibited different behavior thanthe polymeric matrix. In more detail, for PPR/MWCNTs 5 wt%, the area under the peak which corresponds to the melting ofα-crystals is larger than that observed for PPR, indicating oncemore the strong nucleating effect of MWCNTs. This time thehigh filler percentage forces the material to crystallize mainly atthe α-phase, overcoming the effect of calcium pimelate on theβ-phase crystallization of the sample. Once again, the enlargedarea under the peak owing to the α-crystals can be the result ofthe two overlapped meltings of α1 and α2 crystals, and for thisreason, at slower heating rates, the peak is more broadened.The recrystallization phenomenon is not that obvious for this

Figure 1. Second heating of the PPR/MWCNT nanocomposites.

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sample, and the step-scan DSC technique was used for theseparation of the different phenomena.In order to investigate further the recrystallization phenom-

enon that is observed during the melting of β or α phase, thestep-scan (temperature modulated) DSC technique was used.The step-scan DSC technique introduces true isothermal stepsbetween heating steps, and its signal attributes differentcontributions during the heating of semicrystalline polymers.First, the non-reversing endothermic signal is ascribed to themelting of separate lamellae or stacks of lamellae. Second, thereversing endothermic signal is attributed to partial melting oflamellae which are able to recrystallize rapidly on existingsignals. Furthermore, the reversing signal helps in theseparation of events such as the glass transition from othernon-reversing processes such as recrystallization or enthalpyrelaxation. The latter exothermic events are completely absentfrom the phenomena represented by the reversing signal; thus,the resolution of different thermal events is easier. The datawhich provide crystallization exotherms only contribute to thenon-reversing signal; thus, separation of exotherms fromreversing melting or other heat capacity events is achieved.The kinetics of the data which provide crystallization exothermscause a heat flow response which is not in phase with thetemperature oscillation.14 Unfortunately, exothermic and

endothermic non-reversible events can occur simultaneouslyand they cannot be completely separated from each other. Thatis why standard DSC and TMDSC are difficult to interpret,especially for polymers where significant recrystallization takesplace during measurments.15 The results from the step-scanDSC study for PPR and PPR/MWCNTs 5 wt % are shown inFigure 3. The heating rate used during the heating steps of thescans was 5 K/min. The length of the isothermal steps was 0.5min. Also, all heating experiments after isothermal crystal-lization, including step-scan tests, started from the crystal-lization temperature. This is standard practice in order to avoidany possible changes in morphology due to additionalcrystallization on cooling to lower temperatures.The triads of curves which are presented in Figure 3

correspond to reversing heat flow, to raw data (middle curves),and to the non-reversing heat flow signal (lower curves). In thereversing signal curve, it is important to notice that, at highercrystallization temperatures, melting began only a few degreesabove Tc but the melting peaks were not well resolved in theheat flow curve. In the non-reversing signal, broad curves andshallow recrystallization peaks can be seen. Recrystallizationpeaks are more obvious in the case of samples crystallized atlower temperatures, as was expected.Finally, the effect of the crystallization temperature on the

melting profile of samples isothermally crystallized from themelt was studied (Figure 4). It is generally known that thehigher the Tc is, the higher is also the melting temperature, afact which originates from the crystalline perfection due to slowcrystallization processes. Also, for the case of the polymericmatrix, at low crystallization temperatures (Tc < 383 K), singlemelting can be observed (β-crystal melting), while doublemelting occurs only for higher temperatures. The higheramount of β-phase at lower temperatures is caused by theconstant increase of the number of β-nucleating sites, so thegrowth of the α-transcrystalline front faces an early barrier bythe β-spherulites.2c What can also be observed is that themelting peak of α-crystals is constantly rising for Tc > 383 Kdue to the slow crystallization rates and the recrystallizationphenomenon that was described earlier. The particular behaviorhas also been observed by Naffakh et. al.16 On the other hand,the strong nucleating effect of MWCNTs on the β-nucleatedmatrix is obvious once again by the double melting peaks thatthe sample filled with 5 wt % MWCNTs presents for all Tc. Forlow crystallization temperatures, the area under the peak of β-crystals is significantly smaller than that of α-crystals, and whenthe Tc increases and the crystallization occurs at much smallerrates, the area under the peak of β-crystals increases.

3.2. Fast Heating Rates on Conventional DSC. Duringthe study of melting of PPR and PPR/MWCNTs 5 wt %samples, heating rates ranging from 50 to 400 K/min on thePyris Diamond DSC were used, while the cooling rate was 200K/min. As it can be seen in Figure 5 (upper part), in the case ofPPR, the β-crystal population is the one that prevails at thosecooling rates, since the decrease of heating rate induces a shiftof the peak toward lower temperatures, while the peak which isattributed to the α-crystals appears initially as a shoulder at aheating rate of 200 K/min. Furthermore, for heating rates of100 and 50 K/min, it is depicted as a well-defined peak. On theother hand, the sample filled with 5 wt % MWCNTs presents atotally different behavior (Figure 5, lower part). The strong α-nucleating effect of MWCNTs is obvious on the melting of thesample, since the α-crystals prevail this time over β-crystals.However, even at heating rates as high as 400 K/min, the

Figure 2. Melting behavior of (a) PPR and (b) PPR/MWCNT 5%under multiple heating rates.

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presence of β-crystals is obvious on the melting curves, since ashoulder on the α-melting peak appears. With the decrease ofthe heating rate, the peak owing to the melting of β-crystals ismore obvious. The fact that no cold crystallization wasobserved during the experimental procedure leads to theconclusion that higher cooling rates are needed in order toobserve this phenomenon.3.3. Fast Scan DSC Study. Conventional techniques such

as DSC or POM cannot cool down the PPR samples efficientlyenough from temperatures higher than the melting point, inorder to prevent crystallization during cooling. Initially, theresponse of the neat polymer and the sample filled with 5 wt %MWCNTs on the different cooling rates (non-isothermalcrystallization) ranging from 10 to 50 000 K/s was studied.Estimation of the sample mass has been done by comparison ofmeasured absolute heat capacities with specific values availablefrom ATHAS data bank.17 From Figure 6, it is obvious that, atslow cooling rates, the sample has crystallized and the glasstransition step at temperatures from 256 to 270 K is small andbroadened. Also, cold crystallization was observed for PPR at322 K and the melting peak was high due to crystallizationduring the slow rates applied. Regarding the melting peak ofPPR, for cooling rates from 10 to 100 K/s, only one peak isobvious, and according to Mollova et al.,18 it corresponds to themelting of β-crystals. This result is in accordance with theconventional DSC fast heating results that were presentedearlier. Further increase of the cooling rate results in the

broadening and splitting of the melting peak, which is anindication of two different events: melting of α- and β-crystals.Regarding the sample filled with 5 wt % MWCNTs, the obvioussplitting of the peak starts at 1000 K/s, indicating a differentbehavior due to the presence of MWCNTs which act as α-nucleating agent.The results from non-isothermal experiments with DFSC, as

described above, are summarized in Figure 7. The overallenthalpy change represents the sum of the melting enthalpy(ΔHm, positive values) and cold crystallization enthalpy (ΔHcc,negative values). The cold crystallization of PPR increases withincreasing cooling rate and reaches a maximum at 500 K/s,while, from that point and on, presents a decrease. The samplefilled with MWCNTs presents a smaller cold crystallizationpeak, which is mostly pronounced only after applying highcooling rates such as 2000 K/s. Both samples show almost nocold crystallization after cooling at the highest rate (50 000 K/s).The high ΔH values on the enthalpy change graphs represent

the more pronounced effect of melting than cold crystallizationon the two samples. It is also obvious that PPR becomesamorphous at lower cooling rates than the nanocompositesample, since the melting peak and cold crystallization peaks arealmost diminished at a cooling rate of 1000 K/s, while for PPR/MWCNTs 5wt % the critical cooling rate was 5000 K/s. Thecold crystallization enthalpy is dependent on the number ofactive nuclei which can grow during heating, and it is essentially

Figure 3. Temperature-modulated DSC traces for (a, b) PPR and (c, d) PPR/MWCNT 5 wt % samples crystallized at different temperatures.

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considered as a relative indication of the number of nucleipresent before the heating scan. As demonstrated by Mathot19

and Oguni,20 a high number of nuclei are formed when thesamples are cooled at moderate rates, while, at higher rates, thenumber of nuclei is reduced and therefore the coldcrystallization phenomenon that can be observed during theheating scan. The fact that higher cooling rates are needed inorder for the sample to become amorphous, in the case of thenanocomposite, can be explained by the fact that a highernumber and highly efficient nucleating sites are present in thesample, thus promoting crystallization even at very high coolingrates. This is also obvious in the lower part of Figure 7, wheresufficiently fast cooling leads to a significant reduction of theenthalpy of cold crystallization of neat PPR on subsequentheating.The FSDSC device was used afterward in order to observe

the effect of the heating rate and the annealing for 1 min at 400K on the cold crystallization and melting phenomena for

Figure 4. DSC traces for PPR (a) and PPR/MWCNT 5 wt % (b)samples, crystallized at different temperatures. Heating rate 20 °C/min.

Figure 5. Fast heating rates from a conventional DSC for PPR andPPR/MWCNT 5 wt %.

Figure 6. Effect of various cooling rates on the melting behavior at 10000 K/s of PPR and PPR/MWCNTs 5 wt %. The inset shows coldcrystallization of PPR/MWCNT sample after cooling at 5000 K/s(red) and 2000 K/s (blue) compared to heating after fastest cooling(notice the different color code).

Figure 7. Overall enthalpy change on heating (upper graph) and coldcrystallization enthalpy (lower graph) as a function of cooling rate,measured for PPR and PPR/MWCNTs 5 wt %. The colored signs inthe middle x-axis exhibit more clearly the cooling rates presented inFigure 6.

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pristine PPR and the nanocomposite sample filled with 5 wt %MWCNTs. Thus, after several cooling and reheating circles forthe erasure of the thermal history of the samples and the checkof stability, the samples were cooled down at 20 000 K/s,heated with various heating rates (from 1000 to 20 000 K/s) upto 400 K, annealed for 1 min at 400 K, and then heated withthe same rates that were used before. The results are presentedin Figure 8. The cooling rate of 200 K/min (ca. 3 K/s) that was

applied previously with a conventional DSC obviously was notfast enough in order to prevent nucleation during cooling andthe sample was not fully amorphous; thus, no coldcrystallization was observed. The fast cooling rates that wereapplied with DFSC (20 000 K/s) made the samplesamorphous, enabled cold crystallization during heating andthe melting of the existing crystals or crystals grown during theheating procedure could be studied. The fact that the glasstransition is almost identical for both samples in terms of thestep height (heat capacity increment) can be attributed to theabsence of any crystallites after the fast cooling procedure. Inaddition, the large amount of cold crystallization on slowheating rates occurs due to the sufficient amount time that thesamples have in order to develop crystals near the glasstransition region. However, the increase of heating rate resultsin less time for crystal development and low cold crystallization.For PPR, cold crystallization is suppressed for heating rateshigher than 7000 K/s, but this fact seems to affect moresignificantly the melting of β-crystals, since the α-crystalcontent, as represented by the area under the melting peak,starts increasing again from this point and after due to theannealing process at 400 K. This fact can be an indication thatthe α-crystals behave as crystals which were developed duringthe annealing process and cold crystallization is mostly relatedto the formation of β-crystals.3.4. Isothermal Crystallization Kinetics. Crystallization

of polymer melts is usually accompanied by significant heatrelease which can be measured by differential scanningcalorimetry. The isothermal crystallization behavior of the β-nucleated polymeric matrix and the effect of the MWCNTcontent was investigated by means of DSC over a temperaturerange between 373 and 398 K. The relative degree ofcrystallinity X(t) can be obtained if we make the assumptionthat the evolution of crystallinity is linearly proportional to the

evolution of heat released during the crystallization, and it canbe formulated as

∫∫

= ∞X tH t t

H t t( )

(d /d ) d

(d /d ) d

t

0 c

0 c (1)

where dHc denotes the measured enthalpy of crystallizationduring an infinitesimal time interval dt. The limits t and ∞ onthe integrals are used to denote the elapsed time during thecourse of crystallization and at the end of the crystallizationprocess, respectively. In the DSC traces, it was obvious that, asthe supercooling, i.e., the difference between the melting andcrystallization temperature, decreases, the crystallization ratebecomes slower and the exothermic peak becomes broader.Thus, the time to reach the peak increases. The half time ofcrystallization values can be obtained by the degree ofcrystallinity versus time curves, and the inverse of the halftime of crystallization is a measure of the crystallization rate.According to the results which are presented in Figure 9, the

reciprocal half time of crystallization decreases almostexponentially with increasing crystallization temperature. Thestrong nucleating effect of MWCNTs can be observed due tothe simultaneous increase of the reciprocal crystallization halftime and the MWCNT content.21

3.5. Non-Isothermal Crystallization Kinetics. The studyof non-isothermal crystallization is of great significance, sincemost practical processes are performed under non-isothermalconditions. In our previous paper, we studied the effect of a β-nucleating agent on the crystallization kinetics of a propylenerandom copolymer (PPR), and the important role of thenucleating agent on the crystallization temperature and rate wasexhibited.22 This time, the β-nucleated PPR was prepared andselected as the polymeric matrix and carbon nanotubes wereused as fillers during the study of the crystallization behavior.The strong nucleating effect of carbon nanotubes was shownearlier; however, the presence of the β-nucleating agent isalready a factor which ensures high crystallization rates for thematerial. After the non-isothermal experiments at differentcooling rates for β-PPR and its nanocomposites, it was shownthat the temperature of the crystallization peak (Tp) increased

Figure 8. Apparent heat capacity of PPR and PPR/MWCNTs 5 wt %on heating at various heating rates after annealing for 1 min at 400 K.

Figure 9. Reciprocal crystallization half-time of nanocompositesamples. The line is a guide to the eyes only.

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with increasing MWCNT content (Figure 10). This behavior isassociated with the nucleation effect that MWCNTs have on

the polymeric matrix and it is more pronounced as theMWCNT content increases, since the nucleation sites are alsoincreasing significantly.3.6. Effective Activation Energy of Non-Isothermal

Crystallization. The crystallization of polymers is controlledby two factors: one is the dynamic factor, which is related to theactivation energy for the transport of crystalline units across thephase, and the other is the static factor that is related to the freeenergy barrier for nucleation.23

Isoconversional methods which are based on multipleheating rates are recommended, since they can describeaccurately the dependence of the effective activation energyon the degree of conversion and temperature.24 Vyazovkin andSbirrazzuoli have already demonstrated that the major problemof most of the techniques that have been used in the past is thatreversing the sign for β is a mathematically invalid procedureand the equations that were used are inapplicable to theprocesses that occur on cooling.25

Two of the most used isoconversional methods are theintegral method proposed by Vyazovkin and the differentialmethod proposed by Friedman.26 In the present manuscript,Friedman’s method will be used. This method uses thelogarithm of the general kinetic equation

α = −α

αα⎜ ⎟⎛

⎝⎞⎠t

A f aE

RTln

dd

ln( ( ))a (2)

where R is the universal gas constant and Aα, Eα, and Tα are thepreexponential factor, the effective activation energy, and thetemperature at a certain conversion degree, respectively. Theresults from the application of Friedman’s method are visible inFigure 11a. Negative increasing values of the effective activationenergy have been first reported by Vyazovkin and Sbirrazzuolifor the non-isothermal crystallization of PET from the melt.25

Then, this observation has been confirmed several times byother authors for various thermoplastics.8d,27 In agreement withthese results, the effective activation energy increases withincreasing degree of conversion values, suggesting that, as thecrystallization progresses, it is more difficult for the polymer to

crystallize. In addition, the effective activation energy values ofthe nanocomposites are lower than that of the β-nucleatedpolymeric matrix, which is an indication that the process ofcrystallization proceeds easier for those samples due to theincreased number of nucleating sites that MWCNTs offer andthe crystallization phenomenon is promoted. The effectiveactivation energy has here the meaning of the temperaturecoefficient of the crystallization rate.28

In addition, the effective energy can be plotted as a functionof temperature by taking an average temperature associatedwith a certain α value. Figure 11b shows the variation of theeffective activation energy with temperature for the nano-composites. For all samples, the activation energy valuesconstantly decrease with increasing temperature.

■ CONCLUSIONSThe melting behavior and the crystallization kinetics of a β-nucleated propylene/ethylene random copolymer polymericmatrix filled with MWCNTs, under dynamic and isothermalconditions, have been investigated. The concentration of thefiller (MWCNTs) played a very important role on the meltingbehavior of the nanocomposites, since on the one hand the β-

Figure 10. Evolution of the crystallization peak temperature as afunction of the cooling rate for PPR and nanocomposites.

Figure 11. Effective activation energy of PPR/MWCNT nano-composites as a function of relative degree of crystallinity (a) andtemperature (b).

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nucleating agent forced the material to crystallize on the β-form, while the high α-nucleating efficiency of MWCNTs andthe inherent tendency of PPR worked synergistically in order toincrease the α-crystal content. The melting behavior of thesamples has been observed under various conditions, such asdifferent heating rates or different crystallization temperatures,and the effect of each filler on the composition of thepolymorphic samples has been pointed out. The step-scan DSCtechnique was applied in order to confirm the βα-recrystallization phenomenon on the samples, by the separationof the conventional signal in the reversing, non-reversing, andtotal signals. The shallow peaks in the non-reversing signalestablished once again the βα-recrystallization. Fast scan DSCmade it possible to study the cold crystallization phenomenonfor the polymeric matrix and the sample filled with 5 wt %MWCNTs, since the cooling rates were high enough to preventcrystallization during cooling and made the sample amorphous.The nanocomposite samples crystallized faster than the β-nucleated matrix, due to the presence of MWCNTs, as wasconcluded under all crystallization conditions (dynamic andisothermal). The fact that led to the conclusion that the overallcrystallization phenomenon was not driven exclusively byMWCNTs was that the neat polymeric matrix with calciumpimelate (PPR) presented significant differences in its behaviorfrom the samples filled with MWCNTs and especially at lowerfiller content, which indicated the effect of calcium pimelate andthe competitive action of the two components. Thus, theoverall crystallization was affected by both components insidethe polymeric matrix. Finally, the apparent activation energy ofthe nanocomposites was calculated according to Friedman’stheory, and the fact that the nanocomposites presented lowervalues of apparent activation energy than the polymeric matrixis once more an indication that the crystallization phenomenonis promoted for those samples.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +30 2310 998188. E-mail: hrisafis@physics.auth.gr.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research has been cofinanced by the European Union(European Social Fund − ESF) and Greek national fundsthrough the Operational Program “Education and LifelongLearning” of the National Strategic Reference Framework(NSRF) − Research Funding Program: Heracleitus II.Investing in knowledge society through the European SocialFund

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