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Thermochimica Acta 565 (2013) 82– 94

Contents lists available at SciVerse ScienceDirect

Thermochimica Acta

jo ur nal home p age: www.elsev ier .com/ locate / tca

ffect of the type of nano-filler on the crystallization and mechanical propertiesf syndiotactic polystyrene based nanocomposites

.Z. Papageorgioua, D.S. Achiliasa, N.P. Nianiasa, P. Trikalitisb, D.N. Bikiarisa,∗

Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24 Thessaloniki, Macedonia, GreeceDepartment of Chemistry, University of Crete, Vasilika Vouton, Iraklion 71003, Crete, Greece

a r t i c l e i n f o

rticle history:eceived 8 February 2013eceived in revised form 5 April 2013ccepted 26 April 2013vailable online xxx

eywords:yndiotactic polystyreneanocomposites

a b s t r a c t

In this research, syndiotactic polystyrene based nanocomposites containing 3 wt% of nanofiller wereprepared using the melt-mixing technique. Nano-fillers used included multi-wall carbon nanotubes,silver nanoparticles, nanodiamonds and copper-nanofibers. The latter were prepared especially for thisuse. Prior to melt mixing, polymer and nanoparticles were ball-milled to form a physical mixture. TEMand SEM micrographs verified a fine dispersion of the nanoparticles in the polymeric matrix. Formationof nano-filler aggregates was very limited. Tensile testing showed slight improvement of the mechanicalproperties of the nanocomposites compared to the neat sPS. From the WAXD patterns, existence ofmixture of crystals mainly of � and � crystal types of sPS was evidenced. In general, crystals of �-type

echanical propertiesrystallization kinetics

dominated, except in case of the sPS/MWCNT nanocomposite in which mainly �-type crystals appeared.Isothermal and non-isothermal crystallization studies showed that the presence of nano-filler causedenhancement of crystallization rates and that the most effective nucleation was achieved using MWCNT.These results were further reinforced by estimating the effective activation energy of non-isothermalcrystallization using the isoconversional method of Friedman as well as the nucleation activity of thefiller.

. Introduction

The amorphous atactic polystyrene is one of the most impor-ant polymeric materials in the world with exceptional properties.owever, a new versatile thermoplastic was prepared when stere-specific polymerization became attainable due to the discoveriesf Ziegler and Natta that led to a massive number of effectiveatalyst systems [1]. The first syndiotactic polystyrene (sPS) wasynthesized by Ishihara in 1985 [2]. sPS differs from atactic orsotactic polystyrene in that the phenyl rings regularly alternaterom side to side and this regular structure allows sPS to crys-allize readily. An increasing interest in the syndiotactic form isbserved because of its complex polymorphic behaviours [3,4] andther advantageous properties, including rapid crystallization rate,igh melting point (about 270 ◦C), good chemical resistance, lowpecific gravity, good dielectric properties and superior mechanicaltrength [5,6].

The extensively studied polymorphic behaviour can be

escribed in terms of five crystal forms. The �- and �-form,here the molecular chain conformations in crystalline regions are

rans-planar, zig–zag (with trigonal and orthorhombic packing of

∗ Corresponding author. Tel.: +30 2310997812.E-mail address: dbic@chem.auth.gr (D.N. Bikiaris).

040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.tca.2013.04.037

© 2013 Elsevier B.V. All rights reserved.

chains respectively) and the �-, �- and �-forms where the chainsare in the s(1/2)2 helical confirmation. The �- and �-forms withthe TTTT chain conformation can be obtained by thermal treat-ment [7–10] whereas the �-form with a TTGG chain conformationis a complex between the polymer and an organic solvent that wasinduced while the polymer was in the glassy [11,12]. Removal ofthe solvent molecules of the �-form sample or dipping a glassysample into acetone, cyclohexanole, or supercritical CO2 results inthe also monoclinic �-form [13,14]. Chloroform-induced recrystal-lization of samples exhibiting the �-crystalline form, followed bychloroform desorption in suitable condition will result in the �-form of sPS [15,16]. Further studies revealed more classificationsof the above crystalline forms. The disordered forms of �- and �-form (�′- and �′-) and the ordered forms (�′′- and �′′-) [17]. Also the�e-form (empty �-form) that is obtained by extracting the �-formin supercritical CO2 [18] and the intercalated co-crystalline phasesof sPS with low-molecular mass molecules [19]. Mixtures of twodifferent crystalline forms can also be present [20].

The formulation of nanocomposites is a relatively new mate-rial concept, in order to produce new polymeric materials withenhanced properties. While the science of filling thermoplastics to

increase stiffness has been practiced for many years, the need toadd large amounts of filler often increases the stiffness of the ther-moplastic while decreasing other desirable properties, particularlyimpact properties. The preparation of polymer nanocomposites

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G.Z. Papageorgiou et al. / The

sing either inorganic nanoparticles such as silicate clays, SiO2, car-on nanotubes (CNTs), nanodiamonds, etc. drew great attentionue to the tremendous enhancement of their mechanical, thermal,ptical, and structural properties of the pristine material [21–23].ompared to conventional composites, polymer nanocompositesaximize the polymer nanoparticle interactions, since the filler

s dispersed on a manometer scale. This results in lighter mate-ials with higher modulus and reduced linear thermal expansion.lso the addition of nanofillers improves the electrical propertiesnd alters the crystalline modification, morphology, crystallizationinetics and melting behaviour rendering them desirable for manypplications [24]. Nanoscale inorganic particles have obvious effectn sPS crystallization behaviour and physical properties. Clay withcale of 1–2 nm was found to greatly facilitate the formation of-form crystal when sPS was melt- or cold-crystallized, and signifi-antly affect the crystallinity and crystallization behaviour [25,26].arbon nanotubes have a nucleating effect on sPS crystallization,nd improved thermal and conductive properties as compared witheat sPS [27,28].

In this study, several sPS nanocomposites were prepared usinganoparticles with high conductivity, such as multi-wall car-on nanotubes, nano-diamonds, silver nano-powder and copperanofibres. The aim of this paper was to investigate the effect ofhese nanoparticles on the sPS mechanical properties as well as onts crystallization kinetics.

. Experimental

.1. Materials

Syndiotactic polystyrene (sPS) was kindly supplied by Dowhemicals with stereoregularity at 99% and average moleculareight Mw = 2.1 × 105. Purified multi walled carbon nanotubes

MWCNTs) used in this work were synthesized by chemi-al vapour deposition (CVD) process and the average outeriameter is 10–25 nm, they were purchased from Timesnanohengdu Organic Chemicals Co. Ltd. (China). NanodiamondsNDs) of cubic diamond phase and average primary particleize of 4.0 nm were purchased from NanoDiamond S.A. Silveranopowder (Ag) used in this research was of 99.92% puritynd 40–90 nm of average nanoparticle size, supplied by Irframatdvanced MaterialsTM. CuCl2·2H2O (99%), N,N,N′,N′-tetramethylthylenediamine (TMEDA, 98%), cyclohexane (99%), cetyltrimethy-ammonium bromide (95%, CTAB) and glucose (99.5%) wereurchased from Aldrich and used without any further purification.he copper nanofibres (Cu) were produced in the laboratory; theethodology is described in follow.

.2. Synthesis of copper nanofibres

Copper nanofibes were synthesized via the reduction of aopper–amine complex in an aqueous-non polar bilayer systemonsisted of CTAB/TMEDA/H2O/Cyclohexane. In a typical synthe-is 2.4 mL of TMEDA and 8 mmol of glucose were added to 20 mLf a CuCl2 0.2 M aqueous solutions under stirring for 10 min. Then,.6 g of CTAB and 40 mL of cyclohexane were added to this mixture.he reaction solution was transferred into a Teflon-lined autoclaveith a capacity of 80 mL and heated at 120 ◦C for 24 h. The systemas then allowed to cool to room temperature. The product was

btained by filtering, washed with ethanol several times, and driedt 60 ◦C for 12 h.

.3. Nanocomposites preparation

sPS nanocomposites containing 3 wt% of different nanoparti-les (MWCNTs, NDs, Cu-nanofibers and Ag nanopowder) were

imica Acta 565 (2013) 82– 94 83

prepared by melt mixing in a Haake-Buchler Rheomixer (model600) with roller blades and a mixing head with a volumetric capac-ity of 69 mL. Prior to melt-mixing the nanoparticles were dried byheating in a vacuum oven at 130 ◦C for 24 h. The two componentswere physically premixed before being fed in the rheomixer. Inorder to achieve a better dispersion of the nanoparticles in sPS aRETSCH planetary ball mill (model S100) was used. The sPS flakesalong with the proper amount of nanoparticles were fed into the‘C’ type stainless steel grinding jar with a capacity of 25 mL. Fivesteel spheres were also added as a grinding medium. The millingwas set at 500 rpm for a period of 3 h. Melt blending was performedafter ball milling at 270 ◦C and 30 rpm for 15 min. During the mix-ing period the melt temperature and torque were continuouslyrecorded. Each nanocomposite after preparation was milled andplaced in a desiccator to prevent any moisture absorption.

2.4. Mechanical properties

Measurements of tensile mechanical properties of the preparednanocomposites were performed on an Instron 3344 dynamome-ter, in accordance with ASTM D638, using a crosshead speed of5 mm/min. Relative thin sheets of about 300 ± 25 �m were pre-pared using an Otto Weber, Type PW 30 hydraulic press connectedwith an Omron E5AX Temperature Controller, at a temperature of280 ± 5 ◦C. The moulds were rapidly cooled by immersing themin water at 20 ◦C. In order to measure the mechanical propertiesfrom these sheets, dumb-bell-shaped tensile test specimens (cen-tral portions 5 mm × 0.5 mm thick, 22 mm gauge length) were cutin a Wallace cutting press. The values of Young’s modulus, yieldstress, elongation at break and tensile strength at the break pointwere determined.

For the notched Izod impact tests a Tinius Olsen apparatus wasused under ASTM D256. The specimens were prepared in a similarway as described before.

Prior to the measurements, the samples were conditioned at25 ◦C in a 50 ± 5% relative humidity environment for 48 h. At leastfive specimens were tested for each sample and the average valuesare reported.

2.5. Scanning electron microscopy (SEM)

SEM of nanocomposites surface was carried out using a JEOLJMS-840A scanning microscope equipped with an energy disper-sive X-ray (EDX) Oxford ISIS 300 micro-analytical system. For SEMmeasurements thin films were used.

2.6. Transmission electron microscopy (TEM)

TEM images were obtained on a JEOL JEM-2100 instrumentequipped with a LaB6 filament, operating at 200 kV. TEM speci-mens were prepared by supporting thin sections (80–100 nm) ofsamples onto 200 mesh holey carbon copper grids.

2.7. Differential scanning calorimetry (DSC)

Thermal behaviour of the nanocomposite materials was stud-ied using a Perkin–Elmer Pyris Diamond DSC differential scanningcalorimeter. The instrument was calibrated with high purityindium and zinc standards. Samples of about 5 mg were used.

It is important for the crystallization experiments to minimizethe thermal lag, so low mass samples should be used in crystal-lization tests. For isothermal crystallization tests the samples were

first melted to 320 ◦C for 5 min to erase any previous thermal his-tory and then cooled to the crystallization temperature (Tc) at a rate200 ◦C/min. Crystallization temperatures ranged from 240 ◦C to260 ◦C. The samples were held at Tc till the end of crystallization. For

8 rmochimica Acta 565 (2013) 82– 94

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2

waC

3

3

pcadtFfsosstiowatcttaanp

ebtwsesw

Teimbs

0,0 0,4 0,8 1,2 1,6 2,0 2,4 2,80

10

20

30

40

50

60

Te

nsile

Str

ess (

MP

a)

Elongation (%)

sPS 3 % ND s

sPS 3 % Ag

sPS 3 % MWCNT s

sPS 3 % Cu

sPS

4 G.Z. Papageorgiou et al. / The

on-isothermal crystallizations from the melt, the samples wererst melted as in the previous case and then cooled down to −20 ◦Ct various cooling rates, namely 2.5 ◦C/min, 5 ◦C/min, 7.5 ◦C/min,0 ◦C/min, 15 ◦C/min and 20 ◦C/min. It should be noted that theE Pyris Diamond DSC used for raw data collection in this work,s an instrument of low furnace mass and high sensitivity, appro-riate for accurate measurements. A Perkin-Elmer Intracooler IIas used to allow the DSC to achieve high and constant cool-

ng rates. A fresh sample was used in each run. Heating rate wasn most heating scans 20 ◦C/min. If some other method involv-ng different rate was used, this will be discussed in the specificection.

.8. Wide angle X-ray diffractometry (WAXD)

WAXD study of sPS nanocomposites, in the form of thin films,ere performed over the range 2� from 5◦ to 60◦, at steps of 0.05◦

nd counting time of 5 s, using MiniFlex II XRD system from Rigakuo.

. Results and discussion

.1. Mechanical properties

sPS has attracted significant interest because of its uniquehysical properties like high heat resistance, excellent chemi-al resistance, high degree of crystallinity, low specific gravity,nd good dielectric properties. However, sPS has certainly majorrawbacks such as high brittleness poor impact strength and lowear resistance, which limits the areas that can be applied [29].or this reason, enhancement of properties, like mechanical per-ormance, of sPS has gained great interest. The toughening ofPS with rubber particles has been an area of active researchver the past few years [30]. However, by increasing the impacttrength, due to the addition of rubbers, tensile strength degreasesimultaneously. Mechanical properties can be also enhanced byhe addition of nanoparticles. In the case of sPS nanocompos-tes, the mechanical properties improvement depends mostlyn the quantity and type of the nanofiller and the interactionsith the macromolecular chains [31]. The mechanical properties

re also affected by the chain conformation and degree of crys-allinity of the sample. The �- and �-forms with the trans-planarhain conformation provide much better mechanical propertieso sPS than the s(2/1)2 helical chains of the other possible crys-alline forms. The amorphous regions provide certain elasticitynd impact resistance; on the other hand crystal regions require

greater force to deform. In Fig. 1 the stress–strain curves of sPSanocomposites containing different types of nanoparticles areresented.

As can be seen neat sPS exhibits a linear stress–strain behaviournding in a brittle failure and breaks before yielding. The sameehaviour can be also found for the nanocomposites. However,here are some small differences in the maximum tensile stress,hich for almost all nanocomposites is higher than the corre-

ponding of neat sPS. This is an indication of the reinforcementffect of these nanoparticles. From these stress–strain curves ten-ile strength at break, elongation at break and Young’s modulusere calculated and are presented in Table 1.

Neat sPS has a tensile strength at break of about 46.8 MPa.ensile strength is slightly increased in all nanocomposites andspecially when nanodiamonds were used as nanofiller. The

ncrease is higher than 20%. The trend remains the same in Young’s

odulus. Due to the brittleness of all samples, the elongation atreak remains in low rate values. Apart from the increase of tensiletrength, in some nanocomposites an increase is also recorded in

Fig. 1. Stress–strain curves of sPS and its nanocomposites at a crosshead speed of5 mm/min.

impact strength. As can be seen from Table 1 the highest increaseis recorded in samples containing MWCNTs and Cu nanofibers.Compared to the value of neat sPS an augmentation of morethan 30% was measured. It seems that the addition of ND and Agnanoparticles has lower effect, maybe due to the spherical shape ofthese nanoparticles.

3.2. Morphological characterization of the nanocomposites usingSEM and TEM

Apart from the kind of nanofiller, its dispersion into the poly-mer matrix also affects the mechanical properties. For this reasonall samples were characterized by TEM and SEM. In Fig. 2a it wasrevealed that MWCNTs have various lengths from 1 �m to 2.5 �mand the diameter of the nanotubes was 10–20 nm. The micrographsrevealed that most of the MWCNTs are dispersed as individualnanoparticles and aggregates are limited. Maybe, this can explainthe higher increase in impact strength of these nanocomposites.Furthermore, it can be seen that ball milling did not affect the shapeor the length of nanotubes even though in literature the oppositehas been mentioned (Fig. 2b) [32,33].

The morphological examination of the sPS/NDs nanocompos-ite (Fig. 2c) affirms that nanodiamonds have a diameter of 5 nm,but their structure is not perfectly spherical and a cubic diamondphase is characteristic. Also, except of the fine dispersion of NDssome agglomerates can be also formed (Fig. 2d). However, their sizeremains in the nanoscale level. The small size of the nanoparticles(approximately 4 nm diameter) dispersed in a non-polar polymerlike sPS and the strong intermolecular van der Waals interactionsare the main reasons of the formation of these small aggregates.A similar dispersion was also revealed from TEM micrographs insPS/Ag nanocomposite (Fig. 2e). As it can be seen, a fine dispersionof nanoparticles co-existed with a small amount of aggregates. Thehighly conductive silver nanoparticles appear much darker thanthe other nanoparticles.

In the SEM micrograph of the sPS/Cu sample (Fig. 2f) it canbe seen that copper fibres of various sizes are dispersed in thesPS matrix with inconsequent orientation. The length of the fibresvaries from 3 �m to 15 �m with a diameter of approximately150 nm. Furthermore, these fibres do not entangle and thereforeno aggregates are visible. The spherical spots in the micrograph are

mostly the tips of copper fibres vertically aligned to the viewinglevel, or it could be parts of the fibres that were destroyed duringmelt mixing.

G.Z. Papageorgiou et al. / Thermochimica Acta 565 (2013) 82– 94 85

Table 1Mechanical properties of sPS nanocomposites.

Sample Tensile strength at break (MPa) Elongation (%) Young’s modulus (MPa) Impact strength (J/m)

sPS 46.8 ± 0.7 1.93 ± 0.2 2587 ± 170 11.1 ± 1.2sPS/MWCNTs 49.7 ± 0.8 2.06 ± 0.2 2920 ± 150 13.9 ± 1.3

± 0.3 ± 0.2

± 0.3

3

pwp

nitIt

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sPS/ND 55.0 ± 0.4 2.34sPS/Cu fibres 48.0 ± 0.9 1.80sPS/Ag 53.2 ± 0.7 2.65

.3. WAXD patterns and crystal structure of the nanocomposites

Films of the nanocomposites and neat sPS were prepared by meltressing. The temperature of melt pressing was 280 ◦C. The filmsere then rapidly cooled by immersing in cold water. The WAXDatterns of the films were recorded and are shown in Fig. 3.

In accordance to previous studies, it was found that in theanocomposite containing MWCNT formation of �-type crystals

s favoured even by quenching. So in the respective WAXD pat-ern Fig. 3 mainly the reflections of �-type crystals of sPS appear.t should be reminded at this point that neat sPS forms mainly �-ype crystals after melting at high temperatures such as 340 ◦C or

ig. 2. HR-TEM micrographs of (a) sPS/MWCNTs, (b) sPS/3%MWCNTs sample (high magnhe sPS/Ag and (f) SEM micrograph of sPS/Cu sample.

3100 ± 180 11.7 ± 1.1 2785 ± 150 14.6 ± 1.2 2810 ± 180 12.5 ± 1.1

higher and then isothermal crystallization at temperatures higherthan 250 ◦C or at very slow cooling rates. During crystallization atlower temperatures or during fast cooling �-crystals are formed.At moderate crystallization temperatures both polymorphs appear.As a matter of fact, mixtures of crystals of both types appeared inthe WAXD patterns of the nanocomposites but the relative quan-tities varied. In the WAXD patterns of Fig. 3, the crystallographicplanes associated with the crystal reflections are shown. So, one

can see that in contrast to the sPS/MWCNTs nanocomposite, inthe rest nanocomposites studied in this work mainly �-type crys-tals were formed and only a small portion of �-type crystals weregenerated.

ification), (c) sPS/NDs sample (high magnification), (d) sPS/NDs, (e) TEM image of

86 G.Z. Papageorgiou et al. / Thermoch

0 10 20 30 40

Silver

Cop per

ND

MWCNT

Neat s-PS

(041)

(111)

(060)

(120)

(040)

(110)(020)

(211)

(220)(300)(110)

Inte

nsity (

a.u

.)

Angle 2 (deg)

3n

ptc2ettitfipbtbteif

F

Fig. 3. WAXD patterns of neat sPS and the sPS nanocomposites.

.4. DSC crystallization and melting behaviour of theanocomposites

The crystallization and melting behaviour of the sPS nanocom-osites were also tested using DSC. Isothermal crystallizations fromhe melt were performed at various temperatures. After isothermalrystallization at each temperature, the subsequent heating scan at0 ◦C/min was recorded to study the melting behaviour of the gen-rated crystalline structure. In order to evaluate the effect of theype of the material on the melting behaviour, Fig. 4 shows the DSCraces of samples crystallized at 245 ◦C. Three melting peaks appearn these traces. It is interesting that the onset and peak tempera-ures as well as the relative intensities of peak II and III vary withller type. As a matter of fact, peak III seems more intense thaneak II for the nanocomposite containing MWCNT which, as wille discussed below, showed the faster crystallization rates amonghe studied materials. The multiple melting behaviour of sPS haseen discussed in previous works and it was attributed mainlyo the presence of crystals of different types [34,35]. To study the

ffect of the crystallization temperature on the subsequent melt-ng behaviour for each material, sets of isothermal crystallizationsrom the melt were performed at various temperatures and the

245 250 255 260 265 270 275 280

IIIII

I

1W/g

Cu

nanofi bre s

Ag

NDs

MWCNTs

sPS

Norm

aliz

ed H

eat F

low

(W

/g)

Temperature (oC)

ig. 4. DSC heating traces for the various sPS nanocomposites crystallized at 245 ◦C.

imica Acta 565 (2013) 82– 94

subsequent heating scans were recorded. For example Fig. 5 showsthe DSC heating traces after crystallization at different tempera-tures for the sPS/MWCNT nanocomposite and neat sPS. Obviousdifferences in the traces for the different materials were observed.

3.5. Crystallization kinetics

3.5.1. Isothermal crystallization kineticsCrystallization of polymer melts is accompanied by significant

heat release. Based on the assumption that the evolution of crys-tallinity is linearly proportional to the evolution of heat releasedduring the crystallization which can be measured by differentialscanning calorimetry (DSC), the relative degree of crystallinity, X(t),can be obtained according to the following equation:

X(t) =∫ t

0(dHc/dt)dt∫ ∞

0(dHc/dt)dt

(1)

where dHc denotes the measured enthalpy of crystallization duringan infinitesimal time interval dt. The limits t and ∞ are used todenote the elapsed time during the course of crystallization and atthe end of the crystallization process, respectively.

Isothermal crystallization of sPS nanocomposites samples wasperformed at various temperatures from 240 ◦C to 260 ◦C. Fromthe DSC scans it was observed that as the supercooling, i.e. thedifference between the melting and crystallization temperature,decreases, the crystallization rate gets slower and the exothermalpeak becomes broader. Thus, the time to reach the peak increases.Subsequently, the relative crystallinity X(t) at time, t, was deter-mined using Eq. (1) and results appear in Fig. 6a. From these curves,the half time of crystallization, t1/2, was determined as the timeelapsed from the onset of crystallization to the point where thecrystallization is half completed. All the t1/2 values as a functionof temperature or supercooling are illustrated in Fig. 6b. In thisfigure, it is apparent that the t1/2 values increase almost exponen-tially as the crystallization temperature is increased, meaning thatthe crystallization rates decrease with increasing temperature.

For the analysis of the isothermal crystallization, the most com-mon approach is the Avrami method [36–41]. Accordingly, therelative degree of crystallinity, X(t), is related to the crystallizationtime, t, according to:

X(t) = 1 − exp(−ktn) or X(t) = 1 − exp[−(Kt)n] (2)

where, n is the Avrami exponent which is a function of the nuclea-tion process and k is the growth function, which is dependent onnucleation and crystal growth. Since the units of k are a functionof n, Eq. (2) can be written in the composite–Avrami form usingK instead of k (where k = Kn) [42,43]. The values of n, k and K, canbe calculated from fitting to experimental data using the doublelogarithmic form of Eq. (2):

log{− ln[1 − X(t)]} = log k + n log t (3)

Eq. (3) is thus used to fit the experimental data and com-pare crystallization behaviours the various nanocomposites. Plotsof log {− ln[1 − X(t)]} vs logt for neat sPS and the nanocompositecontaining copper nanofibers were constructed and are shown inFig. 7a. In these plots an initial linear part is observed, which wasused for the estimation of the parameters n and k. The deviation,which is observed after this first linear part in the Avrami plots, isoften attributed to secondary crystallization. This limited relativedegree of crystallinity was around 80% for pristine sPS and most

nanocomposites except those based on MWCNTs (Table 2). In thelatter case the limited X started from 80% at low crystallization tem-peratures (i.e. 240 ◦C), while it was as low as 53% when temperaturerose to 260 ◦C. This is most probably reflecting a dual mechanism

G.Z. Papageorgiou et al. / Thermochimica Acta 565 (2013) 82– 94 87

240 250 260 270 280

270 .0oC

262. 4oC

Te=272 .3oCTo=25 6.2

oC

Neat sPS

2 W/g 260oC

255oC

250oC

245oC

240oC

No

rma

lize

d H

ea

t F

low

(W

/g)

En

do

Up

o

240 250 260 270 280

270. 7oC

264 .2oC

Te=273.2oCTo=259 .3

oC

MWCN T

2 W/g

240oC

245oC

250oC

255oC

260oC

No

rma

lize

d H

ea

t F

low

(W

/g)

En

do

Up

o

mposi

othdin

TAt

Temperature ( C)

Fig. 5. DSC heating traces for neat sPS and the sPS/MWCNT nanoco

f crystallization. It seems that initially MWCNTs nucleated crys-allization of sPS, while later crystallization continued by slower

omogenous nucleation or secondary crystallization. As will beiscussed below, MWCNTs showed a significant nucleating activ-

ty, which was much more important than in the case of the restanoparticles.

able 2vrami parameters k, n and K as well as limiting relative degree of crystallinity, Xl , obtai

emperatures.

Sample/temperature Limiting relative degree ofcrystallinity, Xl (%)

Neat sPS240 80

245

250

255

260

sPS/n-silver240 80

245

250

255

260

sPS/n-copper240 75

245

250

255

260

sPS/n-diamonds240 80

245

250

255

260

sPS/MWCNT240 80

245 65

250 60

255 55

260 53

Temperature ( C)

te samples isothermally crystallized at the indicated temperatures.

From the slope and the intercept of the Avrami plots, the val-ues of n and k, were calculated respectively and the results are

summarized in Table 2. In the same table, the values of K areincluded, calculated from the corresponding values of n and k. Itis known that the value of n strongly depends on both the mecha-nism of nucleation and the morphology of crystal growth, and that

ned during isothermal crystallization of neat sPS and nanocomposites at different

k (min−n) n K (min−1)

33.4965 2.46 4.167914.13048 3.07 1.587270.20749 3.44 0.633070.00334 3.16 0.164615.05E−5 2.81 0.02957

312.608 3.27 5.7942510.8893 2.27 2.86305

0.46514 2.65 0.749130.02624 2.59 0.245900.00293 2.10 0.06253

76.9130 2.59 5.3341210.9395 3.41 2.01694

0.28510 3.32 0.685240.00577 3.29 0.209162.86E−5 3.25 0.03993

169.044 3.45 4.423825.02343 2.37 1.977110.29107 3.81 0.723300.01698 2.86 0.240023.81E−4 2.51 0.04343

30.0607 2.01 5.4365514.4211 2.04 3.69945

2.20293 1.75 1.570360.27733 2.21 0.560440.02046 1.78 0.11249

88 G.Z. Papageorgiou et al. / Thermochimica Acta 565 (2013) 82– 94

0 10 20 30 40 50 60

0

20

40

60

80

100a

sPS/Cop per nanofib ers

Re

lative

de

gre

e o

f cry

sta

llin

ity (

%)

Time (min)

240oC

245oC

250oC

255oC

260oC

240 245 250 255 260

0

5

10

15

20

25

30

35b

Cry

sta

lliza

tio

n H

alf T

ime

(m

in)

Temperature (oC)

Nea t

Cop per Na nof ibr es

MWCN T

Nano Diamonds

Sil ver

Ftp

iwstpeidA(wtsts2Ttrh

3

co

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6a

sPS/Copper

log

{-In

[1-X

(t)]

}

log[t(min)]

24 0

24 5

25 0

255

26 0

1.8 8 1.9 0 1.92 1.94 1.960.02

0.05

0.14

0.37

1.00

2.72

7.39b

K(m

in-1)

1000/T K-1

sPS

sPS/Silv er

sPS/ Copp er

sPS/ Dia mon d

sPS/MWCNT

2.5 3.0 3.5 4.0 4.5 5.0

-4

-3

-2

-1

0

1

2

ln(K

)

1000/[R(Tm

-T)]

sP S

sPS/S ilver

sPS/Cop per

sPS/Diam ond s

sP S/MWCN T

c

ig. 6. Relative degree of crystallinity vs time and crystallization half time vsemperature obtained under isothermal crystallization conditions at several tem-eratures.

deally n would be an integer [36]. The n values found in principleere in the vicinity of 3, although those for the faster crystallizing

PS/MWCNT nanocomposites were lower. The Avrami parame-ers k and K are indicative of the crystallization rate. Both of thearameters decrease with increasing temperature. The depend-nce of K on the inverse of the crystallization temperature appearsn Fig. 7b. From this figure it is obvious that the experimentalata do not follow an Arrhenious-type expression. Therefore, thevrami constant, K was plotted as a function of the supercooling

Tmo − T), using an equation in the form, K = K0 exp(−E/R(Tm

o − T))ith K0 and E a pre-exponential factor and an effective activa-

ion energy, respectively. The plots appear in Fig. 7c and very goodtraight lines were observed now. The effective activation energieshus estimated for the neat sPS, as well as the nanocomposites ofPS with silver, copper, diamonds and MWCNT were 2.44 kJ/mol,.28 kJ/mol, 2.40 kJ/mol, 2.28 kJ/mol and 1.96 kJ/mol, respectively.he values calculated for all nanocomposites were lower comparedo neat sPS, meaning an easier crystallization. Moreover, theseesults verified the finding that the addition of MWCNT leads inigher crystallization rates.

.5.2. Non-isothermal crystallization kineticsFrom non-isothermal crystallization experiments, data for the

rystallization exotherms as a function of temperature can bebtained, at each cooling rate, as one can see in Fig. 8a and b for the

Fig. 7. Avrami plots for the isothermal crystallization of sPS/copper nanofibers (a),effect of temperature in the Avrami rate constant (b) and estimation of an effectiveactivation energy of all different nanocomposites (c).

crystallization on cooling of sPS/silver and sPS/MWCNT nanocom-

posite respectively. As it was expected, the crystallization exothermbecomes broader and shifts to lower temperatures with increas-ing cooling rate. Hence, the crystallization peak temperature isdecreased with increasing cooling rate (Fig. 9), meaning that the

G.Z. Papageorgiou et al. / Thermochimica Acta 565 (2013) 82– 94 89

235 240 245 250 255 260 265

a

0.5 W/g

No

rmalize

d H

eat F

low

(W

/g)

End

o U

p

10oC/min

7.5oC/min

5oC/min

2.5oC/min

1oC/min

235 240 245 250 255 260 265

b

0.2 W/g

No

rma

lize

d H

ea

t F

low

(W

/g)

En

do

UP

Temperature (oC)

10oC/mi n

7. 5oC/mi n

5oC/min

2. 5oC/mi n

1oC/min

Fn

hltaMh

230 235 240 24 5 25 0 25 5 260 265 270

0

20

40

60

80

100

sPS/Silver

a

Re

lative

de

gre

e o

f cry

sta

llin

ity (

%)

Temperature (oC)

10oC/min

7.5oC/min

5oC/min

2.5oC/min

1oC/min

0.0 2.5 5. 0 7. 5 10 .0 12. 5 15. 0

0

20

40

60

80

100

sPS/Sil ver

b

Re

lative

de

gre

e o

f cry

sta

llin

ity (

%)

Time (min)

10oC/min

7.5oC/min

5oC/min

2.5oC/min

1oC/min

ig. 8. DSC cooling traces of (a) sPS/silver nanocomposite and (b) sPS/MWCNTanocomposite at various cooling rates.

igher the cooling rate the lower the temperature that the crystal-ization process starts and completes. Furthermore, it was observed

hat crystallization in nanocomposites starts and completes alwayst higher temperatures compared to neat sPS. Moreover, use ofWCNT resulted always in higher Tp compared to other nano-

ybrids.

0 1 2 3 4 5 6 7 8 9 1110

240

244

248

252

256

260

Cry

sta

lliza

tio

n p

ea

k t

em

pe

ratu

re (

oC

)

Cooling Rate (oC/min)

neat

copper

MWCNT

NanoDiamonds

Silver

Fig. 9. Variation of crystallization peak temperature with cooling rate.

Fig. 10. Evolution of relative degree of crystallinity with temperature (a) and time(b) of crystallization sPS/silver nanocomposite on cooling for various cooling rates.

From the data for the crystallization exotherms as a functionof temperature dHc/dT the relative crystallinity as a function oftemperature X(T) can be calculated as follows:

X(T) =∫ Tc

To(dHc/dT)dT

∫ T∞To

(dHc/dT)dT(4)

where To denotes the initial crystallization temperature and Tc, T∞the crystallization temperature at time t and after the completionof the crystallization process, respectively.

The crystallization temperature Tc, can be converted tocrystallization time, t, with the well-known relationship for non-isothermal crystallization processes that is strictly valid when thesample experiences the same thermal history as designed by theDSC furnace [42]:

t = (Tc − To)ˇ

(5)

where ̌ is the constant cooling rate. Fig. 10 shows the evolutionof relative degree of crystallinity with temperature or time of thesPS/silver nanocomposite on cooling at various cooling rates.

To quantitatively describe the evolution of the crystallinity dur-ing nonisothermal crystallization, a number of models have beenproposed in the literature [44]. The most common approaches arethose based on the modified Avrami and Ozawa equations [45,46].

90 G.Z. Papageorgiou et al. / Thermochimica Acta 565 (2013) 82– 94

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-1.5

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

10

7.5

5

2.5

1

log{-ln[1-X(t)]}

log[t(min)]

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

10

7.5

5

2.5

1

log{-ln[1-X(t)]}

log[t(min)]

Fs

o

X

weEi

tlnenptclpspfnapnhb

Table 3Parameters of the modified Avrami model and limiting relative degree of crys-tallinity, Xl where linearity holds obtained from the analysis of the non-isothermalcrystallization of sPS and its nanocomposites.

Sample/coolingrate (◦C/min)

Xl (%) Zt (min−n) n KAvrami (min−1)

SPs1 55 1.51E−4 4.47 0.139762.5 0.00451 4.72 0.318395 0.03945 5.46 0.553177.5 0.17219 5.86 0.74067

10 0.34198 6.72 0.85242

sPS/n-silver1 65 4.17E−5 5.20 0.14382.5 0.00123 5.53 0.29775 0.1750 4.60 0.68467.5 0.6471 4.95 0.9158

10 2.0893 5.16 1.1533

sPS/n-copper1 80 2.04E−5 6.04 0.16732.5 0.00575 5.60 0.39815 0.0837 6.33 0.67597.5 1.0351 5.80 1.0060

10 2.9580 6.45 1.1831

sPS/n-diamonds1 80 0.00126 4.11 0.197582.5 0.03451 4.20 0.448655 0.1233 5.25 0.671217.5 0.5559 5.78 0.90337

10 2.1528 5.78 1.14186

sPS/MWCNT1 55 0.00251 2.95 0.131412.5 0.01754 3.21 0.28377

ig. 11. Avrami plots for nonisothermal crystallization of: (a) SPS–copper and (b)PS at various cooling rates.

According to the modified Avrami method, the relative degreef crystallinity, X, can be calculated from:

= 1 − exp(−Zttn) or X = 1 − exp[−(KAvramit)n] (6)

here Zt and n denote the growth rate constant and the Avramixponent, respectively. Since the units of Zt are a function of n,q. (6) can be written in the composite–Avrami form using KAvraminstead of Zt (where Zt = KAvrami

n) [42,44].Again Zt and n can be calculated by fitting the experimen-

al data to an equation similar to Eq. (3). In Fig. 11 plots ofog{−ln(1 − X)} vs log(t) are shown for the neat sPS and sPS/copperanocomposites. As it can be seen straight lines are obtained inach cooling rate and from the slope and intercept of each line

and log(k) are calculated and the best fitting values for thearameters n, Zt and KAvrami are presented in Table 3. A devia-ion from linearity appeared at high values of relative degree ofrystallinity. The relative degree of crystallinity until which theinearity of the Avrami model holds was near 80% for the nanocom-osites of sPS with either copper or diamonds, much lower whenilver was used (i.e. 65%), while for the neat sPS and nanocom-osite with MWCNT the value was as low as 55% (Table 3). Inact the behaviour of the sPS/MWCNTs nanocomposite and theeat sPS should be associated with the wide temperature ranget which crystallization occurred. For the sPS/MWCNTs nanocom-

osite a plausible explanation is that due to effective heterogeneousucleation on the solid MWCNTs surface, crystallization begins atigh temperatures, however it continued till lower temperaturesecause of secondary crystallization in the confined environment of

5 0.06166 3.83 0.483147.5 0.11912 4.50 0.62299

10 0.38548 4.50 0.80929

the already formed crystals or because of homogenous nucleationwhich always holds at the lower temperature region. Formationof �-crystals is favoured by the higher crystallization tempera-tures. In case of the neat sPS homogenous nucleation is mainlyexpected to govern the phenomenon. So the crystallization tem-perature range is wide since homogenous nucleation rates andthus the overall crystallization rates are slower. In case of the restnanocomposites, moderate nucleation efficiency of the nanopar-ticles leads to rather fast crystallization but at almost the samelow temperature range with that at which homogenous nuclea-tion is also favoured. So in the latter case the crystallization rangeis narrower, because of the larger supercoolings. It also is truethat on crystallization of neat sPS both � and � crystal typesare usually formed. But the � crystals are mainly formed afterthe polymer is melted at 340 ◦C or higher prior to crystallize atalso elevated temperatures, i.e. above 250 ◦C, while �-crystals areformed in the opposite case. In this work it was expected mainly�-crystals to form as the melting temperature was not so high.However, for the slow cooling rates on non-isothemal crystal-lization, or for the high isothermal crystallization temperatures,formation of � crystals cannot be precluded. Finally, some inter-connection between crystallization in the two different � and �crystal types and the dual crystallization mechanisms, as concludedform the Avrami plots, especially in the sPS/MWCNTs nanocom-posite might also be reasonable. Of course, this point needs furtherstudy.

As it was expected the values of the Avrami rate constantincreased with the cooling rate while the values of n did not

presented a clear tendency. The absolute values of the Avrami expo-nent were in between 5 and 6 for most nanocomposites and coolingrates with much lower values (i.e. in between 3 and 4.5) whenMWCNT were used.

rmochimica Acta 565 (2013) 82– 94 91

3

iubp(dscr

ϕ

we

B

wv�ia

sd

l

m(

l

ootaamtrasdk

3

iaoiophdciut

3.0x10-4

4.0x 10-4 5.0x1 0

-46.0x 10

-4 7.0x1 0-4 8.0x1 0

-4

0.0

0.5

1.0

1.5

2.0

2.5

a

ln(

)

1/(2)

Neat sPS

sPS/M WCNT

sP S/NDia mond s

sPS /Si lver

sPS/C opper fibre s

WITHOUT COPPE R NDI AMOND S SILVE R MWCN T

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05b

Nu

cle

atio

n A

ctivity (

)

Filler Typ e

G.Z. Papageorgiou et al. / The

.6. Nucleation activity

Dobreva et al. [47,48] suggested a simple method for calculat-ng the nucleation activity of foreign substrates in polymer meltsing data from non-isothermal crystallization. This method haseen also used for silica nanoparticle-filled PEN, isotactic poly-ropylene and PLA nanocomposites [49–51]. Nucleation activityϕ) is a factor by which the work of three-dimensional nucleationecreases with the addition of a foreign substrate. If the foreignubstrate is extremely active, ϕ approaches 0, while for inert parti-les, ϕ approaches 1. The nucleation activity is calculated from theatio:

= B∗

B(7)

here B is a parameter that can be calculated from the followingquation

= ω�3V2m

3nkBT0m�S2

m(8)

here ω is a geometric factor, � is a specific energy, Vm is the molarolume of the crystallizing substance, n is the Avrami exponent,Sm is the entropy of melting and Tm

0 the infinite crystal melt-ng temperature. Tm

0 was assumed to be equal to 286 ◦C for sPSccording to Woo [52].

Furthermore, B can be experimentally determined from thelope of Eq. (9) obtained by plotting ln(ˇ) vs the inverse squaredegree of supercooling 1/�Tp

2 (�Tp = Tm − Tp) [48–51]:

n ̌ = Const − B

�T2p

(9)

The above equation holds for homogeneous nucleation from aelt, near the melting temperature. By using a nucleating agent, Eq.

9) is transformed to the following for heterogeneous nucleation:

n ̌ = Const − B∗

�T2p

(10)

Plots of ln(ˇ) vs 1/�Tp2 for neat sPS and all sPS nanocomposites

f this work are shown in Fig. 12a. As it can be seen straight lines arebtained in every sample (correlation coefficient R > 0.998). Fromhe slopes of these lines, the values of B and B* for the plain sPSnd the nanocomposites can be calculated. Then, the nucleationctivity is computed from Eq. (7) and is presented in Fig. 12b. Itust be noted that a value of ϕ close to zero indicates high nuclea-

ion activity, while values close to 1 show low activity. From theseesults, it can be seen that in the nanocomposites, MWCNTs werecting effectively as nucleation agents in the sPS matrix. Somethingimilar has been recently found to happen also during the thermalegradation of polymer nanocomposites, where a change in theinetic mechanism has been proposed [53].

.7. Effective activation energy of non-isothermal crystallization

Apart from the aforementioned macroscopic kinetic models its interesting also to evaluate the effective energy barrier, �E, for

non-isothermal crystallization process. Considering the variationf the peak temperature with the cooling rate, ˇ, several mathemat-cal procedures have been proposed in literature for the calculationf �E. However, a major concern has been raised for the use of theserocedures in obtaining �E, since they have been formulated foreating experiments (i.e. positive values of ˛). Vyazovkin [54] hasemonstrated that dropping the negative sign for ̌ is a mathemati-

ally invalid procedure that generally makes the Kissinger equationnapplicable to the processes that occur on cooling. Moreover, these of this invalid procedure may result in erroneous values ofhe effective energy barrier, �E. Accordingly, for the estimation

Fig. 12. Plots of ln(ˇ) vs 1/�Tp2 (a) and variation of nucleation ϕ activity with filler

type (b) for the sPS nanocomposites.

of effective activation energy of crystallization on cooling, use ofmultiple heating rate methods such as isoconversional methodsis recommended [54]. The differential isoconversional method ofFriedman [55] and the advanced integral isoconversional methodof Vyazovkin [54,56] are the most appropriate.

The method of Friedman was used in this work. According to thedifferential isoconversional method of Friedman, different effectiveactivation energies are calculated for every degree of crystallinityfrom:

ln(

dX

dt

)X,i

= Const − �EX

RTX,i(11)

where dX/dt is the instantaneous crystallization rate as a function oftime at a given conversion X, �EX is the effective activation energyat a given conversion X, TX,i is the set of temperatures related to agiven conversion X at different cooling rates, ˇi and the subscript irefers to every individual cooling rate used.

According to the method of Friedman, first the X(t) functionobtained from the integration of the experimentally measured crys-tallization rates is differentiated with respect to time [55]. Thus,the instantaneous crystallization rate, dX/dt are obtained. Then, byselecting appropriate degrees of crystallinity (i.e. from 2% to 98%)the values of dX/dt at a specific X are correlated to the correspond-

ing crystallization temperature at this X, i.e. TX. Following, the lefthand side of Eq. (11) is potted with respect to 1/TX. A straight lineis regularly obtained with a slope equal to �EX/R.

92 G.Z. Papageorgiou et al. / Thermoch

Ft

rowc

Fm

ig. 13. Plot of ln(d˛/dt) vs 1/T according to the Friedman isoconversional methodo estimate the effective activation energy.

By applying the Friedman method, in every X for cooling rates

anging between 1 ◦C/min and 10 ◦C/min almost linear curves werebtained (Fig. 13). The effective activation energy, thus obtainedas subsequently plotted as a function of the relative degree of

rystallinity, as one can see in Fig. 14a. The effective activation

0.0 0.2 0.4 0.6 0.8 1.0-56 0

-52 0

-48 0

-44 0

-40 0

-36 0

-32 0

-28 0

-24 0

-200

Effective A

ctivation E

nerg

y (

kJ/m

ol)

Relative degree of crystallinity

sPS

sPS/Coppe r

sPS/Diamond s

sPS/ MWCN T

sPS/Silver

236 23 8 24 0 24 2 24 4 24 6 24 8 25 0 252 254 25 6-560

-520

-480

-440

-400

-360

-320

-280

-240

-200

Effective A

ctivation E

nerg

y (

kJ/m

ol)

Temperature (oC)

sPS

sPS /Coppe r

sPS /Diamond s

sPS/MWCN T

sP S/Silver

ig. 14. Effective activation energy as a function of (a) X obtained from the Friedmanethod and (b) temperature, for sPS and all nanocomposites.

imica Acta 565 (2013) 82– 94

energy increases with X as it has also been observed in othersystems. Moreover, it was observed that the nanocompositespresent always higher absolute �E values compared to neat sPS.This again means that the crystallization rates of the nanocompos-ites are faster compared to neat sPS as it was observed from theisothermal and non-isothermal DSC measurements.

Furthermore, the effective energy barrier can be plotted as afunction of temperature by taking an average temperature associ-ated with certain a value [56]. The respective plots for sPS and thenanocomposites are presented in Fig. 14b. These plots show that thesPS/MWCNT nanocomposite sample crystallizes at higher temper-ature and that also it has the higher absolute value. This obviouslyhas to do with the higher crystallization temperature and also withthe fact that MWCNT of large length increase the melt viscosityand hinder diffusion. Similarly, copper nanofibers with large lengthshow high activation energy of crystallization. On the other handspheroid nanoparticles like those of silver or nanodiamonds showlower activation energy than MWCNT or copper nanofibers at thesame crystallization temperatures. For the sPS/MWCNT nanocom-posite as can be seen the temperature range of crystallization isbroader than for the rest nanocomposites, although narrower thanthe neat polymer as can also be observed in Fig. 8b.

For critical studies concerning the isoconversional methods ofkinetic analysis and recommendations for performing kinetic com-putations on thermal analysis data one can see two recent veryinteresting papers by Vyazovkin et al. [57] and Criado et al. [58].

3.8. Estimation of the Lauritzen–Hoffman parameters

In this section the Lauritzen–Hoffman parameters are estimatedusing either isothermal DSC data or non-isothermal crystallizationdata and the theory proposed by Vyazovkin and Sbirrazzuoli [59].The latter has been proved to be valid for different polymer systems[60].

Accordingly, the growth rate, G is given as a function of the crys-tallization temperature, Tc, by the following biexponential equationin the context of the Lauritzen–Hoffman secondary nucleation the-ory [61].

G = G0 exp[− U∗

R(Tc − T∞)

]exp

[− Kg

Tc (�T) f

](12)

where, G0 is the pre-exponential factor, the first exponential termcontains the contribution of diffusion process to the growth rate,while the second exponential term is the contribution of the nuclea-tion process; U* denotes the activation energy which characterizesmolecular diffusion across the interfacial boundary between meltand crystals; usually set equal to 1500 cal/mol (6285 J/mol). T∞ isusually set equal to (Tg − 30) K with Tg the glass transition temper-ature of the polymer; Kg is a nucleation constant and �T denotesthe degree of undercooling (�T = Tm

0 − Tc); f is a correction fac-tor given as f = 2 Tc/(Tm

0 + Tc); the equilibrium melting temperature,Tm

0 can be calculated using the linear or nonlinear Hoffman–Weeksextrapolation.

For a secondary or heterogeneous nucleation, Kg can be calcu-lated from:

Kg = n��eb0T0m

�hf�ckB(13)

where, n takes the value of 4 when crystallization takes place inregimes I or III and the value 2 in regime II, �, �e are the side surface(lateral) and fold surface free energies which measure the workrequired to create a new surface, b0 is the single layer thickness,

G.Z. Papageorgiou et al. / Thermoch

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

0

1

2

3

4

5

6

ln(1

/t1

/2)

+ U

* /[R

{Tc-(

Tg-3

0)}

]

105/{T

cf( T)}

neat sPS

sPS/copper

sPS/MWCNT

sPS/nDiamonds

sPS/nSilver

Fig. 15. Lauritzen-Hoffman type plots for neat sPS and all nanocomposites.

Table 4Estimated nucleation parameter Kg using the Lauritzen–Hoffman analysis andisothermal data, as well as the Vyazovkin–Sbirrazzuoli method and non-isothermaldata.

Material Kg (×105 K2) fromthe L–H analysis

Kg (×105 K2) fromthe V–S method

sPS 1.63 1.98sPS/copper 1.58 1.95

�t

b

l

agsTstfiicw

aaAtrf

[

sPS/MWCNT 1.36 1.69sPS/n-silver 1.50 1.70sPS/n-diamonds 1.51 1.71

hf �c = �Hf is the enthalpy of melting per unit volume and kB ishe Boltzmann constant.

In order to estimate the L–H parameters, Kg and U*, Eq. (12) cane written as follows:

n G + U∗

R(Tc − T∞)= ln G0 − Kg

Tc (�T) f(14)

Plotting the left-hand side of Eq. (14) with respect to 1/(Tc(�T)f), straight line should appear having a slope equal to Kg. Fig. 15ives such plots for neat sPS and the sPS nanocomposites. In thistudy U* was set equal to 6285 J/mol, Tm

0 = 286 ◦C and Tg = 100 ◦C.he growth rate G, which should be obtained essentially by thepherulite growth, was calculated by the inverse of crystalliza-ion half time (1/t1/2) [62]. Thus, the following data are mainlyor the purpose of qualitative comparison between sPS matrix andts nanocomposites. The estimated Kg values appear in Table 4. Ast can be seen all nanocomposites presented slightly lower valuesompared to neat sPS with the lowest calculated when MWCNTsere used.

Next, the L–H parameters were estimated using the effectivectivation energy, �Ex, values calculated in the previous sectionnd the method proposed by Vyazovkin and Sbirrazzuoli [59].ccordingly, the procedure for the calculation of �EX is based on

he equivalency between the temperature coefficient of the growthate, G and the overall crystallization rate, dX/dt. After some trans-ormation the following equation holds

d ln(G)dT−1

= d ln(dX/dt)dT−1

(15)

Differentiating Eq. (12) according to T−1 one gets:

d ln(G)dT−1

= −U∗

R

T2

(T − T∞)2− Kg

(T0m)

2 − T2 − T0mT

(T0m − T)

2T

(16)

[

[

imica Acta 565 (2013) 82– 94 93

Hence, by combining Eqs. (11) and (15) with (16) the effectiveactivation energy at a given conversion X can be calculated from[59]:

�EX = U∗ T2

(T − T∞)2+ KgR

(T0m)

2 − T2 − T0mT

(T0m − T)

2T

(17)

Using this formulation, the Lauritzen–Hoffman parameters U*

and Kg can be calculated by fitting of the theoretical Eq. (17) tothe experimental data produced using the differential isoconver-sional method of Friedman and illustrated in Fig. 14b. In order theresults to be comparable with those estimated previously with theL–H method and isothermal data, parameter U* was initially setconstant to a value equal to 1500 cal/mol (6285 J/mol). Then, theestimated Kg values appear in Table 4. The same tendency withprevious results was clear. The actual values were slightly highercompared to the corresponding values estimated with the L–Hmethod and isothermal data. On letting to vary both U* and Kg,the best-fit values for neat sPS were 13,715 J/mol and 2.3 × 105 K2,respectively. U* has a value almost double that of the universal onewhile Kg keeps almost the same value.

4. Conclusions

sPS nanocomposites were prepared by mechanical mixing ofthe polymer and nanofiller, ball milling of the resulting mixtureand finally melt mixing. Multi-walled carbon nanotubes, coppernanofibers, silver nanoparticles or nanodiamonds were combinedwith sPS, to obtain corresponding nanocomposites containing3 wt% filler in every case. It was found that besides the type, theshape also of the nano-filler affects mechanical properties and crys-tallization kinetics. MWCNT promoted formation of �-type crystalsof sPS. MWCNTs also were found to act as the most effectivenucleation agents among the tested nanoparticles. Isothermal andnonisothermal crystallization kinetics was studied and crystalliza-tion rates for the nanocomposites proved to be faster compared tothe pristine polymer. The isoconversional method of Friedmannwas applied in order to estimate the activation energy of non-isothermal crystallization of neat sPS and the nanocomposites.Results showed that the lower value of effective activation energyis associated with use of MWCNTs.

References

[1] J. Schellenberg, Recent transition metal catalysts for syndiotactic polystyrene,Prog. Polym. Sci. 34 (2009) 688–718.

[2] N. Ishihara, T. Seimiya, M. Kuramoto, M. Uoi, Synthesis and properties ofpolystyrene with new stereoregularity, Polym. Prepr. Jpn. 35 (1986) 240.

[3] G. Guerra, V.M. Vitagliano, C. de Rosa, V. Petraccone, P. Corradini, Polymor-phism in melt crystallized syndiotactic polystyrene samples, Macromolecules23 (1990) 1539–1544.

[4] C. de Rosa, G. Guerra, V. Petraccone, P. Corradini, Crystal structure of the a-formof syndiotactic polystyrene, Polym. J. 23 (1992) 1435–1442.

[5] C. de Rosa, Crystal structure of the trigonal modification (a form) of syndiotacticpolystyrene, Macromolecules 29 (1996) 8460–8465.

[6] C. de Rosa, M. Rapacciuolo, G. Guerra, V. Petraccone, On the crystal struc-ture of the orthorhombic form of syndiotactic polystyrene, Polymer 33 (1992)1423–1428.

[7] Y. Chatani, Y. Shimane, Y. Inoue, T. Inagaki, T. Ishioka, T. Ijitsu, Structural studyof syndiotactic polystyrene. 1. Polymorphism, Polymer 33 (1992) 488–492.

[8] V. Vittoria, A.R. Filho, F. de Candia, Structure of syndiotactic polystyrene ther-mally crystallized from the glassy state, J. Macromol. Sci. Phys. B29 (1990)411–428.

[9] F. Decandia, A. Ruvolo Filho, V. Vittoria, Thermally induced crystallization inglassy syndiotactic polystyrene, Colloid Polym. Sci. 269 (1991) 650–654.

10] E.M. Woo, Y.S. Sun, M.L. Lee, Crystal forms in cold-crystallized syndiotacticpolystyrene, Polymer 40 (1999) 4425–4429.

11] V. Vittoria, F. de Candia, P. Iannelli, A. Immirzi, Solvent-induced polymor-phism in syndiotactic polystyrene, Makromol. Chem. Rapid Commun. 9 (1988)765–769.

12] M. Kobayashi, T. Nakaoki, N. Ishihara, Polymorphic structures and molecularvibrations of syndiotactic polystyrene, Macromolecules 22 (1989) 4377–4382.

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13] C. Naddeo, L. Guadagno, D. Acierno, V. Vittoria, Studies of the gamma to alphatransition in syndiotactic polystyrene, Macromol. Symp. 138 (1999) 209–214.

14] W. Ma, J. Yu, J. He, Direct formation of � form crystal of syndiotacticpolystyrene from amorphous state in supercritical CO2, Macromolecules 37(2004) 6912–6917.

15] O. Tarallo, V. Petraccone, R.A. Albunia, C. Daniel, G. Guerra, Monoclinic andtriclinic �-clathrates of syndiotactic polystyrene, Macromolecules 43 (2010)8549–8558.

16] P. Rizzo, C. Daniel, A. de Girolamo Del Mauro, G. Guerra, New host poly-meric framework and related polar guest cocrystals, Chem. Mater. 19 (2007)3864–3866.

17] R.H. Lin, E.M. Woo, Melting behavior and identification of polymorphic crystalsin syndiotactic polystyrene, Polymer 41 (2000) 121–131.

18] D.A. Rani, Y. Yamamoto, S. Mohri, M. Sivakumar, Y. Tsujita, H. Yoshimizu, Struc-ture and properties of the mesophase of syndiotactic polystyrene. II. Effect ofstepwise extraction on the preparation of the mesophase, J. Polym. Sci. Part B:Polym. Phys. 41 (2003) 269–273.

19] S. Malik, C. Rochas, J.M. Guenet, Thermodynamic and structural investigationson the different forms of syndiotactic polystyrene intercalates, Macromolecules39 (2006) 1000–1007.

20] Y.S. Sun, E.M. Woo, Relationships between polymorphic crystals and multi-ple melting peaks in crystalline syndiotactic polystyrene, Macromolecules 32(1999) 7836–7844.

21] S.C. Tjong, Structural and mechanical properties of polymer nanocomposites,Mater. Sci. Eng. R. 53 (2006) 73–197.

22] D.N. Bikiaris, A. Vassiliou, E. Pavlidou, P. Karayannidis, Compatibilisation effectof PP-g-MA copolymer on iPP/SiO2 nanocomposites prepared by melt mixing,Eur. Polym. J. 41 (2005) 1965–1978.

23] K. Chrissafis, G. Antoniadis, K.M. Paraskevopoulos, A. Vassiliou, D.N. Bikiaris,Comparative study of the effect of different nanoparticles on the thermal degra-dation mechanism of in situ prepared poly(�-caprolactone) nanocomposites,Compos. Sci. Technol. 67 (2007) 2165–2174.

24] J.F. Vega, J. Martvnez-Salazar, M. Trujillo, M.L. Arnal, A.J. Móller, S.Bredeau, Rheology, processing, tensile properties, and crystallization ofpolyethylene/carbon nanotube nanocomposites, Macromolecules 42 (2009)4719–4727.

25] C.R. Tseng, H.Y. Lee, F.C. Chang, Crystallization kinetics and crystallizationbehavior of syndiotactic polystyrene/clay nanocomposites, J. Polym. Sci. Polym.Phys. 39 (2001) 2097–2107.

26] C.R. Tseng, S.C. Wu, J.J. Wu, F.C. Chang, Crystallization behavior of syndiotacticpolystyrene nanocomposites for melt- and coldcrystallizations, J. Appl. Polym.Sci. 86 (2002) 2492–2501.

27] S. Kumar, T. Rath, R.N. Mahaling, C.K. Das, Processing and characteriza-tion of carbon nanofiber/syndiotactic polystyrene composites in the absenceand presence of liquid crystalline polymer, Composites Part A 38 (2007)1304–1317.

28] C.-L. Huang, C. Wang, Polymorphism and transcrystallization of syndiotacticpolystyrene composites filled with carbon nanotubes, Eur. Polym. J. 47 (2011)2087–2096.

29] S. Cimmino, E. di Pace, E. Martuscelli, C. Selvestre, Syndiotactic polystyrene:crystallization and melting behaviour, Polymer 32 (1991) 1080–1083.

30] B. Kang, S.V. Nair, H. Yang, H.S. Lim, D.H. Lee, K. Cho, Effect of crystallinemicrostructure near the particle/matrix interface on the toughening of syn-diotactic polystyrene/polyamide-6 blends, J. Appl. Polym. Sci. 108 (2008)2734–2747.

31] C.I. Park, O.O. Park, J.G. Lim, H.J. Kim, The fabrication of syndiotacticpolystyrene/organophilic clay nanocomposites and their properties, Polymer42 (2001) 7465–7475.

32] Z. Tao, H. Geng, K. Yu, Z. Yang, Y. Wang, Effects of high-energy ball milling on themorphology and the field emission property of multi-walled carbon nanotubes,Mater. Lett. 58 (2004) 3410–3413.

33] W.M. Tucho, H. Mauroy, J.C. Walmsley, S. Deledda, R. Holmestad, B.C. Hauback,The effects of ball milling intensity on morphology of multiwall carbon nano-tubes, Scr. Mater. 63 (2010) 637–640.

34] E.B. Gowd, K. Tashiro, C. Ramesh, Structural phase transitions of syndiotacticpolystyrene, Prog. Polym. Sci. 34 (2009) 280–315.

35] E.M. Woo, Y.S. Sun, C.P. Yang, Polymorphism, thermal behavior, and crystalstability in syndiotactic polystyrene vs its miscible blends, Prog. Polym. Sci. 26(2001) 945–983.

[

[

imica Acta 565 (2013) 82– 94

36] M.J. Avrami, Kinetics of phase change I, J. Chem. Phys. 7 (1939)1103–1112.

37] M.J. Avrami, Kinetics of phase change II, J. Chem. Phys. 8 (1940) 212–224.38] M.J. Avrami, Granulation, phase change, and microstructure, J. Chem. Phys. 9

(1941) 177–184.39] A.N. Kolmogorov, A statistical theory for the recrystallization of metals, Izv.

Akad. Nauk. SSSR Ser. Math. 1 (1937) 355–360.40] W.A. Johnson, R.F. Mehl, Reaction kinetics in processes of nucleation and

growth, Trans. Am. Inst. Mining Metall. Eng. 35 (1939) 416–441.41] U.R. Evans, The laws of expanding circles and spheres in relation to the lateral

growth of surface films and the grain-size of metals, Trans. Faraday Soc. 41(1945) 365–374.

42] D.S. Achilias, G.Z. Papageorgiou, G.P. Karayannidis, Isothermal, Non-isothermalcrystallization kinetics of poly(trimethylene terephthalate), J. Polym. Sci. B:Polym. Phys. 42 (2004) 3775–3796.

43] P. Supaphol, Application of the Avrami, Tobin, Malkin, and Urbanovici-Segalmacrokinetic models to isothermal crystallization of syndiotactic polypropyl-ene, Thermochim. Acta 370 (2001) 37–48.

44] M.L. di Lorenzo, C. Silvestre, Non-isothermal crystallization of polymers, Prog.Polym. Sci. 24 (1999) 917–950.

45] A. Jeziorny, Parameters characterizing the kinetics of the non-isothermal crys-tallization of poly(ethylene terephthalate) determined by d.s.c., Polymer 19(1978) 1142–1144.

46] T. Ozawa, Kinetics of non-isothermal crystallization, Polymer 12 (1971)150–158.

47] A. Dobreva, I. Gutzow, Activity of substrates in the catalyzed nucleation of glass-forming melts. I. Theory, J. Non-Cryst. Solids 162 (1993) 1–12.

48] A. Dobreva, I. Gutzow, Activity of substrates in the catalyzed nucleation ofglass-forming melts. II. Experimental evidence, J. Non-Cryst. Solids 162 (1993)13–25.

49] S.H. Kim, S.H. Ahn, T. Hirai, Crystallization kinetics and nucleation activity ofsilica nanoparticle-filled poly(ethylene 2,6-naphthalate), Polymer 44 (2003)5625–5634.

50] G.Z. Papageorgiou, D.S. Achilias, D.N. Bikiaris, G.P. Karayannidis, Crystallizationkinetics and nucleation activity of filler in polypropylene/surface-treated SiO2

nanocomposites, Thermochim. Acta 427 (2005) 117–128.51] G.Z. Papageorgiou, D.S. Achilias, S. Nanaki, T. Beslikas, D. Bikiaris, PLA nanocom-

posites: effect of filler type on non-isothermal crystallization, Thermochim.Acta 511 (2010) 129–139.

52] S.H. Li, E.M. Woo, Miscibility in blends of isotactic/syndiotactic polystyrenes atmelt or quenched amorphous solid state, Macromol. Mater. Eng. 291 (2006)1397–1406.

53] P.E. Sanchez-Jimenez, L.A. Perez-Maqueda, A. Perejon, J.M. Criado, Nanoclaynucleation effect in the thermal stabilization of a polymer nanocomposite: akinetic mechanism change, J. Phys. Chem. C 116 (2012) 11797–11807.

54] S. Vyazovkin, N. Sbirrazzuoli, Isoconversional analysis of calorimetric data onnonisothermal crystallization of a polymer melt, J. Phys. Chem. B 107 (2003)882–888.

55] H.L. Friedman, Kinetics of thermal degradation of char-forming plastics fromthermogravimetry, J. Polym. Sci. C 6 (1964/1965) 183–195.

56] S. Vyazovkin, Modification of the integral isoconversional method toaccount for variation in the activation energy, J. Comput. Chem. 22 (2001)178–183.

57] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Perez-Maqueda, C. Popescu,N. Sbirrazzuoli, ICTAC kinetics committee recommendations for performingkinetic computations on thermal analysis data, Thermochim. Acta 520 (2011)1–19.

58] J.M. Criado, P.E. Sanchez-Jimenez, L.A. Perez-Maqueda, Critical study of theiso-conversional methods of kinetic analysis, J. Therm. Anal. Calorim. 1 (2008)199–203.

59] S. Vyazovkin, N. Sbirrazzuoli, Macromol. Rapid Commun. 25 (2004) 733.60] D.S. Achilias, G.Z. Papageorgiou, G.P. Karayannidis, Evaluation of the isocon-

versional approach to estimating the Hoffman–Lauritzen parameters from theoverall rates of non-isothermal crystallization of polymers, Macromol. Chem.

Phys. 206 (2005) 1511–1519.

61] J.D. Hoffman, G.T. Davis, J.I. Lauritzen, in: N.B. Hannay Jr. (Ed.), Treatise on SolidState Chemistry, 3, Plenum Press, New York, 1976 (chapter 7).

62] T.W. Chan, A.I. Isayev, Quiescent polymer crystallization: modelling and mea-surements, Polym. Eng. Sci. 34 (1994) 461–471.