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Polymer 55 (2014) 6725e6734

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Polymer

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Kinetics of nucleation and crystallization in poly(butylene succinate)nanocomposites

Dimitrios G. Papageorgiou a, Evgeny Zhuravlev b, George Z. Papageorgiou c,Dimitrios Bikiaris d, Konstantinos Chrissafis a, Christoph Schick b, *

a Solid State Physics Department, School of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greeceb University of Rostock, Institute of Physics, Wismarsche Str. 43-45, 18051 Rostock, Germanyc Chemistry Department, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greeced Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki,GR-541 24 Thessaloniki, Macedonia, Greece

a r t i c l e i n f o

Article history:Received 13 September 2014Received in revised form31 October 2014Accepted 4 November 2014Available online 12 November 2014

Keywords:Poly(butylene succinate)Fast scanning calorimetryCrystallization

* Corresponding author. Tel.: þ49 381 498 6880; faE-mail address: christoph.schick@uni-rostock.de (

http://dx.doi.org/10.1016/j.polymer.2014.11.0140032-3861/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Differential fast scanning calorimetry (DFSC) was employed on poly(butylene succinate) nanocompositescontaining silver nanoparticles and multi-walled carbon nanotubes (MWCNT), in order to identify thetemperature range of heterogeneous nucleation caused by both nanofillers. The fast scanning rates alsoallow investigating self-nucleation by recrystallization experiments approaching the crystallizationtemperature from low temperatures. The recrystallization behavior of PBSu and its nanocomposites isdistinct from all other polymers studied so far as only the previously crystallized part of the material isable to recrystallize, independently on the available large number of nuclei. Since full melting of smallcrystals at low temperatures is observed this highlights the importance of ordered structures remainingin the polymer melt. On cooling from the melt the neat polymer did not crystallize at rates higher than70 K/s, while the nanocomposites needed rates of 500 K/s and 300 K/s for silver and MWCNT, respec-tively. Below 280 K the crystallization kinetics of the matrix was almost the same with the nano-composite samples. The nucleation mechanism changes at 280 K from heterogeneous to homogeneous.The study further confirms that below the glass transition nucleation and crystallization appears onlyafter approaching the enthalpy value of the extrapolated supercooled liquid by enthalpy relaxation.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Poly(butylene succinate) (PBSu) is a biodegradable aliphaticpolyester witch has been commercially available over the last years,since the demand for environmentally-friendly products isconstantly increasing. The structural characteristics of PBSu havealready been analyzed by several groups and the discrimination oftwo different crystalline forms (a- and b-form) has been carriedout. The a-form crystal structure has been reported by Ihn et al. [1],while Ichikawa et al. [2,3] confirmed the presence of b-form, whichwas induced by strain. Furthermore, the multiple melting behaviorand the recrystallization of PBSu during heating along with crys-tallization phenomena were studied [4e8]. Both crystal structuresbelong to the monoclinic systemwith the space group of P21/n andthe cell dimensions of the a-crystal form are a ¼ 0.523 nm,b ¼ 0.912 nm, c (fiber axis) ¼ 1.090 nm and b ¼ 123.9�. For the b-

x: þ49 381 498 6882.C. Schick).

form modification, the cell dimensions are a ¼ 0.584 nm,b ¼ 0.832 nm, c (fiber axis) ¼ 1.186 nm and b ¼ 131.6� [2]. PBSugenerally presents interesting physical properties such as goodbiodegradability, thermoplastic processability and balanced me-chanical properties [9,10], however more advanced applicationsdemand enhanced properties, which can be achieved by controllingcrystal nucleation, e.g. by introduction of nanofillers.

Polymer crystallization has attracted the interest of the scientificcommunity for several decades since polymeric crystals are morecomplex than other materials due to their disordered structure. Asa result, various crystallization theories have been proposed such asthe ones by Gibbs [11], LauritzeneHoffman [12], Wunderlich [13],Strobl [14] and others, which all have one thing in common; they allaccept the fact that polymer crystallization is a complex andmultistep process. The major challenge during the analysis ofnucleation and crystallization phenomena is the interpretation andrecognition of the physical mechanisms of different processes thatoccur during each step.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e67346726

The rapid emerge of technology has led to the development ofcharacterization techniques and instruments which are very sen-sitive and give insight in to structure formation of complex mate-rials like semicrystalline polymers. One of these instruments is thedifferential fast scanning calorimeter (DFSC) which has alreadyprovided significant details regarding the nucleation and crystal-lization phenomena of various materials, since it allows cooling andheating at rates as high as 1,000,000 K/s. A detailed description ofthe measurement principle and the instrumental setup has beengiven in earlier publications [15,16]. Fast scanning calorimetry wasapplied to provide an in situ investigation of structural and kineticcharacteristics of the PBSu samples along with size-dependent ef-fects far from thermodynamic equilibrium. The use of rates thathigh can prevent the reorganization or crystallization phenomenaon a sample during heating scans and thus, allows studying themelting of the crystals that were originally present in the samplebefore the heating scan [17e20]. The above mentioned coolingrates include the rates applied in industrial polymer processing,such as injection molding. Therefore the available informationregarding phase transitions at processing relevant conditions canbe considered very significant.

According to the classical nucleation theory, the nucleation of apolymeric material can be either homogeneous or heterogeneous.Homogeneous nucleation is much more difficult than heteroge-neous nucleation to occur since it demands the formation of anuclei in the bulk liquid phase and with that an interface at theboundaries of the new phase. The driving force for nucleation isincreasing with increasing supercooling (i.e. the difference be-tween the crystallization and melting temperatures). Nucleation isfavored when the overall change in free energy DGv is negative. Onthe other hand, heterogeneous nucleation occurs at lower super-cooling much more often than homogeneous nucleation due to thefact that the foreign preexisting surfaces such as phase boundariesor impurities reduce the free energy barrier. Thus, the lower DGvvalues lead to faster nucleation rates. In the current work, hetero-geneous nucleation was evoked by the presence of multi-walledcarbon nanotubes (MWCNTs) and silver (Ag) nanoparticles, whichwere inserted in the polymeric matrix at a concentration of2.5 wt.%.

In the current manuscript, the advantages of DFSC wereemployed to study the nucleation and crystallization kinetics ofpure PBSu and nanocomposite samples. Conventional techniquessuch as polarized optical microscopy (POM) and differential scan-ning calorimetry (DSC) were also applied in order to observe thenucleating effect of the different fillers on the polymeric matrix atsmall supercooling. Ultra-high cooling rates (non-isothermal crys-tallization) were initially applied in order to separate the differentnucleation and crystallization events in each sample. Furthermore,the samples were annealed at various temperatures for differenttime periods, for the evaluation of crystallinity, half-time of crys-tallization and nucleation activity of the added heterogeneities incomparison to homogeneously formed nuclei. Finally, enthalpyrelaxation and crystallization below the glass transition wereinvestigated too.

2. Experimental

2.1. Materials

For the preparation of PBSu succinic acid (SA, purum 99%), 1,4-butanediol (purum 99%) and tetrabutoxytitanium [Ti(OBu)4], ascatalyst (analytical grade), were used, which have been purchasedfrom Aldrich Co. Silver nanoparticles (Ag) were supplied byInframat Advanced Materials (Manchester, USA). Their diameterwas about 10 nm and their purity 99.95% of metal. Neat multi

walled carbon nanotubes (MWCNTs) were synthesized by thechemical vapor deposition (CVD) process and were supplied byTimesnano Chengdu Organic Chemicals Co.Ltd (China). Theirlength was about 10 mm and their diameter 10e20 nm. Chloroformand dimethylformamide of analytical grade were purchased fromAldrich Co.

2.2. Preparation of poly(butylene succinate)

PBSu was prepared by the two-stage melt polycondensationmethod (esterification and polycondensation) in a glass batchreactor. The proper amount of succinic acid and 1,4-butanediol inan acid/diol molar ratio 1/1.1 and the catalyst [Ti(OBu)4](1 � 10�3 mol/mol SA) were charged into the reaction tube of thepolyesterification apparatus. The apparatus with the reagents wasevacuated several times and filled with argon in order tocompletely remove oxygen. The reaction mixture was heated at463 K under an argon atmosphere and constant speed stirring(350 rpm). This first step (esterification) is considered to becompleted after the collection of theoretical amount of H2O (about3 h), which was removed from the reaction mixture by distillationand collected in a graduated cylinder. In the second step of poly-condensation a vacuum (5.0 Pa) was applied slowly over a period of15 min, to avoid excessive foaming and minimize oligomer subli-mation, a potential problem during melt polycondensation. Thetemperature was slowly increased to 493 K while stirring speedwas increased to 720 rpm. The polycondensation continued forabout 60 min at 493 K and after that time the temperature wasincreased to 513 K and the reaction continued for 60 min. Themolecular weight of the prepared polymer was found equal toMw ¼ 1.4$105 g/mol.

2.3. Preparation of PBSu nanocomposites

PBSu nanocomposites with MWCNTs and silver (Ag) nano-particles were prepared using CHCl3 as a mutual solvent withsonication and stirring at different nanofiller loadings. For thefabrication of the PBSu nanocomposites with MWCNTs and silver(Ag) nanoparticles, the desired amounts of the nanofillers were firstadded into CHCl3 at a concentration of 1 mg/mL, and the mixturewas subjected to sonication for 1 h to obtain a uniform dispersion.At the same time, PBSu was completely dissolved in CHCl3 at aconcentration of 20mg/mL. The PBSu solutionwas thenmixedwiththe nanofiller suspension followed by stirring and sonication for30min. The solvents were evaporated at elevated temperature for 2days and the nanocomposite films were obtained. In this way, PBSuwas mixed with the addition of 2.5 wt.% MWCNTs and 2.5 wt.% Agnanoparticles respectively. Nanocomposites containing 2.5 wt.%silver nanoparticles or 2.5 wt.% MWCNTs were prepared with codenames PBSu/Ag and PBSu/MWCNT. The polydispersity of PBSu andthe nanocomposites was evaluated by gel permeation chromatog-raphy (GPC) and it was found equal to 2.2 for neat PBSu and 2.3 forthe nanocomposite samples.

2.4. Polarized optical microscopy (POM)

Spherulite growth rates were measured on freshly made films.The preparation of the specimens and experimental procedurefollowed is described in detail in Supplementary Information.

2.5. Calorimetric experiments

Conventional DSC was initially applied in order to observe theeffect of each filler on the crystallization temperature and the

Fig. 2. Apparent heat capacity of PBSu and nanocomposites after cooling with variouscooling rates and immediate heating at 5000 K/s. The color code on the legend cor-responds only to the top panel graph for brevity reasons, however the general remarkis that the darker the color of the curve, the lower the cooling rate. The reader isreferred to the web version of the article for better interpretation of the curves.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e6734 6727

occurrence of the multiple melting behavior of the preparedpolyester, see Supplementary Information.

One of the main advantages of DFSC is the use of high coolingrates, which enables the suppression of crystallization and evenhomogeneous nucleation on cooling. Thus the study of cold crys-tallization, glass transition and melting can be performed in sam-ples which commonly crystallize during cooling and for whichconventional DSC instruments can give accurate information onlyin a limited temperature range. After different thermal treatmentscovering the temperature range from below Tg to close to Tm, seeSupplementary Information, the samples were reheated with5000 K/s or 10,000 K/s in order to observe their behavior at glasstransition, during cold crystallization and melting.

3. Results

3.1. Polarized optical microscopy

The spherulitic morphology of PBSu with and without theaddition of the nanofillers was evaluated with POM and the resultsare presented in Fig. 1. The neat PBSu samples present well-developed spherulites with distinct boundaries, while the nano-composite samples present significantly smaller spherulites whichimpose higher nucleating density and indicate the stronger het-erogeneous nucleation effect. Even at higher crystallization tem-peratures the nanocomposites form a significantly higher numberof spherulites (or axialites) of smaller final size, due to the highnucleation density, which leads to immediate impingement. Thesmaller radius and the higher density of the spherulites of PBSu/Agsample, is expected to attribute faster crystallization and nucleation

Fig. 1. POM images of PBSu and nanocomposites

phenomena, compared to the other nanocomposite sample, PBSu/MWCNTs. Furthermore, spherulites appeared after much shorterinduction periods in the nanocomposites due to the effectivenucleation, thus crystallization experiments could be performed athigher temperatures.

. The scale bar in all photographs is 50 mm.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e67346728

3.2. Non-isothermal ordering

The influence of different cooling rates on the nucleation andcrystallization of the PBSu and nanocomposite samples wasexamined. Fig. 2 shows the heat capacity curves on reheating with5000 K/s after cooling at various rates down to 100 K, which isfar below the glass transition temperature. For details seeSupplementary Information.

From the graphs presented in Fig. 2 it is obvious that after slowcooling the samples had already developed significant degree ofcrystallinity, and the crystals melt over a quite wide temperaturerange. In addition, the glass transition step was broadened and theCp increase was limited, while cold crystallization was not observ-able. With the increase of cooling rate, the glass transition stepbecamemore pronounced and sharper due to the smaller degree ofcrystallinity for the corresponding samples and reached amaximum at cooling rates of 70 K/s for PBSu, 300 K/s for PBSu/MWCNTs and 500 K/s for PBSu/Ag. In addition, the melting peakdecreased with increasing cooling rate resulting also from lesscrystallinity developed during cooling. At the same limiting coolingrates the melting peak disappears since the high cooling rates donot allow the sample to produce any crystals before the heating,Fig. 3. This is an indication that higher cooling rates were neededfor the nanocomposites in order to suppress crystallization and alsothat the PBSu/Ag sample crystallized faster than the compositewith MWCNTs. The faster rates needed for the PBSu/Ag sample andthe differences with the PBSu/MWCNTs sample, can be attributedto the bundles of nanotubes which are formed in the PBSu/MWCNTs sample due to the strong Van derWaals forces and hinderthe crystallization phenomenon along with the formation of a fillernetwork in the PBSu/Ag sample and the alignment of Ag particles,as it was realized in a previous study of the same samples [21].Additionally, it is also possible that the nucleation ability of thesurface of the three dimensional filler (Ag nanoparticles) mayexhibit higher nucleation activity than the MWCNTs, acceleratingthis way the crystallization phenomena in the specific sample.Finally, the significantly higher nucleation density of the nano-composites, whichwas already observed by POM, plays amajor roleon the crystallization rates of the specific samples, since there is ahigh number of nucleating sites which act as heterogeneous

Fig. 3. Total enthalpy change of PBSu and nanocomposites from DFSC on heating at5000 K/s after cooling with rates ranging from 10 to 10,000 K/s.

nucleators and they accelerate the general crystallizationphenomenon.

Another significant observation is the difference between themelting temperatures of PBSu, as presented in the literature fromconventional DSC, compared to the results from DFSC. During DFSCmeasurements, crystal reorganization or recrystallization wassuppressed due to the high heating rates applied [17]. Τhus, themelting temperature for PBSu in this case, is about 353 K (80 �C),significantly lower than that observed from conventional DSC ofabout 388 K (115 �C).

3.3. Isothermal ordering kinetics

The results from isothermal crystallization experiments areshown in Fig. 4, where the samples were crystallized for differenttime periods at 260 and 350 K. The heating rate applied was10,000 K/s for the acquisition of a better signal, see Supplementary

Fig. 4. Apparent heat capacity of PBSu and nanocomposites from DFSC on heating with10,000 K/s after annealing at different times at 260 K (a) and 350 K (b). The undershootat the end of the sharp melting peaks is an artifact from the power compensationscheme, not influencing the total area of the peak. The color code on the legend cor-responds only to the top panel graph for brevity reasons, however the general remarkis that the darker the color of the curve, the lower the annealing time. The reader isreferred to the web version of the article for better interpretation of the curves.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e6734 6729

Information for further details of the measurement program andsample preparation.

The first isothermal crystallization temperature of 260 K wasselected because it was just above the end of the glass transition ofthe samples (Tg ¼ 241 K). The results from the crystallization at260 K showed two melting peaks; the first one is attributed to theimperfect low temperature crystals, which melt just above thecrystallization temperature and then reorganize even at 10,000 K/sheating rate to more stable crystals melting at 350 K. This tem-perature however, is still low compared to the value of 388 K for theapparent melting point in standard DSC measurements. The verylow temperature related to the first melting (about 310e320 K),reflects the small imperfect crystals grown at large supercooling.Thus, under high supercoolings the recrystallization rates are highand overpass the melting rates. This is the case for the samplescrystallized at 260 K, (given that T0

m ¼ 406 K the supercooling is146 K), which exhibit double melting in accordance with themelting-recrystallization-final melting scheme.

The second isothermal crystallization temperature of 350 K wasselected due to the fact that it was close to themelting temperature.This time, only one melting peak can be observed (at about 375 K,that is still lower than the apparent melting point of 388 K in stan-dardDSC traces), originating from themeltingof the crystals of PBSugrown at 350 K. Only the sample filledwith Ag presented additionalcold crystallization andmelting peaks at lower temperatures of 270and 300 K, respectively. In the specific sample, for annealing timeslonger than 10 s, immediately after the glass transition, cold crys-tallization is initiated, thus producing a number of crystals, whichstart melting after the end of cold crystallization. The effect wasreproducible for several different fresh samples. The reason for thisparticular behavior of the Ag nanocomposite is not yet known.

The PBSu matrix and the nanocomposite samples were alsoannealed at different temperatures in order to observe the effect ofthe annealing time and temperature on the total latent heat of thesample. The results can be seen in Fig. 5, where the total latent heatvalues can be used as a measure of overall crystallization. It can beseen that crystallinity of the pure material does not present sig-nificant differences (within the mass determination error of ±10%)with the crystallinity of the nanocomposite samples, at all crys-tallization temperatures.

4. Discussion

4.1. Nucleation efficiency

Because of the filler nucleation activity, the nanocompositesshowed faster overall crystallization rates. However, fast crystalli-zation always results in formation of poor crystals. In Fig. 5 the

Fig. 5. Total specific enthalpy change of PBSu and nanocomposites as a function of the annthat annealing temperature.

values of total latent heat represent the crystallized material afterthe annealing procedure at each temperature. The development ofDh allows the observation of isothermal crystallization kinetics atTa. The temperature for the fastest crystallization is 310 K for allsamples.

The results from the integration of the experimental data werefitted according to the formalism proposed by Kolmogorov-Johnson-Mehl-Avami (KJMA) [22] (Fig. 6):

Dh ¼ DhC∞

�1� exp

�� tt½

ln 2�n�

þ A2ðln t � ln t½Þ

��12

�jt � t½jt � t½

þ 1��

(1)

where DhC∞ is the final enthalpy of primary crystallization atinfinite time, t is the annealing time, t1/2 is the crystallization halftime, n is the Avrami coefficient of crystallization and A2 is a sec-ondary crystallization parameter. The latter part of secondarycrystallization term is used to make it zero before primary crys-tallization halftime.

Fig. 6 depicts the results from Fig. 5 and the KJMA formalism.Generally it can be concluded that the DhC∞ values of the samplesincrease with increasing crystallization temperature but the fillersdo not influence these values significantly (Fig. 6a). The crystalli-zation half time (t1/2) of PBSu and the nanocomposite samples isplotted against crystallization temperature in Fig. 6b. For crystalli-zation temperatures up to 280 K, the crystallization half times of allsamples, nucleated or not, are similar. This is an indication thathomogeneous nucleation is the dominant nucleationmechanism atthat (low) temperature range and the presence of MWCNTs or Agnanoparticles does not speed up crystallization. The maximum rateof crystallization can be seen at 310 K where the PBSu/Ag samplepresents the fastest crystallization rate. The MWCNTs speed upcrystallization only slightly in the temperature range of the mini-mum t1/2. This changes at higher crystallization temperatures,where the PBSu/MWCNTs and the PBSu/Ag samples both crystallizemore than an order of magnitude faster than the pure PBSu. Asimilar effect was seen in previous DSC crystallization experiments[21], but there, at even higher crystallization temperatures, thePBSu/Ag sample always crystallizes slightly faster than the PBSu/MWCNT sample. Nevertheless, the similarity between the crystal-lization kinetics of both nanocomposites at high temperatures is anindication that there is an increasing nucleating efficiency of theMWCNTs with increasing crystallization temperature above 330 Kfor the particular nanocomposite sample. Additionally, theappearance of cold crystallization for the PBSu/Ag sample forcrystallization temperatures higher than 350 K, Fig. 4, causes a

ealing time at different temperatures. Each point represents a single measurement for

Fig. 6. Avrami fit parameters for PBSu and nanocomposite samples after isothermal crystallization at several temperatures: (a) maximum melting enthalpy of crystallization DhC∞

(b) crystallization half-time t1/2. Dashed lines represent the general trend of the respective curves as guide to the eye.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e67346730

retardation of the overall crystallization process, since a time delaybetween nucleation and growth during annealing can be observed.This particular behavior has been observed in similar MWCNT-based nanocomposite systems [23,24].

4.2. Isothermal nucleation and crystallization

Since heating is so fast that no cold crystallization and corre-sponding melting is seen after fast enough cooling we are sure thatall changes in the total enthalpy that are visible in the heating scansfor long crystallization times, are attributed to the previous heattreatment. The cold crystallization phenomenon isn't obvious dur-ing this procedure and thus, the doublemelting peak is attributed tothemelting of crystals originating from the development during theannealing process and the second one from the reorganization andre-meltingof the samecrystals. This conclusion is in agreementwiththe findings of Qiu et al. [6] who also reported the multiple meltingbehavior of PBSu crystallized isothermally from the melt by usingDSC andwide-angle X-Ray diffraction (WAXD) to prove that there isonly one crystal population in PBSu after isothermal crystallization.In addition, increasing annealing time results in a broadened andshifted to higher temperatures glass transition region, along withthe shift of melting peaks, which is a consequence of the internalstabilization process of the isothermally formed crystals. Themelting peaks of the sample filled with silver nanoparticles, appearat even lower temperatures, indicating lower stability of the crystalsin the sample, since the large number of nuclei is expected to resultin a large number of small and imperfect crystals.

4.2.1. Dependence of recrystallization on heating rateThe effect of heating rate on the recrystallization was investi-

gated in case of the samples isothermally crystallized at 260 K.Fig. 7aec shows the scans recorded at 1,000, 5000 and 10,000 K/s,after isothermal crystallization at 260 K, for neat PBSu and thenanocomposite with MWCNTs, respectively. For both materials, theenthalpy of fusion of the low temperature melting peak increasedwith increasing the heating rate, while the higher temperaturepeak shifts simultaneously to lower temperatures. A recrystalliza-tion peak with decreasing enthalpy appeared between the meltingpeaks, with increasing heating rate. These facts evidenced theenhanced recrystallization rates of the three samples, after crys-tallization at low temperature. PBSu is a very fast crystallizingpolyester and rates as high as 10,000 K/s cannot prevent recrys-tallization. However, it can be seen that the increase of the heatingrate lowers the time that is available for recrystallization. At10,000 K/s recrystallization (difference between the onsets of bothmelting peaks) needs only about 3 ms. In contrast, isothermal

crystallization in the same temperature range around 300 K has ahalf time of about 500ms, Fig. 6c, which is two orders of magnitudelonger. The extensive recrystallization that took place during slowheating resulted in the formation of a sharp melting peak, whichcan be attributed to the melting of crystals with higher perfection,compared to the ones that were formed very fast, when the samplewas heated with 10,000 K/s. These results are in accordance withother polyesters studiedwith fast scanning calorimetry, such as PET[17] and PBT [25].

4.2.2. Identification of different nuclei populations on PBSu/Agsample

The behavior of the nanocomposite with Ag after isothermalcrystallization at 350 K was also investigated by the application ofdifferent heating rates. The results are presented in Fig. 8 wherethree different heating rates, namely 1,000, 5000 and 10,000 K/swere selected.

In contrast to Fig. 7c, after annealing at 350 K, the PBSu/Agsample presents an endothermic peak at temperatures near 300 K,a phenomenon that was already noticed during non-isothermalcrystallization in the same temperature range. Since in the PBSu/Ag nanocomposite crystallization is faster under most crystalliza-tion conditions than in the other materials of this work, metastablestructures are expected to form. Thus, melting at lower tempera-tures for this nanocomposite is due to the poor crystals formed atcold crystallization during the fast heating scan. Furthermore, itwas proved that Ag nanoparticles show higher nucleation effi-ciency at higher rates or lower isothermal crystallization temper-atures, or maybe finer dispersion of the nanoparticles was achievedfor the specific sample, a fact which resulted in a large number ofnuclei. These nuclei are also responsible for the immediate andmore extensive cold crystallization compared to the other twomaterials. In case of MWCNTs it is known that there is a trend foraggregation due to the strong Van derWaal's forces, their small sizeand large surface area, thus nucleation sites are probably fewer inthe respective nanocomposite. The specific conclusion can bestrengthened if we take into account the TEM images for the spe-cific nanocomposite sample, published in an earlier work [21].

Fig. 8 also shows that the decrease of the heating rate induces anincrease of the cold crystallization, as was expected, since thecrystals have more time to develop. The small endothermic peak,which represents the melting of these crystals stays unaffectedfrom the initial decrease of the heating rate from 10,000 K/s to5000 K/s, but moves to lower temperatures when the heating ratewas 1000 K/s, due to the fact that the cold crystallization phe-nomenon starts at significantly lower temperatures. This firstmelting peak increased in heat of fusion with decreasing heating

Fig. 7. Apparent heat capacity of PBSu (a), PBSu/MWCNT (b) and PBSu/Ag (c) nano-composites from DFSC on heating with different rates after annealing at 260 K. Thetimes of annealing are given in the legend.

Fig. 8. Effect of the heating rate on PBSu/Ag nanocomposite samples, after annealing at350 K with increasing annealing time.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e6734 6731

rate as a result of themore extensive cold crystallizationwhich gavelarger crystallinity. Furthermore, with increasing heating rate, thepeak temperature increased and the peak became broader probablydue to superheating. This observation needs further investigationregarding the crystalline structure of the specific samples under theapplied conditions. Therefore, an in-situ synchrotron X-ray scat-tering or an AFM study may provide important informationregarding the morphology of these specific crystals.

4.3. Enthalpy relaxation

Annealing at different temperatures below the glass transition ofPBSu can provide information regarding the enthalpy relaxation ofthe amorphous polymer. Enthalpy relaxation is the process bywhichequilibrium is reached for the amorphous sample and the drivingforce for the phenomenon is the difference between the enthalpy ofthe glass and the (extrapolated) enthalpy of the equilibrium liquid atthat temperature. The polymeric matrix, PBSu was annealed at fivedifferent temperatures for time periods ranging from 1 to 1000 s(Fig. 9). After annealing for 10 s, the enthalpy recovery peak appearsfor the neat polymer (blue (in the web version) curve) at the lowestannealing temperature (200 K), which is located at the lower end ofthe glass transition.With increasingannealing temperature, the timeneeded for the appearance of the enthalpy recovery peak wassignificantly lower (1 s). In the low annealing temperature range,when the annealing temperature increased, the enthalpy relaxationpeak presented a shoulder. At higher temperatures, the main peak,practically exhibited a split, representing two different events; theenthalpy relaxation and the melting of small crystals formed duringthe annealing process. It can be seen that the formation of thosecrystals depends highly on the annealing time, since the two peaksare well-separated at the highest annealing times. The longer asample stays at a specific temperature, beside the increasingenthalpy relaxation, the number of crystals that are formed is greaterand higher perfection of those crystals can be achieved.

Furthermore, when the annealing temperature is closer to theglass transition, after the enthalpy relaxation peak, cold crystalli-zation and then melting occur, as it can be seen at a small per-centage for Ta ¼ 220 K and more obviously for Ta ¼ 225 K. This is aproof that during the annealing of the amorphous phase, a few veryunstable crystals were also formed.

More detailed analysis of the peaks is shown in Fig. 10. Fig. 10ashows the heat capacity, after annealing at 225 K for different time,with subtracted non-annealed curve (‘0 s’ black curve, see inser-tion). The result of integration is shown in Fig. 10b. For integrationthe first undershoot, corresponding to the glass transition shift tohigher temperatures, was neglected. This is needed since in case ofcrystallization the glass transition shifts significantly to highertemperatures and this is then not related to enthalpy relaxation.The black horizontal line in Fig. 10b shows the maximum possibleat this temperature enthalpy relaxation peak area, which can beapproximated by the equation Dhmax ¼ Dcp(Tg)$(Ta � Tg). The ver-tical green (in the web version) line designates the beginning of

Fig. 9. Heating at 10,000 K/s after annealing at different temperatures and different times for neat PBSu. The right part of the figure represents the magnified image of the annealingpeak enthalpy.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e67346732

crystallization on annealing, which can be traced by the meltingpeak at 265 K and 330 K.

The crystallization followed by melting appears only after theenthalpy relaxation reaches its limiting value. The very tiny andunstable crystals formed on annealing, melt at 265 K and recrys-tallize to more stable crystals melting at 330 K. These observationsare in agreement with previous studies on PCL [23] and PLLA [26]. Amore detailed discussion is available from Ref. [23].

5. Conclusions

The nanocomposite samples based on poly(butylene succinate)filled withmulti-walled carbon nanotubes and silver nanoparticles,prepared by the solvent evaporation method, both show remark-able nucleating activity of the nanoparticles. Silver nanoparticlesshow the most enhanced crystallization kinetics. This is confirmedby conventional techniques such as POM and DSC as well as fastscanning calorimetry. Differential fast scanning calorimetry wasemployed for the study of crystallization and melting of PBSu andits nanocomposites for the whole temperature range where crys-tallization occurs (from below Tg to close toTm) covering times from100 ms to 1000 s. From this extended set of experimental data newinsight into the behavior of the nanocomposites was obtained.Before discussing the unexpected new results we list results, whichnicely confirm previous observations with other polymers andnanocomposites.

� The critical cooling rate for avoiding crystallization was deter-mined as 70, 300, and 500 K/s for PBSu, PBSu/MWCNTand PBSu/Ag, respectively, Fig. 3.

� The presence of a small amount of well dispersed nanoparticles(2.5 wt.%) does not alter the limiting degree of crystallization,Figs. 3 and 5.

� Below 280 K homogeneous nucleation overturns nucleation bythe nanoparticles, Fig. 6c.

� At annealing below Tg first enthalpy relaxation, reaching thevalue for the extrapolated supercooled liquid state, occurs andafter that nucleation and formation of very unstable crystals isobserved, Fig. 10b.

� PBSu and its nanocomposites show a very fast recrystallizationkinetics on a ms time scale while isothermal crystallization atcomparable temperatures is about two orders of magnitudeslower, Figs. 7 and 6c.

Even though the recrystallization kinetics and its relation toisothermal crystallization is very similar to previous reports [27],the recrystallization behavior of PBSu and its nanocomposites isvery unique. For DSC experiments and also for fast scanningexperiments it is a common feature that the final melting peak(melting of the recrystallized material, highest melting temper-ature) is essentially independent on the previous thermal treat-ment as long as recrystallization occurs and depends only onheating rate [17]. For PBSu the size of the high temperaturemelting peak of the recrystallized material basically resemblesthe size of the first melting peak, Figs. 2, 4, 7, 9 and 10a. As usual,the size increases with decreasing heating rate since more timefor recrystallization is available. But even a large number ofnuclei, heterogeneous or homogeneous, present in the samplesdoes not allow crystallization everywhere. Recrystallizationessentially proceeds only in the region previously occupied bythe originally formed crystals. This observation supports the

Fig. 10. Enthalpy relaxation and crystallization interplay during annealing at different temperatures below Tg. Measured curves after subtraction of the curve without annealing (0 s)e (a), and results of integration for different temperatures e (b). (see text for details).

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e6734 6733

opinion that the fast recrystallization kinetics and nucleation ofrecrystallization is governed by the remaining order in the meltafter the melting of unstable crystals. The case highlighted inFig. 10a is of particular interest. The tiny crystals formed atannealing below Tg, melt at 265 K, where overall crystallizationfrom the melt is already one order of magnitude slower than at320 K, which is the temperature where the recrystallized crystalsmelt. The fact that no further recrystallization occurs at thistemperature, as seen for other conditions as an exotherm inFig. 7, indicates a particular state of the melt and requires furtherinvestigations.

Acknowledgments

The authors wish to acknowledge co-funding of this research byIKY (Greece) and DAAD (Germany), Action ‘‘IKYDA 2014’’. E.Z.

acknowledges support from a European Union funded Marie CurieEST fellowship (MEST-CT-2005-020986) (ADVATEC).

Appendix A. Supplementary data

Supplementary data related to this article can be found online athttp://dx.doi.org/10.1016/j.polymer.2014.11.014.

References

[1] Ihn KJ, Yoo ES, Im SS. Structure and morphology of poly(tetramethylenesuccinate) crystals. Macromolecules 1995;28:2460e4.

[2] Ichikawa Y, Kondo H, Igarashi Y, Noguchi K, Okuyama K, Washiyama J. Crystalstructures of a and b forms of poly(tetramethylene succinate). Polymer2000;41:4719e27.

[3] Ichikawa Y, Suzuki J, Washiyama J, Moteki Y, Noguchi K, Okuyama K. Strain-induced crystal modification in poly(tetramethylene succinate). Polymer1994;35:3338e9.

D.G. Papageorgiou et al. / Polymer 55 (2014) 6725e67346734

[4] Miyata T, Masuko T. Crystallization behaviour of poly(tetramethylene succi-nate). Polymer 1998;39:1399e404.

[5] Papageorgiou GZ, Bikiaris DN. Crystallization and melting behavior of threebiodegradable poly(alkylene succinates). A comparative study. Polymer2005;46:12081e92.

[6] Qiu Z, Komura M, Ikehara T, Nishi T. DSC and TMDSC study of meltingbehaviour of poly(butylene succinate) and poly(ethylene succinate). Polymer2003;44:7781e5.

[7] Wang X, Zhou J, Li L. Multiple melting behavior of poly(butylene succinate).Eur Polym J 2007;43:3163e70.

[8] Papageorgiou GZ, Papageorgiou DG, Chrissafis K, Bikiaris D, Will J, Hoppe A,et al. Crystallization and melting behavior of poly(butylene succinate) nano-composites containing silica-nanotubes and strontium hydroxyapatite nano-rods. Ind Eng Chem Res 2013;53:678e92.

[9] Xu J, Guo B-H. Poly(butylene succinate) and its copolymers: research, devel-opment and industrialization. Biotechnol J 2010;5:1149e63.

[10] Fujimaki T. Processability and properties of aliphatic polyesters, ‘BIONOLLE’,synthesized by polycondensation reaction. Polym Degrad Stab 1998;59:209e14.

[11] Gibbs JW. Collected works. New York: Dover; 1961.[12] Lauritzen JI, Hoffman JD. Theory of formation of polymer crystals with folded

chains in dilute solution. J Res Nat Bur Stand A Phys Ch 1960;64:73e102.[13] Wunderlich B, Mehta A. Macromolecular nucleation. J Polym Sci Polym Phys

Ed 1974;12:255e63.[14] Strobl G. Crystallization and melting of bulk polymers: new observations,

conclusions and a thermodynamic scheme. Prog Polym Sci (Oxford) 2006;31:398e442.

[15] Zhuravlev E, Schick C. Fast scanning power compensated differential scanningnano-calorimeter: 2. Heat capacity analysis. Thermochim Acta 2010;505:14e21.

[16] Zhuravlev E, Schick C. Fast scanning power compensated differential scanningnano-calorimeter: 1. The device. Thermochim Acta 2010;505:1e13.

[17] Minakov AA, Mordvintsev DA, Schick C. Melting and reorganization ofpoly(ethylene terephthalate) on fast heating (1,000 K/s). Polymer 2004;45:3755e63.

[18] Kolesov I, Mileva D, Androsch R, Schick C. Structure formation of polyamide 6from the glassy state by fast scanning chip calorimetry. Polymer 2011;52:5156e65.

[19] Mollova A, Androsch R, Mileva D, Gahleitner M, Funari SS. Crystallization ofisotactic polypropylene containing beta-phase nucleating agent at rapidcooling. Eur Polym J 2013;49:1057e65.

[20] Mileva D, Androsch R, Zhuravlev E, Schick C. Critical rate of cooling for sup-pression of crystallization in random copolymers of propylene with ethyleneand 1-butene. Thermochim Acta 2009;492:67e72.

[21] Papageorgiou DG, Chrissafis K, Pavlidou E, Deliyanni EA, Papageorgiou GZ,Terzopoulou Z, et al. Effect of nanofiller's size and shape on the solid statemicrostructure and thermal properties of poly (butylene succinate) nano-composites. Thermochim Acta 2014;590:181e90.

[22] Avrami M. Kinetics of phase change. II Transformation-time relations forrandom distribution of nuclei. J Chem Phys 1940;8:212e24.

[23] Zhuravlev E, Schmelzer JWP, Wunderlich B, Schick C. Kinetics of nucleationand crystallization in poly (epsilon caprolactone) (PCL). Polymer 2011;52:1983e97.

[24] Zhuravlev E, Wurm A, P€otschke P, Androsch R, Schmelzer JWP, Schick C. Ki-netics of nucleation and crystallization of poly( 3-caprolactone) e multiwalledcarbon nanotube composites. Eur Polym J 2014;52:1e11.

[25] Kim HG, Robertson RE. A new approach for estimating the recrystallizationrate and equilibrium melting temperature. J Polym Sci Part B Polym Phys1998;36:133e41.

[26] Androsch R, Di Lorenzo ML. Kinetics of crystal nucleation of poly(L-lactic acid).Polymer 2013;54:6882e5.

[27] Minakov AA, Mordvintsev DA, Schick C. Isothermal reorganization of poly(-ethylene terephthalate) revealed by fast calorimetry (1000 K s�1; 5 ms).Faraday Discuss 2005;128:261e70.