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Crystallization and Melting Behavior of Poly(Butylene Succinate) Nanocomposites Containing Silica-Nanotubes and Strontium Hydroxyapatite Nanorods George Z. Papageorgiou,† Dimitrios G. Papageorgiou,‡ Konstantinos Chrissafis,‡ Dimitrios Bikiaris,*,† Julia Will,§ Alexander Hoppe,§ Judith A. Roether,⊥ and Aldo R. Boccaccini§ †

Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, 541 24, Thessaloniki, Macedonia, Greece ‡ Solid State Physics Section, Physics Department, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece § Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany ⊥ Institute of Polymer Materials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martenstrasse 7, 91058 Erlangen, Germany ABSTRACT: The multiple melting behavior of poly(butylene succinate) (PBSu) nanocomposites containing silica nanotubes (SiNTs) and strontium hydroxyapatite nanorods (SrHNRs) was studied with Step Scan DSC. In the reversing signal curves, recrystallization proved to be significant for samples crystallized at low temperature, which lead to large supercooling, favoring fast crystallization and formation of poor crystals that suffer recrystallization on subsequent heating scans. The crystallization study, under both isothermal and nonisothermal conditions, evidenced that only SiNTs act effectively as nucleating agents. Consequently, crystallization of the nanocomposite samples filled with 5 and mainly with 20 wt % SiNTs occurred faster than that of the neat polymeric matrix. The insertion of SrHNRs slightly lowered the crystallization rate of PBSu due to the formation of larger aggregates inside the PBSu matrix, which may inhibit crystallization. The size of crystals was smaller for PBSu/SiNTs samples, as it was proved by wide angle X-ray diffracrion (WAXD), whereas the addition of SrHNRs did not significantly affect the crystalline size of nanocomposites.

1. INTRODUCTION The widespread use of conventional polymers over the last decades has brought up serious environmental problems. Therefore, synthetic aliphatic polyesters have attracted great interest from industry and academia because they can be easily degraded in the environment.1,2 One of the most promising materials in the category of biodegradable polyesters is poly(butylene succinate) (PBSu) due to its relatively low cost and large production volume. PBSu has already been used in various commercial applications like packaging materials, films and fibers due to its superior biodegradability, and thermal and mechanical properties.3,4 In addition, PBSu has been used for more advanced applications such as drug delivery systems and tissue engineering.5−8 Those applications have not stayed unaffected by the continuous growth of polymer nanocomposites and a significant number of biodegradable polyester-based nanocomposites has been tested for possible use as drug delivery systems or as a part of tissue engineering methods. Fillers of different geometries and shapes have been tested for the improvement of the final properties of polymer nanocomposites and nanotube-shaped additives have been proved to induce significant enhancements.9,10 Inorganic nanotubes have been proved an interesting alternative to conventional carbon nanotubes, exhibiting advantages such as easy synthetic access and satisfactory dispersion and adhesion, properties which greatly affect the final materials.11 For this © XXXX American Chemical Society

reason, silica nanotubes (SiNTs) and strontium hydroxyapatite nanorods (SrHNRs) were developed and inserted into a PBSu matrix to observe their effect on the PBSu properties. These nanoparticles were tested for first time in an upcoming publication from our group as appropriate materials for biomedical applications like scaffolds. It was found that SiO2 nanotubes support cell attachment, whereas Sr5(PO4)3OH nanorods decrease cell activity, maybe due to its high release concentrations during hydrolysis. Crystallization and melting of various biodegradable polyesters has been presented in numerous publications.12−17 In our previous study, it was found that the biodegradability of aliphatic polyesters is affected by the polymer crystallinity and, specifically, PBSu exhibited low biodegradation rates compared to other familiar polyesters like poly(propylene succinate) because of its higher crystallinity.18 Furthermore, it is wellknown that the addition of nanofillers, such as those used in the present study, can change the crystallinity of aliphatic polyesters and its hydrolysis rate.19,20 Therefore, it is very important to study the crystalline content of the prepared PBSu nanocomposites and understand the parameters that affect the polymer crystallization for the Received: September 30, 2013 Revised: December 3, 2013 Accepted: December 11, 2013

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dx.doi.org/10.1021/ie403238u | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2.1.2. Preparation of Strontium Hydroxyapatite Nanorods (SrHNRs). In a typical procedure for the preparation of luminescent Sr5(PO4)3OH nanorods (SrHNRs), 3 mmol of Sr(NO3)2, 0.5 g of CTAB, and 10 mL of ammonia solution (NH3·H2O) (used for adjusting the pH value as alkaline solution) were dissolved in 30 mL of deionized water to form solution A. Then, 6 mmol of trisodium citrate (labeled as Cit3−, the molar ratio of Cit3−/Sr2− is 2:1) and 2 mmol of (NH4)2HPO4 were added into 20 mL H2O to form solution B. After solution B was vigorously stirred for 30 min, it was introduced into solution A (dropwise). After additional agitation for 20 min, the as-obtained mixed solution was transferred into a Teflon bottle (80 mL) held in a stainless steel autoclave, sealed, and maintained at 180 °C for 24 h. As the autoclave cooled to room temperature naturally, the precipitate was separated by centrifugation and was washed with deionized water and ethanol in sequence. Then, the obtained product was redispersed in 150 mL of acetone and refluxed at 80 °C for 48 h to remove the residual template CTAB. Finally, the precipitate was again separated by centrifugation and dried in vacuum at 70 °C for 24 h to obtain the final sample.24 According to this procedure, the synthesized nanorods, as was proved by TEM analysis, have a diameter of approximately 20 nm and their length is about 120−150 nm. 2.1.3. Preparation of SiO2 Nanotubes (SiNTs). Synthesis of SiO2 nanotubes was achieved by a sol−gel technique using tetraethoxysilane (TEOS) and a surfactant (hydrochloric ndodecylamine) as a template. The necessary quantity of TEOS was dissolved in heptane (C7H16) and added carefully and slowly in an aqueous solution of LAHC (hydrochloric ndodecyl amine, 0.1 M, pH = 4.5) to not disturb the membrane between organic and aqueous phases. The molar ratio of [TEOS]/[LAHC] was 4, same as the ratio of [H2O]/[C7H16]. The system was left for 7 days, and afterward, the aqueous phase was collected and the product filtered and washed with deionized water. After it was dried at 80 °C for 6 h, it was calcinated at 450 °C for 6 h to receive the inorganic part.25,26 According to the described procedure, silica nanotubes with a length of 5 μm and a thickness of 150 nm are prepared. 2.1.4. Preparation of Nanocomposites. Nanocomposites containing 5 and 20 wt % of nanofillers were prepared by melt mixing in a Haake-Buchler Reomixer (model 600, New Jersey, USA) with roller blades. SiNTs and SrHNRs were used as nanofillers to produce composite materials with PBSu as polymer matrix. The sample codes that will be used throughout the paper will be the following: PBSu, PBSu/SiNTs 5 wt %, PBSu/SiNTs 20 wt %, PBSu/SrHNRs 5 wt %, and PBSu/ SrHNRs 20 wt %. Prior to melt-mixing, the nanofillers were dried by heating in a vacuum oven at 130 °C for 24 h. The two components were physically premixed before being fed in the reomixer, to achieve a better dispersion. Melt blending was performed at 130 °C with 30 rpm for 5 min. During the mixing period, the melt temperature and torque were continuously recorded. Each nanocomposite after preparation was milled and placed in a desiccator to prevent any moisture absorption. 2.2. Characterization of Prepared Materials. 2.2.1. Wide-Angle X-ray Diffraction (WAXD). WAXD studies of nanocomposites, in the form of thin films, were performed over the range 2θ from 5 to 80°, at steps of 0.05° and a counting time of 5 s, using a MiniFlex II XRD system from Rigaku Co. 2.2.2. Differential Scanning Calorimetry (DSC). A PerkinElmer Pyris Diamond DSC differential scanning calorimeter,

optimization of the processing conditions and the properties of the final product. Also, the mechanism of the multiple melting behavior of PBSu should be assessed to evaluate the crystalline perfection and thickness under different testing conditions. The possible origin of the multiple melting endotherms may be listed as follows: (a) melting-recrystallization-remelting procedure during DSC heating scans, (b) the presence of more than one crystalline type in the crystal structure of the main polymeric material (polymorphism), (c) morphology variations such as lamellar thickness, distribution, perfection or stability, (d) physical aging or/and relaxation of the rigid amorphous fractions, and (e) different molecular weights.21−23 In the present manuscript, a detailed analysis of the crystallization and melting behavior of PBSu/SiNTs and PBSu/SrHNRs nanocomposites under isothermal and dynamic conditions has been performed for first time. Temperature modulated differential scanning calorimetry (TMDSC) was employed to comprehend the melting behavior and separate the simultaneously occurring events during the melting of the samples. The effect of the inorganic nanofillers on the nanocomposite samples’ performance was also evaluated. Several macroscopic models were studied to test the validity of the equations that are usually elaborated to describe polymer crystallization. The nucleation activity of the fillers and the effective activation energy were also determined.

2. EXPERIMENTAL SECTION 2.1. Materials. For the preparation of PBSu succinic acid (SA, purum 99%), 1,4-butanediol (purum 99%) and tetrabutoxytitanium [Ti(OBu)4], as a catalyst (analytical grade), were used. For the preparation of strontium hydroxyapatite nanorods [Sr5(PO4)3OH], strontium nitrate (Sr(NO3)2), trisodium citrate (labeled as Cit3−), (NH4)2HPO4, cetyl-trimethyl-ammonium bromide (CTAB), as a surfactant (analytical grade), were used. For the preparation of SiO2 nanotubes, tetraethoxy silicon oxide (TEOS) and a hydrochloric n-dodecylamine surfactant as a template were used. All these materials were purchased from Aldrich Chemical Co. All other materials and solvents that were used for the analytical methods were of analytical grade. 2.1.1. Preparation of Poly(butylene Succinate). PBSu was prepared by the two-stage melt polycondensation method (esterification and polycondensation) in a glass batch reactor. The proper amount of succinic acid and 1,4-butanediol in an 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 the polyesterification apparatus. The apparatus with the reagents was evacuated several times and filled with argon to completely remove oxygen. The reaction mixture was heated at 190 °C under an argon atmosphere and constant speed stirring (350 rpm). This first step (esterification) was considered to be completed after the collection of theoretical amount of H2O (about 3 h), which was removed from the reaction mixture by distillation and collected in a graduated cylinder. In the second step of polycondensation, a vacuum (5.0 Pa) was applied slowly over a period of 15 min to avoid excessive foaming and minimize oligomer sublimation, a potential problem during melt polycondensation. The temperature was slowly increased to 220 °C while the stirring speed was increased to 720 rpm. The polycondensation continued for about 60 min at 220 °C, and after that time, the temperature was increased to 240 °C, and the reaction was continuous for 60 min. B



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equipped with a Perkin-Elmer Intracooler II and calibrated with high purity indium and zinc standards was used for DSC measurements. For nonisothermal crystallizations from the melt, low mass samples (about 5 mg) were first melted at 140 °C for 1 min and then cooled to −30 °C at various cooling rates, namely 2.5, 5, 7.5, 10, 15, and 20 °C/min. For isothermal crystallizations from the melt, the samples were first melted at 140 °C for 1 min and then cooled to the crystallization temperature at a rate of 200 °C/min. The samples then crystallized isothermally till the end of the phenomenon. The end was detected from leveling off. Subsequently, heating scans at 20 °C/min were performed. Modulated-temperature DSC experiments were also carried out using the same Pyris Diamond DSC and the Perkin-Elmer Step Scan software. The scans involved heating steps of 1 °C performed at a heating rate of 5 °C/min between isothermal steps of 1 min. 2.2.3. Scanning Electron Microscopy (SEM). SEM studies of nanocomposites surface were carried out using an ESEM scanning microscope (Quanta 200, FEI, The Netherlands)) at an operating voltage of 20 kV. For SEM measurements, thin films were used, which were sputtered with gold prior to examination.

3. RESULTS AND DISCUSSION 3.1. Melting Behavior of Nanocomposites. 3.1.1. Standard DSC. The thermal behavior of the materials was studied with DSC. The as-received PBSu samples showed a melting peak at 114.8 °C, with a recrystallization exotherm before it and also a shoulder at the low temperature side. The heat of fusion was about 68 J/g, which corresponds to a degree of crystallinity of 33%. The quenched samples showed a glass transition temperature Tg = −37 °C, which was followed by a coldcrystallization peak at about 0 °C and the final melting at 113 °C. In general, a similar picture with small variations was also observed for the composites of this work. To estimate the effect of nanofillers on thermal behavior of PBSu, all nanocomposites were studied with DSC under various conditions. For this reason, initially, the samples were isothermally crystallized and then heated with a heating rate of 20 °C/min. Because the differences in the peak forms between the DSC thermogramms of the nanocomposites are very small, due to space limitations, only the melting behavior of neat PBSu and its nanocomposites, PBSu/SiNTs containing 20 wt % SiNTs will be presented, which can be seen in Figure 1. As can be seen the number of melting peaks depended on the crystallization temperature at this particular heating rate. For instance, for the PBSu sample crystallized at 85 °C, four peaks can be observed (Figure 1b). First, the small melting peak indicated by I can be found in all melting curves and its peak is almost 5 °C higher than the crystallization temperature. This peak has been observed in many semicrystalline polymers such as PET,27 isotactic polystyrene,28 syndiotactic polypropylene29 poly(ethylene naphthalate),30 poly(butylene naphthalate),31 and others and it is usually referred to as “annealing peak”. The annealing peak is always interpreted as the melting of secondary crystals formed during the isothermal crystallization stage, and its temperature and magnitude increase with increasing the melting temperature. Also, this peak is associated with devitrification of the rigid amorphous fraction of the polymer.32 The second endothermic peak, indicated by II in the graph, is observed for samples crystallized at Tcs > 80 °C and increases in peak temperature and heat of fusion with increasing

Figure 1. DSC traces of (a) as received and quenched PBSu and (b) neat PBSu and (c) PBSu/SiNTs 20 wt % crystallized at different temperatures. The heating rate was 20 °C/min.

Tc. The third endothermic peak (III) is present in all the DSC heating traces for samples crystallized at temperatures Tc > 80 °C and, like the second peak, it shifts to higher temperatures with increasing Tc. The peaks II and III are associated with melting of populations of original crystals of different stabilities. C



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Figure 2. Modulated DSC traces for PBSu/SiNTs 20 wt % isothermally crystallized at (a) 85, (b) 90, (c) 100, and (d) 105 °C.

or SrHNRs nanofillers were similar, which is an indication that the kind and the amount of nanofillers do not play an important role on the melting behavior of the samples if the samples are well crystallized. Thus, it should be concluded that the thermal treatment of the samples and mainly the crystallization temperature determine the melting behavior. 3.1.2. Step-Scan DSC. To verify our previous conclusions about the melting-crystallization-remelting phenomenon, which explain the melting behavior of PBSu and its nanocomposites, the Step-Scan (temperature modulated) DSC technique was employed. Step-Scan DSC introduces true isothermal steps between heating steps and its signal attributes different contributions during the heating procedure of semicrystalline polymers. Crystallization endotherms only contribute to the nonreversing signal, thus separation of exotherms from reversing melting or other heat capacity events is achieved. Unfortunately, exothermic and endothermic nonreversible events can occur simultaneously and they cannot be completely separated from each other.33 Thus, TMDSC techniques can separate crystallization and recrystallization exotherms, from glass transition, reversible melting or other heat capacity events. Typical Step-Scan DSC scans for PBSu/SiNTs 20 wt % sample crystallized at different temperatures can be seen in Figure 2. The conventional heat flow signal (total signal) has been analyzed into the reversing and nonreversing signals. As stated earlier, the nonreversing signal is kinetic in nature and can be attributed to nonreversible melting and crystallization on heating, a process known as recrystallization and can be observed in the samples under study. The exothermic peaks in

As a matter of fact, the exothermic recrystallization process, also occurring at its faster rates just at the temperature range of the melting peaks II and III, is responsible for the split of the melting peak of the original crystals to the peaks II and III. In fact, partial melting and recrystallization are two competitive phenomena and the DSC traces show the algebraic sum of the exothermic and endothermic events. In the case of samples crystallized at the low crystallization temperature range, which is when crystallization occurs under large supercoolings, imperfect crystals of low stability and low melting point are generated. After the melting of these crystals, the molten material recrystallizes to more perfect and larger crystals, which finally melt at higher temperatures, like those of peak IV. With increases in the crystallization temperature, isothermal crystallization becomes slower and better approximates the equilibrium. Thus, more perfect structures of increased melting temperatures are formed and only limited recrystallization upon the subsequent heating scan may occur. The ultimate peak, IV, does not shift to higher temperatures and it is almost constant at 114.9 °C. The fact that the peak temperature is stable is an indication that peak IV is associated with melting of crystals perfected upon recrystallization or reorganization in the solid state during heating. For the samples crystallized isothermally at temperatures higher than 95 °C, peak IV coincides with peak III and for those crystallized at Tcs above 100 °C, the left peak (III) coincides with peak II and a single melting peak can only be seen. The findings for PBSu/SiNTs 20 wt % (Figure 1c) and the other nanocomposites containing different amounts of SiNTs D



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Figure 2 confirm the recrystallization phenomenon that was not that obvious with the conventional heat flow signal. For samples crystallized at lower temperatures, the subsequent Step-Scan heating results are shown in Figure 2a,b where two recrystallization peaks are observed in the nonreversing spectra. In Figure 2c,d, the heating Step Scans for samples previously crystallized at high temperatures (100 or 105 °C) are exhibited. In this case and in the nonreversing signal curve, the second recrystallization exotherm, coincides with the first. Then there is one peak left in the nonreversing signal, appearing at the same temperature, where in the reversible signal a shoulder can also be seen in the melting peak. Normally, the second recrystallization peak in the nonreversing signal should correspond to the temperature where the end of melting of primary crystals appears in the reversing signal. Here however, it seems to occur in the shoulder region and not at the end of the melting peak. This is probably due to the fact that recrystallization and melting of the recrystallized species are so dominant that the shoulder region is distorted.28 A narrower recrystallization temperature region is also obvious in the nonreversing signal curve for such samples crystallized at a high temperature. This is because recrystallization is prevented by final melting. In the reversing signal curve, it is obvious that, for these samples, a shoulder appears before the melting peak. The major part of melting occurs at lower temperature. The high melting endotherm corresponds to the melting of the crystallites with high thermal stability formed through the recrystallization of the melt of the crystallites of the low melting endotherms (shoulder peaks).33 The behavior of neat PBSu and the other nanocomposites (data not shown) is similar to that discussed previously for the PBSu/SiNTs 20 wt % sample. This is further evidence that the kind and the amount of nanofillers do not play an important role on the melting behavior of the well crystallized samples. On the other hand, the crystallization temperature is crucial for the formed morphology and thus for the melting behavior. 3.2. Isothermal Crystallization. 3.2.1. Avrami Analysis. To further evaluate the effect of nanofillers, the isothermal crystallization kinetics of PBSu and nanocomposite samples was studied by heating at temperatures higher than the melting point and then cooling the melt rapidly to the crystallization temperature with DSC. The isothermal crystallization temperature range in this study was from 80 to 97.5 °C. The exothermal curves were recorded as a function of crystallization time, which kept becoming longer along with the broadening of the exothermic peaks with increasing Tc. The development of relative crystallinity with crystallization time for the studied samples under different temperatures was obtained because the assumption that the evolution of crystallinity is linearly proportional to the evolution of heat released during the crystallization was made:

Figure 3. Evolution of the relative crystallinity as a function of crystallization time for PBSu (a) and PBSu/SiNTs 5 wt % (b) at various temperatures. The geometrical points represent experimental data whereas the continuous line represents the results of the theoretical Avrami model.

seen that the typical sigmoidal curves were obtained and the crystallization time prolongs with increasing crystallization temperature for both samples, suggesting that the crystallization is slowed down at high Tc. The well-known Avrami method was employed to analyze the isothermal crystallization kinetics of the samples.34−38 This method assumes that the relative degree of crystallinity X(t) develops as a function of crystallization time t as follows: X(t ) = 1 − exp( −kt n) ⇒ X(t ) = 1 − exp[−(Kt )n ]

where X(t) is the relative crystallinity at time t, k is the crystallization rate constant depending on nucleation and growth rate, and n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals. Because the units of k are a function of n, eq 1 can be written in the composite Avrami form using K instead of k (where k = Kn). To take into consideration the whole range of data that correspond to the relative crystallinity of the studied sample, the nonlinear fitting procedure with eq 1 was employed, based on the Marquardt−Levenberg algorithm. The estimated values of n and K for all samples are summarized in Table 1. The theoretical lines that were produced are plotted in Figure 3 (as an example) and compared to the experimental data. As it can

t

X (t ) =

∫0 (dHc/dt )dt ∞

∫0 (dHc/dt )dt

(2)

(1)

where dHc denotes the measured enthalpy of crystallization during an infinitesimal time interval dt. The limits t and ∞ on the integrals are used to denote the elapsed time during the course of crystallization and at the end of the crystallization process, respectively. Figure 3 shows the development of relative crystallinity with crystallization time at different temperatures for PBSu and PBSu/SiNTs 5 wt %. It can be E



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Table 1. Results from the Avrami Analysis of Isothermal Crystallization for PBSu and Nanocomposites PBSu

PBSu/SiNTs 5 wt %

PBSu/SiNTs 20 wt %

PBSu/SrHNRs 5 wt %

PBSu/SrHNRs 20 wt %

Tc (oC)

n

K (min−1)

n

K (min−1)

n

K (min−1)

n

K (min−1)

n

K (min−1)

80 82.5 85 87.5 90 92.5 95 97.5

4.3 4.1 3.6 3.4 3.2 3 2.98 2.92

1 0.92 0.64 0.44 0.29 0.16 0.09 0.04

4.6 4.3 3.9 3.4 3.3 3.1 3.1 2.8

0.92 0.89 0.55 0.46 0.25 0.18 0.09 0.05

4.4 3.8 3.28 3.76 2.87 3.44 2.9 2.8

1.16 1.7 0.94 1.16 0.62 0.57 0.3 0.19

3.89 3.71 3.16 3.38 3 3.2 3.28 3.23

0.91 0.81 0.55 0.44 0.26 0.17 0.09 0.04

4.07 4.13 4.26 3.93 3.35 3.21 3.03 2.87

0.82 0.74 0.45 0.34 0.19 0.11 0.07 0.03

matrix. Some aggregates were formed only in the case of nanocomposites containing 20 wt % SiNTs. In addition, the fact that SiNTs are almost 3 orders of magnitude larger enhances nucleation and adds larger nucleating sites to the nanocomposite samples filled with SiNTs. In the case of PBSu/ SrHNRs nanocomposites, it can be seen that the dispersion of Sr5(PO4)3OH nanorods (appearing also in bright color) depends on its content (Figure 5c,d). In the nanocomposite with 5 wt % Sr5(PO4)3OH nanorods, dispersion seems to be very good, because only a few aggregates are present with sizes 250−1000 nm. However, for 20 wt % content, aggregates appear in a higher extent and exhibit larger sizes (1000−3000 nm). This is mainly due to weak interactions between nanoparticles and the polyester matrix. All the above indicate that isothermal crystallization is a nucleation-controlled process in the crystallization temperatures employed. The kind of nanofiller used and its amount, which reflects the dispersion of the nanofiller into PBSu matrix, affects the nucleation and crystallization behavior of nanocomposites. The Avrami parameters can be used to calculate the activation energy of the crystallization of the samples under isothermal conditions. The Avrami parameter K can be described by an Arrhenius relationship as follows:

be seen, the correlation between the theoretical and experimental values is quite high (R2 > 0.995). Some deviations appear for data which correspond to very low (95%) degrees of crystallinity. The n values that were found in the case of PBSu and nanocomposites were in the vicinity of 3 for most crystallization temperatures and they should possibly be related with threedimensional growth. Also, both n and K parameters decrease with increasing crystallization temperature. Furthermore, the half time of crystallization t1/2, which represents the time needed for a particular sample to achieve 50% of the total crystallinity that the material is capable of developing during the isothermal crystallization process, was determined. The variation of reciprocal t1/2 values with Tc for PBSu and its nanocomposites are presented in Figure 4.

K1/ n = K 0exp( −ΔE/RTc)

(3)

where K0 is a temperature-dependent pre-exponential factor, ΔE is the activation energy, R is the gas constant, and Tc is the crystallization temperature. ΔE/R is determined by the linear regression of the experimental data of K1/n against 1/T, as plotted in Figure 6. The crystallization activation energies of the nanocomposites were calculated: −165.5, −138.5, −93.7, −166.4, and −178.6 kJ/mol for PBSu, PBSu/SiNTs 5 wt %, PBSu/SiNTs 20 wt %, PBSu/SrHNRs 5 wt %, and PBSu/ SrHNRs 20 wt %, respectively. The negative values of the activation energies can be justified if the fact that the samples have to release energy when the molten fluid transformed into the crystalline state is taken into account. These values clearly indicate that SiNTs can act as nucleating agents enhancing the crystallization of PBSu whereas SrHNRs have a slightly negative effect, as was already found from t1/2 and described previously. 3.2.2. Application of Secondary Nucleation Theory. Several nucleation and growth models have been proposed to analyze isothermal crystallization kinetics. One of the most popular is the Lauritzen−Hoffman model, which has been under criticism from many reports.39−41 This is partly due to the fact that this theory cannot explain certain morphological observations on the crystals grown at low supercoolings, and the physical significance of some parameters of the theory cannot be justified. However, the theory is still valid for many cases, because its analytical expression (eq 4) can describe

Figure 4. Reciprocal half-time of crystallization for PBSu and nanocomposites.

The decrease of 1/t1/2 values with the increase of Tc indicates that the overall crystallization rate decreases with the increase of Tc. What is interesting is that in the samples filled with SrHNRs, the filler does seem to negatively affect the crystallization rate of the neat polymeric matrix, because the process is delayed due to the nanorods. On the other hand, the presence of SiNTs evokes faster crystallization for the nanocomposite samples. This is because, as was found from SEM analysis (Figure 5), SrHNRs has poor dispersion in the PBSu matrix and creates a lot of aggregates. In the case of PBSu/SiNTs nanocomposites (Figure 5a,b) the nanotubes (recorded in bright color) are well dispersed in the polyester F



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Figure 5. SEM micrographs of (a) PBSu/SiNTs 5 wt %, (b) PBSu/SiNTs 20 wt %, (c) PBSu/SrHNRs 5 wt %, and (d) PBSu/SrHNRs 20 wt %.

⎡ ⎤ ⎡ Kg ⎤ U* G = G0exp⎢ − ⎥exp⎢ − ⎥ ⎣ R(Tc − T∞) ⎦ ⎣ Tc(ΔT )f ⎦

(4)

where G0 is the pre-exponential factor and the first exponential term contains the contribution of diffusion process to the growth rate and U* is the activation energy for the transport of the chains to the growing front (a value of 1500 cal/mol is usually employed). The second exponential term is the contribution of the nucleation process, and Kg is the activation energy for nucleation for a crystal with critical size and strongly depends on the degree of supercooling. Furthermore, U* is the diffusional activation energy for the transport of crystallizable segments at the liquid−solid interface and it is usually set to 1500 cal/mol, R is the universal gas constant, T∞ is the temperature below which diffusion stops, usually set to T∞=Tg − 30 K.45 Kg is the nucleation parameter, ΔT denotes the degree of undercooling (ΔT = T0m − Tc), f is a correction factor that is close to unity at high temperatures and is given as f = 2Tc/(T0m + Tc). The equilibrium melting point of PBSu was set 133.5 °C.46 The parameter Kg contains the variable n, reflecting the regime behavior, and it can be expressed as45

Figure 6. Plot of (1/n)ln(K) versus 1/Tc from the Arrhenius method for isothermal crystallization activation energy of PBSu and nanocomposites.

Kg =

42−44

experimental data over a large supercooling range well. The theory predicts that the crystallization rate (G(T)) can be expressed as a function of supercooling ΔT according to the expression:45

jb0σσeTm0 kB(Δhf )

(5)

where j = 4 for regimes I and III and j = 2 for regime II, b0 is the thickness of a single stem on the crystal, σ is the lateral surface G



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free energy, Δhf is the enthalpy of fusion, and kB is Boltzmann’s constant. It has been stated that the inverse of the crystallization half times as obtained from DSC isothermal crystallization experiments can be used in place of the spherulite growth rate.45 The nucleation constant Kg is calculated from the double logarithmic transformation of eq 4: Kg U* ln(G) + = ln(G0) − R(Tc − T∞) Tc(ΔT )f (6)

width and the molecular layer thickness, respectively. According to the α-form unit cell of PBSu, if the crystal growth plane is 110, α0 should be 0.525 nm and b0 should be 0.404 nm.50 Generally, the Thomas−Stavely constant α ranges between 0.1 and 0.3. The value α = 0.1 is widely applied in polyethylene and other flexible polymers. Roitman and Marand found that the α value was ca. 0.25 for polypivalolactone. The α value is not at all universal and strongly depends on the chemical structure of the polymer, and it is related to entropy differences between the crystal and the melt interface. Because crystallization of polymers is governed by secondary surface nucleation, σ and σe are the key parameters controlling both crystal nucleation and growth rates. Also, the work of chain folding can be obtained from the fold surface free energy as q = 2σeα0b0. The q value is defined as the work for chain folding by bending the polymer chain back upon itself in the appropriate configuration. q has been found to be a parameter closely related with molecular structure, the inherent stiffness of the chain itself. The results are presented in Table 2. As it can be seen from the results that occurred after the application of the Lauritzen−Hoffman nucleation theory, the presence of silica and strontium nanotubes has some small effect on the nucleation of the nanocomposites. The samples containing silica nanotubes present differences in the values of the activation energy for nucleation, the lateral surface free energy, and the fold surface free energy. The lower values of these samples represent their tendency to promote the nucleation of spherulites in their surface. However, the values of all the other samples are almost the same with PBSu, a fact that leads once again to the conclusion that SrHNRs act as inert fillers in the polymeric matrix. 3.3. Nonisothermal Crystallization. 3.3.1. Nonisothermal Crystallization Kinetics. The nonisothermal crystallization kinetics of PBSu and nanocomposites was investigated under different cooling rates. The peak temperature where the crystallization rate is maximized shifts to the low temperature region when the cooling rate increases. The crystallization peak temperatures for all samples can be seen in Figure 8. From the samples, only PBSu/SiNTs 20 wt % crystallizes at significantly higher temperatures whereas the peaks of PBSu/ SiNTs 5 wt % and PBSu/SrHNRs 5 wt % do not present significant variations from the peak temperature of PBSu, because their crystallization peak differences are in the vicinity of ±0.5−0.8 °C. On the other hand, PBSu/SrHNRs 20 wt % crystallizes at temperatures considerably lower than that of neat PBSu, which is an indication that the high percentage of filler content, along with the aggregates that are formed, induces crystallization at lower Tp. As with isothermal crystallization, an integration of the exothermal peaks during the nonisothermal scan can give the relative degree of crystallization as a function of time. The crystallinity was calculated from the fraction of the enthalpy of

Plotting the left-hand side of eq 6 (where G ≈ 1/t1/2) versus 1/ Tc(ΔT)f a straight line should produce a line whose slope and intercept will be equal to −Kg and G0, respectively (Figure 7).

Figure 7. Plots of ln(G) + U*/R(Tc − T∞) against 1/Tc(ΔT)f for PBSu and nanocomposites.

To obtain the best fit for the secondary nucleation theory, two parameters should be predefined: the equilibrium melting temperature and the equilibrium melting enthalpy. These last were assumed to be Tom= 133.5 °C and ΔHom = 210 J/g or ΔHom = 281.4 × 105 J/m3 whereas Tg = −37 °C for the PBSu homopolymer.46−48 The nucleation constant Kg can be related to the product of lateral and folding surface free energy (σσe) and it represents the free energy that is needed to form a nucleus of a critical size. In general, the critical points in the plot, which are identified by the change in the slope of the curve, are attributed to regime transitions, accompanied by morphological changes in the crystals. However, in the case of this work, the crystallization temperatures fall in the regime II region46 and, as was expected, no break point was observed. The lateral surface free energy (σ) can be estimated by the empirical treatment proposed by Thomas and Stavely:49 σ = αΔhf(a0b0)1/2 where the a0 and b0 factors are the molecular

Table 2. Results from the Application of the Secondry Nucleation Theory Proposed by Lauritzen−Hoffman for PBSu and Nanocomposites material

Kg III (K2)

PBSu PBSu/SiNTs 5 wt % PBSu/SiNTs 20 wt % PBSu/SrHNRs 5 wt % PBSu/SrHNRs 20 wt %

× × × × ×

1.74 1.55 1.18 1.68 1.70

5

10 105 105 105 105

σσe (J2/m4) 4.61 4.11 3.13 4.45 4.50

× × × × ×

−4

10 10−4 10−4 10−4 10−4 H

σ (J/m2)

σe (J/m2)

q (kJ/mol)

0.0136 0.0136 0.0136 0.0136 0.0136

0.0339 0.0302 0.0230 0.0327 0.0331

9.65 8.60 6.55 9.31 9.42



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Figure 8. Variation of crystallization peak temperatures with different cooling rates for PBSu and nanocomposite samples.

fusion of the PBSu and nanocomposite samples with the enthalpy of fusion of 100% crystalline PBSu, which was 210 J/g according to literature.51 Thus, the crystallinity values were 33, 34.6, 32.3, 34.5, and 32.2% for PBSu, PBSu/SiNTs 5 wt %, PBSu/SiNTs 20 wt %, PBSu/SrHNRs 5 wt %, and PBSu/ SrHNRs 20 wt %, respectively. It is interesting to notice that the fillers at 5 wt % loading increase slightly the crystallinity percentage on the nanocomposite samples whereas, in higher amounts (20 wt %), there is a small reduction. Furthermore, even though the type of fillers is different, the crystallinity on both filler contents (5 and 20 wt %) is almost the same. Many different macrokinetic models have been proposed for the analysis of nonisothermal crystallization, and the modified Avrami approach is one of the most used. According to this mehod, the relative degree of crystallinity can be calculated from52 X = 1 − exp( −Ztt n)

(7)

X = 1 − exp[−(KAvramit )n ]

(8)

or where Zt and n denote the growth rate constant and the Avrami exponent, respectively. Because the units of Zt are a function of n, eq 7 can be written in the composite-Avrami form using KAvrami instead of Zt (where Zt = KnAvrami). Thus, the Avrami parameters n and Zt can be obtained by the slope and intercept of linear fitting of the plots of log[−ln(1 − X(t))] vs log(t). The plots were constructed, and they are presented in Figure 9 for PBSu and nanocomposite samples. As it can be seen from the Avrami plots, neat PBSu and the samples filled with 5 wt % SiNTs and SrHNRs present good linearity and the quality of linear fit is high (R2 > 0.998). However, the plots of the samples filled with 20 wt % SiNTs are not completely linear. PBSu/SiNTs 20 wt % presents curvature at high relative degrees of crystallinity. The deviation from linearity can be attributed to the spherulite impigment in the later stage due to the high filler content. The kinetic parameters n and k for PBSu and nanocomposites are presented in Table 3. As stated earlier, the Avrami exponent n is related to the type of nucleation and growth geometry of the crystals. Thus, the higher values of the Avrami exponent that the samples filled

Figure 9. Avrami plots of PBSu and nanocomposites under different cooling rates.

with 20 wt % SiNTs and SrHNRs present from the other samples is another indication for the different crystal growth geometry and heterogeneous nucleation, at least in case of the SiNTs. I



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Table 3. Results of the Avrami Analysis for Nonisothermal Crystallization of PBSu and Nanocomposites PBSu

PBSu/SiNTs 5 wt %

PBSu/SiNTs 20 wt %

PBSu/SrHNRs 5 wt %

PBSu/SrHNRs 20 wt %

Tc (°C)

n

Zt

n

Zt

n

Zt

n

Zt

n

Zt

2.5 5 7.5 10 15 20

3.1 2.9 2.8 3 2.8 2.7

0.32 0.54 0.75 0.88 1.1 1.35

3.3 3.1 3 3.4 2.8 2.6

0.2 0.42 0.57 0.7 1.03 1.37

3.8 4.1 3.8 3.9 3.9 3.8

0.34 0.49 0.75 0.85 1.05 1.17

3.2 3 2.9 2.4 2.7 3.1

0.33 0.57 0.82 0.96 1.38 1.71

3.8 3.5 3.9 4 3.7 3.7

0.31 0.74 0.91 1.12 1.72 1.91

The growth rate constant Zt represents how quickly the crystals grow during the nonisothermal crystallization and increases with increasing cooling rate. The plots of the crystallization peak temperature, along with the growth rate constant for PBSu and PBSu/SiNTs 20 wt %, can be seen in Figure 10. From the plot, it can be stated that the peak

φ=

B* B

(9)

where B is a parameter that can be calculated by the following equation: B=

ωσ 3Vm2 3nkTm0ΔSm2

(10)

where ω is a geometric factor, σ is a specific energy, Vm is the molar volume of the crystallizing substance, n is the Avrami exponent, ΔSm is the entropy of melting, and T0m is the infinite crystal melting temperature. T0m was assumed to be equal to 133.5 °C.46 In addition, B can be experimentally determined from the slope of eq 11 obtained by plotting ln(β) versus the inverse squared degree of supercooling 1/ΔT2p (ΔTp = Tm − Tp): ln β = Const −

B ΔTp2

(11)

The above equation holds for homogeneous nucleation from a melt, near the melting temperature. By using a nucleating agent, eq 11 is transformed to the following for heterogeneous nucleation: Figure 10. Crystallization peak temperature and Avrami growth rate parameter versus cooling rate for PBSu and PBSu/SiNTs 20 wt %. Despite the low supercooling, crystallization of the nanocomposite is fast.

ln β = Const −

B* ΔTp2

(12)

1/ΔT2p

Plots of ln(β) versus for PBSu and nanocomposites were constructed and presented in Figure 11a. Straight lines were obtained in every sample. From the slopes of these lines, the values of B and B*, for pure PBSu and the nanocomposites can be calculated, respectively. Then, the nucleation activity can be computed from eq 9 and is presented in Figure 11b. It must be reminded that a value of φ close to zero indicates high nucleation activity, and values close to 1 show low activity and when φ is higher than one; this is an indication that the crystallization of the neat polymer is retarded. From Figure 11b, it is obvious that silica nanotubes are quite efficient nanofillers because they facilitate the crystallization phenomenon providing more nucleation sites and increasing the crystallization rates, and their nucleation activity increases with increasing nanotube content. On the other hand, strontium nanotubes practically decrease the nucleating activity of the polymeric matrix, because the nucleation activity of PBSu/SrHNRs nanocomposite samples is lower than that of the polymeric matrix. 3.3.3. Effective Activation Energy of Nonisothermal Crystallization. Apart from the nucleation activity, the calculation of the effective activation energy of nonisothermal crystallization is important. Considering the variation of the peak temperature with the cooling rate, several mathematical procedures have been proposed in literature for the calculation

temperatures for neat PBSu are much lower than for the nanocomposite. Keeping in mind that supercooling (i.e., the difference between the melting temperature and the temperature at which crystallization of the polymer occurs) is the driving force for crystallization, it is obvious that crystallization of neat PBSu is taking place under much larger supercoolings than for the nanocomposite and thus, it was expected to be faster. However, the values of constant Zt, that is crystallization rate values, are almost the same for PBSu and the nanocomposite. This is a clear proof of the nucleating activity of the SiNTs. 3.3.2. Nucleation Activity of Nanoparticles. The nucleation activity of foreign substrates in a polymer melt can be estimated using the data from nonisothermal crystallization according to a method proposed by Dobreva and Gutzow.53,54 This method has also been used for nanocomposite samples.55,56 Nucleation activity (φ) is a factor by which the work of three-dimensional nucleation decreases with the addition of a foreign substrate. If the foreign substrate is extremely active, φ approaches 0, whereas for inert particles, φ approaches 1. The nucleation activity is calculated from the ratio: J



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where dX/dt is the instantaneous crystallization rate as a function of time at a given conversion X, ΔEX is the effective activation energy at a given degree of conversion, TX,i is the set of temperatures related to a given conversion X at different cooling rates αi, and the i subscript refers to different cooling rates that were used during the experimental procedure. The X(t) function obtained by the integration of the experimentally measured crystallization rates is initially differentiated with respect to time to obtain the instantaneous crystallization rate dX/dt. In addition, by selecting appropriate degrees of crystallinity (i.e., from 5% to 95%) the values of dX/dt at a specific X are correlated to the corresponding crystallization temperature at this X. Then, by plotting the left-hand side of eq 13 with respect to 1/TX, a straight line must be obtained with a slope equal to ΔEX/R. Thus, the effective activation energy obtained was subsequently plotted as a function of the relative degree of crystallinity, as it can be seen in Figure 12a. It is obvious that for most of the samples, the activation energy remains quite constant with an increasing degree of conversion, and only for PBSu and PBSu/SrHNRs 5 wt % can a significant decrease of E be observed. PBSu/SiNTs 20 wt % presents the higher activation energy of all samples (−70 ± 10

Figure 11. Plots of ln(β) versus 1/ΔT2p for PBSu and nanocomposites and (b) nucleation activity of nanotubes.

of ΔE. Among them, the Kissinger’s method has been widely applied in evaluating the overall effective energy barrier: ⎛ a ⎞ ΔE ln⎜ 2 ⎟ = Const − RTP ⎝ TP ⎠

(13)

However, a major concern has been raised for the use of these procedures in obtaining ΔE, because they have been formulated for heating experiments (i.e., positive values of α). Vyazovkin57 has demonstrated that dropping the negative sign for α is a mathematically invalid procedure that generally makes the Kissinger equation inapplicable to the processes that occur on cooling. Moreover, the use of this invalid procedure may result in erroneous values of the effective energy barrier, ΔE. Isoconversional methods are the most appropriate in this case and especially for cooling.58−60 The isoconversional methods proposed by Friedman61 and Vyazovkin62 are the most used methods for evaluating the effective energy barrier. In the present paper, Friedman’s method was applied: ⎛ dX ⎞ ΔEX ln⎜ ⎟ = Const − ⎝ dt ⎠ X , i RTX , i

Figure 12. Dependence of the effective activation energy on (a) the degree of conversion and (b) temperature for PBSu and its nanocomposites.

(14) K



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kJ/mol), whereas PBSu and the samples filled with 5% SiNTs and SrHNRs have almost the same activation energies (−150 ± 20, −140 ± 5, −150 ± 10 kJ/mol, respectively). Finally, the PBSu/SrHNRs 20 wt % sample presents the lowest activation energy (−170 ± 10 kJ/mol). The negative activation energy values of the samples indicate that the rate of crystallization increased with decreasing the temperature, and the crystallization process of polymers is a spontaneous process. In addition, the calculated values are between the ones calculated with the Arrhenius theory during isothermal crystallization, which is another indication for the accuracy of the results. Finally, the dependence of activation energy on temperature can be estimated if it is assumed that an average temperature for each degree of crystallinity exists.58 Thus, Figure 12b shows the variation of the effective activation energy with temperature. At lower temperature values, the preexisting crystallites play a negative role, reducing chain mobility and thus increasing the effective activation energy. However, from the differences in the activation energy values among the materials, for example, for the PBSu/SiNTs 20 wt %, neat PBSu, and the PBSu/SrHNRs 20 wt %, it is obvious that the value of the activation energy for the materials is strictly related with the temperature window of crystallization. 3.4. X-ray Diffraction. The effect of nanofillers on the crystalline structure was studied by means of XRD and it is presented in Figure 13. Very small changes were obvious at the diffractograms of the samples, thus the introduction of silica

nanotubes and strontium nanorods did not significantly distort the crystalline parameters of the polymeric matrix. X-ray diffraction experiments were also performed for neat silica nanotubes, where a wide peak appeared due to the fact that silica nanotubes are highly amorphous and for strontium nanorods, where their main peaks at 20.4°, 26.6°, and 31° were also observed. The main diffraction peaks of PBSu were observed at 2θ 19.8°, 22°, and 22.6°, corresponding to (020), (021), and (110) reflections, respectively.63 The presence of the nanofillers can be verified by the slight differences in the nanocomposites diffractograms, such as the widening of the peak at 22° due to the presence of silica nanotubes or the peak at 31° due to strontium hydroxyapatite nanorods. The degree of crystallinity (Xc) of all studied samples was calculated by fitting initially the XRD profiles of PBSu and nanocomposites with the Lorentzian function and then calculating the ratio between the crystalline diffraction area (Ac) and the total area of the diffraction profile (At), Xc = Ac/At.64 To further examine the effect of the fillers on the crystalline structure, Scherrer’s equation was applied for the calculation of the sizes of the crystallites (L(hkl)), which are perpendicular to the (hkl) planes. The crystallite size is reflected in the broadening of a particular peak in a diffraction pattern associated with a particular planar reflection from within the crystal unit cell. Scherrer’s theory is represented by the equation:65 L(hkl) =

kλ β cos θ

(15)

where k is a constant which generally equals to 0.9, λ is the Xray wavelength (=0.154 nm), β is the width of the peak (in radian units) and is measured by the full width at halfmaximum, and θ is the peak position. The calculated values of L(hkl) for the crystallites perpendicular to the planes (021) and (110) are presented in Table 4. Also, to calculate the space Table 4. Results from the XRD Analysis of PBSu and Nanocomposites sample

Xc (%)

L110 (nm)

L021 (nm)

d110 (Å)

d021 (Å)

PBSu PBSu/SiNTs 5 wt % PBSu/SiNTs 20 wt % PBSu/SrHNRs 5 wt % PBSu/SrHNRs 20 wt %

32.6 33.3 31.4 33.8 29.9

14.99 14.04 13.47 15.25 15.39

7.12 6.21 5.18 6.89 7.27

3.91 3.92 3.95 3.92 3.94

4.03 4.04 4.07 4.05 4.07

between the structural layers of the nanocomposites, Bragg’s law was used: sin θ =

nλ 2d

(16)

where n is an integer, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes. For the determination of the spacing between the planes in an atomic lattice, the crystalline reflections (110) and (021) were used. From the results presented in Table 4, it can be seen that both crystalline size and d-spacing do not change significantly for the samples filled with SrHNRs and a notable decrease in the crystalline size along with a very small increase in the d-spacing can be observed only for the samples filled with SiNTs. This is an indication that in these samples, the crystal packing becomes

Figure 13. X-ray diffractograms for the polymeric matrix (PBSu), the fillers (SiNTs and SrNRs), and the nanocomposite samples. L



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biodegradable polyester nanocomposites. Eur. Polym. J. 2006, 42, 1330−1341. (3) Mohanty, A. K.; Misra, M.; Hinrichsen, G. Biofibres, biodegradable polymers and biocomposites: An overview. Macromol. Mater. Eng. 2000, 276−277, 1−24. (4) Gan, Z.; Abe, H.; Doi, Y. Crystallization, Melting, and Enzymatic Degradation of Biodegradable Poly(butylene succinate-co-14 mol ethylene succinate) Copolyester. Biomacromolecules 2001, 2, 313−321. (5) Gualandi, C.; Soccio, M.; Govoni, M.; Valente, S.; Lotti, N.; Munari, A.; Giordano, E.; Pasquinelli, G.; Focarete, M. L. Poly(butylene/diethylene glycol succinate) multiblock copolyester as a candidate biomaterial for soft tissue engineering: Solid-state properties, degradability, and biocompatibility. J. Bioact. Compat. Polym. 2012, 27, 244−264. (6) Oliveira, J. T.; Crawford, A.; Mundy, J. L.; Sol, P. C.; Correlo, V. M.; Bhattacharya, M.; Neves, N. M.; Hatton, P. V.; Reis, R. L. Novel Melt-Processable Chitosan−Polybutylene Succinate Fibre Scaffolds for Cartilage Tissue Engineering. J. Biomater. Sci., Polym. Ed. 2011, 22, 773−788. (7) Wei, J.-D.; Tseng, H.; Chen, E. T.-H.; Hung, C.-H.; Liang, Y.-C.; Sheu, M.-T.; Chen, C.-H. Characterizations of Chondrocyte Attachment and Proliferation Scaffolds of PLLA and PBSA for Use in Cartilage Tissue Engineering. J. Biomater. Appl. 2011, 0, 1−23. (8) van Dijkhuizen-Radersma, R.; Roosma, J. R.; Kaim, P.; Métairie, S.; Péters, F. L. A. M. A.; de Wijn, J.; Zijlstra, P. G.; de Groot, K.; Bezemer, J. M. Biodegradable poly(ether-ester) multiblock copolymers for controlled release applications. J. Biomed. Mater. Res., Part A 2003, 67A, 1294−1304. (9) Krause, M.; Mücklich, A.; Zak, A.; Seifert, G.; Gemming, S. High resolution TEM study of WS2 nanotubes. Phys. Status Solidi B 2011, 248, 2716−2719. (10) Zhu, Y. Q.; Sekine, T.; Brigatti, K. S.; Firth, S.; Tenne, R.; Rosentsveig, R.; Kroto, H. W.; Walton, D. R. M. Shock-Wave Resistance of WS2 Nanotubes. J. Am. Chem. Soc. 2003, 125, 1329− 1333. (11) Chin, S. J.; Hornsby, P.; Vengust, D.; Mihailović, D.; Mitra, J.; Dawson, P.; McNally, T. Composites of poly(ε-caprolactone) and Mo6S3I6 Nanowires. Polym. Adv. Technol. 2012, 23, 149−160. (12) Papageorgiou, G. Z.; Bikiaris, D. N.; Achilias, D. S.; Papastergiadis, E.; Docoslis, A. Crystallization and biodegradation of poly(butylene azelate): Comparison with poly(ethylene azelate) and poly(propylene azelate). Thermochim. Acta 2011, 515, 13−23. (13) Papageorgiou, G. Z.; Nanaki, S. G.; Bikiaris, D. N. Synthesis and characterization of novel poly(propylene terephthalate-co-adipate) biodegradable random copolyesters. Polym. Degrad. Stab. 2010, 95, 627−637. (14) Soccio, M.; Lotti, N.; Finelli, L.; Gazzano, M.; Munari, A. Aliphatic poly(propylene dicarboxylate)s: Effect of chain length on thermal properties and crystallization kinetics. Polymer 2007, 48, 3125−3136. (15) Pan, H.; Qiu, Z. Biodegradable Poly(l-lactide)/Polyhedral Oligomeric Silsesquioxanes Nanocomposites: Enhanced Crystallization, Mechanical Properties, and Hydrolytic Degradation. Macromolecules 2010, 43, 1499−1506. (16) Wang, H.; Qiu, Z. Crystallization kinetics and morphology of biodegradable poly(l-lactic acid)/graphene oxide nanocomposites: Influences of graphene oxide loading and crystallization temperature. Thermochim. Acta 2012, 527, 40−46. (17) Ihn, K. J.; Yoo, E. S.; Im, S. S. Structure and Morphology of Poly(tetramethylene succinate) Crystals. Macromolecules 1995, 28, 2460−2464. (18) Bikiaris, D. N.; Papageorgiou, G. Z.; Achilias, D. S. Synthesis and comparative biodegradability studies of three poly(alkylene succinate)s. Polym. Degrad. Stab. 2006, 91, 31−43. (19) Guan, W.; Qiu, Z. Isothermal Crystallization Kinetics, Morphology, and Dynamic Mechanical Properties of Biodegradable Poly(ε-caprolactone) and Octavinyl−Polyhedral Oligomeric Silsesquioxanes Nanocomposites. Ind. Eng. Chem. Res. 2012, 51, 3203− 3208.

looser and the size of crystals drops, because the presence of silica nanotubes enhances crystallization; the geometry and the high percentages of the fillers enable them to act as heterogeneous nucleating agents. The above results reflect once again the previously stated conclusions that only the PBSu/SiNTs 20 wt % sample presents notably different structural and crystallization behavior than the other nanocomposite samples whereas the PBSu/SiNTs 5 wt % presents slight differences. Finally, the crystallinity percentage values that were calculated from the XRD patterns show that at low percentage of fillers the crystallinity increases slightly whereas when the fillers are present at high contents (20 wt %), the crystallinity presents a drop. The values presented at Table 4 are very close to the values calculated by DSC, a fact that indicates the accuracy of the results.

4. CONCLUSIONS Multiple melting behavior was observed for the PBSu/SiNTs and PBSu/SrHNRs nanocomposites and neat PBSu. Using Step Scan DSC, it was found that recrystallization plays an important role especially in the melting behavior of samples isothermally crystallized at large supercoolings. The crystallization study showed that the PBSu/SiNTs nanocomposites crystallize faster than neat PBSu whereas, on the other hand, the SrHNRs do not behave as a nucleating agent but mostly as inert fillers slightly reducing the crystallization rate of PBSu. This was also proved from the crystallization activation energy values, which, in the case of isothermal crystallization of the PBSu/SiNTs nanocomposites containing 5 and 20 wt % SiNTs, are lower (−138.5 and −93.7 kJ/mol, respectively) than that of neat PBSu (−165.5 kJ/mol), whereas those of PBSu/SrHNRs nanocomposites are −166.4 and −178.6 kJ/mol. This is because, due to the poor interactions between SrHNRs and PBSu, large aggregates are formed, which may inhibit crystallization. The nucleation activity study of nanofillers into PBSu, estimated according to Dobreva and Gutzow method, proved that SiNTs can enhance nucleation of PBSu, whereas SrHNRs practically decrease this ability. The overall behavior of the samples filled with SiNTs, in contrast to the samples filled with SrHNRs, can be additionally attributed to the fact that SiNTs are 3 orders of magnitude larger. Finally, the XRD studies showed that the introduction of the nanotubes did not affect substantially the crystalline parameters of the polymeric matrix.

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AUTHOR INFORMATION

Corresponding Author

*D. Bikiaris. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge cofunding of this research by IKY (Greece) and DAAD (Germany), Action “IKYDA 2012”.

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REFERENCES

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