Crystallization Kinetics and Morphology Studies of Biodegradable ...

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Journal of Nanoscience and Nanotechnology Vol. 9, 4961–4969, 2009

Crystallization Kinetics and Morphology Studies of Biodegradable Poly(butylene succinate-co-butylene adipate)/Multi-Walled Carbon Nanotubes Nanocomposites Zhaobin Qiu∗ , Siyu Zhu, and Wantai Yang State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Biodegradable poly(butylene succinate-co-butylene adipate) (PBSA)/carboxyl-functionalized multiwalled carbon nanotubes (f-MWNTs) nanocomposites were prepared through solution casting method with different f-MWNTs contents ranging from 0.5 to 2 wt%. Scanning electron microscopic observations reveal a fine dispersion of f-MWNTs the PBSA matrix. Effect of f-MWNTs Delivered by throughout Ingenta to: on the crystallization behavior of PBSA was investigated in detail via various techniques and difNational Institute for Materials Sciernce ferent crystallization conditions including crystallization at different cooling rates and IP :nonisothermal 144.213.253.16 isothermal crystallization at different crystallization temperatures Thu, 04 Jun 2009 00:51:05in this work. For both nonisothermal and isothermal melt crystallization, the addition of f-MWNTs enhances the crystallization of PBSA apparently due to their heterogeneous nucleation effect. However, the crystal structure of PBSA does not change in the nanocomposites. Moreover, an attempt was made to study the effect of the presence of f-MWNTs and their contents on the nucleation activity and crystallizability of PBSA in the nanocomposites quantitatively.

Keywords: Crystallization, Biodegradable Polymer, Poly(butylene succinate-co-butylene adi-

1. INTRODUCTION Biodegradable polymers have received much more attention in the last two decades due to their potential applications in the fields related to environmental protection and the maintenance of physical health. Poly(butylene succinate-co-butylene adipate) (PBSA), a copolymer of poly(butylene succinate) (PBSU), is one of biodegradable semicrystalline aliphatic polyesters and commercially available. The biodegradability, crystal structure, crystallization kinetics and melting behavior of PBSA have been studied in detail.1–6 Compared with the homopolymer PBSU, PBSA is more susceptible to biodegradation because of its lower crystallinity and more flexible polymer chains. However, practical application of PBSA has been limited because of its softness, low gas barrier properties and so on. Polymer blending is a simple and economic way to modify the physical properties and extend the practical application fields of biodegradable polymers. On the one hand, PBSA is miscible with poly(ethylene oxide) (PEO) and poly(vinylidene fluoride) (PVDF).7–9 ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2009, Vol. 9, No. 8

The spherulitic growth of PEO was found to continue in the spherulites of PBSA in the simultaneous spherulitic growth process of the two components in PBSA/PEO blends, resulting in the formation of interpenetrating spherulites.7 In the PVDF/PBSA blends, the crystallization behavior of PBSA was affected by the presence of pre-existing crystals of PVDF. Three different types of crystalline morphologies for PBSA were found in the PVDF/PBSA blends.8 9 On the other hand, PBSA is immiscible with poly(hydroxybutyrate) (PHB) and poly(Llactic acid) (PLLA).10 11 In the PHB/PBSA blends, crystal structure and crystallinity of PHB and PLLA changed slightly despite blend composition.10 In the PLLA/PBSA blends, the crystallization mechanism of PBSA did not change while the crystallization rate of PBSA decreased with increasing the PLLA content.11 In addition, structure and properties of nanocomposites based on PBSA and organically modified montmorillonite were also investigated by Sinha Ray et al. in detail for extending its wide practical application.12 Carbon nanotubes (CNTs) were first reported by Iijima in 1991.13 There are two types of carbon nanotubes, namely single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). An individual carbon

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doi:10.1166/jnn.2009.1283

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pate), Multi-Walled Carbon Nanotubes, Nanocomposite.

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Crystallization Kinetics and Morphology Studies of Biodegradable PBSA/MWCNTs Nanocomposites

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nanotube is in a macromolecular structure with nanosized nanocomposites, PBSA was mixed with the addition of diameter and micrometre length. Polymer/CNTs composvarious f-MWNTs contents, specified as 0.5, 1, and 2 wt% ites possess high stiffness and good electrical conductivity in the polymer matrix, respectively. Chloroform was used at a relatively low concentration of carbon nanotubes.14 as the mutual solvent. On the one hand, the appropriThe very large aspect ratio of the CNTs imparts a dramatic ate amount of f-MWNTs was added into the chloroform. increase in the modulus of the composites.15 Therefore, Then, the mixture was sonicated with a KQ 3200E ultrathe combination of biodegradable polymers with a very sonic generator to make a uniformly dispersed suspension. small quantity of expensive CNTs must be of great interOn the other hand, PBSA was placed into chloroform and est and use from both academic and industrial viewpoints, stirred for 1 h to dissolve PBSA completely. Next, the which may provide attractive possibilities for improving PBSA solution was added to the f-MWNTs suspension, the physical properties of biodegradable polymers and and sonication was continued, with stirring for 6 h. The thus extend their practical application. Some biodegradPBSA/f-MWNTs solution was poured into a dish to evapable polymer/CNTs nanocomposites have been reported orate the solvent at room temperature. The sample was in the literatures,16–22 especially for PLLA and poly(further dried at 70 C under vacuum for 3 days to remove caprolactone) (PCL). the solvent completely. In this work neat PBSA and its Depending on the crystallization conditions and the three PBSA/f-MWNTs nanocomposites were abbreviated presence of other components, crystallization of polyas 100/0, 99.5/0.5, 99/1 and 98/2, respectively, with the mers may result in different morphology and different first number referring to PBSA while the second number crystalline forms in some cases. The presence of a secreferring to f-MWNTs. Delivered by Ingenta to: ond phase may have the following complicated effects A field emission scanning electron microscopy (S-4700, National Institute for Materials Sciernce on the crystallization of the polymer matrix including Hitachi Co., Japan) was used to observe morphology of the IP : 144.213.253.16 acceleration/retardation of crystallization, changes in the surfaces fractured in liquid nitrogen of PBSA/f-MWNTs Thu, 04 Jun 2009 00:51:05 spherulitic morphology and modification in the crystal nanocomposites. All samples were coated with gold before structure in a few cases. All these observations are usually examination. related to the presence of a second phase in a micrometer Wide angle X-ray diffraction (WAXD) patterns were scale. However, the dimensions of the second phase are recorded using a Rigaku D/Max 2500 VB2t/PC X-ray close to the chain dimensions in the polymer nanocomposdiffractometer. The Cu K radiation ( = 015418 nm) ites; therefore, a number of studies have been performed source was operated at 40 kV and 200 mA. The samples in order to elucidate the effect of nanosize filler on the were first pressed into films with a thickness of around crystallization of polymers.23 0.5 mm on a hot stage at 130 C and then transferred into It is obvious that polymer/CNTs nanocomposites may a vacuum oven at 75 C for 24 hours. WAXD patterns be generally defined as two phase materials with polymer were recorded from 10 to 50 at 5 /min. being the matrix phase and CNTs being the second phase Thermal analysis was carried out using a TA Instrudispersed in the polymer matrix. In the present work, crysment differential scanning calorimetry (DSC) Q100 with tallization of PBSA/MWNTs nanocomposites at different a Universal Analysis 2000. All operations were performed MWNTs loadings has been studied extensively to invesunder nitrogen purge, and the weight of the samples varied tigate the effect of the second phase in nanodimensions between 4 and 6 mg. Two different procedures, i.e., nonon the various aspects of crystallization of PBSA such isothermal crystallization and isothermal crystallization, as crystal structure, nonisothermal crystallization, isotherwere employed to study the crystallization behavior of neat mal crystallization, and spherulitic morphology. Moreover, PBSA and its nanocomposites. In the case of nonisotheran attempt is made to evaluate the presence of MWNTs mal crystallization studies, the samples were first heated to and their contents on the crystallizability and nucleation 130 C at 40 C/min, held at 130 C for 3 min to erase any activity of PBSA quantitatively in the PBSA/MWNTs thermal history, and cooled to −20 C at different constant nanocomposites at different MWNTs contents. cooling rates ranging from 5 to 30 C/min. The crystallization peak temperature was obtained from the cooling traces. In the case of isothermal crystallization, the sam2. EXPERIMENTAL DETAILS ples were annealed at 130 C for 3 min to eliminate any thermal history, cooled to the desired crystallization temPBSA (Mw = 14400), a copolymer with 20 mol% perature (Tc at 40 C/min, and then maintained at Tc until butylene adipate, was obtained from Aldrich Co. The the crystallization was completed. The exothermal traces carboxyl-functionalized multi-walled carbon nanotubes were recorded for the later data analysis. After isothermal (f-MWNTs) samples were purchased from Chengdu Insticrystallization at various Tc s, the samples were heated to tute of Organic Chemistry, Chinese Academy of Sciences. 130 C again at 20 C/min to study the subsequent melting The outer diameter is around 30–50 nm, with lengths rangbehavior. ing between 10 and 20 m. Spherulitic morphology of neat PBSA and PBSA/ The PBSA/f-MWNTs nanocomposites were prepared MWNTs nanocomposites were observed under crossed through a solution mixing method. For the fabrication of 4962

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polarizers by an optical microscope (POM) (Olympus BX51) with a temperature controller (Linkam THMS 600). The samples were first annealed at 130 C for 3 min to erase any thermal history and then cooled to 80 C at 40 C/min.

3. RESULTS AND DISCUSSION 3.1. Dispersion of f-MWNTs and Its Effect on the Crystal Structure of PBSA

(110)

(020) (021)

Intensity (a.u.)

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PBSA/f-MWNTs

100/0 99.5/0.5 99/1

Heat flow (Exo down)

100/0 99.5/0.5 99/1 98/2

20

40

60

80

100

120

Temperature (ºC) Fig. 1. SEM image of fracture surface for a PBSA/MWNTs 99/1 nanocomposite.


Fig. 3. DSC cooling traces of neat PBSA and its nanocomposites at 10 C/min.

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It should be noted that the dispersion of CNTs in the 98/2 polymer matrix plays a dominant role of influencing the physical properties of polymer matrix. A homoge10 15 20 25 30 35 40 neous dispersion of CNTs usually effectively improves 2θ (º) the mechanical, electrical and thermal performances of the polymer matrix. The surface of PBSA/f-MWNTs Fig. 2. WAXD patterns of neat PBSA and its nanocomposites. nanocomposites fractured in liquid nitrogen was examined by SEM in order to examine the distribution ofDelivered f-MWNTs by Ingenta presence to: of MWNTs. The three main diffraction peaks at in the PBSA matrix. Figure 1 showsNational an overview on thefor Materials , 21.7 and 22.6 are assigned to (020), (021), around 19.5Sciernce Institute fracture surface of a 99/1 nanocomposite as an IP example. and (110) planes of PBSA, respectively.3 : 144.213.253.16 The uniformly dispersed bright dots and linesThu, are the ends 04 Jun 2009 00:51:05 of the broken MWNTs, indicative of a fine dispersion 3.2. Effect of f-MWNTs on the Nonisothermal Melt of f-MWNTs throughout the PBSA matrix. Since some Crystallization of PBSA in the PBSA/f-MWNTs nanotubes seem to be pulled out of the section surface, Nanocomposites we can even observe the ends of individual f-MWNTs embedded in the matrix. The homogeneous dispersion of It is of great interest to study the effect of the incorpof-MWNTs are also shown for 99.5/0.5 and 98/2 samples, ration of f-MWNTs and the f-MWNTs contents on the and no severe aggregation of f-MWNTs is found in the crystallization and crystalline morphology of PBSA in the PBSA matrix, indicating that the variation of f-MWNTs nanocomposites. content from 0.5 to 2 wt% does not influence the disperNonisothermal melt crystallization of PBSA and its sion and distribution of MWNTs in the polymer matrix nanocomposites was studied first by DSC at various coolsignificantly. ing rates. Figure 3 shows the DSC cooling traces of PBSA Figure 2 shows the WAXD patterns of neat PBSA and and its three nanocomposites from the melt at 10 C/min its nanocomposites after crystallizing at 75 C. Both neat as an example. As shown in Figure 3, the crystallization PBSA and its nanocomposites exhibit almost the same peak temperature (Tp is around 51.2 C for neat PBSA, diffraction peaks at almost the same locations, indicating that the crystal structure of PBSA does not change in the

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Relative crystallinity, Xt (%)

which shifts to high temperature range in the presence of the f-MWNTs contents. From the above mentioned studies, it is clear that the presence of f-MWNTs and their conf-MWNTs. In the case of 99.5/0.5 sample, Tp shifts to tents have a significant effect on the nonisothermal melt around 54.5 C; however, in the case of 99/1 and 98/2 crystallization behavior of PBSA in the PBSA/f-MWNTs samples, Tp s shift to around 68.3 and 73.2 C, respectively. nanocomposites due to the heterogeneous nucleation agent It is obvious that the increase in Tp is only 3.3 C with effect of f-MWNTs. increasing f-MWNTs content from 0 to 0.5 wt%; however, the increase in Tp is around 13.8 C with increasing f-MWNTs contents from 0.5 to 1 wt%. But the increase is 3.3. Effect of f-MWNTs on the Overall Isothermal only around 4.9 C with further increasing f-MWNTs conMelt Crystallization Kinetics of PBSA in the tent from 1 to 2 wt%. Such results indicate that the presPBSA/f-MWNTs Nanocomposites ence of f-MWNTs enhances the crystallization of PBSA; moreover, the enhancement is affected significantly by the The overall isothermal melt crystallization kinetics of f-MWNTs contents. In brief, Tp of PBSA shifts to high neat PBSA and its nanocomposites was further studied temperature in the presence of f-MWNTs; furthermore, with DSC. Figure 5(a) dipicts the development of relasuch increase in Tp is slight at 0.5 wt% f-MWNTs contive crystallinity as a function of crystallization time for tent and becomes significant at 1 and 2 wt% f-MWNTs a 99/1 sample isothermally crystallized at various cryscontents as compared to that of neat PBSA. tallization temperatures. It is clear that the crystallization In order to show the effect of cooling rate on the time prolongs with increasing crystallization temperature. nonisothermal melt crystallization, Figure 4 Delivered summarizes by Ingenta The well-known Avrami equation is used to analyze the to: the variation of Tp with cooling rateNational for neat PBSA andfor Materials overall isothermal melt crystallization kinetics of both neat Institute Sciernce its three nanocomposites with different f-MWNTs conPBSA and its nanocomposites. According to the Avrami IP : 144.213.253.16 tents. From Figure 4, both the effects of cooling and 2009 00:51:05 Thu,rate 04 Jun f-MWNTs contents on the variation of Tp can be obtained. (a) 100 On the one hand, it is obvious that Tp shifts to low temperature range with increasing cooling rate for both neat 80 PBSA and its nanocomposites. The samples do not have enough time to crystallize at high temperature range with increasing cooling rate, resulting in that the crystallization 60 exotherms shift to low temperature range. On the other hand, Tp s of PBSA in the nanocomposites are higher than 40 that of neat PBSA for a given cooling rate; moreover, 76 ºC Tp s shift to high temperature range with increasing the 78 ºC f-MWNTs contents in the nanocomposites. Such results 80 ºC 20 82 ºC indicate again that the nonisothermal melt crystallization 84 ºC of PBSA is enhanced by the presence of f-MWNTs and the 0 degree of enhancement in Tp is influenced apparently by 0 5 10 15 20 25 30 35 40 Crystallization time (min) 80

100/0 99.5/0.5 99/1 98/2

75 70

(b) 0.4

Log(–ln(1–Xt))

0.0

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Tp (ºC)

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60 55 50

– 0.4

– 0.8

76 ºC 78 ºC 80 ºC 82 ºC 84 ºC

45 –1.2

40 35

–1.6

30

– 0.8

5

10

15

20

25

30

Φ (ºC/min) Fig. 4. Effect of cooling rates on the crystallization peak temperatures for neat PBSA and its nanocomposites.

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–0.4

0.0

0.4

0.8

1.2

1.6

Log t Fig. 5. (a) Development of relative crystallinity with crystallization time for 99/1 nanocomposite, (b) Avrami plots of a PBSA/MWNTs 99/1 nanocomposite at indicated crystallization temperatures.


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equation, the relative degree of crystallinity Xt develops as a function of crystallization time t as follows:

t05 = Table I. Avrami nanocomposites.

parameters

ln 2 k for

1/n

(2)

both

neat

PBSA

and

its

Tc ( C)

n

k (min−n )

t05 (min)

1/t05 (min−1 )

Neat PBSA

66 68 70 72 74

25 24 25 24 24

531 × 10−2 280 × 10−2 117 × 10−2 386 × 10−3 112 × 10−3

275 378 524 848 1409

0364 0264 0191 0118 0071

99.5/0.5

66 68 70 72 74

25 25 23 24 23

533 × 10−1 251 × 10−1 109 × 10−1 475 × 10−2 176 × 10−2

111 151 222 312 496

0900 0663 0450 0320 0202

99/1

76 78 80 82 84

24 22 22 23 21

908 × 10−1 194 × 10−1 527 × 10−2 161 × 10−2 181 × 10−3

089 177 320 521 1605

1121 0564 0312 0192 0062

98/2

80 82 84 86 88

22 20 22 23 23

693 × 10−1 110 × 10−1 458 × 10−2 125 × 10−2 134 × 10−3

100 249 339 581 1504

1000 0402 0295 0172 0067

Sample


nanocomposites were studied with DSC after isothermal crystallization at various Tc s. Parts (a) and (b) of Figure 6 show the melting behaviors of neat PBSU and a 99/1 nanocomposite, respectively. As shown in Figure 6(a), two melting endothermic peaks, denoted as Tm1 and Tm2 from low to high temperatures, are observed despite Tc for neat PBSA. Tm1 shifts from around 82.6 to 87.7 C with increasing Tc from 66 to 74 C. However, Tm2 is almost unchanged at around 96 C. Furthermore, the ratio of the area of Tm1 to that of Tm2 increases with Tc . When Tc is at 72 C and below, Tm1 is the dominant while Tm2 is the minor. However, Tm1 becomes the minor while Tm2 becomes the dominant when Tc is at 74 C. Figure 6(b) shows the melting behviour of a 99/1 nanocomposite crystallized from 76 to 84 C. Simialr to neat PBSA, two melting endothermic peaks or one major endothermic peak with a small shoulder, denoted as Tm1 and Tm2 from low to high temperatures, are observed despite Tc for PBSA in the nanocomposite irrespective of Tc . When Tc is at 80 C and below, two melting peaks are shown with Tm1 being the dominant and Tm2 the minor. With further increasing Tc to 82 C and above, Tm2 becomes a small shoulder and finally merges into one main melting peak. These factc could be explained by the mechanism of melting, recrystallization and remelting of PBSA crystals.27–29 Tm1 is the melting of the crystals formed at Tc which are present prior 4965

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The values of t05 and 1/t05 are summarized in Table I for both neat PBSA and its three nanocomposites at different f-MWNTs contents. As shown in Table I, the values 1 − Xt = exp −kt n

(1) of 1/t05 decrease with increasing Tc for both neat PBSA and its nanocomposites. On the contrary, the values of t05 where Xt is the relative crystallinity at time t, k is the increase with increasing Tc . Such variations suggest that crystallization rate constant and n is the Avrami exponent the overall isothermal crystallization rate decreases with depending on the nature of nucleation and growth geomincreasing Tc for both neat PBSA and its nanocomposetry of the crystals.24 Figure 5(b) shows the Avrami plots ites. The reduced overall crystallization rate with increasfor the 99/1 nanocomposite at various Tc s, from which ing Tc indicates that the isothermal melt crystallization is a the Avrami parameters n and k are obtained from the nucleation control process because of the low supercooling slopes and intercepts of the Avrami plots, respectively. All used in this work. Furthermore, the values of t05 for the the related crystallization kinetics parameters for the overnanocomposites are smaller than those of neat PBSA at a given Tc . On the other hand, the values of 1/t05 increase all isothermal melt crystallization of neat PBSA and its with increasing the f-MWNTs content for the nanocomthree nanocomposites at different f-MWNTs contents are posites and are larger than those of neat PBSA, indicating listed in Table I. The values of n are between 2 and 3 that the presence of f-MWNTs accelerates the crystallizafor both neat PBSA and its nanocomposites in the investion process of PBSA in the nanocomposites. tigated Tc s, indicating that the crystallization mechanism Hoffman of PBSA does not change after nanocomposites preparaDelivered by Ingenta to:and Weeks have shown a relationship between the apparent melting point Tm and the isothermal crystaltion. The result that n varies between 2 and 3 suggests National Institute for Materials Sciernce lization temperature Tc that the crystallization of PBSA may correspond to the IP : 144.213.253.16 three-dimensional truncated sphere growth with athermal Thu, 04 Jun 2009 00:51:05 T = T + 1 − T (3) m c m nucleation.25 The half-life crystallization time t05 , the time required where Tm is the equilibrium melting point, and may be to achieve 50% of the final crystallinity of the samples, is regarded as a measure of the stability, i.e., the lamellar an important parameter for the discussion of crystallization thickness of the crystals undergoing the melting process.26 kinetics. Usually, the crystallization rate can also be easily The equilibrium melting point can be obtained from the described by the reciprocal of t05 . The value of t05 is intersection of this line with the Tm = Tc equation. calculated by the following equation: The subsequent melting behavior of neat PBSU and its


increases with increasing the f-MWNTs contents in the nanocomposites because of their nucleation agent effect. In conclusion, the presence of f-MWNTs and their contents in the PBSA matrix have a significant influence on the spherulitic morphology and the overall crystallization process of PBSA.

(a) 66 ºC

Heat flow (Endo up)

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68 ºC 70 ºC

3.4. Effect of f-MWNTs on the Nucleation Activity and Crystallizability of PBSA

72 ºC 74 ºC

70

80

Heat flow (Endo up)

(b)

From the aforementioned studies, it is obvious that nonisothermal melt crystallization, overall isothermal crystal90 100 110 lization rate and nucleation density of PBSA have been Temperature (ºC) enhanced by the presence of f-MWNTs in the nanocomposites due to the heterogeneous nucleation effect. In this section, the effect of the presence of f-MWNTs and their contents on the nucleation activity and crystallizability of PBSA were evaluated quantitatively in the PBSA/ 76 ºC f-MWNTs nanocomposites. Delivered by Ingenta to: 78 ºC The nucleating activity of a foreign substrate with National Institute for Materials Sciernce respect to the crystallization of a polymer can be estimated 80 ºC IP : 144.213.253.16 with a method developed by Dobreva and Gutzowa.30 31 Thu,8204 Jun 2009 00:51:05 ºC For homogeneous nucleation from the melt, the cooling rate can be written as follows: 84 ºC log = A − 90

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Temperature (ºC) Fig. 6. Melting behaviors of neat PBSA and the PBSA/MWNTs 99/1 nanocomposite after isothermal melt crystallization, (a) neat PBSA and (b) 99/1.

to the heating scan in DSC, and Tm2 is the melting of the crystals formed through the recrystallization and reorganization of the crystals corresponding to Tm1 during the heating process. Therefore, Tm1 is used for the analysis with the Hoffman-Weeks equation. Figure 7 displays the HoffmanWeeks plot for neat PBSA and its three nanocomposites with different f-MWNTs contents. From the Hoffmanweeks plot, the values of Tm are determined to be around 115.7 C for both neat PBSA and its nanocomposites. The incorporation of f-MWNTs and the f-MWNTs contents do not affect Tm of PBSA in the nanocomposites. The spherulitic morphology of neat PBSA and its nanocomposites was studied with POM. Figure 8 illustrates the spherulitic morphology of neat PBSA and its nanocomposite crystallized at 80 C. It can be seen from Figure 8(a) that the well developed spherulites grow to a size of roughly several hundreds of microns in diameter for neat PBSA. Parts (b), (c) and (d) of Figure 8 show that the size of PBSA spherulites becomes smaller with increasing the f-MWNTs contents, indicative of an increased heterogeneous nucleation effect of f-MWNTs. From Figure 8, it is obvious that the nucleation density of PBSA spherulites 4966

(4)

where Tp is defined as Tm − Tp , Tm is the melting point temperature, and Tp is the crystallization peak temperature. A and B are constants. For heterogeneous nucleation, the cooling rate is defined as follows: log = A −

B∗ 2303Tp2

(5)

where B ∗ is a constant. The ratio of B ∗ /B is defined as the nucleating activity N . If the foreign substrate is extremely active, the nucleation activity approaches zero,

120

100

Tm (ºC)

80

B 2303Tp2

80 100/0 99.5/0.5 99/1 98/2

60

60

80

100

120

Tc (ºC) Fig. 7. Hoffman-Weeks plot for the estimation of the equilibrium melting point temperatures for neat PBSA and its nanocomposites.


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Crystallization Kinetics and Morphology Studies of Biodegradable PBSA/MWCNTs Nanocomposites (a)

(b)

(c)

(d)

Delivered by Ingenta to: National Institute for Materials Sciernce IP : 144.213.253.16 Thu, 04 Jun 2009 00:51:05 Fig. 8. Optical micrographs (same magnification, bar = 100 m) of spherulitic morphology of neat PBSA and its nanocomposite after complete crystallization at 80 C, (a) neat PBSA, (b) 99.5/0.5, (c) 99/1, and (d) 98/2.

100/0 99.5/0.5 99/1 98/2

1.6

Log Φ

1.4

1.2

1.0

0.8

0.6 0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

1/∆Tp2 (K–2) Fig. 9. Plots of log versus 1/Tp2 for the estimation of the nucleation activity of PBSA induced by the f-MWNTs at different contents in the nanocomposites.


of N are determined to be 0.934, 0.639 and 0.434 for the nanocomposites at 0.5, 1 and 2 wt% f-MWNTs contents, respectively. All the N values are less than 1, indicating that f-MWNTs act as nucleating agent for the crystallization of PBSA. Moreover, the nucleation activity is improved with increasing the f-MWNTs contents. The nucleation activity of PBSA is enhanced slightly by the presence of low f-MWNTs content less than 0.5 wt%; however, the enhancement in the nucleation activity of PBSA is significant with further increasing f-MWNTs contents, especially at 2 wt% f-MWNTs content. Such results are consistent with POM study in the previous section. After evaluating the effect of f-MWNTs on the nucleation activity of PBSA, we further studied the effect of f-MWNTs on the crystallizability of PBSA in the nanocomposites. In order to rank polymer crystallizability on a single scale, Nadkarni et al. proposed a method to compare nonisothermal experimental data by plotting the degree of undercooling versus cooling rate for investigating the crystallizability.32 According to the Nadkarni mehod, the variation of the degree of supercooling Tc with cooling rate can be fitted to a linear equation: Tc = P ∗ + Tc

(6)

where the intercept Tc signifies the inherent crystallizability, being the degree of undercooling required in the limit of zero cooling rates, and the slop P is a process 4967

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while for inert particles, it approaches 1. However, it is well known that Tm is affected by several factors, like scanning rate, presence of multiple peaks and annealing phenomena. Therefore, we used Tm to calculate the supercooling instead of Tm . Figure 9 shows the plots of log against 1/Tp2 , from which the values of B o for neat PBSA and B ∗ for its three nanocomposites with different f-MWNTS contents are obtained from the slopes, respectively. Thus, the values


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∆Tc (ºC)

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Table II. Summary of P and Tc values for evaluating sensitivity and sensitivity factor that accounts for the kinetic effects. crystallizability of neat PBSA and its nanocomposites during nonisotherThe degree of supercooling was calculated as the differmal melt crystallization. ence between the melting peak temperature in the heatSamples P Tc ( C) ing scan and the temperature at onset of crystallization in the cooling scan. However, it is well known that the Neat PBSA 0.339 50.2 melting peak temperature in a DSC scan is affected by PBSA/MWNTs-99.5/0.5 0.306 46.9 PBSA/MWNTs-99/1 0.264 37.9 several factors, like scanning rate, presence of multiple PBSA/MWNTs-98/2 0.243 34.4 peaks and annealing phenomena.33 Similar to the study of nucleation activity in the previous section, we also employed the equilibrium melting temperature instead of the reduction in the Tc becomes smaller again when the apparent melting peak temperature to diminish the the f-MWNTs content is increased from 1 to 2 wt% influence of the aforementioned factors in the present in the nanocomposites. Such results indicate again that the work. f-MWNTs content plays a remarkable role of influencFigure 10 shows the plots of the variation of Tc with ing the crystallizability of PBSA in the nanocomposites. It cooling rate , from which the values of P and Tc are is obvious that the crystallizability of PBSA is enhanced obtained from the slopes and the intercepts, respectively. significantly at low f-MWNTs contents less than 1 wt%. The values of P and Tc for neat PBSA and its three With further increasing f-MWNTs content above 1 wt%, nanocomposites at different f-MWNTs contents are listed the crystallizability can still be improved to some extent, in Table II. From Table II, it is seen that the values of by Ingenta to: Delivered but the degree of enhancement is not as obvious as that at P decrease from around 0.339 for National neat PBSAInstitute to 0.243for Materials Sciernce low f-MWNTs contents. for the 98/2 nanocomposite with increasing the f-MWNTs IP : 144.213.253.16 contents. Such reduction of P values suggests the 2009 00:51:05 Thu, that 04 Jun crystallization process sensitivity of PBSA decreases with 4. CONCLUSIONS increasing the f-MWNTs contents in the nanocomposites during nonisothermal melt crystallization. In other words, Biodegradable PBSA/f-MWNTs nanocomposites at differincreasing the f-MWNTs contents in the nanocomposites ent f-MWNTs loadings have been prepared successfully reduces the sensitivity of the variation of supercoolings through solution casting method. A homogeneous disperwith cooling rate. Moreover, Tc is around 50.2 C for sion of f-MWNTs throughout the PBSA matrix has been neat PBSU and decreases to 34.4 C for the 98/2 nanocomobserved by SEM. The effect of f-MWNTs on the crysposite with increasing the f-MWNTs contents; suggesting tallization behavior of PBSA in the nanocomposites has that the crystallizability of PBSA has been enhanced sigbeen investigated by DSC, POM and WAXD in detail. nificantly by the presence of f-MWNTs due to the hetThe results show that f-MWNTs act as nucleation agent erogeneous nucleation effect. The reduction in the Tc for the crystallization of PBSA. The crystallization rate between neat PBSA and the 99.5/0.5 sample is small and of PBSA is enhanced significantly while the crystallizaonly around 3.3 C; however, the reduction in the Tc tion mechanism remains unchanged in the presence of becomes larger and is around 9 C with further increasf-MWNTs in the nanocomposites. The enhancement in the ing f-MWNTs content from 0.5 to 1 wt%. In addition, nucleation activity and crystallizability of PBSA was evaluated quantitatively to investigate the effect of the presence of f-MWNTs and their loadings in the PBSA/f-MWNTs 60 nanocomposites. It is found that the nucleation activity and crystallizability of PBSA are enhanced significantly 55 at low f-MWNTs contents less than 1 wt%. With further increasing f-MWNTs content above 1 wt%, the nucleation 50 activity and crystallizability can still be improved to some 45 extent, but the degree of enhancement is not as obvious as that at low f-MWNTs contents. 40 35

100/0 99.5/0.5 99/1 98/2

30 25 5

10

15

20

25

30

Φ (ºC/min) Fig. 10. Plots of the variation of Tc with cooling rates for comparing the crystallizability of neat PBSA and its nanocomposites.

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Acknowledgments: Part of this work is financially supported by the National Natural Science Foundation, China (Grant Nos. 20504004 and 20774013), Program for New Century Excellent Talents in University (NCET-06-0101), Program for Changjiang Scholars and Innovative Research Team in University (IRT0706), and the projects of Polymer Chemistry and Physics, Beijing Municipal Commission of Education (XK100100640). J. Nanosci. Nanotechnol. 9, 4961–4969, 2009

Qiu et al.


References and Notes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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Received: 7 September 2008. Revised/Accepted: 27 September 2008.

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