Synthesis, Crystallization Kinetics, and Morphology of Novel

Article pubs.acs.org/IECR

Synthesis, Crystallization Kinetics, and Morphology of Novel Biodegradable Poly(butylene succinate-co-hexamethylene succinate) Copolyesters Guyu Wang and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Biodegradable poly(butylene succinate) and a series of poly(butylene succinate-cohexamethylene succinate) (P(BS-co-HS)) with hexamethylene succinate (HS) comonomer composition ranging from 14 to 35 mol % were prepared in the present work via a two-stage melt polycondensation method. Basic thermal behaviors, crystal structure, isothermal melt crystallization kinetics, and spherultic morphology and growth of P(BS-co-HS) copolyesters with different HS composition were studied in detail with various techniques and compared with those of neat PBS. With respect to neat PBS, the glass transition temperature of P(BS-co-HS) decreases slightly, while the melting point temperature and equilibrium melting point temperature of P(BS-co-HS) are reduced significantly with an increase in the HS composition. Both neat PBS and P(BS-co-HS) have the same crystal structure; however, the crystallinity values are slightly smaller in P(BS-co-HS) than in neat PES. The overall isothermal melt crystallization kinetics of neat PBS and P(BS-co-HS) were studied in a wide crystallization temperature range and analyzed by the Avrami equation. The experimental results indicate that the crystallization mechanism remains unchanged for both neat PBS and P(BS-co-HS); however, the overall crystallization rates of P(BS-co-HS) decrease with increasing HS composition and crystallization temperature. The spherulitic morphology of neat PBS and P(BS-co-HS) were also investigated in a wide crystallization temperature range; moreover, the spherulitic growth rates of P(BS-co-HS) also decrease with increasing HS content and crystallization temperature.

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INTRODUCTION

In addition, the incorporation of a second comonomer into the backbone of PBS is another effective way of modifying its various properties from both academic and practical viewpoints. Many new biodegradable random and block PBS-based copolyesters have already been developed via the chemical copolymerization method, such as poly(butylene succinate-coethylene succinate) (P(BS-co-ES)), poly(butylene succinate-copropylene succinate) (P(BS-co-PS)), poly(butylene succinateco-butylene adipate) (P(BS-co-BA)), poly(butylene succinateco-ε-caprolactone) (P(BS-co-CL)), poly(butylene succinate-cobutylene terephthalate) (P(BS-co-BT)), poly(butylene succinate-co-butylene fumarate) (P(BS-co-BF)), poly(butylene succinate-co-diethylene glycol succinate) (P(BS-co-DEGS)), poly(ethylene succinate)-b-poly(butylene succinate) (PES-bPBS), poly(butylene succinate-b-poly(ethylene glycol) succinate) (P(BS-b-PEGS)) multiblock copolymers, etc.18−29 Similar to PBS, poly(hexamethylene succinate) (PHS) is also a chemosynthetic biodegradable polyester. PHS has a chemical structure of (−OCH2CH2CH2CH2CH2CH2O2CCH2CH2CO−)n. For PBS and PHS, they are only different in the numbers of methylene groups between the two ether groups, that is, 4 and 6, respectively. Crystal structure, single crystal degradation, and nonisothermal and isothermal melt crystallization kinetics of PHS have been investigated in detail.30,31 Synthesis, crystallization behaviors and thermal

As a commercially available biodegradable polyester, poly(butylene succinate) (PBS) has recently received considerable attention.1 PBS has a chemical structure of (OCH2CH2CH2CH2O2CCH2CH2CO)n. Till now, the crystal structure, nonisothermal and isothermal melt crystallization kinetics, and multiple melting behaviors of PBS have been investigated with various techniques extensively.2−7 The physical properties of PBS may be improved for its wider practical application via many modifications including polymer blending and chemical copolymerization.8−29 Polymer blending is known as a convenient means of preparing novel materials with improved and desired physical properties. Miscibility and crystallization behaviors of the PBSbased polymer blends have been studied extensively.8−17 On the one hand, PBS is miscible with many polymers, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene chloride-covinylchoride) (PVDCVC), poly(ethylene oxide) (PEO), poly(vinyl phenol) (PVPh), poly(butylene adipate) (PBA), and tannic acid (TA).8−13 On the other hand, PBS is immiscible with some biodegradable polyesters, such as poly(hydroxybutyrate) (PHB), poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV), poly(ε-caprolactone) (PCL), and poly(Llactide) (PLLA).13−17 For these PBS-based polymer blends, they not only have improved physical properties but also are completely biodegradable.13−17 However, it should be noted that the PBS-based polymer blends may not be completely biodegradable if they are blended with a nonbiodegradable polymer.8−10,12 © 2012 American Chemical Society

Received: Revised: Accepted: Published: 16369

October 15, 2012 November 8, 2012 November 28, 2012 November 29, 2012 dx.doi.org/10.1021/ie302817k | Ind. Eng. Chem. Res. 2012, 51, 16369−16376

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) Chemical structure of P(BS-co-HS) and (b) 1H NMR spectrum for P(BS-co-24 mol %HS).

filled with nitrogen to remove oxygen completely. The mixture was heated to 150−160 °C and kept at this temperature until all the monomers melted completely under nitrogen atmosphere. The mixture was then heated to 190 °C within 2 h; moreover, the water was removed and collected through a cooling device. During the polycondensation stage, the reaction temperature was further increased to 220 °C at a reduced pressure of around 0.5 mmHg. After the reaction lasted 3−5 h at 220 °C, the synthesized product was dissolved in chloroform (CHCl3) and then precipitated using three times the amount of methanol. The precipitate was washed with methanol for several times and then dried at 30 °C in a vacuum for 72 h. Wide-angle X-ray diffraction (WAXD) experiments were performed on a Riga- kud/Max2500 VB2+/PC X-ray diffractometer at 40 kV and 20 mA from 5° to 45° at 4°/ min. The samples for the WAXD experiments were first pressed into films with the thickness of around 1 mm on a hot stage at 140 °C for 3 min, and then transferred into an oven at 65 °C for 24 h. 1 H NMR spectra of the polyesters were obtained with a Bruker AV 600 spectrometer operating at a frequency of 600 MHz. Deuterated chloroform (CDCl3) was used as a solvent. Gel permeation chromatography (GPC) analysis was performed using CHCl3 as the solvent at a flow rate of 1 mL/min with a Waters 515 HPLC GPC (Waters Company, USA). The weight-average molecular weight (Mw) was calculated by using a calibration curve, which was obtained by polystyrene standards with low polydispersity indices. Thermal analysis was performed using a TA Instruments differential scanning calorimeter (DSC) Q100 with Universal Analysis 2000 software. The samples were annealed at 140 °C for 3 min to erase any thermal history and subsequently quenched to −70 °C at a cooling rate of 60 °C/min. Glass transition temperature and melting point of the melt-quenched samples were measured at a heating rate of 20 °C/min. The crystallization peak temperature was obtained from the DSC cooling traces from the crystal-free melt at 10 °C/min. The

properties of the PHS-based copolymers have also recently been investigated.32−35 Poly(butylene succinate-co-hexamethylene succinate) (P(BSco-HS)), a random copolyester of PBS and PHS, is also biodegradable; however, few works have dealt with the structure and properties of P(BS-co-HS) compared with those of their parent homopolymers, PBS and PHS.34,35 In the present work, a series of P(BS-co-HS) random copolyesters and neat PBS were synthesized via a two-stage melt polycondensation method; moreover, the effect of comonomer composition on the basic thermal behaviors, crystal structure, isothermal melt crystallization kinetics, and spherulitic morphology and growth rates of P(BS-co-HS) was studied with various techniques in detail with respect to neat PBS. The aim of this work is to get a better understanding of the structure and properties relationship of biodegradable polymers from both academic and practical viewpoints.

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EXPERIMENTAL SECTION Neat PBS and its random copolyesters P(BS-co-HS) with different hexamethylene succinate (HS) composition were synthesized from succinic acid (SA) and the proper 1,4butanediol (BD) or mixture of BD and 1,6-hexanediol (HD). SA was bought from Tianjing Fuchen Chemical Solvent Factory. BD and HD were purchased from Tianjing Guangfu Fine Chemicals Research Institution and Sinopharm Chemical Reagent Company, respectively. Tetrabutyl titanate (TBT) was bought from Beijing Chang Ping Jing Xiang Chemical Factory and used as a catalyst. The aliphatic polyesters were prepared via a two-stage melt polycondensation method (including esterification and polycondensation). For the preparation of neat PBS and its copolyesters, the proper amount of appropriate SA and diol(s) (various molar ratios of BD and HD) in an acid/diol(s) molar ratio of 1:1.2 and the catalyst (1 × 10−4 mol TBT/mol acid) were charged into a three-necked 250 mL flask equipped with an agitator. During the esterification stage, the flask was first 16370

dx.doi.org/10.1021/ie302817k | Ind. Eng. Chem. Res. 2012, 51, 16369−16376


Article

isothermal melt crystallization was also examined with DSC. The samples were also annealed at 140 °C for 3 min to erase any thermal history and then cooled at 60 °C/min to the desired crystallization temperature until completely crystallization. After isothermal melt crystallization, the samples were heated to melt again at a rate of 20 °C/min to study the subsequent melting behavior for estimating the equilibrium melting point temperature. All the operations were performed under nitrogen purge, and the weights of the samples were varied between 4 and 5 mg. The spherulitic morphology and growth rates of neat PBS and its copolymers were investigated with a polarized optical microscope (POM) (Olympus BX51) equipped with a firstorder retardation plate and a temperature controller (Linkam THMS 600). The samples were first melted at 140 °C for 3 min to erase any thermal history and then quenched to the desired crystallization temperature at 60 °C/min. The spherulites growth rate (G) was calculated from the variation of radius (R) with time (t), that is, G = dR/dt.

Figure 2. DSC heating curves of neat PBS and its copolyesters at 20 °C/min after quenching from the melt at 60 °C/min.

Neat PBS has a Tg of −33.8 °C and a Tm of 114.0 °C. For the P(BS-co-ES) copolymers, both the Tg and Tm values are found to decrease apparently with an increase in the HS comonomer composition. With increasing the HS content from 14 to 35 mol %, the Tg values are found to decrease apparently from −38.2 to −45.9 °C for the P(BS-co-HS) copolyesters. Tm also shifts downward to low temperature range for the P(BS-co-HS) copolyesters with increasing the HS composition. The Tm values are reduced significantly from 103.6 to 84.3 °C with increasing the HS composition from 14 to 35 mol %. It should also be noted that the heat of fusion (ΔHm) values are dependent on the HS composition for neat PBS and P(BS-coES). In the case of neat PBS, it has a ΔHm of 93.9 J/g; however, the ΔHm values are significantly smaller in the P(BS-co-HS) samples than in neat PBS. For example, the ΔHm values are varied between 65.8 and 59.7 J/g with an increase in the HS content from 14 to 35 mol %. As shown in Figure 2, there is a crystallization exotherm just prior to Tm for all the samples, which also shifts to a low temperature range with an increase in the HS comonomer composition. The crystallization exotherm may arise from the recrystallization of the melting of the crystals with low thermal stability.7,26 Nonisothermal melt crystallization behaviors of neat PBS and P(BS-co-HS) were further investigated with DSC. Figure 3 shows the DSC cooling traces of neat PBS and P(BS-co-HS) with different HS composition at 10 °C/min. For neat PBS, the

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RESULTS AND DISCUSSION Composition of Synthesized Polyesters. As described in the Experimental Section, neat PBS and its copolyesters were synthesized by the two-stage method. Figure 1a shows the chemical structure of P(BS-co-HS). It is of great importance to determine the exact composition of the P(BS-co-HS) copolyesters. In the present work, the composition of P(BSco-HS) was estimated from 1H NMR spectrum using the relative intensities of the proton peaks arising from the butylene succinate (BS) and HS repeating unit. For instance, Figure 1b illustrates the 1H NMR spectrum and peak assignment of (PBSco-24 mol %HS). The peaks at 2.61 ppm originate from the methylene protons H1 of SA. The peaks at 1.69 and 4.10 ppm are attributed to H3 and H2 protons from BS. In addition, the peaks at 1.36, 1.62, and 4.07 ppm are attributed to H6, H5, and H4 protons from HS, respectively. The molar composition of P(BS-co-HS) was calculated by the area ratio of peaks of H2 from BS units and H4 from HS units, or H3 from BS units and H5 from HS units. The obtained results are summarized in Table 1. The Mw values of neat PBS and its copolyesters with different compositions were determined with GPC and are also listed in Table 1. Table 1. Compositions, Molecular Weight and Crystallinity of Neat PBS and Its Copolyesters with Different HS Composition samples

BS/HS molar ratio

neat PBS P(BS-co-14 mol %HS) P(BS-co-24 mol %HS) P(BS-co-35 mol %HS)

100/0 86/14 76/24 65/35

Mw (g/mol)

Xc (%)

× × × ×

49.1 46.9 42.7 39.1

1.6 3.3 1.9 2.4

104 104 104 104

Basic Thermal Behaviors of Neat PBS and Its Copolyesters. It is essential to investigate the effect of the HS content on the glass transition temperature (Tg) and melting point (Tm) of P(BS-co-HS). As introduced in the Experimental Section, Tg and Tm were measured for the meltquenched neat PBS and P(BS-co-HS) samples. Figure 2 shows the DSC heating traces of neat PBS and its copolymers with different HS comonomer composition at a heating rate of 20 °C/min.

Figure 3. DSC cooling curves of neat PBS and its copolyesters at 10 °C/min. 16371



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P(BS-co-HS) were further investigated with DSC to separate the morphological effect from the thermodynamic effect. Hoffman and Weeks proposed a relationship between the apparent Tm and the isothermal crystallization temperature (Tc)

nonisothermal melt crystallization peak temperature (Tcc) is 75.2 °C. For the P(BS-co-HS) samples, the Tcc values are significantly smaller than that of neat PBS. For instance, Tcc shifts from 61.2 to 34.8 °C with increasing HS content from 14 to 35 mol % for the P(BS-co-HS) copolyesters when they were crystallized from the melt at 10 °C/min. It is also interesting to note that the melt crystallization enthalpy (ΔHcc) values are also dependent on the HS composition for the P(BS-co-HS) copolyesters. For neat PBS, it has a ΔHcc of 93.5 J/g. For the P(BS-co-ES) samples, the ΔHcc values become significantly smaller compared with that of neat PBS. With an increase in the HS content from 14 to 35 mol %, the ΔHcc values decrease from 65.9 to 49.8 J/g for the P(BS-co-HS) copolyesters. Both the Tcc and ΔHcc variations suggest that the nonisothermal melt crystallization behaviors of P(BS-co-HS) are suppressed and affected significantly by the HS composition with respect to neat PBS. For a better understanding of the effect of the HS composition on the basic thermal behaviors of P(BS-co-HS), the obtained Tg, Tcc, ΔHcc, Tm, and ΔHm values of P(BS-coHS) with different HS composition are summarized in Table 2

Tm = ηTc + (1 − η)Tmo

(1)

where is the equilibrium melting point, and η may be regarded as a measurement of the stability, i.e., the lamellar thickness, of the crystals undergoing the melting process.36 Tom can thus be obtained from the intersection of this line with the Tm = Tc equation. The subsequent melting behaviors were studied with DSC after isothermal crystallization at various Tc values for neat PBS and its copolyesters with different HS content. Figure 4 shows Tom

Table 2. Basic Thermal Properties for Neat PBS and Its Copolyesters samples

Tg (°C)

Tcc (°C)

ΔHcc (J/g)

Tm (°C)

ΔHm (J/g)

Tom (°C)

neat PBS P(BS-co-14 mol % HS) P(BS-co-24 mol % HS) P(BS-co-35 mol % HS)

−33.8 −38.2

74.6 61.2

93.5 65.9

114.0 103.6

93.9 65.8

136.3 117.5

−42.4

52.9

64.5

94.3

64.4

110.0

−45.9

34.8

49.8

84.3

59.7

103.7

Figure 4. Melting behavior of P(BS-co-35 mol %HS) crystallized at different Tc values.

the melting behavior of P(BS-co-35 mol %HS) as an example. In the investigated Tc region of 45−65 °C, two melting endothermic peaks are observed for P(BS-co-35 mol %HS). Similar melting behaviors are also observed for neat PBS and the other two copolymers. For brevity, they are not shown here. The lower endothermic peak (Tm1) shifts from 64.9 upward to 78.1 °C with increasing Tc from 45 to 65 °C. However the higher endothermic peak (Tm2) is almost unchanged at around 84 °C. It is interesting to find that the magnitude of the area of Tm1 increases while that of Tm2 decreases with increasing Tc, indicating that the ratio of the area of Tm1 to that of Tm2 becomes larger with the increase of Tc. Such double melting behaviors are very common for semicrystalline polymers, which may be well explained by the melting, recrystallization, and remelting mechanism.37−40 Tm1 is attributed to the melting of crystals formed during the isothermal crystallization process at a given Tc, and Tm2 is attributed to the melting of the crystals formed through the recrystallization during the heating process. At lower Tc, the crystals are imperfect; therefore, they will undergo melting, recrystallization, and remelting upon heating to the melt. At higher Tc, the crystals become more perfect; therefore, the recrystallization will be restricted during the heating process. Therefore, Tm1 is used to for the Hoffman− Weeks equation to estimate the Tom values. Figure 5 displays the Hoffman−Weeks plots for neat PBS and its three copolyesters, from which the Tom values may be estimated. For neat PBS, Tom is determined to be 136.3 °C. For the copolyesters, the Tom values are estimated to be 117.5, 110.0, and 103.7 °C with increasing HS content from 14 to 35 mol %. These values are also listed in Table 2 for comparison. The

and compared with those of neat PBS. From Table 2, it can be concluded that the incorporation of the HS comonomer has a significant influence on the basic thermal behaviors of P(BS-coHS) with respect to neat PBS. All the basic thermal behaviors have been suppressed with incorporation of the HS comonomer composition for the P(BS-co-HS) copolyesters. In a comparison of P(BS-co-35 mol %HS) and neat PBS, the Tg value is reduced moderately by around 10 °C, and the Tcc and Tm values are reduced seriously by around 40 and 30 °C, respectively. In addition, the ΔHcc and ΔHm values are also reduced significantly by around 40 and 30 J/g, respectively. It is interesting to discuss the suppression of the basic thermal properties of P(BS-co-HS) with incorporation of the HS comonomer composition. Because the Tg value of PHS is as low as around −50 °C, the incorporation of HS content into the backbone of PBS must reduce the Tg values of P(BS-co-HS) compared with neat PBS. In comparison with that of the PBS homopolymer, the order of the polymer chain will be destroyed in the P(BS-co-HS) random copolyesters; therefore, the crystallization ability will also be reduced, resulting in the suppression of Tcc, ΔHcc, Tm, and ΔHm. Equilibrium Melting Point Temperature of Neat PBS and Its Copolyesters. As described in the above section, increasing the HS composition leads to an apparent depression of Tm for P(BS-co-HS) relative to neat PBS. It is well-known that Tm is influenced by both the thermodynamic factors and the morphological factors such as crystalline lamellar thickness. In this section, equilibrium melting points of neat PBS and 16372



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copolyesters crystallized at 65 °C for 24 h. For neat PBS, it shows three main characteristic diffraction peaks located at 19.4°, 21.9°, and 22.6°, which are assigned to (020), (021), and (110) planes, respectively.2,20 For the copolyesters, they exhibit the similar diffraction peaks at almost the same locations as neat PBS. It can thus be concluded from the WAXD pattern that all the copolyesters with various HS contents have the same crystal structures as neat PBS, indicating that the HS content exists in an amorphous state and is excluded from crystal region of PBS.19,20 The crystallinity (Xc) values were further estimated on the basis of the WAXD patterns. The Xc values were estimated to be 49.1%, 46.9%, 42.7%, and 39.1% for neat PBS and its copolyesters with increasing HS content from 14 to 35 mol %, which are also listed in Table 1 for comparison. In conclusion, compared with neat PBS, the incorporation of the HS content does not modify the crystal structures but reduces slightly the crystallinity values of P(BS-co-HS). The overall isothermal melt crystallization kinetics of neat PBS and P(BS-co-HS) was further studied with DSC in a wide crystallization temperature region. Figure 7 panels a and b show the development of relative crystallinity with crystallization time for both neat PBS and P(BS-co-24 mol %HS) at different Tc values, respectively. As shown in Figure 7, the crystallization time becomes longer with increasing Tc for both neat PBS and P(BS-co-24 mol %HS), suggesting that crystallization is retarded at higher Tc. Similar trends are also found for the other two copolyesters. The well-known Avrami equation is often used to analyze the isothermal crystallization kinetic; it assumes that the relative crystallinity develops with crystallization time as

Figure 5. The Hoffman−Weeks plots for the determination of equilibrium melting point temperatures of neat PBS and its copolyesters.

aforementioned results indicate that the incorporation of the HS composition results in a significant depression of Tom for the P(BS-co-HS) copolyesters relative to neat PBS. Crystal Structure and Isothermal Melt Crystallization Kinetics of Neat PBS and Its Copolyesters. It is of great interest and importance to study the effect of the HS content on the crystal structure of P(BS-co-HS), because both BS and HS are able to crystallize in the PBS and PHS homopolymers. The crystal structures of neat PBS and its three copolyesters with different HS contents were investigated with WAXD. Figure 6 shows the WAXD patterns of neat PBS and its

1 − X t = exp( −kt n)

(2)

where Xt is the relative crystallinity at time t, n is the Avrami exponent depending on the nature of nucleation, and growth geometry of the crystals, and the k is the crystallization rate constant involving both nucleation and growth rate parameters.41 In the case of the DSC experiments, Xt at t is defined as the ratio of the area under the exothermic curve between the crystallization onset time and t to the whole area under the exothermic curve from the crystallization onset time to the crystallization end time. Figure 8 panels a and b show the Avrami plots for neat PBS and P(BS-co-24 mol %HS), respectively. A series of almost parallel lines were obtained for both of the samples at various Tc values, indicating that the Avrami method can describe the isothermal melt crystallization process of neat PBS and its copolyesters very well. The Avrami parameters n and k are

Figure 6. WAXD patterns of neat PBS and its copolyesters.

Figure 7. Development of relative crystallinity with crystallization time at various temperatures for (a) neat PBS and (b) P(BS-co-24 mol %HS). 16373



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Figure 8. Avrami plots for (a) neat PBS and (b) P(BS-co-24 mol %HS).

Table 3. Avrami Parameters for Neat PBS and Its Copolyesters at Various Tc values neat PBS Tc (°C) 85 90 95 97.5 100

n 2.6 2.2 2.7 2.4 2.2

P(BS-co-14 mol %HS) −n

k (min ) 1.62 1.11 1.03 4.28 5.42

× × × × ×

−1

10 10−2 10−4 10−5 10−5

Tc (°C)

n

65 70 75 80 85

2.7 2.4 2.5 2.4 2.4


k (min ) 2.85 3.81 3.52 2.72 1.42

× × × ×

Tc (°C)

n

55 60 65 70 75

2.5 2.6 2.5 2.5 2.5

10−1 10−2 10−3 10−4

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

k (min ) 1.95 5.00 6.75 2.92 8.23

× × × ×

10−1 10−2 10−3 10−5

Tc (°C)

n

45 50 55 60 65

2.7 2.8 2.2 2.6 2.5

k (min−n) 1.70 2.42 1.11 1.98 9.83

× × × × ×

10−1 10−2 10−2 10−4 10−6

with increasing Tc for all the samples, indicating that the overall isothermal melt crystallization rate decreases with increasing Tc. Such results are reasonable, because nucleation becomes more difficult at higher Tc. In addition, at a given Tc of 65 °C, the 1/ t0.5 values decrease with increasing the HS content. The reduction of the overall isothermal melt crystallization rate with increasing HS content may be explained as follows. First, increasing the HS content must make the crystallizable BS unit of P(BS-co-HS) more difficult to crystallize because of the diluent effect induced by the HS composition. Second, the depression of equilibrium melting point temperature of P(BSco-HS) reduces the driving force of isothermal crystallization. In brief, the overall isothermal melt crystallization rate of P(BS-coHS) is reduced with increasing Tc and the HS content. Spherulitic Morphology and Growth for Neat PBS and Its Copolymers. Spherulitic morphology and growth were further studied with POM in a wide crystallization temperature range for neat PBS and P(BS-co-HS) with different HS content, which have important influence on the physical properties and biodegradation behaviors of biodegradable polymers. Figure 10 displays a series of POM images of P(BS-co-24 mol %HS) spherulites isothermally crystallized at various Tc values as an example. As shown in Figure 10, the size of the P(BS-co-24 mol %HS) spherulites becomes larger with an increase of Tc. The average diameter of P(BS-co-24 mol %HS) spherulites is only around 30 μm at 50 °C; however, the average diameter increases significantly to be around 200 μm with a further increase of Tc to 70 °C. Such variation is reasonable because of the difficulty in nucleation and a subsequent decrease in number of nuclei at higher Tc. Similar results are also found for neat PBS and the other two P(BS-co-HS) copolyesters. The spherulitic growth rates were measured by following the variation of radius with time at various Tc values for neat PBS and its copolyesters. All the samples show a linear growth with crystallization time until contact with other spherulites in spite of Tc, indicating that the spherulites growth rates are independent of crystallization time for neat PBS and P(BS-coHS). Figure 11 displays the crystallization temperature dependence of the spherulites growth rates for neat PBS and

obtained from the slopes and intercepts, respectively, which are summarized in Table 3. From Table 3, it is clear that the values of n are varied slightly between 2.2 and 2.8 for both neat PBS and its copolyesters within the Tc range investigated in this work. The average n value is around 2.5 for all the samples, indicating that the crystallization of neat PBS and P(BS-co-HS) copolyesters may correspond to the three-dimensional truncated sphere growth with athermal nucleation.42 The slight variation of n also indicates that the crystallization mechanism does not change. As the unit of k is min−n and n is not constant, it is not suitable to compare the overall crystallization rate directly from the k values. The crystallization half-time (t0.5) is introduced for the discussion of crystallization kinetics of neat PBS and its copolyesters in this work and may be calculated by the following equation:

t0.5 =


(3)

Figure 9 illustrates the variation of 1/t0.5 with Tc for neat PBS and its copolyesters. From Figure 9, the 1/t0.5 values decrease

Figure 9. The Tc dependence of 1/t0.5 for neat PBS and its copolyesters. 16374



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point temperature and equilibrium melting point temperature of P(BS-co-HS) significantly. The WAXD study reveals that P(BS-co-HS) copolyesters have the same crystal structures as neat PBS, indicating that the HS unit remains in the amorphous region; moreover, the crystallinity values are smaller in P(BS-coHS) than in neat PBS because of the incorporation of HS comonomer composition. The nonisothermal melt crystallization behaviors of neat PBS and P(BS-co-HS) with different HS composition were studied with DSC at a cooling rate of 10 °C/ min. Both the nonisothermal melt crystallization peak temperatures and the crystallization enthalpy values of P(BS-co-HS) were reduced apparently relative to neat PBS, suggesting that incorporating the HS composition has retarded the nonisothermal melt crystallization of P(BS-co-HS). The overall isothermal melt crystallization kinetics was further investigated in a wide crystallization temperature range and analyzed by the Avrami equation for neat PBS and P(BS-co-HS). It is found that the crystallization mechanism remains unchanged for neat PBS and P(BS-co-HS) despite crystallization temperature and the HS composition. With increasing crystallization temperature, the overall isothermal melt crystallization rates decrease for both neat PBS and P(BS-co-HS) with different HS composition; in addition, at a given crystallization temperature, increasing the HS composition reduces the overall isothermal melt crystallization rates of P(BS-co-HS). The spherulitic morphology and growth were further studied with POM for neat PBS and P(BS-co-HS) in a wide crystallization temperature range. The experimental results indicate that the spherulitic growth rates of P(BS-co-HS) were smaller than that of neat PBS at the same crystallization temperature; moreover, increasing the HS composition decreases further the spherulitic growth rates of P(BS-co-HS).

Figure 10. Spherulitic morphologies of P(BS-co-24 mol %HS) crystallized isothermally at various Tc values (scale bar = 50 μm): (a) 55, (b) 60, (c) 65, and (d) 70 °C.

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Figure 11. Variations of G with Tc for neat PBS and its copolymers.

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-64413161. E-mail: [email protected].

its copolymers. The spherulitic growth rates decrease with an increase of Tc for neat PBS and the P(BS-co-HS) copolyesters. Spherulitic growth rate is faster in neat PBS than in the P(BSco-HS) copolyesters at the same Tc, indicating that the incorporation of the HS unit has reduced the spherulitic growth rates of the copolyesters. In addition, the spherulitic growth rates decreases with increasing the HS composition for the P(BS-co-HS) copolyesters with respect to neat PBS. The reduction of the spherulitic growth rates of P(BS-co-HS) may be related to the following two factors. On one hand, the incorporation of the HS unit results in the dilution of BS chains at the spherulites growth front and thereby reduces the growth rates of spherulites. On the other hand, the depression of Tom in the P(BS-co-HS) copolyesters drops the thermodynamic driving force required for the growth of spherulites.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this research was financially supported by the National Natural Science Foundation, China (Grant No. 51221002).

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REFERENCES

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CONCLUSIONS In the present work, biodegradable PBS and a series of P(BS-coHS) with HS comonomer composition ranging from 14 to 35 mol % were prepared via a two-stage melt polycondensation method and characterized by 1H NMR. The basic thermal behaviors, crystal structure, isothermal melt crystallization kinetics, and spherultic morphology and growth of neat PBS and the P(BS-co-HS) copolyesters were studied in detail with DSC, WAXD, and POM. In comparison to neat PBS, increasing the HS composition decreases the glass transition temperature of P(BS-co-HS) slightly but reduces the melting 16375



Article

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