A Comparative Study of Crystallization, Melting Behavior, and

Sep 26, 2012 - A comparative study was performed in this work to investigate the crystal structure, thermal behavior, melting behavior, overall isothe...
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A Comparative Study of Crystallization, Melting Behavior, and Morphology of Biodegradable Poly(ethylene adipate) and Poly(ethylene adipate-co-5 mol % ethylene succinate) Huina Wu 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, People’s Republic of China ABSTRACT: A comparative study was performed in this work to investigate the crystal structure, thermal behavior, melting behavior, overall isothermal melt crystallization kinetics, and spherulitic morphology of biodegradable neat poly(ethylene adipate) (PEA) and its novel copolyester poly(ethylene adipate-co-5 mol % ethylene succinate) P(EA-co-5 mol % ES). Both neat PEA and P(EA-co-5 mol % ES) show the same crystal structure. Relative to neat PEA, the incorporation of a small amount of ethylene succinate (ES) composition exerts almost no influence on the glass-transition temperature, reduces the melting point temperature and equilibrium melting point temperature slightly, but suppresses the nonisothermal melt and cold crystallization significantly of P(EA-co-5 mol % ES). Depending on crystallization temperature, both neat PEA and P(EA-co-5 mol % ES) show double melting endotherms or one melting endotherm, which can be well explained by the melting, recrystallization, and remelting mechanism. The overall isothermal melt crystallization kinetics was studied for neat PEA and P(EA-co-5 mol % ES) and analyzed by the Avrami equation. With increasing crystallization temperature, the crystallization rates are reduced for both neat PEA and P(EA-co-5 mol % ES); moreover, the crystallization rate is faster in neat PEA than in P(EA-co-5 mol % ES) at a given crystallization temperature. The crystallization mechanism remains unchanged despite crystallization temperature and the incorporation of ES composition. Depending on crystallization temperature, neat PEA and P(EA-co-5 mol % ES) may form ringbanded or ringless spherulites.

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spherulitic morphology of P(ES-co-EA).19 In this research note, we made a comparative study to investigate the crystal structure, thermal behavior, melting behavior, overall isothermal melt crystallization kinetics, and spherulitic morphology of neat PEA and its novel copolyester P(EA-co-5 mol % ES). The research reported herein should be interesting and helpful for a better understanding of the structure and properties relationship of biodegradable polymers from both academic and industrial viewpoints.

INTRODUCTION Biodegradable poly(ethylene adipate) (PEA) is a type of aliphatic polyester, which has a chemical structure of −(OCH 2 CH 2 O 2 CCH 2 CH 2 CH 2 CH 2 CO)− n . PEA can be made from glycol and diacid.1 Crystallization kinetics, spherulitic morphology, and thermal behavior of PEA have been investigated.2−4 Polymer blending is one of the most economical and convenient ways to develop new materials with desired properties. To improve the physical properties, many polymers have been blended with PEA. PEA is found to be miscible with poly(ethylene oxide) (PEO), tannic acid (TA), and poly(butylene succinate) (PBS).5−7 Similar to PEA, poly(ethylene succinate) (PES) is also a biodegradable polyester, which has a chemical structure of −(OCH2CH2O2CCH2CH2CO)−n. PES is one of the most attractive chemosynthetic biodegradable polymers, because it is commercially available; moreover, its crystal structure, crystallization kinetics, spherulitic morphology, melting behavior, and degradation behaviors have also been investigated extensively.8−13 Until now, most of the investigated biodegradable polymers are homopolymers, such as PEA, PES, PBS, poly(3hydroxybutyrate) (PHB), poly(L-lactide) (PLLA), and poly(εcaprolactone) (PCL).14 Only a few works have focused on biodegradable copolyesters. 15−19 In previous work, we synthesized a series of poly(ethylene succinate-co-ethylene adipate) (P(ES-co-EA)) copolyesters with ethylene adipate (EA) composition ranging from 5.1 mol % to 15.3 mol % and studied the effect of EA comonomer composition on the crystal structure, thermal behavior, crystallization kinetics, and © 2012 American Chemical Society

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EXPERIMENTAL SECTION Neat PEA (Mw = 4.9 × 104 g/mol) and P(EA-co-5 mol % ES) (Mw = 2.5 × 104 g/mol) were synthesized in our laboratory via a two-step melt-polycondensation method.1,19 Figure 1 shows the chemical structure of P(EA-co-5 mol % ES). Wide-angle X-ray diffraction (WAXD) experiments were performed on a Rigaku d/Max2500 VB2+/PC X-ray diffractometer at 40 kV and 20 mA at 4o/min from 5° to 45°. The samples for the WAXD experiments were first pressed into films with a thickness of ∼1 mm on a hot stage at 90 °C and then transferred into a vacuum oven at 35 °C for 72 h. Thermal analysis was performed using a TA Instruments Q100 differential scanning calorimetry (DSC) system with a Universal Analysis 2000 software. The samples were first annealed at 90 °C for 3 min to erase any thermal history and quenched to −80 at 60 °C/min to reach the amorphous state. Received: July 24, 2012 Accepted: September 26, 2012 Published: September 26, 2012 13323

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Figure 1. Chemical structure of P(EA-co-5 mol % ES).

Basic Thermal Behaviors of Neat PEA and P(EA-co-5 mol % ES). As introduced in the Experimental Section, the basic thermal behaviors of neat PEA and P(EA-co-5 mol % ES) were studied with DSC. Figure 3a illustrates the DSC traces of neat PEA and P(EA-co-5 mol % ES) at a heating rate of 10 °C/ min after quenching from the melt at 60 °C/min. For neat PEA, it has a glass-transition temperature (Tg) of −43.5 °C, a cold crystallization peak temperature (Tch) of −4.7 °C with a cold crystallization enthalpy (ΔHch) of 41.5 J/g, and a melting point (Tm) of 48.5 °C with a heat of fusion of (ΔHm) of 50.9 J/ g. For P(EA-co-5 mol % ES), it has a Tg of −42.9 °C, a Tch of 10.5 °C with a ΔHch of 47.8 J/g, and a Tm of 43.3 °C with a ΔHm of 42.9 J/g. Figure 3b shows the DSC cooling traces at 2 °C/min for neat PEA and P(EA-co-5 mol % ES). For neat PEA, it has a crystallization peak temperature (Tcc) of 14.4 °C with a melt crystallization enthalpy (ΔHcc) of 93.3 J/g; however, for P(EAco-5 mol % ES), it has a Tcc of 6.7 °C with a ΔHcc of 72.6 J/g. It is obvious that both the Tcc and ΔHcc values of P(EA-co-5 mol % ES) are smaller than those of neat PEA at the same cooling rate, indicating that the presence of the ES content suppresses the nonisothermal melt crystallization of P(EA-co-5 mol % ES), compared to neat PEA. Melting Behaviors of Neat PEA and P(EA-co-5 mol % ES). Relative to neat PEA, Tm is reduced by ∼5 °C with the introduction of the ES composition for P(EA-co-5 mol % ES) as shown in Figure 3a; however, Tm is affected by not only the thermodynamic factors but also the morphological factors, such as crystalline lamellar thickness. For a better understanding of the effect of the ES content on the melting point depression of the P(EA-co-5 mol % ES), equilibrium melting points of neat PEA and P(EA-co-5 mol % ES) were further investigated with DSC in order to separate the morphological effect from the thermodynamic effect. Figure 4a shows the subsequent melting behaviors of P(EAco-5 mol % ES) after crystallizing at different Tc values at a heating rate of 10 °C/min. Figure 4a clearly shows that P(EAco-5 mol % ES) shows double melting endotherms or one melting endotherm, depending on Tc. When Tc is 26 °C and below, double melting endotherms are found; however, only one melting endotherm is found when Tc is 30 °C and above. In the case of double melting endotherms, the lower endothermic peak (Tm1) shifts upward to the high-temperature range with increasing Tc; however, the higher endothermic peak (Tm2) remains almost unchanged. In addition, the ratio of the area of Tm1 to that of Tm2 increases with the increase of Tc. In the case of one melting endotherm, the endothermic peak shifts upward to the high-temperature range with increasing Tc. Such melting behaviors may be explained by the melting, recrystallization, and remelting mechanism.22,23 Tm1 is the melting of crystals formed during the isothermal crystallization process at a given Tc, and Tm2 is the melting of the crystals formed through the recrystallization during the heating process. When Tc is 26 °C and below, the formed imperfect crystals will

The glass-transition temperature and the melting point were measured on the quenched samples at a heating rate of 10 °C/ min. For the nonisothermal crystallization, the samples were first heated to 90 at 20 °C/min, held for 3 min to erase any thermal history, cooled to −80 at 2 °C/min. The crystallization peak temperature was obtained from the cooling traces. For the isothermal melt crystallization, the samples were first annealed at 90 °C for 3 min to erase any thermal history and then cooled at 60 °C/min to the desired crystallization temperature (Tc) until the crystallization was complete. After isothermal melt crystallization, the samples were heated to the melt again at 10 °C/min to study the subsequent melting behavior for the estimation of equilibrium melting point temperatures. A polarized optical microscope (POM) (Olympus BX51) equipped with a first-order retardation plate and a temperature controller (Linkam THMS600) was used to investigate the spherulitic morphology and growth of neat PEA and P(EA-co-5 mol % ES). The samples were first annealed at 90 °C for 3 min to erase any thermal history and then quenched to the desired Tc at 60 °C/min.

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RESULTS AND DISCUSSION Crystal Structures of Neat PEA and P(EA-co-5 mol % ES). Crystal structures of neat PEA and P(EA-co-5 mol % ES) were investigated with WAXD. Figure 2 shows the WAXD

Figure 2. Wide-angle X-ray diffraction (WAXD) patterns for neat PEA and P(EA-co-5 mol % ES).

patterns of neat PEA and P(EA-co-5 mol % ES). It can be seen from Figure 1 that the three main peaks at 2θ = 20.5°, 21.7°, and 24.6° are assigned to (111), (110), and (020) planes of neat PEA, respectively.2,20 Both neat PEA and P(EA-co-5 mol % ES) exhibit nearly the same diffractions at the same locations, indicating that the introduction of the ES unit does not modify the crystal structure of P(EA-co-5 mol % ES). It can be concluded from Figure 2 that the ES content exists in an amorphous state and is excluded from crystal region of neat PEA.21 13324

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Figure 3. (a) Differential scanning calorimetry (DSC) heating curves of neat PEA and P(EA-co-5 mol % ES) at 10 °C/min after quenching from the melt at 60 °C/min and (b) DSC cooling traces of neat PEA and P(EA-co-5 mol % ES) at a rate of 2 °C/min.

Figure 4. (a) Subsequent melting behaviors of P(EA-co-5 mol % ES) at various Tc values and (b) heating rate dependence of melting behaviors of P(EA-co-5 mol % ES) at 22 °C.

Hoffman and Weeks once proposed a relationship between Tm and Tc as

undergo the melting, recrystallization, and remelting during the heating process; therefore, double melting endotherms are found. When Tc is 30 °C and above, the formed perfect crystals will melt directly without recrystallization during the heating process; therefore, only one melting endotherm is found. Similar melting behaviors are also observed for neat PEA. For brevity, the results are not shown here. The heating rate dependence on the multiple melting behaviors has often been considered as the evidence of the melting, recrystallization, and remelting model.22,23 Figure 4b displays the melting behaviors of P(EA-co-5 mol % ES) crystallized at 22 °C at various heating rates ranging from 5 °C/min to 20 °C/min. It is obvious from Figure 4b that two melting endotherms are found for P(EA-co-5 mol % ES), despite the heating rate; however, the shape of the two melting endotherms is observed to vary with the heating rate. With increasing heating rate, Tm1 shifts upward to a high temperature range, whereas Tm2 shifts downward to a low temperature range; moreover, the ratio of the area of Tm1 and Tm2 increases. The aforementioned results indicate that the recrystallization of P(EA-co-5 mol % ES) must have been restricted during the heating process, because the time for P(EA-co-5 mol % ES) to melt and recrystallize becomes shorter with increasing heating rate. Similar results are also found for neat PEA. Therefore, Tm1 is used to estimate the equilibrium melting points for both neat PEA and P(EA-co-5 mol % ES).

Tm = ηTc + (1 − η)Tmo

(1)

where Tom 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.24 Tom can be obtained from the intersection of this line with the Tm = Tc equation. Figure 5 shows the Hoffman−Weeks plots for both neat PEA and P(EA-co-5 mol % ES), from which the Tom values are estimated to be 64.8 and 62.9 °C, respectively, for neat PEA and P(EA-co-5 mol %ES). In brief, the incorporation of a small amount of ES slightly reduces the Tom value of the P(EA-co-5 mol % ES) copolymer, with respect to neat PEA. For comparison, Table 1 summarizes the basic thermal properties of neat PEA and P(EA-co-5 mol % ES). Relative to neat PEA, the incorporation of a small amount of ES composition exerts almost no influence on Tg, reduces Tm and Tom slightly, and suppresses the nonisothermal melt and cold crystallization significantly of P(EA-co-5 mol % ES). Isothermal Melt Crystallization Kinetics of Neat PEA and P(EA-co-5 mol % ES). As introduced in the Experimental Section, the overall isothermal melt crystallization kinetics of neat PEA and P(EA-co-5 mol % ES) was further studied with DSC and analyzed by the well-known Avrami equation. According to the Avrami equation, it assumes the development 13325

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truncated sphere growth with athermal nucleation.27 The aforementioned results indicate that the crystallization mechanism remain unchanged for both neat PEA and P(EA-co-5 mol % ES), despite the crystallization temperature; moreover, the incorporation of the ES composition does not change the crystallization mechanism of P(EA-co-5 mol % ES), compared to neat PEA. It is not reasonable to compare the overall crystallization rate directly from the k values, because the units of k are min−n and n is not constant in this work. The crystallization half-life time (t0.5), the time required for half completion of the final crystallinity of the samples, is used to discuss crystallization kinetics of neat PEA and P(EA-co-5 mol % ES). The t0.5 value may be calculated through the following equation:

t0.5 =

Figure 5. Hoffman−Weeks plots for the determination of Tom values of neat PEA and P(EA-co-5 mol % ES).

Tg (°C)

Tch (°C)

Tcc (°C)

Tm (°C)

Tom (°C)

neat PEA P(EA-co-5 mol % ES)

−43.5 −42.9

−4.7 10.5

14.4 6.7

48.5 43.3

64.8 62.9

of the relative degree of crystallinity (Xt), as a function of crystallization time (t), as 1 − X t = exp( −kt n)

(3)

Consequently, the crystallization rate can be described easily by the reciprocal of t0.5 (1/t0.5). The 1/t0.5 values were calculated for both neat PEA and P(EA-co-5 mol % ES) at different Tc values, which are listed in Table 2 for comparison. It is obvious from Table 2 that the 1/t0.5 values decrease with increasing Tc for both neat PEA and P(EA-co-5 mol % ES), indicating that the overall isothermal crystallization rates are reduced at higher Tc because of small supercooling. Moreover, at a given Tc, the 1/t0.5 value is greater in neat PEA than in P(EA-co-5 mol % ES), indicating again that the overall isothermal melt crystallization rate is reduced for P(EA-co-5 mol % ES), compared to neat PEA, because of the introduction of the ES composition. Spherulitic Morphology of Neat PEA and P(EA-co-5 mol % ES). In this section, the spherulitic morphology of neat PEA and P(EA-co-5 mol % ES) was further studied with POM in a wide crystallization temperature range. Figure 7 shows a series of POM images of neat PEA and P(EA-co-5 mol % ES) spherulites crystallized isothermally at various Tc values. With increasing Tc, spherulites become larger for both neat PEA and P(EA-co-5 mol % ES), which may arise from the difficulty of nucleation and a subsequent decrease in number of nuclei. It can also be found from Figure 7 that the nucleation density of spherulites is smaller in P(EA-co-5 mol % ES) than in neat PEA at a given Tc. Depending on crystallization temperature, neat PEA and P(EA-co-5 mol % ES) may form ring-banded spherulites. For neat PEA, ring-banded spherulites are found in a crystallization temperature range of 32−38 °C; however, only ringless spherulites are observed when Tc is below 32 °C or above 38

Table 1. Summary of Basic Thermal Behaviors for Neat PEA and P(EA-co-5 mol % ES) sample

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(2)

where n is the Avrami exponent, depending on the nature of nucleation and growth geometry of the crystals, and k is the crystallization rate constant involving both nucleation and growth rate parameters.25,26 Parts a and b of Figure 6 show the Avrami plots of neat PEA and P(EA-co-5 mol % ES), respectively. A series of almost-parallel lines have been observed in Figure 6 for both neat PEA and P(EA-co-5 mol % ES) at different Tc values, suggesting that the Avrami equation may describe the isothermal melt crystallization process very well. The n and k values are acquired from the slopes and intercepts, respectively, and summarized in Table 2 for neat PEA and P(EA-co-5 mol % ES) at different Tc values for comparison. As shown in Table 2, the average values of n are ∼2.84 for neat PEA and the P(EA-co-5 mol % ES) copolymer within the investigated crystallization temperature range, which may correspond to the three-dimensional (3D)

Figure 6. Avrami plots for (a) neat PEA and (b) P(EA-co-5 mol % ES). 13326

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Table 2. Summary of the Isothermal Melt Crystallization Kinetics Parameters for Neat PEA and P(EA-co-5 mol % ES) based on the Avrami Equation at Different Tc Values Neat PEA Tc (°C) 30 32 34 36 38

n 2.8 2.8 2.9 2.7 2.6

P(EA-co-5 mol % ES)

k (min−n) 5.28 1.34 6.65 2.47 5.77

× × × × ×

−3

10 10−3 10−4 10−4 10−5

1/t0.5 (min−1) 1.73 1.04 8.74 5.14 2.85

× × × × ×

Tc (°C)

−1

10 10−1 10−2 10−2 10−2

26 28 30 32 34

n 2.9 3.0 3.0 2.9 2.8

k (min−n) 1.66 2.53 1.36 4.25 1.42

× × × × ×

−3

10 10−4 10−4 10−5 10−5

1/t0.5 (min−1) 1.20 9.46 5.57 3.49 2.01

× × × × ×

10−1 10−2 10−2 10−2 10−2

melting point temperature, but suppressed apparently the nonisothermal melt and cold crystallization of P(EA-co-5 mol % ES), with respect to neat PEA. Both neat PEA and P(EA-co-5 mol % ES) show double melting endotherms or one melting endotherm, depending on the crystallization temperature, which can be well-explained by the melting, recrystallization, and remelting model. The overall isothermal melt crystallization kinetics was investigated and analyzed by the Avrami equation for both neat PEA and P(EA-co-5 mol % ES). The crystallization rates are increased for both neat PEA and P(EAco-5 mol % ES) with decreasing crystallization temperature; furthermore, at a given crystallization temperature, the crystallization rate is slower in P(EA-co-5 mol % ES) than in neat PEA. Despite crystallization temperature and the incorporation of ES composition, the crystallization mechanism remains unchanged. The spherulitic morphology of neat PEA and P(EA-co-5 mol % ES) was studied in a wide crystallization temperature range. It is found that, depending on the crystallization temperature, neat PEA and P(EA-co-5 mol % ES) may form ring-banded or ringless spherulites.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

Figure 7. Spherulitic morphologies of neat PEA and P(EA-co-5 mol % ES) crystallized at various Tc values: (a) neat PEA at 28 °C, (b) P(EAco-5 mol % ES) at 28 °C, (c) neat PEA at 32 °C, (d) P(EA-co-5 mol % ES) at 32 °C, (e) neat PEA at 36 °C, and (f) P(EA-co-5 mol % ES) at 36 °C.

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

°C. For P(EA-co-5 mol % ES), ringless spherulites are observed when Tc is below 28 °C or above 34 °C; however, in a crystallization temperature range of 28−34 °C, ring-banded spherulites are found. The formation of ring-banded spherulites may arise from the twisting of lamellae; however, it is still an open question about the origin of lamellar twisting and needs further investigation.18

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CONCLUSIONS The crystal structure, thermal behavior, melting behavior, overall isothermal melt crystallization kinetics, and spherulitic morphology of biodegradable neat PEA and its copolymer P(EA-co-5 mol % ES) were investigated in detail in this work with WAXD, DSC, and POM. P(EA-co-5 mol % ES) shows the same crystal structure as neat PEA, indicating that the ES content exists in an amorphous state and is excluded from crystal region of neat PEA. The experimental results show that the incorporation of a small amount of ES composition has exerted almost no influence on the glass-transition temperature, slightly reduced the melting point temperature and equilibrium 13327

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