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Crystallization Kinetics, Morphology and Mechanical Properties of Novel Biodegradable Poly(ethylene succinate-co-ethylene suberate) Copolyesters Shoutian Qiu, Zhiqiang Su, and Zhaobin Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02654 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016

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Industrial & Engineering Chemistry Research

Crystallization Kinetics, Morphology and Mechanical Properties of Novel Biodegradable Poly(ethylene succinate-co-ethylene suberate) Copolyesters Shoutian Qiu, Zhiqiang Su, Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, MOE Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected]

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ABSTRACT Through a two-step melt polycondensation method, three poly(ethylene succinate-co-ethylene suberate) (PESSub) copolymers containing different contents of ethylene suberate (ESub) from 4.8 to 15.3 mol% and similar molecular weights were successfully synthesized in this research. To demonstrate the effect of the ESub composition, the crystallization kinetics, morphology, and mechanical properties of PESSub were systematically studied. The crystal structure of PESSub was the same as that of PES; however, with increasing the ESub composition the degree of crystallinity values slightly decreased. The increase of the ESub content led to a depression in the glass transition temperature, cold crystallization temperature, cold crystallization enthalpy, melting point, heat of fusion, and equilibrium melting point of PESSub. Increasing the ESub composition remained the crystallization mechanism but decreased the crystallization rates and spherulitic growth rates. The copolymers with higher ESub component showed greater elongation at break but smaller tensile strength and Young’s modulus. The crystallization behavior and mechanical properties of the synthesized novel copolyesters were well regulated by adjusting the content of the ESub units.

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Introduction Biodegradable polymers have recently received considerable interests for the purpose of solving the problems arisen from the plastic wastes.1−4 Aliphatic polyesters constitute a famous family of biodegradable polymers, because they not only possess comparable physical properties compared to many traditional thermoplastics but also may usually undergo enzymatic or hydrolytic degradation.1−4 Poly(ethylene succinate) (PES) is a promising chemosynthetic linear aliphatic polyester with a relatively higher melting point.5 PES has comparative mechanical properties compared to polyethylene and polypropylene. Many efforts have been focused on the crystallization kinetics, melting behavior, spherulitic morphology, and degradation of PES.4, 6−15 To improve the properties of PES, several methods have been utilized to modify PES, including blending and copolymerization. Polymer blending plays an important role in polymer modification because it is a convenient method to design novel materials to meet various requirements. Many polymers have been used to blend with PES, such as poly(ε-caprolactone), poly(L-lactide), poly(butylene succinate), poly(ethylene oxide), poly(p-dioxanone), poly(3-hydroxybutyrate), and poly(vinyl phenol).16−27 In previous work, we blended PLLA with PES and found the Young’s modulus of PES significantly improved in the PES-rich blends.18 However, polymer blending may confront the problem of immiscibility, thereby affecting the physical properties of the blends. Therefore, copolymerization offers an effective way in the modification of PES by incorporating another component into the molecular chain. Several PES-based copolymers were synthesized by incorporating another monomer or polymer into the

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backbone of PES for a wider practical application, such as poly(ethylene succinate-co-butylene succinate), poly(ethylene succinate)-b-poly(butylene succinate), poly[propylene-co-(ethylene succinate)], poly(ethylene succinate)-b-poly(ethylene glycol), poly(butylene succinate-co-trimethylene succinate), and poly(ethylene succinate-co-ethylene

adipate).28−37

succinate-co-octamethylene succinate–co–decamethylene

Recently,

we

succinate) succinate)

synthesized

poly(ethylene

and

poly(ethylene

copolymers

and

investigated

the

crystallization kinetics, morphology, and mechanical properties of these novel PES-based biodegradable copolyesters.38, 39 We found that the crystallization behavior and physical properties of the two series of PES-based copolyesters may be regulated by the types and contents of the newly used diol monomers. Poly(ethylene suberate) (PESub), a more flexible aliphatic polyester than PES, has been seldom reported.40−42 The crystallization kinetics and crystalline morphology of PESub were systematically studied.42 To our knowledge, the poly(ethylene succinate-co-ethylene suberate) (PESSub) copolyesters based on PES and PESub have not been reported. The aims of this research are as follows. First, by incorporating the ethylene suberate (ESub) unit, the chemical structure and physical properties of PES can be adjusted to meet various requirements for the end use from a practical viewpoint. Second, the influence of the ESub composition on the crystallization behavior and physical properties of PES-based copolyesters is also important and interesting from a fundamental study viewpoint to develop new biodegradable polymeric materials. Therefore, three PESSub copolyesters with

4


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different contents of ESub unit were first synthesized in this work. The effects of the comonomer composition on the crystallization kinetics, morphology, and mechanical properties of PESSub were then systematically investigated. From a practical viewpoint, the research results reported herein may be helpful to better understand the structure and properties relationship of biodegradable copolymers. Experimental Section Ethylene glycol, suberic acid, and succinic acid were bought from Beijing Chemical Works, Sinopharm Chemical Reagent Co. Ltd., and Tianjin Fuchen Chemical Reagents Factory, respectively. Tetrabutyl titanate was purchased from Beijing Chang Ping Jing Xiang Chemical Factory. Through a two-step melt polycondensation method, the copolymers with different contents of ESub were synthesized.32, 43 For simplicity, the detailed synthesis process was not described here but showed in the Supporting Information. Scheme 1 displays the chemical structure of the copolymer. 1HNMR from a spectrometer (Bruker AV 600) was used to determine the composition of the synthesized copolymers using deuterated chloroform (CDCl3) as the solvent. The three copolymers containing 4.8, 9.9, and 15.3 mol% of ESub were named PESSub5, PESSub10, and PESSub15, respectively. O

O

O

O O

O

x

y n

O

O

Scheme 1. Chemical structure of PESSub. The weight average molecular weight (Mw) and polydispersity index (PDI) of

5



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PESSub were measured on a Longzhida microscale gel permeation chromatography (GPC) (Waters Company, USA) using chloroform as a solvent. A Rigaku d/Max2500 VB2+/PC X-ray diffractometer was used to perform the wide-angle X-ray diffraction (WAXD) experiments at room temperature from 5° to 45° at 4 °/min. The films with thickness of around 1 mm were crystallized for 12 h at 54 °C in a vacuum oven, which were pressed after annealing at 140 °C for 3 min. The thermal analysis and crystallization behavior were studied with a differential scanning calorimeter (DSC) Q100. The weights of the samples were around 5 mg. To measure the glass transition temperature (Tg) and melting point temperature (Tm), the amorphous samples were first obtained by quenching from the crystal-free melt to −80 °C at 60 °C/min and were heated to 140 °C at 10 °C/min. For the isothermal crystallization procedure, the samples were quickly cooled to the predetermined crystallization temperature (Tc) at 60 °C/min after erasing any previous thermal history and crystallized isothermally for a considerable time to ensure complete crystallization. After isothermal crystallization, the melting behavior of the samples was studied at 10 °C/min for the calculation of equilibrium melting point temperature. A polarized optical microscope (POM) (Olympus BX51) was used to study the spherulitic morphology and growth rate of the copolymers, which was equipped with a temperature controller (Linkam THMS 600). All the samples were quickly cooled to the desired Tc at 60 °C/min after erasing any previous thermal history. The spherulitic growth rate (G) was calculated through the variation of radius (R) with time (t), i.e., G = dR/dt.

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The mechanical properties of the copolymers were studied at room temperature with an Instron Universal Testing Machine (Model 4302, Instron Engineering Co., Canton, MA) at a speed of 50 mm/min. For each copolyester three specimens were tested, and the average values were obtained. Results and Discussion Molecular Weights and Crystal Structures of PESSub The molecular weights of PESSub were first measured because they may affect the biodegradation and physical properties.44 The Mw and PDI values were measured with GPC. Table 1 lists the obtained data. Regardless of the ESub content, the Mw values of the three copolymers were relatively high and comparable; moreover, they also showed the similar PDI values. Therefore, the novel synthesized PESSub copolyesters may show good mechanical properties and find potential end use as biodegradable polymeric materials, which would be discussed in the following section. In addition, the similar Mw and PDI values were not the essential factor of affecting the different thermal parameters, crystallization behaviors, and mechanical properties of these copolyesters. It is interesting to investigate the crystal structures of the novel synthesized PESSub copolyesters. The WAXD patterns of the three PESSub copolyesters are displayed in Figure 1 after crystallizing at 54 °C for 12 h. As shown in Figure 1, the three copolymers with different ESub contents presented almost the same three main diffraction peaks at 20.1°, 22.7°, and 23.2°, which corresponded to (021), (121), and (200) of PES, respectively.32 Therefore, despite the ESub content, the crystal

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structures of PESSub did not change. The degree of crystallinity values were calculated from the WAXD patterns for the three PESSub copolyesters. The degree of crystallinity values for PESSub5, PESSub10, and PESSub15 were 36%, 33%, and 29%, respectively, which were smaller than that of PES (44%),32 and are also listed in Table 1. In brief, the three PESSub copolyesters shared the same crystal structure, but the degree of crystallinity values gradually decreased with an increase in the ESub content. Table 1. Molecular Weights and Degree of Crystallinity Values of PESSub. Samples

Mw (g/mol)

PDI

Degree of crystallinity (%)

PESSub5

9.5×104

2.2

36

PESSub10

8.2×104

2.3

33

PESSub15

9.0×104

2.7

29

(021) (121)

(200)

Intensity (a.u.)

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PESSub5

PESSub10

PESSub15

5

10

15

20

25 2θ (°)

30

35

40

45

Figure 1. WAXD patterns of PESSub. Basic Thermal Parameters of PESSub The influence of the ESub composition on the Tg and Tm of PESSub was first 8


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investigated. Figure 2 displays the DSC heating traces of the PESSub copolyesters from the amorphous state at 10 oC/min. Tg of PESSub5 was determined to be –13.0 oC. With further increasing the ESub content, the Tg values showed a decreased tendency and shifted downward to –16.3 oC for PESSub10 and –19.6 oC for PESSub15, respectively. Such variation in Tg was explained by the increased chain mobility and flexibility of the copolymers by incorporating a more flexible ESub content.

0.5 w/g

Heat flow (Endo up)

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PESSub5

PESSub10

PESSub15

-40

-20

0

20 40 60 Temperature (°C)

80

100

120

Figure 2. DSC heating curves of the PESSub copolyesters from the amorphous state at 10 °C/min. From Figure 2, with increasing the ESub content Tm of PESSub shifted downward to low temperature range. With increasing the ESub content from 4.8 to 15.3 mol%, Tm shifted downward from 94.9 to 81.4 oC for the copolyesters. The values of heat of fusion (∆Hm) also obviously decreased with increasing the ESub content. With an increase in the ESub content from 4.8 to 15.3 mol%, the ∆Hm values of the three copolyesters significantly decreased from 49.2 to 19.3 J/g. Figure 2 also displays that the PESSub copolyesters exhibited a cold crystallization exotherm, regardless of the 9



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ESub content. The cold crystallization temperature (Tch) values increased from 36.3 to 45.0 oC for the three PESSub copolymers, while the related cold crystallization enthalpy (∆Hch) values apparently decreased from 44.5 to 18.5 J/g. In the above section, the degree of crystallinity as obtained by WAXD showed closely differing values, but the cold crystallization enthalpy values obtained by DSC displayed a major drop in PESSub15 as compared to the others. Such results should be mainly attributed to the different crystallization conditions. In the WAXD experiments, the samples were isothermally crystallized at 54 oC for 12 h from the crystal-free melt. The crystallization time of 12 h was longer enough for each sample to crystallize sufficiently, thereby resulting in the relatively high crystallinity. However, in the DSC measurements, the samples were nonisothermally crystallized from the amorphous state. As shown in Figure 2, the crystallization time was so short that some sample like PESSub15 with low crystallizability did not have enough time to crystallize sufficiently, thereby leading to a sharply decreased crystallinity. PES had a ∆Hch of 49.3 J/g.32 With regard to that of PES, PESSub5, PESSub10, and PESSub15 showed a relative decrease in the crystallinity about 10%, 19%, and 62%, respectively, when they were nonisothermally crystallized from the amorphous state. Table 2 collects all the related thermal parameters data. For comparison, the related data of PES are also listed in Table 2. Relative to the homopolymer PES, the Tg values of PESSub decreased, indicating the increased chain mobility; their Tch values increased while the ∆Hch values decreased, indicating the restricted cold crystallization during the heating process from the amorphous state; the Tm and ∆Hm

10


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values decreased, indicated the decreased crystallinity and the crystal thickness. Table 2. Summary of Basic Thermal Parameters of PESSub. Samples

Tg (oC) Tch (oC) ∆Hch (J/g) Tm (oC) ∆Hm (J/g) Tmo (oC)

PESa

–9.5

36.0

49.3

103.9

64.8

109.5

PESSub5

–13.0

36.3

44.5

94.9

49.2

104.3

PESSub10

–16.3

42.0

40.0

89.0

40.2

98.0

PESSub15

–19.6

45.0

18.5

81.4

19.3

91.9

a

: The data of PES were cited from reference 32.

Tm is one of the most important parameters for crystalline polymers; however, Tm of polymers may vary under different crystallization conditions. Both the morphological and thermodynamic factors may affect Tm of semicrystalline polymers. Therefore, Tmo was further determined to study the influence of the ESub content on the depression of Tm of PESSub in this research. Tmo may be determined from the Hoffman–Weeks equation: Tm = ηTc + (1-η)Tmo

(1)

where Tm corresponds to the apparent melting point at Tc and η describes a measure of the stability, i.e., the lamellar thickness of the crystals undergoing the melting process.45 The subsequent melting behavior of PESSub was studied in detail after isothermally crystallizing at different Tc values. Figure 3 illustrates the melting behavior of PESSub15 as an example. Two melting peaks were clearly observed at indicated Tc values. With increasing Tc, the melting peak at lower temperature (TmL)

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shifted gradually upward to high temperature range, while the melting peak at higher temperature (TmH) remained almost unchanged. The other two copolyesters also presented such double melting behaviors. For brevity, they were not illustrated.

0.5 w/g 39 °C

Heat flow (Endo up)

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42 °C 45 °C 48 °C 51 °C

40

50

60

70

80

90

100

110

Temperature (°C)

Figure 3. Melting behavior of PESSub15 after crystallizing at indicated Tc values. The double melting behavior of PESSub was well described according to the melting, recrystallization, and remelting model.46, 47 TmL corresponded to the melting of the crystals formed during the isothermal melt crystallization at Tc, while TmH should arise from the melting of the crystals formed through the recrystallization of the melt of the crystals formed at Tc.46,

47

Consequently, TmL was used for the

Hoffman–Weeks analysis in this work. The Hoffman–Weeks plots of the three copolyesters are displayed in Figure 4. From Figure 4, the slopes of the lines for PESSub5, PESSub10, and PESSub15 were 0.48, 0.46, and 0.43, respectively. The values of Tmo were determined to be 104.3, 98.0, and 91.9 oC for PESSub5, PESSub10, and PESSub15, respectively, which are also collected in Table 2. The Tmo values of PESSub gradually decreased with increasing the ESub content. In addition, 12


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the Tmo values of PESSub were smaller than that of neat PES (109.5 oC).32 Such apparent depression of Tmo must have a significant influence on the crystallization kinetics and morphology of the PESSub copolyesters. 120 Melting point temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

80

PESSub5 PESSub10 PESSub15

60

40 20

40

60

80

100

120

Crystallization temperature (°C)

Figure 4. The Hoffman–Weeks plots of PESSub. In brief, increasing the ESub content obviously decreased the Tg, Tm, Tmo, ∆Hm, and ∆Hch values, but increased the Tch values. The increment of the flexible ESub unit could make the glass transition easier to happen but make the crystallization more difficult to occur for the PESSub copolyesters. Isothermal Melt Crystallization Kinetics and Spherulitic Morphology of PESSub The isothermal crystallization kinetics of PESSub copolyesters was systematically investigated in this section. Figure 5 illustrates the plots of relative crystallinity (Xt) versus crystallization time (t) of the three PESSub copolyesters. From Figure 5, t prolonged with increasing Tc for each sample, indicating a reduction of crystallization rate at higher Tc. For instance, it required PESSub5 around 10 min to complete crystallization at 57 °C and almost 55 min at 69 °C. 13



100

100

(a)

80

Relative crystallinity (%)


60 57 °C 60 °C 63 °C 66 °C 69 °C

40

20

(b)

80

60

48 °C 51 °C 54 °C 57 °C 60 °C

40

20

0

0 0

10

20

30

40

50

60

0

10

20

100

30

40

50

60

70

Crystallization time (min)



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(c)

80

60

40

39 °C 42 °C 45 °C 48 °C 51 °C

20

0 0

5

10

15

20

25

30

35

40

45

50


Figure 5. Plots of relative crystallinity versus crystallization time at indicated Tc values for (a) PESSub5, (b) PESSub10, and (c) PESSub15. The overall isothermal melt crystallization kinetics of the copolymers was analyzed by the renowned Avrami equation. It assumes Xt develops with t as follows: 1 – Xt = exp (–ktn)

(2)

where n is the Avrami exponent, and k is the crystallization rate constant.48, 49 Figure 6 displays the related Avrami plots of PESSub5, PESSub10, and PESSub15 at indicated Tc values. As shown in Figure 6, within the studied Tc range several almost parallel lines were obtained for each of the studied copolyesters, suggesting that the Avrami method could well fit the isothermal melt crystallization process of the copolymers. The values of n and k were acquired from the slopes and intercepts of the almost

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parallel lines, respectively. Table 3 summarizes all the related kinetics data of the copolyesters with different ESub contents at different Tc values. It was obvious from Table 3 that n varied between 2.3 and 2.7 and was not significantly influenced by Tc; therefore, all of the copolymers may crystallize through the three dimensional truncated sphere growth with athermal nucleation mechanism.50 In brief, despite the variation of the ESub composition, the crystallization mechanism of PESSub copolyesters did not change.

(a)

log (-ln(1-Xt))

(b)

log (-ln(1-Xt))

57 °C 60 °C 63 °C 66 °C 69 °C

-0.4 -0.2

0.0

0.2

0.4

0.6 log t

0.8

1.0

1.2

1.4

1.6

48 °C 51 °C 54 °C 57 °C 60 °C

-0.4 -0.2 0.0

0.2

0.4

0.6 0.8 log t

1.0

1.2

1.4

1.6

1.8

(c)

log (-ln(1-Xt))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39 °C 42 °C 45 °C 48 °C 51 °C

-0.2

0.0

0.2

0.4

0.6 0.8 log t

1.0

1.2

1.4

1.6

Figure 6. Avrami plots of (a) PESSub5, (b) PESSub10, and (c) PESSub15 at indicated Tc values.

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Table 3. Isothermal Crystallization Kinetics Parameters PESSub. Samples

PESSub5

PESSub10

PESSub15

Tc (oC)

n

k (min-n)

t0.5 (min)

1/t0.5 (min-1)

57

2.6

2.7×10-2

3.5

2.9×10-1

60

2.6

1.6×10-2

4.4

2.3×10-1

63

2.5

7.1×10-3

6.0

1.7×10-1

66

2.6

2.7×10-3

8.8

1.1×10-1

69

2.5

1.0×10-3

13.8

7.2×10-2

48

2.4

3.7×10-2

3.4

2.9×10-1

51

2.5

1.0×10-2

5.2

1.9×10-1

54

2.7

2.6×10-3

7.7

1.3×10-1

57

2.5

7.9×10-4

14.7

6.8×10-2

60

2.6

1.5×10-4

25.3

4.0×10-2

39

2.4

7.6×10-3

6.5

1.5×10-1

42

2.3

4.7×10-3

8.5

1.2×10-1

45

2.4

2.7×10-3

9.7

1.0×10-1

48

2.5

1.1×10-3

13.0

7.7×10-2

51

2.6

4.0×10-4

17.2

5.8×10-2

Furthermore, the direct comparison of the crystallization rate with the k values was not acceptable, for the unit of k was min-n while the n values were not constant in this research. Therefore, crystallization half-time (t0.5), the time required to achieve 50% of the final crystallinity, was applied to discuss the overall isothermal crystallization kinetics of the copolymers. The t0.5 value was calculated as follows:

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t 0.5 = (

ln 2 1/ n ) k

(3)

From the t0.5 values listed in Table 3, the effects of Tc and the ESub composition on the crystallization rate of PESSub were discussed as follows. On one hand, t0.5 became greater with increasing Tc for each sample, indicating that the sample crystallized more slowly at higher Tc than at lower Tc. Therefore, the reciprocal of t0.5 (1/t0.5) decreased with increasing Tc, which was used to compare the overall isothermal melt crystallization rate and is also summarized in Table 3. The reduction of the crystallization rate with increasing Tc may be attributed to the smaller degree of supercooling (∆T).32, 33, 42, 43 On the other hand, the effect of the ESub composition on the overall isothermal melt crystallization rate was also studied. However, the crystallization ability of the copolymers with different ESub contents was so different that it was hard to make a direct comparison of the crystallization rate of the three copolymers at a certain Tc. For instance, t0.5 of PESSub5 at 60 oC was 4.4 min, while that of PESSub10 significantly increased to 25.3 min. Similarly, t0.5 of PESSub10 at 48 oC was 3.4 min, while that of PESSub15 apparently increased to be 17.2 min. From the above mentioned results, the copolymers with higher contents of ESub had a greater t0.5 value and a smaller 1/t0.5 value, indicative of weaker crystallization ability. However, the increment of the ESub content may not be the only reason that caused the decrease of the crystallization rates. The three copolymers did not have the same Tmo; therefore, ∆T was different at the same Tc. For example, when PESSub5, PESSub10, and PESSub15 were crystallized at 60, 54, and 48 oC, respectively, they had almost the same ∆T of 44 oC; therefore, the corresponding t0.5 values were 4.4, 7.7, 17



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and 13.0 min, respectively. The following two reasons may explain the weak crystallizability of PESSub with increasing the ESub content. First, the depression of the Tmo values of PESSub reduced the ∆T with an increase in the ESub content, leading to a reduction of the driving force for the isothermal crystallization. Second, the crystallizability of ethylene succinate unit of PESSub became weaker with increasing the ESub content due to its diluent effect. Figure 7 illustrates the POM images for the copolymers with different ESub compositions isothermally crystallized at 48 oC. From Figure 7, despite the ESub composition they displayed the characteristic Maltese cross extinction pattern. With increasing the ESub content the nucleation density of the spherulites decreased; therefore, the size of the spherulites became accordingly bigger. The incorporation of ESub may make the nucleation more difficult for the copolymers with higher ESub content at the same Tc because the ∆T became smaller due to the depression of Tmo in the PESSub copolyesters.32, 33, 42, 43

Figure 7. Spherulitic morphology for (a) PESSub5, (b) PESSub10, and (c) PESSub15 crystallized at 48 oC. The spherulitic growth rates of PESSub were measured in this work. All the copolymers showed a linear growth at all given Tc values. Figure 8 displays the Tc dependence of G for PESSub. From Figure 8, the typical bell-shaped curves were

18


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observed for the three copolyesters. The G values first increased and then decreased with increasing Tc after they reached the maximum value; moreover, the copolymers with lower ESub content had greater G values. From Figure 8, PESSub5 had a maximum G value of almost 5 µm/min, while PESSub10 presented a maximum G value of 2.5 µm/min, and PESSub15 displayed a lowest maximum G value of about 1 µm/min. The diluent effect of the ESub content and the reduced driving force required for the growth of spherulites resulted in the decrease of G with increasing the ESub content. 7 PESSub5 PESSub10 PESSub15

6 5 G (µm/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2 1 0 20

30

40

50

60

70

Tc (°C)

Figure 8. Variation of G with Tc for PESSub. Mechanical Properties of PESSub The study of the mechanical properties of the novel synthesized PESSub copolyesters is important from a viewpoint of practical application. The mechanical properties of PESSub were studied in this work. Figure 9 displays the stress−strain curves and the enlarged portion of the stress-strain curves of PESSub. From Figure 9, all the copolymers displayed the characteristic stress−strain curves of semicrystalline 19



plastics, showing a clear yielding at an elongation value of around 11%. The yield strength values for PESSub5, PESSub10, and PESSub15 were 20.0 ± 1.7, 16.4 ± 1.2, and 13.4 ± 0.7 MPa, respectively. PESSub5 showed a Young’s modulus of 383 ± 44 MPa, a tensile strength of 50.3 ± 2.8 MPa, and an elongation at break of 1055 ± 99 %. With the increase of the ESub component, the copolymers exhibited increased elongation at break and decreased Young’s modulus and tensile strength. The Young’s modulus, tensile strength, and elongation at break for PESSub10 were 318 ± 17 MPa, 46.7 ± 2.8 MPa, and 1214 ± 65 %, respectively. PESSub15 showed the highest elongation at break of 1343 ± 65 % and the lowest tensile strength and Young’s modulus of 46.0 ± 2.6 MPa and 190 ± 7 MPa, respectively. Table 4 summarizes the values of their mechanical properties. In brief, the mechanical properties of the copolymers could be well regulated by adjusting the content of the ESub units. 60

(b)

PESSub10

(a)

PESSub5

50

20

PESSub5

40

Stress (MPa)

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

30

PESSub10

10

20 PESSub15

10

PESSub15

0

0 0

400

800

1200

0

1600

20

40

Strain (%)

Strain (%)

Figure 9. (a) Stress−strain curves and (b) enlarged portion at low strain for PESSub.

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Table 4. Summary of Mechanical Properties of PESSub.

Samples

Young’s

Yield

Elongation

Tensile

Elongation

modulus

strength

at yield

strength

at break

(MPa)

(MPa)

(%)

(MPa)

(%)

PESSub5

383 ± 44 20.0±1.7

11.2±1.0

50.3 ± 2.8

1055 ± 99

PESSub10

318 ± 17 16.4±1.2

10.9±0.8

46.7 ± 2.8

1214 ± 65

PESSub15

190 ± 7

11.4±1.1

46.0 ± 2.6

1343 ± 65

13.4±0.7

Conclusions Through a two-step melt polycondensation method, we successfully synthesized three PESSub copolymers with different ESub contents ranging from 4.8 to 15.3 mol% in this research. All the synthesized copolyesters had high molecular weights, which were ranging from 8.2 × 104 to 9.5 × 104 g/mol. The effects of the ESub composition on the crystallization kinetics, morphology, and mechanical properties of PESSub were systematically studied with several techniques. Regardless of the ES composition, the three PESSub copolyesters presented the similar WAXD patterns, indicating that they displayed the same crystal structures as PES; moreover, with increasing the ESub composition the degree of crystallinity values gradually decreased. The glass transition temperature, cold crystallization temperature, cold crystallization enthalpy, melting point, heat of fusion, and equilibrium melting point of PESSub were found to decrease with an increase in the ESub composition. The isothermal crystallization kinetics of PESSub was investigated at different crystallization temperatures and analyzed by the Avrami equation. Despite the ESub

21



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composition and crystallization temperature, the crystallization mechanism of the three PESSub copolyesters remained unchanged. With increasing crystallization temperature and the ESub composition, the isothermal crystallization rates of PESSub decreased. The spherulitic morphology study revealed a decrease in the spherulites nucleation density of PESSub with increasing crystallization temperature and the ESub composition, resulting in the larger size of the spherulites. The tensile testing results showed that all the synthesized PESSub copolyesters possessed good mechanical properties; moreover, the mechanical properties could be regulated by adjusting the content of the ESub units. Supporting Information Synthesis of PESSub copolyesters. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements The authors thank the National Natural Science Foundation, China (51373020, 51573016 and 51521062) for supporting this research.

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For TOC Use Only

Crystallization Kinetics, Morphology and Mechanical Properties of Novel Biodegradable Poly(ethylene succinate-co-ethylene suberate) Copolyesters Shoutian Qiu, Zhiqiang Su, Zhaobin Qiu*

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Crystallization Kinetics, Morphology, and ... - ACS Publications

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