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Thermal properties and crystallization kinetics of poly(butylene suberate)
Accepted Manuscript Thermal Properties and Crystallization Kinetics of Poly(butylene suberate) Zhiming Cui, Zhaobin Qiu PII:
S0032-3861(15)00406-1
DOI:
10.1016/j.polymer.2015.04.069
Reference:
JPOL 17821
To appear in:
Polymer
Received Date: 20 January 2015 Revised Date:
15 March 2015
Accepted Date: 20 April 2015
Please cite this article as: Cui Z, Qiu Z, Thermal Properties and Crystallization Kinetics of Poly(butylene suberate), Polymer (2015), doi: 10.1016/j.polymer.2015.04.069. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal Properties and Crystallization Kinetics of Poly(butylene suberate)
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Zhiming Cui and Zhaobin Qiu*
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Thermal Properties and Crystallization Kinetics of Poly(butylene suberate) Zhiming Cui and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, MOE Key Laboratory of Carbon
China
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Fiber and Functional Polymers, Beijing University of Chemical Technology, Beijing 100029,
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Fax: +86-10-64413161. E-mail: [email protected].
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ACCEPTED MANUSCRIPT Abstract In this work, we synthesized poly(butylene suberate) (PBSub) with a weight average molecular weight of 3.64 × 104 g/mol from the monomers of butanediol and suberic acid via a
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two-step melt polycondensation method. The basic thermal behaviors, overall isothermal melt crystallization kinetics, crystal structure, spherulitic morphology and growth, thermal stability, and hydrolytic degradation of PBSub were systematically investigated for the first time.
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PBSub has a low glass transition temperature of about −61 oC, a melting point of 55.2 oC, and
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an equilibrium melting point of 61.4 oC. The overall isothermal melt crystallization kinetics of PBSub was investigated at different crystallization temperature values and analyzed by the Avrami equation. PBSub has an average Avrami exponent value of about 3, suggesting that the crystallization mechanism of PBSub may correspond to three-dimensional truncated
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sphere growth with athermal nucleation. An obvious spherulitic morphology was observed for PBSub, and the spherulitic growth rates decrease with increasing crystallization temperature. PBSub exhibits a crystallization regime II to regime III transition on the basis of the
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secondary nucleation theory. The crystal structure study reveals that PBSub is highly
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crystalline, presenting strong diffraction peaks and a great crystallinity of about 55%. The thermogravimetric analysis study demonstrates that PBSub has both a high decomposition temperature at 5 wt% weight loss of about 377 oC and a high temperature at the maximum degradation rate of about 421 oC, suggesting its good thermal stability. PBSub may undergo a hydrolytic degradation, which may be of interest for its end use as a degradable material in some special application fields. Keywords: crystallization behavior; thermal properties; poly(butylene suberate) 2
ACCEPTED MANUSCRIPT 1. Introduction Biodegradable aliphatic polyesters have been attracting considerable research interests from both academic and industrial viewpoints [1−7]. Some typical members of the extensively
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investigated biodegradable polyesters include poly(L-lactide) (PLLA), poly(ε-caprolactone) (PCL), poly(ethylene succinate) (PES), poly(butylene succinate) (PBS), poly(ethylene adipate) (PEA), poly(butylene adipate) (PBA), etc [8−39]. Huneault et al. reviewed the crystallization
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behavior of PLLA and provided comprehensive relationships between crystallization kinetics
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and the molecular structure characteristics [8]. Eastmond reviewed the miscibility, crystallization, and properties of PCL based polymer blends [17]. Qiu et al. reported the preparation,
morphology,
crystallization,
and
properties
of
PES
based
polymer
nanocomposites containing multi-walled carbon nanotubes, thermally reduced graphene, and
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polyhedral oligomeric silsesquioxanes [22, 25, 26]. Guo et al. reviewed the research, development, and industrialization of PBS and its copolymers [27]. Inoue et al. reported the crystallization kinetics and crystalline structure of PEA with low and high molecular weights
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[33]. Gan et al. studied the metastability and transformation of polymorphic crystals in PBA
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[35]. Some of them, such as PES, PBS, PEA, and PBA may be synthesized from the condensation of dicarboxylic acids (such as succinic or adipic acid) with diols (1,2-ethanediol or1,4-butadenediol). Through the characteristic two-stage melt polycondensation method, PES, PBS, PEA, and PBA have been successfully prepared; moreover, PES, PBS, and PBA have become industrialized and commercially available. A lot of works have been devoted to the basic thermal properties, crystallization behaviors, melting behaviors, crystal structures and polymorphism, mechanical properties, and thermal and enzymatic degradation behaviors 3
ACCEPTED MANUSCRIPT of these biodegradable polyesters [21−39]. Some other monomers with more methylene (CH2) groups between dicarboxylic acids or diols have also been used to develop new aliphatic polyesters [40−46]. Among them, suberic
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acid has been used as a new monomer to prepare a series of novel aliphatic polyesters, such as poly(ethylene suberate) (PESub), poly(butylene suberate) (PBSub), poly(hexamethylene suberate) (PHSub), and poly(octanediol suberate) (POSub) [43−46]. Few works have focused
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on PBSub, because it is not commercially available until now. Puiggali et al. reported that
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PBSub adopted an all-trans conformation and pack via an orthorhombic unit cell containing four molecular segments; moreover, the lattice parameters of the orthorhombic unit cell were a = 0.506 nm, b = 1.462 nm, and c = 1.72 nm, respectively [44]. In addition, they also found that the lamellar crystals may be acquired when PBSub was isothermally crystallized from a
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dilute alcohol solution, exhibiting characteristic truncated lozenge morphology [44]. To our knowledge, the thermal properties and crystallization kinetics of PBSub have not been reported so far. The purpose of this work is to study the thermal properties and
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crystallization kinetics of PBSub, as they are interesting and important not only for the
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practical application of PBSub but also for a better understanding of the structure and properties relationship of aliphatic polyesters. In this work, we extensively studied the basic thermal behaviors, overall isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, thermal stability, and hydrolytic degradation of PBSub. The research results reported herein are expected to be useful and helpful to promote the interests in the basic study and practical application of this aliphatic polymer. 2. Experimental Details 4
ACCEPTED MANUSCRIPT Butanediol (BDO) was purchased from Tianjin Fu Chen Chemical Reagent Factory. Suberic acid (SuA) was bought from Sinopharm Chemical Reagent Co., Ltd. Tetrabutyl titanate (TBT), used as catalyst, was purchased from Beijing Chang Ping Jing Xiang
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Chemical Factory. PBSub was synthesized in our laboratory from the condensation of BDO and SuA via a typical two-step melt polycondensation method [23, 29, 30]. During the esterification process,
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the proper amount of appropriate BDO and SuA (SuA/BDO molar ratio = 1:1.1) and the
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catalyst (1×10-4 mol TBT/mol acid) were charged into a three-necked 250 mL flask, which was first filled with nitrogen to completely remove oxygen. The mixture was first heated to 150~160 oC, kept for 20 min under nitrogen atmosphere for all the monomers to melt completely, and was then heated to 190 oC for 3 h. During the polycondensation process, the
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mixture was further heated to 230 oC for 5 h at a reduced pressure. The synthesized product was purified by dissolving in chloroform and then precipitating into a 3 times amount of methanol. Finally, the precipitate was dried in a vacuum at 30 oC for 72 h before use.
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Figure 1 shows the chemical structure of PBSub . O
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O
O
n O
Figure 1. Chemical structure of PBSub.
The molecular weight of PBSub was measured with gel permeation chromatography (GPC) (Waters 515 HPLC, Waters Company) using tetrahydrofuran as the solvent at 1 mL/min. Calibration was performed with narrow polydispersity polystyrene (PS) standards. The thermal behaviors of PBSub were measured using a TA Instruments differential 5
ACCEPTED MANUSCRIPT scanning calorimeter (DSC) Q100. The sample was first annealed at 140 oC for 3 min to erase any previous thermal history and was then nonisothermally crystallized at different cooling rates to –90 oC. The corresponding melting point and glass transition temperature were then
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determined on the nonisothermally crystallized sample at 10 oC/min. For the isothermal melt crystallization kinetics and equilibrium melt point estimation studies of PBSub, the samples were treated through the similar processes as those in our previous works, which were
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described in detail elsewhere [23, 29, 30].
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Wide-Angle X-ray diffraction (WAXD) pattern was recorded from 5 – 45o at 4 o/min on a Rigaku d/Max2500 VB2+/PC X-ray Diffractometer, which was operated at 40 kV and 200 mA.
The spherulitic morphology and growth of PBSub were investigated with a polarized
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optical microscope (POM) (Olympus BX51). As the nucleation ability of PBSub was too strong, the sample was dissolved in chloroform with a concentration of 1 mg/mL and then cast on a cover glass to form a thin film for the POM observation.
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The thermal stability was performed with a TA Instruments thermogravimetric analysis
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(Q50) at 20 oC/min under nitrogen atmosphere. The hydrolytic degradation of PBSub was performed at 37 °C in a sodium hydroxide (NaOH) solution with a pH value of 14 by following the variation of weight loss with degradation time.
3. Results and Discussion It is well-known that the molecular weight and polydispersity index (PDI) values of a polymer must have an obvious influence on its basic thermal behaviors, crystallization 6
ACCEPTED MANUSCRIPT kinetics, crystalline morphology, thermal stability, mechanical properties, etc. PBSub has a number average molecular weight (Mn) of 1.86 × 104 g/mol, a weight average molecular weight (Mw) of 3.64 × 104 g/mol, and a PDI value of about 1.96. In literature, Puiggali et al.
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synthesized a PHSub sample, and the Mn and Mw values were 2.08 × 104 and 3.73 × 104 g/mol, respectively [45]. In the present work, the Mn and Mw values of PBSub are very close to those of PHSub.
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The basic thermal properties of PBSub were first studied in this work with DSC. PBSub
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was subjected to the thermal analysis through the thermal treatment process described in the Experimental Section. After the heating, cooling, and reheating process, the following parameters were acquired for the synthesized PBSub sample, including the nonisothermal melt crystallization peak temperature (Tp), glass transition temperature (Tg), and melting point
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(Tm). Figure 2a shows the crystallization exotherms of PBSub at different cooling rates ranging from 5 to 40 oC/min from the crystal-free melt after a slow heating process to eliminate thermal history. Figures 2b displays the subsequent melting endotherms of PBSub at
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a heating rate of 10 oC/min after the sample finished the nonisothermal melt crystallization
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processes at different cooling rates. As shown in Figure 2a, regardless of cooling rate, PBSub has a very sharp crystallization peak, indicating that the crystallizability of PBSub is strong. With increasing the cooling rate, the crystallization exotherms gradually shift downward to low temperature range. At a cooling rate of 5 oC/min, the Tp value was determined to be 39.1 o
C, and the melt crystallization enthalpy (∆Hc) was measured to be 88.5 J/g. When the cooling
rate was increased to be 40 oC/min, the Tp value was decreased to be 33.2 oC, and the ∆Hc value was estimated to be 87.4 J/g. As shown in Figure 2b, PBSub exhibits a well-defined 7
ACCEPTED MANUSCRIPT single melting endotherm, regardless of cooling rate. From Figure 2b, PBSub displays a Tm value of 55.1 oC with a heat of fusion (∆Hm) of 89.9 J/g, when a cooling rate of 5 oC/min was used. At a cooling rate of 40 oC/min, PBSub shows a Tm value of 54.4 oC with a ∆Hm of 89.3
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J/g. All of the above obtained thermal parameters are summarized in Table 1. It is clear that the variation of cooling rate from 5 to 40 oC/min obviously affects the Tp values but slightly influences the ∆Hc, Tm, and ∆Hm values. Figures 2c displays the enlarged glass transition
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behavior of PBSub at a heating rate of 10 oC/min after the sample was nonisothermally
–61.0 oC.
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crystallized from the melt at 10 oC/min, from which the Tg value was measured to be about
Table 1. Thermal Properties of PBSub at Different Cooling Rates. Tp (oC)
∆Hc (J/g)
Tm (oC)
∆Hm (J/g)
5
39.1
88.5
55.1
89.9
37.3
86.9
55.2
94.2
34.0
90.9
54.7
92.8
33.2
87.4
54.4
89.3
20 40
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10
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Cooling rates (oC/min)
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The low Tg value of PBSub suggests that PBSub may have good flexibility because of its strong chain mobility, which needs further investigation. It is interesting to note that the Tg value of PBSub is very close to that of PCL (∼ –60 oC) [17]. The similarity of the Tg values between PBSub and PCL may be related to the following fact that they have the same CH2/CO ratio value of 5 according to their chemical structures. In addition, it is also interesting to compare the basic thermal properties of PBSub with those of PBS, as they have the similar chemical structures, except that the former possesses four more CH2 groups 8
ACCEPTED MANUSCRIPT between the carbonyl groups than the latter. In other words, the CH2/CO ratio is 5 for PBSub, while that of PBS is 3. PBS has a Tg value of about –34 oC and a Tm value of about 112 oC [28]. It is obvious that both the Tg and Tm values are greater in PBS than in PBSub, suggesting
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that the chain mobility of PBSub is stronger than PBS because of the introduction of more CH2 groups into the chemical structure of PBSub. In brief, the basic thermal properties
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indicate that PBSub is a semicrystalline aliphatic polyester with low Tg and Tm values.
(a)
2 W/g
20 °C/min
10 °C/min 5 °C/min
0
10
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Heat flow (Endo up)
40 °C/min
20
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40
50
60
70
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Temperature (°C)
(b)
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Heat flow (Endo up)
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2 W/g
40 °C/min 20 °C/min 10 °C/min 5 °C/min
20
30
40
50
Temperature (°C)
9
60
70
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Heat flow (Endo up)
(c)
o
-80
-75
-70
-65
-60
-55
-50
-45
-40
-35
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Temperature (°C)
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Tg = -61.0 C
0.01 W/g
Figure 2．DSC curves of PBSub showing (a) nonisothermal melt crystallization exotherms, (b)
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meting endotherms, and (c) enlarged glass transition region.
In the above section, the Tm value was measured to be 55.2 oC, when PBSub was heated at 10 oC/min on a nonisothermally crystallized sample from the crystal-free melt at a cooling rate of 10 oC/min. It is obvious that the Tm value may be affected by both the crystallization
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conditions (nonisothermal or isothermal crystallization) and heating rate. If the sample is crystallized nonisothermally, cooling rate is the key factor of influencing the subsequent
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melting behavior. If the sample is crystallized isothermally, crystallization temperature (Tc) and crystallization time are the main factors of affecting the subsequent melting behavior. In
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addition to crystallization conditions, heating rate is also an important factor of influencing the Tm value. To avoid the aforementioned kinetics and morphology influences on the Tm value, equilibrium melting point is an important and basic thermal parameter to characterize a semicrystalline polymer. Figure 3 demonstrates the subsequent melting behaviors of PBSub after it was isothermally crystallized at a temperature range from 40 to 48 oC. To avoid the influence of crystallization time, all of the samples were crystallized for the same crystallization time (1 h) at different Tc 10
ACCEPTED MANUSCRIPT values. As shown in Figure 3, PBSub presents a well defined single melting endotherm, regardless of Tc, indicating that the formed crystals are so perfect that the well-known melting, recrystallization, and remelting process does not occur. Figure 3 also clearly shows that
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increasing Tc shifts the melting endotherm upward to high temperature range. For instance, at a Tc of 40 oC, PBSub has a Tm of 56.3 oC, while it has a Tm of 58.3 oC at a Tc of 48 oC,
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suggesting that increasing Tc favors the formation of relatively perfect crystals with higher Tm.
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48 °C
46 °C 44 °C 42 °C
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40 °C
35
40
45
50
55
60
65
70
75
Temperature (°C)
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Figure 3. Melting behaviors of PBSub after crystallizing at various Tc values for 1 h.
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Hoffman and Weeks described a relationship between Tm and Tc: Tm = ηTc + (1-η) Tmo
(1)
where T om represents the equilibrium melting point, and η is a measurement of the stability related to the crystal layer thickness of the polymer [47]. The Hoffman–Weeks plot is displayed in Figure 4. PBSub has a Tmo of 61.4 oC and a η value of 0.24. The Tmo value of PBSub seems a little low in this work, which should be mainly related to its molecular weight [44]. In addition, the enthalpy of 100% crystalline PBSub value (∆Hmo) was calculated to be 35 kJ/mol, i.e., 153.5 J/g, on the basis of the values reported by van Krevelen for methylene 11
ACCEPTED MANUSCRIPT (4 kJ/mol) and ester (–2.5 kJ/mol) groups [48].
o T m = 61.4 °C
Tm (°C)
60
40 40
45
50
55
60
65
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50
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70
Tc (°C)
Figure 4. Hoffman−Weeks plot of PBSub. The overall isothermal melt crystallization kinetics of PBSub was investigated with DSC
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and analyzed by the Avrami equation in a Tc range from 44 to 48 oC, which would affect the final physical properties. Figure 5a illustrates the development of relative crystallinity (Xt)
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with crystallization time (t) at different Tc values. All of the Xt − t plots present the characteristic sigmoid curves. Figure 5a clearly shows that the crystallization time required to
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finish crystallization is longer at higher Tc than at lower Tc, suggesting that increasing Tc may suppress the crystallization. The Xt − t plots were further treated with the Avrami equation, which is shown as follows:
1 – Xt = exp (–ktn)
(2)
where n is the Avrami exponent reflecting crystallization mechanism, and k is the crystallization rate constant [49, 50]. Figure 5b shows the related Avrami plots of PBSub. As shown in Figure 5b, the Avrami plots of PBSub at different Tc values illustrate a series of 12
ACCEPTED MANUSCRIPT almost parallel straight lines, suggesting that the n values should be close to each other. In other words, PBSub may crystallize through the same crystallization mechanism, regardless of Tc. In brief, the overall isothermal melt crystallization of PBSub could be described by the
100
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60
44 °C 45 °C 46 °C 47 °C 48 °C
40
20
0 0
10
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Relative crystallinity (%)
(a) 80
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Avrami equation.
20
30
40
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Crystallization time (min)
(b)
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log(-ln(1-Xt))
0
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-1
44 °C 45 °C 46 °C 47 °C 48 °C
-2
-0.4
0.0
0.4
0.8
1.2
1.6
log t
Figure 5. (a) Development of relative crystallinity with crystallization time and (b) Avrami plots of PBSub at indicated Tc values. Table 2 summarizes the n and k values, after PBSub was crystallized at different Tc values. The n values of PBSub vary slightly from 2.9 to 3.2 in the investigated Tc range. Despite Tc, 13
ACCEPTED MANUSCRIPT all of the n values are close to 3, suggesting a possible three-dimensional truncated growth with athermal nucleation mechanism [51]. In previous work, we once studied the isothermal melt crystallization kinetics of PBS and its nanocomposite with carbon nanotubes in a range
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of Tc from 96 to 104 oC, and the n values were also close to 3 [52]. Table 2 clearly demonstrates that the k values decrease with increasing Tc, when the n values are the same at 44 and 48 oC or 45 and 46 oC. The reduction of the k values indicates a slow crystallization
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rate. For an easy comparison of crystallization rates at different Tc values, we calculated the
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crystallization half-time (t0.5), the time at 50% of the final crystallinity (t0.5) through the following equation:
t0.5 = (
ln 2 1/ n ) k
(3)
Therefore, the 1/t0.5 values may be used to compare the crystallization rates, as they are
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normalized rate constants with the same units (min-1) and are independent of the n values. The t0.5 and 1/t0.5 values were acquired and are also listed in Table 2. For comparison, the experimental crystallization half-time (t1/2) values were also directly read from Figure 5a and
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are also summarized in Table 2. Regardless of Tc, the calculated t0.5 value and the
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experimental t1/2 value are close to each other, indicative of the validity of the Avrami equation during the isothermal crystallization kinetics study.
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ACCEPTED MANUSCRIPT Table 2. Isothermal Crystallization Kinetics Parameters of PBSub. Tc (oC)
n
k (min−n)
t1/2 (min) t0.5 (min) 1/t0.5 (min−1)
2.9 6.31×10−2
2.15
2.27
4.41×10−1
45
3.1 1.68×10−2
3.29
3.34
2.99×10−1
46
3.1 0.29×10−2
5.77
5.82
47
3.2 5.56×10−4
9.40
9.41
48
2.9 2.37×10−4
15.73
16.12
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44
1.72×10−1 1.06×10−1
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6.20×10−2
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In the above section, the overall isothermal melt crystallization kinetics study of PBSub was performed with DSC in a Tc range of 44 to 48 oC and analyzed by the Avrami equation. The n values are close to 3, suggesting that PBSub may crystallize according to a crystallization mechanism of spherulite growth with athermal nucleation [51]. To verify the
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crystallization mechanism, the crystalline morphology of PBSub was investigated. Figure 6 displays the POM image of PBSub, which was isothermally crystallized at 46 oC for 50 min. PBSub presents characteristic spherulitic morphology. Several PBSub spherulites grew during
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the crystallization process, impinged against each other later, forming clear spherulites
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boundaries, and finally filled the whole space. The average size of PBSub spherulites are about 65 µm in radius. The crystalline morphology study confirms that PBSub crystallizes via the spherulitic growth mechanism; moreover, the spherulites are relatively regular and perfect.
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Figure 6. POM image of PBSub spherulites after crystallizing at 46 oC.
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In addition, the growth rate (G) was calculated by following the variation of radius (R) of spherulites with crystallization time, i.e., G = dR/dt. The G values of PBSub spherulites at different Tc values were estimated and are shown in Figure 7. It is clear from Figure 7 that the G values decrease significantly with increasing Tc. For example, PBSub presents a great G
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value of 154.9 µm/min at a Tc of 36 oC, while it only displays a small G value of 3.6 µm/min at a Tc of 46 oC. Such an obvious reduction of the G values indicates that Tc is a key factor of determining not only the spherulites nucleation but also spherulites growth, because it is
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related to the important and decisive factor of influencing the crystallization of biodegradable
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polyesters, i.e., supercooling.
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160 140
100 80 60 40 20 0 36
38
40
42
44
46
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Tc (°C)
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G (µm/min)
120
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Figure 7. Growth rates of PBSub spherulites at various Tc values. We further analyzed the spherulitic growth rates of PBSub using the secondary nucleation theory proposed by Lauritzen and Hoffman in this research. The Lauritzen and Hoffman equation is shown as follows:
(4)
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Kg U∗ G = G0 exp − exp− R(Tc − T∞ ) Tc (∆T ) f
where G0 is a preexponential factor, U* is the activation energy for transporting the polymer
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chain segments to the crystallization site, R is the gas constant, T∞ is a temperature below which the polymer chain movement ceases, ∆T is the degree of supercooling, f is a correction
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factor accounting for the variation in the enthalpy of fusion given as f =2Tc /(Tmo+Tc), and Kg is the nucleation constant [53]. The values of U* =17246.3 J/mol and T∞ = T g - 51.6 K were used in this work [54]. Figure 8 shows the Lauritzen–Hoffman plot for PBSub. Figure 8 obviously illustrates that all the data may be fitted with two straight lines with different slopes, indicating a crystallization regime transition from regime II at high Tc to regime III at low Tc. From Figure 8, the nucleation constant values for regime II (KgII) and regime III (KgIII) were determined to be 4.57×104 K2 and 8.85×104 K2, respectively. The KgIII/KgII value is 1.93, 17
ACCEPTED MANUSCRIPT which is close to the theoretical value of 2 [52]. The regime transition temperature (Ttr) is about 41 oC. Similar to PBSub, the spherulites of PES and PBS may also be observed in a wide range of temperature range [23, 29]. In previous work, PES also exhibited a
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crystallization transition from regime II to regime III, and the Ttr value was around 60 oC [23]. Similarly, PBS displayed a crystallization transition from regime II to regime III, and the Ttr value was around 95 oC [29]. It is interesting to note that the crystallization transition from
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regime II to regime III is very common for aliphatic polyesters; moreover, the transition
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temperature shifts to low temperature with decreasing the melting point temperature values of the aliphatic polyesters.
6.0 5.5 5.0
4.0 3.5 3.0
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lnG+U*/(R(Tc−T∞))
4.5
2.5 2.0 1.5 1.0
-4
1.4×10
-4
1.6×10
-4
1.8×10
-4
2.0×10
-2
EP
1/(Tc(∆T)f) (K )
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Figure 8. Regime transition analysis of PBSub.
Similar to other aliphatic polyesters, such as PBS, PES, PCL, etc, PBSub is also a semicrystalline polymer. The crystal structure of PBSub was further investigated in this research. Figure 9 illustrates the WAXD pattern of a PBSub sample, which was isothermally crystallized at 40 oC for 48 h. As shown in Figure 9, PBSub presents several strong diffraction peaks at the 2θ range of 20o to 35o, indicating its highly crystalline feature. Puiggali et al. once determined the crystal structure of PBSub as follows, i.e., an orthorhombic unit cell with 18
ACCEPTED MANUSCRIPT dimension a = 0.506 nm, b = 1.462 nm, and c = 1.72 nm; therefore, the two main characteristic diffraction peaks, which are located at 21.4o and 24.6o, are attributed to the (120) and (040) planes, respectively [44]. In addition, the crystallinity value of PBSub was further
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determined to be about 55% on the basis of the WAXD pattern, which is also summarized in Table 1. The crystallinity value of PBSub is also close to that of PCL (∼50%) [17]. In brief,
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PBSub is a highly crystalline aliphatic polyester.
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from both the two strong characteristic diffraction peaks and the great crystallinity value,
Intensity (a.u.)
(120)
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(040)
15
20
25
30
35
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2θ (°)
Figure 9. WAXD pattern of PBSub.
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From a practical viewpoint of melt processing, the investigation on the thermal stability of PBSub is important. In this work, the thermogravimetric experiment of PBSub was performed at a heating rate of 20 oC/min under nitrogen atmosphere. Figure 10 illustrates the thermogravimetric analysis (TGA) curve and the corresponding derivative thermogravimetric (DTG) curve of PBSub. From Figure 10, the weight loss of PBSub occurred in one-step decomposition process. The decomposition temperature at 5 wt% weight loss (Td) was determined to be about 377 oC, while the temperature at the maximum degradation rate (Tmax) 19
ACCEPTED MANUSCRIPT was estimated to be about 421 oC. Both the TGA and DTG results indicate that PBSub may become one of the promising biodegradable polyesters with excellent thermal stability. In previous work, PBS displayed a Td value of 363 oC and a Tmax value of 406 oC [52], indicating
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that the thermal stability of PBSub is a little higher than that of PBS.
2.5
100
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60
1.0
40 20 0 0
100
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Weight (%)
1.5
Deriv.Weight (%/°C)
2.0
80
200
300
400
500
0.5 0.0 -0.5
600
Temperature (°C)
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Figure 10. TGA and DTG curves of PBSub. As PBSub is an aliphatic polyester, it may undergo a hydrolytic degradation. In the present
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work, we also investigated the hydrolytic degradation process of PBSub, which was performed at 37 °C in a NaOH solution (pH = 14). By following the variation of weight loss
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with degradation time, the weight loss coefficient Wloss(%) was calculated by the following equation:
Wloss (%) = 100×(W0-Wt-dried)/W0
(5)
where W0 is the initial weight and Wt-dried is the weight of sample subjected to hydrolytic degradation for time t and drying in vacuum. Figure 11 illustrates the variation of weight loss with hydrolytic degradation time. As shown in Figure 11, PBSub may degrade quickly during the hydrolytic degradation process, since it lost about 80% weight within 3 days, indicating 20
ACCEPTED MANUSCRIPT that PBSub is a degradable material and may find its end use in some special application fields. PLLA lost its about 80% weight within around 3 weeks in a hydrolytic degradation
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process a NaOH solution (pH = 13) [10], suggesting PLLA may degrade slowly than PBSub.
100
60
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Weight loss (%)
80
40
0 0
1
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20
2 3 4 Degradation time (day)
5
6
Figure 11. Hydrolytic degradation process of PBSub in a NaOH solution (pH = 14).
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4. Conclusions
PBSub was synthesized via a two-step melt polycondensation method in this research from
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the monomers of butanediol and suberic acid. The synthesized PBSub has an average molecular weight of 3.64 × 104 g/mol and a narrow polydispersity index of about 2. The basic
o
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thermal properties studies reveal that PBSub has a glass transition temperature of about −61 C, a melting point of 55.2 oC, and an equilibrium melting point of 61.4 oC. The overall
isothermal melt crystallization kinetics of PBSub was investigated with DSC and described by the Avrami equation. On one hand, regardless of crystallization temperature, the average Avrami exponent value of PBSub is about 3, indicating a crystallization mechanism of three-dimensional truncated sphere growth with athermal nucleation. On the other hand, increasing crystallization temperature may reduce the overall crystallization rate. The 21
ACCEPTED MANUSCRIPT crystalline morphology study displays that PBSub presents obvious spherulitic morphology; moreover, the spherulitic growth rates decrease with increasing crystallization temperature. According to the secondary nucleation theory, PBSub exhibits a crystallization regime
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transition at about 41 oC between regime II and regime III. The WAXD study indicates that PBSub is a highly crystalline semicrystalline polymer, exhibiting strong diffraction peaks and a great crystallinity of about 55%. PBSub has both a high decomposition temperature at 5
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wt% weight loss of about 377 oC and a high temperature at the maximum degradation rate of
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about 421 oC, indicating that PBSub is a novel biodegradable polyester with good thermal stability. In addition, PBSub may undergo a hydrolytic degradation process, which may be of help and interest for its end use in some special application fields as a degradable material.
Acknowledgement
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The authors thank the National Natural Science Foundation, China (51373020 and
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EP
51221002) for the support of this research.
22
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ACCEPTED MANUSCRIPT > We synthesized aliphatic PBSub successfully> The thermal properties and crystallization kinetics of PBSub were investigated > PBSub exhibits a crystallization regime II to regime III transition > PBSub presents strong diffraction peaks and a
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crystallinity of about 55% > PBSub has a high thermal stability and may undergo a
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EP
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hydrolytic degradation process >
S0032-3861(15)00406-1
DOI:
10.1016/j.polymer.2015.04.069
Reference:
JPOL 17821
To appear in:
Polymer
Received Date: 20 January 2015 Revised Date:
15 March 2015
Accepted Date: 20 April 2015
Please cite this article as: Cui Z, Qiu Z, Thermal Properties and Crystallization Kinetics of Poly(butylene suberate), Polymer (2015), doi: 10.1016/j.polymer.2015.04.069. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal Properties and Crystallization Kinetics of Poly(butylene suberate)
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Zhiming Cui and Zhaobin Qiu*
1
ACCEPTED MANUSCRIPT
Thermal Properties and Crystallization Kinetics of Poly(butylene suberate) Zhiming Cui and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, MOE Key Laboratory of Carbon
China
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Fiber and Functional Polymers, Beijing University of Chemical Technology, Beijing 100029,
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Fax: +86-10-64413161. E-mail: [email protected].
1
ACCEPTED MANUSCRIPT Abstract In this work, we synthesized poly(butylene suberate) (PBSub) with a weight average molecular weight of 3.64 × 104 g/mol from the monomers of butanediol and suberic acid via a
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two-step melt polycondensation method. The basic thermal behaviors, overall isothermal melt crystallization kinetics, crystal structure, spherulitic morphology and growth, thermal stability, and hydrolytic degradation of PBSub were systematically investigated for the first time.
SC
PBSub has a low glass transition temperature of about −61 oC, a melting point of 55.2 oC, and
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an equilibrium melting point of 61.4 oC. The overall isothermal melt crystallization kinetics of PBSub was investigated at different crystallization temperature values and analyzed by the Avrami equation. PBSub has an average Avrami exponent value of about 3, suggesting that the crystallization mechanism of PBSub may correspond to three-dimensional truncated
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sphere growth with athermal nucleation. An obvious spherulitic morphology was observed for PBSub, and the spherulitic growth rates decrease with increasing crystallization temperature. PBSub exhibits a crystallization regime II to regime III transition on the basis of the
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secondary nucleation theory. The crystal structure study reveals that PBSub is highly
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crystalline, presenting strong diffraction peaks and a great crystallinity of about 55%. The thermogravimetric analysis study demonstrates that PBSub has both a high decomposition temperature at 5 wt% weight loss of about 377 oC and a high temperature at the maximum degradation rate of about 421 oC, suggesting its good thermal stability. PBSub may undergo a hydrolytic degradation, which may be of interest for its end use as a degradable material in some special application fields. Keywords: crystallization behavior; thermal properties; poly(butylene suberate) 2
ACCEPTED MANUSCRIPT 1. Introduction Biodegradable aliphatic polyesters have been attracting considerable research interests from both academic and industrial viewpoints [1−7]. Some typical members of the extensively
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investigated biodegradable polyesters include poly(L-lactide) (PLLA), poly(ε-caprolactone) (PCL), poly(ethylene succinate) (PES), poly(butylene succinate) (PBS), poly(ethylene adipate) (PEA), poly(butylene adipate) (PBA), etc [8−39]. Huneault et al. reviewed the crystallization
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behavior of PLLA and provided comprehensive relationships between crystallization kinetics
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and the molecular structure characteristics [8]. Eastmond reviewed the miscibility, crystallization, and properties of PCL based polymer blends [17]. Qiu et al. reported the preparation,
morphology,
crystallization,
and
properties
of
PES
based
polymer
nanocomposites containing multi-walled carbon nanotubes, thermally reduced graphene, and
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polyhedral oligomeric silsesquioxanes [22, 25, 26]. Guo et al. reviewed the research, development, and industrialization of PBS and its copolymers [27]. Inoue et al. reported the crystallization kinetics and crystalline structure of PEA with low and high molecular weights
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[33]. Gan et al. studied the metastability and transformation of polymorphic crystals in PBA
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[35]. Some of them, such as PES, PBS, PEA, and PBA may be synthesized from the condensation of dicarboxylic acids (such as succinic or adipic acid) with diols (1,2-ethanediol or1,4-butadenediol). Through the characteristic two-stage melt polycondensation method, PES, PBS, PEA, and PBA have been successfully prepared; moreover, PES, PBS, and PBA have become industrialized and commercially available. A lot of works have been devoted to the basic thermal properties, crystallization behaviors, melting behaviors, crystal structures and polymorphism, mechanical properties, and thermal and enzymatic degradation behaviors 3
ACCEPTED MANUSCRIPT of these biodegradable polyesters [21−39]. Some other monomers with more methylene (CH2) groups between dicarboxylic acids or diols have also been used to develop new aliphatic polyesters [40−46]. Among them, suberic
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acid has been used as a new monomer to prepare a series of novel aliphatic polyesters, such as poly(ethylene suberate) (PESub), poly(butylene suberate) (PBSub), poly(hexamethylene suberate) (PHSub), and poly(octanediol suberate) (POSub) [43−46]. Few works have focused
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on PBSub, because it is not commercially available until now. Puiggali et al. reported that
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PBSub adopted an all-trans conformation and pack via an orthorhombic unit cell containing four molecular segments; moreover, the lattice parameters of the orthorhombic unit cell were a = 0.506 nm, b = 1.462 nm, and c = 1.72 nm, respectively [44]. In addition, they also found that the lamellar crystals may be acquired when PBSub was isothermally crystallized from a
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dilute alcohol solution, exhibiting characteristic truncated lozenge morphology [44]. To our knowledge, the thermal properties and crystallization kinetics of PBSub have not been reported so far. The purpose of this work is to study the thermal properties and
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crystallization kinetics of PBSub, as they are interesting and important not only for the
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practical application of PBSub but also for a better understanding of the structure and properties relationship of aliphatic polyesters. In this work, we extensively studied the basic thermal behaviors, overall isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, thermal stability, and hydrolytic degradation of PBSub. The research results reported herein are expected to be useful and helpful to promote the interests in the basic study and practical application of this aliphatic polymer. 2. Experimental Details 4
ACCEPTED MANUSCRIPT Butanediol (BDO) was purchased from Tianjin Fu Chen Chemical Reagent Factory. Suberic acid (SuA) was bought from Sinopharm Chemical Reagent Co., Ltd. Tetrabutyl titanate (TBT), used as catalyst, was purchased from Beijing Chang Ping Jing Xiang
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Chemical Factory. PBSub was synthesized in our laboratory from the condensation of BDO and SuA via a typical two-step melt polycondensation method [23, 29, 30]. During the esterification process,
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the proper amount of appropriate BDO and SuA (SuA/BDO molar ratio = 1:1.1) and the
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catalyst (1×10-4 mol TBT/mol acid) were charged into a three-necked 250 mL flask, which was first filled with nitrogen to completely remove oxygen. The mixture was first heated to 150~160 oC, kept for 20 min under nitrogen atmosphere for all the monomers to melt completely, and was then heated to 190 oC for 3 h. During the polycondensation process, the
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mixture was further heated to 230 oC for 5 h at a reduced pressure. The synthesized product was purified by dissolving in chloroform and then precipitating into a 3 times amount of methanol. Finally, the precipitate was dried in a vacuum at 30 oC for 72 h before use.
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Figure 1 shows the chemical structure of PBSub . O
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O
O
n O
Figure 1. Chemical structure of PBSub.
The molecular weight of PBSub was measured with gel permeation chromatography (GPC) (Waters 515 HPLC, Waters Company) using tetrahydrofuran as the solvent at 1 mL/min. Calibration was performed with narrow polydispersity polystyrene (PS) standards. The thermal behaviors of PBSub were measured using a TA Instruments differential 5
ACCEPTED MANUSCRIPT scanning calorimeter (DSC) Q100. The sample was first annealed at 140 oC for 3 min to erase any previous thermal history and was then nonisothermally crystallized at different cooling rates to –90 oC. The corresponding melting point and glass transition temperature were then
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determined on the nonisothermally crystallized sample at 10 oC/min. For the isothermal melt crystallization kinetics and equilibrium melt point estimation studies of PBSub, the samples were treated through the similar processes as those in our previous works, which were
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described in detail elsewhere [23, 29, 30].
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Wide-Angle X-ray diffraction (WAXD) pattern was recorded from 5 – 45o at 4 o/min on a Rigaku d/Max2500 VB2+/PC X-ray Diffractometer, which was operated at 40 kV and 200 mA.
The spherulitic morphology and growth of PBSub were investigated with a polarized
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optical microscope (POM) (Olympus BX51). As the nucleation ability of PBSub was too strong, the sample was dissolved in chloroform with a concentration of 1 mg/mL and then cast on a cover glass to form a thin film for the POM observation.
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The thermal stability was performed with a TA Instruments thermogravimetric analysis
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(Q50) at 20 oC/min under nitrogen atmosphere. The hydrolytic degradation of PBSub was performed at 37 °C in a sodium hydroxide (NaOH) solution with a pH value of 14 by following the variation of weight loss with degradation time.
3. Results and Discussion It is well-known that the molecular weight and polydispersity index (PDI) values of a polymer must have an obvious influence on its basic thermal behaviors, crystallization 6
ACCEPTED MANUSCRIPT kinetics, crystalline morphology, thermal stability, mechanical properties, etc. PBSub has a number average molecular weight (Mn) of 1.86 × 104 g/mol, a weight average molecular weight (Mw) of 3.64 × 104 g/mol, and a PDI value of about 1.96. In literature, Puiggali et al.
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synthesized a PHSub sample, and the Mn and Mw values were 2.08 × 104 and 3.73 × 104 g/mol, respectively [45]. In the present work, the Mn and Mw values of PBSub are very close to those of PHSub.
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The basic thermal properties of PBSub were first studied in this work with DSC. PBSub
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was subjected to the thermal analysis through the thermal treatment process described in the Experimental Section. After the heating, cooling, and reheating process, the following parameters were acquired for the synthesized PBSub sample, including the nonisothermal melt crystallization peak temperature (Tp), glass transition temperature (Tg), and melting point
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(Tm). Figure 2a shows the crystallization exotherms of PBSub at different cooling rates ranging from 5 to 40 oC/min from the crystal-free melt after a slow heating process to eliminate thermal history. Figures 2b displays the subsequent melting endotherms of PBSub at
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a heating rate of 10 oC/min after the sample finished the nonisothermal melt crystallization
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processes at different cooling rates. As shown in Figure 2a, regardless of cooling rate, PBSub has a very sharp crystallization peak, indicating that the crystallizability of PBSub is strong. With increasing the cooling rate, the crystallization exotherms gradually shift downward to low temperature range. At a cooling rate of 5 oC/min, the Tp value was determined to be 39.1 o
C, and the melt crystallization enthalpy (∆Hc) was measured to be 88.5 J/g. When the cooling
rate was increased to be 40 oC/min, the Tp value was decreased to be 33.2 oC, and the ∆Hc value was estimated to be 87.4 J/g. As shown in Figure 2b, PBSub exhibits a well-defined 7
ACCEPTED MANUSCRIPT single melting endotherm, regardless of cooling rate. From Figure 2b, PBSub displays a Tm value of 55.1 oC with a heat of fusion (∆Hm) of 89.9 J/g, when a cooling rate of 5 oC/min was used. At a cooling rate of 40 oC/min, PBSub shows a Tm value of 54.4 oC with a ∆Hm of 89.3
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J/g. All of the above obtained thermal parameters are summarized in Table 1. It is clear that the variation of cooling rate from 5 to 40 oC/min obviously affects the Tp values but slightly influences the ∆Hc, Tm, and ∆Hm values. Figures 2c displays the enlarged glass transition
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behavior of PBSub at a heating rate of 10 oC/min after the sample was nonisothermally
–61.0 oC.
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crystallized from the melt at 10 oC/min, from which the Tg value was measured to be about
Table 1. Thermal Properties of PBSub at Different Cooling Rates. Tp (oC)
∆Hc (J/g)
Tm (oC)
∆Hm (J/g)
5
39.1
88.5
55.1
89.9
37.3
86.9
55.2
94.2
34.0
90.9
54.7
92.8
33.2
87.4
54.4
89.3
20 40
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10
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Cooling rates (oC/min)
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The low Tg value of PBSub suggests that PBSub may have good flexibility because of its strong chain mobility, which needs further investigation. It is interesting to note that the Tg value of PBSub is very close to that of PCL (∼ –60 oC) [17]. The similarity of the Tg values between PBSub and PCL may be related to the following fact that they have the same CH2/CO ratio value of 5 according to their chemical structures. In addition, it is also interesting to compare the basic thermal properties of PBSub with those of PBS, as they have the similar chemical structures, except that the former possesses four more CH2 groups 8
ACCEPTED MANUSCRIPT between the carbonyl groups than the latter. In other words, the CH2/CO ratio is 5 for PBSub, while that of PBS is 3. PBS has a Tg value of about –34 oC and a Tm value of about 112 oC [28]. It is obvious that both the Tg and Tm values are greater in PBS than in PBSub, suggesting
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that the chain mobility of PBSub is stronger than PBS because of the introduction of more CH2 groups into the chemical structure of PBSub. In brief, the basic thermal properties
SC
indicate that PBSub is a semicrystalline aliphatic polyester with low Tg and Tm values.
(a)
2 W/g
20 °C/min
10 °C/min 5 °C/min
0
10
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Heat flow (Endo up)
40 °C/min
20
30
40
50
60
70
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Temperature (°C)
(b)
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Heat flow (Endo up)
EP
2 W/g
40 °C/min 20 °C/min 10 °C/min 5 °C/min
20
30
40
50
Temperature (°C)
9
60
70
ACCEPTED MANUSCRIPT
Heat flow (Endo up)
(c)
o
-80
-75
-70
-65
-60
-55
-50
-45
-40
-35
SC
Temperature (°C)
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Tg = -61.0 C
0.01 W/g
Figure 2．DSC curves of PBSub showing (a) nonisothermal melt crystallization exotherms, (b)
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meting endotherms, and (c) enlarged glass transition region.
In the above section, the Tm value was measured to be 55.2 oC, when PBSub was heated at 10 oC/min on a nonisothermally crystallized sample from the crystal-free melt at a cooling rate of 10 oC/min. It is obvious that the Tm value may be affected by both the crystallization
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conditions (nonisothermal or isothermal crystallization) and heating rate. If the sample is crystallized nonisothermally, cooling rate is the key factor of influencing the subsequent
EP
melting behavior. If the sample is crystallized isothermally, crystallization temperature (Tc) and crystallization time are the main factors of affecting the subsequent melting behavior. In
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addition to crystallization conditions, heating rate is also an important factor of influencing the Tm value. To avoid the aforementioned kinetics and morphology influences on the Tm value, equilibrium melting point is an important and basic thermal parameter to characterize a semicrystalline polymer. Figure 3 demonstrates the subsequent melting behaviors of PBSub after it was isothermally crystallized at a temperature range from 40 to 48 oC. To avoid the influence of crystallization time, all of the samples were crystallized for the same crystallization time (1 h) at different Tc 10
ACCEPTED MANUSCRIPT values. As shown in Figure 3, PBSub presents a well defined single melting endotherm, regardless of Tc, indicating that the formed crystals are so perfect that the well-known melting, recrystallization, and remelting process does not occur. Figure 3 also clearly shows that
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increasing Tc shifts the melting endotherm upward to high temperature range. For instance, at a Tc of 40 oC, PBSub has a Tm of 56.3 oC, while it has a Tm of 58.3 oC at a Tc of 48 oC,
SC
suggesting that increasing Tc favors the formation of relatively perfect crystals with higher Tm.
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1 W/g Heat flow (Endo up)
48 °C
46 °C 44 °C 42 °C
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40 °C
35
40
45
50
55
60
65
70
75
Temperature (°C)
EP
Figure 3. Melting behaviors of PBSub after crystallizing at various Tc values for 1 h.
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Hoffman and Weeks described a relationship between Tm and Tc: Tm = ηTc + (1-η) Tmo
(1)
where T om represents the equilibrium melting point, and η is a measurement of the stability related to the crystal layer thickness of the polymer [47]. The Hoffman–Weeks plot is displayed in Figure 4. PBSub has a Tmo of 61.4 oC and a η value of 0.24. The Tmo value of PBSub seems a little low in this work, which should be mainly related to its molecular weight [44]. In addition, the enthalpy of 100% crystalline PBSub value (∆Hmo) was calculated to be 35 kJ/mol, i.e., 153.5 J/g, on the basis of the values reported by van Krevelen for methylene 11
ACCEPTED MANUSCRIPT (4 kJ/mol) and ester (–2.5 kJ/mol) groups [48].
o T m = 61.4 °C
Tm (°C)
60
40 40
45
50
55
60
65
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35
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50
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70
Tc (°C)
Figure 4. Hoffman−Weeks plot of PBSub. The overall isothermal melt crystallization kinetics of PBSub was investigated with DSC
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and analyzed by the Avrami equation in a Tc range from 44 to 48 oC, which would affect the final physical properties. Figure 5a illustrates the development of relative crystallinity (Xt)
EP
with crystallization time (t) at different Tc values. All of the Xt − t plots present the characteristic sigmoid curves. Figure 5a clearly shows that the crystallization time required to
AC C
finish crystallization is longer at higher Tc than at lower Tc, suggesting that increasing Tc may suppress the crystallization. The Xt − t plots were further treated with the Avrami equation, which is shown as follows:
1 – Xt = exp (–ktn)
(2)
where n is the Avrami exponent reflecting crystallization mechanism, and k is the crystallization rate constant [49, 50]. Figure 5b shows the related Avrami plots of PBSub. As shown in Figure 5b, the Avrami plots of PBSub at different Tc values illustrate a series of 12
ACCEPTED MANUSCRIPT almost parallel straight lines, suggesting that the n values should be close to each other. In other words, PBSub may crystallize through the same crystallization mechanism, regardless of Tc. In brief, the overall isothermal melt crystallization of PBSub could be described by the
100
SC
60
44 °C 45 °C 46 °C 47 °C 48 °C
40
20
0 0
10
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Relative crystallinity (%)
(a) 80
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Avrami equation.
20
30
40
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Crystallization time (min)
(b)
EP
log(-ln(1-Xt))
0
AC C
-1
44 °C 45 °C 46 °C 47 °C 48 °C
-2
-0.4
0.0
0.4
0.8
1.2
1.6
log t
Figure 5. (a) Development of relative crystallinity with crystallization time and (b) Avrami plots of PBSub at indicated Tc values. Table 2 summarizes the n and k values, after PBSub was crystallized at different Tc values. The n values of PBSub vary slightly from 2.9 to 3.2 in the investigated Tc range. Despite Tc, 13
ACCEPTED MANUSCRIPT all of the n values are close to 3, suggesting a possible three-dimensional truncated growth with athermal nucleation mechanism [51]. In previous work, we once studied the isothermal melt crystallization kinetics of PBS and its nanocomposite with carbon nanotubes in a range
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of Tc from 96 to 104 oC, and the n values were also close to 3 [52]. Table 2 clearly demonstrates that the k values decrease with increasing Tc, when the n values are the same at 44 and 48 oC or 45 and 46 oC. The reduction of the k values indicates a slow crystallization
SC
rate. For an easy comparison of crystallization rates at different Tc values, we calculated the
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crystallization half-time (t0.5), the time at 50% of the final crystallinity (t0.5) through the following equation:
t0.5 = (
ln 2 1/ n ) k
(3)
Therefore, the 1/t0.5 values may be used to compare the crystallization rates, as they are
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normalized rate constants with the same units (min-1) and are independent of the n values. The t0.5 and 1/t0.5 values were acquired and are also listed in Table 2. For comparison, the experimental crystallization half-time (t1/2) values were also directly read from Figure 5a and
EP
are also summarized in Table 2. Regardless of Tc, the calculated t0.5 value and the
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experimental t1/2 value are close to each other, indicative of the validity of the Avrami equation during the isothermal crystallization kinetics study.
14
ACCEPTED MANUSCRIPT Table 2. Isothermal Crystallization Kinetics Parameters of PBSub. Tc (oC)
n
k (min−n)
t1/2 (min) t0.5 (min) 1/t0.5 (min−1)
2.9 6.31×10−2
2.15
2.27
4.41×10−1
45
3.1 1.68×10−2
3.29
3.34
2.99×10−1
46
3.1 0.29×10−2
5.77
5.82
47
3.2 5.56×10−4
9.40
9.41
48
2.9 2.37×10−4
15.73
16.12
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44
1.72×10−1 1.06×10−1
SC
6.20×10−2
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In the above section, the overall isothermal melt crystallization kinetics study of PBSub was performed with DSC in a Tc range of 44 to 48 oC and analyzed by the Avrami equation. The n values are close to 3, suggesting that PBSub may crystallize according to a crystallization mechanism of spherulite growth with athermal nucleation [51]. To verify the
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crystallization mechanism, the crystalline morphology of PBSub was investigated. Figure 6 displays the POM image of PBSub, which was isothermally crystallized at 46 oC for 50 min. PBSub presents characteristic spherulitic morphology. Several PBSub spherulites grew during
EP
the crystallization process, impinged against each other later, forming clear spherulites
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boundaries, and finally filled the whole space. The average size of PBSub spherulites are about 65 µm in radius. The crystalline morphology study confirms that PBSub crystallizes via the spherulitic growth mechanism; moreover, the spherulites are relatively regular and perfect.
15
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ACCEPTED MANUSCRIPT
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Figure 6. POM image of PBSub spherulites after crystallizing at 46 oC.
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In addition, the growth rate (G) was calculated by following the variation of radius (R) of spherulites with crystallization time, i.e., G = dR/dt. The G values of PBSub spherulites at different Tc values were estimated and are shown in Figure 7. It is clear from Figure 7 that the G values decrease significantly with increasing Tc. For example, PBSub presents a great G
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value of 154.9 µm/min at a Tc of 36 oC, while it only displays a small G value of 3.6 µm/min at a Tc of 46 oC. Such an obvious reduction of the G values indicates that Tc is a key factor of determining not only the spherulites nucleation but also spherulites growth, because it is
EP
related to the important and decisive factor of influencing the crystallization of biodegradable
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polyesters, i.e., supercooling.
16
ACCEPTED MANUSCRIPT
160 140
100 80 60 40 20 0 36
38
40
42
44
46
SC
Tc (°C)
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G (µm/min)
120
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Figure 7. Growth rates of PBSub spherulites at various Tc values. We further analyzed the spherulitic growth rates of PBSub using the secondary nucleation theory proposed by Lauritzen and Hoffman in this research. The Lauritzen and Hoffman equation is shown as follows:
(4)
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Kg U∗ G = G0 exp − exp− R(Tc − T∞ ) Tc (∆T ) f
where G0 is a preexponential factor, U* is the activation energy for transporting the polymer
EP
chain segments to the crystallization site, R is the gas constant, T∞ is a temperature below which the polymer chain movement ceases, ∆T is the degree of supercooling, f is a correction
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factor accounting for the variation in the enthalpy of fusion given as f =2Tc /(Tmo+Tc), and Kg is the nucleation constant [53]. The values of U* =17246.3 J/mol and T∞ = T g - 51.6 K were used in this work [54]. Figure 8 shows the Lauritzen–Hoffman plot for PBSub. Figure 8 obviously illustrates that all the data may be fitted with two straight lines with different slopes, indicating a crystallization regime transition from regime II at high Tc to regime III at low Tc. From Figure 8, the nucleation constant values for regime II (KgII) and regime III (KgIII) were determined to be 4.57×104 K2 and 8.85×104 K2, respectively. The KgIII/KgII value is 1.93, 17
ACCEPTED MANUSCRIPT which is close to the theoretical value of 2 [52]. The regime transition temperature (Ttr) is about 41 oC. Similar to PBSub, the spherulites of PES and PBS may also be observed in a wide range of temperature range [23, 29]. In previous work, PES also exhibited a
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crystallization transition from regime II to regime III, and the Ttr value was around 60 oC [23]. Similarly, PBS displayed a crystallization transition from regime II to regime III, and the Ttr value was around 95 oC [29]. It is interesting to note that the crystallization transition from
SC
regime II to regime III is very common for aliphatic polyesters; moreover, the transition
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temperature shifts to low temperature with decreasing the melting point temperature values of the aliphatic polyesters.
6.0 5.5 5.0
4.0 3.5 3.0
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lnG+U*/(R(Tc−T∞))
4.5
2.5 2.0 1.5 1.0
-4
1.4×10
-4
1.6×10
-4
1.8×10
-4
2.0×10
-2
EP
1/(Tc(∆T)f) (K )
AC C
Figure 8. Regime transition analysis of PBSub.
Similar to other aliphatic polyesters, such as PBS, PES, PCL, etc, PBSub is also a semicrystalline polymer. The crystal structure of PBSub was further investigated in this research. Figure 9 illustrates the WAXD pattern of a PBSub sample, which was isothermally crystallized at 40 oC for 48 h. As shown in Figure 9, PBSub presents several strong diffraction peaks at the 2θ range of 20o to 35o, indicating its highly crystalline feature. Puiggali et al. once determined the crystal structure of PBSub as follows, i.e., an orthorhombic unit cell with 18
ACCEPTED MANUSCRIPT dimension a = 0.506 nm, b = 1.462 nm, and c = 1.72 nm; therefore, the two main characteristic diffraction peaks, which are located at 21.4o and 24.6o, are attributed to the (120) and (040) planes, respectively [44]. In addition, the crystallinity value of PBSub was further
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determined to be about 55% on the basis of the WAXD pattern, which is also summarized in Table 1. The crystallinity value of PBSub is also close to that of PCL (∼50%) [17]. In brief,
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PBSub is a highly crystalline aliphatic polyester.
SC
from both the two strong characteristic diffraction peaks and the great crystallinity value,
Intensity (a.u.)
(120)
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(040)
15
20
25
30
35
EP
2θ (°)
Figure 9. WAXD pattern of PBSub.
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From a practical viewpoint of melt processing, the investigation on the thermal stability of PBSub is important. In this work, the thermogravimetric experiment of PBSub was performed at a heating rate of 20 oC/min under nitrogen atmosphere. Figure 10 illustrates the thermogravimetric analysis (TGA) curve and the corresponding derivative thermogravimetric (DTG) curve of PBSub. From Figure 10, the weight loss of PBSub occurred in one-step decomposition process. The decomposition temperature at 5 wt% weight loss (Td) was determined to be about 377 oC, while the temperature at the maximum degradation rate (Tmax) 19
ACCEPTED MANUSCRIPT was estimated to be about 421 oC. Both the TGA and DTG results indicate that PBSub may become one of the promising biodegradable polyesters with excellent thermal stability. In previous work, PBS displayed a Td value of 363 oC and a Tmax value of 406 oC [52], indicating
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that the thermal stability of PBSub is a little higher than that of PBS.
2.5
100
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60
1.0
40 20 0 0
100
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Weight (%)
1.5
Deriv.Weight (%/°C)
2.0
80
200
300
400
500
0.5 0.0 -0.5
600
Temperature (°C)
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Figure 10. TGA and DTG curves of PBSub. As PBSub is an aliphatic polyester, it may undergo a hydrolytic degradation. In the present
EP
work, we also investigated the hydrolytic degradation process of PBSub, which was performed at 37 °C in a NaOH solution (pH = 14). By following the variation of weight loss
AC C
with degradation time, the weight loss coefficient Wloss(%) was calculated by the following equation:
Wloss (%) = 100×(W0-Wt-dried)/W0
(5)
where W0 is the initial weight and Wt-dried is the weight of sample subjected to hydrolytic degradation for time t and drying in vacuum. Figure 11 illustrates the variation of weight loss with hydrolytic degradation time. As shown in Figure 11, PBSub may degrade quickly during the hydrolytic degradation process, since it lost about 80% weight within 3 days, indicating 20
ACCEPTED MANUSCRIPT that PBSub is a degradable material and may find its end use in some special application fields. PLLA lost its about 80% weight within around 3 weeks in a hydrolytic degradation
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process a NaOH solution (pH = 13) [10], suggesting PLLA may degrade slowly than PBSub.
100
60
SC
Weight loss (%)
80
40
0 0
1
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20
2 3 4 Degradation time (day)
5
6
Figure 11. Hydrolytic degradation process of PBSub in a NaOH solution (pH = 14).
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4. Conclusions
PBSub was synthesized via a two-step melt polycondensation method in this research from
EP
the monomers of butanediol and suberic acid. The synthesized PBSub has an average molecular weight of 3.64 × 104 g/mol and a narrow polydispersity index of about 2. The basic
o
AC C
thermal properties studies reveal that PBSub has a glass transition temperature of about −61 C, a melting point of 55.2 oC, and an equilibrium melting point of 61.4 oC. The overall
isothermal melt crystallization kinetics of PBSub was investigated with DSC and described by the Avrami equation. On one hand, regardless of crystallization temperature, the average Avrami exponent value of PBSub is about 3, indicating a crystallization mechanism of three-dimensional truncated sphere growth with athermal nucleation. On the other hand, increasing crystallization temperature may reduce the overall crystallization rate. The 21
ACCEPTED MANUSCRIPT crystalline morphology study displays that PBSub presents obvious spherulitic morphology; moreover, the spherulitic growth rates decrease with increasing crystallization temperature. According to the secondary nucleation theory, PBSub exhibits a crystallization regime
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transition at about 41 oC between regime II and regime III. The WAXD study indicates that PBSub is a highly crystalline semicrystalline polymer, exhibiting strong diffraction peaks and a great crystallinity of about 55%. PBSub has both a high decomposition temperature at 5
SC
wt% weight loss of about 377 oC and a high temperature at the maximum degradation rate of
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about 421 oC, indicating that PBSub is a novel biodegradable polyester with good thermal stability. In addition, PBSub may undergo a hydrolytic degradation process, which may be of help and interest for its end use in some special application fields as a degradable material.
Acknowledgement
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The authors thank the National Natural Science Foundation, China (51373020 and
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EP
51221002) for the support of this research.
22
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ACCEPTED MANUSCRIPT > We synthesized aliphatic PBSub successfully> The thermal properties and crystallization kinetics of PBSub were investigated > PBSub exhibits a crystallization regime II to regime III transition > PBSub presents strong diffraction peaks and a
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crystallinity of about 55% > PBSub has a high thermal stability and may undergo a
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hydrolytic degradation process >