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. 2020 Feb 6;5(6):3080–3089. doi: 10.1021/acsomega.9b04315

Isothermal Kinetics of Poly(butylene adipate-co-butylene itaconate) Copolyesters with Ethylenediaminetetraacetic Acid

Chin-Wen Chen 1,*, Te-Sheng Hsu 1, Syang-Peng Rwei 1,*
PMCID: PMC7033981  PMID: 32095731

Abstract

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A series of aliphatic copolyesters, poly(butylene adipate-co-butylene itaconate) (PBABI), have been synthesized using melt polycondensation of adipic acid (AA), itaconic acid (IA), 1,4-butanediol (1,4-BDO), and the tetra-functional group of ethylenediaminetetraacetic acid (EDTA, 0.1 mol %) to form partially cross-linking density as novel thermoplastic unsaturated copolyesters in our previous research. The crystal phase of PBABI copolyesters tended to prefer thermodynamics in the presence of a small amount of EDTA. The isothermal crystallization analysis revealed that the PBABI with EDTA exhibited a higher crystallization rate and a shorter half-time of crystallization than neat PBABI copolyesters. All of the sizes of spherulite/sheet crystals in the BA/BI = 9/1 are smaller than at BA/BI = 10/0 with or without a cross-linking agent, which demonstrated that the morphology behavior tended to form a small sheet crystal in the presence of 10 mol % IA, which played a dominant role in determining the average size of the crystal. These results deepen our understanding of the relationship among the cross-linking agent, the crystal form, and solidification time in PBABI copolyesters, making these kinds of polymers applicable to reinforce three-dimensional (3D) air-permeable polyester-based smart textiles.

Introduction

Aliphatic polyester, consisting of different lengths of the CH2 group and ester bonds within the main chain or side chain, is a semicrystalline polymer and widely used as adhesives, fibers, films, scaffolds in biodegradable materials, and so on, which exhibits excellent flexibility in the polymer chain, mechanical properties, and biodegradability.1−4 Poly(butylene adipate) (PBA) is a typical aliphatic polyester that has been commercialized by the BASF company.5−9 PBA is a polymorph polyester, and the crystal phases could be transformed into α-phase, β-phase, and an α/β complex under heat treatment.10−22 The α-phase of PBA is associated with thermodynamic stability, while the β-phase is a metastable phase that can be induced into the α-phase via annealing procedures. The α-phase of PBA through annealing procedures could drive the transition behavior of crystal morphology and size, which leads to biodegradability. Gan et al. adopted thermal annealing procedures to switch the conversion of α/β-phase crystals, indicating that β-phase crystals existed below the temperature of 31.8 °C, while α-phase crystals occurred at temperatures above 29.8 °C. A mixture of α/β-phase crystals has been observed with the crystallization temperature around 30 ± 1 °C.10 At that time, the β- to α-phase crystal transformation has been investigated, observing that the crystal transformation of PBA takes place at a higher temperature for the annealing operation, and this transformation in PBA has been proved as a solid–solid crystal-phase transition process to improve the increase of the thickening in the size of crystals.11,12 Yang and co-workers blended poly(butylene succinate) (PBS) with PBA to examine the α/β-phase crystal transition behavior, signifying that the α-phase crystal of PBA played a dominant role with the content of PBS below 70 wt % under the PBS/PBA blend experiment.13 The orientated PBS crystals resulted in excellent nucleation ability to induce the formation of α-phase PBA crystals in any crystallization situation.23 Furthermore, a nucleation agent has been chosen to induce the transformation of the crystallization behavior between α- and β-phase crystals, such as uracil,14 poly(vinyl alcohol),15 cyanuric acid,16 anodic aluminum oxide,17 anhydrous orotic acid,18 oxalamide derivative,19 sorbitol derivative,21 and so on. Bao and colleagues have modified PBA with the end-functional H-bonding group, 2-ureido-4[1H]-pyrimidinone (UP), revealing that the UP-modified PBAs favored the formation of the thermally stable α-phase crystal at the same temperature, as well as lowering and broadening the temperature range for the β- to α-phase crystal transition of PBA under heating processes.20 Also, a well-known unsaturated polyester, poly(butylene itaconate) (PBI), is synthesized from itaconic acid (IA) and 1,4-butanediol (1,4-BDO) via melt polymerization.24 IA has a dicarboxylic group, is a biomassed, unsaturated, and sustainable material, and has been extensively used in bio-based UV-cured adhesive resin.25,26 A new aliphatic copolyester, poly(butylene adipate-co-butylene itaconate) (PBABI),27 has an unfortunate mechanical property due to the linear conformation of the molecular chain. Hence, cross-linking agents were the best choice to form a stereo network structure to enhance the mechanical and thermal properties of these kinds of aliphatic copolyesters.28

Ethylenediaminetetraacetic acid (EDTA) is widely utilized as a chelating agent for the removal of metal ions, adhesives, and antioxidants, as well as in bioengineering and other fields.29,30 The chemical structure of EDTA has two nitrogen atoms in the center and four symmetrical −COOH groups, which react with the −OH group to produce an ester bond. The toughness and elasticity of copolyesters can be reinforced by copolymerizing with EDTA as a cross-linking agent to form a network architecture,31 which plays a role in improving the thermal stability and mechanical strength of copolyesters.32 As described above, the aliphatic polyester can form a three-dimensional (3D) network architecture through copolymerization with tetra-functional cross-linking agents such as ethylenediaminetetraacetic acid,28 which have been recognized in a strictly controlled content to explore the thermal and mechanical properties and the crystallization behavior in our research.33

Herein, the crystallization behavior of a series of PBABI copolyesters with cross-linking agent EDTA was investigated through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized light microscopy (PLM) analyses. The 3D architecture was observed in the tetrahedral structure of EDTA, implying that different 3D networks were formed, to examine the influence of the cross-linking agent on the crystallization behavior.

Results and Discussion

Figure 1 displays the DSC trace of PBABI copolyesters, without and with EDTA molecules in the temperature range of −50 to 150 °C. In Figure 1a, the increase of the IA composition to 10 mol % could reduce the Tc by about 10 °C compared to the added cross-linking agent at the same BA/BI ratio, suggesting the amount of IA molecules may restrict the molecular chain packing, making it easy to form an ordered state. Tc in BA/BI = 10/0 has a similar trend around 29.5 °C compared to the EDTA system but shows a slight difference at temperatures between 19.2 and 22.7 °C when the content of BA/BI is 9/1, and with EDTA, respectively, indicating that the crystallization behavior was associated with the IA molecule. The most significant value of ΔHc was detected in the presence of EDTA as a node in both BA/BI = 10/0 and 9/1 systems, implying that the EDTA molecule could reduce the freedom of the molecular chain to constrain the molecular chain motion. As can be seen in Figure 1b, the Tm value of PBABI at BA/BI = 10/1, with and without EDTA, was located in the range of 50.2–50.8 °C; however, ΔHm shows a significant reduction from 49.2 to 43.1 J g–1, which demonstrated that EDTA could disturb the chain packing in the crystal state, and the EDTA molecules preferred to form a tetrahedral conformation, which reflects a more flexible molecular chain to form a loose and movable 3D network. All of the data are listed in Table 1.

Figure 1.

Figure 1

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DSC curve of PBABI copolyesters with EDTA at different contents of BA/BI for the (a) first cooling process and (b) second heating process at a rate of 10 °C min–1.

Table 1. Thermal Property and Intrinsic Viscosity (I.V.) of PBABI Copolyesters with EDTA at BA/BI Contents of 10/0 and 9/1.

item Tc (°C) ΔHc (mJ mg–1) Tm (°C) ΔHm (mJ mg–1) I.V. (dL g–1)
BA/BI = 10/0 29.5 –53.1 50.2 49.2 0.75
BA/BI = 10/0—EDTA 29.5 –46.1 50.8 43.1 0.98
BA/BI = 9/1 19.2 –51.9 42.9 46.4 0.94
BA/BI = 9/1—EDTA 22.7 –47.9 42.4 45.9 1.02
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The compared X-ray diffraction (XRD) patterns of PBABI copolyesters, with and without EDTA, at BA/BI contents of 10/0 and 9/1 are displayed in Figure 2. The XRD patterns of PBABI have similar values of 2θ in terms of BA/BI = 10/0 and 9/1, suggesting that the existence of the BI unit did not affect the crystal transformation behavior of the BA unit and was preferred in the α-crystal. These results were also measured in poly(butylene succinate-co-butylene itaconate) copolyesters.34 The XRD peaks in PBABI at BA/BI = 10/0 and 9/1 were measured at the 2θ values of around 21.61, 22.31, and 23.96° for the crystal lattices of (110), (020), and (021), respectively, which corresponded to the α-phase of neat PBA.8,22 When EDTA, 0.1 mol % was copolymerized into BA/BI = 10/0 copolyesters, the β-phase at 2θ = 21.31° appeared slightly and was associated with the (110) crystal lattice. Furthermore, the intensity of the XRD pattern increased with the presence of IA, implying that the metastable β-phase could be induced with an increase of the IA content, but all of the thermodynamic stable α-phase peaks still existed and played a dominant role in the crystallization zone. Furthermore, it is interesting that the majority of crystallization behavior was transformed into an α/β complex to form small sheet crystals when the EDTA was copolymerized into both BA/BI = 10/0 and 9/1. This is due to the fact that the partial cross-linking occurring due to the EDTA molecule could hinder the molecular chain to stack well into noncompleted sheet crystals instead of the integrated ordered state to grow up to spherulite. Actually, PBABI is a linear aliphatic copolyester and the molecular chain tends to be more flexible. Hence, the molecular chain prefers to form a stable crystal α-phase without EDTA to limit the chain stretching and packing. The geometrical structure of EDTA within the PBABI copolyester was represented in tetrahedral steric conformation, indicating that the main-chain rotation exhibited a semirigid property with the EDTA molecule as a node.

Figure 2.

Figure 2

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XRD patterns of PBABI copolyesters with EDTA at different contents of BA/BI.

DSC was adopted to measure the exothermic heat during polymer chain stacking into an ordered state to form a crystallization regime, which could be well determined using Avrami models,34−40 and the detailed procedures have been established in our laboratory.41,42 The evolution of crystallinity is linearly proportional to the evolution of heat released during the crystallization, and the relative degree of crystallinity, Xt (%), can be obtained according to the following equation

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where dHc represents the enthalpy of crystallization at the target temperature during the time interval dt and is measured via DSC. The integral limits of t and ∞ denote the time of occurrence of crystallization and the end of the crystallization procedure, respectively.

The isothermal crystallization behavior of PBABI, with and without EDTA, was investigated in the temperature ranges of 26–42 and 18–26 °C for BA/BI = 10/0 and 9/1, respectively (see Figure 3). The curves indicated that PBABI at a ratio of 10/0 is faster than that at 9/1, even with EDTA, due to the frustration of the IA molecule. The crystallization rate of PBABI copolyesters with EDTA was found to be faster than that of neat copolyester in both of PBABI = 10/0 and 9/1 under the same temperature, implying that the EDTA molecule could enhance the crystallization rate of the PBABI copolyester compared to neat PBABI copolyesters. This is due to the three-dimensional architecture that could be formed into a tetrahedral structure,28 which exhibited the semirigid property to promote the chain motion near the cross-linking spot.

Figure 3.

Figure 3

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Relative crystallinity Xt (%) as a function of crystallization time at various temperatures for PBABI copolyesters with EDTA at different contents of BA/BI. For the synthesized copolyester, (a) BA/BI = 10/0, (b) BA/BI = 10/0 with EDTA, (c) BA/BI = 9/1, and (d) BA/BI = 9/1 with EDTA at various temperatures. The solid lines represent the DSC experimental results of the Avrami equation.

The Avrami plots for PBABI copolyesters, without and with EDTA in the temperature ranges of 26–42 and 18–34 °C for BA/BI = 10/0 and 9/1, respectively, are displayed in Figure 4. All of the experimental results were plotted with log(−ln(1 – Xt (%))) as a function of log(t) at Xt (%) in the range of 20–80% and were well fitted by the Avrami equation at various temperatures. All of the Avrami parameters, n and K, were linearly regressed to obtain from the slopes and intercepts of the curve for PBABI copolyesters at various temperatures. The half-time and growth rate were also calculated, and all of the detailed data are summarized in Table 2.

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Figure 4.

Figure 4

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Avrami plots for PBABI copolyesters with EDTA at different BA/BI contents: (a) BA/BI = 10/0, (b) BA/BI = 10/0 with EDTA, (c) BA/BI = 9/1, and (d) BA/BI = 9/1 with EDTA at various temperatures.

Table 2. Avrami Analysis for Isothermal Crystallization and Half-Time of Crystallization for PBABI Copolyesters with EDTA at Different BA/BI Contents.

temp. (°C) n K (min–n) t1/2 (min) G (min–1) temp. (°C) n K (min–n) t1/2 (min) G (min–1)
BA/BI = 10/0 BA/BI = 9/1
26 1.68 13.2388 0.15 6.71 18 1.65 6.4420 0.22 4.46
28 1.71 10.8161 0.17 5.81 20 1.92 6.0223 0.27 3.68
30 1.59 5.4955 0.24 4.21 22 1.86 4.7220 0.30 3.32
32 2.09 2.7517 0.43 2.34 24 2.07 2.8074 0.42 2.38
34 2.36 1.2439 0.63 1.58 26 2.43 0.6336 0.84 1.20
36 2.94 0.2472 1.11 0.90 28 2.33 0.0991 1.87 0.54
38 3.18 0.0346 2.00 0.50 30 2.39 0.0299 3.01 0.33
40 2.71 0.0163 3.17 0.32 32 2.37 0.0090 5.07 0.20
42 3.13 0.0006 7.42 0.13 34 2.17 0.0046 8.28 0.12
BA/BI = 10/0—EDTA BA/BI = 9/1—EDTA
26 1.49 14.3678 0.12 8.63 18 1.46 13.0951 0.12 8.40
28 1.77 13.0350 0.16 6.15 20 1.46 10.8353 0.14 7.38
30 1.93 8.2410 0.23 4.30 22 1.74 10.4112 0.18 5.55
32 2.54 4.4623 0.38 2.60 24 2.17 7.5100 0.27 3.65
34 3.32 1.3667 0.63 1.59 26 2.52 1.6379 0.57 1.75
36 3.91 0.2003 1.05 0.96 28 2.66 0.3751 1.00 1.00
38 3.51 0.0762 1.44 0.69 30 3.21 0.0550 1.71 0.58
40 2.95 0.0266 2.37 0.42 32 3.22 0.0132 2.66 0.38
42 2.49 0.0071 5.04 0.20 34 2.53 0.0135 3.80 0.26
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Avrami parameters including “n” and “K” are associated with the Avrami exponent and crystallization constant, which are dependent on the growth geometry of nucleation and the shape of the growing crystal and could be obtained from the plot of log[−ln(1 – Xt (%))] as a function of log(t) through linear regression with the slope and intercept in the dimensionless unit in min–n, respectively. As can be seen, the values of n are located between 2 and 3 for PBABI copolymers with EDTA in the target range of temperatures. First, the effect of IA without any cross-linking agent was compared at BA/BI = 10/0 and 9/1 with averaged n values of 2.38 and 2.13 in the temperature range of 26–42 °C, respectively, and demonstrated that the presence of IA could lower the crystal formation and decrease the crystallization rate to form a sheet crystal. Then, a different cross-linking agent with BA/BI = 10/0 was compared, and the averaged values of n were calculated to be 2.38 and 2.66 without and with EDTA, respectively, implying a better crystallization behavior with EDTA than neat copolyester, which is caused by the tetrahedral 3D architecture with a semirigid chain conformation. Moreover, the crystallization rate K at BA/BI = 10/0 with EDTA was larger than that of neat copolyester as a function of temperature even in a cluster of BA/BI = 9/1 experimentally. Furthermore, the n and K values have a similar trend at BA/BI = 9/1 and BA/BI = 10/0, revealing that EDTA plays a significant role in the crystallization rate and morphology even inside the IA molecule. The crystals could be disrupted to form small sheet crystals when both IA and EDTA were copolymerized into the PBABI copolyester. In addition, a significant amount of the IA molecule plays a considerable role in determining the deviation of crystals. t1/2 and G exhibited a faster crystallization rate in the presence of EDTA than in the neat copolyester at the same temperature, indicating that the crystallization rate could be improved in the presence of EDTA.

Figure 5 displays the growth rate of crystallization at different temperatures. The growth curve almost overlapped at BA/BI = 10/0 and with EDTA but shows a slight change at a lower temperature due to the supercooling degree, which suggested that the growth rate was not dependent on the existence of EDTA. When 10 mol % of IA was added, the curve of the growth rate was separated dramatically at a higher supercooling condition and also had a sequencing of EDTA to a neat copolyester. This observation could also give evidence that a higher crystallization growth rate was obtained when EDTA existed as a semirigid node to lower the chain movement, which caused the molecular chain to prefer sheet crystals.

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Figure 5.

Figure 5

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Growth rate of crystallization (G, min–1) as a function of temperature for PBABI copolyesters with EDTA at different contents of BA/BI.

The DSC trace of the heating process after the isothermal experiment for PBABI copolyesters and with EDTA at different BA/BI contents is exhibited in Figure S1. The Hoffman–Week equation was implemented to obtain the equilibrium melting point (Tm0), and the results are displayed in Figure 6. All of the Tm and thickening coefficient (γ) through the slope of the curve are summarized in Table 3. The Tm0 values were revealed in the temperature ranges of 57.12–57.14 and 53.62–51.74 °C for BA/BI = 10/0 and 9/1 copolyesters, respectively. The values of γ were located around 4.69–4.06 and 3.27–3.08 for BA/BI = 10/0 and 9/1 copolyesters, respectively, indicating that the cross-linking agent may raise the limitation in the growth of the crystal to obtain a smaller thickness of the crystal regime. In the literature survey,11 the Tm values of PBA in α- and β-crystals were represented around 64 and 54 °C, implying that the α- and β-crystals coexisted in the presence of the cross-linking agent and was also demonstrated in our XRD results in Figure 2. The PBA in the α-crystal was in a stable thermodynamic phase, whereas the β-crystal was in a metastable phase, indicating that the crystals of PBABI copolyesters tended to form α–β complex crystals coexisting in the presence of EDTA and IA.

Figure 6.

Figure 6

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Plot of melting point as a function of crystallization temperature for PBABI copolyesters at different BA/BI contents: (a) neat BA/BI = 10/0 and with EDTA and (b) neat BA/BI = 9/1 and with EDTA. The black squares and filled red circles represent neat BA/BI and with EDTA, respectively. The extrapolation method was performed to obtain the equilibrium melting point (Tm0).

Table 3. Equilibrium Melting Point (Tm0), Activation Energy (Ea), and Thickening Coefficient (γ) for PBABI Copolyesters with and without EDTA at Different Contents of BA/BI.

item Tm0 (°C) activation energy (kJ mol–1) thickening coefficient (γ)
BA/BI = 10/0 57.12 204.31 4.69
BA/BI = 10/0—EDTA 57.14 205.34 4.56
BA/BI = 9/1 53.62 183.57 3.27
BA/BI = 9/1—EDTA 51.74 187.53 3.08
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Additionally, as shown in Figure 7, the activation energy (Ea) could be obtained through the Arrhenius equation plot and is tabulated in Table 3. The lower values of Ea were found individually without adding EDTA in both BA/BI = 10/0 and 9/1 copolyesters, implying that the PBABI copolyesters exhibited a higher heat sensitivity when EDTA was present. From the point of view of thickening coefficient (γ), the value of γ decreased with increasing content of IA and EDTA, indicating that the comonomer and cross-linking agent could hinder the chain packing to lower the thickness of the crystal regime.

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Figure 7.

Figure 7

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Arrhenius plot for PBABI copolyesters at different BA/BI contents: (a) neat BA/BI = 10/0 and with EDTA and (b) neat BA/BI = 9/1 and with EDTA. The filled black squares and filled red circles represent neat BA/BI and with EDTA, respectively. The curve was regressed to obtain the slope for activation energy.

To the best of our knowledge, the polymer crystallization pathway includes the nucleation and growth mechanism; hence, the second nucleation theory of Lauritzen–Hoffman was performed to investigate the kinetics of initial crystal growth,43,44 where the crystal grows gradually when the nucleus passes through the activation energy of nucleation. The growth behavior could be analyzed through a linear regression to realize the relationship between the growth rate of crystal and temperature, and the formula can be described as follows

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where G0 is the pre-exponential factor, U* is the activation energy of chain movement, Kg is the activation energy of nucleation, R is the gas constant, Tc is the crystallization temperature, T∞ is defined as Tg – 30 K, ΔT is the degree of supercooling (ΔT = Tm0 – Tc), Tm is the equilibrium temperature, f is the modified factor (f = 2 Tc/(Tm0 + Tc)), C1 = 4120 cal mol–1, and C2 = 51.6 K. G0 is associated with chain flexibility and molecular chain regularity. The growth rate, G, is correlated to the diffusion and molecular chain mobility in the crystal regime and was defined in the first exponent term. The second exponent term is related to the second nucleation rate. When the nucleus was formed, the molecular chains tend to pack well to generate a new surface to grow continually. The mechanism of growth rate is also affected by the degree of supercooling, which consisted of three regimes. Regimes I, II, and III were observed in high, medium, and low supercooling degrees, respectively, and the crystallization behavior was controlled by nucleation alone, both nucleation and growth, and growth alone. The Lauritzen–Hoffman plot of ln(G) + U/RTc – T∞ vs 1/(TcΔTf) for PBABI copolyesters, with and without EDTA, at different contents of BA/BI is displayed in Figure 8, exhibiting a two-stage discontinuous transition of regimes III and II obtained and fitted to the slope to gain the nucleation constant (Kg), implying the two temperature regimes of the crystallization growth mechanism. The transition temperatures of regime II → III were obtained to be 308.15 and 308.82 K for BA/BI = 10/0 without and with EDTA and 301.05 and 301.37 K for BA/BI = 9/1 without and with EDTA, respectively. All of the obtained data are listed in Table 4. The obtained Kg(III)/Kg(II) in our research was calculated in the range of 2.05–2.49 for PBABI copolyesters, with and without EDTA at different contents of BA/BI, and the results were approximately equal to the theoretical value of Kg(III)/Kg(II) of 2.43,45,46

Figure 8.

Figure 8

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Lauritzen–Hoffman plot for PBABI copolyesters with and without EDTA at different contents of BA/BI: (a) 10/0 and (b) 9/1.

Table 4. Parameters of the Lauritzen–Hoffman Model for PBABI Copolyesters with and without EDTA at Different Contents of BA/BI.

item Kg(III) × 10–4 (K2) Kg(II) × 10–4 (K2) Kg(III)/Kg(II) TII→III (K)
BA/BI = 10/0 17.52 8.02 2.18 308.15
BA/BI = 10/0—EDTA 17.81 7.15 2.49 308.82
BA/BI = 9/1 26.10 12.74 2.05 301.05
BA/BI = 9/1—EDTA 24.25 10.22 2.37 301.37
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The morphologies of PBABI copolyesters, with and without EDTA at different contents of BA/BI = 10/0 and 9/1 are revealed in Figure 9. All of the isothermal crystallization of PBABI copolyesters was investigated under a hot-stage and cooling system PLM. The sample was melted at 80 °C, held for 5 min, and then cooled quickly at a rate of 150 °C min–1 to target observation temperatures. The PLM images for BA/BI =10/0 at different temperatures are shown in Figure 9a and displayed in the spherulite to sheet crystals when the supercooling degree increased. In the literature survey, PBA (BA/BI = 10/0) was observed in spherulite with lowering molecular weight from 3000 to 7000 g mol–1.47 The PLM images were shown in a similar trend for PBABI copolyesters, demonstrating that the morphology tends to sheet crystals with the existence of IA and EDTA molecules, respectively, and are shown in Figure 9b–d. However, smaller sheet crystals were observed when IA was copolymerized even with or without EDTA. The sheet crystals are exhibited with EDTA inside in both BA/BI = 10/0 and 9/1 at setting temperatures. For the overall compared observation, the spherulite in α-phase was observed at a higher temperature of 38–42 °C under a larger degree of supercooling. On the contrary, the crystal size was decreased with a decrease of temperature under larger supercooling. When the lower temperature of 26–30 °C was achieved, the size of the α/β complex becomes more extensive and more apparent in sheet crystals. From the literature, the α- and β-form crystals were induced at higher and lower temperature requirements, respectively. When the temperature was decreased to 30 °C or below, the crystals grew very fast and immediately indicated that these kinds of crystals tend to form metastable crystals of the β-form in a small sheet shape due to the formation of more nucleation sites. The increase of the IA concentration within PBABI copolyester resulted in sheetlike crystals that grew with a higher stacking density (Figure 9c). This observation suggested that IA served as a nucleation site, which provides other free surfaces to facilitate the nucleation of PBA to achieve heterogeneous crystallization. When the BA/BI = 10/0 copolyesters were copolymerized with EDTA (Figure 9b), the spherulite was obtained at 38–42 °C to pack into α-crystals (see Figure 2). This is due to the effect of chain flexibility in the semirigid conformation for the EDTA inside, which could induce the occurrence of phase transition. Furthermore, all of the size of sheet crystals in BA/BI = 9/1 is smaller than that in BA/BI = 10/0 even with or without EDTA existed, implying that the crystallization behavior tended to form sheet crystals in the presence of IA, which played a dominant role in determining the size and type of crystals.

Figure 9.

Figure 9

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PLM images of PBABI copolyester at (a) BA/BI = 10/0, (b) BA/BI = 10/0 with EDTA, (c) BA/BI = 9/1, and (d) BA/BI = 9/1 with EDTA at different temperatures. The scale bar is 10 μm.

Conclusions

The crystallization behavior of the PBABI copolyesters was studied with EDTA, observing that the thermodynamical α- and metastable β-form crystals coexisted in the presence of IA and EDTA molecules. The EDTA molecules preferred to form a tetrahedral conformation, which reflects a more flexible molecular chain to form a loose and movable 3D network to increase the crystallinity. The half-time and growth rate have a sequence with EDTA to neat copolyester at the same temperature, indicating that the crystallization behavior could be improved in the presence of EDTA. Tm0 has been revealed in temperature ranges of 57.12–57.14 and 53.62–51.74 °C for BA/BI = 10/0 and 9/1 copolyesters, respectively. The values of γ were located around 4.69–4.06 and 3.27–3.08 for BA/BI = 10/0 and 9/1 copolyesters, respectively, indicating the EDTA may raise the limitation in the growth of crystal to obtain the smaller thickness of crystal regime. The transition temperatures of regime II → III were obtained as 308.15 and 308.82 °C for BA/BI = 10/0 without and with EDTA and 301.05 and 301.37 °C for BA/BI = 9/1 without and with EDTA, respectively. A relatively larger spherulite in the α-phase was observed at a higher temperature of 38–42 °C due to a more significant degree of supercooling. Thus, the present study demonstrated the type of crystals could be controlled by adding the tetra-functional group of the EDTA molecule, taking the advantages of the PBABI copolyesters to control the solidification time in 3D smart textile applications.

Experimental Section

Materials

Adipic acid (AA, 99.8%) was obtained from Asahi Kasei Corporation. Itaconic acid (IA, 99%) and 1,4-butanediol (1,4-BDO, 99%) were supplied from the First Chemical Corporation (Taiwan). Ethylenediaminetetraacetic acid (EDTA, 98%) was obtained from Vetec. 4-Methoxyphenol (99%) and titanium(IV) butoxide (Ti(OBu)4, 97%) were acquired from Aldrich. Dibutyltin dilaurate (DBTDL, 95%) was obtained from Alfa Aesar. All of the chemicals were implemented by traditional bulk polymerization without any purification.

Synthesis of PBABI Copolyesters with EDTA

PBABI copolyesters were copolymerized via bulk polymerization, consisting of AA and IA esterified with 1,4-BDO, 4-methoxyphenol as an inhibitor of C=C of IA, DBTDL, and Ti(OBu)4 as the co-catalyst and EDTA as a cross-linking agent under a 2 L steel reactor. All detailed data of the synthesis, characteristics, and thermal and mechanical properties have been published in our previous study.28 EDTA has four carboxylic functional groups, and it has a significant interesting role to discuss in isothermal crystallization kinetics.

Measurement

Differential Scanning Calorimetry (DSC) and Isothermal Crystallization Process

DSC (Hitachi High Tech., DSC-7000, Japan) was adopted to examine the thermal properties, including melting temperature (Tm), crystalline temperature (Tc), melting enthalpy (ΔHm), and crystalline enthalpy (ΔHc) of the PBABI copolyesters. For each experiment, the PBABI copolyesters were detected from −50 to 150 °C at a heating rate of 10 °C min–1 and kept at 150 °C for 10 min to avoid thermal history for further analysis. Then, these PBABI copolyesters were cooled to −50 °C at a cooling rate of 10 °C min–1 for the first cycle. After that, the second cycle of the heating procedure was performed from −50 to 150 °C at the same heating rate of 10 °C min–1 to inspect the melting point. Tc and Tm of the PBABI copolyesters were determined from the maximum exothermic peak and endothermic peak through the first cycle cooling and second cycle heating processes, respectively. All of the DSC measurements were conducted in a nitrogen atmosphere with aluminum pans.

The DSC experiments of the isothermal crystallization process of a PBABI copolyester sample were performed in two stages: (1) the sample was heated to 100 °C at a rate of 10 °C min–1 and kept for 5 min to remove thermal history, and (2) the sample was cooled at a rate of 20 °C min–1 to the given Tc and then held for 60 min. The given temperature range of Tc was set at 26–42 and 18–34 °C for BA/BI = 10/0 and 9/1, respectively.

Intrinsic Viscosity (I.V.)

The PBABI copolyesters (1.0 g dL–1) were dissolved in a mixture of phenol and tetrachloroethane, 50/50 wt %, and measured using an Ubbelodhe viscometer at 25 ± 0.05 °C. The I.V. values for PBABI copolyesters with EDTA at different contents of BA/BI are tabulated in Table 1.

X-ray Diffraction (XRD)

The PBABI copolyesters were prepared as a film type via hot-pressure mechanics, and the XRD pattern of the PBABI film was verified in the 2θ range of 10–40° with a scanning speed of 0.2° min–1 by a Malvern PANalytical X’Pert3 powder diffractometer (Malvern, U.K.) with Cu Kα radiation (λ = 0.154 nm).

Polarizing Light Microscope (PLM)

The crystal morphology of PBABI copolyesters under isothermal crystallization was considered using a polarizing light microscope (PLM) (Nikon, ECLIPSE LV100N POL) equipped with a Linkam THMS Examina/FTIR600 heating stage, a Linkam ECP water cooling control unit, and a Nikon camera with the NIS Elements imaging software. For the crystallization behavior experiment, the operation conditions related to the results of DSC measurements, i.e., the PBABI copolyesters, were initially 80 °C for 5 min at a heating rate of 150 °C min–1, then a rapid cooling to the target crystallization temperature at a cooling rate of 150 °C min–1, and finally, holding at a set temperature for an hour to observe the crystallization growth. All of the crystallization processes were recorded as a video file and captured as an image.

Acknowledgments

The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan (MOST 107-3017-F-027-001).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04315.

  • DSC trace of the heating process after the isothermal experiment for PBABI copolyesters (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b04315_si_001.pdf (224.2KB, pdf)

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Supplementary Materials

ao9b04315_si_001.pdf (224.2KB, pdf)

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