Using the melt polycondensation method, a polyesteramide was prepared based on poly(L-lactic acid) prepolymer and poly(ε-caprolactam) prepolymer and was characterized by Fourier transform infrared spectroscopy and 1H-NMR. Isothermal crystallization behavior at different temperatures and non-isothermal crystallization kinetics at different cooling rates were investigated by differential scanning calorimetry, and non-isothermal crystallization kinetics parameters were obtained using the Mo, Ozawa and Jeziorny methods. It was found that the increased cooling rates led to the broadening of the polyesteramide crystallization peaks and their shift toward lower temperatures. Mo and Jeziorny methods were found to be more suitable than the Ozawa method for the analysis of this system, as shown by the comparison these non-isothermal crystallization analysis methods. In addition, the values of activation energy of non-isothermal crystallization for polyesteramide obtained by the Kissinger and Takhor methods were −155.96 and −149.12 kJ/mol, respectively.
Poly(L-lactic acid) (PLA) is an aliphatic polyester obtained from renewable resources that exhibits excellent biocompatibility and biodegradability (1). Although PLA and its copolymers are the most important biodegradable polymers in the field of biological medicine, the use of PLA is limited by its brittleness and low thermal stability. Copolymerization modification through the control of the chemical structure has attracted widespread attention as a method for improving the PLA properties. Wang et al. (2) and Feng et al. (3, 4) reported block copolymerization of DL-lactide and ε-caprolactone aimed at exploiting the degradability of PLA and the permeability of PCL to drugs. In particular, poly(ε-caprolactone)/poly(L-lactic acid) (PCL/PLA) biodegradable poly(ester-urethanes) were synthesized and characterized. The first step of the synthesis consisted of the ring opening polymerization of L-lactide initiated by the hydroxyl terminal groups of the PCL chain, followed by the chain extension of these PLA-PCL-PLA triblocks using hexamethylene diisocyanate (HDI) (5). Kawasaki et al. (6) performed this synthesis by reacting L-lactic acid with ε-caprolactam in the presence of Fe, Sn or Zn powders as catalysts. It was shown that in contact with an active sludge, the copolyesteramides were degraded by up to 50% after 33 d. Qian et al. (7) synthesized a biodegradable polyesteramide based on lactic acid and aminoundecanoic acid using the melt polycondensation method. It was found that the melting temperature and crystallinity increased with the increasing amount of aminoundecanoic acid units, with the water absorption and degradation rate decreasing accordingly. However, the crystal morphology of PLA was improved by copolymerization, improving its mechanical and heat resistance properties. Therefore, the crystallization behavior of the copolymer was studied in deeply in order to control the structure of the copolymer at the microscopic level and broaden its range of applications. This process can be explained based on the theory of crystallization kinetics. At the same time, obtaining products with good performance has a very important practical significance. At present, the polyesteramide is synthesized by L-lactide, lactic acid prepolymer and caprolactam monomer, but the crystallization properties and crystallization kinetics of polyesteramide aren’t studied. In this paper, the purpose of the project is improvement the crystallization rate and crystallinity of PLA via the intramolecular and intermolecular hydrogen bonding of the amide group in the nylon polymer chains. A kind of polyesteramide copolymer based on PLA was successfully synthesized using PLA prepolymer and poly(ε-caprolactam) prepolymer as the raw materials. The structure of the polyesteramide was characterized by Fourier transform infrared spectroscopy (FT-IR) and 1H-NMR, and the crystallization behavior and kinetic processes were studied by differential scanning calorimetry (DSC).
L-lactic acid (Shenzhen Bright China Industrial Co. Ltd., China) was used without further purification. This solution contains 12 wt% water. ε-Caprolactam was purchased from China Petrochemical Co. Ltd. (Hunan, China), and stannous chloride and p-toluenesulfonic acid were obtained from the Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Phosphoric acid and monosodium glutamate were supplied by the Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
2.2 Synthesis of PLA prepolymer
Sixty grams of L-lactic acid monomer was reacted under reduced pressure at 120°C for 2 h and then was maintained for 2 h under atmospheric pressure. SnCl2 (0.3 g) and p-toluenesulfonic acid (the mole ratio 1:1) acting as catalysts were added in the reaction chamber. The mixture was heated to 145°C for 2 h and was heated to 170°C for 5 h. The product was dissolved in acetone, reprecipitated with cold methanol and finally dried for 8 h at 60°C.
2.3 Synthesis of poly(ε-caprolactam) prepolymer
Forty grams of ε-caprolactam monomer was heated until melting at 100°C, and 5% sodium glutamate and phosphoric acid solutions were added, corresponding 2.5 wt% of the ε-caprolactam. Distilled water was also added in the amount of about 7.5 wt% of the ε-caprolactam. The mixture was placed in a reaction chamber a high temperature and pressure set to heat up to 100°C in 30 min and then heat up to 250°C in 120 min. Finally, the mixture was reacted for 4 h at this temperature.
2.4 Synthesis of polyesteramide
PLA prepolymer, poly(ε-caprolactam) prepolymer and SnCl2 were placed in the reaction chamber at different temperatures and pressures. The molar ratios of PLA prepolymer to poly(ε-caprolactam) prepolymer were 50/50, and the amount of SnCl2 was 0.5 wt%. The air in the reaction chamber was removed by reducing the pressure and pumping in the nitrogen gas. This process was repeated three times. The mixture was heated to 145°C for 2 h and was then heated to 180°C for 5 h. The product was dissolved in chloroform, reprecipitated with cold methanol and finally dried for 8 h under reduced pressure.
A Fourier infrared spectrometer was used to characterize the chemical structure of the polyesteramide. The scanning range was 400–4000 cm−1, and the scan was performed 32 times.
The 1H-NMR spectrum (in CDCl3 or DMSO) was recorded with a Bruker 300 spectrometer (Bruker, Rheinstetten, Germany) using trimethylsilane (TMS) as the internal reference standard.
The thermal performance of polyesteramide was measured using a DSC TA Q800 (TA Instruments, Germany). Isothermal melt-crystallization was performed after melting at 250°C for 2 min and rapid cooling to the indicated temperature at the rate of ca. 100°C/min. The samples were allowed to crystallize for 10 min, and the isothermal crystallization curves were obtained. Each sample was heated to 250°C at the rate of 30°C/min and kept at this temperature for 5 min. The samples were then crystallized at different cooling rates (2.5, 5, 10, 20 and 40 K/min), and the non-isothermal crystallization curves were obtained.
3 Results and discussion
3.1 Chemical structure analysis of PLA-co-PA6
The structure of PLA-co-PA6 was characterized by FT-IR and 1H-NMR spectra. FT-IR spectra of PLA prepolymer (pre-PLA), poly(ε-caprolactam) prepolymer (PA6) and polyesteramide (PLA-co-PA6) are shown in Figure 1. The peak at 3480, 3300, 2930 and 2858 cm−1 indicated the presence of N-H stretching vibrations. The absorption bands at 1660, 1545 and 1482 cm−1 are assigned to amide I, II and III, respectively. Comparison with the FT-IR spectrum of PA6 shows that the stretching vibrations of methylene and amide III remain the same in PLA-co-PA6. The absorption intensity at 3300 cm−1 obviously increased and is mainly secondary amine based. Examination of the spectra shows that the double peak of the primary amine in PA6 was transformed to a single peak. The peak at 1757 cm−1 was assigned as a carbonyl aliphatic peak. The appearance of the characteristic peak shows that the pre-PLA and PA6 were polymerized successfully.
The 1H-NMR spectrums of PLA-co-PA6 are shown in Figure 2. For the PLA unit, two signals of methyl and methylene protons were observed at 5.2 and 1.2 ppm, respectively. For the PA6 unit, the chemical shifts at 3.0 ppm (CH2 of NH2 terminal of PA6), 2.03 ppm (CH2 of COOH terminal of PA6) and 7.3 ppm (NH2 of PA6) were observed. Other protons are indicated by d, e and f in Figure 2. Based on the 1H-NMR spectrum, it can be concluded that pre-PLA and PA6 reacted successfully.
3.2 Isothermal crystallization behavior of PLA-co-PA6
For a normal polymer, isothermal crystallization occurs in a temperature range from the glass transition temperature (Tg) to the melting temperature (Tm). The isothermal melt-crystallization curves of PLA-co-PA6 are presented in Figure 3 and clearly exhibit a single melting temperature peak at various temperatures. The crystallization peaks shift to the right and broaden with increasing crystallization temperature. This implies that the crystallization time of PLA-co-PA6 increased with increasing crystallization temperature due to the increased movement of the active chain segments of PLA with lower crystallization temperature. Molecular chains of poly(ε-caprolactam) move normally due to the hydrogen bonding. Hydrogen bonding is the dominant interaction of the PA6 chain so that the crystallization of the copolymer starts early and crystallized quickly. On the contrary, a higher crystallization temperature corresponds to a slower polymer crystallization. When the crystallization temperature is close to the melting point, the copolymer chains move rapidly and over large distances. This may partially destroy the hydrogen bonding interactions in poly(ε-caprolactam), harming the nucleation and making crystallization more difficult.
3.3 Non-isothermal crystallization behavior of PLA-co-PA6
3.3.1 Basic parameters of non-isothermal crystallization
Currently, isothermal crystallization kinetics for various materials are almost completely understood. Compared with isothermal crystallization, non-isothermal crystallization research has more practical significance because practical production is implemented under non-isothermal conditions. Therefore, many researchers are paying increasing attention to the study of non-isothermal crystallization. To date, dozens of data analysis methods have been used for the study of non-isothermal crystallization kinetics, including the Jeziorny method (8, 9), Ozawa method (8, 10), Mo method (8, 11), Ziabicki method (12) and so on. In this paper, non-isothermal crystallization kinetics were studied using the Mo, Ozawa and Jeziorny methods.
The non-isothermal crystallization curves of PLA-co-PA6 at various cooling rates are shown in Figure 4. It indicated that the non-isothermal crystallization curves exhibit a single peak at various cooling rates. When the melt crystallizes, the maximum crystallization peak of PLA-co-PA6 shifts from higher to lower temperatures with increasing cooling rate and the crystallization peak gradually broadens and its area becomes larger. This is due to the crystallization time becoming shorter with increasing cooling rate, resulting in a harder order arrangement and folding of the lattice for the PLA-co-PA6 molecular chains. In other words, full crystallization is difficult to achieve in short time period, requiring a greater cooling rate. Therefore, the location of the crystallization peak shifts to lower temperatures and the crystallization peak is broadened. This also suggests that the introduction of PA6 into PLA can change the PLA crystallization properties.
Figure 5 shows the curves of relative crystallinity Xt (%) at different crystallization temperatures during the non-isothermal crystallization process of PLA-co-PA6. Figure 5 shows that the required temperature range to achieve the same relative crystallinity is broadened with the increasing cooling rate. Based on these curves, for the non-isothermal crystallization, the relationship between crystallization time (t) and crystallization temperature (T) can be expressed by the following equation (Eq. ):
where T0 and Φ are the temperature at the beginning of the process (at t=0) in units of °C and the cooling rate in units of K/min, respectively. The curves of relative crystallinity Xt (%) at different crystallization times in the non-isothermal crystallization process of PLA-co-PA6 are presented in Figure 6 and show that the time range required to achieve the same relative crystallinity becomes narrower with increasing cooling rate.
The non-isothermal crystallization process involves some important parameters such as the crystallization initial temperature (T0), crystallization terminal temperature (Tf), crystallization peak temperature (Tp), crystallization time of crystallization peak temperature (tmax), relative crystallinity of the crystallization peak temperature (Xt), total enthalpies of crystallization (∆Hc) and so on. Some parameters characterizing non-isothermal crystallization behaviors are listed in Table 1. Figure 7 shows the relationship between T0, Tp, Tf and Φ. As shown in Figure 7, the crystallization temperature decreases in order of T0, Tp, Tf with increasing cooling rate and falls more rapidly (larger slope). This means that the arrangement order of molecular chain become worse with the speeding up of the cooling rate during the crystallization of PLA-co-PA6. Therefore, the nucleation occurs rather late and delays the initial crystallization leading to the shift of the initial crystallization temperature to lower values. On the contrary, as cooling rate is decreased, the activity ability of the molecular chain become stronger so that the chain has plenty of time to form a nucleus and assume an orderly arrangement. This leads to the higher values for the initial crystallization temperature.
3.3.2 Mo method
Considering the influence of the cooling rate, Mo (13) combined the Avrami equation with the Ozawa equation and developed a new theory for the analysis of non-isothermal crystallization kinetics of polymers. The equations for this analysis method are as follows:
where F(T) is a characteristic of the crystallization rate in the units of K·min−1, which specifies the cooling rate required to reach a defined degree of crystallinity in a unit of time, where n is an index of Avrami, m is an index of Ozawa during the non-isothermal crystallization kinetics and Zt is crystallization rate constant in the units of min−n.
Figure 8 shows the plot of logΦ versus logt at various degrees of crystallinity. The values of F(T) and a can be obtained from the linear intercept and slope respectively of the fitted curve and are listed in Table 2.
As shown in Figure 8, plots of logΦ versus logt show a good linear relationship between these two quantities with an essentially perfect linear fit. The ideal result of the linear fitting suggests that the analysis of the non-isothermal crystallization process of PLA-co-PA6 by the Mo method is feasible. The values of the parameter vary slightly between 1.03 and 1.12. The F(T) values increase with increasing relative degree of crystallinity, suggesting that a faster cooling rate is required to reach a higher relative degree of crystallinity in a unit of time. Moreover, the crystallization rate decreases when the relative degree of crystallinity increases at non-isothermal crystallization conditions.
Respectively, where ∆E is the activation energy of non-isothermal crystallization for PLA-co-PA6, Tp is the crystallization peak temperature and R is the ideal gas constant.
Plots of ln(Φ/Tp2)and lnΦ versus 1/Tp are presented in Figures 9 and 10 . The values of ∆E can be determined from the slopes of the fitted lines in Figures 9 and 10 and are equals to −155.96 kJ/mol and −149.12 kJ/mol according to the Kissinger and the Takhor methods, respectively. According to the polymer crystallization theory, the activation energy of a polymer includes the activation energy of the chain and the activation energy of nucleation. The activation energy of nucleation is the crucial factor when the melt crystallizes due to the decreasing temperature. The non-isothermal crystallization activation energies obtained by the two methods are close to each other and are relatively small. This means that PLA-co-PA6 is easy to crystallize and the nucleation rate is very high. It is possible that a chain of polycaprolactam with hydrogen bonding interaction accelerated the crystallization of PLA during the crystallization process.
3.3.3 Ozawa method
Ozawa (10) extended the Avrami equation based on the theory of Evans. He deduced the relation between Xt and Φ at different temperatures and considered the effect of the nucleus formation and spherulite growth on the crystallization from the polymer. The relationship between Xt and Φ is given by
where Xt is the relative crystallinity at the temperature T, F(T) is the cooling function of non-isothermal crystallization, Φ is the cooling rate in units of K/min and m is the Ozawa index.
Figure 11 shows plots of log[−ln(1−Xt)] versus logΦ at the indicated temperature for PLA-co-PA6. As seen in Figure 11, log[−ln(1−Xt)] and logΦ do not exhibit a good linear relationship, implying that the Ozawa method is not ideal for describing non-isothermal crystallization of PLA-co-PA6.
3.3.4 Jeziorny method
To remove the cooling or heating rate, the analysis using the Avrami equation was modified by Jeziorny so that:
where Zc is crystallization rate constant of Jeziorny method, which is revised by the cooling rate. The relation between log[−ln(1−Xt)] and logt by Jeziorny method is shown in Figure 12. The data follow a straight line with the slope and the intercept of the line determining the values of n and logZt, respectively. Zc values were then calculated using the modified equation of Jeziorny with the results listed in Table 3. As shown in Figure 12, the plots of log[−ln(1−Xt)] versus logt show good linear relationships at various cooling rates. This means that the Avrami equation modified by Jeziorny is suitable for describing non-isothermal crystallization kinetics of PLA-co-PA6. The Avrami index can reflect the nucleus formation and growth mechanism: for three-dimensional spherulite growth, n≥3, for two-dimensional photonic crystallization, n=2 or 3, and for one-dimensional crystallization, n=1 or 2. Greater n values correspond to the higher dimensionality of nucleation and growth. As shown in Table 3, the Avrami index of PLA-co-PA6 falls into the range between 3.862 and 5.117 (>3), suggesting that the crystallization mechanism of PLA-co-PA6 is that of three-dimensional spherulite growth by homogeneous nucleation. The value of t1/2 decreases with increasing cooling rate so that shorter crystallization times are obtained for faster cooling rates, suggesting that higher cooling rate is advantageous for the increase in the crystallization rate (18). Because a higher cooling rate leads to a faster process from the melt to the crystallization, the corresponding crystallization rate will increase. The value of Zc increases with increasing cooling rate, suggesting that higher cooling is also advantageous for the increase of the crystallization rate, same as the result obtained for the t1/2 values.
|Φ (K/min)||n||Zt||Zc||t1/2 (min)|
Using the melt polycondensation method, a polyesteramide was successfully prepared based on poly(lactic acid) prepolymer and poly(ε-caprolactam) prepolymer. Non-isothermal crystallization kinetics of PLA-co-PA6 were studied by DSC and were analyzed using the Mo, Ozawa and Jeziorny methods. It found that for the analysis of this system, the Mo and Jeziorny methods were more suitable than the Ozawa method. Based on the Mo analysis methods, the values of a are between 1.03 and 1.12, and the values of F(T) are between 17.87 and 30.99. Based on the modified Jeziorny method, the Avrami index of PLA-co-PA6 is between 3.862 and 5.117, suggesting that the crystallization of PLA-co-PA6 proceeds through three-dimensional spherulite growth caused by homogeneous nucleation. Furthermore, the activation energy of non-isothermal crystallization of PLA-co-PA6 is −155.96 kJ/mol and −149.12 kJ/mol as obtained using the Kissinger and the Takhor methods, respectively. This means that the activation energy of nucleation is very low, so that the rate of nucleation is very high.
The research was supported by the National Natural Science Foundation of China (grant no. 51273060) and the National Undergraduate Training Program for Innovation and Entrepreneurship (grant nos. 201610500004 and 201710500014).
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