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Chapter 9
Effects of Monomer Compositions and Molecular Weight on Physical Properties of Alginic Acid Esters Yusuke Matsumoto,1 Daisuke Ishii,2 and Tadahisa Iwata*,1 1Science of Polymeric Materials, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan 2Department of Chemistry for Life Sciences and Agriculture, Faculty of Life Sciences, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan *E-mail:
[email protected].
Effects of monomer compositions and molecular weights on physical properties were investigated among alginic acid (Alg) esters. Tensile tests of Alg ester films clearly revealed that the molecular weight rather than monomeric composition affects the tensile properties. Furthermore, Alg esters with the shorter side chain showed the higher tensile strength and higher glass transition temperature (Tg) while elongation at break was almost maintained. In particular, Alg propionate prepared from Alg with the mannuronic acid content of 56 mol % and viscosity grade of 1000 cp had a tensile strength of 63 MPa and a Tg of 192 °C. These properties are comparable with or even higher than those of conventional plastics such as polycarbonate. The results suggest the potential of Alg as a starting material for plastics with high thermostability and favorable mechanical properties.
© 2018 American Chemical Society Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Alginic acid (Alg) is a natural polysaccharide that can be extracted from brown algae. Alg consists of (1→4)-linked β-D-mannuronic acid (M) and α-L-gluronic acid (G), the C5 epimer of mannuronic acid (1–3). Because of a carboxyl group linked to the C5 carbon in the sugar backbone, Alg has high hydrophilicity and pH-dependent solubility and is capable of gelation in the presence of multivalent metal cations (4, 5). It is also biocompatible and biodegradable (6, 7). Because of these properties, Alg has been utilized in medicine and the food industry (8, 9). However, to date, there have been no reports of the development of thermoplastic and/or organosoluble materials from Alg. The major obstacle to utilizing polysaccharides as plastic materials is the presence of hydroxy groups in the sugar backbones. Because the hydroxy groups form inter- and intramolecular hydrogen bonds, most polysaccharides are not thermoplastic or soluble in organic solvents. Various derivatization methods have been applied to loosen the interactions within and between polysaccharide backbones. The most popular derivatization method is esterification. Esterification has several advantages over other derivatization methods for the following reasons. First, the thermal, mechanical, and other (e.g., optical) properties of polysaccharide derivatives can be widely controlled by the type, composition, degree of substitution (DS), and regiospecificity of the substituents. Second, the reagents used in the esterification reactions (carboxylic acids and reaction catalysts) are readily available. Third, many polysaccharide esterification reactions proceed at ambient or mildly elevated temperatures. Because of these advantages, polysaccharide ester derivatives such as cellulose acetate have been widely utilized in various industrial fields (10–12). The synthesis and properties of nonionic polysaccharides—namely, cellulose, curdlan, glucomannan, pullulan, and α-(1→3)-glucans—have been reported previously (12–18). However, there have been fewer reports on the synthesis of ester derivatives of ionic polysaccharides, such as chitin, chitosan, and Alg (19, 20). We have succeeded in preparing Alg esters with fully substituted hydroxyl groups (21). By esterifying all the hydroxyl groups, we obtained thermoplastic Alg materials that were soluble in organic solvents, and consequently, we were able to manufacture Alg ester films by hot pressing and solvent casting. The monomer composition and molecular weight have a significant influence on the physical properties of copolymers (22). The M/G composition of underivatized Alg affects its gelation ability and the strength of the gel (23). However, there has been no detailed investigation of the relationship between the M/G composition and the physical properties of derivatized Alg. In the present chapter, we report the effects of M/G composition and molecular weight on the physical properties of Alg ester derivatives. We also report the effect of the length of the alkyl side chain of the Alg ester derivatives.
126 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Experimental Section Materials Four sodium alginates (M/G_viscosity = 39/61_100cp, 56/44_100cp, 69/31_100cp, and 56/44_1000cp) were obtained from KIMIKA CO., Ltd., Tokyo, Japan. The carboxylic acids (acetic acid, propionic acid, hexanoic acid, and octanoic acid) and their anhydrides (except for n-octanoic anhydride) were purchased from FUJIFILM Wako Pure Chemical Corporation, Tokyo, Japan; n-octanoic anhydride was purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. All other reagents were obtained from commercial sources and used without further purification.
Pretreatment of Alg Each sodium alginate (5.0 g) was dissolved in 300 mL of distilled water, and pH of the solution was adjusted to 2.1 by adding 6 M HCl. Nine hundred milliliters of acetone was added to the mixture and stirred for 30 min to remove water and NaCl, and the precipitate was recovered by filtration. The precipitate was stirred in 400 mL of a 3:1 mixed solution of acetone and water and recovered by filtration. The washing process was repeated three times and acid-form Alg were obtained.
Esterification of Alg As shown in Table 1, Alg esters (Alg propionate, hexanoate, and octanoate) were prepared by using four types of Alg, according to a previously reported method (21). First, Alg was completely swollen in water. The water absorbed by the Alg was then removed by stirring in acetone. The acetone-exchanged Alg was recovered by filtration and stirred into 100 mL of carboxylic acid to obtain solvent-exchanged Alg. After solvent exchange, the pretreated Alg was stirred into carboxylic anhydride (50 mL) at 40 °C. The mixture was reacted at 40 °C for 3 h by adding 60% aqueous perchloric acid (0.5 mL) as the catalyst and carboxylic acid (50 mL) as the reaction medium. The reaction mixture was poured into 1 L of distilled water. The precipitate was recovered by filtration, washed several times with distilled water, and dried in vacuo overnight. Four and three-tenths grams of Alg propionate (AlgPr) (56/44_1000cp) were obtained. During the preparation of Alg hexanoate (AlgHe) and Alg octanoate (AlgOc), the reaction mixture was poured into a sodium bicarbonate (100 g)/distilled water (200 mL) solution. The precipitate was filtered, washed with distilled water, and dried in vacuo to obtain the sodium salts of AlgHe and AlgOc. The sodium-salts forms were transformed into their carboxylic-acid forms by dissolving them in propionic acid (100 mL) and precipitating them in distilled water (800 mL). The precipitates were recovered by filtration, washed in water, and dried in vacuo to obtain AlgHe (39/61_100cp, 5.2 g; 56/44_100cp, 4.4 g; 69/31_100cp, 2.9 g; 56/ 44_1000cp, 4.5 g) and AlgOc (56/44_1000cp, 4.6 g) (Scheme 1). 127 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Scheme 1. Preparation of Alg ester derivatives.
Proton Nuclear Magnetic Resonance Measurement Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a JEOL JNM-A500 FT-NMR (500 MHz) spectrometer (JEOL Ltd., Tokyo, Japan), using trifluoroacetic acid-d (TFA-d) (99.5% atom D, Sigma-Aldrich, Co., Ltd., St. Louis, MO, USA). The chemical shifts (δ) are reported as parts per million (ppm). 1H NMR [δ (ppm), in trifluoroacetic acid-d]:AlgPr [1.22 (-OOCCH2CH3), 2.5 (-OOCCH2CH3)], AlgHe [0.9 (-OOC(CH2)4CH3), 1.3 (-OOC(CH2)2(CH2)2CH3), 1.6, 1.7 (-OOCCH2CH2(CH2)2CH3), 2.5 (-OOCCH2(CH2)3CH3)], AlgOc [0.8 (OOC(CH6)CH3), 1.3 (-OOC(CH2)2(CH2)4CH3), 1.6 (-OOCCH2CH2(CH2)4CH3), 2.4 (-OOCCH2(CH2)5CH3)]. DS values were calculated by DS = (CH3/3)/(Ring-H/5). Gel Permeation Chromatography The number- and weight-average molecular weights (Mn and Mw) and polydispersity indices (Mw/Mn) were estimated by gel permeation chromatography (GPC) (LC-10ADVP system, Shimadzu, Co., Ltd., Kyoto, Japan, equipped with an RID-10A reflective index detector) in N,N-dimethylacetamide (DMAc) at 40 °C. The Alg esters were eluted at 0.6 mL/min and permeated through a polystyrene column (KD-804, 7.8 mm i.d. × 300 mm, Shodex Co., Ltd., Japan). Calibration curves were obtained by using pullulan standards (Shodex Co., Ltd.). Preparation of Solvent-Cast Films AlgPr, AlgHe, and AlgOc (250 mg) were dissolved in acetone (10 mL) and cast on a Teflon petri dish (5 cm diameter). The solvent was then evaporated in air at room temperature. The obtained films were dried in vacuo for 1 day to remove the solvent. Tensile Tests Tensile tests were carried out on the Alg ester cast films at room temperature using an EZ-test machine (Shimadzu, Co., Ltd.). The Alg esters films [4 mm (W) × 30 mm (L), initial gauge length of 10 mm] were stretched at a crosshead speed of 20 mm/min. 128 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Thermogravimetric Analysis Thermogravimetric analysis (TGA) was carried out using an STA6000 (Perkin Elmer Inc., Waltham, MA, USA) in a nitrogen atmosphere. Each sample was heated from 30 to 500 °C at 10 °C/min.
Differential Scanning Calorimetry Differential scanning calorimetry (DSC) thermograms were recorded using a DSC8500 system (Perkin Elmer Inc., Waltham, MA, USA) in a nitrogen atmosphere. The Alg esters were initially heated from 30 to 210 °C at 20 °C/min and held at 200 °C for 1 min. Then, the samples were immediately cooled to -30 °C and held at that temperature for 5 min. The second heating scan was run from -30 °C to 200 °C at 20 °C/min. The temperature was calibrated using indium as an external standard.
Dynamic Mechanical Analysis Hot-pressed films were obtained by using a Mini Test Press 10 (Tokyo Seiki Co. Ltd., Tokyo, Japan). The powdered Alg esters were sanded using Kapton films and pressed at 200 °C (AlgPr), 180 °C (AlgHe), and 160 °C (AlgOc). The thickness of the films was adjusted to 500–700 μm using a 0.5-mm-thick stainlesssteel spacer. Dynamic mechanical analysis (DMA) measurements were performed on a DVA-2200S dynamic mechanical analyzer (IT Keisoku Seigyo Co., Ltd., Kyoto, Japan). The temperature sweep scans were performed at 100 Hz from 30 to 250 °C at a heating rate of 2 °C/min in a nitrogen atmosphere. The shearing strain applied to the sample was 0.05%.
Table 1. Characteristics of Alg Ester Derivatives DS
Yield(%) AlgHe
AlgOc
Mw/Mn
39/61_100cp
71.6
2
17.2
1.6
56/44_100cp
47.2
2
14.4
1.6
69/31_100cp
43.1
2
14.1
1.5
60.0
2
—
—
47.2
2
32.9
3.7
42.5
2
29.1
3.6
AlgPr AlgHe
Mw (104)
56/44_1000cp
129 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Results and Discussion Preparation of Alg Esters Solvent exchange is an indispensable process for the complete esterification of the hydroxyl groups in Alg. The swellability of Alg in water depends on the M/G ratio and the molecular weight. In the present study, Alg 39/61_100cp, Alg 56/44_100cp, and Alg 69/31_100cp required 200 mL of water and swelling. Alg 56/44_1000cp required 500 mL of water to achieve maximum swelling Figure 1 shows side chain 1H NMR spectra of AlgHe, that was prepared from Alg with a different M/G ratio and molecular weight. The peaks of the representative side chain protons and ring protons were observed. DS of all the Alg esters calculated from the ratio of peaks area of methyl proton and sugar backbone was 2.0. This indicates that all the hydroxyl groups in the Alg were esterified. The averaged molecular weights are summarized in Table 1. The GPC analysis revealed that the AlgHe samples (39/61_100cp, 56/44_100cp, and 69/ 31_100cp) had similar molecular weights, namely, 17.2 × 104, 14.4 × 104, and 14.1 × 104, respectively. Because of the similar molecular weights of these AlgHe, the influence of molecular weight can be ignored when considering the effect of the M/G ratio. In contrast, AlgHe (56/44_1000cp) had a Mw of 32.9 × 104, which was much larger than the Mw values of the other AlgHe. The effect of molecular weight can be considered by comparing AlgHe (56/44_1000cp) with the other AlgHe samples.
Figure 1. 1H NMR spectra of four different AlgHe. 130 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Solvent-Cast Films and Their Tensile Properties All the AlgHe samples were dissolved in acetone, irrespective of their M/G ratio or molecular weight. The AlgHe solvent-cast films were prepared from the acetone solutions. Figure 2 shows the translucent and self-standing cast films of AlgHe and their stress–strain curves. Whereas the AlgHe films (all 100cp) had similar tensile strengths (16.8–18.2 MPa) and Young’s moduli (0.26–0.30 GPa), the AlgHe (56/44_1000cp) film had a much higher tensile strength (26.8 MPa) and Young’s modulus (0.41 GPa). These results suggest that the M/G ratio does not affect the film properties. However, the molecular weight of the Alg esters has a significant effect on the film properties.
Figure 2. Cast films and their stress–strain curves of AlgHe sample of (a) 39/61_100cp, (b) 56/44_100cp, (c) 69/31_100cp, and (d) 56/44_1000cp.
Figure 3 shows the solvent-cast films of the Alg (56/44_1000cp) esters with different side chain lengths [(a), (b), and (c)] and their stress–strain curves. All the Alg esters were self-standing and transparent. Comparing among the Alg esters (56/44_1000cp), when the length of alkyl side chains decreased, the tensile strength and Young’s moduli of the Alg esters increased, whereas the elongations at break were almost the same. These results suggest that the strength of the Alg ester films increases with a decrease in the number of carbon atoms in the alkyl side chain, whereas the M/G ratio does not affect the film properties. The tensile properties of the Alg esters and conventional plastic materials—namely, polystyrene (PS), poly(methyl methacrylate) (PMMA), and polycarbonate (PC)—are summarized in Table 2 (24). The tensile strength of AlgPr (56/44_1000cp), which was 63 MPa, was higher than the tensile strengths of polystyrene and PMMA and was comparable to the tensile strength of PC. Furthermore, the 56/44_1000cp Alg ester had a larger elongation at break than did PS and PMMA. These results indicate that high-molecular-weight Alg esters have tensile properties that are comparable or even superior to those of conventional plastic materials. 131 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. Cast films and their stress–strain curves of Alg ester derivatives (56/44_1000cp): (a) AlgPr, (b) AlgHe, and (c) AlgOc.
Thermal Properties The thermal behaviors of the Alg esters were analyzed by TGA and DSC. The TGA thermograms are shown in Figure 4. The decomposition temperature at 5% weight loss (Td5%) increased from 170 °C to 200–250 °C following esterification of the Alg hydroxyl groups. However, there was no clear dependence on the M/G ratio or molecular weight. Except for AlgHe (56/44_1000cp), the AlgHe samples did not exhibit any characteristic peaks in the first run of DSC thermogram (data not shown). Although there was a small peak in the first DSC run thermogram of AlgHe (56/44_1000cp), it was too small to be considered a melting point. These results confirm that AlgHe is an amorphous polymer. However, there were no significant glass transitions during the second runs of DSC analysis. A similar absence of glass transition has also been reported based on the DSC results for ester derivatives of xylan, glucomannan, and pullulan (14–17, 25).
Figure 4. TGA thermograms of (1) (a) Alg and AlgHe samples of (b) 39/61_100cp, (c) 56/44_100cp, (d) 69/31_100cp, and (e) 56/44_1000cp and (2) Alg ester derivatives (56/44_1000cp): (A) AlgPr, (B) AlgHe, and (C). AlgOc. 132 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
DMA Results for Alg Ester Hot-Pressed Films Hot-pressed films of the Alg esters were prepared to estimate their glass transition temperature (Tg) values by DMA. Temperature dependence of the dynamic loss tangent (tan δ) is shown in Figure 5. According to the peaks of the loss tangent thermograms, the Tg values of the AlgHe were determined to be within the range of 160–182 °C [Figure 5(1)]. AlgHe (69/31_100cp) had a much higher tan δ value than the other Alg esters. Figure 5(2) shows the loss tangent curves of the (56/44_1000cp) Alg esters. According to the loss tangent curves, the Tg values of the Alg esters were 176, 182, and 192 °C for AlgOc, AlgHe, and AlgPr, respectively. The thermal properties of the polysaccharides are summarized in Table 2. The Tg values of the polysaccharides were much higher than those of conventional lplastics (polystyrene, PMMA, and polycarbonate) (24). These results indicate that the Alg esters are highly thermostable amorphous plastics.
Figure 5. DMA curves of (1) AlgHe samples of (a) 39/61_100cp, (b) 56/44_100cp, (c) 69/31_100cp, and (d) 56/44_1000cp and (2) Alg ester derivatives (56/44_1000cp): (A) AlgPr, (B) AlgHe, and (C) AlgOc. 133 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 2. Tensile and Thermal Properties of Alg Ester Derivatives
AlgHe
Tensile Strength (MPa)
Elongation at break (%)
Young modulus (GPa)
Td5% (°C)
Tga (°C)
39/61_100cp
17 ± 1.1
20 ± 8.8
0.26 ± 0.03
253
160
56/44_100cp
17 ± 1.2
12 ± 2.0
0.27 ± 0.06
221
176
69/31_100cp
18 ± 1.8
10 ± 1.7
0.30 ± 0.05
250
171
64 ± 5
35 ± 12
0.74 ± 0.15
237
192
27 ± 2
35 ± 17
0.41 ± 0.13
204
182
18 ± 2
30 ± 4
0.25 ± 0.04
210
176
AlgPr AlgHe
56/44_1000cp
AlgOc
50
Polystylene
134
PMMA
50 60
Polycarbonate Glass transition temperature were measured by DMA. Onset temperature of weight decrease.
a
b
3
2
3
4
100
(26) Onset temperature of weight decrease.
2 c
ca.
300b
213c ca.
450d
(27) 10% weight decrease temperature.
Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
100 105 145 d
(28)
Conclusions In this research, effects of monomer composition and molecular weight as well as side chain length of Alg esters to their physical properties were investigated. AlgHe were prepared from four kinds of Alg with different ratios of mannuronic acid (M) to gluronic acid (G) and different molecular weights. All the Alg esters formed self-standing film, which enabled us to investigate the effect of M/G ratio and Mw on the tensile properties by solvent-casting method. From tensile test of AlgHe samples, it was revealed that tensile properties were clearly affected by Mw but not by M/G ratio. The effect of side chain length was further investigated by Alg esters prepared from high Mw (1000cp) Alg. It was revealed that the Alg esters with a shorter side chain showed higher tensile strength, while the elongation at break was almost unaffected. In particular, AlgPr (56/44_1000cp) gave the highest tensile strength (63 MPa) and glass transition temperature (192 °C), which are higher than those of most amorphous plastics. The results of the present study indicate that Alg esters have great potential as plastic materials.
Acknowledgments This work was carried out as part of the “Innovative Synthesis of High-Performance Bioplastics from Polysaccharides” project supported by JST ALCA Grant Number JPMJAL1502, Japan.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Draget, K. I.; Smidsrød, O.; Skjåk-Bræk, G. Carbohydr. Polym. 1994, 25 (1), 31–38. Trujillo-Roldán, M. A.; Moreno, S.; Segura, D.; Galindo, E.; Espín, G. Appl. Microbiol. Biotechnol. 2003, 60, 733–737. Venegas, M.; Matsuhiro, B.; Edding, M. Bot. Mar. 1993, 36, 47–51. Matsumoto, T.; Mashiko, K. Biopolymers 1990, 29, 1707–1713. Grant, G. T.; Morris, E. R.; Rees, D. A.; Smith, P. J. C.; Thom, D. FEBS Lett. 1973, 32, 195–198. Remminghorst, U.; Rehm, B. H. A. Biotechnol. Lett. 2006, 28, 1701–1712. Andersen, T.; Strand, B. L.; Formo, K.; Alsberg, E.; Christensen, B. E. Carbohydr. Chem. 2012, 37, 227–258. Vos, P. D.; Haan, B. D.; Schilfgaarde, R. V. Biomaterials 1997, 18, 273–278. Ertesvåg, H.; Valla, S. Polym. Degrad. Stabil. 1998, 59, 85–91. Edgar, K. J.; Buchanan, C. M.; Debenham, J. S.; Rundquist, P. A.; Seiler, B. D.; Shelton, M. C.; Tindal, D. Prog. Polym. Sci. 2001, 26, 1605–1688. Morooka, T.; Norimoto, M.; Yamada, T.; Shiraishi, N. J. Appl. Polym. Sci. 1984, 29, 3981–3990. Crepy, L.; Miri, V.; Joly, N.; Martin, P.; Lefebvre, J. M. Carbohydr. Polym. 2011, 83, 1812–1820. Marubayashi, H.; Yukinaka, K.; Enomoto-Rogers, Y.; Takemura, A.; Iwata, T. Carbohydr. Polym. 2014, 103, 427–433. 135 Cheng et al.; Green Polymer Chemistry: New Products, Processes, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
14. Enomoto-Rogers, Y.; Ohmomo, Y.; Takemura, A.; Iwata, T. Carbohydr. Polym. 2014, 101, 529–599. 15. Enomoto-Rogers, Y.; Ohmomo, Y.; Iwata, T. Carbohydr. Polym. 2013, 92, 1827–1834. 16. Danjo, T.; Enomoto-Rogers, Y.; Takemura, A.; Iwata, T. Polym. Degrad. Stabil. 2014, 109, 373–378. 17. Enomoto-Rogers, Y.; Iio, N.; Takemura, A.; Iwata, T. Eur. Polym. J. 2015, 66, 470–477. 18. Puanglek, S.; Kimura, S.; Enomoto-Rogers, Y.; Kabe, T.; Yoshida, M.; Wada, M.; Iwata, T. Sci. Rep. 2016, 6, 30479. 19. Schweiger, R. G. J. Org. Chem. 1962a, 27, 1786–1789. 20. Schweiger, R. G. J. Org. Chem. 1962b, 27, 1789–1791. 21. Matsumoto, Y.; Ishii, D.; Iwata, T. Carbohydr. Polym. 2017, 171, 229–235. 22. Martinez-Gomez, F.; Encinas, V. M.; Matsuhiro, B.; Pavez, J. J. Appl. Polym. Sci. 2015, 132, 42398–42408. 23. Andriamanantoaninaa, H.; Rinaudob, M. Polym. Int. 2010, 59, 1531–1541. 24. Billmeyer, F. W. J. Textbook of polymer science; J. Wiley: New York, 1984. 25. Fundador, N. G. V.; Enomoto-Rogers, Y.; Takemura, A.; Iwata, T. Polymer 2012, 53, 3885–3893. 26. Mehta, S.; Biederman, S.; Shivkumar, S. J. Mater. Sci. 1995, 30, 2944–2949. 27. Gałka, P.; Kowalonek, J.; Kaczmarek, H. J. Therm. Anal. Calorim. 2014, 115, 1387–1394. 28. Zhou, W.; Yang, H.; Zhou, J. J. Anal. Appl. Pyrol. 2007, 78 (2), 413–418.
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