ARTICLE pubs.acs.org/IECR
Study on EsterAmide Exchange Reaction between PBS and PA6IcoT Zhen Yao,‡ Jia-ming Sun,‡ Qiang Wang,‡ and Kun Cao*,†,‡ †
State Key Laboratory of Chemical Engineering and ‡Institute of Polymerization and Polymer Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: To improve the thermal properties and mechanical properties of synthetic biodegradable polymers, poly(butylene succinate) (PBS) and poly(hexamethylene isophthalamide-co-terephthalamide) (PA6IcoT) are reactively blended in a LIST corotating processor under melting conditions. The products are fractionated into insoluble and soluble parts using chloroform as the solvent. The Fourier transform infrared spectra of these two fractions confirm that the exchange reaction between PA6IcoT and PBS occurs when p-toluenesulfonic acid is used as the catalyst. On the basis of the 13C NMR analysis, the pathway of the exchange reaction is described. The extent of exchange reaction, average length of the various sequences, and degree of randomness in the reactive blends are calculated. It has been found that the randomness of copolymers is increased with the increase in reaction extent. Scanning electron microscopy (SEM) is employed to investigate morphology development of the reactive blends. It shows that the copolymers generated by the exchange reaction act as the compatibilizer to improve the compatibility of PBS and PA6IcoT. The fraction of PA6IcoT- and PBS-rich phases is significantly reduced with increasing reaction extent.
’ INTRODUCTION In recent years, much research has focused on environmentally benign materials, especially biodegradable polymers. Poly(butylene succinate) (PBS), polylactic acid (PLA), and polyhydroxybutyrate (PHB) are the most studied synthetic biodegradable polymers.13 Among these polymers, PBS has good biodegradability and processability.46 However, with the disadvantage of poor thermal and mechanical properties, its practical applications are limited. To solve the problem, various methods, such as blending, copolymerization, and chain extension, have been investigated.724 On the other hand, polyamide has good thermal and mechanical properties. However, most polyamides are nonbiodegradable. Therefore, poly(ester amide)s (PEAs), which combine the properties of both polyamides and polyesters, appeared as promising biodegradable materials. There are two ways of synthesizing poly(ester amide): polymerization of monomers and reaction of polymers.2544 Blending of the polymers is an effective way to produce versatile materials with desired properties. In a melt-mixing process, the exchange reactions may occur between condensing polymer molecules. As a result of exchange reactions, portions of the blends can be transformed into multiblock or random copolymers, which can effectively improve the compatibility of polymers.3146 In this paper, aliphatic semicrystalline PBS and semiaromatic amorphous poly(hexamethylene isophthalamide-co-terephthalamide) (PA6IcoT) are melt-blended in a LIST corotating processor with p-toluenesulfonic acid as the catalyst. The product is fractionated and their structures are studied by FTIR and 13C NMR. Moreover, the effect of the extent of the exchange reaction on the development the morphology of blends is investigated.
in a vacuum oven for 24 h before being used. Dimethyl sulfoxide (DMSO), p-toluenesulfonic acid (TsOH), chloroform, and ethyl alcohol are analytical reagent and purchased from Sinopharm Chemical Reagent Co. Ltd. Reactive Blending Procedure. PBS and PA6IcoT are mixed by a certain weight percentage in a corotating processor (CRP, 2.5 batch) manufactured by LIST AG (Berstelstr, Switzerland). Its volume is 3.0 L. This CRP is specially designed for processing of highly viscous materials. The agitation system, which consists of two hollow intermeshing shafts, provides extensive heat transfer surfaces and intensive mixing to ensure complete uniformity in the polymer melt. A discharge screw is integrated into the outlet section to facilitate sampling and discharge of the final products. The reactor is evacuated and charged with nitrogen alternatively three times before being used. When PBS and PA6IcoT are well melt-mixed at 260 °C, TsOH is added to the reactor as the catalyst. Samples are collected at regular time intervals. The percentages of the polymers and catalyst are based on a weight fraction. Solubility Experiment. Samples of reactive blends are placed in chloroform and stirred for 24 h. The solutions are filtered to obtain insoluble substances, which are dried at 80 °C before being weighted. The filtrates are precipitated with ethanol. The precipitates are also dried and weighted. Characterization. The samples for FTIR measurement are mixed with KBr and pressed into 8-μm-thick films under the conditions of 180 °C and 100 kgf/cm2. FTIR spectra are obtained under a dry nitrogen atmosphere in an IR spectrometer (Nicolet 5700) with Omnic software for data collection and analysis. Samples are dissolved in a mixture of CDCl3/(CF3CO)2O/ CF3COOD (70:17:13 v/v/v) for 13C NMR measurement,40,41
’ EXPERIMENTAL SECTION
Received: October 15, 2011 Accepted: November 29, 2011 Revised: November 29, 2011 Published: November 29, 2011
Materials. The PBS (Biocosafe 1903, Zhejiang Xinfu Pharma-
ceutical Co., Ltd.) and PA6IcoT (Zytel 330, Dupont Co.) used in this study are commercialized polymers. They are dried at 80 °C r 2011 American Chemical Society
751
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Table 1. Solubility of PBS/PA6IcoT Blends in Chloroform Solvent weight fraction (%) original weight ratio PBS/PA/TsOH (time, min)
insoluble substance
soluble substance
70/30/0 (60)
30.48
69.52
70/30/0 (180) 90/10/0 (60)
27.73 10.18
72.27 89.82
90/10/0 (180)
10.58
89.42
70/30/0.5 (60)
15.82
84.18
70/30/0.5 (180)
3.54
96.46
90/10/0.5 (60)
7.57
92.43
90/10/0.5 (180)
7.51
92.49
Figure 1. FTIR spectra of the insoluble substance of PBS/PA6IcoT blends in chloroform solvent.
which is performed in a Bruker Avance 500 M superconducting nuclear magnetic resonance spectrometer with tetramethylsilane as the internal standard. The concentration of the sample solution is 15% (w/v). Morphologies of blends are observed using a SIRON field emission scanning electron microscope (FESEM).
’ RESULTS AND DISCUSSION Solubility Analysis. Pure PBS can be dissolved in chloroform, but pure PA6IcoT is insoluble in chloroform. After being dissolved in chloroform for 24 h, the mass fraction of insoluble substance and soluble substance in the products of melt-blending is obtained, which is shown in Table 1. In the absence of TsOH, the mass fraction of insoluble substance and soluble substance is similar to the original fraction in the blends. This indicates that the exchange reaction between PBS and PA6IcoT is difficult to carry out without catalyst. When 0.5 wt % of TsOH is used, the mass fraction of soluble substance increases rapidly. This indicates that a portion of PA6IcoT can react with PBS and the generated copolymer is soluble in chloroform. To further verify whether an exchange reaction occurred, soluble substance and insoluble substance are analyzed by FTIR. The results are presented in Figures 1 and 2. The absorption band at 1700 cm1 belongs to the ester carbonyl group. The absorption bands for amide group appear at 1634 and 1537 cm1. Figure 1 shows that the absorption peak of the ester group does not appear in the FTIR spectra of PA6IcoT and the insoluble substance of blends without TsOH. In Figure 2, the absorption peak of amide group cannot be found in the spectra of soluble substance of blends without TsOH. These results confirm that reaction between PBS and PA6IocT is insignificant without the catalyst and that the two polymers in the blends can be completely separated. On the other hand, the absorption bands of ester and amide groups can be found in the insoluble and soluble fractions of the blends with TsOH, respectively. This further verifies that an exchange reaction takes place between PBS and PA6IcoT in the presence of TsOH as the catalyst. The generated copolymers contain both ester and amide groups. The PBS-rich part of these copolymers are dissolved in chloroform with PBS, while the PArich part of the copolymers stay insoluble with PA6IcoT. In Figure 2, it can be also found that the height of the amide absorption band is increased with the increasing reaction time. This means that the extent of reaction has increased with longer reaction time.
Figure 2. FTIR spectra of the soluble substance of PBS/PA6IcoT blends in chloroform solvent.
Structure Analysis. There are two important questions in studying the exchange reaction: one is whether reaction occurs and another is how to calculate the average length of the various sequences and the degree of randomness. As to the first question, solubility experiments and FTIR spectra have proved that the reaction occurred. To answer the second question, 13C NMR is employed.4750 To distinguish the difference between homopolymer and reaction product, the 13C NMR of the physical blend is collected. The PBS and PA6IcoT with specified ratio are dissolved in DMSO at 80 °C and precipitated with ethanol to obtain the physical blend. Figure 3 presents the expanded 13C NMR spectra of various blends. The assignments of chemical shifts are shown in Table 2. As seen from Figure 3, there is no significant difference between the NMR spectra of the physical blend and the blends without catalyst. However, new chemical shifts, which are assigned to the new structure generated through the exchange reaction, can be found in the spectra of the blends with TsOH. On the basis of the 13C NMR spectra of the blends, the pathway of the exchange reaction between PBS and PA6IcoT is described in Scheme 1. The exchange reaction between the linear polycondensation polymers yields copolymers with following molecular formula:
—½ðA 1 B1 Þx —ðA 1 B2 Þy m —½ðA 2 B1 Þz —ðA 2 B2 Þω n — 752
ð1Þ
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Figure 3.
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13
C NMR spectra of PBS/PA6IcoT blends.
A1 and A2 unit. The method proposed by Eersels et al.48 and Devaux et al.49,50 are used in this work to calculate these parameters. There will be 100% exchange when all A1B1 and A2B2 units are changed into A1B2 and A2B1units, respectively. The percentage
where A1, A2, B1, and B2 represent the succinic acid, terephthalic acid, butanediol, and hexamethylenediamine monomeric units, respectively. x, y, z, and ω are the average length of the various sequences; m and n are the average lengths of blocks with the same 753
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Table 2. Assignments of Chemical Shifts in the NMR Spectra of PBS/PA6IcoT Blends
of exchange reaction can be calculated as Ψ=% ¼ ðFA1 B2 þ FA2 B1 Þ 100%
z¼
ð2Þ
½A i Bj 2
∑ ½Ai
ði, j ¼ 1, 2Þ
PA 2 B2
½A 2 B1 þ 1 ½A 2 B2
¼
½A 2 B2 þ 1 PA 2 B1 ½A 2 B1 Degree of randomness is
Here, FAiBj represent the fraction of AiBj unit. FA i B j ¼
1
ð3Þ
i¼1
[AiBj] and [Ai] are the concentration of AiBj and Ai units. Different concentrations have the following relations:
ω¼
1
χ¼
1 1 þ x ω
¼
ð9Þ ð10Þ
ð11Þ
The fraction of A1B1, A1B2, A2B1, and A2B2 units can be calculated from NMR spectra using following equations: FA1 B1 ¼
RIα þ RIα00 RI
ð12Þ
FA1 B2 ¼
RIα0 þ RIα000 RI
ð13Þ
ð6Þ
FA2 B1 ¼
RIβ0 þ RIβ00 þ RIγ0 þ RIγ00 RI
ð14Þ
Here, PAi Bj is probability of generating AiBj, which can be calculated as
FA2 B2 ¼
RIβ þ RIβ000 þ RIγ þ RIγ000 RI
ð15Þ
½A i ¼ ½Bi ði ¼ 1, 2Þ
ð4Þ
½A i Bj ¼ ½A j Bi
ð5Þ
The average length of the A1B1 sequences is x¼
1 ½A 1 B1 þ 1 ¼ PA 1 B2 ½A 1 B2
PAi Bj ¼ ½A i Bj =½A j
ð7Þ
1 PA 1 B1
¼
½A 1 B2 þ 1 ½A 1 B1
∑
∑
∑
Here, RI represents the relative intensity of the corresponding chemical shifts and ∑RI is the sum of relative intensity of all carbonyl group peaks. Using the above formula, the fraction of all units are calculated, and they are listed in Table 3. The extent of exchange reaction,
The average length sequences of A1B2, A2B1, and A2B2 are y¼
∑
ð8Þ 754
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Scheme 1. Pathway of Exchange Reaction between PBS and PA6IcoT
Table 3. Relative Mole Fraction of All Segments in the PBS/ PA6IcoT Blends
a
runa
FA1
FA2
FA1B1
FA1B2
FA2B1
FA2B2
90/10/0.5, 180
0.86
0.14
0.83
0.025
0.028
0.11
80/20/0.5, 180 70/30/0.5, 180
0.80 0.71
0.20 0.29
0.76 0.63
0.040 0.080
0.040 0.055
0.16 0.24
60/40/0.5, 180
0.57
0.43
0.49
0.081
0.079
0.35
50/50/0.5, 180
0.48
0.52
0.39
0.093
0.094
0.43
40/90/0.5, 180
0.37
0.63
0.30
0.070
0.065
0.57
30/70/0.5, 180
0.29
0.71
0.24
0.048
0.045
0.66
20/80/0.5, 180
0.19
0.89
0.16
0.029
0.030
0.78
10/90/0.5, 180
0.13
0.87
0.11
0.020
0.022
0.85
70/30/0.5, 60
0.71
0.29
0.66
0.050
0.051
0.24
PBS/PA/catalyst (wt %), mixing time (in min).
Table 4. Extent of Exchange Reaction, Average Sequence Lengths, and Degree of Randomness of PBS/PA6IcoT blends
a
runa
ψ
x
y
z
ω
χ
90/10/0.5, 180
5.30
34.40
1.04
1.25
4.93
0.23
80/20/0.5, 180 70/30/0.5, 180
8.00 13.50
20.00 8.88
1.05 1.13
1.25 1.23
5.00 5.27
0.25 0.30
60/40/0.5, 180
16.00
7.04
1.16
1.25
5.44
0.33
50/50/0.5, 180
18.70
5.16
1.23
1.21
5.53
0.37
40/60/0.5, 180
13.50
5.29
1.23
1.11
9.69
0.29
30/70/0.5, 180
9.30
6.04
1.21
1.08
15.82
0.23
20/80/0.5, 180
5.90
6.55
1.19
1.14
29.67
0.19
10/90/0.5, 180
4.20
6.50
1.18
1.02
39.55
0.18
70/30/0.5, 60
10.10
14.20
1.08
1.21
5.69
0.25
PBS/PA/catalyst (wt %), mixing time (in min)].
effect of catalyst concentration on the extent of reaction is insignificant. The melt-blending process was also conducted at 220, 240, 260, 280, and 300 °C. PA6IcoT melted incompletely at 220 and 240 °C. The degradation of PBS becomes significant at 280 and 300 °C. Therefore, all exchange reactions reported here were conducted at 260 °C. This paper is aimed at investigating the influence of exchange reaction on microstructure and morphology of the reactive blends. The optimum conditions with regard to the extent of reaction will be determined in our future work. Morphology Analysis. The morphology plays an important role in the properties of polymer blends. The blends between incompatible polymers have poor mechanical properties. When PBS is the continuous phase, blends are eroded with chloroform to remove the PBS-rich matrix. The morphologies of the dispersed phase are shown in Figure 4. Without catalyst, the dispersed phases exist in the spherical form as the result of the poor compatibility between PBS and PA6IcoT. The particle size of the dispersed phase is increased with the rise of the PA weight fraction. In the presence of TsOH as the catalyst, the dispersed phase changes into irregular particle shapes and its size becomes smaller evidently. In this case, there is little variation in the particle size of dispersed phase with the change in PA weight fraction. The reason is that copolymers generated by the exchange reaction act as a compatibilizer and reduce the interfacial tension. When PBS is the dispersed phase, reactive blends are brittle and fractured in liquid nitrogen and eroded with chloroform.
average sequences length, and degree of randomness are presented in Table 4. It can be found that the extent of exchange reaction, average sequence length, and degree of randomness all vary with the change in the proportion of the two polymers. Both the extent of exchange reaction and degree of randomness reach a maximum when the weight ratio between two polymers is 50:50. At this ratio, the probability of the collision between an acidolysis and aminolysis group is higher. Furthermore, the extent of reaction increases with increasing time. The exchange reaction was carried out at the catalyst concentration of 0.25, 0.5, 0.75 and 1 wt %. It has been found that the 755
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Figure 4. Morphology of the dispersed phase with PBS as the matrix phase in PBS/PA6IcoT blends [PBS/PA/catalyst (wt %), mixing time]. (3) Evens, B. Polymer finds new use as biodegradable additlves. Plast. Technol. 1990, 36 (5), 41–44. (4) Jeong, E. H.; Im, S. S.; Youk, J. H. Electrospinning and structural characterization of ultrafine poly(butylene succinate) fibers. Polymer 2005, 46 (23), 9538–9543. (5) Ishioka, R., Kitakuni, E., Ichikawa, Y. Aliphatic polyesters: “Bionolle”. In Biopolymers, Vol. 4. Polyesters III Applications and Commercial Products; Doi,Y., Steinb€uchel, A., Eds.; Wiley-VCH, New York, 2002, 275297. (6) Xu, J.; Guo, B. H. Poly(butylene succinate) and its copolymers: Research, development and industrialization. Biotechnol. J. 2010, 5 (11), 1149–1163. (7) Lai, S. M.; Huang, C. K.; Shen, H. F. Preparation and properties of biodegradable poly(butylene succinate)/starch blends. J. Appl. Polym. Sci. 2005, 97 (1), 257–264. (8) Park, J. W.; Im, S. S. Phase behavior and morphology in blends of poly(L-lactic acid) and poly(butylene succinate). J. Appl. Polym. Sci. 2002, 86 (3), 647–655. (9) Yokohara, T.; Okamoto, K.; Yamaguchi, M. Effect of the shape of dispersed particles on the thermal and mechanical properties of biomass polymer blends composed of poly(L-lactide) and poly(butylene succinate). J. Appl. Polym. Sci. 2010, 117 (4), 2226–2232. (10) Shibata, M.; Inoue, Y.; Miyoshi, M. Mechanical properties, morphology, and crystallization behavior of blends of poly(L-lactide) with poly(butylene succinate-co-L-lactate) and poly(butylene succinate). Polymer 2006, 47 (10), 3557–3564. (11) Harada, M.; Ohya, T.; Iida, K.; Hayashi, H.; Hirano, K.; Fukudal, H. Increased impact strength of biodegradable poly(lactic acid)/ poly(butylene succinate) blend composites by using isocyanate as a reactive processing agent. J. Appl. Polym. Sci. 2007, 106 (3), 1813–1820. (12) Qiu, Z. B.; Ikehara, T.; Nishi, T. Poly(hydroxybutyrate)/poly(butylene succinate) blends: Miscibility and nonisothermal crystallization. Polymer 2003, 44 (8), 2503–2508. (13) Nikolic, M. S.; Djonlagic, J. Synthesis and characterization of biodegradable poly(butylene succinate-co-butylene adipate)s. Polym. Degrad. Stab. 2001, 74 (2), 263–270. (14) Tserki, V.; Matzinos, P.; Pavlidou, E; Vachliotis, D.; Panayiotou, C. Biodegradable aliphatic polyesters. Part I. Properties and biodegradation of poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91 (2), 367–376. (15) Mochizuki, M.; Mukai, K.; Yamada, K.; Ichise, N.; Murase, S.; Iwaya, Y. Structural effects upon enzymatic hydrolysis of poly(butylene succinate-co-ethylene succinate)s. Macromolecules 1997, 30 (24), 7403–7407. (16) Cao, A.; Okamura, T.; Nakayama, K.; Inoue, Y.; Masuda, T. Studies on syntheses and physical properties of biodegradable aliphatic poly(butylene succinate-co-ethylene succinate)s and poly(butylene succinate-co-diethylene glycol succinate)s. Polym. Degrad. Stab. 2002, 78 (1), 107–117.
Figure 5. Morphology of the matrix phase with PA6IcoT as the matrix phase in PBS/PA6IcoT blends [PBS/PA/catalyst (wt %), mixing time].
The morphologies of these samples are presented in Figure 5. In the blends with catalyst, the hole left by the eroded PBS is an irregular shape when the fraction of added PBS increases. This is caused by the increasing compatibility provided by the product of the exchange reaction.
’ CONCLUSIONS PBS and PA6IcoT are melt-blended in a LIST corotating processor with TsOH as the catalyst. The solubility and polymer chain structure of reactive blends were investigated. The results indicate that TsOH is an effective catalyst for the esteramide exchange reaction. When the ratio of two polymers was close to 1:1, the degree of randomness of copolymers and reaction extent reach maximum values. The morphology development of the reactive blends shows that the produced copolymers act as the compatilizer and reduce the interfacial tension of the two phases. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China through NSFC Project No. 50873090, the Program for Changjiang Scholars and Innovative Research Team in University, and the Fundamental Research Funds for the Central Universities. ’ REFERENCES (1) Chandra, R.; Rustgi, R. Biodegradable polymers. Prog. Polym. Sci. 1998, 23 (7), 1273–1335. (2) Doi, Y., Fukuda, K. Biodegradable Plastics and Polymers; Elsevier, New York, 1994; pp 150174. 756
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