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Synthesis and characterization of degradable electrically conducting copolymer of aniline pentamer and polyglycolide
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 4244–4252
www.elsevier.com/locate/europolj
Synthesis and characterization of degradable electrically conducting copolymer of aniline pentamer and polyglycolide Caifeng Ding, Yan Wang, Shusheng Zhang
*
College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China Received 18 June 2007; received in revised form 23 July 2007; accepted 24 July 2007 Available online 31 July 2007
Abstract A novel block copolymer of polyglycolide (PGA) and aniline pentamer that is electro-active and degradable was synthesized. The copolymer was prepared by low-temperature polycondensation between carboxyl-capped aniline pentamer and hydroxyl-capped PGA, with N,N 0 -dicyclohexylcarbodiimide (DCC) as dehydrating agent. The structures of monomers and the copolymer were characterized by IR, 1H NMR. The chemical redox process of the as-prepared copolymer was studied by UV–vis spectra and cyclic voltammetry (CV). Thermal analysis and the study of degradation show that the copolymer has good thermal stability and degradability. 2007 Elsevier Ltd. All rights reserved. Keywords: Aniline pentamer; Polyglycolide; Conducting polymers; Degradation
1. Introduction Electrical stimulation can enhance the excretion and expression of growth factors and accelerate cellular proliferation and differentiation. It can also accelerate the healing of bone, cartilage, and peripheral nerves [1–3]. Many attempts have been made to seek new materials that incorporate electrical signals, which would deliver an electrical stimulus at the site of damage and provide a physical template for cell growth and tissue repair. Electrically conducting polymers have attracted much interest in the last 20 years because they not only have the physical and chemical properties of
*
Corresponding author. Tel./fax: +86 532 84022750. E-mail address: [email protected] (S. Zhang).
organic polymers but also have the electrical characteristics of metals. There are many conducting polymers, such as polyaniline, polyacetylene, polythiophene and polypyrrole, were reported in recent years [4–7]. Of all known conducting polymers, polyaniline is applied in many areas, owning to its excellent electrical properties, good environmental stability, facile synthesis, and low cost of the monomer. The structural and electrical properties of aniline oligomer are similar to polyaniline, but the solubility is better. Aniline oligomers can be used in various fields, such as anticorrosion for metals, chemical sensors and catalytic oxidation [8–10]. Degradable electrically conducting polymers possess the unique properties of being electrically conducting and degradable, so they have a promising prospect to be used as neural conducting tissue
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.07.032
C. Ding et al. / European Polymer Journal 43 (2007) 4244–4252
engineering material which can conduct bioelectrical signals in vivo. But the biodegradable electrical conductive materials were reported occasionally. A biodegradable electrically conducting polymer, which was synthesized from conducting oligomers of pyrrole and thiophene that were connected via degradable ester linkages, was studied by Rivers et al. [11]. Some properties, such as conductivity, degradability and biocompatibility of the polymer were researched. Another electrically conductive and biodegradable composite made of polypyrrole nanoparticles and polylactide was researched by Zhang and co-workers [12]. In their work, electrical stability of the composite in a physiological environment was investigated, and hypothesis that direct electron current applied through such a conductive composite can affect cellular activities was tested. Huang et al. [13] synthesized and characterized an electro-active and biodegradable copolymer of polylactide and aniline pentamer. They proved that the copolymer is electroactive, degradable and biocompatible, and concluded that the copolymer can support cells attachment and proliferation. Polyglycolide and polylactide are biodegradable and biocompatible materials that can be used as medical biomaterials. In this work, a block copolymer of polyglycolide (PGA) and aniline pentamer, which is electro-active and degradable, was synthesized. The redox process of the copolymer was studied by UV–vis spectra and cyclic voltammetry (CV). Conductivity, thermal stability and degradability were also evaluated. 2. Experimental 2.1. Materials N-Phenyl-p-phenylenediamine (98%, Sigma, USA), p-phenylenediamine (Shanghai Shengqiang Industrial Co., Ltd., China), glycolide (GA, Beijing Kangan macromolecule Research Institute, China), 1,4-butanediol (Tianjin Bodi Chemical Reagent Company, China), N-methyl-pyrrolidone (NMP), N,N 0 -dimethylformamide (DMF) (Tianjin Ruijinte Chemical Reagent Company, China), stannous octanoate (Shanghai Shengyu Chemical Reagent Company, China), 4-dimethylaminopyridine (DMAP, Yangzhou Feiyang Chemical Reagent Company, China). succinic anhydride and ammonium persulfate, made by Shanghai Aijian Chemical Reagent Plant (China). The chemicals used above were analytical grade. N,N 0 -Dicyclohexylcarbodii-
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mide (DCC, Guoyao Chemical Reagent Company, China) was reagent grade. All chemicals were used as received without further purification. 2.2. Apparatus Infrared spectra were recorded using a Nicolet 510P FT-IR spectrometer (USA) with KBr pellets. The 1H NMR spectra were recorded on a Bruker AV500 NMR spectrometer (Switzerland) and d6DMSO was used as the solvent, tetramethylsilane (TMS) was used as an internal standard. UV–vis spectra were taken by a Cary 50 UV–vis-NIR spectrophotometer (Australia). The thermogravimetric analysis (TGA) was performed with a thermal analyzer of NETZSCH TG209 (Germany) in a nitrogen atmosphere between 295 K and 1173 K with a temperature rate of 10 K/min. Conductivity was measured on a DDS-307 conductometer (China). 2.3. Synthesis of monomers and copolymer The schematic diagrams for the synthesis of monomers and the copolymer are shown in Scheme 1. 2.3.1. Preparation of carboxyl-capped aniline pentamer 4-Oxo-4-(4-(phenylamino)phenylamino)butanoic acid and aniline pentamer were prepared as described previously [14]. Briefly, succinic anhydride (0.05 mol) and N-phenyl-p-phenylenediamine (0.05 mol) were dissolved in dichloromethane, respectively. And then the latter was added dropwise into the former. The reaction was carried out at room temperature for 3 h under nitrogen atmosphere. After filtration, 4-oxo-4-(4-(phenylamino)phenylamino)butanoic acid obtained was washed with diethyl ether and then dried at 30 C under vacuum for 12 h. 4-Oxo-4-(4-(phenylamino)phenylamino)butanoic acid (0.01 mol) and p-phenylenediamine (0.005 mol) were all dissolved in DMF (15 mL). A mixture solution (30 mL DMF; 25 mL distilled water; 5 mL concentrated hydrochloric acid) and ammonium persulfate (0.01 mol in 1 M HCl) were added into the above solution by sequence. The reaction was carried out at 16 C with stirring for 4 h. Then the mixture was poured into distilled water (300 mL). After filtration, the precipitate was suspended in ammonia (300 mL, 1 M) by stirring, hydrazine hydrate was added in the same time. The solution was filtrated after changing the pH of the solution
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Scheme 1. Structure and synthetic reaction scheme for degradable electrically conducting copolymer.
to 2. The precipitate was dried at 45 C in vacuum for 48 h. The solution of the precipitate in DMF was added drop-wise into ethanol. Aniline pentamer was obtained by filtration, dried in vacuum and then soxhleted with 1,2-dichloroethane and THF.
ether. The copolymer was dissolved in chloroform (7 mL), and then added drop-wise into anhydrous diethyl ether (150 mL) quickly. The copolymer isolated by filtration was dried thoroughly at 30 C under vacuum.
2.3.2. Preparation of hydroxyl-capped polyglycolide Glycolide (0.07 mol), 1,4-butandiol (0.035 mol) and stannous octanoate (0.0004 mol) were added in turn to n-butyl acetate (120 mL). Hydroxylcapped polyglycolide was obtained after refluxed at 110 C for 10 h.
2.4. Conductivity measurements
2.3.3. Synthesis of the copolymer The copolymer was synthesized by low-temperature polymerization, as described by Putnam et al. [15]. Under a nitrogen atmosphere, hydroxylcapped polyglycolide (1 mmol) and carboxylcapped aniline pentamer (1 mmol) were dissolved in N-methyl-2-pyrrolidone (NMP, 7.5 mL). DMAP (1 mmol) and DCC (4 mmol) were added into the mixture solution by sequence. The reaction was carried out at 0 C with stirring for 48 h, and then filtered. The filtrate was added drop-wise into anhydrous diethyl ether (75 mL) and stirred for 5 min. The copolymer congealed on the bottom of the flask and was isolated by decanting the diethyl
Aniline pentamer (doped with 0.1 M HCl) and the copolymer (doped with 0.1 M HCl) were dissolved in NMP (0.1 M) respectively. The conductivities of the two solutions and NMP were measured on a DDS-307 conductometer. The conductivity was the difference between solution and solvent. 2.5. Degradation Tests The degradation behavior of the as-prepared copolymer was studied over a period of 4 months. All specimens possess the same dimensions (8mm · 4mm · 0.2 mm). Each specimen was immersed in a vial filled with 20 mL of phosphate buffer solution (pH 7.4) and incubated at 37 C under static conditions. PBS was changed every 7 days. The specimens were taken out one by one at regular intervals, dried at 30 C to constant weight in order to determine the weight loss.
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The percentage of weight loss (WL) was calculated as follows: WLt ¼ ðm0 mf Þ=m0 100%; where m0 is the initial mass (t = 0), mf is the final mass after degradation and drying, t is the degradation time. 3. Results and discussion 3.1. Characterization of synthesized monomers and copolymer 3.1.1. IR of PGA The IR spectrum of PGA is shown in Fig. 1. It can be observed from the spectrum that the characteristic absorption peak of PGA is at 1237 cm1, which is the absorption peak of C–O–C in straight chain, but not at 1297 cm1, the characteristic absorption peak of C–O–C in cycloaliphatics. This indicates that the ring-open polymerization of GA had occurred. 3.1.2. IR spectra of aniline pentamer, PGA and copolymer Fig. 2 shows the IR spectra of the copolymer together with its corresponding precursors as a typical example. As shown in Fig. 2A, the characteristic carbonyl stretching vibrations are observed at 1657 and 1696 cm1, and the bands at 1504 and 1610 cm1 are characteristic of the benzenoid ring of aniline pentamer. An intensive carbonyl stretching band at 1745 cm1 and the characteristic C– O–C stretching vibration characterize the IR spectrum of PGA, as shown in Fig. 2B. All the charac-
Fig. 2. IR spectra of two monomers and copolymer: (A) aniline pentamer, (B) PGA and (C) copolymer.
teristic absorptions both in aniline pentamer and in PGA can be observed in the spectrum of the copolymer, as shown in Fig. 2C. 3.1.3. 1H NMR spectra of monomers and the copolymer Fig. 3 is the 1H NMR spectra of aniline pentamer, PGA, and the copolymer. The signals attributed to aniline pentamer are observed at 12.1 (HOOC–CH2–CH2–), 9.7 (–CH2–CO–NH–C6H4–), 8.0 (–C6H4–NH–C6H4–), 6.5–7.4 (benzenoid ring), 2.9 (HOOC–CH2–CH2–CO), and 2.7 (HOOC– CH2–CH2–CO). The proton signals attributed to PGA are observed at 5.5 (HO–CH2–CO–), 4.7–4.9 (–O–CH2–CO–), 4.1 (–OCH2CH2CH2CH2O–), and 1.6 (–OCH2CH2CH2CH2O–). Fig. 5b shows the 1 H NMR spectrum of the copolymer, all proton signals belonging to both aniline pentamer and PGA blocks are confirmed. The peaks at d 12.1 and d 5.5 are missing, so it can be concluded that the copolymer was synthesized from aniline pentamer and PGA by polycondensation. 3.2. The influence of the ratio between glycolide and 1,4-butandiol
Fig. 1. IR spectrum of PGA.
Ring-open polymerization between glycolide and 1,4-butandiol produced a hydroxyl- capped prepolymer. The molecular weight of the prepared prepolymer is determined by the amount of difunctional compound. The molecular weight of the resulting prepolymer becomes lower with the growth of the ratio of 1,4-butandiol [16,17]. If the molecular weight of the prepolymer were too high,
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Fig. 3. 1H NMR spectra of two monomers and copolymer: (A) aniline pentamer, (B) PGA and (C) copolymer.
it would be insoluble in NMP and difficult to polymerize with carboxyl-capped aniline pentamer. So a soluble hydroxyl-capped PGA is expected. In this work, the mol ratio between GA and 1,4-butandiol was controlled at 2:1, and the prepared prepolymer dissolves easily in NMP. 3.3. UV analysis In order to study the chemical oxidation process of the copolymer, the UV–vis spectra of the copoly-
mer were recorded. The obtained copolymer in the lecoemeraldine oxidation state was dissolved in DMF. Trace amount of (NH4)2S2O8 was added to it. The solution gradually turned to blue and then turned to mauve because of the oxidation of the copolymer. The UV–vis spectra are shown in Fig. 4. When the copolymer was in the lecoemeraldine oxidation state, there was only one peak at 330 nm which is associated with a p–p* transition of the conjugated ring system. The absorption started to undergo a blue shift gradually along with
C. Ding et al. / European Polymer Journal 43 (2007) 4244–4252
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Fig. 4. UV–vis spectra of the copolymer in NMP oxidized with ammonium persulfate: (A) the intensity of absorbance increased while being oxidized and (B) the intensity of absorbance decreased after reaching the maximum.
the oxidation, and the UV–vis spectra shows a new absorption at about 610 nm, which is assigned to the benzenoid to quiniod excitonic transition. The kmax of the second absorption began to undergo a blue shift (from 610 nm to 600 nm) after its intensity reached the maximum. A possible explanation (Scheme 2) of this phenomenon is as following [18]: the copolymer in leucoemeraldine oxidation state reached the first emeraldine oxidation state with each carboxyl-capped aniline pentamer segment containing only one quinoid ring. Then it was oxidized to second emeraldine oxidation state with each carboxyl-capped aniline pentamer segment containing two quinoid rings, which showed increase in intensity of the second absorption. There was a blue shift after the second absorption reaching the maximum in intensity, which showed that a pernigraniline oxidation state was formed. 3.4. CV analysis The redox processes of aniline pentamer and the copolymer were demonstrated by cyclic voltammetry. The cyclic voltammogram (Fig. 5b) of the copolymer shows three pairs of redox peaks. The first oxidation wave at around 0.23 V was ascribed to the transition from the leucoemeraldine oxidation state to the first emeraldine oxidation state, the second oxidation wave at around 0.36 V was assigned to the transition from the first emeraldine oxidation state to the second emeraldine oxidation state, and the last oxidation wave at around
Fig. 5. Cyclic voltammograms of aniline pentamer (a) and the copolymer (b). Measured on GC electrode (working electrode) in DMF (electrolyte: 0.1 M HCl).
0.67 V was due to the pernigraniline oxidation state. The differences of cyclic voltammograms between the copolymer and aniline pentamer in reversibility and peak potential are due to their structural difference. The incorporation of polyglycolide enhances the stability of the copolymer, so it is difficult to be oxidated and easy to be deoxidized.
3.5. Conductivity The conductivities of aniline pentamer (0.1 M in NMP, 0.1 M HCl doped) and the copolymer (0.1 M in NMP, 0.1 M HCl doped) were measured to be 39.6 lS cm1 and 32.81 lS cm1, respectively. The conductivity of the copolymer is a little lower than that of aniline pentamer because of the incorporation of glycolide. The UV–vis spectra of aniline pentamer and copolymer in leucoemeraldine oxidation
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Scheme 2. Molecular structures of electrically conducting copolymer at various oxidation states.
Fig. 6. UV–vis spectra of: aniline pentamer (A, 105 M) and copolymer (B, 105 M) in lecoemeraldine oxidation state in NMP.
state are shown in Fig. 6. There is little difference between the intensity of benzenoid ring absorption of aniline pentamer and the copolymer. So the electrically conducting mechanisms of the two substances might be similar. And the change of the conductivity is little.
Fig. 7. Degradation profile of copolymer at 37 C in PBS (pH 7.4) in terms of weight loss.
significant decline in weight in the first 30 days (from 0% to 40.9%). This can be explained by the cleavage of the ester bond in the backbone. The copolymer degraded rapidly in the first 30 days because of the well hydrophilicity of PGA. The degradation rate was slow in the following 60 days. In the last 30 days, no further degradation was observed, leaving a solid residue (fragments of aniline pentamer) of 49.8% of the initial mass of the specimen.
3.6. Degradation of the copolymer 3.7. Thermal analysis The degradation rate was evaluated by the weight loss of the polymers over predetermined time intervals. As shown in Fig. 7, the copolymer showed a
Conducting polymers can be used in many fields, such as electromagnetic shielding materials, electro-
C. Ding et al. / European Polymer Journal 43 (2007) 4244–4252
Fig. 8. TG and DTG curves of the copolymer: (A) TG curve and (B) DTG curve.
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of DCC and DMAP in NMP. Its chemical structure has been verified by IR and 1H NMR. The conductivity of the copolymer (0.1 M in NMP, 0.1 M HCl doped) is 32.81 lS cm1, which is similar to the aniline pentamer. The redox process of the copolymer was studied by UV–vis spectra and cyclic voltammetry. It was oxidated by oxidizing agent from the leucoemeraldine oxidation state to the first emeraldine oxidation state, and then from the first emeraldine oxidation state to the second emeraldine oxidation state, and last from the second emeraldine oxidation state to pernigraniline oxidation state. TGA result indicates that the copolymer prepared has good thermal stability. Acknowlegements
luminescent materials, artificial muscles and biosensors. So the thermal stability of conducting polymers is very important for their potential use in industry and biology [19]. TG and DTG analysis are significant dynamic ways of detecting degradation behavior; the weight loss of a polymer sample is measured continuously, whereas the temperature is changed at a constant way. In order to evaluate the thermal stability of copolymer, the thermal analysis of as-formed copolymer was tested, as shown in Fig. 8. The thermal analysis was performed under a nitrogen stream in the temperature of 295–1173 K with a heating rate of 10 K min1. When the temperature reached 363 K, the weight loss of the copolymer was nearly equal to about 3.2%. This degradation can be ascribed to water evaporation or other moisture trapped in the copolymer. When the temperature increased up to about 474 K, there was a slight weight loss of 6.15%, which may be caused by the volatilization of oligomers. It can be seen clearly that there are two evident degradation processes for copolymers. They occurred at 538 K and 726 K. These prominent weight losses were caused by the degradation of the skeletal copolymer backbone chain structure. Based on these results described above, it can reasonably draw the conclusion that the copolymer has good thermal stability for the application as a conducting material. 4. Conclusion A novel biodegradable block copolymer has been synthesized by low-temperature polycondensation between PGA and aniline pentamer in the presence
The author acknowledges the financial support of this work by the Natural Science Foundation of Shandong Province, China (Y2006B07), the Key Technologies R&D Programme of Shandong Province, China (2006GG2203024), and the Program for New Century Excellent Talents in University (No. NCET-04-0649). References [1] Ciombor DM, Aaron R K. Influence of electromagnetic fields on endochondral bone formation. J Cell Biochem 1993;52(1):37–41. [2] Aaron RK, Ciombor DM. Therapeutic effects of electromagnetic fields in the stimulation of connective tissue repair. J Cell Biochem 1993;52(1):42–6. [3] Pomeranz B, Campbell JJ. Weak electric current accelerates motoneuron regeneration in the sciatic nerve of 10-monthold rats. Brain Res 1993;603:271. [4] Soares BG, Leyva ME, Barra GMO, Khastgir D. Dielectric behavior of polyaniline synthesized by different techniques. Eur Poly J 2006;42(3):676–86. [5] Erdem E, Karakisßla M, Sac¸ak M. The chemical synthesis of conductive polyaniline doped with dicarboxylic acids. Eur Poly J 2004;40(4):785–91. [6] Waugaman M, Sannigrahi B, McGeady P, Khan IM. Synthesis, characterization and biocompatibility studies of oligosiloxane modified polythiophenes. Eur Poly J 2003;39(7):1405–12. [7] Saidman SB. The potentiometric response of polypyrrole electrosynthesised in alkaline media. Eur Poly J 2005;41(3):433–7. [8] Wang C, Gao JB, Chen CH, et al. Stabilization and anticorrosion property of phenyl-capped and aniline tetramer as additives to common coating. Polym Prepr 2000;40(2):1746–7. [9] MacDiamid AG, Zhang WJ, Feng J, Huang F, Hsieh B. Application of thin films of conjugated oligomers and polymers in electronic devices. Polym Prepr 1998;39(1):80–1.
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[10] Hirao T, Fukuhara S. An organic catalytic system for dehydrogenative oxidation. J Org Chem 1998;63:7534–7. [11] Rivers Tyrell J, Hudson Terry W, Schmidt Christine E. Synthesis of a novel, biodegradable electrically conducting polymer for biomedical applications. Adv Funct Mater 2002;1:33–7. [12] Shi GX, Rouabhia Mahmoud, Wang ZX, Dao Leˆ H, Zhang Z. Synthesis of amino-acid-containing polyacids and their application in self-cured glass-ionomer cement. Biomaterials 2004;25:2477–88. [13] Huang Lihong, Hu Jun, Lang Le, et al. Synthesis and characterization of electroactive and biodegradable ABA block copolymer of polylactide and aniline pentamer. Biomaterials 2007;28:1741–51. [14] Chen L, Yu YH, Mao HP, Lu XF, Zhang WJ. Synthesis of amino-capped aniline pentamer and UV–Vis spectral study. Chem J Chin Univ 2004;25:1768–70.
[15] Putnam David, Langer Robert. Poly(4-hydroxy-L-proline ester): low-temperature polycondensation and plasmid DNA complexation. Macromolecules 1999;32:3658–62. [16] Hiltunen K, Seppa¨la¨ J, Ha¨rko¨nen M. Use of hydroxyl terminated prepolymer in urethane synthesis. J Appl Polym Sci 1997;63:1091–100. [17] Hiltunen K, Ha¨rko¨nen M, Seppa¨la¨ J, Va¨a¨na¨nen T. Synthesis and characterization of lactic acid based telechelic prepolymers. Macromolecules 1996;29:8677–82. [18] Chao DM, Lu XF, Chen JY, et al. New method of synthesis of electroactive polyamide with amino-capped aniline pentamer in the main chain. J Polym Sci: Part A 2006;44(1):477–82. [19] Zhang Shusheng, Hou Jian, Zhang Rui, et al. Electrochemical polymerization of 3,4-ethylenedioxythiophene in aqueous solution containing N-dodecyl-b-D-maltoside. Eur Poly J 2006;42:149–60.
European Polymer Journal 43 (2007) 4244–4252
www.elsevier.com/locate/europolj
Synthesis and characterization of degradable electrically conducting copolymer of aniline pentamer and polyglycolide Caifeng Ding, Yan Wang, Shusheng Zhang
*
College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China Received 18 June 2007; received in revised form 23 July 2007; accepted 24 July 2007 Available online 31 July 2007
Abstract A novel block copolymer of polyglycolide (PGA) and aniline pentamer that is electro-active and degradable was synthesized. The copolymer was prepared by low-temperature polycondensation between carboxyl-capped aniline pentamer and hydroxyl-capped PGA, with N,N 0 -dicyclohexylcarbodiimide (DCC) as dehydrating agent. The structures of monomers and the copolymer were characterized by IR, 1H NMR. The chemical redox process of the as-prepared copolymer was studied by UV–vis spectra and cyclic voltammetry (CV). Thermal analysis and the study of degradation show that the copolymer has good thermal stability and degradability. 2007 Elsevier Ltd. All rights reserved. Keywords: Aniline pentamer; Polyglycolide; Conducting polymers; Degradation
1. Introduction Electrical stimulation can enhance the excretion and expression of growth factors and accelerate cellular proliferation and differentiation. It can also accelerate the healing of bone, cartilage, and peripheral nerves [1–3]. Many attempts have been made to seek new materials that incorporate electrical signals, which would deliver an electrical stimulus at the site of damage and provide a physical template for cell growth and tissue repair. Electrically conducting polymers have attracted much interest in the last 20 years because they not only have the physical and chemical properties of
*
Corresponding author. Tel./fax: +86 532 84022750. E-mail address: [email protected] (S. Zhang).
organic polymers but also have the electrical characteristics of metals. There are many conducting polymers, such as polyaniline, polyacetylene, polythiophene and polypyrrole, were reported in recent years [4–7]. Of all known conducting polymers, polyaniline is applied in many areas, owning to its excellent electrical properties, good environmental stability, facile synthesis, and low cost of the monomer. The structural and electrical properties of aniline oligomer are similar to polyaniline, but the solubility is better. Aniline oligomers can be used in various fields, such as anticorrosion for metals, chemical sensors and catalytic oxidation [8–10]. Degradable electrically conducting polymers possess the unique properties of being electrically conducting and degradable, so they have a promising prospect to be used as neural conducting tissue
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.07.032
C. Ding et al. / European Polymer Journal 43 (2007) 4244–4252
engineering material which can conduct bioelectrical signals in vivo. But the biodegradable electrical conductive materials were reported occasionally. A biodegradable electrically conducting polymer, which was synthesized from conducting oligomers of pyrrole and thiophene that were connected via degradable ester linkages, was studied by Rivers et al. [11]. Some properties, such as conductivity, degradability and biocompatibility of the polymer were researched. Another electrically conductive and biodegradable composite made of polypyrrole nanoparticles and polylactide was researched by Zhang and co-workers [12]. In their work, electrical stability of the composite in a physiological environment was investigated, and hypothesis that direct electron current applied through such a conductive composite can affect cellular activities was tested. Huang et al. [13] synthesized and characterized an electro-active and biodegradable copolymer of polylactide and aniline pentamer. They proved that the copolymer is electroactive, degradable and biocompatible, and concluded that the copolymer can support cells attachment and proliferation. Polyglycolide and polylactide are biodegradable and biocompatible materials that can be used as medical biomaterials. In this work, a block copolymer of polyglycolide (PGA) and aniline pentamer, which is electro-active and degradable, was synthesized. The redox process of the copolymer was studied by UV–vis spectra and cyclic voltammetry (CV). Conductivity, thermal stability and degradability were also evaluated. 2. Experimental 2.1. Materials N-Phenyl-p-phenylenediamine (98%, Sigma, USA), p-phenylenediamine (Shanghai Shengqiang Industrial Co., Ltd., China), glycolide (GA, Beijing Kangan macromolecule Research Institute, China), 1,4-butanediol (Tianjin Bodi Chemical Reagent Company, China), N-methyl-pyrrolidone (NMP), N,N 0 -dimethylformamide (DMF) (Tianjin Ruijinte Chemical Reagent Company, China), stannous octanoate (Shanghai Shengyu Chemical Reagent Company, China), 4-dimethylaminopyridine (DMAP, Yangzhou Feiyang Chemical Reagent Company, China). succinic anhydride and ammonium persulfate, made by Shanghai Aijian Chemical Reagent Plant (China). The chemicals used above were analytical grade. N,N 0 -Dicyclohexylcarbodii-
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mide (DCC, Guoyao Chemical Reagent Company, China) was reagent grade. All chemicals were used as received without further purification. 2.2. Apparatus Infrared spectra were recorded using a Nicolet 510P FT-IR spectrometer (USA) with KBr pellets. The 1H NMR spectra were recorded on a Bruker AV500 NMR spectrometer (Switzerland) and d6DMSO was used as the solvent, tetramethylsilane (TMS) was used as an internal standard. UV–vis spectra were taken by a Cary 50 UV–vis-NIR spectrophotometer (Australia). The thermogravimetric analysis (TGA) was performed with a thermal analyzer of NETZSCH TG209 (Germany) in a nitrogen atmosphere between 295 K and 1173 K with a temperature rate of 10 K/min. Conductivity was measured on a DDS-307 conductometer (China). 2.3. Synthesis of monomers and copolymer The schematic diagrams for the synthesis of monomers and the copolymer are shown in Scheme 1. 2.3.1. Preparation of carboxyl-capped aniline pentamer 4-Oxo-4-(4-(phenylamino)phenylamino)butanoic acid and aniline pentamer were prepared as described previously [14]. Briefly, succinic anhydride (0.05 mol) and N-phenyl-p-phenylenediamine (0.05 mol) were dissolved in dichloromethane, respectively. And then the latter was added dropwise into the former. The reaction was carried out at room temperature for 3 h under nitrogen atmosphere. After filtration, 4-oxo-4-(4-(phenylamino)phenylamino)butanoic acid obtained was washed with diethyl ether and then dried at 30 C under vacuum for 12 h. 4-Oxo-4-(4-(phenylamino)phenylamino)butanoic acid (0.01 mol) and p-phenylenediamine (0.005 mol) were all dissolved in DMF (15 mL). A mixture solution (30 mL DMF; 25 mL distilled water; 5 mL concentrated hydrochloric acid) and ammonium persulfate (0.01 mol in 1 M HCl) were added into the above solution by sequence. The reaction was carried out at 16 C with stirring for 4 h. Then the mixture was poured into distilled water (300 mL). After filtration, the precipitate was suspended in ammonia (300 mL, 1 M) by stirring, hydrazine hydrate was added in the same time. The solution was filtrated after changing the pH of the solution
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Scheme 1. Structure and synthetic reaction scheme for degradable electrically conducting copolymer.
to 2. The precipitate was dried at 45 C in vacuum for 48 h. The solution of the precipitate in DMF was added drop-wise into ethanol. Aniline pentamer was obtained by filtration, dried in vacuum and then soxhleted with 1,2-dichloroethane and THF.
ether. The copolymer was dissolved in chloroform (7 mL), and then added drop-wise into anhydrous diethyl ether (150 mL) quickly. The copolymer isolated by filtration was dried thoroughly at 30 C under vacuum.
2.3.2. Preparation of hydroxyl-capped polyglycolide Glycolide (0.07 mol), 1,4-butandiol (0.035 mol) and stannous octanoate (0.0004 mol) were added in turn to n-butyl acetate (120 mL). Hydroxylcapped polyglycolide was obtained after refluxed at 110 C for 10 h.
2.4. Conductivity measurements
2.3.3. Synthesis of the copolymer The copolymer was synthesized by low-temperature polymerization, as described by Putnam et al. [15]. Under a nitrogen atmosphere, hydroxylcapped polyglycolide (1 mmol) and carboxylcapped aniline pentamer (1 mmol) were dissolved in N-methyl-2-pyrrolidone (NMP, 7.5 mL). DMAP (1 mmol) and DCC (4 mmol) were added into the mixture solution by sequence. The reaction was carried out at 0 C with stirring for 48 h, and then filtered. The filtrate was added drop-wise into anhydrous diethyl ether (75 mL) and stirred for 5 min. The copolymer congealed on the bottom of the flask and was isolated by decanting the diethyl
Aniline pentamer (doped with 0.1 M HCl) and the copolymer (doped with 0.1 M HCl) were dissolved in NMP (0.1 M) respectively. The conductivities of the two solutions and NMP were measured on a DDS-307 conductometer. The conductivity was the difference between solution and solvent. 2.5. Degradation Tests The degradation behavior of the as-prepared copolymer was studied over a period of 4 months. All specimens possess the same dimensions (8mm · 4mm · 0.2 mm). Each specimen was immersed in a vial filled with 20 mL of phosphate buffer solution (pH 7.4) and incubated at 37 C under static conditions. PBS was changed every 7 days. The specimens were taken out one by one at regular intervals, dried at 30 C to constant weight in order to determine the weight loss.
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The percentage of weight loss (WL) was calculated as follows: WLt ¼ ðm0 mf Þ=m0 100%; where m0 is the initial mass (t = 0), mf is the final mass after degradation and drying, t is the degradation time. 3. Results and discussion 3.1. Characterization of synthesized monomers and copolymer 3.1.1. IR of PGA The IR spectrum of PGA is shown in Fig. 1. It can be observed from the spectrum that the characteristic absorption peak of PGA is at 1237 cm1, which is the absorption peak of C–O–C in straight chain, but not at 1297 cm1, the characteristic absorption peak of C–O–C in cycloaliphatics. This indicates that the ring-open polymerization of GA had occurred. 3.1.2. IR spectra of aniline pentamer, PGA and copolymer Fig. 2 shows the IR spectra of the copolymer together with its corresponding precursors as a typical example. As shown in Fig. 2A, the characteristic carbonyl stretching vibrations are observed at 1657 and 1696 cm1, and the bands at 1504 and 1610 cm1 are characteristic of the benzenoid ring of aniline pentamer. An intensive carbonyl stretching band at 1745 cm1 and the characteristic C– O–C stretching vibration characterize the IR spectrum of PGA, as shown in Fig. 2B. All the charac-
Fig. 2. IR spectra of two monomers and copolymer: (A) aniline pentamer, (B) PGA and (C) copolymer.
teristic absorptions both in aniline pentamer and in PGA can be observed in the spectrum of the copolymer, as shown in Fig. 2C. 3.1.3. 1H NMR spectra of monomers and the copolymer Fig. 3 is the 1H NMR spectra of aniline pentamer, PGA, and the copolymer. The signals attributed to aniline pentamer are observed at 12.1 (HOOC–CH2–CH2–), 9.7 (–CH2–CO–NH–C6H4–), 8.0 (–C6H4–NH–C6H4–), 6.5–7.4 (benzenoid ring), 2.9 (HOOC–CH2–CH2–CO), and 2.7 (HOOC– CH2–CH2–CO). The proton signals attributed to PGA are observed at 5.5 (HO–CH2–CO–), 4.7–4.9 (–O–CH2–CO–), 4.1 (–OCH2CH2CH2CH2O–), and 1.6 (–OCH2CH2CH2CH2O–). Fig. 5b shows the 1 H NMR spectrum of the copolymer, all proton signals belonging to both aniline pentamer and PGA blocks are confirmed. The peaks at d 12.1 and d 5.5 are missing, so it can be concluded that the copolymer was synthesized from aniline pentamer and PGA by polycondensation. 3.2. The influence of the ratio between glycolide and 1,4-butandiol
Fig. 1. IR spectrum of PGA.
Ring-open polymerization between glycolide and 1,4-butandiol produced a hydroxyl- capped prepolymer. The molecular weight of the prepared prepolymer is determined by the amount of difunctional compound. The molecular weight of the resulting prepolymer becomes lower with the growth of the ratio of 1,4-butandiol [16,17]. If the molecular weight of the prepolymer were too high,
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Fig. 3. 1H NMR spectra of two monomers and copolymer: (A) aniline pentamer, (B) PGA and (C) copolymer.
it would be insoluble in NMP and difficult to polymerize with carboxyl-capped aniline pentamer. So a soluble hydroxyl-capped PGA is expected. In this work, the mol ratio between GA and 1,4-butandiol was controlled at 2:1, and the prepared prepolymer dissolves easily in NMP. 3.3. UV analysis In order to study the chemical oxidation process of the copolymer, the UV–vis spectra of the copoly-
mer were recorded. The obtained copolymer in the lecoemeraldine oxidation state was dissolved in DMF. Trace amount of (NH4)2S2O8 was added to it. The solution gradually turned to blue and then turned to mauve because of the oxidation of the copolymer. The UV–vis spectra are shown in Fig. 4. When the copolymer was in the lecoemeraldine oxidation state, there was only one peak at 330 nm which is associated with a p–p* transition of the conjugated ring system. The absorption started to undergo a blue shift gradually along with
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Fig. 4. UV–vis spectra of the copolymer in NMP oxidized with ammonium persulfate: (A) the intensity of absorbance increased while being oxidized and (B) the intensity of absorbance decreased after reaching the maximum.
the oxidation, and the UV–vis spectra shows a new absorption at about 610 nm, which is assigned to the benzenoid to quiniod excitonic transition. The kmax of the second absorption began to undergo a blue shift (from 610 nm to 600 nm) after its intensity reached the maximum. A possible explanation (Scheme 2) of this phenomenon is as following [18]: the copolymer in leucoemeraldine oxidation state reached the first emeraldine oxidation state with each carboxyl-capped aniline pentamer segment containing only one quinoid ring. Then it was oxidized to second emeraldine oxidation state with each carboxyl-capped aniline pentamer segment containing two quinoid rings, which showed increase in intensity of the second absorption. There was a blue shift after the second absorption reaching the maximum in intensity, which showed that a pernigraniline oxidation state was formed. 3.4. CV analysis The redox processes of aniline pentamer and the copolymer were demonstrated by cyclic voltammetry. The cyclic voltammogram (Fig. 5b) of the copolymer shows three pairs of redox peaks. The first oxidation wave at around 0.23 V was ascribed to the transition from the leucoemeraldine oxidation state to the first emeraldine oxidation state, the second oxidation wave at around 0.36 V was assigned to the transition from the first emeraldine oxidation state to the second emeraldine oxidation state, and the last oxidation wave at around
Fig. 5. Cyclic voltammograms of aniline pentamer (a) and the copolymer (b). Measured on GC electrode (working electrode) in DMF (electrolyte: 0.1 M HCl).
0.67 V was due to the pernigraniline oxidation state. The differences of cyclic voltammograms between the copolymer and aniline pentamer in reversibility and peak potential are due to their structural difference. The incorporation of polyglycolide enhances the stability of the copolymer, so it is difficult to be oxidated and easy to be deoxidized.
3.5. Conductivity The conductivities of aniline pentamer (0.1 M in NMP, 0.1 M HCl doped) and the copolymer (0.1 M in NMP, 0.1 M HCl doped) were measured to be 39.6 lS cm1 and 32.81 lS cm1, respectively. The conductivity of the copolymer is a little lower than that of aniline pentamer because of the incorporation of glycolide. The UV–vis spectra of aniline pentamer and copolymer in leucoemeraldine oxidation
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Scheme 2. Molecular structures of electrically conducting copolymer at various oxidation states.
Fig. 6. UV–vis spectra of: aniline pentamer (A, 105 M) and copolymer (B, 105 M) in lecoemeraldine oxidation state in NMP.
state are shown in Fig. 6. There is little difference between the intensity of benzenoid ring absorption of aniline pentamer and the copolymer. So the electrically conducting mechanisms of the two substances might be similar. And the change of the conductivity is little.
Fig. 7. Degradation profile of copolymer at 37 C in PBS (pH 7.4) in terms of weight loss.
significant decline in weight in the first 30 days (from 0% to 40.9%). This can be explained by the cleavage of the ester bond in the backbone. The copolymer degraded rapidly in the first 30 days because of the well hydrophilicity of PGA. The degradation rate was slow in the following 60 days. In the last 30 days, no further degradation was observed, leaving a solid residue (fragments of aniline pentamer) of 49.8% of the initial mass of the specimen.
3.6. Degradation of the copolymer 3.7. Thermal analysis The degradation rate was evaluated by the weight loss of the polymers over predetermined time intervals. As shown in Fig. 7, the copolymer showed a
Conducting polymers can be used in many fields, such as electromagnetic shielding materials, electro-
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Fig. 8. TG and DTG curves of the copolymer: (A) TG curve and (B) DTG curve.
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of DCC and DMAP in NMP. Its chemical structure has been verified by IR and 1H NMR. The conductivity of the copolymer (0.1 M in NMP, 0.1 M HCl doped) is 32.81 lS cm1, which is similar to the aniline pentamer. The redox process of the copolymer was studied by UV–vis spectra and cyclic voltammetry. It was oxidated by oxidizing agent from the leucoemeraldine oxidation state to the first emeraldine oxidation state, and then from the first emeraldine oxidation state to the second emeraldine oxidation state, and last from the second emeraldine oxidation state to pernigraniline oxidation state. TGA result indicates that the copolymer prepared has good thermal stability. Acknowlegements
luminescent materials, artificial muscles and biosensors. So the thermal stability of conducting polymers is very important for their potential use in industry and biology [19]. TG and DTG analysis are significant dynamic ways of detecting degradation behavior; the weight loss of a polymer sample is measured continuously, whereas the temperature is changed at a constant way. In order to evaluate the thermal stability of copolymer, the thermal analysis of as-formed copolymer was tested, as shown in Fig. 8. The thermal analysis was performed under a nitrogen stream in the temperature of 295–1173 K with a heating rate of 10 K min1. When the temperature reached 363 K, the weight loss of the copolymer was nearly equal to about 3.2%. This degradation can be ascribed to water evaporation or other moisture trapped in the copolymer. When the temperature increased up to about 474 K, there was a slight weight loss of 6.15%, which may be caused by the volatilization of oligomers. It can be seen clearly that there are two evident degradation processes for copolymers. They occurred at 538 K and 726 K. These prominent weight losses were caused by the degradation of the skeletal copolymer backbone chain structure. Based on these results described above, it can reasonably draw the conclusion that the copolymer has good thermal stability for the application as a conducting material. 4. Conclusion A novel biodegradable block copolymer has been synthesized by low-temperature polycondensation between PGA and aniline pentamer in the presence
The author acknowledges the financial support of this work by the Natural Science Foundation of Shandong Province, China (Y2006B07), the Key Technologies R&D Programme of Shandong Province, China (2006GG2203024), and the Program for New Century Excellent Talents in University (No. NCET-04-0649). References [1] Ciombor DM, Aaron R K. Influence of electromagnetic fields on endochondral bone formation. J Cell Biochem 1993;52(1):37–41. [2] Aaron RK, Ciombor DM. Therapeutic effects of electromagnetic fields in the stimulation of connective tissue repair. J Cell Biochem 1993;52(1):42–6. [3] Pomeranz B, Campbell JJ. Weak electric current accelerates motoneuron regeneration in the sciatic nerve of 10-monthold rats. Brain Res 1993;603:271. [4] Soares BG, Leyva ME, Barra GMO, Khastgir D. Dielectric behavior of polyaniline synthesized by different techniques. Eur Poly J 2006;42(3):676–86. [5] Erdem E, Karakisßla M, Sac¸ak M. The chemical synthesis of conductive polyaniline doped with dicarboxylic acids. Eur Poly J 2004;40(4):785–91. [6] Waugaman M, Sannigrahi B, McGeady P, Khan IM. Synthesis, characterization and biocompatibility studies of oligosiloxane modified polythiophenes. Eur Poly J 2003;39(7):1405–12. [7] Saidman SB. The potentiometric response of polypyrrole electrosynthesised in alkaline media. Eur Poly J 2005;41(3):433–7. [8] Wang C, Gao JB, Chen CH, et al. Stabilization and anticorrosion property of phenyl-capped and aniline tetramer as additives to common coating. Polym Prepr 2000;40(2):1746–7. [9] MacDiamid AG, Zhang WJ, Feng J, Huang F, Hsieh B. Application of thin films of conjugated oligomers and polymers in electronic devices. Polym Prepr 1998;39(1):80–1.
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