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Synthesis, characterization and H2O2-sensing properties of pyrimidine-based hyperbranched polyimides
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 121–127
www.elsevier.com/locate/europolj
Synthesis, characterization and H2O2-sensing properties of pyrimidine-based hyperbranched polyimides Su¨leyman Ko¨ytepe, Aziz Pasßahan, Ergun Ekinci, Turgay Sec¸kin
*
_ ¨ nu¨ University, 44280 Malatya, Turkey Faculty of Science and Literature, Department of Chemistry, Ino Received 4 May 2004; received in revised form 5 August 2004; accepted 6 August 2004 Available online 21 September 2004
Abstract A series of hyperbranched polyimides having pyrimidine core in the backbone was prepared by solution polycondensation of pyromellitic dianhydride (A2 type) and 2,4,6-triaminopyrimidine (B3 type). The resulting polyimides were characterized by IR, GPC, DTA, DSC, TGA, density and viscosity measurement. The results showed that polyimides were obtained in 82–86% yield, had inherent viscosities of 1.58–1.87 dL/g and highly thermal stability. From the solubility tests, it was found that all the polyimides were soluble in NMP, DMAc and DMSO at room temperature. In addition, because of their interesting film properties, attempts were made to prepare H2O2-sensing polyimide material. For this reason, selectivity of film-coated electrodes obtained from the chemically prepared hyperbranched polyimides toward electroactive (ascorbic acid, oxalic acid and hydrogen peroxide) and non-electroactive (lactose, sucrose and urea) species was examined by means of CV, DPV and TB techniques. From the electrochemical data obtained, it has been demonstrated that polyimide (HPI-2) film responded to only hydrogen peroxide among the species examined. In other words, it has been claimed that the mentioned polyimide-coated electrode can be used as H2O2-selective membrane in the presence of the electroactive and non-electroactive interferents. 2004 Elsevier Ltd. All rights reserved. Keywords: Polyimide; Film; Selective membrane
1. Introduction In recent years, polyimide-based materials have attracted great interest, both in industry and in academia, because they exhibit unique properties not shared by conventional polymers. The addition of the functional group into polymeric backbone gives rise to sensible increases in permeability, abrasion and heat resistance. The physical nature of the polyimides necessarily gives
* Corresponding author. Tel.: +90 422 3410037; fax: +90 422 3410758/3410037. E-mail address: [email protected] (T. Sec¸kin).
rise to their application depending of interactions between the polymer and the media [1–14]. The improved permselective behavior of polyimides is only achieved when the proper combination of the functionality is applied in the polymeric backbone. In our recent studies, the permselective properties of polyimides prepared from heterocyclic counterparts and hyperbranched core have been reported elsewhere [15–17]. In applications of the amperometric biosensor based on oxido-reductase enzymes such as glucose oxidase [18–20], alcohol oxidase [21] and galactose oxidase [22,23], it is measured hydrogen peroxide obtained as a result of enzymatic reaction between enzyme and the relevant substrate. However, one of the most important
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.08.007
122
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
solvent. All the electrochemical operations such as cyclic voltammetry (CV), differential puls voltammetry (DPV) and amperometric measurements (TB) were performed by a BAS (Bioanalytical Systems, Inc.) 100 W electrochemical analyzer. The standard three-electrode system consisting of a Pt working electrode (CHI, 2 mm diameter), an Ag/AgCl reference electrode (CHI) and a Pt wire coil auxiliary electrode was used. The pH was measured with a Jenway 3010 pH meter. All chemicals for the synthesis were purchased from Aldrich/Sigma Co. and used after purification. NMP was distilled over CaH2 under ˚ molecular sieves. reduced pressure and stored over 4 A Reagent grade aromatic dianhydride, pyromellitic dianhydride (PMDA), was sublimed at 180 C under reduced pressure for 10 h (Tm = 285 C), and dried under vacuum at 120 C prior to use. 2,4,6-Triaminopyrimidine (Tm = 250 C) was used as received. Ascorbic acid, oxalic acid, hydrogen peroxide, lactose, sucrose and urea were of analytical grade and supplied either by Sigma Chemical Company or E. Merck. Electrochemical tests were made in a PBS (phosphate buffer salts, pH = 7) solution. Ascorbic acid solution was prepared freshly before experiment. All aqueous solutions were prepared with deionized and doubly distilled water.
problems when measuring hydrogen peroxide is the presence of the electroactive and non-electroactive interferents. The mentioned problem can be overcome by using selective-polymeric membranes. Ekinci and coworkers have previously reported the use of the electrochemically-modified polymeric films for this aim [24–26]. In this paper, we report synthesis and characterization of hyperbranched polyimides with pyrimidine core. Moreover, it has been examined that whether chemically-modified polyimide electrode could be used as a permselective membrane for hydrogen peroxide in the presence of the interferents.
2. Experimental 2.1. Apparatus and materials Infrared spectra were recorded as KBr pellets in the range 4000–400 cm 1 on an Ati Unicam Mattson 1000 Fourier Transform Infrared Spectrometer. Differential scanning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetry (TG) were performed with Shimadzu DSC-60, DTA-50 and TGA-50 thermal analyzers, respectively. DSC, DTA and TGA analysis were performed with a heating rate of 10 C/min in nitrogen atmosphere. GPC analyses were performed at 30 C using NMP as eluant at a flow rate of 0.5 mL/ min. A differential refractometer was used as a detector. The instrument (Agilent 1100 series GPC-SEC system) was calibrated with a mixture of polystyrene standards (polysciences; molecular masses between 200 and 1,200,000 Da) using GPC software for the determination of the average molecular masses and the polydispersity of the polymer samples. Inherent viscosities (ginh = lngr/c at polymer concentration of 0.5 g/dL) were measured with an Ubbelohde suspended-level viscometer at 30 C using NMP as the
2.2. Synthesis of polyimides A typical polyimide (HPI-1 to HPI-3) synthesis was performed as follows: 2,4,6-triaminopyrimidine (0.51 g, 4.15 mmol) was dissolved in NMP (15 mL) in a 50 mL Schlenck tube equipped with a nitrogen line, overhead stirrer, a xylene filled Dean–Stark trap, and a condenser. PMDA (the ratio of amine: anhydride was varied as shown in Table 1) was added to the amine solution for a period of 4 h at 30 C and stirred overnight to give a viscous solution. The mixture was heated to 70 C, xylene (5 mL) was added, and the mixture was refluxed for 3 h. Following the removal of xylene by distillation, the reaction mixture was cooled at room temperature
Table 1 Basic properties of polyimides Polymer
n/m
Yield (%)
d (g/cm3)a
g (dL/g)b
Mn · 10
HPI-1 HPI-2 HPI-3
1/1 1/2 1/3
82 83 86
1.43 1.50 1.64
1.58 1.64 1.87
2.23 3.14 4.35
5.91 12.12 28.18
2.65 3.86 6.48
NMP
DMAc
Ether
THF
Hexane
DMSO
DMF
+ + +
+ + +
+ + +
+ +
Solubilityc HPI-1 HPI-2 HPI-3
± ±
5
Mw · 10
5
PDI
a
Determined by suspension method at 30 C. Measured at a concentration of 0.5 g/dL in NMP at 30 C. c Solubility tested at 2% solid concentration; + soluble at room temperature; ± soluble upon heating; molar ratios of amine and anhydride, respectively. b
insoluble. m and n refer the
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
123
Scheme 1. Synthesis of polyimide (HPI-2).
2.3. Preparation of polyimide film-coated electrodes and permselectivity measurements Prior to polymeric film coating, the surface of the Pt disc working electrodes was cleaned according to the standard procedure and polished with consecutive finer grades of aqueous alumina slurry down to 0.5 lm [27]. Polyimide films were prepared onto the electrodes by dropping 10 ll of polyimide solution. Then, polymer-modified surface was allowed to dry at room temperature for three days. Before the permselectivity measurements, the background current was allowed to reduce to steady state. Then, the amperometric behavior of the polymeric electrodes toward the electroactive and non-electroactive substances was recorded as a function of time at a potential of 0.7 V vs. Ag/AgCl.
3. Results and discussion Hyperbranched polymers are generally prepared from AB2 type monomers where A and B represent two types of functional groups, however it is not possible to prepare hyperbranched polyimides from a monomer of this type, since both the anhydride and the amine groups are very reactive to each other. The copolymerization of A2 and B3 type monomers is an alternative method for preparing hyperbranched polyimides, the cross-linking is prevented by controlling the monomer concentration [28]. Pyromellitic dianhydride (PMDA) as the A2 and the 2,4,6-triaminopyrimidine as B3 were used to prepare a series of hyperbranched polyimides. The synthesis involves two-
step polycondensation method in NMP as shown in Scheme 1. The polymerization was influenced by many factors such as monomer addition, monomer molar ratio and concentrations. In order to prevent gelation, the dianhydride monomer should be added so slowly in order to avoid any high local concentration in all cases. It was found that the total solid content should be kept as 0.2 mol/L. The amine terminated polyamic acids were thermally imidized in solution in the presence of xylene. The solubility properties and inherent viscosities of the prepared hyperbranched polyimides were shown in Table 1. All polymers were soluble in NMP, DMAc and DMSO at room temperature. The inherent viscosities
H P I-1
H P I-2 Transmittance (%T)
and the product was precipitated by addition of a large excess of methanol. The dark yellow product was isolated and dried at 100 C under vacuum for 10 h and then at 220 C under nitrogen for 2 h. In brief, polyimides, HPI-1, HPI-2 and HPI-3 were prepared by changing the amine:anhydride ratio from 1:1 to 1:3, respectively.
H P I-3
4000
3400
2200 1600 2800 Wavenumber (cm-1)
Fig. 1. FTIR spectra of polyimides.
1000
400
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S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
DTA uV
TGA %
100.00
PMDA 565.4C
396.4C
286.1C
80.00
504.2C 60.00
HPI-1
599.6C HPI-3
40.00 HPI-2 HPI-1
20.00
496.5C HPI-2
0.00 0.00
606.2C
200.00
400.00
600.00
800.00
Temp [C]
Fig. 3. TGA curves of HPI-1 to HPI-3 at heating rate of 10 C/ min under nitrogen flow.
452. 9C
HPI-3
200
0
40
800
600
Temp [C]
Fig. 2. DTA thermograms of the PMDA and polyimides.
of the polyimides showed that all polymers were in high molecular range, which was also rationalized with GPC data with the dispersity index 2.65 and 3.86 for HPI-1 and HPI-2 respectively, and 6.48 for HPI-3. The increase in polydispersity index is attributed to the high degree of swelling of the polymer backbone in the solvent.
Fig. 1 shows the FTIR spectra of the polyimides, HPI-1 to HPI-3. The characteristic absorption bands of the imide group are observed at 729, 1377, 1720 and 1786 cm 1. The terminal amino groups (N–H stretching of amino groups) generated a broad absorption band at 2700–3500 cm 1 that also ascertains that hyperbranching was achieved effectively. Fig. 2 shows the differential thermal analysis (DTA) spectra of the PMDA and the hyperbranched polyimides, HPI-1 to HPI-3. Two transition regions were observed in all cases, one is at 504 C, 496 C and 452 C, the other transition
Table 2 Thermal properties of polyimides Polymer
On set
End set
% 10a
Charb
IDTc
TGA analysis HPI-1 HPI-2 HPI-3
444 462 468
669 730 708
485 493 503
26.1 34.3 41.0
420 245 437
TDPe
On set
End set
Heat (kJ/g)
565 600 606
518 520 532
617 614 655
3.16 4.56 4.01
Tg (C)f
On set
End set
Transition (mW)
218 190 183
215 161 168
223 194 185
d
DTA analysis HPI-1 HPI-2 HPI-3
DSC analysis HPI-1 HPI-2 HPI-3 a
1.66 1.07 1.03
Temperature of 10% weight loss was assessed by TGA at a heating rate of 10 C/min in N2. Assessed by TGA at 800 C in N2. c IDT (initial decomposition temperature) is the temperature at which an initial loss of mass observed. d DTA thermograms of polyimides with a heating rate of 10 C/min in air. e TDP (thermal decomposition peak). f Determined by DSC in N2. b
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
125
Fig. 4. CVs of 4 mM ascorbic acid at the bare Pt and polyimide-coated electrode.
is observed at 565 C, 599 C and 606 C for HPI-1 to HPI3, respectively. DSC analysis indicated that the highest Tg was 218 C for HPI-1, it might be the reason for the chain entanglement that is more likely to take place when the monomer ratio was 1 to 1. Fig. 3 illustrates the TGA curves of the polyimides, HPI-1 to HPI-3 at a heating rate of 10 C/min under nitrogen flow. The thermal decomposition temperatures of polymers were listed in Table 2. They are all above 564 C. The TGA results showed excellent thermal stability of the prepared hyperbranched polyimides. The slightly higher decomposition temperature than the conventional polyimide might be due to the hyperbranching that is also rationalized with the high density of the polyimides.
On the other hand, one of the most important problems facing in applications of H2O2-based amperometric biosensor is the presence of electroactive and non-electroactive interferents. The electroactive interferents such as ascorbic acid and oxalic acid affect anodic signal of H2O2 while non-electroactive species such as lactose, sucrose and urea foul the electrode surface. In order to overcome this interference problem, polyimide film to be used as permselective membrane must be only permeable to hydrogen peroxide among the mentioned species. In preliminary electrochemical tests, it was observed that polyimide (HPI-1) film responded slightly to ascorbic acid. This means that the mentioned film is not suitable for H2O2 determination in the presence of ascorbic acid. On the other hand, in spite of its resistance to the interferents, the polyimide (HPI-3) film was not also
Fig. 5. CVs of 4 mM oxalic acid at the bare Pt and polyimide-coated electrode.
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Fig. 6. DPVs of 4 mM ascorbic acid at bare and polyimidecoated electrode.
suitable for sensor application due to its irregular responses toward hydrogen peroxide. Therefore, we have naturally focused on the other polyimide (HPI-2) film for this purpose. We first investigated the cyclic voltammetric behavior of the polyimide-coated electrode to electroactive interferents. As can be easily seen in Fig. 4, the electrochemical activity of ascorbic acid in the potential region examined is completely suppressed at the polyimide (HPI-2) electrode when compared with that on the bare electrode. The similar change was also observed for oxalic acid (Fig. 5). In addition, these CV findings were also supported with DPVs as shown in Figs. 6 and 7. From these voltammetric results, it is clearly seen that polyimide
Fig. 8. The permselectivity of the polyimide-coated electrode. Each injection corresponds to 2 mM of the mentioned species. H2O2 injections were made at 100 s intervals (1700–2600 s).
(HPI-2) film does not respond to electroactive ascorbic acid and oxalic acid. Also, to test permselectivity of the mentioned polyimide electrode, electrochemical behavior of polyimidecoated electrode to hydrogen peroxide in the presence of the electroactive (ascorbic acid and oxalic acid) and non-electroactive interferents (lactose, sucrose and urea) was examined as a function of time. As expected and shown in Fig. 8, the polyimide electrode responded to only hydrogen peroxide injections among the species examined. 4. Conclusion Hyperbranched polyimides with pyrimidine core were prepared via two-stage solution polycondensation method from the corresponding monomers in different ratios. The resulting polyimides showed high molecular weight, thermal stability and good solubility in the aprotic media. Furthermore, chemically-modified polyimide films were then tested as hydrogen peroxide-selective membrane in the presence of interferents. From the electrochemical data obtained, it was found that the hyperbranched polyimide (HPI-2) film where the ratio of the amine:anhydride was 1:2 was best suited for permselectivity of hydrogen peroxide. Finally, we claim that polyimide electrode with a fast response time (<3 s) can be applicable to amperometric sensing of hydrogen peroxide in the presence of the mentioned electroactive and non-electroactive interferents.
References Fig. 7. DPVs of 4 mM oxalic acid at bare and polyimide-coated electrode.
[1] Benaglia M, Puglisi A, Cozzi F. Chem Rev 2003; 103(9):3401.
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127 [2] Bergbreiter DE. Chem Rev 2002;102(10):3345. [3] Leadbeater NE, Marco M. Chem Rev 2002;102(10):3217. [4] Mittal KL. Polyimides: synthesis, characterization and applications, vols. 1 & 2. New York: Plenum; 1984. [5] Bessonov MT, Koton MM, Kudryavtsev VV, Laius LA. Polyimides: thermally stable polymers. New York: Consultants Bureau; 1987. [6] Wilson D, Stenzenberger HD, Hergenrother PM. Polyimides. Glasgow: Blackie and Son Ltd.; 1990. [7] Ghosh MK, Mittal KL. Polyimides: fundamentals and applications. New York: Marcel Dekker; 1996. [8] Oishi Y, Ishida M, Kakimoto M, Imai Y, Kurosaki T. J Polym Sci Part A: Polym Chem 1992;1027. _ Des Monomers Polym ¨ zdemir I. [9] Sec¸kin T, Ko¨ytepe S, O 2003;6:175. _ C ¨ zdemir I, [10] Sec¸kin T, Ko¨ytepe S, Demir S, O ¸ entinkaya B. J Inorg Organomet P 2003;13:223. [11] Sec¸kin T, Ko¨ytepe S, C ¸ etinkaya E. J Polym Res 2004;11: 119. [12] Yamanaka K, Jikei M, Kakimoto MA. Macromolecules 2000;33:6937. [13] Fang J, Kita H, Okamoto KY. Macromolecules 2000;33:4639. [14] Seino H, Mochizuki A, Ueda M. J Polym Sci Part A: Polym Chem 1999;37:3584.
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[15] Pasßahan A, Ko¨ytepe S, Ekinci E, Sec¸kin T. Polym Bull 2004;51:351. [16] Ekinci E, Ko¨ytepe S, Pasßahan A, Sec¸kin T, submitted for publication. J Appl Polym Sci. [17] Ekinci E, Emre FB, Ko¨ytepe S, Sec¸kin T, in press. J Polym Res. [18] Ekinci E. Polym Bull 1999;42:693. [19] Foulds NC, Lowe CR. J Chem Soc 1986;82:1259. [20] Pandey PC. J Chem Soc 1988;84:2259. [21] Gu¨lce H, Gu¨lce A, Kavanoz M, Coskun H, Yıldız A. Biosens Bioelectron 2002;17(6–7):517. [22] Gu¨lce H, Ataman Y, Gu¨lce A, Yıldız A. Enzyme Microb Technol 2002;30:41. [23] Yang Y, Mu S, Chen H. Synth Met 1998;92:173. ¨ zden M, Tu¨rkdemir MH, Karago¨zler AE. J [24] Ekinci E, O Appl Polym Sci 1998;70:2227. ¨ zden M, Ekinci E, Karago¨zler AE. J Solid State Electro[25] O chem 1998;2:427. ¨ zden M, Ekinci E, Karago¨zler AE. J Appl Polym Sci [26] O 1999;71:2141. [27] Gileadi E, Kirowa-Eisner E, Penciner J. Interfacial electrochemistry. An experimental approach. Reading: AddisonWesley; 1975. p. 311. [28] Hawker CJ, Lee R, Frechet JM. J Am Chem Soc 1991;113:4583.
European Polymer Journal 41 (2005) 121–127
www.elsevier.com/locate/europolj
Synthesis, characterization and H2O2-sensing properties of pyrimidine-based hyperbranched polyimides Su¨leyman Ko¨ytepe, Aziz Pasßahan, Ergun Ekinci, Turgay Sec¸kin
*
_ ¨ nu¨ University, 44280 Malatya, Turkey Faculty of Science and Literature, Department of Chemistry, Ino Received 4 May 2004; received in revised form 5 August 2004; accepted 6 August 2004 Available online 21 September 2004
Abstract A series of hyperbranched polyimides having pyrimidine core in the backbone was prepared by solution polycondensation of pyromellitic dianhydride (A2 type) and 2,4,6-triaminopyrimidine (B3 type). The resulting polyimides were characterized by IR, GPC, DTA, DSC, TGA, density and viscosity measurement. The results showed that polyimides were obtained in 82–86% yield, had inherent viscosities of 1.58–1.87 dL/g and highly thermal stability. From the solubility tests, it was found that all the polyimides were soluble in NMP, DMAc and DMSO at room temperature. In addition, because of their interesting film properties, attempts were made to prepare H2O2-sensing polyimide material. For this reason, selectivity of film-coated electrodes obtained from the chemically prepared hyperbranched polyimides toward electroactive (ascorbic acid, oxalic acid and hydrogen peroxide) and non-electroactive (lactose, sucrose and urea) species was examined by means of CV, DPV and TB techniques. From the electrochemical data obtained, it has been demonstrated that polyimide (HPI-2) film responded to only hydrogen peroxide among the species examined. In other words, it has been claimed that the mentioned polyimide-coated electrode can be used as H2O2-selective membrane in the presence of the electroactive and non-electroactive interferents. 2004 Elsevier Ltd. All rights reserved. Keywords: Polyimide; Film; Selective membrane
1. Introduction In recent years, polyimide-based materials have attracted great interest, both in industry and in academia, because they exhibit unique properties not shared by conventional polymers. The addition of the functional group into polymeric backbone gives rise to sensible increases in permeability, abrasion and heat resistance. The physical nature of the polyimides necessarily gives
* Corresponding author. Tel.: +90 422 3410037; fax: +90 422 3410758/3410037. E-mail address: [email protected] (T. Sec¸kin).
rise to their application depending of interactions between the polymer and the media [1–14]. The improved permselective behavior of polyimides is only achieved when the proper combination of the functionality is applied in the polymeric backbone. In our recent studies, the permselective properties of polyimides prepared from heterocyclic counterparts and hyperbranched core have been reported elsewhere [15–17]. In applications of the amperometric biosensor based on oxido-reductase enzymes such as glucose oxidase [18–20], alcohol oxidase [21] and galactose oxidase [22,23], it is measured hydrogen peroxide obtained as a result of enzymatic reaction between enzyme and the relevant substrate. However, one of the most important
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.08.007
122
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
solvent. All the electrochemical operations such as cyclic voltammetry (CV), differential puls voltammetry (DPV) and amperometric measurements (TB) were performed by a BAS (Bioanalytical Systems, Inc.) 100 W electrochemical analyzer. The standard three-electrode system consisting of a Pt working electrode (CHI, 2 mm diameter), an Ag/AgCl reference electrode (CHI) and a Pt wire coil auxiliary electrode was used. The pH was measured with a Jenway 3010 pH meter. All chemicals for the synthesis were purchased from Aldrich/Sigma Co. and used after purification. NMP was distilled over CaH2 under ˚ molecular sieves. reduced pressure and stored over 4 A Reagent grade aromatic dianhydride, pyromellitic dianhydride (PMDA), was sublimed at 180 C under reduced pressure for 10 h (Tm = 285 C), and dried under vacuum at 120 C prior to use. 2,4,6-Triaminopyrimidine (Tm = 250 C) was used as received. Ascorbic acid, oxalic acid, hydrogen peroxide, lactose, sucrose and urea were of analytical grade and supplied either by Sigma Chemical Company or E. Merck. Electrochemical tests were made in a PBS (phosphate buffer salts, pH = 7) solution. Ascorbic acid solution was prepared freshly before experiment. All aqueous solutions were prepared with deionized and doubly distilled water.
problems when measuring hydrogen peroxide is the presence of the electroactive and non-electroactive interferents. The mentioned problem can be overcome by using selective-polymeric membranes. Ekinci and coworkers have previously reported the use of the electrochemically-modified polymeric films for this aim [24–26]. In this paper, we report synthesis and characterization of hyperbranched polyimides with pyrimidine core. Moreover, it has been examined that whether chemically-modified polyimide electrode could be used as a permselective membrane for hydrogen peroxide in the presence of the interferents.
2. Experimental 2.1. Apparatus and materials Infrared spectra were recorded as KBr pellets in the range 4000–400 cm 1 on an Ati Unicam Mattson 1000 Fourier Transform Infrared Spectrometer. Differential scanning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetry (TG) were performed with Shimadzu DSC-60, DTA-50 and TGA-50 thermal analyzers, respectively. DSC, DTA and TGA analysis were performed with a heating rate of 10 C/min in nitrogen atmosphere. GPC analyses were performed at 30 C using NMP as eluant at a flow rate of 0.5 mL/ min. A differential refractometer was used as a detector. The instrument (Agilent 1100 series GPC-SEC system) was calibrated with a mixture of polystyrene standards (polysciences; molecular masses between 200 and 1,200,000 Da) using GPC software for the determination of the average molecular masses and the polydispersity of the polymer samples. Inherent viscosities (ginh = lngr/c at polymer concentration of 0.5 g/dL) were measured with an Ubbelohde suspended-level viscometer at 30 C using NMP as the
2.2. Synthesis of polyimides A typical polyimide (HPI-1 to HPI-3) synthesis was performed as follows: 2,4,6-triaminopyrimidine (0.51 g, 4.15 mmol) was dissolved in NMP (15 mL) in a 50 mL Schlenck tube equipped with a nitrogen line, overhead stirrer, a xylene filled Dean–Stark trap, and a condenser. PMDA (the ratio of amine: anhydride was varied as shown in Table 1) was added to the amine solution for a period of 4 h at 30 C and stirred overnight to give a viscous solution. The mixture was heated to 70 C, xylene (5 mL) was added, and the mixture was refluxed for 3 h. Following the removal of xylene by distillation, the reaction mixture was cooled at room temperature
Table 1 Basic properties of polyimides Polymer
n/m
Yield (%)
d (g/cm3)a
g (dL/g)b
Mn · 10
HPI-1 HPI-2 HPI-3
1/1 1/2 1/3
82 83 86
1.43 1.50 1.64
1.58 1.64 1.87
2.23 3.14 4.35
5.91 12.12 28.18
2.65 3.86 6.48
NMP
DMAc
Ether
THF
Hexane
DMSO
DMF
+ + +
+ + +
+ + +
+ +
Solubilityc HPI-1 HPI-2 HPI-3
± ±
5
Mw · 10
5
PDI
a
Determined by suspension method at 30 C. Measured at a concentration of 0.5 g/dL in NMP at 30 C. c Solubility tested at 2% solid concentration; + soluble at room temperature; ± soluble upon heating; molar ratios of amine and anhydride, respectively. b
insoluble. m and n refer the
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
123
Scheme 1. Synthesis of polyimide (HPI-2).
2.3. Preparation of polyimide film-coated electrodes and permselectivity measurements Prior to polymeric film coating, the surface of the Pt disc working electrodes was cleaned according to the standard procedure and polished with consecutive finer grades of aqueous alumina slurry down to 0.5 lm [27]. Polyimide films were prepared onto the electrodes by dropping 10 ll of polyimide solution. Then, polymer-modified surface was allowed to dry at room temperature for three days. Before the permselectivity measurements, the background current was allowed to reduce to steady state. Then, the amperometric behavior of the polymeric electrodes toward the electroactive and non-electroactive substances was recorded as a function of time at a potential of 0.7 V vs. Ag/AgCl.
3. Results and discussion Hyperbranched polymers are generally prepared from AB2 type monomers where A and B represent two types of functional groups, however it is not possible to prepare hyperbranched polyimides from a monomer of this type, since both the anhydride and the amine groups are very reactive to each other. The copolymerization of A2 and B3 type monomers is an alternative method for preparing hyperbranched polyimides, the cross-linking is prevented by controlling the monomer concentration [28]. Pyromellitic dianhydride (PMDA) as the A2 and the 2,4,6-triaminopyrimidine as B3 were used to prepare a series of hyperbranched polyimides. The synthesis involves two-
step polycondensation method in NMP as shown in Scheme 1. The polymerization was influenced by many factors such as monomer addition, monomer molar ratio and concentrations. In order to prevent gelation, the dianhydride monomer should be added so slowly in order to avoid any high local concentration in all cases. It was found that the total solid content should be kept as 0.2 mol/L. The amine terminated polyamic acids were thermally imidized in solution in the presence of xylene. The solubility properties and inherent viscosities of the prepared hyperbranched polyimides were shown in Table 1. All polymers were soluble in NMP, DMAc and DMSO at room temperature. The inherent viscosities
H P I-1
H P I-2 Transmittance (%T)
and the product was precipitated by addition of a large excess of methanol. The dark yellow product was isolated and dried at 100 C under vacuum for 10 h and then at 220 C under nitrogen for 2 h. In brief, polyimides, HPI-1, HPI-2 and HPI-3 were prepared by changing the amine:anhydride ratio from 1:1 to 1:3, respectively.
H P I-3
4000
3400
2200 1600 2800 Wavenumber (cm-1)
Fig. 1. FTIR spectra of polyimides.
1000
400
124
S. Ko¨ytepe et al. / European Polymer Journal 41 (2005) 121–127
DTA uV
TGA %
100.00
PMDA 565.4C
396.4C
286.1C
80.00
504.2C 60.00
HPI-1
599.6C HPI-3
40.00 HPI-2 HPI-1
20.00
496.5C HPI-2
0.00 0.00
606.2C
200.00
400.00
600.00
800.00
Temp [C]
Fig. 3. TGA curves of HPI-1 to HPI-3 at heating rate of 10 C/ min under nitrogen flow.
452. 9C
HPI-3
200
0
40
800
600
Temp [C]
Fig. 2. DTA thermograms of the PMDA and polyimides.
of the polyimides showed that all polymers were in high molecular range, which was also rationalized with GPC data with the dispersity index 2.65 and 3.86 for HPI-1 and HPI-2 respectively, and 6.48 for HPI-3. The increase in polydispersity index is attributed to the high degree of swelling of the polymer backbone in the solvent.
Fig. 1 shows the FTIR spectra of the polyimides, HPI-1 to HPI-3. The characteristic absorption bands of the imide group are observed at 729, 1377, 1720 and 1786 cm 1. The terminal amino groups (N–H stretching of amino groups) generated a broad absorption band at 2700–3500 cm 1 that also ascertains that hyperbranching was achieved effectively. Fig. 2 shows the differential thermal analysis (DTA) spectra of the PMDA and the hyperbranched polyimides, HPI-1 to HPI-3. Two transition regions were observed in all cases, one is at 504 C, 496 C and 452 C, the other transition
Table 2 Thermal properties of polyimides Polymer
On set
End set
% 10a
Charb
IDTc
TGA analysis HPI-1 HPI-2 HPI-3
444 462 468
669 730 708
485 493 503
26.1 34.3 41.0
420 245 437
TDPe
On set
End set
Heat (kJ/g)
565 600 606
518 520 532
617 614 655
3.16 4.56 4.01
Tg (C)f
On set
End set
Transition (mW)
218 190 183
215 161 168
223 194 185
d
DTA analysis HPI-1 HPI-2 HPI-3
DSC analysis HPI-1 HPI-2 HPI-3 a
1.66 1.07 1.03
Temperature of 10% weight loss was assessed by TGA at a heating rate of 10 C/min in N2. Assessed by TGA at 800 C in N2. c IDT (initial decomposition temperature) is the temperature at which an initial loss of mass observed. d DTA thermograms of polyimides with a heating rate of 10 C/min in air. e TDP (thermal decomposition peak). f Determined by DSC in N2. b
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Fig. 4. CVs of 4 mM ascorbic acid at the bare Pt and polyimide-coated electrode.
is observed at 565 C, 599 C and 606 C for HPI-1 to HPI3, respectively. DSC analysis indicated that the highest Tg was 218 C for HPI-1, it might be the reason for the chain entanglement that is more likely to take place when the monomer ratio was 1 to 1. Fig. 3 illustrates the TGA curves of the polyimides, HPI-1 to HPI-3 at a heating rate of 10 C/min under nitrogen flow. The thermal decomposition temperatures of polymers were listed in Table 2. They are all above 564 C. The TGA results showed excellent thermal stability of the prepared hyperbranched polyimides. The slightly higher decomposition temperature than the conventional polyimide might be due to the hyperbranching that is also rationalized with the high density of the polyimides.
On the other hand, one of the most important problems facing in applications of H2O2-based amperometric biosensor is the presence of electroactive and non-electroactive interferents. The electroactive interferents such as ascorbic acid and oxalic acid affect anodic signal of H2O2 while non-electroactive species such as lactose, sucrose and urea foul the electrode surface. In order to overcome this interference problem, polyimide film to be used as permselective membrane must be only permeable to hydrogen peroxide among the mentioned species. In preliminary electrochemical tests, it was observed that polyimide (HPI-1) film responded slightly to ascorbic acid. This means that the mentioned film is not suitable for H2O2 determination in the presence of ascorbic acid. On the other hand, in spite of its resistance to the interferents, the polyimide (HPI-3) film was not also
Fig. 5. CVs of 4 mM oxalic acid at the bare Pt and polyimide-coated electrode.
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Fig. 6. DPVs of 4 mM ascorbic acid at bare and polyimidecoated electrode.
suitable for sensor application due to its irregular responses toward hydrogen peroxide. Therefore, we have naturally focused on the other polyimide (HPI-2) film for this purpose. We first investigated the cyclic voltammetric behavior of the polyimide-coated electrode to electroactive interferents. As can be easily seen in Fig. 4, the electrochemical activity of ascorbic acid in the potential region examined is completely suppressed at the polyimide (HPI-2) electrode when compared with that on the bare electrode. The similar change was also observed for oxalic acid (Fig. 5). In addition, these CV findings were also supported with DPVs as shown in Figs. 6 and 7. From these voltammetric results, it is clearly seen that polyimide
Fig. 8. The permselectivity of the polyimide-coated electrode. Each injection corresponds to 2 mM of the mentioned species. H2O2 injections were made at 100 s intervals (1700–2600 s).
(HPI-2) film does not respond to electroactive ascorbic acid and oxalic acid. Also, to test permselectivity of the mentioned polyimide electrode, electrochemical behavior of polyimidecoated electrode to hydrogen peroxide in the presence of the electroactive (ascorbic acid and oxalic acid) and non-electroactive interferents (lactose, sucrose and urea) was examined as a function of time. As expected and shown in Fig. 8, the polyimide electrode responded to only hydrogen peroxide injections among the species examined. 4. Conclusion Hyperbranched polyimides with pyrimidine core were prepared via two-stage solution polycondensation method from the corresponding monomers in different ratios. The resulting polyimides showed high molecular weight, thermal stability and good solubility in the aprotic media. Furthermore, chemically-modified polyimide films were then tested as hydrogen peroxide-selective membrane in the presence of interferents. From the electrochemical data obtained, it was found that the hyperbranched polyimide (HPI-2) film where the ratio of the amine:anhydride was 1:2 was best suited for permselectivity of hydrogen peroxide. Finally, we claim that polyimide electrode with a fast response time (<3 s) can be applicable to amperometric sensing of hydrogen peroxide in the presence of the mentioned electroactive and non-electroactive interferents.
References Fig. 7. DPVs of 4 mM oxalic acid at bare and polyimide-coated electrode.
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