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Macroscopically structured polymer formation governed by spatial patterns in the Belousov–Zhabotinsky reaction
16 September 2002
Chemical Physics Letters 363 (2002) 534–539 www.elsevier.com/locate/cplett
Macroscopically structured polymer formation governed by spatial patterns in the Belousov–Zhabotinsky reaction Yevhen Yu. Kalishyn a, Vyacheslav O. Khavrus a, Peter E. Strizhak a, Michael Seipel b, Arno F. M€ unster b,* a
L.V.Pysarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, pr.Nauki 31, Kiev, 03039, Ukraine b Institute of Physical Chemistry, University of W€urzburg, Am Hubland, Marcusstrasse 9-11, D-97074 W€urzburg, Germany Received 10 April 2002; in final form 24 July 2002
Abstract We report the formation of macroscopically structured cross-linked polyacrylamide hydrogel in the Belousov– Zhabotinsky (BZ) system (oxidation of malonic acid by bromate catalyzed by ferroin). Here, acrylamide, the crosslinker bis-acrylamide, and polymerization initiator are added into the BZ system. We show that the formation of waves and ripples in the polymer is governed by spatial structures emerging in the BZ system. Without any spatial structures in the BZ system only the formation of a spatially uniform polymer is observed. Without cross-linker, a spatially uniform polymer was observed as well. Structured polymer formation is caused by the interaction of chemical reactions in the BZ system and the polymerization process including gelation and cross-linking of the monomer units. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The application of nonlinear chemical phenomena to the synthesis of macroscopically structured materials is considered as an important field in the modern design of materials, e.g. [1–3]. However, there is a lack of experimental results using such an approach since adding compounds, that might form a material, to autocatalytic reactions in many cases inhibits the desired nonlinear phenomena such as oscillations, wave propagation
*
Corresponding author. Fax: +49-931-888-6302. E-mail addresses: [email protected] (P.E. Strizhak), [email protected] (A.F. M€ unster).
or pattern formation. However, there are some examples of coupling nonlinear chemical phenomena and polymerization, as has been shown recently [4,5]. The chemo-mechanical interplay of a nonlinear reaction and polyacrylamide gel has been studied as well [5–7]. In particular, it has been reported that polyacrylonitrile is periodically formed if the polymerization is coupled to chemical oscillations in the Belousov–Zhabotinsky (BZ) oscillating chemical reaction in a well-stirred reactor [8]. Polymer materials are of extraordinary practical importance. They cover a broad band of material properties which can be deliberately controlled by a proper choice of conditions during the polymerization process. In particular, a novel
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 2 4 1 - 1
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
class of polymer materials called ‘intelligent gels’ has recently been developed [9,10]. These materials respond to external stimuli, such as temperature, chemical environment, value of pH or electric field, by changes of their physical properties. Most of the intelligent gels developed so far are based on polymers of acrylamide or acrylonitrile. The coupling of these gels to a chemical oscillator may lead to novel properties of the resulting material as has been demonstrated recently [11,12]. Here, the authors obtained a self-oscillating gel by running the BZ reaction in a matrix of poly(N-isopropyl acrylamide). In the present Letter we report preliminary results of experimental studies of a polymerization reaction conducted in a spatially distributed system where chemical waves and patterns occur. We performed a systematic search for conditions where the polymerization of acrylamide occurs and macroscopic structures are formed in the Belousov–Zhabotisky reaction at the same time. We speculate that the polymer formation is caused by an interaction of chemical reactions in the BZ system and polymerization process including gelation and cross-linking of the monomer.
2. Experimental All chemicals were of analytical grade. Doubly distilled water was used to prepare solutions. All experiments were carried out in a thermostated Petri dish (inner diameter 96 mm) at 25 °C. The volume of the reaction mixture was 13.2 ml in all experiments. The actual reaction mixture was prepared by mixing two solutions A and B. Solution A contained the monomers of the gel to be formed, namely acrylamide, N,N 0 -methylenbisacrylamide, and triethanolamine. Solution B was prepared by subsequent mixing of solutions containing the components of the BZ reaction, namely potassium bromate, sulfuric acid, malonic acid, and ferroin. Solution A was added to solution B one minute after the preparation of solution B. Then, 0.28 ml of a 0.876 M ammonium peroxodisulfate solution was added into the premixed system. The final solution was placed into the Petri dish and the system was monitored by a digital
535
video camera connected to a PC. Concentrations of substances in stock solutions were chosen in such a way that the final solution always contained 0.364 M H2 SO4 , 0.09 M of malonic acid, 3:5 104 M of ferroin, 1.34 M of acrylamide, 0.0186 M of ammonium peroxodisulfate, and 0.082 M of triethanolamine. Concentrations of N,N 0 -methylenbisacrylamide and potassium bromate were varied. To obtain a polymer we allowed the reaction to run during 2 h. In order to obtain a solid film free of educt chemicals, the polymer was removed from the reaction mixture, washed with water, placed into acetone for 30 min, then placed into ethanol for another 30 min, and finally dried in an exicator containing P2 O5 .
3. Results and discussion Following the idea to find conditions where the BZ system exhibits waves in a Petri dish and adding monomer to the system leads to structured polymer formation. In particular, we have studied the polymerization of acrylamide. If only acrylamide is added to the BZ system only hydrogel of high viscosity is formed at all concentrations of the BZ system we have studied. To obtain a polymer film we additionally added the cross-linker N,N 0 methylenbisacrylamide [13]. Using N,N 0 -methylenbisacrylamide as cross-linker leads to polymer formation; however, the polymer forms very slowly. To accelerate the polymerization we additionally added an initiator of the radical-chain-polymerization reaction (ammonium peroxodisulfate and triethanolamine). The reaction of ammonium peroxodisulfate and triethanolamine produces radicals that are sufficiently reactive to initiate the polymerization [14]. Increasing the concentration of the cross-linker produces a more stable polymer if we increase concentrations of sulfuric acid or bromate ions. Increasing the concentration of sulfuric acid supports the formation of waves in the BZ system but simultaneously inhibits the polymerization process. To achieve the desired result of a macroscopically structured polymer it is necessary to change both, sulfuric acid and N,N 0 methylenbisacrylamide, initial concentrations. For certain concentrations of acrylamide and N,
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N 0 -methylenbisacrylamide, ripples caused by hydrodynamic instability [15] are observed after adding the latter solutions to the BZ reaction mixture. An increase of the initial malonic acid concentration supports the formation of these hydrodynamic structures. Finally we found that polymerization occurs and macroscopically structured polymer is formed in the BZ system for the following fixed initial concentrations of reagents: 0.364 M H2 SO4 , 0.09 M of malonic acid, 3:5 104 M of ferroin, 1.34 M of acrylamide, 0.0186 M of ammonium peroxodisulfate, and 0.082 M of triethanolamine. Initial concentrations of KBrO3 and N,N 0 -methylenbisacrylamide may be varied between 0.053 to 0.16 M (KBrO3 ) and 0.008 to 0.03 M (cross-linker), respectively. Fig. 1a gives an example of a spatially uniform polymer formed in the BZ system under conditions where no spatial self-organization phenomena have been observed. Contrary, if waves appear in the BZ reaction the polymer obtained is characterized by macroscopic spatial nonuniformity. As an example, Fig. 1b shows a typical picture of a polyacrylamide film formed at low concentration of the cross-linker. Both images show the dried polymer film after electronic contrast enhancement using the commercial software OP T I M A S 6.1. The inhomogeneities seen are due to the nonuniform thickness of the film. A comparison of the images presented in Figs. 1a and b illustrates the difference between spatially uniform and spatially nonuniform polymers. Fig. 2 illustrates the correspondence between spatial structures in the BZ system and macroscopic spatial structures in the polymer obtained. The left panels in this figure display waves and ripples observed in the BZ system at the particular moment of time that corresponds to the gelation of polyacrylamide. This moment of time lies between 10 and 12 min after mixing the reaction components according to observations reported previously [16]. The right panels in Fig. 2 present images of dried polymer films obtained for conditions corresponding to those in the left panels. The contrast in the figures emerges from the nonuniform thickness and surface modulation of the film. For the conditions corresponding to the upper panels it is obvious that the polymer structure
Fig. 1. Images showing spatially uniform (a) and spatially nonuniform (b) polymers (dried films) obtained under different conditions. Initial concentrations of variable components: (a) 0.037 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide; (b) 0.053 M KBrO3 and 0.01 M of N,N 0 -methylenbisacrylamide. Initial concentrations of the other components: 0.364 M H2 SO4 ; 0.09 M of malonic acid; 3:5 104 M of ferroin; 1.34 M of acrylamide; 0.0186 M of ammonium peroxodisulfate; 0.082 M of triethanolamine. The size of each frame is 43:3 43:3 mm.
mimics the wave observed in the BZ system. In addition, there are ripples visible in Fig. 2b. Fig. 2d also indicates a reflection of the wave which is little
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
537
Fig. 2. Images showing waves and ripples formed in the BZ system during gelation (a,c) and corresponding dried polymer films formed in these systems (b,d). Initial concentrations of variable components: (a,b) 0.053 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide; (c,d) 0.047 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide. Initial concentrations of the other components: 0.037 M KBrO3 and 0.364 M H2 SO4 ; 0.09 M of malonic acid; 3:5 104 M of ferroin; 1.34 M of acrylamide; 0.0186 M of ammonium peroxodisulfate; 0.082 M of triethanolamine. Size of pictures (a,c) is 94 75 mm, size of the pictures (b,d) is 43:3 43:3 mm.
pronounced comparing to Fig. 2b. However, in this case the ripples are better expressed for the BZ system (Fig. 2c) as well as for the polymer (Fig. 2d). Our experiments have shown that an increase of cross-linker initial concentration leads to the appearance of macroscopically ordered structures in the resulting polymer. These structures are governed by waves and ripples formed in the BZ system. Varying the initial concentration of the cross-linker indicates a critical minimal concentration of 0.008 M of N,N 0 -methylenbisacrylamide below which a macroscopically structured polymer does not form. Probably this finding is associated with insufficient cross-linking of polymer having consequences in its inability to ‘memorize’ (or reproduce) patterns from the BZ system. Varying the initial concentration of bromate ions lead us to the conclusion that a macroscopi-
cally structured polymer may be obtained only if waves or ripples appear in the BZ system. Below a critical initial concentration of bromate ions of 0.053 M there are no spatial structures in the BZ system and a spatially uniform polymer is formed. However, we have also found that there is an upper limit of the initial bromate ions concentration (0.16 M) above which no polymer is formed and only gelation occurs. The experimental data reported here may be supported by the kinetic scheme presented in Table 1. The scheme represents reactions of the BZ system catalyzed by ferroin coupled to the polymerization of acrylamide. The first 12 reactions in Table 1 correspond to the BZ chemistry [17]. If these reactions are coupled with diffusion, they produce a variety of spatial self-organization phenomena observed in the ferroin catalyzed BZ
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Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
Table 1 Kinetic scheme of the BZ reaction catalyzed by ferroin coupled with the polymerization of acrylamide Hþ þ HBrO2 þ HBrO3 () HBrOþ 2 þ BrO2 þ H2 O þ
BrO2
þ H ()
FeðphenÞ2þ 3
þ
ð1Þ
HBrOþ 2
HBrOþ 2
ð2Þ FeðphenÞ3þ 3
()
þ HBrO2
ð3Þ
2HBrO2 () HOBr þ HBrO3 þ
ð4Þ
H þ Br () HBrO2 þ 2HOBr
ð5Þ
Hþ þ Br þ HOBr () Br2 þ H2 O
ð6Þ
þ
H þ Br þ HBrO3 () HBrO2 þ HOBr FeðphenÞ3þ 3
ð7Þ
FeðphenÞ2þ 3
þ
ð8Þ
H2 O þ CBrðCOOHÞ2 () Hþ þ Br þ COHðCOOHÞ2
ð9Þ
þ CHBrðCOOHÞ2 ()
þ H þ CBrðCOOHÞ2
HOBr þ CHBrðCOOHÞ2 () CBr2 ðCOOHÞ2 þ H2 O þ
ð10Þ
Br2 þ CHBrðCOOHÞ2 () CBr2 ðCOOHÞ2 þ H þ Br
ð11Þ
CHBrðCOOHÞ2 þ H2 O () CHOHðCOOHÞ2 þ Hþ þ Br
ð12Þ
M þ CBrðCOOHÞ2 !
R1
þ CHBrðCOOHÞ2
ð13Þ
M þ COHðCOOHÞ2 ! R1 þ CHOHðCOOHÞ2
ð14Þ
S2 O2 8
ð15Þ
!
2SO 4
SO 4
ðcatalyzed by NðC2 H5 OÞ3 Þ
R1
ð16Þ
Rj þ M ! Rðjþ1Þ
ð17Þ
Mþ
Rðiþ1Þ Rk Rj Rj
þ
!
Rðjþ1Þ
þ CrL ! Rðjþ1Þ CrL Rðiþ1Þ
þ Rðjþ1Þ CrL Rðiþ1Þ ! þ
Rk
þ
BrO2
ð18Þ
Rm
ð19Þ
! RðjþkÞ
ð20Þ
! Rj BrO2
reaction. Among them are waves, target patterns, spirals, twinkling patterns [17,18], and even turing patterns which may be realized if the reaction is run in a microemulsion [19,20]. The scheme indicates that the BZ system itself produces free radicals according to reactions (8) and (9). These free radicals initiate the polymerization process according to reactions (13) and (14). Reactions (15) and (16) illustrate another pathway for the initiation of polymerization caused by the triethanolamine catalyzed decomposition of peroxodisulfate ions [14]. These reactions provide a sufficient source of radicals for the initiation of polymerization. Without adding triethanolamine and ammonium peroxodisulfate to the reaction mixture the polymer film does not form but gelation occurs instead. Reaction (17) describes a linear growth of the polymer
ð21Þ
chain whereas reactions (18) and (19) represent cross-linking [13]. Finally, polymerization terminates either due to the recombination process (20) or by BrO2 radicals according to reaction (21). The latter termination of polymerization is probably favorable as has been shown recently [4]. This kinetic scheme gives a qualitative explanation for the phenomena reported in this Letter. Particularly, an increase of bromate inhibits the formation of a polymer film due to termination of the polymerization according to reaction (21).
4. Conclusions Our studies have demonstrated the possibility to obtain macroscopically structured polyacryla-
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
mide where the structure is governed by chemical waves and patterns formed in the BZ system. The formation of the macroscopically structured polymer is caused by only a few factors: first of all, polyacrylamide does not directly interfere with the components of the BZ system. Secondly, the use of cross-linker N,N 0 -methylenbisacrylamide together with a proper initiator of the polymerization (ammonium peroxodisulfate and triethanolamine) allowed us to obtain macroscopically structured polymers. Thirdly, adding acrylamide, cross-linker and initiator does not drastically affect waves emerging in the BZ system. Adding all the components required for polymer formation just decelerates the wave propagation and leads to hydrodynamic patterns (ripples). The emergence of hydrodynamic patterns may be enhanced by the heat released through the exothermic polymerization reaction.
Acknowledgements The work has been supported financially by the NATO Cooperative Science & Technology SubProgramme (Collaborative linkage grant) and the DFG.
539
References [1] I.R. Epstein, J.A. Pojman, An Introduction to Nonlinear Chemical Dynamics: Oscillations, Waves, Patterns, and Chaos, Oxford University Press, New York, Oxford, 1998. [2] J. Maselko, J. Chem. Soc. Farad. Trans. 92 (1996) 2879. [3] P.E. Strizhak, Chem. Phys. Lett. 241 (1995) 360. [4] J.A. Pojman, D.C. Leard, W.W. West, J. Amer. Chem. Soc. 114 (1992) 8298. [5] F. Fecher, P. Strasser, M. Eiswirth, F.W. Schneider, A.F. M€ unster, Chem. Phys. Lett. 313 (1999) 205. [6] O. Steinbock, E. Kasper, S.C. M€ uller, J. Phys. Chem. A 103 (1999) 3442. [7] O. Steinbock, E. Kasper, S.C. M€ uller, Z. Phys. Chem. 216 (2002) 687. [8] R.P. Washington, W.W. West, G.P. Misra, J.A. Pojman, J. Amer. Chem. Soc. 121 (1999) 7373. [9] R. Dagani, Chemical & Engineering News (1997) 26. [10] R. Yoshida, T. Takahashi, T. Yamaguchi, H. Ichijo, J. Am. Chem. Soc. 118 (1996) 5134. [11] K. Miyakawa, F. Sakamoto, R. Yoshida, E. Kokufuta, T. Yamaguchi, Phys. Rev. E 62 (2000) 793. [12] R. Yoshida, E. Kokufuta, T. Yamaguchi, Chaos 9 (1999) 260. [13] S. Hjerten, Arch. Biochem. Biophys. Suppl. 1 (1962) 147. [14] H. Logemann, in: Methoden der Organischen Chemie, Vol. XIV, Thieme, Stuttgart, 1961, pp. 291 ff. [15] A. Cloot, G. Lebon, J. Fluid Mech. 145 (1984) 447. [16] J. Ohmer, Diploma-Thesis, University of W€ urzburg, 2001. [17] A.V. Rovinsky, J. Phys. Chem. 91 (1987) 5113. [18] A.M. Zhabotinsky, A.V. Rovinsky, J. Phys. Chem. 94 (1990) 8001. [19] V. Vanag, I.R. Epstein, Phys. Rev. Lett. 87 (2001) 228301. [20] I.R. Epstein, Faraday Discuss. 120 (2002) 421.
Chemical Physics Letters 363 (2002) 534–539 www.elsevier.com/locate/cplett
Macroscopically structured polymer formation governed by spatial patterns in the Belousov–Zhabotinsky reaction Yevhen Yu. Kalishyn a, Vyacheslav O. Khavrus a, Peter E. Strizhak a, Michael Seipel b, Arno F. M€ unster b,* a
L.V.Pysarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, pr.Nauki 31, Kiev, 03039, Ukraine b Institute of Physical Chemistry, University of W€urzburg, Am Hubland, Marcusstrasse 9-11, D-97074 W€urzburg, Germany Received 10 April 2002; in final form 24 July 2002
Abstract We report the formation of macroscopically structured cross-linked polyacrylamide hydrogel in the Belousov– Zhabotinsky (BZ) system (oxidation of malonic acid by bromate catalyzed by ferroin). Here, acrylamide, the crosslinker bis-acrylamide, and polymerization initiator are added into the BZ system. We show that the formation of waves and ripples in the polymer is governed by spatial structures emerging in the BZ system. Without any spatial structures in the BZ system only the formation of a spatially uniform polymer is observed. Without cross-linker, a spatially uniform polymer was observed as well. Structured polymer formation is caused by the interaction of chemical reactions in the BZ system and the polymerization process including gelation and cross-linking of the monomer units. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The application of nonlinear chemical phenomena to the synthesis of macroscopically structured materials is considered as an important field in the modern design of materials, e.g. [1–3]. However, there is a lack of experimental results using such an approach since adding compounds, that might form a material, to autocatalytic reactions in many cases inhibits the desired nonlinear phenomena such as oscillations, wave propagation
*
Corresponding author. Fax: +49-931-888-6302. E-mail addresses: [email protected] (P.E. Strizhak), [email protected] (A.F. M€ unster).
or pattern formation. However, there are some examples of coupling nonlinear chemical phenomena and polymerization, as has been shown recently [4,5]. The chemo-mechanical interplay of a nonlinear reaction and polyacrylamide gel has been studied as well [5–7]. In particular, it has been reported that polyacrylonitrile is periodically formed if the polymerization is coupled to chemical oscillations in the Belousov–Zhabotinsky (BZ) oscillating chemical reaction in a well-stirred reactor [8]. Polymer materials are of extraordinary practical importance. They cover a broad band of material properties which can be deliberately controlled by a proper choice of conditions during the polymerization process. In particular, a novel
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 2 4 1 - 1
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
class of polymer materials called ‘intelligent gels’ has recently been developed [9,10]. These materials respond to external stimuli, such as temperature, chemical environment, value of pH or electric field, by changes of their physical properties. Most of the intelligent gels developed so far are based on polymers of acrylamide or acrylonitrile. The coupling of these gels to a chemical oscillator may lead to novel properties of the resulting material as has been demonstrated recently [11,12]. Here, the authors obtained a self-oscillating gel by running the BZ reaction in a matrix of poly(N-isopropyl acrylamide). In the present Letter we report preliminary results of experimental studies of a polymerization reaction conducted in a spatially distributed system where chemical waves and patterns occur. We performed a systematic search for conditions where the polymerization of acrylamide occurs and macroscopic structures are formed in the Belousov–Zhabotisky reaction at the same time. We speculate that the polymer formation is caused by an interaction of chemical reactions in the BZ system and polymerization process including gelation and cross-linking of the monomer.
2. Experimental All chemicals were of analytical grade. Doubly distilled water was used to prepare solutions. All experiments were carried out in a thermostated Petri dish (inner diameter 96 mm) at 25 °C. The volume of the reaction mixture was 13.2 ml in all experiments. The actual reaction mixture was prepared by mixing two solutions A and B. Solution A contained the monomers of the gel to be formed, namely acrylamide, N,N 0 -methylenbisacrylamide, and triethanolamine. Solution B was prepared by subsequent mixing of solutions containing the components of the BZ reaction, namely potassium bromate, sulfuric acid, malonic acid, and ferroin. Solution A was added to solution B one minute after the preparation of solution B. Then, 0.28 ml of a 0.876 M ammonium peroxodisulfate solution was added into the premixed system. The final solution was placed into the Petri dish and the system was monitored by a digital
535
video camera connected to a PC. Concentrations of substances in stock solutions were chosen in such a way that the final solution always contained 0.364 M H2 SO4 , 0.09 M of malonic acid, 3:5 104 M of ferroin, 1.34 M of acrylamide, 0.0186 M of ammonium peroxodisulfate, and 0.082 M of triethanolamine. Concentrations of N,N 0 -methylenbisacrylamide and potassium bromate were varied. To obtain a polymer we allowed the reaction to run during 2 h. In order to obtain a solid film free of educt chemicals, the polymer was removed from the reaction mixture, washed with water, placed into acetone for 30 min, then placed into ethanol for another 30 min, and finally dried in an exicator containing P2 O5 .
3. Results and discussion Following the idea to find conditions where the BZ system exhibits waves in a Petri dish and adding monomer to the system leads to structured polymer formation. In particular, we have studied the polymerization of acrylamide. If only acrylamide is added to the BZ system only hydrogel of high viscosity is formed at all concentrations of the BZ system we have studied. To obtain a polymer film we additionally added the cross-linker N,N 0 methylenbisacrylamide [13]. Using N,N 0 -methylenbisacrylamide as cross-linker leads to polymer formation; however, the polymer forms very slowly. To accelerate the polymerization we additionally added an initiator of the radical-chain-polymerization reaction (ammonium peroxodisulfate and triethanolamine). The reaction of ammonium peroxodisulfate and triethanolamine produces radicals that are sufficiently reactive to initiate the polymerization [14]. Increasing the concentration of the cross-linker produces a more stable polymer if we increase concentrations of sulfuric acid or bromate ions. Increasing the concentration of sulfuric acid supports the formation of waves in the BZ system but simultaneously inhibits the polymerization process. To achieve the desired result of a macroscopically structured polymer it is necessary to change both, sulfuric acid and N,N 0 methylenbisacrylamide, initial concentrations. For certain concentrations of acrylamide and N,
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Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
N 0 -methylenbisacrylamide, ripples caused by hydrodynamic instability [15] are observed after adding the latter solutions to the BZ reaction mixture. An increase of the initial malonic acid concentration supports the formation of these hydrodynamic structures. Finally we found that polymerization occurs and macroscopically structured polymer is formed in the BZ system for the following fixed initial concentrations of reagents: 0.364 M H2 SO4 , 0.09 M of malonic acid, 3:5 104 M of ferroin, 1.34 M of acrylamide, 0.0186 M of ammonium peroxodisulfate, and 0.082 M of triethanolamine. Initial concentrations of KBrO3 and N,N 0 -methylenbisacrylamide may be varied between 0.053 to 0.16 M (KBrO3 ) and 0.008 to 0.03 M (cross-linker), respectively. Fig. 1a gives an example of a spatially uniform polymer formed in the BZ system under conditions where no spatial self-organization phenomena have been observed. Contrary, if waves appear in the BZ reaction the polymer obtained is characterized by macroscopic spatial nonuniformity. As an example, Fig. 1b shows a typical picture of a polyacrylamide film formed at low concentration of the cross-linker. Both images show the dried polymer film after electronic contrast enhancement using the commercial software OP T I M A S 6.1. The inhomogeneities seen are due to the nonuniform thickness of the film. A comparison of the images presented in Figs. 1a and b illustrates the difference between spatially uniform and spatially nonuniform polymers. Fig. 2 illustrates the correspondence between spatial structures in the BZ system and macroscopic spatial structures in the polymer obtained. The left panels in this figure display waves and ripples observed in the BZ system at the particular moment of time that corresponds to the gelation of polyacrylamide. This moment of time lies between 10 and 12 min after mixing the reaction components according to observations reported previously [16]. The right panels in Fig. 2 present images of dried polymer films obtained for conditions corresponding to those in the left panels. The contrast in the figures emerges from the nonuniform thickness and surface modulation of the film. For the conditions corresponding to the upper panels it is obvious that the polymer structure
Fig. 1. Images showing spatially uniform (a) and spatially nonuniform (b) polymers (dried films) obtained under different conditions. Initial concentrations of variable components: (a) 0.037 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide; (b) 0.053 M KBrO3 and 0.01 M of N,N 0 -methylenbisacrylamide. Initial concentrations of the other components: 0.364 M H2 SO4 ; 0.09 M of malonic acid; 3:5 104 M of ferroin; 1.34 M of acrylamide; 0.0186 M of ammonium peroxodisulfate; 0.082 M of triethanolamine. The size of each frame is 43:3 43:3 mm.
mimics the wave observed in the BZ system. In addition, there are ripples visible in Fig. 2b. Fig. 2d also indicates a reflection of the wave which is little
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
537
Fig. 2. Images showing waves and ripples formed in the BZ system during gelation (a,c) and corresponding dried polymer films formed in these systems (b,d). Initial concentrations of variable components: (a,b) 0.053 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide; (c,d) 0.047 M KBrO3 and 0.02 M of N,N 0 -methylenbisacrylamide. Initial concentrations of the other components: 0.037 M KBrO3 and 0.364 M H2 SO4 ; 0.09 M of malonic acid; 3:5 104 M of ferroin; 1.34 M of acrylamide; 0.0186 M of ammonium peroxodisulfate; 0.082 M of triethanolamine. Size of pictures (a,c) is 94 75 mm, size of the pictures (b,d) is 43:3 43:3 mm.
pronounced comparing to Fig. 2b. However, in this case the ripples are better expressed for the BZ system (Fig. 2c) as well as for the polymer (Fig. 2d). Our experiments have shown that an increase of cross-linker initial concentration leads to the appearance of macroscopically ordered structures in the resulting polymer. These structures are governed by waves and ripples formed in the BZ system. Varying the initial concentration of the cross-linker indicates a critical minimal concentration of 0.008 M of N,N 0 -methylenbisacrylamide below which a macroscopically structured polymer does not form. Probably this finding is associated with insufficient cross-linking of polymer having consequences in its inability to ‘memorize’ (or reproduce) patterns from the BZ system. Varying the initial concentration of bromate ions lead us to the conclusion that a macroscopi-
cally structured polymer may be obtained only if waves or ripples appear in the BZ system. Below a critical initial concentration of bromate ions of 0.053 M there are no spatial structures in the BZ system and a spatially uniform polymer is formed. However, we have also found that there is an upper limit of the initial bromate ions concentration (0.16 M) above which no polymer is formed and only gelation occurs. The experimental data reported here may be supported by the kinetic scheme presented in Table 1. The scheme represents reactions of the BZ system catalyzed by ferroin coupled to the polymerization of acrylamide. The first 12 reactions in Table 1 correspond to the BZ chemistry [17]. If these reactions are coupled with diffusion, they produce a variety of spatial self-organization phenomena observed in the ferroin catalyzed BZ
538
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
Table 1 Kinetic scheme of the BZ reaction catalyzed by ferroin coupled with the polymerization of acrylamide Hþ þ HBrO2 þ HBrO3 () HBrOþ 2 þ BrO2 þ H2 O þ
BrO2
þ H ()
FeðphenÞ2þ 3
þ
ð1Þ
HBrOþ 2
HBrOþ 2
ð2Þ FeðphenÞ3þ 3
()
þ HBrO2
ð3Þ
2HBrO2 () HOBr þ HBrO3 þ
ð4Þ
H þ Br () HBrO2 þ 2HOBr
ð5Þ
Hþ þ Br þ HOBr () Br2 þ H2 O
ð6Þ
þ
H þ Br þ HBrO3 () HBrO2 þ HOBr FeðphenÞ3þ 3
ð7Þ
FeðphenÞ2þ 3
þ
ð8Þ
H2 O þ CBrðCOOHÞ2 () Hþ þ Br þ COHðCOOHÞ2
ð9Þ
þ CHBrðCOOHÞ2 ()
þ H þ CBrðCOOHÞ2
HOBr þ CHBrðCOOHÞ2 () CBr2 ðCOOHÞ2 þ H2 O þ
ð10Þ
Br2 þ CHBrðCOOHÞ2 () CBr2 ðCOOHÞ2 þ H þ Br
ð11Þ
CHBrðCOOHÞ2 þ H2 O () CHOHðCOOHÞ2 þ Hþ þ Br
ð12Þ
M þ CBrðCOOHÞ2 !
R1
þ CHBrðCOOHÞ2
ð13Þ
M þ COHðCOOHÞ2 ! R1 þ CHOHðCOOHÞ2
ð14Þ
S2 O2 8
ð15Þ
!
2SO 4
SO 4
ðcatalyzed by NðC2 H5 OÞ3 Þ
R1
ð16Þ
Rj þ M ! Rðjþ1Þ
ð17Þ
Mþ
Rðiþ1Þ Rk Rj Rj
þ
!
Rðjþ1Þ
þ CrL ! Rðjþ1Þ CrL Rðiþ1Þ
þ Rðjþ1Þ CrL Rðiþ1Þ ! þ
Rk
þ
BrO2
ð18Þ
Rm
ð19Þ
! RðjþkÞ
ð20Þ
! Rj BrO2
reaction. Among them are waves, target patterns, spirals, twinkling patterns [17,18], and even turing patterns which may be realized if the reaction is run in a microemulsion [19,20]. The scheme indicates that the BZ system itself produces free radicals according to reactions (8) and (9). These free radicals initiate the polymerization process according to reactions (13) and (14). Reactions (15) and (16) illustrate another pathway for the initiation of polymerization caused by the triethanolamine catalyzed decomposition of peroxodisulfate ions [14]. These reactions provide a sufficient source of radicals for the initiation of polymerization. Without adding triethanolamine and ammonium peroxodisulfate to the reaction mixture the polymer film does not form but gelation occurs instead. Reaction (17) describes a linear growth of the polymer
ð21Þ
chain whereas reactions (18) and (19) represent cross-linking [13]. Finally, polymerization terminates either due to the recombination process (20) or by BrO2 radicals according to reaction (21). The latter termination of polymerization is probably favorable as has been shown recently [4]. This kinetic scheme gives a qualitative explanation for the phenomena reported in this Letter. Particularly, an increase of bromate inhibits the formation of a polymer film due to termination of the polymerization according to reaction (21).
4. Conclusions Our studies have demonstrated the possibility to obtain macroscopically structured polyacryla-
Y.Y. Kalishyn et al. / Chemical Physics Letters 363 (2002) 534–539
mide where the structure is governed by chemical waves and patterns formed in the BZ system. The formation of the macroscopically structured polymer is caused by only a few factors: first of all, polyacrylamide does not directly interfere with the components of the BZ system. Secondly, the use of cross-linker N,N 0 -methylenbisacrylamide together with a proper initiator of the polymerization (ammonium peroxodisulfate and triethanolamine) allowed us to obtain macroscopically structured polymers. Thirdly, adding acrylamide, cross-linker and initiator does not drastically affect waves emerging in the BZ system. Adding all the components required for polymer formation just decelerates the wave propagation and leads to hydrodynamic patterns (ripples). The emergence of hydrodynamic patterns may be enhanced by the heat released through the exothermic polymerization reaction.
Acknowledgements The work has been supported financially by the NATO Cooperative Science & Technology SubProgramme (Collaborative linkage grant) and the DFG.
539
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