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OH radical induced depolymerization of poly(methacrylic acid)
Nuclear Instruments and Methods in Physics Research B 151 (1999) 350±355
www.elsevier.nl/locate/nimb
OH radical induced depolymerization of poly(methacrylic acid) Piotr Ulanski a b
a,b
, Eberhard Bothe a, Clemens von Sonntag
a,*
Max-Planck-Institut f ur Strahlenchemie, Stiftstr. 34-36, PO Box 101365, 45413 M ulheim an der Ruhr, Germany Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland
Abstract Hydroxyl radicals (generated pulse radiolytically in dilute N2 O-saturated aqueous solutions) react with poly(methacrylic acid) producing two kinds of radicals. The primary radical is converted into a secondary one by Habstraction (k 3.5 ´ 102 sÿ1 ) as monitored by changes in the UV spectrum. Subsequently, the secondary radicals undergo chain scission (k 1.8 sÿ1 at pH 7±9). This process has been followed both by spectrophotometry as well as by conductometry. In competition with the bimolecular decay of the radicals the ensuing end-chain radicals undergo ef®cient depolymerization resulting in the release of monomer. Since the lifetime of the radicals is much longer at high pH, where the polymer attains a rod-like conformation, depolymerization is most ecient in basic solution. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Poly(methacrylic acid); Ionizing radiation; Pulse radiolysis; Hydroxyl radical; Depolymerization; Crosslinking
1. Introduction Next to poly(acrylic acid), poly(methacrylic acid) is the most simple synthetic polyelectrolyte. Basic data on its synthesis and use [1], its properties and solution behaviour [2] are available. It has widely served as a model in the study of the properties of charged polymers in solution [3±8].
* Corresponding author. Tel.: +49-208-306-3529; fax: +49208-306-3951; e-mail: [email protected]
Poly(methacrylic acid) and its derivatives are used as thickeners and gelling agents, ion-exchange resins, ¯occulating agents, binders, adhesives and soil conditioners. A relatively new and very promising application of synthetic polyelectrolytes is the formation of a new class of biomaterials, the so-called intelligent hydrogels which are at present being tested for possible applications in medicine and pharmacology [9± 12]. Ionizing radiation is an excellent tool for
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 0 7 8 - 6
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
crosslinking such polymeric material, i.e. the formation of hydrogels for medical use. Bene®ts of this process are the high purity of the products and an easy process control. Moreover, at the same time the necessary sterilization is achieved [13,14]. In poly(acrylic acid), free-radical-induced crosslinking competes with chain scission, but conditions can be found where crosslinking dominates over scission [15]. In preceding studies we have noticed that with poly(methacrylic acid) the rate of radical-induced scission is considerably faster than with poly(acrylic acid) [16,17]. It will be shown that this can even lead to a considerable depolymerization. 2. Experimental From poly(methacrylic acid) (Polysciences) residual monomer and solvents were removed by drying in vacuo for 2 h at 40°C, dissolving in water by stirring overnight at R.T., dialysis (tangential ¯ow, Minitan, Millipore; membrane of a nominal molecular weight cut-o of 10 kDa) and ®ltration through a 5 mm-pore-size ®lter (Minisart, Sartorius). The remaining monomer content was below 0.1% (w/w of dry polymer); for further details see Ref. [17]. The weight-average molecular weight was 2.8 ´ 105 Da as determined by low-angle laser light-scattering. Polymer concentrations are given in monomer units. All the glassware and apparatus parts used to prepare, purify, store and irradiate poly(methacrylic acid) solutions were previously soaked overnight in aqueous EDTA solution in order to remove traces of transition metal ions that may cause unwanted side reactions (cf. Ref. [15]). c-irradiations were performed with a panorama 60 Co-c-source (Nuclear Engineering) at a dose rate of 0.092 Gy sÿ1 . Pulse radiolysis measurements were carried out with a van de Graa accelerator, generating 0.4±4 ms pulses of 2.8 MeV electrons, equipped with optical and conductometric detection systems [18,19]. Prior to irradiation, solutions were saturated for 1 h with N2 O puri®ed by an Oxisorb column (Messer±Griesheim). Fricke dosimetry was used for c-irradiations, while for opti-
351
cal and conductometric pulse experiments thiocyanate and dimethyl sulfoxide dosimetries were applied [20,21]. Methacrylic acid was identi®ed and quanti®ed by ion chromatography (Dionex 2010i; Ion-Pac AS9-SC column; eluent: 10ÿ3 mol dmÿ3 NaHCO3 ) after ultra®ltration (Amicon TCF10 system with a Dia¯o YM10 membrane) at pH 3 in order to remove polymeric material. Poly(methacrylic acid) radicals have a very long lifetime. Hence, to avoid their reaction with oxygen, the irradiated samples were kept closed overnight in the dark prior to analysis.
3. Results and discussion 3.1. Formation of poly(methacrylic acid) radicals When dilute, aqueous poly(methacrylic acid) solutions are subjected to ionizing radiation, its energy is absorbed practically only by water. In the radiolysis of water (reaction (1)), hydroxyl radicals, hydrated electrons and hydrogen atoms are formed in Ref. [22]. Nitrous oxide has been used to convert hydrated electrons into OH radicals (reaction (2)).
Hydroxyl radicals react with poly(methacrylic acid) by H-abstraction (reactions (3) and (4)). At pH 3.1, where poly(methacrylic acid) is protonated (the apparent pKa of the polymer is ca. 7 [23]) it reacts with a rate constant (related to monomer units) of k3;4 3.1 ´ 107 dm3 molÿ1 sÿ1 , and at pH 8.4 the nearly completely deprotonated polymer reacts four times faster (k3;4 1.2 ´ 108 dm3 molÿ1 sÿ1 ) [17]. These rate constants refer to a polymer with a weight-average molecular weight of 2.8 ´ 105 Da as used in these experiments.
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P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
The increase in rate at high pH results most probably from a deprotonation-induced change of these polyelectrolytes from a coil to a rod-like conformation. The more open rod-like conformation of the deprotonated structure makes is more accessible to OH-radical attack. In addition, the OH radicals react faster with carboxylate ions than with their corresponding free acids [24,25]. 3.2. Transformation of the primary radical 2 into the secondary radical 1 The absorption spectra of the poly(methacrylic acid) radicals formed initially are very similar at low and high pH and extend up to 350 nm [17]. At pH 7.5, where radicals of negatively charged poly(methacrylic acid) are very long-lived, the absorbance at 320 nm decays faster (half-life 2 ms) than that at 260 nm. This spectral change (inset in Fig. 1) is attributed to an intramolecular transformation of the primary radical 2 into the secondary radical 1 (reaction (5); note the favourable six-membered transition state). Similar radical transfer processes are well established, e.g. in the 2-propanol system the b-hydroxypropyl radical reacts with 2-propanol with a rate constant close to 103 dm3 molÿ1 sÿ1 yielding the a-hydroxyprop2-yl radical [26]. Also in poly(vinyl alcohol), the tertiary hydrogen is transferred even to the secondary radical (k 460 sÿ1 ) [27].
A further change in absorbance is observed in the time range where chain scission occurs (see below).
Fig. 1. Pulse radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at pH 7.5. Decay (short time scale, inset) and subsequent buildup (long time scale, main graph) at 320 nm as a function of time.
3.3. Chain scission One of the main reactions of the poly(methacrylic acid) radicals is chain breakage [28±30]). It probably proceeds by a b-scission of radicals 2 and/or 1 (e.g. reaction (6)). These reactions result in an end-chain radical 3 and an unsaturated terminal structural element such as 4. If our assignment of the optical changes observed in the 10 ms time range, i.e. a relatively fast transformation of radical 2 into radical 1 (reaction (5)) is correct, chain breakage (which occurs within a few seconds, see below) must occur from radical 1 (reaction (6)) rather than from radical 2.
Standard pulse radiolysis with optical detection does usually not allow to follow chain scission, since the absorption spectra of the mid-chain and terminal radicals are very similar. On the other hand, charged polymers oer the possibility to use
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
time-resolved conductance to monitor this process (for a discussion of this approach see Ref. [31]). Typical examples of conductivity changes after OH-radical attack on poly(acrylic acid) and poly(methacrylic acid) are shown in Fig. 2. It can be seen from this ®gure that there is a dramatic increase in the rate of scission on going from poly(acrylic acid) (Fig. 2, inset) to poly(methacrylic acid) (Fig. 2, main graph). It is noted that the kinetics of the slower change in the absorption at 320 nm (cf. Fig. 1) and the conductance change (Fig. 2) agree. This is supported by an identical temperature dependence (Fig. 3), wherefrom Ea 64 kJ molÿ1 (DH# 62 kJ molÿ1 , DS# ÿ29 J molÿ1 Kÿ1 ) are calculated. We therefore conclude that this change in the 320 nm absorption has to be attributed to the dierence of the absorption of the internal and the end-of-chain radical. In the range between pH 6 and 7 the rate of scission increases by a factor of 4, plateaues out in the pH range 7±9, and falls again at higher pH [17]. Details of this behaviour are not
353
Fig. 3. Pulse radiolysis of N2 O-saturated solutions of poly(methacrylic acid) at pH 8. Arrhenius plot of chain scission as followed optically at 320 nm (d; cf. Fig. 1 main graph, i.e. slow component) and conductometrically (n; cf. Fig.2).
yet fully understood. The maximum rate of chain breakage is 1.8 sÿ1 . This value is considerably slower than the value of 1.4 ´ 104 sÿ1 estimated on the basis of the in¯uence of benzoquinone on radiation-induced viscosity changes [29]. Since the present value has been measured with a more direct method, the earlier value has to be revised.
3.4. Depolymerization
Fig. 2. Pulse radiolysis of N2 O-saturated aqueous solution of poly(methacrylic acid) and poly(acrylic acid) (inset), pH 8.5, 2 Gy. Conductance increase as a function of time.
The main part of the conductivity increase is of ®rst-order, i.e. its half-life does not depend significantly on the dose per pulse (0.02±5 Gy per pulse). At later stages some further conductance increase is observed. The main origin of the slow conductivity change is likely due to the ensuing depolymerization (chain unzipping) reaction (7). Support for the occurrence of a very eective depolymerization has been obtained from c-radiolytic experiments. At a dose rate of 0.09 Gy sÿ1 and a dose of 20 Gy G(methacrylic acid) 500 ´ 10ÿ7 mol Jÿ1 (pH 9) has been measured.
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P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
It has been noticed before, that irradiated PMAA solutions bleach iodine and KMnO4 , and this has been attributed to the formation of the end-chain structural elements such as 4 [29]. We now found that besides this expected reaction the monomer, methacrylic acid 5, is formed in surprisingly high yields. This indicates that in this system the depolymerization reaction (7) is very eective. One way of how the depolymerization may come to a halt is an internal H-abstraction (via a six-membered transition state) yielding a mid-chain radical (as shown to occur in poly(acrylic acid) oligomers [32]; cf. also Ref. [15]) and subsequent b-fragmentation towards the end of the chain. The rate of depolymerization strongly depends on pH, and is only very prominent at high pH (Fig. 4).
Fig. 4. Monomer yields at a dose of 22.2 Gy in the c-radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at a dose rate of 0.092 Gy sÿ1 as a function of pH.
As the irradiation proceeds the monomer yield tends to come to a plateau (Fig. 5). At higher doses, the OH radicals react largely with the monomers, and their yield decreases again. We interpret the formation of a plateau by a competition of depolymerization and polymerization, i.e. the equilibrium (7)/(8). This shows that under these conditions the equilibrium is largely on the side of the monomer and explains why methacrylic acid has to be polymerized at low pH [33,34]. 3.5. Attempts to crosslink poly(methacrylic acid) at low pH Since the poly(methacrylic acid) radicals undergo chain scission and even depolymerize very rapidly at high pH, it is impossible to crosslink poly(methacrylic acid) under these conditions by ionizing radiation. In principle, at a high dose rate as can be delivered by electron beam irradiation, crosslinking of poly(methacrylic acid) could occur at low pH, where the radical lifetime is shorter and the rate of scission slower. However, at such very high dose rates a large number of radicals are
Fig. 5. c-radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at a dose rate of 0.092 Gy sÿ1 at pH 9.6. Monomer yields as a function of dose.
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
formed on each polymer molecule. This causes intramolecular crosslinking rather than gel formation. In order to favour the latter high poly(methacrylic acid) concentrations are required (smaller numbers of radicals on one macromolecule). At a 10% solution, some small particles had formed, but a wall-to-wall gel was not yet observed at a dose of 40 Gy per pulse (repetition 20 Hz, total dose up to 200 kGy, natural pH of the polymer). It is worth noting that the rate of scission for the dissociated poly(methacrylic acid) radicals is ca. 70 times faster than that of poly(acrylic acid) radicals under similar conditions [15]. This much slower rate of fragmentation is paralleled by the fact that an unzipping reaction (cf. reaction (7)) was not observed with the poly(acrylic acid)-derived radicals. References [1] J.W. Nemec, W. Bauer, Acrylic and Methacrylic Acid Polymers, Wiley, New York, 1988. [2] P. Molyneux, Water Soluble Polymers. Properties and Applications, CRC Press, Boca Raton, 1987. [3] E.A. Bekturov, Z.K. Bakaouva, Synthetic Water-Soluble Polymers in Solution, Huethig & Wepf, Basel, 1986. [4] M. Mandel, Polyelectrolytes, Wiley, New York, 1988. [5] A.F. Olea, J.K. Thomas, Macromolecules 22 (1989) 1165. [6] B. Bednar, J. Trnena, P. Svoboda, S. Vajda, V. Fidler, K. Prochazka, Macromolecules 24 (1991) 2054. [7] Y. Kurimura, Y. Sairenchi, S. Nakayama, Macromol. Chem. Macromol. Symp. 59 (1992) 199. [8] I. Soutar, L. Swanson, Polymer 35 (1994) 1942. [9] A.S. Homan, MRS Bulletin, September 1991, p. 42. [10] I. Kaetsu, K. Uchida, Y. Morita, M. Okubo, Radiat. Phys. Chem. 40 (1992) 157. [11] O. Hirasa, J. Intelligent Material Syst. Struct. 4 (1993) 538.
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[12] J. Dusek, Responsive Gels: Volume Transitions, Springer, Berlin, 1993. [13] J.M. Rosiak, in: R.C. Clough, S.W. Shalaby (Eds.) Radiation eects on polymers, Am. Chem. Soc. Book Series 475, 1991, p. 271, American Chemical Society, Washington. [14] J.M. Rosiak, J. Controlled Release 31 (1994) 9. [15] P. Ulanski, E. Bothe, K. Hildebrand, C. von Sonntag, J. Chem. Soc. Perkin Trans. II (1996) 13. [16] C. von Sonntag, E. Bothe, P. Ulanski, D.J. Deeble, Radiat. Phys. Chem. 46 (1995) 527. [17] P. Ulanski, E. Bothe, C. von Sonntag, Radiat. Phys. Chem., in press. [18] C. von Sonntag, H.-P. Schuchmann, Methods Enzymol. 233 (1994) 3. [19] E. Bothe, E. Janata, Radiat. Phys. Chem. 44 (1994) 455. [20] R.H. Schuler, L.K. Patterson, E. Janata, J. Phys. Chem. 84 (1980) 2088. [21] D. Veltwisch, E. Janata, K.-D. Asmus, J. Chem. Soc. Perkin Trans. II (1980) 146. [22] C. von Sonntag, The Chemical Basis of Radiation Biology, Taylor and Francis, London, 1987. [23] A. Katchalsky, J. Gillis, Proc. Int. Colloqu. Macromol., Amsterdam, 1950 p. 277. [24] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, J. Phys. Chem. Ref. Data 17 (1988) 513. [25] P. Ulanski, E. Bothe, J.M. Rosiak, C. von Sonntag, J. Chem. Soc. Perkin Trans. II (1996) 5. [26] H.-P. Schuchmann, C. von Sonntag, Radiat. Phys. Chem. 32 (1988) 149. [27] C. von Sonntag, E. Bothe, P. Ulanski, A. Adhikary, Radiat. Phys. Chem., in press. [28] P. Alexander, A. Charlesby, J. Polym. Sci. 23 (1957) 355. [29] T.S. Chen, J.K. Thomas, Radiat. Phys. Chem. 15 (1980) 429. [30] H. Nishide, M.D. Cho, T. Kaku, Y. Okamoto, Macromolecules 26 (1993) 2377. [31] E. Bothe, D. Schulte-Frohlinde, Z. Naturforsch 37c (1982) 1191. [32] B.C. Gilbert, J.R.L. Smith, C.E. Milne, A.C. Whitwood, P. Taylor, J. Chem. Soc. Perkin Trans. II (1994) 1759. [33] G. Blauer, J. Poly. Sci. 11 (1953) 189. [34] G. Blauer, Trans. Faraday Soc. 56 (1960) 606.
www.elsevier.nl/locate/nimb
OH radical induced depolymerization of poly(methacrylic acid) Piotr Ulanski a b
a,b
, Eberhard Bothe a, Clemens von Sonntag
a,*
Max-Planck-Institut f ur Strahlenchemie, Stiftstr. 34-36, PO Box 101365, 45413 M ulheim an der Ruhr, Germany Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland
Abstract Hydroxyl radicals (generated pulse radiolytically in dilute N2 O-saturated aqueous solutions) react with poly(methacrylic acid) producing two kinds of radicals. The primary radical is converted into a secondary one by Habstraction (k 3.5 ´ 102 sÿ1 ) as monitored by changes in the UV spectrum. Subsequently, the secondary radicals undergo chain scission (k 1.8 sÿ1 at pH 7±9). This process has been followed both by spectrophotometry as well as by conductometry. In competition with the bimolecular decay of the radicals the ensuing end-chain radicals undergo ef®cient depolymerization resulting in the release of monomer. Since the lifetime of the radicals is much longer at high pH, where the polymer attains a rod-like conformation, depolymerization is most ecient in basic solution. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Poly(methacrylic acid); Ionizing radiation; Pulse radiolysis; Hydroxyl radical; Depolymerization; Crosslinking
1. Introduction Next to poly(acrylic acid), poly(methacrylic acid) is the most simple synthetic polyelectrolyte. Basic data on its synthesis and use [1], its properties and solution behaviour [2] are available. It has widely served as a model in the study of the properties of charged polymers in solution [3±8].
* Corresponding author. Tel.: +49-208-306-3529; fax: +49208-306-3951; e-mail: [email protected]
Poly(methacrylic acid) and its derivatives are used as thickeners and gelling agents, ion-exchange resins, ¯occulating agents, binders, adhesives and soil conditioners. A relatively new and very promising application of synthetic polyelectrolytes is the formation of a new class of biomaterials, the so-called intelligent hydrogels which are at present being tested for possible applications in medicine and pharmacology [9± 12]. Ionizing radiation is an excellent tool for
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 0 7 8 - 6
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
crosslinking such polymeric material, i.e. the formation of hydrogels for medical use. Bene®ts of this process are the high purity of the products and an easy process control. Moreover, at the same time the necessary sterilization is achieved [13,14]. In poly(acrylic acid), free-radical-induced crosslinking competes with chain scission, but conditions can be found where crosslinking dominates over scission [15]. In preceding studies we have noticed that with poly(methacrylic acid) the rate of radical-induced scission is considerably faster than with poly(acrylic acid) [16,17]. It will be shown that this can even lead to a considerable depolymerization. 2. Experimental From poly(methacrylic acid) (Polysciences) residual monomer and solvents were removed by drying in vacuo for 2 h at 40°C, dissolving in water by stirring overnight at R.T., dialysis (tangential ¯ow, Minitan, Millipore; membrane of a nominal molecular weight cut-o of 10 kDa) and ®ltration through a 5 mm-pore-size ®lter (Minisart, Sartorius). The remaining monomer content was below 0.1% (w/w of dry polymer); for further details see Ref. [17]. The weight-average molecular weight was 2.8 ´ 105 Da as determined by low-angle laser light-scattering. Polymer concentrations are given in monomer units. All the glassware and apparatus parts used to prepare, purify, store and irradiate poly(methacrylic acid) solutions were previously soaked overnight in aqueous EDTA solution in order to remove traces of transition metal ions that may cause unwanted side reactions (cf. Ref. [15]). c-irradiations were performed with a panorama 60 Co-c-source (Nuclear Engineering) at a dose rate of 0.092 Gy sÿ1 . Pulse radiolysis measurements were carried out with a van de Graa accelerator, generating 0.4±4 ms pulses of 2.8 MeV electrons, equipped with optical and conductometric detection systems [18,19]. Prior to irradiation, solutions were saturated for 1 h with N2 O puri®ed by an Oxisorb column (Messer±Griesheim). Fricke dosimetry was used for c-irradiations, while for opti-
351
cal and conductometric pulse experiments thiocyanate and dimethyl sulfoxide dosimetries were applied [20,21]. Methacrylic acid was identi®ed and quanti®ed by ion chromatography (Dionex 2010i; Ion-Pac AS9-SC column; eluent: 10ÿ3 mol dmÿ3 NaHCO3 ) after ultra®ltration (Amicon TCF10 system with a Dia¯o YM10 membrane) at pH 3 in order to remove polymeric material. Poly(methacrylic acid) radicals have a very long lifetime. Hence, to avoid their reaction with oxygen, the irradiated samples were kept closed overnight in the dark prior to analysis.
3. Results and discussion 3.1. Formation of poly(methacrylic acid) radicals When dilute, aqueous poly(methacrylic acid) solutions are subjected to ionizing radiation, its energy is absorbed practically only by water. In the radiolysis of water (reaction (1)), hydroxyl radicals, hydrated electrons and hydrogen atoms are formed in Ref. [22]. Nitrous oxide has been used to convert hydrated electrons into OH radicals (reaction (2)).
Hydroxyl radicals react with poly(methacrylic acid) by H-abstraction (reactions (3) and (4)). At pH 3.1, where poly(methacrylic acid) is protonated (the apparent pKa of the polymer is ca. 7 [23]) it reacts with a rate constant (related to monomer units) of k3;4 3.1 ´ 107 dm3 molÿ1 sÿ1 , and at pH 8.4 the nearly completely deprotonated polymer reacts four times faster (k3;4 1.2 ´ 108 dm3 molÿ1 sÿ1 ) [17]. These rate constants refer to a polymer with a weight-average molecular weight of 2.8 ´ 105 Da as used in these experiments.
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P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
The increase in rate at high pH results most probably from a deprotonation-induced change of these polyelectrolytes from a coil to a rod-like conformation. The more open rod-like conformation of the deprotonated structure makes is more accessible to OH-radical attack. In addition, the OH radicals react faster with carboxylate ions than with their corresponding free acids [24,25]. 3.2. Transformation of the primary radical 2 into the secondary radical 1 The absorption spectra of the poly(methacrylic acid) radicals formed initially are very similar at low and high pH and extend up to 350 nm [17]. At pH 7.5, where radicals of negatively charged poly(methacrylic acid) are very long-lived, the absorbance at 320 nm decays faster (half-life 2 ms) than that at 260 nm. This spectral change (inset in Fig. 1) is attributed to an intramolecular transformation of the primary radical 2 into the secondary radical 1 (reaction (5); note the favourable six-membered transition state). Similar radical transfer processes are well established, e.g. in the 2-propanol system the b-hydroxypropyl radical reacts with 2-propanol with a rate constant close to 103 dm3 molÿ1 sÿ1 yielding the a-hydroxyprop2-yl radical [26]. Also in poly(vinyl alcohol), the tertiary hydrogen is transferred even to the secondary radical (k 460 sÿ1 ) [27].
A further change in absorbance is observed in the time range where chain scission occurs (see below).
Fig. 1. Pulse radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at pH 7.5. Decay (short time scale, inset) and subsequent buildup (long time scale, main graph) at 320 nm as a function of time.
3.3. Chain scission One of the main reactions of the poly(methacrylic acid) radicals is chain breakage [28±30]). It probably proceeds by a b-scission of radicals 2 and/or 1 (e.g. reaction (6)). These reactions result in an end-chain radical 3 and an unsaturated terminal structural element such as 4. If our assignment of the optical changes observed in the 10 ms time range, i.e. a relatively fast transformation of radical 2 into radical 1 (reaction (5)) is correct, chain breakage (which occurs within a few seconds, see below) must occur from radical 1 (reaction (6)) rather than from radical 2.
Standard pulse radiolysis with optical detection does usually not allow to follow chain scission, since the absorption spectra of the mid-chain and terminal radicals are very similar. On the other hand, charged polymers oer the possibility to use
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
time-resolved conductance to monitor this process (for a discussion of this approach see Ref. [31]). Typical examples of conductivity changes after OH-radical attack on poly(acrylic acid) and poly(methacrylic acid) are shown in Fig. 2. It can be seen from this ®gure that there is a dramatic increase in the rate of scission on going from poly(acrylic acid) (Fig. 2, inset) to poly(methacrylic acid) (Fig. 2, main graph). It is noted that the kinetics of the slower change in the absorption at 320 nm (cf. Fig. 1) and the conductance change (Fig. 2) agree. This is supported by an identical temperature dependence (Fig. 3), wherefrom Ea 64 kJ molÿ1 (DH# 62 kJ molÿ1 , DS# ÿ29 J molÿ1 Kÿ1 ) are calculated. We therefore conclude that this change in the 320 nm absorption has to be attributed to the dierence of the absorption of the internal and the end-of-chain radical. In the range between pH 6 and 7 the rate of scission increases by a factor of 4, plateaues out in the pH range 7±9, and falls again at higher pH [17]. Details of this behaviour are not
353
Fig. 3. Pulse radiolysis of N2 O-saturated solutions of poly(methacrylic acid) at pH 8. Arrhenius plot of chain scission as followed optically at 320 nm (d; cf. Fig. 1 main graph, i.e. slow component) and conductometrically (n; cf. Fig.2).
yet fully understood. The maximum rate of chain breakage is 1.8 sÿ1 . This value is considerably slower than the value of 1.4 ´ 104 sÿ1 estimated on the basis of the in¯uence of benzoquinone on radiation-induced viscosity changes [29]. Since the present value has been measured with a more direct method, the earlier value has to be revised.
3.4. Depolymerization
Fig. 2. Pulse radiolysis of N2 O-saturated aqueous solution of poly(methacrylic acid) and poly(acrylic acid) (inset), pH 8.5, 2 Gy. Conductance increase as a function of time.
The main part of the conductivity increase is of ®rst-order, i.e. its half-life does not depend significantly on the dose per pulse (0.02±5 Gy per pulse). At later stages some further conductance increase is observed. The main origin of the slow conductivity change is likely due to the ensuing depolymerization (chain unzipping) reaction (7). Support for the occurrence of a very eective depolymerization has been obtained from c-radiolytic experiments. At a dose rate of 0.09 Gy sÿ1 and a dose of 20 Gy G(methacrylic acid) 500 ´ 10ÿ7 mol Jÿ1 (pH 9) has been measured.
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P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
It has been noticed before, that irradiated PMAA solutions bleach iodine and KMnO4 , and this has been attributed to the formation of the end-chain structural elements such as 4 [29]. We now found that besides this expected reaction the monomer, methacrylic acid 5, is formed in surprisingly high yields. This indicates that in this system the depolymerization reaction (7) is very eective. One way of how the depolymerization may come to a halt is an internal H-abstraction (via a six-membered transition state) yielding a mid-chain radical (as shown to occur in poly(acrylic acid) oligomers [32]; cf. also Ref. [15]) and subsequent b-fragmentation towards the end of the chain. The rate of depolymerization strongly depends on pH, and is only very prominent at high pH (Fig. 4).
Fig. 4. Monomer yields at a dose of 22.2 Gy in the c-radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at a dose rate of 0.092 Gy sÿ1 as a function of pH.
As the irradiation proceeds the monomer yield tends to come to a plateau (Fig. 5). At higher doses, the OH radicals react largely with the monomers, and their yield decreases again. We interpret the formation of a plateau by a competition of depolymerization and polymerization, i.e. the equilibrium (7)/(8). This shows that under these conditions the equilibrium is largely on the side of the monomer and explains why methacrylic acid has to be polymerized at low pH [33,34]. 3.5. Attempts to crosslink poly(methacrylic acid) at low pH Since the poly(methacrylic acid) radicals undergo chain scission and even depolymerize very rapidly at high pH, it is impossible to crosslink poly(methacrylic acid) under these conditions by ionizing radiation. In principle, at a high dose rate as can be delivered by electron beam irradiation, crosslinking of poly(methacrylic acid) could occur at low pH, where the radical lifetime is shorter and the rate of scission slower. However, at such very high dose rates a large number of radicals are
Fig. 5. c-radiolysis of N2 O-saturated aqueous solutions of poly(methacrylic acid) (10ÿ2 mol dmÿ3 ) at a dose rate of 0.092 Gy sÿ1 at pH 9.6. Monomer yields as a function of dose.
P. Ulanski et al. / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 350±355
formed on each polymer molecule. This causes intramolecular crosslinking rather than gel formation. In order to favour the latter high poly(methacrylic acid) concentrations are required (smaller numbers of radicals on one macromolecule). At a 10% solution, some small particles had formed, but a wall-to-wall gel was not yet observed at a dose of 40 Gy per pulse (repetition 20 Hz, total dose up to 200 kGy, natural pH of the polymer). It is worth noting that the rate of scission for the dissociated poly(methacrylic acid) radicals is ca. 70 times faster than that of poly(acrylic acid) radicals under similar conditions [15]. This much slower rate of fragmentation is paralleled by the fact that an unzipping reaction (cf. reaction (7)) was not observed with the poly(acrylic acid)-derived radicals. References [1] J.W. Nemec, W. Bauer, Acrylic and Methacrylic Acid Polymers, Wiley, New York, 1988. [2] P. Molyneux, Water Soluble Polymers. Properties and Applications, CRC Press, Boca Raton, 1987. [3] E.A. Bekturov, Z.K. Bakaouva, Synthetic Water-Soluble Polymers in Solution, Huethig & Wepf, Basel, 1986. [4] M. Mandel, Polyelectrolytes, Wiley, New York, 1988. [5] A.F. Olea, J.K. Thomas, Macromolecules 22 (1989) 1165. [6] B. Bednar, J. Trnena, P. Svoboda, S. Vajda, V. Fidler, K. Prochazka, Macromolecules 24 (1991) 2054. [7] Y. Kurimura, Y. Sairenchi, S. Nakayama, Macromol. Chem. Macromol. Symp. 59 (1992) 199. [8] I. Soutar, L. Swanson, Polymer 35 (1994) 1942. [9] A.S. Homan, MRS Bulletin, September 1991, p. 42. [10] I. Kaetsu, K. Uchida, Y. Morita, M. Okubo, Radiat. Phys. Chem. 40 (1992) 157. [11] O. Hirasa, J. Intelligent Material Syst. Struct. 4 (1993) 538.
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