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Photoinitiated degradation of polystyrene in the presence of low-molecular organic compounds
European Polymer Journal 36 (2000) 1167±1173
Photoinitiated degradation of polystyrene in the presence of low-molecular organic compounds H. Kaczmarek*, A. KaminÂska, M. SÂwiatek, S. Sanyal ` Faculty of Chemistry, Nicolaus Copernicus University, Gagarin 7, 87-100 TorunÂ, Poland Received 31 March 1999; received in revised form 2 June 1999; accepted 18 June 1999
Abstract The in¯uence of low-molecular organic compounds such as benzophenone (BPh), anthraquinone (AQ) and benzoyl peroxide (BPo) on the phototransformations of polystyrene has been investigated. The viscometry, gravimetry, infrared and UV±vis spectroscopy has been used in these studies. The results indicate that additives applied accelerate and increase the eciency of photodegradation, photodestruction and photo-oxidation processes in polystyrene but they hamper the photocrosslinking and formation of the double bonds in this polymer. In all these processes, BPh showed the greater activity as photoinitiator comparing to AQ and BPo. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Photodegradation; Polystyrene; Benzophenone; Anthraquinone; Benzoyl peroxide
1. Introduction Ketones, quinones and peroxides are initiators of dierent reactions (polymerisation, degradation, chemical modi®cation) occurring in organic compounds [1± 11]. They absorb a light up to about 380 nm, which cause their excitation or cleavage into free radicals. These ones may initiate polymer degradation and other transformations by abstraction of hydrogen atom from a macromolecule (PH) and formation of polymer alkyl radical (P ) [12±14]. Eectivity of these photoinitiators action depends on the atmosphere, temperature and solvent [12±15]. It was found that they accelerate the degradation of polyisoprene and polystyrene in benzene solution [16]. Quinones cause increase formation of gel during irradiation of solid polyethylene [17], polystyrene
* Corresponding author. Fax: +48-56-6542477.
[15,18] and poly(methyl methacrylate) [19]. Benzophenone sensitises the photodegradation of cis-1,4 polyisoprene in benzene solution at air, while photocrosslinking of this polymer is dominant at the oxygen-free atmosphere [15]. Moreover, at the presence of benzophenone, dierent way of degradation of poly(vinyl chloride) occurred [20]. It has been shown in previous works [21±26], that polystyrene (PS) degrades upon UV-irradiation through two distinct steps: the initial decomposition of photolabile structures and the other with the photolysis of oxidation products, such as hydroperoxides. Thus, during ®rst stage of degradation, the initial photodecomposition is responsible directly (by the formation of alkoxy radicals) or indirectly (by production of ketonic species from alkoxy radicals) for the secondary, subsequent reactions in polymer. The aim of this work was to answer for the question: how much the chemical structure of the photoinitiators (benzophenone Ð BPh, anthraquinone Ð AQ
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and benzoyl peroxide Ð BPo) in¯uence to photochemical transformations i.e., degradation, crosslinking, oxidation and double bond formation in PS.
2. Experimental 2.1. Materials used Commercial polystyrene (PS±SF, Os wiecim, Poland) ` n 64000 w of average molecular weights M and M 169000 was used after puri®cation. The following low-molecular organic compounds were employed as photoinitiators: . benzophenone (99%, Aldrich, Germany); . 9,10-anthraquinone (sublimed, Chemapol, Praha, Czech Republic); . dibenzoyl peroxide (pure, Argon, /loÂdzÂ, Poland) Ð commonly known as benzoyl peroxide. A 1% solutions of PS and organic compounds in benzene were made. The solution of BPh, AQ or BPo was added to PS solution at suitable ratio. Thin (about 20 mm) PS ®lms with and without guest molecules were obtained by casting of solutions onto horizontal glass plate. After solvent evaporation, samples were dried in a vacuum drier over 24 h. This was found to be adequate to completely remove benzene from ®lms. Film thickness was measured with the help of Thickness Gauge, Ultrameter A-91 (INCO, Poland) composed of electronic supply system and open magnetic circuit (accuracy = 21 mm). 2.2. Irradiation The ®lms were exposed upon low-pressure vapour mercury lamp (TUV30W, Philips, Holland) for 0.25±2 h at ambient temperature in air atmosphere. Lamp emits radiation with wavelength equal to 253.7 nm and intensity 2.2520.06 mW/cm2. 2.3. Analysis Exposed and control samples were analysed for
C1O, OH and CH2 group content by examining their IR spectra, which were obtained with a Perkin±Elmer FTIR spectrometer Spectrum 2000. The formation of the double bonds was observed in the UV±vis absorption spectra, which were obtained by using Spectrophotometer UV-1601 PC (Shimadzu, Japan). Chain breaking was studied by viscometry using an Ubbelohde viscometer. Measurements were performed in 1% polymer solutions in benzene at 20 2 0.18C. Limiting viscosity numbers (LVN) were calculated from double extrapolation of Kramers and Huggins equations to concentration equal zero. Insoluble gel formed in UV-irradiated PS was separated from solution, dried in vacuum at room temperature and weighted. All estimations were repeated three times for each sample.
3. Results and discussion The changes of the LVN caused by photochemical transformation in PS and PS with 0.1 or 0.5% addition of BPh, AQ or BPo are presented in Table 1. The fast drop of LVN is observed in UV-irradiated PS in the presence of BPh while other photoinitiators have no such signi®cant in¯uence on the decrease of LVN in comparison to pure PS. The greatest changes of LVN occur in PS containing 0.1% BPh. All additives restrain a photocrosslinking of PS. It is shown by a mass of insoluble gel, which is formed as eect of this process (Table 2). Here also greater eectivity was observed in PS + 0.1% BPh sample. It is in agreement with the statement, that gel is formed with smaller eciency in polymer with lower molecular weight [27,28]. Thus, above results indicate that photodegradation of PS with breaking up of C±C bonds in main chain is strongly accelerated by BPh. 0.1% content of BPh shows greater in¯uence on eciency of this process in PS than 0.5% concentration. It may be explained by the decrease of component compatibility with increase of BPh content and, in consequence, by the decrease of contact surface between polymer and BPh. Moreover, simultaneously with the photodegrada-
Table 1 In¯uence of UV-irradiation on limiting viscosity number of PS and PS with additives Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.5 1.0 2.0
113.5 109.2 105.8 104.2
114.1 111.0 107.3 106.7
114.0 110.3 108.1 107.0
113.1 103.2 94.5 89.4
113.2 105.3 96.7 91.5
114.4 110.9 109.3 104.5
114.6 111.0 110.9 106.8
H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173
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Table 2 Amount of gel (%) formed in PS and PS with additives after UV irradiation Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.5 1.0 2.0
0 13.6 21.9 28.5
0 8.8 15.3 21.4
0 9.7 18.0 22.6
0 6.3 14.7 18.7
0 7.2 16.1 19.2
0 10.0 16.9 23.1
0 13.3 17.2 24.4
tion and photocrosslinking, photodestruction with breaking up of the pendant groups or fragments from end chains may occur in PS. As the eect of these reactions, the decrease of concentration of CH3, CH2 and CH groups is observed in IR spectra. As can be seen in Fig. 1, these changes (measured as an integral intensity of absorption band due to stretching vibration in 2770±3130 cmÿ1 range) in the samples with BPh, AQ and BPo are higher than in pure PS. The greatest eect on acceleration of PS photodestruction exhibits again 0.1% BPh addition. Macroradicals, which were formed as a result of photodegradation and photodestruction, at the presence of air may react with oxygen forming macroperoxyradicals and then hydroperoxides [12±14]. After their dissociation on alkoxy and hydroxy radicals, ®nally C1O and OH groups are created in the polymer chains:
Fig. 1. Changes of CH3 + CH2 + CH group concentration in PS and PS with BPh, AQ and BPo during UV-irradiation calculated as a surface area of absorption band in 2770±3130 cmÿ1 range.
(1) O2 PH ÿ4 ÿ P1 O±O or P2 O±O ÿ ÿ4 ÿ P1 OOH P1 or P2 ÿ
or P2 OOH P hn POOHÿ ÿ4 ÿ PO HO
2
3
(4) Although the formation of P2 radicals in UV-irradiated PS was suggested [23], their presence is less probable than existence of P1 , which are more stable. The dierent type of free radicals generated during photolysis of PS was recently identi®ed by ESR spectroscopy [26,29]. The results of photo-oxidation processes in investigated samples are presented in Table 3. As can be seen, in prevalent cases, the eciency and the rate of C1O groups formation is greater in PS samples containing initiators. The highest yield of C1O after 2 h UV-irradiation was observed in PS + 0.5% BPh. In this time, the concentration of OH and OOH groups in the samples with additives, in general, is smaller than in pure PS (Table 4). It is necessary to note, that carbonyl groups are created from OOH decay. Therefore, it con®rms that photo-oxidation of PS is accelerated by action of additives used (specially by BPh and AQ). Besides considering the above reactions, conjugated double bonds appear in PS during its UV-irradiation. In ®rst step of degradation, macroallyl radicals are created from alkyl radicals, and then, polyene sequences with dierent number of double bonds appeared in subsequent, chain reactions. Polyene radicals are characterised by relatively high ther-
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Table 3 Percentage changes of integral intensity of CO groups in pure PS and PS with initiators during UV-irradiation (calculated from IR absorption band at 1700±1770 cmÿ1 range) Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.25 0.5 1.0 2.0
0 9 27 69 96
0 10 45 87 103
0 16 18 64 187
0 16 48 89 118
0 35 42 91 195
0 7 80 81 154
0 20 65 99 101
mal stability due to delocalisation of p-electrons [26].
5 They cause increase of absorbance at the UV±vis region and shift the absorption band of degraded PS to long wavelength. This batochromic shift enhances polymer sensitivity onto visible light. This reaction is also seen visually as sample yellowing. As was described earlier, the dierent bands in absorption spectra of degraded polymer can be attributed suitable number of conjugated double bonds [21]. In our case, there are no clear maxima in electronic absorption spectrum of UV-irradiated PS, however, systematic increase of absorbance in the whole UV±vis region was observed. We calculated the percentage changes of absorbance at 312, 328 and 368 nm, which due to polyenes with n 2, 3, 4, respectively. An example of these changes is shown in Fig. 2. It can be seen, that processes of double bond formation occur in pure PS with
greater eciency than in PS doped with BPh, AQ and BPo. Similar trends were obtained for absorbance at 328 and 368 nm but changes occurred are smaller. It means that photoinitiators used hamper reactions leading to unsaturation in polymer chain. Probably radicals from initiators deactivate allyl macroradicals more eciently than atmospheric oxygen, which is known as radical scavenger. Another reason of sample yellowing is formation of quinomethane groups in UV-irradiated PS (reaction (6)) [12,13]. Possibility of creation of dierent type of other chromophores in PS such as acetophenones, benzalacetophenones, a,b-unsaturated ketones, products of ring opening reactions was described by Rabek [12± 14].
6
Table 4 Percentage changes of integral intensity of OH/OOH groups in pure PS and PS with initiators during UV-irradiation (calculated from IR absorption band at 3140±3680 cmÿ1 range) Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.25 0.5 1.0 2.0
0 165 394 724 816
0 142 367 745 756
0 172 272 641 753
0 194 466 505 788
0 199 341 643 697
0 380 473 768 971
0 52 122 438 478
H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173 hn ÿ4 ÿ 2HO H2 O2 ÿ
1171
10
11 The ketyl radical formed from BPh can react with another ketyl radical to produce benzopinacol (1,1,2,2tetraphenyl-1,2-ethanodiol) [30,36]. Such reaction ((11)) is more probable in sample with higher concentration of BPh and it explains smaller eciency of PS degradation in sample containing 0.5% BPh comparing to PS + 0.1% BPh.
Fig. 2. Changes of double bond concentration in PS and PS with BPh, AQ and BPo during UV-irradiation calculated as a percentage changes of absorbance at 312 nm.
12 Above results point out that eectivity of low-organic molecular compound in photoinitiation of PS degradation depends upon their structure and surroundings of C1O groups in BPh, AQ and BPo. The great activity of BPh is probably caused not only by its reaction with polymer (reaction (7)) but also by reaction of ketyl radicals with oxygen (reaction (8)) and creation of active HO and HOO radicals [12,30]. These radicals are known as accelerators of photo-oxidative degradation of polymers [31±35]. Recombination of small HO and HOO radicals with macroalkyl radicals leads to formation of hydroxy/hydroperoxy and, in consequence, to aldehyde groups (shown in reactions (3) and (4)).
2HOO 4 O2 H2 O2
9
AQ biradicals after reaction with PS form macroradicals (P ) and semiquinone radicals which may terminate another radicals [12]. It may cause smaller eectivity of AQ than BPh in photo-oxidative degradation of PS.
Second reason of dierences in reactivity of BPh and AQ is dierent lifetime of their excited states. Lifetime of triplet excited state of BPh (6.9 ms) is over 60 times longer that in AQ (0.11 ms) [37]. Thus, deactivation of long-living excited states of BPh in chemical reactions with macromolecules becomes very probable. BPo under UV-irradiation undergoes decomposition onto pairs of benzoyl radicals, which are very unstable and rapidly eliminate CO2 creating phenyl radicals [38]:
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tiator depends on its chemical structure, concentration, life-time of its excited states and stability of radicals formed during their photolysis. 15 These radicals in contact with polymer molecules form macroradicals and non-reactive benzene molecules. In this way, the action of this photoinitiator may be rapidly diminished.
16 The recombination of radicals generated from BPo with phenyl radicals leading to phenyl benzoate is also possible resulting of cage eect.
17 Since the photoreactions take place at room temperature, which is much below the glass transition temperature of PS, the role of diusion of low-molecular additives and the PS segmental reconformation can be discounted. The assumption, that relative positions of these additives and PS segments are ®xed, seems to be correct. Finally, it is necessary to add that substances used can act as typical sensitisers, which transfer their excitation energy to polymer molecules. However, this mechanism is more important in case of using of long wavelength radiation, which is absorbed by aromatic ketones but not by PS. In our case, radiation with 254 nm wavelength carries enough high energy (470 kJ/mol of photons) for chemical bond breaking, thus, free radical mechanism of initiation is predominant.
4. Conclusions It can be concluded that addition of small amount (0.1±0.5 wt%) of low-molecular compounds such as BPh, AQ and BPo eectively in¯uences the photoprocesses in polystyrene. It has been found that photo-oxidative degradation of PS is more ecient in the presence of these additives and BPh action is strongest, especially in sample with its very low content. Competitive reactions occurring in UV-irradiated PS-crosslinking and double bond formation are retarded in the presence of these initiators. Results obtained suggest, that eectivity of photoini-
References [1] Yamashita T, Watanabe M, Kojima R, Shiragami T, Shima K, Yasuda M. J Photochem Photobiol, Part A: Chem 1998;118:165. [2] Kani R, Nakano Y, Yoshida H, Hayase S. Macromolecules 1998;31:8794. [3] Karakisla M, Sacek M. J Appl Polym Sci 1998;70:1701. [4] Shield SR, Harris JM. Anal Chem 1998;70:2576. [5] Bevington JC, Breuer SW, Huckerby TN, Hunt BJ. Eur Polym J 1998;34:539. [6] Jiang DD, Wilkie CA. Eur Polym J 1998;34:997. [7] Srivastava S, Yourd E, Toscano JP. J Am Chem Soc 1998;120:6173. [8] Lu Z, Huang X, Huang J. J Polym Sci, Part A: Chem 1998;36:109. [9] Oatsis Jr JE, Knapp DR. Tetrahedron Lett 1998;39:1665. [10] Langer NM, Wilkie CA. Polym Adv Technol 1998;9: 290. [11] Dauria M, Racioppi. J Photochem Photobiol 1998;112:145. [12] Rabek JF. Mechanisms of photophysical processes and photochemical reactions in polymers. Theory and applications. Chichester: Wiley, 1987. [13] Rabek JF. Polymer photodegradation Ð mechanisms and experimental methods. London: Chapman and Hall, 1993. [14] Rabek JF. Photodegradation of polymers: Physical characteristics and applications,. Berlin: Springer±Verlag, 1996. [15] Rabek JF. Chem Stosowana 1967;11:53, 73, 89. [16] Mill T, Irvin KC, Mayo FR. Rubber Chem Technol 1968;41:296. [17] Andrushenko DA, Bogdan LS, Kachan AA, Schrutovicz VA. Vysokomol Soed, A 1972;14:594. [18] Rabek JF, Ranby B. J Polym Sci, A1 1974;12:295. [19] Oster G. J Polym Sci, B 1964;2:1181. [20] Owen ED, Bailey RJ. J Polym Sci, A1 1972;10:113. [21] Geuskens G, Baeyens-Volant D, Delaunois G, Lu-Vinh Q, Piret W, David C. Eur Polym J 1978;14:291, 299. [22] Geuskens G. The photooxidation of polystyrene. In: Grassie N, editor. Developments in polymer degradationÐ3. London: Applied Science Publishers, 1982. p. 143 (Chapter 5). [23] Weir A. Photo- and photo-oxidative reactions of polystyrene and of ring subsituted polystyrenes. In: Grassie N, editor. Developments in polymer degradationÐ4. London: Applied Science Publishers, 1981. p. 207 (Chapter 7). [24] Weir NA, Kutok P, Whiting K. Polym Deg Stab 1989;24:247. [25] Prasad AV, Singh RP. J Appl Polym Sci 1998;70:637. [26] Kuzina SI, Mikhailov AI. Eur Polym J 1993;29:1589. [27] Kaminska A. Wplyw masy cz asteczkowej i polimoleku` larnos ci polimeroÂw na ich Wl/ as ciwos ci ®zyczne i mechan-
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[28] [29] [30] [31] [32]
iczne oraz na odpornos c na dzial/ anie promieniowania UV. UMK, Torun, 1975. Charlesby A. In: Atomic radiation and polymers. Oxford: Pergamon Press, 1960. p. 140. Kuzina SI, Mikhailov AI. Eur Polym J 1998;34:291, 1157. Pitts Jr. JN, Letsinger RL, Taylor RP, Patterson JM, Recktenwald G, Martin RB. J Am Chem Soc 1959;81:1068. Kaczmarek H, Linden LA, Rabek JF. Polym Deg Stab 1995;47:175. Kaczmarek H, Linden LA, Rabek JF. J Polym Sci, Part A: Polym Chem 1995;33:879.
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[33] Kaczmarek H. Polym Bull 1995;34:211. [34] Kaczmarek H, Linden LA, Rabek JF. Macromol Symp 1994;84:351. [35] Linden LA, Rabek JF, Kaczmarek H, Kaminska A. Coord Chem Rev 1993;125:195. [36] Beckett A, Porter G. Trans Faraday Soc 1963;59:2038, 2051. [37] Murov SL, Carmichael I, Hug GL. Handbook of photochemistry, 2nd ed. New York: Marcel Dekker, 1993. [38] Rabek JF. J Photochem Photobiol 1968;7:5.
Photoinitiated degradation of polystyrene in the presence of low-molecular organic compounds H. Kaczmarek*, A. KaminÂska, M. SÂwiatek, S. Sanyal ` Faculty of Chemistry, Nicolaus Copernicus University, Gagarin 7, 87-100 TorunÂ, Poland Received 31 March 1999; received in revised form 2 June 1999; accepted 18 June 1999
Abstract The in¯uence of low-molecular organic compounds such as benzophenone (BPh), anthraquinone (AQ) and benzoyl peroxide (BPo) on the phototransformations of polystyrene has been investigated. The viscometry, gravimetry, infrared and UV±vis spectroscopy has been used in these studies. The results indicate that additives applied accelerate and increase the eciency of photodegradation, photodestruction and photo-oxidation processes in polystyrene but they hamper the photocrosslinking and formation of the double bonds in this polymer. In all these processes, BPh showed the greater activity as photoinitiator comparing to AQ and BPo. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Photodegradation; Polystyrene; Benzophenone; Anthraquinone; Benzoyl peroxide
1. Introduction Ketones, quinones and peroxides are initiators of dierent reactions (polymerisation, degradation, chemical modi®cation) occurring in organic compounds [1± 11]. They absorb a light up to about 380 nm, which cause their excitation or cleavage into free radicals. These ones may initiate polymer degradation and other transformations by abstraction of hydrogen atom from a macromolecule (PH) and formation of polymer alkyl radical (P ) [12±14]. Eectivity of these photoinitiators action depends on the atmosphere, temperature and solvent [12±15]. It was found that they accelerate the degradation of polyisoprene and polystyrene in benzene solution [16]. Quinones cause increase formation of gel during irradiation of solid polyethylene [17], polystyrene
* Corresponding author. Fax: +48-56-6542477.
[15,18] and poly(methyl methacrylate) [19]. Benzophenone sensitises the photodegradation of cis-1,4 polyisoprene in benzene solution at air, while photocrosslinking of this polymer is dominant at the oxygen-free atmosphere [15]. Moreover, at the presence of benzophenone, dierent way of degradation of poly(vinyl chloride) occurred [20]. It has been shown in previous works [21±26], that polystyrene (PS) degrades upon UV-irradiation through two distinct steps: the initial decomposition of photolabile structures and the other with the photolysis of oxidation products, such as hydroperoxides. Thus, during ®rst stage of degradation, the initial photodecomposition is responsible directly (by the formation of alkoxy radicals) or indirectly (by production of ketonic species from alkoxy radicals) for the secondary, subsequent reactions in polymer. The aim of this work was to answer for the question: how much the chemical structure of the photoinitiators (benzophenone Ð BPh, anthraquinone Ð AQ
0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 7 5 - 5
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H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173
and benzoyl peroxide Ð BPo) in¯uence to photochemical transformations i.e., degradation, crosslinking, oxidation and double bond formation in PS.
2. Experimental 2.1. Materials used Commercial polystyrene (PS±SF, Os wiecim, Poland) ` n 64000 w of average molecular weights M and M 169000 was used after puri®cation. The following low-molecular organic compounds were employed as photoinitiators: . benzophenone (99%, Aldrich, Germany); . 9,10-anthraquinone (sublimed, Chemapol, Praha, Czech Republic); . dibenzoyl peroxide (pure, Argon, /loÂdzÂ, Poland) Ð commonly known as benzoyl peroxide. A 1% solutions of PS and organic compounds in benzene were made. The solution of BPh, AQ or BPo was added to PS solution at suitable ratio. Thin (about 20 mm) PS ®lms with and without guest molecules were obtained by casting of solutions onto horizontal glass plate. After solvent evaporation, samples were dried in a vacuum drier over 24 h. This was found to be adequate to completely remove benzene from ®lms. Film thickness was measured with the help of Thickness Gauge, Ultrameter A-91 (INCO, Poland) composed of electronic supply system and open magnetic circuit (accuracy = 21 mm). 2.2. Irradiation The ®lms were exposed upon low-pressure vapour mercury lamp (TUV30W, Philips, Holland) for 0.25±2 h at ambient temperature in air atmosphere. Lamp emits radiation with wavelength equal to 253.7 nm and intensity 2.2520.06 mW/cm2. 2.3. Analysis Exposed and control samples were analysed for
C1O, OH and CH2 group content by examining their IR spectra, which were obtained with a Perkin±Elmer FTIR spectrometer Spectrum 2000. The formation of the double bonds was observed in the UV±vis absorption spectra, which were obtained by using Spectrophotometer UV-1601 PC (Shimadzu, Japan). Chain breaking was studied by viscometry using an Ubbelohde viscometer. Measurements were performed in 1% polymer solutions in benzene at 20 2 0.18C. Limiting viscosity numbers (LVN) were calculated from double extrapolation of Kramers and Huggins equations to concentration equal zero. Insoluble gel formed in UV-irradiated PS was separated from solution, dried in vacuum at room temperature and weighted. All estimations were repeated three times for each sample.
3. Results and discussion The changes of the LVN caused by photochemical transformation in PS and PS with 0.1 or 0.5% addition of BPh, AQ or BPo are presented in Table 1. The fast drop of LVN is observed in UV-irradiated PS in the presence of BPh while other photoinitiators have no such signi®cant in¯uence on the decrease of LVN in comparison to pure PS. The greatest changes of LVN occur in PS containing 0.1% BPh. All additives restrain a photocrosslinking of PS. It is shown by a mass of insoluble gel, which is formed as eect of this process (Table 2). Here also greater eectivity was observed in PS + 0.1% BPh sample. It is in agreement with the statement, that gel is formed with smaller eciency in polymer with lower molecular weight [27,28]. Thus, above results indicate that photodegradation of PS with breaking up of C±C bonds in main chain is strongly accelerated by BPh. 0.1% content of BPh shows greater in¯uence on eciency of this process in PS than 0.5% concentration. It may be explained by the decrease of component compatibility with increase of BPh content and, in consequence, by the decrease of contact surface between polymer and BPh. Moreover, simultaneously with the photodegrada-
Table 1 In¯uence of UV-irradiation on limiting viscosity number of PS and PS with additives Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.5 1.0 2.0
113.5 109.2 105.8 104.2
114.1 111.0 107.3 106.7
114.0 110.3 108.1 107.0
113.1 103.2 94.5 89.4
113.2 105.3 96.7 91.5
114.4 110.9 109.3 104.5
114.6 111.0 110.9 106.8
H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173
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Table 2 Amount of gel (%) formed in PS and PS with additives after UV irradiation Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.5 1.0 2.0
0 13.6 21.9 28.5
0 8.8 15.3 21.4
0 9.7 18.0 22.6
0 6.3 14.7 18.7
0 7.2 16.1 19.2
0 10.0 16.9 23.1
0 13.3 17.2 24.4
tion and photocrosslinking, photodestruction with breaking up of the pendant groups or fragments from end chains may occur in PS. As the eect of these reactions, the decrease of concentration of CH3, CH2 and CH groups is observed in IR spectra. As can be seen in Fig. 1, these changes (measured as an integral intensity of absorption band due to stretching vibration in 2770±3130 cmÿ1 range) in the samples with BPh, AQ and BPo are higher than in pure PS. The greatest eect on acceleration of PS photodestruction exhibits again 0.1% BPh addition. Macroradicals, which were formed as a result of photodegradation and photodestruction, at the presence of air may react with oxygen forming macroperoxyradicals and then hydroperoxides [12±14]. After their dissociation on alkoxy and hydroxy radicals, ®nally C1O and OH groups are created in the polymer chains:
Fig. 1. Changes of CH3 + CH2 + CH group concentration in PS and PS with BPh, AQ and BPo during UV-irradiation calculated as a surface area of absorption band in 2770±3130 cmÿ1 range.
(1) O2 PH ÿ4 ÿ P1 O±O or P2 O±O ÿ ÿ4 ÿ P1 OOH P1 or P2 ÿ
or P2 OOH P hn POOHÿ ÿ4 ÿ PO HO
2
3
(4) Although the formation of P2 radicals in UV-irradiated PS was suggested [23], their presence is less probable than existence of P1 , which are more stable. The dierent type of free radicals generated during photolysis of PS was recently identi®ed by ESR spectroscopy [26,29]. The results of photo-oxidation processes in investigated samples are presented in Table 3. As can be seen, in prevalent cases, the eciency and the rate of C1O groups formation is greater in PS samples containing initiators. The highest yield of C1O after 2 h UV-irradiation was observed in PS + 0.5% BPh. In this time, the concentration of OH and OOH groups in the samples with additives, in general, is smaller than in pure PS (Table 4). It is necessary to note, that carbonyl groups are created from OOH decay. Therefore, it con®rms that photo-oxidation of PS is accelerated by action of additives used (specially by BPh and AQ). Besides considering the above reactions, conjugated double bonds appear in PS during its UV-irradiation. In ®rst step of degradation, macroallyl radicals are created from alkyl radicals, and then, polyene sequences with dierent number of double bonds appeared in subsequent, chain reactions. Polyene radicals are characterised by relatively high ther-
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Table 3 Percentage changes of integral intensity of CO groups in pure PS and PS with initiators during UV-irradiation (calculated from IR absorption band at 1700±1770 cmÿ1 range) Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.25 0.5 1.0 2.0
0 9 27 69 96
0 10 45 87 103
0 16 18 64 187
0 16 48 89 118
0 35 42 91 195
0 7 80 81 154
0 20 65 99 101
mal stability due to delocalisation of p-electrons [26].
5 They cause increase of absorbance at the UV±vis region and shift the absorption band of degraded PS to long wavelength. This batochromic shift enhances polymer sensitivity onto visible light. This reaction is also seen visually as sample yellowing. As was described earlier, the dierent bands in absorption spectra of degraded polymer can be attributed suitable number of conjugated double bonds [21]. In our case, there are no clear maxima in electronic absorption spectrum of UV-irradiated PS, however, systematic increase of absorbance in the whole UV±vis region was observed. We calculated the percentage changes of absorbance at 312, 328 and 368 nm, which due to polyenes with n 2, 3, 4, respectively. An example of these changes is shown in Fig. 2. It can be seen, that processes of double bond formation occur in pure PS with
greater eciency than in PS doped with BPh, AQ and BPo. Similar trends were obtained for absorbance at 328 and 368 nm but changes occurred are smaller. It means that photoinitiators used hamper reactions leading to unsaturation in polymer chain. Probably radicals from initiators deactivate allyl macroradicals more eciently than atmospheric oxygen, which is known as radical scavenger. Another reason of sample yellowing is formation of quinomethane groups in UV-irradiated PS (reaction (6)) [12,13]. Possibility of creation of dierent type of other chromophores in PS such as acetophenones, benzalacetophenones, a,b-unsaturated ketones, products of ring opening reactions was described by Rabek [12± 14].
6
Table 4 Percentage changes of integral intensity of OH/OOH groups in pure PS and PS with initiators during UV-irradiation (calculated from IR absorption band at 3140±3680 cmÿ1 range) Time of irradiation (h)
PS
PS + 0.1% AQ
PS + 0.5% AQ
PS + 0.1% BPh
PS + 0.5% BPh
PS + 0.1% BPo
PS + 0.5% BPo
0.0 0.25 0.5 1.0 2.0
0 165 394 724 816
0 142 367 745 756
0 172 272 641 753
0 194 466 505 788
0 199 341 643 697
0 380 473 768 971
0 52 122 438 478
H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173 hn ÿ4 ÿ 2HO H2 O2 ÿ
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10
11 The ketyl radical formed from BPh can react with another ketyl radical to produce benzopinacol (1,1,2,2tetraphenyl-1,2-ethanodiol) [30,36]. Such reaction ((11)) is more probable in sample with higher concentration of BPh and it explains smaller eciency of PS degradation in sample containing 0.5% BPh comparing to PS + 0.1% BPh.
Fig. 2. Changes of double bond concentration in PS and PS with BPh, AQ and BPo during UV-irradiation calculated as a percentage changes of absorbance at 312 nm.
12 Above results point out that eectivity of low-organic molecular compound in photoinitiation of PS degradation depends upon their structure and surroundings of C1O groups in BPh, AQ and BPo. The great activity of BPh is probably caused not only by its reaction with polymer (reaction (7)) but also by reaction of ketyl radicals with oxygen (reaction (8)) and creation of active HO and HOO radicals [12,30]. These radicals are known as accelerators of photo-oxidative degradation of polymers [31±35]. Recombination of small HO and HOO radicals with macroalkyl radicals leads to formation of hydroxy/hydroperoxy and, in consequence, to aldehyde groups (shown in reactions (3) and (4)).
2HOO 4 O2 H2 O2
9
AQ biradicals after reaction with PS form macroradicals (P ) and semiquinone radicals which may terminate another radicals [12]. It may cause smaller eectivity of AQ than BPh in photo-oxidative degradation of PS.
Second reason of dierences in reactivity of BPh and AQ is dierent lifetime of their excited states. Lifetime of triplet excited state of BPh (6.9 ms) is over 60 times longer that in AQ (0.11 ms) [37]. Thus, deactivation of long-living excited states of BPh in chemical reactions with macromolecules becomes very probable. BPo under UV-irradiation undergoes decomposition onto pairs of benzoyl radicals, which are very unstable and rapidly eliminate CO2 creating phenyl radicals [38]:
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tiator depends on its chemical structure, concentration, life-time of its excited states and stability of radicals formed during their photolysis. 15 These radicals in contact with polymer molecules form macroradicals and non-reactive benzene molecules. In this way, the action of this photoinitiator may be rapidly diminished.
16 The recombination of radicals generated from BPo with phenyl radicals leading to phenyl benzoate is also possible resulting of cage eect.
17 Since the photoreactions take place at room temperature, which is much below the glass transition temperature of PS, the role of diusion of low-molecular additives and the PS segmental reconformation can be discounted. The assumption, that relative positions of these additives and PS segments are ®xed, seems to be correct. Finally, it is necessary to add that substances used can act as typical sensitisers, which transfer their excitation energy to polymer molecules. However, this mechanism is more important in case of using of long wavelength radiation, which is absorbed by aromatic ketones but not by PS. In our case, radiation with 254 nm wavelength carries enough high energy (470 kJ/mol of photons) for chemical bond breaking, thus, free radical mechanism of initiation is predominant.
4. Conclusions It can be concluded that addition of small amount (0.1±0.5 wt%) of low-molecular compounds such as BPh, AQ and BPo eectively in¯uences the photoprocesses in polystyrene. It has been found that photo-oxidative degradation of PS is more ecient in the presence of these additives and BPh action is strongest, especially in sample with its very low content. Competitive reactions occurring in UV-irradiated PS-crosslinking and double bond formation are retarded in the presence of these initiators. Results obtained suggest, that eectivity of photoini-
References [1] Yamashita T, Watanabe M, Kojima R, Shiragami T, Shima K, Yasuda M. J Photochem Photobiol, Part A: Chem 1998;118:165. [2] Kani R, Nakano Y, Yoshida H, Hayase S. Macromolecules 1998;31:8794. [3] Karakisla M, Sacek M. J Appl Polym Sci 1998;70:1701. [4] Shield SR, Harris JM. Anal Chem 1998;70:2576. [5] Bevington JC, Breuer SW, Huckerby TN, Hunt BJ. Eur Polym J 1998;34:539. [6] Jiang DD, Wilkie CA. Eur Polym J 1998;34:997. [7] Srivastava S, Yourd E, Toscano JP. J Am Chem Soc 1998;120:6173. [8] Lu Z, Huang X, Huang J. J Polym Sci, Part A: Chem 1998;36:109. [9] Oatsis Jr JE, Knapp DR. Tetrahedron Lett 1998;39:1665. [10] Langer NM, Wilkie CA. Polym Adv Technol 1998;9: 290. [11] Dauria M, Racioppi. J Photochem Photobiol 1998;112:145. [12] Rabek JF. Mechanisms of photophysical processes and photochemical reactions in polymers. Theory and applications. Chichester: Wiley, 1987. [13] Rabek JF. Polymer photodegradation Ð mechanisms and experimental methods. London: Chapman and Hall, 1993. [14] Rabek JF. Photodegradation of polymers: Physical characteristics and applications,. Berlin: Springer±Verlag, 1996. [15] Rabek JF. Chem Stosowana 1967;11:53, 73, 89. [16] Mill T, Irvin KC, Mayo FR. Rubber Chem Technol 1968;41:296. [17] Andrushenko DA, Bogdan LS, Kachan AA, Schrutovicz VA. Vysokomol Soed, A 1972;14:594. [18] Rabek JF, Ranby B. J Polym Sci, A1 1974;12:295. [19] Oster G. J Polym Sci, B 1964;2:1181. [20] Owen ED, Bailey RJ. J Polym Sci, A1 1972;10:113. [21] Geuskens G, Baeyens-Volant D, Delaunois G, Lu-Vinh Q, Piret W, David C. Eur Polym J 1978;14:291, 299. [22] Geuskens G. The photooxidation of polystyrene. In: Grassie N, editor. Developments in polymer degradationÐ3. London: Applied Science Publishers, 1982. p. 143 (Chapter 5). [23] Weir A. Photo- and photo-oxidative reactions of polystyrene and of ring subsituted polystyrenes. In: Grassie N, editor. Developments in polymer degradationÐ4. London: Applied Science Publishers, 1981. p. 207 (Chapter 7). [24] Weir NA, Kutok P, Whiting K. Polym Deg Stab 1989;24:247. [25] Prasad AV, Singh RP. J Appl Polym Sci 1998;70:637. [26] Kuzina SI, Mikhailov AI. Eur Polym J 1993;29:1589. [27] Kaminska A. Wplyw masy cz asteczkowej i polimoleku` larnos ci polimeroÂw na ich Wl/ as ciwos ci ®zyczne i mechan-
H. Kaczmarek et al. / European Polymer Journal 36 (2000) 1167±1173
[28] [29] [30] [31] [32]
iczne oraz na odpornos c na dzial/ anie promieniowania UV. UMK, Torun, 1975. Charlesby A. In: Atomic radiation and polymers. Oxford: Pergamon Press, 1960. p. 140. Kuzina SI, Mikhailov AI. Eur Polym J 1998;34:291, 1157. Pitts Jr. JN, Letsinger RL, Taylor RP, Patterson JM, Recktenwald G, Martin RB. J Am Chem Soc 1959;81:1068. Kaczmarek H, Linden LA, Rabek JF. Polym Deg Stab 1995;47:175. Kaczmarek H, Linden LA, Rabek JF. J Polym Sci, Part A: Polym Chem 1995;33:879.
1173
[33] Kaczmarek H. Polym Bull 1995;34:211. [34] Kaczmarek H, Linden LA, Rabek JF. Macromol Symp 1994;84:351. [35] Linden LA, Rabek JF, Kaczmarek H, Kaminska A. Coord Chem Rev 1993;125:195. [36] Beckett A, Porter G. Trans Faraday Soc 1963;59:2038, 2051. [37] Murov SL, Carmichael I, Hug GL. Handbook of photochemistry, 2nd ed. New York: Marcel Dekker, 1993. [38] Rabek JF. J Photochem Photobiol 1968;7:5.