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Photopolymerization of oxetane based systems
European Polymer Journal 40 (2004) 353–358 www.elsevier.com/locate/europolj
Photopolymerization of oxetane based systems M. Sangermano *, G. Malucelli, R. Bongiovanni, A. Priola Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy Received 19 June 2003; received in revised form 7 July 2003; accepted 30 September 2003
Abstract Oxetane based systems were investigated in cationic photopolymerization process. The effect on the rate of polymerization induced by the presence of epoxy monofunctional monomers and vinyl ether monomers were investigated. In both cases an increase of the rate of polymerization is evidenced either induced by a flexibilization effect due to a copolymerization or to a cross-propagation mechanism. Acrylate comonomers were also investigated and the formation of an IPN tightly cross-linked evidenced. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Cationic photopolymerization; Oxetane monomers; Copolymerization; Interpenetrating networks
1. Introduction The UV-curing process is a polymerization technique in which radiation induces a fast transformation of a liquid monomer into a solid polymer. Radical or cationic species are generated by interaction of UV-light with a suitable photoinitiator, which induces the curing reaction of suitable reactive monomers and oligomers [1]. Cationic photoinduced process presents some advantages over the radical one [2]; in particular lack of inhibition by oxygen, low shrinkage, good mechanical properties of UV-cured materials and good adhesion properties to various substrates. In addition, the monomers employed are generally characterized by low irritation and toxicity properties. Different type of monomers and oligomers have been proposed and reported in literature, mainly epoxides [3– 6], vinyl ethers [7–10] and propenyl ethers [11–15]. More recently attention has been devoted to the synthesis and cationic photopolymerization of oxetane
* Corresponding author. Tel.: +39-011-564-4651; fax: +39011-564-4699. E-mail address: [email protected] (M. Sangermano).
monomers [16–21]. The high ring strain, very similar to that of epoxides, and the higher basicity of the heterocyclic oxygen in oxetanes than that for oxirane oxygen in epoxides, make oxetanes interesting alternative monomers in cationic UV-curing applications. The purpose of this work is to study oxetane based systems in photocured process; the kinetic of curing and the properties of final products were investigated, either for the pure monomer or in mixture with epoxy resins, with vinyl ether monomers and with acrylates.
2. Experimentals 2.1. Materials The following monomers were employed: bis[1ethyl(3-oxetanyl)]methyl ether (OXT-221, DOX) from Toagosei, Japan; Cyclohexene oxide (CY) from Aldrich; bis-(4-vinyl oxy butyl) isophtalate (Vectomer 4010, IDVE) from Allied Signal; Polyethylenglycol-600diacrylate (Ebecryl 11, PEGDA) from UCB Belgium. In Table 1 the structures of the monomers employed are reported. Triphenylsulphonium hexafluoroantimonate (UVI6974, PI) gently supplied from Dow-Union Carbide was employed as photoinitiator.
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.09.026
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M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
Table 1 Monomer employed
3. Results and discussion 3.1. Photopolymerization of oxetane monomer
O
O DOX CH2=CHCOO (CH2CH2)n OOCCH=CH2
O
PEGDA 600
CY
O
O
O
O
O
O
IDVE
2.2. UV-curing The photopolymerizable formulations were prepared by adding to the mixtures always 2 wt% of the cationic photoinitiator. The 100 l thick films were obtained by coating the mixtures on glass substrate and curing them, in air, by using a Fusion lamp (D-bulb), with radiation intensity on the surface of the sample of 280 mW/cm2 , and a belt speed of 6 m/min.
2.3. Analyses and characterization The kinetics of the photopolymerization was determined by FT-IR spectrometry, following the decrease of the IR band related to the reactive groups. A KBr disk was coated with the photocurable mixture and the reaction was followed at different irradiation times with a medium pressure Hg lamp (Italquartz, Milano, Italy) at a light intensity of 10 mW/cm2 . The FT-IR instrument used was a Genesis Series ATI Mattson (USA) Spectrometer. The gel content of the films was determined by measuring the weight loss after 20 h extraction at room temperature with chloroform. DSC measurements were performed using a Mettler DSC30 (Switzerland) instrument, equipped with a low temperature probe. All the film characterizations were performed after storage of the films for 24 h and than a treatment in ammonia saturated atmosphere, in order to stop the acidic species.
As a reference resin DOX monomer was photopolymerized in the presence of sulphonium salt photoinitiator. In Fig. 1 the FT-IR kinetic curve of the pure DOX monomer, obtained by following the decrease of the typical oxetane IR band at 980 cm1 , is reported and compared with the kinetic curves obtained for the cycloaliphatic epoxy monomer (CY) photocured in the same conditions. In this case the decrease of the 780 cm1 IR band was measured. Notwithstanding the high ring strain energy of 107 kJ/mol [22] and high basicity with a pKa of 3.7 [23], the rate of polymerization for oxetane rings results slower compared with cycloaliphatic epoxy monomer (see Fig. 1). Also the conversion is lower; after 1 min of irradiation only 37% of oxirane ring conversion is obtained. This low oxetane ring conversion is probably due to a low propagation rate constant. The polymerization reaction continue in the dark, and the oxetane conversion reaches 48% after 20 min and 65% after 2 h of dark storage. At the end the photocured films presented 90% gel content, indicating that a high cross-linked polymer network has been formed upon photopolymerization. The DSC analysis shows a glass transition temperature at about 30 °C. 3.2. Photopolymerization of oxetane/epoxy systems In order to investigate the effect of an epoxy monomer on the curing kinetic and final properties of the oxetane photocured resin, different mixtures of CY monofunctional epoxy monomer with DOX resin were investigated.
100 80 Conversion %
O
60 40 20 0 0
10
20
30
40
50
60
Time (s)
Fig. 1. FT-IR kinetic curves of the pure DOX (N) and CY ( ) monomers.
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358 100
80
80
Conversion %
60
Conversion %
355
40
60
40
20
20 0
0 0
10
20
30
40
50
60
0
20
Time (s)
40
60
Time (s)
Fig. 2. FT-IR kinetic curves of the pure DOX monomer (r) and in the presence of 20 (N), 30 (j) and 50 ( ) wt% of CY.
Fig. 3. FT-IR kinetic profile for the oxetane ( ) and epoxy (j) groups during photopolymerization of 50:50 mol% DOX/CY mixture.
The CY monomer is very reactive and its presence induce an acceleration to the polymerization of DOX monomer. In Fig. 2 the kinetic curves of the pure DOX monomer is reported and compared with the curves obtained by following the decrease of the oxetane IR band at 980 cm1 for the DOX/CY mixtures. A clear increase of the rate of polymerization and final oxetane conversion is evident by increasing the amount of the monofunctional epoxy monomer in the photocurable mixture. Because one of the factors controlling the rate of polymerization is the easy of hydrogen abstraction [24], any acceleration observed when CY is added in the photocurable mixture could simply be the result of providing more easily abstractable protons. A copolymerization between oxetane and epoxy monomer is suggested by the presence of a single Tg values in the different DOX/CY photocured samples (see Table 2). This is confirmed by the same profile and a close final conversion of epoxy and oxetane groups, for the 50:50 mol% DOX/CY mixture (see Fig. 3).
3.3. Photopolymerization of oxetane/vinyl ether systems
The effect of the presence of a vinyl ether monomer on the rate of polymerization and final properties of the DOX photocured films was investigated. As a vinyl ether monomer bis-(4-vinyl oxy butyl) isophtalate (IDVE) was employed, mixed in different amount with the DOX resin, and photocured in the presence of a sulphonium salt photoinitiator. The conversion versus time curves was obtained following the decrease of the oxetane IR band at 980 cm1 , after different time of irradiation. In Fig. 4 the rate of polymerization for oxetane monomer in the 1:1 molar ratio DOX/VE mixture is reported and compared with that of the pure DOX and pure IDVE monomers. In the presence of the vinyl ether monomer a dramatic increase on the rate of photopolymerization and final oxetane conversion is evident. It is possible to explain this behaviour considering that upon the UV exposure of the monomer blend a copolymer may be formed by a cross-propagation mechanism: the oxonium ion reacting with the vinyl ether double bond and the vinyl ether carbon cation
Table 2 Properties of oxetane/epoxide systems Sample
Tg (°C) DSC
Gel content (%)
DOX Conv. (%)a
DOX Conv. (%)b
Pure DOX DOX/CY 80:20 DOX/CY 70:30 DOX/CY 50:50
30 27 25 10
93 90 88 90
37 58 60 74
70 75 78 80
a b
Final conversion after 1 min of irradiation at low intensity. Final conversion after one passage under Fusion lamp.
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M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
A further evidence in favour of such a mechanism has been obtained by DSC analysis which showed, in any case, a single Tg for the UV-cured blend, with values between the Tg values of the pure UV-cured monomers. The Tg data, together with the gel content, are reported in Table 3. The high gel content values indicate tightly crosslinked networks.
Conversion %
100 80 60 40 20
3.4. Photopolymerization of oxetane/acrylate systems
0 0
20
40
60
Time (s)
Fig. 4. FT-IR kinetic curves of the oxetane ring in the 1:1 DOX/IDVE molar mixture () compared with the kinetic curves of the pure DOX (N) and pure IDVE (r) monomer.
with the oxetane ring. The proposed mechanism is reported below. Arguments in favour of vinyl ether/epoxy monomer copolymerization are reported in literature [25,26] and considering that oxetane groups have equal behaviour as epoxy ring, these arguments can be taken to reinforce our copolymerization hypothesis.
+ CH2 CH
+
O
O
O
O
R
O CH2 CH
O
O+
+
CH2=CHOR
O R
+ CH2 CH CHCH2CHOR O
CH2
R
C(CH2CH3)2 O
O
Dioxetane/diacrylate monomer 1:1 molar mixture (DOX/PEGDA 600) was photocured in the presence of the triphenylsulphonium salt in air. The rate of photopolymerization was obtained by following the decrease of the typical acrylate band at 1634 cm1 and the oxetane band at 980 cm1 , after different time of irradiation. The data are reported in Fig. 5. The acrylate polymerization is clearly induced by free-radicals that are formed by photolisis of the cationic photoinitiator [27]. The observable induction period can be attributed to the inhibitory effect of atmospheric oxygen on radical initiated polymerization. It is particularly important in this case, notwithstanding the high reactivity of PEGDA 600, because of the too slow production of free-radicals upon photolysis of the sulphonium salt. These radicals must first react with the oxygen dissolved in the sample to reduce its concentration low enough to allow the acrylate monomer to compete successfully with oxygen for the scavenging of the initiating radicals [28]. Subsequently the dioxetane build-up network induces an increase in viscosity, which slows down the diffusion of atmospheric O2 into the sample and thus the kinetic of acrylic double bond conversion increase rapidly with a high final conversion (around 65%). The rate of polymerization of the dioxetane is higher, compared with the acrylate monomers, and the network formation is not affected by its presence. It should be noted that even if characterized by a higher rate of polymerization, after 6000 of irradiation only 37% of oxetane groups result converted, but the remaining groups continue to react slowly upon storage of the sample in the dark because of the living character of cationic polymerization. It was observed that the oxetane monomer continue to polymerize slowly after the light has been switched off, from 37% to 45% within the first 20 min, while the acrylate conversion stays constant. A higher conversion of the oxetane group (68%) was achieved after storing the sample 2 h in the dark. The DSC analysis of the photocured films revealed two glass transition temperature respectively, at )60 and 30 °C; a typical DSC thermogram is reported in Fig. 6. This behaviour clearly indicates the formation of an interpenetrating network (IPN). Typically an IPN
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
357
Table 3 Properties of oxetane/vinyl ether systems Sample
Tg (°C) DSC
Gel content (%)
DOX Conv. (%)a
DOX Conv. (%)b
Pure DOX Pure IDVE DOX/IDVE 80:20 DOX/IDVE 70:30 DOX/IDVE 50:50
30 45 33 36 38
93 93 88 92 90
37 86 68 72 84
70 93 73 76 82
a b
Final conversion after 1 min of irradiation. Final conversion after one passage under Fusion lamp.
polymer film obtained after UV-curing of the the DOX/ PEGDA600 mixture, indicates that phase separation does not occur upon photopolymerization and that the two polymer networks are well compatible and tightly interpenetrated.
70
Conversion %
60 50 40 30
4. Conclusions
20
Oxetane based systems were investigated in photopolymerization process, either as pure monomer or in mixtures with epoxy monomer, vinyl ether or acrylate resin. Notwithstanding the high ring strain of the oxetane group and its high basicity, the kinetic of photopolymerization of the pure monomer results slow with a low oxetane conversion after 1 min of irradiation probably due to a low propagation rate constant. Because of the living cationic character, the conversion increases during dark storage of the sample. A clear increase on the rate of polymerization and final oxetane conversion is evident in the presence of epoxy monofunctional additive. The same effect is evident in the presence of vinyl ether monomer, it has been proposed a cross-propagation mechanism supported by DSC analysis and confirmed by previous data reported in literature concerning the cross-copolymerization between epoxy and vinyl ethers. The combination of radical UV-curing of acrylate monomer and cationic UV-curing of oxetane, induced the formation of an IPN network tighlty cross-linked. Because only sulphonium salt is employed as photoinitiator, the acrylate polymerization is clearly induced by free-radicals that are formed by photolisis of the cationic photoinitiator.
10 0 0
10
20
30
40
50
60
Time (s)
Fig. 5. FT-IR kinetic curves of the oxetane ring (r) and of the acrylate double bond (j) for the DOX/PEGDA600 1:1 molar mixture.
Fig. 6. DSC thermogram of the DOX/PEGDA600 1:1 molar mixture photocured film.
network can be synthesized by crosslinking polymerization of two multifunctional monomers that polymerize by different mechanism, e.g. radical and cationic types [29]. In our system the combination of radical UVcuring of acrylate monomer and cationic UV-curing of oxetane was employed. The perfectly transparent
References [1] Fouassier JP, Rabek JC. In: Radiation curing in polymer science and technology, vol. 1. London: Elsevier; 1993. [2] Takimoto Y. In: Fouassier JP, editor. Radiation curing in polymer science and technology, vol. 3. London: Elsevier; 1993. p. 269.
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[3] Pappas SP. UV-curing science and technology. New York: Plenum Press; 1992. p. 2. [4] Crivello JV, Ortiz RA. J Polym Sci Polym Chem 2001;39:3578. [5] Crivello JV, Ortiz RA. J Polym Sci Polym Chem 2001;39:2385. [6] Decker C, Thi Viet TN, Thi HP. Polym Int 2001;50: 986. [7] Roffey CG. Photopolymerization of surface coatings. New York: Wiley; 1982. p. 74. [8] Decker C, Morel F. Polym Mater Sci Eng 1997;76:70. [9] Decker C, Bianchi C, Decker D, Morel F. Prog Org Coat 2001;42:253. [10] Sangermano M, Malucelli G, Morel F, Decker C, Priola A. Eur Polym J 1999;35:639–45. [11] Crivello JV, Jo KD. J Polym Sci Polym Chem 1993;31: 1473. [12] Crivello JV, Jo KD. J Polym Sci Polym Chem 1993; 31:1483. [13] Crivello JV, Lohden G. J Polym Sci Polym Chem 1996;34: 2051. [14] Crivello JV, Jo KD. J Polym Sci Polym Chem 1993;31: 2143.
[15] Sangermano M, Malucelli G, Bongiovanni R, Priola A, Annby U, Rehnberg N. Polym Int 2001;50(9):998–1003. [16] Sasaki H, Crivello JV. J Macromol Sci Pure Appl Chem 1992;A29(10):915. [17] Crivello JV, Saski H. J Macromol Sci Pure Appl Chem 1993;A30(2&3):189. [18] Crivello JV, Saski H. J Macromol Sci Pure Appl Chem 1993;A30(2&3):173. [19] Kato H, Saski H. Polym Preprints 2001;42(2):729. [20] Suzuki H, Sasaki H. Polym Preprints 2001;42(2):733. [21] Sasaki H. Am RadTech Proc 2002:64. [22] Pell AS, Pilker G. Trans Faraday Soc 1965;61:71. [23] Arnett EM. Prog Phys Org Chem 1967;7:243. [24] Dougherty JA, Crivello JV. Polym Mater Sci Eng Prepr 1995;95:410. [25] Decker C. Polym Preprints 2001;42(2):762. [26] Decker C, Thi Viet TN, Decker D, Weber-Koenl E. Polymer 2001;42:5531. [27] Crivello JV. J Polym Sci Polym Chem 1999;37:4241. [28] Decker C, Moussa K, Bendaikha T. J Polym Sci Polym Chem 1991;29:739. [29] Sperling LH. Interpenetrating polymer networks and related materials. New York: Plenum Press; 1981.
Photopolymerization of oxetane based systems M. Sangermano *, G. Malucelli, R. Bongiovanni, A. Priola Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy Received 19 June 2003; received in revised form 7 July 2003; accepted 30 September 2003
Abstract Oxetane based systems were investigated in cationic photopolymerization process. The effect on the rate of polymerization induced by the presence of epoxy monofunctional monomers and vinyl ether monomers were investigated. In both cases an increase of the rate of polymerization is evidenced either induced by a flexibilization effect due to a copolymerization or to a cross-propagation mechanism. Acrylate comonomers were also investigated and the formation of an IPN tightly cross-linked evidenced. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Cationic photopolymerization; Oxetane monomers; Copolymerization; Interpenetrating networks
1. Introduction The UV-curing process is a polymerization technique in which radiation induces a fast transformation of a liquid monomer into a solid polymer. Radical or cationic species are generated by interaction of UV-light with a suitable photoinitiator, which induces the curing reaction of suitable reactive monomers and oligomers [1]. Cationic photoinduced process presents some advantages over the radical one [2]; in particular lack of inhibition by oxygen, low shrinkage, good mechanical properties of UV-cured materials and good adhesion properties to various substrates. In addition, the monomers employed are generally characterized by low irritation and toxicity properties. Different type of monomers and oligomers have been proposed and reported in literature, mainly epoxides [3– 6], vinyl ethers [7–10] and propenyl ethers [11–15]. More recently attention has been devoted to the synthesis and cationic photopolymerization of oxetane
* Corresponding author. Tel.: +39-011-564-4651; fax: +39011-564-4699. E-mail address: [email protected] (M. Sangermano).
monomers [16–21]. The high ring strain, very similar to that of epoxides, and the higher basicity of the heterocyclic oxygen in oxetanes than that for oxirane oxygen in epoxides, make oxetanes interesting alternative monomers in cationic UV-curing applications. The purpose of this work is to study oxetane based systems in photocured process; the kinetic of curing and the properties of final products were investigated, either for the pure monomer or in mixture with epoxy resins, with vinyl ether monomers and with acrylates.
2. Experimentals 2.1. Materials The following monomers were employed: bis[1ethyl(3-oxetanyl)]methyl ether (OXT-221, DOX) from Toagosei, Japan; Cyclohexene oxide (CY) from Aldrich; bis-(4-vinyl oxy butyl) isophtalate (Vectomer 4010, IDVE) from Allied Signal; Polyethylenglycol-600diacrylate (Ebecryl 11, PEGDA) from UCB Belgium. In Table 1 the structures of the monomers employed are reported. Triphenylsulphonium hexafluoroantimonate (UVI6974, PI) gently supplied from Dow-Union Carbide was employed as photoinitiator.
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.09.026
354
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
Table 1 Monomer employed
3. Results and discussion 3.1. Photopolymerization of oxetane monomer
O
O DOX CH2=CHCOO (CH2CH2)n OOCCH=CH2
O
PEGDA 600
CY
O
O
O
O
O
O
IDVE
2.2. UV-curing The photopolymerizable formulations were prepared by adding to the mixtures always 2 wt% of the cationic photoinitiator. The 100 l thick films were obtained by coating the mixtures on glass substrate and curing them, in air, by using a Fusion lamp (D-bulb), with radiation intensity on the surface of the sample of 280 mW/cm2 , and a belt speed of 6 m/min.
2.3. Analyses and characterization The kinetics of the photopolymerization was determined by FT-IR spectrometry, following the decrease of the IR band related to the reactive groups. A KBr disk was coated with the photocurable mixture and the reaction was followed at different irradiation times with a medium pressure Hg lamp (Italquartz, Milano, Italy) at a light intensity of 10 mW/cm2 . The FT-IR instrument used was a Genesis Series ATI Mattson (USA) Spectrometer. The gel content of the films was determined by measuring the weight loss after 20 h extraction at room temperature with chloroform. DSC measurements were performed using a Mettler DSC30 (Switzerland) instrument, equipped with a low temperature probe. All the film characterizations were performed after storage of the films for 24 h and than a treatment in ammonia saturated atmosphere, in order to stop the acidic species.
As a reference resin DOX monomer was photopolymerized in the presence of sulphonium salt photoinitiator. In Fig. 1 the FT-IR kinetic curve of the pure DOX monomer, obtained by following the decrease of the typical oxetane IR band at 980 cm1 , is reported and compared with the kinetic curves obtained for the cycloaliphatic epoxy monomer (CY) photocured in the same conditions. In this case the decrease of the 780 cm1 IR band was measured. Notwithstanding the high ring strain energy of 107 kJ/mol [22] and high basicity with a pKa of 3.7 [23], the rate of polymerization for oxetane rings results slower compared with cycloaliphatic epoxy monomer (see Fig. 1). Also the conversion is lower; after 1 min of irradiation only 37% of oxirane ring conversion is obtained. This low oxetane ring conversion is probably due to a low propagation rate constant. The polymerization reaction continue in the dark, and the oxetane conversion reaches 48% after 20 min and 65% after 2 h of dark storage. At the end the photocured films presented 90% gel content, indicating that a high cross-linked polymer network has been formed upon photopolymerization. The DSC analysis shows a glass transition temperature at about 30 °C. 3.2. Photopolymerization of oxetane/epoxy systems In order to investigate the effect of an epoxy monomer on the curing kinetic and final properties of the oxetane photocured resin, different mixtures of CY monofunctional epoxy monomer with DOX resin were investigated.
100 80 Conversion %
O
60 40 20 0 0
10
20
30
40
50
60
Time (s)
Fig. 1. FT-IR kinetic curves of the pure DOX (N) and CY ( ) monomers.
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358 100
80
80
Conversion %
60
Conversion %
355
40
60
40
20
20 0
0 0
10
20
30
40
50
60
0
20
Time (s)
40
60
Time (s)
Fig. 2. FT-IR kinetic curves of the pure DOX monomer (r) and in the presence of 20 (N), 30 (j) and 50 ( ) wt% of CY.
Fig. 3. FT-IR kinetic profile for the oxetane ( ) and epoxy (j) groups during photopolymerization of 50:50 mol% DOX/CY mixture.
The CY monomer is very reactive and its presence induce an acceleration to the polymerization of DOX monomer. In Fig. 2 the kinetic curves of the pure DOX monomer is reported and compared with the curves obtained by following the decrease of the oxetane IR band at 980 cm1 for the DOX/CY mixtures. A clear increase of the rate of polymerization and final oxetane conversion is evident by increasing the amount of the monofunctional epoxy monomer in the photocurable mixture. Because one of the factors controlling the rate of polymerization is the easy of hydrogen abstraction [24], any acceleration observed when CY is added in the photocurable mixture could simply be the result of providing more easily abstractable protons. A copolymerization between oxetane and epoxy monomer is suggested by the presence of a single Tg values in the different DOX/CY photocured samples (see Table 2). This is confirmed by the same profile and a close final conversion of epoxy and oxetane groups, for the 50:50 mol% DOX/CY mixture (see Fig. 3).
3.3. Photopolymerization of oxetane/vinyl ether systems
The effect of the presence of a vinyl ether monomer on the rate of polymerization and final properties of the DOX photocured films was investigated. As a vinyl ether monomer bis-(4-vinyl oxy butyl) isophtalate (IDVE) was employed, mixed in different amount with the DOX resin, and photocured in the presence of a sulphonium salt photoinitiator. The conversion versus time curves was obtained following the decrease of the oxetane IR band at 980 cm1 , after different time of irradiation. In Fig. 4 the rate of polymerization for oxetane monomer in the 1:1 molar ratio DOX/VE mixture is reported and compared with that of the pure DOX and pure IDVE monomers. In the presence of the vinyl ether monomer a dramatic increase on the rate of photopolymerization and final oxetane conversion is evident. It is possible to explain this behaviour considering that upon the UV exposure of the monomer blend a copolymer may be formed by a cross-propagation mechanism: the oxonium ion reacting with the vinyl ether double bond and the vinyl ether carbon cation
Table 2 Properties of oxetane/epoxide systems Sample
Tg (°C) DSC
Gel content (%)
DOX Conv. (%)a
DOX Conv. (%)b
Pure DOX DOX/CY 80:20 DOX/CY 70:30 DOX/CY 50:50
30 27 25 10
93 90 88 90
37 58 60 74
70 75 78 80
a b
Final conversion after 1 min of irradiation at low intensity. Final conversion after one passage under Fusion lamp.
356
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
A further evidence in favour of such a mechanism has been obtained by DSC analysis which showed, in any case, a single Tg for the UV-cured blend, with values between the Tg values of the pure UV-cured monomers. The Tg data, together with the gel content, are reported in Table 3. The high gel content values indicate tightly crosslinked networks.
Conversion %
100 80 60 40 20
3.4. Photopolymerization of oxetane/acrylate systems
0 0
20
40
60
Time (s)
Fig. 4. FT-IR kinetic curves of the oxetane ring in the 1:1 DOX/IDVE molar mixture () compared with the kinetic curves of the pure DOX (N) and pure IDVE (r) monomer.
with the oxetane ring. The proposed mechanism is reported below. Arguments in favour of vinyl ether/epoxy monomer copolymerization are reported in literature [25,26] and considering that oxetane groups have equal behaviour as epoxy ring, these arguments can be taken to reinforce our copolymerization hypothesis.
+ CH2 CH
+
O
O
O
O
R
O CH2 CH
O
O+
+
CH2=CHOR
O R
+ CH2 CH CHCH2CHOR O
CH2
R
C(CH2CH3)2 O
O
Dioxetane/diacrylate monomer 1:1 molar mixture (DOX/PEGDA 600) was photocured in the presence of the triphenylsulphonium salt in air. The rate of photopolymerization was obtained by following the decrease of the typical acrylate band at 1634 cm1 and the oxetane band at 980 cm1 , after different time of irradiation. The data are reported in Fig. 5. The acrylate polymerization is clearly induced by free-radicals that are formed by photolisis of the cationic photoinitiator [27]. The observable induction period can be attributed to the inhibitory effect of atmospheric oxygen on radical initiated polymerization. It is particularly important in this case, notwithstanding the high reactivity of PEGDA 600, because of the too slow production of free-radicals upon photolysis of the sulphonium salt. These radicals must first react with the oxygen dissolved in the sample to reduce its concentration low enough to allow the acrylate monomer to compete successfully with oxygen for the scavenging of the initiating radicals [28]. Subsequently the dioxetane build-up network induces an increase in viscosity, which slows down the diffusion of atmospheric O2 into the sample and thus the kinetic of acrylic double bond conversion increase rapidly with a high final conversion (around 65%). The rate of polymerization of the dioxetane is higher, compared with the acrylate monomers, and the network formation is not affected by its presence. It should be noted that even if characterized by a higher rate of polymerization, after 6000 of irradiation only 37% of oxetane groups result converted, but the remaining groups continue to react slowly upon storage of the sample in the dark because of the living character of cationic polymerization. It was observed that the oxetane monomer continue to polymerize slowly after the light has been switched off, from 37% to 45% within the first 20 min, while the acrylate conversion stays constant. A higher conversion of the oxetane group (68%) was achieved after storing the sample 2 h in the dark. The DSC analysis of the photocured films revealed two glass transition temperature respectively, at )60 and 30 °C; a typical DSC thermogram is reported in Fig. 6. This behaviour clearly indicates the formation of an interpenetrating network (IPN). Typically an IPN
M. Sangermano et al. / European Polymer Journal 40 (2004) 353–358
357
Table 3 Properties of oxetane/vinyl ether systems Sample
Tg (°C) DSC
Gel content (%)
DOX Conv. (%)a
DOX Conv. (%)b
Pure DOX Pure IDVE DOX/IDVE 80:20 DOX/IDVE 70:30 DOX/IDVE 50:50
30 45 33 36 38
93 93 88 92 90
37 86 68 72 84
70 93 73 76 82
a b
Final conversion after 1 min of irradiation. Final conversion after one passage under Fusion lamp.
polymer film obtained after UV-curing of the the DOX/ PEGDA600 mixture, indicates that phase separation does not occur upon photopolymerization and that the two polymer networks are well compatible and tightly interpenetrated.
70
Conversion %
60 50 40 30
4. Conclusions
20
Oxetane based systems were investigated in photopolymerization process, either as pure monomer or in mixtures with epoxy monomer, vinyl ether or acrylate resin. Notwithstanding the high ring strain of the oxetane group and its high basicity, the kinetic of photopolymerization of the pure monomer results slow with a low oxetane conversion after 1 min of irradiation probably due to a low propagation rate constant. Because of the living cationic character, the conversion increases during dark storage of the sample. A clear increase on the rate of polymerization and final oxetane conversion is evident in the presence of epoxy monofunctional additive. The same effect is evident in the presence of vinyl ether monomer, it has been proposed a cross-propagation mechanism supported by DSC analysis and confirmed by previous data reported in literature concerning the cross-copolymerization between epoxy and vinyl ethers. The combination of radical UV-curing of acrylate monomer and cationic UV-curing of oxetane, induced the formation of an IPN network tighlty cross-linked. Because only sulphonium salt is employed as photoinitiator, the acrylate polymerization is clearly induced by free-radicals that are formed by photolisis of the cationic photoinitiator.
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Fig. 5. FT-IR kinetic curves of the oxetane ring (r) and of the acrylate double bond (j) for the DOX/PEGDA600 1:1 molar mixture.
Fig. 6. DSC thermogram of the DOX/PEGDA600 1:1 molar mixture photocured film.
network can be synthesized by crosslinking polymerization of two multifunctional monomers that polymerize by different mechanism, e.g. radical and cationic types [29]. In our system the combination of radical UVcuring of acrylate monomer and cationic UV-curing of oxetane was employed. The perfectly transparent
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