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Effect of UV-irradiation on fluorescence of poly(methyl methacrylate) films with photosensitive organic compounds
Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Effect of UV-irradiation on fluorescence of poly(methyl methacrylate) films with photosensitive organic compounds Jolanta Kowalonek* , Halina Kaczmarek, Marzanna Kurzawa , 87-100 Torun , Gagarina 7, Poland Faculty of Chemistry, Nicolaus Copernicus University in Torun
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 August 2015 Received in revised form 22 December 2015 Accepted 23 December 2015 Available online 28 December 2015
The simple way of physical modification of poly(methyl methacrylate) (PMMA) for the purpose of formation its photoactivity and fluorescence properties has been described. It was found that aromatic carbonyl compounds (2,2-dimetoxy-2-phenylaceto-phenone, benzoyl peroxide and tert-butyl peroxybenzoate) dispersed in PMMA matrix changed its optical properties, which was studied by absorption and emission spectroscopy. Modified PMMA exhibits fluorescence upon excitation of 220 or 270 nm. The intensity of emission band decreases after samples exposure to short wavelength UV-irradiation, which is evidence of component photolysis. The fluorescence decay is slightly smaller for the irradiated initiators in PMMA matrix, but the trend is similar to that for pure initiators exposed to UV. The protective effect of PMMA on the studied compounds after UV-irradiation, however, is accompanied by photodegradation reactions occurring in the polymer, which was confirmed by viscometric measurements. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Poly(methyl methacrylate) Photoinitiators Fluorescence spectroscopy UV–vis spectroscopy
1. Introduction Photoactive polymeric composites attract much attention because they are potential materials for the new technologies in the fields such as electronics, optics, biotechnology, medicine, pharmacy etc. Poly(methyl methacrylate)(PMMA) is a valuable raw material for the production of photosensitive plastics. The physical and chemical modification of this polymer can lead to obtaining fluorescent or photoconductive materials [1–4]. Moreover, acrylate polymers are also used as a matrix for low-molecular organic or inorganic compounds [5,6]. For example, PMMA doped with fluorescent probes (amino aromatic species) has been successfully obtained by pulsed laser deposition. Such material can act as a sensor for detection of environmental impurities [7]. In other interesting work, the switching properties of fluorescent photochromic PMMA copolymerized with spironaphthoxazine and D-p-A type pyran-based dye have been reported [8]. Atactic and syndiotactic PMMA with anthracene fluorophores attached to the end of side chains or embedded in the polymer backbone has been studied by time-resolved fluorescence spectroscopy. Additionally, isotactic PMMA without anthracene fluorophore was prepared to mix it with labeled PMMA. It was found
* Corresponding author. E-mail address: [email protected] (J. Kowalonek). http://dx.doi.org/10.1016/j.jphotochem.2015.12.017 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
that stereocomplexes between the syndiotactic and the isotactic PMMA in dimethylformamide were formed due to the local conformational motion of polymer segments [9]. Studies of anthracene-labeled PMMA gels by fluorescence anisotropy decay were the subject of the article by Aoki [10]. It was concluded that the local motion of PMMA gel at crosslinks was governed by the segment density of the network chains in the vicinity of crosslinks. Furthermore, it was found that nanocarbon-PMMA composites, prepared by pulsed laser ablation technique, exhibited fluorescence. Spectroscopic studies revealed interactions between nanocarbon particles and polymer matrix due to transfer of p electrons from the carbonyl groups of PMMA to nanocarbon [11]. The possibilities of other applications of modified PMMA and its copolymers in fabrication of nanodevices, in which the photoactivity is demanded, have been reported in numerous publications [3,4,12–18]. Moreover, experimental emission techniques appeared useful in evaluation of component miscibility in polymer blends by means of excimer fluorescence and fluorescence microscopy [19,20]. Recently, PMMA was modified by the addition of small amount of different photosensitive low-molecular weight compounds such as aromatic or aliphatic ketones and peroxides [21–24]. It was found that PMMA doped with these photoinitiators undergoes accelerated photodegradation compared to the unmodified polymer, which is important in designing photodegradable
J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
polymers. Moreover, the thermal resistance of modified polymer was lower than that of unmodified, whereas the surface properties of PMMA with or without additives did not change significantly. The aim of the present work was to study the fluorescence of PMMA physically modified with three organic compounds: 2,2dimetoxy-2-phenylaceto-phenone (Irg), benzoyl peroxide (BP) and tert-butyl peroxybenzoate (tBPB). Physical mixing of photosensitive compounds with polymers is much simpler and cheaper method of preparing fluorescent materials than the complex, often multi-step and long-term synthesis of polymer-based photoactive compounds. In this way it is possible to prepare photosensitive plastics for potential applications because of easy processing and possibility of modification of mechanical properties in a wide range. An important objective of the work was to check whether the fluorescent properties of the photosensitive compounds still remained in the polymer matrix and to what extent these properties changed under the influence of UV radiation. The emission properties of modified PMMA-based materials are important in the case of using such plastics as selective sensors, fluorescent markers as well as the substances for lithography, optics or electronics. 2. Experimental 2.1. Materials Poly(methyl methacrylate), IUPAC name: poly(methyl 2-methylpropenoate), PMMA, has been purchased from Sigma–Aldrich. The average molecular weight of polymer was 120,000. Photoinitiators: (2,2-dimetoxy-2-phenylaceto-phenone); trade name: Irgacure 651 (Irg), benzoyl peroxide (BP) and tert-butyl peroxybenzoate (tBPB) were supplied by Ciba, Basel, Switzerland; Sigma–Aldrich. The formula of these compounds and theirs abbreviations are shown below:
Acetonitrile (ACN) and tetrahydrofuran of high purity (Sigma– Aldrich) were used as solvents.
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Excitation and emission slit widths were 5 nm for the wavelength response of the system. The response time was 0.1 s while a data interval—1 nm. Individual and three-dimensional spectra were recorded in the range of 200–900 nm at a rate of 60 000 nm/min. The spectra were evaluated with the PC software package supplied with spectrophotometer (FL-Solution 2.1 for F-7000). Emission spectra of UV-irradiated samples were recorded directly after exposure. To compare the UV influence on different samples, the relative changes of fluorescence intensity has been calculated:
DIrel ¼
I o It Io
ð1Þ
where Io and It are fluorescence intensity of unirradiated sample before and after t time of irradiation, respectively All fluorescence measurements were performed for samples in a solid state. 2.4. UV–vis spectroscopy Absorption spectra in UV–vis range have been recorded using UV-1601PC spectrophotometer (Shimadzu, Japan). The measurements were performed in acetonitrile solutions (pure initiators) and in a solid state (thin films of photoinitiator-doped PMMA). The molar absorption coefficients, elmax, for photoinitiators in dilute solutions have been calculated from Beer-Lambert law. 2.5. Ultraviolet exposure Polymeric films were UV-irradiated using a low-pressure mercury vapor lamp TUV30W, Philips, Holland. This source emits radiation of 254 nm wavelength. The intensity of the incident light, measured by HD 9021 radiometer, was 32.2 W/m2. The irradiation was carried out at 30 C and at ambient atmosphere. Fluorescence
spectra of UV-irradiated films were systematically measured. 2.6. Viscosity measurements
2.2. Sample preparation 2% (w/v) solutions of polymer and organic compounds in tetrahydrofuran were prepared. The solutions were mixed in a proper volume ratio to obtain the mixtures of PMMA with 5 wt.% of modifier content. Thin films (thickness of 20 mm) of pure PMMA and PMMA with photoinitiators were obtained by casting method. After solvent evaporation, films were carefully dried in a vacuum oven at 30 C for one week. The purity of dried films was checked by spectroscopic analysis. 2.3. Fluorescence measurement Fluorescence measurements were performed on a Hitachi F-7000 Fluorescence Spectrophotometer at room temperature using a Xenon lamp source for excitation and the 1 cm quartz cell.
Unirradiated and UV-irradiated films of PMMA and PMMA with photoinitiators have been dissolved in acetonitrile. Viscosity of these diluted solutions was measured at 25 0.01 C using Ubbelohde capillary viscometer. From the flow times of the solvent and the solutions, relative (hr) and specific (hsp) viscosities were determined for 4–5 concentrations. The flow time of liquid was measured with accuracy of 0.01 s. Intrinsic viscosity (limiting viscosity number, LVN, dl/g) has been calculated by extrapolation of reduced viscosity (hsp/c) to zero concentration (c = 0). The relative percentage changes of LVN of various samples have been calculated according to the formula:
DðLVNÞ ¼
ðLVNÞ0 ðLVNÞt 100% ðLVNÞ0
ð2Þ
20
J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
Fig. 1. Changes in absorption spectra during exposure to UV of photoactive modifiers in ACN solutions: (a) Irgacure (Cm = 3.2 105 M), (b) benzoyl peroxide (Cm = 7.3 106 M), (c) tert-butyl peroxybenzoate (Cm = 7.5 106 M). Arrows in the diagram indicate the direction of the change. Drawings in color are available in the online version.
where (LVN)o and (LVN)t are intrinsic viscosity of unirradiated sample before and after t time of irradiation, respectively. 3. Results and discussions First of all, in order to evaluate the optical properties of the examined systems, absorption spectra of the selected organic compounds in a solution have been collected in the range of UV– vis. The spectra provide information about absorption of chromophoric groups present in the structure of photoinitiators and possibility of their excitation upon UV. It is necessary for occurrence of the fluorescence in these compounds. Moreover, UV–vis absorption spectra of pure PMMA and PMMA with these compounds (in a solid state) have been done. Then, all prepared specimens were UV-irradiated and both absorption and emission (fluorescence) spectra have been collected immediately after exposure. Our previous studies [21–24] have shown that it was not easy to predict the photochemical behavior of polymer doped with low molecular weight organic compounds based on the properties of individual constituents.
The alterations in UV–vis spectra of the exposed photoinitiators are shown in Fig. 1. Absorption bands observed at 252, 234 and 229 nm in Irgacure, benzoyl peroxide and tert-butyl peroxybenzoate, respectively, are due to p,p* transitions in aromatic rings. Additionally, the band of low intensity at about 270 nm appears in all UV-irradiated samples, which can be attributed to n,p* transitions in carbonyl groups. The value of molar absorption coefficient, elmax, determined for the main absorption band was the lowest for Irg (e252 = 15300 dm3 mol1 cm1) and the highest for BP (e234 = 42400 dm3 mol1 cm1), for tBPB it was: e229 = 39300ndm3 mol1 cm1). The systematic fast decrease in the main absorption bands of the initiators spectra during their UV-irradiation clearly indicates their efficient photolysis. Particularly fast decomposition was observed in the case of Irg, just after 10s the compound decomposed. However, a new band at 223 nm was found indicating the formation of a new product, thus, the exposure time was prolonged up to 2 min to observe the formation of this moiety. Time needed to complete decomposition of BP and tBPB was 20s and 40s, respectively. Simultaneously, the broad tail of absorption in the range of longer wavelength has been observed for all exposed photoinitiator solutions.
J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
The mechanism of photochemical reactions in these photoactive compounds is known, so interested readers are referred to the cited references [22,23]. The main absorption band (p,p*) characteristic of photosensitive compounds was also observed for PMMA films with these compounds (not shown in the work). The second band (n,p*) was invisible in PMMA matrix because of low concentration (5%wt.) of photoinitiators. Small shift of absorption band (about 5 nm) was observed only for BP in PMMA, whereas for two other samples (PMMA + Irg and PMMA + tBPB) practically there were no changes in the position of band maximum. UV-irradiation also causes destruction of the photoinitiators dispersed in PMMA matrix but this process is much slower in comparison to that in solutions: 30 min UV exposure was needed for complete disappearance of absorption bands in the spectra. It should be noticed that the changes in the UV–vis spectra of the pure polymer after exposure to UV are insignificant indicating that the formation of products with chromophore groups cannot be detected using this method. The slight increase in absorbance in the whole UV–vis region was caused by some photoreactions occurring in the polymer, mainly photooxidative degradation leading to formation of new type of carbonyl groups and unsaturated bonds. The slower rate of photoinitiator decomposition in PMMA matrix can be explained as follow: the initiator molecules are trapped in the polymer matrix with low macromolecular mobility due to its glassy state at the experimental conditions, the diffusion of chemical moieties is restricted, contrary to solutions where macromolecular movements are less restricted. The formed radicals in a film can be characterized by much longer life time before recombination. Also a diffusion of atmospheric oxygen is more limited in the film than that in a solution, then, the quenching of excited states or direct reactions of radicals with oxygen is less efficient is a solid. Modified PMMA films were exposed to short-wavelength UV radiation and then fluorescence spectra were recorded immediately after each irradiation time. It was found that the maximum of fluorescence intensity depends on the type of modifier, thus, the wavelength at 220 nm was used for excitation of Irg and tBBPcontaining samples and the wavelength at 270 nm was chosen for sample with BP. The energy of this radiation is sufficient to excite the carbonyl groups in PMMA:
The neat PMMA also exhibits a very weak fluorescence but the emission maximum is observed at shorter excitation wavelength (i.e., 210 nm) than for PMMA with modifiers. To confirm this fact, 3D spectrum of pure PMMA was recorded (Fig. 2a). As can be seen, there is only a band coming from Rayleigh scattering [26] in the spectrum (the excitation beam produces a distinct diagonal feature). Additionally, two spectra at excitation wavelength l = 220 nm and at l = 270 nm were recorded for this polymer (Fig. 2a and b). The spectra confirm the lack of bands in the range from 300 nm to 500 nm. In order to estimate the real influence of PMMA matrix on specimen behavior, the emission spectra of pure modifiers
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Fig. 2. 3-D fluorescence spectrum of pure PMMA (a); the emission spectra of pure PMMA at 270 nm and 220 nm of excitation wavelength (b).
irradiated with the same dose under the identical conditions were done (Fig. 3). As can be seen, the emission spectra maximum of Irg and tBPB is slightly shifted to lower wavelength in polymer matrix (about 15 nm). Only in the case of BP the position of
emission band is almost the same (maximum at 300 nm) as for PMMA + BP specimen. It indicates that some interactions appear between PMMA molecules and Irg or tBPB, contrary to PMMA and BP. It seems obvious because BP with completely symmetric geometry is nonpolar compound. Two other modifiers have unsymmetrical structure and electronegative oxygen atoms, which can resulted in dipole interactions between the oxygen atoms of the initiators and carbonyl groups of PMMA side substituents. UV-irradiation of the pure initiators and PMMA modified with the initiators leads to a decrease in fluorescence intensity, which is clearly seen in Fig. 3. This is caused by photochemical decomposition of organic compounds used as modifiers. Such reactions are
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Fig. 3. Fluorescence spectra of modifiers and PMMA with these additives: (a) Irgacure, (b) PMMA + Irgacure, (c) benzoyl peroxide, (d) PMMA + benzoyl peroxide, (e) tert-butyl peroxybenzoate, (f) PMMA + tert-butyl peroxybenzoate. Excitation wavelength: 220 nm (a, b, e, f) and 270 nm (c, d).
known and described in the literature, since these compounds due to the presence of unstable photosensitive groups are used as polymerization initiators [25]. Recently, these compounds were proposed as the agents causing the polymer matrix photodegradability [21–24]. Fluorescence measurements provide interesting information on the progress of decomposition of these compounds in the polymer matrix upon UV irradiation. In order to facilitate the
quantitative comparison of the observed processes, the relative changes in fluorescence intensity were calculated according to the Formula (1). The results are presented in Fig. 4. As can be seen, both initiators containing peroxide bonds (BP and tBPB) are characterized by more efficacious fluorescence decay in comparison to this process for Irg. The same trend is observed for these compounds in the polymer matrix which, although, somewhat retards the degradation of the modifiers. Probably energy transfer processes
J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
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occur as a result of intermolecular interactions between photoinitiator in excited state and the polymer in ground state:
The mutual interactions between constituents and the photodegradation products and energy transfer processes can also occur
This partial protective action of PMMA is accompanied by the polymer photodegradation, which is competitive reaction to initiator photolysis in the polymer matrix. To confirm this assumption, the viscosity measurements have been performed. Neat and modified PMMA films have been UV-irradiated during 0– 30 min and then dissolved in ACN. The intrinsic viscosity values obtained for each system and different exposure times have been used for calculation of the LVN percentage changes, which is shown in Fig. 5. The decrease in the LVN is evidence of main chain scission reactions in the polymer during UV-irradiation. The photodegradation process of PMMA is accelerated by Irg. It is obviously caused by the fastest decomposition of this compound under UV-light, which was proved by UV–vis spectroscopy. However, chain breaking in PMMA is less efficient in the presence of BP and tBPB and even slightly inhibited compared to that in pure PMMA. A different reactivity of the free radicals produced from photoinitiators, having diverse chemical structure, can provide the explanation of above-mentioned processes. Moreover, the photochemical decomposition competes with physical deactivation reactions of excited states. The following scheme (5) can summarize our observations:
in a solid state. It should be added that the other photochemical reactions occurring in PMMA (depolymerization, side chain abstraction) have no significant influence on average molecular weight (thus, also on intrinsic viscosity) of polymer.
Three organic compounds: 2,2-dimetoxy-2-phenylaceto-phenone, benzoyl peroxide and tert-butyl peroxybenzoate, known as polymerization photoinitiators, were applied in this work to make poly(methyl methacrylate) sensitive to UV-light. The changes in photosensitivity of modified PMMA have been studied by emission (fluorescence) spectroscopy. Additionally, absorption spectroscopy in UV–vis range has been applied for better characterization of optical properties of studied samples. The rate of modifiers photolysis depends on the physical state of specimens. Decomposition of initiators is very fast in diluted solutions, almost complete disappearance of the characteristic absorption band of a compound is observed after several seconds of irradiation. In PMMA matrix this process is much slower—the decomposition of absorption band occurs during half of an hour. It
Free radicals formed during initiator photolysis are able to abstract hydrogen atoms from macromolecules, which initiates polymer decomposition. Previously, it was found that this process was less efficient for PMMA in the presence of BP and tBPB. These initiators provide benzoyloxyl radicals, C6H5COO (in the case of tBPB also t-butoxyl radicals are formed). Degradation of polymer containing Irg was accelerated, thus benzoyl radicals (C6H5 C¼O) and/or a,a-dimethoxybenzyl C6H5 C (OCH3)2, created during decomposition of this photoinitiator, were more reactive compared to the radicals form BP and tBPB. It suggests that the reactions of benzoyl and a,a-dimethoxybenzyl radicals with PMMA molecules are privileged, while benzoyloxyl C6H5 C (¼O)O and t-butoxyl (CH3)3 C O moieties undergo also significant secondary reactions (giving unreacted products) or recombine in the polymer cage. Such cage effect is feasible in polymer film due to the limiting molecular movement as mentioned above and because of difficult diffusion through the dense matrix. Thus, the collision of radicals with their nearest neighbors is more probable.
should be emphasized that higher photoresistance of modifiers in PMMA is caused by difficult diffusion of free radicals, oxygen and degradation species in polymer matrix which restricts the molecular mobility of the species. It has been found that fluorescence is negligible process in pure PMMA but can be enhanced in the presence of photosensitive compounds. Thin modified polymeric films showed fluorescence maximum at 220 and 270 nm of excitation wavelength. The exposure to UV radiation of 254 nm wavelength causes the gradual decay of initiator fluorescence. This process is somewhat inhibited in PMMA matrix possibly due to the excitation energy transfer to macromolecules. Deactivation of molecules in excited states occurring by physical processes is competitive reaction to photochemical destruction of modifiers and polymer. The highest efficiency of main chain scission reactions observed for PMMA containing Irgacure corresponds to the smallest decay of fluorescence for this specimen. It means that benzoyl and a,a-dimethoxybenzyl radicals are more efficient species in PMMA
4. Summary and conclusions
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J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
References
Fig. 4. Relative changes in fluorescence intensity versus irradiation time for initiators (a) and PMMA with initiators (b).
Fig. 5. Relative changes in the limiting viscosity number for UV-irradiated: PMMA and PMMA with modifiers.
degradation than those formed during photolysis of benzoyl peroxide and tert-butyl peroxybenzoate. This physical method of polymer modification is useful for preparation of new materials having different properties than pure original PMMA. Fluorescent PMMA-based plastics are important materials which can be applied in modern technologies. Owing to easy modification, low production cost and known processing, the further new applications of this thermoplastic polymer are expected in the near future.
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Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Effect of UV-irradiation on fluorescence of poly(methyl methacrylate) films with photosensitive organic compounds Jolanta Kowalonek* , Halina Kaczmarek, Marzanna Kurzawa , 87-100 Torun , Gagarina 7, Poland Faculty of Chemistry, Nicolaus Copernicus University in Torun
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 August 2015 Received in revised form 22 December 2015 Accepted 23 December 2015 Available online 28 December 2015
The simple way of physical modification of poly(methyl methacrylate) (PMMA) for the purpose of formation its photoactivity and fluorescence properties has been described. It was found that aromatic carbonyl compounds (2,2-dimetoxy-2-phenylaceto-phenone, benzoyl peroxide and tert-butyl peroxybenzoate) dispersed in PMMA matrix changed its optical properties, which was studied by absorption and emission spectroscopy. Modified PMMA exhibits fluorescence upon excitation of 220 or 270 nm. The intensity of emission band decreases after samples exposure to short wavelength UV-irradiation, which is evidence of component photolysis. The fluorescence decay is slightly smaller for the irradiated initiators in PMMA matrix, but the trend is similar to that for pure initiators exposed to UV. The protective effect of PMMA on the studied compounds after UV-irradiation, however, is accompanied by photodegradation reactions occurring in the polymer, which was confirmed by viscometric measurements. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Poly(methyl methacrylate) Photoinitiators Fluorescence spectroscopy UV–vis spectroscopy
1. Introduction Photoactive polymeric composites attract much attention because they are potential materials for the new technologies in the fields such as electronics, optics, biotechnology, medicine, pharmacy etc. Poly(methyl methacrylate)(PMMA) is a valuable raw material for the production of photosensitive plastics. The physical and chemical modification of this polymer can lead to obtaining fluorescent or photoconductive materials [1–4]. Moreover, acrylate polymers are also used as a matrix for low-molecular organic or inorganic compounds [5,6]. For example, PMMA doped with fluorescent probes (amino aromatic species) has been successfully obtained by pulsed laser deposition. Such material can act as a sensor for detection of environmental impurities [7]. In other interesting work, the switching properties of fluorescent photochromic PMMA copolymerized with spironaphthoxazine and D-p-A type pyran-based dye have been reported [8]. Atactic and syndiotactic PMMA with anthracene fluorophores attached to the end of side chains or embedded in the polymer backbone has been studied by time-resolved fluorescence spectroscopy. Additionally, isotactic PMMA without anthracene fluorophore was prepared to mix it with labeled PMMA. It was found
* Corresponding author. E-mail address: [email protected] (J. Kowalonek). http://dx.doi.org/10.1016/j.jphotochem.2015.12.017 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
that stereocomplexes between the syndiotactic and the isotactic PMMA in dimethylformamide were formed due to the local conformational motion of polymer segments [9]. Studies of anthracene-labeled PMMA gels by fluorescence anisotropy decay were the subject of the article by Aoki [10]. It was concluded that the local motion of PMMA gel at crosslinks was governed by the segment density of the network chains in the vicinity of crosslinks. Furthermore, it was found that nanocarbon-PMMA composites, prepared by pulsed laser ablation technique, exhibited fluorescence. Spectroscopic studies revealed interactions between nanocarbon particles and polymer matrix due to transfer of p electrons from the carbonyl groups of PMMA to nanocarbon [11]. The possibilities of other applications of modified PMMA and its copolymers in fabrication of nanodevices, in which the photoactivity is demanded, have been reported in numerous publications [3,4,12–18]. Moreover, experimental emission techniques appeared useful in evaluation of component miscibility in polymer blends by means of excimer fluorescence and fluorescence microscopy [19,20]. Recently, PMMA was modified by the addition of small amount of different photosensitive low-molecular weight compounds such as aromatic or aliphatic ketones and peroxides [21–24]. It was found that PMMA doped with these photoinitiators undergoes accelerated photodegradation compared to the unmodified polymer, which is important in designing photodegradable
J. Kowalonek et al. / Journal of Photochemistry and Photobiology A: Chemistry 319 (2016) 18–24
polymers. Moreover, the thermal resistance of modified polymer was lower than that of unmodified, whereas the surface properties of PMMA with or without additives did not change significantly. The aim of the present work was to study the fluorescence of PMMA physically modified with three organic compounds: 2,2dimetoxy-2-phenylaceto-phenone (Irg), benzoyl peroxide (BP) and tert-butyl peroxybenzoate (tBPB). Physical mixing of photosensitive compounds with polymers is much simpler and cheaper method of preparing fluorescent materials than the complex, often multi-step and long-term synthesis of polymer-based photoactive compounds. In this way it is possible to prepare photosensitive plastics for potential applications because of easy processing and possibility of modification of mechanical properties in a wide range. An important objective of the work was to check whether the fluorescent properties of the photosensitive compounds still remained in the polymer matrix and to what extent these properties changed under the influence of UV radiation. The emission properties of modified PMMA-based materials are important in the case of using such plastics as selective sensors, fluorescent markers as well as the substances for lithography, optics or electronics. 2. Experimental 2.1. Materials Poly(methyl methacrylate), IUPAC name: poly(methyl 2-methylpropenoate), PMMA, has been purchased from Sigma–Aldrich. The average molecular weight of polymer was 120,000. Photoinitiators: (2,2-dimetoxy-2-phenylaceto-phenone); trade name: Irgacure 651 (Irg), benzoyl peroxide (BP) and tert-butyl peroxybenzoate (tBPB) were supplied by Ciba, Basel, Switzerland; Sigma–Aldrich. The formula of these compounds and theirs abbreviations are shown below:
Acetonitrile (ACN) and tetrahydrofuran of high purity (Sigma– Aldrich) were used as solvents.
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Excitation and emission slit widths were 5 nm for the wavelength response of the system. The response time was 0.1 s while a data interval—1 nm. Individual and three-dimensional spectra were recorded in the range of 200–900 nm at a rate of 60 000 nm/min. The spectra were evaluated with the PC software package supplied with spectrophotometer (FL-Solution 2.1 for F-7000). Emission spectra of UV-irradiated samples were recorded directly after exposure. To compare the UV influence on different samples, the relative changes of fluorescence intensity has been calculated:
DIrel ¼
I o It Io
ð1Þ
where Io and It are fluorescence intensity of unirradiated sample before and after t time of irradiation, respectively All fluorescence measurements were performed for samples in a solid state. 2.4. UV–vis spectroscopy Absorption spectra in UV–vis range have been recorded using UV-1601PC spectrophotometer (Shimadzu, Japan). The measurements were performed in acetonitrile solutions (pure initiators) and in a solid state (thin films of photoinitiator-doped PMMA). The molar absorption coefficients, elmax, for photoinitiators in dilute solutions have been calculated from Beer-Lambert law. 2.5. Ultraviolet exposure Polymeric films were UV-irradiated using a low-pressure mercury vapor lamp TUV30W, Philips, Holland. This source emits radiation of 254 nm wavelength. The intensity of the incident light, measured by HD 9021 radiometer, was 32.2 W/m2. The irradiation was carried out at 30 C and at ambient atmosphere. Fluorescence
spectra of UV-irradiated films were systematically measured. 2.6. Viscosity measurements
2.2. Sample preparation 2% (w/v) solutions of polymer and organic compounds in tetrahydrofuran were prepared. The solutions were mixed in a proper volume ratio to obtain the mixtures of PMMA with 5 wt.% of modifier content. Thin films (thickness of 20 mm) of pure PMMA and PMMA with photoinitiators were obtained by casting method. After solvent evaporation, films were carefully dried in a vacuum oven at 30 C for one week. The purity of dried films was checked by spectroscopic analysis. 2.3. Fluorescence measurement Fluorescence measurements were performed on a Hitachi F-7000 Fluorescence Spectrophotometer at room temperature using a Xenon lamp source for excitation and the 1 cm quartz cell.
Unirradiated and UV-irradiated films of PMMA and PMMA with photoinitiators have been dissolved in acetonitrile. Viscosity of these diluted solutions was measured at 25 0.01 C using Ubbelohde capillary viscometer. From the flow times of the solvent and the solutions, relative (hr) and specific (hsp) viscosities were determined for 4–5 concentrations. The flow time of liquid was measured with accuracy of 0.01 s. Intrinsic viscosity (limiting viscosity number, LVN, dl/g) has been calculated by extrapolation of reduced viscosity (hsp/c) to zero concentration (c = 0). The relative percentage changes of LVN of various samples have been calculated according to the formula:
DðLVNÞ ¼
ðLVNÞ0 ðLVNÞt 100% ðLVNÞ0
ð2Þ
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Fig. 1. Changes in absorption spectra during exposure to UV of photoactive modifiers in ACN solutions: (a) Irgacure (Cm = 3.2 105 M), (b) benzoyl peroxide (Cm = 7.3 106 M), (c) tert-butyl peroxybenzoate (Cm = 7.5 106 M). Arrows in the diagram indicate the direction of the change. Drawings in color are available in the online version.
where (LVN)o and (LVN)t are intrinsic viscosity of unirradiated sample before and after t time of irradiation, respectively. 3. Results and discussions First of all, in order to evaluate the optical properties of the examined systems, absorption spectra of the selected organic compounds in a solution have been collected in the range of UV– vis. The spectra provide information about absorption of chromophoric groups present in the structure of photoinitiators and possibility of their excitation upon UV. It is necessary for occurrence of the fluorescence in these compounds. Moreover, UV–vis absorption spectra of pure PMMA and PMMA with these compounds (in a solid state) have been done. Then, all prepared specimens were UV-irradiated and both absorption and emission (fluorescence) spectra have been collected immediately after exposure. Our previous studies [21–24] have shown that it was not easy to predict the photochemical behavior of polymer doped with low molecular weight organic compounds based on the properties of individual constituents.
The alterations in UV–vis spectra of the exposed photoinitiators are shown in Fig. 1. Absorption bands observed at 252, 234 and 229 nm in Irgacure, benzoyl peroxide and tert-butyl peroxybenzoate, respectively, are due to p,p* transitions in aromatic rings. Additionally, the band of low intensity at about 270 nm appears in all UV-irradiated samples, which can be attributed to n,p* transitions in carbonyl groups. The value of molar absorption coefficient, elmax, determined for the main absorption band was the lowest for Irg (e252 = 15300 dm3 mol1 cm1) and the highest for BP (e234 = 42400 dm3 mol1 cm1), for tBPB it was: e229 = 39300ndm3 mol1 cm1). The systematic fast decrease in the main absorption bands of the initiators spectra during their UV-irradiation clearly indicates their efficient photolysis. Particularly fast decomposition was observed in the case of Irg, just after 10s the compound decomposed. However, a new band at 223 nm was found indicating the formation of a new product, thus, the exposure time was prolonged up to 2 min to observe the formation of this moiety. Time needed to complete decomposition of BP and tBPB was 20s and 40s, respectively. Simultaneously, the broad tail of absorption in the range of longer wavelength has been observed for all exposed photoinitiator solutions.
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The mechanism of photochemical reactions in these photoactive compounds is known, so interested readers are referred to the cited references [22,23]. The main absorption band (p,p*) characteristic of photosensitive compounds was also observed for PMMA films with these compounds (not shown in the work). The second band (n,p*) was invisible in PMMA matrix because of low concentration (5%wt.) of photoinitiators. Small shift of absorption band (about 5 nm) was observed only for BP in PMMA, whereas for two other samples (PMMA + Irg and PMMA + tBPB) practically there were no changes in the position of band maximum. UV-irradiation also causes destruction of the photoinitiators dispersed in PMMA matrix but this process is much slower in comparison to that in solutions: 30 min UV exposure was needed for complete disappearance of absorption bands in the spectra. It should be noticed that the changes in the UV–vis spectra of the pure polymer after exposure to UV are insignificant indicating that the formation of products with chromophore groups cannot be detected using this method. The slight increase in absorbance in the whole UV–vis region was caused by some photoreactions occurring in the polymer, mainly photooxidative degradation leading to formation of new type of carbonyl groups and unsaturated bonds. The slower rate of photoinitiator decomposition in PMMA matrix can be explained as follow: the initiator molecules are trapped in the polymer matrix with low macromolecular mobility due to its glassy state at the experimental conditions, the diffusion of chemical moieties is restricted, contrary to solutions where macromolecular movements are less restricted. The formed radicals in a film can be characterized by much longer life time before recombination. Also a diffusion of atmospheric oxygen is more limited in the film than that in a solution, then, the quenching of excited states or direct reactions of radicals with oxygen is less efficient is a solid. Modified PMMA films were exposed to short-wavelength UV radiation and then fluorescence spectra were recorded immediately after each irradiation time. It was found that the maximum of fluorescence intensity depends on the type of modifier, thus, the wavelength at 220 nm was used for excitation of Irg and tBBPcontaining samples and the wavelength at 270 nm was chosen for sample with BP. The energy of this radiation is sufficient to excite the carbonyl groups in PMMA:
The neat PMMA also exhibits a very weak fluorescence but the emission maximum is observed at shorter excitation wavelength (i.e., 210 nm) than for PMMA with modifiers. To confirm this fact, 3D spectrum of pure PMMA was recorded (Fig. 2a). As can be seen, there is only a band coming from Rayleigh scattering [26] in the spectrum (the excitation beam produces a distinct diagonal feature). Additionally, two spectra at excitation wavelength l = 220 nm and at l = 270 nm were recorded for this polymer (Fig. 2a and b). The spectra confirm the lack of bands in the range from 300 nm to 500 nm. In order to estimate the real influence of PMMA matrix on specimen behavior, the emission spectra of pure modifiers
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Fig. 2. 3-D fluorescence spectrum of pure PMMA (a); the emission spectra of pure PMMA at 270 nm and 220 nm of excitation wavelength (b).
irradiated with the same dose under the identical conditions were done (Fig. 3). As can be seen, the emission spectra maximum of Irg and tBPB is slightly shifted to lower wavelength in polymer matrix (about 15 nm). Only in the case of BP the position of
emission band is almost the same (maximum at 300 nm) as for PMMA + BP specimen. It indicates that some interactions appear between PMMA molecules and Irg or tBPB, contrary to PMMA and BP. It seems obvious because BP with completely symmetric geometry is nonpolar compound. Two other modifiers have unsymmetrical structure and electronegative oxygen atoms, which can resulted in dipole interactions between the oxygen atoms of the initiators and carbonyl groups of PMMA side substituents. UV-irradiation of the pure initiators and PMMA modified with the initiators leads to a decrease in fluorescence intensity, which is clearly seen in Fig. 3. This is caused by photochemical decomposition of organic compounds used as modifiers. Such reactions are
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Fig. 3. Fluorescence spectra of modifiers and PMMA with these additives: (a) Irgacure, (b) PMMA + Irgacure, (c) benzoyl peroxide, (d) PMMA + benzoyl peroxide, (e) tert-butyl peroxybenzoate, (f) PMMA + tert-butyl peroxybenzoate. Excitation wavelength: 220 nm (a, b, e, f) and 270 nm (c, d).
known and described in the literature, since these compounds due to the presence of unstable photosensitive groups are used as polymerization initiators [25]. Recently, these compounds were proposed as the agents causing the polymer matrix photodegradability [21–24]. Fluorescence measurements provide interesting information on the progress of decomposition of these compounds in the polymer matrix upon UV irradiation. In order to facilitate the
quantitative comparison of the observed processes, the relative changes in fluorescence intensity were calculated according to the Formula (1). The results are presented in Fig. 4. As can be seen, both initiators containing peroxide bonds (BP and tBPB) are characterized by more efficacious fluorescence decay in comparison to this process for Irg. The same trend is observed for these compounds in the polymer matrix which, although, somewhat retards the degradation of the modifiers. Probably energy transfer processes
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occur as a result of intermolecular interactions between photoinitiator in excited state and the polymer in ground state:
The mutual interactions between constituents and the photodegradation products and energy transfer processes can also occur
This partial protective action of PMMA is accompanied by the polymer photodegradation, which is competitive reaction to initiator photolysis in the polymer matrix. To confirm this assumption, the viscosity measurements have been performed. Neat and modified PMMA films have been UV-irradiated during 0– 30 min and then dissolved in ACN. The intrinsic viscosity values obtained for each system and different exposure times have been used for calculation of the LVN percentage changes, which is shown in Fig. 5. The decrease in the LVN is evidence of main chain scission reactions in the polymer during UV-irradiation. The photodegradation process of PMMA is accelerated by Irg. It is obviously caused by the fastest decomposition of this compound under UV-light, which was proved by UV–vis spectroscopy. However, chain breaking in PMMA is less efficient in the presence of BP and tBPB and even slightly inhibited compared to that in pure PMMA. A different reactivity of the free radicals produced from photoinitiators, having diverse chemical structure, can provide the explanation of above-mentioned processes. Moreover, the photochemical decomposition competes with physical deactivation reactions of excited states. The following scheme (5) can summarize our observations:
in a solid state. It should be added that the other photochemical reactions occurring in PMMA (depolymerization, side chain abstraction) have no significant influence on average molecular weight (thus, also on intrinsic viscosity) of polymer.
Three organic compounds: 2,2-dimetoxy-2-phenylaceto-phenone, benzoyl peroxide and tert-butyl peroxybenzoate, known as polymerization photoinitiators, were applied in this work to make poly(methyl methacrylate) sensitive to UV-light. The changes in photosensitivity of modified PMMA have been studied by emission (fluorescence) spectroscopy. Additionally, absorption spectroscopy in UV–vis range has been applied for better characterization of optical properties of studied samples. The rate of modifiers photolysis depends on the physical state of specimens. Decomposition of initiators is very fast in diluted solutions, almost complete disappearance of the characteristic absorption band of a compound is observed after several seconds of irradiation. In PMMA matrix this process is much slower—the decomposition of absorption band occurs during half of an hour. It
Free radicals formed during initiator photolysis are able to abstract hydrogen atoms from macromolecules, which initiates polymer decomposition. Previously, it was found that this process was less efficient for PMMA in the presence of BP and tBPB. These initiators provide benzoyloxyl radicals, C6H5COO (in the case of tBPB also t-butoxyl radicals are formed). Degradation of polymer containing Irg was accelerated, thus benzoyl radicals (C6H5 C¼O) and/or a,a-dimethoxybenzyl C6H5 C (OCH3)2, created during decomposition of this photoinitiator, were more reactive compared to the radicals form BP and tBPB. It suggests that the reactions of benzoyl and a,a-dimethoxybenzyl radicals with PMMA molecules are privileged, while benzoyloxyl C6H5 C (¼O)O and t-butoxyl (CH3)3 C O moieties undergo also significant secondary reactions (giving unreacted products) or recombine in the polymer cage. Such cage effect is feasible in polymer film due to the limiting molecular movement as mentioned above and because of difficult diffusion through the dense matrix. Thus, the collision of radicals with their nearest neighbors is more probable.
should be emphasized that higher photoresistance of modifiers in PMMA is caused by difficult diffusion of free radicals, oxygen and degradation species in polymer matrix which restricts the molecular mobility of the species. It has been found that fluorescence is negligible process in pure PMMA but can be enhanced in the presence of photosensitive compounds. Thin modified polymeric films showed fluorescence maximum at 220 and 270 nm of excitation wavelength. The exposure to UV radiation of 254 nm wavelength causes the gradual decay of initiator fluorescence. This process is somewhat inhibited in PMMA matrix possibly due to the excitation energy transfer to macromolecules. Deactivation of molecules in excited states occurring by physical processes is competitive reaction to photochemical destruction of modifiers and polymer. The highest efficiency of main chain scission reactions observed for PMMA containing Irgacure corresponds to the smallest decay of fluorescence for this specimen. It means that benzoyl and a,a-dimethoxybenzyl radicals are more efficient species in PMMA
4. Summary and conclusions
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References
Fig. 4. Relative changes in fluorescence intensity versus irradiation time for initiators (a) and PMMA with initiators (b).
Fig. 5. Relative changes in the limiting viscosity number for UV-irradiated: PMMA and PMMA with modifiers.
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