The effect of copper ions on interaction of UV radiation with methacrylic matrix – EPR study

Materials Chemistry and Physics 143 (2013) 440e445

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The effect of copper ions on interaction of UV radiation with methacrylic matrix e EPR study  ska, Alina Zalewska*, Ryszard Krzyminiewski, Bernadeta Dobosz, Justyna Mrozin  ski Zdzis1aw Kruczyn  , Poland Medical Physics Division, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan

h i g h l i g h t s
The carboxyls in matrix of polymer increase the loading capacity of Cu ions.
There is produced much more radicals in Methafilcon A with Cu ions than without them.
The carboxyls coordinated with copper ions play a key role in UV-degradation process.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2013 Received in revised form 13 September 2013 Accepted 18 September 2013

The role of metal ions introduced to polymer matrix in the photochemical degradation of material is not fully understood. In this paper, we considered the effect of copper ions on the photochemical changes in Methafilcon A after UV-irradiation. The presence of methacrylic acid in the structure of Methafilcon A increases the loading capacity of these ions. In result, there is observed the production much more radicals after UV-irradiation than in pure matrix, without copper ions. When the time of UV-exposure increases, the EPR signal of trapped Cu(II) ions in the material decreases. This proves the transformation of Cu(II) to a diamagnetic state of stable Cu(I)-intermediates or copper oxides. Simultaneously, in the first 5-min of UV-irradiation there is observed a rapid increase in intensity of the radical signal, which disappears when the exposure time is extended. This mechanism of radical generating is quite different than for Methafilcon A matrix without copper ions. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Electron paramagnetic resonance (EPR) Irradiation effects Organometallic compounds Polymers

1. Introduction The widespread use of polymeric materials in the outdoor environment and the search for ways of modifying their physical and chemical properties accounts for much of the current interest in the photochemistry of polymers. Each of materials has its own characteristic modes of degradation and resistances to heat, light and chemicals. Polymers which form chelates with transition metal ions are interesting because of great significance in catalytic reactions as selective heterogeneous catalysts [1] or as precursors of high temperature superconductors [2]. The most widely used method for attaching these metallic species to polymeric matrix involves synthesis appropriate functional monomers and their post-synthesis modification (including ion-exchange). One of the most universal monomers for this purpose is methacrylic acid.

* Corresponding author. Tel.: þ48 61 829 5278; fax: þ48 61 829 5189. E-mail address: [email protected] (A. Zalewska). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.025

Cupric ions show a strong tendency to formation of chelates with these materials. This occurs throughout replacement of hydrogen in carboxyls and it is consistent with the reactions [3,4]:

Cu2þ þ ðCOOHÞ2 /ðCOOÞ2 Cu þ 2Hþ

(1)

These systems offer different properties over pure polymer due to formation of chelating ring structure with copper ions. It is well known that certain transition metal complexes prevent thermal and photochemical degradation of polymers (acting as stabilizators) [5e8], on the other side they act as a catalyst of these processes [2,9e11]. The mechanism of photochemical decomposition of copper-monocarboxylate system is according to the chargetransfer reaction [10,12]:

i* h

hn RCOO Cu2þ !R COO þ Cu1þ /R þ CO2 þ Cu1þ

(2)

This free-radical process may be excellently observed by electron paramagnetic resonance spectroscopy. Due to the absence of

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such EPR researches in literature, in this paper we consider the effect of the copper ions on the photochemical changes of polychelate caused by UV radiation. We will try to determine the validity of copper ions in the photodegradation of poly(methacrylic acid-co-2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate), commercially known as Methafilcon A and Polymacon respectively.

formed in these materials. It is now widely adopted in accordance with Symons [17] and Abraham et al. [13], that this 9-line spectrum is due to the two conformations of the same methacrylic radical [e CbH2eCa*(CbH3)(COOR)] formed during the main chain scission. This theory is considered on the basis of an isotropic hyperfine coupling the unpaired electron resided in pz(p)-orbital of carbon adjacent to the protons in the b-positions. The hyperfine splitting constant to b-proton is described using the following equation:

2. Experimental

  A Hb ¼ B$ra $cos2 q

Methafilcon A (Cooper Vision) is a co-polymer of methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) cross-linked with ethylene glycol dimethacrylate (EGDMA). Polymacon (Bausch & Lomb) consists of 2-hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA). Copper (II) chloride dihydrate CuCl2$2H2O (SigmaeAldrich) was used without further purification to preparing initial 10 mM aqueous solutions. Studied materials were impregnated with this solution for a period of 48 h and dried for 24 h in 295 K. Each sample was weighed (average value was about 15 mg for both materials) and put separately in initial solution contains cations of copper. The impregnation experiments were carried out with 1 ml each initial solution per each 10 mg of dry material. After impregnation time, samples were rinsed with distilled water to remove the unbounded metal ions and then were dried for 48 h in 295 K. The dried and crushed samples with (Cu-Methafilcon A, CuPolymacon) and without (Methafilcon A, Polymacon) copper ions were UV irradiated using a medium pressure mercury lamp (EMITA VP-60, 180 W) without a filter. The lamp emits a spectrum ranging from 240 to 580 nm including intense UV spectral lines primarily at 265 nm (Il265/Il366 ¼ 0.24), 297 nm (Il297/Il366 ¼ 0.18), 302 nm (Il302/Il366 ¼ 0.30), 313 nm (Il313/Il366 ¼ 0.63) and 366 nm (Il265/ Il366 ¼ 1.0). The irradiation process took place in an air atmosphere outside of the spectrometer cavity. Samples were irradiated for various lengths of time (1, 5, 10, 15, 35, 45, 245 min). The temperature under the lamp during irradiation in sample place was measured all time and did not exceed 65  C. We checked that there was not any radical signal in EPR measurements when the studied samples with and without copper ions heated to 100  C. The EPR spectra were measured immediately when the irradiation was completed. All EPR spectra were recorded using X-band EPR spectrometer (Bruker EMX-10) with the operation conditions of microwave power 7.95 mW and microwave frequency 9.36 GHz; a second field modulation frequency 100 kHz and amplitude 0.3 or 1 mT; a time constant 20 ms. The magnetic field was calibrated using DPPH standard sample. All sample preparations and experiments were carried out at 295 K. The methacrylic radicals and Cu(II) concentration were referenced to weak pitch sample from Bruker Instruments (1013 spins per centimeter) and copper(II) chloride in aqueous solution (10 mM), respectively. Experimental EPR spectra were simulated using WIN-EPR SimFonia program (Bruker Analytische Messtechnik GmbH). The simulated spectra assumed a solution spectral model (isotropic interaction) for organic free radicals and a powder spectra model for copper ions in matrix of studied materials. Solid state electronic reflectance spectra were recorded in range 200e900 nm on Varian-Cary 100 spectrometer.

(3)

where B is constant with value about 4.6 mT, ra is net spin density at the appropriate nucleus of carbon atom and q is the angle between the carbonehydrogen bond and the symmetry axis of the pz orbital, both projected onto a plane perpendicular to the CaeCb bond. This is illustrated for the methacrylic radical in Fig. 1. The methacrylic radical spectrum has been assigned to two overlapping spectra with carbon-centered g-value (g z 2.0036) derived from different conformation of the b-methylene protons around the Cae Cb: 9-lines spectrum (designated further as R9) and 5-lines spectrum (designated further as R5) with intensity ratios of 1:2:4:6:6:6:4:2:1 and 1:4:6:4:1, respectively [14,18e20]. At room temperature, a freely rotating b-protons of the methyl group lead to averaged value of hyperfine coupling constant (about 2.3 mT). In addition, one conformation favors a symmetrical spacing of the bmethylene protons around the CaeCb bond (q1 ¼ q2 ¼ 60 ) and the other asymmetrical one (q1 s q2, q1 ¼ 75 q2 ¼ 45 ). In the first case, the hyperfine splitting from both protons of methylene group is about 1.15 mT in accordance with equation (3). In the latter case, the hyperfine splitting one of the b-protons of methylene group is smaller than the width of the line, and for a second is z 2.3 mT. The relative intensity of set 9- and set 5-lines reflects the relative probabilities of the both geometrical structures. Fig. 2 shows EPR spectra of the radicals generated in studied materials: Polymacon (HEMA þ EGDMA) and Methafilcon A (HEMA þ MAA þ EGDMA), recorded after 5-min UV irradiation. The spectra of Polymacon and Methafilcon A are very similar, they are characterized by lines in the same positions, but they differ in their intensity ratios. In addition, these spectra are different from spectrum of methyl methacrylate and their other esters, which are presented everywhere in the literature as standard [13e17], although Polymacon and Methafilcon A consist of these monomers. Number of radicals generated in the studied materials is the same order of magnitude per gram of material (w1.4  1016 radicals g1 for Methafilcon A and w4.1  1016 radicals g1). EPR spectrum of Polymacon after 5-min UV exposure will serve only as a reference for further part of discussion, now we focus exclusively on Methafilcon A material. Fig. 3 shows EPR spectra recorded during UV irradiation of Methafilcon A, after 1, 5, 10, 15, 30 and 45 min exposure. The signal intensity of each line increases with time of exposure, but unequally. We therefore assume that there is created more than one kind of radical. With increasing irradiation time, the intensity of two extreme lines (designed by asterix in Fig. 3) remains almost at the same level, while the intensity of other lines (designed by circle

3. Results and discussion 3.1. Photochemical degradation of Methafilcon A and Polymacon EPR spectrum of irradiated polymethacrylic acid and their methyl, ethyl esters is composed of nine lines [13e17]. There is still no consensus on the structure and conformation of the free radicals

Fig. 1. The methacrylic radical: (a) structure and (b) conformation in Newman projection.

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Fig. 2. EPR spectra of Methafilcon A, Polymacon, CueMethafilcon A and CuePolymacon after 5-min UV irradiation.

in Fig. 3) is increasing rapidly. After mathematical subtraction spectrum of well recognized in the literature methacrylic radical from spectrum of UV-irradiated Methafilcon A, there is one which can be a combination of two signals: the quartet with intensity ratios of 1:3:3:1 (R4) and a broad singlet line (R1). Simulated spectra of radical forming the experimental spectrum of Methafilcon A are shown in Fig. 4 and their simulated parameters are summarized in Table 1. The fits had only three adjustable parameters: an isotropic hyperfine interaction of unpaired electron with an appropriate nuclei of hydrogen (A), a linewidth (DB) and shape (Gaussian or Lorentzian). Spectrum of methacrylic radical (R9 and R5) was simulated accurately according to the above concept of two conformations with distinct dihedral angles between the CeH bond and the pz(p)-orbital of radical center, and so with different hyperfine splitting. Radicals R4 and R1 enable reproduce the experimental spectra of materials after UV exposure. Based on the simulation data we can say something about the local environment

Fig. 4. Simulated spectra of individual radicals R9, R5, R4 and R1, which reproduce the experimental spectrum of Methafilcon A after UV-irradiation.

of unpaired electron. For example, these data of radical R4 suggest that the unpaired electron is located near CH3 group (hyperfine coupling to three equivalent protons gives a quartet with intensity ratios of 1:3:3:1) and close to an oxygen atom (delocalization of electron density which reduces the hyperfine coupling constant to 1.35 mT value; higher than 2.0036 value of g-factor). However, the precise interpretation of these radicals requires further investigations and confirmation by computational models of quantum chemistry. Radical R1 is probably an oxygen-centered radical CO* due to g ¼ 2.0056. The exemplary fits of experimental spectra calculated using theoretical spectra R9, R5, R4 and R1 are show in Fig. 5. The fits seem to be good. The recombination rate of radicals signal formed in Methafilcon A after 10 min UV exposure is described using the following equation:

CR ¼ 0:7 þ 1:3$e0:02$t



R2 ¼ 0:997



(4)

where CR is the radical concentration in 1016 g1 and t is the time measured in minutes after irradiation. Number of radicals is referenced to standard sample with a known concentration of radicals and normalized to the weight of the sample. After about 75 min in the absence of illumination, the number of radicals formed in the material decreases to half. Fig. 6 shows effect of exposure time to UV-irradiation on the intensity of EPR signals generated in Methafilcon A. There is observed an exponential increase of radicals (designed by asterix) when the exposure time increases. Initially, the production rate of radicals caused by UV radiation used in

Table 1 EPR parameters of simulated spectra for radicals (R9, R5, R4, R1) generated in Methafilcon A after UV irradiation. Radical R9

Fig. 3. Progressive changes in EPR spectra of Methafilcon A as function of irradiation time (after 1, 5, 10, 15, 30, 45 min).

Intensity ratios 1:2:4:6:6:6:4:2:1

R5

1:4:6:4:1

R4 R1

1:3:3:1 1

DB [mT]

A [mT] AH AH AH AH AH

¼ ¼ ¼ ¼ ¼

2.26 1.15 2.28 0.25 1.35

(3H) (2H) (4H) (1H) (3H)

g-factor

0.80

L

2.0036

0.32

L

2.0036

0.70L 1.60G

2.0042 2.0056

Aehyperfine coupling constant. DBeline width, superscript G or L designed Gaussian or Lorentzian line shape, respectively.

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associated with the quartet and a broad singlet line (R4 þ R1). The number of R9 þ R5 radicals formed in Methafilcon A remains constant, when exposure time is more than 10 min. After this time, there is only a significance increase of R4 and R1 contributions in experimental spectrum. Thus, the mechanism of their formation in Methafilcon A under the influence of photochemical degradation is favored. 3.2. Photochemical degradation of CueMethafilcon A and Cue Polymacon

Fig. 5. The exemplary fits of experimental spectra (solid line) calculated using theoretical spectra R9, R5, R4 and R1 (dotted line) for Methafilcon A after 30-min UV irradiation and for CueMethafilcon A after 5-min UV irradiation.

experiment is faster than the recombination rate of radicals and consequently there (Fig. 6) is obtained the resultant number of radicals described by equation:

CR ¼ 3:3  3:3$e0:08$t



R2 ¼ 0:977



(5)

where CR is the radical concentration in 1016 g1 and t is the exposure time measured in minutes. However, when the exposure time is extended this curve becomes saturated, which means that the production and recombination rates of radicals are comparable. This is probably due to the fact that a higher concentration of radicals per volume unit increases the probability of recombination processes. The contribution of the individual radicals R9, R5, R4, R1 in the resultant spectrum is different and it depends on the irradiation time. In Fig. 6, the square marked contribution from methacrylic radicals (R9 þ R5), while the remaining contribution is

Fig. 6. Concentration of radicals generated during irradiation of the Methafilcon A (asterix) and the individual contributions of radicals to resultant spectrum: R9 þ R5 (square) and R4 þ R1 (diamond).

To determine the loading capacity of copper ions in Polymacon and Methafilcon A, the quantitative measurements of impregnated samples and solution after impregnation were done. The maximum loading capacity for CueMethafilcon A is approximately 6.0  1019 copper ions per gram of material, and for CuePolymacon the maximum quantity is about 3.6  1018 g1. This higher loading capacity of Methafilcon A can be explained by consideration the chemical composition of tested polymers, because it plays the important role in sorption process [21]. In contrast to Polymacon, Methafilcon A has an ionic monomer (methacrylic acid) in matrix. After 5 min UV-irradiation, the number of radicals generated in CueMethafilcon A and CuePolymacon is about 15.8  1016 radicals g1 and 4.1  1016 radicals g1, respectively. The intensity of free-radical signal is about 11 times greater for Cue Methafilcon A compared to Methafilcon A matrix without copper ions and for CuePolymacon is at the same level as for pure matrix of Polymacon. Therefore, the carboxyl groups with attached copper ions play an important role in the free-radical production. Fig. 2 shows EPR spectra of this radicals. In these spectra, there are mathematically cut signals of copper ions, because they were slightly overlapped with the signals of radicals. The radical spectrum of CueMethafilcon A after 5-min UV irradiation can be attributed to typical methacrylic radicals (R9 þ R5), without significant contamination of any other radical, while the radical spectrum recorded for CuePolymacon is not distinguishable from this recorded for Polymacon. The solid state UVeVis spectra of Methafilcon A and Cue Methafilcon A are show in Fig. 7. The presence of copper ions in the

Fig. 7. The solid state UVeVis spectra of Methafilcon A and CueMethafilcon A respectively. The vertical dashed markers indicate UV spectral lines emitted by the UVlamp used in experiment.

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material results in the appearance of broad bands related to de d transition of metal ion in 463 nm and 743 nm. Furthermore, in region 200e330 nm which is typical for p / p* and n / p* intraligand transitions (214 nm, 241 nm, 287 nm), there appear additional band in 306 nm assigned as ligand to metal charge transfer (LMCT) from electronic lone pairs of carboxylate oxygen to the metal ions [22e24]. This band is very intensive in comparison to other bands in spectrum. Thus, the radiation emitted from UV lamp used in the experiment promotes the LMCT transitions in Cue Methafilcon A material (see vertical dashed markers in Fig. 7 which indicate UV spectral lines emitted by the UV-lamp used in experiment) and it in turn enhance photolysis rate. According to our EPR investigation, the LMCT transitions favor the production of radicals R9 and R5. Fig. 8 shows EPR spectra of CueMethafilcon A recorded before UV irradiation and after 5, 15, 45 and 245 min of its effect. There are observed the characteristic anisotropic signals with axial symmetry of Cu2þ ions and four features of hyperfine splitting for parallel orientation. This hyperfine splitting is derived from interaction between copper nuclei with a spin I ¼ 3/2 and unpaired electron. The signal with the highest intensity is assigned to CueMethafilcon A before UV irradiation. Spectroscopic parameters of copper ions obtained from the simulation are gII ¼ 2.354, gt ¼ 2.074 and AII ¼ 143  104 cm1. Observed gII > gt is typical for the copper ions having an unpaired electron in dx2 y2 orbital. The parameters of signal are similar to those reported in the literature [25e28] and indicates that the copper ions coordinate via the oxygen atoms from the carboxyl groups. In the case of the tested materials there is not change of the spectroscopic parameters (gII, gt, AII) after UV exposure suggesting changes in the coordination environment of copper ions. However, in Fig. 8 are noticeable changes in the intensity of the EPR signal originating from Cu(II) ions in CueMethafilcon A. When the time of UV-exposure increases, there is a decrease of detected Cu(II) ions in the material. This apparent loss of signal is due to the transition from the paramagnetic Cu(II) valence state to a diamagnetic ones which can not by detected by EPR technique. This is shown in Fig. 9 and is described using the following equation:

CCu ¼ 2105 þ 4028$e0:031$t



R2 ¼ 0:987



(6)

Fig. 9. Disappearance of copper (II) ions signal in CueMethafilcon A as a function of exposure time.

where CCu is the Cu (II) concentration in 1016 ions g1 and t is the exposure time measured in minutes. It is an irreversible state and a detectable concentration of Cu(II) ions is maintained on constant level after irradiation (there is no transformation from diamagnetic to paramagnetic state). This is consistent with the charge-transfer mechanism described by reaction (2). The recombination rate of radicals signal formed in CueMethafilcon A after 5 min UV exposure is described using the following equation:

CR ¼ 0:8 þ 15:0$e0:025$t

(7)

where CR is the radical concentration in 1016 g1 and t is the time measured in minutes after irradiation. This recombination process is faster than it is for pure matrix of Methacilcon A. After about 30 min in the absence of illumination, the number of radicals formed in the material decreases to half. The situation is quite different during observation of radical signal depending on the exposure time in matrix containing copper ions (Fig. 10) compared to pure matrix (Fig. 6). In the first 5-min of UV irradiation is observed a rapid increase in intensity of the radicals signal in Cue Methafilcon A matrix to value 15.8  1016 g1. When the UV exposure time is longer than 5 min, there is observed the disappearance of the radicals signal in this material which is described by equation:

CR ¼ 1:9 þ 14:3$e0:021$t

Fig. 8. EPR spectra of CueMethafilcon A recorded before UV irradiation (signal of copper ions with the highest intensity) and after 5, 15, 45 and 245 min of its effect.

  R2 ¼ 0:997

  R2 ¼ 0:979

(8)

where CR is the radical concentration in 1016 g1 and t is the exposure time measured in minutes. This disappearance rate of radicals signal is very similar to the recombination rate of radicals signal. This is unexpected, because it may suggest that UV exposure time longer than 5 min apparently does not cause formation of radicals in Cue Methafilcon A or their production rate is much lower than their recombination rate. It should be remembered that the whole process is accompanied by decrease in Cu(II) signal (or rather its transformation from paramagnetic to diamagnetic state). Thus, for Cue Methafilcon A there are two compartments which represent the different behavior of the radical signal (Fig. 10). In the first 5-min of UV irradiation is observed a rapid increase in intensity of the radicals signal. The UV radiation promotes the charge-transfer process from an O atom of carboxyl group to the copper ion. This lead to the formation of a polymeric radical and the transformation of Cu(II) ions in

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different for CueMethafilcon A matrix than pure matrix of Methafilcon A without copper ions. The Cu(II) ions act as elementary catalysts of photochemical destruction and there is observed the production much more radicals. References

Fig. 10. Concentration of radicals generated during irradiation of the CueMethafilcon A (asterix). For comparison, the dashed line is from radicals generated during irradiation of the Methafilcon A.

stable Cu(I)-intermediates [10,12] or copper oxides [2]. When the UV exposure time is longer than 5 min, there is a continued decrease in the intensity of Cu(II) signal in CueMethafilcon A but simultaneously with the disappearance of the radical signals. It may suggest that there must be some mechanisms of radical recombination with involving also Cu(II) ions. However, a comprehensive explanation of this process requires further investigations. 4. Conclusions The carboxyl groups increases the loading capacity of copper ions and play an important role in the free-radical production. The mechanism of radical generating after UV-irradiation is quite

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