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Characterization of ethylene–vinylalcohol copolymer doped with chlorophyll
Polymer Testing 21 (2002) 571–576 www.elsevier.com/locate/polytest
Material Characterisation
Characterization of ethylene–vinylalcohol copolymer doped with chlorophyll N.A. Bakr *, M. Ishra Semiconductor and Polymer Laboratory, Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 7 September 2001; accepted 28 October 2001
Abstract Thin films of ethylene–vinylalcohol copolymer (EVAl) doped with different concentrations of chlorophyll, from 5 up to 20 wt%, have been obtained. Studies were carried out utilizing DSC and X-ray diffraction to characterize the thermal and structural properties of EVAl–chlorophyll systems. Results reveal that addition of chlorophyll destroys the semicrystalline structure of the host polymer. The optical properties of pure and doped samples have been discussed by the analysis of the UV absorption spectra in the wavelength range 200–1000 nm. The dependence of the relaxation processes on both chlorophyll concentration and temperature has been discussed using the data from dielectric and isothermal depolarization current measurements. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer-chlorophyll; Ethylene-vinylalcohol; Characterization; Relaxation; Isothermal depolarization
1. Introduction Many researchers around the world are trying to build reliable electronic devices where suitable materials can be prepared by properly selecting the type of the host polymeric materials and the dye incorporated into it. There are several problems to be dealt with in the design and evaluation of polymeric materials for different applications. Ethylene–vinylalcohol copolymer (EVAl) is one of the materials which has been most investigated in this field particularly for contact lens applications [1]. Light energy absorbed by the photosynthesis reaction centers of chlorophyll and related pigments stimulate several physical and photochemical processes as well as light energy conversion systems [2,3]. Much of the research has been focused on chlorophyll fluorescence investigations [3–7] as a useful method to control the physical
* Corresponding author. E-mail address: [email protected] (N.A. Bakr).
properties. Photoelectric, photoacoustic and energy deactivation of chlorophyll and related pigments embedded in different matrices such as nitrocellulose, nematic liquid crystal and polyvinylalcohol (PVA) have also been studied [7–13]. These chlorophyll molecules belong to a group of biologically important compounds, C55H72O5N4Mg (chlorophyll a) and C55H70O6N4Mg (chlorophyll b), having alternating single and double bonds in the ring structure around the central molecular Mg. Also, from biological observations, it has been proposed that chlorophyll molecules assume a highly ordered state [2]. In certain biological systems, such as the closely arrayed chlorophyll molecules in a polymer, the energy of an absorbed visible or near UV photon is not confined to a single molecular component but spread out over the system. Dielectric measurements and isothermal depolarization processes represent one of the most intensively researched topics in physics. Such measurements are also being used to monitor the progress of chemical reactions in which dipoles are being created or lost [14–16]. To date, no firm evidence is available concerning the relaxation phenomena of chlorophyll molecules
0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 1 2 6 - X
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immersed in a polymer matrix, especially EVAl copolymer (as biological macromolecular complexes).
and the depolarization current Idep was recorded over 1 h. The depolarization current was measured by using a Keithley Electrometer 610C. Conducting electrodes were prepared by using carbon paste.
2. Experimental section 2.1. Materials Ethylene–vinylalcohol (EVAl) copolymer is a commercial product supplied by Polyscience, Inc., USA, in powder form with nominal molecular weight Mw=72,000. Copolymer of EVAl with high content of vinylalcohol group (65%) has been used due to its excellent surface properties for biological purposes [1]. Chlorophyll was extracted from spinach leaves and dissolved in methyl alcohol. 2.2. Sample preparation Thin films of EVAl have been prepared by dissolving a known weight of copolymer in methyl alcohol as a solvent. Appropriate quantities of chlorophyll solutions have been mixed with EVAl solution to obtain films of EVAl doped with different concentrations of chlorophyll (0, 5, 10 and 20 wt%). The resultant solution was stirred at room temperature, in the dark, for 2 days. Thin films have been obtained by casting the solution onto a flat glass petri dish and drying in an air atmosphere for one week in an oven regulated at 380 K to avoid effects from water and remove the solvent traces. Films having a thickness of about 10 µm were found to be homogeneous and optically transparent as observed by an optical microscope.
3. Results and discussion The DSC scans for EVAl copolymer pure and doped with different chlorophyll contents are shown in Fig. 1. All samples show the presence of a clear endothermic peak at 苲160°C corresponding to the melting point (Tm). The glass transition temperature (Tg) of EVAl is found to be around 28°C which agrees with that reported in the literature [17,18]. As the chlorophyll content increases up to 5% there is a slight decrease in Tg and it then increases to about 35°C for higher chlorophyll contents. In addition, the DSC scans of pure EVAl copolymer shows a semicrystalline nature with the appearance of a weak exothermic peak at 苲85°C, corresponding to the crystallization temperature (Tc). As the copolymer is doped with chlorophyll, the exothermic peak of crystallization disappear. These observations are consistent
2.3. Measuring techniques Differential scanning calorimetry (DSC) thermograms were recorded over the temperature range 290–520 K by means of a Shimadzu DSC-50 apparatus. The measurements were carried out at a heating rate 5 K/min. X-ray diffraction patterns were recorded by a Shimadzu DX-30 apparatus using CuKα radiation. UVabsorption measurements were made by a Unicam UV/VIS spectrometer in the wavelength range 200–1000 nm at room temperature. Dielectric measurements have been carried out using a phase detector technique (lock-in-amplifier) at a series of frequencies from 10 Hz to 100 kHz. The measurements were carried out in the temperature range 300– 393 K under vacuum 苲10⫺3 torr. The isothermal depolarization current as a function of time has been measured as follows: (1) samples were heated by an external heating source to a constant temperature and then an external DC constant electric field of 106 V/m was applied for 1 h to polarize the sample. (2) After that, the sample electrodes were short circuited
Fig. 1. DSC scan for EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt%, (c) 10 wt% and (d) 20 wt%.
N.A. Bakr, M. Ishra / Polymer Testing 21 (2002) 571–576
Fig. 2. X-ray absorption spectra for EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt% and (c) 20 wt%.
with the X-ray diffraction results shown in Fig. 2. It is found that the EVAl (curve a) is semicrystalline in structure with the presence of a sharp reflection at 2θ苲23°, superimposed over diffuse scattering in the diffraction pattern (2q苲12° and 20°). On the other hand, no diffraction peaks have been observed for EVAl–chlorophyll systems. A diffraction pattern that reflects the amorphous state of these samples has been recorded (curves b and c). The structural modification of EVAl copolymer by the incorporation of chlorophyll was confirmed by UV absorption spectra as shown in Fig. 3. The strong band edge at 苲220 nm for pure EVAl and doped with chlorophyll up to 10 wt%, is a characteristic band which can
Fig. 3. UV absorption spectra of EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt%, (c) 10 wt% and (d) 20 wt%.
573
be attributed to the carbonyl C=C group [19,20]. It is observed that the abrupt decrease in the absorbency, which corresponds to the principle band gap of EVAl (Eg苲5.6 eV), does not change (up to 10 wt% chlorophyll) and implies that the electronic band structure of EVAl is not affected. Incorporation of chlorophyll in EVAl appears as an increase in the absorbency values as well as the appearance of new bands located at different wavelengths. The shoulder and peak which are situated at 苲420 and 675 nm, respectively, can be attributed to the blue and red absorption of chlorophyll monomer [2,21]. Increasing chlorophyll concentration leads to an increase in there intensities. A new group of broad bands has been obtained as a result of chlorophyll addition to EVAl copolymer matrix. Bands at 苲265, 285, 330 and 360 nm have appeared for chlorophyll concentrations up to 10 wt%. For high chlorophyll concentration, 20 wt%, new bands at 510, 540 and 615 nm have been detected. A series of peaks, for 20 wt% chlorophyll, have been noted at 335, 355, 365 and 375 nm, respectively. These peaks can be explained by the possibility of the appearance of a number of excited states within the EVAl band gap characterized with an exciton bending energy (Eex) in the range between 60 and 200 meV. The presence of such exciton states has been discussed before by many workers in a photosynthetic antenna and structures consisting of pigment molecules such as chlorophyll bound to protein complexes or stacked nucleic acid polymer [22,23]. They regard these exciton states as local molecular excited states (i.e. instantaneously localized states). These observations reflect the probability of the existence of chlorophyll molecules in the polymeric matrix in two forms. One form is the homogeneous distribution of chlorophyll centers through the host polymeric material. These centers of chlorophyll molecules are supposed to have a specific reaction due to participation of hydrogen bonding [2,24]. Therefore, such a tendency may reflect the interaction of chlorophyll centers and the polar groups of the EVAl host polymer. The second form of chlorophyll found in the polymer matrix is the cluster or aggregation form, which is incorporated by physical interaction into the polymer matrix rather than destroying the structure through hydrogen bonding processes. From this analysis, it can be stated that addition of chlorophyll does not change the optical band gap of the material structure but creates many interband levels appearing in the form of trapping and excitonic states. The frequency dependence of the dielectric loss (e⬙) for EVAl pure and doped with different concentrations of chlorophyll, measured at room temperature, 300 K, and for EVAl doped with 5 wt% chlorophyll measured at different temperatures up to 393 K has been illustrated in Fig. 4. The dielectric loss e⬙, was found to decrease with the frequency increase, where no definite relaxation modes can be recognized. The plot of the real part of the
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conductivity increases rapidly. This critical frequency shifts to higher values with increase in the temperature of measurement. Above this critical frequency, the conductivity is proportional to Fs (where s is the slope of the linear part of the relation in Fig. 5 and takes the value 0.5⬍s⬍1.0) and its values increases with decreasing temperature of measurement. On the other hand, the electric loss modulus M∗=1/e∗ has most frequently been employed in studies to investigate the relaxation phenomena in such ionic systems [20,27]. The frequency dependence of loss modulus (M⬙) for EVAl doped with different chlorophyll concentrations measured at 300 K and EVAl doped with 5 wt% chlorophyll measured at different temperatures has been shown in Fig. 6(a) and (b), respectively. It is found that M⬙–Log (F) in Fig. 6 spectra shows more informative results. Fig. 6(a) exhibits double peaks in two frequency domains (i.e. low and high frequency regions) suggesting a relaxation between unequal states. This figure shows an increase in peak height (M⬙max) at 10 wt% chlorophyll. The present dielectric relaxation analysis confirm the presence of two configurations of chlorophyll within the polymer matrix: one may be due to the homogenous distribution sites through the hydrogen bonding process and the other a clustering form. Furthermore, the shape of the M⬙–Log (F) curve, Fig. 6(b), continuously changes from a broad distribution Fig. 4. Dielectric loss ε⬙–Log (F) dependence for EVAl pure and doped with different concentrations of chlorophyll measured at room temperature (a) and for EVAl–5 wt% chlorophyll measured at different temperatures (b).
conductivity (s⬘) as a function of frequency for EVAl–5 wt% chlorophyll measured at different constant temperatures ranging from 300 to 393 K has been illustrated in Fig. 5. This figure shows the characteristic features of the so called “universal dielectric response” [14,25,26]. It is observed that the conductivity increases with increase of both the frequency and the temperature of measurement. There is a critical frequency at which the
Fig. 5. Real part of the complex conductivity dependence on the frequency for EVAl–5 wt% chlorophyll measured at different temperatures.
Fig. 6. Variation of M⬙ with Log (F) for pure and doped EVAl with different chlorophyll concentrations (a) and for EVAl–5 wt% chlorophyll at different temperatures (b).
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at temperatures up to 360 K to a relatively narrow band at 390 K. Investigation of Fig. 6(b) reveals that different positions of loss modulus maximum (M⬙max) are observed at different frequencies where it moves towards higher frequency as the temperature of measurement increased. These loss modulus maximum values were found to decrease as the temperature of measurement increased from 300 to 363 K with an abrupt increase at 393 K. This behavior indicates the probability of charge build up at the interfacial state between chlorophyll and polymer host material. This interfacial state is to be expected for a mixture of two materials with different conductivities and permittivities [20]. The data transformation from complex dielectric constant e* to the corresponding electric modulus M*, gives the possibility of calculating the relaxation time and hence the activation energy Ea [20]. The corresponding activation energies for the samples under investigation are listed in Table 1. It is observed that, as the chlorophyll concentration increases, the activation energy increases, which may be related to the formation of chlorophyll clusters within the polymer matrix. According to Stundzia et al. [15] the depolarization current can be expressed in the form I(t)⫽⫺(e0S/d)Vpot(es⫺e⬁)
dj dt
(1)
where f(t) is the normalized function determined from the ratio of polarization at a given time to its value at t=0. For disordered systems, the relaxation process can be given by Kohlrausch–William–Watts (KWW) [15,28]:
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Fig. 7. Depolarization current–time dependence for pure and doped EVAl with different chlorophyll concentrations (a) and for EVAl–5 wt% chlorophyll at different temperatures (b).
(4)
where b is the slope of Log (Idep) vs. Log (t) relation and b=1⫺b. The results of the isothermal depolarization current (Idep) as a function of time at room temperature for EVAl pure and doped with different concentrations of chlorophyll and at different temperatures for EVAl– 5 wt% chlorophyll, according to the approximated Eq. (4), are presented in Fig. 7. The generalization constant b as a function of chlorophyll concentration and temperature has been illustrated in Tables 1 and 2. It is easy to visualize from Fig. 7(a) and (b) that the depolarization current–time dependence has two different slopes b1 and b2, distinguished for high and low times, respectively. This behavior may be attributed to the co-operation of two different relaxation processes. This means that the relaxation phenomena in our system can be regarded as a consequence of cooperative effect of chlorophyll–poly-
Table 1 The variation of the activation energy ⌬E (eV) and the constants b1 and b2 as a function of chlorophyll concentrations
Table 2 The dependence of constants b1 and b2 on temperature for EVAl–5 wt% chlorophyll
Chlorophyll conc. (%)
Temperature (K)
f(t)⫽exp[⫺(t/t)b] 0⬍b⬍1
(2)
where b is the coefficient describing the shape of the depolarization curve (i.e. stretched exponential) and t is the relaxation time. Hence, the depolarization current Idep according to KWW is rewritten in the form:
冉 冊
S IKWW(t)⫽Idep(t)⫽ e0 Vpot(e0⫺e⬁)bt−btb−1 d
(3)
For tt Idep(t)⫽a(t−b)
0 5 10 20
⌬E (eV) 0.48 0.57 0.65 0.71
b1 0.750 0.786 0.750 0.761
b2 0.48 0.49 0.50 0.55
300 323 343 363 393
b1 0.786 0.835 0.837 0.852
b2 0.49 0.646 0.692
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mer dipole orientation and the chlorophyll clustered molecules with two different relaxation time processes.
4. Conclusion The structural characterization of EVAl copolymer incorporating chlorophyll molecules was confirmed by analysis using DSC, X-ray, UV absorption spectra, dielectric and isothermal depolarization measurements. Results reveal that the incorporation of chlorophyll molecules into EVAl copolymer is achieved as the co-operative chlorophyll–polymer interaction through the hydrogen bonding associated with the cluster formation within the copolymer. This configuration was found to have a significant effect on the relaxation phenomena in the resultant composites without changing its principle electronic band-edge.
Acknowledgements The authors would like to acknowledge Professor Dr. E.M. Reicha for facilitating the dielectric measurements in his laboratory.
References [1] S. Ou, H. Ishida, J.B. Lando, J. Polym. Sci.: Part B: Polym. Phys. 29 (1991) 67. [2] T. Miyasaka, T. Watanabe, A. Fujishima, K. Honda, J. Am. Chem. Soc. 100 (1978) 21. [3] A. Barocsi, L. Kocsany, S. Varkonyi, P. Richter, Z. Csintalan, K. Szente, Meas. Sci. Technol. 11 (2000) 717. [4] G. Agati, Pure Appl. Opt. 7 (1998) 797. [5] J.G. Kornet, H. Griffioen, K. Schurer, Meas. Sci. Technol. 3 (1992) 196.
[6] A. Kowalczyk, A. Walosxek, D. Frackowiak, Photosynthetica 35 (3) (1998) 369. [7] R. Sanders, M.A.M.J. van Zandvoort, A. Draaijer, Y.K. Levine, H.C. Gerritsen, Photochem. Photobiol. 64 (5) (1996) 817. [8] A. Ptak, A. Der, R. Toth, N.S. Naser, D. Frackowiak, Photochem. Photobiol. A 104 (1997) 133. [9] D. Wrobel, Photosynthetica 33 (3-4) (1997) 413. [10] A. Ptak, E. Chrzumnicka, A. Planner, D. Frackowiak, Biophys. Chem. 65 (1997) 165. [11] D. Wrobel, I. Hany, R. Bartkowiak, R.M. Ion, J. Fluorescence 8 (3) (1998) 191. [12] D. Wrobel, I. Hanyz, A. Planner, A. Dudkowiak, T. Sarna, Photochem. Photobiol. B 47 (1998) 165. [13] K. Klaczynska, A. Dudkowiak, D. Frackowiak, A. Planner, M. Hara,J. Miyake, 2nd International Photochemistry and Photobiology Conf., no. 16, 1999. [14] A.K. Jonscher, J. Phys. D: Appl. Phys. 32 (1999) R57. [15] V. Stundzia et al., J. Phys. D: Appl. Phys. 33 (2000) 719. [16] S. Fedosov, J.A. Giacometti, G.F. Leal Ferreira, M.M. Costa, J. Appl. Phys. 82 (9) (1999) 4355. [17] J.A. Wachtel, B.C. Tsai, C.J. Farell, Plastic Eng. 41 (1985) 41. [18] T. Ikegami, K. Nagashima, M. Shimoda, Y. Tanaka, Y. Osajima, Food Sci. 56 (1991) 500. [19] M.A. Moharram, S.M. Rabie, W.S. El-Hamouly, J. Appl. Polym. Sci. 42 (1991) 3025. [20] M.D. Migahed, N.A. Bakr, M.I. Abdel-Hamid, O. ElHanafy, M. El-Nimr, J. Appl. Polym. Sci. 59 (1996) 655. [21] I. Bedja, P.V. Kamat, S. Hotchandani, J. Appl. Phys. 80 (8) (1996) 4637. [22] R.M. Pearlstein, Photochem. Photobiol. 35 (1982) 835. [23] R.M. Pearlstein, in: H. Scheer (Ed.), Theoritical Interpretation of Antenna Spectra in Chlorophylls CRC Press, Boca Raton, 1991, p. 1047. [24] A.F. Janzen, J.R. Bolton, J. Am. Chem. Soc. 101 (1978) 6342. [25] A.K. Jonscher, Nature Lond. 267 (1977) 673. [26] F. Kremer, L. Dominguez, W.H. Meyer, G. Wegner, Polymer 30 (1989) 2023. [27] H.W. Starkweather, P. Avakian, J. Polym. Sci.: Part B: Polym. Phys. 30 (1992) 637. [28] K. Liedermann, A. Loidl, J. Non-Cryst. Solid. 155 (1993) 26.
Material Characterisation
Characterization of ethylene–vinylalcohol copolymer doped with chlorophyll N.A. Bakr *, M. Ishra Semiconductor and Polymer Laboratory, Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 7 September 2001; accepted 28 October 2001
Abstract Thin films of ethylene–vinylalcohol copolymer (EVAl) doped with different concentrations of chlorophyll, from 5 up to 20 wt%, have been obtained. Studies were carried out utilizing DSC and X-ray diffraction to characterize the thermal and structural properties of EVAl–chlorophyll systems. Results reveal that addition of chlorophyll destroys the semicrystalline structure of the host polymer. The optical properties of pure and doped samples have been discussed by the analysis of the UV absorption spectra in the wavelength range 200–1000 nm. The dependence of the relaxation processes on both chlorophyll concentration and temperature has been discussed using the data from dielectric and isothermal depolarization current measurements. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer-chlorophyll; Ethylene-vinylalcohol; Characterization; Relaxation; Isothermal depolarization
1. Introduction Many researchers around the world are trying to build reliable electronic devices where suitable materials can be prepared by properly selecting the type of the host polymeric materials and the dye incorporated into it. There are several problems to be dealt with in the design and evaluation of polymeric materials for different applications. Ethylene–vinylalcohol copolymer (EVAl) is one of the materials which has been most investigated in this field particularly for contact lens applications [1]. Light energy absorbed by the photosynthesis reaction centers of chlorophyll and related pigments stimulate several physical and photochemical processes as well as light energy conversion systems [2,3]. Much of the research has been focused on chlorophyll fluorescence investigations [3–7] as a useful method to control the physical
* Corresponding author. E-mail address: [email protected] (N.A. Bakr).
properties. Photoelectric, photoacoustic and energy deactivation of chlorophyll and related pigments embedded in different matrices such as nitrocellulose, nematic liquid crystal and polyvinylalcohol (PVA) have also been studied [7–13]. These chlorophyll molecules belong to a group of biologically important compounds, C55H72O5N4Mg (chlorophyll a) and C55H70O6N4Mg (chlorophyll b), having alternating single and double bonds in the ring structure around the central molecular Mg. Also, from biological observations, it has been proposed that chlorophyll molecules assume a highly ordered state [2]. In certain biological systems, such as the closely arrayed chlorophyll molecules in a polymer, the energy of an absorbed visible or near UV photon is not confined to a single molecular component but spread out over the system. Dielectric measurements and isothermal depolarization processes represent one of the most intensively researched topics in physics. Such measurements are also being used to monitor the progress of chemical reactions in which dipoles are being created or lost [14–16]. To date, no firm evidence is available concerning the relaxation phenomena of chlorophyll molecules
0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 1 2 6 - X
572
N.A. Bakr, M. Ishra / Polymer Testing 21 (2002) 571–576
immersed in a polymer matrix, especially EVAl copolymer (as biological macromolecular complexes).
and the depolarization current Idep was recorded over 1 h. The depolarization current was measured by using a Keithley Electrometer 610C. Conducting electrodes were prepared by using carbon paste.
2. Experimental section 2.1. Materials Ethylene–vinylalcohol (EVAl) copolymer is a commercial product supplied by Polyscience, Inc., USA, in powder form with nominal molecular weight Mw=72,000. Copolymer of EVAl with high content of vinylalcohol group (65%) has been used due to its excellent surface properties for biological purposes [1]. Chlorophyll was extracted from spinach leaves and dissolved in methyl alcohol. 2.2. Sample preparation Thin films of EVAl have been prepared by dissolving a known weight of copolymer in methyl alcohol as a solvent. Appropriate quantities of chlorophyll solutions have been mixed with EVAl solution to obtain films of EVAl doped with different concentrations of chlorophyll (0, 5, 10 and 20 wt%). The resultant solution was stirred at room temperature, in the dark, for 2 days. Thin films have been obtained by casting the solution onto a flat glass petri dish and drying in an air atmosphere for one week in an oven regulated at 380 K to avoid effects from water and remove the solvent traces. Films having a thickness of about 10 µm were found to be homogeneous and optically transparent as observed by an optical microscope.
3. Results and discussion The DSC scans for EVAl copolymer pure and doped with different chlorophyll contents are shown in Fig. 1. All samples show the presence of a clear endothermic peak at 苲160°C corresponding to the melting point (Tm). The glass transition temperature (Tg) of EVAl is found to be around 28°C which agrees with that reported in the literature [17,18]. As the chlorophyll content increases up to 5% there is a slight decrease in Tg and it then increases to about 35°C for higher chlorophyll contents. In addition, the DSC scans of pure EVAl copolymer shows a semicrystalline nature with the appearance of a weak exothermic peak at 苲85°C, corresponding to the crystallization temperature (Tc). As the copolymer is doped with chlorophyll, the exothermic peak of crystallization disappear. These observations are consistent
2.3. Measuring techniques Differential scanning calorimetry (DSC) thermograms were recorded over the temperature range 290–520 K by means of a Shimadzu DSC-50 apparatus. The measurements were carried out at a heating rate 5 K/min. X-ray diffraction patterns were recorded by a Shimadzu DX-30 apparatus using CuKα radiation. UVabsorption measurements were made by a Unicam UV/VIS spectrometer in the wavelength range 200–1000 nm at room temperature. Dielectric measurements have been carried out using a phase detector technique (lock-in-amplifier) at a series of frequencies from 10 Hz to 100 kHz. The measurements were carried out in the temperature range 300– 393 K under vacuum 苲10⫺3 torr. The isothermal depolarization current as a function of time has been measured as follows: (1) samples were heated by an external heating source to a constant temperature and then an external DC constant electric field of 106 V/m was applied for 1 h to polarize the sample. (2) After that, the sample electrodes were short circuited
Fig. 1. DSC scan for EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt%, (c) 10 wt% and (d) 20 wt%.
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Fig. 2. X-ray absorption spectra for EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt% and (c) 20 wt%.
with the X-ray diffraction results shown in Fig. 2. It is found that the EVAl (curve a) is semicrystalline in structure with the presence of a sharp reflection at 2θ苲23°, superimposed over diffuse scattering in the diffraction pattern (2q苲12° and 20°). On the other hand, no diffraction peaks have been observed for EVAl–chlorophyll systems. A diffraction pattern that reflects the amorphous state of these samples has been recorded (curves b and c). The structural modification of EVAl copolymer by the incorporation of chlorophyll was confirmed by UV absorption spectra as shown in Fig. 3. The strong band edge at 苲220 nm for pure EVAl and doped with chlorophyll up to 10 wt%, is a characteristic band which can
Fig. 3. UV absorption spectra of EVAl copolymer pure (a) and doped with different chlorophyll concentrations (b) 5 wt%, (c) 10 wt% and (d) 20 wt%.
573
be attributed to the carbonyl C=C group [19,20]. It is observed that the abrupt decrease in the absorbency, which corresponds to the principle band gap of EVAl (Eg苲5.6 eV), does not change (up to 10 wt% chlorophyll) and implies that the electronic band structure of EVAl is not affected. Incorporation of chlorophyll in EVAl appears as an increase in the absorbency values as well as the appearance of new bands located at different wavelengths. The shoulder and peak which are situated at 苲420 and 675 nm, respectively, can be attributed to the blue and red absorption of chlorophyll monomer [2,21]. Increasing chlorophyll concentration leads to an increase in there intensities. A new group of broad bands has been obtained as a result of chlorophyll addition to EVAl copolymer matrix. Bands at 苲265, 285, 330 and 360 nm have appeared for chlorophyll concentrations up to 10 wt%. For high chlorophyll concentration, 20 wt%, new bands at 510, 540 and 615 nm have been detected. A series of peaks, for 20 wt% chlorophyll, have been noted at 335, 355, 365 and 375 nm, respectively. These peaks can be explained by the possibility of the appearance of a number of excited states within the EVAl band gap characterized with an exciton bending energy (Eex) in the range between 60 and 200 meV. The presence of such exciton states has been discussed before by many workers in a photosynthetic antenna and structures consisting of pigment molecules such as chlorophyll bound to protein complexes or stacked nucleic acid polymer [22,23]. They regard these exciton states as local molecular excited states (i.e. instantaneously localized states). These observations reflect the probability of the existence of chlorophyll molecules in the polymeric matrix in two forms. One form is the homogeneous distribution of chlorophyll centers through the host polymeric material. These centers of chlorophyll molecules are supposed to have a specific reaction due to participation of hydrogen bonding [2,24]. Therefore, such a tendency may reflect the interaction of chlorophyll centers and the polar groups of the EVAl host polymer. The second form of chlorophyll found in the polymer matrix is the cluster or aggregation form, which is incorporated by physical interaction into the polymer matrix rather than destroying the structure through hydrogen bonding processes. From this analysis, it can be stated that addition of chlorophyll does not change the optical band gap of the material structure but creates many interband levels appearing in the form of trapping and excitonic states. The frequency dependence of the dielectric loss (e⬙) for EVAl pure and doped with different concentrations of chlorophyll, measured at room temperature, 300 K, and for EVAl doped with 5 wt% chlorophyll measured at different temperatures up to 393 K has been illustrated in Fig. 4. The dielectric loss e⬙, was found to decrease with the frequency increase, where no definite relaxation modes can be recognized. The plot of the real part of the
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conductivity increases rapidly. This critical frequency shifts to higher values with increase in the temperature of measurement. Above this critical frequency, the conductivity is proportional to Fs (where s is the slope of the linear part of the relation in Fig. 5 and takes the value 0.5⬍s⬍1.0) and its values increases with decreasing temperature of measurement. On the other hand, the electric loss modulus M∗=1/e∗ has most frequently been employed in studies to investigate the relaxation phenomena in such ionic systems [20,27]. The frequency dependence of loss modulus (M⬙) for EVAl doped with different chlorophyll concentrations measured at 300 K and EVAl doped with 5 wt% chlorophyll measured at different temperatures has been shown in Fig. 6(a) and (b), respectively. It is found that M⬙–Log (F) in Fig. 6 spectra shows more informative results. Fig. 6(a) exhibits double peaks in two frequency domains (i.e. low and high frequency regions) suggesting a relaxation between unequal states. This figure shows an increase in peak height (M⬙max) at 10 wt% chlorophyll. The present dielectric relaxation analysis confirm the presence of two configurations of chlorophyll within the polymer matrix: one may be due to the homogenous distribution sites through the hydrogen bonding process and the other a clustering form. Furthermore, the shape of the M⬙–Log (F) curve, Fig. 6(b), continuously changes from a broad distribution Fig. 4. Dielectric loss ε⬙–Log (F) dependence for EVAl pure and doped with different concentrations of chlorophyll measured at room temperature (a) and for EVAl–5 wt% chlorophyll measured at different temperatures (b).
conductivity (s⬘) as a function of frequency for EVAl–5 wt% chlorophyll measured at different constant temperatures ranging from 300 to 393 K has been illustrated in Fig. 5. This figure shows the characteristic features of the so called “universal dielectric response” [14,25,26]. It is observed that the conductivity increases with increase of both the frequency and the temperature of measurement. There is a critical frequency at which the
Fig. 5. Real part of the complex conductivity dependence on the frequency for EVAl–5 wt% chlorophyll measured at different temperatures.
Fig. 6. Variation of M⬙ with Log (F) for pure and doped EVAl with different chlorophyll concentrations (a) and for EVAl–5 wt% chlorophyll at different temperatures (b).
N.A. Bakr, M. Ishra / Polymer Testing 21 (2002) 571–576
at temperatures up to 360 K to a relatively narrow band at 390 K. Investigation of Fig. 6(b) reveals that different positions of loss modulus maximum (M⬙max) are observed at different frequencies where it moves towards higher frequency as the temperature of measurement increased. These loss modulus maximum values were found to decrease as the temperature of measurement increased from 300 to 363 K with an abrupt increase at 393 K. This behavior indicates the probability of charge build up at the interfacial state between chlorophyll and polymer host material. This interfacial state is to be expected for a mixture of two materials with different conductivities and permittivities [20]. The data transformation from complex dielectric constant e* to the corresponding electric modulus M*, gives the possibility of calculating the relaxation time and hence the activation energy Ea [20]. The corresponding activation energies for the samples under investigation are listed in Table 1. It is observed that, as the chlorophyll concentration increases, the activation energy increases, which may be related to the formation of chlorophyll clusters within the polymer matrix. According to Stundzia et al. [15] the depolarization current can be expressed in the form I(t)⫽⫺(e0S/d)Vpot(es⫺e⬁)
dj dt
(1)
where f(t) is the normalized function determined from the ratio of polarization at a given time to its value at t=0. For disordered systems, the relaxation process can be given by Kohlrausch–William–Watts (KWW) [15,28]:
575
Fig. 7. Depolarization current–time dependence for pure and doped EVAl with different chlorophyll concentrations (a) and for EVAl–5 wt% chlorophyll at different temperatures (b).
(4)
where b is the slope of Log (Idep) vs. Log (t) relation and b=1⫺b. The results of the isothermal depolarization current (Idep) as a function of time at room temperature for EVAl pure and doped with different concentrations of chlorophyll and at different temperatures for EVAl– 5 wt% chlorophyll, according to the approximated Eq. (4), are presented in Fig. 7. The generalization constant b as a function of chlorophyll concentration and temperature has been illustrated in Tables 1 and 2. It is easy to visualize from Fig. 7(a) and (b) that the depolarization current–time dependence has two different slopes b1 and b2, distinguished for high and low times, respectively. This behavior may be attributed to the co-operation of two different relaxation processes. This means that the relaxation phenomena in our system can be regarded as a consequence of cooperative effect of chlorophyll–poly-
Table 1 The variation of the activation energy ⌬E (eV) and the constants b1 and b2 as a function of chlorophyll concentrations
Table 2 The dependence of constants b1 and b2 on temperature for EVAl–5 wt% chlorophyll
Chlorophyll conc. (%)
Temperature (K)
f(t)⫽exp[⫺(t/t)b] 0⬍b⬍1
(2)
where b is the coefficient describing the shape of the depolarization curve (i.e. stretched exponential) and t is the relaxation time. Hence, the depolarization current Idep according to KWW is rewritten in the form:
冉 冊
S IKWW(t)⫽Idep(t)⫽ e0 Vpot(e0⫺e⬁)bt−btb−1 d
(3)
For tt Idep(t)⫽a(t−b)
0 5 10 20
⌬E (eV) 0.48 0.57 0.65 0.71
b1 0.750 0.786 0.750 0.761
b2 0.48 0.49 0.50 0.55
300 323 343 363 393
b1 0.786 0.835 0.837 0.852
b2 0.49 0.646 0.692
576
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mer dipole orientation and the chlorophyll clustered molecules with two different relaxation time processes.
4. Conclusion The structural characterization of EVAl copolymer incorporating chlorophyll molecules was confirmed by analysis using DSC, X-ray, UV absorption spectra, dielectric and isothermal depolarization measurements. Results reveal that the incorporation of chlorophyll molecules into EVAl copolymer is achieved as the co-operative chlorophyll–polymer interaction through the hydrogen bonding associated with the cluster formation within the copolymer. This configuration was found to have a significant effect on the relaxation phenomena in the resultant composites without changing its principle electronic band-edge.
Acknowledgements The authors would like to acknowledge Professor Dr. E.M. Reicha for facilitating the dielectric measurements in his laboratory.
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