Spectrophotometric Analysis for the UV-Irradiated (PMMA)

Transcription

1 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 8 Spectrophotometric Analysis for the UV-Irradiated (PMMA) J. H. Nahida Deptt. of Applied Science, University of Technology, Baghdad Iraq The aim of the present work to investigate the effect of UV-irradiation on the optical properties of PMMA within period of time (4,7,and1hr.). The samples are caste as thin films. These samples were irradiated UV-source for a period of time by (4,7,and1hr.). These samples were evaluated spectrophotometrically before and after UV-irradiation. The absorption spectra of irradiated samples showed radiation induced absorption changes. There was an increment in absorption proportional with irradiation time up to (7hr.). This increment was attributed to interfaces traps formation by photodegradation,which are caused by UV-irradiation. At (1hr.) UVirradiation, there was decreasing in absorption, which was attributed to the degassing process induced by photo degradation. It was found there is decrease in energy gap for irradiated samples for an irradiation time up to (7hr.),then there is increase in energy gap for irradiated samples to (1hr.),which was attributed to the absorption changes induced by photo degradation. The optical constant (α,k,n, εr,and εi)for the samples before and after irradiation were investigated. Key words: UV-Irradiation,PMMA optical properties, Optical constants Introduction: Polymethylmethacrylate is an amorphous thermoplastic that can be delivered PMMA is produced by the addition polymerization of methylmethacrylate [1,].The polymer has very good optical properties but has poor scratch resistance. It has good dimensional stability due to rigid polymer chains. It has good weather resistance, and is stable to acid and alkalis. It is attacked by several organic solvents, and has good impact strength higher, than that of glass or polystyrene [3,4]. It has the best transparency and optical properties of commercially available thermoplastic. It is colorless with a 9% light transmission, an index of refraction of It comes in a full range of transparencies. Some of its applications are lenses, outdoor lighting, and indoor lighting [-8]. When radiation passes through a sample of any substance, the intensity is reduced by the same factor for each equal increase in distance traveled. The intensity thus decays exponentially and, if the decay is due only to absorption, rather than to scattering and reflection, then [7,9]. I=I e -αt (1) Where t: sample thickness I o : intensity of light incident on sample I: intensity emerging from sample. α: absorption coefficient. Equation (1) is the mathematical expression of Lambert s law. The quantity I/ I o is the transmittance, T, of thickness, t, and a quantity, A, given by [1] A=log 1 [I o / I]. () Absorptance is defined as the ratio between absorbed light intensity (I A ) by material and the incident intensity of light ( I o ).

2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 9 A = I A / I o.. (3) Transmittance (T) is given by reference to the intensity of the rays transmitting from the film(i) to the intensity of the rays incident on it (I o ) (T=I/ I o ), and can be calculated by [11]: T = exp [-.33A]. (4) Reflectance can be obtained from absorption and transmission spectra in accordance with the law of conservation of energy by the relation [11] R + T + A = 1 () The optical constants are very important because they describe the optical behavior of the materials. The absorption coefficient of the material is very strong function of photon energy and band gap energy [1]. Absorption coefficient (α) is defined as the ability of a material to absorb the light of a given wavelength α=.33a/t. (6) Where A: is the absorption of the material t: the sample thickness in cm. The Refractive index (n), the index of refraction of a material is the ratio of the velocity of the light in vacuum to that of the specimen [13-1]. R= ((n-1) +k )/ ( (n+1) +k ).... (7) When k R= (n-1) / (n+1). (8) n= (1+R 1/ )/ (1-R 1/ ) (9) The extinction coefficient (k) was calculated using the following equation [16]: K=αλ/4π. (1) Dielectric constant is defined as the response of the material toward the incident electromagnetic field. The dielectric constant of compound (ε) is divided into two parts real(ε r ), and imaginary (ε i ).The real and imaginary parts of dielectric constant (ε r and ε i ) can be calculated by using equations [1, 17, 18] ε=ε r + iε i (11) ε r =n -k (real part). (1) ε i =nk (imaginary part).. (13) Experimental part: The purified polymer was used to cast films of different thicknesses from solution of different concentrations (6-1%wt/vol).The polymers were dissolved in methylene chloride and hand shaken until a homogenous solution was obtained, after which solution was transferred to clean glass Petri dish of(7cm) in diameter placed on plate form. The dried film was then removed easily by using tweezers clamp. The best concentration for film production for casting without bubbling and can be dismounted easily from the petri dish was found to be 7%wt/vol. Digital vernier was used to measure the samples thickness to be, for (.7mm). The samples were irradiated by UV-Radiation using (XENTEST 1 of xenon lamp of(3 nm) for time periods (4,7, and 1 hr.).the samples were evaluated spectrophotomerically before and after irradiation.

3 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 6 The spectra were used to carry out some optical measurements. Energy gap was obtained by plotting (αhυ) 1/r versus hυ with r values equal to 1/,3/,,and 3. The linear portion was best fitted with r=1/, which indicates a transition of direct type. Energy gap of the prepared samples before and after UV- irradiation for time of periods (4, 7, and 1 hr.) were plotted as a function of the time of irradiation. The optical constants were calculated (absorption coefficient, refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant), by using equations (6), (9), (1), (1), and (13), for all prepared samples. Results and Discussion : Ultraviolet and visible spectra for (PMMA) before and after UV- irradiation for time of periods (4, 7, and 1 hr.) are shown in Fig.(1). In the visible region (PMMA), was transparent. These results are in good agreement with Osawa, and Fukuda[19], and Ayash, S.A []. The results showed new peaks for the irradiated samples for 7, and 1hr,which was attributed to photodegradation. Photodegradation induced absorption changes by increasing the interfaces states. There was an increase in absorption spectra with time of irradiation up to 7hr., then decrease due to random photodegration by UV-irradiation[19-]. The reflection spectra for (PMMA) before and after irradiation within periods of time (4, 7, and 1hr.), their behavior is shown in Fig. ().It is seen that it was of the same behavior of the absorption spectra. The best reflection was for the irradiated samples to 7hr. Fig (3) shows the spectral transmittance over the range of (4-64 nm) for (PMMA), and after irradiation within periods of time (4,7,and 1hr.). It was obvious that its behavior was opposite to that of the absorption and reflection spectra. The energy gap (E g ) was obtained by plotting (αhν) 1/r versus (hν) with (r) values equal to 1/,3/,,and 3.The linear portion was best fitted with (r=1/),which indicates a transition of direct type as in Figs.(4) to homopolymer (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) fig.(4). Fig.() shows decreasing energy gaps with time irradiation up to 7hr.,then it increased, which was attributed to random photodegradation []. The variation of (α) versus (λ) given in Fig. (6) shows a sharp edge for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) In fig.(6),it is seen that, the absorption edge shifts towards the longer wavelength side and becomes broader with increasing of time of irradiation up to 7hr.,then decreasing[].also, it can be seen, decrease in α values in these polymer systems, is attributed to decreasing energy gap, which will be discussed. Its behavior was similar to the absorption spectra fig.(7). Fig (8) showed the variation of refractive index as a function of wavelength for the (PMMA), before and after irradiation within periods of time (4,7,and 1hr.).It was found that the refractive index was decreasing with increasing irradiation time, that was attributed to its reflection of the highest value Fig.(3) The variation of the refractive index (n) with wavelength (λ) follows the dispersion formula [1]. n λ=a+b/λ +..+C/λ n +.(14) Where, A, B, and C are constants that account for relationship, where the refractive index decreases with increasing wavelength.finally, from Fig (9), it is seen there is nonlinear relationship between the refractive index and irradiation time []. Fig (1) shows the variation of extinction coefficient for (PMMA), before and after irradiation as a function of wavelength. It was seen that the extinction coefficient (k)

4 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 61 was increasing with increasing of irradiation time up to 7hr., so the highest extinction coefficient (k) was of irradiated samples to (7hr.) and shifted towered the long wavelength. The displacement in (k) was due to the energy gap decreasing in which is the lowest (4.76eV).Then behavior of (k) was nearly similar to corresponding absorption coefficient as shown in Fig. (6). From Fig (11), it was seen that there was nonlinear relationship between the extinction coefficient and the irradiation time, which was attributed to the previous reason [1]. The plots of the real part (ε r ) and imaginary part (ε i ) of (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) are illustrated in Figs. (1) to (13),and Figs. (14) to (1), respectively. These figures show that in all samples the real part behaves like the refractive index, while the imaginary part behaves like the extinction coefficient. The behavior of ε r is similar to refractive index because the smaller value of k comparison of n, while ε i imaginary depends on the k value, The parameters of optical properties of polymer system involved are represented in table(1).it was found decreasing in optical constants by UV-Irradiation,which was attributed to the photodegradation induced damage in the irradiated samples caused that[1]. A% PMMA/1hr./A% PMMA/7hr./A% PMMA/4hr./A% PMMA/A% Fig. (1): UV/Visible absorption spectroscopy of PMMA before and after irradiation within periods of time(4,7,and 1hr.) 1 R% Wavelength(nm PMMA/1hr./R% PMMA/7hr/R% PMMA/4hrR% PMMA/R% Fig. (): Optical reflection spectra of (PMMA) before and after irradiation within periods of time(4,7,and 1hr.)

5 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: T% PMMA/1hr.r/T% PMMA/7hr/T% PMMA/4hr/T% PMMA/T% Fig.(3): Optical transmittance spectra for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) 4 (ahv)(ev/cm)^ hv(ev) PMMA/1hr./(ahv) PMMA/4hr/(ahv) PMMA/7hr/(ahv) PMMA/(ahv) Fig.(4) :( αhυ) versus (hυ) for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) Eg(eV) Eg/uv-irradiation Poly. (Eg/uv-irradiation) Irradiation time(hr.) Fig. (): Energy gap values for for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.)

6 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 63 a(cm^-1) a/pmma/1hr. a/pmma/7hr a/pmma/4hr a/pmma Fig. (6): Absorption coefficient versus wavelength for (PMMA) ), before and after irradiation within periods of time (4,7,and 1hr.). 3 1 a(uv-irrad.)(cm^-1) Irradi.time(hr.) a Poly. (a) Fig. (7 ): The absorption coefficient versus irradiation time. n PMMA/1hr./n PMMA/7hr.7/n PMMA/4hr/n PMMA/n Fig. (8): Variation of refractive index as a function of wavelength for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.).

7 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: n(uv-irrad) Irradiation time(hr.) n Poly. (n) Fig. (9): Variation of the refractive index versus the irradiation time..1.9 k PMMA/1hr./k PMMA/7hr/k PMMA/4hr/k PMMA/k Fig. (1): Variation of extinction coefficient as a function of wavelength for (PMMA) before and after irradiation within periods of time (4,7,and 1hr.). kx1^-(uv-irrad.) kx1^- Poly. (kx1^-) Irradiation time(hr.) Fig. (11): Variation of extinction coefficient versus irradiation time

8 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: εr PMMA/1hr./εr PMMA/7hr/εr PMMA/4hr/εr PMMA/εr Fig.(1): Variation of the real part of the dielectric constant as a function to the wavelength for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.) εi PMMA/1hr/εi PMMA/7hr/εi PMMA/4hr/εi PMMA/εi Fig.(13): Variation of the imaginary part of the dielectric constant as a function to the wavelength for (PMMA), before and after irradiation within periods of time (4,7,and 1hr.). 6. εr(uv-irrad.) 4. 4 εr Poly. (εr) Irradiation time(hr.).(14): The real part of the dielectric constant versus the irradiation time

9 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 1 No: 66 εi(uv-irrad)x1^ εix1^- Poly. (εix1^-) Fig.(1): The imaginary part of the dielectric constant versus the irradiation time. Table (1): Represents the parameters of optical properties of polymer system involved. Polymer. system Time of irradiation (hr.) E g (ev) λ cut (nm) n K 1 - ε r ε i 1 - α (cm -1 ) PMMA PMMA PMMA PMMA Conclusions: There was an increment in absorption proportional with irradiation time up to (7hr.). This increment was attributed to interfaces trap formation by UVirradiation photo degradation. At (1hr.) UV-irradiation, there was decrease in absorption, which was attributed to the degassing process induced by photo degradation. It was found there is decrease in energy gap for irradiated samples for an irradiation time up to (7hr.),then there was increase in energy gap for irradiated samples to (1hr.),which was attributed degassing of impurities induced by photo degradation. It was seen there is decrease in optical constants (α,k,n, εr,and εi)for the samples after irradiation,which was attributed to decrease with energy gap,and the optical constant are inversely proportional with the wavelength. (It was calculated at λ cut ). Acknowledgment I am fully thankful for( Mr.Mohamad T./Al-Nahrain University) for his aids.

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