Study of the effect of -ray irradiation on the optical properties of polyethylene terephthalate polymer
K. Chikaoui
Laboratory for Nuclear Science, Physics Faculty University of Sciences and Technology – USTHB
Algiers, Algeria
chikaoui.khadidja@gmail.comorkchikaoui@usthb.dz
R. Yefsah, M. Siad.
Centre de Recherche Nucléaires d’Alger.
COMENA, 02 Blvd Frantz Fanon, Alger, Algeria
Abstract— The effect of -ray irradiation on optical properties of polyethylene terephthalate (PET) has been studied using UV-Visible spectrophotometry technique. - ray irradiations were performed, using60Co source with mean energy of 1.25 MeV, at Nuclear Research Centre of Algiers, Algeria. The dose rate was about 1.3 kGy/h as measured using the Frick dosimeter. Several PET samples with a thickness of 6 µm were irradiated at room temperature in air atmosphere at dose range 0.05 - 5 MGy.
After irradiation, the colour of the PET thin film, which is initially transparent, becomes slightly yellowish at high - ray dose. The UV–Visible absorption spectra of pristine and irradiated PET polymer have been obtained by a double beam Cintra 40 spectrophotometer in the range 200 – 900 nm. The absorption measurements show a large absorption band extended from 310 to 355 nm assigned to the formation of extended systems of conjugate bonds with the creation of carbon clusters.
The optical absorbance in this range increases linearly with the- ray dose. An empirical relation is established to estimate gamma dose from this correlation. Both direct and indirect energy gaps deduced from the Tauc relation are found to decrease with increasing- ray dose indicating the appearance of new electronic transitions. Moreover, it is also observed that the Urbach energy increases with the- ray dose. This indicates the disorganization of the PET structure after irradiation. Moreover, the linear behavior of the absorption band versus -ray dose observed in the present study suggests the potentiality of using PET polymer as gamma dosimeter.
Keywords—polyethylene terephthalate;-ray; UV-Visible;
optical properties.
I. INTRODUCTION
The aging study of cable insulator and polymers used in nuclear engineering was attracted many researchers in the world. These materials could be submitted to harsh radiation.
Irradiation, humidity, thermal and moisture induce degradation in polymeric materials which leads to the mechanical and electrical failure [1-3]. Particularly, the ionising radiation induces active chemical species (anions, cations, free radicals, electrons) by ionisation and excitation mechanism. The interaction between such kinds of species
gives rise to two main processes: cross linking and chain scission. On the other hand, the oxidation process [4,5]
contribute to formation of smaller molecules. The latter processes induce changes in the optical, electrical and structural properties of the irradiated polymers. This affects their ability to operate safely for a long time.
Polyethylene terephthalate (PET) is one of the most important polymers used for this purpose due to its radiation resistance and chemical and physical properties. It is used as an insulator in the instrumentation and control cables [6] and as nuclear track detector for particles detection [7]. Thus, it is interesting to study the effect of radiation on PET used in the radioactive environments.
Although, there are numerous published papers on radiation damage generated by different particles [8-11] in polyethylene terephthalate, the defects mechanism induced by radiations is not well understood. In our previous work, the damage induced by alpha particle in Polyethylene terephthalate was investigated by FTIR technique [12]. It is found that while the crystalline phase decreases, the amorphous phase increases. This suggests the crystalline to amorphous phase conversion process during alpha particle irradiation. In the present investigation, - ray irradiation effects in the optical properties were investigated in order to understand the defect formation mechanism.
II. EXPERIMENTAL A. Irradiation
Samples about 1 x 1cm2 were cut from commercially available polyethylene terephthalate sheets (Somar International, Inc.) whose thickness was about 6 µm. The samples were irradiated with γ rays. The irradiation was made at CRNA, Algiers, using the 60Co source with the average gamma energy of 1.25 MeV and with a dose rate of 1.3 kGy/h.
The dose rate was determined using the Frick dosimeter taking G(Fe3+) as 15.6 mol/(kg.Gy). The irradiation was performed at room temperature in air atmosphere. The dose was ranged
300 350 400 450 500 550 600 0.25
0.50 0.75
Absorption
(nm)
Pristine
50kGy 1000kGy
100kGy 1500kGy
150kGy 2000kGy
200kGy 2500kGy
250kGy 3000kGy
300kGy 3500kGy
400kGy 4000kGy
500kGy 4500kGy
750kGy 5000kGy
0 1 2 3 4 5
0 5 10 15 20
A
Gamma Dose (MGy) from 0.05 to 5 MGy. The different irradiated doses were
obtained by monitoring time exposures.
The UV–Visible absorption spectra of pristine and gamma irradiated PET polymer have been studied by using a double beam Cintra 40 spectrophotometer in the range 200–900 nm.
III. RESULTAS AND DISCUSSION A. Optical absorption measurements
After irradiation the transparent PET polymer became yellowish and the degree of yellowing increases slightly with increasing gamma irradiation dose. The optical absorption spectra of PET samples before and after- ray irradiation are presented in Fig.1.The slight color change of the polymer after gamma rays irradiation may be due to the creation of new transitions and color center due to gamma irradiation.
The spectra show a change of the absorption edge with the increase of gamma dose. This behavior is due to chemical changes that lead to absorption bands more or less defined in the UV range. The observed changes could be interpreted by the formation of extensive systems of conjugated bonds [13].
The evolution of (310 to 355 nm) band area
(absorbance area of the irradiated films absorbance area of the pristine film) versus gamma irradiation dose is shown in Fig.2. The error estimated at 3%, is mainly related to the experimental conditions of the measure and the calculations of area. As can be seen, the optical absorbance increases linearly with gamma dose. It is interesting to note that this linear behavior could be used as a dosimetric property.
Fig. 1. Absorption spectra of irradiated PET samples at different gamma doses.
Fig. 2. Bande area changeversusirradiation dose.
B. Optical band gap
The Tauc plot method [14] is widely used to determine the optical band gap according to the following equation.
αhhg)n (1)
where B is a proportionality constant. Egis the energy gap and n represents a parameter that characterizes the different electronic transitions generated during the absorption process.
It takes the value 1/2 for direct gap (direct transitions) and the value 2 for indirect gap (indirect transitions).
The method requires the optical absorption coefficient α, which can be calculated from the absorbance (A) by using the formula:
αe (2)
e is the sample thickness in cm and A is defined as
log, where I0 and I are the intensity of the incident and transmitted beams, respectively.
The energy gap Eg was determined from the plot of (αhn versus (h). The linear portion was extrapolated to the h axis as it is showed on figure 3. Only four spectra (Pristine, 1 MGy , 2 MGy , 4.5 MGy) were kept for clarity.
The obtained values of direct and indirect energy gap Egat different gamma doses are presented in Fig. 4a.
0 1 2 3 4 5 0
40 80 120 160
Eu (meV)
Dose (MGy) (b)
0 1 2 3 4 5
3.80 3.84 3.88 3.92 3.96
4.00 Indirect
Direct
Eg (eV)
(a)
3.00 3.15 3.30 3.45 3.60 3.75 3.90 4.05
30 60 90
(h)1/2 (cm-1 eV)1/2
h
eV
Pristine 1MGy2MGy 4.5MGy
Fig. 3. The dependence of (αhν)1/2on photon energy (hv) for pristine and gamma irradiated PET film; represents the absorption coefficient.
Fig. 4. (a) The variation of direct and indirect band gap with gamma dose for pristine and irradiated PET films.(b) The variation of Urbach energy Eu
with gamma dose for pristine and irradiated PET films.
It can be seen the simultaneous existence of direct and indirect energy band gap in PET polymer as it has been observed in other works. [15,16].
It found that both indirect and direct gap optical band decreases linearly with increasing gamma dose. This behavior can be attributed to the possible creation of new intermediate states that allow electronic transitions between molecular orbitals [17]. It is interesting to note that the direct electronic transitions are greater than the indirect electronic transitions.
Similar behaviors have been reported by different authors [18-
19] but no result has been reported at so higher gamma dose for PET polymer.
C. Urbach energy
Urbach energy Euis an energy that represents the width of the tail of localized states in the forbidden band gap. Its origin is considered to be due to the thermal vibrations in the lattice [20]. It is related to the absorption coefficient by the following expression [21].
ααexphu) (3)
The Urbach energy values were extracted from the slop of the linear portion in the lower energy region.
It can be seen on Fig.4b, that the Urbach energy increases with the gamma dose. This indicates the gamma ray induces disorganization of the PET structure. The decrease in Urbach energy may be attributed to the decrease in the crystallinity of the PET polymer.
IV.CONCLUSION
The effects on optical properties of pristine and gamma rays irradiated PET has been studied using UV-Visible spectrophotometry. Various properties, such as, absorption, energy band gap (for direct and indirect transitions), Urbach's energy, of the PET film are studied versus gamma rays dose.
The absorption and the Urbach's energy obtained from UV–vis spectra were found to increase with the gamma rays dose, whereas the optical band gap decreased.
Irradiation of the PET causes the breaking of bonds and new bond formation, giving a combination of degradation and crosslinking, with formation of unsaturated products.
The linear behavior of the increases absorption band at wavelength in the range 310–355 nm with gamma rays dose observed indicates the degradation of the chemical structure and suggests the potentiality of using PET polymer as gamma dosimeter.
Acknowledgment
The authors wish to thank the members of the Nuclear Research Center of Algiers for the irradiation of the samples and their co-operation.
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