In recent years, studies on optical and electrical properties of polymer blend films have increased remarkably in view of their wide applications in radiation dosimetry and in optical and electronic devices. The manufacturing of these materials can be tailored to have unique optical, electrical and electrochemical properties by irradiating with the electromagnetic radiation such as x- and γ-rays or the particulate radiation such as electron and ion beams in order to change their physicochemical properties through grafting and scission or dehydrochlorination[2-4]. Dosimetric characteristics of dyed Polyvinyl Butral (PVB)/chloral hydrate containing chlorine show change colour with increasing dose of γ-rays[2,5]. Detachment of C-Cl covalent bonds, results in the production of free radicals and ions. The subsequent chemical reactions lead to the formation of acids that allow the dye to change its structure and colour.
The increase of ionic species in polymer blends upon irradiation can be studied using absorption spectrometers, which provide details of the optical band gap energy and absorption edge and activation energy. The absorption spectra in UV region increase with increasing dose. In principle, photon with energy greater than the band gap energy will be absorbed. Electromagnetic wave packet interacts with electron in the Valence Band (VB), which is then raised across the band gap on the Conduction Band (CB) via two possible types of electronic transition, i.e. direct transition and indirect transition. In the direct transition the wave vector for the electron remains unchanged, while in the indirect transition the lattice vibration or phonons assist the transition so that the minimum of the CB lies in the different part of k-space from the maximum of the VB. This study was concerned with the changes of optical band gap, absorption edge and activation energy with dose of dyed PVA-CH blend films irradiated with γ-rays.
MATERIALS AND METHODS
The film samples of the polymer blends were prepared using a solvent casting
method. The PVA polymer powders (Mw = 70000, obtained from SIGMA, USA) were
of research grade with the purity of 99%. The polymers were dissolved in double
distilled water at 90°C and cooled at 30°C. Chloral hydrate [CCl3CH(OH)2,
2,2,2-trichloro-1,1-ethanediol] with the blending ratio of 23, 34, 45 and 57%
(w/w) were added to the polymer solution. The blends were doped with cresol
red dye to facilitate the radiation dosimetric study. The polymer
blends were solidified for about 72 h by solvent casting at room temperature.
The average thickness of the films was found to be 70 μm.
The film irradiations were carried out in the 60Co γ-rays chamber at the Malaysian Institute for Nuclear Technology Research Bangi, Malaysia. The absorbed dose rate was calibrated using Fricke dosimetry method to be 9.39 k Gy h-1. The samples were exposed at different doses up to 12 k Gy at room temperature.
The absorption spectra of the unirradiated and irradiated blend films were measured using a scanning double beam UV-visible spectrophotometer (Shimadzu, Model 1601) in the wavelength range 240-600 nm at room temperature. The absorbance A was calculated according to the Beer-Lambert Law equation A = log10 (I/I0), where, I is the transmission intensity through the sample and I0 is the intensity through the air gap as a reference. The absorption spectra are presented as the absorbance versus wave number.
RESULTS AND DISCUSSION
The absorption spectra show an increase and a decrease with increasing dose
for blend concentration at 23% CH. Following irradiation the blend films undergo
colour change from yellow at 438 nm peak to red at 529 nm peak, corresponding
with the amount of acid formation (Fig. 1). These two peaks
were used to determine dosimetric properties of the polymer blends, which have
been discussed elsewhere. The absorption peak at UV region, which
increases with dose were used to study the optical characteristics.
Absorption spectra of dyed PVA-CH film at 23% blend concentration
showing an increase in 529-nm absorption peak and a decrease in 438 nm peak
with the increase of dose when it changes colour from yellow to red. However
the absorption spectra at UV region always increase with increasing dose
||Variation of α(v) vs. hv of the PVA-CH blends at various
doses for (a) 23% and (b) 57% blend concentrations
These bands relate to the excitation of outer electrons, which can provide
information on the electronic transitions between molecules in the blend samples.
They are attributed to the π-π* transitions from donor atoms (HOMO)
to acceptor atoms (LUMO) because of the radiation-induced structural disorder
in the polymer blends by the radiation scission of the blends
and the presence of radiation-induced ions. The absorption coefficient α(v)
of the PVA-CH polymer blends was taken from the optical absorption spectrum
where, d is the sample thickness in cm and A is the absorbance. The plots of
α(v) vs. hv at different doses are shown in Fig. 2a and
b for 23 and 57% CH concentration, respectively. The results
show a continuous increase of α with increasing photon energy hv. Near
the absorption edge, α increases more rapidly with hv. The absorption edge
was determined by extrapolating the linear portion of α(v) vs. hv curve
to zero absorption value. The absorption edge decreases with increasing
dose and increases with CH composition.
||Absorption edge position (eV) of the PVA-CH blends at various
doses and blend concentrations
It was found that the absorption edge of 23% CH and 57% CH samples decrease
from 5.08 to 4.96 eV and 4.85 to 4.62 eV, respectively, when the dose increases
from 0 to 12 k Gy (Fig. 3). Abdel Fatah et al.
have measured the absorption edge of unplasticized poly(vinyl chloride) (UPVC)
and they found that the value decreases from 4.35 to 2.04 eV when the fluence
of 25 MeV proton beam increases from 10+13 to 10+15 ions
Analysis of the optical absorption spectra can reveal the optical energy gap Eg between the CB and the VB due to direct and indirect transitions for both crystalline and amorphous materials. The absorption coefficient α(v) is a function of photon energy and obeys Mott and Davis model.
where, h is the energy of the incidence photon, h is the Planck constant, Eg is the optical energy band gap, B is a constant known as the disorder parameter which is nearly independent of photon energy. Parameter m is the power coefficient with the value that is determined by the type of possible electronic transitions, i.e. m = ½, 3/2, 2 or 1/3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden, respectively.
The direct optical band gap can be evaluated from the linear plots of (αhv)2
versus hv at different doses as shown in Fig. 4a and b
for 23 and 57% CH composition, respectively. The extrapolations of the lines
of (αhv)2 versus hv for which (αhv)2 = 0, give
the direct optical band gap, which is a function of dose as shown in Fig.
5. The optical band gap increases with increasing dose and CH concentration.
The results show Eg decreases with increase of both the dose and
the CH composition. It was found that Eg at 0 Gy reduces from 2.50
eV for 23% to 2.25 eV for 34% CH composition.
||Variation of direct allowed transition (α hv) 2
vs. hv of the PVA-CH blends at various doses for (a) 23% and (b) 57% blend
||Variation of the optical band gaps with increasing dose of
the PVA-CH blends at 57% blend concentrations
At 12 k Gy the value reduces further from 2.38 eV for 23% to 2.22 eV for 34%
CH composition (Fig. 5). A decrease in the band gap energy
with increasing dose may be attributed to an increase in structural disorder
of the irradiated polymer blends. Irradiation the polymer blends induces dehydrochlorination
of chloral hydrate, resulting of loss of Cl and induce ion detachments and unsaturated
||The optical activation energy ΔE of the PVA-CH blends
at various doses and blend concentrations
||The optical activation energy ΔE of PVA-CH blends at
various doses and blend concentrations
These contributed to the structural defects, which reduce the band gap with
increasing dose[1,8]. The decrease in Eg implies an increase
in the conductivity of irradiated polymer blends. Some researchers have reported
the change in band gap energy of some materials irradiated with γ-rays
and protons. The value of Eg varied from 2.59 to 2.49 eV for manganese
Phthalocyanine (MnPc) irradiated with γ-rays up to 25 k Gy,
from 3.75 to 3.45 eV for tellurium dioxide (TeO2) irradiated with
low doses γ-radiation up to 40 Gy. It varied from 4.35 to
2.04 eV for UPVC films irradiated with proton beam of increasing fluence from
10+13 to 10+15 ions cm-2.
The optical activation energy, ΔE, is the energy width of the tail of localized states in the band gap was evaluated using the Urbach-edges method given by the formula:
where, α0 is a constant and ω = 2πv. The activation energy ΔE of irradiated samples were determined from the slope of the straight lines of ln(α) versus photon energy hv of Fig. 6 for 57% CH composition. The results of ΔE values at different doses are shown in Fig. 7. It can be seen that ΔE is strongly dependent on CH composition but is less dependent on dose. It was found that ΔE at 0 Gy increases from 0.24 eV for 23% CH to 0.42 eV for 34% CH composition and then to 0.69 eV for 57% CH composition. At 12 k Gy the value increases from 0.20 eV for 23% CH to 0.40 eV for 34% CH and then to 0.64 eV for 57% CH. It has been reported that ΔE value increased from 0.5 to 0.98 eV for manganese Phthalocyanine (MnPc) irradiated with γ-rays up to 25 k Gy and for UPVC films irradiated with proton beam of increasing fluence from 10+13 to 10+15 ions cm-2 the value increased from 1.18 to 1.93 eV.
The absorption spectra of irradiated PVA-CH polymer blends in UV region increase with the increase of dose. The interaction of gamma radiation with the polymer blends induces dehydrochlorination of chloral hydrate, resulting ion detachments and formation of unsaturated group, which contribute to the structural defects in the polymer blends. The absorption edge and the optical band gap energy (Eg) decrease with increasing dose. However, the activation energy (ΔE), the energy width of the tail of localized states in the band gap, is less influence by the radiation action.