Influence of gamma rays on the optical characteristics of CPVC/Ag nanocomposite film

In the current study carboxylated polyvinyl chloride/silver (CPVC/Ag) nanocomposite film was prepared. Samples from the CPVC/Ag nanocomposite film (1.5cm × 1.5 cm) were treated with 25–200 kGy gamma dosages. The optical characterization of the treated films has been investigated considering dosage by applying UV spectroscopy and colour difference studies. The optical bandgap, refractive index, Urbach energy, optical dielectric parameters, chromaticity coordinates, tristimulus values, colour intensity and colour intercepts have been calculated and interpreted based on gamma dosages. The bandgap decreased from 4.14 to 3.87 eV with augmenting the gamma dosage upto 200 kGy, together with a rise in the Urbach energy. This is attributed to the domination of crosslinks which damage the crystalline phase. Moreover, the non-treated CPVC/Ag is practically transparent. It demonstrated kindliness to change in colour by gamma treatment, as the colour intensity reached a value greater than 5, meaning permanent colour changes.


Introduction
The feasibility of nanocomposites (NCs) application in various fields has drawn the attention of numerous authors to the production of it using blended polymers and nanoparticles (NPs) [1].Owing to the minute size of the implanted NPs, these polymeric NCs exhibit a unique behaviour that differs from that of bulk materials [2].As a result, those NCs can be extensively used in a variety of applications, including, sensors, optoelectronic devices, single electron transistors and solar cells [3,4].
Nanoparticles have a great role in improving the physical and chemical properties of matter for a possibility of several applications [5].Superior membranes with enhanced sensitivity and selectivity are produced by implanting NPs in the matrix of the blended polymers [6].The physical characteristics of the final NC are improved as a result [7].
Polymeric nanocomposite NCP materials have developed excellent research field because of the small size that affects the physical properties of the materials [8][9][10].This allows matter to have special properties than bulk matter [11].The new properties of the synthesized nanocomposite allow it to be a suitable candidate for several industrial applications.Interesting applications as solar cells and optoelectronic tools are dependent on those characters [4].Thermostability of the matter can be augmented using metal-polymers NCPs with extra new optical properties [12].Polymer NCPs presented a rank in the current periods due to its possible applications in the opto-electronic devices fields [13].CPVC has the capability to enable surface adsorption through the carboxylic group [14].Comparing CPVC with impartial PVC, the properties of CPVC are healthier owing to the moderately advanced conductivity [15].NCP matter involved an enormous consciousness due to their potential for refining exact belongings.The recently nearby NCPs may enable the manufacture of novel polymers-inorganic composite to yield advanced excellence films with better sensitivity and selectivity [6].The physical properties of matter can be enhanced by merging metal nanoparticles NPs into the polymer matrix thus the produced matter will be appropriate for diverse applications [16][17][18].Besides, the treatment of polymer can encourage numerous variations in optical characteristics [19].The provoked photochemical possessions alterations in polymeric matter have been gripped care for much application [20].Numerous investigations have been carried out on the alteration of the physical possessions of polymers by means of NPs [21][22][23][24][25][26][27][28][29][30].The current paper investigates the gamma treatment effects on the optical properties of CPVC/Ag NCP with the aim of improving its properties for different applications.

Preparation of CPVC films
0.9 g CPVC was mixed with 0.4 g dioctylphthalate in a glass petri dish (5 cm diameter), after that the mixture was dissolved in 15 mL tetrahydrofuran (THF) [31].The solution was stirred well and heated continuously at 50°C for half an hour until obtained homogeneity.The Petri dish was enclosed with filter paper and left to situate for 1 h to allow slow solvent vanishing.

Preparation of silver nanoparticles
Silver nitrate solution (1 mmol/mL) was prepared and used as a metal salt precursor for preparing silver nanoparticles.Solution of sodium citrate with the same concentration of AgNO 3 was used as a reducing and stabilizing mediator.Conversion of the transparent colourless solution to characteristic pale yellow is a good marker for creation of nano-sized silver.Centrifugation for the purification process of the produced nanoparticles was used, and the colloids containing nanoparticles were washed at least three times using deionized water and nitrogen flow to get rid of excess silver ions.A freeze-drying method was used to obtain a dry dust of nanoparticles.To use the nanoparticles and perform its characterization, the dried dust of the nano-sized silver was suspended in deionized water and a Fisher Bioblock Scientific ultrasonic cleaning container was used to homogenize the suspension [32].

Preparation of carboxylated PVC nanocomposite membranes
The nanocomposite was prepared using the ex-situ technique [31].All the amount of nanoparticles resulted from the previous preparation steps was singly suspended in THF with sonication for 1 h, and then each type was separately added to the casting CPVC/THF solution and the steps to form the CPVC film were completed as mentioned above.
The Obtained NCP films were treated applying 60 Co source (designed by Atomic Energy of Canada Ltd.) A.E.A., Cairo, Egypt.
Transmission Electron Microscopy is carried out using a JEOL model 1200EX instrument of 120 kV.
UV/Vis spectroscopy was applied by means of Tomos UV-1800 spectrophotometer.The CIE methodology was performed for the explanation of coloured samples [34].

TEM of Ag NPs
The TEM image of Ag NPs was categorized in Figure 1.The bit size spreading was fluctuating between 10 and 80 nm through a normal 50 nm particle size [31].

Absorption investigation
The absorbance spectra of treated and non-treated CPVC/Ag films have been demonstrated to estimate the alternations in the bandgap configuration due to gamma treatment (Figure 2).Elevated absorption band appeared at nearly 320 nm which reduces with increasing wavelength up to 820 nm.The absorbance drop may be owed to either the phenyl group (π − π * ) [35] or the creation of colour centres at 290 nm [36].At that wavelength, the photochemical reactions are initiated in the CPVC/Ag matrix as a result of the absorption of UV light triggering the molecules to its single or triple state [35].At longer wavelengths greater than 300 nm, the γ treatment breaks the C-H bond.
Moreover, the absorbance of the CPVC/Ag NCP rose with augmenting the dosage upto 200 kGy.This is owed to the enhancement of conjugated bonding.Thus, the bandgap is expected to reduce on growing dosage [37].
The extinction coefficient (k) is an important parameter that supplies knowledge about the fractional dissipation of the incident rays due to spreading and incorporation means of the penetrated area.The compound part of the refractive index is calculated from [3]: k and n * are the imaginary and real portions of the complex refractive index, correspondingly.k is calculated from [3]: where λ is the wavelength.α is the absorption coefficient that can be considered from the formula (3): where A is absorbance, d is the sample's thickness and α stands for the quantity of the absorbed photons by matter, hence it is used to illustrate the variations in band construction.

Interpretation of bandgap
The values of bandgap (E g ) have been calculated applying Tauc's principle for direct transition that provides information concerning the transitions in bandgap structure [38]: where B is constant, hν is incident photon energy and n is an index of value that signifies the character of microelectronic transition.n equals 3/2 or 1/2 for direct transitions, while n equals to 2 or 3 for indirect rendering allowed or forbidden, correspondingly [39].Using Figures 3-5, E g is valued by scheming (αhν) 1/n versus hν; formerly induce the straight portion of the curvature to meet the hν axis.The variation of the direct bandgap of the CPVC/Ag films with the gamma dosage is shown in Figure 6 (using the data represented in Figure 3).The bandgap decreased from 4.14 to 3.87 eV on growing the gamma dosage to 200 kGy.The suppression of the bandgap is owing to crosslinking.Therefore, the disordered character of the CPVC/Ag NCP is augmented through persuading defects and henceforth localized    states are presented in the bandgap system that causes microelectronic transitions with less energy.The consequential effect of gamma on the CPVC/Ag is the formation of free radicals that causes crosslinkins.On raising the gamma dosage, the free radicals creation rate rises.Therefore, the unsaturated and conjugated bonding increase, and consequently E g decreases [37].
The optical dielectric loss and Tauc's model from Figures 7 and 3-5, respectively, were considered to  exactly deduce E g and the kind of microelectronic transition.This is because of the optical dielectric which is enormously influenced by the band construction of the material.Additionally, the investigation of dielectric loss applying UV spectra is considerably helpful in assuming the whole band construction of matter [39].The imaginary portion of ε is useful to investigate the microelectronic transition among the unoccupied and occupied states [40,41].The ε values were valued by applying the formula: The ε spectra obtained for the non-treated and treated CPVC/Ag films are displayed in Figure 7.The E g values gotten from ε are significantly near to those valued from Tauc's model (Table 1).Therefore, the sort of microelectronic transition is the allowed direct transition [41].

Interpretation of Urbach energy
E u investigates the building of matter via the judgment of the defect altitude in the forbidden E g .The E u values were computed using the formula [42]: α o is constant.E u expresses the degree of disorder [43].By scheming lnα verses hυ, E u can be evaluated from the inverse of the slope of the resultant straight line.E u rose from 0.29 to 0.35 eV on augmenting the dosage up to 200 kGy .(Figure 6).The increase of E u is attributable to the rise of the disorder character via crosslinks [44].

Interpretation of refractive index
The refractive index of the CPVC/Ag films is valued using the rule [1]: where R is the reflectance, is considered from the formula R = 1 − √ Te A (T is the transmission).Figure 8 displays the change of n with wavelength.The previous studies showed that the variation of refractive index with wavelength is essential for monitoring the optical characteristics of matter, and their dispersion.This is an imperative for several applications.Also, Figure 8 explained the dependencies of refractive index on gamma dosage for the CPVC/Ag films.The refractive index rises on growing dosage up to 200 kGy.This trend is in agreement with that of E g meaning the authority of crosslinks.The chain scissions create lively free radicals that allow the creation of covalent bonding via crosslinking.This explanation is in good agreement with that obtained previously [45,46].
Mostly, the dielectric properties provide information concerning the optical characteristics of matter [47].The dielectric constant varies with hν, signifying that certain interactions between photon and electron occur in that energy range.
The dielectric constant valued from the equation [48]: The ε values were estimated and plotted in Figure 9 against wavelength.ε rises with raising the dosage up to 200 kGy.This explains that the gamma treatment rises the density of states inside the forbidden gap of the CPVC/Ag NCP [49].

Interpretation of colour alternation
The transmission spectrum of the treated and nontreated CPVC/Ag films (370-780 nm) is displayed in Figure 10.The real red, green and blue lights are represented by tristimulus values X, Y and Z [50].The chromaticity coordinates and tristimulus values were valued by means of the transmittance 370-780 nm (Table 2).The X, Y and Z values were reduced with a dosage up to 200 kGy.The values of x and y improved on rising the dosage up to 200 kGy.The z coordinate showed an inverted trend.The CIELAB colour intercept a * compares the red (+a * ) and green (-a * ), while the intercept b * compares the yellow (+b * ) and blue (-b * ).L * expresses the lightness.A faultless white has L * of 100, and a faultless black has L * of 0. The accuracy in calculating L * is ±0.05 and ±0.01 for both a * and b * , respectively.The variation of colour intercepts with the gamma dosage is represented in Figure 11.The colour intercept b * showed negative values that augmented with growing the dosage up to 200 kGy.This specifies that the blue colour component rises and turns yellow.The greenred constituent (a * ) is unaffected by the gamma treatment.This was related to the growth in darkness in the CPVC/Ag NCP (−L * ) (Figure 11).E is the colour intensity which represents the difference in colour between the non-treated and treated films is calculated using the formula used before [15].Its variation is displayed in Figure 12 against gamma dosage.The E improved on growing the gamma dosage up to 200 kGy.The values of E attained a significant colour variation which is a reasonable contest in saleable replica on printing presses as E is bigger than 5 [51,52].This investigates that the CPVC/Ag NCP has a retort to colour transformation by gamma treatment.The colour fluctuations are produced by the chemically active free radicals that are shaped by chain scissions.Moreover, the chemically active free radicals  that possess electrons with unpaired spin, origin colour discrepancies [31].

Conclusion
The gamma treatment of CPVC/Ag films leads to the dominance of crosslinks that modifies their optical characteristics.This was reflected in an increase in the absorbance, refractive index and creation of colour centre.Additionally, the optical band gap decreased with increasing the gamma dose up to 200 kGy.This trend may optimize CPVC/Ag films for optoelectronic applications.Furthermore, the CPVC/Ag films can be an adequate match in marketable imitation on printing presses.

Figure 2 .
Figure 2. The absorbance spectra of the treated and nontreated CPVC/Ag films.

Figure 3 .
Figure 3.A plot of (αhν) 2 versus hν for the treated and non-treated CPVC/Ag films.

Figure 4 .
Figure 4.A plot of (αhν) 0.5 versus hν for the treated and non-treated CPVC/Ag films.

Figure 6 .
Figure 6.E g and E u variation with gamma dosage.

Figure 7 .
Figure 7.A plot of optical dielectric loss versus hν for the treated and non-treated CPVC/Ag films.

Figure 8 .
Figure 8.The refractive index spectra for the treated and non-treated CPVC/Ag films.

Figure 9 .
Figure 9.A plot of optical dielectric constant versus hν for the treated and non-treated CPVC/Ag films.

Figure 10 .
Figure 10.The transmission spectra for the treated and non-treated CPVC/Ag films.

Figure 12 .
Figure 12.Color intensity variation with gamma dosage.

Table 1 .
E g (from ε and Tauc's model versus hυ) calculated for the non-treated and treated CPVC/NCP films E g from Tauc's Model (eV) Dose (kGy) E g from ε vs. hν (eV)

Table 2 .
The chromaticity coordinates and tristimulus values for CPVC/NCP films as a function of gamma dosage.