Study the Optical Properties of Polyvinyl Alcohol / Methyl Red Composite Irradiated by Violet Laser

ABSTRACT


INTRODUCTION
Polyvinyl alcohol (PVA) is a linear, semicrystalline compound composed of a hydroxyl (OH) functional group and a carbon chain serving as the backbone [1].This polymer is produced through the polyvinyl acetate (PVAC) hydrolysis, and the degree of hydrolysis has a major impact on the material's physical properties.PVA has a broad molecular weight range of 9000-186,000 g/mol [2].As a result, it is considered a versatile polymer [3].PVA is a very well hydrophilic and water-soluble polymer.Other advantageous characteristics of PVA include its accessibility., non-toxic, Biodegradability, minimal cost, and exceptional film formation, and biocompatibility, good chemical resistance, The PVA is an appealing choice for a variety of applications, including the formation of coatings, hydrogels, fibers, scaffolds, composites, and polymers, due to its thermal resistance and adhesion films [4].PVA is given by the formula (C2H4O) and chemicallyPVA is classified as a homopolymer of vinyl alcohol or ethanol.Figure 1 illustrates the structural formula of PVA [5].
Methyl red is classified as an organic compound due to the carbon and hydrogen in the atomic structure, supplemented by nitrogen and oxygen elements.In the study of organic chemistry, these structural elements are commonly known as functional groups.Two rings within the molecule contain double bonds; these rings are referred to as aromatic or benzene rings [6].
The main application of the MR serves as an indicator of the benzenoid structure (yellow) in a base medium and the quinonoid structure (red) in acidic medium.As a pH indicator, methyl red can be applied to radiochromic material [7].The chemical formula of the Methyl red is C 15 H 15 N 3 O 2 as illustrated in Figure 2.

Revue des Composites et des Matériaux Avancés-Journal of Composite and Advanced Materials
Two or more substances are combined to form a composite material, with the finished assembly possessing attributes superior to the individual parts [10].The laser radiation is transmitted and reflected when it strikes a surface (the air-solid contact).Some of the beams are reflected, some are absorbed, and some are transmitted.According to Beer Lambert's law, it is absorbed when it moves across a new medium [11]: where, Io and I are the intensity of the transmitted and incident photons respectively, (α) represents the absorption coefficient, while (t) represents the sample's thickness.
Examining the optical properties of this combined material has an important impact across various fields for instance pH sensing for food packaging [12].
designing optoelectronic devices [13], manufacturing the biological sensors to detect pollutants and toxins [12] besides crucial research to understand the light-matter interaction [14,15].
The optical characteristics present an information regarding the interaction between light and materials.The energy gap is equivalent to or smaller than the photon energy.
The electron-hole pair is composed of the maximum wavelength of the incoming photon λ, which is defined as [14]: The absorbance spectrum will be used in calculating the absorption coefficient of the films [16]: The absorbance is denoted by A.
The extinction coefficient is calculated by [17]: The ratio of the speed of light in a substance to the speed of light in a vacuum represents the refractive index.It may also be computed utilizing the following formula [18]: For the calculation of reflectance, the following empirical relationship is used [19]: where, T is the transmittance.The real (   ) and imaginary (  ) part of the dielectric constants were obtained using the equations [20]: The materials' band gap is obtained from Tauc's equation of the following form [21]: The B constant is correlated with the structure of the sample and r represents the empirical index that signifies the electronic transition.
The optical conductivity of a material are given by the following relation [22]: where, the velocity of light is denoted by c and  ° is the electrical permittivity of the space (8.854 × 10 −12 F/m).

MATERIALS AND METHODS
Methyl red and polyvinyl alcohol powder were supplied by Sigma-Aldrich Company.In 20 ml of distilled water, 10,000 g/mol molecular weight, precisely 0.008 milligrams of PVA and 0.012 mg of MR with molecular weight (269.3 g/mol) are weighed and diluted at 90℃ separately.The mixtures were subsequently vigorously stirred with a magnetic stirrer for approximately 10 minutes at room temperature, or until the PVA and MR had completely dissolved.Films were obtained by pouring the prepared polymer solutions and methyl red into petri dishes with the same diameter (2.5 cm) and then let them dry under identical atmospheric conditions.The films have a thickness of approximately 3  for PVA and 6  for MR, as determined by weight method.The same steps are repeated to obtain the (PVA/MR) composite with a thickness of about 9  under similar conditions.PVA and MR materials were chosen due to their compatibility in aqueous solutions which assists the MR dispersion in PVA matrix evenly.In addition, the PVA offers a stable and flexible substrate which suitable for applications that require bending and stretching.PVA/MR can response to the wavelengths (UV or visible) that makes it suitable for optical properties tuning.
The thick films are irradiated using a laser with a 405 nanometer wavelength (Violet) and a laser intensity of 20 mW for various applications time intervals (0, 20, 30 and 40) minute.Using the UV-Visible spectrophotometer (Shimadzu, UV-1900 I, JAPAN), the optical properties were measured before and after laser irradiation.An investigation is conducted on absorbance and transmittance spectra within (190 nm to 1100 nm) wavelength range.A local Excel program was used for calculations.

RESULTS AND DISCUSSION
The crystallite structure was determined using XRD (X-Ray Diffraction) characterization.The purified PVA thick films exhibited significant crystalline reflections at approximately 2θ = 19.92°and 42.74° in their X-ray diffraction pattern.Characteristic of PVA, the two peaks represent reflections from a monoclinic unit cell at (200) and (110).The results of the XRD data obtained for MR with a value of 2θ =12.745˚, 16.720˚, 24.773˚, 31.005˚and44.949˚ there are five most substantial intensities, the highest peak at 12.745˚, Obtaining Miller's index from the intensity mentioned above, namely, (-101), ( 200), (211), (-122), and (311) which can show crystal structure, as shown in Figure 3 (a) and (b).
Optical properties of PVA, MR and PVA/MR composite thick film measured before and after laser exposure with different irradiating times (0, 20, 30, and 40) minutes.
Figures 4-6 show the transmittance spectrum in the rang (275_460) nm for PVA, MR, and PVA/MR.It is clear that the transmittance spectra decreased with increasing irradiating time, which attributed to increasing the number of collisions between the incident photons and atoms.show the energy gap that is determined by extrapolating the linear part of (ℎ) 1/2 towards the (ℎ) axis leading to an estimated value that listed in Tables 1-3.
Increasing the band gap with increasing the irradiation time as a result of decreasing the absorption leading to the decrease in the secondary levels between the bands.

CONCLUSION
Absorbance spectra were recorded for PVA, MR and PVA/MR composite to be able to calculate the optical parameters as a response of the laser effect.It was found that , ,   ,   , and   decreased while   increased by the increase of the irradiating time.the irradiation time presents a great opportunity for bonds breakage, thus decreases the thick films absorption, and then increase in the energy gap   .This has a suitable application in manufacturing the optical filters.Although, the laser irradiation can improve the films' structure, controlling the energy gap and the optical parameters.The casting method is a fast and easy deposition method, but there are some experimental limitations have to be avoided in the next research such us the humidity sensitivity and temperature.In addition, the films processing method determines the composite homogeneity which affect some industrial applications.

Figure 9 .
Figure 9. (a) Reflectance spectra of PVA/ MR Composite.(b) Maximum reflectance vs. laser irradiation times at 300nm wavelength Figures 10-12show the energy gap that is determined by extrapolating the linear part of (ℎ) 1/2 towards the (ℎ) axis leading to an estimated value that listed in Tables1-3.

Figure 13 .Figure 14 .
Figure 13.The correlation between polyvinyl alcohol's optical conductivity and wavelength

Figure 15 .
Figure 15.The optical conductivity as a function of the wavelength of PVA /MR

Table 1 .
The optical parameters of PVA composite at a wavelength of 290 nm

Table 2 .
The optical parameters of MR composite at a wavelength of 300 nm

Table 3 .
The optical parameters of PVA/MR composite at a wavelength of 300 nm