Effect of Thickness variation on The Optical Properties of (Poly Vinyl Alcohol : Green Methyl) Films

Poly (vinyl alcohol) doped with different Green methyl concentration (4%) by solvent cast method for different thickness the prepare films were 5, 10, 15,and 20 μm. Transmission and absorption spectrum have been recored in order to study the effect of increasing thickness on some opttical constans such as transmittance, reflectance, absorption coefficient, refractive index and extinction coefficient. The film study reveals that all these parameters affect by increasing the thickness. The films show indirect allowed interband transitions that influenced by the thicknesses , the optical energy gap has been increased from about (3.36 eV) for the (5 μm) , (3.39 eV) for the (10 μm) , (3.41 eV) for the (15μm) and (3.45 eV) for the (20 μm) .


Introduction
Poly(vinyl alcohol) (PVA), is a water-processable polymer, with excellent chemical resistance, optical and physical properties which recommend it and itscomposites for broad industrial uses.
PVA can be combined with other polymers or fillers to enhance its properties [1,2] , which are useful in technical applications including biochemical and medical.The important feature of semicrystalline PVA isthat the presence of crystalline and amorphous regions and its physical properties,which are resulting from the crystal-amorphous interfacial effects.These two regions are well separated by portions of an intermediate degree of ordering, which enhances the macromolecule, producing several crystalline and amorphous phases [3].Optical behavior of the polymer are affected by doping which depends on the interaction between the polymer and the dopant .the doped polymers have been the subjects of interest for the theoretical and experimental studies, because of the physical and chemical properties needed for specific application may be obtained by adding or doping with some dopant.It is observed that doping a polymer with metal salts has significant effect on their physical properties including optical, In the present work is to investigate the optical properties of poly vinyl alcohol (PVA) doped with 4% of Green methyl with different thicknesses by solvent cast method

Experimental Details
Poly Vinyle alcohol polymer (PVA) solution was prepared by adding deionzied distilled water to solid PVA and then stirred by a magnetic stirrer at 70 ˚C for one hour, a solution of (Green methyl) was prepared by adding deionized distilled water to (Green methyl)and then stirred by a magnetic stirrer at room temperature for ½ h.Appropriate mixtures of PVA and (Green methyl) solution were mixed for (4% ) different thicknesses 5 μm , 10 μm , 15 μm and 20 μm.The solution was poured into flat glass dish.Homogenous films were obtained after drying in an oven for (1 h) at 60 ˚C .The film thickness was measured with a digital micrometer (China Hunan E&K Tools Company) and the average area was (4 x 4) cm2.
Absorbance and transmittance measurement were carried out using UV/VIS Spectrophoto meters in the wave length range (190 -1100) nm  The absorption coefficient of PVA Green Methyl films prepared by solvent cast method are determine from the region of high absorption at the fundamental absorption edge of film.The absorption coefficient was calculating using Lambert law [8]:

Conclusions
The detailed study of the PVA Green Methyl films thickness effect on some optical properties has shown that all the optical constant such as transmittance, reflectance, absorption coefficient, refractive index, and extinction coefficient have been affected by increasing the thickness.And the the optical energy gap has been increased from about (3.36 eV) for the (5 µm) , (3.39 eV) for the (10 µm) , (3.41 eV) for the (15µm) and (3.45 eV) for the (20 µm) .

Figure ( 1 )
Figure(1) shows the of Poly Vinyl Alcohol : Green Methyl) Films for different thickness as a function of wavelength we can observe that the transmittance decreases with increasing the thickness This may be attributed to the creation of levels at the energy band by increasing thickness and this leads to the shift of peak to smaller energies.There are no absorption bands in the visible region good absorption at which region the films are transparent and this result agree with previous studies[7]