Study the Effect of Copper Chloride II (CuCl₂) on Optical Properties of Polyvinyl Alcohol (PVA)

Abstract

The optical properties of polyvinyl alcohol (PVA) doped with copper chloride II (CuCl₂) were studied. The copper chloride was added to polyvinyl alcohol with different concentrations (0, 1, 3, 5, 7, and 12 wt.%). The PVA:CuCl₂ films were prepared using the casting techniques. The absorption and transmission spectra have been recorded in the wavelength range (300-800) nm by using UV-VIS spectrophotometer. The fundamental optical parameters such as optical band gap energy, refractive index, extinction coefficient, and dielectric constants have been investigated. Results show that by adding copper chloride to PVA, the optical parameters (refractive index, extinction coefficient, real and imaginary dielectric constant) are increasing with the increase of concentrations of CuCl₂ While the optical band gap energy decreases. The single oscillation energy E₀ and the dispersion energy E_d were determined by using the Wemple-DiDomenico model. The results show that E₀ and E_d values increase with increasing the dopant CuCl₂ concentration.

Keywords:
Polyvinyl alcohol (PVA); casting techniques; optical constant; optical band gap; PVA:CuCl₂ films; Wemple-DiDomenico model

1. Introduction

Composite materials are a material system composed of a combination of two or more materials that differ in form on material composition. The properties of a composite are different from those of its materials¹,². It is also cohesive in structure. The composite comprises two major components: the matrix (the basic material) and the additives. The matrix is the basic material, serving to enclose the composite and give it bulk form. It surrounds other constituents and makes them more cohesive to form a "compact system". Additives are constituents added to polymers to provide them with specific properties and improve basic properties. These constituents are added in granular form or as small particles. Additives can increase the overall conductivity, reduce porosity, improve friction and some magnetic properties, etc³.

The study of the optical properties of polymers increases our knowledge of the type of polymer internal structure, nature of the bonds, and expands the potential scope of polymer application. Knowing the spectrums of absorbance and transmittance of a polymer assists in identifying many optical properties in different ranges of wavelengths. Conducting examination at the ultraviolet spectrum range enables us to know the type of bonds, orbitals, and energy beams. The study at the visible spectrum range provides sufficient information about the behavior of matter for solar applications. The study at the infrared range is very important in knowing the general structure of a polymer and the elements consisting of its chemical structure⁴.

Polyvinyl alcohol (PVA) based polymer electrolyte has attracted enormous attention in view of their satisfactory performance and biodegradability. PVA is a potential semi-crystalline polar polymer having an excellent charge storage capacity, high dielectric strength, good mechanical stability, and it has dopant-dependent optical and electrical properties⁵,⁶. PVA has a carbon chain backbone with hydroxyl (O–H) groups that can be a source of hydrogen bonding, which assists the formation of polymer complexes and makes them as an excellent host material for solid polymer electrolytes (SPEs)⁷.

Poly (vinyl alcohol) (PVA) is the largest synthetic water soluble produced in the world. It is commercially produced by the hydrolysis of poly (vinyl acetate). The physical properties of poly (vinyl alcohol) depend on the method of preparation, as in the case of other polymers⁸,⁹. The final properties are affected by the polymerization conditions of parent poly (vinyl acetate) used as well as the hydrolysis conditions, drying, and grinding.

Many research works carried on the effect of adding salt to polymers, and the optical properties of polymers electrolytic composites. For example, Hashim et al.¹⁰ studied the optical properties of PVA:CrCl₂ composites. The results showed that the absorbance increased with the increase in concentration of CrCl₂, absorption coefficient, extinction coefficient, refractive index and real and imaginary parts of dielectric constants were increasing with increase in CrCl₂ concentration. Salman et al.¹¹ prepared and studied some electrical and optical properties of PVA:NiCl₂ composites. The experimental results for PVA:NiCl₂ films showed that the transmittance decreased with increasing the filler content, while the absorption coefficient increased with increasing the filler content. Moreover the results showed that it allowed indirect transitions, and the energy gap (E_g) decreased with increasing the filler content.

In the present work, the main goal is to investigate the effect of CuCl₂ on the optical properties of polyvinyl alcohol (PVA) films in the UV/VIS region, including the fundamental optical parameters such as optical band gap energy, refractive index, extinction coefficient, and dielectric constants. In particular, the introducing of an inorganic salt such as CuCl₂ into the polymer can improve and modify its optical properties, due to a strong interface interaction between an inorganic salt and the organic polymer. And another goal is to search for the possibility of obtaining the properties required to be appropriate for optoelectronic applications.

2. Experiment Work

2.1. Sample preparation

Films of Polyvinyl alcohol (M_w = 72,000) with copper chloride II (PVA:CuCl₂ (were prepared by using the solution casting method. In this method, 1gm of pure PVA was dissolved in 50 ml of distilled water. CuCl₂ salt was also dissolved in 10 ml distilled water. The resulting blue-green color solution of CuCl₂ was added slowly to the polymer solution with different concentrations (0, 1, 3, 5, 7, and 12 wt.%). To obtain the complete dissolution, a magnetic stirrer was used at 333 K for 12 hours. These homogenous solutions were cast in a glass dish (diameter of 5 cm). The whole assembly was placed in a dust free chamber, and the solvent was allowed to evaporate slowly in open air at room temperature for 48 hours. The thickness of PVA:CuCl₂ films was measured using a digital micrometer screw gauge and found to be of the order of 65 ± 01 μm. It was determined at different places in each film. The transmission and absorption spectra of PVA:CuCl₂ films at room temperature have been recorded in the wavelength range (300-800) nm using the UV-VIS (Cary) spectrophotometer.

2.2. Theoretical background and basic equations

The optical absorbance spectra (A) of the PVA:CuCl₂ films were recorded in the wavelength range of 300-800 nm at room temperature by using UV-VIS spectrophotometer. The absorption coefficient α(ω) was then calculated from the absorbance (A) by using the relation¹²:

$\alpha (\omega )=\frac{2.303}{x}\log \left(\frac{Io}{I}\right)=\frac{2.303}{x}A(\omega )$ (1)

where I₀ and I are the incident and transmitted intensities of UV radiation, respectively, and x is the sample thickness.

A powerful method for determining the optical band gap energy is the plot of the absorption coefficient data with photon energy as:

$$\alpha ħ\omega =\beta {\left(ħ\omega -E_{opt}\right)}^r$$ (2)

where, ħ is Planck’s constant, ω is the photon’s frequency, E_opt is the optical band gap, and β is a proportionality constant¹³,¹⁴. The dependence of the absorption coefficient on the photon energy gives the nature of the optical transition by the value of the exponent r. It was found that for the tested composites (r = 1/2) for the direct allowed transition for electrons energy in k-space. Plotted (α ħ ω)² versus photon energy (ħ ω), that obtain a good straight line, with extrapolation of the linear portion of these lines gives (E_opt).

At lower absorption coefficient level, in the range of 1–10⁴ cm⁻¹, α(ω) is described by the Urbach formula¹⁵:

$$\alpha \left(\omega \right)={\alpha }_{0}exp\left(ħ\omega /\Delta E\right)$$ (3)

where α₀ is a constant and ΔE is the energy gap tail interpreted as the width of the tail of localized states in the forbidden band gap. The Urbach formula was used to calculate the width of the Urbach tail of the localized states due to the defect levels in the transition gap.

The complex refractive index is given by:

$$n*=n–іk$$ (4)

Where, k is the extinction coefficient and n is the refractive index and they can be calculated from the following equations¹⁶,¹⁷:

$k=\frac{\alpha \lambda }{4\pi }$ (5)

$n=\left\{{\left[\frac{4R}{{(R-1)}^2}-k^2\right]}^{1/2}-\right.\left.\frac{R+1}{R-1}\right\}$ (6)

Where, R is the reflectance which determined from the absorption and transmittance spectra data.

On the other hand, if the refractive index and extinction coefficient are known, the real and imaginary parts of the complex dielectric constant of the nanocomposite can be also estimated. The real ε′ and imaginary ε′′ parts of the complex dielectric constant are expressed as¹⁸:

$$\epsilon \text{'}=n^2–k^2$$ (7)

$$\epsilon "=2nk$$ (8)

The refractive index is expressed by a single effective oscillator dispersion equation of the form¹⁹:

${(n^2-1)}^{-1}=\frac{E_0}{E_d}-\frac{1}{E_0{E}_d}{(\hslash \omega )}^2$ (9)

Where, ħω is the incident photon energy, E_o is the single-oscillator energy for electronic transitions, and E_d is the dispersion energy.

3. Results and Discussion

The absorption coefficient α(ω) values are calculated from the absorbance by using equation (1). Figure 1 shows the absorption coefficient of PVA:CuCl₂ films with different concentrations of CuCl₂ salts as a function of the wavelength. It can be seen the absorption is smallest at high wavelength. This means that the possibility of an electron transition is low because the energy of the incident photon is not sufficient to move the electron from the highest unoccupied molecular orbital to the lowest occupied molecular orbital²⁰. The increase in absorbance with the increase in weight percentage of the added CuCl₂ can be explained by the fact that copper chloride ions absorbed the incident light fall on them. In other words, ions absorb the incident light by the free electrons.

Figure 1
The absorption coefficient for PVA:CuCl₂ films as a function of the wavelength.

From the linear part of the plots of (αħω)² against (ħω) for the PVA:CuCl₂ films as shown in Figure 2, the optical energy gap values have been determined according to equation (2). As illustrated in Figure 2, the intercept of the extrapolation of the linear portion of these curves to zero absorption on ħω axis, gives the value of optical band gap energy E_opt.

Figure 2
(αħω)² versus photon energy ħω for PVA:CuCl₂ films.

The determined values of optical band gap energy for all samples are presented in Table 1. From Table 1, the values of E_opt reduced significantly from 3.24 eV for pure PVA to 2.94 eV for PVA complexed with 12 wt% CuCl₂. It was reported that the modification in electronic structure may affect the optical properties of SPEs²¹. The decrease in E_opt values is a consequence of the generation of new energy levels (traps) between the highest occupied molecular orbital and lowest unoccupied molecular orbital, due to the formation of the disorder in the SPEs films. This leads to an increased density of the localized states in the mobility band gap of the PVA matrix.

Table 1
Optical energy results for PVA:CuCl₂ composite films.

CuCl2 wt%	Optical band gap Eopt (eV)	Energy gap tail ΔE (eV)	Eopt + ΔE (eV)
Pure PVA	3.25 ± 0.108	0.67 ± 0.0072	3.92 ± 0.1082
1	3.18 ± 0.091	0.72 ± 0.0074	3.90 ± 0.0913
3	3.12 ± 0.076	0.76 ± 0.0077	3.88 ± 0.0764
5	3.05 ± 0.066	0.81 ± 0.0080	3.86 ± 0.0665
7	3.00 ± 0.058	0.85 ± 0.0082	3.85 ± 0.0586
12	2.92 ± 0.052	0.91 ± 0.0085	3.83 ± 0.0527

To realize the formation of defects in PVA host polymer, the width of the tail of localized states within the forbidden band gap energy was calculated from the exponential region of the UV-visible absorption spectra. At the lower absorption coefficient α(ω) level, the values of α(ω) are described by the Urbach formula (equation 3). Figure 3 presents the plots of ln(α) against photon energy (ħω) for pure and doped PVA films. The ΔE values are calculated from the reciprocal of the slope of the linear part of each curve, and they are inserted in Table 1. It is evident that the energy tails values increase from 0.67 eV for pure PVA to 0.91 eV for 12 wt% CuCl₂ composites. The smallest value of energy tails for PVA can be ascribed to the semi-crystalline nature of PVA.

Figure 3
The plot of ln(α) against photon energy (ħω) for PVA:CuCl₂ films.

Thus, the increase in ΔE for doped samples suggests the increase of the amorphous portion. The addition of salts to polymers may produce many trapped, which reduces the values of optical band gap energy of the SPE films²². The increase of energy tails ΔE as the concentration of CuCl₂ increases is consistent with the drastic decrease of the optical band gap energy E_opt. In general, the sum (E_opt + ΔE) represents the mobility band gap energy; Table 1 contains values of the mobility gap for investigated SPE samples. The decrements in the mobility band gap energy values can be explicated by the fact that increasing in the CuCl₂ content could lead to the formation of ionic complexes, disorder, and imperfections in the structure of the host polymer, leading to create new localized states of various depths in the forbidden band gap energy. This usually contributes to the decrease in the optical band gap energy²³.

The extinction coefficient (k) is calculated using equation (5). The change of the extinction coefficient for the PVA:CuCl₂ films with different concentrations of CuCl₂ salts as a function of the wavelength is shown in Figure 4. It can be noted that the extinction coefficient has lowering values at low concentrations, but it increases with increasing the weight percentage of the added CuCl₂. This is attributed to an increased absorption coefficient with an increased percentage of added CuCl₂.

Figure 4
The Extinction coefficient for PVA:CuCl₂ films as a function of the wavelength.

The refractive index (n) is calculated from equation (6). Figure 5 shows the variation of refractive index for PVA:CuCl₂ films with different concentrations of CuCl₂ salts as a function of the wavelength. From the figure, we can see that the refractive index increases with increasing CuCl₂ concentrations. The reason for this result is an increase in the number of free electrons²⁴. The refractive index decreases with increasing wavelength, this behavior may be a result of the variation of the absorption coefficient which leads to spectral deviation in the location of the charge polarization at the attenuation coefficient due to the losses in the energy of the electron transition between the energy bands.

Figure 5
The variation of the refractive index (n) with UV wavelength.

The real and imaginary parts of the dielectric constant (ε', ε") for PVA:CuCl₂ films with different concentrations of CuCl₂ have been calculated by using equations (7) and (8), respectively. Figure 6 and Figure 7 show the change of (ε') and (ε") as a function of the wavelength. It can be seen that (ε') and (ε") increase with increasing the concentration of added CuCl₂, and this behavior is like (n) and (k) because (ε') depends on (n²) due to low value of (k²), while (ε") is dependent on (k) value that change with the change of the absorption coefficient due to the relation between (α) and (k)²⁵.

Figure 6
The real part of dielectric constant for PVA:CuCl₂ films as a function of the wavelength.

Figure 7
The imaginary part of dielectric constant for PVA:CuCl₂ films as a function of the wavelength.

The dispersion parameters E_o and E_d of the PVA:CuCl₂ films were evaluated by using equation (9) according to the Wemple DiDomenico model. E_d represents the dispersion energy associated with the average strength of the optical transitions and E_o simulates the excitation of electrons²⁶. The dispersion energy E_d, which is a measure of the inter-band optical transition intensity, is a significant factor in calculating the materials dispersion parameters²⁷,²⁸. Plotting (n²-1)^-1 versus (ћω)² for PVA:CuCl₂ films, we have got straight lines as shown in Figure 8. The values of E_o and E_d were calculated from the slope and the intercept (E_o/E_d) on the vertical axis, and then listed in Table 2 which shows that E_o and E_d values increased with the CuCl₂ concentration while the optical band gap values decreased.

Figure 8
The variation of (n²-1)^-1 with (ћω)².

Table 2
The dispersion parameters E₀ and E_d of the PVA:CuCl_2.

CuCl2 wt%	Single-oscillator energy parameter Eo (eV)	Dispersion energy parameter Ed (eV)
Pure PVA	4.14	4.56
1	4.61	10.84
3	4.94	17.61
5	5.14	26.72
7	5.22	32.54
12	5.64	42.53

4. Conclusions

In this work, the effect of CuCl₂ impurity on the optical properties of PVA films has been studied using the solution cast technique. The absorbance and the absorption coefficient for PVA:CuCl₂ films increase with increasing the filler content (wt. %). The optical constants of polyvinyl alcohol (absorption coefficient, refractive index, extinction coefficient, real and imaginary dielectric constant) are increasing with the increase of concentrations of CuCl₂.

In general, the energy band gap of polyvinyl alcohol decreases with the increase of concentrations of CuCl₂, while the energy gap tail ΔE values increase.

The values of dispersion energy E_d and single-oscillator energy E_o parameters according to the Wemple DiDomenico model were determined for the PVA:CuCl₂ thin films. These values increase with the increase of the CuCl₂ concentrations.

5. References

• ¹ Taha TA. Optical properties of PVC/Al2O3 nanocomposite films. Polym Bull. 2019;76(2):903-18. http://doi.org/10.1007/s00289-018-2417-8
  » http://doi.org/10.1007/s00289-018-2417-8
• ² Shehap AM, Akil DS. Structural and optical properties of TiO 2 nanoparticles/PVA for different composites thin films. International Journal of Nanoelectronics & Materials. 2016;9(1):17-36.
• ³ Ali HE, Khairy Y. Optical and electrical performance of copper chloride doped polyvinyl alcohol for optical limiter and polymeric varistor devices. Physica B. 2019;572:256-65. http://doi.org/10.1016/j.physb.2019.08.014
  » http://doi.org/10.1016/j.physb.2019.08.014
• ⁴ Abdullah OG, Aziz SB, Rasheed MA. Structural and optical characterization of PVA: KMnO4 based solid polymer electrolyte. Results Phys. 2016;6:1103-8. http://doi.org/10.1016/j.rinp.2016.11.050
  » http://doi.org/10.1016/j.rinp.2016.11.050
• ⁵ Devi CU, Sharma AK, Rao VN. Electrical and optical properties of pure and silver nitrate-doped polyvinyl alcohol films. Mater Lett. 2002;56(3):167-74. http://doi.org/10.1016/S0167-577X(02)00434-2
  » http://doi.org/10.1016/S0167-577X(02)00434-2
• ⁶ Raj CJ, Varma KBR. Synthesis and electrical properties of the (PVA) 0.7 (KI) 0.3· xH2SO4 (0≤ x≤ 5) polymer electrolytes and their performance in a primary Zn/MnO2 battery. Electrochim Acta. 2010;56(2):649-56. http://doi.org/10.1016/j.electacta.2010.09.076
  » http://doi.org/10.1016/j.electacta.2010.09.076
• ⁷ Singh P, Saroj AL. Effect of SiO2 nano-particles on plasticized polymer blend electrolytes: vibrational, thermal, and ionic conductivity study. Polymer-Plastics Technology and Materials. 2021;60(3):298-305. http://doi.org/10.1080/25740881.2020.1793202
  » http://doi.org/10.1080/25740881.2020.1793202
• ⁸ Awada H, Daneault C. Chemical modification of poly (vinyl alcohol) in water. Appl Sci (Basel). 2015;5(4):840-50. http://doi.org/10.3390/app5040840
  » http://doi.org/10.3390/app5040840
• ⁹ Tweissi M, Khoshman JM, Miqdad H, Adaileh A. Determination of Lorenz number of composite films based on copper chloride (II) in polyvinyl alcohol matrix. Plast Rubber Compos. 2024;53(1):36-42. http://doi.org/10.1177/14658011231212629
  » http://doi.org/10.1177/14658011231212629
• ¹⁰ Hashim A, Marza A, Rabee BH, Habeeb MA, Abd-Alkadhim N, Saad A. Optical Properties of (PVA-CrCl₂) Composites. Chemistry and Materials Research. 2013;3(7):59-64.
• ¹¹ Salman SA, Bakr NA, Mahmood MH. Preparation and study of some electrical and optical properties of (PVA-NiCl₂‎) composites. Journal of Garmian University. 2015;2(38):1152-65.
• ¹² Al-Tweissi M, Ayish IO, Al-Ramadin Y, Zihlif AM, Elimat ZM, Al-Faleh RS. Electrothermal and optical properties of hybrid polymer composites. Journal of Nano and Electron Physics. 2018;10(2):2006. http://doi.org/10.21272/jnep.10(2).02006
  » http://doi.org/10.21272/jnep.10(2).02006
• ¹³ Davis EA, Mott N. Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors. Philos Mag. 1970;22(179):0903-22. http://doi.org/10.1080/14786437008221061
  » http://doi.org/10.1080/14786437008221061
• ¹⁴ Tauc J. Amorphous and liquid semiconductors. New York, NY: Plenum; 1974. http://doi.org/10.1007/978-1-4615-8705-7
  » http://doi.org/10.1007/978-1-4615-8705-7
• ¹⁵ Al-Tweissi M, Tarawneh MA, Owaidat MQ, Alsboul M. optical characterization of thin films poly (ethylene oxide) doped with cesium iodide. Journal of Nano and Electron Physics. 2018;10(5):05016. http://doi.org/10.21272/jnep.10(5).05016
  » http://doi.org/10.21272/jnep.10(5).05016
• ¹⁶ Yahia IS, Farag AAM, Cavas M, Yakuphanoglu F. Effects of stabilizer ratio on the optical constants and optical dispersion parameters of ZnO nano-fiber thin films. Superlattices Microstruct. 2013;53:63-75. http://doi.org/10.1016/j.spmi.2012.09.008
  » http://doi.org/10.1016/j.spmi.2012.09.008
• ¹⁷ Qwasmeh AAH, Abu Saleh BA, Al-Tweissi M, Tarawneh MAA, Elimat ZM, Alzubi RI, et al. Effects of gamma irradiation on optical properties of poly (ethylene oxide) thin films doped with potassium iodide. Journal of Composites Science. 2023;7(5):194. http://doi.org/10.3390/jcs7050194
  » http://doi.org/10.3390/jcs7050194
• ¹⁸ Kadhim KJ, Agool IR, Hashim A. Enhancement in optical properties of (PVA-PEG-PVP) blend by the addition of titanium oxide nanoparticles for biological application. Adv Environ Biol. 2016;10(1):81-8.
• ¹⁹ Wemple SH. Refractive-index behavior of amorphous semiconductors and glasses. Phys Rev, B, Solid State. 1973;7(8):3767-77. http://doi.org/10.1103/PhysRevB.7.3767
  » http://doi.org/10.1103/PhysRevB.7.3767
• ²⁰ Alwan TJ. Refractive index dispersion and optical properties of dye doped polystyrene films. Malaysian Polymer Journal. 2010;5(2):204-13.
• ²¹ Praveena SD, Ravindrachary V, Bhajantri RF, Ismayil. Dopant‐induced microstructural, optical, and electrical properties of TiO2/PVA composite. Polym Compos. 2016;37(4):987-97. http://doi.org/10.1002/pc.23258
  » http://doi.org/10.1002/pc.23258
• ²² Mahendia S, Chahal RP, Tomar AK, Wadhwa H, Kumar S. (2019). Radiation-induced effects on the properties of polymer-metal nanocomposites. In: Kumar V, Chaudhary B, Sharma V, Verma K., editors. Radiation effects in polymeric materials. Cham: Springer. p. 141-92. http://doi.org/10.1007/978-3-030-05770-1_5
  » http://doi.org/10.1007/978-3-030-05770-1_5
• ²³ Sbeih SA, Zihlif AM. Optical and electrical properties of kaolinite/polystyrene composite. J Phys D Appl Phys. 2009;42(14):145405. http://doi.org/10.1088/0022-3727/42/14/145405
  » http://doi.org/10.1088/0022-3727/42/14/145405
• ²⁴ Harun MH, Saion E, Kassim A, Mahmud E, Hussain MY, Mustafa IS. Dielectric properties of poly (vinyl alcohol)/polypyrrole composite polymer films. J. Adv. Sci. Arts. 2009;1(1):9-16.
• ²⁵ Asogwa PU. Band gap shift and optical characterization of PVA-capped PbO thin films: effect of thermal annealing. Chalcogenide Lett. 2011;8(3):163-70.
• ²⁶ Wemple SH, DiDomenico M Jr. Behavior of the electronic dielectric constant in covalent and ionic materials. Phys Rev, B, Solid State. 1971;3(4):1338-51. http://doi.org/10.1103/PhysRevB.3.1338
  » http://doi.org/10.1103/PhysRevB.3.1338
• ²⁷ Abass KH, Latif DM. The urbach energy and dispersion parameters dependence of substrate temperature of CdO thin films prepared by chemical spray pyrolysis. Int J Chemtech Res. 2016;9(9):332-8.
• ²⁸ Zbala AAK, Al-Ogaili AOM, Abass KH. Optical properties and dispersion parameters of PAAm-PEG polymer blend doped with antimony (III) oxide nanoparticles. Neuroquantology. 2022;20(2):62-8. http://doi.org/10.14704/nq.2022.20.2.NQ22026
  » http://doi.org/10.14704/nq.2022.20.2.NQ22026

Publication Dates

• Publication in this collection
  23 May 2025
• Date of issue
  2025

History

• Received
  28 Jan 2025
• Reviewed
  03 Apr 2025
• Accepted
  21 Apr 2025