Spectroscopic Studies on Films of Lead NitrateDoped Polyvinyl Alcohol – Polyvinyl Pyrrolidone Blend

Preeti B. Hammannavar and Blaise Lobo*

Department of Physics, Karnatak University’s Karnatak Science College, Dharwad 580 001, Karnataka, India

Corresponding author Email: blaise.lobo@gmail.com

DOI : http://dx.doi.org/10.13005/msri/150107

ABSTRACT:

Films of polyvinyl alcohol (PVA) – polyvinyl pyrrolidone (PVP) blend doped with lead nitrate (Pb(NO₃)₂) were prepared by solution casting method, with doping level (DL) of Pb(NO₃)₂ in PVA – PVP polymeric blend varying from 2.7 up to 50.5 wt%. The prepared films were characterized using UV-Visible spectroscopy, fluorescence (or photo- luminescence) spectroscopy, FTIR and Raman spectroscopy. Parameters like optical band gaps due to indirect allowed transition (IAT) and direct allowed transition (DAT), as well as the activation energy (E_a) for optical transitions are extracted from the UV- Visible (or optical) spectra of these films. Objective of the FTIR, Raman and Fluorescence spectroscopic study is to understand the molecular chemical changes in PVA and PVP caused by the dopant (Pb(NO₃)₂). There is an enhancement of fluorescence intensity at DLs of 2.7 wt%, and at DL 7.6 wt%, the fluorescence intensity decreases (quenching). PVP acts as a capping agent for the dopant species, and is considered to be responsible for changes in fluorescence intensity. Signature of nitrate ion (NO₃^-) vibration is observed at wavenumber 1037 cm^-1 in Raman spectra, and in the FTIR spectra, the NO₃^- vibration is seen at the wavenumber equal to 1380 cm^-1.

KEYWORDS: Direct allowed transition; Doping; Raman shift; FTIR assignments; Indirect allowed transition; Lead Nitrate; PVA -PVP blend; UV-Visible spectroscopy

Copy the following to cite this article: Hammannavar P. B, Lobo B. Spectroscopic Studies on Films of Lead NitrateDoped Polyvinyl Alcohol – Polyvinyl Pyrrolidone Blend. Mat.Sci.Res.India;15(1)

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Introduction

Polymer composites have drawn the attention of researchers due to their potential applications in different areas.¹ Polymer composites having lead (Pb) salts as a component are important for primary and secondary protection from gamma photons and X-rays, when a shielding material is required to prevent the effect of harmful radiation on personnel, as well as other users and materials (or equipment).^2,3 Research investigations reveal that polymer composites are dual purpose shielding materials, considering the partnership of hydrogen rich polymer for slowing down neutrons⁴ and the metallic additives (for attenuating gamma photons). Hence, a polymer composite with high atomic number (Z) component is considered as a potentially good solution for the radiation shielding problem. Considering all the high – Z components or elements commonly used for shielding applications, Pb is commonly used for gamma shielding applications, because of its availability and cost effectiveness in comparison with other metals.⁵ Keeping this in mind, lead nitrate (Pb(NO₃)₂) was used to dope the selected polymeric blend of polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP).

PVA is a semi-crystalline polyhydric alcohol, which is soluble in water. Due to its several interesting physical properties, PVA (un-doped as well as doped with inorganic salts) has been studied in detail. It has many industrial and technical applications, including those in biochemical and medical fields.⁶ A unique feature of semi-crystalline PVA is the presence of crystalline and amorphous regions, which results from crystalline -amorphous interfacial effects, the presence of inter-molecular and intra-molecular hydrogen bonding. Boundary of the crystalline and amorphous regions is found to have portions with an intermediate degree of ordering, which complicates the microstructure, producing several crystalline and amorphous phases.⁷ The doping of PVA films with suitable chemical agents results in a material having a strong dependence of properties on donor-acceptor mechanism. Commonly, PVA is a poor electric conductor; only on doping or adding suitable filler (say, an inorganic salt), PVA can inherit an improved electrical conductivity. Along with complex formation, the physical interactions between polymer chains and dopant (or filler) via hydrogen bonding with hydroxyl groups is noticed in doped PVA.^8-13

PVP is a water soluble polymer. It is hygroscopic in nature, with properties like adhesiveness, dye complexing ability, film forming nature and behavior as a protective colloid.¹⁴ Due to its wide range of applications in medical domain and a structure which is similar to that of proteins, it is interesting to researchers from a medical (and biological) point of view.¹⁵ The peptide bond in lactum ring results in presence of highly polar side groups in PVP. The presence of rigid pyrrolidone group in PVP results in its amorphous nature. Also, PVP doped with an inorganic salt (cobalt chloride) and iodine can be used as a humidity sensor.¹⁶

The carbonyl group of PVP interacts through hydrogen bonding with the hydroxyl group of PVA, which results in the good miscibility of PVA-PVP blends.^17-19 In fact, interesting properties result from the formation of polymer blends, with modified properties, which combine the features of both constituent polymers. Hydrogen bonding also plays an important role for the solubility of both PVA and PVP in water. In medicine, PVA-PVP blend has application as a skin dressing component, and as membranes. Doped films of polymeric blends with different inorganic salts^20-23 have gained importance due to their modified properties and potential multi-functional applications.

Hitachi U 3310 UV-Vis-NIR spectrometer was used in wavelength range 200 – 800 nm for recording the optical absorption (UV-Vis-NIR) spectra. Details about the procedure adopted for analyzing optical spectra are elaborated by us, elsewhere.^24,25

UV- Visible (Optical) Spectrum of a sample is a plot of Absorbance (A) versus Wavelength (λ). The Absorbance of the sample is given by equation 1. The UV-Visible spectra of Pb(NO₃)₂ doped PVA-PVP blend films are shown in Fig.1.

In equation 1, I and I_o are the transmitted and incident light intensities respectively at a particular wavelength of incident electromagnetic radiation. The absorption coefficient α is obtained using equation 2.

 

In equation 2, d is the thickness of the polymer film, expressed in centimeter (cm).

The absorbance edges (rising and falling edges) of each absorbance peak in the UV-Visible spectra of Pb(NO₃)₂ doped PVA-PVP blend (sample) is linearly fit, and the x-intercept value (that is, wavelength, , corresponding to the absorption edge ), in each case, is determined using equation 3. See Figure 2a and Figure 2b for the illustrative diagrams.

The energy of incident electromagnetic radiation (photon) which is absorbed by the sample to cause an electronic transition from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) or other intermediate energy levels is given by equation 4.

In equation 4, h denotes the Planck’s constant and c is the velocity of electromagnetic radiation in vacuum.

The activation energy (E_a or E_g) for optical transition is calculated using equation 5, wherein, the value of wavelength (λ_g) corresponding to absorption edge is substituted in nanometres (nm), to obtain the value of E_g in electron volts (eV). 

Table 1 lists the values of E_a, for films of Pb(NO₃)₂ doped PVA-PVP blend, with doping level (DL) varying from 2.7 up to 50.5 wt%. The Absorption coefficient obeys the well – known Mott Davis model.

In equation 6, B is the disorder parameter, and E_g1 is the optical energy band gap. The value of m is 1/2, 3/2, 2 and 1/3 for direct allowed transition, direct forbidden transition, indirect allowed transition and indirect forbidden transition, respectively. Taking m = 2, a plot of (αhν)^1/2 Vs hν is obtained, and is found to give the best fit, for Pb(NO₃)₂ doped PVA-PVP blend films. The linear fit, in the high absorption region can be used to obtain the optical band gap for indirect allowed transitions (IAT) in k-space, denoted by E_IAT (= E_g1 of the material for IAT). It was observed that the absorption edge gives a good fit for both indirect allowed transitions and direct allowed transitions in k-space. In equation 6, taking m = 1/2, a plot of (αhν)² Vs hν is obtained, and is also found to give a good fit, for films of Pb(NO₃)₂  doped PVA-PVP blend. The linear fit, in the high absorption region can be used to obtain the optical band gap for direct allowed transitions (DAT) , denoted by E_DAT (= E_g1 of the material for DAT).

Fluorescence Spectra of the composite films have been recorded with F-7000 FL spectrophotometer, in the wavelength range 200-800nm. NXR FT-Raman module spectrometer, at 4 cm^-1 resolution, was used to acquire Raman spectra of the Pb(NO₃)₂ filled PVA-PVP composite films, in the wavenumber range 100-3750 cm^-1.

 FTIR spectra of Pb(NO₃)₂ filled PVA-PVP composite were recorded in the wavenumber range 400-4000 cm^-1, using NICOLET 6700 FTIR Spectrometer, following the potassium bromide (KBr) pellet method.

Results and Discussions

UV-Visible Studies

The UV-Visible spectra of  films of Pb(NO₃)₂ doped PVA-PVP blend are analysed to yield the optical band gaps due to indirect allowed transition (IAT) and direct allowed transition (DAT), as well as the activation energy (E_a or E_g) for optical transitions (see the ‘instrumentation and analysis’ section, for details of UV – Visible spectral analysis procedure). The optical band-gap for inter-band transition is 4.8 eV for film with doping level (DL) equal to 2.7 wt% (concentration of Pb(NO₃)₂ in PVA-PVP blend, expressed in weight percentage (wt%)). The band gap increases to 5.4 eV, at 12 wt% DL and then decreases on further doping, to a low of 4.2 eV at 45.2 wt% DL. The optical spectra show multiple transitions at intermediate doping levels, with activation energies observed between 3.2 eV and 3.4 eV, due to additional energy bands created by the dopant species (Pb(NO₃)₂) in the HOMO-LUMO gap in the band-structure of PVA – PVP blend.

Figure 1: UV-Visible spectra of Pb(NO₃)₂ doped PVA-PVP blend films, with DL varied from 2.7 wt% up to 50.5 wt%.

Click on image to enlarge

 

Table 1: Activation energy (E_a or E_g) for optical transitions, for films of Pb(NO₃)₂ doped PVA-PVP blend, with doping level (DL) varying from 2.7 up to 50.5 wt%.

Doping Level DL (wt%)	Ea1 (eV)	Ea2 (eV)	Ea3 (eV)	Ea4 (eV)
0.0	4.1	2.5	2.2	—
2.7	4.3	3.3	1.9	—
7.6	5.6	5.5	4.8	—
12.1	5.4	5.5	4.8	2.2
16.2	4.7	3.2	—	3.9
19.8	4.7	3.7	—	3.2
23.3	5.2	5.4	4.7	3.3
26.4	5.3	5.3	4.8	4.3
32.0	5.1	5.4	4.7	3.4
36.4	4.4	4.3	4.0	3.2
42.7	5.2	5.3	4.7	3.2
45.5	4.2	3.5	—	1.5
50.5	4.4	3.3	4.0	2.0

 

Figure 2: Determination of activation energies (Eg) for optical transitions from UV-Visible spectra of Pb(NO₃)₂ doped PVA-PVP blend films with DL of  a) 7.6 wt%  and b) 50.5 wt%. Ratio of the negative Y – intercept to the slope gives value of wavelength corresponding to the absorption edge (λ_g or λ_i; I = 1,2,3), from which E_g can be determined using equation 5

Click on image to enlarge

 

Table 2: Optical band gaps (E_d and E_IaT) corresponding to direct allowed transition (DAT) and indirect allowed transition (IAT) in Pb(NO₃)₂ doped PVA-PVP blend films, with doping level (DL) varying from 2.7 up to 50.5 wt%.

DL (wt%)	Ed1 (eV)	Ed2 (eV)	Ed3 (eV)	EIaT1 (eV)	EIaT2 (eV)	EIaT3 (eV)
2.7	5.03	4.0	—	0.9	—	—
7.6	5.6	4.9	—	5.4	—	—
12.1	5.4	4.7	—	5.2	4.6	—
16.4	—	4.7	—	4.6	2.1	—
19.8	—	4.7	—	4.5	1.6	—
23.3	5.3	4.7	—	5.1	4.5	—
26.4	5.4	4.8	—	5.0	4.1	2.5
32.0	5.2	4.6	—	4.9	4.4	2.3
36.7	—	4.4	3.9	4.2	3.9	—
42.7	5.2	4.6	—	4.9	4.5	0.6
45.5	—	4.3	3.9	4.2	2.9	—
50.5	—	4.4	3.9	4.3	3.8	1.2

 

Fluorescence Spectroscopy

The emission (fluorescence or photo-luminescence) spectra of Pb(NO₃)₂ filled PVA-PVP blend films are shown in figure 2. The excitation wavelength was fixed at 350 nm for all the samples. Two emission peaks are observed at 412 nm and 438 nm, along with a just resolved peak at 468 nm. Multi-peak Gaussian fitting method was used to identify the peaks; this analysis fetched important parameters, namely, width of the emission peak, its height and area under the peak (values are listed in table 3). The presence of carbonyl (C=O) group in both PVA (due to un-hydrolyzed acetyl groups) and PVP, enables transfer of electron to the dopant ion (Pb²⁺). The fluorescence peak at DL of 2.7 wt% shows an enhancement in fluorescence intensity, due to formation of micro-crystallites of the dopant species. In the photo-luminescence (PL) spectra, for the film with DL of 7.6 wt% the fluorescence intensity reduces, whereas the fluorescence intensity increases once again at DL 12.1 wt%. For films with DL varying from 19.8 wt% up to 42.7 wt%, there is lot of variation in fluorescence intensity wherein the PVP acts as capping agent, as confirmed from FTIR spectra.    As we move on to higher doping regions, it is noticed that there is quenching of emission (or PL peak) at 438 nm and fluorescence at wavelength of 412 nm is found to have enhanced.

Figure 3: Fluorescence spectra of Pb(NO₃)₂ doped PVA-PVP blend films, with DL varied from 2.7 wt% up to 50.5 wt%.

Click on image to enlarge

 

Table 3: The table shows area under the emission (fluorescence) peak, its width, centre and height, as obtained from Gaussian fitting of the PL spectra of Pb(NO₃)₂ doped PVA-PVP blend films, with DL varied from 2.7 up to 50.5 wt%.

Doping Level,  DL (wt%)	Emission Band No.	Area	Center p (nm)	Peak energy, Ep (eV)	FWHM    λ (nm)	Width of energy band, E (eV)	Height (Counts)
0.0	1	13039	432	2.9	82.0	15.1	127
	2	1149	413	3	13.8	89.9	66
	3	2367	433	2.9	29.8	41.6	63
2.7	1	14309	416	2.9	16.34	75.9	699
	2	17435	436	2.8	21.47	57.8	648
	3	44064	455	2.8	51.14	24.2	687
7.6	1	849	415	2.9	17.88	69.4	38
	2	438	437	2.8	17.34	71.5	20
	3	2370	451	2.7	52.03	23.8	36
12.1	1	14346	416	2.9	16.5	75.2	692
	2	17677	436	2.8	22.5	55.1	628
	3	41872	454	2.7	52.1	23.8	642
16.2	1	3279	415	2.9	17.8	69.7	147
	2	2129	436	2.8	19.2	64.6	88
	3	8341	452	2.7	52.3	23.7	128
19.8	1	1108	415	2.9	19.8	62.6	45
	2	1038	438	2.8	21.8	56.9	38
	3	776	464	2.7	27.9	44.4	22
23.3	1	242	415	2.9	17.8	69.7	11
	2	366	438	2.8	28.4	43.7	10
	3	126	468	2.6	18.5	67.0	6
26.4	1	53	438	2.8	13.1	94.7	3
	2	231	416	2.9	18.8	65.9	10
32.0	1	4948	417	2.9	16.5	75.2	240
	2	7117	437	2.8	22.6	54.9	251
	3	18138	456	2.7	53.4	23.2	271
36.7	1	250	416	2.9	0.3	4133.3	674
	2	4	408	3	0.5	2480	7
42.7	1	969	417	2.9	18.3	67.8	42
	2	912	438	2.8	20.9	59.3	35
	3	1069	462	2.6	35.3	35.1	24
45.5	1	2839	417	2.9	16.7	74.3	135
	2	37776	438	2.8	22.3	55.6	135
	3	8132	456	2.7	50.1	24.8	130
50.5	1	4235	417	2.9	16.6	74.7	204
	2	5640	438	2.8	22.6	55.6	199
	3	12532	457	2.7	51.4	24.1	194

 

It is well known that radiative energy transfer and non-radiative energy transfer are two ways to transfer energy, after molecules of the sample (PVA and PVP) are excited on absorption of incident electromagnetic energy (electronic transition; HOMO to LUMO). The processes are illustrated by the Jablonskii diagram, commonly used in the understanding of fluorescence spectroscopy. In radiative energy transfer, the de-excitation of fluorophore (which loses its energy) from excited state to ground state takes place by ejecting a photon (emission of electromagnetic radiation).²⁶ There is an enhancement of fluorescence intensity at DLs of 2.7 wt%, and at DL 7.6 wt%, the fluorescence intensity decreases (quenching).

Quenching of the fluorophore in Pb(NO₃)₂ doped PVA-PVP blend films could due to transfer of electrons from molecules of the host polymer matrix (PVA and PVP) to dopant ions (Pb²⁺), resulting in formation of Pb (reduction); hence, these electrons are not available for the excitation and de-excitation processes (during PL study).

Figure 4: Variation of peak of the emission (or PL) spectra of Pb(NO₃)₂ doped PVA-PVP films, at three different emission peak wavelengths (λ_i ; i = 1, 2 and 3), relative to DL

Click on image to enlarge

 

Raman Spectroscopy

Raman spectra ofPb(NO₃)₂ doped PVA-PVP films show dominant peaks at 2913 cm^-1 and 1437 cm^-1 due to CH₂ stretching and CH₂ deformation. The signature of the nitrate ion (NO₃^–)  is seen as a sharp peak (with increasing intensity, with increase in DL), with Raman shift lying between 1038 cm^-1 and 1045 cm^-1 (as shown in fig 5) for filler levels varying from 26 up to 45 wt%, suggesting phase separation at these higher dopant concentrations.

CH₂ and C=O stretch, scissor

In the Raman spectra of Pb(NO₃)₂ doped PVA – PVP blend films, a sharp peak in the wavenumber range varying from 2921 cm^-1  up to 2922 cm^-1  is attributed to CH₂ group of PVP. The corresponding wavenumber does not change much on increasing the DL of Pb(NO₃)₂ in PVA – PVP from 2.7 up to 50.5 wt%, but the intensity of this peak increases on increasing from 2.7 wt% to 16.2 wt%, and on further addition of Pb(NO₃)₂ in the polymer matrix (PVA – PVP), the intensity decreases. As PVA and PVP are involved in the formation of charge transfer complex with the dopant ion (Pb²⁺) on interaction with carbonyl (C=O) groups, the prominent peak in the wavenumber range varying from 1629 cm^-1 to 1655 cm^-1 is attributed to C=O group of un-hydrolyzed parts of PVA (acetyl group) and the C=O group of PVP; the changescan be attributed to the change in C=O to C-O group on interaction of Pb²⁺ with C-O‾ (see reaction scheme in figure 7).  The presence of a band around 1437 cm^-1isassigned to CH₂ scissor vibration.

C-C chain, ring and breathing

Three different peaks at 751 cm^-1, 849 cm^-1 and 927 cm^-1 are due to C-C chain in polymer backbone. In doped polymers, the peak frequency varies, as indicated in table 4. Due to inter/intra molecular interaction between dopant ions (Pb²⁺ and NO₃‾) with polymeric molecules (PVA and PVP), the shift in these wavenumbers are observed.

NO₃‾ ion vibration

Peaks ranging from 1037 cm^-1  to 1046 cm^-1   confirms the presence of  NO₃‾  group in the prepared Pb(NO₃)₂ filled PVA-PVP composite.

Table 4: Raman peak assignments of Pb(NO₃)₂ doped PVA-PVP blend films, with DL varied from 2.7 wt% up to 50.5 wt%.

Peak Wavenumber (cm-1)	Raman Assignments	Peak Raman shifts at differentDLs ( cm-1)
292   02912	ν s  CH2 group of PVP	2912-2918
2191	ν s  (CH2)	2118-2203
1661	ν s (C=O)	1629-1658
1436	δ(CH2), δ (OH) Bending	1437-1442
1226	C-C structure of  γω(CH2)	1226-1234
1115	ν (C=O); δ (CO+OH)	1099-1106
1360	COO stretch	1311-1373
1045	NO3‾    ion vibration	1036-1046
927	γ (C-O); δ (CO+OH)	921-938
849	ν (C-C) , CCH aliphatic	845-850
751	ν (C-C) (C=O) deformation	744-748

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