Experimental and DFT studies on the structural and optical properties of chitosan/polyvinyl pyrrolidone/ZnS nanocomposites

Chitosan/Polyvinyl pyrrolidone (CS/PVP) semi-natural polymeric blend involving gradient concentrations of ZnS nanoparticles (ZnS-NPS) was prepared via a simple casting method. In conjunction with computational density functional theory approaches (DFT), prepared samples were characterized by UV/Vis spectrophotometric studies and Fourier transform infrared measurements (FTIR) to take into account a detailed description of the different reaction mechanisms within the polymeric matrices. To conduct all calculations, the Becke three-parameter hybrid functional (B3LYP) correlation function used with the electron core potential basis set LANL2DZ was used. A detailed study of different reaction regimes was studied and reaction via Oxygen was observed to be preferred and compatible with that of the experimental data. UV/vis. Absorption experimental data were used to calculate the optical energy gap using the Mott-Davis equation and observed data was found to follow an indirect transition route.


Introduction
Polymer blending is an enticing route for creating new polymeric materials with customized properties that are preferable to that of each polymer product.A polymer blend (PB) is a combination of at least two polymers or copolymers mixed to form a new substance with specific physical characteristics, in which the content of the ingredient is greater than 2 wt%.This behavior depends on the solubilities between the components of the polymer mixture.The blending of two or more polymers becomes very essential to improve production efficiency by manufacturing products with a complete range of desired properties, improving specific properties, mixing with a more rigid and heat-resistant resin can result in better modulus and dimensional stability but provide means for the regeneration of industrial and/or municipal waste plastic [1][2][3].
The second most frequent biopolymer of Polysaccharides of biological origin is chitosan present in nature (after cellulose).It was derived from shells of shrimp, krill crab, lobster, and prawn and can be synthesized usage of chitin as a raw resource through a deacetylase reaction [4].Chitin contains good chemical composition and strong membrane-forming properties and is soluble in water solutions with low acidity, renewable, and non-toxic.This is a perfect biodegradable that can be deposited underground without disturbing the normal circulation of the environment.Due to the extreme properties of chitosan such as antimicrobial activity, biocompatibility, and non-toxicity, it is widely used in wound healing and dressing.The drawbacks of chitin include the ease of degradation by ultraviolet radiation, the difficulty of rotating leading to a lack of wet strength results, little water solubility, and the absence of processability due to low heat resistance.Chitosan was used in various areas of use such as product packaging, isolation or purification, biomedical, and edible materials [5].
Polyvinyl pyrrolidone (PVP) is an amorphous, synthetic polymer and has high Tg values up to 170 C due appearance of a solid pyrrolidone group, which is heavy with the drawing group and is known to form separate complexes with other polymers.It exhibits high wetting properties and shapes films quickly in the solution.This works nicely as a paint or a coating additive.PVP as just a watersoluble polymer provides positive effects on safety, viscosity, absorbency, solubilization, and condensation, the most important characteristics of which are superior solubility and biological performance.Also, PVP possesses low toxicity and is used in a wide range of fields, including health-related domains, cosmetics, and medical, food packaging.However, issues related to the stable yet delicate nature of the PVP and its loss of robustness have led to difficulties in production.Due to its extremely low cytotoxicity, it is widely used in medicine.The other uses are in biological and pharmaceutical technology and electrochemical instruments (batteries, displays) [6][7][8].
Zinc sulfide nanoparticles (ZnS-NPS) are important group II-IV semiconductors with unique properties, which can be found in one of two structural forms cubic sphalerite or hexagonal wurtzite.The properties of ZnS are highly dependent on their size, structural form, and morphology.This non-toxic material, which is chemically more stable than other semiconductors, is characterized by a wide band-gap energy of ~ 3.7 eV.Because of these properties, ZnS nanoparticles can be used in both biomedical and optoelectronic applications, such as biosensors, bio-composites, cell tagging, light-emitting diode (LED), and apply these technologies to other fields, such as optoelectronics, marking, monitoring agents, information collection, optics, fluorescent probes, and drug distribution [9][10][11][12].

3
Polymer Bulletin (2023) 80:13279-13298 The presented work aims to effect gradually increased zinc sulfide nanoparticles on the structure and physical properties of a semi-natural polymer blend comprising 80% polyvinyl pyrrolidone and 20% of chitosan.

Sample preparation and characterization
Studied samples were synthesized in the form of thin membrane films using the traditional solvent casting and evaporation route.A calculated amount of the Cs was dissolved through vigorous stirring in (50 mL) of a 2% aqueous solution of acetic acid and mixed with a specific amount of PVP and stirred for 60 min.Then the mixed solution was dropped on clean Petri dishes and dried at 50 °C for 24 h.PVP/ CS (80/20) blend was prepared with various loading concentrations of zinc sulfide nanoparticles (ZnS-NPS) as shown in Table 1.All samples were prepared via the same route and dropped on clean Petri dishes and dried at 50 °C.
The FTIR optical absorption spectra were recorded under the spectral spectrum of 4000-400 cm −1 using a single beam Nicolet is 10 spectrometers, absorption mode with 32 scans and a resolution of 2 cm −1 to analyze to recognize vibration bands associated with major chemical groups, detect the molecular structure and intermolecular interaction between polymers.The optical properties of the samples were detected utilizing UV\Vis.spectrophotometer (V-570 UV/Vis-NIR, JASCO Corp) in the 200-1100 nm wavelength range at room temperature.Calculations were achieved using the Gaussian 09 software within the application of DFT.Density functional calculations have been employed to ensure reaction mechanisms through an agreement between the experimental measurement and theoretically calculated data.Both expected structures have been enhanced using Becke, 3-parameter

Fourier transforms infrared analysis (FTIR)
FT-IR spectroscopy is a very powerful technique to recognize vibration bands associated with major chemical groups and detect the molecular structure which means characterizing the assignment of bands for each sample and inter-molecular interaction between polymers.Figure 1 shows FT-IR absorbance spectra in the range 4000-500 cm −1 of pure PVP, CS, and CS/PVP composites films at room temperature are registered.The positions of the FTIR absorption band and their functions are shown in Table 2.For pure PVP; the spectra show broadband at about 3451 cm −1 is delegated the hydroxyl group (OH) stretching vibration because PVP is hygroscopic [13][14][15].In conjunction, four overlapping signals can be applied to the (CH) stretching modes: symmetrical CH 2 stretching at approximately 2885 cm −1 , asymmetrical CH 2 stretching at approximately 2943 cm −1 [16,17], peaks at 1446 cm −1 and 1370 cm −1 also referring to the CH deformation modes of the CH 2 group [18].The stretching vibration near 1662 cm −1 can be due to C = O in the pyrrolidone group [14,19,20].Besides, bending vibrations at 1291 cm −1 , which are connected to the C-N of the pyrrolidone structure, may be identified.It is remembered that PVP is just a bi-substituted amide, and characteristic absorption of amines at about 3400-3500 cm −1 has not been found [13,21].The band at 750 cm −1 corresponds to C-C chain.The bands at 655 cm −1 and 567 cm −1 correspond to N-C = O [15].For pure CS; the hallmark of absorption bands at 3424 cm −1 assigned to (O-H) overlapped with (N-H) stretching vibration [4,22], Also, the characteristic band at 2880 cm −1 assigned to (C-H) stretching [22][23][24], Also, the at 1653 cm −1 assigned to C = O stretching (amide II) O = C-NHR.The peak at 1574 cm −1 is assigned for (NH) bending (amide I) (NH2) [25].The results of the coupling of (N-H) angular deformation and (C-N) axial stretching at 1426 cm −1 and 1382 cm −1 [26], the absorption bands at 1160 cm −1 assign to (C-O-C bridge) anti-symmetric stretching [23], the peaks at 1076 cm −1 , 1035 cm −1 , and 665 cm −1 are characteristic of its saccharide structure (skeletal vibrations involving the C-O stretching) [22][23][24].For the PVP/CS blend; absorption spectra were seen at 3451 cm −1 , attributed to the (-OH) and (-NH) groups.Chitosan (-OH) and (-NH) absorption spectra groups at approximately 3424 cm −1 were observed to be drifting towards a higher frequency region with the inclusion of PVP to form a blended polymer.The prominent peaks at 2943 and 2884 cm −1 were assigned to the methylene group (CH 2 ) in the PVP and CS of asymmetrical and symmetrical stretching vibrations.The position of the characteristic band of chitosan at 1633 cm −1 is shifted in blends to a higher frequency (1662 cm −1 for the blend with composition 80/20).It indicates the presence of hydrogen bonding between PVP and CS.Chitosan, a hydrogen sender, forms a hydrogen bond with the PVP carbonyl group.Pyrrolidone rings in PVP contain a proton that embraces carbonyl moiety, which chitosan introduces as side groups of hydroxyl and amino.Hydrogen bonding interactions between these two chemical fractions can also take place in a mixture of CS and PVP.The existence of hydrogen bonds between two specific macromolecules competes with the formation of hydrogen bonds between molecules of the same polymer.The peak at 1430 cm −1 is the result of the coupling of N-H angular deformation and C-N axial stretching.The peak at 1291 cm −1 was due to C-N stretching vibration and this confirmed the presence of the amine group in pure in the membrane matrix.The peaks at 1076 and 1035 cm −1 (skeletal vibrations involving the C-O stretching) are characteristic of its saccharide framework.
Figure 2 indicates FTIR absorption in the region of 4000-500 cm −1 for comparing the spectra of a blend (PVP/CS) when blending with various concentration of zinc sulfide nanoparticles (ZnS-NPS) revealing that the spectral changes appear as a result of ZnS blending and exhibit the characteristic bands in the Table 3.
First, the stretching vibration of hydroxyl groups (O-H) locates at 3400-3500 cm −1 of (PVP/CS) appears as a sharp band after merging different concentrations of ZnS-NPs with a blend that becomes broad in sample S2 and then splits with increasing concentration of ZnS-NPS up to (0.1%).Second, the band at 1662 cm −1 decreases in intensity, and the width of the band increase with an increase in the concentration of ZnS-NPs which demonstrates the complexation between ZnS-NPs and (PVP/CS) polymer blend by a coordinate bond.Third, there was a strong shift in severity with a rise in the content of ZnS doping.The peaks at 1 3 Polymer Bulletin (2023) 80:13279-13298 1434 cm −1 and 1283 cm −1 become broad and decrease in intensity with increasing the concentration of ZnS-NPS up to (0.025%).

Density function theory (DFT)
The theoretical methodology is used to describe the framework of connection between polymeric matrices and to calculate the degree of agreement with experimental evidence for complicated interactions between components (Polyvinyl pyrrolidone and chitosan).All measurements were calculated using Gaussian 09 software within the DFT system.A blend of (PVP/CS) and blend-ZnS system were designed using the Becke three-parameter hybrid functional (B3LYP) correlation function used with the electron core potential basis collection LANL2DZ.
Figure 3 reveals an optimized 3D structure for Polyvinyl pyrrolidone monomer in combination with both experimental FT-IR and calculated infrared spectra.Experimental FT-IR spectral data when compared with the theoretically obtained data shows an extra new band located at about 3451 cm −1 .Such band can be attributed to water or OH groups results from the hygroscopic nature of the studied polymeric sample.Other bands can be attributed to their respective vibrational groups listed in the Table 2. Table 4 presents the assignment of the theoretical DFT and experimental FT-IR spectral band position of the PVP polymer.It was clear that a small shift was observed in the numerical values that may result from the matrix effect since calculations were made on a single unit (monomer).
Figure 4 reveals an optimized 3D structure for chitosan monomer in combination with both experimental FT-IR and calculated infrared spectra.Table 5 present the DFT and experimental FT-IR spectral band position of the Chitosan polymer.The next step is to study the interaction between the two studied polymers (PVP and Chitosan) through the introduction of both monomers.Figure 5 shows two separate probabilities of interaction between PVP and CS.The optimized structure in both cases was used to calculate the spectral FT-IR data to be compared with  the experimental data of the 80/20 PVP/Cs polymer blend.Such comparison may shed light on the more acceptable probability of the interaction.Figure 6 shows the experimental and theoretical FT-IR data corresponding to the two possible interaction possibilities.Obtained data reveal the appearance of the prominent peaks at their positions with different intensities and with and with higher compatibility in case of interaction between monomers through the (NH) groups (see Table 6).The next step is to study the probabilities of interaction between the ZnS dopant and the blend polymeric matrix.The interaction is supposed to be through both the Oxygen atom and the nitrogen atom as shown in Fig. 7.The figures also illustrate the suggested 2D structure along with the 3D structure obtained from the optimized geometry through DFT calculations for the interaction probabilities between PVP/ CS poly-blend and ZnS-NP.The optimized structure in both cases was used to calculate the spectral FT-IR data to be compared with the experimental data of the PVP/Cs/ZnSNP polymer nanocomposite.Such comparison may shed light on the more acceptable probability of the interaction.Figure 8 shows the experimental and theoretical FT-IR data corresponding to the two possible interaction possibilities.Obtained data reveals the appearance of the prominent peaks at their positions with higher compatibility in case of interaction through the Oxygen atom in the structure (see Table 7).

Optical properties in ultraviolet/visible regions
A valuable method for sample analysis is ultraviolet-visible (UV/Vis.)spectroscopy.it provides an understanding of the importance of optical parameters, including bandgap energy ( E g ) and the absorption of light energies by polymer composites in UV and visible areas, which include the movement of electrons from the ground  state (σ, π, and ƞ orbital) to higher energy states described by molecular orbitals [27].The result of absorption studies with UV/Vis.spectrophotometer in the wavelength 200-1100 nm carried out on pure blend and samples containing ZnS-NPS as illustrated in Fig. 9.The samples display only one absorption peak at approximately 230 nm with no other peak before the end of the measurements.This can be due to the translucent existence of both the PVP and the CS, and the previously reported data [28][29][30] referred to the absorption band at approximately 225 nm as a result of the π → π* electronic transition that comes from unsaturated bonds, mainly C = O bond which identified by in FT-IR at about 1662 cm −1 .Figure 9 reveals the absorption peaks of samples (S1, S2, S3, and S4) show the onset absorption peaks and increase in intensity with increasing concentration of ZnS-NPs this may be due to the interaction between the two (PVP/CS) blend and ZnS-NPs, and the value of wavelength are slightly increased.

Identification of the optical band gap ( E g )
The optical technique can analyze optically induced transitions and provide data on the bonding structure of organic compounds and the energy gap among crystalline and non-crystalline samples.Semiconductors are usually categorized into two kinds: (1) direct band spacing and (2) indirect band spacing.In the direct band spacing, the head of the valence band and the rest of the conduction band have the same momentum magnitude.If the bottom of the conduction band does not equate to zero crystal momentum, it is called indirect band spacing.From the optical absorption spectra using the Beer-Lambert relationship (1), the absorption coefficient can be calculated as follows: ( where A is the absorbance, log (I 0 /I) is defined where I 0 and I are the strength of the incident and transmitted beams, respectively and L is the film thickness in cm.Analysis of the spectral dependence of absorption at the absorption edge can determine the optical bandgap.Concerning optical transitions resulting from energy photons hν > E g , the present optical data can be analyzed for near-edge optical absorption according to the following relationship (2).
where E g is the magnitude of the optical energy gap, h is the energy of the incident photons, and r is the force that characterizes the transformation phase in K-space.In specific, depending on the type of electron transitions responsible for optical absorption, r will take the values 1, 2, 3, 1⁄2, and 3/2.It is valid that the value of r is 2 in the case of a direct electronic transition over a direct energy gap in K space and 1⁄2 in the case of an indirect electronic transition over an indirect energy gap.The factor β is based on the probability of transformation and can be assumed to be stable within the optical frequency spectrum.The usual procedure for calculating the value of E g involves plotting ( h ) r against (h ).The dependence of ( h ) r and photon energy (2) ( )h = (h − E g ) r  (h ) was plotted for the films analyzed using various values of r (1/2, 2).Close to the absorption edge for the present experimental results, the plots of (αhν) 1/2 and (αhν) 2  versus.(hν) of the absorption edge produce a linear fit over a broader range of hν, as seen in Fig. 10.
The E g values of the films were calculated from the linear part of these curves, and given in (see Figs. 11,12;Table 8. From this table, in general, the value of E g decreases with increasing ZnS-NPs content (see Table 9).

Scanning electron microscopy (SEM)
The surface morphology of the PVP/CS blend and PVP/CS/ZnS-NPs were examined by scanning electron microscopy.Figure 13 presents SEM images of a blend (PVP/CS) which show a slightly smooth appearance of the surface, when the addition of (ZnS-NPs) with different concentrations to blend, the surface becomes rough and grain size was formed and varied in their shape according to ZnS-NPs concentration, that's mean that the morphology of PVP/CS/ZnS-NPs is critically affected by the addition ZnS-NPs.

Conclusion
Infrared spectrophotometric measurements by Fourier transform (FTIR) show the maintenance of pure PVP, CS, and (PVP/CS) polymer blend characteristic bands in conjunction with computational density functional theory approaches (DFT) and does not show the existence of any new peaks.The proposed potential reaction mechanisms are performed by computational data and give the percentage compatibility between experimental and computed spectral data through function (B3LYP) and basis set LANL2DZ.Obtained data support a higher possibility of interaction between PVP and Cs monomers through the (NH) groups in the

Fig. 1 3 2
Fig.1FT-IR absorption spectra of pure the blend, pure PVP, and pure CS

Fig. 3
Fig. 3 3D chemical structure of PVP, DFT and experiment FT-IR spectra of PVP monomer

Fig. 6
Fig.6 Experimental data of 80/20 PVP/Cs polymer blend and theoretical FTIR of the two probabilities of interaction using DFT calculations

Fig. 7 3
Fig. 7 Graphical representation of the mechanism of interaction between the blend and ZnS-NP

Fig. 8 FT
Fig.8FT-IR Theoretical and Experimental spectra for the interaction between poly-blend and ZnS-NPs

Table 3
Assignments of the FT-IR characterization bands of the blend (PVP/CS) and PVP/CS/ZnS

Table 4
DFT and experimental FT-IR spectral data of PVP monomer