Experimental and DFT studies on the structural and optical properties of chitosan/polyvinyle pyrrolidone/ZnS nano composites


 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 for 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. Polymer blend (PB) is a combination of at least two polymers or copolymers mixed to form a new substance with speci c physical characteristics, in which the content of the ingredient is greater than2 wt.%. This behavior depends on the solubilities between the components of the polymer mixture. The blending of two or more polymers become very essential to improve production e ciency by manufacturing products with a complete range of desired properties, improving speci c 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 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 which is widely used in wound healing and dressing. The drawbacks of chitin include the ease of degradation by ultraviolet radiation, the di culty of rotating lead to a lack of wet strength results, have little water solubility, and absence of processability due to low heat resistance. Chitosan was used in various areas of use such as product packaging, isolation or puri cation, biomedical and edible materials [5].
Polyvinyl pyrrolidone (PVP) is an amorphous, synthetic polymer and has high Tg values up to 170C 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 lms quickly in the solution. This works nicely as a paint or a coating additive. PVP as just a water-soluble 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 elds, 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 di culties 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) [6-8].
Zinc sul de nanoparticles (ZnS-NPS) are important group II-IV semiconductors with unique properties, which can be found in one of the 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 bandgap 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 elds, such as optoelectronics, marking, monitoring agents, information collection, optics, uorescent probes, and drug distribution [9][10][11][12].
The presented work aims to effect gradually increased zinc sul de nanoparticles on the structure and physical properties of a semi-natural polymer blend comprising 80% poly vinyl pyrrolidone and 20% of chitosan.

Sample Preparation and characterization
Studied samples were synthesized in the form of thin membrane lms using the traditional solvent casting and evaporation route. A calculated amount of the Cs was dissolved through vigorous stirring in (50 mL) of 2% aqueous solution of acetic acid and mixed with a speci c amount of PVP and stirred for 60 min. Then mixed solution dropped on clean Petri dishes and dried at 50°C for 24h. PVP/CS (80/20) blend was prepared with various loading concentrations of zinc sul de 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.  20 20 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 measured and theoretical calculated data.

Fourier transforms infrared analysis (FTIR)
FT-IR spectroscopy is a very powerful technique to recognize vibration bands associated with major chemical groups, detect the molecular structure which means characterizing the assignment of bands for each sample and inter-molecular interaction between polymers. Figures   which are connected to the C-N of the pyrrolidone structure, may be identi ed. It is remembered that PVP is just a bi-substituted amide, characteristic absorption of amines at about 3400-3500 cm − 1 have 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 assign to (O H) overlapped with (N H) stretching vibration [22,4],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].

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 that show a new band which obtained in the experimental data is not obtained in the theoretical data hence it's maybe that the PVP is hygroscopic.     (7) shows a schematic diagram of the 2D and 3D interaction mechanism probabilities between PVP/CS poly-blend and ZnS-NP. it provides an understanding of the importance of optical parameters, including bandgap energy ( ) and the absorption of light energies by polymer composites in UV and visible areas, which include the move of electrons from the ground state (σ, π, and 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 contains ZnS-NPS as illustrated in gure (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] 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 identi ed by in FT-IR at about 1662 cm -1 .  (1), the absorption coe cient can be calculated as follows: where A is the absorbance, log (I 0 /I) is de ned where I 0 and I are the strength of the incident and transmitted beams, respectively and L is the lm 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 hn>Eg, the present optical data can be analyzed for near-edge optical absorption according to the following relationship (2).
Where Eg is the magnitude of the optical energy gap, is the energy of the incident photons, and r is the force that characterizes the transformation phase in K-space. In speci c, depending on the type of electron transitions responsible for optical absorption, r will take the values 1, 2, 3, ¹⁄ , 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⁄ 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 Eg involves plotting (αhv) r against (hv). The dependence of (αhv) r and photon energy (hv) was plotted for the lms analyzed using various values of r (1/2, 2).
Close to the absorption edge for the present experimental results, the plots of (α hv) 1/2 and (α hv) 2 vs. (hv) of the absorption edge produce a linear t over a broader range of hn, as seen in Figure (10).
The Eg values of the lms were calculated from the linear part of these curves, and given in table (8).
From this table, in general, the value of value Eg decreases with increasing ZnS-NPs content.