1. Introduction

2. Materials and Methods

3. Results and Discussion

3.2. Effect of Ethanol– Acetic Acid Ratio on Rheological Properties of PVA–CS Solution

The aqueous solutions containing 4% PVA, 3% CS and a mixture of ethanol and acetic acid solvents in different ^ratios such that the total concentration of the organic solvents was 60% were prepared. The rheological properties of these solutions were then investigated (Figure 2).

The flow curve of a PVA–CS 4–3 solution under the influence of different ratios of ethanol–acetic acid also shows the character of a complex flow with three stages: destruction of the initial structure of the solution when applying shear rates of less than 1.5 s^–1, Newtonian models of fluid flows with shear rates from 1.5 to 10 s^–1 and non–Newtonian models with shear rates greater than 10 s^–1.

Figure 2. Graphs of the dependence of shear rate from the dynamic viscosity of PVA–CS solutions at the different ratios of ethanol–acetic acid.

Figure 2. Graphs of the dependence of shear rate from the dynamic viscosity of PVA–CS solutions at the different ratios of ethanol–acetic acid.

The Carreau–Yasuda model of non–Newtonian liquid flow was applied for a complex description of the flow of solutions in the second and third ranges [30,31].

$$\mathsf{\eta }(\mathsf{\gamma }\text{̇})={\mathsf{\eta }}_0{[1+{\left(\mathsf{\lambda }\mathsf{\gamma }\text{̇}\right)}^{\mathrm{a}}]}^{{\displaystyle \frac{\mathrm{m}-1}{\mathrm{a}}}}$$

where η₀ is the zero shear viscosity, λ is the characteristic time constant, a is the change of flow type, and m is the flow (pseudoplasticity) index. On Figure 2, the fitting curves of the Carreau–Yasuda equation are shown as black lines. The parameters of the Carreau–Yasuda equation for the curves (Figure 2) are presented in Table 2.

The values of the coefficient of determination show that the model describes the considered shear rate ranges almost perfectly. It can be seen that the values of η0, which characterize the total viscosity of the solution in the second range of shear rates, change in a complex manner with an increase of the ethanol–acetic acid ratio.

Table 2. The parameters of the Carreau–Yasuda equation for PVA–CS solutions at the different ratios of ethanol–acetic acid.

Table 2. The parameters of the Carreau–Yasuda equation for PVA–CS solutions at the different ratios of ethanol–acetic acid.

CH3COOH/C2H5OH, (%)	η0, (mPa·s)	λ · 103 (s)	a	m	R2
60/0	3678	13,86	0,93	0,611	0,9999
55/5	3479	7,11	0,84	0,507	0,9995
50/10	3993	11,54	0,80	0,553	0,9999
45/15	3143	13,70	0,92	0,617	0,9999
40/20	4511	7,56	0,68	0,485	0,9999

In this case, the flow rate m represents the dependence from the ratio of ethanol–acetic acid. The flow index m is less than 1, which characterizes the active liquefaction of the system.

The complex variation in the values of viscosity η0 and the flow index m of PVA–CS solutions under the influence of the ethanol–acetic acid ratio is associated with a modification in the structure and combinatorial state of macromolecules in the solution. Alterations in the size of nanoparticles, particle density, and polymer solubility, in addition to adaptations of the values of optical density and turbidity, obviously had a significant effect on the rheological properties of PVA–CS 4–3 solution.

The pH, viscosity and electrical conductivity of PVA–CS 4–3 solutions under the action of ethanol– acetic acid ratios are measured and presented in Table 3 and on Figure 3.

The rheological properties of the solution vary greatly depending on the ratio of ethanol and acetic acid in the solution, where the pH value increases with the decreasing of acetic acid concentration, and the viscosity and conductivity values inconsistent change.

As mentioned above, the heterogeneous jump in viscosity η₀ is caused by two opposing trends in viscosity variations when acetic acid concentration decreases and ethanol concentration increases. The first one is that when the density of hydrogen bonds between acetic acid molecules and polymers reduces, so does the size of macromolecule blocks and viscosity. The second tendency is a drop of CS solubility due to an insufficient pH value for the CS polycation, which enhances the polymer's intermolecular stickiness and viscosity. As a result, the viscosity of PVA–CS solutions changed unevenly.

The electrical conductivity depends on the density of free ions and the viscosity of the solution and therefore also varies heterogeneously. However, in general, the conductivity of the solution decreases with increase of ethanol– acetic acid ratio.

The change of rheological properties leads to an adaptation in the ability to form nanofibers by electrospinning. In a previous study on the fabrication of PVA–CS nanofibers from the solution of 4% PVA and 3% CS without the presence of ethanol, the obtained nanofibers diameter was 326 ± 62 nm at the optimal needle–collector distance of 140 mm, feed rate of 0.1 mL/h and voltage of 27 kV [26]. It will be interested to investigate how the addition of ethanol to the electrospinning solution affects nanofibers production, electrospinning technology parameters, and the properties of the generated PVA CS nanofibers.

3.3. Effect of Ethanol– Acetic Acid Ratio on Fabrication of PVA–CS Nanofibers

3.3.1. The Ability to Fabricate PVA–CS Nanofibers

Solutions containing 4% PVA, 3% CS and 60% acetic acid and ethanol mixtures were electrospinning to investigate their ability to fabricate nanofibers. The influence of the technological parameters, including the distance between the needle tip and the collector being varied from 100 to 150 mm, the voltage being altered from 16 to 30 kV, and the flow rate being adjusted from 0.1–0.2 mL/h on the nanofibers fabrication is presented in Table 4.

Thus, the PVA–CS nanofibers fabrication ability from the ethanol–acetic acid ratio of 15–45 is the greatest.

3.3.2. Intermolecular interactions for the PVA–CS nanofibers fabrication

The impact of intermolecular interactions between polymers and solvents as well as between polymers themselves on the acquired properties of polymer nanofibers is one of the most significant concerns related to the electrospinning process. According to Hildebrand-Scatchard theory, dispersion, polar, and hydrogen forces (f_d, f_p and f_h ) make up intermolecular interactions, with the dispersion component counting for the majority of these interactions. The solubility parameters of the components approach each other as the molar enthalpy of mixing (ΔH) approaches zero. Figure 4 depicts a Teas diagram that accounts for each component's contribution (assuming that their sum equals 100%) and is based on the presumption that all materials are equal.

For every part of the experimental combination, dispersion forces (f_d), polar forces (f_p), and hydrogen bond forces (f_h) were calculated. Table 5 presents these fibers formation parameters.

In our previous work, the influence of acetic acid on the solubility parameters of a 4%PVA-3%CS solution was reported [26]. To better illustrate the differences in effects between the two solvent systems with and without ethanol, Figure 5 reconstructs the Teas diagram for both co-solvent systems.

Table 5. The parameters of fractional solubility in the solution of 4% PVA–3% CS in the mixture of solvents CH₃COOH-C₂H₅OH-H₂O.

Table 5. The parameters of fractional solubility in the solution of 4% PVA–3% CS in the mixture of solvents CH₃COOH-C₂H₅OH-H₂O.

CH3COOH wt.%	C2H5OH wt.%	H2O wt.%	100fd	100fp	100fh	fh-fp	fh-fd	fp-fd
60	0	33	32,19	24,13	43,68	19,55	11,48	-8,06
55	5	33	31,98	23,91	44,11	20,19	12,13	-8,06
50	10	33	31,76	23,70	44,54	20,84	12,77	-8,06
45	15	33	31,55	23,48	44,97	21,48	13,42	-8,06
40	20	33	31,33	23,27	45,40	22,13	14,06	-8,06
35	25	33	31,12	23,05	45,83	22,77	14,71	-8,06

Given that water makes up 33% of the mixtures in this experiment, it was possible to create a three-dimensional graph by the plotting of the concentrations of solely CH₃COOH and C₂H₅OH along the X and Y axes, while the values of f_d, f_p and f_h were drawn along the Z axis and (Figure 5).

Compared to the solution without ethanol, it is evident that the addition of ethanol to PVA-CS-CH₃COOH solutions has the effect of the stabilization of the parameters of 4% PVA–3% CS solutions.

Thus, it can be concluded that binary, trinary, or other multi-component solutions with a “f_h-f_p” value between 19.5 and 21.5% are appropriate for the use in the electrospinning process for PVA-CS nanofibers fabrication.

Figure 4. The Teas plot for PVA-CS electrospinning solutions with CH₃COOH-H₂O and CH₃COOH-C₂H₅OH-H₂O as the co-solvent system.

Figure 4. The Teas plot for PVA-CS electrospinning solutions with CH₃COOH-H₂O and CH₃COOH-C₂H₅OH-H₂O as the co-solvent system.

Figure 5. Values of polar interactions f_h, f_p, f_d for a mixture of solvents CH₃COOH-C₂H₅OH-H₂O in the solution of 4% PVA- 3% CS.

Figure 5. Values of polar interactions f_h, f_p, f_d for a mixture of solvents CH₃COOH-C₂H₅OH-H₂O in the solution of 4% PVA- 3% CS.

3.3.3. Morphology of PVA–CS Nanofibers

Micrographs of nanofibers obtained from 4% PVA–3% CS–ethanol–acetic acid solutions with electrospinning parameters fixed at the needle–collector distance of 140 mm, the voltage 28 kV, the feed rate 0.2 mL/h were used to investigate the effect of ethanol–acetic acid ratios on the morphology and diameter of PVA–CS nanofibes.

From the data presented in Table 6 and on Figure 6 it can be concluded that the addition of ethanol to the electrospinning solution clearly altered the fabrication ability of PVA–CS nanofibers as well as their morphology and diameter. The results of microimage analysis and diameters of nanofibers obtained from solutions with different concentrations of ethanol–acetic acid revealed that the solution with ethanol-acetic acid ratio 15 – 45 has the lowest viscosity and is also the solution for high nanofiber fabrication capability with the finest morphology and the smallest diameter.

The observation of nanofibers formation in the electrospinning as well as the results of the analysis of the micrographs and the diameter distribution of the nanofibers from solutions with different ethanol– acetic acid ratios showed that the solution 4% PVA, 3% CS, 15% ethanol and 45% acetic acid were optimal for the fabrication of PVA–CS nanofibers. As a result, further investigation of electrospinning parameters is required to determine the optimal technological parameters.

3.4. Effect of Ethanol– Acetic Acid Ratio on Optimal Electrospinning Parameters

3.4.1. Parameter of the Distance between the Needle Tip and the Collector

Electrospinning was performed with the solution of PVA–CS ratio 4–3 with the solvent ratio of ethanol– acetic acid 15–45, with the distance between the needle and the collector varying from 100 to 150 mm and the voltage and the speed fixed at 28 kV and 0.2 mL/h, respectively.

Figure 7 and Table 7 show microimages and statistics on fiber diameter distribution. The data demonstrate that the optimal distance between the needle and the collector for electrospinning is 140 mm. At this parameter, the fibrous film has the least number of defects, as well as the diameter of the resulting fibers is also the smallest.

3.4.2. Parameters of Feed Rate

The solution of PVA–CS (ratio 4–3) with ethanol–acetic acid solvent (ratio 15–45) was electrospun with the voltage and the needle–collector distance fixed at 28 kV and 140 mm, respectively, and the rate was adjusted from 0.1 mL/h to 0.4 mL/h. Microimages and fibers diameter distribution statistics are shown on Figure 8 and in Table 8.

Thus, at the needle–collector distance parameters of 140 mm, the feed rate of 0.2 mL/h and the voltage of 28 kV the PVA–CS nanofibers obtained have the smallest diameter of 285 ± 65 nm. At the feed rate of 0.3 mL/h although the PVA–CS nanofibers have less defects, the fiber diameter is larger.

3.4.3. Parameters of the Voltage

The solution of PVA–CS (the ratio 4–3) with the solvent ratio of ethanol-acetic acid 15–45 was subjected to electrospinning with the fixed distance between the needle and the collector of 140 mm, the flow rate set at 0.2 mL/h, and the voltage change between 25 kV and 30 kV. Microscopic images and fiber diameters distribution statistics are shown on Figure 9 and in Table 9.

The morphological analysis and diameter distribution of PVA–CS nanofibers revealed that at 28 kV, the obtained fibers had the smallest diameter with fewest defects. The optimal technological parameters for the electrospinning include the needle–collector distance of 140 mm, the feed rate of 0.2 mL/h and the voltage of 28 kV. The obtained PVA–CS nanofibers have the diameter of 285 ± 65 nm.

Table 9. Diameter dibstribution of PVA–CS nanofibers obtained from solution of PVA–CS (ratio 4–3) with the solvent ratio of ethanol– acetic acid 15–45 at the needle–collector distance 140 mm, the feed rate 0.3 mL/ h and the variation of voltage from 25 to 30 kV.

Table 9. Diameter dibstribution of PVA–CS nanofibers obtained from solution of PVA–CS (ratio 4–3) with the solvent ratio of ethanol– acetic acid 15–45 at the needle–collector distance 140 mm, the feed rate 0.3 mL/ h and the variation of voltage from 25 to 30 kV.

Diameter (nm)	Voltage (kV)
	25	26	27	28	29	30
Mean	348	334	309	285	307	318
Standard deviation	102	99	91	65	96	114
Min	97	139	110	121	131	120
Max	652	734	600	592	644	689

The fabrication of PVA–CS nanofibers without ethanol leads to the nanofiber diameter 326 ± 62 nm, and the optimal feed rate was only 0.1 mL/h. When compared to the acetic acid, the mixed solvent of 45% acetic acid and 15% ethanol not only improved the morphology and diameter of PVA–CS nanofibers, but also doubled or tripled the nanofiber fabrication yield (at the feed rate 0.3 mL/h diameter was 300 ± 79 nm).

Figure 9. Microscopic images at 100x and 1000x and diameter distributions of PVA–CS nanofibers obtained from the solution of PVA–CS (ratio 4–3) with the solvent ratio of ethanol– acetic acid 15–45 at the needle–collector distance 140 mm, the feed rate 0.3 mL/ h and the variation of voltage from 25 to 30 kV.

Figure 9. Microscopic images at 100x and 1000x and diameter distributions of PVA–CS nanofibers obtained from the solution of PVA–CS (ratio 4–3) with the solvent ratio of ethanol– acetic acid 15–45 at the needle–collector distance 140 mm, the feed rate 0.3 mL/ h and the variation of voltage from 25 to 30 kV.

3.5. Fourier–Transform Infrared (FTIR) Spectroscopy

In our previous research [26] it was established that acetic acid entirely separates from the nanofiber matrix and has no effect on the polymeric components of PVA–CS nanofiber. Infrared spectra of nanofibers fabricated from the solutions of PVA–CS and ethanol–acetic acid in different rations were taken in order to analyze the effect of ethanol on the chemical bonds formation in the system of PVA–CS nanofibers.

Infrared spectra of PVA–CS nanofibers (Figure 10) demonstrated that they are the same. The peak positions did not change, indicating that no new chemical bonds were formed when ethanol was added to the electrospinning solution system. Thus, it is evident that during electrospinning, ethanol and acetic acid were entirely separated from the PVA–CS nanofiber mat.

When two polymers are combined in the PVA–CS nanofibers, the IR spectra of the nanofiber systems become simpler and closely resemble that of pure PVA. The oscillations of the C–H alkyl bond in the CS molecule at 2869 cm^–1 [32] take on a shoulder shape when combined with the asymmetric and symmetric stretching bands of the CH₂ groups in the PVA molecule at 2940 cm^–1 and 2910 cm^–1 [33,34]. In the PVA–CS nanofibers, the bands of each polymer in the 1590–890 cm^–1 region become simpler, wider, less multiple, and shift into the space between bands. These are evidences that PVA and CS are closely linked together in the PVA–CS nanofiber system.

Figure 10. Infrared spectra of PVA powder, chitosan powder and PVA–CS nanofibers.

Figure 10. Infrared spectra of PVA powder, chitosan powder and PVA–CS nanofibers.

There was a combination of bands at 3279 cm^–1 (of -OH bonds in PVA molecule), 3300 cm^–1 and 3362 cm^–1 (of -OH and -NH bonds in CS molecule) to form a broad and high intensity band in the region of 3000–3600 cm^–1 common to all -OH and -NH groups in the polymer system [32,33,34,35]. The 896 cm^–1 band of the -OH out–of–plane vibrations and the 690 cm ^{– 1} band of the N–H twist vibrations related to ring stretching in the IR spectrum of CS disappeared in the nanofibers spectra, demonstrating mobility of the -OH and -NH₂ groups of CS are disappeard [36,37,38]. The disappearance of these mobilities, together with the broadening of the absorption region in the range 3000–3600 cm^–1, suggests that significant number of hydrogen bonds formed between the PVA and CS molecules. The similar conclusions were made in the previous publications [34,35].

3.6. X–ray diffraction (XRD) Analysis

The XRD spectral data are shown on Figure 11 and the results on the crystal lattice parameters of the obtained nanofiber systems are presented in Table 10.

Overall, compared to the polymer powders, the XRD spectra of the PVA and PVA–CS nanofibers have stronger and more noticeable peaks. As PVA was converted from a powder to nanofiber, the position and intensity of its peaks altered significantly. The PVA–CS nanofibers' XRD peak intensities varied slightly from those of the PVA nanofibers and from each other, indicating that solvent factors in addition to the presence of CS and the powder–to–nanofiber transitions affect the crystal structure of these nanofibers.

Figure 11. X–Ray diffraction of PVA powder, CS powder, PVA nanofibers from aqueous solution and PVA – CS nanofibers from aqueous solution with C₂H₅OH and CH₃COOH.

Figure 11. X–Ray diffraction of PVA powder, CS powder, PVA nanofibers from aqueous solution and PVA – CS nanofibers from aqueous solution with C₂H₅OH and CH₃COOH.

Table 10. The lattice parameters of PVA–CS nanofibers obtained from electrospun solutions with different ethanol– acetic acid ratios.

Table 10. The lattice parameters of PVA–CS nanofibers obtained from electrospun solutions with different ethanol– acetic acid ratios.

Lattice parameters			Axial Lengths [Å]			Angles [°]			Cell volume [Å3]	Crystal-linity (%)
			a	b	c	α	β	γ
Powder		CS	15.7371	8.3352	3.0609	90	90	90	401.5017	48.29
		PVA	15.2596	5.2416	9.7092	90	97.188	90	770.4844	57.69
Ethanol– acetic acid	0–60		12.6928	3.5873	10.8013	90	95.237	90	489.7613	60.64
	5–55		7.6994	16.7927	6.8328	90	96.878	90	877.0803	54.64
	10–50		15.9762	5.5865	9.3954	90	93.823	90	836.6833	62.36
	15–45		12.7499	7.2561	8.637	90	92.457	90	798.3136	57.45
	20–40		16.0564	4.1401	11.3214	90	93.814	90	750.9244	52.22

The results (Table 10) revealed that the increasing of the ethanol concentration and the decreasing of the acetic acid concentration in the initial electrospinning solution caused a unit cell volume reduction in the PVA–CS nanofibers lattice. However, in the absence of ethanol, the volume of the lattice unit cell is the smallest.

The crystallinity of the nanofiber systems as well as the axial lengths of the irregularly varied crystal cells can be derived from the irregular variation of the viscosity and electrical conductivity of the original solutions.

The variation in crystal structure implies that the ethanol and acetic acid components in the initial electrospun solution also have an impact on the mechanical and thermal properties of the produced PVA–CS nanofiber systems.

3.7. Differential Scanning Calorimetry (DSC) Analysis

Information about the DSC curves of PVA–CS nanofibers that were electrospun from polymer solutions at different solvent ratios is shown on Figure 12 and in Table 11.

The enthalpy change (ΔH), crystallinity (χ), melting temperature (T_m), and glass transition temperature (T_g) results for PVA–CS nanofibers at the different ethanol– acetic acid ratio in electrospinning solutions are presented in Table 11.

Table 11. DSC data for the thermal desorption of PVA – CS nanofibers at the different ethanol– acetic acid ratio in electrospinning solutions.

Table 11. DSC data for the thermal desorption of PVA – CS nanofibers at the different ethanol– acetic acid ratio in electrospinning solutions.

Ethanol– acetic acid ratio	ΔHPVA (J/g)	χPVA (%)	ΔHCS (J/g)	Tg (°C)	Tm(°C)
0–60	57.22	38.15	5.86	75	222, 247
5–55	42.23	28.15	11.19	75	220, 249
10–50	44.09	29.39	9.79	84	220, 250
15–45	38.72	25.81	13.82	75	222, 251
20–40	53.73	35.82	2.56	87	223, 252

The DSC heating curves of PVA–CS nanofibers at the various ethanol– acetic acid ratios are slightly different. Specifically, as the percentage of ethanol increases, both the crystallization temperature and the melting point increase slightly. This indicates that the different solvent ratios had an effect on the linking intensity of the polymers during electrospinning procedure.

There is the correlation between both the results presented in Table 11 and those in Table 1 and on Figure 1. The change of the specific enthalpy of PVA on the DSC curve of PVA-CS nanofibers was decreased when the optical density (A) and turbidity of the solution dropped (at acetic acid-ethanol ratios of 55-5, 50-10, and 45-15). This suggests that the aggregation state of polymer macromolecules in the initial solution influences polymer crystallization during the electrospinning.

The reduction of ΔH_PVA by about 20-30% while ΔH_CS increased by 2-fold suggests that the enhancement of the ethanol ratio in the initial solution mainly affects the structure of CS macromolecules rather than the PVA macromolecules.

Thus, an alteration of the ethanol-acetic acid ratio in the initial solution results in a change in the polymer's aggregation state in the initial solution, which leads to an adjustment of the individual crystal composition of each polymer in the structure of the PVA-CS nanofiber system. This results in a modification in the lattice, which affects the overall crystallinity of the system. In general, increasing the proportion of ethanol decreases the volume of the unit cell in the crystal lattice and raises the temperature of crystallization and melting of the system.

3.8. Thermogravimetric Analysis (TGA)

TGA curves for heating PVA powder, CS powder, and PVA–CS nanofibers at the different solvent ratios are shown in Figure 13.

PVA powder and PVA nanofibers path through three basic stages of heat degradation, whereas CS powder only goes through two. The main stage of the CS powder decomposes at a higher temperature than the other samples, demonstrating that the CS molecules are more heat resistant due to their intricate structure. However, as a result of that, almost all CS bonds breakdown during this stage.

Figure 13. TGA thermogram of PVA powder, CS powder and PVA–CS nanofibers at different ethanol– acetic acid ratios in initial electrospun solution.

Figure 13. TGA thermogram of PVA powder, CS powder and PVA–CS nanofibers at different ethanol– acetic acid ratios in initial electrospun solution.

Table 12. The stages of thermal decomposition and weight reduction of powder and nanofibers samples.

Table 12. The stages of thermal decomposition and weight reduction of powder and nanofibers samples.

Degradation stages		Powder		PVA–CS nanofibers with different CH3COOH/C2H5OH ratios
		PVA	CS	60 – 0	55 – 5	50 – 10	45 – 15	40 – 20
First stage	Range (oС)	25–202	25–177	25–172	25–172	25–172	25–172	25–172
	Peaks (oC)	112	67	52; 114	52; 112	52; 112	52; 107	52; 112
	Weight loss (%)	3.04	5.21	5.42	7.34	5.63	8.58	5.63
Second stage	Range (oС)	202–367	177–462	172–377	172–377	172–377	172–377	172–377
	Peaks (oC)	297	302	267	272	272	267	272
	Weight loss (%)	83.37	77.04	63.79	64.19	62.94	61.40	64.31
Third stage	Range (oС)	367–527		377–547	377–547	377–547	377–547	377–547
	Peaks (oC)	422; 442; 462		429; 459	422; 437	427	422; 437; 457	427
	Weight loss (%)	10.32		17.64	18.43	16.72	16.97	16.97

PVA–CS nanofibers' thermal degradation process resembles a coherence of the thermal decomposition of the two polymers. This again confirms that following the electrospinning procedure, the solvents were virtually entirely removed from the nanofiber system. The decomposition temperatures of these nanofibers, on the other hand, were all lower than those of pure polymers, indicating that their complexes were formed by weak bonds.

Thus, the change of the ethanol–acetic acid ratio has no effect on the thermal decomposition of the PVA–CS nanofibers.

4. Conclusions

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