For the synthesis of the hydrogel of chitosan and polyvinylpyrrolidone, the paper of Risbud, Hardikar and Bhonde (2000) was used as base [23] once it describes a simple method to produce hydrogels with interesting areas of application, like in growth modulation of fibroblasts and membranes [23] [24] .

Thus, an aqueous solution of 196 ml of 0.1 M acetic acid was prepared in a beaker. Such beaker was placed on a magnetic stirrer (360 rpm) and 4 g of chitosan were slowly added to the system. Then, it was covered with film plastic and the stirring continued for 24 hours. After such period, the solution was name solution 1.

In another beaker, 192 ml of distilled water were added and placed on a magnetic stirrer.

Thus, 8 g of PVP were also slowly added. The system was covered with film plastic and allowed to stir for 3 hours. Such solution was named solution 2.

Then, solutions 1 and 2 were mixed in different ratios (1: 1, 7:3 and 3:7 (volume/volume)). These solutions were named as 3A, 3B and 3C, respectively.

An aqueous solution of 3 ml of 25% glutaraldehyde was added to solutions 3 (1 ml each). Such solutions were placed on a magnetic stirrer for 10 minutes at room temperature. After that, the solutions were placed in Petri dishes which were dried at 32˚C for 72 h in a sterile atmosphere. These hydrogels were named Hydrogel 1, Hydrogel 2 and Hydrogel 3, as shown in Table 1.

In order to analyze the behavior of the hydrogels at different pHs, six samples of each hydrogel were collected, their masses were measured and, in pairs, they were immersed in solutions of pH 3, 3; 7.3 and 9.6, as shown in Table 2. These

solutions were prepared with distilled water and the pHs were handled with the aid of NaOH and HCl.

The bodies were removed from the solution after 36 h, the surface of the material was dried with paper and the mass was measured again to determine the percentage of water absorbed. Thus, the following formula was used to determine the water absorbed in the hydrogels:

% ofwaterabsorbed = ( Hydrogelswollenmass Hydrogeldrymass ) × 100 % (1)

There is currently little research related to the molecular simulation of hydrogels at the nanoscale [25] . So, in order to contribute to the studies related to the computational chemistry of hydrogels and obtain a better visual comprehension of the phenomenon observed in the experiment, the modeling of Polyvinylpyrrolidone (PVP), chitosan and hydrogel chitosan/PVP was carried out by using the Avogadro software (2018 version).

It is a free license software that allows to edit, visualize and simulate three-dimensional molecules, in order to reveal and study details at microscopic levels and to provide information about chemical properties, reactions and laws [26] . In addition, it has simpler manipulation tools for software such as VMD, NAMD and LAMMPS, allowing modeling and simulation of chemical structures in a simple way. In addition, as a way of visualizing the reaction of the hydrogel to different pH values, the same software was used to represent the chitosan/ PVP molecule in situations of protonation and deprotonation, as well as its interaction with water molecules.

After prepare solution 1, it was possible to see some areas with chitosan accumulation, what indicates that the polymer wasn’t totally dissolved, as shown in Figure 3.

Consequently, after the synthesis of hydrogels 1, 2 and 3, it was possible to perceive that the higher the amount of chitosan in the hydrogel, more rigid was hydrogel.

The rigidity due high values of chitosan is related to the degree of deacetylation, which consists in the transformation of an acetamino group (-NCOCH₃) into an amine group (-NH₂), as shown in Figure 4 [27] .

Thereby, it is possible to conclude that the chitosan used in the experiment hasn’t got a high degree of deacetylation since the higher the degree of chitosan deacetylation, the greater its flexibility is [28] . This happens because, when such a value is low, there is difficulty in the spatial rotation of the glycosidic bond due to the presence of several-NCOCH₃ units, which causes a high steric hindrance and consequently stiffening of the material. However, when the number of amine units is high, the rotation of the glycosidic bond is favored since the steric hindrance is smaller, allowing for greater flexibility [29] .

It is important to mention that not all acetamine groups turn into amine group, that is why chitosan is a copolymer (N-acetyl-glucosamine + glucosamine).

After the experiment had been carried out, it was also detected that the higher the proportion of chitosan in the hydrogel (in relation to the proportion of PVP), the greater the stiffness and the gelatinous characteristic of it. So, the hydrogel 2 presented the highest stiffness, hydrogel 3 was quite liquid and hydrogel 1 had intermediate characteristics between those previously mentioned.

Such phenomenon is associated with the fact that amine groups are capable of

forming crosslinks. Thus, the more units of such a group are present, the greater the gelation rate and the lower the gelation temperature [30] .

Thereby, it was difficult to handle hydrogel 3 samples because they were quite brittle and sensitive, as it is possible to see in Figure 5 and Figure 6. Moreover, when inserted in the solutions of different pHs, these were broken to numerous pieces, which made the measurement of absorbed water difficult. Hence, it became essential to disregard hydrogel 3 for the analyses, since its results were not reliable and conclusive.

After the experiment presented, the Average Mass of the samples of the same hydrogel that were immersed in the same pH was calculated. Thus, it was possible to determine the percentage of water absorbed, presented in Table 3.

By analyzing Table 3, it is possible to notice that all samples expanded when in contact with solutions of different pHs. To understand such phenomenon, it is necessary to analyze the structure of PVP and chitosan in order to conclude if only one or both of them are responsible for the hydrogel sensibility to pH.

The glucosamine part of the structure of chitosan, approach in Figure 1, shows that it has amino groups capable of protonating and deprotonating. Such aspect is responsible for the material sensitivity to pH variation [31] . In chitosan, at low pHs (usually below 6), its amine groups are protonated as a quaternary ammonium salt with positive charges. And at high pH values (usually above 6), they are deprotonated [6] .

The pka is the magnitude associated with this phenomenon so that this value determines at what pH this transition occurs and depends directly on the degree of deacetylation during chitosan production [6] . In general, the pka of the amino groups of chitosan is between 6.2 and 6.3 [32] .

In addition, the variation in the volume of the material occurs in the sense of minimizing the repulsion of the charges present in the chemical structure [33] .

However, PVP structure approach in Figure 2, shows that it is a non-ionic polymer [34] , what means that it can’t pronotate/deprotonate, and consequently, it is not responsible for the sensibility to pH presented by the hydrogel.

As a way of visualizing the phenomenon, the reaction below was developed using the Avogadro software, based on the hydrogel formation proposed by Zhang, Jin, Li, Zhang & Wu (2018), so that the geometry of the structure was constructed and adjusted according to the optimization tool geometry provided by the software itself, minimizing the total energy of the molecule. Such modelling can be observed in Figure 7 and the red, blue, gray and white spheres represent, respectively, the oxygen, nitrogen, carbon and hydrogen atoms.

The modeling of the chitosan/PVP complex was also developed under pH values of 3.3; 7.3 and 9.6, as approached in Figure 8, where the prominent figures represent the amino groups in situations of protonation and deprotonation.

Analyzing the hydrogels 1 and 2, it is also noted that the volumetric expansion varied according to the different ranges of pHs and there was no pattern in relation to these samples. In this way, bibliographical references were consulted to understand what behavior was appropriate.

Firstly, it was raised the hypothesis that the hydrogel 2 presented the most coherent behavior since, because it possessed more chitosan in its composition, it was more rigid and easier to handle, besides demonstrating a better consistency of gel.

After the search, the hypothesis was confirmed. That is, as in Risbud, HdardikarBhat and Bhonde (2000), it was expected that the volumetric variation of the materials studied would respond negatively as the pH increased.

In addition, it was expected that the higher the proportion of chitosan in the hydrogel, the greater its sensitivity to pH, ie, the greater its swelling mechanism, which could also be confirmed when comparing hydrogels 1 and 2.