# Chitosan Films with Caffeine and Propolis as Promising and Ecofriendly Packaging Materials
## 3 Results and Discussion
### Tensile Properties  
Mechanical testing holds significance in terms of assessing the practicality of the fabricated materials. Hence, the tensile strength (TS) and elongation at break (EB) of the chitosan-based films under investigation have been assessed, and the outcomes are illustrated in Figures 5 and 6, respectively.  
The films obtained had various values of tensile strength, whereby the highest values were found in the samples with acetic acid. The TS for sample A was 38.80 MPa, and for sample AC, it was 35.73 MPa. Sample ACP, which involved the use of additional caffeine and EP, had a TS of 15.22 MPa. Samples where citric acid was used were characterized by the following tensile strengths. Sample C, with pure chitosan in citric acid, had a TS of 2.65 MPa. The addition of caffeine resulted in a minor tensile strength of 5.72 MPa. On the other hand, sample CCP, which is a combination of chitosan dissolved in citric acid, caffeine and EP, obtained a value of 3.40 MPa. The mere addition of caffeine had no  
Figure 5: Tensile strength (TS) for prepared chitosan films. Different letters indicate samples that were significantly different (\(p<0.05\)).  
Figure 6: Elongation at break (EB) for prepared chitosan films. Different letters indicate samples that were significantly different (\(p<0.05\)).  
major effect on changes in tensile strength. However, the solvent used was an important factor. The selection of a natural carboxylic acid is essential for evaluating the potential utility of food packaging materials. Furthermore, investigating the interplay between the structure and properties of chitosan films in conjunction with the type of acid can optimize the attributes of the base film without the need for additional additives. This can offer substantial insights into film selection to meet the requirements of food packaging [78].  
Regarding elongation at break, an opposite relationship to tensile strength (TS) can be observed. Samples C, CC and CCP are characterized by the highest elongation at break (EB) for the prepared chitosan films, while samples ACP and CCP are marked by the highest value of elongation at break at 121.87 and 99.4%, respectively. Therefore, the use of citric acid as a solvent has a positive effect on elongation at break and, thus, the flexibility of the sample. For sample CC, where caffeine was applied, there was a decrease in elongation to 34.71%. Samples in which acetic acid was used were marked by lesser elongation. For pure sample A, EB was 35.5%, while samples AC and ACP had elongation values of 6.92 and 10.46%, respectively.  
The research on the mechanical properties of the prepared films demonstrated that the solvent had a key impact on the results. As is known, acetic acid is a common solvent that acts as a proton donor, creating a dense chitosan film. The study demonstrated no effect of acetic acid on the deterioration of tensile strength in the tested films. The samples with acetic acid were characterized by greater tensile strength than the samples with citric acid. As reported in the literature [78], acidic structure affects the properties of the resulting films through electrostatic interactions, hydrogen bonds, and hydrophobic interactions. Therefore, acids with more carboxyl groups can improve film elongation by means of ionic crosslinking. In this study, citric acid improved the elongation at break of the obtained films. Elongation for the samples with citric acid was significantly greater than for the film where acetic acid was used. Citric acid is recognized for containing three carboxyl groups, which leads to a greater number of hydroxyl groups in the film matrix. This increase in hydroxyl groups promotes enhanced chain mobility and reduces intermolecular forces, resulting in an augmentation of the elongation at break while simultaneously causing a decrease in the tensile strength of the tested samples [79].