## 3 Results
### Mechanical Properties  
The mechanical properties of the films, namely EB, TS, and EM were determined by tensile tests and are summarized in Table 4. Both TPS and TPSDH films exhibited similar stiffness or elastic modulus, though the TPSDH films were slightly more rigid and remarkably more resistant to break but also less extensible. The TPS control film had an EB value of approximately 30%, in line with that observed by [PERSON] et al. [50] for TPS films. Furthermore, the TPSDH film without fibers showed an EB value of 23%, being lower than the neat TPS film. This mechanical embrittlement can be attributed to the changes occurred in the starch chains, such as short-chain amyloses produced by heat-induced hydrolysis of starch [54] or the increase in the content of C=O and -COOH groups during the DH of starch [67]. Other authors [60; 68] also suggested that the conversion of native -OH groups to C=O and -COOH ones in oxidized starch may promote -H bonds between amylose and amylopectin molecules, increasing the integrity of the biopolymer matrix and improving the tensile resistance of the films.  
The incorporation of CMFs at 3 wt% increased notably film stiffness in both kinds of films, being this effect less noticeable at other ratios of fibers. Several studies have also reported higher EM values of starch films containing cellulosic fractions from different  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Formulation** & **EB (\%)** & **TS (MPa)** & **EM (MPa)** \\ \hline TPS & 30 \(\pm\) 4  & 180 \(\pm\) 5  & 4.2 \(\pm\) 1.0  \\ TPS3 & 14 \(\pm\) 5  & 550 \(\pm\) 18  & 14.6 \(\pm\) 0.9  \\ TPSDHH & 23 \(\pm\) 4  & 190 \(\pm\) 3  & 200 \(\pm\) 7  & 10\(\pm\) 1  & 7.2 \(\pm\) 1.2  & 10\(\pm\) 10  & 7.2 \(\pm\) 1.2  & 10\(\pm\) 10  & 7.2 \(\pm\) 1.2  & \multicolumn{1}{c}{} \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mechanical properties in terms of elongation at break (EB%), tensile strength at break (TSsources [36; 56; 69]. In general, all the here-developed films became more resistant and less stretchable as higher loading of CMFs were incorporated. Higher fiber contents, of 3 and 5 wt% CMFs, decreased the TPS film stretchability by about 50%, whereas a similar decrease was observed for the TPSDH film with only 1 wt% CMFs. The lack of stretchability limits the overall extensibility of the films without markedly reducing the resistance to break. As shown in FESEM micrographs, CMFs were strongly bonded to the polymer matrix, which restricted the movement of starch chains, decreasing their stretchability but also reinforcing the TPS matrix [70]. The highest reinforcing effect was observed for the 3 wt% CMFs in both TPS and TPSDH films. These films particularly exhibited the highest EM (about 550 and 590 MPa for TPS and TPSDH, respectively) and resistance to break (approximately 7 and 10 MPa for TPS and TPSDH, respectively), but also with reduced elongation capacity (14 and 5% for TPS and TPSDH, respectively). In this regard, [PERSON] et al. [65] also observed a threshold concentration in cellulose nanofibers from heenquen to reinforce starch films.  
Therefore, the obtained results in tensile behavior of TPS and TPSDH films demonstrated the reinforcing capacity of CMFs for starch films, which indicates good compatibility and dispersion along the TPS matrix. However, contents higher than 3 wt% led to lower mechanical performances, which can be attributed to a loss of cohesion forces or a lower degree of dispersion and certain agglomeration of the fibers in the TPS matrix.