## 3 Results and Discussion
### Characterization of the Rods and Rod with ARP
#### 3.3.6 Mechanical Study  
In this study, the influence of the terpolymer composition, \(M_{n}\), and the presence of ARP in the rods on the mechanical properties was determined under tensile loadings (Figure 10 and Table 5). Figure 10: Exemplary stress–strain (\(\sigma=f(\varepsilon)\)) curves determined for blank rods and rods with ARP: (a) Td, l 21 rod and Td,l 21 rod-ARP, (b) Tl 59 rod and Tl 59 rod-ARP; and (c) Tl 77 rod and Tl 77 rod-ARP. The stress-strain curves (\(\sigma=f(\epsilon)\)) for the analyzed formulations showed the ductile modes of deformation, classified according to the PN-EN ISO 527-1:2020-01 standard, as types B and C. Type B was distinctive for the curves of the Tl 59 rod and Tl 59 rod-ARP, and the Tl 77 rod and Tl 77 rod-ARP. Strain hardening after crossing the yield point was noted for all type B curves. After crossing the yield point, stresses initially decreased and subsequently increased to failure at the highest stress values. Type C was noted for curves of the Tl,l 21 rod and Tl,l 21 rod-ARP when the values of yield (\(\sigma_{y}\)) and maximum stresses (\(\sigma_{m}\)) were comparable. For type C curves, stresses slightly decreased and remained relatively constant until failure of the material (Figure 10). The analysis of mechanical parameters, such as \(\sigma_{y}\), \(\epsilon_{y}\), \(\sigma_{m}\), \(\epsilon_{m}\), \(\sigma_{b}\), and \(E\), showed these to be lower for rods based on Tl,l 21 compared to rods based on Tl 59 and Tl 77 (Table 5). These results indicate a significant relationship of the mechanical properties with LA stereochemistry and the value of the \(M_{n}\). The density of the Tl,l 21 chain packing was lower than for Tl 59 and Tl 77. Amorphous regions of Tl,l 21 weakened the mechanical properties, whereas for Tl 59 and Tl 77, the interactions between l-LA chains enhanced the mechanical stability of the formulations. Moreover, shorter chains of Tl,l 21 may have been the deciding factor in weakening mechanical properties. The presence of ARP in terpolymers resulted in weakening the mechanical properties of the formulations. For the Tl,l 21 rod-ARP, the value of \(E\) was significantly lower compared to the Tl,l 21 rod, which indicates the lower stiffness of the Tl,l 21 rod-ARP, whereas the presence of the drug substance in the Tl 59 rod-ARP caused a decrease in all the analyzed mechanical parameters. The Tl 59 rod-ARP had less strength and was less stiff and deformable at all stages of stretching than the Tl 59 rod. It should be pointed out that the DSC study revealed that the plasticization effect (Figure 6 and Table 3) resulting from the presence of ARP in the Tl,l 21 rod-ARP and Tl 59 rod-ARP may be connected with the reduction in mechanical properties. The plasticization of d,l-PLGA caused by wetting the d,l-PLGA scaffolds with water or ethanol resulted in the same effect [46]. The comparison of the Tl 77 rod-ARP and Tl 77 rod revealed that the presence of the drug substance caused the most significant differences in mechanical behavior; the formulation became significantly stiffer (\(E=221.94\)\(\pm\) 88.04 MPa and 85.01 \(\pm\) 22.09 MPa, respectively) and less extensible (\(\epsilon_{y}=18.7\)\(\pm\) 4.1% vs. 28.4 \(\pm\) 7.6% and \(\epsilon_{m}=350.3\)\(\pm\) 167.1% vs. 436.4 \(\pm\) 100.2%, respectively). It could be pointed out that the presence of ARP blurred the rubbery and flexible behavior of TMC, despite the highest content of this component (\(\sim\)25%) (Table 3). However, it should be added that the ARP did not act as a plasticizer in the case of the Tl 77 rod-ARP (Figure 6 and Table 3). Other failure parameters are comparable for Tl 77 rod and Tl 77 rod-ARP (Table 5). From the mechanical point of view, the most promising formulation for subcutaneous administration is the Tl 59 rod-ARP due to its higher strength and moderate stiffness, whereas, for intramuscular administration, the Tl,l 21 rod-ARP seems to be the most appropriate because of its high flexibility and rubbery character.