## 3 Results and Discussion  
### Micromolding of Mixtures of PLA and the Porogenic Agent  
A preliminary study was done to determine the capability of the ultrasound equipment to mold mixtures of PLA and a water-soluble salt. Sodium carbonate (Na\({}_{2}\)CO\({}_{3}\)), sodium bicarbonate (NaHCO\({}_{3}\)), and sodium chloride (NaCl) were selected due to their low cost, biocompatibility, and wide use in salt leaching applications [44; 45; 46]. In a first attempt, PLA was mixed with a low percentage (e.g., 10 wt-%) of the selected salt. Both carbonates were decomposed during processing giving rise to carbon dioxide (Figure 2a,b) which means that samples could not be successfully molded. Even by varying amplitude, force, and time processing parameters, the PLA was not able to fill completely the mold and the specimens presented a lot of bubbles, especially for the bicarbonate sample (Figure 2c). Gas bubbles inside the molten polymer hindered the diffusion of molecular chains and led to a bad distribution and an imperfect molded specimen. Moreover, bubbles were responsible of cavitation effects that caused some polymer degradation as evidenced by the observed punctual coloration in some parts of the specimens. By contrast, sodium chloride (NaCl) with a particle size of 250-100 \(\upmu\)m allowed obtaining well molded specimens (Figure 2d) without modifying the processing parameters previously determined for the selected commercial PLA [38]. Thus, amplitude, force, and time of 24 \(\upmu\)m, 300 N, and 1.2 s, respectively, were considered optimal and used in most of the performed tests. Tests were also carried out considering different NaCl loads and varying also the particle size. The above indicated processing parameters were appropriate to get specimens up to a maximum NaCl load of 40 wt-% when diameters were in the 250-100 \(\upmu\)m size range. Higher salt contents lead to an overload signal. When smaller particles (e.g., \(<\)45 \(\upmu\)m) were employed, the salt percentage could be increased to 50 wt-% but the applied force should be increased to 400 N. Lower (e.g., 300 N) and higher (e.g., 500 N) forces caused an incomplete filling of the mold (85%) and an overload signal, respectively. GPC chromatograms showed that the molecular weight of the neat PLA sample and that processed with 40 wt-% of salt were practically identical (Figure 3), although a small decrease on the weight average molecular weight (i.e., from 72,200 g/mol to 68,800 g/mol) was detected, suggesting the occurrence of a small degradation caused by ultrasound processing. This was slightly more significant than that detected in previous works [43], being consequently associated to the increased force that was necessary to process the salt rich specimen. In any case, the observed degradation (decrease on Mn less than 5%) was not significant to compromise final mechanical properties. Note also that the influence of the molecular weight decrease should be insignificant considering the porosity of the sample once the incorporated salt is removed. Figure 4 shows cross sections of a PLA specimen prepared under the chosen optimal conditions (i.e., amplitude, force, and time of 24 \(\upmu\)m, 400 N, and 1.2 s, respectively), the maximum allowed  
Figure 3: GPC chromatograms of commercial PLA powder, micromolded PLA, and mixtures of PLA:NaCl at 40 wt-% of salt. Processing parameters were 24 \(\upmu\)m-300 N-1.2 s for neat PLA and 24 \(\upmu\)m-400 N-1.2 s for PLA:NaCl 40%. Figure 2: Polylactide (PLA) micromolded samples loaded with 10 wt-% of sodium carbonate (**a**,**b**), sodium bicarbonate (**c**) and sodium chloride (**d**). Processing amplitude, force and time parameters were in this case 24 \(\upmu\)m, 300 N and 1.2 s, respectively. amount of NaCl (i.e., 40 wt-%) for both the highest and the lowest particle size, <45 um (Figure 4a) and 100-250 um (Figure 4d). Salts were well encapsulated within the specimen with a minimum deposition on the specimen surface.