## 3 Results and Discussion
### Ultrasound Micromolding of PLA/PEG Blends  
In spite of PEG being employed as a sacrifice polymer and thus preservation of molecular weight and mechanical properties not being essential, an analysis of the micromolding suitability of PEG and PLA/PEG blends was undertaken, following the protocol previously done for PLA [38]. It should be noticed that the ultrasound processing of PEG and even its blends with PLA had not been previously evaluated. As such, a further study focused in processing, composition, structure and calorimetric properties of the derived micromolded specimens was carried out. Figure 4: SEM micrographs of the cross section of PLA specimens loaded with 40 wt-% of NaCl before (**(a)** for <45 μm and **(d)** for 100–250 μm) and after immersion in water (**(b,c)** for <45 μm and **(e)** for 100–250 μm). NaCl particles are marked with yellow ellipsoids. PEG samples with two distinctive molecular weights (i.e., \(M_{w}\) of 100,000 and 35,000 g/mol) were selected. Results pointed out that both polymers could be properly molded with the same set-up that was selected for PLA specimens (i.e., 24 um-300 N-1.2 s). Nevertheless, specimens produced from PEG with lower molecular weight had not enough mechanical consistency and were shattered in the mold ejection step. This problem persisted even with the blend containing only 15 wt-% of PEG. In order to get perfect specimens for all compositions, the higher molecular weight sample was selected for all assays despite its higher viscosity could cause potential problems for the mixing process. Blends with 15, 30 and 50% of PEG were assayed. These blends will be referred to as: PLA:PEG 85%, PLA:PEG 70%, and PLA:PEG 50% respectively where the percentage refers to PLA content. Molecular weight of molded PEG was compared with that of the original commercial sample (i.e., before to exposition to the US heating source.) As shown in Figure 5, both GPC chromatograms were similar and the molecular weight of the molded sample was kept practically unaltered. Specifically, \(M_{n}\) and \(M_{w}\) values were 54,200 and 88,600 g/mol for the powdered PEG and 57,400 and 87,700 g/mol for the micromolded sample. GPC chromatograms were also characterized by a narrow distribution that contrasted with the relatively high polydispersity index (PI) determined for micromolded PLA (i.e., 1.5 versus 1.9). GPC chromatograms of micromolded blends were logically wider than those observed for individual polymers and for example PI reached values around 3.1 for a 50 wt-% of PEG. Distribution of PEG and PLA in the processed specimens could not be homogeneous due to the different flow properties of the two involved polymers caused by their different chemical structure and viscosity. This problem was evaluated by analyzing two half parts of each specimen: proximal (P) and distal (D) parts along the flow direction (Figure 6). Figure 6 compares the FTIR spectra of micromolded specimens of PLA, PEG, and representative PLA/PEG blend with 30 wt-% and 15 wt-% of PEG. The PLA spectrum was characterized by the C=O stretching band of the ester group at 1750 cm\({}^{-1}\), whereas PEG was characterized by the ether band at 1090 cm\({}^{-1}\) which is also close to the C(O)-O signal of the ester PLA group. PEG had also a strong signal at 950 cm\({}^{-1}\), which was useful to evaluate its presence since the PLA spectrum was rather clear in the 1000-900 cm\({}^{-1}\) region. Figure 5: Gel permeation chromatography (GPC) profiles of powdered polyethylene glycol (PEG), micromolded PEG and PLA/PEG blends having 30 wt-% and 50 wt-% of PEG. The curve of a micromolded PLA specimen (dashed line) is also given for comparison purposes. FTIR spectra clearly demonstrated that PEG tended to be concentrated in the proximal part (i.e., the closest section from the feeding camera) as consequence of its lower fluidity (higher viscosity and higher molecular weight). Differences between proximal and distal parts become enhanced as the total PEG content in the initial blend increased.