# 4D Printing of Multicomponent Shape-Memory Polymer Formulations
## 3 Extrusion Based 4D Printing
### Fused Filament Fabrication (FFF)
#### 3.1.1 FFF of Polymer-Polymer Blends  
As previously mentioned, PLA is a common thermoplastic material used in FFF that shows a thermo-responsive SME. To modify the shape-memory properties, blending of PLA with other thermoplastics is carried out. [PERSON] et al. [94] prepared mixtures of PLA and TPU at weight ratios of 90/10, 80/20, 70/30, and 60/40 using a twin-screw extruder. The PLA/TPU mixtures were immiscible as observed by scanning electron microscopy images (SEM), with TPU droplets changing to fibrils as the content of TPU increased from 10 to 40 wt.% in the printed specimens thanks to shear elongation. All the samples were able to be fixed into temporary shapes at room temperature without additional heating or cooling cycles (Figure 3a). It was observed that the blend containing 80 wt.% PLA had the highest \(R_{f}\) (\(R_{f}\) = 82% in tension, \(R_{f}\) = 91% in compression, and \(R_{f}\) = 84% in bending) at room temperature, but it also displayed brittle behavior and became more brittle as the number of loading cycles reached three. Here, PLA crystalline domains acted as crosslink points, retaining the original shape, while the TPU phase acted to accommodate strain, preventing the PLA from breaking at high deformations. Once heated beyond their \(T_{g}\) (60 \({}^{\circ}\)C), the PLA molecules displayed increased mobility and released the stored stresses, enabling the specimens to return to their original shapes. In another study by [PERSON] et al. [95], 80/20 TPU/PLA mixtures were compounded via twin-screw extrusion, converted to filaments, and processed via FFF. However, the high content of TPU made the obtained filaments very flexible and difficult to print, necessitating the use of a screw-based MEX system. [PERSON] et al. also observed that PLA tended to fibrillate under certain extrusion conditions when the PLA/TPU viscosity ratio (\(\eta_{PLA}/\eta_{TPU}\)) was close to 1, as observed at extrusion temperatures of lower than 190 \({}^{\circ}\)C in the 3D printer. The PLA fibrils were visualized by dissolving the TPU in dimethylformamide (DMF) and taking scanning electron microscopy (SEM) images of the remaining material. It was observed that PLA fibrils with large aspect ratios were formed in the nozzle of the printer and were oriented in the direction of deposition in the 3D printed specimens. The degree of crystallinity of the printed specimens was much higher than for samples produced by compression molding due to the different heating/cooling rates, and levels of polymer chain orientation. The higher crystallinity resulted in a higher tensile strength and Young's modulus for the printed specimens. The shape memory capabilities of these TPU/PLA polymer-polymer composites were later evaluated by [PERSON] et al. [96]. In this study, the content of PLA was varied from 0 to 30 wt.%. As the content of PLA increased, \(R_{f}\) improved, but the recovery ratio (\(R_{r}\)) decreased. The optimum overall shape memory behavior was achieved by adding 10 wt.% of PLA to TPU (\(R_{f}\) = 91%, \(R_{r}\) = 71%). The orientation of the infill also played a role in the shape memory ratios obtained. When the infill was parallel to the direction of the applied force (i.e., 0\({}^{\circ}\)), the frailty ratio (\(R_{f}\) = 90%) was the highest, followed by when the orientation was 45\({}^{\circ}\) from the loading direction (\(R_{f}\) = 72%). Using an infill pattern perpendicular to the applied force (i.e., 90\({}^{\circ}\)) yield the lowest \(R_{f}\) (\(R_{f}\) = 65%). In contrast, \(R_{r}\) was not significantly affected by the infill orientation, with all specimens having \(R_{r}\) values between 71 to 75% [96] (Figure 3d). Another blend of thermoplastic polymers that has been recently reported to display shape memory capabilities is that of polybutylene succinate (PBS), with PLA at a mass ratio of 9:1. This blend was prepared by melt compounding in a twin-screw extruder, with the resultant filaments used directly for FFF.