# Fabrication of a High-Performance Bending Actuator Made with a PVC Gel
## 3 Results and Discussion  
Because the crystalline region of PVC is a turbid state, increase of the PVC molecular weight decreases the optical transmittance [23; 24]. To show this effect, we prepared three PVC films (1 mm thick) with different molecular weights and measured their transmittance on indium tin oxide (ITO) glass using UV-vis spectroscopy (HP 8452, HP, Palo Alto, CA, USA). Figure 3a shows the transmittance of the prepared pure PVC films. The optical transmittances of the \(\mathrm{PVC_{L}}\), \(\mathrm{PVC_{M}}\) and \(\mathrm{PVC_{H}}\) films at 550 nm were measured and found to be 48.9%, 46.1% and 37.6%, respectively. This result shows that the optical transmittance of pure PVC films decreases as we increase the PVC's molecular weight. The transmittance of the PVC gels was also measured. The measurement system and the procedure were the same as for the pure PVC films. As we mentioned before, we prepared 17 PVC gels. The prepared 17 PVC gels were divided into three groups (\(\mathrm{PVC_{L}}\)-based gel, \(\mathrm{PVC_{M}}\)-based gel and \(\mathrm{PVC_{H}}\)-based gel). We selected the PVC gel with the greatest amount of plasticizer for each PVC gel group. The selected PVC gels were \(\mathrm{PVC_{L}}\)5, \(\mathrm{PVC_{M}}\)13 and \(\mathrm{PVC_{H}}\)13. Normally, the plasticizers extend the gap between PVC chains and increase the free volume of the PVC gel. Due to these effects, the crystalline region of a PVC polymer is changed into an amorphous region with entanglement and physical crosslinking. Therefore, the transmittance of the PVC gel becomes higher than that of pure PVC films. Furthermore, comparing two PVC gels (\(\mathrm{PVC_{M}}\)13 and \(\mathrm{PVC_{H}}\)13) with the same plasticizer content but different molecular weights, the transparency of \(\mathrm{PVC_{M}}\)13 (95.1%) was almost the same as that of \(\mathrm{PVC_{H}}\)13 (95.4%) at 550 nm. This result implies that most of the crystalline regions in the PVC resin changed into amorphous regions during gel formation. Strain-stress relationships of three pure PVC films (\(\mathrm{PVC_{L}}\)0, \(\mathrm{PVC_{M}}\)0 and \(\mathrm{PVC_{H}}\)0) were measured using a Universal Tensile Machine (UTM, Tinius Olsen, H5 KT) to confirm that PVCs with different molecular weights had different mechanical properties. For the experiment, dumbbell-shaped PVC films according to ASTM D638 type-V were prepared. The crosshead speed of the UTM was determined to be 50 mm/min. In Figure 4, we can observe the mechanical properties (including the toughness and the break point) of the three prepared pure PVC films. This result indicates that the crystalline region of the PVC film increases with increasing molecular weight (same as the result for the transmittance). In addition, the strain and the break point of the PVC film increase as the molecular weight of the PVC increases. Because the entanglement and physical crosslinking in the amorphous region of the PVC increase with increasing molecular weight of the PVC, the PVC can stand more applied stress. Taken together, the increase of the PVC molecular weight causes a larger crystalline region, with more entanglement and physical crosslinking, in the PVC. The mechanical property of the PVC gel is one of the most important factors affecting the actuation performance of the PVC gel-based bending actuator. Therefore, an experiment to investigate the mechanical properties of PVC gels was conducted using the same UTM as the one we used in the  
Figure 3: Optical property of the PVC films and gels: (**a**) Transmittance of the PVC films and (**b**) transmittance of the PVC gels. previous experiments. We prepared 14 PVC gel samples: two PVC gels with low molecular weight (PVC\({}_{\text{L}}\)3 and PVC\({}_{\text{L}}\)5), six PVC gels with medium molecular weight (PVC\({}_{\text{M}}\)), and six PVC gels with high molecular weight (PVC\({}_{\text{H}}\)). Figure 5 shows the strain-stress relationships of the PVC gels with different molecular weights and different mixing weight ratios of plasticizer.