Molecularly Rigid Cross-Linking Agent. MDI was used as a cross-linking agent that reinforces Young’s modulus by adding molecules to the PGS matrix at the molecular level due to rigid phenyl groups. MDI links between hydroxyl groups of a PGS chain using the isocyanate groups. In the current study, the molar mixing ratios between glycerol/SA/MDI in THF solution are 1:1:0.05 and 1:1:0.1, denoted as MDI 4.1 wt % and MDI 7.8 wt %, respectively.