Water-Induced Hardening for Shape Memory Polyurethane Based on Molecular Design and Microphase Separation

From the principle of molecular design and microphase separation, B-THDI-MPEG was prepared by the reaction of 1,6-hexyl diisocyanate trimer (THDI) with bis(2-hydroxyethyl) terephthalate (BHET) and methoxy polyethylene glycols (MPEGs). Polyurethane with a hard skeleton hanging PEG soft segment (SPPU) was synthesized by using B-THDI-MPEG as a polyol. SPPU is capable of microphase separation in the aqueous phase, leading to molecular chain rearrangement, thus improving the crystallinity, tensile performance, thermal stability, and triggering shape memory property. The tensile strength of SPPU increases after soaking in water (25 W) compared to a dry environment (25D). Compared with SPPU4-25D (18.1 MPa), the tensile strength of SPPU4-25W is 21.1 MPa, showing a water-induced softening phenomenon. However, the tensile strength of polyurethane with PEG soft segments on the main chains (PPU) decreases after being soaked in water. Moreover, when the hard segment content is 57.5 wt %, the recovery rate of SPPU4 reaches 88.6% and takes a short time under heating in water.