Water-Induced Hardening\nfor Shape Memory Polyurethane\nBased on Molecular Design and Microphase Separation

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