Elaborate fabrication of well-defined side-chain liquid crystalline polyurethane networks with triple-shape memory capacity

Abstract

Targeting triple-shape memory capacity, a series of side-chain liquid crystalline polyurethane networks (SCLCPU-Ns) with well-defined architecture were prepared via an elaborately designed synthetic route: the liquid-crystalline (LC) functionalized monomer was polymerized with diisocyanate to produce a linear polyurethane precursor with a uniform distribution of pendant mesogenic units; and then, tetra-functional pentaerythritol (PTOL) was used as a crosslinker to produce a PU network with a similar backbone length between each crosslinking point. The chemical structures of the monomer and linear polyurethane precursor were characterized using nuclear magnetic resonance (NMR) spectroscopy. Differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide angle X-ray diffraction (WAXD) were employed to verify the nematic nature of the liquid crystalline state. DSC analysis combined with POM observation reveals that almost all the SCLCPU-Ns display two thermal transitions (T_g and T_cl), which can be utilized as T_trans to trigger triple shape memory behavior. The cyclic thermomechanical analysis, which was performed using DMA, reveals that the networks exhibit excellent triple-shape memory properties, although the architecture of the networks influences their performance. Taking advantage of the overlap of T_g and T_cl in the SCLCPU-Ns, which can be regarded as one broad thermal transition, a well-controllable gradual recovery process was realized in a broad temperature range.