Solid-state structure, phase transition, and shape-memory properties of network copolymers with hyperbranched units containing s-benzenetricarbamide cores

This study is aimed at elucidating the origin of the shape-memory characteristics, associated with temperature changes of network copolymers consisting of hyperbranched units with s-benzenetricarbamide cores. For the copolymers used in this study, the trifunctional hyperbranched unit is designated as a hard segment (Hard-Sg), and polyethylene glycol (PEG), which is N–H group-capped at both ends and linked to Hard-Sg, is designated as a soft segment (Soft-Sg). When we use Hard-Sg obtained by linking five units of 4-methylaminobenzoic acid at the core and Hard-Sg with its –COOH terminal carbazidized, the solubility in the general solvents and the high-temperature behavior of the obtained copolymers are significantly different. Using carboazidization of the Hard-Sg terminal, the copolymers could be cured at 140 °C via the Curtius rearrangement, and thus they became insoluble, exhibited elongation and shrinking characteristics above the melting point, and regained their shape when the temperature was raised again. Reciprocal lattice analysis revealed that the PEG chain, i.e. the Soft-Sg, was the main component of crystallization, and the shape-memory characteristics repeat plasticization, structural fixation, and contraction restoration with the phase transition of this Soft-Sg unit.