Effects of Methyl-Substituted Aniline Derivatives on Phase-Separated Bisphenol A-Based-Polybenzoxazine/Polyurethane Two-Way Shape Memory Polymers Behaviors

Abstract Two-way shape memory polymers (2WSMPs) based on gradient bilayer interpenetrating polymer network (IPN) architectures have attracted increasing attention owing to their ability to undergo reversible actuation without external reprogramming, making them promising candidates for applications such as self-folding structures, soft robotics, and safety circuit breakers. However, the molecular and morphological factors governing gradient architecture formation and their influence on reversible actuation behavior remain insufficiently explored. In this work, gradient bilayer IPNs were constructed in bisphenol A-based polybenzoxazine/polyurethane thermosetting systems by tailoring the benzoxazine phase using aromatic amines with different substitution patterns. A clear correlation between curing kinetics, cross-link density, phase morphology, and reversible actuation behavior was established. Meta-substituted systems (BA-3,5x and BA-mt) exhibited lower ring-opening polymerization temperatures, promoting early stage network formation and phase fixation during curing. The resulting higher cross-link density facilitated the formation of homogeneous benzoxazine-rich domains and distinct gradient bilayer architectures, as confirmed by FTIR, surface hardness, and AFM analyses. In contrast, ortho- and para-substituted systems (BA-ot and BA-pt) showed higher curing temperatures, lower network rigidity, and greater phase heterogeneity, resulting in poorly developed gradient architectures and insufficient structural integrity for reliable actuation. The optimized systems (BA-3,5x/PU and BA-mt/PU) demonstrated excellent reversible deformation, with high shape fixity (∼98%) and shape recovery (∼97–98%). Notably, BA-3,5x/PU exhibited faster recovery behavior (32–38 s) than BA-mt/PU (40–43 s), attributed to its higher cross-link density and more effective gradient bilayer architecture. This study provides correlation insights into gradient bilayer IPN formation and establishes a molecular design strategy for advanced two-way shape memory polymers.