Polymer design for high temperature shape memory: Low crosslink density polyimides

Shape memory in polymers is a process whereby mechanical energy is microscopically stored, and reversibly recovered within the polymer. Consideration of the viscoelastic and glassy dynamics necessary for each step of the process reveals key molecular characteristics that may improve performance, including a rigid polymer backbone with narrow molecular weight distribution between a low fraction of crosslinks. With this insight to guide high temperature polymer design, aromatic CP2 polyimide and associated single wall carbon nanotube (SWNTs) nanocomposites are shown to have excellent shape memory performance at 220 °C with rapid recovery (<10 s), excellent fixity (>98%), good cyclability and outstanding creep resistance. A narrow glass transition temperature regime (<10 °C) and high fragility (m ∼ 117) affords a narrow triggering window and the ability to spatially localize recovery with a temperature gradient. The addition of up to 3 vol% of dispersed SWNTs improves the rubbery modulus and blocking force without substantially impacting these crucial characteristics. The structure-performance relationships in this material system reinforce the key molecular characteristics for the design of polymers for shape memory.