Shape Memory Superelastic Poly(isocyanurate-urethane) Aerogels (PIR-PUR) for Deployable Panels and Biomimetic Applications

Abstract

Shape memory polymers (SMPs) remember and return to an original shape when triggered by a suitable stimulus, typically a change in temperature. They are pursued as cost-effective, low-density, higher-strain-tolerant alternatives to shape memory alloys. Arguably, porous SMPs may offer the near-ultimate refinement in terms of density reduction. To that end, shape memory polymeric aerogels (SMPAs) may offer a viable approach. The necessary condition for SMPs is rubber-like superelasticity, which is introduced via cross-linking. Cross-linking is also a necessary condition for inducing phase separation during solution-phase polymerization of suitable monomers into 3D nanoparticle networks. Such networks form the skeletal frameworks of polymeric aerogels. Those principles were explored here with rigid trifunctional isocyanurate cross-linking nodes between flexible urethane tethers from four short oligomeric derivatives of ethylene glycol: H(OCH2CH2)nOH (1 ≤ n ≤ 4). Formation of self-supporting 3D particle networks depended on the solubility of the developing polymer, which translated into specific combinations of the diol, monomer concentration, and composition of the solvent (CH3CN/acetone mixtures). Those parameters were varied systematically using statistical design-of-experiments methods. The skeletal frameworks of the resulting poly(isocyanurate-urethane) (PIR-PUR) aerogels consisted of micrometer-size particles. Bulk densities were in the 0.2–0.4 g cm–3 range, and typical porosities were between 70% and 80% v/v. Glass transition temperatures (Tg) varied from about 30 (n = 4) to 70 °C (n = 1). At and above Tg, all SMPAs showed rubber-like elasticity. They also became stiffer after the first stretching cycle, which was traced to maximization of H-bonding interactions (NH···O═C and NH···O(CH2)2). Below the Tg zone, the elastic modulus of all formulations decreased by about 1000 fold. That property gave rise to a robust shape memory effect (SME), the quality of which was evaluated via several figures of merit that were calculated from tensile stretching data over five temperature cycles between Tg + 10 °C and Tg – 40 °C. All thermomechanical testing was carried out with dynamic mechanical analysis (DMA). The strain fixity was always >98%, pointing to very low creep. After the first cycle, strain recovery (a measure of fatigue) improved from 80−90% to about 100%, and the fill factor, a cumulative index of performance, reached 0.7, which is in the range of fast elastomers. The robust shape memory effect was demonstrated with deployable panels and bionic hands capable of mimicking coordinated muscle function.