Triple-Shape Memory Materials via Thermoresponsive Behavior of Nanocrystalline Non-Isocyanate Polyhydroxyurethanes

Crystallization of long n-alkyl side chains within the confined environment of nonisocyanate polyhydroxyurethane (PHU) networks renders PHUs thermoresponsive, enabling thermomechanical programming of temperature-induced shape changes. Key intermediates of shape memory PHUs are highly branched, semicrystalline polyamidoamine curing agents tailored by amidation of a polyamine-terminated hyperbranched polyethylenimine with semicrystalline long chain behenic acid. Both cure temperature and content of n-alkyl side chains, varied independently, govern crystallization behavior, phase separation and mechanical properties of semicrystalline PHU networks obtained by curing pentaerythritol-based polyfunctional cyclic carbonates with hyperbranched, semicrystalline polyamidoamines. As compared to conventional PHUs, the incorporation of hydrophobic, crystalline n-alkyl side chains significantly lowers hydrophilicity. Typically, the n-alkyl side chains of behenic amides in PHU networks melt at temperatures varying between 40 and 75 °C. According to analyses by means of atomic force microscopy (AFM) and differential scanning calorimetry (DSC) crystallization of the behenic amide side chains accounts for nanophase separation producing nanocrystalline PHUs with programmable shapes. Hence, controlled PHU crystallization and PHU nanostructure formation afford thermomechanical programming of PHU triple-shape memory materials memorizing two different shapes in addition to the original shape within a single shape memory cycle. Opposite to conventional polyurethanes, triple-shape memory PHUs require neither the use of isocyanates nor phosgene.