Synthesis, structure and tunable shape memory properties of polytriazoles: dual-trigger temperature and repeatable shape recovery

Abstract

Thermally induced shape memory polymers (SMPs) are capable of storage, release of energy and shape recovery on the macro-scale. In this work, cross-linked SMPs were synthesized from the monomers tris(4-(1-azido 3-oxy propan-2-ol)phenyl) methane (A₃), and 1,4-bis(propargyloxy) benzene (B₂), and the propargyl functionalized novolac oligomer (B₃). The A₃B₂ polymer exhibited a low trigger temperature of 92 °C, whereas A₃B₃ registered a higher trigger temperature of 125° C. The extent of shape recovery of both the SMPs was above 95% and it was repeatable for at least 10 times. On repeating the shape memory cycles, the response time for shape recovery increased, but nearly a complete shape recovery was observed in both polymers. In addition, a polymer with dual trigger temperature (83 and 113 °C) was synthesized by click polymerization between the A₃ monomer and stoichiometric equivalence of B₂ and B₃ monomers. The polytriazole networks bearing monophenyl and oligomer structures in tandem enabled both low and high trigger temperature in a single polymeric structure. The SMP possesses a shape recovery to an extent as high as 98% at both the trigger points even on the 10th shape memory cycle. These polymers exhibited hydrophobicity with a static water contact angle of about 90° and very low water absorption (<0.05 weight%). The enhanced water repellency is attributed to the inter/intra-chain hydrogen bonding characteristics of triazole bridges. The low/high/dual trigger temperature SMPs are thermally stable (T_10% > 230 °C) and are suitable for actuator applications.