High-Strain Shape-Memory Polymers

Shape-memory polymers (SMPs) are self-adjusting, smart materials in which shape changes can be accurately controlled at specific, tailored temperatures. In this study, the glass transition temperature (T-g) is adjusted between 28 and 55 degrees C through synthesis of copolymers of methyl acrylate (MA), methyl methacrylate (MMA), and isobornyl acrylate (IBoA). Acrylate compositions with both crosslinker densities and photoinitiator concentration optimized at fractions of a mole percent demonstrate fully recoverable strains at 807% for a T-g of 28 degrees C, at 663% for a T-g of 37 degrees C, and at 553% for a T-g of 55 degrees C. A new compound, 4,4'-di(acryloyloxy)benzil (referred to hereafter as Xini) in which both polymerizable and initiating functionalities are incorporated in the same molecule, was synthesized and polymerized into acrylate shape-memory polymers, which were thermornechanically characterized yielding fully recoverable strains above 500%. The materials synthesized in this work were compared to an industry standard thermoplastic SMP, Mitsubishi's MM5510, which showed failure strains of similar magnitude, but without full shape recovery: residual strain after a single shape-memory cycle caused large-scale disfiguration. The materials in this study are intended to enable future applications where both recoverable high-strain capacity and the ability to accurately and independently position T-g are required.