Hygromorphic Polymers: Synthesis, Retro-Michael Reaction, and Humidity-Driven Actuation of Ester–Sulfonyl Polyimides and Thermally Derived Copolyimides

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• 作者：David H. Wang ; Ruel N. McKenzie ; Philip R. Buskohl ; Richard A. Vaia ; Loon-Seng Tan
• 刊名：Macromolecules
• 出版年：2016
• 出版时间：May 10, 2016
• 年：2016
• 卷：49
• 期：9
• 页码：3286-3299
• 全文大小：620K
• 年卷期：0
• ISSN：1520-5835

文摘

With a view toward broadening the adaptive capability of polyimide-based systems that have been shown to be mechanically responsive to light, heat, and thermal-electrical stimuli, a simple diamine containing a highly polar ester–sulfonyl (ES) pendant was synthesized via a two-step route. It was polymerized with five common dianhydrides in N-methylpyrrolidinone to afford poly(amic acid), PAA, solutions, which were subsequently converted to a series of amorphous polyimides containing ester–sulfonyl (−CH₂CH₂SO₂Me) pendants, generically designated as PI-ES, by either chemical imidization at room temperature in the same pot or heat treatment of PAA cast film at 175 °C. The chemically imidized polyimide films are tough and creasable, but the thermally imidized ones are brittle because of much lower molecular weights (GPC results). In addition, a series of thermally derived copolymers designated as PI-ES:A, which contains ES and carboxylic acid (A) pendants, were prepared from PI-ES via a retro-Michael reaction at 250 °C, in which A was formed from ES pendant with the concomitant expulsion of vinyl methyl sulfone molecule. For various comparison purposes, the homopolymers, PI-A containing 100% A pendant and nonfunctional PI-N (i.e., without any stimuli-responders), were also prepared from their respective dianhydrido and diamino monomers. In addition to physical/mechanical characterization by FTIR, thermal analysis, WAXD, and DMA, the thin films of PI-ES, PI-A, and PI-ES:A have shown remarkable locomotion and beam-like oscillation under gradient (nonequilibrium) conditions created by humidity (or methanol vapor) while the PI-N, Ultem, and Nafion films were nonresponsive under the same conditions. While the state-of-the-art humidity-driven actuators have illustrated the innovative bilayer designs and clever utilization of responsive polymeric and nanocomposite systems, in which ionic moieties play the critical role in hosting the water molecules, this work shows that a simple, wholly covalent, and amorphous polymer in monolithic form can be hygromorphic and motile, and specifically this newly found humidity-gradient responsivity would enhance the functional versatility of polyimides.