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Morphology and Proton Transport in Humidified Phosphonated Peptoid Block Copolymers

Sun, Jing, Jiang, Xi, Siegmund, Aaron, Connolly, Michael D., Downing, Kenneth H., Balsara, Nitash P., Zuckermann, Ronald N.
Macromolecules 2016 v.49 no.8 pp. 3083-3090
N-substituted glycines, clean energy, composite polymers, hydrogen fuel cells, molecular weight, phosphonates, photosynthesis, protons, small-angle X-ray scattering, transmission electron microscopy, water content, water uptake
Polymers that conduct protons in the hydrated state are of crucial importance in a wide variety of clean energy applications such as hydrogen fuel cells and artificial photosynthesis. Phosphonated and sulfonated polymers are known to conduct protons at low water content. In this paper, we report on the synthesis phosphonated peptoid diblock copolymers, poly-N-(2-ethyl)hexylglycine-block-poly-N-phosphonomethylglycine (pNeh-b-pNpm), with volume fractions of pNpm (ϕNₚₘ) values ranging from 0.13 to 0.44 and dispersity (Đ) ≤ 1.0003. The morphologies of the dry block copolypeptoids were determined by transmission electron microscopy and in both the dry and hydrated states by synchrotron small-angle X-ray scattering. Dry samples with ϕNₚₘ > 0.13 exhibited a lamellar morphology. Upon hydration, the lowest molecular weight sample transitioned to a hexagonally packed cylinder morphology, while the others maintained their dry morphologies. Water uptake of all of the ordered samples was 8.1 ± 1.1 water molecules per phosphonate group. In spite of this, the proton conductivity of the ordered pNeh-b-pNpm copolymers ranged from 0.002 to 0.008 S/cm. We demonstrate that proton conductivity is maximized in high molecular weight, symmetric pNeh-b-pNpm copolymers.