Tuning Stiffness of Free-Standing Hydrogen-Bonded LbL Films with Fe3+ Coordination

Layer-by-layer (LbL) assembly of polymers through hydrogen bonding is a versatile technique for building ultrathin functional films. However, the hygroscopic nature of the polymer building blocks and hydrogen-bonding propensity of water can render the assembled films sensitive to environmental conditions, especially humidity, resulting in the low stiffness of such films. In this paper, we report a simple approach to cross-link free-standing hydrogen-bonded LbL films by coordination with iron(III). The stiffnesses of these films increased by three orders of magnitude upon cross-linking with Fe3+, allowing the films to be handled and manipulated easily at high humidity. Moreover, we used the photoinduced reduction of the coordinated iron(III) ions to demonstrate the potential for tuning the mechanical properties of these films using light. In sum, this paper demonstrates a simple and effective approach for strengthening and tuning the mechanical properties of hydrogen-bonded LbL films.