Humidity-Driven Transparent Holographic Free-Standing\nPolyelectrolyte Films

Abstract

In\nthe present work, transparent holographic poly­(diallyldimethylammonium\nchloride) (PDADMAC)/heparin and PDADMAC/poly­(styrenesulfonate) (PSS)\nfilms were synthesized via polyelectrolyte coacervates. PDADMAC/heparin\nfilms were obtained without temperature treatment. Thin holographic\nfree-standing films with a 1 μm grating period and uniform surface\nof a polyelectrolyte complex were readily and quickly made by pressing\npolyelectrolyte coacervate, the hydrated viscoelastic fluid-like form\nof polyelectrolyte complex precursor, between a flat surface and holographic\nmask. Heparin replaces PSS in film composition to prepare the sheer\nfilm. Thus, the PDADMAC/heparin holographic film demonstrates transparency\nand reversible response for humidity under diffraction detection.\nIn addition to diffraction humidity signal measurements, the cobalt­(II)\nchloride was impregnated in polyelectrolyte coacervate to make an\nadditional colorimetric signal response. In this case, the free-standing\nfilm serves both as the substrate for the hygroscopic salt and as\na diffraction humidity sensor. The PDADMAC/heparin/Co­(II) chloride\nfilm demonstrates a linear humidity range from 50 to 90%. Additionally,\ndue to hydrated inorganic salt ion size, cobalt chloride prevents\nfilm porosity, which initiates under film swelling. Based on the results\nand calculations obtained, the study proposes the mechanism of water\nincorporation, including the reptation model and polyelectrolyte complex\nbehavior. Results of density functional theory calculations prove\nthat binding of cobalt aqua complexes [Co­(H₂O)₆]²⁺ with the dimeric associates heparin/PDADMAC via noncovalent\ninteractions (hydrogen bonds) additionally is much more energetically\nfavorable compared with the alternative association of heparin/PDADMAC\nwith water molecules.