Precise Solution-Based Deposition of Ultrathin Covalent Molecular Networks by Layer-by-Layer Cross-Linking Polymerization of Tetra- and Bifunctional Amine/Isocyanate Pairs

Ultrathin films of covalent molecular networks were deposited onto substrate surfaces by carrying out layer-by-layer cross-linking polymerization (LBLCP), which enabled the thickness of the resulting polyurea network film to be precisely controlled on the molecular length scale. The LBLCP was performed by alternatingly dipping the substrate into solutions of a tetrafunctional tetrahedral network former and a bifunctional counter-monomer. We investigated how the chemical and geometrical structure of the bi- or tetrafunctional amine or isocyanate monomers used in the LBLCP affected the growth behavior of the network films. Structural characteristics of these monomers such as meta/para phenylene substitution, rigid/flexible linkages, and aliphatic/aromatic nature determined whether molecular networks of these monomers could grow and how the thickness of the network increased from one deposition cycle to the next. The results showed the LBLCP method to be an efficient way of synthesizing ultrathin network fil...