Layer-by-layer Assembly Of Preassembled Precursors

Layer-by-layer assembly (LbL), which involves the sequential deposition of specieson substrates based on complementary chemical interactions, has been proven to be awidely used approach to fabricate composite films. After two decades of worldwideresearch, numerous advanced films have been produced by LbL assembly, which includebut not limited to antireflection coatings, superhydrophobic surfaces, actuators,self-healing films, drug delivery systems, and separation membranes. The rapid progressof LbL assembly benefited mainly from enriching interactions that drive the multilayerformation, broadening types of materials that can be manipulated by LbL assembly, anddeveloping rapid LbL assembly technique that enables the convenient fabrication of thickfilms. However, the LbL assembly has not matured to the point that is powerful enough tosupport the production of films with more complicated structures and unprecedentedfunctions. A retrospective on the LbL assembly will find that there is still a gap betweenthe newest developments in self-assembly and the LbL assembly technique. The seamlessintegration of the LbL assembly with new concepts in self-assembly will provides a wayto further push the progress of LbL assembly technique. In brief, the dynamic nature of thepre-assembled precursors has not be exploited for the fabrication of LbL assembled films.In this dissertation, we designed several precursors with more diverse structures, whichenable multicomponent, multilevel LbL assembly to proceed, producing composite filmswith more complexed structures and optimized functions.In chapter1, we overviewed briefly the developments and challenges of LbLassembly. LbL assembly has advanced from the traditional LbL assembly with one speciesin each dipping solution to LbL assembly of the pre-assembled precursors with multiplecomponents and more complicated structures. By LbL assembly of pre-assembledprecusors, the structures of the resultant films can be conveniently controlled during filmdeposition through adjustment of assembly parameters of the dipping solutions or by post treatment after film deposition. Although pre-assembled precursors have been the idealbuilding blocks of the LbL assembly, the composition and structures of the previouslyused precursors were simple and did not show adaptive and dynamic features.In chapter2, we reported the preparation of polymeric actuators that are capable ofreversible bending/unbending movements and prolonging their bending deformationwithout UV irradiation by releasing thermally cross-linked azobenzene containingpolyelectrolyte films with a limited free volume from substrates. Layer-by-layer assemblyof poly{14[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediylsodium salt}(PAZO) poly(acrylic acid)(PAA) complexes (noted as PAZO PAA) withpoly(allylamine hydrochloride)(PAH) produces azobenzene-containing PAZO PAA/PAHfilms. UV irradiation induces trans cis isomerization of azobenzenes and allowslarge-scale bending deformation of the actuators. The actuators prolong the bendingdeformation even under visible light irradiation because the cis trans back isomerizationof azobenzenes is inhibited by the limited free volume in the actuators. Unbending ofactuators is attained by exposing the actuators to a humid environment at roomtemperature. Film expansion in a humid environment produces a mechanical force that issufficiently strong to enable the cis trans back isomerization of azobenzenes and restorethe bent actuators to their original configuration. The capability of the force produced byfilm expansion for cis trans azobenzene isomerization can be helpful for designing novelpolymeric actuators.In chapter3, multicomponent, multilevel layer-by-layer assembly method has beendeveloped by alternate assembly of poly(allylamine hydrochloride)-methyl red (PAH-MR)complexes with PAA. During alternate deposition of the positively charged PAH-MRprecursors with PAA, MR was released from PAH and simultaneously protonated by theacidic environment provided by PAA. As a result, the π-π interaction of MR moleculesenables the formation of MR nanofibrous layer. We proved that the dynamic nature andthe multiple interactions among the building blocks are crucial to acquire the multi-levelLbL assembly which includes several sub-assemblies within the main assembly for the fabrication of films with complex structures. We further developed the multicomponent,multilevel LbL assembly for the fabrication of PAH-1-Pyrenebutyrate/PAA compositefilms with fibrillar structures.In chapter4, PAA grafted with cyclodextrin (PAA-CD) were LbL assembled withPAH-4-(phenylazo) benzoic acid (PAH-azo) complexes to produce polymeric compositefilms with ring structures. The PAA-CD can form micellar particles on the substrate aftertreatment by acidic water. During deposition of PAH-azo on the substrate with PAA-CDmicellar particles, azo molecules can enter into the cavity of CDs in PAA chains, whichinduce the formation of micrometer-sized ring structures. It is believed that the multilevelassembly of PAA-CD, PAH and azo molecules, which originates from the diverseinteractions among them, produces the rings in the PAA-CD/PAH-azo film.