Layer By Layer Assembly Of Ldhs Based Hybrid Films:Tuning Driving Interactions And Study On Their Physicochemical Properties

Layer-by-layer (LbL) assembly is a widely applied approach to fabricate ultrathin films, which involves the alternate deposition of various species on substrates based on chemical weak interactions. During last decade of worldwide research, numerous advanced functional thin films based on layered double hydroxides (LDHs) have been designed and fabricated by LbL assembly, which have been applied into antireflection coatings, superhydrophobic surfaces, optical sensor and drug delivery systems. However, most ultrathin films were fabricated via electrostatic LBL assembly of positively charged LDHs nanosheets and negative guest molecules. This significantly limits the development of films with more complicated structures and unprecedented functions. To meet specific requirements in applications, how to enrich LbL assembly driving interactions to the fabrication of multilayer and broaden application of films for the rapid development of LDH functional film is an challenge. In this dissertation, a series of functional thin film materials have been fabricated by LBL assembly of LDHs as host material and other guest molecules. A combination study of the experimental techniques and theoretical simulations illustrate the relationship between the two-dimensional (2D) structure, host-guest interactions, molecular orientation and the property of the composite films. Moreover, these 2D structured films demonstrate superior properties in smart electrochemical switch, magnetic and luminescent anisotropy, as well as gas barrier behavior.(1) Temperature-responsive LDH/poly(N-isopropylacrylamide) (pNIPAM) films were fabricated by the LBL assembly method based on hydrogen bonding interaction, which show a reversible ON-OFF transformation in their electrocatalytic performance. A 2D periodic layered structure vertical to the substrate was obtained via the alternate deposition of LDH nanoplatelets and pNIPAM. The resulting films exhibit reversible transformation in surface topography, film thickness and surface wettability by modulating the temperature in the range 20-40℃, owing to the contracted and expanded configuration of pNIPAM polymer. In addition, a temperature-triggered ON-OFF switch for electrocatalytic oxidation of glucose based on the (LDH/pNIPAM)n films was achieved, resulting from the significant change in charge transfer rate. The mechanism of switchable electrochemical behavior of the films was explored by comparison study, which is attributed to the contraction-expansion transition of pNIPAM with low-high impedance. It is expected that the strategy presented here can be employed to fabricate a variety of intelligent materials based on regular arrangements of smart species within a 2D inorganic matrix.(2) Flexible and robust free-standing films were fabricated via LBL assembly of NiFe-LDH nanoplatelets, poly (vinyl alcohol) (PVA) and styrylbiphenyl derivative (BTBS) by hydrogen bonding and electrostatic interaction. The obtained free-standing films show enhanced magnetic and luminescent anisotropy simultaneously. A series of experimental techniques indicate that the films possess a periodic layered and uniform surface morphology. In contrast to the disordered LDH/PVA/BTBS film, the (LDH/PVA/LDH/BTBS)n films with ordered structure exhibit enhanced magnetic and optical performance, including higher saturation magnetization, longer luminescence lifetime and stronger polarized photoemission. Moreover, owning to highly arrayed 2D-organized structure for LDH nanoplatelets, the free-standing film displays enhanced magnetic and luminescent anisotropy. It is expected that this novel material with magnetic and luminescent anisotropy may have potential applications in magneto-optical materials.(3) Multilayer films were fabricated based on LBL assembly of LDH nanoplatelets and several polymer guests by electrostatic, hydrogen and chemical bonding interaction, which exhibit tremendously enhanced oxygen barrier property. The research is divided into three sections. First of all, (CA/LDH)n multilayer films were fabricated by LBL assembly of cellulose acetate (CA) and LDH. The oxygen transmission rate of the resulting (CA/LDH)n multilayer films can be tuned by changing the aspect ratio of LDH nanoplatelets. The obtained (CA/LDH)n multilayer films display excellent oxygen barrier behavior. Molecular dynamics (MD) simulations reveal that a hydrogen bonding network occurs at the interface of highly-oriented LDH nanoplatelets and CA molecules, accounting for the suppression of oxygen transportation and the resulting largely-improved barrier behavior. Then, a self-healing film with high gas barrier performance via LBL assembly of poly(sodium styrene-4-sulfonate) (PSS) and LDH nanoplatelets followed by assembly of PVA. Owing to the high orientation imparted by LDH nanoplatelets, the (LDH/PSS)n-PVA film exhibits largely enhanced oxygen barrier property. Most importantly, the obtained film is capable of self-healing crack area by humidity stimulation, owing to the formation of hydrogen-bonding among hydroxyl groups of PVA with the assistance of water molecules. Finally, ultralow gas barrier (XAl-LDH/PAA)n-CO2 films were fabricated by alternating assembly of various kind of XA1-LDH nanoplatelets (X= Mg, Ni, Zn, Co) and polyacrylic acid (PAA) using the LBL assembly followed by CO2 assembly. This aligned 2D structure creates a long diffusion length to hinder the diffusion of oxygen. And, the CO2 molecules assembled on the surface of LDH nanoplatelets fill the pinholes on the organic-inorganic interface, which enables the prominent reduction of free volume and consequent suppression of oxygen permeation. Therefore, this work provides a facile and cost-effective strategy for the fabrication of LDH-based oxygen barrier material, which can be potentially used in flexible display, druggery and food packaging field.