Layer-by-Layer Assembled Free-standing Films

Layer-by-layer (LbL) assembled multilayer films have attracted much attention in the recent decade because of their potential applications as separation membranes, sensors, optical and electronic film devices, and so forth. The LbL assembled multilayer films are usually deposited on solid substrates which provide firm support but can meanwhile produce an influence on their properties. The preparation of LbL assembled free-standing multilayer films which exist without solid substrates is meaningful because of the following: (i) Free-standing films make it possible to investigate directly the elastomeric properties of the LbL assembled films. (ii) Free-standing films are expected to broaden further the application of LbL assembled multilayer films, especially as separation membrances, sensors, catalytic film, micromechanical devices, or even artificial organs. In this thesis, we mainly focus on the development of a facile method for the preparation of LbL assembled free-standing films, which named as ion triggered exfoliation. And we strudy the functions and applications of the LbL assembled free-standing films obtained by ion triggered exfoliation.In chapter 2, a facile way to prepare sheet- and tubelike free-standing films of poly(acrylic acid) (PAA)/poly (allylamine hydrochloride) (PAH) was developed by exfoliating PAA/PAH multilayer films from substrates in acid aqueous solution containing copper ions. The exfoliation of the PAA/PAH film from the substrate was achieved by breaking the electrostatic interaction of the PAA layer with the underlying substrate while keeping the integrity of the resultant films. Further study shows that thermally cross-linked freestanding PAA/PAH film can be prepared by treating the film in acid aqueous solution with a pH of 2.0. The ion-triggered exfoliation of PAA/PAH multilayer film provides a simple and flexible way to prepare layer-by-layer (LbL) assembled free-standing multilayer films.In chapter 3, a type of humido- and thermo-responsive film has been fabricated by vacuum deposition of a nanoscale-thick aluminum layer on top of the thermally cross-linked poly(acrylic acid) (PAA)/ poly(allylamine hydrochloride) (PAH) free-standing film. The bilayer Al/(PAA/PAH) film can undergo bending/unbending movements when the environmental humidity and/or temperature changes, which mimics the petals of morning glory flowers. The bending/unbending movement of the bilayer films is caused by the large difference in response of the aluminum layer and the PAA/PAH polymeric film toward humidity and temperature. The present study provides a simple and flexible way to prepare novel humido- and thermo-responsive film materials. Most importantly, we believe that the flexibility of the LbL assembly technique for free-standing film fabrication allows the introduction of well designed recognition sites and enables the response of the bilayer films toward target substances. Therefore, the present study will provide a facile way to prepare novel responsive film materials toward not only humidity and temperature changes but also other types of stimuli and substances.In chapter 3, a new type of bilayer films with 3D responsive movements has been fabricated based on the artificial muscle-like PAA/PAH free-standing film and are utilized in humidity sensors and walking robots triggered by humidity. Here, we first show that thermally cross-linked PAA/PAH free-standing films exhibit the amazing ability of transferring chemistry energy to mechanical energy in a highly efficient way. The bilayer films composed a responsive layer and an assistant layer successfully produces fast and energetic 3D responsive movements because of the mismatched of bilayer films combined with the good mechanical strength, flexibility, small scale thickness and large lateral dimensions of the free-standing films. The (PAA/PAH)&NOA63 bilayer free-standing film with 3D responsive movements can perform responding and loading function. Besides its sensitive response to humidity changes, the film can load a heavy burden as high as 119 times of its own weight with a humidity alternation between 11% RH and 40% RH. Furthermore, we analyzed the responsive movements of (PAA/PAH)&NOA63 bistructure free-standing films based on the beam theory. The mechanical analysis confirms the membrane materials micro/nanoscale thickness and macroscopic sizes are promising for the fabricating of sensor membranes and be obtained under the direction of theoretical models of bistructure films. actuator devices. We expect that the parameter optimization of various designed devices will...