Patterning Layer-by-Layer Assembled Polymeric Multilayers By Room-Temperature Imprinting Technique

Layer-by-layer (LbL) assembly technology has attracted extensive attentions of chemists, due to its strongpoints, such as rich components, easy operation, low-cost. And it plays an important role in the preparation of various functional films used in the fields such as anti-reflection coating, nonlinear optics, biosensor, and so on. A critical step which determines the final applications of polymeric multilayer films, specially in integrated optics, plastic electronics, sensors, and optical memory devices is the ability to pattern these films in a simple, flexible and economic way. As the continuously developing of the surface patterning techniques, many other novel patterning methods have been proposed beyond the typical microcontact printing (μCP), photolithography and nanoimprint lithography (NIL). Methods developed for patterning of LbL assembled polymeric films contain selective deposition technology, polymer-on-polymer stamp (POPS), multilayer transfer printing (MTP) and lift-off technology. Whereas, those methods are all based on the chemical properties of polymers used, and sometimes will be of no effect for the patterning some special kinds of polymeric films that is impossible or at least not easy to achieve by using other patterning techniques. Therefore, it is important to develop a universal, easy operation and low-cost approach to pattern layered polymeric multilayers, which determines its final applications in the various industries. In Chapter 2, we first proposed patterning layered polymeric multilayers by room-temperature imprinting method, which is combined layer-by-layer assembly technology with nanoimprint lithography. Polyelectrolyte multilayer films of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) and PAH/poly(sodium 4-styrenesulfonate) based on electrostatic interaction were patterned by room-temperature imprinting method. Under imprinting pressure of 40 bar with a Ni mold, well-defined pattern structures with a line width of～330 nm and a separation of～413 nm were achieved, and the imprinted pattern structure is just a copy of the mold used. Meanwhile, hydrogen-bonding directed multilayer films of poly(vinyl pyrrolidone)/poly(methyl acrylic acid) and poly(4-vinylpyridine)/PAA can also be patterned in a similar way. The successful imprinting of these films originates from the high compressibility and fluidity of the layered polymeric films under high pressure. Room-temperature imprinting method provides a simple and flexible way to pattern layered polymeric multilayer films.In Chapter 3, based on previous work, we further developed room-temperature imprinting method by replacing Ni hard mold with Norland Optical Adhesives (NOA 63) soft polymer mold. Layer-by-layer assembled polyelectrolyte multilayer films of PAA/PAH have been successfully patterned by this improved imprinting method. Proper amount of water in the PAA/PAH multilayer film can decrease the viscosity of the film and facilitate the imprinting. Many factors, such as imprinting pressure, length of imprinting time, the structure and size of the patterns in the polymer mold can produce an influence on the finally imprinted pattern structures on multilayer films. High imprinting pressure of 100 bar and elongated imprinting time of several hours are needed to achieve a patterned PAA/PAH multilayer films with feature size of several tens of micrometers. With a twice imprinting, grid structures can be successfully produced when a NOA 63 mold having line structures is used. Room-temperature imprinting by using polymer NOA 63 mold provides a facile way to fabricate layered polymeric films with various kinds of large area pattern structures. In Chapter 4, as one of the potential applications of these imprinted multilayer films, we further examined the cell adhesion behavior on the patterned PAA (pH 3.5)/PAH (pH 7.5) multilayer surfaces. To evaluate the effect of pattern physical properties (i.e., width and height) on cell attachment and morphology, we compared the responses of two cell types (NIH/3T3 fibroblast and HeLa cell) cultured on various PAA/PAH pattern surfaces. PAA/PAH multilayers are with good biocompatibility and always cytophilic to NIH/3T3 fibroblast and HeLa cells, no matter whether PAA or PAH is the outermost layer. Whereas an imprinted PAA/PAH multilayer surface with defined pattern size and height influences cell adhesion. With the consist pattern height of 1.29μm, a 6.5μm-width/3.5μm-spacing line pattern surface behaves good cytophobic to NIH/3T3 and HeLa cells, while the one with 69μm-width/43μm-spacing pattern remains cytophilic. Further, fixing the pattern size at 6.5μm-width/3.5μm-spacing, the cytophobic surface turns to cytophilic with decreasing the pattern height from 1.29μm to 107 nm. Interestingly, NIH/3T3 cells adapt to the line shape, adopting high aspect ratio to spread on the line pattern surface. This kind of smart transition from cytophilic to cytophobic on the PAA/PAH polyelectrolyte multilayer surface is achievable by simply imprinting multilayer films with an appropriate feature size. In other words, we can use the patterned layered PAA/PAH multilayers by room-temperature imprinting to effectively control and direct the cell adhesion behavior cultured on pattern surfaces.