Design And Structure Tailoring Of Layer-by-Layer Assembled Thick Films

The layer-by-Layer (LbL) assembly is a versatile technique for functional composite film fabrication and surface modification based on supramolecular interactions. The LbL assembly involves multiple interfacial assembly processes in which each layer can be designed individually and multilayers can be combined together according to a pre-designed sequence. During multiple interfacial assembly processes, the deposition solution interacts with the layers already deposited onto the substrate, which can change the degree of complexation between the adjacent layers and affect the entire structure and function of the LbL assembled film. The art of LbL assembly lies in how to design each individual deposition process and the way they are combined. Compared with other film-preparation methods, the LbL assembly is unique in precisely controlling over the chemical composition and structure of the films on micro- and nanoscales by the way of stepwise assembly. Meanwhile, the LbL assembly is particularly suitable for fabricating films with high repeatability on nonplanar substrates or substrates with large surface areas. Furthermore, the LbL assembly also has the prospect in mass production by simplifying the assembly process as alternate deposition and rinsing steps. Finally, the LbL assembly is compatible with existing micro fabrication techniques. Therefore, film materials with patterned structures on micro- or nanoscale could be made. Because of the above-mentioned advantages, the LbL assembly has been widely accepted as a convenient and versatile method to fabricate functional film materials with precise control over film composition and architecture on micro- and nanoscales.LbL assembly is widely utilized in its earlier stage to prepare ultrathin films with thickness less than 100 nm. However, thick films with thickness of micro- or submicro scales are easier to achieve high loading capacity for guest molecules. Meanwhile, micro and nanosized hierarchical structures, ultrastrong mechanical properties, integrated multiple functions can be more easily achieved in LbL assembled thick films than thin ones. In order to achieve the required thickness, films with tens or even hundreds of bilayers are required to be prepared, which is usually quite time-consuming. Several alternative methods have been proposed for the fast fabricating of LbL assembled thick films, including spin LbL assembly, spray LbL assembly, dynamic LbL assembly and exponential LbL assembly. We have realized the fast fabricating of LbL assembled thick films by taking large dimensional polyelectrolyte complexes as building blocks. A common problem for the methods mentioned above is how to control efficiently and conveniently over the film structures. This dissertation will discuss three different aspects concerning the design and structure tailoring of LbL assembled thick films as shown below:I. We took LbL assembled sandwich-like films of [(poly(acrylic acid) (PAA)/ poly(allylamine hydrochloride) (PAH))x-(Au nanoparticles/PAH)y]*n to investigate their imprinting possibility by a NOA 63 polymer mold at room temperature. The loading content of Au nanoparticles could easily be controlled by changing the value of x and y, which stand for the number of bilayers of PAA/PAH and Au nanoparticles/PAH, respectively. This study is important in extending the applicability of the room-temperature imprinting technique for patterning organic-inorganic hybrid films. The patterning process is dependent on the rigidity of the films, which in turn is strongly related to the content of inorganic species in the films. The flexible organic-inorganic films, which have a low content of inorganic species, can be patterned by a room-temperature imprinting process because of the compressibility and fluidity of the films under high pressure. In contrast, highly rigid organic-inorganic hybrid films which have a high content of inorganic species can be patterned by a lift-off process in which the film in contact with the NOA 63 molds is peeled off because of the strong adhesion between the film and the mold and the fragility of the film. We believe that this study of patterning hybrid films is not limited to polyelectrolyte/Au nanoparticle films but can be extended to a large variety of LbL assembled organic-inorganic films. The imprinting and lift-off methods conducted at room temperature are complementary for patterning hybrid organic-inorganic films with different compositions. The combination of room-temperature imprinting and lift-off methods by using polymer NOA 63 molds has opened a general and experimentally simple way to pattern LbL assembled organic-inorganic hybrid films with different compositions and film structures.II. We took Poly(4-vinylphenol) (PVPh)&Poly(4-vinylphyridine) (PVP) complexes formed between PVPh and PVP in ethanol as a proof-of-concept to demonstrate the inherent"living"character of polymer complexes. Such"living"systems could easily grow larger with time during the"aging"process and show quite different LbL assembly behavior due to the changes in particle size and the existence of complex aggregates, making the corresponding film structure tunable by simply changing the"aging"time of the complexes. Therefore, we believe that aging time can be taken as a new but convenient tool to control the growth behavior of the complexes and the corresponding structure of the LbL assembled films, which will endow the films with diverse structures and functions.III. We successfully fabricated honeycomb-like macroporous films from exponentially growing LbL assembled films with high diffusion of the polyelectrolytes by post treatment method. The pore structure. can be tuned in several ways. The size of pores and number of pore layers in one film can be controlled by altering the film thickness. Immersing in an aqueous AgNO3 bath following with the acidic solution treatment can produce double layer large porous structure on micrometer scales even at low film thickness because of the complexation between Ag+ and amino groups and the ion exchange interaction between Ag+ and carboxyl groups. Films with totally closed honeycomb-like macropores on micrometer scales can be fabricated from LbL assembled double-layered films composed of exponentially growing (PAA/PAH)*20 layer and the spin-LbL assembled (PSS/PDDA)*15 layer. It is worth noting that the high diffusion of the polyelectrolytes in the exponentionally growing PAA/PAH films is essential for the formation of the honeycomb-like macroporous structure. Stable honeycomb-like free-standing porous films with satisfactory mechanical stability can be obtained by releasing the films with ion exfoliation method. Such porous free-standing films are believed to find potential applications in filtration, separation and microfluidics, and so forth.