Directed Self-Assembly of Polyelectrolytes & Charged Block Copolymer Micelles in Aqueous Media

Abstract

Polyelectrolyte complexes (PECs) have received a growing interest since the early sixties. PECs have been used for large-scale industrial applications and have demonstrated enormous potentials in various fields such as coatings, binders and flocculants. Using the Layer-bylayer (LbL) deposition technique, an ultrathin polyelectrolyte multilayer (PEMs) coating was first built in 1990 and soon both theoretical and practical interest in these coatings were growing exponentially. In the first chapter of this dissertation, we demonstrate that polyelectrolyte (PE) multilayer thin films deposited on patterned posts with incredibly large numbers of bilayers, which would not be possible with the conventional LbL deposition methods, can be obtained in short process time using alternating polyelectrolyte droplets generated in a microfluidic channel, representing a significant advantage over the conventional processes based on the polyelectrolyte deposition followed by the separation of such substrates (typically colloidal particles) with centrifugation and sonication. Positively- and negatively-charged polyelectrolyte droplets were alternatively generated in a microfluidic channel by controlling the capillary number (Ca) as well as the fraction of dispersed phase over the continuous phase. Patterned posts, serving as the substrates for the PE deposition, were created with photocurable polymers using the optofluidic maskless lithography. The impact of these PE droplets onto the patterned posts allowed the alternative adsorption of PEs, similar to the conventional LbL deposition methods. It was shown that the intensity of fluorescence dye tagged onto (+)-charged PEs adsorbed on the post(s), taken with confocal laser scanning microscopy, increases with deposition time and varies around the post(s). The effect of post shape and interval between the two posts for the droplet-based LbL deposition was also experimentally investigated and analyzed, in connection with the numerical simulations, to elucidate the underlying principles of relevant two-phase flows. In the second chapter, we demonstrated block copolymer micelle (BCM) / BCM multilayer films with a large number of bilayers onto microposts using alternating droplets in crossshaped microfluidic channels. Alternating aqueous droplets containing oppositely charged BCMs were employed for sequential adsorption onto microposts in a specific range of capillary numbers (Ca), resulting in the production of BCM multilayer films with thicknesses of hundreds of nanometers requiring a small amount of the BCM solution in a short process time. To date, it has not been possible to deposit a large amount of BCMs onto colloidal substrates without a multi-step process involving steps such as centrifugation and redispersion by sonication to proceed to next deposition step. In addition, cross-shaped microfluidic channels with BCMs deposited onto posts were utilized for the visualization of selective and continuous destruction of the BCM multilayer films using two different laminar streams, basic distilled water as a release solvent and oleic acid as a non-reactive medium, since the ionization of cationic segments is negligible above a pH of 5.5. As a result, the BCM multilayer films were selectively removed from the posts by contact with the aqueous release solvent. In the case of BCMs touched with the non-reactive medium, the decrease of the fluorescence intensity on the BCMs, which is only modulated by the shear rate, was negligible at the same processing time. In the third chapter, we demonstrated the complexes of oppositely charged polystyreneblock-poly(acrylic acid) PS-b-PAA) micelles and polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) micelles. In generally, complexation of soft colloids can induce novel morphologies unlike packing problem of hard particles since deformable interface of soft colloids can be easily tuned by inter-corona repulsion or packing parameters. In the present study, complex morphology of charged BCMs as controlled by pH of aqueous solvent as well as solvent quality. To determine the effective pH range for the inter-corona combination of PAA and P4VP blocks in aqueous media, we studied the dissociation behavior of both coronas using Fourier Transform Infrared Spectroscopy. Lower pH region (3.6 < pH < 5.0) in aqueous medium offers stronger interactions between oppositely charged corona blocks, resulting in polymeric hexagonal prism omplexes. In the higher pH region (5.5 < pH < 6.5), they first self-assembled into hierarchical bumpy spheres induced by the simple adsorption of small PS-b-PAA BCMs on the surfaces of PS-b-P4VP large compound micelles since the degree of ionization of P4VP blocks is relatively low. However, the crew-cut BCM complex morphology with high aggregation number does not allow the hexagonal prism structure to be formed without rearranging strongly aggregated core blocks. We note that the crew-cut BCM complexation in higher DMF content of mixed solvent (1 % < DMF < 20 % in water) induces inter-corona association leading to the hexagonal prism structure due to the decrease in selectivity of water for PS blocks. In the last chapter, we demonstrate the hierarchical structures of block copolymer blend, mixture of PS-b-PAA and PS-b-P4VP, in various solvent qualities. P4VP & PAA homopolymers mixed in aqueous solution can be preferentially dissociated at specific solution pH region. That is, solution pH is approached to 1, P4VP can be preferentially charged. However, solution pH increase to 14, PAA can be preferentially dissociated. In addition, P4VP segments are soluble at solution pH < 5.5 since P4VP egments can be positively charged in the pH region. Based on this study, morphologies of PS-b-PAA and PS-b-P4VP blends were controlled by initial pH and solvent quality. As a result, when they are mixed together in co-solvent ominant environment, their phase can be separated similar in bulk. However, they are mixed in water-dominant solvent, their blend morphology are only vesicles at extremely high or low pH because their coronas are very sensitive to solution pH.