Assembly and Organization in Nanostructured Polymer Thin Films
Das, Biswa Prakash
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Multilayered polymeric thin films formed by layer-by-layer (LbL) assembly technique, have many advantages over conventional blends and composites such as good control over geometry and composition, and tailored interfacial properties. The films exhibit exceptional structural and functional properties owing to the molecular level interactions between the constitutive components. Open questions about the multilayered materials including the precision of structural organization, pathways for scale–up and tunable response to external conditions have been addressed in this dissertation. The internal structure and macroscopic properties of polyelectrolyte-clay multilayers and polyelectrolyte-clay solvent cast complexes composed of strong polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA) and synthetic clay (Laponite) were compared. The clay platelet organization was related to the surface features and mechanical properties of the multilayer and solvent cast films. The molecular weight of PDDA and deposition pH of Laponite, were established as parameters to tune the structure of multilayers and solvent cast solution complexes. The obtained thickness, surface morphology, and structural organization in (i) PDDA-Laponite multilayers, (ii) PDDA-PSS-PDDA-Laponite multilayers, involving a strong polyelectrolyte, poly (styrene sulfonate) (PSS) and (iii) PDDA-PAA-PDDA-Laponite multilayers, containing a weak polyelectrolye, polyacrylic acid (PAA) revealed exponential growth mechanism in the PAA containing system and linear growth in the other two. Ionization of PAA within the multilayer was established as an essential factor contributing to the exponential growth. Deposition pH of PAA was shown to influence the thickness, surface morphology, composition, and optical properties of PDDA-PAA-PDDA-Laponite multilayers. The growth mechanism, exponential or linear, was also depicted to be a function of deposition pH of PAA for PDDA-PAA-PDDA-Laponite multilayers and PDDA-PAA multilayers. PDDA-(PAA+Laponite) multilayers, where PAA and Laponite were used in combination as a deposition solution, were demonstrated as novel clay containing exponentially growing system. The mutilayers were also investigated as pH responsive films to understand their stability and the mechanism of disassembly. The rate and mechanism of disassembly of PDDA-PAA multilayers and PDDA-PAA-PDDA-Laponite multilaters were demonstrated to be a function of pH of the immersion solution. At extremely low pH values the disassembly was fast and almost complete, as the electrostatic interactions ceased to exist. At intermediate and higher pH values, the disassembly was slow because the polyelectrolytes gained charge to become soluble molecules or soluble complexes. The variation in PDDA molecular weight and the capping with PDDA-Laponite multilayers were demonstrated as strategies to control the film disassembly to various extents. The results on the structural characteristics and properties of multilayers formed under various deposition conditions and upon exposure to different environments provide insights into their behavior and sensitivity to external stimuli during various applications.