Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 3: Control of Properties of Self-Assembled Multilayer Thin Films

Novel azobenzene polyelectrolytes have been used to fabricate biocompatible self-assembled multilayer (SAMU) thin films of variable absorbance, thickness, organization, and morphology. The prepared SAMU films are useful for directed cell growth, and this application relies directly on control of contact and surface energy, and requires the ability to tune the surface characteristics which are critical to their development. The azo polyelectrolytes employed here were similar in their degree of polymerization and repeat unit composition of acrylic acid monomer and azo monomers, and only differ from each other due to the presence of different substituted head R-groups present on the p-position of the aromatic ring of the azo chromophores. Possession of characteristics of both the self-assembly due to acrylic acid groups, and photoswitchability of the azo monomer enable the azobenzene functionalized polyelectrolytes to exhibit novel photo-reversible applications. The azo polyelectrolytes with the substituted R-group pairs of shorter-ionized hydrophilic COOH and SO₃H, shorter-non-ionized hydrophobic H and OC₂H₅, and larger-nonionized hydrophobic octyl C₈H₁₇ and C₈F₁₇ were used as polyanions and counter charge PDAC used as polycation to fabricate the layer-by-layer SAMU films onto glass and silicon substrates. The fabricated SAMU films were also characterized by various techniques. The UV absorption maxima, λ_max p of the SAMU films move to lower wavelength relative to solution to exhibit a blue shift for the hydrophobic R-groups, while this behaviour was not observed for the hydrophilic R-groups. Similarly, the thickness, organization, morphology and other properties of the thin films were found to be dependent on the type of substituted R-groups of the azo polyelectrolytes due to the inter-related factors of ionization, hydrophobicity/hydrophilicity, solubility, and aggregation of azo PEL in the dipping solutions used for fabrication of the SAMU films. Understanding and controlling the adsorption characteristics of azo multilayer thin–film of switchable functionalities are vital to explore their potential for the development and application of new devices in diverse areas of biosensor, drug delivery systems, on-chip microscale chemical process and microfluidics systems.