Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 1: Synthesis, Characterization and Absorption Spectroscopy Studies

Abstract

A new class of a series of amphiphilic polyelectrolytes functionalized with azobenzene chromophores have been synthesized and thoroughly characterized by various techniques. A facile two stage strategy is developed, and first involved the preparation of a precursor base polymer, designated as P(APEA), by the free-radical copolymerization of the monomers of acrylic acid (AA) and 2-(phenylethylamino)ethyl acrylate (PEA). In the second step, precursor PAPEA polymers are reacted and post-polymer modified with the diazonium salts to synthesize azobenzene polyelectrolyte, PAPEA-R with different substituted R-groups present on the para position of the aromatic ring of the azo chromophores. The PAPEA-R polyelectrolytes are same in their degree of polymerization and repeat unit compositions of AA and PEA monomers, and only differ from each other by the type of R-groups. The copolymers were classified on the basis of the characteristics of their R-groups into the hydrophilic-ionizeable smaller pair of ─SO3H and ─COOH, the hydrophobic-non-ionizeable smaller pair of ─ H and ─OC2H5, and the hydrophobic-non-ionizeable larger octyl pair of ─C8H17 and ─ C8F17. The prepared copolymers are also characterized by NMR spectroscopy for structure, GPC for molecular weight, and UV-Visible spectroscopy for absorption determination. In DMF solvent, approximately similar absorption maxima, (λmax) values were observed for azo chromophore-containing monomers and after incorporating these into their corresponding polymers structures. However, for the self-assembled multilayer thin films, λmax moved to lower wavelengths to exhibit a blue shift with hydrophobic R-group of ─C8H17, while this behaviour was not observed for hydrophilic R-group of ─COOH. The shift in λmax is found to be highly dependent on the type of substituted R-group, and attributed to aggregation of hydrophobic azo chromophores in DMF:H2O mixture employed for self-assembly. The presence of the ionizeable AA and light-sensitive azo-chromophore functionalized PEA monomers in the PAPEA-R polyelectrolytes impart self-assembling and photoswitchable characteristics, respectively. Through understanding and controlling the solubility and complex solution aggregation behaviour of the different substituted R-groups of azo PEL, their adsorption, thickness, morphology, wetting, molecular-control, and photoresponsiveness can be tailored to enhance the capabilities of the self-assembled multilayer film process in diversified areas of microfluidics, sensing, and controlled release.