Self-assembly of amphiphilic diblock co-oligomers in aqueous solution

Abstract

Amphiphilic diblock co-oligomers are short block copolymers that maintain some of the desirable features of conventional surfactants such as water solubility and short equilibration times. The aqueous solubility of amphiphilic diblock co-oligomers is significantly better than long block copolymers, while their surface activities are similar to conventional surfactants. Amphiphilic diblock co-oligomers composed of a common acrylic acid hydrophilic block (AA) (5 to 10 units of acrylic acid) and a hydrophobic block (n-butyl acrylate, tert-butyl acrylate or ethyl acrylate) were synthesized by RAFT polymerization. Self-assembly of BA5AA5, tBAxAA5 (x = 5, 7 or 10) and EAyAA5 (y = 5, 10 or 20) co-oligomers into micelles and liquid crystals, i.e. hexagonal phase (H1), cubic (I1) and lamellar phase (L), was studied using polarizing optical microscopy, small angle x-ray scattering (SAXS), fluorescence with pyrene as a probe and small angle neutron scattering (SANS). The self-assembled structures of these co-oligomers were also studied in monovalent and divalent salt solutions using SANS. The morphology of co-oligomer aggregations depend on the added salt concentration and the valency of counter ions, i.e. mono- or divalent. Moreover, these co-oligomer micelles can synergistically mix with sodium dodecyl sulfate micelles, based on the results from pyrene fluorescence and SANS. The self-assembly of these co-oligomers has allow us to design a smart material responsive to stimuli and changes in solution such as the degree of ionization of acrylic acid block, the length and structure of the hydrophobic block, the addition of salt or surfactant. By controlling these parameters, the self-assembly of the co-oligomers can be varied and even be switched from insoluble to soluble in water. These properties have promised for potential applications of these co-oligomers in industry, e.g. surfactants, detergents and drug delivery agents.