Small-Angle Neutron Scattering Study on Selective Solvation of Poly(12-acryloyloxydodecanoic acid-co-acrylic acid) Gels in Mixed Solvents

The roles of hydrophobic ionizable side chains in amphiphilic copolymer gels consisting of 12-acryloyloxydodecanoic acid (ADA, hydrophobic) and acrylic acid (AA, hydrophilic) (ADA/AA) have been investigated by small-angle neutron scattering (SANS). The equilibrium swelling ratio and the structure factor of copolymer gels were obtained as a function of the solvent composition of two mixed solvent systems consisting of water and organic solvents. Propionic acid and 1-propanol were chosen because these solvents have similar values of log P, where P is the partition equilibrium coefficient with respect to octanol/water. This choice made it possible to suppress hydrophobicity effects and to illuminate hydrogen-bonding effects. The ADA/AA gels exhibited different swelling behaviors and disorganization processes of organized structures depending on the kind of mixed solvents in the concentration range of the organic component from x = 2−15 mol %, where x is the mol % of organic solvent in water. SANS results indicated the presence of an organized structure for the ADA/AA gel in water (x = 0 mol %) and up to x = 5 mol % with a long spacing of ca. 55 Å. The organized structure was disorganized by further increase of x, depending on the kind of solvent. The roles of acidity as well as hydrophobicity on organization/disorganization of hydrophobically associated structure were discussed from the viewpoints of swelling behavior and microscopic structure.