Swelling behavior and microstructure of poly(12-acryloyloxydodecanoic acid-co-acrylic acid) gels in the ethanol/water system

The swelling behavior and hierarchical structures of poly(acrylic acid) (AA) homopolymer gels and poly(12-acryloyloxydodecanoic acid (ADA)-co-AA) gels carrying ionizable groups as well as hydrophobic groups have been investigated by equilibrium swelling measurement, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The equilibrium swelling degree (S,) and the structure factors of the copolymer gels were obtained as a function of the ethanol (EtOH)/water (W) solvent composition, x (= vol % of EtOH). S, showed a convex-upward function of x. That is, the gels swelled with increasing x until x = 50-60 vol % for AA gels and x = 70-80 vol % for ADA/AA gels and then decreased gradually. This phenomenon is explained by hydrophobic association of long alkyl chains of ADA and a cosolvency effect of poly(AA) in EtOH and W. The DLS results also indicated that the major component of light scattering was frozen inhomogeneities. SANS intensity functions, I(q), for the copolymer gels when x - 25 vol % showed the presence of an ordered structure in water with a long spacing of ca. 46 Angstrom. These experimental pieces of evidence suggest a strong correlation between the macroscopic properties, such as S, and the microscopic structure as well as the dynamics. The diffusion coefficients were obtained by DLS as well as SANS, of which physical meanings are discussed by comparing the x dependence of S-v.