Small-angle scattering study of tetra-poly(acrylic acid) gels.

Tetra-poly(acrylic acid) (tetra-PAA) gels were fabricated by cross-end coupling of azide (-N3) and alkyne (-C≡CH) terminated tetra-arm poly(tert-butyl acrylate)s. The structures of the tetra-PAA gels and their sols (tetra-PAA sols, i.e., tetra-PAA macromer solutions obtained by deprotection of tert-butyl groups) were investigated by small-angle X-ray and small-angle neutron scattering experiments in methanol, salt-free aqueous media, and salt solutions. The scattering functions, I(q), of the tetra-PAA sols in methanol were described by the Ornstein-Zernike (OZ) function, I q = I ( 0 ) / ( 1 + ξ 2 q 2 ) , whereas those of the gels were represented by the sum of the OZ function and a power-law function, I q ∼ A q b . Here, ξ is the correlation length, q is the magnitude of the scattering vector, A is a constant, and b is the scattering exponent. The OZ and the power-law functions represent the concentration fluctuations and cross-link inhomogeneities characteristic of gels, respectively. In salt-free aqueous media, there appeared a characteristic polyelectrolyte peak in the scattering functions, and the polymer concentration, C, dependence of the peak was similar for both sols and gels. In the presence of salt, both the scattering upturn for gels and the polyelectrolyte peak for both gels and sols were suppressed. Upon increasing the salt concentration, C s, the scattering intensity increased in the low q regions and the scattering functions were well fitted by the OZ function for both gels and sols.