Dissolution and swelling behaviors of random and cross-linked methacrylic acid-ethyl acrylate copolymers

The neutralization behaviors of random and cross-linked methacrylic acid-ethyl acrylate (MAA-EA) copolymers were examined as a function of degree of neutralization (alpha) using potentiometric titration and laser light scattering techniques. The random MAA-EA copolymers exhibit a conformational transition from a compact latex particle to a swollen randomly coiled aggregate upon neutralization over a certain range of alpha. With further addition of NaOH, the swollen aggregates dissociate into several smaller clusters. This conformational change is controlled by the balance between electrostatic repulsion within ionized MAA groups and hydrophobic attraction of EA. The cross-linked MAA-EA copolymers do not undergo a drastic conformational change during neutralization. The polymer latex particles swell slightly upon neutralization, and the extent of chain expansion is proportional to MAA molar composition and inversely proportional to cross-linked density. The electrostatic Gibbs energy (Delta G(el)) obtained from the potentiometric titration data indicates that a higher MAA portion is favorable for the deprotonization of both the random and cross-linked MAA-EA copolymers suggesting that the dissociation is mainly dominated by polymer structure instead of the electrostatic attraction between H+ and -COO-. Moreover, static and dynamic light scattering results confirmed that the cross-linked latex particle exists as monodispersed hard sphere in the collapsed state, whereas in its swollen state the latex particle possesses a core-shell structure.