Light scattering study of the formation and structure of partially hydrolyzed poly(acrylamide)/calcium(II) complexes

The Ca2+ concentration and hydrolysis degree [-COO-] dependence of the self -complexation of partially hydrolyzed poly(acrylamide) (HPAM) chains in CaC l2 aqueous solution was systematically investigated by a combination of sta tic and dynamic laser light scattering. We have, for the first time, reveal ed a transition between the intrachain and interchain complexations. For ea ch given HPAM sample, there exists a critical Ca2+ concentration ([Ca2+](ag g)) at which the interchain HPAM complexation becomes dominant. [Ca2+](agg) is related to [-COO-] by [Ca2+](agg) = 7.46 x 10(-9) [-COO-](-1.4), indica ting that the complexation is not stoichiomeric and many Ca2+ ions are free in water. We also found that even at [Ca2+] > [Ca2+](agg), the complexatio n at the initial stage was mainly an intrachain process, but gradually evol ved into an interchain aggregation. The length of the initial stage increas es as [-COO-] and [Ca2+] decrease. Our results showed that in the complexat ion process, the weight average molecular weight (M-w) of the HPAM/Ca2+ com plexes is scaled to the size (R) of the complexes as M-w proportional to R2 .11+/-0.04 for different [Ca2+] and [-COO-], suggesting that the HPAM/Ca2complexes have a fractal structure. The fractal dimension of 2.11 +/- 0.04 shows that the complexation is a reaction-limited cluster aggregation (RLCA ) process.