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 CaCl2 aqueous solution was systematically investigated by a combination of static and dynamic laser light scattering. We have, for the first time, revealed a transition between the intrachain and interchain complexations. For each given HPAM sample, there exists a critical Ca2+ concentration ([Ca2+]agg) at which the interchain HPAM complexation becomes dominant. [Ca2+]agg is related to [−COO-] by [Ca2+]agg = 7.46 × 10-9 [−COO-]-1.4, indicating that the complexation is not stoichiomeric and many Ca2+ ions are free in water. We also found that even at [Ca2+] > [Ca2+]agg, the complexation at the initial stage was mainly an intrachain process, but gradually evolved into an interchain aggregation. The length of the initial stage increases as [−COO-] and [Ca2+] decrease. Our results showed that in the complexation process, the weight average molecular weight (Mw) of the HPAM/Ca2+ complexes is scaled to the size (R) of the complexes as Mw ∝ R2.11±0.04 for different [Ca2+] and [−COO-], suggesting that the HPAM/Ca2+ complexes have a fractal structure. The fractal dimension of 2.11 ± 0.04 shows that the complexation is a reaction-limited cluster aggregation (RLCA) process.