Chain conformation and intermolecular interaction of partially neutralizedpoly(acrylic acid) in dilute aqueous solutions

The isoionic dilution method was used to measure the intrinsic viscosity, [ eta], and the Huggins coefficient, K-H, of two poly(acrylic acid) samples, 50% neutralized with NaOH, with molecular weights 5.0 x 10(3) (Na50PA5) and 4.8 X 10(4) (Na50PA48) at different ionic strengths. For Na50PA5 at low io nic strength, [eta] is higher than the value predicted by a rigid-rod model . Assuming that the chain has reached its maximum extension, we have combin ed the rigid-rod model with the idea of effective dimensions that depend li nearly on the Debye-Huckel screening length, kappa(-1), to account for a pr imary electroviscous effect, as a means to describe the observed behavior. As a consistent approach, we have also combined the effective dimensions wi th the wormlike chain model to calculate the persistence length of the high er molecular weight sample, Na50PA48. The result is that the electrostatic persistence length, l(c), is proportional to kappa(-1), in the region of hi gh to moderate ionic strength, as is usually observed experimentally for fl exible polyelectrolytes, while at the lowest ionic strengths, l(c) tends to level off, which was predicted theoretically. Both the samples show a cons iderable increase in the Huggins coefficient as the ionic strength is lower ed, with the sample having the lowest molecular weight giving the highest v alue at any particular ionic strength. This is attributed to the increasing intermolecular electrostatic interactions, which have a relatively larger importance in the case of Na50PA5, because the dimensions of the chain are smaller when compared with the range of the electrostatic interactions, as measured by kappa(-1), than for Na50PA48. (C) 1999 Elsevier Science Ltd. Al l rights reserved.