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

The isoionic dilution method was used to measure the intrinsic viscosity, [η], and the Huggins coefficient, K(H), of two poly(acrylic acid) samples, 50% neutralized with NaOH, with molecular weights 5.0 x 10³ (Na50PA5) and 4.8 x 10⁴ (Na50PA48) at different ionic strengths. For Na50PA5 at low ionic strength, [η] is higher than the value predicted by a rigid-rod model. Assuming that the chain has reached its maximum extension, we have combined the rigid-rod model with the idea of effective dimensions that depend linearly on the Debye-Huckel screening length, κ^-1, to account for a primary electroviscous effect, as a means to describe the observed behavior. As a consistent approach, we have also combined the effective dimensions with the wormlike chain model to calculate the persistence length of the higher molecular weight sample, Na50PA48. The result is that the electrostatic persistence length, l(e), is proportional to κ^-1, in the region of high to moderate ionic strength, as is usually observed experimentally for flexible polyelectrolytes, while at the lowest ionic strengths, l(e) tends to level off, which was predicted theoretically. Both the samples show a considerable increase in the Huggins coefficient as the ionic strength is lowered, with the sample having the lowest molecular weight giving the highest value 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 κ^-1, than for Na50PA48.