SPECTROSCOPIC EVALUATION OF MODELS FOR POLYELECTROLYTE CHAIN CONFORMATION IN DILUTE-SOLUTION

Conformational changes of the weak polyelectrolyte poly(acrylic acid) are monitored by polarized Raman spectroscopy as a function of molecul ar weight and concentration when the polymer is ionized under salt-fre e conditions. Persistence lengths are obtained by interpreting the dep olarization ratio for CH stretching in terms of the relative number of gauche and trans conformers and then inserting these conformer statis tics into a simplified rotational isomeric state model. The results sh ow that in semidilute solution, chains extend beyond persistence lengt hs predicted by wormlike chain models that employ the full Poisson-Bol tzmann equation, irrespective of molecular weight. In dilute solution, on the other hand, the data agree best with models that adopt the Deb ye-Huckel approximation. Manning condensation is not followed in eithe r concentration regime for the molecular weights studied inasmuch as t he persistence length increases at ionizations above the condensation threshold. The concentration dependent intrinsic persistence length fo und may be attributed to the amount of cooperativity exhibited by hydr ogen bonding.