Conformational changes of the weak polyelectrolyte poly(acrylic acid) are monitored by polarized Raman spectroscopy as a function of molecular weight and concentration when the polymer is ionized under salt-free conditions. Persistence lengths are obtained by interpreting the depolarization ratio for CH stretching in terms of the relative number of gauche and trans conformers and then inserting these conformer statistics into a simplified rotational isomeric state model. The results show that in semidilute solution, chains extend beyond persistence lengths predicted by wormlike chain models that employ the full Poisson-Boltzmann equation, irrespective of molecular weight. In dilute solution, on the other hand, the data agree best with models that adopt the Debye-Huckel approximation. Manning condensation is not followed in either concentration regime for the molecular weights studied inasmuch as the persistence length increases at ionizations above the condensation threshold. The concentration dependent intrinsic persistence length found may be attributed to the amount of cooperativity exhibited by hydrogen bonding.