Conformation of poly(methacrylic acid) in acidic aqueous solution studied by small angle X-ray scattering

The nature of the contracted form of poly(methacrylic acid) PMA chain in sa lt-free acidic aqueous solution was studied by analyzing scattering curves registered by small-angle X-ray scattering, comparing it with those of PMA in methanol at 26 degrees C and of partially neutralized PMA in aqueous sol ution containing added salt (the concentration of added salt, Cs = 0.1 M Na F). It is shown that the distribution of segments in the contracted form as well as that of PMA in methanol is that of a random-coil in a theta medium and that this distribution of segments is stable over a fair range of degr ees of ionization alpha for Cs below 0.1 M. Moreover, the persistence lengt h of PMA at Cs = 0.1 M (4 +/- 0.5 Angstrom) is substantially constant throu ghout the entire range of alpha, indicating that the contracted form of PMA changes to an expanded random-coil in a higher pH region without a signifi cant change in the chain flexibility. (C) 1999 Elsevier Science B.V. All ri ghts reserved.