LUMINESCENCE STUDIES OF POLYELECTROLYTE BEHAVIOR IN SOLUTION .3. TIME-RESOLVED FLUORESCENCE ANISOTROPY MEASUREMENTS OF THE CONFORMATIONAL BEHAVIOR OF POLY(METHACRYLIC ACID) IN DILUTE AQUEOUS-SOLUTIONS

Time-resolved anisotropy measurements (TRAMS), using synchrotron excit ation of fluorescence, have been used to study the conformational beha vior of poly(methacrylic acid) (PMAA) in dilute (10(-3) wt % in polyme r) aqueous solution. Copolymerized acenaphthylene (ACE) and 1-vinylnap hthalene (1-VN) were used as fluorescent labels (0.5 mol %). In basic media, segmental relaxation of the polysalt is adequately described by a single exponential model of the fluorescence anisotropy, r(t). Howe ver, the dynamics of the acidic form of PMAA are more complex. A minim um of two exponential terms is required for adequate description of r( t). Furthermore, below pH = 4, the relaxation data for PMAA/ACE and PM AA/1-VN become nonequivalent. The rotational correlation time associat ed with the slower motional process [evident upon dual exponential mod eling of r(t)] of the label becomes pH independent for PMAA/ACE. In co ntrast, that for PMAA/1-VN maximizes at ca. pH = 4. These differences might have origins in a hindrance upon backbone motion of the PMAA exe rted locally by the ACE label at the site of chemical attachment to th e chain. Alternatively, the 1-VN label might enjoy greater mobility at lower pH values as a result of ''decoupling'' of its motion independe nt of the chain, from that of the macromolecular segments. This might result from a reduction in carboxylate-carboxylic acid interactions at pH values less than that corresponding to the 'half-neutralization' p oint.