LUMINESCENCE STUDIES OF POLYELECTROLYTE BEHAVIOR IN SOLUTION .2. THE APPLICATION OF STEADY-STATE FLUORESCENCE ANISOTROPY MEASUREMENTS TO THE STUDY OF THE CONFORMATIONAL BEHAVIOR OF POLY(METHACRYLIC ACID) IN DILUTE AQUEOUS-SOLUTION

Steady-state anisotropy and fluorescence-quenching measurements have been used to study the segmental relaxation of poly(methacrylic acid) (PMAA) in dilute aqueous solution. None of the quenchers (CH3NO2, T1+ and I-) employed in this work exhibited truly dynamic quenching of the excited states of the fluorescent species (0.5 mol% copolymerized acenaphthylene and 1-vinylnaphthalene) used to label the PMAA. As a consequence, the dependence of the measured anisotropy r upon the relative, quenched fluorescence intensity could not be used as a means of determination of the intrinsic anisotropy r0. Furthermore, the influence of a static component in the quenching process rendered plots of r-1 as a function of fluorescence lifetime less reliable for estimation of r0 than would be the case with a purely dynamic quencher. (This limitation is a direct consequence of the difficulties encountered in the measurement of r itself in the presence of significant static quenching.) Consequently, under these conditions, steady-state fluorescence anisotropy measurements have been shown to be of limited value in studying the conformational behaviour of PMAA. In contrast, time-resolved measurements have allowed the determination of rotational correlation times which are internally consistent and independent of the choice of fluorescent label at the pH values studied.