Fluorescence depolarization measurements of polymer segmental mobility

Abstract

Measurements of fluorescence lifetime and depolarization have been made on dilute solutions of a number of polymer chains containing fluorescent groups. Bulky xanthydrol groups do not provide a sensitive measure of chain segmental mobility, but anthracene or naphthalene units adequately reflect segmental mobility in methacrylate- and styrene-based polymers. However, comparison with di-electric data suggests that groups of this size fail to reflect changes in mobility in “flexible” chains such as poly(methyl acrylate) and poly(vinyl acetate). It is shown that rotational times for groups at chain ends are less than one fifth of those for the same groups in the chain interiors. The results suggest that non-steady state determinations of rotational relaxation times are more significant than steady-state observations for groups in very “stiff” chains where rotation is markedly anisotropic. Under these conditions use of the Perrin equation where polarization is studied as a function of T/η does not seem justified. It is confirmed that whole molecule rotation (rate-dependent on molecular weight) is the important process in poly(N-vinylcarbazole) of molecular weight below 10⁴. Unambiguous evidence for singlet energy transfer along the dissoved poly(N-vinylcarbazole) chain is presented.