A Reconsideration of the Measurement of Polymer Interdiffusion by Fluorescence Nonradiative Energy Transfer

A general formalism has been developed for quantitative determination of polymer self-diffusion coefficients, D_p, using fluorescence nonradiative energy transfer (NRET). The experimental geometry consists of a “sandwich” of two thin polymer films, one labeled with NRET donor chromophores and the other with NRET acceptor chromophores. D_p can be characterized self-consistently by steady-state fluorescence intensity measurements of donors or acceptors or by transient donor fluorescence intensity decay measurements as a function of interdiffusion time, t. For t < x²/(16D_p), where x is the thickness of the donor-labeled polymer layer, increases in the normalized acceptor intensity and normalized energy transfer efficiency with interdiffusion are the same and equal to k_n(D_pt)^1/2/x, where k_n is a function of the initial acceptor concentration. Similarly, the decrease in the normalized donor intensity with interdiffusion is proportional to (D_pt)^1/2/x. The general formalism presented here has been compared to earlier approaches, revealing that a previous method of analyzing the steady-state acceptor intensity in terms of polymer diffusion is merely a limiting case of the present formalism while a previous method of analyzing the donor intensity decays results in underestimates of D_p.