Fluorescence correlation spectroscopy
(FCS) has become an important
tool in polymer science. Among various other applications the method
is often applied to measure the hydrodynamic radius and the degree
of fluorescent labeling of polymers in dilute solutions. Here we show
that such measurements can be strongly affected by the molar mass
dispersity of the studied polymers and the way of labeling. As model
systems we used polystyrene and poly(methyl methacrylate) synthesized
by atom transfer radical polymerization or free-radical polymerization.
Thus, the polymers were either end-labeled bearing one fluorophore
per chain or side-labeled with a number of fluorophores per chain
proportional to the degree of polymerization.The experimentally measured
autocorrelation curves were fitted with a newly derived theoretical
model that uses the Schulz–Zimm distribution function to describe
the dispersity in the degree of polymerization. For end-labeled polymers
having a molecular weight distribution close to Schulz–Zimm,
the fits yield values of the number-average degree of polymerization
and the polydispersity index similar to those obtained by reference
gel permeation chromatography. However, for the side-labeled polymers
such fitting becomes unstable, especially for highly polydisperse
systems. Brownian dynamic simulations showed that the effect is due
to a mutual dependence between the fit parameters, namely, the polydispersity
index and the number-average molecular weight. As a consequence, an
increase of the polydispersity index can be easily misinterpreted
as an increase of the molecular weight when the FCS autocorrelation
curves are fitted with a standard single component model, as commonly
done in the community