We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of molecular and macromolecular tracers in polystyrene solutions over a broad range of concentrations (c) and molecular weights (M(w,m)) of the matrix polymer. Molecular tracer diffusion scales only with the matrix concentration and superimposes on a single, nonpolymer specific, curve. On the contrary, the diffusion of macromolecular tracers in solutions of matrix polymers with M(w,m) sufficiently larger than the tracer molecular weight scales with c/c(p)*, where c(p)* is the tracer overlap concentration. We further demonstrate that FCS can address local and global dynamics simultaneously.
Fluorescence correlation spectroscopy (FCS) was employed to study the diffusion of molecular tracers in different polymer melts (polydimethysiloxane (PDMS), 1,4-cis-polyisoprene (PI), poly-(vinylethylene) (PVE), and a symmetric PI/PVE blend) as a function of molecular weight (M w ) and temperature (T). The single molecule sensitivity of the FCS technique precludes any modification of the matrix polymer properties. In all studied systems, the small tracer diffusion coefficient D(M w ,T) senses local segmental dynamics depending on the glass transition temperature T g (M w ) of the polymer matrix and not its macroscopic viscosity. From the good representation of the D(T) data by the common non-Arrhenius (VFT) function, we found that the activation energy (B D ) increases with tracer size (R) and for a given tracer the value of B D in PI is almost 2 times bigger than in PDMS. The possibility to establish a direct relation between D(T) and the segmental relaxation time τ(T) of the polymer matrix was critically addressed based on experimental data in dynamically homogeneous (homopolymers) and heterogeneous (miscible blend) systems and discussed in view of recent computer simulations of polymer/penetrant mixtures.
We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of small fluorescence tracers in liquid filled silica inverse opals. The inverse opals consisted of a nanoporous silica scaffold spanning a hexagonal crystal of spherical voids of 360 nm diameter connected by circular pores of 70 nm diameter. The diffusion of Alexa Fluor 488 in water and of perylene-3,4,9,10-tetracarboxylic diimide (PDI) in toluene was studied. Three diffusion modes could be distinguished: (1) Free diffusion limited by the geometric constraints given by the inverse opal, where, as compared to the free solution, this diffusion is slowed down by a factor of 3-4, (2) slow diffusion inside the nanoporous matrix of the silica scaffold, and (3) diffusion limited by adsorption. On the length scale of the focus of a confocal microscope of roughly 400 nm diffusion was non-Fickian in all cases.
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