Chemistry Reference
In-Depth Information
single-molecule tracking was used to collect data. In connection with the above-
mentioned basic notion that the mobility of molecules changes during the
polymerization process, the diffusion times of the macromolecular emitter compound
4
was studied. The dynamic range that can be covered by both methods overlaps,
allowing following the full conversion (denoted here as
U
, ranging from0 to 1, 1 being
full conversion) in the polymerization process based on single-molecule fluorescence
measurements (see Figure 12.4).
Compound
was used for studying the polymerization of styrene in the absence
and presence of a 1,4-divinylbenzene cross-linker [44]. In the absence of a cross-
linker, compound
4
diffuses freely in the surrounding medium (see Figure 12.5).
The FCS autocorrelation curves can be fitted with one diffusion time up to high
conversion (0.83). The resulting values of the diffusion constant
D
, decreasing with
increasing
U
, are shown as gray circles in Figure 12.4a and b (same data set). The
acceptable fits with one
D
value indicate that translational diffusion of the dyes is
rather homogeneous.
A second set of experiments focused on the diffusion in polymer networks. For that
purpose, the above-mentioned experimental conditions were repeated with addition
of 1% and 3% DVB cross-linker, respectively. At low
U
when using 1% of DVB,
autocorrelation curves and diffusion constants were similar to the experiment without
DVB. However, when gelation started, the FCS curves could only be fitted with two
diffusion constants as indicated by the triangles (fast: green, slow: red; black circles:
weighted average) in Figure 12.4. The fast component can be related to still freely
moving reporter molecules while the slowcomponent is related to the trapped reporter
molecules. When using 3% of DVB, gelation started earlier than at 1% and the
D
values that were lower at similar
U
. Also here the FCS curves could be best fitted with
two diffusion constants.
As the speed of diffusion decreases as a function of the conversion rate, a point
will be reached when FCS measurements are no longer adequate and the single
molecules can be tracked by the wide-field detection (dotted line in Figure 12.4). The
yellow tracks in the left-side image of Figure 12.5 indicate the location the individual
molecules of compound
4
for the case when no cross-linker is present. They can be
seen still moving unhindered at 0.64
U
. In the presence of a cross-linker at similar
conversion (0.65
U
), the motion of the compound
4
reporter molecules describes the
onset of heterogeneity that arises during the formation of a network. In the wide-field
measurements this heterogeneity can be directly visualized as a fraction of the
molecules is clearly trapped by its nanoenvironment while the remainder of the
molecules seems to diffuse unhindered at the same time. The latter is depicted in
the right-side image of Figure 12.5 showing reporter molecules barely move (red
tracks) and some that are still mobile (yellow tracks).
These investigations can be extended to other polymerization systems such as
interpenetrating networks and nanocomposites during their formation process. This
particular kind of study will provide a deeper understanding of the factors that control
heterogeneities in a polymerization process. With this knowledge it may be possible
to attain a better control over polymerization and the properties of the resulting
polymers and polymer networks.
4
