The most important parameter in this fitting function is the critical
water content fw,c at which the particle rotation is dramatically
slowing down as a result of the first steps towards film formation.
For dispersion A (Fig. 4) the critical water content was found to be
0.40 w/w. For the other neat dispersion (B), a critical water content
of 0.39 w/w was found.
In Fig. 5, the data obtained for dispersion A with added filmforming
agent are plotted along with evaluation attempts according
to the same models shown in Fig. 4 and described in Eq. (3).
Despite the much higher baseline (resulting from plasticizing
action of the film-forming agent on the polymer), the Fermi-model
can also be applied here to describe the first phase of the drying
process. The less perfect fit of the Fermi model to the data in the
second phase of the drying process is due to the fact that also
the film-forming agent undergoes a slow evaporation process.
The critical water content obtained for both dispersion samples
with added film-forming agent was 0.43 w/w.
An increase in the critical water content in the presence of the
film-forming agent can be expected for two reasons: the increased
volume of the polymer phase due to the presence of the film-forming
agent and an increased stickiness of the polymer particles due
to the plasticizing action.
It should be noted that the strong gradient inside the profiling
NMR mouse may lead to the observation of dramatically shortened
apparent relaxation times for fast-diffusing samples (about 30 ms
instead of over 2 s for plain water under the conditions in our
experiments) [10]. Like this, such effects may also have an impact
on the phenomena described here. However, these phenomena
rather would lead to flattening of the observed relaxation time
decrease than to a stronger decrease at later drying times.