Biomedical Engineering Reference
In-Depth Information
eraged over different wave vectors
k
over a relatively small accessible range,
from 264 cm
-1
(4th order diffraction) to 387 cm
-1
(6th order diffraction). Sec-
ond, with the good solvent monolayers, the data progression with increasing
Π
is such that they start at Limit I, move counterclockwise toward Limits III
and IV and then loop back clockwise from
ε
d,max
either along the same trajec-
tory or move toward the interior of the plot with higher viscosities. These are
illustrated in Fig. 16. In Fig. 17, we divide both paths, counterclockwise and
clockwise progression with
for PVAc (Fig. 16D), to show the details of paths
with error bars. Our rationale for not dwelling much on the clockwise trajec-
tory, however, arises from the fact that this regime entails onset of monolayer
collapse, hence is no longer susceptible to the analysis scheme based on the
dispersion equation. The situation is far more complex with the monolayers
under the poor solvent conditions, all approaching Limit V.
Π
5.1.2
Binary Monolayer: Side Chain Length Effect
Chen et al. [108] reported that poly(vinyl stearate) (PVS) could be studied
only in the dilute regime (
<
5 mN m
-1
), as films compressed to higher sur-
face pressures showed continuous relaxation in the surface pressure. Hence,
Π
-
<A>
Isotherms for PVP/PVAc binary monolayers on water at 25
◦
C. Surface
pressure
Π
for a variety of poly(vinyl palmitate)/poly(vinyl acetate) mixtures as defined
in the plot are shown as a function of area per monomer. Surface concentration was con-
trolled by step-wise compression. The incorporation of PVP, which does not form stable
monolayers alone, condenses the film and also increases the instability of the film.
<A>
= average area per monomer
Fig. 18
Π
