Biomedical Engineering Reference
In-Depth Information
a shorter side chain was examined with poly(vinyl palmitate), but it too gave
rise to unstable films. Thus, the side chain length effect on vinyl polymer is
examined by a binary system of PVP and PVAc [109]. Examples of isotherms
from stepwise compression are shown in Fig. 18. Increasing the PVP compo-
sition causes the film to become more condensed, while the film stability is
decreased. The film instability is well shown in Fig. 19 where both compres-
sion and stepwise addition were employed for a 72 mol % PVP monolayer.
The isotherms are essentially identical up to
5 mN m
-1
, however, beyond
Π
≈
this point
Π
addition
, a clear signature for
long relaxation times of the film upon compression. Thus,
Π
compression
was always greater than
5 mN m
-1
,we
are no longer in thermodynamic equilibrium if we proceed with the compres-
sion method.
Turning to the viscoelastic properties of these binary monolayers, a sim-
ple trend is confirmed. The larger is the content of PVP, the more viscous
becomes the monolayer, starting from the nearly elastic monolayer of PVAc.
We show the trend by means of the polar plots at two compositions, 27 mol %
and 72 mol % but not an intermediate third composition; the latter case
is shown only for results obtained with the successive addition method.
They are shown in Figs. 20 and 21. Although the plots appear to resem-
ble that of PVAc, Figs. 14A and 16D, several key differences emerge. For the
Π
≥
Fig. 19
-
<A>
isotherms for 72 mol % PVP/PVAc mixed monolayers on water at
25
◦
C by stepwise addition and compression. Surface pressure
Π
for 72 mol % poly(vinyl
palmitate)/poly(vinyl acetate) binary mixture as a function of area per monomer. The
surface concentration was controlled as noted in the plot. For the stepwise-addition tech-
nique, lens formation was observed in the region where the two techniques differ for
Π
Π
10 mN m
-1
. For the plot, the mixture required stabilization times considerably
longer than the 1-2 hours allowed between points to form “equilibrium” films.
<A>
=
average area per monomer
>
≈
