Structural Properties and Phase Behavior ... 134. PARTIALLY CHARGED COPOLYMERS
4.1. Symmetric Diblocks
In the presence of charges, the RHS of Eq. 14 should be modified to
include the ionic contribution according to the general result of the RPA given
by Eq. 4. The resulting expression for Saa(q) is obtained as
2(^1) as bs ab
1 1 2
aa as 1
bs
1
1 2 1
(^112)
1
Sq
S S q q f
Sq
. (16)
This is still a simplified expression, which nevertheless, contains the
important ingredients of structural properties and phase behavior for partially
charged DCPs. Analysis of the scattering curves Saa(q) vs qR^22 g permits to
evaluate the effects of ionic parameters such as the degree of ionization f and
the salt concentration salt.
Figure 5 shows that the peak position qm increases while Saa(qm) decreases
with f (panel a). Panel b illustrates the changes of the scattering envelop Saa(q)
versus qR^22 g with f. Knowing that the number of charges carried by each chain
is Nch = Nf/2, we see that only few charges per chain (say, Nch = 10 for
f = 0.2) may lead to significant changes in the scattering curves. The
polyelectrolyte effects expressed by the long range repulsion between charged
monomers promote intra molecular inter blocks enhanced compatibility. The
microstructure shows lower sizes qm^1 and intensities S(qm) as f increases.
On the other hand, increasing the ionic strength reveals opposite trends
due to the screening of electrostatic repulsion between A-charged monomers.
The highly mobile counter ions surrounding the charged monomers tend to
annihilate the polyelectrolyte effects as demonstrated by Figure 6. Panel a
shows qRmg^22 and Saa(qm) versus salt while part b gives Saa(q) vs qR^22 g for
different values of salt. The peak position qm drops quickly with salt while
Saa(qm) increases. Note that these polyelectrolyte effects are modulated by the
polymer concentration and both peak position and height increase with for a
given degree of ionization and ionic strength.