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(a)
(b)
b
c
0
T
i
Coexistence
curve
a
c
0
T
f
c
a
b
c
0
0
Spinodal
c
1
c
s
1
c
s
2
c
2
c
s
1
2
c
1
Concentration of A
Concentration of A
(a) Phase separation in a binary mixture of polymers A and B as a function of concentration of A,
showing the coexistence (binodal) and spinodal curves. Below the binodal curve the mixture is
separated into two phases, and above it is miscible. The temperature where the binodal and spinodal
touch is the upper critical solution temperature (UCST). Starting at high temperature Ti
i
and
quenching the mixture at the final temperature Tf,
f
, we can see a phase transformation through
nucleation and growth in the case where the initial concentration is c
0
;
and via spinodal
decomposition when it is c
0
. c
1
and c
2
are the equilibrium concentrations of the two phases at Tf,
f
,
and c
s
and c
s
are the concentrations on the spinodal line. (b) The corresponding free energy of
mixing versus concentration of A in the mixture A+ B, at temperature Tf
f
determining the
boundaries of the phase diagram.
Figure 10.1
For example, if a solution of nominal concentration around 0.5% separates into a phase
with a volume fraction
ϕ
of 0.4, its effective concentration is now (c/
ϕ
) = 1.25%.
On the other hand, gels formed by
'
synergistic
'
interactions can be divided into two
types: coupled networks, formed by speci
c binding between two different polymers;
and interpenetrating polymer networks (IPNs). The latter comprise two networks, both of
which completely
fill the available space and so mutually interpenetrate but, structurally
at least, appear not to interact or in
uence one another. For example, if images are made,
what will appear is a superimposition of the images of the two separate components. In
modern parlance they can be regarded as nanocomposites. According to Lipatov and
Alekseeva (
2007
), the IPN results from incomplete phase separation of networks, where
the kinetics of phase separation and of gelation proceed in non-equilibrium conditions.
A number of excellent reviews and chapters on mixed gels have been published
(Morris, E. R.,
1990
; Morris, V. J.,
1998
; Tolstoguzov,
1998
,
2006
;Morris,E.R.et al.,
2009
). The aim of the present chapter is to compliment these publications rather than to
present a complete catalogue of systems (which would require several volumes rather than a
single chapter). It is important to stress that comparison between workers is particularly
dif
cult because of variability in samples and in preparative and experimental conditions.
There are very few
studies because most workers have pursued only a few
techniques. For this reason there is still much scope for future research in this area.
In many applications, gelling systems contain mixtures of several biopolymers, and we
shall concentrate almost exclusively on these. Water is generally a good solvent for the
biopolymer species of the blend composition (proteins, polysaccharides), but phase
separation for such systems is still a very common phenomenon (Harding et al.,
1995
).
'
complete
'
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