Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 232

20

B

A

C

Binodal

C1

10 Spinodal

C2

C3

0

0 0.1 0.2 0.3

φ 2

Binodal ( ○ ) and spinodal ( ● ) lines for the system aPS-TD. Reproduced from Arnauts et al.( 1993 )
with permission from John Wiley & Sons.

Figure 8.7

same internal composition until they are large enough to scatter light. Typically, polymer
solutions will exhibit this type of behaviour near the

θ -temperature. If the mobility of the

system is suf ciently low and if the solution is quenched rapidly to a temperature below

the spinodal line, nucleation and growth can be avoided, and the solution can phase
separate by a spinodal mechanism. Provided the volume fraction of the minor component
is high enough, the microphase morphology will consist of two continuous intercon-
nected phases separated by a characteristic distance. In the initial stages of microphase
separation, the same characteristic interphase distance will be maintained with increasing
time. If one of the phases is a polymer-rich phase, the morphology will be frozen in, as
soon as the polymer concentration increases enough to cause the glass transition of this
phase to be below the temperature of phase separation. Interesting morphologies can be
obtained when the solvent is eliminated from the demixed samples.

8.3.1.1 Morphology investigations

For the system aPS-TD (Arnauts et al., 1993 ), three different situations have been
followed through the phase diagram, indicated in Figure 8.8 ; samples were cooled to
− 40°C before morphological observations. Cooling of a solution with c = 4.4% w/w into

the binodal region resulted in the formation of spherical, glassy particles in turbid, freely

flowing solutions, which showed a tendency to agglomerate into larger domains and
eventually deposit on the bottom of the tube ( Figure 8.8a ). More concentrated solutions
(c >5 10% w/w) phase separated into opaque gels at c.10°C. A solution with the critical

polymer concentration (6% w/w) demixed by a spinodal mechanism, resulting in the
formation of the typical spinodal morphology ( Figure 8.8b ). At this overall polymer
concentration, material continuity of the glassy phase was achieved and this phase was
no longer deposited on the bottom of the tube. The morphology represented in

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