Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 235

M w = 3 .0 × 10 4
M w / M n < 1.05

0

One-phase solution

(i)

One-phase
gel

Two-phase solution

(iii) (ii)

− 10

Two-phase gel

(iv)

− 20

0 10 0

Concentration (g L − 1 )

200 300

Phase diagram of a narrowM w distribution aPS in nitropropane. Adapted with permission from Tan
et al.( 1983 ) © 1983 American Chemical Society.

Figure 8.10

High-sensitivity calorimetric measurements were carried out to evaluate the heat
associated with the sol gel transition. A transition was detected with DSC cooling

scans, corresponding to the same temperature as T gel measured by the tilted test tube
method. From the exotherm associated with this transition, a

-

' heat of gelation ' of the

2Jg − 1 was calculated, which is a very low value, probably undetectable by
earlier authors. The phase diagrams ( Figures 8.9 and 8.10 ) for aPS indicate that gelation
occurs as a distinct transition and that the phenomenon is not caused by phase separation.
In this case, gelation behaviour is determined by polymer

order of 1 -

solvent interactions, but does
not correlate in a simple way with the binodal curve. Gel formation does not appear to
correlate with the glass transition of the highly plasticized polymer either, as was the case
for aPS-TD. The critical gelation concentration (in our notation, c 0 ) is strongly dependent
on M w . The authors also conclude that chain overlap is a necessary condition for gel
formation; since the

-

is so small, this may be reasonable.
The gelation behaviour of the polydisperse aPS-CS 2 system was compared with two
narrow M w distribution samples, one of a similar M w and the other of a similar M n .The
narrow fraction with similar M w exhibits a sol

' heat of gelation '

gel transition at higher temperature and
has a lower c 0 than the polydisperse system. On the other hand, the polymer with
similar M n shows very comparable gelation behaviour, i.e. almost identical sol

-

gel
transition temperature and c 0 .Presumablythelow-M w tail of the polydisperse poly-
mers remained in solution at the sol

-

-

gel transition and hence did not in uence the

gelation behaviour.
In order to further clarify the mechanisms of gelation of aPS in CS 2 , Yanxiang and
Deyan ( 1997 ) investigated solutions by IR spectroscopy. They examined the possibility
of a conformational change in the polymer inducing gelation, and made some interesting
observations. In the IR spectra of aPS, the spectral region 500 600 cm − 1 has been found -