Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 230

the mechanism of gel formation. DOP is a plasticizer for PVC but is not a good solvent.
For gels prepared in THF

liquid phase separation
had taken place during THF evaporation in the presence of the poor solvent, but the
authors claim that the gels were transparent. The size of the phase separated domains thus
remained small compared to the wavelength of visible light.
Lozinsky et al.( 2000 ) chose to heat the evaporated initial solutions up to 120°C, then
cool them rapidly to room temperature and follow the increase of viscoelasticity with
time. They therefore followed the build-up of the structure (sol

- DOP mixtures it is likely that liquid -

- gel transition) with time

and concluded that equilibrium values of G 0 and tan for the aged samples were in good

agreement with the corresponding values of the as-prepared samples at room temper-
ature. This means that the two protocols used by Li et al.( 1997 ) and by Lozinsky et al.
( 2000 ) lead to the same rheological state: the same PVC

δ

DOP structure builds up either
during THF evaporation at room temperature or after heating at 120°C and cooling at
room temperature. The samples were aged for 45 to 180 days and the authors found the
samples stabilized after 1 month, in agreement with other publications (Walter, 1954 ;te
Nijenhuis and Dijkstra, 1975 ; Dorrestijn et al., 1981 ). The ultimate values, although
close to each other, are not identical to the as-prepared gels, and the ageing period is
surprisingly long considering these are solutions with concentrations of about 10% w/w.
Aoki et al.( 1998 ) and Barendswaard et al.( 1999 )usedtheWinter

-

-

Chambon criteria,

-

and determined the gel sol transition temperature to be T = 54°C for a PVC of concen-

tration of 70 g L − 1 (7% w/v) or T = T=82°C for the same PVC at a concentration of 100 g L − 1 .
Reviewing this series of experiments on PVC gels, it is dif sol

transition to any thermodynamic solution state, since there is such a range of concentrations
and temperatures where it can be observed. The question of the existence of a real
equilibrium state remains totally open. In reality, the viscoelastic components reported at
any time or temperature probably depend on the whole thermal history. The local hetero-
geneities created during evaporation of the good solvent (THF) or during a rapid quenching
of a hot solution to room temperature (DOP solvent) do not allow any interpretation of the
gelation process in terms of a purely molecular mechanism. The G 0 values, which are
indeed very high in these gels (c.10 4 Pa for c = 20%w/v) at room temperature, appear to be
frequency-independent after suf

cult to relate the gel -

cient ageing times and at low temperatures (Aoki et al.,
1998 ), indicating long-lived junctions or phase morphologies, in contrast to the labile
networks observed in hydrophobic associations ( Chapter 6 ). It must be noticed also that,
unlike polysaccharide or gelatin gels ( Chapters 5 and 7), PVC gels have been studied at
relatively high concentrations (around 80%). In PVC gels various types of solvents point
towards subtle solubility properties and complex formation.

8.3 Gelation in the absence of crystallization

8.3.1 Atactic poly(styrene) in trans -decalin: phase separation and glass transition

Poly(styrene) is a vinyl polymer. Structurally, it is a long hydrocarbon chain with a
phenyl group attached to every other carbon atom.