Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 222

8 Gelation through phase transformation

in synthetic and natural polymers

8.1 Introduction

Physical cross-links can be created when solutions undergo a phase transformation
(Keller, 1995 ), but the gels arising speci

cally from such a phase transformation do
not usually correspond to a stable state. However, phase transformations tend to lead to
network formation under speci c circumstances:

* when phase transformation is incomplete: for instance, a crystalline polymer fraction
coexists with an amorphous part
* when a newly formed macroscopic phase does not develop, owing to kinetic
constraints
* when connectivity is achieved either between individual molecules (chains leading
from one junction site to another) or by the structured phase morphology, not specif-
ically related to the phase transformation itself
* when phase diagrams are needed to understand the gel formation but are insuf

cient to

predict this gelation
* when, overall, physical gels arising through phase transformation are in a non-
equilibrium state.

At its final stage, liquid - liquid phase separation would generate two fluid layers; in a

liquid-to-crystal phase transition the final state would be a crystal with macroscopic

dimensions. In order to obtain a gel, the

final stages of the phase separations are not
reached and, instead, the phase separations are arrested at a certain point where the
network is formed. Every case reported in this chapter illustrates a different situation,
although the majority of the examples examined are of synthetic polymers.

8.1.1 Crystallization

Since crystallization was regarded initially as a principal route for physical gelation,
speci

first recall the
conditions for crystallization to occur from a solution. In the case where polymer and
solvent are miscible in all proportions at high temperatures, under supercooling con-
ditions crystals can form below the liquid-to-crystalline solid phase limit. These crystals
from supercooled solutions normally appear as suspended particles, giving a turbid

cally in the case of synthetic polymers in organic solvents, we