Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 223

appearance to the solution. The state of maximum stability is that of a macroscopic
crystal, and the usual polymer crystallization leads to a crystal

amorphous structure in a
single-component system (polymer melt) in a non-equilibrium state. In the gels, the non-
equilibrium

-

expands throughout the solvent. Such systems, not being in
equilibrium, eventually collapse by expelling the solvent (the so-called syneresis effect),
but the non-equilibrium state can be long-lived, thus retaining the appearance of a gel
state for weeks or months. The explanation for the growth restriction on new crystals, and
the limitation of size for the existing crystals, is a major question which has to be solved
to understand the persistence of the gel state.
In block copolymers, the sequences that can crystallize under certain circumstances
form the network junctions and the rest of the chain the connecting paths. Gelation
should then proceed until all the crystallizable portions of the copolymer have formed
crystal junctions, and so the gel state would approach true equilibrium for the micro-
structure. This is indeed observed for micelle formation in aqueous solutions of some
amphiphilic copolymers under certain conditions ( Chapter 6 ). In a homopolymer the
factors that can in

' structure '

uence the amount of crystallizable material are the tacticity of the
chain, type of solvent, temperature and time (since this is a non-equilibrium state).
Tacticity, which can be measured directly using proton or 13 C NMR spectroscopy, refers
to polymers with one non-hydrogen substituent (R) attached to the carbon chain per
monomer unit. In isotactic macromolecules all these substituents are located on the same
side of the macromolecular backbone; in syndiotactic macromolecules they have alternate
positions along the chain; in atactic macromolecules all Rs are placed randomly along the
chain. In principle atactic polymers cannot crystallize, but this is not always the case. Chain
tacticity will be thoroughly examined for a number of gel systems in this chapter.

8.2 ' Crystallization ' -induced gelation: poly(vinylchloride) (PVC) gels

Poly(vinylchloride) is the third most widely produced commercial plastic, after
poly(ethylene) and poly(propylene). PVC is widely used in construction because it
is cheap, durable and easy to assemble. It can be made softer and more flexible by the

addition of plasticizers, the most widely used being phthalates. In this form, it is used
in clothing and upholstery and to make

flexible hoses and tubing, flooring and

electrical cable insulation.
PVC is usually produced by radical polymerization at 50°C, and under these con-
ditions the material is atactic. Using various heating protocols between − 60°C and +90°

C during synthesis, Ceccorulli et al.( 1977 ) investigated their in uence on PVC tacticity

and degree of crystallinity. They observed that changes of tacticity produced changes of
glass transition temperature (T g ) in the bulk material, and showed that partially syndio-
tactic samples have a higher T g than nearly atactic samples. Such samples are synthesized
at low temperature (low-temperature PVC) and have a higher degree of crystallinity.
After extensive thermal cycling of the bulk material around 230°C, the difference in T g
persisted between low- and high-temperature PVCs, while the crystalline content was
melted out.