Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 224

The formation of PVC gels is strongly related to tacticity. PVC physical gels are
generally placed in the class of crystallization-induced gelation processes. In order to
compare gel properties obtained from various investigations, it is important to specify
the polymer tacticity, molecular mass and solvent. Increasing the proportion of syn-
diotactic sequences, the crystallizable chain portions of the polymer, themselves
analogous to the copolymer sequences mentioned before, are expected to enhance
the elasticity of the gels. Indeed, even in very early investigations it was noticed that
PVC can form gels with relatively high modulus in a large variety of solvents. Reports
on the gelation of PVC date back to the late 1940s (Stein and Tobolsky, 1948 ; Alfrey
et al., 1949 ; Walter, 1954 ).
Guerrero and co-workers (Guerrero et al., 1980 ; Guerrero and Keller, 1981 ) reported
that the crystallinity of PVC gels was strongly in

solvent interac-
tions. Their results suggest that the gelation ability of various PVC compositions was not
only related to stereo-regularity, but also to this polymer

uenced by polymer -

solvent interaction, i.e. the
hydrogen bonding ability of the solvent. The physical junctions of PVC gels could
therefore arise from two different origins:

-

(a) crystalline junctions related to the degree of stereo-regularity
(b) polymer

- solvent complexes.

The former are the most stable, melting at high temperatures, while the latter melt at
low temperatures. Most studies, especially those using X-ray diffraction ( Chapter 2 ),
were carried out on dried gels and stretched films (either wet or dried); such processing

may induce rearrangements of the initial structure of the gel, and should therefore be
con

rmed by in situ measurements of the gel properties. In order to characterize the gel
state, Mutin and Guenet ( 1989 ) synthesized samples with variable proportions of syn-
diotactic sequences (different temperatures of synthesis) and used different solvents.
High-temperature PVC contained 0.33 syndiotactic groups and low-temperature PVC
0.39. The authors first noticed that, in the same solvent, highly syndiotactic PVC

possesses a much higher elastic modulus (

(five times higher) than atactic PVC, consistent

with assumption (a).
These workers established a temperature concentration phase diagram for high-

temperature PVC (HTPVC) gels in diethylmalonate (DEM) gels aged for 24 h, in
which the two types of gels could be identi

-

ed ( Figure 8.1 ).
Their Gel I systems have a range of thermal stability which extends from +50°C to
100

150°C, above which they melt out and the solution returns to a liquid-like (sol) state,
while Gel II systems exist between

-

50°C and +50°C. According to the authors, Gel I
contains only syndiotactic microcrystals and melts at high temperature (gel terminal
melting), this temperature being higher for PVC with long syndiotactic sequences. Gel II
contains two types of junctions, syndiotactic microcrystals and independently an organ-
ized form of the polymer

−

-

solvent complex.
The transition from Gel II to Gel I occurs at a constant temperature, which argues in
favour of the formation of a complex with a fixed solvent/monomer ratio. Mutin and

Guenet propose the formation of a bridge between two carbonyl groups C=O of the
diethyl ester and two H - Cl - C groups which involve the methyne hydrogen of two