Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 236

to be especially sensitive to conformational behaviour of the polymer chains, and
perhaps also to intra-chain and inter-chain interactions. Based on studies of syndiotactic
poly(styrene) (sPS), which could crystallize in the helical ttgg (trans-trans-gauche-
gauche) conformation, it was assumed that the appearance of the band with a peak at
572 cm − 1 indicated the formation of regular ttgg sequences. Therefore, the 572 cm − 1 peak
was one of the conformational sensitive bands.
The IR spectrum of aPS-CS 2 gel, dried as a

film, was measured, and it was almost the

film, but a peak at 572 cm − 1 was clearly observed which did
not exist in the IR spectra of normal aPS

same as that of normal aPS

film. This band did not disappear even after

one month at film, some syndiotactic segments

became segregated in a helical conformation, but short stereo-regular sequence length
in the aPS prevented the formation of long regular sequences of the ttgg conformation
or crystallization, because in its IR spectra only one of the ttgg conformational sensitive
bands, at 572 cm − 1 , could be detected. However, the formation of sequences of a
particular conformation suggested that, compared to glassy polymers, some more
ordered structures existed in the non-crystallizable polymer. Thus, in such

− 26°C. This suggested that, in such a

films,
ordered structures could be formed through stacks of some stereo-regular segments
adopting the ttgg type conformation, and these were strongly dependent on the
interaction between CS 2 and polymer segments. The dynamics of interaction was
studied by measuring IR spectra at increasing temperature. The solvent escaped from
the bulk film, the regular ttgg conformation transferred to the less regular gauche tg

conformation and the molecular chains relaxed. Independently of these studies, Izumi
et al.( 1995 ) performed wide-angle neutron diffraction (WANS) experiments to iden-
tify the local structure in the aPS-CS 2 gels. They observed highly ordered structures far
below the gelation temperature. However, the gel near the gelation temperature con-
tained less distinct order.
The overall picture that emerges is that gelation of aPS in CS 2 is ascribed to
intermolecular associations of chain segments including syndiotactic sequences and
solvent molecules (similar to the polymer solvent complexes mentioned for PVC).
François et al.( 1986 ) adopted a different procedure for investigating this system: using
DSC they examined aPS-CS 2 gels which had been kept in tightly sealed pans, annealed
for several days at 40°C and then kept for at least 2monthsat room temperature to
ensure thorough homogenization. Some DSC measurements were repeated a month
later to test homogenization. All of the DSC experiments were consistent with the
existence of structures possessing some degree of order. Their work shows that the
structures responsible for the physical cross-links form a stoichiometric compound.
From their temperature

- concentration phase diagram, François et al.( 1986 ) found a

fixed polymer concentration of c =0.47gcm − 3 (40% w/w) of intermolecular associ-
ations, and concluded that physical gelation in CS 2 arose from the formation of a
polymer

-

ned between c = 40% and
50% w/w. Consequently the maximum proportion of physical junctions should be
formed in this concentration range. Even if other authors do not agree with this
particular stoichiometry, there is clear evidence that the nature of the solvent favours
polymer complex formation and conformational change.

solvent compound whose stoichiometry was de