Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 240

cases, in the most favoured form the chain structure can be approximated as tt (the
structure with the

α -CH 3 group coplanar with the ester group and oriented away from the

carbonyl).
Watanabe et al.( 1961 ) reported that, in some solvents, mixing solutions of s-PMMA
and i-PMMA led to the formation of aggregates, which yielded pronounced changes in
the rheological properties of the mixture and led to gel formation.
Indeed, long stereo-regular sequences of PMMA have a strong tendency for mutual
interaction (Spevacek and Schneider, 1987 ), and sequences of both equal stereo-regularity
(i (i-s) are capable of this. Consequently, aggre-

gates of stereo-regular PMMA can be divided into three types: (1) self-aggregates of
i-PMMA; (2) self-aggregates of s-PMMA; and (3) the so-called stereo-complex formed
by the mixing of solutions of i-ands-PMMA.
These three types of aggregates differ in their thermal stability and the extent of
aggregation in the given solvent. i-PMMA self-aggregates exhibit the highest thermal
stability and s-PMMA self-aggregates the lowest, with aggregates of the stereo-complex
in between. In the literature, the greatest attention has been paid to aggregates of the
stereo-complex and to s-PMMA self-aggregates. Information about the self-aggregates
of i-PMMA is scarce because the content of these aggregates is always relatively low,
irrespective of the type of solvent. Self-aggregates of i-PMMA decompose at temper-
atures close to the melting temperature of crystalline i-PMMA, which might indicate that
the aggregates in solution are small crystallites.
Self-aggregates of s-PMMA and stereo-complexes of PMMA exhibit a number of
common features. Bosscher et al.( 1982 ) performed X-ray scattering on stretched

- i and s - (i-s) and differing stereo-regularity (i-i -

films, cast from solutions. They found that the X-ray pattern of the stereo-complex
resembles the pattern of s-PMMA more than that of i-PMMA, and assumed that
the syndiotactic chain in the stereo-complex has a conformation comparable to the
conformation of s-PMMA after solvent-induced crystallization of s-PMMA. The
isotactic chain in the stereo-complex takes positions similar to those occupied by
the solvent molecules in pure crystalline s-PMMA. The stereo-complex was found to
be a double-stranded helix in which an isotactic chain with a small radius is sur-
rounded by a syndiotactic chain with a large radius, in such a way that the requirement
i/s = 1/2 can be satis ed.

One of the decisive factors in the self-aggregation of s-PMMA and in stereo-complex
formation is the stereo-regularity of polymer chains. A stable, ordered structure is formed
only in those cases where the length of the interacting sequences is longer than some
minimum length, but the actual values of this minimum length depend on the solvent. In
the case of the stereo-complex of PMMA, it was possible to classify the solvents as
strongly complexing (minimum length of associated s-sequences ~3

4 monomer units)
and weakly complexing (~10 monomer units). At the same time, the thermal stability of
the aggregates is determined by stereo-regularity: the temperature of melting of aggre-
gates formed by long stereo-regular sequences is higher than for aggregates formed by
short sequences. The solvent has a less pronounced effect on the thermal stability of the
aggregates. It was found that ester groups play an important role in the interactions of
stereo-regular sequences leading to aggregation, both with self-aggregates of s-PMMA

-