Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 239

the approximate tube tilting method. According to the phase diagrams established by Tan
et al.( 1983 ), the gels investigated here were also in the transparent gel region.
It is worth mentioning that François et al.( 1986 ) found no molecular mass effect on the
melting points determined by DSC, unlike those measured mechanically by Koltisko
et al.( 1986 ) for this type of gel. The differences in experimental conditions, in particular
the time scale of observation of the gel state adopted in the DSC study, make it dif cult to

compare the two experiments. However, because of the discrepancy between the various
results, the statement by Tan et al.( 1983 ) that the aPS gels are in equilibrium should also
be reconsidered.

8.4 Stereo-complexation and conformational changes: isotactic and

syndiotactic PMMA gels

Polymethylmethacrylate (PMMA) is a member of the (vinyl) acrylate family with an
extra methyl group attached to the

-carbon. PMMA is a strong, hard and transparent
material with the advantage of being more transparent than glass, and so replaces glass in
some applications.
Of the structural parameters affecting PMMA aggregation in solution, the most
important are the con

α

gurational and conformational structure of PMMA and the
mobility of the various PMMA functional groups. A diad containing two α -carbons of

equal con guration is described as isotactic; a diad formed by two α -carbons of unequal

con of

PMMA we mean the distribution of isotactic and syndiotactic diads in the polymer.
Purely i-PMMA consists of diads with a linear backbone structure with all methyl ester
groups situated on one side of the chain, whereas the purely s-PMMA consists of a linear
backbone structure where these groups alternate on either side of the chain. The overall
con

guration is described as syndiotactic. By the term ' con gurational structure '

gurational structure of PMMA can be described most simply by determining the
content of syndiotactic or isotactic diads. This result alone yields no information on the
distribution of diads in longer sequences, but strongly affects the aggregation of PMMA.
In PMMA, conformational isomers can be generated by rotation about three types of
single bonds: (1) the bonds between the methylene group and the quaternary carbon,
CH 2 - C; (2) the bonds between the quaternary and carbonyl carbons, C - CO; and (3) the

bonds between the carbonyl carbon and oxygen, CO C)

determines the conformational structure of the chain backbone, and rotation about bond 2
(C

- O. Rotation about bond 1 (CH 2 -

CO) determines the orientation of the ester groups with respect to the chain. Rotation
about bond 3 (CO

-

O) does not need to be considered in studies of the conformational
structure of PMMA because, in all simple methyl esters, the ester group has been found to
have a planar structure, with the ester methyl group in cis orientation with respect to the
carbonyl; therefore, it may be assumed that this structure of the ester group is also
preserved in PMMA. The structures generated by rotation about CH 2 -

-

C bonds are
often described in terms of trans (t) and gauche (g) forms, even if they deviate somewhat
from the strictly staggered structures. The conformational structures of the PMMA chain
in the energetically most favoured forms of i- and s-PMMA differ very little; in both