Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 241

and with the stereo-complex of PMMA. The mobility of ester groups in the aggregates is
strongly hindered. After removal of the solvent from solutions or gels where self-
aggregates of s-PMMA or stereo-complex of PMMA exist, the structure generated
during aggregation is preserved even in the solid state, similar to the case of aPS-CS 2
presented above (Spevacek and Schneider, 1987 ). Up until this time, structures of the
double helix type had only been considered for biological macromolecules (e.g. carra-
geenans and, agarose; see Chapters 5 and 7). Stereo-regular PMMAs may be the

rst
synthetic polymers in which double helices generated by physical interactions have been
observed.
A more detailed analysis has been published for the self-aggregates of s-PMMA in
o-xylene and toluene, and a two-step mechanism more clearly identi ed. On cooling

solutions of s-PMMA from high temperature (85°C) to room temperature in o-xylene, a
transparent gel is immediately formed (Berghams et al., 1987 ). On standing, no changes
in its optical characteristics were observed, while heating resulted in a transition to a
transparent solution. The corresponding enthalpy change was measured by DSC and
increased with increasing tacticity. The gel formed on cooling was very brittle at any
concentration, so stretching was not possible. When solutions of s-PMMA were trans-
formed into a gel, changes in the IR spectrum were also observed. The most interesting
frequencies were those at 843 and 860 cm − 1 , which are ascribed to the

-

CH 2 - rocking

vibration and correspond, respectively, to tt and tg conformations. An increase of the
860 cm − 1 peak at the expense of absorption at 843 cm − 1 is characteristic for a transition
from a random-coil (predominantly tg) into a regular all-trans conformation, as observed
by Berghmans et al.( 1994 ) in toluene gels. Theoretical calculations designate this
slightly deformed tt conformation as the energetically most stable form on which the
helix conformation of the polymer chain is based. In Figure 8.12 the intensity ratio of

1. 2

1. 0

0.8

0.6

0.4

0.2

0

0 20 40 60 80 10 0

T (°C)

IR intensity ratio I 860 /I 843 as a function of temperature during cooling ( ○ ) and during heating ( ● ) for
s-PMMA in toluene, c = 10 wt%. Reprinted with permission from Berghmans et al.( 1994 ) © 1994
American Chemical Society.

Figure 8.12