Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 165

Carboxymethylcelluloses (CMCell) with dodecylamide groups were synthesized by
Cohen Stuart and co-workers by converting a small fraction of the carboxyl groups into
dodecylamide groups with the degree of dodecyl substitution varying between 0.003 and
0.05 per residue (or glucose unit). The molecular mass of these samples was about
450 000 g mol − 1 , so, although there are dif culties in the synthesis, one chain carries, on

average, between 10 and 200 docecyl groups (Cohen Stuart et al., 1998 ). Information
about the distribution of the dodecyl groups along the chains should also be important,
although such information is usually dif

cult to obtain. Alkyl chain attachment takes
places in hot ethanol, where dodecylamine is quite soluble but CMCell is not, a differ-
ence which may lead to non-randomness in the substitution. From simple statistical
considerations, it can be inferred that, at low degrees of substitution, the sample should
be heterogeneous in the sense that different macromolecules may differ considerably in
their degrees of substitution. A tendency for the alkyl groups to cluster adds to this
heterogeneity.

6.3.3 Random-block copolymers obtained by microemulsion polymerization

Hydrophobically modi ed poly(acrylamide)s (HMPAm) make up an important class of

associating polymers. One method used to copolymerize acrylamide with a hydrophobic
co-monomer is a micellar polymerization technique in which the hydrophobe is solubi-
lized into micelles dispersed in a water continuous medium (Volpert et al., 1996 ). A
schematic view of this micellar copolymerization is given in Figure 6.6 . This process was
shown to be well suited to the preparation of polymers with interesting viscosifying
properties.
Because of the high local concentration within the micelles, hydrophobic groups tend
to be distributed randomly along the poly(acrylamide) backbone in blocks. However,

Hydrophobic monomer
Water-soluble monomer
Surfactant molecule

Figure 6.6

Schematic representation of the micellar copolymerization medium containing water-soluble
monomers, hydrophobic monomers and surfactant molecules. Adapted with permission from
Candau et al.( 1998 ) © 1999 American Chemical Society.