Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 173

concentrations, the breakdown of the intermolecular aggregates decreases connectivity
between the polymer chains and, consequently, the viscosity. The pre-phase separation
surfactant concentration indicates a limit when all the alkyl groups in the mixed micelles
belong to one polymer chain. Further addition of surfactant molecules results in the
formation of more and more micelles free of any polymer alkyl groups, electrostatically
bound to the polymer backbone, and the system approaches phase separation.
In their work, Magny et al.( 1994 ) estimated the number of alkyl groups per micelle as
the surfactant concentration increased: the micelle aggregation number for pure surfactant,
close to 40, increases to 50 upon addition of the modi ed polymer. There is, however, no

fixed stoichiometry between the polymer and surfactant contribution; rather, it depends on
the surfactant concentration. A minimum limit was estimated to be around 6 alkyl groups
per micelle for a PAA solution containing 1% to 3%HMmonomers, with C 12 alkyl groups,
while the limit is lower with C 18 , for the same surfactant. As shown in Figure 6.11 ,atthe
maximum viscosity the numbers of alkyl groups participating in the formation of a mixed
micelle is approximately twice this value.
Similar mechanisms of association of surfactant (SDS) to the hydrophobic groups of
EHEC and HMEHEC solutions were observed by Thuresson et al.( 1995 )bylight
scattering and titration microcalorimetry techniques, and by Nyström et al.( 1995 )
using rheological methods. Structurally signi cant parameters such as the aggregation

number, microstructure of the micelles and interconnectivity of the chains were not
determined in these complex solutions because the heterogeneity of the local compo-
sition makes their determination problematic. All investigations of the polymer
synthesis mentioned above highlight this dif

culty, but also stress the fact that hetero-

geneous or block copolymers are more ef cient viscosity enhancers than homogeneous

samples.

6.6 Thermogelation or phase separation?

In most cases presented above, hydrophobic groups (mainly alkyl) spontaneously asso-
ciate into micellar aggregates at room temperature. When the polymer is well solubilized
at room temperature, the thickening effect appears on heating the solutions. There is a
very important class of associating polymers which exhibit such large thermal effects,
resulting from the hydrophobic interactions between long sequences of copolymers, that
the phenomenon is called . Thermogelling systems, based on neutral block

copolymers with hydrophobic cores such as PEO n -

' gelation '

ed natural
polymers such as methyl cellulose (MC) are also well known for exhibiting such an
increase of viscosity upon heating. We discuss in the following section the mechanisms
of hydrophobic association for these polymers.
We

PPO m - PEO n , and modi

first examine solutions with hydrophobic core copolymers, known by their trade
names as Pluronics ® or Poloxamers. Temperature-dependent micellization and gel for-
mation are two of the most characteristic properties of these block copolymers in aqueous
solution. Because of the marked change in water solubility of the central PPO block,
these copolymers form various aggregates depending on the degree of polymerization of