Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 162

In aqueous solution these associating polymers organize in a similar way to surfactant
molecules, forming micelle-like structures with reversible intermolecular bonds. Such
macromolecules have remarkable rheological properties and are widely used as thick-
eners in cosmetics, pharmaceuticals, detergents, textiles and many other areas.
A minimum concentration is required for intermolecular associations to take place,
equivalent to the CMC of the polymer, but this limit is more dif cult to determine

because intramolecular associations of the hydrophobic moieties can take place in very
dilute solution. At these very low concentrations, intramolecular association is the
dominant effect and molecules adopt a compact conformation, while at higher concen-
trations the molecules form reversible networks.
The replacement of hydrocarbons with per uoroalkyl groups in HM associating poly-

mers is also of potential interest since the hydrophobic character of per

uoroalkyl groups is
more pronounced than with their hydrocarbon analogues. The associative ability of CF 2
groups is 1.7 times higher than that of the CH 2 groups, as shown, for instance, in viscosity
measurements for HMPAA by Zhou et al.( 2001 ) and Shedge et al.( 2005 ).
We now undertake a brief survey of the various types of HM water-soluble polymers
and their synthesis methods.

6.3.1 HM polyelectrolytes

Free radical copolymerization of a hydrophilic monomer, acrylic acid (AA), with
n-alkyl acrylates (n = 8, 12, 18), with the fraction of n-alkyl acrylate in the monomer
mixture not exceeding 20 mol%, forms an interesting class of associating polymers
(Philippova et al., 1997 ). As poly(acrylic acid) (PAA) is a weak acid, its degree of
ionization can be varied over a wide range by changing the pH of the medium. Two
extreme cases exist: uncharged acrylic acid and completely ionized acrylic units,
shown in Figure 6.3 .
In HM polyelectrolytes, the electrostatic repulsion between polymer backbones
tends to counteract the hydrophobic attractive effect. As a result, the

'
behaviour depends on both pH and ionic strength. These polymers were also prepared
as cross-linked hydrogels using N,N 0 -methylene bis(acrylamide) as a cross-linker. At
low degrees of ionization (low pH), HMPAA gels are in a collapsed state, but their
swelling increases with the degree of ionization, since swelling is controlled by the
osmotic pressure exerted by the mobile counterions ( Chapter 4 ). The fraction of
hydrophobic units also controls the swelling transition, the pH of the transition

' viscosifying

An example of MH polyelectrolyte (x is the percentage of grafted monomers): (a) uncharged and
(b) completely ionized acrylic units.

Figure 6.3