Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 166

precise information about the microstructure of these copolymers, such as the number
and length of hydrophobic blocks, tends to be uncertain. In some cases, a drift in
composition is observed which results in heterogeneous copolymer samples, making it
impossible to draw any precise conclusion about the polymer s structure. The drift in

copolymer composition was found to increase strongly upon increasing the number of
hydrophobes per micelle, and the hydrophobe monomer is indeed consumed more
rapidly than the acrylamide.
The authors showed that the replacement of N

'

alkyl bonds avoids
this drift in composition, so that, in N-methyl-N-hexyl acrylamide (MeHexAm) or
N,N-dihexyl acrylamide (DiHexAm) copolymers, no corresponding drift was observed.
In the case of DiHexAm the number of hydrophobes per chain is controlled by
adjusting either the molecular mass of the polymer or the total hydrophobe content.
Here again, in order to maintain solubility in water, the hydrophobe content of the
modi

- H bonds by N -

ed poly(acrylamide)s is limited. These copolymers may be considered as model
systems for random-block associating polymers. The rheological properties of these
multi-block copolymers are strongly dependent on the hydrophobe/surfactant ratio, i.e.
the initial number of hydrophobes per micelle, during the synthesis. This number also
determines the hydrophobic block length, which was varied from 3 to 7. HMPAm
polymers with the same hydrophobe content (1 mol%) and blockiness were synthesized
with various molecular masses between 1.15 × 10 5 and 2.2 × 10 6 g mol − 1 .

6.3.4 Telechelic copolymers

As mentioned in Chapter 4 , linear polymers with functional groups at the two chain ends
are known as telechelic, and one major class consists of the hydrophobic ethoxylated
urethanes (HEURs), associative thickeners made of poly(ethylene glycol) (PEG) chains
extended by di-isocyantes and end-capped by long-chain alkanols. Their description is
found in papers by Jenkins et al.( 1991 ), Lundberg et al.( 1991 ) and Annable et al.
( 1993 ). Interest in these molecules results in part from their use as thickening agents in
coating formulations but, because the positioning of the associating groups is known,
they are also of great importance in more fundamental studies.
The positioning of the hydrophobe at the respective chain ends gives a particular
type of rheology, discussed below, with Newtonian behaviour up to a relatively high
shear rate. Molecular masses of the HEUR molecules are rather low, of the order of
10 000 to 30 000 g mol − 1 , and with a relatively narrow distribution (M w /M n ~1.4).The
end-cap length can be varied from C 12 to C 22 . The increase of the zero-shear viscosity
relative to end-cap chain length is shown in Figure 6.7 . A logarithmic increase of
viscosity with hydrophobe length implies that the activation energy for disengage-
ment (or, for a hydrophobe in the micelle(s), the absolute value of bonding suscept-
ibility) increases with cap length. Extrapolation back to PEG solutions of similar
molecular mass allows the minimum end-cap length required for viscosity enhance-
ment to be determined: for these polymers it appears to be approximately 6 carbons.
The activation energy derived from Figure 6.7 corresponds to 0.9 k B T per methylene
group.