Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 163

300

200

100

0

2 4 6 8 10 12

pH

Figure 6.4

Dependence of the degree of swelling of PAA gels (1) and HMPAA gels: C 8 - 2.5% (2), C 8 - 10% (3)
and C 8 - 20% (4) on the pH of the solution surrounding the gel sample. Equilibrium swelling is
de ned by the relative variation of the gel mass m, compared with that in the dry state, m 0 . Adapted
with permission from Philippova et al.( 1997 ) © 1997 American Chemical Society.

becoming higher with increasing fraction and chain length of the hydrophobic units.
NMR measurements (Philippova et al., 1997 ) have demonstrated that the hydrophobic
microdomains may be reversibly destroyed or created by changes in the pH of the
medium. The swelling behaviour of these pH-responsive HMPAA gels is shown in
Figure 6.4 over a range of pH which can, in turn, be varied between 5 and 8 as the mole
fraction of hydrophobes (n = 8) increases from 0% to 20%.

6.3.2 Polysaccharide derivatives with pendant alkyl side chains

Polysaccharide derivatives are a class of products of interest in many industrial applica-
tions including textiles, printing, food, paper and pharmaceuticals. Typically the back-
bone is a polysaccharide to which lateral alkyl chains can be grafted. Examples include
hydrophobically modi

ed hydroxyethyl cellulose (HMHEC), ethyl(hydroxyethyl) cel-
lulose (HMEHEC) and modi ed alginates (HMalginate) and dextrans (HMdextran).

Figure 6.5 shows some characteristic HM polysaccharides.
In one study of interest (Nyström et al., 2009 ), the HMHEC polymer contained
2mol%ofn-C 16 H 33 . The unmodi

ed EHEC is a non-ionic amphiphilic polymer with
hydrophilic and hydrophobic microdomains distributed randomly along the polymer
backbone and exhibits demixing upon heating. In the HMEHEC sample, a small
proportion of hydrophobic side chains (0.70%) of C 12 or C 14 alkyl chains was grafted
on to the backbone.
As described in Chapter 5 , alginates are marine anionic polysaccharides consisting of

β α- l-guluronic acid (G-block)

units, in which GG-, MG- and MM-blocks of various lengths are arranged along the chain. A
typical HMalginate contains approximately 5 mol% of n-octylamine (30% of C 8 ) groups

-d-mannuronic acid (so-called M-block) and (1 4)-linked

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