Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 166

reach a plateau. This effect has been reported before for physical hydrogels based
on biopolymers [ 143 , 144 ]. The G ′ values of the high molecular-weight chitosan
are lower during the whole experiment, with a final value of 950 Pa, compared
to the low molecular-weight chitosan with a final value of 1,550 Pa, as shown in
Fig. 13 . Dumitriu and colleagues interpreted this observation by less efficient for-
mation of elastically effective crosslinks in hydrogels prepared from high molec-
ular-weight precursors than in hydrogels prepared from low molecular-weight
precursors, without further explanation as to why this is.
To check for history-dependence of the gel mechanical properties, Dumitriu
and coworkers compared rheology experiments on freshly prepared hydrogels (no
coacervation time) to experiments on hydrogels prepared after 24 h of aging of the
coacervates. Whereas the swelling degree of these hydrogels decreases during the
aging process, the storage modulus increases; this observation can be explained
by assuming formation of more homogenous networks during the ageing period,
which increases the number of elastically effective crosslinks in the constituent
polymer networks. This observation agrees with a postulate of Meijer and cow-
orkers, who hypothesized that supramolecular polymer networks show self-repair-
ing of network defects and thus evolve into more homogenous and mechanically
stronger networks with time [ 145 , 146 ].
To conclude the rheological probing of the coacervates and the hydrogels,
Dumitriu and coworkers demonstrated their linear viscoelastic response to increasing
strain. For this purpose, both freshly prepared and aged hydrogel samples, all based
on low molecular-weight chitosan, were probed, and the data were fitted to a first-
order approximation of the Kronig-Kramers equation as modified by Tschoegl [ 147 ].
Dumitriu and colleagues could show that the viscoelastic modulus increases with the
time of preparation of the hydrogels, which is again addressable to formation of a
denser network. The coacervate exhibits a viscoelastic modulus of 2 kPa, whereas the
freshly prepared hydrogel displays a modulus of 9 kPa. The hydrogel prepared after
24 h of coacervation has a modulus of 12 kPa, as shown in Fig. 14 . The values of G ″
calculated with the Kronig-Kramers equation comply to the recorded G ″, showing
that the mechanical spectra lay in the linear viscoelastic domain.

Fig. 13 Time-dependent
evolution of the storage
modulus of polyelectrolyte
hydrogels formed from
different molecular-weight
chitosan precursors during
coacervation. DA represents
the degree of acetylation of
chitosan in mol%. Modified
from Dumitriu et al. [ 141 ].
Copyright 2004 Elsevier

MW: 1,000,000 g mol -1 ; DA: 17%
MW: 600,000 g mol -1 ; DA: 18%