Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 165

molecular-weight chitosan decreased from 3,000 to 2,500 %; this trend progressed
with the gels prepared after 24 h of coacervation time, with mass-swelling degrees
of 250 % in case of the low molecular-weight chitosan and 1,000 % in case of the
high molecular-weight chitosan, as compiled in Fig. 12 a. Dumitriu and cowork-
ers explained this observation by assuming slower diffusion of the longer polymer
chains, resulting in a less ordered hydrogel compared to the material composed of
shorter polymer chains. As a consequence, water molecules can penetrate the less
ordered network easier, causing these gels to exhibit higher swelling degrees.
In addition to the internal structure of the gels, the swelling degree of ionic
hydrogels depends on the pH of the swelling medium. To investigate this inter-
play, Dumtriu and collaborators probed a gel composed of high molecular-weight
chitosan in view of its swelling characteristics as a function of pH. The swelling
degree increases as a function of the pH from 3.5 to 5.8, explainable by gradual
deprotonation of the ammonium moieties in the chitosan polymer, thereby screen-
ing the ionic interactions between xanthan and chitosan. Similarly, at pH ≤ 2, the
degree of swelling is higher due to protonation of the carboxy moieties in the xan-
than polymer, which also attenuates ionic interactions between xanthan and chi-
tosan, as shown in Fig. 12 b.
With the help of shear rheology, Dumitriu and coworkers investigated the for-
mation of coacervates of a low and a high molecular-weight chitosan and xanthan.
In the beginning, both systems showed a steep slope of the storage modulus as a
function of time, indicating progressive formation of ionic interactions between the
polymer chains that can still diffuse easily in the solution. This is followed by per-
colation to a gel state as a consequence of orientation of water molecules toward
the polymer chains, resulting in a rigid network structure that entails hindered dif-
fusion of the polymer chains; in this period, the slope of G ′ as a function of time
decreases, but even after 24 h, the elastic part of the shear modulus, G ′, does not
Fig. 12 Investigation of the swelling degree of polyelectrolyte hydrogels based on chitosan
and xanthan depending on the composition, coacervation time, and pH. a Effect of the molec-
ular weight and coacervation time of chitosan on the swelling charateristics of the resulting
polyelectrolyte hydrogels. b Influence of pH on the swelling of the gel. Swelling degrees were
determined by comparison of the mass of dried hydrogels with the mass of the swollen gels.
DA represents the degree of acetylation of chitosan in mol%. CH79 (79 %) and CH71P (71 %)
represent the degree of deacetylation of chitosan in mol%. Modified from Dumitriu et al. [ 141 ].
Copyright 2004 Elsevier