Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 235

Amphiphilic chitosan-based hydrogels are developed by conjugating hydrophobic
groups and/or hydrophilic groups into chitosan molecules. Amphiphilic chitosan can
aggregate into nano micelles or particles, and these micelles or particles have special pH
sensitivity because their formation and dissociation depend on the pH of the environment.
The rationale for designing pH-sensitive nanoparticles was based on either the protona-
tion or deprotonation of amino groups of chitosan. In general, the structure of micelles
or particles is loose when pH < p K a (chitosan) and becomes compact when pH > p K a
(chitosan) [11]. For example, when the pH is ~4, chains of the pH-sensitive graft copoly-
mer, poly[(2-dimethylamino)ethyl methacrylate]-graft-chitosan (PDMAEMA-g-chitosan),
extend because chitosan and dimethylamino ethyl methacrylate (DMAEMA) segments
are almost fully protonated. When pH values are in the vicinity of the p K a of chitosan,
amino groups of chitosan are deprotonated and their hydrophobicity increases, resulting
in the formation of micelles with chitosan core and the partially deprotonated DMAEMA
segments as the shell. When pH > 7, both DMAEMA and chitosan segments are deproto-
nated, and double-layered hard spheres are formed, as shown in Figure 5.5 [12].
5.2.1.2 Chitosan-Polyanion Polymer Polyelectrolyte Complex
Chitosan-polyanion polymer polyelectrolyte complex (CS-PEC) hydrogels are generally
obtained either by the reaction of chitosan or polyanions. CS-PEC forming reaction can
only occur when the pH values are between the p K a of chitosan and p K a' of polyanion
polymer. That is to say, pH can control the formation and dissociation of CS-PECs.
Therefore, most properties of CS-PEC hydrogels depend on the pH of the environment. In
general, CS-PECs show the lowest swelling degree in the range of p K a' < pH < p K a due to
the strong interactions between chitosan and polyanion polymer. However, CS-PECs swell
obviously at pH < p K a' and/or pH > p K a . Pectin is an acidic polysaccharide that has repeat-
ing units of α (1, 4)-l-rhamnose units. A polyelectrolyte complex is formatted from anionic
pectin and cationic chitosan. The PEC swells obviously at pH < 3 and pH > 8, and does not
swell in the range of 3 < pH < 8. Moreover, its degree of swelling in an acidic medium is
much more than that in an alkaline medium. The swelling of the PEC correlates with its
composition and is also affected by the degree of deacetylation (DD) and the methoxy level
of pectin [13]. Yoshizawa et al. [14,15] found that the chitosan/polyalkyleneoxide-maleic
acid copolymer (CS/PAOMA) PEC films swell at low pH and shrink at pH between 4.8 and
6.5, and the swelling increases after pH 6.5. These swelling behaviors can be attributed to
the electrostatic interaction between the protonation of amino groups in chitosan and the
ionization of carboxyl groups in PAOMA. The p K a of chitosan is 6.5 and the p K a of PAOMA
is 4.8. In different pH media, corresponding groups (e.g., NH 3 + and COO − ) play different
roles between intermolecules.
Figure 5.5
Schematic representation of the microstructure of PDMAEMA-g-chitosan copolymer at different pH values.