Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 249

8

7

6

(a)

(b)
(c)

5

4

3

2

1

0 15

20 25 30

Te mperature (°C)

35 40 45 50

Figure 5.19
Equilibrium swelling ratios as a function of temperature for PNIPAAm/chitosan gels in water. (a) PNIPAAm,
(b) chitosan/PNIPAAm semi-IPN, and (c) chitosan//PNIPAAm IPN.

a PNIPAAm system is well known to increase LCST [63]. During the swelling progress,
there are two competitive phenomena that determine the behaviors of the gel as the tem-
perature rises: the hydration capacity of chitosan increases, whereas that of PNIPAAm
decreases as segments collapse due to the temperature-controlled conformational transi-
tion. Therefore, the temperature-dependent swelling behavior is the result of a fine bal-
ance that is influenced by both the degree of cross-linking and the chitosan to PNIPAAm
ratio [64]. Chitosan/PNIPAAm IPN or semi-IPN hydrogels are prepared in acidic medium
because of insolubility of chitosan in neutral and basic media, which limits the application
of chitosan/PNIPAAm hydrogels to some extent. Soluble chitosan-g-PNIPAAm copolymer
can overcome this disadvantage and provide a more effective method for preparing inject-
able hydrogels. In chitosan-g-PNIPAAm copolymer, chitosan chains could be considered
as pendant chains instead of the backbone. Thus, the initial hydrophilic/hydrophobic
balance of the backbone did not change. Consequently, the LCST of the resulting hydrogels
nearly underwent no change [65]. Although the swelling ratios of chitosan-g-PNI-
PAAm copolymer hydrogels underwent a sharp decrease around LCST, the thermosensi-
tivity is influenced by the grafting ratio and concentrations. The higher the grafting ratio
of PNIPAAm, the sharper the thermosensitivity [66]. Different concentrations of the
chitosan-g-PNIPAAm copolymer solution had different thermosensitivities. In general,
the lower the concentration, the higher the thermosensitivity. This may stem from the fact that
with increasing concentration of the copolymer, the concentration of segments also increases.
As a result, the free movement of segments is restricted, and hydrophobic association between
macromolecules is strengthened to a certain extent, and so gels are rapidly formed [67].
Chitosan/PNIPAAm hydrogels also have excellent pH sensitivity. For example, Lee
et al. [68] found that the swelling ratio of chitosan-g-PNIPAAm hydrogels decreases
with increasing pH value. That is to say, the swelling behaviors of chitosan/PNIPAAm
hydrogels can be controlled by temperature and pH, which endow these hydrogels with