Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 237

than that of chitosan hydrogels [16]. When pH is equal to isoelectric point (p I ) of chitosan-
based amphoteric cross-linking hydrogels, the numbers of -NH 2 and negative groups are
equal and ionic interactions between opposite charges result in the lowest swelling degree.
When pH > p I , a larger concentration of negative groups inside the hydrogels are ionized,
which results in an osmotic pressure and makes the amphoteric cross-linking hydrogels
swell, and the swelling degree increases with increasing pH. When pH < 7, the dissocia-
tion of carboxyl groups is inhibited and -NH 2 is protonated. When pH < p I , the dominant
ionic groups of amphoteric cross-linking hydrogels are -NH 3 + , and the swelling increases
with decreasing pH. The effect of substitution degree (SD) on swelling degree is somewhat
“complex.” The swelling degree is almost unchanged when pH = p I . When pH < p I , the
swelling degree of amphoteric cross-linking hydrogels increases with decreasing SD.
However, when pH > p I , the opposite phenomenon can be observed. For example, the
higher the SD value that N -(2-carboxybenzyl) chitosan cross-linking hydrogels (CBCSG)
possess, the bigger the swelling degree it exhibits in the pH range 7.4-9.0. In the pH range
1.0-5.0, the result is contrary to the above-mentioned results ( cf. Figure 5.7) [17]. Yang et al.
[18] found that poly(acrylic acid)-modified chitosan-based cross-linked hydrogels exhibit
similar swelling behaviors.

5.2.2.2 Chitosan-Based Semi-IPN Hydrogels

At low pH, the swelling process of the chitosan-based semi-IPN involves the protonation
of amino groups in the gels and relaxation. The equilibrium swelling degree of semi-IPN
hydrogels increases and the semi-IPN becomes more pH sensitive with decreasing cross-
linking degree. However, at high pH, where there are no protonation and hydrogen bond-
ing association, there is almost no effect of cross-linking degree on the equilibrium degree
of swelling and pH sensitivity. In addition, with decreasing chitosan content in the semi-
IPN, the semi-IPN becomes less pH sensitive.

16

14 CBCSG2

CBCSG4

CBCSG5

12

CSG

10

8

6

4

2

0 0

2 4 6 8 10

pH

Figure 5.7
Swelling characteristics of the CBCSG with different SD values in different buffer solutions for 24 h. SD: CBCSG2
(2.62 mmol/g), CBCSG4 (2.10 mmol/g), CBCSG5 (1.64 mmol/g), and CSG2 (0).