Mechanical strength is also an important factor for thermo-responsive hydrogel materials used in industrial and biomedical fields. The mechanical properties are highly dependent on the polymer network structure of the hydrogel. Many efforts have been focused on increasing the mechanical strength of the hydrogels with different network structure [11].
16 1 Structure-Function Relationship of Thermo-responsive Hydrogels
0 10 20 30 40 50 60 70 80
Release [%]
20 25 30 35 40 45
Time [min]
Temp. [oC]
0 2 4 6 8 10 12 14 16
Vt/Vo [ - ] Hydrogel No.1-1 Hydrogel No.2-1
0 1 2 3 4 5 6 7 8 9 10 11
Fig. 1.13
Thermo-responsive controlled-release
characteristics of solutes from PNIPAM hydrogels with different internal microstructures and the corresponding dynamic volume deswelling behaviors of NaCl-loaded PNIPAM hydrogels (Reproduced with permission from Ref. [4], Copyright (2006), IOP)
The authors’ group also developed another kind of thermo-responsive MCG hydrogel, which could bear repeated elongations and compressions. This second kind of MCG hydrogels is prepared by copolymerization of modified poly(acrylamide) (mPAM) microgel and NIPAM in water (Fig.1.14). The mPAM microgel with unsaturated vinyl bonds on its surface is prepared by treating PAM microgel with N-methylolacrylamide. Table 1.3 lists the preparation recipes for these mPAM microgel-cross-linked MCG hydrogels.
As shown in Fig. 1.15, the internal microstructure of mPAM microgel-cross- linked PNIPAM hydrogel is similar to the MCG hydrogels constructed by H2O2- treated P(NIPAM-co-AAc) microgels as mentioned above. Such MCG PNIPAM hydrogel also exhibits larger degree of volume change and faster response rate to the ambient temperature change compared to normal PNIPAM hydrogel due to the similar microgel-cross-linked internal microstructure.
Figure1.16shows the compression tests of MCG2-1.5 and NG2-1.5 hydrogels at temperature lower than their LCSTs. It can be clearly seen that MCG hydrogel exhibits better compression resistance than normal hydrogel NG2-1.5. Further- more, such MCG hydrogel cross-linked with mPAM microgels also shows good tensile properties. Figure 1.17 shows the typical elongation process of MCG3- 1.5 and NG2-1.5 hydrogels at temperature lower than their LCSTs. The MCG
Fig. 1.14 The schematic illustration of the preparation of MCG PNIPAM hydrogel cross-linked by mPAM microgels with unsaturated vinyl bonds
Table 1.3 Preparation recipes of mPAM microgel-cross-linked PNIPAM hydrogels Hydrogel
sample no. Water (g)
Cross-linker mPAM microgel (g)
Monomer NIPAM (g)
Cross-linker MBA (g)
Initiator KPS (g)
Accelerator TEMED (L)
MCG2-0.5 19.6 0.4 1.13 0 0.027 30
MCG2-1.5 19.6 0.4 3.39 0 0.081 70
MCG3-1.5 19.6 0.6 3.36 0 0.081 70
NG2-1.5 20 0 3.39 0.0924 0.081 70
Fig. 1.15 SEM images of the internal microstructures of (a) MCG2-1.5 and (b) MCG2-0.5 PNIPAM hydrogels. The scale bars are 5m
18 1 Structure-Function Relationship of Thermo-responsive Hydrogels
Fig. 1.16 Compression tests of MCG2-1.5 and NG2-1.5 hydrogels at temperature lower than their LCSTs: (a, d) before compression, (b, e) under compression, and (c, f) after compression. (a, b, c) NG2-1.5 hydrogel and (d, e, f) MCG2-1.5 hydrogel. The scale bars are 10 mm
Fig. 1.17 The elongation process of MCG3-1.5 and NG2-1.5 hydrogels at temperature lower than their LCSTs. (a) NG2-1.5 hydrogel and (b) MCG3-1.5 hydrogel. The scale bars are 20 mm
Fig. 1.18 Scheme for the preparation of microgel composite PNIPAM hydrogel using active PNIPAM microgels as the cross-linker
Table 1.4 Preparation recipes of the third kind of MCG hydrogels
Hydrogel sample no. NIPAM (g) Water (g) aPN-xx dispersion (g)
MCGxx-1.5 0.8475 (1.5 mmol/gH2O) 0.5 4.5
MCGxx-1.0 0.565 (1.0 mmol/gH2O) 0.5 4.5
MCGxx-0.5 0.2825 (0.5 mmol/gH2O) 0.5 4.5
MCGxx-0.25 0.1413 (0.25 mmol/gH2O) 0.5 4.5 MCGxx-1.5-0.5C 0.8475 (1.5 mmol/gH2O) 2.5 2.5
Note: Preparation temperatureD0ıC, TEMEDD20L. The MCG PNIPAM hydrogels are labeled as MCGxx-yy, in which “aPN-xx” refers to the polymerization time in active PNIPAM microgel preparation and yy refers to the NIPAM concentration
hydrogel could be elongated so much without fracture. More preferably, the mPAM microgel-cross-linked hydrogel can be elongated and compressed repeatedly and can restore to its original shape and size after experienced large strain.
To further improve the mechanical strength of thermo-responsive PNIPAM hydrogels, the authors’ group developed the third kind of MCG hydrogels, which is prepared by copolymerization of NIPAM and active PNIPAM microgels. The schematic illustration of the preparation process is shown in Fig.1.18, and the active PNIPAM microgels with unsaturated carbon bonds are prepared by precipitation polymerization of NIPAM in water at 60ıC using sodium dodecyl sulfate (SDS)as space obstructor. Table1.4lists the preparation recipes of the third kind of MCG hydrogels. The content of unsaturated carbon bonds in active PNIPAM microgels decreases with the increase of polymerization time.
20 1 Structure-Function Relationship of Thermo-responsive Hydrogels
Fig. 1.19 SEM images of the internal microstructures of MCG hydrogels prepared with different aPN microgel concentrations: (a) MCG40-1.5 and (b) MCG40-1.5-0.5C. The scale bars are 1m
Fig. 1.20 Photos of MCG20-1.5-0.5C hydrogel compressed at temperature lower than its LCST:
(a) before compression, (b) under compression, and (c) after compression. The scale bars are 10 mm
The third kind of MCG hydrogels using active PNIPAM microgels as cross- linker presents heterogeneous microstructure like cross-linked threads, as shown in Fig.1.19. So, such MCG hydrogels also exhibit larger volume change and faster response rate to the ambient temperature change compared to normal PNIPAM hydrogel. Especially, this kind of MCG hydrogels is able to restore to their initial shape and size after being compressed, chopped, bent, contorted, knotted, or elongated with large strain, as shown in Figs.1.20,1.21, and 1.22. The high mechanical strength of MCG hydrogels is from the internal microstructure. There exist a large number of long PNIPAM bridge chains between active PNIPAM microgels, which can disperse the stress added on MCG hydrogels. The long PNIPAM bridge chains have sufficient space for configuration change, which could be available to withstand the stress even at large strain.
There exists a hysteresis circle when the third kind of MCG hydrogels restores after being elongated, as shown in Fig. 1.23. The degree of hysteresis for the MCG hydrogels is affected by cross-linking density and decreases when the concentrations of initial NIPAM and aPN microgels increase or the polymerization time in the preparation of aPN microgels decreases.
Fig. 1.21 Photos of MCG20-1.5-0.5C hydrogel cut by a blade at temperature lower than its LCST:
(a) before cutting, (b) under cutting, and (c) after cutting. The scale bars are 10 mm
Fig. 1.22 Photos of MCG20-1.5-0.5C hydrogel demonstrating its elasticity characteristics at temperature lower than its LCST. (a) bending, (b) torsion, (c) knotting, and (d) elongation after knotting. The scale bars are 20 mm
22 1 Structure-Function Relationship of Thermo-responsive Hydrogels
0 3 6 9 12
a15 b
c d
e f
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 MCG40-1.5
MCG40-1.0 MCG40-0.5 MCG40-0.25
0 5 10 15 20 25 30 35 40
e [mm/mm]
e [mm/mm]
e [mm/mm]
e [mm/mm]
e [mm/mm]
* [kPa]s
e [mm/mm]
s* [kPa]
MCG30-1.5 MCG30-1.0 MCG30-0.5 MCG30-0.25
0 2 4 6 8 10
0 0.2 0.4 0.6 0.8 1
s* [kPa]
MCG10-1.0
0 5 10 15 20 25 30 35
s* [kPa]
MCG20-1.0 MCG30-1.0 MCG40-1.0
0 3 6 9 12 15
0 0.5 1 1.5 2 2.5 3 3.5
s* [kPa]
MCG40-1.5 MCG40-1.5-0.5C
0 5 10 15 20 25 30 35 40
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 1 2 3 4 5 6
0 0.5 1 1.5 2 2.5 3 3.5
s* [kPa]
MCG30-1.5 MCG30-1.5-0.5C
Fig. 1.23 The elongation and restoration curve of MCG PNIPAM hydrogels at temperature lower than their LCSTs. (a, b) MCG hydrogels prepared with different NIPAM concentrations, (c, d) MCG hydrogels prepared with different aPN microgel, and (e, f) MCG hydrogels prepared with different aPN microgel concentrations