Preparation and Properties of Monodisperse pH-Responsive Microgels
7.3 Monodisperse Cationic pH-Responsive Microgels
Fig. 7.6 CLSM images of the chitosan microgels with different cross-linking degrees:
(a) CPAD0.1 wt %, QIPD100L/h, QOPD1,000L/h; (b) CPAD0.5 wt %, QIPD100L/h, QOPD1,000L/h. (a1) and (b1): transmission channel, (a2) and (b2): green fluorescent channel, (a3) and (b3): overlay channel. All the bars are 25m
a finite solubility in inner water phase, the chitosan microcapsules could be formed via diffusion of terephthalaldehyde from outer oil phase to inner chitosan droplets in neutral medium to form the Schiff base bondings.
Different chitosan microgels are prepared by controlling the cross-linker con- centration (CPA) in the outer oil phase at same gelation time. The morphology of the chitosan microgel changes from hollow capsule to solid sphere as CPA
increases. Figure 7.6 shows the CLSM images of the chitosan microgels with different cross-linking degree after water evaporation. Obviously, the microgels with low cross-linking degree (CPAD0.1 wt %) have a hollow capsule structure (Fig.7.6a), while the microgels with high cross-linking degree (CPAD0.5 wt %) are solid spheres (Fig.7.6b). Furthermore, it can be clearly seen that these chitosan microgels have small sizes less than 50m. These monodisperse chitosan microgels with small size also have good pH-responsive properties in acidic medium like the chitosan microspheres mentioned above, which have a great potential in smart drug delivery systems and high-throughput screening of enzymes and cells.
7.3 Monodisperse Cationic pH-Responsive Microgels 161
Fig. 7.7 Effects of PVP concentration and molecular weight on the hydrodynamic diameter (solid line) and size distribution (dotted line) of PDM microgels. The dosages of other ingredients are 0.5 mol/L DM, ethanol/water ratio of 1:9 (v/v), 0.5 wt % MBA (Reproduced with permission from Ref. [24], Copyright (2007), Elsevier)
Monodisperse cationic pH-responsive PDM microgels are successfully prepared by dispersion polymerization in ethanol/water mixture using poly(vinyl pyrroli- done) (PVP) as the steric stabilizer and N,N0-methylenebisacrylamide (MBA) as the cross-linker [24]. The control of microgel size and size distribution is achieved by properly varying the polymerization parameters. The research results show that increase of the polarity of the reaction system by increasing the volume ratio of ethanol to water could increase the size and size distribution of microgels. In addition, at a high concentration of the cross-linker, 2 wt %, macrogels instead of microgels may be formed. Increasing the concentration of N,N-dimethylaminoethyl methacrylate (DM) monomer could also increase the microgel size; however, the microgels become unstable when the monomer concentration reaches 1.0 mol/L.
Two kinds of PVP (PVP K30 and PVP 360) with different concentrations are used as the steric stabilizer to prepare PDM microgels. As shown in Fig. 7.7, it is demonstrated that increase of the concentration and the molecular weight of the PVP stabilizer could decrease the size and size distribution of the microgels [24].
The results can be explained as follows. The higher the concentration of stabilizer is, the faster the stabilizer is adsorbed, and consequently the larger the amount of the adsorbed stabilizer is. Hence, for a given duration, a greater number of smaller particles are stabilized during the primary stabilization process. On the other hand, increasing PVP concentration can increase the viscosity of the medium, which leads to a retardation of particles coagulation. Higher molecular weight of PVP is meant to increase the viscosity of the medium and improve the stabilization by longer chains of PVP [25]. Therefore, the size of microgels decreases and the size distribution becomes more monodisperse with an increase in the molecular weight of PVP.
In order to examine the pH-dependent stability of PDM microgels, zeta potentials of these PDM microgels are measured at different pH values as shown in Fig.7.8 [24]. It is determined that the isoelectric point (IEP) of PDM microgels is at around pH 6. At pH lower than 5, the zeta potential of microgels is positive, as expected, due to protonation of the amine groups of PDM. The negative zeta potentials are determined at pH values of 7–11. This may be caused by deprotonation of
Fig. 7.8 Zeta potentials of PDM microgels prepared with 10 wt % PVP K30 (ı), 2 wt % PVP 360 (˙), and 5 wt % PVP 360 () at different pH values at 25ıC.
The dosages of other ingredients are 0.5 mol/L DM, ethanol/water ratio of 1:9 (v/v), and 0.5 wt % MBA.
All the samples are carried out on purified dilute dispersions (Reproduced with permission from Ref. [24], Copyright (2007), Elsevier)
amine groups in the microgels that leads to overall negative charge. The results demonstrate that the prepared PDM microgels are able to maintain electrostatic stabilization at both low and high pHs. The microgels are partially or totally aggregated at pH values in the range of 57, which spans the IEP of the PDM microgels.
The PDM microgels exhibit excellent pH responsivity and significantly swell at low pH values. Figure7.9shows the optical micrographs of PDM microgels with different stabilizer PVP (10 wt % PVP K30, 2 wt % PVP 360, and 5 wt % PVP 360) at pH 2.5 and pH 11 [24]. It can be seen that all the microgels significantly swell at pH 2.5 compared to that at pH 11.
Figure 7.10 shows the hydrodynamic diameters of these PDM microgels at different pH values, and the degree of pH responsivity is determined by the maximum ratio of pH-dependent volume change of PDM microgels (Vmax/Vmin) [24]. The selected three types of microgels with different original hydrodynamic diameters have different maximum ratios of volume change at the tested pH range.
The microgels prepared with 2 wt % PVP 360 have the largest ratio of 11.7. PDM bearing tertiary amine groups in the side chains can be protonated in acidic solution.
The protonation of the amine groups would introduce positive charge to the polymer side chains. As a result, PDM microgels swell in acidic solution by electrostatic repulsion of positively charged amine groups within the gel and hydration of such functional groups. But on the other hand, the conformational entropy elasticity of the cross-linked polymer chains counteracts this swelling. Since the cross-linker is kept constant in all experiments, the large volume tends to consume a larger amount of cross-linker inside the microgels network. Therefore, for large microgels, such as the one prepared with 10 wt % PVP K30, the cross-linked polymer chains might restrict the gel swelling more. Therefore, the microgels prepared with 10 wt % PVP K30 have the lowest degree of pH responsivity among the selected three types of microgels. On the other hand, the small microgels prepared with 5 wt % PVP 360
7.4 Monodisperse Cationic pH-Responsive Hydrogel Capsules 163
Fig. 7.9 Optical micrographs of PDM microgels prepared with 10 wt % of PVP K30 (a, b), 2 wt % of PVP 360 (c, d), and 5 wt % of PVP 360 (e, f) at pH 2.5 and pH 11. The dosages of other ingredients are 0.5 mol/L DM, ethanol/water ratio of 1:9 (v/v), and 0.5 wt % MBA. All the samples are carried out on purified dilute dispersions. Scale barD5m (Reproduced with permission from Ref. [24], Copyright (2007), Elsevier)
have relatively low degree of pH responsivity (Vmax/VminD6.6) because of their low content of amine groups resulting in relatively weak electrostatic repulsive forces.
Consequently, the microgels prepared with 2 wt % PVP 360 with moderate size has the highest degree of pH responsivity (Vmax/VminD11.7).