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Preparation and characterization of novel cationic pH-responsive poly(N,N′ -dimethylamino ethyl methacrylate) microgels
Journal of Colloid and Interface Science 311 (2007) 110–117 www.elsevier.com/locate/jcis
Preparation and characterization of novel cationic pH-responsive poly(N ,N -dimethylamino ethyl methacrylate) microgels Lin Hu a , Liang-Yin Chu a,b,∗ , Mei Yang a , Hai-Dong Wang a , Catherine Hui Niu c a School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China b Institute for Nanobiomedical Technology and Membrane Biology, State Key Laboratory of Biotherapy, Sichuan University, Chengdu, Sichuan 610065, PR China c Department of Chemical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada
Received 19 December 2006; accepted 22 February 2007 Available online 24 February 2007
Abstract Novel monodisperse cationic pH-responsive microgels were successfully prepared by dispersion polymerization in ethanol/water mixture using N ,N -dimethylamino ethyl methacrylate (DMAEMA) as the monomer, poly(vinyl pyrrolidone) (PVP) as the steric stabilizer and N ,N methylenebisacrylamide (MBA) as the cross-linker. The effects of various polymerization parameters, such as medium polarity, concentration of cross-linker, concentration of monomer, and concentration and molecular weight of stabilizer on the final diameter and monodispersity of poly(N ,N -dimethylamino ethyl methacrylate) (PDMAEMA) microgels were systematically studied. The pH-responsive characteristics of PDMAEMA microgels were also investigated. The experimental results showed that these microgels exhibited excellent pH-responsivity and significantly swelled at low pH values. The maximum ratio of volume change of the prepared microgels in response to pH variation was more than 11 times. It was found that the prepared microgels completely aggregated at the isoelectric point (IEP) around pH 6. On the other hand, the microgels were stable in aqueous solution at both low and high pH values. The results can be used for effectively controlled separation of particles. © 2007 Elsevier Inc. All rights reserved. Keywords: pH-responsive; Cationic microgels; Poly(N ,N -dimethylamino ethyl methacrylate); Dispersion polymerization; Hydrodynamic diameter; Size distribution
1. Introduction Environment-responsive microgels have attracted great interests from both therapeutical and biotechnological fields in recent years [1,2], due to their versatile applications such as drug delivery systems [3], bioseparation [1], gene carriers [4], sensors [5], and so on. The microgels are capable of changing their physical-chemical properties and colloidal properties in response to changes in environmental conditions, alone or in combination, including temperature [6–10], pH [11–14], and magnetic field [15]. Within the human body pH differentiation very often exists, for example, between various segments of the gastrointestinal tract [16], tumors and normal tissues [17], and extracellular and endosomal/lysosomal microenvironments * Corresponding author. Fax: +86 28 8540 4976.
E-mail address: [email protected] (L.-Y. Chu). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.02.063
[18]. In addition, to design and artificially control environmental pH stimuli have been easily done in many cases. All of these facts determine that pH-responsive microgels are practically important. There has been a considerable amount of research done on anionic pH-responsive microgels based on (meth)acrylic acid [11,12,19,20], but very little research on cationic pHresponsive microgels has been reported. Such microgels capable of swelling at low pH due to protonation are preferred in many circumstances [3,4]. In the previous works on preparation of cationic particles with N ,N -dimethylamino ethyl methacrylate (DMAEMA) as the monomer, one or two other co-monomers, such as methyl methacrylate (MMA) [21–24], styrene (St) [22,25] and N -isopropylacrylamide (NIPAM) [26,27], were often added in the reaction process. Although these copolymerizations were reported to enhance the glass transition temperature (Tg ) or generate multistimuli-responsive properties, the pH-responsivity of the prepared microgels was
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reduced to a certain extant in most cases. To overcome this problem, in this work DMAEMA was used as the primary monomer to prepare cationic pH-responsive monodisperse poly(N,N -dimethylamino ethyl methacrylate) (PDMAEMA) microgels. The prepared microgels were further characterized. Research on this aspect has not been sufficiently reported. In the present work, PDMAEMA microgels were prepared directly by dispersion polymerization in ethanol/water medium using PVP as the steric stabilizer. Dispersion polymerization is an effective and simple method for preparing monodisperse polymeric micro-particles [28]. The influences of various polymerization parameters on the hydrodynamic diameter and monodispersity of PDMAEMA microgels were examined. In addition, the responsivity and stability of PDMAEMA microgels as a function of pH were also investigated. For characterization of the prepared microgels, optical microscopy (OM) and transmission electron microscopy (TEM) were used to ascertain their morphologies and laser diffraction measurement to determine the hydrodynamic diameters and monodispersity. Furthermore, the zeta potentials of the microgels calculated from electrophoretic mobilities were also measured at different pH values. 2. Experimental 2.1. Materials N,N -Dimethylamino ethyl methacrylate (DMAEMA) (99.5%, Wuxi Xinyu Chemical Co., Ltd., China) was distilled under reduced pressure and stored in a refrigerator before use. Poly(vinyl pyrrolidone) K30 (PVP K30, Mw = 30,000 Da) (Shanghai Yuanju Biotech Co., Ltd., China) and PVP 360 (Mw = 360,000 Da) (Sigma–Aldrich) were commercial products and used as received. N ,N -Methylenebisacrylamide (MBA) (Chengdu Kelong Chemicals, China), N ,N -azobisisobutyronitrile (AIBN) (Tianjin Kermel Chemical Reagent Co., Ltd., China), and ethanol (Chongqing Chuandong Chemical (Group) Co., Ltd., China) were of general reagent grade and used without further purification. Deionized water (18.2 M) used in all experiments was from Millipore Milli-Q purification system. 2.2. Preparation of PDMAEMA microgels The dispersion polymerizations of DMAEMA were carried out according to the conditions given in Table 1. When one parameter was varied, the others were kept constant. For a typical polymerization, the monomer DMAEMA, cross-linker MBA and initiator AIBN with specific feed ratios were directly dissolved in 2 ml of ethanol. After addition of 18 ml of water, the reaction solution was magnetically stirred, fed with stabilizer PVP, purified by bubbling nitrogen gas for 10 min, and transferred to a cuvette. Then the cuvette was rapidly sealed by a stopper, wrapped by polyethylene film, and submerged in a thermostatic water bath at 65 ◦ C for 3 h without agitation. Finally, the reaction mixture was immediately cooled down to room temperature by immersion in cold water.
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Table 1 Recipes for dispersion polymerization of DMAEMA Ingredient
Standard reciped
Experimental variation
DMAEMA a (mol/L) PVP K30 or PVP 360b (wt%)
0.5 10 1/9 0.5
0.25–1.0 0–10 0/10–3/7 0–2.0
Ethanol/water (vol/vol) MBAc (wt%)
Note. Polymerizations were carried out at 65 ◦ C for 3 h using AIBN (0.2 g/L) as initiator after 10 min bubbling with nitrogen gas. a Based on the total volume of the ethanol/water mixture. b Based on the weight of DMAEMA. c Based on the weight of DMAEMA. d When one parameter was changed while other parameters were fixed in the experiments, the values in this column were adopted as the optimum values.
2.3. Purification In order to examine the pH-responsivity of the prepared microgels, the dispersions of the microgels was washed with 200 ml of deionized water using a microfiltration filter with a 0.1 µm Nylon-6 membrane. The microfiltration was carried out under a trans-membrane pressure of 90 kPa at room temperature. In order to reduce the concentration polarization of the dispersion, mechanical stirring was provided at ca. 300 rpm. When the volume of filtrate was about 100 ml, the Nylon-6 membrane was taken out to be cleaned and another 100 ml of fresh deionized water was added to the dispersion. This process was repeated till the viscosity of the filtrate (measured by an Ostwald viscometer) was close to that of pure water. Then the purified dispersion of PDMAEMA microgels was obtained. It was determined that in the purification process microfiltration was preferred to typical centrifugation because the microgels tended to aggregate together in centrifugation process, making it difficult for them to redisperse. 2.4. pH adjustment For the investigation of responsivity and stability of PDMAEMA microgels as a function of pH, 0.2 M HCl or 0.1 M NaOH were added to the purified dispersions (0.025–0.05 wt% PDMAEMA microgels). The pH was adjusted within the range 2–11. All pH adjustments were done using a Mettler-Toledo pH meter (SevenMulti, Mettler-Toledo Instruments), which was calibrated using pH 4.00, 6.86, and 9.18 standard solutions. In addition, 0.005 M NaCl was used as the background electrolyte in all dispersions. 2.5. Observation by optical microscopy A drop of the diluted dispersion was placed on a microscope slide and observed using an optical microscope (BX61, Olympus, Japan) equipped with infinity corrected optics. This technique was used to estimate the size range of microgels in aqueous medium and the dispersion stability in terms of whether the microgels were individual ones or formed by coagulation. Micrographs (magnification ×200) were taken with a digital CCD camera (GB130, Micron Technology).
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2.6. Laser light scattering
where ε is the dielectric constant of the medium, and η is the viscosity of the medium.
Hydrodynamic diameters (Dh ) of PDMAEMA microgels were determined at room temperature by a Malvern Mastersizer 2000 laser diffractometer (Malvern Instruments, UK) using Hydro MU as sample feeder and operating in a suitable standard operating procedure (SOP) (refractive index of particle: 1.59). The pump speed for sample feeding was chosen at 1250 rpm. The average hydrodynamic diameter (D¯ n ), size distribution (expressed by the coefficient of variation, CV), and maximum ratio of pH-dependent volume change (Vmax /Vmin ) were calculated from the following equations: D¯ n =
N
CV = 100% × Vmax = Vmin
(1)
Dhi /N,
i=1
¯ Dn max D¯ n min
N (Dhi − D¯ n )2 i=1 3
1/2
N
,
D¯ n ,
(2)
(3)
where Dhi is the hydrodynamic diameter of the ith microgel, N is the total number of microgels measured, Vmax and D¯ n max are the maximum pH-dependent volume and the maximum average hydrodynamic diameter of microgels, respectively, and Vmin and D¯ n min are the minimum pH-dependent volume and the minimum average hydrodynamic diameter of microgels, respectively. Each measurement was performed in triplicate on aqueous dispersions without pH adjustment for nonpurified 0.1–0.2 wt% PDMAEMA microgels and with pH controlled for purified ones (0.025–0.05 wt%). The pH-adjusted dispersions had been equilibrated for 20–24 h before measurement. 2.7. Transmission electron microscopy The morphology of PDMAEMA microgels were imaged in dry state by a transmission electron microscope (JEM 100CX, JEOL, Japan) operating at an accelerating voltage of 80 kV. To that end, one drop of the purified dispersions was placed on a carbon-coated copper grid and dried at room temperature. Micrographs of the dried samples were then taken with 10,000× magnification. 2.8. Measurement of zeta potential Zeta potentials of PDMAEMA microgels were directly measured at 25 ◦ C with a Malvern Zetasizer Nano ZS (Malvern Instruments, UK) using the folded capillary cells. The experiments were performed in triplicate on the purified dispersions (0.025–0.05 wt% PDMAEMA microgels containing 0.005 M NaCl) with pH ranging from 2 to 11. The adjusted dispersion had been equilibrated for more than one day at each pH value before measurement. The zeta potential (ζ ) used in this work was calculated from electrophoretic mobility, u, employing the Helmholtz–Smoluchowski equation [26,29]: u = εζ /η,
(4)
3. Results and discussion 3.1. Influences of polymerization parameters on the hydrodynamic diameter and size distribution of PDMAEMA microgels 3.1.1. Effect of medium polarity The medium polarity was adjusted by varying the volume ratio of ethanol to water. Fig. 1a shows the appearance of reaction systems after polymerization at different ethanol/water ratios, and Figs. 1b, 1c, and 1d display the optical micrographs of PDMAEMA microgels prepared at the ethanol/water ratios of 1/9, 2/8 and 3/7, respectively. Other parameters of polymerization were kept same. It was found that the ratio of ethanol to water strongly affected the outcome of the reaction. The resulting system was transparent and layered without ethanol. The underlayer material looked like macroscopical hydrogel, and its mechanical strength was poor due to the low cross-linked density. When the ratios of ethanol to water were changed to 1/9 and 2/8, the resulting systems became uniformly opalescent. The corresponding D¯ n and CV of the prepared microgels were 1.73 µm and 20.0%, and 2.22 µm and 23.7%, respectively. There were visible sediments at the bottom of cuvette when the ethanol/water ratio was set to be 3/7. The results may be explained by that ethanol is a more effective solvent for PDMAEMA than water. The increase of the ethanol concentration in the medium increases the solubility of PDMAEMA and its grafts. As a result, oligomeric radicals with longer critical chain length were formed before particle nucleation, which reduced the generation rate of nuclei. Consequently, the larger microgels with broader size distribution were obtained. It was also confirmed by the experimental observation that the reaction system turned uniformly opalescent within 15 min with the ethanol/water ratio of 1/9 or 2/8 while nearly half an hour when the ratio was increased to 3/7. 3.1.2. Effect of concentration of cross-linker Table 2 shows the effect of the cross-linker MBA concentration on the hydrodynamic diameter and size distribution of PDMAEMA microgels. With the increase of MBA dosage, the diameter of the resulting microgels increased gradually, and the size distribution tended to become broader. Moreover, the Table 2 Effect of MBA concentration on the hydrodynamic diameter and size distribution of PDMAEMA microgels MBAa (wt%) 0 0.5 1.0 2.0b
Hydrodynamic diameter (µm)
CV (%)
1.59 1.73 1.87 –
18.1 20.0 22.8 –
a Based on the weight of DMAEMA. b The macroscopical and cross-linked hydrogel but not microgel was formed
with 2 wt% MBA.
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Fig. 1. (a) Appearance of reaction systems after polymerization and (b, c, d) optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with different ethanol/water ratios. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, 10 wt% PVP K30, 0.5 wt% MBA, and 0.2 g/L AIBN.
Fig. 2. Optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with (a) 0.25 mol/L and (b) 1.0 mol/L of DMAEMA. Scale bar = 5 µm. The dosages of other ingredients were ethanol/water ratio of 1:9 (v/v), 10 wt% PVP K30, 0.5 wt% MBA, and 0.2 g/L AIBN.
higher the MBA content, the more partially coagulated microgels observed by optical microscope (micrographs not shown). In particular, when the MBA concentration was increased to 2 wt% based on the weight of DMAEMA, macroscopical and cross-linked hydrogel instead of microgels was formed. One possible reason would be that the higher concentration of the cross-linker, the more molecules of the DMAEMA monomer that can be linked together. As a result, the PDMAEMA gels of bigger sizes were formed. In addition, the increased crosslinker concentration may enhance inter- and intra-cross-linking of PVP molecules [30,31]. Along with this, the effectiveness of PVP stabilizer was reduced. Considering the mechanical strength and stability of the PDMAEMA microgels, 0.5 wt% MBA dosage was chosen for the following experiments. 3.1.3. Effect of concentration of monomer The initial monomer concentration plays a key role in the polymerization process and it has considerable effect on the resulting microgels. When the initial monomer concentra-
tion was about 0.25 mol/L based on the total volume of the ethanol/water mixture, the produced microgels were mildly coagulated as shown in Fig. 2a. Under the experimental conditions, the monomer concentration was relatively low in comparison with that of the cross-linker MBA. Similar to the results observed in the previous section, the microgels were partially coagulated. When the monomer concentration was as high as 1.0 mol/L, stable dispersion was not obtained. Instead, a viscous polymer mixture was formed and serious aggregation was observed as shown in Fig. 2b. The reason was that the increase of monomer concentration also increased the solubility of the medium for PDMAEMA and its grafts, which extended the nucleation period and then caused the serious aggregation. 3.1.4. Effect of concentration and molecular weight of stabilizer In the present system, a water- and ethanol-soluble polymer (PVP) was used as the steric stabilizer. Fig. 3 shows the optical micrographs of PDMAEMA microgels prepared with different
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Fig. 3. Optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with different PVP concentrations and molecular weights. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
quently the larger the amount of the adsorbed stabilizer. Hence, for a given duration, a greater number of smaller particles were stabilized during the primary stabilization process. On the other hand, increasing PVP concentration increased the viscosity of the medium, which led to a retardation of the coagulation of particles. Higher molecular weight of PVP is meant to increase the viscosity of the medium and improve stabilization by longer chains of PVP [31]. Therefore the diameter of microgels decreased and the size distribution became more monodisperse with an increase in the molecular weight of PVP. 3.2. pH-dependent characteristics
Fig. 4. Effects of PVP concentration and molecular weight on the hydrodynamic diameter and size distribution of PDMAEMA microgels. The solid and dotted lines are used to indicate the data of the same group. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
concentration of two kinds of PVP (PVP K30 and PVP 360). As further illustrated in Fig. 4, in general, increasing the concentration and the molecular weight of PVP decreased the diameter of the prepared microgels and narrowed the size distribution. The results can be explained as follows. The higher the stabilizer content, the faster the rate of stabilizer adsorption, and conse-
3.2.1. Effect of pH on the hydrodynamic diameter of microgels Optical micrographs of PDMAEMA microgels at pH 2.5 and 11 are shown in Fig. 5 for three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3). It can be seen that all the microgels significantly swelled at pH 2.5 compared to that at pH 11. Fig. 6 shows TEM micrographs (magnification ×10,000) of PDMAEMA microgels prepared with 5 wt% of PVP 360 at pH 8.65 and pH 2.5. The diameter of the sample at pH 2.5 was found to be much larger than that at pH 8.65. It was noted that the microgels diameter determined by TEM was larger than that measured by laser light scattering. The former one is somewhat unreliable as the soft
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Fig. 5. Optical micrographs (magnification ×200) of PDMAEMA microgels prepared with 10 wt% of PVP K30 (top), 2 wt% of PVP 360 (middle), and 5 wt% of PVP 360 (bottom) at pH 2.5 and 11. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
Fig. 6. TEM micrographs (magnification ×10,000) of PDMAEMA microgels prepared with 5 wt% PVP 360 at (a) pH 8.65 and at (b) pH 2.5. Scale bar = 1 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
microgels may have a tendency to deform or flatten and spread on the grid during the specimen preparation. Many “black dots” were found from TEM micrographs with a high magnification. These “black dots” were mainly distributed on the surface of the microgels and may be the PDMAEMA oligomeric radicals or unstable small PDMAEMA nuclei captured during the particle growth stage. The acid-swelling of these “black dots” also indicated the possibility of the PDMAEMA oligomeric radicals or unstable small PDMAEMA nuclei observed on the surface of microgels. Fig. 7 shows the hydrodynamic diameters of PDMAEMA microgels at different pH values for three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3). An interesting phenomenon was observed that the microgels were partially or totally aggregated at pH values being in the range 5–7, which spanned the isoelectric point (IEP) determined in the following section. The degree of pH-responsivity was determined by the maximum ratio of pHdependent volume change of PDMAEMA microgels as shown in Fig. 8. The selected three microgels with different original hydrodynamic diameters had different maximum ratios of volume change at the tested pH values. The microgels prepared with 2 wt% PVP 360 had the highest ratio of 11.7. The degree of pH-responsivity was associated with the original diameter of microgels. It is well recognized that the equilibrium
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Fig. 7. Hydrodynamic diameters of PDMAEMA microgels prepared with (!) 10 wt% of PVP K30, (E) 2 wt% of PVP 360, and (1) 5 wt% of PVP 360 at different pH values. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
Fig. 9. Zeta potentials of PDMAEMA microgels prepared with (!) 10 wt% PVP K30, (E) 2 wt% PVP 360, and (1) 5 wt% PVP 360 at different pH values at 25 ◦ C. The solid lines are used to indicate the data of the same group. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
repulsive forces. Consequently, the microgels prepared with 2 wt% PVP 360 with moderate size among the selected three types of microgels had the highest pH-responsivity.
Fig. 8. The maximum pH-dependent volume change ratios of PDMAEMA microgels prepared with different PVP additions. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
gel-swelling volume is a balance between the osmotic pressure of the polymer network and the elasticity of the network [12]. PDMAEMA bearing tertiary amine groups in the side chains can be protonated in acidic solution [26]. The protonation of the amine groups introduced positive charge to the polymer side chains. As a result, microgels swelled by repulsion of positively charged amine groups within the gel and hydration of such functional groups. On the other hand, the conformational entropy elasticity of the cross-linked polymer chains counteracted this swelling. For the large microgels, such as ones prepared with 10 wt% PVP K30, since the cross-linker was kept constant in all experiments, the large volume tended to consume a larger amount of cross-linker inside the microgels network, which might limit the gel swelling more strongly. Therefore, the large microgels prepared with 10 wt% PVP K30 had the lowest degree of pH-responsivity among the selected three microgels. On the other hand, the small microgels prepared with 5 wt% PVP 360 had the relatively low degree of pH-responsivity (Vmax /Vmin = 6.6) because of their low content of amine groups resulting in relatively weak electrostatic
3.2.2. Effect of pH on the zeta potential In order to examine the pH-dependent stability of PDMAEMA microgels, zeta potentials of three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3) were measured at different pH values as shown in Fig. 9. It was determined that the IEP of PDMAEMA microgel was at around pH 6. Additionally, observation of partial or total aggregation of microgels within the pH range 5–7 double confirmed the above result. At the pH lower than pH 5, the zeta potential of microgels was positive, as expected, due to protonation of the amine groups of PDMAEMA. The negative zeta potentials were determined at pHs of 7 to 11. This may be caused by deprotonation of groups in the microgels that leads to overall negative charge. The results demonstrated that the prepared microgels were able to maintain electrostatic stabilization at both low and high pHs. 4. Conclusions Novel monodisperse pH-responsive cross-linked PDMAEMA microgels with PVP stabilizer were successfully synthesized by dispersion polymerization in the ethanol/water mixture, and their pH-dependent size changes were systematically characterized by optical microscopy, TEM, and laser light scattering. The control of microgels size and their size distribution were achieved by properly varying the polymerization parameters. Under the experimental conditions, increase of the polarity of the system by increasing the volume ratio of ethanol to water greatly increased the size and its distribution of microgels, so did the cross-linker MBA. At the high concentration of the cross-linker, 2 wt%, macrogels instead of microgels may be formed. In addition, increasing the concentration of DMAEMA monomer also increased the size of the microgels, however, the
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gels became unstable when the monomer concentration reached 1.0 mol/L. On the other hand, it was demonstrated that increase of the concentration and the molecular weight of the stabilizer PVP decreased the size and its distribution of the microgels. The PDMAEMA microgels prepared with 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9, 0.5 wt% MBA, 2% PVP (360 K) and 0.2/L AIBN in this work showed greatly swelling behavior at low pH 2.5 compared to that at high pH 11. Although they were of aqueous stabilization at both low and high pH values, the microgels were partially or totally aggregated at the IEP region 5–7. This effect can be used for effectively controlled separation of particles. Acknowledgments This work was supported by the National Natural Science Foundation of China (20206019, 20674054), the Key Project of the Ministry of Education of China (106131), and Sichuan Youth Science and Technology Foundation for Distinguished Young Scholars (03ZQ026-41). References [1] [2] [3] [4]
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Preparation and characterization of novel cationic pH-responsive poly(N ,N -dimethylamino ethyl methacrylate) microgels Lin Hu a , Liang-Yin Chu a,b,∗ , Mei Yang a , Hai-Dong Wang a , Catherine Hui Niu c a School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China b Institute for Nanobiomedical Technology and Membrane Biology, State Key Laboratory of Biotherapy, Sichuan University, Chengdu, Sichuan 610065, PR China c Department of Chemical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada
Received 19 December 2006; accepted 22 February 2007 Available online 24 February 2007
Abstract Novel monodisperse cationic pH-responsive microgels were successfully prepared by dispersion polymerization in ethanol/water mixture using N ,N -dimethylamino ethyl methacrylate (DMAEMA) as the monomer, poly(vinyl pyrrolidone) (PVP) as the steric stabilizer and N ,N methylenebisacrylamide (MBA) as the cross-linker. The effects of various polymerization parameters, such as medium polarity, concentration of cross-linker, concentration of monomer, and concentration and molecular weight of stabilizer on the final diameter and monodispersity of poly(N ,N -dimethylamino ethyl methacrylate) (PDMAEMA) microgels were systematically studied. The pH-responsive characteristics of PDMAEMA microgels were also investigated. The experimental results showed that these microgels exhibited excellent pH-responsivity and significantly swelled at low pH values. The maximum ratio of volume change of the prepared microgels in response to pH variation was more than 11 times. It was found that the prepared microgels completely aggregated at the isoelectric point (IEP) around pH 6. On the other hand, the microgels were stable in aqueous solution at both low and high pH values. The results can be used for effectively controlled separation of particles. © 2007 Elsevier Inc. All rights reserved. Keywords: pH-responsive; Cationic microgels; Poly(N ,N -dimethylamino ethyl methacrylate); Dispersion polymerization; Hydrodynamic diameter; Size distribution
1. Introduction Environment-responsive microgels have attracted great interests from both therapeutical and biotechnological fields in recent years [1,2], due to their versatile applications such as drug delivery systems [3], bioseparation [1], gene carriers [4], sensors [5], and so on. The microgels are capable of changing their physical-chemical properties and colloidal properties in response to changes in environmental conditions, alone or in combination, including temperature [6–10], pH [11–14], and magnetic field [15]. Within the human body pH differentiation very often exists, for example, between various segments of the gastrointestinal tract [16], tumors and normal tissues [17], and extracellular and endosomal/lysosomal microenvironments * Corresponding author. Fax: +86 28 8540 4976.
E-mail address: [email protected] (L.-Y. Chu). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.02.063
[18]. In addition, to design and artificially control environmental pH stimuli have been easily done in many cases. All of these facts determine that pH-responsive microgels are practically important. There has been a considerable amount of research done on anionic pH-responsive microgels based on (meth)acrylic acid [11,12,19,20], but very little research on cationic pHresponsive microgels has been reported. Such microgels capable of swelling at low pH due to protonation are preferred in many circumstances [3,4]. In the previous works on preparation of cationic particles with N ,N -dimethylamino ethyl methacrylate (DMAEMA) as the monomer, one or two other co-monomers, such as methyl methacrylate (MMA) [21–24], styrene (St) [22,25] and N -isopropylacrylamide (NIPAM) [26,27], were often added in the reaction process. Although these copolymerizations were reported to enhance the glass transition temperature (Tg ) or generate multistimuli-responsive properties, the pH-responsivity of the prepared microgels was
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reduced to a certain extant in most cases. To overcome this problem, in this work DMAEMA was used as the primary monomer to prepare cationic pH-responsive monodisperse poly(N,N -dimethylamino ethyl methacrylate) (PDMAEMA) microgels. The prepared microgels were further characterized. Research on this aspect has not been sufficiently reported. In the present work, PDMAEMA microgels were prepared directly by dispersion polymerization in ethanol/water medium using PVP as the steric stabilizer. Dispersion polymerization is an effective and simple method for preparing monodisperse polymeric micro-particles [28]. The influences of various polymerization parameters on the hydrodynamic diameter and monodispersity of PDMAEMA microgels were examined. In addition, the responsivity and stability of PDMAEMA microgels as a function of pH were also investigated. For characterization of the prepared microgels, optical microscopy (OM) and transmission electron microscopy (TEM) were used to ascertain their morphologies and laser diffraction measurement to determine the hydrodynamic diameters and monodispersity. Furthermore, the zeta potentials of the microgels calculated from electrophoretic mobilities were also measured at different pH values. 2. Experimental 2.1. Materials N,N -Dimethylamino ethyl methacrylate (DMAEMA) (99.5%, Wuxi Xinyu Chemical Co., Ltd., China) was distilled under reduced pressure and stored in a refrigerator before use. Poly(vinyl pyrrolidone) K30 (PVP K30, Mw = 30,000 Da) (Shanghai Yuanju Biotech Co., Ltd., China) and PVP 360 (Mw = 360,000 Da) (Sigma–Aldrich) were commercial products and used as received. N ,N -Methylenebisacrylamide (MBA) (Chengdu Kelong Chemicals, China), N ,N -azobisisobutyronitrile (AIBN) (Tianjin Kermel Chemical Reagent Co., Ltd., China), and ethanol (Chongqing Chuandong Chemical (Group) Co., Ltd., China) were of general reagent grade and used without further purification. Deionized water (18.2 M) used in all experiments was from Millipore Milli-Q purification system. 2.2. Preparation of PDMAEMA microgels The dispersion polymerizations of DMAEMA were carried out according to the conditions given in Table 1. When one parameter was varied, the others were kept constant. For a typical polymerization, the monomer DMAEMA, cross-linker MBA and initiator AIBN with specific feed ratios were directly dissolved in 2 ml of ethanol. After addition of 18 ml of water, the reaction solution was magnetically stirred, fed with stabilizer PVP, purified by bubbling nitrogen gas for 10 min, and transferred to a cuvette. Then the cuvette was rapidly sealed by a stopper, wrapped by polyethylene film, and submerged in a thermostatic water bath at 65 ◦ C for 3 h without agitation. Finally, the reaction mixture was immediately cooled down to room temperature by immersion in cold water.
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Table 1 Recipes for dispersion polymerization of DMAEMA Ingredient
Standard reciped
Experimental variation
DMAEMA a (mol/L) PVP K30 or PVP 360b (wt%)
0.5 10 1/9 0.5
0.25–1.0 0–10 0/10–3/7 0–2.0
Ethanol/water (vol/vol) MBAc (wt%)
Note. Polymerizations were carried out at 65 ◦ C for 3 h using AIBN (0.2 g/L) as initiator after 10 min bubbling with nitrogen gas. a Based on the total volume of the ethanol/water mixture. b Based on the weight of DMAEMA. c Based on the weight of DMAEMA. d When one parameter was changed while other parameters were fixed in the experiments, the values in this column were adopted as the optimum values.
2.3. Purification In order to examine the pH-responsivity of the prepared microgels, the dispersions of the microgels was washed with 200 ml of deionized water using a microfiltration filter with a 0.1 µm Nylon-6 membrane. The microfiltration was carried out under a trans-membrane pressure of 90 kPa at room temperature. In order to reduce the concentration polarization of the dispersion, mechanical stirring was provided at ca. 300 rpm. When the volume of filtrate was about 100 ml, the Nylon-6 membrane was taken out to be cleaned and another 100 ml of fresh deionized water was added to the dispersion. This process was repeated till the viscosity of the filtrate (measured by an Ostwald viscometer) was close to that of pure water. Then the purified dispersion of PDMAEMA microgels was obtained. It was determined that in the purification process microfiltration was preferred to typical centrifugation because the microgels tended to aggregate together in centrifugation process, making it difficult for them to redisperse. 2.4. pH adjustment For the investigation of responsivity and stability of PDMAEMA microgels as a function of pH, 0.2 M HCl or 0.1 M NaOH were added to the purified dispersions (0.025–0.05 wt% PDMAEMA microgels). The pH was adjusted within the range 2–11. All pH adjustments were done using a Mettler-Toledo pH meter (SevenMulti, Mettler-Toledo Instruments), which was calibrated using pH 4.00, 6.86, and 9.18 standard solutions. In addition, 0.005 M NaCl was used as the background electrolyte in all dispersions. 2.5. Observation by optical microscopy A drop of the diluted dispersion was placed on a microscope slide and observed using an optical microscope (BX61, Olympus, Japan) equipped with infinity corrected optics. This technique was used to estimate the size range of microgels in aqueous medium and the dispersion stability in terms of whether the microgels were individual ones or formed by coagulation. Micrographs (magnification ×200) were taken with a digital CCD camera (GB130, Micron Technology).
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2.6. Laser light scattering
where ε is the dielectric constant of the medium, and η is the viscosity of the medium.
Hydrodynamic diameters (Dh ) of PDMAEMA microgels were determined at room temperature by a Malvern Mastersizer 2000 laser diffractometer (Malvern Instruments, UK) using Hydro MU as sample feeder and operating in a suitable standard operating procedure (SOP) (refractive index of particle: 1.59). The pump speed for sample feeding was chosen at 1250 rpm. The average hydrodynamic diameter (D¯ n ), size distribution (expressed by the coefficient of variation, CV), and maximum ratio of pH-dependent volume change (Vmax /Vmin ) were calculated from the following equations: D¯ n =
N
CV = 100% × Vmax = Vmin
(1)
Dhi /N,
i=1
¯ Dn max D¯ n min
N (Dhi − D¯ n )2 i=1 3
1/2
N
,
D¯ n ,
(2)
(3)
where Dhi is the hydrodynamic diameter of the ith microgel, N is the total number of microgels measured, Vmax and D¯ n max are the maximum pH-dependent volume and the maximum average hydrodynamic diameter of microgels, respectively, and Vmin and D¯ n min are the minimum pH-dependent volume and the minimum average hydrodynamic diameter of microgels, respectively. Each measurement was performed in triplicate on aqueous dispersions without pH adjustment for nonpurified 0.1–0.2 wt% PDMAEMA microgels and with pH controlled for purified ones (0.025–0.05 wt%). The pH-adjusted dispersions had been equilibrated for 20–24 h before measurement. 2.7. Transmission electron microscopy The morphology of PDMAEMA microgels were imaged in dry state by a transmission electron microscope (JEM 100CX, JEOL, Japan) operating at an accelerating voltage of 80 kV. To that end, one drop of the purified dispersions was placed on a carbon-coated copper grid and dried at room temperature. Micrographs of the dried samples were then taken with 10,000× magnification. 2.8. Measurement of zeta potential Zeta potentials of PDMAEMA microgels were directly measured at 25 ◦ C with a Malvern Zetasizer Nano ZS (Malvern Instruments, UK) using the folded capillary cells. The experiments were performed in triplicate on the purified dispersions (0.025–0.05 wt% PDMAEMA microgels containing 0.005 M NaCl) with pH ranging from 2 to 11. The adjusted dispersion had been equilibrated for more than one day at each pH value before measurement. The zeta potential (ζ ) used in this work was calculated from electrophoretic mobility, u, employing the Helmholtz–Smoluchowski equation [26,29]: u = εζ /η,
(4)
3. Results and discussion 3.1. Influences of polymerization parameters on the hydrodynamic diameter and size distribution of PDMAEMA microgels 3.1.1. Effect of medium polarity The medium polarity was adjusted by varying the volume ratio of ethanol to water. Fig. 1a shows the appearance of reaction systems after polymerization at different ethanol/water ratios, and Figs. 1b, 1c, and 1d display the optical micrographs of PDMAEMA microgels prepared at the ethanol/water ratios of 1/9, 2/8 and 3/7, respectively. Other parameters of polymerization were kept same. It was found that the ratio of ethanol to water strongly affected the outcome of the reaction. The resulting system was transparent and layered without ethanol. The underlayer material looked like macroscopical hydrogel, and its mechanical strength was poor due to the low cross-linked density. When the ratios of ethanol to water were changed to 1/9 and 2/8, the resulting systems became uniformly opalescent. The corresponding D¯ n and CV of the prepared microgels were 1.73 µm and 20.0%, and 2.22 µm and 23.7%, respectively. There were visible sediments at the bottom of cuvette when the ethanol/water ratio was set to be 3/7. The results may be explained by that ethanol is a more effective solvent for PDMAEMA than water. The increase of the ethanol concentration in the medium increases the solubility of PDMAEMA and its grafts. As a result, oligomeric radicals with longer critical chain length were formed before particle nucleation, which reduced the generation rate of nuclei. Consequently, the larger microgels with broader size distribution were obtained. It was also confirmed by the experimental observation that the reaction system turned uniformly opalescent within 15 min with the ethanol/water ratio of 1/9 or 2/8 while nearly half an hour when the ratio was increased to 3/7. 3.1.2. Effect of concentration of cross-linker Table 2 shows the effect of the cross-linker MBA concentration on the hydrodynamic diameter and size distribution of PDMAEMA microgels. With the increase of MBA dosage, the diameter of the resulting microgels increased gradually, and the size distribution tended to become broader. Moreover, the Table 2 Effect of MBA concentration on the hydrodynamic diameter and size distribution of PDMAEMA microgels MBAa (wt%) 0 0.5 1.0 2.0b
Hydrodynamic diameter (µm)
CV (%)
1.59 1.73 1.87 –
18.1 20.0 22.8 –
a Based on the weight of DMAEMA. b The macroscopical and cross-linked hydrogel but not microgel was formed
with 2 wt% MBA.
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Fig. 1. (a) Appearance of reaction systems after polymerization and (b, c, d) optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with different ethanol/water ratios. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, 10 wt% PVP K30, 0.5 wt% MBA, and 0.2 g/L AIBN.
Fig. 2. Optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with (a) 0.25 mol/L and (b) 1.0 mol/L of DMAEMA. Scale bar = 5 µm. The dosages of other ingredients were ethanol/water ratio of 1:9 (v/v), 10 wt% PVP K30, 0.5 wt% MBA, and 0.2 g/L AIBN.
higher the MBA content, the more partially coagulated microgels observed by optical microscope (micrographs not shown). In particular, when the MBA concentration was increased to 2 wt% based on the weight of DMAEMA, macroscopical and cross-linked hydrogel instead of microgels was formed. One possible reason would be that the higher concentration of the cross-linker, the more molecules of the DMAEMA monomer that can be linked together. As a result, the PDMAEMA gels of bigger sizes were formed. In addition, the increased crosslinker concentration may enhance inter- and intra-cross-linking of PVP molecules [30,31]. Along with this, the effectiveness of PVP stabilizer was reduced. Considering the mechanical strength and stability of the PDMAEMA microgels, 0.5 wt% MBA dosage was chosen for the following experiments. 3.1.3. Effect of concentration of monomer The initial monomer concentration plays a key role in the polymerization process and it has considerable effect on the resulting microgels. When the initial monomer concentra-
tion was about 0.25 mol/L based on the total volume of the ethanol/water mixture, the produced microgels were mildly coagulated as shown in Fig. 2a. Under the experimental conditions, the monomer concentration was relatively low in comparison with that of the cross-linker MBA. Similar to the results observed in the previous section, the microgels were partially coagulated. When the monomer concentration was as high as 1.0 mol/L, stable dispersion was not obtained. Instead, a viscous polymer mixture was formed and serious aggregation was observed as shown in Fig. 2b. The reason was that the increase of monomer concentration also increased the solubility of the medium for PDMAEMA and its grafts, which extended the nucleation period and then caused the serious aggregation. 3.1.4. Effect of concentration and molecular weight of stabilizer In the present system, a water- and ethanol-soluble polymer (PVP) was used as the steric stabilizer. Fig. 3 shows the optical micrographs of PDMAEMA microgels prepared with different
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Fig. 3. Optical micrographs (magnification ×200) of nonpurified PDMAEMA microgels prepared with different PVP concentrations and molecular weights. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
quently the larger the amount of the adsorbed stabilizer. Hence, for a given duration, a greater number of smaller particles were stabilized during the primary stabilization process. On the other hand, increasing PVP concentration increased the viscosity of the medium, which led to a retardation of the coagulation of particles. Higher molecular weight of PVP is meant to increase the viscosity of the medium and improve stabilization by longer chains of PVP [31]. Therefore the diameter of microgels decreased and the size distribution became more monodisperse with an increase in the molecular weight of PVP. 3.2. pH-dependent characteristics
Fig. 4. Effects of PVP concentration and molecular weight on the hydrodynamic diameter and size distribution of PDMAEMA microgels. The solid and dotted lines are used to indicate the data of the same group. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
concentration of two kinds of PVP (PVP K30 and PVP 360). As further illustrated in Fig. 4, in general, increasing the concentration and the molecular weight of PVP decreased the diameter of the prepared microgels and narrowed the size distribution. The results can be explained as follows. The higher the stabilizer content, the faster the rate of stabilizer adsorption, and conse-
3.2.1. Effect of pH on the hydrodynamic diameter of microgels Optical micrographs of PDMAEMA microgels at pH 2.5 and 11 are shown in Fig. 5 for three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3). It can be seen that all the microgels significantly swelled at pH 2.5 compared to that at pH 11. Fig. 6 shows TEM micrographs (magnification ×10,000) of PDMAEMA microgels prepared with 5 wt% of PVP 360 at pH 8.65 and pH 2.5. The diameter of the sample at pH 2.5 was found to be much larger than that at pH 8.65. It was noted that the microgels diameter determined by TEM was larger than that measured by laser light scattering. The former one is somewhat unreliable as the soft
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Fig. 5. Optical micrographs (magnification ×200) of PDMAEMA microgels prepared with 10 wt% of PVP K30 (top), 2 wt% of PVP 360 (middle), and 5 wt% of PVP 360 (bottom) at pH 2.5 and 11. Scale bar = 5 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
Fig. 6. TEM micrographs (magnification ×10,000) of PDMAEMA microgels prepared with 5 wt% PVP 360 at (a) pH 8.65 and at (b) pH 2.5. Scale bar = 1 µm. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
microgels may have a tendency to deform or flatten and spread on the grid during the specimen preparation. Many “black dots” were found from TEM micrographs with a high magnification. These “black dots” were mainly distributed on the surface of the microgels and may be the PDMAEMA oligomeric radicals or unstable small PDMAEMA nuclei captured during the particle growth stage. The acid-swelling of these “black dots” also indicated the possibility of the PDMAEMA oligomeric radicals or unstable small PDMAEMA nuclei observed on the surface of microgels. Fig. 7 shows the hydrodynamic diameters of PDMAEMA microgels at different pH values for three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3). An interesting phenomenon was observed that the microgels were partially or totally aggregated at pH values being in the range 5–7, which spanned the isoelectric point (IEP) determined in the following section. The degree of pH-responsivity was determined by the maximum ratio of pHdependent volume change of PDMAEMA microgels as shown in Fig. 8. The selected three microgels with different original hydrodynamic diameters had different maximum ratios of volume change at the tested pH values. The microgels prepared with 2 wt% PVP 360 had the highest ratio of 11.7. The degree of pH-responsivity was associated with the original diameter of microgels. It is well recognized that the equilibrium
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Fig. 7. Hydrodynamic diameters of PDMAEMA microgels prepared with (!) 10 wt% of PVP K30, (E) 2 wt% of PVP 360, and (1) 5 wt% of PVP 360 at different pH values. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
Fig. 9. Zeta potentials of PDMAEMA microgels prepared with (!) 10 wt% PVP K30, (E) 2 wt% PVP 360, and (1) 5 wt% PVP 360 at different pH values at 25 ◦ C. The solid lines are used to indicate the data of the same group. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN. All the samples were carried out on purified dilute dispersions.
repulsive forces. Consequently, the microgels prepared with 2 wt% PVP 360 with moderate size among the selected three types of microgels had the highest pH-responsivity.
Fig. 8. The maximum pH-dependent volume change ratios of PDMAEMA microgels prepared with different PVP additions. The dosages of other ingredients were 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9 (v/v), 0.5 wt% MBA, and 0.2 g/L AIBN.
gel-swelling volume is a balance between the osmotic pressure of the polymer network and the elasticity of the network [12]. PDMAEMA bearing tertiary amine groups in the side chains can be protonated in acidic solution [26]. The protonation of the amine groups introduced positive charge to the polymer side chains. As a result, microgels swelled by repulsion of positively charged amine groups within the gel and hydration of such functional groups. On the other hand, the conformational entropy elasticity of the cross-linked polymer chains counteracted this swelling. For the large microgels, such as ones prepared with 10 wt% PVP K30, since the cross-linker was kept constant in all experiments, the large volume tended to consume a larger amount of cross-linker inside the microgels network, which might limit the gel swelling more strongly. Therefore, the large microgels prepared with 10 wt% PVP K30 had the lowest degree of pH-responsivity among the selected three microgels. On the other hand, the small microgels prepared with 5 wt% PVP 360 had the relatively low degree of pH-responsivity (Vmax /Vmin = 6.6) because of their low content of amine groups resulting in relatively weak electrostatic
3.2.2. Effect of pH on the zeta potential In order to examine the pH-dependent stability of PDMAEMA microgels, zeta potentials of three selected microgels (10 wt% PVP K30, 2 wt% PVP 360, and 5 wt% PVP 360 presented in Fig. 3) were measured at different pH values as shown in Fig. 9. It was determined that the IEP of PDMAEMA microgel was at around pH 6. Additionally, observation of partial or total aggregation of microgels within the pH range 5–7 double confirmed the above result. At the pH lower than pH 5, the zeta potential of microgels was positive, as expected, due to protonation of the amine groups of PDMAEMA. The negative zeta potentials were determined at pHs of 7 to 11. This may be caused by deprotonation of groups in the microgels that leads to overall negative charge. The results demonstrated that the prepared microgels were able to maintain electrostatic stabilization at both low and high pHs. 4. Conclusions Novel monodisperse pH-responsive cross-linked PDMAEMA microgels with PVP stabilizer were successfully synthesized by dispersion polymerization in the ethanol/water mixture, and their pH-dependent size changes were systematically characterized by optical microscopy, TEM, and laser light scattering. The control of microgels size and their size distribution were achieved by properly varying the polymerization parameters. Under the experimental conditions, increase of the polarity of the system by increasing the volume ratio of ethanol to water greatly increased the size and its distribution of microgels, so did the cross-linker MBA. At the high concentration of the cross-linker, 2 wt%, macrogels instead of microgels may be formed. In addition, increasing the concentration of DMAEMA monomer also increased the size of the microgels, however, the
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gels became unstable when the monomer concentration reached 1.0 mol/L. On the other hand, it was demonstrated that increase of the concentration and the molecular weight of the stabilizer PVP decreased the size and its distribution of the microgels. The PDMAEMA microgels prepared with 0.5 mol/L DMAEMA, ethanol/water ratio of 1:9, 0.5 wt% MBA, 2% PVP (360 K) and 0.2/L AIBN in this work showed greatly swelling behavior at low pH 2.5 compared to that at high pH 11. Although they were of aqueous stabilization at both low and high pH values, the microgels were partially or totally aggregated at the IEP region 5–7. This effect can be used for effectively controlled separation of particles. Acknowledgments This work was supported by the National Natural Science Foundation of China (20206019, 20674054), the Key Project of the Ministry of Education of China (106131), and Sichuan Youth Science and Technology Foundation for Distinguished Young Scholars (03ZQ026-41). References [1] [2] [3] [4]
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