Preparation of nearly monodisperse pH-responsive microgels by radiation-induced dispersion polymerization

Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 58–65

Preparation of nearly monodisperse pH-responsive microgels by radiation-induced dispersion polymerization Wei Zhang a , Mei Tao b , Zhongqing Hu a , Zhicheng Zhang a,∗ a

Department of Polymer Science and Engineering, University of Science and Technology of China, 230026 Hefei, China b Department of Chemistry, Anhui Medical University, 230032 Hefei, China Received 2 March 2007; received in revised form 16 April 2007; accepted 18 April 2007 Available online 24 April 2007

Abstract Nearly monodisperse poly(2-(diethylamino) ethyl methacrylate) (PDEA) microgels were prepared directly by radiation-induced dispersion polymerization in water/ethanol media using poly(vinylpyrrolidone) (PVP) as the stabilizer at room temperature under certain circumstance (appropriate ethanol/water ratio and monomer dosage), which afforded novel pH-responsive behavior with range of 600–2500 nm. This method takes the advantages of radiation-induction (no chemical initiator, temperature independent, uniform initiation with a high efficiency and so on) that may result in the formation of uniform polymer particles. PVP acted as not only a physical stabilizer, but also as a macromonomer to form the grafted copolymer, which was confirmed by the Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1 H NMR) and was important for the stabilization of microgels. The characterization (morphology, size and distribution, and swelling/deswelling kinetics) of PDEA microgels were carried out by the scanning electron microscope (SEM), dynamic light scattering (DLS) and turbidity studies. © 2007 Elsevier B.V. All rights reserved. Keywords: Poly(2-(diethylamino) ethyl methacrylate); pH-responsive microgels; Dispersion polymerization; Radiation

1. Introduction Increasing attention has been paid to the synthesis and applications of novel pH-responsive microgels. Over the past decade or so, several classes of pH-responsive microgels have been reported. These include the follows: (a) methacrylic acid-based alkali-swellable latexes [1,2]; (b) N-isopropylacrylamide-based copolymer microgels containing either acidic or basic monomers with temperature-responsive core and pH-responsive arms [3–5]; (c) acid-swellable latexes based on basic monomers such as vinylpyridine (4VP and 2VP), or tertiary amine methacrylates such as 2-(diethylamino) ethyl methacrylates (DEA) or 2-(diisopropylamino)- ethyl methacrylate (DPA) [6–8]. All the pH-responsive microgels mentioned above were in the range of 50–700 nm (submicron) and synthesized by emulsion polymerization, which were still not large enough to some usage, and the properties of micrometer-level pH microgels is rarely being reported.

∗

Corresponding author. Tel.: +86 551 3601586; fax: +86 551 5320512. E-mail address: [email protected] (Z. Zhang).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.041

Furthermore the preparation of monodisperse particles in the micron range is particularly challenging because it is just between the limits of particles size by conventional emulsion polymerization and suspension polymerization. Various types of methods have been employed to prepare monodisperse particles, such as multistage swollen emulsion polymerization [9,10], successive seed emulsion polymerization [11,12], and suspension polymerization [13,14]. However, the above-mentioned procedures are time-consuming and often difficult to carry out. Dispersion polymerization [15–22] is a simple effective method for preparing monodisperse polymer particles, which suits not only a wide variety of monomers, but also permits easy functionalization. Stabilizer plays a very important role in dispersion polymerization for preparing monodisperse particles. Polymeric stabilizers designed for steric stabilization are anchored on colloidal particles either by chemical graft or by physical absorption to the particles surface. In general, chemical graft is a more effective form of stabilization than physical absorption, due to the permanence of the covalent bond between particle and stabilizer. The mechanism of chemical graft stabilization in dispersion polymerization is well established, copolymer chains

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resulting from the copolymerization of macromonomer and certain monomer grow up until reaching a critical size, after that, they are no longer soluble in the media and then coagulating with other insoluble oligomer or captured by existing particles. In this work, gamma rays from 60 Co source were used to initiate dispersion polymerization. Radiation induced polymerization is one of the most convenient methods owning to its large yield of radicals initiated by ␥-ray irradiation and temperature independent initiation process, which can easily result in a shorter nucleation stage during the dispersion polymerization, it is favorable to prepare the monodisperse polymer particles without the pollution of chemical initiators. Generally, eaq − , H• and OH• are produced when water is irradiated with gamma rays, and it is H• and OH• that initiate the polymerization of monomers. After that, the nucleation process takes place by the formed oligomer clusters stabilized by means of graft-copolymerization or/and absorption of stabilizer. With a higher dose rate (>1 Gy/s) the nucleation process takes place rapidly, which gives a potential possibility to form monodisperse latex particles. In this paper, nearly monodisperse microgels in the range of 0.6–2.5 ␮m were successfully synthesized by radiation-induced dispersion polymerization in ethanol water media using PVP as stabilizer. Then we found some different behavior of these PVP-g-PDEA microgels from PEGMA/PDEA microgels in literature [7-a]. 2. Experimental

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Table 1 Standard recipe for radiation-induced dispersion polymerization of DEA

Monomera Stabilizer Dispersion media Dispersion media

Material

Weight ratio (wt%)

DEA PVP Distilled deionized water Ethanol

10.4 1.7 50.8 37.1

Dose rate: 67.38 Gy/min. a NPGDA was used as cross-linker at 1.0 wt% based on DEA in all cases except specially noted.

2.4. Characterization The proton NMR spectrum was recorded with a Bruker ACF (400 MHz) spectrometer using CDCl3 as a solvent. The infrared spectrum was recorded with VECTOR22 FT-IR using a KBr pellet. The morphology and the size of produced PDEA latexes (in their latexes form at pH 9) were observed by a Nicolet SEM X-65 scanning electron microscopy. 2.5. Microgels characterization All of the solutions were prepared using doubly distilled deionized water. If required, pH adjustments were made using either HCl or NaOH. All of the measurements were made on dispersions that had equilibrated for 30–90 min at the appropriate pH values. Significant pH drift was observed for most microgels on a longer time staying (h). NaCl was used as the background electrolyte in all experiments.

2.1. Materials 2.6. Aqueous electrophoresis 2-(Diethylamino) ethyl methacrylate (DEA) (Aldrich), neopentyl glycol diacrylate (NPGDA) (industrial grade), were purified by vacuum distillation and were stored at −10 ◦ C before usage. Ethanol was chemically pure grade and used without further purification. Poly(vinylpyrrolidone) (PVP) (K30 , Mv = 40,000 g/mol) was supplied by Sino-pharm Chemical Reagent Company. Doubly distilled deionized water was used in all the experiment. 2.2. Microgels synthesis via dispersion polymerization The preparation of microgels was performed by gamma-ray radiation induced dispersion polymerization in ethanol/water media, in which the monomer and stabilizer are easily soluble. Purified nitrogen was bubbled through the mixture for about 20 min to get rid of oxygen. After that the solution was directly fed into a sealed glass ampoule and subjected to the gamma-ray irradiation at room temperature. The irradiation time was 1 h. The standard recipe used in this paper is given in Table 1. 2.3. Purification of microgels The un-grafted PVP and other reagents were removed by ultracentrifugation of latexes at 16,000 rpm for 20 min, then the latexes were dispersed in distilled water, and then repeat this process for at least 10 times.

Zeta potentials were calculated from the measured electrophoretic mobilities using a Nano-ZS90 instrument (Malvern Instruments Ltd.). Measurements (averaged over 20 runs for each test, 6 times for each sample) were made as a function of pH on dilute dispersions (0.01 wt%) in 0.005 M NaCl by gradually adding HCl to induce the latex-to-microgel transition in dispersions equilibrated overnight at about pH 10. 2.7. Characterization of microgels swelling/deswelling transition The kinetics of microgels (de)swelling transition experiments were performed by turbidimetry, the change in transmittance at 500 nm was monitored over time for 0.05 wt% dispersion using UV-2401PC (Sahimadzu Corporation, Japan). Dilute (0.01 M) HCl (or NaOH) was added to adjust the solution pH to 3.5–4.5 (or pH 8–9). Hydrodynamic particles diameters of swelling/deswelling PDEA microgels were measured at 25 ◦ C using Nano-ZS90 instrument (Malvern Instruments Ltd.) equipped with a 4 mW He–Ne laser operating at 633 nm. The scattered light was detected at 90◦ to the incident laser source, and the mean particle diameter was calculated using the Stokes–Einstein equation from the quadratic fitting of the correlation function, with typical analysis times of 20–30 min. These measurements were per-

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formed in triplicate on dilute aqueous dispersions (0.01 wt%) using 0.005 M NaCl as background electrolyte. 2.8. Characterization of microgels size and PDI The produced PDEA latexes size and PDI were measured with by Rise-2006 (based on the Particle Diameter Measure & Analysis System which developed by Jinan Rise Science and Technology Co. Ltd.) equipped with a solid-state laser operating at 632 nm and 200 W. The polydispersity index PDI value was read from the instrument directly, which was defined by the following set of equations (number average, Dn ; weight average, Dw ):  ni Di Dn =  (1) ni  ni Di4 Dw =  (2) ni Di3 Dw PDI = Dn

(3)

where ni is the number of polymer particles with diameter Di . 3. Results and discussion 3.1. Effect of ethanol/water ratio (w/w) on dispersion polymerization The influence of ethanol/water ratio on the size and PDI of PDEA latexes is given in Table 2. The system was heterogeneous when the weight ratio of ethanol/water was less than 1/5; the reaction was carried out similar to emulsion polymerization under such circumstance, which induced a larger PDI. As the weight ratio of ethanol/water increased from1/5 to 1/1.25, the latex diameter increased constantly, while the distribution narrowed first, and then broadened after the ethanol/water ratio reaching to a certain point (at 1/1.37). This behavior can be explained as follows: with the increase of ethanol/water ratio, the solubility of monomer and related polymer increased, so the polymerization system became more homogeneous, which would lengthen the growing period of chains in homogeneous

phase and increase the critical chain length, and decrease the migration rate of PVP-g-PDEA copolymer from the medium to the newborn particles surface due to the higher viscosity, as a result, the larger and more monodisperse micro-latexes were produced; on the other hand, when the ethanol/water ratio was too large, too small amount of nuclei with too large diameter were formed, which was unfavorable to be stabilized and so that the distribution of particles broadened. Much coagulum produced while the ethanol/water ratio increased to 1.2/1; and no latexes particles separated out from the system while the ratio increased to 2/1, in fact, the polymerization was carried out in a perfect solution system under such a circumstance. Hu and coworkers had reported similar result in the PVP stabilized dispersion system of MMA polymerization [23]. 3.2. Kinetic study of dispersion polymerization Fig. 1 shows the polymerization conversion against irradiation time curve. A mixture of DEA, NPGDA and PVP was placed in a 5-mL polymerization tube. Purified nitrogen was bubbled through the mixture for about 20 min to get rid of oxygen. The polymerization was carried out under gamma-ray irradiation at a dose rate of 67.38 Gy/min for a prescribed time at room temperature. After polymerization for a certain time, the reaction mixture with adding several drops of hydroquinone in it was then dried in a vacuum oven at 40 ◦ C. The conversion of monomer was calculated based on following equation: Conv. (%) =

WP − WPVP × 100% WM

(4)

where WP , WPVP and WM are the mass of the polymer obtained, PVP and the monomer added, respectively. The irradiation time dependence of the conversion is represented by S-shaped curves. After a short initial period, the polymerization rate increase to a certain value until a higher conversion. This feature can be explained by the existence of the gel effect. At the early stage, the polymerization mainly occurred in the continuous phase and the formed growing oligomers were

Table 2 Effects of dispersion medium on PDEA latexes’ size and PDI Ethanol/water (w/w)

Latex diameter (␮m)

1/5 1/4.1 1/3.1 1/1.88 1/1.75 1/1.37 1/1.25 1.2/1 2/1

0.12 0.16 0.33 0.75 1.41 2.10 2.48

PDI

1.90 1.86 1.69 1.44 1.35 1.15 1.28 Much coagulum produced No polymer particles separated out

All samples were prepared under similar recipe, i.e., DEA 10.4 wt%, (water + ethanol) 87.9 wt%, and PVP 1.7 wt%, NPGDA was used as cross-linker at 1.0 wt% based on DEA.

Fig. 1. Conversion vs. irradiation time. Dose rate: 67.38 Gy/min; DEA: 10.4 wt%; water/ethanol (w/w): 57.3/30.6; PVP stabilizer: 1.7 wt%; NPGDA was used as cross-linker at 1.0 wt% based on DEA. 298 K.

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mainly staying in the continuous phase until to be nucleated. As conversion increased, the viscosity of system built up to a certain higher level and the number of nucleated microgel increased (which is similar to the nucleation period in emulsion polymerization), both of them are favorable to accelerate the propagation rate by decrease the termination rate, which is quite similar to the case caused by gel effect. Similar behavior has been reported for the dispersion polymerization of MMA in which vinyl terminus PSI-22 macromonomer was used as reactive surfactants [24]. 3.3. Characterization of PDEA latexes by FT-IR and 1 H NMR spectroscopy

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Fig. 3. Resonance assignment of hydrogen in poly(vinylpyrrolidone).

All samples before measurements were ultracentrifugated and extracted for several times to remove the stabilizer absorbed physically on the microgels surface and then dried in vacuum at 60 ◦ C. Fig. 2 shows the FT-IR spectrum of the PDEA based microgels (latexes form). The absorption peak at 1730 cm−1 corresponding to the stretching vibrations of ester carbonyls group is from the main chain of the polymer, while the absorption peak at 1680 cm−1 corresponding to the stretching vibrations of amide carbonyls group indicated that PVP has been grafted onto the latexes. The proton NMR spectrum as shown in Fig. 4 is another evidence to confirm that PVP has been grafted onto the PDEA latexes. The methine and methylene hydrogens in the polymer backbone are indicated as ␣ and ␤, respectively; whereas the CH2 groups in the pyrrolidone ring are addressed by the numbers in Fig. 3 [25]. According to Ref. [26], the peaks of PVP are clearly found in Fig. 4. Therefore, it can be concluded that graft reaction between PVP and PDEA has taken place. Another evidence for the presence of grafting from the experiment is that the latexes can be easily re-dispersed into deionized water after removed un-grafted PVP. But the mechanism of dispersion polymerization is complex and poorly understood. A few reports put the stress on the nucleation and stabilization mechanism. Paine [26] developed

a multipoint kinetic model for aggregation of precipitated radicals or unstabilized particles in dispersion polymerization. He has claimed the exclusive effect of grafted-copolymer stabilizer in particle nucleation, which can be produced in situ and ends up on the particle surface when the steric dispersant contains active sites for chain transfer of radicals. While Shen et al. [27] reported that the steric dispersant itself would be adsorbed onto the surface of the particles and stabilize the particle in addition to the graft-copolymer stabilizer, so a competition would exist between the adsorption of the graft-copolymer stabilizer and the precursor homopolymer. Which one of these mechanisms is dominant is still subject to speculation. In this paper, simply by FT-IR and 1 H NMR results we still cannot say what the dispersion polymerization mechanism is. But one point which can be sure is the graft-copolymer must make its contribution to stabilize the PDEA latexes in whole polymerization process, otherwise the purified latexes could not be easily re-dispersed into the water. The probable mechanism of grafting reaction between PVP and PDEA is shown in Scheme 1. In the gamma ray field, large numbers of PVP and DEA monomer and oligomers radicals are produced directly through the radiolysis of PVP and DEA or indirectly through the reaction with H• and • OH radicals formed by radiolysis of water. These radicals will react with each other, and when the PVP radicals take the reaction with PDEA radicals or initiate the

Fig. 2. FT-IR spectrum of PVP-g-PDEA latex.

Fig. 4. 1 HNMR spectrum of PVP-g-PDEA latex.

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Scheme 1. Grafting reactions between DEA and PVP.

polymerization of DEA monomer, the grafting copolymers can be obtained. At a higher dose rate, the ratio of PVP• among the radicals produced directly by gamma ray radiation could be much larger, which leads to increase the grafting reaction chance [28]. As soon as the copolymer chains are formed, those chains may be as the nuclei of growing PDEA particles (just like the micelles of emulsion system) or migrate to the surface of PDEA oligomers to stabilize them from their coagulation. 3.4. Effect of PVP/DEA ratio (w/w) on latex size and distribution The influence of PVP/DEA ratio on latex sizes and distribution is given in Fig. 5. Latex sizes decreased with the increase of PVP/DEA ratio, while PDI increased. But when the PVP/DEA ratio increased to a certain quantity, the decrease trend of microgels’ size became smoothly. The probable explanation is the conglomeration possibility of oligomers decreased with the increase of PVP/DEA ratio, because of the stabilization effect of PVP on the formed latexes, as the result, the nucleation period was lengthened which induced a larger number of latex particle formation and at the same time induced an increase of distribution. From Fig. 5, it can be noticed that the system (b) can produce more monodispersed microgels with the PVP/DEA

Fig. 6. Variation of latex size and distribution vs. NPGDA/DEA (w/w) ratio; all samples in the same figure were prepared with the similar recipe while the dosage of NPGDA was changed. (a) Recipe, DEA: 10.4 wt%; water/ethanol (w/w): 57.3/30.6; PVP stabilizer: 1.7 wt%.

ratio at 0.137. And if the dosage of PVP is less than this point, then the distribution goes up, that is because the PVP dosage is too small to stabilize the system. The minimum value of distribution for system (a) appears at 0.054 of PVP/DEA ratio, the difference between these two systems comes from their different ethanol/water ratio. 3.5. Effect of NPGDA/DEA (w/w) ratio on latex size and distribution The influence of NPGDA/DEA ratio on latex size and PDI is given in Fig. 6. Latex sizes increased with the NPGDA/DEA ratio, while the PDI decreased. It is because NPGDA as the crosslinking agent can promote the conglomeration of oligomers to form latexes particles, which causes the shortening of nucleation period and at the same time lengthening of growing period of particles. As a result, with the increase of NPGDA/DEA ratio the larger particles and the narrow distribution can be realized.

Fig. 5. Variation of latex size and distribution vs. PVP/DEA (w/w) ratio; all samples in the same figure were prepared with the similar recipe while the dosage of PVP was changed. (a) Recipe, DEA (g): 8.0; water/ethanol (g): 45.0/24.0; PVP stabilizer (g): 0.18–4.45. (b) Recipe, DEA (g): 8.0; water/ethanol (g): 40.0/29.0; PVP stabilizer (g): 0.88–2.78. And NPGDA was used as cross-linker at 1.0 wt% based on DEA. The dose rate for both systems was 67.38 Gy/min. Irradiation time was 1 h for all systems.

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Fig. 7. Scanning electron microscope photographs of monodisperse PDEA microspheres. Recipe, DEA: 10.4 wt%; water/ethanol (w/w): 50.8/37.1; PVP stabilizer: 1.7 wt%. The dose rate was 67.38 Gy/min irradiation time was 1 h the samples were in their latexes form under pH 9.

It is interesting to point out that EGDMA (ethylene glycol dimethacrylate) was proved to be a failure as cross-linker in emulsion polymerization of DEA [7-a]. But its homologous compound NPGDA showed a good performance as cross-linker in dispersion polymerization of DEA. 3.6. Morphology studies of microgels by SEM The monodisperse PDEA based microgels (latexes form) photographed by scanning electron microscopy were shown in Fig. 7. The latexes’ size was estimated to be around 2.15 ␮m. Electron microscopy studies of the morphology of the DEAbased latexes have been proved somewhat problematic due to their soft, film-forming nature [7-a]. But the spherical micronsized features of PVP-g-PDEA latexes are observed clearly. Maybe the grafting of PVP onto the surface of latexes is important for stabilizing the latexes from film-forming. The monodispersity of the colloidal dispersion has been discussed by Lamer and Dinegar previously [29]. They claimed that monodispersity is only obtained when the initiation and nucleation stage is very short compared with the overall reaction period. At the same time, the period of repetitive nucleation must be made so short that monodispersity arise from subsequent uniform growth on the exiting nuclei. In the irradiation-induced polymerization, the gamma ray radiation with a higher dose rate

(e.g. 1 Gy/s) can produce the higher rate of free radicals formation and polymerization, and the formation of nuclei can be finished in a very short time after irradiation [30]. After the nucleation period, the number of particles keeps constant, and the subsequent uniform growth of those particles results in the formation of monodisperse particles in the irradiation induced dispersion polymerization. 3.7. Swelling/deswelling properties of pH-responsive microgels All the synthesized microgels were milky-white dispersions in their latex form at alkaline condition (see Fig. 8). On addition of acid, the PDEA became protonated and the dispersion quickly became optically transparent below pH 6.5 due to microgel swelling. On addition of alkali, visual inspection indicated that the transparent solutions quickly became turbid at pH 9, suggesting the apparent reversibility of the microgel-to-latex deswelling transition. However, more turbidimetry measurements indicated that significant hysteresis effects occurred during deswelling. All microgels could be subjected to at least three pH cycles (between pH 9 and pH 4). DLS and electrophoresis measurements were carried out on dilute aqueous dispersions of PDEA microgels, and the results were shown in Figs. 9 and 10. Three distinct regimes can be

Fig. 8. Digital photos of PVP–PDEA microgels dispersions at 0.05 wt% in 0.005 M NaCl. The solution pH was lowered from pH 9 (latex form) to pH 5 (transparent microgel form) and then returned to pH 9. Visual inspection indicated the apparent reversibility of the swelling transition.

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Fig. 9. Variation of mean hydrodynamic diameter with pH for PVP-g-PDEA microgels (0.01 wt%); the data are averaged over three runs and the solid line is a guide to the eye, rather than to fit the data. (The corresponding confidence interval were in the range of ±5%.)

Fig. 10. Zeta potential vs. pH curve of PVP-g-PDEA microgels (0.01 wt%); the data are averaged over three runs and the solid line is a guide to the eye, rather than to fit the data. (The corresponding confidence interval were in the range of ±5%.)

found for these particles. Above pH 9, the particles are in their latex form. As the pH is reduced from pH 9 to pH 6.5, the PVP stabilizer chains and PDEA cores remain unsolvable. Below pH 6.5, the PDEA cores become protonated and the latex-tomicrogels swelling transition occurs. The accompanying change in hydrodynamic diameter is clearly seen in Fig. 9. According to Figs. 9 and 10, the PVP-g-PDEA microgels are swelled around pH 6.2–6.5, which is different from the PEGMA/PDEA microgels swelled around pH 6.5–7.0 which prepared by Armes and coworkers. The reason for this difference is the nature of graft stabilizer on PDEA microgels surface [7-a]. And the swelling factor (based on the ratio of the swollen microgel diameter at pH 4 to the latex diameter at pH 9) of PVP-g-PDEA microgels is about 10, which is similar to the PEGMA/PDEA microgels (also about 10), but the difference is lying on the latex size (micron range versus nanometer range) produced by the two kinds polymerization system, which means more potential capacity with using PVP-g-PDEA microgels as drug delivery carrier or other applications can be achieved. The PVP-g-PDEA microgels had more quickly swelling/ deswelling transition comparing with PEGMA/PDMA microgels [7-a] by turbidimetry. Fig. 11 shows the kinetics studies of the PVP-g-PDEA microgels, which completed the swelling transition with only several seconds, while the PEGMA/PDMA microgels taken about 12 s and on hours scale to completed swelling/deswelling process, respectively. And the pronounced hysteresis effects are also observed by turbidimetry studies of deswelling process. The reason for this behavior can be illustrated as follows: according to Ref. [7-a], when the microgel contraction takes place, both its water content and also the salt generated during neutralization must be excreted. Contraction occurs initially, since the polymer chains are rapidly deprotonated and therefore no longer cationic. However, this contraction necessarily leads to an increase in the local salt concentration within the microgels, which in turn causes water from the bulk solution diffusing into the microgel to lower the local salt concentration. It is this ingress of water due to the increase in osmotic pressure that retards the rate of microgel deswelling in its latter stages.

Fig. 11. Kinetic study of latex-to-microgel swelling transition (a); kinetic study of microgel-to-latex deswelling transition (b).

W. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 58–65

4. Conclusion Nearly monodisperse PDEA microgels were prepared by irradiation-induced dispersion polymerization of DEA monomer in the water–ethanol media using PVP as stabilizer. Certain conclusions were obtained as follows: PVP acted as not only a physical stabilizer but also a grafting agent to the surface of PDEA microgels, this is favorable to stabilize the system. Latex sizes were decreased with the increase of PVP/DEA (w/w) ratio, while PDI increased, when the PVP/DEA (w/w) ratio increased to a certain quantity, however, the decreased trend of microgels size became smoothly. As the content of ethanol increased, the latex diameter increased constantly, while PDI decreased at first, then increased after the ethanol/water ratio reach a certain point. The dosage of cross-linker also influenced the latex sizes and distribution: latex sizes increased with the NPGDA/DEA ratio, while PDI decreased. The latex-to-microgel transition was observed at around pH 6.2–6.5, and PVP-g-PDEA microgels showed more quick swelling/deswelling transition compared with PEGMA/PDEA microgels. Furthermore within the range of several microns size, the PVP-g-PDEA microgels still shows good swelling properties, which may provide some potential application in various fields. References [1] B.E. Rodriguez, M.S. Wolfe, M. Fryd, Macromolecules 27 (1994) 6642. [2] B.R. Saunders, H.M. Crowther, B. Vincent, Macromolecules 30 (1997) 482. [3] (a) M. Bradley, J. Ramos, B. Vincent, Langmuir 21 (2005) 1209; (b) K. Dirk, C.D. Vo, S.E. Wohlrab, Langmuir 18 (2002) 4263. [4] G. Gan, L.A. Lyon, J. Am. Chem. Soc. 123 (2001) 7511.

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