Swelling Mechanism Investigation of Microgel with Double-Cross

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Swelling Mechanism Investigation of Microgel with Double-CrossLinking Structures Hu Jia,* Qiang Ren, Wan-Fen Pu,* and Jinzhou Zhao State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, People’s Republic of China ABSTRACT: Microgel is a novel conformance control technology that can be injected into high permeability zones and transport into deep area through pore throats. It can efficiently plug large pores to force injection water diversion to achieve conformance control. In this paper, we synthesized the microgel with a double-cross-linking structure of non-labile and labile characteristics. Then, a polarizing microscope, laser particle analyzer, Brookfield DV-III, and atomic force microscopy (AFM) were used to investigate the swelling mechanism of the microgel in high-saline water (total dissolved solids is from 30 to 100 g/ L) at 65 °C. Results show that the swelling ability of the double-cross-linking structure microgel is not sensitive to salinity and the solution viscosity can increase to a certain degree with the maximum value of 18 mPa s. The decomposition of labile cross-linker results in a loose and explosive structure, which seems like a “mushroom cloud”. Because of the high-density cross-linking sites provided by the non-labile cross-linker, the integrity of the spherical feature of the microgel was reserved after aging for 70 days. On the basis of the viscosity increase of microgel solution during the swelling process, a concept to develop a novel water shutoff agent is proposed. That is, the conventional cross-linkers, such as formaldehyde, methenamine, or phenolic or chromic salt, can be added to microgel solution during the injecting process. Thus, the cross-linker can have the potential to cross-link with the groups that released during the swelling process to form a bulk gel system, which can further improve the water shutoff performance.

1. INTRODUCTION As a chemical method of enhanced oil recovery (EOR), injecting a polymer solution together with a cross-linker proposed for conformance control application has been widely used in mature oilfield development.1,2 Conformance control is a technique to block the already well-swept layers of reservoir for mobilizing pockets of unswept oil/gas.3 Cross-linkers, such as chromium(III) salt,4−6 phenol formaldehyde,7−9 polyethylenimine (PEI),10−12 etc., cross-linking with acrylamide-based copolymer or hydrolyzed polyacryamide (HPAM) can form a polymer gel in subterranean formation during a few hours to several days. In this process, gelation performance has been found to depend upon many parameters; therefore, the gelation time and, thus, the depth of the gel penetration are quite difficult to predict. These difficulties result from the uncertainties concerning different factors: shear stresses, in both surface facilities and near-wellbore areas, and also the physical− chemical environment around the well (salinity, temperature, and pH).3 Among all of the parameters, shear degradation of the polymer/cross-linker in porous media seems to be more important.13,14 Furthermore, both polymer and/or cross-linker adsorption in the near-wellbore region and dilution by dispersion during placement can also greatly affect the treatment effectiveness. On the basis of the problems encountered, size-controlled gel,15,16 “bright water”,17,18 preformed particle gel (PPG),19−21 etc. were developed in recent years. Especially for “bright water”, a kind of microgel, in fact, is prepared by emulsion polymerization. “Bright water” of submicrometer/micro size matching with pore throat and in an unswelled state can be easily injected and placed deep into the reservoir; when microgels encounter a high temperature in the © 2014 American Chemical Society

reservoir, they swell, to efficiently block high-permeability zones in the heterogeneous reservoir to achieve the goal of fluid diverting. The swelling characteristic of this microgel was controlled by two cross-linking structures. One is stable enough to make the microgel have a stronger space network structure that is not easy to degrade. However, the other one is labile and can decompose easily. At the reservoir temperature, the labile cross-linker tends to slowly decompose after injected in the formation for more than 10 or dozens of days. At this moment, the microsphere size begins to gradually swell 10−100 times. Moreover, the microgel does not dissolve and still has an integrated and independent structure. The synthesis method of the double-cross-linking structure microgel was reported in refs 22−25, but a systematic research on the swelling mechanism is not conducted. Swelling properties of microgel can affect its field design as well as production performance; therefore, it is necessary to investigate the swelling mechanism of the microgel with a double-crosslinking structure. In this paper, we first synthesize the microgel with a double-cross-linking structure according to the synthetic methods in refs 22−25. Then, four kinds of experimental apparatuses, including polarizing microscope, laser particle analyzer, Brookfield DV-III, and atomic force microscopy (AFM), are used to investigate the swelling mechanism. The polarizing microscope and laser particle analyzer are used to observe the swelling characteristics at different times. Brookfield DV-III is used to periodically measure the viscosity of the Received: May 14, 2014 Revised: September 11, 2014 Published: September 12, 2014 6735

dx.doi.org/10.1021/ef5012325 | Energy Fuels 2014, 28, 6735−6744

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Figure 1. Swelling prosperities of sample 1 microgel. tetraacetic acid (EDTA) (chelating agent), and deionized water. The above-mentioned materials were used throughout this work for polymerization. The rest reagent NaCl used to prepare the saline water was analytical reagent (AR) grade. Among the materials, the labile cross-linking agent is PEG 200 DA

microgel solution. AFM is used to study the microstructure of the swelled microgel, including verifying whether the microgel has two types of cross-linking structures and analyzing the microstructure of the microgel after the decomposition of the labile cross-linker. Finally, the application prospect of the double-cross-linked microgel used for water shutoff is discussed.

) and the non-

(

2. MATERIALS AND METHODS labile cross-linking agent is MBA (

2.1. Materials. The materials used in our experiments without further purification included acrylamide (AM), 2-acrylamide-2methylpropanesulfonic acid (AMPS), NaOH, Tween-80, Span-60, N,N′-methylenebis(acrylamide) (MBA) (non-labile cross-linking monomer), poly(ethylene glycol(200) diacrylate) (PEG 200 DA) (labile cross-linking monomer), initiator 2,2′-azobis(2-methylpropionamide) dihydrochloride (V-50), aviation kerosene, ethylenediamine-

).

2.2. Methods. Microgels are prepared by inverse emulsion polymerization. The representative steps are as follows: (1) Blending 8.38 g of AMPS into 14 g of deionized water, and then solution pH was adjusted to neutral by the addition of NaOH. (2) Blending 20 g of AM and 1.2 g of Tween-80 into 15.6 g of deionized water for fully 6736


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Figure 2. Swelling prosperities of the sample 2 microgel. stirring. (3) Blending 30 g of aviation kerosene and 6 g of Span-60 for fully stirring. Whereafter, the mixed solution was added to the threenecked flask used for polymerization. (4) Blending the mixed solution prepared in steps 1−3, and then 0.2 g of EDTA, 0.03 g of PEG 200 DA, and 0.01 g of MBA were successively added to the mixed solution, which will be loaded in the three-necked flask. Next, the mixed solution is fully stirred accompanied by the aeration of N2 for thorough emulsification and oxygen removal. (5) The three-necked flask is put in the water bath at 50 °C, and then the stirring speed is adjusted to 300 rpm, accompanied by the addition of 0.03 g of V-50 to the solution. The reaction time is controlled at 6−8 h. (6) Cleaning the synthesized emulsified microgel with acetone over and over again to prepare microgel samples for observation study. After that, the above-mentioned experimental apparatus was employed to investigate the expansion mechanism of the microgel. The mean particle size (d50) shows the central tendency of the particle size distribution. In this case, d50 is the particle diameter corresponding to the cumulative weight of 50% on the cumulative particle size distribution curve, which will be used for discussion in subsequent sections.

3. RESULTS AND DISCUSSION 3.1. Dynamic Swelling Ability of the Microgel. The microgel with different particle sizes can be obtained by changing the concentration of the emulsifying agent, rotating speed, or other affecting factors. To investigate the swelling mechanism of the double-cross-linking microgel at 65 °C, three representative microgel samples 1, 2, and 3 with different salinities are chosen. The test samples were prepared by the addition of 2 wt % microgel in different saline water with 30, 50, and 100 g/L NaCl. 3.1.1. Polarizing Microscope Analysis. 3.1.1.1. Sample 1 at 30 g/L Saline Water. The experimental observations are shown in Figure 1, which indicates that the initial diameter of sample 1 microgel generally distributed within 8−15 μm and the shape of the particles was of uniform sphere. After 1 day, the microgel begins to swell obviously. It can be seen from the micrograph that the diameter of the microgel mainly distributed within 17− 50 μm, microgels swelled notably but the spheres were still neat, and the interface between the spheres was very clear. The 6737


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Figure 3. Swelling prosperities of the sample 3 microgel.

that can be easily injected to the high-permeability reservoirs with a permeability of 500−4000 mD. The solution viscosity slightly increased to 3 mPa s after 1 day and reached the highest value of 10 mPa s after 7 days. This also further proved the reason why the polarizing micrographs became blurry. After 7 days, the viscosity of the microgel solution began to reduce and was then kept at around 6.2 mPa s. Because of the swelling, the mechanism of the micrographs mainly depends upon the loosening efficacy of the labile cross-linker and the free water molecules can enter into the inner structure of the microgel, which exhibits a loose and decomposition state to stimulate swelling. The cross-linked microsphere particles also have a certain water absorption swelling ability, but under such a high salinity, the cross-linked microgel simply relying on water absorption swelling performance will be greatly reduced; hence,

micrograph became a bit blurry after 7 days under the same light intensity, indicating that part of the microgel dissolved, while most of the microsphere particles remained full of spherical conformation. At this time, the diameter of the microgel generally distributed within 25−80 μm, which had swelled to a certain degree compared to the swelling performance after 1 day. No obvious changes occurred after 15 and 20 days, in comparison to the swelling performance after 7 days. The diameter of the microgel generally distributed within 30−80 μm, and the spheres were still intact. The swelling ratio is still at a higher level and can efficiently plug the high permeability zone. In addition, the viscosity of microgel solution is increased with time. The initial viscosity of the microgel solution with 2 wt % microgel was 1.2 mPa s, which is close to the water-phase viscosity. Furthermore, the initial diameter was within 8−15 μm 6738


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Figure 4. Particle size dynamic changing versus time for the sample 1 microgel.

decrease of the viscosity. However, there is no continuous increase of the viscosity caused by the loosening efficacy of the labile cross-linker; therefore, the viscosity of the microgel solution tends to be smooth at the end. For the general hydrogels, the swelling ratio usually reduced with the increase of water salinity.3,26−28 The experimental results show that the swelling mechanism of the double-crosslinking structure microgel is mainly due to the loosening efficacy of the labile cross-liker, which is different from general hydrogels. The viscosity of the microgel with a high

the main swelling mechanism is the loosening efficacy of the labile cross-linker. At the same time, the amide groups cross-linked with the labile cross-linker will be released and the partial hydrolysis of the amide groups will give rise to the solution viscosity increase. When the decomposition of the labile cross-linker reaches the greatest degree, the non-labile cross-linker can still make the microgel keep an ideal cross-linking structure. The hydrolysis of the amide groups will be limited at high salinity. In addition, the carboxyl groups hydrolyzed will be curbed and lead to the 6739


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concentration of 2 wt % only increased to the highest value of around 10 mPa s, indicating that the high-strength cross-linking sites formed by the non-labile cross-linker can make the

spherical particle remain in the configuration integrated and not completely decompose. It can also be seen from the polarizing micrographs that the swelled microgel is still in a dense state 6740


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3.1.1.2. Sample 2 at 50 g/L Saline Water. The experimental observations are as shown in Figure 2. Similar to sample 1, the initial diameter of sample 2 generally distributed within 6−12 μm and the shape of the microgel is in a uniform sphere state.

and does not appear with an obvious sparse phenomenon. Therefore, the viscosity increase property of the double-crosslinked microgel will play an important role in mobility control as well as water diversion. 6741


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Figure 7. Schematic diagram of the loosening efficacy of the labile cross-linker.

been shown to be reduced. Of course, with the salinity increases, the viscosity reduces. Furthermore, the structures of the three samples were still integrated after long-term observation, indicating that the long-term stability is very good. 3.1.2. Particle Size Analysis. 3.1.2.1. Sample 1 at 30 g/L Saline Water. The laser particle analyzer was used to test the particle size distribution to understand the swelling ratio of the microgel. The experimental results of sample 1 are shown in Figure 4. It can be seen from Figure 4 that the particle size distribution curves of the original sample and the swelled sample are steep. Furthermore, the curve kurtosis has a tendency of becoming steeper in the late swelling period, indicating that the grain size sorting of the microgel becomes better with time. In addition, the microgel swelled rapidly in the first 7 days, and d50 increased from 13.35 to 76 μm. However, no changes occurred in the next almost 20 days. d50 was about 77 μm at 15 and 20 days. The particle size analysis shows good agreement with the conclusions obtained by the polarizing microscope test. In the later observation, the complete dissolving of the microgel is not seen. 3.1.2.2. Sample 2 at 50 g/L Saline Water. It can be inferred from Figure 5 that the grain size sorting of the microgel (sample 2) becomes better with time. The microgel swelled rapidly in the first 7 days, and d50 increased from 13.35 to 73.37 μm. In the follow-up evaluation, d50 does not increase obviously and remains at about 77 μm. It is similar to the evaluation result of the microgel at salinity of 30 g/L. The microgel was not completely dissolved in the later observation. High salinity can not only curb the swelling ratio of the microgel but can also be beneficial to keep the configuration integrity of the microgel; hence, it will not rapidly decompose. 3.1.2.3. Sample 3 at 100 g/L Saline Water. Figure 6 shows the particle size distribution curve of sample 3 at different times, indicating that the microgel swelled rapidly in the first 1−5 days, d50 reached up to about 70 μm, and the diameter and solution viscosity changed little in the later 2 days. The swelling ratios of the three samples with different salinities from 30 to 100 g/L were almost the same, indicating that the swelling ratios of the double-cross-linking microgel is not sensitive to salinity, while high salinity only has the negative effect on the viscosity. In the later observation of the above-mentioned microgel simples, spherical particles are still visible after aging for half a year; these microgels do not completely decompose along with a certain viscosity. The results show that the double-cross-

The microgel also began to swell after 1 day. It can be seen from the micrograph that the diameter of the microgel generally distributed within 25−44.72 μm. The micrograph became a bit blurry after 7 days under the same light intensity, indicating that part of the microgel dissolved but most of the microsphere particles remained full of spherical conformation. At this time, the diameter of the microgel generally distributed within 29.61−61.44 μm. d50 of the microgel reached the highest value of 70.09 μm, which is not obviously swelled at 15 and 25 days, and the shape of the particles were of uniform sphere. In addition, d50 and viscosity curves show that the median diameter and solution viscosity increased with time in the initial stage. The initial viscosity of 2 wt % microgel solution was 1.2 mPa s, which is close to the water-phase viscosity. The viscosity reached up to the highest value of 18 mPa s at the seventh day, followed by a decrease in later stage, and was kept at 12 mPa s at the 25th day. The phenomenon of completely dissolved did not occur after 4 months. 3.1.1.3. Sample 3 at 100 g/L Saline Water. The experimental observation results of sample 3 are shown in Figure 3, which indicates that the microgel still has a good swelling ability and thermostability. This phenomenon seems to be inconsistent with the mechanism that the polymer molecule chain will crimp because of the shielding effect under high salinity. It can be explained that the main swelling mechanism of the double-cross-linking microgel is the decomposition of the labile cross-linker, not only limited to microgel swelling by absorbing water. The decomposed labile cross-linker can generate many loose network structures around the microsphere particles. Because of the difference of osmotic pressure, a high-salinity solution can freely infiltrate into the internal structure of the microgel to promote swell. After 25 days, its structure was still integrated; after 30 days, d50 increases to about 70 μm. It was almost in the same swelling ratio as the salinity of 30 and 50 g/L. However, high salinity can lead to serious crimping of the hydrolyzed carboxyl groups, which can act as the reason for the slow increase of solution viscosity. The viscosity of the microgel solution was only about 2 mPa s at a high salinity of 100 g/L after aging for 25 days, which can further prove the swelling mechanism of the double-crosslinking microgel mainly because of the decomposition of the labile cross-linker for volume swelling, not only for single water absorption. The above studies proved that the swelling ability of the double-cross-linking structure microgel is not sensitive to salinity. Even under the high salinity, the swelling ratio has not 6742


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Figure 8. Internal skeleton structure of the polymer microgel.

Figure 9. Schematic diagram of the desired novel water shutoff agent.

of the swelled microgel still has a certain thickness of 5−10 nm. This special skeleton structure of the microgel is the nature that can ensure good thermal stability under high salinity.

linked microgel synthesized by the method of inverse emulsion polymerization has an excellent thermal stability. 3.2. Microstructure Study on the Swelled Microgel through AFM. Through some of the swelled microgels dissolved, however, most of them have the characteristics of an internal structure. The sample 1 microgel after 70 days of aging was chosen to characterize the internal structure by the AFM study. The microstructures are shown in Figures 7 and 8. Figure 7 shows that a loose structure, such as a mushroom cloud, is formed after the thermal decomposition of the labile cross-linker, while the structure of the microgel is still compact enough because of the non-labile cross-linker maintaining a high cross-linking density, which ensured the structural integrity. Figure 8 reflects the internal skeleton structure of the microgel. The skeleton chains are full of branches and in free stretching state. The stereo image reflects that the skeleton

4. FUTURE PROSPECTS FOR NOVEL WATER SHUTOFF AGENT The novel water shutoff agent is based on the assumption that the water shutoff agent is a composite system prepared by injection water, which contains a certain concentration of the double-cross-linking microgel and cross-linker.29 The microgel will gradually release a part of carboxyl and amide groups because of the thermal decomposition of the labile cross-linker, which can result in the increase of viscosity of the swelled microgel solution. When injecting microgel, it can be mixed with the cross-linkers, such as formaldehyde, methenamine, or phenolic or chromic salt, and the cross-linker can cross-link 6743


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with the groups released by labile cross-linker decomposition. Therefore, it is expected to form bulk gel to improve the plugging effect. Besides, the microgel will shrink in the oil phase than cannot damage the oil-bearing zone. The desired function diagram of the novel water shutoff agent is shown in Figure 9.

REFERENCES

(1) Seright, R. S.; Liang, J. Proceedings of the SPE Laitin American and Caribbean Petroleum Engineering Conference; Buenos Aires, Argentina, April 26−29, 1994; Paper 26991. (2) Ghannam, M. T.; Esmail, N. J. Chem. Eng. Jpn. 2010, 43, 115. (3) Jia, H.; Zhao, J. Z.; Liu, R.; Pu, W. F.; Zhao, T. H. Adv. Mater. Res. 2012, 524, 1681. (4) Cordova, M.; Cheng, M.; Trejo, J.; Johnson, S. J.; Willhite, G. P.; Liang, J. T.; Berkland, C. Macromolecules 2008, 41, 4398. (5) Seright, R. S. Conformance Improvement Using Gels; United States Department of Energy (U.S. DOE): Washington, D.C., 2004; Annual Technical Progress Report DOE/BC/15316-6, U.S. DOE Contract DE-FC26-01BC15316, pp 72. (6) Seright, R. S. Proceedings of the SPE International Symposium on Oilfield Chemistry; Houston, TX, Feb 5−7, 2003; Paper 80200. (7) Moradi-Araghi, A. J. Pet. Sci. Eng. 2000, 26, 1. (8) Banerjee, R.; Ghosh, B.; Khilar, K. C.; Boukadi, F.; Bemani, A. Energy Sources, Part A 2008, 30 (19), 1779. (9) Jia, H.; Pu, W. F.; Zhao, J. Z.; liao, R. Energy Fuels 2011, 25, 727. (10) Jia, H.; Pu, W. F.; Zhao, J. Z.; Jin, F. Y. Ind. Eng. Chem. Res. 2010, 49, 9618. (11) Reddy, B. R.; Eoff, L.; Dalrymple, E. D.; Black, K.; Brown, D.; Rietjens, M. SPE J. 2003, 8, 99. (12) Al-Muntasheri, G. A.; Nasr-El-Din, H. A.; Hussein, I. A. J. Pet. Sci. Eng. 2007, 59, 73. (13) Shi, J.; Varavei, A.; Huh, C.; Delshad, M.; Sepehrnoori, K.; Li, X. F. Energy Fuels 2011, 25, 5063. (14) Shi, J.; Varavei, A.; Huh, C.; Delshad, M.; Sepehrnoori, K.; Li, X. F. Energy Fuels 2011, 25, 5033. (15) Chauveteau, G.; Tabary, R.; Blin, N.; Renard, M.; Rousseau, D.; Faber, R. Proceedings of the SPE/DOE 14th Symposium on Improved Oil Recovery; Tulsa, OK, April 17−21, 2004; Paper 89390. (16) Chauveteau, G.; Tabary, R.; Renard, M.; Feng, Y. J.; Omari, A. Proceedings of the SPE European Formation Damage Conference; Hague, Netherlands, May 13−14, 2003; Paper 82228. (17) Husband, M.; Ohms, D.; Frampton, H.; Carhart, S.; Carlson, B.; Morgan, J. C.; Chang, K. T. Proceedings of the SPE Improved Oil Recovery Symposium; Tulsa, OK, April 24−28, 2010; Paper 129967. (18) Frampton, H.; Morgan, J. C.; Cheung, S. K.; Munson, L.; Chang, K. T.; Williams, D. Proceedings of the SPE/DOE 14th Symposium on Improved Oil Recovery; Tulsa, OK, April 17−21, 2004; Paper 89391. (19) Tang, H. U.S. Patent Application 11/712,055, 2007. (20) Bai, B. J.; Li, L. X.; Liu, Y. Z.; Liu, H.; Wang, Z. G.; You, C. M. SPE Reservoir Eval. Eng. 2007, 10, 415. (21) Bai, B.; Liu, Y.; Coste, J. P.; Li, L. SPE Reservoir Eval. Eng. 2007, 10, 176. (22) Frampton, H. U.S. Patent 5,701,955, 1997. (23) Chang, K. T.; Frampton, H.; Morgan, J. C. U.S. Patent 6,454,003, 2002. (24) Chang, K. T.; Frampton, H.; Morgan, J. C. U.S. Patent 6,729,402, 2004. (25) Chang, K. T.; Frampton, H.; Morgan, J. C. U.S. Patent 7,300,973, 2007. (26) Hu, Y.; Wang, Q.; Wang, J.; Zhu, J. T.; Wang, H.; Yang, Y. J. Biomicrofluidics 2012, 6, 026502. (27) Ganji, F.; Vasheghani-Farahani, S.; Vasheghani-Farahani, E. Iran. Polym. J. 2010, 19, 375. (28) Hoare, T.; Pelton, R. J. Phys. Chem. B 2007, 111, 11895. (29) Jia, H.; Pu, W. F.; Zhao, J. Z.; Liao, R. Chinese Patent CN102329599A, 2012.

5. CONCLUSION (1) The swelling ability of the double-cross-linking structure microgel is not sensitive to salinity. The swelling mechanism is different from the general hydrogels, and the main mechanism is the loosening efficacy of the labile cross-linker. The swelling ratios of the three representative microgel samples are nearly equal in the high salinity from 30 to 100 g/L. The median particle size (d50) increased with time in the first 7 days, followed by a little change in the later observation. d50 grows to 72.96−77.54 μm after 25 days for the three samples. (2) The solution viscosity increased within a certain range during the swelling process. The solution viscosity with a salinity of 30 and 50 g/L increased in the initial stage and decreased a little in the later stage. The maximum value can reach up to 18 mPa s. The viscosity of the microgel solution slowly increased at 100 g/L saline water and only reached up to 2 mPa s after 25 days. This is mainly because the carboxyl groups produced by the loosening efficacy of the labile cross-linker cannot efficiently stretch in high-salinity solution. It is expected to form bulk gel to improve the water shutoff performance by the addition of cross-linkers, such as formaldehyde, methenamine, or phenolic or chromic salt, when injecting microgel. (3) The AFM analysis shows that the swelled microgel has a loose but integral structure, which seems like a “mushroom cloud”, indicating the characteristic of the loosening efficacy of the labile cross-linker. The internal structure of the microgel exhibits skeleton structures with chains that are in a state of free stretching. Because of the high-strength cross-linking sites formed by the non-labile cross-linker, the entire structure of the microgel maintains integrity. Furthermore, the special internal structure is the essence that can be used to explain why the double-crosslinking structure microgel has an excellent thermal stability at high salinity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the CNOOC Tianjin Branch (No. S10TJPXX049), the National Natural Science Foundation of China (NSFC) (51404202), the Scientific Research Starting Project of Southwest Petroleum University (SWPU) (2014QHZ001), and the Science and Technology Fund of SWPU (2013XJZ007). The special fund of China’s Central Government for the Development of Local Colleges and Universities, the project of National First-Level Discipline in Oil and Gas Engineering, is also greatly appreciated. Special thanks to the assistance of Jun-Zhong Li (SWPU, now in CNOOC) in experimental work. 6744


Swelling Mechanism Investigation of Microgel with Double-Cross

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