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Highly Deformable Nano-Crosslinkers Bridged Nanocomposite Hydrogels for Water Management of Oil Recovery Lizhu Wang, Jiaming Geng, and Baojun Bai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03649 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Highly Deformable Nano-Crosslinkers Bridged Nanocomposite Hydrogels for Water Management of Oil Recovery Lizhu Wang*, Jiaming Geng, Baojun Bai* Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA *To whom correspondence should be addressed. E-mail:[email protected], [email protected]

Abstract: Conventional poly(acrylamide)-based hydrogels suffered from mechanical instability during water flooding, which markedly reduced their performance for water management and oil recovery. In this report, divinylbenzene (DVB) nano-structured crosslinkers bridged nanocomposite hydrogels with high elasticity were described to increase hydrogel mechanical integrity. Precipitation polymerization of DVB monomers generated well-defined DVB nanocrosslinkers having styrenyl moieties on the surfaces as demonstrated by 1H NMR analysis. Frequency sweeps of the hydrogels confirmed the formation of covalent junctions between poly(acrylamide) chains and DVB nano-crosslinkers within the network. The nanocomposite hydrogels with covalent crosslinks showed high degree extensibility greater than 40 times, compared to self-crosslinked N,N'-dimethylacrylamide hydrogels with elongation of 14 and N,N'-methylenebisacrylamide crosslinked PAM hydrogel having stretchability less than 2 times. The concentration of ammonium persulfate initiator showed greater effect on mechanical robustness than DVB nano-crosslinkers. The increase in initiator significantly increased hydrogel extensibility upon stress. In addition, nano-crosslinkers-based hydrogel displayed slow swelling 1 ACS Paragon Plus Environment

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kinetics in brine in comparison with commercially available LiquiBlock 40K gel. Low cost DVB monomer-based nano-crosslinkers bridged highly deformable hydrogels with excellent elasticity rendered hydrogel production in industrial scale feasible. High deformation characteristic facilitated hydrogels propagation through pore throats for in-depth fluid diversion.

Key words: nano-crosslinker, divinylbenzene, deformable, hydrogels

Introduction

Sweep factor and displacement efficiency during water and CO2 flooding governed oil recovery of mature reservoirs. Generally, poor sweep efficiency was ascribed to the reservoir heterogeneity generated by long-term flooding, sand production and un-intentionally hydraulic fracturing.1,2 Reservoir conformance control is a cost-effective approach to increase sweep efficiency in most heterogeneous reservoirs. The injected water preferred to flow through low permeability zones after profile modification, where a large amount of oil reserves remained, thus improving oil recovery and reducing water production.3,4 In the past decades, during the effort to mitigate excess water production and increase hydrocarbon recovery, fracturingplugging and fluid diverting materials were deployed for conformance control. Inorganic materials based gels were employed to water treatment of reservoirs.5 However, implementation of such gels to heterogeneous reservoirs was demonstrated to be less fruitful due to its low selectivity to oil in place. Metallic crosslinkers of Cr3+, Zr4+ and Al3+ crosslinked in-situ gels have been successfully deployed in oilfields for water reduction and oil enhancement by increasing sweep efficiency. However, they are mechanically vulnerable due to the sensitivity of metallic bridges to salinity and acidic environments under reservoirs.6,7 In addition, the gelant of the polymer and 2 ACS Paragon Plus Environment

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crosslinker mixture suffered from water dilution, inadequate control of gelation time and chromatography separation.8-10 Therefore, the resistance to water flow is relatively not robust, particularly in void space conduits containing reservoirs.11 In recently years, preformed particle gels (PPGs) prepared on surface before injection showed high tolerance and resistance to salinity and temperatures.12,13 In particular, PPGs utilized in conformance control overcome inherent drawbacks posed by in-situ gel technology. The gels have been applied to heterogeneous reservoirs with porous channels to decrease water production and increase oil recovery. However, conventional particle gels might break into pieces during transport to fractured matrix due to their mechanically unstable properties under stress. In most cases, radically active crosslinkers of N,N'-Methylenebisacrylamide (MBA) with two acrylamide groups on the short chains and poly(ethylene glycol) diacrylate (PEGDA) were introduced to 3-dimensional polymeric network as covalently chemical bridges to maintain gel integrity. However, conventional crosslinkers bridged hydrogels showed poor elasticity due to the absence of the center of energy dissipation.14 Engineered nano-crosslinkers, responsive nanoparticles with multiple active moieties on the surfaces having similar chemical composition to conventional crosslinkers, imparted unprecedentedly physical properties of the hydrogels. Anti-intuitive nano-gels having volumetric expansion at higher temperatures was obtained by introducing carboxylic moieties to crosslinker.15 In addition, catechol modified nano-crosslinkers and photocatalytic titania (TiO2) nanosheet constructed hydrogels rendered reversible solid-like mechanics and photolatent modulation, respectively.16,17 These results implied that engineered nano-crosslinkers might be a new strategy to tune the dynamic properties of the nanocomposite hydrogels. However, the fabrication of highly stretchable yet thermally stable hydrogels at low cost

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production remains a challenge for scale-up production in industry. In previous reported studies, the introduction of functional surface in divinylbenzene (DVB) microspheres led to their applications as toughing agents and impact modifiers.18-20 The combination of reversible addition fragmentation chain transfer polymerization and hetero-Diels-Alder reaction on DVB microspheres provided functional entities with a host of applications. The accessible styrenyl double bonds on DVB surface allowed the grafting of hydrophilic polymer chains via thiol-ene chemistry. By virtue of its unique properties, the introduction of hard DVB polymeric nanoparticles with high elastic modulus to mechanically soft hydrogel matrix might provide a straight and effective approach to obtain highly stretchable yet robust polymeric network. In this work, we report a facile method to prepare a highly stretchable and robust hydrogel from a nano-structured crosslinker. We will show how nano-crosslinker is synthesized by precipitation polymerization. The residual double bonds on the surface of nano-crosslinker enable us to prepare covalently bridged hydrogel. The introduction of thermally stable nanocrosslinkers could increase hydrogel integrity at elevated temperature. In addition, the nanocrosslinkers as the energy dissipation centers provide robust hydrogel with high elongation compared to conventional MBA crosslinked hydrogel. Furthermore, the swell rate of nanocrosslinker formed hydrogels was delayed, thus facilitating hydrogel deployment to petroleum reservoirs.

Experimental

Materials. All chemicals and reagents were purchased from Sigma-Aldrich (St. Louis, MO) except as noted. Divinylbenzene (DVB, 80%), poly(vinylpyrrolidone) (PVP, average molecular weight 58,000 g mol-1), acrylamide (AM), ammonium persulfate (APS) and anhydrous ethanol


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(99 %) were used without further purification.

Synthesis of PDVB based Nano-Crosslinker. PDVB nano-crosslinker was prepared by modified precipitation polymerization reported by Montoto and co-workers to produce nano-particles in diameter of ~ 500 nm.21 A typical polymerization is as follows: To the stirring solution of poly(vinylpyrrolidone) (3.2 g) in anhydrous ethanol (800 mL) were added DVB (16 g) and 2,2′Azobis(2-methylpropionitrile) (AIBN) (200 mg) at the stirring speed of 170 rpm, bubbled with argon for 30 min and immersed in a preheated oil bath at 70 °C for 6 h. The reaction mixture was cooled to room temperature and centrifuged. The supernatant was decanted and washed with petroleum ether to remove trace amount of DVB. The product was collected, dried in vacuo and used without further purification.

Preparation of DVB Nano-Crosslinker Bridged Hydrogels. In a typical polymerization, to the suspension solution of DVB nano-crosslinkers (108 mg) in de-ionized water (75 mL) under ultrasonication for 30 min was added AM (21.6 g, 300 mmol) and APS (17.10 mg, 0.075 mmol), bubbled with argon for 30 min and immersed in a preheated oil bath at 60 °C under vigorous stirring until the formation of the bulk gel. The bulk gel was cured at 60 °C for 6 h and used without further purification.

Rheological Studies. The rheological properties of hydrogels were based on the average of triplicate measurements using a Haake MARS III rheometer (Thermo Scientific Inc.). The experimental errors from measurements were approximately in the range of ± 20%. The samples were subject to rheological measurements at 25 °C using parallel-plate geometry (P35 Ti L) with


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a gap of 1 mm. Strain sweep experiments were conducted to determine the linear strain regime. Oscillatory frequency sweep was performed from 0.1 to 16 Hz with a shear strain of 1 % within the linear strain range. Oscillation time-dependent experiments were performed at a fixed frequency of 1 Hz and controlled strain of 1 % to obtain shear elastic modulus of G' and viscous modulus of G" as a function of time.

Morphology Characterization. Scanning electron microscopy images were collected on a HITACHI S-4700 FESEM microscope operating at 15.0 kV to elucidate the microstructures of the gels. All images were captured with the samples after freeze-drying in non-hydrated state and sputter coated with Au/Pd prior to imaging.

Dynamic Light Scattering. Dynamic light scattering (DLS) measurements on DVB nanocrosslinker hydrogels were performed by a Malvern Zetasizer Nano-ZS90 (Malvern Instruments Ltd., UK) at a detection angle of 90○ with an incident beam of wavelength at 633 nm at 25 °C. The measurements were based on 3 repeated results for each sample. The intensity size distributions were obtained from analysis of the correlation functions by the Multiple Narrow Modes algorithm in the instrument software.

NMR Analysis. 1H nuclear magnetic resonance (NMR) spectroscopy was conducted on Varian Inova 400 FT-NMR using CDCl3 as solvent and the residue peak at 7.26 ppm as an internal reference.

Swelling Degree Measurements. The disc-shaped samples in thickness of ~ 1.4 mm and diameters of ~ 15 mm were used to determine swelling ratio of the gels. The disks were 6 ACS Paragon Plus Environment

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immersed in deionized water and allowed to swell for 24 h at room temperature to reach the equilibrium state. Then the samples were dried in a freezing-dry system. The swelling ratio was calculated as follows: ୛

Q = ୛౭౛౪ ౚ౨౯

where Wwet is the mass of the wet sample in fully swollen state and Wdry is the weight of the dry sample after lyophilization.

Results and discussion

Scheme 1. Hydrogel preparation using DVB nano-crosslinkers: (A) DVB nano-crosslinkers with residual double bonds on the surface prepared by precipitation polymerization of DVB monomers in ethanol, (B) DVB nano-crosslinkers bridged AM based hydrogels. Preparation of DVB Nano-Crosslinkers Bridged Hydrogels. The fabrication of highly stretchable hydrogels was illustrated in Scheme 1 as prepared from AM monomers in the presence of DVB nano-crosslinkers initiated by APS. Precipitation polymerization of DVB monomers in ethanol generated DVB nano-crosslinkers bearing residual styrenyl double bonds


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on the surface acting as crosslinking center.18,19 Addition of AM monomers to DVB nanocrosslinkers suspension in water initiated by APS generated nano-crosslinkers bridged hydrogels. The multiple crosslinks on the nano-crosslinkers rendered manipulatable physical properties of the hydrogels. The mechanical strength of nano-engineered hydrogels could be controlled by tuning the concentration of nano-crosslinker and initiator. The flexible linear poly(acrylamide) chains between nano-crosslinkers are assumed to exhibit highly stretchable property by dissipating energy through reversible chain elongation and contraction under external stress.22

Figure 1. (A)Hydrodynamic size distribution of the as-obtained nano-crosslinkers in ethanol, diluted to 50 times and measured by DLS at room temperature at the concentration of ~ 0.5 mg mL-1; (B) SEM micrographs of the nano-gel crosslinker in dry state. The images was taken from nano-gel dispersion in ethanol and allowed to dry at room temperature. Synthesis of DVB Nano-Crosslinker. Monodisperse DVB nanoparticles with uniformed geometry were synthesized via precipitation polymerization with the aid of PVP surfactant following previous work.21 As shown in Figure 1A, the average hydrodynamic diameter of DVB nano-crosslinkers in ethanol was 850 nm, ranging from 300 to 900 nm. The hydrodynamic


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diameter of DVB nano-crosslinkers measured by DLS approximately was smaller compared to the size as visualized by scanning electron microscopy (SEM) from several hundred nanometers to microns as illustrated in Figure 1B, where uniform spherical nano-crosslinkers were observed. The difference in sizes may be due to the techniques used for nano-crosslinker measurements. In previous report, bromine titration of DVB spheres confirmed the formation of residual styrenyl double bonds on the surfaces, which permitted the preparation of core-shell microspheres by radical capture of second monomers.23 However, the residual double bonds on DVB nanocrosslinkers in our work were used as nano-crosslinkers to construct highly stretchable hydrogels under radical conditions.

(B) Initiator

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Figure 2. (A) 1H NMR spectrum (400 MHz) of DVB nano-crosslinkers dispersed in CDCl3 and (B)Proposed crosslinking mechanism of DVB nano-crosslinker bridged hydrogels. Furthermore, the nano-crosslinkers used for hydrogel synthesis were subjected to evaluation of proton nuclear magnetic resonance (NMR) spectroscopy, a powerful tool to analyze a particular group, demonstrating the presence of residual double bonds. As revealed in 9 ACS Paragon Plus Environment

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Figure 2A, the signals at δ 6.75, 5.77 and 5.26 ppm in the 1H NMR spectrum corresponded to olefinic protons from styrenyl moieties. The unsaturated double bonds facilitate the synthesis of engineered hydrogels from nano-crosslinkers with branched junctions. In this work, DVB nanocrosslinker was isolated by precipitation from the polymerization system in a controllable manner. However, MBA and NIPAM nano-gels via precipitation polymerization required delicate conditions of relatively short time and lower temperatures to retain acrylate double bonds on the surfaces of poly(N-isopropylacrylamide) nano-gels.22 The retention of radical active styrenyl bonds was obtained under harsh conditions, restricting their application in industry. Styrenyl double bonds involved in radical polymerization of acrylamide monomers, whose function was similar to traditional crosslinker of N,N-Methylenebis(acrylamide), Figure 2B. Poly(acrylamide) chains within the hydrogels were connected by multiple crosslinkable styrenyl double bonds on the surfaces of DVB nano-crosslinkers.

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Figure 3. Dynamic rheology of DVB nano-crosslinker covalently crosslinked hydrogel as a function of frequency (ω = 0.1-100 rad s-1), filled and open symbols denoted for elastic and


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viscous moduli, respectively. The sample was prepared at the nano-crosslinkers concentration of 4 wt%. The DVB nano-crosslinker covalently bridged hydrogels were demonstrated by rheological analysis under frequency sweeps. The hydrogel was prepared from 4 wt% DVB nano-crosslinkers at APS concentration of 0.4 wt% relative to AM monomers. A predominantly elastic modulus of G' greater than viscous modulus of G'' over the entire angular frequency validated the formation of covalently bonded polymeric network, Figure 3. In addition, the elasticity of the hydrogel showed independent feature to frequency sweeps. The mechanical response to frequency was similar to that of MBA interconnected hydrogel in which G' was frequency-independent. However, the elasticity of a 25 wt% PAM solution in the absence of crosslinker had strong independence on frequency, implying its viscoelastic materials nature.24

Figure 4. SEM micrographs of 0.5 wt% DVB nano-crosslinker interconnected hydrogel in dry state after lyophilization. (A) Interconnected polymeric network and (B) Enlarged portion of DVB nano-crosslinker bridged poly(acrylamide) chains. 11 ACS Paragon Plus Environment

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The morphological features of the hydrogel analyzed by scanning electronic microscopy (SEM) showed porously interconnected network with irregular pores distribution, Figure 4A. The DVB nano-crosslinkers bridged network was demonstrated by PAM chains interconnected with DVB nano-crosslinkers, Figure 4B, where DVB nano-crosslinker centers were observed by SEM. The results are similar to previously reported data in which Fe3O4 nanoparticles were well dispersed in the hydrogel network as visualized by SEM microstructures.25 The observed diameter of DVB nano-crosslinkers were in agreement with the average sizes of SEM images, which was the pivot of controlling dynamic mechanical properties of nano-crosslinkers based hydrogels. The energy dissipation center of DVB nano-crosslinkers accounted for highly elastic, stretchable features of the hydrogels.

Dynamic Modulus, G', G" (Pa)

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Figure 5. Gel strength as a function of crosslinker concentration measured at angular frequency of 6.28 Hz with standard errors of ~ ± 20% in each data point. Crosslinker concentration was based on mass ratio of DVB nano-crosslinker to acrylamide monomer. The gel synthesis was conducted at 60 °C initiated by APS in water.


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Concentration of DVB Nano-Crosslinker Effect on Gel Elasticity. DVB nano-crosslinkers nanocomposite hydrogels were subjected to rheometry evaluation to examine hydrogel mechanical strength. As observed in Figure 5, the elasticity of the gel increased with nanocrosslinker concentration, indicating synergistic interactions of covalent crosslinks between nano-crosslinkers and poly(acrylamide) chains and reinforcement of nano-crosslinkers to hydrogels. For example, the hydrogel with nano-crosslinker of 0.5 wt% showed elastic modulus of 18 Pa while a 4-fold increase in elasticity was observed as the concentration of nanocrosslinkers was maintained at 8 wt%. However, further addition of nano-crosslinker concentration resulted in mechanical instability of the prepared hydrogel in water. The results are comparable to previously reported work.26 Apparently, the mechanical strength of the hydrogel could be modulated by nano-crosslinker concentration. The lower elasticity might be ascribed to the relatively less covalent crosslinks due to low amount of ammonium persulfate (APS) initiator, leading to fewer radicals used for polymerization. 300

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Figure 6. Elastic moduli of DVB nano-crosslinkers bridged hydrogels in the fully swollen state responding to initiator concentration based on the mass ratio of APS to AM monomers. The data


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were measured at angular frequency of 6.28 Hz with standard errors of ~ ± 20% in each data point. (A) and (B) the samples were prepared at the crosslinker concentration of 4 and 8 wt% and APS initiator of 0.08 and 0.4 wt%, respectively.

APS Initiator Effect on Gel Strength. To investigate APS initiator effect on the mechanical performance of DVB nano-crosslinkers reinforced nanocomposite hydrogels, the samples initiated by different amount of APS were prepared. As shown in Figure 6A, the increase in APS initiator concentration from 0.08 to 0.4 wt% in the prepolymer solution at the crosslinkers concentration greater than 1 wt% led to a pronounced increase in mechanical strength from 80 to 115 Pa, suggesting more covalent crosslinks formed between DVB nano-crosslinkers and PAM chains in DVB nano-crosslinkers reinforced hydrogels. The addition of DVB nano-crosslinkers concentration to 0.4 wt% at the crosslinkers concentration of 8 wt% approximately increased more than 2 times elasticity from 86 to 182 Pa, Figure 6B. However, when the nano-crosslinkers concentration was below 1 wt%, the elasticity of the samples was irrespective of initiator concentration. The elastic moduli increase was similar to self-crosslinked N, N'dimethylacrylamide-potassium persulfate (KPS) gel system, where the moduli increased from 2.5 to 5.0 KPa as addition of KPS concentration increased from 0.4 to 1.8 mol%.14 The increased moduli was attributed to the increase in covalent cross-links via dimethyl side chain transfer under radical conditions.


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100 80 60 40 20 0 20

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Figure 7. Gel integrity in response to temperatures of measurement. The sample was prepared with 8 wt% DVB nano-crosslinkers at 60 °C. Solid symbol in the graph denotes elastic moduli of the hydrogel while open symbol stands for viscous elasticity of the hydrogel. The data were measured at angular frequency of 6.28 Hz with standard errors of ~ ± 20%.

Gel Integrity Responding to Temperature. To evaluate the hydrogel mechanical integrity under reservoir conditions, the mechanical property of the hydrogel was conducted at temperatures ranging from 25 to 80 °C at strain of 1% under sweep frequency of 1 Hz. As displayed in Figure 7, the independence of hydrogel elasticity on temperatures was demonstrated by nearly constant elastic and viscous moduli of G' and G''. DVB nano-crosslinkers reinforced hydrogels showed rubber-like behaviors due to its elasticity insensitivity to temperatures. As the temperature increased from 25 to 80 °C, the elasticity of the hydrogel maintained approximately 80 Pa.


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Figure 8. Salt effect on hydrogel swelling behaviors of (A) and mechanical strength of (B) measured at the angular frequency of 6.28 Hz at the gap of 1 mm with standard errors of ~ ± 20%. Hydrogels were synthesized in the presence of 0.5 and 8 wt% DVB nano-crosslinkers relative to AM monomers.

Gel Behaviors in Response to Salinity. When the hydrogels were placed to fractures containing mature reservoirs, the physical properties of the hydrogels were affected by reservoir salinity. In most cases, the hydrogels under reservoirs would experience dramatic reduction in swelling ratio, which would reduce the ability of the hydrogels to divert water and CO2 flow. However, in our study, the swelling ratio in mass increased with the addition of NaCl, Figure 8A, as the salinity increased from 1 to 12 wt%. Probably, the gels in different salinity had similar swelling ratio in volume, leading to the increase in swelling ratio in mass. Correspondingly, the mechanical strength of the hydrogels was not influenced by salinity of the reservoirs. The hydrogels with 4 and 8 wt% nano-crosslinkers showed similar elastic moduli of 20 ± 1 and 82 ± 2 Pa, respectively as the salinity ranged from 1 to 12 wt%, Figure 8B. The insensitivity of the 16 ACS Paragon Plus Environment

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hydrogels to salinity could be due to hydrophobic effect of DVB nano-crosslinkers within the hydrogels.27 In addition, in 1 wt% CaCl2 the gel showed approximately ~ 75% swelling ratio of in mass compared to the sample in 1 wt% NaCl. However, the increase in salinity was demonstrated to significantly reduce in-situ gel mechanical integrity. Under such conditions, the hydrogels maintained mechanical integrity in high concentration brine, highlighting nanocrosslinkers reinforced roles in the hydrogels. Based on swelling and mechanical behaviors, the nano-crosslinked reinforced hydrogels had excellent salt resistance to high salinity compared to in-situ gels. 100

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Figure 9. Swelling kinetics of the synthesized samples and LiquiBlockTM 40K gel monitored at room temperature in 1 wt% NaCl. (A) The samples of A, B, C and D in the size of ~ 1 mm were prepared by varied crosslinker concentration of 0.5, 1, 4 and 8 wt% at the initiator concentration of 0.08 wt% relative to AM monomers, (B) LiquiBlockTM 40K gel in the size of ~ 1 mm.

The swelling behavior, a critical parameter for hydrogel delivery to fractured reservoirs, was studied as a function of time. The mass increase of the hydrogels immersed in 1 wt% NaCl was used to monitor the swelling process. The hydrogels of different crosslinkers at the same 17 ACS Paragon Plus Environment

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initiator concentration of 0.08 wt% showed similar swelling behaviors in 1 wt% NaCl, Figure 9. The water absorption of the hydrogels increased rapidly within 1.5 h, approximately 50% of their maximum swelling degree after which absorbed water in a relatively slow fashion. The swelling ratio of the hydrogels at APS concentration of 0.08 wt% reached 60 ± 3 mL/g in 24 h. However, conventional hydrogel of LiquiBlockTM 40K had fast swelling kinetics, which increased to fully swelling state in 15 min.28 To some extent, nano-crosslinker bridged hydrogel had swelling delayed feature compared to conventional particle gel. The swelling behavior of nano-crosslinker hydrogel facilitates deployment of such gel to field applications.

Figure 10. Deformable demonstration of elastic and highly stretchable hydrogel synthesized at DVB concentration of 0.5 wt% and APS concentration of 0.8 wt%. (A) bending, (B) twisting and (C) stretchability at room temperature.

Elastic Behaviors of DVB Nano-crosslinkers Hydrogels. Highly deformable hydrogels with excellent elasticity facilitate hydrogels propagation to fractures through pore throats for diversion of a specific fluid.12 The hydrogels included with branched crosslinkers showed high substantial elongation in the stretching experiment. The DVB nano-crosslinkers bridged hydrogels are 18 ACS Paragon Plus Environment

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highly compliant as displayed in Figure 10. The sample could withstand high level bending (Figure 10A), twisting (Figure 10B) and stretching in one direction (Figure 10C). The elongation of the hydrogels in its un-swollen state was determined by hands until the hydrogels broke. Upon unloading the stress DVB nano-crosslinkers bridged hydrogels exhibited fully spontaneous strains up to 4000 % as shown in Figure 10C. The flexible poly(acrylamide) chains between DVB nano-crosslinkers accounted for highly elastic and stretchable behavior of DVB nano-crosslinker bridged hydrogels.25 The reversible state from coil to elongation of the poly(acrylamide) chains in the DVB nano-crosslinked hydrogels in responding to stress could effectively dissipate energy. Thus, the hydrogels prepared from DVB nano-crosslinkers were highly deformable under stress. In previous work on particle gel transport mechanism through porous media, traditional particle gels were mechanically brittle and broken to pieces under stress13. However, DVB nano-crosslinker particle gels maintained mechanical integrity under large deformation conditions.

5000

2000 0.8%

4000 0.4%

3000

2000

0.08%

1000

0

Elongation at Break (%)

(A) Elongation at Break (%)

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(B)

0.5% 1%

1500

1000

4%

8%

500

0

Initiator Concentration (wt%)

DVB Concentration (wt%)

Figure 11. Tensile behaviors of the as-prepared hydrogels responding to varied synthetic parameters of (A) APS initiator concentration relative to AM monomers at DVB nano-


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crosslinker concentration of 0.5 wt% and (B) DVB nano-crosslinker concentration at APS of 0.08 wt%.

Furthermore, the effect of initiator and DVB nano-crosslinker concentration on mechanical properties was examined. As revealed in Figure 11A, the introduction of more initiators to AM and DVB nano-crosslinker system during polymerization markedly increased mechanical deformation upon stretching. For example, as APS concentration increased from 0.08 to 0.4 wt% relative to AM monomers, the elongation of the hydrogels increased from 1800 to 4000 %, resulting in 2.5- fold increase in stretchability. However, the elongation of the hydrogels decreased to 1000 % as the crosslinkers concentration increased from 0.5 to 8 wt%, Figure 11B. In contrast, MBA crosslinked PAM hydrogel showed less than 2 times elongation and selfcrosslinked DMA hydrogels had substantial elongation of ~ 14. Based on the data, DVB nanocrosslinkers bridged hydrogels were highly stretchable.

Conclusions

In

conclusion,

nanocomposite

divinylbenzene

hydrogels

with

high

(DVB) elasticity

nano-structured

crosslinkers

were

herein.

described

bridged

Precipitation

polymerization of DVB monomers generated well-defined DVB nano-crosslinkers having styrenyl moieties on the surfaces as demonstrated by 1H NMR analysis. Frequency sweeps of the hydrogels confirmed the formation of covalent junctions between poly(acrylamide) chains and DVB nano-crosslinkers within the network. The nanocomposite hydrogels with covalent crosslinks showed high degree extensibility greater than 40 times, compared to self-crosslinked N,N'-dimethylacrylamide

hydrogels

with

elongation

of

14

times

and

N,N'-

methylenebisacrylamide crosslinked PAM hydrogel having stretchability less than 2 times, 20 ACS Paragon Plus Environment

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rendering the ease in penetration of pore throats in fractured matrix. The concentration of initiator showed greater effect on mechanical robustness than DVB nano-crosslinkers. The increase in initiator significantly increased hydrogel deformation upon stress. More importantly, such hydrogels exhibited swelling delayed feature in comparison with commercial LiquiBlockTM 40K particle gel used for water management of mature reservoirs. The nano-crosslinker bridged highly deformable hydrogels with excellent compliant feature offered a candidate to facilitate hydrogels propagation through pore throats for in-depth fluid diversion.

Author Information Corresponding author. *Email: [email protected] Acknowledgements

The authors would like to express their grateful acknowledge to the financial support from DOE under the contract of DE-FE0024558

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Highly Deformable Nano-Crosslinkers Bridged Nanocomposite Hydrogels for Water Management of Oil Recovery Lizhu Wang,* Jiaming Geng, Baojun Bai* Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA *To whom correspondence should be addressed. E-mail:[email protected], [email protected]

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