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Controlled Synthesis of Poly (Acrylamide-co-Sodium Acrylate) Copolymer Hydrogel Micro-Particles in a Droplet Microfluidic Device for Enhanced Properties Dizhu Tong, Gurkan Yesiloz, Carolyn L. Ren, and Chandra Mouli R Madhuranthakam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02949 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Industrial & Engineering Chemistry Research

Controlled Synthesis of Poly (Acrylamide-coSodium Acrylate) Copolymer Hydrogel MicroParticles in a Droplet Microfluidic Device for Enhanced Properties Dizhu Tonga Ϯ, Gurkan Yesilozb Ϯ, Carolyn L. Renb and Chandra Mouli R. Madhuranthakamc* Ϯ Authors have equal contribution * Corresponding author a Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. b Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada. c Chemical Engineering Department, Abu Dhabi University, P.O. Box 59911, Abu Dhabi, UAE. E-mail: [email protected];

Keywords: Polyacrylamide hydrogel, Microfluidic, Micro-particles, photo-polymerization.

*Corresponding Author’s email: [email protected] T +971 2 5015304 | F +971 2 5860182 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

In this study, mono-dispersed poly (acrylamide-co-sodium acrylate) hydrogel micro-particles with the controlled water absorbance capacity were synthesized in a droplet microfluidic device which can be used for enhanced oil recovery application. The experimental method and the microfluidic device were optimized to produce a well-spaced mono-dispersed monomer droplets, and then polymerized by UV initiation in an oil reservoir. The monomer composition (acrylamide to sodium acrylate weight ratio) and the cross-linker concentration was tailored to increase the water absorbance capacity. The copolymer composition was evaluated and confirmed using FTIR spectroscopy measurement. The water absorbance capacity result from the swelling experiments agreed very well with the Flory-Huggins swelling theory that relates swelling to the ionic content and the cross-linker concentration.

1. INTRODUCTION Hydrogels are cross-linked three-dimensional polymeric–structures that can swell and retain a significant amount of water within their structures, while they are insoluble in water1. The properties of hydrogel materials are highly influenced by external changes such as pH, salinity, pressure and temperature, which makes them widely applicable in many processes such as drug delivery2-4, water treatment for heavy metal removal5, tissue engineering6, hygienic products, and plugging agents for enhanced oil recovery by fine tuning their properties. The materials, methods and processes conventionally used to synthesize hydrogels are extensively reported in a recent review1, however, synthetic hydrogels, due to their higher water absorbent capacity and mechanical strength and longer shelf-life, have found more research and industrial applicability

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than the natural hydrogels over the past two decades. Of these, polyacrylamide- and acrylamiderelated copolymers form the major base materials for super absorbance synthetic hydrogels. In recent years, polyacrylamide and polyacrylamide based micro-gels are widely used in enhanced oil recovery (EOR) for water-shut-off treatment and conformance control7, which is a process that improves the uniformity of flood-fluids front both in the vertical direction and area based. The water-flooding technique, which involves injecting water underground to replace the oil, creates many “thief zones” that are highly water permeable. Non-crosslinked polyacrylamide is used to increase the viscosity of the fluids injected during polymer flooding which is a common EOR technique primarily used for mobility control. Both water flooding and polymer flooding result in poor sweep efficiency, which is defined as the percentage of original oil in place displaced from a formation by flooding fluid in a layered system (heterogeneous system). The injected fluid tends to flow into highly permeable zones. As a result, it is difficult to achieve efficient oil displacement by conventional flooding techniques. The polymer micro-gel is used primarily for conformance control when there is a high permeability contrast8. For the information of the case that use gel in EOR—such as bulk gel, weak gel, colloidal dispersion gel and performance gel, readers are referred to a recent review by M. Abdulbaki8. Compared to the non-crosslinked polymer used in polymer flooding, polymer gel has better mechanical stability9, thermal stability8,10, shear resistance10, salinity resistance11 and chemical resistance8. To achieve the conformance control, polymer micro-gel can deform and pass through pore-throats due to the pressure difference. Size, size distribution, and swelling ratio are three important characteristics of the micro-gel that are used for EOR as a plugging agent. The size of the polymer gel needs to match with the size of the pore-throats; otherwise the micro-gel will not



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enter the pore or will just pass by the pore-throats. Either consequence may result in an inefficient conformance control12,13. Currently, the conventional method for hydrogel micro-particle synthesis involves bulk polymerization, solution polymerization, emulsion polymerization, and micro-emulsion polymerization. All of these polymerization methods require either complex mixing agitators in reactors or a significant amount of surfactant and emulsion stabilizer, which are costly. In addition to their production cost, many polymerization reactions are highly exothermic. The control of reaction temperature is critical to controlling the uniformity of polymer properties such as crosslinking density, molecular weight and copolymer composition. In this regard, polymerization in a droplet microfluidic device is a promising method to provide better process controllability, which improves the property of hydrogel micro-particles. Droplet microfluidics allows highly monodispersed nanoliter-sized droplets to be generated at kHz rates in microchannel networks by injecting one fluid into another immiscible fluid14,15,16. Due to its advantages for polymer synthesis, in terms of uniform (mono-dispersed) droplet and thus particle formation and high-surface to volume ratio which enables fast heat dissipation,

droplet

microfluidic platform has been used for many polymerization reactions, for example, free-radical polymerization of Butyl Acrylate17, N-isoproprlacrylamide

18-21

and Divinylbenzene (DVB)22.

This approach allows precise control over reaction conditions, and therefore droplet microfluidic assisted methods produce hydrogel micro-particles with an improved control over size, size distribution, composition, and morphology23. In this study, a droplet microfluidic chip is developed for synthesizing mono-dispersed spherical poly (acrylamide-co-sodium acrylate) hydrogel micro-particles with controlled compositions. Several critical challenges need to be addressed in order to make this chip work


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robustly for manufacturing hydrogel micro-particles with controlled compositions. First, droplets tend to change their shape during polymerization which may result in a change in the local resistance and droplet spacing. Preliminary testing indicates that some polymerized droplets cause channel blockage. Therefore, the generated droplets should be smaller than the channel width and tube diameter so that the polymerized droplets still follow the main flow stream. Secondly, droplet spacing is critical to the process of producing hydrogel micro-particles. If droplet spacing is too small, the polymerized adjacent droplets tend to form a clump creating more dynamic complication to the droplet flow which in turn could cause the chip to fail. If the spacing is too large, the throughput is decreased. Therefore, the spacing must be optimized in the design stage and should be well controlled during the production process meaning not influenced significantly due to polymerization. Details about the strategy to address these challenges are provided below. The concentration of monomer composition and cross-linker are well controlled and their effects on the swelling ratio are carefully evaluated using swelling test, FTIR spectroscopy, size and size distribution.

2. MATERIALS AND METHODS 2.1 Materials Photoresist SU-8 was obtained from Microchem Co. (MA, USA). Pre-polymer Sylgard 184 Silicon Elastomer Kit (poly-(dimethylsiloxane) (PDMS)) was obtained from Dow Corning Corp (Midland, MI, USA). Monomer acrylamide (Am) and co-monomer sodium acrylate (NaA), the photo-initiator

2,2-Diethoxyacetophenone

(DEAP),

the

cross-linker

N,N′-Methylene-

bisacrylamide (BIS) , and surfactant sorbitan monooleate (SPAN 80) were purchased from Aldrich Canada and used as received. The oil phase hexadecane was also purchased from



Aldrich Canada and used after filtration. Hexadecane was chosen because the oil phase needs to be removed after polymerization using centrifugation. Preliminary testing showed hexadecane works better at this stage.

2.2 Microfluidic device fabrication Hydrogel micro-particles were made using a flow focusing generator which enables monodispersed droplet generation in a microfluidic device made of PDMS shown in Figure 1. The microfluidic device was fabricated using standard soft lithographic techniques24. Briefly, the photoresist master was fabricated on silicon wafers using the same protocol reported in the previous study25. In order to make PDMS replica molds, PDMS pre-polymer was mixed at a 10:1 ratio of base to curing agent, degassed and molded against the SU-8/ silicon master and then cured at 95 0C for 2 h. The molds were then peeled off from the master and fluidic access holes were made using a 1.5 mm biopsy punch. Both the finished components and the PDMS mold were then treated with oxygen plasma at 29.7 W, 500 mTorr for 30 s. The plasma treatment process renders PDMS hydrophilic; however, for generating water in oil droplets stably, the PDMS channels need to be hydrophobic which was realized by either changing the channel property by Aquapel injection or keeping the finished microchips on a hotplate for 48 hours for microchannel wall surface treatment. In the experiments, channel width and height were fixed at 200 µm and 50 µm respectively. Hexadecane tends to swell PDMS, which reduces the channel cross-sectional area. To ensure the consistency of the experimental results, hexadecane was kept in PDMS channels for one hour so that it swells PDMS to saturation.


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Figure 1. Schematic diagram of the flow-focusing microfluidic device used to synthesize hydrogel micro-particles. Qd represents the flow rate of the dispersed phase (monomer solution) and Qc represents the flow rate of the continuous phase (oil solution).

2.3 Experimental setup and preparation of hydrogel micro-particles In this work, we introduced a microfluidic approach to synthesize copolymer hydrogel microparticles via photo-polymerization. The schematic diagram of the inverted chip configuration, droplet generation and UV-exposure setup is shown in Figure 2. A computer controlled pressure system (Fluigent MFCS8) is used to pump the dispersed phase fluid (monomer solution) into the center channel and the continuous phase fluid (oil solution) into the two side channels. The estimated flow rates were 1808 and 1829 µL/hr for disperse and continuous phases, respectively. The dispersed phase is an aqueous solution containing two monomers (acrylamide and sodium acrylate), a cross-linker (BIS), and an initiator (DEAP). The total monomer feeding concentration is fixed at 9.28 M and its compositions are listed in Table 1. Solution 1, 2 and 4 represents pure Am, 90% Am and 45% Am solution respectively. The oil phase is a mixture of hexadecane, initiator, DEAP, surfactant and Span 80 (100:5:2 with respect to hexadecane: DEAP: Span80 ratio). The purpose of adding the initiator, DEAP, in the oil phase is to maintain



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the initiator concentration inside the monomer droplet since DEAP is miscible in both water and hexadecane. Table 1. Monomer feeding composition design. Monomer solution number

Am in feeding (g)

NaA in feeding (g)

Cross-linker weight (g)

1

6.60

0

0.20

2

5.90

0.87

0.10

3

5.90

0.87

0.20

4

2.96

4.80

0.10

5

2.96

4.80

0.20

6

0

6.60

0.20

On-chip polymerization of droplets is ideal since no external setup is needed except a UV lamp that needs to be aligned with a long channel or serpentines where droplets are traveling through. However, partial or complete channel blockage often occurs due to the shape and viscosity change of the droplets after polymerization. Blockage results in the change in the local resistance which slows down the coming droplets causing multiple droplets to be polymerized together forming a large polymer slug (see ESI-S1). In this on-chip method, the UV light is focused on the reaction channel. This method suffers from two problems. First, if the UV light intensity is too high, the UV light that is scattered horizontally through the PDMS mold due to the UV transmittance feature of PDMS initiates the monomer solution in the feeding microchannel. This issue causes early initiation of polymerization and results in clogging in the feeding channel. Second, if the UV intensity is relatively at medium or lower level, then the reaction channel is required to be extended using long serpentine design in order to increase the reaction time.


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However, the extension of serpentine channels can cause a large pressure drop in the microchannel which makes it difficult to pump highly viscous droplets after polymerization. Besides,

if

the

100 um

Figure 2. Schematic diagram of the inverted chip method with a cross flow focusing microfluidic device. The green tubing indicates for the dispersed phase inlet and the gray tubing indicates the continuous phase inlets. The serpentine before the junction is for pressure stabilization. polymerization is not completed along the long serpentine channels, then the partially reacted polymer microparticles become as gels and coalesce each other. These merged droplets form a large and highly viscous plugs which come to a stop in the serpentine channel due to large pressure drop (see Figure S1.1 and Figure S1.2). The performance is improved by generating much smaller droplets than the channel width which, however, pushes the droplet generation into



unstable operating regimes26-30. Later, a semi-on-chip polymerization approach is adopted by forming droplets containing monomer solution inside the chip and transporting them into a capillary tubing which is aligned with the UV-source to initiate the reaction. In this approach, the tubing is connected at the outlet of the microfluidic chip which is needed to be bended to be aligned with the UV-source. The bending of the tubing and gravitational force on the droplets caused non-uniform droplet spacing. Although larger and smaller tubing diameters than that of droplet size are used, the droplet spacing issue results in having long polymerized slugs (see ESIS2 for further information). Also, off-chip polymerization of droplets is attempted. However, after reaction, droplets formed a thin-film of polymer (see ESI-S3). Therefore a practical yet useful method is then implemented. The microfluidic chip is inverted upside down, and a short tubing is attached to the outlet of the microfluidic chip. The tubing is a straight PTFE tubing (ID: 250µm) which is immersed into a 10 ml graduated cylinder that serves as the reaction chamber which is filled with hexadecane, surfactant (Span 80) and photo-initiator. Droplets are continuously pumped from the outlet channel to the reaction chamber with this setup. When the droplet generation is stabilized which is confirmed through an inverted microscope (Eclipse Ti, Nikon) and a high speed camera (Phantom v210, Vision Research), the microchip is removed from the microscope and inverted upside down. The generated monomer droplets flowing in the center of cylinder are polymerized via exposure to UV light (ML-3500S, 50W/cm2, Spectroline). The microfluidic chip, outlet tubing and bottom of cylinder are covered with an aluminum foil in order to avoid undesired photo-polymerization of the droplets inside the chip and aggregation of micro-particles at the bottom of the cylinder. A train of droplets in the outlet tubing and reaction chamber can be found in ESI-S4. The reaction time is controlled by varying the length of graduated cylinder and inlet


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pressure. The hydrogel micro-particles are washed with isopropyl alcohol three times and centrifuged at 8000 rpm for 1 minute for three times. The washed micro-particles are dried in a vacuum oven and stored in a vacuum desiccator for further analysis. This process can continuously produce hydrogel micro-particles without clogging issue. An advantage of this method is that it prevents the droplet from merging inside the outlet tubing and aggregation of micro-particles in the oil reservoir. In all the other attempted methods where the outlet tubing points upwards, the speed of droplet changes due to the gravity which often results in droplet merging at the bending location of the tubing. In the inverted chip method, since there is no bending on the outlet tubing and droplets travel downwards instead of upwards against gravity, the droplet velocity is not affected. As a result, the risk of merging at outlet tubing is minimized, and a well-spaced and robust droplet trains are achieved. The well-spaced droplets prevent the aggregation of micro-particles (see Figure S4.1).

2.4 Characterization of micro-particles 2.4.1 Size and size distribution The size of droplets and micro-particles were observed and recorded by an inverted microscope equipped with a CCD and a high-speed camera. The size and size distribution were measured and processed by ImageJ software. The analyzed size distribution was compared with the calculated diameter variation from a MATLAB image processing program.

2.4.2 FTIR Spectroscopy and SEM analysis The Fourier Transform infrared spectra (FTIR) of the dried hydrogel micro-particles is performed using potassium bromide (KBr) pellet method and recorded by Bruker (Vertex 70) FTIR. The FTIR analysis is to confirm the copolymer composition in response to the monomer



composition changes so that the absorbance capacity can be related to the copolymer composition. The scan range is from 400 to 4000 cm-1 for 32 times. A background scan is run every time before a sample scan. SEM images of the dried hydrogel micro-particles are also processed in order to investigate the morphology.

2.4.3 Swelling Test The degree of swelling is evaluated by a swelling test under room temperature. The degree of crosslinking is closely related to the degree of swelling, according to the Flory-Huggins and the equilibrium swelling theories. For the micro-particle swelling test, the dried polymer microparticles are added into a 96 well cell-culture plate, and the sizes of the dried polymer microparticles (particle diameter) are recorded using a microscope and then analyzed with ImageJ® software. The analyzed dehydrated particle size is confirmed via SEM measurement as well (see Figure 3). Then DI water at room temperature (25°C) is added into the corresponding well. The swelling process is recorded by using a microscope. The diameter of the swollen particle (at maximum absorbance capacity) is used to calculate the degree of swelling by the following equation (1) (assuming that the micro-particles are spherical): = = (1)

where V and D are the volume and diameter of the swollen polymer micro-particle, respectively,

and are the volume and diameter of the dehydrated micro-particle, respectively. Bulk

polymer is synthesized in order to compare with the swelling ratio of micro-particles. The bulk polymer was synthesized by using the same monomer solution and the same UV lamp.


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3. RESULTS AND DISCUSSIONS 3.1 Size and size distribution of droplets and micro-particles The size distribution is evaluated using CV%, as shown in equation (2). The CV% value of the dehydrated hydrogel micro-particles increased to 5.68% due to deformation during the drying process. CV% =

(2)

where σdroplet is the standard deviation of the droplet size and µdroplet is the mean droplet size. The droplet sizes are uniformly distributed with a CV% value of 3.3% both in the microchip and the tubing, which indicates that flipping the microchip has no notable effects on the size distribution. The CV% value of the dehydrated hydrogel micro-particles increases to 5.68% due to deformation during the drying process. The droplet size can be controlled by varying the channel dimension, inlet pressure and fluids properties30. Since the focus of this study is to develop a droplet microfluidic system suitable for making uniform hydrogel beads, we did not vary the channel dimensions. The droplet size is shown in S8.



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100 µm

Figure 3. SEM image of dried hydrogel microparticles.

3.2 Effect of monomer feeding on chemical composition of micro-particles The chemical composition of copolymer was measured using FTIR spectrum as shown in Figure 4. The reference peaks are listed in the Table 2. The relative peak area was calculated according to equation (3) to correlate the monomer feeding composition and the cumulative copolymer composition, = where and A

(3)

represent the peak area under the wave number of 1410 cm-1 and 2950

cm-1, respectively. The spectrum is magnified by software at two peaks, 1410 cm-1 and 2950 cm1

. The magnified images and detailed procedures are shown in the supplementary document

(Figure S5 and S6).


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Figure 4. FTIR spectrum for the copolymer micro-particles. From top to bottom: Pure Polyacrylamide, 10% sodium acrylate copolymer, 55% sodium acrylate copolymer and pure Sodium Polyacrylate.

Table 2. Major bands of FTIR spectrum of hydrogel Wavenumber (cm-1)

Assignment

3410-3421

asymmetry vibrations of the NH2 group

3190-3914

symmetry vibrations of the NH2 group

2950

CH and CH2 stretching

3300

O-H stretch

1650-1680

vibration C=O of acrylamide

1410

symmetry vibrations of the COO- group

The acrylate content has a good linear correlation with the relative peak area, which is shown in Figure 5(a). The relation is obtained by a linear-regression method, as follows:



NaA% = %8.6675 + 194.6867A./0 (4) The increase in the calculated relative absorbance is matched with the increase in acrylate content in the monomer feeding. At zero acrylate content in the feed (polyacrylamide homopolymer), the relative absorbance is 0.5124, which corresponds to the weak amine stretch of acrylamide. The y-axis intercepts at -8.6675, which is similar to the reported value of -8.8331. The reported slope is smaller than it is in this study, which could be caused by the cross-linker concentration difference. To verify this hypothesis, two levels of cross-linker concentration were studied, which are 0.1 and 0.2 grams of BIS in the monomer solution. The cross-linker effect is demonstrated in Figure 5(b). The lower cross-linker concentration shows a higher value of Arel. The increasing crosslinker concentration reduces the calculated relative absorbance. The same effect is shown in both 10% and 55% acrylate content in feeding.


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Figure 5. (a) Cumulative copolymer composition and relative peak area (Arel) relation for different copolymer composition. (b) Effect of cross-linker on relative peak area.

3.3 Swelling ratio The dried polymer micro-particles with different monomer compositions are placed in the cellculture well plate, and their original sizes are recorded using the microscope. When DI water is added, the micro-particles swell quickly and the swelling process is recorded by the microscope, as shown in Figure 6. The volume increases from 4 to 36 times for different monomer compositions and different cross-linker concentrations that these results are very promising for enhanced oil recovery (EOR) applications.

Figure 6. Experimental results of hydrogel micro-particles swelling behavior. Average swelling ratio vs. swelling time is demonstrated for the fixed cross-linker concentration at 0.2 w/v%.



Two important parameters of hydrogels are the equilibrium volume-swelling ratio (Q) and the 3 4 )32. The equilibrium volume-swelling number average molecular weight between crosslinks (M ratio is the reciprocal of the polymer-volume fraction in the swollen state, which can be measured by the swelling test. The number average molecular weight between crosslinks determines the crosslinking density and porosity in hydrogel. Combining the equilibrium 3 4 can be calculated based on the swelling theory and Flory-Huggins mixing theory, the M following equation, 5ln81 % 9,: ; + 9,: + ? @89,: ; % 0.589,: ;B + = 0 (5) D

where 9,: is the volume fraction of polymer, ? =

3I 1 2H @1 % B (6) 3I 3J 9H H

where C is the fraction of the ionized polymer structural units in the polymer chain, VK the molar

volume of the monomer unit, and the solvent molar volume. According to the Flory-Huggins theory, the swelling ratio is proportional to the ionic content in the hydrogel and is inversely proportional to the crosslinking density.


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3.4 Effect of monomer feeding and cross-linker on swelling of micro-particles After swelling, the dried micro-particles expand to almost 40 times of its original size for 55% NaA copolymer. The 10% NaA copolymer has a smaller swelling ratio (~14 times). The larger swelling ratio is attributed to the higher ionic content in 55% NaA copolymer, which is in agreement with the Flory-Huggins theory.

3.5 Effect of cross-linker concentration on swelling of micro-particles For different monomer compositions, the effect of cross-linker concentration on the swelling of microparticles is investigated. Two different concentrations of the cross-linker (0.1g and 0.2g) are used in this study. The cross-linker concentration has an opposite effect on the swelling ratio, as shown in 7(a), which is in agreement with the Flory-Huggins theory. The higher cross-linker concentration, the higher crosslinking density and the smaller number of average molecular weight between the crosslinks; which results in less swelling. Compared to the bulk hydrogel, the micro-particles not only swell faster but also possess a higher swelling ratio, especially when the ionic content is at a high level such as 55%. From Figure7(b), the swelling ratio is the same for the polyacrylamide homopolymer obtained from the bulk and the microfluidic chip. At 10% of sodium acrylate content, the swelling ratio of the micro-particles is 47.12%, which is higher than that of the bulk. At 55% of sodium acrylate content, the swelling ratio of the micro-particles is 126.49%, which is higher than that of the bulk as well. The micro-particles have a large surface-to-volume ratio compared to that of the bulk; therefore, micro-particle swell much faster than that in the bulk. During polymerization, the oil 19 Environment ACS Paragon Plus


phase instantaneously removes the heat released from droplet polymerization because the droplets are fully encapsulated in the oil phase (Serra et. al. 2008). Therefore, the micro-particles

a)

b)

have a more uniform crosslink in the surface and core compared to the bulk, which allows water to penetrate to the core of micro-polymer faster than in bulk.


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Figure 7. (a) Effect of cross-linker concentration on swelling ratio. (b) Swelling ratio difference between bulk and micro-particles (weight of cross-linker is fixed at 0.2g).

4. CONCLUSIONS Due to their wide applications in many processes such as drug delivery, tissue engineering and enhanced oil recovery (EOR) as plugging agents, it is important to develop technologies that are capable of fine tuning the properties of hydrogel microparticles. For instance, in oil recovery processes, many conventional flooding techniques that have been employed to increase the oil displacement efficiency have poor sweep efficiency due to highly permeable zones in the medium. Moreover, the conventional methods for hydrogel micro-particle synthesis require either complex mixing units in reactors or a significant amount of surfactant and emulsion stabilizer, which are costly. Additionally, many polymerization reactions are highly exothermic, and the control of the reaction temperature is critical to control the uniformity of polymer properties such as crosslinking density, molecular weight and copolymer composition. Thus, new techniques are highly needed for controlling and enhancing hydrogel production and properties, and the findings in this study on the composition and swelling ratio of the hydrogel particles, which is synthesized using droplet-microfluidics, are important to control the characteristics of the microgels that are used for EOR as a plugging agents. In these regards, a droplet-based microfluidic chip was developed to investigate the impact of controlling parameters such as the concentration of cross-linker and monomer on the uniformity and swelling ratio of poly (acrylamide-co-sodium acrylate) hydrogel microparticles. Microscopic, SEM imaging and FTIR spectrum were used to evaluate the size distribution and swelling ratio of the hydrogel microparticles. It was demonstrated that the developed simple, yet



robust droplet-based microfluidic chip can continuously produce microparticles without aggregation. It was found that the swelling property is controlled by the ionic content and crosslinker concentration. The swelling ratio obtained from the swelling test matched very well with the Flory-Huggins theory and the equilibrium swelling theory. The swelling index was much higher in the hydrogel microparticles compared to that of bulk hydrogel.

ASSOCIATED CONTENT Supporting Information. “Supplementary_Microfluidic hydrogel synthesis.doc”

AUTHOR INFORMATION Corresponding Author Chandra Mouli R. Madhuranthakam Chemical Engineering Department, Abu Dhabi University, P.O. Box. 59911, Abu Dhabi, UAE. E-mail: [email protected];

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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REFERENCES (1)

Ahmed, E.M., Hydrogel : Preparation , characterization , and applications : A review. J. Adv. Res. 2015, 6, 105–121. (2) Capretto, L., Mazzitelli, S., Colombo, G., Piva, R., Penolazzi, L., Vecchiatini, R., Production of polymeric micelles by microfluidic technology for combined drug delivery : Application to osteogenic differentiation of human periodontal ligament mesenchymal stem cells ( hPDLSCs ). Int. J. Pharm. 2013, 440, 195–206. (3) Vladisavljevi, G.T., Khalid, N., Neves, M.A., Kuroiwa, T., Nakajima, M., Uemura, K., Ichikawa, S., Kobayashi, I., Andersson-svahn, H., Industrial lab-on-a-chip : Design , applications and scale-up for drug discovery and delivery, adv. Drug Delivery Rev. 2013, 65, 1626–1663. (4) Zhao, C., He, L., Zhang, S., Middelberg, A.P.J., Nanoparticle synthesis in microreactors. Chem. Eng. Sci. 2011, 66, 1463–1479. (5) Çavuş, S., Gürdag, G., Sözgen, K., Gürkaynak, M.A., The preparation and characterization of poly(acrylic acid-co-methacrylamide) gel and its use in the noncompetitive heavy metal removal. Polym. Adv. Technol. 2009, 20, 165–172. (6) Yeh, C.-H., Lin, P.-W., Lin, Y.-C., Chitosan microfiber fabrication using a microfluidic chip and its application to cell cultures. Microfluid. Nanofluidics 2010, 8, 115–121. (7) Rousseau, D., Chauveteau, G., Rheology and transport in porous media of new water shutoff/conformance control microgels. Symp. Oilf. 2005. (8) Abdulbaki, M., Huh, C., Sepehrnoori, K., Delshad, M., Varavei, A., A critical review on use of polymer microgels for conformance control purposes. J. Pet. Sci. Eng. 2014, 122, 741–753. (9) Al-maamari, R., Al-shabibi, I., Mansoor, A., Al-sharji, H., Zaitoun, A., Mechanical Stability of High-Molecular-Weight Polyacrylamides and an ( acrylamido tert -butyl sulfonic acid )– Acrylamide Copolymer Used in Enhanced Oil Recovery, Appl. Polym. Sci., 2014, 40921, 6–11. (10) Dupuis, G., Al-Maamari, A., Mechanical and Thermal Stability of Polyacrylamide-based Microgel Products for EOR. SPE Int. Symp. Oilf. Chem. Symp. Oilf. 2013, 1–11. (11) Qiu, Y., Wei, M., Geng, J., Wu, F., Successful Field Application of Microgel Treatment in High Temperature High Salinity Reservoir in China 2002, 2016. (12) Lin, M., Zhang, G., Hua, Z., Zhao, Q., Sun, F., Conformation and plugging properties of crosslinked polymer microspheres for profile control. Colloids Surf. A. 2015, 477, 49–54. (13) Yao, C., Lei, G., Hou, J., Xu, X., Wang, D., Steenhuis, T.S., Enhanced Oil Recovery Using Micron-Size Polyacrylamide Elastic Microspheres : Underlying Mechanisms and Displacement Experiments. Ind. Eng. Chem. Res. 2015, 10925–10934. (14) Guzowski J., Korczyk P.M., Jakiela S and Garstecki P, Automated high-throughput generation of droplets. Lab Chip, 2011, 11, 3593-3595. (15) Yesiloz, G., Boybay, M. S. and Ren, C.L. Label-free high-throughput detection and content sensing of individual droplets in microfluidic systems. Lab Chip 2015, 15, 4008– 4019. (16) Wen N., Zhao Z., Fan B., Chen D. Men D, Wang J and Chen J., Development of Droplet Microfluidics Enabling High-Throughput Single-Cell Analysis. Molecules, 2016, 21, 881.



(17) Iwasaki, T., Yoshida, J., Free Radical Polymerization in Microreactors. Significant Improvement in Molecular Weight Distribution Control. Macromolecules 2005, 38, 1159– 1163. (18) Choi, C.-H., Jung, J.-H., Kim, D.-W., Chung, Y.-M., Lee, C.-S., Novel one-pot route to monodisperse thermosensitive hollow microcapsules in a microfluidic system. Lab Chip 2008, 8, 1544. (19) Ikkai, F., Iwamoto, S., Adachi, E., Nakajima, M., New method of producing mono-sized polymer gel particles using microchannel emulsification and UV irradiation. Colloid Polym. Sci. 2005, 283, 1149–1153. (20) Shepherd, R.F., Conrad, J.C., Rhodes, S.K., Link, D.R., Marquez, M., Weitz, D. a., Lewis, J. a., Microfluidic assembly of homogeneous and janus colloid-filled hydrogel granules. Langmuir 2006, 22, 8618–8622. (21) Wei, J., Ju, X.-J., Zou, X.-Y., Xie, R., Wang, W., Liu, Y.-M., Chu, L.-Y., Multi-StimuliResponsive Microcapsules for Adjustable Controlled-Release. Adv. Funct. Mater. 2014 (22) Sugiura, S., Nakajima, M., Itou, H., Seki, M., Synthesis of polymeric microspheres with narrow size distributions employing microchannel emulsification. Macromol. Rapid Commun. 2001, 22, 773–778. (23) Serra, C. A., Chang, Z., Microfluidic-assisted synthesis of polymer particles. Chem. Eng. Technol. 2008, 31, 1099–1115. (24) Xia, Y., Whitesides, G.M., Soft Lithography. Angew. Chemie Int. Ed. 1998, 37, 550–575. (25) Yesiloz, G., Boybay, M.S. and Ren, C.L., Effective Thermo-Capillary Mixing in Droplet Microfluidics Integrated with a Microwave Heater. Anal. Chem., 2017, 89, 1978-1984. (26) Gupta, A., Murshed, S.M.S., Kumar, R., Droplet formation and stability of flows in a microfluidic T-junction, Appl. Phys. Lett., 2009, 94, 164107. (27) Garstecki, P., Fuerstman M.J., Stone H.A. and Whitesides G.M., Formation of droplets and bubbles in a microfluidic T-junction-scaling and mechanism of break-up, Lab Chip, 2006, 6, 437–446. (28) Anna S.L., Bontoux N. and Stone H.A., Formation of dispersions using “flow focusing” in microchannels, Appl. Phys. Lett., 2003, 82, 3, 364-366. (29) Glawdel, T., C. Elbuken, and C. Ren, “Passive droplet trafficking at microfluidic junctions under geometric and flow asymmetries,” Lab Chip,, 2011, vol. 11, no. 22, p. 3774. (30) Glawdel, T., Ren, C.L., Droplet formation in microfluidic T-junction generators operating in the transitional regime. III. Dynamic surfactant effects. Phys. Rev. E: Stat. Nonlinear, Soft Matter Phys. 2012, 86, 1–12. (31) Magalhães, A.S.G., Neto, M.P.A., Quim. Nova, 2012, 35, 1464–1467. (32) Hartmann, J. (1991), Absorbent polymer technology. Edited by Peppas, B and Harland, R. S., Sorbent Polymer Technology. Elsevier. 2008.


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Copolymer Hydrogel Microparticles in a - ACS Publications - American

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