Enhanced PAM Polymer Gels Using Zirconium Hydroxide

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Materials and Interfaces

Enhanced PAM Polymer Gels Using Zirconium Hydroxide Nanoparticles for Water Shutoff at High Temperatures: Thermal and Rheological Investigations Feven Mattews Michael, Arshia Fathima, Eman Alyemni, Jin Huang, Ayman Almohsin, and Edreese H Alsharaeh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04126 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Industrial & Engineering Chemistry Research

Enhanced PAM Polymer Gels Using Zirconium Hydroxide Nanoparticles for Water Shutoff at High Temperatures: Thermal and Rheological Investigations Feven Mattews Michael1, Arshia Fathima1, Eman AlYemni1, Huang Jin2, Ayman Almohsin2, Edreese H. Alsharaeh1* 1College

of Science and General Studies, AlFaisal University, P.O. Box 50927, Riyadh, 11533, Saudi Arabia Advanced Research Center, Saudi Aramco, P.O. Box 5000, Dhahran, 31311, Saudi Arabia *corresponding author: [email protected]

2EXPEC

Abstract: In this work, the effect of using zirconium (IV) hydroxide (Zr(OH)4) nanoparticles in the enhancement of the thermal stability and viscoelastic properties of organically crosslinked polyacrylamide (PAM) hydrogels using hydroquinone (HQ) and hexamethylenetetramine (HMT) was studied. The thermal stability and viscoelastic properties of the PAM gels were studied by DSC and DMA, respectively. From the DSC analysis, the bound and free water contents were analyzed along with the degradation enthalpy to give a preliminary clue about the direct vs indirect interactions taking place in the gel. With the addition of 0.2wt%-0.8wt% Zr(OH)4, the thermal stability of the PAM hydrogel improved by 3-5°C (reaching 187°C) compared to PAM alone. Moreover, the elasticity of the PAM hydrogel increased with the addition of Zr(OH)4 which was also supported by the strong interaction bond formed as demonstrated from the higher degradation enthalpy and free water recorded. These interactions were further explained using FTIR and optical microscope images. The TGA results also revealed the functionalization of PAM gel and showed that the gel was thermally stable even at 400°C. In addition, the effects of calcination, organic cross linker, salinity, and pH on the gel properties were investigated. From DSC and DMA analysis, it was found that the optimal conditions for the polymer gel were as follows: PAM at 4 wt%, Zr(OH)4 at 0.20.8 wt%, organic crosslinkers (OrgCL) at 0.3 wt%, and KCl or MgCl2 at 2 wt%. Keywords: Water shutoff, hydrogels, thermal stability, gel strength, zirconium (IV) hydroxide, Organic crosslinkers, polyacrylamide 1. INTRODUCTION Oil and gas production is often hindered by the presence of water in the hydrocarbon reservoirs. This formation water can invade production areas through fractures already

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present in the reservoirs causing issues such as corrosion of the well facilities. This, along with reduced oil and gas production, eventually leads to damage and shut off of the wells. Billions of dollars are spent yearly to treat this produced water before re-injection or discharge due to environmental regulations. A strategy to save on treatment costs is to apply water shutoff treatments 1. Different mechanical, chemical, and biological technologies are being applied to achieve this. Currently, under the chemical window, several kinds of polymer hydrogels are being used for water shutoff. Hydrogels have gained popularity due to their three-dimensional, hydrophilic, polymeric networks that give them unique properties such as high water content, softness, porosity, and flexibility. Polyacrylamide (PAM) hydrogel is one of the commonly used water soluble polymer gel in water shutoff application due to low cost, high viscosity at low concentrations, and availability 2. However, polymers hydrogels in general have limitations that affect their efficiency including low mechanical strength, relatively low degradation temperature. This can limit their application in the water shutoff treatment application. In addition, when developing a polymer hydrogel for water shutoff application, it is important to consider the environmental conditions (pH, salinity, temperature) 3. This is because, high salinity in the downhole formations can lead to flocculation of the polymer solution due to bridging effect between the divalent ions (such as Ca2+) and the anionic charges on the polymers. Besides, PAM is affected by multiple degradation mechanisms including thermal, free radicals, mechanical, biological, and oxidative pathways 4. The acrylamide segment of PAM is the cause of poor thermal stability and low salt tolerance of the hydrogel. The hydrolysis of acrylamide at high temperatures is a well-known mechanism that may lead to syneresis, precipitation 5, and collapse of the gel due to bridging effects with other ions 4. Excessive crosslinking can also lead to syneresis at higher temperatures 6. Further enhancement in the polymer gel properties has been achieved with the incorporation of nanoparticles within the hydrogel network. Nanoparticles can reinforce and add multiple functionalities via surface functional groups for crosslinking polymeric hydrogels thereby giving them superior physical, chemical, thermal, and mechanical properties

7,8.

Carbon-

based nanomaterials, polymeric nanoparticles, metal/metal-oxide nanoparticles, and inorganic/ceramic nanoparticles are such examples that have been used to enhance polymer gels. Some studies reported that the thermal stability and gel strength of PAM gels were improved upon addition of nanoparticles such as titanium oxide (TiO2) 7, graphene oxide


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(GO)

9

and fly ash

10–12.

In one study, nanosilica particles was used to enhance the PAM

(molecular weight of 12,000,000 g/mol) gel strength along with organic crosslinkers HQ and HMT

13.

The gel was developed with 0.4% PAM, 0.3% HQ and 0.3% HMT with varying

amounts of nanosilica (0.1, 0.2 and 0.3%). Gelation was carried out at 110˚C. Although the gelation time decreased with addition of 0.3% nanosilica from 16 hours to 9 hours, the gel strength (storage modulus and loss modulus) was found to increase by 5-6 times compared to PAM without nanosilica. The thermal stability increased from 137.8 °C to 155.5 °C. In addition, the mass fraction of bound water within the gel increased from 22.5% to 39.9% by addition of silica nanoparticles with a concentration of 0.3 wt%. In addition, Jiang et al.

8

prepared a self-healing PAM hydrogel by incorporating zirconium hydroxide at room temperature. This PAM hydrogel attained tensile strength of 404.3 KPa and compression strength of 36.6 MPa. Also, the hydrogel had self-healing efficiency of 86%. This combination of excellent mechanical properties, good self-healing ability can make zirconium a potential for applications in water shutoff treatments. This is because, zirconium can withstand temperature as high as 400˚F (204.4˚C), while titanium is stable up to 325˚F (162.78˚C). Over the years, different crosslinking agents have been used to produce polymeric gels that are suitable at the harsh conditions of the reservoir, including high temperatures

14.

These

crosslinkers can be either organic or inorganic. Generally, organically crosslinked PAM gels are favorable compared to inorganically crosslinked gels 15. This is could be due to the strong covalent bonds formed between the polymer and the organic cross linker, producing more thermally stable crosslinked PAM gels even at high temperature and under high salinity conditions. In addition, inorganic cross linkers tend to shorten the gelation time of PAM gel at high temperature thus resulting in the gelation of PAM before successfully placing the gel in reservoirs

15.

Hence, organic cross linkers such as phenol-formaldehyde, hydroquinone-

hexamethylenetetramine

(HQ/HMT),

methenamine,

polyethyleneimine (PEI) are preferred to crosslink PAM

terephthal 14.

aldehyde,

and

Furthermore, with addition of

crosslinkers, the concentration of PAM can be reduced which can result in the increase the gelation time. However, this comes at the cost of the mechanical integrity of the hydrogel since lower PAM concentration would form weaker hydrogel. Hence, it is important to find the right balance between the concentration of the crosslinker and the polymer. Many researchers have investigated the role the crosslinker plays in minimizing the concentration of the PAM. For instance, Adewunmi et al. 11, varied the concentration of PAM and PEI from



2.87wt% to 8.4wt% and 0.3wt% to 1.04wt%, respectively, and was able to prepare thermally and mechanically stable PAM hydrogel with 6wt% PAM and 0.67wt% PEI in presence of 2wt% fly ash. Meanwhile, other studies have prepared thermally stable PAM hydrogel with only 0.4 wt% of PAM in presence of 0.3 wt% of HQ/HMT 13 as well as 0.6wt% of PAM in presence of 0.6wt% HQ-HMT 16. Therefore, in this study, the effect of using zirconium (IV) hydroxide (Zr(OH)4) nanoparticles in the enhancement of the thermal and viscoelastic properties of low molecular weight polyacrylamide (PAM with MW 550,000 g/mol) hydrogels will be investigated. The Zr(OH)4 nanofiller was prepared using microwave irradiation and organic cross linkers hydroquinone (HQ) and hexamethylenetetramine (HMT) were added to control gelation. Other factors such as pH and salinity were also investigated using ammonia, KCl, MgCl2, and Arabian seawater. 2.

MATERIALS AND METHODOLOGY

2.1. Materials Low molecular weight (MW of 550,000 g/mol) polyacrylamide (PAM) was purchased from Flotek Company. The hydroquinone (HQ) and hexamethylenetetramine (HMT) used as organic cross linkers were obtained from Loba Chemie while the potassium chloride (KCl) used to adjust the salinity of the gelant solution was purchased from Scharlau. In addition, the chemicals required to prepare zirconium (IV) hydroxide (Zr(OH)4) such as zirconium oxychloride octahydrate (zirconyl chloride), cetyltrimethyl ammonium bromide (CTAB) and 32% ammonia solution (NH3) were purchased from Loba Chemie, Srichem and Scharlau, respectively. All chemicals were used without further modification. 2.2. Methodology 2.2.1. Preparation of Nanofillers (Zr(OH)4 and ZrO2) The synthesis of zirconium (IV) hydroxide was done by microwave technique. Two solutions were prepared separately. In the first solution (Solution A), 5 gm of CTAB was added to a dilution of 32% NH3 (i.e. 62.5 ml of NH3 + 62.5 ml of distilled water). This was then heated to 80°C while stirring. The second solution (Solution B) consisted of 65.375 gm of zirconyl salt in 250 ml of distilled water. While Solution A was stirring, 175 ml of Solution B was added to it drop wise and the solutions were kept to stir for 3 hours. Once this was done, the


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resulting solution was heated in a commercial kitchen microwave for 10 minutes at full power (1100 W). An additional 145 ml of 32% ammonia was then added to it to form a precipitate. Filtration and washing was done several times with distilled water followed by ethanol. The resulting filtrate was then dried overnight at 80°C. At this point, Zr(OH)4 was obtained. The prepared Zr(OH)4 was then calcined to give zirconia (ZrO2). The calcination was performed at 600°C for 2 hours at a ramping rate of 7°C/min. 2.2.2. Preparation of Polymer Gel The prepared Zr(OH)4 (at 0.2 wt%, 0.5 wt%, 0.8 wt%, and 1 wt%) was added to 5 ml of distilled water and sonicated for 15-30 minutes until it was dispersed. While stirring the sonicated solution with a magnetic bar, low molecular weight PAM (4 wt%) was added and left to stir for 1 hour. After 1 hour, the organic cross linkers HQ and HMT (0.3 wt% each) and KCl (2 wt%) were added and left to stir for 15 minutes. The resulting solution was then flushed with nitrogen gas for about 30 seconds and placed in a 150°C oven in glass vials for 48 hours. For comparison purposes, the same procedure was applied for the calcined nanofiller ZrO2 at 0.5 wt%. Furthermore, a procedural change, referred to as P.C., was also investigated to see the effect on the gel properties. In this change, the organic cross linkers were added to the nanofiller before the PAM. That is to say, the nanofiller was added and sonicated, followed by HQ and HMT, followed by PAM, and finally KCl. The solution was then flushed with nitrogen and put in the oven under the same conditions. Moreover, the effect of the organic cross linkers added was investigated by varying their amount (i.e. 0.15% each versus 0.3% each) as well as their ratio (i.e. HQ: HMT as 1:1 and 1:2). Other factors such as pH and salinity were also studied. The pH was increased from 8 to 10-11 by adding few drops of ammonia solution. Meanwhile, the effect of salinity was investigated by adding 2wt% MgCl2 instead of KCl or using 5 mL of sea water in place of the distilled water and not adding additional salts. All the studies were compared to neat PAM as the control sample which was prepared without the use of any nanofiller.



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2.2.3. Characterization The chemical interaction of the PAM and Zr(OH)4 was investigated using Fourier transform infrared spectroscopy (FTIR - Thermo Scientific Nicolet-iS10) and X-ray diffraction (XRD Rigaku MiniFlex 600). The XRD of the nanofiller and PAM gels was studied with Cu radiation [40 kV, 15 mA, Kα radiation (1.54 A°)] and recorded in the range of 5-90° while the FTIR spectra of the PAM nanocomposites were used to determine the functionalization of the nanocomposites as well as their interaction with polymers. The samples were dried first and ground with KBr to make the pellets. The Fourier transform infrared spectroscopy of the prepared nanomaterials and gels was recorded in the range of 4000–500 cm−1 wavenumbers. The optical microscope (SCO Tech) was used to visualize the polymer chains at the micrometer scale. This was done to compare potential morphological changes among the gels prepared. The gels were smeared onto glass slides and dried as wet samples did not show a clear morphology. The polymer gels were viewed at 10x magnification for the objective lens. The microstructural surface morphology of the hydrogels was examined using Scanning electron microscope (SEM, JEOL JSM-IT500LV). The hydrogels were prepared by freezing with liquid nitrogen and cutting with razor blade. The images were taken in low vacuum mode without coating at 250x magnifications. The thermal property of the PAM gels was studied by Differential scanning calorimetry (DSC, Hitachi DSC7020. The samples (5 mg) were first frozen by decreasing the sample temperature from 30°C to -20°C at the rate of 5°C/min under nitrogen flow of 50 ml/min. Then, the DSC measurements were done from -20°C to 350 °C at a heating rate of 5°C/min under a nitrogen flow of 50 ml/min. Moreover, the thermal stability of the polymer nanocomposite hydrogels was studied using thermogravimetric analyzer (TGA, Hitachi STA7200). TGA of the prepared nanocomposites were recorded from 30°C to 500°C at a heating rate of 2°C/min under nitrogen flow of 50 ml/min. Rheology oscillatory tests (also referred to as “dynamic mechanical analysis, DMA) were used to examine the viscoelastic behaviour of prepared hydrogels. The stress-controlled rheometer (MCR 502, Anton Paar) equipped with 40-mm parallel plate geometry with 1-mm gap was used.

For all the

measurements, the temperature was maintained at room temperature (20°C) all the time. At first, several trial oscillatory amplitude sweep tests were conducted under constant shear rate (CSR) mode. This is to determine the linear viscoelastic (LVE) region of the hydrogels. In these tests, the strain (deformation) was ramped from 0.1 to 100% and the angular frequency


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was kept at a constant value of 1 Hz. Gel strength characterization tests were performed after these trial tests by oscillatory frequency sweep. For all the measurements, a constant strain of 2% (The stress response to a strain of 2% was confirmed to be within the linear viscoelastic regime from the trial tests) was used and the applied oscillatory shear frequency ranged from 0.1 to 100 rad s-1. The measured elastic modulus G’ is used as a quantitative indicator of the gel strength and for comparing gel strength of different samples. 3. RESULTS AND DISCUSSION 3.1. Chemical Interaction Usually, crosslinkers are used as bridging agents that would bring the polymer molecules and the filler together. Figure 1 a and b illustrates the chemical reaction between the HMT and HQ prior to reacting to PAM or Zr(OH)4. In this case, the HMT is initially decomposed to form formaldehyde that would then react with the HQ, thus increasing the hydrophilic interactive sites of the OrgCL

16.

This is then expected to increase the PAM molecules and

Zr(OH)4 interaction by attaching the amide group of the PAM molecule (-CONH2) with the hydroxyl group of the OrgCL (i.e. -CH2OH) forming the carbon-nitrogen bond through covalent bond 16, while the Zr(OH)4 will link with the other hydroxyl group (i.e. -OH) of the OrgCL through hydrogen bond 8. The schematic representation of the chemical reaction between the PAM, OrgCL and the Zr(OH)4 is depicted in Figure 1 (c). Meanwhile, the –OH groups of Zr(OH)4 are generally reduced to oxygen upon calcination

17.

As a result, the

interaction between PAM and ZrO2 would reduce due to limited hydrophilic interactions 18. N

N

+ N

H 2O

HO

+

N

Methanediol

Hexamethylenetetramine

HO

OH

OH

H 2C

O

formaldehyde

(a)


+

H 2O

NH3


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OH OH

+

H 2C

HO

OH

HO

OH

O

HO OH

Hydroquinone

2,3,5,6-tetramethylol hydroquinone

(b) OH HO

OH Zr O

*

CH H2C

*

O

C

NH

O

C

NH

*

CH

CH2

*

*

* O

H 2C

CH

*

NH

C

O

NH

C

O

CH

*

H 2C

Zr HO

OH OH

(c) Figure 1: Chemical reaction between HQ and HMT (a) decomposition of HMT and (b) chemical interaction between HQ and HMT 16 and (c) the chemical interaction between between the PAM, organic crosslinkers (HQ/HMT) and Zr(OH)4 These interactions i.e. between the nanofillers (Zr(OH)4 and ZrO2) and the PAM gels were investigated using FTIR as shown in Figure 2. All gels depicted similar peaks. Stretching vibration peaks for the PAM nanocomposites, such as C=O, and primary amide i.e. C-N were detected at 1667 cm-1 and 1390 cm-1 , respectively. A wide band width was detected at 3150 cm-1 representing the stretching vibrations of the hydroxyl group (-OH) due to the presence of water in the PAM nanocomposite gels. These peaks are in agreement with other studies conducted on PAM

19.

In other studies, the Zr-O stretching vibrations for the Zr(OH)4 and

ZrO2 were usually detected around the fingerprint regions 505 cm-1, 590 cm-1, and 750 cm-1 18.

The slight shift in the Zr-O peak detected at 786 cm-1 can be assigned to Zr-O-C due to the


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interaction with the OrgCL. A similar shift in the Ti-O peak was reported upon interaction with PAM and a new Ti-O-C bond was formed 7.

Figure 2: FTIR spectrum of PAM, PAM-Zr(OH)4 and PAM-ZrO2 gels Figure 3 depicts the XRD patterns for the PAM gel nanocomposites in comparison to neat PAM and the nanofillers (calcined and uncalcined Zr(OH)4). The PAM-Zr(OH)4 nanocomposite displayed an amorphous nature with sharp peaks detected at 28°, 40°, 50°, and 66°. The peak at 28° corresponded to the peak found in neat PAM whereas the rest of the peaks were not detected in neat PAM or the nanofillers. This could be due to the fact that the PAM and the nanofillers had an amorphous nature which can be noticed from the widespread peaks detected at the range of 20°- 40°; similar XRD patterns for neat PAM and Zr(OH)4 particles were observed in

19

and

20,

respectively. Upon calcination of Zr(OH)4, an

enhancement in crystallization of ZrO2 was observed in the detection of the monoclinic phase of zirconia at 17o, 24o (011), 28o (111), 31o (-111), 34o (002), 40o (211), 45o (112) (211), 50o (221), 55o (031), 60o (302), 64o (230), and 66o (-222). These peaks correspond the XRD pattern reported for ZrO2 in

21.

Moreover, the PAM nanocomposite gels (i.e. both PAM-

Zr(OH)4 and PAM-ZrO2) had similar XRD pattern whereby sharp peaks were detected at 28o (this was observed for PAM alone as well), 40o, and 50o due to the crystalline nature of ZrO2.



Figure 3: XRD patterns for PAM nanocomposites in comparison to the neat PAM and nanofillers 3.2. Molecular Interaction The properties of polymers are greatly influenced by the average molecular weight (AMW) of the polymer

22.

AMW is an indication of how well the polymer molecules are

interconnected or crosslinked and a higher AMW means a higher degree of molecular chain entanglement. Following that logic, attaining a higher AMW will increase the strength, toughness, and thermal properties of the polymer. As a result, polymers with higher AMW have the ability to stretch more and withstand more stress before the polymer chain breaks. In addition, higher AMW increases the viscosity of the polymer because more energy/enthalpy is required to break the more entangled polymer chains in order to increase polymer mobility. One way to observe the interaction or crosslinking of the polymer molecules is through optical microscope images 23.


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Figure 4 shows the optical images obtained for the PAM nanocomposites with different Zr(OH)4 weight percentage in comparison to PAM alone. The PAM alone hydrogel on the other hand depicted more long chains entangled with one another. These longer chains can provide more crosslinking sites thereby allowing for larger networks. Thus, upon addition of fillers, the PAM-Zr(OH)4 hydrogels at lower amounts of Zr(OH)4 (i.e. 0.2wt%) had longer chains compared to higher amount (i.e. 1wt%) which could imply the gel strength would be better at lower amount of filler due to higher entanglement of the chains. In addition, a well crosslinked PAM with long polymer chains can reduce the fillers ability to fully disperse leading to filler agglomeration which can affect the properties of the polymer, as shown for the higher amount of Zr(OH)4.

Figure 4: Optical microscope images of (a) PAM, (b) PAM-Zr(OH)4 0.2%, (c) PAMZr(OH)4 0.5%, (d) PAM-Zr(OH)4 0.8%, (e) PAM-Zr(OH)4 1% 3.3. Surface Morphology It is usually desired the polymer hydrogels for water shutoff applications have several pore network that are capable of holding water without having significant effect on the viscoelastic properties and thermal stability of the hydrogels

10–12.

Thus, analysing the surface

morphology of the hydrogel using scanning electron microscopy (SEM) is important in providing insight into the pore interconnectivity of the hydrogel. Figure 5 a and b depict the SEM microstructural images of the PAM hydrogels with and without Zr(OH)4. Both SEM



images exhibited porous structure whereby the PAM alone hydrogel has larger pores (Figure 5 a) compared to the PAM-Zr(OH)4 hydrogel which had smaller pores with uniform size (Figure 5 b). The presence of these pores is the key factor for the water holding capacity of the PAM hydrogels during the gelation process. This means that the PAM alone hydrogel is expected to hold or entrap more water within its crosslinked structure, thus have less thermal stability and viscoelastic properties compared to the PAM-Zr(OH)4 hydrogel. Similar observation was made when PAM/PEI hydrogel was incorporated with fly ash 10–12.

Figure 5: SEM microstructure images of (a) PAM and (b) PAM-Zr(OH)4 3.4. Thermal and Viscoelastic Properties Usually, the stability of the bond or strength of the interaction between the PAM and Zr(OH)4 can be determined by how much energy is required to break the network. This energy is referred to as degradation enthalpy and can be measured by differential scanning calorimetry (DSC). The stronger the interaction, the more endothermic energy is required to break down the gel. In addition, the temperature at which the crosslinked chains/bonds of the PAM gel starts to break down is known as the degradation temperature. At this temperature, the chain mobility of PAM is increased. There are many possible direct interactions (strongly crosslinked gel) that can take place between the PAM, Zr(OH)4, and the OrgCL which include PAM-Zr(OH)4, PAM-OrgCLZr(OH)4, PAM-PAM, and Zr(OH)4-Zr(OH)4 interactions. In addition, there are weaker indirect interactions that an occur between the PAM, Zr(OH)4, and the OrgCL. This is as a result of the hydrophilic nature of the materials which could form hydrogen bonds with water molecules. Measuring the water content of the PAM gels is crucial for this reason. It is


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important to understand that the indirect interactions take place in the form of either free water or bound water. The free water refers to the absorbed water, that is, water molecules attached to other water molecules via hydrogen bond. Meanwhile, the bound water refers to water that is chemically bound to the surface as illustrated in Figure 6 and will have a significant effect on the structure of the PAM gels

13.

In other words, formation of bound

water reduces the availability of interactive sites on the surfaces allowing for less direct crosslinking between PAM and Zr(OH)4 and so producing a weaker gel. Therefore, attaining higher bound water would imply less endothermic energy is required to break the bond between PAM and Zr(OH)4. The bound water and free water can be calculated from the endothermic peak between -5˚C and 0˚C through the following equation found in literature 13. 𝑤𝑏 = ∆𝐻/∆𝐻𝑜 𝑤𝑓 = 1 ― 𝑤𝑏 where wb is the fraction of bound water, wf is the fraction of free water, ∆H is the enthalpy required for heating the free water in the gel, and ∆Ho is the 333.5 J/g standard degradation enthalpy of free water.

Figure 6: Types of water on surfaces, their structure and interactions on surfaces (adapted from 24) The interaction between the materials can also influence the viscoelastic strength of the PAM gels. The strength of the PAM-Zr(OH)4 gels is analyzed using the viscoelastic properties measured by DMA. These properties include the storage modulus and loss modulus of the gels. The storage modulus (G’) measures the energy stored by the gel, representing the elasticity of the gel (solid-like), while loss modulus (G”) measures the dissipated energy as heat, representing the viscous portion (liquid-like). Consequently, their ratio (G’/G”) gives information on the gel strength i.e. whether the gels are strong like solids or weak like liquids.



In this study, the conditions for the PAM gels are optimized by finding a good balance between the thermal and viscoelastic properties by correlating the DSC and DMA analysis i.e. bound water (hydrogen bonding) with elasticity (storage modulus) as well as gel strength (G’/G”) with degradation enthalpy (energy to break bond). It is important to note that hydrogen bonding is reversible and dynamic and can enhance the elasticity of the polymer 8. Therefore, it is expected that the gel with the lower amount of bound water would heal and crosslink upon chain breaking due to hydrogen bonding. Jiang et.al. 8, reported that a Zr(OH)4 crosslinked hydrogel encouraged self-healing due to the formation of hydrogen bonds. Moreover, attaining higher degradation enthalpy would suggest the creation of stronger bond between the PAM and Zr(OH)4 thus producing a gel with high strength. Based on this, the effects of Zr(OH)4, organic cross linkers, calcination, procedure change, pH and salinity on the thermal and viscoelastic properties of the PAM gels were studied. The DSC, TGA and DMA thermograms of the PAM-Zr(OH)4

hydrogels in comparison to PAM alone are

presented in detail in Section 3.4.1. Meanwhile, for the rest of the samples, the results are summarized in Table S1. Also, the discussion on the thermal stability and viscoelastic properties will be based on mainly DSC and DMA results. 3.4.1. Effect of Zr(OH)4 Figure 7 depicts the DSC curves of the PAM nanocomposites at different Zr(OH)4 concentration compared to the control. From the figure, two peaks were detected with the first peak indicating the presence of free water in the gel and the second peak representing the Tdeg of the polymer gel. It was observed that at lower Zr(OH)4 concentration (i.e. 0.2wt% and 0.5wt%), the PAM nanocomposites attained 100% bound water while at higher concentration (i.e. 0.8wt% and 1wt%), the PAM nanocomposites had free water present as seen in the Figure 7. This means that lower numbers of water molecules are chemically bound/attached to the surface of the PAM. From the prepared PAM nanocomposites, the 0.8wt% Zr(OH)4 attained the highest free water content (increased by 23.8% compared to PAM alone). Moreover, the presence of free water and bound water was evaluated using TGA, where PAM nanocomposites with higher bound water tend to have higher weight loss at around 100oC as a result of water evaporation. The TGA results are depicted in Figure 8 and the weight loss of all the samples are tabulated in Table S1. From the figure, PAM had the least water bound weight loss, thus indicating less water was attached/linked with the surface of the PAM compared to the PAM-Zr(OH)4 nanocomposites. The 0.2wt% and 0.5wt% Zr(OH)4


Page 14 of 27

Page 15 of 27

had higher loss of bound water compared to 0.8wt%. This could be due to the 100% BW observed by the DSC results. However, the 1wt% of Zr(OH)4 lost more water, also suggesting that at higher weight percentage of Zr(OH)4 (i.e. 1 wt%) more water molecules had sites available for bonding. In addition, between the temperatures of 100oC to 400oC, two degradation peaks were noticed. The first peak (~225oC) is the gel breakdown point while the second one (~350oC) might be related to the breakdown of PAM molecule itself

25.

Within

this range of the temperature, the dried PAM nanocomposite gels recorded a weight loss of 6% (0.2wt% Zr(OH)4), 5% (0.5wt% Zr(OH)4), 6% (0.8wt% Zr(OH)4) and 4% (1 wt% Zr(OH)4) compared to dried PAM alone which was 8%. This shows that the PAM nanocomposites are more thermally stable than PAM alone due to the functionalization of PAM. PAM 4% PAM 4% Zr(OH)4 0.2% PAM 4% Zr(OH)4 0.5% PAM 4% Zr(OH)4 0.8% PAM 4% Zr(OH)4 1.0%

Heat Flow (mW)

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


o

1795 J/g @ 182 C

211 J/g

o

1599 J/g @ 185 C o

1298 J/g @ 187 C o

1838 J/g @ 187 C

260 J/g

o

222 J/g

-25

0

25

1509 J/g @ 155 C

50

75 100 125 150 175 200 225 250 275 300 325 350 o

Temperature ( C) Figure 7: DSC thermograms for PAM-Zr(OH)4 at different Zr(OH)4 weight percentage



100

PAM 4% PAM 4% Zr(OH)4 0.2% PAM 4% Zr(OH)4 0.5% PAM 4% Zr(OH)4 0.8% PAM 4% Zr(OH)4 1.0%

90 80 30

70

25

60

Weight Loss (%)

Weight Loss (%)

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

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50

20

15

40 10

30 5 100

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20

200

250

300

350

400

o

Temperature ( C)

10 0 30 50

100

150

200

250

300

350

400

450

500

o

Temperature ( C)

Figure 8: TGA thermograms for PAM-Zr(OH)4 at different Zr(OH)4 weight percentage Furthermore, from the DSC, the degradation temperature of PAM has shifted by 3-5˚C upon the addition of 0.2-0.5wt% Zr(OH)4. This could be due to the restricted chain mobility of the PAM upon addition of Zr(OH)4. This is evident from the optical microscope images obtained in Figure 4 b and c, where the PAM gels with 0.2wt% and 0.5wt% Zr(OH)4 depicted longer chains requiring higher temperature to break. Nonetheless, even at a high degradation temperature of 185oC and 187oC, less energy is required to degrade these gels as reflected by the lower degradation enthalpy reported. In other words, at higher temperatures, the chain mobility in these two gels increases thereby requiring lower energy to break interactions such as hydrogen bonds. Increasing the Zr(OH)4 percentage to 0.8wt%, on the other hand, increased the degradation enthalpy and degradation temperature compared to PAM alone. This implies that PAM-Zr(OH)4 with 0.8wt% had better interaction (i.e. strongly crosslinked) as observed from the denser networks through branching chains observed under the microscope (Figure 4d). However, a sudden drop in degradation temperature was recorded for Zr(OH)4 1wt% as a result of having shorter chain network which allowed the molecules to move past each other easily (Figure 4e). In addition, the wider degradation enthalpy peak detected for PAM-Zr(OH)4 1wt% could indicate presence of more than one interaction taking place, i.e. filler-filler (the agglomerate) and filler-polymer (PAM-Zr(OH)4) interactions,


Page 17 of 27

among others. The formation of filler agglomerates lowers the reinforcing effect of the filler on the PAM matrix, thus allowing the PAM molecules to move freely at a lower temperature. This justifies the reduction in degradation temperature. Agglomeration can also have a negative influence on the viscoelastic properties of the PAM gels. This is evident from the reduction of gel strength of the PAM gels as the filler weight percentage increased (as shown in the reduction of the ratio of G’ to G”) (Table S1). This is because agglomerates act as a stress concentration site causing the materials to fail/rapture faster instead of withstanding the stress. Hence, even though the PAM gel with 1wt% Zr(OH)4 had attained the highest storage modulus as shown in Figure 9 (i.e.+72% compared to PAM alone), the agglomeration also caused the loss modulus to increase (i.e. Figure 10) leading to a reduction in G’/G” (i.e. Table S1). The PAM-Zr(OH)4 gel with 1 wt%, as such, was observed to have the least gel strength while the PAM gel at lower Zr(OH)4 weight percentage, i.e. 0.2wt% Zr(OH)4, had the highest gel strength even compared to PAM alone. This could be due to the strong reinforcement or entanglement of PAM with Zr(OH)4 at lower weight percentage as depicted by Figure 4 b. 4000 PAM 4% PAM 4% Zr(OH)4 0.2% PAM 4% Zr(OH)4 0.5% PAM 4% Zr(OH)4 0.8% PAM 4% Zr(OH)4 1.0%

3000

Storage Modulus (Pa)

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


2000

1000

0 0.1

1

10

100

Angular Frequency (1/s)

Figure 9: DMA analysis i.e. storage modulus of PAM-Zr(OH)4 at different Zr(OH)4 weight percentage



800 PAM 4% PAM 4% Zr(OH)4 0.2% PAM 4% Zr(OH)4 0.5% PAM 4% Zr(OH)4 0.8% PAM 4% Zr(OH)4 1.0%

600

Loss Modulus (Pa)

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

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400

200

0 0.1

1

10

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Angular Frequency (1/s)

Figure 10: DMA analysis i.e. loss modulus of PAM-Zr(OH)4 at different Zr(OH)4 weight percentage 3.4.2. Effect of Calcination The percentage of calcined zirconia (ZMC) was chosen based on the results from previous optimization i.e. 0.5wt%, considering a good balance between higher degradation temperatures and elasticity, respectively. In general, it is expected that polymer gels with calcined nanocomposites would have lower thermal and mechanical performance due to the reduced number of crosslinking sites for networking through hydrogen bonding with the polymer chains. The results of DSC, TGA and viscoelastic properties are summarized in Table S1 for Zr(OH)4 and ZrO2 gels. From the thermal analysis, it is observed that the thermal properties reduced for the calcined Zr(OH)4. Compared to Zr(OH)4, the degradation temperature for the ZrO2 gel decreased by 3°C while still being higher than that of PAM alone. On the other hand, the degradation enthalpy for both calcined and uncalcined Zr(OH)4 gels was lower with respect to PAM alone, indicating higher hydrogen bonding further complemented by the 100% BW attained. In addition, the TGA results show that the PAM with both the calcined and uncalcined fillers recorded higher bound water weight loss compared to PAM alone (Table S1). However, comparing calcined and uncalcined, the calcined gel had higher enthalpy due to lesser number of hydrogen bonding and potentially


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increased covalent bonding with PAM. This was further complemented by the viscoelastic properties reported in Table S1; compared to the uncalcined gel, the calcined gel attained a higher storage and loss modulus giving a lower G’/G”. The calcined gel was weaker theoretically due to the reduction in hydrogen bonding. 3.4.3. Effect of Procedure Change The procedure change was applied to have the organic cross linkers interact with the nanofiller first, promoting stronger covalent bonding with PAM chains. The procedure was tested for optimized conditions of Zr(OH)4 nanocomposites and the gels formed were evaluated using DSC analysis.. There was a general increase in the free water amount and degradation enthalpy supports the claim for covalent bonding between the nanocomposites and PAM using organic crosslinkers attached on the nanocomposite as the bridging point. However, addition of higher amounts of nanocomposites to the gels had lower degradation temperatures and enthalpy. This could be due to increased and stronger interactions with organic crosslinker functionalized nanoparticles through hydrogen bond once the procedure changed, thus increasing the gel self-healing properties. This in turn enhanced the PAM gel elasticity, recording an increase of 114% and 286% in storage modulus compared to PAM alone and PAM-0.8% Zr(OH)4, respectively. However, the gel strength of the PAM gel reduced (also evident from the reduction in G’/G”) upon change in procedure. This is further complemented from the decrease in degradation enthalpy (weaker interaction) and that the PAM gels degraded at lower temperature. From the TGA results (Table S1), the functionalization of the PAM is evident. 3.4.4. Effect of Organic Crosslinker Cross linkers are chemical compounds used to covalently bond materials. Nonetheless, excess amounts can lead to syneresis i.e. repulsion of water out of the gel structure due to shrinkage in gel volume

26.

In addition, the amount of crosslinker added can determine the

gelation time and gel strength. In one study, Liu et. al

16

investigated the effect of HQ and

HMT weight percentage on the gelation time and strength of PAM. It was reported that upon addition of 0.3 wt% cross linker (HQ/HMT), the PAM gelled within 15.5 hours, however, increasing the amount of OrgCL to 1.2wt% reduced the gelation time significantly (i.e. to 6.3 hours). This is because at lower cross linker percentage, the reaction kinetics are expected to be slower thus prolonging the gelation time. Meanwhile, increasing the gelation time comes



at the cost of the gel strength since at lower OrgCL wt%, less crosslinking sites are available, limiting the interaction between the PAM and the OrgCL. Consequently, the gel strength of the PAM was seen to drop from 0.071 MPa (with at 1.2wt% OrgCL) to 0.033 MPa (with 0.3wt% OrgCL). Hence, Liu et. al

16

found the balance between the gelation time and gel

strength, by adding 0.6wt% PAM with 0.6wt% OrgCL. Initial tests with 0.6wt% OrgCL gave a short gelation time of 1 hour limiting the deep placement of the gels. Consequently, lower OrgCL weight percentages (i.e. 0.15wt% and 0.3wt%) were tested. With 0.15 wt% OrgCL, the PAM alone gel recorded a drastic decrease in gel strength (G’/G”) due to the limited interaction between PAM and the OrgCL. This was also evident from the reduction in amount of free water and degradation enthalpy depicted from Table S1. That being said, the amount of free water and the degradation enthalpy of PAM-Zr(OH)4 improved (Table S1) suggesting that the gels became stronger at a lower cross linker concentration. This could be due to the slowed reaction between the OrgCL and the materials thus allowing the PAM and fillers to interact with one another instead of having to compete with the excess OrgCL. This in turn improved the elasticity of the PAM-Zr(OH)4 as shown in the increase in storage modulus. The TGA results (Table S1) also depicted similar observations: the 0.15 wt% OrgCL gels had higher water loss at 100oC and lower weight loss of the dry gel showing increased functionalization of the gel compared to 0.3 wt% OrgCL. However, the degradation temperature of all gels decreased when the OrgCL reduced indicating the formation of shorter chain that could increase the PAM chain mobility at a lower temperature, thus degrading faster. Increasing the ratio between the OrgCLs (HQ:HMT i.e.1:2) can also influence the gel properties 15. As mentioned in section 3.1, HMT decomposes into formaldehyde that would react with HQ in order to create more interactive sites. Thus, when the HMT amounts are increased, it ensures the presence of excess reactant for the first reaction to occur where water is consumed to produce more aldehydes. This would also affect the condensation reaction where more water is released and potential networking is through weaker hydrogen bonding. This is also observed from the DSC where changing the weight ratio between HQ and HMT from 1:1 to 1:2 depicted a decrease in free water and degradation enthalpy for the PAM and PAM-Zr(OH)4 gels. In addition, the DMA results (Table S1) showed the gel strength was reduced for all the gels when the HMT amount was increased while keeping the HQ constant (i.e HQ:HMT, 1:2).


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3.4.5. Effect of pH The effect of pH on the thermal and mechanical properties of the PAM was studied at different crosslinker amounts. Generally, an alkaline environment is expected to increase the gelation time but at the cost of the gel strength

16.

However, with OrgCL 0.15 wt% and a

higher pH, the PAM alone as well as PAM-Zr(OH)4 did not gel. This could be due to the fact that the amount of HMT was not enough to form effective networks since it did not decompose in the presence of high amounts of ammonia despite the high temperatures, thereby severely retarding the gel formation

16,27.

Therefore, only 0.3 wt% crosslinker is

reported in this section. From the DSC analysis in Table S1, the degradation temperature and enthalpy of the PAM gels were seen to reduce significantly with an increase in pH. This is further complemented from the TGA analysis (Table S1), where at higher pH, the PAM gels had higher amount of bound water evaporated. This is because, as mentioned earlier, even though the OrgCL percentage increased to 0.3 wt%, the alkaline environment demoted the decomposition of HMT to form formaldehyde which in turn lowered the crosslinking sites with the PAM. This leads to formation of shorter and less dense polymer chains thus increasing the chain mobility of PAM at lower temperature. However, the formation of short chains caused the gel strength of PAM alone gel to reduce (as explained in section 3.2). In contrast to PAM alone, the gel strength of PAM-Zr(OH)4 gel increased significantly as observed from the G’/G” calculated in Table S1 even though the degradation temperature and enthalpy reduced drastically. This indicates that although the PAM gels linked using shorter chain, the PAM surface still provided a higher surface area (i.e. higher phase volume) for the fillers to attach to. Moreover, the reduced decomposition of HMT also encouraged more PAM and Zr(OH)4 interaction without competition. 3.4.6. Effect of salinity Salinity is another factor that can affect the gelation time and gel strength where increased salinity can increase the gelation time but reduce the gel strength 15. This is because salts can reduce the hydrolysis by shielding the negative charges of the carboxylate groups of PAM 28. In other words, the cations disrupt the PAM chain interactions and ultimately lowering the gel strength. This effect is stronger with divalent ions (Mg2+, Ca2+) 15. As such, the effect of salinity was studied by using different salts, i.e. KCl and MgCl2. In addition, Arabian Gulf



seawater was also tested instead of deionized water for gelation as this could lower the costs of gelant if gel properties are not affected significantly. From the DSC analysis in Table S1, the free water content and degradation enthalpy of PAM alone gel increased when using MgCl2 compared to KCl. This also caused the gel strength (G’/G”) to increase while KCl produced the weakest PAM gel. This suggests that MgCl2 (divalent metal ions) had a greater influence on the crosslinking sites than KCl (monovalent metal ion) due to higher charge density

15.

Sea water, on the other hand, contains chemical

ions such as chloride, sodium, sulfate, magnesium, calcium, potassium, bicarbonate, bromide, borate and others which could shelter the crosslinking sites. That being said, Arabian sea water had less influence compared to MgCl2 thus producing PAM gel with lower strength compared to MgCl2 even though it was still stronger than KCl. However, at higher OrgCL (0.3wt%), KCl produced stronger gel compared to both MgCl2 and Arabian sea water. This shows that despite the influence of the cations to shield the interactive sites, there were enough sites for the PAM to crosslink since the amount of OrgCL increased. As for the PAM-Zr(OH)4 nanocomposite with 0.15 wt% of OrgCL, both degradation enthalpy and free water amount reduced when MgCl2 and Arabian sea water were used instead of KCl, which also reflected in the reduction of gel strength. This could be due to the MgCl2 cations and sea water ions shielding the PAM chains from interacting with the Zr(OH)4 ultimately lowering the gel strength. When the OrgCL increased to 0.3 wt%, the MgCl2 produced a stronger gel compared to the rest, as revealed from the highest degradation enthalpy and G’/G” obtained compared to the controls (PAM alone and PAM nanocomposites). This could be that, MgCL2 lowered the availability of extra interactive sites on the OrgCL that would have competed with the filler, hence allowing the PAM to interact more with the filler. 4.

CONCLUSION

In this study, polymer gels with low MW PAM and Zr(OH)4 nanocomposites in presence of organic cross linkers (HQ and HMT) were developed with high thermal performance. The properties of these PAM (550,000 g/mol) gels were studied in terms of gel strength and thermal stability. The gel strength was determined based on the water content (i.e. free water and bound water) and the G’/G” measured by DSC and calculated from DMA results, respectively. It was observed that higher bound water promoted indirect hydrogen bonding


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(i.e. filler-water-water-OrgCL-water-water-PAM interaction) over direct covalent bonding between cross linker, filler, and PAM chains. At a high degradation temperature of 187°C, the hydrogen bonds were able to break easily which reflected in the lower enthalpy obtained to degrade the PAM gel with 0.2 wt% and 0.5 wt% Zr(OH)4. However, as hydrogen bonding is reversible, it can allow the PAM gel to be more elastic which was reflected in the DMA result of PAM with 0.2 wt% Zr(OH)4. In addition, other factors influencing the properties of the PAM gels such as calcination of Zr(OH)4, procedure changes, the organic cross linker ratio and weight, pH, as well as salinity have been studied and successfully optimized. It was shown that PAM gels at higher OrgCL wt% (i.e. 0.3 wt%) was preferable as the gel strength, free water content, degradation enthalpy, and temperature were seen to increase, indicating better interaction between the PAM and filler. Increasing the pH to 10-11 was seen to lower the degradation enthalpy (i.e. weaken interactions) due to HMT not decomposing in the presence of a high amount of ammonia despite the high temperature thereby severely retarding gel formation. Last but not least, PAM gels prepared in the presence of MgCl2 produced more crosslinked gel due to a higher charge density as depicted from the DSC and DMA analysis. In conclusion, the optimized PAM gels prepared for water shutoff application were stable until 187°C which is the highest reported to the best of the authors’ knowledge. The optimized conditions were 4 wt% PAM, 0.2 wt% - 0.8 wt% Zr(OH)4, 0.3 wt% OrgCL (HQ and HMT each) in presence of either KCl or MgCl2 gelled at a temperature of 150°C. 5. SUPPORTING INFORMATION In this section, the complete data of thermal and viscoelastic properties of the PAM hydrogels are summarized in tabular format as shown in Table S1. The thermal properties of the hydrogels are expressed in terms of DSC and TGA data obtained while the viscoelastic properties are represented with the DMA data. Furthermore, the gel strength is also calculated from the G’ and G” obtained i.e. in terms of G’/G”. In all the investigations conducted in this study, PAM alone is used as a reference.

ACKNOWLEDGEMENT This study is part of research project agreement no. AFU-01-2017 in collaboration with EXPEC Advanced Research Centre, Saudi Aramco. The authors gratefully acknowledge the continued support from AlFaisal University and its Office of Research.



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TABLE OF CONTENTS GRAPHIC

Figure 11: For Table of Contents Only


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Enhancement in Thermal and Viscoelastic Properties of PAM gels 254x190mm (96 x 96 DPI)


Enhanced PAM Polymer Gels Using Zirconium Hydroxide

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