Smart Cellulose Nanofluids Produced by Tunable Hydrophobic

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Smart cellulose nanofluids produced by tunable hydrophobic association of polymer grafted cellulose nanocrystals Yea Ram Lee, Daehwan Park, Sang Koo Choi, Miju Kim, Heung Soo Baek, Jin Nam, Chan Bok Chung, Chinedum O. Osuji, and Jin Woong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08783 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

Smart cellulose nanofluids produced by tunable hydrophobic association of polymer grafted cellulose nanocrystals

Yea Ram Leea, Daehwan Parka, Sang Koo Choia, Miju Kimb, Heung Soo Baekb, Jin Namb, Chan Bok Chungc, Chinedum O. Osujid,*, Jin Woong Kima,e,*

a

Department of Bionano Technology, Hanyang University, Ansan 15588, Republic of Korea

b

Amore-Pacific R&D Centre, Yongin 17074, Republic of Korea

c

SK Bioland R&D Co. Ltd, Osong 28162, Republic of Korea

d

Department of Chemical and Environmental Engineering, Yale University, New Haven CT

06511, USA e

Department of Chemical and Molecular Engineering, Hanyang University, Ansan 15588,

Republic of Korea

KEYWORDS: Smart Nanofluids, Associative Cellulose Nanocrystals, Hydrophobic Interaction, Sol-Gel Transition, Nanofibrillar Crystallinity

ACS Paragon Plus Environment

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ABSTRACT Cellulose fibrils, unique plant-derived semi-crystalline nanomaterials with exceptional mechanical properties, have significant potential for rheology modification of complex fluids due to their ability to form a physically associated semi-flexible fibrillary network. Here, we report new associative cellulose nanocrystals (ACNCs) with stress-responsive rheological behaviors in an aqueous solution. The surface-mediated living radical polymerization was employed to graft poly (stearyl methacrylate-co-2-methacryloxyethyl phosphorylcholine) brushes onto the nanofibrils, and then 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation was conducted to produce nanoscale ACNCs in the aqueous solution. The ACNCs displayed inter-fibril association driven by the hydrophobic interaction that resulted in the formation of a nanofibrillar crystalline gel phase. We observed that the viscosity of the ACNC fluid showed reversible shear thinning and temperature-induced thickening in response to applied shear stress and thermal shock. Moreover, thanks to generation of a mechanically robust nanofibrillar crystalline gel network, the ACNC suspension showed extraordinary stability to changes in salinity and pH. These results highlighted that the inter-fibril hydrophobic association of ACNCs was vital and played an essential role in regulation of stimuli-responsive sol-gel transitions.

1. Introduction Nanofluids, colloidal suspensions with 1-100 nm sized nano-species in a base fluid,1,2 can display unique rheological behaviors in response to external stimuli, such as heat,3 pH,4 the presence of metals5 and light.6 Reversible intermolecular interactions commonly determine the


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rheological responsiveness and reversible sol-gel transition of the nanofluid. To form a gel phase, building blocks arrange themselves in a three-dimensional network, which is usually driven by electrostatic interactions,7,8 hydrophobic interactions,9,10 or DNA interactions.11,12 Among these, the use of hydrophobic interactions is particularly attractive, since the weak but long-range interaction forces (a few kBT in magnitude) can induce stimuli-responsive association and dissociation between building blocks. For example, hydrophobically modified ethoxylated polymers display robust interactions between the hydrophobic segment ends, giving rise to a remarkable resistance of the rheological properties to changes in pH and salinity.13,14 By extending this approach, it has been reported that associative nanoparticles can be also utilized as a polymer rheology modifier with particulate crosslinks, feasibly regulating particle-mediated sol-gel transitions.15 Imparting anisotropic properties to nano-objects, such as nanotubes,16 nano-rods,17,18 and nanofibrils,19-22 excludes their volume, which critically depends on the applied stress, thus allowing us to achieve unique rheological properties. For this purpose, specific interest has recently been concentrated on the utilization of cellulose nanocrystals (CNCs), since they have long semi-flexible chains with thickness of a few nanometers, with high specific strength and modulus.23-25 Moreover, such long, flexible, nanoscale fibrillary chains may associate each other to form physical crosslinks that readily hinder the flow of the system.26,27 Despite these intriguing rheological properties, their much wider applications have been hampered because of their limited viscosity enhancement performance, which stems from the relatively low possibility for effective inter-chain entanglement. Therefore, in order for CNCs to physically interact each other, a technology that allows for additional inter-chain associations must be developed. More


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importantly, these inter-chain associations should be controllable, such that a given suspension can be engineered with unique rheological properties. The ultimate goal of this study is to create a novel nanofluid system, where associative cellulose nanocrystals (ACNCs) with controlled hydrophobic interactions between the CNC chains can display the reversible sol-gel transitions in response to both shear and thermal stresses. ACNCs are synthesized by chemically grafting a hydrophobically-modified watersoluble polymer, poly (2-methacryloyloxyethyl phosphorylcholine-co-stearyl methacrylate), hereafter poly (MPC-co-SMA). The strength of the hydrophobic interaction can be manipulated in a rational manner by changing the ratio of SMA to MPC in the copolymer brush-SMA is hydrophobic due to its 18-carbon long pendant alkyl chain, whereas MPC is a zwitterionic hydrophilic monomer which gives a strong hygroscopic nature to the copolymer over a broad range of solution pH. Suspension rheology studies in aqueous media are carried out to show that ACNCs exhibit strong viscosity enhancement that is remarkably independent of pH and ionic strength. We also demonstrate that when the surfaces of ACNCs are further grafted with temperature-responsive polymer brushes, the resulting ACNC nanofluid shows a temperatureresponsive flow behavior.

2. Experimental Section 2.1. Materials. Dried cellulose nanocrystals (CNCs) were supplied from Process Development Center of the University of Maine (USA). The CNCs have diameter of 5-20 nm and length of 150-200 nm, with 1.05 wt% sulfur. 3-aminopropyltriethoxysilane (APTES), trichloroacetyl isocyanate (TAI), molybdenumhexacarbonyl (Mo(CO)6), toluene (99.8%), dibutyltin dilaurate (DBTDL), 2,2,6,6-tetramethylpipridine-1-oxyl radical (TEMPO), sodium bromide, and sodium


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hypochlorite solution (12%) were purchased from Sigma Aldrich (USA). SMA and Nisopropylacrylamide (NIPAM) were used as grafting monomers and also purchased from Sigma Aldrich (USA). Hygroscopic zwitterionic monomer, MPC, was kindly supplied by KCI Co. (Korea). All chemicals were reagent grades and used without further purification. Deionized doubled distilled water was used through the experiment.

2.2. Synthesis of poly (MPC-co-SMA) grafted ACNCs. ACNCs were synthesized by using surface-mediated living radical polymerization. In the first step, CNCs (1g) were dispersed in 30 mL toluene in a round glass flask stirring for 30 min, followed by addition of APTES (0.38 mL) to functionalize the surface of CNCs with primary amine groups. Subsequently, the mixture was fluxed with stirring at 110 ˚C for 8 h under argon. Unreacted materials were completely removed by repeated centrifugation with tetrahydrofuran and toluene at 4000 rpm for 5 min. In the second step, the amine functionalized CNCs were again dispersed in toluene (40 mL). Then, TAI (0.12 mL) was incorporated to induce the urea condensation reaction between the amine group on the surface of CNCs and the isocyanate group of TAI in the presence of DBTDL (0.005 ml) as a catalyst. The reaction mixture was refluxed at 80 ˚C for 8 h under argon. The trichloroacetylfunctionalized CNCs were washed by repeated centrifugation with tetrahydrofuran and ethanol at 4000 rpm for 5 min. MPC, SMA, and Mo(CO)6 dissolved in ethanol (40 ml) were added into the CNC dispersion. Then, living radical polymerization was conducted at 70 ˚C with reflux stirring for 12 h under argon. After polymerization, the mixture was washed again with ethanol and rinsed with water. In the case of synthesizing poly (NIPAM-co-SMA) grafted CNCs, NIPAM was copolymerized with varying the copolymerization ratio against SMA.


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2.3. TEMPO-mediated Oxidation. The polymer-grafted CNCs (1 g) were suspended in water (100 mL) containing TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). Then, NaClO solution (0.748 mL, 12 mmol) was slowly added to the mixture solution. The pH was adjusted to 10 by adding 0.5 M NaOH solution until no NaOH consumption was observed. TEMPO-oxidized cellulose dispersions were thoroughly washed with water by dialysis and stored at 4 ˚C before next uses. Finally the oxidized cellulose was disintegrated using an ultrasonic homogenizer at room temperature for overnight.

2.4. Characterization of ACNCs. The monomer conversion was determined by the internal standard analysis method28, in which proton nuclear magnetic resonance (1H-NMR, Bruker) was employed to quantitatively determine the monomer conversion by comparing the integral areas of characteristic chemical shifts. The polymer-grafted CNCs (0.06 g) were dispersed in DMSO (0.6 mL) for the analysis. Surface chemistry of CNCs before and after modification was analyzed by Fourier Transform Infrared Spectroscopy (FT-IR, Varian 1000, USA). The morphology of the ACNCs was confirmed energy-filtering transmission electron microscope (TEM, Carl Zeiss, Germany). TEM samples were prepared by dispersing 0.1 wt% of CNCs and ACNCs in water, respectively. A drop of sample dispersion was then pipetted onto a carbon coated copper grid. Negative staining using uranyl acetate was performed for 30 s at room temperature. After complete drying at room temperature for 5 min, the morphology of fibrils was observed with TEM operated at an accelerating voltage of 120 KV.

2.5. Rheological measurements. Rheological properties of cellulose nanofiber dispersions were characterized using a DHR-1 rheometer (TA instrument, USA) in the stress-control mode with a


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cone-plate geometry, of which diameter was 40 mm and the angle was 2 ˚. The concentration of nanofibers in water was adjusted to 4 wt% by evaporating water. Before operation of the rheometer, equilibration was performed for all rheological measurements for 10 min. To prevent water evaporation, a solvent trap was installed around the sample. The suspension rheology was characterized by running flow sweep, oscillation amplitude sweep, temperature sweep, and time sweep tests. The flow sweep measurement was conducted at a shear rate γ ranging from 0.001 to 100. A strain range of oscillation amplitude was 0.01-100 %. Oscillation amplitude sweeps were performed with sweeping the temperature from 25 to 60 ˚C.

3. Results and Discussion In a particular synthetic procedure, ACNCs were synthesized by cellulose nanocrystal (CNC)mediated living radical polymerization, which enables chemical grafting of associative poly (MPC-co-SMA) brushes, and by consecutive TEMPO oxidation, as schematically illustrated in Figure 1. The polymer brushes were copolymerized with SMA having 18C alkyl chains to introduce a hydrophobic moiety and MPC to give zwitterionic hygroscopic property to the polymer, thereby facilitating complete hydration of the polymer brushes with water molecules. After the graft polymerization, the conversion of monomers was over 90%, as determined by 1HNMR analysis, (Figure S1). Successful grafting of poly (MPC-co-SMA) was also confirmed by assigning the characteristic FT-IR peaks at 2915 cm–1 for –CH3, 1719.7 cm–1 for –COO–, 1236 cm–1 for –POCH2–, and 959 cm–1 for –N+(CH3)3– (Figure S2). TEMPO-mediated oxidation converted the hydroxyl groups to carboxylate groups on the ACNCs. Initially, the CNCs used in this study had high crystallinity and aggregated to form particulate bundles, of which length scale is from hundreds of nanometers to a few micrometers (Figure S3). The oxidation process


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dissociated the CNC bundles to individual CNCs, resulting in a bright bluish, viscous aqueous suspension. The carboxylate content of ACNCs was 1.5 mmol/g, which was assayed by electric conductivity titration method.29 Generation of the individualized nanoscale fibrillary morphology could be clearly confirmed via high-resolution transmission electron microscopy (HR-TEM) analysis (Figure 2A). Grafting poly (MPC-co-SMA) onto ACNCs resulted in nanofibrils coated with a thin polymer layer (Figure 2B), with average thicknesses of ~2 nm. After TEMPO-mediated oxidation, numerous carboxylate ions generate on the surface of CNCs. As the electrostatic repulsive force dominates the CNC dispersion, neat CNCs were readily hydrated but could not form a gel-like phase. By contrast, incorporation of an appropriate amount of SMA, which is typically at φSMA = 7.5×10–3, produced an ACNC gel fluid, as shown in the inset of Figure 3A (see also Movie S1). If φMA is too high, the ACNCs just separated from the system to formed a nanoscale dispersion (Figure S4). The ACNCs suspension with a proper φSMA exhibited a typical shear thinning flow behavior (Figure 3A-B), arising from breakdown of the gel network structure in response to the applied shear stress. Noticeably, the viscosity difference between the two fluids at identical concentrations was approximately one order of magnitude, implying that the viscosity enhancement due to hydrophobic alkyl-alkyl association was independent of concentration in the range of concentrations studied. The alkyl-alkyl association can be synergistically induced by van der Waals force. Although this force is quite weak (~4 kJ·mol–1, which is typically characterized from △Hvap of the CH2 unit), it can effectively generate intermolecular interactions between hydrophobic polymer or oligomer chains. In terms of the solvophobic effect, more favourable alkyl-alkyl interaction is obtained in solvents with high cohesive energy density, CED, which is defined by = √ = (∆ − )/ , where is the Hildebrand solubility parameter, R is the gas constant, and


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Vm is the molar volume of a particular solvent at temperature, T.30 The CED of water is approximately 550 cal·cm–3. Consequently, ACNCs modified with water-soluble associative polymer brushes having an appropriate amount of C18 alkyl chains have a strong tendency toward forming the hydrophobic attraction in water. In principle, the viscosity-shear rate relationship can be represented by η = with |n|≤1, where n is the shear thinning exponent and K is the consistency. It has been commonly understood that n cannot be larger than 1, since it means a system in which it is easier to flow at higher shear rates than at lower shear rates. The neat CNC suspensions and diluted ACNC suspensions produced in our study showed the n values in the range of 0.52~0.88. When ACNCs are concentrated near 4 wt%, interestingly, the suspension showed n ~1.24, implying that the flow may not be steady or homogeneous. From the polarized microscope observation, we were able to figure out that such an unusual flow property was closely related to reversible formation of a CNC crystalline phase. As shown in Figure 4A-C, the ACNC suspension consisted of aligned cellulose crystalline domains in the micrometer scale, which was quite comparable to the neat CNC suspension. Upon applying shear stress, this crystalline phase disappeared, indicating that the extraordinary rheological behavior of the concentrated ACNC suspension stemmed from the secondary shear-responsive phase transition of the fibrillary crystalline gel network, as schematically illustrated in Figure 4D. We further investigated the rheological behaviors of ACNC fluids. While observing oscillation stress vs. oscillation strain, the ACNC fluid showed a much higher stress response against strain than the neat CNC fluid (Figure 5A). These results give an interpretation where the ACNC fluid showed enhanced stress resistance due to formation of a fibrillary crystalline gel network in the low stress regime. In order to demonstrate the structural robustness of our gel network, we


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observed how their viscoelastic properties were affected by strain changes (Figure 5B). The storage (G') and loss (G'') modulus of the ACNC fluid were remarkably higher than those of the neat CNC fluid, with differences of more than two orders of magnitude, resulting in a low loss tangent (tanδ=G''/G') of ~0.1 (Figure 5C). It was noticeable from our observation that although the ACNC fluid formed a robust gel structure in the absence of stress, the structure is so weak that the gel-to-sol transition rapidly occurred even at a low strain of ~10%. We also concerned ourselves with the response of our ACNC fluid to harsh environmental factors. Our suspension rheology studies revealed that, thanks to the formation of a structurally robust fibrillary crystalline gel network, the ACNC fluid exhibited such an excellent resistance to salinity and pH changes (Figure 5D-E). We pursued the incorporation of temperature responsiveness to enhance the functionality of the ACNC suspension. For this purpose, we fabricated ACNCs with poly (NIPAM-co-SMA) brushes using the same synthetic protocol (Figure S5). Taking advantage of the selected intraand inter-chain hydrogen bonding between NIPAM monomer units and water molecules in response to temperature changes,31,32 we regulated the hydrophobic interaction between ACNCs using poly (NIPAM-co-SMA) brushes. Consequently, copolymerization of NIPAM significantly increased the viscosity of the ACNC fluid at high temperatures (Figure 6A, Figure S6). When the same amount of NIPAM-co-SMA against CNCs was polymerized, the most effective hydrophobic interaction between the ACNCs occurred. The ACNCs with NIPAM units also showed the greater attraction, leading to the elastic gel phase with G' > G'' (Figure 6B). This highlights the fact that the hydrophobication of NIPAM units above LCST led to the enhanced inter-fibrillar association.33-36 Notably, the temperature responsiveness of our ACNC fluid exhibited in the range of 35-38 ˚C. It is much higher than that of pure poly (NIPAM-co-SMA)


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polymer, which is detected at 31.5 ˚C (Figure S7). One possible explanation for this behavior is that ACNCs would have a long relaxation time due to their bulk fibrillar structures. We further examined the rheological behavior of the ACNC fluid as a function of temperature. From the strain sweep data below and above LCST, we observed that both G' and G'' above the LCST were larger than the values below the LCST, across the range of stain rates considered, up to 10% (Figure 6C). In addition, time sweep data showed that the gel maintained stable states at both temperatures, as reflected in the effective time-independence of the elastic and viscous moduli at each temperature (Figure 6D). These results highlight that incorporation of NIPAM units into the associative polymer brushes of ACNCs allowed us to successfully impart temperature responsiveness to the fluid (see also Figure S8).

4. Conclusions In summary, this study introduced a facile but truly effective approach for fabricating shear stress- and temperature-responsive ACNC fluids, in which a nanoscale cellulose fibril gel network was reversibly formed due to the controlled inter-fibril hydrophobic attraction. Using ACNCs synthesized by grafting hydrophobically modified water-soluble polymer brushes, we successfully demonstrated that the incorporation of C18 alkyl chains in the polymer brushes facilitated the hydrophobic interaction of the ACNCs in aqueous solution. Suspension rheological studies have elucidated that the ACNCs could not only dramatically enhanced the viscosity of the nanofluid, but also imparted excellent tolerance against pH changes and salt addition. These characteristics highlight that ACNCs have good potential as naturally-derived materials to replace conventional synthetic rheology modifiers. Our continued study also confirmed that grafting poly (NIPAM-co-SMA) onto ACNCs could assign temperature


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responsiveness to the nanofluid. We expect that the ACNCs fabricated in this study can overcome would have great applicability either for development of a variety of smart nanofluids or for fabrication of new types of nanofibrilar surfactants and nanofibrils-mediated catalysts.

AUTHOR INFORMATION Supporting Information The Supporting Information is available free of charge on the ACS Publications website. More information and details about 1H NMR and FR-IR spectra for characterizing surface-mediated living radical polymerization, morphology observation of CNCs, synthetic process and suspension rheology of temperature-responsive ACNC nanofluids, and demonstration movie for reversible sol-gel transition of the ACNC nanofluid.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions Y. R. Lee and D. Park equally contributed to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


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This research was supported by National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (No. 2008-0061891 and 2016R1A2B2016148).

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Figure 1. Schematic illustration for grafting poly (MPC-co-SMA) brushes on CNCs by using surface-mediated living radical polymerization.


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Figure 2. TEM images of (A) neat CNCs and (B) poly (MPC-co-SMA) grafted ACNCs. ϕSMA = 7.5×10-3.


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Figure 3. (A) Viscosity behaviors of CNC suspensions produced with varying ϕSMA in poly (MPC-co-SMA) brushes. The insets are photographs of CNC suspensions: (a) neat CNCs and (b) ACNCs. (B) The effect of the CNC concentration on the viscosity change. All the experiments were carried out at room temperature.


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Figure 4. Polarized microscope images of CNC suspensions: (A) neat CNC suspension at steady state, (B) ACNC suspension at steady state, and (C) ACNC suspension upon shearing. The concentration of CNCs in the suspensions was set to 4 wt%. All the experiments were carried out at room temperature. (D) Interaction between ACNCs via the hydrophobic attraction between poly (MPC-co-SMA) brushes.


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Figure 5. (A) Oscillation stress vs strain of neat CNCs and ACNCs. (B) Storage modulus, G' (closed symbol) and loss modulus, G'' (open symbol) as a function of oscillation strain. (C) Tan delta, tan δ=G''/G', as a function of oscillation strain. (D) Viscosity resistance against salinity. (E) Viscosity resistance against to pH changes. The concentration of neat CNCs and ACNCs was set to 4 wt% in water. ϕSMA = 7.5×10-3.


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Figure 6. Suspension rheology of poly (NIPAM-co-SMA) grafted ACNC fluids. (A) Viscosity behaviors as a function of temperature. G' (closed symbols) and G'' (open symbols) changes as a function of (B) temperature, (C) oscillation strain and (D) time. The concentration of ACNCs was set to 4 wt% in water. ϕSMA = 7.5×10-3.


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Table of Contents Graphic


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Smart Cellulose Nanofluids Produced by Tunable Hydrophobic

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