Doping of γ -Fe3O4 magnetic nanoparticles in Pluronic F127/ DMF LLCs phases to tune the structural transitions and rheological behavior

ABSTRACT A new rapid microwave-assisted approach has been employed to prepare the γ-Fe3O4 magnetic nanoparticles (γ-Fe3O4 MNPs). The formation of spherical γ-Fe3O4 MNPs with an average size of 45 nm and bandgap of 2.3 eV is confirmed by FESEM and UV–Visible spectroscopy. γ-Fe3O4 is doped (0.01 wt% and 0.02 wt%) in the lyotropic liquid crystalline (LLC) lamellar phases of Pluronic F127/N,N dimethylformamide (DMF) to prepare the LLC nanocolloids. Polarizing optical microscopy measurement reveals that γ-Fe3O4 doping does not affect the lamellar LLC structures except 0.02 wt% at 10:90 wt%:wt% system where lamellar to nematic LLC phase transition is observed. Pure LLC phases and nanocolloids are studied for steady and dynamic rheological behavior. LLC phases and LLC nanocolloids possess shear thinning and solid viscoelastic behavior. A complex moduli study of LLCs and LLC nanocolloids is done which indicates the highly elastic strength-based lamellar and nematic phases. Shear-thinning and highly viscoelastic nature of LLCs and LLC nanocolloids have future applications in lubrication, cleaning, cosmetics, and pharmaceutical.


Introduction
In the past decades, LLCs phases derived from nonionic surfactants have found potential applications in the field of the cosmetic industry, drug delivery, cleaning, water treatment, and as a lubricant. [1][2][3][4][5] In the nonionic class, nonionic amphiphilic block copolymers possess one/two head groups that enable them to form the micelles very easily in single and binary aqueous and non-aqueous solvents. [6] Amphiphilic block copolymer is decorated with hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO)/poly(butylene oxide) (PBO)/polystyrene. The micellization process in the block copolymer depends on the temperature and a particular critical micelle concentration. [7] Block copolymers have physicochemical properties such as surfaceactive agent, thermoplasticity, solubility in polar solvents, and are nontoxic and nonimmunogenic. Such properties make them potential candidates for applications in the fields of emulsifiers, detergents, foaming agents, dispersion stabilizers, drug solubilization, nanoparticle synthesis, bioprocessing, dental care, and lubrication. [8][9][10][11] Block copolymers are further classified into two types viz. diblock co-polymer and triblock copolymer. The diblock copolymer consists of one hydrophilic and one hydrophobic block, while the triblock copolymer is decorated with three blocks of homopolymer. [7] The hydrodynamic volume in the diblock copolymer increases faster as compared to the triblock copolymer. The increase in hydrodynamic volume reduces the stability and fast aggregation behavior in diblock copolymer. Hence, triblock copolymers are found to be more stable nonionic amphiphiles in preparing LLCs phases. [12] Pluronic, poloxamer, pluricare, and synperonics are some common commercial nonionic triblock polymers used for this cause. Tuning of LLCs phases in triblock copolymers (having PEO-PPO blocks) depends on the intermolecular, interassembly interactions, and self-assembly process. [6] Apart from this, the polarity of solvents also plays an important role in the formation, stabilization, and packing of micelles into various structures. The higher the polarity of the solvent, the more prominent is the interaction between copolymer and solvent. [7,13] The incorporation of nanoparticles can also tailor the structural, optical, thermal, and rheological properties of triblock copolymer-based LLCs phases. Interactions of nanoparticles in block copolymer solution play a vital role in controlling the self-assembly. In this concern, some reports have been cited in the literature on nanoparticle-doped nonionic triblock copolymer systems. For example, Chiu et al. discussed the control of gold nanoparticles location in poly(styrene-b-2 vinyl pyridine) block copolymer matrix. [14] Wei et al. discovered the floating gate memory devices by the doping of gold nanoparticles in self-assembled poly(styrene-b-2 vinyl pyridine) block copolymer. [15] Lin et al. studied the dispersion of carbon black in Pluronic F127 and water solution to tune the adsorption behavior of nanocolloids solution. [16] Nibali et al. reported the incorporation of titanium dioxide nanoparticles in a self-assembled Pluronic F127 aqueous solution. [17] Hickey et al. studied the doping of magnetic nanoparticles in block copolymer composed of poly(acrylic acid) and polystyrene in water, THF/DMF, and dioxane/THF mixture. Magnetic nanoparticles in a self-assembled structure affect the morphology and enhance the magnetic relaxivity rate through the solvent-nanoparticle and polymer-nanoparticle interactions. [18] In other reports, metallic nanoparticles are added into the lamellar phases of block copolymer to tailor the optical properties and control the spatial and orientational distribution of nanoparticles. [19] Moreover, the incorporation of Fe 3 O 4 in Pluronic F127 hydrogel exhibits viscoelastic behavior and thermal stability in an aqueous solution. The successful incorporation of magnetic nanoparticles leads to the development of materials for biomedical applications. [20] Considering the above literature review and facts presented therein, there is a great scope for experimental research in these systems and to explore new application perspectives. No report is found in the literature on the single nonaqueous solvent-based Pluronic LLC phases. The rheological aspects of such systems have also not been highlighted so far.
Hence, in this context, we are reporting on the LLCs phase of nonionic amphiphilic PEO-PPO-PEO triblock copolymer Pluronic F127 in single solvent systems DMF. Pluronic F127 and DMF are used as good emulsifiers that complement the physicochemical properties of each other and enhance the solubilization process. Furthermore, the prepared LLCs systems are doped with γ-Fe 3 O 4 magnetic nanoparticles. γ-Fe 3 O 4 magnetic nanoparticles are prepared via a rapid microwave process. Microwave synthesis has emerged as a simple, economical, and rapid synthesis approach with better control of the structural and morphological aspects of nanomaterials. The detailed process and factors affecting the nucleation and growth of nanoparticles are highlighted in our earlier publications. [21][22][23] Structural, morphological, thermo-optical, and rheological behavior of pure and doped Pluronic F127/DMFbased LLCs systems are studied and correlated. This study also throws some light on the importance of the rheological parameters for biomedical applications. The presented systems can be explored in future for biomedical, cosmetic, pharmaceutical, cleaning, and lubrication applications.

Synthesis of γ-Fe 3 O 4 MNPs
γ-Fe 3 O 4 MNPs were synthesized by using the domestic microwave in the following steps; firstly, FeCl 2 ·4H 2 O and FeCl 3 ·6H 2 O were dissolved in 100 ml of DI water in a 1:2 molar ratio. The solution was gently stirred at 80°C for 30 min. NaOH pellets were added to the reaction mixture to control the pH ⁓ 9. Furthermore, the precursor solution was cooked in a domestic microwave at 300 W for 60 s. A change in the color of the solution from orange to black confirms the formation of iron oxide. The schematic diagram of the synthesis process of γ-Fe 3 O 4 MNPs is presented in Figure 1. The formation of the particles was further checked by placing a magnet near the surface of the beaker. The grown iron-oxide nanoparticles were filtered and washed thoroughly with DI water for several times. Finally, the prepared particles were dried in a vacuum oven at 100°C for 6 h.

Rheological Study of LLCs and LLCs Nanocolloids
Rheological study was done with a cone plate geometry with a diameter of 45 mm, a gap of 0.08 mm, and an angle of 4° at temperature 25°C. The prepared LLCs and LLC samples of amount 0.5 mL were kept between the cone to plate geometry. The shear viscosity and shear stress were measured with the shear rate in the range of 1-100 s −1 , and the frequency sweep rheology was done in a 1-100 rad s −1 angular frequency range at 0.1% constant strain.

Characterizations
Structural information of γ-Fe 3 O 4 MNPs was studied through an X-ray diffractometer (Bruker D8 Advance) in the 2θ range 15°-70° with Cu-Kα radiations of wavelength λ = 1.5414 Å. Morphological analysis of the particles was done via field-emission scanning electron microscope (FESEM) (Ultra Plus, Carl-Zeiss). Particle size was computed from SEM images using ImageJ software. UV-Vis spectroscopy of iron-oxide nanoparticles was done by Lambda 365, PerkinElmer UV-Visible spectrophotometer in the wavelength range of 200-600 nm. LLC phases were examined by polarizing optical microscopy (POM) (Olympus BX53M) and thermooptical study was done by Linkam hot stage (LTS420E). Rheological study of LLCs and LLCs nanocolloids was carried out by Rheometer (Anton Paar, MCR-72) with a cone plate geometry.

Analysis of Magnetic Nanoparticles
The structural analysis of prepared particles is presented in supplementary information (SI) Figure S1 [24] The FESEM image for γ-Fe 3 O 4 MNPs is presented in Figure S1(b) in the SI.
Well-defined spherical particles with uniform distribution are seen in the SEM image. Particle size is calculated using image J software. The histogram pattern of γ-Fe 3 O 4 MNPs with an average particle size of ~45 nm is shown in Figure S1(c) in the SI.
The absorption spectra of γ-Fe 3 O 4 MNPs taken in an aqueous solution are presented in Figure S2 in the SI. Two absorption peaks are observed at 281 nm and 372 nm. The observed absorbance in γ-Fe 3 O 4 MNPs is matched with the literature. [25,26] The energy bandgap for MNPs was determined using the Tauc equation given by Equation (1) [27] : where α represents the absorption coefficient, h is Planck's constant, ν is the frequency of photons, K is a constant, E g is the energy bandgap, and n is an integer number. The bandgap for MNPs is computed by extending the linear plot of αhv ð Þ 2 versus hv. The calculated bandgap E g of MNPs is ~2.30 eV, which is in line with the literature on the bandgap of γ-Fe 3 O 4 MNPs. [25,28]

Structural Analysis
Homogenous dispersion and stabilization of the nanoparticle-doped systems is a challenging task. A visual confirmation of the MNPs dispersion for the studied systems is presented in Figure 2 Hence, the dense LLCs ordering of tri-block copolymer systems decreases the dispersion ability of MNPs. A possible mechanism for possible interactions is given in the subsequent section.
POM textures of pure LLCs (P1, P2, and P3) and γ-Fe 3 O 4 /LLCs nanocolloids (P1-1, P1-2, P2-1, P2-2, P3-1, and P3-2) are presented in Figure 3. It could be higher viscosity of these systems. Due to the lack of LLC phases and poor dispersibility, these systems are not studied further. Considering these results, it can be said that the doping of γ-Fe 3 O 4 MNPs preserves the LLCs ordering in Pluronic F127/DMF-based systems that could be the result of noncovalent interactions between solvent, solute, and γ-Fe 3 O 4 MNPs. Intermolecular interaction between block copolymer and solvent plays an important role in the formation of pure LLCs lamellar phase. Lamellar phases in pure LLCs systems are formed by the intermolecular interaction of hydrophilic PEO blocks with polar solvent and the solvophobic interaction with PPO block. [6] On the other hand, in LLCs/γ-Fe 3 O 4 MNPs Nanocolloids systems, intermolecular interaction between block copolymer and γ-Fe 3 O 4 MNPs and γ-Fe 3 O 4 MNPs to solvent play a vital role.
A proposed mechanism for the formation of LLCs phases and dispersion of MNPs is given in Figure 4. The formation of pure LLCs phases is governed by the electrostatic interaction between the hydrogen atom of the hydroxyl group of Pluronic F127 and the oxygen atom of the DMF and the interaction between the oxygen atom of Pluronic F127 with the hydrogen atom of DMF. Dispersed γ-Fe 3 O 4 MNPs in Pluronic F127/DMF solution may interact in two ways, i.e., oxygen atom of MNPs with the hydrogen atom of DMF or Fe atom of γ-Fe 3 O 4 interacts with the oxygen functionality of the Pluronic F127. Interaction of the Pluronic with aqueous and non-aqueous solvents is reported in the literature as well. [29] At a lower concentration of γ-Fe3O4 MNPs in LLC systems, particles could be aligned with the plane of lamellar layers, while at a higher concentration of γ-Fe3O4 MNPs (0.02 wt%) in 10:90 wt%:wt% systems undergo a phase transition into nematic lyotropic with effect of particles doping. [30] However, lamellar to nematic transition at 0.02 wt% γ-Fe 3 O 4 MNPs in 10:90 wt%:wt% Pluronic F127/DMF concentration could be the result of a decrease in the ordering and For other reasons, the fluidity of LLCs phases is also responsible for the lamellar phase stability and phase transition. At lower Pluronic F127 concentration in γ-Fe 3 O 4 /LLCs, nanocolloids exhibits more fluidity, whereas, γ-Fe 3 O 4 MNPs can flow freely at the interface. In P3-2 systems, surfactant concentration is higher, whereas the sample is more viscous as compared to the other LLCs systems. The flowability of γ-Fe 3 O 4 MNPs is restricted in LLCs and starts to break the lamellar stacks that gives rise to more defects.
The transition from shear-thinning to Newtonian in pure LLCs and γ-Fe 3 O 4 /LLCs nanocolloids depends on the microstructural changes due to the deformation of layered structure under shear-induced conditions. In LLCs and γ-Fe 3 O 4 /LLCs nanocolloids, lamellar layers are oriented parallel to the vorticity gradient and contain many dislocations and defects at the lower shear rate. [32,33] With further increase in shear rate, lamellar phases start to break with few defects and shift parallelly to the velocity gradient direction. [34] At higher shear, the lamellar stacks of LLCs aligned perpendicular to the velocity gradient and shear thinning behavior transit into the Newtonian region without any defects. On the other hand, the transition from shear-thinning to shear thickening in P2-2 samples could be due to the deformation of planar lamellar structure through the incorporation of γ-Fe 3 O 4 MNPs. [32,34] [35,36] Shear viscosity in all LLCs and γ-Fe 3 O 4 /LLCs nanocolloids systems depend on shear rate. Hence, these systems obtain the cross-fluid behavior under shear. The fluid behavior of LLCs and γ-Fe 3 O 4 /LLCs nanocolloids is examined by the Cross model by using equation [33] where η, η o , η 1 , C, _ γ, and m are the effective shear viscosity, zero-shear viscosity, viscosity at a higher shear rate, flow constant, shear rate, and power-law exponent, respectively. The computed parameters of zero shear viscosity and power law exponent from the equation of cross-model are given in Table 1.
The zero-shear viscosity of pure LLCs and 0.01% γ-Fe 3 O 4 /LLCs nanocolloid systems increased with the rise in the concentration of surfactant in DMF solution. On the other hand, zero-shear viscosity for 0.02 wt% γ-Fe 3 O 4 MNPs doped LLCs, first increased with increase in surfactant concentration and then dramatically decreased for P3-2 γ-Fe 3 O 4 /LLCs nanocolloids. Such a decrease in the zero-shear viscosity may be attributed to the lamellar to nematic phase transition at this concentration. Such dependence of viscosity on LLC phase transitions has already been reported for cesium pentadecafluorooctanoate/water, lecithin/C 12 E 5 /water, and H 3 Sb 3 P 2 O 14 nanosheets/aqueous systems. [37][38][39] The rate constant m is a dimensionless quantity that is used to explain the degree of dependency of viscosity on the shear rate in the shear thinning region. A zero value of m indicates the Newtonian behavior, while a higher value of m infers a more shear thinning fluid behavior. In the presented results, pure LLC systems exhibit th higher shear thinning fluid behavior and follow the order P1 > P3 ˃ P2. It means, LLCs lamellar phases show stable shear thinning behavior with an increase in surfactant concentration. On the other hand, γ-Fe 3 O 4 /LLCs nanocolloids of 0.01 wt% doped γ-Fe 3 O 4 MNPs show trends as P1-1 ˃ P3-1 ˃ P2-1. Interestingly, for higher doping of γ-Fe 3 O 4 MNPs (0.02 wt%) at 10:90 wt%:wt% LLC system, the value of the rate constant is decreased in the sequence P3-2 ˃ P1-2 ˃ P2-2, while the value of m is increased by 23.78%  for P3-2 system due to the lamellar to the nematic transition of LLC phases. In addition, the magnitude of the critical shear stress (τ c ) for LLCs and γ-Fe 3 O 4 /LLCs nanocolloids is computed through the reciprocal of cross time constant (1/C), where C is a dimensionless quantity. The value of the critical shear rate is given in Table 1.
The magnitude of the critical stress for pure and 0.01 wt% γ-Fe 3 O 4 MNPs doped LLCs increased with increase in surfactant concentration, while the critical stress for 0.02 wt% γ-Fe 3 O 4 MNPs doped LLCs first decreased with increase in concentration and then shows the maximum value for P3-2 γ-Fe 3 O 4 /LLCs nanocolloid system. In the studied systems, below the critical stress pure and doped systems are deformed elastically, while, above the critical stress all the systems behave like a fluid. Hence, pure and doped LLC samples have yield stress. The plot between shear stress and shear rate confirmed the yield stress (Fig. 7). The experimental and computed magnitude of yield stress from the plot for pure LLCs P1, P2, and P3 are 1.39 Pa, 2.39 Pa, and 2.46 Pa, respectively, while, the value of yield stress for γ-Fe 3 O 4 /LLCs nanocolloids P1-1, P2-1, P3-1, P1-2, P2-2, and P3-2 are 1.61 Pa, 2.79 Pa, 3.94 Pa, 1.57 Pa, 2.84 Pa, and 2.50 Pa, respectively. The incorporation of γ-Fe 3 O 4 MNPs may enhance the mechanical properties of LLC phases. These data confirm the pseudoplastic behavior of lamellar phases with strong alignment under shear. [40,41] These types of nonionic surfactant-based lamellar phases are good for cosmetic, dermatologic, and ophthalmic applications; coating applications; pharmaceutical industries, and biomedical.

Dynamic Rheology
The frequency sweep rheology of LLCs and γ-Fe 3 O 4 /LLCs nanocolloids is presented in Figure 8. The storage (G′) and loss (G″) modulus as a function of angular frequency (1-100 rad·s −1 ) demonstrates purely elastic, viscous, and viscoelastic behaviors when G′ > G″, G″ > G′, and G′ = G″, respectively. [42] In Figure 8 In addition, pure LLCs and γ-Fe 3 O 4 /LLCs nanocolloid systems except P2-2 system show a constant magnitude of G′ and G″ till angular frequency ˂10 rad·s −1 , while above 10 rad·s −1 , G′ and G″ show frequency-dependent behavior as the magnitude of G′ and G″ increased linearly with the increase of angular frequency. These results are following the same behavior like polystyrene-block-polyethylene/propylene, polystyrene-block-polyethylene/butylene-blockpolystyrene, Pluronic P84, Pluronic L121, poly (2-methyl-2-oxazoline), poly(2-iso-butyl-2-oxazoline) diblock and triblock copolymeric systems. [43][44][45] In the case of P2-2 and P3-2 γ-Fe 3 O 4 /LLCs nanocolloids, the magnitude of G′ and G″ shows dependence on frequency in the studied angular frequency range. In other observations, the magnitude of storage modulus is reduced with increase in the surfactant concentration in DMF for pure LLCs, while the magnitude of the storage modulus is improved by the doping of γ-Fe 3 O 4 MNPs in LLCs phases except P3-2 systems where nematic phase transition is seen. Hence, higher doping of γ-Fe 3 O 4 MNPs in LLCs results from the high storage capacity. Similar trend/behavior of frequency sweep rheology is seen in the LLCs lamellar phases derived from the nonionic surfactant. For instance, an increase in the elastic modulus was reported by Gonclaves et al. in Pluronic F127 aqueous hydrogel. [20] Boucenne et al. reported on the solid viscoelastic behavior of laponite nanoparticles embedded in Pluronic block copolymer micelle solution. [46] Azubuike et al. discussed the improvement in elastic strength by the incorporation of multi-walled carbon nanotubes in styrene-ethylene /butadiene-styrene and styrene-ethylene/propylene block copolymer. [47] Xie et al. studied the effect of the pH on the dependency of storage and loss modulus on angular frequency for highly elastic C18ADPA/NaOH/water-based micellar system. [48] Variation of complex modulus as a function of angular frequency is shown in Figure 9. It is observed that in the case of pure and 0.01 wt% doped γ-Fe 3 O 4 /LLCs nanocolloids the complex modulus is almost independent of the angular frequency (<10 rad·s −1 ). A frequency dependent plateau of the complex modulus for the 0.02 wt% γ-Fe 3 O 4 MNPs doped LLCs phases is observed. After a critical frequency range, all systems exhibit a frequency-dependent variation of the complex modulus. This behavior is further fitted by using the power-law (3). [49] G � ¼ Aω n where, G*, ω, A, and n are the complex moduli, angular frequency, constant (intercept), and power-law exponent, respectively. The computed values of the fitted parameters are given in Figure 9(d).
The gel and liquid-like behavior of any system is confirmed by the value of the power-law index. If the value of n = 0, systems behave like a gel that is independent of the frequency, while if the value of n is greater than zero, the system exhibits elastic behavior. [49] It is noticed that the values of power-law exponent for LLCs decreased with increase in Pluronic F127 concentration in DMF as well as γ-Fe 3 O 4 MNPs doping concentration in LLCs medium. It infers that the systems follow the transition from elastic/solid viscoelastic to liquid/liquid viscoelastic behavior. Such transitions in LLCs and γ-Fe 3 O 4 /LLCs nanocolloids may be obtained by the reduction in the amount of solvent in the LLC systems. On the other hand, the value of exponent n dramatically increases for P3-2 γ-Fe 3 O 4 /LLCs nanocolloids. This could be due to the phase transition in these systems. The lyotropic nematic phase shows a highly elastic mesophase with frequency. In addition, the magnitude of complex modulus at constant angular frequency 1 rad·s −1 decreases with increase in amphiphile concentration in DMF, while, after the doping of γ-Fe 3 O 4 MNPs the magnitude of complex modulus increases with increase in the amount of Pluronic F127. It means that nanocolloid systems have more storage applicability. The opposite behavior is seen for nanocolloids showing lamellar and nematic phase transition. The high elastic modulus and loss modulus of the nematic phase result in a low complex modulus. Such a drop in the magnitude of the complex modulus depicts the flexibility of mesophases. [50] It could be the reason for the behavior seen at 0.02 wt% γ-Fe 3 O 4 nanoparticle doped 10:90 wt%: wt% LLCs.
From the above results, it can be concluded that rheology may help to explore the application of γ-Fe 3 O 4 /LLCs nanocolloids in the fields of pharmaceutical, cosmetic, dental care, biomedical, and lubrication applications. Shear thinning behavior and improved yield stress and elastic to viscoelastic transitions in nanocolloids make them very interesting systems for the field of cosmetics and biomedical science. Viscoelastic pure and LLC nanocolloids presented in this study can be used in drug delivery and lubrication industries as well. [51] Conclusions Spherical γ-Fe 3 O 4 MNPs are successfully prepared using domestic microwave and further utilized to prepare the γ-Fe 3 O 4 /LLCs nanocolloids. γ-Fe 3 O 4 in LLCs phase does not perturb the lamellar ordering of doped systems except 0.02 wt% doping in 10:90 wt%:wt% in LLCs, where system transits from lamellar to nematic phase. This transition may be attributed to the increased defects in lamellar layers and the dominance of particle-particle interaction between the γ-Fe 3 O 4 MNPs. Pure and γ-Fe 3 O 4 MNPs doped LLCs nanocolloids exhibit shear-thinning fluid behavior. An increase in concentration in pure LLCs and LLC nanocolloids gives rise to zero-shear viscosity of the lamellar phase while the zero-shear viscosity dropped for lyotropic nematic phases. Complex moduli analysis of lamellar and nematic phases in the higher range of frequency exhibits strong elastic strength. The strong yield makes nanocolloids highly elastic, stable, and frequency dependent in nature that is good for lubrication applications. This study highlights the importance of the rheological parameters for future bio-medical applications.

Disclosure Statement
No potential conflict of interest was reported by the author(s).

Funding
This work has been financially supported by the Science and Engineering Research Board (SERB), India [Project Number: EMR/2016/005591].