Impact of graphene oxide on the magnetorheological behaviour of BaFe12O19 nanoparticles filled polyacrylamide hydrogel

Polymer 97 (2016) 258e272

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Impact of graphene oxide on the magnetorheological behaviour of BaFe12O19 nanoparticles filled polyacrylamide hydrogel Bablu Mordina a, b, Rajesh Kumar Tiwari a, Dipak Kumar Setua a, *, Ashutosh Sharma b, ** a b

Defence Materials and Stores Research and Development Establishment, Kanpur 208013, India Department of Chemical Engineering & Center of Nanoscience, Indian Institute of Technology Kanpur, Kanpur 208016, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 December 2015 Received in revised form 4 May 2016 Accepted 7 May 2016 Available online 11 May 2016

Development of magneto-sensitive smart hydrogels based on crosslinked polyacrylamide (PA) mixed with either nanoparticles of barium ferrite (BaM) or combinations of both BaM and graphene oxide (GO), as nanosheets, have been carried out. Spherical BaM particles with an average crystallite size of 46.63 nm were synthesized using an ultrasonic assisted novel coprecipitation method. Thin films of isotropic PA hydrogel nanocomposites were obtained by solution casting of in-situ polymerized mixture of acrylamide, N,N methylene bisacrylamide monomer (crosslinker) and a redox initiator and the fillers, as above. Variety of characterization techniques viz., XRD, for determination of crystal structure of nanofillers; X-ray photoelectron spectroscopy, for determination of chemical composition and oxidation state of elements in BaM; FTIR, for interaction of BaM and GO with polymer matrix; DSC, for determination of glass transition temperature and FESEM and TEM, for correlation of morphological features with the magnetorheological (MR) properties were used. MR studies were conducted in a parallel plate rheometer and the composites were found to exhibit both negative and positive MR effects depending on concentration and dispersion of the fillers added to the polymer. High surface energy and permanent magnetization of the nanoparticles lead to formation of large clusters interconnected through bridges. The positive MR effect at very low concentration of BaM is attributed to the localized hardening of the polymer matrix by the heterogeneously dispersed BaM clusters. Addition of GO along with BaM improves further the dispersion of BaM nanoparticles but the negative MR effect still holds up to 10 wt% loading of BaM. However, increase in BaM loading to 20 wt% in presence of GO leads to a positive MR effect. This is due to higher concentration of BaM which formed a continuous network of nanoparticles and the shear stress could not break the connectivity between the nanoparticles resulting in a positive MR effect. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Magnetoresponsive hydrogel Barium ferrite nanoparticle Graphene oxide

1. Introduction Magnetorheological (MR) gels are cross-linked, either chemically or physically, swollen 3D polymer network consisting of embedded magnetic particles [1e3]. They find emerging usages in applications related to active vibration control and damping devices, reversible switches, artificial muscles, soft actuators, clutches, etc. [4e8]. These gels contain different types of microparticles viz., carbonyl iron, Fe3O4, barium ferrite, etc. filled in a soft polymer matrix e.g., carrageenan, polyvinyl alcohol and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.K. Setua), [email protected] (A. Sharma). http://dx.doi.org/10.1016/j.polymer.2016.05.026 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

polyurethane [1,5,9e13]. Generally a substantial increase of modulus is observed by addition of carbonyl iron, but gels containing Fe3O4 microparticles show reduction, although small, of storage modulus (~104 Pa) in case of carrageenan polymer [5,9,11]. However, significant reduction of both storage and complex moduli, of the order of ~107 Pa, has been reported in case of magnetic gels containing barium ferrite microparticles [1,5,10]. Barium ferrite particles possess highly aggregative permanent magnetic behavior and their mean diameter of 1.84 mm in dry stage is increased to 20.8 mm in polymer hydrogel and exist in randomly distributed and physically connected clusters. Small stress and pumping rheological pressure could easily break connectivity of these physical contacts and result in significant reduction of storage modulus by temporal destruction of physical contacts between the agglomerates as well as magnetostriction (permanent demolition

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of particles network) [5]. Although vast literature exists on polymer gel containing magnetic microparticles but those with magnetic nanoparticles are scanty. Zrinyi et al. were the first to study the deformation and shape transition behavior of Fe3O4 nanoparticles filled magnetic hydrogel however they did not study the magnetorheological effect [14,15]. Nanoparticles can experience higher magnetic field induced dipole magnetic moment due to formation of strong three dimensional networks of nanoparticles and as a result induce significant change in the storage modulus at low concentration. Recently we have reported on the magnetorheological effect in case of silicone elastomer containing 5e20 wt% of nanoparticles or nanofibers which show modulus changes comparable to 60e80 wt % loading of microfillers [16,17]. Polyacylamide (PA) is a well-known polymer matrix for gel applications with variable physical and mechanical properties. Depending upon the degree of crosslinking, they can be tuned from flow-able liquid to semi solid (gel like) or to very stiff elastomeric network. In this study we, for the first time, have investigated the magnetorheological properties of BaFe12O19 (BaM) nanoparticles filled PA hydrogel and accomplished both negative and positive MR effect depending on the state of dispersion and concentration of magnetic particles, either individually or in a hybrid in the gel matrix. BaM has been used in the form of nanoparticles has characteristics of higher permanent magnetization as well as high surface energy arises from their nanosizes. Graphene and graphene oxide (GO) have sheet like structures and their use in combination with BaM at very low concentration is expected to improve the dispersion of BaM in the gel by occupying interspaces between the nanoparticles and polymer due to phase emulsification. Although the effect of different carbon based materials (such as carbon black, carbon nanotube and graphite powder, etc.) on the magnetorheological properties of the magnetorheological elastomers were investigated by several researchers [18e20]. However, to the best of our knowledge the effect of GO on magnetorheological properties of the hydrogel has not been explored in the literature to date. BaM nanoparticles were synthesized by ultrasonic assisted coprecipitation technique by a novel synthesis method. GO was produced by modified Hummers method. The polymer gel samples in thin films with varying filler concentration of magnetic particle content were fabricated by solution casting technique. Several analytical techniques were used to characterize the nanoparticles e.g., X-ray diffraction (XRD), High resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscope (FESEM), and Vibrating sample magnetometer (VSM). The MR properties of the hydrogels were determined in a parallel plate rheometer under magnetic field. The observed MR properties of the nanocomposites were correlated with the morphological features obtained under microscopic techniques and magnetization values determined by VSM. 2. Experimental 2.1. Materials Ammonium oxalate was procured from British Drug Houses (India) Private, Ltd. Ferrous sulfate was obtained from E. Merck. Barium carbonate was purchased from Sarabhai M. Chemicals Pvt. Ltd, India. Oxidized graphite flakes of size 45 mm and acetone were procured from Alfa Aesar Co., USA. Concentrated HCl was purchased from Samir Tech-Chem Pvt. Ltd., India. Potassium permanganate (KMnO4), concentrated H2SO4 and sodium nitrate (NaNO3) were obtained from Fisher scientific, India. N, N methylene bisacrylamide, acrylamide, potassium persulphate (K2S2O8) and N, N, N,

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N tetramethylenediamine (TEMED) were procured from Loba Chemie, India. 30% H2O2 was supplied by Ranbaxy Laboratories Ltd., India.

2.2. Synthesis of barium ferrite nanoparticles and graphene oxide nanosheets In a 500 ml beaker 0.25 mole ferrous sulfate was dissolved in desired amount of ammonium oxalate solution in water under continuous stirring by using a magnetic stirrer and ultrasonicator (GEX 750, Power 750 Watt, Cole-Parmer India). Ferrous oxalate (FeC2O4) thus precipitated was dried under vacuum for 4 h at 100
C. Subsequently a measured quantity of FeC2O4 and barium carbonate were dispersed in acetone by using a high speed disperser (T25 D S22, IKA, Germany) and dried in a Petri dish to obtain a homogeneous mixture. This mixture was then subjected to two step heating treatment. During first calcination at 500
C for 4 h, FeC2O4 is decomposed to form Fe3O4 nanoparticles with simultaneous generation of CO and CO2 [21]. In second step, the sample is fired at 1000
C for 6 h, where BaCO3 reacts with Fe3O4 to form BaFe2O4, a-Fe2O3 and CO2 which by further reaction produce BaFe12O19 nano particles [22]. The chemical reactions involved in the different synthesis steps of BaM are represented in the following reaction scheme. FeSO4$7H2O þ (COONH4)2 / FeC2O4 þ (NH4)2SO4 þ 7H2O 3FeC2O4 / Fe3O4 þ 4CO[ þ 2CO2[ BaCO3 þ Fe3O4 / BaFe2O4 þ a-Fe2O3 þ CO2[ BaFe2O4 þ 5a-Fe2O3 / BaFe12O19 Graphene oxide (GO) nanosheets were synthesized from oxidized graphite flakes by modified Hummers method according to the procedure described elsewhere [23].

2.3. Fabrication of BaM -PA and BaM -GO-PA MR-hydrogel MR-hydrogels with varying amount of magnetic nanoparticles, 5, 10 and 20 wt% of the total weight of acrylamide, were prepared by solution casting technique. For 10 and 20 wt% of BaM the MR-gel with 1 wt% graphene oxides were also prepared to investigate the influence of hybrid fillers on MR properties. Measured quantity of GO and BaM were dispersed in 20 ml deionized water by using ultrasonicator and a weighed amount of acrylamide and N, N methylene bisacrylamide monomers were added. Combination of K2S2O8 and TEMED (redox initiator) were used to polymerize the monomers. When the viscosity of the mixture started to rise it was immediately transferred onto the Petri dish to get the cured gel. Table 1 gives the formulation of the MR-gels.

Table 1 Formulation of the MR-gels.a Name of gel

Gel 0

Gel 1

Gel 1-1GO

Gel 2

Gel 2-1GO

BaFe12O19 NPs (g) GO (mg)

0.12 e

0.24 e

0.24 24

0.48 e

0.48 24

a Common ingredients for all the MR-gels are acrylamide (2.4 g), N, N methylene bisacrylamide (24 mg), K2S2O8 (0.12 g) and TEMED (32 ml).

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3. Characterization techniques 3.1. Structural and morphological analysis Structural characterization for BaFe12O19 and GO were done by XRD in X’Pert PRO, PAN-alytical, Netherland. Morphology and particle size of BaM nanoparticles were investigated in TEM, FEI Tecnai 20UT, USA. The samples were prepared by dispersing the nanoparticles in conducting epoxy and then ion milling by PIPS (Gatan) at 2.5 kV up to total time period of 30 min (exposure for 5 min with interval of 5 min). Morphology of GO, selected area electron diffraction (SAED) pattern of both GO and BaM and high resolution transmission electron microscope (HRTEM) images of BaM nanoparticles were obtained from FEI Tecnai G2 12 Twin TEM, USA. FESEM analysis was carried out in FESEM instrument (Quanta 200, Zeiss, Germany). MR gels were investigated in inverted optical microscope, Zeiss, Germany to acquire insight of the magnetic particle distribution within the polymer matrix as well as to explore the influence of addition of GO on the dispersion of BaM. 3.2. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy study of the BaM nanoparticles was carried out in XPS instrument (Model-PHI 5000 Versa Probe II), FEI Inc. to investigate the chemical composition and also to acquire information about oxidation states of different constitutive elements from their binding energy. Full range XPS scan (0e1100 eV) was carried out at room temperature using C 1s peak for calibration of binding energy. 3.3. Fourier transform infrared spectroscopy Fourier Transform infrared (FTIR) spectroscopy of GO was performed in Perkin Elmer Spectrum 100 FTIR instrument to investigate the different functional groups. Attenuated total reflectance (ATR) - FTIR spectra of thin films of hydrogels were obtained in FTIR spectrophotometer (Tensor 27, Bruker, Germany) in the wavenumber range of 400e4000 cm1. 3.4. Particle size determination of grapheme oxide by dynamic light scattering Particle size analysis of GO was performed in Delsa Nano, Beckman, Coulter particle size analyzer by dispersing the GO in water medium. 3.5. Magnetic and magnetorheological analysis Magnetic properties of BaM nanoparticles were investigated at ambient temperature using vibrating sample magnetometer, Model no.-3472-70, GMW Magnet System, USA. Temperature dependent magnetic properties of the nanoparticles were evaluated in Magnetic Property Measurement System (MPMS), Quantum Design, USA. The field cooled curve (FC) and zero field cooled curve (ZFC) were recorded in the temperature range 5e350 K under 100 Oe applied magnetic field. Heating rate during the experiment was fixed at 5 K/min. Magnetorheology of circular disc specimens were carried out in Physica MCR 301 (Parallel plate rheometer, Anton Paar, Germany). MRD 180 device was utilized to perform the rheological investigation under variable magnetic field. Both steady-state and dynamic mechanical properties were examined by shearing the specimen with a circular plate of diameter 20 mm. Direction of magnetic field was perpendicular to the plane of the test specimen. Stress -strain characteristics were measured in the strain range

between 106and 0.5% under variable magnetic fields of 0.386, 0.761, 1.046 and 1.102T, respectively. Dependence of storage modulus on the strain amplitude was determined by varying the strain value from 106 to 102% at fixed frequency of 1 Hz under these above variable magnetic fields. Similarly the effect of frequency on the storage modulus was measured at constant strain of 102% in the frequency range of 1e100 Hz. The change in storage modulus of the gel specimens with respect to magnetic field was examined at fixed strain and frequency of 102% and 1 Hz, respectively in the external magnetic field range between 0 and 1.102 T. 3.6. Thermal analysis Thermo-physical changes of the hydrogels were investigated in Differential scanning calorimeter (DSC) (Model no. Q 200, TA Instruments Co. Ltd., USA) in the temperature range between 30 and 350
C under N2 atmosphere to determine the glass transition temperature (Tg). Heating rate during the experiment was fixed at 10
C/min. Thermogravimetric analysis was performed in the temperature range between 30 and 800
C by heating the samples at a rate of 10
C/min. under N2 atmosphere using a thermogravimetric analyzer (SDT Q600 TA Instruments, USA). In order to find out the organic (polymer) and inorganic (filler) content of the hydrogel gravimetric analysis was carried out. Initially 5 g hydrogel samples were heated in oven at 100
C for 2 h followed by heating of the dried samples up to 800
C at a heating rate of 10
C/min. in a Muffle furnace and then cooled to room temperature. Initial heating at 100
C gives the polymer and filler content of the hydrogel after removal of water and finally heating at 800
C gives only the filler content of the hydrogel through decomposition of the polymer. For weighing purpose electronic balance (Mettler Toledo, USA) with accuracy ±0.05 mg was used. Hence from this polymer and filler content of the hydrogels can be easily calculated. 4. Results and discussion 4.1. X-ray diffraction XRD spectra of BaM and GO are shown in Fig. 1(a) and (b). Crystalline peaks at 2q values 30.32, 30.80, 31.30, 32.16, 34.13, 37.08, 40.37, 42.42, 55.05, 56.51, 63.09 and 72.61 can be accounted for diffraction at (110), (008), (112), (107), (114), (203), (205), (206), (217), (304), (220) and (317) crystal planes, respectively. The analysis of XRD spectra (matched with JCPDS file no. 43-0002) confirms the occurrence of M-type hexaferrite phase with hexagonal crystal structure in the nanoparticles [24]. Evaluating the Equation (1), given below, for diffraction at (107) and (114) planes, lattice parameters (‘a’ and ‘c’) appear to be 5.8999 and 23.01 Å, respectively [25,26].

. . .  3a2 þ l2 c2 1 d2hkl ¼ 4 h2 þ hk þ k2

(1)

Where, dhkl is the interplanar distance and h, k, l are the Miller indices. Average crystallite size (D) of the BaM nanoparticles can be calculated from the highest intensity peak using Scherrer equation D ¼ kl/bcosq, where k is Scherrer constant (0.9), b is the full angular line width in radians at half maximum intensity, l is wavelength of X-ray radiation in nm, q is the diffraction angle in degree. Average crystallite size evaluated from the Sherrer equation arises to be 46.63 nm. GO exhibits a diffraction peak at 10.26
which can be attributed to diffraction at (001) plane. The distance between the GO layers evaluated considering the diffraction at (001) plane arises

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Fig. 1. XRD of (a) BaFe12O19 nanoparticles, (b) GO.

to be 0.861 nm. 4.2. X-ray photoelectron spectroscopy Fig. 2(a) represents the survey spectra of BaM. It is observed that survey spectra confirm the presence of all the constituent elements i.e. Ba, Fe and O of BaM. XPS peak at binding energy of 284.0 eV can be designated to the C 1s and is used for calibration of binding energy and spectrum. The Ba 4d and Ba 4p spectrum are observed at 87.88 and 175.9 eV due to presence of different barium orbitals in BaM whereas the signal for Fe 3p arises at 54.70 eV [27]. Fig. 2(b) represents the deconvoluted spectra for Fe 2p done by utilizing XPS software with a mixture of Lorentzian and Gaussian function and Shirley function as background. The spectra can be resolved into five main peaks and one pre peak. The peak at 710.8 and 717.7 eV belong to main and satellite peaks of Fe3þ 2p3/2 whereas the peak at 724.84 eV attributes to Fe3þ 2p1/2 of Fe2O3 [28,29]. These peaks clearly indicate the presence of Fe3þ. The XPS peak at 708.78 and

722.47 eV can be attributed to the Fe2þ 2p3/2 and Fe2þ 2p1/2 of FeO. Hence the iron is also present as Fe2þ [30,31]. Moreover, the oxidation state of Fe can be confirmed from the position of satellite peak relative to the main 2p3/2 peak. Fe will be in þ2 state if the satellite peak is at 6 eV higher than the 2p3/2 peak position and þ3 state in case of 8 eV [32]. In our case the satellite peak is 6.9 eV higher than main 2p3/2 peak. Therefore, it indicates the presence of both Fe3þ and Fe2þ. Narrow binding energy scan spectra of Ba 3d are exhibited in Fig. 2(c). The spectra can be resolved into two peaks, one at 778 and other at 793.36 eV and can be attributed to the Ba 3d5/2 and Ba 3d3/2 of barium in BaM [27]. Deconvoluted spectra of O1s are shown in Fig. 2(d). It consists of two peaks, one at around 528.23 eV and other at 530.55 eV which may be designated to the oxygen linked to BaO and O2 of BaM, respectively [33]. 4.3. Fourier transform infrared spectroscopy Fig. 3(a) demonstrates the FTIR spectra of synthesized GO. The

Fig. 2. (a) XPS survey spectra of BaM; Deconvoluted spectra of (b) Fe 2p, (c) Ba 3d, (d) O 1s.

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Fig. 3. (a) FT-IR spectra of GO (b) ATR-FTIR spectra of neat PA, gel 1, gel 1-1GO, gel 2 and gel 2-1GO.

peaks at 831, 1228, 3430 cm1 can be accounted for stretching vibration of epoxy group, CeO group and OeH group of carboxylic acid, respectively. Other two peaks at 1640 and 1734 cm1 can be designated to the C]C and C]O stretching vibrations of the carboxylic group. Fig. 3(b) represents the ATR-FTIR spectra of neat PA, gel 1, gel 11GO, gel 2 and gel 2-1GO. The peaks at 1607 and 1653 cm1 can be assigned to NeH bending and C]O stretching vibration of acrylamide [34]. Absorption peaks at 1456 and 2937 cm1 can be designated to bending and asymmetric stretching vibration of CeH. The peaks at 1100 and 1339 cm1 can be attributed to CeO stretching vibration of GO. Characteristics absorption at 438 and 591 cm1 can be assigned to vibration of octahedral and tetrahedral sites of BaM [35,36]. Disappearance of C]O stretching vibration (peak at 1734 cm1) of eCOOH group in GO depicts the chemical reaction of GO with the polymer matrix in case of hybrid hydrogels. 4.4. Particle size analysis GO is characterized by a broad particle size distribution with d10, d90 values of 353.8 nm and 182.489 mm, respectively. The average particle diameter (d50) found to be 76.021 mm (Refer Fig. S1 for particle size distribution histogram). 4.5. Morphological analysis Fig. 4(a) and (b) show the FESEM images of BaM and GO. The image exhibits that the nanoparticles of BaM are nearly spherical in

shape whereas GO has thin sheets appearance. Fig. 5(a) shows that BaM nanoparticles consist of both small and large nanoparticles of nearly spherical shape. GO nanosheet is visible with numerous creases on its surface under the TEM and is electron transparent [Fig. 5(b)]. The inset of Fig. 5(b) shows the selected area electron diffraction (SAED) pattern of GO. It reveals the existence of hexagonal lattice arrangement in GO with 6-fold periodic pattern. Moreover, regular hexagonal electron diffraction pattern also confirms the formation of exfoliated monolayer GO sheet with single crystal geometry. From the particle size distribution histogram [Fig. 5(c)] it can be observed that the size of the nanoparticles ranges from 20 to 65 nm with maximum population in the size range between 45 and 50 nm. The nanoparticles have average particle size of 45.92 nm with 10.88 nm standard deviation and a broad range of particle size distribution. However, the average crystallite size calculated from the Sherrer equation matches with the average particle size observed by TEM analysis. Fig. 5(d) represents the SAED pattern of BaM nanoparticles. It exhibits several discrete bright diffraction spots originating from the well-organized crystalline planes of the nanoparticles. Fig. 5(e) and (f) exhibit the high magnification HRTEM images of BaM at lattice resolution which demonstrate consistent crystal orientation in the nanoparticles. The crystalline planes with interplanar distances 2.62, 3.12 and 3.83 Å can be attributed to the (114), (106) and (104), respectively. The presence of these planes clearly confirms the formation of hexagonal BaM during the synthesis. Fig. 6(a), (b) and (d) show the inverted optical microscopic

Fig. 4. (a) FESEM image of BaM nanoparticles (b) GO nanosheets.

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Fig. 5. TEM images of (a) BaM, (b) GO; (c) particle size distribution of BaM; (d) Selected area electron diffraction pattern of BaM; HRTEM of BaM showing (e) (104) and (114) crystal planes, (f) (106) crystal plane.

images of gel 0, gel 1 and gel 2 which reveal that magnetic nanoparticles form clusters due to physical contact of several primary nanoparticles and are distributed within the polymer matrix as large agglomerates. In gel 1-1 GO the presence of GO helps in breaking-up of these large clusters into smaller size, refer Fig. 6(c). Fig. 6(e) shows at 20 wt% of BaM and 1 wt% of GO, in gel 2-1 GO, a continuous 3D network of embedded magnetic particles inside the PA matrix is generated. Incidentally the neat PA hydrogel is totally transparent as depicted in the optical micrograph Fig. 6(f).

4.6. Magnetic properties Fig. 7(a) exhibits the specific magnetization curve of BaM. The magnetic nanoparticles possess apparent saturation magnetization (Ms) of 50.23 emu/g in the ±15 kOe applied field region. A careful examination of the magnetization curve shows a rising trend of the

magnetic moment values and hence a higher applied field is required for attainment of full saturation magnetization of the magnetic dipoles. The nanoparticles have large hysteresis characteristics with hysteresis ferrimagnetism between 8.5 and þ8.5 kOe applied magnetic field. High remnant magnetization (Mr) and coercivity (Hc) values of 27.04 emu/g and 4153.48 Oe, respectively reveal that BaM is hard ferrimagnetic in nature. The ratio Mr/Ms for magnetic nanoparticles is found to be 0.538. Theoretically Mr/Ms must be within 0.5 for non-interacting magnetic particles having uniaxial single domain structure, as suggested by Stoner Wohlfarth model [37]. Therefore, the synthesized nanoparticles are consisting of multidomain and possess large magneto crystalline anisotropy. Fig. 7(b) depicts the temperature dependence of specific magnetization under constant applied magnetic field. The figure shows that both ZFC and FC curves follow almost similar trend and magnetic moment value gradually

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Fig. 6. Inverted microscope image of (a) gel 0, (b) gel 1 (c) gel 1-1GO (d) gel 2 (e) gel 2-1GO (f) Neat PA.

Fig. 7. (a) Moment versus applied field, b) Moment versus temperature plots of BaM.

decreases with increase in temperature. Moreover, it can be observed that FC curve is always above ZFC in the entire temperature range. This can be attributed to the participation of more magnetic spin to the magnetization process as domain wall unpinning process becomes easier in presence of constant applied magnetic field [38]. 4.7. Magnetorheological properties 4.7.1. Static test Fig. 8(a)e(f) show the shear stress versus strain behavior of gel 0, gel 1, gel 1-1GO, gel 2, gel 2-1GO, and neat PA hydrogel,

respectively. Fig. 9(a) and (b) demonstrate the shear modulus versus BaM loading plots of these hydrogels under variable magnetic field at 0.15% strain value. It is observed that initially shear modulus of gel 0, gel 1 and gel 2 is increased with increasing magnetic field values from 0 to 0.761 T, and at 1.046 T the gels achieve their maximum shear modulus. On the contrary, gel 1-1GO and gel 2-1GO exhibit reduced shear modulus in the magnetic field ranges 0e0.761 T. This can be attributed to the diamagnetic characteristics of gel 1-1GO and gel 2-1GO due to the presence of GO. BaM shows conglomeration in pure water and ensemble in the form of large particle of average size ~20 mm [5]. The nanoparticles retain their agglomerated state even within the gel matrix and exist

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Fig. 8. Shear stress versus strain curve of (a) gel 0, (b) gel 1, (c) gel 1-1GO, (d) gel 2, (e) gel 2-1GO and (f) neat PA hydrogel.

as an interconnected cluster by forming an ordered local network. Inverted microscopic images (see Fig. 6) of the magnetic gels also support this statement. This interconnected structure arises out of simple physical contact between the clusters of magnetic particles and not by specific forces of magnetic interaction [5]. Under the influence of small strain and low magnetic field the particles are repelled by the magnetic field resulting in a temporal demolition of the particles network (See Fig. 10). This is the example of manifestation of Payne effect [39]. Ultimately this destruction of the local structure leads to neat reduction of the shear modulus value. After attaining the highest shear modulus (caused by the complete magnetization of the magnetic particles at 1.046 T, any further increment in the external magnetic field (e.g., at 1.102 T) brings about sharp reduction in the storage modulus which perhaps due to permanent demolition of the particles network structure encouraged by the magnetostriction. Similar type of reduction in storage modulus was observed for BaFe12O19 microparticles filled polyurethane elastomer after magnetization due to permanent deformation (magnitude of several microns) in the direction perpendicular to magnetization direction [40]. Fig. 9(c) and

(d) represent the peak shear stress versus BaM nanoparticle loading plot for neat PA, gel 0, gel 1, gel 2 and gel 1-1GO, gel 2-1GO, respectively at 0.3158% stain value. It is observed that peak shear stress is achieved under 1.046 T magnetic field for all hydrogels except gel 1-1GO. The magnitude of the peak shear stress is observed to be increased linearly with increasing BaM loading. Incorporation of GO further enhances the magnitude of the peak shear stress as observed in gel 1-1GO and gel 2-1GO. Fig. 8(b), (d) and (f) exhibit that neat PA, gel 1 and gel 2 show nonlinear viscoelastic behavior after certain strain value (>0.25%). However, this behavior was not much prominent in cases of gel 1-1GO and gel 21GO containing GO. Incorporation of GO modified the matrix properties which is also reflected by the increase in zero field shear modulus of the hydrogels. However, BaM nanoparticles have the detrimental effect on the zero field shear modulus of the hydrogels in absence of GO. The presence of nanoparticles can also, to some extent, hinders the crosslinking reaction which ultimately leads to a lower crosslinking density of polymer and hence a low shear modulus value of the hydrogels.

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Fig. 9. Shear modulus versus BaFe12O19 NPs loading plot of (a) neat PA, gel 0, gel 1 and gel 2, (b) gel 1-1GO and gel 2-1GO under variable magnetic field; peak shear stress versus BaFe12O19 NPs loading plot of (c) neat PA, gel 0, gel 1 and gel 2, (d) gel 1-1GO and gel 2-1GO.

Fig. 10. Schematic representation of formation of physical contact and its demolition on application of magnetic field.

4.7.2. Dynamic testing 4.7.2.1. Strain sweep test. Strain dependence of the storage modulus is exhibited in Fig. 11(a)e(f) for gel 0, gel 1, gel 1-1GO, gel 2, gel 2-1GO, and neat PA hydrogel, respectively under the variable magnetic field. It is observed that gel 0 and gel 1 show low zero field storage modulus than the neat PA. This may be because of low crosslinking density of the polymer matrix originated from the

heterogeneous distribution of BaM within the gel at low particle loading. However, gel 2 shows higher zero field modulus than the gel 1 as well as neat PA. This can be attributed to the changes in particle distribution pattern from heterogeneous to homogeneous one with increase in the nanoparticle concentration which ultimately leads to the better connectivity between the embedded particles [36]. Hence, the nanoparticles exert high modulus by

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Fig. 11. Storage modulus versus strain plots of (a) gel 0, (b) gel 1, (c) gel 1-1GO, (d) gel 2, (e) gel 2-1GO and (f) neat PA hydrogel.

virtue of their interconnected structure supporting compared to the polymer alone under shear [40]. Sozeri et al. experimentally proved the transformation of heterogeneous filler distribution to homogeneous one with increase in BaFe12O19 NPs concentration by measuring the current density versus swelling ratio of PA hydrogel [36]. In presence of GO the initial zero field modulus of gel 1-1GO and gel 2-1GO is improved due to the additional restriction imparted by the GO nano sheets against the deformation. In general, all the hydrogels show tremendous reduction in storage modulus with increase in strain value. As discussed earlier this may be attributed to the destruction of local particle networks which are fragile to be broken by the application of small strain. Magnetostriction (i.e. magnetic field induced strain) as high as 103 was reported by Mitsumata et al. due to permanent demolition of BaFe12O19 particles network in carrageenan gel matrix [5,40]. This magnetostriction is also believed to be contributing in the giant reduction of storage modulus of the hydrogel. Nevertheless the ultimate modulus of gel 1-1GO and gel 2-1GO becomes almost similar at strain value >103 under experimentally applied magnetic field irrespective of the magnetic particle concentration in hydrogels. This demonstrates that enhanced nonlinear viscoelastic

behavior of hydrogels is originated from the temporal demolition of physical contact between the clusters of nanoparticles. Increasing the magnetic field renders the magnetic gels to become more rigid and progressively large strain is required to start the deformation process. The effect is more prominent for gel 1-1GO and gel 2-1GO than gel 1 and gel 2 as presence GO increases the initial modulus of these hydrogels. Fig. 12(a)e(f) depict the strain dependence of damping factor for gel 0, gel 1, gel 1-1GO, gel 2, gel 2-1GO, and neat PA hydrogel, respectively. In general all the hydrogels (except gel 0 and gel 2) exhibit gradual reduction in damping factor with the rise of strain value and thereafter show almost linear behavior with the strain. Presence of GO in gel 1-1GO and gel 2-1GO reduces the damping factor of these hydrogels and the behavior of the curves is more linear compared to the hydrogels without GO beyond certain strain value. Increase in magnetic field from 0 to 1.046 T gradually increases the damping factors of the hydrogels (except gel 0) keeping the nature of the curves unaltered. However, beyond 1.046 T magnetic field damping factor of the hydrogels exhibits anomalous behavior and even decreases from its zero field value. The hydrogels without GO exhibited more nonlinear characteristics

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Fig. 12. Damping factor versus strain plots of (a) gel 0, (b) gel 1, (c) gel 1-1GO, (d) gel 2, (e) gel 2-1GO and (f) neat PA hydrogel.

compared to the hydrogel with GO. The damping factor in the low strain region (i.e. nonlinear region) is attributed to the particle network destruction energy and hence dependent on the number of physical contacts between the clusters and their brittleness. On the other hand damping factor in the linear viscoelastic region signifies the frictional energy and adhesive force between the magnetic particles [5]. 4.7.2.2. Frequency sweep test. Influence of frequency on the dynamic mechanical properties of the gel 0, gel 1, gel 1-1GO, gel 2, gel 2-1GO, and neat PA hydrogel under variable magnetic fields are shown in Fig. 13(a)e(f). It is observed that all the hydrogels show nonlinear viscoelastic response with the frequency. Storage modulus of the gel 1-1GO and neat PA hydrogel are observed to follow a gradual decreasing trend beyond 25 Hz frequency whereas for gel 0, gel 2 and gel 2-1GO a gradual increase in the storage modulus is observed. Gel 1 shows both linear and nonlinear viscoelastic behavior up to 0.761 T and as the magnetic field value is increased further to 1.046 T the response becomes almost linear. All the hydrogels show increased storage modulus compared to the zero field storage modulus for concomitant increment of the

external magnetic field except for storage modulus curves at 1.102 T in case of gel 1-1GO and gel 2. However, overall pattern of the curves remains unaltered. This increase in modulus can be attributed to the small length ordering or chain formation of the magnetic nanoparticles in the hydrogel under the magnetic field [41]. 4.7.2.3. Magnetic sweep test. Magnetic flux density sweep test is performed to investigate the change in storage modulus under continuously increasing magnetic field. Fig. 14(a) and (b) represent the corresponding curves for gel 0, gel 1 and gel 1-1GO, gel 2, gel 21GO, respectively. The gel 0, gel 1 and gel 2-1GO show positive MR effect. Moreover, magnitude of MR effect is tremendously high for gel 0 and gel 1. This may be because of heterogeneous distribution of BaM within gel 0 and gel 1 at low filler concentration as observed in Fig. 6(a) and (b) [40]. Significant particle clustering is observed in these hydrogels which collectively acts as big magnetic particle and induces increased dipole magnetic moment in the hydrogel under externally applied magnetic field and triggers localized hardening of the hydrogel. Moreover, heterogeneous particle distribution decreases the initial zero field modulus of the hydrogels as shown in Table 2 which also contributed to the increase in absolute and

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269

Fig. 13. Storage modulus versus frequency plots of (a) gel 0, (b) gel 1, (c) gel 1-1GO, (d) gel 2, (e) gel 2 and (f) neat PA hydrogel.

Fig. 14. Storage modulus versus magnetic flux density plots of (a) gel 0 and gel 1, (b) gel 1-1GO, gel 2, and gel 2-1GO.

relative MR effect. The relation between initial zero field storage modulus and absolute MR effect is depicted in the following equation,

DG ¼ GMAX  G0

(2)

Where, G0 is zero field storage modulus, DG is absolute MR effect

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Table 2 MR properties of hydrogels. Sample details

Value of G0 in MPa

Value of G

Gel Gel Gel Gel Gel

0.0148 0.038 5.846 5.642 5.557

5.125 4.366 5.528 5.49 5.706

0 1 1-1GO 2 2-1GO

Max

in MPa

and GMax is the maximum storage modulus obtained under external magnetic field. Similarly, relative MR effect (DGr) is also related to the zero field storage modulus as follows,

DGr ¼ fðGMAX  G0 Þ  100g=G0 ½%

(3)

From these above equations it is clear that lower the value of zero field storage modulus, higher is the magnitude of absolute and relative MR effect. For gel 2-1GO, although the final storage modulus achieved under the 1.102 T magnetic field is higher (i.e.5.706 MPa) than that of gel 0 and gel 1 (i.e. 5.125 and 4.366 MPa), homogeneous particle distribution {compare Fig. 6(a), (b) and (e)} and presence of GO increases the zero field storage modulus in case former. Hence gel 2-1GO exhibits lower absolute and relative MR effect. On the contrary, gel 1-1GO and gel 2 exhibit negative MR effect. If we look into the magnetic particle distribution of these hydrogels {See Fig. 6(c) and (d)} it is observed that in both hydrogels magnetic particles are embedded within the hydrogel forming local network of small clusters (not as much big like in gel 0 and gel 1). These clusters are physically connected by the nanoparticles in bridges. As observed earlier, when a magnetic field is applied under dynamic condition physical contacts between the clusters are destroyed which ultimately lead to the reduction in storage modulus. A strain of 102% (applied in our study) is sufficient enough to cause temporal destruction of the contacts between particle clusters. Hence the storage modulus under magnetic field becomes lower than the zero field storage modulus and thus these hydrogels show negative MR effect. Our experimental observation is further supported by the literature as strain value as low as order of 103 was reported to be sufficient to destroy the physical contacts between the BaFe12O19 clusters in case of carrageenan gel [5]. Moreover, it is noteworthy that with increasing BaM loading from 10 to 20 wt% in presence of GO, a transition from negative to positive MR effect is observed. This can be attributed to the fact that 20 wt% BaM and 1 wt% GO being the optimum combination of hybrid nanofillers, BaM could assume a more preferred form of continuous network of nanoparticles inside the polymer matrix. Hence the connectivity between the nanoparticles is not easily destroyed by the application of shear stress in presence of GO. Thus gel 2-1GO exhibits positive MR effect. However, in case of gel 11GO, although the presence of GO improves the dispersion of BaM but due to low BaM concentration nanoparticles could not form well designed continuous network of nanoparticles inside the polymer matrix. The negative MR effect of gel 1-1GO, likewise others, is due to nature of similar type of destruction of connectivity between the clusters under applied magnetic field. Moreover, increase in magnetic particle concentration increases the magnitude of ultimate storage modulus owing to enhanced dipole magnetic moment under the magnetic field.

Absolute MR effect (MPa) at 1.102 T

Relative MR effect (%) at 1.102 T

5.110 4.328 0.318 0.152 0.149

~34527 ~11389 ~5.44 ~2.69 ~2.68

interaction of BaM nanoparticles and GO with the polymer matrix. Fig. 15 represents the DSC curves for neat PA, gel 0, gel 1, gel 1-1GO, gel 2 and gel 2-1GO. PA homopolymer shows glass transition temperature (Tg) in the temperature range of 80e130
C depending on molecular weight [42]. Gel 2 shows first endothermic peak at 79.46
C for which the enthalpy change (DH) appears to be 6.184 J/g. This peak can be attributed to the Tg of the hydrogel. Tg is not observable in the other hydrogels. This may be because of overlapping of Tg and endothermic peak due to removal of water from the hydrogels. Neat PA hydrogel synthesized in this study exhibits sharp endothermic peak at 99.31
C due to removal of water. The enthalpy change (DH) for this endothermic peak is 1032 J/g. Incorporation of BaM nanoparticles into gel 1 and gel 2 shifted the endothermic peak to 108.61 and 132.62
C due to restriction in water removal in presence of BaM nanoparticles. The corresponding enthalpy change for these endothermic peaks appears to be 1116 and 132 J/g for gel 1 and gel 2, respectively. Higher the BaM loading in hydrogel greater is the resistance in water removal and thus gel 2 shows endothermic peaks at higher temperature compared to gel 1. In gel 1-1GO the endothermic peak is broadened due to presence of GO. This is because of slow water removal rate in presence of GO nanosheets and therefore gel 1-1GO exhibits

4.8. Thermal analysis DSC of hydrogels has been performed to investigate the

Fig. 15. DSC plots of neat PA, gel 0, gel 1, gel 1-1GO, gel 2 and gel 2-1GO.

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271

Fig. 16. (a) TGA curves, (b) DTG curves of hydrogels.

endothermic peak in the temperature range 105.62e130.51
C. The enthalpy change for this endothermic peak is 77.02 J/g. Epoxy and carboxylic functional groups present in GO act as the reaction point for the grafting to occur during radical chain transfer polymerization. Hence, grafting of polyacrylamide macromolecules takes place onto the GO nanosheets [43,44]. This further makes the removal of water more difficult due to increased density of the hydrogel. Endothermic peak at 182.26
C (DH ¼ 16.41 J/g) and in the temperature range 218e260
C may be attributed to the degradation of functional groups (eCOOH or eOH) of GO and polymer chain degradation. Like gel 1-1GO, broadening of endothermic peak is also observed in gel 2-1GO. It shows endothermic peak in the temperature range 70.89e132.54
C. Presence of the second endothermic peak at 232.60
C can be attributed to the polymer chain degradation. Fig. 16(a) represents the TGA curves whereas Fig. 16(b) indicates the derivative TGA plots of hydrogels. Figures show that all hydrogels exhibit three step weight loss process. First weight loss between 25 and 200
C is due to loss of intra and intermolecular water molecules. Second weight loss in the temperature range 200e325
C can be attributed to the loss of ammonia molecules from two amide groups (forming imine compound) and thermal decomposition of hydrophobic side chains [45]. Third weight loss occurs between 325 and 500
C due to decomposition of amide groups and polymer main chain. Beyond 500
C weight loss takes place through decomposition of polymeric backbone [46]. Table 3 represents the polymer and filler content of the hydrogels determined through gravimetric analysis of bulk samples in the Muffle furnace. The results show that polymer and filler content of the hydrogels are in well agreement with the actual weight taken during the preparation of the hydrogels. 5. Conclusions

Acknowledgement

Polyacrylamide and BaM based hydrogels with or without GO (dispersant) have been successfully prepared and characterized for their MR properties. BaM nanoparticles were synthesized via novel coprecipitation method. XPS analysis revealed the presence of all

Table 3 Polymer and filler content of hydrogels. Sample details

Polymer content (wt%)

Gel Gel Gel Gel Gel

10.67 10.61 10.60 10.50 10.49

0 1 1-1GO 2 2-1GO

constituent elements of BaM in the synthesized nanoparticles. Both þ3 and þ 2 oxidation state of Fe were confirmed by the narrow binding energy scan of Fe 2p peak. Specific magnetization curve of the nanoparticles calcined at 1000
C shows increasing trend in magnetization value and hence does not show saturation at ±15 kOe applied magnetic field. This could be the possible reason for low saturation magnetization of the nanoparticles. DSC analysis confirms the interaction of polymer and magnetic nanoparticles. Grafting of polymer molecules onto the GO nanosheets is evident from both FTIR spectra and DSC thermogram of the hydrogels. MR study revealed that barium ferrite filled hydrogel showed mixed MR effect (both negative and positive) depending on the dispersion and ratio of nanofillers inside the polymer matrix. Temporal destruction of physical contacts between the agglomerates of nanoparticles was responsible for negative MR effect in the hydrogels whereas heterogeneous dispersion of nanofillers at low filler concentration led to the positive MR effect. MR effect as high as ~34527% was achieved in 5 wt% BaM filled hydrogel because of localized hardening of polymer matrix owing to heterogeneous filler dispersion in absence of any dispersant. This high MR effect of the hydrogels conforms suitability of their engineering applications in soft actuator, switch, valve, etc. Further, the BaM nanoparticles at low concentration effectively led to higher percentage of MR effect in the hydrogels that could be achieved by addition of much higher concentration of microparticles and therefore much promising for generating low density magnetic hydrogel for futuristic smart applications. Besides, the addition of GO in association with BaM produced synergistic effect in increasing the extent of dispersion of magnetic nanofillers as well as enhancement of initial zero field storage modulus of the polymer matrix and led to conversion from negative to positive MR effect of the designed hydrogels.

± ± ± ± ±

0.15 0.12 0.14 0.12 0.10

The authors are grateful to DST support to the Nanoscience Center at Indian Institute of Technology Kanpur (IITK) and Prof. V. K. Jain and Mr. Sanjeev Kumar Verma of IITK for providing their instrumental facilities in carrying out magnetorheological study. Authors also would like to acknowledge Director DMSRDE for providing the laboratory facilities and Dr. Partha Ghosal, DMRL for performing TEM analysis of nanoparticles.

Filler content (wt%) 0.532 1.059 1.058 2.097 2.095

± ± ± ± ±

0.006 0.014 0.13 0.025 0.02

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.05.026.

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