Nanoclay (Montmorillonite clay surface modified with 15 - 35 wt% octadecylamine and 0.5 - 5 wt% aminopropyltriethoxysilane) was purchased from Sigma Aldrich, USA. Guar gum with molecular weight 4.22 × 10⁶, Ammonium persulphate (Extrapure), NN’-Methylene bis-acrylamide has been procured from SD Fine-Chem Limited, Mumbai. Methyl Violet (10 B, practical grade) was obtained from Himedia Laboratories Pvt. Ltd. Mumbai. All the other solvents and chemicals of analytical grade were purchased from SD Fine-Chem Limited, Mumbai.

1 g of guar gum is dissolved in 100 ml of distilled water. 6 ml of acrylic acid is added to the guar gum solution and stirred for 10 minutes. 0.09 g of ammonium persulphate (APS) initiator and 0.09 g of methylene bisacrylamide (MBA) cross linker was added to the mixture. Then it was subjected to microwave irradiation using locally fabricated Microwave reactor (Enerzi Microwave Systems, India) at different temperatures and varied reaction times. To carry out the reaction power of 800 W was supplied. The reaction mixture of acrylic acid grafted guar gum was cooled to room temperature. In order to remove unreacted acrylic acid and homopolymer, the mixture was neutralized using 1N NaOH till the pH reaches 7. This was followed by precipitation using acetone. The precipitate obtained was washed several times with acetone and kept for drying at 80˚C for 5 - 6 hours.

The procedure for synthesis of superabsorbent with nanoclay is similar to that of control sample with sonication. Varied quantities of nanoclay was added to guar gum solution and sonicated for 30 minutes. After sonication, acrylic acid, MBA and APS were added. Similar process of microwave irradiation was carried out and the dried composite of superabsorbent with nanoclay was obtained. Amount of nanoclay was varied from 0 to 5 wt%.

The reaction parameters i.e., monomer concentration, initiator dosage, cross-linker concentration, nanoclay con- centration, time of reaction and temperature of the reaction has been optimized to obtain the maximum grafting percentage. The effects of these reaction parameters on the percentage swelling were investigated.

0.1 ± 0.01 g of powdered superabsorbent was taken in a test tube and 10 ml of prepared buffer solution was added. For every interval of 10 minutes the solution was decanted using filter paper. The weight of swollen sample was noted. The procedure was repeated till 120 minutes. Equilibrium swelling of sample was calculated using Equation (1).

Equilibrium Swelling in (1)

where, W_s = weight of swollen sample (g);

W_d = weight of dried sample (g).

Dye adsorption was carried out by immersing the 0.05 ± 0.01 g of grafted guar gum into 20 ml of dye solution. All adsorption experiments were examined through a batch method. To study the adsorption kinetics, at specified time intervals, the amount of adsorbed crystal violet dye was evaluated using a UV spectrometer at λ_max= 590 nm [23] . Adsorption capacity (q_t, mg/g) is calculated using following Equation 2 given below. Similar procedure was carried out for the composite with optimal concentration of nanoclay. Standard dye solution of crystal violet dye was prepared for five different concentrations namely 10 mg/l, 20 mg/l, 30 mg/l, 40 mg/l and 50 mg/l. The absorbance of dye solution was measured using UV spectrometer at a wavelength of 590 nm.

Adsorption capacity, (2)

where,

C_o is the initial dye concentration (mg/l);

C_t is the remaining dye concentrations in the solution at time t (mg/l);

V is the volume of dye solution used (l);

m is the weight of superabsorbent (mg).

The Fourier transform infrared spectroscopy (FTIR) analysis of pure guar gum, pure nanoclay and guar gum grafted acrylic acid were carried out using FTIR spectrophotometer (Perkin-Elmer spectrum 1000) between 300 and 4000 cm⁻¹. The samples were coated on a potassium bromide (KBr) plate and dried in a vacuum oven at 120˚C before it was tested.

X-ray diffraction (XRD) measurements for the composites have been performed using advanced diffractometer (analytical, XPERT-PRO) equipped with Cu-Kα radiation source (X = 0.154 nm). The diffraction data were collected in the range of 2θ = 3˚ - 60˚ using a fixed time mode with a step interval of 0.05˚.

The TGA of guar gum and acrylic acid grafted guar gum nanocomposite were carried out by using Perkin-Elmer Pyris Diamond 6000 analyzer in an atmosphere of nitrogen. The sample was subjected to a heating rate of 10˚C /min in a heating range of 20˚C - 900˚C.

The morphological characterization of the specimen was carried out using a scanning electron microscope (SEM) (JEOL, JSM-840A microscope). The specimens were gold sputtered prior to microscopy.

Transmission electron microscopy (TEM) for nanocomposites has been performed using a JEOL, Model 782, operating at 200 kV. TEM specimens were prepared by dispersing the composite powders in methanol by ultrasonication. A drop of the suspension was put on a TEM support grid (300 mesh copper grid coated with carbon). After drying in air, the composite powder remained attachedto the grid and was viewed under the transmission electron microscope.

Figure 1 shows FTIR spectrograms of neat guar gum and acrylic acid guar gum. Neat guar gum (curve a) shows characteristic bands at 3430 and 2945 cm⁻¹ for −O−H stretching vibration along with −C−H stretching. The bands at 1468 and 1098 cm⁻¹ corresponds to −C−H and −O−H bending vibrations [24] . A new band at 1584 cm⁻¹ can be seen for guar gum grafted acrylic acid (curve b) indicating formation of ester group and the presence of carboxylic anion [25] .

Figure 2(a) and Figure 2(b) show the SEM micrographs of grafted guar gum (without nanoclay) and guar gum-g-acrylic acid loaded with 1.75% nanoclay respectively. The grafted guar gum (without nanoclay) shows a smooth but dense surface. The nanocomposite exhibits a coarse dimpled surface indicating dispersion of nanoclay in the matrix. Similar observation has been made by Wang et al. [19] . Rough surfaces lead to enhancement of overall surface area and hence increased the adsorption sites [26] [27] . TEM images of 1.75% NC loaded guar gum are shown in Figure 2(b) and Figure 2(c). TEM images show clusters of various sizes. This agglomeration has been caused during TEM grid preparation due to drying induced clustering [28] [29] . Hence, the presence of uniformly spread both large and small aggregates can be seen. Similar observations have been made by Liu et al. [27] and Giri et al. [28] . Due to intercalation of the nanoclay particles, the interlayer space increased due to which, the water absorbency improved [30] - [32] .

XRD diffractograms for the nanocomposite is shown in Figure 3. In the case of guar gum grafted acrylic acid (without i.e. 0% nanoclay); the peaks appear at 2θ values of 5.2˚ and 19.77˚ respectively. The nanoclay used has crystalline peaks at 2θ values of 7.8˚, 19.73˚, 24.17˚, 26.63˚ and 34.98˚. For the nanocomposite loaded with 1.75% NC, the peaks both in the lower as well as higher range are not to be seen. This indicates that the silane modified

nanoclay has exfoliated in grafted guar gum. The amine groups of silane modified nanoclay can interact effectively with the carboxyl and unsubstituted hydroxyl groups of grafted guar gum.

The TGA thermograms of neat guar gum and the nanocomposite loaded with 1.75% NC is shown in Figure 4. The initial mass loss for neat guar gum is due to removal of absorbed water. The first zone of weight loss of

10% occurs at 260.3˚C due to the onset of decomposition of guar gum. The second major zone with 63% weight loss starts at 328.4˚C due to cleavage of guar gum backbone [20] . For the nanocomposite loaded with 1.75% NC, 63% weight loss occurs at 436˚C indicating enhanced thermal stability due to the presence of ceramic nanoparticles. The initial decrease in decomposition temperature for grafted guar gum is due to anhydride decomposition which forms due to dehydration of acrylic acid [33] [34] .

The influence of monomer (acrylic acid) concentration has been studied by synthesizing series of composites with varied quantities of acrylic acid. Volume of acrylic acid has been varied from 2 ml to 8 ml, Figure 5(a) indicates that addition of 6 ml of acrylic acid gives maximum grafting percentage of 9%, and thereafter it reduces. Co-polymerization between acrylic acid and guar gum increases the grafting percentage thereafter it decreases due to formation of homopolymer of acrylic acid. Similar observation has been reported by Bardajee et al. [35] for salep grafted sodium acrylate/alumina based superabsorbent.

Initiator has greater influence on rate of polymerization as it involves in initiating free radicals. Figure 5(b) depicts that continuous increase in grafting percentage upto 25% is observed till an optimum weight of 90 mg which indicates that initiation of free radicals increases the polymerization which in turn increases the grafting percentage. At higher levels of initiator concentration, grafting is unfavorable and this behavior is attributed to the formation of excess number of free radical sites leading to shorter polymer chains. Yu et al. [36] analyzed that in case of carboxymethyl chitosan/vinyl monomer superabsorbent with Azobis (isobutyl amide hydrochloride) as initiator, the mean kinetic chain length decreased with increase in initiator concentration.

Concentration of crosslinker has considerable effects on water absorbency. In Figure 5(c) it is observed that highest Equilibrium swelling of 6.96 g/g is achieved at 90 mg of MBA, thereafter it is decreases. This behavior indicates that initially crosslinker forms a continuous three-dimensional network which facilitates water to penetrate into polymer network space. Higher crosslinker concentration leads to increased number of crosslink points and crosslink density, due to which network space for water retention is reduced. Similar observation has been reported by Wang et al. [37] in case of guar gum/attalpulgite based superabsorbent with methylene bisacrylamide as crosslinker.

Reaction time is the deciding factor for efficiency of reaction. Series of nanocomposites at time interval of 0 to 6 minutes has been synthesized. From Figure 5(d), it can be seen that the maximum grafting percentage of 14% has been observed at reaction time of 4 minutes thereafter it decreases gradually. This behavior is attributed to increase of co-polymerization at initial intervals of reaction time which favors increase in grafting percentage. However, at higher reaction times (after 4 minutes) formation of short chain polymers is favorable which decreases the grafting percentage. Similar observations are reported in the study carried out by Likitha et al. [38] .

Figure 5(e) shows the effect of temperature on grafting percentage. From figure it can be seen that as the

temperature increased grafting percentage also increased till 60˚C then it has decreased slightly. This behavior indicates that a temperature of 60˚C is desirable for radical formation thereafter excess radical’s forms short chain polymers leading to decreased grafting percentage.

The water absorbency increases with increased nanoclay content upto 1.75% (Figure 6). Further nanoclay loading led to a decrease in equilibrium swelling value. The initial increase in absorbency may be due to inhibition of entanglement of grafted polymer chains due to the addition of rigid nanoclay particles. Further, the hydrophilic groups i.e. −OH and −COOH weakened the hydrogen bonding interactions which allow penetration of water. At higher nanoclay contents, the voids would be filled and increase the hydrophobicity leading to reduction in swelling properties. Similar observations on polysaccharide based superabsorbent nanocomposites have been made by Zhang et al. [39] and Wang et al. [32] .

Figures 7(a)-(c) show the swelling behavior of guar gum-g-acrylic acid in various pH buffer solutions. The

swelling is lowest in acidic medium and highest in neutral medium. The hydrogen bonding interactions between −OH and −COOH led to increase in physical crosslinking and this in turn hindered swelling of the superabsorbent. As the pH increased beyond the acidic media range, the hydrogen bonding interactions are weakened leading to an increase in negatively charged −COO groups causing repulsion of polymer chains. Hence, this allows diffusion of water molecules in the network leading to an increase in swelling properties [40] . The addition of 1.75% NC showed a similar trend in various pH mediums although the magnitude is higher. The nanoclay enhanced the holding capacity of water due to increased surface area as reported by Wang et al. [37]

The swelling behavior with time is indicated in Figures 7(a)-(c), where the rate of swelling is high till 70 minutes and thereafter equilibrium is attained. The swelling kinetics of composites in three different buffer solutions can be evaluated using Schott’s second order swelling kinetic model.

where

E_S is the equilibrium swelling (g/g) at a given time t (minutes);

K_i is the initial swelling rate constant (g∙g⁻¹∙min⁻¹);

E_i is the theoretical equilibrium swelling (g/g).

The t/ES versus t plots of experimental data showed straight lines with linear correlation coefficient (R²) > 0.95, indicating that Schott’s swelling theoretical model is suitable for evaluating the kinetic swelling behavior of the composites. By fitting the experimental data with equation (3), the swelling kinetic parameters like the initial swelling rate constant (K_i) and the theoretical equilibrium water absorption (E), were calculated from the slope and intercept of the lines shown in Figure 8 and are listed in Table 1. Value of Ki for neutral buffer > basic buffer > acidic buffer indicates that the order of swelling capability in neutral medium is higher than that of basic and acidic medium.