XIONG Yao ZHENG Mei-Xia CAI Kun-Qi ZHANG Yi ZHANG Long-Tao ZHENG Bao-Dong

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Grafting of Acrylamide and Trimethylolpropane Triglycidyl Ether onto the Gellan Gum: Synthesis and Characterization①

XIONG Yaoa, b, c②ZHENG Mei-Xiad②CAI Kun-Qia, b, cZHANG Yia, b, cZHANG Long-Taoa, b, c③ZHENG Bao-Donga, b, c

a(350002)b(350002)c(350002)d(350003)

To improve the physiochemical properties of gellan gum (GG), GG was modified with acrylamide and trimethylolpropane triglycidyl ether (TTE). The structure and morphology ofmodified GG were characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The characteristic peaks at 3448, 2788, 1654, 1411, 1117 and 1044 cm-1in the FT-IR spectrum confirm the modification. The XRD and DSC data revealed that the modification enhanced the thermal stability of GG. SEM analysis suggested the modification introduced a porous microstructure, resulting in the adsorption of crystal violet. In addition, the adsorption capacity, thermal stability and swelling property of GGTTE3 were superior to GGTTE1, GGTTE2, GGTTE4 and GGTTE5.

gellan gum, modification, acrylamide, trimethylolpropane triglycidyl ether, crystal violet;

1 INTRODUCTION

Gellan gum (GG) is a new microbial polysac- charide derived from gram-negative bacterium. Due to its functional properties, such as high transparency and resistance to heat and acidic environments, GG has been widely used in foods, pharmaceutical production, and chemical engineering applications[1].

There are many hydrophilic groups in the molecular chains of GG. Its property can be custo- mized and extended by hybridization with synthetic polymers. Vijan et al. (2012) synthesized acrylami- de-grafted-GG and applied it in drug delivery[2].Ferris et al. (2013) modified gellan gum for tissue engineering applications[3].Karthika et al. (2015) synthesizedGG-grafted-poly (2-dimethylamino) ethyl methacrylate, and it was evaluated as an adsorbent to remove methyl orange[4]. Karthika et al. (2015) grafted polyaniline (PANI) onto GG to improve its electrical conductivity[5]. Wang et al. (2005) synthesized the composite hydrogel of polyvinyl alcohol-GG-Ca2+with improved network structure and mechanical properties[1]. The chemical modification of polysaccharide and synthetic poly- mer resulted in new materials with obviously enhanced properties.

Acrylamide (AM) is one of the most important grafting materials. Bakarich et al. (2012) used AM to prepare an interpenetrating polymer network hy- drogel[6]. Karthika et al. (2013) synthesized GG-g- Poly (AAm-co-IA) hydrogel and confirmed that the hydrogel exhibited a reasonable sensitivity to pH and ionic strength[7]. Trimethylolpropane triglycidyl ether (TTE) is usually used as a cross linking agent in the modification of macromolecules[8, 9].Until now, simultaneous grafting of AM and TTE onto GG has not been reported.

Natural biopolymers are important in adsorption of dyes and metal ions[10-12]. Crystal violet (CV) is a common purple dye for textiles, such as cotton, wool, silk, a histological stain and in Gram’s Method of classifying bacteria. Because of its toxicity, dye removal from textile effluents is essential. The common removal methods include photocatalytic degradation[13],electrochemical methods[14], biode- gradation[15], and adsorption[16, 17]. Removal by adsorption shows the most potential and pros- pect[18].

In this work, we focus on the synthesis of a new hydrogel. GG was grafted with AM and modified with TTE as a cross-linking agent using potassium persulfate (KPS) as an initiator. The adsorption capacity of the modified GG for CV from aqueous solution was investigated. The modified GG was characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron micrograph (SEM). Details on the synthesis, characterization and functional are discussed below.

2 EXPERIMENTAL

2. 1 Materials

GG (CAS number 71010-52-1, Sigma-Aldrich, Co.) was purchased from Fuzhou Bona Biotechno- logy co., LTD. AM (AR, 99.0%, PubChem CID: 6579; C3H5NO) was purchased from Sigma-Aldrich with a molar mass of 71.08.TTE (Tech, PubChem CID: 103015) was purchased from Sigma-Aldrich. All other chemical reagents were of analytical grade.

2. 2 Modification of GG

The modified GG was synthesized in a four- necked flask under N2gas environment and equip- ped with mechanical stirring, thermometer, and a reflux condenser. GG (0.3 g) was added to 150 mL distilled water and stirred at room temperature for 60 min until dissolved. Then, AM (10 g) and 85, 170, 260, 345, and 430 μL TTE were added to the GG solution sequentially and stirred under nitrogen. The temperature was held at 60 °C, and 0.3 g of KPS was added and reacted for 4 h to initiate the graft copolymer. The modified GG was precipitated using ethanol. The sediment was filtered, washed thoroughly with ethanol/water mixture (4:1, v/v) for 3 times under high-speed stirring, and soaked in an ethanol/water mixture (4:1, v/v) for 24 h. The modi- fied GG was collected by filtration and lyophiliza- tion. After drying, the modified GG was ground and milled, and sieved through a 250 μm membrane. This was used as GGTTE. For the modified GG, TTE dosages of 85 μL (1%, w/w), 170 μL (2%, w/w), 260 μL (3%, w/w), 345 μL (4%, w/w) and 430 μL (5%, w/w) were recorded as GGTTE1, GGTTE2, GGTTE3, GGTTE4 and GGTTE5, respectively.The percentage of grafting (, %) and that of monomer grafting (, %) were calculated via equations (1) and (2), respectively

(%) = (1–0)/0× 100% (1)

where0,1and2are the weight of GG, GGTTE and AM, respectively.

(%) = (1–0)/2× 100% (2)

where0,1and2are the weight of GG, GGTTE and AM, respectively.

2. 3 Crystal violet adsorption

The GGTTE sample (0.05 g) was transferred into 100 mL CV solution (60 mg/L) and equilibrated at 25 °C for 24 h. The CV working solution was prepared after dilution of the stock solution, which was prepared in distilled water. The concentrations of CV in the initial solution and the residual solutions were determined by recording the absorbance values at 590 nm. The adsorption capacity (Q, mg/g) was calculated via equation (3) and the removal rate (, %) was calculated via equation (4).

Q= (C–C)/m (3)

= (C–C)/× 100% (4)

whereCandCare the initial and final CV concentrations (mg/L), respectively,is the volume of CV (L), and m is the weight in grams of GGTTE used for adsorption measurements.

2. 4 Swelling properties of GGTTE

The swelling properties of GGTTE hydrogel were determined by the water-sorption of dry GGTTE in ultrapure water at 20 °C. The swollen GGTTE was separated by filtering through a mesh nylon bag at predetermined time intervals and weighed immediately. The swelling rations () of GGTTE were calculated via equations (5) and (6), respectively.

SR= (M–M)/M(5)

SR= (M–M)/M(6)

whereMis the initial weight in gram of dry GGTTE,Mis the weight in gram at equilibrium after water-sorption, andMis the weight in gram at timeduring water-sorption of GGTTE.

2. 5 Characterization of GGTTE

2. 5. 1 FT-IR analysis

FT-IR spectra of GGTTE and GG were recorded in solid state using KBr pellets with a FT-IR spectrometer (Nicolet AVATAR 360) from 400 to 4000 cm-1at 2 cm-1resolution.

2. 5. 2 XRD analysis

The different phases of GGTTE and GG were determined using XRD. The XRD measurements were carried out using Empyrean XRD instrument (Rigaku, Miniflex 600) operating in the reflection mode with Curadiation. The X-ray generator was operated at 40 kV and 15 mA. The XRD chromato- grams were recorded over an angular range of 5° to 60° (2) at a scanning speed of 10 °/min and a step size of 0.02 (= 1.5406 nm).

2. 5. 3 DSC analysis

DSC measurements were carried out under N2flow (20 mL/min) using NETZSCH DSC 200F3 instruments from 45～500 °C at a heating rate of 10 °C/min. The sample mass was 10 mg.

2. 5. 4 Microstructure analysis

The microstructures of GGTTE3 and GG were examined with a SEM. SEM analyses were perfor- med using a JSM-6380LV (JEOL) operated with an accelerating voltage of 15.0 kV. The samples were coated with Au prior to imaging.

3 RESULTS AND DISCUSSION

3. 1 Adsorption capacity

The adsorption capacity of GGTTE for crystal violet is shown in Fig. 1A. TheQof GGTTE1, GGTTE2, GGTTE3, GGTTE4 and GGTTE5 for CV were 33.70 ± 0.46, 31.41 ± 0.55, 33.75 ± 0.42, 32.20 ± 0.14 and 33.55 ± 0.12 mg/g, respectively at 25 °C for 24 h. The maximum ofQwas 33.75 ± 0.42 mg/g with GGTTE3. TheQvalues in this work were higher than those of guar gum grafted by acrylic acid/nanoclay (12.00 mg/g)[19]. Theof GGTTE1, GGTTE2, GGTTE3, GGTTE4 and GGTTE5 for CV were 56.17 ± 0.76, 52.35 ± 0.92, 56.75 ± 0.71, 53.20 ± 0.24 and 55.92 ± 0.20 %, respectively, at 25 °C for 24 h. The maximum ofwas also 56.75 ± 0.71 % with GGTTE3.

3. 2 Swelling characteristics

The swelling ratio of GGTTE in ultrapure water was between 17.7 and 41.2, as shown in Fig. 1B. The swelling equilibrium was achieved in 10 min, and the swelling ratio increased with increasing the TTE content. The swelling ratio andQ(mg/g) of GGTTE3 were the highest, compared to GGTTE1, GGTTE2, GGTTE4 and GGTTE5. GGTTE3 samples were used in the subsequent experiments.

Fig. 1: A: Adsorption crystal violet by GGTTE; B: Swelling rations of GGTTE1 (black line), GGTTE2 (red line), GGTTE3 (blue line), GGTTE4 (rose red line) and GGTTE5 (green line) in ultrapure water at 20 °C calculated using Eqs. (5) and (6). The error bars represent the standard error

3. 3 FT-IR analysis

The FT-IR spectra of GG and GGTTE are shown in Fig. 2. There were no obvious spectra changes among GGTTE1, GGTTE2, GGTTE3, GGTTE4 and GGTTE5. To clearly indicate the differencesbetween GGTTE and GG,the spectra of GGTTE3 versus GG were compared. The spectrum of GG had a broad peak at 3422 cm-1due to the stretching vibrations of O–H groups. GG also had peaks at 1609 and 1413 cm-1due to the asymmetrical and symmetrical vibrations of COO- groups. GG had a peak at 1026 cm-1due to the characteristic absorption of C6-OH. Similar observations of GG absorbance spectra have been reported[5, 20]. The GGTTE3 sample had a broad peak at 3448 cm-1due to the stretching vibrations of O–H and N–H. For GGTTE3, the peak at 1654 cm-1was characteristic of absorption of CONH2. The bending vibration peak of C6-OH at 1044 cm-1decreased in the spectrum of GGTTE3. There were some new peaks in GGTTE3, and the peak at 2788 cm-1is due to the stretching vibration of N–H. The peak at 1411 cm-1is due to stretching vibration of C–N, and the absorption spectrum of NH2at 1117 cm-1results from the wagging band of NH2. All the spectral data confirmed the grafting of AM and TTE onto GG.

Fig. 2. A: FT-IR spectra of GG (black line) and GGTTE3 (red line) from 4000 to 400 cm-1; B: FT-IR spectra of GG (black line), GGTTE1 (red line), GGTTE2 (blue line), GGTTE3 (gray line), GGTTE4 (rose red line) and GGTTE5 (purple line) from 4000 to 400 cm-1

3. 4 XRD analysis

XRD spectra of GG and GGTTE are shown in Fig. 3. There were two wide diffraction peaks at 2= 20.073°and 9.534°, which showed the amorphous nature of GG. There was only one wide diffraction peak roughly at 2= 22°, which meat the amor- phous nature of GGTTE. GGTTE1 and GGTTE4, for example, have wide diffraction peaks, and the ratios of peak area to curve area of GGTTE1 and GGTTE4 were larger than that of GG, suggesting that GGTTE had more regularity. The XRD parameters for GGTTE and GG are shown in Table 1. The crystalline interplanar spacing () decreased after GG was modified with AM and TTE, and the crystallinity of GGTTE increased, compared with GG. It could be inferred that the rearrangement in the morphology of GGTTE strengthened the GG intra- and inter-molecular hydrogen bond force and improved the regularity of the molecular arrange- ment. The XRD data confirmed the success of the hydrogel synthesis. The result is consistent with the FT-IR spectra, which indicate the chemical modifier reacted with -OH groups of GG and the polymer chain is regular, thus making it difficult for GGTTE to crystallize.

Table 1. XRD parameters for GGTTE and GG

Fig. 3. X-ray diffraction spectra of GG (black line), GGTTE1 (red line) and GGTTE4 (blue line). X-ray generator was operated at 40 kV and 15 mA. The XRD chromatograms were recorded over an angular range of 5° to 60° (2) at a scanning speed of 10 °/min and a step size of 0.02 (= 1.5406 nm)

3. 5 Thermal analysis

The DSC thermogram of GG and GGTTE is shown in Fig. 4, in which an endothermic curve at 50.1 °C and an exothermic peak at 253.3 °C were formed, respectively indicative of the moisture loss and thermal decomposition of GG[21, 22].

Fig. 4. DSC curves of GG (black line), GGTTE1 (red line), GGTTE2 (blue line), GGTTE3 (rose red line), GGTTE4 (green line) and GGTTE5 (purple line) from 45 to 500 °C at a heating rate of 10 °C/min under N2flow (20 mL/min)

Multiple endothermic peaks were observed in the DSC thermogram of GGTTE. The first broad endo- thermic peak at 50～70 °C is due to the loss in absorbed moisture in the samples. Melting of GGTTE1, GGTTE4 and GGTTE5 samples at 136～150°C as well as GGTTE2 and GGTTE3 samples at 190 °C resulted in the second peak. Furthermore, loss of ammonia was indicated by a third peak at 315.0 °C. The peak at 343.1 °C resulted from the decomposition of the imide group formed via cyclization. The last peak at 430 °C represented the decomposition of cyclized imide groups. Thus, we can conclude that the thermal stability of GG was improved by grafting AM and TTE.

3. 6 SEM

The morphological features of GG and GGTTE3 are shown in Fig. 5. The microstructure of GGTTE3 was more porous than that of GG. The changes in surface morphology supported the graft copolymeri- zation. These results also supported the results of FT-IR and XRD.

Fig. 5. SEM micrographs of GGTTE3 (A) and GG (B). The illustrations are the macrostructures of GGTTE and GGTTE, respectively. SEM analyses were performed using a JSM-6380LV (JEOL) operated with an accelerating voltage of 15.0 kV. The samples were coated with the Au prior to measurement

3. 7 Mechanism of GGTTE production

The GGTTE hydrogel was prepared by grafting AM onto GG in the presence of cross-linking agent TTE caused by free radical initiator KPS. Scheme 1 outlines the proposed mechanism of grafting and chemical cross-linking. During polymerization, the KPS initiator decomposed to generate sulfate anion- radicals under heating. Sulfate anion-radicals reacted with the ring structure of D-glucose in the GG chain and formatted active groups such as alkoxy radicals. Sulfate anion-radicals simultaneously attacked AM and TTE molecules and formed AM and TTE-based radicals. Monomers of GG, AM and TTE near the reaction site became acceptors for radicals and ignited the chain initiation. Subsequently, the monomers became free radical donors to the neighboring molecules, which caused the graft chain to grow indefinitely. The polymer chains reacted with the end vinyl groups of TTE during chain propagation. The main chain of GG was extended with the reaction between the hydroxyl groups of GG reacting with AM. The TTE was connected to the polymer chain and became the cross-linking point because of the amino group of AM and the epoxy group of TTE participating in a ring-opening reaction upon heating. The copolymer GGTTE was comprised of a cross-linked and network structure.

Scheme 1. Ｍechanism of GGTTE using TTE as a cross-linker. Sulfate radicals were generated under heating, and the sulfate radical abstracts hydrogen atoms from the GG molecules producing GG free radicals. The monomer molecules, which were in close vicinity to the reaction sites, become acceptors of the GG radicals resulting in chain initiation.Thereafter, they become free radical donors to the neighboring molecules causing the grafted chains to grow

Table 2 summarizes the formulation details used in the synthesis. The G (%) varies from 3394% to 3717%, and the E (%) falls in the 102%～111% range.

4 CONCLUSION

GG was modified with AM and TTE to promote higher regularity, better thermal stability, and a more porous structure. The resulting hydrogel is capable of absorbing crystal violet. This implies that the modified GG (GGTTE1, GGTTE2, GGTTE3, GGTTE4 and GGTTE5) could have utility in the adsorption of heavy metal ions and dyes. This could purify the contaminated water.

ACKONWLEDGMENT

The work was financially supported by the International Science and Technology Cooperation and Exchange Program of Fujian Agriculture and Forestry University (KXGH17001). We thank LetPub (www.LetPub.com) for its linguistic assistance during the preparation of this manuscript.

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26 December 2017;

13 April 2018

① The work was financially supported by the International Science and Technology Cooperation and Exchange Program of Fujian Agriculture and Forestry University (KXGH17001). We thank LetPub (www.LetPub.com) for its linguistic assistance during the preparation of this manuscript.

②These authors contribute equally to this work

. Prof. Dr. Zhang Long-Tao (1979~). E-mails: zlongtao@fafu.edu.cn and zlongtao@hotmail.com

10.14102/j.cnki.0254-5861.2011-1935