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View Article Online View Journal RSC Advances This article can be cited before page numbers have been issued, to this please use: X Shi, W Wang, Y Zheng and A Wang, RSC Adv., 2014, DOI: 10.1039/C4RA10866C This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available You can find more information about Accepted Manuscripts in the Information for Authors Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content The journal’s standard Terms & Conditions and the Ethical guidelines still apply In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains www.rsc.org/advances Page of 32 RSC Advances View Article Online Utilization of hollow Kapok fiber for the fabrication of a pH-sensitive superabsorbent composite with improved gel strength and swelling properties Xiaoning Shia,b, Wenbo Wanga, Yian Zhenga , Aiqin Wanga* a Academy of Sciences, Lanzhou, 730000, P.R China b Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Gansu University of Traditional Chinese Medicine, Lanzhou, 730000, P.R China 10 11 12 13 14 15 16 17 18 19 20 21 * Corresponding author Tel.: +86 931 4968118; fax.: +86 931 8277088 E-mail address: aqwang@licp.cas.cn (A.-Q Wang) 22 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C RSC Advances Page of 32 View Article Online ABSTRACT: Pretreated kapok fiber (PKF) with hollow tube structure was introduced into copolymerization fiber-g-poly(sodium acrylate) (PNaA/PKF) superabsorbent composite The network characteristics and surface morphologies of the PNaA/PKF superabsorbent composite were investigated by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), as well as by determination of mechanical properties, swelling and stimuli responses to salts, pH The results showed that the incorporation of proper amount of PKF not only improved the water absorption, but also gel content and 10 gel strength The composite with 10 wt.% PKF shows the best water absorption, gel 11 content and gel strength The swelling kinetics of the composite followed Schott’s 12 second-order kinetics model, and the swelling rate constant enhanced 2.63 folds after 13 adding 10 wt.% PKF The swelling and deswelling behaviors in various saline and pH 14 solutions revealed that the stimuli-sensitivity of PNaA/PKF composite to salt 15 concentration, ionic charge and external pH, and a remarkable time-dependent swelling 16 process with an overshooting characteristic was observed in pH solutions poly(sodium acrylate) and network crosslinking by a reaction simultaneous free-radical to a fabricate novel graft kapok 17 18 Keywords: Kapok fiber, Acrylic acid, Graft copolymerization, Superabsorbent 19 composite, Gel strength 20 21 22 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C Page of 32 RSC Advances View Article Online DOI: 10.1039/C4RA10866C Introduction Superabsorbent (SAP) is a kind of lightly crosslinked hydrophilic functional polymer materials with three-dimensional (3D) network structure It can absorb, swell and retain aqueous solutions up to hundreds of times their own mass, and has been extensively applied in many fields, such as agriculture and horticulture,1~3 hygienic products,4,5 wastewater treatment,6~8 drug delivery system9~11 and biotechnology,12,13 etc Currently, the design of SAP from renewable bioresources becomes a preferred approach to alleviate the environment and resource problems resulting from the excessive consumption of petroleum-based raw materials Thus far, many naturally available 10 materials, such as starch,14,15 chitosan,12,16 cellulose, 10,17,18 gum,11,19 protein20,21 and 11 some crops residues,22,23 have been used to produce eco-friendly SAP through their 12 free-radical graft polymerization with vinyl monomers It has been confirmed that the 13 properties of SAP are highly dependent on the structure of natural polymers Therefore, 14 the design and development of new SAP by introducing natural polymers with special 15 structure becomes the sustainable research subject in the future 16 Kapok fiber (KF) is a silky fiber that encloses the seeds of the ceiba tree of the family 17 Bombacaseae It has a homogeneous hollow tube shape with a wall thickness of ca 18 0.8~1.0 µm and is composed of 64% cellulose, 13% lignin, 23% pentosan and some 19 wax cutin on its surface.24 KF is traditionally used as a stuffing, especially for life 20 preservers, bedding, upholstery, and for insulation against sound and heat Besides, it 21 exhibits greater potential as a filter product for oily water treatment because of its lower 22 density, higher porosity, greater specific surface area and hydrophobic-oleophilic RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 RSC Advances Page of 32 View Article Online physicochemical characteristics.25~27 However, the intrinsic hydrophobic properties of raw KF make it quite difficult to be dispersed in aqueous solution for graft polymerization Because NaClO2 can break some of the phenolic compounds,28 it can be used to treat KF to reduce the lignin content and change the surface properties of KF from hydrophobic to hydrophilic The moderate modification of KF renders it potential as a matrix to fabricate new materials by a grafting copolymerization reaction Kang et al reported the graft of glycidyl methacrylate onto pretreated KF under irradiation with 60 suitable for graft copolymerization However, there are no related researches on the 10 preparation of a KF-based composite with stimuli-responsive properties in aqueous 11 solution by a graft copolymerization reaction Co gamma rays,28 which confirms that the surface groups of KF are active and 12 Acrylic acid (AA) is an important hydrophilic vinyl monomer and is widely used for 13 the fabrication of superabsorbent hydrogels By graft polymerization of partially 14 neutralized AA onto biomacromolecular backbones, the three-dimensional network can 15 be formed and the synthesized superabsorbent composites can find improved swelling 16 properties, biodegradability and biocompatibility.23, 29~31 Especially, the incorporation of 17 carboxyl groups endows these bioresources-based superabsorbent hydrogels with 18 response to various external stimuli, such as salts and pH, and thus extended their 19 applications to many fields, such as drug release formations, adsorption of liquid and 20 metal ions or dyes.8, 10, 30 21 Based on above background, in this study, KF was pretreated by NaClO2 and used to 22 synthesize superabsorbent composite using green solution polymerization technology RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C Page of 32 RSC Advances View Article Online The synthetic process of the superabsorbent is eco-friendly and no organic solvent was used The effect of PKF content on the water absorption, gel content, gel strength and time-dependent swelling ratio was investigated systematically The network structure of the PNaA/PKF superabsorbent hydrogel was characterized by FTIR and SEM, and a possible synthesizing mechanism was proposed The swelling behaviors of the superabsorbent in various salt and pH solutions were investigated to expand its potential application in adsorption, separation, and drug release system 10 Experiment section 2.1 Materials 11 Kapok fiber (KF) was obtained from Shanghai Panda Co Ltd (Shanghai, China) 12 Acyclic acid (AA, C.P grade) was purchased from Shanghai Wulian Chemical Factory 13 (Shanghai, China) Ammonium persulfate (APS, A.R grade) was purchased from Xi’an 14 Chemical Reagent Factory (Xi’an, China) N, N'-methylene-bis-acrylamide (MBA, A.R 15 grade) was purchased from Shanghai Chemical Reagent Corp (Shanghai, China) 16 Sodium Chlorite (NaClO2, C.P grade) was purchased from Beijing HWRK Chem Co 17 Ltd (Beijing, China) Other agents used are all analytical grade and all solutions were 18 prepared with distilled water 19 20 2.2 Pretreatment of KF 21 The crude KF was smashed with a high-speed grinder for 20 s and then dipped in 22 wt.% of NaClO2 solution at 70~80 oC for h The treated KF was drained and washed RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C RSC Advances Page of 32 View Article Online with large amount of distilled water until pH~7, and then dried in an oven at 100 oC for 24 h to obtain the pretreated kapok fiber (PKF) This treatment process may transform hydrophobic KF as hydrophilic.28 2.3 Preparation of PNaA/PKF superabsorbent composite A certain amount of PKF, 30 mL distilled water and 7.2 g AA (firstly neutralized with 8.8 mL of mol/L NaOH solution in an ice bath) were added into a 250-mL four-necked flask equipped with a mechanical stirrer, a condenser, a thermometer and a nitrogen line The mixture was heated to 60 oC with an oil bath and stirred under 10 nitrogen purging for 30 to remove the dissolved oxygen Afterward, 10 mL of the 11 aqueous solution containing 100 mg of APS and 14.4 mg of MBA was added, and the 12 reactor was slowly heated to 70 oC and kept for h to complete the polymerization The 13 resulting product was dried in an oven at 70 oC for 72 h The dried product was ground 14 into particles and passed through a 40-80 mesh sieve 15 16 2.4 Measurement of gel content and gel strength 17 In order to determine the gel content, 0.10 g (M1) of dry sample was soaked in 300 18 mL of distilled water for 72 h, and then filtered out The extracted swollen gels were 19 firstly dewatered by anhydrous ethanol and then dried for 12 h at 70 oC The dried 20 samples were reweighed (M2) and the gel content (Gel%) can be calculated by Eq.(1): 21 22 Gel% = M2/M1×100% (1) The gel strength was measured on a Physica MCR 301 Rheometer (Germany) by a RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C Page of 32 RSC Advances View Article Online rheological method, and expressed as the rheological curves of storage modulus (G′, Pa) versus angular frequency (ω, rad/s) The constant deformation strain is 0.5% and the angular frequency (ω) is in the range of 0.1~100 rad/s 2.5 Measurement of equilibrium water absorption and swelling kinetics 0.050 g dried samples were immersed in 200 mL of distilled water or other swelling media at room temperature for h to reach swelling equilibrium The swollen hydrogels were separated from the unabsorbed water by a 100-mesh sieve, and then gently dabbed the hydrogels with filter paper to remove the residual water on surface The equilibrium 10 water absorption (Qeq, g/g) was calculated by Eq (2) Qeq = ( M s − M d ) / M s 11 (2) 12 Here, Md and Ms are the mass of dried sample (g) and swollen hydrogel (g), respectively 13 The measurements were repeated for three times to obtain a mean value of Qeq, and the 14 ± SD value is less than 3% 15 The swelling kinetics was measured as the follow procedure: 0.050 g of samples were 16 soaked in 200 mL of solution, and the swollen gels were filtered out using a sieve at 17 different time intervals (1, 3, 5, 7, 10, 15, 20, 30, 60 and 120 min), the swelling rate (Qt) 18 at the given time t was measured by weighing the swollen (Mst) and dry (Md) samples 19 and then calculated according to Eq (2) 20 The deswelling kinetics of the swollen superabsorbent in different salt solutions was 21 also measured as above procedure The fully swollen gels were soaked in 200 mL of salt 22 solution At the set time intervals, the samples were taken out from the salt solution and RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C RSC Advances Page of 32 View Article Online weighed after removing the residual fluids Water retention (WR) in the hydrogels was calculated as: WR = ( M t − M d ) / M s × 100 (3) where Mt is the mass of sample after deswelling at time t Md and Ms are the same as defined in Eq (2) 2.6 Characterization FTIR spectroscopy was recorded on a Nicolet NEXUS FTIR spectrometer in 4000-400 cm-1 region, using a KBr pellet The morphologies of the PNaA/PKF 10 composites were examined using a S-4800 scanning electron microscopy (SEM) after 11 coating the samples with gold film 12 13 Results and discussion 14 3.1 Synthesis of superabsorbent composite 15 The mechanism of free-radical graft copolymerization of polysaccharides, such as 16 chitosan, guar gum and cellulose, with AA was described elsewhere 16, 29, 31 According 17 to the literature, a proposed reaction mechanism for the grafting and chemical 18 crosslinking of the PNaA/PKF superabsorbent is depicted in Scheme Firstly, for the 19 purpose of enhancing electrostatic repulsion and expanding the polymer network, AA 20 was treated with NaOH solution to convert COOH groups to COO- groups Then, PKF 21 was dispersed in the partially neutralized AA solution, and the thermal decomposition of 22 APS generated sulfate anion radicals, which abstracts hydrogen from the hydroxyl RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C Page of 32 RSC Advances View Article Online group of PKF backbone to form alkoxy radicals, resulting in active centers on the PKF backbone to initiate a radical graft copolymerization of AA with PKF In the presence of the crosslinker MBA, the end vinyl groups of MBA crosslink polymer chains and finally form a 3D polymer network of PNaA/PKF The PKF chain interpenetrated and entangled within the 3D network of PNaA [Scheme 1] 3.2 FTIR spectroscopy analysis The FTIR spectra of PKF, PNaA, the physical mixture of PKF and PNaA, and 10 PNaA/PKF (10%) superabsorbent composite are shown in Figure PKF shows a broad 11 band at 3412 cm-1 due to the stretching vibration of O-H groups The band at 2917 cm-1 12 is the C-H stretching vibration of methyl and methylene groups in cellulose.27 The 13 relatively intense band at 1742 cm-1 is assigned to the C=O stretching vibration from the 14 carbonyl group, and the bands in the region of 1598~1426 cm-1 are ascribed to the 15 skeletal C=C stretching vibrations of the residual aromatic ring in PKF.28 The 16 characteristic absorption band at 1055 cm-1 is attributed to C-O stretching vibrations of 17 cellulose, which obviously weakened after reaction (Figure 1d) However, it can still be 18 observed in the spectrum of the physical mixture of PKF and PNaA with almost no 19 varying intensity (Figure 1c) In Figure 1d, new bands at 1731, 1571, and 1457 cm-1 are 20 related to the stretching vibration of C=O, asymmetrical stretching vibration, and 21 symmetrical stretching vibration of –COO-, respectively,29,30 indicating the presence of 22 –COO- groups in the composite network These results indicate that AA monomers were RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C RSC Advances Page 18 of 32 View Article Online concentration of salt solution and the valence of cations Their swelling ratio decreased with an increase of the salt concentration on the whole The time-dependent deswelling rate in 15 mmol/L of NaCl, CaCl2 and AlCl3 solution is in the order: Al3+ > Ca2+ > Na+ Furthermore, the water absorption of PNaA/PKF superabsorbent is sensitive to external pH values The time-dependent swelling in pH solution exhibits a remarkable “overshooting effect” It confirms that the ionic repulsion among electriferous groups existed in the polymeric network can be modulated by altering the external pH, and becomes the main driving force for the change of water absorption 10 11 12 Acknowledgements The authors gratefully acknowledge jointly supporting of this 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X.M Ji, H Dong, Y Ying and H Zheng, Carbohydr Polym., 2008, 71, 10 682 11 12 21 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C RSC Advances Page 22 of 32 View Article Online Figure captions: Scheme A proposed mechanism for the formation of PNaA/PKF superabsorbent Scheme A schematic diagram of the swelling process with overshooting effect in pH solution Figure FTIR spectra of (a) PKF, (b) PNaA, (c) physical mixture of PKF and PNaA and (d) PNaA/PKF (10 wt.%) Figure The SEM images of (a) crosslinked PNaA, (b) PKF and (c) PNaA/PKF (10 wt.%) Figure Effect of PKF content on the storage modulus (G') of the PNaA/PKF 10 superabsorbents 11 Figure (a) Time-dependent water absorptions of the PNaA/PKF superabsorbent 12 composite in distilled water, and (b) the plots of t/Qt versus t 13 Figure (a-c) Water absorption of PNaA/PKF superabsorbents as a function of PKF 14 content in various concentrations of NaCl, CaCl2 and AlCl3 solutions; (d) The 15 time-dependent deswelling behaviors of the superabsorbent (PKF 10 wt.%) in 15 16 mmol/L of NaCl, CaCl2 and AlCl3 solutions 17 Figure (a) Water absorption of PNaA/PKF superabsorbents with different amount of 18 PKF in pH 2~13 solutions, and (b) Dynamic swelling curves of the PNaA/PKF (10 19 wt.%) in pH 2, and 12 solutions 20 21 22 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 DOI: 10.1039/C4RA10866C Page 23 of 32 RSC Advances View Article Online DOI: 10.1039/C4RA10866C Table Variations of water absorption and gel content as a function of PKF content PKF content (wt.%) Qeq (g/g) SD Gel% SD 195 1.1314 84.8 0.4431 270 1.9799 88.2 0.6881 10 356 0.1414 95.7 0.8045 15 292 1.4142 86.6 0.6205 20 247 0.9899 81.5 0.6364 23 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 RSC Advances Page 24 of 32 View Article Online DOI: 10.1039/C4RA10866C Table Swelling kinetic parameters of PNaA/PKF as a function of PKF content Q∞ (g·g-1) kis (g·g-1·s-1) R 196 1.8471 0.9994 271 3.0450 0.9996 10 360 4.8556 0.9998 15 292 4.2189 0.9992 20 248 3.1845 0.9996 PKF content (wt.%) 10 11 24 RSC Advances Accepted Manuscript Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Scheme 10 11 12 13 14 15 16 17 18 25 RSC Advances Accepted Manuscript Page 25 of 32 RSC Advances DOI: 10.1039/C4RA10866C View Article Online Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Scheme 10 11 12 13 14 15 16 17 18 26 RSC Advances Accepted Manuscript RSC Advances View Article Online Page 26 of 32 DOI: 10.1039/C4RA10866C Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 10 11 12 13 14 15 16 27 RSC Advances Accepted Manuscript Page 27 of 32 RSC Advances DOI: 10.1039/C4RA10866C View Article Online Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 28 RSC Advances Accepted Manuscript RSC Advances View Article Online Page 28 of 32 DOI: 10.1039/C4RA10866C Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 10 11 12 13 14 15 29 RSC Advances Accepted Manuscript Page 29 of 32 RSC Advances DOI: 10.1039/C4RA10866C View Article Online Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 4 30 RSC Advances Accepted Manuscript RSC Advances View Article Online Page 30 of 32 DOI: 10.1039/C4RA10866C Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 5 31 RSC Advances Accepted Manuscript Page 31 of 32 RSC Advances DOI: 10.1039/C4RA10866C View Article Online Published on 24 September 2014 Downloaded by State University of New York at Stony Brook on 08/10/2014 09:23:56 Figure 32 RSC Advances Accepted Manuscript RSC Advances View Article Online Page 32 of 32 DOI: 10.1039/C4RA10866C