Temperature and pH Responsive Hydrogels Using

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 1763−1771

pubs.acs.org/journal/ascecg

Temperature and pH Responsive Hydrogels Using Methacrylated Lignosulfonate Cross-Linker: Synthesis, Characterization, and Properties Can Jin,†,‡ Wenjia Song,‡ Tuan Liu,‡ Junna Xin,*,‡ William C. Hiscox,§ Jinwen Zhang,*,‡ Guifeng Liu,† and Zhenwu Kong*,† †

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, National Engineering Laboratory for Biomass Chemical Utilization, Nanjing, 210042, China ‡ School of Mechanical and Materials Engineering, Composite Materials and Engineering Center, Washington State University, P.O. Box 641806, Pullman, Washington 99164, United States § Nuclear Magnetic Resonance Center, Washington State University, P.O. Box 4630, Pullman, Washington 99164, United States S Supporting Information *

ABSTRACT: In this work, biobased hydrogels with temperature and pH responsive properties were prepared by copolymerizing N-isopropylacrylamide (NIPAM), itaconic acid (IA), and methacrylated lignosulfonate (MLS), where the multifunctional MLS served as a novel macro-cross-linker. The network structures of the lignosulfonate-NIPAM-IA hydrogels (LNIH) were characterized and confirmed by elemental analysis, Fourier transform infrared, and 13C nuclear magnetic resonance. The equilibrium swelling capacity of the LNIH hydrogel decreased from 31.6 to 19.1 g/g with MLS content increasing from 3.7 to 14.3%, suggesting a strong dependence of water absorption of the gel on MLS content. LNIH hydrogels showed temperaturesensitive behaviors with volume phase transition temperature (VPTT) around the body temperature, which was also influenced by MLS content. Moreover, all LNIH hydrogels exhibited pH sensitivity in the range of pH 3.0 to 9.1. Rheological study indicated that mechanical strength of the gel also increased with MLS content. The results from this study suggest that lignosulfonate derivative MLS is a potential feedstock serving both water-absorbing moiety and cross-linker for preparation of biobased smart hydrogels. KEYWORDS: Lignosulfonate, Hydrogel, Temperature-sensitive, pH-sensitive, Cross-linker

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INTRODUCTION Smart hydrogels, which are sensitive to environmental stimuli including temperature,1 pH,2 light,3 and certain chemical triggers4 have received considerable attention in the fields of biomedicine, tissue engineering, agriculture, and materials science.5−7 In particular, temperature- and pH-sensitive hydrogels are mostly studied because both parameters are important environmental factors in typical biological, physiological, and chemical systems. On the other hand, multiple environmental stimuli may occur at the same time in many cases. Therefore, numerous smart hydrogels have been prepared by combining a temperature-sensitive component (e.g., N-isopropylacrylamide, pluronics, and 2-oxazoline) and a pH-sensitive component (e.g., alginate, acrylic acid, and © 2017 American Chemical Society

orthoester amide) to control their dual stimuli-responsive behaviors.8−10 Very recently, there has been growing interest in the preparation of smart hydrogels from natural polymers including cellulose,11 chitosan,12 and starch.13 Natural polymers endow smart hydrogels many distinct advantages such as biocompatibility and biodegradability. Besides, natural polymers may also offer excellent mechanical strength to the hydrogel networks.14 Lignosulfonate, accounting for 90% of commercial lignin, is an abundant (∼1.8 million tons per year) and inexpensive Received: September 7, 2017 Revised: November 16, 2017 Published: December 20, 2017 1763

DOI: 10.1021/acssuschemeng.7b03158 ACS Sustainable Chem. Eng. 2018, 6, 1763−1771

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ACS Sustainable Chemistry & Engineering byproduct generated from the sulfite pulping industry.15 Lignosulfonate is a branched and aromatic polymer with hydrophilic, chemically reactive, and bioactive features, which enable it to be utilized as dispersant,16 flocculant,17 ion-exchange resin,18 and antioxidant.19 In recent years, lignosulfonate has been chosen as a renewable candidate for fabricating “green” hydrogels. Sun et al. synthesized lignosulfonate-modified graphene hydrogel for Pb(II) adsorption through one-step method.20 Wang et al. designed lignosulfonate-grafted poly(acrylic acid-co-poly(vinylpyrrolidone)) hydrogel for drug delivery.21 Fu et al. reported the application in dye removal using lignosulfonate-graf ted-acrylic acid hydrogels as a new adsorbent.22 Moreover, our group recently developed a new approach to prepare lignosulfonate amine-PEG hydrogel without any cross-linker.23 Interestingly, our study indicated that the utilization of lignosulfonate derivative as macro-cross-linker could eliminate use of a traditional cross-linker (e.g., N,N’-methylenebis(acrylamide)) which was made from petrochemical feedstock with high cost and serious environmental impacts.24 Despite lignosulfonate shows promising prospects in “green” hydrogels by taking its advantages of biocompatibility, biodegradability, and abundance, lignosulfonate-based hydrogel with stimuli-responsive behavior has been rarely reported. In this work, a series of stimuli-responsive hydrogels which contained biocompatible N-isopropylacrylamide (NIPAM) and itaconic acid (IA) were chemically cross-linked by a lignosulfonate derivative, where NIPAM and IA were imported as the temperature- and pH-sensitive components, respectively. Methacrylate groups were introduced into the structure of lignin, and the resulted methacrylated lignosulfonate (MLS) was used as a macro-cross-linker. Effects of MLS content in the reaction on the composition and properties of lignosulfonate-NIPAM-IA hydrogels (LNIH) were investigated. Temperature- and pH-sensitive swelling behaviors of MLS cross-linked hydrogels were systematically studied. Moreover, the mechanical properties of designed hydrogels were improved due to the incorporation of lignosulfonate backbone. The findings from this study may set up a framework for preparation of smart hydrogels from lignin feedstock.

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was added into the aqueous solution to start the polymerization. The reaction was carried out at 70 °C for 4 h and then the solution was placed and cooled down to room temperature to obtain a LNIH hydrogel. Subsequently, to remove non-cross-linked copolymer and/or residual monomers, the LNIH hydrogel was cut into small pieces and washed thoroughly with excess deionized water, which was changed twice every day for at least 5 days. Finally, the washed LNIH hydrogels were separated, cut into pieces, and dried in an oven to dry gel state with constant weight. Characterization. Gel Content. The gel content (G%) is calculated according to eq 1:

G% = Wa /Wb × 100

(1)

where G is the gel content of LNIH, Wa is the weight of dry gel (washed), and Wb is the weight of unwashed hydrogel. Three replicates were performed to determine the average gel content of each sample. Swelling Behaviors of Hydrogels. The dry gel (washed) with known weight was placed into a teabag and then immersed into deionized water for sufficient swelling at 25 °C. Next, the hydrogel sample was taken out from aqueous solution and weighted after the removal of free water on surface with filter paper. The equilibrium swelling capacity (SC) of hydrogel is calculated using eq 2: SC(g/g) = (WS − W0)/W0

(2)

where WS and W0 represent the weights of the swollen hydrogel and dry gel (washed), respectively. Duplicates were performed to determine the average SC value of each sample. The pH sensitivity of LNIH hydrogels (washed) was examined in buffer solution at 25 °C. The pH values of buffer solutions were adjusted by HCl and NaOH and ranged from 3.0 to 9.1, and the ionic strength of buffer solution was 50 mmol/L by adding appropriate amount of NaCl. After the hydrogel samples were sufficiently immersed in buffer solutions, the equilibrium swelling capacities of hydrogels were obtained. FT-IR Spectroscopy. FT-IR spectra of dry gel samples (washed) were recorded on a Nicolet iS50 spectrometer (Thermo-Fisher Corporation, USA) using KBr disc method and collected ranging from 4000 to 500 cm−1 for 64 scans at a 4 cm−1 resolution. Nuclear Magnetic Resonance (NMR). 1H NMR, 13C NMR, and 31 P NMR spectra were recorded on a Varian 400 MHz NMR spectrometer. Particularly, the hydroxyl values of SLS and MLS were analyzed by 31P NMR spectra using TMDP as a phosphitylating reagent according to our previous work.25 Solid-state cross-polarization magic angle spinning (CPMAS) 13C NMR spectra for dry gel (washed) and MLS were acquired at a frequency of 100.63 MHz on a Bruker Avance DRX-400 NMR spectrometer fitted with a Chemagnetics triple resonance 5 mm MAS probe, with a spectral width of 5000 Hz, relaxation delay of 4 s, contact time of 0.5 s, and spinning rate of 8000 Hz. The signal averaged over a total of 20 000 scans and then Fourier transformed after applying 50 Hz line broadening. Elemental Analysis. Carbon, hydrogen, nitrogen, and sulfur contents of dry LNIH samples (washed) were determined on a LECO CHN analyzer (LECO TruSpec, St. Joseph, MI). Scanning Electron Microscopy (SEM) Measurement. The swollen hydrogel was frozen in liquid nitrogen and then freeze-dried. The freezedried hydrogel was surface-coated with Au and examined on a Quanta 200F instrument (FEI, US).

EXPERIMENTAL SECTION

Materials. Sodium lignosulfonate (SLS, 2.97 mmol −OH/g, Mw = 62200 g/mol from GPC analysis) was purchased from Borregaard LignoTech USA Inc. and used as received. N-Isopropylacrylamide (NIPAM, 98%), itaconic acid (IA, 99%), and sodium bisulfite (SBS) were purchased from Acros-Organics. Methacrylic anhydride (MAA, 94%), potassium persulfate (KPS, 99%), triethylamine (TEA, 99%), and 2-chloro-4,4,5,5-tetramethyl-1,2,3-dioxaphospholane (TMDP, 95%) were purchased from Sigma-Aldrich. All reagents were used as received unless otherwise specified. Synthesis of Methacrylated Lignosulfonate (MLS). SLS (3.5 g, 10.4 mmol −OH groups), MAA (8.5 g, 52 mmol), and TEA (0.1 g) were dissolved in 60 mL deionized water. The solution was heated to 70 °C and remained under stirring for 18 h. After cooled down to room temperature, the reaction mixture was poured into ethanol and washed three times with excess ethanol to obtain a brown precipitate. The precipitate was freeze-dried to obtain methacrylated lignosulfonate (Mw = 71400 g/mol). Synthesis of Lignosulfonate-NIPAM-IA Hydrogels (LNIH). To endow the hydrogels with temperature- and pH-responsive properties, the content of NIPAM and IA monomer as temperature- and pH-sensitive components were maintained at above 80 and 4 wt % in copolymerization reaction, respectively. Therefore, NIPAM (1000 mg), IA (50 mg), a certain amount of MLS (up to 200 mg) and KPS (2 wt % on the basis of total weight of reactants) were weighed and dissolved in deionized water (5 mL/1 g reactant) in a glass vial under an argon atmosphere. After stirring at 100 rpm for 10 min, SBS (50% equiv of KPS)

Scheme 1. Schematic Synthesis Route of Methacrylated Lignosulfonate (MLS)

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Figure 1. 1H NMR spectra of SLS (top) and MLS (bottom) in D2O. Differential Scanning Calorimetry (DSC) Analysis. The volume phase transition temperature (VPTT) of hydrogel sample (washed) was determined on a DSC 1 instrument (Mettler-Toledo, Switzerland). The sample was immersed into deionized water of 10 times its weight and allowed to reach an equilibrium swelling. Then, the swollen sample was cut into pieces (6−10 mg), placed into aluminum pan, and sealed. The sample was scanned from 12 to 80 °C at a heating rate 5 °C/min under a continuous N2 flow (40 mL/min). Rheological Measurement. The hydrogel sample for rheology test was prepared in a cylinder-shaped mold with a diameter 25 mm. The resulting LNIH hydrogel was sliced into discs with a thickness of ∼2 mm for tests. Rheological properties were measured on a Discovery HR-2 rheometer (TA Instruments, USA) with a parallel plate

geometry (diameter 25 mm), and the sample was scanned from 0.1 to 160 rad/s at a 1% strain and 25 °C. Shear modulus was tested from stress−strain curve in the strain range of 0−10% at 25 °C.

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RESULTS AND DISCUSSION Preparation of Methacrylated Lignosulfonate (MLS). Sodium lignosulfonate (SLS) was modified through methacrylation reaction using TEA as a catalyst in aqueous media to give MLS (Scheme 1). In comparison with the unmodified SLS, 1 H NMR spectrum of MLS clearly confirmed the methacrylation by the appearance of characteristic peaks that corresponded to protons of double bond CH2 (δ 5.6−6.7 ppm)

Figure 2. 31P NMR spectra of SLS (top) and MLS (bottom). 1765


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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis Route of LNIH Hydrogel from the Reaction of NIPAM, IA, and MLS

and methyl groups −CH3 (δ 2.1 ppm) from the methacrylate moiety (Figure 1), respectively. Quantitative 31P NMR spectrum were also performed to determine the reaction degree through the change of −OH values of SLS and MLS (Table S1). As shown in Figure 2, the content of methacrylate groups in MLS was determined to be 1.61 mmol/g, which was calculated by comparing the total hydroxy content of SLS before (2.97 mmol/g) and after (1.36 mmol/g) the methacrylation reaction.25 Preparation and Characterization of Hydrogels. LNIH hydrogels were synthesized through radical polymerization of NIPAM and IA in the presences of MLS as cross-linker and KPS as initiator at 70 °C (Scheme 2). The control experiment for the synthesis of LNIH hydrogel without use of MLS failed to receive a gel product, revealing that MLS played a cross-linker role in the formation of hydrogel network. Determinations of carbon (C%), hydrogen (H%), nitrogen (N%), and sulfur (S%) contents by elemental analysis further confirmed the formation of the LNIH hydrogels (Table 1). It was noted that the MLS content (MLS%) in the washed hydrogel product increased with the concentration of MLS cross-linker in the reaction, ranging from 3.7 to 14.3 wt %. These results are roughly in good agreement with the theoretical values of MLS% by assuming complete conversion of MLS, suggesting the polymerization condition in this study was sufficient. Hereafter, the hydrogels are named as LNIH-3.7%, LNIH-5.7%, LNIH-6.9%, LNIH-8.6%, and LNIH14.3% in which the numbers correspond to the actual MLS contents in individual hydrogels, respectively. The evidence for the successful preparation of cross-linked hydrogels could also be found from the FT-IR analysis (Figure 3). Several characteristic peaks of MLS, including hydroxyl groups (−OH) stretching at 3470 cm−1, benzene ring vibration at 1506 cm−1, and C−H bending from benzene ring at 1017 cm−1, were found in the spectrum of the LNIH-8.6% sample, respectively. Meanwhile, the peak around 1545 cm−1 corresponding to the

Figure 3. FT-IR spectra of IA, NIPAM, MLS, and LNIH-8.6%.

amide groups (−NHCO, amide II band) from NIPAM was also noted in the spectrum of LNIH-8.6%. In addition, the LNIH-8.6% sample exhibited an obvious shoulder peak around 1724 cm−1 attributed to the carboxyl groups (−COOH) from the IA moiety. Figure 4 shows the 13C NMR spectra of IA (in solution), NIPAM (in solution), MLS (in solid), and LNIH-8.6% (in solid). The peaks at 126−134 ppm ascribed to the double bond CH2 from IA, NIPAM, and MLS, respectively, nearly disappeared in the spectrum of LNIH-8.6%, suggesting a better completion of the copolymerization. Since NIPAM was a major comonomer in the hydrogel, the characteristic signals at 21.26 ppm (−CH3) and 41.66 ppm (−CH) from the NIPAM moiety could be seen in the LNIH-8.6% spectrum, respectively. Furthermore, the signal at 173.63 ppm related to the carbons (CO) from NIPAM,

Table 1. Elemental Compositions of MLS and LNIHs elemental composition sample

a

MLS LNIH-3.7% LNIH-5.7% LNIH-6.9% LNIH-8.6% LNIH-14.3%

MLS (mg) amount in copolymerization

C%

H%

N%

S%

MLS% (measured)b

MLS% (calculated)c

30 50 75 100 200

48.98 58.86 58.26 58.89 58.98 58.81

5.68 9.08 9.06 8.91 8.93 8.70

0.18 10.37 10.28 10.24 10.16 9.61

4.05 0.15 0.23 0.28 0.35 0.58

3.7 5.7 6.9 8.6 14.3

2.8 4.5 6.7 8.7 16.0

a

The hydrogels were prepared using NIPAM (1000 mg), IA (50 mg), various amounts of MLS and KPS (2 wt % of total weight of reactants) in deionized water (5 mL/1 g reactants) under an argon atmosphere. The hydrogels were then washed with deionized water. bMLS content (wt %) in hydrogel was calculated from the measured S content by referring the S content of pure MLS (4.05 wt %). cCalculated MLS content (wt %) in the gel by assuming complete conversion for each comonomer. 1766


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Figure 4. 13C NMR spectra of IA (in solution), NIPAM (in solution), MLS (in solid), and LNIH-8.6% (in solid).

solvent by sublimation which left voids in place where the solvent previously occupied. In addition, it was noted that the pore size of hydrogel became larger with MLS content. A similar phenomenon was also previously noted in the study of another lignin-based hydrogel material.23 This result was likely due to the increased cross-link density that caused faster phase separation during freezing, resulting in formation of more coarse structure. Swelling Properties. In Figure 6, the gel content increased from 62.4 to 80.8% with MLS content increasing from 3.7 to 14.3% in the reaction. Meanwhile, the swelling capacity decreased

MLS, and IA could also be easily recognized in the spectrum of LNIH-8.6%. Both the FT-IR and 13C NMR results confirmed the successful copolymerization of MLS, NIPAM, and IA. The morphologies of the freeze-dried hydrogel samples were investigated by SEM (Figure 5). The obtained hydrogel samples exhibited porous and mesh-like morphological structures after the removal of water from the hydrogel networks under the freeze-drying condition. This observed porous structure of freeze-dried hydrogel was due to the phase separation of the gel during rapid cooling (freezing) and subsequent removal of the

Figure 5. SEM micrographs of LNIH hydrogels (a) LNIH-3.7%, (b) LNIH-5.7%, (c) LNIH-6.9%, (d) LNIH-8.6%, and (e) LNIH-14.3% crosslinked with different MLS contents. 1767


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Table 2. Kinetic Parameters of Different LNIHs for Absorption of Deionized Water sample

SCeq (g/g)a

SCexp (g/g)b

LNIH-3.7% LNIH-5.7% LNIH-6.9% LNIH-8.6% LNIH-14.3%

32.6 28.9 25.4 22.7 19.4

31.6 28.4 24.5 22.5 19.1

K [g/(g·s)] 3.6 6.0 3.7 11.5 13.1

× × × × ×

10−5 10−5 10−5 10−5 10−5

a Calculated equilibrium swelling capacity bExperimental equilibrium swelling capacity.

for each LNIH, it is noted that the most cross-linked LNIH14.3% possessed the lowest calculated and experimental values of swelling capacity (SCeq and SCexp), but the highest swelling rate constant (K). This result is likely the consequence of two competing mechanisms in the hydrogel. As the sulfonatecontaining MLS is a strong water-absorbing moiety in the resulting hydrogel, thus increasing MLS content would result in increase of swelling capacity; on the other hand, the cross-link density increases with MLS content, which would result in drastic decrease in swelling capacity. According to the assumption of pseudo-second-order model, the swelling rate of the cross-linked LNIH hydrogel was controlled by relaxation process of polymer chains.28 Temperature Responsive Behavior. The NIPAM-type hydrogel exhibits volume-phase transition performance due to the conformation variation of NIPAM chains on network while the temperature is set below or above the volume phase transition temperature (VPTT).29,30 In Figure 8a, like other typical poly-NIPAM hydrogels,31,32 LNIH-3.7%, LNIH-5.7%, and LNIH-6.9% remained swollen in deionized water at lower temperatures but experienced a sharp deswelling process with increase in temperature. However, the temperature responsive behavior was not clearly noted for both the LNIH-8.6% and LNIH-14.3% samples. The more rigid architecture of hydrogel was formed with higher MLS content, which overwhelmed the volume-phase transition performance of NIPAM chains in network above VPTT. In Figure 8b, DSC analysis indicated the VPTTs of LNIH3.7%, LNIH-5.7%, and LNIH-6.9% were around 34.5, 34.5, and 35.5 °C, respectively, which were close to the human physiological temperature (37 °C). Increasing cross-linker content in polymeric network resulted in slightly higher VPTT values for LNIHs than that of a typical poly-NIPAM hydrogel (∼32 °C),33

Figure 6. Swelling capacity (g/g) and gel content (G%) of LNIH hydrogels.

from 32.1 to 20.2 g/g with increasing MLS content. Based on these results, it can be assumed that the decrease in swelling capacity was associated with the increase in cross-link density of the gel. The swelling rate of the hydrogel was also investigated as a function of time at 25 °C (Figure 7a). It was noted that the amount of absorbed water increased rapidly during the initial swelling for each hydrogel and then increased slowly until reached equilibrium. The swelling kinetics of LNIHs in deionized water were explored using a pseudo-second-order model based on eq 3.26 t 1 t = + SCt SCeq K · SCeq 2

(3)

where SCt is the swelling capacity of swollen hydrogel at swelling time t, K is the swelling rate constant, and SCeq is the swelling capacity at equilibrium time. As displayed in Figure 7b, all plots of t/SCt versus t exhibited an ideal straight line with high correlation coefficients, which demonstrated that the swelling behaviors of hydrogels could be well described by the pseudo-second-order model.27 Based on the fitting curves, the kinetic parameters including swelling rate constant (K) and the theoretical equilibrium swelling capacity (SCeq) in swelling process were obtained and are listed in Table 2. The values of calculated equilibrium swelling capacity were in good agreement with the experimental data for all hydrogels. By analyzing all the values of SCeq, SCexp, and K

Figure 7. (a) Swelling rate and (b) pseudo-second-order kinetics of hydrogels in deionized water. All the values of correlation coefficient (R2) > 0.998. 1768


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Figure 8. (a) Temperature-sensitive behaviors of LNIH hydrogels and (b) DSC curves of LNIH hydrogels in the temperature range from 12 to 80 °C.

Figure 9. Swelling capacities of LNIH hydrogels at different pH values under (a) 25 and (b) 42 °C.

condition (pH > 7.4) for screening effect, leading to the shrinking of hydrogels, thus their swelling capacities decreased subsequently. At 42 °C, all LNIH hydrogels exhibited relatively lower swelling capacities compared with that at 25 °C due to their temperature-responsive shrinking behaviors above VPTT (Figure 9b). Meanwhile, it should be noted that LNIH hydrogels still maintained pH responsive behaviors at 42 °C in the experimental pH range which were consistent with the observations at 25 °C,35 as has been discussed above. These results suggest that the swelling behavior of LNIH hydrogels can be manipulated by varying pH of the solution. Rheological Properties. Rheological properties of LNIH hydrogels with different MLS contents were measured at 25 °C. Because of the hydrogel network structure, the storage modulus (G′) of all LNIH hydrogels were higher than the corresponding loss modulus (G″) over the whole selected angular frequency range (Figure 10a and b). Besides, G′ showed a monotonous increase with MLS content in the gel. This result was most likely attributed to the rigid structure of lignin and increased cross-link density,23,36 leading to the increase of stiffness of the network structure. G″ also increased with MLS content in the gel, which was a consequence of higher cross-link density of the gel leading to more heat dissipation for chain segment movement. The positive effect of MLS content on the mechanical properties of LNIH hydrogels could also be observed in their stress−strain curves (Figure 10c), where LNIH-14.3% presented much higher stress values than the other hydrogels over the entire examined

revealing that the higher temperature was needed to drive the disruption of hydrogen bonds strengthened by the increased MLS content. Furthermore, LNIH-8.6% and LNIH-14.3% showed no obvious VPTT peak in DSC measurement, which were consistent with the results of temperature responsive swelling test. pH Responsive Behavior. The pH responsive behaviors of LNIH hydrogels were studied within the range of pH 3.0 to 9.1 in buffers below and above VPTT (Figures 9, S2, and S3). At 25 °C which was below VPTT, all hydrogels exhibited lower swelling capacities in 20 mM buffers with a 50 mM ionic strength than in pure water, owing to the higher ionic strength of external solution.2 The swelling capacities of all hydrogels increased with pH increasing up to 7.4 and then decreased with further increase of the pH value (Figure 9a). In view of the chemical structure of the hydrogel, introduction of itaconic acid (pKa1 3.85 and pKa2 5.44) mainly contributed to the pH sensitivity of hydrogels. Under strong acidic condition, the carboxyl groups in the polymeric network existed in acid form (−COOH) and could form intermolecular hydrogen bonds. This internal hydrogen bonding resulted in the unfavorable swelling behavior and lower swelling capacity for hydrogels. As the pH value increased to 7.4, the carboxyl groups gradually transformed into the ionized carbonate form (−COO−) which led to stronger hydrophilicity and higher electrostatic repulsion of the network, triggering the increased water absorption capacity.34 However, the repulsion of the negative −COO− groups from itaconic acid and −SO3− groups from MLS would be shielded by more Na+ ions in the basic 1769


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dual temperature and pH responsive properties. Use of methacrylated lignosulfonate as cross-linker enabled efficient cross-linking of the copolymer hydrogels with acceptable gel contents. Study of swelling kinetics revealed that the pseudo-second-order model was suitable for describing the water absorption of LNIH hydrogels. LNIH-3.7%, LNIH-5.7%, and LNIH-6.9% hydrogels showed temperature-sensitive behaviors around 35 °C, which were very close to the physiological temperature (37 °C). Additionally, LNIH hydrogels also exhibited pH-sensitive due to the carboxyl groups from IA moieties. Moreover, shear strength and rheological properties of LNIH hydrogels increased with MLS content in the polymeric network. The lignosulfonate-based hydrogels with stimuli-responsive behaviors may have great potential for controlled release of some pesticides or drugs in various conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03158. Additional supporting figures (Figure S1−S3) for LNIH hydrogels under different pH and temperature conditions and 31P NMR results (Table S1) for samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.X.). *E-mail: [email protected] (J.Z.) *E-mail: [email protected] (Z.K.). ORCID

Tuan Liu: 0000-0001-7868-9424 Jinwen Zhang: 0000-0001-8828-114X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 31400516) and Fundamental Research Funds for the Central Nonprofit Research Institution of Chinese Academy of Forestry (No. CAFYBB2014QB043).

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Figure 10. Curves of (a) storage modulus (G′, 1% strain), (b) loss modulus (G″, 1% strain), and (c) stress−strain for LNIH hydrogels with various cross-linker contents. All the hydrogel test specimens contained 83.3 wt % water.

strain range. Together with the observation of G′ and G″ values, these results indicate that increasing the content of the multifunctional MLS cross-linker can dramatically improve the mechanical properties of the hydrogels.

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CONCLUSIONS We demonstrated that a methacrylated lignosulfonate derivative (MLS) was utilized as a novel cross-linker in the preparation of lignosulfonate-NIPAM-IA copolymer hydrogels (LNIH) with 1770


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