Facile fabrication and characterization of highly stretchable lignin-based hydroxyethyl cellulose self-healing hydrogel

Accepted Manuscript Title: Facile fabrication and characterization of highly stretchable lignin-based hydroxyethyl cellulose self-healing hydrogel Authors: Siqi Huang, Shuyi Su, Haibo Gan, Linjun Wu, Chaohui Lin, Danyuan Xu, Haifeng Zhou, Xuliang Lin, Yanlin Qin PII: DOI: Article Number:

S0144-8617(19)30747-7 https://doi.org/10.1016/j.carbpol.2019.115080 115080

Reference:

CARP 115080

To appear in: Received date: Revised date: Accepted date:

15 May 2019 4 July 2019 12 July 2019

Please cite this article as: Huang S, Shuyi S, Gan H, Linjun W, Lin C, Danyuan X, Zhou H, Lin X, Qin Y, Facile fabrication and characterization of highly stretchable ligninbased hydroxyethyl cellulose self-healing hydrogel, Carbohydrate Polymers (2019), https://doi.org/10.1016/j.carbpol.2019.115080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile fabrication and characterization of highly stretchable lignin-based hydroxyethyl cellulose self-healing hydrogel Haifeng Zhou c, Xuliang Lin a, *, Yanlin Qin a, *

School of Chemical Engineering and Light Industry, Guangdong University of

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a

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Siqi Huang a, Shuyi Su a, Haibo Gan a, Linjun Wu a, Chaohui Lin b, Danyuan Xu a,

Technology, Guangzhou, China

School of Electro-mechanical Engineering, Guangdong University of Technology,

Key Laboratory of Low Carbon Energy and Chemical Engineering, College of

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c

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Guangzhou, China

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b

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Chemical and Environmental Engineering, Shandong University of Science and

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Technology, Qingdao, China * Corresponding Author: Tel.: 86-20-39322812; Fax: 86-20-39322812;

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E-mail: [email protected] (X.L. Lin); [email protected] (Y.L. Qin)

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Highlights:

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Highly stretchable hydrogel was prepared from biomass lignin.

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LCP hydrogel performed excellent viscoelasticity and stretchability.



Compared with PVA hydrogel, the elongation rate of LCP increased by 20 times.



LCP hydrogel showed good thermosensitivity and electrical conductivity.

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Abstract： In this study, hydroxyethyl cellulose (HEC) and polyvinyl alcohol (PVA) as the framework, borax as the cross-linker, and biomass lignin from pulping black liquors

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and biorefinery as the plasticizer were used to synthesize the lignin-based HEC-PVA

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(LCP) physical self-healing conductive hydrogel with highly stretchable and thermosensitive properties by the one-step fabrication method. Compared with the

PVA hydrogel, the maximum storage modulus and the elongation rate was increased by

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7 times and 20 times, respectively. Uniformly distributed lignin could increase the

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mobility and distance of polymer molecular chains, therefore improve the

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viscoelasticity and stretchability of the LCP self-healing hydrogel. The LCP hydrogel could recover to the original state in 12 s after 10000% shear strain for 4 cycles. The

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LCP hydrogel also presented good thermosensitivity and electrical conductivity, and

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were very promising for applications in the fields of 3D printing and wearable

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electronic devices, that broadening the efficient utilization of biorefinery lignin.

Keywords: Rheology; PVA; borax; lignin; thermosensitive

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1. Introduction Hydrogels with self-healing properties can restore their structure and function

after injury, that have great potential to address microcracks and recessive damage of polymer structural materials (White et al., 2001). The ability to repair microcracks made it have a broad range of potential applications in drug delivery (Bastings et al., 2

2014), wound dressing (Z. Li et al., 2018), artificial tissue engineering (Yang, Abe, Biswas, & Yano, 2018) and disc nucleus replacement (Reitmaier et al., 2012). Self-healing hydrogels were mainly prepared from unsustainable fossil materials via dynamic chemical bonds, including dynamic covalent bonds (Wei et al., 2014),

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hydrophobic interaction (Tuncaboylu, Sari, Oppermann, & Okay, 2011), host-guest

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interaction (Jia & Zhu, 2015), hydrogen bonding (W. Lu, Le, Zhang, Huang, & Chen,

2017) and so on. Dynamic noncovalent interactions with reversible nature contribute to the synthesis of physically self-healing hydrogels (Niu, Wang, Dai, Shao, & Huang,

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2018). Polyvinyl alcohol (PVA) with good biocompatibility, non-toxicity and favorable

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mechanical properties was widely used to prepare self-healing hydrogels with borax by

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diol bond and hydrogen bonding (Koga, Takada, & Nemoto, 1999). These hydrogels exhibited physically crosslinked network via the hydrogen bond and reversible

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didiol-borax complex (B. Lu et al., 2016). Typical PVA-borax self-healing hydrogels

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had limited applications due to the poor mechanical strength. However, the

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introduction of biomass polysaccharide could overcome these issues of the PVA hydrogel.

Cellulose nanomaterial was an ideal polymer reinforcing filler to improve the

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mechanical properties of PVA-based hydrogels (Eichhorn et al., 2010; Moon, Martini, Nairn, Simonsen, & Youngblood, 2011). The incorporation of nanofibrillated cellulose, highly-crystalline cellulose nanoparticle and ball milling pretreated microcrystalline cellulose could improve the non-Newtonian behavior, flow properties and thermal 3

stabilities of the PVA-borax crosslinked system (J. Han, Lei, & Wu, 2013, 2014; J. Q. Han et al., 2017; B. Lu et al., 2016). Nevertheless, the major issue associated with these cellulose nanomaterial-based hydrogels was their inherent lamellar deposition structure and irreversible self-aggregation behavior (Bian, Jiao, et al., 2018), which

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lead to the unsatisfactory stretchability and plasticity.

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Lignin is the most abundant aromatic natural polymer, which is made up of three kinds of phenylpropane structural units, such as syringylpropane units (S type), guaiacyl propane units (G type) and p-hydroxyphenylpropane units (H type)

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(Laurichesse & Avérous, 2014; Qian et al., 2018). Lignin could serve as reinforcing

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fillers, UV adsorbents, antioxidants, antibacterial agents, carbon precursor and

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biomaterials for tissue engineering and gene therapy (Fortunati et al., 2016; H. Li et al., 2016; Tang, Zhou, Li, Qiu, & Yang, 2017). Acetonitrile extracted lignin and modified

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lignin with alkyne group could be used to prepare self-healing hydrogels with high

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stretchability crosslinked with polyethylene glycol diglycidyl ether (Cui et al., 2018)

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and poly(5-acetamidopentyl acrylate) (H. L. Liu & Chung, 2016). Lignin-based hydrogels were expected to play an important role in adhesives, coatings, medical materials (Larrañeta et al., 2018), dye and metal scavengers (Domínguez-Robles et al.,

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2018; J. Li et al., 2019; Thakur et al., 2017), but the fabrications often involved toxic chemicals，relatively complicated synthesis processes (Kai et al., 2015; H. L. Liu & Chung, 2016) and poor yields (Cui et al., 2018). Based on the consideration of previous works upon the lignin-based polymers,, it is reasonable to hypothesize that lignin can 4

improve the stretchability, viscoelasticity and other characteristic of PVA-borax hydrogels. Lignocellulose hydrogels were usually regenerated by dissolving lignocellulose in the solvent such as N-methylmorpholine-N-oxide (NMMO) (L. Zhang et al., 2019),

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ionic liquid or NaOH/Urea (Ciolacu, Oprea, Anghel, Cazacu, & Cazacu, 2012), or

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prepared by chemical or physical crosslinking from lignin containing cellulosic fiber

(Bian, Wei, et al., 2018), that could be used for the removal of heavy metals and dyes and the release of polyphenols. Despite the recent progress in developing

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lignocellulosic hydrogels, the current preparation methods of lignocellulosic hydrogels

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had critical issues of high toxicity of organic solvent, expensive ionic liquid, high

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energy cost at low temperature, and harsh synthesis conditions. In this study, hydroxyethyl cellulose (HEC) and PVA were used as the framework,

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borax as the cross-linker, biomass lignin from pulping black liquors and biorefinery as

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the plasticizer to synthesize the lignin-based HEC-PVA (LCP) physical self-healing

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hydrogel with highly stretchable, conductive and thermosensitive properties by the one-step fabrication method. The effects of lignin amount, HEC and PVA molecular structure on the stretchability and rheological properties of the LCP self-healing

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hydrogel were first studied. Biorefinery lignin with various molecular weights fractionated by alkaline solvent was used to study the effect of lignin structure on the stretchability of the LCP hydrogel. What’s more, the self-healing, conductive and thermosensitive properties of the LCP physical self-healing hydrogel were investigated, 5

and the structure mechanism of the LCP self-healing physical hydrogel was proposed. Overall, the present research broadened the effective utilization filed of biorefinery residue lignin.

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2. Materials and Methods 2.1 Materials

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Alkali lignin was obtained from the black liquor of pine and eucalyptus soda

pulping, supplied by Xiangjiang Paper Co., Ltd. (Hunan province, China). The weight

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average molecular weight (Mw) of alkali lignin was 4,300 g/mol, the polydispersity was

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2.76, and the phenolic hydroxyl group content was 1.69 mmol/g.

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Corncob lignin was the biorefinery residual of furfural and glucose production

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from corncob, supplied by Shandong Vland Biotech Co., Ltd. The acid-insoluble

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lignin (klason lignin) content and cellulose content of corncob lignin raw material was 20.4% and 60.1%, respectively.

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Hydroxyethyl cellulose (HEC, Mw = 90,000, 720,000, 1,300,000 g/mol,

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hydroxyethyl substitution degree is 2.5) was purchased from Sigma-Aldrich, polyvinyl alcohol (PVA, Mw = 47,000, 145,000, 205,000 g/mol, the degree of alcoholysis is 99%, 99%, and 88%, respectively), borax (sodium tetraborate

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decahydrate, AR 99.5%, Na2B4O7•10H2O, Mw = 381.37 g/mol) reagents were purchased from Shanghai Aladdin Chemical Co., Ltd., and all solutions were prepared using deionized water.

2.2 Fractionation of corncob lignin 6

Corncob lignin with various molecular weight fractions were fractionated with different concentrations of sodium hydroxide. 10 g of Corncob lignin was dissolved in 100 mL of 0.1 M sodium hydroxide solution at room temperature, the cellulose insoluble material was removed by centrifugation, and the pH was adjusted with dilute

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hydrochloric acid, then the precipitate was centrifuged and washed with water several

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times, and vacuum drying to obtain Lignin I fraction ( CCL1); The insoluble material of

first step was dissolved in 100 mL of 0.5 M sodium hydroxide solution at 80 °C, centrifugally precipitated, acid precipitated, washed with water, and vacuum drying to

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obtain Lignin II fraction (CCL2); the remainder was mainly a mixture of cellulose and

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lignin (CCL3).

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2.3 Characterization of Lignin

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The Mw of alkali lignin, CCL1, and CCL2 were determined using GPC on an ICS-3000 system (Dionex, Sunnyvale, CA). Lignin sample (5 mg) was dissolved in 1

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mL of HPLC-grade tetrahydrofuran (THF) without stabilizer, and 50 μL of the

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solution was injected onto the GPC columns. THF was used as eluent at a flow rate of 1 mL/min. The column temperature was 30 °C. Polystyrene standards with the Mw of

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170, 1,300, 2,200, 5,200, 13,000, 25,000 and 50,000 g/mol were used for calibration. Lignin samples and polystyrene standards were detected by a mutiple wavelength detector at 280 and 254 nm, respectively (N. Li et al., 2018). Phenolic hydroxyl contents of the lignin samples were determined by the Folin– Ciocalteu colorimetric method with vanillin as the reference substance (X. Lin et al., 7

2014).

2.4 Preparation of LCP hydrogels The LCP hydrogels with 1% of HEC, 3% of PVA, 0.8% of borax and desired

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amount of lignin were prepared via one-pot reaction. 0.05 g of hydroxyethyl cellulose powder, a certain amount of alkali lignin was dissolved in 0.8% borax solution to

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make the total mass of 4.85 g. The mixture was placed in an oil bath at 90 °C and

stirred at 400 rpm for 10 min until the lignin and HEC dissolved uniformly. After the

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mixture was taken out, 0.15 g of polyvinyl alcohol was added, and the PVA was

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uniformly dispersed in the solution at room temperature with magnetic stirring to

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prevent agglomeration. The mixture was heated at 90 °C in an oil bath with magnetic

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stirring for 60 min. The pH of the mixture was about 9.2. The viscous liquid obtained

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by the reaction was frozen under 0 °C for 12 h. As the mixture cooled down to room temperature, the homogeneous and stable LCP hydrogel was finally formed. PVA

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hydrogel with 3% of PVA and 0.8% of borax was prepared as the control, the

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composite hydrogel containing 1% of HEC, 3% of PVA and 0.8% of borax was named as the CP hydrogel, and the CP hydrogels containing lignin with the concentration of

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0.5%, 1%, 2% and 3% were labelled as LCP-0.5%, LCP-1%, LCP-2% and LCP-3%, respectively.

2.5 Characterization of LCP hydrogels The FTIR spectrum of pure PVA, HEC, AL powder and the LCP hydrogels was measured in a transmittance mode at room temperature using an FTIR analyzer (iS50R, 8

Themor-Fisher, America) with a universal attenuated-total-reflection (ATR) probe. A total of 64 scans were performed for each test using air as the background with a resolution of 0.09 cm-1 and a spectral range of 4000-500 cm-1. PVA, CP and LCP hydrogels were used directly for FTIR analysis after freeze-drying.

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The rheological behavior of PVA, CP, LCP hydrogels were analyzed using an

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Anton Paar MCR302 rheometer (Anton Paar, Ashland, VA, USA) using a parallel

plate of 25 mm diameter. Before the test, hydrogel was set on the platform, when the gap between the plate and platform was 1 mm before measurement, the excess

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hydrogel out of the plate was scrapped off. The samples were analyzed straight twice

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after one freeze-thawing cycle, and the data was reported when the error was less than

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5%. The dynamic strain sweep was set at a fixed angular frequency of ω=10 rad/s at room temperature of 25 °C, the shear strain γ varying from 0.1% to 100%, and the

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storage modulus (G′) and loss modulus (G″) were recorded; Dynamic frequency

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sweep was measured over the ω range of 0.1–100 rad/s at 25 °C with shear strain γ=1%

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in the linear viscoelastic region. For the shear stress, the damping factor tan δ (G" / G′) was set as the ratio of loss modulus to storage modulus. The dynamic temperature sweep was measured through a heating-cooling-heating process (20-50-20-50 °C) at

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ω=10 rad/s and γ=1%. The heating and cooling rates were 2.0 and 1.0 °C/min, respectively (J. Han et al., 2014).The self-recovery properties of the LCP hydrogel were evaluated using an oscillatory deformation and recovery cycle test. The 3ITT ladder test (oscillation mode, continuous step strain) was used to study the structural 9

damage and recovery process of self-healing hydrogel. The programmed procedures was performed as follows: shear strain γ=1% (60 s) → γ=10000% (60 s) → γ=1% (60 s) → γ=10000% (60 s) → γ=1% (60 s) → γ=10000% (60 s) → γ=1% (60 s) → γ=10000% (60 s) → γ=1% (72 s) at the angular frequency ω=10 rad/s.

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The samples were making in a size of 40 mm × 5 mm × 5 mm before tensile

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testing. The hydrogel was stretched from both ends until it was break, and the initial

length and final length of the hydrogel were recorded. The elongation rate of LCP hydrogel was evaluated by Equation 1, where L0 and L are the initial and final length

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of the LCP hydrogel (cm), respectively. ε represents the elongation rate. Each sample

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was repeatedly stretched 3 times and the average was reported. (1)

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ε=(L-Lo )/Lo

The conductivity of the LCP hydrogel was evaluated by using a circuit with the

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hydrogel, 3 V voltage batteries and a light-emitting diode (LED). The resistance of the

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hydrogel was measured by a universal meter. Each measurement was repeated three

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times and the average was recorded.

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3. Results and discussion 3.1 Preparation of LCP hydrogels Hydroxyethyl cellulose and lignin with rich hydroxyl groups had good compatibility with PVA, and the LCP self-healing hydrogels were formed by dynamic 10

hydrogen bonding between HEC, lignin and PVA with borax. As shown in Fig. 1 (a), the LCP hydrogels self-healed quickly without marks after cutting, while the mixture of lignin, HEC and PVA without borax could not show the self-healing behavior. The incorporation of HEC and lignin could improve the viscoelasticity and stretchability

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of the LCP hydrogel. As shown in Fig. 1 (b), PVA, CP and LCP hydrogels can be

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stretched from the initial of 4 cm to 10.5 cm, 62 cm, and 104 cm, respectively; the

corresponding elongation rate was increased from 2.8 to 15.5 and 26. The LCP hydrogels were intertwined by the attraction of hydrogen bond and borax-diol bond,

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and could be shaped into different shapes by external force, as shown in Fig. 1 (c).

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The plasticity, injectability and moldable processability demonstrates the LCP

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hydrogels are very promising for application in the 3D printing field.

Fig. 1 (a) Self-healing diagram, (b) stretching diagram and (c) plasticity of PVA, CP and LCP hydrogels

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3.2 FTIR analysis of LCP hydrogels As shown in Fig. 2, the peak around 3449 cm-1 obviously shifted to the lower wavenumber at 3345 cm-1, which probably indicated the formation of hydrogen bonding between -OH groups of PVA chains, HEC chains and lignin molecules. In

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addition to C-O stretching vibration of HEC at 1050 cm-1, the absorption peak at 1515

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cm-1, 1425 cm-1 and 1600 cm-1 in the LCP hydrogel was assigned to the aromatic

skeleton vibration of lignin, and the absorption peak at 1375 cm-1 was attributed to the stretching vibration of aliphatic C-H and phenolic hydroxyl group of lignin.

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Characteristic peaks, such as 661 cm-1 (bending of the B-O-B bond in the borate

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network), 833 cm-1 (B-O stretching from residual B(OH)4-), confirming the presence of

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borax and borate (D. Liu, Ma, Wang, Tian, & Gu, 2014). The new absorption peak at

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1333 cm-1 (asymmetric stretching relaxation of B-O–C) was correlated to the hydroxyl groups that have complexed (formed crosslinks) with borate ions (Spoljaric, Salminen,

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Luong, & Seppälä, 2014). All these results suggested that the successful complexation

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of PVA, HEC, lignin and borate ion took place which lead to the physically

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1515 1375 1333

3345

1050

661

1425

Transmittance (%)

3449

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833

1600 PVA

HEC powder

CP

AL powder

LCP-2%

4000

3000

1500

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PVA powder

1000

500

-1

Wavenumber (cm ) crosslinked hydrogel.

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Fig. 2 FTIR spectra of pure PVA, HEC, AL powder and the PVA, CP, and LCP

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hydrogels

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3.3 Effect of composition structure on the rheological and stretchability

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properties of LCP self-healing hydrogels

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3.3.1 Effect of the concentration of lignin The LP hydrogel prepared with 1% of lignin, 3% of PVA and 0.8% of borax or the

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LC hydrogel prepared with 1% of lignin 1% of HEC and 0.8% of borax cannot form hydrogel under the same conditions. The effect of lignin concentration on the

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rheological properties and stretchability of the LCP hydrogel was shown in Fig. 3. The storage modulus (G′) of the PVA hydrogel was less than the loss modulus (G″) at γ=0.1~100%, indicated that the PVA hydrogel was fluid-like trend. After the addition of HEC or HEC and lignin, the maximum storage modulus in the linear viscoelastic region (LVR) of the CP or LCP hydrogel was increased from 110 Pa to 170 Pa and 1000 13

Pa, respectively, as shown in table 1; and Gˊ is greater than G′′, which indicated the CP and LCP hydrogel performed elastic-like behavior. When the shear strain was increased, LCP hydrogel performed Gˊ < G′′, indicating the fluid-like properties. The corresponding frequency of the PVA, CP and LCP-1% hydrogels exhibited G' > G"

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was higher than 15.8 rad/s, 2.51 rad/s and 1.58 rad/s, as shown in Fig. 3(b). The results

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indicated that the LCP hydrogel could maintain their shape at a low frequency with the addition of lignin. The storage modulus and elongation rate of the LCP hydrogel was increased rapidly with increasing the lignin concentration. When the lignin

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concentration was 1%, the storage modulus of LCP hydrogel reached a maximum of

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1,500 Pa in the high frequency region. The damping factor between 0.1 rad/s and 100

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rad/s was less than 1, Gˊ > G′′, and the elongation rate was up to 34.4, which was larger than that of molecularly engineered dual-crosslinked hydrogel (>7 times) (P. Lin, Ma,

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Wang, & Zhou, 2015) and dual physically crosslinked hydrogels triggered by

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nanosheets and iron ions (Fe3+) (circa 21 times) (Hu et al., 2016). When the lignin

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concentration continued to increase, the storage modulus and elongation rate of the LCP hydrogels was decreased, however, it was still larger than that of the CP hydrogel. When the lignin concentration was 3%, the elongation rate of the LCP hydrogel was

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lower than that of the CP hydrogel. The CP or LCP hydrogels with the lignin concentration less than 1% were sticky at low frequency and performed elastic-like behavior at high frequency. The LCP hydrogels with the lignin concentration higher than 2% exhibited elastic-like behavior at ω=0.1 rad/s to 100 rad/s, implying a rather 14

stable and strong network of the hydrogel thanks to the interaction of lignin molecules and flexible PVA chains or HEC chains via hydrogen bonds and physical junctions in the presence of borax.

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(a)

100 G'

G" PVA CP LCP-0.5% LCP-1% LCP-2% LCP-3%

1

10

100

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0.1

PVA CP LCP-0.5% LCP-1% LCP-2% LCP-3%

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G',G"(Pa)

1000

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Shear strain (%)

(b)

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100

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G',G"(Pa)

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1000

G'

10 1

10

Angular frequency (rad/s)

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0.1

G" PVA CP LCP-0.5% LCP-1% LCP-2% LCP-3%

15

PVA CP LCP-0.5% LCP-1% LCP-2% LCP-3%

100

10

(c)

1

0.1 0.1

1

10

100

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Angular frequency (rad/s)

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tanδ

PVA CP LCP-0.5% LCP-1% LCP-2% LCP-3%

Fig. 3 The LCP hydrogels at different lignin concentrations: (a) dynamic strain sweep curve at ω = 10 rad / s; (b) dynamic frequency sweep curve at γ = 1%; (c) tan δ at

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dynamic frequency sweep curve at γ = 1%

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Table 1 Effect of the lignin concentration on the elongation rate and rheological

elongation

maximum Gˊ with

G″ with LVR at

high frequency

rate

LVR at ω=10 rad/s,

ω=10 rad/s(Pa)

plateau of Gˊ,

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hydrogel

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properties of the LCP hydrogels

Gˊ∞（ω=100 rad/s, Pa）

2.8±1.1

110

140

510

16.5 ± 1.5

170

120

550

24.0 ± 0.8

480

390

800

34.4 ± 1.5

1,000

670

1,500

LCP-2%

17.3 ± 3.0

470

350

1010

LCP-3%

13.8 ± 1.3

320

180

560

CP LCP-0.5%

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LCP-1%

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PVA

Gˊ max (Pa)

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3.3.2 Effect of the molecular weight of HEC The LCP hydrogels with 1% of HEC, 3% of PVA, 0.8% of borax and 2% of lignin

were synthesized. The effect of HEC molecular weight on the rheological properties and stretchability of the LCP hydrogels was shown in Table S1. In the linear 16

viscoelastic region, the maximum storage modulus of the hydrogels with HEC molecular weight of 90,000, 720,000, and 1.300000 were 320 Pa, 470 Pa and 950 Pa, respectively, and could be stretched to 4.1, 17.3 and 21.5 times beyond the initial hydrogel, respectively. The storage modulus and elongation rate of the LCP hydrogels

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was increased with the increase of molecular weight of HEC. The LCP hydrogels with

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different molecular weights of HEC exhibited elastic-like behavior in the range of ω=

0.1 rad/s to 100 rad/s. The results showed that the HEC linear polymer in the LCP

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hydrogels could continue to stretch when the molecular weight of HEC increased.

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3.3.3 Effect of the molecular weight of PVA

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The LCP hydrogels with 1% of HEC, 3% of PVA, 0.8% of borax and 2% of lignin

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were synthesized. The effect of PVA molecular weight on the rheological properties

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and stretchability of LCP hydrogels was shown in Table S2. The results showed that PVA with the molecular weight of 47,000 could not form a self-healing hydrogel,

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exhibited fluid-like behavior. In the linear region, PVA with the molecular weight of

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145,000 and 205,000 formed the hydrogel performing the maximum storage modulus of 620 Pa and 950 Pa, and could be stretched to 30.9 and 21.5 times beyond the initial

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hydrogel, respectively. The LCP hydrogels of various PVA molecular weight appeared elastic-like behavior at high frequency, but viscous-like behavior at low frequency. Apparently, PVA with low molecular weight exhibited better stretchability, which may cause by the lower degree of crosslinking.

3.3.4 Effect of the structure of corncob lignin 17

Refined Corncob lignin CCL1 and CCL2 with various molecular weight fractions were fractionated with different concentrations of sodium hydroxide from enzymatic hydrolysis lignin of corncob biorefinery residual. The weight average molecular weight, the hydroxyl group content and the yield based on klason lignin of corncob

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lignin was showed in table 2.

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Lignin-based self-healing hydrogels were prepared from refined corncob lignin

CCL1, CCL2 and CCL3. CCL, CCL1, CCL2 and CCL3 lignin-based self-healing hydrogels could be stretched to 27.8, 39.3, 45.3 and 23.5 times beyond the initial

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hydrogel, respectively, as shown in table 2. The results showed that CCL1 and CCL2

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with high purity and low molecular weight had better solubility and reactivity, and thus

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showed better compatibility with HEC and PVA. The uniform lignin-based self-healing hydrogel had excellent viscoelasticity and stretchablity. The stretchability of refined

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corncob lignin-based hydrogel was better than that of alkali lignin.

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Table 2 Effect of corncob lignin molecular weight on the elongation rate of LCP

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Fractionation

yield

self-healing hydrogels Mw

PDI

(%)

OH

Klason

(mmol/g)

lignin

glucan

elongation rate

CCL

—

—

—

—

60.1%

20.4%

27.8 ± 1.5

CCL1

23.7

1870

2.43

5.16

100%

—

39.3 ± 1.8

CCL2

61.3

2610

4.00

4.53

100%

—

45.3 ± 2.8

CCL3

8.8

—

—

—

24.4%

50.4%

23.5 ± 4.0

Note: HEC concentration was 1%, PVA concentration was 3%, Na2B4O7 concentration was 0.8%, lignin concentration was 2%, Mw (HEC) = 720,000 g/mol, Mw (PVA) = 205,000 g/mol. 18

3.4 Self-recovery of LCP self-healing hydrogels The recovery and thixotropic properties of the LCP hydrogels were evaluated using an oscillatory deformation and recovery cycle test. Fig. 4 showed the storage modulus was higher than the loss modulus at γ=1%, and the LCP hydrogel exhibited

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elastic-like behavior; when the shear strain γ was increased to 10000%, the storage

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modulus and loss modulus was decreased sharply, and the storage modulus was less than the loss modulus. The network structure of the hydrogels was destroyed; the

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rotor agitation was so intense that the storage modulus of PVA hydrogel was

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decreased from 50 Pa to 0 Pa. The storage modulus of the CP and LCP-1% hydrogels

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was decreased from 210 Pa, 310 Pa to near 0 Pa, respectively. After 60 s, when the

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strain was returned to 1%, due to the self-recovery of the hydrogel structure, Gˊ

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quickly returned to the initial value and Gˊ > G′′. After 4 cycles, both the CP and LCP hydrogels quickly returned to their original state. The storage modulus of the PVA, CP

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and LCP hydrogels finally reached to 50 Pa, 200 Pa and 430 Pa, respectively. And the

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storage modulus of the LCP hydrogel was significantly higher than the initial value. The results showed that the LCP hydrogel could recover to the original state in 12 s after 10000% shear strain for 4 cycles, which implying the immediate recovery of the

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internal network and mechanical properties of the LCP hydrogels (Y. Zhang, Tao, Li, & Wei, 2011). The self-recovery of hydrogels may be attributed to the dynamic and reversible didiol-borax complexes (B. Lu et al., 2016).

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0

60

120

180

240

300

360

420

480

540

100

1

0.1

G' PVA

CP

LCP-1%

PVA

CP

LCP-1%

G"

0.01 0

60

120

180

240

300

360

420

480

540

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Time (s)

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G',G" Pa

10

Fig. 4 Continuous step-strain measurements of PVA, CP and LCP hydrogel at 25 °C with high-amplitude oscillatory parameters: γ = 1%, ω = 10 rad / s and

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low-amplitude oscillatory parameters: γ = 10000%, ω = 10 rad / s

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3.5.1 Thermosensitivity

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3.5 Responsive properties of LCP self-healing hydrogels

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When PVA, CP and LCP hydrogels were heated to 90 °C, the hydrogels

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preformed uniform and stable fluid-like behavior. After cooled down to room temperature, hydrogels exhibited solid-like characteristic again. After 10 cycles, the

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fluid state could still return to the hydrogel state, as shown in Fig. 5(a). The LCP-1% was taken as an example to demonstrate the thermo-reversibility of hydrogels (Fig.

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5b). During the entire heating-cooling-heating circle, G′ was throughout higher than G″, suggesting an elastic hydrogel network. Starting from 20 to 40 °C in the first heating process, G′ decreased with elevation of temperature. The G′ increased gradually with further increase of temperature from 40 to 50 °C, which might be caused by the formation of a denser network at higher temperatures and inevitable 20

water evaporation inside the hydrogels. For the following cooling process, G′ increased rapidly from 50 to 40 °C and reached a temperature-independent plateau between 40 and 20 °C. During the second heating process, at lower temperature range from 20 to 40 °C, G′ curve basically overlapped the cooling curve, revealing that

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hydrogels stiffening during cooling was almost completely thermo-reversible. The

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results indicated that the LCP hydrogels showed reversible thermosensitive property,

which probably due to thermal-induced water evaporation, reversible and exothermic

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reactions between hydroxyl groups of PVA, HEC, lignin and B(OH)4-.

2000

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G' G' G'

1600

G',G"(Pa)

(b) 2 nd heating

800 Cooling

400

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1200

G" G" G"

1 st heating

20

25

30

35

40

45

50

Temperature (°C)

Fig.5 (a) Thermosensitive property of PVA, CP and LCP hydrogels; (b) 21

temperature dependence of G′ and G″ for the LCP-1% hydrogel during a heating-cooling-heating circle (20-50-20-50 °C) at ω=10 rad/s and γ=1%.

3.5.2 Conductivity

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A circuit with the LCP hydrogel and 3V voltage batteries efficiently illumed a connected light-emitting diode (LED). The resistance of the LCP initial hydrogel was

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1000 Ω. When the LCP hydrogel was sequentially cut off and healed, the diode corresponded to a change in the extinction-luminescence, and the resistance

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corresponded to a change from 105 Ω to 1050 Ω. When the LCP hydrogel was

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stretched to 10 cm and the strain reached 5.7 times beyond the initial length, the

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resistivity of the hydrogel was increased up to 16600 Ω and thus the brightness of the

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diode was decreased, as shown in Fig. 6. The results showed that the LCP self-healing

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hydrogels performed ionic conductivity, which were promising for application in wearable electronic materials (Zhou et al., 2019). Their conductivity was probably

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due to surface Na+ and B(OH)4- ions.

Fig. 6 Brightness changes of LEDs during the cutting off-healed-extension of the 22

LCP hydrogels

3.6 The structure mechanism of LCP self-healing hydrogels As shown in Fig. 7, HEC and PVA polymers were intertwined to form a basic

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framework by mutual hydrogen bonding and physical cross-linking via borax.. The incorporation of HEC reduced the excessive cross-linking of PVA chains, which

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increased the ductility of the CP hydrogel to some extent. Lignin with rich phenolic

hydroxyl groups, alcoholic hydroxyl groups and carboxyl groups was likely

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responsible for the highly stretchable performance. Uniformly distributed lignin could

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form dynamic hydrogen bonding with HEC and PVA, probably crosslink with borax,

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reduce the force and wrap between PVA and PVA, PVA and HEC, HEC and HEC

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polymer molecular chains that increased the mobility and distance of polymer

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molecular chains and improved the viscoelasticity and stretchability of the LCP self-healing hydrogel. After adding appropriate amount of lignin to the LCP hydrogel,

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it could be stretched up to 34.4 times beyond the initial hydrogel; Once the amount of

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lignin was excessive, a large amount of hydrogen bond between lignin and HEC or PVA and dynamic hydrogen bonding of borax limited the free mobility of polymer

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molecular chains, resulting in the decrease in the storage modulus and elongation rate of the LCP hydrogels. Using HEC and PVA as the framework, borax as the crosslinker, lignin as the plasticizer, the lignin-based self-healing conductive hydrogels with highly stretchable and thermosensitive properties were synthesize by the one-step fabrication method, that was expected to be used in emerging fields such as 3D 23

printing and wearable electronic devices to broaden the efficient utilization of

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biorefinery residue lignin.

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Fig. 7 The structure mechanism of lignin-based self-healing hydrogels

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3. Conclusion

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Using HEC and PVA as the framework, borax as the crosslinker, biomass lignin

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from pulping black liquors and biorefinery as the plasticizer, the lignin-based LCP

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self-healing conductive hydrogels with highly stretchable and thermosensitive properties were synthesize by the one-step fabrication method. Lignin molecules

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likely accounted for the satisfactory stretchability of up to 34.4 times beyond the initial hydrogel. The LCP hydrogel could recover to the original state in 12 s after

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10000% shear strain for 4 cycles. In addition, the LCP hydrogels showed good thermosensitivity and conductivity, which were expected to be used in emerging fields such as 3D printing and wearable electronic devices to realize the efficient utilization of biorefinery residue lignin.

24

Acknowledgments The authors acknowledge the financial support of the National Natural Science Foundation of China (21706038, 21808042), Guangdong Province Office of Education Youth Innovation Personnel Training Project (262524146) and the start-up funding of

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Guangdong University of Technology (220413187).

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