Facile preparation of lignin-based underwater adhesives with

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Facile preparation of lignin-based underwater adhesives with improved performances Congying Wei, Xiangwei Zhu, Haiyan Peng, Jianjun Chen, Fang Zhang, and Qiang Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06731 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Sustainable Chemistry & Engineering

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Facile preparation of lignin-based underwater adhesives with

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improved performances

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Congying Wei,‡ Xiangwei Zhu,‡ Haiyan Peng, Jianjun Chen, Fang Zhang, and Qiang Zhao*

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Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education,

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School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology,

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1037 Luoyu Road, Wuhan 430074 China. E-mail: [email protected]

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KEYWORDS: Lignosulfonate, Value-added bioprocessing, Underwater adhesive, Coacervate,

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Self-curing

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ABSTRACT: Bioinspired wet adhesives have demonstrated versatile applicability in humid

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conditions, but the attainment of catecholic protein mimics comprises multi-step synthesis and use

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of complex chemical components. Advanced wet adhesives derived from inexpensive bio-

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resources and green processing are highly expected. We report a straightforward means to

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underwater-implemented adhesives from aqueous mixing of lignosulfonate (LS) and

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polyamidoamine-epichlorohydrin (PAE-Cl) solution. The formation of fluidic LS-PAE complex

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was driven by a delicate balance between electrostatic attraction and hydrophilic stabilization. The

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obtained adhesive highlights instant wet adhesion on diverse submerged surfaces and spontaneous

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curing in water. More importantly, it demonstrates robust and stable bonding strength over the

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alkali, salty, high-temperature and long-time soaking conditions. This work advanced the

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development of lignin into functional wet adhesives through a green and sustainable approach.

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ACS Paragon Plus Environment

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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INTRODUCTION

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Wet adhesion is indispensable to various applications spanning from surgery closure,

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underwater fixing, to aqueous coating and surface engineering etc.1–3 Conventional

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adhesives lack sufficient hydrophilicity and frequently fail to submerged hydrophilic

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substrates because of reduced interfacial bonding in the presence of surface water

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molecules.4,5 Marine mussels and sandcastle worms are known for their exceptional ability

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of underwater adhesion through the synergy of catecholic protein sequence and controlled

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coacervate.6 Oppositely charged proteins were mixed to form catechol-containing

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coacervate, a water immiscible fluid which spreads over submerged substrates and allows

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for wet adhesion through the combination of supramolecular interfacial interactions and

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self-oxidative crosslinking.7,8 Despite catechol’s enabling roles, the synthesis of catechol-

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containing polymers is hard to scale and involves complicated functionalization, and/or

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protection treatments because catechol groups are prone to self-oxidation.9–11 Moreover,

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sequence control of catechol groups in synthetic polymers remains unresolved challenges

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so far.12 To improve the sustainability of facile processing to wet adhesives, a catechol-free

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coacervation strategy compatible with low-cost resource biopolymers is highly demanded.

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Lignosulfonate (LS) is a major byproduct from the pulping residues of lignocellulose biomass,

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accounting for 90% of the total market of commercial lignins.13,14 Regardless of the large

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availability (1.8 million tons annual production worldwide), LS valorization is still in the early

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development stage, representing only 2% of the lignin production from industries.15 The vast

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majority of LS is inefficiently utilized as industrial additives or even burnt as a low-value fuel,

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imposing resources and environmental constrains.16,17 Value-added processing of LS, thus,

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represents as an important task in advancing its sustainable utilization.18 Concomitantly, LS-based

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composites have been developed by blending with various polymeric formulations.14 However,

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owing to the abundant sulfonate groups, LS features high polarity and exhibits poor miscibility

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with many other polymers when fabricating the moldable and functional materials.17 One effective

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solution is to deprotonate or desulfonate LS to reduce their polarity.19,20 Nevertheless, rational

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utilization of negatively charged sulfonate groups of LS has been less realized.

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Alternatively, liquid-liquid phase separation driven by electrostatic complexation between

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oppositely charged polyelectrolytes is a major route to obtain the bio-inspired coacervate

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adhesive.21 From this perspective, LS represents a potential candidate to complex coacervate once


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paired with appropriate cationic polyelectrolytes. Given the easy availability and bio-compatibility

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of LS resources, the obtained coacervate could serve as an economical and green platform for

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advanced wet adhesives. Despite these potential benefits, however, randomly blending of LS with

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poly-cations (e.g., poly(N-methylaniline) or poly(allylamine)) can hardly produce any complex

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coacervate; instead, solid precipitations or homogeneous solutions were observed.21,22 The

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formation of complex coacervates requires exquisite maneuver of polyelectrolyte charge

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properties and optimization of the solution-mixing conditions.23 For this reason, LS-based

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complex coacervates remained unreported.

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In this study, we screened and selected polyamidoamine-epichlorohydrin (PAE-Cl), an

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economic commercial additive applied in the cellulose and fiber industry,24 as the cationic

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candidate, and induced its electrostatic complexation with LS to develop a fluidic and underwater

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implemented wet adhesive. Both ingredients boost highly water solubility, which facilitates their

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green processing using water as the solvent. The complexation was confirmed by Fourier-

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transform infrared (FT-IR) spectroscopy and Elemental analysis. Contact adhesion and curing

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behavior of obtained adhesives were characterized by underwater pull-off test, rheological test and

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1

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pH, salt, temperature and long-time soaking.

H NMR. Robust wet adhesion was achieved on various substrates and found to be stable against

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EXPERIMENTAL SECTION

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Materials. Polyamidoamine-epichlorohydrin (PAE-Cl) was kindly donated by Golden

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Huasheng Paper ltd., Suzhou, China. The solid content and charge density of the PAE-Cl solution

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(pH 4.2) were measured to be 12.5 wt % and 2.5 meq/g. Sodium lignosulfonate (molecular weight

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~ 52 KDa, charge density 1.8 meq/g), Poly(allylamine hydrochloride) (average Mw ~ 50 Kda),

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Poly(ethylene imine) (average molecular weight ~ 70 KDa) and Poly(diallyldimethylammonium

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chloride) (molecular weight 200-350 KDa) were purchased from Sigma-Aldrich, St. Louis, MO,

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USA. Chitosan quaternary ammonium salt was provided by Zhejiang golden-shell pharmaceutical

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co., ltd, Yuhuan, All chemicals were used as received. Deionized water (conductivity: 0.6 µS/cm)

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was used in this work.

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Preparation of LS-PAE adhesive. Lignosulfonate (LS) solution was prepared at 100 mg/mL

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and added into equal volume of PAE-Cl solution (125 mg/mL) dropwise under magnetic stirring


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(400 rpm). Turbidity was observed once the LS solution was added. After 20-minute settling,

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fluidic wet adhesive was obtained at the bottom of the glass beaker and applied directly.

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Characterizations. Fourier-transform infrared spectroscopy (FT-IR) was carried out on a

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Bruker Vertex 80V FT-IR spectrometer with 32 scans at a resolution of 4 cm–1 by using KBr pellet.

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Elemental analysis (C, H, N, S) was performed on a Flash EA1112 from ThermoQuest Italia S.P.A.

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Nuclear Magnetic Resonance (1H NMR) spectra of PAE were recorded on a Bruker Avance 400

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spectrometer at room temperature. Turbidity of LS-PAE complex at different salt concentration (0

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M to 1.0 M) was measured at 600 nm by UV–vis spectrophotometry (Shimadzu UV-1900, Japan).

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The relative turbidity is defined as 100 − 100 × 10−A (600). Rheological measurements were carried

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out on an Anto Paar MCR 302 rheometer at 25 °C. Two parallel stainless steel plates (diameter:

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25 mm, gap: 0.5 mm) were used to sandwich the sample during characterization. Oscillatory strain

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sweep was run from 0.1 to 1000, and 1% as the amplitude was the optimized condition within the

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linear viscoelastic regime. Oscillatory frequency sweep data were collected in the range from 0.1

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to 100 rad/s. All measurements were repeated for at least three times.

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Adhesion tests. The pull-off adhesion of LS-PAE was tested according to the modified version

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of the ASTM standardized method.25 Typically, two substrate panels were pre-submerged in water,

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and adhesive (0.25 g) was sandwiched between the two panels in-situ in water, with a lap shear

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joint of 1.0 cm ×1.0 cm. External pressure (40 KPa, 0.4 N) was applied to the bonding area for 5

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seconds once the adhesive was underwater dispensed, and a small portion of coacervate (~ 0.028

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g, 11.2% to the applied coacervate 0.25 g) was squeezed out during this process. Afterwards, the

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coacervate adhesive was spontaneously cured underwater with no pressure applied. The adhesion

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measurement was performed in a water-filled chamber on a Suns Tech UTM2103 universal testing

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machine with a 1000 N loading cell at a strain rate of 20 mm/min. Adhesion strength was obtained

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from the maximum force at joint failure divided by the overlap area. Each test was replicated at

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least 5 times and averaged.

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Instant underwater adhesion of LS-PAE was tested on different substrates, including glass,

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aluminum, stainless steel, ceramics, and poly tetra fluoroethylene (PTFE). The adhesive was

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uniformly dispensed on a submerged glass surface (1.0 cm × 1.0 cm), which was pre-fixed to a

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stainless steel baseboard by commercially available Glue (Gorilla Glue). Then, the second glass

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panel (1.0 cm × 1.0 cm) was overlapped with a stabilized steel dolly. Immediately, the dolly was

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hooked to the sensor of a Universal Testing Machine for pull-off tests.


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For the adhesion stability, the adhesive was underwater implemented in between two glass

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substrates and cured in the acid condition (0.2 M PBS buffer at pH 3 and 5), DI water, alkali

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conditions (0.2 M PBS buffer at pH 8-11), salty solutions (0.2 M, 0.5 M, 0.8 M and 1.0 M NaCl)

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and elevated temperatures (50 °C, 80 °C and 100 °C) respectively. The adhesion strength was

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measured after 2-day conditioning. Soaking stability of LS-PAE adhesive was measured by

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consecutively recording the wet adhesion strength from 0 to 30 days.

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RESULTS AND DISCUSSION

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Preparation of Lignosulfonate-PAE complex (LS-PAE) adhesive. The LS-PAE wet adhesive

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was prepared through the solution mixing of lignosulfonate (LS) aqueous solution (1.5 mL, 100

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mg/mL) with PAE-Cl aqueous solution (1.5 mL, 125 mg/mL). Liquid-liquid phase separation, i.e.,

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coacervation, occurred immediately, which after settling, yield a highly viscous but fluidic

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coacervate phase with brown color (Figure 1a,b and viscosity test in Figure S1). Electrostatic

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complexation between the sulfonate and azetinidium group is responsible for the chain

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aggregations. Meanwhile, the counter-balance effect by PAE’s intrinsic hydrophilicity (from the

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hydroxyls and amide linkage) reconciles colloidal stability (Figure 1c), and with their delicate

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interplay, fluidic adhesive formed instead of uncontrolled precipitates.26

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Figure 1. (a) Chemical structures of PAE-Cl and LS. (b) LS-PAE coacervate prepared by solution

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mixing PAE-Cl (1.5 mL, 125 mg/mL) and LS (1.5 mL, 100 mg/mL) aqueous solution. (c)

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Represented functional groups involved in the coacervate formation. (d) Complexations of LS with

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four types of reference polyelectrolytes: i) QCS: chitosan quaternary ammonium salt, ii) PDDA:

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poly(diallyldimethylammonium chloride), iii) PEI: poly(ethylene imine) and iv) and PAH:

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poly(allylamine hydrochloride). The LS solution (1.5 mL, 100 mg/mL) was mixed with equal

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volume of cationic polyelectrolyte solutions (125 mg/mL) respectively. Glass vials were inverted

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after 20-min conditioning.

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For comparison, LS solution was mixed with other cationic polyelectrolytes (QCS, PDDA, PEI,

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PAH, Figure 1d) to observe their complexation behaviors. For QCS and PDDA comprising stiff

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pyran rings or hydrophobic alkyls, precipitation occurred when mixing with anionic LS, indicative

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of strong complexation. On the other hand, the weak cationic polyelectrolytes, PEI and PAH,

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manifested no detectable phase separation when mixed with LS solution, denoting their

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insufficient complexation. Taking the flexible polymeric structures and cationic charge density

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into consideration, PAE-Cl represented an ideal candidate to prepare LS-based coacervate

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adhesive through aqueous mixing.

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Rheological measurements show that LS-PAE complex exhibit fluidic nature as its G’’ value is

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relatively higher than G’ under full range of oscillatory stain sweep (Figure 2a). Characteristic

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peaks for both the LS (1510 cm−1 for benzene rings and 1035 cm−1 for sulfonate) and PAE (1635

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cm-1 for amide linkage) were found from the FT-IR spectra of LS-PAE complex coacervate (Figure

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2b). From their thermal degradation curves, a new peak appeared at 340 °C for LS-PAE (Figure

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S2). Moreover, the nitrogen element content in LS-PAE coacervate is 6.6 wt%, on basis of which

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the LS content in LS-PAE is calculated to be 61.8 wt% (Elemental anylisis, Table S1). All these

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results indicate that coacervate was obtained as a result of the complexation between PAE and LS.


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Figure 2. (a) Storage (G’) and loss (G’’) moduli versus strain of LS-PAE coacervate (Figure 1b),

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(b) FT-IR spectra of the lyophilized PAE-Cl, LS, and LS-PAE coacervate. Characteristic peaks of

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each component were labeled.

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Underwater adhesion performance. The instant contact wet adhesion of LS-PAE was measured

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by underwater pull-off test (Figure S3) and showed substrate-dependent (Figure 3a) performances,

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i.e., 122.3 ±14.3 KPa for glass, 104.8 ±17.5 KPa for aluminum, 80.1 ±8.6 KPa for stainless steel,

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42.6 ±9.4 KPa for ceramics, and 14.2 ±3.7 KPa for polytetra fluoroethylene (PTFE), respectively.

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After 2-day conditioning, the adhesion strength increased drastically to ~ 400 KPa for glass,

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aluminum and stainless steel, and ~ 300 KPa for ceramics. Optical examinations shown that

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cohesion failure occurred for glass, aluminum, stainless steel and ceramics substrates, while

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interfacial glue exfoliation was dominant for PTFE (Figure S4). LS-PAE experienced spontaneous

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curing in the water (mechanism discussed at Figure 5), and thus the increased adhesion strength

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was reasonably attributed to the reinforced cohesion of LS-PAE coacervate. However, on PTFE

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substrate, the adhesion strength of LS-PAE only experienced a minor increase to 50.6 ±16.2 KPa,

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suggesting the limited interfacial interaction of LS-PAE with hydrophobic PTFE surface. A

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demonstrative example was shown whereby two pieces of underwater bonded steel sheet was able

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to lift a 2.0 kg weight (Figure 3c).

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Here the catechol-free LS-PAE features instant wet adhesion comparable to state-of-the-art

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performances,27,28 and more importantly, self-curing ability rarely observed for catechol-free

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coacervates. Catechol-containing polymers are widely exploited to develop robust underwater

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adhesives, and organic solvents are needed for materials processing and/or the polymer synthesis

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that normally involves multiple complicated steps to reconcile the sensitive catechol groups for


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Page 8 of 18

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improved adhesion.29 In this work, not only the raw materials for LS-PAE coacervate (61.8 wt%

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LS, Table S1) are from economic resources and renewable lignin byproducts, but also the

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processing is straightforward and waterborne. Thus, the environment constraints have been largely

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alleviated by this sustainable strategy.

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Figure 3. (a) Adhesion strength of LS-PAE adhesive on different substrates. Note: pull-off tests

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were conducted immediately (instant adhesion) after the underwater implementation of LS-PAE

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adhesive, or after 48h-conditioning in DI water (cured adhesion). Al: aluminum, SS: stainless steel,

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PTFE: poly(tetra fluoroethylene). The bonding strength development with the curing time was

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plotted in Figure S5. (b) Cartoon illustrating the underwater implementation of LS-PAE adhesives.

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(c) A demonstration of underwater implemented LS-PAE adhesive (48 h conditioning) bearing 2

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kg load.

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The underwater curing effect of LS-PAE was confirmed from rheological (Figure 4) and 1H

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NMR (Figure 5) data. Consistent with strain sweep (Figure 2a), frequency sweep of pristine LS-

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PAE exhibited relatively higher G’’ to G’, indicative of its liquid-like feature. After conditioning,

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the G’ value experienced significant increase and exceeded G’’, suggesting the curing effect to the

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adhesive.


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Figure 4. Storage (G’) and loss (G”) moduli versus frequency of LS-PAE adhesive before (pristine)

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and after 2 days curing in the water.

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From 1H NMR, little changes were observed when PAE was conditioned at pH 3 (Figure 5a,

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PAE-3). In contrast, two distinct peaks appear at 3.23 ppm (g’) and 2.62 ppm (h’) when PAE was

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cured at pH 7 and pH 9, respectively. These two peaks denote the ring-open crosslinking reaction

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between the azetinidium and amine groups (Figure 5b).30,31 As such, the self-curing behavior of

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LS-PAE adhesive was attributed to its reactive azetinidium groups from PAE resin, whose self-

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crosslinking reaction was inhibited in the acid, and trigged in the neutral and/or base

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conditions.30,32 On the other hand, LS plays structural role for the reinforced adhesion through the

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electrostatic complexation with PAE.33 A control coacervate adhesive was made by mixing PAE

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with sodium dodecyl sulfonate, and show much reduced wet adhesion compared to LS-PAE

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coacervate (Figure S6) due to the lack of network complexation. It is noteworthy that catechol

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groups are efficient in improving both the interfacial adhesion and cohesion strength through

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multiple supramolecular interactions (e.g., hydrogen bonding, metal chelation, cation-π, etc) and

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self-oxidative cross-linking, respectively.2,34,35 Here the LS-PAE system in this work provides a

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materials platform simultaneously integrating high interfacial bonding, self-curing properties,

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inexpensive resources and green processing in one single system. Yet the LS-PAE coacervate is

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catechol-free, and highlights waterborne preparation that is straightforward, sustainable, and labor-

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efficient.


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Figure 5. (a) 1H NMR spectra of PAE solutions conditioned at pH 3, 7, 9 (named as PAE-3, -7,

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and -9) for 2 days, and the corresponding structural changes. (b) NMR spectra of pristine PAE was

4

indicated as the reference.31,33

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The performance of wet adhesive is frequently challenged by complicated practical

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conditions.8,26,36,37 Adhesion stability of LS-PAE against pH, salt concentration, temperature, and

7

soaking time was studied. LS-PAE adhesive exhibits moderate adhesion strength in acidic

8

condition (pH 3 ~ 6) and a much higher performance in near neutral and base conditions (Figure

9

6a, pH 6 ~ 11), in stark contrast to the literature results suffering from reduced adhesion in the

10

alkali conditions.35,38–40 The stable adhesion in base condition is reasonably due to the self-curing

11

properties of PAE (Figure 5). For better comparison, wet adhesion strength of LS-PAE was

12

benchmarked against representative commercial glues in both water and pH 9 PBS buffer (Figure

13

6b). The PVA and Starch glue were unable to survive even in DI water. Polyurethane-based

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commercial adhesive displayed underwater adhesion of 360.2 ± 52.6 KPa for Gorrila glue and

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400.6 ± 60.5 KPa for 3M Marine. However, both of them are not alkali-resistant, as the wet

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adhesion strength decreased massively when the pH of bulk solution went to 9. Collectively, the

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LS-PAE adhesive represented a suited candidate for alkali-demanding conditions.4,27,35,39


10

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Figure 6. (a) Adhesion strength of LS-PAE adhesive at different pH. (b) Underwater adhesion

3

strength (48 h conditioning) of LS-PAE adhesive compared to commercial glues in DI water and

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pH 9 condition. Glass was bonded as the substrate.

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In addition, the adhesion strength of LS-PAE was maintained ~ 380 KPa when the NaCl

6

concentration of bulk solution was increased to 0.8 M (~1.5 times of common seawater salinity),

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and experienced a 10 % decrease with increasing salt concentration to 1.0 M (Figure 7a). Both LS

8

and PAE-Cl are strong polyelectrolytes, and their inter-polyelectrolyte complexation was less

9

affected by salts,41 as supported by their turbidity curves (Figure 7b). When it comes to temperature,

10

it is interesting to observe that the LS-PAE adhesive is more than simply surviving high

11

temperature conditions. Instead, the adhesion strength reached 623 ±48.5 KPa with increasing the

12

water bath temperature to 100 oC (Figure 8). Wet adhesives are relatively hydrophilic, and the

13

elevated temperature could enhance adhesive swelling and chain relaxation, both of which should

14

decrease adhesion strength.42–44 Here for LS-PAE system, the self-curing effect of azetinidium

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groups could be enhanced at elevated temperature,45 which plays a major role dominating over the

16

negative effect of polymer swelling.


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Figure 7. (a) Adhesion strength of LS-PAE adhesive on glass substrates after underwater

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deposition and 48 h conditioning in DI water and salty conditions (0.2M-1.0 M NaCl solution). (b)

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Turbidity of LS-PAE complex at 0 M, 0.5 M and 1.0 M NaCl.

5

6 7

Figure 8. Adhesion strength of LS-PAE adhesives at elevated temperatures on glass substrate.

8

The adhesion of LS-PAE was tough and stable under both neutral and base conditions in a

9

30-day underwater soaking test (Figure 9). Mildly increased adhesion was observed at the first 10

10

days, followed by stable bonding performance over the rest of time. Due to the heterogeneous

11

aromatic structures, LS demonstrated improved structural stability over other biopolymers, such

12

as proteins and polysaccharides.46–49 Thus, in the presence of consecutively underwater soaking,

13

hydrolysis or degradation tended to weaken the adhesion strength for most bio-based adhesives,

14

while LS-PAE remained less affected.50,51


12

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1 2

Figure 9. Adhesion strength of the soaked LS-PAE was consecutively recorded from 0-30 day.

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Glass was used as the substrate. Values in Day 0 are the adhesion strengths of LS-PAE cured in

4

water and pH 9 buffer from Figure 6a.

5

Conclusions

6

A waterborne and catechol-free strategy was proposed to prepare robust underwater adhesives by

7

exploiting a biomass pulping residues, lignosulfonate (LS) and an economic industrial additive,

8

polyamidoamine-epichlorohydrin (PAE-Cl). Upon solution mixing of LS and PAE-Cl,

9

electrostatic complexation and hydrophilic stabilization worked synergeticly to induce the fluidic

10

yet water-immiscible adhesive.The LS-PAE adhesive allows for instant underwater adhesion to

11

various substrates, and experiences spontaneous curing for substantially improved wet adhesion

12

strength. The wet adhesion was stable against pH (pH 6 ~ 10), increased salt concentration (1.0 M

13

NaCl), high temperature (100 ℃), and elongated soaking time (30 days). These properties are

14

benificial for practical applications and originated from the self-curing of PAE as well as the strong

15

complexation between PAE and LS. Collectively, the LS-PAE adhesive exquisitly integrated

16

value-added bioprocessing with high-performance underwater adhesion, and opened new doors to

17

the green and sustainable development of LS-based materials.

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ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the ACS

20

Publications website. The following files are available free of charge.


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Page 14 of 18

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Viscosity measurement of LS-PAE adhesive and commercial PVA glue, differential thermal

2

gravity and elemental analysis of LS, PAE and LS-PAE, in-situ underwater adhesion test of LS-

3

PAE adhesive, fracture surfaces of different bonding substrates.

4

AUTHOR INFORMATION

5

Corresponding Author

6

*E-mail: [email protected] (Q. Zhao).

7

Author Contributions

8

The manuscript was written through contributions of all authors. All authors have given approval

9

to the final version of the manuscript. ‡These authors contributed equally.

10

Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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The authors gratefully acknowledge financial support from the 1000 Young Talent program and

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the Huazhong University of Science and Technology (No. 3004013118). Authors thank Prof. J.T.

15

Zhu (HUST) for generous help with turbidity measurements.

16

REFERENCES

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TOC:

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Synopsis:

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A green route to robust underwater adhesive was exploited through the lignosulfonate-based complex coacervation in water.


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Facile preparation of lignin-based underwater adhesives with

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