A Dual Cross-Linked Strategy to Construct Moldable Hydrogels with

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A Dual Cross-Linked Strategy to Construct Moldable Hydrogels with High Stretchability, Good Self-Recovery, and Self-Healing Capability Yang Qin, Jinpeng Wang, Chao Qiu, Xueming Xu, and Zhengyu Jin J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Journal of Agricultural and Food Chemistry

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A Dual Cross-Linked Strategy to Construct Moldable Hydrogels with High Stretchability,

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Good Self-Recovery, and Self-Healing Capability

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Yang Qin,†,‡,§ Jinpeng Wang,†,‡,§ Chao Qiu,†,‡,§ Xueming Xu,†,‡,§ and Zhengyu Jin*,†,‡,§

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†State

Key Laboratory of Food Science and Technology, ‡School of Food Science and

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Technology, and §Synergetic Innovation Center of Food Safety and Nutrition, Jiangnan

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University, Wuxi, Jiangsu 214122, People’s Republic of China.

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* Corresponding Author, E-mail: [email protected]. ABSTRACT

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Most conventional synthetic hydrogels suffer from poor mechanical properties, despite recent

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significant progress in fabricating tough hydrogels, it is still a challenge to simultaneously realize

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high stretchability, self-recovery, and self-healing capability in a hydrogel. In this work, a new

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type of starch/PVA/borax hybrid dual cross-linked (DC) hydrogel was synthesized by “one-pot”

13

method. The as-prepared DC hydrogels exhibited mechanical properties of remarkable

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extensibility (ca. 2485%), excellent toughness (ca. 290.5 kJ∙m-3), high compression strength (ca.

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547.8 kPa), rapid recoverability (81.9% energy recovery after 30 min), and free-shapeable

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behavior. More impressively, the DC gels sustained approximately 300 times of its own weight,

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and exhibited an outstanding self-healing capability at room temperature both in air and

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underwater. Furthermore, the adsorption amount of methylene blue onto the anionic DC gel

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(144.68 mg/g) was much higher than that of corn starch gel. Consequently, the eco-friendly,

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stable, and biodegradable hydrogels will have a great potential application in removing anionic

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dyes from the wastewater produced by agriculture and industry.

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KEYWORDS: dual cross-linked network, starch matrix, hydrogels, free-shapeable behavior,

ACS Paragon Plus Environment


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swelling-resistant, methylene blue

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INTRODUCTION

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One of the most promising soft and hydrophilic materials, hydrogels have a

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three-dimensional cross-linked network structure composed of polymer chains and adjustable

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physical and chemical properties.1,2 These smart materials have received considerable attention

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from academia and industry due to their unique applications in a diverse range of fields, such as

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tissue-engineered scaffolds,3−5 coating materials,6 water treatment,7,8 drug delivery,9−11 and

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biosensors.12,13 Due to the intrinsic structural heterogeneity and/or lack of effective energy

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dissipation mechanism, most conventional hydrogels suffer from poor mechanical properties, 14-16

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which severely limits their application in industrial and commercial fields. High mechanical

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strength, stretchability, and self-healing capacity are highly desirable, but are inversely related

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properties of gel materials. Superior mechanical strength and stretchability avert breakage of the

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materials, while the self-healing capacity enables the materials to spontaneously recover their

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functionality and mesh structure after being damaged.1 The presence of diverse mechanical

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properties in hydrogel systems has recently become a hotspot for the research focused on in the

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field.11,17,18 However, most hydrogels possess only one of these properties and very few possess all

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properties. Therefore, it is still a challenging task to combine high stretchability and strength with

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good self-healing capacity in a single material.19

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Great efforts have been made to develop versatile hydrogels to solve this predicament by

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introducing new network structures and/or energy dissipation strategies to strengthen the

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mechanical properties of hydrogels,2,11 such as double-network (DN) hydrogels, nanocomposite

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hydrogels, ionically cross-linked hydrogels, and hydrogen bonded hydrogels20–24 etc. First


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proposed by Gong et al.25 in 2003, DN hydrogels are a promising way to achieve this goal. The

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DN hydrogel is composed of two different polymer networks with asymmetric structures: a rigid

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and brittle first network, which provides a skeleton structure to the hydrogel, and a soft and

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stretchable second network, which provides elasticity.26,27 Owing to the unique dual-network

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structure, DN hydrogels have exhibited desirable mechanical properties and energy-dissipating

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performance. DN hydrogels are considered impressive materials because they demonstrate

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excellent mechanical properties.12,19 Although these DN hydrogels demonstrate improved

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properties over their corresponding single-network (SN) parent hydrogels, the mechanical

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properties do not improve as significantly as expected.

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Recently, dual cross-linked (DC) network hydrogels, as a special DN gel, have attracted

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widespread interest from academia by optimizing and combining different properties of each

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network structure, including dual physically cross-linked hydrogels,2,16 dual chemically

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cross-linked hydrogels,28 and hybrid cross-linked hydrogels.29 However, chemically cross-linked

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points lack the ability to efficiently recover from damage caused by stretching and compressing,

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resulting in a loss of mechanical properties due to the permanent breaking of chemical bonds.30,31

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In contrast, physically cross-linked points were reversible, which allowed hydrogels to recover or

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self-heal after a large deformation or disruption, i.e. the first network unzips and dissipates energy

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upon deformation, thus, the physical bonds can reform after removal of the stress.

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Afterward, many efforts have been made to endow a self-healing capacity and

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multi-functional properties to hydrogels. The research works have focused on the construction of

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DC network hydrogels by dynamic and reversible physical bonds (e.g. hydrogen bond,

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hydrophobic interaction, π−π stacking, or ionic bond).14-16,24,32 However, there are a number of



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self-healing hydrogels that are frequently plagued with problems such as long self-healing time

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and stimulus dependence in application.6,9,11,14,16 Moreover, it is still worth evaluation of the

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bioactivity of the synthetic hydrogels owing to a number of vinyl monomers as the building block

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for the DC network. Recently, rising needs in the biochemical, medical, and functional food fields

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spurred considerable interest in developing an efficient strategy for the preparation of non-toxic,

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environment-friendly,

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replacements for synthetic materials. To address these concerns, as a scaffold or functional matrix,

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biomacromolecules have been used to construct bioactive DC hydrogels (e.g. gelatin,17 cellulose,33

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chitosan,34 carrageenan,31 and protein2,35). Due to its great cross-linking ability in the presence of

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abundant hydroxyl groups, starch is known to be a good precursor material for preparing

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hydrogels.36 However, the starch-based hydrogels exhibit poor mechanical and brittleness, and

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thus hamper the development of unmodified starch as a stable hydrogel.36,37 Due to the -OH

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groups in each of the repeating molecular units, poly(vinyl alcohol) (PVA) are proven to

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simultaneously have good gel- and film-forming capability.38 Thus, the introduction of PVA

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improve mechanical property and stability of starch based gels.39,40 Furthermore, studies have

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demonstrated the introducing cross-linking agents (epichlorohydrin) markedly strengthen the

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mechanical and thermal properties of starch/PVA composite films through cross-linking the

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chains of starch and PVA.41 Borax (B), as a reversible cross-linker, dissolves and dissociates into

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borate ions (B(OH)4-) in water. The PVA chains and borate ions can easily form a single

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cross-linked (SC) network structure. Based the above strategy, Sreedhar et al. investigated the

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effect of B concentrations on the thermal, mechanical, and surface characterization of the soluble

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starch and PVA blend films.42 Moreover, boron complexes of PVA and starch blend films were

biocompatible,

and

biodegradable


multifunctional

hydrogels

as

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also prepared by Bursali et al., who investigated the effect of the cross-linking agent

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(glutaraldehyde) on the antimicrobial activity of the composite films.43 These studies had prepared

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PVA matrix composite film as the starting point, and mainly investigated the effect of the addition

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of starch and cross-linking agent on the physicochemical properties of the films and not the

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mechanical behaviors and the interaction mechanisms. Starch, as a poly-hydroxyl macromolecule,

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is similar to PVA. Inspired, can we simultaneously introduce the PVA chains and borate ions to

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starch-based gels for constructing a novel DC hydrogel with excellent mechanical properties,

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self-recovery and self-healing abilities at room temperature? Moreover, it is necessary to

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understand whether cross-linked interactions occurred among the starch and PVA chains and the

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borate ions. To the best of our knowledge, the starch-based DC hydrogels with high stretchability,

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mechanical strength, and self-healing properties have never been reported in the previously

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literature.

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In this paper, we report a new type of hybrid physically-chemically DC starch-based

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hydrogel through a “one-pot” method, i.e. the retrogradation technology combined with

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freezing-thawing technology (Scheme 1). The as-prepared DC hydrogels consist of a hydrogen

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bond-associated starch network and a dynamic borate bond cross-linked network. Within the DC

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gels, the borate ions further assured the interpenetration, tangling, and cross-linking of corn starch

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(CS) and/or PVA chains by covalent bonds (Scheme 1b). Consequently, the hydrogen bonds and

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borate ions were jointly promoted the strong interaction between CS chains and PVA chains,

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making the interconnection of the CS and PVA networks denser. Among the networks, CS

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networks provide a firm platform to bear stress, and the dynamic PVA/B networks endow the

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hydrogels with a self-healing capacity. As a result, the DC hydrogels achieved high stretchability



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and toughness as well as self-recovery and self-healing abilities at the same time. Additionally,

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because of the presence of numerous borate ions (B(OH)4-) in DC gel, the anionic DC gel

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exhibited an efficient adsorption of MB from aqueous environment through electrostatic

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attraction. This work provide a new design strategy to prepare starch-based gels with highly

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mechanical, stable, self-healing, and high adsorption properties, consequently, we hope the

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renewable, biocompatible, and biodegradable DC gel will have a promising application in the field

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of agriculture, biomedicine, and pollutant adsorbent.

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MATERIALS AND METHODS

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Materials. Corn starch (CS, 26.9% amylose content) was purchased from Ingredient China

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Ltd (Guangdong, China). Poly(vinyl alcohol) (PVA, DS = 1750±50, degree of deacetylation,

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99.0% Mw = 75,00080,000 g/mol) and sodium tetraborate decahydrate (Borax, over 99.5%

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purity, Mw = 381.37 g/mol) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai,

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China). Methylene blue (MB) was supplied by Aladdin and used as adsorbate. All reagents were

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of reagent-grade.

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Synthesis of CS/PVA/B DC Hydrogel. CS/PVA/B hydrogels with corn starch, PVA, and B

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were prepared via one-pot tandem reactions as shown in Scheme 1. PVA powders were dispersed

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in deionized water, and stirred at 95 °C for 30 min to make the PVA powders dissolved enough.

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The PVA solution and the gelatinized starch solution was well mixed in a PVA solution with

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continuous magnetic stirring (300 rpm/min) to form a mixture of homogeneous solutions of 10

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wt% (starch) and 4 wt% (PVA) in concentration, respectively. Afterward, the mixed solution was

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heated at 95 °C in a water bath with magnetic stirring for 1 h. Subsequently, B powders were

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slowly added to the CS/PVA mixed solution with the concentration of 0.026 mol/L, which were


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stirred vigorously (300 rpm/min for 2 h) until a formation of homogeneous sol. In order to avoid

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the evaporation of the water, the heating and stirring process was conducted in a hermetically

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sealed beaker. Moreover, the sol with ultrasonic treatment for 5 min to remove air bubbles.

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Finally, the resultant uniform sol was poured into a cylindrical plastic mold (45 mL), and firstly

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stored at 4 °C for 12 h to form the gel network of starch. Afterward, to further enhance the

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network of the hydrogels, the samples frozen at −18 °C for 4 h and unfrozen at 25 °C for 8 h, and

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the freezing-thawing process performed one time. In addition, all the other hydrogels were

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prepared according to the conditions processed for the CS/PVA/B hydrogels. The control sample,

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the starch, PVA, and starch hydrogels with PVA or B were synthesized by using the same

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experiment process. The eventual samples were named CS, PVA, CS/B, PVA/B, and CS/PVA.

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All specimens with various compositions were listed in Table 1.

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Characterization. FTIR spectra of the hydrogels were obtained on a Nicolet IS10

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spectrometer (Thermo Nicolet, Inc., Waltham, MA, U.S.A.) equipped with an attenuated total

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reflection (ATR) accessory over the wavenumber range of 4000−400 cm-1. XRD diffraction of the

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hydrogels were obtained using an XRD diffractometer (D2 PHASER, Bruker AXS, Inc.,

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Germany) equipped with Cu Kα radiation in the range of 4−40° at a scan rate of 0.02°/min. All

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samples were freeze-dried prior to analyses. The morphology of the specimens was taken with the

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scanning electron microscope (SEM, 7500F, JEOL, Japan) at an accelerating voltage of 5 kV. The

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hydrogels were freezed in liquid nitrogen, snapped immediately, and further dried 24 h in vacuum.

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Afterward, the cross sections of specimens were coated with gold vapor, observed, and

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photographed. Water content and degree of swelling of the hydrogel were determined by the

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gravimetric method. The weight of the cylindrical hydrogel (25 mm×16 mm) was determined



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(W1). Then, the samples were dried at -80 ºC for 48 h to obtain the dry mass (Wd). The water

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content is defined as (W1Wd)W1. The gels were continuously retrieved from the deionized water

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at room temperature. The swelling ratio of the hydrogel was measured gravimetrically as a

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function of time (0−96 h) after removing excess water on the surface of the gels with dry paper.

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The swelling ratio of hydrogels can be calculated as (WtWd)Wd, where Wt is the weight of the

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swollen gel. The zeta potential of the swollen DC gel solution was determined by a dynamic light

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scattering instrument (Zetasizer Nano ZS, Malvern, U.K.) under different pH conditions adjusted

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by using NaOH and HCl solutions.

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MB Adsorption Test. The adsorption of MB from aqueous solution onto gel adsorbent was

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systematically evaluated. A certain amount of gel adsorbent was added into 100 ml of MB

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solution (150 mg L-1) and placed in an isothermal water bath shaker (25 °C) at 250 rpm. After a

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period of time, the supernatant was centrifuged and the concentration was measured with a

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UV-Vis spectrophotometer (UV-1800, Shimadzu Corporation, Japan) at 663 nm as λmax of MB.

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Effects of contact time (0−48 h), pH level (2−10), hydrogel dosage (25−250 mg), initial MB

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concentration (50−250 mg/L), and temperature (25, 37, 50, and 55 °C) have been explored to

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evaluate the performance of sorbents. Then, the adsorption capacity was calculated using

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equation:

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qt = (C0 ― Ct)V/m

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where qt (mg/g) is the adsorbed amount after time t, C0 and Ct (mg/L) are the initial

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concentration and remaining concentration after time t, respectively. V (mL) is the volume of MB

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solution and m (mg) is the weight of gels.

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Mechanical Property Measurement. Compressive Tests: The compressive properties of the


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hydrogels were performed using a texture analyzer (TA-XTplus, Stable Micro Systems, Surrey,

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U.K.). Both uniaxial and cyclic compressive tests were all conducted on cylindrical sample (25

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mm in diameter and 16 mm in height). Afterward, placed it on the lower plate and compressed

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with a P36R probe at a strain rate of 0.2 mm∙s−1 at the room temperature (25 °C) in air. The

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compressive modulus was calculated by fitting the initial slope of the stress-strain curves.44

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Compression strain and stress data were recorded during the experiments. Hysteresis Tests: The

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cylindrical hydrogel specimens were first compressed by a loading cycle to a maximum strain of

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70% with a speed of 0.2 mm∙s−1 and then unloaded at the same rate. As for the recovery

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experiments, the gel samples were relaxed at 25 C for a certain waiting time (0, 30 min, and 5 h)

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before next loading process, during which the specimens were coated with a silicon oil surface and

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then being encased with a plastic wrap to avoid the water evaporation to minimize water

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evaporation. The energy dissipation was calculated from the area between the loading-unloading

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curves.45 The residual strain ratio was calculated by the ratio of height change along the direction

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of stress after removal of stress with respect to the original height of the specimen.46 Tensile Tests:

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The hydrogels were cut into a dumbbell shape with a gauge length of 20 mm, a width of 10 mm,

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and a thickness of 5 mm. Subsequently, the samples were elongated to breakage at a rate of 100

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mm∙min−1. The tensile strain was determined as the elongation divided by the initial length. The

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tensile stress was defined as the load force divided by the original specimen cross-sectional area.

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The elastic modulus of gels was calculated by fitting the initial linear regime of stress–strain

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curve.47 For the self-healing experiments, the strip hydrogels were cut into halves, and then put the

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two parts contacted at room temperature in air. Moreover, there was no other outside stimulus

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applied during the healing process. After self-healing, the tensile test was carried out again to



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calculate the healing efficiency. In addition, the hardness of the hydrogel were carried out using a

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texture analyzer fitted with a P 0.5 probe. The test speed was 0.5 mm∙s−1, and the hydrogels were

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compressed twice to a strain of 70%.

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RESULTS AND DISCUSSION

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Fabrication Process and Formation Mechanism. Scheme 1 schematically indicates the

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fabrication process and formation mechanism of the CS/PVA/B DC hydrogel. When the

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gelatinized starch solution was well mixed in a PVA solution completely, PVA chains transferred

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into the interstitial water phases of starch gels. Afterward, the resulting pastes stored at 4 C, and

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the amylose and/or amylopectin chains in starch cross-linked via numerous hydrogen bonds and

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formed a strong physical cross-linking framework in the gel.48,49 Neighboring PVA chains formed

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crystallites through the freezing-thawing method,50 and the crystallites served as linked points for

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the PVA chains to fabricate another soft network. Borate ions further assured the interpenetration,

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tangling, and cross-linking of CS and/or PVA chains by covalent bonds and hydrogen bonds

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(Scheme 1b). Due to the high affinity towards hydroxyl groups, the borate ions formed complexes

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rapidly and reversibly in aqueous media with the poly-hydroxyl polymers,51,52 and then the borate

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ions acted as an ionic cross-linker between the CS and/or PVA chains. Moreover, Hydrogen bonds

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and borate ions were jointly promoted the strong interactions between CS and PVA chains, then,

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the two networks were connected more close. The two networks become entangled and sustain

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each other via hydrogen bonds and ionic interactions, resulting in the strong DC hydrogel.25,27

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Structures and Morphologies of the Hydrogels. To confirm whether there were ionic

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interactions between CS and PVA chains in the presence of the cross-linking agent (B), FTIR was

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carried out to gain insight into the vibration of molecules. As shown in Figure 1a, a large peak


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attributed to O–H stretching was observed and located at 3100–3700 cm-1 in all spectra. The C–O–

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C bond of starch showed discernible absorption peaks at 1160 and 1080 cm-1.53,54 In the pure

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spectrum of PVA, the absorption peak at 1100 cm-1 was related to the C–O in stretching mode for

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PVA.55. Compared with the CS/PVA gel, the -OH stretching peak of the CS/PVA/B DC hydrogel

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shifted to a lower wavenumber (blue-shift). Stronger H-bonding leads to a more significant shift in

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vibrational frequency.50 Because borate ions have probably attached with –OH groups of PVA,

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amylose and/or amylopectin chains via H-bonds, the interactions of H-bond were enhanced.

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Therefore, the broad band from 3100 to 3700 cm-1 may be assigned to the –OH groups of starch

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and PVA, as well as the complexes that formed among starch and/or PVA with borate ions.

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Similar result was reported by Manna and Patil (2009) 56 After the addition of B molecules within

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the hydrogels, they displayed several distinct peaks characteristic of B and borate: i.e. 661 cm-1

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(bending of B–O–B linkages within borate networks), 833 cm-1 (B–O stretching from residual B),

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and 1330 and 1420 cm-1 (asymmetric stretching of B–O–C from the complexes).57 When borate

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ions were introduced into the networks of CS, PVA, and CS/PVA gels, respectively, there were

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two new vibrational absorption peaks observed at 1330 and 1420 cm-1. The CS/PVA/B DC

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hydrogel spectra showed the characteristic absorption peaks of starch, PVA, and B. This

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confirmed not only the occurrence of complexes between borate ions and CS or PVA chains, but

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the presence of cross-linking networks among CS chains and PVA chains with borate ions within

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the hydrogels.

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The XRD profiles of SN (CS and PVA), SC (CS/B and PVA/B), DN (CS/PVA), and DC

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(CS/PVA/B) gels are presented in Figure 1b. The retrograded CS SN gel at low temperature

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exhibited a B-type with diffraction peaks at 2θ of 17.1 and 22.2.58 With the addition of B, the



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typical peaks of CS crystallites disappeared, with only a blunt amorphous peak centered at 2θ =

244

22.2°. These results revealed that the strong interaction between CS chains and borate irons

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inhibited the retrogradation of the amylose and/or amylopectin in starch, and led to the decrease of

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CS crystallites. The PVA gel showed two typical peaks at 2θ of 19.4 and 22.9°.59 Borate irons

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destroyed the organized configuration of PVA, and thus the diffraction peaks of PVA/B gel

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became weaker.51 The intensity of the diffraction peak at 17.1 of CS/PVA gel decreased

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significantly, indicating that the hydrogen bonding interactions between CS and PVA chains were

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disadvantageous to improving the ordering arrangement of the amylose and/or amylopectin

251

chains. However, the CS/PVA/B DC gel displayed very low crystallinity, even lower than the

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CS/PVA gels. The result indicates that borate ions could weaken the self-hydrogen bonding of the

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DC gel to a considerable degree by forming new cross-linking bonds with CS and/or PVA chains,

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and thus further restrains the crystallization of CS/PVA gels. It was thus concluded that the

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presence of borate ions and PVA chains would inhibit the retrogradation of amylose and

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amylopectin in starch, and in turn affected on the starch crystal structure of DC hydrogels.

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Morphology of the Hydrogels. SEM images of the inner morphologies of freeze-dried

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hydrogels are shown in Figure 1(c-h). The pure CS gels showed a lamellar and loose

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microstructure. After introducing borate ions, CS/B hydrogel structure became dense in pore wall.

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The fracture surfaced of PVA gel was rather flat and smooth without holes, while the addition of

261

B to the hydrogel resulted in a fine, micron porous network structure. The phenomenon may be

262

caused by the formation of densely cross-linked complexes among starch (amylose and/or

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amylopectin) or PVA chains with borate ions. Moreover, CS/PVA gel exhibited a comparatively

264

coarse and porous surface, which combined the characteristics of both CS and PVA. Compared


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with two different cross-linked gels (CS/B and PVA/B), the surface image of the CS/PVA/B DC

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gel in Figure 1h formed an enhanced micro-structure with a large number of small, regular cellular

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nests. From the micro-morphology, the internal skeleton of the DC gel was more orderly and

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compact which could lead to its better mechanical properties.

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Steadiness of the Hydrogels. The cylindrical shaped samples were kept at room temperature

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for 60 s, and a shape change was observed (Figure S1). In particular, the shape of the PVA/B SC

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hydrogel showed significant changes, proving the bonds of borate ions and PVA hydroxyls are

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dynamic.32 The prepared pure PVA specimen was in sol states and exhibited good fluidity.

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Compared with the PVA/B SC hydrogel, the shape of the DC hydrogel remained the same. The

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reason for the higher stability of the hydrogel is the greater number of hydrogen bonds and the

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stronger cross-linked interactions obstructing the motion of the PVA chains.60,61 Moreover, the

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first starch network also provided an additional platform to firm the structure, thus improving the

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stability of the second network. Although all the CS based hydrogels (CS, CS/B, CS/PVA, and

278

CS/PVA/B) could remain at the same initial shape, significant differences in the mechanical

279

strengths were exhibited.

280

Mechanical Properties. Then, we initially compared the compression behavior of CS-based

281

hydrogels. While the hydrogels were compressed, the cylindrical CS SN and CS/PVA DC gels

282

exhibited different degrees of damage (Figure 2ab). Because the polysaccharide-based hydrogels

283

exhibited poor mechanical strength and a brittle network structure,62,63 we speculated the breakage

284

of the SN and DN hydrogels were related to the first starch networks formed through hydrogen

285

bonds of amylose and/or amylopectin chains (Figure 2ef). As the borate ions were introduced into

286

the hydrogels, the CS/B SC and CS/PVA/B DC hydrogels could be compressed using high strain



287

without breaking (Figure 2cd), indicating that introducing borate ions markedly strengthen the

288

networks of the two hydrogels, and endow extraordinary mechanical properties. In spite of

289

withstanding high-level deformation compression (over 50%), the CS/B and CS/PVA/B gels only

290

exhibited deformation behavior, while no stiff CS network or flexible PVA network destruction

291

was observed. The mechanical strength and toughness of the SC and DC gels was an enhanced

292

result owing to the cross-linked interactions of CS and/or PVA chains with borate ions. More

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importantly, upon removal of the deformation force, the compressed DC gels could recover their

294

initial height and shape (after repeating five times), while the CS/B SC hydrogels partly returned

295

to their initial height (approximately 60%). CS supported many hydroxyl groups on their surfaces

296

that formed hydrogen bonds with PVA chains, forming a cross-linked network with chain

297

entanglement. The borate ions cross-linked the chain entanglement of CS and PVA and further

298

reinforced the networks, which enhanced the ductility of the DC gels (Figure 5kl). The cyclic

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compression and stress relaxation processes revealed that SC and DC gels exhibited excellent

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shape-recovery properties.30,64 To further evaluate whether CS-based hydrogels could withstand

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continuous compression stress, optical photographs of the mechanical performance of the

302

hydrogels are shown in Figure S2. The CS, CS/B, and CS/PVA hydrogels break down directly

303

when the gels were subjected to weight (~1 kg) for 5, 60, and 15 s, respectively. By contrast, the

304

DC hydrogel with a slight deformation sustained a loading weight of ~1 kg (approximately 300

305

times its own weight) for more than 5 min. Upon removing the load, the DC hydrogel rapidly

306

recovered its original shape within 15 s. These results determined that introducing PVA chains

307

and borate ions reinforced DC hydrogels and made the DC gels able to withstand continuous


Page 14 of 50

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308

compression, consequently, possessing higher stiffness and compression strength, which far

309

exceed those of the parent gels (CS, CS/B, and CS/PVA).

310

To better understand how the second PVA/B network compositions affect the mechanical

311

properties of DC gels, we conducted a series of compressive tests to investigate the effect of PVA

312

chains or borate ions on the mechanical properties of DC hydrogels (Figure S3 and Figure S4).

313

When increasing the PVA concentrations from 2 to 4 wt%, the DC gels exhibited a monotonically

314

enhanced compression stress, compression modulus, and hysteresis energy. The result was

315

possibly due to more CS chains cross-linked with PVA chains via borate ions, thus, leading to an

316

enhancement in the mechanical properties of the DC hydrogel. However, a further increase in the

317

PVA content resulted in the decrease of mechanical strength in the DC gel, which might be

318

attributed to the inhibited formation of the first starch networks by residual PVA chains. Similar to

319

the PVA concentration effect, when the DC gels contained 0.026 mol/L of B, both compression

320

stress and hysteresis energy achieved maximal values of 219.615.7 kPa and 5790450 J∙m-3,

321

respectively (Figure S4bd). However, at the high B concentration (0.039 mol/L), the mechanical

322

behavior of the DC gels exhibited a decreasing trend. Previous studies reported that excessive

323

cross-linking could cause the first and second networks to become brittle and lose ductility.17

324

Therefore, the CS/PVA/B DC gels prepared under optimal conditions were used to the subsequent

325

experiments.

326

Favorable mechanical properties are of great significance for hydrogels in practical

327

applications. The pure CS SN and CS/PVA DN hydrogel exhibited brittle behavior (Figure 3a)

328

and fractured at a strain of 32.3 and 29.4% (Figure S5a), respectively, which meant the

329

cross-linked networks of the hydrogen bonds were not able to withstand large deformations. The



330

load force changed the orientation and relative position of amylose and amylopectin chains in the

331

CS gel. Meanwhile, the interstitial waters were squeezed out, thus, the compression stress of the

332

CS increased with the increasing strain. However, the interspace for chain movement determined

333

the increase of the compression strain. Once the compressive deformation reached its limit, further

334

increase in the load resulted in much more breakage of the gel networks. By contrast, the

335

reinforced CS/B and PVA/B SC hydrogels did not break at strains as high as 70%, suggesting a

336

soft yet tough behavior. Moreover, incorporation of borate ions and PVA chains to form the

337

PVA/B SC gel also conquered the low modulus of the PVA SN gel. It can be clearly seen that

338

when the second PVA/B network was combined with the CS network, the mechanical strength of

339

the DC gel increased dramatically. For instance, the compression stress reached up to 209.816.8

340

kPa for the DC gel, which was 1090%, 1200%, and 1410% higher than the corresponding CS SN

341

(19.22.3kPa), CS/B SC (17.41.6 kPa), and CS/PVA DN (14.71.3 kPa) hydrogels, respectively

342

(Figure S5b). Moreover, the compression modulus were 13.54 kPa for the DC hydrogel, 8.61 kPa

343

for the CS SN hydrogel, and 7.21 kPa for the CS/PVA DN hydrogel (Figure 3c), indicating the

344

mechanical strength of the DC gel exceeded those of either of its parent gels. Furthermore, the

345

photographs of the DC hydrogel for the presentation of mechanical properties have been recorded

346

as well. As shown in Figure 3d, the DC gels can partly return to their initial shapes after

347

compression with a strain of 70%–80%, showing much improved toughness than conventional

348

hydrogels.65 Cyclic compressive assays were performed to evaluate the energy dissipation

349

capacity of the hydrogels (Figure 3bc). The CS/PVA/B DC hydrogel had a larger hysteresis loop

350

than the CS-based and PVA/B SC hydrogels, confirming the more efficient energy dissipation of

351

the DC gels. Moreover, the energy dissipation of the DC gels reached as high as 5750450 J/m-3,


Page 16 of 50

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352

which was 4.5×, 8.0×, and 11.5× higher than the CS, CS/B, and CS/PVA gels, respectively

353

(Figure 3c). Intriguingly, the values of the cumulative energy dissipation of all the control

354

specimen gels (CS, CS/B, CS/PVA, and PVA/B) were approximately two times lower than the

355

DC hydrogel. The results further illustrated that incorporating PVA chains and borate ions

356

improved the mechanical properties of the starch hydrogels. We speculated that the reason for the

357

notable hysteresis loop could stem from the much more reversible dissociation-association of CS

358

and PVA chains cross-linked by the ionic bridge of the borate ions upon compression. Meanwhile,

359

the reversible dissociation-association process enabled an efficient stress transfer and energy

360

dissipation,46 contributing more significant viscous dissipation and shock absorbing capacity than

361

those of H-bonds in the SN gels.

362

Self-Recovery Behaviors of DC Hydrogels. In addition to the reinforcement of mechanical

363

strength, another advantageous feature of the DC network structure is the ability to undergo

364

repeated association to endow excellent self-recovery behaviors of gels (Figure 2d). To evaluate

365

the self-recovery efficiency of DC hydrogels, the recovery performance of the gels at a strain of

366

70% with different interval times (0–5 h) was tested. The compression strength and hysteresis

367

loop of the DC hydrogels gradually increased with the extension of the resting time (Figure 4ab).

368

Moreover, we used the hysteresis energy to calculate the recovery efficiency of the DC hydrogel,

369

and the recovery efficiency of DC gel exceeding 81.9% and 93.5% after 30 min and 5 h,

370

respectively. As the resting time was prolonged from 0 min to 5 h, the hysteresis energy increased

371

from 3970190 to 5360310 J∙m-3. Owing to the dynamic nature of the coordination interactions

372

(hydrogen bonds, borate ion cross-link interactions, etc.) within the DC hydrogel, they can be

373

rapidly re-established when the external load is removed. Therefore, the gels could recover all or



374

part of the original mechanical properties if the second load was delayed for a longer time,

375

highlighting their efficiency as an energy dissipating mechanism. The morphology of the DC gels

376

after recuperating for 30 min and 5 h is shown in Figure 4c. The consecutive compression curves

377

with gradient increases in the maximum strain on DC hydrogels are shown in Figure 4d. Upon

378

increasing the strain (70%99%), the compressive strength of the gels increased from 211.7 to

379

547.8 kPa. The result suggested that the abovementioned DC gels could sustain the high-level

380

compressive deformation (strain up to 99%), and the breakage surface of the gel was not observed

381

after removing the load.

382

Free-Shapeable Properties and Stretchability of DC Hydrogels. Figure 5 shows that the

383

DC hydrogels displayed a milky white color as well as strong and free-shapeable properties. As

384

shown in Figure 5a to e, DC gels were highly free-shapeable to form different fine structures, such

385

as a starfish, shells, conch, and cherry blossom. This is an important to extend the starch-based

386

hydrogel's application, especially in the bioengineering field where artificial tissues with specific

387

geometrical shapes are used to achieve their biologically mimicking functions.13,28 Additionally,

388

DC gels are highly ductile and flexible to withstand stretch winding (Figure 5f), convolving

389

(Figure 5g), knotting (Figure 5h), knot stretching (Figure 5i), and stretching (Figure 5j). Moreover,

390

an intact DC gel was easily stretched up to approximately 4-fold its original length (Figure 5j).

391

Particularly upon removal of the deformation force, the convolving hydrogels could recover their

392

initial shapes, which further confirmed that the gels exhibit excellent shape-recovery properties.

393

Afterward, the recuperative DC gel was knotted again, which also bore a large deformation of

394

350% without breakage (Figure 5i). As a proof-of-concept, Figure 5kl shows the tensile stress–

395

strain properties of the DC hydrogel. The pure starch matrix hydrogels were too brittle to be tested


Page 18 of 50

Page 19 of 50


396

for mechanical tensile properties, while the obtained DC hydrogel exhibited much better tensile

397

properties. The fracture strain of the DC gel reached up to 2480% with a tensile strength of

398

~12500 Pa, which are the top values in ever reported in starch hydrogel systems. In addition, the

399

visual inspection of the stretching demonstrated that the DC hydrogels had excellent mechanical

400

tensile properties (Figure 5i). The enhanced stretchability of the DC gels was due to the chain

401

entanglement of amylose and/or amylopectin chains and PVA chains through ionic interaction of

402

the borate ions (Figure 5m). The fantastic integration of good ductility, flexibility, and

403

free-shapeability would broaden the scope of application for the fabricated composite hydrogels.33

404

Self-Healing Capacity of DC Hydrogels. The novelty of this DC hydrogel is not only

405

reflected in its excellent mechanical and free-shapeable properties, but also in its outstanding

406

automatic self-healing capacity. We conducted a macro self-healing experiment: the cylindrical

407

DC gel was cut into three pieces, and the pieces kept in close proximity, as represented in Figure

408

6a. After contact for 15 s, it should be noted that the pieces of hydrogel joined together to form a

409

single entity without a visible joint interface, then, the integrated gel could be lifted without falling

410

(Figure 6a2a3), supported its own weight by placing it on a bridge (Figure 6a4), and withstood the

411

weight of ~50 g weight (approximately 5-fold its own weight). If the contact time of the pieces

412

was delayed for 30 s, we found that the self-healed DC hydrogels did not break under a static load

413

of ~1 kg (more than 100-fold its own weight). Moreover, the self-healing property of the conch-

414

and shell-shaped DC hydrogel was also proven by lifting with fingers, standing upright on a table,

415

and by bending the gel in a U shape (conch-shaped: reverse and lateral; shell-shaped: up-down

416

and left right) as shown in Figure 6bc (b1-b4 and c1-c4) where the self-healed pieces are intact. The

417

point of self-healing can be noted from the remaining scar (red circle). Afterward, we further



418

challenged our DC gels to inspect their self-healing properties. As shown in Figure 6c, the

419

self-healed shell-shaped DC hydrogels were cut into multiple fragments again (c5) and put into a

420

shell- and crab-shaped mold, respectively. It is noteworthy that the fragments could also be

421

remolded and form an unabridged shell- and crab-shaped hydrogel both in air and underwater after

422

contact for 5 h (c6, c7), which suggested excellent self-healing performance of the DC hydrogels.

423

Possibly the dynamic cross-linking of CS and/or PVA chains with borate ions was still carried out,

424

these chains intertwined with each other, and thus the hydrogels could heal. With the rapid

425

self-healing capacity both in air and underwater, the DC gels will have a huge potential

426

application, especially in fields that require a fast response or working.

427

Healing efficiency is an important parameter to characterize the degree of self-healing,

428

therefore tensile tests were conducted on the original and self-healed samples.66,67 Fracture strain

429

and stress recovery percentages of the self-healed gels were defined as the self-healing

430

efficiency.68 The fractured parts were put back together, in contact, for different lengths of time

431

(15, 30, 60, and 600 s) in the air at room temperature. The healed hydrogel was stretched to test

432

the tensile strength (Figure 7). In the case of 15 s healing time, the cut gels were partially

433

self-healed; the healed gel could sustain a tensile stress of 6300 Pa with elongation strain of 985%

434

before fracture. Moreover, the elastic modulus, fracture strain, and healing efficiencies

435

significantly increased (p < 0.05) with an increased healing time. When the healing time

436

exceeding 600 s, the fracture energy of the healed gel was 199.121.1 kJ∙m-3 with a strain over

437

2000% (the fracture energy and fracture strain of the initial gel were 290.519.4 kJ∙m-3 and

438

2485%). The healing efficiency was 68.5% based on stress or 81.2% based on strain. The tensile

439

strain recovered to more than 80%, thus, we speculated that the CS network could be responsible


Page 20 of 50

Page 21 of 50


440

for the healing process. Therefore, we also investigated the self-healing behaviors of the parents of

441

the DC hydrogel. Although the CS SN and CS/PVA DN hydrogels did not exhibit self-healing

442

(Figure S6), the introduced borate ions cross-linked CS/B hydrogel possessed a self-healing

443

behavior (Figure S7). As shown in Figure S7, the two pieces of CS/B SC hydrogel formed an

444

integrated gel which was lifted and could withstand its weight (b, c3). Afterward, the two broken

445

starfish-shaped SC hydrogels healed completely without a visible joint interface after 12 h at 25

446

C. The healed gel could withstand gentle stretching (c4). These results further confirmed the

447

occurrence of cross-linking networks between CS chains with borate ions within the CS/B SC

448

hydrogels. In addition, we predicted the healing mechanisms of the DC gel (Figure 7a). When the

449

two fracture surfaces were put in contact at room temperature, the borate bonds within the network

450

were able to re-crosslink the broken CS and/or PVA chains by forming new complexes. The CS

451

chains were dragged by PVA chains and tangled with each other via hydrogen bonds to form a

452

new network. Self-healing capabilities were demonstrated by uniting the two pieces into one.69

453

Once repaired with the reversible ionic crosslink and hydrogen bond mechanism, the hydrogel

454

samples could regain their mechanical properties. Overall, the fast self-healing of the DC gels

455

resulted from the re-establishment of the network structure.

456

Swelling Properties of Hydrogels. Figure 8d shows the time dependence of the water

457

absorbency of the hydrogels. Due to the loose network structure of the hydrogel, the SN (CS and

458

PVA) and SC (CS/B and PVA/B) aerogels exhibited fast initial swelling. Compared with the SN

459

and SC gels, the CS/PVA DN and CS/PVA/B DC gels were capable of absorbing fewer quality

460

water molecules. Moreover, the equilibrium water content (EWC) of DC and DN were only 4.52

461

and 3.15 g/g, respectively (Figure 8a). These results suggested that the DC hydrogel exhibited



462

good swelling resistance as compared with those of either of its parent gels. This phenomenon is

463

possible due to the stronger double cross-linked structure and more chain entanglements via

464

hydrogen bonds making the network closer, and hindering the water molecules from accessing the

465

hydrogel cage, resulting in lower EWC. When introducing borate ions into network, we noticed

466

the EWC of the cross-linked network gels (CS/B, PVA/B, and CS/PVA/B) were larger than the

467

corresponding control (CS, PVA, and CS/PVA). The borate ions may inﬂuence the formation of

468

the crystalline of the starch and PVA networks and result in an increased amorphous region of the

469

hydrogels; thus, their water-holding abilities increased. This observation was also noted in the

470

X-ray results (Figure 1b). At the same time, Figure 8b exhibits the dried and swollen CS, CS/B,

471

CS/PVA, and CS/PVA/B gels, which nearly hold their initial shapes in a swollen state. However,

472

the swollen PVA and PVA/B gels were collapsed and could not retain their shapes, thus, the PVA

473

and PVA/B were not shown in the figure.

474

Then, we further inspected the compressive properties and textural hardness of the

475

pillar-shaped hydrogels in their swollen states (Figure 8c and Figure S5). The mechanical

476

properties of the swollen CS and CS/B gels were slightly decreased compared with the original

477

levels. This result was due to the destruction of the network structure during the freeze-drying

478

process, causing the SN and SC gels to exhibit a weaker mechanical property. As listed in Table 1,

479

the water content of the gels remained at a high level (70%95%) owing to the strong H-bonds

480

and sufficient interaction between the hydroxyl groups of adjacent polymer strands.70 Therefore,

481

we speculated that the reason for the decreased strain was more water molecules introduced into

482

the CS network from the swelling, thereby leading to decreased interactions between the CS

483

strands. The freeze-drying process enhanced the crystalline and cross-linking density of PVA


Page 22 of 50

Page 23 of 50


484

within the hydrogel, consequently, the mechanical strength of the CS/PVA gel increased. It is

485

noteworthy that although the strength of the swollen DC hydrogels was lower than the initial DC

486

gels upon addition of borate ions, the swollen DC hydrogels still exhibited excellent mechanical

487

properties. Figure 8c shows the compression stress–strain curves of the swollen hydrogels and one

488

can see the DC hydrogel demonstrated high mechanical performance with 166.2 kPa compression

489

stress, 70% compressive strain, and 1610.7 g hardness (Figure S5). The mechanical strength

490

recovered to a great extent, i.e. the recovery efficiency of swollen DC gel reached as high as

491

79.2% and 100% based on the compression stress and strain, respectively. In addition, the

492

CS/PVA/B DC aerogel was light-weight, and could be supported by a tender leaf (Figure 8c).

493

Absorption of MB. Contact time is an important factor that indicates whether the adsorbent

494

removes the target contaminant to reach the equilibrium. It is apparent that the adsorption amount

495

rapidly increased at initial time and then gradually slowed down until equilibrium (Figure 9a).

496

This may be ascribed to the higher MB concentration, faster diffusion, and more adsorptive sites

497

at initial adsorption phase.71 With increasing adsorption time, the vacant adsorptive sites of the DC

498

gel decreased, which lowered the probability of interactions between MB and adsorptive site,

499

consequently, the adsorption gradually slowed down and eventually tended to balance. When the

500

adsorption of hydrogels was achieved equilibrium, the MB adsorption capacities of CS/B, PVA/B,

501

and CS/PVA/B respectively reached up to 80.32, 65.26, and 144.68 mg/g, which were higher than

502

the corresponding CS (16.43 mg/g), PVA (38.29 mg/g), and CS/PVA (26.41 mg/g) gels.

503

Considering the anionic characteristic of the hydrogels, a possible reason for the increase of the

504

adsorbed ionic dyes might attribute to the electrostatic interactions.71 Furthermore, the porous

505

crosslinked structure would be beneficial for adsorption process since it increased surface area and



506

facilitated diffusion of dyes into the adsorbent.72 Thus, a significant increase in the adsorption

507

capacity of CS/PVA/B was observed. Moreover, the color of solution with CS/PVA/B DC gel

508

turned to transparent significantly (Figure S8), especially in comparison with CS, indicating the

509

beneficial effects of borate ions and PVA on MB adsorption.

510

The effects of temperature and pH on the removal amount are important factors to evaluate

511

dye adsorption in agricultural and industrial wastewater at different temperatures and pH values.

512

When the temperature increased, the adsorption capacity of MB on DC gel decreased, however, it

513

was still higher than that of the gels (CS, CS/B, PVA, PVA/B, and CS/PVA) at 25 °C. As the

514

solution pH was changed from acidic to basic condition, the zeta potential of the swollen DC gel

515

solution decreased from 0.24 mV to 25.60 mV (Figure S9). It can be speculated that the

516

isoelectric point (pI) for the DC gel was less than pH 1.0. When the solution pH > pI, the DC gel

517

is negatively charged, which was favorable for adsorbing MB owing to the electrostatic attraction.

518

It can be seen that the MB adsorption of DC hydrogels was highly pH-dependent. Close to

519

neutrality or under alkaline conditions, an obvious increase in the adsorption amount was

520

observed. Moreover, the adsorption amount of MB on DC gel markedly increased from 56.33 to

521

144.29 mg/g as the solution pH increase from 2.0 to 8.0 (Figure 9b). Under acidic conditions (pH

522

is of 1.0–5.0), the protonation of boric acid (B(OH)3) groups was unfavorable for the adsorption

523

process due to the fewer hydrogen bonds between the DC gel and MB. At basic conditions, the

524

de-protonation of B(OH)4 resulted in the destruction of hydrogen bonding interactions and the

525

“charge screening effect” of excess Na+ ions also prevented effective electrostatic interactions

526

between the borate ions of gel and MB. Under neutral conditions, the moderate residual B(OH)4

527

and B(OH)3 groups may be conducive to the formation of electrostatic interactions and hydrogen


Page 24 of 50

Page 25 of 50


528

bonds between DC gel and MB, leading to the optimal adsorption capacity of DC gel for MB. As

529

depicted in Figure 9c, because the initial MB concentration is fixed, when the gel dosage

530

increased, the total adsorption amount of MB on the hydrogel increased, but the adsorption

531

amount per unit mass of DC gel was decreased, and the utilization of adsorbent would be greatly

532

reduced. In addition, the adsorption amount was significantly affected by the initial concentrations

533

of dye solution. With increasing the MB concentrations, the adsorption amount exhibited an

534

initially increasing and then an equilibrium trend (Figure 9c). The removal amount was 143.66

535

mg/g at 150 mg/L MB dye initial concentration, which increased to 170.29 mg/g at 200 mg/L

536

initial concentration.

537

In summary, we have designed and fabricated a one-pot method, which is a simple,

538

time-saving, and easily controlled process to prepare hybrid DC hydrogels. Within the DC gel,

539

hydrogen bonds cross-linked to CS chains as the rigid and brittle first network, and dynamic

540

borate bond cross-linked to PVA chains as the second network. The first network provides

541

reversible sacrificial bonds and dissipates energy, whereas the second network provides elasticity

542

to the DC hydrogel. Because of the distinct hybrid physically-chemically cross-linked structure,

543

the DC hydrogels showed notably improved mechanical properties over the single physically

544

cross-linked starch-based hydrogels. The tensile fracture strains and toughness of the DC

545

hydrogels could be as high as 2485% and 290.5 kJ∙m-3, respectively. Moreover, the compression

546

strength of the DC gels reached 547.8 kPa at a compression strain of 99%, significantly higher

547

than the starch matrix hydrogels previously reported in the literature. They retained their original

548

shapes after continuous loading−unloading compressive cyclic tests, demonstrating their superior

549

self-recovery ability. Furthermore, the DC gels exhibited remarkable self-healing properties both



Page 26 of 50

550

in air and underwater at room temperature. Specially, the healed DC hydrogel specimens still

551

showed ~20 times elongation at break. What’s more, because of its unique reversible network

552

structures, the gel system is able to produce any complex shape. The combination of good

553

mechanical properties, self-recovery of strength and toughness, and free-shapeable ability, along

554

with an easy synthesis method, allow the materials potential applications in the field of

555

biomaterials and load-bearing artificial soft tissues. In addition, the introduction of borate ions and

556

PVA play an important role in the adsorption of MB by DC hydrogel whose maximum removal

557

amount reaches 144.68 mg/g which is much higher than that of CS gel. Consequently, the

558

prepared DC gels could be served as eco-friendly, stable, and efficient adsorbents for cationic dyes

559

from the agricultural and industrial effluents.

560

Supporting Information

561

Compressive

stress-strain

curves,

compression

stress

and

modulus,

compressive

562

loading-unloading curves and corresponding hysteresis energy of hydrogels with different PVA

563

and B concentrations; hydrogel steadiness images; photographs of the mechanical performance of

564

CS-based hydrogels; self-healing properties of CS, CS/PVA, and CS/B hydrogels; compressive

565

strain, stress, and textural hardness of hydrogels in the initial and swollen states; images of the MB

566

dye solution adsorption on different hydrogels. (PDF)

567

ACKNOWLEDGMENTS

568

This work was supported by National First-Class Discipline Program of Food Science and

569

Technology (JUFSTR20180203).

570

Notes

571

The authors declare no competing financial interest.


Page 27 of 50


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Nanocomposite Physical Hydrogels Based on the Synergistic Effects of Dynamic Hydrogen

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762

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763

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764

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766

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767

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768

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769

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770

Figure Captions

771

Scheme 1. Schematically fabrication of the method and formation mechanism of CS/PVA/B

772

double cross-linked (DC) hydrogel (a). Single and double cross-linked network (SC and DC)

773

formed by several specific interactions among corn starch (CS), PVA, and borax (B): hydrogen

774

bond, borate bond (reversible ionic crosslink), and chain entanglement (b).

775

Figure 1. (a) FTIR spectra and (b) X-ray diffraction pattern of CS, CS/B, PVA, PVA/B, CS/PVA,

776

and CS/PVA/B hydrogels; SEM images exhibiting the microstructure of (c) CS, (d) CS/B, (e)

777

PVA, (f) PVA/B, (g) CS/PVA, and (h) CS/PVA/B hydrogels.

778

Figure 2. Photographs showing the processes of compressing and loosening behavior of shapeable

779

CS-based hydrogels: a) CS single network hydrogel, b) CS/PVA double network hydrogel, c)

780

CS/B single cross-linked hydrogel, and d) CS/PVA/B double cross-linked hydrogel. Schematic

781

illustration of the possible mechanisms of the CS-based gel networks compression and loosen

782

process.

783

Figure 3. Mechanical properties of hydrogels at room temperature: (a) Compressive stress-strain

784

curves, (b) compressive loading-unloading curves, and (c) compression modulus and hysteresis

785

energy of CS, CS/B, PVA/B, CS/PVA, and CS/PVA/B hydrogels. (d) Photographs revealing the

786

notable compression resistance of CS/PVA/B DC hydrogel.

787

Figure 4. Self-recovery of CS/PVA/B DC hydrogels at room temperature: (a) Compressive curves,

788

(b) loading-unloading curves, and (c) time-dependent hysteresis recovery and hysteresis energy

789

histogram of DC hydrogels and the corresponding recovery photographs (inserted image). (d) The

790

consecutive compression curves with gradient increased in maximum strain on CS/PVA/B DC

791

hydrogels: the DC hydrogel withstand the compressive strain of 99% without breakage.


Page 36 of 50

Page 37 of 50


792

Figure 5. CS/PVA/B DC hydrogels exhibiting excellent free-shapeable property to form any

793

complex shape with very fine structures, including starfish (a), shell (b), conch (c), and cherry

794

blossoms (d, e). DC gels are highly strong and flexible to withstand stretch winding (f),

795

convolving (g), knotting (h), knot stretching (i), and stretching (j) without breakage. Tensile

796

stress−strain curves of DC hydrogel: The DC gel achieved the highest strain of 2485% (k). The

797

image of stretched hydrogel (l), and schematic diagram of original and stretched DC gel network

798

structure (m).

799

Figure 6. Visual evidence of self-healing of a CS/PVA/B DC hydrogel: Cubic DC hydrogel which

800

is cut into three pieces (a1). Two or three parts of hydrogels are contacted for 15 s, (a2, a3) the

801

whole hydrogel is lifted without falling; (a4) the self-healed hydrogel on a bridge support and can

802

withstand the weight of 50g weight (approximately 5-fold its own weight). After contacting for

803

30 s, the self-healed hydrogel can withstand 1000g weight (approximately 100-fold its own

804

weight) without breakage. Conch shaped hydrogel: The cut sample of conch hydrogel (b1). The

805

self-healed gel can bear its own weight (b2); and sustain bending without breaking (b3, reverse;

806

b4, lateral). Shell shaped hydrogel: Strip shaped hydrogel (c1), the self-healed shell hydrogel also

807

can withstand its weight (c2) and bent in a U shape without breaking (c3, up-down; c4, left right).

808

The shell hydrogel is cut into small pieces (c5); the pieces are put in the shell or crab shaped mold

809

and molding in air or water for 5 h; the gel pieces heal as an integral shell and crab shaped

810

hydrogels (c6, c7). The points of contact are encircled by red circle.

811

Figure 7. Schematic illustration of the mechanism of CS/PVA/B DC hydrogel self-healing process

812

in air at room temperature (a).The tensile stress-strain curves of the self-healed DC hydrogels with

813

various healing time at room temperature (b), the corresponding fracture stress and fracture strain



814

(c), and elastic modulus and toughness (d) of self-healed DC hydrogels for indicated times.

815

Numbers denote the healing efficiency of DC gels.

816

Figure 8. Swelling properties of hydrogels: Photos of the shapeable CS, CS/B, CS/PVA, and

817

CS/PVA/B aerogels before (a) and after (b) swelling in deionized water. Light-weight CS/PVA/B

818

DC aerogel could support by a tender leaf (c). Swelling kinetics of CS, CS/B, PVA, PVA/B,

819

CS/PVA, and CS/PVA/B aerogels in the deionized water at room temperature: The PVA, CS/B,

820

and PVA/B aerogels reach equilibrium within 9 h, while the CS, CS/PVA DN, and CS/PVA/B DC

821

aerogels can reach equilibrium at approximately 36, 30, and 60 h, respectively (d). Compression

822

stress−strain curves of the CS, CS/B, CS/PVA, and CS/PVA/B gels in the swollen state (e).

823

Figure 9. Effects of contact time, pH, temperature, hydrogel dosage, and initial MB concentration

824

on the adsorption amount of MB onto gels. (a) Contact time (t = 0−48 h, C0 = 150 mg/L, V = 100

825

mL, T = 25 °C, pH = 6.45, dos. = 100 mg), (b) temperature (CS/PVA/B DC gel, C0 = 150 mg/L,

826

V = 100 mL, t = 24 h, pH = 6.45, dos. = 100 mg) and pH (CS/PVA/B DC gel, C0 = 150 mg/L, V

827

= 100 mL, t = 24 h, T = 25 °C, dos. = 100 mg), and (c) initial concentration (CS/PVA/B DC gel, t

828

= 24 h, V = 100 mL, T = 25 °C, dos. = 100 mg) and hydrogel dosage (CS/PVA/B DC gel, C0 =

829

150 mg/L, t = 24 h, V = 100 mL, T = 25 °C, pH = 6.45). qe represents the adsorption amount of

830

MB onto gels at equilibrium.


Page 38 of 50

Page 39 of 50

831


Table 1 Hydrogels with various compositions and the corresponded water contents. Sample

Starch (wt%)

PVA (wt%)

Borax (mol/L)

Water content (%)

CS

10.0

0.0

0.0

89.142.01

PVA

0.0

4.0

0.0

94.991.39

CS/B

10.0

0.0

0.026

86.790.95

PVA/B

0.0

4.0

0.026

93.351.32

CS/PVA

10.0

4.0

0.0

83.971.41

CS/PVA/B

10.0

4.0

0.026

79.460.79

Values represent the mean ± standard deviation of triplicate tests.



a

Mixing

Starch chains

Page 40 of 50

Borax

95 C, 1h

95 C, 2 h

PVA 4 °C, 12 h

chains

Freezing and thawing

Idealized DC network model

CS/PVA/B DC O

O

CS/B SC

B-

O

O

O

O

B-

O

O

O

B-

O

O

O

b

PVA/B SC

Complex types

Chain entanglement Hydrogen bond

832 833 834 835

Borate bond

PVA

crystallite Scheme 1. Schematically fabrication of the method and formation mechanism of CS/PVA/B double cross-linked (DC) hydrogel (a). Single and double cross-linked network (SC and DC) formed by several specific interactions among corn starch (CS), PVA, and borax (B): hydrogen bond, borate bond (reversible ionic crosslink), and chain entanglement (b).


Page 41 of 50


CS

a

CS/B

PVA

PVA/B

CS/PVA

CS/PVA/B

19.4

b

17.1

Intensity (%)

T (%) 4000

CS CS/B PVA PVA/B CS/PVA CS/PVA/B

22.9

3500

3000

2500

2000

-1

1500

1000

500

5

10

15

Wavenumber (cm )

c

d

f

g

20 m

2 (°)

25

30

35

40

e

20 m

20 m

20

20 m

h

20 m

20 m

836

Figure 1. (a) FTIR spectra and (b) X-ray diffraction pattern of CS, CS/B, PVA, PVA/B, CS/PVA,

837

and CS/PVA/B hydrogels; SEM images exhibiting the microstructure of (c) CS, (d) CS/B, (e)

838

PVA, (f) PVA/B, (g) CS/PVA, and (h) CS/PVA/B hydrogels.



CS

Page 42 of 50

CS/B

Compressing

Loosening

c

a First

CS/PVA

Compressing

Fifth

After addition of borax

CS/PVA/B

Loosening

d

b First

e)

Fifth

Schematic illustration of CS and CS/B hydrogel network Network destruction

No network destruction 1

2

Initial

Compressing strain: 30%

CS hydrogel 2 Single network

f)


Borate ions cross-linked

CS/B hydrogel 2 Single cross-linked network

Schematic illustration of CS/PVA and CS/PVA/B hydrogel network Network destruction

Initial

Starch chain

No network destruction



CS/PVA hydrogel Double network

839 840 841 842 843

Partial recovery

Borate ions cross-linked

PVA chain

Hydrogen bond

Recover to the initial height

CS/PVA/B hydrogel Double cross-linked network PVA crystallite

Borate bond

Figure 2. Photographs showing the processes of compressing and loosening behavior of shapeable CS-based hydrogels: a) CS single network hydrogel, b) CS/PVA double network hydrogel, c) CS/B single cross-linked hydrogel, and d) CS/PVA/B double cross-linked hydrogel. Schematic illustration of the possible mechanisms of the CS-based gel networks compression and loosen process.



200 20

160

b)

CS CS/B PVA/B CS/PVA CS/PVA/B

Stress (kPa)

40

Stress (kPa)

240

Stress (kPa)

a)

120 0 0

80

5

10

15

Strain (%)

20

25

40 0

240

CS CS/B CS/PVA PVA/B CS/PVA/B

40

200 20

160

Stress (kPa)

Page 43 of 50

120 0

80

0

10

20

0

10

20

30

40

50

60

0

70

50

60

70

0

10

6000

12

5000

10

4000

8

3000

6

2000

4

0

50

60

70

d)

Initial

50%

Release

70%

/P

A/

40

CS

PV

30

VA CS /P VA /B

0 B

1000

/B

2

Hysteresis energy (Jm-3)

7000

14

CS

20

Strain (%)

16

CS

Compression modulus (kPa)

844 845 846 847

40

40

Strain (%)

c)

30

Strain (%)

Figure 3. Mechanical properties of hydrogels at room temperature: (a) Compressive stress-strain curves, (b) compressive loading-unloading curves, and (c) compression modulus and hysteresis energy of CS, CS/B, PVA/B, CS/PVA, and CS/PVA/B hydrogels. (d) Photographs revealing the notable compression resistance of CS/PVA/B DC hydrogel.



250

a)

Initial

0 min

30 min

5h

250

(kPa)

150

Stress

Stress (kPa)

200

100 50 0

b)

Initial

0

10

20

30

40

50

60

3000 2000 1000 0

100

0

10

20

100

c)

Photographs

80

30 min 40

d)

500

Initial

60

30

40

Strain (%)

70

80

90

50

60

70

100

400 300 200

5h

20 0

600

100

0 min

30 min

5h

0

0

20

Time

848 849 850 851 852

5h

150

0

70

Stress (kPa)

4000

30 min

50

Hysteresis recovery (%)

Hysteresis energy ( Jm-3 )

5000

0 min

200

Strain (%) 6000

Page 44 of 50

40

60

80

100

Strain (%)

Figure 4. Self-recovery of CS/PVA/B DC hydrogels at room temperature: (a) Compressive curves, (b) loading-unloading curves, and (c) time-dependent hysteresis recovery and hysteresis energy histogram of DC hydrogels and the corresponding recovery photographs (inserted image). (d) The consecutive compression curves with gradient increased in maximum strain on CS/PVA/B DC hydrogels: the DC hydrogel withstand the compressive strain of 99% without breakage.


Page 45 of 50


c

b

a

e

d

f g

i

h

j

l

k

m CS chains

PVA chains

Stress (Pa)

15000

10000

5000

0

853 854 855 856 857 858 859

Photograph 0

500

1000

1500

2000

Tensile strain (%)

2500

Initial

Network

Stretch

Network

Figure 5. CS/PVA/B DC hydrogels exhibiting excellent free-shapeable property to form any complex shape with very fine structures, including starfish (a), shell (b), conch (c), and cherry blossom (d, e). DC gels are highly strong and flexible to withstand stretch winding (f), convolving (g), knotting (h), knot stretching (i), and stretching (j) without breakage. Tensile stress−strain curves of DC hydrogel: The DC gel achieved the highest strain of 2485% (k). The image of stretched hydrogel (l), and schematic diagram of original and stretched DC gel network structure (m).



a1

a3

a5

a4

50 g

a2

Gravity

b1

Page 46 of 50

a6 1 kg

Gravity

b2

b3

b4

c3

c4

Gravity

c1

c2

c5

c6

Self-healing in air

c7

Self-healing in water

860 861 862

Figure 6. Visual evidence of self-healing of a CS/PVA/B DC hydrogel: Cubic DC hydrogel which is cut into three pieces (a1). Two or three parts of hydrogels are contacted for 15 s, (a2, a3) the whole hydrogel is lifted without falling; (a4) the self-healed hydrogel on a bridge support and can

863 864 865 866 867 868 869 870 871

withstand the weight of 50g weight (approximately 5-fold its own weight). After contacting for 30 s, the self-healed hydrogel can withstand 1000g weight (approximately 100-fold its own weight) without breakage. Conch shaped hydrogel: The cut sample of conch hydrogel (b1). The self-healed gel can bear its own weight (b2); and sustain bending without breaking (b3, reverse; b4, lateral). Shell shaped hydrogel: Strip shaped hydrogel (c1), the self-healed shell hydrogel also can withstand its weight (c2) and bent in a U shape without breaking (c3, up-down; c4, left right). The shell hydrogel is cut into small pieces (c5); the pieces are put in the shell or crab shaped mold and molding in air or water for 5 h; the gel pieces heal as an integral shell and crab shaped hydrogels (c6, c7). The points of contact are encircled by red circle.



Cutting

a)

Self-healing

12000

b)

15 s 30 s 60 s 600 s

Tensile stress (Pa)

10000

Chain entanglement

PVA chain

PVA crystallite

Fracture strain (%)

c) 3000 39.6% 2500

59.5%

10000

62.9% 43.1% 52.2% 67.1%

2000

8000

1500

6000

1000

4000

500

2000

0

15 s

30 s

60 s

600 s

Healing time

872 873 874 875 876

12000

69.4% 81.1%

Initial

00

6000 4000 2000 0

Hydrogen bond

d) 600 Elastic modulus (Pa)

Borate bond

Fracture stress (kPa)

CS chain

8000

0

39.8%

1000

Tensile stain (%)

50.2% 61.7%

24.3%

37.1% 47.7%

2000

350

79.9%

300

68.6%

250

400

200 150 200

100 50

00

15 s

30 s

60 s

600 s

Initial

Toughness (kJm-3)

Page 47 of 50

0

Healing time

Figure 7. Schematic illustration of the mechanism of CS/PVA/B DC hydrogel self-healing process in air at room temperature (a).The tensile stress-strain curves of the self-healed DC hydrogels with various healing time at room temperature (b), the corresponding fracture stress and fracture strain (c), and elastic modulus and toughness (d) of self-healed DC hydrogels for indicated times. Numbers denote the healing efficiency of DC gels.



Page 48 of 50

a c b

CS/B

CS

CS/PVA CS/PVA/B

0.7 g of weight

d

e

200

PVA/B

10 8 6 4

CS

CS/B

CS/PVA

CS/PVA/B

150

100

50 2 0

877 878 879 880 881 882 883

CS CS/B PVA CS/PVA CS/PVA/B

Stress (kPa)

Swelling ratio (g/g)

12

0

20

40

60

Time (h)

80

100

0

0

10

20

30

40

Strain (%)

50

60

70

Figure 8. Swelling properties of hydrogels: Photos of the shapeable CS, CS/B, CS/PVA, and CS/PVA/B aerogels before (a) and after (b) swelling in deionized water. Light-weight CS/PVA/B DC aerogel could support by a tender leaf (c). Swelling kinetics of CS, CS/B, PVA, PVA/B, CS/PVA, and CS/PVA/B aerogels in the deionized water at room temperature: The PVA, CS/B, and PVA/B aerogels reach equilibrium within 9 h, while the CS, CS/PVA DN, and CS/PVA/B DC aerogels can reach equilibrium at approximately 36, 30, and 60 h, respectively (d). Compression stress−strain curves of the CS, CS/B, CS/PVA, and CS/PVA/B gels in the swollen state (e).


Page 49 of 50


a

CS/PVA/B CS/PVA CS/B CS PVA/B

160

PVA

140

qe (mg/g)

120 100 80 60 40 20 0

0

5

10

15

20

25

30

35

40

45

50

Time (h)

35

40

45

50

55

Temperature (C)

160 140 120 100 80 60 40 20 0

884 885 886 887 888 889 890 891

2

4

pH

6

8

10

60

180 170 160 150 140 130 120 110 100 90 80 70 60

c

200

0

50

100

150

200

250

300 200

Dosage (mg)

180

180

160

160

140

140

120

120

100

100

80

80

60

60

40

40

20

20

0

50

100

150

200

250

qe (mg/g)

30

qe (mg/g)

25

qe (mg/g)

qe (mg/g)

b

20 180

0

MB concentration (mg/L)

Figure 9. Effects of contact time, pH, temperature, hydrogel dosage, and initial MB concentration on the adsorption amount of MB onto gels. (a) Contact time (t = 0−48 h, C0 = 150 mg/L, V = 100 mL, T = 25 °C, pH = 6.45, dos. = 100 mg), (b) temperature (CS/PVA/B DC gel, C0 = 150 mg/L, V = 100 mL, t = 24 h, pH = 6.45, dos. = 100 mg) and pH (CS/PVA/B DC gel, C0 = 150 mg/L, V = 100 mL, t = 24 h, T = 25 °C, dos. = 100 mg), and (c) initial concentration (CS/PVA/B DC gel, t = 24 h, V = 100 mL, T = 25 °C, dos. = 100 mg) and hydrogel dosage (CS/PVA/B DC gel, C0 = 150 mg/L, t = 24 h, V = 100 mL, T = 25 °C, pH = 6.45). qe represents the adsorption amount of MB onto gels at equilibrium.



Graphic Abstract Self-healing capacity

Initial 60 s

15000

Stress (Pa)

892

Page 50 of 50

15 s 600 s

30 s

10000

Initial

70%

Release

5000

0 0

Shapeable ability

Self-recovery


500

1000 1500 2000 2500 Photograph

Tensile strain (%)

Initial and healed hydrogels

A Dual Cross-Linked Strategy to Construct Moldable Hydrogels with

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