Self-Healing Cellulose Nanocrystals-Containing Gels via ... - MDPI

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Self-Healing Cellulose Nanocrystals-Containing Gels via Reshuffling of Thiuram Disulfide Bonds Wenyan Li 1 , Shengchang Lu 1 , Mengchan Zhao 1 , Xinxing Lin 1 , Min Zhang 1 , He Xiao 1 , Kai Liu 1 , Liulian Huang 1 , Lihui Chen 1 , Xinhua Ouyang 1, *, Yonghao Ni 1,2, * and Hui Wu 1, * 1

2

*

College of Material Engineering, Fujian Agriculture and Forestry University, No. 63, Xiyuangong Road, Fuzhou 350108, China; [email protected] (W.L.); [email protected] (S.L.); [email protected] (M.Z.); [email protected] (X.L.); [email protected] (M.Z.); [email protected] (H.X.); [email protected] (K.L.); [email protected] (L.H.); [email protected] (L.C.) Department of Chemical Engineering, Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada Correspondence: [email protected] (X.O.); [email protected] (Y.N.); [email protected] (H.W.); Tel.: +86-18005906759 (X.O.); +1-506-4534547 (Y.N.); +86-18649784585 (H.W.)

Received: 16 November 2018; Accepted: 12 December 2018; Published: 15 December 2018

 

Abstract: Self-healing gels based on reshuffling disulfide bonds have attracted great attention due to their ability to restore structure and mechanical properties after damage. In this work, self-healing gels with different cellulose nanocrystals (CNC) contents were prepared by embedding the thiuram disulfide bonds into gels via polyaddition. By the reshuffling of thiuram disulfide bonds, the CNC-containing gels repair the crack and recover mechanical properties rapidly under visible light in air. The thiuram disulfide-functionalized gels with a CNC content of 2.2% are highly stretchable and can be stretched approximately 42.6 times of their original length. Our results provide useful approaches for the preparation of dynamic CNC-containing gels with implications in many related engineering applications. Keywords: cellulose; disulfide; gel; self-healing

1. Introduction Self-healing materials have received significant attention because of their ability to restore structure and mechanical properties after damage, which can be applied to various applications such as coatings/sealants, tissue adhesives, and drug/cell delivery [1–5]. Healing agents including cross-linking reactants and catalysts were applied in self-healing systems initially [6]. Upon mechanical damage, these agents, in the encapsulation of nanotubes and microcapsules, were released and subsequently polymerized within the crack so that the damages were fixed [6,7]. Recently, materials using reversible chemical bonds to repair damages in polymeric materials were explored extensively [1–5]. Generally, non-covalent interactions and dynamic covalent bonds were employed in the creation of reversible self-healing systems. The non-covalent bonds usually include coordination interactions [8–10], hydrogen bonds [11–13], hydrophobic interactions [14], electrostatic interactions [15], host–guest interactions [16], and π–π stacking [17]. Metallosupramolecular polymers comprising the hard phase of metal–ligand complexes and soft domains of the hydrophobic core were fabricated [8]. The cracks can be healed by photothermal conversion. An autonomic self-healing material with high stretchability was synthesized by the introduction of a Fe(III)-2,6-pyridinedicarboxamide coordination complex in a poly(dimethylsiloxane) matrix [9]. Thermoplastic elastomers prepared using polystyrene backbone as hard phase and polyacrylate amide brushes as soft phase were healable through hydrogen bonding [11]. A pressure- and flexion-responsive

Polymers 2018, 10, 1392; doi:10.3390/polym10121392

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material obtained by embedding nickel nanostructured microparticles into supramolecular organic polymer can heal the crack via hydrogen bonds [12]. Although the non-covalent interactions offer intensive reactivity and repeatability of the repair reaction, the gels composed of physical crosslinks are weaker [18]. In contrast, reversible covalent bonds, such as disulfide bonds [2], imine bonds [19], acylhydrazone bonds [20], and didiol−borax complexations [21], provide relatively stronger molecular interactions and strength for self-healing materials. Under external stimuli such as heat, light, and pH, the dynamic reactions allow the breakage and re-forming of bonds to achieve repair. In particular, disulfide chemistry, one crucial part of dynamic covalent chemistry, is unique in the controllability of exchange reactions [22–31]. A self-healing copolymer with dual disulfide bonds and supramolecular crosslinked network was prepared [22]. The samples restore its original shape and stretch to two times after the two separated pieces contacted tightly for 24 h. By introducing the cyclic disulfide, the reactivity of disulfide bonds enhances, and the hydrogel heals under neutral/mildly acidic conditions [23]. Self-healing polymers were fabricated by the reshuffling of trithiocarbonate units under UV irradiation [24] and the reforming of disulfide bonds under visible light [25]. However, the introduction of non-environmentally friendly chemicals and complex reaction condition are usually requested to construct self-healing systems, which is not practical for the environment and mass production. Cellulose is an abundant green resource in nature. Owing to the high intensity of hydroxyl groups along the skeleton of cellulose, cellulose is allowed to generate functional materials via physical and chemical modification [32–47]. In particular, the cellulose nanocrystals (CNC) have attracted a tremendous amount of interest owing to its fascinating physicochemical properties such as adaptable surface chemistry and high mechanical strength. Nanocellulose was widely used as a reinforcing agent to enhance the mechanical properties of materials, which have potential applications in nanocomposites, electronics, membranes, and supercapacitors [48]. However, reports on CNC-containing gels with rapid self-healing and high stretchable properties are still rare. Therefore, it is highly desired to explore natural resource-derived gels incorporating disulfide bonds with rapid self-healing and high stretchability using CNC as building blocks. Herein, we report a CNC-containing gel by incorporating dynamic disulfide covalent bonds prepared via polyaddition. The CNC-containing disulfide-functionalized gels with high stretchability are capable of self-healing and recovering the mechanical properties under visible light rapidly. 2. Experiments 2.1. Materials Bamboo dissolving pulp was purchased from Sichuan Tianzhu Resources Development Co., Ltd. (Yibin, Sichuan, China). Hexamethylene diisocyanate (HDI), triethanolamine (TEA), N,N-dimethylformamide (DMF), ditin butyl dilaurate (DBD), carbon disulfide (CS2 ), iodine (I2 ), tris(2-carboxyethyl)phosphine hydrochloride (TCEP) were purchased from Aladdin Industrial Corporation (Shanghai, China). 2-(ethylamino)ethanol was obtained from TCI (Shanghai) Development Co., Ltd. (Shanghai, China). Chloroform (CHCl3 ) was supplied by Sinopharm Group Chemical Reagent Co., Ltd. (Tianjin, China). 2.2. Preparation of Cellulose Nanocrystals (CNC) CNC were prepared from bamboo pulp by hydrolysis with sulfuric acid. Bamboo pulp was immersed in water and stirred for 1 day. The concentrated sulfuric acid (95%) was added dropwise into the mixture under ice bath until the 64% acid concentration was reached. After the suspension was stirred at 45 ◦ C for 3 h, the mixture was diluted with deionized water and centrifuged at 9000 rpm for 30 min repeatedly. The resulting suspension was dialyzed against deionized water until neutrality was reached. Finally, sample was freeze-dried to give a white CNC powder.

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2.3. Preparation of Thiuram Disulfide (TDS) TDS was synthesized according to the previous report [49]. 2-(ethylamino)ethanol (35.6 g, 0.4 mmol) and CHCl3 (200 mL) were charged into a 500 mL round-bottom flask. Then, I2 (25.4 g) and CS2 (12 mL) were added slowly, stirring under ice bath for 3 h. The reaction mixture was washed with cold deionized water repeatedly to remove aminehydroiodide. The organic layer was evaporated under vacuum. The crude product was purified by a column chromatography (silica gel, [hexane]/[ethyl acetate] = 3/2, v/v) to yield a yellow oil (14.04 g, 43% yield). 1H NMR spectroscopic measurements were recorded at 25 ◦ C on a 400 MHz Bruker instrument (BRUKE AVANCE III, Karlsruhe, Germany). 1 H NMR (400 MHz, CDCl ): δ = 4.1 (multiplet, 6H, CH ), 3.2 (singlet, lH, OH), 1.4 (triplet, 3H, CH ). 3 2 3 2.4. Preparation of CNC-Containing Gels To prepare the gels with different CNC contents, a determined amount of CNC (0 mg, 50.0 mg, 70.0 mg, 100.0 mg, 120.0 mg and 188.4 mg) was dispersed in 3.6 mL of DMF by stirring. TDS (472.8 mg, 1.44 mmol), TEA (168.5 mg, 1.13 mmol), HDI (481.6 mg, 2.87 mmol) and DBD (3 drops, 0.5% for isocynate units) were charged in sequence. After stirring for 10 min, the solution was transferred into 1 mL of injection syringe for 1 day to give cylindrical organogels with CNC content of 0%, 1.1%, 1.5%, 2.2%, 2.6%, and 4.0%, respectively. 2.5. Characterization The morphology of CNC was observed using transmission electron microscopy (TEM). Two drops (about 10 µL by using micropipette) of 0.006 wt % CNC ethanol suspension were deposited on a carbon-coated copper grid and then dried under ethanol atmosphere at room temperature overnight. The sample was then observed using a FEI tecnai G2 F20 (FEI Company, Hillsboro, OR, USA) with 200 KV acceleration voltages. The healing process of CNC-based gels was monitored by an optical microscopy (Nikon Eclipse E200, Tokyo, Japan). The CNC-containing gel was split into two parts using a blade. The detached samples were contacted together without applying additional pressure and healed by exposing to the light of the optical microscopy system with power of 6 W and sample-to-light distance of 12 cm. The tensile performances of CNC-containing gels were tested by a tensile tester (INSTRON 3365, Norwood, MA, USA). The gels sheets were molded into dumbbell shapes (2 mm in thickness, 2 mm in width, 12 mm in length). To measure the healing efficiency, the healed samples with dumbbell shape were prepared by cutting into halves by a blade, re-contacting and exposing in the light source (10 W, distance to the samples = 10 cm) for 2 min in air. All the samples were tested at a rate of 30 mm/min at room temperature. Real time recording was used to get stress-strain curve. The healing efficiency of tensile strain (HEt ) and stress (HEs ) are expressed as [35] HEt =

Lh × 100% Lp

HEs =

Sh × 100% Sp

where Lh and Sh are the healing strain and stress of the healed samples at the breaking point, Lp and Sp are the strain and stress of pristine samples at the breaking point, respectively. The rheological behaviors of CNC-containing gels were performed with a stress-controlled rheometer Rotational Rheometer MARS III Haake (Thermo Scientific, Karlsruhe, Germany) with a parallel plate of 35 mm diameter. Samples with a diameter of 30 mm and a thickness of 5 mm were subjected to carry out a strain sweep test for five measurements. The value of the strain amplitude was selected as 1% to ensure that all measurements were determined within a linear viscoelastic region.

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was selected as1392 1% to ensure that all measurements were determined within a linear viscoelastic Polymers 2018, 10, 4 of 13 region. Resultsand andDiscussion Discussion 3.3.Results The cross-linked cross-linked CNC-containing CNC-containinggels gels incorporating incorporatingTDS TDS units units were were synthesized synthesized via via the the The polyadditionbetween betweenCNC, CNC,TDS, TDS, and HDI. CNC were obtained from sulfuric hydrolysis polyaddition and HDI. CNC were obtained from thethe sulfuric acidacid hydrolysis of of bamboo pulp. The of size of CNC ± 68 in length 25 ± nm inrespectively width, respectively bamboo pulp. The size CNC is 199is± 199 68 nm in nm length and 25 and ± 8 nm in 8width, (Figure (Figure 1a). Because of anumber large number of hydroxyl groups on the CNC surface, TDS, andTEA, TEA,the the 1a). Because of a large of hydroxyl groups on the CNC surface, TDS, and isocynate groups in HDI reacted with CNC, TDS and TEA via polyaddition to form cross-linked isocynate groups in HDI reacted with CNC, TDS and TEA via polyaddition to form cross-linked gels.Both BothTEA TEAand andCNC CNCcan canbe beacted actedas ascrosslinkers crosslinkersduring duringnetwork networkformation. formation.The Thedynamic dynamicTDS TDS gels. unitswith withdisulfide disulfidebonds bondswhich whichare arecapable capableofofreshuffling, reshuffling,were wereincorporated incorporatedininthe thecrosslinked crosslinked units gel,asasshown shownininFigure Figure1b. 1b.ItItisiscrucial crucialtotoregulate regulatethe themole moleratio ratioofofCNC, CNC,TDS, TDS,TEA TEAand andHDI HDItoto gel, designCNC-containing CNC-containinggels gelswith withhigh highself-healing self-healingefficiency efficiencyand andstretchable stretchableproperty. property.InInour ourstudy, study, design CNC-containinggels gelswith withaaCNC CNCcontent contentfrom from0% 0%to to4.0% 4.0%were werefabricated. fabricated. CNC-containing 20

Intensity

a

15 10 5 0

50

100 150200

Length (nm) 500 nm

b

Figure Figure1.1. (a) (a)TEM TEMimage imageofof cellulose cellulose nanocrystals. nanocrystals. (b) (b)Schematic Schematicrepresentation representationofofself-healing self-healing CNC-containing CNC-containinggel gelvia viapolyadditation polyadditationreaction. reaction.

TheCNC-containing CNC-containinggels gelsshow showaaremarkable remarkableself-healing self-healingability abilityunder undervisible visiblelight lightatatroom room The temperature,without withoutthe theneed needfor forapplying applyingany anycatalyst catalystfor forhealing. healing.Figure Figure2a 2ashows showsthe thegel gelwith with temperature, CNCcontent contentofof2.2% 2.2%before beforecutting. cutting.The Thegel gelisisflexible flexibleunder underbending bending(Figure (Figure2b). 2b).For Forthe thestudy studyofof CNC self-healingbehavior, behavior,the thecylindrical cylindricalsample sampleofofgel gelwas wascut cutinto intotwo twopieces pieces(Figure (Figure2c) 2c)with witha ablade. blade. self-healing After placing placing the in close contact by hand, the re-contacted samples were exposed After the two twoseparated separatedpieces pieces in close contact by hand, the re-contacted samples were to visible in light air forin2air min temperature. As shown Figure the 2d, twothe pieces exposed tolight visible forat2room min at room temperature. As in shown in 2d, Figure two became pieces a single piece essentially and no apparent boundary can be observed. Furthermore, the self-healed

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became a single piece essentially and no apparent boundary can be observed. Furthermore, the self-healed CNC-containing gelintegrity retainedunder its integrity bending without2e). break The CNC-containing gel retained its bendingunder without break (Figure The(Figure healing2e). process healing process reversible, the gelsmultiple can experience is reversible, andisthe gels can and experience healingmultiple cycles. healing cycles. To further further study study the the self-healing self-healing behavior behavior of of the the CNC-containing CNC-containing gels, gels, the the self-healing self-healing process process To was monitored monitored by by optical optical microscopy. microscopy. Figure 3 shows the photographic photographic sequence of the the healing healing was process of of gels gels with with CNC CNC content content of of 0%, 0%, 1.0%, 1.0%, 1.5%, 1.5%, 2.2%, 2.2%, 2.6%, 2.6%, and and 4.0%. 4.0%. The red arrows point point out out process the cracks cracks during during the the healing healing process. process. After the gel was cut by a blade, blade, the two two separated separated pieces pieces were re-contacted re-contacted immediately immediately and healed healed under under the the visible visible light light of of optical optical microscopy. microscopy. The gels gels were with different CNC contents show different times for healing. For the gel without CNC (CNC with different CNC contents show different times for healing. For the gel without CNC (CNC content content of 0%), 350 s is heal the the CNC content increases to 2.2%,time the of 0%), 350 s is needed to needed heal theto sample. Assample. the CNCAs content increases to 2.2%, the healing healing time shortens. 120for s is needed for disappearance the complete disappearance the rupture when the shortens. Only 120 s is Only needed the complete of the ruptureofwhen the CNC content CNC content toa2.2%, a rapid self-healing However, increasing (2.6%, CNC reaches to 2.2%,reaches showing rapid showing self-healing process. However, process. increasing CNC concentration concentration 4.0%) in the gels appear to crack It isofknown that the 4.0%) in the gels(2.6%, appear to tardy crack recovering. It istardy known that recovering. the aggregation nanoparticles is aggregation of nanoparticles and the number of non-dispersed will ascend unavoidable, and the number is ofunavoidable, non-dispersed particles will ascend in the CNC particles solution with higher in the CNC solution with higher which might lead to thedisulfide longer healing time. To concentration, which might lead toconcentration, the longer healing time. To verify that bonds served as verify that disulfide bonds as the main self-healingofsystem and re-forming of the main self-healing systemserved via breaking and re-forming bonds,via webreaking also measured the healing bonds, we also measured the healing behavior of the sample in the absence of TDS. As expected, behavior of the sample in the absence of TDS. As expected, the gel with a 2.2% CNC concentration the gel with a 2.2% CNC concentration any when disulfide was unable towere integrate when without any disulfide units, was unable without to integrate twounits, separated samples in contact, two separated were in contact, even for a long period (30 min). even for a longsamples period (30 min).

a

b 5 mm

c

5 mm

e

d 5 mm

5 mm

5 mm

Figure 2.2.Photographs Photographsofofgels gels with CNC content of 2.2%: (a) before cutting, (b) bending before Figure with CNC content of 2.2%: (a) before cutting, (b) bending before cutting, cutting, (c) after cutting, (d) self-healing, and (e) bending after self-healing. (c) after cutting, (d) self-healing, and (e) bending after self-healing.

To To achieve achieve self-healing self-healing properties, properties, materials materials containing containing disulfide disulfide bonds bonds usually usually need need rigorous rigorous external stimulus to trigger trigger the the reversible reversible reaction, reaction, such such as as ultraviolet ultraviolet light, light, heat, heat, ultrasound, ultrasound, and and electricity. It is well-known that the bond energy of disulfide is less than that of C–C, which favors electricity. well-known disulfide less than that of C–C, which favors the breaking breaking and and re-forming re-forming of of S–S S–S bonds. bonds. Although the exchange of aromatic disulfides is faster faster than the corresponding corresponding aliphatic disulfides [22], 2 h are needed needed to repair repair the the crack crack of of polyurethane polyurethane prepared prepared by by aromatic aromatic disulfides disulfides [29]. [29]. However, our our results results showed showed that that the the gels gels with with CNC CNC content content of 2.2% 2.2% can can achieve achieve self-healing self-healing within 2 min min under under ambient ambient visible visible light, light, indicating indicating the disulfide disulfide bonds in TDS TDS units units require require less less energy energy to to dissociate dissociate than than aliphatic aliphatic or or aromatic aromatic disulfides. disulfides. Thanks Thanks to the electronegativity electronegativity of C=S in TDS units, the dissociation and reformation of S–S bonds are more more likely to occur occur under under mild mild conditions. conditions.

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Figure 3. Images of gels with CNC content of (a) 0%, (b) 1.1%, and (c) 1.5%, (d) 2.2%, (e) 2.6%, and Figure 3. Images of gels with CNC content of (a) 0%, (b) 1.1%, and (c) 1.5%, (d) 2.2%, (e) 2.6%, and (f) (f) 4.0% 4.0% during duringthe theself-healing self-healingprocess process (Red arrows point cracks). (Red arrows point outout cracks).

ToTo evaluate the mechanical property of the CNC-containing gels, tensile tests of gels with different evaluate the mechanical property of the CNC-containing gels, tensile tests of gels with contents of CNC were out. carried As shown Figure 4, ainsimilar that both elongation and different contents of carried CNC were out.inAs shown Figure trend 4, a similar trend that both stress increases first and then decreases can be observed with increasing CNC concentration. The gels elongation and stress increases first and then decreases can be observed with increasing CNC with concentration of gels CNCwith below 1% are brittle one-fold its original length. concentration. The concentration of and CNCcannot belowstretch 1% aretobrittle andofcannot stretch to Theone-fold gel without shows the minimum stress CNC in all shows the gels. the concentration of the CNC goes of itsCNC original length. The gel without theAs minimum stress in all gels. Asup, thethe stress of the samples increases When of CNC increases up to 2.2%, elongation concentration of CNC goesgradually. up, the stress theconcentration samples increases gradually. When CNC attains to about 4260% of the showing the gel is highly stretchable. The elongation concentration increases up original to 2.2%,length, elongation attains to about 4260% of the original length, is showing the 42.6 gel istimes highly The elongation is is approximately timesCNC. and the stress at approximately andstretchable. the stress at breaking point ten times that42.6 without Nevertheless, breaking point isisaten times withoutthe CNC. Nevertheless, it turns out there is a drop with it turns out there drop withthat increasing concentration of CNC from 2.2% to 4.0%. The greater increasing the concentration of CNC from 2.2% to 4.0%. The greater increase in CNC concentration increase in CNC concentration (2.6%, 4.0%) results in a decrease in the tensile stress (4.4%, 19.4%) as (2.6%, results strain in a decrease the tensile stress (4.4%, 19.4%) as with well as in the fracture strain well as in4.0%) the fracture (32.3%,in 99.6%) as compared to the sample 2.2% CNC concentration.

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4000 4000

240 240

3000 3000

180 180

2000 2000

120 120

1000 1000

60 60

0 0

Stress/ kPa / kPa Stress

ElongationatatBreak Break/ % /% Elongation

(32.3%, 99.6%) as compared to the sample with 2.2% CNC concentration. Therefore, the gels with Therefore, the gelsaswith CNC concentration 2.2% achieved maximum strain. These results (32.3%, 99.6%) compared to the sample of with 2.2% CNC concentration. Therefore, the gelsindicate with CNC concentration of 2.2% achieved maximum strain. These results indicate that CNC significantly CNC concentration ofenhances 2.2% achieved maximum strain. These results indicate that CNC significantly that CNC significantly the mechanical properties of gels containing thiuram disulfide [50]. enhances the mechanical properties of gels containing thiuram disulfide [50]. As the CNC enhances the mechanical properties of gels containing thiuram disulfide [50]. As the CNC Asconcentration the CNC concentration thein increase in the of gels be ascribed the enhancing increases, increases, the increase the stress of stress gels can be can ascribed to the toenhancing of concentration increases, the increase in the stress of gelsbycan be ascribed to the the enhancing of a of mechanical mechanical strength by CNC and the energy dissipation disulfide bonds when gels suffer strength by CNC and the energy dissipation by disulfide bonds when the gels suffer a mechanical However, strength byhigh CNC and concentration the energy dissipation by disulfide bonds when the gels suffer a deformation. CNC leads to the mechanical degradation of gels due deformation. However, high CNC concentration leads to the mechanical degradation of gels due to to deformation. However, high CNC concentration leads to the mechanical degradation of gels due to thethe formation of heterogeneous gels;gels; homogenous gels are to formtobecause of the incomplete formation of heterogeneous homogenous gelsdifficult are difficult form because of the the formation of heterogeneous homogenous difficult to form because of the reaction of polyaddition at high CNCgels; [51].gels are[51]. incomplete reaction of polyaddition atconcentration high CNC concentration incomplete reaction of polyaddition at high CNC concentration [51].

0 0 0 0

1

2

3

1 2 3 CNC Content / wt% CNC Content / wt%

4 4

Figure Elongation at at break break (black (black square, square, indicated and stress (red circles, Figure 4. 4.Elongation indicatedby bythe theblack blackarrow) arrow) and stress (red circles, Figure 4. Elongation at break (black square, indicated by the black arrow) and stress (red circles, indicated by the red arrow) of CNC-containing gels. indicated by the red arrow) of CNC-containing indicated by the red arrow) of CNC-containing gels.

HealingEfficiency Efficiency/ % /% Healing

100 100 80 80 60 60

Stress / kPa Stress / kPa

investigate healing behavior of cellulose-containing gels,self-healed the self-healed was ToTo investigate thethe healing behavior of cellulose-containing gels, the gel wasgel prepared To investigate the healing behavior of cellulose-containing gels, the self-healed gel was by cutting a blade, re-contacting and in exposing in the visible inset 5 byprepared cutting into halves into by ahalves blade, by re-contacting and exposing the visible light. Thelight. insetThe in Figure prepared by cutting into halves by a blade, re-contacting and exposing in the visible light. The inset in Figure showed the typicalcurves stress-strain curves ofand theself-healed pristine and with CNC showed the 5typical stress-strain of the pristine gelself-healed with CNCgel content of 2.2%. in Figure 5 showed the typical stress-strain curves of the pristine and self-healed gel with CNC content of 2.2%. The of stress-stain curve thesimilar healedto gels was similar togel. thatThe of pristine gel. The The stress-stain curve the healed gels of was that of pristine strain value of the content of 2.2%. The stress-stain curve of the healed gels was similar to that of pristine gel. The strain value of the healed gels attains to 4260%, showing that the healed gel can be stretched healed gels attains to 4260%, showing that the healed gel can be stretched approximately 42 times their strain value of the healed gels attains to 4260%, showing that the healed gel can be stretched approximately 42 times their original length. The stress of which healed isgel is 215.7 ± 7.2 kPa, which is original length. The stress of healed gel is 215.7 ± 7.2 kPa, close to that of pristine sample approximately 42 times their original length. The stress of healed gel is 215.7 ± 7.2 kPa, which is close to that of pristine sample (221.6 ± 9.0 kPa). These results indicate that the healed gels nearly (221.6 9.0that kPa). These results healed nearly restored functionalities and close±to of pristine sampleindicate (221.6 ±that 9.0 the kPa). Thesegels results indicate that their the healed gels nearly restored their functionalities and structures after damage. structures after damage. restored their functionalities and structures after damage. 200 200 100 100 0 00 0

HEs HEs HEt HEt

Pristine Pristine Healed Healed 2000 4000 Strain 2000/ % 4000 Strain / %

40 40 20 20 0 0

0 0

1

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1 Content 2 / wt% 3 CNC CNC Content / wt%

4 4

Figure 5. Self-healing efficiency of gels with different CNC content. Inset is the stress–strain curves Figure Self-healing 5. Self-healing efficiency gels withdifferent differentCNC CNCcontent. content.Inset Insetisisthe thestress–strain stress–strain curves curves of Figure efficiency ofofgels of the5.pristine and self-healed gel withwith CNC content of 2.2%. of the pristine and self-healed gel with CNC content of 2.2%. the pristine and self-healed gel with CNC content of 2.2%.

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To Tobetter betterunderstand understand the the healing healing efficiency, efficiency,the theHE HESSand andHE HEtt were were evaluated evaluated and and the the calculated calculated results were given in Figure 5. Similarly, the self-healing efficiency of samples appears to an increase results were given in Figure 5. Similarly, the self-healing efficiency of samples appears to an along with concentration of CNC from 0% to 2.2% and decline to less than 20% when it comes to increase along with concentration of CNC from 0% to 2.2% and decline to less than 20% when4% it concentration. The HES and HE gels with 2.2% CNC concentration are 97.3% and 93.3% respectively, t ofHE comes to 4% concentration. The S and HEt of gels with 2.2% CNC concentration are 97.3% and showing an excellentshowing healing efficiency. These demonstrate the self-healed CNC-containing gels 93.3% respectively, an excellent healing efficiency.that These demonstrate that the self-healed with CNC contentgels of 2.2% intact crosslinking network withstand stress among extensive CNC-containing withformed CNC content of 2.2% formed intacttocrosslinking network to withstand elongation. However, the lower or higher concentration of CNC cannot impart gels with high stress among extensive elongation. However, the lower or higher concentration of CNC healing cannot efficiency. This may result from inefficient chain movement and limited surface disulfide radicals impart gels with high healing efficiency. This may result from inefficient chain movement and exchange in brittle gels with lowerexchange or higherinCNC content. limited surface disulfide radicals brittle gels with lower or higher CNC content. We further studied the influence of separation time on the healing ability We further studied the influence of separation time on theof cellulose-containing healing ability of gels. A gel sample made of 2.2% CNC concentration and was cut into two sections; at various cellulose-containing gels. A gel sample made of 2.2% CNC concentration and was cut intotimes two after their separation, they were then brought into contact and exposed to a visible light treatment sections; at various times after their separation, they were then brought into contact and exposed to for 2 min.light As treatment shown in for Figure 6, As theshown healinginefficiency of strain decreases asofthe separation time a visible 2 min. Figure 6, the healing efficiency strain decreases as increases, supporting the conclusion that the time gel was time cut, isafter a key for the separation time increases, supporting theseparation conclusion thatafter the the separation thefactor gel was the self-healing property; gels separated for a very long time cannot self-heal as efficiently as those cut, is a key factor for the self-healing property; gels separated for a very long time cannot self-heal newly separated. This may beseparated. caused by This the reduced the thiuram disulfide bonds as efficiently as those newly may beconcentration caused by theofreduced concentration of the or radicals in the fractured surface due to the loss or migration of the TDS radicals to the of interior of thiuram disulfide bonds or radicals in the fractured surface due to the loss or migration the TDS gel [25]. This will alleviate their accessibility to disulfides on another part when the surfaces are in radicals to the interior of gel [25]. This will alleviate their accessibility to disulfides on another part contact again. Notably, HEt was higher than 60% fort the separated for 4for h, indicating a high when the surfaces are inthe contact again. Notably, the HE wasgel higher than 60% the gel separated healing ability. for 4 h, indicating a high healing ability.

100

HEt / %

80 60 40 20 0

0

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25

Figure 6. 6. The The healing healing efficiency efficiency of of the the gel gel for for various various separation separation times. times. Figure

To gain gain insight insight into into the thedynamic dynamicrheological rheological behavior, behavior,the theangular angularfrequency frequency(ω) (ω) dependence dependence To 0 ) and of storage storage modulus modulus (G (G′) and loss loss modulus modulus (G”) (G″) of CNC-containing gels with CNC content of 1.5%, of 2.2% and and 2.6% 2.6% were were shown shown in in Figure Figure 7. 7. It should be noted that the gels with CNC content of 0%, 2.2% 1.1% and and 4.0% 4.0% are are too too fragile fragile to to perform perform rheology rheology test. test. The The G G′0 curves curves of of all all the the gels gels exhibited exhibited the the 1.1% plateau-like behavior, behavior,indicating indicating rubbery plateau region thenetwork. gels network. can be plateau-like thethe rubbery plateau region of theof gels This canThis be ascribed ascribed to the crosslinking structurebygenerated by TEA and CNC.the Interestingly, the G″ atcurve to the crosslinking structure generated TEA and CNC. Interestingly, G” curve decreases low decreases and at low frequency and fluctuates at high-frequency might be due to reversible frequency fluctuates at high-frequency which might be due towhich reversible thiuram disulfide bonds thiuram disulfide bonds in resulting cellulose-containing resulting in variations inchains. the relaxation of in cellulose-containing gels, in variationsgels, in the relaxation of polymers Generally, polymerschains chains. Generally, polymer requirethemselves significant to time to rearrange polymer require significant timechains to rearrange minimize the freethemselves energy. It to is minimizethat the the freedithiocarbamyl energy. It is reported that the remarkable dithiocarbamyl radicals have longevity reported radicals have longevity and are remarkable stable for more than and weeks are stable more the than two weeks [30]. Thus, the to chains sufficient timewhich to relax at low two [30].forThus, chains have sufficient time relaxhave at low frequency, result in frequency, result in decrease in G″ atat low frequency. However, at a rearrangement high frequency,might the chain decrease in which G” at low frequency. However, a high frequency, the chain not rearrangement might not be completed in a short timescale, leading to fluctuation in G″. In the whole frequency range, the G′ values were significantly greater than G” values, showing a gel network with elastic behavior rather than viscous nature. The G′ of gels containing 2.2% CNC (6865

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G', G''/Pa

G', G''/Pa

be completed a short timescale, leading fluctuation in 1.5% G”. InCNC the whole frequency range,that thethe G0 kPa) shows a in 332% increase in contrast to to that containing (at 10 Hz), indicating Polymers 2018, 10, x FOR PEER REVIEW 9 of 13 values werehigh significantly greater thaninG” values, showing a gel network behavior rather CNC with mechanical strength the well-formed gels enhance thewith gel’selastic mechanical property 0 than viscous G of gels containing 2.2% CNC (6865 kPa) shows 332% in contrast significantly. However, further the concentration of CNC to indicating aincrease decrease in G″ kPa) showsnature. a 332%The increase in increases contrast tointhat containing 1.5% CNC (atleads 10a Hz), that the to to that containing 1.5% CNC (at 10 Hz), indicating that the CNC with high mechanical strength in 3532 kPa (at 10 Hz) for gels containing 2.6% CNC. This may be due to the brittle gel network CNC with high mechanical strength in the well-formed gels enhance the gel’s mechanical propertythe well-formed gels enhance the gel’s mechanical significantly. significantly. However, further increases in theproperty concentration of CNC However, leads to a further decreaseincreases in G″ to in structure caused by the higher CNC content. the3532 concentration CNC to a decrease G” toThis 3532may kPa be (at due 10 Hz) for brittle gels containing 2.6% kPa (at 10 of Hz) for leads gels containing 2.6%inCNC. to the gel network structure caused CNC content. CNC. This may be by duethe to higher the brittle gel network structure caused by the higher CNC content. 4 10 G'-2.2% 4 10 G'-2.6% G'-2.2% G'-1.5% G'-2.6% G''-2.2% 3 G'-1.5% 10 10

G''-2.2%

3

G''-2.6% G''-2.6%

10

2

10

G''-1.5% 2

G''-1.5%

0.1

1

10

100

ω / Hz 0.1 1 10 100 ω / Hz Figure 7. Storage modulus G′ and loss modulus G″ of the gels with CNC content of 1.5%, 2.2%, and 0 and loss modulus G” of the gels with CNC content of 1.5%, 2.2%, and 2.6% against angular frequency. Figure 7. Storage modulus GG′ Figure 7. Storage modulus and loss modulus G″ of the gels with CNC content of 1.5%, 2.2%, and 2.6% against angular frequency. 2.6% against angular frequency.

To verify that the dynamic disulfide bonds control self-healing points of network structure, the ToTo verify that disulfidebonds bondscontrol control self-healing points of network structure, verify thatthe the dynamic disulfide self-healing points of network structure, the gel-forming property ofdynamic the cellulose-containing gels under reducing condition were investigated. the gel-forming property of the cellulose-containing gels under reducing condition were investigated. gel-forming property of the cellulose-containing gels under reducing condition were investigated. As shown in Figure 8, after adding the reductant TCEP solution into the tube containing a gel, the As shown in Figure 8, completely after adding thethe TCEP solution containing a agel, the shown in Figure 8, after adding reductant TCEP intothe thetube tube containing gel, thegel gelAs was decomposed into areductant turbid solution 1 solution h later.into Because TCEP is a strong reducing gel was completely a turbid 1 hbonds later. Because is a strong reducing was decomposed completely into ainto turbid solution 1 h later. Because is abonds strong reducing agent, the decomposed phosphine nucleophile attacked thesolution disulfide andTCEP the TCEP S-S cleaved toagent, form agent, the phosphine nucleophile attacked the disulfide bonds and the S-S bonds cleaved to form the phosphine nucleophile attacked the disulfide bonds and the S-S bonds cleaved to form two thiol two thiol (-SH) groups [52]. As a result, the network of gels broke gradually, and eventually became two thiol (-SH) groups [52]. As a result, the network of gels broke gradually, and eventually became (-SH) a sol. groups [52]. As a result, the network of gels broke gradually, and eventually became a sol. a sol.

TCEP TCEP 00min min

60 min

60 min

Figure under reducing conditions. Figure 8.8.Cellulose-containing Cellulose-containing gels under under reducing reducingconditions. conditions. Figure8. Cellulose-containing gels

The self-healing gels is to reshuffling the reshuffling reshuffling thiuram disulfide self-healing ofofCNC-containing gels is attributed to the of thiuram disulfide bonds, The self-healing ofCNC-containing CNC-containing gels is attributed attributed to the ofofthiuram disulfide bonds, asin Figure 9.9.visible Under the light, the the dithiocarbamate dithiocarbamate ester bondsin inthethe as shown Figure 9.inUnder the the dithiocarbamate ester bonds inester the CNC-containing bonds, asshown shownin Figure Underlight, the visible visible bonds CNC-containing gels are ofofhomolytic cleavage. Thus, thedormant dormant dithiocarbamyl radical gels are capable of homolytic cleavage. Thus, the dormant dithiocarbamyl radical intermediates are CNC-containing gels arecapable capable homolytic cleavage. Thus, the dithiocarbamyl radical intermediates are generated, which theis unpaired electron delocalized over dithiocarbamate intermediates are generated, ininwhich the unpaired electron is delocalized overthe the dithiocarbamate generated, in which the unpaired electron delocalized over is the dithiocarbamate structure. After the structure. After the fractured surfaces come inother, contact each dithiocarbamyl structure. After the fractured surfaces come contact with eachother, other,the the dithiocarbamyl fractured surfaces come in contact with eachin thewith dithiocarbamyl radicals combine radicals toradicals reform combine to reform identical covalent S–S bonds arbitrarily across the re-contacted surface through combine to reform identical covalent S–S bonds arbitrarily across the re-contacted surface through identical covalent S–S bonds arbitrarily across the re-contacted surface through radical crossover and radical crossover and degenerative radical transfer reactions [25]. of of radical crossover and degenerative radical reactions [25]. Consequently, thetwo twopieces pieces degenerative radical transfer reactions [25].transfer Consequently, the twoConsequently, pieces of the the organogel merged the organogel merged intointegrity integrity bythe the dynamic reversible bond. self-healing fuse into the organogel merged into by dynamic bond. The self-healing gels fuse into into integrity by the dynamic reversible bond. The reversible self-healing gelsThe fuse into their gels original shape their original shape after damage and restore their original properties. Therefore, the their originalandshape damage properties. and restore their the original properties. Therefore, the after damage restoreafter their original Therefore, bond-shuffling reactions of thiuram bond-shuffling reactionsofofthiuram thiuramdisulfide disulfide enable reorganization linking units in in thethe bond-shuffling the reorganization ofthe the linking units disulfide enablereactions the reorganization of the linkingenable units the in the re-contactedofgel surfaces and successful re-contacted gel surfaces and successful self-healing of the damaged gels, which hold great re-contacted of the hold areas great self-healing ofgel the surfaces damagedand gels,successful which holdself-healing great potential to bedamaged applied ingels, manywhich application potential to be applied in many application areas including engineering adhesives, sensors for potential be applied in manysensors application areas including engineering adhesives, sensors for including to engineering adhesives, for radicals, vulcanizates, storage, and microelectronics. radicals, vulcanizates, storage, and microelectronics. radicals, vulcanizates, storage, and microelectronics.

Polymers 2018, 10, 1392 Polymers 2018, 10, x FOR PEER REVIEW

SS

Light

SS

S S S S

SS

SS

S S S

S S

S

Heal

S

S

S S

SS

SS

S S

SS

S S

SS SS

S S S S S S

Contact

S

SS

S S

SS

SS S

S S

SS

S S

S S

S

SS

Cut

SS

S

S

SS

SS

S

SS

SS

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: CNC

Figure 9. Schematic illustration of the self-healing process of CNC-containing gel.

4. Conclusions 4. Conclusions In summary, self-healing cellulosed-containing gels with different CNC contents were fabricated by In summary, self-healing cellulosed-containing gels with different CNC contents were incorporating thiuram disulfide bonds. Via thiuram disulfide reshuffling reactions, the CNC-containing fabricated by incorporating thiuram disulfide bonds. Via thiuram disulfide reshuffling reactions, gels healed and restored their structures and mechanical properties after damage at room temperature the CNC-containing gels healed and restored their structures and mechanical properties after under visible light rapidly. The thiuram disulfide-functionalized gel with a CNC content of 2.2% damage at room temperature under visible light rapidly. The thiuram disulfide-functionalized gel showed high mechanical and self-healing properties. The gel was highly stretchable and could with a CNC content of 2.2% showed high mechanical and self-healing properties. The gel was be stretched nearly 42.6 times of their original length. This study can provide versatility in the highly stretchable and could be stretched nearly 42.6 times of their original length. This study can development of self-healable CNC-containing gels with implications in many related engineering provide versatility in the development of self-healable CNC-containing gels with implications in applications, such as adhesives, sensors, vulcanizates, storage, and microelectronics. many related engineering applications, such as adhesives, sensors, vulcanizates, storage, and microelectronics. Author Contributions: H.W. and W.L. designed the experiments; W.L. and M.Z. (Mengchan Zhao) performed the experiments; H.W., W.L., X.O. and Y.N. wrote the paper; S.L., X.L., M.Z. (Min Zhang), H.X., K.L., L.H. and L.C. contributed to the analysis andand discussion of the experimental results. Author Contributions: H.W. W.L. designed the experiments; W.L. and M.Z. performed the experiments; H.W., W.L., and Y.N. wrote the paper; S.L., Natural X.L., M.Z., H.X., K.L., L.H.ofand L.C. contributed to the Funding: ThisX.O. research was funded by the National Science Foundation China (31470598, 21774021), analysis and discussion of the experimental results. the Award Program for Minjiang Scholar Professorship, International Science and Technology Cooperation and Exchange Project of Fujian Agriculture and Forestry University (KXb16002A), and Scientific and technological Funding: This research was funded by theand National Natural Science Foundation ofKF2015002). China (31470598, 21774021), innovation funding of Fujian Agriculture Forestry University (CXZX2017040, the Award Program for Minjiang Scholar Professorship, International Science and Technology Cooperation Acknowledgments: Thanks to Renhui Qiu for kindly providing tensile testing instrument. and Exchange Project of Fujian Agriculture and Forestry University (KXb16002A), and Scientific and Conflicts of Interest: Thefunding authors of declare conflict of interest. technological innovation FujiannoAgriculture and Forestry University (CXZX2017040, KF2015002).

Acknowledgments: Thanks to Renhui Qiu for kindly providing tensile testing instrument.

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