1MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy Beijing Forestry University Beijing 100083 China2Beijing Shoufa Tianren Ecological Landscape Co Ltd Beijing 102600 China
Correspondence should be addressed to Jiufang Duan duanjiu99163com
Received 30 October 2014 Revised 12 January 2015 Accepted 21 January 2015
Academic Editor Ning Lin
Copyright copy 2015 Jiufang Duan et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Molecules that associate to form cross-links by hydrophobic association are designed and synthesised Hydrogels based oncellulose nanowhiskers (CNWs) acrylamide (AM) and stearyl methacrylate (C18) were synthesised bymicellar copolymerisationusing ammonium peroxydisulfate as an initiator CNWs composite hydrogels were characterised by Fourier transform infraredspectroscopy (FTIR) and their morphologies were investigated by scanning electron microscope (SEM) The system shows theoriginal extensibility up to about 2500 the tensile strength and compressive strength have maximum values of 1338MPa and2835MPa respectively Besides excellent mechanical properties CNWs composite hydrogels also have the ability to self-heal andremould this is mainly attributed to the dissociation and reassociation of the associated micelles In contrast to conventionalcellulose hydrogels these systems when broken or cut can be simply repaired by bringing together fractured surfaces to self-healat room temperature
1 Introduction
The preparation and application of hydrogels based oncellulose have been reviewed by some groups because of theirapplications across several technologies such as hygienicproducts horticulture gel-actuators drug delivery systemswater blocking tapes and coal dewatering [1ndash5] Celluloseas the most abundant renewable source on earth offersan answer to maintaining hydrogel materialsrsquo sustainabledevelopment as an economically and ecologically attractivetechnology Although a wide range of applications have beenproposed for such hydrogels due to their unique propertiesmost hydrogels suffer from a lack of strength and self-healingability [1]
An attractive method of designing multifunctionalhydrogels is to use the concept of supramolecular polymersNanocomposite gels are a useful strategy when seeking toimprove hydrogel mechanical strength CNWs have attractedmuch attention not only because of their unsurpassedquintessential physicochemical properties but also becauseof their inherent renewability and sustainability in addition to
their abundance They have been the subject of a wide arrayof research projects as reinforcing agents in nanocompositesdue to their low cost nanoscale dimension renewabilityavailability and unique morphology [6] Various nanocom-posite gels with high mechanical strength have been widelyinvestigated [7ndash9] Considering the excellent properties ofCNWs the fabrication moulding and application of hydro-gels containing CNWs have many advantages compared withother solid nanocomposite systems [10 11]The application ofsuch gels is typically limited by their poor mechanical prop-erties CNWs used for the reinforcement of polyacrylamidematrices have attracted much attention [11 12] howevermultifunctional hydrogels reinforced with CNWs are lesswell-studied [13 14] Cellulose hydrophobically associatinghydrogel with high mechanical strength and self-healingability has seldom been reported
Here we report a new type of CNWs composite hydrogelthat can overcome the aforementioned shortcomings Thereported CNWs composite hydrogel is a hydrophobic asso-ciation hydrogel which was prepared by micellar copoly-merisation AM and CNWs acted as the main monomer to
Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 963436 8 pageshttpdxdoiorg1011552015963436
2 Journal of Nanomaterials
form a hydrophilic backbone C18 dodecyl 2-methylacrylate(C12) and tridecyl methacrylate (C13) respectively acted ashydrophobic monomers to form associated micelles that arethe physical cross-linking points in a network of CNWs com-posite hydrogels because of the unique network structureCNWs composite hydrogels exhibit excellent strength andrubber-like properties the most remarkable properties beingthat CNWs composite hydrogels are self-healing
2 Experimental Work
21 Materials andMethods Acrylamide sodium dodecylsul-fate (SDS) stearylmethacrylateammoniumpersulfate (APS)NNNN1015840-tetramethylethylenediamine (TEMED) tride-cyl methacrylate dodecyl 2-methylacrylate and NaCl werecommercially available and used as received
IR spectra were recorded by FTIR (Nicolet iN10 ThermoFisher Scientific China) over the region from 4000 to400 cmminus1 The CNW structure was analysed by atomic forcemicroscopy (Shimadzu SPM-9600)
For morphological characterisation the hydrogels wereanalysed by scanning electron microscope (SEM) (S-3400NHitachi Japan) with an acceleration voltage of 40 kV A thinlayer of the sample was cast on a silica wafer and freeze-dried overnight in a lyophiliser A layer of gold was sputter-coated over the sample by vacuum spray to form a conductivesurface
Images of CNWs were obtained using atomic forcemicroscopy in intermittent contact mode Samples for AFMwere prepared by placing a drop of dilute CNW suspensionon freshly cleaved mica followed by rinsing in deionisedwater and drying under a gentle flow of N
2
Tensile stress-strain measurements were performed byusing an Instron 3365 Universal Testing Machine (NorwoodMA USA) with the following parameters sampling rate10000 ptssec beam speed 100mmmin full scale loadrange 01000 kN humidity 25 and temperature 24∘CThe strip-shaped gel samples measured 100mm times 10mm times3mm and the original length between top and foot clampswas 25mm Each data point was measured on six samplesand the average value of five measurements was takenStatistical analysis of data was performed by one-way analysisof variance assuming a confidence level of 95 (119875 lt 005) forstatistical significance
The gravimetricmethodwas used tomeasure the swellingratios of the gels After immersion in distilled water forapproximately 48 hr at 25∘C to reach swelling equilibriumthe gel samples were weighed The average value of threemeasurements was takenThe equilibrium swelling ratio (SR)was calculated as SR = 119882
119904119882119889 where119882
119904is the weight of the
swollen gel and119882119889is the weight of the gel in its dry state
22 Preparation of CNW Nanocomposite Hydrogels
221 Preparation of CNWs The cotton fibres were firstlysoaked in dimethyl sulphoxide (60mL) for 6 h at roomtemperature After this the fibres were washed in deionisedwater The CNWs were obtained through acidic hydrolysis of
the cotton fibres using 45wt H2SO4(a celluloseH
2SO4
ratio of 120 gmL) at 75∘C for 10 h under vigorous magneticstirring After this process the resulting solution was cen-trifuged at 10000 rpm for 5 min and washed thoroughly withdeionised water until a pH of 7 was reached The resultantmaterial was lyophilised at 57∘C for 48 h
222 Synthesis of CNW Nanocomposite Hydrogels The gelswere synthesised by micellar copolymerisation The reac-tion system generally consisted of CNWs water-solublemonomers (AM) hydrophobicmonomers (C12 C13 or C18)surfactants (SDS) and water SDS (07 g) and NaCl (030 g)were dissolved in 99mL of dispersion of CNWs at 35∘C toobtain a transparent solution Then hydrophobic monomerC18 (009 g 43 wt relative to the amount of solid content)was dissolved in this solution under stirring for 2 h at atemperature of 35∘C After adding and dissolving AM (090 g459 wt relative to the amount of solid content) for 30minTEMED (25 120583L) was added to the solution Finally 008 g ofAPS was added to initiate the reactionThe copolymerisationreactions were carried out at 25∘C for 24 h
3 Results and Discussion
31 Synthesis of CNWs Composite Hydrogels CNWs canbe isolated from various renewable sources (wood cottonwheatrice straw etc) Figure 1 shows AFM images of cotton-derived CNWs The cotton-derived CNWs have a rod-likemorphology (diameter 10 to 67 nm length 100 to 250 nm)The main process for the isolation of CNWs from cellulosefibres is based on acid hydrolysis The cotton cellulose chainshave a more ordered structure with a crystallinity of about70 [5] The amorphous regions appear as imperfectionsin the cellulose microfibrils [5] The crystalline regions ofcellulose having a higher resistance to acid attack remainintact disordered regions in the cellulose are preferentiallyhydrolysed Followed by an acid treatment that hydrolyses thecellulose colloidal particle CNWs were obtained [5]
We used CNWs AM and metacrylic acid ester (C12C13 or C18) as reactor monomers and added SDS to formsolubilised micelles or comicelles with the hydrophobicmonomers in an aqueous solution Copolymerisation wasinitiated by ammonium persulfate herein and the hydrogelsthus obtained were transparent and resilient (Figure 1)
In this reaction system CNWs and AM acted as the mainmonomer to form a hydrophilic backbone C12 C13 and C18acted as hydrophobic monomers to form associated micelles(Figure 1) that form the physical cross-linking points in anetwork of cellulose gels The salt (NaCl) led to micellargrowth and solubilisation of large hydrophobes within the as-grown worm-like SDS micelles [15] However the micelleswere stable enough to avoid destruction under the appliedconditions
32 Characterisation of the CNWs Composite Hydrogels Thestructure of hydrogels of acrylamidewith different hydropho-bic monomers (stearyl methacrylate (a) tridecyl methacry-late (b) or dodecyl 2-methylacrylate (c)) and CNWs were
Journal of Nanomaterials 3
Cellulose nanowhisker
Poly(acrylamide-co-alkylmethacrylate) chain
Micelle
Hydrophobic side chains
CH2OHCH2OH
OHOH
OH
OH
OO O
ONH4S2O8
CH2 = CCH3COORCH2 = CHCONH2
R = (CH2)11CH3(CH2)12CH3(CH2)17CH3
CH3
CCH2
COORm n
CHCH2
CONH2
HO
HOOH
OH
OH
OH
O O
OO
n
000000000000000000000000000000 00
6731
(nm
)
000000
000
020
040
060
080 020
040
080
060
100000times100000 (nm) 20
00ndash6731 (nm)
200 120583m 800 times 800 120583m 000
11561(nm)
Figure 1 Scheme of the CNWs composite hydrogel matrix
characterised by FTIR (Figure 2) It was observed that thehydrogen bond stretched at 3345 cmminus1 The CndashOndashC CndashCndashO andCndashCndashHdeformationmodes and stretching vibrationsin which the motions of the C-5 and C-6 atoms are at898 cmminus1 and the CndashOH out-of-plane bendingmode around670 cmminus1 also occurred According to Figure 2 absorptionwas observed at 1730 cmminus1 (stretching vibration C=O)1060 cmminus1 (asymmetrical stretching vibration of the etherbond) 2940 cmminus1 and 2830 cmminus1 (stretching vibration CndashH bond in methylene) 1651 cmminus1 and 792 cmminus1 (bendingvibration NndashH stretching vibration CndashN) The intensityof the stretching vibration of methylene increased with theincrease of the carbon chain length of the hydrophobicmonomer
33 Mechanical Properties of the CNWs Composite HydrogelsThe effect of the concentration of cellulose acrylamidecontent and content (and types) of hydrophobic monomerson the gel mechanical properties was analysed First we
29402830
(c)
(b)
(a)
4000
29402830
(c)
(b)
(a)
500 0100015002000250030003500
Wavenumbers (cmminus1)
Figure 2 FTIR spectra of CNWs composite hydrogels (stearylmethacrylate (a) tridecyl methacrylate (b) or dodecyl 2-methy-lacrylate (c))
synthesised the gel at the same concentration and experi-mental conditions with only CNW concentration changed
4 Journal of Nanomaterials
0005
01015
02025
03035
04045
05
0 00005 0001 00015 0002 00025 0003 00035 0004
Tens
ile st
reng
th (M
Pa)
The concentration of cellulose (gmL)
(a)
0
05
1
15
2
25
3
0 00005 0001 00015 0002 00025 0003 00035 0004
Com
pres
sive s
treng
th (M
Pa)
The concentration of cellulose (gmL)
(b)
(c)
Figure 3 Effect of CNW concentration on CNWs composite hydrogel mechanical properties ((a) tensile strength (b) compressive strength(c) compression strength test images of hydrogel with a CNW concentration of 00014 gmL)
001020304050607
30 35 40 45 50 55 60
Tens
ile st
reng
th (M
Pa)
The content of acrylamide ()
(a)
0002004006008
01012014
30 35 40 45 50 55 60
Com
pres
sive s
treng
th (M
Pa)
The content of acrylamide ()
(b)
Figure 4 Effect of acrylamide content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
Figure 3 shows the mechanical properties of the CNWscomposite hydrogel specimens using C18 as hydrophobicmonomer When increasing the concentration of CNWsfrom 00007 gmL to 00035 gmL the tensile strengthshowed a maximum value of 04550MPa and a compres-sive strength of 28MPa at a cellulose concentration of00014 gmL A possible reason is that high-aspect ratiofibres have the ability to sustain mechanical stress in a uni-form manner When the cellulose content used exceeds thecritical concentration thereof it will be difficult to avoidaggregation during their dispersion in the matrix because oftheir tendency to entangle In principle nonhomogeneousfiller dispersion caused by entanglement usually results thisin turn leads to poor composite properties and an inefficientstrengthening effect The hydrogel cannot be crushed as
shown in Figure 3(c) (a hydrogel with a CNW concentrationof 00014 gmL)
We synthesised gel of C18 under the same concentrationand experimental conditions with only the dosage of AMchanged from 345 wt to 544 wt Figure 4 shows themechanical properties of those gel specimens using C18 ashydrophobic monomer With increasing amounts of AMthe tensile strength and compressive strength increased from00028MPa to 05998MPa and 00211MPa to 01175MParespectively The increased tensile and compressive strengthwere consistent with the expected increases in cross-linkingdensities in the three-dimensional network structure As aresult the intertwining of polymeric chains was promotedand the hydrogen-bonding interaction between hydrophilicgroups such as ndashCONH
2and ndashOH among others was
strengthened Thus the degree of cross-linking increased afact that was favourable to the improvement of mechanical
Journal of Nanomaterials 5
002040608
1121416
2 3 4 5 6 7
Tens
ile st
reng
th (M
Pa)
The content of stearyl methacrylate ()
(a)
0
05
1
15
2
25
3
2 3 4 5 6 7
Com
pres
sive s
treng
th (M
Pa)
The content of stearyl methacrylate ()
(b)
Figure 5 Effect of stearylmethacrylate content onCNWs composite hydrogelmechanical properties ((a) tensile strength and (b) compressivestrength)
0001002003004005006007008
Dodecyl 2-methylacrylate
Tens
ile st
reng
th (M
Pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(a)
0
05
1
15
2
25
3
Dodecyl 2-methylacrylate
Com
pres
sive s
treng
th (M
pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(b)
Figure 6 Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
propertiesThe same influences will also arise with increasingcross-link density in traditional polymers cross-linked bychemical bonds
Figure 5 shows the mechanical properties of those gelspecimens using C18 as hydrophobic monomer The tensileand compressive strengths hadmaximumvalues of 1338MPaand 2835MPa respectively when the C18 content was350wt then with further increases in the amount ofhydrophobic groups the tensile and compressive strengthsdecreased It seemed that the action of hydrophobic groupson polymer chains was similar to that on the chemical cross-linking points [16]
Figure 6 shows the mechanical properties of gel speci-mens using C12 C13 and C18 as hydrophobic monomer Wesynthesised gels with hydrophobic monomer C12 C13 andC18 under identical experimental conditionsWith increasinghydrophobic side chain length the tensile and compressivestrengths increased from 00084MPa to 00708MPa andfrom 00483MPa to 24800MPa respectively The reasonfor this was that the hydrophobic side chain length becamelarger while the intermolecular hydrophobic interaction
became stronger In other words increasing the length ofhydrophobic side chain between backbones will induce animprovement in the mechanical properties of these gels
34 Surface Morphology of CNWs Composite HydrogelsFigure 7 shows SEM micrographs of freeze-dried nanocom-posite hydrogel filled with cellulose nanowhiskers (00007 gmL 00014 gmL 00021 gmL 00028 gmL and 00035 gmL) after being swollen to equilibrium It was seen that thehydrogels had a heterogeneous pore distribution and itseemed that the addition of CNWs would change the mor-phology CNWs composite hydrogel (Figure 7(a) 00007 gmL) had a relatively large-pored structure as well as theCNWs composite hydrogel (Figure 7(e) 00035 gmL) whichhad a relatively dense surface structure This may have beencaused by the different intermolecular forces arising fromthe alteration of the cellulose content This kind of surfaceallows a higher influx of water into the polymeric matrixwhich would then benefit the water absorbency by suchsuperabsorbent composites
6 Journal of Nanomaterials
(a) (b)
(c) (d)
(e)
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
form a hydrophilic backbone C18 dodecyl 2-methylacrylate(C12) and tridecyl methacrylate (C13) respectively acted ashydrophobic monomers to form associated micelles that arethe physical cross-linking points in a network of CNWs com-posite hydrogels because of the unique network structureCNWs composite hydrogels exhibit excellent strength andrubber-like properties the most remarkable properties beingthat CNWs composite hydrogels are self-healing
2 Experimental Work
21 Materials andMethods Acrylamide sodium dodecylsul-fate (SDS) stearylmethacrylateammoniumpersulfate (APS)NNNN1015840-tetramethylethylenediamine (TEMED) tride-cyl methacrylate dodecyl 2-methylacrylate and NaCl werecommercially available and used as received
IR spectra were recorded by FTIR (Nicolet iN10 ThermoFisher Scientific China) over the region from 4000 to400 cmminus1 The CNW structure was analysed by atomic forcemicroscopy (Shimadzu SPM-9600)
For morphological characterisation the hydrogels wereanalysed by scanning electron microscope (SEM) (S-3400NHitachi Japan) with an acceleration voltage of 40 kV A thinlayer of the sample was cast on a silica wafer and freeze-dried overnight in a lyophiliser A layer of gold was sputter-coated over the sample by vacuum spray to form a conductivesurface
Images of CNWs were obtained using atomic forcemicroscopy in intermittent contact mode Samples for AFMwere prepared by placing a drop of dilute CNW suspensionon freshly cleaved mica followed by rinsing in deionisedwater and drying under a gentle flow of N
2
Tensile stress-strain measurements were performed byusing an Instron 3365 Universal Testing Machine (NorwoodMA USA) with the following parameters sampling rate10000 ptssec beam speed 100mmmin full scale loadrange 01000 kN humidity 25 and temperature 24∘CThe strip-shaped gel samples measured 100mm times 10mm times3mm and the original length between top and foot clampswas 25mm Each data point was measured on six samplesand the average value of five measurements was takenStatistical analysis of data was performed by one-way analysisof variance assuming a confidence level of 95 (119875 lt 005) forstatistical significance
The gravimetricmethodwas used tomeasure the swellingratios of the gels After immersion in distilled water forapproximately 48 hr at 25∘C to reach swelling equilibriumthe gel samples were weighed The average value of threemeasurements was takenThe equilibrium swelling ratio (SR)was calculated as SR = 119882
119904119882119889 where119882
119904is the weight of the
swollen gel and119882119889is the weight of the gel in its dry state
22 Preparation of CNW Nanocomposite Hydrogels
221 Preparation of CNWs The cotton fibres were firstlysoaked in dimethyl sulphoxide (60mL) for 6 h at roomtemperature After this the fibres were washed in deionisedwater The CNWs were obtained through acidic hydrolysis of
the cotton fibres using 45wt H2SO4(a celluloseH
2SO4
ratio of 120 gmL) at 75∘C for 10 h under vigorous magneticstirring After this process the resulting solution was cen-trifuged at 10000 rpm for 5 min and washed thoroughly withdeionised water until a pH of 7 was reached The resultantmaterial was lyophilised at 57∘C for 48 h
222 Synthesis of CNW Nanocomposite Hydrogels The gelswere synthesised by micellar copolymerisation The reac-tion system generally consisted of CNWs water-solublemonomers (AM) hydrophobicmonomers (C12 C13 or C18)surfactants (SDS) and water SDS (07 g) and NaCl (030 g)were dissolved in 99mL of dispersion of CNWs at 35∘C toobtain a transparent solution Then hydrophobic monomerC18 (009 g 43 wt relative to the amount of solid content)was dissolved in this solution under stirring for 2 h at atemperature of 35∘C After adding and dissolving AM (090 g459 wt relative to the amount of solid content) for 30minTEMED (25 120583L) was added to the solution Finally 008 g ofAPS was added to initiate the reactionThe copolymerisationreactions were carried out at 25∘C for 24 h
3 Results and Discussion
31 Synthesis of CNWs Composite Hydrogels CNWs canbe isolated from various renewable sources (wood cottonwheatrice straw etc) Figure 1 shows AFM images of cotton-derived CNWs The cotton-derived CNWs have a rod-likemorphology (diameter 10 to 67 nm length 100 to 250 nm)The main process for the isolation of CNWs from cellulosefibres is based on acid hydrolysis The cotton cellulose chainshave a more ordered structure with a crystallinity of about70 [5] The amorphous regions appear as imperfectionsin the cellulose microfibrils [5] The crystalline regions ofcellulose having a higher resistance to acid attack remainintact disordered regions in the cellulose are preferentiallyhydrolysed Followed by an acid treatment that hydrolyses thecellulose colloidal particle CNWs were obtained [5]
We used CNWs AM and metacrylic acid ester (C12C13 or C18) as reactor monomers and added SDS to formsolubilised micelles or comicelles with the hydrophobicmonomers in an aqueous solution Copolymerisation wasinitiated by ammonium persulfate herein and the hydrogelsthus obtained were transparent and resilient (Figure 1)
In this reaction system CNWs and AM acted as the mainmonomer to form a hydrophilic backbone C12 C13 and C18acted as hydrophobic monomers to form associated micelles(Figure 1) that form the physical cross-linking points in anetwork of cellulose gels The salt (NaCl) led to micellargrowth and solubilisation of large hydrophobes within the as-grown worm-like SDS micelles [15] However the micelleswere stable enough to avoid destruction under the appliedconditions
32 Characterisation of the CNWs Composite Hydrogels Thestructure of hydrogels of acrylamidewith different hydropho-bic monomers (stearyl methacrylate (a) tridecyl methacry-late (b) or dodecyl 2-methylacrylate (c)) and CNWs were
Journal of Nanomaterials 3
Cellulose nanowhisker
Poly(acrylamide-co-alkylmethacrylate) chain
Micelle
Hydrophobic side chains
CH2OHCH2OH
OHOH
OH
OH
OO O
ONH4S2O8
CH2 = CCH3COORCH2 = CHCONH2
R = (CH2)11CH3(CH2)12CH3(CH2)17CH3
CH3
CCH2
COORm n
CHCH2
CONH2
HO
HOOH
OH
OH
OH
O O
OO
n
000000000000000000000000000000 00
6731
(nm
)
000000
000
020
040
060
080 020
040
080
060
100000times100000 (nm) 20
00ndash6731 (nm)
200 120583m 800 times 800 120583m 000
11561(nm)
Figure 1 Scheme of the CNWs composite hydrogel matrix
characterised by FTIR (Figure 2) It was observed that thehydrogen bond stretched at 3345 cmminus1 The CndashOndashC CndashCndashO andCndashCndashHdeformationmodes and stretching vibrationsin which the motions of the C-5 and C-6 atoms are at898 cmminus1 and the CndashOH out-of-plane bendingmode around670 cmminus1 also occurred According to Figure 2 absorptionwas observed at 1730 cmminus1 (stretching vibration C=O)1060 cmminus1 (asymmetrical stretching vibration of the etherbond) 2940 cmminus1 and 2830 cmminus1 (stretching vibration CndashH bond in methylene) 1651 cmminus1 and 792 cmminus1 (bendingvibration NndashH stretching vibration CndashN) The intensityof the stretching vibration of methylene increased with theincrease of the carbon chain length of the hydrophobicmonomer
33 Mechanical Properties of the CNWs Composite HydrogelsThe effect of the concentration of cellulose acrylamidecontent and content (and types) of hydrophobic monomerson the gel mechanical properties was analysed First we
29402830
(c)
(b)
(a)
4000
29402830
(c)
(b)
(a)
500 0100015002000250030003500
Wavenumbers (cmminus1)
Figure 2 FTIR spectra of CNWs composite hydrogels (stearylmethacrylate (a) tridecyl methacrylate (b) or dodecyl 2-methy-lacrylate (c))
synthesised the gel at the same concentration and experi-mental conditions with only CNW concentration changed
4 Journal of Nanomaterials
0005
01015
02025
03035
04045
05
0 00005 0001 00015 0002 00025 0003 00035 0004
Tens
ile st
reng
th (M
Pa)
The concentration of cellulose (gmL)
(a)
0
05
1
15
2
25
3
0 00005 0001 00015 0002 00025 0003 00035 0004
Com
pres
sive s
treng
th (M
Pa)
The concentration of cellulose (gmL)
(b)
(c)
Figure 3 Effect of CNW concentration on CNWs composite hydrogel mechanical properties ((a) tensile strength (b) compressive strength(c) compression strength test images of hydrogel with a CNW concentration of 00014 gmL)
001020304050607
30 35 40 45 50 55 60
Tens
ile st
reng
th (M
Pa)
The content of acrylamide ()
(a)
0002004006008
01012014
30 35 40 45 50 55 60
Com
pres
sive s
treng
th (M
Pa)
The content of acrylamide ()
(b)
Figure 4 Effect of acrylamide content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
Figure 3 shows the mechanical properties of the CNWscomposite hydrogel specimens using C18 as hydrophobicmonomer When increasing the concentration of CNWsfrom 00007 gmL to 00035 gmL the tensile strengthshowed a maximum value of 04550MPa and a compres-sive strength of 28MPa at a cellulose concentration of00014 gmL A possible reason is that high-aspect ratiofibres have the ability to sustain mechanical stress in a uni-form manner When the cellulose content used exceeds thecritical concentration thereof it will be difficult to avoidaggregation during their dispersion in the matrix because oftheir tendency to entangle In principle nonhomogeneousfiller dispersion caused by entanglement usually results thisin turn leads to poor composite properties and an inefficientstrengthening effect The hydrogel cannot be crushed as
shown in Figure 3(c) (a hydrogel with a CNW concentrationof 00014 gmL)
We synthesised gel of C18 under the same concentrationand experimental conditions with only the dosage of AMchanged from 345 wt to 544 wt Figure 4 shows themechanical properties of those gel specimens using C18 ashydrophobic monomer With increasing amounts of AMthe tensile strength and compressive strength increased from00028MPa to 05998MPa and 00211MPa to 01175MParespectively The increased tensile and compressive strengthwere consistent with the expected increases in cross-linkingdensities in the three-dimensional network structure As aresult the intertwining of polymeric chains was promotedand the hydrogen-bonding interaction between hydrophilicgroups such as ndashCONH
2and ndashOH among others was
strengthened Thus the degree of cross-linking increased afact that was favourable to the improvement of mechanical
Journal of Nanomaterials 5
002040608
1121416
2 3 4 5 6 7
Tens
ile st
reng
th (M
Pa)
The content of stearyl methacrylate ()
(a)
0
05
1
15
2
25
3
2 3 4 5 6 7
Com
pres
sive s
treng
th (M
Pa)
The content of stearyl methacrylate ()
(b)
Figure 5 Effect of stearylmethacrylate content onCNWs composite hydrogelmechanical properties ((a) tensile strength and (b) compressivestrength)
0001002003004005006007008
Dodecyl 2-methylacrylate
Tens
ile st
reng
th (M
Pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(a)
0
05
1
15
2
25
3
Dodecyl 2-methylacrylate
Com
pres
sive s
treng
th (M
pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(b)
Figure 6 Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
propertiesThe same influences will also arise with increasingcross-link density in traditional polymers cross-linked bychemical bonds
Figure 5 shows the mechanical properties of those gelspecimens using C18 as hydrophobic monomer The tensileand compressive strengths hadmaximumvalues of 1338MPaand 2835MPa respectively when the C18 content was350wt then with further increases in the amount ofhydrophobic groups the tensile and compressive strengthsdecreased It seemed that the action of hydrophobic groupson polymer chains was similar to that on the chemical cross-linking points [16]
Figure 6 shows the mechanical properties of gel speci-mens using C12 C13 and C18 as hydrophobic monomer Wesynthesised gels with hydrophobic monomer C12 C13 andC18 under identical experimental conditionsWith increasinghydrophobic side chain length the tensile and compressivestrengths increased from 00084MPa to 00708MPa andfrom 00483MPa to 24800MPa respectively The reasonfor this was that the hydrophobic side chain length becamelarger while the intermolecular hydrophobic interaction
became stronger In other words increasing the length ofhydrophobic side chain between backbones will induce animprovement in the mechanical properties of these gels
34 Surface Morphology of CNWs Composite HydrogelsFigure 7 shows SEM micrographs of freeze-dried nanocom-posite hydrogel filled with cellulose nanowhiskers (00007 gmL 00014 gmL 00021 gmL 00028 gmL and 00035 gmL) after being swollen to equilibrium It was seen that thehydrogels had a heterogeneous pore distribution and itseemed that the addition of CNWs would change the mor-phology CNWs composite hydrogel (Figure 7(a) 00007 gmL) had a relatively large-pored structure as well as theCNWs composite hydrogel (Figure 7(e) 00035 gmL) whichhad a relatively dense surface structure This may have beencaused by the different intermolecular forces arising fromthe alteration of the cellulose content This kind of surfaceallows a higher influx of water into the polymeric matrixwhich would then benefit the water absorbency by suchsuperabsorbent composites
6 Journal of Nanomaterials
(a) (b)
(c) (d)
(e)
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
Figure 1 Scheme of the CNWs composite hydrogel matrix
characterised by FTIR (Figure 2) It was observed that thehydrogen bond stretched at 3345 cmminus1 The CndashOndashC CndashCndashO andCndashCndashHdeformationmodes and stretching vibrationsin which the motions of the C-5 and C-6 atoms are at898 cmminus1 and the CndashOH out-of-plane bendingmode around670 cmminus1 also occurred According to Figure 2 absorptionwas observed at 1730 cmminus1 (stretching vibration C=O)1060 cmminus1 (asymmetrical stretching vibration of the etherbond) 2940 cmminus1 and 2830 cmminus1 (stretching vibration CndashH bond in methylene) 1651 cmminus1 and 792 cmminus1 (bendingvibration NndashH stretching vibration CndashN) The intensityof the stretching vibration of methylene increased with theincrease of the carbon chain length of the hydrophobicmonomer
33 Mechanical Properties of the CNWs Composite HydrogelsThe effect of the concentration of cellulose acrylamidecontent and content (and types) of hydrophobic monomerson the gel mechanical properties was analysed First we
29402830
(c)
(b)
(a)
4000
29402830
(c)
(b)
(a)
500 0100015002000250030003500
Wavenumbers (cmminus1)
Figure 2 FTIR spectra of CNWs composite hydrogels (stearylmethacrylate (a) tridecyl methacrylate (b) or dodecyl 2-methy-lacrylate (c))
synthesised the gel at the same concentration and experi-mental conditions with only CNW concentration changed
4 Journal of Nanomaterials
0005
01015
02025
03035
04045
05
0 00005 0001 00015 0002 00025 0003 00035 0004
Tens
ile st
reng
th (M
Pa)
The concentration of cellulose (gmL)
(a)
0
05
1
15
2
25
3
0 00005 0001 00015 0002 00025 0003 00035 0004
Com
pres
sive s
treng
th (M
Pa)
The concentration of cellulose (gmL)
(b)
(c)
Figure 3 Effect of CNW concentration on CNWs composite hydrogel mechanical properties ((a) tensile strength (b) compressive strength(c) compression strength test images of hydrogel with a CNW concentration of 00014 gmL)
001020304050607
30 35 40 45 50 55 60
Tens
ile st
reng
th (M
Pa)
The content of acrylamide ()
(a)
0002004006008
01012014
30 35 40 45 50 55 60
Com
pres
sive s
treng
th (M
Pa)
The content of acrylamide ()
(b)
Figure 4 Effect of acrylamide content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
Figure 3 shows the mechanical properties of the CNWscomposite hydrogel specimens using C18 as hydrophobicmonomer When increasing the concentration of CNWsfrom 00007 gmL to 00035 gmL the tensile strengthshowed a maximum value of 04550MPa and a compres-sive strength of 28MPa at a cellulose concentration of00014 gmL A possible reason is that high-aspect ratiofibres have the ability to sustain mechanical stress in a uni-form manner When the cellulose content used exceeds thecritical concentration thereof it will be difficult to avoidaggregation during their dispersion in the matrix because oftheir tendency to entangle In principle nonhomogeneousfiller dispersion caused by entanglement usually results thisin turn leads to poor composite properties and an inefficientstrengthening effect The hydrogel cannot be crushed as
shown in Figure 3(c) (a hydrogel with a CNW concentrationof 00014 gmL)
We synthesised gel of C18 under the same concentrationand experimental conditions with only the dosage of AMchanged from 345 wt to 544 wt Figure 4 shows themechanical properties of those gel specimens using C18 ashydrophobic monomer With increasing amounts of AMthe tensile strength and compressive strength increased from00028MPa to 05998MPa and 00211MPa to 01175MParespectively The increased tensile and compressive strengthwere consistent with the expected increases in cross-linkingdensities in the three-dimensional network structure As aresult the intertwining of polymeric chains was promotedand the hydrogen-bonding interaction between hydrophilicgroups such as ndashCONH
2and ndashOH among others was
strengthened Thus the degree of cross-linking increased afact that was favourable to the improvement of mechanical
Journal of Nanomaterials 5
002040608
1121416
2 3 4 5 6 7
Tens
ile st
reng
th (M
Pa)
The content of stearyl methacrylate ()
(a)
0
05
1
15
2
25
3
2 3 4 5 6 7
Com
pres
sive s
treng
th (M
Pa)
The content of stearyl methacrylate ()
(b)
Figure 5 Effect of stearylmethacrylate content onCNWs composite hydrogelmechanical properties ((a) tensile strength and (b) compressivestrength)
0001002003004005006007008
Dodecyl 2-methylacrylate
Tens
ile st
reng
th (M
Pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(a)
0
05
1
15
2
25
3
Dodecyl 2-methylacrylate
Com
pres
sive s
treng
th (M
pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(b)
Figure 6 Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
propertiesThe same influences will also arise with increasingcross-link density in traditional polymers cross-linked bychemical bonds
Figure 5 shows the mechanical properties of those gelspecimens using C18 as hydrophobic monomer The tensileand compressive strengths hadmaximumvalues of 1338MPaand 2835MPa respectively when the C18 content was350wt then with further increases in the amount ofhydrophobic groups the tensile and compressive strengthsdecreased It seemed that the action of hydrophobic groupson polymer chains was similar to that on the chemical cross-linking points [16]
Figure 6 shows the mechanical properties of gel speci-mens using C12 C13 and C18 as hydrophobic monomer Wesynthesised gels with hydrophobic monomer C12 C13 andC18 under identical experimental conditionsWith increasinghydrophobic side chain length the tensile and compressivestrengths increased from 00084MPa to 00708MPa andfrom 00483MPa to 24800MPa respectively The reasonfor this was that the hydrophobic side chain length becamelarger while the intermolecular hydrophobic interaction
became stronger In other words increasing the length ofhydrophobic side chain between backbones will induce animprovement in the mechanical properties of these gels
34 Surface Morphology of CNWs Composite HydrogelsFigure 7 shows SEM micrographs of freeze-dried nanocom-posite hydrogel filled with cellulose nanowhiskers (00007 gmL 00014 gmL 00021 gmL 00028 gmL and 00035 gmL) after being swollen to equilibrium It was seen that thehydrogels had a heterogeneous pore distribution and itseemed that the addition of CNWs would change the mor-phology CNWs composite hydrogel (Figure 7(a) 00007 gmL) had a relatively large-pored structure as well as theCNWs composite hydrogel (Figure 7(e) 00035 gmL) whichhad a relatively dense surface structure This may have beencaused by the different intermolecular forces arising fromthe alteration of the cellulose content This kind of surfaceallows a higher influx of water into the polymeric matrixwhich would then benefit the water absorbency by suchsuperabsorbent composites
6 Journal of Nanomaterials
(a) (b)
(c) (d)
(e)
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
Figure 3 Effect of CNW concentration on CNWs composite hydrogel mechanical properties ((a) tensile strength (b) compressive strength(c) compression strength test images of hydrogel with a CNW concentration of 00014 gmL)
001020304050607
30 35 40 45 50 55 60
Tens
ile st
reng
th (M
Pa)
The content of acrylamide ()
(a)
0002004006008
01012014
30 35 40 45 50 55 60
Com
pres
sive s
treng
th (M
Pa)
The content of acrylamide ()
(b)
Figure 4 Effect of acrylamide content on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
Figure 3 shows the mechanical properties of the CNWscomposite hydrogel specimens using C18 as hydrophobicmonomer When increasing the concentration of CNWsfrom 00007 gmL to 00035 gmL the tensile strengthshowed a maximum value of 04550MPa and a compres-sive strength of 28MPa at a cellulose concentration of00014 gmL A possible reason is that high-aspect ratiofibres have the ability to sustain mechanical stress in a uni-form manner When the cellulose content used exceeds thecritical concentration thereof it will be difficult to avoidaggregation during their dispersion in the matrix because oftheir tendency to entangle In principle nonhomogeneousfiller dispersion caused by entanglement usually results thisin turn leads to poor composite properties and an inefficientstrengthening effect The hydrogel cannot be crushed as
shown in Figure 3(c) (a hydrogel with a CNW concentrationof 00014 gmL)
We synthesised gel of C18 under the same concentrationand experimental conditions with only the dosage of AMchanged from 345 wt to 544 wt Figure 4 shows themechanical properties of those gel specimens using C18 ashydrophobic monomer With increasing amounts of AMthe tensile strength and compressive strength increased from00028MPa to 05998MPa and 00211MPa to 01175MParespectively The increased tensile and compressive strengthwere consistent with the expected increases in cross-linkingdensities in the three-dimensional network structure As aresult the intertwining of polymeric chains was promotedand the hydrogen-bonding interaction between hydrophilicgroups such as ndashCONH
2and ndashOH among others was
strengthened Thus the degree of cross-linking increased afact that was favourable to the improvement of mechanical
Journal of Nanomaterials 5
002040608
1121416
2 3 4 5 6 7
Tens
ile st
reng
th (M
Pa)
The content of stearyl methacrylate ()
(a)
0
05
1
15
2
25
3
2 3 4 5 6 7
Com
pres
sive s
treng
th (M
Pa)
The content of stearyl methacrylate ()
(b)
Figure 5 Effect of stearylmethacrylate content onCNWs composite hydrogelmechanical properties ((a) tensile strength and (b) compressivestrength)
0001002003004005006007008
Dodecyl 2-methylacrylate
Tens
ile st
reng
th (M
Pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(a)
0
05
1
15
2
25
3
Dodecyl 2-methylacrylate
Com
pres
sive s
treng
th (M
pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(b)
Figure 6 Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
propertiesThe same influences will also arise with increasingcross-link density in traditional polymers cross-linked bychemical bonds
Figure 5 shows the mechanical properties of those gelspecimens using C18 as hydrophobic monomer The tensileand compressive strengths hadmaximumvalues of 1338MPaand 2835MPa respectively when the C18 content was350wt then with further increases in the amount ofhydrophobic groups the tensile and compressive strengthsdecreased It seemed that the action of hydrophobic groupson polymer chains was similar to that on the chemical cross-linking points [16]
Figure 6 shows the mechanical properties of gel speci-mens using C12 C13 and C18 as hydrophobic monomer Wesynthesised gels with hydrophobic monomer C12 C13 andC18 under identical experimental conditionsWith increasinghydrophobic side chain length the tensile and compressivestrengths increased from 00084MPa to 00708MPa andfrom 00483MPa to 24800MPa respectively The reasonfor this was that the hydrophobic side chain length becamelarger while the intermolecular hydrophobic interaction
became stronger In other words increasing the length ofhydrophobic side chain between backbones will induce animprovement in the mechanical properties of these gels
34 Surface Morphology of CNWs Composite HydrogelsFigure 7 shows SEM micrographs of freeze-dried nanocom-posite hydrogel filled with cellulose nanowhiskers (00007 gmL 00014 gmL 00021 gmL 00028 gmL and 00035 gmL) after being swollen to equilibrium It was seen that thehydrogels had a heterogeneous pore distribution and itseemed that the addition of CNWs would change the mor-phology CNWs composite hydrogel (Figure 7(a) 00007 gmL) had a relatively large-pored structure as well as theCNWs composite hydrogel (Figure 7(e) 00035 gmL) whichhad a relatively dense surface structure This may have beencaused by the different intermolecular forces arising fromthe alteration of the cellulose content This kind of surfaceallows a higher influx of water into the polymeric matrixwhich would then benefit the water absorbency by suchsuperabsorbent composites
6 Journal of Nanomaterials
(a) (b)
(c) (d)
(e)
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
Figure 5 Effect of stearylmethacrylate content onCNWs composite hydrogelmechanical properties ((a) tensile strength and (b) compressivestrength)
0001002003004005006007008
Dodecyl 2-methylacrylate
Tens
ile st
reng
th (M
Pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(a)
0
05
1
15
2
25
3
Dodecyl 2-methylacrylate
Com
pres
sive s
treng
th (M
pa)
Hydrophobic monomer
Tridecylmethacrylate
Stearylmethacrylate
(b)
Figure 6 Effect of hydrophobic monomer on CNWs composite hydrogel mechanical properties ((a) tensile strength and (b) compressivestrength)
propertiesThe same influences will also arise with increasingcross-link density in traditional polymers cross-linked bychemical bonds
Figure 5 shows the mechanical properties of those gelspecimens using C18 as hydrophobic monomer The tensileand compressive strengths hadmaximumvalues of 1338MPaand 2835MPa respectively when the C18 content was350wt then with further increases in the amount ofhydrophobic groups the tensile and compressive strengthsdecreased It seemed that the action of hydrophobic groupson polymer chains was similar to that on the chemical cross-linking points [16]
Figure 6 shows the mechanical properties of gel speci-mens using C12 C13 and C18 as hydrophobic monomer Wesynthesised gels with hydrophobic monomer C12 C13 andC18 under identical experimental conditionsWith increasinghydrophobic side chain length the tensile and compressivestrengths increased from 00084MPa to 00708MPa andfrom 00483MPa to 24800MPa respectively The reasonfor this was that the hydrophobic side chain length becamelarger while the intermolecular hydrophobic interaction
became stronger In other words increasing the length ofhydrophobic side chain between backbones will induce animprovement in the mechanical properties of these gels
34 Surface Morphology of CNWs Composite HydrogelsFigure 7 shows SEM micrographs of freeze-dried nanocom-posite hydrogel filled with cellulose nanowhiskers (00007 gmL 00014 gmL 00021 gmL 00028 gmL and 00035 gmL) after being swollen to equilibrium It was seen that thehydrogels had a heterogeneous pore distribution and itseemed that the addition of CNWs would change the mor-phology CNWs composite hydrogel (Figure 7(a) 00007 gmL) had a relatively large-pored structure as well as theCNWs composite hydrogel (Figure 7(e) 00035 gmL) whichhad a relatively dense surface structure This may have beencaused by the different intermolecular forces arising fromthe alteration of the cellulose content This kind of surfaceallows a higher influx of water into the polymeric matrixwhich would then benefit the water absorbency by suchsuperabsorbent composites
6 Journal of Nanomaterials
(a) (b)
(c) (d)
(e)
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
Figure 7 Scanning electron microscopy of CNWs composite hydrogels ((a) cellulose concentration of 00007 gmL (b) celluloseconcentration of 00014 gmL (c) cellulose concentration of 00021 gmL (d) cellulose concentration of 00028 gmL and (e) celluloseconcentration of 00035 gmL)
35The SR of the CNWs Composite Hydrogels Hydrogels canabsorb large amounts of water and release the absorbed waterin dry conditions A decrease in the water uptake capacity ofthese CNWs composite hydrogels was observed (Figure 8) Itcould be seen that the incorporation of 00007 gmLofCNWsproduced the maximum water uptake in CNWs compositehydrogels However greater amounts of CNWs decreasedthe availability of functional groups from the matrix thatwere able to interact with the water correspondingly theinteraction between hydrophilic groups from the filler andthe hydrogel network caused a decline in osmotic swellingpressure finally the influx of water into the hydrogel matrixdecreased [11] Furthermore the excessive amount of CNWs
particles may physically stack in the network voids decreas-ing the water-holding capability consequently the hydrogelscan absorb less water as the CNW content increased [11]
36 The Self-Healing Behaviour of CNWs Composite Hydro-gels CNWs composite hydrogels exhibit excellent strengthand rubber-like properties in a unique network structure(Figure 1) the most remarkable properties are that cellulosegels have the ability to self-heal (Figure 9) After cutting agel sample into two parts and then bringing them togetherfor 60min at room temperature it was found that the cuthad healed (Figure 9) Not only can it be shape-formed ina polymerisation system but also it would be reformed by
Journal of Nanomaterials 7
CNWs content (gmL)0
Swel
ling
ratio
()
2600
2400
2200
2000
1800
1600
1400
1200
100043 352 251 1505
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
Figure 8 Swelling ratios of CNWs composite hydrogels
Cut
Cut
Healed
Surface
Figure 9 Self-healing behaviour of CNWs composite hydrogels
a remoulding process as thermoplastic resins would if theprocess continued for sufficient time This behaviour wasascribed to the rearrangement of cross-linking structuresby dissociation and reassociation of hydrophobic groupsThis endowed these gels with the ability to self-heal and beremoulded in a fashion similar to a thermoplastic resin
Gels with SDS exhibited a self-healing efficiency of nearly100 after a healing time of 60min However when swollenin water no self-healing ability was observedThis wasmaybebecause the gels swelled in water leading to the extraction ofSDS micelles from the gel network the surfactant SDS con-trolled the hydrophobic associations formed in the hydrogelsThis suggested that the key factor leading to self-healing wasthe weakening of strong hydrophobic interactions due to thepresence of surfactant molecules [17]
4 Conclusions
A simple method of making CNWs composite hydrogels thatare transparent and soft materials containing a large amountof water has been proposed In many applications the useof hydrogels is often severely limited by their mechanicalproperties Using CNWs to prepare ldquosmartrdquo hydrogels couldnot only improve their self-healing and reforming capacitybut also improve their mechanical strength CNWs compos-ite hydrogels exhibited excellent mechanical properties boththe tensile strength and the compressive strength stronglydepended on hydrophobic monomer content hydrophobicside chain length and AM and cellulose contents C18 in theCNWs composite hydrogels played an important role in thedomination of the physical cross-linking points formed byhydrophobic association leading to self-healing and reform-ing capacity whereas CNWs and AM mainly contributedto the increasing equilibrium between mechanical strength
and swelling ratio thereof Their unique self-repairing prop-erties the simplicity of their synthesis their availability fromrenewable resources and the low cost of their raw ingredients(cellulose) bode well for future applications
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
This paper is supported by ldquo2015 BeijingConstruction Projectof Scientific Research and Cultivation of Graduate Studentrdquoand ldquoResearch Program of Circulation Utilization of PlantWaste 2014HXKFCLXY034rdquo
References
[1] J-Y Sun X ZhaoW R K Illeperuma et al ldquoHighly stretchableand tough hydrogelsrdquo Nature vol 489 no 7414 pp 133ndash1362012
[2] MM Abeer M C I M Amin AM LazimM Pandey and CMartin ldquoSynthesis of a novel acrylated abietic acid-g-bacterialcellulose hydrogel by gamma irradiationrdquo Carbohydrate Poly-mers vol 110 no 1 pp 505ndash512 2014
[3] X Qiu and S Hu ldquolsquoSmartrsquo materials based on cellulose a reviewof the preparations properties and applicationsrdquoMaterials vol6 no 3 pp 738ndash781 2013
[4] R Cha Z He and Y Ni ldquoPreparation and characterization ofthermalpH-sensitive hydrogel from carboxylated nanocrys-talline celluloserdquo Carbohydrate Polymers vol 88 no 2 pp 713ndash718 2012
[5] A Sannino C Demitri and M Madaghiele ldquoBiodegradablecellulose-based hydrogels design and applicationsrdquo Materialsvol 2 no 2 pp 353ndash373 2009
[6] Y Habibi L A Lucia and O J Rojas ldquoCellulose nanocrystalschemistry self-assembly and applicationsrdquo Chemical Reviewsvol 110 no 6 pp 3479ndash3500 2010
[7] K Haraguchi and T Takehisa ldquoNanocomposite hydrogels aunique organic-inorganic network structure with extraordi-nary mechanical optical and swellingde-swelling propertiesrdquoAdvanced Materials vol 14 no 16 pp 1120ndash1124 2002
[8] L Xiong X Hu X Liu and Z Tong ldquoNetwork chain densityand relaxation of in situ synthesized polyacrylamidehectoriteclay nanocomposite hydrogels with ultrahigh tensibilityrdquo Poly-mer vol 49 no 23 pp 5064ndash5071 2008
[9] J Yang C-R Han J-F Duan et al ldquoSynthesis and characteriza-tion of mechanically flexible and tough cellulose nanocrystals-polyacrylamide nanocomposite hydrogelsrdquo Cellulose vol 20no 1 pp 227ndash237 2013
[10] C Zhou Q Wu Y Yue and Q Zhang ldquoApplication of rod-shaped cellulose nanocrystals in polyacrylamide hydrogelsrdquoJournal of Colloid and Interface Science vol 353 no 1 pp 116ndash123 2011
[11] C Spagnol F H A Rodrigues A G V C Neto et al ldquoNano-composites based on poly(acrylamide-co-acrylate) and cellu-lose nanowhiskersrdquo European Polymer Journal vol 48 no 3 pp454ndash463 2012
8 Journal of Nanomaterials
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
[12] F H A Rodrigues C Spagnol A G B Pereira et al ldquoSuperab-sorbent hydrogel composites with a focus on hydrogels contain-ing nanofibers or nanowhiskers of cellulose and chitinrdquo Journalof Applied Polymer Science vol 131 no 2 Article ID 39725 pp1ndash13 2014
[13] M Larsson Q Zhou and A Larsson ldquoDifferent types ofmicrofibrillated cellulose as filler materials in polysodiumacrylate superabsorbentsrdquo Chinese Journal of Polymer Sciencevol 29 no 4 pp 407ndash413 2011
[14] M Harini and A P Deshpande ldquoRheology of poly(sodiumacrylate) hydrogels during cross-linking with and withoutcellulose microfibrilsrdquo Journal of Rheology vol 53 no 1 pp 31ndash47 2009
[15] D C Tuncaboylu M Sari W Oppermann and O OkayldquoTough and self-healing hydrogels formed via hydrophobicinteractionsrdquo Macromolecules vol 44 no 12 pp 4997ndash50052011
[16] G Jiang C Liu X Liu G Zhang M Yang and F Liu ldquoCon-struction and properties of hydrophobic association hydrogelswith high mechanical strength and reforming capabilityardquoMacromolecular Materials and Engineering vol 294 no 12 pp815ndash820 2009
[17] D C Tuncaboylu M Sahin A Argun W Oppermann and OOkay ldquoDynamics and large strain behavior of self-healinghydrogels with and without surfactantsrdquo Macromolecules vol45 no 4 pp 1991ndash2000 2012
