Versatile Surface Modification of Cellulose Fibers and Cellulose Nanocrystals through Modular ...

12 Pages·2017·7.39 MB·English

Article
CiteThis:Chem.Mater.2018,30,2424−2435 pubs.acs.org/cm
fi
Versatile Surface Modi cation of Cellulose Fibers and Cellulose
Nanocrystals through Modular Triazinyl Chemistry
Ayodele Fatona,† Richard M. Berry,‡ Michael A. Brook,† and Jose M. Moran-Mirabal*,†
†
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1,
Canada
‡
Celluforce Inc., 625 President-Kennedy Avenue, Montreal, Quebec H3A 1K2, Canada
*
S Supporting Information
ABSTRACT: The ability to tune the interfacial and functional
properties of cellulose nanomaterials has been identified as a
critical step for the full utilization of nanocellulose in the
development of new materials. Here, we use triazine chemistry
in a modular approach to install various functionalities and
chemistries onto cellulose fibers and cellulose nanocrystals
(CNCs). The surface modification is demonstrated in aqueous
and organic media. Octadecyl, monoallyl-PEG, benzyl, and
propargyl triazinyl derivatives were grafted onto cellulose/
CNCs via aromatic nucleophilic substitution in the presence of
baseashydrochloricacidscavenger.Thecovalentnatureanddegreeofsubstitutionofgraftedaliphatic,polymeric,alkynechains,
and aromatic rings were characterized through Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy,
elementalanalysis,andthermogravimetricanalysis.Inaddition,AFMandDLSanalysisshowedminimalchangeinthegeometry
and individualized character of CNCs after surface modification. X-ray diffraction analysis confirmed that the modification
happened only at the CNC surface, while the bulk crystalline core remained unmodified. Modified cellulose/CNCs showed
hydrophilic or hydrophobic properties depending on the grafted functionality, which resulted in stable colloidal suspensions of
CNCs in polar and nonpolar organic solvents. Furthermore, the reactive nature of propargyl-modified cellulose was
demonstratedbythesuccessfulgraftingofanazido-fluoresceindyeviacopper-catalyzed Huisgen1,3-dipolarcycloaddition.The
triazinyl chemistry thus presents a versatile route for tuning the interfacial properties of nanocellulose, with the possibility of
postmodification for applications that require the conjugation of molecules onto cellulose through bio-orthogonal chemistries.
■
INTRODUCTION on our ability to tune their interfacial properties to enhance
dispersionorintroducereactivefunctionalgroupsthatcanform
Cellulose is an attractive green material due to its intrinsic
covalentbondswiththehostmatrix.
mechanicalandchemicalproperties,whichcanbeusedtobuild
ThesurfacechemistryofisolatedCNCsisdependentonthe
hierarchical structures that enhance the performance of tradi-
tional polymeric materials.1−3 The partial hydrolysis of the acidusedduringhydrolysis.Sulfuricacidhydrolysisofcelluloseis
networksofcellulosemicrofibrilsinplantcellwallswithstrong stillthemostcommonprocessforCNCisolationandresultsin
the grafting of sulfate half-ester groups onto the surface of the
acids, which attack preferentially the disordered or paracrystal-
line regions of the microfibrils, allows the isolation of highly isolatedCNCs.Whilethesenegativelychargedgroupsallowthe
colloidal stabilization of CNCs in aqueous media through
crystalline rodlike nanoparticles, called cellulose nanocrystals
(CNCs). These environmentally friendly and noncytotoxic4 electrostatic repulsion, they, along with the abundant hydroxyl
groupsontheCNCsurface,preventtheirsuspension/dispersion
nanoparticles have unique characteristics, including very high
surfacearea(150−300m2g−1),5,6hightensilestrength(7.5−7.7 inmostorganicsolventsandpolymersystems.5,9Thislimitsthe
GPa),7highelasticmodulus(110−220GPa),7highaspectratio reinforcementcapabilitiesofCNCsaswellastheirprocessability
(∼20−70),5 and interesting optical and electrical properties.8 fornewnanotechnologicalapplications.
Efforts have been made to convert electrostatically stabilized
Such characteristics make them attractive for a broad range of
CNCsintostericallystabilizedCNCsthroughthemodification
applications, including materials reinforcement, drug delivery,
of the surface-reactive hydroxyl groups along the cellulose
foamstabilization,supercapacitordevelopment,andrheological
property modification, among many others. However, a major backbone. This would permit the CNCs to be dispersed in
hurdle to the widespread application and utilization of CNCs, nonaqueoussolventsandenhancetheircompatibilitywithawide
and nanocellulose in general, is their inability to form stable
suspensionsinarangeoforganicsolventsorbecompatiblewith Received: February3, 2018
polymer matrices. Thus, the effective use of CNCs in select Revised: March 16, 2018
applications, such asmaterials reinforcement, dependsstrongly Published: March 16, 2018
©2018AmericanChemicalSociety 2424 DOI:10.1021/acs.chemmater.8b00511
Chem.Mater.2018,30,2424−2435
ChemistryofMaterials ■ Article
range of hydrophobic polymer matrices. Esterification,10 EXPERIMENTALSECTION
cationization,11 carbamination,12−14 silylation,15 amidation,16,17
Materials. Octadecylamine (C18), benzylamine, propargylamine,
polymer grafting,18,19 and etherification20 have all been cyanuricchloride,triethylamine,N,N-diisopropylethylamine(DIPEA),
successfully used to modify the surface of CNCs. However, sodiumhydroxide(NaOH),Whatmangrade1cellulosefilterpaper(10
such approaches are rarely suitable for large-scale implementa- mm),andpotassiumcarbonate(KCO)werepurchasedfromSigma-
2 3
tion or have limitations in the functionalities that can be Aldrich(Oakville,ON,Canada).Poly(ethyleneglycol)monoallylether
introduced. The present work explores triazine chemistrya (MW388gmol−1)wasagiftfromEnRouteInterfaces,Inc.(Hamilton,
ON,Canada).5-Fluoresceinazide(5-FAM-Azide)waspurchasedfrom
well-known technology in the textile, agriculture, polymer, and
LumiprobeCorp.(HuntValley,MD).Spray-driedcellulosenanocryst-
paper industries for the production of dyes,21 herbicides,22 alshydrogensulfatesodiumsalt(CNCs)wereprovidedbyCelluForce
optical brighteners,23 and dendrimers24as a simple, versatile, Inc.(Montreal,QC,Canada).Allsolventsincludingdichloromethane
andmildapproachtocarryoutthemodularsurfacemodification (DCM), acetone, ethanol, N,N-dimethylformamide (DMF), tetrahy-
of cellulose, especially CNCs, with potential for large-scale drofuran (THF), and chloroform were purchased from Caledon
implementation. Haller and Heckendorn21 pioneered the Laboratories (ON, Canada) and used as received. Bacterial micro-
triazinechemistryforaffixingamino-reactivedyesontocellulose cforyosdtsatluliffneNcaetllaudloeseC(oBcMo.CC)wasisolatedinhousefromthecommercial
fibersforcoloration.Helbertetal.,25Ringotetal.,26andWalczak Characterization: Nuclear Magnetic Resonance (NMR). (1H
et al.27 further extended this process for the grafting of and 13C NMR) spectra were obtained on a Bruker AV600-600 MHz
fluorescent dyes, porphyrin, and N-lipidated oligopeptides NMRspectrometerusingdeuteratedCDCl andCDCl assolvents.
3 2 2
onto cellulose fibers, for applications involving the study of Mass Spectrometry (MS). Mass spectra of synthesized dichloro-
triazinyl derivatives were recorded on a Micromass Ultima (LC-ESI/
cellulase activity, endowing cellulose with antibacterial proper-
ties,andformetaboliteprofiling,respectively. APCI) Triple Quadrupole mass spectrometer and Micromass Global
UltimaESIQuadrupoleTimeofFlight(Q-TOF)massspectrometer.
Ourchoiceof2,4,6-trichloro-1,3,5-triazine(cyanuricchloride)
FourierTransformInfraredSpectroscopy(FTIR).FTIRspectra
as a linker molecule derives not only from its commercial ofunmodifiedandmodifiedCNCswererecordedonaThermoNicolet
availability and low cost, but also from the ease of derivatizing 6700FTIRspectrometerintransmissionmodeusingKBrdisksloaded
cyanuric chloride with nucleophiles to generate a myriad of with1wt%sampledriedundervacuumovernightat60°C.
mono- or disubstituted 1,3,5-triazines. In our approach, the X-rayPhotoelectronSpectroscopy(XPS).Spectrawererecorded
surface functionality of CNCs is strategically tuned by first with a Thermo Scientific Theta Probe XPS spectrometer with
monochromaticAlKαradiation(50W)atatakeoffangleof45°and
grafting the targeted molecules (small or polymeric) onto the aspotsizeof200μm.Surveyandhigh-resolutionspectrawerecollected
cyanuricchloridelinker,andthengraftingtheresultingtriazinyl withpassenergiesof280and26eV,respectively,anddatawereanalyzed
derivativesontoCNCsinasinglestep.Inthisway,thetriazinyl with the software provided with the instrument. Unmodified and
linker can be used either as a standalone modification or as a modifiedCNCsamples,1wt%,weredepositedviadropcastingonto
building block for secondary modifications with biorthogonal cleansiliconwafersandair-driedatroomtemperaturebeforeanalysis.
ElementalAnalysis.Massfractionsofcarbon,hydrogen,nitrogen,
chemistries.Anadditionaladvantageofthischemistryisthatthe
andsulfurweredeterminedforvacuum-driedunmodifiedandmodified
selective chlorine substitution of cyanuric chloride can be
CNCs by Micro Analysis Inc. (Wilmington, DE). The samples were
controlled with moderate temperatures, making it a highly
combusted in a pure oxygen environment where product gases were
predictable procedure for nanocellulose modification. More separated and detected by thermal conductivity. Triplicates were
importantly,tothebestofourknowledge,thisisthefirstarticle measured for each sample, and averages are reported. The results
detailing the use of triazine chemistry in tuning the interfacial obtained from this technique were used to determine the degree of
substitution(DS),givenbythenumberofhydroxylgroupssubstituted
propertiesofnanocellulose.
We report the grafting of four different triazinyl derivatives bydichlorotriazinylderivativesperunitofanhydroglucose.TheDSwas
calculatedusingeq1below;asreported28
onto crystalline cellulose fibers or CNCs. These consist of
nonpolar aliphatic chains (octadecylamine, C18), polymer 72.07−C×162.14
(DS)=
chains(poly(ethyleneglycol)monoallylether,APEG),aromatic M ×C−M
rings (benzylamine), and alkyne functionalities (propargyl- g Cg (1)
amine).Dependingonthechemicalmoietycoupledtocyanuric whereCistherelativecarboncontentinthesample;thenumbers72.07
chloride, the surface modification of CNCs was carried out in and162.14correspondtothecarbonmassoftheanhydroglucoseunit
polaror nonpolar solvents. Withthis chemistry, the polarity of (CH O)andthemolecularweightofanhydroglucose,respectively;
6 10 5
theCNCsurfacehasbeentunedtoenabletheirdispersionina MgandMCgcorrespondtothemolecularweightofthedichlorotriazinyl
range of solvents to form homogeneous colloidal suspensions derivatives(i.e.,compounds1,2,3,and4)linkedtotheanhydroglucose
thatarestabilizedthroughelectrostaticandstericinteractions.In unit, and their carbon masses, respectively. Experimental values were
correctedbyaconversionfactorof1.077consideringunmodifiedCNCs
addition,twotriazinederivativesareamenabletofurtheraddition
as pure cellulose, which correlates with a relative carbon content of
chemistries such as click reactions. Thus, as proof-of-concept, 44.44%,followingapreviouslyreportedprocedure.29
nanocellulose modified with propargyl-triazine derivatives was ThermogravimetricAnalysis(TGA).Analyseswerecarriedouton
furthermodifiedwithfluoresceinthroughanazide−alkyneclick aTAInstrumentsQ50thermogravimetricanalyzer.Datawerecollected
reaction. afterplacingca.10mgofavacuum-driedsampleinacleanplatinumpan
Ourintentionistoleveragethetriazinechemistryasaversatile and heating from ambient temperature to 800 °C under argon
and low-cost technique for surface modification of CNCs. It is atmosphere(heatingrateof20°Cmin−1).
FluorescenceMicroscopy.FluorescenceimagesofBMCClabeled
anticipatedthatthissurfacechemistrywillaidintheproduction
usingtriazine-alkyne−azideclickchemistryweretakenusingaNikon-
of functional CNCs that can be deployed in high-volume
Eclipse LV100N POL microscope, equipped with excitation and
nanocompositeprocessing,rheologicalmodification,cosmetics, emission filters for fluorescein dye, a Retiga 2000R CCD mono-
andsensingapplications. chromatic camera (QImaging, Surrey, BC, Canada), and CFI LU P
2425 DOI:10.1021/acs.chemmater.8b00511
Chem.Mater.2018,30,2424−2435
ChemistryofMaterials Article
Scheme1.(A)Synthesisof4,6-Dichloro-1,3,5-triazineDerivatives,(B)ChemicalGraftingof4,6-Dichloro-1,3,5-triazine
DerivativesontoCellulose,and(C)FluoresceinAzideClickGraftingontoAlkyne-ModifiedNanocellulose
10×/0.25 NA and 40×/0.65 NA objectives (Nikon Canada Inc., temperaturebeforeanalysis.CNClengthsweremeasuredforindividual
Mississauga,ON,Canada). particles within the images, and anaverage was calculated for n > 50
Dynamic Light Scattering (DLS). The apparent hydrodynamic measurements.TheerrorassociatedwiththeCNClengthsisreportedas
sizewasmeasuredfornativeandsurface-modifiedCNCsin0.025wt% thestandarddeviationofthemeasurements.
dispersions in water (except for DTC18-modified CNCs, where the X-ray Diffraction (XRD). Two-dimensional diffraction patterns
measurementsweredonein1:1chloroform:DMSO)usingaMalvern werecollectedusingaD8Davincidiffractometer(Bruker,Billerica,MA)
ZetasizerNanoparticleanalyzer.Triplicatesamplesweremeasured15 equipped with a sealed tube cobalt source. Analysis of the resulting
timeseach,andtheaveragehydrodynamicsizeisreported.Thestandard patternswasconductedusingtheBrukerTOPASsoftwareandMatlab
deviation from the triplicate samples is reported as the error for the scripts.Thebeamwascollimatedtoadiameterof0.5mm(35mA,45
measurements. kV). Cellulose samples were drop-cast and oven-dried onto clean Si
Atomic Force Microscopy (AFM). AFM images were obtained wafer pieces for the analysis. A still frame of a blank piece of Si was
using an Asylum Research MFP-3D ClassicTM scanning probe initiallyexaminedtocorrectforbackground.Thebackgroundintensity
microscope (Santa Barbara, CA). Images were acquired in tapping wassubtractedfromeachsampleframepriortointegrationofthedata.A
mode with aluminum reflex coated silicon cantilevers (FMR, Nano- 2θ range 13−42° was used for the crystallinity index (CrI) analysis.
worldAG,Neuchat̂el,Switzerland)withnominalspringconstantof2.8 Integrationalongrelativeangleχforevery2θvaluewasperformedto
Nm−1andresonantfrequencyof75kHz.Sampleswerepreparedby obtain one-dimensional diffraction plots of intensity versus 2θ. The
spincoatingsolutionscontaining0.001wt%unmodifiedCNCsor0.01 background corrected intensity versus 2θ plots were fitted to five
wt%modifiedCNCssamplesontopiranha-cleanedsiliconwafers(for symmetric Lorentzian peaks; four peaks corresponding to the (100),
unmodifiedCNCs,thesubstrateswerepretreatedbyspincoatingwitha (010),(002),and(040)crystallineplanes,30andonebroadamorphous
0.1% PAH solution) at 4000 rpm for 30 s and air-dried at room peakfixedat24.1°.TheCrIwascalculatedbythepeakdeconvolution
2426 DOI:10.1021/acs.chemmater.8b00511
Chem.Mater.2018,30,2424−2435
ChemistryofMaterials Article
methodastheratiooftheareaforthecrystallinepeaksoverthetotalarea Chemical Grafting of Dichlorotriazinyl Derivatives onto
forthediffractionplots. Nanocellulose. See Scheme 1b. Two methods were used in the
ContactAngleMeasurements.Thewettabilityvaluesofmodified graftingofdichlorotriazinylderivativesontoCNCs:Inmethod1,an
cellulosefibers(Whatmanfilterpaper)wereassessedbystaticcontact aqueous suspension of CNCs (20 mL, 1 wt %) in a 100 mL round-
anglemeasurementthroughsessiledropmethodviaanOCA20Future bottomedflaskwassolvent-exchangedintoDMF(30mL)byvacuum-
Digital Scientific system (Garden City, NY) equipped with a CCD assistedrotaryevaporationremovingasmallportionofwater(10mL).
camera.Water(1μLof18.2MΩcm−1,A10-Merck-Milliporesystem, Thiswasfollowedbysuccessivecentrifugation(10000g,5°C,10min)
Darmstadt,Germany)wasdroppedontomodifiedcellulosepaperwhile andresuspensioninacetoneandthendryDCM,withsonication(10
digital images were captured. The average of 3 replicate filter paper min, 5 °C) in between each solvent-exchange step to prevent CNC
aggregation. KCO (0.25 g, 1.81 mmol) was added to a stirred
samplesisreportedasthecontactangle. 2 3
Synthesisof4,6-Dichloro-n-octadecyl-1,3,5-triazine-2-amine suspensioncontainingCNCsindryDCM(0.5g,1wt%).After20min,
(1,DTC18).SeeScheme1a.NaCO (2.86g,0.027mol)wasaddedtoa 5mmolof1,2,or3(10:1excesstothesurfaceOHpergofCNCs)in
2 3
stirredsolutionofcyanuricchloride(5.0g,0.027mol)indryTHF(50 dryDCM(10mL)wasaddedandlefttostiratroomtemperaturefor24
mL)cooledto0°C.After20minofstirring,asolutioncontainingn- h.ModifiedCNCswerethensubjectedto3successivecentrifugation
and sonication steps in DCM to remove excess dichlorotriazinyl
octadecylamine(7.28g,0.027mol)indryTHF(100mL)wasadded
dropwise over a period of 45 min. The end of reactioncomplete derivatives.Thiswasfollowedbystirred-celldialysistoremoveinorganic
disappearanceofstartingmaterialwasmonitoredviaTLC(silicagel, baseandanyleftoverunboundderivativesinthepresenceofacetone(3
DCM).Thereactionmixturewasfiltered,andTHFwasremovedunder cycle runs), acetone−water (50%, 3 cycle runs), and water (14 cycle
runs). In method 2, since compound 4 is hydrophilic in nature, the
vacuumtoyieldawhiteprecipitatewhichwassuspendedinwaterfor1h
tfioltrreamteowvaesadnryierdematairnoinogmNteam2CpOer3a.tTuhreistsousgpiveenstihoenfiwnaaslfiplrtoerdeudc,ta(nwdhtihtee sgoralvfteinntg-erxecahcatniogne pwraoscecdaurrrieedinomutethinoda170w%aswnaottern−eacceesstoarnye, asnydstetmhe.
precipitate,10.9g,97%yield).1HNMR(600MHz,CDCl,ppm)δ: Compound 4 (5 mmol) in acetone (20 mL) was added to a stirred
5.96(s,1H),3.51−3.38(m,2H),1.69−1.58(m,2H),12.43−2 1.23(m, suspensioncontainingCNCsinMilli-Qwater(0.5g,1wt%).Then,
NaOH(10mL,0.2N)wasaddedtothemixturedropwiseover30min
31H),0.90(t,J=7.0Hz,3H).13CNMR(151MHz,CDCl,ppm)δ:
2 2 andlefttoreactundercontinuousstirringatroomtemperaturefor24h.
170.20,166.53,41.94,32.32,30.44,27.01,23.08,14.26.HRMS(ESI-
A similar cleanup procedure as method 1 above was utilized for
QTOF):calcdforC21H38Cl2N4,417.46;found,417.2543(M+). DTAPEG-modifiedCNCs.
Synthesis of 4,6-Dichloro-n-propargyl-1,3,5-triazine-2- FluoresceinLabelingofNanocellulosebyanAzide−Alkyne
amine (2, DTP). SeeScheme 1a. Propargyl amine (0.59 g, 0.69 mL, ClickReaction.SeeScheme1c.AnaqueoussuspensionofBMCC(10
10.76mmol)wasaddedtoastirredsolutionofcyanuricchloride(2.00g,
mL 0.3 wt %) was solvent-exchanged by successive centrifugation
10.85mmol)indryTHF(25mL)cooledto−20°C.DIPEA(1.54g, (10000g,5°C,5min)andresuspensioninacetoneandthendryDCM,
2.08mL,11.94mmol)indryTHF(5mL)wasthenaddeddropwise withsonication(10min,5°C)inbetweeneachsolvent-exchangestepto
overaperiodof2hwiththehelpofasyringepump.Thereafter,reaction preventBMCCaggregation.KCO wasadded(15mg,0.11mmol)toa
2 3
mixturewasstirredforanother3hmaintainingthereactiontemperature stirredsuspensioncontainingBMCC(30mg,0.3wt%)indryDCM(10
between −20 and 0 °C. After this time, THF was removed under mL).After20min,compound2(30mg,0.15mmol)indryDCM(2
vacuum to yielda residue that was dissolved in EtOAc (50 mL)in a mL)wasaddedandlefttostiratroomtemperaturefor24h.Modified
separatingfunnelandwashedwithwater(3×15mL),followedbybrine BMCCwasthensubjectedto3successivecentrifugationandsonication
solution(30mL)andfinallydriedoversolidanhydrousNa2SO4toyield stepsinDCMtoremoveexcessDTPderivatives.Thiswasfollowedby
apureproduct(whiteprecipitate)inquantitativeyield(2.18g,99%) stirred-celldialysistoremoveinorganicbaseandanyleftoverunbound
underreducedpressure.1HNMR(600MHz,CDCl,ppm)δ:6.00(s, derivatives in the presence of acetone (3 cycle runs), acetone−water
3
1H),4.23(dd,J=5.7,2.5Hz,2H),2.26(t,J=2.5Hz,1H).13CNMR (50%,3cycleruns),andwater(14cycleruns).ThemodifiedBMCC
(151MHz,CDCl,ppm)δ:170.62,166.10,78.16,72.73,31.61.MS wasthenusedintheazide−alkyneclickreactionasfollows.Asolution
2 2
(ESI):calcdforCHClN,203.03;found,203.0(M+). containing 5-FAM-azide (2 mg, 4.36 μmol) in ethanol (1 mL) was
6 4 2 4
Synthesisof4,6-Dichloro-n-benzyl-1,3,5-triazine-2-amine(3, addedtoastirredsuspensioncontainingDTP-modifiedBMCC(6mg)
DTB).SeeScheme1a.NaCO (0.58g,5.4mmol)wasaddedtoastirred in Milli-Q HO (2 mL), followed by the addition of a solution
2 3 2
solutionofcyanuricchloride(1.00g,5.4mmol)indryTHF(20mL) containingCuSO·5HO(0.2mg)andascorbicacid(0.6mg)inMilli-Q
4 2
cooled to −20 °C. After 20 min of stirring, a solution containing HO(100μL).Thereactionmixturewasstirredovernightinthedarkat
2
benzylamine (0.58 g, 5.5 mmol) in dry THF (10 mL) was added room temperature. The fluorescently labeled BMCC was cleaned-up
dropwiseoveraperiodof60min.Theendofreactionwasmonitoredvia withMilli-Qwaterthroughstirred-celldialysisuntiltheeffluentgavea
TLC (silica gel, DCM). The product was isolated by centrifugation U■Vabsorbancevaluesimilartothatofwater.
followedbyfiltrationofthesupernatant,andevaporationundervacuum
toyieldapureproduct(whiteprecipitate)inquantitativeyield(1.35g, RESULTSANDDISCUSSION
98%).1HNMR(600MHz,CDCl,ppm)δ:7.45−7.28(m,5H),6.10(s,
1H),4.67(d,J=6.0Hz,2H).13C3 NMR(151MHz,CDCl,ppm)δ: Triazine-Coordinated Chemical Grafting onto Nano-
3 cellulose.Cyanuricchloridewasutilizedasalinkertotunethe
169.81,165.50,136.07,129.03,128.25,127.76,45.52.MS(ESI):calcd
forC HClN,255.10;found,255.3(M+). interfacial properties of nanocellulose in a modular fashion. In
10 8 2 4
Synthesisof4,6-Dichloro-2-poly(ethyleneglycol)monoallyl- the synthetic approach, one of the three chlorine atoms of
1,3,5-triazine-2-ether(4,DTAPEG).SeeScheme1a.Poly(ethylene cyanuricchloridewassubstitutedwithoctadecylamine(DTC18,
glycol) monoallyl ether (4.21 g, 10.85 mmol) dissolved inDCM (20 1), propargylamine (DTP, 2), benzylamine (DTB, 3), or
mL)wasaddedtoastirredsolutionofcyanuricchloride(2.00g,10.85 poly(ethylene glycol) monoallyl ether (DTAPEG, 4), to yield
mmol)inDCM(30mL)cooledto−20°C.After20minofstirring,a four distinct triazinyl derivatives (Scheme 1a) with different
NaOHsolution(50%,1gin2mLofwater)wasaddeddropwiseover30
physicochemicalcharacteristics.Afterthoroughcharacterization
min with constant stirring. After 2 h of stirring while keeping
of the derivatives (NMR and MS spectra presented in Figures
ttewmicpeewraittuhrewanteearr(02°×C1,0thmeLo)r,gadnriicedlaoyveerrwNaas SseOparaanteddcaonndcewntarsahteedd S1−S4), the triazine chemistry on cellulose was first demon-
undervacuum,resultinginanoilyproduct(5.02g,864%yield).1HNMR strated by grafting DTC18 onto filter paper. This one-step
(600MHz,CDCl,ppm)δ:5.99−5.84(m,1H),5.31−5.12(dd,2H), reactionproceededrapidlyatroomtemperatureandallowedus
3
4.72−4.54(m,2H),4.03(d,J=5.5Hz,2H),3.91−3.81(m,2H),3.78− to tune the hydrophilicity of the cellulose surface (Figure 1).
3.5(m,29H).13CNMR(151MHz,CDCl,ppm)δ:172.50,171.06, Under the reaction conditions selected for the triazine
134.78,117.07,72.36,71.32,69.40,68.42. 3 substitution, the filter paper presented water contact angles of
2427 DOI:10.1021/acs.chemmater.8b00511
Chem.Mater.2018,30,2424−2435
ChemistryofMaterials Article
dialysis.Thiscleanupprocedurehasbeenpreviouslyshownasan
effective way to obtain reproducible and predictable DTAF
graftingresults.33
FTIRspectroscopywasusedtoconfirmthecovalentgrafting
of the triazinyl derivatives onto CNCs. Figure 2 shows a
Figure 1. Tunable hydrophobization of filter paper through the
controlledgraftingofDTC18derivatives.Insetsshowthesessilewater
dropletcontactanglemeasuredonthefilterpaperaftertreatment.Error
barsrepresentthestandarddeviation,n=3.
∼125° after 3 h. The contact angle did not increase even after
doubling the reaction time, which points to a saturation of the
cellulosesurfacewithDTC18.Furthermore,itwasobservedthat
thewaterdropletsremainedintactonthesurfaceofthemodified
paperevenafterprolongedincubation,indicatingthatthesurface
functionalization turned the filter paper into an effective water
barrier. However, plasma oxidation of the modified cellulose
surfacereturnedthefilterpapertoahydrophilicstate,allowing
fullpenetrationofthewaterdropletintothefilterlumen.These
resultsshowthatthetriazinechemistrycanbeusedtotunethe
interfacial properties of cellulosic materials, where a DTC18-
based modification can be used to define hydrophilic/hydro- Figure2.FTIRspectraof(A)unmodifiedCNCand(B−E)modified
phobicareas,withapplicationinthefabricationofchannelsfor CNCs.ThemodifiedCNCsbear(B)DTC18,(C)DTP,(D)DTB,and
paper-basedmicroanalyticaldevices. (E)DTAPEGmoieties.Insetshowsthetriazineringtautomericshiftin
Having demonstrated the triazine-mediated grafting for DTB-andDTAPEG-modifiedCNCs.
macroscopic cellulosic materials, we focused our attention on
thesurfacemodificationofCNCs.Thechemicalmodificationof comparison between the FTIR spectra of unmodified and
CNCs with nonpolar moieties presents a challenge, due to the chemically modified CNCs, demonstrating the successful
charge imparted by the sulfate half-ester groups introduced grafting of C18, APEG, propargyl, and benzyl functionalities
during the production of CNCs via sulfuric acid hydrolysis.9,28 onto CNCs. The data show the appearance of distinct and
Therefore,forthesuccessfulgraftingofDTC18,DTP,andDTB unique absorption bands around 1630−1560 cm−1 in each
ontoCNCs,aninitialsolventexchangewasperformedfroman modifiedsample,whichcorrespondtotheCNandCNsp2
aqueoussuspensiontodichloromethane (DCM).Thisenabled vibrations present in the triazine rings chemically grafted onto
the chemical modification of CNCs without aggregation using CNCs. The modified CNCs also exhibit a narrower OH
nonpolartriazinylderivatives1,2,and3atambienttemperature stretchingband(3360cm−1)withbroadshoulders(overlapping
in the presence of solid potassium carbonate as a hydrochloric NH,CCH,andCCHstretchingvibrations3300−
acidscavenger(Scheme1b).Conversely,thechemicalgraftingof 3010 cm−1) due to the depletion of hydroxyl groups and the
4(apolartriazinylderivative) onto CNCswascarried outina introduction ofnewchemical bondsatthecellulosesurface.In
straightforward fashion in aqueous solutions, where sodium addition, the appearance of characteristic aliphatic CH
hydroxidewasusedasthebase.TheetherificationofCNCsby stretching absorption bands (2960−2850 cm−1) in modified
the triazinyl derivatives involved the nucleophilic aromatic CNCs (most prominent in C18) further confirms chemical
substitutionofthechlorineatomspresentincompounds1−4by graftingofthedifferenttriazinylderivativesontoCNCs.Itisalso
the numerous surface hydroxyl groups along the crystalline worth noting that a tautomeric shift within the triazine ring is
cellulosebackbone.Ithasbeenreportedthattheetherlinkages observed in both benzyl-modified (prominent) and APEG-
are stable, as shown by the imaging of bacterial cellulose modified(minor)CNCs,whichcorrespondstotheformationof
microfibrils and CNCs fluorescently labeled with DTAF amidebonds(1725 cm−1,newCONHvibrations) withα,β-
(dichlorotriazinyl-aminofluorescein).25,31−33 To ensure that no unsaturation. This results from the substitution of the last
unboundtriazinylderivativeswerenonspecificallyadsorbedonto chlorineatom(6-Cl)inthe4,6-dichlorotriazinylderivativesbya
theCNCsafterthemodificationreactions,theyweresubjected hydroxylgroupafterinitialchemicalgraftingontotheCNCsthat,
toarigorouscleanupprocedureinvolvingseveralwashingsteps in turn, involves the shiftof aproton fromthe introduced OH
through centrifugation (to remove the bulk of the unreacted (enolform)tooneoftheadjacentnitrogenatoms.Theoutcome
material and base), followed by several cycles of stirred-cell is a delocalization of the π-electrons within the triazine ring to
2428 DOI:10.1021/acs.chemmater.8b00511
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ChemistryofMaterials Article
Figure3. (A)Low-resolution XPSspectra,and high-resolution C1s peakdeconvolution of(B)unmodifiedCNCandCNCs modifiedwith (C)
DTAPEG,(D)DTC18,(E)DTB,and(F)DTP.InsetsinpanelashowtheS2pandCl2psignals,respectively,indicatingthepresenceofsulfurand
chlorine.
formamidebonds(ketoform,Figure2).Thisphenomenonwas decreasedincomparisontothoseforunmodifiedCNCs,aclear
found to occur while drying the modified CNCs at elevated demonstration of the increase in carbon content as chemical
temperatures, as any bound water molecule can force the modificationwascarriedout.Asmallamountofsulfurwasalso
substitution of the third chlorine atom, hence the tautomeric detectedat∼168eV,whicharosefromsulfatehalf-estergroups
shiftintoitsmoststableform.34,35 grafted onto CNCs during production via sulfuric acid
SurfaceGraftingEfficiencyofTriazinyl-ModifiedCNCs. hydrolysis. In addition, modified CNCs exhibited distinct
Thesurfacemodificationefficiencyofthetriazinechemistrywas
peaks at 400 and 200 eV, corresponding to nitrogen (N) and
established using X-ray photoelectron spectroscopy (XPS) and
chlorine(Cl)atoms,respectively,thatarenon-nativetocellulose.
elementalanalysis(EA),whileX-raydiffraction(XRD)measure-
Thesenewchemicalsignaturesarisefromthesuccessfulgrafting
mentswereusedtoestablishtheretentionofthecrystallinityof
unmodified and modified CNCs. Full scan low-resolution XPS oftriazinylderivativesandwereconsistentlyseeninallmodified
CNCs, with the exception of DTAPEG-CNCs where chlorine
spectra are shown in Figure 3a, highlighting the characteristic
was not present in a detectable amount. This finding can be
peaksobservedaroundbindingenergies287and534eV,which
correspond to carbon and oxygen atoms, respectively, as the explained by the displacement of all chlorine atoms on the
maincomponentsintheanhydroglucoseunitsofcellulose.The triazine ring when the grafting reaction was performed under
value of 0.80 oxygen-to-carbon (O/C) ratio (Table S1) for basic conditions in an aqueous environment. Under these
unmodifiedCNCsisclosetothosereportedforpurecellulose.36 conditions,thelastremainingchlorineatomcouldbedisplaced
As expected, the O/C ratios for all triazinyl-modified CNCs via hydrolysis after the initial DTAPEG grafting. The low-
2429 DOI:10.1021/acs.chemmater.8b00511
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ChemistryofMaterials Article
a
Table1.CNCCharacterization
sample %C(corrected) %H %N %S DS CI(%) length(nm) AHPR(nm)
CNCs 41.28(44.44) 6.07 0.05 0.72 95±1 150±40 90±40
DTAPEG-CNCs 42.41(45.67) 6.37 1.88 0.65 0.06±0.01 90±4 150±30 130±40
DTB-CNCs 44.26(47.67) 5.95 2.50 0.70 0.16±0.01 87±1 140±30 140±50
DTP-CNCs 42.49(45.76) 5.81 3.70 0.73 0.18±0.04 87±3 170±30 240±120
DTC18-CNCs 46.13(49.68) 7.08 2.54 0.59 0.110±0.003 78±4 280±80 300±100
aThecompositionofallCNCmaterialswasdeterminedbyelementalanalysis(correctedvaluesinparentheses).CNCcrystallinityindex(CI)was
quantified by X-ray diffraction. CNC lengths were obtained from AFM images (histograms shown in Figure S5). The apparent hydrodynamic
particleradius(AHPR)wasobtainedfromDLSmeasurements(representativemeasurementsshowninFigureS5).Valuesforallmeasurementsare
means of n> 3replicate samples, except for AFM, where n> 50 particles were measured. Errors represent standard deviations.
resolution XPS results thus confirmed the surface grafting of content from the grafted triazinyl derivatives. The numbers of
triazinylderivativesontoCNCs. triazinyl derivatives grafted per 100 anhydroglucose units, as
An additional advantage of XPS analysis is that detailed calculatedfromthecarbonelementalanalysisdata,were11,18,
informationaboutthesurfacechemistryofmodifiedCNCcanbe 16, and 6 for DTC18-, DTP-, DTB-, and DTAPEG-grafted
retrievedfromthedeconvolutionofhigh-resolution(surface)C CNCs,respectively.Thesenumberscorrespondto4−6%ofthe
1ssignals.Thehigh-resolutionC1sspectrafromunmodifiedand hydroxyl groups bearing triazine ether linkages in DTC18-,
modifiedCNCsweredeconvolvedintothevariouscarbonpeaks, DTP-, and DTB-modified CNCs, and 2% in the case of
whichreflecttheirlocalenvironments.TheC1,C2,C3,andC4 DTAPEGgrafting.TheDSvaluesreportedherecorrespondtoa
peaks,withcorrespondingbondenergiesof285.0,286.3,287.7, highgraftingefficiency,andarecomparabletovaluesreportedfor
and288.9eV,wereassignedtoCC/CHaliphaticlinkages, otherchemistriesusedinthesurfacemodificationofCNCs.37,38
CO linkages in alcohols and ethers, OCO/CO Furthermore,theDScould betunedbymodifying thegrafting
linkages in acetals, and OCO linkages in esters, conditionstotunethesurfaceenergyofCNCs,sinceonlyasmall
respectively.28,36 Changes in the intensities of the C 1s signals numberofgraftedmoleculesareneededtomodifytheinterfacial
(Figure 3) reflect changes in the relative proportions of the propertiesofCNCs.39
(surface)chemicalbondsinvolvingcarbonatoms(includingnew Structural Characterization of Modified Cellulose
typesofbonds)associatedwiththegraftedtriazinylderivative.A Nanocrystals. X-ray diffraction (XRD) measurements were
markedincreaseinC1intensitywasobservedforDTC18-CNCs performed to assess the crystallinity of native and modified
and DTAPEG-CNCs, confirming the successful grafting of CNCs. The crystallinity index (CI) was determined using 2-
aliphatic/ethylene chains onto CNCs through the triazine dimensional diffraction intensity patterns (Figure S6). Integra-
chemistry.Ontheotherhand,onlyasmallincreasewasrecorded tionalongtherelativeangleχforevery2θvaluewasperformedto
for benzyl- and propargyl-grafted CNCs, since their aliphatic/ obtainone-dimensionalplotsofintensityversus2θ.Background
CCbondcontributionisminimal.AllmodifiedCNCsshowed corrected intensity versus 2θ plots for native CNCs were then
anexpectedincreaseinC3peakintensity,whichresultsfromthe fitted to five symmetric Lorentzian peaks, four peaks
newOCNorNCNbondsintroducedbythecovalent correspondingtothe(100),(010),(002),and(040)crystalline
linkagebetweenthetriazinylderivativesandtheCNCsurface. planes,30andonebroadamorphouspeakfixedat24.1°.Thefour
Whilethelow-resolutionXPSresultscorroboratethepresence crystallinepeakswereusedtocreateabasecrystallinecellulose
ofnitrogenandchlorineonmodifiedCNCs,itwasimpossibleto scatteringfunction,andtheCIwascalculatedastheratioofthe
reliably determine the degree of surface substitution from the areaofthisscatteringfunctiontothetotalareaunderthecurve.
high-resolution data since the binding energies of the newly ThismethodologyhasbeenpreviouslyvalidatedagainstCNC−
introducedOCOandNCNbondsoverlapwiththose polymer mixtures with well-defined compositions.40 This
ofacetalgroupspresentinunmodifiedCNCs.Thus,elemental method yielded a CI value of 95% for unmodified CNCs
analysiswascarriedouttodeterminethedegreeofsubstitution (Table 1), which is in agreement with previous measure-
(DS)inmodifiedCNCs.DSvalueswerecalculatedformodified ments.40,41
CNCsfromtheirrelativeatomiccompositions(C,H,N,andS The scattering plots for CNCs modified with triazinyl
presented in Table 1) according to eq 1. A corrected carbon derivatives (Figure S6) were then fitted to the base crystalline
contentwasusedinthecalculationtoaccountforthedifference scattering function, where only the overall amplitude of the
betweenthe theoretical value(44.44%) andthe experimentally functionwasallowedtovary.Thisallowedthecrystallinesignal
obtainedvalue(41.28%)forunmodifiedCNCs,whichisinline to be fitted keeping the relative proportion of each of the four
withreportedvalues.28,29Thedifferencebetweentheoreticaland crystalline peaks found in native CNCs constant. This practice
experimental values is ascribed to organic impurities adsorbed was important because the triazinyl modifications could
onto the surface of CNCs during storage or processing. A introducesignalsthatoverlappedwiththecrystallinepeaksand
significant increase in nitrogen content over the baseline of couldbiasthecalculatedcrystallinityifeachoneofthecrystalline
unmodified CNCs was observed in all modified materials, as peaks was fitted independently. The CI for the modified
expected from the introduction of the triazinyl moiety. In materials wasthen calculated asthe ratio ofthearea underthe
addition,smallamounts ofsulfurfromsulfatehalf-ester groups crystallinescatteringfunctionoverthetotalareaunderthecurve.
were observed in all samples, indicating that no significant In all cases, it was observed that the introduced triazinyl
desulfationoccurredduringthegraftingprocedure.Therelative modification moderately lowered the CI value (Table 1), since
decrease in %S for modified CNCs, most noticeable in those the grafted materials contributed to the signal considered
bearing the bulkier DTC18 and DTAPEG derivatives, was amorphous. The changes in crystallinity were dependent not
expectedasaresultoftheproportionalincreaseinC,H,andN only on the degree of substitution, but also on the size of the
2430 DOI:10.1021/acs.chemmater.8b00511
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grafted molecule, as made evident by the largest decrease for bearingDTAPEG,DTB,andDTP,andawiderdistributionof
DTC18-grafted CNCs. A quick calculation using published sizesforthosemodifiedwithDTC18(FigureS5).Theaverages
valuesforCNClengthandwidth(130and8nm,respectively),41 andstandarddeviationsforsamplesmeasuringn>50particles
and crystalline cellulose density (1.5 g cm−3),42 and using the arepresentedinTable1.
degree of substitution obtained from elemental analysis (Table Thermal Stability of Triazinyl-Modified CNCs. The
1),indicatedthatthemassofthecrystallinecellulosecorewould thermaldegradationofCNCsduringnanocompositeprocessing
represent ∼79% of the total mass of the DTC18-modified canseverelylimittheirapplicability.Thus,maintainingthermal
nanoparticles. This is in good agreement with the CI values stability after surface modification is a key requirement for the
obtained from XRD, suggesting the crystallinity of the core useofmodifiedCNCs.Thermogravimetricanalysis(TGA)was
CNCswasnotaffectedbythemodification,andthatthechanges performedonunmodifiedandmodifiedCNCstostudytheeffect
inthescatteringintensityarisesolelyfromthesurfacegraftingof thatthechemicallygraftedtriazinylderivativeshaveonstability.
thetriazinylderivatives. The unmodified CNCs showed a thermal degradation profile
AFMimagingwasperformedondilutespincoatedsamplesof characteristic to commercial CNCs41 (sodium salt form) with
the unmodified and modified CNCs to confirm the individual the onset of degradation occurring at ∼300 °C and the fastest
natureofthenanoparticles.Figure4showsthatthenativeCNCs, degradation at ∼330 °C (Figure 5). We observed that all
Figure 5. TGA thermograms and derivative curves (inset) of
unmodifiedandmodifiedCNCs.
modified materials had a similar onset, but showed a slight
enhancementinthermalstability,asmadeevidentbythefastest
degradationofthecellulosecoreobservedat∼350,355,360,and
365°CforDTC18-,DTB-,DTP-,andDTAPEG-graftedCNCs,
respectively(Figure5,inset).Thisindicatesthatthesulfatehalf-
ester groups on the backbone of the crystalline cellulose are
further protected by the surface modification with the triazinyl
derivatives. Nevertheless, the DTAPEG-modified particles
showed partial degradation at lower temperatures, which is
ascribedtothedegradationofthegraftedpolymerchain,whose
thermalstabilityislower.
Dispersion of Modified CNCs in Aqueous/Organic
Figure 4. AFM height images of (A) unmodified CNC and CNCs Media. One of the goals of this work was to enhance the
modifiedwith(B)DTAPEG,(C)DTB,(D)DTP,and(E)DTC18.All dispersion and stability of CNCs in a wide range of solvents
imagesacquiredatthesamemagnification. through their modification with triazinyl derivatives. The
blending of CNCs into materials for the formation of
nanocomposites benefits from the compatibilization of the
aswellasthosemodifiedwithDTAPEG,DTB,andDTP,appear CNCsurfacechemistrywiththereceivingmatrix,whichensures
well-dispersedandcanbeidentifiedasindividualparticles,with reduced aggregation, more uniform dispersion, and better
only a few aggregates appearing in the modified materials, and transfer of the intrinsic CNC properties to the composite. To
beingmorepredominantintheDTPsample.Ontheotherhand, demonstratethis,themodifiedCNCsweredispersedinaqueous
CNCsmodifiedwithDTC18showedaproportionofthesample andorganicmedia,andthesuspensionswerefollowedovertime
intheformofsubmicronaggregates, wheremultipleindividual toinvestigatetheimpactoftheintroducedsurfacefunctionalities
particlescouldbevisualized.Quantificationofthelengthofthe on the stability of the suspensions. Unmodified and modified
particlesinthe AFMimagesconfirmedthe visualobservations, CNCsweredispersedinsolventsataconcentrationof0.5wt%
and yielded histograms with narrow dispersions for CNCs through15minpointprobesonicationinanicebath.Figure6
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derivative made the CNCs stable and compatible with water
andpolarorganicsolvents.
Similarly,thegraftingofDTC18,DTB,andDTPintroduced
alkyl,benzyl,andpropargylmoieties,respectively,thatrendered
the CNCs hydrophobic. Consequently, CNCs modified with
suchtriazinylderivativesweredispersibleandremainedstablein
chloroform, while unmodified CNCs immediately aggregated
and crashed out of this solvent. On the other hand, when
suspended in water, DTC18-CNCs rapidly crashed out of
solution,whileDTB-CNCsandDTP-CNCsshowedmoderate
stability and slow sedimentation. Further characterization
through DLS showed that DTB showed similar characteristics
to unmodified CNCs and DTAPEG-modified CNCs. On the
other hand, the average apparent hydrodynamic radii of DTP
and DTC18 were 2−3-fold larger than those of unmodified
CNCs, which suggests short-range clustering of the modified
CNCs over time. This confirmed the observations from AFM
imagesofDTC18-CNCs(Figure4E),wheresmallclusterswere
easily discernible. Despite the short-range clustering observed,
theintroductionofDTB,DTP,andDTC18modificationsmade
theCNCscompatiblewithnonpolarorganicsolvents.Itmustbe
highlightedthatthosesuspensions,exhibitingstabilityafter24h,
were found to remain stable even after months of storage in
appropriately sealed containers. The compatibilization of
modified CNCs with a range of organic solvents could open
thedoorfortheiruseasfillersandreinforcingagentsinarangeof
polymericandnonpolymericmatrices.
FluorescentLabelingofDTP-ModifiedCelluloseFibers
through “Click” Reactions. Finally, as proof-of-concept, we
aimedtodemonstratetheversatilityofthetriazinechemistryby
usingitasamodularplatformforthesecondarymodificationof
crystalline cellulose through “click” chemistry. To this end, we
first grafted DTP onto bacterial microcrystalline cellulose
(BMCC) fibrils to introduce alkyne functionalities and carried
outasecondaryazide−alkynecycloadditionreactionwithazido-
fluorescein (Scheme 1c). This yielded uniformly and brightly
labeled BMCC fibrils that could be readily imaged through
fluorescence microscopy (Figure 7). This procedure not only
demonstrates the ability to use the triazinyl chemistry as a
building block for secondary modifications, but also opens the
Figure6.PhotographsofunmodifiedandmodifiedCNCssuspensions
(0.5wt%)inaqueousandorganicmediaatT=0handT=24hafter
sonication. (A) DTAPEG-CNCs, (B) DTC18-CNCs, (C) DTB-
CNCs,and(D)DTP-CNCs.
shows pictures of the suspensions obtained immediately after
sonication (T = 0 h) and after being left undisturbed on the
benchtopfor24h.ItwasobservedthatDTAPEG-CNCswere
dispersible in aqueous and polar organic solvents (methanol,
ethanol, and isopropyl alcohol), and yielded homogeneous
suspensionsthatremainedstableovertimethroughelectrostatic
and steric interactions. In contrast, unmodified CNCs almost
immediately aggregated and crashed out of all polar solvents,
with the exception of water. Further characterization through
DLS (Table 1 and Figure S5) showed that unmodified CNCs
and APEG-modified CNCs remained suspended as individual
particles, with average hydrodynamic radii of 90 and 140 nm,
respectively.Thelackofnoticeableaggregation,fromDLSand
AFM measurements, supports the notion of colloidal stabiliza-
tion through steric and electrostatic repulsion. Thus, the Figure 7. DTP-grafted BMCC fibrils labeled through a secondary
introduction of polar polyether chains in the DTAPEG azide−alkynecycloadditionreactionwithfluoresceinderivatives.
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9
refs 43 44 45 18,19 46,47;48,4 16,17 −1214 11,20 thiswork
reactionconditions −°CO,124h94C,inertatmosphere,DMF,K23 °105Cfor2min °80C,inertatmosphere,drytoluene,24h roomtemperature(RT),DMAP,TEA,dryTHF,24h;°°110C,HMTETA,neat,12h;90C,chlorobenzene,HMTETA,24h−O,24h,RT,drytoluene,imidazole,16h;RT,H2°110C,8h −−O,DMF,pH=7.58;24hRT,H2 −°80110C,inertatmosphere,TEA,from30minto7days −°O,NaOH,56.5h65C,H2 O/acetone,NaOH,24h;RT,DCM,KCO,RT,H22324h
fffiofDierentChemistriesUsedintheModicationofNanocelluloseSurfaces processconsiderations “ff”Reactioninvolveslow-costreagents,andproductsarestable.However,reactionconditionsarehighlysusceptibletopeelingeectofCNCs,ffffiwheretheCNScrystallinestructure/morphologyisadverselyaected.Reaction/degreeofsubstitutionarediculttocontrol.OnlyacetylatedfiCNCsareproduced,norouteforselectivepostmodication.fiReactioninvolveshigh-costreagents(alkenylsuccinicanhydrides)andinvolvesafreezingdryingstepbeforemodication.ApproachavailableforfithemodicationofCNCswithalkenylsuccinicanhydrides. Reactioninvolveshigh-costreagents(lactide,tincatalyst),andrequiressolventexchangeanddistillation.ThereactionallowsgraftingofpolymerchainsthroughROPfromCNCs. Reactioninvolvesalaborioustwo-stepprocessthatuseshigh-costreagents(2-bromoisobutyrylbromide,Cu(I)Br,styrene,MMAZO,etc.)and−fiinvolvesfreezethawcyclesbeforemodication.TheresultingmaterialscanbeusedtograftpolymerchainsfromtheCNCsurfacethroughATRP. Simpleandversatileprocedure,withmanypotentialfunctionalitiesthroughcommerciallyavailablesilanes.Reactioninvolvesverycostlyreagents(chloro/alkoxysilanes)thatcouldmakeitprohibitiveforlarge-scaleapplications.Solventexchangeisrequiredwhenusingchlorosilanesandafifreeze-dryingstepinadditiontoanannealingprocessisnecessarywhenusingalkoxysilanes.Cellulosecrystallinitycouldbemodiedduringthesilylationprocess.fiReactioninvolvesalaborioustwo-stepprocess,wheretheCNCisrstoxidized,andthenEDCcouplingisperformed.Theprocessinvolveshigh-costreagents(TEMPO,NaCIO,EDC,NHS,etc.),andthedegreeofoxidationmustbecarefullycontrolled.Dependingonthefunctionalityfiintroduced,theCNCmayrequiresolventexchangebeforemodication. Reactioninvolvesalaboriousone-/three-stepprocess.Theprocessinvolvescostlyreagents(isocyanates-phNCO,diisocyanates-2,4-TDI).ThefifiprocedurerequiressolventexchangebeforeCNCmodication.Allowsgraftingwell-dened,amine-terminatedpolymersontoCNCswithoutfimodicationoftheCNCcrystallinity.fiThereactionissimpletoimplement,butthecostdependsontheepoxideusedformodication.Completeactivationofsurfacehydroxylgroupsffiwithbasicsolutionsrequiredforecientfunctionalization.Basecontentusedneedstobecarefullycontrolledtoavoidlossofcrystallinity.Mayfirequireadesulfationstepbeforemodicationreaction. Thereactionissimple,isperformedundermildconditions,andinvolveslow-costreagents(cyanuricchloride<$10/kg),andtheresultingproductsfiarestable.Theprocessishighlytunable,andtheresultingmodiedcellulose/CNCscanbeeitherpolarornonpolarandamenabletofipostfunctionalization.Dependingonthefunctionalityintroduced,theCNCmayrequiresolventexchangebeforemodication.
n
Table2.Compariso strategy acetylation fiesterication(ring-openingandATRPpolymeriza-tion) silylation amidation/carbamination fietherication triazinyl
2433 DOI:10.1021/acs.chemmater.8b00511
Chem.Mater.2018,30,2424−2435