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Please cite this article in press as: Siqueira, E.J., et al., The effects of sodium chloride (NaCl) and residues of cellulosic fibres derivedfrom sodium carboxymethylcellulose (NaCMC) synthesis on thermal and mechanical properties of CMCfilms. Ind. Crops Prod. (2015),http://dx.doi.org/10.1016/j.indcrop.2015.01.017

ARTICLE IN PRESSG Model

INDCRO-7764; No.of Pages10

Industrial Crops and Products xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Industrial Crops and Products

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The effects of sodium chloride (NaCl) and residues of cellulosic fibresderived from sodium carboxymethylcellulose (NaCMC) synthesis onthermal and mechanical properties of CMC films

E.J. Siqueira a,b,c, M.-C. Brochier Salon a,b,c, E. Mauret a,b,c,∗

a Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, Franceb CNRS, LGP2, F-38000 Grenoble, Francec Agefpi, LGP2, F-38000 Grenoble, France

a r t i c l e i n f o

Article history:

Received 28 October 2014Received in revised form 6 January 2015Accepted 11 January 2015Available online xxx

Keywords:

Cellulose productsSodium carboxymethylcelluloseFilm castingGlass transitionThermo-mechanical analysis

a b s t r a c t

The main aim of this work is to provide an accurate determination of the morphological, thermal andmechanical properties of NaCMC films. Indeed, their main applications require good thermal stabilityand mechanical strength and the literature lacks a good description of these properties. We analyse twotypes of NaCMC, an analytical (Fluka) and a crude (Niklacell) grade. For both grades, optical micrographsof NaCMC solutions show partially soluble cellulosic fragments. As evidenced bySEM, films prepared withNiklacell display the formation of opaque (CMC-fNo) and semi-transparent (CMC-fNt) regions due to thecombined effect of the formation of a NaCl layer and the presence of fragments of cellulosic fibres. Afterthe elimination of NaCl by precipitation of the acid form of the crude grade of CMC, the films (CMC-fNp)are slightly opaque compared to films prepared with analytical NaCMC grade (CMC-fF). This is due to theresidues of cellulosic fibres not eliminated during the precipitation and filtration procedures. The TGAcurves of the NaCMC films display a three-step degradation. The results of DSC analyses show that NaClhas an impact on the water content of films due to its hygroscopic behaviour. As evidenced by DSC andDMA, NaCl does not influence the T m (which is determined at ca. 240 ◦C) of NaCMC films. However, NaCl

has a considerable impact on the thermal and mechanical properties close to the T g. CMC-fF (analytical)and CMC-fNp (purified grade) exhibit similar T g values at ca. 40 ◦CandCMC-fNt and CMC-fNo (crude gradesamples) show the highest temperature values for T g and a remarkable broadening of this transition. NaClalso influences the E’ modulus of NaCMC films in the temperature range studied as observed from DMA.The CMC-fF has the highest E’ values.

© 2015 Published by Elsevier B.V.

1. Introduction

Carboxymethylcellulose (CMC) is the most widely usedwater-soluble cellulose derivative (Heinze and Pfeiffer, 1999;Pushpamalar et al., 2006; Li et al., 2009; Jung-Feng et al., 2010;El-Sayed et al., 2011; Singh and Khatri, 2012). It is prepared fromcellulose macromolecules by the reaction between cellulose andmonochloroacetate (or monochloroacetic acid), yielding a partialsubstitution of the hydroxyl groups at the 2, 3 and/or 6 posi-tions in the cellulose structure by carboxymethyl groups. Fig. 1Ashows the CMC structure. The kinetics of carboxymethylation canbe described by a nucleophilic reaction scheme. The decrease of the carboxymethylation rateduring CMCsynthesisis causedby the

∗ Corresponding author. Tel.: +33 476826917.E-mail address: [email protected](E. Mauret).

decline of thereactivity of theremaining hydroxyl groupsdue to anexponential decrease of the kinetic constants (Salmi et al., 1994).Different degrees of substitution (DS) can be obtained dependingon the modification route and the raw materials used, but the DSvalues generally vary between 0.6 and 0.95 from the usual hetero-geneous reaction (Heinze and Pfeiffer, 1999; Pushpamalar et al.,2006;Lietal.,2009;Käuperetal.,1998;Capitanietal.,2000;HeinzeandKoschella,2005). Ingeneral,allchemicalmodificationreactionsof cellulosic fibresshow at least some regioselectivity, i.e. in no casedoes an equal reactivity at positions 2, 3 and 6 appear. The reactiv-ity of the free hydroxyl groups depends on many factors includingthe state of activation and dissolution, the composition of the reac-tion mixture, the reagents used, and so on (Heinze and Koschella,2005).

The CMC structure is made up of linear -(1→4) linked gly-caneswhich in aqueous medium exhibit polyelectrolytic propertiesdue to the presence of weakly acidic groups (Mutalik et al., 2007;

http://dx.doi.org/10.1016/j.indcrop.2015.01.0170926-6690/© 2015 Published by Elsevier B.V.

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2 E.J. Siqueira et al./ Industrial Crops andProducts xxx(2015) xxx–xxx

Fig. 1. (A) NaCMC molecular structureand (B) labelling of theanhydroglucose moietyfor NMR studies.

Tong et al., 2008). CMC is in its free acid form (neutral) at pH val-ues close to 3 and the acid groups are ionized (negatively charged)at pH values close to 7.0 (Wang and Somasundaram, 2005). The

pK a value of CMC was determined to be 3.2 (Hoogendam et al.,1998). However,itshouldbenotedthatthispK a representsanaver-age value because it depends on the distance between carboxylicgroups in the polymer backbone, i.e. it can be controlled by thepatternoffunctionalization.Forexample,CMCwithapreferredcar-boxymethylation at the O-2 position possesses a pK a of 3.0, whilethe preferred O-6 modified samples have a pK a of 3.3 (Koetz et al.,1990).

CMC is generally used in its sodium salt form (NaCMC). Itis known that during the first step of the carboxymethylationreaction, i.e. the formation of alkali cellulose, low molar masscomponents may be extracted from the cellulosic fibres (Barbaet al., 2002). These components which are mainly oligosaccha-rides also undergo carboxymethylation and play a role on thephysical–chemical properties of the NaCMC. Depending on thegrade of NaCMC,the reaction by-products such as NaCl mayormaynot be removed by further industrial purification steps. Finally, asNaCMC is prepared from cellulosic fibres, insoluble materials mayexist in the final NaCMC aqueous solutions. These are partially car-boxymethylated fibre fragment residues and it is claimed that theyare present in significant quantities if the DS is less than 0.2 (Borsaand Racz, 1995).

For a long time, NaCMC has been used such as a stabilizerand protective colloid in the detergent, paper coating, pharma-ceutical, cosmetic and food industries (Heinze and Pfeiffer, 1999;Singh and Khatri, 2012; Barba et al., 2002; Shakun et al., 2013;Ambjörnsson et al., 2013). However, today there is an increasingdemand for environmentally-friendly products that are renew-able and biodegradable. More recently, cellulose derivatives and

among them NaCMC have been studied to produce new bio-basedmaterials with tailored properties, such as composites (Myllytieet al., 2010; Sasso et al., 2010; Neto et al., 2012; Pahimanolis et al.,2013; Yadav et al., 2013), films and membranes (El-Sayed et al.,2011; Ghanbarzadeh et al., 2010; Bochek et al., 2010; Bochek et al.,2012; Kibar and Us, 2013) or fibres, obtained by the electrospin-ning process (Frenot et al., 2007; Lin et al., 2013). In these works,theblendingof NaCMCwithparticles or other polymersis generallyinvestigatedmainlytoincreaseitsthermalstabilityandmechanicalproperties.

The properties of some NaCMC blended films were alreadystudied. These included corn-starch/CMC(Kibar andUs, 2013), chi-tosan/CMC (Shang et al., 2008), PVA/CMC (El-Sayed et al., 2011),poly-N -vinylformamide/CMC(Bocheketal.,2012), andsoon.Some

characteristics of NaCMC or its blends have also already beendescribed in the literature, such as the degree of swelling in water( Jung-Feng et al., 2010; Barba et al., 2002), water vapour sorption

(Bochek et al., 2012; Ma et al., 2008), GPC studies (Lin et al., 2013),viscosity in aqueous medium (Mutalik et al., 2007; Bochek et al.,2010), degree of substitution (Pushpamalar et al., 2006; Singh andKhatri, 2012; Barba et al., 2002), and so on. The influences of themolar mass distribution and DS values on the viscosity of NaCMCsolutions werestudied by Barba et al.(2002), andthe water solubil-ity and vapour permeability of NaCMC films by Tong et al. (2008).The role of hydrogen bonds in the NaCMC structure was studiedby Li et al. (2009) using infrared spectroscopy and DSC analysis.Some other studies concerning NaCMC, such as the adsorption of CMC at the solid–liquid interface(Wangand Somasundaram, 2005)and the use of NaCMC in the preparation of nanocomposites havebeen described in the literature (Neto et al., 2012; Yadav et al.,2013).

The purity of NaCMC depends on the application requirements.Large amounts of NaCMC are produced on a crude commercialgrade without any purification for application, for example, inthe papermaking industry. NaCMC is used to improve the drystrength of the paper ( Jung-Feng et al., 2010; Shang et al., 2008;Laine et al., 2000, 2002, 2003) or in combination with cationicpolyamideamine epichlorohydrin (PAE) resins during the prepa-ration of PAE-based wet strength papers (Gärdlund et al., 2003;Gernandt et al., 2003; Gärdlund and Norgren, 2007; Enarssonand Wägberg, 2007; Ankerfors et al., 2009). As previously dis-cussed, these grades may contain high quantities of sodium saltsas sodium chloride (NaCl) and other by-products, such as partiallycarboxymethylated cellulosic fibre residues or unmodified fibres,which can directly have an impact on the thermal and mechani-cal properties of the prepared materials. Highly purified products,

i.e. with a low salt content, are mainly used in the food and thepharmaceutical areas.

In this context, the main objective of this work is to providean accurate characterization of NaCMC films considering both: (i)a crude grade used in the papermaking industry to prepare PAE-based wet strength papers (Niklacell NaCMC) and (ii) an analyticalgrade homologue (Fluka NaCMC). The morphological, thermal andmechanical properties ofNaCMCfilmsas well asthe influence oftheby-products such as NaCl and cellulosic fibre fragments, on thesepropertiesare investigated. Thus,in thispaper, we highlight impor-tant properties of NaCMC films that have not yet been describedin the literature. These properties can determine the final appli-cation and the appropriate NaCMC grade for a specific industrialapplication.

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E.J. Siqueira et al./ Industrial Crops andProducts xxx (2015) xxx–xxx 3

2. Experimental

2.1. Materials

Two NaCMC salts were tested in this study. The first is a crudeconventional product used for the industrial production of wetstrength papers (Niklacell P70 UC from Mare; DS= 0.5–0.65), andthe second is an analytical grade product (Fluka from Biochemica;DS= 0.6–0.95). Hydrochloric acid (HCl), sodium hydroxide (NaOH),and ethanol were purchased from Sigma–Aldrich and were usedwithout further purification.

2.2. Methods

2.2.1. Preparation and optical characterization of Niklacell and

Fluka NaCMC solutions

Solutionscontaining1% NaCMC werepreparedby dissolvingthepowder in deionized water at 25◦C(or80 ◦C)for 24h usinga mag-netic stirrer with a Teflon-coated stir bar. NaCMC solutions wereobserved on an optical microscope (Axio Imager Min) betweenglass thin plates.

2.2.2. Purification of Niklacell NaCMC

A purification of this crude grade was carried out by acidifyingwith HCl a 1% Niklacell NaCMC solution to achieve a pH value closeto2 under moderatemagnetic stirring.One litre ofethanolwasthenaddedandthesolutionwaslefttorestfor30min.Whilethesodiumsalt form of CMC (NaCMC) is water soluble, treating the polymerwith mineral acid leads to the water-insoluble carboxylic acid CMCform. The protonation of the carboxylate groups (COO−) of theionized NaCMC formed in aqueous medium minimizes the elec-trostatic repulsion between polymer chains. Numerous and stronginter-chain hydrogen bonds are formed between CMCchains, turn-ing it insoluble in water (Bochek et al., 2010, 2012). The insolubleacid Niklacell CMC form was then filtered on Whatman paper(grade 4) and washed several times with distilled water until thewashingsreached a neutral pH.Due to itswater insolubilityand the

formation of aggregates insoluble in hot water (at 80◦C) even withvigorous magnetic stirring, the acid CMC form was re-solubilizedusing an ultra-sound treatment at 80◦Cfor1handatapHvalueof 10 continuouslyadjustedwith NaOH reforming the soluble NaCMCsalt form. Finally, the purified Niklacell NaCMC form was dried inan oven (at 60◦C for 24h) and stored in a desiccator.

2.2.3. Preparation of NaCMC films

After preparation of the NaCMC solutions, the films used in thiswork were obtained from the procedure described hereafter. Onepercent aqueous solutions of NaCMC were prepared by dissolvinganalytical and crude NaCMC powders in distilled water at 25◦Cfor24h using a magnetic stirrer with a Teflon-coated stir bar. CMCfilms were cast in Teflon moulds (diameter 5 cm) under controlled

conditions(25◦

Cand50%RH)inanenvironmentalchamberforoneweek.Filmswithathicknessofca.0.2mmwerethusobtained.Afterpreparation, films were conditioned in an environmental chamberunder controlled conditions (25◦C and 50% RH).

2.2.4. Drying kinetics

The drying kinetics of the NaCMC films were determined byweighing NaCMC aqueous solutions in the Teflon moulds undercontrolled conditions (25◦C and 50% RH) to a constant weight.

2.2.5. 13C nuclearmagnetic resonance at liquid and at solid states

NMR spectra of NaCMC films were recorded at 25◦C on Varianspectrometers, UNITY400 and MERCURY400. 13C and 23Na experi-mentswereperformedwitha10mmBB(BroadBand)probe.Heavy

water solutions were prepared for performing NMR in solution.

The concentrations were ca. 10% (w/w) for the carbon and sodiumexperiments.

The spectral widths were adjusted to the area of signals, relax-ations delays andpulses usedallowingquantitativemeasurements.Zero-filling functions, but no apodization functions, were appliedbefore FT. Chemical shifts were referenced to TMS (0ppm). The13C quantitative spectra were obtained at 100.580 MHz with pro-ton decoupling applied only during acquisition time to avoid NOE(Nuclear Overhauser Effect) and with an 11s relaxation delay and30◦ pulse. A 4 Hz line broadening was applied before FT.

The 23Na spectra were obtained at 105.802 MHzwithout protondecoupling. Relaxation times (T 1) were measured by the recoverymethod. Chemical shifts are referenced to NaCl (0ppm).

The chemical structures of insoluble materials were studied bysolid-state 13CNMR experiments. The13Cspectrawererecordedona Bruker AVANCE400spectrometer equipped with a 4 mm CP/MASprobe and operating at 79.490MHz. Samples were placed in 4 mmZrO2 rotors, with a 12kHz spinning rate. All of the spectra pre-sented were recorded using a combination of cross-polarization,high-powerprotondecouplingandmagicanglespinning(CP/MAS).The recycle delays and contact times were 5s and 2ms, respec-tively. A 10Hz line broadening was applied before FT. Chemicalshifts are referenced to TMS (0ppm) with a glycine sample (C Osignal at 176.03ppm) (Brus et al., 2002).

2.2.6. Fourier Transformed Infrared spectroscopy (FTIR)

The FTIR spectra of NaCMC powder and films were collectedusing an FTIR spectrometer (Paragon 1000 PerkinElmer) on trans-mission mode. CMC samples (ca. 2mg) were dried in an oven at105 ◦C for 24h, thoroughly mixed with 200mg of spectroscopicgrade potassium bromide (KBr), and pressed into pellets. A scan-ning range from 400 to 4000cm−1 at a resolution of 4cm−1 wasused, and each spectrum represents an average of 16 consecutivescans.

2.2.7. Scanning Electron Microscopy (SEM) and Energy Dispersive

Spectroscopy (EDS)

An environmental scanning electron microscope (Quanta 200)was used to examine representative regions of the surface andcross-section of NaCMC films. Samples were conditioned in desic-cators andcoated with gold under vacuum (Emitech K550x) beforeanalyses. The X-ray microanalysis was carried out with X-Flash5010 detector (silicon drift detector).

2.3. Thermogravimetric analyses (TGA)

Thermogravimetric analyses were performed using a TG anal-yser (TA Instruments) under a flow rate of nitrogen or synthetic air(50mL min−1) during scans. NaCMC samples (ca. 5 mg) were stud-ied in aluminium pans with a heating rate of 10◦Cmin−1 between25 and 700 ◦C.

2.3.1. Differential Scanning Calorimetry (DSC)

DSC analyses of the NaCMC films were performed using aDSC analyser Q800 (TA Instruments) under a nitrogen flow rate(50mL min−1) during scans. NaCMC samples (ca. 5 mg) were stud-ied in open aluminium pans with a 5◦Cmin−1 heating rate. Twoconsecutive scans were performed for each sample: the first from−90up 120 ◦C and the second scan from−90up 300 ◦C. An isother-mal step at 120 ◦C for 10min was carried out between scans.

2.3.2. Dynamic Mechanical Analyses (DMA)

DMA of NaCMC films (10×5 mm2) were carried out using aRheometric System Analyser III (TA Instruments) on tension modeat a 1Hz scanning frequency and with a 5◦Cmin−1 heating rate A

nitrogen flow rate was used during analyses. The glass transition



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Fig. 2. Micrographs obtained by optical microscopy of 1% (A) Niklacell and (B) Fluka NaCMC solutions.

temperature (T g) was determined as the temperature on the peakof loss tangent curves (tan ı).

3. Result and discussions

FlukaandNiklacellNaCMCsolutionswerepreparedasdescribedin the Section 2.2 and were observed by optical microscopy. Fig. 2presents micrographs for 1% NaCMC solutions. The micrographsshow residual fibres that are apparently partially carboxymethy-lated. Niklacell NaCMC solution shows these cellulosic residues toa great extent when compared with theFluka NaCMC solution.Ele-mentary fibrils of cellulose consist of crystallites and intercalatedless-ordered (amorphous) domains. Cellulosicfibresdiffer in termsof their crystalline content, size and orientation as well as in theamorphous domains, size and shape of voids and number of inter-fibrillar lateral tie molecules (Stanna-Kleinschek et al.,2001). Theseproperties have an influence on the accessibility of monochloroac-etate to the cellulose structure during NaCMC preparation (Barbaet al., 2002; Borsa and Racz, 1995). Monochloroacetate more eas-

ilypenetrates the amorphous domains, giving rise to the formationof modified and/or partially modified phases within the cell wallof cellulosic fibres. After contact with water (at 25 or 80 ◦C for24h), more and less swollen regions alternate along the mainaxis of the carboxymethylated fibres. As observed on the micro-graphs, themodified domains of thefibresstructure swell to a greatextent. Unmodified and/or slightly modified domains do not swellas much, which gives rise to the formation of small rings along theremaining fibres. To the best of our knowledge these structuresare described for the first time. Solutions containing 0.01, 0.5 and2% (w/w) NaCMC also showed these structures even after heat-ing the NaCMC solutions at 80◦C for 24h. For the NaCMC samplesused in this work, the presence of residual cellulosic fibres wasnot expected because the DS (0.5–0.95) values was considered to

be high enough (Barba et al., 2002). Under these conditions, car-boxymethylation was supposed to allow a complete solubilizationin water (Borsa and Racz, 1995). Therefore, we can postulate thatinsoluble or partially soluble cellulosic fragments seem to be a typ-ical characteristic of NaCMC grades in contact with water even forDS within the range of 0.5–0.95 and regardless of the consideredgrade (analytical or crude).

FilmspreparedwiththeFlukaNaCMCsolution(analyticalgrade)are flexible, transparent, and easily removed from the Teflonmould without breaking whereas thosepreparedwiththe NiklacellNaCMC solution (crude grade) are more brittle (although withoutvisual cracks) and visually heterogeneous, showing the formationof two domains: (i) an opaque and (ii) a semi-transparent region.To explain these morphological differences, NaCMC powders and

NaCMC films(designedas f samples)preparedtherefromwereana-

Table 1

Quantitative data obtained by solid state CP/MAS 13 C NMR of NaCMC samples (C6urepresents un-substituted C6 of AGU and C6s substituted C6 of AGU).

Region (inppm)assignment

190–170C O

110–95C1

95–67C4,C2 ,C3,C5,C7,C6s

67–50C6u

FlukaCMC-F 0.88 1.00 5.15 0.89CMC-fF 0.95 1.00 5.21 0.90NiklacellCMC-N 0.87 1.00 4.64 1.23CMC-fNo 1.10 1.00 4.66 1.20CMC-fNp 0.86 1.00 4.59 1.04CMC-fNt 0.53 1.00 4.58 0.93CommercialCMC-0.7 0.83 1.00 5.05 0.88CMC-0.9 0.90 1.00 5.20 0.86CMC-1.2 1.26 1.00 5.73 0.82

lysed:(i) powdered Fluka NaCMC (CMC-F); (ii) filmof Fluka NaCMC(CMC-fF); (iii) powdered Niklacell NaCMC (CMC-N); (iv) opaqueregion of Niklacell NaCMC film (CMC-fNo); (v) semi-transparent

region of Niklacell NaCMC film (CMC-fNt); and (vi) film of purifiedNiklacell NaCMC (CMC-fNp).

TheNaCMCsampleswerestudiedbyNMR analyses. Fig.3showsthe solid state CP/MAS 13C NMR spectra at 298K of CMC samples,and Table 1 shows the quantitative integrals data (quantitativemeasurements determined from CP curves). Three other commer-cial analytical grade NaCMC with known DS values of 0.7, 0.9 and1.2 were added for comparisons.

In Table1, the intensities ofthe carbonpeaksassociatedwiththeanhydroglucose moiety are given according to the labelling of thisunitary group,as shown inFig.1B. From theCP/MAS 13CNMRspec-tra,DS valuescan becalculatedfromtheratio ofthe carboxyl(C O)to the C1 signal area. CMC-F (powder of Fluka NaCMC) and CMC-fF (film of Fluka NaCMC) exhibit close DS values of 0.88 and 0.95,

respectively, which are in the range of the supplier data (0.6–0.95)and are close to commercial analytical grade CMC-0.9 (added forcomparisons). However, theDS values of theNiklacell NaCMC sam-ples (CMC-N, CMC-fNo, CMC-fNt and CMC-fNp) vary to a greaterextent(0.53–1.10)comparedtothoseassociatedwiththeanalyticalgrade product (Fluka NaCMC). Only the DS of the CMC-fNt sample(0.53) is in the range of the supplier data (0.5–0.65). A possibleexplanation for these DS variations of the Niklacell NaCMC sam-ples is the presence of by-products, such as NaCl, which affect all of the resonancesintensities by spuriouseffects such as concentrationand ionic strength (Capitani et al., 2000).

Although the presence of NaCl is expected in crude-gradesamples, such as Niklacell NaCMC, some liquid state 23Na NMR measurements wereperformedon NaCMCaqueous solutions.23Na

has a nuclear spin of 3/2, and its relaxation is mostly controlled



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Fig. 3. CP/MAS 13 C NMR spectra at 298K of NaCMC samples.

Fig. 4. Curvesof thelongitudinal relaxation time (T l) andwidthat half-height(1/2)versus %Nain the formof COO−Na+ as deduced by liquid-state 23Na NMR analyses.

by the interaction between its nuclear quadrupole moment andlocal electric field gradients. The line width of the peak was con-ventionally used as a measurement of mobility and/or symmetryof the electric field gradient around the sodium ion. The line widthat half height (1/2) is related to the transversal relaxation time(T 2) through an apparent relaxation timeT ∗2 by the equation:

1T 2

=1T ∗2

+ C and1/2 =1

(+ T ∗2)

where C is a constant.Serrai et al. (2000) investigated 23Na relaxation data (T1, T ∗2 or1/2) as a function of sodiumconcentration andfitted their exper-imental curves with monoexponential functions. In the same way,Minato and Satoh (2004) studied evolution of 23 Na line width athalf height as a function of CH3COO−Na + concentration as calibra-tion curves. So we have applied an exponential fit toour results (seeFig. 4), which show similar profile to Minato’s calibration curves.

The width at half height (1/2) and longitudinal relaxation time(T 1) were measured. These two parameters depend on the sodiumbond type: (i) Na+Cl− or (ii) COO−Na+. Some measurements werealso carried out after hydrochloric acid addition to release Na+.These results are presented in Table 2. For the Fluka NaCMC sam-ples (CMC-F and CMC-fF), practically of all the sodium is present in

sodium carboxylate bonds. After HCl addition, sodium is present in

Table 2

Line width at half height(1/2) andthe longitudinalrelaxationtime (T l) ofsodiumions, as obtained by liquid-state 23 Na NMR experiments.

Samples 1/2 Na (Hz) T l Na (ms)

NaCl 10.0 46.9FlukaCMC-F 50.2 9.5CMC-F-HCl 10.5 45.1CMC-fF 49.2 9.6CMC-fF-HCl 10.4 45.1NiklacellCMC-N 34.4 14.1CMC-N-HCl 16.3 45.4CMC-fNo 23.0 21.2CMC-fNo-HCl 19.0 45.2

CMC-fNt 28.7 17.6CMC-fNt-HCl 21.0 45.0

salt form (Na+Cl−) as indicated by 23Na NMR studies of CMC-F-HCland CMC-fF-HCl samples because the sodium parameters are simi-lar to those of standard NaCl. For Niklacell NaCMC samples, sodiumis present either in COONa or in NaCl salt forms. Regarding Fig. 4,it appears that CMC-N (Niklacell NaCMC in powder form) contains25% of Na as NaCl and 75% as COONa forms. During the film forma-tion, there is a segregation of phases and the salt concentration inthe opaqueregion(CMC-fNo) shows Na values of 50% NaCl and50%COONa and in the semi-transparent region (CMC-fNt), it shows Navaluesof 30% NaCl and70% COONa.These results confirmthe initial

statements about free NaCl as a by-product derived from synthe-sis in the Niklacell NaCMC samples. The results obtained by liquidstate 23NaNMR arepromisingtovisualize,forexample,counter-ionbinding at the molecular level of charged gels derived from carbo-hydrate polymers. NMR spectroscopy of metal cations seems to bea powerful tool because chemical shift of an alkali metal cation, forexampleNa+,sensitivelychangesonthecontaction-pairformation.However, only a few NMR studies on the counter-ion binding of charged polymer gels are available in the literature, even for singlecounter-ion systems (Bai et al., 2014; Naumann and Kuchel, 2010;Kummerlowe et al., 2010; Medronho et al., 2006). Other experi-mentalevidenceofNaClinNiklacellNaCMCsampleswereobservedbyconductivityvaluesofsolutionsmeasuredwithaconductimeter.One-percent Flukaand NiklacellNaCMCsolutionsshow conductiv-

ity values of 190 and 4800S cm−1

, respectively.



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Table 3

Main absorption bands of Fluka and Niklacell NaCMC samples obtained from FTIR analyses in the ATR mode (Pushpamalar et al. , 2006; Tong et al. , 2008; Wang andSomasundaram, 2005).

Band (cm−1) Assignments

580 Ring stretching and ring deformation of –D–(1–4) and–D–(1–6) linkages1050 CH2 twisting vibrations, primary alcoholic CH2OH s tretching a nd C H O CH2 stretching1328 OH bending vibration and symmetrical deformations of CH21426 symmetric vibrations of COO− group a nd CH2 scissoring1606 Anti-symmetric vibrations of COO− groups;and ring stretching of glucose and ofnon-hydrated C O groups

2904 C H stretching of the CH2 groups3424 O H stretching and intermolecular/intramolecular hydrogen bonds

NaCMC powder and films were studied by FTIR analyses in theATR mode. Table 3 shows the main assignments. Bands associatedto Niklacell NaCMC samples are larger compared to bands of FlukaNaCMC probably because of the presence of NaCl, as already evi-denced from 23Na NMR studies and conductivity measurements.Characteristic signals of ester groups in the region of 1600cm−1

(C O) and 1100–1000cm−1 (O R) were shifted between 5–10%for higher wavenumbers for Niklacell NaCMC (CMC-N, CMC-fNo,CMCfNt and CMC-fNp) compared with Fluka (CMC-F and CMC-fF)NaCMC samples. However, for both grades, the main characteristicabsorption bands were observed (see Table 3).

Fig. 5A and B show micrographs of CMC-fNo obtained by SEManalyses of surface andcross-section, respectively. Saltsare presenton the film surface whereas the cross-section displays phase seg-regation. During films preparation in Teflon moulds by casting wehave two surfaces formed, namely, the upper side (onclose contactwith atmosphere of environmental chamber) and bottom side (onclose contact with surface of Teflon mould). The observed uppersurface of the film on the cross-section micrograph (Fig. 5B)wasincontact with Teflon mould during film preparation (bottom side).Due to water elimination from the Niklacell NaCMC solution, saltsprecipitated and deposited on the mould surface. The observedlower surface of the film on the micrograph (Fig. 5B)was in contactwith the atmosphere of the environmental chamber (upper side).X-ray microanalysis shows that the bottom side is rich in Na and

Cl atoms, whereas the upper side of the film contains mostly C, O,Na and Cl atoms. Thus, we can affirm the presence of NaCl in theNiklacell NaCMC mainly on the film surface in close contact withthe mould surface (bottom side). During the casting of the films,there is a segregation of at least two phases: an NaCl-rich layer anda mixtured Niklacell NaCMC/NaCl layer. CMC-fNt showed a similarbehaviour.

Fig. 5C–E present micrographs obtained by SEM of CMC-fNp(film of purified Niklacell NaCMC) of both surfaces (upper and bot-tomsides) and the cross-section, respectively. The surface that wasin contact with the Teflon mould (bottom side: Fig. 5.D) shows anetwork of fibres, which is not present at the surface in contactwith the atmosphere of the environmental chamber (upper side:Fig. 5C). A cross-section micrograph (Fig. 5E) also shows fibres, butnottheclearformationofsaltlayersorphasesegregationcomparedto CMC-fNo and CMC-fNt samples. X-ray microanalyses confirmthe presence of C, O and Na atoms as the major components onboth sides (upper and bottom). These micrographs and EDS anal-yses demonstrate that purification procedure used in this work(precipitation, filtration and re-solubilization of Niklacell NaCMC)allowed the removal of NaCl butdid not allow the removal of cellu-losic fibres or partially carboxymethylated, thus preserving someof the original constituents of the Niklacell NaCMC powder. Thiswas important to understand the individual influences of NaClandresidualcellulosicfragments on themorphological,mechanicaland thermal properties of the NaCMC films. Fig. 5F shows a cross-section micrograph of CMC-fF (analytical grade). As expected, theFluka NaCMC films appearto be homogeneous andit is notpossibleto observe the presence of salt layers and/or segregation of phases.

X-ray microanalysis shows that C, O and Na are the major elementsforming NaCMC structure and confirms the degree of purity of thischemical.

Even if the presence of NaCl and partially carboxymethy-lated fibres in films of unpurified Niklacell NaCMC (CMC-fNo andCMC-fNt) was proved by 23Na NMR analyses, conductivity mea-surements and optical micrographs and SEM, respectively, theformation of an opaque and a semi-transparent region in the mor-phology of films prepared with unpurified Niklacell NaCMC seemsto be a combined effect of the thickness of the NaCl layer (which ishigher for CMC-fNo when compared to CMC-fNt) and the remain-

ingfibresonthesurfaceofthefilmsonclosecontactwiththeTeflonmouldduring itspreparation.On theother hand,due tothe elimina-tion of NaCl during the purification of the Niklacell NaCMC and theabsence of NaCl in analytical grade Fluka NaCMC, the differences inthe transparencyof films prepared from these chemicals (CMC-fNpand CMC-fF) seems to be mainly due to the remaining fibres (FlukaNaCMC films are visually more transparent than purified NiklacellNaCMC films).

Fluka and Niklacell NaCMC samples were analysed by thermo-gravimetric analyses. Table 4 shows the results obtained. NaCMCsamples show a tree-step degradation under air or N2 flow rate: (i)at 200–400 ◦C (organic volatiles with low molecular weight); (ii)at 400–600◦C (decarboxylation of NaCMC with elimination of CO2)(Linetal.,2013); and(iii) at 600–700◦C (residualorganic fractions).

Jung-Feng et al. (2010) also observed a three-step degradation pro-file of NaCMC samples. An initial weight loss of up to 12% between30and180◦Cwas observed dueto theeliminationof moisture con-tent. Curves of drying kinetics under controlled conditions (at25◦Cand 50% RH) of both NaCMC powder and films showed sigmoidaltracings and also ca. 10% water content.

Fluka and Niklacell NaCMC samples (CMC-F, CMC-fF, CMC-N,CMCfNt, CMCfNo and CMCfNp) were studied by DSC analyses andthecurvesobtained (heat flowx temperature) showedsimilar trac-ings. Fig. 6 shows, only as an example, DSC tracings correspondingto Fluka NaCMC powder (CMC-F). We chose this sample as rep-resentative of Niklacell and Fluka NaCMC because of its analyticalgrade and also because of the form in which NaCMC is generallysupplied (powder form). To clarify the transitions we observed,we magnified its temperature range during the first and secondscans (see the graphical scales on Fig. 6). Table 5 summarizes thetemperature ranges of the thermal transitions obtained by DSCanalyses. The main transitions observed are: (i) an endothermictransitionduringthefirstscanbetweenca.30and120◦Cattributedto water evaporation; (ii) an endothermic transition during thesecond scan between ca. 90 and 220◦C related to residual waterprobably attached to NaCl and not eliminated during first scan;(iii) an endothermic transition during second scan close to 250◦Cattributed to the melting temperature (T m) of crystalline regions of NaCMC structure; and (iv) an exothermic transition between 250and 320 ◦C attributed to the thermal degradation of the NaCMCstructure. For CMC-fF and CMC-fNp, because of a very low concen-tration of hygroscopic salts (NaCl), the water elimination occursonly during first scan and as expected, the CMC-fNp and CMC-fF



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Fig. 5. Micrographs obtained by SEM analyses of CMC-fNo (A and B) surface in close contact with theTeflonmould (bottom side) during film preparation and cross-section,respectively;(CandD)surfaceofCMC-fNpinclosecontactwiththeenvironmentalchamber(upperside)andinclosecontactwiththeTeflonmould(bottomside),respectively,and (E) cross-section of CMC-fNp; (F)cross-section of films prepared with Fluka NaCMC (CMC-fF).



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Table 4

Thermogravimetric analyses of NaCMC samples.

Initial weight loss First decomposition Second decomposition Third decomposition Final weight700 ◦C(%)

Air N2 Air N2 Air N2 Air N2 Air N2FlukaCMC -fF 8% up 120 ◦C 9% up 120 ◦C 250–330 ◦C

35%230–360 ◦C40%

340–450 ◦C10%

350–500 ◦C10%

500–690 ◦C12%

510–650 ◦C8%

12 22

CMC-F 10% up110◦

C 12% up 130◦

C 250–350◦

C35% 210–390◦

C43% 350–440◦

C6% 360–560◦

C10% 540–670◦

C16% 570–740◦

C5% 15 21

NiklacellCMC-fNO 9%up 180 ◦C 10% up 180 ◦C 240–350 ◦C

36%200–330 ◦C66%

380–410 ◦C3%

350–510 ◦C8%

550–590 ◦C35%

590–770 ◦C7%

29 40

CMC-N 9% up 110 ◦C 10% Up 190 ◦C 210–350 ◦C37%

200–390 ◦C70%

350–460 ◦C12%

400–560 ◦C8%

600–700 ◦C14%

640–780 ◦C5%

34 40

CMC-fNt 9%up 110 ◦C 12% up 110 ◦C 240–340 ◦C40%

250–370 ◦C43%

350–450 ◦C10%

480–630 ◦C5%

500–610 ◦C23%

620–780 ◦C10%

23 28

CMC fNP 8%up 120 ◦C 9% up 170 ◦C 240–350 ◦C47%

220–380 ◦C45%

350–450 ◦C7%

390–550 ◦C6%

540–660 ◦C18%

560–780 ◦C5%

18 28

-50 0 50 100 150

-0,6

-0,4

-0,2

0,0

0,2

H e a t F l o w ( W / g )

Temperature (

o

C)

(A)

-100 -50 0 50 100 150 200 250 300 350 400

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

H e a t F l o w ( W / g )

Temperature (

o

C)

(B)

Fig. 6. DSC analysis of Fluka NaCMC powder (CMC-F) during: (A) first and (B) secondheating scans.

(purified and analytical grades, respectively) show very close ther-mal transition temperatures in the DSCanalyses (seeTable5). Someliterature shows similar results for the NaCMC thermal transitionvalues obtained by DSC analyses (El-Sayed et al., 2011; Neto et al.,2012; Yadav et al., 2013; Bochek et al., 2012; Lin et al., 2013).

Jung-Feng et al. (2010) and Kibar and Us (2013), based on DSCstudies, determineda T g valueof NaCMC atca. 55◦C.Itwasnotpos-sible to observe T g values in this work on DSC curves probably dueto the presence of water in the samples. As we are mainly inter-

Table 5

DSC analyses of Fluka and Niklacell NaCMC films (first scan −90 up to 120 ◦C andsecond scan −90up to 300 ◦C).

Endothermic (◦C) Exothermic (◦C)

CMC-fNo First scan 35–120Secondscan

95–210 255–320250

CMC-fNt First scan 30–120Secondscan

100–215 215–305250

CMC-fNp First scan 25–120Second scan 265 270–310

CMC-fF First scan 30–120Second scan 255 260–300

ested in the effects of NaCl and residual cellulosic fibres (the mainby-products of CMC preparation) on the morphology and thermaland mechanical properties of NaCMC films, they were storedundercontrolled conditions (at 25◦C and 50% RH) and not under abso-lutely anhydrous conditions. Thus, theT g (represented by a slightinflexion of the base line on the DSC curves) could be overlappedby other thermal transitions as, for example, water elimination.

To determine the thermal dynamics transitions and effects of by-products on the thermal and mechanical properties of NaCMCfilms (CMC-fF, CMC-fNo, CMC-fNt and CMC-fNp), we used dynamicmechanical analysis (DMA). As described in Section 2.2, NaCMCfilms were stored in controlled conditions (at 25◦C and 50% RH)before analyses. Preparation of anhydrous NaCMC films for DMAisnot possible because this polymer is hygroscopic, and when dried,NaCMCfilmsareverybrittle.Fig.7A shows the storage modulus (E’)and loss tangent (tan ı) curves of CMC-fF (film of analytical gradeFluka NaCMC) and CMC-fNp (film of purified Niklacell NaCMC).CMC-fF shows a highest E’ in the temperature range studied whencompared with CMC-fNp probably because of its analytical grade(low amount of unmodified or partially modified cellulosic fibresand NaCl). Additionally, crude grade Niklacell NaCMC used in thiswork was purified using a precipitation/filtration/re-solubilizationprocedure (see Section 2.2.2). Due to the insolubility in water of the acid form of Niklacell CMC after precipitation/filtration, a re-solubilization step using ultrasound treatment at 80◦C for 1 h at



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Fig. 7. Storage modulus (E’) and loss tangent (tan ı) curves obtainedby DMA anal-

yses of:(A) CMC-fF and CMC-fNp; (B) CMC-fNt and CMC-fNo.

pH 10 was carried out, which could also contribute to a slightdegradation of the structure of the purified Niklacell NaCMC, thusdecreasing the storage modulus of the films prepared therefrom(CMC-fNp) compared with films prepared with analytical FlukaNaCMC (CMC-fF).

CMC-fF presents two relaxations on a tanı curve: (i) at ca. 38◦Cattributed to T g as also observed by Mutalik et al. (2007); and (ii)at 260 ◦C, attributed to T m. An endothermic transition observedon the DSC curves at this temperature range supports this assign-ment even if the films break down under stress during DMA afterT m because of their degradation. CMC-fNp presents three relax-ations: (i) at ca. 40◦C, attributed to T g; (ii) at 240 ◦C, attributed to

T m; and (iii) between 90 and 120◦

C, attributed to the effects of water elimination on the thermo-mechanical propertiesof the film.When observing the E’ curve of CMC-fNp, there is an increase of E’between 90 and 120 ◦C. Generally, the storage modulus decreaseswhen the temperature increases (Siqueira and Botaro, 2013). Prob-ably because of waterelimination, there is a cooperative segmentalmobility in the amorphous phase of neighbouring regions and thepolymer chains can be in closer contact, thus increasing inter-actions between the chains (hydrogen bonds and Van der Waalsforces) that restrict polymer chain motions and increasing E’. Hereagain,the effects of watereliminationwerrenot observed forCMC-fF probably because of the high purity of this chemical (a low NaClcontent) and its low moisture content (see TGA data).

CMC-fNt and CMC-fNo (Fig. 7B) present two relaxations on

the tan ı curves in the temperature range between 0 and 250◦

C:

(i) a relaxation between 50 and 100◦C and between 50 and170 ◦C of CMC-fNt and CMC-fNo, respectively. These transitionsare attributed to T g and water elimination effects on the structureof NaCMC films. The broadening of T g of CMC-fNt and CMC-fNocompared to CMC-fF and CMC-fNp is probably due to the higheramount of NaCl in the structure of these NaCMC films (and conse-quently a higher moisture content) that forms a layer, as observedby SEM analyses restricting NaCMC chain motions in the amor-phous regions. Bochek et al. (2012) also describe T

g values at ca.

190 ◦C for NaCMC; and (ii) a relaxation at approximately 240 and250 ◦C of CMC-fNt and CMC-fNo samples, respectively, attributedto T m of the crystalline regions of NaCMC structure. Therefore, wecan conclude that the presence of NaCl or an NaCl layer in the mor-phology of NaCMCfilmshas a considerableimpacton themolecularmotionof NaCMC macromolecules in the amorphousdomains, andconsequently, on T g values. However, in the range of T m (highertemperatures), these influences on mechanical properties can beconsidered negligible.

4. Conclusions

In this study, the influences of by-products such as NaCl and

partially carboxymethylated cellulosic fibres, on the thermal andmechanical properties of NaCMC films prepared with an analytical(Fluka) and a crude (Niklacell) NaCMC grade were studied. Opti-cal micrographs of NaCMC solutions show insoluble or partiallysoluble cellulosic fragments, which are typical residues in NaCMC,even for DS within the range of 0.5–0.95. The combined effect of NaCl layer formation and cellulosic fibre fragments in the crudeNaCMC grade films is responsible for its semi-transparency com-pared with films prepared by analytical NaCMC grade, which arefully transparent. DS values were calculated by the ratio of car-boxyl to C1 signal area from CP/MAS 13 C NMR spectra, and NaClaffects all resonances intensities by spurious effects, such as ionicstrength. The results, obtained by liquid state23Na NMR, show thatfor Fluka NaCMC samples (CMC-Fand CMC-fF), practically allof the

sodium is present on COONa bonds and for Niklacell NaCMC sam-ples (CMC-N, CMC-fNo, CMC-fNt, CMC-fNp) as NaCl or COONa. ForNiklacell NaCMC samples, there was segregation of the NaCl andNaCMC phases during film preparation, as observed by SEM andEDS. On the other hand, for the Fluka NaCMC film, the main ele-ments forming NaCMC structure determined by EDS analyses wereC, O and Na confirming its purity grade. NaCl not only considerablyaffects the thermal degradation of NaCMC samples, as observed byTGA,butmainlythewatercontentduetoitshygroscopicbehaviour.Niklacell and Fluka NaCMC samples showed a three-step degra-dation profile. NaCl and residual cellulosic fibres did not show aneffect on thermal transition values close toT m. However, they haveremarkable influences on T g and on the thermal and mechanicalproperties close to T g. Based on the obtained results, if mechanicaland optical properties are not the main properties of interest whenconsidering NaCMC film preparation, the crude NaCMC grade maybe an appropriate candidate, mainly due to its low cost comparedto analytical grade.

Acknowledgements

This project was funded by ANR (French National ResearchAgency) in the framework of the MAT&PRO program (PAPREHproject). Siqueira, E.J. also would like to thank CNPq-Brazil(National Center for Scientific and Technological Developments) –Process 200626/2013-2.

This research was made possible, thanks to the facilities of theTekLiCell platform fundedby the RégionRhône-Alpes (ERDF: Euro-

pean Regional Development Fund).



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