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Polymer 53 (2012) 5779e5787

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Polymer

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Ionic liquid: A powerful solvent for homogeneous starchecellulose mixing andmaking films with tuned morphology

Weiqing Liu, Tatiana Budtova*

Centre de Mise en Forme des Matériaux e CEMEF1, Mines ParisTech, UMR CNRS 7635, BP 207, 06904 Sophia-Antipolis, France

a r t i c l e i n f o

Article history:Received 26 July 2012Received in revised form16 October 2012Accepted 20 October 2012Available online 29 October 2012

Keywords:CelluloseStarchIonic liquid

* Corresponding author. Tel.: þ33 (0)4 93 95 74 70E-mail address: Tatiana.Budtova@mines-paristech.

1 Member of the European Polysaccharide Networkepnoe.eu.

0032-3861/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2012.10.043

a b s t r a c t

Corn starch and cellulose were mixed in common solvent, ionic liquid 1-ethyl-3-methylimidazoliumacetate (EMIMAc). The properties of mixtures in the liquid state (flow, viscosity, miscibility) and offilms obtained via wet casting were investigated. No phase separation in mixtures was observed even athigh polymer concentration studied (up to 10wt%). Mixture zero-shear rate viscosity plotted as a func-tion of composition followed the mixing law, allowing concluding, in the first approximation, on theabsence of any special interactions between the components. Hybrid films from corn starchecellulosemixtures were obtained by coagulation in water and in ethanol. FT-IR analysis of dry films confirmedthe absence of any new bonds formed between the components, and XRD showed a significant decreaseof crystallinity. When coagulated in ethanol, starch was trapped in 3D cellulose network; when coag-ulated in water starch was partially leached out, creating pores and channels. The morphology of freeze-dried wet films showed that pore size can be tuned by altering mixture composition and varyingcoagulation bath. Wet film water permeability was from 10 to 60 L m�2 s�1 bar�1.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Being renewable and biodegradable, cellulose, the most abun-dant polysaccharide, is an important natural resource for makingvarious materials. For example, cellulosic films and membranes arebiocompatible and of high chemical stability. Blending with othercomponents allows bringing additional functionalities and varyingmaterial properties and morphology. Starch, as well as its variousderivatives, is a very interesting candidate for blending with cellu-lose, opening new routes for making multifunctional films, mem-branes and templates in food, pharmaceutical, biomedical andwatertreatment applications. However, the limited amount of cellulosesolvents have always been a challenge not only for cellulose pro-cessing and performing chemical modifications, but also forblending with other polymers.

Very few common solvents have been reported for mixingcellulose with other polysaccharides and then making “hybrid”membranes, beads, or fibers via dissolution-mixing-coagulationprocess. One common solvent is alkali-based: aqueous NaOH

; fax: þ33 (0)4 92 38 97 52.fr (T. Budtova).of Excellence (EPNOE), www.

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solution. It dissolves cellulose, but solutions are gelling even ifcellulose is pre-treated or in the presence of additives such as urea,thiourea, or zinc oxide [1e3]. Another disadvantage of NaOH-wateris that it does not dissolve cellulose at concentrations higher thanw7e8wt% [4,5] and of DP higher than 500e600, which obviously isan obstacle for making materials with good mechanical properties.With this solvent, cellulose and celluloseestarch blendedmembranes/films have been prepared, but cellulose concentrationand molecular weight was always restricted [6e9]. The secondsolvent is N-methylmorpholine-N-oxide monohydrate (NMMO),which is the only one used for making man-made cellulose fibers(Lyocell technology) besides viscose. Lyocell process is environ-mental friendly, has a closed solvent recovery system, and providesmore processing options than alkali-based solvents [10]. Thedisadvantages of NMMO as cellulose solvent are the relatively highenergy consumption needed for dissolution and the complexity ofhandling solutions which are sensible to water and not thermallystable [11]. It was recently demonstrated that it is possible todissolve starch in NMMOmonohydrate [12], but we did not find anyliterature reporting on blending starch with cellulose using NMMOas a common solvent. Finally, imidazolium based ionic liquids (ILs)is a family of solvents to efficiently dissolve relatively larger amountof cellulose, up to 20wt% and of high molecular weight [13]. It wasrecently reported that starch can also be dissolved in the same ionicliquids as cellulose, e.g., 1-butyl-3-methylimidazolium chloride

W. Liu, T. Budtova / Polymer 53 (2012) 5779e57875780

(BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl)[14]. Kadokawa et al. successfully used BMIMCl as a commonsolvent to blend cellulose and starch [15]. We have demonstratedthat it is possible to dissolve waxy corn starch in 1-ethyl-3-methylimidazolium acetate (EMIMAc) and reported starch flowand hydrodynamic properties in this solvent [16]. The commer-cialized EMIMAc has very lowvapor pressure, high thermal stabilityand a relatively low viscosity at room temperature which enableseasy cellulose dissolution and thus makes this solvent an attractivemedium for blending polysaccharides.

The goal of this work is to understand the properties ofcelluloseestarch mixtures in ionic liquid, EMIMAc, and to charac-terize the morphology of materials made from these blends via wetcasting. We start with a brief presentation of starcheEMIMAcsolution properties. Then we perform a rheological study ofcelluloseestarcheEMIMAcmixtures of various compositions and atdifferent temperatures. We check if the mixing rule can be appliedand discuss if there are any special interactions between twopolysaccharides in solution. Wet films of various compositionswere prepared via the dissolution-mixing-coagulation process.Films were characterized with FTIR, XRD, SEM and tested for waterpermeability, in order to understand structureeproperties rela-tionships. Cellulose films prepared with the same method werealso studied for comparison.

2. Experimental

2.1. Materials

Avicel� PH-101 microcrystalline cellulose (“cellulose” in thefollowing) purchased from SigmaeAldrich was used. It is a purified,partially depolymerized a-cellulose, with a mean degree of poly-merization (DP) of 180, as given by the manufacturer. We used thislow DP cellulose because its solution properties in ionic liquids hadalready been extensively studied (see, for example [17e20],) andthose results will be used here. Waxy corn starch (amylose-free,“starch” in the following) was also from SigmaeAldrich. Ionicliquid, 1-ethyl-3-methylimidazolium acetate (EMIMAc) ofpurity � 90% was provided by BASF. Both polysaccharides weredried before use in vacuum oven at 50 �C for 1 h to remove waterresidues. EMIMAc was used as received without furtherpurification.

2.2. Methods

2.2.1. Making solutions and mixturesPolysaccharideeEMIMAc solutions of different concentrations

were prepared by mixing polymer and solvent in a sealed reactionvessel filled with nitrogen at 80 �C for 24e48 h to ensure completedissolution. CelluloseestarcheEMIMAc mixtures were prepared bymixing ready solutions in various proportions and stirring underthe same condition for additional 8 h. All concentrations are inwt%.

2.2.2. Making filmsFilms from cellulose and cellulose/starch mixtures were

prepared via dissolution-mixing-coagulation route. Few millilitersof solution or mixture was placed on a Petri dish and a thin layerwas obtained using a spin-coater SPIN150-NPP (SPS, theNetherlands) at 300e400 RPM for 120 s. The polymer was thencoagulated in distilled water or in ethanol followed by washing insuccessive baths at room temperature until a stable conductivityof the bath. Bath volume was 10 times higher than solutionvolume, thus theoretically all EMIMAc was washed out. Theobtained wet films were either freeze-dried in liquid nitrogen tobe used for SEM observation, or dried between two clipped glass

slides in vacuum oven at 70 �C for 24 h for XRD and FTIR tests.Because it was not possible to obtain pure starch films withouta plasticizer, to perform FTIR and XRD analysis starch was simplyprecipitated in ethanol, washed repeatedly and then dried in thesame conditions as described above.

2.2.3. Rheological measurementsWe performed the rheological measurements on a Bohlin

Gemini� 150 rheometer (Malvern Instruments, UK) with a Peltierplate for temperature control. A cone-plate measuring systemwith4� angle and 40 mm diameter was used. Steady state viscosity wasrecorded at shear rates from 0.01 to 500 s�1 at constant tempera-ture which was varied from 10 �C to 100 �C with 10 �C step. Siliconeoil (DC 200, SigmaeAldrich) was placed around the edge of thecone to prevent water absorption. The oil had a much lowerviscosity (h20 �C ¼ 9.5 mPa s) than all studied systems and was notmiscible with polysaccharide solutions. A solvent trap covering themeasuring cell and wrapped with Parafilm� (Pechiney PlasticPackaging, US) was also used to protect solutions from waterabsorption from the air.

2.2.4. X-ray diffractionX-ray diffraction was performed on X’Pert PRO (PANanalytical,

Netherlands) with the following conditions: Cu Ka ¼ 1.5406 Å,operating at 30 mA and 40 kV, and 2q varied from 6� to 50�.Samples for X-ray diffraction were initial polysaccharides (Avicelmicrocrystalline cellulose and waxy corn starch granules), drycoagulated cellulose and starch and celluloseestarch filmsprepared via wet casting.

2.2.5. Fourier transform infrared spectroscopyFourier transformed infrared spectroscopy (FTIR) was per-

formed with a PerkinElmer Spectrum�. One spectrometer in therange of 4000e400 cm�1 with 8 scans. The resolution was 4 cm�1.The samples studied were the same as described in the previoussection. About 150 mg sample was milled andmixed with 1 mg KBrand transformed into pellets.

2.2.6. Scanning electron microscopeThe PHILIPS XL30 SEM was set in environmental mode, with

back-scattered electron (BSE) detector, and the acceleration voltageat 15 kV. Samples were placed on a Peltier device at �8 �C, and thepressure was maintained at 0.4e1.0 mbar in the vacuum chamber.Before being placed in the measuring chamber, all wet filmsunderwent freeze-dried treatment: they were frozen in liquidnitrogen and immediately transferred to the vacuum chamber; asa result the liquid in the pores (water or ethanol) was sublimated.This treatment preserved the three-dimensional morphology of thewet films.

2.2.7. Weight loss during coagulationIn order to estimate material weight loss from films during

coagulation due to starch (amylopectin) partial leaching-out causedby its dissolution in water, the total weight of polymer in the filmwas compared with the weight known from the preparation, Wini.Filmswere dried in a vacuum oven at 70 �C for 24 h, their weightWf

was measured and weight loss W% calculated as follows:

W% ¼ 100%��Wini �Wf

�.Wini (1)

2.2.8. Permeability measurementsWater permeability of wet films was evaluated using experi-

mental set-up of X-Flow (Enschede, the Netherlands). A round film

W. Liu, T. Budtova / Polymer 53 (2012) 5779e5787 5781

of diameter 74 mm was placed into the measuring chamber. Thepermeability, K, was calculated according to Equation (2), in whichV is the volume of permeate in L, t is flow time in hour, P is appliedpressure in bar, and S is the film surface in m2. The measurementswere carried out with pressure from 1 to 6 bar at roomtemperature.

K ¼ Vt � P � S

(2)

3. Results and discussion

3.1. Properties of starchecelluloseeEMIMAc mixtures in the liquidstate: flow and mixture viscosity

In order to understand the behavior of starchecelluloseeEMIMAc mixtures, the flow of the individual components shouldbe first evaluated. A detailed study and discussion of amylopectin-EMIMAc and microcrystalline celluloseeEMIMAc solution proper-ties in dilute and semi-dilute regimes (flow, viscosity-concentrationdependence, intrinsic viscosity as a function of temperature, etc.)have been reported in refs [16e18]. Here we shall show only somestarcheEMIMAc flow properties that are needed for the under-standing of the viscosity of starchecelluloseeEMIMAc mixtures.

An example of the flow of 5% starcheEMIMAc solution atdifferent temperatures is shown in Fig. 1. Similar results were ob-tained for other polymer concentrations. The solutions are shear-thinning with a Newtonian plateau at low shear rates. Theviscosity-shear rate dependence at a fixed temperature wasapproximated by Carreau-Yasuda equation [21]:

h�g��� hN

h0 � hN¼

h1þ

�lg��ain�1

a(3)

where h0 is zero-shear rate viscosity, hðg�Þ is viscosity measured ata given shear rate, hN is solvent viscosity, l is the relaxation time, nis power law index and a is the fitting parameter. Zero shear rateviscosity values will be used in the following analysis of starchecelluloseeEMIMAc mixtures.

0.1

1

10

100

0.01 0.1 1 10 100 1000

shear rate, s-1

, Pa.s

10°C

30°C

50°C

70°C

90°C

η

Fig. 1. Viscosity-shear rate dependence for 5% amylopectin-EMIMAc solution atdifferent temperatures. Solid lines are the best fits to Eq. (3). Errors are smaller or equalto the size of symbols.

CelluloseeEMIMAc solution viscosity at various concentrationsand temperatures was also measured as a function of shear rate.Contrary to starcheEMIMAc solution, 5% celluloseeEMIMAc flowwas Newtonianwithin a few decades of shear rates at temperaturesfrom 0 �C to 100 �C. These results are not shown here as far as verysimilar to the ones that have been previously reported inRefs. [17,18].

An example of the flow of 1:1 ¼ cellulose:starch mixtures inEMIMAc with total polymer concentration 5% at different temper-atures is shown in Fig. 2. Mixtures were shear thinning; theviscosity-shear rate dependence was successfully fitted with Eq. (2)(solid lines in Fig. 2). A comparison of the flow of the initialcomponents with their mixtures, at a fixed temperature, is given inFig. 3. It seems that mixture is behaving simply as a “sum” of thecomponents, with the relaxation times corresponding to the onesof each individual polymer. This is the first indication that there areno specific interactions between two polymers.

In order to check, in the first approximation, if there are anyspecial interactions between different mixed macromolecules,a simple mixing rule was applied for zero shear rate viscosities.Because polymer concentration in the mixture is above the overlapconcentration of each polymer (for amylopectin it varies from0.8wt% to 1 wt% [16] and for cellulose from w1wt% to w2wt%[17,18] for temperatures from 20� to 100 �C, respectively), a log-additive dependence was used:

ln hmix ¼ F1ln h1 þ F2ln h2 (4)

where hmix is the calculated viscosity, F1 and F2 are the weightfractions of each component in themixturewithF1þF2¼1, and h1and h2 are the viscosities of each component at F1 ¼ 1 and F2 ¼ 1,respectively. Roughly, the comparison between the experimentaland calculated viscosity allows concluding on the formation of newstructures in solution. If experimental values of viscosity are lowerthan the calculated ones, this should indicate that new “compact”systems (interpolymer complexes) are formed. If the experimentalviscosities are higher than the calculated ones, the components aremaking “loose gel-like” or “branched” structures with loops anddangling ends.

The experimental and calculated values of mixture zero shearrate viscosity as a function of composition (here, as a function of

0.1

1

10

100

0.1 1 10 100 1000

shear rate, s-1

η, Pa.s

20°C

40°C

60°C

80°C

100°C

Fig. 2. Viscosity-shear rate dependence for 1:1 ¼ cellulose:starch mixture, total poly-mer concentration 5%, at different temperatures. Solid lines are the best fits to Eq. (3).

1

10

100

0.01 0.1 1 10 100 1000

shear rate, s-1

η, Pa.s

1

4

3

2

Fig. 3. Viscosity-shear rate dependence of 5% starcheEMIMAc (1), 5% celluloseeEMI-MAc (4) and their mixtures starch:cellulose ¼ 4:1 (2) and 1:1 (3) at 30 �C.

W. Liu, T. Budtova / Polymer 53 (2012) 5779e57875782

cellulose weight fraction Fcell) at different temperatures from 20 �Cto 100 �C is shown in Fig. 4. With the increase of celluloseconcentration in the mixture, the viscosity smoothly decreases, asfar as celluloseeEMIMAc solution zero shear rate viscosity is lowerthan the one of starcheEMIMAc. The calculated additive viscositycoincides with the experimental values indicating that there are nospecial interactions leading to the formation of any new celluloseeamylopectin structures. These two polysaccharides coexist in thecommon solvent without any phase separation as far as totalpolymer concentration used, 5%, is far from the limit of theirdissolution in ionic liquid: the maximum solubility is not reportedin literature for these polymers, but it is known that it is possible to

-3

-1

1

3

5

0 0.2 0.4 0.6 0.8 1

cell

ln η0

20°C

40°C

60°C

80°C

100°C

Φ

Fig. 4. Viscosity vs. mixture composition at different temperatures; total polymerconcentration is 5%. Symbols are experimental data; lines are calculated according toEq. (4).

dissolve cellulose up to 20% in EMIMAc [13,22], and corn starch upto 15% in 1-butyl-3-methylimidazolium chloride, as well as 19%amylose can be dissolved in 1-allyl-3-methyl imidazole formate[23,24]. Celluloseestarch mixtures of 10wt% total polymer concen-tration that we prepared for making films were also not phaseseparating. As it will be shown in the following sections, celluloseand starch phase separate when coagulated in a non-solvent.

Zero-shear rate viscosity-temperature dependence of mixturesis presented as Arrhenius plot in Fig. 5 where T is temperature in Kand R the universal gas constant. The experimental dependences oflnh0 vs.1/RTcan be considered as linear (with a reasonable standarddeviation of 0.985), despite a slightly concave shape. The non-lineardependence of lnh0 on 1/RT is due to EMIMAc specific temperaturebehavior as demonstrated in refs [17,18] for celluloseeEMIMAcsolutions. Vogel-Fulcher-Tamman (VFT) approach has been shownto fit better the experimental data; however, here, in the firstapproximation, we shall use the classical Arrhenius approachh w exp (Ea/RT), where Ea is the activation energy.

The values and the dependence of the activation energy onmixture composition are shown in Fig. 6. Within the experimentalerrors, Ea linearly varies as a function of mixture composition,which is another indirect proof of the absence of any specialinteractions between the components.

3.2. Properties of starchecellulose films

3.2.1. FTIRIn order to conclude on the absence of any new chemical bonds

formed between cellulose and amylopectin mixed and coagulatedfrom a common solvent, FTIR spectra of the initial components,their coagulated counterparts, and films made from the mixturewere recorded.

First, FTIR spectra of native and coagulated cellulose (Fig. 7)were compared. During the dissolution-coagulation process(transformation of cellulose I to cellulose II), IL broke the hydrogenbonds of cellulose molecules, and increased the content of freehydroxyl group, resulting in the reduction of the OeH stretching

-3

-1

1

3

0.31 0.33 0.35 0.37 0.39 0.41 0.43

1/RT, mol/kJ

ln η0

12345

Fig. 5. Mixture zero-shear rate viscosity as a function of inverse temperature forvarious compositions, with total polymer concentration 5%: 5%starcheEMIMAc (1),4:1 ¼ starch:cellulose (2), 1:1 ¼ starch:cellulose (3), 1:4 ¼ starch:cellulose (4) and 5%celluloseeEMIMAc (5). Lines are Arrhenius linear fits.

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

Φcell

Ea,kJ/mol

Fig. 6. Activation energy of starchecelluloseeEMIMAc mixtures as a function ofcomposition. Total polymer concentration is 5%. Dashed line is a linear fit.

W. Liu, T. Budtova / Polymer 53 (2012) 5779e5787 5783

peak from 3374 cm�1e3408 cm�1. The peak of CeOeC antisym-metrical stretching was also relocated from 1170 cm�1e1158 cm�1.CeH stretching peak was also affected and slightly shifted (from2900 cm�1e2892 cm�1), also CH2 scissoring peak was shifted from1432 cm�1e1420 cm�1, indicating the split of intramolecularhydrogen bond concerning O at C6. The CeOH skeletal vibrationpeak at 1112 cm�1 wasmasked due to the strong band at 1068 cm�1

on the spectra of coagulated cellulose. The above changes of thepeaks are consistent with the results reported for the FTIR spectraof native and coagulated cellulose from other solvents, such asNaOH/thiourea [2], AMIMCl [25] and BMIMCl [26]. As suggested byNelson and O’Connor [27], the intensity ratio of CeH bending peak

8001600240032004000

Wavenumber (cm-1

)

1

2

3

4

Fig. 7. FTIR spectra of native microcrystalline cellulose (1), coagulated cellulose (2),native amylopectin (3) and 1:1 ¼ cellulose:starch film (4), the spectrum of coagulatedamylopectin was not shown to avoid overloading the graph, it was identical to nativeamylopectin within the experimental errors.

at 1376 cm�1 and CeH stretching peak at 2900 cm�1 is linearlyproportional to the crystallinity of cellulose. In our case, this ratiovaried from 0.61 to 0.41 for native and coagulated cellulose,respectively, indicating a decrease of crystallinity from cellulose I tocellulose II. This trend was also confirmed by the XRD analysis asdescribed in the next section.

The spectra of native and coagulated amylopectin were prac-tically identical within the experimental errors: characteristictriplet peaks were at 1158 cm�1, 1082 cm�1, and 1014 cm�1 asreported in Ref. [28]. FTIR spectra of the blends were a superpo-sition of the spectra of neat components. As far as both are verysimilar, the only conclusion that can be made is that when mixedno new strong bonds between amylopectin and cellulose wereformed.

3.2.2. X-ray diffractionFig. 8 shows the XRD spectra of the initial and coagulated

polysaccharides, and of their blend 1:1 ¼ cellulose:starch obtainedfrom coagulated in water 7.5%cellulosee7.5%starcheEMIMAcsolution (total polymer concentration in solution 15%). Microcrys-talline cellulose (Fig. 8, spectrum 1) shows classical diffractionpeaks characteristic to cellulose I at 2q ¼ 14.8�, 16.3�, and 22.6�

[2,25,29]. After the dissolution in EMIMAc and coagulation, cellu-lose II polymorph shows three characteristic diffraction peaks at2q ¼ 12.1�, 19.8�, and 22.0� (Fig. 8, 2), also previously reported forcellulose dissolved in EMIMAc [29]. The smaller area underdiffraction peaks of coagulated cellulose compared to the nativeone indicates a reduced crystallinity after dissolution and coagu-lation, as expected.

Filmsmade from dissolved amylopectinwithout a plasticizer areknown to be completely amorphous [30,31], which is confirmed bythe spectrum of coagulated starch (Fig. 8, 4). The spectrum ofcellulose:starch ¼ 1:1 film (Fig. 8, 5) is a superposition of thespectra of neat coagulated polymers and reflects an amorphousstructure. Two characteristic diffraction peaks of cellulose II at 19.8�

and 22.0� are “hidden” under the amorphous halo of coagulatedstarch.

3.2.3. Loss of starch during coagulation and washingWhen coagulated in water or in ethanol from solution in ionic

liquid, starch becomes amorphous, forms flocks and precipitates,

10 15 20 25 30

2 θ , °

1

2

5

4

3

Fig. 8. XRD spectra of native microcrystalline cellulose (1), cellulose II (2), nativeamylopectin (3), coagulated amylopectin (4) and 1:1 ¼ cellulose:starch film (5).

W. Liu, T. Budtova / Polymer 53 (2012) 5779e57875784

but does not make a 3D network structure as coagulated cellulosedoes. Amylopectin becomes partially soluble in water. A similarphenomenon, i.e. fast dispersion in water of amylopectin filmsprepared via starch gelatinization-dissolution in water withsubsequent drying, has been reported [31]. Thus it is very difficult,if impossible, to make amylopectin films via wet casting fromEMIMAc. The mechanical properties of celluloseeamylopectin wetfilms with high starch content are also poor. In the following weshall thus focus on films made from celluloseestarchmixtures withcellulose being in the major phase.

We assume that when the dissolved in EMIMAc celluloseestarch mixtures were coagulated in water, amylopectin partiallyleached out because of solubility in water and the absence of“cohesion” between cellulose and starch molecules. If coagulatingstarchecellulose mixtures in ethanol, amylopectin should betrapped in cellulose network. The hypothesis of amylopectinleaching out during coagulation and washing in water was checkedby measuring material weight loss, as described in Section 2.2.7.Fig. 9 shows the weight loss W% as a function of the theoreticalstarch concentration in the film, Cstarch, theor, supposing totalmaterial conservation. The same measurement was performed forfilms obtained in ethanol coagulation bath.

The loss of starch during coagulation in water was confirmed:for example, about 35% of amylopectin was leached from1:1 ¼ cellulose:starch mixture. As a result, starch concentration inthe dry filmwas about 30% instead of 50%. The inset of Fig. 9 showsreal starch concentration in the film, Cstarch, real, recalculatedconsidering the weight loss, as a function of the theoretical starchconcentration in the film. When coagulating in ethanol, starch wasnot leaching out and thus real starch concentration in the filmcoincides with the bisector of Cstarch, real vs Cstarch, theor which cor-responds to no weight loss. We may expect that depending on thetype of coagulation bath and mixture composition, films withvarious morphologies and porosities can be prepared. This will bediscussed in the next section.

3.2.4. Films morphology as probed with the scanning electronmicroscope

Cellulose films and films obtained from celluloseestarch blendscoagulated inwater (Figs. 10 and 11) and in ethanol (Figs. 12 and 13)were analyzed using SEM. Three compositions were selected: 100%cellulose (“a” images in Figs. 10e13), cellulose:starch ¼ 2:1 (“b”

0%

10%

20%

30%

40%

0% 10% 20% 30% 40% 50%

W,%

ethanol

water

Cstarch, theor

, wt%

0

10

20

30

40

50

0 10 20 30 40 50

Cstarch, theor

, wt%

Cstarch, real

wt%

Fig. 9. Starch weight loss as a function of the theoretical starch concentration in thefilm. Inset: real starch concentration in the film vs theoretical starch concentration. Seemore details in the text.

images) and 1:1 (“c” images). The initial total polymer concentra-tion in EMIMAc was 10%. The morphology of surface (Figs. 10 and12) and cross-section (Figs. 11 and 13) of freeze-dried wet (neverdried) films is compared and discussed below.

The first striking difference between cellulose and celluloseestarch films observed for all series of samples is the increasedheterogeneity, roughness and porosity of films made frommixtures, seen both for surface and cross-section (compare all “a”images corresponding to cellulose films on Figs. 10e13 with “b” and“c” images corresponding to hybrid films). Amylopectin and cellu-lose coexist when dissolved in EMIMAc, but phase separate duringcoagulation, with cellulose making a 3D network and starch beingtrapped in it or partially washed out (when coagulated in water).This is one of the reasons of celluloseestarch films heterogeneousmorphology.

Another significant difference between cellulose and celluloseestarch films is the influence of coagulation bath, water vs. ethanol,on sample morphology. For coagulated cellulose films, themorphology in water is similar to the one in ethanol (compareFig. 10a vs Fig. 12a and Fig. 11a vs Fig. 13a). On the contrary,celluloseestarch films coagulated in water are of much higherporosity as compared with the ones coagulated in ethanol; this isvalid for both surface and cross-section observations (for example,compare Fig.10b vs Fig.12b, Fig.11b vs Fig.13b). As demonstrated inthe previous section, amylopectin is leaching out into water duringcoagulation and washing. This creates large pores and channelsseen on the surface and even larger pores seen on the cross-section.Samples obtained via coagulation in water are of open porosity.

With the increase of starch content in the mixture (“b” imagesfor 2:1 ¼ cellulose:starch mixtures vs “c” images for 1:1 mixtures),larger amount of starch is “lost” when coagulated in water (seeFig. 9). A similar trend was observed for films prepared fromsolutions with total polysaccharides concentration of 15% (notshown). Pores in 1:1 ¼ cellulose:starch films are thus larger than inthe one of 2:1 ¼ cellulose:starch films (see Fig. 11b vs c). In reality,starch concentration in these films is 33% instead of 50% (1:1mixture) and 29% instead of 33% (2:1 mixture) because of amylo-pectin leaching out during coagulation and washing in water.

The surface of cellulose and celluloseestarch films obtained viacoagulation in water and in ethanol is quite dense and smooth(Figs. 10a and 12a). The formation of such a rather dense skin isknown for cellulose dissolved in other solvents, for example, in N-methylmorpholine-N-oxide monohydrate (NMMO) and coagulatedfrom the hot solutions [32,33]. This morphology may result fromthe increase of cellulose concentration stimulated by rapid solventdepletion on the surface of the sample during coagulation. Similarobservations were reported for cellulose membranes regeneratedfrom cuoxam [34]. The cross-section of wet cellulose films (Figs. 11and 13) displays a rather homogeneous porous structure, with poresize of about fewmicrons for cellulose coagulated inwater (Fig.11a)and smaller pores for cellulose coagulated in ethanol (Fig. 13a). Themechanism of cellulose phase separation during coagulation fromsolutions, either hot celluloseeNMMO or celluloseeEMIMAc, wasshown to proceed via spinodal decomposition [33,35]. In thesecases regular slightly “hairy” beads of about 0.5e1 micron areformed, as shown in detailed studies in Refs. [33,35]. The beads areorganized in a “network” for low cellulose concentrations of 3-5wt% used to make aerogel-like materials, so-called aerocellulose, fromeither celluloseeNMMO or celluloseeEMIMAc [35]. In cellulosefilms of higher cellulose concentrations, 10e12wt%, the observa-tions made with environmental SEM showed a “globular texturemade by the juxtaposition of small spheres” [33]. As far as there areno new bonds formed between cellulose and starch, we supposethat when coagulating from EMIMAc solutions both polymers arephase separating via spinodal decomposition.

Fig. 11. Cross-section of the same films as in Fig. 10.

Fig. 10. SEM images of films surface prepared from cellulose (a), cellulose:starch ¼ 2:1 (b), and 1:1 (c) mixtures coagulated in water. The initial polymer concentration in EMIMAcwas 10%. The scale bar is 20 mm.

W. Liu, T. Budtova / Polymer 53 (2012) 5779e5787 5785

3.2.5. PermeabilityWater permeability of wet (never dried) cellulose and

celluloseestarch films was measured using a set-up described inSection 2.2.8. As far as the permeability of cellulose and celluloseestarch films obtained from EMIMAc wet-casting had never beenreported, we shall first present our experimental findings, and thencompare them with known results for cellulose films made fromother solvents.

First, the influence of pressure on water permeability wasinvestigated. An example for a film prepared from cellulose dis-solved in EMIMAc and coagulated in water is shown in Fig. 14. Thefilm was subjected to several cycles of consecutively step-wiseincreasing pressure: first, from 1 to 6 bar, then immediately from2 to 6 bar and again 2 bar. Then there was a 45 min interval for thefilm to relax before the permeability at 2 bar was measured again.Permeability dropped with increasing pressure and did not fullyrecover when a new cycle was started immediately (Fig. 14). Thedecrease of permeability is most probably due to the flexibility ofwalls of the 3D network of coagulated cellulose, making the overallsample rather soft and compressible. The loss of permeability

Fig. 12. SEM images of films surface prepared from cellulose (a), cellulose:starch ¼ 2:1 (b), awas 10%. The scale bar is 20 mm.

turned out to be partially reversible: it was recovered after a 45minrelaxation between the last two tests.

In order to exclude the influence of film compressibility, thepermeability offilms of different compositions prepared in differentconditions (coagulation in ethanol and in water) was performed atthe same fixed applied pressure. Fig. 15 shows the results for cellu-lose coagulated in water and in ethanol, and for 3:1 ¼ celluloseestarch films coagulated in ethanol. In all samples the total polymerconcentration in solutionwas 10%. The permeability of cellulosefilmcoagulated in water is twice lower that the one coagulated inethanol, at any pressure studied. A similar result, 30 vs.150 L m�2 s�1 bar�1, was reported for cellulose films prepared viadissolution inNaOH-urea-water [7]. For this alkali solvent the size ofthe pores in films coagulated in ethanol is almost twice higher thatthe size when coagulated water, which can be the reason of higherpermeability. In general, the permeability of our cellulose filmsprepared via dissolution in EMIMAc followed by coagulation inwater or ethanol falls in the same range as the values previouslyreported for cellulose dissolved in alkali solvents and coagulated invarious aqueous solutions, from10 to80Lm�2 s�1 bar�1 [1,6,34]. The

nd 1:1 (c) mixtures coagulated in ethanol. The initial polymer concentration in EMIMAc

Fig. 13. Cross-section of the same films as in Fig. 12.

0

10

20

30

40

1 2 4 6 2 4 6 2 2pressure, bar

permeability, L/(m2

.h.bar)

45 minutes

between the

last two tests

Fig. 14. Permeability of cellulose films coagulated from 10% celluloseeEMIMAc solu-tion in water, with 20% errors. See more details in the text.

0

10

20

30

40

50

60

70

80

1 2 4 6Pressure, bar

cell (coag. water)

cell (coag. ethanol)

cell:starch=3:1 (coag. ethanol)

Permeability, L.m-2

.h-1

.bar-1

Fig. 15. Permeability of cellulose film coagulated in water and ethanol and of hybridfilm based on 3:1 ¼ cellulose:starch coagulated in ethanol, with 20% errors. Totalpolymer concentration in EMIMAc solution was 10%.

W. Liu, T. Budtova / Polymer 53 (2012) 5779e57875786

permeability of cellulose films from EMIMAc, coagulated in waterand equilibrated under pressure (4e6 bar) is very similar to theconventional ultrafiltration membranes prepared from cellulosedissolved in cuprammonium (10e15 L m�2 s�1 bar�1 [36],). Whencoagulated in ethanol, the permeability of cellulose films from ionicliquid is close to the one made from cellulose dissolved in NMMO(70e80 L m�2 s�1 bar�1, [37]).

A few tests were performed on checking the molecular weightcut-off of wet (never dried) films made from cellulose coagulated inwater from 10%celluloseeEMIMAc solution. Very low rejection of2000 kDa dextran, within 10%, was obtained. The flow rate of 1 g/Ldextran solution was practically the same as that of water. Exceptsome special cases when the dissolved matter is adsorbed on cellu-lose surface, we suppose that wet cellulose films may be used for theseparation from undissolved solid particles (i.e. in microfiltration).

The permeability of celluloseestarch films coagulated in ethanoldecreases with the increase of applied pressure from 1 to 2 bar,similar to what was obtained for cellulose films (Fig. 15). This is notsurprising: the major phase forming the film is cellulose, which is“pressure-sensitive”. However, further pressure increase does notdecrease celluloseestarch film permeability: at 2, 4 and 6 bar it ispractically the same, contrary to a continuous permeabilitydecrease for cellulose films. Starch, being blocked in cellulosematrix, is “reinforcing” it. The permeability of celluloseestarchfilms coagulated in ethanol is slightly lower than that of theircounterpart, cellulose coagulated in ethanol, despite the presenceof some rather large pores in hybrid films (see Fig. 13“a” vs “b” and“c”). The reason can be again the same: starch is located in the poresin coagulated cellulose and thus the permeability is decreased.

4. Conclusions

Cellulose and starch were mixed in the common solvent, ionicliquid. The polymers are not phase separating at the concentrationsstudied (total polymer concentration in the mixture was 10% andlower); mixing in ionic liquid allows processing and modificationsof cellulose and starch at the same time. The analysis of mixtureflow and zero shear rate viscosity suggested the absence of anyspecial interactions between cellulose and starch.

Celluloseestarch films were obtained viawet casting. Water andethanol were used as coagulating liquids. FTIR confirmed that nonew bonds were formed between cellulose and amylopectin. XRDshowed a strong decrease of crystallinity in all coagulatedmaterialsand their mixtures as compared with the native components.Amylopectin was partially leaching out inwater during coagulationand washing steps because of dissolution. On the contrary, whencoagulated in ethanol, starch was trapped in the 3D cellulosenetwork.

The morphology of the films obtained depends on starchcontent in the mixture. While cellulose films have a smooth surfaceand rather homogeneous porous inner structure, celluloseestarch

W. Liu, T. Budtova / Polymer 53 (2012) 5779e5787 5787

films are heterogeneous, with rough surface and large pores.Cellulose films showed dense skin and porous core morphology,similar to what was reported for cellulose dissolved in NMMOmonohydrate. When coagulated in water, celluloseestarch filmsare of open porosity, with large pores and channels due to starchpartial leaching during coagulation.

Finally, the permeability of cellulose and celluloseestarch filmswas evaluated. It is similar to the one reported for cellulose filmsobtained via dissolution in alkali solvents and in NMMO mono-hydrate. Because cellulose pore walls are soft and elastic, perme-ability depends on the applied pressure, in particular for cellulosefilms. Celluloseestarch films are less pressure-sensitive because ofstarch trapped in cellulose network.

The results obtained demonstrate the possibility of mixing twomost abundant polysaccharides, cellulose and starch, in a commonsolvent, ionic liquid, in a large range of concentrations and com-positions, and making materials with tuned morphology. Thisopens new ways of making multifunctional films, separationdevices and templates with controlled properties.

Acknowledgments

The research leading to these results has received funding fromthe European Union Seventh Framework Program (FP7/2007-2013)under grant agreement No 214653, project “SurFunCell”. We thankSuzanne Jacomet for the help with SEM, Gabriel Monge for XRD,Romain Sescousse for testing cellulose membranes molecularweight cut-off in the laboratory of Volker Ribitsch (University ofGraz, Austria) and Prof. Song Guilan of University of Jinan (China)for FTIRmeasurements. We gratefully acknowledge Dr. Jens Potreckand Stefan Koel from X-Flow, the Netherlands, for their help withthe water permeability test. We should like to thank John Mitchell,University of Nottingham (UK), for a fruitful discussion.

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