biobased ﬁlms prepared from naoh/thiourea aqueous solution ... · springer science+business media...

Biobased films prepared from NaOH/thiourea aqueoussolution of chitosan and linter cellulose

D. L. Morgado • E. Frollini • A. Castellan •

D. S. Rosa • V. Coma

Received: 1 September 2010 / Accepted: 11 February 2011 / Published online: 8 March 2011! Springer Science+Business Media B.V. 2011

Abstract In the present study, films based on linter

cellulose and chitosan were prepared using anaqueous solution of sodium hydroxide (NaOH)/thio-

urea as the solvent system. The dissolution process of

cellulose and chitosan in NaOH/thiourea aqueoussolution was followed by the partial chain depoly-

merization of both biopolymers, which facilitatestheir solubilization. Biobased films with different

chitosan/cellulose ratios were then elaborated by a

casting method and subsequent solvent evaporation.They were characterized by X-ray analysis, scanning

electron microscopy (SEM), atomic force microscopy

(AFM), thermal analysis, and tests related to tensile

strength and biodegradation properties. The SEM

images of the biofilms with 50/50 and 60/40 ratio ofchitosan/cellulose showed surfaces more wrinkled

than the others. The AFM images indicated that

higher the content of chitosan in the biobasedcomposite film, higher is the average roughness

value. It was inferred through thermal analysis thatthe thermal stability was affected by the presence of

chitosan in the films; the initial temperature of

decomposition was shifted to lower levels in thepresence of chitosan. Results from the tests for tensile

strength indicated that the blending of cellulose and

chitosan improved the mechanical properties of thefilms and that an increase in chitosan content led to

production of films with higher tensile strength and

percentage of elongation. The degradation study in asimulated soil showed that the higher the crystallin-

ity, the lower is the biodegradation rate.

Keywords Biobased composites ! Linter cellulose !Chitosan ! NaOH/thiourea aqueous solution

Introduction

A rapid progress has been observed in the develop-

ment and commercialization of biobased materials,which can reduce the widespread reliance on

petroleum because they are attractive from both

D. L. Morgado ! A. Castellan ! V. Coma (&)Laboratoire de Chimie des Polymeres Organiques,IPB/ENSCBP, Universite de Bordeaux, 16 avenuePey-Berland, 33607 Pessac Cedex, Francee-mail: [email protected]

D. L. Morgado ! A. Castellan ! V. ComaCentre National de la Recherche Scientifique,Laboratoire de Chimie des Polymeres Organiques,33607 Pessac Cedex, France

D. L. Morgado ! E. Frollini (&)Instituto de Quımica de Sao Carlos, Universidade SaoPaulo, Caixa Postal 780, Sao Carlos,Sao Paulo 13560-970, Brazile-mail: [email protected]

D. S. RosaUniversidade Federal do ABC, Santo Andre,Sao Paulo 09210-170, Brazil

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Cellulose (2011) 18:699–712

DOI 10.1007/s10570-011-9516-0

environmental and economic points of view (Moh-anty et al. 2002). In this context, there is a substantial

interest regarding both the comprehension of the

properties of cellulose and its application, due to itsimportance as a renewable and biodegradable

material.

Due to its crystallinity, cellulose cannot be solu-bilized in conventional solvents (Jin et al. 2007), and

extensive efforts have been directed toward intensi-

fying studies on solvent systems already known, inaddition to identifying new cellulose solvents (Ramos

et al. 2005; Ass et al. 2006; Ciacco et al. 2008, 2010;

Qi et al. 2008; Liu and Zhang 2009; Quan et al.2010). At the same time, research on films based on

cellulose has received great attention (Nishino et al.

2004; Park et al. 1993; Nayak et al. 2008; Liu andZhang 2009; Retegi et al. 2010), the aim being the

development of materials with good mechanical

properties, chemical stability, and biological compat-ibility (Kamide and Iijima 1994; Zhang et al. 1995,

2005; Nues and Peinemann 2001), among other

properties.In studies related to the dissolution of cellulose

and/or preparation of films, solvents systems based on

aqueous solutions of NaOH/urea or NaOH/thioureahave been considered. Many researches have been

carried out to understand the action of urea and

thiourea in alkaline aqueous solutions, consideringtheir ability to increase cellulose solubility (Las-

zkiewicz and Cuculo 1993; Isogai and Atalla 1998;

Zhang et al. 2002; Qi et al. 2008; Liu and Zhang2009). The advantages of these solvents systems are

that they are less expensive and less toxic for the

environment than other solvents normally used suchas N-methylmorpholine-N-oxide (NMMO), lithium

chloride/N,N-dimethylacetamide or ionic liquids (Jin

et al. 2007). The NMMO/H2O system seems the mostpowerful in attaining exceedingly high concentrations

solutions, however, it requires high temperature for

dissolution (Zhang et al. 2010). The dissolutionof polymers at low temperature as for the system

NaOH/thiourea or NaOH/urea, can be considered asfriendly-environmentally process, because of the

avoidance of evaporation of the chemical agents

during the process (Lue et al. 2007). It is well-knownthat the dissolution of cellulose in such solvent

depends on the temperature. These authors reported

that water is not a simple liquid and it exists as acluster. A stable inclusion complex associated with

NaOH, thiourea, water cluster and cellulose thatcould occur at low temperature. In other words, this

inclusion complex is hosted by thiourea and NaOH,

in which cellulose chain associated with NaOHhydrates as guest is encaged. The NaOH ‘‘hydrates’’

could be thus more easily attracted to cellulose chains

through the formation of new H-bonded network atlow temperature (Cai et al. (2008). Yan et al. (2007)

investigated the morphology of the surface films

obtained in a NaOH/thiourea aqueous solution ofcellulose, and the results showed a homogeneous

distribution of NaOH and thiourea around the cellu-

lose fibers, which could lead to synergic interactionsthat can influence the dissolution process of cellulose.

In spite of the considerable interest devoted to

finding appropriate conditions to solubilize celluloseand derive other materials from it, biodegradable

products based on cellulose have some disadvantages,

such as poor mechanical properties, when they arecompared to thermoplastic polymers (Liu 2006).

However, cellulose has other advantages resulting

from its sustainability and worldwide availability. Thepossibility of associating cellulose with other biopoly-

mers—also obtained from renewable resources—for

film elaboration for instance, should be thereforeconsidered. In this direction, the possibility of creation

of biobased films from cellulose and chitosan is one of

the possible strategies (Almeida et al. 2010).Chitin [(1-4)-linked-2-acetamido-2-deoxy-b-D-

glucan] is found in crustaceous shells or in cell walls

of various fungi and algae. Industrial applicationsof chitin are relatively less, although it is available on

a large scale at low cost, because chitin is insolu-

ble in most solvents (Brugnerotto et al. 2001). Thebiopolymer chitosan is the deacetylated form

[(1–4)-linked-2-amino-2-deoxy-b-D-glucan, after 100%deacetylation] of natural chitin. Chitosan and itsderivatives have been widely studied as biobased

materials for various applications, owing to the

favorable properties of this natural polymer (Kweonand Kang 1999; Coma et al. 2002; Zhang et al. 2002;

Wu et al. 2004; Trindade Neto et al. 2005; Coma2008; Almeida et al. 2010).

There are some structural differences to account

for the different properties and behaviors of celluloseand chitosan. In the case of chitosan, instead of the

hydroxyl group at C2, there are amino or acetamido

groups in each pyranose ring, depending on thedegree of deacetylation. These amino groups have a

700 Cellulose (2011) 18:699–712

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pKa value of *6.5, which leads to a protonation inacidic solutions with a charge density dependent on

the pH and deacetylation degree. This makes chitosan

soluble in acidic conditions (i.e. acetic, lactic, adipic,formic, malic, malonic, propionic, pyruvic, succinic

acids). The possibility of strong intra- and intermo-

lecular hydrogen bonds both in cellulose and chitosancan favor and enhance their compatibility (Guan et al.

1998), resulting in favorable interactions and forma-

tion of miscible polymeric blends (Hasegawa et al.1994).

Blending of polymers may offer a simple and

relatively inexpensive method to develop new mate-rials with a number of valuable properties. For

example, a brittle polymer mixed with a rubbery

polymer may result in a material with good rigidityand toughness (Luo et al. 2008). Chitosan already

showed excellent association ability with polyvinyl

alcohol (Hasegawa et al. 1992). The blend, whichmay be used for dialysis applications (Hasegawa

et al. 1992), reached the maximum value of mechan-

ical strength with 15–30% of chitosan content (Limaet al. 2005). In recent years, some efforts have been

made toward the study of chitosan–cellulose blends.

Twu et al. (2003) used N-methylmorpholine-N-oxideas the solvent to prepare chitosan and cellulose

blends with potential applications for odor treatment

and metal-ion adsorption. Phisalaphong and Jatupai-boon (2008) prepared films from cellulose and low-

molecular-weight chitosan. They showed that the

presence of 0.75% (w/v) chitosan yielded films withsmaller pore diameter and higher surface area,

compared to the bacterial cellulose films.

To create new films based on biopolymer-blends,an approach may be the use of solvent systems

suitable for the dissolution of both biopolymers.

NaOH/thiourea based solvent is an appropriate sys-tem in the case of sisal cellulose and chitosan

(Almeida et al. 2010). It is well-known that cellulose

dissolution depends on its structural characteristics asits degree of polymerization (DP), crystallinity (Ic),

as well as of the characteristics of ‘‘micro-, macro-pores’’ present in its structure. In a previous study, it

was found that microcrystalline cellulose dissolved

easily in lithium chloride/dimethylacetamide (LiCl/DMAc) due to a combination of its small molar mass,

and presence of large pores in its supra-molecular

structure (Ramos et al. 2005). However, cotton-linters could be completely dissolved only after the

reduction of both Ic and crystallite size, besides themodification of its pore size distribution, which were

achieved through mercerization. Probably because

sisal has Ic and crystallite size smaller than those ofcotton-linters, besides macro-pores in its supra-

molecular structure, unmercerized sisal dissolved in

LiCl/DMAc (Ramos et al. 2005). In addition, cellu-loses obtained from lignocellulosics fibers, e.g. sisal,

are normaly associated with hemicellulose, which

also influences the dissolution process (Ciacco et al.2010). Thus, the characteristics of the starting

cellulose influence the process of its dissolution, not

only in LiCl/DMAc, but also in other solventsystems, which in turn has influence on the processes

carried out from these solutions, whether a derivati-

zation or preparation of films.In this context, in previous studies (Ramos et al.

2005; Ciacco et al. 2008, 2010), different celluloses

have been subjected to the same process, in order toevaluate how the characteristics of the starting

cellulose influences the course of the process, and

then the properties of the final product. It can beconsidered that this is an important point in studies

involving cellulose, and hence in the expansion of its

use and application.It should still be noted that when the process under

investigation consists of preparing a blend-based

film, such as in this study, besides the previouslymentioned factors, starting celluloses with different

characteristics, e.g. molecular weight, presence of

hemicellulose, may develop interactions with differ-ent intensities with the other component (in this case,

chitosan), which in turn can lead to materials with

different properties.To preserve renewability and biodegradability and

to improve the mechanical resistance of the final

films, associations between linter cellulose andchitosan were investigated. Compared to our previous

study based on the association between sisal cellulose

and chitosan (Almeida et al. 2010) and following ourapproach of evaluating the performance of celluloses

with different characteristics in the same process, thepresent work assessed how the different proportions

of chitosan and cotton-linter cellulose, may affect the

morphology and mechanical properties of filmsprepared from a NaOH/thiourea aqueous solution.

The biobased films were characterized by X-ray

diffraction, scanning electron microscopy (SEM),atomic force microscopy (AFM), thermal analysis

Cellulose (2011) 18:699–712 701

123

[thermogravimetry (TG) and differential scanningcalorimetry (DSC)], and tests related to tensile

strength and biodegradation properties.

Experimental

Materials

Cotton linters with low average molar mass (used inthe textile industry), kindly provided by Industria

Fibra S/A, Americana, Sao Paulo, Brazil, were used

as the cellullose raw material. Chitosan-244 waskindly provided by France Chitine, Marseille, France.

Characterization of cellulose and chitosan

Degree of polymerization

The degree of polymerization (DP) of cellulose was

determined by measuring the viscosity (at 25 "C)of cotton-linter cellulose solution in 0.5 M cuprie-thylenediamine, using an Ostwald shear-dilution

Cannon–Fenske viscometer coupled to a thermostatic

bath and circulator (Masterline Forma Scientific,Marietta, OH), following TAPPI standard T230

om-04. The average molar mass is given by Mv =

DP 9 162 (TAPPI PRESS 2004).The DP of chitosan, the same previously used, was

determined as already described (Almeida et al.

2010). The intrinsic viscosity [g] (milliliters pergram) and the Huggins constants were calculated

from a plot gsp/c versus concentration ‘‘c’’ of AcONa

0.2 M/AcOH 0.3 M chitosan solutions. The capillaryviscosimetry was carried out in an Ubbelohde-type

viscometer at 25 "C on the concentration series.

Considering the degree of acetylation (DA, deter-mined as follows in this text) the parameters K and a

(Mark-Houwink parameters) were selected from

Rinaudo et al.(1993), namely, 0.074 and 0.76,respectively, which corresponds to the DA value

closest to the determined in the present study.

Degree of acetylation

The degree of acetylation (DA) of chitosan was deter-

mined by proton nuclear magnetic resonance (1H-NMR)

on a Bruker AC-200 spectrometer, running at200 MHz at 80 "C. Before analysis, 10 mg of chitosan

was dissolved in D2O/HCl (100/1, v/v) and stirred for24 h at room temperature before NMR analysis.

TheDAvaluewas calculated using the ratio between

the area corresponding to the proton resonance of theacetamide methyl groups (ACH3

, d = 2.00 ppm) and

the corresponding resonance for the proton linked to

the C2 of glycosamine ring (AH2, d = 3.12 ppm):

%DA " ACH3=3AH2

# $ % 100 #1$

Elaboration of films

Cellulose films

Films were elaborated according to the method

described by Cai et al. (2004), with some modifica-tions. Briefly, a mixture of an aqueous solution was

prepared by directly mixing NaOH/thiourea/H2O in

the ratio 5.0/6.0/89.0 (w/w/w). The desired amount ofcellulose (3 g) was added to 97 g of solvent at room

temperature under vigorous stirring for 5 min. To

improve dissolution, the solution was then stored at4 "C for 12 h. The solution was subjected to centri-

fugation at 8,000 rpm for 20 min at 10 "C to exclude

the very low content of undissolved part (e.g.chitosan impurities) left over and to carry out the

degasification. The resulting transparent solution was

immediately cast onto a polypropylene plate to get afilm with a final thickness of about 10–20 lm (dry).

The films were air-dried at room temperature for

18 h. The resulting films were washed with runningwater until the pH was equal to 6.5 and were finally

dried at 20 "C under 65% relative humidity (RH) for

2 days. The materials thus obtained were subse-quently conditioned at 23 ± 1 "C and 50 ± 5% RH

for 5 days before analyses.

Chitosan films

Chitosan films were elaborated by adding 1.5 gchitosan to 98.5 g NaOH/thiourea solution, as

described elsewhere (Almeida et al. 2010). The

solution was stirred for 5 min before following thesame steps as mentioned for linter-cellulose films.

Biobased chitosan/cellulose films

An aqueous solution containing NaOH/thiourea (5/6,

w/w) was used as the solvent system for preparation

702 Cellulose (2011) 18:699–712

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of chitosan/cellulose films of ratios 50/50, 60/40,70/30, 80/20, and 90/10. The solutions of cellulose

and chitosan were separately prepared and com-

pletely dissolved within 5 min. The chitosan/cellu-lose films were prepared using the same conditions

described for the pure polysaccharide-based films.

Characterization of films

Crystallinity index (Ic)

X-Ray diffraction analysis was used to evaluate the

crystallinity index (Ic) of films and the originalsamples of cellulose and chitosan. The diffractogram

patterns were obtained in a VEB CARL ZEISS-JENA

URD-6 Universal Diffractometer operating withCuKa (k = 1.5406 A) generated at 40 kV and

20 mA (Ramos et al. 2005; De Paula et al. 2008;

Ciacco et al. 2008, 2010; Ass et al. 2006). Thecrystallinity index (Ic) was calculated as previously

described by Buschle-Diller and Zeronian (1992):

Ic " 1& Imin=Imax# $ #2$

where, Imin is the minimum intensity, attributed to the

noncrystalline region of the sample, and Imax is themaximum intensity, attributed to the crystalline

region of the sample.

Sulfur and sodium determination

Aiming at to verify the possible presence of residuesof the components of the solvent system (thiourea/

NaOH), the films and the original samples of

cellulose and chitosan were characterized by Induc-tively Coupled Plasma Atomic Emission Spectros-

copy (ICP-AES, from Spectro, ARCOS-SOP, sulfur

analysis) and by atomic absorption (Varian, modeloAA 240 FS, sodium analysis).

Morphological surface analysis

SEM studies Film surfaces and sectional viewswere examined by SEM using a LEO 440 ZEISS/

LEICA model, operating at 15 kV with a tungsten-

filament electron source and scanned at roomtemperature. Samples were metalized with gold

(Ramos et al. 2005; Ciacco et al. 2008, 2010; Ass

et al. 2006).

AFM studies To better understand the surface

topography, films were analyzed by AFM. AFM

measurements were carried out using a NanoscopeIII-A microscope (VEECO Digital Instruments) in the

intermittent contact mode in air at room temperature,

using silicon cantilevers with a resonance frequencyclose to 190 kHz. The image size was 10 9 10 lm2,

with a resolution of 512 9 512 pixels. The images

were only height-corrected after measurements. Theroot-mean-square (RMS) can be considered as the

standard deviation of the distribution of surface

heights, corresponding to an important parameter todescribe the surface roughness (Gadelmawla et al.

2002). RMS processing and determination of the

(RMS) values were carried out using the Gwyddionsoftware of the AFM microscope.

Thermal analyses

TG and DSC studies TG measurements of films

made with cellulose, chitosan, and chitosan/cellulosemixtures were carried out using a Shimadzu TGA-50

instrument. Approximately 8.0 mg of sample was

weighed on a platinum pan and heated from 25 to600 "C at 20 "C min-1 under a flow of nitrogen at a

rate of 20 mL min-1.

DSC analyses were carried out with a ShimadzuDSC-50 WSI instrument using approximately 5.0 mg

of material (sample on an aluminum pan) under a

flow rate of 20 mL min-1 of nitrogen. The sampleswere heated from 25 to 450 "C at a heating rate of

20 "C min-1.

Mechanical properties

Tensile strength (in MPa) and elongation-at-breakpoint (Eb, %) were evaluated with a Dynamic

mechanical analyzer (DMA) model 2980, operating

in tension film mode (23 ± 1 "C). A force ramp rateof 1 N min–1 was applied until 18 N or failure. At

least five repetitions were carried out for each type of

film. Biobased films, 6.3 mm in width, 10 mm ingauge length and thickness (lm, assessed from SEM

images): 50/50: 18.5 ± 1.5; 60/40: 13 ± 1; 70/30:

23 ± 1; 80/20: 13 ± 2; 90/10: 12 ± 1, were used forthe tests. The standard deviations (Table 3) were

calculated from using Student’s t test at 95%

probability.

Cellulose (2011) 18:699–712 703

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Potential biodegradability

Biodegradation experiments were carried out byageing the films under simulated conditions (Megiatto

et al. 2008). Samples were weighed and buried in

simulated soil containing 23% loamy silt, 23% organicmatter (cow manure), 23% sand, and 31% distilled

water (w/w). Biodegradation was monitored every

30 days for 180 days by measuring mass retention.Periodically, the samples were washed with distilled

water, dried, weighed, and then returned to the flask

containing the simulated soil. The standard deviations,calculated from using Student’s t test at 95% proba-

bility (three repetitions), have varied from ±1 to ±5.

Results and discussion

Characterization of cellulose and chitosan

An aqueous solution of sodium hydroxide (NaOH)containing thiourea dissolves cellulose more easily than

that containing urea (Zhang et al. 2002). Consequently,

NaOH was selected as the sole solvent systems forcellulose and chitosan association. The first part of this

study is thus the determination of the impact of the

solvent system on linter cellulose and chitosan.During the course of dissolution of both biopoly-

mers using NaOH/thiourea as solvent systems, these

polysaccharides are exposed to variable conditions.The dissolution mechanism in aqueous NaOH/thio-

urea solution depends mainly on the destruction of

the hydrogen bonding (El-Wakil and Hassan 2008).The average molar masses of cotton-linter cellulose

both before and after dissolution inNaOH/thioureawere

measured; the values were 66,200 and 40,500 g mol-1,respectively. This result showed that the dissolutionwas

followed by a decrease in the average molar mass of

cellulose. Lue et al. (2007) showed that the presence ofNaOH mainly cleaves the close chain packing

of cellulose through the formation of new hydrogen-

bonding between cellulose and the small molecules inthe solvent. For cotton-linter cellulose, the decrease

is less drastic than that for sisal cellulose,

118,200–18,500 g mol-1, taking into account theresults of Almeida et al. (2010). In the same previous

study, we showed that, for chitosan also, a decrease of

the average molar mass was observed (22,000–6,500;Almeida et al. 2010). Thus, under the conditions used,

the ease of dissolution of cellulose and chitosan in

NaOH/thiourea can also be related to the depolymer-ization of the polysaccharidic chains in this solvent

system. The important differences between sisal-pulp

cellulose and cotton-linter cellulose are attributed to theless-crystalline form of cellulose polymer in sisal

(crystallinity index, Ic = 54; Almeida et al. 2010) than

the form in cotton (Ic = 79, Table 1).The DA of chitosan was determined by 1H-NMR

spectrometry. As in our previous study (Almeida et al.

2010), the average DA values of chitosan before andafter dissolution were 30 and 26%, respectively, indi-

cating that dissolution of chitosan in NaOH/thiourea led

to a slight deacetylation of chitosan, probably due tosome reaction of the acetamido groups with hydroxyl

anions.

Characterization of the films

Crystallinity index (Ic)

The Ic values of cellulose, chitosan, and chitosan/cellulose films are listed in Table 1. Original cellu-

lose and chitosan were also analyzed. The X-ray

diffraction patterns of the films (figures not shown)showed an amorphous–crystalline structure.

Dissolution processes should result in rearrange-

ment of the crystal packing from that of cellulose I(before dissolution) to cellulose II (after dissolution)

and film elaboration, due to the presence of NaOH in

the solvent system; however, the crystallinity waspractically not affected from the starting stage of

cellulose to the final form of the film. These results are

in accordance with the study of Chen et al. (2006),which was conducted on cotton-linter cellulose.

Table 1 Crystallinity index (Ic) of cellulose, chitosan, cellu-lose film, chitosan film, and biobased chitosan/cellulose films

Sample Ic (%)

Cellulose 79

Cellulose film 77

Chitosan 64

Chitosan film 57

50/50 Chitosan/cellulose film 76





704 Cellulose (2011) 18:699–712

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Dissolution of chitosan in aqueous solution ofNaOH/thiourea and, further, production of the film

led to a decrease in crystallinity. The decrease in both

average molecular weight and DA of chitosan duringdissolution, as mentioned previously, can lead to

different arrangements of the chains, compared to

the raw material cellulose. The crystallinity of thebiobased chitosan/cellulose films was close to or

higher than that of chitosan film, but no correlation

was observed between the Ic values and content ofchitosan and/or cellulose.

Sulfur and sodium determination

To verify the possible presence of residual solvent in

the films, the contents of sulfur (from thiourea) andsodium (from NaOH) were evaluated by Inductively

Coupled Plasma Atomic Emission Spectroscopy and

atomic absorption, respectively. The starting materi-als, cellulose and chitosan (control samples), were

initially analyzed. Sulfur was not found in the controlsamples, as expected, the same occurring for all the

films (cellulose, chitosan, chitosan/cellulose).

The content of sodium in cellulose film was thesame as that for the control sample (0.15 ± 0.02%)

and in chitosan film increased 0.02%, when compared

to the control sample (0.14 ± 0.01%). The chitosan/cellulose films presented contents of sodium ranging

from 0.14 ± 0.01 to 0.17 ± 0.04%. Considering the

errors, all the values were very close, and this can beconsidered as an indication that there was no residual

solvent in the films.

Morphological surface analysis

SEM studies The cellulose, chitosan, and chitosan/cellulose films were analyzed by SEM to investigate

their supramolecular aspects. The SEM images were

collected from both surface and sectional views(Figs. 1, 2).

Fig. 1 SEM micrographs (sectional views) of a cellulose; b chitosan; and chitosan/cellulose films of ratios c 50/50, d 60/40, e 70/30,f 80/20, and g 90/10

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The images show that the cellulose film has a

heterogeneous structure in the sectional view,

whereas the chitosan film displayed a relativelysmooth morphology. In the mixed chitosan/cellulose

films, most of the films showed a rugged surface in

the transversal cut, and the 70/30 and 50/50 ratios ofchitosan/cellulose materials appear to be less uni-

form. Figure 2 shows the SEM images of the surfaces

of cellulose, chitosan, and biobased films.Chitosan film has a more homogeneous surface

compared to the cellulose film, and the surfaces of

chitosan/cellulose films appear smooth initially(Fig. 2). At higher magnification (Fig. 3) of chito-

san/cellulose films, for the 60/40 and 70/30 percent

ratios, there are some structures suggesting that fibersare present. This is in favor of interactions of chains,

leading to a separation of domains in the mixed films.

AFM studies AFM imaging has advantage overSEM because of the higher resolution and absence of

damage due to electron beams. Figure 4 shows the

topographic and three-dimensional AFM images of

chitosan and biobased chitosan/cellulose films, in

addition to their corresponding RMS surface-roughness values. The pure cellulose film was not

analyzed because this technique requires a regular

surface, and this is not the case in this type of film.The AFM images (Fig. 4a) show that the surface

of the chitosan film is more homogeneous compared

to those of chitosan/cellulose films (Fig. 4b–d). TheAFM and SEM results are in very close agreement

(Figs. 2, 3, 4). The chitosan/cellulose films are

rougher than the chitosan film (0.21 lm). The 90/10film (chitosan/cellulose) shows a greater asperity

thickness (1.2 lm) compared to the others (Table 2).

The results of theRMS surface-roughness values forchitosan and chitosan/cellulose films indicate amarked

influence of the cellulose content in the films, which

alters their roughness values. This influence is moresignificant starting from 40% of cellulose content.

When cellulose chains are present, the interactions

between them can form aggregates, which can give

Fig. 2 SEM micrographs (surface views) of a cellulose; b chitosan; and chitosan/cellulose films of ratio c 50/50, d 60/40, e 70/30,f 80/20, and g 90/10

706 Cellulose (2011) 18:699–712

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rise to the fiber structures in the films. When lowercontents of cellulose are present, it appears that the

interactions between cellulose/cellulose chains are

more favorable than those between cellulose/chito-san, generating fiber structures under higher exten-

sion, when the solvent is eliminated.

Thermal analyses

TG and DSC results Figure 5 shows the TG andDTG curves of cellulose and 50/50 chitosan/cellulose

films. The TG and DTG curves of the other films

(figures not shown) showed the same general

Fig. 3 SEM micrographs (surface views) of chitosan/cellulose films of different ratios: a 50/50, b 60/40, c 70/30, d 80/20, ande 90/10

Fig. 4 AFM images of the topographic and three-dimensionalstructures obtained for a chitosan film; and biobased chitosan/cellulose films of ratios b 50/50; c 60/40; d 70/30; e 80/20 and

f 90/10 (10- 9 10 lm2); the root mean square (RMS)roughness values of the films are also shown

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behavior. The chitosan film was prepared under theconditions as described by Almeida et al. (2010) and

has properties similar to those described in the

thermal analysis section.As shown in Fig. 5, the films show two stages of

weight loss: the first one between room temperature

and 110 "C, related to the evaporation of residualabsorbed water (Trindade Neto et al. 2005; Szczes-

niak et al. 2008). The second step is related to the

thermal decomposition of the polymer chain, withvaporization of the volatile compounds (aldehydes,

ketones, furans, and pyrans). For cellulose, this step

involves dehydration and leads to anhydrocellulose,which can result from either inter- or intraring

dehydration (Scheirs et al. 2001).

According to the results obtained by TG analysis,the cotton-linter cellulose film shows a higher

maximum temperature (Tp = 361 "C, Fig. 5a) com-

pared to the sisal-cellulose film (Tp = 278 "C;

Almeida et al. 2010). The biobased 50/50 chitosan/cellulose film shows an intermediate maximum decom-

position temperature (Tp = 311 "C, Fig. 5b), when

compared to the maximum decomposition tempera-tures of cellulose films (Tp = 361 "C, Fig. 5a) and

chitosan films taking into account previous study

(Tp = 278 "C, Almeida et al. 2010). The less intensepeak observed at 482 "C for the film of chitosan/

cellulose (Fig. 5b) is probably a consequence of the

volatiles generated by the decomposition of thebyproducts produced in the previous stage.

Figure 6 shows the DSC curves for the cotton-

linter cellulose and the film with 50/50 chitosan/cotton-linter cellulose. The DSC curves of the other

films (figures not shown) showed the same general

behavior.In Fig. 6, the DSC curves of the films show peaks

between 220 and 380 "C, which are attributed to the

decomposition processes involving the main chainsof the polymers, as also observed by TGA

experiments.

The thermal decomposition of cellulose consists ofa series of reactions. In general, the thermolysis

reactions of cellulose occur by the cleavage of the

glycosidic bonds, C–H, C–O–C bonds, in addition tothe effects of dehydration, decarboxylation, and

decarbonylation. The pyrolysis of cellulose results

in the formation of levoglucosan, which is producedby the scission of the 1,4-glucosidic linkage in

cellulose, followed by the intramolecular rearrange-

ment of the monomer units (Li et al. 2001). Theinfluence of the cellulose source on the thermal

Table 2 Root mean square (RMS roughness) values and dataof asperity thickness of chitosan film and biobased chitosan/cellulose films

Films RMS(nm)

Asperitythickness (lm)

Chitosan film 24.0 0.21

50/50 Chitosan/cellulose film 42.2 0.37





Fig. 5 TG and DTG curves of the a cotton-linter cellulose and b chitosan/cellulose 50/50 films (N2-atmosphere: 20 mL min-1;heating rate: 20 "C min-1)

708 Cellulose (2011) 18:699–712

123

decomposition of cellulose films should additionallybe considered. The results showed that cotton-linter

cellulose film (exothermic peak at 376 "C, Fig. 6a) isthermally more stable than the sisal-cellulose film(exothermic peak at 305 "C; Almeida et al. 2010),

probably because of the presence of some remaining

hemicelluloses and the lower degree of crystallinityof sisal cellulose, thus shifting the thermal decom-

position of sisal cellulose to lower temperatures.

The DSC curve for the chitosan film obtained afterdissolution in NaOH/thiourea (Almeida et al. 2010)

showed two peaks, an endothermic peak at 225 "Cand an exothermic peak near 275 "C. These peaks areprobably related to the deacetylation of the chain and

to decompositions involving the main chain (Ou et al.

2008).The DSC curve related to the biobased 50/50

chitosan/cellulose (Fig. 6b) film shows an exothermic

peak near 310 "C (high intensity) and an endothermicpeak at 228 "C (low intensity), and they are related to

the decomposition processes. The shift of the exo-

thermic peak of the 50/50 chitosan/cellulose film,when compared to the same peak for the chitosan and

cellulose films, can be considered an indication thatthe interactions between chitosan and linter-cellulose

influenced the decomposition process of their

polysaccharides.

Mechanical properties

The maximal tensile strength (TS) and percentage of

elongation-at- break point (Eb) of biobased films were

determined from typical stress–strain curves and aresummarized in Table 3. The stress–strain curves of

chitosan and linter-cellulose films have not been

obtained, because the initial mechanical stress wassufficient to bring these films to fail. It is thus clear

that the biobased films show better mechanical

properties than the original polymers.These results show the impact of cellulose content

on both tensile strength and percentage of elongation

(Table 3). It appears likely that the increase of themechanical strength of chitosan/cellulose films is

closely connected with the formation of fiber struc-

tures at high extension, favored by a lower content ofcellulose, as already mentioned. These results suggest

that the enhancement of tensile strength may be a

consequence of the interactions between cellulosechains, which can lead to formation of aggregates and

then to micro- or nanofibers, which in turn can act as

reinforcers of the films. As showed by Eriksson et al.

Fig. 6 DSC curves of the a cotton-linter cellulose and b chitosan/cellulose 50/50 films (N2-atmosphere: 20 mL min-1; heating rate:20 "C min-1)

Table 3 Tensile strength (TS) and percentage of elongation-at-break point (Eb) of chitosan/cellulose films, followed by thestandard-deviations (SD) calculated from five repetitions

Films TS(MPa) ± SD

Eb

(%) ± SD

50/50 Chitosan/cellulose film 4.0 ± 1.0 0.76 ± 0.02





Cellulose (2011) 18:699–712 709

123

(2007), mechanical properties of composites weredependent not only on the filler/filler interaction, but

also on the quality of the dispersion. Wittaya (2009)

studied rice starch films reinforced with microcrys-talline cellulose from palm pressed fiber (MCPF).

This author showed that addition of MCPF higher

than 25%, resulted in a decreased elongation at break,which could be due to a decrease in homogeneity and

aggregate formation.

Potential biodegradability

Due to environmental concerns, biodegradable poly-mers constitute a newly emerging field. They have

attracted attention because of the positive environ-

mental impact when compared to synthetic polymers.Hence, for application in packaging films, selection

of biomaterials that show high biodegradation rate is

important to relieve environmental problems.The biodegradation behavior of films was ana-

lyzed by biological biodegradation in simulated soil.

Figure 7 shows the results obtained from biodegra-dation of films with different contents of chitosan and

cotton-linter cellulose.

After 180 days of exposure to microorganisms insimulated soil, the 60/40 chitosan/cellulose film

showed a higher biodegradation rate. This film had

the lowest crystallinity index value (Ic = 57%). Thereis a correlation between the biodegradation rate and the

crystallinity index of the films. This observation has to

be related to the fact that the rate of biodegradation is

associated with the noncrystalline regions of thesample, which are more accessible to water and

microorganisms. Enzyme hydrolysis, associated with

biodegradation, is mainly observed in amorphousregions (Ciolacu et al. 2008). The higher the crystal-

linity the lower is the biodegradation rate (Fig. 7).

Conclusion

Improvement of cotton-linter cellulose films by

association with chitosan was investigated in this

study. Cotton-linter cellulose was chosen for its highdegree of crystallinity, compared to sisal cellulose,

for example. It limited both depolymerization of

cellulose chains in NaOH/thiourea solutions and filmbiodegradability. SEM analysis of biobased films

indicated that their surfaces were quite wrinkled

when the chitosan/cellulose ratios were 50/50 and60/40. AFM analysis showed that the roughness

values increased with the chitosan content of the

blend. Blending cotton-linter cellulose with chitosanimproved the mechanical properties of films. A

higher content of chitosan improved the tensile

strength and elongation-at-break point of the films.This study confirmed that blending high-crystalline

cellulose with chitosan resulted in the formation of

films with promising properties.

Acknowledgments E. F. is grateful to CNPq (NationalResearch Council, Brazil) for a research productivityfellowship and financial support. The authors also thankFAPESP (The State of Sao Paulo Research Foundation,Brazil) for financial support and for the doctoral fellowshipawarded to D. L. Morgado.

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