Mechanically improved polyvinyl alcohol-composite films using modified cellulose nanowhiskers as nano-reinforcement

Carbohydrate Polymers 191 (2018) 25–34

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Mechanically improved polyvinyl alcohol-composite films using modified
cellulose nanowhiskers as nano-reinforcement

T

⁎

Cristiane Spagnola, Elizângela. H. Fragala, Maria A. Witta,b, Heveline D.M. Follmanna, ,
⁎
Rafael Silvaa, Adley F. Rubiraa,
a
b

Universidade Estadual de Maringá (UEM), Av. Colombo 5790, CEP 87020-900, Maringá, Paraná, Brazil
Pontifícia Universidade Católica do Paranỏ (PUCPR), Imaculada Conceiỗóo Street, CEP 80215-901, Curitiba, Paranỏ, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords:
Cellulose nanowhiskers
Chemical surface modification
PVA films

Nanocomposites
Mechanical properties

Cellulose nanowhiskers (CWs) extracted from cotton fibers were successfully modified with distinct anhydrides
structures and used as additives in poly(vinyl alcohol) (PVA) nanocomposite films. The surface modification of
CWs was performed with maleic, succinic, acetic or phthalic anhydride to compare the interaction and action the
carboxylic groups into PVA films and how these groups influence in mechanical properties of the nanocomposites. CWs presented a high degree of crystallinity and good dispersion in water, with average length at the
nanoscale. The addition of specific amounts (3, 6 and 9 wt.%) of modified-CWs increased up to 4.4 times the
storage modulus (PVA88-CWSA 9 wt.%), as observed from dynamic mechanical analysis (DMA), compared to
the bare PVA films. A significant increase in mechanical properties such as tensile strength, elastic modulus, and
elongation at break showed a close relationship to the amount and chemical surface characteristics of CWs
added, suggesting that these modified-CWs could be explored as reinforcement additives in PVA films.

1. Introduction
The use of cellulose nanowhiskers (CWs) in nanocomposites is a
promising research field related to the development of mechanically
responsive materials. In addition to the low cost of the raw material, the
use of cellulose particles as a reinforcement phase in nanocomposites
include advantages such as low density, low abrasiveness, and energy
consumption during processing, biodegradability, and a reactive surface that can be chemically modified by specific groups. However, the
high hygroscopic characteristic of unmodified CWs can result in poor
adhesion to non-polar polymeric matrices. Depending on the polymer
matrix used, the presence/addition of pure CWs can present some disadvantages, such as high water/moisture adsorption and poor adhesion
to non-polar polymer matrix caused by the polar differences/interactions of the CWs. Hence, chemical modification of CWs bearing specific
groups has been employed to tune phase to additive interaction/adhesion. (Abraham et al., 2016; Arjmandi, Hassan, Haafiz, & Zakaria,
2017; Fragal et al., 2017; Paralikar, Simonsen, & Lombardi, 2008;
Wang, Shankar, & Rhim, 2017).
Cellulose is the most abundant biopolymer, so a special attention
has been paid to its physicochemical properties, associating them to the
development/obtainment/production of higher value-added polymerbased materials from sustainable and renewable resources (de Melo, da

⁎

Silva Filho, Santana, & Airoldi, 2009; Dufresne, 2017; Follain, Marais,
Montanari, & Vignon, 2010; Kim & Kuga, 2001; Klemm, Heublein, Fink,
& Bohn, 2005). The repeating unit of cellulose (known as cellobiose) is
composed of two glucose molecules linked by β-1,4-glycosidic bonds.
The presence of six HO-groups in such structure permits intra and intermolecular hydrogen bond interactions, resulting in a strong tendency
for cellulose to form crystals completely insoluble in water and most
organic solvents (George & Sabapathi, 2015; Klemm et al., 2005; Silva,
Haraguchi, Muniz, & Rubira, 2009). However, it is possible to prepare
aqueous suspensions of these crystalline forms of cellulose (cellulose
whiskers) through acid hydrolysis. Because permeability is higher in the
amorphous phase, the kinetics of such hydrolysis is faster there than in
the crystalline region, so one can—under controlled conditions—destructs amorphous regions around and among cellulose
microfibrils while crystalline segments remain intact (Fragal et al.,
2016; Silva et al., 2009). In this sense, the hydrolysis using hydrochloric
acid can provide CWs with a minimum surface charge that allows for
the chemical grafting maintaining its initial morphology and the mechanical properties.
Cellulose chemical modification using cyclic anhydrides such as
succinic, maleic or phthalic anhydride provides ester-based surfaces
bearing carboxylic groups that can be additionally reacted (Klemm
et al., 2011; Liu et al., 2007). Some applications for the modified

Corresponding authors.
E-mail addresses: (H.D.M. Follmann), (A.F. Rubira).

/>Received 4 October 2017; Received in revised form 28 February 2018; Accepted 1 March 2018
Available online 03 March 2018
0144-8617/ © 2018 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 191 (2018) 25–34

C. Spagnol et al.

cellulose include the preparation of chelating materials for the adsorption of heavy metals and cations from aqueous solution (Gurgel &
Gil, 2009; Malik, Jain, & Yadav, 2016), the preparation of adsorbent
materials for the removal of dyes present in water (Fan, Liu, & Liu,
2010), and the fabrication of capacitive humidity sensors (Ducéré,
Bernès, & Lacabanne, 2005). Moreover, due to its excellent biocompatibility, CWs have been studied for applications in drug delivery
systems (de Oliveira Barud et al., 2016; Jackson et al., 2011), packaging
and barrier films (Siró & Plackett, 2010), filtration membranes (Cao,
Wang, Ding, Yu, & Sun, 2013; Ma, Burger, Hsiao, & Chu, 2011), medical
implants (de Oliveira Barud et al., 2016; Dugan, Collins, Gough, &
Eichhorn, 2013), and especially as reinforcement for polymer matrices
(Arjmandi et al., 2017; Cho & Park, 2011; Iyer & Torkelson, 2015;
Wang, Wang, & Shao, 2014). Also, tensile strength and Young's modulus of CWs are comparable to other engineered materials such as glass
fibers, carbon fiber and Kevlar 49® (Wang, Sain, & Oksman, 2007; Wang
& Chen, 2014), being promising options to increase mechanical properties of composites. For instance, the addition of CWs (0–15 wt.%) to a
PVA (88–98% of hydrolyzed groups) polymer matrix produced via
electrospinning increased up to 3 times the storage modulus of the final
material (Peresin, Habibi, Zoppe, Pawlak, & Rojas, 2010). Likewise,
other studies showed an increase in elasticity modulus of PVA nanocomposites containing progressive amounts of CWs (1, 3, 5 or 7 wt.%)
incorporated in the bulk matrix (Cho & Park, 2011).
Among different techniques explored to prepare nanocomposites
the solvent evaporation by casting has been the most used procedure to
incorporate aqueous suspensions containing cellulose whiskers into the
organic polymeric matrix (Habibi, Lucia, & Rojas, 2010). In this sense,
in order to obtain polymer/nanowhiskers nanocomposites with better

mechanical properties, one has to consider the dispersion process of the
cellulose nanowhiskers into the polymer matrix as a crucial step
(Habibi et al., 2010; Iyer, Flores, & Torkelson, 2015; Iyer, Schueneman,
& Torkelson, 2015; Iyer & Torkelson, 2015).
Poly(vinyl alcohol) (PVA) is a water-soluble hydrophilic polymer
with excellent film-forming property (Chiellini, Cinelli, Imam, & Mao,
2001; Pingan, Mengjun, Yanyan, & Ling, 2017), and due to its excellent
chemical resistance, physical properties, and biodegradability, it has
been used in a large number of industrial applications (Dai, Ou, Liu, &
Huang, 2017; Tao & Shivkumar, 2007). Besides, PVA uses in biomaterials have attracted considerable attention due to its biocompatibility
and biodegradability. The broad biomedical and pharmaceutical applications are due to non-toxicity, non-carcinogenic, bioadhesive and
hemocompatible, and ease of processing properties (Tao & Shivkumar,
2007). In fact, a review paper reported by Villanova et al. (Villanova,
Oréfice, & Cunha, 2010) featured PVA-based materials comprising hydrogels, contact lenses, dialysis membranes, membranes for the replacement of wounded tissues, artificial components of the organism
and controlled release of drugs.
In this study, cellulose-rich cotton fibers were used to obtain CWs
through acid hydrolysis. Chemical surface modification of the resulting
CWs was performed using distinct anhydrides in order to investigate
their influence on the crystalline structure and thermal stability. PVA
nanocomposite films containing modified-CWs (3–9 wt.%) were prepared by casting. The influence of the hydrolysis degree of PVA
(polymer matrix), and the amount of CWs (pure and modified) added,
on the mechanical properties of the final material were especially
evaluated through tensile strength (kPa), elastic modulus (kPa) and
elongation at break (%) measurements.

88% and 98% of hydrolyzed groups (Mw 13,000–23,000 g/mol), were
purchased from Sigma-Aldrich (USA). Hydrochloric acid (HCl) was
acquired from F. Maia (Cotia, Brazil), maleic anhydride, phthalic anhydride, sodium hydroxide from Vetec (Brazil), and N,N-dimethylacetamide (DMAC) from Nuclear (Brazil). All the reactants and solvents
were used as received without further purification.

2. Materials and methods

Nanocomposite films were prepared by initially mixing 100 mL of a
PVA solution (60 g/L) (88 or 98% of hydrolyzed groups) and 8 wt.% of
glycerol as plasticizer. Then, this mixture was stirred for 10 min and
distinct amounts of as-prepared CWs (3, 6 or 9 wt.%) were added with
the medium being kept under magnetic stirring for additional 15 min.
This suspension was sonicated for 2 min and transferred to a glass mold
(15 × 24 cm), and kept at 35 °C for 24 h so the nanocomposite films

2.2. Cellulose nanowhiskers selective extraction
Cellulose-rich cotton fibers (2 g) was immersed in a NaOH solution
(2% w/v) and kept under magnetic stirring for 1 h. This mixture was
poured into a glass vial containing distilled water (excess) and kept at
80 °C under magnetic stirring (for 1 h) until the material has a neutral
pH. The resulting cotton fibers were dried in a circulating air oven at
room temperature to constant weight.
To obtain the cellulose nanowhiskers (CWs), 1 g of “cleaned” cotton
fibers were hydrolyzed using concentrated HCl (20 mL, 37%) at 45 °C
for 1 h, under magnetic stirring. The resulting suspension was centrifuged (10.000 rpm) for 5 min and washed several times with distilled
water in order to remove the excess of acid (final pH ∼6–7). Then, the
final material was frozen and lyophilized.
2.3. CWs surface modification with maleic or succinic anhydride
8 g of maleic anhydride (MA) was added to a one-neck round flask
(50 mL) and kept at 120 °C until complete melting. 1 g of the as-prepared CWs was added and the medium was allowed to react for 24 h
under magnetic stirring. Then, 20 mL of dimethylacetamide (DMA) was
added to the mixture and stirred for 20 min so the unreacted anhydride
dissolves and can be removed from the reaction medium. It was filtered,
washed with distilled water and dried at 110 °C for 24 h. A similar
procedure was used to obtain modified-CWs with succinic anhydride,

with the melting temperature for succinic anhydride being adjusted to
130 °C. The modified-CWs were named as CWMA and CWSA, respectively.
2.4. CWs surface modification with acetic anhydride
A mixture consisting of acetic anhydride (AA) (10 mL) and CWs
(1 g) was added to a one-neck round flask (50 mL) and kept under
magnetic stirring at 110 °C for 24 h. The medium was filtered, washed
with distilled water and dried at 110 °C for 24 h. The modified-CWs was
named as CWAA.
2.5. CWs surface modification with phthalic anhydride
The present procedure was adapted from item 2.3. Here, a mixture
consisting of 9 g of melted phthalic anhydride (PA, 131 ° C), 8 mL of
DMA and 0.8 g of CWs was added to a one-neck round flask (50 mL) and
kept at 135 °C under magnetic stirring for 20 h. Then, extra 20 mL of
DMA was added to the mixture and stirred for 20 min so the unreacted
anhydride dissolves and can be removed from the reaction medium. It
was filtered, washed with distilled water and dried at 110 °C for 24 h.
The modified-CWs was named as CWPA. For the route of synthesis of
modified-CWs with all different anhydrides, see Supporting Information
(Fig. S1).
2.6. PVA/CWs nanocomposite films

2.1. Materials
Cellulose-rich cotton fibers were purchased from Cocamar
(Agroindustrial Cooperativa of Maringá, Brazil). Acetic anhydride,
succinic anhydride, glycerol (99%), and poly(vinyl alcohol) containing
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C. Spagnol et al.

would form by casting. As a control, a blank sample of PVA films with
glycerol was prepared by casting and no CWs added. Nanocomposite
films were labeled depending on the PVA hydrolyzation degree (88 or
98% of hydrolyzed groups) and the amount (wt.%) of modified-CWs
added. So, PVA88CW3 represents the formulation composed of PVA
(88% of hydrolyzed groups) and 3 wt.% of unmodified CWs, while
PVA88CWMA3 contains 3 wt.% of modified-CWs with maleic anhydride (MA) instead. For additional details as well as for the nanocomposite films prepared using PVA 98% of hydrolyzed groups, please
check Supporting Information (Table S1).

character at the same time that these groups can react to each other to
form covalent ester bonds, with no changes of the original CWs mechanical properties (Hakalahti, Salminen, Seppälä, Tammelin, &
Hänninen, 2015). Considering the hydrophilic nature of PVA, specific
chemical modification of originally insoluble CWs can improve interfacial compatibility between polymer matrix/modified-CWs enhancing
mechanical properties of the final composite (Rescignano et al., 2014).
The CWs are extracted from cellulose fibers through hydrolysis using
hydrochloric acid, which attacks the amorphous regions of cellulose
keeping its crystalline regions. Such process does not accumulate/add
surface charge over CWs, so eOH groups present on the cellulose
chemical structure can react with anhydrides (MA, SA, AA or PA). The
chemical modification steps to obtain the modified-CWs are described
in detail in Fig. S1 (Supporting information, SI).
From TEM and AFM images (Fig. 2(a) and (b)), it was possible to
observe that CWs extracted from cellulose-rich cotton fibers showed
defined form with elongated shapes, like needles. The average length
for CWs estimated from TEM images was 200 ± 63 nm. The presence
of CWs aggregates after hydrolysis using hydrochloric acid is expected
due to the high surface area responsible for hydrogen bonds and Van
der Waals interactions among the as-prepared nanowhiskers (Li, Chen,

& Wang, 2015), being same behavior is observed for modified-CWs
(Fig. S2). Combined with the fact that these CWs are free of surface
charges and so, present poor colloidal stability—different from hydrolysis using sulfuric acid that generates sulfate groups on CWs surface
(Araujo, Rubira, Asefa, & Silva, 2016). Fig. S3 shows the stability of the
suspensions of the pure and modified-CWs obtained in different solvents.
Chemical modification of CWs had the intention to tune surface
charge, colloidal stability, and matrix-to-additive compatibility.
Fig. 3(a) shows the FTIR spectra of pure (Fig. 3(a)-i) and modified-CWs
(Fig. 3(a)-ii to (a)-v) with the range from 2000 to 630 cm−1. The presence of bands at 1429, 1163, 1111 and 897 cm−1 in the spectra indicates that CWs are mainly in the form of Iβ cellulose (a crystalline
form of cellulose type I, monoclinic unit cell) (Leung et al., 2011).
Despite similarities among the anhydride structures used for the chemical modification procedures, each modified-CWs is discussed separately. The modified-CWs with maleic anhydride (CWMA) can be observed in Fig. 3(a)-ii that show intense bands at 1718 and 1734 cm−1
representing the coupling stretching of carboxyl and ester groups, respectively (Nishino, Matsuda, & Hirao, 2004). The bands at 1637 and
1235 cm−1 are assigned to ν(α,β C]C) and to ν(COeOH) present in the
CWMA structure (de Melo et al., 2009). At Fig. 3(a)-iii (CWSA) one can
observe the appearance of a band at 1740 and 1725 cm−1 related to
ester and carboxyl group stretching, respectively. The band at
1425 cm−1 is due to the coupling ν(C]O) and δ(OeH) groups (Chang
& Chang, 2001), while the band at 1160 cm−1 corresponds to ν(C]O)
and ν(O]CeOeR) stretching of ester segments. Fig. 3(a)-iv shows a
band at 1735 cm−1 related to the ester carbonyl group present in CWAA
structure (Braun & Dorgan, 2009), at 1232 cm−1 a band corresponding
to ν(CeCeO) stretching associated to acetate fragment and at
1370 cm−1 to the presence of eCeCH3 groups (Fan et al., 2010). Finally, Fig. 3(a)-v displays the spectrum of CWPA-modified nanowhiskers with a band at 1713 cm−1 associated to ν(C]O) stretching for
ester and carboxylic acid moieties. The bands at 1585, 1450 and
740 cm−1 correspond to eCeH deformation out of the ring plane and
assigned to aromatic vibrations. The band centered at 1270 cm−1 corresponds to ν(CeO) stretching characteristic of aromatic carboxylic
acid ester (de Melo, da Silva Filho, Santana, & Airoldi, 2010; Follmann
et al., 2016). When we took the whole range from 4000 to 630 cm−1
(data not shown) rather than just the more limited range in Fig. 3(a),
cellulose main chain shows one broad band between 3100 and

3600 cm−1 assigned to eOH stretching (Liu, Dong, Bhattacharyya, &
Sui, 2017), and the band between 3000 and 2800 cm−1 associated to
ν(CeH) stretching of methyl groups and symmetric and asymmetric
vibrations of eCH2 groups, respectively (Kloss et al., 2009; Shang et al.,

2.7. Characterization
2.7.1. Cellulose nanowhiskers characterization
CWs surface topography and morphology were examined through
Transmission Electron Microscopy (TEM) using a TOPCON 002B
equipment (accelerating voltage 200KV) and Atomic Force Microscopy
(AFM) using a Shimadzu SPM-9500J3 equipment, with AFM image of
81.70 nm. For TEM images, a dilute suspension of CWs was dropped on
an ultra-thin copper substrate (coated with a carbon thin film, 400
mesh) and allowed to dry at room temperature. For AFM images, CWs
suspension was previously sonicated in order to avoid aggregates. Then,
it was dropped/deposited on a freshly cleaved mica surface and allowed
to dry under vacuum.
Chemical modification of CWs was analyzed through Infrared
Spectroscopy with Fourier Transform (FTIR-ATR). The spectra were
acquired using a BOMEM Spectrometer (model MB-100) in the range
from 4000 to 630 cm−1, with a resolution of 4 cm−1 (32 scans).
Carbon-13 Nuclear Magnetic Resonance (13C NMR CP-MAS) also was
used to characterize the chemical modification of CWs. The spectra
were acquired using a Varian Mercury Plus BB 300, MHz spectrometer,
operating at 75.457 MHz for 13C (contact time of 3 ms, waiting time for
recycling (d1) and signal accumulation of 1024 repetitions).
Thermogravimetric (TG) analyses were obtained using a Shimadzu
TGA-50 Instrument. The measurement was acquired under a nitrogen
atmosphere, with a heating rate of 10 °C/min from room temperature to
600 °C. X-ray diffractograms (XRD) were obtained using a Shimadzu

(Model XRD-7000) diffractometer with a 40 kV voltage applied and a
current of 30 mA (radiation CuKα; α = 1.5418 Å), in the range of
2θ = 10–50° and a scanning speed of 2° min−1. The crystallinity index
was determined by the empirical method described by Segal et al.
(Segal, Creely, Martin, & Conrad, 1959).
2.7.2. Nanocomposite films characterization
The Dymanic-Mechanical Analysis (DMA) of the composites films
(25 mm × 7 mm) were characterized using a TA Instruments DMA
analyzer (model Q800), under tension module, according to ASTM
D5026-01. The mechanical properties of the films (50 mm × 10 mm)
were evaluated by tensile tests using a texturometer equipment (model
TA-XTplus-Texture Analyser, England), based on ASTM D882-10. The
obtained data were subjected to analysis of variance and mean Tukey
test at 5% probability by using the software aid StatView Version 5.0.1
(SAS Institute Inc. Cary, NC, USA). For all these measurements, the film
thickness was determined using a digital micrometer ( ± 0.001 mm)
from Mitutoyo, model IP65. Each film thickness value resulted from an
average of ten measures took from distinct areas of the sample (See
supporting information for further experimental details).
3. Results and discussion
An illustration scheme of PVA88 (88% of hydroxyl groups) composite films containing pre-determined amounts of the distinct nanoreinforcements (0, 3, 6 or 9 wt.% of modified-CWs) with the intention
to improve final mechanical response is shown in Fig. 1. It has been
reported that modified-CWs bearing groups such as hydroxyl (eOH)
and/or carboxyl (eCOOH) could increase cellulose hydrophilic
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C. Spagnol et al.

Fig. 1. Illustration scheme of PVA88/modified-CWs nanocomposite films containing distinct amounts of cellulose additives.

Fig. 2. (a) TEM and (b) AFM images of CWs extracted from cellulose-rich cotton fibers.

(shoulder peak) also decreases as modification reactions occur, suggesting a higher reactivity associate to the amorphous phases and the
surface of the biopolymer structures. The region between δ 69–75 ppm
refers to the C2, C3, and C5 carbon atoms of cellulose structure, while δ
105 ppm refers to C1. For the sample, CWMA (Fig. 3(b)-ii) the signal at
δ 166 ppm is related to the ester carbonyl group (C7 and C10), and the
signal between δ 125–132 ppm is related to α, β-unsaturated C9 and C8
from the anhydride moiety. Fig. 3(b)-iii still shows the spectrum of the
CWSA with a broad signal at δ 174 ppm related to the ester carbonyl
groups (C7 and C10) and eCH2 (C8 and C9) signal at δ 29 ppm. CWAA
spectrum, Fig. 3(b)-iv, shows characteristic signals at δ 172 ppm associated to the ester group (C7) and δ 20.6 ppm related to the methyl
group (C8). Last, at Fig. 3(b)-v one can observe CWPA spectrum with
signals at δ 184 and δ 173 ppm corresponding to a carboxyl group (C14)

2016). The FTIR spectra of PVA88 and PVA98 is in the Supporting
Information (Fig. S5).
Structure modification of the CWs was also evaluated through
13
CeCP/MAS NMR, as shown in Fig. 3(b) (See the individual spectra
with their structures in the Supporting Information, Fig. S4) The resonance signal at δ 65 ppm is associated with C6 at the crystalline phase
of cellulose and the less evident/discrete signal (shoulder peak) at δ
61.2 ppm is assigned to C6 at the amorphous phase of cellulose (de
Melo et al., 2009). The signal intensity at δ 61.2 ppm decreases as pure
CWs (Fig. 3(b)-i) are modified with anhydrides (Fig. 3(b)-ii to (b)-v)
suggesting that esterification reaction occurred mainly at C6 at the
amorphous phase. At δ 89 and δ 83 ppm is possible to observe crystalline and amorphous phase regions of C4 at cellulose structure, respectively. The intensity of C4 amorphous phase signal at δ 83 ppm

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Fig. 3. (a) FTIR and (b)

13

C-CP/MAS NMR spectra of (i) CW, (ii) CWMA, (iii) CWSA, (iv) CWAA and (v) CWPA.

and carbonyl ester group (C7). In the region between δ 124–137 ppm, it
is possible to observe the carbon atoms from the aromatic ring present
in the original phthalic anhydride structure. So, both FTIR and 13C-CP/
MAS NMR spectra confirmed the chemical modification of CWs surface
once characteristic peaks associated with the anhydride structures were
observed at the nanowhiskers samples.
The crystalline structure of bare and modified-CWs (CWMA, CWSA,
CWAA or CWPA), as well as the PVA nanocomposite films containing
specific amounts of CWs additives (3, 6 or 9 wt.%), was evaluated from
the XRD diffraction patterns as presented in Fig. 4. Fig. 4(a)-i to (a) iv
displays peaks at 14.5° associated to the plane (101), at 16.5° to the
plane (101′), at 20.4° to the plane (021), the broad peak at 22.6° to the
plane (002) and at 34.1° to the plane (040), representing the typical
crystalline form of cellulose I (cellulose native) (Wang et al., 2017; Ye &
Yang, 2015). Despite the anhydride used during modification procedure, the specific peaks of CW kept at constant degree, suggesting that
the esterification reaction did not cause significant changes in the
crystalline phase of the as-prepared CWs. Yet, the XRD diffraction

pattern for CWPA showed peaks at 18.4°, 26.8° and 30.6° that could
correspond to the formation of new crystal planes possibility revealing
a transition from crystalline cellulose I to cellulose II form (allotropic
form) (Yin et al., 2007). Still, from XRD data, it is possible to estimate
the crystallinity index (Icr%) (Table 1) along the 002-reflection peak, as
well as the average crystallite size (L) along planes 002, 101 and 101′
for pure and modified-CWs. Icr% values were calculated using Segal’s
empirical method (Segal et al., 1959) and the average crystallite size by
using the Scherrer’s equation (Eq. (1), Supporting Information). Pure
CW had an Icr% of 90.3%, which after chemical modification with MA,
SA and AA anhydrides dropped ca. of 1.7%, 2,4%, and 3.2%, respectively, indicating some disruption generated by the presence of these
moieties. The decrease in Icr was more significant for the modified-CWs
with PA representing ca. of 18.3% (Icr% of 73.8%, Table 1). In this
particular case, the presence of an aromatic ring in the anhydride
structure (bulky and rigid group) could reduce the density of hydrogen
bonds, partially destroying the crystalline structure while modifying
CWs. The crystallinity reduction of this sample suggests that the surface
layer becomes more disordered and with the amorphous characteristic.
As the cellulose chains present within the cellulose crystallite becomes
probably more disordered after the modifications the average size (L) of
crystallites also decrease (Garvey, Parker, & Simon, 2005).
The XRD diffraction patterns for the PVA88 nanocomposite films
show 4 diffraction peaks at 12.5°, 19.4°, 22.5°and 40.3° (Fig. 4(b)–(e))
characteristic of the ordered structure of the CWs (Panaitescu, Frone,
Ghiurea, & Chiulan, 2015). In addition to the respective diffraction
peaks at 14.8° and 16.5° (CWs crystalline planes 101 and 101′,

respectively), the intensity of the peak at 22.5° increased as the amount
of CWMA, CWSA or CWAA added also increased (3, 6 and 9 wt.%)
proving that modified-CWs were successfully incorporated into the

composite. Differently, for the composite material containing CWPA
(Fig. 4(e)) no significant increase at 22.6° peak intensity was observed,
at the same time that the diffraction peaks at 14.8° and 16.5° are no
longer visible. Due to the fact the CWPA has a low crystallinity, it was
expected that its presence would reflect on diffraction peaks at 14.8°and
16.5° with lower intensities. Anyway, this XRD diffraction pattern also
proves the presence of cellulose nanowhiskers within the PVA88
polymeric matrix. The results/data about PVA98 composite films are
shown in the Supporting Information (Fig. S6). Additional information
about the thermal stability of the nanocomposite films and the different
CWs are shown in the Supporting Information (Fig. S7 and S8).
The influence of modified-CWs within PVA nanocomposite films
was analyzed through dynamic mechanical analysis (DMA), with storage modulus (E’) measurements presented in Fig. 5. Pure PVA88 shows
typical behavior of a semicrystalline polymer with two transition regions, and the first module drop observed at 30–60 °C associated with
the amorphous phase feature of glass-rubber transition. At the temperature range of 60–200 °C the E' value slowly decreases until film
breaks around 200 °C due to the melting of the PVA crystalline regions
(Uddin, Araki, & Gotoh, 2011). Essentially, the addition of biopolymerbased CWs (3, 6 and 9 wt.%) increased the storage modulus (E') of the
films as the temperature also raised, and it was proportional to the
amount of nano-additives into the composite. Furthermore, it was
found that with increasing additions of the biopolymers, the formed
films have generated high storage modulus even at high temperatures.
For PVA88 nanocomposite films containing 9 wt.% of CW, CWMA,
CWSA, CWAA or CWPA there was an increase in storage modulus (at
30 °C) of up to 1.4, 3.3, 4.7, 2.0 and 4.4 times, respectively, compared
to pure PVA88 film (Table 2). The results to the PVA98 films with
different CWs are found in Table S2 and Fig. S9, Supporting information. The PVA98 composites obtained had lower mechanical properties
than the PVA88 films.
From these data (Table 2) it is possible to verify that the storage
modulus (E’) for all the samples decreased with temperature. Even
though, PVA88 films containing even small amounts of CWs showed an

increase in the storage modulus compared to pure PVA88, demonstrating its significant effect on the mechanical resistance of this polymeric matrix (Li, Yue, & Liu, 2012). In the present case, the higher is the
amount of CWs the greater is the interaction between cellulose nanowhiskers and PVA, restricting main chain movement of the PVA88
(George, Ramana, Bawa, & Siddaramaiah, 2011) and causing the increase in composite stiffness.
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Fig. 4. XRD diffractograms: (a) (i) CW, (ii) CWMA, (iii) CWSA, (iv) CWAA, (v) CWPA; (b) PVA88 containing CW (3, 6, 9 wt.%), (c) PVA88 containing CWMA (3, 6, 9 wt.%), (d) PVA88
containing CWSA (3, 6, 9 wt.%), (e) PVA88 containing CWAA (3, 6, 9 wt.%), and (f) PVA88 containing CWPA (3, 6, 9 wt.%).

Additional mechanical analysis deals with tensile strength (kPa),
elastic modulus (kPa) and elongation at break (%) of these nanocomposite films are shown in Fig. 6. Only PVA88 nanocomposite films
containing CW, CWSA or CWPA (3, 6 and 9 wt.%) showed a linear
behavior profile (P < 0.01) for tensile strength (kPa). The addition of
9 wt.% of CW, CWSA, and CWPA to the polymer matrix increased the
tensile strength values up to 16.6, 25.8 and 20.5% with respect to the
bare PVA88 films, respectively. For PVA88 films incorporated with
CWMA and CWAA cellulose-based additives, it was not possible to

Table 1
Crystallinity index (Icr%) and average crystallite size (L) of modified-CWs.
Samples

Icr%

L002 (nm)

L101(nm)

L101′(nm)

CW
CWMA
CWSA
CWAA
CWPA

90.3
88.8
88.1
87.4
73.8

6.1
6.0
6.0
6.0
5.8

6.7
6.7
6.6
5.0
3.1

4.9
4.9

4.7
4.5
4.0

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C. Spagnol et al.

Fig. 5. Storage modulus (E') of PVA88 nanocomposite films (a) containing CW (3, 6, 9 wt.%), (b) containing CWMA (3, 6, 9 wt.%), (c) containing CWSA (3, 6, 9 wt.%), (d) containing
CWAA (3, 6, 9 wt.%) and (e) containing CWPA (3, 6, 9 wt.%).

adjust a specific model, so the results were evaluated through Tukey’s
test (Table S3). In this case, the addition of 6 wt.% of CWMA and CWAA
increased tensile strength values up to 33.1 and 22.3%, proportionately.
These results showed the influence of additive (CW) chemical modifications as well as their amount present in the PVA88 polymer matrix,
providing films with higher tensile strength. The best result corresponded to the PVA88 nanocomposite film containing 6 wt.% of CWMA
additive, with tensile strength values in the order of 70 kPa.
The tensile strength increase in nanocomposite films containing
such cellulose-based additives (CW, CWSA, CWPA, CWMA and CWAA)
are due to the effective strain transfer occurring at the CW-polymer
interface (Khan et al., 2012) associated with a good interaction
(Ibrahim, El-Zawawy, & Nassar, 2010) between biopolymers and
PVA88 matrix. It also indicates a proper biopolymer-based additive

Table 2
Storage modulus (E') of PVA88 nanocomposite films containing modified-CWs at specific
amounts (wt.%).

Nanocomposite

E' (MPa)
at 30 °C

E' (MPa)
at 100 °C

Nanocomposite

E' (MPa)
at 30 °C

E' (MPa)
at 100 °C

pure PVA88
PVA88CW3
PVA88CW6
PVA88CW9
PVA88CWMA3
PVA88CWMA6
PVA88CWMA9
PVA88CWSA3

654
710
893
903
1470

1680
2146
817

120
158
152
204
197
225
242
164

PVA88CWSA6
PVA88CWSA9
PVA88CWAA3
PVA88CWAA6
PVA88CWAA9
PVA88CWPA3
PVA88CWPA6
PVA88CWPA9

1388
3086
1469
1460
1303
1574
2876
2877

171
307
188
242
289
144
164
190

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C. Spagnol et al.

rigid. Such effect is probably due to a homogeneous distribution of such
biopolymers crystalline reinforcements within the matrix (Iyer &
Torkelson, 2015). The nanocomposites with CWMA and CW displayed
higher values of elastic modulus. The nanowhiskers-to-polymer interactions between the CWMA/Polymer and CW/Polymer looks like better
and stronger when compared with others nanowhisker modified and
the polymer matrix. These films have smaller amounts of voids. The
CWPA/Polymer matrix showed smaller elastic modulus when compared
with all others nanocomposite films. If we analyze the structure of the
CWPA we realized that it presents an aromatic ring in its structure.
Probably this group removes the polymer chains by increasing the
amount of voids.
All kinds of modified-CWs increased tensile strength and elastic
modulus of nanocomposite films. Also, the increase in films stiffness

may be associated with the strong interactions between biopolymer
additives and polymer matrix, which may be decreasing the voids
within the polymer matrix. In other words, it seems that the interactions between the CW and CWMA with the polymer matrix are stronger
when compared to the other nanowhiskers, as reported above (Fig. 6b).
To CWSA, CWAA, CWPA the interactions appear to be smaller, because
they have groups that seem to move away the polymer chains, especially the CWPA which has an aromatic ring, increasing the amount of
voids. This makes the nanocomposite films less rigid. For CWMA/
Polymer matrix it seems that the presence of the double bond in the
nanowhisker chain is making the nanocomposite films more rigid
(Fig. 6a and b). However, when we analyzed the tensile strength to CW/
Polymer matrix we can not relate the data to the above reported. Another explication can be, as reported by Erden, Sever, Seki, and
Sarikanat (2010), that the increase in the modulus and tensile strength
may be a result of enhanced adhesion between nanowhiskers and the
polymer matrix, that indicates a greater interface, and enable improved
stress transfer between the components of the composite (Erden et al.,
2010).
Elongation at break (%) for PVA88 nanocomposites films with different amounts of CW, CWMA, CWSA, and CWPA presented a linear
model profile (p < 0.01), Fig. 6c and Table S5 (Supporting information). PVA88-CWAA films did not show a specific model profile, thus
the results were evaluated by the Tukey’s test. For CW, CWSA, and
CWPA elongation at break decreased down to 44.6, 52.3 and 41.6%,
respectively, with the increase of bio-additive (9 wt.%), which can be
the result of poor stress transfer from matrix polymer to filler resulting
in stress concentration points and failure points. For example, Iyer et al.
(2015) reported that the elongation at break values in the composite
decreased due the void formation and severe filler agglomeration
throughout the melt processing (Iyer et al., 2015). An additional interpretation for the reduction in elongation at break and enhanced
elastic modulus of the nanocomposite films could be due the increase in
the viscosity of film solution with the increase of nanowhisker amount
and the orientation of the nanowhiskers into the nanocomposite films
(Ugbolue, 2017).

In fact, it has been reported that films typically tend to become
brittle with the increase of reinforcement particle concentration (Rhim,
2011). This has been no different for nanocomposite films (Khan et al.,
2012). Interestingly, composites films containing 9 wt.% of CWMA and
CWAA showed an increase of elongation at the break up to 41.5 and
50.6%, respectively. This results could be related with the efficiency of
interface bonding between the polymer matrix and the nanowhiskers to
allow stress transfer (Erden et al., 2010). See supporting information for
additional results of PVA98 composite films (Fig. S10).
In general, nanocomposite films containing modified-CWs additives
show improved mechanical properties compared to the pure polymer
films, with the mechanical properties of such composites strongly depending on additive particle size and interaction of the particle-matrix
interface (Fu, Feng, Lauke, & Mai, 2008). It is true that the mechanical
tests performed in the present study demonstrate modified-CWs being
effective on stabilizing film structures improving reinforcement and

Fig. 6. (a) Tensile strength (kPa), (b) Elastic modulus (kPa) and (c) Elongation at break
(%) of PVA88 nanocomposites films containing specific amounts of CW, CWMA, CWSA,
CWAA, or CWPA additives (3, 6 and 9 wt.%). Note: In the X-axis, PVA88 have no additives.

distribution within the polymer matrix. In the specific case of CWAA,
the increase of 33.1% in film tensile strength values may be related to a
better additive-matrix interaction once more acetyl groups are presented in the CWAA chemical structure. Under stress-strain the acetyl
groups in the CWAA start to fill the empty spaces within the PVA matrix, which can probably increase the effect of reinforcement.
All nanocomposite films (3, 6 and 9 wt.%, Table S4) presented a
linear model profile (P < 0.01) of elastic modulus (kPa) values
(Fig. 6b). At 9 wt.% of additive, it was possible to verify an increase of
this mechanical property up to 114.9, 139.6, 94.1, 85.9 and 17.1% for
CW, CWMA, CWSA, CWAA, CWPA, respectively. It is worth mentioning
that PVA88-CWMA films had an increase of 2.4 times, and even the

lowest value observed (films containing CWPA) was of ca. 1.2 times.
The addition of CW, CWMA, CWSA, and CWAA to PVA88 generated
nanocomposite films with high elastic modulus and with features more
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C. Spagnol et al.

toughening effects. Digital images of some nanocomposite films are
show in the supporting information (Fig. S11). The films showed the
presence of light brownish coloration with increase the modified-CWs
percentage. That coloring is due the presence of modified-CWs, which
have brown coloration. The nanocomposite films with CW (without
modification) displayed transparent appearance.

Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
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4. Conclusions
Highly crystalline cellulose nanowhiskers (CW) were successfully

obtained from cotton fibers through acid hydrolysis, and its subsequent
chemical modification using different anhydrides was confirmed by
FTIR, NMR, and XRD analysis. TEM and AFM images of CW showed a
defined form with elongated shape—like needles—with an average
length of 200 ± 63 nm. XRD analysis indicated that the chemical
modification procedure did not cause significant changes in the crystalline phase of the as-prepared CWs, showing in some cases the formation of new crystal planes possibly revealing a transition from
crystalline cellulose I to cellulose II form (allotropic form).
Nanocomposite films composed of PVA88 and the modified-CWs revealed the presence of such bioadditives without significant changes of
the crystalline domains.
Mechanical analysis of such nanocomposites films presented a
proportional increase in the storage modulus (E’) to the amount of CWs
within the composite (3, 6 and 9 wt.%). In specific cases, (9 wt.% of
CWPA or CWSA) it reached values (at 30 °C) up to 4.4 and 4.7 times,
respectively, in comparison to the pure PVA88 film. Although E’ decreased with temperature (at 100 ° C), all PVA88 films containing even
small amounts of CWs (3 wt.%) showed improved mechanical performance in comparison to the bare polymeric matrix. This clearly demonstrates its positive effect on the mechanical resistance of PVA88,
where the higher the amount of CWs the greater is the interaction between cellulose nanowhiskers and PVA, decreasing the amount of voids
and consequently increasing the stiffness of nanocomposite films.
Tensile strength (kPa), elastic modulus (kPa) and elongation at
break (%) emphasize the reinforcement effect of modified-CWs on the
PVA88 nanocomposite films. Tensile strength and elastic modulus increased up to 33% and 140%, respectively, depending on the CW
chemical surface and amount (wt.%), suggesting a good additive-topolymer interaction with an effective strain transfer at the CW-polymer
interface. In general, elongation at break (%) decreased with the
amount of bioadditive (9 wt.%), which can be the result of poor stress
transfer from matrix polymer to filler resulting in stress concentration
points and failure points (Iyer et al., 2015). Interestingly, composites
films containing 9 wt.% of CWMA and CWAA showed an increase of
elongation at break. It could be associated to additive particle size,
additive-to-matrix interface interaction, and distribution while stressstrain test.
Altogether, the present data strongly indicate that the presence of
such biopolymer-based additives had a reinforcement effect on the

PVA88 matrix, with the increase in its mechanical properties depending
on the additive amount (wt.%) and CW chemical modification. These
nanocomposite materials have promising applications as biodegradable
composites, at the same time that modified-CWs could be explored as
polymer matrices reinforcement.

Acknowledgements
CS and EHF acknowledge Coordenaỗóo de Aperfeiỗoamento de
Pessoal de Nível Superior (CAPES, Brazil) for doctoral fellowships.
HDMF, MAW, RS, and AFR acknowledge the financial support given by
Conselho Nacional de Desenvolvimento Cientớco e Tecnolúgico
(CNPq, Brazil), CAPES and Fundaỗóo Araucỏria (Brazil).

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