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.
Trang 1Contents lists available atScienceDirect Carbohydrate Polymers journal homepage:www.elsevier.com/locate/carbpol
cellulose nanowhiskers as nano-reinforcement
Cristiane Spagnola, Elizângela H Fragala, Maria A Witta,b, Heveline D.M Follmanna,⁎,
Rafael Silvaa, Adley F Rubiraa,⁎
a Universidade Estadual de Maringá (UEM), Av Colombo 5790, CEP 87020-900, Maringá, Paraná, Brazil
b Pontifícia Universidade Católica do Paraná (PUCPR), Imaculada Conceição Street, CEP 80215-901, Curitiba, Paraná, Brazil
A R T I C L E I N F O
Keywords:
Cellulose nanowhiskers
Chemical surface modification
PVA films
Nanocomposites
Mechanical properties
A B S T R A C T Cellulose nanowhiskers (CWs) extracted from cottonfibers were successfully modified with distinct anhydrides structures and used as additives in poly(vinyl alcohol) (PVA) nanocompositefilms The surface modification of CWs was performed with maleic, succinic, acetic or phthalic anhydride to compare the interaction and action the carboxylic groups into PVAfilms and how these groups influence in mechanical properties of the nanocompo-sites 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 PVAfilms 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 researchfield 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
sur-face 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
dis-advantages, such as high water/moisture adsorption and poor adhesion
to non-polar polymer matrix caused by the polar
differences/interac-tions of the CWs Hence, chemical modification of CWs bearing specific
groups has been employed to tune phase to additive
interaction/ad-hesion (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
polymer-based 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 byβ-1,4-glycosidic bonds The presence of six HO-groups in such structure permits intra and in-termolecular 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 con-ditions—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 me-chanical 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
https://doi.org/10.1016/j.carbpol.2018.03.001
Received 4 October 2017; Received in revised form 28 February 2018; Accepted 1 March 2018
⁎ Corresponding authors.
E-mail addresses: hevelinefollmann@hotmail.com (H.D.M Follmann), afrubira@uem.br (A.F Rubira).
Available online 03 March 2018
0144-8617/ © 2018 Elsevier Ltd All rights reserved.
T
Trang 2cellulose include the preparation of chelating materials for the
ad-sorption 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
bio-compatibility, CWs have been studied for applications in drug delivery
systems (de Oliveira Barud et al., 2016;Jackson et al., 2011), packaging
and barrierfilms (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
mod-ulus 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
prop-erties 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 thefinal
material (Peresin, Habibi, Zoppe, Pawlak, & Rojas, 2010) Likewise,
other studies showed an increase in elasticity modulus of PVA
nano-composites 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 excellentfilm-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
bioma-terials have attracted considerable attention due to its biocompatibility
and biodegradability The broad biomedical and pharmaceutical
ap-plications 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
hy-drogels, contact lenses, dialysis membranes, membranes for the
re-placement of wounded tissues, artificial components of the organism
and controlled release of drugs
In this study, cellulose-rich cottonfibers 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
pre-pared 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
2 Materials and methods
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
88% and 98% of hydrolyzed groups (Mw13,000–23,000 g/mol), were purchased from Sigma-Aldrich (USA) Hydrochloric acid (HCl) was acquired from F Maia (Cotia, Brazil), maleic anhydride, phthalic an-hydride, sodium hydroxide from Vetec (Brazil), and N,N-dimethylace-tamide (DMAC) from Nuclear (Brazil) All the reactants and solvents were used as received without further purification
2.2 Cellulose nanowhiskers selective extraction Cellulose-rich cottonfibers (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 cottonfibers 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 cen-trifuged (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 roundflask (50 mL) and kept at 120 °C until complete melting 1 g of the as-pre-pared 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 wasfiltered, 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, respec-tively
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 wasfiltered, 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 roundflask (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 wasfiltered, 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 nanocompositefilms Nanocompositefilms 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
Trang 3would form by casting As a control, a blank sample of PVAfilms 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
anhy-dride (MA) instead For additional details as well as for the
nano-compositefilms prepared using PVA 98% of hydrolyzed groups, please
check Supporting Information (Table S1)
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 for13C (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 Nanocompositefilms characterization
The Dymanic-Mechanical Analysis (DMA) of the compositesfilms
(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, thefilm
thickness was determined using a digital micrometer ( ± 0.001 mm)
from Mitutoyo, model IP65 Eachfilm 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)
com-posite films containing pre-determined amounts of the distinct
nano-reinforcements (0, 3, 6 or 9 wt.% of modified-CWs) with the intention
to improve final mechanical response is shown inFig 1 It has been
reported that modified-CWs bearing groups such as hydroxyl (eOH)
and/or carboxyl (eCOOH) could increase cellulose hydrophilic
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 me-chanical properties (Hakalahti, Salminen, Seppälä, Tammelin, & Hänninen, 2015) Considering the hydrophilic nature of PVA, specific chemical modification of originally insoluble CWs can improve inter-facial compatibility between polymer matrix/modified-CWs enhancing mechanical properties of thefinal composite (Rescignano et al., 2014) The CWs are extracted from cellulosefibers 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 cottonfibers 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 hydro-lysis 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 sol-vents
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 pre-sence of bands at 1429, 1163, 1111 and 897 cm−1in the spectra in-dicates that CWs are mainly in the form of Iβcellulose (a crystalline form of cellulose type I, monoclinic unit cell) (Leung et al., 2011) Despite similarities among the anhydride structures used for the che-mical modification procedures, each modified-CWs is discussed sepa-rately The modified-CWs with maleic anhydride (CWMA) can be ob-served inFig 3(a)-ii that show intense bands at 1718 and 1734 cm−1 representing the coupling stretching of carboxyl and ester groups, re-spectively (Nishino, Matsuda, & Hirao, 2004) The bands at 1637 and
1235 cm−1are assigned toν(α,β C]C) and to ν(COeOH) present in the CWMA structure (de Melo et al., 2009) AtFig 3(a)-iii (CWSA) one can observe the appearance of a band at 1740 and 1725 cm−1related to ester and carboxyl group stretching, respectively The band at
1425 cm−1is due to the couplingν(C]O) and δ(OeH) groups (Chang
& Chang, 2001), while the band at 1160 cm−1corresponds toν(C]O) andν(O]CeOeR) stretching of ester segments.Fig 3(a)-iv shows a band at 1735 cm−1related to the ester carbonyl group present in CWAA structure (Braun & Dorgan, 2009), at 1232 cm−1a band corresponding
to ν(CeCeO) stretching associated to acetate fragment and at
1370 cm−1to the presence ofeCeCH3groups (Fan et al., 2010) Fi-nally, Fig 3(a)-v displays the spectrum of CWPA-modified nano-whiskers with a band at 1713 cm−1associated toν(C]O) stretching for ester and carboxylic acid moieties The bands at 1585, 1450 and
740 cm−1correspond toeCeH deformation out of the ring plane and assigned to aromatic vibrations The band centered at 1270 cm−1 cor-responds 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 inFig 3(a), cellulose main chain shows one broad band between 3100 and
3600 cm−1assigned toeOH stretching (Liu, Dong, Bhattacharyya, & Sui, 2017), and the band between 3000 and 2800 cm−1associated to ν(CeH) stretching of methyl groups and symmetric and asymmetric vibrations ofeCH2groups, respectively (Kloss et al., 2009;Shang et al.,
Trang 42016) The FTIR spectra of PVA88 and PVA98 is in the Supporting
Information (Fig S5)
Structure modification of the CWs was also evaluated through
13CeCP/MAS NMR, as shown inFig 3(b) (See the individual spectra
with their structures in the Supporting Information, Fig S4) The
re-sonance signal atδ 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 atδ 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 Atδ 89 and δ 83 ppm is possible to observe
crys-talline and amorphous phase regions of C4 at cellulose structure,
re-spectively The intensity of C4 amorphous phase signal at δ 83 ppm
(shoulder peak) also decreases as modification reactions occur, sug-gesting a higher reactivity associate to the amorphous phases and the surface of the biopolymer structures The region betweenδ 69–75 ppm refers to the C2, C3, and C5 carbon atoms of cellulose structure, whileδ
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 betweenδ 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 atδ 174 ppm related to the ester carbonyl groups (C7 and C10) andeCH2(C8 and C9) signal atδ 29 ppm CWAA spectrum,Fig 3(b)-iv, shows characteristic signals atδ 172 ppm asso-ciated to the ester group (C7) andδ 20.6 ppm related to the methyl group (C8) Last, atFig 3(b)-v one can observe CWPA spectrum with signals atδ 184 and δ 173 ppm corresponding to a carboxyl group (C14)
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.
Trang 5and carbonyl ester group (C7) In the region betweenδ 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 and13C-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 nanocompositefilms containing
specific amounts of CWs additives (3, 6 or 9 wt.%), was evaluated from
the XRD diffraction patterns as presented inFig 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
proce-dure, 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%,
respec-tively, indicating some disruption generated by the presence of these
moieties The decrease in Icrwas 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 compositefilms are shown in the Supporting Information (Fig S6) Additional information about the thermal stability of the nanocompositefilms 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 sto-rage modulus (E’) measurements presented inFig 5 Pure PVA88 shows typical behavior of a semicrystalline polymer with two transition re-gions, and thefirst module drop observed at 30–60 °C associated with the amorphous phase feature of glass-rubber transition At the tem-perature 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 biopolymer-based 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 nanocompositefilms 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 informa-tion The PVA98 composites obtained had lower mechanical properties than the PVA88films
From these data (Table 2) it is possible to verify that the storage modulus (E’) for all the samples decreased with temperature Even though, PVA88films containing even small amounts of CWs showed an increase in the storage modulus compared to pure PVA88, demon-strating its significant effect on the mechanical resistance of this poly-meric matrix (Li, Yue, & Liu, 2012) In the present case, the higher is the amount of CWs the greater is the interaction between cellulose nano-whiskers and PVA, restricting main chain movement of the PVA88 (George, Ramana, Bawa, & Siddaramaiah, 2011) and causing the in-crease in composite stiffness
Fig 3 (a) FTIR and (b) 13 C-CP/MAS NMR spectra of (i) CW, (ii) CWMA, (iii) CWSA, (iv) CWAA and (v) CWPA.
Trang 6Additional mechanical analysis deals with tensile strength (kPa), elastic modulus (kPa) and elongation at break (%) of these nano-compositefilms are shown inFig 6 Only PVA88 nanocompositefilms 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
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.%).
Table 1
Crystallinity index (I cr %) and average crystallite size (L) of modified-CWs.
Samples I cr % L 002 (nm) L 101 (nm) L101′(nm)
Trang 7adjust 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 mod-ifications as well as their amount present in the PVA88 polymer matrix, providing films with higher tensile strength The best result corre-sponded to the PVA88 nanocompositefilm 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
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.%).
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 654 120 PVA88CWSA6 1388 171
PVA88CW3 710 158 PVA88CWSA9 3086 307
PVA88CW6 893 152 PVA88CWAA3 1469 188
PVA88CW9 903 204 PVA88CWAA6 1460 242
PVA88CWMA3 1470 197 PVA88CWAA9 1303 289
PVA88CWMA6 1680 225 PVA88CWPA3 1574 144
PVA88CWMA9 2146 242 PVA88CWPA6 2876 164
PVA88CWSA3 817 164 PVA88CWPA9 2877 190
Trang 8distribution within the polymer matrix In the specific case of CWAA,
the increase of 33.1% infilm tensile strength values may be related to a
better additive-matrix interaction once more acetyl groups are
pre-sented in the CWAA chemical structure Under stress-strain the acetyl
groups in the CWAA start tofill the empty spaces within the PVA
ma-trix, 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
nanocompositefilms with high elastic modulus and with features more
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 inter-actions between the CWMA/Polymer and CW/Polymer looks like better and stronger when compared with others nanowhisker modified and the polymer matrix Thesefilms have smaller amounts of voids The CWPA/Polymer matrix showed smaller elastic modulus when compared with all others nanocompositefilms 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 nanocompositefilms 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 interac-tions 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, espe-cially 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 An-other 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 nanocompositesfilms with dif-ferent amounts of CW, CWMA, CWSA, and CWPA presented a linear model profile (p < 0.01),Fig 6c and Table S5 (Supporting informa-tion) PVA88-CWAAfilms 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 tofiller 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 in-terpretation for the reduction in elongation at break and enhanced elastic modulus of the nanocompositefilms could be due the increase in the viscosity offilm solution with the increase of nanowhisker amount and the orientation of the nanowhiskers into the nanocompositefilms (Ugbolue, 2017)
In fact, it has been reported thatfilms 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, compositesfilms 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 compositefilms (Fig S10)
In general, nanocompositefilms containing modified-CWs additives show improved mechanical properties compared to the pure polymer films, with the mechanical properties of such composites strongly de-pending 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
ad-ditives.
Trang 9toughening 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
4 Conclusions
Highly crystalline cellulose nanowhiskers (CW) were successfully
obtained from cottonfibers 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
crys-talline phase of the as-prepared CWs, showing in some cases the
for-mation 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
re-vealed 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’
de-creased with temperature (at 100 ° C), all PVA88films containing even
small amounts of CWs (3 wt.%) showed improved mechanical
perfor-mance in comparison to the bare polymeric matrix This clearly
de-monstrates its positive effect on the mechanical resistance of PVA88,
where the higher the amount of CWs the greater is the interaction
be-tween 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 nanocompositefilms Tensile strength and elastic modulus
in-creased up to 33% and 140%, respectively, depending on the CW
chemical surface and amount (wt.%), suggesting a good
additive-to-polymer 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 tofiller 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
stress-strain 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 Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior (CAPES, Brazil) for doctoral fellowships
HDMF, MAW, RS, and AFR acknowledge thefinancial support given by
Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq, Brazil), CAPES and Fundação Araucária (Brazil)
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