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

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.

Carbohydrate Polymers 191 (2018) 25–34 Contents lists available at ScienceDirect 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 Pontifớcia Universidade Catúlica Paranỏ (PUCPR), Imaculada Conceiỗóo Street, CEP 80215-901, Curitiba, Paraná, 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, and wt.%) of modified-CWs increased up to 4.4 times the storage modulus (PVA88-CWSA 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 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: hevelinefollmann@hotmail.com (H.D.M Follmann), afrubira@uem.br (A.F Rubira) https://doi.org/10.1016/j.carbpol.2018.03.001 Received October 2017; Received in revised form 28 February 2018; Accepted 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 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, or 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 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 wt.% of glycerol as plasticizer Then, this mixture was stirred for 10 and distinct amounts of as-prepared CWs (3, or wt.%) were added with the medium being kept under magnetic stirring for additional 15 This suspension was sonicated for 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 h This mixture was poured into a glass vial containing distilled water (excess) and kept at 80 °C under magnetic stirring (for 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), g of “cleaned” cotton fibers were hydrolyzed using concentrated HCl (20 mL, 37%) at 45 °C for h, under magnetic stirring The resulting suspension was centrifuged (10.000 rpm) for 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 g of maleic anhydride (MA) was added to a one-neck round flask (50 mL) and kept at 120 °C until complete melting 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 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 g of melted phthalic anhydride (PA, 131 ° C), 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 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 26 Carbohydrate Polymers 191 (2018) 25–34 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 wt.% of unmodified CWs, while PVA88CWMA3 contains 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 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 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 × 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) Results and discussion An illustration scheme of PVA88 (88% of hydroxyl groups) composite films containing pre-determined amounts of the distinct nanoreinforcements (0, 3, or wt.% of modified-CWs) with the intention to improve final mechanical response is shown in Fig It has been reported that modified-CWs bearing groups such as hydroxyl (eOH) and/or carboxyl (eCOOH) could increase cellulose hydrophilic 27 Carbohydrate Polymers 191 (2018) 25–34 C Spagnol et al Fig Illustration scheme of PVA88/modified-CWs nanocomposite films containing distinct amounts of cellulose additives Fig (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 28 Carbohydrate Polymers 191 (2018) 25–34 C Spagnol et al Fig (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, or wt.%), was evaluated from the XRD diffraction patterns as presented in Fig 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 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, and 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 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, and 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 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 29 Carbohydrate Polymers 191 (2018) 25–34 C Spagnol et al Fig XRD diffractograms: (a) (i) CW, (ii) CWMA, (iii) CWSA, (iv) CWAA, (v) CWPA; (b) PVA88 containing CW (3, 6, wt.%), (c) PVA88 containing CWMA (3, 6, wt.%), (d) PVA88 containing CWSA (3, 6, wt.%), (e) PVA88 containing CWAA (3, 6, wt.%), and (f) PVA88 containing CWPA (3, 6, wt.%) Additional mechanical analysis deals with tensile strength (kPa), elastic modulus (kPa) and elongation at break (%) of these nanocomposite films are shown in Fig Only PVA88 nanocomposite films containing CW, CWSA or CWPA (3, and wt.%) showed a linear behavior profile (P < 0.01) for tensile strength (kPa) The addition of 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 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 30 Carbohydrate Polymers 191 (2018) 25–34 C Spagnol et al Fig Storage modulus (E') of PVA88 nanocomposite films (a) containing CW (3, 6, wt.%), (b) containing CWMA (3, 6, wt.%), (c) containing CWSA (3, 6, wt.%), (d) containing CWAA (3, 6, wt.%) and (e) containing CWPA (3, 6, wt.%) adjust a specific model, so the results were evaluated through Tukey’s test (Table S3) In this case, the addition of 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 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 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 31 Carbohydrate Polymers 191 (2018) 25–34 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 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 (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, and 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, and wt.%, Table S4) presented a linear model profile (P < 0.01) of elastic modulus (kPa) values (Fig 6b) At 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 32 Carbohydrate Polymers 191 (2018) 25–34 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 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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, and 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 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This suspension was sonicated for 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... 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