Citric Acid as Green Modifier for Tuned Hydrophilicity of Surface

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Citric acid as green modifier for tuned hydrophilicity of surface modified cellulose and lignin nanoparticles Xiaoyan He, Francesca Luzi, Weijun Yang, Zefang Xiao, Luigi Torre, Yanjun Xie, and Debora Puglia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01202 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Sustainable Chemistry & Engineering

Esterified CNC and esterified/etherified LNPs altered by citric acid treatment showed tuned dispersability, preserved morphology and enhanced thermal stability 243x95mm (300 x 300 DPI)

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Citric acid as green modifier for tuned hydrophilicity of surface modified cellulose and lignin nanoparticles

Xiaoyan He a,b, Francesca Luzib, Weijun Yangb, Zefang Xiaoa, Luigi Torreb, Yanjun Xiea, Debora Pugliab* a

Northeast Forestry University, College of Material Science and Engineering, Key Laboratory of Bio-based Material Science and Technology (Ministry of Education), 26 Hexing Road, Harbin 150040, People’ s Republic of China b University of Perugia, Civil and Environmental Engineering Department, Materials Engineering Center, UdR INSTM, Strada di Pentima 4, 05100, Terni, Italy

Xiaoyan He, [email protected] Francesca Luzi, [email protected] Weijun Yang, [email protected] Zefang Xiao, [email protected] Luigi Torre, [email protected] Yanjun Xie, [email protected] Debora Puglia, [email protected]


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Abstract

This article proposes one straightforward route for citric acid modification of two different lignocellulosic products, cellulose nanocrystals (CNC) and lignin nanoparticles (LNP). Modified cellulose nanocrystals (MCNC) and lignin nanoparticles (MLNP) were characterized by means of Fourier Transform Infrared Spectroscopy (FT-IR), 13C and 1H nuclear magnetic resonance (NMR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Ultravioletvisible/near-infrared (UV-VIS/NIR) Spectrometer and thermogravimetric analyses (TGA). The reaction mechanism between citric acid and both CNC and LNP was discussed. The resultant MCNC exhibited improved dispersion in polar solvents, better thermal stability as compared with CNC while, in the case of LNP, a slight increase in thermal stability and alteration of MLNP dispersability in polar solvents were proved. These results confirmed how esterified (MCNC) and etherified (MLNP) biobased nanoparticles with tuned hydrophilicity, obtained by a treatment with a low cost, sustainable and easily soluble cross-linker, can found widespread applicability in the field of polymeric based nanocomposites having different polarity.

*Corresponding author: [email protected]; Tel +390744492916; Fax +390744492950

Key words: cellulose nanocrystals. Lignin nanoparticles, citric acid; esterification; solvent dispersion



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Introduction Cellulose nanocrystals (CNC) and lignin nanoparticles (LNP), both of them primarily extracted from biodegradable and renewable lignocellulosic biomass, possess large specific surface area, as well as impressive physicochemical properties.1-3 CNC obtained by acid hydrolysis are particles with rod-like or needle-shaped morphology, along with high crystallinity. The geometrical dimensions of CNC may differ according to bioresource origins and selected hydrolysis conditions.4 Nevertheless, the typical width is within 5-10 nanometers, whereas the length is usually comprised between 100 and 300 nanometers. Cellulose nanocrystals display attractive advantages, such as low thermal expansion coefficient, low density (about 1.57g/cm3), and high elastic modulus (up to 150 GPa), that endow extensive potential for their use as efficient reinforcement agents at low filler loading levels in polymeric systems.5 Other outstanding merits include optical transparency, biocompatibility,

biodegradability,

non-toxicity,

stiffness,

renewability,

sustainability,

gas

impermeability, adaptable surface chemistry.6-8 Extensive literature presenting CNC as the reinforcement on different polymer materials have been published recently, that demonstrated how dispersability of CNC in the polymer matrix has an important role in obtaining functional nanostructured polymers with tunable behavior.9-15 It is known that every cellulose unit bears 3 hydroxyl groups that, combined with CNC large specific area, reveal abundant occurrence of hydroxyls on CNC surface, responsible for CNC agglomeration and poor compatibility and interfacial contact with non-polar matrices, especially when CNC loading is higher than 3 wt.%. Self-aggregation and limited dispersion of hydrophilic nanocellulose in hydrophobic polymers limit its efficiency as reinforcing element.15-16 Meanwhile, these hydroxyl groups open the possibility of suitable chemical modifications, such as polymer grafting, esterification, etherification, silytation and oxidation,8 that hinder aggregation and get better dispersion and interfacial adhesion, while maintaining the original CNC crystalline arrangement-.17 Surface oxidation of primary alcohols to carboxylate groups has been carried out upon cellulose, which leads to good dispersability.18-21 Chemical modification techniques have been used to develop new surface modified CNCs with exceptional properties,22-24 in particular CNC hydrophobicity can be radically modified by formation of ester groups. Acetylation by anhydride is, for example, an effective method that improves dispersion in organic solvents and hydrophobic matrices.25 Surface carboxylated CNC have been obtained by using simultaneous HCl catalyzed hydrolysis and


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esterification by means of biobased functional organic acids (malic, citric, malonic), demonstrating that the two concurring reactions have moderate effect on CNC morphology and crystallinity of CNC.26 A similar procedure the combined citric and hydrochloric acid hydrolysis was even reported for the extraction of carboxylic CNC from MCC, where higher crystallinity, increased carboxylic content and better thermal stability were achieved. On the other hand, these samples showed also remarkable coagulation flocculation performance.27 It has been proposed that when hydroxyl groups and acid carbonyls in polycarboxylic acid (i.e. 1,2,3,4-Butanetetracarboxylic acid, citric acid, usually used for cotton fabrics and pulped paper esterification) react with the cellulose, ester linkages and carboxylic acid functionalities will appear.28-31 As a result, the treated cellulose possesses strength retention superior to that obtained by treatment with cellulose etherification by cross-linking agents. Recently, it has been proved that citric acid can be used as an effective catalyst in anhydride acetylation of CNC and that acetate groups present on CNC surface

significantly

modified their hydrophilicity.8, 32 Differently from cellulose, lignin is a high-branched polyphenolic polymer, which constitutes 2030% of the wood and accumulates between cellulose micro fibrils.33-34 The chemical structure of lignin varies depending on its source. Generally, lignin is derived from dehydrogenative polymerization, followed by re-aromatization of p-hydroxycinamyl, coniferyl, and sinapyl alcohols, which give rise to H (p-hydroxyphenyl), G (guaiacyl), and S (syringyl) phenylpropanoid units. While softwood species contain lignins primarily composed of G units, in the case of hardwood lignins additional S units in different amounts, depending on the botanic source and the extraction processes35 The prominent linkages between G units in softwood kraft lignin obtained from 1D NMR and HSQC model compounds are summarized in Figure 1.35-37 Hydroxyl groups in the aromatic rings give the most representative functionality in lignin; they impact on its reactivity and create reactive for further macromolecular chemistry approaches. The limited solubility of lignin in water is the major limitation for its industrial processing at large scale. Chemical functionalization has been considered as an effective route to be applied for improving lignin reactivity, adding functionalities to its structure for an easier incorporation into bio-based thermoplastic and thermosetting polymers. Different acetylation procedures have been described in the literature as suitable ways for reactive modification of lignin solubility.38-40 On the other hand, it has been recently revealed that it is possible to prepare aqueous LNP dispersions41 by



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different techniques, such as self-assembly technology, sonication, high homogenization, nanoprecipitation.1, 42-49 In addition, lignin can be converted into low molecular weight fragments, by cleavage of β−O−4 bonds) and can condensate by free radical-induced polymerization between aromatics.37 The utilization of LNP as a nanofiller in polymer matrix often follows two global approaches: (1) use of lignin without any chemical pretreatment; (2) its esterification and etherification , to make hydroxyl functions more accessible, Interestingly, cellulose triacetate biopolymeric nanocomposite films containing lignin as a filler were prepared, and improved mechanical resistance was obtained by using acetylated lignin.50-51 In spite of the clear scientific and technological thought of esterified or etherified LNP as fully bio-based filler with improved water dispersability, few examples of its application in polymeric

nanocomposites recently

appeared in the literature. Despite the well-known potential of modified CNC and LNP, to our best knowledge, there has not been before any documented study on utilization of citric acid as a reagent for CNC and LNP hydrophobization, accompanied by sodium hypophosphite (SHP) adopted in this study, that is considered as the most efficient catalyst in the esterification reaction of wood and cellulosic fabrics in presence of citric acid.52-53 The main aim of this research was to prepare chemically modified nanoparticles (MCNC and MLNP) by means of a oversimplified, sustainable, mild and low energycosting procedure, exploring the possible reaction mechanism between citric acid and CNC (and LNP). The twofold purpose was even related to the possibility of opening an opportunity to downstream application of functionalized nanofillers as novel biobased reinforcements in polymer matrix having variable hydrophilicity. Experimental Materials: Microcrystalline cellulose (MCC, dimensions of 20 mm) and alkali lignin were used as raw materials for the preparation of nanoparticles. Materials and chemicals, including microcrystalline cellulose, alkali lignin, sulfuric acid (H2SO4, 98%), Dowex Marathon MR-3 hydrogen and hydroxide form, hydrochloric acid (HCl, 35%), ethylene glycol (99.8%), citric acid (99.5%), and sodium hypophosphite (99%) were supplied by Sigma-Aldrich. All of the chemicals were used as received without additional purification.

Synthesis of Cellulose nanocrystals: CNC aqueous suspension was prepared by H2SO4 hydrolysis ACS Paragon Plus Environment

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of microcrystalline cellulose (MCC) according to the procedure previously reported.54-57 Briefly, MCC was dispersed in 64% (v/v) aqueous H2SO4 and stirred constantly for 30 min at 45 °C. After hydrolysis, the resultant was diluted into 20-fold deionized water to terminate the reaction. The suspension was subjected to successive centrifugation for 20 min to remove excess acid and to obtain concentrated cellulose. Subsequently, precipitated material was dialyzed against deionized water until approximate neutrality. Mixed bed ion exchange resin was mixed and put in contact with the cellulose suspension for 24 h, then removed by a filtration process. An ultrasonic treatment was performed for 5 min to allow good dispersion of CNC into water and finally, a loose powder was obtained by freeze drying the CNC suspension.

Synthesis of lignin nanoparticles: LNP suspension was obtained from alkali lignin by hydrochloric acid treatment based on the procedure reported in previous papers.1, 45, 48 4% (m/v) of alkali lignin in ethylene glycol was stirred for 2 h at 35 °C. Afterwards, hydrochloric acid (8 mL, 0.25 M) was mildly added to the solution at a rate of 3-4 drops/min, after that the suspension was stirred again for other 2 h. The product was filtered to eliminate soluble impurities from lignin. The solution was then dialyzed against deionized water up to neutrality to obtain the LNP suspension. Chemical modification of CNC and LNP: Under the condition of magnetic stirring at room temperature, freeze-dried CNC were added into 5% (w/w) of citric acid aqueous solution and 1% (w/w) sodium hypophosphite (SHP) used as the catalyst. One hour later, the mixture solution was put in a vacuum oven at 0.6 bar for 2h and then at room temperature for 12h. The surplus water in the suspension was removed by leaving it in the air dry oven at 60 °C (48h) and the colloidal material was then maintained at 130 °C (4h) afterwards. After the reaction, the product was redispersed into water and centrifuged three times, the precipitated material was then dialyzed against deionized water for removal of unreacted reagents, catalysts and incomplete reaction products. The same procedure was also applied in the case of LNP particles. The suspension was finally uniformly dispersed into water after ultrasonic treatment. Lastly, the powdered modified CNC (MCNC) and modified LNP (MLNP) derivatives were obtained by freeze drying at -50 °C and 0.12 mbar.

CNC and LNP yield: After dialysis, the total weight of CNC (and LNP suspension) was measured. 2mL of the suspension was then transferred to a weighing beaker, weighed and dried at 105 °C for



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2h. After cooling down to room temperature, the sample and the beaker were weighed by an analytical balance with 0.01 mg readability. The final result for each sample was obtained as the mean value of five duplicates. The yield (%) was calculated as follows, Eq. (1):

(1) where ms is the total mass of as-prepared CNC suspension (g), m2 is the total mass of oven-dried CNC and beaker (g), m0 is the mass of the weighed beaker (g), m1 is the total mass of CNC suspension and beaker to be oven-dried (g), mi is the mass of raw materials (MCC or alkali lignin) used for nanoparticles preparation (g). The same procedure and calculating principle has been considered for the evaluation of weight gain in CNC and LNP after modification with citric acid, where ms is the total mass of MCNC/MLNP suspension after dialysis (g), m2 is the total mass of oven-dried MCNC/MLNP and beaker (g), m0 is the mass of the weighed beaker (g), m1 is the total mass of MCNC/MLNP suspension and beaker to be oven-dried (g), mi is the mass of raw materials (CNC/LNP) used for modification (g).

Characterization of MCNC and MLNP Fourier transform infrared (FT-IR) spectroscopy: Fourier transfer infrared (FTIR) spectra of the powdered CNC, MCNC, LNP and MLNP, were recorded on a Jasco FTIR 615 spectrometer (Japan). The samples were measured using a KBr-pellet method (resolution of 4 cm-1, wavenumber range 4000–400 cm−1). NMR measurements: Solid-state

13

C CP-MAS NMR spectroscopy of CNC and MCNC was

performed by using a Bruker Avance III 400 NMR spectrometer (BrukerBioSpin AG, Switzerland) equipped with a 4-mm MAS probe. The measurements were done at 298.3 K using the ramp

13

C

CP/MAS pulse sequence (cross-polarization and magic angle spinning) with proton decoupling during acquisition. The contact time during CP was set as 2ms. The SPINAL64 sequence (small phase incremental alternation with 64 steps) was used for hetero nuclear decoupling during acquisition with a proton field H1H. The spinning rate contact time during CP and relaxation delay were separately set as 12 kHz, 2 ms and 2.0 s.


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The spectra of LNP and MLNP were recorded on a Bruker Avance 400 MHz spectrometer (Bruker BioSpin AG, Switzerland) at room temperature. The method and parameters were selected according to previous literature.58 Quantitative 13C NMR were obtained at a sample concentration of 20 wt. % in dimethyl sulfoxide (DMSO-d6), the operating parameters including a 90o pulse width, a 1.4 s acquisition time, and a 1.7 s relaxation delay were used. Chromium (III) acetylacetonate (0.01 M) was added to the solution to guarantee the relaxation of nuclei, 20000 scans were collected. Quantitative 1H NMR spectrums of LNP and MLNP were separately recorded at the sample concentration of 2% in CDCl3, with a 90° pulse width and a 1.3 s acquisition time. A relaxation delay of 7 s was applied to provide complete relaxation of aldehyde protons. A total of 128 scans were collected. All NMR spectra were processed in MestreNova 9.0 software with zero filling, phase correction, and a Bernstein polynomials baseline correction. Morphological characterization: Drops of aqueous suspensions of CNC and MCNC, LNP and MLNP were deposited on silicon substrates, air dried for 24h, gold coated by using an ion sputter coater, and observed by using a scanning electron microscope (FESEM, Supra 25-Zeiss) and field emission gun operated at 3 kV. The length and width of CNC and MCNC, the diameter of LNP and MLNP were determined by using digital image analysis software (Nikon NIS-Elements Basic Research), 50 duplicates have been adopted to obtain rational average values. The morphological differences of the CNC and MCNC, LNP and MLNP were also evidenced by means of transmission electron microscope (TEM). The samples were separately ultrasonically dispersed in absolute ethanol to obtain a uniform suspension. Small drop of 0.1 wt. % solution was deposited on a bacitracin-pretreated surface of a carbon-coated copper grid. TEM observations were carried out on a Tecnai F30 electron microscope operated at an acceleration voltage of 300 kV. Thermal analysis: Thermogravimetric analysis of CNC, MCNC, LNP, MLNP samples was performed by using a thermo gravimetric analyzer (TA Instruments, Seiko Exstar 6300). The samples weighed approximately 8 mg, were initially heated from 30 °C to 800 °C under nitrogen atmosphere (250 mL min–1) at a heating rate of 10 °C min−1. The mass loss (TG) and derivative mass loss (DTG) curves were calculated. Onset degradation temperature (Tonset), defined as the ACS Paragon Plus Environment


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point of intersection of tangents to two branches of the thermogravimetric, was evaluated. Maximum thermal degradation temperature (Tmax) was also collected from DTG peaks maxima, along with the residual weight percent at 800 °C. Dispersion in methanol: The CNC, MCNC, LNP and MLNP were re-dispersed into 10 ml of methanol, varying the product quantity in order to obtain a stepwise concentration of 0.05 g/L, 0.1 g/L, 0.3 g/L, 0.5 g/L. The resulting suspensions were sonicated for 10 min. Stability and absorbance of

suspensions

were measured

using an ultraviolet-visible/near-infrared

(UV-VIS/NIR)

spectrometer (Varian Cary 4000) from 900 nm to 250 nm at a scanning speed of 240 nm/ min. The dispersability of samples was observed by taking photographs after standing for certain time (3 min, 10 min, 60 min and 21 days). Contact angle measurements: Treated and untreated CNC and LNP powders were compressed in order to obtain a pellet with a flat surface. Contact angles of sessile drops of deionized water (volume 3 µl) were measured using a contact angle measurement device ((FTA2000, First Ten Angstroms, Inc. Portsmouth, UK) equipped with a camera and Drop Shape Analysis SW21; FTA32 2.0 software (First Ten Angstroms, Inc., Portsmouth, UK)). Five measurements were considered for each sample. Measurements were taken for 25 s at 3 s intervals for the first measurement and after 5 s for the last four measurements, starting 1 s after drop placement on the pellet surface. Results and Discussion The solid content of MCNC and MLNP after dialysis determined by Eq. (1) was, respectively, increased by 11% and 17%, when compared to initial content of CNC and LNP. This observed weight gain confirmed that adopted modification and purification protocols by means of citric acid can be considered as a feasible procedure for nanoparticles green functionalization.

FT-IR spectroscopy and NMR analysis: The chemical structure of CNC and LNP particles after modification and purification by dialysis was examined by FTIR, related spectra are shown in Figure 2 (arbitrary units for transmittance signals). A new absorption band appeared both in MCNC_BD and MCNC_AD at 1751 cm-1 (I) in the spectra reported in Figure 2a, which was assigned to the carbonyl C=O stretch vibration of formed ester groups and unreacted carboxylic groups. Compared with MCNC_BD, the intensity of band at 1751 cm-1 decreased in the spectrum of ACS Paragon Plus Environment

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MCNC_AD, due to the elimination of free carboxylic groups from citric acid and hydrolytically cleavage of ester groups during dialysis (absence of free carboxylic groups in MCNC_AD will be also further confirmed by TGA analysis). The bands in MCNC_AD spectrum raised in the range of 1222 cm-1 (IV) to 1030 cm-1 (V), which are related to ether bond (C-O-C) stretching, (C-OH) and pyranose ring (C-O-C) vibration of CNCs, gave further proof for formation of ester linkages.59 The absorbance at 3400 cm-1 (not shown) was ascribed to OH stretching , while the absorption at 1640 cm-1 (II)was related to adsorbed water molecules in CNC60. These two typical signals for hydroxyl groups decreased in MCNC_BD spectrum, when compared to CNC spectrum, owing to hydroxyl groups blocking by reaction with citric acid. On the other hand, hydroxyl groups increased in MCNC_AD spectrum, in contrast to MCNC_BD and CNC, since more hydrophilic surface was revealed after removal of excess wrapped products. In the meanwhile, the citric acid modification introduced new hydroxyl groups for CNCs, confirming that the esterification principally occurred at the surface. A new absorption at 1607 cm-1 (III) appeared after modification and dialysis (MCNC_AD), originating from conjugated carbonyl groups, indicating that the hydroxyl groups of the pyranose ring in CNC have been oxidized, to carbonyl and carboxyl groups. These groups are chromophores, able to capture visible radiation and responsible for the yellowing of MCNC (see detail in Figure 3)..61-62 In the case of lignin nanoparticles (Figure 2b), intensity decrease and shape change were noted in the peak corresponding to O-H (3000–3600cm-1), related to the consumption of OH during the esterification reaction. A new carbonyl peak at 1745 cm-1 (VI) and an increase in the intensity of the ether band at 1097 cm-1 (XI) confirm the existence of ester linkages. The methoxylene groups band located at 2952 and 2865 cm-1 (not shown) increased after modification, thereby indicating that crosslinking reaction between citric acid and lignin was effective. Three peaks at 1613, 1530, and 1480 cm-1 arise from the benzene skeleton structure, and are typical lignin absorptions. Two additional peaks detected at 1671 (VII) and 1347 cm-1 (VIII) in MLNP_BD, which are absent in the spectra of LNP ( shoulder at 1671 cm-1) and MLNP_AD, were associated with conjugated carbonyl groups and in-plane bending vibration of phenolic hydroxyl groups. They proved the occurrence of oxidation of benzoyl hydroxyl to carbonyl group and oxidative cleavage of ether linkages in the main chains of MLNP_BD, leaving a large quantity of monomeric phenols.63 In addition, generated phenols contained many functional groups, such as methoxyl and alkyl groups.64. Due to the



complexity of lignin structure, lignin oxidation reactions might induce the formation of carbonyl groups, including side-chain elimination and ring-opening reactions in phenols.65 Finally, MLNP_AD presented typical G (guaiacyl) type lignin bands at 1289 cm-1 (IX, C-O stretch) and 859 cm-1 (XII, C-H out-of-plane). Aromatic C-H-in-plane deformations in S units turned to a weaker shoulder at 1140 cm-1(X) in MLNP_AD, induced by destruction and draining of S units during acid treatment and dialysis. Moreover, the bands at 1289-1097 cm-1,composed of many overlapping groups, such as ether stretching in C–O-C bond and C-O in

(C-OH), are more intense in

MLNP_AD in comparison to LNP. It indicated that MLNP may also undergo etherification stimulated by oxidative carbonyl groups and hydroxyl groups of citric acid and accompanied by the formation of ester linkages, as also suggested by 13C and 1H NMR.

CP-MAS solid state NMR has been shown to be an effective tool for the characterization of esterified CNC.26 The signal assignment of CNC and MCNC has been summarized in Table 2. In details, amorphous and crystalline region for C6 have been attributed to the signals at 61.3 and 64.2 ppm, while the signals at 70.9 and 73.7 ppm can be assigned to C2, C3 and C5. Signals of amorphous region and crystalline region for C4 were located at 82.8 and 87.9 ppm, as well as the signal for C1 at 104.7 ppm. When compared with CNC, new signals located at 178 and 45 ppm emerged in MCNC spectra, confirming the presence of carbons from carbonyl (C=O) of ester groups and methylene groups of citric acid. NMR results further proved that esterification successfully occurred between citric acid and hydroxyl groups. The peak ratio of C4 (79–85ppm)/ C4 (85–96ppm) in Table 1, which is often used to evaluate the cellulose crystallinity,66 had a slight increase after modification. This fact indicated that a minor part of crystalline region might be changed by the esterification.67 According to the mechanism towards polycarboxylic acid to cotton fabrics and paper proposed in previous articles,28-31 the esterification occurs by the formation of the five-membered cyclic anhydride intermediate and at least one of the carboxyl groups will be regenerated as ester group. The schematic mechanism of the reaction is shown in Figure 3. Partial signal assignments and integral in the 1H spectra and 13C NMR spectra of LNP and MLNP are reported in Table 2 and Table 3.. It can be observed that the aromatic region of the spectra for alkali lignin (Table 2) can be divided into protonated (CAr-H, 125–106 ppm), condensed (CAr-C, 140– 125 ppm), and oxygenated aromatic (CAr-O, 160–140 ppm) regions. The C3 and C4 carbon atoms on ACS Paragon Plus Environment

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the aromatic ring correspond to the oxygenated aromatic region includes, while the protonated region (125–106 ppm) comprises C2 and C6 methine carbon atoms and protonated C5 carbons. On the other hand, condensed aromatic region includes C1 and C5 carbon atoms from condensation of aromatic ring linkages.68 The integral of the region δC 160-102 ppm was taken set as the reference, assuming that it includes six aromatic carbons and 0.12 vinylic carbons. It means that the integral value for other moieties are expressed per aromatic ring (Ar).58, 69 The chemical shift of δC 62.8 and δC 60.2 ppm peaks (C-α and C-γ in β-O-4 in primary hydroxyl groups) cannot be detected by the instrument in MLNP, suggesting the consumption of hydroxyl groups during modification. The introduction of citric acid, is clearly shown by new signals at δC 44.4 ppm and an increase of methylene (R2CH2) at δH 1.2 ppm (not shown) in MLNP, which are the intrinsic methylene groups of citric acid. Meanwhile, the appearance of δC 171.4 ppm, assigned to carbonyl carbon (C=O) of carboxylic acid derivatives or ester groups could originate from the esterified lignin in MLNP70, which it is in agreement with the increased hydrogen content of carboxylic groups (13.50- 10.50) in 1

H-NMR spectra. Ester groups also contribute to an increase of the signal in the range of 116-114

ppm. Additionally, MLNP displays an arisen signal at δC 89 ppm (Alk-O-Ar), demonstrating the formation of more β aryl ether structures, and ether covalently bonded between LNP and citric acid. Hence, the postulated reaction mechanism was is shown in Figure 4. As shown in Table 3, MLNP has more phenolic OH and aliphatic CH-O and C-H moieties, while LNP is enriched in more aromatic rings (δC/δH 116-114/ 8.00-6.00) and methoxyl groups (δC/δH 56-54/4.05-3.45). The remained low molecular weight of citric acid may be a contributing factor to this increased aliphatic content, as well as the introduction of aliphatic reaction products. This indicates that acid chains were fragmented from aromatic rings. This may be attributed to the degradation of aliphatic side chains during treatment, together with opening and partial degradation of the aromatic ring, which it is consistent with decreased integral of CAr-o (δC 151-146 ppm). Overall, the 1H NMR results provided quantitative analysis of the change in functional groups. Morphological analysis: Morphological aspect of nanoparticles was observed by FESEM and TEM. As shown in Figure 5a and Figure 5b, both CNC and MCNC showed similar short rod-like features. Length and width of CNC were in the range of 100-300 nm and 10-25 nm (Figure 6a and Figure 6b). 75% of as-prepared CNC locate at 140-260 nm in length and 15-20 nm in width. However, the mean dimensions of MCNC resulted towards shorter (60-260 nm) and wider (10-35 ACS Paragon Plus Environment


nm) when compared with that of CNC. MCNC with the length range of 100-180 nm take more than 75% and the majority of widths distribute regularly in the range of 15-35 nm. In addition, while CNC appeared smooth, MCNC showed a rougher morphology, which might be dependent on the penetration and swelling effect of crosslinking resultants and limited solubilization of cellulose during esterification.67,

71

Generally, MCNC maintain the similar shape as CNC and well

individualized from each other. LNP and MLNP showed compact range size distribution (Figure 5c and Figure 5d). The average diameter of LNP and MLNP were found to be 60±15 nm and 80±17 nm, respectively (Figure 6c). The vast majority of LNP distributes in the diameter region from 30 nm to 90 nm, in agreement with LNP prepared by same procedure.45 MLNP present larger spherical nanoparticles as a consequence of attachment, entrapment and interposition of the synthesis on LNP, thus endow MLNP larger dimension and confirm the occurrence of crosslinking reaction. The changes in MCNC and MLNP morphologies upon modification were characterized by TEM and the images of CNC, MCNC, LNP and MLNP are shown in Figure 7 (a-d). It’s noteworthy that in the sample preparation procedure, CNC without negative staining exhibits low density in the final images. Nevertheless, similar threadlike shapes for both CNC and MCNC are visible, which it is in agreement with the result of the SEM investigation. It can be seen that MCNC had superior network ability in comparison to CNC, which may come from cross linking between CNC and citric acid. Nevertheless, the inherent morphology and structure of the nanocrystals were preserved. Agglomeration was moderately low for treated samples, signifying that surface modification induced hydrophobicity on CNC. Figure 7 (c) and (d) showed that both LNP and MLNP were sphere-like structures in the nanometer size, with MLNP having larger spherical particle size with respect of unmodified LNP. Aggregation of LNP and MLNP occurred to form larger particles: in the case of LNP, the aggregative tendency was more obvious, ascribing it to the reduced negative charge on the surface of LNP in comparison with MLNP. This phenomenon indicated that hydrophilicity of LNP, due to the abundant hydroxyl groups, was weakened by substitution by hydrophobic groups after modification. The results from water contact angle measurements for unmodified and modified CNC and LNP powders (Figure S1) confirmed the observations from NMR and morphological characterization: in particular, while a general decrease of the value in the proposed time scale was observed for all the nanofillers, in the case of CNC the citric acid


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modification induced a variation of the angle towards higher values for all the time sampling, with an angle variation from 41.4° up to 54.8° after CA modification at 20s); in the case of LNP, the variation was not so obvious, since a certain decrease of the angle towards lower values was detected after 8s in the case of MLNP. Thermal analysis: TG and DTG curves of citric acid, CNC and MCNC are reported in Figure 8a, while data of Tonset, Tmax and the residue at 800 °C are summarized in Table 4. It was observed that the decomposition of citric acid started at lower temperature (207 °C), achieved the maximum degradation rate at 220 °C and degraded fairly rapidly in a narrow temperature range. There is no observation of the typical degradation temperature of citric acid in MCNC, which suggests the efficient removal during dialysis. As shown in Figure 8a, both CNC and MCNC decomposed in a three stages process. It is clear that during initial stage from room temperature to 120 °C, both the samples contain small amount of moisture, as the weight loss decreases at a maximum peak upon heating, but MCNC had a minor weight loss due to increased hydrophobic behavior after modification. In the second stage, from 250 °C to 390 °C, crystalline region started to decompose, with a visible weight loss mainly due to gas phase transition and tar formation.72 During the third stage from 390 °C to 550 °C, the crystalline structure has been completely destructed and the cellulose decomposes into D-glucopyranose monomeric units, furtherly decomposed into free radical and converted into volatiles and tar.73 The onset degradation temperature was considered to start at 249 °C for CNC, 298 °C for MCNC and the highest rate of weight loss of CNC occurred at 265 °C, while for MCNC happened at 341°C. The increasing tendency of decomposition temperature indicated that the thermal stability of MCNC is higher than that of untreated CNC. Increased thermal stability of esterified samples can be ascribed to the replacement of hydroxyl groups by more stable ester groups.17, 22, 74 On the other hand, high surface area of CNC provides faster heat transfer and efficient pathways for small phonons scattering, leading to higher thermal conductivity.73, 75. In this context, the increase in thermal stability of CNC is in agreement with previous characterization, which evidenced that CNC esterification took place in their outmost layers, while kept the ultrastructure unaffected. It’s important to note that alteration of surface chemistry did not decrease thermal stability, that it extremely important when MCNC are used as reinforcement for polymers whose melting process requires elevated temperatures. In agreement with these findings, the residue of MCNC was 5% higher than CNC, implying that CNC structure



was further enhanced by reaction with citric acid, with formation of more thermo stable components. TG and derivative thermogravimetric analysis (DTG) curves of citric acid, LNP and MLNP are shown in Figure 8b. The DTG curves’ peak temperatures, maximum decomposition rates and the TGA residual values are given in Table 4. The decomposition step of both LNP and MLNP take takes place in the wide temperature range between 150 and 700 °C. When the temperature was 400 nm), indicating decreased solubility of MLNP in methanol. In addition, it should be noticed that the UV transmittance (0.3g/L), which seems to be related to the chromic effect of aromatic components in MLNP. This demonstrates that, compared to LNP, higher UV absorption capability of MLNP was noted for concentrations exceeding 0.3g/L. Both LNP and MLNP are stable and well dispersed in methanol in first 5 min during UV-vis measurement, LNP at higher concentrations (0.3 and 0.5g/L ) show a tendency to sedimentation when the measurement come to the end and the time counts about 4 min. In order to better compare the dispersability of CNC, MCNC, LNP and MLNP, images of the different suspensions are reported in Figure 9e. MCNC and MLNP are, as expected, stable and well dispersed in methanol after 10 min, while phase separation was observed in case of native (unmodified) systems. At the tested concentrations lower than 0.3 g/L, all resuspended freedried samples formed a stable suspension in methanol, whereas 0.5 g/L suspensions sedimented quickly. This result is consistent with UV-vis analysis. Unmodified CNC and LNP are not stable and show a faster sedimentation in the used solvent after 60 min. The high stability of the MCNC might be due to the substitution of OH groups by ester groups. On the other hand, the partial



repulsive force was amplified after modification, therefore the action of hydrogen bonds was considerably decreased. It is worth mentioning that a concentration higher than 0.3 % might be less unstable because of the lower interaction between nanoparticles. The observed behavior inspires the use of esterified CNC in non-polar polymeric matrices, where the alterated hydrophilicity of CNC can potentially enhance the dispersion and improve overall nanocomposite properties. MLNP have certain amount of ester groups and carboxylic hydroxyl groups that contribute to its ability to disperse in methanol. Morever, the quantity of guaiacyl phenolic groups is more in MLNP, denotating more efficient deprotonation if compared to LNP, that most likely contain irregular C-C and C-O-C in most of the positions.81 However, MLNP composed of more extent of etherified linkages compromised its solubility in the polar solvent. As a result, those formed soluble fractions substitute the strong hydrogen bond induced by abundant hydroxyl groups and form more stable suspension. Meanwhile, higher concentrations (0.3g/L and 0.5g/L) of MLNP exhibits weakened solubility, as illustrated by higher absorbance both in visual and ultraviolet light region. In fact, esterified and etherified lignin could be readily dispersed within the hydrophilic polymer matrix poly (vinyl alcohol) (Figure 10) and heeded the call for higher value of LNP as an abundant renewable industrial nanofiller.

Conclusion Functional MCNC manufactured by esterification and MLNP have been synthesized by the simultaneous esterification and etherification using citric acid as reagent. The occurrence of the chemical modification was proved by appearance of new signals for ester (MCNC and MLNP) and ether groups (MLNP), detected in the FTIR and

13

C NMR (MCNC and MLNP) and 1H NMR

(MLNP) spectra of modified samples. The new ester groups covalently attached to the surface of MCNC altered its hydrophilicity, as evidenced by better dispersion in methanol with respect of CNC. However, MLNP favors more etherification rather than limited esterification. Hence, MLNP exhibit reduced dispersability in methanol as compared to LNP. On the other hand, morphological and thermal analysis indicated that only small parts of chemical structure were affected by modification, the intrinsic shapes of MCNC and MLNP preserved, whereas the thermal stability of both moderately improved. Finally, it is concluded that the modification on CNC and LNP using citric acid as reagent was efficient and the approach adopted in this study could provide a feasible


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and convenient method to prepare functionalized nanofillers. All in all, studies on MCNC and MLNP reinforced water soluble polymers are underway to gain insight effects on nanocomposite properties and will be reported elsewhere.

Acknowledgements The authors gratefully acknowledge receiving the generous financial support from Chinese Scholarship Council (CSC). Supporting Information Figure S1: Water contact angle measurements for unmodified and citric acid modified CNC (a) and LNP (b)

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10.1016/j.biortech.2012.09.133 (68) Nair, S. S.; Sharma, S.; Pu, Y.; Sun, Q.; Pan, S.; Zhu, J. Y.; Deng, Y.; Ragauskas, A. J. High Shear Homogenization of Lignin to Nanolignin and Thermal Stability of Nanolignin-polyvinyl Alcohol Blends. Chemsuschem 2014, 7 (12), 3513-20, doi: 10.1002/cssc.201402314 (69) El Hage, R.; Brosse, N.; Chrusciel, L.; Sanchez, C.; Sannigrahi, P.; Ragauskas, A., Characterization of Milled Wood Lignin and Ethanol Organosolv Lignin from Miscanthus. Polym. Degrad. Stabil. 2009, 94 (10), 1632-1638, doi: 10.1016/j.polymdegradstab.2009.07.007 (70) Thielemans, W.; Wool, R. P. Lignin Esters for Use in Unsaturated Thermosets: Lignin Modification and Solubility Modeling. Biomacromolecules 2005, 6 (4), 1895-1905, doi: 10.1021/bm0500345 (71) Hill, C. A.; Khalil, H. A.; Hale, M. D. A study of the Potential of Acetylation to Improve the Properties of Plant Fibres. Ind. Crop. Prod. 1998, 8 (1), 53-63, doi: 10.1016/S0926-6690(97)10012-7 (72) Yang, Z.; Xu, S.; Ma, X.; Wang, S. Characterization and Acetylation Behavior of Bamboo Pulp. Wood Sci. Technol. 2008, 42 (8), 621-632, doi: 10.1007/s00226-008-0194-5 (73) Hu, W.; Chen, S.; Xu, Q.; Wang, H. Solvent-free Acetylation of Bacterial Cellulose under Moderate Conditions. Carbohyd. Polym. 2011, 83 (4), 1575-1581, doi: 10.1016/j.carbpol.2010.10.016 (74) Ramírez, J. A. Á.; Suriano, C. J.; Cerrutti, P.; Foresti, M. L. Surface Esterification of Cellulose Nanofibers by a Simple Organocatalytic Methodology. Carbohyd. Polym. 2014, 114, 416-423, doi: 10.1016/j.carbpol.2014.08.020 (75) Shimazaki, Y.; Miyazaki, Y.; Takezawa, Y.; Nogi, M.; Abe, K.; Ifuku, S.; Yano, H. Excellent Thermal Conductivity of Transparent Cellulose Nanofiber/epoxy Resin Nanocomposites. Biomacromolecules 2007, 8 (9), 29762978, doi: 10.1021/bm7004998 (76) Jakab, E.; Faix, O.; Till, F. Thermal Decomposition of Milled Wood Lignins Studied by Thermogravimetry/mass Spectrometry. J. Anal. Appl. Pyrol. 1997, 40, 171-186, doi: 10.1016/S0165-2370(97)00046-6 (77) Hosoya, T.; Kawamoto, H.; Saka, S. Role of Methoxyl Group in Char Formation from Lignin-related Compounds. J. Anal. Appl. Pyrol. 2009, 84 (1), 79-83, doi: 10.1016/j.jaap.2008.10.024 (78) Mihai Brebu, C. V. Thermal Degradation of Lignin-A Review. Cell.Chem. and Technol. 2009, 44 (9), 11. (79) Lin, X.; Sui, S.; Tan, S.; Pittman, C. U.; Sun, J.; Zhang, Z. Fast Pyrolysis of Four Lignins from Different Isolation Processes Using Py-GC/MS. Energies 2015, 8 (6), 5107-5121, doi: 10.3390/en8065107 (80) Li, H.; McDonald, A. G. Fractionation and Characterization of Industrial Lignins. Ind. Crop. Prod. 2014, 62, 6776, doi: 10.1016/j.indcrop.2014.08.013 (81) Nagy, M.; Kosa, M.; Theliander, H.; Ragauskas, A. J. Characterization of CO2 Precipitated Kraft Lignin to Promote Its Utilization. Green Chem. 2010, 12 (1), 31-34, doi: 10.1039/B913602A



Page 24 of 37

Table 1: Signal assignment and partial integral of CNC and MCNC CP MAS 13C NMR

No.

Integral Range

Assignment

(ppm)

Chemical

Integral

shift (ppm)

Chemical

Integral

shift (ppm)

CNC

MCNC

1

C=O

--

178

2

C1

104.7

104.5

3

85-96

C4crystalline

87.9

0.96

87.8

1.00

4

79-85

C4amorphous

82.8

0.60

83

0.73

5

73.7

73.7

70.9

70.8

C2-C3-C5 cluster

6

C6crystalline region

64.2

64.3

7

C6amorphous

61.3

61.4

8

CH2Citric acid

--

45


Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Assignment and quantification of signals in the 13C NMR spectrum of LNP and MLNP

No.

Chemical shift (ppm)

Assignment

Integral LNP

MLNP

1

171.4

ester groups (OCOCH3)

--

new

2

151-146

CAr-O C4 (G etherified)

0.87

0.81

3

129-126

C2/C6 H

0.44

0.44

4

120-118

Ar-H (C6 G)

0.55

absent

5

116-114

C5 G, C3/C5 PC ester CAr-H

0.84

0.88

6

109.5

acetals

present

absent

7

89

Alk-O-Ar (Cβ in β-O-4, Cα in β5 and ββ)

--

new

8

62.8

R-CH2OH ( Cγ in PC OHprim)

0.03

absent

9

60.2

R-CH2OH (Cγ in β-O-4)

0.19

absent

10

56-54

OCH3 in G

1.77

1.66

11

44.4

CH2Citric acid

--

new

PC = p-coumaryl, G = guaiacyl, Ar = Aromatic ring



Page 26 of 37

Table 3: Partial assignment and hydrogen content (%) of different functional groups in the ratio of all H containing functional groups as determined by 1H NMR spectrum of LNP and MLNP

No.

Chemical shift (ppm)

Assignment

Hydrogen content of selected groups (%) LNP

MLNP

1

13.50-10.50

Carboxylic acid

1.92

3.00

2

9.35-8.00

phenolic

1.92

3.00

3

8.00-6.00

Aromatic, vinyl

26.92

18.18

4

6.00-4.05

Aliphatic CH-O

7.69

15.15

5

4.05-3.45

Methoxyl –OCH3

48.08

30.3

6

2.25-0.00

Aliphatic C-H

13.46

30.3

Hydrogen content of selected groups was calculated based on the integral value of 1H-NMR spectrum of LNP and MLNP.

Table 4: Thermal characteristics from TGA for citric acid, CNC, MCNC, LNP, and MLNP

Tonset (°C)

Tmax (°C)

Residue (%)

Citric acid

207

220

0.36

CNC

249

265

17

MCNC

298

341

22


Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tonset (°C)

Tmax1 (°C)

Tmax2 (°C)

Tmax3 (°C)

Residue (%)

LNP

253.1

--

371

629

25.6

MLNP

227.3

215.6

381

--

34.2

For Table of Contents Use Only

Esterified CNC and esterified/etherified LNPs altered by citric acid treatment showed tuned dispersability, preserved morphology and enhanced thermal stability



Figure 1: Prominent units and major linkages among units of lignin, (A) β-O-4, (B) β—β, (C) β-5, (D) 4-O-5, (E) β-1, (F) 5-5, (G) α-O-4 516x313mm (96 x 96 DPI)


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Figure 2: FTIR spectra of pristine CNC, modified CNC before dialysis (MCNC_BD), and modified CNC after dialysis (MCNC_AD) are shown in (a), pristine LNP, modified LNP before dialysis (MLNP_BD) and modified LNP after dialysis (MLNP_AD) are shown in (b). 1751 cm-1 (I), 1640 cm-1 (II), 1607 cm-1 (III), 1222 cm1(IV), 1030 cm-1(V), 1745 cm-1 (VI), 1671cm-1 (VII), 1347 cm-1 (VIII), 1289 cm-1 (IX), 1140 cm-1 (X), 1097 cm-1 (XI), 859 cm-1 (XII). 450x303mm (96 x 96 DPI)



Figure 3: A schematic diagram illustrating esterification of CNC and carboxyl groups of citric acid 481x373mm (96 x 96 DPI)


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Figure 4: Proposed major units transformations of LNP during modification and schematic mechanism between functional groups of LNP and citric acid 559x187mm (96 x 96 DPI)



FESEM images of CNC (a), MCNC (b), LNP (c) and MLNP (d) 195x140mm (150 x 150 DPI)


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Figure 6: Dimensions of CNC and MCNC, length ((a), ranging from 60-300 nm) and diameter ((b), ranging from 10-35 nm) in the different distribution are statistically fitted to the nonlinear Gaussian curves; (c) Diameter of LNP and MLNP in various distribution (30-120 nm), LNP and MLNP were taken as spherical shapes and the diameters were fitted into Gaussian to obtain statistical analysis 522x193mm (96 x 96 DPI)



Figure 7: TEM images of CNC (a), MCNC (b), LNP (c) and MLNP (d) 305x304mm (96 x 96 DPI)


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Figure 8: TG and DTG curves of CNC, citric acid, and MCNC(a), LNP, citric acid and MLNP (b) 415x214mm (96 x 96 DPI)



Figure 9: UV-vis absorbance spectra of various concentrations (0.05, 0.1, 0.3, 0.5 g/L) of CNC(a), MCNC (b), LNP (c) and MLNP (d), methanol used as solvent. Photographs of CNC, MCNC, LNP and MLNP samples after standing for certain time (0, 5 min, 10min, 60 min, 21 days), at concentrations corresponding to UVvis (e). 374x232mm (96 x 96 DPI)


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Figure 10: Photographs of CNC/PVA, MCNC/PVA, LNP/PVA and MLNP/PVA films at various loading levels (from 1 to 5 % wt.) showing the color difference and dispersion variability of CNC and LNP nanofiller in poly (vinyl alcohol) (PVA)matrix 374x194mm (96 x 96 DPI)


Citric Acid as Green Modifier for Tuned Hydrophilicity of Surface

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