nanomaterials

Article Reinforcement of Natural Rubber Latex Using Jute Carboxycellulose Nanofibers Extracted Using Nitro-Oxidation Method

Sunil K. Sharma 1 , Priyanka R. Sharma 1,* , Simon Lin 1, Hui Chen 1, Ken Johnson 1, Ruifu Wang 1, William Borges 2, Chengbo Zhan 1 and Benjamin S. Hsiao 1,* 1 Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, USA; [email protected] (S.K.S.); [email protected] (S.L.); [email protected] (H.C.); [email protected] (K.J.); [email protected] (R.W.); [email protected] (C.Z.) 2 Roslyn High School, Roslyn, NY 11576, USA; [email protected] * Correspondence: [email protected] (P.R.S.); [email protected] (B.S.H.); Tel.: +1-631-5423506 (P.R.S.); +1-631-6327793 (B.S.H.)


Received: 28 February 2020; Accepted: 29 March 2020; Published: 8 April 2020


Abstract: Synthetic rubber produced from nonrenewable fossil fuel requires high energy costs and is dependent on the presumed unstable petroleum price. Natural rubber latex (NRL) is one of the major alternative sustainable rubber sources since it is derived from the plant ‘Hevea brasiliensis’. Our study focuses on integrating sustainably processed carboxycellulose nanofibers from untreated jute biomass into NRL to enhance the mechanical strength of the material for various applications. The carboxycellulose nanofibers (NOCNF) having carboxyl content of 0.94 mmol/g was prepared and integrated into its nonionic form (–COONa) for its higher dispersion in water to increase the interfacial interaction between NRL and NOCNF. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analyses of NOCNF showed the average dimensions of nanofibers were length (L) = 524 203 nm, diameter (D) 7 2 nm and thickness 2.9 nm. Furthermore, fourier transform ± ± infra-red spectrometry (FTIR) analysis of NOCNF depicted the presence of carboxyl group. However, the dynamic light scattering (DLS) measurement of NRL demonstrated an effective diameter in the range of 643 nm with polydispersity of 0.005. Tensile mechanical strengths were tested to observe the enhancement effects at various concentrations of NOCNF in the NRL. Mechanical properties of NRL/NOCNF films were determined by tensile testing, where the results showed an increasing trend of enhancement. With the increasing NOCNF concentration, the film modulus was found to increase quite substantially, but the elongation-to-break ratio decreased drastically. The presence of NOCNF changed the NRL film from elastic to brittle. However, at the NOCNF overlap concentration (0.2 wt. %), the film modulus seemed to be the highest.

Keywords: natural rubber latex; NOCNF; jute fibers; nitro-oxidation

1. Introduction Synthetic and natural rubber are a staple commodity for numerous industrial applications [1–6]. The International Rubber Study Group (IRSG) [7] reported that the U.S. consumed 2.7 million metric tons of rubber ranging from automotive parts to sealants in 2013. These products consist of mostly synthetic rubber derived from petroleum sources and natural rubber derived from Hevea trees (Hevea brasiliensis). The use of petroleum-derived synthetic rubber causes several concerns [8,9]. The reliance on nonrenewable resources causes a dependent and unstable price for synthetic rubber costs. Synthetic rubber production also requires a higher energy consumption and is environmentally

Nanomaterials 2020, 10, 706; doi:10.3390/nano10040706 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 706 2 of 13 intensive when compared to using natural rubber [8]. Natural rubber in its unprocessed or raw form has low strength and that limit their applications. The strength of the natural rubber latex is improved mostly by the vulcanization process, where the long chains of rubber molecule are cross-linked through the chemical process which ultimately transform the natural rubber latex into a strong elastic product (natural rubber) with reversible deformability, good mechanical strength, excellent dynamic properties and fatigue resistance [10]. The mechanical properties of natural rubber latex can be improved by addition of varying types of reinforcing fillers. Nanocellulose is a most abundant, inexpensive and renewable nanomaterial that has potential in many different applications including pharmaceuticals, food, energy storage, water purification, biomedical, 3D printing, anti-bacterial, carbon nanotubes stabilizer, electronics and tissue engineering. Owing to its exceptional mechanical properties, nontoxicity, biodegradability and tunable chemistry of surface hydroxyl groups, nanocellulose has garnered tremendous levels of attention over the past decades [11–13]. There are many methods reported for preparation of nanocellulose from various biomass sources, for example, carboxymethylation, acid hydrolysis to produce cellulose nanocrystals, 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation, nitro-oxidation etc. A potential reinforcing agent for the latex rubbers is the derivatives of natural cellulose polymers [14–19]. These polymers are called carboxycellulose and have been widely used for biomedical applications such as surgical sutures [20–22]. Recent developments of carboxycellulose in the nanoscale have further expanded their uses in making strengthened nanocomposite materials [23–25]. Since carboxycellulose nanofibers is derived from cellulose microfibril building blocks, it is readily able to be extracted from a variety of biomass materials [26,27]. Some of these biomass materials include jute, spinifex, agave and agricultural wastes [28]. Our study primarily focuses on jute-derived carboxycellulose nanofibers extracted using the recently developed nitro-oxidation method [28–32]. The nitro-oxidation method is found to be a simple, cost-effective process to extract the carboxycellulose nanofibers from any type of raw biomass that does not require any pretreatment steps. However, the other methods—TEMPO oxidation and carboxymethylation processes—are fully efficient in extracting the carboxycellulose nanofibers from delignified pulp, which requires prior treatment of raw biomass. Nitro-oxidation produces carboxycellulose nanofibers with residual lignin and hemicellulose impurities; however, it requires less chemicals, processing time and steps for their extraction. Additionally, the unused effluent of the reaction has potential to be converted into nitrogen-rich plant fertilizer. The nitro-oxidation method involves the reaction of nitric acid with sodium nitrite to create nitroxonium + ions (NO ) which attacks the hydroxyl group on cellulose to produce a nitrite ester (R–CH2–O–NO). The nitrite ester then decomposes and generates nitroxyl (HNO) and aldehyde groups which is further oxidized into carboxyl (COOH) groups. This oxidation cycle continues at the presence of excess HNO and HNO2 to create the saturated carboxyl groups which provide the function sites for further chemical reaction. Since this is a recently developed method [28,31,33–35], there are also interests in applying these carboxycellulose nanofibers materials for further testing. The primary focus of this study is to integrate this low-cost nitro-oxidized carboxycellulose nanofibers (NOCNF) into natural rubber latex sources to observe the enhancement effect of the samples at various concentrations. Since latex are primarily composed of cis 1,4 polyisoprene emulsions in water [36], we chose the carboxylate functional group (–COO−) to induce a high dispersity during the integration stage. The carboxylate functional group is hydrophilic which along with dispersity, could allow for better interfacial interactions between the fibers and the isoprene molecules. Other latex enhancement studies also indicate the use of a hydrophilic carboxycellulose nanofibers state to be effective for making enhanced nanocomposites with enhanced tensile modulus [23]. Nanomaterials 2020, 10, 706 3 of 13

2. Methodologies

2.1. Materials Untreated jute fibers were provided by Toptrans Bangladesh Ltd. (Dhaka, Bangladesh). Fibers were cut to 3–5 cm in length and further grinded by an IKA MF 10 basic grinder at 1000 rpm (IKA Works Inc., Wilmington, NC, USA). Analytical-grade nitric acid (ACS reagent 60%) and sodium nitrite (ACS reagent 97%) were purchased from Sigma-Aldrich (Allentown, PA, USA); sodium bicarbonate was ≥ purchased from Fischer Scientific (Fairlawn, NJ, United States). Processed polygen liquid latex (NRL) nonvulcanized with 60% concentration was obtained from UK suppliers (ReAgent, Runcorn, UK).

2.2. Experimental Method

2.2.1. Preparation of Carboxycellulose Nanofibers (NOCNF) Fifteen grams of untreated grinded jute fibers were placed in a 3 L three-neck, round-bottom flask with 210 mL of 60 % nitric acid. Fibers in the flask were allowed to completely soak before adding 14.4 g of sodium nitrite. The addition of sodium nitrite forms red gas inside the flask due to generation of NO2 gas. Hence, the mouths of the round bottom flask were sealed with stoppers sealed with parafilm. The reaction was performed at 40 ◦C for 16 h and was then quenched by adding 1 L distilled (DI) water. The supernatant liquid was discarded to remove excess acid and decantation with 70% ethanol and DI water in the ratio of 80:20 was performed 4–5 times until the suspended fibers stopped settling down. The fibers suspension was then transferred to a dialysis bag (Spectral/Por, molecular weight cut-off (MWCO): 6–8 kDA) and equilibrated until the conductivity of the water reached below 5 µS. The fibers were then treated with 8% sodium bicarbonate up to pH 7.5, to transform the initially generated carboxyl group (COOH) to carboxylate groups (COO−). To remove the excess bicarbonate from the fibers, the suspension was again dialyzed using the dialysis bag until the conductivity of the water reached below 5 µS. The 0.2 wt. % of fibers suspension then passed through homogenizer (GEA Niro Soavi Panda Plus Bench top homogenizer, Columbia, MD, USA), at 250 bar for one cycle.

2.2.2. Nanocomposite Preparation A 50 mL closed glass vial was used to integrate NRL and NOCNF. Ten milliliters of 60% solid NRL was measured for each 0–0.4 wt. % sample. A 0.26 wt. % NOCNF suspension with different volume was added into NRL solution to prepare the solution containing different (0.1, 0.2 and 0.4 wt. %) of NOCNF. The solutions were set to stir for 16 h followed by 1 h of sonication. Afterwards the prepared solution was casted evenly on a petri dish and degassed to prevent bubbles forming.

2.2.3. Characterization of Carboxycellulose Nanofibers (NOCNF)

Fourier Transform Infra-Red Spectrometry (FTIR) The FTIR curve was measured with a PerkinElmer Spectrum One instrument (product model, 1 city, country) with the transmission mode set between 450 and 4000 cm− . Three scans were taken per 1 sample with a resolution of 4 cm− .

Conductometric Titration Method

The carboxylate (–COO−) group in NOCNF was determined by measuring the conductivity throughout a base titration experiment. A calculated volume containing 0.3 g of nanofibers was dispersed in 55 mL of DI water. The pH was set between 2.5 and 3.0 by adding 0.1 M HCl. The solution was then titrated with 0.4 M NaOH at a rate of 0.1 mL/min until pH reached 11. Throughout the titration, the pH was measured along with the conductivity. The carboxylate content was calculated using the conductivity and pH curves. Nanomaterials 2020, 10, 706 4 of 13

Lignin and Hemicellulose Analysis in Raw Jute Fibers and NOCNF Lignin and hemicellulose (total sugar) analysis of the samples was performed by Celignis (Limerick, Ireland). The following analytical procedures were used: (1) acid hydrolysis of samples, (2) determination of acid-soluble lignin (ASL) using Ultraviolet–Visible (UV-Vis) spectroscopy, (3) gravimetric determination of klason lignin (KL) and (4) chromatographic analysis of hydrolysate. A detailed explanation of the analytical procedure is provided in Supplementary Materials.

Transmission Electron Microscopy (TEM) The TEM image was obtained with a FEI Tecnai G2 Spirit Bio TWIN instrument (Columbia, MD, United States). The instrument is equipped with a digital camera which allowed it to take photographic film. The instrument is also equipped with tilt stage and electron diffraction capabilities. The samples were prepared using a 10 µL aliquot sample of 1 mg of NOCNF in 10 mL DI water deposited on carbon coated Copper grids (300 mesh, Ted Pella Inc., Redding, CA, United States). The prepared grid was then stained with 2 wt. % aqueous uranyl acetate solution.

Atomic Force Microscopy (AFM) AFM of NOCNF was performed using a Bruker Dimension ICON scanning probe microscope (Bruker Corporation, Billerica, MA, USA) equipped with a Bruker OTESPA tip (tip radius (max.) = 10 nm). In this measurement, a 10 µL of 0.005 wt. % NOCNF suspension was deposited on the surface of a silica plate, where the air-dried sample was measured in the tapping mode.

Zeta Potential Measurements Zeta potential of the NOCNF sample was measured by Zeta probe Analyzer (Colloid Dynamics). This instrument consisted of a built-in titration setup equipped with pH electrode and Electrokinetic Sonic Amplitude (ESA) sensor probe. Before analyzing the sample, the pH electrode was calibrated using three different pH buffer standards (pH = 4.01, 7.01 and 10.01), followed by a standard titration solution. The ESA sensor was calibrated using the standard zeta probe polar solution (KSiW solution). Upon the completion of calibration test, the NOCNF suspension (0.26 wt. %, 250 mL) was filled in the sample holder, where the ESA sensor was then introduced into the sample under magnetic stirring to analyze the zeta potential.

Dynamic Light Scattering (DLS) The DLS of NRL sample was measured using Nano Brook 90 Plus particle size analyzer (Brookhaven, Holtsville, NY, USA) The DLS graph was set to lognormal plot and the data is an average of four total runs to obtain the polydispersity index (PDI).

Contact Angle Measurement Static contact angle of NRL and composite films prepared by NOCNF and NRL were measured using the FDS-contact angle measurement instrument (Model no. OCA 15 EC). Films of NRL alone and composite films containing NRL and NOCNF were prepared by solvent casting method. A flat portion of film was cut and fixed onto sample holder. A 10 µL drop of deionized water was dropped onto film through a syringe needle operated automatically by syringe pump. The contact angle was measured 20 s after the drop casting to ensure that the water droplet reached its equilibrium position.

Tensile Test The tensile data was obtained using the INSTRON model 4442 device (Instron, Norwood, MA, USA). Clamps were calibrated with a 1.75 cm gap and rectangular samples of 2 cm wide by 5 cm long were clamped evenly in the device. Elongation rate was set to 60 mm/min with data recording every second. Each data plot obtained is an average of three sets of experiments. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 13 Nanomaterials 2020, 10, 706 5 of 13 Scanning Electron Microscopy (SEM) ScanningA Zeiss Electron LEO Microscopy 1550 SFEG-SEM (SEM) instrument (White Plains NY, USA) was used to record SEM imagesA Zeiss of the LEO samples. 1550 SFEG-SEM The instrument instrument was(White comprised Plains of NY, an USA)in-lens was secondary used to recordelectron SEM detector images in ofaddition the samples. to the The standard instrument E-T wasdetector, comprised and a of Ruther an in-lensford secondarybackscatter electron electron detector detector. inaddition It was also to theequipped standard with E-T detector,an EDS and (energy a Rutherford dispersive backscatter X-ray electronspectroscopy) detector. system, It was alsoprovides equipped elemental with ancompositions EDS (energy and dispersive X-ray maps X-ray of spectroscopy) the various phases system, of provides the materials elemental examined. compositions Images and of surface X-ray mapsmorphology of the various of NRL phases and NRL of the composite materials films examined. were taken Images to observe of surface the morphology NRL film surface of NRL change and NRLon addition composite of filmsNOCNF. were taken to observe the NRL film surface change on addition of NOCNF.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Characterization Characterization of of NOCNF NOCNF TheThe characterization characterization on on the the surface surface functionalization functionalization of of NOCNF NOCNF extracted extracted from from untreated untreated jute jute fibersfibers was was first first carried carried out out by by FTIR FTIR and and conductometric conductometric titration.titration. FigureFigure1 1(i)i demonstrates demonstrates the the FTIR FTIR spectraspectra of of jute jute fibers fibers and and prepared prepared NOCNF. NOCNF. The The characteristic characteristic peaks peaks of cellulose of cellulose are ascribed are ascribed at (i) 3327 at (i) 1 1 cm3327− to cm O–H-1 to stretching O–H stretching vibrations, vibrations, and at (ii) and 2904 at cm(ii)− 2904to CH cm and−1 to CH CH2 stretching. and CH2 Thestretching. prominent The 1 peakprominent in NOCNF peak atin 1591NOCNF cm− atpresented 1591 cm-1the presented carboxylate the carboxylate groups (–COONa) groups (–COONa) appeared inappeared NOCNF, in whichNOCNF, represents which therepresents oxidation the of oxidation anhydroglucose of anhydroglu unit atcose C6 position. unit at C6 Additional position. peaksAdditional at 1372, peaks 1150, at 1 1100,1372, and 1150, 1030 1100, cm −andwere 1030 due cm-1 to were stretching due to and stretching bending and vibrations bending invibrations glycosidic in bondsglycosidic in cellulose. bonds in 1 Othercellulose. peaks Other in the peaks FTIR in of the jute FTIR fibers of such jute asfibers 1512; such 1732, as 1456, 1512; 1235 1732, and 1456, 808 1235 cm− andare 808 because cm−1 ofare aromaticbecause symmetricalof aromatic symmetrical streching of Cstreching=C bonds of in C=C the bonds lignin andin the in hemicelluloselignin and in hemicellulose units respectively. units Interestingly,respectively. the Interestingly, peaks belonging the peaks to lignin belonging and hemicellulose to lignin and completelyhemicellulose disappear completely or significantly disappear or reducesignificantly in NOCNF, reduce indicating in NOCNF, that the indicating nitro-oxidation that the was nitro-oxidation resonably effective was inresonably removing effective the lignin in andremoving hemicellulsoe the lignin impurities and hemicellulsoe from raw jute fibers.impuriti Thees quantitativefrom raw jute determination fibers. The of quantitative lignin and hemicelulosedetermination in of NOCNF lignin and was hemicelulose also perfomed in NOCN to findF the was exact also amountperfomed of to lignin find andthe exact hemicellulsoe, amount of whichlignin is and explained hemicellulsoe, in the next which section. is explained in the next section.

NOCNF 0.15 wt. %

FigureFigure 1. 1.( i()i) Fourier Fourier transform transform infrared infrared spectrometry spectrometry (FTIR) (FTIR) of of carboxycellulsoe carboxycellulsoe nanofibers nanofibers (NOCNF) (NOCNF) andand jute jute fibers, fibers, and and (ii) conductometric(ii) conductometric titration titration graph graph to determine to determine the carboxylate the carboxylate group on group NOCNF on (volumeNOCNF of (volume NaOH consumedof NaOH consumed= 0.705 mL), = 0.705 inset mL), the photograph inset the photograph of NOCNF of suspension. NOCNF suspension.

TheThe quantitativequantitative determinationdetermination ofof carboxylatecarboxylate groupsgroups inin NOCNFNOCNF waswas performedperformed byby conductometricconductometric titration titration method.method. The following eq equationuation (Equation (Equation 1) (1)) was was used used to todetermine determine the thedegree degree of ofoxidation oxidation (DO (DO): ): MX (V2 V ) DO = − 1 (1) w (1) where M is the molarity of NaOH in mol/L, V2 and V1 is the final and initial volume of NaOH in mL, w iswhere the weight M is the of the molarity NOCNF of NaOH dried fibers in mol/L, added V2 inand grams. V1 is the final and initial volume of NaOH in mL, w is the weight of the NOCNF dried fibers added in grams.

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The conductometric titration plot of NOCNF shown in Figure 1(ii) indicates its calculated DO valueThe which conductometric is 0.94 mmol/g. titration This plotDO ofvalue NOCNF shows shown that NOCNF in Figure 1containsii indicates a moderate its calculated degree DO of valueoxidation, which which is 0.94 is mmol further/g. Thisconfirmed DO value by showszeta potential that NOCNF measurement. contains a moderateThe zeta degreepotential of oxidation,measurement which demonstrates is further confirmed the presence by zeta of potential−115 ± 4 measurement.mV charge on The the zeta NOCNF potential surface. measurement NOCNF demonstrates the presence of 115 4 mV charge on the NOCNF surface. NOCNF showed good showed good dispersion in water− (inset± photograph in Figure 1(ii)) because of the repulsion caused dispersionin between inthe water fibers (inset due to photograph similar charges. in Figure 1ii) because of the repulsion caused in between the fibersThe due lignin to similar and hemicellulose charges. analysis of raw jute fibers is presented in Table 1. It shows that the total Thehemicellulose lignin and (sugar hemicellulose content) analysis and total of rawlignin jute content fibers is(klason presented lignin in (KL) Table +1 .acid It shows soluble that lignin the total(ASL)) hemicellulose in the raw jute (sugar fibers content) was 68.9% and totaland lignin17.55%, content respectively. (klason However, lignin (KL) the+ totalacid solublehemicellulose lignin (ASL))and lignin in the content raw jutein NOCNF fibers was was 68.9% found and as 65% 17.55%, and respectively.1.94% respectively. However, The results the total indicate hemicellulose that the andreasonable lignincontent hemicellulose in NOCNF and residual was found lignin as 65% are andstill present 1.94% respectively. in the NOCNF The after results the indicatenitro-oxidation. that the reasonable hemicellulose and residual lignin are still present in the NOCNF after the nitro-oxidation. Table 1. Characteristic of carboxycellulsoe nanofibers (NOCNF) obtained from jute. Table 1. Characteristic of carboxycellulsoe nanofibers (NOCNF) obtained from jute. Carboxylate Zeta Residual Residual Carboxylate Zeta Residual Residual Length/Width Thickness Sample Content Potential Lignin (%) hemicellulose Length/Width Thickness Sample Content Potential Lignin (%) hemicellulose (nm) (nm) (mmol/g) (mV) KL/ASL a (%) (nm) (nm) (mmol/g) (mV) KL/ASL a (%) 524 ± 203/ NOCNF 0.94 −115 ± 4 0.58/1.36 65 524 203/ 2.9 NOCNF 0.94 115 4 0.58/1.36 65 ±7 ± 2 2.9 − ± 7 2 a KL = klason lignin, ASL = acid soluble lignin. ± a KL = klason lignin, ASL = acid soluble lignin.

The TEM image of NOCNF is shown in Figure 2i. The average fiber length observed for The TEM image of NOCNF is shown in Figure2i. The average fiber length observed for NOCNF NOCNF was 524 ± 203 nm and width were in the range of 7 ± 2 nm. However, the AFM of the was 524 203 nm and width were in the range of 7 2 nm. However, the AFM of the NOCNF indicated NOCNF± indicated the average fibers thickness was± 2.9 nm. In this study, the NOCNF obtained has the average fibers thickness was 2.9 nm. In this study, the NOCNF obtained has greater width and greater width and thickness as compared to the cross section of most of the cellulose fibers where thickness as compared to the cross section of most of the cellulose fibers where width is in the range of width is in the range of 4–5 nm and thickness ~1.5 nm.[37] This is probably due to the chosen 4–5 nm and thickness ~1.5 nm [37]. This is probably due to the chosen nitro-oxidation conditions are nitro-oxidation conditions are relatively mild, where the presence of residual hemicellulose and relatively mild, where the presence of residual hemicellulose and lignin contents were still high. lignin contents were still high.

Figure 2. (i) Transmission electron microscopy (TEM) and (ii) atomic force microscopy (AFM) of Figure 2. (i) Transmission electron microscopy (TEM) and (ii) atomic force microscopy (AFM) of NOCNF extracted from raw jute fibers. NOCNF extracted from raw jute fibers. The dynamic light scattering (DLS) data measurement of NRL is presented in Figure3. The data The dynamic light scattering (DLS) data measurement of NRL is presented in Figure 3. The data shows that the effective diameter of NRL molecule is 637 nm with polydispersity value of 0.005. shows that the effective diameter of NRL molecule is 637 nm with polydispersity value of 0.005. This This indicates that the NRL is composed of polyisoprene molecules with almost the same size. The TEM indicates that the NRL is composed of polyisoprene molecules with almost the same size. The TEM measurement of NOCNF indicated its fibers length in the range of 524 203 nm, which is almost like measurement of NOCNF indicated its fibers length in the range of 524± ± 203 nm, which is almost like the NRL particles’ size. We have assumed that similar sizes of two interacted molecules NOCNF and the NRL particles’ size. We have assumed that similar sizes of two interacted molecules NOCNF and NRL will provide the better chances of their physical interaction. NRL will provide the better chances of their physical interaction.

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Figure 3. Dynamic light scattering (DLS) data of natural rubber latex (NRL) (averagediameter = 637 nm Figurewith size 3. Dynamic polydispersity light scattering (PDI) = 0.005. (DLS) data of natural rubber latex (NRL) (average diameter = 637 nm with size polydispersity (PDI) = 0.005. 3.2. Characterization of Natural Rubber Latex (NRL) and Composite Films 3.2. CharacterizationThe contact angle of Natural measurement Rubber wasLatex performed (NRL) and onComposite the films Films prepared by NRL and the composite filmsThe made contact of NRL angle and varyingmeasurement content ofwas NOCNF performe (0.1,d 0.2 on and the 0.4 films wt. %),prepared which isby shown NRL in and Figure the4. compositeThe contact films angle made measurement of NRL ofand pure varying NRL has conten shownt of the NOCNF average (0.1, angles 0.2 (left and= 0.463.8 wt.◦ and %), right which= 65 is◦) shownclearly in indicate Figure that4. The it hascontact hydrophobic angle measurement surface. An of earlier pure NRL study has of shown the chemically the average crosslinked angles (left NRL = 63.8°film (usingand right potassium = 65°) persulphateclearly indicate (KPS) that as an it initiator)has hydrophobic reported thatsurface. the contactAn earlier angle study should of be the in chemicallythe range ofcrosslinked 94◦, which NRL was film clearly (using more potassium hydrophobic persulphate than the (KPS) noncrosslinked as an initiator) film inreported the present that thestudy contact [38]. However,angle should the smallbe in additionthe range of of NOCNF 94°, which (0.1 wt.was %) clearly into NRL more has hydrophobic changed the than average the noncrosslinkedcontact angle of film composite in the present membrane study to be[38]. around However, 49.5 ◦the(left small= 51 addition◦ and right of =NOCNF48◦). The (0.1 decrease wt. %) intoin contact NRL has angle changed indicates the theaverage appearance contact ofangle hydrophilic of composite behavior membrane in composite to be around film because 49.5 ° (left of the = 51°presence and right of NOCNF = 48°). whichThe decrease has more in hydrophiliccontact angle groups indicates such the as hydroxyl appearance and of carboxylate. hydrophilic The behavior further inaddition composite of NOCNF film because (0.2 and of 0.4thewt. presence %) into of NRL NOCNF has further which reducedhas more the hydrophilic contact angle groups of composite such as hydroxylfilms (e.g., and film carboxylate. with 0.2 wt. The % further NOCNF: addition left = of48.4 NOCNF◦ and right (0.2 and= 44 0.4◦; film wt. with%) into 0.4 NRL wt. has % NOCNF: further reducedleft = 48 the◦ and contact right angle= 43◦ ).of Interestingly,composite films not (e.g., much film change with 0.2 in thewt. contact% NOCNF: angle left of = composite 48.4° and right films =containing 44°; film with 0.2 and0.4 wt. 0.4 % wt. NOCNF: % of NOCNF left = 48° was and observed. right = 43°). This Interestingly, is probably not because much 0.2 change wt.% in is the the contactoverlapping angle concentration of composite of films NOCNF containing [31], where 0.2 someand 0.4 aggregations wt. % of NOCNF might have was occurred observed. in theThis NRL is probablymatrix at andbecause above 0.2 overlapping wt.% is concentrationthe overlapping (i.e., concentration 0.2 and 0.4 wt. of %) ofNOCNF NOCNF, [31], which where resulted some in aggregationsdecrease in hydrophobicity might have occurred of the composite in the NRL films. matrix at and above overlapping concentration (i.e., 0.2 and 0.4 wt. %) of NOCNF, which resulted in decrease in hydrophobicity of the composite films.

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FigureFigure 4. Contact angle measurementsmeasurements of of ( i()i) NRL NRL film film (control), (control), and and composite composite films films made made of NRL of NRL and andNOCNF NOCNF with with varying varying concentration concentration of NOCNF of NOCNF (ii) 0.1(ii) wt. 0.1 %,wt. ( iii%,) ( 0.2iii) wt.0.2 %,wt. ( iv%,) ( 0.4iv) wt.%.0.4 wt.%.

3.3.3.3. SEM SEM Images Images SEMSEM images images of of film film made of of NRL and compos compositeite films films consisting of of NRL and varying concentrationsconcentrations NOCNF NOCNF are are presented presented in in Figure Figure 5.5 .Th Thee image image of ofNRL NRL in inFigure Figure 5i 5indicatesi indicates that that the filmthe filmpossesses possesses uniform uniform surface surface roughness. roughness. This This was was due due to tothe the agglomeration agglomeration of of similar similar granular granular shapeshape NRL NRL particles. particles. This This was was consistent consistent with with the the size size characterization characterization of of NRL NRL using using the the DLS DLS techniquetechnique (Figure(Figure3 ,3, NRL NRL possessed possessed an an average average particle partic sizele size of 640 of nm640 with nm thewith polydispersity the polydispersity of 0.005). of 0.005).The morphology The morphology of NRL of filmNRL on film addition on addition of 0.1 of wt. 0.1 % wt. of % NOCNF of NOCNF (nanofibers) (nanofibers) has nothas shownnot shown any anysignificant significant changes changes on theon the surface surface appearance. appearance. However, However, the the addition addition of of 0.2 0.2 wt. wt. %% and 0.4 wt. wt. % % nanofibersnanofibers into NRL (Figure 55iiiiii,iv) and resulted iv) resulted in significant in significant changes changes in surface in surface appearance appearance in terms in termsof roughness. of roughness. TheThe roughness roughness of of the the latex latex film film decreases decreases on on increasing increasing the the amount amount of of nanofibers nanofibers concentration concentration aboveabove 0.1 wt.wt. %% thatthat could could be be because because of of the the well well distribution distribution of NOCNF of NOCNF with with the latex the particles.latex particles. These Thesedata correlate data correlate well with well the with contact the anglecontact measurement, angle measurement, as the drastic as the decrease drastic in decrease contact anglein contact from angle63◦ to from 43◦ was 63° observedto 43° was for observed the NRL for film the on NRL addition film ofon 0.1 addition and 0.2 of wt. 0.1 % and of NOCNF.0.2 wt. % However, of NOCNF. the However,cracks start the beginning cracks instart composite beginning films in containingcomposite the films (highest) containing 0.4 wt. the % of(highest) NOCNF. 0.4 The wt. probable % of NOCNF.reason for The the probable crack could reason be thefor phasethe crack separation could be in betweenthe phase NRL separation and NOCNF in between because NRL of theirand NOCNFcorresponding because hydrophobic of their corresponding and hydrophilic hydrophobic behavior. and hydrophilic behavior.

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Figure 5. Scanning electron microscopy (SEM) images taken at scale bar = 200 nm on (i) NRL film Figure(control), 5. Scanning and composite electron films microscopy made of (SEM) NRL and images NOCNF taken with at scale varying bar concentration= 200 nm on ( ofi) NOCNFNRL film ( ii) (control),0.1 wt.%, and (iii composite) 0.2 wt.%, films (iv) 0.4 made wt.%. of NRL Red circlesand NOCNF indicate with the cracksvarying in concentration the film. of NOCNF (ii) 0.1 wt.%, (iii) 0.2 wt.%, (iv) 0.4 wt.%. Red circles indicate the cracks in the film. 3.4. Mechanical Properties of Latex and Composite Films 3.4. Mechanical Properties of Latex and Composite Films The mechanical properties of NRL and composite films containing NRL and varying concentrations of NOCNFThe mechanical is shown inproperties Table2. Theof stress–strainNRL and composite curve was plottedfilms containing for all the compositeNRL and films varying and is concentrationspresented in Figure of NOCNF6. The stressis shown on the in y-axis Table was 2. calculatedThe stress–strain by dividing curve the loadwas byplotted the cross-sectional for all the compositearea of the films film. and The is thickness presented and in lengthFigure of 6. the The film stress is measured on the y-axis using was a caliper calculated and all by nanocomposites dividing the loadshowed by the the cross-sectional same thickness area of of 0.08 the film. cm2. The There thickness could beand a length slight variationof the film of is thickness measured due using to thea caliperincreased and amountall nanocomposites of NOCNF added, showed but thethe disamefference thickness would beof negligible0.08 cm2. sinceThere the could NRL be solid a amountslight variationcontributes of thickness to most of due the to sample’s the increased volume. amou Thent stress–strain of NOCNF curves added, for but the the NRL difference and composite would filmsbe negligiblealso indicates since theirthe NRL ultimate solid tensileamount strength contributes (UTS). to most of the sample’s volume. The stress–strain curves for the NRL and composite films also indicates their ultimate tensile strength (UTS). Table 2. Young’s modulus (Ym), ultimate tensile strength (UTS), and maximum elongation (λmax). TableEach 2. value Young’s is the modulus average of(Ym), three ultimate replicates tensile samples. strength NRL—pure (UTS), naturaland maximum rubber latex, elongation NRL 0.1—NRL (λmax). Eachcontaining value is 0.1 the wt. average % of NOCNF; of three NRL replicates 0.2—NRL samples. containing NRL—pure 0.2 wt. %natural of NOCNF; rubber NRL latex, 0.4—NRL NRL 0.1—NRLcontaining containing 0.4 wt. % 0.1 of NOCNF.wt. % of NOCNF; NRL 0.2—NRL containing 0.2 wt. % of NOCNF; NRL 0.4—NRL containing 0.4 wt. % of NOCNF. Sample Ym (kPa) UTS (MPa) λmax (%) SampleNRL Ym (kPa) 3.3 UTS (MPa) 0.77 λmax (%) 234 NRL 3.3 0.77 234 NRL 0.1 79.6 2.5 31.4 NRL 0.1 79.6 2.5 31.4 NRL 0.2 2080 5.2 2.5 NRL 0.2 2080 5.2 2.5 NRL 0.40.4 1770 1770 6.2 6.2 3.5 3.5

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(i) (ii) (iii) (iv)

FigureFigure 6.6.Stress–strainStress–strain curves curves on on (i) NRL(i) NRL film film (control), (control), and composite and composite films made films of made NRL andof NRL NOCNF and withNOCNF varying with concentration varying concentration of NOCNF of ( iiNOCNF) 0.1 wt.%, (ii) (0.1iii) wt.%, 0.2 wt.%, (iii) (0.2iv) wt.%, 0.4 wt.%. (iv) 0.4 wt.%.

ItIt waswas foundfound thatthat thethe NRLNRL filmfilm exhibitedexhibited thethe UTSUTS valuevalue ofof 0.770.77 Mpa.Mpa. However,However, thethe additionaddition ofof NOCNFNOCNF increasedincreased thethe UTSUTS valuevalue ofof thethe filmfilm quitequite notablynotably (e.g.,(e.g., thethe filmsfilms withwith 0.1,0.1, 0.20.2 andand 0.40.4 wt.wt. %% ofof NOCNFNOCNF showedshowed thethe UTSUTS valuevalue ofof 2.5, 2.5, 5.25.2 andand 6.26.2 MPa,MPa, respectively).respectively). WeWe notenote thatthat thethe ultimateultimate tensiletensile strengthstrength reportedreported forfor thethe purepure NOCNFNOCNF extractedextracted fromfrom jutejute fibersfibers usingusing thethe nitro-oxidationnitro-oxidation methodmethod waswas 108 108 MPa MPa [31 [31]] and and the the chemically chemically crosslinked crosslinked NRL NRL film typicallyfilm typically exhibited exhibited the UTS the value UTS ofvalue 27 Mpa of 27[39 ].Mpa In the[39]. above In the study above [39], vulcanizedstudy [39], (chemicallyvulcanized crosslinked)(chemically NRLcrosslinked) was reinforced NRL was by additionreinforced of by cellulose addition nanocrystals of cellulose (CNC) nanocrystals where the(CNC) 3 wt. where % of CNCthe 3 waswt. % found of CNC to be was most found effective to be tomost increase effective the ultimateto increase tensile the strengthultimate oftensile NRL (bystrength about of 29%). NRL In (by this about study, 29%). the authors In this havestudy, used the theauthors term cellulosehave used nanofibers the term tocellulose describe nanofibers nanocellulose to describe isolated nanocellulose from coconut isolated spathe using from thecoconut acid hydrolysisspathe using method. the acid We hydrolysis believe that method. the length We believe of such that nanocellulose the length particlesof such nanocellulose extracted by theparticles acid hydrolysisextracted by approach the acid shouldhydrolysis be shorter, approach the should cross-sectional be shorter, dimensions the cross-sectional larger and dimensions the crystallinity larger higherand the than crystallinity those in NOCNF, higher than and theythose should in NOCNF, be termed and they CNC. should It is interesting be termed to CNC. note that It is the interesting overlap concentrationto note that the of CNC overlap usually concentration varies between of CNC 1.5 wt. usually % to 3varies wt. %, between depending 1.5 onwt. the %source to 3 wt. of the%, biomassdepending [40 ].on We the hypothesize source of thethe ultimatebiomass tensile [40]. strengthWe hypothesize of vulcanized the ultimate CNC-NRL tensile also takesstrength place of nearvulcanized the overlap CNC-NRL concentration also takes of CNC.place near the overlap concentration of CNC. OnOn thethe otherother side,side, thethe maximummaximum elongationelongation (λ(λmaxmax,, %)%) observedobserved inin the the testedtested filmsfilms showedshowed anan oppositeopposite trend.trend. ForFor example,example, thethe purepure NRLNRL filmfilm exhibitedexhibited λλmaxmax at about 234234 %,%, wherewhere 0.10.1 wt.%wt.% ofof NOCNFNOCNF inin the the composite composite film film decreased decreased the λthemax λvaluemax value to 31.4 to %.31.4 The %. increase The increase in theNOCNF in the NOCNF content furthercontent decreased further decreased the λmax valuethe λmax (e.g., value 0.2 (e.g., and0.4 0.2 wt. and % 0.4 of NOCNFwt. % of filmsNOCNF showed films the showedλmax value the λ ofmax aroundvalue of 2.5% around and 2.5% 3.5%), and rendering 3.5%), rendering the films tothe be films quiteto brittle. be quite These brittle. results These were results consistent were consistent with the SEMwith imagesthe SEM (Figure images5), which(Figures showed 5), which that the showed addition that of NOCNFthe addition decreased of NOCNF the roughness decreased of thethe NRLroughness film and of increasedthe NRL film the contentand increased of cracks the owing conten tot phaseof cracks separation owing to between phase separation NOCNF and between NRL. NOCNFThe ductile–brittleand NRL. transition was also noticeable from the Young’s modulus (Ym) evaluation shownThe in Figureductile–brittle7. The pure transition noncrosslinked was also NRLnoticeable was very from ductile, the Young’s showing modulus an Ym value(Ym) ofevaluation merely 3.5shown KPa, in where Figure 0.1 7. wt.The % pure of NOCNF noncrosslinked addition NRL increased was very the ductile, Ym value showing to 79.6 an KPa Ym and value 0.2 of wt. merely % of NOCNF3.5 KPa, additionwhere 0.1 increased wt. % of theNOCNF Ym value addition maximum increased to 2080the Ym kPa. value The to further 79.6 KPa increase and 0.2of NOCNFwt. % of contentNOCNF decreased addition theincreased Ym value the to Ym 1770 value kPa. maximum As a result, to the 2080 higher kPa. NOCNF The further content increase would of not NOCNF lead to anycontent property decreased enhancement, the Ym value where to 1770 the best kPa. content As a result, of NOCNF the higher addition NOCNF appeared content to would occur not near lead its overlapto any property concentration enhancement, (0.2 wt.%). where the best content of NOCNF addition appeared to occur near its overlap concentration (0.2 wt. %). Nanomaterials 2020, 10, 706 11 of 13 Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 13

FigureFigure 7. GraphGraph represents represents the the relationship relationship between between the the Young’s Young’s modulus (kPa) and the NOCNF concentrationconcentration in in the the composite films. films.

4.4. Conclusion Conclusions ThisThis studystudy showedshowed that that the the nitro-oxidized nitro-oxidized carboxycellulose carboxycellulose nanofibers nanofibers (NOCNF) (NOCNF) extracted extracted from fromraw jute raw fibers jute fibers could could be incorporated be incorporated into theinto NRL the NRL matrix matrix to increase to increase the mechanical the mechanical properties properties even evenin a nonvulcanized in a nonvulcanized state. The state. optimal The amountoptimal ofam theount NOCNF of the for NOCNF the overall for propertythe overall improvement property improvementseems to take seems place aroundto take place the overlap around concentration the overlap concentration of NOCNF (around of NOCNF 0.2 wt.(around %). 0.2 This wt. is %). not Thissurprising is not assurprising the overlap as concentrationthe overlap concentratio of nanocellulosen of nanocellulose represents the represents transition pointthe transition from a viscous point fromstate toa viscous a gel state. state Addition to a gel state. of NOCNF Addition into of non-vulcanized NOCNF into non-vulcanized rubber has changed rubber the has NRL changed film from the NRLelastic film to brittle.from elastic The more to brittle. schematic The studymore onschematic latex composite study on preparation latex composite can explore preparation the use can of explorethese inexpensive the use of and these sustainable inexpensive nanofibers and sustainabl materiale into nanofibers the preparation material of into other the rubber-based preparation (e.g., of otherGuayule) rubber-based composite (e.g., materials. Guayule) composite materials.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/4/706/s1. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Author Contributions: Methodology, H.C. and K.J.; software, S.L.; validation, P.R.S., S.K.S. and B.S.H.; formal Authoranalysis, Contributions: R.W. and W.B.; Methodology, data curation, H.C. C.Z.; and writing—original K.J.; software, draft S.L.; preparation,validation, P.R.S., S.K.S. andS.K.S. S.L.; and writing—review B.S.H.; formal analysis,and editing, R.W. P.R.S. and and W.B.; B.S.H.; data supervision, curation, P.R.S., C.Z.; S.K.S. writing—original and B.S.H.; funding draf acquisition,t preparation, B.S.H. S.K.S. All authors and haveS.L.; writing—reviewread and agreed toand the editing, published P.R.S. version and ofB.S.H.; the manuscript. supervision, P.R.S., S.K.S. and B.S.H.; funding acquisition, B.S.H.Funding: All authorsThe financial have supportread and for agreed this work to the was published provided version by a grant of the from manuscript. the Polymer Program of the Division of Materials Science of the National Science Foundation, United States (DMR-1808690). Funding: The financial support for this work was provided by a grant from the Polymer Program of the DivisionAcknowledgments: of MaterialsAuthors Science of would the National like to acknowledge Science Foundation, ThINC facilityUnited atStates AERTC, (DMR-1808690). Stony Brook University for the AFM, TEM, DMA, SEM characterizations of the samples. Acknowledgements: Authors would like to acknowledge ThINC facility at AERTC, Stony Brook University for Conflicts of Interest: The authors declare no conflict of interest. the AFM, TEM, DMA, SEM characterizations of the samples.

ConflictsReferences of Interest: The authors declare no conflict of interest.

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