Polylactic acid biocomposites reinforced with nanocellulose fibrils with

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Polylactic acid biocomposites reinforced with nanocellulose fibrils with high lignin content for improved mechanical, thermal and barrier properties Sandeep Sudhakaran Nair, Heyu Chen, Yao Peng, Yanhui Huang, and Ning Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01405 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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ACS Sustainable Chemistry & Engineering

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Polylactic acid biocomposites reinforced with nanocellulose fibrils

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with high lignin content for improved mechanical, thermal and

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barrier properties

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Sandeep S. Nair, † Heyu Chen, † Yao Peng, ϗ Yanhui Huang, ǁ, Ning Yan*, †, ‡

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†

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‡

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Toronto, Ontario, M5S3E5

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ϗ

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East Road 35, Haidian, Beijing 100083, China.

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Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ontario M5S3B3, Canada Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street,

MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Qinghua

ǁ

College of Materials Science and Technology, Beijing Forestry University, 100083 Beijing, China

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*Corresponding Author. Tel: +14169468070. Fax: +14169783834.

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E-mail: [email protected]

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Keywords: Nanocellulose fibrils with high lignin content, Polylactic acid cellulose composite, Atomic

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force microscope infrared spectroscopy, Mechanical property, Thermal stability, Water vapor

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transmission rate

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ABSTRACT: Lignin, the second most abundant natural polymer on earth after cellulose,

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contains both hydrophilic and hydrophobic groups. In this study, the use of nanocellulose fibrils

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with high lignin content (NCFHL) have been explored to make polylactic acid (PLA)

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biocomposites with excellent mechanical, thermal, and barrier properties. Different amounts of

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NCFHL aqueous suspensions (5-20 wt%) were wet mixed with PLA latex to form composite 1

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films by casting and hot pressing. The presence of lignin imparted a strong compatibility

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between NCFHL and PLA matrix, overcame the major issue of poor interfacial bonding

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associated with nanocellulose fibrils without lignin reported by literature studies. Atomic force

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microscope infrared spectroscopy (AFM-IR) characterization results showed an effective

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coupling between the NCFHL and PLA at the nanoscale. With 5-10 wt% of NCFHL additions to

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PLA matrix, a significant improvement in mechanical, thermal, and water vapour barrier

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properties was observed for the resulting biocomposites. The addition of 10 wt% of the NCFHL

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increased the modulus and strength by 88 % and 111 %, respectively, and the water vapour

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transmission rate was reduced by 52 %, compared to neat PLA.

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INTRODUCTION

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Petroleum based polymers are widely used as packaging materials due to their large availability,

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low cost, and good mechanical and barrier performance. These materials are non-biodegradable

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and highly durable and, therefore persist or accumulate in the environment for decades or

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centuries causing serious environmental issues. Packaging materials usually get contaminated by

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food stuff and other chemical substances during usage, as a result, their disposal is typically by

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landfill or incineration. However, landfills can cause serious soil and ground water

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contamination and incineration of plastics can release hazardous substances into the

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atmosphere.1–3 A 100% bio-based packaging material, that is based on polymers obtained from

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sustainable natural resources and is also completely biodegradable, is highly desirable by the

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packaging industry.

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Some major biopolymers derived from natural resources that have been explored as packaging

materials

include

starch,

cellulose,

corn-derived

2


polylactic

acid

(PLA),

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microorganisms-derived polyhydroxy alkanoates (PHA) and polyhydroxy butyrate (PHB), and

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bacterial cellulose.4,5 PLA is increasingly used as a packaging polymer for various fresh food

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products such as fruits and vegetables in the forms of films, containers, and coatings.5,6 Despite

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extensive research efforts, the thermal, mechanical, and barrier properties of PLA are still

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insufficient for some demanding food packaging usages. One major disadvantage of PLA is that

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it is thermally unstable. PLA, an aliphatic polyester with a hydrolysable backbone, is easily

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degraded by thermal processing and hydrolytic reactions. The degradation could result in

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decrease of the molecular mass which negatively affect the mechanical properties.8 In addition,

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PLA shows moderate water barrier properties and poor oxygen properties compared to major

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petroleum derived polymers with high barrier property, such as ethylene vinyl alcohol (EVOH)

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and polyvinylidene chloride (PVDC).9,10

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One approach to overcome this problem of PLA is by using bio-based nanofillers.

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Nanocellulose is a renewable nanofiller obtained from plants or other microorganisms.

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Nanocellulose includes crystalline nano rods known as nanocellulose crystals (NCC) and

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nanocellulose fibrils with both crystalline and amorphous regions, known as NCF. Nanocellulose

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have exceptional mechanical and optical properties and, therefore are used as a building block

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for a variety of high performance products.11,12 The individual crystalline NCC possesses

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Young’s modulus as high as 250 GPa.12 These nanosized cellulosic fillers are excellent materials

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for barrier applications by forming a dense network within the composites due to strong

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hydrogen bonding and make it very hard for the molecules to pass through. 12,13 However, due to

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high polarity and poor compatibility, nanocellulose reduces moisture resistance and yield poor

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mechanical properties when it is used in hydrophobic polymers, such as PLA.14–23 Different

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surface treatments, such as grafting with monomers,14 silylation,15–17 surfactants,18 acetylation,19– 3



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and esterification,22,23 have been used to decrease the hydrophilicity of nanocellulose to

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facilitate a better dispersion in PLA. While these treatments may enhance dispersion due to

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increased hydrophobicity, they can also result in reduction in hydrogen bonds between the fibrils

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or interfacial bonding with various matrices to adversely affect the final properties.23,24 Solvent

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casting process has been used to achieve good dispersion of nanocellulose in PLA matrix. The

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process involves exchanging water in nanocellulose suspension with low polarity organic

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solvents prior to mixing with PLA dissolved in the same organic solvent. These mixtures are

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then casted to form films, followed by extrusion and molding. 19,25 However, these processes are

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laborious and time consuming and produce significant chemical waste.

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Recently, our group have been successful in preparing nanocellulose fibrils with high amounts

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of lignin (about 20-23 wt%) from barks of various coniferous species.26,27 Strongly attached to

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cellulose fibers with covalent bonding, lignin was shown as a barrier for micro fibrillation of

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cellulose fibers to nano-scale fibrils in most of the previous studies.28,29 However, previous

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studies showed that mild alkaline treated bark fibers could be effectively fibrillated to

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individualised nanofibrils with a high lignin content simply by micro grinding.26,27 Lignin, the

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highly branched and crosslinked polyphenolic macromolecule with a high molecular weight is

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thermally more stable than polysaccharides in plants.30 The presence of lignin enhanced the

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thermal stability of nanocellulose fibrils compared to fibrils made from bleached pulp.26 In

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addition to polar hydroxyl groups, lignin also contains a high amount of non-polar hydrocarbon

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and benzene groups. Lignin strongly crosslinks with most of the polysaccharides and shields the

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polar hydroxyl groups of cellulose from bonding with water molecules.31 Nair et al

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that the presence of lignin enhanced the water barrier function of cellulose nanofibrils. The

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amphiphilic behaviour of lignin (having both polar and non-polar groups) could play a 4


26

showed

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significant role in enhancing the compatibility between nanocellulose and various hydrophobic

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polymers.32–35 Winter et al.32 prepared solution cast films of PLA reinforced with 1 wt% NCF

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containing a high amount of lignin and hemicellulose. Their results showed that these fibrils

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showed reduced polarity and improved dispersion and reinforcing efficiency in PLA than NCF

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made from bleached pulp. Artificially coated lignin on cellulose nanocrystals have shown a

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better dispersion of crystals in PLA matrix, enhancing the rheological and thermomechanical

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properties of the PLA composites.33 The use of 20-36 wt% of NCFHL in neat epoxy almost

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doubled the mechanical properties of the composites.34 Also, Gindl-Altmutter et al. showed that

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the lignin containing NCF had more compatibility with various hydrophobic polymers such as

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polystyrene (PS) and polycaprolactone(PC).35

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The objective of this study is to investigate the effect of lignin containing nanocellulose fibrils

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(NCFHL) on the mechanical, thermal, and barrier properties of resulting polylactic acid (PLA)

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biocomposites. In all previous studies,32–35 the NCFHL were either solvent exchanged with low

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polarity solvents or dried before casting or melt processing. Over the last few decades, latex

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forms of various hydrophobic polymers have been used as suitable matrices for nanocellulosic

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fillers for processing various bionanocomposites.36–39 The aqueous suspensions of nanocellulose

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can be wet mixed with these latices, thereby preventing the agglomeration problem of

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nanocellulose fillers during dry mixing. In this study, nanofibrils with a high lignin content were

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prepared from the red cedar (Thuja plicata) bark fibers using the same procedure reported in the

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earlier studies.26,27,34 The impact of these nanofibrils with high lignin on the performance of the

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resulting PLA biocomposites have been investigated.

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MATERIALS AND METHODS

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Materials, bark extraction and fibrillation. Western Red Cedar (Thuja plicata) bark was

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supplied by Terminal Forest Products. The PLA latex used was a commercial latex (Landy PL-

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2000, Miyoshi Oil & Fat Co., Ltd., Japan) with a mean particle size of 2 µm and solids content

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of 40 wt%. All other chemicals were from Caledon labs (Georgetown, ON, Canada). The air-

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dried barks were extracted using 1% NaOH solution at a temperature of 90 oC for 120 min with a

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liquor to bark weight ratio of 10:1. The extracted bark was rigorously washed with hot water

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prior to fibrillation. The mechanical fibrillation was done using a Super-MassColloider

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(MKZA10-15J, Masuku Sangyo Co., Ltd, Japan) at 1500 rpm. The mechanical fibrillation was

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operated at contact grinding and the extracted bark at 2 wt% solid consistency was fed

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continuously for 20 passes through the grinder. The resulting fibrils obtained after fibrillation

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were termed as nanocellulose fibrils with high lignin content (NCFHL). The extracted bark fibers

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prior to fibrillation and NCFHL were analyzed to determine the amount of holocellulose,

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cellulose, klason lignin, and ash contents. Browning’s method was used to determine the amount

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of holocellulose and cellulose.40 A modified procedure in wood was used to determine the

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amount of klason lignin.41 Ash contents were obtained using the ASTM D1102-84 method.

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Preparation of PLA/NCFHL biocomposites. NCFHL suspensions with 0.2 wt% solids

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consistency and PLA latex were first mixed through manual stirring and degassed under vacuum

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for at least 1h by magnetic stirring. The mixtures were poured into a glass petri dish and air dried

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overnight. The total solid weight was the same for all mixtures. The cast films were further dried

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at 120 oC in the oven for 3 h before they were compression-molded in a mold template having

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diameter of 150 mm. The molding was done using a Carver Laboratory Press (Carver Inc.,

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Wabash, IN) at 160 °C for 5 min at 1 MPa. The mass amounts of NCFHL used in PLA latex 6


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were 5 wt%, 10 wt%, 15wt%, and 20 wt%. The resultant composites were named as 5 wt%, 10

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wt%, 15 wt%, and 20 wt% NCFHL composites. For neat PLA films, pure latex was degassed,

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dried and compression-molded as discussed above. The thickness of the neat PLA and composite

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films ranged between 168 to 234 µm. NCFHL films were made by vacuum filtration of 0.2 wt%

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suspensions on a filter membrane of 0.22-µm pore size. (Millipore GVWP14250, Bedford, MA,

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USA). The films obtained after dewatering were placed between metal caul plates under a

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pressure of 23 kg and allowed to air dry for 48 h. The films were then hot pressed at 160 °C for 5

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min at 1 MPa. The average thickness of NCFHL films was between 30 to 46 µm.

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Characterizations. For mechanical testing, films were cut into dog-bone shape tensile

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specimens according to ASTM D 638-Type V specifications using a specimen cutter with the

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same dimensions. The narrow section of dog-bone shaped tensile specimen had length and width

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of 9.5 mm and 3.2 mm, respectively. Two sets of samples for neat PLA and composites were

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tested. One set was stored in a desiccator at 50% RH and the other at 90% RH for 4 days prior to

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testing. The NCFHL films were also cut with the same specification as that of the composites.

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Tensile tests of the films were performed using an Instron machine (Model 3367) equipped with

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a 2 kN load cell, in which a span of 2.5 cm and an elongation speed of 1 mm min−1 were used.

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Dynamic mechanical analysis (DMA) was performed in tension mode using a TA Instruments

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Q800 analyzer (New Castle, DE). Test specimens were 6-7 mm wide and the span was 20 mm.

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The frequency and amplitude of oscillation were set at 1 Hz, and 14 µm, respectively. The

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specimens were tested in air at a heating rate of 3 °C/min, from 25 °C to 120 oC.

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Microtomed cross sections (1µm thickness) of the composite films were examined using a

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confocal laser scanning microscope (LSM 880 Elyra system, Carl Zeiss, Oberkochen, Germany)

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to observe the distribution of NCFHL. The excitation wavelength of 410-700 nm was used for 7



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imaging lignin autofluorescence. Scanning electron microscopy (SEM) images were taken on the

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cross-sectional fracture surfaces of the composite films with a JEOL 6610LV (Seal Laboratories,

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El Segundo, CA, USA) operated at 15 kV. For the extracted bark fiber and NCFHL morphology,

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samples at 0.01% solids consistency were quickly frozen using liquid nitrogen and freeze dried.

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All samples were mounted with carbon tapes on an aluminum stub and sputtered with gold to

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provide adequate conductivity prior to SEM imaging. Environmental scanning electron

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microscopy (ESEM) (Quanta 250 FEG, FEI Inc., Oregon, USA) was also used to obtain the

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morphology of NCFHL. The field emission gun (FEG) systems of the ESEM contained an

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S/TEM detector for obtaining higher-magnification images. Samples at 0.01% solids consistency

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were put on a copper grid and then air dried prior to imaging. Attenuated total reflectance-

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fourier transform infrared spectroscopy (ATR-FTIR) was used to characterize the chemical

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structure of NCFHL, neat PLA, and composites ((ATR-FTIR, Nicolet iS50 FT-IR, ThermoFisher

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Scientific, USA). The sample spectra were recorded between 600 and 4000 cm−1 at a resolution

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of 4 cm−1 and 64 scans were obtained for each spectrum. X-ray diffraction crystalline peaks for

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NCFHL, neat PLA, and 10 wt% NCFHL composite were obtained using a PW 1830 Generator

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(Philips, Almelo, The Netherlands) with a monochromatic CuK source (λ = 1.54 Å). The

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diffraction was obtained in the step scan mode (step of 0.02) with a 2θ angle ranging from 10o to

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40o and a scanning time of 50 min.

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AFM-IR. The composite samples embedded in LR white resin were microtomed using a

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diamond knife and an ultra thin slice of several nanometer thickness was obtained for imaging.

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The slices were then transferred onto a flat silicon substrate with approximate dimensions of 10

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mm × 10 mm and were mounted on the sample stage of the AFM-IR instrument. A nanoIR2

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AFM-IR instrument (Anasys Instruments Corporation, Santa Barbara, USA) was used for 8


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collecting the accurate location IR spectra. The AFM-IR spectra were recorded in tapping

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contact mode from 800 cm-1 to 4000 cm-1 at a resolution of 4 cm-1 and a scan rate of 0.1 Hz using

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a gold coated silicon nitride probe. Also, the IR laser source from the AFM-IR instrument was

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fixed at two particular wavenumbers, and separate absorption images were obtained for each

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wavenumber. The absorption images obtained with the IR laser were tuned to 3330 cm-

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1

(characteristic peak of NCFHL) and for 1750 cm-1 (characteristic peak of neat PLA).

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Thermogravimetric analysis (TGA). The thermal stability of the NCFHL, neat PLA, and

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composites were obtained using a TGA-Q500 (TA Instruments, New Castle, DE, USA). The air-

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dried samples (8–10 mg) were heated from room temperature to 800 ◦C in platinum crucibles at a

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rate of 10 ◦C/min in a nitrogen atmosphere. The Tonset, defined as the temperature at which the

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sample weight loss became apparent was determined using the tangent method.26,34 The Tmax,

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defined as the temperature at which the degradation rate is fastest, corresponds to the maximum

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value of the derivative weight curve (dm/dTmax).

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Water vapour transmission rate (WVT). WVT was measured using ASTM E-96-95 water

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method. The PLA and composite films were cut into 6.4 cm diameter circles and conditioned to a

200

constant weight in a 50% RH atmosphere. The films were attached to a test dish containing 50

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ml of distilled water. Room conditions were held constant at 23 ◦C and 50% RH. The weight of

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the dish assembly was taken every 30 min and the slope of the generated weight-loss curve was

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used to determine the WVT. WVT was determined using the following equation

204 205

WVT=∆G/(∆t x A)

(1)

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9



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Here, ∆G is the weight change of water obtained from curve, ∆t is the time during the G

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occurred, and A is the test area (dish mouth area).

209 210

RESULTS AND DISCUSSION

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Morphology of the extracted fibers, NCFHL, and biocomposites. The average diameters of the

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bark fibers after alkaline extraction were between 25 µm and 40 µm (Figure 1a). Inset image

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(Figure 1a) shows the rough surface of the fibers with a clear indication of microfibrils

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(markings on the surface) inclined at an angle to the axis of growth. The extraction resulted in

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the mild delignification and removal of hemicellulose from the cellulose-lignin-hemicellulose

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networks within the fibers. This process was found to be very helpful in increasing the porosity

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of fibers for effective nano-fibrillation process.26,27 Figure 1b shows the complete disintegration

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or fibrillation of fibers into finer NCFHL after 20 passes through the micro grinder. Table S1

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shows the chemical composition of the extracted fibers and the NCFHL obtained after

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fibrillation. The high-resolution SEM images show the presence of several globule shaped

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particles (as indicated by arrows in Figure 1c) on the NCFHL. Several studies have shown this as

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coalesced lignin particles formed during autohydrolysis at higher temperatures.42,43 Presence of

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these globule shape particles on fibrils during the nano-fibrillation process has also been reported

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in earlier studies.26,27 Figure 1c shows micron sized sheets which are aggregated structures

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formed by finer nanofibrils during the freeze-drying process.44 Figure 1d shows the high

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resolution ESEM image of the NCFHL obtained after 20 passes. A quantitative image analysis

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was conducted on the ESEM image using Gwyddion software and found that most NCFHL were

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between 20-40 nm in diameter.

10


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Varying amounts of NCFHL and PLA were wet mixed followed by casting. The cast films

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were successfully hot-pressed to obtain a series of biocomposites. Figure 1e-h shows the cross-

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sectional surfaces of the neat PLA and the biocomposites. The white spots present on the neat

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PLA surface (Figure 1e) mostly appeared as globule shaped or spherical particles. The fibrils

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were mostly embedded within the PLA with a strong attachment or interphase with matrix (as

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indicated by arrows in Figure 1f and Figure 1g). The scanty distribution of fibrils in 5 wt%

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NCFHL biocomposite (Figure 1f) can be attributed to the limited amount of NCFHL used in

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bulk PLA phase. The number of fibrils increased with further additions. The 15 wt% composites

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showed large number of voids or porosity on the surface (Figure 1h). Enlarged fracture surfaces

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for 15 wt% and 20 wt% composites are shown in the supporting information (Figure S1).

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ATR-FTIR. ATR-FTIR was used to explore the possible interactions between the PLA and

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NCFHL. Figure 2a displays the FTIR spectra of the PLA, NCFHL, and their biocomposites. The

241

assignments of peaks corresponding to neat PLA and NCFHL are listed in Table S2. With

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increasing NCFHL concentration in the biocomposites, the relative strength of peaks due to neat

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PLA decreased and the peak associated with the vibrational stretching of O−H groups in NCFHL

244

increased and broadened. The peak position at 1750 cm-1 corresponding to C=O stretching of

245

neat PLA shifted to 1752 cm-1, 1753 cm-1, and 1754 cm-1 with the addition of 5 wt%, 10 wt%

246

and 15 wt% of NCFHL, respectively. This relative shift could be due to intermolecular hydrogen

247

bonding between the hydroxyl groups of NCFHL and carbonyl groups of PLA molecules or due

248

to some mechanical interaction between the molecules.45,46 Also, the peak position at 3330 cm-1

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for the O-H stretching of pure NCFHL changed to 3383 cm-1, 3359 cm-1, and 3347 cm-1 with the

250

addition of 5 wt%, 10 wt% and 15 wt% of NCFHL to PLA, respectively. All these results

251

indicated that there was considerable interactions between the NCFHL and PLA molecules.45–47 11



252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282

Figure 1. SEM images of (a) Extracted bark fibers prior to fibrillation (b) NCFHL after 20 passes (c) NCFHL after 20 passes (high resolution) (d) ESEM images of NCFHL after 20 passes (e) crosssectional fracture surface of neat PLA (f) cross-sectional fracture surface of 5 wt% NCFHL biocomposite (g) cross-sectional fracture surface of 10 wt% NCFHL biocomposite (h) crosssectional fracture surface of 15 wt% NCFHL biocomposite 12


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283

AFM-IR and confocal microscopy. AFM-IR spectroscopic images were obtained on the

284

microtomed fracture surface for the 10 wt% biocomposite to characterize the dispersion of

285

various phases and the interfacial regions. AFM tip can sense the thermal expansion from IR

286

absorptions and record AFM-IR spectra with a high spatial resolution at tens of nanometers,

287

while conventional IR microscopy has limited spatial resolution of 2-14 µm.48 Figure 2b shows

288

the AFM topography image (cross marks represent the area with minimum roughness selected

289

for obtaining AFM-IR spectrum) and figure 2c shows the corresponding AFM-IR spectra

290

obtained on these regions. The spectra obtained were more of a blend between NCFHL and PLA

291

suggesting that they were obtained on localized interfacial regions on the cross section. It

292

showed most of the PLA peaks (2994 cm-1, 2944 cm-1, 2880 cm-1, 1752 cm-1, 1452 cm-1, 1381

293

cm-1, 1365 cm-1, 1180 cm-1, 1128 cm-1, 1080 cm-1, and 1043 cm-1)49,50 and few of NCFHL peaks

294

(3347-3357 cm-1, 1508 cm-1, and 1266 cm-1),34,51 suggesting that the selected regions were much

295

closer to PLA than to fibrils, or interfacial zone with more proximity towards PLA. Also, the

296

carbonyl peak in point D showed double bands, one at 1752 cm-1 and at 1770 cm-1. Kister et al 52

297

showed that this was formed due to differences in chiral unit enchainment in the amorphous

298

PLA.

299 300 301 302 303 304 305

13



306 307 308 309

Figure 2. (a) FTIR spectra for NCFHL, neat PLA, and NCFHL/PLA biocomposites (b) AFM topography image of the microtomed cross-sectional surface for 10 wt% NCFHL/PLA biocomposites (cross marks (A to D) represent the area with minimum roughness selected for obtaining AFM-IR spectrum) and (c) corresponding spectra obtained for areas A-D 14


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310

The IR laser source from the AFM-IR instrument was fixed at two wavenumbers, and separate

311

absorption images were obtained for each wavenumber on the same locations on the sample.

312

Figure 3a shows the absorption images obtained with the IR laser tuned to 3330 cm-1 (a

313

characteristic peak of NCFHL) and Figure 3c for 1750 cm-1 (a characteristic peak of neat PLA).

314

The green color within the images corresponds to the regions with strongest IR absorption, and

315

indicates the presence of higher concentrations of either NCFHL (Figure 3a) or neat PLA (Figure

316

3c). The images were thresholded to binary images using image J software to separate the pixels

317

with strongest IR absorption. The darker region within the figures corresponds to the NCFHL

318

(Figure 3b) and neat PLA (Figure 3d) on the fracture surfaces. Figure 3b indicates that the

319

nanofibrils were well dispersed even at a low concentration of 10 wt%. A quantitative analysis

320

was done to calculate the area of the respective components. Results showed that around 22% of

321

total areas were occupied by fibrils. Image analysis was done to find the size distribution of

322

fibrils in figure 3b and it was found that most of the fibrils in the cross-section were less than 70

323

nm in size. Also, very few aggregates were also seen on the sample and most of them were less

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than 200 nm in size. However, considering its high concentration in the sample, neat PLA did

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not appear as a continuous phase on the cross section, and occupied about 32% of the total areas.

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This means that almost 46% of the total area within the image corresponds to reacted areas or

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interfacial regions. The highly dispersed NCFHL could react with PLA molecules and change

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the chemical structure of interphase/interface regions. Also, during the pressing of the

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composites at 160 oC, lignin can soften and flow to fill in the voids between the fibril and PLA

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polymer, acting as a strong compatibilizer.34,53 All these could result in the increase in interfacial

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regions within the sample. Figure 3e shows the fluorescence mapping image obtained on the

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cross-section of 10 wt% composite using confocal microscopy. Lignin has its specific auto 15



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fluorescence at 530 nm34 and Figure 3e shows fluorescence throughout the cross-section,

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confirming the well dispersed fibrils throughout the surface.

335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362

Figure 3. (a) AFM-IR absorption images obtained with the IR laser tuned to 3330 cm(characteristic peak of NCFHL) (color scale is +0.3 to -0.3) (b) corresponding thresholded binary image (c) AFM-IR absorption image with the IR laser tuned to 1750 cm-1 (characteristic peak of neat PLA) (color scale is +0.3 to -0.3) (d) corresponding thresholded binary image (e) Confocal laser microscope image of the cross-sectional fracture surface (enclosed within arrows) of 10 wt% NCFHL/PLA biocomposite 1

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Mechanical Properties. The average mechanical properties of the biocomposites are shown

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in Figure 4 and Table S3. For the composites, there is a clear increase in mechanical properties

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compared to neat PLA. The mechanical properties progressively increased with the addition of

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NCFHL up to 10 wt%. Compared to neat PLA, the 5 wt% addition of NCFHL increased the

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modulus and tensile strength by 56 % and 69 %, respectively. Meanwhile, the addition of 10

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wt% of the fibrils increased the modulus and strength by 88 % and 111 %, respectively. The

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percentage of mechanical enhancement achieved in our study is superior to most of the other

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studies based on low lignin containing nanocellulose fibril (NCF) reinforced PLA

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composites.18,19,21,22,54 Wang

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composites. However, they could not achieve good mechanical properties at lower

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concentrations (

Polylactic acid biocomposites reinforced with nanocellulose fibrils with

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