Electrospun Poly(lactic acid)-Based Fibrous Nanocomposite

Subscriber access provided by MT ROYAL COLLEGE

Article

Electrospun poly(lactic acid)-based fibrous nanocomposite reinforced by cellulose nanocrystals: Impact of fiber uniaxial alignment on microstructure and mechanical properties Siqi Huan, Guoxiang Liu, Guangping Han, and Long Bai Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00023 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Biomacromolecules is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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

Biomacromolecules

Electrospun poly(lactic acid)-based fibrous nanocomposite reinforced by cellulose nanocrystals: Impact of fiber uniaxial alignment on microstructure and mechanical properties Siqi Huan1, Guoxiang Liu1, Guangping Han*, Long Bai* Key Laboratory of Bio-based Material Science and Technology (Ministry of Education), Northeast Forestry University, Harbin 150040, PR China

ABSTRACT Uniform PLA/CNC fibrous mats composed of either random or aligned fibers reinforced with up to 20 wt% cellulose nanocrystals (CNCs) were successfully produced by two different electrospinning processes. Various concentrations of CNCs could be stably dispersed in PLA solution prior to fiber manufacture. The microstructure of produced fibrous mats, regardless of random or aligned orientation, was transformed from smooth to nanoporous surface by changing CNC loading levels. Aligning process through secondary stretching during high-speed collecting can also affect the porous structure of fibers. With the same CNC loading, fibrous mats produced with aligned fibers had higher degree of crystallinity than that of fibers with random structure. The thermal properties and mechanical performances of PLA/CNC fibrous mats can be enhanced, showing better enhancement effect of aligned fibrous structure. These results from a synergistic effect of the increased crystallinity of fibers, the efficient stress transfer from PLA to CNCs, and 1 ACS Paragon Plus Environment

Biomacromolecules 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

Page 2 of 31

the ordered arrangement of electrospun fibers in the mats. This research paves a way for developing an electrospinning system that can manufacture high-performance CNC-enhanced PLA fibrous nanocomposites. Keywords: Cellulose nanocrystals, poly(lactic acid) fibers, electrospinning nanocomposite, nanoporous structure INTRODUCTION Poly(lactic acid), PLA, a biodegradable thermoplastic polyester derived from sustainable resources,1 has been considered to be one of the most attractive biopolymers due to its renewability, biodegradability, biocompatibility, and superior physical properties.2 Furthermore, hydrophobic PLA naturally displays higher hydrophilicity than common hydrophobic thermoplastic polymers due to a better access between water molecules and the polar oxygen linkages of its backbone,3 providing a promising opportunity to forming bio-grade products. PLA-based materials can be fabricated by a number of commercially technologies including solvent casting, particulate leaching, membrane lamination, and melt molding, as well as electrospinning and precise extrusion.4 Among these techniques, consecutive electrospinning to manufacture PLA-based micro- or nano-fibers attracts considerable attention due to its simplicity, high efficiency, and desirable microstructure (e.g., porosity).5 However, electrospun pure PLA fibers normally have the drawbacks of showing weak mechanical properties and low thermal stability,6 narrowing their industrial applications. Enhancing the mechanical performances of electrospun PLA nanofibers, therefore, is highly desired for consumer applications, especially for PLA-based fibers that have to meet controllable mechanical requirements during transportation, reprocessing, and recovery. 2 ACS Paragon Plus Environment

Page 3 of 31 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

Biomacromolecules

Fabrication of nanocomposites that occur at nanoscale has been a facile method to develop or modify novel structural and functional heterogeneous materials.7-9 Incorporating mechanically robust nanoscale fillers, e.g., bioactive glass,10 into electrospun PLA matrices, therefore, is a promising approach to fabricate high-performance PLA fibrous nanocomposites. Since PLA is an environmental-friendly material, it is important from the perspective of sustainability to select suitable additives, resulting in a fact that bio- or green-nanocomposite of PLA-based electrospun fibers made from renewable or naturally derived nanofillers is in great necessity. Recently, one strong trend is to utilize nanocellulose acting as green nanofillers to improve the properties and versatility of polymer-based nanocomposites achieving a combination of desired properties and environmental benefits.11-14 The biocompatible and biodegradable cellulose nanocrystals (CNCs) that are readily isolated via controlled acid hydrolysis of cellulosic raw materials are short crystalline nanorods,15 showing a novel enhancement effect. The driving force to explore CNCs as reinforcing agents in electrospinning technique, therefore, is their large specific surface area, low density, high surface charge, and the ability to enhance properties at low loading levels, as well as adaptability to processing conditions.16 Particularly, CNCs are usually used to reinforce the mechanical properties of electrospun polymer materials due to their high Young’s modulus (~138 GPa)17 and high mechanical strength (~7 GPa)18 obtained from the densely and orderly crystallized structure after acid hydrolysis.19 Accordingly, CNCs are one of the most promising green nanofillers to modify the properties of electrospun PLA fibers. The effective enhancement of incorporating CNCs into hydrophobic polymer-based electrospun nanofibers, including polystyrene,20 poly(methyl methacrylate),21, 22 and polymer mixture containing PLA,23 has been demonstrated, however, full discovery of the unique contribution of CNCs to controllably design

3 ACS Paragon Plus Environment


Page 4 of 31

and tailor customized microstructure and performances of nanocomposites under electrospinning process remains to be a challenge. In electrospun CNC-based nanocomposites, the local orientation of CNCs in fibers has been systematically reported,24,

25

which can be considered as the main reason for significant

modification of mechanical performance, since efficient stress transfer within electrospun fibers and CNCs is expected only as CNCs are oriented along the fiber axis.26 Furthermore, it has also been verified that the alignment of electrospun fibers could further increase their performances and endow novel properties than those of randomly oriented ones.27 Chen et al. reported that the alignment of electrospun protein fibers reinforced with cellulose nanowhiskers greatly improved their mechanical properties,28 demonstrating the function of fiber alignment in final performance. Consequently, a combination of the enhancement effect of CNCs and the formation of uniaxially ordered fiber arrangement can offer a synergetic function to create high-performance electrospun PLA/CNC fibrous nanocomposite. However, although alignment of electrospun polymers functionalized with CNCs has been studied recently,29 few studies have investigated the influence of CNCs loading levels on the formation of aligned PLA/CNC fibers. Moreover, the structure-property

relationship

of

uniaxially

aligned

electrospun

PLA/CNC

fibrous

nanocomposites, particularly the effects of fiber alignment on their mechanical properties, have not been fully investigated and understood. More importantly, to our knowledge, this is the first report to combine CNC and fiber alignment process during electrospinning to controllably tune the surface microstructure (nanopores) of produced fibers. In this study, a series of PLA/CNC fibrous nanocomposite with random and aligned fibers were fabricated via an electrospinning process using PLA solutions with varying CNC loading levels (Figure 1). The main objective of this study was to investigate the influence of CNC 4 ACS Paragon Plus Environment

Page 5 of 31 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

Biomacromolecules

loading levels and fiber alignment on the surface morphology and properties of electrospun PLA/CNC fibers. Surface microstructure of electrospun fibers was explored by AFM and SEM. Performance evaluations of PLA/CNC fibrous nanocomposite mats, especially mechanical and thermal properties, were undertaken to elucidate the structure-property relationship. The longterm perspective in this research is to develop a bio-grade electrospinning system to manufacture fully biocompatible, biodegradable, and environmental-friendly high-performance fibrous nanocomposites for potential biological applications.

Figure 1. Schematic drawing (not to scale) of (a) preparation of PLA/CNC as-spun solutions and electrospinning process of PLA/CNC fibrous mats with (b) random and (c) aligned fiber arrangement.

EXPERIMENTAL Materials. Poly(lactic acid) (PLA) particles ( 270,000) were purchased from Nature Works company in USA, and dried in a vacuum oven at 60 for 24 h prior to use. Commercial 5 ACS Paragon Plus Environment


Page 6 of 31

microcrystalline cellulose (MCC, KY100S, 75%moisture, Daicel, Japan) was used as raw material for producing cellulose nanocrystals (CNCs). 98% sulfuric acid was purchased from Aladdin (Shanghai, China). N,N-dimethylformamide (DMF) and chloroform (CHCl3) (Kermel Co., Tianjin, China) were of analytical grade and used as received without further purification. Deionized water was used throughout the study. Preparation of cellulose nanocrystals. CNCs were prepared via sulfuric acid hydrolysis of MCC according to our previous study.8 Briefly, 170 mL of 64 wt% sulfuric acid aqueous solution was heated to 45 in a water bath before adding 40 grams of MCC under vigorous magnetic stirring. After 1h, the hydrolysis process was stopped by adding additional water (10 times). The resulting suspension was then cooled down to ambient temperature and washed with deionized water by successive centrifugations (10000 rpm, 15 min) until neutral pH. Dialysis against deionized water was performed for 7 days to remove free acid molecules from the suspension. After dialysis, the yield was calculated by withdrawing a known, small amount of the sample and obtaining its oven-dried weight. The pelleted CNCs obtained after dialysis were dried using a freeze dryer (Scientz-10N, Xin Zhi Co., China). The shape and microstructure of produced CNCs were characterized and analyzed by atomic force microscopy (AFM) with scanning area of 3 µm×3 µm. Preparation of as-spun solutions. Spinning solutions were prepared from CNCs and a solution mixture of PLA, DMF and CHCl3. After comparing the morphologies by electrospinning different concentrations of PLA solution without adding CNCs (shown in Figure S1), 10 wt% PLA was chosen to investigate the influence of fiber alignment during electrospinning on microstructure and properties of CNC/PLA fibrous nanocomposites. In brief, certain amount of dry PLA particles were dissolved in the solvent mixture of CHCl3 and DMF 6 ACS Paragon Plus Environment

Page 7 of 31 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

Biomacromolecules

(3:1), and the obtained solution was vigorously stirred in 65 water bath for 2 h to obtain 10 wt% PLA solution. Then, varied amounts of CNC powder produced as above were loaded into PLA solutions, and the mixtures were sonicated with high amplitude for 20 minutes to obtain final as-spun solutions. All the as-spun solutions were freshly made shortly before electrospinning experiments. The CNC loading concentrations in relation to the adding weight of PLA were 0, 5, 10, 15, and 20 wt%. The samples were designed as X-PLA/CNC-y, where X represents the random (R) or aligned (A) fiber arrangement, and y corresponds to CNC loading level. The conductivity, viscosity, and surface tension of prepared PLA/CNC as-spun solutions were characterized at ambient temperature using a conductivity meter (DDSJ-318, Lei Ci Co., Shanghai, China), digital rotational viscometer (SNB-1, Heng Ping Co., Shanghai, China), and a surface tension meter (JK99B, Zhong Chen Co., Shanghai, China), respectively. Electrospinning set-up. The set-up of electrospinning apparatus was purchased from Yong Kang Le Ye Company, China, as described in our previous research.20 The as-spun solutions were loaded into a 10 mL plastic, disposable syringe (Zhi Yu Co., Shanghai) with a stainless steel needle (i.d.: 0.6 mm). The positive voltage generator applying a potential of 16 kV was fixed on the needle, while the negative electrode applying -3 kV potential was connected with the collector in order to collect the electrospun fibrous mats. For the collection of non-alignment fibers, a rectangular piece of aluminum foil (240 mm length and 150 mm width) was covered on the cylinder with rotating speed of 80 rpm. The aligned fibrous nanocomposites were collected on a smaller cylinder (480 mm length and 50 mm width) with ultra-high rotating speed of around 2800 rpm. The flow rate of the as-spun solutions was automatically controlled by a syringe pump. The temperature and humidity in the chamber during electrospinning were controlled at 7 ACS Paragon Plus Environment


Page 8 of 31

25 and 22%, respectively. The obtained fibrous mats were carefully detached from the aluminum foil and stored at ambient for 24 h prior to characterization. Characterization. The surface as well as the cross-section of PLA/CNC fibers were observed by scanning electron microscope (SEM, QUANTA-200, FEI, Hillsboro, OR, United States). For the surface morphology characterization, fibrous mats collected on the aluminum foil were cut into small pieces, and then were mounted on metal stubs with double sided adhesive tape. For the cross-section characterization, fibrous mats with aligned morphology were immersed in epoxy resin with addition of initiator, dried in an oven under 100 , and then cut by a glass knife. Gold-palladium was coated on both the samples of surface and cross-section before observation at an accelerating voltage of 12.5 kV. The diameter of the electrospun fibers were analyzed from SEM images by ImageJ.30 The mean diameters, diameter distribution and surface pore size of the fibers were calculated based on at least 100 nanofibers from the corresponding SEM micrographs. Surface topological structure in the bulk of PLA/CNC fibrous mats was determined via using atomic force microscope (AFM). Images were obtained from a commercial instrument (Dimension Icon, Bruker, Billerica, MA, United States) under tapping mode at a scan rate of 0.999 Hz. To obtain typical images for the sample, scanning was made over a large area. After selecting the typical area, higher magnified images were obtained in the area of 5µm×5µm. The digital resolution of all pictures was 256×256 points. Roughness data were analyzed by NanoScope software. The neat PLA and PLA/CNC fibrous mats were analyzed using differential scanning calorimetry (DSC-204, Netzsch, Germany) to study the phase transitions and crystalline behavior. 8 ACS Paragon Plus Environment

Page 9 of 31 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

Biomacromolecules

Samples of about 5 mg were loaded into aluminum crucibles sealed using a crimping tool. An empty aluminum crucible served as a comparison sample. All DSC measurements were performed with respect to the signal base line obtained for the two empty crucibles. The samples were heated from room temperature to 200 , held for 2 min to eliminate the thermal history, and then cooled down to room temperature, and finally heated to 200 . All the processes were conducted at 10 /min heating rate in a 50 mL/min dynamic N2 atmosphere. The accuracy of the temperature measurement was 0.1 , and the accuracy of the enthalpy measurement for phase transitions was 0.2 J/g. All tests were carried out in duplicate. The crystallinity degree of neat PLA and PLA/CNC fibers was calculated using the following equation:31

χc =

∆Hm -∆Hcc ω∆H0m

×100%

where ω is the weight fraction of PLA in fibrous mats, ΔH (J/g) is the heat of fusion from the second heating circle, ∆H (J/g) is the heat of cold crystallization, and ΔH is the heat of fusion

of 100% crystalline PLA which has a value of 93.6 J/g.32 Thermogravimetric analysis (TGA, TGA-209, Netzsch, Germany) was conducted to study thermal decomposition of PLA/CNC fibrous mats. Samples of 5-10 mg were heated from room temperature to 700 at a rate of 5 /min under Ar atmosphere. The onset decomposition temperature and the maximum thermal decomposition temperature of samples were defined as Tonset and Tmax, respectively. The surface wettability of PLA/CNC fibrous mats was characterized by a contact angle meter (OCA20, Dataphysics, Bad Vilbel, Germany) at ambient temperature. Contact angle (CA) was measured using a sessile drop method with droplet volume being 5 µL. The CA values of the 9 ACS Paragon Plus Environment


Page 10 of 31

right side and the left side of the water droplet were both measured and averaged. All the CA data were an average of at least five measurements at different locations on the surface. Mechanical properties of PLA/CNC fibrous mats were determined from stress-strain curves of tensile test. The tensile test was carried out on a Model 3365 universal testing machine (Instron Co., Norwood, MA, United States) with a tensile rate of 10 mm/min according to the ASTM D 882-09 at ambient temperature and 30% humidity. The size of each sample was 15 mm length and 5 mm width. All tests were carried out at least in triplicate. Notably, the tensile for fibrous mats with aligned fibers was performed along the fiber direction.

RESULTS AND DISCUSSION Characteristics of as-spun solutions. The microstructure of CNCs prepared by acid hydrolysis was observed by AFM (Figure 2). As shown in Figure 2, the pristine CNCs showed homogeneous morphology of rod-shaped, well-dispersed nanoparticles, which can be attributed to the electrostatic repulsion from the negative sulfate half-ester groups introduced from acid hydrolysis process.33 Individual CNCs with negligibly lateral or longitudinal association were also clearly identified, ensuring a prerequisite for fine enhancement effect. From AFM images, the average dimensions of CNCs were approximately 260±32 nm in length and 42±18 nm in width, indicating that the corresponding aspect ratio (L/W) was around 7. The obtained CNCs were further freeze-dried and re-dispersed in 10 wt% PLA solution to prepare as-spun solutions. Photograph of as-spun solutions with different CNC loading levels was shown in Figure S2. With increasing CNC amount, the transparency of prepared PLA/CNC solutions was gradually decreased, showing the highest turbidity at 20 wt% CNCs. On the other hand, the as-spun solution containing 20 wt% CNCs was still homogeneous without any sedimentation before and 10 ACS Paragon Plus Environment

Page 11 of 31 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

Biomacromolecules

during electrospinning, which may be beneficial from the strong electrostatic repulsion of CNCs and the high-energy sonication process. Furthermore, no nozzle blockage for spinning solution with 20 wt% CNCs during electrospinning (Section 3.2) was observed, directly demonstrating the well-dispersion of CNCs in PLA solution. Since physicochemical parameters of as-spun solution play crucial roles in determining the properties of electrospun fibers, therefore, viscosity, surface tension, and conductivity of as-spun solutions containing various CNC concentrations were measured (Table 1). The viscosity significantly increased with increasing CNC loading levels, possibly attributing to the increased entanglement between CNCs and PLA chains originated from their interconnection during sonication. The surface tension of as-spun solutions was similar, showing slightly increase compared with that of pure PLA solution. Conductivity was also observed to increase with loading more CNCs into PLA solutions. This can be ascribed to the sulfate half-ester groups on CNC surface. The increased conductivity can provide positive effect on the continuous spinning of smaller diameter and more homogeneous fibers in PLA/CNC systems because the high charge density could increase the electrical force imposed on the ejected jets. Taken together, in spite of combining hydrophobic PLA and hydrophilic CNCs in single spinning solution system, the prepared PLA/CNC as-spun solutions display fine electrospinnability.



Page 12 of 31

Figure 2. AFM (a) height (19.4 nm light to dark) and (b) phase (15° from light to dark) topographic images of pristine CNCs. The scanning area was 3 µm×3 µm. Table 1 Characteristics of as-spun solutions with varying amounts of CNCs Code

Viscosity (mPa·s)

Surface tension (mN/m)

Conductivity (F/m)

PLA

385.3

32.804

1.458

PLA/CNC-5

399.5

33.625

1.794

PLA/CNC-10

444.3

33.912

2.377

PLA/CNC-15

484.8

34.212

2.734

PLA/CNC-20

511.6

34.993

3.176

Fiber morphology. The morphology of electrospun PLA/CNC fibrous mats with random or aligned fibers loading with various amounts of CNCs are shown in Figure 3. As shown in Figure 3R1-R5, all fibers produced showed similar diameter with being around 2 µm, and obvious beads or bead-on-string structure within single fibers were scarcely visible. In Figure 3R2, PLA/CNC fibers containing 5 wt% CNCs showed smooth surface but less uniform as fibers without adding CNCs. Since the solvent (DMF/CHCl3) used was identical for all formulations in current study, it can be concluded that the main change of fiber morphology was caused by varied CNC loading levels. As the viscosity and conductivity of spinning solutions increased with loading more CNCs while surface tension kept similar, the change of fiber morphology can 12 ACS Paragon Plus Environment

Page 13 of 31 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

Biomacromolecules

be attributed that the higher conductivity resulted from incorporating CNCs could increase the electrical force applied on the ejected fluid, which can counteract the viscoelastic forces more efficiently, thereby stretching the PLA/CNC jets to a greater extent during electrospinning.34 Meanwhile, the higher viscosity of spinning solutions could provide sufficient resistance to reduce rapid shape transformation and recovery, leading to a steady electrospinning process. Increasing CNC concentrations, the surface of produced fibers showing random arrangement become porous with pore size being around 90 nm (Figure 3R3-R5, and Figure S3). The formation of porous structure on fiber surface may be caused by the poor compatibility between hydrophilic CNC nanorods and hydrophobic PLA chains at interfaces. CNCs that were well dispersed in as-spun solutions may weaken the inter-connection of PLA chains, transforming this effect from interior to surface as increasing addition of CNCs. On the other hand, the differences on solvent evaporation rate of DMF and CHCl3 can also promote the formation of porous structure. Since the boiling point of DMF (153 ) is much higher than that of CHCl3 (61), therefore, PLA dissolved in solvent mixture can form a main fiber network after the rapid evaporation of CHCl3 as soon as spinning droplets ejected. Meanwhile, DMF may be trapped inside fibers transitorily and afterwards evaporated during electrospinning, thereby inducing a secondary structure formation on fiber surface. Taken together, it can be reasonably assumed that the formation of nanopores on fiber surface is controlled by the synergistic effect of CNC addition and solvent evaporation rate. When depositing PLA/CNC fibers on a collector with high spin speed, all the formed fibers can be aligned along the rotating collector, leading to a possibility to form fibrous mats with aligned fiber arrangement. The fiber alignment in PLA/CNC fibrous mats was microscopically visualized and the obtained micrographs were shown in Figure 3A1-A5. As shown in Figure 13 ACS Paragon Plus Environment


Page 14 of 31

3A1-A5, the PLA/CNC fibers produced displayed fine oriented arrangement with uniform fiber diameter distribution. Notably, compared with the electrospun PLA/CNC fibers showing random morphology, the diameter of aligned fibers was much smaller and more uniform at the same CNC loading levels, which may be attributed that the high-speed stretching during fiber orienting process could further extend fiber lengthways and decrease PLA deposit laterally. After aligning, nanopores were also observed on PLA/CNC fibers but only occurred at high CNC loading levels of 15 and 20 wt% (Figure 3A4-A5, and Figure S3). We speculated this discrepancy to be caused by the secondary stretching after fibers were deposited onto the high-speed collector. When CNC addition was less than 15 wt%, the secondary stretching was sufficiently strong to induce the merging of nanopores during fiber deformation at high speed. With loading 15 wt% or more CNCs, although the high-speed secondary stretching still applied on fiber formation, it can hardly prevent the nanopore generation since more CNC nanorods were dispersed within PLA matrix and physically connected with PLA chains. It should be noted that in general, increasing the conductivity of spinning solutions can lead to smaller fiber diameter. However, in current study, the variation trend of fiber diameter shows different behavior as increasing CNC loadings, irrespective of random or aligned arrangement. It can be observed that at low CNC concentrations, fiber diameter was first decreased (Figure 3R2, A2 and A3), ascribing to the increased solution conductivity. Further increasing CNC loading levels, fiber diameter reversed to increase again (Figure 3R3-R5 and A4-A5), but showing porous fiber surface. Since surface pores were generated and their size was approximately 90 nm, it can be likely deduced that the anomalistic increase of fiber diameter for higher CNC concentrations was originated from the widened fiber lateral dimension by forming nanopores. In summary, the microstructure of electrospun PLA/CNC fibers, that is, random vs aligned 14 ACS Paragon Plus Environment

Page 15 of 31 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

Biomacromolecules

morphology and non-porous vs hierarchical multi-porous, can be controllably manufacturing by changing CNC additions and fiber arrangement, demonstrating a finely tunable electrospinning system to produce fibrous bio-based nanocomposites.

Figure 3. SEM images of electrospun PLA/CNC fibrous mats with random (R1-5) and aligned (A1-5) fiber arrangement. The loading level of CNCs was 0, 5, 10, 15 and 20 wt%, from 1 to 5. The scale bar was 2 µm.

Further confirmation of electrospun PLA/CNC fibers with multiple microstructures was revealed by AFM (Figure 4). From the phase image (Figure 4a), it can be clearly found that nanopores appeared regularly on the surface of produced fibers for both random (R5) and aligned (A5) structure, while electrospun fibers without adding CNCs (R1) were non-porous. These results again verified the generation of nanopores on PLA/CNC fibers during electrospinning. Notable, as observed from Figure 4a, no significant differences in surface roughness for electrospun fibers with or without aligned were observed, which might be due to the sensitivity of AFM at relatively large scanning area. However, the roughness measurements and corresponding mean values shown in Figure 4b and c clearly indicated that electrospun fibers in all formulations had rough surfaces but displaying different degree. The mean roughness value of R-PLA (without loading CNCs) collected on low-speed roller was around 250 nm, and the 15 ACS Paragon Plus Environment


Page 16 of 31

roughness of fibers with adding 20 wt% CNCs (R5) increased to 325 nm. After aligning the fibers during electrospinning, the mean roughness value of A5 decreased to around 290 nm. We speculated that the variation of roughness for electrospun PLA/CNC fibers was caused by a combination effect of component compatibility, nanoporous structure, and secondary stretching process during collecting fibers on the high-speed roller. By comparing the roughness values of R1 and R5, it can be seen that incorporating CNCs into PLA matrix via electrospinning could increase the surface roughness of obtained fibers, attributing that a heterogeneously coarsening process could be induced during solvent evaporation and fiber formation process, which has been shown above. On the other hand, the formation of nanopores in R5 could also be responsible for the higher surface roughness. For electrospun PLA/CNC fiber containing aligned structure, as electrospun fibers were aligned by a high-speed collector, the produced fibers would undergo an additional stretching by the collector, leading to a flattening process to smooth the surface with decreasing the roughness (Figure 4c). As discussed above, at highest CNC addition, the nanopores can be retained although they may be affected accordingly, which was also demonstrated by AFM results.


Page 17 of 31 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

Biomacromolecules

Figure 4. AFM images of electrospun PLA/CNC fibrous mats with (a) random fibers, R-PLA (R1) and R-PLA/CNC-20 (R5), and aligned fibers, A-PLA/CNC-20 (A5). (b) and (c) mean roughness measurements and corresponding values of PLA/CNC fibers in (a). The red line in height images indicated the measuring position of (b). The scanning area was 5 µm × 5 µm.

In order to investigate the interior structure of electrospun PLA/CNC fibers with nanopores, cross-section of the aligned fibers containing 10 and 20 wt% CNCs were observed by SEM (Figure 5). It should be noticed that since the fibrous mats were immersed into epoxy resin and cured at high temperature, so the fusion between fibers and resin could be occurred, however, individual fibers can still be identified. Furthermore, the wave-like structure in all images was artificially caused by cutting. As shown in Figure 5, the arrangement of produced fibers was well oriented along the same direction, again demonstrating the successful fabrication of aligned fiber by high-speed collecting method. Another interesting feature obtained from Figure 5 was that compared the cross-section of fibers with (20 wt%) or without (10 wt%) surface nanopores, both of electrospun PLA/CNC fibers showed porous structure inside the fibers (Figure 5c and d, and Figure S4), irrespective of surface structure of produced fibers. Since there is no difference 17 ACS Paragon Plus Environment


Page 18 of 31

between electrospun fibers with 15 and 20 wt% CNC loading levels, it can be concluded that the generation of inner pores was a generalized process during electrospinning for aligned PLA fibers with incorporating CNCs, which may be induced by aforementioned compatible difference between PLA and CNCs and solvent evaporation effect. Therefore, it can be reasonably assumed that the different evaporation rate of DMF and CHCl3 and poor interfacial adhesion of CNC-PLA generate the pores firstly inside fibers during electrospinning process. As aligning process was performed, when the amount of CNCs was insufficient to support the nanoporous structure during secondary stretching, the nanopores would be merged, but interior pores could be retained. As incorporating enough CNCs into PLA matrix, the pores distributed both on and inside the fibers would keep stable and visible after secondary stretching.

Figure 5. SEM images of the cross-section of electrospun aligned PLA/CNC fibers with (a, c) 10 wt% and (b, d) 20 wt% loading level of CNCs.


Page 19 of 31 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

Biomacromolecules

Thermal property. The effect of fiber arrangement on the crystallization and melting behavior of PLA/CNC fibrous mats was studied by DSC, and the results are reported in Figure 6a and b and Table 2. The Tg was not obviously affected by the incorporation of CNCs into PLA, either through random or aligned method. It is likely attributed that the large size of CNCs may be not sufficient to change the mobility of PLA chains within the glass transition region,35 and poor interaction between CNCs and PLA matrix would also weaken the effect on changing glass transition. Furthermore, this result also implied that the fiber arrangement showed little impact on tuning the Tg of PLA/CNC fibrous mats, ascribing that the aligning process during electrospinning could only adjust spatial arrangement of fibers rather than their inherent properties. As shown in Table 2, the degree of crystallinity ( ) of R-PLA/CNC fibrous mats was found to increase by adding 5 wt% CNCs, which can be due to that CNC nanorods could perform as a nucleating agent at low adding level to promote PLA crystallization.18 Further increasing CNC concentrations from 10 to 20 wt%, the reduced gradually with being smallest at R-PLA/CNC-20. This can be ascribed that the poor dispersion of CNCs within PLA matrix and the occurrence of CNC aggregates at higher CNC loading levels could inhibit the nucleation effect of CNCs, and interrupt the intrinsic arrangement of PLA chains to some extent. In Figure 6b and Table 2, compared with random fibers, the crystalline behavior of A-PLA/CNC fibrous mats was similar with that of R-PLA/CNC mats, but showing higher values at the same CNC concentrations, which can be explained that aligning process of fibers during electrospinning might further facilitate orderly stack of PLA chains, namely endowing secondary enhancement on fiber crystallization. It should be noted that the results obtained in current study follow similar trend compared with recent report,18 however, two obvious melting peaks were observed at high CNC concentrations in Shi`s report, which was not detected in current results. This dissimilarity 19 ACS Paragon Plus Environment


Page 20 of 31

can be reasonably deduced to be caused by differences on raw materials (e.g., properties of CNCs and PLA), instrument used, measuring conditions, etc. TGA was used to investigate the effect of CNCs on the thermal stability of electrospun PLA/CNC fibrous mats. Observed from Figure 6c and Table 2, the Tonset of neat R-PLA was 293 , while all electrospun R-PLA/CNC fibrous mats had Tonset of greater than 300 . Furthermore, the Tmax of R-PLA/CNC mats displayed a similar variation trend, showing increased value as loading CNCs into PLA matrix. The electrospun PLA fibrous mats containing 5 wt% CNCs presented the highest Tonset and Tmax, indicating the most significant improvement of heat resistance, which can be ascribed to increased crystallinity of electrospun R-PLA/CNC nanocomposites (Table 2).36 From Figure 6d, compared with the TGA curves of R-PLA/CNC mats, those of A-PLA/CNC fibrous mats were fairly similar but showing higher Tonset and Tmax values at the same CNC loading levels, indicating that the fibrous mats with aligned fibers had better thermal stability, possibly attributing to higher crystallinity of A-PLA/CNC fibers than that of R-PLA/CNC. Summarily, it can be concluded that electrospinning to produce aligned fibrous structure under identical components exhibits better effect on modifying the thermal properties of PLA/CNC fibrous mats than that of random fibers, that is, additional high-speed aligning process of fibers during electrospinning offers a secondary enhancement contribution for controllably tailoring thermal performance of PLA/CNC nanocomposite.


Page 21 of 31 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

Biomacromolecules

Figure 6. DSC and TGA curves of electrospun PLA and PLA/CNC fibrous mats composed of (a, c) random and (b, d) aligned fiber arrangement with different CNC loading levels. The inserts in (c) and (d) were corresponding DTG curves of samples. Table 2. Results of thermal properties of electrospun PLA/CNC fibrous mats with or without aligned fiber arrangement. Code

Tonset ()

Tma ()

Tg ()

Tcc ()

Tm ()

∆Hm (J/g)

∆Hcc (J/g)

χc (%)

R-PLA

293.1

348.1

59.8

120.4

164.7

35.2

35.1

0

R-PLA/CNC-5

310.1

360.7

60.8

122.4

165.3

31.1

24.0

8.0

R-PLA/CNC-10

308.8

360.1

60.4

119.3

165.6

27.8

22.8

5.9

R-PLA/CNC-15

308.4

359.4

60.3

121.5

166.1

22.6

19.1

4.3

R-PLA/CNC-20

306.2

357.2

60.5

118.5

164.3

19.2

16.5

3.5

A-PLA

293.3

347.8

60.1

120.8

164.8

36.1

35.9

0

A-PLA/CNC-5

311.2

361.9

60.7

120.3

165.1

32.2

24.1

9.1

A-PLA/CNC-10

310.5

360.2

60.4

121.3

164.8

27.1

21.9

6.1

A-PLA/CNC-15

309.2

360.7

60.6

122.1

166.2

23.3

19.6

4.5

A-PLA/CNC-20

308.8

358.8

60.2

121.3

165.4

19.1

15.9

4.1



Page 22 of 31

Surface wettability. Contact angle (CA) is a convenient indicator to evaluate surface hydrophilic-hydrophobic nature. The surface hydrophobicity of electrospun PLA/CNC fibrous nanocomposite is an important factor for its application, however, the hydrophobicity difference of CNCs and PLA chains and the changeful microstructure of obtained fibers make it difficult to simply predict the surface properties of PLA/CNC mats. Therefore, the CA values of electrospun PLA/CNC fibrous mats with either random or aligned structures at varied loading amount of CNCs were explored (Figure 7). It can be observed that neat PLA fibrous mats containing random and aligned fibers had values of 127° and 123 ° , respectively. With increasing the addition of CNCs from 5 to 15 wt%, the CA values gradually increased to over 130° for both of fibrous mats, dispalying similar variation trend. Significantly, after loading 20 wt% CNCs, the CA value of the mat made with random fibers attained around 150°, while the CA for fibrous mat with aligned fibers was 140°. Based on these results, a mechanism combining surface microstructure of fibers and spatial arrangement of electrospun fibers was proposed to interpret the change of surface hydrophobic property of electrospun PLA/CNC fibrous mats. Surface roughness is a decisive factor in determining the surface hydrophobicity of materials since rough surface can trap more air, whose CA value is considered to be 180°.37 With adding CNCs into PLA matrix, the surface roughness could be increased (shown in Figure 4c), thereby, leading to an increase in CA values, regardless of random or aligned fibrous mats. Additionally, the structure of electrospun PLA/CNC fibrous mats was turned to be porous after reaching certain amount of CNCs, further increasing uneven property of fiber surface, which also contributed to the change of CA. Interestingly, compared with R-PLA/CNC-20, A-PLA/CNC-20 showed smaller CA although they both had identical CNC concentrations and similar surface porous morphology. This is likely attributed that the porous structure of aligned fibers at 20 wt% CNC 22 ACS Paragon Plus Environment

Page 23 of 31 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

Biomacromolecules

addition underwent a secondary stretching process during high-speed collecting, which may reduce the pore density on fiber surface, leading to a decreased surface roughness (Figure 4c). On the other hand, the ordered spatial arrangement of fibers may present a negative effect on the hydrophobicity of fibrous mats since randomly packed fibers could be considered as criss-cross morphology, allowing water drops to be squeezed out,20 which leads to a relatively smaller CA for A-PLA/CNC fibrous mats at the same CNC concentrations, especially at 20 wt% CNC loading level.

Figure 7. Contact angle (CA) of electrospun PLA/CNC fibrous mats with either random or aligned fiber arrangement. Corresponding images of water droplet on surface of A-PLA/CNC fibrous mats at certain CNC concentrations were inserted accordingly.

Mechanical property. Typical stress-strain curves of neat PLA and PLA/CNC nanocomposite mats with either random or aligned fibers are presented in Figure 8, and their maximum tensile stress ( ), the elongation at break ( ), and Young’s modulus (E) are summarized in Table 3. As shown in Figure 8a and Table 3, with increased CNC contents, the and E of electrospun PLA/CNC nanocomposite mats initially increased at low CNC 23 ACS Paragon Plus Environment


Page 24 of 31

loading levels, showing 4.7±0.1 MPa and 70.6 MPa at 10 wt% CNCs, respectively. Further increasing CNC concentrations, the and E of obtained fibrous mats decreased accordingly. Furthermore, the of all R-PLA/CNC fibrous mats was approximately similar. At low CNC loading levels, the enhanced tensile properties of PLA/CNC fibrous mats should be attributed to the function of CNCs by increasing crystallinity for composite (Table 2) and the reduced loading force on PLA through efficient stress transfer from PLA to stiffer CNCs.38 Moreover, during the electrospinning process, part of CNCs can be aligned into ordered structure to disperse within PLA matrix by electric field force, which provides larger interfacial adhesion,39 leading to a possibility to achieve accelerated physical aging to transform PLA/CNC fibrous mats to a more stable and dense structure.8 At higher CNC concentrations, decreased tendency of mechanical properties of mats can be likely attributed to the aggregation of CNCs at high loading levels in electrospun fibers, especially on fiber surface, decreasing crystallinity of composites (Table 2) and weakened the cohesion between fibers. On the other hand, the obvious surface and interior pores generated during electrospinning may be acted as “defect” that can negatively influence the mechanical performance of mats by forming cracks prematurely during tensile test. For electrospun A-PLA/CNC fibrous mats (Figure 8b and Table 3), the tensile properties have been dramatically improved, with highest (15.3 MPa) and E (126.8 MPa) at 5 wt% CNCs. Increasing CNC contents, the tensile properties of fibrous mats gradually decreased but all showing higher and E than those of electrospun R-PLA mats. From Table 3, it can also be seen that the properties of A-PLA/CNC fibrous mats followed similar varying trend as RPLA/CNC, although the values were significantly higher. These results suggested that adding CNCs into PLA through aligning method during electrospinning could effectively enhance the mechanical performance of fibrous mats. On one hand, since the tensile test was along the fiber 24 ACS Paragon Plus Environment

Page 25 of 31 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

Biomacromolecules

direction, the stretching force was also applied along fibers, leading to a fact that stronger stretching force could be exerted on the oriented fibers, thereby enabling much larger stress onto A-PLA/CNC fibrous mats. Furthermore, loading CNCs into PLA matrix also contributed to the improvement of mechanical properties, either by tuning the crystallinity of PLA or transferring the stress from PLA to CNCs. On the other hand, secondary stretching process during high-speed collecting may also promote fiber deformation to more tightly structure, thereby, increasing the tensile performance.40, 41 Similarly, lower crystallinity of PLA chains resulted from higher CNC additions could also weaken the tensile properties of mats, giving rise to smaller and E. Interestingly, the of mats were decreased with increasing CNC addition. This is also likely attributed to the interior and surface porous structure of fibers. Since the surface pores were distributed on the fibers but not all along the fiber direction, therefore, it can be reasonably speculated that during stretching, the surface pores might be transformed to large cracks, which promoted the rupture of bulk fibers, resulting in shorter elongation at break. In summary, with the more ordered fiber organization in fibrous mats, significant enhancement of tensile properties of A-PLA/CNC fibrous mats is achieved, directly confirming the microstructure of fibers originated from the addition of CNCs and aligning method during electrospinning can be correlated with their mechanical properties.



Page 26 of 31

Figure 8. Stress-strain curves of electrospun PLA/CNC fibrous mats with (a) random and (b) aligned fiber arrangement. Table 3 Tensile properties of electrospun PLA/CNC fibrous mats with different fiber arrangement Code

(MPa)

(%)

Young’s modulus (MPa)

R-PLA

3.9 ± 0.3

87.6

43.8

R-PLA/CNC-5

3.8 ± 0.2

67.4

42.3

R-PLA/CNC-10

4.7 ± 0.1

80.2

70.6

R-PLA/CNC-15

2.9 ± 0.3

81.4

41.1

R-PLA/CNC-20

1.8 ± 0.1

81.8

34.1

A-PLA

4.5± 0.5

27.4

62.7

A-PLA/CNC-5

15.3 ± 1.1

32.5

126.8

A-PLA/CNC-10

10.4 ± 0.6

26.8

82.7

A-PLA/CNC-15

7.6 ± 0.3

22.6

68.1

A-PLA/CNC-20

8.5 ± 0.4

18.6

74.6

CONCLUSIONS A series of CNC-enhanced PLA fibrous nanocomposite mats both with random and aligned fiber arrangement were successfully electrospun by loading different concentrations of CNCs into PLA matrix. Uniform and fine PLA/CNC fibers with expected fiber stacking forms can be produced under experimental conditions due to the synergetic effect of electric conductivity, interfacial tension, and viscosity of spinning solutions. Morphological investigation of obtained 26 ACS Paragon Plus Environment

Page 27 of 31 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

Biomacromolecules

fibrous mats indicated that the microstructure of fibers, regardless of random or aligned orientation, transformed from smooth surface to surface containing nanopores by changing CNC loading levels. Nevertheless, porous surface of fibers can also be influenced by aligning process through secondary stretching during high-speed collecting. The incorporation of CNCs into PLA matrix changed the crystallinity of PLA, showing higher crystallinity for aligned PLA/CNC fibers. It was also found that the thermal properties of PLA/CNC fibrous mats, especially for thermal stability of aligned mats, can be also enhanced. The mechanical properties of electrospun PLA/CNC fibrous mats were significantly improved by aligning fibers during electrospinning. This can be ascribed to a combined effect of the increased crystallinity of fibers, the efficient stress transfer from PLA to CNCs, and the ordered arrangement of electrospun fibers in the mats. Moreover, the surface porous structure also showed impact on the mechanical properties of fibrous mats, particularly for the elongation at break, demonstrating tunable properties of electrospun fibrous mats by controlling fiber microstructure. This study presented for the first time that combining CNCs and fiber aligning process during electrospinning can finely tune the surface microstructure of produced fibers. Finally, this research opens an avenue for developing an electrospinning system that can manufacture environmental-friendly and biocompatible highperformance CNC-enhanced fibrous nanocomposites.

ASSOCIATED CONTENT Supporting Information Additional materials are provided related to SEM images of electrospun fibrous mats with different PLA concentrations without loading CNCs; visual appearance of the as-spun solutions with varying CNC loading levels; SEM images of random and aligned fibers as well as crosssection of aligned fibers at higher magnification. 27 ACS Paragon Plus Environment


Page 28 of 31

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel.: +86-451-8219-1871; *E-mail: [email protected], Tel.: +358-(0)50-339-4495. ORCID Long Bai: 0000-0003-3356-9095 Notes 1

These authors contributed equally. The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Gant No. 31470580). REFERENCES (1) Pirani, S.; Abushammala, H.; Hashaikeh, R. Preparation and Characterization of Electrospun PLA/nanocrystalline Cellulose‐based Composites. J. Appl. Polym. Sci. 2013, 130, 3345-3354. (2) Scaffaro, R.; Botta, L.; Lopresti, F.; Maio, A.; Sutera, F. Polysaccharide Nanocrystals as Fillers for PLA Based Nanocomposites. Cellulose 2017, 1-32. (3) Jalvo, B.; Mathew, A. P.; Rosal, R. Coaxial Poly(lactic acid) Electrospun Composite Membranes Incorporating Cellulose and Chitin Nanocrystals. J. Membr. Sci. 2017, 544, 261-271. (4) Gupta, B.; Revagade, N.; Hilborn, J. Poly(lactic acid) Fiber: An overview. Prog. Polym. Sci. 2007, 32, 455-482. (5) Zhou, C.; Shi, Q.; Guo, W.; Terrell, L.; Qureshi, A. T.; Hayes, D. J.; Wu, Q. Electrospun Bionanocomposite Scaffolds for Bone Tissue Engineering by Cellulose Nanocrystals Reinforcing Maleic Anhydride Grafted PLA. ACS Appl. Mater. Interfaces 2013, 5, 3847-3854. (6) Li, L.; Hashaikeh, R.; Arafat, H. A. Development of Eco-efficient Micro-porous Membranes via Electrospinning and Annealing of Poly(lactic acid). J. Membr. Sci. 2013, 436, 57-67. (7) Paul, D.; Robeson, L. M. Polymer Nanotechnology: Nanocomposites. Polymer 2008, 49, 3187-3204. (8) Huan, S.; Bai, L.; Cheng, W.; Han, G. Manufacture of Electrospun All-aqueous Poly(vinyl alcohol)/cellulose Nanocrystal Composite Nanofibrous Mats with Enhanced Properties Through Controlling Fibers Arrangement and Microstructure. Polymer 2016, 92, 25-35. (9) Bai, L.; Gu, J.; Huan, S.; Li, Z. Aqueous Poly(vinyl acetate)-based Core/shell Emulsion: Synthesis, Morphology, Properties and Application. RSC Adv. 2014, 4, 27363-27380. (10) Noh, K.-T.; Lee, H.-Y.; Shin, U.-S.; Kim, H.-W. Composite Nanofiber of Bioactive Glass Nanofiller Incorporated Poly(lactic acid) for Bone Regeneration. Mater. Lett. 2010, 64, 802-805. (11) De France, K. J.; Chan, K. J.; Cranston, E. D.; Hoare, T. Enhanced Mechanical Properties in Cellulose Nanocrystal-poly (oligoethylene glycol methacrylate) Injectable Nanocomposite 28 ACS Paragon Plus Environment

Page 29 of 31 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

Biomacromolecules

Hydrogels Through Control of Physical and Chemical cross-linking. Biomacromolecules 2016, 17, 649-660. (12) Mittal, N.; Jansson, R.; Widhe, M.; Benselfelt, T.; Håkansson, K. M.; Lundell, F.; Hedhammar, M.; Söderberg, L. D. Ultrastrong and Bioactive Nanostructured Bio-Based Composites. ACS Nano 2017, 11, 5148-5159. (13) Chaabouni, O.; Boufi, S. Cellulose Nanofibrils/polyvinyl acetate Nanocomposite Adhesives with Improved Mechanical Properties. Carbohydr. Polym. 2017, 156, 64-70. (14) Huan, S.; Yokota, S.; Bai, L.; Ago, M.; Borghei, M.; Kondo, T.; Rojas, O. J. Formulation and composition effects in phase transitions of emulsions co-stabilized by cellulose nanofibrils and an ionic surfactant. Biomacromolecules 2017, 18, 4393-4404. (15) Han, G.; Huan, S.; Han, J.; Zhang, Z.; Wu, Q. Effect of Acid Hydrolysis Conditions on the Properties of Cellulose Nanoparticle-reinforced Polymethylmethacrylate Composites. Materials 2013, 7, 16-29. (16) Xu, X.; Wang, H.; Jiang, L.; Wang, X.; Payne, S. A.; Zhu, J.; Li, R. Comparison Between Cellulose Nanocrystal and Cellulose Nanofibril Reinforced Poly(ethylene oxide) Nanofibers and Their Novel Shish-kebab-like Crystalline Structures. Macromolecules 2014, 47, 3409-3416. (17) Nishino, T.; Takano, K.; Nakamae, K. Elastic Modulus of the Crystalline Regions of Cellulose Polymorphs. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1647-1651. (18) Shi, Q.; Zhou, C.; Yue, Y.; Guo, W.; Wu, Y.; Wu, Q. Mechanical Properties and In Vitro Degradation of Electrospun Bio-nanocomposite Mats from PLA and Cellulose Nanocrystals. Carbohydr. Polym. 2012, 90, 301-308. (19) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-assembly, and Applications. Chem. Rev. 2010, 110, 3479-3500. (20) Huan, S.; Bai, L.; Liu, G.; Cheng, W.; Han, G. Electrospun Nanofibrous Composites of Polystyrene and Cellulose Nanocrystals: Manufacture and Characterization. RSC Adv. 2015, 5, 50756-50766. (21) Dong, H.; Strawhecker, K. E.; Snyder, J. F.; Orlicki, J. A.; Reiner, R. S.; Rudie, A. W. Cellulose Nanocrystals as a Rreinforcing Material for Electrospun Poly(methyl methacrylate) Fibers: Formation, Properties and Nanomechanical Characterization. Carbohydr. Polym. 2012, 87, 2488-2495. (22) Li, C.; Li, Q.; Ni, X.; Liu, G.; Cheng, W.; Han, G. Coaxial Electrospinning and Characterization of Core-Shell Structured Cellulose Nanocrystal Reinforced PMMA/PAN Composite Fibers. Materials 2017, 10, 572-587. (23) Arrieta, M.; López, J.; López, D.; Kenny, J.; Peponi, L. Biodegradable Electrospun Bionanocomposite Fibers Based on Plasticized PLA-PHB Blends Reinforced with Cellulose Nanocrystals. Ind. Crops Prod. 2016, 93, 290-301. (24) Song, W.; Liu, D.; Prempeh, N.; Song, R. Fiber Alignment and Liquid Crystal Orientation of Cellulose Nanocrystals in the Electrospun Nanofibrous Mats. Biomacromolecules 2017, 18, 3273-3279. (25) Wanasekara, N.; Santos, R.; Douch, C.; Frollini, E.; Eichhorn, S. J. Orientation of Cellulose Nanocrystals in Electrospun Polymer Fibres. Journal of Materials Science 2016, 51, 218-227. (26) Xue, J.; Xie, J.; Liu, W.; Xia, Y. Electrospun Nanofibers: New Concepts, Materials, and Applications. Acc. Chem. Res. 2017, 50, 1976-1987. (27) Liao, H.; Wu, Y.; Wu, M.; Zhan, X.; Liu, H. Aligned Electrospun Cellulose Fibers Reinforced Epoxy Resin Composite Films with High Visible Light Transmittance. Cellulose 2012, 19, 111-119. 29 ACS Paragon Plus Environment


Page 30 of 31

(28) Wang, Y.; Chen, L. Cellulose Nanowhiskers and Fiber Alignment Greatly Improve Mechanical Properties of Electrospun Prolamin Protein Fibers. ACS Appl. Mater. Interfaces 2014, 6, 1709-1718. (29) He, X.; Xiao, Q.; Lu, C.; Wang, Y.; Zhang, X.; Zhao, J.; Zhang, W.; Zhang, X.; Deng, Y. Uniaxially Aligned Electrospun All-cellulose Nanocomposite Nanofibers Reinforced with Cellulose Nanocrystals: Scaffold for Tissue Engineering. Biomacromolecules 2014, 15, 618-627. (30) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 years of image analysis. Nat. Methods 2012, 9, 671-675. (31) Frone, A. N.; Berlioz, S.; Chailan, J.-F.; Panaitescu, D. M. Morphology and Thermal Properties of PLA-cellulose Nanofibers Composites. Carbohydr. Polym. 2013, 91, 377-384. (32) Mihai, M.; Huneault, M. A.; Favis, B. D.; Li, H. Extrusion Foaming of Semi-Crystalline PLA and PLA/Thermoplastic Starch Blends. Macromol. Biosci. 2007, 7, 907-920. (33) Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I. Modulation of Cellulose Nanocrystals Amphiphilic Properties to Stabilize Oil/water Interface. Biomacromolecules 2011, 13, 267-275. (34) Huan, S.; Liu, G.; Han, G.; Cheng, W.; Fu, Z.; Wu, Q.; Wang, Q. Effect of Experimental Parameters on Morphological, Mechanical and Hydrophobic Properties of Electrospun Polystyrene Fibers. Materials 2015, 8, 2718-2734. (35) Wei, L.; Agarwal, U. P.; Matuana, L.; Sabo, R. C.; Stark, N. M. Performance of High Lignin Content Cellulose Nanocrystals as Filler in Poly(lactic acid). Polymer 2017, 135, 305-313. (36) Kumar, A. P.; Depan, D.; Tomer, N. S.; Singh, R. P. Nanoscale Particles for Polymer Degradation and Stabilization-trends and Future Perspectives. Prog. Polym. Sci. 2009, 34, 479515. (37) Cassie, A.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546551. (38) Zhou, C.; Chu, R.; Wu, R.; Wu, Q. Electrospun Polyethylene oxide/cellulose nanocrystal Composite Nanofibrous Mats with Homogeneous and Heterogeneous Microstructures. Biomacromolecules 2011, 12, 2617-2625. (39) Ansari, F.; Salajková, M.; Zhou, Q.; Berglund, L. A. Strong Surface Treatment Effects on Reinforcement Efficiency in Biocomposites Based on Cellulose Nanocrystals in Poly(vinyl acetate) Matrix. Biomacromolecules 2015, 16, 3916-3924. (40) Lundahl, M. J.; Klar, V.; Wang, L.; Ago, M.; Rojas, O. J. Spinning of Cellulose Nanofibrils into Filaments: A Review. Ind. Eng. Chem. Res. 2016, 56, 8-19. (41) Lundahl, M. J.; Cunha, A. G.; Rojo, E.; Papageorgiou, A. C.; Rautkari, L.; Arboleda, J. C.; Rojas, O. J. Strength and Water Interactions of Cellulose I Filaments Wet-Spun from Cellulose Nanofibril Hydrogels. Sci. Rep. 2016, 6, 30695.


Page 31 of 31 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

Biomacromolecules

For Table of Contents Use Only

Electrospun poly(lactic acid)-based fibrous nanocomposite reinforced by cellulose nanocrystals: Impact of fiber uniaxial alignment on microstructure and mechanical properties Siqi Huan1, Guoxiang Liu1, Guangping Han*, Long Bai*


Electrospun Poly(lactic acid)-Based Fibrous Nanocomposite

Recommend Documents