Sustainable and High Performing Biocomposites with Chitosan

Jul 24, 2018 - Biocomposites encompassing biopolymers and natural fibers represent potential candidates for the replacement of fossil-based polymers i...
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Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Sustainable and High Performing Biocomposites with Chitosan/ Sepiolite Layer-by-Layer Nanoengineered Interphases Daniele Battegazzore,* Alberto Frache, and Federico Carosio Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, sede di Alessandria, Viale Teresa Michel 5, 15121 Alessandria, Italy

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S Supporting Information *

ABSTRACT: Biocomposites encompassing biopolymers and natural fibers represent potential candidates for the replacement of fossil-based polymers in many application fields. However, due to poor matrix/fiber interphase produces insufficient mechanical properties for practical application. In this Letter, we use the Layer-by-Layer assembly technique in order to modify the surface of natural fibers and produce a nanostructured interphase capable of improving the mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/hemp fibers biocomposites. Chitosan and sepiolite nanorods have been selected as interphase constituents. When assembled on hemp fibers, this chitosan/sepiolite system conformally coats every fiber yielding a nanostructured coating that subsequently becomes the matrix/fiber interphase during composite preparation. Thanks to the LbL assembled interphase, the biocomposites achieve impressive mechanical properties with elastic moduli up to 2.6 GPa, which is 70% and 30% better of the neat matrix and the composite prepared with unmodified fibers, respectively. The achieved performances would allow for the use of these LbL engineered biocomposites in load bearing applications, thus opening up new opportunities for the exploitation of biobased resources. KEYWORDS: Biocomposite, Chitosan, Sepiolite, Layer-by-Layer, Mechanical properties, Adhesion, Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), Natural fiber

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INTRODUCTION

drawback of not being appealing from the sustainability point of view.9−12 In the present Letter, we present a novel promising approach where the Layer-by-Layer (LbL) assembly is exploited as surface nanoengineering tool to finely control and enhance the fiber/matrix interaction. The LbL allows for the construction of nanostructured coatings on a substrate poses the great advantages of being highly tunable and versatile since for every substrate it is possible to find matching components and favorable assembly conditions.13 In addition, the LbL can be considered a green technique as the deposition occurs at room conditions, water is used as solvent and the solutions/ suspensions are diluted (≤1 wt %). Here we exploit the electrostatic interactions occurring between the positively charged chitosan (CH) and the negatively charged sepiolite (SEP) nanoparticles for engineering the interphase of poly(3hydroxybutyrate-co-3-hydroxyhexanoate) (PHB)/hemp fibers biocomposites. Hemp fibers have been selected as they possess the highest mechanical properties among natural fibers with a specific modulus higher than glass fibers.4,14 The fibers have

In recent years, the polymer science field witnessed a growing interest toward the use of biobased and biodegradable polymers in order to reduce the global environmental impact of plastic.1 However, as far as practical applications are concerned, bioplastics cannot compete with currently adopted polymers made from fossil resources due to generally poorer mechanical properties.2 In order to overcome this, the development of green thermoplastics composites encompassing short or long natural fibers as reinforcements is normally pursued. Short fibers are the most commonly studied, although they do not guarantee good enough performances for a possible industrial breakthrough.3 On the other hand, only a few studies tried to exploit the benefits of the load carrying capacity of long fibers.4−7 This latter appears to be the best approach for the production of sustainable and high performing biocomposites. Unfortunately, preliminary studies on biocomposites had to face the poor bonding strength between natural fibers and biopolymers, which led to detrimental results due to the premature matrix breakage and debonding of fibers/ matrix during mechanical deformation.8 Conventional adhesion promoters or coupling agents can be added to partially improve the fabric−matrix interactions with the major © XXXX American Chemical Society

Received: June 20, 2018 Revised: July 23, 2018 Published: July 24, 2018 A

DOI: 10.1021/acssuschemeng.8b02907 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. LbL assembly characterization and coating morphology on hemp fabrics: (a) schematic representation of the adopted LbL assembly; (b) coating growth as a function of each deposited layer by infrared spectroscopy of restricted IR region between 800 and 2000 cm−1 of SEP and CH on silica surface; (c) intensity vs layer number plot of the signal at 1014 and 1080 cm−1; FESEM magnification and wettability of neat fabric (d) and LbL treated fabric (e).

fabrics show substantial improvements in both young modulus and tensile strength allowing for practical application in load bearing components.23

been treated with the LbL assembly prior to their employment as reinforcing agent. The hypothesis is that the deposited LbL coating would act as beneficial interphase promoting better adhesion and mass transfer, thus resulting in improved mechanical properties. This is based on the fact that LbL films comprising polyelectrolytes interacting with high charge densities or polyelectrolyte/nanoparticles have been reported to reach impressive mechanical properties with elastic moduli above 10 GPa.15,16 Such results coupled with the ability to finely nanostructure the surface employing a green and sustainable process make the LbL the perfect tool for designing and constructing highly performing interphases in biocomposites. Although the LbL has been used in several application fields including antibacterial-antifouling,17,18 wastewater dye removal,19 flame retardancy and gas barrier properties,20−22 to the best of the authors’ knowledge this approach to interphase engineering through LbL has never been attempted before. The assembly of CH/SEP and the resulting morphology on hemp fibers has been monitored by infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM), respectively. This CH/SEP coating can conformally coat every surface of the hemp fabric producing a nanostructured interphase where SEP nanorods are embedded in a CH matrix. PHB composites prepared with LbL modified

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RESULTS AND DISCUSSION The coating growth of the CH/SEP assembly has been first evaluated on model Si wafer surfaces and followed step by step by infrared spectroscopy. The same procedure has been then adopted on hemp fabrics (Figure 1a) evaluating the changes in surface morphology by FESEM. Figure 1 summarizes the collected results. As reported in Figure S2, neat CH shows characteristic vibration bands of a polysaccharides at 3370−3297 cm−1 (assigned to O−H and N−H stretching); 2931, 2888, 1411, and 1324 (assigned to C−H bond); 1634 and 1567 cm−1 (asymmetric and symmetric stretching vibration mode of the protonated amine NH3+); and 1085 and 1155 cm−1 (assigned to pyranose rings and amino groups).24 On the other hand, neat SEP yields broad bands 3600−3000 cm−1 and 1700− 1600 cm−1 that are ascribed to the zeolitic and coordinated water molecules.24 The Mg−OH bands are found at 3686 cm−1. The strong band at 1014 cm−1 is assigned to Si−O−Si bond. When the two components are LbL assembled together, the resulting spectrum shows signals characteristic of both B

DOI: 10.1021/acssuschemeng.8b02907 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 2. PHB composite characterization: (a, b, c) FESEM micrographs of treated hemp fabric embedded in the PHB matrix; (d) stress−strain representative plots of PHB-based composites; (e) average modulus (E), maximum strength (σmax), deformation at max stress (ε at σmax) histograms and standard deviation bars from the tension tests; (f) tensile strength (σ) versus modulus (E) of PHB-based composites and classes areas according to EN 312 standard for particleboards.

weight fraction was found to average 18% while the void percentage was found to be lower than 1% (Table S1). The morphology and mechanical properties of prepared composites are reported in Figure 2. Once embedded in the polymer matrix, the fibers are immersed at the center of the composite thickness (Figure 2a). The fabric warp and weft structure is apparent. High magnification pictures (Figure 2b,c) point out that the surface of the fibers is still covered by the CH-SEP layers that thus were not destroyed during composite preparation. The deposited thin homogeneous LbL coating wraps around the hemp fibers and produces a modified interphase as evidenced in Figure 2c and Figure S3. By analyzing the fibers/matrix interphase, it is possible to observe that there are no gaps in the composite (Figure 2c). Composite facture during sample preparation occurred within the LbL assembled interphase that is thus playing a predominant role in stress transfer efficiency. During tensile tests, the unmodified polymeric matrix (PHB) yielded a maximum average stress of 27.6 MPa at a 4.86% of elongation with an elastic modulus of 1.5 GPa. The average collected data and standard deviations from the tension tests are listed in Table S2, whereas the representative stress−strain curve for each composite type is plotted in Figure 2d. The deformation at the maximum stress and not at break is reported for the neat PHB matrix. On the other hand, for all the composite materials, no difference between elongation at break and at maximum load is registered, thus they can be used indiscriminately. The inclusion of hemp fibers result in an increase of the Young’s modulus (E) and maximum tensile strength (σmax) (Figure 2e). The reduction of deformation at max stress (ε), that is typical of fiber reinforced composites, is kept to a minimum (−1% with respect to the neat PHB). The beneficial

components (Figure 1b). The most intense peaks are mainly ascribed to SEP while the presence of CH is evidenced by a shoulder at 1080 cm−1 and a broad band centered around 1400 cm−1. Most importantly, the above-mentioned signals grow proportionally to the number of deposited BL, thus confirming the occurrence of a LbL assembly. By plotting the intensity of SEP Si−O−Si and CH C−O−C signals as a function of layer number (Figure 1c), it is possible to devise a linear growth for this CH/SEP assembly that is mainly constituted by SEP nanorods as pointed out by the predominant role of SEP signals. When the coating is transferred to hemp fabrics, a clear change in surface morphology is clearly visible. Indeed, the roughness and inhomogeneity typical of natural fibers (Figure 1d) is replaced by a continuous and homogeneous coating that produces a nanotexture on the fiber surface (Figure 1e). High magnification micrographs further highlight how the assembly can coat each fiber with a thin coating comprising SEP nanorods kept together by CH. This morphology is in agreement with a previous work by Darder et al. dealing with the preparation of microfibrous CH/SEP nanocomposites.25 A practical and fast test was also performed to characterize the wettability of the fabrics. It consists in the observation of a water drop on the fabric surface. As shown in Figure 1d, the drop is instantly absorbed by the neat fabrics; whereas in the case of LbL treated (Figure 1e), the drop steadly remains on the surface. This behavior resembles the one of a superhydrophobic surface and can be ascribed to the physical effect of the CH/SEP nanotexture that can therefore potentially induce a better adhesion when in contact with a polymer matrix. The LbL modifed hemp fabrics have been then used for the preparation of PHB composites exploiting a simple compression molding procedure.5 Composites with unmodified fabrics have been prepared as well. The fabric C

DOI: 10.1021/acssuschemeng.8b02907 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering effect of the LbL treatment is apparent in Figure 2e. Indeed, thanks to the modified CH/SEP interphases, prepared composites achieved elastic moduli of 2.6 GPa and a tensile strengths up to 47 MPa. Such performances are 25−30% better of those achieved with unmodified fibers. Similar results are only possible by employing synthetic coupling agents through organic solvent based processes.5 This further highlights the importance and potential environmental impact of the proposed approach. The possibility of practical application of prepared composites has been evaluated according to EN 312 standard for particleboards in furniture application.23 Classes are defined of the basis of minimum E and σ that define colored areas in the σ vs E plots in Figure 2f, higher classes have more restrictive requirements but open up for an increased number of applications. It is apparent that the presence of the CH/SEP LbL functionalization is mandatory in order to meet the requirement for load bearing components (green area in Figure 2f). Such impressive result can be ascribed to the unique nanotexture imparted by the LbL assembly and the strong ionic interactions occurring at the nanoscale between chitosan and sepiolite. This unique combination results in a strong interphase with greatly improved stress transfer ability capable of controlling the composite mechanical properties.

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AUTHOR INFORMATION

Corresponding Author

*D. Battegazzore. Tel/Fax: +390131229343/+390131229399; e-mail address: [email protected]. ORCID

Daniele Battegazzore: 0000-0002-6829-5605 Federico Carosio: 0000-0003-4067-503X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Amir Noori is acknowledged for the help in the experimental work. REFERENCES

(1) Nagarajan, V.; Mohanty, A. K.; Misra, M. Perspective on polylactic acid (PLA) based sustainable materials for durable applications: Focus on toughness and heat resistance. ACS Sustainable Chem. Eng. 2016, 4 (6), 2899−2916. (2) Dicker, M. P. M.; Duckworth, P. F.; Baker, A. B.; Francois, G.; Hazzard, M. K.; Weaver, P. M. Green composites: A review of material attributes and complementary applications. Composites, Part A 2014, 56, 280−289. (3) Battegazzore, D.; Alongi, J.; Frache, A. Poly (lactic acid)-based composites containing natural fillers: thermal, mechanical and barrier properties. J. Polym. Environ. 2014, 22 (1), 88−98. (4) Pickering, K. L.; Efendy, M. G. A.; Le, T. M. A review of recent developments in natural fibre composites and their mechanical performance. Composites, Part A 2016, 83 (Supplement C), 98−112. (5) Battegazzore, D.; Frache, A.; Abt, T.; Maspoch, M. L. Epoxy coupling agent for PLA and PHB copolymer-based cotton fabric biocomposites. Composites, Part B 2018, 148, 188−197. (6) Porras, A.; Maranon, A. Development and characterization of a laminate composite material from polylactic acid (PLA) and woven bamboo fabric. Composites, Part B 2012, 43 (7), 2782−2788. (7) Couture, A.; Lebrun, G.; Laperrière, L. Mechanical properties of polylactic acid (PLA) composites reinforced with unidirectional flax and flax-paper layers. Composite Structures 2016, 154, 286−295. (8) Sawpan, M. A.; Pickering, K. L.; Fernyhough, A. Improvement of mechanical performance of industrial hemp fibre reinforced polylactide biocomposites. Composites, Part A 2011, 42 (3), 310−319. (9) George, J.; Sreekala, M. S.; Thomas, S. A review on interface modification and characterization of natural fiber reinforced plastic composites. Polym. Eng. Sci. 2001, 41 (9), 1471−1485. (10) Gurunathan, T.; Mohanty, S.; Nayak, S. K. A review of the recent developments in biocomposites based on natural fibres and their application perspectives. Composites, Part A 2015, 77, 1−25. (11) Sawpan, M. A.; Pickering, K. L.; Fernyhough, A. Improvement of mechanical performance of industrial hemp fibre reinforced polylactide biocomposites. Composites, Part A 2011, 42 (3), 310−319. (12) Hu, R.; Lim, J.-K. Fabrication and mechanical properties of completely biodegradable hemp fiber reinforced polylactic acid composites. J. Compos. Mater. 2007, 41 (13), 1655−1669. (13) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd ed.; Wiley-VCH, 2012. (14) Kalia, S.; Kaith, B. S.; Kaur, I. Pretreatments of Natural Fibers and their Application as Reinforcing Material in Polymer CompositesA Review. Polym. Eng. Sci. 2009, 49 (7), 1253−1272. (15) Li, Y.-C.; Schulz, J.; Grunlan, J. C. Polyelectrolyte/nanosilicate thin-film assemblies: influence of pH on growth, mechanical behavior,

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CONCLUSIONS For the first time, biocomposites consisting of poly(3hydroxybutyrate-co-3-hydroxyhexanoate) matrix and Layerby-Layer functionalized hemp fabrics have been successfully prepared. Chitosan and sepiolite nanorods have been selected as biobased components for the assembly. The coating growth shows a linear behavior and it is capable of conformally cover each hemp fiber producing a nanotextrure where sepiolite is embedded in a chitosan matrix. During biocomposite preparation, the deposited chitosan/sepiolite structure results in a strong and dense interphase capable of efficiently controlling the stress transfer and thus positively affecting mechanical properties. Prepared composites achieve substantial improvements in both elastic modulus (+31%) and tensile strength (+24%); the achieved properties allow for the employment as load bearing structures in furniture applications. This is obtained with a simple and green approach to nanotechnology that exploits biobased components using water as solvent and room deposition conditions. The reported impressive results coupled with the ability to finely nanostructure the surface employing biobased components make the LbL the perfect tool to design and construct high performing and multifunctional interphases in biocomposites. This could potentially extend the application fields of biopolymers and allow for the replacement of fossil-based conventional composites. Finally, the reported approach makes it possible to develop novel materials design principles for new biocomposites where sustainability and mechanical properties are optimized.

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fabrics, composite preparation, characterization techniques; compositions, thickness, density and porosity percentage, hemp fabric characterizations; infrared spectroscopy of sepiolite and chitosan; tension test data (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02907. Detailed materials and methods: Layer-by-Layer buildup on silicon wafers, Layer-by-Layer deposition on D

DOI: 10.1021/acssuschemeng.8b02907 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering and flammability. ACS Appl. Mater. Interfaces 2009, 1 (10), 2338− 2347. (16) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and stiff layered polymer nanocomposites. Science 2007, 318 (5847), 80−83. (17) Zhu, X.; Loh, X. J. Layer-by-layer assemblies for antibacterial applications. Biomater. Sci. 2015, 3 (12), 1505−1518. (18) Xu, G.; Liu, P.; Pranantyo, D.; Neoh, K.-G.; Kang, E.-T. Dextran-and Chitosan-Based Antifouling, Antimicrobial Adhesion, and Self-Polishing Multilayer Coatings from pH-Responsive LinkagesEnabled Layer-by-Layer Assembly. ACS Sustainable Chem. Eng. 2018, 6 (3), 3916−3926. (19) Guo, R.; Jiao, T.; Li, R.; Chen, Y.; Guo, W.; Zhang, L.; Zhou, J.; Zhang, Q.; Peng, Q. Sandwiched Fe3O4/Carboxylate Graphene Oxide Nanostructures Constructed by Layer-by-Layer Assembly for Highly Efficient and Magnetically Recyclable Dye Removal. ACS Sustainable Chem. Eng. 2018, 6 (1), 1279−1288. (20) Pan, Y.; Zhan, J.; Pan, H.; Wang, W.; Tang, G.; Song, L.; Hu, Y. Effect of fully biobased coatings constructed via layer-by-layer assembly of chitosan and lignosulfonate on the thermal, flame retardant, and mechanical properties of flexible polyurethane foam. ACS Sustainable Chem. Eng. 2016, 4 (3), 1431−1438. (21) Carosio, F.; Alongi, J. Ultra-Fast Layer-by-Layer Approach for Depositing Flame Retardant Coatings on Flexible PU Foams within Seconds. ACS Appl. Mater. Interfaces 2016, 8 (10), 6315−6319. (22) Carosio, F.; Colonna, S.; Fina, A.; Rydzek, G.; Hemmerle, J.; Jierry, L.; Schaaf, P.; Boulmedais, F. Efficient Gas and Water Vapor Barrier Properties of Thin Poly(lactic acid) Packaging Films: Functionalization with Moisture Resistant Nafion and Clay Multilayers. Chem. Mater. 2014, 26 (19), 5459−5466. (23) CSN EN 312. (24) Darder, M.; Aranda, P.; Ruiz-Hitzky, E. Bionanocomposites: A New Concept of Ecological, Bioinspired, and Functional Hybrid Materials. Adv. Mater. 2007, 19 (10), 1309−1319. (25) Darder, M.; López-Blanco, M.; Aranda, P.; Aznar, A. J.; Bravo, J.; Ruiz-Hitzky, E. Microfibrous Chitosan−Sepiolite Nanocomposites. Chem. Mater. 2006, 18 (6), 1602−1610.

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DOI: 10.1021/acssuschemeng.8b02907 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX