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Cellulose-based Nanocomposites – The Relationship between Structure and Properties
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-1776-2725
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanocellulose materials extracted from various types of biomass have recently attracted significant attention. Due to their remarkable mechanical properties, large surface area and biodegradability, they can be promising reinforcements in nanocomposites. Cellulose-based nanocomposites constitutive of nanocellulose reinforcements and biodegradable polymer matrices have great potential to be used in environmentally friendly applications to replace fossil-based materials. However, the challenge of controlling their nanoscale structure, especially achieving good dispersion of nanocellulose in hydrophobic polymer matrices, still poses significant obstacles to producing high-performance nanocomposites. Therefore, this thesis reports several methods for structural modification of cellulose-based nanocomposites toward the objectives of improving the dispersion of nanocellulose and enhancing the properties of the nanocomposites. The methods include in situ emulsion polymerization in the presence of nanocellulose, crosslinking of polymer matrix, grafting of polymer brushes to nanocellulose and drawing of nanocomposites to obtain aligned structures. The resulting mechanical, thermal and other related properties are investigated, and the relationship between structure and properties of the nanocomposites are discussed.

To address the challenge of achieving good dispersion of nanocellulose in hydrophobic matrices, in situ emulsion polymerization of vinyl acetate monomer in the presence of cellulose nanocrystals has been developed. Microscopy results show that the in situ method improves the compatibility between nanocellulose and hydrophobic polymers, which consequently improves the dispersion of nanocellulose in the nanocomposites. Compared with direct mixed polymer/nanocellulose composites, the in situ synthesized nanocomposites exhibit higher stiffness and strength arising from their superior interphase volume, which is confirmed theoretically and experimentally. Crosslinking of partially hydrolyzed poly(vinyl acetate) by borate additives under different pH conditions has been studied to further enhance mechanical properties of the nanocomposites. Moreover, the “grafting to” modification method also helps to overcome this challenge. It is revealed that poly(ethylene glycol)-grafted cellulose nanofibers disperse better in poly(lactic acid) matrix than unmodified cellulose nanofibers, which is attributed to the improved compatibility and steric effect provided by the covalently grafted poly(ethylene glycol) brushes.

To substantially enhance the unidirectional mechanical properties of cellulose-based nanocomposites, a highly aligned structure in the materials is obtained through the drawing process. Drawing conditions including temperature, speed and draw ratio show considerable effects on the mechanical and thermal properties of the nanocomposites. Furthermore, the aligned nanocomposites consisting of poly(lactic acid) matrix and ultra-low weight fraction of poly(ethylene glycol)-grafted cellulose nanofibers demonstrate competitive strength, superb toughness and interesting optical behaviors compared with other aligned nanocellulose-based materials reported in the literature, indicating their potential to be further developed for large-scale environmentally friendly applications.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Nanocellulose, Nanocomposite, Dispersion, Poly(vinyl acetate), Poly(lactic acid), Alignment, Mechanical characteristics
National Category
Composite Science and Engineering Nano Technology Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
URN: urn:nbn:se:ltu:diva-70411ISBN: 978-91-7790-182-2 (print)ISBN: 978-91-7790-183-9 (electronic)OAI: oai:DiVA.org:ltu-70411DiVA, id: diva2:1239016
Public defence
2018-09-19, E632, Luleå, 10:00 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg FoundationBio4EnergyAvailable from: 2018-08-16 Created: 2018-08-15 Last updated: 2019-09-11Bibliographically approved
List of papers
1. Crosslinked polyvinyl acetate (PVAc) reinforced with cellulose nanocrystals (CNC) – structure and mechanical properties
Open this publication in new window or tab >>Crosslinked polyvinyl acetate (PVAc) reinforced with cellulose nanocrystals (CNC) – structure and mechanical properties
2016 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 126, p. 35-42Article in journal (Refereed) Published
Abstract [en]

The structure of cellulose-based nanocomposites significantly influences their final mechanical properties. However, obtaining a good dispersion of hydrophilic nanocellulose materials in a hydrophobic polymer matrix is challenging. In this study, two unique methods were developed to improve the dispersion of cellulose nanocrystals (CNC) in a poly(vinyl acetate) (PVAc) matrix. One method was the crosslinking of PVAc by sodium tetraborate (borax), which is expected to prevent agglomeration of CNCs during the drying process, and the other method was the in-situ polymerization of vinyl acetate in the presence of CNCs to generate good compatibility between CNC and PVAc. The results showed that the crosslinking degree of PVAc could be varied by tuning the pH. The atomic force microscopy images illustrate that after drying, the in-situ polymerized PVAc/CNC composite was much better dispersed than the composite produced using mechanical mixing. The mechanical and thermo-mechanical characterizations indicate that the in-situ nanocomposite with 10 wt% of CNC had a higher strength and storage modulus compared with the mixed composite with the same CNC concentration. Further investigations of the restriction effect caused by the crosslinker are required.

National Category
Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
urn:nbn:se:ltu:diva-5856 (URN)10.1016/j.compscitech.2016.02.013 (DOI)000372682000005 ()2-s2.0-84957917026 (Scopus ID)40c8220b-8531-4b49-a0be-236507082a62 (Local ID)40c8220b-8531-4b49-a0be-236507082a62 (Archive number)40c8220b-8531-4b49-a0be-236507082a62 (OAI)
Note
Validerad; 2016; Nivå 2; 20160211 (krioks)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-08-15Bibliographically approved
2. Plasticizing and crosslinking effects of borate additives on the structure and properties of poly(vinyl acetate)
Open this publication in new window or tab >>Plasticizing and crosslinking effects of borate additives on the structure and properties of poly(vinyl acetate)
Show others...
2017 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, no 13, p. 7483-7491Article in journal (Refereed) Published
Abstract [en]

As an environmentally friendly, low-cost and widely used polymer, poly(vinyl acetate) (PVAc) is worth modifying to achieve better properties. Here, we report on the influence of borate additives on the structure and properties of partially hydrolysed PVAc. In addition to the general crosslinking function of borate additives, an extraordinary plasticizing effect was found. By controlling the pH from 4 to 11 during sample preparation, the plasticizing and crosslinking effects can be shifted. In alkaline conditions, the degree of crosslinking in the PVAc/borate sample is increased; however, this increase declines gradually with an increase in the borate additive content, which impacts the morphology of the PVAc latex particles, as well as the mechanical and thermal properties of the PVAc/borate films. In contrast, in acidic conditions, the PVAc/borate films are plasticized by borate additives; thus, their ultimate mechanical strength, elastic moduli and thermal stabilities decrease, while the water diffusivities increase.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2017
National Category
Physical Chemistry Bio Materials
Research subject
Wood and Bionanocomposites; Chemistry of Interfaces
Identifiers
urn:nbn:se:ltu:diva-61616 (URN)10.1039/C6RA28574K (DOI)000393757100006 ()2-s2.0-85010648470 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-02-13 (andbra)

Available from: 2017-01-24 Created: 2017-01-24 Last updated: 2018-11-16Bibliographically approved
3. Well-dispersed cellulose nanocrystals in hydrophobic polymers by in situ polymerization for synthesizing highly reinforced bio-nanocomposites
Open this publication in new window or tab >>Well-dispersed cellulose nanocrystals in hydrophobic polymers by in situ polymerization for synthesizing highly reinforced bio-nanocomposites
Show others...
2018 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 10, no 25, p. 11797-11807Article in journal (Refereed) Published
Abstract [en]

In nanocomposites, dispersing hydrophilic nanomaterials in a hydrophobic matrix using simple and environmentally friendly methods remains challenging. Herein, we report a method based on in situ polymerization to synthesize nanocomposites of well-dispersed cellulose nanocrystals (CNCs) and poly(vinyl acetate) (PVAc). We have also shown that by blending this PVAc/CNC nanocomposite with poly(lactic acid) (PLA), a good dispersion of the CNCs can be reached in PLA. The outstanding dispersion of CNCs in both PVAc and PLA/PVAc matrices was shown by different microscopy techniques and was further supported by the mechanical and rheological properties of the composites. The in situ PVAc/CNC nanocomposites exhibit enhanced mechanical properties compared to the materials produced by mechanical mixing, and a theoretical model based on the interphase effect and dispersion that reflects this behavior was developed. Comparison of the rheological and thermal behaviors of the mixed and in situ PVAc/CNC also confirmed the great improvement in the dispersion of nanocellulose in the latter. Furthermore, a synergistic effect was observed with only 0.1 wt% CNCs when the in situ PVAc/CNC was blended with PLA, as demonstrated by significant increases in elastic modulus, yield strength, elongation to break and glass transition temperature compared to the PLA/PVAc only material.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2018
National Category
Composite Science and Engineering Biomaterials Science Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
urn:nbn:se:ltu:diva-68465 (URN)10.1039/C7NR09080C (DOI)000437761500010 ()29675528 (PubMedID)2-s2.0-85049520332 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-08-07 (rokbeg)

Available from: 2018-04-23 Created: 2018-04-23 Last updated: 2018-08-15Bibliographically approved
4. Aligned plasticized polylactic acid cellulose nanocomposite tapes: Effect of drawing conditions
Open this publication in new window or tab >>Aligned plasticized polylactic acid cellulose nanocomposite tapes: Effect of drawing conditions
2018 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 104, p. 101-107Article in journal (Refereed) Published
Abstract [en]

Aligned nanocomposite tapes based on plasticized polylactic acid (PLA) and 1 wt.% cellulose nanofibers (CNF) were prepared using uniaxial solid-state drawing, and the effects of drawing conditions including temperature, speed and draw ratio on the material were studied. Microscopy studies confirmed alignment and the formation of ‘shish-kebab’ morphology in the drawn tape. Mechanical properties demonstrate that the solid-state drawing is a very effective way to produce stronger and tougher PLA nanocomposites, and the toughness can be improved 60 times compared to the undrawn tape. Additionally, the thermal properties, i.e. storage modulus, glass transition temperature and degree of crystallinity were improved. These improvements are expected due to the synergistic effect of CNF in the nanocomposite and orientations induced by the solid-state drawing.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Nanocomposites, Mechanical properties, Thermal properties, Microstructural analysis
National Category
Composite Science and Engineering Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
urn:nbn:se:ltu:diva-64862 (URN)10.1016/j.compositesa.2017.10.019 (DOI)000418966900010 ()2-s2.0-85032722515 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-11-14 (andbra)

Available from: 2017-07-13 Created: 2017-07-13 Last updated: 2019-09-13Bibliographically approved
5. High-strength, High-toughness Aligned Polymer-based Nanocomposite Reinforced with Ultra-low Weight Fraction of Functionalized Nanocellulose
Open this publication in new window or tab >>High-strength, High-toughness Aligned Polymer-based Nanocomposite Reinforced with Ultra-low Weight Fraction of Functionalized Nanocellulose
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Multifunctional lightweight, flexible, yet strong polymer-based nanocomposites are highly desired for specific applications. However, the control of orientation and dispersion of reinforcing nanoparticles and the optimization of the interfacial interaction still pose substantial challenges in nanocellulose-reinforced polymer composites. In this study, poly(ethylene glycol)-grafted nanocellulose fibers (TOCNF-g-PEG) has demonstrated much better dispersion in a poly(lactic acid) (PLA) matrix as compared to unmodified nanocellulose fibers. Through a uniaxial drawing method, aligned PLA/nanocellulose nanocomposites with high strength, high toughness, and unique optical behavior are obtained. With the incorporation of only 0.1 wt% of TOCNF-g-PEG in PLA, the ultimate strength of the nanocomposite reaches 343 MPa, which is significantly higher than that of other aligned PLA-based nanocomposites reported previously. Compared with the aligned nanocomposite reinforced with unmodified nanocellulose, the ultimate strength and toughness are enhanced by 39% and 70%, respectively. Moreover, the aligned nanocomposite film is highly transparent and possesses an anisotropic light scattering effect, revealing its significant potential for optical applications.

Keywords
nanocellulose, nanocomposite, alignment, mechanical characteristics, light scattering
National Category
Composite Science and Engineering Nano Technology Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
urn:nbn:se:ltu:diva-70439 (URN)
Funder
Knut and Alice Wallenberg FoundationBio4Energy
Available from: 2018-08-16 Created: 2018-08-16 Last updated: 2018-08-16

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Citation style
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Output format
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