Deformation and fracture mechanisms in nanocellulose reinforced composites
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School of Chemical Technology |
Doctoral thesis (article-based)
| Defence date: 2012-10-05
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Authors
Date
2012
Major/Subject
Mcode
Degree programme
Language
en
Pages
131
Series
Aalto University publication series DOCTORAL DISSERTATIONS, 109/2012
Abstract
Cellulose is the main constituent of plants. In the cell wall of plants, cellulose nanofibrils act as a reinforcing agent embedded in a matrix of hemicelluloses and lignin, forming a nanocomposite material. Manmade nanocellulose reinforced composites began to receive attention approximately two decades ago when isolation methods for nanocellulose were developed. However, studies on the deformation of these novel materials have been limited. The effect of the composites' preparation method on the mechanical properties was investigated and compared with theoretical models. Deformation mechanisms in composites reinforced with low weight fractions of different types of nanocellulose were investigated along with the effects of acetylation. Then the stress-transfer and micromechanics of composites reinforced with higher weight fractions of nanocellulose were studied using Raman spectroscopy. Finally, the effect of nanocellulose on thermomechanical properties of the composites and their behaviour in moist environment were addressed. The results show that the preparation method has an influence on the final mechanical properties of composites. Degassing of the nanocellulose/polymer mixture showed a positive effect on the Young's modulus and tensile strength at lower weight fractions of nanocellulose due to the lower viscosities of the mixtures. However, degassing had no effect on the density of the composites. Chemical modification significantly improved the dispersion of nanocellulose in non-polar media as Raman imaging revealed. In turn, the mechanical properties and deformation of the composites was different with respect to the degree of substitution. The toughening of poly(lactic) acid by the addition of low weight fractions of nanocellulose was attributed to extensive polymer crazing which was also dependent on the morphology and degree of substitution of the nanocellulose. Using Raman spectroscopy it was shown that the deformation micromechanics at high weight fractions of nanocellulose are network dominated. This leads to a stress transfer mechanisms similar to a composite within a composite, where composite strength is dependent on stress transfer within the dense network. The mechanical properties of the composites were improved as well as the glass transition temperature. The crystallization behaviour and, in turn, crystallinity of the composites was observed to be impeded at large weight fractions of nanocellulose. Furthermore, the composites had better mechanical properties in humid environments compared to the pure PLA matrix and the pure nanocellulose film. Thus embedding of hydrophilic fibrils in a hydrophobic matrix improves the performance of these materials in humid environments.Description
Supervising professor
Hughes, Mark, Professor, Aalto University, FinlandKeywords
nanocellulose, micromechanics, deformation, composites
Other note
Parts
- [Publication 1]: M. Bulota, A. S. Jääskeläinen, J. Paltakari, and M. Hughes. 2011. Properties of biocomposites: influence of preparation method, testing environment and a comparison with theoretical models. Journal of Materials Science, volume 46, number 10, pages 3387-3398. DOI 10.1007/s10853-010-5227-4.
- [Publication 2]: Mindaugas Bulota, Kätlin Kreitsmann, Mark Hughes, and Jouni Paltakari. 2012. Acetylated microfibrillated cellulose as a toughening agent in poly(lactic acid). Journal of Applied Polymer Science, volume 126, number S1, Special Issue: Polysaccharides, pages E448-E457. DOI 10.1002/app.36787.
- [Publication 3]: Mindaugas Bulota and Mark Hughes. 2012. Toughening mechanisms in poly(lactic) acid reinforced with TEMPO-oxidized cellulose. Journal of Materials Science, volume 47, number 14, pages 5517-5523. DOI 10.1007/s10853-012-6443-x.
- [Publication 4]: Mindaugas Bulota, Supachok Tanpichai, Mark Hughes, and Stephen J. Eichhorn. 2012. Micromechanics of TEMPO-oxidized fibrillated cellulose composites. ACS Applied Materials & Interfaces, volume 4, number 1, pages 331-337. DOI 10.1021/am201399q.
- [Publication 5]: Mindaugas Bulota, Arja-Helena Vesterinen, Mark Hughes, and Jukka Seppälä. Mechanical behaviour, structure and reinforcement processes of TEMPO-oxidized cellulose reinforced PLA. Submitted.