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Cellulose nanofibril materials with controlled structure: the influence of colloidal interactions
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. (Fiberteknologi)
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Nanoparticles are very interesting components. Due to their very large specific surface area they possess properties in between molecules and macroscopic materials. In addition, a material built up of hierarchically assembled nanoparticles could obtain unique properties, not possessed by the nanoparticles themself.

A very interesting group of nanoparticles is the cellulose nanofibrils. The fibrils are found in various renewable resources such as wood, bacteria and tunicates. In this work fibrils extracted from wood is studied. In wood the fibrils are the smallest fibrous component with the approximate dimensions; 4 nm in width and length in the micrometer range, providing a high aspect ratio. In addition, they have a crystallinity above 60% and, hence, a high stiffness. These fibrils are hierarchically ordered in the wood fiber to give it its unique combination of flexibility and strength.

The properties of the fibrils make them very suitable to be used as reinforcement elements in composites and, due to their ability to closely pack, to make films with excellent gas barrier properties. The key aspect to design materials, efficiently utilizing the properties of the individual fibrils, is to control the arrangement of the fibrils in the final material. In order to do so, the interactions between fibrils have to be well characterized and controlled. In this thesis the interaction between fibrils in aqueous dispersions is studied, where the main interactions are attractive van der Waals forces and repulsive electrostatic forces. The electrostatic forces arise from carboxyl groups at the fibrils surface, which either are due to hemicelluloses at the fibrils surfaces or chemically introduced to the cellulose chain. This force is sensitive to the chemical environment. It decreases if the pH is reduced or if the salt concentration is increased. If it is strongly reduced the system aggregates. In dilute dispersions aggregation causes formation of multiple clusters, whereas in semi-dilute dispersions (above the overlap concentration) a volume filling network, i.e. a gel, is formed. The tendency of aggregation, i.e. the colloidal stability, can be predicted by using the DLVO theory. In this thesis DLVO predictions are compared to aggregation measurements conducted with dynamic light scattering. Good agreement between experiments and the designed theoretical model was found by including specific interactions between added counter-ions and the carboxyl groups of the fibrils in the model. Thus, the surface charge is both reduced by protonation and by specific interactions. This emphasizes a much larger effect of the counter-ions on the stability then generally thought. Hence, this work significantly improves the understanding of the interfibril interactions in aqueous media.

As mentioned above, the fibrils can be physically cross-linked to form a gel. The gelation is an instant process, occurring at pH or salt levels causing the interfibril repulsion to decrease close to zero. If a well dispersed stationary dispersion is gelled, the homogenous and random distribution of the fibrils is preserved in the gel. These gels can be used as templates to produce composites by allowing monomers or polymers to enter the network by diffusion. In an effort to mimic processes occurring in the tree, producing materials with fibrils aligned in a preferred direction, the ability to form gels with controlled fibril orientation were studied. Such networks were successfully produced by applying strain to the system prior or past gelation. Orientation prior gelation was obtained by subjecting the dispersion to elongational flow and freezing the orientation by “turning off” the electrostatic repulsion. Orienting the fibrils after gelation was achieved by applying shear strain. Due to the physical nature of the crosslinks, rotation in the fibril-fibril joints can occur, enabling the fibrils to align in the shear direction. This alignment significantly increased the stiffness of the gels in the shear direction.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , v, 24 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2011: v, 24
Keyword [en]
Colloidal stability, cellulose nanofibrils, nanofibers, nanostructured materials, biocomposite, DLVO, gel
National Category
Physical Chemistry
URN: urn:nbn:se:kth:diva-49159ISBN: 978-91-7501-183-7OAI: diva2:459359
2011-12-19, K1, KTH, Teknikringen 56, 100 44, Stockholm, 14:40 (Swedish)
Swedish Research Council, 63512
QC 20111205Available from: 2011-12-05 Created: 2011-11-25 Last updated: 2011-12-07Bibliographically approved
List of papers
1. Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions
Open this publication in new window or tab >>Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions
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2011 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 18, 11332-11338 p.Article in journal (Refereed) Published
Abstract [en]

Cellulose nanofibrils constitute an attractive raw material for carbon-neutral, biodegradable, nanostructured materials. Aqueous suspensions of these nanofibrils are stabilized by electrostatic repulsion arising from deprotonated carboxyl groups at the fibril surface. In the present work, a new model is developed for predicting colloidal stability by considering deprotonation and electrostatic screening. This model predicts the fibril-fibril interaction potential at a given pH in a given ionic strength environment. Experiments support the model predictions that aggregation is induced by decreasing the pH, thus reducing the surface charge, or by increasing the salt concentration. It is shown that the primary mechanism for aggregation upon the addition of salt is the surface charge reduction through specific interactions of counterions with the deprotonated carboxyl groups, and the screening effect of the salt is of secondary importance.

microfibrillated cellulose, polyelectrolyte, cylinders, surfaces, fibers
National Category
Materials Chemistry
urn:nbn:se:kth:diva-41786 (URN)10.1021/la201947x (DOI)000294790500010 ()2-s2.0-80052713583 (ScopusID)
Swedish Research Council

QC 20150630

Available from: 2011-10-03 Created: 2011-10-03 Last updated: 2015-06-30Bibliographically approved
2. Microstructure control of physically cross-linked nanocellulose gels for biocomposite templates
Open this publication in new window or tab >>Microstructure control of physically cross-linked nanocellulose gels for biocomposite templates
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry Materials Chemistry
urn:nbn:se:kth:diva-50392 (URN)
QC 20111205Available from: 2011-12-05 Created: 2011-12-05 Last updated: 2011-12-05Bibliographically approved

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