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Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Biofibre Materials Centre, BiMaC.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
Department of Forest Products Technology, Aalto, Finland.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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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.

Place, publisher, year, edition, pages
2011. Vol. 27, no 18, 11332-11338 p.
Keyword [en]
microfibrillated cellulose, polyelectrolyte, cylinders, surfaces, fibers
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-41786DOI: 10.1021/la201947xISI: 000294790500010Scopus ID: 2-s2.0-80052713583OAI: oai:DiVA.org:kth-41786DiVA: diva2:445267
Funder
Swedish Research Council
Note

QC 20150630

Available from: 2011-10-03 Created: 2011-10-03 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Cellulose nanofibril materials with controlled structure: the influence of colloidal interactions
Open this publication in new window or tab >>Cellulose nanofibril materials with controlled structure: the influence of colloidal interactions
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.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011: v, 24
Keyword
Colloidal stability, cellulose nanofibrils, nanofibers, nanostructured materials, biocomposite, DLVO, gel
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-49159 (URN)978-91-7501-183-7 (ISBN)
Presentation
2011-12-19, K1, KTH, Teknikringen 56, 100 44, Stockholm, 14:40 (Swedish)
Opponent
Supervisors
Funder
Swedish Research Council, 63512
Note
QC 20111205Available from: 2011-12-05 Created: 2011-11-25 Last updated: 2011-12-07Bibliographically approved
2. Colloidal interactions and orientation of nanocellulose particles
Open this publication in new window or tab >>Colloidal interactions and orientation of nanocellulose particles
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanoparticles are very interesting building blocks. Their large surface-to-bulk ratio gives them different properties from those of larger particles. Controlling their assembly can greatly affect macroscopic material properties. This often happens in nature, resulting in macroscopic materials with properties far better than those of similar human-made materials. However, in this fast-growing research field, we may soon compete with nature in certain areas. This thesis demonstrates that the distribution and orientation of nanocellulose particles can be controlled, which is crucial for many applications.

Nanocellulose is an interesting nanoparticle, for example, because of its high strength, low thermal expansion, and high crystallinity. Nanocellulose particles are called nanofibrillated cellulose (NFC) or cellulose nanocrystals (CNCs). NFC is obtained from wood by mechanically shearing apart fibrils from the fiber wall and to obtain CNCs, parts of the cellulose are broken down by hydrolytic acidic reactions, most commonly, prior to homogenization. NFC particles are longer and less crystalline than are CNCs, but both are similar in width. The particles attract each other in aqueous dispersions and have a high aspect ratio and, thus, a large tendency to aggregate. The rate at which this occurs is typically reduced by charging the particles, generating an electrostatic repulsion between them.

To fully utilize the many interesting properties of nanocellulose, the aggregation and orientation of the particles have to be controlled; examining this delicate task is the objective of this thesis. The limits for particle stability and aggregation are examined in papers 2–3 (as well as in this thesis) and orientation of the particles is investigated in papers 3–5. In addition, the liberation of the nanoparticles from different types of wood fibers is studied in papers 1 and 2.

It was found that the liberation yield improved with increased fiber charge. In addition, the charge of the fibrils is higher than the charge of the original fibers, indicating that the fibrils were liberated from highly charged parts of the fibers and that the low-charge fraction was removed during processing.

Aggregation was both theoretically predicted and experimentally studied. A theoretical model was formulated based on Derjaguin–Landau–Verwey–Overbeek theory, which is intended to predict the influence of salt, pH, and particle charge on the colloidal stability of the NFC. To predict the experimental trends, specific interactions between salt counterions and the particles charges had to be included in the model, which greatly increased the effect of salt on the NFC stability. Below the particle overlap concentration, instability induced by pH or salt created small sedimenting flocs, whereas above the overlap concentration the system gelled. Increasing the particle concentration further also gels the system.

Orientation of nanocellulose was first achieved by shearing, salt- or acid-induced NFC gels. This oriented the fibrils and increased the gel modulus in the direction of shear. The orientation persisted after the shear strain was released and did not cause breakdown of the macroscopic gel. The orientation is probably due to rotation in the interfibril crosslinks, which is possible because the crosslinks are physical, not covalent.

     Second, orientation was also induced by elongational flow. Shear and acceleration forces were combined to align fibrils in the direction of the flow. The orientation was then frozen by gelation (adding salt or reducing the pH). Drying the gel threads created filaments of aligned fibrils with a higher specific strength than that of steel.

     Finally, CNC particles could be aligned on flat surfaces. The particles were first forced to align due to geometrical constraints in grooves on a nanowrinkled surface. The CNCs were then transferred to a flat surface using a contact-printing process. This created surfaces with lines of highly aligned CNCs, where the line–line spacing was controlled with nanometer precision.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vi, 49 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:47
Keyword
NFC, CNC, CNF, MFC nanocellulose, colloidal stability, orientation
National Category
Chemical Sciences Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-133941 (URN)978-91-7501-912-3 (ISBN)
Public defence
2013-12-06, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20131114

Available from: 2013-11-14 Created: 2013-11-13 Last updated: 2013-11-14Bibliographically approved

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