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Elasticity of Cellulose Nanofibril Materials
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The demand for renewable load-carrying materials is increasing with increasing environmental awareness. Alternative sources for materials manufacturing and design have to be investigated in order to replace the non-biodegradable materials. The work presented in this thesis investigates structure-property relations of such renewable materials based on cellulose nanofibrils. Cellulose is the most abundant polymer on earth and exists in both ordered and disordered phases, where the ordered crystalline cellulose shows excellent mechanical properties. The celluloses nanofibril is composed of partly crystalline cellulose where the stiff crystal regions, or crystallites, are orientated in the axial direction of the fibrils. The cellulose nanofibrils have a high aspect ratio, i.e. length to diameter ratio, with a diameter of less than 100 nm and a length of some micrometres. In the presented work, different properties of the cellulose nanofibril were studied, e.g. elastic properties, structure, and its potential as a reinforcement constituent. The properties and behaviour of the fibrils were studied with respect to different length scales, from the internal structure of the cellulose nanofibril, based on molecular dynamic simulations, to the macroscopic properties of cellulose nanofibril based materials. Films and composite materials with in-plane randomly oriented fibrils were produced. Properties of the cellulose nanofibril based materials, such as stiffness, thickness variation, and fibril orientation distribution, were investigated, from which the effective elastic properties of the fibrils were determined. The studies showed that a typical softwood based cellulose nanofibril has an axial stiffness of around 65 GPa. The properties of the cellulose nanofibril based materials are highly affected by the dispersion and orientation of the fibrils. To use the full potential of the stiff fibrils, well dispersed and oriented fibrils are essential. The orientation distribution of fibrils in hydrogels subjected to a strain was therefore investigated. The study showed that the cellulose nanofibrils have high ability to align, where the alignment increased with increased applied strain.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 60 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1215
Keyword [en]
Cellulose nanofibrils, Elastic properties, Micromechanics, Composites
National Category
Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
URN: urn:nbn:se:uu:diva-240250ISBN: 978-91-554-9135-2 (print)OAI: oai:DiVA.org:uu-240250DiVA: diva2:776120
Public defence
2015-02-13, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2015-01-22 Created: 2015-01-06 Last updated: 2015-03-09
List of papers
1. Prediction of elastic properties of nanofibrillated cellulose from micromechanical modeling and nano-structure characterization by transmission electron microscopy
Open this publication in new window or tab >>Prediction of elastic properties of nanofibrillated cellulose from micromechanical modeling and nano-structure characterization by transmission electron microscopy
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2013 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 20, no 2, 761-770 p.Article in journal (Refereed) Published
Abstract [en]

Cellulose-based materials have a great potential in terms of mechanical performance, since crystalline cellulose is known to have excellent stiffness along the main axis. This potential is not completely fulfilled in structural wood materials and in composite materials, due to structural inhomogeneities, misalignment, voids etc. on several length scales. This study investigates the difference in stiffness of nanofibrillated cellulose (NFC) compared to that of cellulose crystallites, based on nanostructural characterization, image analysis and micromechanical modeling. Nanofibrillated cellulose is believed to be composed of a distribution of crystallites in an amorphous matrix, and it is assumed to represent the distribution of the crystalline allomorph I-beta. To predict the elastic properties of NFC, a micromechanical model based on a Mori-Tanaka approach and self-consistent scheme was used. The input data, i.e. orientation distribution, aspect ratio and volume fraction of these crystalline regions, were estimated from image analysis of transmission electron micrographs. The model predicts a ca. 56 % loss of stiffness of NFC compared to that of cellulose crystals along the main axis.

Keyword
Nanofibrillated cellulose, Elastic properties, Micromechanics, Modelling, Transmission electron microscopy
National Category
Natural Sciences Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-197625 (URN)10.1007/s10570-013-9868-8 (DOI)000315480400017 ()
Available from: 2013-04-03 Created: 2013-04-02 Last updated: 2017-12-06Bibliographically approved
2. Stiffness contribution of cellulose nanofibrils to composite materials
Open this publication in new window or tab >>Stiffness contribution of cellulose nanofibrils to composite materials
2014 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, no 5, 945-953 p.Article in journal (Refereed) Published
Abstract [en]

Nanocomposites, reinforced by different types of cellulose fibrils, have gained increased interest the last years due to the promising mechanical properties. There is a lack of knowledge about the mechanical properties of the cellulose fibrils, and their contribution to the often claimed potential of the impressive mechanical performance of the nanocomposites. This paper investigates the contribution from different types of cellulose nanofibril to the overall elastic properties of composites. A multiscale model is proposed, that allows back-calculation of the elastic properties of the fibril from the macroscopic elastic properties of the composites. The different types of fibrils used were nanofibrillated cellulose from wood, bacterial cellulose nano-whiskers and microcrystalline cellulose. Based on the overall properties of the composite with an unaged polylactide matrix, the effective longitudinal Young's modulus of the fibrils was estimated to 65 GPa for the nanofibrillated cellulose, 61 GPa for the nano whiskers and only 38 GPa for the microcrystalline cellulose. The ranking and absolute values are in accordance with other studies on nanoscale morphology and stiffness estimates. Electron microscopy revealed that in the melt-processed cellulose nanofibril reinforced thermoplastics, the fibrils tended to agglomerate and form micrometer scale platelets, effectively forming a microcomposite and not a nanocomposite. This dispersion effect has to be addressed when developing models describing the structure-property relations for cellulose nanofibril composites.

Keyword
Nanocomposite, Cellulose nanofibrils, Elastic properties, Multiscale modeling, Inverse modeling
National Category
Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-220777 (URN)10.1016/j.ijsolstr.2013.11.018 (DOI)000331158400004 ()
Available from: 2014-03-24 Created: 2014-03-20 Last updated: 2017-12-05Bibliographically approved
3. Elastic models coupling the cellulose nanofibril to the macroscopic film level
Open this publication in new window or tab >>Elastic models coupling the cellulose nanofibril to the macroscopic film level
2015 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, no 71, 58091-58099 p.Article in journal (Refereed) Published
Abstract [en]

The mechanical behaviour of cellulose nanofibrils is typically characterized by casting thin films and performing tensile tests on strips cut from these films. When comparing the stiffness of different films, the stiffness of the nanofibrils is only qualitatively and indirectly compared. This study provides some schemes based on various models of fibre networks, or laminated films, which can be used to assess the inherent stiffness of the nanofibrils from the stiffness of the films. Films of cellulose nanofibrils from different raw materials were manufactured and the elastic properties were measured. The expressions relating the nanofibril stiffness and the film stiffness were compared for the presented models. A model based on classical laminate theory showed the best balance between simplicity and adequacy of the underlying assumptions among the presented models. Using this model, the contributing nanofibril stiffness was found to range from 20 to 27 GPa. The nanofibril stiffness was also calculated from mechanical properties of nanofibril films found in the literature and compared with measurements from independent test methods of nanofibril stiffness. All stiffness values were found to be comparable and within the same order of magnitude.

National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-240247 (URN)10.1039/c5ra04016g (DOI)000357805500100 ()
Available from: 2015-01-06 Created: 2015-01-06 Last updated: 2017-12-05Bibliographically approved
4. Nanorobotic Testing to Assess the Stiffness Properties of Nanopaper
Open this publication in new window or tab >>Nanorobotic Testing to Assess the Stiffness Properties of Nanopaper
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2014 (English)In: IEEE Transactions on robotics, ISSN 1552-3098, E-ISSN 1941-0468, Vol. 30, no 1, 115-119 p.Article in journal (Refereed) Published
Abstract [en]

This paper deals with the nanorobotic and nondestructive assessment of the stiffness properties of nanopaper made of microfibrillated cellulose. Back-calculations of the Young's modulus show the agreement of the newly found results with conventional tensile testing results, therewith proving nanorobotics as a reasonable complement for conventional testing.

Keyword
Automatic testing, bending testing, elastic properties, materials testing, mechanical properties, mechanical testing, microfibrillated cellulose, nanorobotics, scanning electron microscopy (SEM), tensile testing
National Category
Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-221005 (URN)10.1109/TRO.2013.2283409 (DOI)000331301900013 ()
Available from: 2014-03-25 Created: 2014-03-24 Last updated: 2017-12-05Bibliographically approved
5. Thickness variability of cellulose nanofibril films: Measurement and implications for mechanical characterization
Open this publication in new window or tab >>Thickness variability of cellulose nanofibril films: Measurement and implications for mechanical characterization
(English)Article in journal (Other academic) Submitted
Abstract [en]

To investigate the properties of different types of cellulose nanofibrils (CNFs), films are often produced. CNF films are easy to manufacture and relatively straightforward to characterize. Accurate measurements of the tensile properties of the films are often performed from which the mechanical properties of the films are determined. However, accurate measurment of the film thickness is often neglected which is an important property when defining the elastic modulus and strength of a CNF film. Many papers dealing with CNF films measure the film thickness with a micrometer screw gauge. The thicknesses that are measured in such way are the maximum thicknesses resulting in a significant error when determine mechanical properties of films with a high variability of local thickness, such as unsmooth CNF films. In this paper, the statistical distribution of CNF film thickness has been investigated by scanning electron microscopy of film cross-sections. Scale factors are proposed to relate the average thickness to the maximum thickness over a cross-section segment for films of different CNFs. Such scale factors are applicable to estimate the strength and stiffness, which are usally underestimated when using thickness measurements undertaken with a micrometer screw gauge. The maximum thickness, measured with a micrometer screw gauge, was as much as 95% higher than the average thickness. This effect is significant for films that show uneven surfaces resulting from inefficient CNF fibrillation. For chemical pretreatment and several passes through the homogenizer in the CNF manufacturing process, the effect is considerably smaller, but not negligible in characterization of mechanical properties. For further investigation of the thickness distribution, various three-parametric distributions with a lower bound and an upper tail all gave a fitting approximation to the experimental data.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-240248 (URN)
Available from: 2015-01-06 Created: 2015-01-06 Last updated: 2015-03-09
6. Fibril orientation redistribution induced by stretching of cellulose nanofibril hydrogels
Open this publication in new window or tab >>Fibril orientation redistribution induced by stretching of cellulose nanofibril hydrogels
2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 21, 214311Article in journal (Refereed) Published
Abstract [en]

The mechanical performance of materials reinforced by cellulose nanofibrils is highly affected by the orientation of these fibrils. This paper investigates the nanofibril orientation distribution of films of partly oriented cellulose nanofibrils. Stripes of hydrogel films were subjected to different amount of strain and, after drying, examined with X-ray diffraction to obtain the orientation of the nanofibrils in the films, caused by the stretching. The cellulose nanofibrils had initially a random in-plane orientation in the hydrogel films and the strain was applied to the films before the nanofibrils bond tightly together, which occurs during drying. The stretching resulted in a reorientation of the nanofibrils in the films, with monotonically increasing orientation towards the load direction with increasing strain. Estimation of nanofibril reorientation by X-ray diffraction enables quantitative comparison of the stretch-induced orientation ability of different cellulose nanofibril systems. The reorientation of nanofibrils as a consequence of an applied strain is also predicted by a geometrical model of deformation of nanofibril hydrogels. Conversely, in high-strain cold-drawing of wet cellulose nanofibril materials, the enhanced orientation is promoted by slipping of the effectively stiff fibrils.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-240249 (URN)10.1063/1.4922038 (DOI)000355925600038 ()
Available from: 2015-01-06 Created: 2015-01-06 Last updated: 2017-12-05Bibliographically approved

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