Rotational Diffusion of Cellulose Nanofibrils.
Cellulose nanofibrils, CNF, is a technical term for the smallest crystalline constituents
of wood. Namely particles of cylindrical shape measuring 20-40 nm in
diameter and 1-2 um in length. These fibrils display high ultimate tensile strength
and a Young's modulus comparable to Kevlar. The intermutual orientation of fibrils
within a wood fibre decides the strength of the fibre; fibrils pointing in all directions
yield weak fibres and brils pointing in the direction defined by the length of the
fibre yield strong fibres.
A flow channel has been conceived and made, such that an acceleration of a CNF
dispersion align the fibrils it contains. An ensuing fixation of the fibrils, all pointing
in the same direction, allows for the making of fibres of exceptional strength. In
other words, wood is taken apart and rebuilt even stronger. The material, if large
scale production is made possible, could be used for high-performance biomaterials
such as durable textiles.
During the time between alignment and fixation in the channel, a certain increase
in disorder occurs whereby the fibrils dealign. Our thesis focuses on the study of this
process. Since the fibrils are not visible, we have resorted to studying the birefringence,
which is induced by the optical anisotropy of aligned fibrils. The birefringence
has the property of changing polarization of light having travelled through the material.
This property is used to relate a measured intensity to birefringence which in
turn is related to alignment. Using this, we examine how the dealignment is affected
by parameters such as pH of the dispersion and acceleration.
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