Properties of hemp fibre polymer composites -An optimisation of ...

Teleman, 1997). The crystal structure of the formed cellulose is called cellulose Iβ
(Nishiyama et al., 2002; Sarko and Muggli, 1974). Cellulose Iβ has a monoclinic unit
cell with space group P21 and the cellulose chains arranged along the unique c-axis
shown in Figure 12b. Bacteria can also form cellulose Iα with a triclinic unit cell. By
treatment with NaOH, the more chemically stable cellulose II is formed by
rearrangement of the chains. Cellulose II is not formed in many plant fibres since the
structure is determined by the enzyme complex in the plant cell were cellulose synthesis
occurs. However, Cullulose II is formed in some algae and bacteria.
X-ray powder diffraction is a suitable method to study the cellulose crystallinity due to
the separate able diffraction from peaks caused by cellulose crystals and diffraction from
amorphous cellulose, hemicellulose, lignin, pectin and minerals (Paper III). The
crystallite diameter can also be estimated with X-ray diffraction based on the width of
peaks representing directions perpendicular to the fibre axis (Miller indices: 110, 1 1 0
and 200). The crystallite length can be estimated based on the width of peaks
representing directions parallel with the fibre axis (Miller index: 004).
Both the X-ray measurement geometries reflection mode and transmission mode were
investigated to get an accurate procedure for determination of cellulose crystallinity.
Reflection mode was found suitable since air scattering thereby can be neglected, which
in transmission mode contributed to the amorphous scattering.
The most common sample preparation procedure is plane loading by pressing on top of
the sample. This was not suitable for cellulose fibres due to underestimation of the
diffraction peaks representing the fibre direction, mentioned as preferred orientation.
Therefore side loading of samples was used, which gives more random fibre orientation
in the sample and reduced effect of preferred orientation (Figure 13).
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