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

By TEM microscopy, the single fiber middle lamellae (ML) and primary wall (P) were
found to have thickness of 30-50 nm and 70-110 nm, respectively (Figure 3A, B). The
secondary cell wall was composed of a 100-130 nm thick S1 layer and a 3-13 μm thick
S2 layer. Both SEM and TEM observations showed that the S2 layer had a laminate
structure of 1 to 4 major concentric layers of 1-5 μm in thickness. The major layers were
constructed of 100 nm thick lamellae (Figure 1C-D, Figure 3C). Thin layers of 200-240
nm in thickness were located between the major concentric layers (Figure 3A).
Confirmation for a concentric orientation of the layers was further obtained by SEM
observations on the ends of P. radiata Cel 26 treated fibers fractured under tension
(Figure 4). The major layers separated from each other, further indicating that they were
weakly attached to one another.
Microfibril angles of the single fibers
Some single fibers in the H2O2 delignified hemp showed evidence of cavities made by
naturally occurring fungi before harvest. It was possible to measure the microfibril angle
in two adjacent cell wall layers since several parallel cavities crossed each other (Figure
5A). The microfibrils had Z helical orientation with angles in the intervals 0-5° and 25-
30°. Superficial attack of other single fibers showed evidence of MFAs in the range of
70-90° with an S helical orientation (Figure 5B), which was confirmed by polarized light
microscopy (data not shown). Single fibers from H2O2 delignified and P. radiata Cel 26
treated hemp stems showed regions where the S1 layer was partly peeled off (Figure
5C). The cellulose aggregates in the S1 and S2 layer beneath were orientated
perpendicular and parallel to the fiber axis, respectively. The fracture experiments
(Figure 4) showed evidence for a main S2 MFA of 0-10° agreeing with 4° measured
previously by X-ray diffraction (Fink et al., 1999). The microfibril model of the single
fibers shown in Figure 2B is based on the obtained data.
Figure 4. Fractured end of a single fiber showed at increasing magnification. A: Splits
developed by radial tension between the major concentric layers due to the weak
interface. B: Cellulose aggregates (arrows) apparent at site of fracture.
Risø-PhD-11 97