64 Chapter 4
The overall performance of the fibres is dependent on the complete hier-
archical structure, from the cellulose polymer chains to the ultimate bundles.
Nonetheless, it is the fibrous cellulose set within the hemicellulose matrix that is
the key structural component of the fibre composite. The functional properties
of cellulose can be best understood by reference to a simple model, that of the
‘fringed micelle’, which is often used to account for the mechanical behaviour
of thermoplastics. This model presents a picture of a semi-crystalline polymer
aggregate, in which nano-crystallites are embedded within an amorphous matrix
(Figure 6). At the molecular level, each polymer chain passes through several
ordered regions, which result from the close packing of neighbouring chains,
and through several amorphous regions, in which it adopts an unordered, ran-
dom coil arrangement. The strength of the material is dictated by the interlinked,
inextensible crystalline component, while any visco-elastic and plastic behav-
iour is determined by the amorphous regions. Linen fibres exhibit elastic recov-
ery if subjected to only a few percent elongation, but with increased loading the
chains in the amorphous zones will slip past each other effecting first recov-
erable and then irrecoverable creep. In the extreme, of course, the aggregate
will disrupt and the fibre will fracture.
3.2 The Mechanical Performance of the VictorySail
Linen is particularly suited to the production of sailcloth due to its good
mechanical properties, including a high strength and low extensibility; fur-
thermore, the strength of the fibres actually increases on wetting. Inevitably,
however, linen will gradually deteriorate through natural causes and its per-
formance will reduce.
The linen fibres of the Victorysailcloth and, in particular, the cellulose chains
are bound to have aged in the intervening 200 years. This could have pro-
gressed to the point at which the sail cannot even support its own weight. An
appreciation of the sail’s current condition is therefore crucial in deciding
how it should be handled and may even influence the choice of display. With
this in mind, we set about the mechanical testing of the cloth. However, since
Figure 6The fringed micelle model of polymer microstructure