Draw ratio fibre orientation

Drawing to high draw ratios Pre-oriented fibres can be drawn after a few weeks of storing at RT [201]... 

It has been shown that the anisotropy depends on the orientation of the diagonals of indentation relative to the axial direction 14). At least two well defined hardness values for draw ratios A. > 8 emerge. One value (maximum) can be derived from the indentation diagonal parallel to the fibre axis. The second one (minimum) is deduced from the diagonal perpendicular to it. The former value is, in fact, not a physical measure of hardness but responds to an instant elastic recovery of the fibrous network in the draw direction. The latter value defines the plastic component of the oriented material. 

A particularly interesting property of Durham polyacetylene is that it can be stretched to draw ratios of up to 20 during the transformation, to yield a polyacetylene sample with high levels of orientation. This effect was reported by Bott et al. 378) for thin films in the electron microscope and then by Leising et al. 379), who drew single fibres of polyacetylene to a highly oriented /rani-state with a density of 1.06 g cm-3. 

If the orientation process in semi-crystalline fibres is carried out well below the melting point (Tm), the thread does not become thinner gradually, but rather suddenly, over a short distance the neck. The so-called draw ratio (A) is the ratio of the length of the drawn to that of the undrawn filament it is about 4-5 for many polymers, but may be as high as 40 for linear polyolefins and as low as 2 in the case of regenerated cellulose. 

It is not excluded that this mechanism is observed during the formation of fibres from X-500 The authors of this work pointed out that when the fibre was heated to 250-300 °C, its spontaneous elongation took place. Note that to attain higher orientation of a polymer in a fibre, it is necessary not only to transfer it to the liquid crystalline state but also to orient the liquid crystalline domains formed along the axis of the fibre. This orientation of the domain in which the macromolecules have been already mutually ordered requires not too high a draw ratio (the theoretical value must be <2). Indeed, experiments have shown that at the draw ratio of 1.53 to 1,70 the modulus (E) and the tensile strength (a) of the fibre at thermal treatment increase, which can be seen from the table compiled according to the results of this work. 

Long substituents on PPV will allow dissolution of the polymer in common solvents and processing by the usual methods. MEHPPV (Figure 1.43) has been obtained by Motamedi el al. [287] and spun into fibres from THF solution at room temperature. The x-ray orientation is high (/> 0,92) for a moderate draw ratio... 

For semi-crystalline polymers, the average orientation function 2 for the crystal c axes can be calculated from X-ray diffraction measurements (Chapter 3). Figure 8.15 shows how 2 increases linearly with the draw ratio, for polypropylene fibres and films, while the spherulitic microstructure survives. At 2 = 0.9, where the spherulites are destroyed and replaced by a microfibrillar structure, there is an increase in the slope of the 2 versus true strain relationship. It is impossible to achieve perfect c axis orientation... 

Figure 15.6 shows how the crystal and amorphous orientation increases with the fibre draw ratio. A fibre draw ratio of 4.5 aligns the polymer crystal c axes almost perfectly with the fibre length axis (perfect alignment would be fc = 1.0). The pseudo-affine model (Section 3.4.10) for the crystalline phase predicts a rapid increase in orientation factor with draw ratio, while the affine rubber elasticity model for the amorphous phase predicts a less rapid increase. 

Fig. 3.10 A fibre pattern from oriented syndiotactic polypropylene, drawn to a draw ratio of about 5 at 109 °C. (Courtesy of Dr A. P. Unwin.)...

From a combination of X-ray difiraction, isochronous modulus and dynamic mechanical studies Wardand coworkers explained the anomalous behaviour of (0) at low draw ratio (see Fig. 7) in terms of shear parallel to oriented crystalline chains. ° Thus, at low draw ratio, the crystalline units become preferentially oriented with their c-axes conically disposed about the fibre axis, the half-angle of the cone being approxi-... 

For intermediate modulus (IM) fibres, the precursor fibre is provided with a higher draw ratio prior to carbonization. The higher molecular orientation is maintained during carbonization to give a higher modulus while retaining the failnie strain of Type A fibres. 

The extent of orientation of a fibre or a slit film is defined by the degree of cold drawing or of orientation X, which is the ratio of the length after cold drawing L to the original length Lq ... 

Confirmation of this idea comes from the observation that the natural draw ratio observed for melt-spun fibres is sensitive to the degree of molecular orientation introduced during the spinning process. It appears that the molecular network is formed as the polymer freezes from the melt, is subsequently stretched in the rubber-like state before the polymer cools below Tg and is eventually collected as a frozen stretched rubber. The amount of stretching in the threadline can be measured by shrinking these spun fibres back to a state of zero strain, i.e. isotropy. These results then can be combined with measurements of the natural draw ratio to give the limiting extensibility of the network [33]. 

A similar kind of orientation has been well known for many years in some naturally occurring fibrous proteins such as silks (Geddes et al. 1968). The perpendicular orientation of chain axes with respect to the fibre axis, described as cross-p, occurs at low draw ratio and has been explained by the fact that in these soft silks the small crystallites are elongated along the hydrogen bond directions, which run perpendicular to chain axes (Geddes et al. 1968). The cross-p orientation in iPP may be attributed to the simultaneous occurrence of two kinds of shp processes at low... 

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