Orientation draw ratio effects

The factor having the strongest effect is the elongation imparted in the process of production stretching. Second, the overall orientation is affected by the stretching rate. For the same draw ratio, the overall orientation grows with an increase in the stretching rate. The effect of the draw ratio on the value of Hermans function of orientation is illustrated by the values of/o, established by the authors and depicted in Table 7. 

The fibrillar structure of crystalline polymers is determined by molecular characteristics, the initial morphology and orientation conditions. Recently, a complex investigation of the effect of molecular parameters (MW, MWD and degree of branching) and orientation parameters (temperature and draw ratio) on the morphology of PE and its thermomechanical behaviour has been reported 181 185). 

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. 

The effect of an orientation process on an isolated elliptic flaw is depicted in Figure 7A. Let < i and a2 be the permanent stretch (or draw) ratios (ratio of drawn to undrawn length) to which the master sheet is subjected in two orthogonal directions. If an elliptic flaw is originally at right angles to the ai direction (i.e.y p = ir/2) and a2 = 1, then R will increase with until a particular value of is reached at which R = 1. For larger values of i, R will then decrease but the major axis of the ellipse is now at ft = 0. The critical draw ratio at which the orientation P jumps from tt/2 to 0 and for which the ellipse is circular (R = 1) is described later. 

Figure 7. A. Schematic of an elliptic flaw initially at right angles to the draw direction. For a uniaxial draw, the draw ratio D = at. At a critical draw ratio D = I/R the ellipse becomes circular. For higher draw ratios (D > I/R), the ellipse forms with a 90° change in its orientation. B. Schematic of the effect of increasing draw ratio on an ellipse initially at some angle other than 90° to the draw direction. C. A general biaxial draw (al9 a2) acting on an ellipse at initial orientation pto the, major drawm direction (a. > as).

Physically, the flaws with R > 80° are more effectively blunted than are flaws at a lower angle to the draw direction. Above the critical draw ratio, they are oriented close to the draw direction (low R ) and account... 

Hot-Drawn ABS Polymer. Draw ratios were varied between 1.0 and 2.9. In order to determine the extent of orientation produced during drawing, specimens were sectioned and etched the shapes of the initially spherical rubber particles are a measure of orientation (8). In general, the effective draw ratios calculated from particle shapes were close to the actual draw ratios. 

To obtain an insight into the effect of the draw ratio on the molecular orientation of the Vectra phase, wide angle x-ray diffraction, WAXD, was utilized and the results for the Ultem/Vectra 70/30 sheet drawn in the first zone are shown in Figure 11. For the sample with the lowest draw ratio, a sharp Debye ring corresponding to a d-spacing of 0.46 nm is observed. This observation is in agreement with previous data on Vectra reported in the literature (10). As the draw ratio increases, the azimuthal dependence of the... 

As seen in Table 1, the decrease in permeability can be directly attributed to a dramatic reduction in the effective diffusion coefficient, while there is a much smaller effect on the apparent solubility. A similar dependence of the solubility and diffusion coefficients on the draw ratio has been observed in other uniaxially oriented polymers (35-37). Because the glass transition and density of the polystyrene samples were found independent of the draw ratio, they concluded that the reduction in diffusivity was due to anisotropic redistribution of the free volume during drawing. Using an expansion coefficient related to draw ratio, the polystyrene data were successfully correlated using the Cohen-Turnbull free volume theory. However, the situation was found to be more complex for PVC (i ) ... 

The effect of orientation on oxygen permeability of the medium and high barrier resins is seen to be dependent upon the morphological nature of the barrier resin prior to orientation. A plot of the oxygen transmission rates as a function of the overall draw ratio (figure 3) illustrates this clearly. While the semicrystalline polymers, VDC copolymer, and aromatic nylon MXD-6, show little change in the permeability with moderate amounts of orientation in the solid state, orientation of the amorphous polymers SELAR PA 3426 and XHTA-50A causes reduction in the permeability by 5-30% in both resins, depending upon the overall level of orientation. 

The most important parameter controlling the final properties is the draw ratio, X. There is a direct relationship between X and the mechanical properties, in particular the modulus. The maximum draw ratio is generally limited to about 5 for PET and most PA s, about 10 for PP whereas values of 15-20 are common for HDPE. Crystallinity also has a strong effect on final properties. Strain-induced crystallization develops during the orientation process as the molecular order progressively increases upon drawing. 

Blends of PEO/PMMA were oriented by solid-state coextrusion in a capillary rheometer [Kim and Porter, 1988]. Addition of PMMA to POM dramatically decreased its drawabUity from a draw ratio A, = 36 for pure PEO to A. =5 for a blend of PEO/PMMA = 40/60 wt%. Drawing has a strong effect on crystallinity, %, and melting point, T. In pure PEO, % increases from 83% at A, = 1 to X = 95% at = 36, while increased from 62.4 to 70°C, indicating an increase in crystal size and perfection. A different behavior was found in the blends. The crystallinity remained at = 70%, while T decreased not only with increasing PMMA content but also with X. This may be taken as an indication that in POM/ PMMA blends the latter polymer gets trapped between PEO lamellae and disrupts the crystalline morphology. 

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