Draw ratio value

The mechanical properties can be studied by stretching a polymer specimen at constant rate and monitoring the stress produced. The Young (elastic) modulus is determined from the initial linear portion of the stress-strain curve, and other mechanical parameters of interest include the yield and break stresses and the corresponding strain (draw ratio) values. Some of these parameters will be reported in the following paragraphs, referred to as results on thermotropic polybibenzoates with different spacers. The stress-strain plots were obtained at various drawing temperatures and rates. 

Isothermal draw resonance is found to be independent of the flow rate. It occurs at a critical value of draw ratio (i.e., the ratio of the strand speed at the take-up rolls to that at the spinneret exit). For fluids that are almost Newtonian, such as polyethylene terephthalate (PET) and polysiloxane, the critical draw ratio is about 20. For polymer melts such as HDPE, polyethylene low density (LDPE), polystyrene (PS), and PP, which are all both shear thinning and viscoelastic, the critical draw ratio value can be as low as 3 (27). The maximum-to-minimum diameter ratio decreases with decreasing draw ratio and decreasing draw-down length. 

In the Ref [49] the two deformation-strength characteristics prediction was carried out strain up to fracture and fracture stress 0. For the value theoretical estimation two methods can be used. The first from them does not include in the calculation of molecular characteristics and, hence, does not take into account their change, in any case, directly [51], This method is based on the cluster model of polymers amorphous state structure notions [ 14], taking into account the order availability, and the limiting draw ratio value in this case is given as follows [51] ... 

The copolymer fiber shows a high degree of drawabiUty. The spun fibers of the copolymer were highly drawn over a wide range of conditions to produce fibers with tensile properties comparable to PPT fibers spun from Hquid crystalline dopes. There is a strong correlation between draw ratio and tenacity. Typical tenacity and tensile modulus values of 2.2 N/tex (25 gf/den) and 50 N/tex (570 gf/den), respectively, have been reported for Technora fiber (8). 

The existence of the amorphous phase of the fiber is confirmed in x-ray examination by the occurrence of a distinct intensity maximum of the radiation scattered diffusively at 2Q = 21.6 . The fraction of the amorphous phase in the fiber depends on manufacturing conditions and a possible further refining treatment. It is estimated to vary from 0.25 to 0.60. With an increase of the draw ratio and following the thermal treatment of the fiber, the proportion of the amorphous phase only reaches the lower values of this interval,... 

The crystallite orientation in PET fibers depends first and basically on the applicated draw ratio and sec ond on the stretching rate. The values off. characteristics for PET fibers as established by the authors are in Table 5. 

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. 

Fig. 5. Values of P2 ir for the 875 cm"1 band as a function of opt. samples drawn to a series of draw ratios at 80 °C. O samples drawn to draw ratio 4 1 at different temperatures. Reproduced from Journal of Polymer Science by permission of the publishers, John Wiley Sons Incs (C)...

Kashiwagi et al.10) determined the second moment anisotropy for the one-way drawn polyethylene terephthalate sheets discussed above. The three lattice sums S00, S2q and S4o were calculated from the crystal structure determination of Daubeny et al., the proton positions being calculated on the basis of known bond angles and lengths. The isotropic lattice sum S00 was adjusted to a value consistent with the measured isotropic second moment of 10.3G2. The values for P200, P220 etc. were then used to predict the optical anisotropy. The predicted refractive indices for the sheets of draw ratio 2 1 and 2.5 1 are shown in Fig. 10, together with the experimental... 

Fig. 19a. Microhardness values parallel ( ) and perpendicular (X) to the orientation axis as a function of draw ratio, for PE drawn at different temperatures 60 °C (O), 100 °C ( ) 120 °C (A) 130 °C ( )12 b Microindentation anisotropy of the above drawn samples vs. draw ratio...

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. 

According to his deduction the common finding of ellipsoidal deformation of the reflections is indicative for affine deformation. Moreover, he arrives at an equation that permits to determine with high accuracy the microscopical draw ratio, Xd, of the structural entities from the ellipticity of the deformed Debye sphere. This value can be compared to the macroscopical draw ratio. Even the intensity distribution along the ellipsoidal ridge is predicted for a bcc-lattice of spheres, and deviations of experimental data are discussed. 

This model is incorrect because the linear thermal expansivity for both components in the isotropic and oriented state is assumed to be the same. The concequences of this assumption are quite different for Px and For px, it does not play any essential role, because in the isotropic and oriented state perpendicular to the draw axis the thermal expansivity is determined solely by intermolecular interactions. For P, this suggestion may lead to a principal inconsistency. This conclusion is evident from comparison of the calculated and experimental -dependences of P and P for PE and PP according to Eqs. (107) and (108). For Px, the agreement between the model calculation and the experiment is quite satisfactory for all draw ratios. On the other hand, Eq. (108) does not describe the X-dependence of P( at all. This equation does not yield negative values of P even in case of a limited orientation of crystallites (fc = 1) because it is based on the suggestion that Pam is always positive and pam > Pcr. ... 

The electrostriction constant of roll-drawn PVDF was measured by Nakamura and Wada (1971) and the result is given in Fig. 25. The value of k is greatest when stretched along the draw-axis. The ratio of e values at 6 = 0° and 0 = 90°, (e13/e12), is about 8 for the film of draw ratio a = 1.6,... 

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

Figure 8.9 Stress-strain isotherms of the PDMS-PS composites described in the preceding figure.59 Values of the draw ratio and testing directions are indicated on each curve. The symbols with small tabs attached represent data used to test for reversibility.

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