Transverse uniaxial orientation

Isotropization in the Case of Fiber Symmetry. If methods for the analysis of isotropic data shall be applied to scattering patterns with uniaxial orientation, the corresponding isotropic intensity must be computed. By carrying out this integration (the solid-angle average in reciprocal space) the information content of the fiber pattern is reduced. One should consider to apply an analysis of the longitudinal and the transversal structure (cf. Sect. 8.4.3). 

Uniaxially oriented films generally have high strength in only one direction, whereas biaxially oriented films are relatively strong in both machine direction (MD) and transverse direction (TD). Although intuitively one might expect biaxially oriented films to be preferred over uniaxially oriented films. 

Figure 11.6. Schematic illustrations of brittle fracture, (a) Idealized limiting case of perfectly uniaxially oriented polymer chains (horizontal lines), with a fracture surface (thick vertical line) resulting from the scission of the chain backbone bonds crossing these chains and perpendicular to them. This limit is approached, but not reached, in fracture transverse to the direction of orientation of highly oriented fibers, (b) Isotropic amorphous polymer with a typical random coil type of chain structure. Much fewer bonds cross the fracture surface (thick vertical line), and therefore much fewer bonds have to break, than for the brittle fracture of a polymer whose chains are perfectly aligned and perpendicular to the fracture surface, (c) Illustration of a defect, such as a tiny dust particle (shown as a filled circle), incorporated into the specimen during fabrication, which can act as a stress concentrator facilitating brittle fracture.

A uniaxially oriented sample of linear polyethylene (77) also showed the y maximum. In addition, a study (48) has been made on a rodlike sample of Marlex 50 prepared by heating at 175°C. for 119 hours under 40,000-p.s.i.g. pressure it is presumed that this treatment leads to a sample containing principally extended chains. Using a transverse vibration... 

A particular sample of a uniaxially oriented polymer is composed of structural units each of which is transversely isotropic with respect to an axis Oz within the unit. The value of (cos 0) is 0.65, where 6 is the angle between the Oz axis of a typical unit and the draw direction. There are 4.2 x 10 structural units per m in the polymer and the polarisabilities of an individual unit for light polarised parallel and perpendicular to Oz are 2.6 x 10 and 1.9 x 10 F m, respectively. Calculate the polarisabilities of the sample for light polarised parallel and perpendicular to the draw direction and hence the birefringence of the sample. 

Orientation A process of drawing or stretching of as-spun synthetic fibers or hot thermoplastic films to orient polymer molecules in the direction of stretching. The fibers are drawn uniaxially and the films are stretched either uniaxially or biaxially (usually longitudinally or longitudinally and transversely, respectively). Oriented fibers and films have enhanced mechanical properties. The films will shrink in the direction of stretching, when reheated to the temperature of stretching. 

In the uniaxially oriented case, the system is transversely isotropic and the dielectric constant tensor reduces to... 

TD = Transverse direction, and direction of uniaxial orientation. See the appendix for English to metric conversions. 

Transverse direction n. (1) In extruding sheet or film, the direction of the width, crosswise to the direction of extrusion. (2) In a uniaxially oriented plastic, either direction perpendicular to the direction of orientation (stretching). In a biaxially oriented sheet, the direction perpendicular to both axes (the plane) of orientation. (3) In a fiber-reinforced laminate the thickness direction or, in a laminate with unidirectional reinforcement, either of the two directions perpendicular to the fiber lengths. 

Biaxial Orientation. Biaxial orientation substantially improves the physical properties of the film and increases its commercial value. Biaxially oriented films are produced by stretching polymers in directions perpendicular to each other in the melt or rubbery state. The improvement over uniaxially oriented films is due to the redistribution of c-axis chain orientation (36). This results in a film with enhanced physical properties in both directions. The direction perpendicular to the MD is defined as the transverse direction (TD). 

Film Formation and Propoties.— Work on film preparation and subsequent processing of 2GT has been limited to studies on the transverse constriction and stresses observed during uniaxial orientation and the problems associated with the maintenance of planarity during heat relaxation. Orientation and shrinkage have been discussed in a number of papers with studies on orientational setf-ieinforcement and relaxation and its effects on orientation. The effect of the interaction of films with organic liquids has promoted studies on the structural arrangements in liquid-induced crystallization of cold-drawn 2GT films, and the effect of crystallization on adhesion and cohesion. ... 

It should be noted that the properties of oriented films show considerable dependence on the processing conditions (e.g., biaxially oriented films are obtained by blow molding, while extrusion produces uniaxially oriented films). This is important because the mechanical properties of oriented specimens are considerably different in the flow and transverse directions of the films as shown hi Fig. 4.22 (measurements were done by a traditional tensile tester). When the film samples in the form of strips are prepared for TMA measurements, the strips should always be cut from the same orientation (Menczel et al. 1997). When preparing such samples, film widths of >2 mm are recommended in order to minimize edge effects. 

Figure 4.23 schematically shows TMA curves in the machine and transverse directions for film stretched in one dimension to produce uniaxial orientation. In the machine or stretch direction, the film gradually shrinks up to the melting... 

Torsion of oriented polymer sheets was undertaken by Raumann [14] to determine the shear compliances S44 and See for uniaxially oriented (transversely isotropic) low-density polyethylene. Torsion of oriented sheets can also be used to determine the shear compliances S44, 555 and See for sheets possessing orthorhombic symmetry. As this situation is more... 

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