Semicrystalline polymers oriented

Semicrystalline polymers, VDC copolymer and aromatic nylon MXD-6 (Table II) showed little if any reduction in permeability at these moderate orientation levels. In fact, recent unpublished work has shown that aromatic nylon MXD-6 exhibits an initial increase in permeability up to 3X orientation followed by a significant reduction in permeability at higher orientation levels. The VDC copolymer also showed higher permeability with moderate biaxial orientation — 1.5 times the permeability of the unoriented film. This is believed to be due to orientation of the polymer after crystallinity is fully developed. If the orientation of VDC copolymers is induced prior to full development of crystallinity in the material, one would not expect to see an increased oxygen permeability. In commercial practice, therefore, forming of VDC copolymer structures is normally done on rapidly quenched polymer to orient it while still in the amorphous state at temperatures near or above the Tm of VDC copolymer. 

A., Boiteux, G., Seytre, G., and Schonhals, A. (2003) Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). 11. Semicrystalline oriented films. Polymer, 44, 4311-4323. 

The presence of crystalline regions tends to rednce the level of light transmission, and pnre semicrystalline polymers in moderate thickness are generally translucent. They include the polyolehns, polyamides, and thermoplastic polyesters. However, several crystallizing polymers can be made into highly transparent, relatively thick prodncts. They are polymethylpentene and polyethyleneterepthalate. Films of many crystallizing polymers, particnlarly oriented hhns, can also be transparent See also optical properties. 

The most useful synthetic fibers for textile applications are linear, semicrystalline, oriented polymers, whose properties are defined by molecular structure and molecular organization. The first level of molecular organization is the chemical structure that defines the structure of the repeating imit in the base polymer... 

Drawing products in their solid state (monofilament production, uniaxially oriented film, or biaxi-ally oriented film) maximizes molecular orientation and directional properties. In semicrystalline polymers drawing can lead to additional crystallinity development through alignment of the polymer molecules. The ability to successfully draw and process the polymer into oriented fibers, films, and profiles is often dependent on the quenching conditions used to form the precursor fiber, sheet, or profile. 

Fibers are thin threads produced by extruding a molten polymer through small holes in a die, or spinneret. The fibers are then cooled and drawn out, which orients the crystallite regions along the axis of the fiber and adds considerable tensile strength (Figure 31.3). Nylon, Dacron, and polyethylene all have the semicrystalline structure necessary for drawing into oriented fibers. 

Stretched Polymers MF membranes may be made by stretching (Fig. 20-68). Semicrystalline polymers, if stretched perpendicular to the axis of crystallite orientation, may fracture in such a way as to make reproducible microchannels. Best known are Goretex produced from Teflon , and Celgard produced from polyolefin. Stretched polymers have unusually large fractions of open space, giving them very high fluxes in the microfiltration of gases, for example. Most such materials are very hydrophobic. 

The properties of a semicrystalline polymer are controlled by its degree of crystallinity, the alignment of crystallites relative to one another, the number and type of links between the crystallites and amorphous regions, and the overall orientation of molecules within the material. 

Oriented polypropylene is used for clear films used as food wrap. What advantageous properties do the oriented films have over standard films How can a polymer that is, typically semicrystalline be transparent ... 

Nylons are semicrystalline polymers whose properties are controlled primarily by their amide concentration, molecular orientation, crystallization conditions, and the level of absorbed water. As discussed earlier, the level of crystallinity and hence product stiffness, is maximized by high concentrations of amide groups, high orientation, slow cooling, and the absence of absorbed water. 

We have adapted a commercially available x-ray diffractometer normally used for structure determinations on single crystals to operate as a very flexible device for performing x-ray pole figure determinations and related studies on polymeric materials. Descriptions of crystallite orientations, as provided by pole figures, are useful in studying many aspects of the behavior of products made from semicrystalline polymers. This paper describes the software that we have written for our pole figure facility. Except for some vendor-provided routines to drive the hardware Interface all of our software is written in FORTRAN. Menu driven operation is provided to maximize user convenience. 

Processes such as film extrusion, fiber spinning, injection molding, and drawing tend to impart orientation to products made from semicrystalline polymers. Mechanical, dielectric, and optical properties, to mention only three, are often strongly influenced by orientation. X-ray diffraction offers a direct approach to studying crystallite orientation because the Intensity that is diffracted into a detector placed at an appropriate position is directly proportional to the number of crystal lattice planes that are in the correct orientation for diffraction. The principles of such measurements are well described in textbooks 0,2). 

Although the (P2) coefficient is a convenient index to characterize the orientation and is commonly measured by WAXD for semicrystalline polymers, it is only an average value and different distributions of orientation could give the same (P2) coefficient (Section 2). It has been shown that higher (P ) coefficients are required to fully characterize the distribution of orientation [83]. This is... 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

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