Stretching orientation

Carbon and Graphite Fibers. Carbon and graphite fibers (qv) are valued for their unique combination of extremely high modulus and very low specific gravity. Acrylic precursors are made by standard spinning conditions, except that increased stretch orientation is required to produce precursors with higher tenacity and modulus. The first commercially feasible process was developed at the Royal Aircraft Fstablishment (RAF) in collaboration with the acrylic fiber producer, Courtaulds (88). In the RAF process the acrylic precursor is converted to carbon fiber in a two-step process. The use of PAN as a carbon fiber precursor has been reviewed (89,90). 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

Sheet forming processes, such as vacuum forming, do have effects on the product. The designer should be aware that these will affect the performance of one s product and one should learn how to modify the design to minimize any deleterious effects. Probably the most serious problem encountered in formed film or sheet products results from the fact that the materials are made from film or sheet at temperatures well below the melt softening point of the plastic, usually near the heat distortion temperature for the material. Forming under these condition when the draw down ratio is exceeded for a specific plastic can result in over stretched orientation of the material, the production of frozen-in stresses, poor product reproducibil-... 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

Figure 5. Room-temperature, wide-angle X-ray diffraction (WAXD) pictures of stretch-oriented samples of PDHS (left) and PDPS (right).

Durham polyacetylene occurs in a highly disordered state on conversion from the precursor polymer [90], but using stretch orientation techniques during the conversion reaction, a high degree of order with long conjugated sequences can be achieved [91-93],... 

EL with 4>el = 0.05% was observed in multilayer LB-film PLED ITO/399/A1, with a ratio in EL between the parallel and perpendicular orientations of 1.3 [523]. An even higher ratio of 2.4 was achieved in ITO/422/Ca/Al diode made from a stretch-oriented polymer film [524]. For more information on polarized LEDs, see Chapter 5 in this book. 

The polycarbonate membranes are stretch-oriented during fabrication in order to improve their mechanical properties. If the membrane is subsequently heated above its glass-transition temperature ( 150°C), the polymer chains relax to their unstretched conformation and the membrane shrinks. This shrinking of the membrane around the Au nanowires in the pores causes the junction between the nanowire and the pore wall to be sealed. This is illustrated in Fig. 5, which shows voltammograms for tri-methylaminomethylferrocene (TMAFc+) before (Fig. 5A) and after (Fig. 

The structures of crystalline polymer-salt complexes provide insight into the structure of the more conducting amorphous materials. To date, large single crystals of polymer-salt complexes have not been prepared, but it has been possible to obtain structural information from single crystal X-ray diffraction applied to stretched oriented fibres in the PEO NaI and PEOiNaSCN systems (Chatani and Okamura, 1987 Chatani, Fujii, Takayanagi and Honma, 1990). One of the most detailed studies is of (PEO)3 NaI, Fig. 5.11(a). The sodium ion in this structure is coordinated to both the polymer and to the iodide ion and the polymer is coiled in the form of an extended helix. 

It is difficult to prepare stretched oriented fibres and such fibres may differ in their polymer chain conformation compared with the unstretched materials. Furthermore the quality of the single crystal X-ray data is poor and difficult to interpret. In contrast power X-ray data of relatively high quality may be obtained from polycrystalline polymer films. Lightfoot, Mehta and Bruce (1992) have obtained the first crystal structure of a polymer-salt complex, PEOjiNaClQ from powder X-ray data. Fig. 5.11(h). The structure is similar to the corresponding PEOjiNal structure, the PEO chains are wrapped around the Na ions with each Na ... 

The largest component of the Y-tensor is in the conjugation direction. Therefore,.even though no particular bulk symmetry is required for nonzero x > a medium in which all conjugated polymeric chains align in the same direction should have a larger x value along the chain direction relative to that in an amorphous or disordered form of the same polymer. Studies of x in ordered or stretch-oriented polymers as discussed below confirm this prediction. Finally, the polymeric chains should pack as closely as possible in order to maximize the hyperpolarizability density and hence x ... 

Fig. 2.2. Polarized transmittance (dashed line) and near-normal-incidence reflectance (continuous line) spectra of highly stretch-oriented PPV...

Extensional viscometers are useful to measure tensile viscosity in processes such as stretch orientation. 

Stretch blow molding holds the parison above its glass transition temperature (7 ) and stretch-orients it to increase modulus, strength, impact resistance, transparency, and impermeability. This is most important for PET, and is also used for PVC, polypropylene, and polyacrylonitrile. 

Sheet can be produced by extrusion or calendering. For use in thermoforming, the sheet must be extremely uniform. For improved properties, the sheet may be biaxially stretch-oriented up to 300 percent. For large automatic production runs, it is fed continuously off a roll for short runs, large, and/or specialty jobs, it may be cut into individual sheets and fed one at a time. 

PVOH (or tradename PVAL) is a crystalline, white powder soluble in water and alcohols. It is characterized by water solubility, low gas permeability barrier, high resistance to organic solvents other than alcohol, and crystallinity when stretch oriented. Crystallinity allows the material to polarize light. A series of hydrolysis levels of the plastic are available that range from room temperature solubility to those not soluble at all. The major applications of the PVOHs are in elastomeric products, adhesives, films, and finishes. Extruded PVOH hoses and tubing are excellent for use subjected to contact with oils and other chemicals. PVOH is used as a sizing in the manufacture of nylon. 

An interesting aspect of the work on UHMW polyethylene was the possibility of studying the anisotropy of the conductivity since aligned samples of stretch-oriented fibers could be produced and the effect of fiber orientation investigated. These experiments showed charge transport to be favored by a factor of approximately 20 along the direction of the oriented fibers compared with perpendicular to the fibers. This represented the first measurement of anisotropic conduction using the PR-TRMC technique. 

These results support current interpretations of the bathochromic shifts observed in dialkyl-substituted poly silane. Experimental results for poly(di-n-hexylsilane) indicate that as the temperature is cooled below a transition temperature of roughly -35 °C, the major absorption peak shifts from a broad peak at about 310-320 nm (3.9-4.0 eV) to a narrower peak at about 350-370 nm (3.3-3.5 eV), with the red shift being attributed to a transition from a disordered system with a large population of gauche bond twists in the silicon backbone and in the alkyl substituent to a planar dll-trans backbone conformation (5-8, 15). Results from polarized absorption spectra of stretch-oriented samples for the cooled samples exhibit absorbance only for polarizations parallel to the stretch (and presumably the chain axis) direction (22). 

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