Thermal shaping

An example of such a catalyst system is racemic isopropylene bis(l-indenyl) zirconium dichloride in combination with an alumi-noxane (21). The reaction is carried out in hydrocarbon solvents, e.g., toluene. A solution of norbornene in toluene with the catalyst is degassed and then pressurized with ethene. The polymerization is carried out while stirring at 70°C under constant ethylene pressure at 18 bar. After completion, the polymer is precipitated in acetone and filtered (21). The cycloolefin copolymers obtained in this way have a high thermal shape stability and it is possible to use the polymers as thermoplastic molding compositions. 

The electron field emission photograph of a tungsten point formed by field evaporation at low temperatures, shown in Fig. 58, is dominated by the extensive nonemitting (110) plane. The emission comes primarily from the higher index planes around the 100 s with a much smaller contribution from the triangular llljs. The 100 poles, as well as the (211 s, are only ill defined. This is in contrast to the usual thermally shaped emitters, in which the 100, as well as 211 poles appear as definite and prominent areas of low emission. 

TABLE 16.2 Comparison between Two Mechanical Properties of Different Actuating Materials Skeletal Muscles, Thermomechanical (Thermal Liquid Crystals and Thermal Shape Memory Alloys), Electrochemomechanical (Conducting Polymers and Carbon Nanotubes) and Electromechanical (Ionic Polymer Metal Composites, Field Driven Liquid Crystal Elastomers, Dielectric Elastomers)... 

At this time all commercial vena cava filters are fabricated firan ehber nitinol (nickel, cobalt, and titanium alloy), phynox (nidcel, cobalt, chromium, iron, and molybdenum alloy), or stainless steel wire in various catfaeter sheath sizes raiigii fiom 7-14 French (23 - 4.7 mm diameter). These metals were chosen because fiiey are biostable and have low thrombogenicity. This means that blood is less likely to form clots cm the surfiice, and cells are not encouraged to grow around the device. Moreover, nitinol is a thermal shape memory alloy that is self-expanding afier deployment and exposed to txxfy temperature(l]. 

When the material is heated above the melting point of the soft segment, but below glass transition temperature of the hard segment, the stresses and strains are relieved and the material returns to its original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabU-... 

Thermal shape fluctuations are determined by the interplay between available thermal energy and the energetic cost of deformation. The lower part of Figure 7.5 shows the energy curves for quadrupole ( 2) and octupole deformation as compared to the... 

A thermal shaping and recovery study of the D,L-PLA/PCL PDO blends was conducted by immersing the coupons in OO C saline for I minute and then shaping the coupons to a predetermined curvature. The thermo-shaped coupons were then placed in 37°C saline and the curvature recovery was recorded for up to 30 hours. 

Moreover, the addition of PCLrPDO and its subsequent low levels of crystallinity affect the plate s thermal shaping and creep recovery character. One would like to have plates that when shaped resist creeping back to their original shape. Thus, flex coupons were shaped by immersing in 60°C water for one minute over a 2 inch diameter mandrel. One end of each of the flex coupons was then attached to the mandrel with a metallic screw and creep recovery changes were recorded. Results are shown in Table 1. 

Light activation has a significant advantage in comparison with the thermal shape memory behaviour,as photo-activation does not produce tissue damage as could be produced by heat treatments. Ihis fad makes light activation attradive... 

Figure 9 is a stress-strain curve for a single-crystal specimen of a Cu-39.1Zn shape-memory alloy deformed in tension at about 50 °C above its temperature (Schroeder and Wayman, 1979). Yielding at an essentially constant stress (upper plateau) corresponds to the formation of 9R stress-induced martensite (SIM) from the B2 parent. At about 9 7o strain the specimen becomes fully martensitic. When the stress is released, the strain follows the lower plateau and fully recovers as the SIM reverts to the parent. This behavior corresponds to a mechanical (as opposed to a thermal) shape memory. A stress-strain relationship such as that shown in Figure 9 is frequently referred to as a superelastic stress-strain loop. The stress necessary to... 

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