Stiffness polyimide

Nylon, polyacetal, polycarbonates, poly(2,6-dimethyl)phenylene oxide (PPO), polyimides, polyphenylene sulfide (PPS), polyphenylene sulfones, polyaryl sulfones, polyalkylene phthalates, and polyarylether ketones (PEEK) are stiff high-melting polymers which are classified as engineering plastics. The formulas for the repeating units of some of these engineering plastics are shown in Figure 1.15. 

These plastics have high strength and stiffness and are particularly suitable for the manufacture of microwave dishes, hot water containers and other household articles. The uptake of water by polysulfones influences the mechanical properties in a similar manner to PA. One obtains particularly good heat resistance with polyimide plastic... 

In a rubbery polymer with flexible macromolecular chains (PDMS for example) the cavities forming the free-volume are clearly separated from each other. The detailed evaluation of the movement of a penetrant particle from cavity (1) to the neighboring (2), did not show any immediate back jumps (2) — (1). This is mainly do to the fact that the channel between (1) and (2) closes quiet quickly. In a polymer with stiff chains (glassy polyimide (PI) for example) the individual cavities are closer to each other and a rather large number of immediate back jumps ocurred during the time interval simulated (120). This indicates that once a channel between two adjacent cavities in a stiff chain polymer is formed it will stay open for some 100 ps. This makes the back jump (2) - (1) of the penetrant more probable than a jump to any other adjacent hole (3). This process seems to be one cause for the general tendency that the diffusion coefficient of small penetrants in stiff chain glassy polymers is smaller than in flexible chain rubbery polymers. 

New heterocyclic polymers designed especially for service at elevated temperatures have intriguing properties, some of which are in contrast to properties usually associated with linear noncrystalline polymers. These polymers have sometimes been described as stiff chains because of the long inflexible repeat units of which they are comprised. Relatively few quantitative studies have yet appeared in the dilute solution properties or the viscoelastic behavior of the new heterocyclic polymers—partly because of the difficulties inherent in working with the poorly soluble materials. Some studies on the polyimide with the (idealized) structure ... 

The chemical and physical properties of each of these window materials vary widely. For example, polyimide is flexible, semitransparent, and chemically inert, but it has an upper working temperature of 673 K (for information about the properties of Kapton see http //www2.dupont. com/Kapton/en US / assets / downloads / pdf/ summaryofprop.pdf). Beryllium is stiff, has a low density, high thermal conductivity, and a moderate coefficient of thermal expansion it can be machined and is very stable mechanically and thermally. It also retains useful properties at both elevated and cryogenic temperatures. However, it does require a few safety-related handling requirements that are well documented (for detailed environmental safety and health information about beryllium see http //www.brushwellman.com). Nonetheless, as is stated in the Brush Wellman literature (for detailed environmental safety and health information about beryllium see http //www.brushwellman.com), "handling beryllium in solid form poses no special health risk."... 

Teff = Too + uj((T)/Aa for Too > T. A high Tg results in better long-term stability, c.f. polymer Fig. 3.24(b) and Table 3.4, but at the expense of lower non-linearity due to the greater difficulty in poling polymers with stiff backbones, such as polyimides. The application of these materials in devices is considered in Chapter 10, Section 10.2.3. 

In this section, we describe a solution that employs a net-shaped structure to make flexible electronic film devices conformable to three-dimensional surfaces [18]. Although the base films we presently use are of polyimide and poly(ethylenenaph-thalate) (PEN) — materials that are stiff and not inherently stretchable in a rubberlike sense — our solution includes struts of network structures that twist with the application of tension, as can be seen in Figure 6.3.7. Due to this three-dimensional strut deformation, the whole network structure functions electrically with a unidirectional extension of 25%. We have implemented the pressure sensor network on the surface of an egg and have obtained pressure images in this configuration. 

TABLE 20.4-6 Permeabilities and Pemseabilily Rales at 30°C for Various Stiff-Chein Polyimides... 

There is a great diversity of polyimides (PI) having = 180-420°C. Several were blended with PC to improve its stiffness, HDT and strength. PEEPC blends were commercialized in 1992 as Ultem LTX, for injection molding or extrusion. They show higher impact resistance than PEI and higher heat resistance than PC, as well as they retain the stain, chemical resistance, and the hydrolytic stability of PEI. 

The polyester-imides constitute a class of modified polyimide. These are typified by the structure shown in Figure 4.23. Polyether-imides form yet another class of modified polyimide. These are high-performance amorphous thermoplastics based on regular repeating ether and imide linkages. The aromatic imide units provide stiffness, while the ether linkages allow for good melt-flow characteristics and processability. 

To clarify the mechanisms of plastic flow of linear-chain polymers we also consider polyimides (PIMs), which make up a large family of stiff and thermally stable polymers of outstanding performance (Bessonov et al. 1987). Polyimides contain cyclic imide groups as shown in Fig. 2.4 in the main chain. From this very... 

However the constant effort of polyimide suppliers Is to provide materials which will flow during fabrication of parts below 400 F, but which will retain structural stiffness in use of that part at 550 F. The polymer chain stiffness which affords 550 F physicals wrecks havoc with 350-400 F processing. Even low molecular weight prepolymers which can build during cure to higher molecular weight crossllnked structures often do not flow below 400°F unless a species is chosen which sacrifices 550°F thermo-physical properties. 

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