Polyimides crystals

Liquid-crystal (79), advanced polyimide, and amorphous-nylon resin technologies (60) will be appHed. 

Over the years polymers have been produced suitable for use at progressively higher temperatures. Where this is a requirement, it is usual first to decide whether a rubbery or a rigid material is required. If the former, this has been dealt with by the author elsewhere." If the latter, it is usually convenient to look in turn at polycarbonates, PPO-based materials, polyphenylene sulphides, polysul-phones, polyketones such as PEEK and PEK, polyamide-imides, poly-phthalamides, fluoropolymers, liquid crystal polymers and polyimides. 

Crystalline polyimide powders, 304 Crystalline transition temperature. See Melting temperature (Tm) Crystallization rate, for processing semicrystalline polymers, 44 CTE. See Coefficient of thermal expansion (CTE)... 

Solution blending Polar as well as nonpolar solvents can be used in this method. The polymer is solubilized in a proper solvent and then mixed with the filler dispersion. In solution, the chains are well separated and easily enter the galleries or the layers of the fillers. After the clay gets dispersed and exfoliated, the solvent is evaporated usually under vacuum. High-density polyethylene [24], polyimide (PI) [25], and nematic hquid crystal [26] polymers have been synthesized by this method. The schematic presentation is given in Scheme 2.2. 

NAJM Van Aerie, M Barmentlo, and RWJ Hollering, Effect of rubbing on the molecular orientation within polyimide orienting layers of liquid-crystal displays, J. Appl. Phys., 74 3111-3120, 1993. 

Aerosol spray delivery, 23 196 Aerosol sprays, 7 773-774 Aerospace applications aluminum alloys, 2 340 artificial graphite in, 72 740-741 for high performance fibers, 13 397-398 of liquid-crystal polymers, 20 85 metal-matrix composites in, 16 191 polyimide matrix composites in, 20 284 Aerospace bearings, corrosion resistance of, 74 452... 

Structural steels, tellurium in, 24 425 Structure(s), see also Chain structure Chemical structures Cocontinuous structures Controlled structure Crystal structure Molecular structure Morphology Phase structure of carbon fibers, 26 737-739 detersive systems for, 8 413t HDPE, 20 157-162 LLDPE, 20 182-184, 203-205 polyesterether elastomer, 20 72-73 polyester fiber, 20 21 polyether antibiotics, 20 137-139 polyimide, 20 276-278 polymer, 20 395-405 protein, 20 449 PTT, 20 68t... 

Next we consider the dispersion polymerization by polyaddition. In a typical method to prepare polyimide particles, polyamic acid solution is first obtained by coupling of pyromellitic dianhydride and oxy-dianiline, and then by heating the solution. The condensation reaction on heating causes crystallization of polyimide in a spherical form (Fig. 11.2.5, left) (33). However, on the contrary to this conventional method, polyamic acid microspheres could be obtained by dispersion polymerization if an appropriate medium is chosen (34). When a solvent that has a solubility parameter around 17 Mpa is used, submicrometer-sized monodisperse polyamic acid parti-... 

The structural anisotropy in crystalline or structurally ordered BPDA-PFMB films was studied in this laboratory with wide-angle X-ray diffraction (WAXD) methods. In brief, WAXD experiments were designed to examine both the reflection and transmission modes of thin-fihn samples. In addition, uniaxially oriented polyimide fiber WAXD patterns were obtained to aid in the identification of the film structure. The film WAXD pattern obtained from the reflection mode corresponded well to the fiber pattern scanned along the equatorial direction (Figure 16.3), " which indicates that the reflection mode pattern represents the (hkQ) diffractions. On the other hand, as shown in Figure 16.4, the (001) diffractions were predominant in the film WAXD pattern obtained via the transmission mode. This pattern corresponded to the fiber pattern scanned along the meridian direction. These experimental observations clearly indicate that the c-axes of the crystals are preferentially oriented parallel to the film surface however, within the film, they are randomly oriented. 4.2 5 j( should be pointed out that the WAXD experiments are only sensitive to crystalline or ordered structures in polyimide films. They do not provide any information on the amorphous regions. 

PFMB can be used to prepare aromatic polyimides that display solubility in ketone, ether, and polar aprotic solvents. This unusual solubility can be utilized in die facile preparation of thin films that display anisotropy in their structures and properties. The anisotropy in the optical properties of the films makes them promising candidates for use as compensation layers in liquid-crystal displays. Their low dielectric constants and CTEs in combination with their outstanding thennal and thermooxidative stabilities make diem candidates for dielectric layers in microelectronics applications. 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

Fig. 31. Photosensitivity spectra of the photoconductive polyimide-licjuid crystal modulator for positive right) and negative (left) bias on the photoconductor. Voltages in V 80 (/,/ ) 20 (2,2) [259]...

Fig. 33. Diffraction efficiency versus voltages for spatial frequencies in mm-1 10 (/), 28 (2), 51 (i). Switch on time versus voltages at spatial frequencies in mm"110 (/ ), 36 (2 ), 60 (S ). Commutational regime for polyimide-liquid crystal modulator with controlled birefringence [256]...

Fig. 34. Diffraction efficiency versus spatial frequency for electrically controlled birefrigence liquid crystal-polyimide modulator in pulse regime [257]...

Fig. 35. Modulation function of the polyimide modulator with thickness of the liquid crystal in pm 20 (/), 15 (2), 10 (3), 3 (4). Anisotropy of the dielectric permittivity of 12 (2,2,3) 0.05 (4). Polyimide thickness - 2 pm. Pulse regime [14]...

The polymer-dispersed liquid crystal was used as a modulating medium in optically controlled modulators instead of the liquid crystal [261-264], The sandwiched structure from polyimide photosensitive film and the polymer dispersed liquid crystal film - i.e. the optically controlled solid state modulator -had the characteristics presented in Fig. 36 [261]. Contrast ratio 35 1, response time > 400 ps, decay time 80 ms and sensitivity 5 x 10 sJcm 2 were obtained. 

Besides polyimides, photoconductive polymers with conjugated bonds also can be successfully used in liquid crystal spatial light modulators [262-264], The high resolution, mechanical and electrical stability and possibility for sensitization of the photosensitivity make the use of organic polymer photoconductors in spatial light modulators very attractive. Devices such as phase... 

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