Polyimides pressure, effect

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

VDP of polyimides is usually performed in vacuum (pressures <10 Pa). Just as in any CVD process, the deposition parameters greatly influence the properties of the polyimide thin films. The effect of a few notable ones viz., substrate temperature and the relative fluxes of the precursor vapors is the focus of the next section. 

Table 6. Effect of upstream pressure on selectivity for hydrogen relative to carbon monoxide in assy polyimide film...

Table 7. Effect of upstream pressure on the permeability of methane and oxygen in glassy polyimide and glassy Mylar...

In oil processing, separation of aromatic isomers Cg (ethylbenzene 7b= 136°C,p-xylene 7b= 138.3°C, m-xylene Ty, = 139.1°C, >-xylene T], = 144.4°C) is required. According to the literary data, the following isomers of hydrocarbons are separated p-xylene/m-xylene, p-xylene/o-xylene, -hexane/2,2-dimethylbutane, -hexane/3-methylpentane, and n-butane/f-butane [8,83,130-137]. Pervaporation method is the most effective for this purpose. To separate the isomers, membranes based on various polymers were used. Good separation for aU isomer mixtures was attained by the polyimide Kapton film (fip = 1.43-2.18) but parylene films and cellulose acetate also exhibited a relatively high separation factor (fip = 1.22-1.56 and /3p = 1.23-1.56, respectively). Temperatures >200°C were required to obtain a reasonable flux through the polyimide film and a pressure of about 20 atm was necessary to keep the feed stream liquid [8]. 

A quaternary ammonium salt, tetraoctylammonium bromide (TOABr) and doco-sane were used as solutes and toluene was used as the solvent. The membrane used in this study was again the solvent-resistant polyimide membrane, STAR-MEM 122. A dual-cell crossflow filtration rig, similar to the one described in Section 4.3.1 was used in all the experiments with an effective membrane area of 78 cm. The stage cut was between 0.01 and 0.3% over the whole concentration and pressure range. Ideal mixing is assumed throughout the system. 

Aromatic nitriles may be trimerized at moderate temperature and pressure with p-toluenesulfonic acid as catalyst. Studies were conducted to establish the effect of the reaction temperature, pressure, time, and catalyst concentration on yield of the trimerized product. Trimerization studies were also conducted to establish the effect of substituting electron donating or withdrawing groups on benzonitrile. Preliminary results of using the catalytic trimerization approach to prepare s-triazine cross-linked polyimide/graphite fiber composites are presented. 

S. Matsui, T. Nakagawa, Effect of ultraviolet light irradiation on gas permeability in polyimide membranes. II. Irradiation of membranes with high-pressure mercury lamp, J. Appl. Polym. 

Snch shape of the Pip) curves is typical for glassy polymers. The left, low pressure branch of the curve is explained by sani-empirical dual-mode sorption and mobility model. The right branch that shows increases in permeability with increasing pressure is usually explained by the plasticization effects (see Ref. 19, p. 24). There is, however, one interesting peculiarity of PTMSN in conventional glassy polymers such as polyimides, polycarbonates, etc. the pressure p a where the curve passes through a minimum amount... 

C. Hibshman, M. Mager, E. Marand, Effects of feed pressure on fluorinated polyimide - orga-nosilicate hybrid membranes, J. Membr. ScL, 229, 73-80 (2004). 

Figure 11.15 The effect of an addition of hexane to a mixture of methane (90%) and carbon dioxide (10%) at a feed pressure of 1025kPag. (a) CO2 permeability, (b) CH4 permeability, (c) CO2/CH4 selectivity. Reprinted with permission from Ind Eng Chem Res., The effect of hydrocarbons on the separation of carbon dioxide from methane through a polyimide gas separation membrane by Hasan, R., C. A. Scholes, C. W. Stevens and S. E. Kentish, 48, 5415-5419, Copyright (2009) American Chemical Society...

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